Fuels and Lubricants Handbook: Technology, Properties, Performance, and Testing George E. Totten, Editor Section Editors Steven R. Westbrook Rajesh J. Shah
ASTM Manual Series: MNL37WCD
ASTM International 100 Barr Harbor Drive PO Box C700 # iMTBttmrmtM. West Conshohocken, PA 19428-2959 Printed in the U. S. A.
Library of Congress Cataloging-in-Publication Data
Fuels and lubricants handbook: technology, properties, performance, and testing/George E. Totten, editor; section editors, Steven R. Westbrook, Rajesh J. Shah, p. cm.—(ASTM manual series; MNL 37) Includes bibliographical references and index. ISBN 0-8031-2096-6 1. Fuel—Testing—Methodology. 2. Fuel—Analysis. 3. Lubrication and lubricants—Analysis. I. Totten, George E. II. Westbrook, Steven R., 1956-III. Shah, Rajesh J., 1969-IV. Series. TP321.F84 2003 662',6—dc21
2003049604
Copyright © 2003 ASTM International, West Conshohocken, PA. All rights reserved. This material may not be reproduced or copied, in whole or in part, in any printed, mechanical, electronic, film, or other distribution and storage media, without the written consent of the publisher.
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Printed in Glen Bumie, MD June 2003
Foreword This publication, Fuels and Lubricants Handbook: Technology, Properties, Performance, and Testing, was sponsored by ASTM Committee D02 on Petroleum Fuels and Lubricants and edited by George E. Totten, G. E. Totten & Associates, LLC, Seattle, Washington. The section editors were Steven R. Westbrook, Southwest Research Institute, San Antonio, Texas; and Rajesh J. Shah, Koehler Instrument Company, Bohemia, New York. This publication is Manual 37 of ASTM's manual series.
Contents Preface—George E. Totten, Steven R. Westbropk, and Rajesh J. Shah
ix
I. PETROLEUM REFINING PROCESSES FOR FUELS AND LUBRICANT BASESTOCKS—^o/esfe / . Shah, Section Editor Chapter 1—Petroleum Oil Refining Marvin S. Rakow
3
II. FUELS: PROPERTIES AND PERFORMANCE— Steven R. Westbrook, Section Editor Chapter 2—Liquefied Petroleiun Gas Roberts. Falkiner
31
Chapter 3—Motor Gasoline B. Hamilton and Robert J. Falkiner
61
Chapter 4—Aviation Fuels Kurt H. Strauss
89
Chapter 5—^Automotive Diesel and Non-Aviation Gas Turbine Fuels Steven R. Westbrook and Richard he Cren
115
Chapter 6—Introduction to Marine Petroleum Fuels Matthew F. Winkler
145
III. HYDROCARBONS AND SYNTHETIC LUBRICANTS: PROPERTIES AND PERFORMANCE—/?a;es/i/. Shah, Section Editor Chapter 7—Hydrocarbon Base Oil Chemistry Arthur J. Stipanovic
169
Chapter 8—Hydrocarbons for Chemical and Specialty Uses Dennis W. Brunett, George E. Totten, and Paul M. Matlock
185
Chapter 9—Additives and Additive Chemistry Syed Q. A. Rizvi
199
Chapter 10—Synthetic Lubricants: Nonaqueous Thomas F. Buenemann, Steve Boyde, Steve Randies, and Ian Thompson
249
Chapter 11—Environmentally Friendly Oils Hubertus Murrenhoff and Andreas
267 Remmelmann
Chapter 12—Turbine Lubricating Oils and Hydraulic Fluids W. David Phillips
297
vi
CONTENTS Chapter 13—Hydraulic Fliiids Willie A. Givens and Paul W. Michael
353
Chapter 14—Compressor Lubricants Desh Garg, George E. Totten, and Glenn M. Webster
383
Chapter 15—Refrigeration Lubricants—Properties and Applications H. Harvey Michels and Tobias H. Sienel
413
Chapter 16—Gear Lubricants Vasudevan Bala
431
Chapter 17—Automotive Lubricants Shirley E. Schwartz, Simon C. Tung, and Michael L. McMillan
465
Chapter 18—Metalworking and Machining Fluids Syed Q. A. Rizvi
497
Chapter 19—Petroleum Waxes G. Ali Mansoori, H. Lindsey Barnes, and Glenn M. Webster
525
Chapter 20—Lubricating Greases Thomas M. Verdura, Glen Brunette, and Rajesh Shah
557
Chapter 21—Mineral Oil Heat Transfer Fluids John Fuhr, Jim Oetinger, George E. Totten, and Glenn M. Webster
573
Chapter 22—Non-Lubricating Process Fluids: Steel Quenching Technology Bozidar Liscic, Hans M. Tensi, George E. Totten, and Glenn M. Webster
587
rv. PERFORMANCE/PROPERTY TESTING PROCEDURES— Steven R. Westbrook and Rajesh J. Shah, Section Editors Chapter 23—Static Petroleum Measurement Lee Oppenheim
635
Chapter 24—Hydrocarbon Analysis James C. Fitch and Mark Barnes
649
Chapter 25—Volatility Rey G. Montemayor
675
Chapter 26—Elemental Analysis R. Kishore Nadkami
707
Chapter 27—Diesel Fuel Combustion Characteristics Thomas W. Ryan HI
717
Chapter 28—Engineering Sciences of Aerospace Fuels Eric M. Goodger
729
Chapter 29—Properties of Fuels, Petroleum Pitch, Petroleum Coke, and Carbon Materials Semih Eser and John M. Andresen
757
CONTENTS Chapter 30—Oxidation of Lubricants and Fuels Gerald J. Cochrac and Syed Q. A. Rizvi
787
Chapter 31—Corrosion Maureen E. Hunter and Robert F. Baker
825
Chapter 32—Flow Properties and Shear StabiUty Robert E. Manning and M. Richard Hoover
833
Chapter 33—Cold Flow Properties Robert E. Manning and M. Richard
879 Hoover
Chapter 34—Environmental Characteristics of Fuels and Lubricants Mark L. Hinman
885
Chapter 35—Lubrication and Tribology Fundamentals Hong Liang, George E. Totten, and Glenn M. Webster
909
Chapter 36—Bench Test Modeling Lavem D. Wedeven
963
Chapter 37—Lubricant Friction and Wear Testing Michael Anderson and Frederick E. Schmidt
1017
Chapter 38—Statistical Quality Assurance of Measurement Processes for Petroleiun and Petroleum Products Alex T. C. Lau
1043
Index
1061
vii
Preface There are many books on various aspects of fuels and lubricant chemistry, applications, and testing. However, few focus on testing and none provide extensive, in-depth coverage on fluid properties and testing methodologies together. And while there are numerous national and international standards that deal with specific testing procedures appropriate for fuels and lubricants, it is generally beyond the scope of these procedures to provide an extensive discussion of the principles behind the tests and their relationship to the properties themselves. Therefore, there is a strong need to address these informational shortcomings in the Fuels and Lubricants industry, which is one of the most significant tasks undertaken in this work. The ASTM Fuels and Lubricants Handbook: Technology, Properties, Performance, and Testing is an extensive, in-depth, well-referenced handbook that provides a detailed overview of various testing methodologies and also provides a thorough overview of the applications-related properties being tested. Since this manual provides an overview of all of the ASTM and important non-ASTM test procedures relating to the application areas addressed, it is an excellent companion text to the Annual Book of ASTM Book of Standards, or it is an invaluable reference manual on its own. The organization of the ASTM Fuels and Lubricants Handbook: Technology, Properties, Performance, and Testing is based approximately on the committee structure of the ASTM D.02 Petroleum Fuels and Lubricants Committee and the standards for which each committee is responsible. The information in this text is subdivided into four sections: I-Petroleum Refining Processes for Fuels and Lubricant Basestocks; II-Fuels; Ill-Hydrocarbons and Synthetic Lubricants; and IV-Performance/Property Testing Procedures. This manual contains thirty-eight chapters covering the following topics: • An overview of petroleum oil refining processes • Liquified petroleum gas (LPG) • Aviation, automotive diesel, non-aviation gas turbine, and marine fuels • Gasoline and oxygenated fuel blends • Petroleum hydrocarbon base oil chemistry • Synthetic hydrocarbons • Environmentally friendly fluids including those formulated from vegetable oil and synthetic ester basestocks • Lubricating oils for turbines, compressors (industrial and refrigeration), gears, and automotive applications • Metalworking fluids • Petroleum waxes • Lubricating greases • Oils used in non-lubricating applications: heat transfer fluids and metal quenchants • Detailed discussion on: static petroleum measurement, volatility, elemental analysis, fuel combustion characteristics, oxidation, corrosion and viscosity • Properties of coke, petroleum pitch, and carbon materials • Hydrocarbon structural analysis procedures • An extensive discussion of lubrication and wear • Environmental and toxicity testing • Statistical quality assurance testing procedures Essentially, all of the numerous important applications and test methods involved in the Fuels and Lubricants industry are discussed and referenced here. We strongly believe that this book will be an invaluable resource for anyone working in this industry, especially since this information is not available in any other single source. ix
X
PREFACE The preparation of a text of this scope was an enormous task involving many people. The editors are deeply indebted to the authors for their h a r d work and incredible patience. Specicd thanks go to Monica Siperko and Kathy Demoga at ASTM for their continued support and encouragement throughout the development of this text. The editors are especially indebted to their families for many evenings and weekends lost to this project. We Eilso acknowledge the vital support of Southwest Research Institute and the Koehler Instrument Corporation. George E. Totten General Editor G.E. Totten & Associates, LLC. Seattle, WA, USA Steven R. Westbrook Section Editor-Fuels Southwest Research Institute San Antonio, TX, USA Rajesh J. Shah Section Editor-Lubricants Koehler Instrument Company Bohemia, NY, USA
Section I: Petroleum Refining Processes for Fuels and Lubricant Basestocks Rajesh J. Shah, Section Editor
MNL37-EB/Jun. 2003
Petroleum Oil Refining Marvin S. Rakow^
THIS CHAPTER PRESENTS AN OVERVIEW of t h e m o d e m , integrated
oil refinery, a n d how it separates a n d processes crude oil and other hydrocarbon feedstocks into the required array of liquid fuels a n d other products. A description of an overall, simplified refinery flow plan serves to introduce the subject and terminology unique to the industry. I m p o r t a n t crude oil properties a n d test methods, along with a perspective on past, current, a n d likely future fuel product quality and demand, is presented. Each important refinery process is described in sufficient detail to appreciate its purpose, operating characteristics, yield a n d quality parameters, a n d current and future utilization in light of anticipated fuels product trends.
GENERIC REFINERY FLOW PLAN Figure 1 is a simplified block flow diagram of a fully integrated refinery utilizing the major process options in genereJ use. Each actual plant will incorporate those options that meet crude throughput and quality, as well as product dem a n d and quality, while striving for m i n i m u m capital and operating cost. Since most refineries reach their current configuration by periodic revamp and expansion, it is both likely and economically feasible for two refineries with similar feed and product slates to have a different mix of processes to satisfy all technical a n d economic parameters. Crude oil first undergoes physical separation by distillation (ASTM Test Method for Distillation of Petroleum Products, D 86-99a) to yield various boiling range streams as dictated by the chemical processing to follow (Table 1). The atmospheric distillation unit usually separates about one-half of the crude oil into the indicated cuts ranging from the low boiling gases through a gas oil. The final boiling point of the atmospheric gas oil can range from as low as 340°C t o as high as 410°C. Vacuum distillation of the atmospheric distillation unit bottoms (ASTM Test Method for Distillation of Petroleum Products at Reduced Pressure, D 1160-95) usually separates another 30% of the crude oil as vacuum gas oils having a fined boiling point ranging from 500-575°C. Selection of downstream processing units is greatly dictated by demand and specifications of the different products, and, particularly, increasing sulfur removal requirements. One of the key considerations is gasoline demand. For exam-
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pie, in the United States, one hundred volumes of crude oil are converted into about fifty volumes of gasoline (the ratio is about 42% o n a weight basis). Yet, crude oil typically has only 15 to 20% of its hydrocarbons in the gasoline boiling ramge. The fluid cataljrtic cracking unit (FCCU) is the main process that provides the additional "gasoline" and is thus a key process unit in the U.S. refinery flowsheet. The gasoline to crude oil ratio is substantially lower in all other parts of the world; thus FCCU, while in place worldwide, plays a significantly lesser role in non-U.S. refineries. Similar comparisons are summarized for other refinery processes in Table 2. In order of increasing boiling range, description and utilization of the distillation fractions are as follows: • CI to C4 gases—The crude oil gases, along with gases created by the downstream process chemistry, cu^e separated by carbon n u m b e r and hydrocarbon type in one o r more light ends plants (also called vapor recovery units, o r "sats" and "unsats" gas plants). Impurities, the main one being H2S, a r e removed, with final separation providing t h e product disposition shown in Table 3. • LSR—The light (straight run) gasolines, in most refineries, are blended directly t o t h e gasoline pool. If a refiner is squeezed for octane number, o r if LSR octane quality diminishes d u e t o refiner response to changing gasoline specifications, the octane n u m b e r of this stream cem be increased by the isomerization process. • HSR—The heavy (straight r u n ) naphtha, whether from crude oil or other process units, is too low in octane number to be economically blended in gasoline. Thus, all HSR is processed through the catalytic reforming unit (CRU) wherein the hydrocarbon composition is changed or "reformed" to a higher octane n u m b e r stream. • Kerosine—This fraction, after appropriate cleanup (treating), is directly sold to the kerosine and jet fuel markets and is also used as a blending component for the lighter fuel oils and diesel fuels. Since the jet fuel market now exceeds the kerosine market in parts of the world, many refiners now label this stream the "jet" cut. • Atmospheric Gas Oil (AGO)—The gas oils are generally those distillable streams heavier than kerosine. The name was derived in the early industry days when these oils were thermcJly cracked to produce olefin-containing "illuminating" gases for street lighting, for example. Depending on the final boiling point of the AGO a n d the downstream process options, t h e atmospheric distillation unit m a y jdeld one AGO c u t o r two. The lighter portion m a y b e hydrotreated to produce low sulfur diesel, while the heavier cut, hytrotreated or not, is typically processed in the FCCU.
www.astm.org
4
MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
C, + C, GASES
REF. FUEL
PROPANE
•LPG •ALKYLATE -MTBE •GASOLINE
•LPG
BUTANES
CRUDE
LSR
A T M D I
S T
ATM V
ISOMERIZATION
-GASOLINE
NAPH
HYDROTREATING
KERO
HYDROTREATING
GAS OIL
•
CAT REFORMING
•JET FUEL
HYDROTREATING
GASES
RESID
— No. 2 FO/DIESEL
Q
MTBE ALKYLATION
L V A C D I
s
•GASOLINE
GASOLINE
GASOLINE FLUID
LVGO
LCO
No. 2 FO
CATALYTIC HCO CRACKING
HVGO
DO
T
No. 6 FO •CARBON BLACK H, PLANT
HYDROCRACKING
— No. 6 FO — ASPHALT VAC RESID
•REFORMER FEED •JET FUEL
•VISBREAKING •DEASPHALTING •COKING •HYDROCRACKING
FIG. 1—Refinery block flow diagram.
Vacuum Gas Oils (LVGO and HVGO)—i:\iese cuts are usually processed in the FCCU. They can be processed in a hydrocracking unit (HCU) wherein they can be converted into catalytic reformer feed, jet and diesel fuel, and for production of lubricating oil base stocks. Vacuum Resid—The residuum portion of crude oil is the non-distillable cut, boiling above, say, 575°C. It cannot be distilled without causing substantial thermEd destruction of its hydrocarbons. This cut is sold to the asphalt and heavy fuel oil markets, or processed by the options shown in Fig. 1.
The need for sulfur removal is on an ever-increasing path as average crude oil sulfur content continues to increase and product sulfur is forced to lower levels by environmental legislation. Sulfur removal, today, is accomplished in "hydroprocessing" units. Hydrotreating, wherein the goal is to remove sulfur (and other heteroatoms) with minimum conversion of the hydrocarbon feed stream to lower boiling molecules, is the most widely utilized. Alternatively, hydrocracking is utilized to remove heteroatoms and concurrently convert varying amounts of "high boilers" to lower boiling streams.
CHAPTER 1: PETROLEUM TABLE 1—Crude section cuts. Carbon Number Name/Other Names Gases Light straight run gasohne light n a p h t h a LSR gas condensate Heavy straight run naphtha Heavy naphtha Naphtha Reformer feed Kerosine or kerosine jet Light atmospheric gas oil furnace oil diesel Heavy atmospheric gas oil AGO gas oil Light and heavy vacuum gas oils LVGO and HVGO Vacuum resid" vacuum bottoms short resid vacuum reduced crude asphalt
C1-C4 C5-C6
Approximate Boiling Range, "C -160-0 25-90
C6-C10
85-190
C9-C15
160-275
C13-C18
250-340
C16-C25
315^10
C22-C45
370-575
C40+
500+-565 +
"The corresponding bottoms from the Atmospheric Crude Tower is called: Atmospheric Resid Reduced Crude Atmospheric Bottoms Topped Crude Long Resid
OIL REFINING
5
the maximum possible hydrogen. To compUcate matters, in crude oil some carbon atoms are also bonded to sulfur, nitrogen, oxygen and metals, as well as trace amounts of other elements. The four hydrocarbon types, based on the above characteristics, are: Name Paraffin Olefin Naphthene Aromatic
Chain/Ring
Saturated/Unsaturated
Example
Chain Chain Ring Ring
Saturated Unsaturated Saturated Unsaturated
Hexane Hexene Cyclohexane Benzene
The four types are listed in the order shown after chemical analysis, and are usually referred to by the term PONA analysis (ASTM Test Method for Hydrocarbon Types in Liquid Petroleum Products by Fluorescent Indicator Adsorption, D 1319-98). In chain hydrocarbons, all carbon atoms can be linked in a straight row; these are the "normal" versions, such as the example of a six-carbon hydrocarbon, normal hexane (n-hexane). "Isomers" of n-hexane exist as branched-chain versions; as a group, these are labeled isoparaffins, for example i-hexanes. They are commonly connotated as nC6 and iC6s. When a hydrocarbon type analysis is made that includes a normal/isoparaffin split, the normals keep the letter P, the isos take the letter I, and the olefin/aromatic order is reversed, resulting in the acronym PIANO. This same split is not attempted for the normal and branched-chain olefins because of the greater number of potential olefin structures and the lesser impact on refinery process options.
HYDROCARBON TERMINOLOGY There cire four basic tjrpes of hydrocarbon chemical structures. Their differences relate to the bonding between carbon atoms and whether the carbon bonds create a chain- or ringshaped molecule. The number of electrons in the outer shell of a carbon atom requires each atom to have four bonds to be stable. This four-bond requirement can be peirtially met by a carbon atom forming single or multiple bonds with adjacent carbon atoms. Nearly all hydrocarbon molecules encountered in refining chemistry consist of all singly bonded carbons, or contain pairs of doubly bonded adjacent carbons; the remaining bonds cu-e satisfied with hydrogen. When eill carbon-carbon bonds in a molecule are single, it is called saturated, because it contains the maximum possible amount of bonded hydrogen. When there cire double bonds present, the hydrocarbon is labeled unsaturated, since it holds less than
Region Asia/Pacific Western Europe Eastern Europe/FSU Middle East Africa North America So. America/Caribbean
TABLE 3—Disposition of refinery C1-C4 gases. Carbon Number
Chemical Name
CI C2
Methane Ethane Ethylene Propane Propylene
C3 C4
End Uses
n-Butane i-Butane Butylenes
Refinery Fuel Gas (RFG) RFG RFG, PetrochemicEils Liquefied Petroleum Gas (LPG) Petrochemicals, Alkylation Unit, Polygas or Dimerization Unit LPG, Isomerization Unit, Gasoline Blending Alkylation Unit Alkylation Unit, MTBE, Polygas or Dimerization Unit, Petrochemicals, Isooctane Unit
TABLE 2—^Worldwide refining capacity as percent of crude. Weight Percent on Crude Capacity Crude Oil Distillation, Vacuum Hyd retreating Million mt/year Distillation FCCU CRU + Hydrocracking
1020 740 540 300 160 1010 340
23 36 35 34 15 46 43
4110 Source: Oil & Gas Journal Worldwide Refining Report for January, 2001.
13 15 8 5 6 33 19
9 13 12 9 11 19 5
41 58 38 37 25 68 29
Coke
0.4 0.5 0.8 0.3 0.2 4.5 1.8
6
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
By chemical naming convention, all saturated hydrocarbons, the paraffins and naphthenes, end in the letters "ane," while all unsaturated hydrocEirbons, the olefins and aromatics, end in the letters "ene." Each of the four species has different properties and reactivity, and this plays a major role in refinery unit selection and operation. The amount of the various heteroatoms and the m a n n e r in which they are bound to carbon also is critical to refinery operation. These factors will be apparent throughout the remainder of this chapter as well as in other parts of this manual.
C R U D E OIL
almost always less dense than water, which has a specific gravity of 1.0. Worldwide, average API Gravity is about 31 (sp. gr. = 0.87) and ranges from a high of about 45 for "light" crudes to a low of 8-15 for the "heavy" crudes, tar sand oils, and the like. Aromatics are the densest of the hydrocarbon types and are therefore lowest in API, while peiraffins Eire generally highest in API gravity. As boiling point of the crude oil increases, a r o m a t i c s content increases. Thus, high API crudes likely have lower concentrations of high boilers and most likely less aromatics, while low API crudes are generally higher in aromatics and in high boilers. Sulfur
Hydrocarbon Composition Crude oil is composed of aromatic, paraffin and naphthene hydrocarbons. Minor amounts of olefins may be present. Average aromatics content is about 50%; however, it can range from as low as 2 5 % in light paraffinic crudes to as m u c h as 75% in heavy oils, such as those recovered from Canadian tar sands. The paraffin/naphthene ratio is widely ranging and is qualitatively identified via the labeling of many crudes as "paraffinic" or "naphthenic." Hydrocarbon composition affects many considerations in selecting processing options, or meeting product specifications, as shown by the following examples:
Worldwide, weight average crude oil sulfur content is about 1.25% with commercial production ranging from 0.1-6%. Crudes low in sulfur are labeled "sweet" while those higher in sulfur £ure designated "sour." The terms derive mainly from the foul, or sour, odor of one of the sulfur species, the mercaptans, as well as the corrosiveness of hydrogen sulfide and the mercaptans. The split between sweet and sour was at 0.5% when average sulfur was m u c h lower than today; now, the crossover is more likely in the 0.5-1.5% range and is usually designated by each refinery depending on its history and capability to process higher sulfur crudes. Barring the discovery of major low sulfur crude oil pools, it is expected that average sulfur will continue to modestly increase.
Aromatics Because of their inherent molecular stability, they are the bane of refining. Aromatics are the most difficult compounds to thermally crack, hydrogenate, and desulfurize. They are increasingly "unwanted" species in fuel products because they are the hardest to completely combust, thus contributing more to pollution and equipment maintenance. As product aromatics content is regulated to lower levels, refinery process severity and costs will increase. Aromatics do have value in the refinery flow plan as a high-octane gasoline blending component from the catalytic reforming unit and in the recovery of chemical grade aromatic feedstocks for the petrochemical industry. Paraffins
and
Naphthenes
These constituents are more readily processed in the refinery. Each offers "positive" and "negative" features, but the effects of these species are not as severe as for the aromatics. As an example, paraffins crack cleanly and thus yield m i n i m u m coke, but, in fuel blends, they raise pour point and are more difficult to octane-upgrade in the CRU. Density The density of chemiceJ compounds is expressed as specific gravity (sp. gr.). In the petroleum industry, this weight/volu m e relation is expressed worldwide as "API (American Petroleum Institute), or commonly as "API Gravity", per the formula "API = (141.5/sp.gr.) - 131.5 (ASTM Test Method for API Gravity of Crude Petroleum and Petroleum Products, D287-92). The formula was adapted from the Baume density equation used for sulfuric acid. Its value is the creation of a whole n u m b e r scale to more readily discern one crude oil from another, rather than using decimals, since crude oils are
Nitrogen Generally, nitrogen content is about 5-20% of sulfur content in most crudes. This is a helpful circumstance because nitrogen removal requires greater process severity than sulfur removal since nitrogen is mostly bound in ring form while a greater percent of sulfur is bound in chain form. Heteroatom removal from rings is more difficult than from chains. Many California crude oils are high in nitrogen content, often equaling sulfur content, and thus require greater process severity than other crudes of equal sulfur content. Metals Organometallic compounds are predomineintly found in the vacuum resid. Important metals are nickel, vanadium, copper and iron, with nickel (Ni) and vanadium (V) usually in highest concentration. Ni + V levels in resid range from a few p p m to as m u c h as 1000 ppm. The metals, particularly Ni, have an adverse effect on catalyst performance and have specified maximum limits in products such as heavy fuel oil and petroleum coke. This can affect crude selection options for many refinery process configurations and product slates, as well as in process selection for future refinery revamp/expansion projects. Oxygen Acids, particularly naphthenic acids, and phenols constitute the main organic oxygen compounds in crude oil. A substantial portion are in the naphtha, kerosine, and gas oil boiling ranges. These compounds are readily eliminated via hydrogenation, and will pose less concern as more hydrotreating is
CHAPTER added to the flow plan to achieve greater sulfur removal. If these streams are not hydrotreated, the acids are usucJly removed by many variations of caustic treating.
PRODUCTS The main refinery products, except for the gas streams shown in Table 3, are automotive gasoline, kerosine/jet fuel. No. 2 and No. 6 fuel oils, and No. 2 diesel fuel. Changes to the refinery flow plan have mainly been driven by changes in crude oil quality, and in specifications and demand for these products. It should be noted that all industry fuels and lubricants are presented in this manual including a complete presentation and discussion of test methodology and properties. Gasoline The first meaningful specifications evolved in the 1920s. From this beginning until 1990, the two key control properties have been "octane number" and "volatility." Octcine number, the measure of the engine to resist the rapid "pinging" noise, called "knock," due to improper combustion, steadily increased as engines became larger and more powerful, peaking in the eeirly 1970s. The refinery met the required octane appetite of the car population by instcJling processes that produce high octane streams, such as the FCCU, CRU, and alkylation and isomerization. In addition, organolead compounds, mainly tetraethyllead (TEL), were used to add about eight octane numbers to the gasoline pool. Two laboratory octane n u m b e r test methods are used to measure and control this gasoline quality parameter, the Research Octane N u m b e r (RON) and Motor Octane N u m b e r (MON), per ASTM Test Methods for Research Octane Numb e r a n d Motor Octane N u m b e r of Spark-Ignition Engine Fuel, D 2699-97a and 2700-97, respectively. Their different operating conditions, detailed in this manual, act to delineate and bracket the potential for knock in the automobile population. In most of the world, RON is the value the consumer sees at the "pump," while in the United States and Canada, the average of both the RON and MON is the guiding specification. The first significant response to pollution from vehicular emissions came with the passage of the U.S. Clean Air Act in 1973. This legislation stipulated that the m a i n pollutants emitted from the automobile tailpipe; namely, u n b u m e d hydrocarbons, carbon monoxide and nitrogen oxides, had to be reduced by 90%. The automotive industry chose to use a catalyst to comply with this regulation. This "catalytic converter" requires platinum, or other noble metal such as palladium, to effect the desired emissions reduction chemistry. Platinum's activity, however, is adversely affected by lead. As a result the oil industry had to remove lead from gasoline and would have lost the eight octane numbers from the addition of TEL. The Act also required an increase in fuel economy from 12.5 mpg (5.35km/l) to 27.5 mpg (11.77km/l). This led to a reduction in vehicle weight and power, which in turn reduced octane appetite by about four numbers. Thus, the net effect on the U.S. oil industry from 1974 through the mideighties was to add about four octane numbers to the base gasoline pool. This was achieved mainly by the addition of
1: PETROLEUM
OIL REFINING
7
process technology such as catalytic reforming and alkylation. However, the industry also introduced organic oxygenates to gasoline blends, initially alcohols and then ethers, since such compounds have high octane quality (Table 4). Of the three utilized alcohols, the only surviving member is ethanol, which is used in the U.S. through the help of government price subsidy. Concern for water solubility, corrosion, and emissions problems from the alcohols, verified or not, drove the U.S. refining industry to the ethers. The first, a n d still dominant, ether was methyl tertiarybutyl ether (MTBE), which was initially provided by ARCO Chemical converting tertiarybutyl alcohol, a by-product from their propylene oxide process, to the isobutylene feed needed to make the ether. Volatility requirements can be described as the control of the boiling range of the gasoline to assure that the automobile operates properly during startup, warm-up, and hot operation. Gasoline specifications assure that there is a proper balance among the C5 through C l l hydrocarbons that constitute the gasoline boiling reuige, and that the final boiling point is limited. Volatility control also includes the need to add liquefied n-butane to provide sufficient vapor pressure (ASTM Test Method for Vapor Pressure of Petroleum Products (Reid Method), D 323-99a) to ensure cold engine startup. This results in "seasonal" volatility grades wherein butane content and boiling range Eire varied to match expected ambient temperature. As industry used the oxygenates to meet octane number requirements, data showed oxygenates can reduce pollutants such as carbon monoxide (CO) and nitrogen oxides (NO^). And, as analysis of the effect of various gasoline constituents on tailpipe emissions became better defined, it became apparent that other compositional changes would be beneficial. This was addressed by passage of the U.S. Clean Air Act Amendments in 1990. This legislation, and subsequent regulations such as those enacted in Ccdifomia, has substantively changed the entire composition of gasoline (Table 5), per ASTM Specification for Automotive Spark-Ignition Engine Fuel, D 4814-99. As a result, the refinery has undergone continuous change to meet the cleaner fuel product requirements. These changes, initiated in the U.S., are being adopted throughout most of the world; in some instances, legislation in other locales is resulting in even more severe specifications (Table 6).
TABLE 4—The oxygenates. Blending Actual RVP, kPa RON MON
Source
Name
Alcohols Methanol Natural Gas Ethanol Crops t-butyl alcohol Propylene Oxide By-Product Alcohol Ethers MTBE TAME ETBE TAEE DIPE
133 130 109
99 96 93
54 27 17 6
118 111 118
100 98 102
Olefin
Methanol Isobutylene Methanol Isoamylene Ethanol Isobutylene Ethanol Isoamylene (Propylene + Water)
"Average of RON and MON.
32 16 12
104"
8 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK Jet Fuel
TABLE 5—History of U.S. gasoline specifications. Pre-1974 Regular Grade RON MON Pb, g/1 Premium Grade RON MON PB, g/1 RVP, kPa Oxygen, %w Benzene, %w, max 90% Dist., °C, max Aromatics, %w Olefins, %w Sulfur, p p m w
94 86 0.8 100 90 0.9 62-103 0
1992-1994
1974-1991 Remove Pb, Increase Fuel Economy
91 83 0.1
"
98 88 0.1
1995+ 91 83 0 98 88 0 50-90 2.0-2.7* 0.8-1.0 165-180 20-35 4-10 30-150
2.7*
0.3-1.0"
185-190 30-45 8-15 300-500
'^Optional use for octane number improvement. 'Mandatory in locales requiring "oxy fuel" or reformulated gasoline.
TABLE 6—Gasoline specifications in U.S. and Europe. California
United States 1998-2000 2004
Locale Year Oxygen, %w MTBE, %w Benzene, %w Sulfur, p p m w Aromatics, %v Olefins, %v
2.0-2.7 11.0-14.8 0.95-1.0 150-350 25-32 6-12
0.0-2.7 Unknown 0.8-1.0 30-150 20-30 4-9
Europe
1996
2000
2005
2.0 11.0" 0.8-1.0 30 25 4-6
2.7 14.8 1.0 150 35 14
2.7 14.8 1.0 10-30 30-35 12-14
''MTBE banned by 2003, but enforcement is currently on hold.
Today's military and commercial aircraft use jet fuels that are produced almost exclusively from the kerosine fraction of crude oil or similar boiling range cuts from certain refineiy processes (Table 7). Key requirements focus on sulfur and aromatics contents, clean burning characteristics, and storage stability (ASTM Specification for Aviation Turbine Fuels, D 1655-99). Increasingly, refineries use hydrotreating technology, as well as crude slate selection, to meet specifications and aircraft power demand. Legislative reduction in sulfur and aromatics is not currently anticipated. F u e l Oil The various fuel oil grades are shown in Table 8 (ASTM Specification for Fuel Oils, D 396-98). The major products are No. 2 and No. 6 fuel oils. No. 2 fuel oil, commonly called furnace oil or home heating oil, is the fuel used for residential and small commercial oil heating systems. No. 6 fuel oil is commonly called industrial or heavy industrial fuel oil when used by utilities for electricity and steam generation. It is called Bunker, or Bunker C, fuel when used by the marine industry for those ships powered by "furnaces." Nos. 1, 4, and 5 are specialty products. For example, No. 1, also known as stove oil, is a kerosine-type blend for small stovelike non-atomizing b u r n e r s a n d some farm equipment, while Nos. 4 and 5 are lower viscosity industrial fuel oils for smaller commercial furnaces such as those used by schools and hospitcds. Sulfur content is an important quality parameter. While most No. 2 fuel oil is specified at 0.5%, maximum, (ASTM Test Methods for Sulfur in Petroleum Products, D 129-95, D 1266-98, and D 2622-98, for example) m u c h product has considerably lower sulfur because of the n e w low sulfur diesel requirement. Many refiners have found that the added cost to desulfurize No. 2 fuel oil to match diesel specifications may be offset by reducing the n u m b e r of separate products, providing increased product blending options and reducing storage and distribution costs. No. 6 fuel oil sulfur ranges from 0.25-3.5% depending on end use and locale. It is anticipated that average "six-oil" sulfur content will decrease over time.
TABLE 7—Jet fuel specifications. 775-840 Density, kg/m^ 204 10% Dist, °C, max Final Boiling Point, °C, max 288-300 Flash Point, °C, min 38-60 Aromatics, %v, max 20-25 Sulfur, %w, max 0.3-0.4 MercaptEin Sulfur, %w, max 0.002 Freeze Point °C, max - 4 0 to -47
TABLE 8—Fuel oil specifications. Grade Uses Density, kg/m^, max 90% Dist, °C, max Flash Point, °C, min Pour Point, °C, max Sulfur, %w, max Viscosity mm^/s @ 40°C min max mm^/s @ 100°C min max
No. 1 Stove and Farm
No. 2 Home Heating
850 288 38 -18 0.3-0.5
876 338 38-55 -6 0.3-0.5
1.3 2.1
1.9 3.4
No. 4
No. 5
No. 6
Light Industrial
Medium Industrial
Heavy Ind'l & Marine
55 -6
55
60 0.25-3.5
5.5 24.0 5-9 9-15
15 50
CHAPTER TABLE 9—Diesel fuel specifications. No. 1
No. 2
No. 4
Uses
Farm, City Bus
Automobile Truck, Railroad
Railroad, Marine, Stationary Engine
90% Dist, °C, max Sulfur, ppmw, max Cetane No., m i n Flash Point, °C, min Aromatics, %w, max
288 10-5000 40-55 38-55 10-35
338 10-5000 40-55 52-55 10-35
20 000 30 55
Grades
Diesel Fuel Key diesel fuel specifications and major uses eire shown in Table 9 (ASTM Specification for Diesel Fuel Oils, D975-98b). The U.S. Clean Air Act Amendments also required the reduction of sulfur to 0.05% (500 ppmw), maximum, for over-theroad usage; namely, buses, trucks, and automobiles. The legislation did not require lower sulfur for other use such as the railroad and marine industry. Again, as with the fuel oils, m u c h No. 1 and 2 diesel is made "low sulfur" for all markets to minimize product distribution costs. Driven initially by European and California legislation, diesel fuel sulfur is being further reduced toward 10-50 ppmw, requiring more severe hydrotreating. While the U.S. law did not substantially constrain aromatics content, California law did, requiring reduction of aromatics to as low as 10%. Worldwide adoption of this level of dearomatization will be difficult to achieve near-term, but a trend toward lower aromatic content diesel is occurring. This is being affected in part by requiring higher Cetane N u m b e r diesel fuel (ASTM Test Method for Cetzuie Number of Diesel Fuel Oils, D613-95). Aromatics decrease "cetane rating." P o u r point (ASTM Test Method for Pour Point of Petroleum Products, D97-96a) and cloud point (ASTM Test Method for Cloud Point of Petroleum Products, D5771-95) are importctnt parameters to assure proper flow of fuel oils and diesel fuels under all anticipated ambient temperatures. Pour point must be low enough to prevent solidification of the entire fuel while cloud point must be low enough to assure that normal paraffins will not solidify and block fuel flow in piping or through nozzles.
CRUDE OIL PREPARATION AND SEPARATION A refinery may process just one crude oil or u p to as many as a few dozen, depending on factors such as captive crude supply, access to ocean and river delivery, and pipeline supply logistics. At a refinery handling multiple crudes, individucil crude oils are usually fed sequentially over variable lengths of time. Some of the crude oils may first be blended in intermediate crude storage tanks to reduce the effects of substcintial changes in crude oil quality on the process units. In almost all refineries, the crude oil slate is processed as in Fig. 2. Crude oil is first "descJted" to reduce entrained and dissolved salts, mainly sodium, m a g n e s i u m and calcium chlorides, to a n acceptable level a n d to remove "debris" such as mineral matter resulting from transportation. The crude oil is then physiccJly separated by distillation in atmospheric
1: PETROLEUM
OIL REFINING
9
a n d v a c u u m towers/columns/crude u n i t s to provide the streams in Table 1. Many refineries incorporate a preflash column before the atmospheric tower to remove a portion of the lower boiling components. This improves subsequent cut point control and flexibility in the atmospheric tower, and can increase m a i n c o l u m n capacity. A complex heat exchange network is used to warm the crude to desalter temperature and then to furnace inlet temperature. Desalting is accomplished by mixing 100 parts of crude oil with about 3-10 parts of water at about 120-140°C. Emulsifier or demulsifier additives may be used to aid either the mixing of crude and water or separation of the two after mixing. Electrically charged plates may also be used to coalesce water to aid in separation. The desalted water is then sent to wastewater treatment and, if necessary, for stripping of undesired contaminants such as benzene. After further heat exchange, usually to 210-240°C, the crude is sent through multi-pass furnaces to attain atmospheric column inlet temperature in the range of 340-410°C. Then, by control of top column reflux a m o u n t and temperature, reboil, and p u m p a r o u n d / p u m p d o w n parameters, the desired cuts are obtained. Atmospheric column bottoms are usually sent to the vacu u m tower to recover the vacuum gas oils to a final boiling point (FBP) as high as 575°C, although 540-565°C may be more common. The ability to reach desired FBP depends on m a x i m u m furnace outlet temperature a n d its concomitant coke laydown on the furnace coils vs. economic run length, and attainable vacuum. Vacuums in the range of 7-25mm Hg absolute are usually achieved using steam ejector systems or vacuum p u m p s .
PROCESSES FOR GASOLINE AND DISTILLATE YIELD A N D QUALITY Fluid Catalytic Cracking This is the predominant process for converting high boiling gas oils a n d resid into lower boiling streams, mostly gases, a gasoline blending component and a light gas oil for No. 2 fuel oil blending, fts place in the refinery is mainly driven by the yields and compositions of the product components, the ability of the process to vary product yield/composition, a n d lower capital investment cind operating costs compared to alternative process options. Figure 3 depicts the process operating principles. Many equipment configurations and catalysts are employed, but the essence of the process is reactor and regenerator "vessels," vapor phase fluidization of the catalyst in each vessel, flow of catalyst between the two vessels, thermal cracking of the feed in the reactor and removal of coke from the catalyst in the regenerator. Much of the design variations cire related to engineering improvements and the increased processing of poorer quality feed, which in t u r n has been enabled by greatly increased activity of today's catalysts. Fresh feed and, if desired, heavy product recycle, are introduced into the reactor. The feed vaporizes and thermally cracks in about 1-3 s at about 525-550°C, to yield a potential variety of product s t r e a m s exemplified in Table 10. Coke forms in the reactor section due to rapid polymerization of olefins a n d aromatics as cracking chemistry occurs. This
10 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK Condenser
—•0—iS^S—I I
Atmospheric Distillation
*•'Wet Gas •*LPG
Reflux
• • Light "B-' Water r'stabifeation'l Straight Run -*• and. ^ L.SpJtttinjtJ_*
xr
~ ) P Stripper
Naphth: Kerosene
Crude
t
Stripper Steam
-Water
* Diesel
Preheat Desaiteri "Gas Oil
Steam
Brim Preheat L f i j J Preflash Drum
Vacuum Distillation
Furnace^»i=J -••Topped Crude
Steam
Non-condensat)les and steam
PreCondenser High Pressure Steam NonC.W. condensables O
P
-
T
QHot Wbll Accumulator [) ^ Heavy Gas Oil
» Naphtha
Sour Water
Topped Crude Q Ejectors
Coil Steam
Stripping > Vacuum Resid Steam FIG. 2—Atmospheric and vacuum distillation units.
coke "lays down on" the fluidized catcJyst particles, diminishing their activity. The "coked-up" catalyst is continuously removed from the reactor section and transported with air to the regenerator wherein controlled combustion converts the coke into a "flue gas" consisting mainly of carbon monoxide (CO), carbon dioxide (CO2), water (H2O), and sulfur and nitrogen oxides (SOx and NOx) resulting from the presence of sulfur and nitrogen in the feed. This combustion raises the catalyst temperature to 680-760°C which, in turn, supplies most or all of the heat to raise fresh feed to its reactor temperature as the "hot" regenerated catalyst is continuously removed from the regenerator and re-mixed with fresh feed. In fact, many FCC units operate in heat balance, meaning that all heat to raise the feed to reactor temperature is supplied by the regenerated catalyst.
CO, a pollutant, must be removed from the regenerator flue gas. Even if CO emissions were not an issue, economics of maximizing heat recovery would dictate oxidation of CO to CO2. This is accomplished by the use of combustion additives in the regenerator, improved regenerator design and operation, and external secondary combustion in a CO or waste heat boiler. SOx and NOx pollutants face refinery emission limits. Acceptable levels in the flue gas can be achieved with regenerator additives, various flue gas scrubbing processes, or by installing "cat feed hydrotreaters" to reduce the sulfur and nitrogen content in the FCCU feed. Run length between scheduled turnarounds for inspection and maintenance is 3-5 years. Longer runs have resulted from ongoing improvements to the overall design that include multistage regeneration, improved cyclones and feed
CHAPTER nozzles, and taking advantage of favorable FCCU process economics. Factors favoring economics include the ability to produce high yields of high-value gas and gasoline from low value feed, substantial yield flexibility resulting from catalyst selection and variable operating parameters, operation at essentially atmospheric pressure, and no addition of hydrogen as is required in the hydr©cracking process. The cyclone systems in both the reactor and regenerator are mechanical devices to collect the catalyst carried out of the dense phase fluidized beds by the respective cracked product and flue gas vapors. Although thousands of tons of catalyst circulate each day, cyclone efficiency is so close to 100% that losses are kept in the 1-10 ton-per-day range. The losses from the reactor are returned via the downstream product recovery train. Losses from the regenerator are collected and then disposed, thus creating the need for daily catalyst replacement. Early catalysts were silica-alumina "clays" incapable of processing resid and limited in activity. Today's catalysts contain various highly active zeolites, combined with appropriate silica-alumina bases, which permit the processing of heavier, dirtier feed, as well as provide increased yields of
Reactor Vapor
Reactor
Reactor Riso'
Regenerator /
/
Spent-Catalyst Stripper
Combustor Riser Catalyst Cooler (Future) Combustor
Spent-Catalyst Standpipe
RecirculatedCatalyst Standpipe
RegeneratedCatalyst Standpipe
CooledCataiyst Standpipe (Future)
Chargestock
t Combustion Air UitGas FIG. 3—Fluid catalytic cracking unit.
\
1: PETROLEUM
OIL REFINING
11
TABLE 10—FCCU yields. Operating Mode
Gasoline
Gas
Distillate
Reactor Products, %w C1-C2 C3s C4s Gasoline Light Cycle Oil Heavy Cycle Oil Coke on Catalyst
3 6 11-12 46-52 15-19 7-8 6
6 22 20 27 12 5 8
2 4 8 38 35 9 4
light products without the overproduction of coke. Catalyst formulations can be chosen to maximize gasoline yield and octane number, gas yield and olefin content; tolerate resid metals; and resist higher regenerator temperature resulting from additional coke p r o d u c t i o n from the higher carbon residue level in the resid portion of the feed. Reactor effluent is taken to the "cat main fractionator" wherein the main separation occurs to yield the olefin containing gases, gasoline blending streams, a light gas (cycle) oil, and a heavy gas (cycle) oil. If the heavy gas oil has the right properties, it, or a heavier portion of it, may be sold as feed to produce carbon black for the manufacture of tires and printing inks. If so, it will be labeled carbon black oil, decant oil, slurry oil, or clarified oil. It is the heavy gas oil that is used as the "washing" stream at the b o t t o m of the fractionator to remove the catalyst to the settler and then back to the reactor. The gases undergo further absorption and distillation to achieve the desired separation for various end uses. Some refiners select feed, catalyst and operating conditions to maximize olefin gas yield at the expense of gasoline and/or light cycle oil yield. This has been done to meet growing demand for propylene as a petrochemical feedstock and to provide butylenes for MTBE and alkylation feed. The "cat" gasoline, a major gasoline blending component used throughout the world, is treated for mercaptan sulfer removal prior to product blending for odor and corrosion control. Removal of other sulfer compounds, or just the sulfur atoms themselves, will be required to meet future demand for low sulfer gasoline. While desulfurization via hydrotreating will r e m a i n a d o m i n a n t technology for some time, other sulfer removal technologies, for example utilizing extraction or oxidation, are in various stages of commercicdization. The light cycle oil is mainly blended to No. 2 fuel oil. As feeds to the "cat" unit get dirtier, and as demand for cleaner fuels grows, the refiner will be faced with an increasing need to reduce the sulfur, nitrogen, and aromatics content of this stream, if for n o other reason than to provide blending flexibility into the diesel fuel markets. Heavy cycle oil yield is very low in today's operation; most refiners blend this cut to No. 6 fuel oil, while some recycle it to extinction in the reactor. It was this recycle of the FCCU gas oil in the early days of this process that gave rise to labeling them light and heavy cycle oil in addition to light and heavy gas oil. Key fluid catalytic cracking process variables include: • Catalyst circulation rate, commonly called the "cat-to-oil ratio" • Reactor temperature • Catalyst activity • Feed residence time
12
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
The cat to oil ratio, in the weight range of 4 to 15, can be varied within the Umits of maintaining the necessary pressure balances and vessel vapor velocities so that catalyst flow and bed fluidization are properly maintained. Reactor temperature will vary with cat-to-oil ratio, regenerator temperature and catcdyst cooling, and from feed preheat for those units having feed furnaces. Process severity is mainly defined in terms of conversion of the heavy feed into gas and gasoline, and is expressed as 100 minus the product streams boiling above gasoline. Conversion typically ranges from 50-85%. Lower conversion is desired to meiximize light cycle oil yield for blending No. 2 fuel oil during the oil heating season, while higher conversion is set to meet gasoline a n d olefin gas deniEmd. Catalytic R e f o r m i n g This process is an essential part of every refinery that desires to use the heavy straight run naphtha fraction from crude, or similar boiling range material from a n o t h e r process unit such as the hydrocracker, as a gasoline component. The feed, usually a C6-C10 cut, is too low in octane n u m b e r to be economiccdly blended to the gasoline pool. The "cat reformer" converts low octane n u m b e r peiraffins and naphthenes in the feed into "high octane" aromatics in the product. Typical feed unleaded RON is 40-65 while product is 94-103. There Eire three main reactor configurations, two of which are shown in Fig. 4; all accomplish essentially the same chemistry. • Semiregenerative—Fixed bed reactors, all operating in series. • Cyclic—Fixed bed reactors in series; one out of service on rotating basis for catalyst regeneration. • Continuous—Moving bed reactors in series with "continuous" catalyst regeneration. The evolution from the original semiregenerative reformer to today's continuous unit was driven by "competing chemistry" within the process and by the need for higher octane n u m b e r product to help replace the lost octanes from lead. Hydrogen must be added with the naphtha feed to minimize coke formation and laydown on the catcdyst and achieve economic run length, since catalyst performance is readily diminished by coke. Higher operating pressure aids in reducing coke formation chemistry. However, the goal is to maximize the conversion of naphthenes to aromatics since higher eiromatics content translates into higher octane number reformate. Lower operating pressure increases aromatics yield since it makes it easier for hydrogen to "escape" from the naphthene. The continuous process is thus able to operate at lower pressure because the resulting higher coke make can be removed "continuously." In actuality, a catalyst particle tcikes about five days to migrate from the top of the first reactor to the bottom of the last; such r u n length in the fixed bed "semi-regen" reformer would be unacceptable. The original catalyst that enabled this chemistry consisted of an alumina base promoted with 0.25-0.75% platinum. Today's catalysts may contain just the platinum promoter; may be bi- or tri-metallic using additional metals such as rhenium and tin; may have the alumina base converted to the more acidic chloride form by treatment with chlorine or chloroform, for example; or may utilize appropriate zeolites. In any
case, the sensitivity of the catalysts to coke, and to deactivation from heteroatoms such as sulfur and nitrogen, requires that the naphthas first be hydrotreated. Fresh hydrotreated naphtha feed plus recycle hydrogen is heated to reactor temperature of about 500-530°C in a furnace and fed to the first reactor. The over-riding chemistry is endothermic dehydrogenation of naphthenes to aromatics. Thus, as the feed progresses down through the catalyst, as m u c h as 60°C cooling occurs from top to bottom in the first reactor, reducing activity and hence aromatics yield. The net effect is that octane n u m b e r gcdn in one reactor is not enough to make the process attractive. Therefore, the reactor effluent is reheated in a second furnace, or in a second coil within the same furnace, back to 500-530°C and fed to a second reactor, and so on, until desired octane product is attained. Cat reformers usually consist of three reactors although as many as five are utilized. Typical yields at two product RON levels are shown in Table 11. The net hydrogen produced from aromatics formation is the m a i n source of hydrogen in the refinery for its desulfurization and related needs. Refineries are in "hydrogen balance" if all such hydrogen can be provided by the reformers. If not, refiners purchase hydrogen or build their own hydrogen plants. Figure 5 depicts the main process parameters of catalytic reforming; namely, the "yield-octane" relationship. Higher octane n u m b e r d e m a n d requires higher reactor temperature and/or longer residence time. Both will cause more of the naphtha feed to crack into gas and thus reduce gasoline 3rield. "Reformer severity" is uniquely defined as the unleaded RON of the C5 + liquid product, the second major gasoline blending component. In fact, the gasoline streams from the fluid cat cracker emd cat reformer make u p about two-thirds of the world's gasoline. Significant changes in feed composition to the reformer a n d in downstream utilization of the reformate are occurring, related to the move toward cleEmer fuels. The major source of benzene in gasoline is catal3^ic reformate. The trend is to reduce benzene to a m a x i m u m of 1%, with the potential for even lower limits being legislated in parts of the world. When the p u s h for lower benzene content began, worldwide benzene levels ranged from as low as 1.6% in the U.S. to as high as 5% in pEirts of Europe and Asia. Refiners can reduce benzene concentration as follows: 1. Remove benzene precursors from the feed. This is accomplished by increasing the initial boiling point of the heavy naphtha feed, or, in essence, raising the cut point between light and heavy straight run naphtha. This results in less feed available to the reformer and lower octane quality in the resultant higher boiling range light straight r u n stream. 2. Leave the benzene precursors in the feed. In this case, refiners can then distill the C6 cut from the reformate. Benzene can be extracted from this cut for chemical sale, or the benzene can be reacted with olefins to produce alkylbenzenes. The C6 cut can also be hydrotreated to convert benzene to cyclohexEine with a resultant loss in octane number. Clcciner gasoline also mcindates lower final boiling point and lower total aromatics. This requires the refiner to eliminate a substantial portion of high boiling fraction from the naphtha feed and, in some cases, to r u n the reformer at lower
i
CHAPTER 1: PETROLEUM OIL REFINING Semiregenerative Catalytic Reforming Unit Net hydrogen Compressor
Recycle hydrogen
Off Gas
^^-^c^O
Heavy Naphtha
C3s&C4s
Separator
Reformate
Continuous Catalytic Reforming Unit
Spent Catalysi Product 'Separatiofi Naphtha & Recycle Gas
FIG. A—Semiregenerative and continuous catalytic reforming units.
13
14 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
severity to yield less total aromatics. Beside the loss of octane number and yield, these changes likely yield less by-product hydrogen, a costly ingredient needed for desulfurization chemistry. Catalytic reformate also plays an important role in the petrochemical industry by being a major source of benzene, para-xylene, ortho-xylene, and toluene primary feedstocks. The recovery of these aromatics is illustrated in Fig. 6. Total reformate, or a narrower C6-C8 cut, is fed to a liquid-liquid extraction column where it is contacted countercurrently with a solvent with known selective solubility for aromatics. The main solvents are sulfolane or di- to tetra-ethylene glycols. The aromatics, called the extract, are then distilled from the reusable solvent. The rejected paraffins and naphthenes, called raffinate, must find a new home which can include the solvent/spirits market or as feed to ethylene plants running on naphtha; recycling the raffinate to a high severity catalytic reformer is usually not economically attractive. The "pure" aromatics in the extract are then separated in a series of distillation columns, except for meta- and para-xylene, which have virtually identical boiling points. Their separation is accomplished either by crystallization, which takes advantage TABLE 11—Catalytic reforming unit yields. Reformer Severity
102
94
Yield, %w Hydrogen CI + C2 C3 iC4 nC4 C5+ Reformate
3.0 3.7 3.5 1.8 2.5 85.5
1.7 1.0 1.8 1.4 2.1 92.0
of their widely different freeze points, or by adsorption with molecular sieves, which takes advantage of their different molecular diameters. Isomerization This process mainly converts normal paraffins into isoparaffins. The technology is employed to increase the octane quality of light straight run gasoline/naphtha by converting npentane and n-hexane into mixed C5-C6 isoparaffins. The process is also used to convert n-butane into isobutane to either provide feed to the alkylation process or as a step in the process to make MTBE when isobutylene is not available. A simplified process flow diagram is shown in Fig. 7. Fresh feed, along with recycle hydrogen, is heated to reactor temperature of 180-260°C by heat exchange or a furnace. The feed then flows down through a fixed bed of catalyst that is similar to reforming catalyst. Operation may be oncethrough, or with peirtial or total recycle of unconverted nparaffin. The recycle can be recovered using distillation to separate lower boiling isoparaffin product from the normals, or adsorption with molecular sieves to separate smaller diameter n-paraffins from isoparaffins. Recycle yields a greater octane number increase at the expense of higher capital investment and operating cost. Octane improvement generally ranges from 5-25 numbers depending on feed composition and process configuration. Because of the low reactor temperature, thermal cracking of the feed is very low, in the order of 1-2% compared to 12-20% in the CRU. However, the sensitivity of the catalyst to coke laydown still requires the presence of hydrogen to minimize coke formation and hence provide satisfactory activity and run length.
96
High Aromatics Feed
2 g>
Low Aromatics Feed
+
O
76 94
96
98
100
RON without Pb FIG. 5—CRU yield/octane relationship.
102
104
CHAPTER
1: PETROLEUM
OIL REFINING
15
'^Raffinue
Bxtfictio ^Tnadngind Fnctiamtioii Feed
LSGBflD E«EXTRACTOR S'STRlPfCR RC - RECOVERY COLUMN SR - SOLVB^TREOENERATOR FIG. 6—Solvent extraction unit for btx recovery.
Alkylation The reaction of an olefin with an aromatic or isoparaffin is labeled alkylation. Benzene is alkylated with ethylene to eventually produce styrene, and with propylene to eventually produce phenol. These "petrochemical" technologies are found in some refineries. But the "alky" process that is part of the typical refinery flow plan (Fig. 1) is the reaction of C3-C5 olefins with the paraffin isobutane. The process requires that the paraffin be branched to provide a reactive tertiary carbon atom, bonded to only one hydrogen. Normal paraffins, having only primary or secondary carbon atoms, are not sufficiently reactive. Isopentane could be used, but it is already a clean, high-octane liquid contained in the C5 part of the gasoline pool. Thus, the gas isobutane becomes the obvious choice since it is reactive, not part of the gasoline pool, and reasonably available in the refinery as a crude oil component, as well as in the C4 product from processes s u c h as the FCCU, CRU, a n d hydroprocessing units. The chemistry can be accomplished with any olefin. Ethylene is not used because it is expensive to recover, operating pressure would have to be considerable since the process is carried out in the liquid phase, and the resulting alkylate octane quality is poor compared to processing higher molecular weight olefins. Propylene is a n "alky" feed in many units, but its use competes against its value as a petrochemical feed for polypropylene, propylene oxide, and linear alpha olefins. The butylenes are the predominant alky feed because of their
availability, cost t o separate into chemical grade components, and high octane quality of the alkylate. However, as the light olefin content of gasoline diminishes due to evolving legislation, refiners may also find it economically attractive to use amylenes as alky feed, instead of converting them to lower octane pentanes in a hydrotreater. In many refineries, there is insufficient isobutane to match the availability of olefins from the FCCU and cokers. The missing iC4 can be produced by isomerizing available nC4, by purchase from the natural gas condensate pool, or from nearby refineries lacking an alky plant. The catalysts that are commercially employed are 98-99% sulfuric acid (H2SO4), or concentrated hydrofluoric acid (HF) because the chemistry requires a very strong acid. Commercialization of solid-type catalyst is in progress. The process in concept is simple: pressurize the system to liquefy the hydrocarbon feeds, mix them with the strong liquid acid, and separate the alkylate from the catalyst. The process in reality is complex because of acid carryover into the alkylate; formation of acid-hydrocarbon molecules; significant exothermic heat of reaction; the need for substantial excess isobutane to suppress olefin polymerization; high catalyst consumption and utility cost in the case of H2SO4; and safety considerations, especially with hydrofluoric acid. Sulfuric acid alkylation m u s t r u n at low temperature, namely, about 5-10°C, in order to minimize olefin polymerization, acid consumption, and by-product yield. This requires refrigeration of the catalyst and feed, and substantial
16 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK
£
4—Mafenp Hydiogra
O
• • [ ^
ri c:^
X .J^
iall:^^
OM
a
?
sr
LEGEMD D-DRYER R-REACrOR ST • STABILIZER
C5-C
booMnli FIG. 7—Isomerization unit.
mixing energy because of the acid's high viscosity. Most alky units used sulfuric acid from the commercialization of the process in the 1940s until the 1970s, the period when energy cost was low. In addition, H2SO4 consumption is substantial at about 0.1 kg per kg alkylate. Since the early 1970s, most new units have been H F alky plants to counteract higher energy cost. H F alkylation operates at about 25-50°C negating refrigeration, acid viscosity is so much closer to that of the hydrocarbons that designs can accomplish the chemistry in piping or in simpler mixing vessels, and acid consumption is only about 0.001 kg per kg alkylate. However, the use of H F poses a more serious safety concern than H2SO4 in the event of a n acid release. A sulfuric acid spill will cause soil and g r o u n d w a t e r contamination; however, hydrofluoric acid, which boils at 20°C, will release a vapor cloud of H F as it escapes emd depressurizes. Releases of H F at a n u m b e r of U.S. refineries in the 1980s led to significant re-engineering to assure process safety and "save" the process from legislative extinction. A simplified flow diagram is shown in Fig. 8 for an H F alkylation unit. Key highlights include separation of the acid from
alkylate, cooling of recycle acid, recovery and recycle of isobutane, cleanup of alkylate, and separation of products. Typical product yields, quality, and disposition are shown in Table 12. MTBE Oxygenates were introduced into the gasoline pool in the 1970s to help replace the lost octanes from lead additives. Their use has now been mandated in many parts of the world based on data showing their ability to reduce harmful vehicle emissions. The majority of worldwide use of MTBE is by commodity chemical p u r c h a s e . However, a substantial a m o u n t is made within the refinery gate, in particular in the United States. Typically, the MTBE unit is installed in front of a n alkylation unit. The C4 cut from the FCCU is first fed to the MTBE unit where the isobutylene is reacted with purchased methanol. The effluent C4 paraffin and olefin stream, minus isobutylene, is then sent to the alky plant. MTBE has helped the refiner meet gasoline d e m a n d and octane, and is being adapted for blending worldwide. In fact, during the 1980s and much of the 1990s, it was the fastest
CHAPTER 1: PETROLEUM OIL REFINING FEED PRElKEATMEm-
REACTION SECTION
BOSIRIFFER
DEFKOPANIZER
17
HFSnUFPER
Mixed Batane*
OUfin Feed "j
»
DiyingAnd Diokfin Saianlioa
FIG. 8—HP alkylation unit. TABLE 12—HF alkylation unit yield and quality. Olefin Feed
C3 + C4
C4
Yields, %w 4-10 Propane n-Butane 2-5 3-6 80-93 Alkylate 76-90 Alky Bottoms" 3-8 3-8 1 Alky Tar* 1 Alkylate Dist, °C Initial BP 40 40 75 10% 70 100 30% 90 105 100 50% 110 70% 105 125 90% 120 195 Final BP 190 96 RON, without Lead 93 MON, without Lead 92 94 ^Too high boiling for gasoline. Usually blended to diesel and fuel oil. ' A "waste" stream of high molecular weight. May be processed in cracking units, burned as fuel, or disposed.
growing "chemical," with usage now surpassing 20 million mt per year. However, the use of this ether and perhaps other ethers could be reduced or banned because of concern for contamination of drinking water from gasoline storage tank leakage, as detected in parts of the U.S. Should this happen, refiners will again have to find substitute streams to replace lost octane and "barrels." This issue could become economically critical to the refiner and consumer as the industry simultaneously reduces gasoline and diesel fuel sulfur, as well as gasoline aromatics and olefins. TAME, the ether produced by reacting isoamylene with methanol, has entered the refinery picture because it can provide an opportunity to reduce the C5 olefin content of gasoline. A number of MTBE plants have been revamped to handle the higher boiling olefins. However, if MTBE use is
banned, it is likely that TAME will suffer the same fate. Ethanol continues to be used, particularly in the United States and Canada, and some refiners are substituting ethanol for the ethers. Other technologies utilizing isobutylene, beside alkylation, are being added to the refinery flowsheet. One example is the dimerization of isobutylene to isooctene, followed by its conversion to isooctane via hydrogenation.
PROCESSES FOR HETEROATOM REMOVAL AND AROMATIC/OLEFIN SATURATION The main technology for removing unwanted heteroatoms such as sulfur and nitrogen, and for reducing aromatics euid olefin concentration in refinery liquid streams, is by contacting the stream with hydrogen in the presence of a catalyst, and at appropriate temperature and pressure. The vocabuIciry to identify the numerous applications is extensive and not universally consistent. The most common generic name is hydroprocessing. This has been further divided into three categories; hydrotreating, hydrorefining, and hydrocracking, to delineate process severity for desired product cleanliness, as well as feed and product boiling range. Currently, hydrotreating and hydrocracking are the two most widely used names for this technology, with the trend to place hydrorefining within the hydrotreating category. Process severity, with respect to the above vocabulary, is defined by percent conversion (Table 13), or how much of the feed is thermcdly cracked into lower molecular weight, lower boiling product. Conversion is usually defined as 100 minus the amount of product in the same boiling range as the feed. The vocabulary also expands to include the word desulfurization, since sulfur reduction has been and remains the key need for hydroprocessing, in addition to denitrogenation and dearomatization. Some of the myriad names and abbrevia-
18
MANUAL
37: FUELS
AND LUBRICANTS
HANDBOOK
TABLE 13—Hydroprocessing terminology. Conversion, %w Hydrotreating (HT) Hydrorefining (HR)
"0" As little as possible
Hydrocracking (HC) Additional Terminology HDS HDT HDN HDA
25 +
NHT DDS MHC ARDS VRDS
Objectives Cleanup without conversion Cleanup of higher boiling streams with minimum conversion Cleanup + Conversion
Hydrodesulfurization Hydrotreating Hydrodenitrogenation Hydrodearomatization
(HDA is also used for Hydrodealkylation of Toluene to Benzene)
Naphtha Hydrotreater Distillate Desulfurizer Mild Hydrocracking Atmospheric Resid Desulfurization Vacuum Resid Desulfurization
tions for this technology are also listed in Table 13. The refinery streams that can/must undergo hydroprocessing, and the reasons for the process selection are summarized in Table 14. An example of a simplified process flow diagram is shown in Fig. 9. Fresh feed, with hydrogen recycle and make-up hydrogen for the a m o u n t c o n s u m e d by the hydrogenation chemistry, are heated to reaction temperature in a furnace and then fed downflow through one or more fixed catalyst bed reactors in series. After the desired reactions have occurred, the remainder of the process involves the recovery of a high purity hydrogen recycle stream, separation of any cracked hydrocarbon gases from the desulfurized liquid product, as well as distillation of the liquid into appropriate cuts. Start-of-run r e a c t o r t e m p e r a t u r e ranges from 2 8 5 400°C a n d pressure from 900-20 000 kPa; generally, the higher the boiling range axid "dirtiness" of the feed and the greater the desired cracking severity and dearomatization, the greater is the required temperature, pressure and n u m b e r of reactors, as well as chemical hydrogen consumption. Excess hydrogen via the recycle is needed to minimize coke formation and laydown on the catalyst; this recycle is about 3-5 times the expected hydrogen consumption.
cess of hydrogen to minimize coke production, this runaway can increase reactor temperature by u p to 60°C per minute, creating an unsafe operating condition. Refineries therefore usually cap furnace outlet temperature at about 435-440°C. Thus, processing heavier, dirtier feed at higher severity, which requires higher start-of-run temperature, results in shorter r u n length and/or multiple reactors. The metal promoters on the alumina base end up in their oxide form from the drying of the finished catalyst. After being in the reactor for a short time the metal oxides will convert to sulfides from the hydrogen reducing atmosphere, combined with the production of hydrogen sulfide. The oxide form is overactive and can cause temperature excursion, increased coking, and partial catalyst deactivation from changes in the alumina and metal promoter structure. Thus, cobalt-moly and nickel-moly catalysts must be "presulfided" or "presulfurized" prior to the start-of-run. This can be done in-situ by adding compounds such as dimethyldisulfide to a feed to condition the catalyst, for about one to two days, or it can be done ex-situ wherein the catalyst is pretreated by the manufacturer, and then shipped and loaded to assure that the metals remain in the sulfide form.
The catalysts comprise the widest selection among all the refinery processes because this technology is used to "clean" all refinery streams from light straight r u n gasoline through vacu u m resid. The two most common formulations consist of combinations of cobalt and molybdenum or nickel and molybd e n u m promoters on an alumina base. The "cobalt-moly" catalyst, as it is typically called, is preferred for sulfur removal while the "nickel-moly" combination is active for nitrogen removal and some aromatics saturation. Some catalysts also utilize nickel-tungsten. Noble metal catalysts such as platinum as well as zeolites are now employed, in particular for deep deciromatization. Cobalt-moly and nickel-moly catalysts contain in the range of 3-25% promoter metals depending on feed properties and planned hydroprocessing severity. Hydroprocessing is exothermic and increases with hydrogenation severity. Reactor outlet temperature in a fixed bed reactor can be as m u c h as 35°C greater than inlet temperature when processing vacuum gas oils and resid. Further, as the run proceeds, slow b u t inevitable loss of catalyst activity due to coke and metals laydown occurs and is compensated by raising furnace outlet temperature. Hydrogenation reaction rate increases with temperature and at about 470°C, can result in a "runaway" reaction. Since there is always an ex-
At the end of a run, burning the coke off can regenerate the "coked up" catalyst. This also is accomplished in-situ or exsitu. Ex-situ removal more efficiently removes the coke, but at increased cost. If performed in-situ, the catalyst will usually remain in the reactor for 2-4 runs, after which refiners will d u m p the spent catalyst and replace it with fresh material. Spent catalyst is usually then taken to hazardous waste disposal. If the catalyst has been used to process resid, it may be sent for nickel and vanadium recovery or to a process that can remove enough of the feed metals to reactivate the catalyst and allow its reuse. Hydrogen consumption and throughput rate, often called "space velocity," are two of the key process and economic variables. As the boiling range of the feed, and thus its heteroatom and aromatics content, increases, it can be expected that hydrogen consumption will increase a n d throughput will need to decrease to increase residence time. A similar response occurs for a given feed as conversion increases. Examples of yield, hydrogen consumption and residence time for different feeds are shown in Table 15. Hydroprocessing will continue to play a n ever-increasing role in worldwide refining. The factors driving this conclusion are:
CHAPTER 1: PETROLEUM OIL REFINING
19
TABLE 14—Hydroprocessing options. Stream Lt Str Run Gasoline Hvy Str Run Naphtha Coker Naphtha
B.P.°
Sulfur
For the Reduction of Nitrog. Arom.
Diolef.
Acids X X
FCCU GasoHne Catalytic Reformate Kerosine/Jet Lt Attn Gas Oil Atm Gas Oil Coker Gas Oil
X
Vac Gas Oil Vac Gas Oil
Resid
End Use Isomerization Unit Feed Cat Reformer Feed Gasoline Blending, Cat Reformer Feed Low Sulfur Gasoline Low Benzene Gasoline Jet Fuel, Low Sulfur Diesel Low Sulfur Diesel Gasoline Blending, Cat Reformer Feed, Jet Fuel, Diesel Fuel FCCU Feed Gasoline Blending, Cat Reformer Feed, Jet Fuel, Diesel Fuel, Lube Oil Base Stocks Cat Reformer Feed, Jet Fuel, Diesel Fuel, FCCU Feed, Coker Feed, No. 6 Fuel Oil
"Boiling Point Range HEATER IffTSTQ. REACTOR
lEAlER 9 0 STTO. REACTOR
STABIUZER HlACnONATOR LightEmli
FIG. 9—Fixed bed hydroprocessing unit. The sulfur content of crude oil has been increasing over the years and is expected to continue that trend, barring major discoveries of low sulfur crudes. The sulfur content of fuel products, as we know them today, has been decreasing and this trend is expected to accelerate as the world seeks cleaner fuels.
3. The aromatics content of fuel products has been decreasing and this trend should continue and expand worldwide. The technology and catalysts to achieve these results are available; the only missing ingredient is money. If gasoline, jet fuel, and diesel fuel continue as the major sources of transportation fuels, then the industry will be forced to uti-
20 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK TABLE 15—Hydroprocessing unit yields. Feed Density, kg/m^ Sulfur, p p m w Nitrogen, p p m w Initial Boiling Ft., °C 50% Distilled Final Boiling Ft. Products Naphtha Yield, %w Density Sulfur Nitrogen Jet/Diesel Yield Density Sulfur Nitrogen Atm Gas Oil Yield Density Sulfur Nitrogen Hydrogen Consumption, %w of Feed
FCCU LCO + Coker Gas Oil
FCCU LCO
Crude VGO
940 22 400 940 220 275 375
935 28 000 1 000 365 470 575
980 8600
5
24 730 <1 <1
27 770 <1
35 810 <20 1
68 865 <20
94" 900 500 120
215 280 385
38 825 <20 <2 1.4
2.8
3.1
"Includes AGO
lize hydroprocessing for virtually every such product, worldwide.
PROCESSES FOR RESID CONVERSION AND UPGRADING Processing of resid has its unique pEirameters and constraints because of its composition. It, among the crude oil cuts, is: • the highest in carbon residue resulting in high rates of coke production or laydown on catalyst. • the highest in sulfur, contaminEtnt metals, aromatics and usually nitrogen, resulting in high operating severity and/or hydrogen consumption. The easiest way for the refiner to handle resid is to sell it. Its two m a i n markets Eire asphalt for road paving, roofing, and the like, and No. 6 fuel oil for utility and marine energy. To a lesser extent, resid is used by some refiners as a feed for the production of hydrogen. The asphalt market is finite, and the "six oil" market is limited to the a m o u n t that can be sold economically to compete with other fuels such as cocJ and gas. The net result is that the average refiner cannot dispose of Eill of its resid via the above scenarios. In addition, the concern for air pollution is driving the world toward the use of lighter, cleaner fuels. In response, a group of technologies have established their place in the refineiy flowsheet to clean resid and convert it into lower boiling streams. These processes yield products of a higher hydrogen to carbon ratio than that of the resid feed, essentially the result of the cracking and/or saturation of its aromatics, which have the lowest hydrogen to carbon ratio among the hydrocairbon types cind crude oil fractions. The industry has categorized these technologies as accomplishing this desired chemistry by carbon rejection or by hydrogen addition. There Eire dozens of resid
upgrading processes in commercicd operation; however, six have established a prominent position, as follows: Carbon Rejection Hydrogen Addition Visbreaking Fixed Bed Hydrocracking Solvent Deasphalting Fluidized Bed Hydrocracking Delayed Coking Fluid Coking/Flexicoking Table 16 shows a regional s u m m a r y of the worldwide installations for these technologies. Visbreaking This is a thermEtl cracking process using heat alone to partially crack the resid feed to lower boiling products. Most commercial units operate in the 20-30% conversion range, thus, 70-80% of the product r e m a i n s in the resid boiling range. The attractiveness of the process is alluded to in its name; although most of the product is in the same boiling reinge as the feed, this material is significantly lower in viscosity than the feed. Hence, the heat treatment reduces or "breciks" the viscosity of the remaining resid, leading to the process name. The value of visbreciking is in utilizing the unconverted resid to blend No. 6 fuel oil. This product is made by blending lower boiling, higher VEJUC streams, commonly named cutter stocks, to reduce or cut the high viscosity of the resid in order to meet product specifications. By lowering resid viscosity, less cutter stock is required. There are very few visbreakers in North America since this area has substantial gas production and is oriented toward lighter fuels. The opposite pattern exists in Europe, for example, where there are over 60 million metric tons of visbreaking capacity to meet that region's greater utilization of heavier fuels, in general. Furthermore, until recently, Europe has been a gas-poor region. There are two m a i n vEtriations t o the visbreaking flowsheet: one is the soEiker coil visbreaker, the other the soaker drum visbreaker, as in Fig. 10. Both subject the resid feed to heating in a furnace; in the soaker coil unit, all thermal cracking tcikes place in the furnace coil, while in the soaker d r u m unit, the feed is partially cracked in the furnace at about 30°C lower temperature and the cracking is completed by providing increased residence time in the drum, as follows: Residence Time, min. Soaker Coil Soaker Drum
Temperature, "C 465^85 435^55
1-2 4-8
The original driving force for the sociker drum process was to minimize coking in the furnace coil and thus increase run length. However, improvements in furnace design have narrowed the differences in process severity so that either process may be selected. TABLE 16—Resid process capacity. Capacity, Million mt/year Process
Worldwide
North America
Western Europe
AsiaPacific
Visbreaking Delayed Coking Fluid Coking Fixed and Fluid Bed Hydrocracking
159 155 24
11 96 8
62 13 1
21 16 3
110
35
10
48
CHAPTER 1: PETROLEUM FRACTIONATION TOWER HEATER
START
^>-4vvv1-
••ftd
•
I i
QUENCH
f RESID
FRACTIONATION TOWER
X-
SOAKER
HEATER
START
TT
(§)--^^-^
QUENCH
RESID
FIG. 10—Visbreaking unit.
TABLE 17—Visbreaking unit yields. Feed Initial Boiling Pt., °C Yield, %w Density, kg/m^ Sulfur, %w Viscosity, m^/s @ SOX
345 970 4.1 720
Resid Product 345 78 990 4.3 250
Feed 540
Resid Product
1015
540 72 1035
100 000
45 000
Typical product yields are shown in Table 17. The thermally cracked gases contain olefins, but less than FCCU gases, and provide an added source of petrochemiccd and alkylation unit feeds. Visbroken light naphtha can be blended into gasoline after removal of diolefins and mercaptan sulfur, and the heavy naphtha can be sent to the cat reformer hydrotreater (often called the naphtha hydrotreater) for olefin saturation and sulfur/nitrogen removal, followed by octane number upgrading in the reformer. The visbroken gas oils will usually be blended to the fuel oil pool, but can be hydrocracked for diesel and jet fuel blending, or hydrotreated to yield additional FCCU feed. Solvent Deasphalting This is the only process of the six in which no chemical changes to the molecules occur. The process is based on the low solubility of asphaltene-type residuum oil molecules in low molecular weight paraffins. Asphaltenes are highly aromatic, difficult to crack, and contain the majority of the sulfur, nitrogen, and metals heteroatoms that are found in resid. In the deasphalting process, feedstock, usually vacuum resid, is mixed with a paraffin solvent, either propane, butane or
OIL REFINING
21
pentane. The "cleaner" resid components dissolve in the paraffin while the dirtier asphaltenic ones do not. Thus the process yields "clean resid feed" that can be economically processed in another unit, typically the FCCU. In fact, the process is oft-times defined as being used to provide incrementally clean "cat feed." As the molecular weight of the paraffin increases, more of the resid dissolves in the solvent. The part that dissolves in the solvent is most commonly labeled deasphalted oil (DAO) since the solvent has rejected the asphaltenes. The reject raffinate, mostly asphaltenic, is labeled pitch, tar, or simply asphaltenes. Figure 11 depicts the typical separation of heteroatoms between the DAO and pitch. The effect is usually most pronounced for the metals, with lesser benefit for nitrogen and less yet for sulfur. Percentages of recovered DAO for the chosen paraffin solvents are: Solvent
Percent DAO
C3 C4 C5
25-40 50-65 80-95
The process consists of a mixing vessel or column running at sufficient pressure to liquefy the solvent and dissolve the DAO, separation of the DAO/solvent from the pitch, and recovery of the recyclable solvent from the DAO by distillation and stripping. The process can also be designed with two contact vessels operating at different temperatures to yield a third, intermediate-cleanliness cut, usually labeled resin. This may be done to give the refinery a more flexible scheme for selling and further upgrading the various resid cuts. An engineering feature that has played a role in reducing solvent recovery cost sets the solvent recovery at temperature and pressure such that the solvent is at or near its critical conditions. This significantly reduces solvent heat of vaporization as well as its solubility in the DAO. Today's fluid cataljrtic cracking cateJysts, if chosen for high metals tolerance, can accept cat feed with as much as 30 ppm Ni 4- V. A resid at 300 ppm metals permits the addition of 10% resid as incremental cat fee, ignoring the effects of carbon residue, nitrogen, and sulfur. However, based on typical data (Fig. 11), the deasphalter can yield up to 75% "cleaner" resid feed, at least from the constraint of metals that could poison the catalyst. It is such performance that leads to the incorporation of the deasphalting process into the refinery flowsheet.
Delayed Coking This is the dominant resid upgrading process in the U.S. and is showing growth in Europe and, in particular. South America. Like visbreaking, this is also a thermal cracking process. Unlike visbreaking, this process runs at essentially 100% conversion; all of the resid feed is converted into other products; namely, cracked gases, coker naphthas and gas oils, and coke. While the cracked gases and liquids are assumed to have higher market value than the resid feed, it is the market utilization and pricing of some of the coke that further helps to drive favorable economics. A schematic of the process is shown in Fig. 12. The cracking chemistry takes place in pairs of "drums." Cracked gases and liquids leave the top of each drum, while the product
22 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK
100
80
o <
60
40
/y
o 20
^C
f. I
^
i^^^A—
^
C^C^^ ,
^'^^"" 40
20
60
80
100
DEASPHALTED-OIL YIELD, WT. % FIG. 11—Resid solvent deasphalting unit: DAO quality.
Quench
XZ
^r^^ (nh L5^
/
J
I Coke Drums
^
L—
^
^
^
Fumace Steam
'Gasoline
Stripper Steam —* Light Gas Oil ••Heavy Gas Oil
Rc \
'Gas
^
Fresh Feed
Main Column ^^ ~ ^''^^'^ recycle
FIG. 12—Delayed coking unit.
CHAPTER coke r e m a i n s in the d r u m s . At start-up, both d r u m s are empty. The resid is fed to one d r u m for u p to a day while the other drum remains idle. On the second day, feed is cracked in the other d r u m while the coke that has filled the first d r u m is removed (decoked). This back and forth coke removal creates the continuous process. Drums have t3rpically been sized to hold 24 h of coke make, since it required that amount of time to decoke early units. Improvements to the decoking operation have enabled many units to r u n in 12-18 h cycles. This has led to increased capacity revamps in existing units or the opportunity to build smaller drums in new plants. In a typical design, the resid fresh feed enters the bottom section of the coker product m a i n fractionator. This provides for heat balance in the distillation column bottom, the picku p of coke fines carried from the drums with the cracked products, and the removal of a heavy coker gas oil product for recycle to improve lighter product yield. Some fractionator designs also recover a lower boiling gas oil recycle. The feed/recycle is then heated to cracking temperature, about 490-510°C, in the coker furnace. Steam is usually added to the feed to increase furnace coil velocity and thus minimize coke laydown on the furnace tubes as the feed reaches cracking conditions. The purpose is to delay coking as much as possible until the feed enters the bottom of the drum, hence the process name. The time-temperature parameters in the drum cause complete conversion of the resid. As coke is produced, steirting at the bottom, the feed, which is part vapor-part liquid, continues to crack. As coke forms, it provides surface-induced cracking as the feed passes through it. This operation succeeds because most of the coke that is produced is amorphous and spongy and is actually named sponge coke; thus, the fresh feed is able to pass through. The cracked p r o d u c t s are recovered and separated in equipment similar to other processes. An example of 5rields and quality is shown in Table 18. Unlike the FCCU, changes in operating parameters, in this case temperature, pressure and recycle ratio, have only a minor effect on product yields, to the point that yields can be approximated based solely on resid feed properties, in particular aromatics content, by using either carbon residue (ASTM Test Method for Conradson Carbon Residue of Petroleum Products, D189-97) or API gravity as the predictor. Coker n a p h t h a has lower octane quality compared to cat gasoline. In addition, it must be selectively hydrogenated because non-catalytic cracked naphtha has a m u c h higher diolefin content, making it thermally
TABLE 18—Delayed coking unit yields. Density, kg/m'
Resid Feed Products H2S
C1-C4 Naphtha Atm Gas Oil Vac Gas Oil Coke
1025
Sulfur, %w
Nitrogen, ppmw
2.9
3000
0.5 0.95 1.9 3.7
70 600 2200
Carbon Residue, %w
22
Ni + V , ppmw
250
Yield, %w
1.1 11.1 10.4 32.4 12.0 33.0
740 850 960
2.5
2 750
1: PETROLEUM
OIL REFINING
23
unstable and subject to polymerization and gum formation even during storage at ambient conditions. Because of its inherent instability, it is often call wild naphtha. The coker gas oils may be blended to fuel oil or hydrotreated for unsaturates and sulfur reduction. Removal of the sponge coke from a full drum involves the following steps: • Switch and Steam-out—Feed flow is switched from one d r u m to the other. The coke is then stripped with steam to remove entrained vapor and liquid product. The stripping steam-hydrocarbon mix is also fed to the main fractionator. The coke typically contains 4-10% liquids after stripping. This "green" or raw coke may require further "drying" to below 0.5% liquids, via calcining, to meet market needs. • Cooling—The d r u m is next filled with water to cool the coke to about 90-95°C. This is required to prevent spontaneous combustion of hot coke and allow for safe handling when the drum is opened. The resultant steam production is recovered either in the main fractionator when hot, or in a unit called a blowdown system when it is at lower temperature. • Coke Removal—In most designs, the d r u m heads are removed, a hole is drilled through the center of the coke, and then high pressure water jets are used to break up the coke. The drilling/jet equipment in most existing units operates from drilling rigs placed on top of the drums. Large capacity coker drums can be as much as 8 m in diameter and 30-38 m in height. The drilling equipment would add Einother 38 m to the elevation, making this unit easily recognized. In early units, the coke was dumped into railroad hopper cars directly below the elevated drums. In today's units, coke is dumped into pits and the coke is then moved by front-end loader or conveyor system to the transport cars. Most of the engineering improvements in today's designs focus on reducing the time and complexity in this step of the process. • Rehead—The heads are rebolted to the drum, the system is pressure tested, and then hot gas such as cracked product vapors from the operating drum, or flue gas from the coker furnace, is passed through the empty d r u m to warm the steel shell sufficiently to avoid thermal shock when fresh hot feed is reintroduced. Three types of coke can be m a d e . In usual operation, 80-98% of the coke will be sponge coke. The others are shot coke and needle coke. Shot coke, which is hard and pelletlike, has no value other than as fuel and its yield must be minimized. Needle coke, which consists mainly of crystalline elongated "needles," is highly valued and some cokers can be run, if given the proper resid feed properties and unit operating parameters, to produce mostly needle coke. About one-third of delayed coker sponge coke, after calcining, is sold to the aluminum industry to form the "carbon" anode to produce the pure metal from aluminum oxide by electrolysis. Depending on its electrical conductivity, strength, and contaminant levels of sulfur and resid metals (Ni + V), calcined sponge coke c o m m a n d s a selling price about 2-5 times its fuel value. Needle coke may be sold to the steel industry for operation in arc furnaces to yield high quality metallurgical steels. This coke may sell for as m u c h as 5-12 times its fuel value.
24
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
Fluid Coking/Flexicoking These technologies also convert all of the resid feed into cracked products and coke. The chemistry takes place, however, in a reactor containing vapor phase fluidized coke particles. The heated feed is sprayed onto the coke through feed nozzles around the reactor circumference. The product coke deposits on the fluidized coke, which is continuously removed from the reactor. In the fluid coking process, sufficient coke is burned in a separate "burner" vessel to raise its temperature so that this hot coke will maintain heat balance upon its recirculation to the reactor. In the Flexicoking process, essentially all of the coke is converted to a low heat content gas in a third "gasifier" vessel, the hot gas then reheats the circulating coke via heat transfer in what was the fluid coker burner vessel. As an example, if the chemistry yields 30% coke in the reactor, net coke from the fluid coker will be a b o u t 20-24%, b u t will only be a b o u t 0.5%, as a p u r g e stream, from the Flexicoker. The net coke from the fluid coker is in the form of shot coke and can only be used as fuel. Because the delayed coking process yields a higher average value of coke, it accounts for about 85% of worldwide coking capacity. The Flexicoking process provides the refiner the option of generating a fuel gas that can be scrubbed of H2S, instead of coke laden with sulfur, nitrogen and metals. Because the gasifier operates on steam and air, rather than steam and oxygen, the resulting gas heat content is in the range of 3000 to 4500 kJ/m^. Cracked gas and liquids yields from the fluid coking process are similar to yields from the delayed coking process. A washing section is installed above the reactor to prevent carryover of coke fines to the coker main fractionator, produce a coker gas oil recycle and, as a result, control coker gas oil final boiling point.
an equilibrium catalyst activity can be maintained. This eliminates the need to raise feed inlet temperature as run length progresses, as required in fixed bed units. In addition, the a m o u n t of recirculating liquid product required for proper catalyst bed operation is usually sufficient to provide a complete heat sink for the exothermic heat of reaction; thus, the reactor essentially operates isothermally. This permits startof-run temperature as high as 435°C, which is not technically or economically feasible in fixed bed reactors. This temperature, in turn, provides enough thermal energy to substantially crack vacuum resid, and, from the presence of hydrogen at high pressure, typically 15000-18000 kPa, enough hydrogenation reactivity to yield substantially desulfurized lower boiling products.
TABLE 19—ARDS unit yields. Products Feed Naphtha Boiling Range, °C 405 + 30-175 Yield, %w 1.1 Density, kg/m^ 990 740 Sulfur, ppmw 44 000 200 Nitrogen, ppmw 2800 20 Viscosity, 105 m2/s @ 100°C
LGO 175-345 8.8 865 300 200
HGO 345-550 52.7 910 1300 400 10
Vac Resid 550-134.i 980 11000 2800 2700
Fixed Bed Hydrocracking Any boiling range material from crude oil can be hydrogenated. As the boiling range increases, required operating severity increases. In order to desulfurize, dearomatize, and crack resid-containing feed, multiple fixed bed reactors in series are required. Commercial installations have utilized from 2-6 reactors to attain desired process severity. However, most commercial units are usually designed to operate on feeds no dirtier than atmospheric resid, based on capital investment and operating cost, since running on v a c u u m resid substantially increases severity. Existing atmospheric resid hydrocrackers, oft times labeled ARDS units (Table 13), produce clean products as exemplified in Table 19. Fluidized B e d Hydrocracking This is a liquid phase, fluidized catalyst bed process and is more commonly called expanded or ebullated bed hydrocracking. In this process, depicted in Fig. 13, fresh resid feed, along with hydrogen, is introduced into the bottom of the reactor. Liquid product is also recycled to the reactor bottom to provide needed hydraulic lift to expand the catalyst bed by about one-third. When this occurs, the catalyst goes into random motion to create fluid bed behavior. This allows "spent" catalyst to be withdrawn from the reactor each day and replaced with fresh material. Therefore, unlike fixed bed units.
Recycle Oil
FIG. 13—Fluidized bed resid hydrocracking reactor.
CHAPTER 1: PETROLEUM OIL REFINING
25
TABLE 20—Fluidized bed resid hydrocracking unit yields. Products @ 66% Conversion Boiling Range, °C Yield, %w Density, kg/m' Sulfur, p p m w Nitrogen, p p m w
Products @ 80% Conversion
Feed
Naph
LGO
HGO
Vac Res
Naph
LGO
HGO
Vac Res
565 +
C5-175 9 710 300 85
175-345 18 860 800 480
345-565 32 920 2500 1900
565 + 34 1010 12 400 5200
C5-175 12 710 900
175-345 23 860 2700 630
345-565 38 930 11800 2500
565 + 20 1060 27 800 7000
1037 54 000 4000
Conversion levels of 50-90% have been commercially attained. And, the unit can process any quality feed, recognizing that heavier, dirtier feeds will require lower throughput rate (space velocity), and increased hydrogen consumption and catalyst replacement rate. Examples of yields and quality for a representative resid feed are shown in Table 20.
PROCESSES FOR LUBE OIL BASE STOCK PRODUCTION Lubricating oil base stocks are produced from the vacuum gas oils. While lube oil quality requirements are extensive, process requirements are mainly driven by base oil viscosity index, "cleanliness," and pour point. With respect to hydrocarbon composition, the overall requirements are that finished base stocks cannot contain aromatics, and are limited in normal paraffin content. Aromatics must be removed or converted to naphthenes because of their adverse effect on oxidation stability and hot lube performance. Viscosity index (VI) is the measure of the change in viscosity with temperature. High VI, which equates to the least change of viscosity, is most desired. Normal paraffins provide the highest VI among the hydrocarbon types. However, n-paraffins are also highest in pour point; hence, their concentration in finished lubes must be limited to avoid solidification. There are two main process routes to produce the base oils. The original, and still predominant, scheme incorporates three process steps: first, aromatics are removed by solvent extraction; secondly, normeJ paraffin content is reduced via crystallization (dewaxing), and, thirdly, the remaining isoparaffins and naphthenes are hydrotreated to remove other components such as organic acids, sulfur compounds and the like that would adversely affect long term "cleanliness." A newer process utilizes one-step high severity hydrocracking at about 18000 kPa. Under such high pressure, and with the proper catalyst, the aromatics are completely saturated to naphthenes, the other heteroatom-containing components are hydrotreated as above, and a reasonable amount of the normal paraffins are isomerized to isoparaffins, via the ability of the catalyst to effect isomerization activity as well as hydrogenation chemistry, to meet pour point requirements. If the refiner is running a highly paraffinic crude slate, it may still be necessary to dewax the hydrocracked product. Aromatic extraction of vacuum gas oil is similar in concept to extraction of catcJytic reformate, except the solvents are different. The most widely used solvent had been phenol, but it has been banned because of its carcinogenicity. Solvents used today include furfural and n-methylpjrrolidone. Dewaxing is accomplished by chilling the gas oil until the n-paraffins solidify. The process name derives from the end
use of the n-paraffins as wax product. The two largest markets for this wax are the maJcing of candles and the coating of cardboard cartons for the preservation of fruits and vegetables from farm to table. Separation of the solid paraffin crystals from the mother liquor is aided by solvents such as methyl ethyl ketone and urea. The refiner usually supplies some 4 to 9 lube base oils, that vary mainly in viscosity, to lube plants that blend, package, and ship finished product. The largest single market for lube oil base stocks is for the production of motor oils. Hundreds of other products, including greases, are also prepared for a myriad of industrial needs. Overall, this is still a specialty area of refining, accounting for only 1% of crude oil capacity.
OTHER REFINERY PROCESSES while the refinery cannot run without the processes included in this section, they are usually placed in the "other" category since they do not contribute directly to the production of hydrocarbon streams for fuel product blending. Hydrogen Production Petroleum refineries, industry wide, long ago lost their ability to satisfy the need for hydrogen via recovery from the catalytic reforming process. The hydrogen shortfall can be satisfied either by buying hydrogen "across the fence," or by building a refiner-owned plant inside the gate. Worldwide, refinery hydrogen plant capacity has surpassed 300 million m^/day. There are two process routes to hydrogen: the catalytic reaction of low molecular weight hydrocarbons with steam, and the non-catalytic gasification of heavier hydrocarbons with oxygen. The former is known as steam reforming or steam-methane reforming, while the latter is oft-times called partial oxidation. Steam Reforming This process basically utilizes two-step or three-step chemistry, depending on when the plant was built. Older units typically involve three separate catalytic reactions, while process innovation has reduced many of today's flowsheets to two catalytic steps, followed by physical separation of product components. The first reaction, which gives rise to the process name, "reforms" the hydrocarbon feed into a hydrogen-rich synthesis gas, per the following theoreticeJ equation: CH4 + H2O = 3H2 + CO The reforming takes place at about 800°C over a nickel catalyst placed inside tubes within a furnace. This design is re-
26 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK quired due to the high endothermicity of the reaction. The high temperature also results in coke formation, but the coke reacts with excess steam to yield additional synthesis gas. With methane as the feed, the coke usucJly reacts as fast as it forms to keep the catalyst and tubes clean. Heavier feedstocks such as LPG, refinery fuel gas particularly containing C3 +, or light naphtha, can cause coke buildup and limit r u n length. A n a p h t h a hydrocarbon feed may also require hydro treating to remove heteroatoms, such as sulfur and chlorine, to protect the reforming catalyst. The second reaction, called "water gas shift," catalytically converts or shifts the CO to CO2, per the following equation:
Partial Oxidation
CO + H2O = CO2 + H2 This exothermic reaction takes place at about 350°C over a fixed bed of iron oxide catalyst. Some plants employ a lower temperature second catalytic shift using a copper-zinc catalyst to convert any remaining CO. The product gas is then treated for CO2 removed; the carbon dioxide is then vented or recovered. Unfortunately, the hydrogen product gas may still contain u p to 1-3% CO/CO2 which will adversely affect catalyst in a hydrotreater. Thus, a third reaction, called methanation, may be employed to reduce impurities to acceptable levels. This step is carried out in a fixed bed using a nickel catalyst and uses a small amount of the product hydrogen, per the following chemistry:
Sulfur Recovery Hydrogen sulfide is removed from refinery gas and liquid streams via contact with liquid or solid absorbents and adsorbents. Among the dozen or so choices, the ones most widely used in refineries are the ethanolamine liquid absorbents. As a result, this step on the sulfur recovery flowsheet is commonly ccilled "amine treating." After the H2S has been separated from the hydrocarbons, it is converted into elemental sulfur in the sulfur recovery unit (SRU) by the Claus process and its m3rriad variations. Recovery of sulfur from refineries and from natural gas production now accounts for the majority of sulfur supply to its end users. The most commonly used amine treating absorbents, or solvents as they are normally called, are the primary amine, monoethanolamine (MEA); the secondary amine, diethanolamine (DEA), and the tertiary amine, methyldiethanolamine (MDEA). Selection is based on concentration of H2S in the hydrocarbon gas stream, a m o u n t of other acid gas components such as carbonyl sulfide and carbon dioxide, and the relative solvent activity compared with useable concentration of amine in water. The hydrocarbon feed stream is contacted countercurrent to a "lean" solvent, free of H2S, in absorbers (Fig 14). The sol-
CO/CO2 + H2 = CH4 + H2O Most of the newer steam reformers substitute adsorption for third-step chemistry to remove impurities from the hydrogen. The gases from the shift reactor pass through beds of molecular sieve that alternately adsorb and desorb unconverted CO and the CO2, as well as other impurities such as nitrogen and methEine, but not the hydrogen. The product hydrogen gas usually exits the beds at 99.9% purity. The beds are most commonly desorbed by reducing pressure; hence, the process name pressure swing adsorption. The desorbed material is usueilly then blended as part of the fuel required for the reformer furnace. Other sieve beds can be used to separate CO2 for product sale.
Sweet Gas Lean Amine
-^M—Q-J'
(POX)
The advantage of this process is that the chemistry requires no catalyst, and zmy hydrocarbon can be used as feed including heavy gas oils, resid, coke and coal. Its disadvantages are the need for oxygen, high reactor temperature, and more complex gas cleaning. On balance, most hydrogen plants are steam reformers; partial oxidation will be chosen when there is no access to natural gas or other low moleculeir weight feeds, or when sale or utilization of high molecular weight refinery streams Eind coke is not feasible or economically attractive. The POX process reacts hydrocarbon and oxygen at about 1300-1550°C to yield a H2/CO synthesis gas. The oxygen is usually p u r c h a s e d over-the-fence. The "syngas" is next treated for removal of sulfur compounds, which will include carbonyl sulfide in addition to H2S. The scrubbed syngas is then sent to catalytic shift conversion and hydrogen purification in similar manner to steam reforming.
^Ll
Mal
Amine Stripper Sour Gas
Amine Contactor
Flash tank
Rich Amine
^
—CKl
Lean Amine FIG. 14—^Amine treating unit.
Acid 'Gas
CHAPTER 1: PETROLEUM OIL REFINING vent picks u p the acid gases and exits the absorber as a "rich" solvent. This stream is then sent to a regenerator where solvent reboil provides the heat to strip the H2S from the solvent. The solvent is t h e n cleaned a n d recycled to the absorber. The solvent may be filtered at one or more places in the flowsheet to remove impurities, such as iron sulfides, that can cause equipment fouling and loss of solvent efficiency in the absorber. The conversion of H2S to sulfur involves two chemical reactions; namely:
27
chemical. Other outlets include its use as a cross linking agent for the manufacture of tires via the vulcanization process for synthetic rubber, and as a blending component in the manufacture of road asphalt. However, the increasing production of sulfur from refinery hydrotreating and from the cleanup of natural gas has helped depress sulfur prices and not every refinery is able to sell all of its production.
REFINERY POLLUTION CONTROL
H2S + 3/2O2 = SO2 + H2O 2H2S + SO2 = 3S + 2H2O Thus, Claus plants consist of a b u r n e r to convert about one-third of the H2S to SO2, followed by multiple fixed bed catalytic reactors to react the SO2 with the remaining hydrogen sulfide to produce elemental sulfur, as in Fig. 15. Older Claus plants leave enough H2S in the vent gas that a second catalytic process is needed to convert this dilute H2S stream. This added step is commonly labeled "tail gas cleanup." Some of the newer Claus plant catalysts and designs claim no need for this last cleanup step. The Claus plant sulfur is a vapor as it leaves the reactors. It is cooled to produce liquid or molten sulfur and can be taken to market in that form via tank trucks or railroad cars, or it is further cooled to yield a solid yellow powder. This powder can be sold or stored in this form, or converted to prills or into blocks, depending on end use. Sulfur has many markets; the largest is for the production of sulfuric acid, which in turn is the world's largest selling
AIR COMPRESSOR
THERMAL REACTOR
Many parts of the world have imposed very strict control on refinery pollutants. The three categories are air, wastewater, and particulates. Air pollution has mainly concentrated on sulfur oxides and nitrogen oxides and refineries will have a limit on allowable emission levels, established over some time frame, with higher levels cJlowed for short periods due to unit upsets. Three of the key emissions "points" during normal operations are furnaces, boilers, and the FCCU; flare systems become part of the formula when unit upsets take process gases and liquids to flare from the opening of relief valves. Wastewater treatment involves many process steps and parts of its complexity are related to refinery unit selection and operations. Many waste streams will first be sent to settling ponds and API separators to allow solids and various hydrocarbon liquids to be separated from the water. The water will then undergo some n u m b e r of biological treatment steps until "clean" water is produced. Some of the wastewaters are recycled back to parts of the refinery while the rest is sent to disposal via deep wells, for example.
SULFUR SULFUR CONVERTER CONDENSER REHEATERS
R«cycle HydfOKWi SIMMB CM
AeUQu Fran Ajidiit RMNnnlioQ
AddOM From Sour Water Stripper
Com. FIG. 15—Claus unit.
28
MANUAL
37: FUELS
AND LUBRICANTS
HANDBOOK
Particulate emissions can result from improper operation of the catalyst recovery section on the cat cracker regenerator. This Ccin occur from loss of cyclone efficiency that in turn will overload the flue gas solids removal system, or from malfunction in the solids removal system itself. Obviously, any furnaces and boilers running on coke or coal require control of particulate emissions.
COGENERATION Refineries continue to add "cogen" plants to the flow sheet. They offer the opportunity to convert various low value fuels or blend components into electricity. The electricity, in turn, can be used to supply part or all of the refinery's needs, or the plant can be large enough to permit the refiner to sell electricity to nearby utilities. With the advent of utility supply and price deregulation, this option can take on added attractiveness. Cogen feeds range from gases to heavy fuel oils to delayed and Quid coke.
PRODUCT BLENDING Blending equations have been developed over the decades to allow the refiner to optimally blend the fuel products with a high degree of accuracy. The equations may be relatively simple for properties such as viscosity, or relatively complex for predicting blended gasoline octane number. These equations have been built into the refinery computer models with the goal that each batch of product will be blended correctly and will meet specifications at m i n i m u m cost. While the computer models and test methods continue to improve, most product is still blended by mixing the computer-indicated components in a product storage tank, sampling the tank, and then waiting for appropriate laboratory data to certify that the blend meets specifications. Some refineries do online product blending, meaning that components are mixed directly into a product pipeline, for example, with key tests being performed using on-line instrumental analysis. On-line octane analyzers are being used for gasoline blending, and newer test methods such as near and mid-infrared are beginning to replace m o r e complex or time-consuming older methods.
REFINING OPTIONS FOR THE FUTURE The key issues facing the refiner will be mainly driven by: • Crude oil quality • Clean fuels legislation • Product demand and mix • Alternative transportation options • Crude oil and product pricing Crude oil quality is expected to slowly worsen. The most critical parameter continues to be sulfur, but any increase in the percentage of resid from increased use of heavier crude oils will also play an important role as the world moves towEird a lighter fuel product mix. The rate of change is obviously related to the crude oil assay quality of new discoveries.
Based on historic data and recent findings, it is not likely that enough sweet, light crudes can be b r o u g h t to market to counter the expected declining trend. Another factor is crude pricing. When higher prices, say, $145/metric ton ($20/barrel) and above, prevail, the economics of producing heavier, higher sulfur crude obviously improves and, if sufficient price spread between heavy and light crude results, then the refiner has a greater economic incentive to process poorer qucJity crude. In addition, as the industry adds desulfurization capacity to meet future low sulfur fuels demand, some of this process capacity can be designed or revamped to handle dirtier crude. If the demand for today's fuel products continues to increase, then more and more refiners will have no choice but to process poorer quality crude oils. Gasoline and No. 2 diesel will be the products that face the near term drive to cleaner fuels. The key changes to gasoline specifications, and their impact on refining, are: • Boiling Point—Continuing reduction to the E50, E70, E80 and E90, (the maximum temperature when 50, 70, 80, or 90% of the gasoline has evaporated by distillation), for example, will diminish product supply at existing refinery crude capacity and flow sheet configuration. To meet gasoline demand, refining capacity and/or cracking units will have to be expanded just to replace the higher boiling cuts being removed from the blending pool. • Aromatics—Reduction in total aromatics and, in particular, the higher boiling aromatics, can only effectively come from removal of the heavy portion of straight r u n naphtha feed to catalytic reformers, and from lower severity reformer operation. Incorporation of b o t h options m a y maintain gasoline yield, but will result in less hydrogen byproduct. Dearomatization of heavy reformate is not expected to be economically attractive. • Benzene—^Among the benzene reduction options presented earlier, it is expected that extraction for petrochemical Scile or removal of the C6 cut from reformer feed will prevail. The latter will increasingly require isomerization of the deeper-cut, lower octane number, light straight run gasoline. • Olefins—The olefins in the C5-C6 fraction of cat gasoline will be reduced. Likely process options are to install "light cat gasoline" hydrotreaters, which will reduce octane number followed by isom units to octane-upgrade, to take at least the C5 cut to alkylation units, or to convert the isoamylenes to TAME, provided ethers are not banned. • Sulfur—Cat gasoline provides 85-100% of gasoline sulfur and most of this sulfur is concentrated in the higher boiling fraction. Refiners will have the choice of hydrotreating some portion or all of the cat gasoline, or deeply desulfurizing cat feed. Process selection will eJso depend on final realized gasoline sulfur specifications. Capitcd and operating costs to remove sulfur from "cat feed" are so m u c h higher that, on the surface, the only apparent choice is to treat the cat gasoline. However, cat gasoline hydrotreating reduces octane while cat feed h5ftrotreating can improve FCCU conversion without increasing coke on catalyst. Catalyst manufacturers and petroleum refiners continue to develop improved catalysts that will desulfurize cat gasoline with m i n i m u m octane loss. The overall effect is that either process option, or both, may be selected, based in part on
CHAPTER the refinery's crude slate and overall fuel product demand balance. It is also euiticipated that some of the new sulfur removal processes will gain substantial commercial acceptance, offering a n opportunity to meet future gasoline sulfur specifications with p e r h a p s less octane n u m b e r loss and less hydrogen consumption. • Oxygenates—As worldwide legislation for cleaner fuels spreads, every region or country appears to be requiring the use of oxygenates for pollution abatement, with the concomitant octane benefit. It is not possible to know if the expected b a n on MTBE in California and the consideration of a countrywide ban by the U.S. Congress will cause a similar worldwide response. Removal of MTBE will likely place a b a n on TAME, increase interest in ethanol, require an increase in cat reforming, cat cracking and edkylation capacity, increase the need for isomerization of light straight r u n gasoline and introduce the chemical conversion of isobutylene to isooctane. The quality of No. 2 diesel fuel will mainly focus on sulfur and aromatics. As the world likely adopts a m a x i m u m sulfur specification in the 10-50 p p m w range and some reduction in aromatics concentration, refiners will in turn install or revamp middle distillate hydrotreaters that have the flexibility to achieve deeper aromatics saturation with perhaps only the need for a catalyst changeout. Future product demand may be virtually impossible to predict. For example, in the U.S., the rapid acceptance of the van and sport utility vehicle in place of the standard car led to a decrease in overEiU vehicle fuel economy because of lack of regulation for such vehicle tjrpes. This is in the process of being corrected but, meanwhile, gasoline demand in the U.S. is increasing at rates exceeding earlier estimates. Alternative sources of vehicle power create a long-term uncertainty for the refiner. Natural gas and LPG are available in varying degree in m a n y parts of the world; LPG is a readily available automotive fuel in European countries, for example. Ignoring availability, the main detriment to rapid expansion of these fuels is the needed supply and distribution infrastructure. Electricity-driven vehicles, via the battery, are not likely to play any substantive role unless a major b r e a k t h r o u g h in energy efficiency is found. Vehicles powered o n externally-supplied hydrogen have little hope of p e n e t r a t i n g the marketplace because of high p r o d u c t i o n cost, delivery infrastructure, and safety. However, hybrid battery/gasoline powered vehicles will be available shortly. The fuel cell may have an impact on future refineries. This would be intriguing if it becomes technically and economically feasible t o produce a hydrogen/CO synthesis gas by steam reforming of naphtha "under the hood," since this liquid can take the place of gasoline or diesel fuel without any changes at the "pump." Should this occur, octane n u m b e r becomes a non-issue, the value of catalj^ic reforming, fluid catalytic cracking and, hence, alkylation would diminish, and hydrotreating and hydrocracking to supply additional "clean" n a p h t h a would increase, also requiring a major increase in refinery hydrogen. Crude a n d product pricing has become a substantive unknown for the refiner. On the crude oil side, the difficulty has been unexpected changes in price, as well as the inability to predict future prices or the price spread between
1: PETROLEUM
OIL REFINING
29
sweet/light crudes and sour/heavy crudes. On the product side, refiners have not always been able to obtain a reasonable return on investment for the legislated changes in fuels specifications. The net effect in the late 1990s resulted in low refining margins. This, in turn, has led to a sustained period of unusually high refinery utilization rates—^where 80-90% had been considered the norm, rates have recently ranged between 93-98%. Thus, refineries have opted for longer r u n length, which, in turn, has pushed many of the process units to their limit. It is expected that this mode of operation will continue until the cleaner fuels program is in place, refinery capacity is in better balance with demand, and crude prices stabilize.
ASTM STANDARDS Number
Title
D 86-99a D 1160-95
Method for Distillation, of Petroleum Products Method for Distillation of Petroleum Products at Reduced Pressure D 1319-98 Method for H y d r o c a r b o n Types in liquid Petroleum Products by FIA D 287-92 Method for API Gravity of Crude Petroleum and Petroleum Products D 2699-97a Method for Research Octane Number D 2700-97 Method for Motor Octane Number D 323-99a Method for Vapor Pressure of Petroleum Products (Reid Method) D 4814-99 Specification for Automotive Spark-Ignition Engine Fuel D1655-99 Specification for Aviation Turbine Fuels D 396-98 Specification for Fuel Oils D 975-98b Specification for Diesel Fuel Oils D 129-95 1 D 1266-98 f Methods for Sulfur in Petroleum Products D 2622-98 J D 613-95 Method for Cetane Number of Diesel Fuel Oils D 97-96a Method for Pour Point of Petroleum Products D 5771-95 Method for Cloud Point of Petroleum Products D 189-97 Method for Conradson Carbon Residue of Petroleum Products NOTE: The above are listed in order of appearance in text.
BIBLIOGRAPHY [1] Wauquier, J.-P., Crude Oil. Petroleum Products. Process Flowsheets, 1st edition. Editions Technip, Paris, 1995. [2] Wauquier, J.-P., Separation Processes, 1st edition, Editions Technip, Paris, 2000. [3] Leprince, P., Conversion Processes, 1st edition. Editions Technip, Paris, 2000. [4] Gary, J. H. and Handwerk, G. E., Petroleum Refining Technology and Economics, 4th Edition, Marcel Dekker, Inc., NY, 2001. [5] Kaes, G. L., Refinery Process Modeling, 1st edition, The Athens Printing Co., Athens, GA, 2000. [6] Meyers, R. A., Ed., Handbook of Petroleum Refining Processes, 1st and 2nd editions, McGraw-Hill Book Co., NY, 1986 and 1997, respectively.
30 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK [7] Maples, R. E., Petroleum Refinery Process Economics, 2nd edition, PennWell Books, Tulsa, OK, 2000. [8] Chang, T., "Worldwide Refining Report," Oil & Gas Journal, Vol. 98.51, December 2000, pp. 56-120. [9] "Refining Processes 2000," Hydrocarbon Processing, Vol. 79, No. 11, November 2000, pp. 85-142. [10] Tromeur, P., Guibard, I., Harle, V., and Pike, M., "Hydroprocessing Atmospheric and Vacuum Residues," Petroleum Technology Quarterly, Vol. 5, No. 1, Spring 2000, pp. 21-27. [11] Shorey, S. W., Lomas, D. A., and Keesom, W. H., "Use FCC Feed
Pretreating Methods to Remove Sulfur," Hydrocarbon Processing, Vol. 78, No. 11, November 1999, pp. 43-51. [12] Swaty, T. E., Nocca, J. L., and Ross, J., "What are the Options to Meet Tier 11 Sulfur Requirements?," Hydrocarbon Processing, Vol. 80, No. 2, February 2001, pp. 62-70. [13] Law, D. v . , "Hydrocracking: Past, Present and Future," Petroleum Technology Quarterly, Vol. 5, No. 4, Winter 2000/2001, pp. 55-63. [14] Chang, T., "Worldwide Catalyst Report," Oil & Gas Journal, Vol. 97, No. 39, September 1999, pp. 45-68.
Section II: Fuels: Properties and Performance Steven R. Westbrook, Section Editor
MNL37-EB/Jun. 2003
Liquefied Petroleum Gas Robert J. Falkiner^
utilize the butane. Temperate climates with large seasonal temperature changes could use propane in the winter and BP mixtures in the summer. However, the logistics of seasonal distribution and air/fuel calibration changes, coupled with a more than adequate propane supply, generally favors the use of propane throughout the year. The same properties that make LPG so useful contribute to some of the unique challenges in using it safely. Water/ice/hydrate properties are unique. Pressurized systems are more prone to leaks, even when the equipment is idle. The heavier than air vapor density allows accumulation in low points and cavities in the absence of ventilating air flow. When mixed with air in the right ratio in the narrow range, it has the potential for high destructive power in the event of a n ignition source that results in an explosion or fire. Consequently, persons handling LPG and installing equipment are tjrpically required to be trained and licensed. A variety of safety devices and procedures are used, and LPG is odorized to help detect potentially dangerous leaks.
LIQUEFIED PETROLEUM GAS ( L P G ) IS A GENERIC TERM FOR
ETHANE (C2) TO BUTANE (C4) h y d r o c a r b o n mixtures t h a t can exist as liquids u n d e r modest pressures at a m b i e n t temperatures. Methane (CI, natural gas) must be refrigerated to less than —259°F to be condensed by compression to Liquefied N a t u r a l Gas (LNG). Pentane a n d heavier hydrocarbons (C5 + , condensate) are liquids at ambient temperature and pressure, and are used in the manufacture of gasoline, n a p h t h a fuels, and solvents. Ethame, propane, and butane are gases at standard temperature and pressure, but can be liquefied by compression and condensation of the vapor at ambient temperature. Propane (C3), Butane (C4), and butane/propane mixtures (C3/C4 or B-P mix) have ideal properties for a fuel, widely used throughout the world in an amazing variety of applications. They are stable, high-energy content, relatively low sulfur, clean burning fuels that can be transported economically as a liquid, and be used either as a liquid or a gas. Pure component properties are given in Figs. 1-4. Propane can be used from about -40°C to 45°C, and butane from 0°C to about l l O X (about 0-250 psig vapor pressure) or higher depending upon equipment pressure ratings. The LPG tank is always under pressure at normal operating temperatures above the normal boiling point of about — 42°C, so there is no need for a fuel p u m p or electrical components for most applications. This maJses LPG ideally suited to a wide variety of portable, mobile or remote applications, using mechanically reliable and simple equipment. Propane applications tend to be robust and reliable as a result.
HISTORY The LPG industry did not start until about 1904, more than 40 years after the start of North American crude oil and natural gas production a r o u n d 1860. Natural gas (methane) cooking and lighting appliances were commonplace by 1900, but it was difficult to make the gaseous fuel portable. It was not feasible to transport and store compressed natural gas (CNG) in the bulky pressure vessels of the day. LPG is the ideal fuel for mobile and remote gas applications. It is a high BTU content liquid at typical ambient temperatures and modest (<250 psig) pressures, that is practical for storage and transportation. It can be used like natural gas once vaporized, in natural gas appliances adapted to gaseous LPG mixtures with small adjustment for air/fuel ratio. The early years of the industry were cheiracterized by the need to solve the immediate problems of the day, without any standardization or regulatory controls. LPG was one of the first c o n s u m e r goods transported and sold in pressure vessels. There were many economic and technical challenges to produce, transport, and t h e n sell it to the public. The industry was at the forefront of many areas of research and development, ranging from processes, equipment and appliances for m a n u f a c t u r e and use, to analytical test methods for composition a n d properties. The results (in retrospect) were predicable, a n d excesses abounded. However, in a short n u m b e r of years, the emerging industry was compelled by marketing a n d regulatory forces to develop
This chapter deals mostly with ASTM D1835 "Commercial Propane" and "Special Duty Propane" grades. The same distribution equipment, rail/truck tanks and storage vessels can be used for propane or B-P mixtures so it is not outwardly apparent what grade of LPG is being used in any region or application. The t e r m s "propane," "LPG," and "HD5" are c o m m o n l y used interchangeably (although this is not technically correct). ASTM D 1835 and GPA 2140 specifications exist for "Commercial B-P Mixtures," but this is rarely used for consumer applications in North America. There is n o current Canadian (CGSB) specification for B-P Mixtures, as the winter temperatures are too cold, and winter butane d e m a n d is high for winter gasoline production. Polar climates must use propane year round for low temperature operability. Tropical climates (no winter temperatures, no w i n t e r gasoline) tend to use B-P mixtures year r o u n d to ' Imperial Oil Ltd., Quality Assurance Operations and Development, Toronto, Ontario M5W 1K3, Canada.
31 Copyright'
2003 by A S I M International
www.astm.org
32
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
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products and practices that were acceptable to the pubhc. Most of the standards and regulations that were developed in the early years endure today. History of t h e LPG Industry LPG was originally derived from both natural gas or crude oil production, so the industry has two related historical roots. Refining a n d various petroleum conversion processes became additional sources later. The Chinese first transported natural gas, as well as brine and oil, during the Shu Han dynasty (AD 221) from shallow wells and seepages, through simple pipelines made of hollowed b a m b o o logs [1]. The m a i n sources of early North American gas were from numerous surface "seeps" and "gas springs" in Ontario, Pennsylvania and Southern California. The "burning spring" at Niagara Falls, below the cataract, was described early in the 1800s, and was due to a flow of gas from the Niagara shales, which underlie the limestone of the Falls [2]. The first small scale commercial distribution in North America was probably in Fredonia, New York in 1820 using small bore lead pipe distributing gas from a purposebuilt shallow well. There are records of oilfield gas being supplied to several Pennsylvania towns by 1872. The first
large scale commercial application was the piping of gas to Pittsburgh in 1883, about 20 yeEirs after the discovery of oil in Pennsylvania. By the early 1900s, a 20 in. pipeline 165 miles long, was being used for natural gas production [3], and longer pipelines soon followed. The use of petroleum pitch from surface "seeps" date back to recorded time [4], and these were the first source of North American crude production around the Lake Erie basin, at Enniskillen township in Southern Ontario a n d Titusville, Pennsylvania in 1859/60. Early cable "drilling" consisted of "punching a hole in the earth by repeatedly lifting and dropping a heavy cutting tool hung from the end of a cable," and collecting the crude in a wooden crib [5]. It produced virtually no associated gas or LPG, these having weathered off from the shallow formation. Advances in drilling technology quickly allowed deeper wells that hit shallow high-pressure oil and gas bearing formations, at first with near disastrous consequences. The first "gusher," in the spring 1862, "spewed oil above the treetops and covered 50 acres with oil 1-3 feet deep, and p o u r e d enough oil into Black Creek to blacken Lake St. Clair"[6]. The flow disappeared after several months as suddenly as it began, after m o r e t h a n a n estimated 5 million barrels "floated off the waters of Black Creek"[7]. Additional discoveries were made in rapid succession all over
CHAPTER 2: LIQUEFIED PETROLEUM
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34
CHAPTER 2: LIQUEFIED PETROLEUM the United States, in virtually every state from the Appalacha to the Pacific. The industry remained regionalized around these local sources, until the legendary Spindletop East Texas, Panhandle and Oklahoma discoveries, and the post1900 development of crude oil and gas pipelines. The discovery of large oil and gas fields in the early 1900s resulted in an immediate excess production of LPG and light naphtha hydrocarbons, far beyond the capability of developing markets to consume them. The ratio of gas to liquid and the quality of production varied widely between different production formations. At one extreme was low sulfur content "dry" or "non-associated" gas, essentially pure methane with little or no condensable liquid in the underground formation. This was ideal for distribution by gas pipeline, especially in areas such as southern California and the central United States where the gas wells were close to cities, and the terrain allowed easy pipeline excavation. At the other extreme was "sour" and "wet" gas that contained both high sulfur and high propane and heavier content that condensed to a hydrocarbon liquid with even modest compression. "Wet" gas was generally "associated" with an underlying crude reservoir, either as a gas cap or dissolved in the crude at high underground temperature and pressure. Wet gas from either crude or gas wells produced a liquid condensate (NGL, natural gas liquid) when the gas was compressed for distribution. Condensate had few uses, and for more than 50 years was burned or vented as a byproduct of crude and natural gas production. The wasteful practice of flaring enormous quantities of gas from oil wells was eventually realized to be extremely detrimental to the amount of oil that could be recovered (see retrograde condensation). The gas pressure in a formation helped force the oil to the surface. Excessive venting and depressurization left large portions of the crude unrecovered. Moreover, the practices of one gas or crude producer affected the yield of others in the same formation, since the crude and gas could flow underground between producers whose ownership rights were determined by surface surveys. This resulted in several decades of litigation and regulatory development to establish fair and equitable means to balance the often-conflicting social and economic goals [8]. The regulated recovery of liquids from ever increasing gas and crude production resulted in a steadily increasing NGL and LPG production, without adequate markets to consume them. This was aggravated by increasing production of deeper and higher pressure crude and gas reservoirs with higher liquid condensable content. Whole industries appeared (and disappeared) based on the chronically depressed price of LPGs and NGLs, and seasonal demand of natural gasoline. Gas liquids were transported long distances to find new outlets, with even some marine exports in large drums from California. In the 1930s almost all of the world's carbon black was manufactured from cheap Texas panhandle gas and gas liquids (eventually replaced by various "coking" processes). Hundreds of butane/air, propane/air, and LP-gas distribution plants were built to distribute gas to smaller towns and cities. Most were gradually replaced by expansion of natural gas pipeline distribution system, but a few remain in isolated or remote areas of North America. This is commonplace in some areas of the world with low domestic natural gas pro-
GAS
35
duction, where imported LPG is vaporized centrally, and distributed locally in gas pipelines. A balanced North American C3/C4 supply and demand was achieved about 1950. This was mostly due to increased industrial or "chemicals" demand rather than the more modest growth of consumer fuels markets. The alkylation process converted C3/C4 into gasoline, and large-scale seasonal storage in underground salt caverns, natural formations and large storage spheres balanced out the seasonal swings in heating demand. Later, LPG became a preferred feedstock for gas cracking and plastics manufacturing. The LPG fuels industry has always been in excess supply. The "chemicals" market is now more than twice the size of the LPG fuels market. This tends to make LPG fuels economically attractive in a wide variety of niche markets, wherever the properties of LPG offer an advantage, and/or the economic return is higher than alternate BTU or chemicals value. History of LPG The first uses of LPG mixtures were for heating, cooking, and lighting in remote or mobile applications that could not be serviced by natural gas or manufactured gases (town gas, coal gas, water gas, etc.). Natural gas distribution was limited to mainly urban markets, that were supplied by relatively short low pressure pipelines from regional gas wells or "town gas" plants. Mobile applications included railway coach lighting and heating, where high BTU liquid storage was a great advantage over compressed gas (Pinsch gas) or carbide gas (acetylene). Remote applications included smaller cities and suburban areas, where underground pipelines were not economically or technically feasible. In fact, "Rockgas" was an early reference to the alternative to pipeline gas in areas where buried pipelines were not feasible due to bedrock or rocky terrain [9]. The earliest consumer use of LPG was "Blaugas" in Germany about 1904. Several plants were established in the United States afterward [10]. This was a foul mixture obtained by thermal cracking of hydrocarbon fuel oils, similar to town gas production, but at lower temperatures that favored greater condensable liquid yield. Composition was essentially uncontrolled, and the mixture contained a large amount of dissolved hydrogen, methane, LPG, gasoline range hydrocarbons and heavy ends, with a vapor pressure of up to 750-1800 psi at ambient temperature [11]. The first steel tanks imported from Germany were called "bottles," a term still used today in the "bottle gas" market. The early high-pressure cylinders were bulky and extremely heavy. Up to 7 lbs. of steel was transported for every pound of relatively low BTU fuel. The earliest "Blaugas" sets used in United States had two replaceable liquid cylinders and a third permanent gas cylinder that fed the appliance through a low-pressure gas regulator. The sets were expensive, since three "bottles," valves and regulators were required instead of one. Liquid flowed via a dip tube in the liquid cylinders through a valve or a "wet" pressure regulator, where it vaporized into the third cylinder. This maintained a relatively constant composition of gas and avoided the problems associated with "weathering" or multicomponent distillation with the broad cut liquid being used (although this process was not well understood at the time). It was common in 1912 for such sets to lose many pounds of
36
MANUAL
3 7: FUELS
AND LUBRICANTS
HANDBOOK
gas daily. Stoves leaked too [12]. Even so, the industry was modestly successful. Some "Blaugas" plants remained in operation into the 1920s b u t were all gradually displaced by LPG recovered from crude/natural gas production a n d refining. Small regional businesses were built up around individual supply sources and different gas mixtures and gas sets. While the sets were relatively successful, they did experience problems from residues and ice causing regulators to malfunction. Gasoline and heavier hydrocarbons collected in the gas bottle, and had to be periodically drained out. Success depended more on the ability of the equipment to tolerate the mixture than anything else. There were no specifications to control dryness, icing, residues, or composition. The first commercial use of LPG produced from crude oil or natural gas was in 1912, indirectly as result of a burgeoning demand for gasoline due to mass production of automobiles after 1908. Condensable liquids were reported as early as 1893 in natural gas (methane) cryogenic drying processes for natural gas distribution. There are indications of a lowpressure condensate from crude oil p r o d u c t i o n being distributed in wooden barrels in the 1900-04 era, but it was the automobile that changed everything. Automotive gasoline had to be liquid in atmospherically vented tanks, but volatile enough to evaporate when mixed with air in a carburetor. The automobile created an i m m e d i a t e d e m a n d for naphtha from crude oil as well as condensate from natural gas or crude p r o d u c t i o n . Customers w h o b o u g h t a quart of l a m p oil now w a n t e d gallons of gasoline. The liquid condensate from gas production, called "casinghead gasoline," became a preferred product for sale to early refiners for blending into gasoline, and a new industry was b o m . LPG was displaced in importance by the frantic scramble to produce casinghead gasoline, which was m o r e profitable, at least in the wintertime, when there was a strong demand. Initicdly, a high-pressure condensate was allowed to "weather" in standing tanks, until enough boiled off that it was "acceptable" for use in gasoline. The limits of acceptability were only loosely defined at the time, but these fundamental requirements eventually led to the "weathering test" and "vapor tension" specification requirements many years later. Propane a n d b u t a n e continued to be problematic byproducts from both crude and gas production. Propane was too volatile to be a significant portion of gasoline, and not volatile enough for even moderately pressurized natural gas distribution networks pipelines. It was essentially worthless, so there was a large economic incentive for producers to include as m u c h of the volatile propane/butane as possible into casinghead gasoline. While m a n y of the blends provided good fuels for the engines of the day, "a frantic scramble to meet demands, coupled with not a little ignorance, began to create significant problems" [13]. Many serious incidents occurred due to excessive volatility and evaporative losses that sometimes exceeded 40%, and from boiling and over pressuring rail cEirs. In 1915, a rail car explosion killed 47 persons, injured more than 500, and virtually destroyed the town of Ardmore, Oklahoma. Casinghead gasoline was almost universally condemned, and developed such a poor reputation that one major refiner stopped using it in gasoline a n d advertised it as a consumer benefit. By 1919, the last major refiner announced that it was n o longer in the market for casinghead gasoline, and declared, "specifications were urgently
needed" [14]. As late as 1921, there were more than 100 different specifications for casinghead gasoline, most of them meaningless and many based on what a producer may have been producing on a particular day [15]. In April 1921, representatives from 31 companies formed the Association of Natural Gasoline Manufacturers (ANGM), following upon two previous efforts to organize gas processors. This Association later became the Natural Gasoline Association of America (NGAA), and now the Gas Processors Association (GPA). Its purpose was to impose both business a n d technical controls on the unruly new industry. They created a new, m o r e acceptable product, called "natural gasoline" to replace "casinghead gasoline." The ANGM moved quickly to develop technical standards and related tariff agreements, a n d to improve operations. Within three months, the first specifications were written for seven grades of natural gasoline A-G, and four grades of blended natural gasoline m o t o r fuel. The Natural Gasoline specifications were based on density (API gravity) and percent recovered (by evaporation). The lower gravity grades were preferred for gasoline blending a n d c o m m a n d e d a higher price. The blended gasoline grades were defined by a wider set of specifications, including gravity, end point, initial boiling point, vapor tension, color, and an important new concept, percent recovery in a standard distillation test. Boiling point criteria and the distillation test marked significant advances in the development of m o d e r n specifications [16]. The blended gasoline specifications were intended to eliminate practices such as "dumbbell" blending of kerosene with "wild" casinghead gas to give a blend with a naphtha range gravity that did not work well in automobiles. The blended gasoline specifications were eliminated two years later, deferring to specifications developed elsewhere by the refining industry. The natural gasoline specifications were modified several times both for definition for the grades and for test methods as they were developed, and are still published today by the GPA. In 1910, a Pittsburgh motorcar owner walked into U.S. Bureau of Mines chemist Dr. Walter Snelling's office, complaining that the gallon of gasoline he had purchased was only half a gallon by the time he got home. He thought the government should look into why consumers were being cheated because the gasoline was evaporating at a rapid and costly rate. Dr. Snelling took u p the challenge and discovered that the evaporating gases were propane, butane, and other hydrocarbons. Using coils from an old hot water heater and other miscellaneous pieces of laboratory equipment. Dr. Snelling built a still that could separate natural gasoline into its liquid and gaseous components [17]. This resulted in a 1913 patent for a continuous process "to obtain the most volatile ingredient or mixture in the form of a liquid gas under pressure" by fractional condensation of compressed heated vapors of natural gasoline [18,19]. This virtually eliminated b o t h excessive pressure and residue problems, and became the keystone of the m o d e m LPG industry. The first m o d e m LP business venture, American Gasol Co., was founded by Snelling, and three prominant "gas men" of the time, Frank Peterson and two cousins, C. L. a n d A. N. Kerr. The products were called "Gasol" (propane) a n d "Gasolite" (butane). The company failed for lack of markets and the burden of cylinder and distribution costs, and was sold two years later. Snelling went back to the U.S. Bureau of Mines explosives research, and the
CHAPTER three "gas men" went into the casinghead gasohne business, which was more profitable at the time. The three "gas men" returned to the LPG business after WWI. A. N. Kerr became president of the Casinghead Gasohne Association about 1915, one of the failed predecessors of the ANGM (GPA). The Kerrs founded the Rockgas Products Co. in Pittsburgh, and in 1925 the Imperial Gas Company in Los Angeles. They tried to develop other markets for the light ends, such as LP-gas/air town gas distribution plants and for cutting steel. In 1932, the Kerrs authored the first chapter of the Handbook of Butane Propane Gases on "A Chronology of Liquid Gas Development"[20]. In 1912, F r a n k Peterson of the Bessemer Gas Engine Company p u r c h a s e d rights to a 1909 patent to inject naphtha gasoline into hot compressed natural gas to absorb casinghead gasoline. He later patented a multistage compression/liquefaction process in 1912 [21]. These "hot blending" and "compression" processes were licensed to customers using Bessemer compressors and gas engines. In 1914 he patented "a liquefied fraction of n a t u r a l gas containing ethane, propane and butane, substantially free of heavier hydrocarbons and from methane"[22]. He also developed one of the first tests to estimate the "wetness" of a gas sample based on absorbing/desorbing the sample in a mineral seal oil. Details of the test were held closely by the Bessemer Company, which ran the test for $5 per sample. The fee was returned to any prospective producer who purchased Bessemer equipment [23]. The close association of the liquids producers and equipment suppliers has endured and evolved into today's GPA and GPSA (Gas Processor Suppliers Association). The LPG industry grew very slowly prior to 1932. In the early years, it was inhibited by a shortage of tanks and materials due to WWI, by the wide variety of LPG mixtures and equipment, and by competition from the casinghead and natural gasoline business. The entire infrastructure for LPG was new, and there were many technical problems (and costs) associated with production, transportation, equipment, and appliances. E q u i p m e n t a n d transportation costs were of p a r a m o u n t importance, since the cost of the fuel itself was so low. During this period there was litigation on several patents associated with the production (and therefore the use) of LPGs of various compositions. Most notable were the Saybolt patent for absorbtion, held by Standard Oil, and the "stabilization" patent, held by the Carbon and Carbide Company (now Union Carbide). Both patents were eventually held to be invalid. In addition, the first significant regulations on tanks, rail cars, equipment, tariffs, production limits, etc., were p u t in place, t h r o u g h Government agencies such as the B u r e a u of Explosives, Interstate Transport Commission, Texas Railroad Commission, National Fire Protection Association, and others. Many now-familiar company and brand names appeared, including Blaugas, Phillips (Philgas), Rockgas, Carbide (Pyrofax), Skelly (Skelgas), Shell (Shellane), and S t a n d a r d (Flamo). Phillips and Carbide were two of the founding members of the ANGM (NGAA, GPA), and are generally credited with developing the m o d e m LPG business. Frank Phillips, founder of Phillips Petroleum, purchased the Snelling patent rights, and marketed "Philgas" in the mid 1920s. Phillips established an active research department, and did much of the early laboratory research on liquefied gas. George G.
2: LIQUEFIED
PETROLEUM
GAS
37
Oberfell and Richard C. Alden, co-authors of the then classic reference book "Natural Gasoline," were recruited by Phillips to defend against the Carbide patent infringement litigation on the pipestill "stabilization" process. The company also benefited from the close proximity and association with the USBM research station in Bartlesville, OK. Prior to 1920, Oberfell had studied hydrocarbon behavior at the USBM in Pittsburgh, and after early association with Frank Petersen, spent five years pioneering development of low pressure absorber systems for a leading producer of natural gasoline [24]. The Phillips research group grew to include K. H. Hachmuth, (dryness, CoBr test), T. W. Legatski (author of the original GPA LPG specification) and D. L. Katz (hydrocarbon phase behavior, hydrates). The company was instrumental in developing acceptable rail cars, freight rates equivalent to gasoline, a n d large consolidated facilities that made production more cost efficient. The NGAA (GPA) developed additional specifications for commercial propane and butane after the expiration and/or settlement of the composition and processing patents. These were first reported in the Proceedings of the 11th annual Convention 1932 [25], and later published as the forerunner of GPA 2140-Liquefied Petroleum Gas Specifications. However, there were no test methods attached to the first specifications. These were added over the years as the technology was developed. At the same time, the California producers and marketers were active in related technical Eireas. The California Natural Gasoline Association formed in 1925 as a technical society of individuals, rather than a trade association of corporate members [26], and provided high caliber technical leadership to the emerging industry. Western Gas published many technical articles in Butane-Propane News [27], Eind the first edition of the "Handbook Butane-Propane Gases" in 1932. While not a standard or specification, the handbook contained a large amount of technical information on LPG production, distribution and use. It was an important reference book for the entire industry, including the "downstream" distribution and marketing segments not covered by the GPA. These handbooks were maintained into the 1980s. They are no longer in print, but are still widely available in technical libraries. Additioneil information may be available from other gas industry associations that have been active over the years, such as the National Bottled Gas Association (now NPGA [28]), Compressed Gas Association (CGA [29]), or other regional gas marketing associations such as the Pacific Gas Marketers Associations, Pacific Gas Association, and others. These were involved in development of related areas, such as equipment, pressure vessel, appliance, installation and transportation codes, standards and regulations. However, they do not appear to have been active in development of either product standards or test methods for consumer LPG products that today is under the jurisdiction of ASTM.
History—LPG Properties a n d Thermodynamics Many people consider the work conducted in this area during the 1920s and 30s to be synonymous with the development of Chemical Engineering as a separate discipline. High pressure crude, condensates, and LP gases have physical properties and a commercial importance that made them
38 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
the principle target of early research in thermodynamics and chemical processing. LPG and NGL were probably the first commercially important, large volume materials that were handled and processed near or above the critical temperature of components or mixtures. Critical phenomena had been observed in single components as early as 1823. In 1861, Thomas Andrews observed the similar phenomena when he found that carbon dioxide could not be liquefied at any pressure at temperatures higher than 31°C. He hypothesized that such a state existed for all fluids and called it the "critical point" [30]. Andrews's experiments were first made public in the 1863 edition of W. A. Miller's textbook entitled "Elements of Chemistry" [31]. The phenomena of isothermally condensing a gas mixture to form liquid by reducing pressure and isobarically heating a gas-liquid mixture to form a single-phase liquid were first observed by J. D. Kuenen in 1892 when he was studying the phase behavior of the binary system (carbon dioxide + chloromethane). He called these phenomena "retrograde condensation" of the first kind and the second kind, respectively [32]. In the mid 1920s Dr. Walter Podbielniak developed the first practical low temperature fractional distillation test that allowed the composition of NGL mixtures to be accurately determined [33]. The "Pod" column was quickly adopted by the industry and improved over the years both in accuracy and automated operation. It was the industry standard until it was eventually replaced by gas chromatographic analysis in the mid 1950s. "Downhole" samples from high pressure formations taken at underground temperature and pressure were found to contain substantial amounts of dissolved "liquid" methane. This violated the then accepted principle within the industry that no material could be a liquid above its critical temperature. The theories on retrograde condensation in gas processing by Dr. Lacy in a 1932 API paper were greeted with a great deal of skepticism by the gas processing industry [34], but it certainly put technology development into high gear. This caused quite a stir at the time, as retrograde condensation was, as the name implies, the exact opposite of what was expected. This created a great need to understand the phase behavior of high-pressure and supercritical mixtures, both for crude oil reservoir pressure management and for production of high-pressure condensate. Within a decade of intensive research and development, cycling plants for methane re-injection, liquefaction (refrigerated liquid natural gas), LPG dehydration, and sweetening processes were all commercialized. A large amount of technical data generated in this era is still used today. For example, the tables and graphs in the last published edition of the "Handbook Butane-Propane Gases" were essentially unchanged from the first edition, published in 1932. Similarly, the GPSA Engineering Data Book has been in continuous publication, and the 11th edition is now available in U.S. engineering or metric versions, and on CDROM. Tables of information for installers that are derived from this data is still in use today in publications such as NFPA 58 and CAN/CSA-B 149.2 (the respective United States and CDN "propane installation codes"). There is a large amount of related work on other vapor-liquid equilibria for the LPG gases, their mixtures, and other materials that are
commercially important for manufacture and use of LPG. This includes water [35], H2S, C02, COS, N2, mercaptan, and others [36]. Development of Thermodynamic Equation of State (EOS) methods now allows calculation of most properties of even complex mixtures directly from composition [37]. In most cases, properties such as vapor pressures, enthalpies, bubble points, dew points, adiabatic flash compositions and the like, can be calculated about as accurately as they can be measured directly. This includes LPG mixtures around their critical points, and "retrograde" condensation, and more recently temiaiy solid/liquid/vapor and hydrate systems [38].
LPG S P E C I F I C A T I O N S History of ASTM LPG Standards Virtually all of the technical developments relating to ASTM standards were done through GPA/GPSA member companies, and Government/Industry sponsored Research at organizations such as the USBM, Institute of Gas Technology (IGT) and Gas Research Institute (GRI). The GPA research reports and standards and the GPSA Engineering Data Book have been a primeiry source of technical information for the industry. These are available on a commercial basis, both to GPA members and non-members [39]. The GPA maintained the LPG and NGL standards from first publications in 1921-32 until 1960. By this time, LPG had become an important consumer fuel, widely available and with an ever increasing diversity of applications. National Standards were needed to ensure universal acceptance throughout the downstream industry, and to address potential regulatory and liability issues inherent to producer only-specifications. In 1961, ASTM published D1835 specifications for commercial propane and butane that were technically identical to the GPA specifications, including GPA test methods. In 1964, ASTM published D2154 Special Duty Propane, which was similar to GPA HD-5, but later incorporated this grade into D1835 in 1975, adding butane-propane mixtures. The GPA 2140 specifications are still maintained by the GPA, and remain technically equivalent to ASTM D1835. ASTM test methods adapted from the original GPA methods are now used in both standards. Specifications and many test methods for LPG/NGL products not covered in D1835 are still maintained by the GPA. This includes GPA 3132, which is the current version of the original 1932 Natural Gasoline specification, and a variety of other NGL products, chemical feedstocks, sampling methods, and test methods. The GPA 2140 LPG specification is widely used for contractual purposes within the upstream gas and fractionation segments of the industry. The products covered under ASTM D1835 and GPA 2140 specifications are often referred to as "spec" products. All of the others tend to be called NGL's, such as C2+ ("see-two plus," meaning ethane and heavier), fC4 (field butane), etc. For example, "spec butane" means that it conforms to one of the consumer products in D1835 or GPA 2140, or another industry specification or purchase/sale contract. The composition of "NGL Mix," or "C2-I-" or "condensate "can vary over wide ranges, and are not manufactured to strict composition limits.
CHAPTER 2: LIQUEFIED PETROLEUM GAS NGL mixes and other "non-spec" products are not sold to the pubUc. They are intermediate products, suitable for transport and storage, but intended for further processing. They are manufactured and sold between companies with the technical resources to define and control the quality within the available processing capabilities at the time. They usually have additional test methods and limits for both purity and trace components that may be deleterious in downstream processing [40]. This can vary over wide limits under contract terms. The sampling and test methods are still important for "non-spec" products, but not for the same reasons as for the "spec" products in D1835. For example, the market value of an NGL mixture is determined by the composition, since once fractionated, the ethane, propane, iso-butane, butane and C5-I- condensate fractions each have their own market value. The market value of an NGL mix is therefore dependent on the accuracy of the analysis, but the composition may vary over wide limits as agreed to by the purchaser and seller. As a result, no ASTM product specifications are necessary for these materials, but many ASTM/GPA sampling and test methods are used. The accuracy and precision of these methods are still very important to the industry. ASTM D1835 specifications for "commercial propane" and "butane-propane mix" do allow the purchaser and seller to agree on composition limits for propane/propylene and butane/propane. These grades could be used as specialty consumer fuels, such as brazing gas (or higher BTU heating fuels, or aerosol propellants), and are therefore included in the ASTM standard. The ASTM standards are rarely used for commercial contracts such petrochemical feedstocks and the like, as even nominally similar processing units can have different feedstock requirements. Processing contractual agreements tend to be less restrictive on composition, and more restrictive on trace contaminants, depending upon the capability and tolerance of the intended processing unit. In North America, propane is the only widely available LPG fuel for consumer use. Ethane and butane are used in other fuels and chemicals applications. Ethane is converted to ethylene and used predominately in the manufacture of plastics and chemicals. Butane is used predominately as a blending component in winter gasoline, an alkylation feedstock to make high octane unleaded gasoline, gas cracker feedstock for chemicals production, or as a refinery fuel gas when the price for butane drops below the industrial price of natural gas. Propane is sometimes referred to as "butane" in the North American retail market, and several companies still use "butane" in their corporate name. The reason has its roots in history [41 ], as butane was more prevalent in the early years, but has since been replaced by propane. Similarly, "HD5" propane is inherently a reference to GPA 2140 grade HD5 (Heavy Duty, 5% max propylene). HD5 grade was developed to limit minimum octane number for engine applications, and similar grades are found in all international specifications (See AutoPropane). The ASTM D 1835 "special duty" propane, and CGSB 3.14 "Type 1" propane are generally taken to be functionally equivalent to GPA 2140 "HD5" propane, differing only in some sampling and test method options. Even when contracts are written around specifications that make no reference to "HD5," the designation "HD5" tends to be used in conversation, because it is more recognized.
39
The ASTM and GPA LPG specifications have withstood the test of time remarkably well. They have undergone numerous revisions in response to improved technology and/or industry needs [42]. The vast majority of these have been associated with sampling procedures and test methods, and only occasionally with minor changes to the limiting values [43]. The actual composition and properties of LPG produced in 1932 is essentially unchanged from LPG produced today. The ASTM D 1835 standard has formed the basis for many international standards, including ISO 9162, and at this time is the framework for virtually all the world-wide commerce in LPG [44]. Sampling There are three sampling methods commonly referenced in North America: ASTM D 3700 (floating piston cyhnder), D 1265 (80% fill "open" cyhnder), and GPA 2174 (floating piston cylinder and on-line composite sampler). None of the sampling procedures are guaranteed to obtain a representative sample for all specifications or test methods, under all conditions. For example, a floating piston cylinder is capable of taking a liquid sample with no vapor generation that can deplete light ends and dissolved gases. However, machined interior metal surfaces can make this unreliable for trace levels of corrosive materials that can react with the interior surfaces, especially for longer term storage or transport. An 80% fill cylinder can have an interior coating to make it less prone to loss of trace corrosive molecules. However, the composition is slightly changed by venting liquid to establish the 20% vapor space that is necessary to protect against over pressure from thermal expansion of the liquid. ASTM and CGSB are addressing this by allowing the use of coated D 1265 cylinders for routine use for "specification" products, where the small but predictable loss of light ends is not significant. Laboratory measurements and Equation of State calculations confirm that the vapor pressure reduction between using D 3700 and D 1265 is less than 1.6 psi at the 208 psig specification limit. This is not significant with respect to the reproducibility of the vapor pressure test method, and would only be of concern at or near the specification limit, at high (7-8%) ethane contents [45,46]. The D 2163 hydrocarbon composition by gas chromatograph (GC) method is being revised to allow D 1265 "for composition," with a recommendation that D 3700 floating piston cylinder method be used for highly accurate results or when trace gas analysis is required. ASTM D 3700 is currently being revised to limit the scope to ASTM D 1835 "spec" products. This avoids the complications from sampling high pressure NGL's that may contain separated phases, inert gases, and corrosive materials (for example, from production prior to processing). A new appendix documents the various problems that must be addressed for sampling "non-spec" products. GPA 2174 (Obtaining liquid hydrocarbon samples for GC) will be recommended for sampling "non-spec" NGLs and related materials. This also facilitates revision of D 2163 (composition by GC) to include options for using Flame Ionization Detectors (FID) for specification products. FIDs are very linear detectors for hydrocarbons, but do not detect "inerts" (N2, Ar, CO2, etc.) that do not combust in a hydrogen
40
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flame. These are not normally present in "spec" products, except at very low p p m levels t h a t are not significant for intended use (see Composition below).
Composition and Calculated Properties (Octane, Density, Vapor Pressure) D 1835 products composition limits are set at levels that are stringent enough to satisfy the vapor pressure, octane (olefin), a n d volatility/residue r e q u i r e m e n t s of the vast majority of existing applications and equipment. Small composition differences within the bounds of the specification are generally not significant, a n d do not affect either the value or the end use. For example, propane containing 1-2 ethane has the same value and uses as propane with 5-6% ethane. Similarly, propane containing 1% propylene has the same commercial value a n d uses as 5% propylene. As a result, "Specification" products in D 1835 are relatively noncritical for hydrocarbon composition. The D 1835 products are relatively simple mixtures of a small n u m b e r of components. This makes it feasible to estimate several properties that tend to blend linearly by simple linear sum calculations. Calculation based on weighted linear s u m of component values per D 2598 can be used as an alternate to D 1267 Reid vapor pressure, density and octane. Similarly linear sum of C4+ (butane and heavier) can be used as an alternate to D 1837 volatile residue. C o m p o s i t i o n b y GC Very little history was found on ASTM D 2163, Method for Analysis of Liquefied Petroleum (LP) Gases a n d Propane Concentrates by Gas Chromatography The D 2163 GC method was originally based on u p to four different packed columns and thermal conductivity detectors using calibrant gas calibration procedures. Two of the colu m n packings were discontinued due to toxicity. The method was expanded to allow other capilliary/PLOT^ columns and alternate detectors, but retained the original calibration procedures and precision statement. The method is currently being u p d a t e d to allow columns based on resolution performance, as well as FID detectors using response factors referenced to methane, or other detectors of equivalent performance using calibrant gas procedures. D 3700 will be the recommended sampling procedure, but D 1265 will be allowed for routine use, except where highly accurate results are required for trace gases (see sampling above). Octane
Propylene has a low pure component octane value, and requires individual control (5% max) because it can be varied widely in commercial grade. Traces of the other olefins are effectively controlled by the m a x i m u m vapor pressure (ethylene) a n d m a x i m u m C4-t- content (butanes, butylenes and heavier). The defining specification for ASTM D1835 "Special Duty" propane (GPA 2140 HD5) is the m a x i m u m 5 vol% propylene content, intended to control the m i n i m u m octane n u m b e r for severe service engine applications (HD5 is "Heavy Duty, 5% maximum propylene"). Propane meeting the special grade (HD5) specification would have a n octane rating of 95 or greater by the D 2623 LP-method (if it were run). Commercial grade with higher propylene content can still be used for lower severity engine applications. The true HD5 (propane optimized engines) market has yet to materialize. This has resulted in decades of debate for the need for actual octane n u m b e r determination versus a linear sum calculation. Gasoline and LPG blending is known to be non-linear, but the LPG calculation method has prevailed This is probably more because the composition limits of "HD5" propane are restrictive enough that non-linear blending effects are too small to be significant. The same cannot be expected for blends of significantly broader composition [47], or for optimized engine applications that are more optimized for the fuel. In addition, there are some discrepancies in the pure component octane values used in ASTM D 2598, EN 589 and the original ASTM Motor LP Knock Test Method Development as documented in SAE 670055. These differences are relatively small, generally within 1 octane number. However, it may be necessary in the future to optimize either the calculation or the engine determined octane number, or as more recently suggested, by methane number. The methane n u m b e r of a gaseous fuel is a knock engine test using conditions similar to D2699 MON, but using methane and hydrogen as the primary reference fuels. The method has not been standardized to date. The advantage of the methane n u m b e r is that the test can be extended to include all gaseous fuels, including hydrogen, natural gas, LPGs cuid their mixtures, inert diluent gases such as N2 and CO2, and potentially other combustible gases such as CO and HH3. In principle, it is possible to relate methane n u m b e r to octane number, but there are several difficulties in practice. The octane scale, with iso-octane having a value of 100, is extended to 120 with the addition of about 6 mL/usg tetraethyl lead. This tends to make the test increasingly insensitive to non-linear fuel effects in the high range, and the use of TEL even in the lab is problematic. Similarly, the methane n u m b e r does not respond well to fuel effects at the low end of its range. Even so, a reasonably good correlation has been found to exist [48,49].
ASTM D 2623, Test Method for Knock Characteristics of Liquefied Petroleum (LP) Gases by the Motor (LP) Method was originally used to rate octane n u m b e r of LPG mixtures. This used a standard CRC knock engine fitted with an LP-gas carburetor. It was withdrawn because the octane n u m b e r of these simple mixtures could be accurately estimated by linear blending of component octanes based o n GC analysis per D 2598.
Methane n u m b e r also appears to lend itself reasonably well to calculation based on composition. If so, then the calculation method would be preferred for routine operations, similar to D 2598 for LPG today. This is a potential area of future research and standardization.
^ PLOT: Porous Layer Open Tubular (usually KCl deactivated alumina for LPG analysis).
Some recent publications indicate that the olefin content of propane relate to cleanliness, stability or tailpipe emissions
Olefins
CHAPTER in a similzir m a n n e r to gasoline [50,51]. However, there are some significant differences, and there is no consensus at this time. Propane tanks are not atmospherically vented, so there is no oxidative stability concerns for propane, as there is for gasoline and other fuels. LPG carburetor vaporizer deposits are associated with non-volatile residues, and not oxidative gums from fuel olefins, thiophenes and other trace reactive c o m p o n e n t s of gasoline. However, developers of m o d e r n LPG electronic fuel injection (EFI) systems have experienced significant difficulties with both inorganic and organic deposits that interfere with the proper operation of injectors. Propane derived from crude oil and natural gas contains essentially n o olefins. Propane from refinery cracking processes contain olefins, which may can either processed further (alkylation, polygas), separated for propylene sales, or blended into propane u p to specification or contractual limits. Refinery propane usucJly requires removal of propylene to meet the ASTM "Special Duty" or GPA HD5 specification. A recent study and review sponsored by the California Air Resources Board (CARB) r e c o m m e n d e d relaxation of the propylene a n d volatile residue specifications for a u t o propane, citing that a 10% propylene/5% butane fuel gave the best equivalent emissions performance [52]. The Board also recommended a new limit of 0.5% m a x i m u m butenes, pentanes, and heavier. These changes were proposed to preserve and enhance the current supply of auto propane fuel in California, while maintaining adequate emissions performance. About half the propane produced in California is a byproduct of refining processes [53]. This is a possible area of future research and standardization Vapor Pressure Historically, vapor pressure was the most critical LPG specification, being responsible for most of the serious problems in the early days of the industry (see History of LPG). Vapor pressure is invEiriably tied to pressure vessel and safety valve certification and transportation regulations, so it is generally viewed to be critical for regulatory compliance. However, m o d e m pressure vessel standards as well as LPG production e q u i p m e n t a n d analyzers have all b u t eliminated vapor pressure as a significant operational problem. Certification standards vary by jurisdiction, but typically, a large safety margin is built into the system. For example, a typical consumer cylinder pressure rated for 250 psig "working pressure" would be pressure tested after manufacture to twice this (500 psig) and equipped with a "pressure relief valve" (pressure safety valve or vent) 250-500 psig, 50-100% of the test pressure. Propane, at the maximum specification pressure, would have to be heated to over 140°F to even reach the opening ("cracking") pressure of the safety relief valve. In addition, the actual burst pressure is controlled to typically at least three times the working pressure (manufacturers are required to pressure test 1 cylinder out of each lot to destruction as a quality control procedure for the quality of steel and welds). It is common practice for the manufacturer to "overbuild" cylinders to eliminate any uncertainty in the costly certification process. For example, sample cylinders rated for 1800 psig working pressure are often tested at about 5000 psig, a n d have burst pressures far in excess of m i n i m u m
2: LIQUEFIED
PETROLEUM
GAS
41
three times the working pressure, often in the 12-15 000 psig range. Finally, all consumer cylinders, storage vessels, trucks, and rail cars require periodic inspection and recertification. These considerations have essentially eliminated LPG vapor pressure (VP), per se, as a significant problem. The LPG VP specifications indirectly control the maximum concentration of light ends, principally ethane in propane and propane in butane. The manual "Reid" VP test method was developed in a competition in the 1920s to improve upon the original U.S. Bureau of Mines "vapor tension" method (essentially a pressure gage on a length of 2 in. pipe). The competition was won by Dr. Reid (D323 Reid method), and an adaptation of this is used in D 1267 for higher VP (>26 psi) LPGs. The test has changed little over the years. Many laboratories are no longer equipped to r u n the LPG Reid Method. It is now much more common to calculate VP from composition by D 2163 using calculations in D 2598 than it is to run a manned D 1267 Reid VP. However, D 2598 method has no component VP data for m e t h a n e (if it is present), which may force the use of the manual method in some circumstances. In addition, D 2598 VP calculation is known to be biased high (e.g., conservative) at high ethane contents, due to use of a component VP extrapolated from above the critical point of ethane. By historical convention, the VP of LPG is reported in "gage" or "gauge" pressure in pounds per square inch relative to atmospheric pressure (psig). The pressure gauge used in the D 1267 LPG Reid method reads zero at the start of the test, but unlike D 323 or D 4953 the apparatus starts out liquid filled, and no air is introduced into the test apparatus during the test. The test result is essentially the partial pressure of the LPG relative to atmosphere, a n d the absolute pressure is higher by the barometric pressure at the time of the test. This has caused significant confusion over the years when using pressure transducers calibrated in psia units, or when reporting in metric units (kPa), which by definition would be interpreted as being an absolute pressure. As a result, the specifications require reporting in kPa (gage), which is not a true SI unit. Also, this difference is sometimes confused with the high bias at high ethane contents when using D 2598 calculation. ASTM is currently developing a s t a n d a r d test m e t h o d based on newer instrumentation similar to D 5191 and D 6378 for gasoline. This method uses an absolute pressure transducer, so this requires that atmospheric pressure be subtracted from the result to report a VP (gage) final result. There is no universally accepted criteria for setting the maximum VP specification for propane, and this is an area for future research and international standardization. The first GPA VP specification was 225 psig at 105°F set to meet the insulated rail tank car requirements of Interstate Commerce Commission (ICC). In 1955, the specification was revised to 215 psig at 100°F, mainly to use a single water bath for both natural gasoline and propane. Still later in 1955, the VP was lowered to 210 psig at 100°F to meet United States DOT safety standards for rail tank cars [54]. A variety of other regulatory limits also exist. This includes DOT 173.301 (f) (2), which limits VP in cyhnders to 300 psig at 1 SOT, Section 170.314 (c) 225 psig at 105°F, and Section 173.315 (c) (1) cargo and portable tanks at 250 psig at 115°F [55]. Other regulatory requirements would be expected in different jurisdictions. The origined HD5
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VP specification was 200 psig to promote heavy-duty engine applications, but was reset to the current 208 psig limit when this market failed to develop as expected. Higher vapor pressure specifications allow slightly more ethane to be included in the blend, which increases supply slightly, and is potentially beneficial for very low (<-35°C) temperature operations. However, higher ethane content in the gas phase can cause overly lean air/mixtures for some appliance applications with first vapor withdrawal from a full tank, and rich operation from essentially pure propane when the tank is near empty. This wide swing in air/fuel ratio and the attendant problems of calibrating the air/fuel ratio in vapor withdrawal service also prompted the industry to reduce the VP from 215 to 210 psig in 1955. Current ISO 9162 specifications have a maximum VP of 1550 kPa at 40°C, compared to 208 psig (1435 kPa) at 100°F for ASTM, GPA, and CGSB. This difference is larger than the difference in VP due to the small difference in test temperature. Equation of state calculations indicate that the ISO standard would correspond to about 213 psig at 100°F, making the ASTM standard more conservative by about 5 psig. This difference is generally not significant in day-to-day operations The practical maximum for a universal multi-use fuel probably lies somewhere between the current ASTM limits of 208 and the estimated ISO hmit of 213 psig at 100°F. This is a possible area of future research and standardization Volatility Residue ("Weathering") Most consumer propane applications withdraw vapor from the storage tank (vapor withdrawal) utilizing ambient air temperature to provide the latent heat of vaporization. This works well down to about — 35°C or lower, until the VP with auto-refrigeration of propane is not sufficient to satisfy appliance demand. Even thermal radiation to a black night sky can take away several degrees of operating temperature, which can be important when arctic nights can be up to six months long. The Volatility of Liquefied Petroleum (LP) Gas test (ASTM D 1837) is an indicator of the nonvolatile materials present in LP gas [56], and is intended to ensure that most of the LPG is available for use, and that low volatility components do not accumulate with repetitive tank fills. The tank will lose all pressure and "suck a vacuum" below the normal boiling point of the remaining components, requiring supplemental heating, pumps and/or external vaporizers. If, for example, excessive butane were collected in a tank after repetitive fills, the user could experience low pressure when the level indicator showed plenty of "propane" (actually butane) still left in the tank. This is aggravated by the strong temperature dependence of the "k" factor, or enrichment of the propane in the vapor relative to butane. The selective vaporization of the lighter components and retention of heavier components becomes more efficient as the temperature decreases, making this an increasing concern in colder climates. The original weathering test was called the Mercury Freeze Test, but was commonly called the weathering test, or the "tinkle and thud" test. The mercury was used as a temperature indicator, since no thermometers were then available for
-40°C. One hundred milliliters of pre-cooled liquid propane was allowed to boil off ("weather") in a glass tube with a large drop of liquid mercury. Initially, the liquid propane temperature would be at or below the normal boiling point of propane (—42°C), and the mercury would be a solid (melting point = —39°C), and then maJce a "tinkle" sound in the glass when shaken. The temperature of the remaining liquid would increase as the lighter components boiled off. If a sufficient quantity of butane or heavier components were present, the mercury would melt as the temperature of the remaining liquid rose above -39C, and "go thud" (a colloquialism for "failure" in some circles). The pass/fail criterion was that no more than 2 ml of liquid (mainly butane and heavier) remained when the mercury melted. One artifact of the test was that propane near the specification limit could pass at high altitude (low barometric pressure), but could fail at low altitude (higher barometric pressure). The boiling point of the mixture changed with pressure, but the melting point of mercury did not. The test was modified when suitable low temperature alcohol thermometers became available. The 1960 version of the method reported the temperature when 5 ml of liquid remained, corrected for barometric pressure. The original pass/fail limit of + 36°F was retained for butane and b-p mixtures, and a new limit of -37°F (-38.3°C) was set for propane. The advent and wide use of the chromatograph permitted addition of a quantitative limit of 2.5 vol% C4+ in propane and 2.0 vol% C5-Ifor butanes, based on GPA tests showing equivalence with the weathering test. The weathering test was at one time going to be withdrawn, but it is still widely used as a field check, and it was retained. These limits are known to be conservative, even for cold weather operations, confirmed by GPA tests at the University of North Dakota in the winter of 1977-78 using 5-15 vol% butane. However, there has never been any compelling technical or economic justification to change this specification [57], and the current limits are essentially unchanged from the original intent in 1932. Dryness of Propane Consumers need reliable operations when it is cold, but this is when icing is most prone to occur. As a result, "dryness" of propane is probably the most critical of all specification for downstream marketers and users. However, "dryness" is a difficult property to quantify in the LPG world. The "dryness" of propane is determined by specification performance tests that indicate the suitability of use of the LPG, but do not quantitatively measure water content. The dryness criteria are so stringent that "on-spec" propane generally avoids the free water, ice, and hydrate formation regions under the conditions of most end use applications. However, some applications, such as an inadequately heated liquid vaporizer, may require even lower water contents, and/or use of cryostatic de-icers such as methanol to avoid icing with propane that meets the specification "dryness" criteria. Propane and propane-butane mixtures are the only consumer fuels with a "dryness" specification that requires the water content to be well below the equilibrium saturation level. It is not adequate to use visual rating criteria, such as D
CHAPTER
2: LIQUEFIED
PETROLEUM
GAS
43
4176 "Clear and Bright," or "visually free of undissolved water" that is used for butane (and other fuels). It takes about 20-40 p p m of free water in a fuel to visually detect a haze with the unaided eye. In essence, visual ratings only require t h a t the fuel does not contain a n excessive a m o u n t of undissolved ("free") water. The butane test relies on visual detection of water collected in the bottom of the sample cylinder, and not a haze rating. The less stringent criteria is because auto-refrigeration of butane does not cause temperatures low enough to cause ice/hyd r a t e formation. As a result the same criteria applies to b u t a n e as for other fuels. Simple tank draining, which leaves the remaining propane saturated with water, is not nearly enough to meet the LPG "dryness" criteria. The presence of free water at any time in a propane system is sufficient to put the product severely off spec and unusable without dehydration or the addition of methanol (see cautions below). This is critical for proper operation of LPG handling equipment and appliances. Ice accumulation can block vapor or liquid fuel lines, Eind disrupt proper operation of mechanical equipment, such as pumps, meters, filters, VEJVCS (fuel lock-offs), and especially regulators (vaporizor/regulators). Icing can occur very quickly, and can be difficult to diagnose. "Wet" propane is anything that does not pass the "dryness" criteria, including the approximately 30-100% saturation range, even when there is no free water. For example, a storage tank of LPG that is deemed to be "wet" may or may not have any free water at the bottom of the tank. Water saturated LPG (but n o free water) fails all of the dryness criteria by a wide margin. The presence of any free water amounts to "gross" contamination. "Wet" propane that does not contain free water responds extremely well to low levels of methanol. Unlike ethanol, IPA, and higher moleculetr weight alcohols, methanol does not increase the solubility of water in propane (or any other fuel). It is a popular misconception that methanol is an effective de-icer because it increases the solubility of water in the fuel. Methanol decreases the solubility of water in fuels, and causes phase separation when it is injected into water-saturated fuel. The phase-separated liquid is non-freezing, so it will not cause ice/hydrate blockage. When the LPG is sub-saturated with water, the methanol remains dissolved in the propane along with the soluble water. It phase separates along with the water whenever the temperature gets cold enough, and prevents the separated water/methanol mixture from freezing. If the m e t h a n o l concentration is high enough in the separated phase, it wiU not freeze at temperatures down to the normal boiling point of propane of —43°C (i.e., nominal 50/50 vol% methanol/water mixtures Eire commonly used as windshield de-icing fluid good down to —40°C).
in LPG service, mostly because they lack the volatility to provide icing protection in the vapor space. The meaningfulness of the valve freeze test becomes questionable when the LPG contains anti-icing agents. The ASTM D 2713 method is stated to be "non-applicable" when antiicing agents are present. The current ISO and ASTM wording of the "not applicable" footnotes are currently different, and subject to interpretation. This is Ein area of never ending controversy, a n d is probably the highest priority area for research and standardization. None of the current LPG dryness methods (valve freeze, CoBr, Dew Point) have valid precision statements. The pass/fail criteria Eire set at levels that historically have been shown and are known to be suitable for most end uses. The most c o m m o n test for LPG dryness (D2713) is based on a "valve freeze" criteria, where flow is restricted by ice/hydrate formation in a restrictive orifice in a standard valve. The pass/fail criterion is intended as an indicator, to prevent a broader range of problems, and not just freezing of a small orifice valve flashing liquid p r o p a n e at —42°C. The other common test methods are based on the color of the hydrate of Cobalt Bromide exposed to equilibrium vapor, and the water dew p o i n t of the equilibrium vapor. Similarly, the pass/fail criteria are set at a stringent enough level to avoid a broad range of problems based on field experience.
Methanol is very effective in LPG systems at preventing freezing of separated phases, but if mis-handled can aggravate the a m o u n t of separated phase that can accumulate at tank b o t t o m s , and can propagate phase separated phases through the distribution system. Other alcohols or glycols form "glass" phases with water, becoming increasingly viscous on t e m p e r a t u r e drop, b u t without freezing in the traditional sense. They are often described as forming "slushy ice" that is very resistant to accumulation or blockage. However, these are reirely if ever used
It is initially counter-intuitive that the concentration of water in equilibrium propane vapor, withdrawn from the vapor space of a storage tank, increases as the t e m p e r a t u r e decreases. This is the key point of understanding water phase behavior in propane, and the related problems of dryness of propane. Water has a low solubility in LPG, similar to other "pure" parafins such as pentane, isomerate or alkylate. Since so little water is present, it is often confusing why dr3rness is so important for propane, much more so than for other fuels that
It is relatively easy to meet the dryness criteria for propane with m o d e m production processes. However, the occasional process failure of individual producers can affect m a n y others in pipeline gathering systems. Seasonal brine cavern storage, field dehydration facilities, steam/air cleaning of storage tanks, hydrostatic testing, a n d use of c o m m o n marine, pipeline, rail and truck facilities at EJI stages of distribution provide many opportunities for sporadic water contamination. In the occurrence of an off-spec dryness event, the technical difficulties of sampling and analysis can be enormous, and never certain. It is easy to show that propane is "on-spec," because the requirement is so stringent. However, when you are trying to figure out why the fuel is "off-spec," the "dryness" of propane quickly becomes the most exasperating specification, even if it is not the most critical [58]. There is a general correlation between water content and the "drjmess" criteria, but even this is not certain due to nonlinearity of water solubility in propane with temperature, relative linearity of the K ratio with temperature, and the possible presence of alcohol (especiEilly methanol), ice, and hydrates. There are additional factors that mEike the dryness of LPG nvuch m o r e complicated t h a n simple solubility and freezing for the other fuels. These are discussed below. Phase Behavior
of Dissolved
Water in
Propane
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contain more water. This section assumes no hydrate formation, similar to other fuels. Hydrates will be considered immediately following. The formation of "free" or undissolved water from a drop in temperature is similar to other fuels, except that it occurs at a lower concentration level due to lower solubility. The solubility of water in fuels increases with the temperature and aromatics content of the fuel, ranging from about 100 p p m w in low aromatics parafinic fuels to over 500 weight p p m in high aromatics content gasoline or diesel fuels at 70°F [59], A drop in temperature of water-saturated fuel causes the phase separation of a bulk (undissolved or "free") water phase to form as a "haze" of small water droplets. These tend to coalesce into larger droplets and gravity settles them to the bottom of the storage tank. Pure water ice (not hydrate) will only freeze if the temperature is below freezing. Since the solubility of water in propane is already low, there is m u c h less water available to phase separate with a drop in temperature. As a result, classical "fuel line freezing" is not very prevalent in liquid propane systems, and most problems are associated with smaller, m o r e easily plugged orifices in valves a n d regulators than in m u c h larger diameter fuel lines. If free w a t e r is present (e.g., grossly c o n t a m i n a t e d propane), dispersed droplets can "super-cool" below freezing in cold gases or liquids, including propane. This can result in rapid ice accumulation that tends to be most severe in the - 4 to — 10°C range, very similar to automotive carburetor icing and aircraft wing or propeller icing ("rime" icing). As each super-cooled droplet impinges on a surface, a portion of the water spontaneously freezes, adding to the surface ice deposit at the impingement point. The remaining liquid water droplet is heated to 0°C by the heat of fusion, but is rapidly cooled again by the surrounding cold fuel. This causes the remaining water to either freeze, or to form another smaller super-cooled liquid droplet, which can repeat the process to extinction. Flow patterns can create a small impingement point, and filter/screen surface area that is already plugged diverts flow to unobstructed areas. This can result in a very fast and efficient plugging mechanism. Ice accumulation in pipe elbows, meters, filters and screens is often associated with "snap" cooling to just below freezing, a condition that favors formation of super-cooled water hazes and "rime icing." Ice accumulated under these conditions often looks like a translucent "milky white" thick coating t h a t appears to have been "painted" on. These conditions are more commonly found in production facilities, prior to final drying, but can occur during downstream distribution if water has gotten into the system. Methanol is very effective at eliminating rime icing, since only a very small concentration of methanol in the separated phase is required to reduce the freezing point to less than about -10°C. Propane fuel systems are sealed under pressure, and not vented to the atmosphere. Tanks in the distribution system are never allowed to be totally liquid filled for safety reasons. An equilibrium vapor phase is therefore always present in downstream distribution (but not necessarily in production facilities). Most end use applications draw equilibrium vapor from the top of the tank, and large tanks always have a Pressure Relief Valve (Pressure Safety Valve or vent in other jurisdictions). These are safety devices that vent vapor in the event of an overpressure situation. As a result, the water con-
tent a n d freezing behavior of the vapor phase is just as important as the liquid phase. A c o m m o n phrase in the literature is that propane vapor has a much higher capacity for water than propane liquid. This is true, but doesn't provide much clarity. Water, with a molecular weight of 18, would have physical properties closer to niethane (mw 16) and Neon (mw 20) if it were not for hydrogen bonding. Hydrogen bonding results in a very dense, high boiling liquid water phase instead of a light low molecular weight gas. However, when water is dissolved in propane at low p p m levels, there is little hydrogen bonding between the water molecules, a n d it behaves like a low molecular weight dissolved gas. Dissolved water in propane is much more volatile t h a n propane, and becomes enriched in the vapor phase. The "K" ratio, or ratio of water concentration in the equilibrium propane vapor vs. the water concentration in the liquid propane decreases with increase in temperature, ranging from over 20 (mole/mole) at low temperatures down to about 5 at high ambient temperatures. Essentially the VP of propane increases faster with increasing temperature than water, in essence "diluting" the water vapor at higher temperatures. Unfortunately, this makes the propane vapor in the tank more h u m i d when it is cold t h a n when it is hot, making propane vapor more prone to icing. Several operational artifacts come as result of this phase behavior that are not intuitively obvious. For example, this also tends to moderate the absolute a m o u n t of water per unit volume in the vapor phase over a wide temperature range. Propane vapor tends to be more prone to water condensation/freezing than p r o p a n e liquid. "Wet" off-spec propane can be brought on-spec by venting equilibrium vapor, essentially distilling the water out [60], and this process will be more efficient at cool or cold temperatures. Large sample sizes are needed to do a valve freeze or CoBr test, because the sample changes water content as you withdraw equilibrium vapor during the test. The CoBr and Dew Point tests are more stringent at cold storage tank temperatures than hot. The valve freeze test (on the liquid) is not affected when the tank is full, but could be affected when the tank is nearly empty (large vapor space = large volume to enrich water in vapor, depleting or drying out the remaining small volume of liquid). Under some circumstances, it is very difficult to obtain a representative sample. Simply exasperating! LPG Gas
Hydrates
It is not widely appreciated how prevalent hydrates can be in LPG systems. Few people outside of research laboratories have ever seen a gas hydrate, even though thermodynamically they are the most stable form of solid ("ice") in propane systems. In fact, pure water ice is unlikely to form in liquid propane systems at all, unless the water content is very high, and free water is present in the propane liquid, either on the tank bottom or as a dispersed haze in the liquid. Gas hydrates are a class of ice-like clathrate solids formed from water and low molecular weight C1-C5 hydrocarbons, including propane [61]. They consist of water "cages" that surround or "enclathrate" the hydrocarbon molecules. Hydrate formation can be a significant problem for cold weather operations for gas and gas liquids production and processing, especially prior to dehydration. They can form in LPG process-
CHAPTER ing equipment, pipelines, and even under deep-sea conditions due to gas seepage into cold ocean water (with the potential of becoming a significant "new" energy resource) [62,63]. Hydrate blockages in pipes can be very hazardous. Depressurizing one side of the line can result in the hydrate blockage becoming a ballistic projectile, with the capability of causing considerable damage. The propane-water and methane-water systems have been studied in great detail due to their commercial importance. Several GPA and other research reports are available detailing the phase behavior of hydrates [64]. The c o m m o n industry "rule of t h u m b " is that hydrate formation requires free water to be present, and that it cannot occur with sub-saturated, on-spec propane. Technically, this is not true, but it is good enough for most applications, with some caveats to cover high dissolved water contents and high pressures. Technically, free water does not have to be present for propane hydrate to form, but in such cases, there is so little soluble water available for hydrate formation that it is rarely a problem, or at least is not a more severe problem t h a n if only water ice were formed. The presence of free w a t e r in the hydrate formation t e m p e r a t u r e range can cause severe problems because m u c h larger quantities of hydrate can form. Below freezing, there is no practical reason to differentiate between hydrates and water ice in most applications. Propane hydrates are stable above the freezing point of water. The higher the pressure, the higher the temperatures at which they are stable. They can form at any temperature below about 5.5°C (42°F) in propane at its saturated VP. No externally applied pressure (for example, a pump) is required for propane hydrates to form. Externally, the symptoms of having ice or hydrates in LPG systems are virtually identical. They both form only when it's cold, they both accumulate and plug lines and orifices, and they both are "melted" by methanol, leaving the same a m o u n t of liquid methanol/water mixture at the bottom of the tank. The problems are the same, the fix is the same, and the result is the same. Most of t h e time, it doesn't matter, a n d there is n o need to differentiate between water ice and propane hydrate. Gas hydrates are the "phantoms" of the LPG world, and are notoriously difficult to "diagnose" when they do occur. They decompose (sublime) to water and hydrocarbon gas at ambient pressures very quickly, since they are essentially solids with a VP of about 80 psi. The sublimation rate can be high enough to support a burning flame [65]. Often they are gone by the time the e q u i p m e n t is depressurized and opened for inspection. It is always uncertain whether hydrate, pure water ice, or both are involved when icing occurs in LPG systems. The biggest difference is that "icing" can occur between 0 and about 4.5°C in LPG systems, due to hydrate formation. It is r a r e for the t e m p e r a t u r e to be stable in this range long enough during a n icing episode to make it obvious that hydrate and not water ice is the cause. The day/night temperature difference is generally m u c h larger than this. However, even w h e n this occurs, people will generally first tend to think that the thermometer is off by a couple of degrees, or that there is some auto-refrigeration somewhere, rather than a hydrate event occurring above 0°C. Hydrates can form both in the liquid and vapor phases, but there are some differences in behaviors that can be important
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in some situations. Firstly, hydrates tend to form at free-water surfaces. This is because propane type II hydrate has a composition of a b o u t 17.1:1 mole ratio of waterrpropane, which greatly exceeds the propane solubility in water. As a result, when a tank containing free water is cooled towards zero, it will tend to form a thin layer of hydrate on the water surface, like the first thin skin of ice on a pond, puddle, or ice cube tray. The hydrate layer becomes a barrier for propane diffusion, effectively limiting the rate of further hydrate formation. As the t e m p e r a t u r e drops further below 0°C, the underlying "metastable" water can freeze to water ice. Thermodynamically, the hydrate and water ice should not exist together; they are only there due to a kinetic rate limitation. Under thermodynamic equilibrium conditions, water ice would not form until all of the propane is consumed first forming hydrate, leaving water or water ice in equilibrium with hydrate, and no propane liquid. As a result, mixing and turbulence are invariably factors in hydrate formation from propane liquid. Secondly, hydrates can form very quickly on tank surfaces exposed to the vapor space by deposition from the vapor (i.e., the opposite of sublimation). The equilibrium concentration of water is always higher in the vapor phase, which favors hydrate formation from the vapor. This can be a very fast and "efficient" process, as there is no kinetic barrier, and the water vapor and propane are already mixed. Only the rate of diffusion and convection of the water vapor limit the rate of hydrate formation. Hydrate formation from saturated equilibrium vapor depletes water from the vapor phase, essentially dehydrating it. Additional water will vaporize from the liquid, and contribute to additional hydrate formation. If liquid water were in the tank bottom, and one waited long enough, all of the water would eventually form hydrate at the tank bottom, or dissolve into the liquid propane, vaporize, and form hydrate on the vessel walls that are exposed to the vapor. This process would continue until all the free water was consumed, leaving only hydrate on the tank bottom and vapor walls. The liquid propane in equilibrium with the hydrate can still be "off spec" near the high end of the hydrate formation range. The real fun begins when the tank gets emptied out and re-filled with dry or warmer propane, which immediately goes "offspec" for no apparent reason, or the temperature changes rapidly, and slugs of water or chunks of "ice" show up. This can result in some truly exasperating situations when investigating water related problems in cold LPG systems. You are never sure if free water is present, or if the liquid or vapor is in equilibrium, or if any sample is representative. The basic concept of obtaining representative samples is s o m e w h a t uncertain. You generally don't have a b o t t o m drain, and you can't see inside of the tank liquid or vapor space. When you open it up, it's all gone. This, plus the inherent difficulties of sampling/measuring low p p m levels of water and methanol in LPG makes reliable quantitative measurement of water virtually impossible in cool/cold weather, without extraordinary attention to sampling and analytical detail. Throw in a couple of different test methods with no precision statements, no calibrants, no reference standards, no QCs and uncertain correlation, and "exasperating" is an understatement. It is left to the reader's imagination the many hours of stimulating conversation possible between the lab, the process
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engineer, the plant manager, and the receipt terminal. This is a potential area of future reseetrch and standcirdization. Fortunately, the high VP of methanol makes it a very effective means to eliminate ice or hydrate formation in both the liquid a n d vapor phase. Methanol, the lowest molecular weight alcohol, partitions between liquid and vapor phases closest to water, a n d tends to protect b o t h phases from freezing. Unfortunately, the specifications are silent on the use of methanol, the test methods are inconsistent in response to methanol, and there is no consensus between the upstream and downstream industries on the intent and extent of use of methanol in LPG. This is a potential Eirea of future research and standardization. Recently, some non-cryoscopic kinetic "hydrate inhibitor" additives have been commercialized for processing/pipelining applications, but these are not applicable to specification products [66]. Auto-refrigeration Like any gas (except hydrogen) near ambient temperature, p r o p a n e vapor will undergo Joule-Thomson (JT) cooling when depressurized (flashed) across a veJve, orifice, pressure regulator, or other device. The cooling effect is dependent on the process, whether it is adiabatic (a valve) or isentropic (turbo-expander) or polytropic (something in between). The JT cooling effect is not very large, about 0.25°F per psi drop in the 30--40°F range for propane, but this can be significant under certain conditions. In small regulators, the JT cooling is offset by heat gained from the environment, tending to be isothermcd at low flow rates (high heating from ambient air, low JT cooling from low flow). However, as the appliance dem a n d and gas flow increases, there may not be enough convective heating to overcome the JT cooling, and the system tends toweird adiabatic conditions. If the inlet propane vapor is near water saturation (off-spec) and only slightly above the freezing point, the cooling effect can be sufficient to cause phase separation and ice formation at the expansion point. Similarly, if the valve or regulator body is cooled below freezing, then the high-pressure propane vapor at the regulator inlet could be cooled below freezing by the cold valve body. This results in inlet freezing, upstream of the point of vaporization, where the actual cooling occurs. A commercial technical bulletin (LP-24) is available from a regulator manufacturer dealing with vapor regulator inlet line freezing from these effects [67]. Ice will tend to accumulate over time in a flowing system, until a blockage is eventually formed. If the propane liquid is substantially sub-saturated (i.e., on-spec), the cooling from expansion of equilibrium vapor is generally not enough to cool the vapor to the water saturation temperature (dew point), and no ice/water is formed (one reason why HD5 and functionally equivalent specs are where they are). When liquid propane is depressurized or "flashed," a fraction of the liquid will vaporize, cooling the entire mixture down to the boiling point and dew point of the liquid/vapor mixture (they are the same if the mixture is in equilibrium). This cooling effect is large, for example resulting in liquid and gaseous propane at about —42°C at atmospheric pressure. MoUier diagrams (pressure-entropy), pressure enthalpy diagrams, or tables of thermodynamic properties can be used to quantify temperatures, pressures, enthalpies and entropies
for a variety of conditions. Flashing liquid propane into even moderate vacuums can attain temperatures below — 80°C. The vapor pressure and solubility of water in propane below —42°C is extremely low, so essentially all the dissolved water that was present is converted to hydrate/ice. Liquid propane vaporization systems almost always require external heating, b o t h to convert all of the liquid to vapor, and to prevent eventual freeze u p from ice. Transient conditions, for example startup of automobile vaporizer/regulators in very cold weather, t e n d to be p r o n e to icing, until the engine coolant gets w a r m enough to keep the vaporizer above hydrate/freezing temperatures. The methanol addition rates t h a t are r e c o m m e n d e d in various codes and standcirds for new tanks or for when moisture is present) are intended to remove the free water in the tank, and leave only a small a m o u n t of methanol/water mixture prior to putting into service. They are not intended to provide protection where gross water contamination has occurred or to allow weak water/methanol mixtures with high freezing points to remain in the tank. For this reason, the standards generally require inspection and removal of free methanol/water prior to putting the tank into service. Methanol is effective at preventing freeze-ups to very low temperatures, but there is no universally accepted criteria for how much methanol to add to propane during downstream distribution as a preventative m e a s u r e . Many companies have optimized methanol addition rates based on actual temp e r a t u r e s and/or dryness test results. These are effective strategies with a relatively sound technical basis (see phase behavior of m e t h a n o l ) . This is a potential area of future research and standardization. Butane, with a normal boiling point of neetr 0°C is not capable of creating ice freezing conditions by auto-refrigeration u n d e r normeJ conditions of use, and is treated as in other fuels with only a "visually cleeir" requirement. However, if butane is used below ambient pressure, the same precautions may have to be used, as the boiling point at reduced pressure is below the freezing point of water, a n d ice/hydrate cam form. Butane, propane and their mixtures are widely used as the working fluid in large industrial and chemical process refrigeration applications, such as industrial chillers, gas processing, refinery gas processing, or reactor cooling in alkylation processes. These are usucJly integrated into the production processes themselves, so they are n o t very "visible," even though they are very large and very commonplace. All generally require some provision for w a t e r removal and icing protection. Phase Behavior
of Methanol
in
Propane
Methanol is a unique material in the LPG world. It is the only alcohol with a lower molecular weight (31) than propane, and the only one to be concentrated into the vapor (like water). As a result, it tends to partition and "travel" with the water, making it very effective in preventing freeze-up in both the liquid, vapor, and condensed vapor phases. The freezing point depression properties of methanol in water are very good, providing protection down to -40°C [68] and below. Methanol, being a small molecule, has a high diffusion coefficient and is very effective at dissolving water and hydrates. No other alcohol has this broad range of beneficial properties, so methanol is the only alcohol that is commonly used in LPG systems.
CHAPTER The use of methanol is mandated under certain codes and standards to minimize free water from tanks prior to putting t h e m into service or if water is reasonably expected to be present [69]. Methanol works by creating either a second non-freezing separated phase in the liquid, in the vapor (hydrate), or the vapor if re-condensed. Methanol/water mixtures will not freeze, but will gravity settle, and tend to collect at tank bottoms, where it can be difficult to remove. This can promote other problems such as corrosion and resulting sediment and filter plugging, which is undesirable in the downstream applications. As a result, there is a general consensus that propane should be manufactured as dry as necessary for "normal" operations, and that methanol should be used as needed for new tank commissioning, and cold weather operations in downstream (post refining) distribution and use. Methanol is equally difficult to detect as water at low p p m levels, especially in the field. As a result, the actual amounts of water or methanol present are rarely measured directly. It is also recognized that methanol interferes with some of the c o m m o n "dryness" test methods once it has been added to propeme. D2713 "valve freeze" test is quite explicit in stating that the test does not apply to LPG containing antifreezing agents (although it is commonly used for this purpose in spite of this caveat). The CoBr test is believed to be relatively i m m u n e to methanol, but this test is not widely used. The chilled mirror dew point test is affected by methanol, since it phase separates with water. Electronic hygrometers (dew point meters) m a y or may not be affected by m e t h a n o l depending upon the principle of operation. In colder climates, it is common for methanol to be added to bulk storage during distribution as a preventative measure. Long-term field experience indicates that this is a n effective strategy to minimize regulator freeze-up problems, so long as addition is controlled to the m i n i m u m required. Different companies have applied mzmy different criteria, in the absence of reliable field tests that can be used to monitor and control water/methanol. This is an area of r e c o m m e n d e d focus for future research and standcirdization A 50 wt% methzmol/water mixture (common windshield wiper Emti-freeze in cold climates) will not freeze down to below — 40°C. Therefore, as long as any separated methanol/water phase is at least this concentrated, no freezeu p is possible under typical conditions of LPG use. Methanol is more soluble in propane than water, so more remains in the propane phase in equilibrium with the methanol/water separated phase. Limited low temperature solubility data indicates that the methanol solubility in propane in contact with a 50 wt% methanol/water solution at -40°C is about 400-800 p p m w [70]. This indicates that it would take about 400-800 p p m w methanol to prevent freezing down to about —40°C at ambient pressures for w a t e r saturated p r o p a n e (about 100 p p m w at 70°F). This is smaller than the solubility of methanol in propane at ambient temperature, so methanol use tends to be "robust" so long as gross overdoses are avoided (and a dry grade of methanol is used). Slightly lower treat rates would be expected for lower water contents, or if freezing protection was only required to a higher temperature. The lower limit to eliminate all solid formation at -40°C would be expected to correspond to the 400-800 p p m w solubility of methanol in propane u n d e r these conditions. The ab-
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solute values of water/methanol solubility in LPG below the saturation levels and at varying olefin levels are not accurately known. This is a suggested area of research a n d standardization. Ethanol and iso-propanol have a higher solubility and lower vapor pressure than methanol in liquid propane. They require a significantly higher treat rate per p p m w of water present to prevent formation of solids at — 40°C in both the liquid and equilibrium vapor. To date, methanol is technically and economically the best de-icer for propane. In general, over-addition of methanol should be avoided, as drops in temperature or contact with water can result in most of the methanol phase separating in distribution and storage and causing problems. For example, if w a r m propane containing 1% methanol either cooled down to — 20°C or were contacted with water, then more than half of it will phase separate and settle to the bottom of the storage tank as a non-freezing water/methanol mixture. This would amount to 500-1OOOL for a large rail car. This large volume of methanol/water mixture has the potentieJ to accumulate in downstream storage, to the point where individual small deliveries could contain gross quantities of water/methanol. It is generally accepted that methanol should not be used as an alternative to proper dehydration during production, and that periodic use should be minimized. For the producer, the answer is to consistently make LPG very dry, and avoid periodic "slugs" of water. For the marketer, who is subject to the downstream sources of water, the answer is to monitor, and only use methanol when required during cold weather. This is also an area of r e c o m m e n d e d research a n d standardization. Sources
of
Water
Water can be introduced into propane distribution systems from a vairiety of sources. Faulty production can leave excess dissolved water or free water in the LPG due to inadequate dehydration. Water is used for hydrostatic pressure testing of large pressure vessels including tanks, trucks, and rail cars. Steam is used to clean and "inert" tanks for maintenance and change of service. Water can become trapped in tanks from previous wet loads due to internal "dead volume" at the bottom of tanks due to valves or piping that is recessed for physical protection, a c o m m o n requirement of safety codes. Exposure of tanks to the atmosphere can result in moist air condensing water on inside surfaces, where it becomes trapped, and accumulates with each diurnal heat/cool cycle. Free water in trucks or rail czirs tends to be persistent because of the low water solubility of water in p r o p a n e . A small a m o u n t of water in a truck or rail car can render m a n y subsequent loads "off-spec." Recent changes to rail car pressure vessel certification requirements provide another opportunity to further reduce the use and occurrence of steam cleeming and hydrotesting of large pressure vessels (truck/rail). Work is ongoing in this cirea and several programs are planned. This is a suggested area of research and standardization. Measurement
of Water in LPG
There has cJways been a desire in the LPG industry to have better test methods for "dryness," especially for production and terminal bulk receipts, which are the two main control points
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(See "dryness" sections above). This has been an elusive goal for many years, due the technical difficulties involved. The criteria for pass/fail on the CoBr test are as follows [71]: Color
Per Cent Humidity^
Green Blue Lavender Pink
Very Dry From nearly 0% to 30% From 30% to 52% From 52% to 100%
Pass (rcirely seen) Pass Fail Fail
It is generally accepted that a 60 s valve freeze pass is about equivalent to a CoBr pass, or a —26°C dew point and less than about 35 p p m w water in the LPG liquid phase. However, these are not exact relationships, and there are several complicating factors due to differences in phase behavior with temperature and composition. Different people have made different assumptions in estimating equivalence between water content, the liquid propane phase, and the CoBr test results. During the CoBr test, the equilibrium vapor sample is preconditioned with an ice bath to 32°F and 50 psig, slightly below the saturation vapor pressure at 32°F. Most people assume the humidity values in the table to apply at 32°F in doing the equivalence calculations. The a u t h o r noted that "the values in this table were determined by series of tests on propane vapor containing various amounts of water over a temperature range of from 35-70°F. The percents at which the changes of color take place seem to be somewhat affected by t e m p e r a t u r e . However, the determinations were not accurate enough to assign closer values in the table." In the test, the actual humidity of the sample is determined by the bulk tank or bulk sample temperature, not the 32°F test t e m p e r a t u r e . The humidity of the equilibrium vapor varies inversely with temperature at constant liquid water content, due to the change in the "K" ratio. Therefore, there is no single p p m water level of water in liquid propane that corresponds to a CoBr (or Dew Point) pass/fail level. The equivalence level is inherently temperature dependent. Both the CoBr and Dew Point test methods are most severe (conservative) at the lower temperature ranges. One can estimate the amount of water in liquid propane that is required to pass the test on the known dependence of "K" ratio and water dew point (vapor pressure) in gases. This depends on what assumptions are made about the humidity level for failure condition (32% or 40%) and to what temperature the humidity shown in the table applies (32°F versus 60°F). A propane sample taken at 32°F has variously been estimated to require less than 10-20 p p m w water in the liquid to obtain a passing result. A sample taken at 100°F has been estimated to require less than 20-35 p p m w in the liquid to obtain a passing result. This temperature dependence is not recognized in the test methods. However, the pass/fail criteria are severe enough that propane obtaining a passing result is considered suitable for use regardless of the temperature of the sample.
^ The humidity of the equilibrium vapor is dependent on temperature, since the wt. "K ratio varies from about 10 at 60°F to about 15 at 32°F. As a result, both the CoBr and Dew Point test methods are more severe (conservative) at 32°F than at 60°F (or higher). This temperature dependence is not considered in the test methods, and the same pass/fail criteria is applied regardless of the temperature of the storage tank and equilibrium vapor.
Similarly, the CoBr sind Dew Point tests would be expected to be somewhat influenced by gas composition, and especially by olefin content. Olefins have a higher affinity for water, and a correspondingly lower "K" ratio. Directionally, higher propylene content would make the test less severe, requiring higher liquid water contents to obtain a test failure. This difference has not been quantified, and is generally not considered to be significant for commercial propane. There are a n u m b e r of other anal3^ical techniques that can be used for the anedytical determination of water in LPG. Some methods, such as the Karl Fisher, can be used but are difficult to apply to LPG, because of the physical properties. Others, such as vapor phase determinations, can be influenced by sample size due to the very high "K" ratio of dissolved water in propane. The cobalt bromide test, for example, recommends at least 15 gallons of LPG sample if the test cannot be done at the storage tank with the equilibrium vapor. In other cases, such as terminals supplied from underground cavern storage, there is n o source of equilibrium vapor available on-site, because all the vessels are liquid filled. Electronic hygrometers and "dew point" meters [72,73] have been successfully used, and while they have the necessary sensitivity, and are widely used in the natural gas industry [74], they are not currently recognized in the LPG standard. Care must be taken with their use, especially for calibration a n d QC procedures to ensure that results are valid. Several new ASTM procedures are now available that allow adoption of on-line analytical techniques to product certifications, and LPG "dryness" is a prime candidate for these new methods. Many companies have installed solid state electrometric probes at production facilities to monitor drying operations, and there is emerging interest in having these included as an alternate test method in the standard. This is a recommended area of research and standardization. Sampling and sample introduction also needs special care and attention. Formation of hydrates can metke the storage time prior to analysis a factor. At high temperatures and humidity levels, the air in the lab can contain 20 000-40 000 ppmv water vapor, and there can be more water adsorbed on sample lines a n d cylinders t h a n is in an on-spec sample. More than one analyst has falsely reported the humidity level of a plastic sample transfer line as an off spec LPG result. Recently, a new stain tube was developed for use as a field test in natural gas pipelines that has a lower sensitivity to trace glycol used in natural gas dehydrating systems [75]. These stain tubes still have an interference from methanol, but may still have some application as a field test, for example on terminal receipts prior to the addition of methanol. These calibrated in the range 2-10 Ib/MMscft (traditional units for natural gas), which is equivalent to about 40-220 ppmv (multiply by mole weight ratio 18/44 = 0.41 to convert to mass p p m assuming ideal gas at low pressure). This is in the correct range for use on LPG equilibrium vapor, so it can likely be adapted to use for LPG. This would be very similar to the GPA and CGSB field methods for mercaptan odorant (see odorization), that use similar stain tubes calibrated in traditionetl natural gas industry units, with appropriate conversion factors. At present there is no standard test method for the use of stain tubes, or electronic hygrometers or dew point meters for LPG specifications. The ASTM natural gas standards that use these m e t h o d s currently do not include LPG in
CHAPTER the scope. This is another area of recommended research and stEmdardization. R e s i d u a l Matter (Oil Stain & R e s i d u e ) Non-volatile materials that are soluble in liquid propane tend to a c c u m u l a t e at the point of p r o p a n e vaporization as a residue or gum that can interfer with the proper operation of some equipment. The oil stain test attempts to concentrate non volatile residues that can be visually detected by putting it dropwise on a clean filter paper. This test is quite subjective a n d can be influenced by the grade of filter paper, or color of the residue. Historically, the main purpose of the test was to detect traces of compressor oils, a c o m m o n occurence in the early days of the industry, but less c o m m o n today. Other sources of heavy naaterials can be heat transfer fluids from distillation processes, caustic, amine and glycols used in some processes, greases and "pipe dope," sample cylinders, hose plasticizers, or cross contamination with other hydrocarbon fuels and lubricants if any c o m m o n equipment is used in distribution. Contaminants that form "oily residues" or "waxy solids" with auto-refrigeration of vaporizing propane, especially in association with fine particulates, can interfere with proper operation of safety lock-offs, overpressure vents and regulators. Non volatile additives are generally not recommended for use in propane intended for vaporizing service, and specific warnings are included in D130 copper corrosion test pertaining to corrosion inhibitors ("masking agents"). Some higher boiling or polymeric materials may interfere with catalytic heaters or automotive catalysts if present in the liquid propane. ASTM specifications currently do not include limites for higher t e m p e r a t u r e evaporation limits. This is a potential area of future research a n d standardization. Propane is a very poor solvent for parafin wax (n-parafins or near parafins) or for asphaltenes, and has been used in commercial de-waxing and de-ashpalting processes. Contamination of LPG with "black" oils that contain asphaltenes (IFO, HFO, Bunker, Pitch, Crude) or distillate fuels that contain wax (diesel, furnace oil, marine gas oils, kerosene, Jet) can result in the precipitation and collection of waxy or asphaltene solids. This can cause m u c h more severe operational problems than an oily liquid because of more severe impacts on equipment. Care must be taken to prevent cross contamination of these materials. Sources for such contamination can include leaking heat exchangers, pipeline "dead legs" or idle p u m p loops, meter provers or other distribution equipment that may be inadequately cleaned from a previous service. Nonvolatile additives may be beneficial for certain applications, such as detergent/dispersant additives for autopropane, similar to gasoline. However, this can introduce the need for a segregated fuel, since they can still be detrimental to vaporizing services. Maintaining a single universal grade would require additive injection at the point of dispensing. This is a potential area of future research and standardization.
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door use. The original "lamp" method is still listed in most specifications, but rarely, if ever used. A variety of newer instrument methods are in common use, and are under consideration for inclusion in LPG specifications. Recently, the requirement was changed to include the sulfur contribution from added mercaptan odorant, which is effectively a tightening of the standard. The California ARB "HDIO" standard has an 80 p p m maximum sulfur limit. All LPG specifications have some provision for the control of corrosive sulfur compounds to protect copper and copper based alloys commonly used in fittings and connections in storage, transportation, a n d appliances. ASTM D1838 is essentially a modified ASTM D130 copper coupon corrosion test using pressure rated equipment for LPG. The same grade of copper strip and preparation procedures are used, and the strip is suspended on a hook fixed to the outage (ullage) tube in a purpose built stainless steel test cylinder with an O-ring removable top closure. Sulfur based corrosion in LPG is a surprisingly complex subject, due to the chemistry of the reactive sulfur species that can be present, smd the nature of the test [76]. Mixtures of certain sulfur compounds are more corrosive than each alone, even if the individual compounds were present at higher concentrations. This can lead to occasionally bizarre situations where two on-spec propane batches go "off-spec" upon mixing. The classic case is w h e r e one batch contains a small amount of dissolved elemental sulfur (S8), and the other a trace of H2S (hydrogen sulfide) or COS (carbonyl sulfide). Each batch can be on spec by a wide margin, but the mixture can show a persistent fail, or even worse, an occasional fail. There is nothing like a persistent occasional fail to go into "dispute mode," with no hope of short-term technical resolution. Many hours of stimulating conversation are made possible between shippers on a c o m m o n pipeline system or storage facility with such events. Each may sanctimoniously claim the others to b e at fault, presenting credible data to prove their innocence with consistent Cu corrosion passes. Try devising a "round robin" to resolve that dispute. In the vast majority of cases, corrosion problems in LPG are associated with H2S, Sg (elemental sulfur, eight member ring), COS (carbonyl sulfide), RSSxR (alkyl polysulfides), HSSxH (polysulfanes), H2O (water) and O2 (in air). The most important are usually H2S (improper treating), Sg (usually from oxidation of H2S) and COS hydrolysis (reaction with H2O to form CO2 and H2S). Mixtures of H2S and elemental sulfur (Sg) form very reactive polysulfur intermediates that are very corrosive, and react very quickly with most common metals. Trace H2S and Sg £U'e legendary for their difficulty to sample and analyze. Fortunately, lead acetate paper is very sensitive as a field test for H2S. Sampling and analysis for trace Sg is always problematic. Almost all copper corrosion problems are a direct result of inadequate treating at the time of manufacture, and/or the highly non-linear blending characteristics of different sulfur types in propane from different sources.
Corrosion, H2S, a n d Sulfur
Contziminants
Historically, the total sulfur content of LPG (sometimes called the volatile sulfur content) was always much lower than for other fuels, and this was important, especially for in-
Magnetic
Residues
("Black Deposits")
and
Particulates
The occurance of iron oxide/sulphide magnetic residues was studied by the Propane Gas Association of Canada (PGAC
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[77]) as a result of widespread problems especially in automotive LPG fuel systems [78]. Iron particulates either from heat treating "mill scale" or corrosion processes can adversely affect the operation of components that magnetically attract and trap these materials in the working mechanisms (e.g., magnetically coupled level gauges and electronic solenoid valves). Collection of gross a m o u n t s of solid residues (magnetic or otherwise) can adversely affect operation of filters, filter lock-offs, regulators, mixers, pressure release valves, etc., by mechancial blockage. Two to ten micron (absolute. Beta 5000) filters have been found to be effective to remove these materials with good filter life (granular well behaved filter calie). Ammonia A m m o n i a is not used in the production process, a n d is generally removed by gas stripping and distillation if it is present. Anhydrous a m m o n i a is widely used in agriculture as a fertilizer, and is transported in the same pressure rated equipment as LPG. It is c o m m o n for rail and truck cargo vessels to transport "anhydrous" during the spring and s u m m e r growing season. The s a m e rail or truck cargo tanks are converted to LPG service for the fall and winter heating season. This is advantageous for optimal use of transportation equipment, but offers increased chance of cross contamination without adequate cleaning in the change of service procedure. Anhydrous e q u i p m e n t a n d LPG p r o d u c t i o n e q u i p m e n t generally use only carbon or stainless steel fittings. Brass (copper alloy) veilves and fittings are commonly used in LPG systems, particularly those at the dealer and consumer level. Ammonia can cause stress crack corrosion in brass and copper fittings at concentrations above about 5-10 ppmv [79]. There is no firm specification or guideline on the maximum allowable ammonia concentration of propane for either short or long term exposures, Eind a level of < l p p m (detection by field litmus paper test) is usually recommended. Field tests based on color change of wet red litmus p H paper are adequate to detect ammonia down to about 1 ppm, and provide an effective go/no-go test after change of service or at any time during distribution [80]. The time required for the color change due to the field test can also be used to judge the severity of the contamination. Very high (>20 ppm) concentrations cause a strong color change in only a few seconds. Moderately high (5 ppm) will cause a medium color change in about 10 s and low ( < l p p m ) will cause slight change in about 30 s. The litmus test is very sensitive to any material with even a slightly higher than 7.0 pH. This includes saliva (depending on what was last eaten), high p H potable ("tap") water in some regions, or soaps or detergents that may have been used to wash the hands or equipment. Controlled tests indicated that p r o p a n e with low concentrations of a m m o n i a would have n o effect on the concentration of ethyl mercaptan odorant [81]. Halocarbons
(Refrigerants)
There have been rare occasions of contamination of LPG with halocarbon refrigerants or fire retardant (Freon ^M or Halon '•'*'). These materials are miscible with propane, and will form noxious gases upon combustion.
Naturally Occurring Radioactive Materials (NORM) Sludges and tank scale from propane storage tanks, trucks and rail cars, and filters/screens may contain Naturally Occurring Radioactive Material (NORM) in the form of lead 210 (Pb'^'"). Similarly, equipment used for the transfer of propane such as product pipelines, pumps and compressors may have detectable levels of radioactive lead 210 on inner surfaces. Workers involved in cleaning, repair or other maintenance of inner surfaces of such equipment should avoid breathing dust generated from such activities. Suitable codes of practice should be developed for these activities detailing appropriate occupational hygiene and disposal practices [82].
VOLUME CORRECTION FACTORS (VCF) FOR LPGS AND NGLS [83] LPG has the lowest density, largest thermal expansion and compressibility and highest operating pressure of the comm o n fuels. It is the only fuel where compressibility is significant e n o u g h that it can be taken into account both for calibration of composition test methods and custody transfer metering. The original set of Volume Correction Factors (VCF) were developed in an NGAA (GPA) sponsored research p r o g r a m in 1942. It was first published as GPA standard 2142. These factors were later adopted as national and international measurement standards as ASTM/IP 1250. The Volume Correction Factors (VCFs) are used to correct observed volumes to the volume that would be observed at standard conditions (60°F, 14.969 psia). These standard conditions serve as a way to equitably use volumetric measures in general commerce. They are derived from actual density measurements on a wide range of LPG/NGLs, and are essentially the average values of the coefficients of thermal and pressure expansion. These factors are strongly correlated to the density of the material (e.g., the lower density the mixture, the more compressible it is). As a result, the calculations are iterative, and separate tables are published, each covering a narrow range of densities. The differences are small, but they do have large economic impacts because of the large volume of material involved. D 1250 is the ASTM standard for calculating the VCF of a petroleum mixture. This standard essentially directs the use of procedures published in the American Petroleum Institute (API) Manual of Petroleum Measurement Standards (MPMS). As of 1999, there are several procedures that may be used to calculate the VCFs: • Chapter 11.1, Volume Correction Factors, gives procedures to determine the VCFs due to temperature for crude oils, refined products, and lubricating oils. However, since the upper limit on API gravity is 100° it applies only to the heaviest of LPG. • Chapter 11.2.1, Compressibility Factors for Hydrocarbons: 0-90 "API Gravity Range, and 11.2. IM, Compressibility Factors for Hydrocarbons: 638-107 kg/m^ Range, give procedures to determine the VCFs due to pressure. Like Chapter 11.1, since the upper limit on API gravity is 90° it applies only to the heaviest of LPG.
CHAPTER • Chapter W.l.l, Compressibility Factors for Hydrocarbons: 0.350-0.637 Relative Density (60°F / 60"F) and -50°F to 140 °F Metering Temperature, and 11.2.2M, Compressibility Factors for Hydrocarbons: 350—637 kg/m^ Density (ISV) and —46°C to 60°C Metering Temperature, also give procedures to determine the VCFs due to pressure. These procedures cover the lower densities indicative of most LPG and NGL. Until 1998, the VCFs due to temperature for the LPG and NGL not covered by Chapter 11.1 were calculated from a variety of sources. The major source was a set of Petroleum Measurement Tables published in 1953 by ASTM International and the Institute of Petroleum (IP). New implementation procedures for liquids with relative densities (60760°) of 0.3500 to 0.6880 (272.8 to 74.2°API) were p u b h s h e d in 1998 as Technical Publication TP-25 Temperature Correction For The Volume Of Light Hydrocarbons. This is a GPA publication, but GPA, API, and ASTM have jointly adopted it [84]. These procedures are based on an experimental program sponsored by all three above organizations. Based on this expanded database the TP-25 factors are different by as much as 0.2-0.8% from the 1953 Table factors. The underlying procedure in TP-25 is quite different from the fairly empirical equation forms used in the other standards. The TP-25 factors are determined from the interpolation of the properties of reference fluids via a corresponding states formulation. Corresponding states theory has a rich history for the use of correlating various t h e r m o d y n a m i c properties. This particular formulation is unique in that it uses the relative density at 60°F as the interpolating variable. The calculation of the density at a temperature is based on a comparison of the properties of the reference fluids at the same reduced conditions. All the reference materials and critical properties required to do the calculations are available in the standard. The organization of the VCF standards in the API Manual of Petroleum Measurement Standards is being changed. The revised Chapter 11.1, Temperature and Pressure Volume Correction Factors for Generalized Crude Oils, Refined Products, and Lubricating Oils, will combine the previous Chapter 11.1 and 11.2.1 temperature and pressure correction procedures into a single unified procedure. The nominal API gravity range will be —10° to 100°, so it will still only cover the heaviest of LPG. Two important changes will be that the single set of procedures will be valid for the customary and metric standard temperatures (60°F, 15°C, and 20°C) and for measurem e n t temperatures and pressures in customary or metric units. Chapter 11. Iwas approved for D 1250-2002 and will be published in 2003. The revised Chapter 11.2, Temperature and Pressure Volume Correction Factors for Light Hydrocarbons, will use the TP-25 procedure as the basis for the temperature portion of its VCF procedures. It will cover the range of relative densities (60°/60°) of 0.3500-0.6880 (272.8-74.2°API), just like TP25. Like the revised Chapter 11.1, this will contain a single set of procedures valid for the customary and metric standard temperatures (60°F, 15°C, and 20°C) and for measurement temperatures and pressures in the customary or metric units. Technical work is being done to incorporate pressure correction procedures. It is expected that this revised Chapter 11.2 will also be published in 2003.
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The VCFs are always based on averages on a range of materials, or by interpolation between reference standards. Individual mixtures that have similar density but different composition than either the average or reference may have significantly different expansion coefficients. Use of the standard VCFs will give a biased result on these mixtures. Provisions are made in the standards for contracting parties to determine and use VCFs based on density measurements on the individual material or mixture. Alternatively, to minimize uncertainty, contracting parties may specify that the density determination used for custody transfer be obtained as close to the standard conditions as possible. This is easily done for temperature, but is more complicated for pressure. In the past, pressure correction was often omitted or ignored, or based on measurements done at the time and temperature a n d pressure of custody transfer. To accommodate existing industry practices, the new standard TP-25 contains two sets of tables, one for temperature correction alone, and the other for combined temperature and pressure correction.
AUTO PROPANE LPG has been used as a spark ignition motor fuel for over 40 years, especially in fleet and workplace applications. The primary advantages over gasoline in these markets are related to availability, cost, and tailpipe emission levels. Fleet operations can operate from centrally located refueling sites, avoiding the need for widespread availability at retail gasoline service stations. The lower operating cost for fuel may offset the cost of conversion, with a net benefit for high mileage applications such as taxi or delivery fleets. The breakeven mileage generally varies between 5000 to 25 000 km/year, depending mostly upon the presence and level of Government incentives for conversion cost, registration and differential fuel taxation. The tailpipe and environmental benefits depend mostly upon the technology and calibration of the engine using the fuel and to a lesser extent on the properties of the fuel itself. However, with equivalent levels of technology, LPG tends to have a significant advantage with emissions and emissions toxicity. Historically, both gasoline and propane employed similar carburetor and intake manifold configurations. This allowed widespread "aftermarket" conversion of engines from gasoline to LPG, using engine coolant driven vaporizer/regulators and various gas injector devices. These could be plates that could be "bolted on" above or below the existing liquid gasoline carburetors for dual fuel operation, or gas carburetors for dedicated LPG use. LPG carburetor engines generally have lower tailpipe hydrocarbon and carbon monoxide (HC,CO) emissions than the equivalent technology gasoline engine, when no computer feedback engine control strategies are used. This is primarily due to reduction/elimination of fuel enrichment for idle, throttle transient and high power engine operation. This made propane a preferred fuel for such indoor engine applications as forklifts, where indoor CO emissions must be minimized. The widespread introduction of tailpipe catalysts and electronic fuel injection systems to meet increasingly more stringent emission standards has made aftermarket conversions increasingly challenging on recent model automobiles to
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LPG. Second generation conversion systems attempted to use the onboard sensors to control a parallel LPG injection system, often with very good results. However, the increased cost and engineering sophistication tends to limit application to a few high volume platforms. Each has to be re-engineered with every change to the gasoline system, and then maintained in the field. Even so, several of these conversions have been quite successful, but the overall rate of North American conversions declined. Recently, the highly integrated fuel injector/intEike manifold designs and more complex onboard control and diagnostic systems have become the norm [85], but these significantly increase the cost and complexity of conversion. Current Tierl, Tier2 and more recently hybrid gasoline electric vehicles offer exceptional emissions performance. On the horizon are mandated lower emission standards and gasoline reformulation that will further reduce or possibly eliminate the tailpipe emission advantage once enjoyed by LPG conversions. With reduced environmental credits. Governments have tended to reduce incentives for conversion, resulting in longer break even time for operators [86]. As a result, the size of the North American LPG conversion fleet is declining rapidly, OEM built LPG dedicated or dual fuel vehicles is increasing. Major North American OEMs offer current technology LPG powered low emissions light duty cars, trucks, and vans. Recently, there has been renewed interest in development of low emission LPG vehicles in the Pacific Rim and in Europe. OEM vehicles are predominately factory conversions of truck and van platforms aimed at fleet use [87,88]. The North American carmaker's message to the p r o p a n e industry is clear: The carmakers expect the s u p p o r t of the major p r o p a n e industry players if they are to produce, sell and maintain OEM vehicles [89]. This experience is different in other parts of the world, depending mostly upon fuel cost and tax structure. There is a general agreement that the technical problems either can or have been solved (depending upon who you tcJk to). This includes gaseous or liquid fuel injection systems that address many of the problems of earlier systems, including tolerance to trace contaminants, same as gasoline or diesel fuel. Several manufacturers of advanced fuel LPG injector systems have experienced problems with particulate/residues in various areas of the country. Magnetic residues (predominately magnetite "mill scale" and tank cortosion products) have been especially disruptive to some electronic solenoid/ injector designs. Widespread introduction of these systems is likely to put additional emphasis on "cleanliness" of propane, similar to that required for gasoline for fuel-injected cars. Several OEMs have expressed concern for the m a x i m u m sulfur contents of "HD5" propane, although typical sulfur contents are significantly less than current specification levels. These are now higher than some reformulated gasoline specifications (RFG) This is a potential area of future research and standardization. The use of a sulfur containing odourant (ethyl mercaptan) adds u p to 30 p p m w sulfur to propane, which is significant with respect to future RFG regulations. This is another area of recommended research and standardization. There is an opportunity for LPG fuels to compete favorably, and even preferentially in future "clean fuels" markets, especially if factored into future engine/drivetrain design.
LPG could be an ideal fuel for emerging micro-turbine applications a n d high-temperature partial oxidation (POx) fuel cells. LPG can be produced in high purity and low sulfur that may be necessary for these applications. LPG—SAFETY AND HANDLING LPG F l a m m a b i l i t y Propane is flammable in the a t m o s p h e r e at ambient conditions between about 2-9.5 vol%, the Lower and Upper Explosive Limit (LEL a n d HEL), respectively. This corresponds to about 0.3 psia peirtial pressure for the LEL, and about 1.4 psia partial pressure for the HEL. LPG is normally handled at pressures m u c h higher than this. LPG must be refrigerated to less than — 100°C before the equilibrium vapor pressure enters the explosive range. This is virtually unattainable in normal operations and would only be a concern in some research or specialty applications. Thusly, physical mixtures of gases, especicdly in air, is always more important for LPG t h a n "flash point" considerations. Flammability is not a fundamental or t h e r m o d y n a m i c property of any material. It depends upon the circumstances at the time, including gas mixture, temperature, pressure, ignition energy, turbulence, aerosol, catalyst and even the nature of the surface and shape of the containment vessel. The lower explosive limit is less sensitive to these effects than the higher explosive limits. For example, the LEL is almost totally insensitive to the a m o u n t of "diluent" gas added [90,91]. The HEL is the most impacted with dilution by inert gas, essentially decreasing almost linearly to near the LEL at about 40-50% dilution (10-13% oxygen vs. 2 1 % undiluted). The flammable envelope terminates at the point that the HEL meets the LEL. The explosive envelope tends to enlarge at higher temperatures, but again more on the HEL side vs. the LEL side. The change in LEL is in fact quite small, with only an 8% relative change in LEL with a 100°C increase in temperature from ambient [92]. In general, it appears that any increase in temperature, pressure, gas composition, ignition energy, or configuration has a m u c h more profound effect on the HEL than the LEL. Use of the LEL determined at ambient conditions can reasonably be expected to apply over a very wide range of conditions in typical LPG handling situations. This is not true for the HEL, which varies widely with similar changes in gas composition, temperature, and pressure. LPG H a n d l i n g Most jurisdictions require that persons involved with the handling or transport of LPG or installing LPG equipment be trained and licensed. This generally does not include final connection of portable cylinders to consumer appliances using approved fittings, but usually does include filling of the portable cylinders or automobiles at the point of retail sale. LPG, like any fuel, is safe if properly handled. It has a relatively n a r t o w explosive range, so explosions are rare. However, spills and fires are similar to other volatile fuels and solvents. Certain properties of propane should be understood
CHAPTER to p r o m o t e safety in handling the fuel, and a p p r o p r i a t e responses in the event of a spill or emergency response. LPG vapor is about 50% heavier t h a n air (molecular weight 44 versus 29). Vapors will tend to settle in low points and cavities in the absence of a ventilating air flow. Propane/air mixtures have densities closer to air, and are more easily mixed and dispersed from thermal or a m b i e n t air currents. For example, a mixture of one volume of propane vapor (MW 44) diluted 9:1 with air will only be 5% heavier than air (about the same relative difference as sea water a n d fresh water). It will still be above the explosive range, but will be prone to dispersion from ambient air currents. The heavier than air vapor density allows a potential and usually temporary pooling. Leaking gases tend to exit from high-pressure sources at a sufficient velocity to stir the air mass and cause mixing. They will only tend to accumulate in low spots only in very quiescent conditions. When propane or propane/air mixtures (above the HEL) collect in low spots, there will cilways be an interface at the top of the vapor pool that is in the flammable range. This will b u m very similarly t o any other flammable liquid. A very good safety demonstration is to fill a tall (high 1/d) nominal 500 m L beaker with pure LPG vapor, and leave it open in full view for the duration of the training session. When asked after several hours, most attendees generally believe that the "volatile gas" has escaped and is rapidly dispersing itself through the cosmos. However, lowering a match slowly below the rim convinces everyone that not very much gas has escaped, a n d for approximately the next 15 s, you can see the flame level drop in the beaker, as the pool fire consumes the propsme. This is a "worst case" demonstration, as ambient air currents cannot penetrate and mix a tall vessel very far. However, it does serve to show that propane rich pockets of gas can be stable for hours, days, and even weeks in the absence of strong wind or air currents of a velocity sufficient to mix the ambient atmosphere. It is important to vacate and properly ventilate an enclosed area that may have been subject of a propane leak in the absence of ventilating airflow. Small liquid leaks can turn large volumes of area into explosive mixtures under "ideal" circumstances. At room temperature, one liquid liter of propane expands to about 240 Liters of gas, which at 2% LEL is sufficient to create 12 000 L of explosive atmosphere. It takes only a small liquid leak to create a n explosive atmosphere in a confined area, if the propane becomes well mixed. Since the entire system is under pressure, leaks can occur even when equipment is idle, and collect over a long period of time. Gas-Air mixtures may be brought below the flammable limit by dilution with large volumes of non-flammable gasses such as nitrogen, carbon dioxide, steam or air [93] (see flammability above). Some precautions should be taken to prevent ignition by static discharge with some CO2 fire extinguishers. A gas inerted area may still present an asphjrxiation hazard. The rapid venting of gas and especially of gas/liquid aerosols can create static chcirging situations. As a result, ignition of vented "vapors" can occur at any time, depending u p o n circumstance. Gas mixtures inside of the flammable region do not "bum" like a pool fire. They forcefully explode, generating a shock wave a n d a sharp "firecracker" type of sound. The largest energy release is when the mixture is at stoicheometric, and the
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potential of the resulting explosion is at its most powerful point. A one foot (25 cm) diameter round balloon filled with a stoicheometric mixture will provide a bang loud enough to startle m o s t people in a typical small training room. The same bcJloon filled with p u r e propane vapor (no air) will produce a fireball that will rise 6-8 feet. A larger balloon will leave a black scorch mark from the pool fire on the ceiling (measure carefully, do your safety checks, and have a fire extinguisher at hand). The flammability of n e a r explosive mixtures has not been extensively studied in the presence of a stable combustion source. Witness descriptions and injuries indicate that very large h e a t energy releases are possible in n e a r explosive atmospheres. Very few people have ever seen liquid propane. It is a clear colorless liquid that can easily be mistaken for water under certain circumstances, particularly in cold weather. When propane liquid is sprayed into a collection container, only a portion of the liquid initially vaporizes—^just enough to provide the heat of vaporization and cool the remaining liquid down to the — 42°C boiling points at ambient pressure. For example, if liquid propane at 70°F is sprayed into a collection container, only about 40 wt% of the propane will vaporize, and the majority will be collected as a liquid in the container. The lower the starting temperature, the higher the yield of liquid. Propane liquid must be over 200°F (near the critical point) before all of the propane will vaporize upon flashing to ambient pressure. Once collected, the liquid propane in the open container will stay at its boiling point, and boil at a rate determined by the heat transfer into the container. If the container is made of a material that is a good insulator, such as a plastic cup or pail, it can take several hours for the liquid propane to evaporate even at high ambient temperatures. Under stagnant air conditions, the heavier than air cold vapor itself provides additioned insulation as it spills over the top of the container. If the container is metal, then the boiling rate is generally very high initially, until a layer of accumulated ice or frost provides some insulation. If a spill occurs at low temperature onto frozen ground or snow, the resulting liquid pool can be persistent for hours. If the spill occurs at r o o m temperature onto a concrete floor, the evaporation rate is extremely fast. Liquid propane at room temperature on a horizontal surface tends to exhibit the "Liedenfrost effect" where the liquid behaves like water droplets bouncing on the top of a hot stove, essentially riding on its own rapidly forming vapor. A spill under such circumstances, for example a severed hose or inadvertently opened valve, can barely leave time for occupants to vacate the area. Some jurisdictions require "excess flow" valves on consumer cylinders to limit the flow rate of a free flowing propane discharge in the absence of a backpressure. Any propane or BP mix liquid at atmospheric pressure has a normeJ boiling point that is far below the freezing point of water (and flesh and skin) and can cause "frost bite" or freezing "bums." Many a finger has been superficially frozen with LPG liquid inadvertently sprayed inside of a difficult to remove glove. The rapid removal of vapor from a tank will cause the remaining liquid in the tank to sub-cool. For example, in the "worst case," if the valve is opened wide venting to the atmo-
54
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AND LUBRICANTS
HANDBOOK
sphere at 70°F, the hquid will cool down to the boiling point (-42°C) after about 40 wt% of the liquid has vaporized. High appliance demand and rapid vapor withdrawal rates generally results in a large amount of condensation or frost accum u l a t i o n on the exterior of the c o n t a i n e r due to this refrigeration effect. LP-Gas vapor upstream of the regulator can condense to a liquid if the lines external to the tank are at a lower temperature than the tank, and this requires special attention to detail, especially in cold climates. For example, during snap cool situations, outside vapor lines cool down faster than the bulk storage tank, due to the higher thermal mass of the tank. The higher storage tank pressure causes condensation of liquid propane in the vapor lines. This can result in vapor lines becoming filled with condensed liquid propane under snap cool conditions. Propane has a m u c h higher coefficient of thermal (and pressure) expansion than other hydrocarbon fuels. Heating an overfilled propane vessel can result in thermal expansion of the liquid until the cylinder is fully liquid filled (the vapor condenses as the liquid expands). Very high pressures, exceeding several thousand psi, can be generated with further heating and liquid expansion in a liquid filled ("hydraulically locked") cylinder. Pressure safety vents will generally prevent this, b u t the resulting leak of liquid can become a fire hazard. All transportation codes require cylinders to be only partially filled for this reason, but it is not commonly appreciated that this is relative to a reference temperature (traditionally 80vol% fill at 60°F). This protects against hydraulic lock u p to over 140°F for liquid fill at 60°F. If the cylinder is filled at a temperature lower than 60°F, it may be necessary to further reduce the fill density for storage and transport. Consult the authority having jurisdiction for the applicable requirements.
LPG S a m p l e C y l i n d e r A p p r o v a l s Consult the Authority having Jurisdiction for pressure vessel certification r e q u i r e m e n t s for LPG sample cylinders (US DOT in United States and Transport Canada in Canada). At present there is no international approval process for pressure cylinders. Authorities in adjacent or other jurisdictions m a y not approve cylinders approved in one jurisdiction. Cylinders must be approved in all the jurisdictions in which they are used for sample transport. Cylinders approved by the Authority having Jurisdiction u n d e r "equivalent level of safety" criteria are acceptable provided that they are used in accordance with the applicable permits or exemptions. For example, users may not subsequently alter valves or pressure venting devices that are part of a permit or exemption. Periodic cylinder inspections may be required [94]. LPG E x p o s u r e The health effects caused by exposure to LP Gases are much less serious than its fire and explosion risk. LP gases at low concentrations are non-toxic, and liquids have limited ingestion routes due to the physical properties of the materials. The Occupational Exposure Limits (GEL) are based mostly on the need to avoid the explosive hazards by a wide margin. This is t5^ically 20% of the LEL or lower, which is a much lower level than would be set for short or long term toxicity, or Eixphyxiants.
LP Gases at very high (explosive) concentrations are believed to be non-toxic, fast acting anaesthetic agents, as well as being simple asphj^iants. The terminology "propane narcosis" is sometimes used, but this is not accurate. Narcotics generally refer to drugs that act at opiate receptor sites, which is not where general anaesthetic gases act. However, the term "narcosis" is quite generic. The c o m m o n definition is "to put to sleep, partially or totally" due to a general depression of the Central Nervous System (CNS), even though sleep, anaesthesia and narcotics are distinctly different in medical terms. There are some early references to LP gas being tried or used as a medical anaesthetic, and one of the LPG patent claims by F.P Peterson in 1914 was for use "in surgery as an anesthetic" [95]. However, only cyclopropane, nitrous oxide, ethers, and various halocarbons were used clinically as anaesthetic gases, usually in combination with other drugs. Persons exposed to high levels of LPG gas and experiencing CNS depression effects are in a very dangerous situation, since they are breathing explosive gas mixtures. Any ignition source could produce a fire or explosion, resulting in injury or death. Individuals vary widely in their susceptibility and tolerance to these effects. At high concentrations, the effects of narcosis and h3^oxia will be confounded, as they are outwardly similar in many respects, and will occur simultaneously. There is a range of symptoms with increasing concentration from the onset of CNS effects, hypoxia, and asphyxiation: disorientation, lightheadedness, dizziness, drowsiness, loss of physical coordination, impaired judgment, drunkenness, unconsciousness, and death. The CNS effect is believed to be dependent upon the concentration of the gas in the lipid bi-layer (cell wall m e m brane) of nerve cells. Different gases would be expected to have a different potency, and require higher or lower exposures to obtain the same CNS effect. The relative solubility of different gases in olive oil (surrogate for lipid), water (surrogate for blood), can be compared to a very wide range of materials that have well known physiological responses. By this method, the onset of einaesthetic effects is estimated to be n e a r the lower explosive limit (—2%) for butane, a n d about twice the LEL (—4%) for propane. Iso-butane causes drowsiness in a short time in concentrations of 1%, with no apparent injuries from i-C4 or n-C4 after 2-hour exposure at concentrations u p to 5% [96]. The rapidity of both the onset and recovery of CNS effects from gas exposure depends upon the speed at which the gas is transported into and out of the body. This is dependent upon the concentration of the gas in air, the breathing and blood circulation rate, and the relative solubility of the material in lipid (oil) and blood (water). Gases with low water (blood) solubility and high oil (lipid) solubility tend to be the fastest acting. The large volume of blood/water in the body tends to act as a "buffer" for water-soluble compounds, slowing accumulation and depletion from the nerve cells. The solubility properties of LPG lie close to cyclopropane, between those of nitrogen and nitrous oxide/diethyl ether, where the rates are well known. Cyclopropane anaesthetic, which is 4-5 times more soluble in water than propane, is not altered or combined in the body; the major part is exhaled within 10 min, while full desaturation takes several hours [97]. LPG exposure above the explosive limit would be expected to in-
CHAPTER duce CNS effects quickly, certainly in less than 15 min, and probably in less than 5 min. Persons installing new equipment, flushing lines, repairing, or re-lighting appliances, or investigating gas leaks are also the most likely to be exposed to high LP-gas concentrations. A Consumer Products Safety Commission (CPSC) study noted that new LPG installations are more prone to accidents than new natural gas installations and sponsored a n u m b e r of studies relating to LPG odorization [98]. While the lighter than air density and higher LEL of methane are undoubtedly major factors, the CNS effects may help explain the occasionally bizarre circumstances of some LPG accidents. The knowledge of possible impairment can be critical in making decisions effective in reducing the accident rate. For example, a person flushing a line might realize the need to take a fresh air break instead of a "smoke" break, or ventilate the area if it is u n d e r s t o o d t h a t the light-headed or tired feeling may be due to explosive gas concentrations. Anaesthetics also sensitize the myocardium to the effects of epinephrine. Cardiac a r r h y t h m i a can occur when very high concentrations of LP gases (above the HEL for propane) are inhaled when the epinephrine level is elevated (e.g., anxiety, exertion) cind possibly during hypoxic conditions. Ccirdiac arrhythmia can have a sudden onset even with brief exposures a n d may result in sudden death. This is an extreme condition that is usually associated with substance abuse (commonly called "huffing"). Similar problems have been reported in the nitrous oxide bottle gas industry. Additional information is available on the web site of the Compressed Gas Association [99]. LPG Odorization LPG odorization requirements reside in different jurisdictions in different countries. The U.S. requirements are not in the ASTM D1835 specifications, but in NFPA 58 and 19 CFR, and State Regulations. Canadian requirements are in the CGSB LPG product specifications that are referenced in various Provincial and Federal Regulations. Historically, manufactured gas was odorized as a warning aid due to the risk of carbon monoxide poisoning. Manufactured gas often contained high levels of CO, which was a higher risk than flammability. Over time, a variety of matericils, including mercaptans, acetylene, and mixed refinery sulfides were used as odoureints. LPG production from sweet natural gas wells had no olefins and a low sulfur content, so had a n intrinsically low odor. Initially, there was no requirement to odorize gas distributed for fuel purposes, as it was non-toxic relative to manufactured gases. However, a number of fires and explosions with non-odorized natured gas and LPG highlighted the need to odorize as a warning aid to detect leaks. LPG odorization is now a regulated requirement in all North American jurisdictions. Natural Gas Euid LPG odorization has been the subject of substantial product liability litigation, stretching over several decades of work on the technical, political, and legcd issues involved. These are summarized in a series of IGT sponsored technical symposia [100], CPSC, GPA, NPGA, PGAC, and BERC sponsored rescEirch, as well as transcripts of various product liability cases. Current North American Odorization practices are based on 1977 BERC smalysis [101] indicating ethyl mercaptan (EtSH, or EM) to be a preferred odorant
2: LIQUEFIED
PETROLEUM
GAS
55
over tetrahydrothiophene (THT). N u m e r o u s other studies continue to indicate the EtSH remains the odorant of choice, particularly for cold climates, where the relatively high volatility of EtSH is the closest to that of propane at low temperature [102]. The volatility of E t S H and other potential odorants has been extensively studied and reported in IGT and GPA research reports. Odorants are not effective wEiming agents in all cases. Propane vapor is heavier than air and initially tends to settle Eind accumulate in low points and cavities. Persons in the vicinity of low points and cavities may not be exposed to localized gas pockets, b u t m a y b e at risk from an ensuing fire or explosion. S u b s e q u e n t diffusion or convection can distribute LPG vapors t h r o u g h o u t a n area. Certain odorants are polar and/or chemically reactive, and can be depleted by reaction or adsorption. People differ in their ability to smell, and the sensitivity to odors generally decreases with age or with impaired physical conditions such as colds or respiratory allergies. Prolonged exposure to odorants can cause olfactory desensitization. Other odors or distractions can reduce the effectiveness of odorants as warning agents [103]. Mercaptan odorants are susceptible to oxidation u n d e r certain conditions, especially at high temperature and in the presence of water, oxygen, or high oxidation state (red) rust [3]. This is commonly referred to as "odorant fade"[103]. It is now genercdly accepted that propane tanks in continuous service do not experience odorant fade since there is no source of oxidant [104,105,106]. Much industry activity continues to focus on identifying a n d eliminating situations that can result in odorant fade. In general there is a m u c h higher awareness in the industry for the detrimental effects of air, rust, and water in pressure vessels [107-109]. Recent changes to rail car pressure vessel certification requirements provide another opportunity to further reduce the use and occurrence of steam cleaning and hydrotesting of large pressure vessels (truck/rail). Work is ongoing in this area and several programs are being planned. Odourants can be partially or fully removed by absorption, for example in cases where the LPG has permeated through clay-like soils that have high surface absorption capacity. GPA, ASTM, and CGSB have developed field tests for mercaptan odorants using stain tubes, and this allows reliable semi-quantitative monitoring of odorant levels during distribution. ASTM D 6273-98 Standard Test Method for Natu r a l Gas Odor Intensity (odor m e t e r "sniff test") can be applied to LPG as well natural gas. These are not necessary for monitoring odourization equipment, as verification of proper tj'pe and quantity of odourant can be done by a variety of means. These include measures of weight, volume, injection pulses, on-line analysis, calibrated "day pots," etc. and these features are generally included in commercially available odorant injection systems. An NPGA/PGAC Joint Task Force on LP-Gas Odorization developed a set of recommendations in 1991 to assist in their desire to improve LPG safety. All producers and terminal operators w h o odorize LPGas should establish formal, rigid procedures and responsibilities for proper odorization [110]. These sire advisory in n a t u r e a n d compliance is voluntary. I n d e p e n d e n t crosschecks have found to be effective to detect several "single fault" injector system failure m o d e s that could otherwise have gone unnoticed [111].
56
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HANDBOOK
USES OF LPG Industrial fork lifts, indoor carburetor engines (floor sweepers, washers, etc.) oxy/propane cutters, heating/annealing, high pressure welding "plumber" torches, portable soldering irons, small torches hand torches (pallet shrink wrap, localized heating/burning) worksite heating/ventilating (lineman's tents, below grade sites) refrigeration (compressor cycle), evaporation, or specialty (vap pipes) pavement heating for grinding/recycling, roofing tar heating ground thawing, ice/snow melting, concrete heating, etc. propellant for aerosol sprays, cosmetics, shaving cream, perfumes, etc. propellant for paint sprayers, paint airbrush (including PB mix) solvent for cleaning, reaction, diluent/chilling, precipitation remote "smart" gen sets with remote start/stop, telemetry integrated standalone packages (power generation, ventillate, heat/cool) portable soil/trash reclamation, sterilization jackhammers (construction road work)
• portable generator sets (fixed on larger RVs) • golf carts, maintenance equipment, turf and lawn mowers • pyrotechnic/flame special effects Aviation • airport r a m p equipment (i.e., airstairs, luggage tugs, etc.) • 12/24 Vdc piston engine pre-heat (indirect fire) • hot air balloon Marine • LPG powered "jet" boats • "overslung" barbecue/cooking Emergency • standby generators, compressors, heat/cool • stable long term emergency fuel supply Mining • mineshaft ventilation air heating • mine site cookhouse stoves, quarters heating • water pumps to liberate coal seam methane Medical • • • •
Power
cyclopropane anesthetic microturbine and distributed power systems stationary generators standby fuel for gas interruptible supply
Transportation autopropane (conversions) and OEM cargo heaters, engine coolant pre-heaters, ice removal heavy duty truck alternate fuel (fumigation and OEM) hydrocarbon fuel for reforming fuel cell applications retrofit of small engines to tighter emission standards utility right of way maintenance micro-turbine hybrid vehicles Agriculture grain drying, bam/brooder heaters, orchard heaters, weed control general shop heat/power/oxy welders, etc. Colorado Potato Bug eradication, potato guns farm animal m a n u r e dehydration/sterilization (esp. swine/ chicken) weed control as alternative to herbicides to control runoff "scare crow" devices irrigation water pumps Utility
Gas
bulk or refrigerated storage for peak shaving in NG systems bulk vaporizer, distribution of LPG like NG
Acknowledgments The author acknowledges review and significant technical input from Dr. H. ICing, ExxonMobil Research (hydrates), John Jachura (VCF/density), Dr. D.Y. Peng (Peng-Robinson EOS), Dr. H.J. Ng, DBRA Research Associates (Ng-Robinson EOS, hydrates), Larry. R. Robertson, DaimlerChrysler (autopropane), Alex Spatura, Adept Group, (autopropane a n d extensive review), Ian Drummond, lOL Industrial Hygiene (exposure/CNS), Dr. Byrick, Uof Toronto (CNS), Dr. Roth, University of Calgary (CNS), Nancy Bourque, lOL Research Library (literature and patent searches), Tim Eaton, Superior Propane Inc.(uses). Thanks to Mark Sutton and Ron Brunner (GPA) for access to draft publication TP16 on history of LPG specs (hopefully to continue to publication). Special thanks to Ron Cannon (GPA, retired) for input, review, and liberal reference to his excellent book "The Gas Processing Industry—Origins and Evolution." This is highly recommended reading for anyone in the industry, and a good book for both libraries and reception areas. The author acknowledges the support of Imperial Oil Ltd. for the information/communications resources used in the preparation of this chapter.
Residential cooking, heating, clothes drying, hot water catalytic/radiant/pipe radiant heaters, mantel lights h o m e barbeques, refillable lighters, hair curlers, b b q lighters fixed installation mobile home heating/cooking Recreation mobile RVs/trailers cooking/heating, absorption cycle refrigeration
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CHAPTER 2: LIQUEFIED PETROLEUM GAS 57 [4] Redwood, Sir Boverton, A Treatise on Petroleum, 5th ed., Charles Griffon & Company, Ltd., London, Lippincott, Philadelphia, 1926, p. 1. [5] Purdy, G. A., Petroleum, Prehistoric to Petrochemicals, Copp Clark Publishing Co., Toronto, 1958, p. 96. [6] Purdy, G. A., Petroleum, Prehistoric to Petrochemicals, Copp Clark Publishing Co., Toronto, 1958, p. 27. [7] Redwood, Sir Boverton, A Treatise on Petroleum, 5''' ed., Charles Griffon & Company, Ltd., London, Lippincott, Philadelphia, 1926, p. 77. [8] Railroad Commission of Texas, Austin, TX, Available: http://www.rrc.state.tx.us/history/hist.html. [9] Handbook Butane-Propane Gases, First ed., G. H. Finley, Ed., Western Gas, Los Angeles, 1932, p. 16. [10] Handbook Butane-Propane Gases, First ed., G. H. Finley, Ed., Western Gas, Los Angeles, 1932, p. 11. [11] Little, A. S., "Blaugas in the United States of America," Gas World, Vol. 60, pp. 563-4. [12] Handbook Butane-Propane Gases, First ed., G. H. Finley, Ed., Western Gas, Los Angeles, 1932, pp. 10,11,18. [13] Cannon, R. E., "The GAS Processing Industry—Origins and Evolution," Gas Processors Association, Tulsa, OK, 1993, p. 61, Available: http://www.gasprocessors.com/default.asp, March 7, 2002. [14] Cannon, R. E., "The GAS Processing Industry—Origins and Evolution," Gas Processors Association, Tulsa, OK, 1993, p. 64, Available: http://www.gasprocessors.com/default.asp, March 7, 2002. [15] Cannon, R. E., "Liquefied Petroleum Gas Quality/Quality Control," GPA Draft Technical Publication TP23, Gas Processors Association, Tulsa, OK, 1999, p. 8. [16] Cannon, R. E., "The GAS Processing Industry—Origins and Evolution," Gas Processors Association, Tulsa, OK, 1993, p. 80, Available: http://ww.gasprocessors.com/default.asp, March 7, 2002. [17] "About Propane Gas," Available: http://www.propanegas.com. Energy Source Inc., Feb. 20, 2003. [18] Snelling, W. O., "Process of Refining Natural-Gas Gasoline," U.S. Patent 1,056,845, U.S. Patent and T r a d e m a r k Office, Washington, D.C., Mar. 25, 1913. [19] "Liquid Gas Made by M o d e m Prometheus," including reprint from Pittsburgh Sunday Post, Sept. 8, 1912, Butane-Propane News, June 1999, p. 22. [20] Chapter 1, Handbook Butane-Propane Gases, First ed., G. H. Finley, Ed., Western Gas, Los Angeles, 1932, p . 16. [21] Peterson, F. P., "Art of Condensation of Gases and Vapors into their Liquid Forms," U.S. Patent 1,031,664, U.S. Patent and Trademark Office, Washington, D.C., 1912. [22] Peterson, F. P., "Gas," U.S. Patent 1,094,864, U.S. Patent and Trademark Office, Washington, D.C., 1914. [23] Cannon, R. E., "The GAS Processing Industry—Origins and Evolution," Gas Processors Association, Tulsa, OK, 1993, p. 28, Available: http://www.gasprocessors.com/default.asp, March 7, 2002. [24] Cannon, R. E., "The GAS Processing Industry—Origins and Evolution," Gas Processors Association, Tulsa, OK, 1993, p. 113, Available: http://www.gasprocessors.com/default.asp, March 7, 2002. [25] Legatski, T. W., "The Properties of Liquefied Petroleum Gases as Affecting Specifications with a Discussion of the Problem of Odorization," Proceedings of the NGAA Convention, Tulsa, OK, 1932. [26] Cannon, R. E., "The GAS Processing Industry—Origins and Evolution," Gas Processors Association, Tulsa, OK, 1993, p. 74, Available: http://www.gasprocessors.com/default.asp, March 7, 2002. [27] http://v\rww.bpnews.com, Feb. 18, 2003. [28] http://www.npga.org, Feb. 18, 2003.
[29] http://www.cganet.com, Feb. 18, 2003. [30] Peng, D. Y., Professor of Chemical Engineering, University of Saskatchewan, co-author of the Peng-Robinson Equation of State, personal communication. [31] Miller, W. A., Elements of Chemistry, Theoretical and Practical, Vol. I, 3rd ed., J. W. Parker and Son, London, 1867, p. 328. [32] Kuenen, J. D., Theorie der Verdampfung und Verflussigung von Gemischen, E a r t h Verlag, Leipzig, 1906. [33] Podbielniak, Walter J., Ph.D. Thesis, Chemical Engineering Department, University of Michigan, 1928. [34] Cannon, R. E., "The GAS Processing Industry—Origins and Evolution," Gas Processors Association, Tulsa, OK, 1993, p. 167, Available: http://www.gasprocessors.com/default.asp, March 7, 2002. [35] Kolayashu, R. and Katz, D. L., "Vapor-Liquid Equilibria for Binary Hydrocarbon-water Systems," Industrial and Engineering Chemistry, Feb. 1953, p. 440. [36] Engineering Data Book, Gas Processors Supply Association (GPSA); Technical Standards a n d Research Reports, Gas Processors Association (GPA). Available http://www.gasprocesssors.com/default.asp, March 7, 2002. [37] GPA research reports and c o m p u t e r programs. Available http://www.gasprocessors.com/default.asp, March 7, 2002. [38] Robinson, D. B., Ng, H. J., DBR Software, Available: www. dbra.com/Engineering_Software/sm_introduction.htm, Feb. 18,2003. [39] Gas Processors Supply Association (GPSA), Engineering Data Book and GPA Technical Standards and Research Reports, http://www.gasprocessors.com/default.asp, March 7, 2002. [40] Cannon, R. E., Liquefied Petroleum Gas Quality/Quality Control, Draft GPA Technical Publication TP-23, Gas Processors Association, Tulsa, OK, 1996, p. 6. [41] Handbook Butane-Propane Gases, Vol. 1, 6th ed., ButaneP r o p a n e News, Inc., Arcadia, CA, 1983, p. 5, Available: http://www.bpnews.com. [42] Sutton, C , Cannon, R. E., "The Evolution of LP-Gas Specifications," Presented at a Joint Meeting of the Canadian Natural Gas Processing Association and Propane Gas Association of Canada, Calgary, Alberta, Sept. 9, 1977., p. 2 [43] Cannon, R. E., Liquefied Petroleum Gas Quality/Qusdity Control, Draft GPA Technical Publication TP-23, Gas Processors Association, Tulsa, OK, [44] Sutton, C. and Cannon, R. E., "The Evolution of LP-Gas Specifications," presented to a Joint Meeting of the Canadian Natural Gas Processing Association and Propane Gas Association of Canada, Calgary, /Uberta, Sept. 9, 1977, p. 2. [45] ASTM c o m m i t t e e D02-H meeting minutes, 1998, 1999, ASTM International, West Conshohocken, PA, Available: www.astm.org. [46] Falkiner, R. J., Pickard, A. L., et al., "Sampling and Shipping Liquid Propane," presented at t h e IGT Odorization Symposium, Chicago, IL, 1998. [47] Shanerberger, E., Ford Motor Company (ret.), ASTM Committee D02-SCA, 2001, personal communication. [48] Kubesh, J., King, S., and Liss, W., Effect of Gas Composition on Octane N u m b e r of Natural Gas Fuels, SAE #922359, Society of Automotive Engineers, Warrendale, PA, 1992. [49] Callahan, T., Kakockzi, R, et al., "Engine Knock Rating of Natural Gases—Expanding the Methane Number Database," ASME Proceedings, ICE-Vol. 27-4, Book No. G l O l l D , American Society of Mechanical Engineers, NY, 1966. [50] Hewitt, J., "Finding a Premium for Propane," Propane Vehicle, Oct. 1998, p. 17. [51] Hewitt, J., "Quality Begins with an 'H'," Propane Vehicle, June 1998, p. 17. [52] Brasil, T, et al., "Proposed Amendments to the Specification for LPG used in Motor Vehicles (Staff Report and Initial Statem e n t of Reasons)," California Air Resource Board (CARB) Sta-
58 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK tionary Source Division, October 1998, Available: http:// www.arb.ca.gov/regact/lpgspecs/isor.pdf. [53] "CARB to Require HD-10 Propane Spec for Motor Fuel Use," Butane-Propane News, November 1998, p. 17. [54] Sutton, C. a n d Cannon, R. E., "The Evolution of LP-Gas Specifications," presented at a Joint Meeting of the Canadian Natural Gas Processing Association and Propane Gas Association of Canada, Inc., Calgary Alberta, Sept. 9, 1977. [55] Cannon, R. E., Liquefied Petroleum Gas Quality/Quality Control, Draft GPA Technical PubUcation TP-23, Gas Processors Association, Tulsa, OK, 1999, p. 18. [56] ASTM Manual on Significance of Tests for Petroleum Products, MNL 1, 5 * ed., G. V. Dyroff, Ed., ASTM International, West Conshohocken, PA, 1989. [57] Cannon, R. E., Liquefied Petroleum Gas Quality/Quality Control, Draft GPA Technical Publication TP-23, Gas Processors Association, Tulsa, OK, p. 23. [58] Cannon, R. E., Liquefied Petroleum Gas Quality/Quality Control, Draft GPA Technical Publication TP-23, Gas Processors Association, Tulsa, OK, 1999, p. 30. [59] Aviation Fuel Properties, Coordinating Research Council Docu m e n t No. 530, 1983, Available: Society of Automotive Engineers, www.sae.org/servlets/index, Feb. 18, 2003. [60] H a c h m u t h , K. H., "Dehydrating Commercial Propane," Butane-Propane News, Jan. 1932. [61] Davidson, D. W., Water: A Comprehensive Treatise, Vol. 2, F. Franks, Ed., Plenum Press, NY, London, 1973, p. 115. [62] Sloan, H. A., "The Phase Behavior of the Propane-Water System: A Review," Canadian Journal of Chemical Engineering, Vol. 68, Feb. 1990. [63] Smelik, E. A. and King, H. E. Jr., "Crystal Growth Studies of Natural Gas Clathrate Hydrates Using a Pressurized Optical Cell," American Mineralogist, Vol. 82, 1997, p p . 88-98. [64] Gas Processors Association, Research Reports, Available: www.gasprocessors.com/default.asp. [65] Suess, E., B o h r m a n n , G., et al., "Flammable Ice," Scientific American, Vol. 281, No. 5, Nov. 1999, p. 76. [66] "Application of Proprietary Kinetic Hydrate Inhibitors in Gas Flow Lines," OTC 11036, Offshore Technology Conference, Houston, May 6, 1999. [67] "Plain Facts about Freezing Regulators," Technical Bulletin LP-24, Fisher Controls, McKinney, TX, Available: www.fishercontrols.com, Feb. 18, 2003. [68] "Concentration Properties of Aqueous Solutions: Methanol Table 32," CRC Handbook of Chemistry and Physics, 75th ed., CRC Press, Cleveland, OH, p. D231. [69] CAN/CSA B149.2-00: Propane Storage and Handling Code, Appendix A.4.2, "Moisture Removal," Canadian Standards Organization, CSA International, Available: csa-intemational.org. [70] Fig. 20-52, Engineering Data Book, 11th ed.. Vol. 2, GPSA, Gas Processors Suppliers Association, 1998, Available: http://www. gasprocessors.com/gpsa„book.html. [71] H a c h m u t h , K. H., "Dehydrating Commercial P r o p a n e , " Butane-Propane News, Jan. 1932. [72] http://www.shawmeters.com, http://www.panametrics.com, http://www/meeco.com [73] Ardis, M., "Moisture Sensors for Process Organic Liquids, Sensors," Helmers Publishing Inc., Peterborough, NH, Nov. 1988. [74] Mychajliw, B. J., "Determination of Water Vapor Content in Natural Gas," IGT Symposium on Natural Gas Odorization, Natural Gas Quality and Energy Measurement, Chicago, IL, July 26-28, 1999. [75] Roberson, R., "New Low Range Pipeline Dew Point Detector Tube," Acadiana Flow Measurement Society (ASMS), April 1998, Sensidyne, Inc. [76] Pybum, C. M. and Lennox, R. K., "The Effect of Sulfur Compound Interactions on the Copper Corrosion Test in Propane,"
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presented at the 57th Annual GPA Convention, Mar. 20-22, 1978. "The Occurrence of Magnetic Residues in Vehicle Propane Tanks," Sept. 2002, Available: http://www.propanegas.ca Hohenberg, E. G., "The Occurrence of Magnetic Particulate Residues in Vehicle Propane Tanks," M.Sc. Thesis and report to the PGAC Residue and Odorant Committee, University of Manitoba, 1992, Available: http://www.propanegas.ca. Operational Reference Note #173 "Recommended Procedures for Placing Interchangeable Anhydrous Ammonia," and Operational Reference Note #172 "Recommendations for Preventing A m m o n i a C o n t a m i n a t i o n of LP-Gas," NPGA Technical Publications T122, T123, Available: www.npga.org. "Suggested Field Tests for Contamination in Propane," NPGA #151-99, Available: www.npga.org. Goetzinger, J. W. and Ripley, D. L., "Final Report Effect of Ammonia on LP-Gas Odorant," Research Report NIPER/BDM0220, prepared for the U.S. DOE Bartlesville Project Office, Available from the Gas Processors Association, http://gasprocessors.com. Pickard, A. L., Chairman, PGAC Residue and Odorant Committee (ret.), submitted to the Canadian General Standards Board (CGSB) committee o n Gasoline a n d Alternate Fuel Standards, private communication. Juchura, J., M a r a t h o n Oil, and Kopen, E., (InterProvincial Pipelines), ASTM SC E o n static petroleum measurement, personal communication. Beaty, R. E. and Brown, D. R., "Longstanding Partnership Results in Review of Critical Petroleum Tables," ASTM Standardization News, Dec. 1998, p. 26. HoUemans, B., "LPG E q u i p m e n t Technology in Light Duty Vehicles," presented at The World LPG Forum, Paris, France, 1997, Available: http://www.planet-interkom.de/sven. geitmann/ LPG.htm. Annual Windsor Workshop Symposia, N a t u r a l Resources, Canada a n d the U.S. D e p a r t m e n t of Energy, Available: http://www.gasprocessors.com/default. Mar. 7, 2002. Yates, S. E., "Technical Highlights of the Dodge Propane (LPG) R a m Van/Wagon," P r o p a n e Technology Toptec, Toronto Colony Hotel, Toronto Ontario, Canada, fttp:\www.daimlerchrysler.com, http://www.ford.coni, search "LPG." P r o p a n e Vehicle Resource Guide, 2nd ed., RP Publishing, Denver, CO, 1997. Coward, H. F. and Jones, G. W., "Limits of Flammability of Gases and Vapors," U.S. Bureau of Mines Bull. No. 503, U.S. Government Printing Office, Washington, D.C., 1952. MoUer, W. O., Molname, M., and Sturm, R., "Limiting Oxygen Concentration: Recent Results and their Presentation in Chemsafe," presented at The 9 * International Symposium on Loss Prevention and Safety Promotion in the Process Industries, May 1998, Barcelona, Available: http://ptb.de/de/org/3/33/331/moeller.htm. Affens, W. A., "Flammability Properties of Hydrocarbon Fuels Part 1.," Report #6270, U.S. Naval Research Laboratory, 1965. Handbook/Butane-propane Gases, Sixth ed., Butane Propane News Inc., Arcadia, CA, 1983, p. 8. Falkiner, R. J. and Pickard, A. L., et eil., "Sampling and Shipping Liquid Propane," presented at the IGT Odorization Symposium, Chicago, IL, July 1998. Peterson, "Gas," U.S. Patent 1,094,864, U.S. Patent a n d Trademark Office, Washington, D.C., April 28, 1914. Encyclopedia of Chemical Technology, 4th ed.. Vol. 13, John Wiley and Sons, NY, 1994, p. 823. Handbook of Compressed Gases, Second ed.. Compressed Gas Association, Inc., NY, p. 312.
CHAPTER 2: LIQUEFIED PETROLEUM GAS 59 [98] "Nitrous Oxide Safety and Security," Consumer Product Safety Commission (CPSC), Feb. 26, 2003, Available: http://www.cganet.com/N20/default.htm. [99] Compressed Gas Association, Available: http://www.cganet.com, Feb. 18, 2003. [ 100] Symposia on Natural Gas and LPG Odorization, Institute of Gas Technology, the following years: 1980, 1987, 1990/92, 1994, 1995, 1996, 1997, 1998, 1999, Available: http://www.gasprocessors.com, search "odorization." [101] Wiseman, M. L., et al., "A New Look at Odorization Levels for P r o p a n e Gas," Bartlesville Energy Research Centre, Bartlesville, OK, 1977, BERC/Rl-77/1, Available: U.S. Energy Research and Development Administration (ERDA), Technical Information Centre, [102] "Humcin Response Research Evaluation of Alternate Odorants for LP-Gas," GPA Research Report RR-129, Gas Processors Association, Tulsa, OK, Available: www.gasprocessors.com/default.asp. [103] CAN/CGSB 3.14 Liquefied Petroleum Gas (Propane). Sect. 7.3, "Precautionary Notes-Odourant," 1988. [104] Campbell, I. D.," Factors Affecting Odorant Depletion in LPG," presented at the First Symposium on LP-Gas Odorization Technology, Dallas, TX, April, 1989. [105] Hines, W. J. and Hefley, C. G., "Field Test Program of Measuring Odorant in Continuous Use Tank LP-Gas Tanks," Symposia
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on Natural Gas and LPG Odorization, Chicago, IL, Institute of Gas Technology/GAS Research Institute, 1990. Faulconer, H., "Field Testing for Ethyl Mercaptan in LP-Gas Storage Tanks in Four Marketing Areas," Symposia on Natural Gas and LPG Odorization, Chicago, IL, Institute of Gas Technology/GAS Research Institute, 1990. Marshall, M. D. jind Palladino, C. A., "Ethyl Mercaptan Stability after Refill of New and Air-Exposed used LP-Gas Storage Tanks," Symposia on Natural Gas and LPG Odorization, Chicago, IL, Institute of Gas Technology/GAS Research Institute, 1990. Parclay, A. T., et al., "Rail Car Field Test Evaluation Ethyl Mercaptan Odorant in Propane," Symposia on Natural Gas and LPG Odorization, Chicago, IL, Institute of Gas Technology/GAS Research Institute, 1992. Poirer, M. A., Falkiner, R. J., and Pickard, A. L., "Odorant Fade in Stenched Rail Car Cleaning," Symposia on Natural Gas and LPG Odorization, Chicago, IL, Institute of Gas Technology/GAS Research Institute, 1998, Available: www.npga.org. www.adspec.net/safety/report/report.htm, April 8, 2003. Bull, D. E. and Falkiner, R. J., "Risk Management: Verifying LPG Odourization Rates," Symposia on Natural Gas and LPG Odorization, Chicago, IL, Institute of Gas Technology/GAS Research Institute, 2001.
MNL37-EB/Jun. 2003
Motor Gasoline B. Hamilton^ and Robert J. Falkiner-^
nology levels, and have produced the "World Wide Fuel Charter" (WWFC) on this basis. The WWFC receives serious consideration when nations and economic regions are revising fuel specifications, even though it is not an accredited consensus standard, and is the subject of ongoing technical debate. ASTM Standard Practices, Test Methods and Specifications continue to be key to domestic and International Specifications and Regulations, and are used globally through affiliated ISO Standard Writing Organizations. Specifications for n e w generations of engines (electric/ hydraulic hybrids, gasoline direct injection, fuel cells, microturbines) and catalysts (lean b u r n , heated, selective reduction, HC absorption, particulate trap) have yet to be developed. The trend continues to be more restrictive in composition and variation than in previous gasolines.
T H E SPARK IGNITION ENGINE HAS REMAINED THE POWER PLANT OF
CHOICE for personal transport for almost a century, and the fuel specifically formulated for SI engines is called gasoline (U.S.) or petrol (UK). Gasoline h a s evolved continuously since it was first produced in quantity to meet the burgeoning demand from mass production of the automobile in the early 1900s. It was first produced from light naphtha batch distilled from crude oil and liquid condensate from natural gas production, and in the early days, had no test methods or specifications at all (see Chapter 2, Liquefied Petroleum Gas). It became a carefully formulated mixture of hydrocarbons and additives, seasonally blended to m a t c h local ambient conditions, providing good performance and efficiency for constantly changing vehicle fleet. More recently, gasoline has been reformulated to reduce various emissions and minimize the impact of automobiles on urban air quality. For the first 60 years of gasoline production, formulation and manufacture were focused on optimizing desirable performance properties, cost, and yield. Increased octane allowed increased engine power and fuel efficiency. Volatility was controlled to minimize vapor lock, carburetor icing, stalls, sag, hesitation, hard starting, and other measures of "driveability" performance. Stability additives were developed to minimize auto-oxidation, and detergents to keep carb u r e t o r s clean. Increased yield from crude oil and other sources made gasoline more available and affordable. Gasoline properties are increasingly influenced by regulatory requirements intended to reduce air and water pollution during manufacture, distribution, and use. Gasoline is controlled at many regulatory levels, including national, state/provincial, and in some cases, city or municipal.
HISTORY OF GASOLINE L e a d e d Gasoline ("Pre-control") In the late 19th century, fuels, for the automobile were coal tar distillates and the lighter fractions from the distillation of crude oil. It's believed that a gasoline fraction was separated by Joshua Merrill in Boston, and was used for lighting. It was also used in Nicolaus Otto's first four-stroke cycle engine in 1876. By the late 1800s the early refiners were producing volatile gasoline by batch distillation from crude oil. Natural gas producers were making volatile naphtha from the hydrocarbon condensed from compressed natural gas (condensate, casinghead, or "natural" gasoline). There were no standard test methods or national specifications, or definition of quality. Some of the earliest standard test methods were not developed until the 1920s, allowing development of the first widely accepted gasoline specifications. Some test methods developed in the 1920s are still in use today, such as the Reid Vapor Pressure (originally developed for casinghead gasoline) and Distillation (the oldest currently used ASTM gasoline test method, first published in 1930). The D86 was based on tests developed for casinghead or "natural gasoline" by the predecessor organization of the Gas Processors Association (GPA). The data triggered a n intensive discussion of the relationship between gasoline distillation profile (volatility) and engine performance, such as vapor lock, oil dilution, knock, and "sooting."
The current generation of North American "Tier 0" and "Tier 1" vehicles for current emissions standards have converged towards electronic fuel injected engines and "3-way catalyst" emission control systems. Globally, automakers are facing increasingly stringent "Tier 1" emissions limits for the 2004-2009 time period. While it has been recognized that meeting the Tier 2 emission standards will be a challenge for gasoline vehicles, the U.S. EPA does not expect t h a t any major technological innovation will be required to achieve compliance. However, low sulfur gasoline must be available for these standsirds to be feasible. In turn, the automakers are promoting global fuel specifications based on vehicle tech' Research Associate, Industrial Research Limited, P.O. Box 31310, Lower Hutt, New Zealand. ^ Fuels Technical Associate, Quality Assurance and Development Group, Imperial Oil Ltd., I l l St. Clair Ave. W., Toronto Ontario, Canada.
Automotive engines were rapidly being improved and required a more consistent and suitable fuel. New test methods, specifications, and refining processes were developed to better separate and increase the octane of components. Com61
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2003 by A S I M International
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pression absorption and continuous fractional distillation in "pipestills" replaced batch distillation with a continuous process in the 1920s. The first "sweetening" processes were developed to remove smelly and corrosive sulfur compounds. Thermal cracking was introduced just prior to World Weir I, followed by steam cracking, coking, fluid catalytic cracking, reforming, catalytic hydrogenation (hydrotreating), a n d many others. Gasoline specifications were developed to control the composition, properties and performance of the finished gasoline, with the three m a i n areas of octane, volatility, and cleanliness. Fixed bed cracking/reforming was introduced in the early 1930s, which significantly increased the octane of "cat-naphtha" relative to "straight-run" or thermally cracked naphtha, with a good alkyl lead response. Higher octane of the gasoline components allowed production of some of the first "premium" gasoline, called "high-test" at the time, a reference to their higher octane n u m b e r test result. During World War II, continuous fluid cat cracking and semi-continuous reforming processes were developed, along with cateJytic desulfurization, which further increased the alkyl lead response. Processes were developed to convert C3 and C4 olefins from cat cracking into gasoline by polymerization (polygas) and alkylation (alkylate). Highly leaded cracked naphtha, reformate, and alkylate blends were ideal for aviation gasoline for high compression and turbo/supercharged aviation engines developed during WWII, using "performance number" octane ratings based on the increased percent power attained above the baseline of 100% iso-octane fuel. With rare exception, all post WWII North American automotive gasoline contained TEL. In the early 1950s changes in engine designs and driving patterns required the re-introduction of the Research Octane Rating. Automakers utilized military technology to continue to increase the engine compression ratio, requiring higher octane n u m b e r fuels. The octane race occurred during the 1960s, as high performance engines required fuels with increased octane. The octane increase was non-linear with alkyl lead concentration, with diminishing returns, so there was an economic limit to the maximum amount used. The m a x i m u m permitted concentration in gasoline was 1.14g Pb/1, which was well above the optimum alkyl lead response for most refinery streams. Highly leaded premium gasoline was produced for use in high-compression ratio big-block "muscle cars," with RON as high as about 105. For the interested reader, a more detailed discussion of the history of gasoline is presented elsewhere [1-3].
H i s t o r y o f Alkyl L e a d i n G a s o l i n e During the 1910s, laws prohibited the storage of gasoline on residential properties, so Charles F. Kettering modified a Spark Ignition (SI) engine to r u n on kerosene. However the kerosene-fueled engine would "knock" and crack the cylinder head and pistons. He assigned Thomas Midgley, Jr. to confirm that the cause was the kerosene droplets vaporizing on combustion, as he presumed. Midgley demonstrated that the knock was caused by a rapid rise in pressure after ignition, not during pre-ignition as believed. The combination of TML and TEL along with alkyl bromide and chloride alkyl lead scavengers were widely used stEirting shortly after discovery by teams led by Thomas Midgley Jr. in 1922 [4-6]. Several
e m i n e n t public health officials campaigned against the widespread introduction of alkyl leads [7], but after a review, the U.S. Surgeon General decided in favor of use in 1926. The toxicity of TEL soon became apparent, as it was originally handled and added in concentrated form at the point of sale. This was changed to point of production to eliminate handling of alkyl lead concentrates outside of refineries, where it could be better controlled. Alkyl lead is unique among organometallic additives in that alkyl halides were effective "alkyl lead scavengers" to prevent buildup of alkyl lead oxides and related solids in the engine and exhaust system. This allowed lead Eilkyls to be used at m u c h higher concentrations than any other organometallic. The alkyl lead chloride and bromide had a lower melting point and higher vapor pressure than the oxide, resulting in the alkyl lead being "scavenged" and expelled with the exhaust. About 1-1.5 times the molar amount ("theories") to make the alkyl lead tetrahalide was used to provide good scavenging. Aviation gasoline uses only one theory of alkyl bromide to protect against chloride induced corrosion of the hotter exhaust valves. Tricresyl phosphate (TCP) has also been used as a alkyl lead scavenger (mostly in avgas), to form alkyl lead phosphate that has a lesser tendency to form glowing deposits, foul spark plugs, or accumulate deposits in aviation exhaust side turbochargers.
History of Octane Ratings In 1921, the Co-operative Fuel Research Committee (CFRC) was formed to research fuels and engine performance [8]. The CFR engine was developed to evaluate knock performance of fuels under controlled conditions, measuring knock intensity with a bouncing pin knockmeter. In 1927 Graham Edgar developed a n octane rating scale based on high and low reference fuels, and showed that fuels had Octane Ratings between 40-60. This became the C.F.R. Research Octane N u m b e r in 1930. The RON test method was first published in 1932, became a tentative method in 1947, and was adopted as an ASTM standard in 1951. Graham Edgar used two pure hydrocarbon isomers that could be produced in sufficient purity and quantity as reference fuels for rating the anti-knock ability of fuel. These were "normal heptane," which was commercially available in sufficient purity from the distillation of Jeffrey pine oil, and "an octane, named 2,4,4-trimethylpentane" that he first synthesized (commonly Ccdled isooctane today, although there are 17 different isomers possible, and about five prevalent C8 isomers from C4 alkylation). The heptane and iso-octane were given the arbitrary octane n u m b e r s of 0 and 100, respectively. These have similar volatility properties (see Table 1), specifically boiling point, thus the varying ratios 0:100 to 100:0 would have consistent vaporization behavior. The volu m e percentage of isooctane in the binary blend that gave the same knock intensity as the fuel being tested was assigned as the Octane Number of the sample. The early work quickly identified that the "highest useful compression ratio" (efficiency, power) was related to the CFR Research octane number. The sulfur-containing gasoline was initially restricted because sulfur in gasoline inhibited the octane-enhancing effect of the alkyl lead. In addition, different fuels had different "susceptibility" or octane response to the
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TABLE 1—Selected properties of normeil heptane and iso-octane. Normal heptane Iso octane
Melting Point, °C -90.7 -107.45
Boiling Point, "C 98.4 99.3
alkyl leads. Parafins have the best alkyl lead response, followed by napthenes, olefins, and aromatics, while the response of alcohols is negative. The RON of the gasoline could be increased from 60 to 75, and the "highest useful compression ratio" increased from 5.3 to 6.8, increasing the constantspeed fuel economy by about 30%. The Motor Octane N u m b e r (MON) was developed in 1932, using conditions that better matched actual vehicle knock performance climbing the long constant grade hill at Uniontown, Pennsylvania. This method is similar to the operating conditions of the current Motor Octane procedure [9,10]. The MON test was a tentative method from 1933 to 1939, when it was adopted as a n ASTM standard. During the late 1940s through the middle 1960s, the Research method became the m o r e i m p o r t a n t rating because milder conditions m o r e closely represented the octane requirements of the vehicle current engines and driving conditions. Most retail fuels were marketed according to their research octane rating. 1 9 6 3 C l e a n Air A c t , V e h i c l e E m i s s i o n S t a n d a r d s a n d Alkyl l e a d P h a s e d o w n During the late 1950s and 1960s, ambient air quality deteriorated, especially in densely populated urban areas. ReseEirch quickly identified the "primary pollutants" in raw exhaust (HC, CO, and NOx) and sunlight driven photochemical reaction to be a major source of SMOG (SMoke + fOG). Ground level ozone and other "secondary poUutcints" are formed by photochemical reactions driven by sunlight. The concentrations of pollutants followed a daily pattern determined by driving cycles ("rush hours"), daytime sunlight intensity, and local geography and wind patterns, with the most severe area in the U.S. being southern California. The original Clean Air Act was passed in 1963 "to protect and enhance the quality of the Nation's air resources." The first vehicle emissions standards were imposed in California in 1966 a n d Federally in 1968. The early attempts to reduce HC and CO with carburetor calibrations and thermal reactor ("thermactor," air + time delay) type exhaust systems were only about 60% effective for HC. However, this was still a larger drop (est. 10.6 g/mile to 4.1 g/mile HC) than all of the emission reductions since, due to the high emission rates at the time. A n u m b e r of attempts were made to develop "lead-tolerant catalysts" and "alkyl lead traps" that would allow continued use of leaded gasoline. However, several new potential problems with continued widespread use of leaded gasoline were found, and R&D to develop viable exhaust oxidation catalysts went into high gear. Alkyl lead is a toxin by itself, and there was a drive to reduce exposure levels from m a n y sources, ranging from alkyl lead solder in pipes, food cans, toothpaste tubes, alkyl lead pigment in paints, and edkyl leads in gasoline. In addition, the alkyl lead scavengers ethyene dibromide and ethylene dichloride could react with u n b u m e d hydrocarbons in the exhaust
Density, g/ml 0.684 0.6919
Heat of Vaporization, MJ/kg 0.365 at 25°C 0.308 at 25X
to form traces of organo-halogen c o m p o u n d s , including dioxins [11]. It is unlikely that high levels of TEL use would have continued even if a lead-tolerant catalyst h a d been found. Separate regulations were put in place to minimize the amount of alkyl lead (primeirily TEL) used in the remaining leaded gasoline produced and sold, creating "low lead" in a series of reductions in 1985/86. A smaller fill nozzle diameter was adopted for nonleaded gasoline to prevent misfueling catalyst cars with leaded gasoline. In the United States, alkyl lead could not be knowingly added to nonleaded gasoline, and the maximum allowable concentration of 0.05 g/usgal (13 rng/L) was applied as a contamination limit for "incidental contact" with leaded fuels during distribution. Other jurisdictions have slightly different legal definitions and cont a m i n a t i o n limits for nonleaded gasoline, b u t the t e r m s "nonleaded" and "unleaded" tend to be used interchangeably, even if this is not technically or legally correct in all cases. Octane ratings of leaded gasoline decreased from the "octane race" levels, a b o u t 1.5-3 AKI lower for RUL a n d PUL, respectively. Some predominately smEJl engines were designed for 87 AKI nonleaded fuel, but were "alkyl lead tolerant," because they could meet the emission standards of the time without a catalytic converter, so could use either leaded or nonleaded gasoline. The last p r e m i u m leaded gasoline was sold in 1981, and the last regular leaded in 1996 in the United States, and in 1990 in Canada. Alkyl alkyl lead has been credited variously with allowing rapid development of high power Eind efficiency SI engines, and even being a deciding factor in aviation in World War II. It is not widely appreciated that Thomas Midgley's other famous discovery, perfluorocarbon refrigerEint, had the same dichotomous history [12] as alkyl lead. It provided enormous social benefits, b u t was eventually b a n n e d d u e to stratospheric ozone depletion. It was replaced with less long-lived halocarbons that are decomposed at lower altitudes. Nonleaded Gasoline Introduction of the first Conventional Oxidation Catalyst (COC) systems required nonleaded a n d low phosphorous gasoline to protect the exhaust catalyst over the useful lifetime of the car. Alkyl lead Euid phosphorus are potent catalyst "poisons" that slowly decreased catalytic activity, eventually resulting in near total deactivation of the converter. Phosphorous contents were reduced to sub-ppm levels by eliminating the use of phosphate ester based corrosion inhibitors. Alternate corrosion inhibitor additives were readily available, so phosphorus reduction did not pose a problem to fuel manufacturers. It did require the automzikers to significEintly reduce oil consumption, as the only remaining significant source of phosphorous was from crankcase lubricant. Re-
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placing the octane boost from alkyl lead was a big problem for the fuel manufacturers, as there was n o readily available replacement. This required significant changes to both fuel and engine design. Gasoline manufacturers found that it was possible to produce, on average, a nonleaded regular gasoline with an 87 AKI and about 91 RON in the United States, and a premium 95 RON in Europe and elsewhere. United States automakers built engines with EGR and reduced compression ratios initially around 8:1 for use on 87 AKI non-leaded gasoline to reduce engine-out NOx to regulated levels. Air injection reduced HC and CO by oxidation over a COC. Efficiency and fuel economy were reduced because of the compression ratio reduction. However, this was more t h a n offset by Corporate Average Fuel Economy (CAFE) regulations in 1975 that required average LDV fuel economy determined in standard tests to increase over a period of years u p to 27.5 mpg. This was accomplished with a variety of technologies. Vehicle size, weight, and rolling/wind resistance were dramatically reduced ("downsizing"). The majority of the fleet converted to front wheel drive, using more efficient transverse L4 and V6 engines and transmissions, both using lower viscosity and friction modified lubricants.
The United States EPA may grant waivers for nonleaded gasoline formulations that are shown by testing to not cause or contribute to emission device failures. A waiver request is granted if the EPA does not decline the application after 180 days. In 1978, the EPA granted the first waiver for 10% by volu m e of ethanol. The EPA subsequently issued waivers for gasoline containing a variety of alcohols, ethers, and mixtures, of which MTBE and ethanol (gasohol) are the most commercially important today. A complete technical assessment (including extensive references) for the application of alcohols and ethers as fuels and fuel components is available from API [14]. In 1981 the EPA ruled that fuels with u p to 2 wt% oxygen from aliphatic alcohols or ethers were "substantially similar" and thus did not need a waiver. In 1991 the maximum oxygen content was increased to 2.7 wt% oxygen. To ensure sufficient base gasoline was available for ethanol blending, the EPA also ruled that gasoline containing u p to 2 vol% of MTBE could subsequently be blended with 10 vol% of ethanol.
Premium unleaded (PUL) gasoline was introduced in the United States later, in response to the higher Octane Requirement Increase (ORI) relative to historical experience with leaded fuels. This allowed production of "premium preferred" or "premium required" cars, predominately turbocharged and knock sensor equipped engines that could take advantage of the increased octane. ASTM specifications do not m a n d a t e octane number, and historically U.S. fuel producers set octane numbers in response to market forces. In North America the most c o m m o n octane grades are now RUL87, MUL89, PUL91/92, and SUL93/94.
As the tailpipe emissions were reduced by improved exhaust emission control systems, other h y d r o c a r b o n emission sources became more important. This includes hydrocarbons from evaporation or fugitive emissions of gasoline during distribution and storage, vehicle refueling, vehicle at rest, or vehicle during use. The United States CAA required summer reductions in VP in two phases in 1989 and 1992, to reduce a number of fugitive emission sources (running loss, diurnal, refueling.. .). Summer vapor pressure controls have virtually eliminated hot driveability problems. They tend to be design related, appearing as "select model" problems when they occur. Stage 1 vapor control systems reduce truck loading vapor emissions at terminals and at service stations during loading and unloading of trucks. At the service station, the vapor space of the truck is connected to the vapor space of the underground tank, whose vapor is displaced ("vapor balanced") back into the truck during fuel delivery. The truck returns to the terminal, where the hydrocarbon saturated air delivery truck are collected and sent to a Vapor Recovery Unit (VRU) or to a Vapor Destruction Unit (VDU, combustor, flare) as the truck is loaded. Stage 2 systems reduce point of sale vehicle refueling emissions. Vapors from the vehicle tank are vapor balanced with the underground storage tank, or sent to an absorber or combustor. Newer systems incorporate vapor p u m p s to remove the exact amount of air from the fill pipe as volume of gasoline dispenses, and do not require the rubber nozzle "snout" to make a vapor tight seal. These systems are recognized by air slots near the nozzle tip to remove air from the fill spout via an annular tube in the delivery hose. Vehicles are also equipped with enhanced activated carbon canisters (bigger capacity than required for diurnal control) to treat the vehicle tank vapor while refueling. These are subsequently desorbed by a small airflow and burned in the engine during normal use, once specified vehicle speeds and coolant temperatures are reached. Some activated carbons used in older vehicles do not function efficiently with oxygenates, but mod-
1 9 7 0 U S E P A C l e a n Air A c t a n d 1 9 7 7 A m e n d m e n t The 1970 U.S. Clean Air Act (CAA) established the Environmental Protection Agency and required a 90% reduction in automotive emissions by 1975 (HC/CO) and 1976 (NO^). This resulted in the widespread introduction of conventional oxidation catcilysts and nonleaded gasoline in 1975 in response to the HC and CO standards. These regulations were "technology forcing" especially for NO^, and the U.S. Congress put an interim NOx standard in place until 1978. The 1977 Clean Air Act Amendments required reduction in HC in 1980 and CO and NOx in 1981. This resulted in the widespread introduction of sophisticated "three-way catalysts" computer controlled closed loop air-fuel ratio control with exhaust oxygen sensors in most 1981 model year cars. The 1977 CAA amendments contained the first provisions that controlled the properties of "future" gasoline. It had a general requirement that gasoline be "substantially similar" to fuel or fuel additives used to certify 1975 or subsequent years vehicles [13]. The intent was that future gasoline would not render the new catalysts or emission controls ineffective over time. The EPA subsequently promulgated the "substantially similar" ruling, which had a more detailed definition of the meaning. It eilso provided that additives containing only C, H, O, N, S (which combust to the regulated emissions) did not require a waiver at concentrations below 0.25 vol%.
Vapor Controls
CHAPTER ern carbon canister systems can reduce evaporative emissions by more than 95% from uncontrolled levels. On-board canister systems tend to make Stage 2 balancing redundant as the on-road population equipped with enhanced canister systems increases. 1 9 9 0 CAA A m e n d m e n t s ( R e f o r m u l a t e d a n d Conventional Gasoline) A more complete technical discussion on Reformulated Gasoline (RFG) is contained in ASTM Research Report D02:1347 [15], and in numerous other Government and Industry Publications [16]. Officially published sources of information should always be used for regulatory compliance purposes. A summary of the United States EPA gasoline, additive, and related regulations is available at http://www. epa.gov/otag/gasoline.htm. A chronology of EPA emissions control regulations is available at http://www.epa.gov/ otag/12-miles.htm. The 1990 Clean Air Act (CAA) amendments and California Air Resources Board (CARB) Phase 2 (1996) specifications required further reductions in tailpipe emissions, more stringent test procedures and vehicle on-board monitoring technology, and new "clean fuels" programs. These established emission limits for reformulated gasoline, compared with typical 1990 "baseline" gasoline. The first stage, the "Simple Model" was an interim stage between 1 Jan. 1995 to 31 Dec. 1997. The second stage, the "Complex Model" h a d two phases: Phase I (1995-1998) and Phase II (2000+). Metropolitan regions with severe ozone air quality problems were required to use reformulated gasoline containing at least 2.0 wt% oxygen to reduce 1990 volatile organic carbon compounds by 15% (1994), and reduce specified toxic emissions by 15% (1995) and 2 5 % (2000). Because of a lack of data, the EPA was unable to define the required parameters for the CAA, so an advisory committee containing representatives of regulatory, petroleum industry, environmental, and consumer interests recommended a two-stage system for implementation ("reg-neg"). The EPA also gave the refining industry a choice of standard, based on either "per gallon," (Never To Exceed, NTE, or "cap" limit), or "averaged," based on volumetric averages over an annual or seasonal production period (Yearly Pool Average, YPA, or summer/winter). In some jurisdictions, regulations include requirements around sampling, test methods, and validation/demonstration of test proficiency, as well as certified third party testing, auditing, and field "attest" monitoring. Areas in attainment with air quality objectives continued to use "conventional gasoline." The "anti-dumping" provisions of the regulation ensured that the quality of conventional gasoline was not degraded to facilitate production of RFG. Conventional gasoline must be blended to provide equivalent emissions of the national baseline gasoline (default for domestic or import), or individual refiner's baseline based on prior production. The intent of these "anti-dumping" restrictions was to ensure emission improvements and non-attainment did not result in emission increases in areas currently in attainment with the National Ambient Air Quality Standards (NAAQS). Emission improvements had to be made by increased refinery processing for the manufacture of RFG. The Auto-Oil Air Quality Improvement Research Program (AQUIRP, "Auto-Oil") provided the basis for the "simple" and
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"complex" models used in EPA and most other North American gasoline regulations. Measured emissions were correlated to gasoline properties in a wide variety of vehicle emission system types. These spreadsheet type correlation models predict emissions from all sources associated with using gasoline in vehicles. They tcike into account the time of year (winter/summer), gasoline composition as measured by eight parameters (called the model parameters), and an assumed on-road vehicle fleet with a mixture of various generations of emission control tjrpes. A copy of the complex model spreadsheet ("Final.exe") can be downloaded from the EPA web site. California Phase 2 RFG allows a "Predictive Model," similar to the EPA complex model, as an alternative to the original Phase 2 composition based requirements. For Phase I, for example, the CAA specifies no increase in NOx emissions, reductions in VOC by 15% during the ozone season, and specified toxins by 15% all year. These criteria indirectly established vapor pressure and composition limits that refiners had to meet. The percent reductions are calculated with the complex model, which predicts volatile organic compounds (VOC), specified toxic air pollutants (TOx), and nitrogen oxides (NOx) as a function of eight composition related model inputs. Sulfur, benzene, vapor pressure, %oxygen and aromatics have the largest impact on predicted emissions. D86 evaporation volumes E200 and E300, and olefin content have lesser, but still significant impact. Federal regulations reduced vapor pressure and benzene directly, however, aromatics or other parameters could be limited to meet emissions criteria [17]. For example, a fuel with the maxim u m permitted 1.0% benzene requires total aromatics to be limited to about 27% to meet the emissions reduction requirements. Federal (EPA) RFG and California Phase 2 RFG composition limits require the use of oxygenate. The California Phase 2 RFG requires the hydrocarbon composition of the RFG to be significantly more modified than the existing oxygenated gasoline to reduce unsaturated hydrocarbons, volatility, benzene, sulfur, and the reactivity of emissions. California Phase 2 does not require the use of oxygenates in the summertime if using the "Predictive Model" except for federal ozone noncompliance areas of the state where the EPA RFG limits apply as well. Oxygenates were first added to gasoline as a high percentage level blend component (as opposed to a p p m level deicer) to extend gasoline supply supported by some State and Provincial Governments with a reduced gasoline tax rate. In 1992, 2 wt% oxygen gasoline was mandated in cold, high altitude areas starting in 1994, as a means to "lean" the mixture, and reduce unburned HC and CO by over 10% in carb u r e t e d vehicles. However, some oxygenates such as methanol, ethanol, and light ethers increase the vapor pressure of the gasoline, causing increased evaporative emissions that reduced or reversed the benefit. Newer Tier 0 and later technology cars have very fast adaptive feedback loops to control the oxygen content in the exhaust to near stoichiometric using the oxygen sensor [18]. These vehicles are much less sensitive to fuel effects, so the net benefit of oxygenates decreases with time, as the older predominately carbureted engines are retired from service. Other researchers have observed similar reductions when oxygenates are added to reformulated gasoline on older and newer vehicles, but have
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also shown that NOx levels may increase, as also may some regulated toxics [19-21]. By 2000, MTBE was the preferred oxygenate in RFG, but concerns about toxicity and water pollution were mounting. MTBE has a m u c h higher (4.3 wt%) solubility in water t h a n hydrocarbons, a relatively low bio-degradability, and very low o d o r threshold. This makes MTBE objectionable in potable water at trace concentrations, and a concern for water contamination from spills, leaks and marine engines using gasoline-containing MTBE. In 1999, California applied to the Federal government for a waiver from the Federal RFG requirement concerning oxygen content to eliminate MTBE. However, that application was declined in J u n e 2001. In March 2000, the EPA provided advance notice of its intent to initiate rulemaking pursuant to section 6 of the Toxic Substances Control Act (TSCA) to eliminate or limit the use of MTBE as a fuel blend component. Several U.S. FederEil and individual state bills have proposed elimination of MTBE since t h e n including two U.S. Federal Energy bills. At the time of this writing, use of MTBE is likely to be either banned or significantly restricted. Canadian refiners m u s t meet either a 1.0 wt% benzene NTE, or optionally a 0.95 wt% YPA and 1.5 wt% NTE, effective July 1, 2000. In addition, a 72/91 summer/winter Benzene Emission N u m b e r (BEN) requirement is based on the complex model, not including olefins. The BEN is the s u m of exhaust a n d n o n e x h a u s t benzene, excluding calculation terms of the complex model using olefins. There eire several measurement and reporting differences between the U.S. and Canadian RFG regulations. One Canadian province (BC) has VOx and NOx requirements based on the full complex model, but using a BC baseline gasoline for the calculation. Severed other Canadian Provinces have standalone s u m m e r Vapor Pressure regulations. Canadian vehicle emission standards have tended to follow closely u p o n U.S. 49 state requirements, with some differences in phase in schedules.
2 0 0 4 + Vehicle and Fuel Regulation (Future RFG) In April 2000, new Federal Tier 2 criteria for vehicle emissions and fuels were announced phased in beginning in 2004 and extending to 2007+. OfficieJly published sources of information should always be used for regulatory compliance purposes. Full details of the regulations cire available on the United States EPA website at http://www.epa.gov/otaq/cert/veh-cert/bOOOOl.pdf (Document EPA420-B-00-001). Copy of the final rulemaking is available at http://www.epa.gov/OMS/regs/ld-hwy/tier-2/frm/fr-t2pre.pdf. These stcindards apply to both diesel and gasoline powered light duty vehicles, and to fuels for these vehicles. The regulations allow early-adopters to obtain credits that cEin be used later and traded, encouraging rapid introduction of low sulfur gasoline, starting in 2000, well ahead of the 2006 deadline. Temporary, less stringent standards will apply to a few small refiners t h r o u g h 2007. I n addition, temporary, less stringent steindards will also apply to a limited geographic cirea in the western U.S. for the 2004-2006 period. Most U.S. refiners and importers must meet a corporate average gasoline sulfur standard of 120 p p m and a cap of 300 p p m beginning in 2004. In 2005 the cap remains at 300 ppm, with the corporate average reducing to 90 p p m and refinery
average at 30 ppm. In 2006, the average will be 30 p p m and cap 80 p p m , with some areas and refineries having until 2006-2009 to comply. The EPA also n o t e d that gasoline should not be advertised as "low sulfur gasoline" unless it contains 95 p p m sulfur or lower. California has legislated faster implementation of a 30 YPA, 80 NTE sulfur standard, and others, such as North Carohna's SB 953. Part II of that law, NCOS 119-26.2, proposes the same for after January 1, 2004. The Tier 2 LDV stcindards are the same for cars and light duty trucks, whether diesel or gasoline, so include both nonmethEuie organic gases (NMOG) and Particulate Matter requirements (PM essentially for diesels and NMOG essentially for gasoline). Different phase in periods apply for different size vehicles. Light duty car a n d truck phase-in starts in 2004 with 100% compliance by 2007. M e d i u m duty passenger vehicles and heavy light duty trucks will be phased in beginning in 2008, with full compliance in 2009. A new class of medium-duty passenger vehicles (MDPV), < 10,000 GVW, includes large Sport Utility Vehicles (SUV) and vans intended to carry less thzin 12 passengers. These will be treated the same as HLDT and have interim emission standards that expire in 2008. The standard defines 11 "Bins" rEinging from Bin #1 "Zero Emission" to Bin #11, which is compEirable to current LDV standards. The average Bin #5 NOx = 0.07 g/mile which represents a reduction of about 70% from current levels. The highest emissions Bin #9 to #11 expire after the phase in period, contributing to a lower long-term average. More stringent particulate matter emission standards are established in 2004 for most light trucks. The NMOG standards vary depending on which of various individual sets of emission standards manufacturers choose to use in complying with the average NOx standard. The Tier 2 Vehicle emission program increases the useful life of the vehicle to 120 000 miles and introduces new, more severe evaporative emission test procedure. This is expected to result in widespread adoption of "deadhead" gasoline fuel systems, that place the fuel p u m p , filter, and pressure regulator inside of the gasoline tank, with n o liquid gasoline return line from the engine compartment back to the teink. This eliminates a n u m b e r of external mechanical connections that contribute to the fugitive emissions etnd life of vehicle compliance. The Canadian Environmental Protection Act (CEPA) requires refiners and importers to meet either a 170 ppmw sulfur NTE over a 30 month interim period ending Dec 31, 2004, or a 150 p p m average with an 300 NTE in the last half of the interim period. Gasoline is required to be 30 p p m YPA, 80 p p m NTE starting Jan 1, 2005. One Province (Sask.) has impending oxygenate (ethanol) requirements for 2005.
GASOLINE PROPERTIES Gasoline Composition Hydrocarbons Gasoline can contain over 500 different hydrocarbons with between 4 and 14 carbon atoms per molecule. Detailed descriptions of structures can be found in any chemical or petroleum text discussing gasoline [22]. The C3 and lighter
CHAPTER hydrocarbons are too volatile to be used as a blending component of gasoline. Both methane and propane are widely used as an SI engine fuel using pressurized fuel systems and various designs of gaseous carburetors or fuel injectors. Hyd r o c a r b o n s larger t h a n about C14 are too heavy (nonvolatile), and tend to contribute inordinately to oil dilution and incomplete combustion a n d "soot" related problems. These functional limitations result in gasoline having a typical boiling range of about 25-225°C at atmospheric pressure. Vapor pressure, Driveability Index (DI) a n d model based emission criteria all tend to narrow this boiling range by removing the highest and lowest volatility ranges. The type and concentration of the various hydrocarbons in any given gasoline blend can be determined by high resolution gas chromatography, typically using 50-100 meter capillary columns to obtain the high resolution required. The n u m b e r of different hydrocarbons in any given blend depends upon the tj^e and severity of processing at any given refinery. Composition surveys [23] on nonleaded gasoline showed that gasoline blended with high percentages alkylate or low severity reformate could contain less than 175 different hydrocarbon types. Gasoline blended with catalytically cracked naphtha tended to contain more than 350 different molecules. U.S. regulations use a n ASTM external calibration GCMass Spec method for regulatory benzene and aromatic reporting. Current versions of the ASTM GC method peak tables identify over 400 different hydrocarbons by formula, and a n additional 50+ as "unknown." Most methods also measure the c o m m o n oxygenates if present. Since the FID detector is linear, the percentages of parafins, olefins, napthenes, indanes, aromatics, and unknowns can be estimated by linear sum (commonly called a PONAU or PIANO methods, which are acronyms of the first letter of the hydrocarbon t5rpes detected). With over 400 compounds identified,
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there is usually less than 2.5% "unknown" content in a typical RUL and less than 1% in a typical PUL. There are small concentrations of additional isomers that are undoubtedly present below the nominal 50 p p m w detection level of the standard methods using Flame Ionization Detectors. These tend to be highly branched CIO to C14 olefins, isoparafins, and napthenes because of the large n u m b e r of isomers that can be produced within each carbon n u m b e r in this range. The magnitude of the "undetected" hydrocarbons can be inferred to be less than about 0.5% for a complex gasoline by comparing results of high and low resolution methods. The m u c h smaller number of isomers possible for aromatics makes the measurement of aromatics by FID detectors more feasible since essentially all of the possible aromatics up to about CI2 are identified. As a result, Canadian regulations use a Cap-GC/FID method for both benzene and total aromatic reporting. Saturated
Hydrocarbons
(Paraffins,
Alkanes)
Saturates are the most chemically stable species, and comprise 20-80% of gasoline, with 30-60% being typical. The octane ratings of these compounds depend on the branching and n u m b e r of cEirbon atoms. The octane primary reference fuels are both saturated hydrocarbons (0 = heptane, 100 = 224 trimethylpentane). Additional information is presented in Table 1. The interested reader is directed to a standard organic chemistry text for further detail on these and the other classes of hydrocarbons. Unsaturated
Hydrocarbons
Unsaturated hydrocarbons are less stable as compared to saturates. The upper limit of both olefins and aromatics can be limited by emissions or other specifications. Table 2 contains information about the various classes of unsaturated hydrocarbons.
TABLE 2—Unsaturated hydrocarbons. Alkenes
Dienes
Alkynes Arenas (Aromatics)
PNA (Polynuclear Aromatics) PAH (Polycyclic Aromatic Hydrocarbon)
• • • • • • • • • • • • • • • •
Olefins, contain single carbon-carbon double bonds Usually stable, depends upon molecular type Clean burning, reduces VOC but increases NMOG reactivity More reactive than alkanes, higher octane ratings. Present in gasoline 0-25% (10-15% typical) Olefins containing more than one carbon-carbon double bond Usually unstable especially if conjugated (n, n+2 double bonds) More be significantly higher reactivity, toxicity and air reactivity <<1 % generally required for stability Acetylenes, contain carbon-carbon triple bonds Considered very reactive, only present in trace amounts, and usually in some poorly-refined gasolines <<0.1% generally required Very stable, high octane. 10-50 vol% (25-35 vol% typical). Tend to be limited by emission models Benzene > > > toxicity than other aromatics, regulated separtately Alkyl benzenes not considered smoke precursors, as they have vaporized and are not present at the end of combustion, (see PNA) • Condensed ring aromatics (naphthalenes, alkyl napthalene, trace 3 ring) • Generally controlled to < < 1 vol% by boiling point or other specifications (220-225°C vs. 260°C dimethyl napthalene) • Di-alkyl PNA are potent smoke precursors, and are very detrimental to spark ignition engine operations. • Naphthalene and methylnaphthalene boil at 218°C and 230°C and are not considered to be potent smoke pre-cursors. Di-methylnaphthalenes boiling above allows it to ring close on de-hydrogenation to form a larger PNA. It turns it from a "chain ender" molecule to a "smoke precursor". The extra methyl group on the aromatic allows this process to continue to completion of a multi-ring smoke molecule. Many other control measures can be used to control the same property.
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TABLE 3—Properties of selected oxygenates.
Methanol Ethanol MTBE ETBE TAME Gasoline
Oxygenates
Energy Content, Net MJ/kg
Heat of Vaporization MJ/kg
Oxygen Content Wt%
19.95 26.68 35.18 36.29 36.28 42-44
1.154 0.913 0.322 0.310 0.323 0.297
49.9 34.7 18.2 15.7 15.7 0.0
(as Blend
Components)
See History of Gasoline (nonleaded and RFG). Table 3 provides properties of selected oygenates that are used as gasoline blend components. Gasoline Additives Additives are compounds added in trace a m o u n t s (<2500 p p m in the EPA "substantially similar" ruling), to enhance some property of the fuel, or performance of the engine, vehicle, or emissions. Specifications and regulations allow for additive use, and in some cases, require additive use, for example deposit control additives (DCA), which are beneficial for in-use emissions. Oxygenates
(De-icers,
Gas Line
Anti-Freeze)
Aliphatic alcohols and ethers added at 5-15 vol% are considered to be blending components and not "additives." However, at lower concentrations, low molecular weight alcohols, especially methanol, ethanol, and iso-propanol, are effective de-icing additives, commonly available as "gas line antifreeze." Alcohols are typically used in the 500-2500 ppmv (0.05-0.25 vol%) range for this purpose, and function by preventing traces of water (typically from temperature drop and condensation) from freezing in the vehicle fuel system. A higher rate may be required for larger water contaminations, or to prevent carburetor icing, which is accumulation of ice in the carburetor throat and throttle plate. The cooling of high humidity combustion air is caused by throttling (Joule Thompson cooling) and fuel evaporation heat of vaporization (auto-refrigeration). Gasoline with higher VP, Eind lower TIO and T50 (high "front end" volatility) has a higher propensity for producing carburetor icing IPA is approved as aviation fuel system icing inhibitor (FSII Type 2) at a treat rate of 1 vol% in aviation gasoline (ASTM D 4171). Other alcohols and glycols have also been used for carburetors or fuel line icing, including hexylene glycol, propylene glycol, dipropylene glycol, and various glycol ethers. Methanol is the most commonly used for gasoline fuel line freezing protection in fuel-injected vehicles in cold climates. De-icers are not commonly used in automotive fuels in warm/cool climates, due to the low population of carbureted engines susceptible to carburetor icing. Some carbureted utility engines may require alcohols for operation in severe icing conditions ( > 8 5 % relative humidity, 0-10°C). Alkyl Lead
Compounds
Alkyl lead compounds are n o longer used in North American automotive gasoline, because of the detrimental effects on exhaust catalysts and concerns for health effects of alkyl lead
in the environment (see History of Gasoline). Nonleaded specifications and regulations limit alkyl lead contamination to 5 mg/L (Canada) and 0.05 g/gal (13 mg/L) (U.S.). ASTM D 910 Aviation Gasoline lOOLL (low lead) has been reformulated from the original 100/130 grade with the addition of aromatics (toluene) to reduce TEL to the lowest level necessary to maintain the octane number and aircraft performance. Avgas is generally handled in segregated distribution systems, so is generally not a significant source of alkyl lead contamination in automotive gasoline. With no other source of alkyl lead contamination, typical trace alkyl lead levels in North American gasoline are usually undetectable by standard test methods. Methylcyclopentadienyl
Manganese
Tricarbonyl
(MMT)
As the toxicity of the alkyl lead and the halogenated scavengers became of concern, alternatives were considered. The most notable of these is methylcyclopentadienyl manganese tricarbonyl (MMT), an organo-manganese compound with similar antiknock properties as TEL, but at m u c h lower concentrations to obtain about 1-2 octane n u m b e r increase. It was used in the United States in nonleaded gasoline in the mid 1970s, b u t a n application for an EPA waiver was declined in October 1978, effectively banning it in nonleaded gasoline in the United States. In December 1995, the EPA was forced to grant the waiver in U.S. conventional gasoline (not RFG), and MMT can be used in conventional gasoline up to 1/32 g Mn/usg (8.3 mg/L) in all states except California, where it is regulated separately [24]. In a separate case, the court held that EPA's new rules that require pre-meirket testing of new gasoline additives did not apply to MMT. The EPA has stated it intends to monitor MMT use, but MMT was used in only 0.02% of gasoline in 1998. MMT has a high toxicity in concentrated form, but the combination of relatively low volatility and its miscibility with aromatic hydrocarbon solvents allows it to be stored and handled with tj^ical equipment. MMT is light unstable with a half life of about 30 s in strong sunlight, forming a characteristic red "rust" colored precipitate (Mn02). This oxide is softer t h a n iron oxides, and is not considered to be abrasive even at much higher concentrations than typically used in gasoline. MMT has been in continuous use in Canada since the first introduction of nonleaded gasoline in 1972 to a maximum of 18 mg/L Mn, without major incidents. Most problems attributed to MMT are associated with excessive exhaust or catalyst temperatures. These cause the manganese oxide to "ceram" and form a gas impermeable glass that can coat and effectively deactivate the catalyst [25] This is also associated with front face catalyst plugging, due to the "sticky" nature of the glass. Field experience indicates that this is not a serious problem in tj^jical driving cycles, including Tier 0 and 1 vehicles equipped with OBDII emission control monitoring systems. Two provincial inspection a n d m a i n t e n a n c e (I&M) programs using IM240 (d3mo driving cycle) and ASM2525 (25 m p h steady state dyno) test cycles have not found or identified any emissions related problems due to MMT on model years 1982 to present. Proponents claim benefits in NOx reduction, protecting the catalyst from long term deactivation by sequestering catalyst poisons (Zn, P, Pb) or coke, fuel cost benefits, emissions re-
CHAPTER duction from reduced refining severity, and lack of adverse health effects. Opponents cite high exhaust temperature catalyst plugging, exhaust gas sensor "rich shift" for the NOx effect, spark plug tracking, and the potential for adverse health effects. Dozens of controlled and noncontrolled tests and surveys over 35 years have failed to conclusively resolve the debate [26,27,28], probably because the test result depends upon the specific automobile year and model being tested and how the test is done. Several countries are considering MMT as an alkyl lead replacement additive to facilitate alkyl lead phasedown. Iron based
Organo-metallics
Other compounds that enhance octane have been suggested, b u t usually have significant problems such as toxicity, cost, a n d increased engine wear, especially if used at high concentrations. Dicyclopentadienyl iron (ferrocene) a n d nickel carbonyl have been promoted as alternatives to alkyl lead and MMT. For example, the addition of 0.02-0.04 g/L ferrocene with 0.05-0.10 g/L tertiary butyl acetate is reported to increase the AKI of susceptible gasoline hydrocarbon feedstocks by 4-6. More recently, ferrocene has been promoted as an octane enhancer at lower levels of 9 mg/L Fe to avoid any significant deposition or wear problems. The benefit is reduced to about 1-1.5 octane n u m b e r at this level (similar to MMT). Unlike m a n g a n e s e oxides, iron oxides do not ceram at high temperature. Iron pentacarbonyl (Fe(CO)5) was used in Germany at levels of 0.5% or less in gasoline during the 1930s. While the cost was low, the carbonyl decomposed rapidly when the gasoline was exposed to light to form iron oxides. It also has a relatively high vapor pressure, and is extremely toxic in concentrated form, so must be handled with care. Use of the additive at high concentrations in gasoline caused excessive deposition of iron oxide (Fe304) on the spark plug insulator, causing short circuits. The precipitation of iron oxides in the lubricating oil also led to excessive wear rates [29]. Iron picrate (trinitro phenate) and related oil soluble iron chelates are sometimes sold as aftermarket additives with various performance claims, but are not used in commercial production. Performance
Additives
Most other additives are added to address specific safety or p e r f o r m a n c e properties of gasoline. Typical examples include: 1. Oil-soluble dye was originally added to leaded gasoline at about 10 p p m to prevent misuse of leaded gasoline as an industrial solvent, and is now also used to identify grades or brand of product. Red dyes are currently used in the U.S. and Canada to mark fuels that have different taxation rates for different uses. 2. Antioxidants (oxidation inhibitors) typically phenylene diamines (PDA) or hindered alkyl phenols, are added to slow down oxidation of hydrocarbons, especially unsaturated hydrocarbons. These additives are "sacrificial," and once consumed, the oxidation reactions rapidly become autocatalytic, increasing exponentially in rate to form soluble and insoluble gum ("varnish").
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3. Metal Deactivators, typically about 10 p p m of chelating agent such as N,N'-discJicylidene-l,2-propanediamine is added to inhibit copper catalyzed air oxidation of gasoline 4. Ferrous Corrosion Inhibitors, about 5-20 p p m of oilsoluble surfactants, are added to prevent aqueous-based corrosion caused either by water condensing from watersaturated gasoline, or from condensation from air onto the walls of almost-empty gasoline tanks that drop below the dew point. Many pipelines require addition of corrosion inhibitor for protection of pipeline and storage tanks. The NACE 0172 "rust" test is commonly used to establish addition rate for corrosion protection. A variety of comp o u n d s are effective as corrosion inhibitors, including dimer carboxylic acid esters, amides, alkanolamides, and hydroxyesters. Phosphorus based corrosion inhibitors are no longer used in nonleaded gasoline because phosphorus is a potent catalyst poison, and is controlled by both ASTM specification and regulation. ASTM specifications allow, but do not require, corrosion inhibitor or corrosion testing. 5. Lubricity additives are occasionally used in gasoline, and are usually similar in chemistry to corrosion inhibitors, but at higher treat rates. 6. Copper corrosion inhibitors are occasionally used to mitigate sulfur-based corrosion of copper or other easily corroded base metals if/when used in fuel systems. 7. Anti-wear additives are used to control wear in the upper cylinder and piston ring area that the gasoline contacts, and for valve sticking. They are usually very light hydrocarbon oils ("top" oils, "solvent" oils), although synthetics are now preferred, due to potential problems with combustion chamber deposits. If permitted, phosphorus-based additives can also be used in engines without exhaust catalyst systems. 8. Cleanliness or Deposit-modifying Additives are used to mitigate (keep clean) or reduce (clean up) deposits. Some specific types of deposits are discussed below. • Carburetor Deposits—Additives were required w h e n crankcase blow-by positive Crankcase Ventilation (PCV) and exhaust gas recirculation (EGR) controls were introduced (in the 1950s on some engines). Some fuel components reacted with these gas streams to form deposits on the throat, throttle plate and idle air bleeds of carburetors and some throttle body injectors. • Fuel Injector Deposits—Deposits can form in the annulus or sealing surfaces in some pintle needle or poppet valve injector designs during hot soak (>100C), suspected to be mainly from the oxidation and polymerization of the least volatile or unsaturated hydrocarbons. The additives that prevent and unclog these tips are usually polybutene succinimides, polyisobutylamines, or polyether amines, and are often used in combination with "fluidizer" or "solvent" oil. • Intake Valve Deposits—IVDs caused major problems in the mid-1980s with widespread introduction of electronic fuel injectors a n d three way catalyst systems. Some vehicles experienced severe driveability problems even when fully warmed, even though the amount of deposit was considered acceptable by historical carbureted engine standards. It was proposed those new fuels and engine designs were producing a more porous deposit
70
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HANDBOOK
on the valve tulip (fuel side) face. The close proximity of the liquid fuel injector caused some liquid fuel to be absorbed in the deposit, causing lean/rich transients resulting in hesitation, misfire, and excess emissions. Deposit Control Additives (DCA) were mandated in 1992 in California and Federally in 1995 and by CGSB specification in 1994 due to the emissions increase. Intake valves operate about 300°C, and if the valve is kept wet with thermally stable oil in conjunction with a dispersant/detergent, deposits tend not to form. Solvent oil/dispersant/detergent additives that are effective with IVD also tend to be effective for fuel injector deposits. Gasoline factors implicated in these deposits include unsaturates and alcohols, as well as oil leakage through the valve guides. • Combustion Chamber Deposits—CCD were targeted in the 1990s, as they are responsible for significant increases in emissions. Severe deposit accumulation resulted in CCD interference (CCDI) where the deposit contacted the cylinder head at TDC, resulting in noise, and eventual engine damage. Additives have a limited ability to remove pre-existing CCD, so DCA additives are tested for their propensity to avoid formation of CCD relative to the base gasoline. Gasoline factors that contribute to heavier deposits are the presence of alcohols or olefins. Gasoline manufacturers now routinely use additives that prevent IVD and also maintain the cleanliness of injectors. These usually include surfactant and light oil to maintain the wetting of important surfaces. Intake valve deposits can also have significant adverse effects on emissions, and deposit control additives are now standard to reduce emissions and provide clean engine operation. Trace Constituents
and
Contaminants
1. Free Water—"haze" is considered to be a contaminant, associated with corrosion, filter plugging, and freezing problems. The large difference in density usually allows water to settle out from traditional gasoline, and most additive packages are checked for any propensity to stabilize a water haze and prevent water dropout. Gasoline components are often dried at the refineries supplying gasoline into very cold winter distribution systems, to prevent water to phase separate due to condensation on cooling down. Most specifications have a dryness, "clear and bright," (such as ASTM D 4176 for distillate fuels) or some similar "suitable for use" criteria. 2. Sulfur, nitrogen, and oxygen are naturally occurring trace constituents of crude oil, and are present in fuels at the p p m level. These CEin be present as ashless, combustible, additives. Nitrogen is typically not regulated in gasoline formulation because the source of nitrogen in NOx emissions is from atmospheric N2, and not the fuel. The maxim u m sulfur specification prior to regulation was t3^ically 0.1-0.15 wt%. Current California a n d future U.S. a n d Canadian regulations will reduce sulfur to a yearly pool average 30 p p m by 2005/6. Regulated changes to the baseline gasoline used in any predictive model based regulation equates to a change in the gasoline production required for equivalence, including sulfur, which has a large impact on the predicted emissions. This would restrict the use of any
sulfur containing additives in the future. (See Reformulated Gasoline for more detail.) 3. Gasoline may be contaminated by other partially soluble or miscible liquids, such as kerosene, diesel, and oxygenates. Oxygenated fuel can have a higher solubility of water-soluble contaminants, as well as allowing water-soluble c o m p o n e n t s to c o n t a m i n a t e any fresh water that comes into contact with it. 4. "Adulterants" or "extenders" have occasionally been deliberately added to gasoline to increase the volume sold. Materials used have included new or used solvents, used lube oil, formic/acetic acid, heJogenated solvents, refrigerants, dry cleaning solvents, silicon oils/caulking/adhesives, and even PCBs. Deliberate adulteration is usually involved with a criminal activity for illegal waste disposal or fuel tax fraud. Some jurisdictions now have "contaminated fuels regulations" that prohibit these activities. 5. Particulate contaminants, such as dirt and corrosion products, can be readily removed by standard filtration procedures, and increasingly stringent cleanliness requirements are resulting in a n increasing use of microfiltration at truck loading and/or point of sale.
GASOLINE COMBUSTION Spark Ignition
Engines
Spark ignition engines are ideally suited for light duty transportation uses. Their relatively lightweight (compared to other internal combustion engines), wide REM range, broad torque band, and high maximum RPM/power provide good performance with a m i n i m u m number of transmission gears. Vehicle spark ignition engines are air throttled to control power, a n d have inherent limitations on peak cylinder temperatures and pressures due to engine "knock," so are thermally less efficient than compression ignition engines. As a result, diesel engines d o m i n a t e the high torque, low r p m heavy-duty truck, marine, locomotive, and stationary power applications. Smaller high-speed diesel engines comprise only a small percentage of the light duty vehicle market in North America, but are more prevalent in Europe. This is driven mainly by their significantly higher fuel efficiency, which provides a significant market advantage, especially in regions with high fuel prices. Electronic fuel injection has replaced carburetors on essentially all Tier 0, Tier 1, and later model automobiles. This facilitates "closed loop" computer control of air/fuel ratio, to maintain exhaust composition within a n a r r o w range required for optimal operation of a "3-way" catalyst. The name "3-way" is a reference to effectiveness for all three regulated exhaust emissions: u n b u r n e d hydrocarbons (HC), carbon monoxide (CO) by air oxidation, and oxides of nitrogen (NOx) by chemical reduction with HC/CO. These systems require the engine to operate in a narrow air-fuel ratio band on both rich and lean side of stoichiometric, to maintain both the regulated emissions HC, CO, and NOx and O2 in the optimal range where they react to extinction. Hybrid gasoline-electric vehicles couple internal combustion engines with an electric motor for peak power and transient engine operations to obtain ultra-low "steady state" emissions with no significant loss in performance. A smaller,
CHAPTER generally m u c h higher voltage battery set stores enough power for several minutes of high power electric motor operation for cold start or accelerations. This allows a n u m b e r of very efficient and ultra low emission driving cycles, since the majority of SI engine emissions are due to transient operations and cold start, before the catalyst heats up. The SI engine can shut down (zero emissions) during cold start or if required for urban ZEV emissions. It can run at steady state (for example on the highway) letting the electric motor handle variable power requirements (hills, wind load, comers, etc.) that would otherwise cause increased emissions from transient SI operation. It can be r u n at a higher steady state power, using the electric motor as a generator to recharge the batteries. Elimination of transients tends to provide highway fuel efficiency in urban driving cycles, and ultra low emissions (lower t h a n the Super Ultra Low Emission Vehicle (SULEV) level. The 2004 Tier 2 LDV Bin #2 NMOG standard of 0.01 g/mile is about a 90-95% reduction from current levels (0.125-0.25 g/mile), over 99.8% reduction from the 1990 levels (4.1 g/mi), and 99.9% reduction from precontrol (est. 10.6 g/mile). Other hybrid vehicle types have been proposed, such as diesel-electric or gasoline-hydraulic, or gasoline-flywheel, but only gasoline-electric are being commercialized at this time. It is anticipated that both current RFG and 30 p p m sulfur future RFG will be compatible with hybrids, but that the lower sulfur will provide slightly lower in-use emissions. Gasoline Direct Injection (GDI) engines (Spark Ignition Direct Injection, SIDI, etc.) have thermodynamic efficiencies between SI and diesel engines, and may be commercialized in the near future. Fuel requirements for GDI have not been established. Exhaust particulate matter may be more critical for GDI than for current engine/emissions systems. Engine
Management
Systems
Engine management systems are now an important part of the strategy to reduce automotive pollution. A typical m o d e m engine system would monitor and control: mass airflow (or MAP/RPM calculated air flow), exhaust oxygen sensor (lambda sensor), knock (vibration/noise) sensor, EGR, throttle plate angle, manifold air temperature and pressure, crank angle, and transmission gear. These systems can compensate for altitude (MAP), ambient air temperature (MAT), fuel oct a n e (knock sensor), humidity, a n d gasoline composition (oxygen sensor, adaptive map). The engine control module (ECM, computer) then sets the ignition timing and fuel injector open time (fuel flow), and fuel injection timing with crank angle that are appropriate for the current engine operating conditions. Almost all TierO/1 engine control systems incorporate "adaptive memory maps" or adaptive learning strategies. The fuel m a p (or schedule) conceptually consists of three blocks of data organized like three sheets in a spreadsheet, with the rows and columns representing different RPM and manifold vacuum, which together define the engine load. The three sheets contain fuel injector open time for the factory "baseline" settings, the long-term block learn, and the short-term block learn. Conceptually, the data is a c o m p u t e r binary n u m b e r such as 0 to 255 for an 8-bit computer. This might represent, for example, how many milliseconds the fuel injector should be held open, since open time is proportional to
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71
fuel volume delivered at constant pressure. The adaptive maps would start out at mid-range (128), and the total number of milliseconds injector open time would be the sum of the corresponding cells of the three tables for the engine speed and load at that instant of time. The ECM would modify them according to the response of the oxygen sensor. If it was consistently lean for several seconds (too much oxygen in the exhaust) then the ECM will add one n u m b e r to the fast learn block, making it 129, and adding one unit of time to the injector open time, slightly enriching the mixture. This continues until the fast learn block hits its upper limit, when the ECM t3?pically adds one to the long term block cell, and resets the fast learn to its mid value. The long term and fast learn blocks are similar to a "course and fine" control, where the fast learn steps are smaller, but occur faster. The long term map, as the n a m e implies, adjusts for long term trends, for example a slow drop in fuel rail pressure due to a p u m p wearing out over the lifetime of the car, a partially plugged fuel filter, or slowly fouling fuel injectors. The fast learn block takes care of smaller but faster changes, such as changing altitude, humidity, or tank to tank gasoline composition differences, such as switching between high and low aromatic or oxygenated gasoline. This is also the reason that composition mandates are less effective for newer cars than older cars, especially oxygenates for cold high altitude CO control, as the ECM quickly adjusts the air-fuel ratio according to the oxygen sensor readings at the time. One artifact of this system is that a dead or disconnected battery can cause memory loss of the adaptive maps, so that the vehicle drivability can be very poor while the ECM reloads the m a p with data appropriate for the car, engine, fuel, and ambient conditions at the time. All U.S. a n d Cdn LDVs n o w have on-board diagnostics version II (OBDII) that monitor the critical emission system components for conditions that cause high tailpipe emissions or are detrimental to continued operation. The ECM lights the Malfunction Indicator Light (MIL) light on the dash, commonly called the "check engine" light. OBDII warns of malfunctions such as engine misfires, exhaust catalyst failure (inferred from oxygen sensor time delays), and evaporative emissions failure such as a leaking gas cap (fuel tank pressure). Chronic misfire, for example causes combustion of unb u m e d air/fuel on the catalyst, with the potential to generate so much heat that the catalyst activity drops, or the catalyst substrate melts. The air-fuel ratio is controlled at part throttle by a closed loop system using the oxygen sensor in the exhaust and the knock sensor (if equipped). Typical engine calibrations enrich the air-fuel ratio for smoother, stall free idle when in closed loop control, and hold a faster idle after cold start when in open loop, until the catalyst becomes heated and "lights off." The a m o u n t of idle enrichment used is often adjusted according to the air t e m p e r a t u r e at the time of engine start (similar to the function of a choke on a carburetor acting against a bimetal temperature spring that held the choke on longer when cold until the engine heated up). Air - Fuel Ratio and
Stoichiometry
Ideally, hydrocarbons and oxygenates in gasoline combust smoothly to form water, carbon dioxide, and heat energy, with no other by-products. Non-ideal combustion affects
72 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK emissions, efficiency, engine durability, and vehicle operation. Severe engine knock is associated with loss of efficiency a n d engine damage. If there is excess fuel ("rich" air/fuel ratio), the combustion is not complete, and carbon monoxide (CO) will be formed, Eilong with u n b u m e d or partially burned hydrocarbons (HC). As CO can be burned to produce CO2, it is " u n b u m e d " fuel, so t h e r e is a d r o p in efficiency a n d increase in emissions. Nitrogen oxides are formed as a result of conversion of nitrogen (N2) in air to various nitrogen oxides (NOx). High peak combustion temperatures promote NOx ("lean" air/fuel ratio, high intake air temperature, low humidity, high barometric pressure, high load). The required mass or volume of air to provide sufficient oxygen to achieve this complete combustion is the "stoichiometric" mass or volume of air. Insufficient air creates "rich" mixtures, and excess air creates "lean" mixtures. The stoichiometric mass of air for a gasoline is related to the carbon: hydrogen ratio of the fuel. The procedures for calculating the stoichiometric air-fuel ratios are fully documented in SAE J 1829 [30]. However, oxygen (20.9476 vol%). Nitrogen (78.084%), and Argon (0.934%) comprise 99.966 vol% of the atmosphere. The use of nominal masses (O2 = 16) versus natural abundance masses (O2 = 15.994) and ideal gas assumption (vol% = mole) will result in errors of less than 0.5%, because the errors tend to cancel (nominal MW of nitrogen lower than natural abundance offset by higher MW of Argon vs. nitrogen). As a result, it is c o m m o n to assume that the non-oxygen component of the air is nitrogen, which can be added to the equations when the exhaust compositions are required. The error is generally m u c h smaller than the analytical uncertainty of the actual carbon hydrogen ratio of a real gasoline, by elemental analysis or cap GC. If needed, exact molecular weights can be calculated from molecular formula and standard periodic tables weight, or from ASTM GC method peak tables for individual hydroccirbons. For n o r m a l h e p t a n e C7H16 with a molecular weight = 100.204: C7H16 + 1 1 0 2 ^ 7CO2 + 8H2O The chemical stoichiometric combustion of hydrocarbons with oxygen can be written as: CxHy + (x + (y/4))02 -^ XCO2 + (y/2)H20 t h u s 1.000 kg of CyHife requires (1/100.204) * 11 * 2 * 16 = 3.513 kg of O2 and using approximation, (3.513 kg of O2 + (3.513/32) * (28*79.0524/20.9476) = 15.113 kg of air. (Air-fuel ratio = 15.113 vs 15.179) Gasoline
Energy
Content
The energy content (heat of combustion) is the total amount of energy obtained by combustion of gasoline under standard conditions, usually at STP, as if the fuel and air started at STP, and the combustion products are cooled down to STP. Energy captured at other conditions will be different. For example, emitting a hot exhaust from an engine will not capture the heat energy from cooling the exhaust back to STP. The heat of combustion is useful to compare fuels under the same standard conditions. It is measured by combusting
all the fuel inside a b o m b calorimeter and measuring the heat released as evidenced by a t e m p e r a t u r e increase u n d e r controlled conditions. If the combustion is not complete, carbon monoxide (CO) and/or unburned or partially combusted fuel will be formed, resulting in a lower than true energy content. The reported heat of combustion depends on what happens to the water produced from the combustion. If the water remains as a gas, then it cannot release the heat of vaporization, thus producing the net (or lower) heat of combustion. If the water were condensed back to the original fuel temperature, then the gross (or higher) heat of combustion is reported. For internal combustion engines, the net heat of combustion is more appropriate, as the water is emitted as vapor. The engine cannot utilize the additional energy available when the steam is condensed back to water. This is sometimes confused for lower emd higher thermal efficiency of diesel versus spark ignition engines or the lower/higher net heating value of diesel fuel versus gasoline. However, the net/gross or lower/higher heating value oiany fuel is simply a reference to whether the water of combustion is condensed and captured by the process, or exhausted as a vapor and lost to the process. Standard heats of combustion for hydrocarbon molecules and for reactions involved in combustion are available from a number of engineering and chemistry texts and references. This includes ASTM Dataseries DS4B, Physical Constants of Hydrocarbon and Non-Hydrocarbon Compounds, and others. Because all the data are available, the calorific value of fuels can be estimated quite accurately from calculation detailed GC analysis or by correlation with hydrocarbon fuel properties such as the density, sulfur content, and aniline point (which indicates the aromatics content). Oxygenates contain oxygen that has already reacted in the production process, and cannot provide additional energy at the point of use in the engine, so they have significantly lower energy contents. For an engine that can be optimized for oxygenates, more volumetric fuel is required to obtain the same power. However, the decrease in volumetric fuel efficiency (mpg) tends to be less than the energy content decrease. See Table 4 for properties of selected oxygenates. For a water-cooled SI engine with 2 5 % useful work at the crankshaft, the losses may consist of 35% coolant, 33% exhaust, and 12% surroundings. SI engines have inherently lower thermal efficiencies than diesel engines, mainly due to throttling of the engine at partial power, and higher diesel engine compression ratio. Even at wide open throttle, equivalent displacement CI engines have higher thermal efficiency because of higher compression ratio, so this advantage extends over the entire operating range. Octane rating is fundamentally not related to the energy content, although the actual hydrocarbon a n d oxygenate components used in the gasoline will determine both the energy release and the antiknock rating. However, it is possible to make correlation between energy content and octane that are mostly meaningless and often confusing. For example, addition of ethanol to a blend can increase the octane, while decreasing the energy content, while adding pure aromatic increases both. Add to this the difference between energy per fuel liquid volume versus energy per fuel weight, and one can generate octane versus energy graphs that go up, down, or
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TABLE 4—ASTM research and motor octane number test conditions. ASTM D 2700 MON ASTM D 2699 RON Cooperative Fuels Research Cooperative Fuels Research Engine (CFR) (CFR) 600 RPM Engine Speed 900 RPM Variable with Barometric P. 38°C Intake Air Temperature 88 kPa at 19.4X, 101.6 kPa at 52.2°C (Temperature Tuned with Toluene Standardization Fuel ± 22C from lAT) Intake Air Humidity 3.56-7.12 g HzO/kg dry air 3.56-7.12 g HaO/kg dry air 149°C (Temperature Tuned with Not Specified Intake Mixture Temperature Toluene Standardization Fuel 141-163''C) lOOX 100°C Coolant Temperature 57°C Oil Temperature 57°C Ignition Advance Variable with Comp. Ratio 13 degrees BTDC (fixed) 14-26 degrees BTDC Carburetor Venturi 14.3 mm Variable with Altitude 14.3 mm at 0-500 m Test Method
sideways, dependent entirely on what materials and units are chosen. Gasoline Octane Performance Properties Combustion
and Knock
in
Engines
The critical fuel property of gasoline for internal combustion engines is resistance to engine "knock," expressed as the octane n u m b e r of the gasoline. During a normal (no knock) combustion cycle, a flame front travels smoothly from the point of ignition at the spark plug outweird toward the cylinder walls. While this is occurring, the "end gas," or u n b u m e d fuel/air mixture ahead of the flame front is heated and compressed. If the end gases ignite before the flame front arrives, the resulting sudden pressure wave reverberates across the combustion chamber, causing an audible engine knock. This adversely affects output power and dramatically increases heat transfer to the piston and other combustion chamber surfaces. While this can cause damage on its own if severe enough, knock induced preignition can cause rapid catastrophic engine failure. This tends to be a runaway condition. Once started, it gets progressively worse until eventual engine failure, unless the throttle/load is cut quickly, as failure can occur in less than a few minutes. High heat transfer during heavy knock can cause deposits or sharp edges (for example combustion chamber deposit or exposed thread of a spark plug) to overheat. This can act as a "glow plug" type ignition source, causing ignition of the charge before the spark on the next combustion stroke "preignition." This leads to excessively high combustion chamber temperature and pressure from combustion closer to TDC, and rapidly increasing knock intensity. Initial stages of knock damage look like pitting on the piston top, as if it had been attacked with an ice pick or awl. The increased temperature and heat transfer to the piston top eventually causes melting of the crown land down to the rings. The final stage is either catastrophic engine failure from thrown connecting rods (metal softening near the piston wrist pin, causing the piston to separate from the connecting rod), or engine seizure (expansion of the piston, excessive friction heat, and loss of lubrication). Either
condition is sufficient to ruin one's day (in addition to the engine). The octane (and autoignition temperature) of various hydrocarbons is related to their ability to withstand preflame conditions without decomposing into species t h a t could auto-ignite before the flame-front arrives. Unburned "end gases" ahead of the flame front encounter 700°C temperatures due to compression and heat transfer, and commence a series of complex preflame reactions. These reactions occur at different t h e r m a l stages, with the first stage (around 400°C) commencing with the addition of molecular oxygen to alkyl radicals. The internal transfer of hydrogen atoms within the new radical forms an unsaturated, oxygen-containing species following this stage. These new species are susceptible to chain branching involving the HO2 radical during the intermediate temperature stage (400-600°C), mainly through the production of OH radicals. Above 600°C, the most important reaction that produces chain branching is the reaction of one hydrogen atom radical with molecular oxygen to form O and OH radicals. ' Common antiknock additives work by interfering at different points in the preflame reactions. The alkyl lead antiknock compounds interfere with hydrocarbon chain branching in the intermediate temperature range, where HO2 is the most important radical species. Alkyl lead oxide, either as solid particles, or in the gas phase, reacts with HO2 and removes it from the available radical pool. This reduces the major chain branching reaction sequence that results in undesirable, easily auto-ignitable hydrocarbons [31,32]. Oxygenates retard the progress of the low t e m p e r a t u r e or cool-flame reactions, consuming radical species, particularly OH radicals and producing u n s a t u r a t e d hydrocarbons like isobutene. The iso-butene would, in turn, consume additional OH radicals and produce unreactive, resonantly-stabilized radicals, such as allyl- and methyl allyl-, as well as stable species such as allene, which resist further oxidation [33,34]. Anti-knock
Ratings
of Fuels
The Anti Knock Index (AKI) is the average of the Research Octane Number (RON, D 2699) and Motor Octane Number
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(MON, D 2700), sometimes expressed as (RON+MON)/2 or (R+M)/2. The on-line Comparitor Engine method D2885 determines octane numbers that are equivalent to "lab" RON and MON values, and can be used for blend certification and release. The RON and MON methods use two primary reference fuels, n-heptane and 224 trimethyl pentcine, assigned octane numbers of 0 and 100, respectively. The knocking intensity of the test fuel is c o m p a r e d to that of reference fuel blends. The volume percent of iso-octane in the blend that gives the same knock intensity as the test fuel is taken as being the octane n u m b e r of the test fuel (See History of Octane Rating). The same standard engine is used for both tests, but r u n at different test conditions. The conditions of the MON engine tend to simulate hot, highway driving, whereas the RON test is more like heavy acceleration from a traffic light. The difference between the RON a n d MON for a given gasoline ("sensitivity") depends upon the composition of the blend. The RON and MON of parafins and isoparafins is almost the same, so high isoparafins (low aromatic, low olefin) blends tend to have low sensitivities, sometimes below four numbers. Aromatics have intermediate sensitivities and olefins have the highest sensitivity, so gasolines that have high catalytically cracked content (high olefins) will have high sensitivity (RON-MON), as high as about 14. ASTM D 4814 recommends a m i n i m u m MON of 82 for Regular grade gasoline heavy-duty engine applications, but uses AKI for all other recommended uses. Two different 87 AKI gasolines with extreme compositions could have a RON/MON of 94/80 or 89/85. On average they would give similar perform a n c e over a range of engine operating conditions, but for certain applications one may be preferred over the other. For a n engine operating under more MON like conditions, the 87/85 blend will provide knock free operation at more severe conditions than the 97/80 blend. In the late 1960s select European automakers experienced catastrophic engine failures on high speed Autobahn runs, even though the Research Octane of the fuel was within specification. They discovered that either the MON or the Sensitivity (the numerical difference between the RON and MON) also had to be specified to ensure adequate performance under severe high speed Autobahn condition. Similarly, many marine and utility engines run at high load and WOT, and are susceptible to knock damage from too low a MON. In the late 1970s and early 1980s several automotive and marine OEMs promoted development of an even more severe octane method, due to concerns that the high octane contribution from alcohol in the MON test did not appear under their most severe wide open throttle engine operating conditions. However, this was not pursued, Eind some OEMs recommended higher octane 89-91 AKI grades or no oxygenates or methanol for severe marine, snowmobile, outboard, or utility engine applications.
and cylinder casting can be adjusted relative to the crankshaft using a crank handle to obtain the desired compression ratio. The engines have a special four-bowl carburetor that permits individual bowl air-fuel-ratio adjustment and provides rapid switching between reference fuels and samples. A magnetorestrictive detonation sensor in the combustion chamber now measures the rapid changes in combustion chamber pressure caused by knock, and the amplified signal is measured on a "knockmeter" with a 0-100 scale. Only one company currently manufactures these engines, the Waukesha Engine Division of Dresser Industries, Waukesha, Wisconsin, 53186. Two related methods can be used to obtain the octane n u m b e r of the test fuel, the bracketing procedure or the "compression ratio" procedure. For the bracketing method, the engine compression ratio is adjusted to produce a m i d range knock intensity of about 50 on the knockmeter, with the air-fuel ratio adjusted on the carburetor bowl to obtain m a x i m u m knock. At least two blends of primary reference fuels are made, one that is one octane n u m b e r above the expected rating, and another that is one octane number below the expected rating. The PRF blends are placed in different bowls with their air-fuel ratios adjusted for maximum knock during rating. The higher-octane reference fuel should produce a reading of around 30-40, and the lower reference fuel should produce a reading of 60-70. The actual fuel rating is interpolated from the knockmeter readings of the sample and the two PRFs. For the CR procedure, a calibration curve is prepared of knock intensity versus compression ratio for a PRF blend that has a n octane n u m b e r within prescribed limits from the sample. The octane n u m b e r of the sample is the octane number of the single PRF used to calibrate the engine, corrected for barometric pressure and difference in knock intensity using the calibration curve. The D2885 "comparator" method uses the CR procedure, but using a Standard Reference Fuel (SRF) instead of a PRF (see D 2885). Motor Octane Rating ASTM D 2700 The conditions of the Motor method represent severe, sustained high engine speed, high load (but not wide-open throttle) driving. For most hydrocarbon-fuels, including those with either alkyl lead or oxygenates, the Motor Octane Number (MON) will be lower than the Research Octeine Number (RON). Research
For Anti-Knock
Rating
Fuels
Automotive octane ratings are determined in a standardized single-cylinder engine with a variable compression ratio (CR 4:1 to 18:1), operated under standard conditions (Table 4) Cooperative Fuels Research (CFR) engine. The cylinder bore is 82.5 m m ; the stroke is 114.3 m m , giving a displacement of 612 cm^. The piston has four compression rings, and one oilcontrol ring. The intake valve is shrouded. The single head
ASTM D 2699
The research m e t h o d settings represent typical high load (throttle opening) and low to medium engine speeds resulting in low inlet mixture temperatures and moderate loads on the engine. See Table 5 for test conditions On-Line
Procedure
Octane Rating
Analyzer
Octane Rating ASTM
D 2885
On-line octane rating analyzers used in refineries are described in ASTM D 2885, taking a continuously flowing slip stream of gasoline from current production, and measuring the RON and MON continuously. The same engines and conditions are used, so consequently results Eire equivalent to the laboratory MON or RON procedure. Instead of "bracketing" the test fuel with primary reference fuels, D 2885 uses "Standard Reference Fuels," commonly
CHAPTER called "protos" or "gold protos" that are production gasoline that have been rated by about 20 different laboratories, and assigned the average value determined by the group. The SRF is required to be close in octane to the test fuels, so that the difference in octane n u m b e r can be derived from the difference in knock intensity between the SRF and the test fuel. Standard Reference Fuels are developed through cooperative exchange programs, usuedly administered in association with an accredited Standcirds Writing Organization. Examples in North America are National Exchange Group under the auspices of ASTM, the Canadian Cooperative Exchange under the auspices of the CGSB, and the Rocky Mountain Exchange, representing refiners in high altitude areas (now associated with the NEG). Octane Distribution
Throughout
Fuel Boiling
Range
Severe knock can be experienced in some engines if octane level is not distributed throughout the boiling range of a fuel, for example a low octane front end with a high octEine back end. This "octane distribution" is most critical during changes in mixture flow in manifolds, such as sudden full throttle acceleration at low speed and high load. Under these conditions, manifold pressure drops suddenly at low airflow. The fuel can segregate by boiling range in the manifold, with the very volatile fraction reaching the combustion chamber first and, if that fraction is deficient in octane, then knock will occur until the less volatile, higher-octane liquid fractions eirrive. Historically, small displacement engines tend to be the most affected, as they experience the largest changes in manifold conditions during n o r m a l u r b a n driving. Large displacement engines equipped with "hot spot" manifolds, intake mEinifold runners, etc., tended to be the least affected. Most urbcin driving was at relatively low throttle range and at high manifold vacuum that tended to moderate these effects. The delta RON ratings Eire not currently used in the U.S. M o d e m engines with short inlet meinifolds and Port Fuel Injection are usually less sensitive to fuel octane distribution. However, the addition of nonhydrocarbon components into reformulated gasoline of different volatility may affect sensitive engines. Leaded fuel octane distribution was good, as the tetramethyl alkyl lead and tetraethyl alkyl lead octane volatility profiles were well characterized, and TML-TEL ratio could be adjusted to provide distribution of octane throughout the boiling range. Nonleaded gasoline must be properly blended without using TML to obtain acceptable results. The "Delta RON" test was developed to measure the difference in octane between the front £ind back "half" of the gasoline. The fuel is distilled to a specified temperature, which is usually 100°C. Both the parent fuel and the distillate fraction are t h e n rated using the Research Octane m e t h o d . The difference between these is the delta RON (100°C), usually just called the delta RON. During the 1990s, there have been some concerns in Europe about the high sensitivity of some commercially available nonleaded fuels. Octane Number
Requirement
(ONR) of
Vehicles
The actual octane requirement of a vehicle is called the Octane N u m b e r Requirement (ONR). It is determined by monitoring for the onset of knock while accelerating the vehicle
3: MOTOR
GASOLINE
75
using a series of progressively lower octane fuels. Three sets of fuels are used: the primary reference fuels (PRF), and two full boiling range fuels with high and low sensitivity (RONMON). In Europe, delta RON (100°C) fuels are also used. The lowest octane fuel that provides knock free acceleration is the ONR of the vehicle under the conditions of the test. Results are then used to predict the performance of typical commercial gasolines. The original "Uniontown" road octane rating procedure used in development of the original MON test (see History of Octane Ratings) has been replaced by a procedure developed by the Coordinating Research Council (CRC). In the CRC E-15 procedure, the vehicle is tested under a proscribed set of environmental conditions and loads. The procedure "trained raters" to detect audible knock during a series of part throttle accelerations using decreasing octane fuels until trace knock is detected. Three different series of gasolines were used to establish the RON versus MON response of each vehicle, a zero sensitivity primary reference fuel, and two full boiling range fuels with low and high Severity (difference between RON a n d MON). These three fuels represent the extremes of RON and MON blending. By testing a large n u m b e r of cars, it is possible to statistically predict what percentage of the on-road car population would be "satisfied" (no knock) with different RON/MON fuel blends. It is comm o n to apply a correction factor or offset between technical raters and typical drivers, who have a m u c h lower threshold for detecting knock and perceiving it to be a problem. Historically, operating conditions that require maximum octcine are not consistent, but most often occur during fuUthrotde acceleration from low starting speeds using the highest gear available. These can be difficult to reproduce on a chassis dynamometer, so a distinction was always made between an on-road and a dyno result. M o d e m dynomometers with better simulation of inertial, road, and wind loads and engine cooling, produce equivalent results. A benefit is that the conditions can be more tightly controlled, so results do not require corrections for temperature, pressure, and humidity differences from the standard conditions at the time of the test. Historical engine management systems used a system of centrifugal "bob weights" in the distributor for speirk advance and a "vacuum break" to delay spark advance/retard at low speed, high load. With only two parameters to change in addition to the basic spark timing of the engine, one or two areas of high knock anywhere o n the speed/load range resulted in conservative timing at other conditions. If the critical area was under MON like conditions, then the car would be found to respond most to MON. M o d e m engine management systems can adjust the octane requirement by modifying engine conditions that will vary the octane n u m b e r requirment, so adaptive learning systems are preconditioned prior to testing. The m a x i m u m ONR is of most interest, as that usually defines the r e c o m m e n d e d fuel. However, it is recognized that trained raters are more sensitive to engine knock than typical drivers, and that the general populace will have a higher threshold for both detecting knock and perceiving it as being severe. As a result, it is common to apply a correction factor or offset from technical ratings to predict consumer satisfaction.
76
MANUAL
ONR—Engine
3 7 ; FUELS AND LUBRICANTS Design
HANDBOOK
Parameters
The design of the engine and vehicle significantly affect the fuel octane r e q u i r e m e n t for b o t h RON a n d MON. In the 1930s and 40s, most vehicles would have been sensitive to the Research Octane of the fuel, almost regardless of the Motor Octane. In North America, this tended to change after WWI with widespread adoption of automatic transmissions, which made engines operate under more MON like conditions more of the time. For many years, the CRC did octane surveys of each new model year of cars, and the results were widely used to estimate the on-road octane requirement, and the best RON/MON combinations to satisfy the most cars (on average) with the lowest octane blends. Different weightings were p u t on RON a n d MON, for example (R+3M)/4 to better match the measured octane n u m b e r to the average octane requirement of the on-road car population. The advent of electronic adaptive engine controls and use of knock sensors tends to have a more consistent spark and fuel m a p over the entire engine speed and load range, including transients like transmission shift points. This tends to m a k e adaptive feedback control cars equally likely to knock under any condition and equally responsive to RON and MON. This tends to make the AKI a good predictor of field performance in recent model cars. Retarding spark timing for the purpose of knock reduction causes loss of power a n d efficiency that can be incorporated into knock rating procedures, for example by measuring increase in acceleration times with progressively lower octane rating fuels. ONR—Manifold
Air Temperature
and
Pressure
Increasing the air-fuel charge temperature and pressure increases the peak cylinder temperature and pressure, and thus the tendency to knock. M o d e m engines generally pre-heat intake air to a controlled temperature above ambient. Throttling the air to a lower manifold pressure results in a much lower peak cylinder pressure/temperature, and a lesser tendency to knock. Fuel evaporation and PVT expansion across the throttle plate cool the intake mixture, making it slightly sensitive to the engine temperature, which adds heat in the manifold and during the compression stroke. Increasing the coolant temperature in a CFR engine by about 10°C increases the octane n u m b e r r e q u i r e m e n t by about 2 AKI, so the coolant temperature must be tightly controlled for octane rating. Increasing the intake air (RON) or intake mixture (MON) temperature increases the octane requirement (increases the severity of the test). This is used to advantage in the RON and MON tests to "temperature tune" engines in different laboratories using Toluene Standardization Fuels (TSF). This ensures that the test severity of different engines is a relative constant over a broad range of fuel compositions, since PRF are low aromatic, and TSF are high aromatic. ONR—Compression
Ratio
An increase in Compression Ratio will require an increase in fuel octane for the same engine design. Increasing compression ratio increases the theoretical thermodynamic efficiency of an engine according to the standard equation Efficiency = 1 — ((l/compression ratio)^(gamma—1)), where G a m m a = ratio of specific heats at constant pressure
and constant volume of the working fluid (for most purposes air is the working fluid, and is treated as an ideal gas). Thermal efficiency reaches a maximum at a compression ratio of about 17:1 for gasoline fuels in a SI engine, and fuel-injected engines with carefully designed mixture charge concentrations and intake/combustion chamber design has reached 12:1 on retail fuels with 98-100 RON. Continued advances in engine technology have allowed gradual increase in compression ratio without increasing ONR from 87 AKI. This has been used to either increase fuel efficiency in economy cars or to have higher power engines at the same fuel efficiency in larger cars. These engines have substantially higher compression ratio t h a n the "historical" curve established with the CRC engine (see History of Octane Rating]. ONR—The
Air-Fuel
Ratio
Stoichiometric combustion (air-fuel ratio = 14.7:1 for a typical non-oxygenated gasoline) is neither maximum power, which occurs around air-fuel 12-13:1 (Rich), nor maximum thermal efficiency, w h i c h occurs a r o u n d air-fuel 16-18:1 (Lean). Air-fuel ratio can also be used as a knock control strategy, although at a severe emissions debit for a three-way catalyst emission system. Enrichment is also used during full throttle operation to reduce knocking while providing better power and driveability. Recent changes in U.S. emissions testing cycles will promote more control of wide-open throttle mixture settings. Enrichment has also been used as a n "emergency" strategy in turbocharged cars, as an alternative to suddenly dropping turbo boost (power) to prevent runaway knock during an emergency or passing maneuver. On average, there is an increase or decrease in ONR of about 2 AKI for each unit's increase (leaning) or decrease (enriching) of the air-fuel ratio, respectively. This response varies considerably between engines and at different RPMs. In the CFR octane n u m b e r tests, the carburetor float bowl level is adjusted to change the air-fuel ratio. The air-fuel ratio that produces the maximum knock intensity for that fuel is then used to determine the octane n u m b e r of that fuel. ONR—Engine
Spark
Timing
Advancing the spark timing dramatically increases the tendency to knock. The tendency to knock increases as spark advance is increased. For an engine with recommended 6° BTDC (Before Top Dead Center) timing and 93 RON fuel, retarding the spark 4° lowers the octane requirement to 91, whereas advancing it 8° requires 96 octane fuel. This response varies considerably between engines and is most pronounced at high RPM for any given engine (mostly an artifact of delay time being measured by crank angle rotation rate). In older engines, the basic timing was set by rotating the distributor until the #1 cylinder spark was set at a prescribed crank angle. Spark was advanced for RPM with a centrifugal "bob weight" on the distributor shaft, and retarded for load with a manifold vacuum diaphragm actuator acting against the spring loaded distributor plate. The timing of the spark is advanced sufficiently to ensure that the fuel-air mixture b u m s in such a way that maximum pressure of the burning charge is about 15-20° after TDC. Heavy knock will tend to occur if peak temperature/pressure occurs earlier than this
CHAPTER 3: MOTOR GASOLINE point early in the power stroke, involving a larger quantity of end gases igniting at high pressures (gas densities). Light knock usually occurs later in the power stroke, involving less end gas and a lower cylinder temperature and pressure (gas densities). A good analogy is that larger waves in higher density mercury inherently carry more energy and destructive power than smaller waves in lower density water. Knock sensors equipped engine management systems retard ignition timing if knock is detected. If very low octane fuels are used, several octane numbers below the vehicle's ONR at the time, the ECM will have to retard spark timing so much to eliminate the knock that both performance and fuel economy will significantly decrease. Maximum efficiency occurs when the engine is at incipient knock for the fuel being used at the time, so trace or incipient knock is sometimes referred to as "the sound of economy." Unlike fuel maps, knock sensor data is generally not used to advance timing, only to retard it as an engine protection strategy. The ECM will retard timing ("knock back spark") quickly until knock sensor indicates that it has stopped. It will then slowly advance the timing toward baseline, until trace knock re-appears, and store that value in the memory map. Strategies to actively advance timing are more risky, since a failed knock sensor could cause the ECM to advance timing into sustained heavy knock and engine damage. Some systems are capable of "testing" the knock sensor by advancing timing on individual cylinder firings to create a single or small number of knocking cycles, to see if the knock sensor is still functioning. A "dead" knock sensor can be particularly damaging on a turbocharged car, or a "premium required" car, if run on too low an octane fuel. ONR—Effect of Engine Deposits Typical new engines using nonleaded gasoline have an ONR of about 4-6 AKI lower than the same engine at 25 000 km, although some may experience 6-9 ONI, and some engines can experience ONI of over 12 octane, depending upon driving cycle, engine, and fuel/lube. This Octane Requirement Increase (ORI) is attributed to the formation of a mixture of organic and inorganic deposits resulting from both the fuel and the lubricant. They build up and tend to reach an equilibrium quantity in about 10-15 000 miles. However, differences in driving cycles can result in different equilibrium levels, with sustained hot engine operation tending toward the low end, and cooler "stop and go" driving cycles tending toward the high end. Combustion chamber deposits produce the ORI by several mechanisms: • Reducing the combustion chamber volume, effectively increasing the compression ratio • Reducing wall thermal conductivity, increasing the combustion chamber temperatures • Catalyzing undesirable pre-flame reactions that produce end gases with low auto-ignition temperatures.
GASOLINE SPECIFICATIONS. ASTM or CGSB gasoline specifications are not legal requirements unless they are required (referenced) in a regulation by a government authority having jurisdiction.
77
In the United States and Canada, a mixture of Federal, State, and Provincial Government legislation and attendant regulations proscribes gasoline requirements. Some states and associations of states may also specify regional gasoline properties to achieve regional environmental objectives that are within their jurisdiction. These regulations may reference accredited ASTM consensus standards, with or without modifications. In some cases, ASTM sampling and test methods are reproduced in regulations. This can cause update problems, such as use of the wrong procedures for regulatory purposes, when either party changes procedures. The U.S. gasoline specifications and test methods are listed in several readily available publications, including the Federal Register, and the current Annual Book of ASTM Standards. ASTM D 4814, Specification for Automotive Spark-Ignition Engine Fuel The scope of the standard states that "This specification guides in establishing requirements of automotive fuels for ground vehicles equipped with spark-ignition engines." It was first published as ASTM D 439 in 1937, and significantly revised to include oxygenates including "gasohol" (10% ethanol) and other alcohol blends that had EPA waivers (see 1970 EPA Clean Air Act). Several test methods were developed or modified for use with oxygenated fuels, such as Dry Reid Vapor Pressure, Tv/1, and new methods added such as water tolerance and phase separation temperature requirements. It is a complex specification with many details that are beyond the scope of this chapter. However, in all cases, the requirements and the test method for each requirement fall into one of two categories, those that measure the properties or quality at the time of sampling, and those that predict some future condition or performance. For example, a "Clear and Bright" procedure evaluates whether the fuel is "suitable for use" at the time of custody transfer or sale to end consumer. An oxidation stability result, on the other hand, is a "predictive" test, intended to predict if the fuel will be acceptable in the future, after some time in storage prior to sale or use. Many tests are "rig" tests that rate relative performance of the gasoline to provide a reasonable estimate of how the gasoline will perform in actual use. For example, D 130 (Test Method for Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test) measures the relative degree of corrosivity under a proscribed set of conditions. It uses a static coupon of a specified copper alloy, prepared in a specified way, exposed to the product for a specified period of time at a specified temperature. In the vast majority of cases, a "pass" on the D 130 test means that the fuel will not cause significant corrosion to any copper alloy parts exposed in the fuel system, and will be widely acceptable to a broad range of vehicle fuel systems. However, if the in-use conditions for any fuel system component are more severe than the D 130 test conditions, then damage could occur even with a fuel that passes D 130. This could happen if the component is heated hotter than test conditions, or the part is in a "rubbing or rolling" tjfpe of service that continually removes the protective oxide/sulfide
78
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
layer, or the part metallurgy is more easily corroded than the D 130 copper strip. ASTM has two publications that are highly recommended to those not familiar with details or intent of the test methods or specification. ASTM Manual on Significance of Tests for Petroleum Products [35] and Guide to ASTM Test Methods for the Analysis of Petroleum Products and Lubricants [36] are both instructive on reasons for the test and explanation of what and how the test is intended to measure. IdecJly, performance or "rig" tests Eire representative of actual in-use vehicle requirements. Hopefully these two goals always coincide, but it is always possible to "pass the test and fail the field" as any experienced fuels chemist will agree. Passing all the tests does not guarantee that the fuel is "suitable for use" in all cases, especially in cases such as unusual contamination, or new car technology for which the existing standard tests don't consider, or when in-use conditions are more severe than in the standard test method. Specifications Emd test methods are in a continual state of being updated as technology continually advances. Some gasoline requirements pertain to all volatility classes (see Table 5). Anti-knock
Index
(AKI)
Anti-knock Index (AKI), also k n o w n as (RON+MON)/2, "Pump Octane," ((R+M)/2), or "Road Octane" is the average of the Research Octane Number and Motor Octane Number. Dispensing p u m p "posting" requirements are based on AKI. The only accurate m e t h o d of m e a s u r i n g knock is to use standard knock rating engines in ASTM D 2699/D 2700 or D 2885, although several "field test m e t h o d s " are used for screening purposes. The ASTM gasoline standard does not mandate octane levels, but lists levels appropriate for different applications. While limits are not specified, changes in engine requirements according to season and location are discussed. Fuels with an AKI of 87, 89, 91 (nonleaded), and 88 (leaded) are listed as typical for the U.S. at sea level. However, higher altitudes may specify lower octane numbers. Altitude derating of octane is m u c h less for nonleaded versus leaded gasoline because Tier 0/1 cars use manifold absolute pressure/temperature (MAP/MAT) sensors to calculate air density, or absolute mass air flow sensors to control air fuel ratio.
TABLE 5—Detailed requirements for all volatility classes. Lead Content, max, g/L (g/U.S. gal) Non leaded Leaded Copper Strip Corrosion, max Solvent-Washed Gum Content, mg/100 mL, max Sulfur, max, mass% Non leaded Leaded Oxidation Stability, minimum, minutes Water Tolerance
0.013 (0.05) 1.1 (4.2) No. 1
0.10 0.15 240 Water tolerance limits in terms of maximum temperature for phase separation are given in Table 13 of the specification.
Volatility Volatility is measured by the ASTM D 86 distillation, the Vapor/Liquid ratio Temperature (Tv/1 = 20), and one of several vapor pressure test methods, most commonly ASTM D 5191, which is used for regulatory reporting. All of the volatility measurements are somewhat related thermodynamically, being various combinations of vapor pressure and cumulative boiling volumes. However the relationships are not exact, because the standard tests are not ideal thermodynamic processes. Some examples of various ASTM D 86 distillation effects are given in Figs. 1-3. An illustration of approximate vapor pressure and volatility relationships is given in Fig. 4, comparing typical fuels with pure hydrocarbons. Distillation, Evaporation Temperatures, and Driveability Index—The distillation volatility of a finished gasoline can be expressed in several ways. One set uses the temperature at fixed distillation points, such as the TIO, T50, and T90, related indices such as Volume Average Boiling Point (VABP) or ASTM Driveability Index (DI) Driveability Index = DI = 1.5 * TIO + 3 * T50 + 1.0 * T90 Maximum DI limits are specified in ASTM D 4814 for each Volatility Class, ranging from 569-597°C, or 1200-1250°F. Note that the conversion of DI from Centigrade to Farenheit units is DI(F) = DI(C) * 9/5 + 176, where 176 is the sum of the coefficients (1.5 + 3.0 -f- 1.0) * 32. Many companies use volume percent evaporated (%Evap, or D-l-L) at fixed temperatures, which have an advantage over Txx controls, because they blend more linecirly, and are more easily adapted to linear programming (LP) optimization controls. Both are in c o m m o n use at the operational level, because ASTM requires Txx type controls while the complex model requires E200 a n d E300. The complex model spreadsheet has a conversion from Txx to Exxx built into the model. Vapor Pressure and Tvfl—^ASTM vapor pressure specifications are based on VP measured under standard conditions of 4:1 vapor liquid ratio at 100°F and Tv/1 20 to control gasoline front-end volatility. The Tv/120 is the temperature at which 20 volumes of vapor cire formed from one volume of the original liquid, measured by ASTM D 2533 or D 5188. The intent is to produce an isobaric flash prediction, something that is well within the capabilities of m o d e m thermodynamic equation of state (EOS) calculations. A Tv/120 corresponds to about 10% of the gasoline vaporized under conditions of the test (a fully vaporized gasoline would have a V/L of about 220-280 depending upon the blend). As a result, there is a good correlation between measured Tv/1 and linear sums of the VP a n d TIO, T20 u p to T50, so it is much more common to calculate Tv/1 than to measure it. Some companies use an index based on VP emd %Evap at lower temperatures, historically 70°C (158°F). These are often called Vapor Lock Indices or "Hot Fuel Handling," a reference to the origined purpose of these specifications. A v/1 of 20 was originally chosen because, on average, a carbureted engine diaphragm type fuel p u m p could handle about 20 volumes of vapor along with the 0.9 volume of the remaining liquid (Tv/1 = 20). Any higher temperature (more vapor) would cause the p u m p to become "vaporlocked," and not be able to satisfy engine fuel dememd, cans-
CHAPTER 3: MOTOR GASOLINE -Base
Max Summer Dl
Msx Dl High Mid-fill
79
-High Volatility
250
JMax Final Boiling Point (FBP) = 225 C
200 Max Summer T90 = 190 J
150u p
I 100 Typical Summer RUL Volatility Dl = 1150 .
IMin / Max Summer T50 Range = 77-121
50
I
Higliest Summer Volatility T50 = 70 Max Summer T10 = 70
10
20
30
40
li 50
60
70
80
90
100
Volume % Evaporated FIG. 1—ASTM D 86 distillation and driveability index.
ing stalls or rough engine operation. The temperature when this occurs is called the Vapor Lock Temperature. The ASTM specification has vapor-lock protection temperatures based on a regional database of ambient temperature and elevation (barometric pressure). Fuel injected vehicles generally have submerged in-tank fuel pumps, and operate at much higher liquid fuel line (fuel rail) pressures, so eire much less prone to vapor lock problems. However, certain forms of vapor lock can occur.
but are usually due to other mechanical factors and not high ambient temperature. For example, a worn out fuel pump may not put out enough pressure to prevent vapor formation in the fuel rail during a hot shutdown. The vehicle could experience a hard hot starting or no-start condition, because much lower mass vapor flow through the injector would cause an over lean air-fuel ratio. Similarly, a partially blocked primary filter ("pick-up sock") can cause vapor lock on the suction side of the submerged pump at
80 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK -Ethanol
MTBE
Base
250
200
-
/ / 150
o
100
50 Significant decrease in T50 caused by azeotrope effect of ethanol
0
10
20
30
40 50 60 Volume % Evaporated
70
80
90
100
FIG. 2—ASTM D 86 distlllation-ethanol and MTBE.
high temperatures. Routing fuel lines in areas that have high shutdown heat soak temperatures can also cause a hot restart problem. Gasoline and Gasohol Blending The distillation volatility is controlled within specification limits by varying the relative ratios of blend components to make a finished gasoline. These range from butane (normal boiling point 0°C) to the highest boiling components with Final Boiling Points slightly higher than the 225°C FBP specifi-
cation for the finished gasoline. The vapor pressure is controlled below specification maximums by limiting the amount of butane in winter and northern summer grades, and butane plus some pentanes in the lowest vapor pressure summer grades (below about 8 psi VP.) Ethanol at 10 vol% (gasohol) increases the measured VP at 100°F by about 0.8 in a high aromatic gasoline to about 1.2 psi in a low aromatic gasoline. Ethanol is more soluble in aromatics than saturates so the vapor pressure increase is slightly influenced by aromatic content of the base fuel.
CHAPTER 3: MOTOR GASOLINE
81
- A S T M D 8 6 — P R EOS 250 ^
TypiCcil inoroase in ASTM DBG final di&hllalioM tuiiippidluic due to higher tlian one theoretical plate and icflux due to he.it lush at higci toinpci.ituics
JThcmiodynaniic Bubble Point 200 -
I Sciniplcs of this gasoline will not flash oi boil .it toinpcratuicR bolow the bubble point, even if it is at .1 tcnipciaturp iiighcr than thn ASTM D86 Initial ' Boiling Point
O 150 ^ rhcoieticnl Single pl.itc distilkition cuivc
ffl 100
'Typical liiiti.il volume loss .iiid "hool«" on start of ASTM D8b distilljtiuii b.iuscd by cumbin.ition of.
50
- gipatiii than one theoictii..)! pl.itp distillation finin licitiiig of glassw.irc .iiid reilux duiing starl of thn distilLition - nun condensing ,iir HC v.ipor mixturi> at st.iit of test
AS1MD86lnili.ii Boiling Point iJBP)
20
10
40
30
50
70
60
90
80
100
Volume % Evaporated
FIG. 3—ASTM D 86 versus Peng-Robinson Equation of State.
100000 ASTM V.ipur Pri
10000
1000
..•-• Ntiiiii.il riu'lini) Piiiiil rii-.<.-iir>-
100
10
£
£ 0.
CJ A|iuiuxiiii.1li R.m.ii-••! V.i|iiii l>ii-.-iiii'
'C5 CM'
•il -W" i" Fl.i .'1 ''•HI.; T.>ii,|.i 1...SII11
C6
£•7
Cq
CIO - 0.1
-0.01
-100
-80
100
-50
200
Temperature ( C ) FIG. 4—Vapor pressure relationships for fuels. Log P versus 1/T.
300
82
MANUAL
37: FUELS
AND LUBRICANTS
HANDBOOK
Several jurisdictions allow a nominal 1 psi waiver to accommodate "splash bending" of ethanol with a base gasoline. EPA has issued a waiver that does not require conventional gasoline with 9-10% ethanol to meet the required specifications between 1 May and 15 September (see Nonleaded Gasoline). ASTM
Volatility
Class
Specifications
ASTM D 4814 gasoline specification defines six volatility classes of increasing vapor pressure, Tv/1 and Driveability Index (DI). The TIO, T50 min/max and T90, FBP max limits define the allowable range of individual parameters within each volatility class. VP and DI are generally the key blending p a r a m e t e r s for b o t h regulatory and specification compliance. Insufficient volatility may result in difficult starting in cold weather, poor cold start and warm-up driveability, engine deposits and crankcase oil dilution, and increased tailpipe emissions. Excessive front-end volatility can produce poor fuel economy, poor hot driveability in fuel injected engines, vapor lock a n d c a r b u r e t o r icing in older engines, and increased r u n n i n g loss and evaporative emissions. The higher boiling fractions of the gasoline have significant effects on the emission levels of undesirable hydrocarbons and aldehydes. A reduction of 40°C in the final boiling point will reduce the levels of benzene, butadiene, formaldehyde, and acetaldehyde by 25%, and will reduce HC emissions by 20% [37]. Similar emission effects were found in the auto-oil program, and as a result, there are three volatility model parameters in the complex model,VP, E200, E300 (Vapor pressure, percent evaporated at 200°F and 300°F, respectively) that influence predicted evaporative, tailpipe, and toxics emissions. The E200 and E300 model parameters have the least influence of all the model inputs on the predictive model emissions, and are often set at "worst case" values for the purpose of blend planning (although actual values must be used for reporting). Maximum
Alkyl lead
Content
The alkyl lead limits remain to cover fuels for off-road vehicles. Leaded fuels can contain u p to 0.1 g Pb/usg (equivalent 0.029 g Pb/L low alkyl lead in Canada), but none are made commercially. Nonleaded gasoline can contain u p to 0.013 g Pb per liter in the U.S. (contamination limit with no deliberate addition) and 0.005 g Pb/L in Canada to protect catalytic converters. (See sections on alkyl lead phasedown and nonleaded gasoline for more detail). Copper
Corrosion
Copper, silver, brass, and other "soft metal" alloys are susceptible to corrosion from reactive sulfur molecules. The ASTM D 130 "copper corrosion" test is intended to ensure that fuels are not corrosive to these trace components under normal use conditions. More severe conditions could require even lower levels of corrosive sulfur. The total sulfur content is not correlated with D 130 test performance, and in fact, some additives that are effective at mitigating copper corrosion themselves contain sulfur (see additives). Only the most reactive types of sulfur molecules cause or contribute to D 130 copper corrosion. The copper strip test responds to reactive sulfur, whereas the sulfur content reports the total sulfur content.
The D 130 test is a "static" coupon test, and may overpredict durability for more easily corroded alloys such as silver, or for rubbing, rolling, or heated soft alloy fuel system components. Field problems have been experienced with some copper fuel p u m p commutators, silver alloy fuel sender unit resister arrays and silver plated crankcase bearing cages with gasoline that passes D 130. In general, silver alloys should not be used in gasoline fuel systems without provisions for protection from sulfiding. Copper alloy components that are exposed to heat or rolling/rubbing action that removes the protective oxide layer should be tested for long-term durability with a gasoline containing traces of corrosive sulfur (H2S, elemental sulfur). Maximum
Sulfur
Content
Current ASTM and CGSB standards allow 0.10% mass maxim u m sulfur content for nonleaded gasoline, but lower levels are required by various sulfur regulations (California or Canadian Sulfur in Gasoline regulation). Sulfur is oxidized to SO2 over the catalyst, so essentially competes for reactive sites that could otherwise be effective for HC, CO or NOx conversion. This appears as a temporary decrease in catalyst activity at high fuel sulfur levels, and leads to the requirement for ultra low sulfur fuels to attain ultra low emissions. Sulfur can be indirectly limited by RFC composition controls based on emission models (U.S. EPA complex model, BC TOx/NOx a n d Canadian CEPA Benzene in Gasoline regulation Benzene Emission N u m b e r (BEN) requirements. Both Canada and the United States have regulations in place that will reduce sulfur to 30 p p m YPA phased in between 2002 and 2010, coincident with introduction of low emission Tier 2 vehicles (See "Reformulated Gasoline" for more details). Maximum
Phosphorus
Content
The EPA limits phosphorus in all gasoline to 1.3 mg/L P, because phosphorus is a potent catalyst poison, and this limit is reflected in the ASTM specification (see alkyl lead phasedown section for additional details). Typical levels of phosphorus in nonleaded gasoline are undetectable by the standard test methods. Oxidation
Stability
Like all organic material, hydrocarbons are susceptible to air oxidation over long periods of time. The peroxides formed from hydrocarbon oxidation are auto-catalytic, leading to "runaway" reaction if not controlled. Reaction rates are generally higher with u n s a t u r a t e d hydrocarbons, and are catalyzed by parts per billion levels of soluble metals such as copper. Motor gasoline may be stored up to six months, or m o r e . They must not form soluble g u m that collects at the point of vaporization, or precipitated insoluble g u m ("varnish"), or form soluble peroxides that will attack rubber/elastomers in the fuel system. Antioxidants are added to most gasolines to slow down the rate of oxidation and prevent the oxidation from becoming autocatalytic, by forming a stable radical oxidation product that does not react further. The antioxidant is sacrificial, and is gradually consumed during this process. Once depleted, the reactions will become autocatalytic, and the gasoline will eventually "break" and form peroxide gums as fast as oxygen
CHAPTER can diffuse into the liquid (mass transfer controlled oxidation). Some gasolines also require metal deactivators to mitigate catalysis from 10-100 p p b traces of copper (most commonly from trace level corrosion of copper and brass alloys in contact with the gasoline during manufacture, distribution, or use). Oxidation stability tests generally expose a heated sample of gasoline to oxygen for a prescribed period of time to accelerate the rate of oxidation and predict on a relative basis how long the fuel will be stable under more tj^ical conditions. In the ASTM D 525 Oxidation Stability test, the fuel sample is heated with oxygen inside a pressure vessel, and the time until significant oxygen uptake (pressure drop, "break point") occurs is a relative measure of stability. The criteria are where the pressure drop exceeds 2 psi per 15 min. The autocatalytic reaction rate increases exponentially, becoming mass transfer controlled, and consuming all of the remaining oxygen in only 15-30 min. ASTM D 4814 requires a m i n i m u m of 240 min (4 h) breakpoint, which is sufficient for most storage and distribution systems. This may require a higher level at the point of manufacture, since the measured oxidation stability will decrease over time as the sacrificial anti-oxidant is consumed. Other procedures not used in D 4814 are to weigh the oxidative g u m formed after a breakpoint test, (commonly termed a "4 hour gum"), or longer term storage time tests at milder conditions to better simulate field storage conditions. It is common practice to add sufficient antioxidant to meet a 360-600 min oxidation stability at the point of production, so that 240 min is obtained at the point of sale. Factory fill gasoline generally requires a much-fortified gasoline for this purpose, as storage times of automobiles prior to Scile can be several months. This is especially true for some specialty fleet applications, such as police fleets, that may be made in large production runs, with the cars stored for six m o n t h s or more before going into service. Hydrocarbons air oxidation has been extensively studied, and is well documented in the literature. Soluble Gum")
Gum
(Unwashed
and Solvent
Washed
"Existent
ASTM D 4814 limits unwashed gum to 5 mg/100 mL, and has no washed (Existent Gum) requirement (used for aviation gasoline). D 381 measures the amount of fuel soluble oxidative gums and nonvolatile additives remaining after evaporation in the air (i.e., total nonvolatile materials in solution). Solvent washed "Existent Gum" measures the a m o u n t of gums remaining in fuel evaporated u n d e r air and t h e n washed with heptane to remove additives and heavier hydrocarbons. Heptane is a poor solvent for highly oxidized, high molecular weight gums. The heptane soluble portion is relatively low molecular weight material that can accumulate at any point of gasoline vaporization (intake system deposits) and react further to form insoluble gums. The heptane insoluble portion is representative of the more fuel-insoluble gum that can contribute to gum/varnish residue problems, for example filter plugging, sticking carburetor float bowl pintle needles where there is no vaporization, as well as intake system deposits where there is vaporization. Washed and unwashed gums measure the amount of gum formed in the fuel u p to the time of the test, as no significant
3: MOTOR
GASOLINE
83
additional gum is formed under the relatively mild conditions of the test procedure. It is not predictive of future quality. It can identify help identify stale "peroxidized" fuels in the marketplace; Washed Existent gum is often reported to verify that a high unwashed gum is due to the presence of Deposit Control Additives that contribute to unwashed gum. A fuel that has both high ( > 1 0 mg/lOOmls) washed and u n w a s h e d g u m are generally considered suspect or unacceptable. Water
Tolerance
D 6422 Water Tolerance (phase separation) method measures the highest temperature at which phase separation of gasoline - alcohol blends occur. It is not applicable to hydrocarbon or ether oxygenated fuels. Water tolerance is particularly critical for gasoline containing alcohol. The separated lower phase alcohol-water-hydrocarbon phase settles to the bottom of the tank where the p u m p suctions are, and tends to get moved down the distribution system and into cars if not found and removed. The remaining gasoline layer is often off-spec after phase separation due to loss of octane from alcohol and aromatics extracted into the alcohol layer. If dispensed into a vehicle tank, it will not burn in engines, and so it causes immediate stalls, and requires a complete fuel system draining and cleaning. Like water, aqueous alcohol separated phases contribute to corrosion and pickup of rust/dirt, etc., and are also associated with accelerated filter plugging. Methanol has a low solubility in gasoline and requires a cosolvent if used in gasoline blending for that reason. Adding methanol to an oxygenated fuel will increase the measured separation temperature while adding ethanol, IPA, or other low MW alcohols will decrease the measured separation temperature. Water tolerance of gasoline with alcohols is very temperature dependent; consequently diverse limits are established for the U.S. The limits vary according to location and month. For Alaska, North of 62 latitude, it changes from — 41°C in Dec-Jan to 9°C in July, but remains 10°C all year in Hawaii, since it never gets that cold. This is the same control strategy as for s u m m e r t i m e high temperature and Tv/1 or VP, but using the daily low t e m p e r a t u r e s versus the daily high temperatures. Other Nonspecification Properties Autoignition
Temperature
The autoignition temperature of a fuel or even a single molecule is not a thermodynamic property. It is the temperature at which high temperature decomposition products will spontaneously c o m b u s t without a n external ignition source. It is heavily dependent on the conditions at the time of the test, including such factors as what type and shape of container is used, and how long to wait to see if autoignition occurs. The conditions can be varied to simulate a specific situation, and it is not intended to reflect engine conditions. There is a general correlation between the octane n u m b e r of a fuel and the auto ignition temperature, because both are determined by molecular structure. Long straight chain hydrocarbons produce large amounts of easily auto-ignitable pre-flame decomposition, and tend to have low auto ignition temperatures and octane numbers. Branchy and aromatic
84
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
hydrocarbons are more resistant, and have higher auto ignition temperatures and octane numbers. However, correlation is not precise enough to be useful, and several lab rig tests based on auto ignition temperatures or related "cool flame" or "pre-flame" properties to predict engine octane numbers have failed to date. Autoignition temperature of gasoline and other fuels can be measured by ASTM E 659, Standard Test Method for Autoignition Temperature of Liquid Chemicals. Heat of Combustion
and Adiabatic
Flame
Temperature
See section on Gasoline Energy Content for a discussion of the gross and net heat of combustion (He). The He of gasoline can be measured by ASTM D 4809, Heat of Combustion of Liquid Hydrocarbon Fuels by Bomb Calorimeter (Precision Method), or estimated by a n u m b e r of ASTM and other correlations. The adiabetic flame temperature is the highest temperature that combustion gases can attain under standard starting conditions u n d e r adiabatic (no heat loss) conditions, where all of the energy of combustion goes toward heating the combustion gases. There are only slight differences in combustion temperatures of blended gasolines, and the actual temperature in the combustion chamber is determined by other factors, such as load, spark timing, and engine design. The exhaust gas temperature is much below the adiabatic flame temperature because m u c h of the energy combustion has been extracted as PVT work. The adiabatic flame temperature can be measured directly as a peak flame t e m p e r a t u r e (no applicable ASTM test m e t h o d ) , or estimated from the carbon/hydrogen ratio, stoichiometric formula and heat of combustion, or calculated from composition with a suitable thermodynamics calculation, or estimated from density correlations (similar to the He). Flammability
(LEL and
HEL)
The Lower and Higher Explosive limits (LEL, HEL) are the m i n i m u m and maximum hydrocarbon vapor concentrations that will support combustion. The LEL is related the flash point of distillate fuels. Below the LEL there is too little hydrocarbon fuel to support combustion, while above the HEL there is too little air (oxygen) to support combustion. The stoichiometric mixture lies between the LEL and HEL, and represents the mixture of complete combustion, maximum adiabatic flame temperature, and maximum energy release from combustion of explosion. Distillate fuels are handled below their LEL (Flash Point). Gasoline and naptha fuels are normally handled above their HEL. Mixtures of gasoline and distillate fuels, or fuels with equilibrium vapor pressures that place the vapor space in the explosive range are extremely dangerous. Any ignition source can cause Ein explosion in the vapor space or in fuel lines and vents. Once ignited, a flame can propagate down any pipe or vent that contains a combustable mixture. Fuels with vapor spaces in the explosive range require specialized equipment and procedures for safe handling (flame arrestors, anti-static provisions, gas inerting, etc.). The higher and lower explosive limits for gasoline are only rarely a concern. The vapor pressure of gasoline keeps it well over the HEL most of the time. It is only in extremely cold weather that a properly formulated winter gasoline can enter
the HEL explosive range. This can be estimated from PengRobinson Equation of State calculations, since the LEL will occur when the vapor phase concentration of butane/pentane hydrocarbons will be in the 1.5-2 vol% range, and the HEL in the 7-8 vol% range. At 14.7 psia barometric pressure, this corresponds to a range of 0.1-0.2 psia and 1.0-1.2 psia hydrocarbon vapor pressure for the LEL and HEL respectively. Using these values, a typical 14.5 VP winter gasoline would be expected to have an LEL ranging from about —75 to - 6 0 ° F and HEL from about —25 to — 15°F, depending upon composition. A t5^ical 7.8 VP s u m m e r gasoline would be expected to have an LEL ranging from about —50F to —35F and an HEL from 5-15°F. Canadian CGSB gasoline standards have a m i n i m u m VP to provide a wide margin between the HEL and the VP at extreme cold temperatures for various times of year. Diluent gases are much more effective at reducing the HEL than increasing the LEL for all flammable hydrocarbon gas mixtures. The flammable region disappears with the addition of about 30% CO2 or 45% N2, and almost all of this change is due to reduction in HEL and not increase in LEL hydrocarbon concentration [38,39]. From this it can be estimated that N2 gas containing less than about 10 vol% oxygen is nonflammable with hydrocarbon vapors, a "rule of t h u m b " that is often useful for developing procedures for safe handling of gasoline in nonstandard conditions. Heat Capacity, Thermal and Heat of Vaporization
Conductivity,
The thermodynamic bulk properties of gasoline are rarely a concern for automotive gasoline, but are occasionally required for the sizing of specialized handling equipment. Gasoline has a heat capacity of about 0.5 BTU/lb.F (0.5 cal/gm.C), which is about half that of water. Hydrocarbons would be considered to be a good insulator in the absence of convective flow heat transfer with a thermal conductivity of about 0.08 BTU/hr.ft.F near room temperature, about 20% that of water (.35 BTU/hr.ft.F). Because they are essentially nonpolar, the heat of vaporizations of gasoline hydrocarbons are only about 10-20% that of water on an enthalpy per mass basis. Light aliphatics, such as butane, are in the low range, while branched isoparafins, etc. are in the high range, with single ring aromatics in mid range. As a result, a fully blended gasoline has a heat of vaporization near a midrange of about 15% of water, or about 140 BTU/lb (80 kcal/kg). More detailed information is available in standard chemistry and engineering handbooks and manuals. There are no standard ASTM tests for these properties, but they are well documented in the thermodynamics literature. Viscosity
and
Lubricity
The viscosity of gasoline is rarely a concern for automotive gasoline. It is considered to be a very low viscosity fluid at typical a m b i e n t t e m p e r a t u r e s with a typical viscosity of about 0.5 cSt at room temperature, which is only slightly lower than water (0.9 cSt) at the same temperature. Viscosity does not change very much with temperature, similar to density, thermal conductivity, etc. Unlike more viscous lubricants and heavier fuels, inherently low viscosity liquids such as gasoline and LPG are not very sensitive to temperature. For example, a 0.7 cSt liquid would only be expected to de-
CHAPTER crease viscosity to about 0.1 cSt, with an increase of 100° centigrade [40]. Lubricity is determined by trace polar molecule content, b u t is generally very p o o r for gasoline. Clean absorbent treated gasoline range hydrocarbons (low polar content) are often used as the "poor" standard in lubricity test calibrations. Pumps for gasoline service must be chosen carefully due to the low lubricity and propensity for accelerated p u m p wear. Centrifical, gerotor, internal gear, and other low contact p u m p designs are preferred over ordinary gear pumps, which are seldom suitable for gasoline service. Similar wear problems can occur in some positive displacement meters. The high vapor pressure of gasoline can result in localized cavitation in some p u m p designs, with the damage easily misdiagnosed as corrosion and/or wear. Engine mounted fuel p u m p s are usually diaphragm pumps, and low pressure in-tank fuel p u m p s are usually turbine (centrifical) p u m p s for these reasons. High pressure automotive in tank fuel p u m p s are usually two stage turbine/positive displacement pumps, with the second stage tjrpically usually being a roller vane or gerotor type for use in a very "dry" (low lubricity) fuel. There are a n u m b e r of standard ASTM viscosity test methods suitable for gasoline, such as D 445, S t a n d a r d Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (the Calculation of Djoiamic Viscosity). Adaptations of several lubricity test m e t h o d s originally developed for distillate fuels can be applied to lighter hydrocarbons, although not covered with the scope (naptha range solvents are used as calibrants). These include D 6078 and D 6079, Scuffing Load Ball on Cylinder and High Frequency Reciprocating Rig Tests. Conductivity
and Static
Electricity
Conductivity is rarely a concern for automotive gasoline, but is occasionally m e a s u r e d if there is a concern for static buildup during p u m p i n g . Retail dispensing hoses/nozzles and vehicle fill spouts Eire internally grounded, so there is generally enough static dissipation to avoid problems. Some static incidents have occurred with very low conductivity gasoline in plastic fuel lines (replaced by more conductive plastic). Recently, ignitions have occurred during cold weather with containers filled while sitting in plastic bed liners (insulators) of pick-up trucks. Ignitions from static discharge from gloves/clothing during refueling are believed to be due to static accumulation on clothing, and not to charge accumulation from gasoline. Conductivity dissipating additives are effective in gasoline, but do not provide protection if the charging rate is excessively high, and hardware changes are required. The same principles of electrostatics apply to gasoline as to other hydrocarbon fuels, except that the vapor space of gasoline tanks are normally well above the HEL at the time and temperature of use [41]. This is rarely a factor for automotive, as there is little opportunity for vapor accumulation a n d ignition. In generzJ, gasoline should only be transferred under conditions where there is free flow of air and good ventilation. Several standard ASTM test methods are available to measure conductivity of gasoline and other fuels. Static cheirging tendency is highly dependent upon equipment configuration. Consult ASTM D 4865, Guide for Generation and Dissipation of Static Electricity in Petroleum Fuel Systems for more information.
Density,
Refractive
Index,
3: MOTOR and Dielectric
GASOLINE
85
Constant
Density, Dielectric Constant, and Refractive Index are all "bulk properties" of hydroccirbons that depend upon the n u m ber of atoms, charges, and electrons per unit volume. When the composition is limited to only hydrogen and carbon in hydrocarbon fuels, there are strong scientifically based relationships between these properties, and a strong correlation between them for all fuels. Dielectric constant is rarely a concern for gasoline, but is occasionally needed for some capacitance gauging systems. For reasons of composition, the dielectric constant of hydrocarbon fuels (not oxygenates) can be estimated as the square of the refractive index at the required temperature. The dielectric constant is a "bulk property" of the fluid, similar to refractive index and density. All can be estimated with reasonable accuracy for oxygenates blends by volumetrically averaging the oxygenate and the hydrocarbon component dielectric constants, refractive index, or density. Both refractive index and dielectric constant can be used to estimate the density of a blend, which in turn is strongly correlated to Eiromatics content for typiccil blends. Both dielectric constant and refractive index have been used as control parameters for racing fuels for these reasons. Some commercially available field instruments use these principles to estimate oxygenate contents from optical measurements. Gasoline vapor (predominately butane/pentane) is heavier than air and can accumulate in low spots and cavities in the absence of a ventilating flow of air (see Chapter 2, Liquified Petroleum Gas). Several s t a n d a r d ASTM test m e t h o d s are applicable to liquid a n d gas density, refractive index, a n d dielectric constant.
ASTM/IP METHODS ASTM No. D 4806
IP No.
D 4814 D86
123
D 130
154
D 323 D 3 81
131
D 525
40
D665
135
D 1266
107
Title Specification for Denatured Fuel Ethcinol for Blending with Gasolines for Use as Automotive Spark-Ignition Engine Fuel Specification for Automotive SparkIgnition Engine Fuel Test Method for Distillation of Petroleum Products Test Method for Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test Test Method for Vapor Pressure of Petroleum Products (Reid Method) Test Method for Existent Gum in Fuels by Jet Evaporation Test Method for Oxidation Stability of Gasoline (Induction Period Method) Test Method for Rust-Preventing Characteristics of Inhibited Mineral Oil in the Presence of Water Test Method for Sulfur in Petroleum Products (Lamp Method)
86 MANUAL 37: FUELS AND LUBRICANTS D 1298
160
D 1319
156
D2276
216
D2533 D2622 D2699
237
D2700
236
D2709
D2885
D3116 D3120
D3227
342
D3231 D3237 D3341 D3348
D3606
D3710
D3831
D4052
D4045
365
HANDBOOK
Test Method for Density, Relative Density (Specific Gravity), or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method Test Method for Hydrocarbon Types in Liquid Petroleum Products by Fluorescent Indicator Adsorption Test Method for Particulate Contaminant in Aviation Fuel Test Method for Vapor-Liquid Ratio of Spark-Ignition Engine Fuels Test Method for Sulfur in Petroleum Products by X-Ray Spectrometry Test Method for Knock Characteristics of Motor Fuels by the Research Method Test Method for Knock Characteristics of Motor and Aviation Fuels by the Motor Method Test Method for Water and Sediment in Distillate Fuels by Centrifuge Test Method for Research and Motor Method Octane Ratings Using OnLine Analyzers Test Method for Trace Amounts of Lead in Gasoline Test Method for Trace Quantities of sulfur in Light Liquid Petroleum Hydrocarbons by Oxidative Microcoulometry Test Method for Mercaptan Sulfur in Gasoline, Kerosine, Aviation Turbine, and Distillate Fuels (Potentiometric Method) Test Method for Phosphorus in Gasoline Test Method for Lead in Gasoline by Atomic Absorption Spectroscopy Test Method for Lead in GasolineIodine Monochloride Method Test Method for Rapid Field Test for Trace Lead in Unleaded Gasoline (Colorimetric Method) Test Method for Benzene and Toluene in Finished Motor and Aviation Gasoline by Gas Chromatography Test Method for Boiling Range Distribution of Gasoline and Gasoline Fractions by Gas Chromatography Test Method for Manganese in Gasoline by Atomic Absorption Spectroscopy Test Method for Density and Relative Density of Liquids by Digital Density Meter Test Method for Sulfur in Petroleum Products by Hydrogenolysis and Rateometric Colorimetry.
D 4053 D 4057 D 4294
336
D 4420 D 4815 D 4953 D 5059
228
D 5188 D 5190 D 5191 D 5453 D 5482 D 5580
D 5599
408
D5845
D 6422
Test Method for Benzene in Motor and Aviation Gasoline by Infrared Spectroscopy Practice for Manual Sampling of Petroleum and Petroleum Products Test Method for Sulfur in Petroleum Products by Energy Dispersive X-ray Fluorescence Spectroscopy Test Method for Aromatics in Finished Gasoline By Gas Chromatography Test Method for Determination of C, to C, Alcohols and MTBE in Gasoline by Gas Chromatography Test Method for Vapor Pressure of Gasoline and Gasoline-Oxygenate Blends (Dry Method) Test Methods for Lead in Gasoline by X-Ray Spectroscopy Test Method for Vapor-Liquid Ratio Temperature Determination of Fuels (Evacuated Chamber Method) Test Method for Vapor Pressure of Petroleum Products (Automatic Method) Test Method for Vapor Pressure of Petroleum Products (Mini Method) Test Method for Total Sulfur in Light Hydrocarbons, Motor Fuels, and Oils by Ultraviolet Fluorescence Test Method for Vapor Pressure of Petroleum Products (Mini Method Atmospheric) Test Method for Benzene, Toluene, p,/m xylene, o-xylene, C9 and Heavier Aromatics in Finished Gasoline by Gas Chromatography Test Method for Oxygenates in Gasoline by Gas Chromatography and Oxygen Selective Flame Ionization Detection Test Method for MTBE, ETBE, TAME, DIPE, Methanol, Ethanol, and tert - Butanol in Gasoline by Infrared Spectroscopy Water Tolerance (Phase Separation) of Gasoline - Alcohol Blends
BIBLIOGRAPHY [1] Owen, K. and Coley, T., Automotive Fuels Reference Book, 2nd ed.. Society of Automotive Engineers, Warrendale, PA, 1995. [2] Gibbs, L. M., "Transportation Fuels—Automotive Gasoline," Encyclopedia of Energy Technology and the Environment, John Wiley and Sons, NY, 1995, pp. 2675-2698. [3] Poulton, M. L., Alternative Fuels for Road Vehicles, Computational Mechanics Publications, Wit Press, Southampton, UK, 1994. [4] Goodger, E. M., Hydrocarbon Fuels, Macmillan, London, 1975. [5] Goodger, E. M., Alternative Fuels, MacmillEin, London, 1980. [6] "Alcohol Fuels," "Gasoline and Other Motor Fuels," "Hydrogen
CHAPTER 3: MOTOR GASOLINE Energy," and "Fuel Cells," Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed., M. Howe-Grant, Ed., Wiley, NY, 1993. [7] The Automotive Handbook, any edition, Bosch, Society of Automotive Engineers, Warrendale, PA. [8] Automotive Electrical/Electronic Systems, Society of Automotive Engineers, Warrendale, PA. [9] Heywood, J. B., Internal Combustion Engine Fundamentals, 1st ed., McGraw-Hill, NY, 1988. [11] Heisler, H., Advanced Engine Technology, Edward Arnold, London, 1995. [12] Poulton, M. L., Alternative Engines for Road Vehicles, Computational Mechanics Publications, Wit Publishers, Southampton, UK, 1994. [13] SAE J312 and J1297, SAE Handbook, Vol. 1, Society of Automotive Engineers, Warrendale, PA, 1994. [14] Proceedings of the International Symposium on Alcohol Fuels, Held every two years and most of the ten conferences have good technical information, especially the earlier ones, various publishers. [15] Alternative Transportation Fuels: An Environmental and Energy Solution, D. Sperling, Ed., Quorum Books, Wetport, CT, 1989. [16] Hunt, V. D., The Gasohol Handbook, Industrial Press, NY, 1981. [17] Part 4, "Detonation and Combustion," The Science of Petroleum, Oxford University Press, 1938. [18] Hobson, G. D., Modem Petroleum Technology, any edition, Wiley, NY. [19] DS 4B, Physical Constants of Hydrocarbon and Non-Hydrocarbon Compounds, ASTM International, West Conshohocken, PA, 1991. [20] Technical Data Book for Petroleum Refiners, American Petroleum Institute, Washington, D.C. [21] Engineering Data Book, Gas Processors Suppliers Association, Tulsa, OK. [22] Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL. [23] Chemical Engineers Handbook, McGraw Hill, NY. [24] Chemistry Webbook, National Institute of Standards and Technology (NIST), Gaithersburg, MD, www.webbook.nist.gov. [25] An Index of Selected Thermodynamic Data Handbooks, Thermodex, http://thermodex.lib.utexas.edu. [26] "Chemiccd and Physical Properties," LSU Libraries, Louisiana State University, Baton Rouge, LA, http://www.lib.lsu.edu/sci/ chem/properties .html. [27] TRC Thermodynamic Tables—Hydrocarbons, Thermodynamics Research Center, Texas A&M University, College StationTexas, NIST, Boulder, CO.
REFERENCES [1] Owen, K. and Coley, T., Automotive Fuels Reference Book, First ed.. Society of Automotive Engineers, Inc., Warrendale, PA, 1990. [2] Gibbs, L. M., "How Gasoline Has Changed," SAE Report No. 932828, Society of Automotive Engineers, Warrendale, 1993. [3] Purdy, G. A., Petroleum, Prehistoric to Petrochemicals,Copp Clark Publishing Co., Toronto, 1958. [4] Kaufman, K. B., "Midgley: Saint or Serpent?," Chemtech, December 1989, pp. 717-725. [5] Midgley, T. and Boyd, T. A., Industrial and Engineering Chemistry Research, Vol. 14, 1922, pp. 589, 849, 894. [6] Midgley, T., "Problem + Research -I- Capital = Progress," Industrial and Engineering Chemistry Research, Vol. 31, 1939, pp. 504-506. [7] Dying for Work: Workers' Safety and Health in 20th Century America, D. Rosner and G. Markowitz, Eds., Indiana University Press, Bloomington, IN, 1987. [8] Goodger, E. M., Petroleum and Performance, Butterworths Scientific Publications, London, 1953, p. 166
87
[9] Campbell, J. M. and Boyd, T. A., "Methods of Knock Rating. 15. Measurement of the Knocking Characteristics of Automotive Fuels," The Science of Petroleum. Oxford University Press, Oxford, Vol. 4, 1938, pp. 3057-3065. [10] Gibbs, L. M., "How Gasoline Has Changed," SAE Report #932828, Society of Automotive Engineers, Warrendale, PA, 1993. [11] Kaiser, E. W., Siegl, W. O., Cotton, D. F., and Anderson, R. W., "Effect of Fuel Structure on Emissions from a Spark-Ignited Engine. 2. Naphthene and Aromatic Fuels," Environmental Science and Technology., Vol. 26, 1992, pp.1581-1586. [12] Berstein, M., "Thomas Midgley and the Law of Unintended Consequence," Invention and Technology, Vol. 17, No. 4, Spring 2002. [13] Gibbs, L. M, "Transportation Fuels—Automotive Gasoline," Encyclopedia of Energy Technology and the Environment, John Wiley and Sons, NY, 1995, pp. 2675-2698. [14] Gibbs, L. M., et al, Alcohols and Ethers A Technical Assessment of Their Application as Fuels and Fuel Components, API Publication #4261, American Petroleum Institute, Washington D.C. [15] "Research Report on Reformulated Spark-Ignition Engine Fuel," ASTM RR: D02-1347, ASTM International, West Conshohocken, PA (updated regularly). [16] "Federal Reformulated Gasoline," Chevron Technical Bulletin FTB 4, Chevron, San Ramon, CA, 1994. [17] Gibbs, L. M, "Transportation Fuels—^Automotive Gasoline," Encyclopedia of Energy Technology and the Environment, John Wiley and Sons, NY, 1995, pp. 2675-2698. [18] "Motor Gasolines Technical Review (FTR-1)," Chevron Products Company, San Ramon, CA, 1996. [19] "Initial Mass Exhaust Emissions from Reformulated Gasolines," Technical Bulletin No. 1, Auto/Oil Air Quality Improvement Research Program, Coordinating Research Council, Inc., Atlanta, GA, December 1990, pp. 5-7. [20] "Mass Exhaust Emissions Results from Reformulated Gasolines," Technical Bulletin No. 4, Auto/Oil Air Quality Improvement Research Program, Coordinating Research Council, Atlanta, GA, May 1991 [21] "Exhaust Emissions of Toxic Air Pollutants Using RFGs," Technical Bulletin No. 5, Auto/Oil Air Quality Improvement Research Program, Coordinating Research Council, June 1991. [22] Kettering, C. F., "The Effect of the Molecular Structure of Fuels on the Power and Efficiency of Internal Combustion Engines," Industrial Engineering and Chemistry Research, Vol. 36, 1944, pp. 1079-1085. [23] Composition of Canadian Unleaded Gasoline, available from Canadian Petroleum Products Institute, Ottawa Ontario, Canada, 1994. [24] Reisch, M., "EPA Told Not To Ban Ethyl's Fuel Additive," Chemical & Engineering NewsM, 24 April 1995, p. 8. [25] "Gasoline and Other Motor Fuels," Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.. Vol. 12, M. Howe-Grant, Ed., Wiley, NY, 1993. [26] Kozole, K. H., "1996 In-Use Vehicle Emission Survey Program," CPPI Report No. E12-B005248-Final, Canadian Petroleum Products Institute, Ottawa, 1996, Available: http://www.cppi.ca. [27] "Response to the Sasol report of 23 May 01 and the Ethyl report of July 2001," presented at the IPIECA, September 2001, Delta Motor Corporation, Shannon, Ireland, Aug. 24, 2001. [28] "MMT Global Update to IPIECA," presented at the IPIECA, Shannon, Ireland, September 2001. [29] Calingaert, G., "Section 11, Anti-knock Compounds," The Science of Petroleum, Vol. 4, Oxford University Press, Oxford, 1938, pp. 3024-3029. [30] SAE J 1829: Stoichiometric Air-Fuel Ratios o f Automotive Fuels—Recommended Practice, SAE Handbook, Vol. 1, Society of Automotive Engineers, 1994.
88 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK [31] Westbrook, C. K. a n d Pitz, W. J., "The Chemical BUnetics of Engine Knock," Energy and Technology Review, Feb/Mar 1991, pp. 1-13. [32] Westbrook, C. K. and Pitz, W. J., "The Chemical Kinetics of Engine Knock," Energy and Technology Review, Feb/Mar 1991, pp. 562-566. [33] Westbrook, C. K. and Pitz, W, J., "The Chemical Kinetics of Engine Knock," Energy and Technology Review, Feb/Mar 1991, pp. 1-13. [34] Westbrook, C. K., "The Chemistry Behind Engine Knock," Chemistry & Industry (UK), 3 Aug 1992, pp. 562-566. [35] Dyroff, G., Manual on Significance of Tests for Petroleum Products, 5th ed., MNL-1, ASTM International, West Conshohocken, PA, 1993. [36] Nadkami, K., Guide to ASTM Test Methods for the Analysis of Petroleum Products and Lubricants, MNL-44, ASTM International, West Conshohocken, PA, 2000. [37] Schuetzle, D., Siegl, W. O., Jensen, T. E., Dearth, M. A., Kaiser, E. W., Gorse, R., et al., "The Relationship between Gasoline
[38]
[39]
[40] [41]
Composition a n d Vehicle Hydrocarbon Emissions: A Review of Current Studies and Future Research Needs," Environmental Health Perspectives Supplements, Vol. 102, No. 4, 1994, pp. 3-12. Coward, H. F. and Jones, G. W., "Limits of FlammabiHty of Gases and Vapors," U.S. Bureau of Mines Bulletin 503, U.S. Government Printing Office, Washington, D.C., 1952. MoUer, W. O., Molname, M., and Sturm, R., "Limiting Oxygen Concentration: Recent Results and their Presentation in Chemsafe," Presented at the 9th International Symposium on Loss Prevention and Safety Promotion in the Process Industries, May 1998, Barcelona, Available: http://www.ptb.de/de/org/3/33/331/ moeIler.htm. Perry, R. H. and Chilton, C. H., Chemical Engineer's Handbook, Fifth ed., pp. 3-346. Bustin, W. M. and Dukek, W. G., "Electrostatic Hazards in the Petroleum Industry," Electronic and Electrical Engineering Research Studies, Research Studies Press Ltd., UK, John Wiley and Sons, NA, 1983.
MNL37-EB/Jun. 2003
Aviation Fuels Kurt H. Strauss^
publication of a specification for a n 82 octane unleaded grade, intended for the certification of new engines having low octane requirements. Table 1 lists the pertinent aviation gasoline properties in D 910, while Table 2 contains the requirements for D 6227, Grade 82 UL.
HISTORY Aviation Gasoline EARLY AIRCRAFT ENGINES WERE OPERATED o n ordinary straight-
r u n motor gasoline well into the 1920s [ 1 ]. Research then isolated uncontrolled combustion as a major source of engine overheating and failures, prompting a search for ways to cure the problem. The big step came in 1921 with the invention of tetraethyl lead (TEL), a n unequaled knock resistance enhancer. The same period saw the development of the heptane-isooctane scale still in use today for rating antiknock properties in terms of octane numbers. For aviation gasoline, the concept of rating knock resistance in special single cylinder engines resulted in the Aviation Octane Test Method (D 614), which tested fuels under lean fuel mixture conditions simulating cruise operation. (In 1970 this method was replaced in the specification by the Motor Octane Method, D 2700.) An aviation gasoline specification, issued by the U.S. Air Corps in 1938, listed a 68 octane grade containing n o lead and a 92 octane grade with a m a x i m u m of 6 mL TEL/gallon. As engine power output was increased by supercharging the fuel/air mixture, a second rating method, D 909, came into use to evaluate performance u n d e r rich take-off conditions, so that by World War II both rating methods were required. Engine designers soon discovered the performance benefits of high octane, which permitted a higher octane fuel t o develop more power in a given engine or allowed a reduction in engine size with the same power output. Research on high octane fuel thus received a high priority and resulted in a 100 octane fuel by the beginning of World War II. While t h e heroic performance of the RAF is widely recognized, it would have been impossible without the 100 octane fuel.
Jet Fuel Because of different combustion requirements, jet fuel started as a completely different portion of the petroleum barrel [2]. Early British engines used domestic kerosine, a fuel that has not changed drastically over the years. German engines, also military, were operated on a mixture of kerosine and naphtha, probably to extend fuel availability. The progression of U.S. military fuels is illustrated in Table 3. Both JP-3 and JP-4, which are blends of kerosine and naphtha, reflected Air Force concern over fuel availability, while JP-5 is tailored t o Navy r e q u i r e m e n t s for carrier combat safety. JP-7 is a supersonic fuel in very limited use. Today, the Air Force operates almost entirely on JP-8; in fact, this grade has become the primary battlefield fuel to be used in gas turbine and diesel powered ground vehicles as well as combat aircraft. For civil use, ASTM issued specification D 1655 in 1959, containing three grades. Jet A and A-1 were kerosine-type fuels differing only in freezing point, while Jet B was the civil version of JP-4. Currently consideration is being given to removing Jet B from D 1655 and placing it into a separate specification. The key properties of Jet A and A-1 are illustrated in Table 4. Corresponding specifications are issued by the UK Ministry of Defence as defence standards (Def Stans), with Def Stan 91 -91 [3] being the technical equivalent of ASTM Jet A-1. Jet A meeting D 1655 is the civil aviation fuel in the U.S.; Jet A-1 meeting the combined requirements of D 1655 and Def StcUi 91-91 is the civil fuel elsewhere. There is no British grade corresponding to Jet A. The latest effort at a true international jet fuel standard is a series of guidelines [4] issued in 1999 by the IntemationEd Air Transport Association (lATA), which includes Jet, A, Jet A-1, the Russian TS-1, and a Jet B grade.
The dramatic j u m p in aviation activity in the late '30s and '40s witnessed a n increasing n u m b e r of aviation gasoline grades, ultimately standardizing on five grades ranging from 80/87 to 115/145. ASTM recognized this situation by issuing D 618 as a civil aviation gasoline specification in 1944 to be replaced in 1947 by D 910. However, jet engine introduction forced a steady decrease in gasoline volume and reduced the n u m b e r of grades over time so that by the '70s only two grades, a low lead content 80 and a high lead 100 grade, remained in commercial production. The 100 Low Lead grade (lOOLL) was introduced late in that period, permitting a single fuel t o satisfy the requirements of both low and high octane engines. The latest change in this picture was the 1997
AIRCRAFT A N D ENGINE DESCRIPTIONS Aviation Gasoline Although a n u m b e r of military and some commercial aircraft engines were liquid cooled, all engines produced after World War II have been air cooled. These engines are generally of
' 69 Brookside Rd., Portland, ME 04103. 89 Copyright'
2003 by A S I M International
www.astm.org
90 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 1—Key specification requirements for aviation gasoline." Knock Value, lean mixture Motor method Octane number, min tCnock value, rich mixture Superchange rating Octane number, m i n Performance number, min Tetraethyl lead, mL TEL/L max gPb/L max Dye content Blue dye, mg/L max Yellow dye, mg/L max Red dye, mg/L max
Grade 80
Grade lOOLL
Grade 100
80.0
99.5
99.5
ASTM Test MeAod D2700 D909
87.0 0.13 0.14 0.2 none 2.3
130.0 0.53 0.56
130.0 1.06 1,12
2.7 none none
2.7 2.8 none
D 3341 or D 5159
Requirements for All Grades Density at 15°C, kg/m^ Distillation Initial boiling point, ' Fuel evaporated 10%vat°C 40%v at °C 50%v at °C 90%v at °C Final boiling point, °C Sum of 10% + 50% evap. temperatures. "C Recovery volume, % Residue volume, % Loss volume, % Vapor pressure, 38°C, kPa Freezing point, °C Sulfur, %m Net specific energy, MJ/kg Corrosion, copper strip, 2 h @ 100°C Oxidation stability (5 h aging) Potential gum, mg/100 mL Lead precipitate, mg/lOOmL Water reaction, vol change, mL Electrical conductivity, pS/m
Report
D 1298 or D 4052 D86
Report max min max max max min min max max min max max max min max
75 75 105 135 170 135 97 1.5 1.5 38.0 49.0 -58 0.05 43.5 No. 1
max max max max
6 3
D 323 or D 5190 or D5191 D2396 D 1266 or D 2622 D 4529 or D 3338 D323 D873
D 1094 D2624
±2 450
''For complete specification requirements, refer to D 910.
TABLE 2 --Key requirements for grade 82 UL aviation gasoline." Property
Knock value, lean mixture Motor method octane number Color Dye content Blue dye, mg/L Red dye, mg/L Distillation temperature, °C at % (svaporated 10%v 50%v 90%v End point Residue, %v Recovery, %v Loss, %v Net speciflc energy, MJ/kg Freezing point, °C Vapor pressure, kPa Lead content, g/L Cu strip corrosion, 3 h @ 50°C Sulfur, %m Potential gum (5 h aging) mg/L Alcohols and ether content Combined methanol and etheinol, %ni Combined aliphatic ethers, methanol and ethanol as %m oxygen "Refer to D 6227 for complete specification requirements.
Requirement
ASTM Test Method
D2700 min
82.0 purple
max max
7.5 1.9
max
70 66-121 190 225 2 95 3.0 40.8 -58 62 38 0.013 No. 1 0.07
D2392 D86 max max max min max min max max min max max max
D 3338, D4529 or D 4809 D2386 D 4953, D 5190, D 5191, or D 5482 D 3237 or D 5059 D130 D 1266, D 2622, D 3120 D 4294 or D 5453 D873 D4815, D 5999, or D 5845
0.3 2.7
CHAPTER 4: AVIATION FUELS 91 TABLE 3—U.S. military jet fuel chronology. Date of Issue
Grade
Specification
1944
JP-1°
1946 1947
JP-l" JP-S"
1951
JP-4
1952
JP-5
1956
JP-6''
AN-F-32, changed to MIL-F-5616 not issued AN-F-58, changed to MIL-F-5624 MIL-F-5624, now MIL-PRF-5624 MIL-F-5624, now MIL-PRF-5624 MIL-F-25656
1980
JP-7
1979
JP-8
MIL-T-38219, now MIL-PRF-38219 MIL-T-83133, now MIL-DTL-83133
NATO Symbol
the intermittent combustion, four cycle variety with a wet crankcase oil system. In radial engines the cylinders are arranged in a circle with 7 o r 9 cylinders in a row and a n engine having one or m o r e rows. Alternately, in flat engines the cylinders lie horizontally and oppose each other. Most of today's production engines are the flat or pancake type with 4 to 8 cylinders. Some engines contain a supercharger to compress the air and thereby increase the mass flow through the engine. The engine drives a propeller directly from the crankshaft or through a set of speed reduction gears. Fuel is carried either in rubber bladders or removable metal tanks, often in the wings, and flows to cEirbureted engines by gravity or is pumped to a fuel-injected engine by a tank-mounted submerged boost p u m p . A low pressure engine p u m p then supplies the fuel into the carburetor to be mixed with air or injects it into the intake manifold under low pressure. In either case the fuel has to be evaporated by the time it reaches the cylinder because only vapors will b u m . Each cylinder contains two spark plugs for safe operation. Water or a water-alcohol mixture may be injected both to help cool the en-
Volatility and Freeze Point
l l O ^ F m i n flash —60°C max freeze 2 psi max RVP 5-7 psi RVP F-40
2-3 psi RVP
F-44
140°F min flash - 4 6 ° C max freeze High thermal stab. - 5 4 ° C max freeze 140°F min flash - 4 3 ° C max freeze 100°F min flash - 4 7 ° C max freeze
F-34
"Canceled.
TABLE 4—Key specification requirements for jet fuel grades Jet A, A-1, and B.' Property
Composition Acidity, total, mg KOH/g Aromatics, %v Sulfur, mercaptan, %m Sulfur, total, %m Volatilitv Distillation temp., °C 10% recovered, temp. 20% recovered, temp. 50% recovered, temp. 90% recovered, temp Final boil, point, temp Distill, residue, %v Distill, loss, %v Flash point, "C Density at 15°C, k g W Vapor pressure, 38°C, kPa Fluiditv Freezing point, °C Viscosity, -20°C, mm^/s Combustion Net specific energy, MJ/kg One of following shall be met: (1) Smoke point, m m (2) Smoke point, m m and Naphthalenes, %v Corrosion Copper strip, 2 h @ 100°C Stability Thermal, 2.5 h at 260°C Filter press, drop, m m Hg Tube deposit less thsm Contaminants Existent gum, mg/100 mL Water reaction Interface rating Additives Electrical conductivity, pS/m If Additive is used If required by purchaser
max max max max
Jet A or Jet A-1
JetB
0.1 25 0.003 0.3
25 0.003 0.3
ASTM Test Method
D3242 D1319 D3227 D 1266, D 1552, D 2622 D 4294 or D 5453 D86
max max max max max max max min
205 report report 300 1.5 1.5 38 775-840
meix max max
- 4 0 Jet A - 4 7 Jet A-1 8.0
145 190 245 1.5 1.5 751-802 21 -50
D 56 or D 3828 D 1298 or D 4052 D 323 or D 5191 D 2386, D 4305, D 5901 or D 5972 D445
min
42.8
42.8
D 4529, D 3338 or D 4809
min min max
25 18 3.0
25 18 3.0
D1322 D1322 D 1840
max
No. 1
No. 1
D 130
max
25 Code 3
25 Code 3
D3241
max max
7 lb
7
D381 D 1094
lb D2624
max min
"For complete specification requirements refer to D 1655.
450 50
450 50
92 MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
gine and to increase power output. In addition to these traditional engines, other designs, particularly for ultralight aircraft, are being introduced but are not described in detail in this chapter. Exhaust gases are discharged through mufflers, turbochargers or directly into the atmosphere. Compared to today's automobile engines, all these aircraft engines are relatively simple and unsophisticated. With rare exceptions, aircraft piston engines are operated with separate throttle and manual fuel/air mixture controls, whether carbureted or fuel injected. On high performance aircraft the pilot also controls the engine speed by means of separate manual propeller controls. During takeoff and climb the fuel/air mixture is maintained "rich;" the purpose of the excess fuel is to help cool the engine at high power conditions. The fuel/air mixture is "leaned out" during cruise for optimum fuel consumption. "Rich" in this context refers to a mixture in which the fuel/air ratio is well above the stoichimetric ratio, while lean operation is run as close as possible to stoichimetric without overheating the cylinders. Jet Fuel Jet engines operate on a completely different principle. Air flows through a compressor, is guided into a combustion section where fuel is injected and burned and the resulting exhaust gases are expanded in the turbine section, and then expelled into the atmosphere. The turbine section drives the compressor, while the remaining energy in the exhaust stream either reacts on the engine parts to push the engine by "jet propulsion" or is used to drive a propeller in "turboprop" fashion. For more efficient operation, a large single stage
compressor or "turbofan" may be mounted in front of the engine. This fan is driven by its own turbine section by a shaft running through the core engine. Military engines may add extra thrust by injecting fuel into the exhaust stream and burning it in an afterburner at the expense of greatly increased fuel consumption. Unlike reciprocating internal combustion engines, these engines operate with continuous combustion where an igniter serves only to start combustion and is then turned off. Gas turbine engines operate with lean fuel/air ratios well below stoichimetric (with considerable excess air) to provide turbine inlet gas temperatures below the normal combustion temperatures. The fuel system of a modem transport aircraft is illustrated in Fig. 1. The fuel is carried mostly in the wing structure where metal junctions are sealed with an elastomer and the entire inside surface is coated with epoxy paint to prevent corrosion. Elastomeric bags are generally not used for fuel storage, mostly to save weight. From the tankage, fuel is pumped by boost pumps to the engine high pressure pump. The engine pump normally builds fuel pressure in two stages with the first stage a centrifugal pump and the second stage a gear pump capable of delivering fuel at pressures as high as 10 MBCa (1400 psi.). The fuel control, the heart and brains of the fuel system, then meters the proper amount of fuel to the combustion system with the excess fuel being bypassed back to the second stage pump inlet. Fuel enters the combustion system through a fuel manifold, which distributes fuel around the engine and into fuel nozzles in the combustion chamber. Note that the fuel passes through a series of heat exchangers, which reject heat from the hydraulic system, from the engine oil system, and from the auxiliary power sys-
Fueltank Fuel quantity gage
Nozzles
uiuaiiuu umiij lanifold / ssembly /
A/Cheat exchanger Fuel bypass
iLPress Stage\HPress Stage|
Engine oil heat exchanger tkiti
Fuel Control
FilterX Engine Pump Assembly FIG. 1—Turbofan fuel system.
Integrated gen. drive oil heat exch.
I
CHAPTER 4: AVIATION FUELS
93
Horizontal Stabilizer I Tank (optional) Surge Tank
Reserve Tank No. 3 Main Tank No. 3
Main Tank No. 4
Reserve Tank No.2
\y<
>y
\
DryBay
Center Wing Tank Surge Tank\ Main Tank N o . 2 /
/ MainTankNo.l f
Tank
Fuel Tank Volumes Liters
Main Tanks No. 2 and 3 Main Tanks No. 1 and 4 Center Wing Tank Reserve Tanks No. 2 and 3 Horizontal Stabilizer Tank Total Usable Volume
Gallons
47 675 17 030 65 220 5020 12 540
12 594 4499 17 229 1326 3313
217 211
57 381
FIG. 2—Boeing 747-400 fuel tankage.
tern. (In some systems the heat from the hydraulic system is rejected in the fuel tanks.) Additional heat is added to the fuel by energy losses in the pump and by fuel recirculation through the pump. More heat goes into the fuel from the compressor discharge air, which the fuel passes through to get into the combustion system. This heat path highlights another important function of the fuel, to serve as the heat sink that carries away excess heat from other systems and allows other systems to operate. Figure 2 illustrates the tankage arrangement of a Boeing 747-400 aircraft. At takeoff, each engine is supplied fuel from its own tank. After takeoff, fuel is taken selectively from the center wing tank first to control wing stresses. For a typical flight each engine is started by rotating the compressor spool to start air flowing. When airflow is sufficient to start combustion, fuel is injected and the fuel/air mixture is ignited. The engine then accelerates under its own power to idle operating speed and above. Maximum power at take-off is limited by either throttle demand or turbine inlet temperature limits. Take-off power is reduced as the aircraft climbs and is further reduced at cruise altitude to minimize fuel consumption. At the end of cruise, power is further decreased to near idle for descent. Because heat rejection is almost constant but fuel flow is near minimum, the descent portion of the
flight creates the highest fuel temperatures. In this entire flight the pilot controls all engine power settings with a single throttle lever, while the engine fuel control adjusts, for all the variables.
PERFORMANCE REQUIREMENTS AND THEIR IMPLEMENTATION A review of aviation gasoline and jet fuel specifications will show that all are based on Vcirious performance tests rather than closely defined product compositions. In turn, most of the performance tests are empirical and do not define a property on an absolute basis; indeed many were developed to solve specific operating problems. The following sections outline vetrious challenges faced by aviation fuels and their solutions. Low Temperature—Fuel Related Jet Fuel Aviation fuels have to operate in an extremely wide temperature environment. In particular, jet fuel is exposed to very low temperatures due to operating conditions and fuel system de-
94
MANUAL
3 7: FUELS
AND LUBRICANTS
HANDBOOK
sign. This leads to two differing problems, one being normal operation at cJtitude, the other involving steirting at very low temperatures. In the wing tanks the fuel is in direct contact with the lower wing skin which in turn is exposed to ambient t e m p e r a t u r e s as extreme as — 100°C (—148°F). Although aerodynamic heating at cnaise will wEirm the bottom wing surface some 20-25°C or 36-45°F, the resultant fuel temperatures at the wing skin Ccin still be very low. At these low temperatures certain types of hydrocarbons in jet fuel come out of solution. The most critical molecules are straight chain paraffins [5] because they have the highest crystallization temperatures of the hydrocarbons normally in jet fuel. When these compounds come out of solution, they form a wax crystal matrix that can prevent the liquid fuel within the matrix from flowing into a boost p u m p inlet [6]. Only 8-10% of normal paraffins in the fuel are required to form such a matrix. For safe operation the fuel freezing point is defined as the temperature at which the fuel is 100% liquid. This temperature is determined by D 2386 in which a sample is cooled in an isopropyl-dry ice bath until a crystal haze appears. The sample is then weirmed until the last crystal disappears. That melting temperature is reported as the "freezing point". Using the melting point avoids possible super cooling of the sample during the cooling cycle. At this writing, three other tests are permitted as alternates. They include D 5901 (Freezing Point of Aviation Fuels—^Automated Optical Method), D 5972 (Freezing Point of Aviation Fuels—Automatic Phase Transition Method) and D 4305 (Filter Flow of Aviation Fuels at Low Temperatures). However the last method cannot be used when fuel viscosity exceeds 5.5 cs at —20°C (—4°F). In case of dispute, D 2386 is the referee method.
-50
-30
-10
10
In-flight aircraft operation is not permitted below a fixed fuel temperature differential above the specification maxim u m freezing point. This limit is called out in the flight manual of each aircraft. For Boeing aircraft the differential is 3°C or 5°F, and for Airbus aircraft it is 4°C or 7°F. Operation at lower fuel t e m p e r a t u r e s is avoided by aircraft routing through w a r m e r air, by using a fuel of lower freezing point, or in flight by increasing aircraft speed or decreasing altitude when this condition appears imminent [7]. Limited U.S. Air Force research also shows possible promise for additives that lower fuel freezing points [8]. However, the practicality of such additives in unproven. Viscosity is the limiting property for jet engine starting because of its influence on fuel droplet size during atomization. Viscosity is measured by D 445 and typical fuel viscosities are plotted against temperature in Fig. 3 [9]. Engines are designed to start with fuel viscosities u p to a m a x i m u m of 12 cs [10], a viscosity reached around - 4 0 ° C ( - 4 0 ° F ) for either Jet A or A-1. However, specifications allow a m a x i m u m viscosity of 8 cs at —20°C (—4°F), which is taken as equivalent to 15 cs at —40°C. Starting problems are therefore occurring at far N o r t h e r n latitudes, particularly in helicopters, and are avoided by the use of Jet B or JP-4 fuel, which has lower viscosity and increased volatility. Aviation
Gasoline
Aviation gasoline low temperature performance tends to center on engine starting. While fuel volatility is the key to fuel evaporation a n d engine starting, the tight volatility limits allow little flexibility and force solutions other than fuel selection. Starting difficulties at low temperatures are com-
30
50
70
Ten^rature, °C FIG. 3—Typical viscosities vs. temperature.
90
110
CHAPTER 4: AVIATION FUELS 0.10
-
0.06
h
95
0.04
0.02
Jet A, Jet A-1. JP-8 Aviation Gasoline
:§ •o "2
II
0.007 0.006
20
40
60
80
100
Ten^rature, °C FIG. 4—Water solubility vs. temperature.
pounded by increased lubricating oil viscosities and decreased battery performance. High altitude operation does not normally present a low temperature problem because gasoline has a very low freezing point and most piston engine aircraft do not fly at extremely high eJtitudes. Low Temperature—Water Related All hydrocarbons can dissolve a very limited amount of water. Saturation limits for jet fuel and for aviation gasoline are shown in Fig. 4 [9]. When water-saturated fuel is cooled, water comes out of solution as free water. These low levels of free water do not present cin operating problem. However, as the fuel temperature goes below 0°C or 32°F, this free water freezes into ice crystals that can plug filters and other fine passages. The problem is avoided by placing filters into a warmer part of the fuel system, by applying heat to critical filters when water freezing temperatures are noted, or by including an anti-icing additive in the fuel. Diethylene glycol monomethyl ether (diEGME), as described by specification D 4171, Type III, can be added to jet fuel or aviation gasoline as an anti-icer in concentrations of 0.10-0.15% by volume. Ethylene glycol monomethyl ether (EGME) is no longer listed in D 4171, but is still in limited use. Isopropyl alcohol, per D 4171 Type II, may only be added to aviation gasoline. Here specification D 910 warns that the addition of isopropyl alcohol to the 100 grade may lower its knock ratings. Both additives only keep limited amounts of free water from freez-
ing and neither affects wax precipitation. Water concentrations significantly exceeding saturation levels Ccin overwhelm these additives. Techniques for monitoring free water levels etnd keeping excess water out of aircraft are discussed later under Quality Control. The freezing of atmospheric moisture on the throttle plate causes carburetor icing in piston engines; it is due to evaporative cooling by gasoline and trace levels of water in fuel play no part. High Temperature Exposure to high temperatures represents one major difference between aviation gasoline and jet aircraft fuel systems, in fact, high temperature exposure has not occurred in aviation gasoline-powered aircraft. Jet fuel presents a completely different picture. The first incidents, in the early 1950s, took place in military J57 engines that suffered plugging of nozzle filter screens by fuel-caused carbon deposits [11]. Such plugging radically altered the exhaust gas temperature pattern at the turbine inlet and resulted in turbine blade failures. In this engine the fuel manifold that distributed fuel to the individual nozzles was located in the hot compressor discharge air and some fuels formed carbon deposits in the manifold under reduced fuel flow conditions. These deposits broke off and plugged fuel nozzles. The particulsir problem was solved by manifold redesign, but it also alerted the industry to the need for tailoring high temperature performance into the fuel.
96 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
FIG. 5—Jet Fuel Thermal Oxidation Tester (JFTOT).
Extensive research has pinpointed trace levels of certain molecules and metals to be the criticeJ peirticipants in high temperature oxidation reactions. Heteroatoms containing sulfur and/or nitrogen have been investigated in a number of model studies [12], which have found them to be as much as 20 times more reactive than the base fuel. Oxidation transforms these molecules into higher moleculctr weight materials, which become insoluble and can plug filters or screens or deposit on hot surfaces. In turn, such deposits have broken loose or have caused sliding parts to stick. Thus, these reactions involve only a tiny fraction of the total fuel going through the engine, but their effect becomes significant because of the very large fuel volumes handled over a long period of time. Ultimately, the basic reason for fuel degradation is the fuel's use as a heat sink as pointed out under Engine Descriptions. To curb the problem, the engine design approach has been to limit liquid fuel temperature to a maximum of 190°C (325°F) [13]. However, fuel oxidation is a function of both temperature and exposure time so that lengthy residence times at temperatures below this maximum have resulted in operating difficulties. Paradoxically, improving engine design over the years has increased the severity of fuel exposure be-
cause more efficient engines have reduced fuel flows and therefore cause higher heat inputs into the fuel [14]. Fuel constraints have also been instituted. Although specific heteroatoms have been found to oxidize rapidly in model studies, the large variety of materials involved in high temperature oxidation as well as their low concentration precludes their analytical identification on a routine basis. Therefore, the jet fuel specification requires a performance test, the Jet Fuel Thermal Oxidation Tester (JFTOT) by D 3241, to screen out product with unsatisfactory characteristics. The unit is illustrated in Fig. 5. In the apparatus, fuel is pumped over a heated tube under pressure and then through a metal screen with 17 micron openings, with both tube and filter being maintained at a temperature of 260°C (500°F). Filter pressure drop is monitored during the test, and at the end of the test, tube deposit color is competred to a color standard. Normally the test is run only at the specification minimum requirement conditions and the quality margin available above this limit is unknown. Like many other timedependent tests the thermal stability test has poor precision, but the visual rating of deposit color plays a major part in the poor precision [15]. Over the years, differing methods for rat-
CHAPTER 4: AVIATION FUELS ing of tube deposits have been proposed and rejected. Most recently, a method of measuring deposit volumes by an ellipsometric technique [16] has been developed as an alternate to the visual rating but has not been adopted. An interesting side issue has been the catalytic effect of certain metals on these high temperature reactions. This topic is further discussed in the section on Corrosivity. Combustion More than any other performztnce requirement, combustion requirements influence the differences between aviation gasoline and jet fuel. Aviation Gasoline The requirements of spark-ignited reciprocating engines have shaped aviation gasoline since its earliest uses. As the mixture of air and fuel vapor enters the cylinder and is compressed, the mixture must be ignited at a critical time in the engine cycle to provide maximum power through the expansion of exhaust gases. Mixture autoignition, either through compression ("dieseling") or by glowing deposits, will cause a loss of power and can result in detonation with severe mechaniccd damage. Aircraft engines Eire particularly vulnerable because of the extensive use of aluminum alloys and other low melting point meteJs. Compression ignition cjin be alleviated by spark timing retardation as well as maximum power reduction. However, either approach causes a performance loss. Thus the importance of fuel knock resistance through adequate octanes cannot be overemphasized. Gasoline knock resistance is measured by the lean and rich engine tests mentioned earlier. In the lean mixture method, D 2700, the test variable is compression ratio, which is the ratio of the total volume displaced by the piston to the volume above the piston at the top of the compression stroke. When testing Ein unknown fuel in the single cylinder D 2700 engine, this ratio is increased until knock is detected. The condition is then rated by making blends of n-heptane and isooctane and adjusting the ratio of the components until a match is noted with the test fuel. The percent isooctane in the matching blend is the test fiiel's octane number. If fuel octane exceeds 100, the match is made with pure isooctane containing differing levels of TEL. Because the specification requires
ratings by the old Aviation method, D 2700 contains conversion tables from the Motor method to the Aviation method. It is noted in passing that the Motor Method is the only rating method used for both aviation and motor gasolines. The rich rating method, D 909, is more complex. Engine output is measured and controlled by a dynamometer. Air into the engine is compressed Eind cooled. Both fuel and air flow to the engine are metered. Engine loading by the dynamometer is increased at constant fuel/air ratio until knock occurs. The same process is repeated at increasing fuel/siir ratios to develop a curve of knock-limited engine output against fuel/cdr ratio. This curve is bracketed by similar curves using isooctane with varying concentrations of TEL as reference fuels. The test fuel rating is established by interpolation between reference fuel curves cind is usually expressed in terms of performzince numbers, which are directly proportional to the engine power output. Thus, with a fuel having a performance number of 130 the engine developed 30% more power than on straight isooctane. There are currently several research programs [17] related to knock testing and combustion requirements of aviation piston engines. The first program is establishing the octane requirements of a great variety of current production engines to get an up-to-date definition of the relationship of the single cylinder laboratory engines to fl5'ing hardware. As part of this effort, an Engine Octane Rating method has been issued as D 6424. Preliminary results indicate that, while the old, large radicd engines are sensitive to both lean and rich knock characteristics, the flat or pancake engines seem to be critiCcJ in the cruise or lean operating mode only. The second program is pointed toward the development of a high octane, unleaded aviation gasoline. That search is driven by the fact that 100 octane aviation gasoline is the only lead-containing fuel left in the U.S. petroleum inventory and there is environmental pressure to eliminate lead wherever it exists. Ultimately, the two programs should indicate the practicality of developing an unleaded gasoline, which will satisfy the current inventory of engines requiring high octane fuel. Jet Fuel A typical jet fuel combustion system is illustrated in Fig. 6. Air is discharged from the compressor at pressures up to 20
Outer combustor casing Fuel nozzle
Inner liner
Turbine inlet
Jet igniter
Primary comlnistion zone
97
Secondary zone
FIG. 6—^Turbofan combustion system.
9 8 MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
atmospheres (300 pounds per square inch) and temperatures as high as 450°C (840°F) into the combustion system between the outer and inner liners. The air then enters the inner Hner through holes and slots carefully placed and sized to control the air and gas flow patterns. Fuel is injected and atomized through one or more nozzles into the primary combustion zone, where, after ignition, it will bum at temperatures approaching 2000°C (3500°F). Such gas temperatures exceed the operating limit of the turbine section and therefore more air is added in the secondary zone to reduce exhaust gas temperatures to maximum levels of 800-1200°C or 1500-2200°F. Thus, air provides oxygen for combustion, creates local turbulence to stabilize flame location, cools the inner liner surface, and finsJly reduces exhaust gas temperatures. Note that the fuel line to the nozzle must go through the hot compressor dischcirge air and the engine designer must guctrd against excess heat flow into this line. In the early days much effort was spent to discover a jet fuel property that would improve engine efficiency the way that octane improves piston engine performance [18]. No such performance parameter has been found for jet engines. Fuel specification tests therefore serve primarily to limit undesirable components that could shorten engine life. In general, paraffinic molecules provide the cleanest combustion while aromatics tend to create the opposite conditions. Several tests are used to measure fuel combustion quality. The simplest test is the Smoke Point, D 1322. A lamp with a fuel reservoir contains a wick that can be lowered and raised and furnishes fuel to a flame. When the wick is raised, the flame increases in height, but at some height the flame begins to smoke. This height varies with fuel composition. The maximum flame height at which no smoke occurs is defined as the fuel smoke point. Although the smoke lamp represents a very simple combustion process, a large amount of research has validated the good relationship between this test and engine combustion, with fuels having higher smoke points showing cleaner combustion with reduced engine smoke or soot as well as decreased flame radiation in older engines. Engine studies have highlighted the poor combustion quality of double ring aromatics or naphthalenes and these are therefore measured by ultraviolet absorption at 245 nanometers with a spectrophotometer (D 1840). Another much more complex test, the Luminometer (D 1740), was designed to rate fuels by measuring the radiation from a smoke lamp flame, but the close correlation between this test and the smoke lamp has caused the test to fall into disuse. Lastly, the measurement of total aromatics by fluorescent indicator absorption (D1319) or by high performance liquid chromatography (D 6379) is also listed as a combustion test. However, this measurement does not correlate as closely with engine performance as does the Smoke Point and its prime significance is as an indication of elastomer compatibility. Hydrogen content by D 3701 has been proposed as a combustion quality indicator and a substitute for aromatic content but has not been accepted in civil specifications because its relationship to elastomer compatibility is not established. Specification combustion requirements are listed in Table 3. Minor fuel composition changes have little or no effect on combustion quality [19]. Therefore, low concentrations of organic materials as additives have little, if any, effect on combustion. Additives containing heavy metals have reduced en-
gine smoke or soot but are not permitted because of ash formation. [20]. The level of sulfur oxides in the exhaust is directly related to the total sulfur content of the fuel and any required reduction of these oxides has to be achieved by reducing total sulfur content below currently allowed levels. Nitrogen oxides, on the other hand, are purely an engine design function depending primarily on the maximum combustion temperatures, the amount of mixing in the primary combustion zone, and the time at maximum temperature. Fuel characteristics play no part [19]. The same can be said for carbon monoxide emissions in which fuel does not play a significant role [19]. Because the ultimate purpose of fuel is to furnish heat energy, the minimum heat content or specific energy is controlled. Water vapor formed during combustion is not condensed in any aviation engine, therefore the net specific energy is specified. As mentioned in D 910, the minimum specification level for aviation gasoline is high enough to effectively limit the total amount of aromatics to about 25%. Specific energy (formerly called heat content) can be measured in a calorimeter per D 4809, where a small amount of fuel is burned in a pressure vessel, which in turn is immersed in a water bath. The released energy is calculated from the temperature increase of the water bath. Extremely precise thermometry is required for the desired accuracy of the results. Specific energy can also be estimated by equations based on pertinent fuel characteristics. One estimation method, D 4809, uses fuel density and aniline point; another, D 3338, includes density, boiling point and aromatic content as variables. D 3338 is gaining in popularity because of aniline toxicity and because the required variables are already being determined to assure overall specification compliance. Volatility and Flammability Volatility, a fuel's tendency to evaporate, plays a significant part on the ground and in the air. Some performance requirements overlap between the two fuel t5rpes, others do not. Aviation Gasoline Aviation gasoline volatility is a major factor in engine operation because proper ignition requires fuel in vapor form. The fuel, therefore, has to evaporate in the intake manifold before reaching each cylinder. This is particuleirly important during starting. Volatility also affects vapor formation in fuel lines, which occurs when line pressure drops below the vapor pressure of the fuel, usually due to local overheating at low line pressures. Vapor lock takes place when the combined volumes of vapor and liquid in the line exceed the volumetric capacity of the engine pump so that the engine no longer gets the required amount of fuel. As a result, the engine will not run until liquid fuel flow is restored. Lastly, volatility determines the flammability of the vapor space (ullage space) above the fuel. This property is discussed later in this section. Jet Fuel Fuel volatility tends to have different effects on jet fuel operation. Too high a volatility CcUi cause excessive evaporation or even boiling in aircraft tankage during rapid climb when fuel vapor pressures equal the rapidly decreasing atmospheric pressures at altitude. That problem caused the 1950s shift
CHAPTER 4: AVIATION FUELS 280
260 240 220 200 180 160
140 120 100 80 60 40 20 0
99
• •
Jet A
• •
.
\
JetA-l,JP-8
\
JetB,JP-4
• \ \
'
1 20
1_ 40
Avgas
•
•
60
80
100
Percent Evaporated
FIG. 7—Fuel boiling ranges.
from JP-3 to JP-4 fuel by the U.S. Air Force [2]. Ullage flammability in aircraft tankage is of concern and depends on fuel type as well as in-tcink temperatures and pressures. Aside from some effect on engine stcirting at very low temperatures, fuel volatility is not a major factor in jet engine operation. Fuel volatility is normally defined by the product's boiling range and its vapor pressure. The boiling range is measured in a simple distillation apparatus, D 86, in which fuel is boiled off and then condensed at atmospheric pressure under closely controlled conditions. The test result is a relationship of boiling temperatures to fuel percent evaporated. Typical results for Jet A [21 ], Jet A-l/JP-8 [23], Jet B/JP-4 [21 ] and aviation gasoline [22] are plotted in Fig. 7. The boiling range distribution can also be measured by gas chromatography, D 2887, where the molecular distribution of a sample is compared to a standard mixture of known boiling points. Fuel vapor pressure is routinely measured at 38°C (100°F) by D 323 or D 5191. The resuh of the test, called the Reid vapor pressure (RVP), is the sum of the partial pressures of the fuel constituents plus the partial pressures of dissolved air and water measured at a vapor to liquid ratio of 4:1. A true, absolute vapor pressure where the vapor space approaches 0 can be obtained at any temperature with D 6378. A summary of typical absolute vapor pressures against temperature is shown in Fig. 8 [9], but absolute vapor pressure is not determined on a routine basis. Reid vapor pressure is specified for aviation gasoline and Jet B but not for Jet A or A-1 because for these fuels the RVP results are too low to be reliable. Instead, volatility for the kerosine-type fuels is measured by the flash point test. Fuel in a closed cup is immersed in a water bath, which is warmed at a specified rate. Starting 10°C (18°F) below the expected flash point temperature, a small flame is repeatedly inserted into the cup, which is closed between flame immersions. The fuel temperature at which a flash of flame appears and disappears is reported as the flash point. The standard test in D 1655 is the Tag Closed Cup method, D 56, but other flash point tests are also in use. Because of differing surface to volume ratios and different heating rates these tests often give consistently different results and these differences have to be considered when comparing test results [24].
Vapor pressure and flash point, while related, do not measure the same property. The former measures the pressure exerted by a fuel at a specific temperature, while the latter indicates the lowest temperature at which a flammable fuel/air mixture is created. Typical fuel flammability ranges at sea level are summarized in Table 5 [9]. Below the lean limit there is not enough fuel vapor in the air for combustion, while above the rich limit there is not enough air. Only within the flammability limits is the mixture ignitable with an energy source like a spark or flame. The drastic differences between aviation fuels are apparent. These flammability limits are not absolute but are influenced by the test equipment and test conditions and serve only as general guides with regard to vapor space flammability. For example, the fuel/air ratio in a large storage tank is maximum at the fuel surface and decreases with increasing distance above the fuel. Tank vapor space or ullage flammability therefore can vary in a tank, particularly because temperature is also not usually uniform. Flammability limits are also strongly influenced by the effect of ambient pressure on evaporation rate. Thus the change in flammability limits with increasing altitude (decreasing pressure) is shown in Fig. 9. The subject of aircraft tank flammability is being revisited as the result of the 1997 crash of TWA Flight 800, which is attributed to the explosion of the "empty" center tank in the fuselage [25]. Fuel Metering The measurement of fuel quantity and fuel flow is important both for ground and for flight operations. On the ground
TABLE 5—TyplcEil fuelflammabilitylimits at sea level. Flammability Concentration Limits, %v Lower (lean) limit Upper (rich) limit
Avgas
JetB
Jet A
JP-5
1.2 7.0
1.3 8.0
0.6 4.7
0.6 4.6
Flammability Temperature Limits, °C Lower (lean) limit Upper (rich) limit
-44 -12
-23 18
53 77
64 102
100 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK
I
>
I
20
40
60
80
100
120
HO
Temperature, "C FIG. 8—Typical vapor pressures vs. temperature.
product is bought, sold, or transferred on a volume basis. Fuel volume changes with temperature, so the volume is usually corrected to a standcird temperature using the equations or tables in D 1250. Use of the corrections results in consistent inventories and product transfers. In the U.S. the standard temperature is 60°F, outside the U.S. it is 15 or 20°C. Although different only in the fourth significant place, the volume difference between 60°F and 15°C becomes significant for leirge fuel volumes. All aircraft fuel measurement is on a mass basis: whether desiling with total aircraft take-off and landing weight, maintaining mass balances and trim in flight or when metering fuel flow into the engine and its combustion system. Volume is measured in U.S. gallons or in liters. Mass units include density in kg/m^ at 15°C, relative density - the ratio of fuel density to water density, usually at 15°C, and API Gravity,
an arbitreiry scale, at 60°F. All are measured by hydrometer, D 1298, or by digital density meter, D 4052. Fuel height in most jet aircraft tcuiks is measured by gages based on fuel dielectric constant. These fuel heights are converted into tank volumes eind transformed electronically into mass units during aircraft loading. Other aircraft gaging system use ultrasonic signals to measure tank fuel height. To deliver the proper fuel flow to the combustion system the engine fuel control is programmed to meter volume and to correct for fuel density and fuel temperature as well as other parameters. Aircraft Range Aircraft range is controlled by the total heat content of the fuel in the aircraft. Maximum range is limited either by the maximum allowable aircreift take-off weight (weight-limited)
CHAPTER 4: AVIATION FUELS or by the total volume of tankage on the aircraft (volume limited). Either one is determined by fuel density and, in special cases, an optimum fuel density has been selected for a particular long distance flight. In normal operations such an approach is not feasible. Aircraft operations have to EJIOW for the density range permitted by specifications and routes, which require maximum range, are usually avoided, particularly because atmospheric conditions such as winds Eind payload variability play a major part in determining fuel quantity requirements. CoiTOsivity
Copper One of the early compatibility problems has been fuel attack of system metals, particulcirly copper or copper-containing materials. In aircraft fuel systems this frequently involved fuel attack on bronze pump bearings. The primary fuel constituents involved in copper corrosion are elemental sulfur and hydrogen sulfide, although acidic components may also play a part. The most common source of hydrogen sulfide has been incomplete stripping of the gas after refinery hydrotreating. Elemental sulfur (S), which is sulfur uncombined with other elements, has tended to result from the chemicEd reduction of water-soluble sulfates in tank bottoms by sulfate reducing bacteria or by the oxidation of hydrogen sulfide. Elementcil sulfur has also been a byproduct of older
101
mercaptan conversion processes such as Bender or Doctor treating. Both elemental sulfur and hydrogen sulfide are corrosive to copper at levels of around 1 ppm [26], but exact levels at which unacceptable corrosion occurs seem to depend on the presence of other trace materials such as mercaptans. Rather them limit either elemental sulfur or hydrogen sulfide, copper corrosion is therefore minimized by the copper corrosion test, D 130, where pure copper is exposed to fuel at 100°C (212°F) for 2 h. Copper appearance is compared to a standard color chart and only minimal discoloration is permitted. In addition, contact with copper or copper-containing eilloys is avoided as much as possible in aviation fuel systems and good housekeeping will prevent the proliferation of sulfate reducing bacteria. For jet fuel systems there is zdso great concern over copper content in the fuel as the result of copper corrosion. Copper concentrations as low as 15-20 ppb have adversely affected thermal stability and copper levels above 30-50 ppb have decreased this property below the specification minimum [27]. Copper sources have included refinery heat exchangers, bronze heating coils in marine tankers as well as bronze fuel systems in Navy aircraft carriers. The reactant in an old mercaptan conversion process, Linde sweetening, can also be a copper source. Unfortunately, the relationship between the corrosion of copper or bronze parts and the pickup of copper in jet fuel is not clesu-ly established. Compatibility with copper-containing parts in the fuel system is controlled by the copper corro-
80 Temperature, °C FIG. 9—Flammability limits vs. altitude.
100
102 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
sion requirement in the specification, but fuel passing tfiis requirement can dissolve the very low levels of copper, which in turn can wreck thermal stability. At present the most reliable way to avoid pickup is to avoid contact with the metal or to add an approved metal deactivator to chelate and deactivate the trace levels of copper in fuel. Cadmium The corrosion of cadmium-plated parts has been attributed to mercaptan sulfur compounds, particularly in the presence of water [28]. Mercaptan sulfurs are measured by D 3227 or are limited qualitatively by the Doctor test, D 4952. Steel A form of corrosion in which the fuel plays an indirect role is the rusting of mild steel by free water in fuel. Such corrosion shortens the life of steel components but also is the source of solid contamination, which ultimately has to be removed from aviation fuels. U.S. military jet fuels require the presence of fuel-soluble corrosion inhibitors, generally dibasic acids, which form a protective molecular layer on the steel surface. MIL-PRF-25017 defines the requirements for this t3^e of additive and approved additives are listed in the Qualified Products List for the specification. The dual function of these additives as lubricity agents as well as corrosion inhibitors is discussed later in this chapter. Because these materials are surface-active and can interfere with water removal in the filter-coalescing process, their use in civil fuels is very limited. Multi-product pipelines are protected by placing the additive into the non-aviation products. Fuel systems at airports are usually internally coated to prevent rusting. Galvanizing or zinc coating is not an acceptable method for rust prevention in aviation systems because of the sacrificial nature of zinc, which can react with the TEL in aviation gasoline or dissolve into jet fuel to form an ash during combustion. Storage Stability Petroleum products in long term storage are subject to low temperature oxidation, which may cause the formation of high molecular weight products that become insoluble and precipitate out. Olefins formed during cracking are particularly prone to such degradation and are avoided in aviation fuels. Straight-run jet fuels have excellent storage stability due to the presence of natural oxidation inhibitors. However, processes such as hydrocracking or high pressure hydrotreating destroy these inhibitors, leaving the product vulnerable to oxygen attack. In jet fuels such peroxide formation [29] has led to repeated localized incidents of nitrile rubber deterioration in military service [28]. Such elastomer attack has not been reported with civil fuel in the U.S., probably due to the greater amount of fuel commingling. Similar degradation tends to occur in aviation gasoline, which contains no natural oxidation inhibitors and is therefore routinely inhibited with oxidation inhibitor additives. Peroxide formation is prevented by oxidation inhibitors, normally hindered phenols, which scavenge free radicals that are the first step in the complex oxidation process [12]. Acceptable inhibitors are listed by composition in the jet fuel and aviation gasoline specifications. To be effective, these in-
hibitors must be injected into the fuel as early as possible, preferably into the run-down line from the processing unit. Aviation gasoline susceptibility to oxygen attack is evaluated in the Oxidation Stability test, D 873, in which fuel is exposed to pure oxygen under pressure at 100°C (212°F) for 5 or 16 h. The fuel is then evaporated and the residue ("gum"), together with any lead precipitate, is weighed. Based on storage tests conducted during World War II, passing test results are considered equivalent to one year of desert storage. Tests on straight-run jet fuels in the 1960s showed oxidation at high temperatures to occur before oxidation at storage temperatures. In fact, storage stability has not been a problem for such fuels. The D 873 requirement was therefore removed from D 1655. However, heavily hydrotreated or hydrocracked jet fuels should contain oxidation inhibitor to assure the absence of peroxide formation. A test to establish peroxide-forming tendencies has been published by CRC [30], but is not yet approved by ASTM and has not been included in specifications. Lubricity In jet engine fuel systems a number of sliding metal surfaces are lubricated by fuel. Under low contact pressures (hydrodynamic lubrication conditions) fuel viscosity assures separation of these parts. Under increased contact pressures or higher sliding velocities (boundary lubrication) fuel viscosity alone will not prevent metal to metal contact and a thin layer of adsorbed polar material is also required to prevent surface damage. Such material is present in straight-run fuels in very low but adequate concentrations, but it is destroyed by high pressure hydrotreating or hydrocracking. Such fuels have therefore allowed metal to metal contact resulting in destructive wear of gears in high pressure pumps and sticking of close-fitting servo valves in flow controls [31]. "Harsh" or "hard" fuels can be identified in the Ball on Cylinder Lubrication Evaluator (BOCLE test), D 5001. Under a specified load, a stationary steel ball rubs against a rotating cylinder for 30 min. The contact area is submerged in the test fuel and the entire test atmosphere is maintained at a temperature of 25°C (77°F) and a relative humidity of 10%. Afterward the ball wear scar is measured under a microscope. There are currently no specification limits for this test, but a wear scar above 0.85 mm represents a very "hard" fuel. Today the test is used primarily by engine accessory manufacturers who have to qualify their components on very hard fuels to assure satisfactory field performance. However, under pressure from the New Zealand airworthiness authorities, the latest issue of DefStan 91-91 is the first international specification to include a lubricity requirement for certain fuels. In service, very hard fuels can be rendered acceptable by the addition of as little as 10% straight-run product [32]. This effect illustrates why lubricity problems have not been more wide spread, having generally been averted by the extensive commingling in most jet fuel systems. Problems have occurred in locations where hard fuels from a single supply point were not mixed with other fuels and were supplied directly to aircraft. Even then different components responded differently and failures occurred only on certain parts, mostly on older units not qualified on hard fuel.
CHAPTER 4: AVIATION FUELS Aside from hardwEire improvements, problem solutions include blending of hcird fuels with strciight-run product or the addition of lubricity agents. Experience has shown fuel soluble corrosion inhibitors to perform as lubricity agents, although minimum dosages may differ for each use. Acceptable additives are covered by two military specifications and their qualified products lists. U.S. specification, MIL-PRF-25017, and British Standard Def Stan 68-251 define additives suitable as both corrosion inhibitors and lubricity agents. Neither of these additive types are currently listed in civil specifications. Static Electricity Hydrocarbons are extremely poor electriccJ conductors but usually contain trace levels of ionizable materials, which carry charges of static electricity. With the fuel at rest the charges neutralize each other. When fuel is moved, ions of opposite charge are separated and an electrical charge is generated [33]. Local turbulence generated by pumps or filters increases the charge, which is carried along unless the flow velocity is reduced sufficiently for the charges to recombine. When the fuel comes to rest, the charge leaks to ground. The time required for the charge to bleed off depends upon the fuel's conductivity. If conductivity is low, the charge may not leak off gradually but be dissipated instead in the form of a spark, which can become an ignition source. This process takes place independent of system bonding or grounding. However, all components of the fuel system, including the aircraft, are electrically interconnected or "bonded" to eliminate the possibility of spEirks due to stray currents or lightning. Static electricity problems can be minimized by slowing the flow in lines, particularly after a filter, by providing an inert atmosphere over the fuel or by adding a conductivity improver additive to the fuel. Stadis™ 450 is the only conductivity improver approved for jet fuel and for aviation gasoline. Outside the U.S. the additive is required in all jet fuel. In the U.S., conductivity improver use in jet fuel is limited because commonly used clay treaters in terminals remove the additive so that the fuel would require additive reinhibition. Instead, the emphasis is on providing charge relaxation by system design and operation. Guide D 4865 provides an excellent review of the subject. Other Properties Data on other jet fuel properties not covered by specifications such as thermal conductivity, specific heat, enthalpy, heat of vaporization, thermal expansion, dielectric properties, and bulk modulus will be found in the CRC Handbook on Aviation Fuel Properties [9]. This handbook includes data on both commercial and military fuels. FUEL MANUFACTURING Aviation Gasoline Low octane aviation gasoline can be straight-run gasoline (grade 80) or based on premium grade motor gasoline (grade 82UL). The high octane grades (100 LL or 100) are based on alkylate made by the process described in the following equa-
103
tion: Catalyst
C4 olefins + isobutcuie
^
Alkylate
The olefins are separated from catalytic cracker products while the catalyst is hydrofluoric or sulfuric acid. Alkylate consists of branched chain paraffins, mostly iso-octctne. This composition makes it a very high octane material of high veilue to the refinery which can use the material to increase the octane of either aviation or motor gasoline. In view of this competition, as well as the low demeind for high octane aviation gasoline compared to motor gasoline, there is concern in the aviation community regarding the long term availability of the high octane aviation gasoline. This concern is sharpened by the uncertain future of tetraethyl lead. Like other petroleum products, the ultimate availability of this fuel is expected to be determined by market forces. Some blending is usually cEirried out for the high octane gasoline. Isopentane increases vapor pressure, while toluene increases the "rich" rating. Tetraethyl lead, along with the required scavenger and dye, is normally needed to reach the necessary minimum octane levels. Oxidation inhibitor is always added to obtain storage life. Overall, aviation gasoline has become a specialty product and, in the U.S., it is the only remaining fuel containing lead. Jet Fuel Jet fuel processing starts with its separation from crude oil by distillation, with the jet fuel fraction in the 150-290°C (300-550°F) boiling range. Depending on the crude, a few such streams are directly suitable as jet fuel, but most require further treating. Mercaptan sulfurs are converted to disulfides primarily by Merox treating; alternatively, they Eire converted by hydrotreating to hydrogen sulfide which is stripped from the product. Other sulfur compounds may be removed by more severe hydrotreating. An increasing number of refineries are hydrocracking heavier streams to make high qucdity jet fuel with good low temperature characteristics. To control thermal stability, streams from catalytic or thermal cracking Eire excluded or are saturated by severe hydrotreating. Contacting by activated clay removes polar materials that deteriorate fuel-water separation and which may also adversely affect thermal stability. Salt driers may be used to remove free water. Unless a refinery makes jet fuel by hydrocracking, jet fuel yield and quality are dependent on product boiling range and on the crude slate. Table 6 shows the general effect of changing boiling range on jet fuel properties when the product is made by distillation, followed by mercaptan treating [34]. Availability is maximized by making product with the widest possible boiling range. In turn, the boiling range tends to be limited on the low distillation end by flash point, while the maximum distillation end point is controlled by the specification freezing point. The wide rEinge of yield and resultant qucdity for some typical crudes are illustrated in Table 7 [35]. The table also reflects the difficulty of simultaneously improving critical properties when using a single crude oil. As pointed out in the section on History, commercial kerosine was the original jet fuel. Today, mEiny refineries make a single product saleable as either jet fuel, commerciEil kero-
104
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
TABLE 6—Effect of boiling range changes on product quality. Lowering Initial Boiling Point Increases Decreases Smoke point Density Hydrogen content Flash point MJ per kg Aromatic content Vapor pressure Naphthalenes Freezing point Viscosity MJ per m^
Increasing Final Boiling Point Increases Decreases Density Smoke point Hydrogen content Aromatic content MJ per kg Naphthalenes Freezing point Viscosity MJ per m^
Leading to Better combustion quality Better low temperature properties Higher gravimetric heat content Poorer handling safety Lower volumetric heat content
Leading to Lower combustion quality Poorer low temperature properties Lower gravimetric heat content Higher volumetric heat content Unchanged handling safety
sine, or diesel blending stock by having this product meet the specifications for the other products. Such a product is referred to as dual branded kerosine and is divided and suppHed to the differing uses at the distribution terminal. However, this approach has become more complex by differing maximum sulfur limits for the two products in many parts of the world. An additional complication in the U.S. is an EPA requirement for red dye in high sulfur diesel fuel. A current source of controversy and ongoing discussion is the effect of non-petroleum crude starting materials and new processing on jet fuel quality. Jet fuel specifications are performance based and reflect past experience. Therefore, they cannot guarantee satisfactory performance when a major change from past practice occurs. For example, using coal or biomateriaJs as starting materials raises questions that can only be answered by additional testing beyond current specifications.
compatibility with system materials. However, the procedures do not disclose whether an additive will affect performance that has to be evaluated by other testing. A similar protocol does not exist for aviation gasoline, which has seen very few additive changes over the years. It should be noted that ASTM does not confer additive approval. Such approval must be initiated by the airframe and engine manufacturers and approved by the airworthiness agencies. Additive listing in ASTM aviation fuel specifications simply recognizes such approvals. Specific Additives Most additives are described in earlier sections of this chapter under specific properties, but Table 8 lists all jet fuel additives contained in D 1655 together with their permitted dosages. Oxidation inhibitors, metal deactivator, and conductivity improver may be added at the discretion of the refiner. Other additives, such as fuel system icing inhibitors, can only be added with the agreement of the purchaser. The fuel manufacturer or fuel handler must declcire all additives and their concentration. Several military specifications make mandatory the addition of oxidation inhibitor, conductivity improver, icing inhibitor, and corrosion inhibitors. Two new additive categories are u n d e r active consideration by the ASTM committee and could be in the specification by the time this article is printed. These additives are leak detection materials smd biocides.
ADDITIVES General Allowable additives in aviation fuels are tightly controlled for safety and operational reasons. The procedures for evaluating the compatibility of additives with aviation turbine fuels a n d with aircraft fuel system materials are spelled out in ASTM Practice D 4054. The primary purpose of this practice is to standardize additive testing requirements so that an additive manufacturer does not have to repeat the process for each engine or airframe manufacturer. The testing procedures are very detailed and lengthy to assure satisfactory
A new jet fuel additive category in military use are thermal stability improvers, termed JP-8 -1-100 because they are intended to add 100°F (55°C) to the u p p e r thermal stability limit of the fuel [13]. The additive combination consists of ox-
TABLE 7—Effect of crude oils on straight-run product quality. Crude Oil Source Property
Alaska
California
Boiling Range, °C Yield, %v Density, kg/m^ Sulfur, %m Smoke point, m m Freezing point, °C
Indonesia
Arabia
Libya
8.7 801 0.009 28 -32
12.3 817 0.214 24 -40
13.5
200-260 10.3 828 0.093 20 -44
5.7 0.43 17 -70
0.04 23 -40
CHAPTER TABLE 8—Jet fuel additives listed in D 1655-98b. Antioxidants, 24 mg/L maximum 2,6-ditertiary-butyI phenol 2,6-ditertiary-butyl-4-methyl phenol 2,4-dimethyl-6-tertiary-butyl phenol 75% min. 2,6-ditertiary-butyl phenol, plus 25% max. mixed tertiary and tritertiary-butyl phenols 55% min. 2,4-dimethyl-6-tertiary-butyl phenol, plus 15% min, 2,6ditertiary-butyl-4-methyl phenol, remainder as monomethyl and dimethyl tertiary-butyl phenols 72% min. 2,4-dimethyl-6-tertiary-butyl phenol, 28% max. monomethyl and dimethyl tertisiry-butyl phenols Metal deactivator, 2mg/L max at mainufacture of fuel, for retreatment to total of 5.7 mg/L max N,N-disalicylidene-] ,2-propane diamine Electrical conductivity additive, 3 mg/L max at manufacture of fuel, for retreatment to total of 5 mg/L max Stadis™ 450 Fuel system icing inhibitor, 0.10-0.15%v Diethylene glycol monomethyl ether (diEGME) Leak detection additive, 1 ppm max Tracer A
idation inhibitor, metal deactivator and proprietary detergents and dispersants. Because the combination contains powerful surfactants, currently only water-absorbing type filter media (commonly called filter-monitors) will operate in the presence of water and the additive. However, a major effort to develop filter-separators capable of handling the additive holds considerable promise. Table 9 shows the aviation gasoline additives and their permitted dosages as listed in D 910. They include tetraethyl lead fluid, which must contain two atoms of ethylene dibromide per atom of lead as well as one or more of three different color dyes. Blending two or more of the approved dyes makes additional colors. While dyes were originally required to identify any fuel containing lead, today they serve both to identify lead-containing fuel and to distinguish between the several grades of aviation gasoline. Oxidation inhibitors, conductivity improver and fuel system icing inhibitors are also permitted. Note that aviation gasoline can include isopropyl alcohol as an icing inhibitor, but jet fuel Cctnnot.
TRANSPORTATION Pipelines In the U.S. most jet fuel is moved from the refinery by large multi-product pipelines handling all fuels from gasoline to diesel fuel. Lines with diameters as large as 40 in (100 cm) are employed. These lines are normally operated on a fungible basis where each product meets a single specification. All suppliers meet that specification, the products then commingled and supplier identity is lost. Differing products are not physically separated from each other, but operating the line under turbulent flow conditions minimizes product mixing at the interface between products. Each pipeline product cycle has its own product sequence to minimize interface material, which is an off-specification mixture of at least two dissimilar fuels. Pipeline flow is maintained by controlling pressure a n d flow volume while p r o d u c t s are constantly added and removed. Products move at a speed of approximately 5 m p h (8 km/h) and, in one pipeline, take about two
4: AVIATION
FUELS
105
weeks to get from Texas to New Jersey, a distance of about 1500 miles (2500 km). Along each pipeline there are breakout terminals, which are collections of tanks serving as distribution centers. Such tanks are also used to temporarily hold product where pipeline diameters change. In a number of instances jet fuel is moved from a terminal to an airport by pipeline as well. In most instances such pipelines are smaller and are usually restricted to jet fuel. Marine Transport Outside the U.S., jet fuel tends to move from the refinery in so-called "clean" tankers—as distinguished from black oil ships, which carry crude oil or residual fuel oil. Tankers vary tremendously in size, although the largest ones carry only crude oils. As in pipelines, product sequencing is carefully controlled to minimize mixing. To reduce contamination by other products, shippers try to maintain certain compartments or the entire vessel in jet fuel. Tanker compartments today are usually epoxy coated to extend their life and also to make t h e m easier to clean. E a c h c o m p a r t m e n t contains cleaning equipment, which directs a high pressure spray of hot or cold sea water against the inside of the compartment. However, cleaning procedures must be CEirefuUy controlled to avoid contaminating jet fuel with cleaning compounds. A complicating factor in the handling of jet fuel is the inerting of compartment ullage space with engine flue gases. Such compartments are kept seeded under slight overpressure and are normally not open for inspection or thorough cleaning. Marine transport on inland waterways is usually by barges, which may carry their own propulsion equipment or may be moved by tugs. Their construction tends to be more basic than that of tankers and usually does not include internal cleaning equipment. As a result operators tiy to keep barges in the same fuel grade as m u c h as possible.
TABLE 9—Aviation gasoline additives listed in D 910-98. Anti-knock agent, maximum dosage depends on grade Tetraethyl lead plus sufficient ethylene dibromide to provide two atoms of bromine per atom of lead Dyes, maximum dosage depends on grade Blue: 1,4-dialkylamino-anthraquinone Yellow: p-diethylamino-azobenzene or 1,3-benzenediol 2,4bis[(alkylphenyl)azo-] Red: alkyl derivatives of azobenzene-4-azo-naphthol Antioxidants, 12 mg/L max 2,6-ditertiary-butyl phenol 2,6-ditertiary-butyl-4-methyl phenol 2,4-dimethyl-6-tertiary-butyl phenol 75% min. 2,6-ditertiary-butyl phenol, plus 25% max. mixed tertiary and tritertiary-butyl phenols 75% min. di- and tri-isopropyl phenyls plus 25% mstx. di- and tritertiary butyl phenols 72% min. 2,4-dimethyl-6-tertiary-butyl phenol, 28% max. monomethyl and dimethyl tertiary-butyl phenols N,N'-di-isopropyl-para-phenylenediamine N,N'-di-secondary-butyl-para-phenylenediamine Fuel system icing inhibitors Isopropyl alcohol (IPA, propein-2-ol), max dosage recommended by aircraft manufacturer Diethylene glycol monomethyl ether (diEGME), 0.10-0.15%v ElectricjJ conductivity additive, 3 mg/L max, for retreatment 5 mg/L max total dosage Stadis™ 450
106 MANUAL 37: FUELS AND LUBRICANTS
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Product is loaded into tankers by shore-based pumps or by gravity if shore tanks have sufficient elevation above the tanker. Off-loading, on the other hand, is by pumps located in the lowest part of the tanker or barge. This mode of operation tends to leave a small amount of product at the bottom of a compartment when the pump loses suction. The effect of mixing this residue with the next product is covered in more detail in the section on quality control. Road Transport Smaller volumes of product, particularly when moved relatively short distances, are handled by road transport. Such tanker trucks have several compartments and do not have pumps aboard. Product is unloaded by gravity into underground tanks or into a pump that lifts the product into aboveground tanks. Truck size and weight is limited by road restrictions. When possible aviation products are isolated to specific trucks to minimize contamination. Most aviation gasoline is moved by truck because the volumes are too small for pipeline movements. Compared to large pipelines or water transport, truck transport is the most expensive mode of product transportation. Rail Transport In some areas of the world aviation fuel is moved in rail Ccirs. Rail transport does not have the flexibility of road transport; in the U.S. it is limited to Alaska. Airports Airport fuel systems vary widely depending on the type of product and the volumes handled by the airport. Low octane aviation gasoline at small airports is handled through cabinet dispensers similar to those in service stations. These units contain a meter, a hose, and a simple filter. The aircraft is parked at the dispenser and fuel is placed into the aircraft "over wing" through the hose and a hand-held nozzle. At larger airports, high octane gasoline or jet fuel is loaded into tank trucks called fuellers, which drive to the aircraft. These fuellers incorporate a fuel tank divided into several compartments, two or more hoses, a filter, a meter, a pump, and other ancillary equipment. The hose may make a solid connection with the aircraft through a pressure coupling or the fuel may be placed into smaller aircraft over wing with a hand-held nozzle. To avoid misfuelling, gasoline and jet fuel should not be carried on the same vehicle. Also, where possible, different nozzle sizes with matching filler restrictors are used for aviation gasoline and jet fuel. The advantage of fueller vehicles is their flexibility of positioning; the disadvantage is ramp congestion, particularly when as many as four or five truckloads are required for a jumbo jet. Airport congestion is minimized by the use of hydrant systems. Two or more storage tanks are linked to the passenger terminal by a large diameter pipeline, which runs underground alongside the aircraft loading gates. Each gate has at least one hydrant pit containing an underground pressure connection. Trucks, termed hydrant vehicles, have one large hose, which is coupled to the underground connection. Fuel moves through that hose, a meter, a filter and other plumbing and ultimately
gets into the aircraft through one or more hoses coupled directly to the aircraft fuel system. The entire hydrant system, from the storage onward, is pressurized by a series of pumps that maintain operating pressure right up to the aircraft. Underground pressures of 700-1000 kPa (100-150 psi) are common but must be reduced to 315 kPa (45 psi) maximum at the aircraft inlet [36]. Unlike a fueller there is no pump abocird the hydrant vehicle. A hydrant system avoids ramp congestion but permzinently fixes the location of fueling stations. All major airports and many smaller ones incorporate hydrant systems. VirtUcdly all airport fuel systems are commingled systems where each fuel grade meets the same specification and there is no segregation by individual suppliers. B o n d e d Fuel A separate fuel designation, applicable only to international transportation, is "bonded fuel." Legally such fuel enters a country under bond and must leave the country in international transportation use. Bonded fuel is not subject to domestic taxes such as excise or sales taxes. Depending on customs regulations, bonded fuel must be handled as a completely segregated product in its own handling systems while the fuel is in the country or it can be handled in regular distribution systems as long as its entire volume is accounted for. While originally developed for marine transport, the concept is equcdly applicable to aviation and is used in some airports for intemationsJ flights. Its use is limited by economics as well as local customs regulations.
QUALITY C O N T R O L General Philosophy Quality control is intended to protect the fuel properties needed for safe aircraft operation. Toweurd that purpose aircraft certification requires the fuel to fully meet the specification to which the aircraft was certified. This process starts at the refinery where aviation fuel must fully meet the pertinent specification. In addition, fuel quality also has to meet other requirements not in the specification. In their simplest form those requirements are expressed as "clean and dry." However, on its way from the refinery to the airport fuel is exposed repeatedly to water, solid contaminants and to other fuels in multiproduct systems. Quality control therefore becomes an exercise in avoiding that type of contamination, which cannot be corrected in the field, and removing other contaminants at reasonable expense by placing appropriate removal equipment at critical points in the system. Contaminants Any material not originally part of the fuel is considered a contaminant. That definition includes free water, solids such as rust or sand, microbial material, other fuels, unapproved additives as well as more unusual materials such as solvents, fertilizers, plastics, or paints soluble in aviation fuels. Because aviation gasoline and jet fuel have different critical properties, quality control procedures as well as contamination removal equipment differ for each fuel type and are described separately in the following sections.
CHAPTER 4: AVIATION FUELS Aviation Gasoline While all properties must meet pertinent specifications at the aircraft, the most sensitive properties of high octane aviation gasoline are the octane ratings and the volatility. Octane ratings are readily degraded by contamination with other distillate products. The lower the octane of the distillate, the greater the adverse effect. Experience has shown the "rich" rating to be the more sensitive, particularly because the "rich" rating of most aviation gasolines tends to have less margin above the specification minimum. This problem can only be avoided by eliminating possible contact with other fuels such as might occur in switching between different products in transports. Volatility degradation occurs through evaporation in storage and results in vapor pressure decreases and changes in distillation cheiracteristics. Depending on which component is lost through evaporation, there may also be a decrease in octane rating. Unfortunately, none of these effects are readily predictable. Instead it is necessary to reestablish the specification compliance status at specified locations or time intervals. Solids and free water removal is carried out primarily by settling in tankage at a rate of 15 min per ft (45 min per m) of depth [37] and by further removal of solids with micronic filters. Because of the lower density and viscosity of gasoline compcired to jet fuel, contaminant removed from aviation gasoline is simpler and less complex as will be seen in the jet fuel section. There are no industry-wide quality control procedures for aviation gasoline. Instead, each company foUows its own procedures, which tend to be similar throughout the industry. Gasoline is fully tested against specification requirements at the refinery and is then subjected to limited retesting upon arrival at the distribution terminal. The tests normally run at that location are listed in Table 10, but they can differ between suppliers. Sample handling must be done in accordance with D 5842 to avoid the loss of volatile components, which can result in a low vapor pressure. Gasoline is again inspected on airport arrival for appeeirance, color, and density. Only a single product grade should be carried in the transport to the airport and in the fueling vehicle on the airport. In addition, product that has been in storage with no product movement in or out should be checked for specification complicince at six-month intervals. Jet Fuel Jet fuel contamination can be divided into two broad categories: that which can be removed in the field and that which cannot. The first category includes various solid contaminants and water, while the second category includes other fu-
TABLE 10—Terminal receipt tests for aviation gasoline. Knock rating by ASTM D 909 (If not available, knock rating by ASTM D 2700) Distillation by ASTM D 86 Reid vapor pressure by ASTM D 323, D 5191, or D 5190 Tetraethyl lead content by ASTM D 3341 or D 5059 Color by ASTM D 2392 Existent gum by ASTM D 381 Electrical conductivity (if required) by ASTM D 2624
107
els or other materials soluble in jet fuel. Unlike aviation gasoline, a major concern is over contamination with more volatile materials that will lower the flash point. Solid and water removal are more difficult with jet fuel because both the density and viscosity of jet fuel are higher than those of aviation gasoline. The equipment to remove contaminants as well the quality control procedures are therefore more elaborate. In addition, a number of specialized field contamination detection procedures are in use. Contamination Removal Equipment Filter-Separators The heart of contamination removal equipment is the filtersepcirator, shown diagrammatically in Fig. 10. Two sets of cylindrical filter elements are mounted on a deck plate or a meinifold in the case. First stage elements are termed coalescers, the second stage are separators. Both types usually have a 6 in (15 cm) diameter; the total element length is proportioned to the maximum rated flow. Fuel flow in the first stage elements is from inside to outside and outside to inside in the second stage. In the first stage, at least one filtration layer consists of tightly packed, hydrophobic glass fibers, which force smeJl free water droplets to coalesce into leirger droplets as fuel pushes the water droplets through the layer [38]. Other layers remove and hold solid particulates. Most of the coeJesced droplets slide down the outside of the first stage elements. Those water droplets too small to settle between stages Eire prevented from exiting with the fuel by the second stage, which is also hydrophobic. This second stage is made of Teflon-coated wire mesh or a special synthetic fabric supported by a perforated meted cylinder. The coalesced free water collects in a sump at the bottom of the case and is removed by manually opening a drain valve. A high water indicator, usually a float or an electronic water level probe in the sump, shuts off the entire fuel flow if the water level rises too high and threatens to be carried out with the clean fuel. A direct reading differential pressure gauge measures the total pressure drop across both sets of elements. Filter-separators are designed to keep operating when subjected to contaminated fuel. Their minimum performance is governed by an industry specification issued by the American Petroleum Institute as API Specification 1581. This specification delineates the solid and free water concentrations added during qualification testing and dictates the maximum levels of both contamineint types permitted in the effluent. The latest version of the specification, Issue 4, was published in early 2000 and to take effect in January 2002. It contains three test fuel types. Category C is Jet A plus a conductivity improver, a corrosion inhibitor aind a sulfonate. Category M is JP-8 or JP-5 containing military additives. Category MlOO is Category M plus the JP-8 + 100 additive. Category C is intended for civil use, while the other two categories are for the U.S. military. Two performance levels are called out. Type S is to be used where significant solids and water are expected. Type S-LD, with a lower dirt holding capacity, is intended where significcint water but low solids are expected. The standard solid contaminant is 90%m A-1 ultrafine dust and 10% m Copper as red iron oxide. These are major changes from Issue 3 which specified Fischer red iron oxide of about 0,1 micron particle size as the solid contamineint and did not in-
108 MANUAL 3 7: FUELS AND LUBRICANTS
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iST STAGE COALSECER ELEMENTS (INSIDE TO OUT fLOW)
2ND STAGE SEPARATOR ELEMENTS (OUTSIDE TO IN FLOW)
INLET MANIFOLD UTLET MANIFOLD
INLET
OUTLET
FIG. 10—Cutaway of filter-separator.
elude military fuels. Nominal coalescer pore size is expected to be between 0.3 to 0.5 microns. Second stage pore size is much coarser and is not designed for dirt removal. Approvals cover only specific elements in specific housings and are not transferable to other housings. While the U.S. military has issued its own specifications in the past, the current trend is toward the use of equipment meeting the industry specification, API 1581. Water-Absorbing Cartridges or Monitors Sometimes called fuses, these units are designed as a last chance safeguard, which shuts down a system in the presence of water in the fuel. Their primary component is a cellulose layer that normally expands when wetted. By tightly constraining this layer, it compresses when wetted and shuts off fuel flow. Monitor elements are 2, 4, or 6 in. in diameter, depending on the application. The total length of elements is directly proportional to the rated flow of the filter. Because of their operating mode, monitors are located only at the last filtration point just ahead of the aircraft, where they may be the only final filter or be placed after a filter-separator. Alternatively, some operators use them as a second stage in place of the separator elements. Monitors can also be the final filters in aviation gasoline system. They are not normally placed
further upstream because the elements have to be replaced in case of a shutdown by free water. Two specifications govern monitor performance. MIL-M81380 is issued by the U.S. Navy. A specification issued by the Institute of Petroleum (IP) is similar but requires a higher solids holding capacity. This specification is being replaced by a joint API/IP specification, API 1583. Both specifications severely limit the amount of free water that Ccin pass through the monitor before it closes off. These units generally have a nominal 1 micron pore size rating. Latest specification revisions eJso guard against media migration under greatly reduced flow rates. Clay Treatment Vessels While similar in external appearance to filter-separators, the elements in clay treatment vessels are filled with activated clay to adsorb polar materials such as surfactants. This clay is the same as mentioned earlier in the Manufacturing section, the big difference being in the depth of clay as well as the rate at which fuel passes through the clay. This difference results in a 50 times reduction in contact time and a similar reduction in the vessel's capacity to remove surfactants. Field clay treatment vessels are therefore considered "polishing" units with rather limited adsorption capacity. They are nor-
CHAPTER 4: AVIATION FUELS mally intended to remove traces of polar additives picked up by jet fuel in multiproduct transportation systems. However, this adsorptive capacity has mitigated against the general use of conductivity additive because the wide presence of clay field units in the U.S. would remove the additive and require constant additive reinhibition. Conversely, outside the U.S. the additive is in general use and clay treaters are not. There are no government or industry performance specifications covering clay treatment vessels. Micronic Filters (also Microfilters or Prefilters) Another weapon in the cleanup arsenal consists of filter elements, usually made of pleated paper and mounted in a pressure vessel. These units remove only solid particles, but they do so at a much lower cost than filter-coalescers and are therefore placed ahead of filter-separators when significcmt solid contamination occurs. Nominal pore sizes rajiging generally from 0.3-2 microns are in widest use in jet fuel and from 0.5-5 microns for aviation gasoline, with the optimum pore size selected on the basis of operating experience. Here again, there are no government specifications for these units. A specification for microfilters dated April 2002 has been issued as API/IP 1590.
109
fuel usually being a water white color. Color rating methods include Saybolt color by D 156, or the more recent tri-stimulus method, D 6045. Either method is seriously handicapped by the fact that on-specification jet fuel can have a range of colors and color, as such, is not a jet fuel performance parameter. Only a change from the original fuel color is therefore useful as a possible indicator of contamination, which has to be ruled out by specification tests. A requirement peculiar to the U.S. involves the use of red dye to identify high sulfur diesel fuels and heating oils as well as untaxed diesel fuel and heating oil. While the dye is not required in jet fuel, a number of cases of jet fuel contaminated with red dye have occurred by the mixing of jet fuel with dyed product. Because of possible adverse effects on thermal stability, the use of dye-contaminated fuel in aircraft is not permitted. A Boeing service letter [40] specifies the use of white bucket appearance as the measure of fuel acceptability. A portable spectrometer capable of measuring dye levels quantitatively is under development in ASTM but is not currently approved. An extensive program is also underway at the Southwest Research Institute to establish acceptable dye concentrations in high temperature engine components [41]. Solid Particles
Filter Locations Although there are no industry standards for terminal installations, fuel receipts tend to go through a micronic filter into tankage. When going out of terminaJ tankage to an airport the fuel passes through em optional micronic filter, followed by a clay treatment vessel and last through a filter-separator. At the airport the minimum filtration is through a filter-separator into storage, a filter-separator out of storage into fueling trucks, or into a hydrant system and through a final filterseparator on the fueling vehicle into the aircraft. Filtration into storage may be supplemented by a micronic or a clay unit, while the final filtration unit may be a monitor in place of or in addition to the final filter-separator. Contamination Detection Equipment Much of the following equipment, while in extensive worldwide use, has not been standardized by ASTM, but is described in detail in ASTM Manual 5 [39]. Visual Appearance The appearance of jet fuel is routinely checked in either a one-liter jar made of clear glass or a white porcelain or stainless steel bucket. The glass is more likely to disclose the presence of fine, suspended material, while the bucket allows the examination of a larger sample easily taken from a large drain line. In addition, the white bucket has proven most sensitive to product color, which can indicate contamination with other darker or dyed fuels. However, such appearance checks are only a gross measure of possible contamination and do not disclose levels of solids or free water that are not visible to the neiked eye. More sensitive tests for contamination are described in the following sections. Product Color Quantitative color measurement can be used for the detection of other petroleum products having darker colors, jet
Solid particles are collected on special membrane filters made of mixed cellulose acetate esters with an absolute pore size of 0.8 microns. Filter membrane requirements are detailed in ASTM Research Report RR-D02-1012. For particle collection the membranes are mounted in plastic monitors that are encased in a brass housing. A fixed volume of fuel is bled through the membrane off a flowing line under pressure. The solids content of the sample can be established quantitatively by weighing the dried membrane or the dirt level can be described by comparing the membrane color to a standard chart. Both approaches are described in D 2276. Because of the normal variability in particle size, color, and density, there is no direct relationship between the two types of assessments. The mass measurement must be done under laboratory conditions, while the color rating is carried out in the field. Free Water A number of water detection methods, ranging from coarse to precise, are in general use. Water finding paste, which changes color on contact with a water layer, is smeared on gaging sticks or tapes to disclose the depth of the water layer in a storage tank or ship's compartment. This process is often called a "water cut." Suspended, free water is detected by several methods. Three methods used at plane-side include the Shell Detector, the Velcon Hydrokit, and the Metrocator test [39]. Each test is calibrated to show a visible color change at the presence of either 15 or 30 ppm of free water, these levels being called out in quality control procedures. These tests not only eliminate the operator judgement involved in visual examinations, but are also more sensitive than the eye of a trained operator who normally can detect a water haze down to the 50 ppm level. The most sensitive test for undissolved water is the Aqua-Glo test, D 3240, which can detect undissolved water down to 2-3 ppm. This test is based on the reaction of free water with a fluoresceine dye, which impregnates a fiberglass disk. After passing 500 mL of fuel through
110 MANUAL 37: FUELS AND LUBRICANTS
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the disk, einy increased level of fluorescence due to water is measured under ultraviolet light and is translated into ppm of free water. Water content can also be established in the laboratory by the Karl Fischer titration procedure, D 6304. However, this procedure measures total water content or the sum of dissolved and free water. Because aviation fuel filters do not remove dissolved water, measuring only free water by the preceding tests is a more effective way of checking the operation of filter-separators or monitors. Surfactants Surfactants, or more properly surface active agents, are molecules in which one portion is polar and is water soluble (hydrophilic). The other portion is non-polar and is oil soluble (hydrophobic). As a result surfactants affect the surface characteristics of fuel-water systems. Their concentration is highest at the interface between water and hydrocarbon and results in a decrease of the interfacial tension. Mixing the fuel-water-surfactant system will form emulsions, which can prevent small water droplets from settling out. In addition, surfactants can affect the surfaces of fibers in filter-coalescers and prevent them from coalescing small water droplets. Such cocdescers Eire considered disarmed. On the other hand, surfactants do not appear to affect monitor performance. Surfactants are widely used in the petroleum industry as corrosion inhibitors, lubricity agents, conductivity improvers, gasoline detergents, and cutting oil emulsifiers. While coalescers are designed to operate with certain specific jet fuel additives, which are surfactants, other surfactants will disarm the elements and have to be detected and removed. Of two available tests the less sensitive test, D 1094, measures the ability of water and fuel to separate cleanly after mild shaking. Because of its dependence on ultra-clean glassware this test should be performed in the laboratory. A much more sensitive test, D 3948, evaluates the ability of a miniature coalescer to remove very fine fuel droplets from a fuel-water emulsion. By being self-contained and utilizing disposable components this procedure can be conducted in the field and avoids possible contamination by sample containers. As already mentioned, surfactants are removed from jet fuel by clay treaters, which adsorb polcir materials. Microorganisms Many microorgstnisms can metabolize hydrocarbons, others can utilize sulfur. Some require oxygen, others need its absence. Mciny exist naturally in soil or ground water. All have one common characteristic—they must have undissolved, free water to grow and reproduce. As a result, most microbial growth is at the fuel-water interface. Metabolic products, usually acidic, tend to be corrosive to metals, particularly aluminum. These products may also act as surfactants. Microorganisms commonly form slimes or mats that can plug screens or filters. Unfortunately for jet fuel, the problem microbes prefer to metabolize normEil paraffins in the C12 and higher range [42], molecules that are a common constituent in jet fuels. The most reliable defense against microbicil growth is the elimination of free water in jet fuel systems. Frequent draining of all sumps and system low spots is sai important part of
such precautions. An alternative is the use of approved biocides mentioned earlier, but the possible adaptation of specific microbes to a biocide has to be kept in mind. The simplest form of detection is a visual and olfactory examination of water samples from jet fuel systems. Dark or black water, together with a foul or rotten odor, are usually indicative of microbial activity. Laboratory testing can indicate the viability and type of microorganisms, but system inspection is usually the best indicator of whether cleaning is required. Much more detailed information on the subject of microorganisms in hydrocarbon systems will be found in a new standard guide, D6469. Quality Control Procedures U.S. Procedures Similcir to aviation gasoline, jet fuel is fully certified against pertinent specifications at the time of manufacture. In the U.S. this is normally D 1655, Grade Jet A. Quality is often rechecked after arrivEil in the terminal with the tests listed in Table 11. However, this test lineup is experience-based and Ccin differ significantly from one company to einother; thus, some shippers require complete specification recertification of product introduced into their system. Product is settled in storage for one hour per foot of fuel depth [37]. The pressure drop across each filter is checked daily. Sumps are drained at least weekly and after receipt of product. Periodically, usually once a month, the proper operation of the filter-separator is checked by rurming a membrane test emd a free water check. Once approved and settled, the product is shipped to the airport by transport or pipeline. However, none of these operations are governed by industry standcirds and it is the responsibility of the operating company to furnish on-specification, "clear and bright" fuel to the airport. Airport quality control is more systematic. The Air Transport Association of America (ATA) has issued a specification, ATA 103, which is recommended for its member airlines or virtually EJI U.S. eind Canadian airlines. Additionally, member airlines can require the specification to be followed by independent airport biel dealers who supply these airlines. All jet fuel delivered to U.S. airports must meet ASTM D 1655 in order to meet the certification requirements of the various aircraft. As indicated in Table 12, when ATA 103 is to be followed, certain tests must be furnished by the shipper on the specific shipment. Other tests are conducted at the airport during and after product receipt. Additional periodic testing to be carried out at the airport is listed in Table 13. Failure to meet any of these requirements is cause for immediate investigation.
TABLE 11—Terminal receipt tests for jet fuel. Density by ASTM D 1298 or D 4052 Distillation by ASTM D 86 Flash point by ASTM D 56 or D 3828 Freezing point by ASTM D 2386, D 4305," D 5901, or D 5972 Existent gum by ASTM D 381 Water reaction by ASTM D 1094 Electrical conductivity (if required) by ASTM D 2426 "If D 4305 is used it must be accompanied by measuring viscosity at -20"'C by ASTM D 445.
CHAPTER It should be noted that ATA 103 includes other aspects of airport quality control as well, but these do not directly involve fuel quality. International
Procedures
Somewhat similar procedures are followed outside the U.S, the primary difference being the issuing authority. Here the procedures are being issued as guidelines by the Interna-
TABLE 12—Airport quality control per ATA 103. Airport Receipt Requirements Specification: ASTM D 1655, Jet A or A-1 Pipeline or Marine Delivery Truck or Rail Delivery Tests from Shipper
Visual-white bucket API Gravity @ 60°F (15°C) Distillation Flash point Freeze point Water reaction Copper corrosion Existent gum
Visual-white bucket API gravity ©eO^FClS^C)
FUELS
111
tional Air Transport Association (lATA) to be used by individual airline members as part of contractual requirements with suppliers. Ultimately, it is hoped to merge ATA 103 with the LATA Guidelines to create a single set of worldwide airport quality control procedures, but the success of this endeavor cannot be predicted at this time. Aircraft
Procedures
Small amounts of water can become a problem if the water is not removed from tank sumps. Older aircraft require a routine sumping procedure to remove the settled free water while newer aircraft have a water scavenging system that performs this function automatically. If small quantities of water are allowed to accumulate, boost p u m p inlet screens can ice over, biological growths can develop and, if large quantities are allowed to accumulate, exposure to engine flame-out is possible.
OVERVIEW AND CONCLUSIONS
Testing During Receipt at Airport
When: Beginning (after line displacement) Every 2 h Near end What: Visual-white bucket API gravity (within 1°) Membrane color 2 dry max/1 gsillon 3 wet max/1 gallon A msix particles Free water 15 ppm msix Flash point (multiproduct line only)
4: AVIATION
From compartment before unloading Visual-white bucket API gravity (within 1°)
After Receipt on Tank Contents
Visual-white bucket API Gravity @ eCF (15°C), within 1° Distillation, within 8°C Flash point, within SX" Freezing point, within 3°C° ''Variability allowed from supplier's certificate.
For safety reasons, specifications and handling procedures for aviation fuels are more tightly controlled than for other products. Minor changes in fuel properties, cleanliness, or contamination levels can have unanticipated effects on jet engine performance because of the complexity of m o d e m jet engines and the massive amounts of fuels that flow through the engines. The current aviation fuel supply and delivery system is based on accumulated experience, which has evolved over a long period of time. There is a reluctance to make changes because the impact of such changes cannot always be predicted without a thorough investigation. The challenge for the future will be for the aviation technical community to anticipate, identify, and eliminate possible fuel-related problems. These problems could arise from the incorporation of new non-traditional supplies, changes in refinery processing, the reformulation of transportation fuels resulting from tough, new environmental regulations, changes in the transportation of fuel to the aircraft, and new engine emd aircraft requirements. An additional challenge could come from measures required to improve aircraft safety. Resolution of prob-
TABLE 13—Airport fuel quality requirements per ATA 103, other required checks. Daily Required Checks Filter pressure drop Change elements at 15 psi Monitor pressure drop Change elements at 25 psi Drain filter sumps Inspect with white bucket—one gallon sEimple minimum Drain storage tank sumps Inspect with white bucket—one gallon sample minimum Drain fueling truck Inspect with white bucket—one gallon sample minimum Monthly Required Checks
Downstream each airport filter
Free water Membrane color Particulate assessment
15 ppm max 2 dry/gal or 3 wet/gal A max
Drain hydrant system low points Other Requirements
Filter element specification Monitor element specification Replace coalescer elements Replace monitor elements Inspect storage tanks Suggested settling time
API 1581, Group B (Group C for final vehicle) IP specification for monitors Yearly, with one year extension Every two years Yearly 1 h per ft (3 h per m) of vertical depth
112 MANUAL 37: FUELS AND LUBRICANTS
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lems can have an impact on legal issues such as the assignment of liability and can have large financial consequences. The complexities of the interactions between the technical, legal, and financial aspects associated with aviation fuels can therefore make it difficult at times to resolve problems, but such problems must be addressed to maintain and, if possible, improve the reliability of the entire system.
ASTM STANDARDS Unless otherwise indicated all of the following standards are current. No.
Title
D 56 D 86 D 130
Test Method for Flash Point by TAG Closed Tester Test Method for Distillation of Petroleum Products Test Method for Detection of Copper Corrosion From Petroleum Products by Cu Strip Tarnish Test Test Method for Saybolt Color of Petroleum Products Test Method for Vapor Pressure of Petroleum Products (Reid Method) Test Method for Kinematic Viscosity of Transparent cind Opaque Liquids Test Method for Knock Characteristics of Aviation fuels by the Aviation Method (discontinued 1970) Specification for Aviation gasoline (discontinued 1946) Test Method for Oxidation Stability of Aviation Fuels (Potential Residue Method) Test Method for Knock Characteristics of Aviation gasoline by the Supercharge Method Specification for Aviation Gasoline Guide for Petroleum Measurement Tables Practice for Density, Relative density (Specific Gravity) or API Gravity of Crude and Liquid Petroleum Products by Hydrometer Method Test Method for Hydrocarbon Types in Liquid Petroleum Products by Fluorescent Indicator Adsorption Test Method for Smoke point of Aviation Turbine Fuels Specification for Aviation Turbine Fuels Test Method for Luminometer Numbers of Aviation Turbine Fuels Test Method for Naphthalene Hydrocarbons in Aviation Turbine Fuels by Ultraviolet- Spectrophotometry Test Method for Freezing Point of Aviation Fuels Test Method for Color of Dyed Aviation Gasolines Test Method for Knock Characteristics of Motor and Aviation Fuels by the Motor Method Test Method for Mercaptan Sulfur in Gasoline, Kerosine, Aviation Turbine and Distillate Fuels (Potentiometric Method) Test Method for Undissolved Water in Aviation Turbine Fuels Test Method for Thermal Oxidation Stability of Aviation Turbine Fuels (JFTOT Method) Test Method for Estimation of Net Heat of Combustion of Aviation Fuels
D 156 D 323 D 445 D 614 D 618 D 873 D 909 D 910 D 1250 D 1298 D 1319 D 1322 D 1655 D 1740 D 1840 D 2386 D 2392 D 2700 D 3227 D 3240 D 3241 D 3338
D 3701 Test Method for Hydrogen Content of Aviation Turbine Fuels by Low Resolution Nuclear Magnetic Resonance D 3948 Test Method for Determining Water Separation Characteristics of Aviation Turbine Fuels by Portable Separometer D 4052 Test Method for Density and Relative Density of Liquids by Digital Density Meter D 4171 Specification for Fuel System Icing Inhibitor D 4305 Test Method for Filter Flow of Aviation Fuels at Low Temperatures D 4809 Test Method for Heat of Combustion of Liquid Hydrocarbon Fuels by Bomb Calorimeter (Precision Method) D 4865 Guide for Generation and Dissipation of Static Electricity in Petroleum Fuel Systems D 4952 Test Method for Qualitative Analysis of Active Sulfur Species in Fuels and Solvents (Doctor Test) D 5001 Test Method for Measurement of Lubricity of Aviation Turbine Fuels by the Ball-On-Cylinder Lubricity Evaluator (BOCLE Test) D 5191 Test Method for Vapor Pressure of Petroleum Products (Mini Method) D 5842 Practice for Sampling and Handling of Fuels for Volatility Measurement D 5901 Test Method for Freezing of Aviation Fuels (Automatic Optical Method) D 5972 Test Method for Freezing of Aviation Fuels (Automatic Phase Transition Method) D 6045 Test Method for Color of Petroleum Products by the Automatic Tristimulus Method D 6227 Specification for Grade 82 Unleaded Aviation Gasoline D 6304 Test Method for Determination of Water in Petroleum Products, Lubricating Oils, and Additives by Coulometric Karl Fisher Method D 6378 Test Method for Determination of Vapor Pressure (VPx) of Petroleum Products, Hydrocarbons and Hydrocarbon-Oxygenate Mixtures (Triple Expansion Method) D 6424 Test Method for Octane Rating Naturally Aspirated Spark Ignition Aircraft Engines D 6469 Standard Guide for Microbial Contamination of Fuels and Fuel Systems OTHER STANDARDS U.S. Military Aviation Turbine Fuel and Additive Specifications^ No.
Title
MIL-PRF-5624
Turbine Fuel, Aviation Grades JP-4, JP-5, and JP-5/JP-8 DT MIL-PRF-25017 Inhibitor, Corrosion/Lubricity Improver, Fuel Soluble MIL-DTL-27686 Inhibitor, Icing, Fuel System MIL-PRF-38219 Turbine Fuel, Low Volatility (Grade JP-7) ^ These standards are available from Defense Printing Service Detachment Office, 700 Robbins Ave., Piiiladelphia, PA 19111-5094.
CHAPTER 4: AVIATION FUELS 113 MIL-M-81380 MIL-DTL-83133
Monitor, Contamination, Aviation Fuel Dispensing System Turbine Fuel, Aviation, Kerosine Type (Grade JP-8)
U. K. M i l i t a r y A v i a t i o n T u r b i n e F u e l a n d A d d i t i v e Specifications These standards are available from Ministry of Defence, Directorate of Standardization, 65 Brown Street, Glasgow G2 SEX. No. Title ATF High Flash AVCAT/FSII ATF Wide Cut AVTAG/FSII AVGAS Gasoline Aviation 80 and lOOLL (low lead) Turbine Fuel, Aviation Kerosine Defence Standard 91-91 Type, Jet A-1 Defence Standard 91-251 Fuel System Icing Inhibitor (FSII) Defence Standsird 68-251 Fuel Soluble Pipeline Corrosion Lubricity Improving Additive Defence Standard 91-86 Defence Standard 91-87 Defence Standard 91-90
Industry Specifications API/IP 1581 Specifications and Qualification Procedures for Aviation Jet Fuel Filter/Separators API/IP Specification for Similarity for API/IP 1581 Aviation Jet Fuel Filter/Separators API/IP 1583 Specifications and Qualification Procedures for Aviation Fuel Filter Monitors with Absorbent T5rpe Elements API/IP Specifications and Qualification Procedures for Aviation Fuel Microfilters ATA 103, Standards for Jet Fuel Quality Control at Airports LATA Guidance Material for Aviation Turbine Fuels Quality Control and Operating Procedures, Incorporating Joint Inspection Group Guidelines for Joint Airport Depots and Joint into Plane lATA Guidance Material for Aviation Turbine Fuels
REFERENCES [1] Ogsten, A. R, "A Short History of Aviation Gasoline Development, 1903-1980," Preprint No. 810848, SAE, West Coast International Meeting, Seattle, WA, 3-6 August 1981, Society of Automotive Engineers, Warrendale, PA. [2] Dukek, W. G., "Aviation Fuel: Its Energy has Knit the World," Standardization News, ASTM International, West Conshohocken, PA, April 1998. [3] Defence Standard 91-91: Turbine Fuel, Aviation Kerosine, Type Jet A-1, Issue 3, Ministry of Defence, Directory of Standardization, Glasgow, FG2 SEX, United Kingdom, November 1999. [4] Guidance Material for Aviation Turbine Fuel, fourth edition. International Air Transport Association, Montreal, Canada, March 1999. [5] Winkle, T. L., Affens, W. A., Beal, E. A., Hazlett, R. N., and DeGuizeman, R., "The Distribution of Higher n-Alkanes in Partially Frozen Middle Distillate Fuels," NRL Report 8869, Naval Research Laboratory, Washington, DC, 10 April 1985. [6] Smith, M., Aviation Fuels, G. T. Foulis & Co Ltd, Henley-onThames, Oxfordshire, United Kingdom, 1970, pp. 272-278.
[7] Honsberger, B. A., "Jet A Exposure to Freezing on Very Cold Temperatures," Boeing Company CommercicJ Airplane Group, presented to CRC Group on Low Temperature Performance of Aviation Turbine Fuel, June 1991. [8] Unpublished minutes of the CRC Low Temperature Performance of Aviation Turbine Fuels, Coordinating Research Council, Atlanta, GA, May 1999. [9] Aviation Fuel Properties, CRC Report No. 530, Coordinating Research Council, Atlanta, GA, 1983. [10] Final Report, Fuel Tank Harmonization Working Group of Aviation Rulemaking Advisory Committee, Federal Aviation Administration, Washington, DC, July 1998. [11] Rogers, J. D., "Turbine Fuel Thermal Stability," SAE Preprint 103T, Society of Automotive Engineers, National Aeronautic Meeting, Los Angeles, CA, 5-9 Oct. 1959. [12] Thermal Oxidation Stability of Aviation Turbine Fuels, Monograph /, R. N. Hazlett, Ed., ASTM International, West Conshohocken, PA, 1991, pp. 72-91. [13] Heneghan, S. P., Zabamik, S., Ballal, D. R., and Harrison, W. E., Ill, "JP-8 + 100: TheDevelopmentof High-Thermal-Stability Jet Fuel," Transactions oftheASME, Vol. 118, September 1996. [14] Strauss, K. H., "Survey of Current Engine Conditions," CRC Report 573, Coordinating Research Council, Atlanta, GA, 1983. [15] Strauss, K. H., "Thermal Stability Testing of Jet Fuel - A Critical Review," Technical Paper 881532, Society of Automotive Engineers, Aerospace Technology Conference, Anaheim, CA, 3-6 Oct. 1988. [16] Baker, C, David, P., Taylor, S. E., and Woodward, A. J., "Thickness Measurement of JFTOT Tube Deposits by EUipsometry," Proceedings of 5th International Conference on the Stability and Handling of Liquid Fuels, Rotterdam, 1994, pp. 433^47. [17] Unpublished minutes of the Groups on Aviation Engine Octane Rating and Unleaded Avgas Development, Coordinating Research Council, Atlanta, GA, 1995. [18] Sherwood, W. D., "SST Fuels—A Major Airline Concern," Preprint 911C, Society of Automotive Engineers, National Aeronautic and Space Meeting, Los Angeles, CA, 5-9 Oct. 1964. [19] Gleason, C. C, Oiler, T. L., Shayeson, M. W., and Bahr, D. W., "Evaluation of Fuel Character Effects on FlOl Combustion System," AFAPL-TR-79-2018, General Electric Company, Cincinnati, OH, June 1979. [20] Lefebre, A. H., "Influence of Fuel Properties on Gas Turbine Combustion Performance," AFWAL-TR-84-2104, Purdue University, Lafayette, IN, January 1985. [21] Dickson, C. L., "Aviation Turbine Fuels, 1998," National Institute for Petroleum and Energy Research, NIPER-175 PPS, March 1998. [22] Blade, O. C, "Aviation Fuels, 1964," U.S. Dept. of Interior, Bureau of Mines, Petroleum Products Survey No. 39, 1964. [23] Rickard, G. K. and Fulker, R., "The Quality of Aviation Fuel Available in the United Kingdom Annual Survey 1997," Technical Report DERA/MSS1/TR980069/1.0, Version 1.0, Department of Defence, United Kingdom, May 1998. [24] Gibbons, T. R., "Flash Point Methods Applicable to Jet Fuel," Factors in Using Kerosine Jet Fuel of Reduced Flash Point, STP 688, ASTM International, West Conshohocken, PA, 1979. [25] Final Report, Fuel Taiik Harmonization Working Group of Aviation Rulemaking Advisory Committee, Federal Aviation Administration, Washington, DC, July 1998. [26] Smith, M., Aviation Fuels, G. T. Foulis & Co. Ltd., Henley-onThames, Oxfordshire, United Kingdom, 1970, pp. 369-371. [27] Hazlett, R. N., Thermal Oxidation Stability of Aviation Turbine Fuels, Monograph 1, ASTM International, West Conshohocken, PA, 1991, pp. 111-113. [28] Smith, M., Aviation Fuels, G. T. Fouhs & Co. Ltd., Henley-onThames, Oxfordshire, United Kingdom, 1970, pp. 117-118. [29] "Determination of the Hydroperoxide Potential of Jet Fuels," CRC Report 559, Coordinating Research Council, Atlanta, GA, 1988.
114 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK [30] Pande, S. G, Black, B. H., and Hardy, D. R., "Development of a Test Method for t h e Determination of the Hydroperoxide Potential and Antioxidant Effectiveness in Jet Fuel During Long Term Storage," CRC Report 614, Coordinating Research Council, Atlanta, GA, 1998. [31] "Aviation Fuel Lubricity Evaluation," CRC Report 560, Coordinating Research Council, Atlanta, GA, 1983. [32] Defence Standard 91-91: Turbine Fuel, Aviation Kerosine, Type Jet A-1, Annex B, Ministry of Defence, Directory of Standardization, Glasgow, FG2 8EX, United Kingdom. [33] Bustin, W. M. and Dukek, W. G., Electrostatic Hazards in the Petroleum Industry, Research Studies Press Ltd, Letchworth, England, 1983. [34] Strauss, K. H., "Future U.S. Jet Fuel—A Refiner's Viewpoint," AIAA-81-0770, AIAA/SAE ASCE 1981 International Air Transportation Conference, Atlantic City, 26-28 May 1981. [35] "Crude Assays," Oil and Gas Journal, April through August 1983
[36] Smith, M., Aviation Fuels, G. T. Foulis & Co. Ltd., Henley-onThames, Oxfordshire, United Kingdom, 1970, pp. 192-196. [37] Robertson, A. G., "Jet Fuel Settling," Shell Aviation News, Vol. 351, 1967, pp. 17-28. [38] Hughes, V. B., "Aviation Fuel Handling: New Mechanistic Insight into the Effect of Surfactants on Water Coalescer Performance," Proceedings of 6th International Conference on Stability a n d Handling of Liquid Fuels, Vancouver, 12-17 Oct. 1997. [39] Manual of Aviation Fuel Quality Control Procedures, Second Edition, ASTM Manual 57, R. Waite, Ed., ASTM International, West Conshohocken, PA, 1995. [40] "Aircraft Use of Dyed Fuel," Boeing Service Letter for 707, 727, 737, 747, 757, 767, and 777 Series, 22 Nov. 1996. [41] Unpublished minutes of the CRC Group o n Dye In Aviation Turbine Fuels, Coordinating research Council, Atlsinta, GA, 1996 to date. [42] Churchill, A. V., "Microbial Fuel Tank Corrosion: Mechanisms andContributoryFactors,"MaferJaisProtecft'ora, Vol. 2,1963, p. 18.
MNL37-EB/Jun. 2003
Automotive Diesel and Non-Aviation Gas Turbine Fuels Steven R. Westbrook^ and Richard LeCren^
The diesel engine is now fully established in a variety of applications on land and in marine use. On land, it serves to power trains, buses, trucks, and automobiles; to run construction, petroleum drilling, and other off-road equipment; and to be the prime mover in a wide range of power generation and pumping applications. At sea, it serves both to provide main propulsion power and run auxiliaries. Gas turbine engines also serve in a wide range of applications. Over half the larger industrial gas turbines are in electric-generation use. Other uses include gas pipeline transmission, co-generation systems, and transportation. In the military, gas turbines power a number of combatant ships both as main propulsion units and as the power source for auxiliary uses. Gas turbines are also used to power some military ground vehicles such as main battle tanks. The quality criteria and methods for testing fuels for land and marine diesel engines and for non-aviation gas turbines are sufficiently similar to address in a common chapter. Obviously, certain criteria and tests will apply to one or the other rather than both. For example, the cetane number, which is a critical property for diesel fuels, is of limited significance for gas turbine fuels. Conversely, sodium and vanadium content are important in assessing gas turbine fuels, but not those used in diesel engines. This chapter presents information regarding fuels for both automotive diesel engines and gas turbines. The petroleum industry started in the 1850s. Two factors were especially important as impetus. Machines were being developed and they needed oil for lubrication. Also, oil lamps were being used to light homes and offices and the whale oil traditionally burned in these lamps was growing increasingly expensive. James Young, a Scotsman, had recently patented a process for converting coal to coal oil. Coal oil was less expensive than whale oil but smoked and produced a foul smell when burned. Then, in 1857, lamp maker A.C. Ferris produced clean-burning kerosine that did not have a bad smell [1]. The oil used for making the kerosine was being skimmed from natural crude oil seeps until Colonel Edwin Drake drilled into one of the seeps, near Titusville, Pennsylvania, using a rig of the type used for drilling brine wells. An oil boom was bom. During the 1860s, oil exploration and drilling expanded to Ohio, Termessee, New York, Kentucky, Colorado, and Califor-
nia. From this time until about 1900, kerosine for oil lamps was the most valuable fraction of the petroleum barrel. The fraction now known as gasoline was considered surplus and burned. The heavier residual fraction was typically dumped into pits or other convenient dumping grounds. The middle distillate fraction, which would eventually be used in diesel engines, was used in the town gas industry. It was used "either as a source of domestic gas, or for enriching or carburetting water gas, or as an absorbing agent or 'wash oil' for removing condensable constituents from coal gas" [2]. This is the reason that this fraction is still often referred to as gas oil. Throughout this time, the middle distillate fraction of the petroleum barrel was primarily a leftover product with no leirge market. The invention of the diesel engine changed that for the better. Compression Ignition Engines
' Manager, Petroleum Products Technology, Southwest Research Institute, P.O. Drawer 28510, San Antonio, TX 78228. ^ Consultant Engineer, Gas Turbine Combustion & Fuels, Retired, 3760 Sioux Avenue, San Diego, CA 92117.
Since it's beginning in the 1890s, the diesel engine quickly became the engine of choice for many industrial and marine applications, applications where the larger, heavier, slower speed engine excelled in cost and performance when compared to alternative power plants of the time. However, the diesel engine saw only limited use in automotive and aviation applications. After World War I, shortages of gasoline in Germany helped stimulate work on the diesel engine, especially as an automotive engine. Diesel engines were used for propulsion on the Hindenburg. Suppljdng fuel to the engine was the major obstacle to a higher-speed, lower-weight engine suitable for automotive use. Compressed air injection was complicated and the required air pump could not be substantially reduced in either size or weight. In 1922, Robert Bosch set out to develop a fuel-injection system for diesel engines [3]. By early 1923, about a dozen different designs had been developed and initial trials were underway by mid-1923. A final design was approved in 1925 and the first series production fuel-injection pump was on the market. The fuel-injection system opened the door for wide spread use of the diesel engine in previously untried applications. By the mid-1930s there were large numbers of diesel-powered trucks and buses in service; but only small numbers of dieselpowered automobiles. By the beginning of World War II in 1939, the diesel engine was in widespread use as an automobile engine. The German military also used diesel engines for many of their vehicles, including some aircraft [4]. Gasoline engines, because of the logistics problems faced with a twofuel supply system, powered the Allied Forces almost exclusively. Following World War II; the greater fuel economy of
115 Copyright'
2003 by A S I M International
www.astm.org
116 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK
>. (0 Q
a.
m CD
m
0 J
1950
1960
1970
1980
1990
2000
Year Source: U.S. Energy Information Administration FIG. 1—Diesel fuel supplied to the transportation sector.
the diesel engine caused a rapid increase in the use of the engine for automotive applications throughout Europe. The abundance of inexpensive gasoHne in the United States lowered incentives for adopting the diesel engine in trucks and automobiles. Even still, throughout this period, the diesel engine climbed in popularity for use in railroad, marine, and heavy-construction equipment applications. The use of the diesel engine increased until virtually 100% of current marine, trucking, railroad, construction, and much of the stationary power generation applications operate on diesel engines. The fuels used in these engines have, over time, varied from natural gas to crude oil but the middle distillate, number 2 grade liquid fuel, is the fuel of choice for many of these applications. Figure 1 shows the increase in diesel fuel usage for transportation since 1949 [5].
DIESEL FUEL SPECIFICATIONS From the turn of the twentieth century until the mid-1930s there existed no widely used consensus specification for fuel for diesel engines. Those needing diesel fuel likely bought whatever was available on the market that seemed like a usable fuel. In 1934, ASTM Committee D2 published D 396, Standard Specification for Burner Fuels. This specification covers fuels for fuel-oil burning equipment (such as home heating oils). However, fuels of grades 1 and 2 in this specification would have probably been acceptable for most diesel engines of the time. D 396 was used in many purchase contracts as the controlling specification, thereby allowing both buyer and seller to agree on the minimum quality of the fuel. Starting in the eeirly to mid-1930s, the demand for diesel fuel began to rise at a steady rate in most of the industriEilized nations of the world. (During World War II, this demand slowed some owing to the essentially 100% use of gasoline by the Allied Forces.) Diesel engines were becoming increasingly sensitive to the quality of the fuel in order to maintain optimum engine performance.
Following World War II, it was recognized that D 396 did not sufficiently specify the quality of the fuel needed for newer diesel engines. D 396 contains no specification for the ignition quality of the fuel, as an example. So, in 1948, Committee D2 published D 975-48 T, Tentative Specifications for Diesel Fuel Oils. Fuel Grades When first published, D 975 contained specifications for "three grades of Diesel fuel oils suitable for various t5rpes of Diesel engines." The grades were No. 1-D, No. 2-D, and No. 4D. Table 1 provides a description, of each grade, from the 1948 standard and the 1996 standard. It is interesting to note that the description of Grade 2-D in the 1948 version did not include automotive applications, reflecting the fact that diesel engines were for heavy-duty use and large trucks. Fuel grades Low Sulfur No. 1-D and Low Sulfur No. 2-D were added in 1993. These latter grades were added to comply with 40 CFR Part 80—Regulation of Fuels and Fuel Additives: Fuel Quality Regulations for Highway Diesel Fuel Sold in 1993 and Later Calendar Years. The limiting properties included in the 1948 specification were flash point, pour point, water and sediment, carbon residue, ash, distillation, viscosity, sulfur, copper strip corrosion, and cetane number. Since then, pour point has been removed from the specification table. Other changes to the specification table have also occurred over the years. Table 2 is a summary of many of the major changes.
DIESEL FUEL PROPERTIES Table 3 contains the specific requirements for the fuels covered by D 975. The properties listed in that table, along with other selected properties of importance, are discussed below.
CHAPTER 5: GAS TURBINE FUELS TABLE 1—Descriptions of the fuels covered by Specification D 975. Grade
D 975-48 T
D 975-98b
No. 1-D
A volatile distillate fuel oil for engines in service requiring frequent speed and load changes.
A special-purpose, light distillate fuel for automotive diesel engines in applications requiring higher volatility than that provided by Grade No. 2-D fuels.
Low Sulfur No. 1-D
Not Applicable
A special-purpose, light distillate fuel for automotive diesel engines requiring low sulfur fuels and requiring higher volatility than that provided by Grade Low Sulfur No. 2-D.
No. 2-D
A distillate fuel oil of low volatility for engines in industrial and heavy mobile
A general-purpose, middle distillate fuel for automotive diesel engines, which is also suitable for use in non-automotive applications, especially in conditions of frequently varying speed and load.
Low Sulfur No. 2-D
Not Applicable
A general-purpose, middle distillate fuel for automotive diesel engines requiring low sulfur fuel. It is also suitable for use in non-automotive applications, especially in conditions of varying speed and load.
No. 4-D
A fuel oil for low and medium speed engines.
A heavy distillate fuel, or a blend of distillate and residual oil, for low- and medium-speed diesel engines in nonautomotive applications involving predominantly constant speed and load.
Density Density is the mass per unit volume of the fuel. This property is not specified in D 975. However density is a fundamental physical property that can be used in conjunction with other properties to characterize both the light and heavy fractions of petroleum cind petroleum products. Accurate determination of the density of petroleum products is also necessary for the conversion of measured volumes to volumes at the standard temperature of 15°C (60°F). While density is a factor governing the quality of crude petroleum, it is an uncertain indication of petroleum product quality unless correlated with other properties. The two methods most commonly used to measure density are: • ASTM D 1298, Standard Test Method for Density, Relative Density (Specific Gravity), or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method—The sample is brought to the prescribed temperature a n d transferred to a cylinder at approximately the same temperature. The appropriate hydrometer is lowered
117
into the sample and zJlowed to settle. After temperature equilibrium h a s been reached, the hydrometer scale is read, and the temperature of the sample is noted. If necessary, the cylinder and its contents may be placed in a constant t e m p e r a t u r e b a t h to avoid excessive t e m p e r a t u r e variation during the test. • ASTM D 4052, Standard Test Method for Density and Relative Density of Liquids by Digital Density Meter—^A small volu m e (approximately 0.7 mL) of liquid sample is introduced into an oscillating sample tube and the change in oscillating frequency caused by the change in the mass of the tube is used in conjunction with calibration data to determine the density of the sample. API gravity (D 1298) is another measure of the density of fuel that has been used for years. A fuel with a high API gravity is a low-density fuel and a low API gravity fuel is a high-density fuel. It is calculated with the following formula: API gravity, deg = (141.5/S)-131.5 Where 5 = density, kg/nr' at 15°C
TABLE 2—Summary of the major changes" to Specification D 975 since it was first published. Year Changes 1948 First published as a tentative standard. 1950 Changes to pour point and viscosity limits. 1953 Further clzirification of pour point and copper strip corrosion limits. 1959 Table 1 was revised to add a 90% distilled minimum temperature for Grade 2-D, and add a revised minimum viscosity at 100°F for Grade 2-D. 1960 The nomograph for the calculated cetane index method (then an appendix to the specification) was revised. 1964 Section 2 was added to specify that the properties of the fuel apply at the time and place of delivery. Section 4 was amended to permit use of Method D 56 as Ein alternate for Method D 93 to determine flash points of Grade 1-D fuels. 1965 The viscosity and 90% distillation requirements for Grade 2-D were revised. 1966 The sulfur limit for Grade 2-D was lowered from 1.0 wt% to 0.7 wt%. The cetane index method was removed from the appendix and became D 976. 1967 The standard was adopted without revision and was no longer a tentative method. 1968 The sulfur limit for Grade 2-D was lowered from 0.7 wt% to 0.5 wt%. 1973 The allowable amount of water and sediment in Grade 1-D was raised from trace to 0.05 volume percent. 1974 The cloud point requirement and the U.S. 10th percentile temperatures were added as an appendix. These are used to help set cloud point limits. 1978 The viscosity limits were lowered for all grades. 1988-1990 Several changes in format. Standard D 4737 replaced D 976 for the calculation of cetane index. 1992 Grades 1-D low sulfur and 2-D low sulfur were added to the table. A footnote on the use of blue dye in high sulfur fuel was also added to Table 1. 1994 An appendix on the lubricity of diesel fuel was added. 1996 The footnote regarding the use of blue dye was changed to reflect changes in the federal regulations, which now require red dye. "Not all changes are included. Editorial revisions and most of the changes to appendices were left oft this list.
118 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
TABLE 3—Detailed requirements for diesel fuel oils."
Property Flash Point, °C, min. Water and Sediment, % vol, max Distillation Temperature, °C 90 % % vol Recovered min max Kinematic Viscosity, mm^/S at 40°C min max Ash % mass, max Sulfur, % mass, max^ Copper strip corrosion rating mjix 3 h at SOX Cetane number, min'' One of the following properties must be met: (1) Cetane index, min. (2) Aromaticity, % vol, msix Cloud point, °C, max Ramsbottom carbon residue on 10% distillation residue, % mass, max
ASTM Test Method^
Grade Low Sulfur No. 1-D^
Grade Low Sulfur No. l-D"''
Grade No. 1-D'
Grade No. 2-0*'
38 0.05
52 0.05
38 0.05
52 0.05
D93 D2709 D 1796 D86
Grade No. 4-D' 55 0.50
282'^ 338
288
288
282^* 338
D445 D482 D 2622« D129 D130
1.3 2.4 0.01 0.05
1.9 4.1 0.01 0.05
No. 3
D613 916f 1319'" 2500 524
1.3 2.4 0.01
1.9 4.1 0.01
5.5 24.0 0.10
0.50 No. 3
0.50 No. 3
2.00
No. 3
40"
40"
40'
40'
40 35
40 35
;
;
0.15
0.
0.15
30'
0.35
"To meet special operating conditions, modifications of individual limiting requirements may be agreed upon between purchaser, seller, and manufacturer. ''The test methods Indicated are the approved referee methods. Other acceptable methods are Indicated in 3.1. "Under United States regulations. If Grades Low Sulfur No. 1-D or Low Sulfur No. 2-D are sold for tax exempt purposes then, at or beyond terminal storage tanks, they are required by 26 CFR Part 48 to contain the dye Solvent Red 164 at a concentration spectrally equivalent to 3.9 lbs per thousand barrels of the solid dye standard Solvent Red 26, or the tax must be collected. ''when a cloud point less than — 12°C is specified, the minimum flash point shall be 38°C, the minimum viscosity at 40°C shall be 1.7 mm^/s, and the minimum 90% recovered temperature shall be waived. 'Under United States regulations. Grades No. 1-D, No. 2-D, and No. 4-D are required by 40 CFR Part 80 to contain a sufficient amount of the dye Solvent Red 164 so Its presence Is visually apparent. At or beyond terminal storage tanks, they are required by 28 CFR Part 48 to contain the dye Solvent Red 164 at a concentration spectrally equivalent to 3.9 lbs per thousand barrels of the solid dye standard Solvent Red 26. '^Other sulfur limits can apply In selected areas in the United States and In other countries. ^These test methods are specified in 40 CFR Part 80. ''Where cetane number by Test Method D 613 Is not available, Test Method D 4737 can be used as an approximation. 'Low ambient temperatures as well as engine operation at high altitudes may require the use of fuels with higher cetane ratings. 'It is unrealistic to specify low temperature properties that will ensure satisfactory operation at all ambient conditions. In general, cloud point (or wax appearance point) may be used as an estimate of operating temperature limits for Grades Low Sulfur No. 1; Low Sulfur No.2; and No.l and No. 2 diesel fuel oils. However, satisfactory operation below the cloud point (or wax appearance point) may be achieved depending on equipment design, operating conditions, and the use of flow-improver additives as described in X4.1.2. Tenth percentile minimum air temperatures for U.S. locations are provided in Appendix X4 as a means of estimating expected regional temperatures. This guidance Is general. Some equipment designs or operation may allow higher or require lower cloud point fuels. Appropriate low temperature operability properties should be agreed upon between the fuel supplier and purchaser for the Intended use and expected ambient temperatures.
Even though, as use of the International System of Units (SI) becomes more common, use of API gravity decreases, API Gravity is arguably the most widely used cheiracteristic for "mass" of petroleum. Ignition and Combustion Characteristics (Cetane Number) The first diesel engines were large and slow-speed and were not particularly sensitive to the quality of the fuel they burned. As steady improvements were made to the engine, there was a need to improve fuel quality as well. Gradually, the heavier, more viscous, diesel fuels disappeared with lighter and higher speed engines. The higher speed engines are more sensitive to the ignition quality of the fuel; therefore, cetane numbers became the property of greatest concern to both producers and users. Diesel engine performance is a function of compression ratio, injection timing, the manner in which fuel and air are mixed, and the resulting ignition delay or time from the start of injection to the beginning of
combustion. The nature of the fuel is an important factor in reducing ignition delay. Physical characteristics such as viscosity, gravity, and mid-boiling point are influential. Hydrocarbon composition is also important as it affects both the physical and combustion characteristics of the fuel. Straightchain paraffins ignite readily under compression, but branched-chain peiraffins and aromatics react more slowly. The first widely used measure of ignition quality was the diesel index, calculated as: Diesel Index = [(API Gravity)(Aniline Point)] /100 The aniline point is the lowest temperature at which equal amounts of fuel and aniline are completely miscible. Aromatic compounds tend to exhibit higher density than paraffinic compounds and they are better solvents. Therefore, a fuel that is high in aromatics will generally have a lower aniline point than a fuel that is high in paraffins. So, a high paraffin fuel will tend to have a higher API Gravity and a higher aniline point. This combination results in a higher diesel index, which indicates better starting characteristics.
CHAPTER 5: GAS TURBINE FUELS The diesel index equation may be misleading, especially with blended fuels and fuels treated with ignition-improver additives. By the mid-1930s, it was determined that a better measurement of ignition quality was needed. The result was an engine test: • ASTM D 613 Standard Test Method for Ignition Quality of Diesel Fuels by the Cetane Method—The cetane number of a diesel fuel is determined by comparing its ignition quality with those for blends of reference fuels of known cetane numbers under standard operating conditions. Varying the compression ratio for the sample and each reference fuel to obtain a fixed "delay period," that is, the time interval between the start of injection and ignition does this. When the compression ratio for the sample is bracketed between those for two reference fuel blends differing by not more than five ceteine numbers, the rating of the sample is calculated by interpolation. This test involves operating a standard, single cylinder, variable compression ratio engine using a specified fuel flow rate and time of injection (injection advance) for the fuel sample and each of two bracketing reference fuels of known cetane number. The engine compression ratio is adjusted for each fuel to produce a specified ignition delay, and the cetane number is calculated to the nearest tenth by interpolation of the compression ratio values. The cetane number scale uses two primary reference fuels. One, n-hexadecane (normal cetane), has excellent ignition qualities and, consequently, a very short ignition delay. This fuel was arbitrarily assigned a cetane number of 100. The second fuel, a-methylnaphthalene, has poor ignition qualities and was assigned a cetane number of zero. The a-methylnaphthalene was later replaced with heptamethylnonane, which was calibrated against the original fuels and assigned a cetane number of 15. The cetane number scale is now defined by the following equation for volumetric blends of the two primary reference materictls: Cetane Number = % n-cetane -F 0.15 (percent heptamethylnonane) In practice, the primary reference fuels are only utilized to calibrate two secondary reference fuels. These are selected diesel fuels of mixed hydrocarbon composition, which are designated as "T" and "U". "T" fuel typically has a cetane number of approximately 75 while "U" fuel is usucilly in the low 20 cetane number range. Each set of "T" and "U" fuels are paired and test engine calibrations define the cetane numbers for volumetric blends of these two secondaries [6]. In general, the contribution of various fuel components to bulk fuel cetane number can be described as follows [7]: Normal Paraffins > Branched Paraffins > Normal Olefins > Branched Olefins > Naphthenes > Aromatics => Decreasing Cetane Number => Higher cetane number fuels tend to reduce combustion noise, increase engine efficiency, increase power output, start easier (especially at low temperatures), reduce exhaust smoke, and reduce exhaust odor. To assure acceptable cold weather performance, most modern diesel engines require a
119
minimum cetane number of 40 [8] and this is the requirement in D 975. Many engine manufacturers are now pushing to raise the specification minimum limit to 50 cetane in order to meet new engine emission requirements (see the discussion of the Worldwide Fuels Charter below). Cetane Index Two ASTM test methods for calculating approximate cetetne numbers were also developed for situations when performing the engine test was not feasible. Two ASTM methods, and the equations for each, are: • ASTM D 976, Standard Test Method for Calculated Cetane Index of Distillate Fuels Calculated Cetane Index = 454.74 - 1641.416 D + 774.74 0^-0.554 5 + 97.803 (log 5)^ Where: D = density at 15°C, g/mL, determined by Test Method D 1298, and B = mid-boiling temperature, °C, determined by Test Method D 86 and corrected to standard barometric pressure. • ASTM D 4737, Standard Test Method for Calculated Cetane Index by Four Variable Equation CCI = 45.2 + (0.0892)(rioN) + [0.131 + (0.901)B][r50N] + [0.0523 - (0.420)B][r90N] + [0.00049][(rioN)^ - (T'OTN)^] + 1075 -H 605^ Where: CCI - Calculated Cetane Index by Four Variable Equation, D = Density at 15°C, determined by Test Method D 1298, DN =D- 0.85, B
= [e(-3-5XDN)] _ 1^
Tio = 10% recovery temperature, °C, determined by Test Method D 86 and corrected to standard barometric pressure, TioN — Tio — 215, Tso = 50% recovery temperature, °C, determined by Test Method D 86 and corrected to standard barometric pressure, T50N = Tso ~ 260, Tgo = 90% recovery temperature, °C, determined by Test Method D 86 and corrected to standard barometric pressure, TgoN = T90 — 310 Both methods utilize fuel density and distillation values in their calculations. Standard D 4737 is the more widely used method since it is the most recent and it better represents current diesel fuels. Some limitations of calculated cetane index include: 1. It is not applicable to fuels containing additives for raising cetane number. 2. It is not applicable to pure hydrocarbons, synthetic fuels, alkylates, or coal tar products. 3. Correlation is fair for a given type of fuel but breaks down if fuels of widely different composition are compared. 4. Appreciable inaccuracy in correlation may occur when used for crude oils, residuals (or blends containing residuals), or products having end points below 260°C.
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Numerous other cetane index equations/models have been developed, used, and compzired to the ASTM equations but they have never been formaHzed as a n ASTM standard. Additional information concerning some of these non-ASTM methods can be found elsewhere [9-11].
Volatility / Distillation The distillation characteristics of a diesel fuel exert a great influence on performance. Two methods are commolily used to measure distillation characteristics: • ASTM D 86, Standard Test Method for Distillation of Petroleum Products—^A 100 mL sample is distilled u n d e r prescribed conditions that are appropriate to its nature. Systematic observations of thermometer readings and volumes of condensate are made, and from these data, the results of the test are calculated and reported. • ASTM D 2887, Standard Test Method for Boiling Range Distribution of Petroleum Fractions by Gas Chromatography— The boiling range distribution determination by distillation is simulated by the use of gas c h r o m a t o g r a p h y . A non-polar packed or open t u b u l a r (capillary) gas chromatographic column is used to elute the hydrocarbon components of the sample in order of increasing boiling point. The column temperature is raised at a reproducible lineeir rate and the area u n d e r the c h r o m a t o g r a m is recorded throughout the analysis. Boiling points are assigned to the time axis from a calibration curve obtained under the same chromatographic conditions by analyzing a known mixture of hydrocarbons covering the boiling range expected in the sample. From these data, the boiling range distribution can be obtained. Method D 86 is the specified method in D 975 for measuring distillation characteristics. Figure 2 is a plot of distillation data for a single, typical diesel fuel using both methods. It is obvious from this plot that the two methods can give quite different results. For most fuels, results like these are typical;
with more deviation at the beginning and end of the distillation Eind less deviation at the center. For this reason, D 2887 is not Ein accepted eJtemative to D 86 for specification purposes. That will remain the case until a suitable correlation (correction factor) between the two is determined. Figure 3 is the D 2887 gas c h r o m a t o g r a m for the fuel shown in Fig. 2. The chromatogram illustrates the usefulness of the D 2887 method. The time axis can be correlated to carbon n u m b e r by using a known standard. Therefore, it is possible to obtain a "picture" of the carbon n u m b e r distribution in a given fuel. This method is also useful for identifying contaminants such as gasoline a n d engine oil. The average volatility requirements of diesel fuels vary with engine speed, size, cind design. However, fuels having too low volatility tend to reduce power output Eind fuel economy through poor atomization, while those having too high volatility m a y reduce p o w e r o u t p u t a n d fuel economy through vapor lock in the fuel system or inadequate droplet penetration from the nozzle. I n general, the distillation range should be as low as possible without adversely affecting the flash point, burning quality, heat content, or viscosity of the fuel. If the 10% point is too high, poor starting may result. An excessive boiling range from 10-50% evaporated may increase warm-up time. A low 50% point is desirable to minimize smoke and odor. Low 90% cind end points tend to ensure low carbon residuals and minimum crankcase dilution. The temperature for 50% evaporated, known as the midboiling point, usucdly is taken as a n overall indication of the fuel distillation chziracteristics when a single numerical value is used alone. For example, in high-speed engines a 50% point above 302°C might cause smoke formation, produce objectionable odor, cause lubricating oil contamination, and promote engine deposits. At the other extreme, a fuel with excessively low 50% point would exhibit too low a viscosity eind heat content per unit volume. Therefore, a 50% point in the range of 232-280°C is desirable for the majority of higher speed diesel engines. This t e m p e r a t u r e range usually is broadened for Icirger, slower speed engines [12].
100
D86 D2887
-
80
I 40
on
20 -
0-" 100
150
200
250
300
350
400
450
Distillation Temperature, °C FIG. 2—Distillation curves for a typical no. 2 diesel fuel (methods D 86 vs. D 2887).
CHAPTER 5: GAS TURBINE FUELS
Time FIG. 3—D 2887 gas chromatogram of a typical diesel fuel.
For the above reasons, some points on the distillation curve are considered more important and are included in fuel specifications more often. Specification D 975 contains only a limit on the 90% point. Other specifications include requirements for initial boiling point (more so for gasoline), 10%, 50%, and to a lesser degree, 95% and final boiling point. Viscosity The method for measuring viscosity of diesel fuel is: • ASTM D 445, Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (and the Calculation of Dynamic Viscosity)—The time is measured for a fixed volume of liquid to flow under gravity through the capillary of a calibrated viscometer under a reproducible driving head and at a closely controlled and known temperature. The kinematic viscosity is the product of the measured flow time and the calibration constant of the viscometer. The unit of measurement for this method is square millimeters per second (mm^/s), also known as centistokes (cSt). This is currently the most widely used unit of fuel viscosity measurement in the United States. Other units used in the past, and occasionally still used in the present, include Saybolt Universal Seconds (SUS) and centipoise. For some engines it is advantageous to specify a minimum viscosity because of power loss due to injection pump cuid injector leakage. Maximum viscosity, on the other hand, is limited by considerations involved in engine design and size, and the characteristics of the injection system. Fuel viscosity exerts a strong influence on the shape of fuel spray. High viscosities can cause poor atomization, large droplets, and highspray jet penetration. With high viscosities, the jet tends to be a solid stream instead of a spray of small droplets. As a result, the fuel is not distributed in, or mixed with, the air required for burning. This results in poor combustion, accompanied by loss of power and economy. In small engines, the fuel spray may impinge upon the cylinder walls, washing away the lubricating oil film and causing dilution of the crankcase oil. Such a condition contributes to excessive wear [13]. Low fuel viscosity results in a spray that is too soft and does not penetrate far enough in the combustion chamber for
121
good mixing. Combustion is impaired and power output and economy are decreased. Low viscosity can lead to excessive leakage past the injection pump plunger. Fuel metering becomes inaccurate and engine efficiency is reduced. Wear of the fuel system components may increase because lubricating properties of fuels tend to decrease with viscosity. Fuel viscosities for high-speed engines range from 1.8-5.8 centistokes (cSt) at 38°C. Usually the lower viscosity limit is established to prevent leakage in worn fuel injection equipment as well as to supply lubrication for injection system components in certain types of engines. During operation at low-atmospheric temperatures, the viscosity limit sometimes is reduced to 1.4 cSt at 38°C to obtain increased volatility etnd sufficiently low pour point. Fuels having viscosities greater than 5.8 cSt usually are limited in application to the slowerspeed engines. The very viscous fuels commonly used in large stationary and mcirine engines normally require preheating for proper pumping, injection, and atomization. Cloud Point All diesel fuels contain dissolved paraffin wax. As the temperature of the fuel decreases, so does the solubility of the wax in the fuel. At some point wax crystals will begin to precipitate. If enough wax precipitates the crystals can block fuel flow through screens, filters, and other restricted passages in the fuel system. The temperature at which the wax precipitation occurs depends upon the origin, type, refining, and boiling range of the fuel. This temperature is known as the cloud point of the fuel. As the cloud point goes up, the suitability of the fuel for low-temperature operation decreases. The cloud point of the fuel can be measured by the following methods: • ASTM D 2500, Standard Test Method for Cloud Point of Petroleum Products—The specimen is cooled at a specified rate and examined periodically. The temperature at which a cloud is first observed at the bottom of the test jar is recorded as the cloud point. The following methods are variations of D 2500, including, both automatic and automated methods: • ASTM D 3117, Standard Test Method for Wax Appearance Point of Distillate Fuels • ASTM D 5771, Standard Test Method for Cloud Point of Petroleum Products (Optical Detection Stepped Cooling Method) • ASTM D 5772, Standard Test Method for Cloud Point of Petroleum Products (Linear Cooling Rate Method) • ASTM D 5773, Standard Test Method for Cloud Point of Petroleum Products (Constant Cooling Rate Method) For all grades of fuel listed in D 975, any of these test methods may be used to measure cloud point. Method D 2500 is the specified method and the others are considered alternates. In case of dispute. Test Method D 2500 is the referee method. Cloud point is the only control for low temperature operability listed in the diesel fuel specification, D 975. Footnote J to Table 1 in D 975 reads as follows: It is unrealistic to specify low temperature properties that will ensure satisfactory operation at all ambient conditions. In general, cloud point (or wax appearance point) may be used as an estimate of operating temperature limits for Grades Low Sulfur No. 1: Low Sulfur No. 2; and No.
122 MANUAL 3 7: FUELS AND LUBRICANTS
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October—10th Percentile Minimum Temperatures FIG. 4—Example 10th percentile minimum ambient temperature map from D 975.
1 and No. 2 diesel fuel oils. However, satisfactory operation below the cloud point (or wax appearance point) may be achieved depending on equipment design, operating condition, and the use of flow-improver additives... Tenth percentile minimum air temperatures for U.S. locations are provided.. .as a means of estimating expected regional temperatures. This guidance is general. Some equipment designs or operation may allow higher or require lower cloud point fuels. Appropriate low temperature operability properties should be agreed upon between the fuel supplier and purchaser for the intended use and expected ambient temperatures. The tenth percentile temperatures Eire presented for each U.S. state, except Hawaii, for each month from October to March. They are presented in both tabular format and in maps. Figure 4 is an example of one of the maps. Realistic options to reduce the cloud point include: 1. Dilute the fuel with a low wax fuel such as grade No. 1 or kerosine. 2. Treat the fuel with special additives. 3. Refine the fuel from crude(s) with lower wax content or refine the fuel to a lower end point. 4. It is also possible to add insulation and heaters to the vehicle fuel system. This will not change the cloud point of the fuel but will improve the low temperature operation.
Pour Point Before a fuel can be burned in an engine it must first be pumped from the fuel tank. The lowest temperature at which a fuel can be pumped is known as the pour point of the fuel. Test methods for measuring pour point include: • ASTM D 97, Standard Test Method for Pour Point of Petroleum Products—After preliminEiry heating, the sample is cooled at a specified rate and examined at intervals of 3°C for flow characteristics. The lowest temperature at which movement of the specimen is observed is recorded as the pour point. The following methods are variations of D 97, including, both automatic and automated methods. • ASTM D 5949, Standard Test Method for Pour Point of Petroleum Products (Automatic Pressure Pulsing Method) • ASTM D 5950, Standard Test Method for Pour Point of Petroleum Products (Automatic Tilt Method) • ASTM D 5985 Standard Test Method for Pour Point of Petroleum Products (Rotational Method) The pour point should be considered only a guide to the lowest temperature at which a fuel can be used. In general, pour points are from 3-6°C below the cloud point for a given fuel; however, it is not uncommon for the difference to be as much as 11 °C.
CHAPTER 5: GAS TURBINE FUELS For any given fuel, there will be no wax precipitation problems at temperatures above the cloud point. At temperatures below the pour point, it is highly unlikely that the fuel will give satisfactory performance. It is not unusueJ to obtain satisfactory engine performance with a fuel at ambient temperatures between the cloud point and pour point. The degree of performance and the temperature depend on the engine, the vehicle design, and the fuel system configuration. Vehicles and fuel systems with small diameter lines, constrictions, small porosity strainers and filters, and fuel lines exposed to ambient temperatures or wind will tend toward poorer performance. Systems with insulation, supplemental heaters, or large sections of the fuel system in close proximity to engine heat can probably expect better performance at lower temperatures [14]. Low-Temperature Flow Test As discussed above, the mere presence of wax crystals in a fuel does not guarantee the fuel will plug filters or other fuel system components. The tendency of a fuel to plug screens and filters at low temperatures is a dynamic property dependent on the size and shape of the wax crystals. (Vehicle fuel system design is also a factor as discussed earlier.) For this reason, numerous dynamic tests for low temperature operability have been developed. ASTM standardized one such test in 1985: • ASTM D 4539, Standard Test Method for Filterability of Diesel Fuels by Low-Temperature Flow Test (LTFT)—The temperature of a series of test specimens of fuel is lowered at a prescribed cooling rate. Commencing at a desired test temperature and at each 1°C interval thereafter, a separate specimen from the series is filtered through a 1 l-fjun screen until a minimum LTFT pass temperature is obtained. The minimum LTFT pass temperature is the lowest tempera-
ture, expressed as a multiple of 1°C, at which a test specimen can be filtered in 60 s or less. Alternatively, a single specimen may be cooled and tested at a specified temperature to determine whether it passes or fails at that temperature. Figure 5 is a diagram of the LTFT test apparatus. The LTFT was designed to yield results indicative of the low temperature flow performance of the test fuel in some diesel vehicles. The test method is especially useful for the eveiluation of fuels containing flow improver additives. Please refer to Report No. 528 from the Coordinating Research Council for a more detailed discussion [15]. The LTFT was developed in the United States to simulate the low temperature behavior of diesel fuel in the fuel tank of a diesel truck left overnight, in a cold environment, with its engine off. J. E. Chandler, zmd others, have demonstrated that this test has shown excellent correlation with field studies [16-18]. In spite of the demonstrated correlation to field performance, the LTFT is not as widely used as some other tests. The primary reason for this is the slow cooling rate (1°C per hour), which means that the time required completing an analysis can range from 12-24 h. Such test times are generally impractical for routine fuel testing. Cold FUter Plugging Point The Cold Filter Plugging Point, CFPP, was developed for use in Europe. The Institute of Petroleum publishes the method under the designation IP 309. It is similar to the LTFT with the following exceptions: • The fuel is cooled by immersion in a constant temperature bath, making the cooling rate non-linear but comparatively much more rapid (about 40°C per hour). • The CFPP is the temperature of the sample when 20 mL of the fuel first fails to pass through a wire mesh in less than 60s.
Glass Tubing 6 10 Min Throughout Vacuum Gage Point A
^
Heavy Rulit>er Ttibing 4 10 15 OD 2 Vacuum System Rubber Stopper
Storage Lid
200 ML Sample Glass Stem Quality Flex lObing
Sample Container 300 ML Tall-Form Beaker
123
Sample Receiver 400 ML Tall-Form Beaker
All Diminensions in Millimeters
t-TFT Sample Filtration Assembly FIG. 5—LTFT sample filtration assembly (D 4539).
124 MANUAL 37: FUELS AND LUBRICANTS
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While the CFPP is the preferred method in Europe and is used in several European specifications, it appears to overestimate the benefit of using some additives, most especially for vehicles manufactured in North America [19,20]. Cleanliness Diesel fuel cleanliness can mean many things to many people. It is safe to say that most users would consider any fuel that is visually free of undissolved water, sediment, and suspended matter a clean fuel. Indeed, this is the cleanliness (workmanship) requirement stated in D 975. However, it is known that microscopic particulates in the fuel can lead to problems just as serious as the visible contaminants. The three most common methods of measuring cleanliness of diesel fuel are: • ASTM D 2709, Standard Test Method for Water and Sediment in Middle Distillate Fuels by Centrifuge—A 100-mL sample of the undiluted fuel is centrifuged at a relative centrifugal force of 800 for 10 min. at 21-32°C (70-90°F) in a centrifuge tube readable to 0.005 mL and measurable to 0.01 mL. After centrifugation, the volume of water and sediment that has settled into the tip of the centrifuge tube is read to the nearest 0.005 mL and reported as the volumetric % water and sediment by centrifuge. D 2709 is the method currently specified in D 975. It is used to measure the amount of visible water, sediment, and suspended matter. This method gives no effective measurement of the presence or amount of microscopic particulates. Many would consider the level of contamination sufficient to produce readable results in this test to be gross contamination. However, this is an extremely sensitive test for contamination. The human eye is capable of seeing very small macroscopic particles and the presence of 1 or 2 such particles could be considered a failure. In practice, the person conducting the test must exercise judgment based on experience and the requirements of the end use for the fuel. • ASTM D 4860, Standard Test Method for Free Water and Particulate Contamination in Mid-Distillate Fuels (Clear and Bright Numerical Rating)—Visual inspection of the fuel sample for free water and particulate matter is performed immediately when the sample is taken. A glass container is used to view for water haze, and the fuel sample is swirled to create a vortex to detect the presence of particulate matter. A numerical rating for free water is obtained by filtering a portion of the fuel sample at a programmed rate (50 mL/45 s) through a standard fiberglass coalescer/filter. A portion of the effluent is used to establish a reference (100) level by a light transmittance measurement. Another portion of the unprocessed (unfiltered) fuel sample is then compared to the 100 reference-level. The results are reported on a 50-100 scale to the nearest whole number. A test can be performed in 5-10 min. D 4860 was developed to give a means to quantify the level of cleanliness of a fuel. As with D 2709, D 4860 only provides a valid measurement of visible contamination. The benefit of this method is that the measurement is now an objective measure of cleanliness. While this test does provide a quantitative result, it is far less sensitive than D 2709. Three other methods used to determine cleanliness of diesel fuels are D 6217, D 2068, and D 6426. D 6217 could be
considered a combination of the best features of the two previous methods. It is sensitive to small amounts of particles and is even capable of detecting microscopic particulates. It is also a quantitative measure of the cleanliness of the fuel. The quantitative measure is milligrams of particulate (contamination) per liter of fuel. Currently there is no consensus standard with a specification limit for D 6217. Many user specifications, including some federal and military diesel fuel specifications, include a limit of 10 mg per liter for the results of this analysis. Most users have found fuel that meets this limit to give satisfactory performance in the vehicle. • ASTM D 6217, Standard Test Method for Particulate Contamination in Middle Distillate Fuels by Laboratory Filtration—^A measured volume of about 1 L of fuel is vacuum filtered through one or more sets of 0.8 /u,m membranes. Each membrane set consists of a tared nylon test membrane and a tared nylon control membrane. When the level of particulate contamination is low, a single set will usually suffice; when the contamination is high or of a nature that induces slow filtration rates, two or more sets may be required to complete filtration in a reasonable time. After the filtration has been completed, the membranes are washed with solvent, dried, and weighed. The particulate contamination level is determined from the increase in the mass of the test membranes relative to the control membranes, and is reported in units of glw? or its equivalent mg/L. • ASTM D 2068, Standard Test Method for Filter Plugging Tendency of Distillate Fuel Oils—A sample of the fuel to be tested is passed at a constant rate of flow (20 mL/min) through a glass fiber filter medium. The pressure drop across the filter is monitored during the passage of a fixed volume of test fuel. If a prescribed maximum pressure drop is reached before the total volume of fuel is filtered, the actual volume of fuel filtered at the time of maximum pressure drop is recorded. The primary weakness of visual and gravimetric method is that there is no generally accepted correlation between the results of the test and the performance of the fuel in a vehicle fuel system. That is, how long could the vehicle operate on that fuel before the fuel filter plugs or the water separator fails? The British Navy first developed D 2068 as a dynamic test of the cleanliness of fuel for shipboard gas turbine engines [21]. The test was designed around the specific requirements of gas turbine powered ships in the British Navy. The most important requirement was that the shipboard fuel filters had a nominal porosity of 1 /am. As such, a 1 /u,m pore size glass fiber laboratory filter is used in the test. Over the years the test apparatus was upgraded, making it more automated. • ASTM D 6426, Standard Test Method for Determining Filterability of Distillate Fuel Oils—^A sample is passed at a constant rate (20 mL/min) through a standard porosity filter medium. The pressure drop across the filter and the volume of filtrate are monitored. The test is concluded either when the pressure drop across the filter exceeds 170 kPa (25 psi) or when 300 mL have passed through the filter. Results are reported as either the volume that has passed through the filter when a pressure of 170 kPa (25 psi) has been reached or the pressure drop when 300 mL have passed through the filter. In the latter case, the volume, if and when 105 kPa (15 psi) was exceeded, is also recorded.
CHAPTER 5: GAS TURBINE FUELS D 6426 is a modification of Method D 2068. The first difference between the two methods is the pump used in each: D 2068 uses a piston pump whereas D 6426 uses a peristaltic pump. The second difference that D 2068 uses a 13-mm diameter, 1 fim pore size fiher and D 6426 uses a specially constructed test specimen, which is called an F-cell Filter Unit. It is a disposable, precalibrated assembly consisting of a shell and plug containing a 2 5-mm diameter nylon membrane filter of nominal 5.0/am pore size, nominal 60% porosity, with a 17.7-mm^ effective filtering area. Figure 6 is a schematic diagram of the complete test apparatus. Despite the differences in equipment, both D 2068 and D 6426 have the same pass/fail criteria. A fuel fails the test if the pressure drop across the filter reaches 105 kPa (15 psi) before 300 mL of test fuel passes through the filter.
(D)
FLOW ADJUSTMENT KNOB
DISPLAYS PRESSURE VOLUME VOLUME (psi) ((Si 0-25 psi) ((a! 0-15 psi)
PRESSURE TRANSDUCER PERISTALTIC PUMP \ PULSE \ DAMPENrNG /lECIIANISM
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PLA,STIC TIP TYGON® TUBING RUBBER STOPPER (VENTED) FILTER CELL
METAL TUBE (STAINLESS STEEL) TUEL RESERVOIR
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FUEL FLOW COLLECTION CONTAINER
PROCESSED FUEL
FIG. 6—Schematic diagram of filterability apparatus (D 6426).
At this writing, neither D 2068 nor D 6426 enjoys widespread use in the U.S. However, of the two methods, D 6426 is gaining the greatest acceptance in the U.S. This is due to two primary reasons: 1) the equipment to conduct D 6426 is more readily available than the equipment for D 2068; and, 2) the F-cell test specimen is regarded as more representative of automotive fuel filters than is the filter in D 2068. Despite the general recognition that the F-cell is more representative of automotive fuel filters, numerous attempts to correlate the results from D 6426 with fuel filter plugging/filter life have failed [22].
Stability For the purposes of this discussion, fuel stability is defined as the resistance of the fuel to physical and chemical changes brought about by the interaction of the fuel with its environment. The chemistry of diesel fuel stability (instability) is complex. In general, it involves the chemical reaction of precursor species to form molecules of higher molecular weight. The higher molecular weight species then become insoluble deposits in the form of either gums or particulates. The precursor species are usually sulfur and/or nitrogen containing compounds, olefins, and organic acids. The reactions are often initiated by the oxidation of the precursors. Some dissolved metals, especially copper but also zinc and even iron, are known to catalyze the oxidation reactions. In the case of copper, as little as 10-30 parts per billion of dissolved copper is sufficient to catalyze deleterious reactions, depending on the fuel. The mechanism proposed by Pedley [23] (shown in Fig. 7) is perhaps the most cited example of these types of reactions. Although this mechanism explains many of the earlier findings on diesel stability, it does not account for the for-
oxldatlon
Phenalenes
125
1 Phenalenones
Indoles
Acid
Indolylphenalenes
Acid/Oxidation
Fuel Insolubles
FIG. 7—Proposed mechanism of diesel fuel insolubles formation.
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m a t i o n of all diesel fuel sediments. Other_useful reaction schemes have been proposed [24-27]. The solvent strength of the fuel is also a factor in that a high-solvency fuel can keep some of the high molecular weight species in solution. Because the reactions and interactions Eire so numerous and complex, it is quite possible to blend two very stable fuels and obtain a very unstable fuel. In this case, each fuel possesses one or more of the required precursor compounds. When the fuels are mixed, the reactions proceed, and sediment is formed. Yet, had these two fuels never been blended, neither single fuel would have significantly degraded in storage. There are three tjrpes of stability usually of concern for diesel fuel. They are thermal, oxidation, and storage. Each of these will be discussed separately.
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T h e r m a l stability is the resistance of the fuel to change caused by thermal stress (elevated temperature). The recognized ASTM test method is: • D6468, Standard Test Method for High Temperature Stability of Distillate Fuels—Two 50-mL volumes of filtered middle distillate fuel are aged for 90 or 180 min at 150°C in open tubes with air exposure. After aging and cooling, the fuel samples are filtered. The amount of filterable insolubles is estimated by measuring the light reflectance of the filter pads. An unused filter pad and a commercial black standard define the 100 and 0% extremes of the reflectance rating range, respectively. This method is based on a test that has been known by several names, such as: the Nalco 300°F Test, the EMD-Diesel Fuel Stability Test, the Union Pacific Diesel Blotter Test, the du Pont F 21 Test, and the Octel F 21 test [28]. This test differs from most other accelerated stability tests in that the test temperature is much higher and the amount of insolubles formed is estimated rather than measured gravimetrically. In the earlier versions of the test, the a m o u n t of insoluble residue formed during aging was estimated by comparing the test filter to a set of visual stEuidards called reference blotters. Filter pads from the test were rated on a scale from 1 to 20 with a rating of 1 assigned to a filter pad that is essentially free of sediment. A rating of 7 or less was typically considered a pass. The reference pads were produced in both gray and brown-tone prints. The reference pads were also produced to represent samples that were filtered through a Biichner funnel or through a standard m e m b r a n e filter holder. The reference blotters were generally considered satisfactory to rate test filters that had sediment similar in appearance (color and shape) to that depicted on the reference blotter. However, the visual ratings were subjective and different raters had been found to rate the same pad as everything from a 4 to an 11. Also, raters within the same laboratory tended to train each other, thus compounding the possibility for error. Henry [29] described a rating technique (originally proposed by Chevron Oil Company) t h a t used a reflectance m e t e r rather than a visual comparison. Figure 8 shows the relationship of visual and reflectance pad ratings. As expected, the reflectance meter method yielded greatly improved precision compared to the reference blotters. The reflectance meter was the technique selected when the method was standardized [30].
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MULTILABORATORY VISUAL RATINGS
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VISUAL RATINGS IN ONE LABORATORY
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PERCENT REFLECTION (PHOTOVOLT MODEL 670 REFLECTION HETER WITH SEARCH UNIT W)
FIG. 8—Relationship of visual and reflection pad ratings.
Arguably, this test is the most often used method to monitor/predict fuel stability. The primary reason for its popularity is the short test time and simple equipment requirements. This has made the 150°C test, in one form or another, especially popular with many pipeline companies and others with the need to monitor the quality of fuel but do it rapidly. Stavinoha and Westbrook demonstrated that this test has poor correlation to the generally accepted test for storage stability of diesel fuel (see D 4625 below) [31]. Bacha [32] and Henry [33] reported that the 150°C test provides an indication of thermal stability of distillate fuels exposed to high temperatures. The test method can be useful for investigation of operational problems related to fuel thermal stability. When D 6468 is used to m o n i t o r fuel in production or storage, a change in filter rating can indicate a relative change in inherent stability of the fuel. No quantitative relationship exists between pad ratings and the gravimetric mass of filterable insolubles formed during the test. Additional information on the interpretation of results is found in Appendix XI of the test method.
Oxidation Stability Oxidation stability is the resistance of the fuel to change under severely oxidizing conditions. In addition to exposing the fuel to excess amounts of oxygen, oxidation stability tests usueJly incorporate elevated test temperatures to accelerate reaction rates. The method commonly used to measure oxidation stability of diesel fuel is: • ASTM D 2274, Standard Test Method for Oxidation Stability of Distillate Fuel Oil (Accelerated Method)—^A 350-mL volu m e of filtered middle distillate fuels is aged at 95°C (203°F) for 16 h while oxygen is bubbled through the sample at a rate of 3 L/h. After aging, the sample is cooled to ap-
CHAPTER proximately room temperature before filtering to obtain the filterable insolubles quantity. Adherent insolubles are then removed from the oxidation cell and associated glassware with trisolvent. The trisolvent is evaporated to obtain the quantity of adherent insolubles. The sum of the filterable and adherent insolubles, expressed as milligrams per 100 mL, is reported as total insolubles. Because for m a n y years this was the only standardized method to measure stability of middle distillate fuels, it became a part of n u m e r o u s fuel specifications (mostly government/military) and was widely used as a predictor of fuel storage stability. Despite the wide acceptance as a storage stability test, most researchers agreed that, in fact, the results of D 2274 had very poor correlation to actual, ambient storage of the fuel. In addition, the precision of the test was extremely poor. Many researchers have worked to improve the test. Chief a m o n g these is E. W. White of the U.S. Navy [34,35]. Some of the factors he identified as having an effect on the results of the test are: the purity of the oxygen, stray light impinging the sample during aging (especially ultra-violet light), contact with metal surfaces (especially copper), filter drying time, and heating bath configuration. Based on the work of White and others, many needed improvements were made to the method. The precision of the test is better now than when it was first published by ASTM. However, despite the improvements, the results continue to have poor correlation to ambient storage. Because this poor correlation is now more widely known, the use of the method to assess the stability of diesel fuels is very low.
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127
TABLE 4—Factors that influence the quality of diesel fuel in storage. Can lead to tank corrosion, Presence of Water microbiological growth, and entrained water. Can accelerate or slow chemical Storage temperature reactions in the fuel. (ambient temperature) Fixed roof tanks allow greater tank Vented, fixed roof tank vs. floating roof tank breathing, which promotes water condensation, oxygen replenishment in the ullage, and increases airborne contaminants in the fuel. Lined vs. unlined storage tanks
Lined tanks (those with an interior coating) reduce or prevent exposure of the fuel to metal surfaces. This reduces corrosion of the tank. Metal surfaces can also catsdyze some fuel degradation reactions.
Microbiological contamination
Leads to degradation of the fuel by metabolic by-products, corrosion of metallic storage tanks, and emulsification of water and fuel.
Agitation of the fuel through tank filling and/or circulation of the fuel Crude oil source and refining techniques
Promotes increased concentrations of air/oxygen and water in the fuel. These factors influence the chemical composition of the fuel.
Commingling of fuels
The mixing of two stable fuels can, on rare occasions, result in antagonistic chemical reactions.
Storage Stability Storage stability is generally defined as the fuel's resistance to change during storage at ambient temperature and conditions. Ambient storage is storage of the fuel in drums, storage tanks, vehicle fuel tanks, or similar containers in the out-ofdoors. There are many factors that can influence the storage stability of a given fuel. The most c o m m o n factors are listed in Table 4. The two tests for storage stability are: • ASTM D 4625, Standard Test Method for Distillate Fuel Storage Stability at 43°C (nO°F)—Four-hundred mL volumes of filtered fuel are aged by storage in borosilicate glass containers at 43°C (110°F) for periods of 0, 4, 8, 12, 18, and 24 weeks. After aging for a selected time period, a sample is removed from storage, cooled to room temperature, and analyzed for filterable insolubles and for adherent insolubles. • ASTM D 5304, Standard Test Method for Assessing Distillate Fuel Storage Stability by Oxygen Overpressure—A 100-mL aliquot of filtered fuel is placed in a borosilicate glass container. The container is placed in a pressure vessel that has been preheated to 90°C. The pressure vessel is pressurized with oxygen to 800 kPa (absolute) (100 psig) for the duration of the test. The pressure vessel is placed in a forced air oven at 90°C for 16 h. After aging and cooling, the total amount of fuel insoluble products is determined gravimetrically and corrected according to blank determinations. All petroleum fuels u n d e r g o chemical reactions during storage. The results of these reactions are usually g u m s , acids, and/or particulates. The n u m b e r and rate of these deleterious reactions depends on the concentrations of reaction precursors, the concentration of oxygen available for oxida-
tion reactions, the presence of catalytic species such as metals, the a m o u n t of light, and the storage temperature. As mentioned above, the resistance of the fuel to these reactions is defined as its storage stability. Most researchers cJso go a step further and define inherent storage stability as the resistance to change in the absence of environmental factors such as metals and contaminants (i.e., in the absence of catalytic effects). The inherent stability is important because it is a measure of the useful storage life of the fuel. For most diesel fuels under normal storage conditions,it may take several months or even years for the deleterious reactions to generate enough by-products to cause fuel-related p r o b l e m s . A user w h o would like to store fuel would like to know what that length of time is prior to putting the fuel in storage. To predict the storage stability, therefore, we must simulate the storage and measure the quantities of insolubles formed. To accelerate the process, the test must be performed at an elevated temperature. The question is how high above ambient to conduct the test so as to accelerate the reactions and still keep the results representative of storage at ambient temperature. The Arrhenius [36] equation described in Fig. 9 can be used to estimate the reaction rates for fuels stored at ambient to slightly above ambient temperatures. Under this relationship, the reaction rate is accelerated approximately 1.7-3 times for every 10°C rise in temperature. The problem is that, as the temperature rises, reactions that would not normeJly occur during ambient storage (or would occur with very low probability) now begin to occur. The high-temperature reactions result in other types of compounds, which can skew the
128
MANUAL
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k = Ae'^''
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HANDBOOK
k=A,J^
where: Ea = activation energy R = gas constant T = absolute temperature A = frequency factor (collision frequency) Taking the natural logarithm of both sides and rearranging gives: lnfe=(-Ea/RXl/T) + lnA Therefore, a plot of In A: versus 1/T gives a straight line whose slope is equal to -Ea/R and whose intercept is In A FIG. 9—Arrhenius equation.
results of the aging test. Two early studies [37,38] demonstrated the correlation between storage at 43°C and ambient storage. They determined that 13 weeks at 43°C is equivalent to one year at ambient. Over the years, this relationship has been redefined as 1 week at 43 °C is equal to 1 month at ambient. These studies also established storage at 43°C as the standard accelerated storage test, against which all other accelerated storage stability tests are compared. This test works so well because the test temperature is such a small increase over ambient that the relative importance of the various reactions that create insolubles are not appreciably changed. This makes the 43°C storage test an ideal research tool for studying storage stability of diesel fuels. Ironically, that which makes the 43 °C test such a useful research tool also makes it useless for quality assurance and quzility control applications because of the long test times. For this reason, m u c h effort has been expended over the past 25-30 years to develop a rapid test for fuel storage stability. In this context, rapid is generally defined as test times of 16 hours or less. Very little success came of these efforts, with one exception. The U.S. Navy developed a test method that was eventucdly standardized as ASTM D 5304 \39'\. The applicability of the test method was demonstrated in a project conducted during 1992-1993 [40]. Flash Point Flash point is specified in D 975 primEuily for safety during transport, storage, and handling. A low flash point fuel can be a fire hazard, subject to flashing, with possible continued ignition and explosion. Low flash point can also indicate contamination with low-flash fuels such as gasoline. The flash point of a fuel has no significant relation to the performance of the fuel in the engine. Auto-ignition temperature is not influenced by variations in flash point. There are five ASTM test m e t h o d s for measuring flash point of diesel fuels: • ASTM D 56, Standard Test Method for Flash Point by Tag Closed Tester—The specimen is placed in the cup of the tester and, with the lid closed, heated at a slow constant rate. An ignition source is directed into the cup at regular interveJs. The flash point is taken as the lowest temperature at which application of the ignition source causes the vapor above the specimen to ignite.
• ASTM D 93, Standard Test Methods for Flash Point by Pensky-Martens Closed Cup Tester—^A brass test cup of specified dimensions, filled to the inside msirk with test specimen and fitted with a cover of specified dimensions, is heated and the specimen stirred at specified rates by either of two defined procedures (A or B). An ignition source is directed into the test cup at regular intervals with simultaneous interruption of the stirring until a flash is detected. • ASrM D 3828, Standard Test Methods for Flash Point by Small Scale Closed Tester Method A—Flash/No Flash Test— A specimen of a sample is introduced by a syringe into the cup of the selected apparatus that is set and maintained at the specified temperature. After a specific time a test flame is applied and an observation made as to whether or not a flash occurred. • Method B—Finite (or Actual) Flash Point—^A specimen of a sample is introduced into the cup of the selected apparatus that is maintained at the expected flash point. After a specified time a test flame is applied and the observation made whether or not a flash occurred. The specimen is removed from the cup, the cup cleaned, and the cup temperature adjusted 5°C (9°F) lower or higher depending on whether or not a flash occurred previously. A fresh specimen is introduced and tested. This procedure is repeated until the flash point is established within 5°C (9°F). The procedure is then repeated at 1°C (2°F) intervals until the flash point is determined to the nearest 1°C (2°F). If improved accuracy is desired the procedure is repeated at 0.5°C (1°F) intervals until the flash point is determined to the nearest 0.5°C (1°F). • ASTM D 6450, Standard Test Method for Flash Point by Continuously Closed Cup (CCCFP) Tester—The lid of the test chamber is regulated to a temperature at least 18°C below the expected flash point. A 1 mL test specimen of a sample is introduced into the sample cup, ensuring that both specimen and cup are at a temperature at least 18°C below the expected flash point, cooling if necessary. The cup is then raised and pressed onto the lid of specified dimensions to form the continuously closed but unsealed test chamber with an overall volume of 4.0 ± 0.2 mL. After closing the test chamber, the temperatures of the test specimen and the regulated lid are edlowed to equilibrate to within 1°C. Then the lid is heated at a prescribed, constant rate. For the flash tests, an arc of defined energy is discharged inside the test chamber at regular intervals. After each ignition, 1.5 ± 0.5 m L of air is introduced into the test chamber to provide the necessary oxygen for the next flash test. The pressure inside the continuously closed but unsealed test chamber remains at ambient barometric pressure. The exceptions are for the short time during the air introduction and at a flash point. After each arc, the instantaneous pressure increase above the ambient barometric pressure inside the test chamber is monitored. When the pressure increase exceeds a defined threshold, the temperature at that point is recorded as the uncorrected flash point. Each of these three test methods contains an introduction that is similar to the following: This flash point test method is a dynamic test method and depends on definite rates of temperature increase to control the precision of the test method. The rate of heating may
CHAPTER not in all cases give the precision quoted in the test method because of the low thermal conductivity of certain materials. To improve the prediction of flammability, Test Method D 3941, an equilibrium method was developed in which the heating rate is slower. This allows the vapor above the test specimen and the test specimen to be at about the same temperature. • ASTM D 3941, Standard Test Method for Flash Point by the Equilibrium Method with a Closed-Cup Apparatus—This method covers the determination of the flash point of hquids in which the specimen a n d the air/vapor mixture above it are approximately in temperature equilibrium. The test method is limited to a temperature range from 32-230°F(0-110°C). Method D 3941 should b e used to measure Eind describe the properties of material, products, or assemblies in response to heat and flame u n d e r controlled laboratory conditions. It should not be used to describe or appraise the fire hazard or fire risk of materials, products or assemblies under actUcJ fire conditions. However, results of this test may be used as elements of a fire risk assessment that takes into account all of the factors that are pertinent to a n assessment of the fire hazard of a particular end use. It is important to note that while D 3941 may be an acceptable alternative m e t h o d for some applications, the method(s) listed in a given specification, such as D 975, should not be replaced without prior knowledge and consent of all interested parties. This is because the measured flash point of a fuel is dependent not only on the composition of the fuel but also on the apparatus used to make the measurement. ASTM Manual 9 describes many flash point test methods, both U.S. and international [41]. It also lists pertinent specifications/standards worldwide. Lubricity Diesel fuel functions as a lubricant in certain items of fuel injection equipment such as rotary/distributor fuel p u m p s and injectors. In limited cases, fuel with very specific properties can have insufficient lubricating properties that can lead to a reduction in the normal service life of fuel p u m p s or injectors. Two fuel chciracteristics that affect equipment wear are low viscosity and lack of sufficient quantities of trace components, which have a n affinity for metal surfaces. If fuel viscosity meets the requirements of a particular engine, a fuel film is maintained between the moving surfaces of the fuel system components. This prevents excessive metal-to-metal contact and avoids premature failure due to wear. Simileirly, certain surface-active molecules in the fuel (such as acids and heteroatomics) adhere to, or combine with, metallic surfaces to produce a protective film that also can protect surfaces against excessive wear. The concern about fuel lubricity is limited to the use of fuels with viscosities lower than those specified for a particulcir engine. Also of concern is the use of fuels that have been processed in a manner that results in the elimination of the trace levels of the surface-active species that act as lubricating agents. Presently, the only fuels of the latter type shown to have lubricity problems resulted from sufficiently severe processing to reduce aromatics substantieJly below current levels. Research is in progress to identify the chciracteristics of such fuels and where the use of a lu-
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129
bricity improver additive is required, to ensure satisfactory operation in the sensitive areas of the vehicle fuel system [42]. Work in the area of diesel fuel lubricity is ongoing by several organizations, such as the International Organization for Standardization (ISO) and the ASTM Diesel Fuel Lubricity Task Force u n d e r Subcommittee E of Committee D2 [43]. These groups include representatives from the fuel injection equipment manufacturers, fuel producers, and additive suppliers. The charge of the ASTM task force has been the recommendation of test methods and a fuel specification for Specification D 975. At this writing, two test methods are approved: • ASTM D 6078, Standard Test Method for Evaluating Lubricity of Diesel Fuels by the Scuffing Load Ball-on-Cylinder Lubricity Evaluator (SLBOCLE)—^A 50-mL test specimen of fuel is placed in the test reservoir of an SLBOCLE and adjusted to the test temperature of 25°C. When the fuel temperature has stabilized, 50% relative humidity air is used to aerate the fuel at 0.5 L/min while 3.3 L/min flows over the fuel for 15 min. During the remainder of the test sequence, the 50% relative humidity air flows over the fuel at a rate of 3.8 L/min. A load a r m holding a non-rotating steel ball and loaded with a 500 g mass is lowered until it contacts a partially fuel immersed polished steel test ring rotating at 525 rpm. The ball is caused to rub against the test ring for a 30s break in period before beginning an incremental-load or a single-load test. Wear tests are conducted by maintaining the beJl in contact with the particJly immersed 525- rpm test ring for 60 s. For incremental load tests, the test ring is moved at least 0.75 m m for each new load prior to bringing a new ball into contact with the test ring. The tangential friction force is recorded while the ball is in contact with the test ring. The friction coefficient is calculated from the tangential friction force. In the incremental-load test, the m i n i m u m applied load required to produce a friction coefficient greater than 0.175 is an evaluation of the lubricating properties of the diesel fuel. In the single-load test, a friction coefficient less than or equal to 0.175 indicated the diesel fuel passes the lubricity evaluation, while a friction coefficient greater than 0.175 indicated the diesel fuel fails the lubricity evaluation. • ASTM D 6079 Standard Test Method for Evaluating Lubricity of Diesel Fuels by the High-Frequency Reciprocating Rig (HFFR)—^A 2-mL test specimen of fuel is placed in the test reservoir of an HFRR and adjusted to either of the staxidard temperatures (25 or 60°C). The preferred test temperature is 60°C, except where there may be concerns about loss of fuel because of its volatility or degradation of the fuel because of the temperature. When the fuel temperature has stabilized, a vibrator arm holding a non-rotating steel ball and loaded with a 200-g mass is lowered until it contacts a test disk completely submerged in the fuel. The ball is caused to r u b against the disk with a 1-mm stroke at a frequency of 50 Hz for 75 min. The ball is removed from the vibrator a r m and cleaned. The dimensions of the major and minor axes of the wear scar are measured under lOOX magnification and recorded. A diagram of the test apparatus a n d a list of the test conditions for D 6078 and D 6079 are given in Figs. 10 and 11, respectively.
130 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK OUTFUTTO rmcTioM necxMUMNO DEVICE
LOAD . " ^ Q
P'O.
3 ^ ^ COtfTACT U>Ap
MEAW-OUTT FNCUMATIC PWTOM S c h e m a t i c D i a g r a m of t h e Scuffing L o a d Ball-on-Cylinder Lubricity Evaiuator (not including Instrumentation)
MANDREL ASSEMBLY •
SCREWS W REQ'a BUTTON HEAD ••8-32
DRIVESHAFT ASSEMBLY
Ring and Mandrel Assembly (Cylinder) Parameter
Test Conditions Value
Fluid volume 50 ± 1.0 mL Fluid temperature 25 ± 1°C Conditioned air^ 50 ± 1 % relative tiumidity at 25 ± 1°C Fluid pretreatment: 0.50 U min air flowing through and 3.3 L/min air flowing over the fluid for 15 min Fluid test conditions: 3.8 L/min air flowing over the fluid Cylinder rotational speed 525 ± 1 rpm Applied Load Break-in period 500 g Incremental-load test 500 to 5 000 g Single-load test user defined' Test Duration Break-in period 30 s Wear tests 60s *Fifty percent humidity should be achieved using equal volumes of dry and saturated air. The SLBOCLE has a water column through which air passes and it is assumed to be saturated when it exits this column. "The applied load for the single test Is set at the pass/fail requirement for the fuel being evaluated. FIG. 1 0 — T e s t e q u i p m e n t a n d t e s t c o n d i t i o n s f o r s c u f f i n g l o a d b a l l - o n cylinder lubricity e v a i u a t o r ( D 6 0 7 8 ) .
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131
Lubricity Task Force is also working on possible revisions to the standard HFRR test to make it more sensitive to low levels of lubricity additive.
Test Plata Loading
Aromatics
Schematic Diagram of HFRR (not including instrumentation) Test Conditions Fluid volume Stroke length Frequency Fluid temperature
Relative humidity Applied load Test duration Bath surface area
2 ± 0.20 mL 1 ± 0.02 mm 50 ± 1 Hz 25 ± 2°C or 60 ± 2°C >30% 200 ± 1 g 75 ± 0.1 min 6 ± 1 cm^
FIG. 11—Test equipment and test conditions for high frequency reciprocating rig (D 6079).
The inclusion of a single fuel specification in the main table of specification D 975 for Grade No. 2 requires further research because: 1. the correlation of the data among the two test methods and the fuel injection equipment needs further clarification, 2. both methods in their current form do not apply to all fueladditive combinations, 3. the reproducibility values for both test methods are large. In the meantime, the following information may be of use and serve as a general guideline to fuel suppliers and users. Westbrook and coworkers recommended that users monitor their fuel injection pumps for possible trends of abnormal wear rates if the fuel has a scuffing load value between 2000 and 2800 g in Test Method D 6078 [44]. According to this paper, fuels with values below 2000 g will, in all probability, cause accelerated wear in fuel lubricated rotary-t5Tpe fuel injection pumps. It should be noted that a properly-additized fuel might provide protection for fuel-wetted components and yet not produce significant D 6078 test results as compared to the non-additized fuel. Work at ISO indicates that a fuel with a 450-micron wear scar diameter or lower value at 60°C in Test Method D 6079 (380 micron at 25°C) should protect all fuel injection equipment [45]. Other SAE publications present data to show that some fuels and fuel/additive combinations can have values above this level and still provide sufficient lubricity protection to the equipment. Pump stand testing of fuels, although more expensive and time consuming, is a more accurate means of evaluating the lubricity of diesel fuel. At the time of this writing, the ASTM Diesel Fuel Lubricity Task Force is working on the development and standardization of a pump stand test method. The
Diesel fuel contains many types or classes of compounds including paraffins, naphthenes, olefins, and Eiromatics. Compounds that contain heteroatoms such as sulfur, nitrogen, and oxygen are also present. Aromatics warrant discussion because 1) they have an effect on the combustion quality of the fuel, 2) typically, they are the only hydrocarbon type listed in diesel fuel specifications (including D 975), and 3) increased amounts of aromatics can have a negative impact on vehicle emissions. It is well known that an increase in the total aromatics content of a diesel fuel can (and usually does) have an adverse effect on the ignition quality, i.e., cetane number of the fuel. Several methods are available for the measurement of aromatic content. They eire described below: • ASTM D1319, Standard Test Method for Hydrocarbon Types in Liquid Petroleum Products by Fluorescent Indicator Adsorption—Approximately 0.75 mL of sample is introduced into a special glass adsorption column packed with activated silica gel. A small layer of the silica gel contains a mixture of fluorescent dyes. When the entire sample has been adsorbed on the gel, alcohol is added to desorb the sample down the column. The hydrocarbons are separated in accordance with their adsorption affinities into aromatics, olefins, and saturates. The fluorescent dyes are also separated selectively, with the hydrocarbon types, and make the boundaries of the aromatic, olefin, and saturate zones visible under ultraviolet light. The volume percentage of each hydrocarbon type is calculated from the length of each zone in the column. This test method was originally developed for the analysis of gasoline (spark ignition engine fuel). It is for determining hydrocarbon types over the concentration ranges from 5-99 volume % aromatics, 0.3-55 volume % olefins, and 1-95 volume % saturates in petroleum fractions that distill below 315°C. The test method may apply to concentrations outside these ranges, but the precision has not been determined. Samples containing dark-colored components that interfere in reading the chromatographic bands cannot be analyzed. D 1319 was applied to and specified for diesel fuel usually because no other suitable method was available. As suitable methods became standardized they grew in use but have not replaced D 1319 in D 975. This is because the requirement for aromaticity currently included in D 975 comes from the requirement in 40 CFR Fart 80. Since federal law requires D 1319, it is the method listed in D 975. Method D 5186, described below, is more appropriate for diesel fuel and is often used in place of D 1319. However, in case of dispute, D 1319, by virtue of it's status as the legislated method, is considered the referee method. • ASTM D 5186, Standard Test Method for Determination of Aromatic Content and Polynuclear Aromatic Content of Diesel Fuels and Aviation Turbine Fuels by Supercritical Fluid Chromatography—A small aliquot of the fuel sample is injected onto a packed silica adsorption column and
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HANDBOOK
eluted using supercritical carbon dioxide mobile phase. Mono-aromatics and polynucleEir aromatics in the sample are separated from non-aromatics and detected using a flame ionization detector. The detector response to hydrocarbons is recorded t h r o u g h o u t the analysis time. The chromatographic areas corresponding to the mono-aromatic, polynuclear aromatic, and non-aromatic components are determined and the mass % content of each of these groups in the fuel is calculated by area normalization. This test method covers the determination of the total amounts of mono-ciromatic and polynuclear aromatic hydrocarbon compounds in motor diesel fuels, aviation turbine fuels, and blend stocks by supercritical fluid chromatography (SFC). The range of aromatics concentration to which this test method is applicable is from 1-75 mass %. The range of polynuclear aromatic hydrocarbon concentrations to which this test method is applicable is from 0.5-50 mass %. • ASTM D 6591, Standard Test Method for Determination of Aromatic Hydrocarbon Types in Middle Distillates, High Performance Liquid Chromatography Method with Refractive Index Detection—A known mass of sample is diluted in the mobile phase, a n d a fixed volume of this solution is injected into a high performance liquid chromatograph, fitted with a polar column. This column has little affinity for the non-aromatic hydrocarbons while exhibiting a pronounced selectivity for aromatic hydroccirbons. As a result of this selectivity, the aromatic hydrocarbons are separated from the non-aromatic hydrocarbons into distinct bands in accordcince with their ring structure. At a predetermined time, after the elution of the di-aromatic hydrocarbons, the column is back flushed to elute the polycyclic aromatic hydroccirbons as a single sharp band. Method D 2425 offers a more detailed analysis but requires considerable investment in i n s t r u m e n t a t i o n a n d sample preparation time. For these reasons, it is not typically used for routine ctnalysis of diesel fuel. • ASTM D 2425, Standard Test Method for Hydrocarbon Types in Middle Distillates by Mass Spectrometry—This test method covers an analytical scheme using the mass spectrometer to determine the hydrocarbon types present in virgin middle distillates 204-343°C (400-650°F) boiling range, 5-95 volume % as determined by Method D86. Samples with average carbon n u m b e r value of paraffins between C 12 and C 16 and containing petraffins from C I O and C 18 can be analyzed. Eleven hydrocarbon types are determined. These include: paraffins, non-condensed cycloparaffins, condensed di-cycloparaffins, condensed tricycloparaffins, alkylbenzenes, indans or tetraJins, or both, C„H n.io (indenes, etc.), naphthalenes, CnH n.14 (acenaphthenes, etc.), CnH „.i6 (acenaphthylenes, etc.), and tri-cyclic aromatics. Method D 5292 also offers more information than D 1319 or D 5186. However, the results are reported in mole% rather than mass or volume percent, which are normally required in specifications. • ASTM D 5292, Standard Test Method for Aromatic Carbon Contents of Hydrocarbon Oik by High Resolution Nuclear Magnetic Resonance Spectroscopy—This test method cov-
ers the determination of the aromatic hydrogen content (Procedures A and B) 8ind aromatic carbon content (Procedure C) of hydrocarbon oils using high-resolution nuclear magnetic r e s o n a n c e (NMR) spectrometers. Applicable samples include kerosines, gas oils, mineral oils, lubricating oils, coal liquids, and other distillates that are completely soluble in chloroform and Ccirbon tetrachloride at ambient temperature. For pulse Fourier-transform (FT) spectrometers, the detection limit is t5^ically 0.1 mol % aromatic hydrogen atoms a n d 0.5 mol % aromatic carbon atoms. For continuous wave (CW) spectrometers, which are suitable for measuring aromatic hydrogen contents only, the detection limit is considerably higher and typically 0.5 m o l % aromatic-hydrogen atoms. The reported units are mole percent, aromatic- hydrogen atoms and mole% aromatic-Ccirbon atoms. This test method is not applicable to samples containing more than 1 mass % olefinic or phenolic compounds. This test method does not cover the determination of the percentage mass of aromatic compounds in oils since NMR signals from both saturated hydrocarbons a n d aliphatic substituents o n aromatic ring compounds appear in the same chemical shift region. For the determination of mass or volume% aromatics in hydrocarbon oils, chromatographic or m a s s spectrometry methods can be used. It should be noted that there are several standard methods for the analysis of aromatics. Each method yields a slightly different result and each is considered appropriate in different situations. One reason for this apparent inconsistency is that since a single molecule can contain several chemical moieties, it is possible to include it in several hydrocarbon classes. For example, a molecule could contain an aromatic ring, a pciraffinic side chain, and a naphthenic ring. How should this molecule be classified? A hierarchy was established to address this situation. Under this hierarchy, aromatics are on top, then olefins, followed by naphthenes, and finally paraffin. Using this hierarchy, the example compound would be considered an aromatic compound. The level of aromatics in the fuel is also important as it relates to the potential for elastomer and seed swell problems. This is especially true for older vehicles/fuel systems. Depending on the type of elastomer, prolonged exposure to relatively high levels of aromatics, followed by a sudden decrease in the amount of aromatics, can cause elastomeric seals to shrink and thus leak. If the elastomers are too old and have taken a set, they Cctn also crack or break. This phenomenon was widely seen in late 1993 and early 1994 when mcindated reductions in fuel sulfur and aromatics content went into effect. In most instances, the problems were solved by installing new seals made of less sensitive elastomer. Heat Content The heat content or heat of combustion of a fuel is the amount of heat produced when the fuel is burned completely. Gross and net heats of combustion are the two values measured for the heat of combustion. The gross heat of combustion is the quantity of energy released when a unit mass of fuel is burned in a constJint volu m e enclosure, with the products being gaseous, other than
CHAPTER 5: GAS TURBINE FUELS 133 water that is condensed to the hquid state. The fuel can be either hquid or soUd, and contain only the elements carbon, hydrogen, nitrogen, and sulfur. The products of combustion, in oxygen, are gaseous carbon dioxide, nitrogen oxides, sulfur dioxide, and liquid water. • The net heat of combustion is the quantity of energy released when a unit mass of fuel is burned at constant pressure, with all of the products, including water, being gaseous. The fuel can be either liquid or solid, and contain only the elements carbon, hydrogen, oxygen, nitrogen, and sulfur. The products of combustion, in oxygen, are carbon dioxide, nitrogen oxides, sulfur dioxide, and water, all in the gaseous state. For c o m p o u n d s with the same n u m b e r of c a r b o n atoms, heat content increases as you go from aromatics to naphthenes to paraffins, on a weight basis. The reverse order is correct if you measure on a volume basis. The same is true for fuels. Denser fuels, such as diesel, have higher heat content on a volume basis. Less dense fuels, such as gasoline, have higher heat content on a weight basis. Chevron has reported typical heat content values that demonstrate this relationship (see Table 5). Heat of combustion is usually reported in units of megajoules per kilogram (MJ/kg). Conversion factors to other units are given in Table 6. Heat of combustion can be estimated by calculation from selected properties or measured using b o m b calorimetry. The m e t h o d s typically used for diesel fuel are discussed below. • ASTM D 4868, Standard Test Method for Estimation of Net and Gross Heat of Combustion of Burner and Diesel Fuels— This test method covers the estimation of the gross and net heat of combustion of petroleum fuel. The calculations use the fuel density, sulfur, water, and ash content. The equations for estimating net a n d gross heat of combustion used in this method were originally published in the National Institute of Standards and Technology (NIST) Publication No. 97. The equations are: Calculate the gross heat of combustion of the fuel corrected for the sulfur, water, and ash content in accordance" with the following equation: Q^ (gross) = (51.916
8.792^2 X 10- ') [1 (x+y
+s)] + 9.420s
where: Q
d X y 5
= gross heat of combustion at constant volume, MJ/kg, = density at 15°C, kg/m^, = mass fraction of water (% divided by 100), = mass fraction of ash (% divided by 100), eind = mass fraction of sulfur (% divided by 100).
TABLE 5—Typical density and heat content value of different fuels. Fuel
Density, g/cm^
Regular gasoline Premium gasoline Jet fuel Diesel fuel
0.735 0.755 0.795 0.850
Net Heat of Combustion, Btu/lb
18 18 18 18
630 440 420 330
Net Heat of Combustion, Btu/gal
114 116 122 130
200 200 200 000
TABLE 6—Conversion factors for heat of combustion values. 1 cal (International Table calorie) = 4.1868 J 1 Btu (British thermal unit) = 1055.06 J 1 cal (I.T.)/g = 0.0041868 MJ/kg 1 Btu/lb = 0.002326 MJ/kg
Calculate the net heat of combustion of the fuel corrected for the sulfur, water and ash content in accordance with the following equation: Qp (net) = (46.423 - 8.792^^ x 10"* + 3.70d X 10"^) X[l
- (x+y
+ s)] + 9.4205 + 2.449;c
where: Qp = net heat of combustion at constant pressure, MJ/kg, d = density at 15°C, kg/m^, X = mass fraction of water, y = mass fraction of ash, and s = mass fraction of sulfur. This test method is useful for estimating, using a minimum n u m b e r of tests, the heat of combustion of burner and diesel fuels for which it is not usually critical to obtain very precise heat determinations. This test method is purely empirical. It is applicable only to liquid hydrocarbon fuels derived by normal refining processes from conventional crude oil. This test method is valid for those fuels in the density range from 750 to 1000 kg/m^ and those that do not contain an unusually high aromatic content. High aromatic content fuels will not normally meet fuel specification criteria for this method. This test method is not applicable to pure hydrocarbons. It is not intended as a substitute for experimental measurements of heat of combustion. According to the m e t h o d the estimation of the heat of combustion of a hydrocarbon fuel from its density, sulfur, water, and ash content is justifiable only when the fuel belongs to well-defined classes for which a relationship between these quantities have been derived from accurate experimental measurements on representative samples of these classes. Even in these classes, the possibility that the estimate may be in error by large amounts for individual fuels should be recognized. This test method has been tested for a limited number of fuels from oil sand bitumen and shale oil origin and has been found to be valid. The classes of fuels used to establish the correlation presented in this test method are represented by the following specifications: 1. D 396 Fuel Oils Grades 1, 2, 4 (light), 4, 5 (hght), 5 (heavy), and 6 2. D 975 Diesel Grades 1-D, 2-D, and 4-D 3. D 1655 Aviation Turbine Jet A, Jet A-1, and Jet B 4. D 2880 Gas Turbine Grades 0-GT, 1-GT, 2-GT, 3-GT and 4GT 5. D 3699 Kerosine Grades 1-K and 2-K • ASTM D 4809, Standard Test Method for Heat of Combustion of Liquid Hydrocarbon Fuels by Bomb Calorimeter (Precision Method)—This test method covers the determination of the heat of combustion of hydroccirbon fuels. It was
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designed specifically for use with aviation turbine fuels when the permissible difference between duplicate determinations is of the order of 0.2%. It can be used for a wide range of volatile and nonvolatile materials where slightly greater differences in precision can be tolerated. Under normal conditions, the method is directly applicable to such fuels as gasoline, kerosine, Nos. 1 and 2 fuel oil, Nos. 1-D and 2-D diesel fuel and Nos. 0-GT, 1-GT, and 2-GT gas turbine fuels. The increased precision is obtained through the improvement of the CcJorimeter controls and temperature measurements. • ASTM D 240, Standard Test Method for Heat of Combustion of Liquid Hydrocarbon Fuels by Bomb Calorimeter—This test method covers the determination of the heat of combustion of liquid hydrocarbon fuels ranging in volatility from that of light distillates to that of residual fuels. Under normal conditions, this test method is directly applicable to such fuels as gasoline, kerosine, Nos. 1 and 2 fuel oil, Nos. 1-D and 2-D diesel fuel, and Nos. 0-GT, 1-GT, and 2GT gas turbine fuels. This test method is not as repeatable and not as reproducible as Test Method D 4809. In this method the net heat of combustion is represented by the symbol Q n and is related to the gross heat of combustion by the following equation: Q„ {net, 25°C) = Qg {gross, 25°C) - 0.2122 X H where: Q„ (net, 25°C) = net heat of combustion at constant pressure, MJ/kg Q„ (gross, 25°C) = gross heat of combustion at constant volume, MJ/kg H = mass % hydrogen in the sample
Total Sulfur The test methods for measuring total sulfur in diesel fuel, as prescribed in D 975 are: • ASTM D 2622, Standard Test Method for Sulfur in Petroleum Products by Wavelength Dispersive X-ray Fluorescence Spectrometry—D 2622 is prescribed for the measurement of total sulfur in Grades Low Sulfur No. 1-D and No. 2-D. This test method covers the determination of total sulfur in liquid petroleum products and in solid petroleum products that can be liquefied with moderate heating or dissolved in a suitable organic solvent. The applicable concentration range will vary to some extent with the instrumentation used and the nature of the sample. Optimum conditions will allow the direct determination of sulfur in essentially paraffinic samples at concentrations exceeding 0.0010 mass%. • ASTM D 129, Standard Test Method for Sulfur in Petroleum Products (General Bomb Method)—The sample is oxidized by combustion in a bomb containing oxygen under pressure. The sulfur, as sulfate in the bomb washings, is determined gravimetrically as barium sulfate. D 129 is the prescribed method for the determination of total sulfur in Grades No. 1-D, No. 2-D, and No. 4-D. This test
method covers the determination of sulfur in petroleum products, including lubricating oils containing additives, additive concentrates, and lubricating greases that cannot be burned completely in a wick lamp. The test method is applicable to any petroleum product sufficiently low in volatility that it can be weighed accurately in an open sample boat and containing at least 0.1% sulfur. • ASTM D 4294, Standard Test Method for Sulfur in Petroleum Products by Energy-Dispersive X-Ray Fluorescence Spectroscopy—This test method covers the measurement of sulfur in hydroccirbons such as naphthas, distillates, fuel oils, residues, lubricating base oils, and nonleaded gasoline. The concentration range is from 0.05-5mass%. • ASTM D 5453, Standard Test Method for Determination of Total Sulfur in Light Hydrocarbons, Motor Fuels and Oils by Ultraviolet Fluorescence—This test method covers the determination of total sulfur in liquid hydrocarbons, boiling in the range from approximately 25—400°C, with viscosities between approximately 0.2 and 10 cSt (mm/s^) at room temperature. This test method is applicable to naphthas, distillates, motor fuels and oils containing 1.0 to 8000 mg/kg total sulfur. • ASTM D 1266, Standard Test Method for Sulfur in Petroleum Products (Lamp Method)—This test method covers the determination of total sulfur in liquid petroleum products in concentrations from 0.01-0.4 mass %. A special sulfate analysis procedure is described in the method that permits the determination of sulfur in concentrations as low as 5 mg/kg. • ASTM D1552, Standard Test Method for Sulfur in Petroleum Products (High-Temperature Method)—This test method covers three procedures for the determination of total sulfur in petroleum products including lubricating oils containing additives, and in additive concentrates. This test method is applicable to samples boiling above 177°C (350°F) and containing not less than 0.06 mass% sulfur. Two of the three procedures use iodate detection. One employs an induction furnace for pyrolysis, the other a resistance furnace. The third procedure uses IR detection following p5Tolysis in a resistance furnace. The sulfur content of diesel fuel is known to affect particulate matter (PM) exhaust emissions because some of the sulfur is converted to sulfate particles in the exhaust. The amount that is converted varies by engine; but reducing total sulfur produces a linear decrease in PM in nearly all engines. Fuel sulfur can also adversely affect cylinder wear (through the formation of acids) and deposit formation (many sulfur compounds are known deposit precursors). Copper Strip Corrosion The test method for copper strip corrosion is D 130. • ASTM D 130, Standard Test Method for Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test—^A polished copper strip is immersed in a given quantity of sample and heated at a temperature and for a time characteristic of the material being tested. At the end of this period the copper strip is removed, washed, and compared with the ASTM Copper Strip Corrosion Standards (this is an adjunct available from ASTM Headquarters).
CHAPTER 5: GAS TURBINE FUELS 135 The copper strip corrosion test covers the detection of the corrosiveness to copper of aviation gasohne, aviation turbine fuel, automotive gasohne, natural gasoline, or other hydrocarbons having a Reid vapor pressure no greater than 124 kPa(18psi). Crude petroleum contains sulfur compounds, most of which are removed during refining. However, of the sulfur compounds remaining in the petroleum product, some can have a corroding action on various metals and this corrosivity is not necessarily related directly to the total sulfur content. The effect can vary according to the chemical types of sulfur compounds present. The copper strip corrosion test is designed to assess the relative degree of corrosivity of a petroleum product. It is very rare to find a commercially available diesel fuel that fails the D 130 test.
such as amyl nitrate, hexyl nitrate, or octyl nitrate, causes a higher carbon residue value than observed in untreated fuel, which can lead to erroneous conclusions as to the coke-forming propensity of the fuel. Test Method D 4046 can detect the presence of alkyl nitrate in the fuel. The carbon residue value of burner fuel serves as a rough approximation of the tendency of the fuel to form deposits in vaporizing pot-type and sleeve-type burners. Similarly, provided alkyl nitrates are absent (or if present, provided the test is performed on the base fuel without additive) the carbon residue of diesel fuel correlates approximately with combustion chamber deposits. The carbon residue value of gas oil is useful as a guide in the manufacture of gas from gas oil. In a gas turbine it can be an indication of the tendency to form carbon deposits in the combustor.
Carbon Residue
Ash
Carbon residue is the residue formed by evaporation and thermal degradation of a carbon containing material. The residue is not composed entirely of carbon but is a coke that can be further changed by carbon pyrolysis. The term carbon residue is retained in deference to its wide common usage.The test method for carbon residue, as listed in the diesel fuel specification is D 524. • D 524, Standard Test Method for Ramsbottom Carbon Residue of Petroleum Products—The sample, after being weighed into a special glass bulb having a capillary opening, is placed in a metal furnace maintained at approximately 550°C. The sample is thus quickly heated to the point at which all volatile matter is evaporated out of the bulb with or without decomposition while the heavier residue remaining in the bulb undergoes cracking and coking reactions. In the latter portion of the heating period, the coke or carbon residue is subject to further slow decomposition or slight oxidation due to the possibility of breathing air into the bulb. After a specified heating period, the bulb is removed from the bath, cooled in a desiccator, and again weighed. The residue remaining is calculated as a percentage of the original sample, and reported as Ramsbottom carbon residue. Provision is made for determining the proper operating characteristics of the furnace with a control bulb containing a thermocouple, which must give a specified time-temperature relationship. This test method covers the determination of the amount of carbon residue left after evaporation and pyrolysis of an oil, and is intended to provide some indication of relative coke-forming propensity. This test method is generally applicable to relatively nonvolatile petroleum products that partially decompose on distillation at atmospheric pressure. Petroleum products containing ash-forming constituents as determined by Test Method D 482 will have an erroneously high carbon residue, depending upon the amount of ash formed. Values obtained by this test method are not numerically the same as those obtained by Test Method D 189, or Test Method D 4530. Approximate correlations have been derived (Fig. 12) but need not apply to all materials that can be tested because the carbon residue test is applicable to a wide variety of petroleum products. The Ramsbottom Carbon Residue test method is limited to those samples that are mobile below 90°C. In diesel fuel, the presence of alkyl nitrates
Ash is the non-combustible material in a fuel oil. It can be present as either solid material or oil or water-soluble metallic compounds. These solid particles are the same as those often designated as sediments. The concern for fuel systems is that these solid particles can result in wear and erosion ultimately resulting in substandard or failing performance. The test method for ash is D 482. • ASTM D 482, Standard Test Method for Ash from Petroleum Products—The sample, contained in a suitable vessel, is ignited and allowed to bum until only ash and carbon remain. The carbonaceous residue is reduced to an ash by heating in a muffle furnace at 775°C, cooled and weighed. This test method covers the determination of ash in the range 0.001-0.180 mass %, from distillate and residual fuels, gas turbine fuels, crude oils, lubricating oils, waxes, and other petroleum products, in which any ash-forming materials present eire normally considered to be undesirable impurities or contaminants. The test method is limited to petroleum products that are free from added ash-forming additives. Knowledge of the amount of ash-forming material present in a product can provide information as to whether or not the product is suitable for use in a given application. Ash can result from oil or water-soluble metallic compounds or from extraneous solids such as dirt and rust. Low-Sulfur Diesel Fuel and Dyed Diesel Fuel The Clean Air Act Amendments of 1990 established standards for highway diesel fuel. The standards, in part, made it illegal as of October 1, 1993, to manufacture, sell, supply, or offer for sale diesel fuel for highway use that has a sulfur content greater than 0.05% by weight (this amount is also commonly expressed as 500 ppm). Similarly, it is illegal for any person to use fuel that has sulfur content greater than 0.05% by weight in any highway vehicle. EPA also requires diesel fuel not intended for use in highway vehicles be dyed in order to segregate it from highway fuel. Internal Revenue Service (IRS) regulations require that tax-exempt diesel fuel be dyed regardless of the sulfur level of the fuel. The original EPA regulation mandated the addition of a blue dye to fuel with greater than 500-ppm total sulfur. However, the Federal Aviation Administration soon ex-
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100 80 60 40 30 20
6
i s s t—
CJ
£Q_ i.r
^—^. *^
4 3 2
• « •
is ^5
l±i *o
eo Q=
1.0 0.8 0.6
2 §
0.4 0.3
^ S
0.2
g
;
1
u
0.02 0.03
0.06
I
0.10 0.08 0.06 0.04 0.03 0.02
0.01 0.01
0.10
0.2 0.3 0.4 0.60.81.0
2
3 4
6 8 10
20
30 40 60 80 100
CONRAOSON CARBON RESIDUE, PER CENT BY WEIGHT (ASTM D 189)
NOTE 1—^All dimensions are in millimetres. FIG. 12—Correlation of conradson carbon residue (D 189) with ramsbottom carbon residue (D 524).
pressed their concern that blue-dyed fuel might be confused with the most c o m m o n grade of aviation gasoline, which is also dyed blue. Based on this, the EPA changed the requirements to the use of red dye. The EPA now requires "visible evidence of the presence of red dye" to identify high sulfur fuels intended for off-road use. This typically requires that oil companies add a level of red dye equivalent to 0.75 pounds per 1000 barrels of a solid Solvent Red 26 dye standcird. Solvent Red 26 was selected as the dye standard because it is a unique chemical and available in pure form. Diesel fuels are actually dyed with liquid concentrates of Solvent Red 164 because this dye is more fuel soluble and less costly than the standard. Solvent Red 164 is a mixture of isomers that are very similar to Solvent Red 26, except the former incorporates alkyl chains to increase the solubility in petroleum [46]. Under the EPA regulations, any red dye seen in the fuel of a vehicle operating on-road triggers a n analysis of the fuel's total sulfur content. Penalties are assessed based on the measured sulfur content of the fuel, rather than the mere presence of red dye. The IRS tcikes a slightly different path with its regulations. They require that tax-exempt diesel fuels, both low sulfur and
high sulfur, have a m i n i m u m level of Solvent Red 164 that is "spectrally equivalent to 3.9 pounds per 1000 barrels" of Solvent Red 26. This is over five times the a m o u n t required under the EPA regulations. The IRS holds that the excessive dye amount is required to allow detection of attempted tax evasion even after a five-fold dilution of the dyed fuel with undyed fuel. In practice, diesel fuel is taxed as soon as it leaves a terminal unless it has been dyed. The change, in 1993, to low sulfur diesel fuel for on-road use brought numerous problems. Some of these include: • A marked increase in the number of fuel lubricity related failures of fuel-wetted engine components. This is primarily attributed to the fact that the hydrotreating required to remove the sulfur also removes naturally occurring fuel components that would have improved the lubricity of the fuel. • The requirements to dye the fuel at the early stages of the distribution process m e a n that dyed fuel is often transported through pipelines. While not a c o m m o n occurrence, red dyed diesel fuel has been known to contaminate other fuels in the system. This occurs either through actual mixing of the diesel with the other product or contamination of the other product with red dye residue on the wzJls of the
CHAPTER pipeline following a shipnient of dyed fuel. In the cases where the contaminated fuel has been aviation fuel, the result is usually the requirement to dispose of the contaminated fuel since most jet engines are not certified to operate on fuel with red dye. • Under the EPA regulations, kerosine used for home heating and other off-road applications must contain the red dye. Since this fuel is not tcixed, the IRS does not require the presence of the dye. Unfortunately, the evidence regarding the possible health effects of using red-dyed fuel in un-vented kerosine heaters is minimal. Therefore, m a n y users of these heaters are reluctcuit to use red-dyed kerosine. However, if the user wants un-dyed kerosine, they must pay the tax so that it will not have to be dyed u n d e r the IRS regulations. Understandably, m a n y kerosine heater users are upset about having to pay the tcix. The laws do allow a refund of the tax, however, and many users (and in some cases, suppliers) are taking the necessary steps to reclaim those monies. There is also work underway to develop information on the potential health effects of burning the dyed fuel in un-vented heaters. Users are encouraged to check with their fuel suppliers for additional information. • Seal swell and elastomer compatibility problems brought about by the reductions in ciromatic content in low sulfur fuels. This is discussed in the section on aromatics. • It should be noted that the reductions in allowable sulfur have also had some positive effects. Aside from the obvious improvements in engine emissions, the hydrotreating required to remove the sulfur often means that the fuel has significantly better stability characteristics (through the removal of precursors). The concomitant removal of aromatics can also bring some improvement to the ignition quality of the fuel. The requirement to reduce sulfur levels in diesel fuel is now a "fact-of-life" throughout the world. At the time of this writing, the EPA is proposing legislation to reduce the maximum allowable sulfur level to I S p p m b y J u n e 1, 2006 [47]. The primary impetus for this continued reduction in sulfur is the need to protect exhaust-treatment devices installed on diesel engines, many of which are poisoned by sulfur. World Wide Fuel Charter The World-Wide Fuel Charter was jointly developed by the E u r o p e a n Automobile Manufacturers Association (ACEA), the Allieuice of Automobile Manufacturers, the Engine Mcinufacturers Association (EMA), the Japan Automobile Manufacturers Association (JAMA), and numerous associate mem-
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137
bers and supporting organizations [48]. In a letter dated April 2000, the members stated that the "Charter was first established in 1998 to promote greater understanding of the fuel quality needs of motor vehicle technologies and to harmonize fuel quality world-wide in accordance with vehicle needs." The Charter contains four categories of gasoline and diesel fuel as follows (see Table 7 for a listing of selected diesel fuel properties): • Category 1: Markets with n o or minimal requirements for emission control, based primarily on fundamental vehicle/engine performance concerns. • Category 2: Markets with stringent requirements for emission control or other market demands. For example, meirkets requiring US Tier 0 or Tier 1, EURO 1 and 2, or equivalent emission standards. • Category 3: Markets with advanced requirements for emission control or other market demands. For example, markets requiring US California LEV, ULEV, and EURO 3 and 4, or equivalent emission standards. • Category 4: Markets with further advanced requirements for emission control, to enable sophisticated NOx and particulate matter after-treatment technologies. For example, markets requiring US California LEV-II, US EPA Tier 2, and EURO 4. Premium Diesel Fuel As discussed above, environmental regulations promulgated under the Clean Air Act Amendments have resulted in significant changes to the automotive diesel fuel manufactured and sold in the United States. These changes, coupled with rapidly changing engine technology, created the need to address several fuel properties to ensure proper performance, while cJso minimizing engine maintenance problems. There is also a segment of the automotive diesel fuel market that believes that they can benefit from a fuel supply with properties different from or in addition to, the m i n i m u m ASTM D 975 specifications. Many fuel suppliers sell such fuels at a higher price. As a marketing tool, this fuel is often called "premium diesel fuel." Other terms or descriptions have also been used. At the time of this writing, two major groups have proposed definitions for premium diesel. Those two groups are the Nationeil Conference on Weights and Measures (NCWM) and the Engine Manufacturers Association (EMA). In both cases, the proposed premium diesel is based on varying one or more fuel properties. To ensure that the fuel consumer gets a "premium" product for the higher price, the National Conference on Weights and Measures (NCWM) took steps to develop a standardized
TABLE 7—Selected property specifications from world-wide fuel charter. Property
Category 1
Category 2
Category 3
Category 4
Cetane Number, min Cetane Index, min Sulfur, max, mass % Lubricity, HFRR scar dia @60°C, (xm Particulates, mg/L Total Aromatics, mass %
48 45 0.50 400 No Requirement No Requirement
53 50 0.030 400 24 25
55 52 0.003 400 24 15
55 52 Sulfur-free 400 24 15
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TABLE 8—Diesel fuel properties referenced in NCWM definition of premium diesel fuel. Heating Value, Gross, Btu/gallon, min
D240
Cetane number, min Low temperature operability
D613 D 2500 or D4539
Thermal stability, 180 min. 150°C, reflectance, min Fuel injector cleanliness Flow loss, % max CRC rating, % max
D6468
138 700
47.0 2°C maximum above the D 975 tenth percentile minimum ambient air temperature 80%
L-10 Injector 6.0 10.0
definition of premium diesel. An NCWM task force composed of representatives from the oil industry, additive manufacturers, independent labs, and government agencies, with the assistance of ASTM, prepared a set of requirements to define premium diesel. In late 1997, the NCWM task force recommended that a fuel must meet any two of the five criteria listed in Table 8 before it can be labeled "premium diesel." The NCWM adopted the plan at its 84th conference in 1999 [49]. The definition became a model law and was automatically adopted by some states; elsewhere, it will only become effective if a state specifically adopts it. The EMA issued a Recommended Guideline (FQP-IA) for a premium diesel fuel. The proposed values are listed in Table 9. From the EMA consensus position: This diesel fuel is considered to be "premium" insofar as it may assist in improving the performance and durability of engines currently in use and those to be produced prior to 2004. It is not intended to enable diesel engines to meet any emissions standard or, in general, to improve engine exhaust emissions... It is intended as a "living document" in that, as other needs or test procedures are identified, the recommendation will be upgraded [50]. The most significant aspects of this Consensus Position are its requirements for a minimum fuel lubricity, increased cetane number, improved cold weather performance, detergency, thermal stability, minimum energy content, and specifications regarding overall fuel "cleanliness." Alternative Fuels As in the early 1980s, research with alternative fuels is again on the increase. Whereas the earlier work centered on the suitability of alternative fuels to power diesel engines; most of the current work is evaluating the potential these fuels offer to reduce engine emissions [51]. Fischer-Tropsch Liquids and Biodiesel are the fuels that seem to show up the most in the current literature and reports. Fischer-Tropsch Synthesis is the process whereby natural gas or coal is converted into hydrocarbons. The product hydrocarbons are usually upgraded to middle distillate products such as kerosine and diesel fuel. Typically Fischer-Tropsch diesel fuels have high cetane numbers, often greater than 70 cetane, no aromatic compounds, no sulfur, and a density of around 0.78 kg/L [52]. The Fischer-Tropsch liquids have
been evaluated as a diesel fuel and as a blend component with conventional petroleum diesel fuel. Schaberg, et al. [53] tested two variations of the Sasol distillate fuels, a 2-D diesel fuel, a CARB (California Air Resources Board) diesel, and three blends of the Sasol fuel with the 2-D fuel. The Sasol fuels produced significantly lower engine emissions than the 2-D and CARB fuels. The fuel blends reduced emissions in proportion to the amount of the Sasol fuel in the blend. Other resesirchers have shown similar improvements in regulated emissions, with the use of FischerTropsch fuels, as well [54-56]. The most significant potential problem associated with the use of these fuels is lubricity. Fischer-Tropsch fuels have very poor lubricity properties. There may also be some elastomer/seal swell problems, especially in older fuel systems, since these fuels have no aromatic compounds. Biodiesel is also a potential alternative to conventional, petroleum-derived diesel. Biodiesel is a renewable source of energy. In the United States, Biodiesel is a fuel comprised of mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats, designated BIOO. Biodiesel is registered with the U.S. EPA as a fuel and a fuel additive under Section 211(b) of the Clean Air Act. There is, however, other usage of the term biodiesel in the market place. Biodiesel is typically produced by a reaction of a vegetable oil or animal fat with an alcohol such as methanol or ethanol in the presence of a catalyst to yield mono-alkyl esters and glycerin. The finished biodiesel derives approximately 10% of its mass from the reacted alcohol. The alcohol used in the reaction may or may not come from renewable resources. Biodiesel blend is a mixture of biodiesel fuel with petroleum-based diesel fuel designated BXX, where XX is the volume % of biodiesel.
TABLE 9—EMA recommended guideline (FQP-IA). Property
Flash point, °C, min Water Eind sediment, % vol, meix Water, ppm max Sediment, ppm max Distillation, °C, % vol recovery 90% max 95% max Viscosity, 40°C, cSt Ash, % mass, max Sulfur, % mass, max Copper corrosion, max Cetane number, min Cetane index, min Ramsbottom carbon on 10% residue, % mass, max API gravity, max Lubricity, g, min Accelerated stability, mg/L, max Detergency CRC rating, max Depositing test, % flow loss, max Low temperature flow, °C
Test Method
D93 D2709 D 1744 D 2276 or D5452 D86 D445 D482 D2622 D 130 D613 D4737 D524 D287 D 6078(1) D2274 L-10 Injector D 2500 or D4539
Requirements
52 0.05 200 10 332 355 1.9^.1 0.01 0.05 3b 50 45 0.15 39 3100 15 slO.O s6.0 (2)
NOTE: Alternatively, lubricity can be measured by D 6079 with a maximum wear scar diameter of 0.45 Jim at 60°C. Diesel fuels must pass the Cloud Point (D 2500) or Low Temperature Flow Test (D 4539) at the use temperature.
CHAPTER 5: GAS TURBINE FUELS 139
OH O
JL \ ^ ^
J. +
"^^lalyst
HO
3(GH30H)
^
3(RCOOCH3) + OH
O.^ ^ O
T
Triglyceride
+
3 iVIetlianol
"^^*^'^'
3 methyi-estiiers +
Glycerol
R is usually 1 6 - 1 8 carbons with 1 - 3 0 = 0 bonds.
FIG. 13—Reactions of vegetable oil to form methyl-esthers.
TABLE 10—Detailed requirements for biodiesel (BlOO)." Property
Flash point (closed cup) Water and sediment Kinematic viscosity, 40°C Sulfated ash Sulfur^ Copper strip corrosion Cetane number Cloud point Carbon residue" Acid number Free glycerin'^ Total glycerin^
Test Method''
Limits
Units
D93
100.0
min°C
D2709
0.050
max % volume
D445
1.9-6.0"
mm 2/s
D874 D2622 D 130
0.020 0.05 No. 3
max % mass max % mass max
D613 D2500
40 Report to customer 0.050 0.80 0.020« 0.240«
min °C
D4530 D664
max % mass max mg KOH/g % mass %mass
"To meet special operating conditions, modifications of individual limiting requirements may be agreed upon between purchaser, seller, and manufacturer. ''The test methods indicated are the approved referee methods. Other acceptable methods are indicated in 5.3. "SeeXl.3.1. '^Other sulfur limits can apply in selected areas in the United States and in other countries. "Carbon residue shall be r u n on the 100% sample (see 5.2.10). 'See Annex 1 for test method. A gas chromatographic technique is being converted to a standard test method. *The test m e t h o d is u n d e r ASTM consideration by Subcommittee D02.04.OL.
Soybean oil is the largest source of biodiesel in the United States, however, oil from other plants is sometimes used. Biodiesel is a mixture of fatty acid methyl esters. The oils are combined with methanol in a process known as transesterification (Fig. 13). The resulting mixture of fatty acid methyl esters has chemical and physiccJ properties similar to those of conventional diesel fuel. Provisional Specification 121 is the ASTM specification for Biodiesel Fuel (BlOO) Blend Stock for Distillate Fuels. (At this writing, ASTM is working to approve PS 121 as a standard specification.) Table 10 contains the detailed re-
quirements for BlOO as found in PS 121. Diesel engines can run on B1 GO; however, most of the testing in this country has been done on blends of biodiesel a n d low sulfur diesel. A blend of 20% biodiesel with 80% low sulfur diesel (B20) has been tested in numerous applications across the country. The limited testing thus far completed has shown that this fuel produces lower emissions of particulate matter, hydrocarbons, a n d carbon monoxide t h a n conventional diesel fuel. NOx emissions can be slightly higher thcin with conventional diesel, unless the fuel system injection timing is optimized for B20. BlOO has good lubricity properties and contains essentially no sulfur or aromatics. However, it has a relatively high pour point, which could limit its use in cold weather. Biodiesel is biodegradable, but this property may lead to increased biological growth during storage. Biodiesel is also more susceptible to oxidative degradation than petroleum diesel. Other eJtemative fuels that have been investigated for use in diesel engines include ethers, alcohols, naphtha, and various gaseous fuels. Each of these has some advantage (such as reduced engine emissions) associated with its use. However, much work remains to be done with these fuels, including building a distribution infrastructure, before they will be widely used in diesel engines.
GAS TURBINE FUELS This section discusses the fuels used in non-aviation (industrial) gas turbine applications. The specification for industrial gas turbine fuels is D 2880, Standard Specification for Gas Turbine Fuel Oils. Table 11 contains the specific requirements for the fuels covered by D 2880. Comparison of the specifications for diesel fuels and gas turbine fuels shows that gas turbine fuels actually have fewer requirements. Many of the individual property requirements of both specifications are equivalent for corresponding grades. This is demonstrated in Tablel2. The main differences are due to operational difference of diesel versus gas turbine. As an example, diesel fuel has a cetane number requirement whereas
140 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
TABLE 11—Detailed requirements for gas turbine fuel oils at time and place of custody transfer to user.' ASTM Test Method"
Property
Graded'' No. 0-GT f 0.05
No. l-GT'^
No. 2-Gr
No. 3-GT
No. 4-GT
38 (100) 38 (100) 55(130) Flash point °C (°F) min D93 66 (150) 0.05 0.05 Water and sediment D2709 1.0 % vol max D1796 1.0 Distillation Temperature °C (°F) D86 90 % volume recovered 282 min max 288 338 Kinematic viscosity 2 mm/s^ D445 At40°C(104°F) f 1.3 1.9 5.5 min 5.5 max 2.4 4.1 50.0 At 100°C (212°F) max 50.0 Ramsbottom 0.15 0.35 Carbon residue on 10% distillation D524 0.15 Residue % mass, max 0.01 0.01 D482 0.01 0.03 Ash % mass, max Density at 15°C k g W D 1298 max 850 876 -18 D97 -6 Pour point«°C (°F) max "To meet special operating conditions, modifications of individual limiting requirements may be agreed upon between purchaser, seller, and manufacturer. ''Gas turbines with waste heat recovery equipment may require fuel sulfur limits to prevent cold end corrosion. Environmental limits may also apply to fuel sulfur in selected areas in the United States and in other countries. "The test methods indicated are the approved referee methods. Other acceptable methods are indicated in 6.1. ''No. 0-GT includes naphtha, Jet B fuel and other volatile hydrocarbon liquids. No. 1-GT corresponds in general to specification D 396 Grade No. 1 fuel and D 975 Grade 1-D diesel fuel in physical properties. No. 2-GT corresponds in general to Specification D 396 No. 2 fuel and D 975 Grade 2-D diesel fuel in physical properties. No. 3-GT and No. 4-GT viscosity range brackets specification D 396 Grades No. 4, No. 5 (light). No. 5 (heavy), and No. 6, and D 975 Grade No. 4-D diesel fuel in physical properties. "Under United States regulations. Grades No. 1-GT and No. 2-GT are required by 40 CFR Part 80 to contain a sufficient amount of dye Solvent Red 164 so its presence is visually apparent. At or beyond terminal storage taniks, they are required by 26 CFR Part 48 to contain the dye Solvent Red 164 at a concentration spectrally equivalent to 3.9 lbs per thousand barrels of the solid dye standard Solvent Red 26. 'when the flash point is below 38°C (100°F) or when kinematic viscosity is below 1.3 mm^/s at 40''C (104°F) or when both conditions exist, the turbine manufacturer should be consulted with respect to safe handling and fuel system design. *For cold weather operation, the pour point should be specified 6°C below the ambient temperature at which the turbine is to be operated except where fuel heating facilities are provided. When a pour point less than — 18°C is specified for Grade No. 2-GT, the minimum viscosity shall be 1.7 mm^/s and the minimum 90% recovered temperature shall be waived.
TABLE 12—Comparison of specification requirements for selected distillate fuels. Parameter
D396
D975
D 2069
D 2880
D 3699
Flash point Water & sediment Distillation Viscosity Carbon residue Ash Copper strip corrosion Density Pour point Sulfur Cetane # Cloud pt. Freezing point Burning quality Saybolt color NOTE: An asterisk indicates the property is included in the specification.
gas turbine fuel does not. Because the specifications are so similar, most fuels sold under one specification would also meet the requirements of the other. For this same reason, the properties and test methods for diesel fuel, as discussed above, are equally applicable to gas turbine fuels. Gas Turbine Fuel Requirements Industrial gas turbines are basically the same as aviation gas turbines in operation. A simple gas turbine has three major
components: compressor, combustor, and turbine. The purpose of the compressor is to raise the pressure of the operating fluid usually a ratio of 10 to 20 to 1. It is desirable to accomplish this pressure increase as efficiently as possible to maximize the available thrust or horsepower, because the efficiency determines how much horsepower is required for the compression. The purpose of the combustor is to raise the temperature of the operating fluid. The combustor inlet temperature depends on the pressure ratio and efficiency of the compressor. In more complicated cycles some of the heat in the exhaust is recovered and used to increase the combustor inlet temperature, which reduces the required temperature rise across the combustor and thus the amount of fuel required. Current combustor outlet temperatures are in the 1093-1482°C (2000-2700°F) ranges. The combustor must accomplish this temperature rise efficiently. Current combustion efficiencies are in the 99%-f range at most operating conditions. Additionally the outlet temperature profiles have specific requirements. A gas turbine combustor operates at fuel-air ratios less than stoichiometric and below the lower flammability level. Figure 14 is a simplified diagram of how the air is introduced in a conventional combustor i.e., not a low emission combustor. The figure shows the distribution of air for a combustor with an overall air to fuel ratio of 70 to 1. About four parts of air per part of fuel are introduced in the swirler to help stabilize the flame zone. Then, in the primary zone, 12 parts of air per part of fuel are introduced to provide enough
CHAPTER 5: GAS TURBINE FUELS
141
COMBUSTOR AIR DISTRIBUTION FULL LOAD
46
Parts Air
OVERALL AIR-FUEL RATIO = 70
P = PRIMARY S = SECONDARY D = DILUTION OR TERTIARY SW = SWIRLER F = FUEL
FIG. 14—Simplified diagram of gas turbine.
air to provide approximately the stoichiometric amount of air required, making a total of 16 parts of air per part of fuel. In the secondary zone an additional 6 parts of air per of part of fuel are introduced making a total of 22 parts of air per part of fuel, which completes the reaction. The rest of the air (48 parts of air per part of fuel) is used to cool the combustor walls and to dilute the air-stream temperature down to the design turbine inlet temperature. The major difference between an aviation gas turbine and an industrial gas turbine is that the aviation generates thrust to propel the airplane by exhausting hot gases at high temperatures and velocities. The aviation turbine section only has to generate enough horsepower to drive the compressor. In an industrial gas turbine the gases that would be exhausted in an aviation gas turbine are expanded across additional turbine stages to generate shaft horsepower that can be used to drive generators, pumps, gas compressor, etc. Many industrial gas turbines use gaseous fuels but others use a variety of liquid fuels ranging from naphtha to residual oils. Aviation gas turbine fuel requirements are quite narrow because of the varying operating conditions (altitude, temperature, etc.), which impose limitations on volatility, viscosity, distillation range, etc. Industrial gas turbines are usually stationary. This means atmospheric conditions do not change as drastically as with aviation gas turbines. The operating conditions for the combustors for the two applications vary. Many industrial gas turbines operate at or necir design point for extended periods where an aviation gas turbine operates at take-off (full power) for a short time eind then the power level is reduced to cruise for the duration of the flight. In the following paragraphs, selected requirements specific to gas turbine operation will be discussed. The first consideration is light-off (initiating the combustion process and accelerating the engine to idle). Factors such as the tjrpe of fuel injection system, fuel viscosity, and fuel volatility are important. In earlier gas turbines, the fuel injectors or nozzles were of the pressure atomizing type simileir
to those used in some heaters. Lower flow nozzles of this type are particularly sensitive to viscosity, which is why many engine company specifications have a meiximum viscosity limit regardless of temperature. More modern fuel injection systems, which utilize air to assist the atomization of the fuel, are less sensitive to viscosity but some limit is still required. Another factor to be considered for light-off is the volatility of the fuel. The initial boiling point of the fuel must be considered because, even if the fuel is well atomized, if it is too heavy, light-off might not consistently occur. Smaller gas turbines (<10kW) tend to use distillates, while larger engines also use a mix of distillates and residuals or pure residuals. Additional problems resulting from residual fuels include carbon deposits, increased combustor skin temperatures, and exhaust smoke. Trace Metal Limits The effects of trace metals such as sodium, potassium, vanadium and lead on gas turbines are discussed in Appendix X2 of D2880. The sulfur content also is important because of its interaction with sodium and potassium as well as the effects on exhaust emissions. These trace metals rarely occur in distillate fuels at the refinery. Since they can be introduced at some point after the refinery, D2880 specifies that the limits apply at the entrance of the combustor. Trace metals refer both to those metals present as metallic compounds in solution and to metals present in particulates like rust. They are dissolved or suspended either in the fuel hydrocarbons or in free water present in the fuel. Although lower levels of trace metals in a fuel will promote longer turbine service from a corrosion standpoint, the specification of excessively low levels may limit the availability of the fuel or materially increase its cost. Table 13 suggests levels of trace metals that would probably yield satisfactory service. Sodium and potassium can combine with vanadium to form eutectics, which melt at temperatures as low as 566°C
142
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
TABLE 13—Trace metal limits of fuel entering turbine combustor(s).'' Trace Metal Limits, mg/kg Designation No. 0-GT No. 1-GT No. 2-GT No. 3-GT No. 4-GT
Vanadium 0.5 0.5 0.5 0.5
Sodium plus Potassium
Calcium 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 (Consult turbine manufacturers)
Lead 0.5 0.5 0.5 0.5
"Test Method D 3605 may be used for determination of vanadium, sodium, calcium, and lead.
and can combine with sulfur in the fuel to yield sulfates with melting points in the operating range of the gas turbine. These compounds produce severe corrosion, and for turbines operating at gas inlet temperatures above 650°C, additives that control such corrosion are not yet in general use. Accordingly, the sodium-plus-potassium level must be limited, but each element is measured separately. Some gas turbine installations incorporate systems for washing oil with water to reduce the sodium-plus-potassium level. In installations where the fuel is moved by sea transport, the sodium-pluspotassium level should be checked prior to use to ensure that the oil has not become contaminated with sea salt. For gas turbines operating at turbine inlet gas temperatures below 650°C (1200°F), the corrosion due to sodium compounds is of minor importance and can be further reduced by siliconbase additives. A high sodium content is even beneficial in these turbines because it increases the water-solubility of the deposits and thereby increases the ease with which gas turbines can be water-washed to obtain recovery of the operating performance. Calcium is not harmful from a corrosion standpoint; in fact, it serves to inhibit the corrosive action of vanadium. However, calcium can lead to hard-bonded deposits that are not self-spalling when the gas turbine is shut down, and that are not readily removed by water washing of the turbine. The fuel washing systems used at some gas turbine installations to reduce the sodium and potassium level will also significantly lower the calcium content of fuel oil. Vanadium can form low melting compounds such as veinadium pentoxide that melts at 691°C, and causes severe corrosive attack on all of the high temperature alloys used for gas turbine blades. If there is sufficient magnesium in the fuel, it will combine with the vanadium to form compounds with higher melting points and thus reduce the corrosion rate to an acceptable level. The resulting ash will form deposits in the turbine and will require appropriate cleaning procedures. When vanadium is present in more than trace amounts either in excess of 0.5 mg/kg or a level recommended by the turbine manufacturer, it is necessary to maintain a weight ratio of magnesium to vanadium in the fuel of not less than 3.0 in order to control corrosion. An upper limit of 3.5 is suggested since larger ratios will lead to unnecessarily high rates of ash deposition. In most cases, the required magnesium-to-vanadium ratio will be obtained by additions of magnesium-containing compounds to the fuel oil. The special requirements covering the addition of and type of magnesium-containing additive, or equivalent, shall be specified by mutual agreement between the various interested parties. The additive will
vary depending on the application, but it is always essential that there is a fine and uniform dispersion of the additive in the fuel at the point of combustion. For gas turbines operating at turbine inlet gas temperatures below 650°C, the corrosion of the high-temperature EJloys is of minor importance, and the use of a silicon-base additive will further reduce the corrosion rate by absorption and dilution of the vEinadium compounds. Lead Ccin cause corrosion and, in addition, it can spoil the beneficial inhibiting effect of magnesium additives on vanadium corrosion. Since lead is only rarely found in significant quantities in crude oils, its appearance in the fuel oil is primarily the result of c o n t a m i n a t i o n during processing or transportation.
ASTM STANDARDS No. D 56 D 86 D 93 D 97 D 129 D 189 D 240 D 445
D 482 D 524 D 613 D 976 D 1266 D 1298
D 1319
D 1552 D 2068 D 2274 D 2425 D 2500 D 2622
D 2709
Title Test Method for Flash Point by Tag Closed Tester Test Method for Distillation of Petroleum Products Test Methods for Flash Point by Pensky-Martens Closed Cup Tester Test Method for Pour Point of Petroleum Products Test Method for Sulfur in Petroleum Products (General Bomb Method) Test Method for Conradson Carbon Residue of Petroleum Products Test Method for Heat of Combustion of Liquid Hydrocarbon Fuels by B o m b Calorimeter Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (and the Calculation of Dynamic Viscosity) Test Method for Ash from Petroleum Products Test Method for Ramsbottom Ceirbon Residue of Petroleum Products Test Method for Ignition Quality of Diesel Fuels by the Cetane Method Test Method for Calculated Cetane Index of Distillate Fuels Test Method for Sulfur in Petroleum Products (Lamp Method) Test Method for Density, Relative Density (Specific Gravity), or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method Test Method for H y d r o c a r b o n Tjrpes in Liquid Petroleum Products by Fluorescent Indicator Adsorption Test Method for Sulfur in Petroleum Products (High-Temperature Method) Test Method for Filter Plugging Tendency of Distillate Fuel Oils Test Method for Oxidation Stability of Distillate Fuel Oil (Accelerated Method) Test Method for Hydrocarbon Types in Middle Distillates by Mass Spectrometry Test Method for Cloud Point of Petroleum Products Test Method for Sulfur in Petroleum Products by Wavelength Dispersive X-ray Fluorescence Spectrometry Test Method for Water and Sediment in Middle Distillate Fuels by Centrifuge
CHAPTER D2887
Test Method for Boiling Range Distribution of Petroleum Fractions by Gas Chromatography D 3117 Test Method for Wax Appearance Point of Distillate Fuels D 3828 Test Methods for Flash Point by Small Scale Closed Tester D 3941 Test Method for Flash Point by the Equilibrium Method with a Closed-Cup Apparatus D 4052 Test Method for Density a n d Relative Density of Liquids by Digital Density Meter D 4294 Test Method for Sulfur in Petroleum Products by Energy-Dispersive X-Ray Fluorescence Spectroscopy D 4530 Test Method for Determination of Ccirbon Residue (Micro Method) D 4539 Test Method for Filterability of Diesel Fuels by Low-Temperature Flow Test (LTFT) D 4625 Test Method for Distillate Fuel Storage StabiUty at 43°C(110°F) D 4737 Test Method for Calculated Cetane Index by Four Variable Equation D 4809 Test Method for Heat of Combustion of Liquid Hydrocarbon Fuels by Bomb Calorimeter (Precision Method) D 4860 Test Method for Free Water and Particulate Cont a m i n a t i o n in Mid-Distillate Fuels (Clear a n d Bright Numerical Rating) D 4868 Test Method for Estimation of Net and Gross Heat of Combustion of Burner and Diesel Fuels D 5186 Test Method for Determination of Aromatic Content and Polynuclear Aromatic Content of Diesel Fuels and Aviation Turbine Fuels by Supercriticcd Fluid Chromatography D 5292 Test Method for Aromatic Carbon Contents of Hydrocarbon Oils by High Resolution Nuclear Magnetic Resonance Spectroscopy D 5304 Test Method for Assessing Distillate Fuel Storage Stability by Oxygen Overpressure D 5453 Test Method for Determination of Total Sulfur in Light Hydrocarbons, Motor Fuels and Oils by Ultraviolet Fluorescence D 5771 Standard Test Method for Cloud Point of Petroleum Products (Optical Detection Stepped Cooling Method) D 5772 Test Method for Cloud Point of Petroleum Products (Linear Cooling Rate Method) D 5773 Test Method for Cloud Point of Petroleum Products (Constant Cooling Rate Method) D 5949 Test Method for Pour Point of Petroleum Products (Automatic Pressure Pulsing Method) D 5950 Test Method for Pour Point of Petroleum Products (Automatic Tilt Method) D 5985 Test Method for Pour Point of Petroleum Products (Rotational Method) D 6078 Test Method for Evaluating Lubricity of Diesel Fuels by the Scuffing Load Ball-on-Cylinder Lubricity Evaluator (SLBOCLE) D 6079 Test Method for Evaluating Lubricity of Diesel Fuels by the High-Frequency Reciprocating Rig (HFFR) D 6217 Test Method for Particulate Contamination in Middle Distillate Fuels by Laboratory Filtration
D 6426 D 6450 D 6591
5: GAS TURBINE
FUELS
143
Test Method for Determining Filterability of Distillate Fuel Oils Test Method for Flash Point by Continuously Closed Cup (CCCFP) Tester Test Method for Determination of Aromatic Hydrocarbon Types in Middle Distillates—High Perform a n c e Liquid Chromatography Method with Refractive Index Detection
REFERENCES [1] Hyne, N. J., "Petroleum," Comptons Encyclopedia, Vol. 18, Encyclopedia Britannica, Inc., Chicago, 1991, pp. 263-264. [2] Hobson, G. D. and Pohl, W., Modem Petroleum Technology, Fourth Edition, Applied Science Publishers, Ltd. on behalf of The Institute of Petroleum, London, 1975. [3] Diesel Fuel Injection, 1st edition, U. Adler, Ed., Robert Bosch, GmbH, 1994. [4] Owen, K. and Coley, T., Automotive Fuels Handbook, Society of Automotive Engineers, Inc., Warrendale, PA, 1990. [5] Energy Information Administration, "Annual Energy Review," U.S. Department of Energy, 1999. [6] Manual on Significance of Tests for Petroleum Products, 6th ed., G. V. Dyroff, Ed., ASTM International, West Conshohocken, PA, 1993. [7] Dom, P., et. al., "The Properties and Performance of Modern Automotive Fuels," SAE Report No. 861178, Society of Automotive Engineers, Warrendale, PA, 1986. [8] Clerc, J. C, "Cetane Number Requirements of Light Duty Diesel Engines at Low Temperatures," Report No. 861525, Society of Automotive Engineers, Warrendale, PA, 1986. [9] "Diesel Fuels: Performance and Characteristics," SAE SP-675, Society of Automotive Engineers, Warrendale, PA, 1986. [10] Pande, S. G. and Hardy, D. R., " Ap Practical Evaluation of Published Cetane Indices," Fuel, Vol. 69, No. 4, April 1990, pp. 437-442. [11] Gulder, O. M., et. al,, "Ignition Quality Rating Methods for Diesel Fuels—A Critical Appraisal," SAE No. 852080, Society of Automotive Engineers, Warrendale, PA, 1985. [12] Manual on Significance of Tests for Petroleum Products, 6th ed., G. V. Dyroff, Ed., ASTM International, West Conshohocken, PA, 1993. [13] Manual on Significance of Tests for Petroleum Products, 6th ed., G. V. Dyroff, Ed., ASTM International, West Conshohocken, PA, 1993. [14] Westbrook, S. R., et. al., "Fuel System Design Considerations for Diesel and Gas Turbine Engine Powered Military Vehicles," Proceedings of the Second International Conference on Long-Term Storage Stabilities of Liquid Fuels, Southwest Research Institute, San Antonio, TX, 1986, pp. 416-425. [15] "Diesel Fuel Low-Temperature Operability Field Test," Coordinating Research Council Report No. 528, Coordinating Research Council, Inc., Atlanta, GA. [16] Chandler, J. E., "Comparison of All-Weather Chassis Dynamometer Low-Temperature Operability Limits for Heavyand Light-Duty Trucks with Standard Laboratory Methods," SAE Report No. 962197, Society of Automotive Engineers, Warrendale, PA, 1999. [17] Chandler, J. E., "Evaluation of Faster LTFT and SFPP for Protection of Low Temperature Operability in North American Heavy Duty Diesel Trucks," SAE Report No.932769, Society of Automotive Engineers, Warrendale, PA, 1993. [18] Chandler, J. E. and Zechman, I. A., "Low-Temperature Operability Limits of Late Model Heavy-Duty Diesel Trucks and the Effect Operability Additives and Changes to the Fuels Delivery
144 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK System Have on Low-Temperature Performance," SAE Report No.2000-01-2883, Society of Automotive Engineers, Warrendale, PA, 2000. [19] "The Cold Filter Plugging Point of Distillate Fuels," A European Test Method, CEC Report No. P-01-74, Coordinating European Council, Leicester, UK, 1974. [20] Coley, T., et. al., "New Laboratory Test for Predicting Low-Temperature Operability of Diesel Fuels," Journal of the Institute of Petroleum, Vol. 52, No. 510, June 1966. [21] Hiley, R. W., "Filterability of Degraded Fuels," Proceedings of the 2nd International Conference on Long-Term Storage Stabilities of Liquid Fuels, San Antonio, TX, July 29-August 1, 1986. [22] Beal, E. J., Hughes, J. M., and Hardy, D. R., "An Improved Fuel Filterability Test," Proceedings of the Sixth International Conference on Stability and Handling of Liquid Fuels, Vancouver, B.C., Canada, October 1997. [23] Pedley, J., et. al., "Storage Stability of Petroleum-Derived Diesel Fuel," Fuel, Vol. 68, 1989, pp. 27-31. [24] Taylor, W. F. and Frankenfeld, J. W., "Chemistry and Mechanism of Distillate Fuel Stability," Proceedings of the Second International Conference on Long Term Storage Stabilities of Liquid Fuels, San Antonio, TX, July 1986. [25] Beaver, B., "Long Term Storage Stability of Middle Distillate Fuels from a Chemical Mechanistic Point of View. Part 1," Fuel Science and Technology International, Vol. 9, No. 10, 1991, pp. 1287-1335. [26] Beaver, B., "Long Term Storage Stability of Middle Distillate Fuels from a Chemical Mechanistic Point of View. Part 2," Fuel Science and Technology International, Vol. 10, No. 1, 1992, 1-37. [27] Wechter, M. A. and Hardy, D. R., "The Use of Macromolecular Oxidatively Reactive Species (SMORS) to Predict Storage Stability of Mid-Distillate Diesel Fuels," Proceedings of the 4th International Conference on Stability and Handling of Liquid Fuels, Orlando, FL, November, 1991. [28] Henry, C. P., "The du Pont F21 149°C (300°F) Accelerated StabiUty Test," Distillate Fuel Stability and Cleanliness, ASTM STP 751, L. L. Stavinoha and C. P. Henry, Eds., ASTM International, West Conshohocken, PA, 1981, pp. 22-33. [29] ibid. [30] Research Report RR:D02-1463, ASTM International, West Conshohocken, PA. [31] Stavinoha, L. L. and Westbrook, S. R., "Accelerated Stability Test Techniques for Middle Distillate Fuels," Distillate Fuel Stability and Cleanliness, ASTM STP 751, L. L. Stavinoha and C. P. Henry, Eds., ASTM International, West Conshohocken, PA, 1981, pp. 3-21. [32] Bacha, J. D. and Lesnini, D. G., "Diesel Fuel Thermal Stability at 300°F," Proceedings of the 6th International Conference on Stability and Handling of Liquid Fuels, Vancouver, B.C. Canada, October 13-17, 1997, pp. 671-684. [33] Henry, C. P., "The du Pont F21 149°C (300°F) Accelerated StabiUty Test," Distillate Fuel Stability and Cleanliness, ASTM STP 751, L. L. Stavinoha and C. P. Henry, Eds., ASTM International, West Conshohocken, PA, 1981, pp. 22-33. [34] White, E. W., " A Study of Test Variables Affecting Results Obtained in the ASTM D 2274 Accelerated StabiUty Test," Proceedings of the Second International Conference on Long-Term Storage Stabilities of Liquid Fuels, San Antonio, TX, October 1986. [35] White, E. W. and Bowen, R. J., "A Study of Variables Affecting Resuhs Obtained in the ASTM D 2274 Accelerated Stability Test; Parts 2 & 3-Effects of Selected Chemical and Physical Factors," Proceedings of the Third International Conference on Stability and Handling Liquid Fuels, London, U. K., September 13-16, 1988. [36] Chang, R., Chemistry-5th ed., McGraw-Hill, New York, 1994. [37] Anon., "Storage Stability of Distillate Fuels and Blends," Bureau of Mines Summary Report 1, Petroleum Experiment Station,
[38]
[39]
[40]
[41] [42]
[43]
[44]
[45]
[46]
[47]
[48]
[49]
[50] [51]
[52]
[53]
[54]
[55]
[56]
Bartlesville, OK; WPRA-BuMines Cooperative Distillate Fuel Storage Stability Program, published by Western Petroleum Refiners Association, Tulsa, OK, 1956. Garner, M. Q. and White, E. W., "Correlation of Long-Term Storage and Accelerated Stability Tests," Distillate Fuel Stability and Cleanliness, ASTM STP 751, L. L. Stavinoha and C. P. Henry, Eds., ASTM International, West Conshohocken, PA, 1981, pp. 34-46. Hardy, D. R., Hazlett, R. N., Beal, E. J., and Burnett, J, C , "Assessing Distillate Fuel Storage Stability by Oxygen Overpressure," Energy and Fuels, Vol. 3, No. 1, 1989, pp. 20-23. Turner, L. M., et. al., "Use of ASTM D 5304 in Assessing Unstable Diesel Fuel," Proceedings of the Fifth International Conference on Stability and Handling of Liquid Fuels, Rotterdam, the Netherlands, October 3-7, 1994. Wray, H. A., "Memual on Flash Point Standards and Their Use," Manual #9, ASTM International, West Conshohocken, PA, 1992. ASTM D 975-98b, Appendix X3, Annual Book of ASTM Standards, Vol 05.01, ASTM International, West Conshohocken, PA, 1998. Nikanjam, M., Crosby, T., Henderson, P., Gray, C , Meyer, K., and Davenport, N., "ISO Diesel Fuel Lubricity Round Robin Program," SAE Paper No. 952372, SAE Fuels and Lubricants Meeting, October 16-19, 1995, Toronto, Canada. Westbrook, S. R, et. al., "Survey of Low Sulfur Diesel Fuels and Aviation Kerosines from U.S. Military Installations," SAE Technical Paper 952369, Society for Automotive Engineers, Warrendale, PA 1995. Nikanjam, M., Crosby, T., Henderson, P., Gray, C , Meyer, K., and Davenport, N., "ISO Diesel Fuel Lubricity Round Robin Program," SAE Paper No. 952372, SAE Fuels and Lubricants Meeting, October 16-19, 1995, Toronto, Canada. Bacha, J., Blondis, L., Freel, J., Hemighaus, G., Hoekman, K., Hogue, N., et al., Diesel Fuels Technical Review (FTR-2), Chevron Products Company, Sein Francisco, CA, 1998. "Proposed Heavy-Duty Engine and Vehicle Standards and Highway Diesel Fuel Sulfur Control Requirements," United States Environmental Protection Agency, EPA420-F-00-022, May 2000. The interested reader can find additional information at the following internet sites: www.acea.be; www.autoalliance.org; www.engine-manufacturers.org. Handbook 130, "Uniform Laws and Regulations in the Areas of Legal Metrology and Engine Fuel Quality" (2000 Edition), as adopted by the 84th National Conference on Weights and Measures, 1999. EMA Consensus Position, Joint EMA/TMC P u m p Grade Specification for Premium Diesel Fuel. Jett, B. T. and Kirkpatrick, A. T., "Alternative Fuels for DI Engines," SAE No. SP-1412, Society of Automotive Engineers, Warrendale, PA, 1999. Grimes, G., "Economics and Experience of Blending FischerTropsch Diesel at Paramount Petroleum," presented at Gas-toLiquids Processing 99, May 17-19, 1999, San Antonio, TX. Schaberg, P. W., et al, "Diesel Exhaust Emissions Using Sasol Slurry Phase Distillate Process Fuels," SAE No. 972898, Society of Automotive Engineers, Warrendale, PA, 1997. Clark, N. N., et. al., "Transient Emissions Comparisons of Alternative Compression-Ignition Fuels," SAE No. 1999-01-1117, Society of Automotive Engineers, Warrendale, PA, 1999. Clark, N. N., et. al., "On-Road Use of Fischer-Tropsch Diesel Fuels," SAE No. 1999-01-2251, Society of Automotive Engineers, Warrendale, PA, 1999. Mintz, M. M., Wang, M. Q., and Vyas, A. D., "Fuel-Cycle Energy and Emissions Impacts of Propulsion System/Fuel Alternatives for Tripled Fuel-Economy Vehicle," SAE Report No. 1999-011118, Society of Automotive Engineers, Warrendale, PA, 1999.
MNL37-EB/Jun. 2003
Introduction to Marine Petroieum Fuels Matthew F. Winkler^
T H E MODERN PETROLEUM INDUSTRY began during the late 19th
century, although small quantities of petroleum were obtained as early as the 14th century. In Pennsylvania, Edwin Drzike drilled the first oil well in 1859, striking oil at a depth of 21 m. Drake's oil well was generally accepted as the start of the petroleum industry. Kerosine or lamp oil was the object of the early refining industry. The simple distillation process was conducted to obtain as m u c h lamp oil as possible from the Pennsylvania crude oil. In 1870, the S.S. Constantine was the first of many sea vessels converted from coal to fuel oil. While trading in t h e Caspian Sea, the vessel burned the residue from refined Russian crude oil. Early oil burners were primitive a n d ineiBcient, b u t the crude oil was plentiful a n d n o attempt was m a d e to b u m the fuel oil economiccJly. In the early years of oil firing, t h e marine industry a n d Lloyd's Register considered oil-fired instcJlations as experimental, with the vast majority of vessels still powered by coal at t h e end of the 19th century. Early 20th century fuel oils were used to a greater extent in steam boilers and the marine industry experimented with oil-fired diesel engines in the M/T Vulcanus in 1910 and the MA'^ Seletndia in 1911. Then, during the period u p to the outbreak of the first World War, fuel oil was used increasingly in Marine Merchant vessels, predominantly in steam boilers. In the early 1920s, zJmost all motor ships r a n on a distillate fuel CcJled "diesel oil" while the oil-fired steam ships burned a residual fuel called "boiler fuel." The quality difference between the diesel oil and the boiler fuel was considerable. In t h e mid-1920s, some m o t o r ships h a d experimented with burning boiler fuel in the m a i n a n d auxiliary engines. However, there was no commercial benefit in burning boiler fuel, since the smedl price differential between diesel fuel cind boiler fuel was offset by the increased mainteneince costs (resulting from burning boiler fuel). By the beginning of World W a r II, about half of the world marine fleet was oil fired. Of those ships, half were steam a n d half were diesel powered. As society increased the usage of distillate fuels for automotive, truck, railroad, and aircrzift, the increasing demand for distillate fuels increased the cost differential between distillate fuels and residuals. This economic incentive prompted the Mcirine Industry to improve t h e technical feasibility of burning residual fuels (boiler fuels) in marine diesel engines. ' Vice President, Seaworthy Systems, Inc., P.O. Box 965, Essex, CT 06426.
In the late 1940s and throughout the 1950s, motor ship fuel systems a n d marine diesel engines struggled to operate on residual fuels a n d to control reliability a n d m a i n t e n a n c e costs. The successful combination of modified diesel engines, improved fuel handling, a n d purification systems, together with, new, reformulated cylinder lube oils led to the economic use of residual fuels in motor ships. By the mid-1960s, about half of the world merchant fleet tonnage was powered by diesel engines. Then, the distillate and residual fuel costs were jolted sharply upward in 1973 and again in 1979 by t h e Middle Eastern oil suppliers restricting oil supplies and raising crude oil prices. This caused the metrine fuel expenditure to become a significemt factor in the ship owner's operating costs, and caused overeJl propulsive efficiency to take on renewed significance. The result was the rapidly declining popularity of steam-powered vessels. The m u c h m o r e thermally efficient a n d economical diesel engine led to the dominant position in merchant vessels that it still enjoys today. The goal of m o d e m m a r i n e diesel engines is to operate with fuel efficiency a n d reability on cracked a n d heavier residual fuels. Marine residual fuels provide the main source of energy that is used to transport the world's sea trade. More ton-miles are moved by ship than by all other transportation means combined. Therefore, marine fuels have become even more importcint today to world trade.
PETROLEUM CRUDE OIL REFINING Crude oil must be carefully refined to produce the main usable products, such as gases, gasoline, kerosine, jet fuels, diesel fuels, lubricating oil base stocks, petrochemical feed stocks, waxes, bitumens, residucJ fuels, and even petroleum coke. The method and degree of the refining of crude oil a n d its inherent properties determine the quality and quantity of any petroleum product produced. Refining, design, a n d production of main petroleum products are dictated by the local, national, or international requirements for light and middle distillate products as well as for petrochemiceJ feed stocks, which are the higher profit products produced from crude oil. Recently, govemmentcJ regulations to reduce air pollution have controlled sulfur, carbon/aromatic a n d oxygen content, etc., of distillate and residual fuels. The basic refining processes are distillation, cracking, blending, and storage.
145 Copyright'
2003 by A S I M International
www.astm.org
146 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
Distillation Fractional or atmospheric distillation is the oldest and most basic refining process. The heated crude is charged to the tower under pressure and a portion flashes to vapor while the rest stays in the tower as a liquid residue. It consists of boiling the crude oil in a pipe-still at atmospheric pressure up to 371°F. Then the hydrocarbon vapors are directed to a fractionating tower, as shown in Fig. 1. As the various lighter hydrocarbons of the crude oil vaporize at different rates, the lighter, more volatile vapors rise high in the tower before condensing and being drawn off. The heavier, less volatile vapors condense and are collected lower in the tower. The residual fraction is drawn off at the bottom. The fractions drawn from the tower are called straight-run or virgin products. In the 1950s, '60s, and early '70s, straight-run fuels produced from atmospheric distillation were the main residual fuels used in marine diesel engines. Straight-run fuels provided good ignition quality, clean combustion, ease of fuel handling, ease of water separation, storage stability, and solid compatibility. Since the density was usually below 985 Kg/m^, the shipboard settling tanks and old style centrifugal purifiers provided adequate treatment to remove water and sediment from the fuel. At complex refineries, the next refining process is vacuum distillation of the heavy bottoms from the atmospheric distillation unit. This process is a modified atmospheric process where the pressure in the fractionating tower is reduced below atmospheric to a partial vacuum, as in Fig. 2. The atmospheric tower residual is the feed stock for the vacuum distillation process to yield additional heavy distillates, and will further concentrate the metals and carbon content into the vacuum bottoms. The vacuum bottom residuals are generally used as feed stocks for other refinery processes. They are generally not available as marine fuels due to the very high viscosity. The vacuum bottoms can be further refined by using
another (secondary) process called "viscosity breaking" or "visbreaking." In this process, the vacuum bottoms are heated to a higher temperature and higher pressure for cracking, although not as high as in the thermal cracking process. The high viscosity feed stock is converted to a residual fuel that is much lower in viscosity. While this visbroken fuel could be used as marine residual fuel with little or no blending with a lower viscosity distillate, it has higher density, higher carbon content, higher asphaltenes, and poorer ignition quality characteristics. Visbroken fuels are usually less stable and less compatible with other residuals than the original vacuum feed stock. These characteristics can present problems for the fuel handling, fuel purification, and diesel combustion process, which could lead to vessel delays and increased maintenance for the diesel engine. Cracking When heat is applied to hydrocarbons, their molecular energy increases and these molecules move faster. Additionally, if pressure is applied to the liquid hydrocarbon and temperatures are increased further, the energy within the molecules can rupture the Ccirbon-carbon and carbon-hydrogen bonds, and the molecules "crack" into more, smaller molecules that have different chemical structures. Cracked products are hydrogen deficient compared to straight-run products of the same boiling range and some of the new, smaller molecules are unsaturated. Further, these unsaturated molecules tend to be more volatile and can be more unstable. Two basic cracking processes can be used. The older of the two cracking processes is thermal cracking, which typically occurs at about 13 bar and 520-560°C. The long-chain residual oil molecules are cracked or broken, producing some short chain molecules and additional long-chain molecules. These cracked hydrocarbons are then vaporized in a flash chamber and flow to the fractionating tower where they are
GASEOUS
FRACTIONING TOWER
GASOLINE NAPHTHA KEROSENE CRUDE HEATER
NO. 2 DISTILLATE
f
REDUCED CRUDE NO. 5 - 6 RESIDUE
FIG. 1—Atmospheric distillation of crude oil.
_.CRUDE
OIL
CHAPTER 6: INTRODUCTION
TO MARINE PETROLEUM FUELS
147
VACUUM DISTILLATION CHARGE HEATER
HEAVY ^ DISTILLATE
REDUCED - CRUDE OIL
VACUUM BOTTOMS
GASOLINE
"A,^
VISBREAKER BLENDED WITH CUTTING STOCKSNO. 6 EUEL OIL
DISTILLATE-
"^y" RESIDUUM FIG. 2—Vacuum distillation.
condensed to produce additional light and heavy distillates. Therefore, thermal cracking increases the yield of distillate fuels from crude oil and reduces the yield of residual fuels. Thermal cracking concentrates metallic content, carbon content, and asphaltene content into the residual fuel, which reduces combustion quality, increases carbon deposits, increases sulfur emissions, and decreases stability and compatibility of the thermally cracked residual. The more modem cracking process uses a catalyst to reduce the temperatures and pressures at which hydrocarbon cracking occurs and, therefore, has been named (fluidic) catalytic cracking, or FCC, as shown in Fig. 3. Catalytic cracking used a powdered (Quidized) catalyst, typically an alumina-silica based material, which is in direct contact with the liquid hydrocarbon feed stock. By definition, a catalyst is a substance that aids and accelerates a chemical reaction, but which itself undergoes no permanent compositional change. When kept in constant agitation and circulation by air, steam or hydrocarbon vapor, the fine powdered catalyst flows like a fluid. In the beginning of the FCC process, hot, preheated hydrocarbon feed stock enters the reactor and is mixed with hot, regenerated, powdered catalyst. Within the FCC reactor, the hot feed stock vaporizes and fluidizes the catalyst at the same time. Cracking then takes place within the reactor and the hot, cracked hydrocarbon vapors pass into the fractionating tower. These lighter, cracked vapors are then separated into gases, gasoline components, light gas oil, heat oil, and a heavier product known as cycle oil. The catalyst works in a continuous cycle. During the cracking reaction, the powdered catalyst particles become Ccirbon
coated and fall to the bottom of the reactor. The carbon coating is removed in the regenerator, since the catalyst is expensive. Once cleaned, the regenerated catalyst returns to the incoming hydrocarbon feed stock to start the cycle over. During the FCC process, a small amount of slurry oil is produced that contains small particles of catalyst known as cat fines. The slurry oil is normally decanted to reduce the concentration of "cat fines" and then the slurry oil is blended into the residual fuel stream. A number of cases have been documented where catalyst fines have been delivered to ship owners in large quantities. These catalyst particles Eire very heird and abrasive and have caused rapid and severe wear to fuel injection pumps, fuel injectors, cylinder liners, piston rings, piston grooves, and stuffing box seals. This is most evident where conventional centrifugation and filtration have been ineffective. Effective five to seven micron filtration of the residual fuels has proven to control damages from cat fines. Hydrocracking is a process that uses high pressures and the addition of hydrogen to crack refinery residues and to upgrade the resulting products through the addition of hydrogen. Lighter hydrocarbon products exhibit lower viscosities and lower sulfur levels because of hydrocracking. Since hydrogen is added during the process, this process is more expensive than thermal or catalytic cracking. Refinery Blending and Storage Marine fuel oils are blends of products from several different processes. Combustion characteristics, handling properties, stability characteristics, and storage stability are all affected
148 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK
FRACTIONING TOWER GASOLINE REACTOR
NO 2 DISTILLATE - -e-ATALYST-
HEAVY GAS OIL
FIG. 3—Fluidic catalytic cracking.
by the composition of the blend. The blending processes begin at the refinery. Heavy products produced at the refinery cire expected to be in the residual fuel blend to varying degrees. The blending process allows lower quality residuEils to be converted into more suitable fuel oils by the addition of small qucintities of lighter cutter stock, such as light or heavy distillates, or by adding cycle oils. When distillate/residual blends eire made, the solubility of the different components in each other is important. When components Eire completely soluble, the blend is considered compatible. Whenever a blend is contemplated, the compatibility of the resulting blend must be eveduated beforehand to prevent incompatibility problems that can cause excessive sludge accumulation in storage, centrifugation, and filtration, and stratification in the storage tanks. As the refinery processes improve, and as crude oil distillate yields improve, a continuing decrease in the overall quality of cracked residuals used to produce blends for marine fuels can be expected. In many refineries in the United States, all residual products are directed to the petroleum cokers, which convert the residual products into light distillates and solid hydrocarbon coke. At these refineries, n o residual fuels are sold or made for the marine fuel industry. In the future, marine fuels could compete with "cokers" for their feed stock requirements. Ano t h e r use for residual fuel for U.S. a n d E u r o p e a n power plants is decreasing due to environmental restrictions, thereby decreasing overall availability of residual fuel. A large percentage of fuel contamination can be traced to the hydrocarbon storage and transportation process. Water and debris (solids and fibers) are frequently introduced into the marine fuel originating from barge and tanker transport, pipe line transport, and shore side tank storage. A more de-
tailed description of contamination can be found in the next section.
PETROLEUM FUEL OIL CHARACTERISTICS Fuel Properties—Physical Viscosity
(D-445)
The viscosity of a fuel is a measure of the fuel's resistance to flow. As a fuel is heated, the viscosity decreases, and as fuel is cooled, the viscosity increases. Marine residual fuels Eire still selected and purchased on the basis of a limiting viscosity due to restrictions on fuel storage, handling, or engine fuel injection equipment. However, viscosity aJone is not the sole quality indicator, despite the fact that many purchasers of m a r i n e fuels believe this to be t r u e . When purchasing straight-run (distilled, but not cracked) residuals, this practice of associating low viscosity with high quality became established. This false sense of quality assurance was then cEirried over into bu5ring m o d e m blended and cracked marine residuals. As more intensive cracking and processing procedures are used to produce future marine fuels, the relationship between fuel viscosity and fuel qucJity becomes more and more misleading. For example, some heavily cracked 180 cSt residuals have poorer overedl quality than the less-processed 380 cSt residuals. Meirine residual fuels are preheated to lower the viscosity for more efficient fuel bunkering, trsinsfer, settling, centrifugation, fuel injection, and atomization. With preheating, the viscosity of residual fuels drops and it is easier to separate solids and water from the fuel. However, a limitation of 99°C should not be exceeded because the water may flash to steeim
CHAPTER
6: INTRODUCTION
and, therefore, the centrifuge water seal would be lost. As a general rule, after centrifugation, fuel should not be heated above 150°C, since some chemical changes may occur, gases may be given off, or water may vaporize to form steam pockets in suction piping. An increase in primary fuel p u m p pressure could be necessary to prevent water/fuel vapor from forming the more volatile fuel components in the fuel returns piping from the injection pumps. Additionally, all m o d e m fuel systems should be closed (pressurized) systems with n o access to atmospheric pressure in mixing or vent tanks. Older ships with open mixing tank systems should be upgraded to closed and pressurized fuel systems at the next yard period to better cope with m o d e m , cracked, blended residual fuels [1,2]. Another important parameter would be the viscosity index (VT) of the fuel, which is determined according to ASTM D 2270. The viscosity index is the linear relationship between viscosity and temperature. As a general rule, the lower the viscosity index the higher the fuel density for a given fuel viscosity because of the higher concentration of cracked residuals present. A m o d e m cracked residual fuel could have a VI of 10-15, whereas the older straight-run residuals can have a VI as high as 80. Many of the published temperature viscosity/temperature curves and tables are based on residual fuels with a VI of 65-70. Care should be exercised w h e n using these charts and tables since cracked residual blends can deviate widely from these data, since they are more viscous at lower temperatures. It is very important that a viscosity monitoring and control system is used and calibrated periodically. By using these viscosity controllers, variations in viscosity indices from stem to stem will be automatically compensated [3]. Density
(D
1298)
The density of a fuel is the mass-per-unit volume and is expressed as grams-per-milliliter (g/mL) for liquid fuels. In reality, density and specific gravity have very similar values and, for approximate purposes, could be considered the same. Originally, specific gravity at 60°F was predominantly used but, currently, meuine fuels will specify density at 15°C superceding specific gravity as the common usage. In 1977, the Worldwide Marine Industry officially adopted SI units, although it has tciken many years for the Marine Bunker Industry to adjust to the SI units. Since all liquid hydrocarbon fuels expand their volume when heated, their weight-per-unit volume decreases. Therefore, the density (and specific gravity) must be reported at a standard temperature even though another temperature may have been used in the test method. ASTM D 1550 contains volume correction tables for reference. The significance of fuel density revolves around the waterfuel separating technology, such as centrifugal separators or settling tanks that operate on the difference of the densities between the water and the fuel. Older centrifugal sepeirators (purifiers) were limited to fuels at or below 991 density at 50°C to control water removal efficiencies. The removal of free water and salt water are important to control damage, corrosion, and deposits in fuel injection systems; turbo chargers and boilers in diesel; and gas turbine engines. The more m o d e m centrifugal separators can efficiently remove water u p to a fluid density of 1.012 at 50°C.
TO MARINE
PETROLEUM
FUELS
149
A marine fuel with a high density indicates that the fuel is heavily aromatic from being heavily cracked. Higher density fuels are likely to possess poor ignition and/or combustion qualities and could cause increased wear to the mechanical components of the diesel engine. Ignition Quality (D 613 for Distillates) (ISO 8217, Annex B for residuals) The ignition quality of all marine residual and distillate fuels is most important for diesel engines, since combustion starts each cycle of operation. This is not critical for continuous combustion cycles, such as those used for steam boilers and gas turbine engines. In a diesel engine cycle there is always a delay between the beginning of fuel injection and the start of ignition or the initial combustion of fuel. In a distillate fuel, the ignition queJity is indicated by the cetane number/cetane index. The ignition quality in a residual or blended fuel oil is indicated by the Calculated Carbon Aromaticity Index (CCAI) number. The lower the cetane number/index of a fuel or the higher the CCAI number, the poorer the ignition quality of the fuel. This translates into a longer ignition delay, a lower period of time between the start of fuel injection and the beginning of fuel ignition, and the rapid pressure rise associated with fuel ignition. The results of poorer ignition quality can be loss of power, poorer fuel economy, higher NOx emissions, and even diesel engine damage [4,5]. The characteristics of the crude oil and the degree of refinery cracking are largely responsible for the ignition quality of a marine fuel. The combination of highly cracked fuels with high densities and blended or visbroken fuels with low viscosities results in the highest CCAI numbers and the poorest ignition qualities for diesel engines. These high CCAI n u m b e r fuels can cause serious operational limitations on medium speed and high speed diesel engines that have the greatest sensitivity to ignition quality. Large, medium speed diesels and slow speed diesel engines operating at less than 400 RPM are m u c h less sensitive to the ignition quality of the marine fuel. Although m e a n piston speed has been considered a more precise method to categorize diesel engine speed, the Marine Industry generally accepts a slow speed diesel engine to operate below 249 RPM, a medium speed diesel engine between 250 u p to 899 RPM, and a high speed diesel engine above 900 RPM. The ignition quality of a marine fuel can be calculated by a CCAI nomograph or an equation. Most people use the nomograph to derive the CCAI, which is found in Fig. 4. The International Mcirine Fuel Testing Laboratories calculate and list CCAI based on analyzed viscosity and density from the fuel analysis reports. Heating
Value (D
4868)
Heating value is the quantity of heat produced by the combustion of a unit of marine fuel under specified conditions. The gross heating value is the sum of the heat produced by the total combustion of the fuel and the heat released by the condensation of the water and SOx gases formed during combustion. This is primjuily applicable to a steam boiler power plant. The net heating value is the gross heating value minus the heat released by the condensation of the water (vapor) produced during combustion and the sulfur and metals con-
150 MANUAL 37: FUELS AND LUBRICANTS ^1
800 820
HANDBOOK ^800 810
840 -820 860 830
-3
880 -840 900 -850 920 940
860
960
870
980
880
1000
890
1020 -900 1040 910
-920 5000 930 20000 50000
(added) test flame but before it will bum continuously (fire point). The flash point is an important indicator of the fire and explosion hazards associated with a marine fuel. The test equipment and test procedure are very important in obtaining accurate test results. A carefully conducted test will not miss light fuel contamination that can depress the real flash point. For most marine fuels that require heating, a minimum flash point of 60°C is a prerequisite for storage safety. Even though marine fuels may test above 60°C for a flash point, after fuels are heated in storage tanks, vapors can collect in the head space of the storage tanks that can flash below 60°C, which can cause safety concerns. Volatile vapors can be produced at the surface of the hot heating coils so controlling steam temperature (and pressure) of the heating coils and maintaining flash arrester screens on aJl tank vents are critical for shipboard safety [6]. Pour Point (D 97) Pour point is an indicator of the ability of a marine fuel to flow at low temperatures. It is considered the lowest temperature at which the fuel will flow when cooled under controlled conditions. Pour point is not related to fuel quality but depends on the type of crude oil, refining process used, and the use of fuel additives. As the fuel temperature drops, the wax components begin to crystallize and impede the flow characteristics of the fuel. When stored, marine fuels should never be permitted to reach their pour points, because once the fuel congeals, pumping may not be possible until the fuel is heated 10°C above its pour point. If a fuel has congealed, much more time must be allowed for heating since the congealed fuel will not naturally flow to the heating coils.
-950
FIG. 4—Nomograph for determining tlie CCAI. © International Organization for Standardization (ISO). This material is reproduced from ISO 8217:1996 with permission of the American National Standards Institute on behalf of ISO. No part of this material may be copied or reproduced in any form, electronic retrieval system or otherwise or made available on the Internet, a public network, by satellite or otherwise without the prior written consent of the American National Standards Institute, 25 West 43rd Street, New York, NY 10036.
tent. The net heating value is apphcable to a diesel engine. To obtain the full net heating value from a marine fuel, the diesel engine must burn the fuel completely. As marine fuels increase in density and carbon content, it becomes increasingly difficult to completely "bum-out" the fuel. To achieve complete combustion ("bum-out"), all carbon must bum to carbon dioxide, all hydrogen must bum to water vapor, and all sulfur must bum to sulfur dioxide. The net heating value will be primairily influenced by density, water content, and sulfur content. As the fuel density increases, the carbon-to-hydrogen ratio increases, causing there to be relatively less hydrogen with its higher heating value per unit weight. This results in a decrease in the net heat released during combustion. Water and sulfur also cause the net heating value to be lower. Flash Point (D 93) The flash point is the lowest temperature at which a marine fuel will support instantaneous combustion (a flash) from the
Ash (D 482) Ash from the marine fuels includes the inorganic metallic content, other non-combustibles, and miscellaneous contamination. Metallics can contain a mixture of aluminum, calcium, potassium, iron, vanadium, nickel, silicon, and sodium. Ash deposits can cause localized overheating and corrosion to the metal surfaces where they adhere, especially to the exhaust valves. Excessive ash can cause abrasive wear of the cylinder liners, piston rings, injection pumps, fuel valves, and injection tips. Catalyst fines can result in just such abrasive damage. In marine fuels, some of the dispersed and insoluble metallic compounds can be removed onboard by settling, centrifugation, and filtration. However, fuel soluble metallics cannot be reduced by shipboard fuel handling equipment [7-9]. Cat Fines (ISO 10478) Catalyst (CAT) fines are small particles of aluminum silicate used as a fluidic catalyst in the catalytic cracking units at modem refineries. Cat fines can be carried over from the catalytic cracker and can be found in marine fuels. Cat fines are very hard and abrasive and can cause excessive wear of engine components, particularly fuel pumps, cylinder liners, piston rings, and piston ring grooves. To prevent severe abrasive damage, cat fines must be limited by fuel specification restrictions or by shipboard removal settlement, centrifugation, or fine filtration of five to seven micron control. Cat fine control can be monitored by testing the treated fuel for aluminum/silicon levels [10,11].
CHAPTER Water (D 95) (D
6: INTRODUCTION
4176)
Water is a contaminant that enters mEuine fuels during transport, usually when taken by tanker or barge over water. Salt water can cause greater problems in marine fuels then fresh water. Salt water has been considered the greatest single cause of fouling, deposits, and corrosion, especially in the higher temperature regions of the power plant. The presence of water can initiate microbial growth in marine fuels. These simple organisms must live in the water at the tank bottom and feed on the marine fuel at the water/fuel interface. Water in the marine fuels can cause large volumes of centrifuge sludge as a result of water-sludge emulsification during centrifugal separation. Because of upcoming IMO and EPA exhaust gas emission regulations, diesel engines may start using water in fuel emulsions (made with potable water) to reduce oxidizes in nitrogen (NOx), particulates and smoke. These water-in-fuel emulsions can consist of u p to 50% additional potable water and must be intentionally-made emulsions with a tightly controlled water droplet size and a control on all larger size droplets. Lastly, whenever the diesel engine is shut down, all emulsion must be purged out of the fuel system such that the shut down diesel fuel injection system contains no water-infuel emulsion [12].
TO MARINE
PETROLEUM
FUELS
151
360
340
320 1500 P S I , 900 P S I x
300
I1
300 F*SI
280
260
OPSI^
Fuel Properties—Chemical Sulfur
(D
240
4294)
Sulfur is a nonmetallic element that is chemically bound into the marine fuel. Sulfur originates in crude oil and is concentrated into the higher density residual fractions. The mEirine residual fuel has capped sulfur content at 4.5 weight percent to begin to control oxides of sulfur (SOx). When sulfur is burned in the diesel engine it causes several problems, primarily in promoting corrosive wear of the piston rings and cylinder liners and by causing deposits in the ring zone. During the combustion process, sulfur dioxide and sulfur trioxide form in the cylinder. These sulfur compounds then combine with water vapor to form sulfurous and sulfuric acid, which can cause aggressive corrosion. When sulfuric acid vapor leaves the cylinder and contacts low t e m p e r a t u r e surfaces of the heat recovery boiler, the gaseous sulfuric acid condenses and forms highly corrosive (liquid) sulfuric acid. So, in addition to ring and liner damage, the sulfuric acid can attack valve guides, as well as the cooler parts of the heat recovery boiler. Diesel engines can be designed to prevent low temperature corrosion by maintaining surface temperatures above the sulfuric acid condensation temperature, as shown in Fig. 5. Sulfur in marine fuels is normally neutralized by using an alkaline (TBN) lube oil additive. The fuel sulfur level should be balanced against the lube oil TBN additives to just neutralize the sulfur. If too m u c h alkaline additive (TBN) is used, a harmful (abrasive) level of alkaline material is produced that can increase the wear of the cylinder liner and the piston rings. The average sulfur level in marine fuels today is 2.8-3.0 weight percent. Carbon Residue
(D 524)
Carbon residue is the coked material that remains after the marine fuel has been exposed to high temperatures under
220 0
1
2
3
4
5
Sulfur [%] FIG. 5—Acid dew point for diesel engine fuels. Reprinted with the permission of the Society of Naval Architects and Marine Engineers (SNAME). Material originally appearing in SNAME publications cannot be reprinted without written permission from the Society, 601 Pavonia Ave., Jersey City, NJ 07306.
controlled conditions—the laboratory test for Micro Carbon Residue gives an overall guide to the carbon forming tendencies of the marine fuel. As heavier crude oils are refined with the more intensive, secondary cracking process, the carbon concentration in the marine fuel can approach 18-20 weight percent. Further, as the carbon content increases, the asphaltene content typically increases as well. Asphaltenes are high molecular weight hydrocarbons that can adversely affect combustion, residue formation, a n d the compatibility of marine fuels. These high Ccirbon content marine fuels have poor combustion chciracteristics; increased carbon deposit forming tendencies on injection nozzles; pistons and ports of two stroke engines; increased ignition delays; and slow burning constituents that can increase the thermal loading of diesel engine components [13]. The asphaltene content in marine fuels influences the compatibility of blends of light and heavy fuels and blends of marine fuels from different liftings. An increase in purifier sludge discharge or filter fouling is an early indication of incompatibility of fuels being mixed and/or high asphaltene levels. Therefore, fuels taken on-board in different ports should be segregated in separate tanks (as on-board m u c h as possible)
152 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
and mixing fuels from different tanks on a single vessel should be avoided. Compatibility (ISO 10307-2) Compatibility is the ability of a fuel to form a homogeneous mixture that is neither separated nor altered by chemical, time, or temperature interaction. Marine residual fuel blends can be considered a colloidal dispersion of high molecular weight substances (asphaltenes) held in a chemical and/or physical equilibrium in the fuel. If the equilibrium forces are disturbed, the high molecular weight components (usually asphaltenes) are precipitated and dropped out of solution to form a sludge or sediment. This can result in rapid tank sludge buildup, clogged strainers, fouled filters and purifiers, and damaged viscometers. Once into the diesel engine, incompatibility can cause rapid fuel injection pump sticking, fuel injector deposits, exhaust valve deposits, turbo-charger turbine nozzle and blade deposits, and carbon fouled waste heat boilers [14]. Vanadium and Nickel (ISO 14597) Vanadium and nickel are metallic elements that exist as fuelsoluble organometallic compounds. Vanadium and nickel, originate in the crude oil and are concentrated in the marine residual fuels by the various refinery operations. Depending on the origin of the crude oil, the vanadium and nickel levels of the fuel can vary widely. In the combustion process, vanadium and nickel can combine (bond) with sodium (from salt water contamination) to produce very aggressive, low melting point compounds that are responsible for accelerated deposit formation and high temperature corrosion of diesel engine components such as piston crowns, exhaust valves, and turbo charger turbine nozzles. Figure 6 shows for the relationship between vanadium and sodium. In steam boilers, these contaminants, together with sodium, can cause accelerated fouling of the screen tubes and the superheater tubes as well as serious high temperature corrosion of the superheater tubes. Gas turbine engines are the most sensitive to both contaminants due to the very high temperatures of the turbine section. These contaminEints are fuel soluble, therefore they cannot be removed from the fuel on the ship. If they are troublesome to a particular engine, they must be limited at the time of fuel purchase. The effects of vanadium and nickel can be neutralized by using a metallic fuel additive such as magnesium or calcium.
Fig. 7, which shows the critical ratios of metcdlic components and the large effect upon the ash melting temperature. It is the molten form of sodium, vanadium, and nickel that causes the severe hot corrosion and deposits to form. No power plant is beyond this problem since it affects steam boilers, diesel engines, and gas turbine engines. Oxidation (Storage) (ISO 3735) Marine residual fuels are tjrpically stored in heated tankage on shore and in the ship. The higher storage temperatures and the presence of air in the head space above the fuel in the storage tank both combine to age the fuel. As marine fuels age, they oxidize and begin to polymerize, forming sludge, gum, and resin that can foul heaters and filters as well as fuel injectors, combustion chambers, and the exhaust system. Oxidation products can vary in form: some are soluble in marine fuels while others are insoluble and result in the formation of an organic sediment [11]. Sediment (ISO 3735) The insoluble organic products that result from oxidation of marine fuels can combine with inorganic insolubles, such as rust, cat fines, and sand, to produce sediment. This sediment 1600
Sodium (ISO 10478) Sodium is a metallic element that is extremely active chemically. Generally, marine fuels are essentially sodium free when leaving the refinery. The vast bulk of the sodium in marine fuels originates through Scilt water contamination during transport by tank ship or barge and can be removed by settling tanks or by centrifugal separator. Sodium or salt (NaCl), if present in significant quantities, will become involved in severe high temperature corrosion and deposit problems. When unfavorable ratios of vanadium combine with sodium in the fuel, they react during combustion (at high temperatures) to produce eutectic compounds with ash melting points within the engine's operating temperatures. The relationship between sodium and vanadium can be seen in
100
VA ITO
55
'
B5
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3D
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Mixture Proportion [%]
FIG. 6—Bonding relationship for vanadium and sodium. Reprinted with the permission of the Society of Naval Architects and Marine Engineers (SNAII/IE). IVIaterlal originaiiy appearing in SNAIVIE publications cannot be reprinted without written permission from the Society, 601 Pavonia Ave., Jersey City, NJ 07306.
CHAPTER 6: INTRODUCTION
TO MARINE PETROLEUM FUELS
153
1600
1500 /
3Na20*V205
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PRACTICAL FO ULING ZONE
30 P
40
50
60
70
80
90
NajO, MOL Percent
INCREASED FOULING RATE FIG. 7—Phase diagram for sodium and vanadium. Reprinted with permission from ABS, Houston, TX.
can damage high-pressure fuel injection pumps and fuel injectors. Sediment can be removed by gravity settling, filtration, and by centrifugal separators. Fuel Additives A fuel additive can be any material that, when added to a marine fuel, changes its properties, characteristics or performance. As secondary refinery processing cracks fuels more severely, the resultant meirine fuel quality becomes more difficult to handle, store, and combust efficiently. The increased amounts of metallic components, ash levels, and carbon contents adversely affect the performance of the fuel treatment system and the power plant itself. Fouling and corrosion of engine components and increased tank, centrifuge, and filter sludge levels can present costly challenges for the ship's engineers. As fuel cracking intensifies, the fuel density increases and it can make the centrifugal removal of free and emulsified water more difficult. In addition to the conventional methods of settlement, centrifugation, and filtration, a chemical (additive) approach C£in sometimes offer a solution to a particular problem. The
chemical additives that have been the most effective are the stabilizers and dispersants, ash modifiers, biocides, demulsifiers, and emulsifiers [14]. Stabilizers and Dispersants Stabilizers and dispersants tend to control the formation of new sludge and to aid in the dispersion of existing sludge. By limiting sludge, more of the hydrocarbon content of the sludge can be burned and there is less sludge to incinerate or to offload in the next port. Asphaltenes can significantly contribute to sludge production. By dispersing the asphaltenes into very small particles with dispersant additives, sludge can be reduced. Some additives work by using surfactants to facilitate a surface film formation at the fuel/asphaltene interface to stabilize the asphaltenes. The additional surface tension reduction tends to break the asphaltenes into minute particles and stabilizes the fuel. Additive additions are typically known as "tank treatments." Usually, when this additive is first used, it dissolves/disperses existing sludge and organic sediment and it places a heavy burden upon the centrifugal separators and filters as it begins to clean up tanks, piping, etc.
154
MANUAL
Ash
Modifiers
37: FUELS AND LUBRICANTS
HANDBOOK
These ash modifying materials can react with corrosive elements of the fuel during combustion to raise the melting temperatures above the critical temperatures, thereby avoiding high temperature corrosion. Magnesium compounds represent one of the more popular ash modifiers since they are available in many forms and are relatively inexpensive. Prior to combustion, these chemicals are added so that the magnesium combines with the vanadium (from the fuel) to form harmless, high-melting compounds that no longer adhere to and corrode the exhaust valves and seats, piston crowns and turbo charger nozzles in diesel engines and the superheater tubes in boilers. It is preferred to know the vanadium-plusnickel content of the fuel to determine the proper additive usage. Overdosing of ash modifiers could produce h a r d deposits within the cylinder of a diesel engine, which can lead to abrasive wear of the piston rings and cylinder liners. Demulsifiers These chemical additives cause small water droplets to coalesce and separate more readily from the marine fuel. They are typically added to the fuel storage tank before bunkering fuel. They allow better water settling in storage, in the settling tank, and in the centrifugal purifier. These demulsifiers are most effective when the water in the fuel exists as a fine dispersion or as an emulsion. Biocides These chemicals are toxic to microbiological growth in marine fuels, tanks, and piping. This growth typically lives in the water at the bottom of a fuel tank and feeds on the fuel at the interface between the fuel and the water. Since this growth must live in water, the goal of good housekeeping practices is to keep the tank bottoms drained of water. This is a challenge since fuel transport is usually by barge or tanker over seawater. A small quantity of biocide will keep the overall fuel system free of microbiological growth. Biocides should be selected carefully since some are fuel soluble a n d some are water soluble.
F u e l Oil I m p a c t o n E x h a u s t E m i s s i o n s There are several marine fuel properties that can impact exhaust gas emissions. Sulfur is the first fuel property to be regulated by international and domestic rules. The International Maritime Organization (IMO) has limited sulfur to a maxim u m of 4.5%. Some localized regions, such as the Baltic Sea, have imposed even lower sulfur levels. The next emission to be regulated will be Oxides of Nitrogen (NOx). The IMO will restrict new marine diesels installed after 2000 to NOx limits based upon the rotational speed of the engines. Slow speed engines have the highest limits, medium speed next, and high speeds have the lowest limits. The nitrogen content of marine fuels does contribute to NOx emissions, b u t to date, no organization has attempted to limit fuel nitrogen. Ignition quality, cetane for distillate fuels and CCAI for residual fuels, has an impact on NOx produced in a diesel engine. Usually, the lower the ignition quality of the fuel, the higher the NOx exhaust emissions.
Exhaust particulates and smoke b o t h relate to the ability of the diesel engine to completely and efficiently b u m the marine fuel so little or no solid carbonaceous material remains. Therefore, the higher the micro carbon residue and asphzJtene content, the higher the Ccirbonaceous pcuticulates and smoke that can be expected. Lastly, if stack opacity is regulated or if excessive smoking is prohibited, the marine fuel should not contain used lube oil (either supplied with the fuel at bunkering or as the result of the disposal of used lube oil from the diesel engines on board). Used lube oil in marine fuels results in a blue colored haze in the exhaust gases that cannot be reduced until all of the marine fuel with used lube oil has been burned. P e t r o l e u m F u e l Oil T o x i c i t y Marine fuel oils present a minimal health hazard when the engineers practice high standards of personal and industrial hygiene. A caution, however, is that extended exposure to marine fuels can cause skin dermatitis and even skin cEuicer. To minimize this possibility, these precautions should be considered. • Minimize skin and eye contact with marine fuels by modifying work procedures, if necessary. • Always wear protective clothing, especially impermeable gloves and eye protection. • If clothing becomes fuel soaked, remove it and thoroughly wash the affected skin with soap and water. • Avoid carrying fuel soaked rags in pockets. • Do not use light hydrocarbon solvents for removing fuel from the skin. Always avoid inhaling hydrocarbon solvent vapors. • Avoid inhaling hot fuel mist or vapors since the fuel could contain contaminants such as waste solvents, acids, and strong base mixtures that can sometimes get dumped into marine fuel tanks. • Avoid accidental injection of marine fuel into the skin during the testing of fuel injection equipment by following test equipment directions and maintaining protective shields in place.
PETROLEUM FUEL SPECIFICATIONS When the combined requirements of the commercial and military marine fuels are considered, marine fuels represent a small fraction of the refinery capacity. Marine fuels and, in particular residual fuels, are the end products of the major shore side fuel producers' needs. Therefore, the refineries' production is driven by shore side needs and the remainder is delivered into the marine industry. It is little wonder that the unique needs of the marine industry have little impact upon the quality of marine fuels [15]. Commercial m a r i n e fuels range from gas oil to m a r i n e diesel fuel, to blended marine diesel fuel, to blended residuals, to heavy residuals. Gas oils are clean, high quality, 100% distillate diesel fuels that are shipped and handled clean. If gas turbines are used in commercial service, the specifications will likely cedl for gas oil to maintain the very, very low trace metallic contamin a n t levels required for long life. Gas oils are typically used for emergency and lifeboat diesels.
CHAPTER
6: INTRODUCTION
Marine diesel oils (MDO) can be two very different fuels. MDO-light is similcir to gas oil but is handled in dirty barges that carry residucd fuels. This produces a dark colored fuel, which is essentially all distillate, with a little additional sedim e n t and a carbon residue below 0.2% (by mass). MDO-heavy is roughly 85% MDO-light blended with 15% residual fuel and handled dirty in a residual fuel barge. Since MDO-heavy contains about 15% residual fuel content, this should only be used in diesel engines that are designed to have a blended residual fuel capability. The carbon residue of MDO-heavy is typically above 1.5% (by mass). Heavy Residual Fuels (HFO) have high viscosity and density and, in many situations, they must be blended to a lower viscosity and a lower density to be used in marine vessels. Most marine residual fuels are blends of different residual fuel stocks with lighter stocks, such as MDO and cycle oil. The blending process can r e d u c e sulfur, c a r b o n content, vanadium and ignition quality, as well as viscosity and density. Residual fuels are still generally classed by viscosity, however, m a n y other properties are considered before a residual fuel selection is made. International Specifications The International Orgcinization for Standardization (ISO) has written a basic International Marine Fuel Specification 8217 and revised it several times since the original document was published in 1987. The most recent revision was published in 1996. The ISO relies on the guidance Eind advice of the International Council on Combustion Engines (CIMAC), Heavy Fuel Committee, which consists of engine builders, major oil companies, bunker suppliers, treatment equipment builders and consultants. CIMAC also publishes a distillate and residual fuel specification but, since it is not voted upon by an international membership, it truly doesn't carry an international importance. The International Maritime Organization (IMO) has proProperty Density at 15°C, kg/m^ Kinematic viscosity at 40°C, cSt * Flash point, °C Pour point (upper), °C" • Winter quality • Summer quality Cloud point, °C Carbon residue on 10% bttns, Ramsbottom, % mass Carbon residue, Ramsbottom, % mass Ash, % mass Sediment by extraction, % mass Water, % vol Cetane number Visual inspection Sulfiir, % mass Vanadium, mg/kg
TO MARINE
PETROLEUM
FUELS
155
duced a document called Annex VI of MARPOL 73/78, the regulations for the prevention of air pollution from ships, which caps the sulfur level in marine fuels at 4.5 wt. percent and has a provision that will restrict the NOx production from marine diesel engines after Jan 1, 2000. As time moves forward, the IMO will continue to regulate and restrict air pollution from marine diesel engines. In some cases, as in the SOx emissions, the air pollution control will be regulated by restricting fuel properties. Therefore, although the IMO MARPOL regulations are not strictly fuel specifications, they will achieve some of their regulation by restricting marine fuel properties, as has already been done with a fuel sulfur cap. ASTM Marine Fuel Specifications The ASTM D 2069-98 Fuel Specifications are similar to the ISO 8217 standard in many ways. Figure 8 shows the ASTM detailed requirements for marine distillate fuels, and Fig. 9 shows the ASTM requirements for marine residual fuels. Military F u e l Specifications Military marine fuels consist of all distillate fuels that meet an individual country's fuel specifications but most also conform to North Atlantic Treaty Organization (NATO) fuel specifications as well. Military fuels place additional restrictions u p o n these distillate fuels to support the unique, multipurpose military requirements, as well as to provide for extended storage times. In the U.S. Mihtary, the MIL-F-16884 fuel specification requires a clean, stable distillate, somewhat similar to marine gas oil, with additional properties and testing required before acceptance. Therefore, MIL-F-16884, Diesel Fuel Marine (DFM), and NATO F-76 are the same fuel that requires dedicated clean handling to prevent transport contamination, which can cause the fuel to tail out of specification. Limit max. min. max. min. max. max. max. max. max. max. max. max. min. max. max.
DMX a
1.40 5.50 43
DMA 890.0 1.50 6.00 60
DMB 900.0
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11.0 60
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"There may be a maximum limit in some locations. *lcSt=.lmm^/s "Purchasers should ensure that this pour point is suitable for the equipment on board, especially if the vessel operates in both the northern and southern hemispheres. *rhis fuel is suitable for use without heating at ambient temperatures down to -15°C. TTiis fuel shall be visually clear and bright. FIG. 8—ASTM D-2069-98, Detailed Requirements for lUlarine Distillate Fuels.
156 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK The U.S. Navy uses a ciistillate fuel, JP-5, for aircraft gas turbine engines anci selected high speed diesel engines. This fuel is lighter than MIL-F-16884. It differs from other aircraft fuel primarily by its higher flash point requirements (60°C vs. 38°C). JP-5 fuel must be handled and stored clean.
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A known fuel quality has been determined to be fundamental to reliable marine diesel engine operation. A comprehensive fuel sampling and testing program is essential to ensure the quality of the marine fuel lifted, and to ensure that the ship's fuel treatment system can properly clean, heat, and pump this fuel to the diesel engine for efficient combustion, good fuel economy, and low exhaust stack emissions. Even if the marine fuel is refined clean, shore side handling and storage, as well as marine fuel movements, can and frequently do contaminate the fuel such that poor engine operation, accelerated wear, or serious exhaust gas violation can occur. It is critical that the ship's fuel treatment system can properly treat the bunkered marine fuel so that clean, high quality fuel can flow to the diesel engines.
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Fuel Sampling Fuel sampling is the critical first step in obtaining knowledge about the newly bunkered marine fuel. For a fuel test and analysis to be meaningful, it is critical that a representative sample be taken during the delivery of the fuel. A representative sample requires continuous sampling throughout the entire bunkering operation, since marine fuels can stratify or can vary the blending ratios during the delivery period. Most marine fuel testing services provide detailed directions and equipment to take a continuous, representative sample. Also, ASTM D 4057 can provide detailed instructions on taking a representative sample. The old practice of taking a "spot or grab" sample is no longer acceptable.
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Sending the representative sample to a shore side laboratory as soon as possible for accurate and complete analysis is strongly recommended. In addition to a comprehensive analysis, a shore side laboratory can confirm that a fuel meets the ISO purchase specifications and can alert the crew to contaminants, incompatibility, abrasives, excessive water content, and other damaging aspects of the bunkered fuel. Lastly, since these independent shore side laboratories provide an unbiased analysis, they can facilitate the resolution of disputes quickly and at low cost.
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CHAPTER 6: INTRODUCTION On Board Testing Since the shore side testing can take 24-72 h, some operators prefer to supplement the shore side testing with quick, simple on board tests that are helpful in the short term. These quick on board tests provide the immediate results often needed to support decisions on fuel treatment system alignments, adjustments, and operations. The alignments and adjustments of the centrifugal separators can be effected by fuel density, water content, compatibility, abrasive content, etc. Typical on board tests include those for viscosity, density, water, compatibility, and particle content.
FUEL STORAGE SYSTEMS Steam, diesel, and gas turbine marine fuel storage, settling, and transfer systems are very similar. Figure 10 illustrates a steam propulsion plant with a 600 cSt residual fuel and diesel oil fill, transfer, and storage system. A diesel engine propulsion plant with its fill, storage, transfer, and centrifuged purification system is illustrated in Fig. 11. The marine fuel transfer system provides the following functions: • To transfer marine fuel from any storage tank to any other tank, such as to the settling tank. • To strip solids and water from the bottom of the settling tank(s) through the stripping and drain connections. The marine residual fuel and marine diesel fuels are bunkered through deck fill connections that should have sampling connections installed to allow continuous, representative fuel samples to be taken as the fuel is taken aboard. Heated tanks are used for storing marine residual fuels, and unheated tanks for marine diesel fuels and gas oils. From the residual fuel storage tanks, the transfer pumps forward the fuel to the settling tank(s). The residual fuel settling tanks begin the fuel treatment process by settling gross water and solids to the bottom. As soon as the settling tank is filled, it is heated to 80°C, but not higher than 10°C below the fuel's flash point. By heating the fuel, the viscosity is reduced, the gravity settling process is enhanced, and the fuel deaerates as well. Once settled, the fuel is forwarded to the boiler fire front, after additional heating, as illustrated in Fig. 12. The settling tank bottom drain connections permit the removal of settled water, sludge, and solids to the sludge tank. In the fuel transfer system, all piping should be traced, heated, and insulated to prevent the marine residual fuel from solidifying in the piping. The residual fuel storage tanks all have heating coils to control tank temperature and to maintain the fuel at 10°C above the pour point until it is transferred to a settling tank. From the safety standpoint, experience has proven that explosive atmosphere can collect in the tank headspace even though the fuel oil temperature is well below the flash point. This condition exists when cracked marine fuels contact hot heating coils that are significantly hotter than the bulk fuel temperature. Further, if shore side petroleum waste products have been mixed with the marine fuel, the lower flash point of the waste products could greatly lower the flash point of the bulk marine fuel. Even worse, this lower flash point may not be reported if the shore side sample was taken prior to the introduction of the waste. Therefore, the crew must fre-
TO MARINE PETROLEUM FUELS
157
quently inspect and maintain the condition of the flame arrester screens on the fuel tank vent lines, and careful and safe use of the ullage equipment is imperative. In a steam propulsion plant system, a duplex heater set heats the fuel to the atomizing viscosity prior to the fuel reaching the boiler fire front. In a diesel engine propulsion system, the settling tank delivers heated residual fuel to the purifier heater set prior to the centrifugal separator(s). It is important that the residual fuel temperature to the separator stably remain at 98-99°C to maintain high separator efficiency and to prevent boiling of the water (in the fuel) within the separator. Service tanks or day tanks provide an additional opportunity to further settle water and solids from the heated residual fuel. The service tank can be filled by the centrifugal separators and provides additional time for deaeration.
FUEL TRANSFER SYSTEMS The fuel transfer pump(s) is provided to move fuel from storage to settling tank(s). The positive displacement transfer pump(s) is protected by coarse suction strainers, pressure relief valves, and pump bypass lines. The flow rate of the transfer pump is established by the engine's fuel consumption rate and the capacity of the settling tank. The operational flexibility of the transfer system is provided by the arrangement of the valves in the system. This valving can permit the fuel to be pumped from any storage tank to any settling tank or to other storage tanks.
STEAM PLANT FUEL SERVICE SYSTEM The simplest of the service systems is that of a steam plant. It consists of storage, settling, and service tanks; transfer and service pumps; and heater sets (see Fig. 10). The treatment system consists of a heated settling tank that allows solids and gross water to fall to the bottom of the tank [4]. The service system is also very simple, consisting of a pump, a heater, and a pressure (or flow) regulating controller. Most modern systems incorporate a quick-closing fuel valve to shut down fuel flow if there is a flame out, loss of combustion air, or loss of fuel pressure. Two fuel service pumps, with one in standby, are provided and each is capable of supplying the total fuel flow plus an additional msirgin, with the excess flow diverted back to the settling tank. Service pumps are typically of the positive displacement type that are fitted with a pressure relief bypass, remote shut downs, and isolation valves (for servicing). The pressurized fuel flows to the service heater sets where the temperature is increased to provide for the proper fuel viscosity for atomization. The two heater sets (one in standby) are steam heated shell and tube or plate type heat exchangers, each with the capability of increasing the fuel to 145°C. It is important to use properly sized heat exchangers and lowpressure steam supply to prevent overheating the fuel as it passes through the heat exchanger. After the heater sets, the fuel passes through duplex strainers, a viscometer, and a flow meter and then to the burner management system at the boiler Are front. All residual fuel piping must be trace heated (usually by steam) and insulated.
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CHAPTER 6: INTRODUCTION The marine diesel fuel storage (see Fig. 11) provides a separate fuel supply for cold boiler start up and for the emergency diesel generator. A small diesel fuel service tank is used to provide clean fuel for emergency operation when neither steam nor electricity is available. This diesel service tank is located high in the machinery space so that gravity will supply enough fuel pressure to start the boilers and diesel generators in an emergency.
DIESEL PLANT FUEL SYSTEM Diesel engine power plants require more intensive and complex fuel systems than those of steam plants. The contaminants, water, solids, and debris that may have been allowable in a steam plant must be removed prior to diesel engine operation. When burning residual fuels in diesel engines, the basic principles of settling, pumping, heating, and treating are common to slow speed and medium speed diesels alike. A typical residual fuel and diesel fill, transfer, storage, and treatment system for diesel engine power plants can be seen in Fig. 11. All residual fuel piping should be insulated and, if the viscosity exceeds 380 cSt at 50°C, trace heating should be included under the insulation. Whenever emergency shutdowns occur to a diesel engine operating on residual fuel, the viscous fuel will remain in the piping and heaters, and also in the injection pumps and high pressure piping. If all piping and components have trace heating and insulation, it is not a problem since the residual fuel can be reheated and its temperature controlled by recirculation prior to restart. If the emergency will keep the engine shut down for a lengthy time, the residual fuel should be purged from the piping with marine diesel fuel and this will eJlow the trace heating system and the recirculating pump to be secured. Additionally, if Einy maintenance is planned for the fuel injection system, the work will be faster and easier with marine diesel fuel in the piping system. When bunkering residual fuel and marine diesel fuel, both enter through sepjirate deck connections. Both deck connections must incorporate sampling equipment to permit continuous, representative sampling during the entire lifting. This is considered the point of custody transfer and these samples will be key should a quality dispute arise. As was the procedure in steam plants, marine residual fuels for diesel plants are transferred to the settling tank(s) by a transfer pump through a coarse strainer. The best practice is to duplex transfer pumps to prevent pumping problems. If a demulsifier chemical is to be used to aid in water or particulate removal, the demulsifier should be metered into the suction strainer ahead of the transfer pump. Demulsifier chemicals work by facilitating the separation of water and solids from the residual fuel, and this process begins in the settling tank(s). After the settling tank(s), the centrifugal purifiers and clarifiers, from two to four centrifuges, are typically installed to treat the residual fuel. These units include supply pumps, heaters, and automated controllers. Centrifugal separators, set up as purifiers and clarifiers, are widely used. They are considered a reliable and efficient method for treating and cleaning marine diesel fuel and marine residual fuels if properly maintained and adjusted. Centrifugal purifiers have the clear advantage of being capable of removing large quantities of water and particulates. The centrifuge ser-
TO MARINE PETROLEUM FUELS
161
vice pumps, fuel heaters, sludge tank, and interconnecting piping must be designed to match and support the needs of the centrifuge. The piping system must be configured to allow centrifuge operation in parallel or in series as either purifier/purifier, clarifier/clarifier, or purifier/clarifier. The centrifugal processed residual fuel then passes through very fine duplex filtration units that remove abrasive catalytic particles (cat fines) that pass through the centrifuges. After these fine filters, the residual fuel enters the service/day tank. From the service tank, the residual fuel is forwarded to the diesel engine through the residual fuel service system as seen in Fig. 13. The service system raises the residual fuel temperature up to the fuel injection temperature as controlled by a viscometer. An advanced service system will include a homogenizer to treat the residual fuel just prior to injection into the diesel engine. The service system flows two or three times the maximum fuel consumption of the diesel engine. Older fuel service systems are atmospheric pressure mixing tanks, while modem practice is to pressurize the entire system, remove the mixing tank, and add a mechanical deaerator to eliminate gases and water vapor from the injection pump discharge flow. The marine diesel fuel system moves fuel from storage to the service tank by a transfer pump or by a centrifugal purifier. The fuel can be transferred from the service tank to the emergency diesel generator service tank by either transfer pump or centrifugal purifier.
DIESEL FUEL SYSTEM COMPONENTS Centrifuges The recommended centrifuge flow capacity is the quantity that can be treated at the highest separating efficiency. This flow capacity is based upon the d5Tiamic viscosity of the residual fuel at the temperature of separation. The maximum separation temperature has an upper limit of 98°C. There is a risk of the loss of the water seal in the centrifuge due to the formation of steam bubbles. A thorough review of each centrifuge manual will determine recommended maximum flow rates for any given residual fuel. The rule is to reduce the fuel flow through the centrifuge to slighfly above the fuel consumption of the diesel engine. Avoid the temptation to rush the fuel through the centrifuge and into the service tank so it overflows back to the settling tank and is recentrifuged. Peak centrifuge operation efficiency occurs when the flow is reduced to increase the residence time of the fuel within the centrifuge while maintaining a stable fuel temperature of 98°C. Two properly sized, correctly adjusted and operated, selfcleaning centrifuges are considered absolutely necessary to provide a reliable diesel fuel treatment system. Most diesel engine warranties become invalid if adequate centrifuges are not used effectively. The following fundamental principles are necessary to establish and maintain effective centrifugal separator procedures for marine residual fuels. The centrifugal separators are the foundation of the diesel fuel treatment system. • To treat contaminated marine fuels, supplementary systems are required in addition to the centrifuges. These supplementary systems consist of fine filtration, demulsifier
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chemicals, combustion catalyst chemicals, and homogenizer/emulsifiers. • Each centrifuge must be supplied with a complete manual, all necessary parts to operate as a purifier and as a clarifier, a complete set of spare parts, seals, gaskets, etc., and a complete set of specialized tools. • The combined centrifuge flow rate (for parallel operation) and the single centrifuge flow rate (for series operation) must not exceed the diesel engine demand by more than 10%. • All residual fuel centrifuges should be operated continuously, even in port. This reduces fuel contaminants and increases the effective time for fuel treatment. • To properly adjust and operate a centrifuge, these residual fuel properties must be known: 1. viscosity 2. density 3. fuel oil compatibility 4. water content 5. ash content (or bottom sediment content) 6. catctlyst fines content (as measured by aluminum plus silicon content) With this residual fuel information, fuel treatment decisions can be made. • Whenever a residual fuel from a different bunkering is transferred to the settling tank, a density check of this new origin fuel should be made to determine if a gravity disk change is needed in the purifier. Since the centrifugal separator is the foundation of the residual fuel treatment system, its reliable and efficient operation is critical to the durability and reliability of the diesel engines. The operation of the centrifuge must be thoroughly understood by all shipboard engineers so they cein immediately troubleshoot any problems. The following conditions can cause mal-operation of the centrifuges. • Improper residual fuel handling such as improper or inadequate blending, incompatible residual fuels, and emulsified fuels. • Improper flow rate, such as varying flow rates, excessive flow rates, or varying densities. • Improper temperature control, such as varying temperatures or temperatures below 98 °C. • Improper water/fuel interface inhibiting a uniform flow of fuel through all disks, usually caused by the installation of an improper gravity disk. As the residual fuel properties change, the gravity must be checked and changed (if necessary) to control the water/fuel separation zone for maxim u m efficiency. • Improper operation with a water-in-fuel emulsion blocking efficient water and solids removal through the sludge zone. • Overloading the centrifuge with a n accumulation of sludge, which is usually caused by extended desludging intervals or by incompatible residual fuels. Another area of concern is the centrifuge valving to prevent contamination from passing through the centrifugal separators during start up. Immediately after the separators, valving should allow the recirculating of residual fuel back to the settling tank during the centrifuge start up process. During start up, a considerable time must be given to allow the flow rate and the fuel temperature to be increased and stabilized
UCTION
TO MARINE
PETROLEUM
FUELS
163
before the separator efficiency has optimized. If residual fuel is recirculated to the settling tank during this time, no contamination will be passed on to the service tank. Once stabilized, the valving can be switched to allow properly centrifuged fuel to flow to the service tank. Proper practice requires at least two centrifugal separators to be properly sized, adjusted, and operated. When operating at the recommended flow rate for m a x i m u m separating efficiency, each of the two separators must be capable of treating the maximum fuel rate of the diesel engine(s) plus approximately a 10% margin. For normal operation, the series method of a purifier followed by a clarifier is the preferred flow path. The first separator operates as a purifier to remove sludge, sediment, a n d water. The second separator is arranged as a clarifier to remove any remaining sediment or catalyst fines from the residual fuel. Additionally, the clarifier provides a backup to the purifier should it malfunction. Parallel centrifugal separator operation is recommended when the residual fuel contains a high water content. The parallel operation allows each sepsirator to operate as a purifier at one-half the normal fuel flow rate. By doubling the residence time in each purifier, the water removal efficiency of each purifier is enhanced. In the parallel configuration, both purifiers provide the highest cleaning efficiency and, therefore, the cleanest fuel to the diesel engine. There is just one problem: if either purifier should malfunction, there would be n o second stage of centrifugal separation to prevent contamination from directly entering the diesel engine. The decision to operate in series or parallel is a judgment that the ship's engineers must make based upon reviewing the analysis of the representative sample taken during the entire bunkering operation. The fuel analysis must be examined for viscosity, density, water content, sediment, ash, catalyst fines, and compatibility in addition to the expected voyage fuel flow rate. Filters a n d Strainers All marine residual fuel systems use coarse strainers to provide gross protection from very large fuel contaminants. But in diesel power plants, fine filtration serves a major function in controlling highly abrasive solids from reaching the engine. It is because of the high ash, sediment, and catalyst fines content of marine residual fuels that supplemental fine filtration is recommended. Even though the fine filtration is considered as backup to the centrifugal separator system, these filters are essential to ensure that the fuel delivered to the diesel engine is free of damaging abrasive particles. All fine filtration should be duplexed to allow cleaning and replacement of filter elements without a n interruption in fuel flow. An effective fine filtration system can provide positive protection from abrasive particles that would normally damage high pressure injection pumps, injection VcJves, injection nozzles, cylinder liners, piston rings, and piston ring grooves in the piston. Typically, these fine filters require the filter elements to be replaced at 2 0 0 0 ^ 0 0 0 h intervals under normal operating conditions. Additionally, fine residual fuel filters can be used with homogenizers to reduce the asphaltene and sludge removal by filters. In fact, most of the matter collected in fine filters is non-particulate, organic hydrocarbon material. The basic
164
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
function of a homogenizer is to disperse the sludge and asphaltenes in the residual fuel to prevent the removal of these hydrocarbons while passing through the fine filter. Therefore, the homogenizer can greatly increase the life of the filter elements as well as reduce the total quantity of sludge removed from the residual fuel. The reduction in sludge burden reduces the problems and expense of sludge removal and decreases the wasted heating value of the lost sludge. Some fine filters have the ability to remove trace quantities of free water missed by the purifiers. Even though trace water removal might seem insignificant, shipboard testing has shown a 100% increase in injection p u m p life after the addition of fine filters to remove the trace water from the marine residual fuel. It is critical that the water sumps in fine filters are drained daily to prevent water from rising above the sump level and wetting the filter elements. Self-cleaning or auto-flushing filters contain metallic screens that automatically or periodiccdly back flush to clean sections of the screen to allow for continuous operation. The sludge and debris back flushed must be treated and finally discarded. A problem can arise when filtering even a mildly incompatible fuel since the increased sludge volume can require the filter to back flush u p to 400 times a day. This can cause increased wear on back flushing components and on the screen, as well as produce a Icirge volume of sludge for disposal. There are two t3rpes of replaceable, cjirtridge-type fuel filters: the surface type a n d the depth type. Surface elements are similar to metal screens that have a limited thickness. They can be made from cellulose, fiberglass, felt, or metal. Surface filter elements are frequently pleated to increase the filtration surface area within a given space and to increase the element's life. A problem with surface filters is that organic sludge and/or asphtdtenes, together with solids, cEin accumulate on the surface of the filter and seal the surface. This results in rapidly increasing pressure across the filter, which, in turn, limits the useful filter life. A depth filter is designed so that the entire depth of the element works as a progressive filter. This allows the contaminants to be controlled throughout the element's depth Eind reduces the buildup of contaminants on the surface that can reduce filter life. This feature is especially important for fuels with residual components that can cause problems for surface filtration.
Boost Systems and Deaeration The boost or service system consists of two p u m p s (one running, one standby) providing fuel to the boost heater sets, the viscometer, the homogenizer, the final filter, and to the diesel engine injection system. Duplex strainers of 20 to 40 mesh protect the boost p u m p s from tank and piping debris. Modern fuel systems use deaeration units t h a t remove gaseous bubbles from the return fuel from the engine. These systems are closed and pressurized to prevent fociming and frothing of the hot residual fuel. Older systems were open to the atmosphere in the mixing t a n k where the r e t u r n fuel mixed with the incoming fuel and the gaseous bubbles were intended to dissipate within the tank. All older/boost systems should be upgraded to closed, pressurized systems at a convenient yard period. Closed, pressurized systems should use
deaerator units to remove any gaseous or vapor bubbles from the r e t u r n fuel before the fuel gets back to the injection p u m p s . Closed, pressurized boost systems stre the best to prevent operating problems when operating on high viscosity residual fuels, b u t they do require additional circulating p u m p s to be effective. Fuel Service Heaters Two fuel heaters are needed in the service or booster system, with one clean heater on standby. Steam is the normal heating m e d i u m for the t5rpical shell a n d tube or plate style heaters. Frequently, duplex 20 to 40 mesh strainers protect the heater sets from piping debris. The heater sets should have the capability of heating the fuel to 150°F without thermally stressing the fuel in the heater (and coking the heaters). Heaters made from stainless steel, titanium, and aluminum are preferable over copper-based materiEds. The temperature control for the heater(s) should be the automatic viscometer. Viscosity Control The automatic viscometer continuously samples and measures the viscosity of the fuel and controls the steam to the heaters to maintain a preset injection viscosity to the engine's fuel injection system. This is critical since viscosity control is a major factor in efficient fuel injection into the diesel engine. Real time, accurate viscosity measurement is very important because of the low viscosity index of m o d e m , highly cracked residual fuels. Since the viscometer is critical to correct fuel viscosity control, the units should be checked and ccdibrated periodically. A valved bj^Dass line, together with isolation vEdves before and after the viscometer, can provide the means to service the unit without a main engine shutdown. Fuel Supply Flowmeter To accurately measure the engine's fuel consumption, a fuel supply flowmeter is needed. A mass flowmeter or a positive displacement type of meter that is reliable and accurate is recommended. A corrosion resistant material should be used on EJI oil wetted internal surfaces. Some fuel system arrangements require the use of a second fuel flowmeter to measure the return fuel from the engine's injection system. In these systems, the return flow must be subtracted from the fuel flow to determine the fuel used by the engine. The return flowmeter should be the same type a n d model as the fuel flowmeter. Fuel Final Filter Since the high pressure injection p u m p s are sensitive to any debris, the boost system should be fitted with a final simplex 10 micron filter located just before the supply to the fuel injectors. This filter should be fitted with a differential pressure indicator. Some operators opt to use self-cleaning final filters to minimize manpower. B a c k Pressure Control To minimize foaming, cavitation, and vapor formation in the fuel returning from the fuel injections pumps, a positive pres-
CHAPTER 6: INTRODUCTION sure of 350 kPa will be needed for high viscosity residucil fuel that requires a high preheat for proper injection viscosity. Homogenizer As residual fuels become more dense with a higher viscosity and higher carbon and asphaltene contents, homogenizers will become more important to aid in the preparation of the fuel for injection, complete combustion, and the reduction of exhaust peirticulates cmd opacity (smoke). Very simply, a homogenizer disperses insoluble hydrocarbon clusters in the residual fuel, completely disperses all free water and cem produce a stable, water-in-fuel emulsion (when potable water is added to the fuel prior to the homogenizer). The homogenizer breaks up the insoluble hydrocarbon materials and sludges into very fine particles that aid ignition and burnout of the high boiling range, high molecular weight materials. There are several different types of homogenizers available, but the most effective operate on cavitation and shear principles, and require significant power to operate. Other units operate on some ultrasoniccJly. As fuels become more heavily cracked, the use of homogenizers will increase. When used regularly, homogenizers can reduce the unbumed fuel particles that cause waste heat boiler fires (from unbumed fuel particles).
TO MARINE PETROLEUM FUELS
165
engines operate at sustained metcJ temperatures over twice those found in diesel engines, the cleanliness requirements of the fuel are much more severe than those for diesel engines. In gas turbine engines, it is criticcJ to control metallic compounds, insoluble and soluble fuel contaminants, dissolved water soluble metallic contaminants, fire water, solids, and sometimes microbial contaminants [16,17]. The gas turbine fuel treatment system is similar to the diesel engine system except for the more intensive treatment needed to all but eliminate metallic contamination in the fuel. Where some metallics, such as vanadium and nickel cannot be (practically) removed by fuel treatment, they can be neutralized using fuel additives. For example, marine diesel engines can operate well at hundreds of parts-per-million of metallic contaminants, whereas gas turbines must have metallic contaminants in the single digit parts-per-million range, or less. This critical requirement places great emphasis on all aspects of fuel purchase, sampling, delivery, storage, and treatment. The fuel treatment system must consistently and reliably control fuel contaminzmts without upsets or malfunctions. Additioneilly, most gas turbine manufacturers strongly recommend that starting, warming up, and shutting down be done with a clean, contaminant-free, light distillate fuel. Once the engine has been warmed up and stabilized, the residual/crude oil can then be introduced into the fuel system.
Emulsifiers Emulsifiers are units that produce a controlled, stable, waterin-fuel emulsion of residucd fuel. Emulsifiers can be homogenizers with the addition of potable water. The IMO has considered water-in-fuel emulsions as the most cost-effective method to reduce NOx exhaust emissions for existing marine diesel engines. If marine diesel NOx reductions ever mandate the control of NOx on existing ships, water emulsion retrofits will be the primary technology utilized. Waste Heat Economizers As diesel engines become more efficient and slower ship speeds result in reduced diesel engine loads, the heavily cracked fuels are not completely burned out in the diesel engines. The residue of unbumed cylinder lube oil added to the organic fuel residue causes the waste heat economizer to experience a greater rate of tube fouling, soot fires, and major economizer (iron or hydrogen) fires. Since the mid 1980s, the incidence of economizer fires has steadily increased. The homogenizer Ccm minimize the unbumed residual fuel residues and, if potable water is added to produce water in fuel emulsions, the economizer can be maintained in a cleaner condition. Chemical additives used in the residucd fuel can also benefit the cleanliness of the economizer. As a minimal precaution, waste heat boilers should instcJl cind monitor the economizer exit temperature and the pressure drop across the unit to warn the engineers about an impending fire.
GAS TURBINE FUEL SYSTEM Gas turbine engines can operate on gaseous and liquid fuels, including limited residual fuels and crude oils. Since these
Tankage The fuel treatment system actually begins with the fuel tamks, since the initial settlement for free water and solids occurs in storage. Gas turbine fuel tanks should be fitted with floating suctions and fill line diffusers. The floating suctions draw the cleanest fuel from the top of the tank continuously, allowing more time for the settlement of contaminants in the mid and lower regions of the tanks. A fill line diffuser reduces the velocity of the fuel and redirects it during tank filling to prevent reentrainment of tank bottom settlement. Also, it all but eliminates currents in the tank that hinder settling. Since metallic contamincmts are to be minimized, coating the inside tank surfaces with an inert, non-metallic coating, such as an epoxy, is recommended. Under no circumstances should a zinc type coating be considered. Fuel Treatment As with all marine fuel treatment systems, the centrifugal separator is the foundation and keystone of the system. But, with gas turbine systems, this separator system becomes much more comprehensive and complicated [18]. A modem residual fuel or crude oil burning gas turbine usually requires two stages of water washing, centrifuging at reduced flow rates, the use of demulsifying chemical additives, and the use of metallic neutralizing fuel additives (see Fig. 14). Demulsifying chemicals reduce the tendency of residual fuels to readily emulsify with water. Since these emulsions are difficult to break in the sepjirator, the use of a demulsifier is recommended to prevent a problem during water washing of the gas turbine fuel. Figure 14 shows a typical two-stage water washing gas turbine system using a demulsifying additive. The staged water
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washing system is used to reduce the sodium level to below one part-per-million. The system operates by use of a metering pump that adds a demulsifying additive to the untreated residual fuel. The in-line heater set increases the fuel temperature up to 97-99°C. The demulsifier and heated residual fuel are thoroughly mixed with 98°C recycled wash water in the first mixing tank before flowing to the first centrifugal separator. Five to ten percent clean water wash water at 98°F is then metered into the discharge of the first centrifuge and mixed in the second mixing tank. The residual and water mixture then flow to the .second centrifugal separator for final water and solids removal. To maintain an effective gas turbine treatment system, all treated fuel must pass through a monitor that reads the water content in the residual fuel after the second separator. The separators must also remove solid contaminants such as catalyst particles, sand, rust, and debris from the fuel to prevent damage and wear to the gas turbine fuel nozzles. Additionally, these aggressive solids can cause severe erosion to the turbine blades and turbine nozzles. The final step in the gas turbine fuel treatment process is to inhibit/neutralize the vanadium and nickel with a chemical additive. These additives are metered into the clean fuel stream just prior to gas turbine usage. Oil soluble additives, which are the easiest to use, are cost effective for vanadium and nickel levels below 150 PPM. Above 150 PPM, additive cost dictates more economical water soluble or oil dispersable magnesium additives as an alternative. These alternative additives have low operating costs but higher capital costs. (0 Heater sets for gas turbine residual fuels should be low a density designs to prevent heater deposits and to prevent destabilizing of any additives used with these fuels. Gas turbine treatment systems require filtration to remove (5 all solids above five microns. This level of filtration of residual fuel requires a specialized filtration medium that is an all organic, depth type, system. This is critical since gas turbines have a lower tolerance than diesel systems for solid contaminants. Lastly, a final filter, not necessarily a five micron unit, should protect the gas turbine from any piping or system related debris. In addition to using a clean distillate for starting and stopping the gas turbine engine, a fast acting bypass system should be installed to allow the distillate to purge out the residual fuel from the gas turbine's lines in case of an emergency shutdown, stall, or trip. If residual fuel solidifies in the piping of the gas turbine, the engine will require disassembly prior to restart.
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CONSTRUCTION MATERIALS In a typical gas turbine treatment system, iron-based materials, steel, and stainless steel are all valid material choices. It is important that no copper, copper alloys, zinc or any of its alloys (such as galvanized) be allowed to come into direct contact with the fuel. Copper and zinc alloys or coating must never be used in tubes or coils within heaters nor in any coating or cladding for fuel storage tanks. It is essential to avoid the use of any galvanized piping or tank plating, as well as the
CHAPTER
6: INTRODUCTION
popular zinc-rich tank coatings. The preferred tank coating would be an epoxy type coating.
TO MARINE
D396 D2880 (also see D 4418)
PETROLEUM
FUELS
167
Fuel Oils (Steam Plant Spec) Gas Turbine Fuel Oils (G.T. Spec.)
Conclusions The supply of marine residual fuels will experience pressure from three eireas. Refineries will receive heavier crude oils, which will result in higher density, higher carbon content marine residual fuels. Then as refineries upgrade, there is more pressure to increase the intensity of cracking that will result in fuels with more ignition problems and higher carb o n contents. Lastly, the shore side (and to a lesser extent marine) environmental regulations will require cleaner burning fuels with less sulfur and less aromatics. This will divert more difficult, heavier fuels into the marine market. The net result will be that more difficult fuels will increase in the marine market. Then, from the engine manufacturers, come new diesel engines with higher Brake Mean Effective Pressure (BMEP), higher fuel injection pressures, that are more demanding of ignition quality and are more sensitive to fuel contaminants. The combustion of the residual fuel supply quality and the trend in m o d e m diesel engine design will demand larger, more efficient fuel treatment equipment to be assembled into more comprehensive fuel treatment systems to preclude operating problems and schedule interruptions. Older diesel ships should upgrade fuel treatment systems to prevent problems caused when older fuel treatment equipment/systems try to cope with m o d e m cracked fuels. As gas turbine engines continue to grow in the military marine and in cruise ships, their fuel treatment systems must maintain the highest fuel quality and least contamination of any marine engine. Since gas turbine engines use only 100% distillate fuels, the problems are initially easier, but one load of contaminated fuel can cause severe hot corrosion damage, so continued vigilance is needed to prevent fuel related problems.
ASTM STANDARDS No. D 445 D 1298 D 613 for Distillates D 4868 D 93 D D D D
97 482 95 4176
D 4294
D 524 D 2069-98
Title Kinematic Viscosity of Transparent & Opaque Liquids Density by Hydrometer Cetane N u m b e r of Diesel Fuels Estimation of Net and Gross Heat of Combustion of Petroleum Fuels Flash Point by Pensky-Martens Closed Cup Pour Point of Petroleum Products Ash from Petroleum Products Water by Distillation Free Water and Particulate Contamination in Distillate Fuels Sulfur in Petroleum Products by Energy-Dispersive X-ray Fluorescence Spectroscopy Carbon Residue, Conradson, of Petroleum Fuels Marine Fuel Specifications
OTHER STANDARDS ISO ISO 8217, Annex B for Residuals: Calculated Carbon Aromaticity Index Calculation ISO10478, Cat Fines: Digestion & Analysis of Catalyst Particles ISO 10307-2, Compatibility: Total Sediment Potential of Petroleum Fuels ISO 14597,Vanadium and Nickel: Determination of Vanadium and Nickel in Liquid Fuels—^Wavelength Dispersive X-Ray Fluorescence Spectrometry ISO 10478, Sodium: Sodium Content of Petroleum Fuels ISO 3735, Storage Oxidation: Storage Oxidation of Petroleum Fuels ISO 3735/10307-1, Sediment: Total Sediment Existent of Petroleum Fuels ISO 8217, Petroleum Products—Fuels (Class F): Specifications of Marine Fuels Military F u e l Specifications MIL-F-16844: All Purpose Marine Distillate Fuel
REFERENCES [1] Winkler, M. F., Residual Fuel Oil User's Guidebook, Volume 4, Diesels; and Volume 3, Gas Turbines; Southwest Research Institute for Electric Power Research Institute, May 1988. EPRIAP-5826. [2] Leigh-Jones, C, A Practical Guide to Marine Fuel Oil Handling, The Institute of Marine Engineers, London, 1998. [3] Winkler, M. F., The Influence of Fuel Quality on the Performance, Operation, and Maintenance of Diesel Propulsion Engines, Maritime Administration, U.S. Dept. of Commerce, March 1979. [4] Winkler, M. F., Relationship of Fuel Quality to Marine Boilers and Diesel Engine Performance, The Society for Marine Port Engineers, NY, January 1978. [5] Barnes, G. K., Liddy, J. P., and Marshall, E. G., "The Ignition Quality of Residual Fuels," CIMAC Paper 25, International Council of Combustion Engines, 1 Birdcage Walk, Westminster, London SW1H911, UK, June 1987. [6] The Flammability Hazards Associated with the Handling, Storage and Carriage of Residual Fuel Oil, Oil Companies International Marine Forum (OCIMF), 27 Queen Anne's Gate, London, SWIH, 9BU, December 1989. [7] Winkler, M. F., "Coping with Today's Fuels—Distillate and Residual—Diesel and Steam," Society of Naval Architects and Marine Engineers, Jersey City, NJ. Presented at the SNAME Chesapeake Marine Engineering Symposium, Washington, D.C., January 1984; Society of Marine Port Engineers, NY, March 1982; National Maritime Show, Baltimore, March 1982. [8] Winkler, M. F., "Fuel Management: Coping With Future Fuels," Marine Engineering/Log Magazine, March 1983. [9] Newbery, P. J., Davies, T. A. C, and Chomske, K. M., "Heavier Residual Fuels for Marine Diesel Engines," MotorShip Magazine, West Sussex RH16 3DH, UK. Presented at the 6th International Motorship Conference, London, March 1984.
168 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK [10] Winkler, M. F., Systems Design for Future Fuels-Steam and Diesel, Society of Marine Port Engineers, Avenel, NJ, 1983. [11] Winkler, M. F., "Operational Problems and Marine Fuel Oils," Presented at the International Conference and Long-Term Storage Stabilities, San Antonio, TX, Jixly 1986; Society of Automotive Engineers Conference, Washington, D.C., May 1986. [12] Winkler, M. F., "Shipboard Fuel Handling and Treatment For Diesel Engines," presented at the ASTM International SjTnposium on Marine Fuels, Miami, FL, December, 1983; Society of Maritime Arbitrators, NY, October 1983; Fort Schuyler Forum sponsored by Society of Marine Port Engineers, NY, March 1981. [13] Fisher, C. and Lux, J., BUNKERS: An Analysis of the Practical. Technical and Legal Issues, Second Edition, 1994, Lloyd's of London Press, NY, 1994.
[14] Winkler, M. F., "Diesel Fuel Systems to Prevent Operational Problems," presented at the S.A.E. Marine Propulsion Technology Conference, 14 May 1986, Washington, D.C. [15] ISO 8216-0: Petroleum Products - Fuels (Class F) - Classification - Part 0: General, International Organization for Standardization, Geneva, Switzerland, 1986. [16] Winkler, M. F., "Management of Gas Turbine Fuel Systems," Gas Turbine International Magazine, March-April 1977. [17] Winkler, M. F., "Management of Residual Fuel Systems for Gas Turbines," Gas Turbine International Magazine, May-June 1977. [18] Winkler, M. F., "Management of the Gas Turbine Fuel Systems," ASTM International, October 1973; Society of Automotive Engineers, Canada, August 1971; Society of Automotive Engineers, Philadelphia, November 1970.
Section III: Hydrocarbons and Synthetic Lubricants: Properties and Performance Rajesh J. Shah, Section Editor
MNL37-EB/Jun. 2003
Hydrocarbon Base Oil Chemistry Arthur J. Stipanovic^
chemistry: • provide a brief overview of the base oil market and identify emerging trends, • outline the refinery processing scenzirios typical of current base oil production in the United States, • review the chromatographic and spectroscopic techniques most commonly used to characterize base oil composition, • provide a comparison of hydrocarbon tjrpe distributions for base oils obtained by several different refinery processes, including re-refined oils, • review important chemical mechanisms leading to the oxidation and degradation of base oils, • outline composition/performance relationships for base oils in various automotive and industrial lubricant applications.
HYDROCARBON BASE FLUIDS COMMONLY USED in the formulation
of engine oils, industrial lubricants, greases, and other products, are composed of a broad spectrum of molecular species including aromatic, paraffinic, a n d cycloparaffinic (naphthenic) molecules [1-4]. Over the past severeil decades, new separation methods and analytical techniques have made it possible t o accurately characterize and quantify the hydrocarbon t5^es that exist in base oil while relating these compositional pEirameters to crude oil source and refining conditions. F o r example, ASTM test m e t h o d D 2549 exploits column chromatography to separate a base oil into saturate and aromatic fractions while D 2786 and D 3239, mass spectrometry techniques, further sub-divide each fraction into a group of molecular types. As a result, the physical and chemical properties of base fluids are now m u c h better understood and, in some cases, it has become possible to predict the performance of a formulated lubricant directly from base oil composition using statistical m e t h o d s o r neural network modeling [3-8].
LUBRICANT B A S E OIL M A R K E T Refining Capacity i n the United States
From the perspective of lubricant applications, among the most important properties that depend directly on base oil composition include viscosity index (VI), oxidative/thermaJ stability, low temperature fluidity, a n d additive solubility. Other significant parameters such as kinematic viscosity and volatility are influenced primarily by molecular size although composition may play a role as well. As will be discussed in this chapter, each of these performance parameters is influenced differently by subtle changes in the hydrocarbon type distribution of the base oil and, as a result, they are sensitive to crude oil source and refinery processes. Further, since base oil refining technology is changing at a revolutionary pace, it is increasingly important that lubricant formulators appreciate the impact of base oil chemistry on product performance. Although synthetic fluids such as polyalphaolefins a n d polyesters have grown in their importance in many lubricants, this chapter will focus on mineral oil base fluids derived from crude oil via a n u m b e r of refinery processes. For most lubricant product lines such as engine oils and industrial process oils, mineral basestocks account for greater than 90% of the commercieJ product volume, although the marketplace penetration of S5rnthetics is increasing modestly [9]. The organization of this chapter will focus on the following issues in base oil technology a n d hydrocarbon lubricant
Since most lubricants typically contain in excess of 80% base oil by volume, their physical and chemical properties play a key role in defining the ultimate performance profile of a lubricant and the additive technology that must be employed in formulation. In addition, the marketplace availability, cost, and qucdity of base oil are critical considerations in defining the economics and performance of a finished product. As a result, lubricant development requires a n understanding of both the base oil meirket and the techniccil issues surrounding specific product requirements. On a global basis, the market for hydrocarbon-based mineral oils a n d wax exceeds 10 billion gallons per year [10] while annual U.S. production is approximately 3.5 billion gallons [11], of which about 20% is designated as "naphthenic" and 80% "paraffinic" (these terms will be defined below). A profile for the eleven largest base oil refiners in the U.S. is provided in Table 1. Petro-Canada, a major base oil producer in Canada, provides a n additional 230 MM gallons/year from its Mississauga, Ontario plant while Imperial Oil in Samia, Ontario refines a n additional 92 MM gallons/year. Annual base oil re-refining capacity in the U.S. and Camada is estimated to be approximately 62 MM gallons and 32 MM gallons, respectively, as outlined in Table 2 [11]. B a s e Oil M a r k e t P r i c e s
' Director, Analytical and Technical Services, Faculty of Chemistry, State University of New York, College of Environmental Science and Forestry, 123 Jahn Laboratory, One Forestry Drive, Syracuse, NY 13210.
Although the market price of base oil varies dynamically over time in response to crude cost and availability as well as re169
Copyright'
2003 by A S I M International
www.astm.org
170 MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
TABLE 1—Production of lubricating oils and waxes—1999 [11]. Refiner Exxon Equilon" Sun Motiva* Excel Paralubes" Chevron Pennzoil-Quaker State Mobil Valero Energy Corp.'* Cit-Con Marathon—Ashland
Location Baton Rouge, LA and Baytown, TX Deer Park, TX, Martinez, CA and Wood River, IL' Tulsa, OK and Yabucoa, PR Port Arthur, TX Westlake, LA Richmond, CA Rouseville, PA and Shreveport, LA Beaumont, TX Paulsboro, NJ Lake Charles, LA Catlettsburg, KY
Capacity (mm GallonsA'ear) 577 298 271 269 269 214 199 180 176 146 130
"Shell/Texaco joint venture. Formerly Shell base oil refineries. ^Shell/Texaco/Star Enterprise joint venture. Formerly a Star Enterprise refinery. "Conoco/Pennzoil joint venture. ''Formerly a Mobil refinery. 'Since the 1999 NPRA report, the Wood River facility no longer produces lubricants.
TABLE 2—Re-refined base oil production in the U.S. and Canada [11]. Refiner Evergreen Oil Inc. Safety-KUeen Corp. Hub Oil Company Mohawk Lubricants Ltd. Safety-Kleen Corp.
Location Newark, CA East Chicago, IL Calgary, Alberta N. Vancouver, BC Breslau, Ontario
Total
Capacity (mm GallonsATear) 12.3 50.6 3.1 6.1 23.0
95.0
gional refinery supply/demand issues, Table 3 provides a general reflection of base oil prices in early 1999, when crude oil averaged less than $20/bbl compared to 2000 when crude cost climbed above $25/bbl [12 A,B]. Table 3 also illustrates that base oil prices generally increase with increasing viscosity grade due to several factors including: the "natural" molecular weight range of the crude sources used for lubricant manufacture and the extra refining required for heavier distillate streams, since they are t3rpically richer in aromatics and sulfur-containing molecules. Based on an average price of $1.00 per gallon, domestic base oil refiners generate revenues in excess of $3.5 billion annually. Future Refining Trends Although basestock demand is growing only modestly in North America at less than 3% per year [9], other regions of the world are expanding more quickly [10]. Specifically, certain South and Central American and Asian countries are experiencing lubricant growth demand in excess of 5% per year. In all cases, an evolution to higher quality base stocks, such as Group II and III oils, defined below, is evident on a worldwide basis [13]. As a result of extreme competition to produce and utilize Group II base stocks and an overall reduction in demand for naphthenic stocks, the base oil refin-
ing capacity in North America has decreased by approximately 8% from 1980 to 1995 [10].
BASE OIL REFINING AND PROCESSING TECHNOLOGY Crude oil sources from which lubricant-grade mineral oils are refined contain thousands of molecular species differing in chemical constitution and size [14]. Base oil manufacture is, therefore, a very complex process that fractionates molecules based on molecular weight (size) and composition to yield a product with appropriate viscosity and chemical stability. At least five processing stages may be employed to produce a lubricant base oil: 1) crude distillation; 2) deasphalting to remove very heavy fractions of the crude; 3) solvent extraction or hydrogen refining to remove undesirable aromatics and heteroaromatic species; 4) solvent or catalytic dewaxing to remove linear paraffins (wax), which can crystallize, causing poor flow properties at low temperatures; and 5) hydrogen finishing or clay treatment to remove trace quantities of heteroaromatic species, olefins, or other molecules that could cause stability problems [10]. In addition to removing undesirable molecular species, refining generally increases the viscosity index (VI) of a distillate. VI reflects the relationship between base oil viscosity and temperature and is calculated from kinematic viscosities recorded at 40°C and 100°C (ASTM D 445) using an empirically derived procedure defined in ASTM D 2270. All hydrocarbon-based mineral oils decrease in viscosity with increasing temperature as shown in Table 4. Comparing two oils that have an identical viscosity at 100°C (4 cSt), a lower VI oil will have a lower viscosity above 100°C and a higher viscosity below 100°C than a higher VI oil. In general, higher VI products are considered more desirable because they maintain a greater fraction of their viscosity and lubricating properties at high temperatures (100-150°C) while exhibiting lower viscosity, better fluidity, and improved pumpability at very low temperatures (0°C and lower) when treated with pour point depressant additives. The specific molecular components of base oil responsible for high VI will be discussed below.
TABLE 3—Market prices of paraffinic base oil in the U.S. ($/g£d). Base Oil Viscosity
Average Cost ('99/'00)
Range of Prices ('99/'00)
Low (90-150iV) Medium (200-250yV) Heavy (500-600A^) Bright Stock (Very Heavy)
0.85/1.17 0.86/1.18 1.00/1.27 1.19/1.52
0.79-1.04/1.07-1.23 0.80-1.04/1.07-1.27 0.94-1.17/1.20-1.37 1.16-1.31/1.41-1.53
TABLE 4—VI: The effect of temperature on viscosity. Temperature Oil VI
CC
40°C
100°C
150°C
75 100 125 150
192 155 117 92
21.0 19.5 17.7 16.3
4 4 4 4
1.87 1.91 1.97 2.03
CHAPTER 7: HYDROCARBON
BASE OIL CHEMISTRY
171
TABLE 6—Relationship of SUS to kinematic viscosities for a 100 VI base oil.
Distillation Two distillation units are typically required to effectively fractionate crude oil for use in lubricants [10]. First, an atmospheric still is employed to selectively remove gases and other light products such as gasoline and naphtha from the lube distillate stream. Secondly, the remaining stock is distilled under reduced pressure in a vacuum distillation unit yielding a number of viscosity fractions appropriate for various applications. Typical boiling points for each crude oil fraction are shown in Table 5. The distillation cut corresponding to lubricating oils typically consists of molecules that range in size from 20-40 carbon atoms per molecule. Most refineries fractionate the distillate into 3-5 distinct viscosity grades, historically called "neutrals," for which 100°C kinematic viscosity values range from 3-5 cSt for so-called "light neutral oils," 6-9 cSt for "medium neutrals" and > 10 cSt for "heavy neutrals." In commercial practice, the viscosity grade designation for base oils, such as 100 Neutral (lOON), was based on the Saybolt Universal Second (SUS) system of measuring viscosity that is no longer widely utilized. Table 6 illustrates the relationship between "historic" SUS viscosity grades and kinematic viscosities obtained from ASTM D 445.
thenic molecules are insoluble in the extraction solvent while multi-ring aromatics (polynuclear aromatics), sulfur and nitrogen compounds, olefins, and other undesirable species are dissolved in the polar solvent phase. After phase separation, the non-polar phase containing the base oil is stripped of residual polsir solvent and is usually hydrogen treated as discussed below. Furfural, phenol, and N-methyl-2-pyrolidone (NMP) are commonly used as extraction solvents [10,11].
Propane Deasphalting
Hydrogen Refining/Hydrocracking
For certain heavy distillates and vacuum residua, base oils of very high viscosity (Bright Stocks) can be refined through propane deasphalting [10]. In this process, liquefied propane under relatively high pressure is used to precipitate insoluble, high molecular weight aromatic hydrocarbons such as asphaltenes and to extract other compounds (typically termed "resins"), which could be deleterious to the performance of the base oil being refined. Sulfur and nitrogen compounds as well as certain metals can also be removed by propane deasphalting.
Although solvent refining is effective in improving the quality of distillates in the manufacture of base oils, a significant yield loss is normally associated with the extraction process. This loss can be economically unacceptable for distillates derived from relatively poor crude oils that contain high concentrations of heavy aromatic compounds. Fortunately, it is also possible to improve the quality of a raffinate through various catalytic processes in which high-pressure hydrogen (1500 - 4000-1- psi) is employed to saturate aromatic molecules while cracking other large molecules to smaller compounds of molecular weight appropriate for base oils [10]. Olefins as well as sulfur and nitrogen containing compounds are also reacted and removed by hydrogen processing. Because aromatics are converted to saturated compounds, hydrogen processing normally increases VI.
Solvent Refining For basestocks of low to moderately high viscosity, solvent refining has been employed extensively and on a worldwide basis; this refining technology is probably the most popular method. Solvent refining removes undesirable polar and highly condensed aromatic molecules from the distillate and, in doing so, significantly increases VI. In this process, distillate is mixed with an insoluble polar solvent that creates a two-phase system. Ideally, desirable paraffinic and naphTABLE 5—Distillation cuts of crude oil [14, 15]. Fraction
Boiling Range (°C)
Ethane, butane, propane gases Light naphtha Heavy n a p h t h a Gasoline Kerosine Stove oil Lights gas oil Heavy gas oil Lubricating oils Vacuum gas oil Resid
<0 to 150 150-205 to 180 205-260 205-290 260-315 315-425 >400 425-600 >600
Kinematic Viscosity at 100°C (cSt) 3 4 6 8 10 12 14 16 18
SUS Viscosity at 100°F 70 100 200 310 430 560 710 870 1000
Dewaxing Essentially, all crude oil and base oil distillates contain a small fraction of linear paraffins (wax) [14] that can crystallize at low temperatures. For engine oils, transmission fluids, and other lubricants that must remain liquid and pumpable to temperatures approaching -40°C, it is necessary to remove as much wax as possible from the basestock. To accomplish this task, at least two techniques are commonly used to remove wax: solvent dewaxing (SDW) and catalytic dewaxing (CDW). For SDW, the base oil is first diluted in a solvent (toluene, for example) and then the mixture is added to a wax non-solvent (typically ketones such as methyl ethyl ketone-MEK). The mixture is then chilled to low temperatures to precipitate wax as a solid and is then collected by filtration. SDW is a batch process. In the CDW procedure, a shape selective catalyst is exploited to "crack" the paraffins into smaller, volatile segments that can be fractionated by distillation. CDW is a continuous process and, as a result, en-
172
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
joys m a n y advantages over SDW. However, since CDW is more efficient in removing wzix and other branched pEiraffins than is SDW, the resulting base oil VI is usually reduced since paraffins contribute to high VI. More recently, several processes have been developed that isomerize wax to brsmched isomers that do not crystallize as readily as linear peiraffins. As a result, VI is preserved by the hydroisomerization process cind very high VI oils (>120 VI) can be made directly from wax as a feedstock. Several API Group III base oils, to be defined below, are synthesized from a pEiraffin hydroisomerization procedure. Hyfinishing (Hydrofinishing) At the conclusion of the refining steps detailed above, base oils still may contain small quantities of sulfur, nitrogen, organic acids, and partially hydrogenated aromatics that can cause the base oil to exhibit poor color stability and an increased tendency to form sludge or other insolubles [10]. Historically, base oils were "finished" using either a sulfuric acid procedure or clay contacting. Unfortunately, both of these procedures jield potentially hazardous byproducts that require careful disposal to avoid environmental contamination. As a result, most m o d e m refineries use a catal3^ic hydrogenation process called hyfinishing to remove trace levels of impurities from base oil. Hydrogen pressures are typically 500-1000 psi, which are below the levels required for hydrocracking. Hyfinishing decreases the aromatic content of a base oil slightly while significantly lowering sulfur and nitrogen levels. Byproducts of hyfinishing include H2S, NH3, and CO2, which result from the cracking and hydrogenation of undesirable organic molecules containing S, N, or O heteroatoms. B a s e Oil R e - r e f l n i n g The reclamation of base oil from used engine oils and industrial lubricants has become a n important process from both an environmental protection and economic perspective. In the U.S., a Presidential Executive Order spurred the development of re-processing technology in October 1993, requiring that all federal agencies establish procedures to purchase lubricants containing at least 2 5 % re-refined base oil. Many municipalities have adopted this requirement for contract purchases of lubricants as well. Although a n u m b e r of processing m e t h o d s can be used to recycle spent lubricants, most commercial re-refiners utilize at least three stages: heating to remove water and residual low molecular weight hydrocarbons (fuels), distillation to separate hydrocarbons from polar oxidation products (sludge, varnish, other insolubles), residual fuel components, organometallic additives, etc., followed by hydrotreating to reduce sulfur, nitrogen, and polynuclear (multi-ring) aromatic levels. In at least one commercial re-refining process, a p r o p a n e deasphalting stage is employed to remove undesirable by-products etnd additives prior to re-distillation. N a p h t h e n i c vs. Paraffinic B a s e Oils The widely used term "naphthenic" base oils represents those stocks produced from naphthenic crude as defined by the U.S. Bureau of Mines Classification of Crude Oils [16]. In this
system, crude fractions that boil in the range typical of base oils (250-275°C and 275-300°C; at 1 atm) are naphthenic if they have a n API gravities of < 3 3 a n d < 2 0 , respectively. Paraffinic oils for the same boiling ranges provide API gravities of > 4 0 and > 3 0 . API gravity (ASTM D 287) is sensitive to the relative ratio of paraffins, cycloparaffins, and aromatics in a crude or base oil. Higher paraffinic cheiracter increases API gravity. A significant feature that distinguishes most naphthenic crudes is a very low natural level of linear paraffins or wax, which enables the production of base oils with very low pour points and excellent low temperature fluidity, essential in some applications such as refrigeration oils. Naphthenic base oils also have a relatively high aromatic content, which has limited their use in other applications where higher toxicity is a concern. Due to improvements in dewaxing methodology and concern over the potenticd adverse health effects of naphthenic base oils, the market share of these stocks has steadily decreased over the past decade (see Ref 11 and earlier issues of the annual NPRA survey).
B A S E OIL CHARACTERIZATION Hydrocarbon Type Analysis The chemiCcd composition of lubricant base oil is strongly dependent o n a n u m b e r of factors. These include crude oil source, molecular weight range (generally higher molecular weight crudes are richer in multi-ring aromatics), the refining process (solvent vs. hydroprocessing), the degree of refining, and the effectiveness of the finishing process. Determining the chemical compositional profile of base oil is usually initiated with a chromatographic separation procedure to isolate the aromatic and saturate fractions of the sample. ASTM method D 2549 involves an open colu m n procedure using a bauxite column packing with polar and non-polar organic solvents to elute the aromatic and saturate fractions, respectively. In this procedure a sample of base oil is applied to the top of the column, which is then flushed sequentially with solvents of increasing polarity. Saturated molecules, including both paraffins and naphthenic molecules, bind to the column less strongly than ciromatic compounds and, as a result, they elute from the column with n-pentane (non-polar) while the aromatics require "stronger" polar solvents (chloroform/ethyl alcohol) to be eluted. Typical aromatic and saturate levels determined by D2549 for a series of base oils, refined by different methods, are given in Table 7. It may generally be concluded that base oils contain a preponderance of saturate compounds and that the level of aromatics for a solvent refined base oil increases with viscosity grade largely because heavier distillates contain higher
TABLE 7—Composition of base oils. Refining Process - SUS Viscosity Grade
% Saturates
% Aromatics
Solvent Refining (SR)—lOOA? SR Medium Viscosity—320N SR High Viscosity—850A^ Bright Stock Hydrocracked 100 Hydroisomerized 100 SR—Naphthenic Crude
70-90 65-75 60-70 50-60 90-100 95-100 60-70
10-30 25-35 30-40 40-50 0-10 0-5 30^0
CHAPTER 7: HYDROCARBON levels of aromatics. Further, hydroprocessing techniques, specifically hydrocracking and hydroisomerization, drastically reduce aromatic level. In addition, base oils processed from naphthenic crudes typically exhibit a high aromatic content and a very low level of paraffins (<8%). A related column chromatography method, ASTM D 2007, is based on a clay-silica gel adsorption procedure that resolves a base oil sample into aromatic, saturate, and polar fractions. In this procedure, the following solvents are used to elute the hydrocarbon fractions: n-pentane (elutes saturates), toluene (elutes aromatics), and toluene/acetone (elutes polars). ASTM D 2007 was chosen as the preferred method for base oil characterization in the so-called "Base Oil Interchangeability Guidelines" established by the American Petroleum Institute (API) in API Publication 1509 - Engine Oil Licensing and Certification System (API Address: 1220 L Street NW, Washington, DC 20005-4070, www.api.org). In this publication, the API concludes that "Not all base oils have similar physical or chemical properties or provide equivalent engine performance in engine testing The API Base Oil Guidelines were developed to ensure that the performance of engine oil products is not adversely affected when different base oils are used interchangeably by engine oil blenders" (API 1509). Under these guidelines base oils are grouped into three categories according to composition, VI, and sulfur content as shown in Table 8. Group IV is reserved for polyalphaolefins and Group V includes other basestocks not included in Groups I-IV. API Publication 1509 also defines which engine tests are required when interchanging a base oil in an approved gasoline or diesel engine oil formulation using the base oil definitions above. Currently in the U.S., approximately 1.76 billion gallons of annual production of paraffinic base oil is Group I while 870 million gallons/year is Group II [11]. The ratio of Group II to Group I has increased significantly since the early 1990s and most new base oil plants utilize refining technology that provides Group II stocks. In reference to the ASTM D 2007 procedure, it should be noted that a hydrocarbon analysis technique based on ThinLayer Chromatography with Flame-Ionization Detection has TABLE 8 --API lubricant base stock categories API Group I II III
% Saturates
% Aromatics
VI (1)
% Sulfur (2)
<90 >90 >90
>10 <10 <10
<120 >80, <120 >120
>0.03 <0.03 <0.03
"Viscosity Index by ASTM D 2270. ^Sulfur content by any of the following: ASTM D 2622, D 4294, D 4927, D3120.
TABLE 9—Typical saturates distribution of lOON (4 cSt) base oils. Relative Volume % Hydrocarbon Type
Group I
Group II
Group III
Paraffins Monocycloparafflns DlcyloparafHns Tricycloparafflns TetracycloparafHns Pentacycloparafflns Hexacycloparaffins
23.9 14.7 11.3 8.1 9.9 6.6 4.2
22.0 23.1 19.2 11.7 9.3 7.2 5.6
70.2 11.8 5.3 1.8 1.6 0.6 2.5
Total
78.7
98.0
93.7
BASE OIL CHEMISTRY
173
TABLE 10—Typical aromatic distribution of lOON (4 cSt) base oils. Relative Volume % Hydrocarbon Type
Group I
Group II
Group III
Monoaromatics Diaromatics Triaromatics Tetraaromatics Pentaaromatics Thiophenaromatics Unidentified Aromatics Total
11.7 3.1 1.1 0.8 0.4 1.6 2.7 21.4
0.9 0.6 0.2 0 0 0.1 0.2 2.0
4.0 0.9 0.2 0.1 0.1 0.4 0.6 6.3
TABLE 11—Typical aromatic sub-categories for a 70A^ group I base oil. Hydrocarbon Type
Volume %
Monoaromatics
Total 13.2
Alkylbenzenes Naphthenebenzenes Dinaphthenebenzenes Diaromatics
6.94 3.37 2.92
Naphthalenes Acenaphthenes, dibenzofurans Flourenes Triaromatics
0.91 0.89 0.77
Phenanthrenes Napthenephenanthrenes Tetraaromatics
0.45 0.22
Pyrenes Chrysenes Pentaaromatics
0.08 0.17
Perylenes Dibenzanthracenes Thiophenoaromatics
0.06 0.03
Benzothiophenes Dibenzothiophenes Naphthobenzothiophenes Unidentified Aromatics
0.23 0.50 0.0
2.6
0.7
0.3
0.1
0.7
Total Aromatics
1.5 19.0
demonstrated that "the ASTM method does not always yield pure fractions of each hydrocarbon type and cross contamination introduces considerable inaccuracies in the results" [17]. These deviations are attributed to column overloading, incomplete separation, and incomplete recovery of fractions from the column for D 2007. Spectrometric Identification of Base Oils Components Although Nuclear Magnetic Resonance (NMR) [18,19], Ultra Violet (UV; ASTM D 2008), and Infrared Red (IR) spectroscopies [20], along with a number of chromatographic techniques [21] have been successfully employed to characterize the compositional profile of lubricant base oils, several mass spectrometry (MS) techniques have evolved into standard ASTM methods suitable for base oil analysis. Once base oil saturate and aromatic fractions are isolated by the column chromatography procedures discussed above, each fraction can be subjected to further analysis and sub-catergorization by mass spectrometry using ASTM Methods D 2786 and D 3239. Tables 9-11 contain the major sub-categories of satu-
174 MANUAL 37: FUELS AND LUBRICANTS
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rated and aromatic molecular groups resulting from these MS techniques for typical Group I, II, and III base oil types. Representative molecular structures for the hydrocarbon types outlined in Tables 9 and 10 eire given in Tables 12 and 13. It should be noted that actual "base oil" molecular structures would contain a greater number of carbon atoms than shown in Tables 12 and 13. The structures shown are illustrative of the chemical functionality of base oil, however. A more general discussion on the characterization of petroleum fractions by MS is the subject of an excellent review article by Altgelt and Boduszynski [22]. ASTM D 3239 further subdivides the aromatic fraction detailed in Table 10 into sub-categories as illustrated in Table 11 for a Group I base oil. NMR Spectroscopy of Base Oils Although a detailed review of NMR spectroscopy as applied to base oil characterization is beyond the scope of this chapter, it should be emphasized that '^C-NMR spectroscopy is an extremely useful method to distinguish aromatic from saturated carbon atoms in base oil while quantifying the length and degree of branching for peiraffinic components. Recent
papers by Sarpal [18,23] and Sahoo [24] demonstrate the potential of this method in characterizing base oil. Aromatic carbon atoms exhibit resonances at chemiced shifts ranging from 100-160 while saturated carbons (paraffins and naphthenes) provide shifts in the range of 10-60 ppm [24]. From the information in Table 14, it is also possible to estimate the average peiraffinic chain length of base oils by developing ratios of chain ends (-CH3 groups) to -CH2- groups well separated from chain ends. A relatively new pulse protocol called DEPT expands the utility of ^^C-NMR for base oils in that it provides individual spectra for each carbon atom t3?pe CH„ where n = 0-3 thereby increasing resolution and the ability to quantify carbon species [23]. In using NMR to study base oils, care must be taken, however. '^C-NMR is inherently non-quantitative since each carbon atom relaxes at a rate depending on its surroundings and complete relaxation is required for integration data to be quantitative. Since carbon nuclei relaxation times can be on the order of seconds, total relaxation between NMR pulses is not typically achieved. As a result, the use of a so-called relaxation agent, such as chromium acetylacetonate, is essential with base oils to ensure complete relaxation enabling quantitative integration.
TABLE 12—Molecular structure of base oil hydrocarbon types. Hydrocarbon Types
Paraffins (inclndes isoparaffins)
Mononaphthenes
Dinaphthenes
Trinaohthenes
Monoaromatics
Structures
\A/^/\
-0oCc;r V^A^A^-JY^
^
Indanes: Aromatic / Naphthene "Hybrid"
^
Diaromatics
/lL^js*^j,,Jk^
CHAPTER 7: HYDROCARBON
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175
TABLE 13—Molecular structure of base of hydrocarbon types (Continued). Stmctures
Hydrocarbon Types
Triaromatics
Tetraaromatics
Thiophene
^
-o^ 0
Benzothiophene
CO
Dibenzothiophene
0:p
Napthobenzothiophene s
TABLE 14—Structural parameters derived from '^C-NMR. R-Group CHa CH3 CH2 CH2 CH2 CH2
Description (1) Short Branch E n d Long Branch E n d (a) Adjacent to Long Chain E n d (p) Near Chain End (7) Adjacent to Long Chain CH2 (8) Middle of Long Chain (e)
Typical Chemical Shift (ppm) 11.4 14.1 22.7 32.0 30.1 29.7
B A S E OIL C H E M I S T R Y Oxidation Reactions Hydroccirbon oxidation chemistry is of great importance in the ukimate appHcation of base oils and lubricants, since at the high temperatures typical of most lubricating environments, oxidation can lead to undesirable oil thickening, the formation of insoluble sludge and varnish deposits, and the creation of potentially corrosive organoacid compounds. Although this subject has been extensively covered in many ex-
cellent reviews and texts [2,25-31 ], it is worthwhile to discuss several key mechanisms. As shown in Fig. 1, hydrocarbon oxidation proceeds through a number of steps involving free radicals: The initiation stage can be triggered by peroxide or hydroperoxide impurities in the oil [32] or by the thermal cracking of hydrocarbons on hot metal surfaces in cin engine, for example. Once a radiceJ is formed it can readily extract a hydrogen atom from hydrocarbon molecule (R-H), which is subsequently very reactive toward oxygen (O2) 3rielding a peroxide radical (R02»). Table 15 compares the relative hydrogen atom abstraction reaction rates for primary, secondary, and tertiary carbon atoms when attacked by a peroxy radical (sec-alkyl- COO») and a t-butoxy radical ((CH3)3CO») [33]. These data show that hydrogen atoms cffe removed more easily (faster reaction rate) in the order tertiary > secondary > primEiry, which is directly related to the stability of the radical that is formed. This observation suggests that molecular structures such as linear parafBns will have greater resistance to oxidation compared to other branched paraffins or cycloloparafKns that contain a relatively greater proportion
176 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK Initiation:
I*
I-I R-H + !•
Propagation:
^
I-H + R*
->
R02* •
R02» + R-H Termination:
ROOH + R*
R« + R02» "
-> ROOR
2R02»
-•R-O4-R
R-O4-R
-p. O2 +Non-Radical
Products FIG. 1—Hydrocarbon oxidation by free radicals.
TABLE 15-—Relative hydrogen abstraction rates. Molecule Radical
CeHs CH3
sec-alkyl-COO«
1 R-CH3 1
(CH3)3CO.
Cj6^5^'l2^H3
9 R2CH2
12
C6H5CH-(CH3)2
14 R3CH 44
TABLE 16—Reactivity of substrate molecules toward the phenyl radical. Relative Reactivity per a-Hydrogen Substrate Molecule 1 Primary Aliphatic 10 Cyclopentane 9 Cyclohexane 72 Indane (See Table 12) 92 Acenapthalene
of secondary or tertiary hydrogen atoms. The rate of free radical propagation is Eilso very dependent on ring geometry and adjacent function groups as shown in Table 16 for phenyl radicals [33]. As oxidation proceeds, propagation reactions involving peroxy radical addition to other hydrocarbon molecules typically result in the formation of polar compounds and higher molecular weight species, which may ultimately increase the viscosity of the oil. The combination of oligomeric and polymeric radicals during the termination phase of oxidation can also cause a molecular weight increase and viscosity enhancement. Studies on the autoxidation of n- hexadecane (linear CI8 paraffin) at temperatures of 160-180°C have illustrated that Ccirboxylic acids, hydroperoxides, ketones, alcohols, and esters Eire all formed over a period of hours. It is interesting to note, however, that this "model system" exhibited a significant increase in viscosity while only a "modest" increase in molecular weight was observed [31]. A useful oxidation model for thin hydrocarbon films on metal surfaces has been developed by Professor E.E. Klaus a n d his co-workers at the Pennsylvania State University [34-36]. They have shown that if Molecule A is oxidized it forms Molecule B that initially has the same molecular weight as A. At this stage, both A and B can volatilize to A'
and B', respectively, which can Hmit further reactivity by evaporation from the hquid phase system. As oxidation continues, B undergoes condensation-type polymerization reactions to yield molecule P that has a high molecular weight but is still soluble in the hydrocarbon liquid phase. As a result, P is capable of significantly increasing the viscosity of the lubricant being oxidized. As a polymer, P is not volatile and remains in bulk solution and in contact with the metal surface. Through further oxidation and thermal decomposition, P becomes an insoluble deposit of very high molecular weight on metal surfaces. This model has been effective in predicting deposits in both gasoline and diesel engine tests as well as high temperature oxidative oil thickening in many applications. Although hydrocarbon oxidation can have a profound effect on the properties of a lubricant, this process can, fortunately, be controlled by the use of antioxidant additives. These molecules will be discussed more extensively in another chapter of this manual. Very generally, antioxidants function either by scavenging radical species or by decomposing peroxides to unreactive products. Radical scavengers include hindered phenols such as t-butylphenol, quinines, and certain amines. Zinc dialkyldithiophosphate, a common antiwear compound, also serves in a peroxide decomposing capacity as well.
I m p a c t o f S u l f u r C o m p o u n d s o n B a s e Oil Chemistry Throughout the evolution of base oil refining and processing technology, sulfur content has been employed as an indicator of product quality and a predictor of lubricant performance. In this context, the t e r m "sulfur" relates to soluble, organosulfur compounds that occur naturally in crude oil in contrast to elemental, yellow sulfur. Current domestic base oil sulfur concentrations typically range from <0.05 wt% to 0.4% for light base stocks and u p to 0.8-1.0% for heavier viscosity grades. In other regions of the world, it is not uncomm o n to observe sulfur levels in excess of 1.5-2.0%. From the perspective of base oil chemistry, how the organosulfur content impacts lubricant performance is very complex and depends, to a large extent, on the ultimate application and the nature of the additives used in the formulation. Although current manufacturing trends suggest that a very low sulfur content is most desirable for base oil, it has historically been thought that higher sulfur levels directionally improve base oil oxidation stability as well as friction a n d wear performance. The pioneering work of Dennison and Conduit [37] in the 1940s concluded that "the oxidation stability of refined petroleum lubricating oils is the result of small quantities of natural sulfur compounds and not of any inherent stability of the hydrocarbon fraction itself. In the absence of the natural sulfur compounds the hydrocarbon fraction oxidizes rapidly and, in the initial stages of the reaction, autocatalytically." This work clearly illustrated that certain sulfur containing compounds, specifically monothioethers containing at least one aliphatic or cycloaliphatic group attached to the sulfur atom (sulfides), were very potent antioxidants in lubricants. These authors were successful in selectively removing the sulfur compounds from a mineral oil while keeping other
CHAPTER
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TABLE 17—Oxygen uptake of base oil—impact of sulfur compounds. Oxygen Uptake at 1 h, 171°C (cm^ per 100 g oil) Sulfur Compound Added Base Fluid 0.1% Natural Organosulfur Base Oil E (102 VI, 0.1% S) 100 Desulfurized Base Oil E None >2000 Desulfurized Base Oil E +1.4% cetyl phenyl sulfide 190 Desulfurized Base Oil E + 1% bis(phenylethyl) sulfide 50 0.53% Organosulfur 170 Base Oil A (8 VI, 0.53%S) Desulfurized Base Oil A None >5000 Desulfurized Base Oil A 1% dicetyl sulfide 90 Desulfurized Base Oil A 2% dicetyl sulfide <10 Desulfurized Base Oil A 3% dibenzothiophene >5000
compositional variables constant. They observed that the high temperature oxidation rate of the sulfur-free oil was m u c h higher than the original sulfur-containing base fluid. In these experiments oxygen uptake by base oil was monitored over time (hours) at 171 °C under atmospheric pressure. Typical data are provided in Table 17. Table 17 illustrates that desulfurized base oils exhibit very poor oxidation inhibition compared to the original base oil and that certain sulfur compounds, specifically organosulfides, can significantly reduce the rate of oxidation at high temperature. Sulfur-containing aromatic molecules, however, such as dibenzothiophene, did not appear to act as antioxidants and may actually increase the rate of oxidation slightly (data not shown). Further work reviewed by Harpp et. al. [32] revealed that sulfides in themselves are not antioxidants but rather they become active when oxidized to — SO2, — SO3 and related organo-sulfonic acids. These authors also report that thiophenes (or thioaromatics) have "no stabilizing effect on hydrocarbon oil oxidation" and they were not able to identify a molecular mechanism through which antioxidation could occur. More recently, using both statistical and neured network modeling techniques, Stipanovic, Smith, and co-workers [3,4] have shown that base oil thioaromatic content can be directly correlated to an increase in oxidation and deposit formation level observed for crankcase engine oils in the ASTM Sequence HIE and VE gasoline engine tests. These observations were attributed to an increase in the rate of free radical propagation in the engine oil at high temperature based on data reported by Russel [33]. As shown in Table 18, the free radical hydrogen a t o m abstraction rate is m u c h higher for thiophenes than other hydrocarbons for at least two types of radicals that could occur in hydrocarbon systems. As a result, it is reasonable to conclude that propagation reactions can be accelerated by the presence of thioaromatic molecules, such as benzothiophene, although data specific to alkylperoxy radicals, of significance in hydrocarbon oxidation, are not readily available. The role of sulfur compounds as natureJly occurring base oil stabilizers has some very interesting implications. It is well known that untreated, sulfur-free synthetic base oils, such a polyalphaolefins, oxidize more readily in tests such as the Rotary Bomb Oxidation Test (RBOT;ASTM D 2272) than conventional mineral oils that contain sulfur compounds. However, if appropriate antioxidants are added to both t5^es of base fluids, the PAOs typically respond with better longterm oxidation stability. Severely hydrocracked, low sulfur Group II and III base oils exhibit similar behavior.
TABLE 18—Relative reactivity of hydrocarbon radicals. Substrate Species: Radical
CH3 - X X =
Phenyl* Phenyl* Phenyl* Phenyl*
Alkyl Phenyl Benzothiophene 2-Thiophene
RCH2* RCH2*
Phenyl 2-Thiophene
Relative Reaction Rate per Hydrogen Atom 1 9 14 15 65 210
TABLE 19—Impact of basic nitrogen compounds on the oxidation of hexadecane (Cu catalyst, 0.6% butylated hydroxy toluene (BHT) added as an antioxidant). Compound Hexadecane (HD) HD + 3-n-butylpyridine HD -1- 2,4-dimethylquinoline HD + 2-methylindole
HD + Carbazole HD + Phenanthridine
Effective Nitrogen Concentration (ppm)
RBOT Lifetime (min.)
10 5 2 9 4.5 2 11 6 0.2 10 10
596 216 248 296 306 324 405 501 516 578 576 440
Nitrogen C o m p o u n d Reactivity Nitrogen containing molecules found in base oils can also accelerate oxidation and deteriorate the useful lifetime of lubricants. More specifically, "basic nitrogen" compounds (socalled proton acceptors) such as various pyridine derivatives, can act at very low concentrations (below 10 p p m based on N) in deteriorating oxidative stability [6,38,39]. Oxidation lifetime for the straight chain paraffin hexadecane, a base oil "model compound," in the presence of small quantities of "basic nitrogen" are shown Table 19. The data clearly illustrate that such species can promote oxidation rate in the ASTM D 2272 RBOT procedure. These results illustrate that the molecular structure of the "basic nitrogen" compound influences the oxidation reaction. More specifically, Yoshida et. al [38] have found that reducing the pKb of the nitrogen group enhances the overall rate of oxidation. In addition to these model compound studies, statistical and neural network modeling methods have demonstrated that RBOT lifetimes for prototype industrial oils (hydraulic
178
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fluids and turbine oils) also decrease with increasing base oil basic nitrogen content. Further, thermal sludge formation and oxidation onset determined by a high pressure differential scanning calorimetry technique indicated a deterioration with higher levels of basic nitrogen [6]. Olefins Although crude oils Eire generally relatively low in olefin content [10], base oil processing techniques can introduce olefins, especially at high temperatures, due to "cracking" reactions. In the presence of heat or UV light, olefins can polymerize to form higher molecular weight products that can color the base oil or actually cause sediment. In general, olefins can be removed during the process of hydrofinishing or by clay treatment discussed above [10].
CHEMICAL COMPOSITION/LUBRICANT PERFORMANCE CORRELATIONS As previously discussed, base oils contain a broad spectrum of paraffinic, cycloparaffinic, and aromatic molecules, the distribution of which varies with crude source and refinery processing. More importantly to the lubricant chemist and engineer, each of these hydrocarbon types can exert a different effect on the ultimate physical and chemical properties of the base oil and, ultimately, the lubricant from which it is formulated. Table 20 illustrates the relationship between hydrocarbon type and performance for several important properties. B a s e Oil R h e o l o g y a n d C o m p o s i t i o n The flow properties, or rheology, of a lubriccint strongly reflect the composition of the base oil used in its formulation. Comparing hydrocarbons of similar molecular weight, paraffins (especially linear paraffins) provide the highest positive contribution to base oil VI while aromatics and naphthenes, particulcirly multi-ring structures, strongly decrease VI. Base oils of high VI Eire generally preferred for lubricants because they provide higher viscosity at high temperatures and lower viscosities at low temperatures, provided they are properly treated to inhibit wax crystallization. In the absence of effective pour point depression, higher VI oils containing elevated levels of wax display poor low temperature fluidity as wax crystals form network structures that Eire resistant to flow. In most cases, polymeric pour point depressant additives eliminate wax crystallization and, u n d e r these circumstances, base oils rich in paraffins can exhibit excellent rheological properties at low temperatures.
In engine oils, poor cold temperature fluidity can reduce pumpability to the extent that oil stcirvation causes catastrophic engine failure. To protect against this occurrence, the SAE J300 Engine Oil Viscosity Classification specifications include two ASTM tests, D 3829 and D 5293, which measure both the high shear viscosity and pumpability, respectively, of engine oils to assure that a motor will start and have sufficient oil fluidity to assure good lubrication. A low temperature viscosity limit is also included in the specifications for many geeir oils and transmission fluids. This viscosity is determined using a very low shecir Brookfield Viscometer operating at —5 to —40°C, depending o n viscosity grade. The specific test method is described in ASTM D 2983. Composition/Performance Correlations for Engine Oils Gasoline cind heavy duty diesel engine oils probably represent the most sophisticated lubricant formulations in terms of physical a n d chemical requirements and, subsequently, their additive packages are very complex. Engine oils must provide a fluid lubricating film for sliding metal-to-metal interfaces at high t e m p e r a t u r e s while neutralizing acidic combustion gases; minimizing oxidation and corrosion; suspending insoluble combustion a n d lubricant oxidation byproducts; reducing wccir and the tendency to foam; etc. Since each of these characteristics can be influenced by base oil composition, considerable attention has been focused on relating specific molecular components to engine performance. Murtay and co-workers [1,2] were among the first to use base oil compositional data derived from mass spectrometry to develop statistical correlations between oxidation in the ASTM Sequence IIIC and HID oxidation engine tests and base oil hydrocarbon type distribution. These authors observed that a regression function including saturate content plus total sulfur concentration predicted viscosity increase very well for a series of engine oils formulated to be API SE quality. In this function, higher levels of saturates and sulfur enhance performance. Viscosity Index edone was not found to be a good predictor of performance for oils originating from different crude, sources, but it did correlate reasonably well for samples derived from a single crude. Murray also demonstrated that during the course of oxidation reactions, the level of both saturates and aromatics decreased while the concentration of polcir compounds increased significantly. For example, in the ASTM Sequence IIIC engine test, a SAE lOW-30 oil that was originally 74.1% saturates, 13.6 % aromatics, and 4.9% polars exhibited an end-of-test composition of 46.1% saturates, 7.9% aromatics, cind 33% polars.
TABLE 20—Performance characteristics of base oil components. Base Oil Proprety
ParaiBns
Naphthenes
Aromatics
Viscosity Index (VI) Low Temperature Fluidity" Low Temperature Fluidity* Pour Point Oxidation/Thermal Stability Solvent for Additives
Excellent Poor Excellent Poor Excellent Poor
Poor-Good Good Good Good Poor-Good Good
Poor Good Good Excellent Poor Excellent
"Unadditized. ''Treated with a pour point depressant.
CHAPTER Roby and co-workers were also successful in developing statistical correlations between base oil composition and performance in the ASTM Sequence HID and VD gasoline engine tests [40]. For high temperature oxidation in the HID procedure, it was learned that no single base oil parameter provided a good correlation to viscosity increase but a regression equation including nitrogen content (ASTM D 4629), olefin n u m b e r (ASTM D 460), sulfur content (ASTM D 1552) and saturates (column gradient elution chromatography) provided an excellent fit (R-squared = 0.97) to the observed data. For oxidation control, high levels of saturates and sulfur compounds were beneficial while elevated olefin and nitrogen levels contributed to poor performance. In the Sequence VD gasoline engine test, which eveJuates deposit formation at relatively low levels of oil oxidation, the individual variables discussed above all showed good correlations (R-squared values > 0.79) to varnish formation with higher nitrogen, olefin, and sulfur content being detrimental. Average engine varnish ratings in the Sequence VD improved with increasing base oil saturates content, however. More recently, base oil effects in the Sequence HIE and VE engine tests have been studied [3,4] using Partial Least Squares (PLS) and neural network modeling methods. These authors evaluated the engine test performance of a group of different base oils (approx. 12 oils) formulated into engine oils using similar additive technology. For the Sequence HIE oxidation test, viscosity increase and piston skirt varnish ratings generally improved with paraffin content while reacting negatively to high levels of multi-ring naphthenes, multi-ring aromatics, and thioaromatics. Total sulfur content was found to reduce viscosity increase and piston deposits in the Sequence HIE, consistent with its antioxidant effect discussed earlier. For the Sequence VE test, total sulfur, thioaromatics, and multi-ring aromatics all enhanced the formation of varnish deposits. Using the PLS modeling protocol described in Ref 3, it is possible to predict the engine test performance of lubricants, assuming a c o m m o n passenger car engine oil additive technology, using base oil compositional features as input. In this fashion, the potential Sequence VE varnish ratings for engine oils formulated from a population of base oils representing typical U.S. production were predicted. Ratings are based on a visual examination of a n u m b e r of engine pairts where 10.0 corresponds to a totally clean part, 5.0 is the m i n i m u m "passing" rating, and <4.0 corresponds to a very poor engine oil formulation. API Group I oils gave varnish values of 5.0 or less. Group II oils exhibited improved varnish ratings in the range of 5.5-5.8, while Group III and IV oils provided ratings of 6-6.5. In a separate study, a group of re-refined oils (RR) were evaluated and, on average, predictions suggested that they would perform at levels comparable to "virgin" oils (VO) assuming normeJ test-to-test vEiriability. These results are summarized in Table 2 1 . Several generalizations can be made about the role of base oil composition in heavy duty diesel engine oils where oil thickening due to the accumulation of soot cind piston ringland deposits are important considerations in several industry standard tests. Base oils of comparatively high saturates content have been found to perform better in the Mack T-8 engine test than oils of higher aromatic content. This test
7: HYDROCARBON
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OIL CHEMISTRY
179
TABLE 21—Predicted engine test results for a re-refined and virgin base oils in formulated engine oils. Test Rating Sequence HIE Piston skirt varnish (10 = "Clean"; 8.9 = "Passing") % Viscosity increase ( < 3 7 5 % = "Pass") Sequence VE Average engine varnish (10 = "Clean"; 5 = "Passing")
High Result (VO/RR)
Average Result (VO/RR)
Low Result (VO/RR)
9.21/9.34
8.92/8.95
8.63/8.7
283/219
192/163
73/65
5.45/5.51
4.93/5.08
4.32/4.8
measures engine oil viscosity increase as a function of soot level under conditions that result in very little oil oxidation, and it is very sensitive to the chemical nature of the dispersant used in the additive package. It has been speculated that the strong base oil effect could be due to the impact of base oil solvency on the dispersant and its ability to interact with and stabilize soot particles from aggregation. Base oils with high aromatic concentration strongly solvate the dispersant molecule and perhaps, the solvation shell inhibits the dispersant from properly interacting with soot. For oil ringland deposits, evidence suggests that base oils high in saturates content are not effective in washing away deposit precursors in the ring belt region of the piston and, as a result, deposits form more readily. Since this mechanism could be very dependant on additive chemistry, its universality has not been established. API document 1509 also details the relationship between base oil group and the engine testing required when base oils are interchanged in heavy duty diesel engine oil formulations. A more detailed analysis of the impact of base oil composition on the performance of heavy duty diesel engine oils has recently been published by Cherrillo and Huang [41].
B a s e Oil E f f e c t s i n I n d u s t r i a l L u b r i c a n t s Although base oil composition can play an important role in defining the performance of engine oils, additive chemistry and concentration can largely overcome potential deficiencies of a pEirticular basestock. Typically a 10-15% treat level of dispersant/inhibitor package is added to a motor oil to control oxidation, deposits, etc. In industrial lubricants, however, formulation economics dictate very low additive treat levels, usually less than 2% for products such as hydraulic fluids, turbine oils, quenching oils, and other process fluids. As a result, product quality is linked more directly to base oil composition. The work of Murray cited above [1] also demonstrates that industrial oil oxidation lifetimes in the D 943 oxidation test a n d the D 2272 Rotary B o m b Oxidation Test (RBOT) increased with increasing saturate levels for 150 N oils containing an oxidation inhibitor. Sulfur content played a minor role, however, presumably because the inhibitor effectively controlled oxidation. For the D 943 test, Firmstone and coworkers [4] showed, using Partial Least Squares models, that increases in base oil peiraffin content, sulfur concentration, VI, and aniline point enhance oxidation lifetime, while cycloparafHns, multi-ring aromatics, and nitrogen content have
180 MANUAL 37: FUELS AND LUBRICANTS
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a negative impact. In a study by Mooken on the stability of turbine oils, D 943 eind D 2272 results improved with higher levels of saturates and sulfur while lower concentrations of aromatics and basic nitrogen were most beneficial [39]. The impact of evolving base oil quality toward Group II and III stocks on the performance of greases and other industrial oils has been recently reviewed [42]. For base oils containing only negligible quantities of aromatic compounds and essentially no heteroatomic species, oxidation stability was found to be influenced positively by paraffin content and negatively by polycyclic naphthenes. Influence of Base Oil Composition o n the Viscosity-Pressure Coefficient Most organic fluids, including lubricant base oils, exhibit a reversible viscosity increase with the application of high pressure due to the molecular mobility restrictions imposed by the forces being exerted [43]. This phenomenon is especially important for lubricants, since in most mechanical applications, films of fluid are compressed between sliding surfaces under very high loads. The degree to which a fluid thickens under high pressure is given by the following expression [43,44]: log Tjl 7)0 = P a where: T) = Viscosity at pressure P Tjo = Viscosity at atmospheric pressure P = Pressure a = Viscosity — Pressure Coefficient In this expression, if pressure is given in MPa (megaPascals), the viscosity-pressure coefficient for a base oil is t5^ically 9-15 GPa~'' The importance of a in the process of lubrication is multifold. In the so-called hydrodynamic regime of lubrication, the film thickness of fluid that protects the mechani-
cal device from high friction and premature weeir is dependent on the high shear viscosity of the lubricant and its viscosity-pressure coefficient [45]. Thicker lubricant films are, therefore, produced by fluids exhibiting a higher a value. However, in the design of many fuel economy enhancing "energy conserving" lubricants, a less viscous film is desired since mechanical energy can be wasted if the film is too robust. In these cases, lubricants of lower a are favored. In other applications such as Continuously Variable Transmissions (CVTs), where the fluid must provide high levels of traction, a very high a is needed [46]. Based on the importance of a in the design and application of lubricants, effort has been focused on determining the relationship between base oil structure and its response to high pressure [44]. Figure 2 illustrates the magnitude of viscosity increase that can be expected for typical lubricant base oils, over pressure ranges not uncommon in certain mechanical devices, plotted by API base oil groups. These data were recorded using a falling body viscometer [47]. In general, Group IV (polyalphaolefin) fluids provide the lowest increase in viscosity with increasing pressure, followed by Group III, Group I, Group II, and finally napthenic base oils. In this specific example, a statistical analysis [48] revealed that the Group III and IV oils provided the lowest a values because these products were very high in paraffin content (>60%), while Group I and II products provided intermediate performance because they contained less paraffins and higher levels of naphthenic ring compounds (>40%). Since the cycloparaffin ring structure is sterically very bulky, it is especially sensitive to applied forces and contributes significantly to increasing the viscosity-pressure coefficient for base oils. Planar aromatic rings appear to pack with less difficulty. Under pressure, paraffins are easily compressed and can actually be induced to crystallize if pressures are sufficiently high. In the design of CVT fluids, molecules are synthesized to optimize their steric bulkiness [46]. A number of other procedures are also available to calculate the viscosity-pressure coefficient for base oils from com-
Group I -*- Group 11 -A- Group III -¥r Group IV —- Naphthene
100
200
300
400
500
600
Pressure (MPa) FIG. 2—Viscosity - pressure relationships for base oils at 100°C (nominaiiy 4 est at 100°C, 1 atmosphere pressure).
CHAPTER positional data and/or other bulk fluid properties [44,49]. Roelands [44] has shown that the viscosity-pressure relationship for base oil can be predicted solely from atmospheric viscosity (TJO) a n d a knowledge of the percentage of carbon atoms in a aromatic ring structure (CA) and the percentage of naphthenic (cycloparaffinc) carbons (Cn). Johnston determined that the pressure-viscosity coefficient could be calculated from ambient pressure fluid density and the viscosity/temperature relationship (specifically viscosity at two temperatures is needed) [49]. More recently, Spikes and co-workers have shown that the thickness of a lubricant film under conditions of elastohydrodynamic lubrication (EHD) can be related to a by the following expression obtained from a high-speed ultrathin film interferometry technique [45]:
7: HYDROCARBON
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OIL CHEMISTRY
181
and solubilize this plasticizer is an important consideration. In other cases, certain base oil molecular fractions can actually dissolve into the rubber matrix causing it to swell. Although most lubricant base oils are relatively inert in their ability to deleteriously interact with a variety of elastomeric materials commonly in use, high aniline point products, such as Group IV polyalphaolefin (PAO), can cause elastomer cracking after long periods of exposure at high temperatures due to a loss in plasticity. In many cases, a so-called seal swell agent can be successfully added to PAO to maintain good seal characteristics. At the other extreme, low aniline point naphthenic base stocks can cause seals to swell excessively also creating operational problems. As a result, for any base oil system, rubber compatibility should be evaluated carefully in formulating a lubricant product.
h oc U 0-*^ 7, 0*" a 0 "
where: h = film thickness measured by interferometry, U is the mean entrainment speed, and TJ is the low pressure dynamic viscosity. B a s e Oil S o l v e n c y E f f e c t s In any lubricajit formulation, base oils are required to dissolve polar additive compounds and to ultimately disperse polar oxidation products that are formed during use. For this reason, the so-called aniline point of a base oil can be a critical parameter in defining its compatibility with additives and the byproducts of use. Aniline point is determined by ASTM D 611 a n d represents the t e m p e r a t u r e at which aniline, (C6H5MNH2), a polar aromatic compound, becomes miscible with a hydrocarbon base oil. At low temperatures, base oil and aniline are not miscible but as temperature is raised they become a single phase at the aniline point, commonly expressed in °F. In general, base oils of low aromatic/high saturates content have high aniline points (>230°F, 110°C), conventional solvent refined base oils have m o d e r a t e aniline points (200-215°F, 93-102°C) and naphthenic base oils have a very low aniline point (<150°F, 65°C). As a result, aniline point can be viewed as a sensitive index of the "solvent power" of a base oil t o w a r d polymeric VI improvers, organometallic friction modifiers and anti-wear agents, detergents, anti-oxidants, and other molecules. In severe cases, additives soluble in low to medium aniline point base oils might experience limited solubility in high aniline point products. In these cases, improved solubility can be attained by adding a small fraction of a very polar synthetic basestock such as polyolester or an alkylbenzene to the high aniline point base oil thereby increasing its "solvent power." Due to concerns over the potential toxicity of aniline, some effort has been devoted to eliminating this procedure. It has been found using chemometric techniques that proton and '^C-NMR spectral data can be correlated to aniline point with a high degree of accuracy, possibly creating an opportunity to use NMR directly as an index of base oil solvency [48]. Rubber Compatibility In many service applications, lubricant products come in contact with rubber (elastomer) seals, gaskets, o-rings, and other components. Since most elastomers include a plasticizer to soften the material, the ability of a base oil to remove
Biological P r o p e r t i e s of B a s e Oils Since lubricants can come in direct contact with humans in some applications, the biological toxicity of base oils is a concern especially for those products containing high concentrations of polycyclic aromatic hydrocarbons (PAHs). These compounds have been found to be mutagenec and carcinogenic axid the International Agency for Research on Cancer has classified 18 PAHs as cancer causing [50 and references therein]. From a molecular structure perspective, these 18 molecules each have in common a 4-6 fused aromatic ring system and a so-called 3-sided concave "bay region" similar in structure to the geometries shown for triaromatic and tertaaromatic molecules shown in Table 13. Probably the most well established method to assess the potential mutagenicity/carcinogenicity of base oils is the IP 346 Method that uses dimethylsulfoxide (DMSO) extraction to isolate PAHs. By correlation to mouse skin painting studies, it was determined that base oils exhibiting DMSO-extractable levels exceeding 3 wt % should be considered carcinogenic [50]. F u r t h e r work by Nessel a n d co-workers indicated that a level of 5% IP 346 extractables might be a more appropriate target [50]. This group also developed a NMR-based method to characterize the concentration of hydrogen atoms in the so-called "bay region" of PAHs and they have compared these data to IP 346 results and t u m o r incidence results obtained from skin painting studies as shown in Table 22. Based on these results, Nessel proposes that a limit of 1500 p p m of bay region hydrogen atoms be established as a limit of carcinogenicity [50]. NMR enjoys the advantage of being much faster and less expensive than other methods of assessing the toxicological effects of base oils. Unpublished results generated by the author indicate that the IP 346 method correlates reasonably well to the multiring aromatic content obtained from ASTM D 3239 for relatively light to m e d i u m viscosity base oils (70-150N or 3-5 cSt at 100°C). However, it underestimates the aromatic content of m e d i u m to heavy grades (320-850N). Presumably the DMSO extraction procedure of IP 346 is not effective in solubilizing higher molecular weight aromatics. It was not established, however, if these heavier aromatics promoted a mutagenic or carcinogenic response in animal tests. It is possible that their relatively large size restricts their molecular mobility and ability to be adsorbed into the tissue of animals.
182
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
TABLE 22—Base Oil Carcinogenicity: A comparison of techniques. Base Oil/Petroleum Stream
NMR - Bay Region Hydrogens (ppm)
IP 346 - % DMSO Extractables
Tumor Incidence - %
Extracted/Hydrotreated paraiEnic oil Extracted/Nonhydrotreated paraffinic oil Hydrotreated naphthenic oil Straight-run hydrotreated gas oil Processed paraffinic blend Unrefined vacuum distillate Vacuum distillate aromatic extract Catalytically cracked gas oil
Up to 39 Up to 86 100-600 100-200 1500 1500-3200 3800-9000 3800-5600
<1.0 <1.4 2.5-6 5-6 5-6 5-12 12-15 16-43
0 0 0 0 36-54 20-60 60-90 4-40+
TABLE 23—Joint ASTM/IP lubricant test methods. ASTM Test Number
IP Test Number
D 445-97
71
D 611-82 D 2270-93
2 226
D 4629-96
379
are: Institute of Petroleum: www.petroleum.co.uk, ASTM International: www.astm.org
Method Description
Kinematic viscosity cmd cal'c of dynamic viscosity Aniline point determination Cal'c of viscosity index from kinematic viscosity Organically bound nitrogen
Biodegradability The ability of a lubricant to degrade naturally in the environment is becoming an increasingly important property that is directly related to the composition and molecular weight of the base oil employed in its formulation [52,53]. Currently, several standcird experimental protocols are widely utilized to evaluate the biodegradability of a lubricant: ASTM D 5864 (fully formulated lubricants), D 6006 (hydraulic fluids), and the European method CEC-L-33-T-82. Voltz has shown that hydraulic fluids based on hydrocarbon mineral oils only degrade to an extent of 40% after 25 days in the CEC test while synthetic, ester-based fluids are degraded almost entirely (>90%) [53]. Synthetic hydrocarbons and polyethylene glycols were found to display intermediate behavior with maxim u m degradation levels of 70% and 55%, respectively. In addition to composition, the size of a base oil molecule can influence its biodegradability. Weller has reported using the CEC protocol, that 2 and 4 cSt PAOs degrade at reasonably high levels, 75% and 65% respectively, while 6, 10, and 100 cSt PAO samples degraded less than 20% [54]. Similar results have been reported for other types of base oil suggesting a limiting molecular size exists above which an organism cannot adsorb and biodegrade hydrocarbon molecules.
INTERNATIONAL STANDARDS As shown in Table 23, many of the ASTM experimental test procedures outlined in this chapter have analogs in the Institute of Petroleum (IP) Standard Test Methods published in the UK. Several examples are given below where the ASTM and IP organizations share a single test method: in m a n y other cases, the ASTM and IP have developed methods to determine the same lubricant property (sulfur content, low temperature viscosity, base oil aromatics, etc) through the application of somewhat different procedures. Each organization maintains an web site with extensive capabilities of searching for specific methods. The addresses for each site
CONCLUDING REMARKS It was the objective of this chapter to demonstrate that hydrocarbon base oils are a complex collection of molecules whose diverse individual properties combine to determine the performance characteristics of a formulated lubricant. Examples and references have been provided that support this objective and serve to highlight the overall importance and complexity of base oil technology as an integral part of the lubricant industry. In addition, an effort was made to highlight those anal34ical techniques and predictive modeling procedures that enable the chemical composition of a base oil to be determined and then exploited to predict how it will behave in actual product applications. Although further research and method development is still needed, a future can be envisioned where actual engine tests, bench oxidation tests, or biological assays are not needed to confirm the quality/toxicology of a base oil relying, instead, on predictive modeling.
STANDARDS ASTM No. D 92-01
IP No. 36
D 93-00
34
D D D D
97-96a 189-01 297-92 445-97
15
D D D D D D D D
482-00 524-00 611-82 892-01 943-99 1492-96 1500-98 2007-01
D 2008-91
71
2 146 157
Method Description Flash and Fire Points by Cleveland Open Cup Flash Point by Pensky-Martens Closed Cup Tester Pour Point of Petroleum Products Conradson Carbon Residue API Gravity Kinematic Viscosity and Cal'c of Dynamic Viscosity Ash in Petroleum Products Ramsbottom Carbon Residue Aniline Point Determination Lubricant Foaming Oxidation of Inhibited Oils Bromine Index ASTM Color Aromatics, Saturates, Polars in Base Oil UV Absorbance of Petroleum Products
CHAPTER 7: HYDROCARBON BASE OIL CHEMISTRY D 2270-93
226
D 2272-98 D 2500-99 D 2549-91
219
D 2622-98 D 2710-99 D 2786-91 D 2887-01 D 2983-01 D 3228-96 D 3239-91 D D D D D
3339-95 3829-93 4289-97 4530-00 4624-93
D 4629-96 D 4636-99 D 4683-96
379
D 4741-00 D 4742-96 D 4871-00 D 4927-96 D 5293-99 D 5480-95 D 5481 -96 D D D D D
5483-95 5776-99 5800-00 5864-00 5949-96
D 5950-96 D 5985-96 D 6006-97 D 6074-99 D 6082-00 D 6366-99 D 6616-01 E 1687-98
Cal'c of Viscosity Index from Kinematic Viscosity Oxidation Stability-Rotating Vessel Cloud Point of Petroleum Oils Aromatic / Non-Aromatic Separation Sulfur by X-Ray Fluorescence Bromine Index by Electrometric Titration Hydrocarbon Types of Saturates Simulated Distillation (Volatility) Low Temperature Brookfield Viscosity Kjeldahl Nitrogen Aromatics Molecular Types in Base Oil Acid Number Borderline Pumping Temperature Elastomer Compatability Carbon Residue-Micro Method High Temp / High Shear Viscosity by Capillary Viscometry Organically Bound Nitrogen Corrosiveness of Hydraulic Oils High Temp / High Shear Viscosity by TBS High Temp / High Shear Viscosity by Tapered Plug Oxidation by Thin Film Uptake Guide For Universal Oxidation Test Elemental Analysis Cold Cranking Viscosity of Engine Oils at - 5 t o - 3 5 ° C Volatility by Gas Chromatography High Temp / High Shear Viscosity by Capillary Viscometry Oxidation of Grease by DSC Bromine Index Noack Volatility Aquatic Aerobic Biodegradation Pour Point by Automatic Pressure Pulse Pour Point by Automatic Tilt Pour Point by Rotation Biodegradability of Hydraulic Fluids Guide to Characterizing Hydrocarbon Base Oils High Temperature Foaming Trace Nitrogen High Temp / High Shear Viscosity byTBSatlOOX Carcinogenic Potential of Metalworking Fluids
REFERENCES [1] Murray, D. W., MacDonald, J. M., White, A. M., and Wright, P. W., "The Effect of Basestock Composition on Lubricant Oxidation Performance," Petroleum Review, February 1982, pp. 36-40.
183
[2] "Base Oils for Automotive Lubricants," Special Publication SP526, Society of Automotive Engineers, Warrendale, PA, 1982 (and references therein). [3] Stipanovic, A. J., Smith, M. P., Firmstone, G. P., and Patel, J. A., "Compositional Analysis of Re-Refined and Non-Conventional Lubricant Base Oils: Correlations to Sequence VE and IIIE Gasoline Engine Tests," SAE Publication 941978, Society of Automotive Engineers, Warrendale, PA, 1994. [4] Firmstone, G. P., Smith, M. P., and Stipanovic, A. J., "A Comparison of Neural Network and Partial Least Squares Approaches in Correlating Base Oil Composition to Lubricant Performance in Gasoline and Industrial Oil Applications," SAE Publication 952534, Society of Automotive Engineers, Warrendale, PA, 1995. [5] Smith, M. P., Stipanovic, A. J., Firmstone, G. P., Cates, W. M., and Li, T. C, "Comparison of Mineral and Synthetic Base Oils for Bench and Engine Tests," Lubrication Engineering, Vol. 52, No. 4, 1995, pp. 309-314. [6] Yoshida, T., Igarashi, J., Stipanovic, A. J., Thiel, C. Y., and Firmstone, G. P., "The Impact of Basic Nitrogen on the Oxidative and Thermal Stability of Base Oils in Automotive and Industrial Applications," SAE Publication 981405, Society of Automotive Engineers, Warrendale, PA, 1998. [7] Stipanovic, A. J., Firmstone, G. P., and Smith, M. P., "Having Fun with Base Oils: Predicting Properties Using Neural Networks," Preprints (ACS Division of Petroleum Chemistry, Inc.), Vol. 42, No. 1, 1997, pp. 284-286. [8] Patel, I. A., Smith, M. P., Powers, J. R., Whiteman, J. R., and Prescott, G. P., "Composition and Performance of a Hydroisomerized Wax Base Oil, Preprints (ACS Division of Petroleum Chemistry, Inc.), Vol. 42, No. 1, 1997, pp. 200-203. [9] "1997 Report on U.S. Lubricating Oil Sales," National Petroleum and Refiners Association (NPRA), October, 1998. [10] Sequeira, A., Jr., Lubricant Base Oil and Wax Processing, Marcel Dekker, Inc., NY, 1994. [11] "1999 Lubricating Oil and Wax Capacities of Refiners and ReRefiners in the Western Hemisphere," National Petroleum and Refiners Association (NPRA), 1999. [12] A) Lubricants World, March 1999, p. 8.; B) Lubricants World, March 2000, p. 8. [13] Comitius, T., "Basestock Market Outlook," Lubricants World, November 1998, pp. 16-23. [14] Speight, J. G., The Chemistry and Technology of Petroleum, 2"^ Edition, Marcel Dekker, Inc., NY, 1991. [15] BP Petroleum, "Petroleum Refinery Products and Processes," Our Petroleum Industry, 1911, p. 230. [16] Lane, E. C. and Garton, E. L., "Base of a Crude Oil," U.S. Bureau of Mines-Report of Investigation 3279, Washington, DC, 1935. [17] Barman, B. N., "Hydrocarbon-Type Analysis of Base Oils and Other Heavy Distillates by Thin-Layer Chromatography with Flame-Ionization Detection and by the Clay-Gel Method," Journal of Chromatography Science, Vol. 34, 1996, pp. 219-225. [18] Sarpal, A. S., Kapur, G. S., Mukherjee, S., and Jain, S. K., "Characterization by 13C-NMR Spectroscopy of Base Oils Produced by Different Processes," FMS/, Vol. 76, No. 10, 1997, pp. 931-937. [19] Adhvaryu, A., Pandey, D. C, and Singh, I. D., "Effect of Composition on the Degradation Behavior of Base Oil," Preprints (ACS Division of Petroleum Chemistry, Inc.), Vol. 42, No. 1, 1997, pp. 225-228. [20] Coates, J. P. and Setti, L. C, "Infrared Spectroscopic Methods for the Study of Lubricant Oxidation Products," ASLE Transactions, Vol. 29, No. 3, 1985, pp. 394-401. [21] Altgelt, K. H. and Gouw, T. H., Chromatography in Petroleum Analysis, Marcel Dekker, NY, 1979. [22] Altgelt, K. H. and Boduszynski, M. M. "Composition and Analysis of Heavy Petroleum Fractions," Marcel Dekker, Inc., NY, 1994.
184 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK [23] Sarpal, S. A., Kapur, G. S., Chopra, C , Jain, S. K., Srivastava, S. P., and Bhatnagar, A. K., Fuel, Vol. 75, No. 4, 1996, pp. 4 8 3 490. [24] Sahoo, S. K., Sharma, B. K., and Singh, I. D., "NMR Studies of Lube Distillates for Making Quality Base Oils," Preprints (ACS Division of Petroleum Chemistry), Vol. 45, 1999. [25] Igarashi, J., "Oxidative Degradation of Engine Oils," Japanese Journal of Tribology, Vol. 35, No. 10, 1990, pp. 1095-1104. [26] Korcek, S. and Jensen, R. K., "Relation Between Base Oil Composition and Oxidation Stability at Increased Temperatures," ASLE Transactions, Vol. 19, No. 2, 1975, pp. 83-94. [27] Korcek, S., Johnson, M. D., Jensen, R. K., and Zinbo, M., "Determination of High-Temperature Antioxidant Capability of Lubricants and Lubricant Components," Industrial Engineering Chemistry, Prod. Res. Dev., Vol. 25, 1986, pp. 621-627. [28] Colclough, T., "Role of Additives and Transition Metals in Lubricating Oil Oxidation," Industrial Engineering Chemistry, Vol. 26, 1987, pp. 1888-1895. [29] Hsu, S. M., Ku, C. S. and Lin, R. S., "Relationship between Lubricating Basestock Composition and the Effects of Additives on Oxidation Stability," SAE Publication SP-526, Society of Automotive Engineers, Warrendale, PA, 1982, pp. 29-56. (Also see references therein, especially 1-4). [30] Ofunne, G. C , Maduako, A. U., and Ojinnaka, C. M., "High Temperature Oxidation Stability of Automotive Crankcase Oils and Their Base Oils," Tribology International, Vol. 23, No. 6, 1990, pp. 407-412. [31] Blaine, S. and Savage, P. E., "Reaction Pathways in Lubricant Degradation.2. n-Hexadecane Autoxidation," Industrial Engineering Chemistry, Vol. 30, 1991, pp. 2185-2191. [32] Harpp, D. N., Robertson, J., Laycock, K., and Butler, D., "Organosulfur Antioxidants in Hydrocarbon Oils," Sulfur Reports, Vol. 4, No. 6,1985, pp. 195-227. [33] Russel, G. A., in Free Radicals, Vol. I, J. K. Kochi, Editor, John Wiley and Sons, NY, 1973, pp. 275-331. [34] Palekar, V. M., "Bench Scale Evaluation of Automotive Crankcase Lubricants," Ph.D. Thesis, The Pennsylvania State University, August 1996, pp. 57-61. [35] Lee, C. J., Klaus, E. E., and Duda, J. L., "Evaluation of Deposit Forming Tendency of Mineral and Synthetic Base Oils Using the Penn State Micro-Oxidation Test," Lubrication Engineering, Vol. 49, No. 6, 1993, pp. 4 4 1 ^ 4 5 . [36] Palekar, V. M., Duda, J. L., Klaus, E. E., and Wang, J., "Evaluation of High Temperature Liquid Lubricants Using the Penn State Micro-oxidation Test," Lubrication Engineering, Vol. 52, No. 4, 1996, pp. 327-334. [37] Dennsion, G. H. and Conduit, P. C , "Oxidation Of Lubricating Oils—Mechanism of Sulfur Inhibition," Industrial Engineering Chemistry, Vol. 37, No. 11, 1945, pp. 1102-1108. [38] Yoshida, T., Igarashi, J., and Watanabe, H., "Pro-Oxidant Properties of Basic Nitrogen Components in Base Oil," Proceedings of the 11'^ International Colloquium Tribology, Esslingen, Germany, January 1998.
[39] Mooken, R. T., Saxena, D., Base, B., Satapathy, S., Srivastava, S. P., and Bhatnagar, A. K., "Dependence of Oxidation Stability on Base Oil Composition," Lubrication Engineering, 1997, p p . 19-24. [40] Roby, S. H., Supp, J. A., Barrer, D. E., and Hogue, C. H., "Base Oil Effects in t h e Sequence HID and Sequence V-D Engine Tests, SAE Publication 892108, Society of Automotive Engineers, Warrendale, PA, 1989. [41] Cherrillo, R. A. and Huang, A., "The Increasing Significance of Base Oils in the Evolution of Heavy-Duty Diesel Engine Oils," 1999 National Petroleum Refiners Association (NPRA), LW-99130, Houston, TX, 1999. [42] Kramer, D. C , Ziemer, J. N., Cheng, M. T., Fry, C. E., Reynolds, R. N., Lok, B. K., et al., "Influence of Group II and III Base Oil Composition on VI and Oxidation Stability," National Lubricating Grease Institute (NLGI), Paper 9907, Tucson, AZ, 1999. [43] Fresco, G. P., Klaus, E. E., and Tewksbury, E. J., "Measurement and Prediction of Viscosity-Pressure Characteristics of Liquids," Journal of Lubrication Technology (ASME Transactions), Paper No. 68-Lub-8, 1968, pp. 1-7. [44] Roelands, C. J. A., Vlugter, J. C , and Waterman, H. I., "The Viscosity-Temperature-Pressure Relationship of Lubricating Oils and Its Correlation with Chemical Constitution," Journal of Basic Engineering, December 1963, pp. 601-610. [45] Guangteng, G. and Spikes, H. A., "Boundary Film Formation by Lubricant Base Fluids," Society of Tribologists and Lubrication Engineers, Presentation 95-MP-7D-3, Chicago, IL, 1995. [46] Tsubouchi, T., Abe, K., and Hata, H., "Quantitative Correlation Between Molecular Structure of Traction Fluids and Their Traction Properties (Part 2): Precise Investigation into Molecular Stiffness," Japanese Journal of Tribology, Vol. 39, No. 3, 1994, pp. 373-381. [47] Bair, S., "An Experimental Verification of the Significance of the Reciprocal Asymptotic Isoviscous Pressure," ASLE Tribology Transactions, April, 1993. [48] A. J. Stipanovic, Unpublished Results. [49] Johnston, W. G., "A Method to Cedculate the Pressure-Viscosity Coefficient from Bulk Properties of Lubricants," ASLE Transactions, Vol. 24, No. 2, 1980, pp. 232-238. [50] Nessel, C. S., Coker, D. T., King, A. G., and Mumford, D. L., "Carcinogenic Assessment of Lubricant Base Oils by Proton NMR," Preprints (ACS Division of Petroleum Chemistry), Vol. 42, No. 1, 1997, pp. 251-254. [51] Trauth, E. J., "Future Product Development Needs-Comprehensive Life-Cycle Analyses," Lubricants World, July 1998, pp. 20-25. [52] Tocci, L., "Clean Hydraulic Fluids," Lubes'n'Greases, November 1998, pp. 4 2 ^ 4 . [53] Voltz, M., "Biodegradability of Lubricant Base Stocks and Fully Formulated Products," Proceedings, ACS Petroleum Division Symposium, San Diego, CA, 1994. [54] Weller, D. E., Perez, J. M., and Keay, R. E., "Biodegradable Gear Oils," Preprints (ACS Division of Petroleum Chemistry), Vol. 42, No. 1, 1997, p p . 242-245.
MNL37-EB/Jun. 2003
Hydrocarbons for Chemical and Specialty Uses Dennis W. Brunett, ^ George E. Totten, ^ and Paul M. Mattlock^
ETHYLENE (C2), PROPYLENE (C3), AND THE C4 PRODUCTS (butene-
DISCUSSION
1, butene-2, isobutylene and 1,3-butadiene) are monomers of enormous industrial importance. Among the numerous uses of ethylene are the synthesis of ethylene glycol, which is used for antifreeze formulation, and the production of polyethylene terephthalate. Ethylene is also used for polyethylene manufacture and as a co-monomer in the production of various synthetic rubbers. The primary method for the production of ethylene and other light olefins is steam cracking. Steam cracking is performed both on liquefied petroleum gases (LPGs) and on liquid feedstocks. LPGs include ethane, propane, and/or butane. Liquid feedstocks are generally paraffinic or naphthenic and include n a p h t h a and petroleum condensates. When these feedstocks are thermally "cracked," the resulting product stream contains a complex mixture of hydrocarbon comp o u n d s . The process challenge is to separate the various product components at the desired purity. The purity of the individual products is set by their ultimate use. For example, some polymerization processes are especially sensitive to particular contaminsLnts in monomer/ co-monomer feeds. Some of these impurities poison catalysts or reduce their efficiency. Others may degrade the physical or performance properties of the polymer products. Because olefin purity is so important to downstream processing, it is essential that producers and consumers alike utilize a standard set of well-tested and widely accepted antdytical procedures to measure and control the quality of these chemicals. The ASTM D.02.0D Subcommittee on Hydrocarbons for Chemical and Specialty Uses has developed such a series of analytical standards for the petrochemical industry.
Commercial Production of Synthetic Hydrocarbons Basestocks
In this chapter, a brief overview of the most common industrieJ production methods of C2, C3, and C4 synthetic hydrocarbons is provided. Typical purity standards and comm o n classes of impurities are also discussed. The chapter also includes various types of ASTM analytical procedures and the separation principles involved.
' The Dow Chemical Company, 2501 North Brazosport Blvd., Freeport.TX 77541. ^ G. E. Totten & Associates, LLC, PO Box 30108, Seattle, WA 98103. ^ The Dow Chemical Company, 771 Old Saw Mill River Road, Tarrytown, NY 10591.
2003 by A S I M International
Hydrocarbon
Production
The unsaturated synthetic hydrocarbons such as ethylene, propylene a n d butenes m a y be produced either by steam cracking saturated hydrocarbons (ethane, propane, Eind butane), or by steam cracking feedstocks such as virgin crude oil fractions (naphtha, kerosine, and gas oil). In addition, ethylene may also be produced from crude oil directly [1]. Different types of feedstocks may be used for the production of synthetic hydrocarbons. The primary feedstocks are the "gas liquids." Gas liquids include ethane, propane, and butane, all sepeiration products from natural gas. The second class of feedstocks is the refinery off-gases. Refinery off-gases are mixtures of hydrogen, m e t h a n e , ethylene, ethane, propane, a n d others. A typical off-gas contains approximately 20 mol% ethane, 16 mol% ethylene, 3.6 mol% C3 derivatives, with the remaining components being hydrogen, methane, and acid gases. Off-gases are obtained from refinery operations. Liquid feedstocks include various petroleum fractions derived from crude oil processing (naphtha, kerosine, atmospheric gas oil and vacuum gas oil) as well as liquid condensates from natural gas processing (natural gasolines) and natural gas condensates such as Algerian and Brega condensates. They are defined by their boiling points. The approximate boiling ranges of the petroleum fractions are provided in Table 1 [2]. Raffinates are residues from extraction processes such as for aromatics recovery. These feedstocks t3^ically possess a broad range of compositions and boiling ranges. The composition of the crude oil fractions varies with the source as shown in Table 2 [1]. The type of feedstocks will affect the composition of the product being produced. Generally, the heavier the feedstock, the greater the amount of by-products. In addition to feedstock composition, the composition of the product stream resulting from steam cracking is also d e p e n d e n t on t e m p e r a t u r e , residence time, and steam/hydrocarbon ratio. The optimization of the residence time, coil outlet temperature and coil outlet pressure (cracking severity) for a given feedstock are the most critical variables in determining the product mix. The second most frequently used method of ethylene production is recovery from refinery gas. Alternate methods are
185 Copyright'
for Synthetic
www.astm.org
186 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 1—Approximate boiling point ranges for different liquid feedstocks.
mined based on the product mix required. Table 3 provides a list of the different types of feedstocks typically used in steam crackers for the production of ethylene.
Boiling Range (°F)
Liquid Feedstock
Ethylene
Naphtha Full range Light Heavy Kerosine Atmospheric gas oil Light Heavy Vacuum gas oil Gas condensates
80-430 80-300 250-430 400-500
Dependency of Reaction Product and By-Product Complexity on Feedstock Structure—Ethylene may be produced commercially by thermal cracking of hydrocarbons, recovery from refineiy off-gas, recovery from coke-oven gas, or dehydration of ethanol. In practice, almost all of the high purity ethylene produced in the U.S. is the result of steam cracking. Table 4 [3] lists various ethylene manufacturing facilities a n d the method of ethylene generation.
400-680 600-800 800-1100 80-530
TABLE 2—Typical composition of feedstocks used for synthetic hydrocEirbon production. Component Origin Density (g/mL) Sulfur content (mass %) ASTM boiling range (°F) IBP 50% EP BMCI PONA analysis (mass %) Paraffins Olefins Monocyclic naphthenes Polycyclic naphthenes Monocyclic aromatics Polycyclic aromatics
Naptha
Kerosine
Gas Oil
Kuwait
North Sea
Kuwait, North Sea 0.839 0.1
0.724 0.04 28 121 175
0.801 0.03 180 206 242 29.7
220 290 380 27.7
72.3
50.9
41.8
16.7
15.8 10.5 14.9 7.9
19.2 14.1 20.9 4.0
11.0
known using feed streams from coal, ethanol, coke oven gas and other sources, but these are rarely used. Therefore, S5T1thetic hydrocarbon p r o d u c t i o n from these alternative sources will not be discussed in detail here. The value of BMCI shown in Table 2 refers to the Bureau of Mines Correlation Index. BMCI is used to estimate the amount of ethylene in feedstocks. Higher numbers indicate m o r e aromatic character. BMCI is calculated from the following equation: BMCI = VABP +
48640 473.7(^.5)
456.8
Production
[1]
Where: VABP is the volume average boiling point (°K) and dis is the density in g/mL at 15°C. The BMCI value estimates the extent of naphthenic and aromatic content of a hydrocarbon feedstock emd is calibrated for hexane to yield 0% and pure benzene to yield 100%. Synthetic hydrocarbon feedstocks are typically low in sulfur content. High sulfur feedstocks may be used, but these materials produce larger quantities of acid gases, which must be removed from the resulting products. Low chloride feedstocks should be used as chlorides may cause irreparable damage in the furnace tubes due to chloride stress cracking and corrosion. It is possible to obtain any of the synthetic hydrocarbons from any feedstock by steam cracking. Higher paraffinic content materials yield higher conversion to short-chain olefins. The choice of feedstock for a steam cracker is typically deter-
In the steam cracking process, steam is added to the hydrocarbon feed stream to: 1. Reduce the partial pressure of the hydrocarbon 2. Reduce the high-temperature residence time 3. Reduce coke formation in the reactor. (Carbon monoxide and hydrogen are formed by reaction of steam with carbon at the high temperatures encountered in the reactor. This is the famous "water-gas" reaction.) Ethylene product yield, like that of other synthetic hydrocarbons, correlates with the carbon/hydrogen ratio of the components in the feedstock being steam cracked. Generally, as the feedstock becomes heavier (which is typically accompanied by increasing amounts of naphthenes and aromatics and a corresponding lowering of the C/H ratio), the reaction products become heavier and more aromatic, while conversion yields to lighter olefins decrease. E t h a n e m a y be produced by Ci chemistry by thermal cracking of methane at 1000°C as shown in Scheme 1 [4]. This is not the preferred commercial route to ethane, however, because the bond dissociation energy of a C—H bond of 93.3 kcal/mol is considerably higher than a C—C bond-dissociation process of 71.0 kcal/mol [9], making this an energy inefficient process. CH4 -» CHs* 2CH3* -^ CH3 - CH3 S c h e m e 1—Formation of ethane from methane. Ethylene m a y be produced by the dehydrogenation of ethane as shown in Scheme 2. This reaction process, like all thermal cracking processes, is accompanied by side reactions, which lead to the formation of aromatic and naphthenic hydrocarbon ring structures. CH3 - CH3 ^ CH2 = CH2 + H2 S c h e m e 2—Ethylene formation by dehydrogenation of ethane. TABLE 3—Selected feedstocks for synthetic hydrocarbon production. Gases or Gas Liquid Products Ethane Propane Butane Refinery off-gases
Liquid Feedstock Naphtha Kerosine Raffinates from extraction processes Atmospheric gas oil Vacuum gas oil Natural gasoline Gas condensates
CHAPTER 8: HYDROCARBONS FOR CHEMICAL AND SPECIALTY USES 187 T A B L E 4 — S u m m a r y o f ethylene process m e t h o d s u s e d ;in t h e U n i t e d States. PRODUCT ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLfeNE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE ETHYLENE
COUNTRY USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA USA
COMPANY BPCHEM. BP CHEM. COND.VISTA CPCHEM CPCHEM CPCHEM CPCHEM CPCHEM CPCHEM DOW DOW DOW DOW DOW DOW DOW DOW DUPONT EQUISTAR EQUISTAR EQUISTAR EQUISTAR EQUISTAR EQUISTAR EQUISTAR EQUISTAR EXXONMO.CH EXXONMO.CH EXXONMO.CH EXXONMO.CH EXXONMO.CH FORMOSA PL FORMOSA PL HUNTSMANC HUNTSMANC HUNTSMAN C JAVELINAC SHELL CHEM SHELL CHEM SHELL CHEM SHELL CHEM SHELL CHEM SUNOCO CH. SUNOCO CH. TEXAS EAST WESTLAKECA WESTLAKEPC WESTLAKEPC WILLIAMS 0
SITE
AVJimx ALVIN/TX LAKECWLA OEDAR BAYO PORTAR/TX SWEENY/TX SWEENYm( SWEENY/TX SWEENY/TX FREEPORT/T FREEPORT/T PLAQUEMINE PUQUEMINE SEADRIFT/TX TAFT/U TAFT/U TEXAS CITY/rx ORANGE/TX CHANNELVIE CHANNELVIE CHOC.BAYOU CLINTON/IA CORPUS/TX LfiPORJEfTX LAKECH/LA MORRIS/IL BATON R/LA BAYTOWN/TX BAYTOWN/TX BEAUMON/TX HOUSTOWrX POINT CX5MF/TX POINT C»MF/TX ODESSAAJS PORTAR/TX PORTNECHE CORPU^TX DEER PA/rX DEER PA/rX NORCO/LA NORCO/U NORCO/LA BRANDENBUR CLAYMONT/D LONGVIE/TX CALVERT CI LAKECHAA LAKECHflJ^ GEISMAR/LA
ROUTE HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBOI HYD-CARK)N HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARODN HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON RFRY-QAS HYD-CARBON RFRY-GAS HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON HYD-CARBON
TECHNOLOQY STCRACKIN ST CRACKIN ST CRACKIN STCRACKIN ST CRACKIN STCRACKIN STCRACKIN STCRACKIN STCRACKIN STCRACKIN ST CRACKIN ST CRACKIN ST CRACKIN STCRACKIN STCRACKIN STCRACKIN STCRACKIN STCRACKIN STCRACKIN ST CRACKIN ST CRACKIN ST CRACKIN STCRACKIN ST CRACKIN STCRACKIN STCRACKIN STCRACKIN STCRACKIN STCRACKIN STCRACKIN ST CRACKIN ST CRACKIN ST CRACKIN STCRACKIN STCRACKIN STCRACKIN ETH RECOV ST CRACKIN N.A. STCRACKIN STCRACKIN STCRACKIN STCRACKIN ST CRACKIN STCRACKIN STCRACKIN ST CRACKIN STCRACKIN ST CRACKIN
LICENSOR START NA. 1975 1977 BRAUN N.A. 1^8 1977 LUMMUS 1970 LUMMUS 1957 N.A. • N.A. 1967 BRAUN 1978 BRAUN 1991 DOW 1972 1994 BRAUN HA. 1972 DOW 1980 1957 N.A. 1967 LUMMUS• 1978 LUMMUS N.A. 1969 LUMMUS 1967 1976 UMIMUS LUMMUS 1977 1980 N.A. 1^8 KELLOGG S&W 1980 KELLOGG 1991 N.A. 1971 LUMMUS 1972 1972 LUMMUS KELLOGG 1979 1997 EXXON N.A. 1961 N.A. 1978 1994 KELLOGG 2001 KEUOGG N.A. 1965 1979 S&W N.A. 1978 N.A. 0 1978 KELLOGG N.A. 0 KELLOGG 1975 1981 KELLOGG N.A. 2000 1952 N.A. 1961 N.A. 1974 KELLOGG BRAUN 1964 KELLOGG 1991 1997 LUMMUS 1968 LUMMUS
188 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK The structural complexity of the product stream increases with the carbon chain length of the feedstock. This is illustrated by the thermal cracking of propane to jdeld ethylene a n d propylene shown in Scheme 3 [15]. The first two reactions in Scheme 3 yield the primary products [5,6]. Fair proposed that butenes and methane were the most probable decomposition products [15]. C3H8 -^ C2H4 + CH4
40%
C3H8 -^ C3H6 + H2
36%
C3H8 -^ 14C4H8 + CH4
10%
C3H8 -^ VzCjUe + Yi C3H6 + ^CH4
14%
S c h e m e 3—Ethylene formation from propane. The primary thermal cracking reaction products from nbutane and i-butane are shown in Scheme 4 [8]. It was shown by Fair that no ethylene is formed from the branched substrate, t'-butane [8]. n - C4H10 -^ CH4 + C3H6 n - C4H10 -^ C2H6 + C2H4 n — C4H10 —> H2 + n — C4H8
Additional reaction processes shown in Scheme 6 were proposed for temperatures greater than 800°C [11]. C2H3* -> C2H2 + H» C2H4 <-> C2H2 + H2 C2H2 + C2H4 —> condensation products + CH4 C2H4 -^ carbon black + H2 S c h e m e 6—^Additional ethane thermal decomposition reaction processes occurring above 800°C. Laidler determined the reaction mechanism for the formation of ethane from the thermal decomposition of propane shown in Scheme 7 [12]. Laidler proposed that only the H», CH3» and CsH?* radicals are involved in the chain propagation steps and that C2H5», formed by ethane decomposition, is not regenerated. Termination occurs by methyl and propyl radical recombination or by the recombination of two methyl radicals. C3H8 -> CH3» + C2H5» C2H5' + CjHg ^ C2H6 + C3H7* H» + C3H8 ^ H2 + C3H7»
n - C4H10 ^ H2 + 2C2H4
CH3» + CjHg ^ CH4 + C3H7*
i - C4H10 -^ CH4 + C3H6
C3H7* -^ CH3» + C2H4
/ - C4H10 -^ H2 + i - C4H8
C3H7« ^ H» + C3H6
S c h e m e 4—Primary reaction products formed by thermal cracking of n-butane and i-butane. Thermal Cracking Reaction Mechanism—Hydrocarbon thermal cracking reactions have been studied in detail for many years [5,7,9-13]. The Kalinenko cmd Brodski reaction mechanism for the thermal decomposition of ethane below 800°C is provided in Scheme 5. C2H6 ^ 2CH3» H» + [C2H6, CH4, C2H4, C3H8, C4H10] -> H2 + [C2H5», CH3», CzHj*, C3H7», C4H9*} CH3» + [C2H6, H2, C2H4] -^ CH4 + [C2H5», H», CzHj*] CjHs* + [H2, CH4, C2H4] ^ C2H6 + [ H», CH3», C2H3»] C2H3* + [H2, CH4, C2H6} ^ C2H4 + [ H», CH3», CzHs*] CH3» + C2H5* <-> C3H8 2C2H5* <-> C4H10 C2H3« + [C2H5», CH3», C2H3»] ^ [C4H8, C3H6, C4H6 } C2Hs» -^ C2H4 + H» C2H5* -t- C2H4 O C4H9* C4H9» <^ C3H6 + CHj* C2H3» + C2H4 ^ C4H7* C4H7* -^ H» + C4H6 C4H10 -> CH3» + C3H7* S c h e m e 5-Kalinenko and Brodski ethane thermal decomposition reaction mechanism (less than 800°C).
CH3* + C3H7* -^ CH4 + C3H8 or CH3» + CH3* -^ C2H6 C3H7* + C3H7* -^ C S H M or C2H6 + CjHg S c h e m e 7—Laidler mechanism of the thermal decomposition of propane. The mechanism of formation of ethane from the thermal decomposition of butane [13] and higher alkanes [15] have also been studied but will not be discussed here. There are n u m e r o u s reactions involved in hydrocarbon thermal decomposition. In the absence of equilibrium processes, the initial chain scission reaction of the hydrocarbon substrate is unimolecular. The primary decomposition products lead to olefins and by-products. Activation energies for thermal decomposition of hydrocarbons vary from 70-79 cal/g-mol at 500-1000°C [16]. Thermal decomposition of propylene is also unimolecular with reported activation energies varying from approximately 59-67 cal/g-mol [5,7,17-19]. Reaction mechanisms and kinetics have been reported for other potential ethylene feedstocks including; propylene [17,20], n-butene [17], n-butane [8,17,20,21], j-butane [8,17,20], n-butylene [20], and ibutene [17]. Commercial Production of Ethylene—Steam cracking is the most common method used to produce ethylene. The desired feedstock is diluted with steam and passed through a high-temperature furnace. There are numerous patented designs employing vertical or horizontal gas-fired tubes (direct heating), fluidized bed heated tubes (indirect heating), and other heat sources. Figure 1 provides a schematic illustration of a steam cracking unit used to produce ethylene. The reaction products and yields are controlled by the feedstock, cracking temperature, residence time, and other plant-specific variables.
CHAPTER 8: HYDROCARBONS FOR CHEMICAL AND SPECIALTY USES 189 Example of Front End Depropanizer/Front End Reactor Process
>
r:
Acid Gas Removal
Compression
Quench
•
Fuel Oil Product
*• Pyrogasoline Product
Chilling Train
Dryers
Secondary Dryers
Acetylene Reactors
H2. CH4.
Compression
Depropanizer C-4+
Ethylene Recovery
Demethanizer
(H2, CH4) _w Off-Gas
Deethanizer
I
Ethylene Refiigeration
Ethylene " Product
C-2 Splitter
Propylene Product
T}
Pyrogasoline Product
-*\ C-3 Splitter C-2's, C-3's
r
prude C-4 Product
C3,C3=,MA, PD Ethane Recycle
Propane Recycle
FIG. 1—Schematic illustration of a steam-cracking unit used for the production of ethylene. The reaction is quenched (stopped) by cooling the product stream rapidly to about 600°F (315°C), a temperature where olefinic products are typically stable. The cooling process is performed through a heat exchanger or by coinjection with water or oil. The by-products formed during ethylene synthesis by steam cracking include: 1. Acetylene—The a m o u n t of acetylene that is formed depends on the feedstock a n d cracking severity. Although acetylene can be isolated in high purity, it is usually converted to ethylene. This is performed over a catalyst. Catalyst selectivity is very important. If the catalyst is too active ethane will be produced not only from the hydrogenation of the acetylene but also some of the ethylene (negative selectivity) yielding a net loss of ethylene production. Because acetylene cannot be distilled out of the ethylene, hydrogenation is the preferred technique for removal. During the hydrogenation process, a liquid product, green oil, is produced composed of hydrocetrbons increasing in chain length by multiples of 2. This material must be removed because of the possibility of downstream fouling in low temperature portions of the distillation process (demethanizer, C2 splitter, and cold train). This green oil is typically returned to the process mixed with the fuel oil stream also produced in the cracking process. 2. Propylene—Propylene is also produced by hydrocarbon pyrolysis (steam cracking or catalytic cracking). Other methods are used reirely. Some processes upgrade refinery material to a more pure, polymer grade, but the production process still occurs in a refinery. The third m e t h o d is propane dehydrogenation, but this is currently not being used to any appreciable extent in North America or South America. Some dehydrogenation is being done in Western Europe and Southeast Asia. There is a very small amount of propylene being produced in South Africa from coal. 3. Methylacetylene and Propadiene—^Depending on the plant, methylacetylene and propadiene (allene) can be removed
from the propylene in the same reactor as acetylene if the cracked gas is first "depropanized" rather than "de-ethanized." The stream containing Cj to C3 hydrocarbons is then sent to the reactors, followed by various distillations for purification of both ethylene and propylene. The C4 and heavier material is generally not hydrogenated, but simply distilled after the C3 and lighter material is removed. There are, however, some facilities that hydrogenate the raw gas stream after removal of the heavier components in the quench system. Methylacetylene and propadiene are typically of very low concentrations and are typically not isolated. However, they can be isolated and sold as welding gases. 4. Butadiene andButenes—1,3-Butadiene is produced by distillation of the crude C4 material from the mixture followed by solvent extraction of the m o n o m e r . In North America, butadiene is derived from the steam cracking of LPGs or liquid feeds, followed by extractive distillation of the crude C4 cut of the cracked gas. 5. Pyrogasoline—In ethylene production, one of the by-products is called pyrogasoline, which is commonly known as "pyrolysis gasoline" or "pygas." The lightest components in this stream are the C5 compounds, which include isoprene and piperylene. Cyclopentadiene is dimerized in this process and later recovered as dicyclopentadiene. Pyrolysis gasoline is used as a feedstock in C5 S3Tithetic hydrocarbon production. (The subject material for this chapter is limited to C2, C3, and C4 synthetic hydrocarbons.) Pjrolysis gasoline is also the primary feedstock for the extraction of benzene. The total product stream from the cracking process is purified before distillation by multi-step treatments, which may include: • Oil and/or aqueous quenching of the hot cracked gas for removal of heavier components (C5 and heavier), • Compression of the remaining cracked gas to remove the remainder of the heavier hydrocarbons, • Conversion of residual sulfur to hydrogen sulfide Eind subsequent removal by zeolite absorption,
190 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK • Acid gases removal by caustic extraction or amine absorption, • Carbon dioxide removal by absorption or extraction, • Drying to remove residual water after compression, • Acetylenic materials removal by selective hydrogenation or extraction, • Methane removal by low temperature fractionation or selective adsorption or selective absorption. The process challenge in the production of ethylene, like other synthetic hydrocarbons, is obtaining pure ethylene from the complex reaction mixture after cracking. This is accomplished primarily by distillation after the product stream has been treated. Table 5 provides a listing of typical specifications for ethylene produced in this way. Table 6 summarizes the typical physical properties of ethylene and other synthetic hydrocarbons.
Propylene Production Propylene is most often produced by either steam cracking or cataljrtic cracking. Propylene is a major by-product from ethylene production when feedstocks heavier than ethane are used. In steam cracking, generally, the heavier the feedstock, the greater the a m o u n t of propylene produced relative to ethylene. It is not unusual for propylene production to amount to 40-70% of ethylene production [1]. It should be noted, however, propane and low boiling liquid feedstocks may not conform to this pattern if the cracking severity is low. This is why such plants are more often called "olefin plants." Since ethylene and propylene are co-produced in the same units, propylene production will not be detailed separately here. Table 7 provides a summary of typical product specifications for polymer grade propylene. The physical properties of propylene are summarized in Table 6. Butylene and Butadiene Production Reaction Chemistry and Mechanism—Butadiene may be synthesized by dehydrogenation of n-butenes at 1100-1250°F (593-677°C) over a metal catalyst as shown in Scheme 8 [21]. However, due to the relatively high temperatures required for conversion, the reaction selectivity is often poor. Reaction
TABLE 5—Typical product specification for ethylene. Composition 99.9 mol% min 0.1 mol%max 1 mol ppm max 1 mol ppm max 10 mol ppm max 1 mol ppm max 1 mol ppm max 1 mol ppm max 2 mol ppm max 1 mol ppm msix 1 mass ppm max
Component Ethylene Methane and ethane Hydrogen Acetylene Propylene and propane Oxygen Carbon monoxide Carbon dioxide Water Methanol Total sulfur
C4H8 O C4H6 + H2 Scheme 8—Butadiene synthesis by butene dehydrogenation. temperatures may be reduced to 750-1100°F (399-593°C), thus permitting improved selectivity, by introducing air (steam + oxygen) into the reaction (oxidative dehydrogenation) as shown in Scheme 9. Catalysts that have been reported to produce good yields include: bismuth molybdate, mixed ox-
TABLE 6—Thermophysical properties of C2, C3 and C4 hydrocarbons." Property Molecular weight Triple point Temperature, °C Pressure, kPa Latent heat of fusion, J/mol (at melting point) Normal boiling point Temperature, °C Latent heat of vaporization, J/mol Density of the liquid, mol/dm^ Specific heat of the liquid, J m o l ^ ' K " ' Viscosity of the liquid, Pa«s Surface tension of the liquid, N/m Specific heat of the ideal gas (25°C),
Ethylene [1] 28.05 -169.19° 0.11 3350
-103.71 13 540 20.27 67.4 1.61 X 10"" 0.0164 42.84
Propylene 42.08 -185.26 9.54 X 10"^ 3003
-47.6 18 420 14.47
1-Butene
Cis-2-Butene
Trans-2-Butene
1,3-Butadiene
56.11
56.11
56.11
54.09
-185.35
-138.91
-105.55
-108.92
3848
7309
9757
7980
1.92 X lO""* .0167 62.49
-6.25 21 917 11.15 118 1.98 X lO"'* .0160 88.49
3.72 23 349 11.43 127 1.73 X 10"'' .0156 76.89
91.8 4.62 5.52 0.2750 2.058
146.4 4.02 4.153 0.2760 2.719
0.88 22 757 11.17 1.79 X 10"^ .0160 86.00
-4.4 22 597 12.03 123.6 1.98 X 10"^ .0176 77.13
162.4 4.21 4.277 0.2720 2.712
155.5 4.10 4.207 0.2740 2.707
151.9 4.33 4.525 0.2700 2.541
1.6 10.0 385
1.6 9.7 325
1.6 9.7 324
2.0 11.5 420
297.1
213.7
234.1
281.1
Jmor^K"' Critical point Temperature, °C Pressure, MPa Density, mol/dm^ Compressibility factor Gross heat of combustion of the gas (25X), MJ/mol Limits of flammability of atmospheric pressure at 25°C Lower limit in air, mol% Upper limit in air, mol% Autoignition temperature in air at atmospheric pressure, "C Vapor pressure at 25°C, kPa
9.2 5.042 7.635 0.2813 1.411
2.7 36 490 7065
"Values from a variety of sources and are believed accurate.
1.9 11.1 455 1156
CHAPTER 8: HYDROCARBONS FOR CHEMICAL AND SPECIALTY USES TABLE 7—Typical product specification for propylene. Component Propylene Paraffins Hydrogen Ethylene Acetylene Methylacetylene Propadiene C-4 Hydrocarbons Carbon dioxide Carbon monoxide Methanol Oxygen Water Sulfur
Concentration 99.5 mol% min 0.4 inol% max 5 mol ppm max 100 mol ppm max 1 mol ppm max 5 mol ppm max 5 mol ppm max 5 mol ppm max 5 mol ppm max 5 mol ppm max 5 mol ppni max 2 mol ppm max 10 mass ppm max 1 mass ppm max
stream is quenched by cooling, washed in a wash tower to remove oxygenated bjrproducts, and finally scrubbed with an oil to extract the organic products from nitrogen and carbon dioxide. The hydrocarbons Eire removed by stripping them from the oil a n d then using extractive distillation to recover the butadiene. Most of the butadiene production in the world today is obtained as a by-product of ethylene manufacture (see Table 8). When there is excess capacity, butadiene may be selectively hydrogenated to butenes. Alternatively, excess butadiene
POSSIBLE MODIFICATION OF EXISTING COMMERCIAL OPERATIONS TO INCLUDE OXIDATIVE DEHYDROGENATION OF BUTENES
PHILLIPS PETROLEUM COMPANY
ides of tin and antimony, mixed oxides of molybdenum and tellurium and nickel, calcium and chromium phosphates [27].
Shchukin studied the mechanism and reaction kinetics of butadiene synthesis by oxidative dehydrogenation and reported the mechanism illustrated in Scheme 10 [24]. K ( 0 ) and K ( ) represent the oxidized and reduced state of the catalyst respectively. The values xj, X2, Vi, and V2 are reactant and product coefficients. The reaction order was reported to be approximately 0.5 with respect to 1-butene and oxygen [25].
PETRO-TEX CHEMICAL COMPANY
n-BUTANE
C4H8 + 1^02 ^> C4H6 + H2O S c h e m e 9—Butadiene synthesis by oxidative dehydrogenation of n-butane and n-butenes.
n-BUTANE
i
BUTANE CATALYTIC DEHYDROGENATION (HOUDRY)
BUTANE CATALYTIC DEHYDROGENATION BUTANE
BUTANE AND BUTENES BUTENES SEPARATION (FURFURAL EXT. DIST.)
; BUTANE ONLY ;
'-—in BUTADIENE SEPARATION 1 BUTENES SEPARATION i (FURFURAL EXT. DIST.)
BUTADIENE I BUTENES i
BUTENE DEHYDROGENATION
K ( 0 ) + C4H8 -^ K(OC4H8)
^
1
*
CAT^kYTIC
r 'o)(YDiHYDROGENATibNi
j f STEAM
K(OC4H8) -^ K(C4H8) + H2O -^ C4H6 + K ( )
I
I
i.-i
K(OC4H8) + xiK(O) -^ V1CO2 + V1H2O + (xi + 1)K( ) K(0) -f- C4H68 <-> K(OC4H6)
191
BUTADIENE SEPARATION (FURFURAL EXT. DIST)
K(OC4H6) + X2K(0) -^ V2CO2 + y4V2H20 + (X2 + 1)K( )
AIR
*.
BUTENE OXYDEHYDROGENATION CATALYTIC
• 4 . . — I BUTADIENE SEPARATION
K ( ) + O2 ^ K ( 0 2 ) + K ( ) ^ 2 K ( 0 ) S c h e m e 10—Mechanism of butylenes synthesis by oxidative dehydrogenation. The first reaction in Scheme 10 can act to isomerize 1butene into cis- and trans-2-butene [26]. The reactivity of the butene isomers on an aluminum oxide catalyst decreases in the following order: 1-butene > cis-2-butene > trans-2b u t e n e [27]. Unsaturated carbonyl c o m p o u n d s can b e formed as byproducts. Adzhamov reports that these carbonyl compounds inhibit the oxidative dehydrogenation process and butene isomerization [28]. Other by-products that are possible include furan, acetaldehyde, acetone, acrolein, formaldehyde, methyl ethyl ketone, methyl vinyl ketone, and other carbonyl compounds and carbon dioxide [27]. Commercial Butadiene Production Processes—^Two industrial processes for butadiene production by oxidative dehydrogenation are the Phillips Petroleum Company Process [24,25] and the Petro-Tex Chemical Company Process [22]. These two processes are illustrated schematically in Fig. 2 [30,31]. The butene feed stream is typiccJly mixed with steam and air and passed through a fixed-bed reactor. The reaction product
BUTADIENE Note: BROKEN LINES INDICATE AN ENLARGEMENT OF AN EXISTING OPERATION OR THE ADDITION OF A NEW OPERATION.
FIG. 2—Comparative illustration of Butadiene production by the Phillips and the Petro-Tex processes.
TABLE 8—Typical product composition for C4 Hydrocarbon product stream. Component C3 Hydrocarbons 1,3 Butadiene Ethyl acetylene Vinylacetylene cis-Butene-2 fran5-Butene-2 Isobutene 1-Butene n-Butane Isobutane C5 Hydrocarbons Total Sulfur
Composition 0.1 mol% max 35-75 mol% 0.2-0.5 mol% 0.7-1.4 mol% 4-5 mol% 4.5-6.5 mol% 20-30 mol% 15-25 mol% 5-15 mol% 0.5-1 mol% 0.1 mol% max 5 mass ppm max
192 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
may be completely hydrogenated to butanes. In recent years, many of the dedicated butadiene production units have been shut down. Butadiene separation from an oxidative dehydrogenation product stream or from the product stream of an ethylene steam cracker is typically performed by an extractive distillation. The butadiene boiling point is too close to other products to permit effective separation by fractionation in a conventional distillation process. There is no consensus on the best solvent system but extractive distillation systems that have been reported include N-methylpyrrolidone, dimethylformamide, furfural, acetonitrile and dimethylacetamide [32-34]. Analytical Test Methods The above discussion has provided a general overview of the production of ethylene, propylene and C4 hydrocarbons including butanes and 1,3-butadiene. Included in this discussion is the generation of various by-products, many of which exhibit deleterious effects on downstream processes. This section provides a review of the ASTM procedures for product characterization. This will be done by dividing these methods into those that are applicable for ethylene, propylene, C4 hydrocarbons, and other related methods. Ethylene Characterization Guide for Analysis of Ethylene (Guide D 5234)—^When various producers and users of ethylene deal with anal3rtical results, inconsistency of units and test methods may cause major errors. D5234 provides an overview of the typical concentrations of possible components found in ethylene, analytical methods, and units of measure. Although this guide cannot be used as a specification, it can provide a starting point for specification development. It can also be used as a starting point for identifying suitable test methods for ethylene product characterization. Determination of Hydrocarbon Impurities in Ethylene (Test Method D 6159)—This method determines the amounts of certain impurities that could impair the performance of high purity ethylene. Test Method D 6159 is used for the determination of methane, ethane, propane, propylene, acetylene, iso-butane, propadiene, butane, trans-2-butene, 1-butene, iso-butene, cis-2-butene, methyl acetylene and 1,3 butadiene by a gas chromatographic procedure. This test method does not determine all possible impurities. Carbon monoxide, carbon dioxide, water, alcohols, nitrogen oxides, carbon disulfide, and hydrocarbons higher than decane are not determined by D 6159. These products must be determined by other procedures. This test is conducted by injecting the as-received ethylene sample into a capillary gas chromatograph that is equipped with a 6-port sampling valve and two wide bore capillary columns connected in series. The columns are a dimethyl silicone column and a porous layer open tubular (PLOT) AI2O3/KCI column. A flame ionization detector is used. The hydrocarbon impurities are determined and the total impurities are used to calculate the ethylene content. Determination of Ethylene, Other Hydrocarbons and Carbon Dioxide (Test Method D 2505)—Test Method D 2505 is used to
determine carbon dioxide, methane, ethane, acetylene and other hydrocarbons in high-purity ethylene. Hydrogen, nitrogen, oxygen, and carbon monoxide are determined in accordcince with Test Method 2504. Ethylene concentration is determined by subtracting the sum of the percentages of hydrocarbon and non-hydrocarbon impurities from 100%. This method is applicable over the range of impurities from 1-500 parts per million by volume. In this method, the sample is sepetrated in a gas chromatograph utilizing four different packed columns with helium as a carrier gas. Methane and ethane are determined using a silica gel column. Propylene and heavier hydrocarbons are determined using a hexamethylphosphoramide (HMPA) column. Acetylene is determined by using, in series, a hexadecane and a squalene column. Carbon dioxide is determined using a column packed with activated charcoal impregnated with a solution of silver nitrate in /3,/3'-oxydipropionitrile. Calibration data are obtained using standard samples containing the impurities, carbon dioxide, methane, and ethane in the range expected to be encountered. Calibration data for acetylene are obtained assuming that acetylene has the same peak area response on a weight basis as methane. Calculations for carbon dioxide, methane, and ethane are conducted using the peak-height measurement. Propylene Characterization Guide for Analysis of Propylene Concentrates (Guide D 5273)— Standard Guide D 5273 lists the major grades of propylene produced in North America. Standard Guide D 5273 is intended to provide information on the likely composition of propylene concentrates and on probable ways to test them. Since there are currently no ASTM test methods for determining all components of interest. Standard Guide D 5273 provides information on other potentially available test methods. This guide is not intended to be used as a standard for any grade of propylene concentrate. Method for Liquefied Petroleum (LP) Gases and Propylene Concentrates (Test Method D 2163)—D-2163 determines the composition of LP gases and propylene concentrates. The components measured are ethane, propane, propylene, n-butane, isobutane, butylene, and isopentane. The components can be measured so long as their concentration is above 0.1%. This information is useful when the product is to be sold either as a chemical intermediate or as a fuel. The component distribution data of LP gases and propylene concentrates can be used to calculate physical properties such as relative density, vapor pressure, and motor octane. Test Method D 2163 involves the separation of the components of LP gas by gas chromatography emd comparing them to corresponding components separated under identical operating conditions from a reference standard or from pure hydrocarbons. The chromatogram of the sample is interpreted by comparing peak heights or areas with those obtained on a reference standard mixture or pure hydrocarbon of interest. Determination of Trace Hydrocarbon Impurities in Propylene Concentrates (Test Method D 2712)—Test Method D 2712 is used to determine ethylene, total butylenes, acetylene, methyl acetylene, propadiene, and butadiene at levels of 5-500 ppm each in propylene concentrates. Trace levels of
CHAPTER 8: HYDROCARBONS these components may affect the commercial use of propylene concentrates. In Test Method D 2712, a relatively large volume of sample is charged to a gas partition chromatograph equipped with a column that will separate trace hydrocarbon constituents from the major components. Any column or combination of columns may be used providing that they exhibit the necessary resolution. Various columns that have been used successfully are provided in Table 1 of Test Method 2712. Determination of Trace Methanol in Propylene Concentrates (Test Method D 4864)—Methanol is a common impurity in propylene. It can have a deleterious effect on various processes that use propylene as a feedstock. Test Method D 4864 determines methanol in propylene concentrates in the range of 4 ^ 0 parts per million by weight. For Test Method D 4864, a known weight of water is measured into a sample cylinder containing a known amount of liquefied propylene. The contents in the cylinder are shaken and the water/methanol phase is withdrawn. A reproducible volume of the extract is then injected into a gas chromatograph equipped with either a thermal conductivity or a flame ionization detector. The methanol concentration is calculated from the area of the methanol peak using calibration and extraction factors obtained from synthetic blends of known methanol content. Determination of Trace Carbonyl Sulfide (COS) in Propylene (Test Method D 5303)—This test method measures traces of COS in propylene in the range of 0.5-4.0 parts per million by mass. In propylene producing processes, COS usually remains with the C3 and must be removed since it affects product quality. COS acts as a poison to commercial polymerization processes, resulting in deactivation and costly process downtime. Accurate gas chromatographic determination of trace COS in propylene involves unique analyticcJ problems because of the nature of COS and idiosyncrasies of trace level analyses. These problems result from the reactive and adsorptive nature of COS, the low concentrations being measured, the tj^e of detector needed, and interferences from the propylene sample matrix. Test Method D 5303 addresses these analytical problems and ways to properly handle them to assure accurate and precise analysis. Separation of the COS from propylene is accomplished by gas chromatography. A relatively large volume of sample is injected onto a gas chromatograph equipped with a single packed column and flame photometric detector. The separation is achieved isothermally at 10-50°C. Calibration data, based on peaJi areas, are obtained using a known gas standard blend of COS in the range expected for the sample. The COS peak area in the sample is measured and the concentration of COS is calculated. C-4 Product Characterization Guide for Analysis of 1,3-Butadiene (Standard Guide D 5274)—Standard Guide D 5274 covers the analysis of I,3-butadiene products produced in North America. Standard Guide D 5274 is intended to provide information on the likely composition of 1,3-butadiene products and possible ways to test them. Since there are currently no ASTM test methods for determining all components of interest. Standard Guide D 5274 provides information on other available test methods.
FOR CHEMICAL AND SPECIALTY USES
193
This guide is not intended to be used as a specification for butadiene products. Butylene Analysis by Gas Chromatography (Test Method D 4424)—This is a gas chromatographic analysis of commercial butylenes, butylene concentrates, and butane-butylene mixtures. This test method is designed to determine: propane, propylene, isobutane, n-butane, 1-butene, isobutene, trans-2-butene, cis-2-butene, 1,3-butadiene, iso-pentane and n-pentane at concentrations of about 0.05% or greater. It does not cover high-purity 1-butene or high-purity isobutene streams. Test Method D 4424 involves the gas chromatographic separation of a sample on a packed column with either helium or hydrogen carrier gas. The separated components are detected using a thermal conductivity detector or a flame ionization detector. Calibration data are obtained using either relative response factors or by using a standard ceJibration blend. Determination of Butadiene Purity and Hydrocarbon Impurities (Test Method D 2593)—Test Method D 2593 determines 1,3-butadiene impurities such as propane, propylene, iso-butane, n-butane, 1-butene, isobutylene, propadiene, trans-2butene, cis-2-butene, 1,2-butadiene, 1,4-pentadiene, methyl-, dimethyl-, ethyl-, and vinyl-acetylene in polymerization grade butadiene by gas chromatography. Impurities including butadiene dimer, carbonyls, inhibitor, and residue are measured by other procedures, and can be used with this procedure to give a complete composition of the butadiene. Determination of Butadiene Dimer and Styrene in Butadiene Concentrate (Test Method D 2426)—Test Method D 2426 measures butadiene dimer (4-vinylcyclohexene-l) and styrene in "recycle" and specification grade butadiene concentrates by gas chromatography. This is done by injecting the sample into a gas chromatograph equipped with either a thermal conductivity or flame ionization detector with sensitivity sufficient to detect 0.01% of butadiene dimer. Any packed column may be used that is capable of resolving the butadiene dimer and styrene as discrete peaks. The quantity of the components of interest is determined from the chromatogram by comparing their peak areas or heights with those of a synthetic sample. Determination of Nonvolatile Residue in Polymerization Grade Butadiene (Test Method D 1025)—This method covers the determination of nonvolatile material in polymerization grade butadiene. Test Method D 1025 involves taking a measured volume of liquid butadiene and allowing it to evaporate at room temperature from a small glass evaporating dish until only a residue remains. Heating the dish to a constant weight completes the evaporation. The weight gain in the evaporation dish is the nonvolatile residue. Determination of Peroxides in Butadiene (Test Method D 5799)—Butadiene polyperoxide is a very dangerous reaction product of butadiene and oxygen. This peroxide has been reported to cause violent explosions in vessels that are used to store butadiene. Due to the inherent dangers of peroxides in butadiene, specification limits are usually set for their presence. Test Method D 5799 determines the peroxide content of a sample of commercial butadiene in the concentration range of 1-10 ppm by mass.
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37: FUELS AND LUBRICANTS
HANDBOOK
For this method, a known mass of butadiene sample is placed in a flask and evaporated. The residue is then refluxed with acetic acid and sodium iodide. The peroxides oxidize iodide to iodine, which is then titrated with standard sodium thiosulfate solution using visual end-point detection. Interfering traces of iron are complexed with sodium fluoride. Determination of Carbonyls in CA Hydrocarbons (Test Method 4423)—Test Method D 4423 covers the determination of carbonyls (ketones and aldehydes) in butadiene. The presence of carbonyl species can have a deleterious effect upon butadiene polymer properties or on the butadiene polymerization reaction. Test D 4423 measures carbonyl in the range of 0-50 ppm, calculated as acetaldehyde. A measured a m o u n t of the sample is added to an alcoholic hydroxylamine hydrochloride solution t h a t has been adjusted to a given p H indicator coloration using either alcoholic acid or base. The carbonyls will react with the hydroxylamine hydrochloride releasing an equivalent a m o u n t of hydrochloric acid. The solution is then titrated with standard base to the original coloration. A blank containing only methanol and sample is titrated and the sample's results are calculated using the blank adjustment. Results are reported as mg/kg carbonyls as acetaldehyde. Determination of Total Inhibitor Content (Test Method D 1157)—Test D 1157 measures p-?ert-butylcatechol (TBC) in pol3mierization or recycle grades of butadiene or other C4 hydrocarbon mixtures. TBC is commonly added to commercial butadiene in amounts of 50-150 mg/kg as an oxidation inhibitor. It is important that the sample not contain other phenolic material, as they will be measured as catechol. In general, all phenols and their quinone oxidation products are included in the calculated catechol content. Small amounts of polymer do not interfere. This test method is applicable over the range of TBC from 50-500 mg/kg. The catechol is separated from butadiene in Test Method D 1157 by evaporation. The residue is dissolved in water and an excess of ferric chloride is added. The intensity of the yellow-colored complex is compared in a photoelectric colorimeter with that produced by a known concentration of the catechol. Determination of Trace Volatile Chlorides in Butane-Butene Mixtures (Test Method D 2384)—Test Method D 2384 is used to determine trace amounts of volatile chlorides in butanebutene mixtures. Such information is valuable in cases where the chloride is deleterious in the use of this product. Chloride contributes to corrosion problems in processing units in instances where further processing of this material is involved. The test methods included in Test Method D 2384 cover the determination of the total volatile organic chlorides in concentrations from 10-100 p p m in buteine-butene mixtures. The sample is first combusted in one of two ways. In lamp combustion, the sample is burned in an atmosphere of carbon dioxide and oxygen or in purified air. The halogen-containing combustion products are absorbed in dilute sodium carbonate solution. In oxy-hydrogen combustion, the sample is burned in an oxy-hydrogen atomizer burner, and the combustion products are absorbed in a dilute solution of sodium carbonate. Chloride determination may be performed by amperometric titration or spectrophotometrically. For amperometric titration, the chloride ion in aqueous solution is titrated am-
perometrically with standard silver nitrate solution, using a standard saturated calomel electrode as the reference electrode. The diffusion currents are plotted against the corresponding volumes of silver nitrate solution used and the end point is taken as the intersection of the straight-line portions of the curve. This method is not directly applicable in the presence of other substances that combine with silver ion or oxidize chloride ion in dilute acid solution. The chloride ion concentration may also be obtained spectrophotometriccilly by reaction of the chloride ion in the absorber solution with mercuric thiocyanate to release thiocyanate, which forms a reddish-orange complex with Fe^^. The intensity of the color is measured at 460 n m with a spectrophotometer or filter photometer. Bromides, sulfides, ammonia, tobacco smoke, and more than 25 fx,goi hydrogen peroxide in the test solution interfere in the spectrophotometric procedure. ASTM Butadiene Measurement Tables (Standard D 1550)— ASTM D 1550 consists of a series of tables allowing the specific gravity of butadiene or concentrated butadiene (60% butadiene or greater) at 15.6/15.6°C to be calculated from the density measured at a different temperature. The density is measured by means of a hygrometer. General Methods Microcoulometric Determination of Sulfur in Petroleum Gas (Test Method D 3246)—Test Method D 3246 determines sulfur in petroleum gas. Trace quantities of sulfur in hydrocarbon products can be harmful to many catalytic chemical processes. M a x i m u m permissible levels of total sulfur are normally included in specifications for such hydrocarbons. It is recommended that this test method be used to provide a basis for agreement between two laboratories when the determination of sulfur in hydrocarbon gases is important. For liquefied petroleum gas, total volatile sulfur is measured on an injected gas sample. For such a material, a liquid sample must be used to measure total sulfur. Test Method D 3246 covers the determination of sulfur in the range of 1.5-100 p p m by mass in hydrocarbon products that are gaseous at normal room temperature and pressure. The test procedure involves injecting a sample of the hydrocarbon into a c o m b u s t i o n tube m a i n t a i n e d at a b o u t 800°C having a flowing stream of gas consisting of 80% oxygen and 20% inert gas. Oxidative pyrolysis converts the sulfur to sulfur dioxide which then flows into a titration cell where it reacts with triiodide ion present in the electrolyte. The triiodide is reduced by the sulfur dioxide, and is coulometrically replaced. The toted current required to replace the triiodide is a measure of the sulfur present in the sample injected. The reaction occurring in the titration cell as sulfur dioxide enters is: li" + SO2 + H2O -^ SO3 + 3I~ + 2H+ The triiodide consumed is generated coulometrically from: 31" ^ 1 3 + 2 e ' These microequivalents of triiodide (iodine) are equal to the number of microequivalents of titratable sample ion entering the titration cell. A liquid blend containing a known amount
CHAPTER
8: HYDROCARBONS
of sulfur is used for calibration. This is the same chemistry as is used in the well-known Karl Fisher water titration. Gas Chromatographic Determination of Noncondensahle Gases in C2 and Lighter Hydrocarbon Products (Test Method D 2504)—Test Method D 2504 covers the determination of hydrogen, nitrogen, oxygen, and carbon monoxide in parts per million by volume range in C2 and lighter hydrocarbon products. In this test, the sample is separated in a gas-solid chromatographic system using molecular sieves as the solid adsorbant. The concentration of the gases to b e analyzed is calculated from peak heights or areas. Argon can be used as a carrier gas for the determination of hydrogen concentrations below 100 p p m by volume. Argon, if present in the sample, interferes with oxygen determination. Determination of Organic Hydrogen in Petroleum Fractions (Test Methods D 1018 and D 4808)—Test D 1018 determines the percent organic hydrogen contained by a sample. Knowledge of the organic hydrogen content of petroleum products can be helpful in assessing performance characteristics. For Test Method D1018, the petroleum product is burned from a cotton wick in an a t m o s p h e r e of purified air. The water formed is collected from the combustion gases by a desiccant (CaCli) and weighed. The a m o u n t of organic hydrogen can be calculated based on the water formed during combustion. Continuous wave, low-resolution nuclear magnetic resonance spectroscopy provides a simpler, more precise alternative method for determination of organic hydrogen content of petroleum products. It is described in Test Method D4808. The NMR spectrum of the petroleum oil to be analyzed is compared to a reference. The spectrometer nondestructively records the absolute concentration of hydrogen atoms in the reference and test sample. The result is expressed as hydrogen content (on a mass basis) of the sample. Table 9 summarizes which test variants that eire typically used with different petroleum products within D 4808. Determination of Paraffin, Naphthene and Aromatic Hydrocarbons (Test Method D 5443)—Test Method D 5443 provides for the determination of paraffins, naphthenes, and aromatics in low olefinic hydrocarbon streams having final boiling points of 200°C or less. The m e t h o d uses multi-dimensional gas chromatography. Hydrocarbons with boiling points greater t h a n 200°C and less t h a n 270°C are reported as a single group. Olefins, if present, are hydrogenated and resultant saturates are included in the paraffin and naphthene distribution. Aromatics boiling at C9 and above are reported as a single a r o m a t i c group. This test m e t h o d is not intended to determine individual components except for benzene and toluene that Eire only Cg and C7 aromatics, respectively, and cyclopentane, that is the only C5 naphthene. The lower limit of detection for a single h y d r o c a r b o n c o m p o n e n t is 0.05 mass %.
AND SPECIALTY
USES
195
This is a complicated gas chromatography method. Molecular size is determined by using boiling point columns or molecular sieve columns. Polar columns separate by polarity. Hydrogenation is done on column using a platinum lined column a n d hydrogen gas. In addition, t e m p e r a t u r e programming is used for separation. Determination of Boiling Range (Test Methods D 2892 and D 2887)—Test Method D 2892 details procedures for the production of a liquefied gas, distillate fractions, and residuum of' standardized quality o n which analytical data can be obtained, and the determination of yields of the above fractions by b o t h mass and volume. From the above information, a graph of temperature versus mass-percent distilled can be produced. This distillation curve corresponds to a laboratory technique, which is defined at 15/5 (15 theoretical plate column, 5:1 reflux ratio) or TBP (true boiling point). This test m e t h o d can also be applied to any petroleum mixture except liquefied petroleum gases, very light naphthas and fractions having initial boiling points above 400°C. Alternatively, the boiling range distribution of petroleum fractions may be determined by gas chromatography using Test Method D 2887. This test m e t h o d is applicable to petroleum products and fractions having a final boiling point of 538°C (1000°F) or lower at atmospheric pressure and is limited to samples having a boiling range greater than 55°C (100°F) and having a vapor pressure sufficiently low enough to p e r m i t sampling at a m b i e n t t e m p e r a t u r e . For m o r e volatile p e t r o l e u m fractions, including oxygenated compounds. Test Method D 3710 should b e used.
ASTM STANDARDS No. D 1018 D 1025 D 1157 D 1550 D 2163
D 2384 D 2426 D 2504
D 2505 TABLE 9—ASTM D 4808 test procedures available for different petroleum products. Approximate Boiling Range "C (-F) Petroleum Products Test Method A Light distillates 15-260(60-500) B Middle distillates 200-370 (400-700) B Gas oil 370-510(700-950) C Residual 510+ (950+)
FOR CHEMICAL
D 2593 D 2712 D 2887
Title Standard Test Method for Hydrogen in Petroleum Fractions Test Method for Nonvolatile Residue of Polymerization Grade Butadiene Test Method for Total Inhibitor Content (TBC) of Light Hydrocarbons ASTM Butadiene Measurement Tables Test Method for Liquefied Petroleum (LP) Gases and Propylene Concentrates by Gas Chromatography Test Methods for Traces of Volatile Chlorides in Butane-Butene Mixtures Test Method for Butadiene Dimer and Styrene in Butadiene Concentrates by Gas Chromatography Test Method for Noncondensahle Gases in C2 and Lighter H y d r o c a r b o n Products by Gas Chromatography Test Method for Ethylene, Other Hydrocarbons, and Carbon Dioxide in High-Purity Ethylene by Gas Chromatography Test Method for Butadiene Purity a n d Hydrocarbon Impurities by Gas Chromatography Test Method for Hydrocarbon Traces in Propylene Concentrates by Gas Chromatography Test Method for Boiling Range Distribution of Petroleum Fractions by Gas Chromatography
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MANUAL
D 2892 D 3246 D 3710
D 4423 D 4424 D4808
D 4864
D D D D
5234 5273 5274 5303
D5443
D 5799 D 6159
3 7; FUELS AND LUBRICANTS
HANDBOOK
Test Method for Distillation of Crude Petroleum (15-Theoretical Plate Column) Test Method for Sulfur in Petroleum Gas by Oxidative Microcoulometry Test Method for Boiling Range Distribution of Gasoline a n d Gasoline Fractions by Gas Chromatography Test Method for Determination of Carbonyls in C4 Hydrocarbons Test Method for Butylene Analysis by Gas Chromatography Test Method for Hydrogen Content of Light Distillates, Middle Distillates. Gas Oils, and Residua by Low-Resolution Nuclear Magnetic Resonance Spectroscopy Test Method for Determination of Traces of Methanol in Propylene Concentrates by Gas Chromatography Guide for Analysis of Ethylene Product Guide for Analysis of Propylene Concentrates Guide for Analysis of 1,3-Butadiene Product Test Method for Trace Carbonyl Sulfide in Propylene by Gas Chromatography Test Method for Paraffin, Naphthene, and Aromatic Hydrocarbon type Analysis in Petroleum Distillates Through 200°C by Multidimensional Gas Chromatography Test Method for Determination of Peroxides in Butadiene Test Method for Determination of Hydrocarbon Impurities in Ethylene by Gas Chromatography
OTHER STANDARDS D e u t s c h e s I n s t i t u t fur N o r m u n g ( D I N )
ISO 6684:1982
ISO 6792:1982
150 7381:1986
ISO 7382:1986
ISO 8174:1986
ISO 8175:1986
ISO 8176:1986
International Organization for Standardization (ISO) ISO 6377:1981
ISO 6378:1981
ISO 6379:1981
ISO 6380:1981
ISO 6381:1981
Light Olefins for Industrial Use—Determination of Impurities by Gas Chromatography—General Considerations Butadiene for Industrial Use—Determination of Hydrocarbon Impurities— Gas Chromatographic Method Ethylene for Industrial Use—Determination of H y d r o c a r b o n Impurities— Gas Chromatographic Method Propylene for Industrial Use—Determination of Hydrocarbon Impurities— Gas Chromatographic Method Ethylene and Propylene for Industrial
Ethylene and Propylene for Industrial Use—Determination of Acetone, Acetonitrile, Propan-2-ol, and Methanol— Gas Chromatographic Method Propylene for Industrial Use—Determ i n a t i o n of Oligomers—Gas Chromatographic Method Butadiene for Industrial Use—Determination of Active tert-butyl-catechol (TBC)(4-(1,1 -dimethylethyl)-! ,2-benzenediol)- High Performance Liquid Chromatography Method
UOP UOP 212-77
DIN 51872-4 (1990)
Testing of gaseous fuels and other gases; determination of the components; gas Chromatographic procedure DIN 51872-5 (1996) Testing of gaseous fuels and other gases; determination of the components - Part 5; capillary gas chromatographic procedure
Use—Determination of Traces of Carbon Monoxide and Carbon Dioxide— Gas Chromatographic Method Butadiene for Industrial Use—Determination of tert-butyl-catechol (TBC) (4-( 1,1 - d i m e t h y l e t h y l ) - 1 , 2 - b e n z e n e diol)—Spectrometric method Butadiene for Industrial Use—Determination of Oxygen and Argon in the Gaseous Phase Above Liquid Butadiene—Gas Chromatographic Method Butadiene for Industrial Use—Determ i n a t i o n of Oligomers—Gas Chromatographic Method Ethylene for Industrial Use—Determination of Traces of Polar Comp o u n d s — P r e p a r a t i o n of Condensate Samples by Low-temperature Technique
UOP 344-87 UOP 496-63T UOP 542-88
UOP 569-79 UOP 603-88
UOP 772-77 UOP 791-94
UOP 834-82
UOP 845-90
Hydrogen Sulfide, Mercaptan Sulfur and Carbonyl Sulfide in Hydroccirbon Gases by Potentiometric Titration Moisture in Hydrocarbon Streams Using a n On-line Analyzer Carbonyl Compounds in Light Hydrocarbon Gases Trace Diolefins, Acetylenes, and Noncondensable Hydrocarbons in LPG by Gas Chromatography Methanol in Petroleum Distillates and LPG by Gas Chromatography Trace Carbon Monoxide and Carbon Dioxide in Hydrogen and Light Gaseous Hydrocarbons by Gas Chromatography Mercaptan Sulfur in LPG by Microcoulometry Sulfur Components in LPG or C5 Minus Hydrocarbon Fractions by GCSCD Arsine in Ethylene by Electrothermal Atomic Absorption Spectrophotometry Trace Alcohols in LPG by Gas Chromatography
CHAPTER 8: HYDROCARBONS FOR CHEMICAL AND SPECIALTY USES 197 REFERENCES [1] Kniel, L., Winter, O. and Stork, K., Ethylene: Keystone to the Petrochemical Industry, Marcel Dekker Inc., NY, 1980. [2] Ma, J. J. and Scheeline, H. W., "Ethylene," Process Economics Program, Report No. 28B, Stanford Research Institute, Menlo Park, CA, March 1978. [3] World Light Olefins Analysis, Vol II, Chemical Market Associates, Inc., Houston, TX, December 2000, pp. 8-11. [4] Germain, E. and Maurel, R., "Thermal Decomposition of Pure Methane at 1000°C in a Pipe Still," Genie Chimique, Vol. 88, 1962, pp. 122-127 [5] Sherwood, P. W., "Production of Ethylene from Petroleum Sources, Part I.," Petroleum Refiner, Vol. 30, No. 9, 1951, pp. 220-225. [6] Sherwood, P. W., "New Advances in Ethylene Production-I," Petroleum, Vol. 19, May, 1956, pp. 161-164. [7] Martin, R., Dzierzynski, M., and Niclause, M., "Thermal Decomposition of Propane, 1. Introduction a n d Experimental Study of the Pyrolysis of Pure Propane," Journal of Chemie Physique, Vol. 61, No. 3, 1964, pp. 286-297. [8] Fair, J. R., Perkins, T. K., and Rice, H. F., "Comparing Olefin Unit Feedstocks," Petroleum Refiner, Vol. 34, No. 11, 1955, pp. 185-189. [9] Rice, F. O., "The Thermal Decomposition of Organic Compounds from the Standpoint of Free Radicals. I. Saturated Hydrocarbons," Journal of the American Chemical Society, Vol. 53, May 1931, pp. 1959-1972. [10] Davis, H. G. and Williamson, K. D., "The Kinetics of Pyrolysis of E t h a n e and Related Hydrocarbons," World Petroleum Congress Proceedings, Vth, Section 4, Paper 4, New York, 1959, pp. 1-17. [11] Kalinenko, R. A. and Brodskii, A. M., "A Kinetic Scheme for E t h a n e Pyrolysis as an Aid to Optimization of Processes Yielding Ethane," Kinetika I Kataliz, Vol. 6, No. 5, 1965, p p . 826-830. [12] Laidler, K. J., Sagert, N. H., and Wojciechowski, B. W., "Kinetics and Mechanisms of the ThermjJ Decomposition of Propane. 1. The Uninhibited Reaction," Proceedings of the Royal Society of London, Series A, 1962, London, Vol. 270, pp. 242-266. [13] Pumell, J. H. and Quinn, C. P., "The Pyrolysis of n-Butane," Proceedings of the Royal Society of London, Series A, Vol. 270, London, pp. 267-284. [14] Greensfelder, B. S., Voge, H. H., and Good, G. M., "Catalytic and Thermal Cracking of Pure Hydrocarbons," Industrial and Engineering Chemistry, Vol. 4 1 , No. 11, 1949, pp. 2573-2584. [15] Fair, J. R., Mayers, J. W., and Lane, W. H. "Commercial Ethylene Production by Propane Pyrolysis in a Molten Lead Bath," Chemical Engineering Progress, Vol. 53, No. 9, 1957, p p . 433^38. [16] Bartlit, J. R. and Bliss, H., "ICinetics of Ethane Pyrolysis," American Institute of Chemical Engineers Journal, Vol. 11, No. 3, 1965, p p . 562-572.
[17] Fair, J. R. and Rase, H. F., "Process Design of Light Hydrocarbon Cracking Units," Chemical Engineering Progress, 1954, Vol. 50, No. 8, pp. 415-420. [18] Schutt, H. C , "Optimum Temperature Profile for Endothermic Conversions in Tubular Reactors," Zeitschrift fuer Elektrochemie. Vol. 65, 1961, pp. 245-255. [19] Buell, C. K. and Weber, L. J., "Ethylene Production by Cracking of Ethylene-Propylene Mixtures," Petroleum Processing, Vol. 5, March 1950, pp. 266-272. [20] Kamptner, H. K., Krause, W. R., and Shilken, H. P., "High-Temperature Cracking," Chemical Engineering, Feb. 28 1966, pp. 93-98. [21] Schutt, H. C. and Zdonik, S. B., "Processing Scheme-Pyrolysis Methods," Oiland Gas Journal, Vol. 54,2 April 1956, pp. 99-103. [22] Haddeland, G. E., "Butadiene," Process Economics Program, Report No. 35, Stanford Research Institute, Menlo Park, CA, March 1968. [23] Anonymous, "New Plants and Facilities, CE Construction Alert," Chemical Engineering, April 5 1971, pp. 111-116. [24] Nolan G. J., Hogan R. J., and Farba F., Jr., "2-Butene Dehydrogenation Catalyst," U.S. Patent 3,501,547, 17 March 1970. [25] Friedman, L., Womeldorph, D. E., a n d Stevenson, D. H., "Houdry Dehydrogenation for Olefin Production," Proceedings of the American Petroleum Institute, Sect. Ill, Vol. 38, 1958, pp. 203-212. [26] Shchukin, V. P., Ven'yaminov, S. A., and Boreskov, G. K. "IronAntimony Oxide Catalysts of Partial Oxidation. III. Kinetics and Mechanism of the Oxidative Dehydrogenation of Butylenes," Kinetika I Kataliz, Vol. 11, No. 5, 1970, p p . 1236-1242. [27] Echigoya, E., Watanabe, T., and Sano, H., "Studies on Oxidative Dehydrogenation of Butylenes over Bismuth Molybdate Catalyst," Journal of the Chemical Society of Japan, Industrial Chemistry Section, Vol. 73, No. 5, 1970, pp. 933-937. [28] Skarchenko, V. K., "Oxidative Dehydrogenation of Hydrocarbons," International Chemical Engineering, Vol. 9, No.l, 1969, pp. 1-23. [29] Adzhamov, K. Y., Alkhazov, T. G., Belen'kii, M. S., and Lisovskii, A. E., "Effect of Carbonyl Compounds on the Oxidative Dehydrogenation of Butenes," Kinetika i Kataliz, Vol. 9, No. 6, 1968, pp. 1279-1284. [30] Newman, F. C , "Process for Butadiene Manufacture by Oxydehydrogenation of Butenes," Industrial and Engineering Chemistry, Vol. 62, No. 5, 1970, p p . 42-47. [31] Husen, P. C , Deel, K. R., and Peters, W. D., "Phillips Butadiene Process is Success," Oil and Gas Journal, 2 Aug 1971, pp. 60-61. [32] Reis, T., "Compare Butadiene Recovery Methods Processes, Solvents and Economics," Petro/Chem Engineer, August 1969, pp. 12-22. [33] Reis, T., "Butadiene Extraction," Chemical and Process Engineering, March 1970, pp. 65-69, 72-76. [34] Baron, T., "Precise Separations and Selective Extractions are Keys to Petrochemicals," Oil and Gas Journal, 1971, July 5, Vol. 67, No. 27, p p . 100-102.
MNL37-EB/Jun. 2003
Additives and Additive Chemistry Syed Q. A. Rizvi^
M O S T UNTREATED OR NON-FORMULATED LUBRICANTS, i.e., mineral
base oils and synthetic basestocks, do not possess the properties necessary to perform effectively in today's demanding lubricating environments. To function in such environments properly, base fluids need the help of chemicals called additives. Additives improve the lubricating ability of base oils, either by enhancing the desirable properties already present or by adding n e w properties. Such properties include suitable viscosity, slipperiness, high film-strength, low corrosivity, low pour point, good cleansing and dispersing ability, low toxicity and low flammability [1]. Most of today's lubricants are formulated lubricants, and additives are their integral part. The 1998 world consumption of lubricant additives is estimated at 2.588 million metric tons, of which North America a n d Western Europe consumed 67%. This estimate is based on 1995 SRI data [2] and the estimated growth rate.
LUBRICANT COMPOSITION A formulated lubricant comprises a base fluid and a performance package, and in the case of multigrade oils, an additional viscosity modifier. The performance package contains a n u m b e r of additives, the quantity of which is a function of their quality (the ability to deliver the necessary performance), the quality of the base fluid (governed by its physical and chemical properties), and the lubricant's intended use. The performance package can make u p to 30%, and sometimes even higher, of the total lubricant composition, depending upon the desired performance level and the severity of the end-use requirements [3]. In general, poor-quality base fluids need better additives a n d in larger amounts than base fluids of good quality. Likewise, applications such as engine oils a n d automotive gear oils, which place a higher demand on the lubricant, require superior additives than those that are less demanding. Engine oils account for the largest shaire of the additive use.
LUBRICANT BASESTOCKS Lubricants perform a variety of functions to prolong the life of the equipment. These include lubrication (reduction of friction and wear), dissipation of heat, corrosion control, prevention of excessive deposit formation, dispersion of use-gen' Research and Development Manager, Lubricant Additives Division, King Industries, Inc., Science Road, Norwalk, CT 06852.
erated contaminants, and water separation or demulsibility. While some of these functions are common to all lubricants, others are more specific to the equipment being serviced. M o d e m lubricants are formulated from a range of base fluids and chemical additives. The primary function of the base fluid is to facilitate lubrication; that is, to provide a fluid layer between moving surfaces to minimize friction, hence heat and wear. The base fluid in addition functions as a carrier of additives; hence, it must be able to keep them in solution under normal operating conditions. The base fluid can be of mineral origin, synthetic chemical origin, or biological origin. While mineral oil basestocks are obtained directly from petroleum fractionation, synthetic basestocks are manufactured through transformations of petroleum-derived organic chemicals. Partly synthetic (semisynthetic) basestocks are compatible mixtures of mineral oil and synthetic basestocks. Recently, a great deal of interest has surfaced regarding the use of basestocks of biological origin (vegetable a n d animal oils). Their biodegradable nature and non-petroleum origin are two major reasons for this impetus. For some industrial applications, such as metalworking, even water is used to deliver chemicals to parts that need protection. Section 2 of the ASTM Manual contains discussion on basestocks. Although many base fluid properties are modified or enhanced by the use of additives, knowledge of such properties is critical to the formulator. These include density, viscosity (both at low and high temperatures), foaming characteristics, seal compatibility, oxidation resistance, corrosivity, the viscosity-temperature relationship (VI), low-temperature properties (such as cloud point and pour point), and high-temperature properties (such as volatility and flash point) [4a]. Viscosity, defined as an oil's resistance to flow, is critical to its function as a lubricant. Inappropriate viscosity leads to ineffective lubrication and can lead to equipment damage. A n u m b e r of parameters, such as temperature, pressure, and shear forces, affect viscosity. Of these, temperature has the most profound effect; therefore, knowledge of viscosity-temperature relationship or VI of an oil is important to a formulator. The low-temperature properties of a lubricant include cloud point and pour point. When a sample of oil is cooled, its viscosity increases in a predictable manner until wax crystals start to form. On further cooling, the matrix of wax crystals becomes sufficiently dense to cause an apparent solidification of oil. Although this oil does not p o u r u n d e r the influence of gravity, it can be moved if sufficient force is applied: for example, by applying torque to a rotor that is sus-
199 Copyright'
2003 by A S I M International
www.astm.org
200
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
pended in oil. Cloud point is the temperature at which the first signs of wax formation (haziness) can be detected. Pour point is the lowest temperature at which the sample of oil flows by gravity alone. High viscosity oils may cease to flow at low temperatures because of further increase in viscosity rather than because of wax formation. Pour points may be higher thsin cloud points. The high-temperature properties, such as volatility and flash point, of a base oil are governed by its distillation temperature, or the boiling range. Volatility is important because it is a n indication of the oil's tendency to be lost during service due to vaporization, for example, in a hot engine. The methods used to determine volatility include ASTM Distillation Curves, thermogravimetric analysis (ASTM D 6375), Eind Noack volatility (ASTM D 5800). Flash point is the lowest temperature at which auto-ignition of the vapor above the heated sample occurs. Flash point of an oil is important from a safety point of view and is used to classify flammable liquids into hazard grades. Density is important because oils may be formulated by weight but measured by volume. Demulsibility is the ability of a n oil to separate water. Foaming characteristics determine the tendency of the oil to form foam and the stability of the foam once it is formed. Seal compatibility of the oil is also important because the oil in most applications comes in contact with elastomer seals. If seals are damaged, the lubricant will be lost and the equipment will be left unprotected. Oxidative resistance of the base oil depends largely upon the structure of the hydrocarbons present. Lubricants that do not contain aromatic structures, or structures with unsaturation, have better oxidative stability than those that do. Oxidation of the base oil results in the formation of polar compounds that are either corrosive or lead to the formation of resin, deposits, and sludge, which can impair proper functioning of the equipment. Previously, sulfur content of base oils was used as an indicator of their natural resistance to oxidation. This is because many naturally occurring organosulfur compounds in crude oils are moderately effective in destroying organic peroxides and breaking the oxidation chain mechanism. However, m o d e m refining processes, used to enhance other desirable properties of base oils, result in the removal of these beneficial compounds. The base oil should not contain components that promote corrosion. Corrosion tests usually involve bringing the oil sample in contact with a metal, such as iron, copper, or silver, u n d e r controlled conditions. Discoloration of metal, changes in its surface condition, or weight loss reflect corrosive tendency of the oil.
PERFORMANCE PACKAGE Lubricant additives are usually supplied as performance packages that are blended in basestocks to yield formulated lubricants. This is not the case in some applications, such as metalworking fluids, where the end user may purchase the individual additives and blend them in base oil prior to use. In the U.S., foixaulated lubricants are expected to meet performance requirements generally established by the Society of Automotive Engineers (SAE), American Petroleum Institute (API), U.S. Military, Original Equipment Manufacturers
(OEMs), and the end-users. In Europe, ACEA (Association des Constructeurs Europeens de I'Automobiles), CEC (Conseil Europeens de Coordination pour les Developments des Essais de Performance des Lubrifiants et des Combustibles pour Moteurs), ATC (Technical Committee of Petroleum Additive Manufacturers), and ATIEL (Association Technique de rindustries Europeenne des Lubrifiants) have collaborated in a m a n n e r similar to their American counterparts to come u p with their own engine oil classification system, called the E u r o p e a n Engine Lubricant Quality Management System (EELQMS). The system does include some individual OEM requirements. In Japan, the Japanese Automobile Standards Organization (JASO) and, in India, Bureau of Indicin Standards perform these functions. The performance package contains many classes of additives [5,6]. The primary functions of each class are briefly described below. Stabilizers/Deposit Control Agents—Minimize the amount of deposit formation. Oxidation Inhibitors control oxidative decomposition of lubricant and additives. Dispersants keep normally insoluble c o n t a m i n a n t s dispersed in the lubricant. Detergents prevent metal attack by acidic byproducts of combustion and oxidation and keep metal surfaces free of deposits. Film-Forming Agents—Either increase durability of the lubricant film or form chemical films on metal surfaces. Friction modifiers generally lower the coefficient of friction, thereby leading to improved fuel economy and wear control. Antiwear a n d extreme pressure agents protect metal surfaces against wear and equipment seizure. Rust and corrosion inhibitors prevent corrosion and rusting of metal parts that come in contact with the lubricant. Polymeric Additives—^Alter physical properties, such as viscosity and pour point, of lubricants. Viscosity modifiers minimize the rate of viscosity decrease with an increase in temperature. Pour point depressants enable a lubricant to flow at low temperatures. Emulsifiers promote mixing of water and oil to form an emulsion. Some hydraulic and metalworking lubricants are of emulsion type. Demulsifiers enhance water separation from oil c o n t a m i n a t e d with water. Foam inhibitors prevent the lubricant from forming persistent foam. Other Additives—Perform miscellaneous other functions. Seal swell agents swell elastomer seals. Dyes are used to color code lubricants and fuels. Biocides prevent degradation of high water-based lubricants due to microbial attack. Couplers help stabilize water/organic microemulsions. Most lubricant additives, except perhaps some viscosity modifiers and p o u r point depressants, comprise a n oleophilic ("hydrocarbon-loving") hydrocarbon group and a polar functionality [7]. The polar functionality typically contains oxygen, nitrogen, sulfur, and or phosphorus. Figure 1 identifies polar and non-polar portions in the oleic acid molecule. All additives must initially be oil soluble; hence, the presence of a hydrocarbon group of sufficient carbon chain length is essential. Lubricant additives perform their function either in the bulk lubricant or on surfaces. The former type includes oxidation inhibitors, detergents, and dispersants and the latter type includes rust inhibitors, antiwear agents, and extreme pressure (EP) agents. Additives may be designed either by al-
CHAPTER 9: ADDITIVES AND ADDITIVE CHEMISTRY
POLAR MOIETY Surface Active Group
NON-POLAR MOIETY Oleophilic Group Consistsof a hydrocaibon chain
Contains oxygen, nitrogen, sulfur, or plK^honis
POLAR GROUP Surface-active
NONPOLAR GROUP Oil-solubllizing H H H H H . H H
H
H3C' X X X W
H
HH
H H
HH
H!
>^ >^ '>^ ^i ^^°"
HHHHHH|!,,!,HHHHHH
;o
Oleic Acid FIG. 1—A general representation of a typical additive molecule.
tering the strength of the polar functionahty or by changing the size of the hydrocarbon chain. Changing the strength of the polar functionality alone is difficult and has its limitations. Changing the size of the hydrocarbon group, on the other hand, is much easier. In practice, both strategies are used. Whether an additive performs its function on the surface or in the bulk lubricant depends on its polar to non-polar ratio. With the strength of the polar moiety constant, additives with small hydrocarbon groups have higher polar to non-polar ratio than those with large hydrocarbon groups. As a consequence, EP agents and rust inhibitors, which require more surface activity, have small hydrocarbon groups; and dispersants and detergents, which require a higher solubility in oil, contain large hydrocarbon groups. Except in very few cases, a connecting group or a link is necessary to connect ("tie") the two functionalities together. The importance of such a group is described in detail in the dispersants section. StabilizersADeposit Control Agents This class of additives controls deposit formation. These additives do so by inhibiting oxidative breakdown of the lubricant to deposit precursors and by suspending those already formed in the bulk lubricant. Oxidation inhibitors intercept the oxidation mechanism and dispersants and detergents do the suspending part. Oxidation Inhibitors All lubricants, by virtue of being hydrocarbon based, are susceptible to oxidation [7,8]. Many lubricants contain three major components: base oil, additives, and viscosity modifier, all of which have the susceptibility to oxidize. Each tj^e of basestock, mineral, vegetable, or synthetic, has a stable threshold beyond which stabilizers or oxidation inhibitors are needed to retard oxidation. In terms of oxidative stability, synthetic oils eire the most stable and vegetable oils are the least stable. The oxidative stability of mineral oils is intermediate between the two. Most lubrication applications expose lubricants to oxygen in some manner. Oxygen reacts with hydrocarbon molecules
201
that make up the lubricant. The reaction sites, in order of decreasing ease of attack, are benzylic, allylic, tertiary alkyl, secondary alkyl, cind primary alkyl hydrogens. The result is the formation of hydroperoxides and peroxy or other radicals. The rate of oxidation of hydrocarbons in addition depends upon the amount of oxygen, presence or absence of nitrogen oxides (NOx, a general term used for NO and NO2), ambient temperature, and the presence or absence of metal ions. Hydrocarbon oxidation is a three-step process, and comprises initiation, propagation, emd termination (Fig. 2). Initiation involves the attack of atmospheric oxygen or of nitrogen oxides (NOx) on hydrocarbon molecules. The result is the formation of hydroperoxides (ROOH) and alkyl (R*) and peroxy (ROO') radicals. During the propagation stage, hydroperoxides decompose either on their own or in the presence of metal ions to alkoxy (RO') and peroxy radicals. These react with the lubricant hydrocarbons to form a variety of additional radicals and oxygen containing compounds, such as alcohols, aldehydes, ketones, and carboxylic acids [see the chapter on Oxidation]. Aldehydes and ketones are highly reactive and can form polymers in the presence of acids, such as nitric and sulfuric acids. These acids result from the interaction of nitrogen oxides and sulfur oxides, products of combustion, with water. Carboxylic acids attack iron, copper, and lead, present in the mechanical equipment, to form metal carboxylates that further increase the rate of oxidation. During the termination stage, the radicals either self-terminate or terminate by reacting with oxidation inhibitors [8]. Oxidation inhibitors (InH) circumvent the radical chain mechanism by promoting deRH
+
O2
ROOH
RH
+
O2
ROO-+H
RH
+-NO,
'
Initiation
R- + HNOx
ROOH
>v
RO- + OH-
ROOH + Fe+2
-*-
RO- •»• OH' + Fe+3
ROOH + Fe*'
-*•
ROO- + Fe*^ + H*
ROO - ••• RH
-*•
ROOH ••• R-
ROO-
-*-
R- ••• O2
RO - •!• RH
-*•
R O H -I- R-
OH- + RH
•*-
H2O
ROO- + ROO-
•*-
RR + 2O2
ROO- + InH
•*-
ROOH + In-
RO- + InH
-^
ROH + In-
R- + InH
-*•
R H ••• I n -
+
> Propagation
R-
y
>-
Termination
RH = Hydrocarbon
ROOH = Hydroperoxide
RO- = Alkoxy Radical
R - = Alkyl Radical
InH = Inhibitor
In- = Inhibitor Radical FIG. 2—Oxidation mechanism.
202
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HANDBOOK
composition of hydroperoxides and taking reactive radicals out of the oxidation process by reacting with them. Oxidation inhibitors can be classified as hydroperoxide decomposers, radical scavengers, or metal deactivators, depending upon the mode of their controlling action. Sulfur containing compounds, such as sulfides a n d dithiocarbamates, and phosphorus containing compounds, such as phosphites and dithiophosphates, act as hydroperoxide decomposers. Nitrogen a n d oxygen containing c o m p o u n d s , such as arylamines and hindered phenols, act as radical scavengers [8,9]. These chemicals convert chain-propagating hydroperoxides and radicals to innocuous products. Some inhibitors, such as dithiophosphoric acid derivatives and dithiocarbamates are extremely potent oxidation control agents. This is because they act both as hydroperoxide decomposers and as radical scavengers. They in addition possess antiwear properties. Transition metals can act as both oxidation initiators (promoters) and oxidation inhibitors, depending upon their oxidation state [8]. They act as promoters if they facilitate the formation of free radicals, and they act as inhibitors if they remove free radicals from the oxidation process [10]. For example, heavy metals, such as iron and lead, and their salts are well known as oxidation promoters [11,12]. Metal deactivators, another class of oxidation inhibitors, are used to control oxidation under such circumstances. These inhibitors, primarily used in fuels, form complexes with metal ions via chelation, thus taking t h e m out of the chain reaction. Ethylenediaminetetraacetic acid derivatives and N,N-disalicylidene-l,2-propanediamine represent the m o s t p o p u l a r members of this class. The structures of common inhibitors
are provided in Fig. 3. Synthetic methods for polysulfides, phosphites, and dithiophosphoric acid and dithicarbamic acid derivatives are described in the antiwear and extreme pressure agents section. Those for synthesizing alkylphenols are provided in the detergents section. Hindered phenols, such as 2,6-di-?-butyl-4-methylphenol (BHT), a n d diaryla m i n e are p r e p a r e d by reacting phenol, alkylphenol, or diphenylamine with an olefin in the presence of a Lewis acid catalyst. The salicylidene meted deactivator is a product of phenol, formaldehyde, and ethylenediamine. Some oxidation inhibitor combinations are synergistic. That is, they reflect an effect greater than the additive effect of two or more inhibitors. Such combinations usually, but not always, consist of compounds that intercept oxidation by two different mechanisms. An example is the combination of a sulfur compound with an arylamine or a hindered phenol [13]. Oxidation inhibitors cire used in almost all lubricants, with gasoline and diesel engine oils and automatic transmission fluids accounting for ~ 60% of the total use. High temperature and high air exposure applications require a higher level of oxidation protection. Zinc diaJkyl dithiophosphates are the primary inhibitor tj^pe, followed by eiromatic amines, sulfurized olefins, and phenols. A n u m b e r of tests are used to assess a lubricant's oxidation stability under accelerated oxidation conditions. ASTM Sequence IIIE/IIIF (viscosity increase), ASTM Sequence VE/VG (sludge and varnish formation), and CRC L-38 (bearing corrosion) tests are used for engine oils. The CRC L-60-1 test is used for gear oils a n d the ASTM D 943 and ASTM D 2272 tests are commonly used for turbine oils.
1. Hydroperoxide Decomposers R-S-(S)rS-R
X = 0 or 1; R = alkyl or functionalized alkyl
DialkyI polysulfide RQ.,,S (R0)2R, H
:>-c'
Zn
RO'%
CHo
R DialkyI hydrogen phosphite
Alkylphenol
Zinc dialkyi dithlophosphate
2. Radical Scavengers
Methylene coupled dlthlocarbamate
3. lUletal Deactivators
2,6-DI-f-butyl-4-methylphenol
Alkylated diphenylamine
[BHT (Butylated hydroxytoiuene)] N, N-Disalicylldene-1,2-propanediamine FIG. 3—Commonly used oxidation inhibitors.
CHAPTER 9: ADDITIVES AND ADDITIVE CHEMISTRY
203
Hydroperoxide
RCH2CH(CH3)2 + O2 CH3 _,OH R
+
O2
»-
'^"-X^-^*^^
Hydroperoxide
*-
RCH2CH(CH3)2 +
Alkene RCH2CH(CH3)2 +• N O ,
HNO,
o-
R ^ ^ C H 3
R^^N^CHj
+
. OH
CH3 CH3 Hydroperoxide O
Alltoxy radlcai
.OH
O+ • NO,
R ^ x k , ^ ^
+
MONO
Allyloxy radical
Hydroperoxide
o-
ONO /
•
1
RCH2CH(CH3)2
I RCH2CH(CH3)2
Nitrite Ester
RCH2CH(CHi)2 +• NO2
V
»•
RCHiCHCCHjlj-
RCH=CH(CH3)2
NO2 NItroalkane
O
ONO2 + • NO2 — * •
Aliyioxy radical
^
FIG. 4—Formation and decomposition of nitrite and nitrate esters.
Dispersants Dispersants are additives that suspend oil-insoluble resinous oxidation products and particulate contaminants in the bulk oil. By doing so, they minimize sludge formation, particulaterelated abrasive wear, viscosity increase, and oxidationrelated deposit formation. Dispersants perform these functions by: • solubilizing polar contaminants in their micelles. • stabilizing colloidal dispersions in order to prevent aggregation of their particles and their separation out of oil. • suspending such products, if they form, in the bulk lubricant. • modifying soot to minimize its aggregation and oil thickening. • lowering surface/interfacial energy of dirt to decrease its tendency to adhere to surfaces. Dirt is a generic term that describes undesirable materials that result from oxidative degradation of the lubricant, the reaction of chemically reactive species (such as carboxylic acids) with the metal surfaces in the engine, or the decomposition of thermally unstable lubricant additives, such as extreme pressure agents.
In gasoline-fueled engines, nitrogen and oxygen present in the air-fuel mixture react at high temperatures to form NO and NO2. These oxides can react with hydrocarbons of the fuel and the lubricant to form nitroalkanes and nitrite and nitrate esters [14,15]. The mechanism of their formation is shown in Fig. 4. These esters and the hydroperoxides generated from the direct oxidation of the lubricant either thermally or hydrolytically decompose to form highly oxygenated monomeric species [14]. These can polymerize to a slight degree under the influence of heat to form resin, varnish, lacquer, deposits, and sludge (Figs. 5 and 6). Resin is made up of oligomeric molecules with an approximate molecular weight of 500-700. Varnish, lacquer, and deposits comprise materials of much higher molecular weight. The sludge, depending upon the severity of the operation, can be watery in consistency or thick like a semi-solid. In diesel-fueled engines, soot from the combustion chamber is the key component of carbon and lacquer deposits that occur on pistons, and sludge. These deposits result when soot combines with resin. In general, lacquer is rich in resin, and carbon is rich in soot. Sludge results when soot combines with the oxygenated species, oil, and water [16]. Local piston temperatures and the lubricant's ash-producing tendency
2 0 4 MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
Partially Combusted Fuel
Fuel
JNOx Liquid Nitrated and Oxygenated lUlonomers
Oil Insoluble Products (Resin) \ \
Lubricant
Carbon Water \ Solids
/ Varnish
Resin
»-Sludge
-•" Varnish Accumulate as deposits in areas of low oti velocity
Resin-coated Soot Particles Resin + Soot Soot-coated Resin Particles Resin + Soot + Oil + Water
-*-
Lacquer
Sludge
FIG. 5—Mechanism of deposit formation.
*
^ A
Soot
^
Resin
o-
Additive
FIG. 6—Mechanism of soot-resin-additive interaction.
(ASTM D 482) have a profound effect on the composition of carbon deposits. High temperatures and lubricants with high metal content primarily produce deposits with high residue and low organic content [17]. Metals are the main source of ash. Basic detergents contain meted and therefore are considered to contribute towards ash. Zinc dialkyl dithiophosphates also contribute, but only slightly.
Because of low oil-solubility, resin tends to separate out as amber lacquer on hot piston surfaces. If oil contains soot, soot separates with resin to form "resin-coated soot particles," which appear as black lacquer. As the soot level increases, more and more soot associates with resin to form "sootcoated resin particles." These events are shown in parts A and B of Fig, 6. The size and the composition of these particles do not allow them to adhere to metal surfaces. However, they can collect in areas of low oil flow, such as piston grooves, as deposits. Dispersants suppress the interaction between resin and soot particles, by preferentially associating with them and, at the same time, keeping them suspended in the bulk lubricant. Since both resin and soot particles are polar in character, either by their very nature or due to adsorbed polar impurities, the dispersant associates with these particles via its polar end. As mentioned earlier, all additive tj^es, except some viscosity modifiers and pour-point depressants, contain a polar functional group and a non-polar oleophilic hydrocarbon moiety. The polar group, which in dispersants is oxygen or nitrogen-based, attaches itself to oxidation products, such as resin, soot, and sludge peirticles. The oleophilic hydrocarbon group keeps them suspended in oil [16]. These situations are depicted in parts C and D of Fig. 6. The performance of dispersants can be explained by considering the concepts of steric stabilization and electrostatic stabilization. According to steric stabilization, once the dispersant molecules attach themselves to either resin or soot particles, their long hydrocarbon chains prevent agglomeration of the particles [16]. This is depicted in Fig. 7. The electrostatic mechanism is based on repulsion between particles
CHAPTER 9: ADDITIVES AND ADDITIVE CHEMISTRY Hydrocarbon Chains of Adsorbed Dispersant
Separation Distance FiG. 7—iVIeclianism of steric stabiiization.
c : ^ ^
{
Particle ) ^
^
cAi,
c^^
FIG. 8—Tlie meclianism of eiectrostatic stabiiization. (a) by ionized dispersant. (b) by un-ionized dispersant.
205
with like charges, which minimizes agglomeration. The development of charged particles in a low-dielectric medium, such as oil, can be explained in two ways [18]. According to the first explanation, the dispersant is already ionized and through adsorption imparts a chcirge to the particle (depicted by mechanism A in Fig. 8). According to the second explanation, the non-ionized dispersant adsorbs onto the surface of the particle, an acid-base reaction involving the transfer of a proton or another ion occurs, and the counter-ion desorbs resulting in the formation of a charged particle. Particles with like charges repel one another, thereby preventing agglomeration. This is shown in part B of the exhibit. Neutral and high soap detergents, discussed in next section, are believed to perform via this mechanism. It is important to note that for certain classes of dispersants, both mechanisms might be operating simultaneously. Interestingly, empirical data suggest that the ionic mechanism, although it seems unlikely due to the organic nature of the lubricant medium, does operate [18]. The essential performance features in a dispersant are its thermal and oxidative stability, its low-temperature properties, and above all its ability to disperse soot, deposits, and deposit precursors. Poor thermeJ stability can impair a dispersant's ability to disperse polar oxidation and decomposition products in the bulk lubricant. Poor oxidative stability can make a dispersant contribute towzirds deposit formation. Low temperature properties of the modem lubricants are gaining importance. The use of light base oils to meet low temperature requirements in finished lubricants is no longer possible because of the volatility specifications (ASTM D 5480 and D 5800). The use of pour point depressants for paraffinic oils has limited utility because it only affects some low temperature properties, such as pour point, but not others, such as cold cranking. Base oil manufacturers therefore use a number of strategies to develop base oils with good low temperature properties. These include isomerization via hydrocracking and use of special synthetic oils as additives. Because dispersant is a major component in engine oil formulations, its structure can antagonize low-temperature properties of these oils. And of course soot and deposit control, or dispersancy, are the primary functions of a dispersant. If a dispersant does not possess these properties, it is not effective as an additive. A dispersant contains three structural features: a hydrocarbon group, a polar group, and a group that connects the two together, the connecting group or a link. These are depicted in Fig. 9 by a graphic representation of a disperscuit molecule. While each structured feature affects a different property or properties in a dispersant, all three are essential to a dispersant's oversdl performance. The nature of the hydrocarbon
Connecting Group
Nitrogen or o)^gen _ Derived Functionality
FiG. 9—Graphiic representation of a dispersant molecule.
206
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
group impacts all key properties, namely, thermal and oxidative stability, low-temperature performance, and dispersancy. Its structure can influence a dispersant's thermal and oxidative stability and low-temperature properties. Its degree of branching can improve or hurt the low temperature properties and compatibility with the additive package. Its molecular weight can affect dispersancy. The hydrocarbon radical in dispersants is oligomeric or polymeric and is usually aliphatic in nature. It contains 70 to 200 or more carbon atoms to ensure good oil solubility. A polyisobutenyl alkyl group is the most commonly used hydrocarbon group. In some dispersants, the hydrocarbon moiety is derived from a high molecular weight polymer, such as olefin copolymer, polyacrylate, polymethacrylate, or styrene-ester polymer. Such dispersants can also function as viscosity modifiers and are appropriately called dispersant viscosity modifiers. The link is the weakest portion of the dispersant molecule because of its chemical structure. It can fall apart thermally or chemically, making a dispersant ineffective. The most commonly used polar groups are alcohol and amine derived. They can oxidize, hydrolyze, and or thermally degrade and make dispersant molecule lose its structural integrity. The size and the nature of the polar group (whether it is oxygen or nitrogen-based) determine dispersancy. Chemical classes suitable for use as dispersants include cilkenylsuccinimides, alkenylsuccinate esters, high molecular weight amines, Mannich bases, and phosphonic acid derivatives. Polyisobutenylsuccinic acid derivatives (succinimides and succinate esters) are commercially the most commonly used dispersant types. Both succinimides and succinate esters are prepared from alkenylsuccinic anhydrides. Polyisobutylene (the most com-
monly used polyalkylene) of molecular weight between 440 and 5000 is reacted with maleic anhydride, either thermally or in the presence of chlorine, to yield a succinic anhydride. Reaction of the succinic anhydride with amines (polyalkylene-polyamines and heterocyclic polyamines) results in the formation of succinimide dispersants. The corresponding reaction of the succinic anhydride with alcohols, especially polyhydric alcohols, results in the formation of succinate ester dispersants. Figure 10 summarizes the syntheses of alkenylsuccinimides and alkenylsuccinates. Mannich dispersants are produced by the condensation of a high molecular weight alkylphenol (polyisobutylphenol), an aldehyde, and polyalkylene-polyamines. Phosphonic acid ester dispersants are prepared by reacting phosphonic or thiophosphonic acids with ethylene oxide or propylene oxide. The starting acids are obtained from the hydrolysis of olefin-phosphorus pentasulfide adducts. The preparation of Mannich products and phosphonic esters is described in Fig. 11. While in most cases starting polyalkylene has a molecular weight of 1000 or 2000, the resulting succinimide and succinate dispersants have molecular weights that are 3-7 times higher. This is due to the bifunctionality of the alkenylsuccinic anhydride and the polyfunctionality of the alcohols and amines that are used to make dispersants. The result is the formation of bridged structures that increase a dispersant's dirt-suspending ability via more extensive interaction. The bridged structure of a dispersant is graphically depicted in Fig. 12. Dispersants have major uses in gasoline engine oils, diesel engine (heavy-duty and railroad) oils, natural gas engine oils, and aviation piston engine oils. Dispersants are also used in automatic transmission fluids and gear lubricants.
PIB
•<" Polyisobutylene
PIB
Polyisobutenyl Succinic Anhydride
Maleic Anhydride
""'^ ~ |
H2NCH2CH2NHCH2CH2N
,NCH2CH2NHCH2CH2l<^
\ Alkylenepolyamlne Polyisobutenylsucclnlmlde
Polyisobutenyl Succinic Anhydride
H2
R'
,C
PIB
I
PIB
HOH2C-C—CH2OH
I
R Polyisobutenyl Succinic Anhydride
Polyol
V o
R'
/ Y~CH20 R
Polylsobutenylsuccinate Ester FIG. 10—Synthesis of alkenylsuccinimides and alkenylsuccinates.
CHAPTER 9: ADDITIVES AND ADDITIVE CHEMISTRY
207
MANNICH DISPERSANTS
?"
H
/
Js.,.CH2NCH2CH2NHCH2CH2Nv
CH2O + H2NCH2CH2NHCH2CH2N R'
'^
Alkylenepolyamine
Polyaminomethylalkylphenol
Alkylphenol
R=Polyisobutyl Group PHOSPHONIC ACID DISPERSANTS
PIB
+
Polylsobutylene
*•
P2S5
Adduct
— ? — •
PIB—P-OH
Phosphorus Pentasulfide
or
OH
PIB—P-OH OH
Polyisobutylenethiophosphonic Acids
S 11 PIB—P-OH I OH
+
Alkenylphosphonic Acid
Pro
-
Propylene Oxide
S II
CH3 I
PIB—P—OCH2CH — O H I OCH2CH — O H
Bis-hydroxypropyl Alkenylthiophosphonate
I CH,
FIG. 11—Synthesis of Mannich and phosphonic acid dispersants.
Nitrogen or Oxygen Derived Functtonallty \
Aikenylsuccinic acid or Alkylphenol Derived Connecting Group
Alkylene or Pdyalkyleneamine Derived Bridging Group
Hydrocarbon Groups
Nitrogen or Oxygen Derived Functtonality
Altenylsuccinic acid or Alkylphenol Derived Connecting Group
FIG. 12—A representation of the bridged structure of a dispersant.
208 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK Gasoline a n d heavy-duty diesel engine oils account for 75-80% of the toted dispersant use. Succinimide and succinate ester (pentaerythritol esters of polyisobutenylsuccinic anhydride) dispersants are used in both gasoline and diesel engine oils. High molecular weight amines and Mannich dispersants are used in gasoline engine oils only. Succinimide dispersants are often used in automatic transmission fluids, power steering fluids, and, to a limited extent, in gear oils. Both types are also used as fuel additives. The use of phosphonate dispersants is generally restricted to non-engine lubricants. This is because engine oils have a phosphorus limit due to its adverse effect on catalj^tic converters. The formulators prefer to use zinc dialkyl dithiophosphates instead because they are potent oxidation inhibitors as well as good antiwear agents. In gasoline and diesel engine oils, the effectiveness of a dispersant is determined by its ability to suspend oil-insoluble products of combustion, oxidation, and oil degradation. In the laboratory screen tests, this is assessed by a dispersant's ability to either disperse lamp black or used engine oil sludge. However, in the laboratory engine tests, such as the ASTM Sequence VEA'^G Test and various diesel engine tests (Caterpillar IK, IM-PC, IN, IP, and IR), a pass rating in varnish, sludge, and deposits is required. Detergents
low. The excess base per equivalent of acid in metal hydroxide-containing detergents is generally lower t h a n t h a t in metal carbonate-containing detergents. Detergents are described chemically in terms of their metal ratio, soap content, sulfated ash (ASTM D 874), degree of overbasing or conversion, and total base n u m b e r (TBN) [5]. Let us represent a detergent, for example a basic calcium sulfonate, by the general formula (RS03)vCaw(C03)a:(OH)y , where v, w, x, and y denote the n u m b e r of sulfonate groups, the n u m b e r of calcium atoms, the n u m b e r of carbonate groups, and the n u m b e r of hydroxyl groups, respectively. The metal ratio is defined as the total equivalents of metaJ per equivEdent of acid. Since calcium is a divalent metal, each atom of calcium represents two equivedents. Hence, the metal ratio can be Ccilculated by the equation. Metal Ratio =
The degree of overbasing describes the ratio of equivalents of the metal base to equivalents of the acid substrate and is usually expressed as conversion. Conversion indicates the a m o u n t of inorganic materied relative to that of the organic material and is expressed as the n u m b e r of equivalents of base per equivalent of acid times 100. Conversion
Detergents perform functions similar to those of dispersants; that is, they suspend oxidation products and sludge particles in the lubricant. Detergents, in addition, have the ability to neutralize acidic combustion and oxidation products; therefore they control rust, corrosion, and resinous build-up in the engine. Detergents, like dispersants, contain a surface-active polar functionality a n d a n oleophilic hydrocarbon group with an appropriate n u m b e r of carbon atoms to ensure good oil solubility. Sulfonate, phenate, carboxylate, salicylate, and phosphonate are the common polar groups present in detergent molecules. Detergents eire metal salts of organic acids. Common acids that are used to sjrathesize these compounds include alkylbenzenesulfonic acids, alkylphenols, carboxylic acids, and alkenylphosphonic and alkenylthiophosphonic acids. The quantity of metal used may be equal to or in excess of the precise amount (stoichiometric amount) necessary to completely neutralize the acid function. When the metal is present in the stoichiometric amount, the detergents are called "neutral." When it is present in excess, they are called "basic, overbased, or superbased." The general formulas for basic metal sulfonate, metal phenate, and metal carboxylate are given below. (RS03)a M • xMfcCOs • yM(OUX Basic Sulfonate (RPhO)aM • xMfcCOa • yM{OU)c Basic Phenate (RCOO)aM • xMbCOs • >'M(OH)<; Basic Carboxylate Where a,b, and c = 1 or 2, depending upon if the metal M is monovalent or divalent. Detergents may contain the excess base as metal hydroxide, metal carbonate, or both. For noncarbonate detergents, X in the above formulas is zero. For carbonate detergents, y is
2\v
N u m b e r of equivalents of base ^ . „ „ Number of equivalents of acid 2w
X 100
In other words, conversion is the metal ratio times 100. Neutral detergents have a conversion of 100 because the ratio of equivalents of base to the equivalents of acid is 1. Soap content refers to the a m o u n t of neutral organic acid salt and reflects a detergent's cleansing ability, or detergency, and dirt suspending ability. The soap content can be determined by the use of the following formula. Percent soap
Formula weight [(RS03)2 Ca] X 100 Effective formula weight
Effective formula weight is the combined weight of all atoms that make u p the formula (RS03)vCaw(C03);c(OH), plus that of the diluent. The percent sulfate ash is the ash obtained after treating the detergent with sulfuric acid and ignition. The knowledge of ash-producing tendency of a lubricant is important to the formulator (ASTM D 482). This is because combustion of the lubricant results in ash that can contribute to deposits in those areas of the engine that are in close proximity to the combustion chamber, such as the piston top land and the groove behind the top piston ring. The total base number, or TBN, of the detergent indicates its acid neutralizing ability and is expressed as mg KOH/g of additive. Both the acid n u m b e r and the base number are expressed in this fashion. The convention regarding its use and the details on how to calculate acid and base numbers are described in the ASTM Standard D 974. TBN can be calculated from the number of equivalents of the excess metal after salting the acid, that is (2w — v), as follows. TBN (mg KOH/gm)
(2w - v) X 56100 Effective formula weight
CHAPTER 9: ADDITIVES AND ADDITIVE CHEMISTRY For a monovalent metal, (w — v) will be used instead. In addition to the alkylbenzenesulfonic acids, alkylphenols, carboxylic acids, and alkenylphosphonic acids that were mentioned earlier, sulfur-coupled and methylene-coupled alkylphenols are also used as detergent substrates. The methods of synthesis of these substrates are presented in Fig. 13. Alkylbenzenesulfonic acids are obtained by sulfonating alkylbenzenes. The products obtained by sulfonating alkylaromatics from petroleum refining are referred to as "natural" sulfonates and those obtained by sulfonating cilkylaromatics from the catalytic alkylation process are referred to as "3501thetic" sulfonates. Alkylphenols eire prepared from phenol and an olefin by the use of an acid catalyst. These alkylphenols can be further reacted with sulfur, sulfur dichloride, or formaldehyde to form sulfur-bridged and methylene-bridged alkylphenols. Alkylsalicylic acids are prepared by the use of the Kolbe process, which involves reacting an alkali metal phenate with carbon dioxide. Alkenylphosphonic acids are the hydrolysis products of polyisobutylene-phosphorus pentasulfide adducts. Detergents are prepared by reacting an organic acid with an appropriate metal base in the presence of a polar promoter. Overbasable metals that can be used to prepare basic detergents are lithium, sodium, and potassium in Group I and magnesium, calcium, strontium, and barium in Group II of the periodic table. Aluminum is the only overbasable metal
in Group III. The ability to overbase is directly related to a metal's base strength. For Group I metals, basicity increases from lithium to sodium to potassium. For Group II metals, it increases from magnesium to calcium to strontium to barium. It is, therefore, most difficult to overbase lithium in Group I and magnesium in Group II. Metals commonly used to make detergents are sodium, magnesium, calcium, and barium. Of these, calcium and magnesium are used most widely, with a clear preference for calcium due to its lower cost. The use of barium is being limited due to both the toxicity and the economic concerns. The bases of choice are caustic (sodium hydroxide) for sodium detergents; lime (calcium hydroxide) for calcium detergents; magnesium oxide for magnesium detergents; and barium hydroxide for barium detergents. For basic detergents that contain excess base, the base may be present as is or as metal carbonate. In practice, virtually all commercial detergents are overbased to some extent. For example, commercial "neutral" sulfonates have a TBN of 30 or less. "Basic" detergents typically have a TBN of 200 to 500. Sulfonate, salicylate, and carboxylate detergents are commercially available as calcium and magnesium salts. Phenate detergents are available as calcium salts and phosphonate detergents are available as barium salts only. Basic calcium sulfonates make up ~ 65% of the total detergent market, followed by phenates at ~ 31%. SO3H
SO, !< R Alkylbenzenesulfonic Acid
Alkylbenzene
COOK Olefin Plienol
R=Alkyl Group
Sulfur CoupledAlkylphenol or Phenol sulfide
PIB Polyisobutylene
Methylene Coupled Alkylphenol
H,0
P,S 2*»5
Adduct
Phosphorus Pentasulfide
209
••
RIB—P-OH I OH
Polyisobutenylphosphonic Acid
X = O or S FIG. 13—Detergent substrates.
210
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK with these compounds and keep t h e m in solution. The mechanism by which detergents perform is analogous to that of dispersants, which is shown in Figs. 7 and 8. Depending upon the lubricant's end-use, insoluble by-products may be soot, acidic decomposition products, or deposit-forming resinous species [14,16]. Detergents control the buildup of these undesirable contaminants by keeping equipment surfaces clean and neutralizing acidic products from lubricant oxidation and decomposition. They keep surfaces clean by forming a protective film via adsorption and by suspending depositforming species in the bulk oil through association [7]. Detergents can also act as oxidation inhibitors, depending u p o n the n a t u r e of their functional group. For example, phenates, sulfurized phenates, and salicylates possess oxidation-inhibiting properties, primarily due to the presence of the phenolic functional group.
The method of preparation of neutral and basic detergents is outhned in Fig. 14 and the general structures for neutral detergents are shown in Fig. 15. Basic detergents can be considered neutral detergents that contain excess base in an associated form. The structure of detergents can be envisioned as a reverse micelle, with a n a m o r p h o u s metal carbonate molecule encapsulated by metal soap molecules with their non-polar ends extended into the oil. This is depicted in Fig. 16. Detergents are used in lubricants to perform functions similar to those of dispersants: that is, to keep oil-insoluble byp r o d u c t s of c o m b u s t i o n and oil oxidation in suspension. These products, which are polar organic compounds, include alkyl nitrites and nitrates, nitroalkanes, carbonyl compounds (aldehydes, ketones, and carboxylic acids), and oligomers resulting from their thermal reaction. Detergents associate
Substrate + Metal Oxide or Hydroxide M„0 M{OH)„
-*-
Neutral Salt
stoichiometric Amount
Substrate + IMetal Oxide or Hydroxide M„0 iW{OH)„
^ ^ »• Basic or Overbased Salt
No CO,
Stoicliiometnc Excess
Basic Salts = Neutral Salts • M„0, M(OH)„ or MnCOs R/l = Na, Mg, Ca, Ba
n « 1 or 2
FIG. 14—Synthesis of detergents.
0)xlWI
^?\.S03)xM
0)xlWI R
R Metal Salicylate
Metal Sulfonate
or
Sulfur and Methylene Bridged Phenates
s R—P-O I I O-IW
r. n « R—P-O ' '
Metal Phosphonate
Metal Thiophosphonate
0-iw
o
o
II II R_p_s-p-R I I
o^ .o
X = 1 or 2 Y = S or CHa lUI = Na, lUlg, Ca
M Metal Thiopyrophosphonate
FIG. 15—Idealized structures for neutral detergents.
CHAPTER 9: ADDITIVES AND ADDITIVE CHEMISTRY
211
O II
-s-o
'6 \ II
/
-s-o II
o
Sulfonate Head Group
Hydrocarbon Group
Basic Sulfonate Inverse Micelle Structure — Size: 100-150A°
Neutral Calcium Sulfonate FIG. 16—Micellar structure of detergents.
Additive It/lolecule
Metal Surface
Mack T-6, T-7, T-8/E, T-9, and T-10). Detergents have additionEil use in automatic transmission fluids and tractor hydraulic fluids. However, their use in these applications is not to control deposits but to modify the fluid's frictional properties. Film-Forming Agents
Physisorption
Cliemisorption
o o o o o o o o o o o o Chemical Reaction FIG. 17—Modes of additive - surface interactions.
Detergents have major use in crankcase lubricants. Gasoline and diesel engine oils account for over 75% of the total detergent consumption. Detergent treatment levels in engine lubricant formulations are fairly high, with marine diesel engine lubricants containing the highest detergent concentrations. Marine engines use high-sulfur fuel, which leads to acidic combustion products (sulfuric acid). Therefore, lubricants for these engines need the base reserve of the "basic" detergents for neutralization. Detergency in diesel engine oils is evaluated by using both single cylinder engine tests (Caterpillar IK, IM, IN, IP, and IR) and multicylinder engine tests (GM 6.2L and 6.5L; and
Lubrication is necessary to facilitate the counter-movement of two sliding surfaces. This function, which is usually performed by the base oil, can be enhanced by using high viscosity oils [19]. However, beyond a certain threshold temperature, the lubricant fails to form an effective film, and friction and wear can result. The lubricant's film-forming ability under such circumstances can be made more efficient by using film-forming agents. Such agents can interact with metal surfaces either through adsorption or chemical reaction. PhysicaJ adsorption, or physisorption, is a weaker association of the additive with metal than chemical adsorption, or chemisorption, which in turn is weaker than chemical reaction. During adsorption, an additive molecule generally keeps its structural integrity. In a chemical reaction, however, it has the tendency to lose it because it gets converted into new molecules. The modes of interaction of additive molecules with the surface [20] are depicted in Fig. 17. Friction Modifiers Friction modifiers are agents that modify the frictional properties of a lubricant. These are long-chain molecules with a polar end group and a non-polar linear hydrocarbon chain. The polar end groups either physically adsorb onto the metal surface or chemically react with it while the hydrocarbon chains extend into the lubricant. These chains associate with one another and the lubricant to form a strong lubricant film [21]. This is shown in Fig. 18. These additives are used to decrease or increase friction, depending upon the application. In engine lubricants and gear oils, their primary function is to reduce friction, minimize wear and noise, and improve fuel economy by lowering the power loss. In transmission and hydraulic fluids, these additives are used to facilitate timely engagement and disengagement of clutches and bands to assure smooth and noise-
212 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK Cohesion or Association with Lubncant
\ ^
Ifl \!*
^
"
"
^
^ ^ ^
" X ^ ^ " A ^ ^V^ H ^
^ H
HtOT fVSft<
HHZH" H^^HH ^ ^ H ^ QH^H
o
H-4HQ „ H ^
» ^ Hfi^
p
(A) Physisorption (B) Chemisorption FIG. 18—Adsorption of polar additives on the metal surface.
Crankcase Temperature (°C) FIG. 19—The effect of friction-modified lubricant on engine power loss.
free operation. The effect of friction modifiers on the engine power loss [22] is depicted in Fig. 19. At low ambient temperatures, where lubricant viscosity is high enough to form an effective lubricating film, friction modifiers have little effect in minimizing power loss due to friction. However, at high temperatures, friction modifiers become more effective because of the loss of lubricant viscosity, which may lead to
boundary lubrication. Figure 20 shows the friction-reducing capability of these additives in automatic transmission fluids. At low sliding speeds, where metal to metal contact is likely; friction modifiers interact with surfaces in the manner discussed above and minimize such contact, and the associated rough shifting. However, at high speeds, their effect is not as dramatic because such speeds promote hydrodynamic lubrication. These additives minimize brake chatter in tractors in an analogous manner. Due to government regulations, increasing fuel costs, and efforts to conserve resources, lubricants that can provide better fuel economy are becoming more desirable. Energy Conserving engine lubricants are designated as Energy Conserving I (EC-I) or Energy Conserving 11 (EC-II), depending upon the extent of fuel conservation [23]. While the use of friction modifiers helps, other factors such as the lubricant viscosity grade, the type of viscosity modifier used, and the nature of the other additives present in the lubricant, all play an important role [24]. Frictional properties of transmission and hydraulic fluids are extremely important. The frictional compatibility of the fluid with the functioning parts of transmissions and wet brakes and wet clutches in tractors assures their smooth and noise-free (chatter-free) operation. Frictional properties play an equally important role in gear oils. In standard applications, friction-modified gear oils not only protect gears and axles against frictional damage but they also contribute to-
CHAPTER 9: ADDITIVES AND ADDITIVE CHEMISTRY
213
TABLE 1—The effect of friction modifiers in gear oils.
0.16
LFW-1 Machine
Coefficient of Friction
GL-5 gear oil (80W-90) GL-5 gear oil (80W-90) + 1% friction modifier GL-5 gear oil (75W) GL-5 gear oil (75W) + 1% friction modifier
0.057 0.054 0.055 0.052 Coefficient of Friction
20
30
40
50
60
Sliding Speed (cm/s) FIG. 20—Friction reducing ability of an automatic transmission fluid.
LVFA Machine
Static
Dynamic
GL-5 gear oil (75W) GL-5 gear oil (75W) + 1% friction modifier GL-5 gear oil (75W) + 1% oleic acid GL-5 gear oil (75W) + l%ArmeenHT
0.070 0.050
0.052 0.042
0.052
0.043
0.062
0.050
O SAE 7SW Oil + Friction Modifier • SAE 75W Semi-syntlietic Oli
wards fuel economy. For limited-slip axles, which contain friction clutches and cones, friction-modified gear lubricants minimize noise arising from stick-slip. LFW-1 (Alfa Laval Load, Friction, and Wear Test) and LVFA (Low-Velocity Friction Apparatus) data for gear oils showing the effect of friction modifiers on friction [25] are presented in Table 1. As can be seen from the table, friction modifiers decrease static and dynamic coefficient of friction in both the single grade and the multigrade oils. In general, friction-modified gear oils show an increase in axle-efficiency relative to normal gear oils. This is shown in Fig. 21. Axle efficiency is better both for urban and highway driving. Axle efficiency is believed to have a positive correlation with fuel economy. Under heavy loads, the EP agent replaces the friction modifier and takes over the function of preventing damage. As the load decreases, friction modifiers again come into action. Friction modifiers have a finite life, which is related to their oxidative and thermal stability. These additives are commonly used in gasoline engine oils, automatic transmission fluids, tractor hydraulic fluids, power steering fluids, shock absorber fluids, and metalworking fluids. In passenger car applications, these additives help in meeting federal government-mandated fuel economy requirements. In ATFs and limited slip axle lubricants, friction modifiers are used to control clutch and band engagement. Antisquawk additives, which are functionally similar to friction modifiers, are used to reduce objectionable mechanical noise, such as squawk and chatter. Antisquawks are primarily used in automatic transmission and tractor hydraulic fluids, automotive gear oils, and some industrial oils. This class of additives includes fatty alcohols, fatty acids, fatty amides, molybdenum compounds, and graphite. For the fatty alcohol and fatty acid families, friction-modifying properties are a function of the length and the structure of the hydrocarbon chain and the nature of the functional group. Long and linear-chain materials reduce friction more effectively than short and branched-chain materials. Also, fatty acids are better than fatty amides, which in turn are better than fatty alcohols. Saturated acids, containing a 13 to 18 carbon chain, are generally preferred. Lower molecular weight fatty acids are avoided because of their corrosivity. Fatty acid derivatives are the most commonly used friction
110
Sump Temperature (°C) FIG. 21—The effect of a friction modifier on axle efficiency of a gear oil.
modifiers. Graphite presents problems when used in conventional lubricants; hence, at present, its use is limited to greases. The effectiveness of friction modifiers and antisquawk agents is judged by the lubricant's performance in LVFA and SAE # 2 tests. Their load-carrying capacity is determined by the film strength tests, such as the Timken test. Antiwear and Extreme Pressure Agents Wear occurs in all equipment that has moving parts in contact. Three conditions that can lead to wear are surface-tosurface contact, surface contact with foreign matter, and erosion due to corrosive materials. Wear resulting from surface-to-surface contact is frictional or adhesive wear, from contact with foreign matter is abrasive wear, and from contact with corrosive materials is corrosive wear. Fatigue wear is an additional tjrpe of wear that is common in equipment where surfaces are not only in contact but also experience repeated stresses for prolonged periods. Abrasive wear can be prevented by installing an efficient filtration mechanism to remove the offending debris. Corrosive wear can be controlled by using additives that neutralize the reactive species that attack the metal surfaces. The control of adhesive wear requires the use of additives called antiwear and extreme pressure (EP) agents.
214
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
Under normal conditions of speed and load, two metal surfaces are effectively separated by a lubricant film; a condition identified as hydrodynamic lubrication. An increase in load or a decrease in speed promotes metal-to-metal contact. This causes a temperature rise in the contact zone due to frictional heat, which results in the loss of lubricant viscosity a n d hence its film-forming ability. With a progressive increase in load and/or a decrease in viscosity, the nature of lubrication changes from hydrodynamic to mixed-film to boundary lubrication. Antiwear additives and EP agents offer protection under mixed-film andboundary lubrication conditions [26]. This is shown in Fig. 22, Part B [27]. Antiwear and EP additives both provide protection by a similar mechanism, except that EP additives typically require higher activation temperatures and load than the antiwear additives. Simply stated, antiwear additives perform under mild conditions and EP additives perform under severe conditions. The severity of conditions is determined by the "load factor" experienced by the additive and the temperature at which the additive functions. Load is a function of equipment speed and service. Heavy loading requires the use of extreme pressure agents and mild loading requires the use of antiwear agents. Thus, it is important to consider both the load and the temperatures that the equipment is likely to experience before selecting these additives. Antiwear agents are commonly used in engine oils, automatic transmission fluids, power steering fluids, and tractor hydraulic fluids. Extreme pressure agents are used in other power-transmitting fluids, gear oils, shock-absorber fluids, and metalwork-
ing fluids. For GF-2 and API SH and SJ quality engine oils, ASTM Sequence IIIE and VE engine tests are used to determine the effectiveness of antiwear agents. However, for API SL and ILSAC GF-3 categories (approved in 2001), ASTM Sequence IIIF, IVA, and VG engine tests are used. To determine the performance of EP agents in gear oils, the CRC L-37 and CRC L-42 axle tests are used. The general effectiveness of these additives in metalworking fluids is determined by Timken, Four-Ball, and Falex tests. Extreme pressure additives are usually supplemented with antiwear additives to make the formulations effective at lower temperatures and under milder loading conditions as well. Most antiwear and extreme pressure agents contain sulfur, chlorine, phosphorus, boron, or combinations thereof. The classes of compounds that inhibit adhesive wear include alkyl and aryl disulfides and polysulfides, dithiocarbamates, chlorinated hydrocarbons, and phosphorus compounds such as alkyl phosphites, phosphates, dithiophosphates, and alkenylphosphonates. Both antiwear and extreme pressure additives function by thermal decomposition and by forming products that react with the metal surface to form a solid protective layer. This solid metal film fills the surface cavities (asperities) and facilitates effective film formation, thereby reducing friction and preventing welding and surface wear. The meted films consist of iron halides, sulfides, or phosphates, depending upon the antiwear and EP agents used. Friction modifiers differ from antiwear and extreme pressure agents in that they form the protective film via physical Eind chemical adsorption instead of chemical reaction.
LubricantFilm FLUID-FILM LUBRICATION Surfaces well separated by the bulk lubricant film
1
A
1
.. * J c
Lubricant Film
1 Boundary Lubrication MIXED-FILM LUBRICATION Both the bulk lubricant and the boundary film play a role
! \ / - ^ Mixed-film Lubrication
u.
"5 c a>
8 o
—-
» Fluid-film Lubrication
.ZN. P (A) RELATIONSHIP OF VISCOSITY (2), SPEED (AO, AND LOAD (P) TO FRICTION AND FILM THICKNESS
FIG. 22—Types of lubrication.
BOUNDARY LUBRICATION Performance essentially depends upon the quality of the boundary film
(B) GRAPHIC REPRESENTATION OF
LUBRICANT FILM THICKNESS IN DIFFERENT LUBRICATION REGIMES
CHAPTER 9: ADDITIVES AND ADDITIVE CHEMISTRY
4R0H
+ P2S5
- •
2
.^' (R0)2P,
\
215
+ H2S
SH DialkyI Dtttiiophosphoric Acid
2 (RO)2P.
+
.^'
(R0)2P\ .
ZnO
SH DiaikyI Dithjophosphoric Acid Activated Olefin
Zn
+
H2O
Zinc DialkyI Dithioptiosphate
2 CH2=CH—< OR
i^°l<
2 (ROhP,
Zn J2
5 Cn2 CH2
Zinc Diaryl Oithiophosptiate
C.
OR DialkyI Ditliiophosphate Ester THERMAL STABILITY:
Aryl > Primary Alkyl > Secondary Alkyl ANTIWEAR ACTION: Secondary Alkyl > Primary Alkyl > Aryl FIG. 23—Syntiiesis of dialkyi dithiophosphoric acid derivatives.
The film-formation by these additives is a two-step process. The adsorption of the chemical onto the metal surface occurs first. It is followed by the formation of chemically reactive species due to thermal decomposition or hydrolysis. Antiwear Agents—Zinc salts of dithiophosphoric acids are the most widely used antiwear agents. These salts, in addition to providing antiwear protection, act as oxidation and corrosion inhibitors. They find major use in gasoline and diesel engine oils and industrial lubricants. Zinc dialkyi dithiophosphates or zinc diaryl dithiophosphates cire S3aithesized by reacting the respective dithiophosphoric acids with zinc oxide. The dithiophosphoric acid derivatives that do not produce ash on combustion (ashless) can be prepared by reacting the dithiophosphoric acids with alkylene oxides, such as ethylene oxide or propylene oxide, or with materials that contain activated double bonds, such as cilkyl acrylates and methacrylates. The synthetic scheme to prepare these materials is shown in Fig. 23. Dithiophosphoric acids are products of reaction of an alcohol or a phenol with phosphorus pentasulfide. Thermal and hydrolytic stability of these products depends upon the nature of the organic group. Dialkyi dithiophosphates derived from primary alcohols are more thermally stable than those derived from secondary alcohols, and are used extensively in formulating gasoline and automotive diesel engine oils. Diaryl dithiophosphates, although thermally the most stable in this family, are hydrolytically the least stable and, with some exceptions, are not very effective antiwear agents. Therefore, they do not get much use.
Dithiophosphoric acid derivatives decompose, generally below 200°C, to form thiols, olefins, polymeric alkyl thiophosphates, and hydrogen sulfide [28,29]. The antiwear performance of these derivatives depends upon their thermal stability, which in turn depends upon whether the alkyl groups are primary or secondary. Primary dialkyi dithiophosphates decompose via an alkyl transfer mechanism to form zinc monoalkyl dithiophosphate and an alkyl thiophosphate ester. Through a series of steps, these materials are converted into zinc phosphate and trialkyl tetrathiophosphate, along with a variety of other products [28]. Trialkyl tetrathiophosphate appears to be the major thermal decomposition product, as shown by ^'P nuclear magnetic resonance (NMR) spectroscopy. Secondary alkyl zinc dithiophosphates lose an olefin via ;8elimination to form a product with free dithiophosphoric acid functionality. This product can further decompose by the loss of hydrogen sulfide or another olefin to form a thioanhydride and a variety of other products. Trialkyl tetrathiophosphate is again the major product. The aromatic zinc dithiophosphates are believed to decompose by a free radical mechanism to phenol and a number of phosphorus and sulfur-containing products. Besides the thermal mechanism described above, these additives can also decompose oxidatively to form products that are potent oxidation inhibitors. The details of their oxidation-inhibiting properties were discussed in the oxidation inhibitors section and the oxidation chapter of this manusil. It is important to note that the oxidation inhibiting action of these additives is indepen-
216
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
dent of the nature of the alkyl group, but their antiwear action is not. Aliphatic zinc dialkyl dithiophosphates have better antiwear performance t h a n a r o m a t i c derivatives. And among ahphatics, the secondary alcohol derived are better than those that are primary alcohol derived. Extreme Pressure Agents—^Alkyl and aryl disulfides and polysulfides, dithiocarbamates, chlorinated hydrocarbons, dialkyl hydrogen phosphites, a n d salts of alkyl p h o s p h o r i c acids are the c o m m o n extreme pressure (EP) agents. Polysulfides are synthesized from olefins either by reacting with sulfur or sulfur halides, followed by dehydrohalogenation. Sulfurization of olefins with elemental sulfur, or sulfur and hydrogen sulfide, yields organic sulfides and polysulfides [28,30]. Dialkyldithiocarbamates are prepared either by neutralizing dithiocarbamic acid (resulting from the low-temperature reaction of a dialkylamine and carbon disulfide) with bases, such as zinc oxide or antimony oxide, or by its addition to activated olefins, such as alkyl acrylates [31]. The synthesis of these materials is described in Fig. 24 and Fig. 25, respectively. Alkyl and aryl phosphites are obtained by reacting an alcohol or a phenol with phosphorus trichloride or by a transesterification reaction [32]. Alcohols a n d phenols react with phosphorus pentoxide to yield a mixture of an alkyl (aryl) phosphoric acid and a dialkyl (diaryl) phosphoric acid [33]. These acids, when treated with bases, form salts. Alkyl phosphates can also be prepared by the oxidation of phosphites. The preparation of eJkyl phosphites is outlined in Fig. 26 a n d of alkyl phosphates is outlined in Fig. 27. The extent of EP protection in equipment depends upon the conjunction temperature of the two metal surfaces in contact [34]. Figure 28 shows a direct correlation between the conjunction temperature and the degree of EP protection needed. The equipment that operates at low speeds and high loads generally requires more EP protection than equipment that operates at high speeds and low loads. This is because
the former generates higher temperatures as a consequence of the increased friction. Disulfides and polysulfides decompose o n metal surfaces at t e m p e r a t u r e s above 200°C to form a protective sulfide layer. The thickness of this layer depends on the quantity and the lability of sulfur in the additive. Sulfurized fatty oils and sulfurized olefins are the most commonly used products in this class. Chlorine-containing compounds provide protection under boundary lubrication conditions, via the formation of a metal chloride film. A detrimental aspect of chlorine-based E P agents is the formation of hydrogen chloride in the presence of moisture, which can cause severe corrosion problems. Chlorinated paraffins with 40-70% chlorine by weight were once popular. However, environmental concerns about the negative effects of chlorine are limiting the use of these additives. Phosphorus c o m p o u n d s react with the metal surface to make a metal phosphite or a metal phosphate protective film. Such a film forms at a m u c h higher temperature than that formed by sulfur EP agents. Tricresyl phosphate is the bestknown phosphorus EP agent. Dialkyl hydrogen phosphites [35] and phosphonic and phosphoric acid salts are other examples of such EP agents. As mentioned earlier, the EP mechanism can be considered a two-step process. The first step involves adsorption of the EP agent onto the metal surface and the second step involves its chemical reaction with metal to form the EP film. After being adsorbed on the surface, these materials thermally decompose to reactive m e r c a p t a n s or p h o s p h o r u s compounds that form the EP film. Probable mechanisms by which zinc dialkyl dithiophosphates form EP films is depicted in Fig. 29. Organic halides, often not used in m o d e m formulations because of the environmental concerns, form an iron halide protective film, by reacting with the metal surface via a similar mechanism [36]. Organic polysulfides are
R—CHa—CH=CH2 Olefin
+
Sg
I Heat R—CH=CH—CHa
R—CH=CH—CHz
RCHz
Sx
+
I +
Sa
I
I R-CH2-CH-CH3
R—CH2—CH—CH2
f
RCH Organic Poiysutfides + R
Dithiolethione FIG. 24—Olefin sulfurization.
R—CH2—CH—CH3
CHAPTER 9: ADDITIVES AND ADDITIVE CHEMISTRY
RjNH
+
RzN —ct
CSj
Oialkylamine
217
SH
Carbon Disulfide
Ditliiocarbamic Acid
lUletai Oxide or IHydroxidey
CH2=CH—COOR
Activated Olefin
/•/
M = Zn or Sb X = 2 or 3
R2N — C '
R2N—C
M
SCH2—CH5-COOR Metal Dithiocarbamate
Dithiocarbamate Ester
FIG. 25—Synthesis of dithiocarbamic acid derivatives.
P
PCI,
+
3 HCI
Triaryl Phosphite
.^ (R0)2P.
PCI3
3 ROH Alcohol
{PhO)3P
+
2 H C I + RCI
DialkyI Hydrogen Phosphite
+
2 ROH
+
H2O
•
^ (R0)2P.
+
DialkyI Hydrogen Phosphite
Triphenyl Phosphite
(CH30)2P^
+
2 ROH
H Dimethyl Hydrogen Phosphite
(RO)2p^
+
3PhOH Phenol
2CH3OH
Diallcyl Hydrogen Phosphite
Methanol
FIG. 26—Synthesis of alkyl and aryl phosphites.
converted into dicJkyl disulfide, which reacts with the metal to form the metcJ sulfide EP film [37,38]. These inorganic films, which are only a few molecules thick, have low shear strength and are removed during the movement of the surfaces in contact. This situation is represented in Fig. 30. Removal of the EP film can expose fresh metal, and the film-forming process is repeated. Each time the film is removed, the metal is removed with it. One way of looking at the process of EP protection is the controlled wear of rough surfaces, as shown in Fig. 31. In general, formulators use different tjrpes of EP agents in combination because of the possible synergism [36,39]. The synergism between sulfur and chlorine-containing EP agents is shown in Fig. 32, where average scar diameter is plotted as
a function of the applied load [36]. When only disulfide is used, weld occurs at a load of 250 kg, and the scar diameter is about 2.15 mm (Graph A). When a similar level of alkyl chloride is used, the weld load stays the same but the scar diameter improves to 1.74 mm (Graph B). Combining the two types of EP agents to deliver an amount equal to that in the previous cases increases the weld load to 350 kg and decreases the scar diameter to 1.6 mm (Graph C), thereby indicating a synergism between the two chemistries. A further increase in the amount of disulfide and chloride shows weld resistance beyond the load of 500 kg (Graph D). Similar sjoiergism exists between phosphorus and sulfur chemistries [39]. The formation of active new compounds may be responsible for the synergism.
218
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
o
3R0H Alcohol
+
P2O5
—
II RO—P—OH +
Phosphorus Pentoxide
OR
o
II RO—P—OH I OH
DialkyI and MonoalkyI Phosphoric Acids
R'NHa
Metai Base
• / Metal Salts
Amine Salts
P=0 Triaryl Phosphite
Triaryi Phosphate
FIG. 27—Synthesis of alkyl and aryl phosphates. 40C-
Many effective extreme pressure and antiwear additives are corrosive to metals. Therefore, lubricants using them are typically formulated to optimize a balance between corrosivity and extreme-pressure and antiwear protection.
O S^35fl|
52 3 (B a> 30C Q.
E
ffi I-
I 25q
lilHPI^a
u c 5*200
o u
[ZESSIS 15a-
FIG. 28—Extreme pressure (EP) protection requirements vs conjunction temperature.
Some additives in a formulation can diminish the effectiveness of EP/AW agents. These include surface-active additives, such as certain friction modifiers, oxidation inhibitors, rust inhibitors, metal deactivators, detergents, and dispersants. These components either irreversibly adsorb on the surface and interfere with the EP mechanism, or they form complexes with EP agents, thereby rendering them inactive [40,41]. The same is true of some highly polar basestocks. This type of antagonism is quite common for some lubricants, such as gear oils, where EP agents form the core of the formulation. Antiseize additives are a separate class of antiwear additives, which perform independently of temperature. They improve boundary lubrication by forming the protective film through deposition. Molybdenum disulfide and graphite, common additives of this type, are generally used in greases, some industrial oils, and various break-in lubricants.
Rust and Corrosion Inhibitors Corrosion is a general term used to describe the destructive alteration of metal by chemical or electrochemical action of its environment. It primarily involves a heterogeneous reaction, which causes a metal to change from its nascent form (metallic state) to an oxidized form (ionic state). All metals except noble metals are thermodynamically unstable under atmospheric conditions and get converted into their oxidized form. On the other hand, noble metals, such as gold, platinum, iridium, and palladium, are resistant to attack by the environment and are therefore found in nature in the free form. There are many types of corrosion, but a lubricant supplier is primarily concerned with corrosion in the presence of electrolytes (electrochemical corrosion) and in the absence of electrolytes (chemical corrosion). Common electrolytes that lead to electrochemical corrosion include water, acids, alkalis, and salts. Chemical substances that cause chemical corrosion include acids, alkalis, and sulfur. In alloys, corrosion can be selective or nonselective. It is selective if a particular metal is corroded in preference to others. It is nonselective if all metals in the alloy are corroded at the same rate. Electrochemical corrosion involves the reaction of metals in the presence of electrically conducting solutions, or electrolytes, and occurs in two stages: the anodic process and the cathodic process. In the anodic process, metal goes into solution as ions with extra electrons left over. The process can be considered an oxidation. The cathodic process involves the reaction of thus generated electrons with water and oxygen to form the hydroxide ions. The process can be considered a reduction. In solution, metal ions combine with hydroxide ions to form metal hydroxides, or hydrated oxides.
V
RO.
RO
K RO.
FeO Fe OFe Fe FeO Fe OFe Fe fFeO Fe sii„„ OFe Fe FeO Fe OFe Fe FeO Fe \^''''' OFe Fe FeO Fe
S>>ii
RS
-Olefin
J
OR I S=P—S—Zrw I O
EP Film containing Sulfur and Phosphorus Compounds
^
K s=p—s—ziw-X Fe
Fe or Fe203
RO
POLYMER
Adsorption
Fe
or Fe203
Reaction
FIG. 29—The mechanism of boundary film formation by zinc dialkyi dithiophosphates.
FIG. 30—Protective boundary film vs shear.
Original Profile
Worn Profile FIG. 31—Controlled wear of asperities to produce submicron debris.
200
300
Applied Load (l
FIG. 32—Wear - load diagram, the ASTM procedure, showing synergism between chlorine and sulfur extreme pressure (EP) agents.
220
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
The rate of electrochemical corrosion depends upon the nature of the metal oxide film, the presence or absence of a polar solvent such as water, the presence or absence of an electrolyte (salts, acids, or bases), and the temperature. Chemical corrosion, on the other hand, does not need an electrolyte and can occur both in aqueous and organic media. It involves the attack of corrosive species, such as acids, bases, and sulfur, on metals. The damage occurs when the resulting compounds are removed. Factors that affect corrosion include internal factors and external factors [42]. Internal factors are directly related to the metal itself, and include its composition, structure, surface condition, oxidation potential, a n d the presence of stresses. External factors pertain to the environment and include the nature of the oxide film, acidity, alkalinity, the presence of electrolytes, the presence and reactivity of the aggressive species, and the temperature. While the role of these factors is considered elsewhere [42], we are especially interested in the oxidation potential of various metals. Oxidation potentials of the elements that make u p the metallurgy of m o d e m mechanical equipment are provided in Fig. 33. The elements that are primarily used to fabricate metal parts are shown by bold italic symbols. The potentials reported in the table are relative to hydrogen with a value equal to 0. Positive values indicate ease of oxidation and negative values indicate resistance to oxidation. Because of hydrogen being the reference point, we can directly compare these values to assess the relative oxidative tendency of various metals. The higher the oxidation potential, the easier it is for the metal to oxidize. Metals in the left column of the table with smaller values are therefore less susceptible to oxidation, and corrosion, than those in the right column. The values with slash indicate a metal's first and second oxidation potentials. For example, for iron, 0.44 represents the oxidation potential of the Fe° to Fe"^^ transition and 0.04 represents the oxidation potential of the Fe° to Fe^^ transition. The same is true for copper. Of the metals of interest, aluminum is the easiest to oxidize, followed by iron and lead. Copper with negative values is the most resistant of the metals listed in the table. Of the external factors listed above, acidity, alkalinity, the presence of the reactive species, and the temperature have the largest impact. The presence of acids and bases can accelerate corrosion. In lubricant applications, acids result
from the oxidation of the fuel sulfur and the basestock. Lubricant contains additive derived bases, both organic and inorganic, which are usually weak a n d therefore pose little problem by themselves. However, the reaction between the acidic and the basic species results in salts, which can promote corrosion because they are electrolytes. The lubricant also contains additives that can lead to acidic decomposition products. Some of these additives are sulfur and phosphorus compounds that are added to the lubricant to improve its oxidative stability and antiwear performance. These can corrode metals either directly or by forming aggressive chemical species via decomposition. Higher temperatures, which are typically encountered in the internal combustion engines, can accelerate corrosion as well. Metallurgy in automotive equipment commonly contains iron (Fe), copper (Cu), lead (Pb), chromium (Cr), manganese (Mn), antimony (Sb), a l u m i n u m (Al), vanadium (V), zinc (Zn), nickel (Ni), and tin (Sn). Protecting four of these elements against corrosion is of primary interest: iron, which is the principal metal used to forge the engine and the auxiliary equipment; copper, which is present in bearings and seals; lead, which is also present in bearings; and aluminum that is used in newer cars and trucks to make them lighter for better fuel economy. Protection against corrosion is necessary because it can lead to a loss of metal thereby lowering the integrity of the equipment and resulting in its malfunction. In addition, corrosion exposes fresh metal that can wear at an accelerated rate and result in metal ions that can act as oxidation promoters. Corrosion of iron and its alloys, sometimes referred to as ferrous corrosion or rust, is primarily electrochemical in nature. It can occur b o t h in liquid and vapor phase and needs water, electrolyte, and oxygen. In an internal combustion engine, water results from fuel combustion, oxygen comes from the air, and electrol3?tes are metal salts that form by the reaction of metals and certain additives with combustion and oxidation acids. This type of corrosion mainly occurs in engines that are run in short cycles (stop-and-go driving). Rust initiates when water sets u p a localized electrochemical reaction between the surface iron and the iron oxide layer. Nascent iron acts as the anode, and the iron oxide layer acts as the cathode. Iron emits electrons and forms ferrous ions that are re-
Electrode Reaction M
Metal
*•
M"* +
Lithium = Eo of +3.05 Hydrogen = Eg of 0.00
nQ
Eo Volts
Metal
Eo Volts
Lead(Pb)
0.13
Zinc (Zn)
Tin (Sn)
0.14
Chromium (Cr)
0.86/0.74
0.25
Manganese (Mn)
1.18/0.283
Nickel (Ni) Iron (Fe) Copper (Cu)
0.76
0.44/0.04
Vanadium (V)
1.18
-0.S2l-O.34
Aluminum (Al)
1.66
FIG. 33—Oxidation potentials of common metals found In steel.
CHAPTER 9: ADDITIVES AND ADDITIVE CHEMISTRY leased into the medium. The electrons migrate to the cathode (oxide layer) and form hydroxide ions by reacting with oxygen and water. Ferrous ions and hydroxide ions then combine to form ferrous hydroxide, which is subsequently oxidized to ferric hydroxide, loses water, and becomes rust [12]. The mechanism of rust formation is depicted in Fig. 34, Part A. Corrosion of copper and bronze, or yellow metal corrosion, is chemical in nature and occurs due to the attack of aggressive species on metals. Such species result from the oxidation and the combustion of hydrocarbon materials such as fuel, base oil, and sulfur-containing additives. Yellow metal corrosion results when chemically reactive materials such as acids, bases, or sulfur, attack copper or copper oxide. The result is the formation of ionic copper compounds that are removed, thereby causing metal damage. This is shown in Fig. 35, Part A. Lead corrosion involves a preferential removal of lead from copper-lead beeirings, sometimes referred to as lead-leaching, and primarily occurs in diesel engines. While its mechanism is not well understood, it may partly be due to the attack of chemically aggressive species on metal. Certain classes of additives, such as dispersants, appear to aggravate the situation. Aluminum corrosion is very slow because the aluminum oxide film is tenacious and is not easy to remove. For protection, corrosion-inhibiting additives are used. These additives are of two types: acid-neutralizers and filmformers. The acid neutralizing agents are additives that neutralize aggressive acidic materieds and make them innocuous. Film-formers attach themselves to metal surfaces to form an impenetrable protective film [12]. Film formation can occur via physical adsorption or chemical adsorption. In the first case, the resulting film is of a somewhat transient nature. In the second case, however, it is more persistent. Film formation occurs when these additives interact with the metal surface via their polsir ends and associate with the lubricant via their non-polar ends, in a manner similar to that of the friction modifiers. Since these additives have high surface affinity, they can compete with extreme-pressure and antiwear agents and impede their function. The rust-inhibiting mechanism and the corrosion-inhibiting mechanism via film-formation are shown in Parts B of Figs. 34 and 35, respectively. Figure 36 depicts the probable nature of the copper corrosion-inhibiting film. Part A shows copper inhibition by a commercially available dimercaptothiadiazole (DMTD) derivative and part B shows inhibition by tolyltriazole, another commercial product. In the former case, it is the lubricant associated with the hydrocarbon chain that acts as a protective film and in the latter case, it is the adsorbed additive that primarily acts as a protective film [12]. Chemical types used to inhibit corrosion of ferrous metals include polyethoxylated alkylphenols, neutral and basic sulfonates, alkenylsuccinic acids, alkyl phosphites and phosphates, alkanolamines, and polymeric amines. Those used to inhibit yellow metal corrosion mainly include oil-soluble heterocyclic compounds such as triazoles and dimercaptothiadiazole (DMTD) derivatives. For lead corrosion, no effective inhibitors are known. The only way to control lead-leaching appears to be through proper balancing of the additive package. Structures of the common corrosion inhibitors are shown in Fig. 37. Long chain organic molecules, such as alka-
221
PROBLEM >- Lubricant
Part A Rust
Cathodic Reaction (C) O2 + 2H20 + 4e
^ 40H"
Anodic Reaction (A) Fe
Fe^* + 2e
SOLUTION y Lubricant
Parte Protective Layer
Iron ' Metal
FIG. 34—The mechanism of rust Inhibition.
PROBLEM
S S S S S S S S S
ss s s sss s
Corrosive Materials
>- Lubricant
Part A
mmxm
'^^•^^^^'^^^^^^^^^'^i^^'^^^^^^'^^^i^ Copper Oxide
SOLUTION
s s s s SSSSS
Corrosive Materials
•^^ s ^SSs
Parte Deactivator Film
s '^ss <.• , s ^ \
. Copper Metal
Ss s s sss s
^§s^
"* Lubricant
^ ^ • ^ i * ^ ^ ^ ^ - ^ ^ " ^ ^ ' ^ ^ ' ^ ^ * ^ ^ ^ ^ ^ ^ ' ^ - ^ ' ^ ^ ^ ^ ^ Copper Oxide
www^
. Copper Metal
FIG. 35—The mechanism of copper deactivation.
nolamines, cire examples of physical adsorbers. Phosphoric acid, dithiophosphoric acid, and succinic acid derivatives are examples of chemical film-formers. Basic detergents are excellent rust and corrosion inhibitors since they provide protection both by neutralizing acids and by forming physically-
N-N >-Lubricant
"SSR Cu O Cu O Cu O Cu O Cu O Cu OCu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O
Oxide l^yer
(a)
vProtective '^Film Nc. JJ-R
KyN-fl
N>yN-R
KyN-R
1 < ~Cu 0"Cu 0"Cu b'Cu <5 Cu'd Cii'dCu O'Cu'CJCu O Cu D Cii O Cu" O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Oxide Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu L^yer O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O
(b) FiG. 36—Copper passivators and mode of thieir action.
LCT^I
^^\.0(CH2CH20)xCH2CH20H
Ca
2
Polyethoxyiated Ptienol
Neutral Calcium Sulfonate
O
Lpr-]
II
R—CH — C - O H CH—C—OH
Ca. xCaCOs 2
II
O
Basic Calcium Sulfonate
Alltenylsuccinic Acid
0
II
) - - P - O H . R'N
.CH2CH2OH
1
R—N
Aiitylammonlum Phosphates
OR \H2CH2OH
0
R O — P - O H . R'NHa^
Diethanolamine
I
OH
RSS^^^S \\ // N-N
SSR
RS
SH N N-N
Dimercaptothiadiazole Derivatives FIG. 37—Corrosion iniiibitors.
X Alicylbenzotriazole
CHAPTER adsorbed films. For many applications such as gear oils, rust, and corrosion, inhibitor systems are required to provide both vapor-phase and liquid-phase protection, i.e., for surfaces above and below the lubricant level. Corrosion inhibitors have major uses in engine oils, gear oils, metalworking fluids, and greases. Thiadiazole and triazole derivatives are especially useful in protecting against nonferrous or yellow metal corrosion. Metal corrosion is measured by a variety of tests, depending upon the application. For GF-2 and API SH and SJ quality oils, CRC L-38 Test (copper/lead), the ASTM Sequence IID Rust Test (ASTM D 5844), and Cummins Bench Corrosion Test (copper/lead/tin) are used. For GF-3 and API SL quality oils, ASTM Sequence VIII (unleaded L-38 Test; ASTM D 6709) and Ball Rust Test (ASTM D 6557) are used. For gear oils, CRC L-13, L-21, and L-33 Tests are used for rust and the ASTM D 130 Test is used for copper. For industrial products, the ASTM D 665 Test is used for rust and the ASTM D 130 Test is used for copper. For hydraulic and metalworking fluids, a n u m b e r of ASTM specified tests are utilized [3]. Polymeric Additives Materials with polymeric structures are the key components in high-performance lubricants. They can be used as lubricant basestocks (see synthetic lubricants) or to enhance a lubricant's inherent properties, such as viscosity and p o u r point. They can also be used as starting materials to prepare certain classes of additives, such as dispersants and detergents. The materials used for this p u r p o s e are m o r e oligomeric than polymeric, i.e., their molecular weight is relatively lower. Polymeric additives used in lubricants comprise viscosity modifiers, p o u r point depressants, emulsifiers and demulsifiers, and foam inhibitors. Most polymeric materials are compositions that consist of polymer chains of varying sizes. The bulk properties of these compositions depend upon the average molecular weight of the polymer chains. The molecular weights of poljoneric materials are expressed as n u m b e r average, weight average, z-average, (z-l-1) average, and viscosity average molecular weights [43,44]. Of these, number-average (Mn), weight-average (Mw), and z-average (Mz) molecular weights are most often used to describe polymer compositions. The numberaverage molecular weight (Mn) represents chemical stoichiometry cind is useful in carrying out chemical reactions involving polymers. The weight-average molecular weight (Mw) cortelates with mechanical properties, such as tensile strength and modulus in plastics, films, and fibers, and the viscosity-improving behavior of the polymers. The z-average molecular weight (Mz) largely influences the polymer's viscoelastic properties, such as melt elasticity. While average molecular weight information is important, it is the molecular weight distribution that is more useful in understanding the polymer properties. The molecular weight distribution is m e a s u r e d by polydispersity index or heterogeneity index, which is the ratio of the weight-average molecular weight to the number-average molecular weight, or Mw/Mn. For monodisperse polymers, which contain molecules of essentially the same chain length, the value of this index is close to 1. For polydisperse samples, it is greater than 1 because of
9: ADDITIVES
AND ADDITIVE
CHEMISTRY
223
the greater contribution of the higher molecular weight fractions in the molecular weight-determining process. The polydispersity index is a function of the polymerization method. It is closer to 1 for polymers derived from anionic polymerization, 1.5-2.0 for polymers derived from step growth polymerization, between 2 and 5 for polymers derived from radical polymerization, and above 5 for polymers derived from polymerization using coordination catalysts. Viscosity
Modifiers
Mineral oils, which are effective lubricants at low temperatures, become less effective at high temperatures. At these t e m p e r a t u r e s , their film-forming ability (in the hydrodynamic lubrication regime) diminishes due to a drop in viscosity. Prior to the use of viscosity modifiers, this problem was partly overcome through seasonal oil changes. In winter, low-viscosity oils were used and in s u m m e r high-viscosity oils were used. The invention of viscosity modifiers led to the introduction of multigrade oils thereby relieving the owner from seasonal oil changes. The principal function of a viscosity modifier is to minimize viscosity variations with temperature. Previously, viscosity index was used as a measure of an oil's response to temperature changes. Viscosity index (VI), which is derived from the viscosity of the oil at 40°C and 100°C, is no longer meaningful. This is because most m o d e m equipment operates at extreme temperatures ( - 4 0 to +150°). At these temperatures, the viscosities do not conform to those predicted by the viscosity index [4b,45]. Viscosity modifiers are polymers with average molecular weights of 10000 to 150000; but those with molecular weights between 10000 and 20000 are used most often. These chemicals are added to low-viscosity oils to improve their high-temperature lubricating characteristics. Viscosity modifiers minimize viscosity change with a change in temperature. This is a practical m e a n s of extending the operating range of mineral oils to higher temperatures, without adversely affecting their low-temperature fluidity. Viscosity modifiers cause an increase in oil's viscosity at cJl temperatures, except that thickening at lower temperatures is less than that at higher temperatures. At low temperatures, the polymer molecules occupy a small volume and therefore have a m i n i m u m association with the bulk oil, or a small hydrodynamic volume. Hydrodynamic volume is the volume of the polymer and the associated oil. The effect is a marginal viscosity increase. At high temperatures, however, the situation is reversed because the polymer chains extend or expand as a consequence of the added thermal energy. This increases polymer's association with the bulk oil due to an increase in its surface area [7]. The result is an effective increase in viscosity. Another way to describe this phenomenon is that at higher t e m p e r a t u r e s the polymer becomes more soluble, thereby causing viscosity to increase. The effect of a polymer on the viscosity-temperature (VT) properties of an oil is depicted in Fig. 38. Note that the VT line for the viscosity-modified oil has a lower (shallower) slope than that for the base oil, thereby indicating a lower drop in viscosity with increasing temperature. Figure 39 illustrates the mechanism of oilthickening by viscosity modifiers. Variable thickening of oil by viscosity modifiers at low and high temperatures allows the formulation of mutigrade oils.
224 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
The multigrade oils are designed to provide adequate viscosity at high temperatures for engine protection and low viscosity at low temperatures for easy startability. Figure 40 shows the VT characteristics of single grade and multigrade oils. It is technically inaccurate to assess low-temperature viscosities of the Vl-improved oils by linearly extrapolating the VI (or VT) curves based on 40° and 100°C viscosities. This is because such oils can show an inflection at low temperatures thereby leading to erroneous viscosity estimates [4a]. The low-temperature viscosities of the Vl-improved oils should be determined experimentally. It is also important to note that superior viscosity-temperature relationship of the multigrade oils, as expressed by the viscosity index, is not solely due to a greater preferential swelling of the polymer at higher temperatures. The use of low-viscosity oils with good viscositytemperature behavior to prepare these oils makes a substantial contribution towards the overall effect [46]. In addition to affecting the VT relationship, viscosity modifiers affect a lubricant's other properties. These include pour point, dis-
Viscosity-modified Oil
Base Oil
Temperature FIG. 38—The effect of viscosity modifier on VT relationship of an oil.
persancy, fuel economy, and, indirectly, the extreme pressure performance. Commercially available viscosity modifiers belong to two general classes: olefin-based polymers and ester polymers. The olefin-based polymers include polyisobutylenes (PIBs), olefin copolymers (OCPs), and hydrogenated styrene-diene (STDs) polymers. OCPs from ethylene-propylene mixtures are called EPRs and those from ethylene-propylene-diene mixtures are called EPDMs. Ester polymers include polymethacrylates (PMAs) and styrene ester polymers (SEs). Viscosity modifiers find major use in multigrade engine oils and gear oils, transmission fluids, power steering fluids, greases, and some hydraulic fluids. Olefin copolymers are of the most popular type, followed by polymethacrylates, styrene-diene polymers, and styrene-ester polymers. OCPs, EPRs, and EPDMs find extensive use in engine lubricants. Olefin-based
Polymers
These include polyisobutylenes, butyl rubbers, and olefin copolymers. Polyisobutylenes (PIBs) and butyl rubbers are both isobutylene-derived, except that PIBs are of somewhat lower molecular weight than butyl rubbers. Unlike PIBs that are derived from pure isobutylene, butyl rubbers Eire made from isobutylene containing 3-8% diene in the monomer mixture. Both types are manufactured by Lewis acid-catalyzed polymerization. Olefin copolymers (OCPs) are block polymers with rubberlike properties. They are prepared from olefin mixtures by vanadium-based Ziegler-Natta catalysis. In ethylene-propylene polymers (EPRs and EPDMs), the ethylene to propylene ratio and their proper distribution in the backbone are critical to the polymer's low-temperature properties. OCPs are inferior to polymethacrylates in this regard and generally require pour point depressants to improve the low-temperature performance of lubricants containing them. These polymers find wide use in passenger Ccir motor oils and diesel engine lubricants. Alpha-olefin copolymers, the lower molecular weight analogues of OCPs, are prepared by polymerizing aolefins in the presence of a Lewis acid. These polymers find use in lubricants, such as power steering pump lubricants and gear oils, which require enhanced shear stability. The PIBs of 2000 to 3000 molecular weight are used as viscosity modifiers in gear lubricants, hydraulic fluids, and industrial oils.
Effective Volume in Oil Flow s Volume of Polymer Molecule + (Hydrodynamic Volume) Volume of Associated Oil Polymer Molecule
Oil Associated with Polymer Poor-4^ Low ' ^
SOLUBILITY TEMPERATURE
Good High
FIG. 39—The mechanism of oil thickening by viscosity modifiers.
CHAPTER 9: ADDITIVES AND ADDITIVE CHEMISTRY
SAE30
SAE 10W-30
-20 0 20 Temperature (°C)-
120
FIG. 40—Viscosity-temperature characteristics of single grade and multigrade oils.
Styrene-diene polymers (STDs) can be of the diblock or random-block type and are produced by anionic polymerization of stjrene and butadiene or isoprene. This type of poljrmerization produces polymers with a narrower molecular weight distribution than those obtained by the use of Ziegler-Natta and Friedel-Crafts catalysts or the free radical initiators. That is, their Mw/Mn is closer to 1. Because of the narrow molecular weight distribution, these polymers possess the best thickening power of the types discussed so far. However, the presence of the aromatic rings and the double bonds makes these polymers more susceptible to oxidation. This problem is somewhat overcome by catalytically hydrogenating the double bonds. Figure 41 schematically presents the synthesis of these types of polymers. Recently, a different type of polymer, labeled a "star" polymer, also called radial isoprene, has become commercially available. It has thickening power similar to that of the styrene-diene type, but has better shear stability. A clustered polyanion prepared from divinylbenzene and styrene forms the center of the star, and the rays are made up of the polymerized diene monomer units. Commercial olefinbased polymers include Exxon's Paratone series, Texaco's TLA products, and Shell's Shell Vis polymers. Ester Polymers These include polymethacrylates and styrene-ester polymers. Polymethacrylates (PMAs) are produced by free radical polymerization of alkyl methacrylate. Since free radical polymerization produces polymers with relatively broad molecular weight distribution, such polymers have low thickening efficiency. Because of this, polymethacrylates have only moderate viscosity-improving ability. However, the alkyl group in the ester portion of the polymer can be altered to obtain products that have the best oil solubility and optimum viscosityimproving properties. In addition, these polymers have good
225
compatibility with a large number of refined and synthetic basestocks and eire superior to olefin copolymers both in oxidative and thermal stability and in low-temperature properties. Because of these attributes, PMAs find extensive use in a number of lubricants. Such lubricants include automotive engine oils, gear oils, automatic transmission fluids, hydraulic fluids, industrial oils, and greases. Styrene-ester polymers are prepared by first copolymerizing styrene and maleic anhydride and then esterifying the intermediate alternating copolymer using a mixture of alcohols. Normally, the esterification step is taken to about 90% or more, followed by post neutralization using a bifunctional or polyfunctional amine. Because of the presence of the basic nitrogen, these polymers function both as a dispersant and as a viscosity modifier. Figure 42 summarizes the methods of synthesis for polyacrylate and styrene ester type viscosity modifiers. Commercially available ester polymers include Rohm and Haas's Acryloid series and Lubrizol's 3700 series products. Polymers containing both OCP and ester based functionalities are also available. These are made either via grafting of an ester on to the OCP or by copolymeriztion of an alkyl acrylate and axi olefin. Rohm GMBH's Viscoplex polymers belong to this class. Thickening Efficiency—Thickening efficiency and shear stability are the two most important considerations for selecting a polymer for use as a viscosity modifier. Thickening efficiency is a direct function of a polymer's molecular weight. More specifically, it is a function of the length of the pol5rmer backbone. Basically, the longer the backbone, the greater the thickening efficiency. On an equal weight basis, a high molecular weight polymer provides higher viscosity than a low molecular weight polymer, as long as structural features, such as branching, are similar. Since thickening efficiency of a polymer depends upon its molecular weight, different polymers must therefore be compared on an equal molecular weight basis. For a given molecular weight, OCP has greater thickening power than styrenediene polymer, which in turn has greater thickening power than polymethacrylate. The relationship between the polymer's molecular weight and the viscosity improving effect, or thickening, is presented in Fig. 43. Shear-Related Viscosity Loss—The viscosity loss in a viscosity-modified lubricant can result from mechanical, thermal, and oxidative degradation of the polymer. Unlike mineral oils, which primarily exhibit Newtonian rheology, polymerthickened oils exhibit viscoelastic rheology. That is, their viscosity depends upon the degree of mechanical stress (shear). When oils containing viscosity modifiers are subjected to moderate shear stress, viscosity decreases until it approaches the viscosity of the polymer-free oil. The mechanical or shear viscosity loss is generally encountered in those equipment parts that intermesh. Journal bearings, vane pumps, and gear pumps are examples of such parts. The speed of the moving surfaces also influences the shear rate and hence viscosity. The viscosity loss of a lubricant is directly proportional to the applied shear rate, as shown in Fig. 44. The higher the shear rate, the greater the loss in viscosity. Piston rings experience low shear rates; hence, the lubricant in this region experiences a low viscosity loss. Conversely, rod bearings cause high shear; hence the lubricant in this region undergoes a high viscosity loss. The viscosity loss in the regions of main
1. Polyolefin
n
'^3^N C=CH2 H3C
CH3
Acid -=SS».
-CH2-C— CH3 J n Polyisobutylene
Isobutylene 2. Olefin Copolymer
CH3
Ziegler m CH2=CH2
+
n
CH3—CH=CH2
Ethylene
~f~ CH2 \
— • Natta cat.
Propylene
CH2'~i CH—CH2'm
Ethylene Propylene Copolymer
3. Styrene-Diene Poilymers CH3 -CH—CH2-
CH=CH2
CH—CH=C—CH2-
CH, +
m
n CH9—CH
Styrene
Anionic ^—1 Initiator
C-—CHo
m
Isoprene
Styrene-dlene Polymer
FIG. 41—Synthesis of olefln-based polymers.
CHa
CH3 Radical Initiator
H2C=c—COOR Alkyl Methacrylate
-CH,—0I COOR Alkyl Methacrylate Polymer
He=CH2
-CH-CHj—CH—CHRadical Initiator
Styrene
% Maleic Anhydride
R
'I
o^ "o" ^o
Styrene-maleic anhydride Polymer
-CH-CH2—CH—CH0=C I NH
II
R' 1. ROH / 2. H2NCH2(CH2)xNI
C=0 I OR
R'
Styrene Ester Polymer FIG. 42—Synthesis of alkyl methacrylate and styrene ester polymers.
\
CHAPTER bearings a n d cylinder walls that have intermediate shear rates falls in between. The viscosity loss can be temporary or permanent. If the viscosity bounces back to the original viscosity when the stress is removed, it is termed as temporary viscosity loss. This t j ^ e of loss is due to the reversible deformation of the polymer u n d e r the influence of shear forces, which minimizes the association between the polymer and the lubricant. Temporary viscosity loss, shown in Fig. 45, is desired in lubricants because it decreases the viscous drag at low temperatures and therefore could contribute towards fuel economy [47]. The viscosity loss is considered p e r m a n e n t if after the shear forces are removed, the viscosity does not revert to its prior value [47]. Permanent viscosity loss occurs when the polymer in the viscosity-modified oil breaks down to the lower molecular weight fragments u n d e r the influence of shear [48]. This type of viscosity loss is not desired because a formulated lubricant will not stay in its viscosity grade. Whether temporary or p e r m a n e n t , viscosity loss depends upon the shear stability of the polymeric viscosity modifier used, which is a function of its molecular weight. More
9: ADDITIVES
AND ADDITIVE
nii-mf X 100 mi-mo
m;
FIG. 43—Thickening as a function of molecular weight.
227
specifically, it is the function of the size of the polymer backbone. This relationship, which holds within a polymer type, is shown in Fig. 46. High molecular weight polymers generally lose viscosity at a higher rate u n d e r shear than low molecular weight polymers. Hence, the lubricants thickened with low molecular weight polymers are more likely to maintain their viscosity in the desired viscosity range than the lubricants thickened with high molecular weight polymers. Figures 47 and 48, respectively, depict the temporary and the permEinent viscosity loss in oils containing viscosity modifiers of two different molecular weights [47]. As can be seen, in both cases the lower molecular weight polymer experiences a lower viscosity loss. The problem of viscosity loss due to shear can be alleviated by pre-shearing the high molecular weight polymer prior to blending, or by choosing a more shear-stable polymer. Shear stability, which can be defined as the ability of a lubricant to resist viscosity loss under the influence of shear, primarily relates to the permanent viscosity loss. Shear stability of a polym e r is inversely related to its molecular weight: the lower the molecular weight, the higher the shear stability. The percent loss of viscosity in polymer-thickened oils is expressed in terms of their shear stability index, SSI, which can be represented as follows. SSIr^
Molecular Weight -
CHEMISTRY
= Initial viscosity of lubricant with the viscosity modifier, cSt mi = Final viscosity (viscosity after shear) of lubricant with the viscosity modifier, cSt rrio = Viscosity of the lubricant without the viscosity modifier, cSt T = Temperature in °C at which the viscosities are measured Shear stability requirements for different applications parallel the severity of the lubricating environment. Engine lubrication is mostly hydrodynamic in nature; hence, the lubri-
Shear Rate ^SZ.\ FIG. 44—The effect of shear rate on viscosity in different parts of an engine.
228
MANUAL
3 7: FUELS
AND LUBRICANTS
HANDBOOK
cants of low shear stability are adequate. Gear lubrication, on the other hand, is boundary in nature and therefore requires lubricants of high shear stability. Transmission and hydraulic fluids fall in between the two in terms of these requirements. This is shown in Fig. 49. The relationship between the polymer's molecular weight and its viscosity-improving effect, and the relationship between the polymer's molecular weight and its shear stability are presented in Fig. 50 [4]. Specific viscosity TJ^^/C can be used as a measure of a polymer's molecular weight since the solution viscosity of the polymer-thickened lubricant is directly related to the molecular weight of the polymer. Specific viscosity is the absolute viscosity of a liquid relative to that of water at the same temperature. As depicted in the exhibit, both the polymer's viscosity-improving ability and the shear sensitivity increase with an increase in its molecular weight (an increase in the specific viscosity). Higher shear sensitivity implies lower shear stability.
polymer molecules react with oxygen to form hydroperoxides and peroxy radicals (see the oxidation inhibitor section). These species can disproportionate to form a n u m b e r of lower molecular weight oxygenated compounds. This type of degradation not only causes a loss in viscosity but the resulting polar compounds also form varnish and coke deposits, in a manner similar to that of lubricants. The mechanism of oxidative degradation of pol5rmers is shown in Fig. 52. If one compares various classes of polymers in terms of desirable properties, OCPs are the best with respect to treated cost, but st5Tene-diene types (SB and SI polymers) are the best with respect to overall performance. OCP-g-PMA is the best choice if both the cost and the performance are considered, which is usually the case. Dispersant
Viscosity
Modifiers
Dispersant polymers, or dispersant viscosity modifiers (DVMs), are hydrocarbon polymers that have a dispersant moiety attached to them. These materials are used as viscos-
A n u m b e r of tests are available to measure the viscosity improving properties and the shear stability of polymers in lubricants. The shear stability of a lubricant is measured by the CRC L-38 Test, FZG Shear Test, Sonic Shear Test (ASTM D 2603), Orbahn Shear Test, Bosch Injector Test, and the Tapered Bearing Simulator Test (ASTM D 4683, D 4741, and D 6616). In terms of severity, the Tapered Bearing Test is the most severe, followed by the FZG Test, Orbahn Test, and the CRC L-38 Test. Viscosity Loss Due to Polymer Degradation—In addition to mechanical breakdown (shear), polymers can undergo thermeJ and oxidative degradation. Thermal degradation occurs when the polymerization process is reversed under the influence of heat. Polymers break down via chain scission to form lower molecular-weight fragments. The consequence is a loss in viscosity. This type of degradation, which is more prevalent in aromatic polymers due to their ability to form more stable allylic and benzylic radicals, is shown in Fig. 51. Oxidative degradation occurs when weak carbon hydrogen bonds in the
Molecutar Weight — —
>•
FIG. 46—Shear stability as a function of polymer molecular weight.
Vjvelogt^ f ^ OH mm
-
• ^ ~ "
Polvmer-containina Oil
I
N. [Temporary \ w Shear Loss ^
Base Oil
^
lU
—
102
1
1
10*
10«
1
1
106 to* Shear Rate,-=(3-1)
FIG. 45—Temporary viscosity loss due to shear.
1 .
107
108
CHAPTER 9: ADDITIVES AND ADDITIVE CHEMISTRY
A
1
'
1
1
\Mn=20,000 Mn=40,000\
2"
\
u >
Base Oil 1
1
io3
1
^&^
1
t
io5 10^ Shear Rate (s-i)
io7
I
lo^
FIG. 47—Temporary viscosity loss due to shear.
1
1
^
r
^
1
f
-
/ /
n
^
'
^=40,000
^
229
or 4-vinylpyridine, imparts dispersancy directly. Grafting of maleic anhydride, on the other hand, leads to a substituted succinic anhydride or succinic acid that must be reacted with polyalkylene-polyamines to introduce a dispersant functionality. At present, no commercial products made in this manner appear to be on the market. Antioxidant functionality can also be introduced in either of these two ways. Grafting is common in the case of OCP and SD type polymers, but for PMAs, copolymerization is more prevalent. Commercially available DVMs have two distinct functionalities, a polymeric backbone (the VM portion) and a dispersant moiety. Thus, their physical, mechanical, and chemical properties will depend upon both the inherent properties of the VM portion of the molecule and the dispersant functionality. In addition, some properties will depend upon both these functionalities, either due to synergism or antagonism. The properties that relate to the VM portion of the molecule are thickening efficiency, shear performance, thermal and oxidative stability, and low temperature properties, and those pertaining to the dispersant portion are oxidative and chemical stability, and dispersancy. Considering the theorized mechanism of the dispersant action, it is reasonable to assume that the dispersancy will relate to both the basicity and the molecular weight of the DVMs. Based on thepKa values of the different functional groups, one would expect dimethylamino group to be more basic than pyridyl group, which in turn is expected to be more basic than 2-keto pyrrolidinyl group. Hence, one would anticipate dispersancy to follow the same order.
^ ^
Mn=20,000
1 10
1 20
lOOT
30
80 Gear Oil
Shear Time (min) FIG. 48—Permanent viscosity loss due to shear. •O . C60
ity modifiers with added dispersancy. The molecular weights of these materials are usually much higher than those of the polymeric dispersants and can range 25000-500000. These additives are derived from high molecular weight olefin copolymers (OCPs), polymethacrylates (PMAs), styrenedienes polymers (SD) of linear or star configuration, and styrene-ester polymers (SEs). Dispersancy in these polymers is introduced by including basic nitrogen or surfactant type oxygen-containing monomers during the polymerization process. The monomers that are commonly used for this purpose are shown in Fig. 53. The dispersant functionality is introduced either by grafting or through copolymerization. Grafting involves attaching a dispersant moiety, or its precursor, to an already formed polymer. Copolymerization, on the other hand, involves the use of a monomer, which can provide dispersancy during the formation of the polymer. Grafting of or copolymerization with a nitrogen-containing monomer, such as 2-
Tractor Transmission Fluids
|40|
to Hydraulic Fluids
Automatic Trimsmissiori Fluids
20
Engine Oils ' 0.1
04 1 3 Weight Average Molecular Weight (IMWxIO-^
8
FIG. 49—Shear stability requirements for various lubricants.
230
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
i2or
0.01
0.05
0.10
0.15
specific Viscosity % ^ c (cm^/g)
^
0.20
0.25 —
>
•
FIG. 50—The effect of specific viscosity on oil viscosity and shear stability.
FIG. 51—Mechanism of thermal degradation of styrene-diene polymers.
Pour Point
Depressants
The p o u r point is the lowest temperature at which a fuel or oil will pour when cooled under defined conditions. In general, pour point is indicative of the a m o u n t of wax (straightchain paraffins) in an oil. At low temperatures, wax tends to separate as crystals with a lattice type structure. These crystals can trap a substantial amount of oil via association, inhibit oil flow, and ultimately hinder proper lubrication of the equipment. Base oil suppliers remove most of the wax during petroleum refining. However, complete dewaxing of base oils is not practical because of process limitations, economics, and the desirable presence of wax due to its high VI character [7]. For mineral oils to function effectively at low temperatures, the additives called pour point depressants are used.
Current practice favors mild dewaxing in combination with the use of p o u r point depressants. A good p o u r point depressant can lower the pour point of a lubricant by as much as 40°C. These additives are commonly used in mineral oil-based lubricants that are designed for applications with operating temperatures usually below 0°C. Pour point depressants have virtually n o effect on the temperature where wax crystals start to precipitate (cloud point) or the amount of wax that separates. They essentially act as wax-crystal modifiers and function by altering crystal shape and size. They do this either by absorption onto the surface of the newly formed crystals or by co-crystallizing with the precipitating wax. Both mechanisms inhibit lateral crystal growth and keep the bulk oil fluid. Of the commercial pour point depressants, edkylaromatics are believed to perform via the absorption mechanism and aliphatic polymers via co-crystallization [45]. The molecular weight and the structure of the poljTneric pour point depressants enable them to be effective over a wider range than their low molecular weight counterparts. The extended range of performance in the case of polymers is believed to be due to their limited solubility in petroleum fractions. As the temperature decreases, different polymer segments become successively co-crystallizable. A good p o u r point depressant must possess one or more of the following structural features. • Polymeric structure • Waxy and nonwaxy components • Comb structure—comb structure meajis a short backbone with long pendent groups • Broad molecular weight distribution Most commercial pour point depressants are organic polymers, although some nonpolymeric substances have been shown to be effective. Tetra (long-chain)alkyl silicates, phenyltristearyloxysilane, a n d pentaerythritol tetrastearate are examples of the nonpolymeric t3rpe. Commercial pour
CHAPTER 9: ADDITIVES AND ADDITIVE CHEMISTRY
231
[-H-] CH3 O2
.0-
Bond Cleavage Other Oxidation Products (Tliese include Carboxyiic Acids and Esters) IHeat, Oj, and Cataiysts
Varnish a n d C o k e Deposits FIG. 52—Mechanism of oxidative degradation of polymers.
CH3
CH,
I
CH2=C—C—OCH2CH2N ^
II
'
"
/ \
CH, R
1-Vinyl-2-pyrrolidinone or N-Vinylpyrrolidinone
Dimethylaminoethyl Methacrylate
R'
R
Alkylnaphthalene
Alkylphenol R, R' = Waxy Alkyl Groups
0\
CH3
I
A-A
CH2=C—C—OCH2CH2N
2- or 4-Vinylpyridlne
O
Morpholinoethyl Methacrylate
CH3
CH,
CH-CH2—OH—OH-
°i r Styrene Ester
CH3 -OH,—0— COOR Alkyl Methacrylate Polymer
FIG. 54—Structures of c o m m o n pour point depressants.
CH2=C—C—0(CH2CH20|-[-CH2CHO-]R O Polyether Ester of Methacryllc Acid FIG. 53—Monomers used in synthesizing dispersant viscosity modifiers.
point depressants include alkylated naphthalenes, poly(alkyl methacrylates), poly(alkyl fumarates), styrene esters, oligomerized alkylphenols, phthalic acid esters, ethylene-vinyl acetate copolymers, and other mixed hydrocarbon polymers. Figure 54 contains the structures of poly(alkyl methacrylates), alkylaromatics, and stjrene-ester polymers, which are the most commonly used chemical types. High molecular weight polymethacrylate derivatives can act both as viscosity
modifiers and pour point depressants. When this chemistry is used for viscosity improvement, the need for a pour point depressant is minimized. Pour point depressants are used at treatment levels of 1% or lower. In nearly all cases, there is an optimum concentration above and below which the pour point depressants become less effective. The key structural difference between pour point depressants and viscosity improvers of the same class is that viscosity improvers consist of long backbones with short pendent groups and pour point depressants consist of short backbones with large pendent groups. This difference is depicted in Fig. 55. Pour point depressants are used in engine oils, automatic and power transmission fluids, automotive gear oils, tractor and industrial hydraulic fluids, and circulating oils. The per-
232
MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
VISCOSITY MODIFIER— Long Backbone, Short Branches
POUR POINT DEPRESSANT— Short Backbone, Long Branches
FIG. 55—Viscosity modifier vs pour point depressant - structural ditlerence.
formance of a pour point depressant is determined in each basestock by one or more of the following tests: • ASTM D 97—For pour point of petroleum oil. • ASTM D 3829—Borderline pumping test. • ASTM D 2602—^Apparent viscosity at low temperature using cold-cranking simulation. • ASTM D 5949—Automatic Pressure Pulsing Method for pour point of petroleum products. • ASTM D 5950—Automatic Tilt Method for pour point of petroleum products. • ASTM D 5985—Rotational Method for pour point of petroleum products.
Emulsifiers and Demulsifiers Emulsifiers are chemical compounds that enable two immiscible fluids to form an intimate raixture, known as an emulsion. Water-oil mixtures are often used as lubricants in many industries and for a variety of applications. This is because such lubricants are low cost, easier to dispose of, and have fire retardant properties. Emulsions of water and mineral oil have primary use in metalworking and hydraulic applications. To be effective, emulsions must possess a number of desirable properties. They should be stable over long periods of time, possess good lubricating properties, not attack seals and metals, and be easy to demulsify for disposal. In the presence of water, certain lubricant formulations have an increased tendency to form emulsions. This is due to the presence of chemical additives that act as surfactants. Demulsifiers are added to such formulations to enhance water separation and suppress foam formation. Emulsifiers and demulsifiers are basically surfactants and are made up of hydrophilic and hydrophobic moieties. The hydrophilic moiety is nitrogen, oxygen, sulfur, or phosphorus derived polar functional group, which is attached to a hydrophobic hydrocarbon group. The hydrocarbon group must
be of sufficient chain length to provide proper solubility or dispersibility in the oil phase. Emulsifiers and demulsifiers can be classified as nonionic or ionic, depending upon whether the polar part is uncharged or charged. Ionic compounds can be subdivided further into cationic if the charge is positive and anionic if the charge is negative [49]. It is important to note that only the charge on the functional group attached to the carbon chain is used in this classification. The charge on the counterion, which is usually inorganic in origin, is ignored. The term amphoteric applies to a group of additives that contain both the cationic and the anionic groups of organic origin, preferably within the same molecule. They possess the structural features and the properties of both the cationic and the anionic materials grouped together. Generalized structures for emulsifiers and demulsifiers [50] are given in Fig. 56. Emulsifiers reduce the surface tension of water and, therefore, facilitate thorough mixing of oil and water to form an emulsion. The efficiency of an emulsifier depends upon its molecular weight (usually less than 2000), its HLB (hydrophile-lipophile balance) value, water pH and hardness, the nature of the oil, and operating conditions, such as temperature. Emulsifiers with an HLB value of 3-6 are suitable for water-in-oil emulsions and those with an HLB value of 8-18 are suitable for oil-in-water emulsions. The manner in which these additives form emulsions is shown in Fig. 57. Water-in-oil emulsions form when these additives associate with water via their polar ends and with oil and other additive molecules via the non-polar ends. This is shown in part A of the exhibit. The result is water miscibility in oil, or water-in-oil emulsion. The mechanism of oil-in-water emulsion is similar, except that the additive molecules associate in the reverse manner. This situation is shown in part B of the exhibit. Demulsifiers perform the opposite function and enhance water separation. Structurally, most demulsifiers are oligomers or polymers with a molecular weight of up to 100 000 and contain 5-50% polyethylene oxide in a combined form. They are commonly block copolymers of propylene oxide or ethylene oxide and initiators, such as glycerol, phenolformaldehyde resins, siloxanes, polyamines, and polyols [51]. For water-in-oil emulsions, polymers containing 20-50% ethylene oxide are suitable. These materials concentrate at the water-oil interface and create low viscosity zones, thereby promoting droplet coalescence and gravity-driven phase separation. Low molecular weight materials, such as alkali metal or alkaline earth metal salts of dialkylnaphthalene sulfonic acids, are also useful in some lubricant-related applications. As a general rule, nonionic emulsifiers are used in metalworking fluids based on naphthenic stocks, and fatty acid carboxylates are used in those based on paraffinic stocks. Polyalkylene glycols (hydroxyalkyl ethers) are sometimes avoided because their enhanced solubility in water does not allow clean separation for disposal. Polyethylene oxide derivatives and salts of carboxylic and sulfonic acids are the most commonly used emulsifiers, primarily in metalworking fluids. Demulsifiers are used in applications where water contamination of the lubricant is a problem and quick separation of water is desired. Automatic transmission fluids, hydraulic fluids, industrial gear oils, and some engine oils, are examples of such lubricamts.
CHAPTER Foam
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OIL
Inhibitors
Foam forms when a large a m o u n t of gas is entrained in a liquid. While foaming is desirable in certain applications, such as flotation, washing, and cleaning, it is undesirable in others, such as distillation and pumping of fluids. In lubricant-related applications, foam can act as an impediment and must be controlled. Almost every lubricant application involves some kind of agitation that encourages foam formation through air entrainment. Excessive foaming will result in ineffective lubrication and, over time, will cause oxidative degradation of the lubricant. The viscosity and the surface tension of a lubricant determine the stability of the foam. Low-viscosity oils produce foams with large bubbles, which tend to break quickly. High-viscosity oils, on the other hand, generate stable foams that contain fine bubbles and are difficult to break. The presence of surface-active materials, such as dispersants and detergents, further increases the lubricant's tendency to foam.
^^(OCW^A
OIL
(a)
Foam inhibitors control foam formation by altering the surface tension of the oil and by facilitating the separation of air bubbles from the oil phase. In general, these additives have limited solubility in oil, hence they are added as very fine dispersions. Foam inhibitors are effective at very low levels (3-150 parts per million). Silicones (polysiloxanes), poly(alkyl acrylates), and poly(alkyl methacrylates) are the commonly used foam inhibitors, with silicones being more popular. The ASTM D 892 and D 6082 Tests are used to assess a lubricant's foaming tendency. The structures of the two common types of additives are shown in Fig. 58.
H2O
v^«0
O^^^
H2O
Other Additives In addition to the major classes of additives described above, lubricants contain a n u m b e r of other additives. These include seal-swell agents, dyes, biocides, and couplers. Seals are used in m o d e m machinery for a variety of reasons. In lubrication systems, their functions are to: • Isolate various lubrication environments from harmful elements.
H2O
FIG. 57—A representation of (a) water-in-oil emulsion, (b) oil-ln-water emulsion.
Non-ionic CH2CH2OH R—N
RO{CH2CH20)xCH2CH20H NH(CH2CH20)xH N-Hydroxyalkylamide
CH2CH2OH Poiyethoxylated Alcohol (HydroxyalkyI Ether)
DIettianolamlne
R"
R—C
^CpNa®
1 1
R" Sodium Carboxylate
Anionic
(b)
Trialkylammonium Salt
R' R—N—CH2CH2 C, 1 1 R" Tetraalkylammonium Carboxylate
Cationic FIG. 56—Emulsifiers and demulsifiers.
Amplioteric
234 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK CHa
CH,
- 0 - S I - O—SI—0--SI I CH, CH, CHa DImethylslloxane Polymer
CH3
CH, -CH2-
-C I COOR
Alkyl Methacrylate Polymer
FIG. 58—Common types of foam Inhibitors.
• Help maintain hydraulic pressure. • Allow removal and replacement of the malfunctioning parts without the need to totally dismantle equipment. • Minimize contamination and the loss of lubricant. Seals are commonly made from polymeric materiails such as fluoroelastomers, nitrile rubber, polyacrylates, and silicones. Lubricants containing certain basestocks and additive systems can cause shrinkage, brittleness, and deterioration of seals, and impair the performance of the lubricating system. Seal-swell agents are additives that maintain the integrity of these seals. Additives belonging to this class include polyesters, some phosphorus derivatives, and proprietary chemicals. Seal-swell agents are commonly used in transmission and hydraulic fluids. Dyes are used to color-code lubricants to ensure their use in the proper application and as a leak detection aid for the consumer. ATFs contain a red dye, and the two-stroke cycle oils contain a blue or a purple dye. These dyes are oil-soluble organic compounds, mostly with an azo structure (contains nitrogen-nitrogen double bond). To a limited extent, dyes are also used to impart color or fluorescence to lubricants that were historically perceived to indicate good performance. This is because today's refining processes remove compounds that imparted this characteristic. In general, mineral oil based lubricants resist microbial attack because of their high-temperature operation and the presence of additives, many of which have biocidal action. High water-based lubricants, such as certain metaJworking fluids and hydraulic fluids, are easily attacked by microbes and fungi. The control of bacterial and fungal growth is essential to minimizing product deterioration and possible health hazards. This is done by the use of water-soluble triazine, morpholine, imidazoline, and thiazoline derivatives, which possess biocidal properties. Triazines, which owe their biocidal action to their formaldehyde-releasing ability, find extensive use in this application. Couplers are additives used in water-based lubricants to help stabilize microemulsions. Glycol and its derivatives are commonly used for this purpose. Metalworking fluids use a number of other additives. These include alkalinity buffers, odor masks, and antimisting agents. Alkalinity agents and odor masks are used to control acidity and odor in water-based systems. Acidity and odor results from the breakdown of oil and additives due to bacterial and fungal attack. Amines and inorganic bases are used to control acidity and natural and synthetic aromatic materials are used to control odor. Antimisting agents are used to suppress mist formation, primarily in oil-based fluids, which if not controlled can be harmful to workers. Polymers of various types are used for this purpose.
MULTIFUNCTIONAL NATURE OF ADDITIVES A number of additives perform more than one function. Zinc dialkyl dithiophosphates, known mainly for their antiwear action, are also potent oxidation and corrosion inhibitors. Polyacrylates and styrene-ester polymers can act as viscosity modifiers, dispersants, and pour point depressants. Basic sulfonates, in addition to acting as detergents, perform as rust and corrosion inhibitors. They do so by forming protective surface films and by neutralizing acids that arise from fuel combustion, lubricant oxidation, and additive degradation.
ENVIRONMENTAL IMPACT OF ADDITIVES ATC (the Technical Committee of Petroleum Additive Manufacturers in Europe) carried out a study that traced engine oil additives across nineteen OECD (Organization for Economic Cooperation and Development) countries from cradle to grave perspective. The objective was to assess the impact of additives on the consumer and the environment. The report [52] reviews the nature, development, health and safety aspects, benefits to the consumer, and the ultimate fate of engine lubricant additives. An accompanying report [53] examines the same for fuel additives.
THE INTRODUCTION OF A NEW ADDITIVE The development of a new additive is initiated after a new product (lubricant) need is identified. The need for a new product is usually expressed by the OEMs and the end-users and relates either to the inadequate performance of the existing products in the current equipment or the perceived needs of the equipment under development. To fulfill this need, various organizations, such as the SAE, API, ASTM, AGMA, and OEMs, initiate the development of new performance specifications and test methods. Additive companies, either alone or in collaboration with a lubricant supplier, try to satisfy the performance requirements established for the new product. If the additive company is unable to develop the additive system using their existing technology base, they initiate a project to develop and test a new additive. Newly developed additives are blended with other additives in a customer's base oil and are screened in a number of proprietary bench tests. Bench tests, also called screen tests, are accelerated tests that are devised to closely simulate conditions the lubricant is likely to experience in actual service. This kind of testing is quite common because it allows the evaluation of a large number of additives quickly and inexpensively. Once a lubricant satisfies the performance criteria of the bench tests, full-fledged testing using actual equipment is carried out. This may be done in a laboratory or in collaboration with an end-user. For additives used in automotive products, field trials may also be necessary. The costs associated with the development and testing of the new additives can be phenomenal. The performance package that has successfully met all the performance requirements is ready to be marketed either through factory-fill or service-fill lubricant blenders.
CHAPTER
PERFORMANCE TESTING Reliable lubricant performance is a function of the quality of its components. This implies both the base oil quality and that of the performance package. Because most additives are reactive components, their quality is assessed both individually as well as in an additive package, which comprises a collection of additives.
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1500), flash point (ASTM D 92 and D 93), volatility (ASTM D 1078 and D 5800), melting point, boiling point, odor, clarity a n d water content (ASTM D 1744, D 4928, and D 4377). While some ASTM standards are designed to analyze only low quantities of the described elements, in many instances these methods can be used and are used to analyze larger concentrations via dilution. Tests for F i n i s h e d F o r m u l a t i o n s
Tests for Individual Additives The first and foremost concern after an additive is manufactured is to establish its structural identity and purity. This is essential both from the perspective of conserving testing resources and for developing chemicals with optimal performance. Structural identity can be established by the use of analytical techniques available to a chemist. These include elemental analysis, wet chemical methods, functional group determination, molecular weight determination, and spectroscopic techniques. Some of the methods are provided in the ASTM standards while others are not. Elemental analysis is used to determine the amount of certain elements in the additive, the finished lubricant, or the used lubricant. Such elements include chlorine (CI), bromine (Br), nitrogen (N), sulfur (S), phosphorus (P), and boron (B), and metals, such as sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), barium (Ba), zinc (Zn), copper (Cu), and molybdenum (Mo). The m e t h o d s to analyze these are provided in the ASTM Standards D 4951, D 5185, D 4927, D 5291, D 4047, D 1091, D 129, D 874, D 2622, D 808, D 4628, D 6443, and D 6481. Functional group analysis involves the use of both the wet chemical m e t h o d s a n d the spectroscopic techniques. Commonly used wet chemical methods include neutralization n u m b e r determination (ASTM D 974, D664, D 3339, D4739, D2896), saponification n u m b e r (ASTM D 94), bromine n u m b e r (ASTM D 1159 and D 2710), iodine value, acetylation reaction, reaction with Grignard reagent, and Kjeldahl method. The neutralization numbers and saponification n u m b e r provide information on acidic functional groups, such as sulfonic acid, carboxylic acid, and phosphoric acid. The neutralization numbers also provide information on basic functional groups, such as amino groups and inorganic bases, and metal oxides, hydroxides, and carbonates. Bromine n u m b e r and iodine value are used to assess unsaturation in additives and lubricants. Acetylation and reaction with Grignard reagent are used to determine the hydroxyl content of additives. Grignard reagent is primarily used for situations, for example in the case of hindered phenols, where steric crowding does not allow a complete reaction with the acetylation reagent. Kjeldahl method (ASTM D 3228) is used to determine elemental nitrogen. Spectroscopic methods include infrared (IR), nuclear magnetic resonance (NMR), and mass spectrometry (MS). These techniques are well established and are aptly covered in many publications [54]. Purity is determined by gas liquid-phase chromatography (GLC or GC), thin layer chromatography (TLC), liquid chromatography (LC), and gel permeation chromatography (GPC). Additional tests include molecular weight determination by vapor phase osmometer (VPO) and MS, oil solubility/package compatibility (ASTM D 501), copper strip activity (ASTM D 130), viscosity (ASTM D 445), color (ASTM D
As mentioned earlier, lubricant additives are supplied as packages, especially for automotive applications, which are blended in mineral oil or synthetic basestocks to yield finished lubricants. The additives, being reactive chemicals, can react with one another in the package either synergistically or antagonistically [55]. The formulator's challenge is to deliver the intended performance by minimizing antagonistic effects and maximizing synergistic effects through careful balancing. The viscosity modifier and the performance package are usually sold separately. This is because the two are not always compatible in the concentrate form [45]. For applications needing a viscosity modifier, the viscosity modifier is blended in the base fluid, along with the performance package, to formulate the finished lubricant. Lubricant additive suppliers develop general-purpose performance packages that meet industry specifications using widely available basestocks and may fine tune them for an individual company's use in its basestocks. Table 2 shows classes of additives used to formulate engine lubricants, and Table 3 contains classes that are used to formulate non-engine lubricants. It is important to note that all formulations do not contain all classes of additives identified in these tables. The quality of the additive package is determined by its ability to meet established performance standards. Examples of such standards include: • SAE's viscosity classification system • API's, ILSAC's, ACEA's, and U.S. Military's engine oil standard for gasoline engine and diesel engine lubricajits • GM's DEXRON® standard and Ford's MERCON® standard for automatic transmission fluids • API's standard for gear lubricants. The standards result from the joint efforts of many organizations. In the United States, such organizations include the Society of Automotive Engineers (SAE), American Petroleum Institute (API), American Society of Testing and Materials (ASTM), American Automobile Manufacturers Association (AAMA), Engine Manufacturers Association (EMA), U.S. Original Equipment Manufacturers (OEMs), the U.S. Military, and Chemical Manufacturers Association (CMA). In Europe, the organizations include ACEA (Association des Constructeurs E u r o p e e n s de I'Automobiles), CEC (Conseil Europeens de Coordination pour les Developments des Essais de Performance des Lubrifiants et des Combustibles pour Moteurs), ATC (Technical Committee of Petroleum Additive Manufacturers), and ATIEL (Association Technique de I'lndustries Europeenne des lubrifiants). The ILSAC (International Lubricant Standardization and Approval Committee) standard is a result of a collaborative effort of the American Automobile Manufacturers Association of the United States, Inc. (AAMA) and the Japan Automobile Manufactur-
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ers Association, Inc. (JAMA). It is important to note that many OEMs and end-users have performance requirements that are over and above those prescribed in these standards. The testing of packages involves prehminary testing and full scale testing. Preliminary tests include physical, chemical, and machine tests. Full scale testing entails testing in a laboratory via accelerated tests in real world equipment and simulating actual service conditions. That is, the tests eire carried out using actual engines, transmissions, axles, hydraulic pumps, and so on. These tests usually eveJuate more than one lubricant property at a time. The equipment is disassembled and its parts cire rated based on different criteria. Laboratory tests, performed under standard conditions, ascertain that lubricants meet the performance requirements established by various organizations. Standardized test methods for lubricants are published in Federal Republic of Germany's DIN standards, the USA's ASTM standards, the Britain's IP Standards, and the France's NF Standards. The equivalent test methods for important lubricant properties in different standards are available elsewhere [4c,56]. In the U.S., such tests are run according to procedures prescribed by ASTM and OEMs. The lubricants that pass these tests are sometimes tested under field conditions as well.
D 4741, and D 4684), emulsion-forming tendency, foaming characteristics (ASTM D 3427, ASTM D 892, and ASTM D 3825), refractive index (ASTM D 1218), flash point (ASTM D 3828, ASTM D 93, and ASTM D 92), cloud point (ASTM D 2500), pour point (ASTM D 97), volatility (ASTM D 6417), and evaporation loss (ASTM D 972). Standard ASTM test procedures are used to evaluate these properties (See individual additive testing section). Chemical tests, in conjunction with spectroscopic methods, are used to characterize the lubricant. Important chemical tests are: • Structural analysis. • Hydrolytic stability. • Tests to determine carbon residue, water content, sulfur, and ash content. The carbon residue test help determine the coke-forming tendency of the lubricants. • Acidity, alkalinity, and alkaline residue. • Corrosion and corrosion protection tests. • Seal compatibility tests. • Tests to determine a lubricant's aging characteristics to predict its service life. Machine tests help assess a lubricant's performance in laboratory tests that are designed to simulate actual service conditions. Machine tests, such as SAE #1, Four-Ball, Falex, FZG (ASTM D 4998), and Ryder gear tests, evaluate a lubricant's effectiveness u n d e r mixed-film a n d b o u n d a r y lubrication conditions [4a]. The details of the test are provided in Chapter 18, Metalworking and Machining Fluids.
Physical tests, commonly used for finished lubricants, include those relating to density (ASTM D 1298), viscosity (ASTM D 445), viscosity index (ASTM D 2270), shear stability (ASTM D 1298, D 4624, D 5481, D 5133, D 2603, D 4683,
TABLE 2 -- C o m m o n additive types for engine lubricants.
Additive
Gasoline Engine Oils
Diesel Engine Oils
k
•
•
k
•
•
Dispersant Detergent Antiwear/EP Agent Oxidation Inhibitor Corrosion Inhibitor/Metal Deactivator Friction Modifier Pour Point Depressant Foam Inhibitor Viscosity Modifier Other'
k
»
Stationary Gas Engine Oils
Two-stroke Cycle Engine Oils
Aviation Engine Oils
•
•
•
• •
• •
• • •
•
•
• •
'Other additive include couples, dyes, diluents, and emulsifiers.
TABLE 3—Common additive types used in non-engine lubricants.
Additive Dispersant Detergent Antiwear/EP Agent Oxidation Inhibitor Corrosion Inhibitor/ Metal Deactivator Friction Modifier Pour Point Depressant Foam Inhibitor Viscosity Modifier Other^
Automatic Transmission Fluids
Gear Oils Automotive Industrial Gear Oils Gear Oils
• • •
• • •
• • • •
• • • •
'Other additives include couplers, dyes, diluents, and emulsifiers.
Hydraulic Fluids Tractor Hydraulic Fluids
Industrial Hydraulic Fluids
Metalworking Fluids
Greases
• • • •
• « • •
• • • •
• • • •
• •
• •
•
•
• • •
•
CHAPTER Viscosity Tests Viscosity plays a crucial role in forming effective lubricating films, which makes it one of the most important properties of a fluid [57]. Viscosity, defined as a fluid's resistance to flow, is mainly a consequence of the internal friction of the fluid [4]. Viscosity requirements or specifications are therefore prescribed for almost all lubricants. Meeting these is absolutely critical for automotive lubricants, such as engine oils, transmission fluids, and tractor hydraulic fluids. For automotive engine oils, viscosity requirements are described in the SAE Standards J300 [58] and J1536 [59]. The SAE Standard J300 deals with the viscosity of lubricants for four-stroke cycle engines, of both compression ignition (CI) and spark ignition (SI) types, and two-stroke cycle CI engines. The SAE Standard J1536, on the other hand, specifies viscosity of oils for two-stroke cycle SI engines only. The SAE Standard J1536 describes the miscibility and fluidity grades for two-stroke cycle engine oils. The basic viscosity grade categories for engine oils are determined by the Crankcase Classification System devised by the SAE, which uses test methods approved by the ASTM. For transmission fluids and tractor hydraulic fluids, the SAE grades are the same as those used for engine oils. However, for automotive gear oils, these are different and are described in the SAE Standard J306 [60]. Viscosity of non-automotive lubricants, such as industrial gear lubricants and industrial oils, is described by the American Gear Manufacturers Association (AGMA) and International Standardization Organization (ISO) Standards, respectively. ACEA uses SAE viscosity classification system, described in SAE J300. However, it has additional requirements relating to shear stability, evaporation loss, elastomer compatibility, sulfated ash, foaming tendency, and changes in viscosity due to high shear and high temperature. Special laboratory test procedures are used to determine if a lubricant meets the desired performance requirements in these areas. These procedures are described in the SAE J2227 report [61]. The ASTM D 445 method is used to determine kinematic viscosity of a lubricant at 40°C and 100°C. This procedure employs a capillary viscometer. The ASTM methods D 2602, D2983, D 3829, D 4684, and D5293 are used to determine low temperature viscosity of lubricants. ASTM D 2602 uses cold cranking simulator to determine apparent viscosity at 0 to - 4 0 ° C range. The ASTM D 3829 and D 4684 procedures use mini-rotary viscometer and are used to measure borderline pumping temperature (BPT) and apparent yield stress and apparent viscosity, respectively. ASTM D 5133 is scanning Brookfield technique that is used to measure gellation point, point at which sample viscosity reaches 30,000 cP. ASTM D 2983 is also a Brookfield procedure that is used primarily for low temperature properties of automotive gear oils, automatic transmission fluids, torque and tractor fluids, and hydraulic fluids. The measurement is carried out at a constant temperature in the range of 0 to —40°C. The ASTM procedures D 5624, D 4683, D 4741, and D 5481 are used to measure high temperature/high shear viscosity (HTHS). The ASTM D 5624 procedure uses a capillary viscometer where force is applied to a lubricant that is at 150°C to generate a high shear rate (10 s"'). The ASTM D 1092 procedure, used to measure viscosity of greases, also uses pressure [57b]. The ASTM D 4683 and
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D 6616 methods, on the other hand, use tapered bearing simulator to determine HTHS viscosity. Two procedures that are used to measure grease viscosity are ASTM D 1092 and ASTM D 4693. The first measures apparent viscosity and the second measures pumpability. The procedures ASTM D 2270 and D 2161 do not directly measure viscosity but are used for conversion purposes. ASTM D 2270 or D 39 B is used to calculate viscosity index (VI) from 40°C and 100°C kinematic viscosities and ASTM D 2161 is used to convert viscosities measured in now obsolete units of Saybolt Universal Seconds (SUS) and Saybolt Furol Seconds (SFS) into centistokes (cSt). For two-stroke cycle SI engine oils, viscosity grades are based on miscibility with fuel and fluidity. This is to ensure that the lubricant is miscible with fuel and that the blend meets the low-temperature viscosity requirements. The rationale behind these requirements is that two-stroke cycle SI engines usually do not have an oil sump, and the lubricant is mixed with the fuel in the fuel tank. The testing involves mixing lubricant with fuel and determining its pour point (ASTM D 4682). The performance of the new oil is compared with that of the reference oils to assign a viscosity grade. Transmission and tractor hydraulic fluids, like engine oils, have low t e m p e r a t u r e and high t e m p e r a t u r e viscosity requirements. Low temperature requirements are base upon ASTM D 2983 (Brookfield test) and ASTM D 445 is the basis for 40°C and 100°C kinematic viscosities. Performance Tests As mentioned earlier, each tjrpe of lubricant must meet certain performance requirements prior to use. Such requirements are described in service classifications or specifications (ASTM D 4485). The following section describes these based on the lubricant type. Engine
Oils
Engine oil performance requirements for North America are based on standard tests and are described by the API Engine Service Classification System, which was first introduced in 1969/1970 [62]. The system comprised separate performance classes for gasoline engine lubricants and diesel engine lubricants. The system has been revised many times. At present, classes for gasoline engine lubricants range from SA to SL and for diesel engine lubricants they range from CA to CI4. Of these, SA to SG categories for gasoline engine oils and CA to CE categories for diesel engine oils are obsolete and their r e q u i r e m e n t s are satisfied by API SH a n d CF (and higher service classes), respectively [63]. However, at present, the use of SH designation is limited to lubricants recommended for both the heavy-duty diesel and the gasoline engines, such as those designated as CF/SH. The ILSAC's performance categories, GF-1 and GF-2, issued in 1992 and 1996, use the API SH performance criteria but include a fuel economy ASTM Sequence VI Test. A new ILSAC category, ILSAC GF-3, was introduced in the year 2001. This category incorporated a n u m b e r of new tests that use engines of current design and operate on unleaded fuel, so as to conform to the growing trend towards the use of unleaded gasoline. The tests include ASTM Sequence IIIF for oxidation, ASTM Sequence IVA for valve train wear, ASTM Sequence VG for sludge, varnish, and deposits, ASTM Sequence VII for
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bearing corrosion, and ASTM Sequence VIII for rust and corrosion [74]. API has stopped licensing GF-2 passenger car oils since April 2002 [64]. The work on the next ILSAC upgrade, GF-4, has already started. The new category will require oils that are emission system catalyst compatible, have better fuel economy, and provide better engine protection [65]. For diesel engine oils, API CF, CF-2, CF-4, CG-4, CH-4, and CI-4 categories are active. API CF-2 is for severe-duty twostroke cycle engines. API CF is for four-stroke cycle engines that are either turbocharged or use high sulfur fuel. Such engines are often for off-highway use. API CF-4 and CI-4 categories are for four-stroke cycle engines for on-highway use. API CF-4 is for 1991 low emissions engines, API CG-4 is for 1994 low emissions engines, and API CH-4 is for 1998 low emissions engines. API CI-4, the 2002 upgrade of the diesel engine oil standard, is suitable for use in engines that are equipped with exhaust gas recirculation and other exhaust emissions control mechanisms/components. Such engines are designed to meet the 2004 exhaust emissions standard. In general, oils meeting higher service requirements are suitable for use in the lower service class within the same category. New proposed category, PC-8, for next generation heavy-duty diesel engine oils has been under consideration for some time. It is designed to address the Japanese OEM concerns over the ability of the API CH-4/CI-4 qualified oils to protect their engines that use slider followers. The American engines use roller followers. The CI-4 category includes eight fired engine tests and seven bench tests [66]. The diesel engine tests include Caterpillar IR, Cummins M i l , MackTSE (ASTM D 5967) and T-10, and GM 6.5L Roller Follower Test (ASTM D 5966). This category accommodated two major OEM specifications, Cummins CES 20076 and Mack EO-M Plus, both of which require performance beyond that delivered by API CH-4. In addition to API, two other organizations actively participate in establishing engine oil performance. These are U.S. Military and original equipment manufacturers (OEMs). The needs of the U.S. Military are different from those of the commercial users. Its fleets consist of vehicles that not only vary in size (ranging from 2 to over 1000-hp), but they also operate under a variety of conditions. U.S. Military specifications are designated by the prefix MIL. While largely similar to API service designations, there can be subtle differences. Similarly, certain OEMs have additional requirements to qualify oils for use in their equipment. Mack EO-M and Cummins CES 20076 specifications are examples of such requirements. ACEA establishes oil quality for use in Europe and JASO and Bureau of Indian Standards (BIS) establishes oil quality for use in Japan and India, respectively. ACEA specifies the use of tests developed by the ASTM and CEC (Conseil Europeen de Coordination pour les Developments des Essais de Performance des Lubrifiants et des Combustibles pour Moteurs; Coordinating European Council). ACEA 2002 Standard consists of four lubricant classes for gasoline engines, Al-02, A2-96 (issue 3), A3-02 and A5-02; five lubricant classes for light-duty diesel engines, Bl-02, B2-98 (Issue 2). B3-98 (Issue 2), B4-02, and B5-02; and four lubricant classes for commercial diesel engines, E2-96 (Issue 4), E2-96 (Issue 4), E4-99 (Issue 2), and E5-02. Current Japanese specifications include JIS K 2215 for gasoline engine oils and JASO DH-1 for diesel engine oils. Each active category has associated tests. These tests pri-
marily pertain to frictioucJ characteristics, oxidation resistance, deposit controlling ability, corrosion and wear preventing ability, and fuel efficiency of the oil. The current engine and bench tests for gasoline engine lubricants are as follows. U.S. and European Tests • Rust and corrosion—CRC L-38, ASTM Sequence VIII (ASTM D 6709), ASTM Sequence IID (ASTM D 5844), and Ball Rust Test (ASTM 6557) • Oxidation—ASTM Sequences HIE (ASTM D 5844) and IIIF, CRC L-38, and Peugeot TU-3M High-temperature • Thermal and Oxidative Stability—Thermo-oxidation Engine Oil Simulation Test (TEOST; ASTM D 6335), Hightemperature Deposit Test • Sludge, varnish, deposits, and v\;ear—CRC L-38, ASTM Sequence HIE (ASTM D 5533), ASTM Sequence VE (ASTM D 5302), and VG (ASTM 6593), M-B Ml 11 Black Sludge, VW 1302, and Peugeot TU-3M High-temperature • Valve Train Wear—ASTM Sequence IVA and Peugeot TU3M Wear • Fuel Efficiency—ASTM Sequence VI, VIA (ASTM D 6202), and VIB (For GF-3), and M-B M i l l Fuel Economy • Extended drain capability—VW T-4 Japanese and Indian Tests • Rust and corrosion—CRC L-38/Petter Wl, ASTM Sequence IID (ASTM D 5844) • Oxidation—Toyota. IG-FE, CRC L-38/PetterWl, and ASTM HID and HIE (ASTM D 5533) • Sludge, varnish, deposits, and wear—Toyota IG-FE, Nissan VG-20E, Nissan SD22 (JASO 336-90), Nissan TD25 (JASO 336-97), CRC L-38/Petter Wl, CLR LTD, ASTM HID and HIE, ASTM VD and VE (ASTM D 5302) • Valve train wear—Toyota 3A, Toyota 3E, Nissan KA-24E The current diesel engine tests are as follows. These tests are devised to measure protection against rust, varnish, deposits, sludge, wear, high-temperature oil-thickening, ring sticking, fuel economy, and foaming tendency. U.S. and European Tests • Piston deposits, ring sticking, oil consumption, and piston liner and ring distress—Caterpillar IK, IM-PC (ASTM D 6618), IN, IP (ASTM D 6681), and IR; Detroit Diesel 6V 92TA (two-stroke cycle engine; ASTM D 5862)); Mack T-6; VW 1.6TC Diesel Intercooler and VW Direct Injection; Peugeot XUDl 1 ATE and XUDl IBTE; MAN 5305; and M-B OM 364A/LA and M-B OM 441LA • Roller follower wear—Roller Follower (lifter pin wear), GM 6.2L and GM 6.5L (ASTM D 5966) • Soot thickening—Mack T-6, T-7, T-8/T-8E (ASTM D 5967), T-9 (ASTM D 6483), and T-10. • Engine oil corrosiveness—^ASTM D 5968 • Oxidative stability/Viscosity increase—VW Direct Injection and Peugeot XUDl lATE and XUDl IBTE • Sludge, varnish, and wear/cleanliness—M-B OM 602A, M-B OM 364A/LA, M-B OM 441 LA, and Cummins M-11 • Fuel Economy—M-B M i l l Fuel Economy • Foaming tendency—Navistar 7.3L EOAT [Hydraulically Actuated Electronic Controlled Unit Injector (HEUI)] • Bore Polishing—MAH 5305, M-B OM 364A/LA, M-B OM 441 LA
CHAPTER Indian Tests • Piston deposits, sludge, varnish, and wear—Caterpillar 1H2/MWM-B, Caterpillar 1G2/MWM-B, M-B OM 364A, MBOM616 • Soot Thickening—Mack T-7 • Bore Polishing—M-B OM 616 Service classifications for two-stroke cycle engine oils are described in the SAE Standard J2116 [67]. The API designations for oils approved for use in two-stroke cycle engines are TA and TC, and NMMA (National Marine Manufacturers Association) designations are TC-W, TC-WII®, and TC-W3®. TA oils are designed for use in low performance engines and TC oils are designed for use in high performance engines, except outboard engines (ASTM D 4859). TC-W, TC-WII®, and TCW3® oils, on the other hand, are intended for use in watercooled outboard engines. The quality of two-stroke cycle oils is assessed on the basis of their ability to prevent piston scuffing, exhaust system blocking, ring sticking, and deposit-induced pre-ignition ASTM D 4857 and D 4858). JASO and ISO based two-stroke cycle categories are at present being developed. These will be based upon oil's lubricity and detergency, and its ability to control exhaust smoke and exhaust system blocking. The engine test requirements for NMMA's most recent category TC-W3® are listed below. • Piston varnish, deposits, and ring sticking—OMC 40-hp, OMC 70-hp • Scuffing, bearing stickiness, and compression loss—Mercury 15-hp • Tightening and lubricity—Yamaha CE50S • Preignition—^Yamaha CE50S Many changes have occurred for water-cooled engine specifications since writing this article. NMMA's categories TCW, TC-WII®, and TC-W3® are now obsolete and a recertified TC-W3® category was introduced. At present, the availability of the OMC 70-hp and OMC 40-hp engine test parts is in question. Transmission
Fluids
The important functions of these fluids are lubrication, cooling, and to act as a hydraulic medium to transmit power. These fluids are of three types: automatic transmission fluids, manual transmission fluids, and power transmission fluids. There is no official API classification system for these fluids. Performance requirements for transmission fluids are established by the OEMs [68]. The most important features of these fluids are their frictional consistency (durability) and frictional compatibility with the transmission's components. In a u t o m a t i c transmissions, such c o m p o n e n t s include clutches and bands; in manual transmissions and manual transaxles, they include cone or plate type synchronizers. Unlike automatic transmissions, which use transmission fluids recommended only by the OEMs, manual transmissions use a variety of fluids. Such fluids include automatic transmission fluids, engine oils (5W-30), some gear lubricants, and specialty fluids. In addition to frictional properties, certain OEMs require transmission fluids for their equipment to have improved shear stability, low-temperature fluidity, and other specific characteristics. Automatic Transmission Fluids—The frictional compatibility of automatic transmission fluids (ATFs) with the transmis-
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sion's clutch and band system is their most important feature. DEXRON® and MERCON® are the two major types of a u t o m a t i c transmission fluids that are presently in use. DEXRON® fluids meet General Motors' performance specifications and are primarily designed for use in their transmissions. MERCON® fluids meet Ford's performance requirements and are used in its transmissions. DEXRON®-III, MERCON® , and MERCON®-V fluids are the most current specifications for transmission fluids. DEXRON®-III, introduced in 1994, is suitable for use in GM transmissions that were designed to improve fuel economy and emissions [69]. Design changes and operating environment for these transmissions called for fluids with good low-temperature properties, good thermo-oxidative stability (ASTM D 5579), and frictional durability. DEXRON®-III specification was designed to improve shift-feel smoothness, antiwear characteristics, low-temperature fluidity, and oxidation resistance of the fluid. DEXRON®-IV specification, proposed but not yet in use, will require fluids of lower kinematic and Brookfield viscosities so as to minimize energy losses and further improve fuel efficiency. Tests and evaluation criteria for automatic transmission fluids are provided below. • Friction and wear—ASTM D 2882, Band Clutch, Plate Clutch Friction • Thermo-oxidative stability—4L60 Transmission, ABOT Oxidation • Frictional consistency and durability—Cycling (ASTM D 5579) • Smooth shifting of gears—Shift Feel Power Transmission Fluids—Power transmission fluids (PTF) aire used in heavy-duty automatic transmissions and torque converters in off-highway equipment. Such equipment is commonly used in agriculture and construction industries. Viscosity and frictional properties of these fluids are critical to their performance. Just like ATFs, the SAE and OEM performance specifications are used to describe these fluids [70]. Power transmission fluids are classified on the basis of their performance in Allison C-4 and Caterpillar TO4 friction tests. Fluids meeting Allison C-4 requirements are designed for equipment that has both torque conversion and automatic transmission features. Gear Oils Gear oils are primarily formulated to provide extreme pressure protection for gears and axles to prevent fatigue, scoring, and wear under boundary lubrication conditions. Gears are used in automobiles and industry to perform a number of diverse functions. Because of differences in design, each gear type places different demands on the lubricant. Automotive Gear Oils—Performance specifications for automotive gear oils are established by the API, U.S. Military, and the OEMs. The API service designations for automotive gear oils range from GL-1 to GL-6, specifying oils in increasing order of load-carrying capacity [71,72]. The abbreviation "GL" stands for gear lubricant. The GL-6 classification, which was previously used to describe antiscoring performance over and above that provided by the GL-5 lubricants, is technically obsolete. The GL-4 and GL-5 categories correspond to U.S. Military specifications MIL-L-2105 and MIL-L-2105D, respectively, and define oils for service-fill only. The specifica-
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tion MIL-PRF-2105E, issued in 1995, combines the GL-5 requirements of MIL-L-2105D and thermal oxidation stabihty, antiwear, and seal compatibility requirements of the newly released API specification MT-1. The new specification defines lubricants for nonsynchronized manual transmissions used in buses and heavy-duty trucks. Factory-fill oils are defined by major car and truck manufacturers. Such oils have performance characteristics that are critical to the satisfactory operation of a particular drive train and may include break-in, bearing preload, and limited slip durability. Mack GO-G/S and GO-H/S are examples of such specifications. European OEMs use API GL-5 and MIL-L-2105D to define minimum performance requirements for oils used in their equipment. They have additioned requirements pertaining to surface fatigue, component cleanliness, synchromesh durability, and viscometrics, depending upon their specific need/s. Japanese OEMs recommend API GL-5 lubricants for vehicles fitted with h3rpoid and spiral bevel axles and API GL-3 and GL4 lubricajits for cars and trucks equipped with manual transmissions. Most modem cars use transaxle drive train arrangement and hence do not need rear axle lubricants. A new performance specification, PG-2, for automotive geetr oils is presently under consideration [73]. It is a temporary designation for axle lubricants to be used in heavy-duty truck and bus final drives that employ spiral bevel and hypoid gears. This category is designed to qualify lubricants with higher thermal durability, seal compatibility, and surface fatigue performance than the existing GL-5 lubricants. The API and the military specifications have associated bench and axle tests [74]. These include tests to evaluate the oil's ability to protect gears and axles against fatigue, scoring, and wear. Additional tests deal with a gear oil's oxidative stability, foaming tendency, and the ability to protect against rust and corrosion. Axle tests are run according to methods established by the American Coordinating Research Council (CRC). Bench and axle tests used to qualify automotive gear oils (ASTM D 5760) are given below. • Scoring resistance under high-speed shock load conditions— CRC L-19 and FTM 6504T, CRC L-42, Vehicle Rear Axle Score • Resistance to gear distress under high-torque low-speed conditions—CRC L-20, CRC L-37 (ASTM D 6121) • Antiwear properties—kSlU D 5182, FZG (A/8.3/90), and (A/10/90) • Gear surface fatigue—Mack Spalling • High-temperature stability of lubricant—ASTM D 5763, Mack Transmission Test T-2180, CRC L-60, and CRC L-60-1 • Lubricant stability in motored axle test—Ford BJ 15-1 • Corrosion resistance in the presence of water—CRC L-13 or FTM 5313.1 and CRC L-21, CRC L-33 • Copper corrosion—^ASTM D 130 • Oil seal compatibility—ASTM D 471, ASTM D 5662 • Foaming tendency—ASTM D 892, CRC L-12 • Transmission cyclic durability—ASTM D 5579 • Material separated on centrifugation after 30 day storage at room temperature—FTM 3440, modified • Channeling characteristics of lubricant—FTM 3456.1, modified • Compatibility with existing gear lubricants—SS & C FEDSTD-791 Method D 3430 and D 3440
Industrial Gear Oils—Service requirements of industrial gear oils are established by AGMA, DIN, and a variety of other organizations, such as U.S. Steel, Cincinnati Milacron, and Alcoa [74]. The important function of these lubricants is to reduce friction and wear. Tests pertaining to these include Timken (ASTM D 2782), Four-Ball EP (ASTM D 2783), FourBall Wear (ASTM D 4172), U.S. Steel S-205, and FZG (A/8.3/90). Additional tests deal with rust and yellow metal corrosion protection (ASTM D 665, IP 135, and ASTM D 130), demulsibility (ASTM D 1401, IP19, and ASTM D 2711), oxidation resistance (ASTM D 2893, U.S. Steel S-200), foaming tendency (ASTM D 892 and IP 146), and air release time (DIN 51381, IP313). Hydraulic
Fluids
The primary function of these fluids is to transmit power efficiently and control wear. Hydraulic fluids are of two general types: those used to lubricate tractor hydraulics and those used to lubricate industrial hydraulic equipment. Tractor Hydraulic Fluids—Tractor hydraulic fluids (THFs) are multipurpose lubricants that are used to lubricate transmissions, final drives, hydraulic systems, wet brakes, and wet clutches [75]. To perform these functions properly, THFs must combine hydraulic and transmission properties with extreme-pressure properties. Their function as a transmission fluid and as a lubricant for wet brakes and wet clutches requires them to possess proper frictional characteristics. Tractor hydraulic fluids differ widely in performance requirements because OEMs can not agree on common specifications for a universal tractor hydraulic fluid. The specifications for these fluids, in general, deal with extreme pressure (EP) and antiwear properties, and with matching the frictional requirements of the equipment. The quality of these fluids is assessed on the basis of their ability to meet individual OEM specifications as well as API GL-4 (for EP) and Allison C-4 (for friction, oxidation, and wear) performance requirements. There are eight major OEM specifications and most tractor fluids are formulated to meet them. These specifications are: • JI Case MS 1207 • John Deere J20C/D, and J27 • New Holland FNHA-2-C-201.00 and M2C159B/C • AGCO Massey-Ferguson Ml 135, Ml 139, and Ml 141 Tests associated with these specifications include: • Wear and Extreme-pressure Tests—Denison T-50 Vane pump and P-46 Piston Pump; Vickers 35VQ-25 Vane and Vickers V104C Vane Pump, Constant Volume Vane Pump (ASTM D 2882); FZG EP/antiwear [DIN 51354 (Part 2)]; Four-Ball EP (ASTM D 2783); and Four-Ball Wear (ASTM D4172) • Oxidation Tests—Tnrbme Oil (ASTM D 943); Sludge (ASTM D 4310); and Rotary Bomb (ASTM D 2272); Universal Oxidation Test (ASTM D 5846) • Corrosion Tests—Turhins Oil Rust (ASTM D 665) and Copper Strip (ASTM D 130) • Miscellaneous Tests—Turbine Oil Demulsibility (ASTM D 1401); Hydrolytic Stability (ASTM D 2619); Cincinnati Milacron Thermal Stability (ASTM D 2070); Denison TP 02100 Filterability; Foam ASTM D 892; Air Separation (DIN 51381); and Seal Compatibility [DIN 53538 (Part 1)]
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In addition to a hydraulic fluid, farm tractors and related equipment need an engine oil and a transmission fluid. In order to reduce the number of lubricants handled by the farmer, the concepts of Universal Tractor Transmission Oil (UTTO) and Super Tractor Oil Universal (STOU) were developed. UTTOs have the ability to lubricate transmissions, wet brakes, and hydraulic systems. STOUs have the additional ability to be used as engine oils and meet major performance criteria of the leading equipment manufacturers. Such oils must therefore pass additional tests. For UTTO oils, these include L-20 High Torque Low-speed Test, Vane and Piston Pump Test, and Vickers 35VQ25 Test. For STOU oils, these include engine oil tests, such as ASTM Sequence IID, HID, VD, CRC L-38, Mack T-6, and T-7; L-20 High Torque Low-speed Test; Vane and Piston Pump Test; and Vickers 35VQ25 Test. Industrial Hydraulic Fluids—These lubricants help transmit and control power in equipment used in industries, such as automotive, manufacturing, material handling, construction, chemical, mining, textile, food, rubber, and agriculture. These fluids fall under three categories: antiwear hydraulic fluids, rust and oxidation-inhibited oils (R & O oils), and fireresistant fluids. Performance criteria for industrial hydraulic fluids are defined by OEMs. Each hydraulic pump manufacturer has its own performance requirements that pertain to lubricant viscosity, antiwear properties, demulsibility, and ability to inhibit rust, oxidation, corrosion, foam, and filter plugging. Tests for these fluids include:
every other aspect, they resemble soluble oils. Because of the lower oil content, these fluids appear clear. However, some high semisynthetic fluids contain >3.0% oil and are translucent. Their appearance is a function of the particle size. The performance specifications of metalworking fluids are established by the OEMs and end-users. Test methods to evaluate performance of these fluids are not well standardized. Tests that are presently used or can be used to judge the suitability of metalworking fluids include: • Corrosion Teste—Copper Strip (ASTM D 130), Turbine Oil Rust (ASTM D 665), Aqueous Cutting Fluid (IP125), Filter Paper Chip Breakpoint (IP287), Aluminum Cup Stain, Humidity Cabinet Rust (ASTM D 1748), Salt Spray (MIL-B117-64), Cleveland Condensing Humidity Cabinet (ASTM D 2247) •Extreme Pressure—Four-Ball Wear (ASTM D 4172), Timken (ASTM D 2782), Four-Ball EP (ASTM D 2783), Falex EP (ASTM D 3233) • Stability~¥oam Tendency/Stability (ASTM D 892, IP312), Panel Coker, Demulsibility (ASTM D 1401), Emulsion Stability (IP263), Aquarium Biostability • Miscellaneous—Color (ASTM D 1500), GM Quenchometer (ASTM D 3520), Thread Tapping (Lubrizol test). Pipe Threading (Lubrizol test). Stick-slip (Cincinnati Milacron test), Bijur Filtration, Falex #8, SLT (Draw Bead Simulator), Reichert Test, and Tapping Torque Test (ASTM D 5619)
• Wear and Extreme-pressure Tests—Denison T-50 Vane pump and P-46 Piston Pump; Vickers 35VQ-25 Vane and Vickers V104C Vane Pump; FZG EP/antiwear [DIN 51354 (Part 2)]; Four-Ball EP (ASTM D 2783); and Four-Ball Wear (ASTM D 4172) • Oxidation Jesfs—Turbine Oil (ASTM D 943); Sludge (ASTM D 4310); and Rotary Bomb (ASTM D 2272) • Corrosion Tesfs—Turbine Oil Rust (ASTM D 665) and Copper Strip (ASTM D 130) • Miscellaneous Tests—Turbine Oil Demulsibility (ASTM D 1401); Hydrolytic Stability (ASTM D 2619); Cincinnati Milacron Thermal Stabihty; Denison TP 02100 Filterabihty; Foam ASTM D 892; Air Separafion (DIN 51381); and Seal Compatibility [DIN 53538 (Part 1)]
Miscellaneous Industrial Oils Compressor oils, refrigeration oils, turbine oils, circulating oils, slideway lubricants, and rock drill lubricants make up this group. Compressor and refrigeration oils are used in compressors to reduce friction and act as a seal separating low and high-pressure areas. Turbine and circulating system oils are used to lubricate steam and gas turbines for marine and stationary applications. Circulating oils are used in systems where large quantities of heat must be removed and where heavy contamination of oil occurs. As a result, these oils must possess excellent air and water separation properties and good oxidative stability over the duration of their use. Slideway and drill lubricants are used to lubricate guiding surfaces on the bed of a machine along which a table or a carriage moves and to lubricate pneumatic equipment. These oils must perform under extreme temperatures, high loads, moisture, and poor ambient air quality. They therefore possess both the EP activity and the rust and corrosion-inhibiting properties. ISO viscosity grades and U.S. Military and OEM performance requirements usually specify these lubricants. Turbine oils are classified as R & O oils, non-EP oils, and EP oils. R & O oils are formulated to provide rust and oxidation protection and EP oils are formulated to provide EP protection, depending upon the intended end use. EP oils contain R & O packages enhanced with antiwear additives. R & O oil performance specifications are established by OEMs, such as U.S. Steel, Cincinnati Milacron, Denison, General Electric, and organizations, such as U.S. Military, AFNOR (Association Francais Petroles de Normalisation) and DIN (Deutsche Industrie Norm). Specifications for non-EP oils are established by General Electric, British Government, and DIN and for EP oils the OEM Brown Boveri plays an active role [74]. Common tests for these oils are as follows.
Metalworking
Fluids
Metalworking fluids are used to convert metal into a component or a piece. These fluids are of four basic t3^es, straight oils, soluble oils, semisynthetic fluids, and synthetic fluids. Soluble oils, semisynthetic fluids, and synthetic fluids are waterbased and differ from one another, mainly in their oil content and emulsion type [76]. Straight oils are devoid of water. They are mineral oil-based (primarily hydrotreated naphthenic basestocks) and usually contain sulfur, chlorine, and phosphorus-derived additives. Soluble oils are water emulsions of mineral and/or fatty oils. An emulsifying agent or surfactant is used to form these emulsions. Synthetic fluids are oil-free and are simply solutions of additives in water. Since most organic materials are hard to dissolve in water, high polarity of the additives is necessary for solubility. Soaps or other surfactants are sometimes added to help in this regard. Semisynthetic fluids are in between soluble oils and synthetic fluids as far as their oil content is concerned. In
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R & O Oils • Wear Tests—Vane P u m p (ASTM D 2882), Denison P-46 Piston Pump, Four-Ball Wear (ASTM D 4172) • Oxidation Tests—Rotary Bomb (ASTM D 2272), Turbine Oil (ASTM D 943), 1000-hour Sludge (ASTM D 4310), Cincinnati Milacron Heat, FTMS 5308.6 • Corrosion Tests—Rust (ASTM D 665), Copper Strip (ASTM D 130) • Miscellaneous Tests—Turbine Oil Demulsibility (ASTM D 1401), Neutralization N u m b e r (ASTM D 974 and D 664), Foam (tendency/stability) (ASTM D 892), and Air Release (DIN 51381, ASTM D 3427) Turbine Oils • Oxidation Tesfs—Turbine Oil (ASTM D 943), 1000-hour Sludge (ASTM D 4310), IP280 TOP, IP280 Sludge, Rotary Bomb (ASTM D 2272), FTMS 5308.6, and Universal Oxidation Test (ASTM D 5846) • Corrosion Tests—Rust (ASTM D 665), Copper Strip (ASTM D130) • Wear Test—FZG (A/8.3/90), a n d Four-Ball EP (ASTM D 2783), Falex (ASTM D 2670), Ryder gear tests to fulfil U.S. Military requirements. • Miscellaneous Tests—Viscosity Index (ASTM D 2270), Flash Point (ASTM D 93), Pour Point (ASTM D 97), Neutralization N u m b e r (ASTM D 974 and D 664), Air Release (DIN 51381, ASTM D 3427), F o a m (tendency/stability) (ASTM D 892), Demulsibility (ASTM D 2711), and Turbine Oil Demulsibility (ASTM D 1401) Greases The use of this lubricant goes back to ancient times [62]. Lubricating grease is defined as a "solid-to-semifluid products of dispersion of a thickening agent in a liquid lubricant. Other ingredients imparting special properties may be added (ASTM D 288)." Such ingredients include additives that impart other desirable properties, such as EP, water resistance, etc. The lubrication function is carried out by the small amount of oil that is released during equipment operation. Because of their semisolid nature, greases are used when fluid lubricants are inefficient, the need for lubrication is infrequent, and/or the lubricant is required to maintain its original position in a mechanism. Greases are formulated from both synthetic and mineral oil basestocks by using a thickening agent and selected additive packages. The thickener, usually a metal soap (a carboxylic acid salt) and sometimes a gelled basic sulfonate,
serves to immobilize the lubricant until service application causes it to be released. The lubricant contains additives which reduce friction and prevent wear. Greases perform the same basic functions as their fluid counterparts but, in view of their high viscosity, they do not perform cooling and cleaning functions efficiently. Based on thickener, greases can be classified as simple-soap, complex-soap, and nonsoap. Simple-soap greases contain lithium (Li), sodium (Na), calcium (Ca), barium (Ba), or aluminum (Al) fatty acid carboxylates. Complex-soap greases contain metal salts of fatty and nonfatty acid mixtures. Nonsoap greases may contain either inorganic compounds or orgeinic compounds as thickeners. Greases are described by the National Lubricating Grease Institute (NLGI) consistency grades and NLGI Service Classification System for automotive use, first implemented in 1991 [77], Consistency grades are 000, 00, 0, and 1-6 and are based on degree of hardness and ASTM Worked penetration rcinge @ 25°C. NLGI service classifications are LA and LB for chassis use and GA, GB, and GC for use in wheel bearings. Prior to this classification, the SAE recommended practice, published in the SAE information report J310, was used for this purpose. The report, first introduced in 1951, had several revisions, the most recent of which occurred in 1993 [78]. Tests associated with NLGI Service Classes, provided in the ASTM 4950 Automotive Grease Specification, are listed below. • Shear Stability—Mukistroke Penetration (ASTM D 217), Roll Stability (ASTM D 1831), Wheel Bearing Leakage (ASTM D 4290 and D 1263) • Oxidation Resistance—Bomb Oxidation (ASTM D 942), High Temperature Life (ASTM D 3527), High-Temperature Performance (ASTM D 3336) • Water Resistance—Water Washout (ASTM D 1264), Water Spray-off (ASTM D 4049) • Bleed Resistance—Oil Separation, static (FTM 321.3), Pressure Oil Separation (ASTM D 1742) • Extreme Pressure/ Antiwear—Four-ball EP (ASTM D 2596), Timken Method (ASTM D 2509), Four-ball Wear (ASTM D 2266), Fretting Protection (ASTM D 4170), SRV Test (ASTM D 5706 and D 5707) • Corrosion—Rust Test (ASTM D 1743), E m c o r (IP 220), Copper Corrosion (ASTM D 4048) • Pumpability—Low-temperatu re Torque (ASTM D 4693) Mobility (US Steel LT37) • Msce/ZaneoMS—Elastomer Compatibility (ASTM D 4289) Dropping Point (ASTM D 566 or D 2265)
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ASTM AND OTHER STANDARDS ISO No. 2592:1973 2719:1988 6293:1983 3016:1994
IP No. 36/84 (89) 34/88 136/89 15/95
2160:1985
154/95
D 217-97 D 445-01
2137:1985 3104:1994
50/88 71/97
D 482-00 D 566-97 D 664-95 (2001)
2176:1995 6619:1988
132/96 177/96
D 665-99
7120:1987
135/93
D D D D
808-00 874-00 892-01 942-90 (1995)
3987:1994 6247:1998
163/96 146/82 (88) 142/85 (92)
D D D D
943-99 972-97 974-01 1078-95
4263:1986
280/96
6618:1997
139/93 195/90
3839:1978
130/92
D 1298-99
3675:1993
160/96
D D D D
6614:1994 2049-1996
412/96 196/97
D D D D D D
ASTM No. 92-01 93-00 94-00 97-96a 129-00 130-94(2000)
D 1091-00 D 1092-99 D 1159-01 D 1218-99 D 1263-94 (1999) D 1264-00
1401-98 1500-98 1742-94 (2000) 1743-01
D 1744 366/84
D 1748-00 D 1831-00 D 2070-91 (2001) D 2161-93 (1999) D 2265-00
239/97
D 2266-91 (1996) D 2270-93 (1998) D 2272-98
2909:1981
226/91 (95)
D 2500-99 D 2509-93 (1998)
3015:1992
219/94 326/83 (88)
D 2596-97 D 2602-86 D 2603-01
Test Test Method for Flash and Fire Points by Cleveland Open Cup Test Method for Flash Point by Pensky-Martens Closed Cup Tester Test Method for Saponification N u m b e r of Petroleum Products Test Method for Pour Point of Petroleum Products Test Method for Sulfur in Petroleum Products (General B o m b Method) Test Method for Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test Test Method for Cone Penetration of Lubricating Grease Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (the Calculation of Dynamic Viscosity) Test Method for Ash from Petroleum Products Test Method for Dropping Point of Lubricating Grease Test Method for Acid Number of Petroleum Products by Potentiometric Titration Method Test Method for Rust Preventing Characteristics of Inhibited Mineral Oil in the Presence of Water Test Method for Chlorine in New and Used Petroleum Products (Bomb Method) Test Method for Sulfated Ash from Lubricating Oils and Additives Test Method for Foaming Characteristics of Lubricating Oils Test Method for Oxidation Stability of Lubricating Greases by the Oxygen B o m b Method Test Method for Oxidation Characteristics of Inhibited Mineral Oils Test Method for Evaporation Loss of Lubricating Greases and Oils Test Method for Acid and Base Number by Color-Indicator Titration Test Method for t h e Determination of Distillation Characteristics of Volatile Organic Liquids (ASTM Procedure Now Obsolete) Test Method for Phosphorus in Lubricating Oils and Additives Test Method for Measuring Apparent Viscosity of Lubricating Greases Test Method for B r o m i n e N u m b e r s of Petroleum Distillates and Commercial Aliphatic Olefins by Electrometric Titration Test Method for Refractive Index a n d Refractive Dispersion of Hydrocarbon Liquids Test Method for Leakage Tendencies of Automotive Wheel Bearing Greases Test Method for Determining the Water Washout Characteristics of Lubricating Greases Practice for Density, Relative Density (Specific Gravity), or API Gravity of Crude Petroleum and Liquid Petroleum Products Test Method for Water Separability of Petroleum Oils and Synthetic Fluids Test Method for ASTM Color of Petroleum Products (ASTM Color Scale) Test Method for Oil Separation from Lubricating Grease During Storage Test Method for Determining Corrosion Preventive Properties of Lubricating Greases Test Method for Determination of Water in Liquid Petroleum Products by Karl Fischer Reagent (Discontinued 2000) Standard Test Method for Rust Protection by Metal Preservatives in the Humidity Cabinet Test Method for Roll Stability of Lubricating Grease Test Method for Thermal Stability of Hydraulic Oils Practice for Conversion of Kinematic Viscosity to Saybolt Universal Viscosity or to Saybolt Universal Viscosity Test Method for Dropping Point of Lubricating Grease Over Wide Temperature Range Test Method for Wear Preventive Characteristics of Lubricating Grease (Four-Ball Method) Practice for Calculating Viscosity Index from Kinematic Viscosity at 40 and 100°C Test Method for Oxidation Stability of Steam Turbine Oils by Rotating Pressure Vessel Test Method for Cloud Point of Petroleum Products Test Method for Measurement of Load-Carrying Capacity of Lubricating Grease (Timken Method) Test Method for Measurement of Extreme-Pressure Properties of Lubricating Grease (Four-Ball Method) Test Method for Apparent Viscosity of Engine Oils at Low Temperature Using Cold-cranking Simulator (Replaced in 1993 with D 5293) Test Method for Sonic Shear Stability of Polymer-Containing Oils
(Continues)
244 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK ASTM No.
ISO No.
IP No.
D 2619-95 D 2622-98 D 2670-95 (1999) D 2710-99 D2711-01a D 2782-01 D 2783-88 (1998)
293/97
D 2882-00 D 2893-99 D 2896-01
3771:1994
D 2983-01
276/95 267/84
D 3228-96 D 3233-93 (1998) D 3336-97 D 3339-95 (2000)
7537:1989
BS7393
3679:1983 3680:1983
303/83 (88)
4265:1986
149/93
D 3427-99 D 3520-88 (1998) D 3527-95 D 3825-90 (2000) D 3828-98 D 3829-93 (1998) D 4047-00 D D D D
4048-97 4049-99 4170-97 4172-94 (1999)
293/97
D 4289-97 D 4290-94 (1999) D 4310-98 D 4377-00 D 4485-01 D 4624-93 (1998)
356/93
D 4627-92
287/94 125/82
D 4628-97 D 4682-87 (1996) D 4683-96 D 4684-99 D 4693-97 D 4739-96 D 4741-00 D 4857-Ola D 4858-00
276/95 417/96
Test
Test Method for Hydrolytic Stability of Hydraulic Fluids (Beverage Bottle Method) Test Method for Sulfur in Petroleum Products by Wavelength Dispersive X-Ray Fluorescence Spectrometiy Test Method for Measuring Wear Properties of Fluid Lubricants (Falex Pin and Vee Block Method) Test Method for Bromine Index of Petroleum Hydrocarbons by Electrometric Titration Test Method for Demusibility Characteristics of Lubricating Oils Test Method for Measurement of Extreme-Pressure Properties of Lubricating Fluids (Timken Method) Test Method for Measurement of Extreme-Pressure Properties of Lubricating Fluids (Four-Ball Method) Test Method for Indicating Wear Characteristics of Petroleum and Non-Petroleum Hydraulic Fluids in Constant Volume Vane Pump Test Method for Oxidation Characteristics of Extreme-Pressure Lubricating Oils Test Method for Base Number of Petroleum Products by Potentiometric Perchloric Acid Titration Test Method for Low-Temperature Viscosity of Automotive Fluid Lubricants Measured by Brookfield Viscometer Test Method for Total Nitrogen in Lubricating Oils and Fuel Oils by Modified Kjeldahl Method Test Method for Measurement of Extreme Pressure Properties of Fluid Lubricants (Falex Pin and Vee Block Methods) Test Method for Life of Lubricating Greases in Ball Bearings at Elevated Temperatures Test Method for Acid Number of Petroleum Products by Semi-Micro Color Indicator Titration Test Method for Air Release Properties of Petroleum Oils Test Method for Quenching Time of Heat-Treating Fluids (Magnetic Quenchometer Method) Test Method for Life Perforraance of Automotive Wheel Bearing Grease Test Method for Dynamic Surface Tension by the Fast Bubble Technique Test Method for Flash Point by Small Scale Closed Tester Test Method for Predicting the Borderline Pumping Temperature of Engine Oil Test Method for Phosphorus in Lubricating Oils and Additives by Quinoline Phosphomolybdate Method Test Method for Detection of Copper Corrosion from Lubricating Grease Test Method for Determining the Resistance of Lubricating Grease to Water Spray Test Method for Fretting Wear Protection by Lubricating Greases Test Method for Wear Preventive Characteristics of Lubricating Fluid (Four-Ball Method) Test Method for Elastomer Compatibility of Lubricating Greases and Fluids Test Method for Determining the Leaking Tendencies of Automotive Wheel Bearing Grease Under Accelerated Conditions Test Method for Determination of the Sludging and Corrosion Tendencies of Inhibited Mineral Oils Test Method for Water in Crude Oils by Potentiometric Karl Fischer Titration Specification for Performance of Engine Oils Test Method for Measuring Apparent Viscosity by Capillary Viscometer at HighTemperature and High-Shear Rates Test Method for Iron Chip Corrosion for Water-Dilutable Metalworking Fluids Test Method for Analysis of Barium, Cadmium, Magnesium, and Zinc in Unused Lubricating Oils by Atomic Absorption Spectrometry Specification for Miscibility with Gasoline and Fluidity of Two-Stroke Cycle Gasoline Engine Lubricants Test Method for Measuring Viscosity at High Shear Rate and High Temperature by Tapered Bearing Simulator Test Method for Determination of Yield Stress and Apparent Viscosity of Engine Oils at Low Temperature Test Method for Low-Temperature Torque of Grease-Lubricated Wheel Bearings Standard Test Method for Base Number Determination by Potentiometric Titration Test Method for Measuring Viscosity at High Temperature and High Shear Rate by Tapered-Plug Viscometer Test Method for Determination of the Ability of Lubricants to Minimize Ring Sticking and Piston Deposits in Two-Stroke-Cycle Gasoline Engines Other Than Outboards Test Method for Determination of the Tendency of Lubricants to Promote Preignition in Two-Stroke-Cycle Gasoline Engines
CHAPTER 9: ADDITIVES AND ADDITIVE CHEMISTRY ASTM No. D 4859-97
ISO No.
IP No.
D 4863-00 D 4927-96 D 4928-00 D 4950-95 (2000) D 4951-00 D 4998-95 D 5133-99 D 5182-97 D 5185-97 D 5291-96 D 5293-99a D 5302-01
D 5480-•95 (1999) D5481- 96 D 5533-•98 D 5579- 01 D 5 6 1 9 00 D 5662 D 5706- 97 D 5707. D 5760- 95 D 5763- 95 D 5800 00a D 5844. 98 D 5846- 99 D 5862- 99a D 5949- 96 D 5950- 96 D 5966- 99 D 5967- 99a D 5968 00a D 5985- 96 D 6082- 00 D6121- 01 D 6202 -01 D 6335 D 6375 99a D 6417 99
334/93
245
Test Specification for Lubricants for Two-Stroke-Cycle Spark-Ignition Gasoline Engines-TC Test Method for Determination of Lubricity of Two-Stroke-Cycle Gasoline Engine Lubricants Test Methods for Elemental Analysis of Lubricant and Additive Components— B a r i u m , Calcium, P h o s p h o r u s , Sulfur, and Zinc by Wavelength-Dispersive X-Ray Fluorescence Spectroscopy Test Methods for Water in Crude Oils by Coulometric Karl Fischer Titration Classification and Specification for Automotive Service Greases Test Method for Determination of Additive Elements in Lubricating Oils by Inductively Coupled Plasma Atomic Emission Spectroscopy Test Method for Evaluating Wear Characteristics of Tractor Hydraulic Fluids Test Method for Low T e m p e r a t u r e , Low Shear Rate, Viscosity/Temperature Dependence of Lubricating Oils Using a Temperature Scanning Technique Standard Test Method for Evaluating the Scuffing Load Capacity of Oils (FZG Visual Method) Test Method for Determination of Additive Elements, Wear Metals, and Contaminants in Used Lubricating Oils and Determination of Selected Elements in Base Oils by Inductively Coupled Plasma Emission Spectroscopy (ICP-AES) Test Methods for Instrumental Determination of Carbon, Hydrogen, and Nitrogen in Petroleum Products and Lubricants Test Method for Apparent Viscosity of Engine Oils Between - 5 and — 35°C Using the Cold Cranking Simulator Test Method for Eveduation of Automotive Engine Oils for Inhibition of Deposit Formation and Wear in a Spark-Ignition Internal Combustion Engine Fueled with Gasoline and Operated Under Low-Temperature, Light-Duty Conditions (Sequence VE) Test Method for Engine Oil Volatility by Gas Chromatography Test Method for Measuring Apparent Viscosity at High-Temperature and HighShear Rate by Multicell Capillary Viscometer Test Method for Evaluation of Automotive Engine Oils in Sequence IIIE, SparkIgnition Engine Test Method for Evaluating the Thermal Stability of Manual Transmission Lubricants in a Cyclic Durability Test Test Method for Comparing Metal Removal Fluids Using the Tapping Torque Test Machine Test Method for Determining Automotive Gear Oil Compatibility with Typical Oil Seal Elastomers Test Method for Determining Extreme Pressure Properties of Lubricating Greases Using a High-Frequency, Linear-Oscillation (SRV) Test Machine Test Method for Measuring Friction and Wear Properties of Lubricating Greases Using a High-Frequency, Linear-Oscillation (SRV) Test Machine Specification for Performance of Manual Transmission Gear Lubricants Test Method for Oxidation and Thermal Stability Characteristics of Gear Oils Using Universal Glassware Test Method for Evaporation Loss of Lubricating Oils by the Noack Method Test Method for Evaluation of Automotive Engine Oils for Inhibition of Rusting (Sequence IID) Test Method for Universal Oxidation Test for Hydraulic and Turbine Oils Using the Universal Oxidation Test Apparatus Test Method for Evaluation of Engine Oils in Two-Stroke Cycle Turbo-SuperCharged 6V92TA Diesel Engine Test Method for Pour Point of Petroleum Products (Automatic Pressure Pulsing Method) Test Method for Pour Point of Petroleum Products (Automatic Tilt Method) Test Method for Evaluation of Engine Oils for Roller Follower Wear in Light-Duty Diesel Engine The Method of Evaluation of Diesel Engine Oils in T-8 Engine Test Method for the Corrosiveness of Diesel Engine Oil Test Method for Pour Point of Petroleum Products (Rotational Method) Test Method for High Temperature Foaming Characteristics of Lubricating Oils Test Method for Evaluation of the Load Carrying Capacity of Lubricants Under Conditions of Low Speed and High Torque Used for Final Hypoid Drive Axles Test Method for Automotive Engine Oils on the Fuel Economy of Passenger Cars and Light Duty Trucks in t h e Sequence VIA Spark Ignition Engine Test Method for Determination of High T e m p e r a t u r e Deposits by ThermoOxidation Engine Oil Simulation Test Test Method for Evaporation Loss of Lubricating Oils by Thermogravimetric Analyzer (TGA) Noack Method Test Method for Estimation of Engine Oil Volatility by Capillary Gas Chromatography (Continues)
246 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK ASTM No.
D 6443-99 D 6481-99 D 6483-99 D 6557-00 D 6593-00
D 6616-01 D 6618-00 D 6681-01 D 6709-01
ISO No.
IP No.
Test
Test Method for Determination of Calcium, Chlorine, Copper, Magnesium, Phosphorus, Sulfur, and Zinc in Unused Oils and Additives by Wavelength Dispersive X-ray Fluorescence Spectrometry (Mathematical Correction Method) Test Method for Determination of Phosphorus, Sulfur, Calcium, and Zinc in Lubrication Oils by Energy Dispersive X-ray Fluorescence Spectrometry Test Method for Evaluation of Diesel Engine Oils in T-9 Engine Test Method for Evaluation of Rust Preventive Characteristics of Automotive Engine Oils (Ball Rust Test) Test Method for Evaluation of Automotive Engine Oils for Inhibition of Deposit Formation in a Spark-Ignition Internal Combustion Engine Fueled with Gasoline and Operated Under Low-Temperature, Light Duty Conditions (Sequence VG) Test Method for Measuring Viscosity at High Shear Rate by Tapered Bearing Simulator Viscometer at 100°C Test Method for Evaluation of Engine Oils in Diesel Four-Stroke-Cycle SuperCharged IM-PC Single Cylinder Oil Test Engine Test Method for Evaluation of Engine Oils in a High Speed, Single-Cylinder Diesel Engine—Caterpillar IP Test Procedure Test Method for Evaluation of Automotive Engine Oils in the Sequence VIII SparkIgnition Engine (CLR Oil Test Engine)
REFERENCES [1] Obert, E. F., "Lubrication," Ch. 16, Internal Combustion Engines and Air Pollution, Intext Educational Publishing, NY, 1968, pp. 633-677. [2] Modler, R., Anderson E., and Yoshida Y., "Lubricant Oil Additives," Specialty Chemicals, Strategies for Success, Vol. 9, SRI International, December 1996. [3] Rizvi, S. Q. A., "Additives: Chemistry and Testing," Tribology Data Handbook—An Excellent Friction, Lubrication, and Wear Resource, CRC Press, Boca Raton, FL, 1997, pp. 117-137. [4] Klamann, D., Lubricants and Related Products—Synthesis, Properties, Applications, International Standards, Verlag Chemie, Hamburg, 1984. (a) "Analysis and Testing," Ch. 10, pp. 218-247. (b) "Additives," Ch. 9, pp. 177-217. (c) Appendix A, pp. 437-442. [5] Gergel, W. C, "Lubricant Additive Chemistry," Presented at the International Symposium on Technical Organic Additives and Environment, Interlaken, Switzerland, 24-25 May 1984. [6] Ford, J. F., "Lubricating Oil Additives—^A Chemist's Eye View," Journal of the Institute of Petroleum, Vol. 54, July 1968, pp. 188-210. [7] Schilling, A., Motor Oils and Engine Lubrication, Scientific Publications, Great Britain, 1968. [8] Ingold, K. U., "Inhibition of Autoxidation of Organic Substances in Liquid Phase," Chemical Reviews, Vol. 61, 1961, pp. 563-589. [9] Johnson, M. D., Korcek, S., and Zinbo, M., "Inhibition of Oxidation by ZDTP and Ashless Antioxidants in the Presence of Hydroperoxides at 160°C," Lubricant and Additive Effects on Engine Wear, SP-558, Fuels and Lubricants Meeting, San Francisco, CA, 31 Oct.-3 Nov., 1983, pp. 71-81. [10] Al-Malaika, S., Marogi, A., and Scott, G., Journal ofApplied Polymer Science, Vol. 33, 1987, pp. 1455-71. [11] Abou El Naga, H. H. and Salem, A. E. M., "Effect of Worn Metals on the Oxidation of Lubricating Oils," Wear, Vol. 96, 1984, pp.267-283. [12] Vijh, A. K., "Electrochemical Mechanisms of the Dissolution of Metals Eind the Contaminants Oxidation of Lubricating Oils Under High-temperature Friction Conditions," Wear, Vol. 104, 1985,pp.l51-158. [13] HambUn, P. C, Kristen U., and Chasan D., "A Review: Ashless Antioxidants, Copper Deactivators, and Corrosion Inhibitors, Their Use in Lubricating Oils," Lubrication Science, Vol. 2, 1990, pp. 287-318.
[14] Kreuz, K. L., "Gasoline Engine Chemistry as Applied to Lubricant Problems," Lubrication, Vol. 55, 1969, pp. 53-64. [15] Lachowicz, D. R. and Kreuz, K. L., "Peroxynitrates. The Unstable Products of Olefin Nitration with Dinitrogen Tetroxide in the Presence of Oxygen. A New Route to a-Nitroketones," Journal of Organic Chemistry, Vol. 32, 1967, pp. 3885-3888. [16] Kreuz, K. L., "Diesel Engine Chemistry as Applied to Lubricant Problems," Lubrication, Vol. 56, 1970, pp. 77-88. [17] Covitch, M. J., Graf, R. T., and Gundic, D. T., "Microstructure of Carbonaceous Diesel Engine Piston Deposits," Lubricant Engineering, Vol. 44, 1988, p. 128. (b) Covitch, M. J., Richardson, J. P., and Graf, R. T., "Structural Aspects of European and American Diesel Engine Piston Deposits," Lubrication Science, Vol. 2, 1990, pp. 231-251. [18] Kombrekke, R. E., Personal Communication, Research and Development, The Lubrizol Corporation, Wickliffe, OH. [19] Boner, C. J., "Theory of Action and Performance," Ch. 8, Gear and Transmission Lubricants, Reinhold Publishing Company, NY, 1964. [20] Bhushan, B. and Gupta, B. K., "Physics of Tribological Materials," Ch. 3, Handbook of Tribology; Materials, Coatings, and Surface Treatments, McGraw-Hill, Inc., NY, 1991. (c) Buckley, D. H., "Properties of Surfaces," CRC Handbook of Lubrication, (Theory and Practice of Tribology), Vol. II, Theory and Design, Richard E. Booser, Ed., CRC Press, Boca Raton, FL, 1983, pp. 17-30. [21] Lansdown, A. R., "Extreme Pressure and Anti-wear Additives," Ch. 12, Chemistry and Technology of Lubricants, R. M. Mortier and S. T. Orszulik, Eds., VCH Publishers, Inc., NY, 1992, pp. 269-281. [22] O'Brien, J. A., "Lubricant Additives," CRC Handbook of Lubrication, (Theory and Practice of Tribology), Vol. II, Theory and Design, Richard E. Booser, Ed., CRC Press, Boca Raton, FL, 1983, pp. 301-315. [23] "Engine Service Classification System and Guide to Crankcase Oil Selection," API Publication 1509, American Petroleum Institute, Washington, D.C., 1996. [24] Oliver, C. R., Renter, R. M., and Sendra, J. C, "Fuel Efficient Gasoline-Engine Oils," Lubrication, Vol. 67, 1981, pp. 1-12. [25] Jayne, G. J., Matthews, B. M., and Thomas, A. S., "Hypoid Gear Oils for the 1980s," Ch. 19, Performance and Testing of Gear Oils and Transmission Fluids, R. Tourret and E. P. Wright, Eds., Heyden and Son, 1981, pp. 307-319.
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247
Encyclopedia of Polymer Science and Engineering, Second Edition, John Wiley and Sons, NY, 1988, Vol. 11, p. 22. [46] MuUer, H. G., "Mechanism of Action of Viscosity Index Improvers," Tribology International, June 1978, pp. 189-192. [47] Watson, R. W. and McDonnell, T. F., Jr., "Additives—The Right Stuff for Automotive Engine Oils," Fuels and Lubricants Technology: An Overview, SP. 603, Society of Automotive Engineers, Warrendale, PA, October 1984, pp. 17-28. [48] Stambaugh, R. L„ "Viscosity Index Improvers and Thickeners," Chemistry and Technology of Lubricants, R. M. Mortier and S. T. Orszulik, Eds., VCH Publishers, Inc., NY, 1992, pp. 124-159. [49] Becher, P., Emulsions: Theory and Practice, American Chemical Society Monograph Series, Ch. 6, Reinhold Publishing Corporation, NY, 1957, pp. 209-231. [50] Karsa, D. R., "Industrial Applications of Surfactants," Industrial Applications of Surfactants—An Overview, D. R. Karsa, Ed., Published by Royal Society of Chemistry, Cambridge, England, 1987. [51] Hancock, R. I., "Macromolecular Surfactants," Surfactants, T. F. Tadros, Ed., Academic Press, San Diego, CA, 1984, pp. 287- 321. [52] Lubricant Additives and the Environment, CEFIC, Brussels, Belgium, 1993, an ATC (Technical Committee of Petroleum Additive Manufacturers) Technical Publication. [53] Fuel Additives and the Environment, CEFIC, an ATC (Technical Committee of Petroleum Additive Manufacturers) Technical Publication, Brussels, Belgium, 1994, [54] De Hoffmann, E., Charette, J., and Stroobant, V., Mass Spectrometry: Principles and Applications, John Wiley & Sons, NY, December 1996. (b) Colthup, N. B., Daly, L. H., and. Wiberley, 5. E., Introduction to Infrared and Raman Spectroscopy, 3rd edition. Academic Press, San Diego, CA, September 1990. (c) Macomber, R. S., A Complete Introduction to Modem NMR Spectroscopy, John Wiley & Sons, NY, December 1997. (d) Derome, A. E., "Modem NMR Techniques for Chemistry Research," Vol. 6, Tetrahedron Organic Chemistry Series, J. E. Baldwin and P. D. Magnus, Eds., Pergamon Press, Oxford, 1993. [55] Hsu, S. M. and Cummings, A. L., "Interactions of Additives and Lubricating Basestocks," Lubricant and Additive Effects on Engine Wear, SP - 558, Fuels and Lubricants Meeting, San Francisco, CA, 31 Oct.-3 Nov. 1983, pp. 61-70. [56] Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, 1998, and the later revisions. [57] Rein, S. W., "Viscosity-I," Lubrication, Vol. 64, No. 1, 1978, pp. 1-12. (b) Rein, S. W., "Viscosity-II," Lubrication, Vol. 64, No. 1, p p . 13-32, 1978. (c) "Viscosity," Lubrication, Vol. 52, No. 3, 1966, pp. 2 1 ^ 8 . [58] SAE J300: "Engine Oil Viscosity Classification," Society of Automotive Engineers, Warrendale, PA, 1995, and the later revisions. [59] SAE J1536: "Two-stroke Cycle Engine Oil Miscibility/Fluidity Classification," Society of Automotive Engineers, Warrendale, PA, 1995, and the later revisions. [60] SAE J306: "Axle and Manual Transmission Lubricant Viscosity Classification," Society of Automotive Engineers, Warrendale, PA, 1985. (b) "Revision to SAE J306 Approved," Lubrizol NewsLine, Vol. 16, No. 3, June 1998. [61] SAE J2227: "International Tests and Specifications for Automotive Oils," Surface Vehicles Information Report, Society of Automotive Engineers, Warrendale, PA, July 1998. (b) "ACEA Issues New Engine Oil Specification," Lubrizol NewsLine, Vol. 16, No. 3, June 1998. [62] Rizvi, S. Q. A., "History of Automotive Lubrication," SAE Technical Paper 961949, Presented at Fuels and Lubricant Meeting, San Antonio, TX, 14-17 Oct. 1996, Society of Automotive Engineers, Warrendale, PA. [63] SAE J183: "Engine Oil Performance and Engine Service Classification (Other Than "Energy Conserving")," SAE 1995 Hand-
248 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK
[64] [65] [66] [67]
[68]
[69]
[70]
book, Society of Automotive Engineers, Warrendale, PA, 1995, and the later revisions. Sullivan, T., "API Snuffs out GF-2," Lube Report—Industry News from Lubes-n-Greases, Vol. 2, No. 14, 2002. McFall, D., "GF-4 Oil Due in One Year," Lube Report—Industry News from Lubes-n-Greases, Vol. 2, No. 14, 2002. McFall, D., "CI-4 Diesel Oil: On Time, On Target," Lubes-nGreases, Feb. 2002, p p . 6-12. SAE Standard J2116: "Two-stroke-Cycle Gasoline Engine Lubricants: Performance and Service Classification," Approved July 1993, Society of Automotive Engineers, Warrendale, PA, 1994. Deen, H. E. and Ryer, J., "Automatic Transmission Fluids— Properties and Performance," Fuels and Lubricants Technology: An Overview, SP. 603, Society of Automotive Engineers, Warrendale, PA, October 1984, pp. 117-127. Artman, D. M. and Copes, R. G., "ATF From Performance Challenges to Market Opportunities," Presented at the 1994 NPRA National Fuels and Lubricants Meeting, 3-4 Nov. 1994. Graham, R. and Oviatt, W. R., "Automatic Transmission Fluids—Developments Toward Rationalization," Presented at CEC 1985 International Symposium, Wolfsburg, Germany, 7 J u n e 1985.
[71] "Lubricant Service Designations for Automotive Manual Transmissions a n d Axles," API Publication No. 1560, American Petroleum Institute, Washington DC, 1981. [72] Sutherland, J. M., "Proposed Automotive Gear Lubricant Categories: Their Impact on the Industry," Presented at NPRA National Fuels and Lubricants Meeting, Houston, TX, 2-3 Nov. 1989. [73] "Progress is Slow on PG-1 and PG-2," Lubrizol NewsLine, Vol. 11, No. 1, January 1993. [74] (a) "Ready Reference for Lubricant and Fuel Performance," Publication 1288 240-94R1, The Lubrizol Corporation, Wickliffe, OH, 1999/2002. (b) Reference Library, Lubrizol web site, www.lubrizol.com. [75] "Tractor Wet Brake and Wet Clutch Friction Properties," W. K. S. Cleveland, NLGI Sopkesman, July 1987, p p . 135-138. [76] Laemmle, J. T., "Metalworking Lubricants," American Society of Metals Handbook, Friction, Lubrication, and Wear Technology, S. D. Henry, Ed., ASM International 1992, Vol. 18, pp. 139-149. [77] "NLGI Lubricating Grease Guide," National Grease Institute, Kansas City, MO, 1987. [78] SAE J310; "Automotive Lubrication Greases," SAE 1995 Handbook, Society of Automotive Engineers, Warrendale, PA, 1995.
MNL37-EB/Jun. 2003
Synthetic Lubricants— Non Aqueous Thomas F. Buenemann, and Ian Thompson^
^ Steve Boyde, ^ Steve Randies, ^
POLYOL AND DIESTERS
Chemistry and Manufacturing General Features
ALTHOUGH THE DEVELOPMENT OF SYNTHETIC ESTER LUBRICANTS is
In addition to their good properties at extreme temperatures, synthetic esters have other desirable characteristics including good lubricity, high viscosity index, low volatility, and compatibility with s t a n d a r d lubricant additives a n d basefluids. The fundamental chemistry is flexible and a wide range of raw materials is available, which means that ester base fluids can be designed having a wide raxige of viscosities. Other key properties such as biodegradability can also be controlled by molecular design [3]. Consequently, synthetic esters have found use in many applications outside aviation. Examples include automotive crankcase a n d gear oils, 2stroke lubricants, industrial gear oils, hydraulic fluids, textile yarn lubricants, metal cutting and rolling fluids, air compressor lubricants, and refrigeration compressor lubricants [4].
Raw
' Senior Application Manager and ^ Technology Manager, Uniqema Lubricants, P.O. Box 2, 2800 AA Gouda, Buurtje 1, Gouda, BE 2802, Netherlands.
Materials
The raw materials used in the manufacture of synthetic esters for lubricant applications are derived from a variety of sources, both natural and synthetic. Aromatic acids or anhydrides are manufactured by the oxidation of the corresponding hydrocarbons [6,7]. Alpha, omega diacids are also generally produced by oxidation. The diacids used most widely to synthesize ester lubricant fluids are adipic, azelaic, sebacic, and dodecanedioc acids, as well as C36 dimer acid. Adipic and dodecanedioc acids are derived from petrochemical feedstocks.
249 2003 by A S I M International
Groups
Esters are defined as the class of chemical compounds containing the ester functional group. They are normally manufactured by reaction of a carboxylic acid with an alcohol (Fig. 1), optionally in the presence of an esterification catalyst, and with elimination of water [5]. The properties of the product esters can be controlled by appropriate selection of the raw materials used. The final product properties are mainly dependent on the molecular weight, the n u m b e r of ester groups per molecule, and the degree of branching in alkyl substituent groups. A mixture of raw materials may be used to deliver the precise combination of properties required. There are three main classes of synthetic esters currently in use as lubricant base fluids: aromatic esters, aliphatic diesters, and polyol esters. Aromatic esters, shown in Fig. 2, are manufactured by the reaction of an aromatic di or poly acid or anhydride, such as phthalic anhydride, tiimellitic anhydride, or pyromeUitic anhydride, with a monoalcohol or mixture of monoalcohols. Diesters, shown in Fig. 3, are manufactured by the reaction of an aliphatic alpha, omega diacid with a monoalcohol or mixture of monoalcohols. Polyol esters, shown in Fig. 4, are manufactured by reaction of a diol or polyol having the neopentyl structure, such as neopentyl glycol, trimethylol propane or pentaerythritol, with a monoacid or mixture of monoacids. Oligomeric esters, generally known as complex esters, can be manufactured by reaction of a diol or polyol with a di or polyacid/anhydride, with a monoacid or monoalcohol to act as capping reagent. This allows preparation of materials having higher average molecular weights and consequently higher viscosities, than can be achieved with simple diesters or polyol esters. However, complex esters also have a distribution of molecular weights, which means that their volatility characteristics are not as good as simple esters of the same number average molecular weight.
relatively recent, the use of esters as lubricating fluids is as old as h u m a n technology. Before suitable mineral oils became widely available as a b3^roduct of the petroleum based fuels industry, lubricants were based on natural fats or oils, which are either triesters of glycerine with natural fatty acids, e.g., tallow and olive oil, or long chain monoesters of fatty acids with fatty alcohols, e.g., sperm whale oil. The use of synthetic esters as high performance lubricating fluids was originally driven by the development of the gas turbine or jet engine in aviation. Aviation turbines have a higher operating temperature than the piston engines that preceded them, and jet aircraft are capable of operation at m u c h higher altitudes, where the ambient temperature is very low. Consequently, lubricants for aviation turbines are required to have both very good high temperature stability and good low temperature flow properties. It was found that mineral oils and synthetic hydrocarbons did not deliver the required combination of properties, and diesters were adopted as the lubricant base fluid of choice for early aviation turbines [1]. As gas turbine technology developed, the operating temperatures increased further, and diesters have been largely substituted in aviation applications by polyol esters, which have even better thermal stability [2]. Despite intensive research into alternative chemistries, polyol esters remain the base fluid of choice for aviation turbine lubricants.
Copyright'
and Product
www.astm.org
250
MANUAL 37: FUELS AND LUBRICANTS
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whereas azelaic, sebacic, and C36 dimer acids are derived from natural fatty acids [8]. The monoalcohols used in diesters are mainly manufactured by carbonylation of olefin feedstocks. These alcohols are also used in the manufacture of plasticizers and ethoxylate surfactants. They are generally commercially available as a mixture of isomers and/or carbon chain numbers. Exceptions include 2-ethylhexanol, a derivative of butanol, which is a single isomer, and the linear C-even alcohols (eg Cg, Cio alcohol), which are manufactured by chemical reduction of naturally occurring fatty acids [9], The mono acids (fatty acids) used in polyol esters may be derived either from petrochemical or natural sources. Naturally occurring fatty acids are almost always linear and have an even number of carbon atoms. Lower carbon numbers
Polyol esters
Q
R'"
R"-
I
CK
O ^
R = C4-C17linearor branched aikyi groups
^R
R" PE; Pentaerythritol; R' = R" = R'" = CH20COR TMP; Trimethylol propane; R"» R" = CH20C0R, R" = Et NPG; Neopentyl glycol; R' = CH20C0R, R" = R" = Me FIG. 4—Examples of polyolesters.
Acid
^
£st(^r:'
OH Fsterififaqtion ^ ^ ' t ^ R' n
nWater
FIG. 1—Chemistry of esteriflcatlon.
Aromatic esters o
0
I
I ^ I
II Phthalate O
0
Trimellitate
II °
R = C8 - CI8 linear or branched alkyl groups FIG. 2—Examples of aromatic esters.
Diesters
R'
>^' % C — ( ^ " 2 > n - \ . /
0 ~ R '
n = 4 - adipates n = 7 - azelates n - 8 - sebacates n = 10 - dodecanedioates R' - C8 - C13 linear or branched alkyl groups FIG. 3—Examples of diesters.
(Cs—Cift) are normally fully saturated, while higher carbon numbers (Cig—C22) have olefinic unsaturation. Petrochemical fatty acids may be either linear or branched. Linear petrochemical acids are manufactured by the oxidative carbonylation of linear olefins, which are themselves derived from ethylene and are therefore C-even (an even number of C-atoms/molecules). Carbonylation adds a single carbon atom, so linear petrochemical acids are normally C-odd (an odd number of C-atoms/molecules) [8]. The neopentyl polyols used in polyol esters are manufactured by reaction of an aldehyde with formaldehyde, followed by chemical reduction [10]. As noted below, the neopentyl structure confers superior thermal and oxidative stability as compared to other polyols, such as butane diol or glycerol, and neopentyl polyol derived materials are preferred in lubricant applications for this reason. The only significant exception to this rule is for glycerol esters, which are also widely used in lubricating applications, though only those where thermal and oxidative stability is not a key performance requirement. Some of these glycerol esters are simply purified vegetable oils, which are not normally classified as synthetic esters. Others, known as mid-chain triglycerides (MCTs), are produced by chemical reaction of purified short chain natural fatty acids with glycerol. The oxidative stability of MCTs is inferior to that of neopentyl polyol esters, but they are used in applications where very high biodegradability is required, or where products are required to be manufactured only from renewable raw materials. Manufacturing Technology Esters are manufactured by reacting the desired acid and ailcohol with an esterification catalyst, if required, and reacting at an elevated temperature [11]. The esterification reaction is reversible, and consequently it is necessary to drive the equilibrium over to the desired product by removal of water. The reaction temperature and pressure are therefore selected so that water can be removed by distillation as it is formed. High temperatures also increase the rate of reaction, but very high temperatures may lead to undesirable side reactions or discoloration. Esterification reactions are, therefore, normally conducted at a temperature in the range 200-250°C. For diesters and aromatic esters, the reaction is generally conducted in the presence of an excess of the monoalcohol. ASTM D 974-97 Standard Test Method for Acid and Base
CHAPTER 10: SYNTHETIC Number by Colorimetric Titration (DIN 51 559 Part 1) is used to measure the acid value, and when the acid value of the reaction mixture has reached the target value, implying that all of the acid groups have reacted, the excess monoalcohol is removed by distillation. For polyol esters, the reaction is usually carried out in the presence of an excess of the monoacid. In this case, the progress of the reaction is controlled by monitoring the hydroxy] value according to ASTM E 326 Standard Test Method for Hydroxyl Groups by Phthalate Esterification. When the target value is achieved, the excess monoacid is removed by distillation. Esterification catalysts are normally used to accelerate the rate of the esterification reaction. Conventionally used homogeneous catalysts include complexes of transition metals, especially titanium and tin, and strong acids, e.g., toluenesulfonic acid. Heterogeneous catalysts such as acid ion exchange resins may also be used [11]. The fact that acids catalyze the esterification process can be exploited in the manufacture of polyol esters, where the mono acid reagent, present in excess, may itself serve as the catalyst. The crude ester produced by chemical reaction and distillation of excess raw materials normally requires further processing to render it suitable for use as a lubricant base fluid. It is particularly important to remove all traces of the esterification catalyst, since the esterification catalyst is also capable of acting as a hydrolysis catalyst, which would have a detrimental effect on the hydrolytic stability of the product. Where titanium catalysts are used it is normal to wash the crude ester with water, which hydrolyzes the catalyst to form insoluble titanium oxide, which can be removed by subsequent filtration. Further treatments may be applied to reduce acid value; reduce water content, which is determined by ASTM E 1064, Standard Test Method for Water in Organic Liquids by Coulometric Karl-Fischer Titration (DIN 51 777 Part 1); or to remove color, which is measured by ASTM D 1209-97, Standard Test Method for color of clear liquids (Platinum-Cobalt scale) formed during reaction. Finally, the product is filtered to remove particulate impurities. Physical Properties Viscosity Probably the most important characteristic of a lubricant base fluid is the viscosity measured by ASTM D 445-97, Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (DIN 51 550). The ester group does not significantly increase viscosity as compared to a hydrocarbon chain. Consequently, esters typically have viscosities similar to those of hydrocarbons of comparable molecular weight and degree of branching [12]. The viscosity of ester fluids can be controlled over a wide range by appropriate choice of molecular structure, and current commercially available materials cover the range of viscosities at 40°C from 5 cSt for simple diesters up to >1000 cSt for complex esters [13,14]. As for all fluids, the temperature dependence of viscosity (Viscosity Index, VI) is calculated by ASTM D 2270-93, Practice for Calculating Viscosity Index from Kinematic Viscosity at 40°C and 100°C (DIN/ISO 2909) which is a function of the extent and type of branching in the molecule. VI can be controlled by molecular design as for viscosity [12],
LUBRICANTS—NON
AQUEOUS
251
Low Temperature Characteristics The low temperature properties of esters, as for all lubricating fluids, depend on the VI, and on the tendency to form waxy solids [15], As noted above, the VI can be controlled by appropriate molecular design. Linear substituents are desirable to give a high VI and therefore control the increase in viscosity with increasing temperature. Wax formation tendency is dependent on the presence of saturated linear hydrocarbon chains above a critical chain length. Linear saturated hydrocarbon substituents containing approximately eight or more carbon atoms will generally lead to tendency to wax formation in the temperature region of interest, i.e., — 50°C to 0°C. Ester base fluids intended for low temperature applications generally have a balance of linear and branched substituents to achieve both performance criteria which is measured by ASTM D 97-96a, Standard Test Method for Pour Point of Petroleum Products (DIN 51 597). Volatility The ester group carries a permanent electric dipole. This means that an intermolecular force due to dipolar interaction exists between the ester molecules, in addition to the normal molecular van der Waals forces which are present between hydrocarbon groups. Consequently, intermolecular forces are stronger in ester fluids, and esters have significantly lower vapour pressure, and therefore higher flash points than hydrocarbons of similar molecular weight and viscosity [4] determined by ASTM D 92-97, Standard Test Method for Fire and Flash Point by Cleveland Open Cup (DIN 51 376). Chemical Characteristics Thermal Stability Sjmthetic esters all show very good thermal stability as compared to mineral oil, but diesters are significantly less stable than polyol esters. This is because diesters can decompose via a /3-elimination reaction, which forms an acid and an olefin. This pathway is not possible for polyol esters due to the neopentyl structure, which has no hydrogen atoms, attached to the /3 carbon of the alcohol residue [4]. Hydrolytic Stability As noted above, the esterification reaction is reversible, and esters can, in principle, react with water to regenerate the acid and alcohol raw materials. This reaction is known as hydrolysis. The rate of hydrolysis under normal service or storage conditions is very low, and hydrol3?tic stability is rarely an issue in practice. However, it can be a cause of concern in some applications as the acids produced by hydrolysis could potentially act as corrosive agents. Test methods have been developed to characterize the rate of hydrolysis, i.e., ASTM D 2619, Standard Test Method for Hydrolytic Stability of Hydraulic Fluids. In this test, 75 g of fluid and 25 g of water eire sealed in a beverage bottle with a copper strip present for 48 h at 93°C (200°F). At the end of the test, the oil and water layers are separated and insolubles are weighted. Viscosity and acid numbers are also determined. However, these test methods must be used with caution as the rate of hydrolysis depends on factors such as the structure and purity of the ester, reaction conditions, and the nature of additives present. An understanding of these rela-
252
MANUAL
3 7: FUELS AND LUBRICANTS
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tionships can be used to select appropriate ester lubricant formulations for a given application [16]. Biodegradability Esters generally exhibit higher aerobic biodegradability than corresponding hydrocarbons, because microorganisms produce lipase enzymes which catalyze ester hydrolysis. This converts the ester into alcohol and acid, which have higher water solubility and can be further degraded by enzyme catalyzed oxidation reactions. The degree of initial biodegradability of ester fluids is highly correlated with the hydrolytic stability. Ultimate biodegradability is also dependent on the linearity of the hydrocarbon subsituents. Esters derived from reactants containing fully linear alkyl substituents (and in particular those containing olefinic unsaturation) such as polyol esters derived from natural fatty acids, e.g., trimethylolpropane oleate (TMPO), generally biodegrade particularly rapidly. In contrast, a r o m a t i c esters, particularly those containing highly branched alkyl groups, biodegrade more slowly. However, even some branched aromatic esters can meet the criteria for ready biodegradabilitiy according to OECD 301B [17]. OECD 301B is currently the most widely used test method for ready biodegradability of waterinsoluble organic substances [18]. However, another closely related test m e t h o d is ASTM D 5864-95, S t a n d a r d Test Method for Biodegradability of Organic Substances, which is less stringent in that the latter allows for preacclimation of the microbial inoculum, and therefore represents a less severe test regime.
Application and Performance Characteristics Although slightly more polar, most esters are fully miscible with mineral oils or synthetic hydrocarbons. Due to their higher polarity, they are generally better solvents for polar materials, such as many standard lubricant additives used in mineral oil based formulations. Consequently, esters are frequently used as a component of mixed base fluids containing polyalphaoleflns, which are poor solvents for polar additives. However, the relatively higher solvency of esters may be a disadvantage in formulations containing polar additives, such as antiwear and EP agents which are required to adsorb on the metal surface. The additive treat rate in an ester-based fluid may need to be higher than in a mineral oil formulation in order to ensure an effective surface concentration of the additive [19]. The miscibility of esters with more polar materials also allows their use as mixtures with other polar synthetic basefluids, e.g., poly alkylene glycols, which are not generally miscible with mineral oil. The inherent oxidative stability of esters is similar to that of synthetic hydrocarbons. They therefore require the use of antioxidant additives to limit the rate of oxidation in service. However, esters generally show very good response to standard antioxidants and can easily be formulated to give good oxidative stability [4]. Four stroke cycle crankcase formulations require good thermal and oxidative stability, low volatility, and good low temperature pumpability. These requirements can be met by the use of mixed poly (alphaolefin)/ester base fluids. Diesters
and polyol esters are typically used. The more polar ester serves to enhance solubility of less polar additives in PAO, and confer seal swelling properties, as well as superior thermal stability. PhthaJate esters have been used where a low cost seal swellant is required. Diesters based on C36 dimer acid and polyol esters are frequently used in two stroke cycle formulations to provide low smoke properties and biodegradability for spilled or uncombusted oil, particularly in marine applications. Hydraulic fluids are formulated using synthetic polyol esters or diesters where a combination of good oxidative stability and biodegradability is required. Organic esters also have better flame retardancy than mineral oils, although not as good as phosphate esters. As noted above, polyol esters remain the lubricant base fluid of choice for aviation turbine lubricants, even in military applications. Polyol ester- derived aviation turbine lubricants are generally formulated containing aminic antioxidants and ashless phosphate ester antiwear agents. There is continuing interest in increasing the upper temperature limit for aviation turbine lubricants, but the thrust of current research has moved away from investigation of alternative base fluids towards optimization of ester chemistry. The relatively recent development of hydrofluorocarbon (HFC) compatible refrigeration compressor lubricants has led to increased demand for polyol esters [20]. Concern over the environmental impact of the ozone depleting chlorofluorocarbons (CFCs) previously used as refrigerant fluids has led to international legislation prohibiting their use. This has resulted in a change to alternative refrigerants including the zero ozone depletion potential HFCs. The HFCs are not miscible with the hydrocarbon lubricants that were generally used in CFC systems. Lubricant immiscibility was found to cause poor oil return leading to lubricant starvation in the compressor and fouling of low temperature heat exchange surfaces. Consequently, it has been necessary to develop new HFC-miscible synthetic lubricants. Polyol esters have been generally adopted as the preferred basefluid for most HFC compatible refrigeration applications.
POLYALKYLENE GLYCOLS Polyalkylene glycol (PAG) is a generic name used to describe a family of products formed from the polymerization of one or more alkylene oxides. Such products are also known as polyethers, polyoxyalkylene glycols, polyalkylene glycol ethers, polyglycols, and PAGs [21,22]. PAGs are an extremely versatile family of products, which can exhibit a wide range of physical and chemical properties. They are excellent lubricants in their own right [22-25], which makes them the fluid of choice for a large n u m b e r of engineering and lubricant applications. The history of PAGs is a long one. The earliest reported polymerization of ethylene oxide dates back to 1863 [26], with the first commercial products (polyethylene glycols) available in 1939 [27]. The range of viscosities now covered by PAGs is vast, making these products suitable as lubricants in their own right, but also as thickeners and lubricity improvers in water based systems.
CHAPTER 10: SYNTHETIC LUBRICANTS~NON AQUEOUS 253
4
4.5
5
log (molecular weight)
FIG. 5—^Typical molecular weight distribution for a polyalkylene glycol. Chemistry and Manufacturing PAGs are linear or branched chain polymers, which contain ether linkages in their main polymer structure. They are produced by the polymerization of one or more alkylene oxides, such as ethylene oxide (EO, C2H4O) cind propylene oxide (PO, CsHgO), though butylene oxide (BO, C4H10O) and higher oxides can also be used. They can be considered to consist of three parts: the initiator, the alkylene oxide or polyether section, and the terminal or end group as shown below. initiator
polyether section (CH2-CH(R)-0)„
end group -
R2
(R is H or alkyl; R' and R^ are H, alkyl, acyl, etc; X is O, N, S, etc; n is an integer 1, 2, 3, etc.) The three distinct parts of the polymer are variable, and it is therefore possible to produce an infinite n u m b e r of different products, each with its own unique properties. The skill in making PAGs is to select the appropriate combination of initiator, polyether section and terminal group to give the desired properties. Since the PAG is in fact a "statistical polymer," the product will contain a series of polymers w i t h a distribution of polyether chain lengths, centered around "n," and distribution of molecular weights, as shown in Fig. 5. The molecular weight distribution is measured by the modified ASTM D 5296-97, Standcird Test Method Gel Permeation Chromatography (GPC). As "n" and the length of the polymer increases, so does the viscosity, thus giving access to a wide range of viscosity. The R group is H for EO, CH3 for PO, etc. The initiator must be an "active hydrogen" containing compound R ' X — H ; thus X must be an atom of the type O, N, S, etc. Alcohols are the most extensively used initiators, and include monohydric alcohols (i.e., containing 1 OH group), such as butanol, or polyols (i.e., containing two or more OH groups), such as ethylene glycol, glycerol—water can be considered as a dihydric alcohol. Increasing the functionality of the initiator is particularly useful when trying to build high molecular weight, as there are more reactive OH ends for addition, and thus the reaction rate is increased. The higher OH concentration also increases the polarity of the polymer, which is useful for particular applications; the OH value is typically measured using ASTM E326, Standard Test Method for Hydroxyl Groups by Phthalate Esterification. Lower polarity can be achieved by the use of long chain aliphatic or aromatic alcohols; this is an option for achieving mineral oil miscibility of PAGs [28].
The polyether section may contain one or more alkylene oxides. Single oxide derived PAGs ("homopolymers") are widely used and relatively simple to manufacture. When two or more alkylene oxides are used—"copolymers"—these will be incorporated as blocks ("block copolymers," e.g., EO—EO—PO—PO—EO—EO—EO, by sequential reaction of the oxide), or r a n d o m l y ("random copolymers," e.g., PO—EO—EO—PO—EO—EO—PO—EO, by the reaction of a mixture of oxides). These different polyether configurations lead to somewhat different properties. For example, blocks copolymers tend to have worse low temperature properties than r a n d o m copolymers. The principle benefits from including EO are increased water solubility, reduced solubility with non-polar species (e.g., hydrocarbons), and increased stability. The principle benefits from including PO are improved low temperature performance, reduced polarity, and thus increased solubility of lubricant additives a n d nonpolar species. In most PAGs, the end group is a hydrogen atom, and this is adequate for most applications. Reacting this active hydrogen further (endcapping), by etherification or esterification, can yield beneficial effects. Typical end groups of this form include aliphatic and aromatics (by etherification) and alipathic ester groups [29]. Nuclear magnetic resonance (NMR) spectroscopy is a key analytical technique for the chemical structural analysis of PAGs, including the determination of EO/PO ratio, initiator and end group analysis, polymerization sequencing (random/block), and molecular weight [30,31]. Synthesis Most PAGs are manufactured via a three step process involving: catalyzation of the initiator; reaction with the alkylene oxide(s) ('alkoxylation'); and post-treatment to remove the catalyst or adjust the p H (see Fig. 6), the reaction of a n alcohol (ROH) with EO. The initiator is typically reacted with a catalytic amount of a base, such as potassium hydroxide, to produce the alcoholate, RO — K + . The water produced may either be left in at this stage, or may be removed under vacuum. The oxide addition Catalyzation: R-O-H + KOH-
- * R-O-K+ + H2O
Reaction with oxide:
R-OK* + CH2-CH2"
•R-0-CH2-CH,-0-K+
AK R-O-CH2 - CH2 - O- K+ + n-1 (CHj - CH2 ) Post-treatment: R-(0-CH2 - CHj )„ - O - K+ + 'H+'
R-(0-CH2-CH2)„-0-H FIG. 6—Reaction scheme for PAG production.
254
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
reaction is normally carried out in a stainless steel reactor at elevated temperatures (tjrpically 80-150°C) and at high pressures (up to 10 bar). The alcoholate is a strong nucleophile, and this approaches the carbon adjacent to the ring oxygen of the alkylene oxide. An O—C bond is formed, and the epoxide ring is opened; the ring oxygen then acts as the nucleophile for the next alkylene oxide addition. As the oxide is consumed in forming the polymer, more oxide is fed into the reactor, generally at a rate to maintain reactor pressure. A key side reaction that occurs during propoxylation is the formation of unsaturation by the isomerization of the PO unit. This reaction limits the viscosity attainable with pure propoxylates. In terms of reactivity, EO has a higher reaction rate than PO and higher oxides, and reactivity is enhanced by the use of more basic catalysts. Depending on the requirements of the final application, when the poljTiierization reaction is complete, the final product may be subjected to a catalyst removal step. This is achieved by demineralization, using a magnesium silicate, and neutralization with an acid, both of which are followed by filtration. Both techniques work by replacing the metal ion (in this example K"*") with a hydrogen ion (H"*"). Sometimes the catalyst is left in the product, and the product is treated with an acid to adjust its acid value, which is measured by ASTM D 974, Standard Test Method for Acid and Base Number by Colorimetric Titration (DIN 51 559 Part 1); here, no filtration is then used. Typical quality control measurements, carried out during production of PAGs, include determination of viscosity by ASTM D 445, Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (DIN 51 550), density by ASTM D 70, Test Method for Density of Semi-Fluid Bituminous Materials (Pycnometer method), flash point by ASTM D 97, Standard Test Method for Fire and Flash Point by Cleveland Open Cup (DIN 51 376), pour point by ASTM D 92, Standard Test Method for Pour Point of Petroleum Products (DIN 51 597), OH value by ASTM E326, Standard Test Method for Hydroxyl Groups by Phthalate Esterification, and water content by ASTM E 1064, Standard Test Method for Water in Organic Liquids by Coulometric Karl-Fischer Titration (DIN 51 777, Part 1).
PAGs show a broad viscosity range, from a few mm^/s at 40°C up to a few hundred thousand, as a result of the build in molecular weight from a few hundred to several tens of thousands. Over the wide range of PAG viscosity (for a consistent structural t3^e), the trends in the other physical properties tend to vary in a fairly linear fashion. Their key properties are water solubility, and excellent viscosity index (VT) determined by ASTM D 2270, Practice for Calculating Viscosity Index from Kinematic Viscosity at 40°C and 100°C (DIN/ISO 2909) together with very good pour point and flash point. The main route to adjusting the physical properties is by adjusting the initiator and the EO/PO ratio, as shown in Table 1. For example, by changing the initiator from a diol, to a mono hydric alcohol (compare products a and b in Table 1), there is an increase in viscosity index. Likewise, increasing the EO/PO ratio also results in an increase in VI. This is important in applications relying on little change in viscosity over a range of temperatures (e.g., worm gear lubrication). Excellent pour and flash points are maintained across a wide viscosity range, but are again influenced by EO/PO ratio. Increasing the molecular weight leads to increased VI and flash point, but also increased pour point. PAGs may be either water soluble or water insoluble depending on their structure. Increasing the EO content increases the water solubility. Linked with water solubility is the so called "cloud point," (or inverse solubility temperature), which is measured by ASTM D 2024, Standard Test Method for Cloud Point of Petroleum Products—PAGs show inverse solubility in water. Below the cloud point, normal solubility is observed, but at the cloud point, the PAGs come out of solution and eventually separate. The solubility and the cloud point increase as the EO content increases, and decrease with increasing molecular weight. PAGs tend to be soluble in relatively polar materials and their excellent solubility in the ozone benign hydrofluorocarbons, such as Rl 34a (1,1,1,2-tetrafluoroethane), has considerably aided the transition from the chlorofluorocarbons in automotive air conditioning systems [32]. Chemical Characteristics
Physical Properties The physical properties of a selection of commercially available PAGs are shown in Table 1, to illustrate the range of physical properties.
Compared to mineral oils, PAGs have good thermal stability, but poor oxidative stability as shown by thermal gravimetry (23,25,28,30). However, one of the key characteristics of PAGs is that when they decompose, the final decomposition
TABLE 1—Typical physical properties of PAGs (with ASTM methods).
VI
Mol Weight
Density (g/cm^) @20°C
Cloud Point CC)
Flash Point Open Cup ASTM D 92 (°C)
D2270 103 204 184 242 157 225 287 408 489
350 1900 2000 2600 1200 1650 4500 12500 25000
D70 0.9573 0.9940 1.0035 1.0031 1.0951 1.0564 1.0574 1.0908 1.0905
D2024 insol insol insol insol >100 59 53 81 76
D92-97 80 225 230 232 254 230 230 240 240
Viscosity (mm?/s)
Functionality of Initiator
EO/PO Ratio
40°C
100°C
ASTM Method Mono Mono (a) Di(b) Di Tri (c) Mono (d) Mono Di Tri
0:1 0:1 0:1 0:1 3:1 1:1 1:1 3:1 3:1
D445 11 126 142 387 127 132 1050 19500 45000
D445 3 22.5 22.2 65 18 25 180 2400 6500
NOTE: the molecular weight is calculated from Mol Weight = functionality of initiator * 56100/OH value.
Point Pour Point CO D97 -53 -36 -36 -23 -28 -42 -28 4 7
CHAPTER 10: SYNTHETIC products are all volatile, with the result that there is no carbonaceous solid or liquid residue, which can be proven by ASTM D 189-97, Standard Test Method for Conradson Carbon Residue of Petroleum Products. This is a major advantage in many functional fluid applications such as high temperature chain oils and compressor fluids. In the absence of air, PAGs are stable up to around 250°C. At this point polymer chain scission occurs, which releases free radical ends, which further decompose by depolymerization to produce volatile components (eddehydes, ketones, alcohols, alkenes, alkanes, CO2) and lower molecular weight polymers [30]. In the presence of air, PAGs are stable up to around 180°C. Decomposition is initiated by an oxygen molecule attaching itself randomly along the polymer chain. A radical is then formed, which rearranges, causing the polymer chain to break and lose its end group. This again leads to a reduction in molecular weight and production of similar volatile components to the thermal breakdown. By adjusting the chemical composition and by the use of antioxidants, the stability of the PAG can be improved significantly. Increasing the EO content (compare the flash points of product "c" and "d" in Table 1) and increasing the molecular weight of the polymer tend to increase its stability. Aminic and phenolic antioxidants, especially in combination, provide significant increase in stability to PAGs [33]. PAGs have good hydrolytic stability, which is measured by ASTM D 2619-88, Standard Test Method for Hydrolytic Stability of Hydraulic Fluids, since they do not contain hydrolytically labile chemical groups. When released into the environment, PAGs tend to slowly biodegrade, helped by their affinity for water (and thus ability to disperse in the environment) and their ability to oxidize and fragment into smaller and more biodegradable structures [22]. Rapid biodegradability of PAGs, as measured by such tests as the OECD 301 and 302, is not inherent across the range, but, is achievable by careful structural modification, especially towards the lower viscosity end of the range [34]. In general, biodegradability is favored by linear components (i.e., EO rather than PO), and by reduction in molecular weight. In terms of general toxicity, PAGs are generally classified as low hazard, and some have approvals as indirect food contact lubricants [35]. Due to their highly polar nature, which gives them a very strong affinity for metal surfaces, PAGs have excellent inherent lubrication properties [23]. The lubricating film formed between moving metal parts remains intact even during very difficult operating conditions, such as high temperatures and loads. However, their polar nature does require that care must be taken in the choice of compatible materials, particularly paints and some elastomers [25], which is determined by ASTM D 471-98el, Standard Test Method for Rubber Property-Effect of Liquids. Formulations using standard additive chemistry can also be hampered by solubility problems and potential reduction in activity—the PAG may compete for the metal surface with the additive. Increasing the molecular weight and the PO content does improve this situation by reducing the polarity. Their hygroscopicity requires care to be teiken regarding ferrous metal corrosion, which is measured by ASTM D 665-99, Standard Test Method for Wear Preven-
LUBRICANTS—NON
AQUEOUS
255
tive Characteristics of Inhibited Mineral Oil in the Presence of Water. Anti-corrosion agents are available. Application and Performance Characteristics Due to the flexibility in their chemical structure and properties, and their inherent lubricity, PAGs are used in a wide range of application areas, as illustrated by Fig. 7. Very high viscosity PAGs are used extensively in HFC (Hydraulic Fluid C type) water based fire resistant hydraulic fluids [36,37] where they perform the function of thickening the water, suppressing its pour point, and providing lubricity determined by ASTM D 2882, Standard Test Method for Indicating the Wear Characteristics of Petroleum and NonPetroleum Hydraulic Fluids in Constant Volume Vane Pumps. The water provides the fire protection [38]. The key parameter is fire resistance and the performance standards are developing apace [39,40]. A similar lubricity improving function is delivered by PAGs in water based metal working fluids, which additionally use the cloud point phenomenon of the PAG to boost lubricity: as lubricity of the fluid begins to fail, metal-metal contact causes the temperature to increase, which heats the solution above the cloud point of the PAG, which is determined by ASTM 2024-65, Standard Test Method for Cloud Point of Nonionic Surfactants. This temperature rise results in the PAG coming out of solution and being released into the contact to provide enhanced lubricity. The low coefficient of friction and excellent viscosity/temperature or VI characteristics of the PAG lend it to the lubrication of the gecirs and bearings, particularly to heavily loaded worm gears used within the plastics, rubber and paper industries [41-43]. Tests for gear oil performance include the FZG Test [DIN 51 354] and ASTM D 2782-94, Standard Test Method for Measurement of Extreme-Pressure, better known as the Timken Test. The efficiency of the worm gear is related to the friction between the wohn drive and the gear wheel. Contact between these involves a high level of sliding resulting in increases in operating temperatures—ideal operating conditions for the PAG. Combining this with their high level of tolerance for water ingress delivers excellent performance. The compression of process gases presents a unique technical challenge [44]. Due to the intimate contact between lubricant and gas, there can be dissolution of the gas into the lubricant, resulting in a reduction in viscosity of the lubricant, and as a worst case, washing of the lubricant from the metal surfaces requiring lubrication. Owing to the polar nature of PAGs, they perform extremely well in the lubrication of compressors for low polarity process gases (e.g., methane, ethylene, nitrogen). Since there is very little dissolution of the gas into the lubricant, the viscosity and lubricity is maintained [44]. PAGs are also suitable for use as air compressor lubricants, especially when combined with esters [46,47]. Since any build up of sludges or deposits in the compressor can lead to ignition of the lubricant (for the proof of low carbon residues ASTM D 189-97 Standard Test Method for Conradson Carbon Residue of Petroleum Products can be used), the clean decomposition characteristics of the PAG (ASTM D 189) make this a very attractive alternative to mineral oil based air compressor lubricants.
256 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK
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CHAPTER 10: SYNTHETIC LUBRICANTS—NON AQUEOUS 257 POLY (ALPHA-OLEFINS)
/-I
Polyalphaolefiin fluids, or PAOs, are synthetic, saturated hydrocarbons that are manufactured by a two-step process from hnear alpha-olefins, that are themselves manufactured form ethylene. These synthetic hydrocarbons are generically described by their viscosity at 100°C. The most common commercially used PAOs are 2, 4, 6, 8, and 10 cSt and the higher viscosity grades 40 and 100 cSt. PAOs have excellent physical properties when compared with conventional mineral oils, a wide operating temperature range, including high flash and firepoints, high viscosity indices and low volatilities. When compared with certain natural and synthetic esters, PAO fluids have excellent thermal, oxidative, and hydrolytic stability. Since the mid 1980s PAOs have quickly gained market share in the synthetic lubricant base fluid market, particularly in Europe [48]. The major application area was and continues to be the automotive sector for crankcase oils answering the quest for tighter specifications for lower oil volatility. Today, automotive crankcase lubrication is still the main application area for PAOs globally. Other automotive application sectors are now two-stroke cycle engine oils, automatic transmission oils, gear oils (multigrade), and greases. Industrial applications include: hydraulic oils, compressor oils, heat transfer fluids, and food grade oils and greases. The milestones of the technical history are marked by three patents, one by Brennan [49] at Mobil Oil in 1968 and two by Shubkin [50,51] of Ethyl Corporation in 1973. Brennan first described a process for oligomerization of alpha-olefins using a BF3—ROH catalyst system [49] whereby the combination of the selected process conditions and the catalyst system yielded a product consisting of a mixture of oligomers with a high concentration of trimers. Shubkin showed that instead of R—OH as a co-catalyst, H2O [50] or alcohols and carboxylic acids [51] could be also used in combination with BF3 to produce oligomers of uniform quality. The U.S. Army and U.S. Navy took an early interest in this new synthetic base fluid. In July 1970 the MIL-H-83282 specification for fire resistant hydraulic fluids based on PAOs was established in cooperation with the industry [52]. Chemistry and Meinufacturing As the name implies, Polyalphaolefines are synthesized up from alphaolefines. The starting material of the chemical
CH2 = CH2
Catalyst >•
BF3
Unsaturated
R-CH=CH2
>•
ROH
Unsaturated Oligomers + H2
aigomeric Mixture
Dimer Trimer Tetramer Pentamer Higher
NIorPd
DIstilatlon
Oligomers
DImer Trimer Tetramer Pentamer Higher
Saturated Paraffinic Hydrocarbons
Viscosity Grades
FIG. 8—Chemistry of PAO manufacture [55].
Qimsr
FIG. 9—Typical PAO components [55].
synthesis is ethylene as shown in Fig. 8. The reaction products of the first step are linear unsaturated alphaolefins (LAO). These LAOs are used for the manufacture of a variety of chemicals, mainly as detergents. For the synthetic lubricant base fluids (PAOs 2-10 cSt), mainly 1-Decene is oligomerized with the catalyst BF3 and a protic co-catalyst such as water or an alcohol. For the higher viscosity PAO 40 and PAO 100 Ziegler-Natta (Aluminium chloride based), catalysts are used. The unsaturated oligomer mixture is hydrogenated using either Nickel or Palladium catalysts. Finally, a distillation step is applied to remove unreacted monomers and to separate the various product grades. Sometimes the distillation step is carried out prior to hydrogenation [54]. The tjrpical molecular structures of a dimer, trimer and a tetramer of 1-decene are shown in Fig. 9. Every oligomer is branched and is present as a number of different isomers. Though 1 -decene is the pre-dominantly used alphaolefine for PAO manufacturing, shorter or longer chain length olefines may also be used. In particular, 1-dodecene has recently gained importance for the synthesis of PAO 5 and PAO 9 [57]. Tailor made products can also be obtained by changing reaction vsiriables such as temperature, time, catalyst concentration, co-catalyst t3^e, and concentration and distillation conditions [57]. Physical Properties The physical properties of the common commercially available PAOs are shown in Table 2. The given properties are typical values and do not represent values for PAOs from a particular supplier. The low viscosity PAOs 2-10 have excellent low temperature properties which make them very suitable for applications in cold climate. The average viscosity index (VI) calculated by ASTM D 2270, Practice for Calculating Viscosity Index from Kinematic Viscosity at 40°C and lOOX (DIN/ISO 2909) for PAOs is 135. This has the advantage that the viscosity changes much less with increasing temperature compared to a product with low VI. For PAO 2 no VI is given because VI is undefined for fluids having a kinematic viscosity ofless than 2.0 cSt at 100°C. The advantage of a high VI is that addition of viscosity index improvers is not required for the formulation of lubricants for many applications. Also, the addition of pour point depressants is often not necessary as the pour points for PAOs are very low. In Table 2 some other physical properties are given which are important to the lubricant formulator. The flash point is important for safety reasons and the given flash points in Table 2 are at least as high
258
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
TABLE 2—Properties of polyalphaolefines. Parameter
Test Method"
PA0 2
PA0 4
PA0 6
PAO 10
PAO 40
PAO 100
KV @ 100 °C cSt KV @ 40 °C cSt KV @ - 4 0 °C cSt Viscosity Index Pour Point, °C Flash Point, °C Noack, % Loss
ASTM D445 ASTM D445 ASTM D445 ASTM D2270 ASTM D79 ASTM D92 DIN 51581
1.80 5.5 310
3.90 16.8 2540 122 -69 215 12.0
5.90 31.0 7800 137 -63 225 6.7
9.60 45.8 19000 134 -54 264 2.0
40.0 395
100 1250
150 -34 280 0.8
170 -20 290 0.6
-63 >155 99
"ASTM D 6375 and D 5800 are alternate procedures.
TABLE 3—The oxidative stability of motor oil qualities studied using High Pressure Differential Scanning Calorimetry (PDSC). Additive-free Base Oils
Mineral oil based lube Synthetic lubricants Polyalphaolefin Polyolester Diester 1 Blends of synthetic lubricants Polyalphaolefin/polyolester 80/20 blend Polyalphaolefin/diester 80/20 blend Base oils with additive Mineral oil based lube Mineral oil/polyalphaolefin based lube 55% mineral oil/22% PAO Mineral oil based diesel engine lube Polyalphaolefin/esteroil 80/20 blend
T (onset) [°C]
187 187 210 198 196 196 254 260 262 274
as those of mineral oil of the same viscosity. Volatility is measured by the standard NOACK volatility test at 250°C for 1 h. Except for PAO 2, all other PAOs have very low volatility, which makes them very suitable for high temperature applications and engine oils to reduce the need for "topping-up." Low volatility is also an important property for a fluid to retain the original viscosity during its working life. The highly viscous PAO 40 and PAO 100 hsted in Table 2 are similar to low viscosity PAOs and have good viscometric properties and allow operation over a broad temperature range. New PAOs with m e d i u m viscosity grades 5, 7, and 9 and very high viscosity grades, u p to 3000 cSt, and have recently been developed as custom-synthesized products [56]. Chemical Properties and Performance Characteristics Oxidation stability is one of the most important properties of automotive lubricants, which are mainly responsible for the oil renewal time. Essiger [57] investigated the oxidation stability of different motor oil qualities with High Pressure Differential Scanning Calorimetry (PDSC) as shown in Table 3. PAO without anti-oxidants is as stable as mineral base oil, blends with synthetic ester, a n d shows superior oxidation stability. In the presence of anti-oxidants, such blends were significantly more stable than mineral oil based engine lubes. The use of PAO is also c o m m o n today in high performance greases. The lifetime of such greases is said to be three to five fold c o m p a r e d to mineral oil based products. Wunsch recommends operating temperatures of u p to 150-160°C for Lithium-12-hydroxystearate greases based on PAO [57].
Low toxicity in general, and biodegradability of the low viscous PAOs according to the CEC L-33-A-93 test, are other important benefits making these products versatile for incidental food contact and environmentally acceptable lubricants
OTHER SYNTHETIC BASE STOCKS Although mineral oils and the synthetic base fluids described in the sections above, together with solid lubricants, satisfy the performance requirements of the majority of lubricating applications, there r e m a i n m a n y applications where a different combination of properties would be desirable. A wide variety of other chemical classes have been proposed as candidate synthetic base fluids to cover the real or perceived shortcomings of more established fluids. Many of these developments have been targeted towards high performance aerospace applications, e.g., military aircraft and satellites, where satisfactory performance under extreme conditions of t e m p e r a t u r e , high vaccuum, a n d high radiation m a y be required, and unit cost is less significant [3]. Most of the proposed chemistries, e.g., silicate esters, silahydrocarbons, titanate esters and phosphazines, appear not to have found any real application. However, others, in particular silicones, perfluoroalkyl ethers, polyphenyl ethers and cycloaliphatic hydrocarbons have found significant niche applications and are discussed briefly in this section.
Silicones Silicones are polymers containing the siloxane (Si—O—Si) backbone structure with pendant alkyl side chains, normally methyl groups. The chemistry is well established and silicone fluids have been commercially available since the 1940s [59,60]. They are manufactured by hydrolysis of dialkyldichlorosilane themselves p r e p a r e d by reaction of methyl chloride (or other alkyl chlorides) with silicon metal. Trialkyl monochlorosilanes are introduced into the reaction in controlled stoichiometry to act as end capping reagents and control the molecular weight. Silicone fluids can be prepared with viscosities ranging from < 1 to > 500 000 cSt. They are characterized by low pour points, low surface tension, high compressibility and little change in viscosity with temperature. The standard viscosity index calculation is not appropriate for silicones and other materials with such low temperature coefficients of viscosity. Methyl groups m a y be substituted by other functional groups to modify the inherent properties of the basefluid.
CHAPTER 10: SYNTHETIC These substituents include phenyl groups (for improved oxidative stability) and trifluoropropyl or tetrachlorophenyl groups (for lubricity) Silicones are highly fire resistant and have very low volatility, good thermal stability, and very good chemical resistance. This makes them well suited to some highly demanding functional fluid applications, e.g., heat transfer oils and transformer dielectric fluids. However, the load bearing characteristics of poly (dimethyl siloxane) Eire very poor and metal contacts lubricated by silicones tend to seize under boundary lubrication conditions [59]. For this reason, simple silicones are rarely used as base fluids, except in some speciality greases where they serve as stable carriers for additives and solid lubricants. Such greases are used in aviation and automotive industries to lubricate linkages, bearings and bushings, and instrument components [3]. Silicones are also used in textile applications as fibre, thread and yam lubricants, and modified silicones are used as textile treatments for improved resistance to staining. Traditional lubrication applications of silicones are further limited by the fact that they are not generally miscible with mineral oils, or other synthetic base fluids. This does, however, make them suitable for use as both profoaming and antifoaming additives in more conventional base fluids. Use of silicone fluids or additives in some manufacturing environments where goods are painted or coated is discouraged because minute traces of silicones can interfere with the spreading and adhesion of paints. Silicone fluids are essentially non biodegradable and although of low toxicity, they are likely to be persistent in the environment. Consequently, the environmental impact of dispersive applications is a cause of some concern. Perfluoroalkyl Ethers (PFAEs) PFAEs have structures basically similar to those of PAGs, but with all the hydrogen atoms replaced by fluorine. They were originally developed in the 1960s for aerospace applications and have subsequently found some use in other applications, particularly where resistance to oxidation or other chemically aggressive environments is required. Four different classes of PFAEs have been commercialized, for which the abbreviations D, K, Y, and Z have become generally accepted. These differ somewhat in their chemical structure, and consequently show slight differences in performance, but all show broadly the same characteristics as compared to other classes of base fluids. PFAE-K and PFAE-Y have quite similar structures, consisting of a —(CFa—CF(CF3)—O—) repeat unit, although with different manufacturing routes, which lead to some differences in properties. PFAE-K is prepared by anionic polymerization of hexafluoropropylene oxide, whereas PFAE-Y is prepared by polymerization of hexafluoropropene in the presence of oxygen. PFAE-Z contains a —(CF2—CF2)—O—) repeat unit and is prepared similarly to PFAE-Y, using tetrafluoroethene, rather than hexafluoropropene. PFPE - D contains a —(CF2—CF2—CF2—O—) repeat unit, and is manufactured by ring-opening polymerization of tetrafluorooxetane, followed by exhaustive fluorination to convert the remaining C—H bonds to C—F bonds.
LUBRICANTS—NON
AQUEOUS
259
All types are manufactured as poly disperse polymers, which are then fractionated to give the molecular weight and viscosity ranges of interest. PFAEs are available with viscosities ranging from about 5 to about 500 cSt 40°C. Viscosity indices vary according to the chemical type, with the K and Y types having lower VT than the D and Z types. Pour points are generally low. High temperature volatilities depend on the molecular weight spread of the particular grade, but are generally low [60]. The main chemical characteristic of PFAEs is outstanding resistance to oxidation by air. They are also resistant to chemical attack by corrosive chemicals including strong acids and alkalis and oxidants such as fluorine and hydrogen peroxide. As oxidative degradation is one of the most frequent limiting factors for lubricant life, this means that PFAEs can have very long lifetimes in service, and are well suited for sealed for life applications. Like silicones, PFAEs have a tendency to exhibit seizure under boundary conditions, and, also like silicones, they are not miscible with standard lubricant additives, so the problem cannot be alleviated by formulation with conventional antiwear agents [3]. Thermal stability of PFAEs under ideal conditions in contact with glass is very good, but the benefit is often not realized in real life applications as thermal decomposition is catalyzed at lower temperatures by metcil fluorides, particularly those of aluminium and ferrous metals. Metal fluorides are apparently produced by reaction of PFAEs with metal oxides under conditions of tribological contact. The same effect can lead to oxidative corrosion of some metals by PFAEs under air. Both thermal decomposition and oxidative corrosion are inhibited in the absence of oxygen [59]. The very high cost of PFAEs has restricted their use to high value applications where their properties are essential or where greatly extended oil life can justify their use on economic grounds. The initial applications were in spacecraft, where the very high chemical resistance of PFAEs was required. The largest current application is in specialty vacuum pump oils for use in contact with reactive chemicals in electronics manufacture. PFAEs are cJso used in formulation of greases for some sealed for life bearings.
Polyphenyl Ethers Polypheny] ethers (PPEs) consist of benzene rings joined by ether links, with the ether links in bridging monomer units being arranged in meta geometry. They were developed during the 1950s for high thermal, oxidative, and radiation stability [61]. PPEs are manufactured by reaction of phenols and halides. The simplest member of the family is diphenyl oxide; longer chain analogues are available up to six benzene rings and are coded according to the number of benzene rings and ether groups they contain; thus 5P4E contains five benzene rings and four ether linkages. PPEs have very high pour points and most are not liquids at room temperature. For example, the only example which is currently commercially available, 5P4E, crystallizes at 43°C in the pure state, and therefore must be mixed with other fluids to extend the liquid range and inhibit crystallization, or used only in applications where it will re-
260
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
main at elevated temperature [62]. In addition, PPEs have very low viscosity indices, so it is necessary to accept high viscosity at lower temperatures in order to have adequate viscosity at the elevated temperatures of operation. [3] PPEs are also characterized by high surface tension (ca. 50 dyn/cm, as compared to 30 dyn/cm for typical mineral oil). This means that they do not wet metal surfaces, or migrate from the point of application. Thermal stability of PPEs is good, but oxidative stability is not outstanding. PPEs have been used as aviation gas turbine lubricants for supersonic military aircraft [61]. The main current application for PPEs is as ultra-high vacuum diffusion pump fluids, where lubricity is not important and the good high temperature characteristics can be exploited. They are also used in formulation of radiation-resistant greases. Alkylated Cyclopentanes Multiply-alkylated cyclopentanes (MACs) were introduced as candidate lubricating fluids during the 1980s. MACs are manufactured by reaction of cyclopentadiene, in the form of dicyclopentadiene, with an alcohol, followed by catalytic reduction of alkylated cyclopentadiene initial product [63,64]. A wide range of MACs have been reported, although apparently only one member of the class, tris(octyl dodecyl) cyclopentane, has been commercialized. Viscometric properties are typical for hydrocarbons and similar to those of PAOs. The main distinguishing characteristic of MACs is that they are essentially monodisperse and their volatility is therefore substantially lower than mineral oil or PAO of similar viscosity. Viscosity and viscosity index can be adjusted by appropriate selection of the chain length and degree of branching of the alcohol raw material, and by the degree of substitution obtained. MACs are miscible with mineral oil and with standard lubricant additives. Tris(octyl dodecyl) cyclopentane has a particularly low pour point and low volatility, and is used in spacecraft applications [63]. Cyclohexane Derivatives The cyclohexane derivatives are a class of fluids which are targeted, not at high temperature applications, but rather at automotive transmission applications requiring high traction. A range of such fluids were commercialized during the 1970s, although only one such material is currently available [65], namely 2,4-dicyclohexyl-2-methylpentane, which is manufactured by catalytic hydrogenation of the linear dimer of alpha methyl styrene [63]. In most respects, the cyclohexane derivatives behave similarly to other synthetic hydrocarbons of similar molecular weight, but have the distinctive characteristic that they exhibit high traction coefficients under the very high pressures experienced in an elastohydrodynamic (EHD) contact [63]. It is believed that the rigid regular structure of the cyclohexyl component leads to a very high volume change on melting, and consequently a very strong pressure dependence of the temperature of solidification. Thus, under extreme pressure, these fluids transiently solidify in the contact, forming a solid pad in the contact, which readily transmits lateral force and resists formation of shear slip planes.
SUJVIMARY Synthetic lubricants possess superior performance capabilities compared to mineral oils. There are three main reasons why synthetic lubricants are selected in preference to mineral oils: • improved stability, usually oxidative, • improved lubricity, usually at very high or very low temperatures and • reduced environmental impact, usually high biodegradability or reduced toxicity. Oxidative
Stability
The superior oxidative and thermal stability of synthetics over mineral oil is historically the main reason why they are used. It is very difficult to give precise figures for the exact temperature at which decomposition for a synthetic lubricant occurs as this is affected by several variables, the main ones being: • exposure time • tj^e and amount of additives present (antioxidants, metal passivators/chelators etc.) • availability of oxygen (air) • ability of the lubricant to remove heat • which metals the lubricant is in contact with (catalysis and potential deposit formation at metal surfaces) and • the presence of system contaminants (e.g., acidic components) There are several different ways of measuring the oxidative stability of a lubricant: • Decomposition temperature as measured by a change in: viscosity, acid value, hydroxyl value (ASTM D 943, Turbine Oil Stability Test), amount of oxygen consumed (ASTM D 2272, Rotary Bomb Oxidation Test), heat flow change (Differential Scanning Calorimetery), etc., under specified conditions • Temperature and nature of deposit formed on decomposition (e.g.. Panel Coker FTM3462 or Wolf strip tests UK 359, ASTM D 189, Conradson Carbon Residue, DIN 51 551, Pneurop DIN 51352-2) • Volatility of a lubricant at set temperature and times (e.g., ASTM D 5800, Noack test, Thermogravimetric Aneilysis) The life of oil at a particular temperature depends on the amount and type of degradation, which is acceptable, which in turn depends on how much performance can be allowed to deteriorate. Any chart comparing the relative stability of synthetic lubricants will therefore be quite arbitrary and highly dependent on the specific oil, test conditions, and application [65]. For example, oxidation tests on pure basestocks can show that esters have an oxidative stability similar or slightly worse than that of mineral oil. The reason for this is that mineral oil contains impurities that can act as anti-oxidants. Esters tend to only show their remarkably better stability when compared to mineral oil, provided the oils are formulated with antioxidants. Figure 10 gives an indication of oil life versus temperature for a range of formulated synthetic lubricants. Useful lifetimes are based on filed experience from a range of applications and should only be used as a crude approximation of actual service life in a specific application. As the previous dis-
CHAPTER 10: SYNTHETIC LUBRICANTS—NON AQUEOUS 261 cussion has shown, oil hfe will change dependent on the exact temperature regime. Table 4 gives a useful qualitative overview of the volatility a n d deposit forming tendencies of various oils. Depositforming tendencies of synthetics can be highly dependent on several p a r a m e t e r s not necessarily connected with chemistry. For example, lubricants with a high polarity can help solubilize and disperse decomposition products leading to lower deposits. Polymers should have a narrow dispersion of molecular weight to avoid the lower molecular weight component volatilizing. Polymers such as Polyalkylene Glycols (PAGs) and Poly Iso Butylene (PIBs) tend to be very stable u p to a certain t e m p e r a t u r e a n d then rapidly degrade. The volatility and deposit forming tendencies can be highly dependent on the presence of metals. Metals act as a catalyst a n d therefore aid decomposition for specific chemistries.
Their volatility is therefore highly dependent on the test temperature. The types of additives used also play an important role. Therefore, the table should only be used as a rough guideline. The generation of heat from friction causes the temperature of the oil film to increase. This higher temperature reduces the viscosity of the oil. As the oil's ability to remove heat is increased, this may lead t o lower operating temperatures. A lower temperature will reduce the decrease in viscosity of the lubricant and also reduce the oxidative degradation of the lubricant, potentially increasing the life of other components in the system. The heat transfer of various lubricants can be compared by using a simplified version of the Sieder and Tate equation given below [69]. This equation is applicable to areas of turbulent flow. The equation can be further simplified for areas of laminar flow. LrO.bl pO.8^0.33
ha snnHEtic HnmocAfaoNS Where: h = K = p = X= Cp =
SUGON^
too
aoo
300
TEMPERA-niRE ' C FIG. 10—A comparison of oil lifes for a range of synthetic lubricants [66].
TABLE 4—An overview of the volatility and depo forming tendencies of a range of synthetic lubricants evaluated using a variety of tests. Lubricant Mineral oil WON PA0 6 Alkyl benzene 150 Esters PAGs PIBs Silicones Fluorocarbons
Volatility at 250 "C Poor Good Fair Very good Poor Very poor Excellent Good to excellent
Deposit Formation at 250°C Poor Fair Fair Excellent Very good Good Very good Very good
heat transfer coefficient of lubricant components thermal conductivity density viscosity specific heat capacity at constant pressure
Lubricants with good heat-transfer characteristics generally have high specific heat capacity, high thermal conductivity, high density, and a low working viscosity. T5rpical data for several lubricant classes are given in Table 5. PAGs and polyol esters, due to their polar nature and their superior lubricity, should be able to lubricate the system at lower bulk viscosities than their mineral oil equivalents, improving heat transfer still further [69]. Lubrication As with oxidation, the lubricity of a synthetic lubricant is highly dependent on the operating regime. Most wear tends to occur during; start-up, slow-down to stop, overheating, or overloading. Polar lubricants such as esters have greater affinity for metal surfaces than mineral oil and are less likely to drain to the sump [69]. Such lubricants are therefore more likely to maintain a lubricant film on start-up. Low temperature viscosity is also an important technical criterion. Cold starts, for instance, is the prime cause of engine wear and an effective lubricant film can be only be maintained by immediately effective lubrication circulation. Lubricants that have poor low temperature flow properties can take a significant time to reach the parts that require lubrication. For automotive applications, this can be evaluated using the cold crank
TABLE 5-
Lubricant Type
Specific Heat Capacity at100°C Calg-' °C^' (ASTM E 1269)
Thermal Conductivity at 100°C m W m " ' °C"'
Density at 100°C gcni~^ (ASTM D 70)
Viscosity at 100°C in cPs (ASTM D 445)
Lube Heat Transfer Coeft.
Mineral Oil PAO 6 [67] Polyol ester [68] PAG
0.52 0.55 0.55 0.46
127 144 150 185
0.82 0.77 0.93 1.02
6.49 4.54 5.58 7.75
7.33 9.14 9.91 9.21
262 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK simulator test (ASTM D 2602). An example of the benefits synthetics can bring is given in Table 6. The viscosity of a lubricant has a marked effect on wear (viscosity being related to film thickness). Viscosities of lubricating oils are often quoted at 40°C (ISO grade) or 100°C (ASTM D 445). In reality the viscosity u n d e r operating conditions is the controlling factor. Provided the stability of the lubricant is sufficient, the ability of the lubricant to resist viscosity dilution is dependent o n the Viscosity Index (ASTM D 2270) and the ability of the lubricant to remove heat. The viscosity index provides a measure of the rate of reduction of viscosity with temperature. The viscosity pressure coefficient measures the rate of viscosity increase with increased load. The ability of the lubricant to resist overloading is therefore highly dependent on the viscosity pressure coeffecient of the lubricant. Table 7 gives an overview of the viscosity indices and viscosity pressure coefficients for a range of synthetic lubriccints. In certain compressor applications, a proportion of the gas u n d e r compression can become dissolved in the lubricant (e.g., refrigerants, hydrocarbons), thereby reducing the lubricant viscosity below the recommended viscosity required for lubrication. This problem can be solved to a certain degree by using high viscosity grade lubricants (up to ISO 680 mineral oil). However, at low temperatures these lubricants are difficult to p u m p . In addition, gas streams may wash the lubricant off the cylinder walls, resulting in wear. PAGs and PIBs have been used in hydrocarbon applications because of their ability to resist this dilution. Where good solubility of the lubricant is required, e.g., refrigeration applications, the excellent lubricity of PAGs and polyol esters has been used to compensate for the reduction in viscosity. Table 8 gives Ein TABLE 6—Review of cold crank simulator viscosities for fluids with a viscosity of 4cSt fluids at 100°C.
Lubricant
Viscosity at 100°C icSt) (ASTM 445-97 DIN 51550)
Cold Crank Simulator Viscosity at -25°C (MPa.s) (ASTM D 2602) DIN 51377)
Mineral Oil SN 100 PA0 4 Polyol Ester
3.8 3.9 4.5
1300 500 550
TABLE 7—^Viscosity indices and viscosity pressure coefficients for a range of synthetic lubricants. Viscosity
Type of Lubricant
Index (ASTM 2270)
Naphthenic Mineral oils Parafflnic Mineral oils PAO DI & Tri esters Polyol esters PAGS Alkyl benzenes
0-80 80-120 120-150 50-150 50-170 150-280 <0-110
Viscosity Pressure Coefficients (GPa~" 18.0™ -is.ot^" -10.5^"] -5.3[™] -5.3^=5 _7 7[72] _11.8™--
36.0^^" 23.0C72] 12.6™ 19.9™ 21.1™ 19 jpo] 33.4™
TABLE 8—Field experience on the suitability of synthetic lubricants with a variety of gases. Lubricant
Mineral Oil PAOs Esters PAGs
Suitable
Air, Hydrocarbons, CFC Air, Hydrocarbons Air, CFC, HFC, HCFC, CO2 Air, Ammonia, HFC, CO2
Not Suitable
HFC/HFCs, CO2 HFCs Ammonia Hydrocarbon
overview of the suitability of a variety of gases in a range of synthetic lubricants. The polarity of the lubricant can also be very important. Recent work suggests that when a small amount of a high viscosity polar lubricant (ester) is added to a low viscosity non-polar base fluid (PAO), the polar component will preferentially stick to the surface [76]. When the two metal surfaces are far apart, the bulk viscosity is controlled by the PAO. When the surfaces come closer together, the PAO is squeezed out of the contact zone. The polar ester sticks to the surface and stays in the contact area. As the ester has high viscosity, the bulk viscosity of the oil will increase as the surfaces come closer together. As viscosity of the lubricant is reduced, or if shear rate or load is increased, the chance of boundary lubrication is increased. This is especially true under conditions of start-up where the lubricant film may not yet have formed [77]. The general properties of the lubricant that affect boundary lubrication are: the degree of branching/aromaticity, molecular weight, polarity, a n d additives present in the lubricant. Polar molecules are very effective boundary lubricants as they tend to form physical bonds with metal surfaces (i.e., they stick to the surface better than mineral oil). Because of these different interactions, the lubricity of a polar lubricant in a fully formulated fluid is not always easy to predict. Poleir synthetics such as esters and PAGs can compete with antiwear or EP agent for the metal surface. When polar basefluids are used they can cover metal surfaces in preference to the antiwear additives. This can result in higher wear characteristics because, eJthough esters have superior lubricity properties to mineral oil, under high load conditions they are certainly less efficient than antiwear additives. It is therefore very important to choose the cortect additive and to optimize its concentration to get the full lubricity benefit of using polar basestocks. Often, more polar antiwear agents or the same antiwear agent at a higher dose rate is used to offset this factor. Alternatively, the lubricant can be modified to decrease its polarity. The use of bench tests is an attractive approach to wear testing. Their low cost, short duration, and ease of operation make them a desirable research tool. Many wear tests only evaluate a particular wear regime. For example, the Shell four ball (ASTM D 4172) and Falex (ASTM D 2670/D 3233) tests measure the boundary weeir while the FZG (CEC L-071-71) and four ball (ASTM D 2783) tests look at failure load. Due to this narrow assessment, there has been justifiable reluctance to rely on lubricant performance data obtained from the use of bench wear tests. Nevertheless, they are useful as an initial screening tool to spot crude trends that can later be checked by system trials. Environmental
Acceptability
In 1985, some 4.5 million tons of lubricants were used in the EEC. Of these, some 2.5 million tons were consumed in service, leaving 2.0 million tons of waste oil. Of this waste oil, 0.7 million tons were re-used as a fuel supplement and 0.7 million tons were recycled, leaving 0.6 million tons or 13% unaccounted for [79]. This loss is the equivalent of one Exxon Valdez disaster per m o n t h [80]. Increasing interest is therefore being taken in the ultimate environmental impact of the lubricant.
CHAPTER Eco-labels for lubricant applications are starting to appear in both the United States and Europe. Initially these have been focused a r o u n d hydraulic fluids (Blue Angel in Germany, Nordic Swan in Scandinavia, and ASTM D 6046 in the U.S.) but this is likely to be extended to other application areas in the future. The schemes are voluntary and have been set u p to allow products to be differentiated using an agreedupon environmentally friendly labelling classification. The biodegradability of the lubricant is an important part of the legislation and can be measured by a variety of different test methods (CEC-L-33-A-94, OECD 301B, ASTM D 5864, etc.). The biodegradability of a variety of synthetic lubricants is shown in Table 9. Even within a class of synthetics, the biodegradability can cover a large range. Different biodegradability tests can also give very different results for the same chemistry. Results can also be highly dependent on where the activated sludge used in the tests is obtained from. However, in general high viscosity, aromaticity, and a high degree of branching all have a negative impact on biodegradability. ASTM D 6006 and D 6384 are useful guidelines to biodegradability testing and the terminology used to describe the results. Food and Drug Administration (FDA) approved low toxicity lubricants are required increasingly for applications connected to the food industry. The FDA food approval system is undergoing major changes and will be replaced shortly by a new system. However, the old FDA classification system is still being used. See Table 10. The cost of registration of a new lubricant for food use can be considerable. For many of the lubricants, several limitations of use are listed, such as maximum dose rate or effect (e.g., can only be used at a dose rate of bellow 0.5% in another FDA approved oil). Relative
Cost of
Synthetics.
Synthetic lubricants are made from a range of relatively expensive raw materials via a whole range of chemical interacTABLE 9—Biodegradability of synthetic lubricants. Lubricant Type
% Biodegradability by CEC-L-33-A-94 Test After 21 Days
% Biodegradability by OECD 301B Test After 28 Days
Mineral oil PAOs Diesters Aromatic Esters Polyol Esters Alkyl Benzenes PAGs Polybutenes
10-45 20-80 75-100 0-95 0-100 5-20 5-70 5-20
10-40 5-60 25-80 5-45 0-80 0-20 5-80 0-20
TABLE 10—An overview of FDA incidental food contact approval (CFR 178.3570). Lubricant Mineral oil PAD Ester PAG Polybutene
FDA Registered No Yes Glycine ester (CAS No 110-25-8) and Isopropyl oleate Several water soluble and insoluble PAGs > RMM = 1000 Hydrogenated polybutenes listed under 178.3740
10: SYNTHETIC
LUBRICANTS—NON
AQUEOUS
263
TABLE 11—Relative cost of synthetic lubricants versus mineral oil. Lubricant Mineral oil PAOs Esters PAGs PIB Alkyl benzene Silicones Polyphenylethers Fluorocarbons
Relative Cost to Mineral Oil 1 2-5 2-10 2-6 2-4
2-3 10-50 50-250 75-300
tions and transformations. Synthetics will therefore always cost more than mineral oil. Their relative prices as opposed to mineral oil are given in Table 11. Although more expensive than mineral oil, synthetics can lead to a major reduction in system production and running costs and thereby quickly repay their initial costs. For example, synthetics can lead to reduced lubricant consumption, lower maintenance and less plant downtime, reduced disposal costs, and longer equipment lifetimes. The growth of mineral oil has been stagnating over the last few years. The higher performance, reduced environmental impact, and the increasing potential to reduce costs using synthetics has allowed them to show strong year on year growth in many sectors. In short, the commercial importance of synthetic lubricants is set to increase significantly in the future.
ASTM STANDARDS No. D70 D86 D91 D92 D93 D94 D97 D 130 D 189 D445 D471 D524 D664 D665 D873 D892 D 892 (Option A) D943 D972 D974 D D D D
1209 1218 1298 1401
Subject Density Distillation Precipitation n u m b e r (Sludge Formation) Flash Point - Cleveland Open Cup Flash Point - Pensky-Martins Saponification Number Pour Point Copper Strip Corrosion Carbon Residue - Conradson Viscosity - Kinematic Elastomer Seal Compatability Carbon Residue - Ramsbottom Acid Number, Potentiometric Rust Prevention Potential Residue Foaming Characteristics (Sequence IHI) Foaming Characteristics (Sequence IIII) (Option A) Turbine Oil Stability Test (TOST) Volatility - Evaporation loss Acid/ Base N u m b e r by Color Indicator Titration Color - APHA Refractive Index Density Emulsion Characteristics
264 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK D 1500 D 1744 D 1748 D2070 D2270 D2272 D2500 D2602 D2619 D6375 D2670 D2711 D2780 D2782 D2783 D2983 D3233 D3427 D3827 D4052 D4172 D4739 D4742 D5001 D5133 D5185 D5191 D5481 D5800 D6079 D6082 D 6082 (Option A) E326 E 1064 E 1269
Color - ASTM Water Content by Karl Fisher Rust Protection Thermal Stability Viscosity Index Rotating B o m b Oxidation Test Cloud Point Cold Cranking Simulator Hydrolytic Stability (93°C) Volatility - NOACK at 250°C / TGA method Falex PinA^ee Demulsibility Characteristics Gas Solubility Timken OK Load EP, 4-Ball E P Viscosity -Brookfield Falex Pin/Vee Air release Gas Solubility Specific Gravity Four ball wear Base N u m b e r Thin Film Oxygen Uptake Test Ball on Cylinder Lubricity Evaluation Brookfield - Scanning Elemental Analysis Vapour Pressure, Reid Viscosity - High T e m p e r a t u r e High Shear by capillary Volatihty - NOACK at 250°C / TGA method High Frequency Reciprocating Rig Foam, High Temperature Foam, High Temperature (Option A) TMC Certified Phthalate Esterification Coulometric Karl-Fischer Titration Specific Heat Capacity
OTHER STANDARDS No. D I N 51 5 5 0 DIN/ISO 2909 D I N 51 5 9 7 D I N 51 3 7 6 D I N 51 5 5 9 P a r t 1 DIN 51 777 Part 1
Subject K i n e m a t i c Viscosity K i n e m a t i c Viscosity Index P o u r P o i n t of P e t r o l e u m P r o d u c t s Cleveland Open C u p Calorimetric Titration Coulometric Karl-Fischer Titration
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[4] Randies, S. J., "Refrigeration Lubes," Synthetic Lubricants and High Performance Functional Fluids, R. L. Rudnick and R, L. Shubkin, Eds., Marcel Dekker, NY, 1999, p . 563. [5] Randies, S. J., "Esters," Synthetic Lubricants and High Performance Functional Fluids, R. L. Rudnick and R. L. Shubkin, Eds., Marcel Dekker, NY, 1999, p . 63. [6] Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 18, 4th Edition, John Wiley & Sons, Inc., NY, 1991, p. 991. [7] Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 18, 4th Edition, John Wiley & Sons, Inc., NY, 1991, p . 991. [8] Johnson, R. W., "Dibasic Fatty Acids," Fatty Acids in Industry, R. W. Johnson and E. Fritz, Eds., Marcel Dekker, NY, 1982, p. 327. [9] Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 1, 4th Edition, John Wiley & Sons, Inc., NY, 1991, p . 893. [10] Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 1, 4th Edition, John Wiley, NY, 1991, p . 913 [11] Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 9, 4th Edition, John Wiley, NY, 1991, p . 755. [12] Briant, J., Denis, J., a n d Pare, G., Rheological Properties of Lubricants, Editions Tecnip, Paris, 1989. [13] Emkarate Esters for Synthetic Lubricants, Product Brochure, ICI Performance Chemicals, Middlesbrough, UK, 1997. [14] Lubricant Esters, UnichemaBV, Gouda, Netherlands, 1997. [15] Boyde, S., Journal of Synthetic Lubrication, Vol. 18, 2001, p . 99. [16] Boyde, S., Journal of Synthetic Lubrication, Vol. 16, 1999, p . 297. [17] Scholz, N., Diefenbach, R., Rademacher, 1., and Linnemann, D., "Biodegradation of DEHP DBF DINP," Bulletin of Environmental Contamination and Toxicolology, Vol. 58, 1997, p. 527. [18] OECD Guideline for t h e Testing of Chemicals, Ready Biodegradability, OECD 301, Organisation for Economoic Cooperation a n d Development (OECD), Paris, adopted 17 July 1992. [19] Boyde, S., Randies, S. J., and Gibb, P., "The Effect of Molecular Structure on Boundary and Mixed Lubrication by Synthetic Fluids—an Overview," Lubrication at the Frontier, D. Dowson, et al., Eds., Proceedings of the 25th Leeds-Lyon Symposium on Tribology, 1998, Elsevier, NY, 1999, p. 799. [20] Randies, S. J., "Refrigeration Lubes," Synthetic Lubricants and High Peformance Functional Fluids, L. R. Rudnick and R. L. Shubkin, Eds., Marcel Dekker, Inc., NY, 1999, p. 563. [21] Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed., Vol. 6, John Wiley & Sons, Inc., NY, 1991, pp. 225-269. [22] Matlock, P. L. a n d Clinton, N. A., in 'PAGs' in Synthetic Lubricants and High Performance Functional Fluids, R. L. Shubkin, Ed., Marcel Dekker, Inc., NY, 1999, pp. 101-124. [23] Klamann, D., Lubricants and Related Products, Verlag Chemie, Weinheim, Germany, 1984. [24] Emkarox Physical Properties, ICI Corp., London, 1997. [25] Polyalkylene Glycols, Properties and Applications, ICI Corp., London. [26] Wurtz, A., Annales de Chimie et Physique, Vol. 69, 1863, p. 330. [27] McClelland, C. P. and Bateman, R. L., Chemical Engineering News, Vol. 23, No. 3, 1945, p . 247. [28] Thompson, R. I. G., Eastwood, J., and Stroud, P. M., "The Development of High Performance Carrier Fluids for Detergent Fuel Additive Packages," Proceedings of the 11th International Colloquium, Technische Akademie, Esslingen, Germany, 1998, p. 2485. [29] Briant, J., Denis, J., and Pare, G., Rheological Properties of Lubricants, Editions Technip, Paris, 1989, pp. 155-163. [30] Yang, L., Heatiey, F., Blease, T., and Thompson, R. I. G., "A Study of the Mechanism of the Oxidative Thermal Degradation of Poly(ethylene oxide) a n d Poly(propylene oxide) Using ' H and '^C-NMR," European Polymer Journal, Vol. 32, No. 5, 1996, pp. 535-547. [31] Headey, F., Luo, Y.-Z., Ding, J.-F., Mobbs, R. H., and Booth, C , Macromolecules, Vol. 21, 1988, p . 2713.
CHAPTER 10: SYNTHETIC LUBRICANTS—NON AQUEOUS 265 [32] Sanvordenker, K. S., "Materials Compatibility of R134a in Refrigerant Systems," presented at The American Society of Heating, Refrigeration and Air-Conditioning Engineers, Winter Meeting, Jan. 1989. [33] Hamblin, P., "Oxidative Stabilisation of Synthetic Fluids and Vegetable Oils," Journal of Synthetic Lubrication, Vol. 16, No. 2, 1999, pp. 157-181. [34] Moxey, J. R., "Process for the Preparation of Polyoxylalkylene Block Copolymers," European Patent 0 570 121B1, Europena Patent Office, Munich, Germany, 10 Feb. 1999. [35] Chapter I, Food and Drug Administration, Department of Health and H u m a n Services (Continued), Part 178—Indirect Food Additives: Adjuvants, Production Aids, and Santizers— S u b p a r t D—Certain Adjuvants a n d P r o d u c t i o n Aids—Sec. 178.3570, Lubricants with Incidental Food Contact, Food and Drug Administration, U.S. Government Printing Office via GPO Access, Pittsburgh, PA, April 2002. [36] ISO 6743: Industrial Oil Class L Classification Part 4: Family H (Hydraulic Systems), International Organization for Standardization, Geneva. [37] Hodges, P., Hydraulic Fluids, John Wiley & Sons, Inc., NY, 1996. [38] Doc. No. 4746/10/91 EN, Requirements and Tests AppUcable to Fire-Resistant Hydraulic Fluids used for Power Transmission and Control, 7th ed., Comite Europeen des Transmissions Oleohydrauliques et Pneumatiques, Luxembourg, 1994. [39] Bock, W., "Moderne Schwerentflammbare u n d Umweltschonende Hydraulikflussigkeiten in Industrie und Bergbau," Tribologie und Schmierungstechnik, Vol. 46, 1999, pp. 22-28. [40] Test Standard for Specification Test Standard for Flammability of Industrial Fluids—Class Number 6930, Factory Mutual Research Corporation, Johnston, RI, 2000. [41] Kussi, S., "Eigenschaften von Basisflusigkeiten fur Synthetische Schmierstoffe," Tribologie und Schmierungstechnik, Vol. 33, 1986, pp. 33-39. [42] MoUer, U. J., "Grenzen and Moglichkeiten fur Syntheseole und Konventionelle Oele," Erdoel und Kohle, Vol. 23, 1970, p p . 667-673. [43] Moller, U. J. a n d Boor, U., Lubricants in Operation, Verlag, Duesseldorf, 1986. [44] Lilje, K. C , Short, G. D., and Miller, J. W., "Compressors and Pumps," Synthetic Lubricants and High Performance Functional Fluis, R. L. Shubkin, Ed., Marcel Dekker, Inc., NY, 1999, pp. 539-562. [45] Short, G. D., "Development of Synthetic Lubricants for Extended Life Rotary-Screw Compressors," Lubrication Engineering, Vol. 40, 1984, pp. 463-470. [46] Carswell, R. and McGraw, P. W., "Rotary Screw Compressor Lubricants," U.S. Patent 4,302,343, granted to the Dow Chemical Company, Midland, MI, 1981. [47] Ward, E. L., McGraw, P. W., and Appleman, T. J., "Lubricants for Reciprocating Air Compressors," U.S. Patent 4,751,012, granted to the Dow Chemical Company, Midland, MI, 1988. [48] Benda, R., BuUen, J., and Plomer, A., "Base Fluids for High-Performance Lubricants," Journal of Synthetic Lubricants, Vol. 13, No. 1, 1997, p. 41. [49] Brennan, J. A., "Polymerisation of Olefins with BF3" U.S. Patent 3,382,291, to Mobil Oil, Washington DC, 1968. [50] Shubkin, R. L., "Process for Producing a Ce-Cie N o r m a l ALPA-Olefin Oligomer Having a Pour Point Below About 50°F," U.S. Patent 3,763,244, to Ethyl Corp., Washingotn DC, 1973. [51] Shubkin, R. L., "Synthetic Lubricants by Oligomerisation and Hydrogenation," U.S. Patent 3,780,128 to Ethyl Corp., Washington DC, 1973. [52] Szydywar, J., "Synthetische Kohlenwasserstoffe, Speziell Polyalphaolefine," Schmiertechnik + Tribologie, Vol. 28, No. 4, 1981, p. 124.
[53] Plomer, A., Senior Scientist, Personal communication, BP Amoco Chemicals, Feluy, Belgium, October 1999. [54] Benda, R., Market Development Manager, Personal communication, BP Amoco Chemicals, Feluy, Belgium, October 1999. [55] Rudnick, L. R. and Shubkin R. L., "Lubrication at the Frontier," D. Dowson et al., Eds., Proceedings of the 25th Leeds-Lyon Symposium on Tribology, 1998, Elsevier, NY, 1999, p. 10. [56] "PAO a n d Synthetic Esters," Product Literature, Mobil Chemicals, Chemical Products, Edison, NJ, 1998; "Polyalphaolefins," Product Literature, BP Amoco Chemicals, Chicago, IL, 1987. [57] Wunsch, F., "Synthetische Schmierstoffe - Heute und Morgen," Proceedings of the 9th International Colloquium, Technische Akademie Esslingen, 1996, p. 2271. [58] Randies, S. J., Lubrication Engineering, Vol. 22, 1957, p. 82. [59] Quinn, C , Traver, F., and Murthy K., "Silicones," Lubrication at the Frontier, D. Dowson et al., Eds., Proceedings of the 25th Leeds-Lyon Symposium on Tribology, 1998, Elsevier, NY, 1999, p. 267. [60] Bell, G. A., Howell, J., and DelPasco, T. W., "Perfluoroalkyl Polyethers," Synthetic Lubricants and High Performance Functional Fluis, R. L. Rudnick a n d R. L. Shubkin, Eds., Marcel Dekker, Inc., NY, 1999, p. 215. [61] Joaquim, M., "Polyphenyl Ether Lubricants," Synthetic Lubricants and High Performance Functional Fluis, R. L. Rudnick and R. L. Shubkin, Eds., Marcel Dekker, Inc., NY, 1999, p. 239. [62] "OS 124, Polyphenyl Ether Fluid by Santovac," Commecial Literature, Santovac Fluids, St Charles, MO, 1999. [63] Casserly, E. W. and Venier, C. G., "Cycloaliphatics," Synthetic Lubricants and High Performance Functional Fluis, R. L, Rudnick and R. L. Shubkin, Eds., Marcel Dekker, Inc., NY, 1999, p. 325. [64] Venier, C. G. and Casserly, E. W., Lubrication Engineering, 1991, Vol. 47, p. 586. [65] "Santotrac Traction Lubricants," Commercial Literature, Findett Corporation, St Charles, MO, 1999. [66] Landsdown, A. R., High Temperature Lubrication, Mechanical Engineering Publications Ltd., London, 1988. [67] M. J. Neale, Ed., Tribology Handbook, Butterworth, London, 1973. [68] Shubkin, R. L., "Polyalphaolephins," Presented at the Advanced Synthetic Lubricants Education Course, STLE National Meeting, Calgary, Alberta, Canada, May 1993. [69] "Midel 7131," Transformer Fluid Brochure, M & I Materials, 1993. [70] Randies, S. J. and Whittaker, A. J., "Compressor Fluids—^Value Creation Using Synthetics," International Conference on Compressors and Their Systems, Paper C542/029/99, ImechE Conference Transactions, 1999, p . 127. [71] Aderin, M., Spikes, H. A., and Caporiccio, C , "The Elastrohydrodynamic Properties of S o m e Advanced Non-Hydrocarbon Based Lubricants," Lubrication Engineering, Vol. 48. No. 8, August 1992, pp. 633-638. [72] CRC Handbook of Lubrication, Vol. 2, CRC Press, Boca Raton, FL, 1988, p. 235. [73] Guangteng, G. and Spikes, H. A., "Boundary Film Formation by Lubricant Base Fluids," Paper 95-NP-7D-3, Presented at The 50th Annual Meeting, STLE, Chicago, IL, 14-19 May 1995. [74] Chang, H. S., Spikes H. A., and Bunemann, T. F., "The Shear Stress Properties of Ester Lubricants in Elastrohydrodynamic Contacts," Journal of Synthetic Lubricants, Vol. 8, No. 3, 1991, p. 258. [75] Gunsel, S., Lockwood, F. E., and Westmorland, T., "Engine Oil Oxidation—Correlation of ASTM III-D and III-E Sequence Engine Tests to Bench Tests" Presented at the SAR International Fuels and Lubricants Meeting and Exposition, Baltimore, MD, 1989, SAE Paper No. 892164, Society of Automotive Engineers, Warrendale, PA.
266 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK [76] Gunsel, S. and Pozebanchuk, M., "Elastrohydrodynamic Lubrication with Polyester Lubricants and HFC Refrigerants," The Air Conditoinaing and Reirigeration Technology Institute, Project No. 670-54400, Report No. DOE/CE/23810-102, April 1999. [77] Smeeth, M. and Spikes, H. A., "The Formation of Viscous Surface Films by Polymer Solutions: Boundary or Elastrodynamic Lubrication?," Paper 95-NP-7D-2, Presented at the 50th Annual Meeting, STLE, Chicago, IL, 14-19 May 1995. [78] Summers-Smith, D., An Introduction to Tribology in Industry, The Machining Publishing Co., Brighton, England. [79] CONCAWE (Conservation of Clean Air and Water Europe), "The Collection, Disposal and Regeneration of Waste Oil and Related Materials," Report 85/33, The Hague, 1985. [80] Betton, C. I., "Lubricants and Their Environmental Impact," Ch. 13, Chemistry and Technology of Lubricants, 2"^ Edition, R. M.
Mortier and S. T. Orszulik, Eds., Blackie Academic and Professional, London, 1997. [81] Smeeth, M. and Spikes, H. A., "The Formation of Viscous Surface Films by Polymer Solutions: Boundary or Elastrodynamic Lubrication?," Paper 95-NP-7D-2, Presented at the 50th Annual Meeting, STLE, Chicago, IL, 14-19 May 1995. [82] Summers-Smith, D., An Introduction to Tribology in Industry, The Machining Publishing Co. Ltd., Brighton, UK, 1969. [83] CONCAWE (Conservation of Clean Air and Water Europe), The Collection, Disposal and Regeneration of Waste Oil and Related Materials, Report 85/33, The Hague, 1985. [84] Betton, C. I., "Lubricants and Their Environmental Impact," Chemistry and Technology of Lubricants, 2nd Edition, R. M. Mortier and S. T. Orszulik, Eds., Blackie Academic and Professional, London, 1997.
MNL37-EB/Jun. 2003
Environmentally Friendly Oils Hubertus Murrenhoff^ and Andreas Remmelmann^
A Cs
d FM
g P Q T V V
P DIN EP HEES HEPG HETG HFA HFC NZ ppm TMP
VI WGK C CI CO2 H H2O N O VCI
Units ,2 m area J/(kgK) heat capacity mm diameter N force gravity m^/s bar (lO^N/m^) pressure flow m^/s °C temperature m^ capacity Ns/m^ dynamic viscosity mm'^/s kinematic viscosity kg/m^ density Deutsches Institut fiir Normung Extreme Pressure Hydraulic-Environmental group ES (synthetic ester) Hydraulic-Environmental group PG (polyglycol) Hydraulic-Environmental group TG (tri-glycerid) Hydraulic-Fire-resistant group A (water-based) Hydraulic-Fire-Resistant group C (water glycol) Neutralization N u m b e r Parts-Per-Million tri-methylol-propan Viscosity-Index Water Hazard Class Carbon Chlorine Carbon Dioxide Hydrogen Water Nitrogen Oxygen Verband Chemischer Ingenieure
Pressure Media In a variety of technical systems, hydraulic drives are the preferred alternative because of their versatility and efficiency. To perform the drive functions over the life of the system, the hydraulic fluid must be considered as a system component during the design stage. Due to a societal increase in ' Executive Director, Institute of Fluid Power Transmission and Control, IFAS, University of Teciinology Aachen, Steinbachstr. 53, D52074 Aachen, Germany. ^ Product Engineering Hydraulics, John Deere Works Mannheim, Windeckstr. 90, 68163 Mannheim, Germany.
environmental consciousness in the late 1980s, regulatory agencies began d e m a n d i n g that these fluids be rapidly biodegradable and nontoxic. This excluded mineral oil from consideration. Currently vegetable oils, some synthetic esters, and polyglycols are being used for these applications [1-10]. Since the properties of those base fluids differ substantially from those of conventional mineral oils, extensive testing is required. To qualify the fluids for the intended applications, special laboratory tests are required to ensure that they will withstand the pressures and t e m p e r a t u r e s encountered. Many of the chemical and physical test procedures were developed for mineral oils and aren't applicable for these new types of hydraulic fluids. Parallel to the development of base fluids, new additives are being developed. This is necessary for two reasons: 1. Existing additives used so far in mineral oil based pressure media lead to a deterioration of the performance properties in rapidly biodegradable oils. 2. Existing additives contain toxic substances leading to a significant deterioration of the biodegradability of the fluid [11,12-14]. Functions and Requirements In a hydraulic system, the pressure media has many functions. Fig. 1. Transmitting the hydraulic power is the main function. For this purpose, the pressure media connects the generator (pump) and the motor (cylinder) via the enclosed volume and carries the pressure that is determined by the load [15]. This main function of a pressure media can be accomplished in principle with any fluid; but additional requirements result in a substantial limitation of the fluids to be used. To ensure a long lifetime of the hydraulic components, wear protection is of central importance. Moreover, the fluid must protect the component surfaces against corrosion and it must be compatible with the elastomers used as sealing material. Based on the above-mentioned tasks, a variety of demands are m a d e on hydraulic fluids, see Fig. 2. To transmit hydraulic power and guarantee high load stiffness, the fluid must exhibit a high bulk modulus. This is another prerequisite for optimal control in open and closed loop hydraulic systems [14]. The task of protecting hydraulic components against wear and corrosion creates very high demands on the fluid [33]. It must exhibit a high increase of viscosity vs. pressure, thus producing a self-enhancing effect in the tribological contact
267 Copyright'
2003 by AS'I M International
www.astm.org
268
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
FIG. 1—Functions of hydraulic fluids.
FIG. 2—Demands on hydraulic fluids.
for high loads [16]. In addition, the fluid must provide good lubricating film wetting between surfaces moving relative to each other that reduce wear and stick slip. Materials used in hydraulic components such as bronze or other alloys are susceptible to corrosion. Consequently, the fluid should contain only a m i n i m u m a m o u n t of free acid, if any. The by-products created by the aging process must be neutralized or adsorbed by additives contained in the fluid to eliminate any acid build-up. In addition, the fluid properties must vary only slightly over extended time [31]. Elastomers and other non-metal components may also be attacked by these by-products. Increased elastomer swelling may result in complete decomposition and sealing failure. Mechanical and volumetric losses in hydraulic units are responsible for heat generation. This heat is partially released to the environment by convection. The pressure media itself, however, accounts for the major part of the heat loss. To limit the temperature increase of the fluid, it must possess high heat capacity and thermal conductivity. These demands are directly related to the use of the pressure media in a hydraulic system. The development of fluids is significantly influenced by external requirements. For ex-
ample, fluids used in underground mining or in steel mills must be fire resistant to reduce the dcuiger of fire. An additional requirement is environmental compatibility, which has led to the development of another pressure media group [19,22,35,38]. Types of Pressure Media Different types of hydraulic fluids are shown in Fig. 3. Each one is developed for a specific application. The most comm o n type in use today is based on mineral oil. Fluids are blended with additives to inhibit aging, weeir and to reduce friction and corrosion. For special application requirements, additives with detergent and dispersent characteristics are used to suspend solid particles as well as water, in some cases u p to 5% water. By utilizing special refining methods, it is possible to improve the viscosity t e m p e r a t u r e performance of the base stock. Group II API Base Oils can be blended to produce good tribological characteristics and long-term stability. State of the art refining technology ensures constant production of prime quality base stocks. The technology to develop additives is readily available.
CHAPTER 11: ENVIRONMENTALLY
FRIENDLY OILS
269
FIG. 3—Different types of hydraulic fluids [24,25].
TABLE 1—Groups of biologically fast degradable pressure media. Fluid
Base Fluid
Saturation
Origin
HETG HEES
Native ester Synthetic ester
Unsaturated Unsaturated Saturated
HEPG
Poly-glycol
Native materials Native materials Chemical industry Chemical industry
New groups of fluids have been developed to provide good fire resistance, rapid biodegradability, and low toxicity. Mineral oils are classified based on their performance, which is different from readily biodegradable pressure media, which are classified according to base fluid composition. There are three groups of biodegradable fluids: natural esters (type HETG), synthetic esters (type HEES), and poly-glycols (type HEPG) as listed in Table 1. HETG base stocks are derived from vegetable oils such as rapeseed and sunflower. These base fluids have a limited temperature range. New additives have been developed to meet the expected application temperature ranges. Their poor thermal stability is due to the amount of unsaturated carbon containing acids found in these natural esters. HEES base stocks can be produced from various materials including natural esters. The use of natural esters usually leads to unsaturated synthetic esters. Performance of the fluids is superior to HETG fluids due to a more uniform molecular structure and the use of different alcohols. Using completely saturated esters, the resulting fluids exhibit very stable aging characteristics. Additives used speciflcally for this type of fluid have also been developed. Based on the many choices of acids and alcohols available
for the production of synthetic esters, a wide variety of technical performance properties is possible. Early synthetic esters possessed a chemical structure similar to rapeseed oil. HEPG fluids is the third fluid type and is the only type that is water-soluble. This can be an advantage for the biological degradation in water. On the other hand, there is the danger of fluid-contaminated water penetrating more deeply into the soil layers, thus reaching ground water. For that reason, in some countries, polyglycols are not considered environmentally friendly fluids.
CHEMICAL B A S E S O F N A T I V E A N D SYNTHETIC ESTERS Functional Groups and Elementary Compound Pressure media based on native and synthetic esters consist of carbon-hydrogen bonds as do mineral oils. However, the structure differs significantly from that of mineral oil, which explains the different properties and performance characteristics exhibited by these fluids. Examples of the different functional groups can be seen in Fig. 4. The characteristic group for an alcohol is the hydroxy group (OH-group). In a case where the OH-group of aliphatic alcohols is connected to a carbon-atom, the alcohol is described as primary alcohol. In a case where the OH-group is connected to a carbon-atom in the center of the molecule, it is considered secondary alcohol. Tertiary alcohols are those with an OH-group connected at a branch site in the molecule. The hydroxy group of alcohols is responsible for their higher boiling points compared to those of the comparable
270
MANUAL
37: FUELS AND LUBRICANTS hydroxyl-group
carbonyl-group
HANDBOOK
alkyl-group
Mechanism of Esterification
H
0
—c—
C—H
OH
1
H carboxyl-group
aldehyde O
O
C—H
C—OH
ketone 0
II
carboxylic acid
R1-C—R2
alcohol
ester
O
O
R 2 - C — OH
R^-OH
R^-O—C — R2
FIG. 4—Important functional groups of organic chemistry.
R^ \
-.1
O—H
+
\
R' O—H
In Fig. 8, the general equation of a reaction for the production of esters is shown. This chemical reaction produces ester and water. Water must be removed during reaction to achieve a complete conversion of the alcohol reaction with the acid to produce a n ester (by shifting the equilibrium to the right). The equilibrium constant (K) for the ester producing reaction is [30]:
\
-.1 R' +\ * O—H - - 0 — H
-
FIG. 5—Hydrogen bond of alcohols, [22-30].
alkanes, because of the shared hydrogen bond of the hydroxyl groups. A strong polarity of the OH-connection is based on the electro-negativity of oxygen. The result is a positive shifting of the hydrogen a t o m so that the hydrogen bonding connection shown in Fig. 5 becomes feasible. The possibility for hydrogen bonding is the reason for the still unlimited miscibility of simple alcohols with water. However, with increasing size of the non-polar organic alcohol residue, this characteristic decreases. The abbreviation R used in Fig. 5 represents a n alkylgroup. This is a n acylic saturated carbon-hydrogen compound, called an aJkane, from which a hydrogen molecule is split off. According to their bonding capability, alcohols are considered to be monovalent, bivalent, or trivalent. To produce a synthetic ester hydraulic fluid with good performance properties, carboxylic acids are used. The carboxy g r o u p COOH is the functional g r o u p of carbon acids. Its nomenclature is based on the combination of the carbonyl and hydroxy group. Carboxylic acids with long-chain Rgroups attached are called fatty acids. The capability of carbon acids to hydrogen bond as with alcohols, is based on their chemical structure. This is the reason for their relatively high boiling point, which is comparable to alcohols due to its dimeric structure resulting from the hydrogen bonds shown in Fig. 6. Small carbon acids like the corresponding small alcohols are still soluble in water. However, the solubility diminishes with increasing molecular size. When carboxylic acids contain only single bonds, they are considered saturated. Unsaturated acids contain at least one double bond. The n u m b e r of double bonds influences chemical properties. Furthermore, carboxylic acids are distinguished by different isomers resulting in different chemical and physical properties (Fig. 7).
[ester] • [water] [acid] • [alcohol]
K
Where: "[ ]" indicates the concentration of the reactants (acid and alcohol) or products (ester and water) in mole/liter. It is desirable to maximize the concentration of ester and to minimize the concentration of acid and alcohol. One way to do this is to remove water during the reaction. Thus, to maintain equilibrium, more water must be produced, which decreases the concentration of acid and alcohol resulting in increased water production. Similarly, increasing the alcohol concentration will decrease the acid and increase the desired ester content. The rate of reaction is increased by the increasing temperature or by the addition of an acid catalyst such as H2SO4 (typically 5-10% based on the weight of the acid carboxylic). Other, even stronger, acids may also be used. The rate of the esterification reaction increases with increasing catalyst concentration The objective of ester production is to quantitatively complete transition of acid into ester. This means that the concentration of ester should be as high as possible. The removal of water from the reaction mixture may be facilitated by the addition of a solvent such as toluene. In this case, water is removed from the reaction mixture by azeotropic distillation. H —O
R ' - Cy .
+ O— H
,0—H—O C — R^
•^
R^ - C
O
C—R^ O—H—O
FIG. 6—Hydrogen bond of carbon acids.
cis-configuration
trans-configuration
H
H
H
I
I
I
C=C
H FIG. 7—Isomers of carboxylic acids, cis and trans structure.
carboxylic acid
aicohd
ester
water H
O R^-C — OH
R} - O H
^
R' - O — C — R 2
FIG. 8—Chemical reaction of ester production.
I
O—H
CHAPTER 11: ENVIRONMENTALLY The mechanism of the acid catalyzed production of ester from carboxyhc acids and primary or secondary alcohols is based on the addition of a proton to the oxygen of the carboxyl group. Producing a mesomer-stabilized cation and the Catom of the carboxyl group becomes positive. By adding the nucleophilic oxygen of the alcohol to this carbon atom, the ester is finally produced by separation of water and protons; see Figs. 9a and 9b. Tertiary zJcohols are not esterified in this way because the hydroxyl group will undergo acid catalyzed elimination producing undesirable alkenes, as shown in Fig. 10.
FRIENDLY OILS
271
1. step: alcohol protonation H R3C-OH
+
R3C-O
H
2. step: carbenium formation H R3C-0
- >
R3C
+
H2O
H 3. step: electrophilic carboxyl group attack 1. step: carbonyl oxygen protonation
o 1 II
O
OH'
^
R^ - C
+
^ H
1 II —*•
R^ - C — O H
I
~*'
R^ - C — O H
4. step:proton migration 0^—CR,
OH*
OH
R^ - G
+
1
R 2 --OH
R^- - c — 0
~*
H
/
R^ - C — O *
- C = 0 H " '
O—CR3
O—CR3
R^-C=O
+
H
R^1 - CI — O — R 2
Alcohol Bonds
L V 4. step: water split off OH R1 ^ - CI — O — R^
Ri
FIG. 10—Ester synthesis with tertiary-alcohols.
OH
H
O—CR3
-
5. step: proton removal
R^-C=OH'
3. step: proton shifting OH
R1^ - CII— O H
+
FIG. 9a—Ester synthesis with primary and secondary alcohols under acid conditions [22-30].
O—R"" R,1' - C
+
H.O
II . OH 5. step: proton split off 0 —R-" 1
R^ - C
//
\ OH
Rfi^
I OH
2. step: alcohol addition
R' - C
"^
R^ - C
OH
I
o'^CRj
+
H
O—R"
FIG. 9b—Ester synthesis with primary and secondary alcohols under acid conditions.
By variation of the acid and ester reagents, a vast array of possible ester products and a wide variety of chemical properties is achievable. These varieties, however, are limited by the demand for a good biological degradability as well as by other technical requirements with regard to viscosity, viscosity temperature dependence, and its stability under hydraulic load. The different alcohols depicted in Fig. 11 are used for the production of hydraulic fluids. Trimethylolpropane is a colorless, crystalline substance easily soluble in water. This is the stacking material used for polyols by reaction with ethylene oxide and propylene oxide, which are other reacted to form polyurethanes. By esterification with carboxylic acids, ester lubricants with a high viscosity index (VI) are produced. Glycerin is also a trifunctional alcohol and is very soluble in water. This is the main reason for its wide use in drugs and cosmetics. When esterified with carboxylic acids, ester-based lubricants with a high VI are produced as well. These esters can also be produced from natural materials such as rapeseed oil through a pressing process.
272
MANUAL 37: FUELS AND LUBRICANTS
trimethylolpropane HH
H
I
I
I
I
ditives. Mono and glycerin esters, however, exhibit limited chemical stability at high temperatures and low temperature performance. High temperature stability and low temperature performance properties can easily be achieved with di-carboxylic acid and polyol ester. Carbon-6 to carbon-12 di-carboxylic acid esters are increasingly important as engine and compressor lubricants. Polyol-esters are used in huge quantities as turbine oils for aviation applications where they are exposed to extreme temperatures. Good oxidative stability is
glycerin OH
OH
I
I
C— H
H—C — H H
I
H-C—C H
HANDBOOK
-C — O H
-OH
i
H
H H—C—H
H—C—H
I
I
OH
OH
trimethylolethane
pentaerythritol OH
OH
I
vinegar acid
I
H— C— H
stearic acid
H—C — H
O H
H
H
I
I
I
-C — OH
-c-
I
I
H
H
OH-C-
-OH
I
OH neopentylglycol
I
-C
I C
C—H
I
H
"*^17'^35
oleic acid 1x unsaturated cis-configuration O
H
H
OH-C—C^H,-^H
I
-c-
H
H—C—H
I
II
I
H
OH
H
O
H—0—C—C—H
I
"T"
-C—H
H
H H — C—H
"•^8^17
linoleic acid 2x unsaturated cis-configuation O
H
H
H
OH-C—C,H;
OH
For esterification of those alcohols to lubricants, carboxylic acids of vegetable and animal origin are used. Examples of the different acids used in hydraulic fluids are illustrated in Fig. 12. Ester basestocks, which are suitable as lubricants or pressure media typically possess a carbon number of about 18-30. These may be derived from natural sources. Achievable viscosities are within the range of about 22-100 mm'^/s. For this application, esters based on fatty acids are considered. They can be divided into five ester groups: mono, glycerin, dicarboxylic, polyol, and complex, as shown in Fig. 13. Mono-esters are prepared from linear monocarboxylic acids, which are reacted with branched or linear alcohols. The main applications of monoesters are in metaJ working such as for lubricants for cold and hot rolling. Of special importance within the group of mono-esters are the unsaturated methyl esters from which extreme pressure (EP) additives may be produced by reaction with sulfur or sulfur-containing functionality. Glycerin esters of rapeseed oils, particularly when the rapeseed oil has a high iodine number (high unsaturation), are used to synthesize such ad-
'^5^n
H
FIG. 11—Survey of alcohols for lubricants.
Carboxylic Acids
H
-C—C^C-
I
Pentaerythritol is a tetrafunctional alcohol with five carbon atoms. The four primary hydroxyl groups allow a relatively easy esterification with different acids. These esters are very stable against biological degradation in an aqueous solution.
H
FIG. 12—Survey of acids for lubricants.
dicarboxylic acid ester
mono aster
O
0
0
II
II-
II
R1-0—C—R2
R1-0—C—R2-C—0—R3
polyol ester (TMP-ester) O
glycerin ester 0
II
II
CHj-O—C—R1
CHj-O—C—R1
C,H,-C•*
Q[_|__Q
Q
H—C
-R1
CI H j - O — C — R 1
CJ H j - O — C — R 1 complex ester 0
0 II II CH^-O—0—R1
C^riy - 0 - - C — R 2 0
C,H,-( ; — C H j - o — c — -R1
C,Hg<
II
-CH,- 0 — C — R 2
I
CH,-0—C—R1 - C — O — C H ,
II
O
FIG. 13—Different ester types for lubricants [30].
CHAPTER 11: ENVIRONMENTALLY FRIENDLY OILS 273 achieved because of the unsubstituted beta-carbon atom and the primary hydroxyl groups.
CHEMICAL PROPERTIES OF NATURAL AND SYNTHETIC ESTERS Properties of esters are determined by the alcohol and acids from which they are derived. Various polyol-esters have been prepared by using alcohols for the production of lubricants similar to the triglycerides of rapeseed oil. Viscosity and Viscosity Properties Viscosity properties are important when esters are used as lubricants. The acid being esterified does not only determine the viscosity of an ester molecule. The viscosity of esters products increases with increasing molecular weight of the alcohols or their number of hydroxyl groups. This is due to the increasing structural possibilities for carboxylic acid components (Fig. 14). Pour points for these esters exhibit the same dependency as observed with viscosity. Lowering the pour point requires a short-chain branching of the alcohol with tertiary carbon or hydrogen-atoms. On the other hand, those molecular structures lead to a decreased oxidative stability of the alcohol. Therefore, neo-pentyl-polyols are especially advantageous for the production of lubricants, since they contain exclusively primary hydroxyl groups and they are branched. Acids used for the production of lubricants may also be the same as those of natural oils and fats with 16-18 carbon atoms, which produce the required classes of hydraulic fluid viscosity ISO VG 22 to 68. To obtain high oxidation stability, unsaturated acids are not desirable, since they represent a preferred point of attack for reaction with oxygen. However, double bonds also create a distortion of the ester molecule. This distortion positively influences the cold flow properties of the ester. The ester produced from acid with a monofunctional alcohol and acids containing double bonds will still flow at a temperature of 5°C, whereas completely saturated ester become solid at a temperature of 75°C (Fig. 15).
The position of the double bond within the fatty acid has no significant influence on the pour point. However, slight differences can be observed depending on the degree of distortion imparted by the double bond of the molecule. Depending on the position of the double bond, the distance between molecules increases, resulting in slightly different pour points (Fig. 16). Besides the number of double bonds, their steroconfiguration has a decisive influence on the viscosity properties. Ester with cis- double bonds will flow at low temperatures, whereas fluids with trans- double bonds exhibit pour point at comparable temperatures. Branching exerts similar influence as double bonds. With increasing branching at a constant carbon number, the cold flow behavior of the ester increases and viscosity decreases. In contrast, an increased chain length with the same structure results in an increase of viscosity. Reduction of the pour point is also obtainable by using esters with acid mixtures. These mixtures do not crystallize as readily thus leading to a decreased pour point. With the more complex esters there is no correlation between viscosity and carbon number, since the viscosity increase is compensated by branching, due to increasing carbon number.
g 60"o Q. 3 O
ziU
-40 -
6:0
8:0
10:0 12:0 14:0 16:0 18:0 18:1 18:2 18:3 fatty acid [C-number: double bond]
FIG. 15—Pour-point of different esters with the same alcohol [20].
45 40
"
'*
^
—
^>
<• - -
4*
''
35 B
O 30
cis
j
.e 25 o a. S 20 o °- 15 10
pentaerithritol
trimethylolhexane
trimethylolpropane
trimethylolethane
glycerin
neopentylglycol
FIG. 14—Viscosity of different polyol-esters with the same acid [46].
D5
D7
D9
D11
D13
position of the double bond
FIG. 16—Influence of double bonds on the Pour-point [20].
274
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
Hydrolysis The chemical reaction for ester production is an equihbrium reaction. Consequently, all esters-based fluids cleave into their alcohol and acid components upon hydrolosis. This directly influences the ester bond. This reaction, also called "hydrolytic splitting," which continues until the chemical equilibrium is restored. Thus, hydrolytic stability of an ester is influenced by its chemical structure. Steric hindrance of the ester bond improves hydrolj^ic stability. This protection is due to the presence of methyl groups instead of hydrogen atoms relative to the ester group. Those methyl groups sterically protect the ester bond against water attack. Depending on the n u m b e r of methyl groups positioned around the ester group, the reaction rate may be reduced many times. Table 2 provides a survey of the reaction rate as a function of the amount of branching. With mono-esters, hydrolytic stability is primarily increased using linear short-chain alcohols. Even short alkyl chains in these alcohols yield a significant increase of hy-
TABLE 2—Influence of a-carbon branching of the oxygen ester on reaction speed [13]. Alkyl-substitution CH3 CH3CH2 (CH3)2CH (CH3)3C
Relative Reaction Speed 30 1 0,03 «0
drolytic stability. Only when using branched alcohols larger than C8, does resistance increase to the level of saturated linear mono-alcohols. With glycerin esters, the saturated esters are more stable than unsaturated esters. Their stability can be compared to the stable mono-esters. In general, di-carboxylic acid esters are very stable against hydrolysis. Their stability is almost independent of the chain length, branching, and the alcohol components used. Saturated polyol-esters also achieve a comparably good stability. The stability of unsaturated polyol-esters is comparable to unsaturated glycerin esters. Figure 17 depicts hydrolytic stabilities of different ester structures. Low hydrolytic stability corresponds to a high acid n u m b e r in the figure. Molecular features that improve hydrolytic stability also reduce the biodegradability rate. This is of special importance since hydrolosis is the starting reaction for biological degradation. It follows that protecting an ester bond might be a disadvantage with regard to the ecotoxicologiccd properties. In any case, the formulation of rapidly biodegradable pressure media requires a confirmation of the ecotoxicologic properties of the fluid. Oxidation Stability Oxidation stability of synthetic esters is decisively influenced by their structure. With alcohols, especially short-chain branched structures with tertiary H-molecules as they occur in rapeseed oil, which is a triglyceride, results in a significant decrease of oxidation stability. The degree saturation of the ester molecule, however, has a much larger impact on oxida-
FIG. 17—Hydrolytic stability of different ester structure [15].
CHAPTER tion stability. As a rule, saturated alcohols are used for the production of lubricants and pressure media. For that reason, the oxidative stability is mainly determined by the degree saturation of the carboxylic acids. In general, the tendency of an ester molecule towards oxidation continually increases with increasing amounts of unsaturated carboxylic acids. Figure 18 shows this for three different saturated polyol-esters a n d rapeseed oil. The oxidation stability of those fluids was determined by the viscosity increase after an aging process according to the Baader-test (DIN 51587). The dependency of the oxidation time for vegetable oils can be taken from Fig. 19. These results were obtained by the R a n k i m a t method, which provides a significant improvem e n t in the precision of the test. Rapeseed oils, which are low in euricic-acid content and sunflower oils, which are high in oleic acid content were used as hydraulic fluids. Mixing those oils provided the required degrees of saturation. The strong dependency of the stability on the a m o u n t of double bonds is evident. Oxidative stability may be enhanced by sterically protecting the double bond, e.g., by branchings. The effects of steric b r a n c h i n g on oxidative stability are comparable to the
180 160 140 10
'1 F >•
120
•rapseed oil •unsaturated polyol-ester partial saturated polyolestsr •saturated polyol-ester
10(1
+ j
(A O O
an
ID
2
3 4 5 aging time [days]
FIG. 18—Influence of saturation on oxidation stability [34].
^\l
le" 1R (1>
14 -
•— •^
19 IZ -
o T" o o 5 fi0 •O .E 4 O
03
3.25
3.5
3.75
4
4.25
4.5
4.75
FRIENDLY
OILS
275
amount of increased hydrolytic stability obtained. In addition to the amount of unsaturated carboxylic acids (relative to s a t u r a t e d esters), the n u m b e r of double bonds in carboxylic acid also influences oxidative stability. In general, increasing the number of soluble bonds in a carboxylic acid (increasing unsaturation) decreases oxidative stability. The foregoing discussion provides only an overview of structure-performance consideration of ester base stock. However, the generalizations m a d e t h u s far will aid the reader in understanding lubricant formulation to provide high load capacity and good chemical and physical properties. The summary of the effects is illustrated in Fig. 20.
PRODUCTION OF ENVIRONMENTALLY FRIENDLY FLUIDS BASED ON NATURAL AND SYNTHETIC ESTERS Native Esters Oil is removed from rapeseeds by milling, pressing, or extraction. These seeds contain approximately 40% oil and 8-10% water by weight. In the milling process the seeds are fed into rollers to break down the cells so the oil can be released. Next, the milled seeds are conditioned by heating to 80-90°C to control the humidity to a m a x i m u m of 8% (Fig. 21). By this procedure, the vegetable oil becomes lighter and thinner. The seeds are then squeezed in a screw press under a pressure of 200 bcir. This procedure yields between 40-60% of oil. A higher yield can be obtained by either a finishing or by solvent extraction. The finishing process removes 90-96% of the oil. For economic reasons the extraction method is the procedure of choice. A higher oil yield can be obtained either by another pressing process, the finish pressing, or by extracting the prepressed coarse-ground grain. By finish pressing, the oil concentration decreases to values between 4 and 10%. An extraction reduces the residual oil contents to below 2% in the seeds. The individual production steps are shown schematically in Fig. 22. For extraction, a solvent is used to dissolve the glycerides from the seeds, not the undesirable accompanying substances like gums or resins, slime, and dye materials. Moreover, the solvent must not contain any toxic substances that are nonvolatile and thus unremovable. The solvent also must be separated from the extracted material. Aliphatic hydrocarbons are used for this purpose, especially n-hexane, which can, due to its boiling point of 55-70°C, be separated easily from the coarse-ground grain. After those steps, the extracted raw material contains between 1 and 2% free fatty acids as well as 300 p p m phosphatides and 0.35 to 0.5% sterines. To achieve the desired quality of the vegetable oil, a refining process follows the extraction of the crude oil. Refining serves the purpose of reducing the contents of free fatty acids to 0.05% and that of the phosphatides to 0.02 ppm. The following refining steps are necessary:
60
>
11: ENVIRONMENTALLY
5
double compound amount [mmol/g] FIG. 19—Oxidation stability vs. amount of double bonds with Rankimat method [34].
1. Precleaning to remove phosphatides (desliming). 2. Deacidizing and neutralization. 3. Decolorization (bleaching) by means of absorbents, e.g. bleaching earth. 4. Filtration and deodorization.
276
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
branching of branching of saturation of molecule alcohol acid acid weight
hydrolysis staWlity
viscosity
low temperature viscosity oxidation stability
viscosity index
air release property
L ,\
linearity
1
1
11
i
1,
I
L
. I
I
i,
i
1
i,
I
1,
I
L
n
k
, 1
I
1
1 I
I
I
1
1 1
»
It
*
FIG. 20—Summarized table of the influence of structure on chemical and physical properties.
Synthetic Esters Raw materiaJs for the production of synthetic esters are derived mainly from petro-chemistry. However, it is also possible to use natural raw materials or their transformation products. In any case, this requires re-esterification or an additional chemiced treatment, e.g., hydrogenation. Besides the esterification procedure described previously, ester synthesis can be subdivided into the production of different alcohols and acids. The great majority of those products originate from other applications. The systematic production of lubricants can be considered rather insignificant at this time. Quantitatively, the largest portion of esters is the softener sector. Long-chain alcohols and fatty acids cire mainly used for the production of washing and cleaning agents, surfactants, and detergents. The fundamental path of synthesis will be explained using a few examples. MultifunctioncJ alcohols are produced using the so-called "aldol condensation". This is a catalytic addition of carbanions by bases to the carbonyl group of aldehydes or ketones. The carbanions are produced from activated methylene groups. After this reaction step, water is eliminated. Figure 23 shows this process for the alcohol pentaerythritol. The production process for trimethylol propane is very simple except when formaldehyde and n-butanal are used. For the production of neopentyl glycol, the aldol reaction
is conducted in a slightly different way utilizing formaldehyde and i-butanal. However, as a catalytic agent, sodium hydroxide is used (Fig. 24). The production of carboxylic acids can be divided into two major processes. On the one hand, from natural oils and fats the corresponding fatty acids can be generated directly by hydrolysis. On the other hand, an alternative process is to direct re-esterification of natural oils with synthetic alcohols. In addition to those two possibilities, acids may also be produced using chemical processes. Figure 25 shows this process for adipic acids being used for the production of di-carboxylic acid esters. Large-scale production of synthetic esters are typically batch processes. Depending on the desired degree of esterification, different catalysts are added to the process, leading to an accelerated conversion. During esterification, undesirable water is continuously removed from the alcohol and fatty acid reactants to optimize the conversion process. The reaction is stopped once the desired acid number, a measure for the completeness of the esterification, has been attained. To achieve very low acid numbers (<0,5), another refining step is necessary. Undesired substances are eliminated from the ester by a subsequent bleaching earth treatment. Finally, a filtration step is required to eliminate the bleaching earth from the ester. A survey of the ester production process can be taken from Fig. 26.
CHAPTER 11: ENVIRONMENTALLY
FRIENDLY OILS
277
seeds
milling
u.1
f steam ^
conditioning
squeezing
n
pelletextraction
n-hexane
j r drying
n-tiexane
9^i:r^.':''i-'^'-''''-'• -.y'-^wl
Vi
misceiia
• .•••i a
1
filtration
L_n_ steam ^
toasting
D used n-hexane
drying
1
cooling
h'
n
vaporization
p
J
"P
filter backlog
disposal
feeding stuff
FIG. 21—Production of rapeseed crude oils.
LOADS O N P R E S S U R E MEDIA IN OPERATION Oxidation During operation, the pressure media goes through a continuous change of its chemical properties due to the hydraulic load in the system. Over the long service life of the pressure media, its properties should change very slowly; this requires aging stability. During operation, the hydraulic load parameters such as pressure and temperature as well as shear stress affect the fluid. During operation, different types of contamination influence the service life of the pressure media. These can be liquids such as water or foreign oils, and solid particles such as abrasive materials or environmental dust. Those loads can affect the fluid chemically as well as physically. Of special importance for the stability of a pressure medium is its resistance against oxidative attack. Oxidative stability problems are aggravated by contact of the fluid with the ambient air in the tank of the system or with the air dissolved in the fluid, leading to a change of the chemical and physical properties. For the stability of lubricEints based on natural or sjmthetic esters, their resistance against the so-called autoxidation processes is of importance, since those reactions already start at
very low temperatures as they occur in hydraulic systems. Moreover, autoxidation represents the initial stage for polymerization processes leading to surface films such as varnish or lacquer [27,28]. Autoxidation starts with an induction period where the first carbon-radicals are initiated. To make this possible, activation is required, e.g., by mechanical supply of power through shearing or by a temperature increase. Furthermore, different catalytic agents, such as hydro-peroxides, peroxides or heavy-metal ions, accelerate this process, since these molecules undergo bond scission to produce free radical initiators. The necessity of a good fluid filtration is necessary because heavy-metal ions produced from the metallic wear debris produce those free radicals. When hydro-peroxides are subsequently produced from peroxide and another carboxylic acid, then a free C-radical is created. This radical reacts with the oxygen in the air to form peroxides, thus starting a chain reaction (initiation). The hydro-peroxide produced by this reaction is chemically unstable. It transforms easily into epoxides, keto-bonds, di-carboxylic acids, or other polymerization products. The reaction mechanism of autoxidation can be taken from Figs. 27 and 28. The autoxidation mechanism stops once the radical concentration exceeds a threshhold concentration, starting a reaction between the radicals (Fig. 28).
278 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK
I
NaOH )
Jp^ slime removal (90°C)
neutralisation
,
Tt. H3P04 (75%
ik
rotational separation
separation
i r H?n
}
}
H-RBntnnil
crude oil
1
M .•••iiJ 1
washing
I
l ' " ' . - „ „ y vacuum drying ' steam-V| iombar/90°C
HCI active
K
^
•
bleaching
soapy contents ^
I
H2SO4(30%)j & steam I
^
fatty acid raffination
I
H steaming
filtration
H I
steam J
P(
L ^ desodoration 2mbar/250°C
drying
filter backlog disposal
bleach disposal
PI
refined oil
1
FIG. 22—Refining process of rapeseed oil. CHj-OH + Ca(0H)2 H—C—H
+
CH3-C—H
OH-CH2-C—C—H + NaOH
9^3 O formaldehyde
9^30
acetal H—C—H
+
H—C—C—H
I
CH2-OH
HO-CH^C—C-
CH, formaldehyde
CH,
i-butanal
CHj-OH + HCHO, H2O O OH-CH2-C—C—H
-HCOOH
CH2-OH OH-CH2-C—CH2-OH CHj-OH
CH2-OH
pentaerythrltol
FIG. 23—Synthesis of alcohols by aldol condensation.
9H3O HO-CHj-C—C—H CH,
+ H,
CH, HO-CHj-C—CHj-OH CH,
FIG. 24—Synthesis of neopentylglycol.
CHAPTER
11: ENVIRONMENTALLY
FRIENDLY
OILS
279
+
H2O
1. Start
Hydrolysis Hydrolj^ic cleavage of the pressure media based on natural and sjTithetic esters represents a reaJ danger for the fluid, since the reaction to produce synthetic esters is an equilibrium reaction producing water. Since equilibrium reactions may proceed in either the forward or reverse directions, esters exposed to water cleave again into their alcohol and acid components. At room temperature and without mechanical load, this reverse reaction proceeds very slowly with synthetic esters so that no direct damage of the fluid contaminated with water is expected. However, u n d e r mechanical load and increased temperatures such as the t5rpical conditions that occur in a hydraulic system, the hydrolysis process is accelerated. In general, hydrolosis may follow two different paths: saponification or acid catalyzed hydrolysis [17]. Alkali saponification starts by adding a hydroxide ion to the carbonyl carbon atom of the ester. The hydroxide ion can be added to the reaction, e.g., by mixing sodium or potassium hydroxide to it. The reaction intermediate produces a very
+
-> XH
+
R
2. Chain building
R
+
O
-> ROO
ROD
+
RH
->• ROOH
+
R
RO
+
RH
-> ROH
+
R
+
RO
3. Chain splitting
ROOH ROOH
cyclohexanol
RH
-> RO +
+
ROOH
OH • ROO
adipic acid
FIG. 27—Autooxidation process of ester based fluids.
^CH, CH-OH
HNO, or
o.
O
H
H
H
II
I I
I I
I I II C—C—C—OH I I
HO—C—C—C— H
CH,
H
H
H
O
H
4. Chain termination adipic acid
cyclohexanone ^CH, CH,
HN03or —•
o,
O
II
H
I
H
I
H
I
H
I
O
II
R
+
R
>
R-R
R
+
ROO
- • ROOR
HO—C—C—C — C—C—C—OH
I
H
I
H
I
H
I
H
FIG. 25—Synthesis of adipic acids.
FIG. 28—Termination of oxidation process.
alcohol alcohol synthesis
carbon acids acid synthesis bleaching mineral acid
FIG. 26—Schematic ester synthesis.
280
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
1. step: hydroperoxide ion addition O
+
R1-C—O—R2
R1-C—O—R2
OH
HO 2. step: all^oxide ion formation 0~
O
R1-C—O—R2
~*"
I
R1-C—OH
-h
O—R2
HO 3. step: alkoxide deprotonation O -h _ II _ ' O—R2 R1-C—O FIG. 29—Alkalic saponification of esters.
R1-C—OH
1. step: ester protonation 0—R2
1
R1-C
-f-
R2-0H
0 — R2 +
H
R1 - c
•
II .
OH
0 2.step: water addition
H-- O — H
0 — R2 R1-C
HP
-h
il .
^
OH
OH
R1 - C — 0 — R2
H
R1-C—O*
1
I
OH
I
OH
R2
+
H
3. step: alcohol split off OH H R1-C—O*
—»•
1 1 OH R2
OH 1 1 R1-C
+
R2-0H
II . OH
4. step: hydrogen split off OH
0
R1-C M
-f
R2-0H
R1-C—OH
+
R2-0H
+
OH FIG. 30—Ester splitting under acid conditions.
basic cJcoxide ion during the subsequent elimination step. This alcoxide ion reacts with the carboxyhc acid instantly to ydeld a non-reactive carboxylate ion. This reaction is called saponification, and is shown in Fig. 29. During use, the precondition of the acid catalyzed hydrolosis of ester-based fluids is met because the acidity of the fluid increases during use due to fluid oxidation. However, con-
trary to saponification, acid catalyzed hydrolosis is reversible unless the water produced during hydrolosis is removed from the systems. Figure 30 shows the process steps of acid catalyzed hydrolosis. The proton of an acid is necessary to catalyze the addition of water. Following a very fast and reversible proton displacement, splitting of the alcohol takes place. Acid
CHAPTER is produced after a H^-ion is removed. The H"^-ion is then available again for hydrolyzing another ester bond. The reaction continues until the chemical equilibrium of the four components is restored again. In a pressure medium, free fatty acids represent the basic material for chemical reactions. They react with metallic abrasive particles contained in the fluid. From this reaction, so-called metallic saponification agents arise, leading to a possible functional i m p a i r m e n t of the hydraulic system. Metallic saponification agents deposit on the surfaces of the hydraulic components leading to a premature blockage of the filters.
ALTERNATION OF TECHNICAL AND CHEMICAL PROPERTIES BY AGING PROCESSES Oxidation Stability The aging stability of pressure media based on natural a n d synthetic esters is influenced by its oxidation stability. Oxidation stability is a fundamental quality used to differentiate between the different performance classes of pressure media. For a precise rating, different laboratory test procedures are available. Laboratory
Aging
The following describes different tests to evaluate oxidation stability which are performed with the modified Rotary Pressure Vessel Test (RBOT) according to ASTM D 2272. The RBOT test is conducted to determine the time necessary to react with a certain a m o u n t of oxygen. This test is illustrated in Fig. 31. The test fluid and a copper coil are put into a breaker and sealed in a steel container. The copper coil performs as a catalytic agent to accelerate the oxidation reaction. Thus, in the pressure vessel, metal catalytic oxidation takes place (see the Alcohol Bonds section above). The pressure vessel is filled with a specified a m o u n t of oxygen and put into a heated bath. Temperature control equipment provides for an adjustment between 40°and 150°C. The temperature is maintained at ± 1°C. An electronic sen-
11: ENVIRONMENTALLY
FRIENDLY
OILS
281
sor connected to a computer for data storage measures the pressure in the steel container. For hydraulic fluids based on esters, no water is added to the pressure vessel, since hydrolysis could occur. Oxidation stability is defined by the time required for maximum pressure to decrease 1.75 bar. This pressure drop depends on the amount of oxygen with which the lubricant reacts as a result of the oxidation process. To differentiate between the different fluids, tests are conducted at a temperature of 150°C. Three different base oils were evaluated. They are distinguished mainly by their degree of saturation and the alcohol used. Figure 32 depicts the test results for these three base fluids. Fluid G S l , an unsaturated trimethylolpropane ester, reacts quickly with the oxygen in the test vessel, showing only an extremely low resistance against oxidation. Continuous improvement of the oxidation stability is achieved by using partly a n d completely saturated esters. This can be read from the test results for the fluids GS2 and GS3. Each of these base fluids represents the next developmental phase of synthetic esters. Although the first generation of esters contained unsaturation, the next generation of fluids were saturated. The current generation of hydraulic (GS4) fluids are saturated, but they are produced by more economical procedures. For comparison, a mineral oil without any additives was also tested. Its oxidation stability is comparable to that of saturated synthetic esters. However, the test times exhibited by those fluids are not sufficient for use as pressure media. Therefore, the use of additives is necessary to increase aging coxidative stability. Corresponding improvements by using additives can be observed in Fig. 33. Results are shown for the oxidation of the base fluids with additives according to their special requirements. The fluids SI to S4 are each formulated from base fluids GSl to GS4, respectively. The mineral oil HLP is a formulated fluid with the base of the fluid "H." The low oxidation stability of the fluid GSl is improved significantiy by using additives. However, in comparison to HLP mineral oil and to the partly and fully saturated synthetic esters, the oxidation stability obtained is too low for use in high performance hydraulic systems. Adequate oxidation stability is provided with a test time above 200 min. Used oils should show a test time of at least 180 min. to provide adequate oxidation stability. Further development of the base fluids and especially the
1——1
i. 2
'1,75 bar
Z3 W CO
oxidationstability
£ a. TQ copper coii FIG. 31—Rotary bomb test rig.
time [min] T,
282
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK test parameter
40
35
35 30 25 20 15 H 10 o 5 0
20
20
1 11 3
GS1
1 1 1 1 GS2
1 1 1 11 11 11
temperature:
1 1
II 1
t 20
1 1
GS3
t 0 0
150 iC
GS4
base oil
atmosphere: 6,25 bar oxygen
limit:
1,75 bar pressure drop
catalyst: ^ g copper, 3m S 1,5 mm
FIG. 32—Oxidation stability of different base fluids (ASTM D 2272). test parameter
300' != 250!& 200-
1 150• | 100"
8
500-
267 220
tn HLP
1 1 1 1 1 1
fr
213
1 T
W
temperature:
24
150iC
atmosphere: / * ^ 6,25 bar \^Joxygen
70
limit: ^ T ^ 1,75 bar ^ ^ pressure x^P^drop catalyst:
SI
S2 nuid
S3
S4
^ ^
copper, 3m 1,5 mm
FIG. 33—Oxidation stability of different ester formulations (ASTM D 2272).
use of fully saturated synthetic esters results in a significant increase of oxidation stability. However, fully saturated base fluids are relatively expensive raw materials. Oxidation stability is also influenced by temperature. Oxidation stability decreases by one hcilf with every 10°C temperature rise. This rule of thumb is roughly confirmed for the examined fluids. Fluids with additives exhibit a slightly greater increase of reaction rate with increasing temperature than the base fluids (Fig. 34). Besides temperature, the catalyst being in contact with the fluid also influences oxidation stability. In hydraulic systems, many different materials are used for different tasks for a variety of hydraulic components. To evaluate the usability of a fluid, the influence of those materials on the aging performance of the fluid is important. In hydraulic systems, iron and copper alloys and different coated materisils are used as construction materials. Oxidation is significantly accelerated by the presence of certain metallic catalysts, as shown in Fig. 35. The very strong influence of copper on the oxidation sta-
bility compared to the remaining tested materials can easily be recognized. For the pure metals that were tested, these results coincide very well with the position of the different metals in the periodic systems. The position in the periodic system gives preliminary information on the reactivity of the metals. With decreasing periodic number, the catal5tic effect of the metals diminishes. Material coating and their base materials exhibit a very positive influence on the oxidation stability in comparison to the standard construction materials. For the use of rapidly biodegradable pressure media based on synthetic esters, a significant improvement in oxidative stability with increasing temperature is obtained by exchanging traditional construction materials with newly developed material coatings. Test Stand Aging Laboratory tests cire conducted to evaluate oxidation stability and the effects of aging pressure media. In a hydraulic system, different parameters that affect oxidation stability usu-
CHAPTER 11: ENVIRONMENTALLY ally occur in combination, thus accelerating the overall aging process. Therefore, to evaluate the fluid performance, custom aging test stands are required, which model a hydraulic system as precisely as possible. To obtain test results within a reasonable period, fluid loads in such test stands are greater than those of practical applications. The primary task of the test stand consists of examining fluid properties, thereby, the test stand design should be very simple. Figure 36 shows a test rig circuit fulfilling this requirement [8]. The hydraulic test rig circuit is operated as an open circuit. Test stands are constructed using mass-produced hydraulic components to model the loads activating the pressure media in a real system as accurately as possible. Variations of the pressure media are determined by analyzing fluid samples taken periodically. Acid number (AN) and viscosity are very important characteristic parameters to describe the aging condition. The load during operation is primarily responsible for the production of free acids. Those
FRIENDLY OILS
acids can attack non-ferrous metals, thus endangering the hydraulic system during use. In addition, viscosity is an excellent indicator of fluid quality. The molecular structure of the fluid influences its viscosity and its viscosity temperature performance. Depending on the reactions occurring in the fluid, viscosity increases or decreases. As a lifetime criterion for rapidly biodegradable pressure media derived from esters, a viscosity increase of 20% is applicable [8,23]. Results of the Test Stand Aging Studies For the test stand aging studies, the same fluids were used that were already tested with regard to their oxidation stability. The influence of different loads on the fluid was realized by a variation of system pressures and temperatures. The influence of those system parameters on the aging behavior is determined by the variation of acid number and viscosity at 40°C, after an aging time of 1000 h as shown in Fig. 37 [8].
test Darameter 10000 •
E
i 1
• GS1
temperature:
QS1
t.
variable
1 0 0 0 - ^^^^^^^= atmosphere:
10"
9
1 -
90 j C
Z * ^ 6,25 bar
\jjl
oxygen
limit:
^H
^H
_•
^^^^^1
^^^1
^ - 1 ^ 1 , 7 5 bar ^ S f j pressure >«_>'drop
•
1H H
1
120jC temperature [\C]
catalyst: ^ p copper, 3m ^ ^ 1,5 mm
15C>iC
FIG. 34—Temperature influence on oxidation stability.
test parameter
150
temperature:
• S1
-.125 E
110
>!ioo « 75
64
o
1 50 •a "S o 25
84
77
.a
i
150 iC
atmosphere:
RH
6,25 bar oxygen
46 limit:
24
®
1,75 bar pressure drop
catalyst:
Cu
Fe
283
Cr CrN Ti TiN materials and coatings
without
FIG. 35—Catalyst influence on oxidation stability.
^S variable
284
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
FIG. 36—Oil aging test rig [8].
test parameter: temperature: variable
I
pressure: S y\ variable volume flow:
® 300 bar 300 bar 150 bar 300 bar 300 bar 150 bar 90°C 60°C eO-C 90°C 60°C 60°C
GS1
GS1
GS1
S1
SI
20 l/min
water content: ^ < 0,03 % 4 HaO test duration:
S1
1000 hrs
FIG. 37—Change of TAN and viscosity after aging test runs.
The influence of high temperature and pressure indicates a high specific load on the fluid. Comparing the aging influence of pressure and temperature shows that the temperature exhibits the greatest influence on the aging. The influence of pressure is evidently less, at least as determined from tests performed at reservoir temperatures of 90°C and 60°C. Decreasing the reservoir temperature by 30°C reduces the aging process four-fold in the fluid as indicated by viscosity change. On the other hand, a 50% reduction of pressure yields only a very small reduction of the aging processes and rates. Absolute variation of fluid properties during tests are greater than conventional mineral oil based fluids.
Test runs on the aging test bench confirmed the very good oxidation stability of the more advanced fluids formulations (Fig. 38). Fluid S3, in particular, exhibits an aging stability comparable to mineral oils. The excellent stability is indicated by the constant viscosity throughout testing time. A comparison of results of the formulated fluids with their base fluids shows that the additives used and the base fluid correlate well with each other. Additives increase the aging resistance of the base fluid significantly. Table 3 illustrates the effect of additional fluid characteristics. Variations of these characteristics are only of minor importance with respect to the aging performance. Therefore, a graphical presentation is not shown.
CHAPTER
Aging
Rapidly biodegradable pressure media are primarily used in mobile hydraulic equipment, since they are often used in ecologically sensitive environments. These machines are often exposed to atmospheric influences so that there is always the danger of water ingression into the hydraulic system. Due to their chemical structure, synthetic esters tend to be hydrolytically unstable in the presence of water (see the Hydrolysis section above). Thus, evaluation of hydrolytic stability is of central importance for the assessment of the usability of rapidly biodegradable pressure media.
28
1
test parameter:
1
temperature: 90 °C
• viscx3sity
24
EBAN •5 X
IV.
g20
CO
•§12 >,
1^-
^°g.
1 8
I
o
o_
m
FRIENDLY
u
o
O 00_
1
lOCM
1^
o o"
GS1 GS2 GS3 S1 S2 S3 fluid generations
S4
pressure: 300 bar
0)
volume flow:
(0
®
o> c
V
I 4
z <
20 l/min
water content: ^ <
0,03 %
W
H,0
test duration:
HLP
1000 hrs
FIG. 38—AN and viscosity changes of different fluid generations after aging test runs.
TABLE 3—Important fluid properties. Property
Density Viscosity @40X Viscosity @ 100°C Viscosity index Pour-point Flash-point FZG-test Vickers pump test AN
Corrosion Steel
Alternative Test Methods'" ASTM^ ISC^
Unit
HEES 46
HLP 46
DIN Standard"
kg/m3 nim2/s
ca. 920
ca. 880 46
51 757 51562T1
D 1298 D445
3104
7,1
51562T1
D445
3104
100
ISO 2909
D 2270
2909
-27 220 12 <120 <30
ISO 3016 ISO 2592 51 354 T2 51389T1 51 389 Tl 51 558 D 664
D92 D97 D5182 D 2882
3016 2592 14635 20763'*
D 974 D 664 D 5770
3371 6618 7537
46
mm2/5
["C] [°C] Load level [mg] ring [mg] vane [mg KOH/g]
9,3 184 -42 290 12 <120 <30 1.2
0,3
D 665A 7120 D665B 7120 1-100A3 1-100 A24 D130 2160 Copper 51759 "The DIN test procedures shown here were used to obtain the values shown. * Although DIN test procedures were used for this work, equivalent ASTM tests may also be used. '' In some cases, the ASTM tests are not truly equivalent but the ASTM procedures cited may be used. '' This ISO Standard is still under development and has not yet been published. 0-A
OILS
285
The effect of hydrolytic stability was evaluated using the same procedure described above for oxidation stability in a laboratory test procedure. A procedure developed at IFAS (Institute of Fluid Power Transmission and Control, Aachen, Germany) is used. A scheme of the test equipment is provided in Fig. 39. In this case, a pressure-proof and lockable stainless steel container is used. The container features a stirring device that mixes the oil turbulently with the water used to contaminate the system. The container is heated by a heating bath so that the tests are run u n d e r actual operating conditions. During the test, a defined amount of test oil is mixed over 72 h together with a specified amount of water. Metallic catalyst
Hydrolysis Stability Laboratory
11: ENVIRONMENTALLY
0-A
51 585
286
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
test fluid (150 g) + catalyst (iron or copper) +
fluid analysis
(variable)
FIG. 39—Hydroiytic stability test.
3,0
1
1
test parameter:
•—
^^^^1,5% iron |^
temperature:
•32,5
Br 90°C
X
§2,0 E ft. "I-5 en c n u1.0 z < 0.5
fluid: GS1 test duration: 72hrs catalyst:
0,0 0,0
0,2
0,4
0,6 0,8 1,0 water content [%]
1,2
1,4
1,6
^ p
iron, 1,5%
FIG. 40—Hydroiytic stability of the GSI ester fluid vs. water content.
is added in the form of pulverized copper or iron to accelerate the hydrolosis reaction. Thereafter, the AN of the fluid is measured and the deposited hydrolysis products are investigated by means of SEM and element determination. Viscosity measurement is not helpful because it doesn't provide reproducible results due to the hydrolysis byproducts and deposits. Figure 40 provides the test results obtained with varied water content. It is evident that water contents below the saturation point only causes hydrolysis. As free water appears in the fluid, an accelerated attack on the fluid occurs. At water contents above 1.5%, the effect no longer increases linearly with increasing water content. Like the oxidation processes, hydrolysis is also strongly influenced by the presence of metallic particles. In addition, with this reaction, the effectiveness of the individual elements is different. Figure 41 shows the influence of the metallic catalysts on the hydrolysis of the GSI ester. Adding iron or copper chips considerably accelerates the reaction process. Iron exhibits the greatest effect relative to copper. Increasing the temperature also accelerates the hydrolosis reaction. The effect is exponential. Above 110°C, oxidation
processes also occur since the reaction vessel was not purged with nitrogen. Contrary to the oxidation tests reported earlier, the oxygen supply is very low since the container is only filled with atmospheric pressure air. Besides the determination of AN, deposit formation during the tests was determined. This was done by SEM microscopy to determine structure as shown in RBOT tests reported earlier, Fig. 42. The structure of the deposited particles differs most with regard to particle size. The particle size of the deposits was significantly smaller. The medium particle size of the contaminants measures about 10 ^i,m vs. about 60 /xm for the iron test dust of the laboratory investigations. After ultrasonification, there was no significant difference in the particle size and amount between those generated in the test rig cind those generated in the RBOT laboratory tests. In addition, the compositions of the deposits from the two tests were essentially the same. In addition to iron and copper, the deposits taken from the test rigs contained small amounts of tin and zinc, which are alloying elements of the non-porous materials used for construction of the test rig system components. The presence of phosphorous or sulfur.
CHAPTER which would be attributable to the additives used for fluid formulation, was not observed, which indicates that the data obtained from the test rigs correlates well with results obtained from laboratory tests such as RBOT. The analyses techniques, however, were unable to detect hydrogen. Based on the chemical reactions occurring, the presence of hydrogen would be expected, since metal saponification occurs with the metal catalyzed hydrolysis. Ester hydrolysis follows the reaction process described earlier. The faster hydrolytic reaction of iron vs. copper is due to the stronger acidity of iron relative to copper. Hydrolytic
Stability
Test With Test
Stands
Further assessment of the hydrolytic stability of different ester-based fluids was conducted using the test stand. These
1
„5 X 04
11: ENVIRONMENTALLY
FRIENDLY
test parameter: temperature:
1
I
variable
fluid:
1
GSi 0,8 % H2O
to O O)
c
test duration
10
72lirs
^1
catalyst:
40
287
studies were performed with significantly lower loads relative to those used for the studies discussed previously to prolong the use lifetime of the hydraulic components used for the test stand. Initially, the first generation of ester-based fluids (vegetable oils), were evaluated using varying water content u p to a maximum of 0.1%, which is the maximum allowable value r e c o m m e n d e d by VDMA (Association of German Machine Manufacturers) [36,37]. (This is an empirical recommendation, which was not derived experimentally.) For this work, the water contaminant was injected slowly into the suction pipe of the p u m p on the test stand. Excellent mixing of the fluid a n d water c o n t a m i n a n t was obtained from the "whirling" action of the oil flow in the p u m p and the subsequent turbulent motion of the fluid on the pressure re-
- C ^ 1,5% iron -0— 1,5% cxDpper
20
OILS
60 80 temperature [°C]
100
120
140
variable
FIG. 41—Hydrolytic stability influence of a catalyst on the hydrolysis of the GSI ester fluid.
ultra sonic treatnmnt latoratory test
?m*
I FIG. 42—Hydrolytic splitting products.
'•:"|l
288 MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK test parameter: temperature: 60-0
I 0;
pressure: 150 bar
volume flow:
®
20 l/min
water content: ^ ^ variable %0 see figure
0,03% GS1
0,1% GS1
0,5% GS1
0,03% SI
0,1% SI
0,5% SI
test duration: 1000 lirs
FIG. 43—AN and viscosity changes after aging tests in the test rig with water contamination.
test parameter: temperature: 60 °C
1
pressure:
0-
isobar
volume flow:
®
20 l/min
water content:
^
GS1
GS2
GS3
S1
S2
S3
o.syoHjO
test duration: 1000 hrs
FIG. 44—AN and viscosity changes after aging tests in the test rig with water contamination and using different generations of fluid development. The oldest was GSI and the most recent fluid candidate is S3.
lief valve. Water content was determined at 24 h intervals and corrected, if necessary, to maintain the water content in the fluid for the duration of this test. Figure 43 shows the results of the investigations conducted with the fluids GSI and 81. The influence of the water content during the aging (hydrolysis) of the fluid is clearly evident from the variation of the AN of the fluid during the test. All the tests using these base fluids always showed a decrease of AN, whereas the tests in which water was added for the first time exhibited an increase of AN. This is an indication of increased hydrolysis of the fluid into alcohol and acid components. Figure 44 illustrates the results obtained for different fluids, which were evaluated at 0.5% water. The fluids shown in the figure vary in order from the earliest candidate (GSI) to
the most recently developed fluid candidate (SI). The results showed that the susceptibility to hydrolysis decreases with increasing advances in ester-base stock fluid structure. The observed improvement in hydrolytic stability is due to the greater stability offered by the saturated versus unsaturated structure and the improvement of the additives used. For example, fluids S2 and S3 exhibit significantly better demulsifying characteristics (ASTM D 2711) relative to the SI fluid, which causes the water to separate to the bottom of the fluid in a short time. This reduces the amount of water available for hydrolytic degradation throughout the test, thus reducing the overall hydrolysis rate. Varying the viscosity and AN of water contaminated fluids is relatively unimportant compared to the impact of the effects
CHAPTER of increased system pressure and reservoir temperature. In addition, it is also important to consider system component compatibility. For example, water is soluble in the ester-based fluid u p to 0.1% b.w. and 60%. Under these conditions no deleterious system effects were observed. However, water contents of 0.5% b.w. are insoluble in the fluid and significant attack on the system is observed. Although an oil-water emulsion is formed initially, phase separation does subsequently occur depending on the demulsification properties of the fluid. The extent of fluid hydrolysis was greatest with the GSI fluid, which contained no additives. All of the fluid candidates contained emulsifies deposits at the b o t t o m of the reservoir and the filter elements during the test run. These deposits are formed due to the emulsification of the fatty acids, which react with the corrosion metals from the system components. This process may result in filter plugging even after only a few hours of operation during the test. This occurs because the filter elements are equipped with a by-pass valve that opens once a preset differential pressure is exceeded at which time the fluid invades the system without filtration. This results in an increase of solid abrasion particles (contamination) in the system. The tests didn't show any impairment of the lubricating properties of the fluids caused by the water as contaminant. However, indirect consequence of water contamination is that 3-body-abrasion causes increased wear on the hydraulic components. The improved hydrolytic stability of more recently developed fluids produces smaller amounts of deposits on the bottom of the reservoir. A comparison of the composition of these deposits relative to those found in laboratory tests was discussed earlier.
LOW TEMPERATURE PROPERTIES OF ESTER BASED FLUIDS For the use of pressure media in mobile applications, the performaiace at low temperatures is of great importance to prevent failure on the hydraulic system during cold weather operation (such as in the winter). Viscosity Properties To evaluate the viscosity properties of a pressure media at low temperatures, two characteristically different rates of cooling can be distinguished. In a case of fast cooling, the change of viscosity with respect to temperature is important. This also determines the viscosity class that can be used to fill the hydraulic system (ASTM D 6080). The low temperature behavior during slow cooling, exposure to low temperatures, a prolonged time is important, since esters often show a time-dependent solidification. This starts with a separation of paraffin crystals, such as that which occurs in diesel fuel. These crystals may cause clogged filter elements. Figure 45 shows the viscosity-temperature dependence as well as the viscosity index (VI) of different fluids. Viscosity index is measured at the variation of viscosity with temperature. The greater the VI, the less sensitive the fluid viscosity is to variation in temperature. Comparison to a mineral oil is provided. Synthetic esters show significantly better viscosity properties due to their higher VI. At low temperatures, the
11: ENVIRONMENTALLY
FRIENDLY
OILS
289
viscosity of synthetic esters is considerably lower than mineral oil, providing a safe operation of a hydraulic system after changing over from a mineral oil to a synthetic ester of the same viscosity class. At high temperatures, viscosity of the same ISO viscosity class (ISO 3448, ASTM D 2422) is significantly higher, so that oil of the next lower viscosity class might be used. Consequently, synthetic esters perform like a multi-grade oil. ASTM D 6080 provides a procedure for esterbased hydraulic oil as a function of use temperature. Crystallizing P r o c e s s e s With Slowly Decreasing Temperatures With very slowly decreasing temperatures a first formation of crystals may occur at approximately - 2 0 ° C in synthetic esters. As experienced with vegetable oils, the degree of crystallization is dependent on time [8,23,26,32]. For the fluid GSI, the completely crystallized condition is achieved after a four day storage time at a temperature of — 45°C. Consequently, the temperatures necessary for crystallization are about—30°C lower than those for rapeseed oils and are below the temperatures where mineral oils are no longer able to flow. Thus, a safe operation of hydraulic systems operated with synthetic esters instead of mineral oil is guaranteed at low temperatures. From Fig. 46 the start of crystallization can be taken for the generations of fluids for the fluids SI, S2, and S3, which are branched, saturated diester-based fluids. Moreover, different filterability tests were conducted to assess the low temperature performance. Good filtering properties of the pressure media are a precondition for a safe operation of the hydraulic system because if the fluid is excessively viscous, the p u m p will be starved for fluid and cavitation will result. If the fluid viscosity is too low inadequate lubrication will result. For the filterability tests the experimental system shown in Fig. 47 was used that was developed at IFAS. The fluid volume enclosed in the cylinder is pressed through a membrane filter by means of compressed air at different temperatures. Measures for filterability are either the time necessary for a certain volume to pass the filter or the volu m e flow through the filter element in dependence of time. The diagram shows the filterability properties of fluid GSI, since no significant differences were found between the individual fluids based on synthetic esters. Compared to mineral oil HLP 46, those fluids show a considerably better filterability. In the case of mineral oil, the filter m e m b r a n e clogs at temperatures of — 10°C after a test time of only 60 s. The volu m e flow of fluid GSI through the filter element diminishes within the same time by only 25%. However, when the SJTIthetic ester is contaminated with 0.5% water, the good filtering properties are lost, causing the filter element to clog after a very short time. Since the water is no longer dissolved, but finely dispersed in the Quid, ice crystals can form resulting in clogged filter elements. ECOLOGICALLY TOXIC PROPERTIES Test Methods For the investigation and development of environmentally friendly, biologically fast degradable pressure media discussed above, various methods currently in use in Europe
290 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK test oarameter: 10^
'
^^\*-
'
•
'
'
'
'—'—'—'—
^«
A.^
SN VkV m
\ ', \^ KX ^ ^3
•^
GS1
•
S2
•f-
GS3
+
P3
—
HLP46
fluid
^fe
? \ \
slope
•1 • i 3.34
S2
VI
• •
^M WM• i ^ in
• 1
80 .
^ % § «
63,550 • 40 • 31,5.
>
• •
25 . 20 • 15,81
12,5-
^ \
in _
1
u -W
^\ 6 5-60
-zto
-2 0
(3
20
A0
60
80
100
temi:jera ture [iC]
FIG. 45—Viscosity vs. temperature of ester fluids.
a
-10
aging parameter: temperature: 90 °C
i Hi ni 11 Mq HI I I MM nl I ! I 111
pressure:
/ * ^ 300 bar volume flow:
®
water content: / ^ < 0,03 %
new fluid 1000 h aging GS1
GS2
GS3 fluid
S1
20 l/min
W S2
S3
H,0
aging time: 1000 h
FIG. 46—Crystallization of esters at low temperatures. (Crystallization properties are determined by IFAS lab test procedure).
CHAPTER 11: ENVIRONMENTALLY FRIENDLY OILS 291 test parameter: temperature.:
GS1 GS1 0,5% HP — X - -HLP46
20
—A—
I
~~~~-J
,,___^
-10 °C
compressed air (6 bar) "^
"i 5 10. o 5=
E
-~
50
<•
100
150 time [s]
200
ZO filter [5|jm]
FIG. 47—Filtering properties of esters at low temperatures. (Flexibility was determined by IFAS lab test procedure)
were employed. The examination of the toxicological properties of rapidly biodegradable fluids is a critically important parameter in evaluation of the use-properties of these fluids. This includes the self-classification of the test fluids into water hazard classes [40], since exclusively finished products and no individual components were tested. These properties were determined for this work using both new and used fluids. Aged fluids were evaluated to determine impact of aging on biodegradability and toxicity. The VCI (Verband Chemischer Ingenieure) classification system [6,29,32] comprises the tests shown in Fig. 48 and Table 4. Test Results Table 4 provides an insight into the test-substance specific investigations, results, and (preliminary) WGK-classifications (WOK = water hazard class) [40]. ASTM International has developed various standards to provide guidance in determining the biodegradability and ecotoxicity properties of hydraulic fluids. A brief summary of the slope of these tests will be provided here and more detailed discussion is provided in Chaper 34, Environmental Characteristics of Fuels and Lubricants. Biodegadability is one of three characteristics that are assessed when evaluating the environmental impact of a hydraulic fluid. The other two characteristics are ecotoxicity and bioaccumulation. D 6006—Guide for Assessing Biodegradability of Hydraulic Fluids This guide provides information to assist in planning a laboratory test or series of tests that would provide information about the degradability of an unused fully formulated hydraulic fluid in its original form. Biodegradability may be considered by type of environmental compartment: aerobic fresh water, aerobic marine, aerobic soil, and anaerobic media. Test methods for aerobic fresh water, aerobic soil, and
anaerobic media have been developed that are appropriate for each compartment and aire reviewed here. D 5864—Test Method for Determining the Aerobic Aquatic Biodegradation of Lubricants or Their Components This test method covers the determination of the degree of aerobic biodegradation of fully formulated lubricants or their components on exposure to an inoculum under laboratory conditions. This test method is intended to specifically address the difficulties associated with testing water insoluble materials and complex mixtures such as are found in many lubricants. D 6046—Classification of Hydraulic Fluids for Environmental Impact This classification covers all unused fully formulated hydraulic fluids in their original form. Classifications for categories for the impact of hydraulic fluids in different environmental compartments is provided. Fresh water and soil environmental compartments are addressed. D 6384—Terminology Relating to Biodegradability and Ecotoxicity of Lubricants Terminology relating to biodegradability and ecotoxicity of lubricants is provided in this standard.
SUM]VIARY AND OUTLOOK ON FUTURE DEVELOPMENTS IN THE FIELD OF PRESSURE MEDIA The overall broad grouping of esters offers a variety of different fluids from which pressure media can be produced, making a uniform description impossible. However, due to their basic structure, ester-based fluids differ substantially
292 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK
WGK classification
r pre test
. J'"flS'Ctlr,nV-,l compulsory
i--.taxids-.i'J
daphni=•<•:••>-.'*'/
mm^ test of components
H
special case
additional toxicidy tests
biodegradability & mobility
FIG. 48—Ecological test for German water hazard classification [18,19,39,41-45].
Germ-toxic Test method Unit GS2 Unused GS2 1000 h Aging S2 Unused S2 1000 h Aging
TABLE 4—Ecological properties of a formulation and the base fluid. Biodegradability Fish-toxic Mammal-toxic Biodegradability
DIN 38412 T 8
OECD 203
ECio [mg/L] > 10.000 > 10.000 >10.000 > 10.000
LC50 [mg/L] > 10.000 > 10.000 > 10.000 > 10.000
OECD 401 Limittest LD50 [mg/L] >2.000 78 >2.000 84 >2.000 89 >2.000 89
CEC L-33-A-93 Depletion [%] N/A N/A 76 64
BODIS ISO/DIN 10708 Depletion [%] 0 0 0 0
transfer of triboligical properties from the fluids to the materials
OH complete addrtive package
strategy - coatings - eccological acceptable fluids - data bases & simulation tools - examinations on trSbological modells - examinations on machines
FIG. 49—Strategies for future fluid and material developments [21].
Oil less addrtivs
Biodegradability WGK VCI-strategy
CHAPTER 11: ENVIRONMENTALLY FRIENDLY OILS 293 from mineral oil-based lubricants. The different basic structures and their physical and chemicEil properties were explained in detail. When examining their fluids, properties of the esters were found to be both similar and different to those of mineral oils. Esters offer considerable improvement relative to mineral oils with respect to low temperature properties and lubricity. Oxidation stability, hydrolytic stability and, in general, aging stability were deficiencies relative to mineral oil, especially for unsaturated fluids. However, the use of completely saturated fluids eliminated those deficiencies. All tests distinctly demonstrated the application-related advantage of completely saturated fluids in high-load hydraulic systems. The direct assessment of the different developmental stages of the fluids allowed an excellent coordination of the additives with the respective basic fluid. Future development in the field of lubricants will increasingly consider the entire tribological system with the aim of making it ecologically more compatible. This will require the development of environmentally compatible lubricants as well as the use of materials complementing those lubriccints with regard to their tribological properties, to provide increasingly more tribological functions of the lubricants. For example, a further reduction in the toxicological properties of biodegradable hydraulic fluids is necessary. Additional improvements in ester-basestocks chemistry is also important to provide further improvement in film-forming and antiweeir properties. It's likely that the focus will remain on ester-basestock development in view of their overall excellent biodegradable and toxicological properties. However, further and substantial improvement is needed in their oxidation properties for t h e m to rival those exhibited by mineral oil. These functions may potentially be provided by additive development. Calculation or simulation of friction and wear are still not possible. Therefore, work will continue to model tribological systems by analogy to those already known, using laboratory tests. Of course, this process necessitates subsequent validation in actual hydraulic systems.
D 665
D 974 D 1298
D 15 3 3 D 2270 D 2422 D 2619 D 2882
D5182
D 5770
D 5864
D 6006 D 6046 D 6080 D 6158 D 6384
ASTM STANDARDS No. D 2272
D 92 D 95
D 97 D 130
D 445
D 664
Title Standard Test Method for Oxidation Stability of Steam Turbine Oils by Rotating Pressure Vessel S t a n d a r d Test Method for Flash a n d Fire Points by Cleveland Open Cup Tester Standard Test Method for Water in Petroleum Products and Bituminous Materials by Distillation S t a n d a r d Test Method for Pour Point of Petroleum Products Standard Test Method for Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test Standard Test Method for BCinematic Viscosity of Transparent and Opaque Liquids (the Calculation of Dynamic Viscosity) S t a n d a r d Test Method for Acid N u m b e r of Petroleum Products by Potentiometric Titration
E 203
S t a n d a r d Test Method for Rust-Preventing Characteristics of Inhibited Mineral Oil in the Presence of Water Standard Test Method for Acid and Base Number by Color-Indicator Titration Standard Practice for Density, Relative Density (Specific Gravity), or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method Standard Test Methods for Water in Insulating Liquids by Coulometric Karl Fischer Titration Standard Practice for Calculating Viscosity Index from Kinematic Viscosity at 40 and 100°C Standard Classification of Industrial Fluid Lubricants by Viscosity System Standard Test Method for Hydrolytic Stability of Hydraulic Fluids (Beverage Bottle Method) Standard Test Method for Indicating the Wear Characteristics of Petroleum and Non-Petrole u m Hydraulic Fluids in Constant Volume Vane P u m p S t a n d a r d Test Method for Evaluating the Scuffing Load Capacity of Oils (FZG Visual Method) S t a n d a r d Test Method for Semiquantitative Micro Determination of Acid Number of Lubricating Oils During Oxidation Testing Standard Test Method for Determining Aerobic Aquatic Biodegradation of Lubricants or their Components Standard Guide for Assessing Biodegradability of Hydraulic Fluids Standard Classification of Hydraulic Fluids for Environmental Impact Standard Practice for Defining the Viscosity Characteristics of Hydraulic Fluids Standard Specification for Mineral Hydraulic Oils Standard Terminology Relating to Biodegradability and Ecotoxicity of Lubricants Standard Test Method for Water Using Volumetric Karl Fischer Titration
OTHER STANDARDS International Organization for Standardization (ISO) Title Petroleum products; determination of flash and fire points: Cleveland open cup method Petroleum products; determinaISO 3016 tion of pour point Petroleum p r o d u c t s and bitumiISO 3733 nous materials; determination of water, distillation method ISO 6743/4 -1982 Part 4: Lubricants, industrial oils and related products (class L); classification. Family H (hydraulic systems) No. ISO 2592
294
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
D e u t s c h e s Institut fur N o r m u n g (DIN) No. DIN 51 354 Part 2
DIN 51 381
DIN 51 389 Part 2
DIN 51 502
DIN 51 517 Part 1 DIN 51 519 DIN 51 524 P a r t i DIN 51 550 DIN 51 558 P a r t i
DIN 51 561
DIN 51 562 Part 1
DIN 51 566
DIN 51 569
DIN 51 585
DIN 51 587
DIN 51 592
DIN 51 599
DIN 51 757
Title Testing of lubricants: mechanical testing of lubricants by the FZG gear test rig method; gravimetric m e t h o d for A/8, 3/90 lubricating oils Testing of lubricants a n d hydraulic fluids; d e t e r m i n a t i o n of air release properties; impinger method Testing of lubricants; mechanical testing of hydraulic fluids by the vanep u m p method: m e t h o d A for anhydrous hydraulic fluids Lubricants a n d related materials; designation of lubricants and marking of lubricant containers, lubrication equipment and lubrication points Lubricants; lubricating oils; C lubricating oils; m i n i m u m requirements Lubricants; ISO viscosity classes for industrial liquid lubricants Pressure fluids; hydraulic oils; HL hydraulic oils: m i n i m u m requirements Viscometry; determination of viscosity; general principles Testing of mineral oils; determination of neutralization number; color indicator titration Testing of mineral oils, liquid fuels and related liquids; measurement of viscosity using the Vogel-Ossag viscometer: temperature range: approximately 10 to 1 50°C Viscometry; determination of kinematic viscosity using the standeird design Ubbelohde viscometer (at present at the stage of draft) Testing of lubricating oils: determination of foaming characteristics Testing of mineral oils, liquid fuels and related liquids; measurement of viscosity using the Vogel-Ossag viscometer: t e m p e r a t u r e range: —55 to approximately + 10°C Testing of lubricants; testing of corrosive effect of steam turbine oils and hydraulic oils containing additives Testing of lubricants: determination of aging behavior of steam turbine oils and hydraulic oils containing additives (at present at the stage of draft) Testing of lubricants; determination of the content of u n dissolved matter in lubricating oils; membrane filter method Testing of lubricating oils; determination of demulsification capacity by the stirring method Testing of petroleum products and related materials; determination of density
DIN 51 759
DIN 51 848 Part 1
DIN 53 505 DIN 53521
DIN 53 538 Part 1
Testing of liquid hydrocarbons; determination of the effect of corrosion on copper; copper strip test Testing of petroleum products; precision of test methods; general introduction; concepts and their application to petroleum s t a n d a r d s specifying requirements Testing of elastomers; Shore A and D hardness testing Testing of rubber and elastomers: determination of their resistance to liquids, vapours and gases Standard reference elastomers; peroxide-cross-linked acrylonitrilelbutadiene rubber (NBR) for characterizing service fluids with respect to their action on NBR
REFERENCES [1] Backe, W., The Present and Future of Fluid Power, Proceedings of the Institution of Mechanical Engineers, Institution of Mechanical Engineers, Milwaulkee, WI, Dec. 1993. [2] Backe, W. and Busch, C, "Biologisch Schnell Abbaubare Hydraulikflilssigkeiten," Tribologie + Schmierungstechnik, No. 1, 1995, pp. 30-35. [3] Becker, R. and Knorr, A., "Antioxidantien fiir Pflanzliche Ole," Tribologie + Schmierungstechnik, No. 5, 1995, pp. 232-240. [4] Bongardt, F., Einfluji der Chemischen Struktur aufdas Luftabscheidevermogen und die Hydrolytische Stahilitdt von Estem, Teil 1, Fat Science Technology, No. 12, 1990, pp. 607-613. [5] Bongardt, F., Einfluji der Chemischen Struktur aufdas Luftabscheidevermogen und die Hydrolytische Stabilitat von Estem, Teil 2, Fat Sci. Technology, No. 12, 1990, pp. 614-619. [6] Bongardt, F., "Native Ester: Basisole fiir leistungsfahige und Umweltvertragliche Hydraulikfliissigkeiten," Tribologie 2000 Band 1, TAE, Esslingen, 1992. [7] Busch, C, "Biologisch Schnell abbaubare Hydraulikfliissigkeiten," Tribologie und Schmierungstechnik, Vol. 41, No. 1, 1994, pp. 23-31. [8] Busch, C, "Untersuchung und Analyse der Eigenschaften und Eigenschaftsanderungen einer Rapsolbasischen Druckfliissigkeit in ihrer Funktion als Druckiibertragungsmittel," Dissertation RWTH Aachen, Aachen, Germany, 1995. [9] Busch, C, Rapsol auf dem Priifstand - Alterungsstabilitat und VerschleiySschutzvermogen, 10th AFK, Aachen, Germany, 1992. [10] Dimming, T., Hartmann, J., Wunderwald, U., and Schafer, V., "Additivkomponenten auf Basis nativer Ole," Tribologie + Schmierungstechnik, No. 6, 1994, pp. 375-380. [11] Feldmann, D. G. and Remmelmann, A., Biologisch Schnell Abbaubare Hydraulikfliissigkeiten—Ergebnisse von Priifstandstests und Folgerungen fiir die Anwendung, 12th AFK, Aachen, Aachen, Germany, 1996. [12] Feldmsinn, D. G. and Hinrichs, J., Biologisch Schnell Abbaubare Hydraulikfliissigkeiten—ein Neuartiges Konstruktionselement fiir Hydrostatische Getriebe, Konstruktion, VDI Verlag, Dusseldorf, Germany, 1995. [13] Fessenbecker, A. and Korff, J., "Additive fiir Okologische Unbedenklichere Schmierstoffe," Tribologie + Schmierungstechnik, No. 10, 1995, pp. 623-629. [14] Mang, T., Umweltvertragliche Hydraulik, 12th AFK, Aachen, Germany, 1996 [15] Fessenbecker, A., "New Additive for the Hydrolytic Stabilisation
CHAPTER 11: ENVIRONMENTALLY FRIENDLY OILS 295 of Ester Lubricants," Tribology Solving Friction and Wear Problems, Technische Akademie Esslingen, Ostfildern, Germany, 1996. [16] Gold, P. W., "Tribologie 1," Umdrucke zur Vorlesung, RWTH, Aachen, Germany, 1996. [17] Francke, W. and Walter, W., Lehrbuch der Organischen Chemie, 23, S. Auflage, Ed., Hirzel Verlag Stuttgart, Leipzig, Germany, 1998. [18] Hund, K. Fabig, W., and Btinemann, T. F., "Biologische Abbaubarkeit von Synthetischen Schmierstoilkomponenten," Tribologie + Schntierungstechnik, No. 1, 1993, pp. 48-56. [19] Freitag.R., "Gesetzliche VorschriftenfiirdieEntsorgungvonAltol," Okologische und Okonomische Aspekte der Tribologie Band 1, Technische Akademie Esslingen, Ostlildem, Germany, 1994. [20] Hart, H., Organische Chemie, ein Kurzes Lehrbuch, VCH Verlag, Weinheim, Germany, 1989. [21] Holderich, W. F., "Umweltvertragliche Tribosysteme," Entwicklung Neuartiger Schmierstoffe und Druckiibertragungsmedien mit Hilfe Chemischer Methoden, Teilprojekt Al im SFB 442 RWTH, Aachen, Germany, 1997. [22] Ihrig, H., "Umweltvertragliche Schmierstoffe in den 90er Jahren," Tribologie + Schmierungstechnik, No. 3, 1992, p p . 103-110. [23] Kempermann, C. and Remmelmann, A., Umweltschonende Hydraulikflussigkeiten, o +p. No. 1, 1996, pp. 67-73. [24] Mang, T., "Schmierstoffe u n d Druckiibertragungsmedien im Maschinenbau," Umdruck zur Vorlesung, RWTH, Aachen, Germany, 1998. [25] Murrenhoff, H., "Grundlagen der Fluidtechnik, Teil 1 Hydraulik," Umdruck zur Vorlesung, RWTH, Aachen, Germany, 1998. [26] Murrenhoff, H. and R e m m e l m a n n , A., "Environmentally Friendly Pressure Media Based on Synthetic Esters—New Changes for Mobile Hydraulics," SAE International Paper 98185, Presented at t h e Earthmoving Industry Conference, Peoria, IL, 1998, Society of Automotive Engineers, Warrendale, PA. [27] Paetzold, P., Einfiihrung in die Allgemeine Chemie, 2, Auflage Friedrich Vieweg & Sohn, Braunschweig, Wiesbaden, Germany, 1988. [28] Schmitt, H.-G., "Komplexester aus Pflanzlichen Olen," Tribologie -h Schmierungstechnik, No. 1, 1994, pp. 38-43. [29] Schiilert, G., Bernhard, U., Ude, G., a n d Geiger, G., "Alterungsverhalten von Umweltschonenden Hydraulikfliissigkeiten," Okologische und Okonomische Aspekte der Tribologie Band 1, Technische Akademie Esslingen, Ostfildern, Germany, 1994. [30] Schwetlick, K. U. A., Organikum, 20, Auflage, Johann Ambrosius Barth Verlag Heidelberg, Leipzig, Germany, 1996.
[31] Spilker, M. and Bock, W., Die Umwelt als Ideologietrdger, FluidTechnik,No. 4, 1995. [32] Vetter, J., "Synthetische u n d Nachwachsende Grundolkomponenten—Produktsicherheit, Eigenschaften, Gesundheits- und Umweltaspekte," Biologisch Schnell Abbaubare Schmierstoffe und Arbeitsfliissigkeiten, Technische Akademie Esslingen, Ostfildern, Germany, 1995. [33] Wendorf, J., "Daueruntersuchung von Hydraulikolen auf Rapsolbasis in Mahdreschem und Selbstfahrenden Feldhackslem," Biologisch Schnell Abbaubare Schmierstoffe und Arbeitsfliissigkeiten, Technische Akademie Esslingen, Ostfildern, Germany, 1995. [34] Streitwieser, H. and Heathcock, J., Organische Chemie, 1. Auflage VCH-Verlag, Weinheim, Germany, 1990. [35] DIN 51 524 T1-T4 Hydraulikole, Mindestanforderungen, Deutsches Institut fur Normung e. V., Beuth Verlag, 06.1985, Berlin, Germany. [36] VDMA-Einheitsblatt 24 568: Biologisch Schnell Abbaubare Hydraulikfliissigkeiten—Technische Mindestanforderungen, 3, Beuth Verlag, Berlin, 1994. [37] VDMA-Einheitsblatt 24 569: Biologisch Schnell Abbaubare Hydraulikfliissigkeiten—Umstellungsrichtlinien, Beuth Verlag, Berlin, 1994. [38] Bundes Imimissionsschutz-Gesetz, Law of the Federal Republic of Germany, 1998. [39] AbschluySbericht zum Forschungsvorhaben, "Synthetische Ester," VDMA, Frankfurt, Germany, 1997. [40] KBwS: Bewertungsmuster zur Stoffeinstufung in Wassergefahrdungsklassen im Sinne von § 19 Wasserhaushaltsgesetz; Chemiereport VCI, 1989. [41] OECD: Guidelines for Testing of Chemicals; OECD 203, OECD 401, Organization for Economic and Co-operation and Development, Paris 1981 (adopted 1983, 1984, 1986, 1987, 1989 und 1992). [42] DIN 38412 T 8: Testverfahren mit Wasserorganismen L8, Beuth-Verlag, Berlin, 1993. [43] ISO/DIS 10708: Water Quality—Evaluation in an Aqueous Medium of the "Ultimate" Aerobic Biodegradability of Orgsinic Bonds—Method by Determining the Biochemical Oxygen Demand (two-phase closed bottle test). International Organization for Standardization, Geneva, 1995. [44] CEC L-33-A-93: Biodegradability of Two-Stroke Cycle Outboard Engine Oils in Water; Commission for Environmental Cooperation, 1995. [45] DIN 51828-2: B e s t i m m u n g der Schnellen Biologischen Abbaubarkeit—Infrarotspektrometrisches Verfahren; Gelbdruck, Beuth-Verlag, Berlin, 1995. [46] UUmanns Enzyklopddie der Technischen Chemie, Vol. 11, Auflage Verlag Chemie, Weinheim, Germany, 1976.
MNL37-EB/Jun. 2003
Turbine Lubricating Oils and Hydraulic Fluids W. David Phillips^
back or outsourced. Operators are trying to extend the time between overhauls and are therefore seeking longer component lives. This review of turbine oils and fluids is therefore set against a background of significant technological change. Perhaps, not surprisingly, the same pressures that are changing the commercial landscape are also driving developments in lubricant and hydraulic fluid technology.
T H E POWER GENERATION INDUSTRY IS CURRENTLY UNDERGOING
the most radical changes it has seen in its relatively short history. The very conservative, slow moving industry dominated by state utilities has, in the last ten years, seen a major transition for both political and commercial reasons. Many areas of what used to be a highly regulated market have become very competitive with the privatization of large, governmentowned power generators and a developing global energy market [1,2]. With the break-up of the state monopolies, smaller, independent power producers have appeared seeking to install plant at the lowest cost and to make a rapid return on their investment [3]. At the same time, intense competition in the turbine industry has resulted in the development of equipment with substantially increased efficiency as the participants seek to increase their meirket share. On the political front, apart from the decision to deregulate the power industry, there has been environmental pressure to halt the construction of, and even replace, nuclear power stations and to reduce the effect of emissions on global warming by switching from coal and oil to gas and 'green' energy such as wind power. In Europe, safety of machinery legislation [4] has been introduced which, although not specifically directed at the p o w e r industry, nevertheless imposes constraints upon it. As might be expected, such changes, while determining the commercial shape of the industry well into the next century, also have a major impact on its technical requirements. For example, increased competition has driven improvements in equipment efficiency. This, in turn, has resulted in turbines operating at ever higher temperatures [5]. According to one report, for each increase of 100°F in firing temperatures of gas turbines, the output is increased by 10-13% with a gain of 2-4% in simple cycle efficiency [6]. This has the effect of placing greater thermal stress on the system components— including the lubricant—and an adverse impact on operating life. Such pressures have forced the steam turbine builders to consider technology previously used only in aero-derivative gas turbines, w h i c h operate at the highest t e m p e r a t u r e s found in the industry [7].
TURBINE TYPES The source of the kinetic energy for conversion into power depends on the turbine type. In a steam turbine it is the rapid expansion of high-pressure steam, and in a gas turbine, the expansion of fuel combustion products. With wind emd water turbines, the movement of a mass of air, or water under pressure, is the energy source. To convert the rotational energy into a more useful form of power, turbines are coupled to alternators for electricity generation or to a pump, compressor, or fan when used as a mechanical drive [8]. Depending on the speed of the turbine rotation it may be necessary to introduce a reduction gear between the turbine and driven equipment for steam, industrial gas, and wind turbines. Aero-engine gas turbines are not directly coupled to other equipment, but instead the energy of the expanding gases is used to provide forward motion, as in aviation usage, or to drive a second turbine wheel, sometimes called a power turbine, which is independent of the power source and attached to an alternator or compressor. To summarize, turbines are manufactured in a wide variety of sizes and for several different applications as follows: • For power generation, where they may drive a generator, either directly or indirectly via a gearbox. • As a direct mechanical drive for p u m p s and compressors. • As gas generators for propulsion (in the aircraft industry), or when used with an independent power turbine, for driving p u m p s and compressors. Steam Turbines
At the same time commercial pressures on the utilities to lower operating costs have resulted in a reduction of personnel (some units are now operating with a third of staff originally employed). Where possible, maintenance has been cut
Steam turbines are currently manufactured in the following approximate ranges: • Small (0.5-80 MW) units, which are used for mechanical drive and industrial power applications. As these turbines normally operate at high speed (2800-16000 rpm), they are used together with reduction gears and, for power generation, with air-cooled generators or alternators.
' Marketing and Technical Manager, Great Lakes Cliemical Corporation, Performance Additives and Fluids, Tenax Road, Trafford Park, Manchester M17 IWT, England. 297 Copyright'
2003 by A S I M International
www.astm.org
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• Medium size units (100-300 MW) with rotor speeds of 1500-3600 rpm directly driving air/water-cooled generators. • Large sets of up to 1500 MW, also with direct drive and normally used with hydrogen-cooled generators. These are favored by the large utilities for base-load power—particularly for nuclear operations. Gas Turbines In the past gas turbines were so-called because of their sole use of gas as a fuel, but developments in design now enable "gas" turbines to operate on volatile liquid fuels such as kerosene or naphtha. They are perhaps more correctly, and less confusingly, known as combustion turbines, but for the purposes of this chapter will be referred to as gas turbines. Gas turbines can be also be subdivided into three categories: • Micro-turbines of 25-300 kW output, which are currently of current interest to the automotive industry, in small power generation packs and as a mechanical drive [9]. • Industrial (heavy duty) units of 5-300 MW used for power generation either in simple cycle or combined cycle operation (see below) and driving all designs of alternators either via a reduction gear, as is the case with small units up to about 70 MW output, or directly for larger sets. Small industrial sets up to 15 MW are also used for mechanical drive applications, e.g., gas pumping. • Aero-gas turbines with outputs of 2000-6000 lb thrust (turbo-propeller types) and 11000-110000 lb thrust (turbofan and turbojet types) for aviation applications. In industrial and marine applications a reduced range of engines is used and their power output is slightly lower (13-51 MW) than when used in aviation applications. The non-aviation uses require, of course, an independent power turbine to convert the energy generated by the combustion gases into power or for use as a mechanical drive. When gas turbines are used alone for power generation or for the production of both heat and power (combined heat and power—CHP—or co-generation applications [6,10]), they are said to be used in 'simple' cycle operations with operating efficiencies of 35-50%. A demand for higher efficiency has resulted in combining the operation of gas and steam turbines in what are known variously as combined cycle units (CCGT) or "steam and gas" (STAG) or, in Germany, as Gas und Dampf ("GuD"). In this application the waste heat from the gas turbine is used to raise steam for the steam turbine via a heat recovery steam generator thereby raising the overall efficiency of the process to 50-60% [11-13]. This is currently the most popular method of installing new utility or independent power production (IPP) capacity—providing there is a source of gas available. As the investment required is lower than for a single steam turbine of the same total output and installation more rapid, the financial return is faster. The latest developments in this field have been to design the steam, gas turbine and generator (possibly also a pump or compressor) on a single shaft [14]. This has the advantage of lower capital costs, a simpler design, and a layout with a smaller footprint. This is, however, at the cost of flex-
ibility of operation unless a clutch is used between the steam turbine and the generator. In this development a common lubrication system for all components may be used which means that the oil has to withstand the extremes of operation required by the individual pieces of equipment, that is wet operation in the steam turbine and high temperatures in the combustion turbine. A typical lubricating oil system for a single shaft CCGT unit is shown schematically in Fig. 1. Water Turbines Water or hydro-turbines are available in several different designs, such as the Francis or Kaplan turbines, and selection of the most appropriate design depends largely on the available head of water. A fixed blade propeller (Francis) or a variable pitch propeller (Kaplan) uses the hydrostatic head to drive a generator. For high heads (e.g., of 30-300 m) the Francis turbine is preferred while for heads of 15-30 m the Kaplan design is normally used. The power output for water turbines is almost as wide as for steam turbines with a current range of about 1-800 MW. The function of the turbine oil system is to lubricate the bearings of the turbine and the generator, which normally sits above the turbine on the same shaft. A forced lubrication system can be used with the main oil pump driven from the rotor shaft. In such cases an auxiliary pump is necessary to provide adequate oil pressure during start-up and shutdown. An alternative technique is self-lubrication by the suitable design of the bearing pads and pad supports.^ Oil pressures vary between 2-10 bar and the lubrication system capacities vary from 1-10 m^ for turbines of 10-300 MW. In addition to the lubrication system, there is a hydraulic circuit for operating the governor and inlet valve system. In the past the same oil was used for both the hydraulics and bearing lubrication with hydraulic pressures of 20-25 bar. However, pressures in the hydraulic system have increased over the years to their current levels of 100-160 bar with a consequent reduction in system volume from about 3-4 m^ to 0.8-1.5 m^. Today, where hydraulic pressures are >60 bar, separate hydraulic and lubrication systems are used. Where the design involves separate systems, conventional turbine oils of ISO viscosity grades (VG) 32-68 are used for bearing lubrication and hydraulic oils of ISO VGs 32 and 46 for the hydraulic system-except for low temperature environments where oils of ISO VGs 10 and 22 may be used in the gate hydraulic control. (ISO Standard 3448 or ASTM D 2422, Standard Classification of Industrial Fluid Lubricants by Viscosity System, classify industrial lubricants by viscosity grade. The grade number corresponds to the mid-point of a viscosity range extending to ±10 % of the mid-point value and is measured in centistokes at 40°C.) Bulk oil temperatures are in the range of 40-55°C for the lubrication system. Under these conditions the stress levels in the system are low and the oil normally lasts the life of the turbine.
^ Private communication with H. Moeller, Elsam, Nordjyllandsvaerket, Denmark.
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12: TURBINE
LUBRICATING
AC main AC auxiliary DC emergency iube oil lube oil pump lube oil pump pump
OILS AND HYDRAULIC
FLUIDS
299
Oil purifier
FIG. 1—Lubricating oil system for a single-shaft combined-cycle turbo generator. Removal of excess water by centrifuge takes place every six months. Wind Turbines Wind turbines are a relatively recent development in which the rotation of a propeller is coupled to a generator either directly or via a gearbox. The power output currently varies according to location. Typically, land turbines have an output of 300-1000 kW while offshore turbines are larger with a capacity of u p to 2-3 MW [15]. Units of 5 MW output are currently under development [16]. Two basic types of propeller design are in use: those with a horizontal axis (otherwise k n o w n as p i t c h turbines) a n d those with a vertical axis (stall turbines). The former type uses one (or even two) gearbox(es) [17] and a generator. The gearbox is oil lubricated while the roller bearings of the generator are grease lubricated. A separate hydraulic system containing a conventional ISO VG 46 hydraulic oil (40-60 L capacity with a pressure of about 60 bar) for altering the pitch of the propeller may also be included. Some designs have demonstrated it is possible to avoid the use of a gearbox, and with these the propeller directly drives a multi-pole generator. The early gearboxes on machines of < 5 00 kW had a capacity of about 125 L and relied on splash lubrication with a n ISO VG 220 gear oil at temperatures of 80-90°C. For turbines with an output of >500 kW, forced lubrication systems are used a n d system capacities for the larger machines have since risen to about 200 L.
Problems initially arose with the use of conventional turbine oils in t e r m s of reduced oil life, deposit formation, micro-pitting and bearing failure and led to the use of polyalphaolefin (PAO)-based oils owing to their better high temperature stability. In spite of this the average life of the gearbox oil in the small units was still only about 1-2 ycctrs. The oil life has increased since temperatures were reduced to <70°C but the problems of micro-pitting smd bearing failures have not yet been totally eliminated. However, all new gear oils for this application are required to pass an FZG micropitting test (FVA test 54/I-IV, Test Procedure for the investigation of the Micro-Pitting Capacity of Gear Lubricants). Higher viscosity products (ISO VG 320) are also used in an attempt to further minimize the problems. Even with these changes, the current oil life expectations are, as yet, only 3-5 years. With additional research on the effects of oil cleanliness, moisture, and metal contamination, further improvements are thought possible.'^ As well as the operating problems mentioned above, other factors have to be taken into consideration during operation, for example the inaccessibility of the equipment in the event of a failure (especially if located offshore). Power generation without producing carbon dioxide can also have a higher profit margin, so downtime losses/MW output are also higher for this tjrpe of equipment. As well as the gearbox, almost all wind turbines have a small hydraulically operated braking system of 2-4 L capacity and working at about 100 bar pressure. Since conventional turbine oils are not used in wind turbines there will be n o further discussion of this application.
300 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK THE FUNCTION OF THE LUBRICATING OIL AND HYDRAULIC FLUID The primary function of the turbine oil is to lubricate the turbine and generator bearings. However, it may also fulfill the following secondary functions: • Acting as a hydraulic fluid in steam/gas valve operation and in the jacking systems for the rotor shaft and gear wheel • Lubricating the reduction gear (if used) • Lubricating the exciter bearings • Acting as a shaft seal for hydrogen-cooled generators • Lubricating the coupling between the turbine and the alternator In addition, the oil acts as a coolant for the lubricated surfaces, minimizes friction and wear; removes any wear particles to the filters, and protects the system from rust and corrosion. The hydraulic control or governor oil system controls the rate at which steam is admitted to the turbine cylinders eind hence the turbine rotational speed. The hydraulic fluid is also used to operate emergency stop valves.
THE OPERATING ENVIRONMENT FOR LUBRICATING AND HYDRAULIC OILS Although the equipment varies considerably in its size and complexity, the basic design of the hydraulic and lubricating
oil systems is essentially the same across the range of turbine types. With the exception of totcil loss systems in some military aviation applications, the lubrication systems are closed circuits involving a reservoir, a pump train with main, auxiliary, and emergency pumps, filters, a cooler and possibly a centrifuge or purifier (on a bypass to the oil tank) to remove excess water and dirt. The oil is supplied under a slight pressure to the bearings. Fig. 2 shows, in diagrammatic form, the supply of lubricating oil to a steam turbine generator. In high pressure hydraulic systems there may additionally be accumulators to "store" the higher pressure until it is needed for valve displacement. When using fire-resistant hydraulic fluids, the oil purifier's prime function is to remove acid that would otherwise catalyze further degradation of the fluid. Where mineral oil is used, which is still the most common situation, lubricating and hydraulic oil systems are normally combined with lubricating oil system supply pressures at about 3-4 bar and a pressure at the bearing inlet of about 1-2 bar. Hydraulic pressures are somewhat higher at 7-20 bar and both the hydraulic system and lubricating oil for the turbine, gearbox (if used), and generator are fed from the same tank. Exceptionally, there are separate hydraulic systems containing mineral oil where the pressure is high at 160 bar. In modern large turbines, with steam temperatures now reaching 600°C, the presence of mineral oil in the hydraulic system presents a major fire hazard. The escape of mineral oil and contact with steam pipes in the vicinity of the control
- EXCITER
ELECTHIC GENERATOR AND
B" LOW PRESSURE TURBINE
" A ' X O W PRESSURE TURBINE
HIGH PRESSURE AND REHEAT TURBINE MAIN SHAFT OIL PUMP
GRAVITY DRAIN AND GUARD PIPE PRESSURIZED OIL PIPES OIL VAPOR EXTRACTOR OIL RETURN SCREENS
OIL COOLERS
OIL RESERVOIR AND OILE.IECTORS
FIG. 2—Shaft driven lubrication system of a steam turbine generator. Reproduced by permission of the Electric Power Research Institute, Palo Alta, CA.
CHAPTER
12: TURBINE
(Eh
Fluid returns
>
'^—-r
LUBRICATING
OILS AND HYDRAULIC
FLUIDS
301
4x}
©-|J_yent
z Fluid conditioning
^M%
t
Cooling water>
llling using loling pump
C( Dllng water
-M-
Emptying>
illing using ^ s n ianual pump
FIG. 3—Typical steam turbine control fluid system. valves has caused many turbine fires [18,19]. As a result the industry, as long ago as the mid-1950s, introduced the concept of a separate hydraulic system containing a fire-resistant fluid [20]. Although there are alternative ways of reducing the fire hazard, e.g., by providing pipe-in pipe systems, fire-resistant hydraulic fluids are now widely used in electro-hydraulic governor control and emergency stop-valve systems. In order to reduce the volume of fluid used and the capital cost of a separate system, the pressures in these circuits are normally m u c h higher than when oil is used. Currently 160 bar is the m a x i m u m found, but an increase to 200 b a r is being considered. A typiccJ hydraulic control circuit diagram is shown in Fig. 3. Fire-resistant hydraulic fluids based on triaryl phosphates are now used in over 1000 large steam turbines worldwide. More recently the same fluids have been developed for use as a combined hydraulic fluid and main bearing lubricant for both small and large steam turbines u p to 1000 MW and the operating experience obtained since the early 1980s has shown them to be highly successful in this application [21]. Although most industrial gas turbines use mineral oil or synthetic hydrocarbons as the hydraulic fluid and lubricant in a single system, there are applications where fire-resistant hydraulic fluids are used in a separate system (in mediumlarge sized units) or as the operating media for both hydraulic and lubrication systems. As an example of the latter, their use in gas pipeline turbo-compressors has provided in-
creased safety and reduced downtime since 1958 and they are now widely used in this application in North America [22]. In the 1970s they were introduced for use in the control and lubrication systems of 70 MW sets for power generation [23]. A t5rpical lubrication system for an industrial gas turbine is shown diagrammatically in Fig. 4. Aero-derivative gas turbines were introduced during the Second World Weir and, while mineral oils were initially used for lubrication, it was soon realized that both the low temperature properties and the high temperature stability of the oils then available were inadequate for the more powerful engines that were being developed. Synthetic ester fluids were therefore introduced a n d have remained the most widely used type of fluid in this equipment for both aviation and industrial applications where they function as a combined hydraulic fluid and lubricant for the turbine [24]. Mineral oils are still used in some of the older or smaller aero-engines for military aviation, and in industrial applications where the thermal/oxidative stresses are lower. Where aero-derivative units are used for mechanical drive applications, the hot exhaust gases drive a power turbine that is attached to a compressor or a gearbox/generator, etc. In this application, a synthetic ester product lubricates the engine, while the power turbine and the "driven" equipment are normally lubricated with mineral oil in a separate system. When used for pumping natural gas, the power turbine cind compressor Eire often lubricated with a fire-resistant fluid.
302
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IEGBO):
A A A • H
^
Q
^
Mr
mm MSuwIlF DfMn
->
Q )f>RESSUnE 'TRANSMITTEfl
TgwlflCSTiKfOMAt
FIG. 4—Typical lubrication system for an Industrial gas turbine. Reproduced with permisson of Solar Turbines, Inc., San Diego, CA.
There has also been some limited use of fire-resistant fluids in the hydraulic systems of aero-derivative gas turbines, but as yet there is no significant experience with these fluids as lubricants for this equipment. The severity of lubricating oil service (and hence operating life) varies considerably and depends on a number of factors including: • System design • The duty cycle, for example continuous or intermittent operation • Oil stability • The quality of system maintenance • The quality of oil or fluid maintenance • Top-up rates.
The system design determines the degree of thermal and oxidative stress to which the fluid or oil is subjected. In most turbines the thermal loading on the lubricant Eirises through heat conducted along the rotor to the bearings (shaft temperatures in large steam turbines can reach 320°C at the bearings), as a result of the heat generated through frictional and viscous losses in the bearings, and during compression in the pumps. Additionally, in gas turbines, Icirge quantities of sealing air at temperatures of 200-350°C [26] are drawn into the bearing and form an aerated mixture with the lubricating oil as it drains back to the tank—an ideal environment for promoting oxidation and foaming. The inlet temperatures of industrial gas turbines have risen over the years from around 700°C in
CHAPTER 12: TURBINE LUBRICATING 1950 to 1500°C in 1998 [27,28], as shown in Fig. 5 [28], while compressed air temperatures have also increased with the trend toward higher compression ratios. Together this would suggest a significant increase in the heat being dissipated via the engine structure. Certainly gas turbine bearings are now operating at high stress levels. For example, temperatures of up to about 115°C are found for plain bearings and up to about 300°C for roller beeirings. The various sources of heat and its removal for a t5rpical gas turbine bearing are shown in Fig. 6. Unfortunately, although the heat input at the bearings has steadily increased, the volume of fluid available for heat removal has not. If anything, oil volumes have been reduced as a consequence of reducing the size and therefore the cost of the system. This has been achieved by increasing circulation rates (with an adverse effect on aeration) and, where possible, by more effective cooling. In most cases however, the result has been an increase in oil return line temperatures, in the case of recent estimates for steam turbines, by 10-15°C [29]. As oxidation rates approximately double with every ten degrees rise in temperature, it is hardly surprising that tur-
1940
1950
1960
OILS AND HYDRAULIC FLUIDS
303
bine oil lives, particularly for gas turbines, are now giving cause for concern. Since the thermal/oxidative stress on the oil or fluid is a factor of temperature, time and the extent of air contact, the faster the oil temperature (and the air content) can be reduced, the lower the amount of resulting fluid degradation. A cooler in the return line is therefore preferred. The presence of air depends to a large extent on tank design (including the location of return lines, baffles, and sieves to remove entrained air), but it also depends on fluid circulation rates, the level of fluid/oil in the tank, etc., Guidance on the basic system design parameters of turbine lubricating oil systems (including the use of suitable materials of construction; design features of the reservoir and pump train; the appropriate use and location of coolers, valves and filters, etc.) is available for steam and industrial gas turbines when using conventional turbine lubricating oil. This appears in such standards as ASTM D 4241, Standard Practice for the Design of Gas Turbine Generator Lubricating Oil Systems, ASTM D 4248, Standard Practice for Design of Steam Turbine Generator Oil Systems, and the American Petroleum Institute (API) Standard
1970 Year
1980
1990
2000
FIG. 5—Development of gas turbine capacity, inlet/outlet temperature and efficiency. Reproduced by permission of Verelnigung der Grosskraftwerk Betreiber, Essen, Germany.
304
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614, Lubrication, Shail-Sealing, and Control-Oil Systems for Special-Purpose Applications, b u t is t h e n sometimes ignored. In fact, tank residence times have decreased in recent years in the pursuit of lower costs, making it more difficult for the air to be released and with an adverse effect on oxidative stability [29]. The duty cycle is important where the turbine experiences frequent stops and starts or where the unit is used at maxim u m power for only a short period, as in aviation applications. If aircraft gas turbine oils were continually subjected to the same stress as found on take-off, the oil life would be substantially reduced. By contrast, the long periods of continuous operation in base-load thermal or nuclear power generation are less demanding. The stability of the oil or fluid will obviously play an important peirt in determining its operating life and may help identify the most appropriate type of product for the application. Depending on the quality of fluid maintenance (which is discussed in more detail later) and also system maintenance, for example minimizing air and water leeiks into the oil, the life of the lubricant can be extended considerably. The importance of reguleir, planned, maintenance cannot be overemphasized in the pursuit of trouble-free operation. Lastly, top-up rates will determine the rate at which new fluids, and therefore additives, are replenished. Normally, top-up rates are fairly lo\? for steam turbines (about 3-10% per year), but much higher veJues have been reported in the past (up to 27%) and the beneficial effects of such high topu p rates in extending oil life have been investigated [30]. In
industrieJ gas turbines with some mineral oil types, top-up rates can rise to as high as 3 3 % per year while aero-engines in aviation consume on average about 0.25 L per hour or a complete replacement charge in about 24-130 h of engine operation. In industrial operation the c o n s u m p t i o n may be lower due to the fitting of more efficient oil demisters. In the past, an oil change normally took place when equipment builders' recommendations on used fluid performance were exceeded. These would normally include limits on acidity, water, viscosity increase, and dirt levels. When these values were reached or exceeded, the oil or fluid was considered too degraded or contaminated for continued use. Today, filtration techniques are available which can readily reduce the levels of some contaminants, e.g., water and particulates. This improvement, together with the possibility of monitoring the depletion of additives and then re-inhibiting when necessary, makes it possible to extend life substantially in some applications. The volume of oil used in t u r b i n e lubrication systems varies considerably. While a figure of 270 L per MW used to be quoted as typical, steam turbine lubrication systems today contain in the region of 1 0 0 ^ 0 0 L per Megawatt for turbines of <200 MW and from about 50-80 L per Megawatt for sets in the range 250-1500 MW. There is considerable variation as the builders frequently use one size of tank for a range of turbine outputs and the configuration of the unit, i.e., the type of driven equipment, will also influence the volume required. Such large volumes of oil are used to provide adequate time for the release of air from the bulk liquid in the tank.
Heat from shaft/bearing housing (power output/duty cycle)
Lube oil inlet temperature
Lube oil vapour/air to secondary breather
i i
Heat from seal air (on external housing and internally into bearing) - compression ratio - take-off point in compressor
Lube oil outlet temperature hot air to drain - flow rate - thermal conduction - specific heat - density
Heat transferred along shaft or via housing FIG. 6—Heat input and removal for a gas turbine bearing.
CHAPTER 12: TURBINE LUBRICATING
OILS AND HYDRAULIC FLUIDS
305
TABLE 1—Typical operating conditions for turbine lubricating oils. Turbine Type
System Property
Medium-Large Steam (250-1500 MW)
Small Industrial Gas (1-14 MW)
Medium-Large Industrial Gas (30-300 MW)
Lube system capacity (m^.) Circulation rate (changes/hour) Bulk e u i d temp. (°C) Bearing oil return temp. (°C).
19-115 5.5-10 60-65 70
1-3 17-21 50-70" 60-115
15-34 6-10 45-80" 55-85
Aeroderivative-Gas Aviation Industrial (11 000-110 000 lb. thrust) (31 MW) 0.006-0.02 60-240 60-70 120-130
0.9 5 ISO"" 150-180
Marine (4MW) 0.11 60 70 150
''The bulk fluid temperature In the tank may be similar to bearing return oil temperature if the cooler is placed on the pressure side of the pump. In this case the bearing oil supply temperature will be significantly lower than that in the tank.
Much smaller volumes and higher circulation rates are found in small industricd and aero-derivative gas turbines (Table 1) because of restrictions on space and weight. The volume of hydraulic fluid in steam turbines similarly varies from about 1 m^ to 20 m^ depending on system design and pressure and is substantially smaller in gas turbines. With the emphasis on reducing costs, the size and volume of hydraulic systems have been steadily decreasing, but at the expense of increasing pressure. Today, system pressures range from 40-160 bar, but are starting to increase to 200 bar. Some of the operating conditions under which lubricating oil is used in the different tjqjes of equipment are given in Table 1. The data given is representative of the major applications and equipment but cannot possibly encompass all the equipment variations currently available. However, with respect to the impact of these conditions on fluid life, the following generalizations can be made: • For steam turbines, the life of mineral oil and synthetic hydrocarbon ranges from 5-25 years (up to the life of the turbine) while currently averaging about 12 years. • For industricd gas turbines, the lives vary between 1.5-6 years, but for new generation combined cycle gas turbine (CCGT) applications, operating lives to date are in the region of about 4-5 years. In aviation gas turbines, the synthetic ester oil is continually made up and therefore the whole charge is not normally replaced. In industrial applications it may be changed annually or after 8000 operating hours. • For both fire-resistant hydraulic fluids and lubricants the latest data suggests lives of 5 years to more than 20 years depending on the type of in situ conditioning or purification used.
TURBINE OIL TYPES AND THEIR FORMULATION While the turbine oil market used to be almost exclusively the domain of solvent-refined paraffinic mineral oils, the changing technical requirements have resulted in a move to other types of lubricants, both hydrocarbon-based and sjrnthetic. For example, in industrial gas turbines and combined cycle units, the high operating temperatures have resulted in a reduced life for products based on solvent-refined oils. Lives of a "few thousand" hours for such oils in high stress situations have been reported [31,32] and replacement by special "GT" oils—in reedity hydrocracked oils—increases the operating life up to 50000 h [31]. In turbines operating with high temperature steam (>500°C), mineral hydraulic oil has been almost completely
replaced by fire-resistant fluids based on triaryl phosphates in order to avoid fires arising from oil escaping at high pressure and coming into contact with steam pipes. The following is a brief summary of the basestock types and additives currently in use and their advantages/disadvantages. A more detailed account of the different oil types used in hydraulic fluids and their properties, which is also largely applicable to turbine oils, is available from other references [33]. BASESTOCKS Hydrocarbon Oils Petroleum oils are complex mixtures of many different chemicals and their relative amounts vary considerably from one crude source to another. The components do, however, fall into a limited number of categories, principally straight- and branched-chain saturated hydrocarbons (also known as paraffins), cyclic saturated hydrocarbons (also known as cycloparaffins or naphthenes), and unsaturated cyclic hydrocarbons (otherwise known as aromatics), examples of which are shown in Fig. 7. Additionally, small amounts of impurities, consisting mainly of cyclic derivatives of nitrogen, sulfur and oxygen, and polar materials such as naphthenic acids, may be present. Fig. 8 shows typical examples of such heterocyclic compounds. Each hydrocarbon component influences the properties of base oil to an extent dependent on its concentration (Table 2 [34]). The above impurities are present in solvent-refined oil in small quantities, typically 0.1-0.5 %, but occasionally in much larger amounts, and they can also impact the performance of the fluid particularly in terms of stability and lubrication. Much lower amounts of impurities, if any, are present in the hydro treated oils. The need to quantify the different components in lubricating oils has resulted in the development of several analj^tical test procedures. Currently these include ASTM D 2425, Standard Test Method for Hydrocarbon T5rpes in Middle Distillates by Mass Spectrometry; ASTM D 3238, Standard Test Method for Calculation of Carbon Distribution and Structural Group Analysis of Petroleum Oils by the n-d-M Method; and ASTM D 5443, Standard Test Method for Paraffin, Naphthene, and Aromatic Hydrocarbon Type Analysis in Petroleum Distillates Through 200°C by Multi-Dimensional Gas Chromatography. While the D 3238 method is easiest to apply, being based on refractive index, density and molecular weight, it is more restrictive in its application with limits on the total ring content and the ratio of aromatics to naph-
306
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HANDBOOK
Type
n-paraffin
Viscosity index
Structure
\
/
\
/
^
/
^
high
^Ho
high
CH, CH,
iso-paraffin
CH, CHo
CH,
CH
naphthene
moderate
aromatic
low
FIG. 7—Typical hydrocarbon structures.
^ - - ^
Dibenzothiophene
^"^^
I
(High T e m p e r a t u r e Method); ASTM D 2622, Standard Test Method for Sulfur in Petroleum Products by X-Ray Spectrometry; or ASTM D 4927, Standard Test Method for Elemental Analysis of Lubricant and Additive Components— Barium, Calcium, Phosphorus, Sulfur a n d Zinc by Wavelength-Dispersive X-Ray Fluorescence Spectroscopy. In order to provide general guidance on the identification and selection of lubricants, ASTM has issued D 6074, Standard Guide for Characterizing Hydrocarbon Lubricant Base Oils. This document recommends methods for analyzing the composition of base oils, describes their important chemical properties, and discusses the toxicological requirements, including regulations covering the presence of undesirable components such as polynuclear aromatics.
H
Conventional Solvent-Refined Types Pyrrole
1,7- phenanthrone
FIG. 8—Impurities typically present in solvent refined mineral oils. thenics. The levels of impurities such as nitrogen and sulfur are not available from these tests and this information has to obtained from other procedures, e.g., ASTM D 3228, Standard Test Method for Total Nitrogen in Lubricating Oils a n d Fuel Oils by Modified Kjeldahl Method and ASTM D 1552, S t a n d a r d Test Method for Sulfur in Petroleum Products
In order to produce oils that meet industry requirements it is necessary to remove or substantially reduce the level of those components that adversely affect performance, and this has been achieved by various refining techniques. As indicated above, the oil produced by the solvent refining of a crude paraffinic basestock (Fig. 9 [34]) is still in major use for both steam and industrial gas turbine applications. In this basestock, about 45-60% of hydrocarbons are in the form of saturated straight- or branched-chain paraffins and monocycloparaffins, but there is still a significant a m o u n t of
CHAPTER 12: TURBINE LUBRICATING OILS AND HYDRAULIC FLUIDS 307 TABLE 2—Effect of composition on base stock properties. Chemical Component n-Paraffin Iso-Paraffin Naphthene Aromatic Polar compounds
Viscosity Index Very High High Low Low Low
Pour Point (High/Low) High Low Low Low Low
Response to Antioxidants Good Good Good Some poor Poor
Oxidative Stability Good Good Average Average/poor S is antioxidant; N and O are pro-oxidants
Volatility (High = Poor, Low = Good) Good Good/average Average Poor Poor
ReprintedfromHandbook for Hydraulic Fluid Technology, 2000, p. 716, courtesy of Marcel Dekker Inc., NY.
Crude oil
Dewaxing
Solvent extraction
Atmospheric/vacuum distillation
Conventional base oil
Gas oil
i
T
Wax
Extract
FIG. 9—Solvent refining process. Reproduced with permission of Petro-Canada Lubricants, Mississauga, Canada. TABLE 3—Chemical composition of lubricant base oils. Oil Reference
A
B
C
D
E
F
G
API Category
I
II
II
II
III
III
Description
I Solvent Refined
Solvent Refined
Hydro-Cracked
Hydro-Cracked
Hydro-Cracked
Dewaxing
Solvent
Solvent
Solvent
Solvent
Iso-
Solvent
Iso-
Mass Spec. Analysis Paraffins, n- & isoMonocycloparaffins Polycycloparaffins Aromatics Thiophenes Paraffins -1Monocycloparaffins
25.7 20.8 27.9 24.9 0.7 46.5
29 25 31.7 14.2 0.1 54
23.7 30.8 39.1 6.4 0.0 54.5
21.6 32.8 37.6 8 0.0 54.4
30.2 30.5 35.3 4 0.0 60.7
32.6 34.2 32.9 0.6
76.1 14.7 9.2
66.7
90.8
Severely Hydro-Cracked
Severely Hydro-Cracked
Reprinted with permission of Petro-Canada, Mississauga, Canada unsaturated ring structures (Table 3 [35]). The refining process removes wax (mainly high molecular weight paraffinic compounds), most of the aromatic hydrocarbons, as well as some of the polar compounds containing oxygen and nitrogen, products that would otherwise have significantly reduced the stability of the oil. However, small amounts of sulfur-containing c o m p o u n d s , e.g., thiophenes, r e m a i n and these can be beneficial in terms of increasing the stability of the base. These basestocks are classified as Group 1 products by the API Classification of Base Oils according to their viscosity index, sulfur content and the content of saturated hydrocarbons (Table 4 [36]).
Hydrocracked/Hydrotreated Basestocks In view of the demand from industry for oils with better oxidation stability, as operating conditions become more severe, attention has turned to processes that can remove yet more
TABLE 4—American Petroleum Institute classification of base-stocks. Base Stock Sulphur, Saturates, Viscosity Group (wt%) (wt.%) Index Group I >0.03 and/or <90 80-120 Group II <0.03 and >90 80-120 Group III <0.03 and a90 >120 Group IV All Poly-Alphaolefins (PAOs) Group V All Basestocks not included in Groups I-IV of the aromatic content and the residual impurities. Since the mid-1980s, therefore, oils produced by hydrotreating, hydrocracking or hydro-refining processes, involving the reaction of hydrogen with the feedstock in varying degrees of severity, have resulted in the availability of much purer basestocks (Table 3 [35]). Using the API Classification [36] these fall into Group II (mildly hydrocracked) or Group III (severely hydrocracked or hydrotreated) depending on the low sulfur levels and increasing viscosity indices (Table 4). In addition to the hydrogenation of the lubricant feedstock.
308
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the process m a y also include hydro-isomerization of the straight-chain paraffinic components of the wax removed during solvent refining or hydrotreating (Fig. 10) [37]. This process involves a molecular rearrangement and converts the straight chain hydrocarbons to branched-chain or iso-paraffins that are then blended into the hydrotreated basestock to give better low temperature properties. As a result of the different processes, a range of improved basestocks is currently available with which to formulate turbine oils. These are sometimes referred to as High Viscosity Index (HVI) oils, which are classified as Group II products; Very High Viscosity Index (VHVI) oils (Group III); or Ultra Pure Base Oils (also Group III products). Levels of aromatics, sulfur, etc., decrease significantly on moving from Group I to Group III products but significant variability in composition still exists within these groups [35]. Group II products appear currently favored for formulation purposes but with a slight trend towards Group III. The technical advantages of the new basestocks [32,38] include: • Excellent oxidation stability when formulated • High thermeJ stability • Very low deposit-forming tendency • Improved demulsibility • Reduced volatility • Very low toxicity • Environmentally "friendly" There are few technical disadvantages of these new basestocks but, depending on the level of hydrotreating, they m a y be less oxidatively stable compared with solvent-refined products in view of the removal of small amounts of sulfur and aromatic compounds that act as naturally occurring stabilizers.
This is, however, more than compensated for by better response to antioxidants when carefully selected [32,37]. The loss of aromatics also results in reduced additive solvency necessitating a careful choice of stabilizers emd the continued presence of small amounts of aromatics may be beneficial in such cases [39]. The loss of sulfur also reduces the antiwear/extreme-pressure properties of these oils although these can easily be improved by incorporating suitable additives. These base materials vary in price from a level which is comparable to that of solvent refined stocks to about three times their price, depending on material type a n d local availability. As a result of the favorable price/performance characteristics it is likely that their use in conventional turbine oil formulations will spread.
Polyalphaolefins (PAOs) These synthetic fluids are normally m a n u f a c t u r e d by the oligomerization of Eilpha-olefins, particularly a-decene, but also by a-octene and a-dodecene and, until recently, were available in a range of viscosities u p to the ISO 100 grade. The latest development [40] reports the availability of very high viscosity products ranging from 150-3000 cSt at 100°C and intended mainly as blending components for other hydrocarbon oils. They are currently more expensive than other hydrocarbon types. PAOs are free of aromatic hydrocarbons, sulphur, oxygen, and nitrogen compounds, and show excellent response to antioxidants [41], although they are now being challenged by some of the severely hydro-cracked oils [42]. They also have high viscosity indices (as assessed by ASTM D 2270, Standard Practice for Calculating Viscosity Index from Kinematic
H, I •
Atmospheric/vacuum distillation
n«.,,„i„„
H2 I »
- > Ultra pure base oils Crude oil
Atmospheric/vacuum distillation
HTU = hydrotreatment unit
"Wax
HTU2
Hydro Isomerization
FIG. 10—Hydrotreatment plus hydroisomerizatlon process. Reprinted with permission of Petro-Canada Lubricants, Misslssauga, Canada.
CHAPTER 12: TURBINE LUBRICATING OILS AND HYDRAULIC FLUIDS
309
TABLE 5—A comparison of some physical properties of 6.0 cSt PAOs with solvent-refined and hydrocracked oils of similar viscosity. Parameter
Test Method
API Group Viscosity, cSt at 100°C at 40°C at - 4 0 ° C Viscosity Index Pour Point (°C) Flash Point (°C) Evaporation Loss (NOACK), %
ASTM D445
ASTM D 97 ASTM D 92 DIN 51581
PAO IV 5.98 30.9 7830 143 -64 235 6.1
160 Hydro-Treated
240 Neutral
200 Solvent Neutral
II
I
I
5.77 33.1 SoUd 116 -15 220 16.6
6.98 47.4 SoUd 103 -12 235 10.3
6.31 40.8 Solid 102 -6 212 18.8
Very High Viscosity Index Oils III III 5.14 24.1 Solid 149 -15 230 8.8
5.9 NA 127 -12 225 6
NOTE—Reprintedfrom:Synthetic Lubricants and High Performance Functional Fluids, 1992, p. 13, courtesy of Marcel Dekker Inc., NY. Viscosity at 40°C and 100°C) and flash points (by ASTM D 92, Standard Test Method for Flash and Fire Points by Cleveland Open Cup); very good low temperature viscosities (ASTM D 445, Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids) and p o u r points (ASTM D 97, Standard Test Method for Pour Points of Petroleum Products), and a wide operating temperature range [42]. A comparison of the physical properties of PAOs with other hydrocarbon products of the same viscosity is given in Table 5 [43]. In view of the lack of aromaticity, additive solubility can be a problem. PAOs also have limited dispersency and do not penetrate rubber seals to cause swelling (as assessed by FEDSTD-791, methods 3604 and 3633, or by ISO 6072). As aresult, it is necessary to blend PAOs with small a m o u n t s of a sealswelling agent (usually a carboxylate ester) to avoid leakage. PAO-based gas turbine oils have been available for many years [39] where the higher prices could be justified in terms of smaller volume and better stability. They have not, as yet, m a d e a n y significant penetration of the steam turbine oil market [41].
Synthetic Ester Fluids Apart from the very early days of operation, carboxylate or synthetic esters (possibly in c o m b i n a t i o n with polyglycolether thickeners) have been the only products used for the lubrication of aero-derivative gas turbines in aviation applications. This market, like the others, is fragmented into different applications, this time in terms of the lubricant requirements of the turbopropeller, turbofan (civil) or turbojet (principally military) engines (Table 6) and into the different technical requirements of civil and military aircraft operation. The turbopropeller engine, for example, employs a reduction gear to accommodate the propeller. This necessitates the use of a higher viscosity fluid to provide a thicker lubricating oil film with the ability to reduce wear at heavy loads. At the other end of the spectrum, the military requirement to start an engine quickly at low temperatures requires the use of "lower" viscosity oils for turbojet engines. As a result, specifications for these oils currently cover four viscosity levels: 3, 4, 5, and 7.5 cSt at 100°C. Diesters, the lowest viscosity oils used in this application, were introduced after the World War II, initially for military applications. They were also used for the early commercial turbo-jets and in blends with polyglycolethers, for turbopropeller aircraft [24,26]. Diesters are produced by the reaction of an alcohol and an acid [44], viz
TABLE 6—^Aviation lubricant types currently commercially available. Engine Type Turbo-propeller Turbofan Turbojet
Engine Lubricant Composition Diester -I- Thickener 'High' Viscosity Polyol Ester 'Low-Medium' Viscosity Polyol Ester
TABLE 7—A comparison of minersil oil and carboxylateester properties. Mineral
Mineral Lubricant
Oil
Ester
Oil
Kinematic viscosity (cSt)at210°F 2.5 3.3 -40°F 3000 1700 -65°F 25 000 11000 Viscosity Index 75 154 Pour Point ("F) <-75 -80 Vapor Pressure (psi) at 250°F 0.15 350°F <0.01 1.3 400°F 3.5 0.016 0.52 550°F Ryder gear machine 600 2000 failure load lb/in. tooth face width Flash point 295 430 (COC, ''F)
20.1 Solid Solid 95 0
<0.01 0.065 -3000
485
Ester 7.5
12 000 Solid 150
-65
<0.01 0.28 3200 465
NOTE—Reproduced with permission of Elsevier Science, Oxford, UK.
2ROH -1^ HOOC(CH2)nCOOH alcohol diacid
ROOC(CH2)nCOOR diester
where, for aircraft turbine oils, n = 7-8 and R = Cs-g straight or branched chain The resulting diesters have a viscosity of about 3 ^ . 5 cSt at 100°C (ASTM D 445). In comparison with a well-refined mineral oil these products have a similar thermal and oxidative stability, better load-carrying performance, lower volatility (ASTM D 972, Standard Test Method for Evaporation Loss of Lubricating Oils and Greases) and a m u c h wider operating temperature range (see Table 7 [45]). They are also very responsive to the addition of antioxidants. They do, however, slowly hydrolyze in the presence of water and, like most esters, are not compatible with n e o p r e n e seals, w h i c h are widely used with mineral oil. The introduction of more highly-powered engines operating at higher temperatures and the requirement in com-
310
MANUAL
3 7: FUELS AND LUBRICANTS
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mercial applications for extended drain intervals necessitated the development of more stable products, and the socalled "polyol" or "hindered" esters were introduced at the beginning of the 1960s [24]. These esters, based on the reaction products of neopentylglycol, trimethylolpropane, or pentaerythritol with monobasic acids as shown in the reaction scheme below, have thermal degradation temperatures about 40°C higher than the diesters and are more oxidatively stable [46]. They have since been the basis for all commercial engine lubricants and an increasing proportion of military requirements where low temperature limitations permitted. C2H5
I C2H5 3RCOOH + HOH2C—C—CH2OH -^ I I RCOOH2C —C—CH2OOCR CH2OH I CH2OOCR monoacid trimethylopropane ester CH2OH I CH2OOCR 4RCOOH + HOH2C—C—CH2OH + I I RCOOH2C—C—CH2OOCR CH2OH I monoacid pentaerythritol CH2OOCR ester where R = C5_^9 The continual pressure on equipment builders to increase engine power in both military and civil applications has resulted in a need to periodically review and, where necessary, improve the stability (oxidative and thermal) and deposition performance of these oils. New versions of specifications, normally calling for improvements in these areas, have appeared at intervals of a few years, particularly in the military arena. That continual improvements in oil performance have been possible is due to the skills of the formulators and the availability of more effective additives [47]. The impact of changes in additive and basestock composition on oil stability is shown in Fig. 11 [48]. In the oxidation test from which the data has been taken, the B^^'^ temperature is the temperature at which the insoluble content of the oil exceeds 0.5mg/g over an 8-day test period when the oil is oxidized in the absence of metals, and is used as a measure of bulk oil stability. The data shows the initial performance level of diesters in the 1950s followed by improvements to the stabilizer package; the development of more stable fluids (the hindered or polyol esters) followed by yet more additive developments. More information on the various additive t3^es in use and their function is given in the following section. Long chain polyol esters have also been promoted as "less flammable" hydraulic fluids for steam turbine control systems on account of their relatively high flash and fire points. The esters used in this application are normally reaction products of trimethylolpropane and oleic acid (a C18 unsaturated monobasic acid) and in some respects, are significantly different to the short-chain products used in aviation gas turbines oils. For example, they are far less stable due to the unsaturation (or the presence of double bonds) in the molecule and their low temperature properties are inferior. When sold
as fire-resistant fluids they contain a polymeric thickener (also known as a viscosity-index improver), which increases the droplet size in a spray. This reduces the surface area accessible to oxygen and renders the fluid more difficult to ignite under spray test conditions (combustion is an oxidation process). However, as the polymer breaks down due to the shearing action in p u m p s and gears, this advantage is lost and the viscosity (and hence fire-resistance) tends toward that of the basestock. This temporary improvement in fire-resistance is limited to spray flammability and has no impact on the ignitability of the fluid under other hazard conditions, e.g., when it contacts a hot surface [49]. This is therefore an artificial approach which, to date, has been rejected by the major turbine builders for new large steam turbines, although small amounts are used in industrial units and in refurbished equipment. A comparison of the fire resistance properties of mineral oil, polyol esters, and phosphate esters is given in Table 8.
Tiiaryl Phosphates In common with the other two groups of fluids there are variations in the chemistry of triaryl phosphates that have led to the commercial availability of different products. In this case, however, it has been the past limitations on suitable raw materials that has resulted in the diversity of products available. Traditionally, phosphate esters were manufactured from the distillation products of coal tar, which were mixtures of cresols and xylenols and known collectively as "cresylic acids." The phosphate esters produced from these feedstocks were referred to as "natural" products and were chiefly tricresyl and trixylyl phosphate (TCP/TXP). When natural gas became widely available, the distillation of coal tar rapidly declined and attention turned to the production of phosphate esters from synthetic feedstocks, specifically those based on alkylated phenols [50]. The reaction scheme for the manufacture of triaryl phosphates, is given below: ArOH +
POCls^
a phenol
phosphorus oxychloride
(ArOsPO
+
3HC1
a triaryl phosphate
The reaction of propylene or butylene with phenol produces mixtures of alkylated phenols and these became the feedstocks for a range of "synthetic" phosphates with viscosities falling into ISO VGs 22-100. Since their introduction, the raw material supply position for the production of the "naturcil" fluids has eased with the introduction of synthetic processes for the production of cresols and xylenols. As a result the distinction between "natural" and "synthetic" phosphates has become blurred but the terms are still used today to distinguish between TCP/TXP and the synthetic products based on isopropylphenols and tertiarybutylphenols. Although manufactured to the same viscosity level, commonly ISO VG 32 or 46, commercially available fluids do have somewhat different levels of performance relative to one another as indicated in Table 9. Therefore, depending on the requirements of the application, one type of phosphate may be preferred. For example, in steam turbines where contact with water is most likely, the phosphate that is most hydrolytically stable, trixylyl phosphate, may be preferred. In
CHAPTER 12: TURBINE LUBRICATING OILS AND HYDRA ULIC FLUIDS 311
Ester limit?
Additional additive effect
Additive effect
1950
1960
1970
1980
1990
2000
year FIG. 11—Developments In the stability of gas turbine lubricants. Reprinted with permission of B. Rayner, consultant.
TABLE 8—^A comparison of the fire-resistance properties of mineral turbine oil, polyol ester, and phosphate ester fluids. Phosphate Ester Polyol Ester Mineral Oil Property Test Method ISO VG 46 ISO VG 68 ISO VG 46 270 258 ASTM D 92 220 Flash point-COC (°C) ASTM D 92 245 316 350 Fire point ( X ) ASTM D 2155 340 425 580 Autoignition temperature (°C) Hot manifold ignition (°C) AMS 3150C 350 430 >800 ISO 14935 Fail Fail Pass Wick ignition MIL-PERF-19457D Compression ignition -Ignition ratio 10 >42 Spray ignition -Persistence of burning F.M. Std 6930 Fail Pass Pass Group 3 (worst) Group 1-2 Group 1 F.M. Std 6930 - S p r a y Flammability Parameter -Persistence of burning ISO 15029-1 Fail Pass Pass Class H (worst) Class H Class D/E ISO 15029-2 -Ignitability Parameter
312 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 9—Impact of chemical composition on phosphate ester performance. Phosphate Type Trixylyl phosphate Tritertiarybutyl phenyl phosphate Triisopropyl phenyl phosphate
Fire Resistance
Oxidation Stability
Good
Good
Good Moderate
Hydrolytic StabiUty
compares the principal physical properties of an ISO VG 46 mineral turbine oil and triaryl phosphate ester.
Air Release Excellent
Excellent
GoodExcellent Poor
Moderate
Moderate
Moderate
Moderate
TABLE 10—^A comparison of some typical properties of an ISO VG 46 mineral turbine oil and phosphate ester hydraulic fluid/lubricant. Mineral Phosphate Oil Ester Test Method Property Kinematic viscosity ASTM D 445 (cSt) at 100°C 6.7 5 40''C 46 43 0°C 450 1700 -9 Pour point (°C) ASTM D 97 -20 0.15 Acid number ASTM D 664 0.05 (mg KOH/g) Specific gravity 0.87 ASTM D 1298 1.135 (20/20°C) Air release (min) ASTM D 3427 3 1 Foaming tendency/ 250/0 ASTM D 892 30/0 stability at 24°C (ml) Water content (%) ASTM D 1744 0.001 0.06 Rust preventing Pass ASTM D 665 Fail characteristics (distilled water) Thermal conductivity 0.134 0.134 (W/mK) Specific heat at 20°C (J/gK) 1.76 1.69
gas turbines, high oxidative stability is needed and hence a tertiarybutylphenyl phosphate could be selected. These latter fluids are at least as oxidatively stable as the best hydrocarb o n oils available and do not normally require antioxidants [27]. The m a i n reason for selecting phosphates is, of course, t h e i r fire resistance. While they are n o t completely nonflammable (most organic materials will ignite in the presence of sufficient energy) they do require a substanticilly greater energy input for ignition than hydrocarbons and most other non-aqueous fluids in commercial use, and they are self-extinguishing once they have moved away from the ignition source. Additionally they are excellent lubricants as demonstrated in p u m p tests, e.g., ASTM D 2882, Standard Test Method for Indicating the Wear Characteristics of Petroleum and Non-petroleum Hydraulic Fluids in a Constant Volume Vane Pump, and have very low volatility in the ASTM D 972 test procedure. Viscosity/temperature properties are, however, poor [51]; they have a significantly higher density than oil and require more power for circulation. As with all esters they are sensitive to moisture and require different seals and paints to those used with conventional turbine oils. Table 10
ADDITIVES AND THEIR EVALUATION Additives play an important part in improving the performance of all the above fluid types, but no more so than with the aviation esters. As the composition of the aviation lubricant basestocks has remained stable for many years, the improvements in performance have arisen principally as a result of developments in additive technology—particularly in oil stabilization. In comparison, major improvements in hydrocarbon oil stability have been due to improvements in refining techniques and additive response rather than to breakthroughs in additive technology. The move from Group I to Group III has, for example, resulted in a substantial increase in oxidation stability for the same level of antioxidant (Table 11 [52]). While some additives, for example stabilizers, can be of benefit in phosphates, these fluids are also used uninhibited. As will be seen later, in situ conditioning techniques are used with phosphates to remove degradation products as they are formed and this can remove the need for stabilizers. However, corrosion and rust inhibitors are normally highly polar products that would be removed by the conditioning medium (tj^ically an activated clay) and therefore cU"e not usually recommended. Fortunately phosphates, as polar materials, have some inherent rust preventative performance as they tend to be adsorbed on to the metal surface and there form a "barrier" to water. Some of the acidic degradation products are also effective inhibitors. As a result rusting of the system is not normally a problem except where wetting of the surface does not occur. Phosphate esters are good lubricants and are widely used as antiwear additives in hydrocarbon oils. It is therefore not usually necessary to improve this aspect of the performance of fire-resistant fluids. Typical additive packages designed to meet turbine lubricating and hydraulic oil specifications, whether for hydrocarbon or synthetic ester-based fluids, are less complicated than, for example, automotive engine oils and would contain a balanced mixture of antioxidants, metal passivators, rust, and corrosion inhibitors, anti-wear and/or extreme pressure additives, and possibly an antifoam. These types and the various methods used for their evaluation in turbine oils and fluids are now considered in more detail.
Antioxidants The actual mode of oxidative decomposition of both hydrocarbon a n d synthetic oils is a complex process involving many different mechanisms a n d a variety of intermediate products depending largely on the temperature and the structure of the substrate. Fig. 12 [52] is a model of lubricant breakdown at high temperatures, concisely illustrating the different processes involved. Antioxidants are chemicals that extend the life of the fluid by interrupting the oxidation process, e.g., by decomposing hydroperoxide intermediates (ROOH, where R is an alkyl chain) and scavenging free radicals [52]. In the past the dominant stabilizer type for hydrocarbons in both automotive
CHAPTER 12: TURBINE LUBRICATING OILS AND HYDRAULIC FLUIDS 313 TABLE 11—Comparison of oxidation stability of turbine oils formulated with two different antioxidants in a hydrotreated and solvent neutral base stock. Test fluids
H O — / ( ^ V - R-S-R1
0.25%
0.25%
mw = 492
0.25%
HO—/QV-CH3
0.05% balance
Corrosion inhibitor Mineral oil Oxidation stability TOST (ASTM D 943) (95°C; water, Fe and Cu catalysts; 3 L air/hour) - t i m e to total acid n u m b e r of 2.0 mg KOH/g oil -sludge (mg) (ASTM D 4310) Basestock characteristics ISOVG CA (aromatic hydrocarbon) Sulfur
4300 6
0.05% balance
2400 39
0.25%
0.05% balance
2200 28
hydrotreated 32 0 0
% %
1100 47 solvent neutral 32 6.5 0.54
NOTE—Reprinted from: Chemistry and Technology of Lubricants, courtesy of Kluwer Academic Publishers, Dordrecht, The Netherlands.
volatile low molecular weight hydrocarbons
- volatile low molecular weight oxidation products - carbon dioxide - carbon monoxide
volatile hydrocarbon fragmentsi
0.05% balance
air (with/without metal)
polycondensation polymerisation (with/without metal)
high molecular products
polycondensation polymerisation
sludge
FIG. 12—Model of lubricant degradation under high temperature conditions. Reprinted with permission of Kluwer Academic Publishers, The Netherlands.
314
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
and industrial oils has been a zinc dihydrocarbyl diphiophosphate (Fig. 13). This is a multi-functional additive in that it also possesses antiwear/extreme pressure properties and under certain conditions can provide protection against "yellow" metal corrosion. Different types of zinc dialkyldithiophosphates (or diaryl) are available depending on the alcohol or phenol used in their manufacture, and the hydrocarbyl group present strongly influences their activity; the greater the thermal stability of the product, the lower the antioxidant activity [33]. There is, however, a move away from this tj^e of antioxidant in turbine oils and hydraulic oils as the thiophosphates are sensitive to moisture and the hydrolysis products, (zinc oxide and hydroxide), can precipitate. Some tur-
RO
bine builders are therefore placing strict limits on the zinc content of approved oils, and so-called "ashless" antioxidants are being used in their place. These are either hindered phenols or aromatic amines (Fig. 13) that are used separately or in combination [53]. The antioxidant most widely used in turbine oils in the past was 2,6-ditertiarybutyl-p-cresol (also known as BHT). While it is still featured in some turbine oil formulations on account of its relatively low cost and good efficiency, its high volatility and solid form has lead to its replacement by higher molecular- weight products, preferably those that are liquids for ease of blending. Although, in general, aromatic amines tend to be more active than hindered phenols across the tem-
OR
S
where R is typically C3 - Cg
\ . Zn
OR
zinc dialkyldithiophosphate
2, 6 - ditertiarybutyl - 4 - methylphenol (BHT)
O CH2—CH2—C —OR
where PL is Cg-Cig
long-chain esters of substituted hydroxybenzenepropionic acid
where R is C4-C9
dialkyldiphenylamine FIG. 13—The structure of some common turbine oil antioxidants.
CHAPTER
12: TURBINE
perature spectrum [52], there are exceptions. Some hindered phenols are excellent high t e m p e r a t u r e antioxidants, but even these can be outperformed by certain amines or S3Tiergistic mixtures of amines and phenols [52]. However, radical scavenging efficiency is not the only criterion used in selecting an antioxidant. Cost is obviously important as are the tendencies of amines to color the basestocks in which they are used and also because they are more likely to form deposits during oxidation. The type of mineral oil basestock, e.g.. Group 1 or Group 2, etc., can also influence the choice of antioxidant as hindered phenols are preferred in oils with a low aromatic and low sulfur content. Synergism is also displayed between radical scavengers and hydroperoxide decomposers, for example a r o m a t i c amines are used with phenothiazine derivatives (a sulphurcontaining antioxidant) in aviation turbine oils. Table 12 shows the synergism between an amine antioxidant and an organosulfur hydroperoxide decomposer [52]. The superior performance of aromatic amines, particular the alkylated derivatives [52], at high temperatures has resulted in their exclusive use in aviation gas turbine oils. Here, in addition to their efficiency in reducing fluid viscosity increase and acidity development (which are the two most c o m m o n parameters used in assessing oxidation stability), they have been able to control deposit formation on hot surfaces. This is important because it is more likely that an engine failure would take place as a result of deposits them as a consequence of a rise in viscosity and acidity, especially as the aero-engine oils are topped-up so frequently. Other stabilizers, for example phosphites, which are used in some oil applications as sjTiergists with hindered phenols, are not used widely in general industrial turbine oils, possibly because these materials are usually very sensitive to the presence of moisture with the resulting development of acid. They have been used in aviation gas turbine oils where the temperatures would limit the water level in the fluid, but as extreme pressure additives rather t h a n as antioxidants. The activity of antioxidants is normally assessed, as indicated above, by their effect in minimizing the physical changes that occur in oil on oxidation, i.e., an increase in viscosity and the development of acidity and sludge. In tests where metals are present, either to catalyze the oxidation or
TABLE 12—Oxidation stability of a lubricant stabilized with a synergistic antioxidant combination. Test Fluid Alkylated diphenylamine S(CH2CH2COOR)2 Rust inhibitor Basestock
Oxidation Stability TOST (ASTM D 943) (95°C; water; Fa and Cu catalyst 3 L air/h) time (h) to total acid number of 2 mg KOH/g sludge (mg) (ASTM D 4310) Basestock characteristics
0.25% 0.05% balance
0.25% 0.05% balance
2000
200
0.2% 0.05% 0.05% balance 3300
172
>5000 89 ISO VG 32 CA (aromatic carbon) = 6.5% S = 0.54%
NOTE—Reprinted from Chemistry and Technology of Lubricants, courtesy of
Kluwer Academic Publishers, Dordrecht, the Netherlands.
LUBRICATING
OILS AND HYDRAULIC
FLUIDS
315
to represent the materials of construction of the system, measurements are also made of the weight changes of the metal specimens. Oxidation is accelerated by holding the fluid at a high temperature in the presence of air, which is usually bubbled through the sample at a fixed rate. In some tests, particularly for steam turbine oils, water is also present to represent the effect of c o n t a m i n a t i o n t h a t is a fairly c o m m o n occurrence in this application. The use of water can limit the temperature of the test in order to avoid loss due to volatility. It can also preferentially solubilize acids that are generated in the oil on oxidation and, if present as a separate layer, may remove t h e m from the oil so reducing their impact o n oil oxidation and acidity measurements. On the other hand the water will promote rusting and the generation of sludge and may also result in the partition of some additives into the water layer. The oxidation tests that are used to assess stability of turbine oils fall into two categories; those which use water to accelerate the testing and/or make the test more representative of service, and those where the fluid is tested "dry." In spite of the fact that gas turbine oils are used at higher temperatures than steam turbine oils and that service conditions are generally drier, "wet" tests are also used for evaluating the stability of these products—though sometimes in combination with a dry test. The two principal tests used today for assessing the stability of inhibited turbine oils are indicated below. 1. ASTM D 2272, Standard Test Method for Oxidation Stability of Steam Turbine Oils by Rotating Bomb—In this test (also known as the RBOT test), the fluid, water and a copper catalyst are contained in a glass vessel. This is placed in a pressure vessel equipped with a gauge or other means of monitoring the change in internal pressure. The "bomb" is charged with oxygen to a pressure of 6.2 bar (90 psi) and heated to 150°C. The time taken for the pressure to fall by 1.75 bar (25.4 psi) is monitored and recorded as the life of the sample. Fig. 14 shows the b o m b and it's component parts while Fig. 15 shows a typical pressure chart after a test. This is a highly accelerated test that is normally completed well within 24 h. It does not represent what happens in practice as the pressure prohibits the antioxidant (and water) from volatilizing. However, it is maintained that it shows good correlation with service [54]. It is also widely used in fluid development as a screening test for quality control purposes and in monitoring the stability of used fluid. Typical limits on mineral turbine oil would be >350 m i n for ISO 32 or 46 grade products (ASTM D 4304). Higher values, e.g., > 4 5 0 min are required by some turbine builders. 2. ASTM D 943, Standard Test for Oxidation Characteristics of Inhibited Mineral Oils, also known as the TOST (turbine oil stability) test, is the second principal test for steam turbine oils. This involves heating the fluid together with water at 95°C in the presence of iron and copper catalysts, while air is passed through. At regular intervals the acid n u m b e r in the fluid is measured and the test terminated when the level reaches 2.0 mg KOH/g. In contrast to the RBOT procedure, and depending on the base stock and activity of the stabilizers, this test can take several thousand hours to complete. Minimum requirements for ISO 32/46 viscosity grades are normally 2000-2500 h. Fig. 16 shows
316 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK
FIG. 14—A view of the ASTM D 2272 pressure vessel, recorder, and sample container with catalyst.
FIG. 15—Typical chart recording pressure changes in the RBOT test.
CHAPTER 12: TURBINE LUBRICATING the arrangement of fluids and metal specimens in the oxidation cell used for this test. As the length of the D 943 method can make product development with this procedure a very time-consuming process, some specifications (e.g., MIL-L-17331) have reduced the duration to 1000 h but added limits on metal content and sludge. To meet the requirements for a procedure in which sludge was determined, ASTM introduced D 4310, Standard Test Method for Determination of the Sludging Tendencies of Inhibited Mineral Oils. This procedure utilizes the apparatus of the D 943 method but after 1000 h the test is concluded and the fluid filtered through a 5-fim filter in order to determine the weight of sludge produced. Optionally, the change .'S'^ia k^
OILS AND HYDRAULIC FLUIDS
317
in weight of the catalyst coil may be measured and, of course, the increase in acidity may be determined. Another goal of test method development in this Eirea has been to find a method that could be used for both steam and gas turbine oils. Although the FED-STD-791 oxidation test methods are quite widely used (see below), they are regarded primarily as tests for aviation oils in view of the types of metal catalysts used and the high temperatures normally specified. When these conditions are used with mineral oils, the results are regarded as "somewhat inconsistent" [54]. For these reasons, it was eventually decided to specify a dry test procedure, which involved passing air through the oil or fluid in the presence of copper and iron catalysts, but at the lower temperature of 135°C. This formed the basis for ASTM D 5846, Universal Oxidation Test for Hydraulic and Turbine Oils. In this case the acidity and sludge are monitored and the test is terminated when the acid number reaches 0.5 mg KOH/g or the level of sludge (determined by rating the deposits left on a filter paper after placing a drop of used fluid at its center), becomes unacceptable. Since its introduction, the test has been of particular interest for gas turbine oil development in view of the availability of more stable basestocks. The stability testing of gas turbine oils, particularly aeroengine oils, has traditionally been at much higher temperatures than for steam turbine oils in view of their more severe operating environment. Most of the tests used in this area have been variations on the FED-STD-791, Method 5308, Corrosiveness and Oxidation Stability of Light Oils, which, in its original version, involved oxidizing the fluid in the presence of six different metal specimens for 164 h at 120°C. A modified version of this test still features in U.S. Navy specification MIL-PRF-23699F in which test conditions of three days at 175°C, 204°C, or 218°C are required. Although initially used in the U.S. Air Force specification MIL-L-7808 it was later replaced by FED-STD-791 method 5307, which has a test duration of four days and possible temperatures of 248-680°F (120-360°C) in the presence of seven metal specimens. The test (slightly modified) also features in the latest USAF specification, MIL-L-27502, for a high temperature engine oil in which conditions of two days at 220°C and 232°C are specified. Unlike the 5308 test in which the fluid and catalyst coupons are evaluated only at the end of the test, in the 5307 procedure, fluid samples are taken during the test and monitored for viscosity and acidity increase. ASTM D 4636, Standard Test Method for Corrosiveness and Oxidation Stability of Hydraulic Oils, Aircraft Turbine Engine Lubricants, and Other Highly Refined Oils, was later issued as a combined 5307/5308 method offering three alternative procedures and the ASTM method is now also specified as an alternative to the 5307 procedure in MIL-PRF-7808L.
FIG. 16—Metal catalyst, fluid, and water arrangement in the ASTM D 943 oxidation test.
The metal specimens used in the oxidation tests are normally (but not cJways) in electrical contact as this is how they are found in the system and corrosion is frequently accelerated by bringing together metals of different electrical potential. The arrangement of the metal specimens in the FEDSTD-791, method 5308 and Alternative Procedure 2 in ASTM D 4636 is shown in Fig. 17. Mineral gas turbine oils are also tested by a modification of the 5308 procedure. General Electric, for example, specifies this method for its high temperature gas turbine oil specifi-
318
MANUAL
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HANDBOOK bine oils meeting ISO VGs 32 and 46. To increase the test severity the duration was extended from 3-9 days for most oils. This resulted in the mineral oils starting to degrade significantly.
Rust and Corrosion Inhibitors
FIG. 17—Metal catalyst arrangement used in the FEDSTD-791 method 5308 oxidation test.
cations, GEK 32568 and GEK 101941, which call for a temperature of 175°C while reducing the duration to three days. Because of the importance of deposit formation in aircraft lubricants, the accurate measurement of the amount of dirt or sludge produced during the high temperature oxidation tests is as important a feature of these tests as the generation of acidity and increase in viscosity. Oxidation test conditions for phosphate esters tend to fall between those of mineral oil axid synthetic esters though, because of their tendency to hydrolyze, the tests specified are dry tests. Typical test conditions are 164 h at 120°C in the presence of copper and iron catalysts (ISO 15595) or three days at 175°C (FED-STD-791, m e t h o d 5308 modified) as found in b o t h ASTM D 4293, S t a n d a r d Specification for Phosphate Ester Based Fluids for Turbine Lubrication and the General Electric specification GEK 28136 for phosphate ester fire-resistant gas turbine lubricants. Table 13 [27] compares the oxidation stability under stemdard and more severe FED-STD-791 test conditions of uninhibited butylatedphenyl phosphate esters and commercially available mineral gas tur-
Rust is a continuing problem in turbines when carbon steel is present because water contamination of steam turbine oils and fluids is very difficult to avoid. Acid is also invariably present in degraded oils and this can attack certain metals. Although the engine in aero-gas turbines is usually constructed of stainless steel or exotic alloys, carbon steel can still be present in the auxiliary equipment and the oil tank. In order to avoid rusting and corrosion, certain chemicals have been found to protect the metal surfaces. For steel, such inhibitors are usually highly polar materials, e.g., organic acids, esters or amides (Fig. 18), which form an adsorbed film on the surface of the metal that physically hinders the transfer of water to the metal surface. This necessitates careful formulation to avoid interaction with other surface-active materials, such as antiwear additives, and to minimize the impact on foaming/air release properties. In view of the fact that many turbines operate in saline environments, rust protection is frequently required against salt water. This is a more severe requirement than distilled water protection. Rust inhibitors may also be used in polyol and phosphate esters. However, the polarity and film-forming tendency of the latter when uninhibited already offer some protection against rusting, which is enhanced by acidic degradation products. If the phosphate is used in conjunction with adsorbent media for controlling the level of acidic degradation products, careful selection of the inhibitor is required as the adsorbent solids can remove both acidic and basic additives. The most widely used procedure for evaluating rust inhibitors is ASTM D 665, Standard Test Method for Rust Preventing Characteristics of Inhibited Mineral Oil in the Presence of Water. This involves stirring together 300 mL of the fluid and 30 mL of either distilled or synthetic seawater for 24 h at 60°C. A cylindrical test specimen of carbon steel is immersed in the mixture and, at the conclusion of the test, is assessed for the amount of rust produced. Ratings vary from a "pass" where no rust is found, through light rust (<6 spots) to moderate rust ( < 5 % of the surface is covered by rust), and to heavy rust ( > 5 % covered). If rusting is going to occur it is thought that it will appear within the first 4 h of the test and proposals are currently being considered by ASTM to reduce the test duration to this period. In addition to steel, there are other metals that can be attacked by degraded oils and by some additives, such as sulfur-containing extreme-pressure additives. Of these, copper is by far the most important, not only because of its common use in system construction but also because it may catalyze the breakdown of oils and fluids when present as soluble salts at concentrations of < 4 0 p p m [52]. The mechanism normally involves acid attack on the metal with the formation of metal salts, which then dissolve in the oil. However, copper is also susceptible to attack from sulfur either present in the oil or, more commonly, released from extreme-pressure additives as they degrade. This can result in the formation of sulfidecontaining deposits. Fortunately certain types of chemicals
CHAPTER 12: TURBINE LUBRICATING
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319
TABLE 13--Comparison of the oxidation stability of mineral gas turbine oil with butylated phenyl phosphates under extended FED-STD-791 test conditions. Test Duration (days)
Viscosity Increase
ISO VG 32 phosphate (TBPP)
3 6 9
ISO VG 46 phosphate (TBPP)
3 6 9
Gas turbine oil A (ISO VG 32)
3 6 9
3 4.4 3.8 6 6.4 7.3 3.6 4.2 25
Gas turbine oil B (ISO VG 46)
3 6 9 3 6 9
Fluid
Gas turbine oil C (ISO VG 46) Gas turbine oil D (ISO VG 46) Turbine industry limits ASTM D 4293 limits
3 6 3 3
(%)
18.3 24.3 49.5 23.4 28.6 80.6 13.4 21.2 - 5 to +15 - 5 to +20
Acidity Increase (mg KOH/g) 0.11 0.37 0.85 0.27 0.33 1 0.44 0.8 5.3 2.2 3.1 5 3 4.5 7 2 5.1 2.5 max 3.0 max
C12H22—CH — C O O H
Metal Weight Change (mg/cm^) Fe
Cu
Cd/Fe
Al
Mg
-0.11 0.02 -0.01 -0.01 0.11 -0.04 0.0 0.03 0.14 0.03 0.21 0.56 0.02 0.04 0.18 0.06 0.24
-0.17 0.04 0.1
-0.03 0.02 0.03 -0.01 0.17 0.18
-0.07 0.0 0.0 -0.11 0.08 -0.11 0.0 0.0 0.09
-0.03 0.02 -0.02 -0.02 0.13 0.07
where R, is
0.03 0.29 -0.02 0.02 0.04 0.01 0.08 0.13 0.04 -0.03 -0.1 -0.47 0.02 -0.13
0.05 -0.05 -11.7
0.03 0.03 0.05
0.01 0.03 0.06 0.06 0.07 0.33 0.02 0.01 0.04
-0.04 0.02
0.0 0.52
0.01 0.35
-0.04 0.02 -0.15 -0.1 0.07 0.96
0.05 0.02 0.32
+-CH2CH2O-J-H
I CH2—COOR1
C12H22—CH — C O O H
where R2 is a polyamine residue
CH2—CONHR2 FIG. 18—Typical structures for turbine oils rust inhibitors.
can protect the metal from either type of attack by the formation of a protective layer of up to 5000A thick [55]. These products, known as metal passivators are, for turbine oils and fluids, chiefly of the triazole family (Fig. 19), and the effect of their activity is to substantially extend the life of fluids in copper-catalyzed oxidation tests. Their effectiveness is readily demonstrated in oxidation tests such as ASTM D 2272 and ASTM D 943. Apart from their performance in these and other metal-catalyzed oxidation tests, the corrosive tendencies of oils towards copper is most frequently assessed by ASTM D 130, Standard Method of Test for Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test. In this test, a copper strip is normally immersed in the oil for 3 h at lOO'C, but more severe conditions are also used depending on the application. Following the immersion period the strip is solvent cleaned cind visually rated for corrosion by comparing the surface color or tarnish with the ASTM Copper Strip Corrosion Standards, which represent increasing levels of tarnish axid corrosion. Normally turbine oil specifications call for a maximum rating of lb, which is slight tarnish.
Unfortunately, triazole derivatives titrate as acid and therefore contribute slightly to the acidity of the fresh fluid, a factor that has to be considered when meeting specification limits on fresh fluid. Antiwear and Extreme-Pressure Additives Antiwear additives are compounds added to an oil to reduce the wear occurring between the surfaces in sliding contact. They are normally effective at light to medium loads, for example in pump operation. These additives, of which neutral triaryl phosphates are perhaps the most well known examples, initially form strongly adsorbed layers on the surface. As the temperature increases due to the relative movement of the surfaces, a chemical reaction takes place with the surface. The mechanism early suggested involved the formation of a lower melting eutectic of iron and iron phosphide, which flowed into the gaps between the asperities and therefore helping to provide a greater surface area to cany the load. This idea was later rejected in favour of a metal phosphate layer that assisted lubrication [56]. The latest work,
320 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
methylene-bis-benzotriazole
tolutriazole
N N
I CH2NR2
N-alkylated benzotriazole
N-alkylated 1,2,4, triazole
FIG. 19—Structures of some metal passivators used in turbine oils and fluids. however, suggests the formation of a self-regenerating polyphosphate layer possibly in the presence of amorphous carbon [57]. As the temperature increases still further a point is reached where the phosphate film breaks down and cannot prevent metcil to metal contact. In order to sustain even greater loads, for example to provide adequate gear lubrication, it is necessary to generate higher melting films, which are normally achieved by adding extreme pressure (EP) additives, particularly sulfur in the form of a sulphur carrier, e.g., ZDTP, sulfurized fats or olefins. Combinations of phosphorus and sulfur are also widely used and cover a wider temperature range than each element individually, providing both antiwear properties and EP performance over a wide range of applied loads. Figure 20 shows the structures of some typical antiwear and extreme pressure additives. Most specifications for hydrocarbon turbine oils do not require antiwear performance. There are also normally no load-carrying requirements for steam turbine oils except for extreme pressure steam turbine oils, e.g., in marine applications, but they do exist for gas turbine oils, and where the oil is used for the lubrication of both a steam and gas turbine as in some combined cycle applications. These requirements are not particularly onerous, typically lying between FZG failure load stage values of 5-8 while EP turbine oils require a failure load stage of 12 + . Micro-pitting resistance may aJso be required for this more severe application and is also ceirried out on the FZG gear rig using the Forschungsvereinigung Antriebstechnik (FVA) method 54/I-IV. The other application to require gear testing is aero-engine
lubrication. These oils have traditionally required performance on the Ryder Gear Tester (ASTM 1947—now discontinued—or FED-STD-791, method 6508). Increasingly, because of better precision, the test devised by the German Forschungsinstitut fiir Zahnrad und Getriebe (FZG) is being used and this method has been standardized by ASTM as D 5182, Standard Test Method for Evaluating the Scuffing Load Capacity of Oils. Both the FED-STD 791 method and the FZG procedure involve "a recirculating power loop principle, also known as a four square configuration, to provide a fixed torque (load) to a pair of precision test gears." A schematic is shown in Fig. 21. The drive gearbox and the test gearbox are connected through two torsional shafts. Shaft 1 contains a load coupling used to apply the torque through the use of known weights hung on the loading arm. The procedure involves operating the machine at a constant speed (1450 rpm) and oil temperature (90°C) for a fixed period at successively increasing loads until the failure criteria are reached. This is "when the summed total width of scuffing or scoring damage from all 16 teeth is estimated to equal or exceed one gecir tooth width." The FVA micro-pitting test referred to above is in two parts. First, a load stage test is carried out in which the test oil is run at each load stage between 5 and 10 for a period of 16 h. This is followed by endurance test involving 80 h at Load Stage 8 followed by 5 periods of 80 h at Load Stage 10. An assessment is made of the amount of pitting or profile deviation on each tooth at the end of each test period. For aviation applications, the Ryder Gecir Test is still used with the MIL-PRF-23699F specification in accordance with
CHAPTER 12: TURBINE LUBRICATING FED-STD-791 method 6508, in which the performance of the test oil is evaluated in comparison with reference oils giving known failure loads. In this case the load-carrying ability is defined as the "gear tooth load at which the average percent of tooth area scuffed is 22.5 %." The variability of the results on this equipment requires eight determinations of the reference oil and six determinations of the test oil. The performance is judged acceptable if the average of the six results is not less than 102% of the reference oil. In the MIL-PRF7808L standard, however, it is now possible also to use the ASTM D 5182 method where a minimum load stage failure of 5 is required. Only an average of two tests is required on this equipment.
OILS AND HYDRAULIC FLUIDS
Antifoams The inhibition of foaming is essential to ensure the correct lubrication of pumps and bearings and, in hydraulic systems, the transmission of power. Foam can also result in a rapid loss in oxidation stability of the oil or fluid and, under the worst conditions, can cause fluid loss from the tank. It is therefore important that air is lost quickly from the fluid and this can be assisted by the addition of very small quantities (usually a few parts per million) of specific chemicals known as antifoams that reduce the surface tension on the bubble envelope [58]. The most widely used of these are the silicone fluids. Chemically, these are polydimethylsiloxanes, which
where R is typically CjjCj orC4
triaryl phosphate
dialkyl phosphite where R is C4-C8
triphenylphosphorothionate
RO^
OH.HNR'2
321
R'zNH.HO'
OH.HNR'2
amine phosphates FIG. 20—Structures of typical antlwear/extreme pressure additives used in turbine oils and fluids.
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are available in a range of viscosities, b u t the most active products are those that are not soluble but dispersible in the oil or fluid. Soluble siloxanes do not have the same activity at the air/fluid interface. Other problems associated with these additives include the difficulty in obtaining a homogenous dispersion—the polymers tend to accumulate at the surface and then deposit on the walls of the tank so that over time the efficiency of the additive is lost. At "high" concentrations they can also have an adverse effect on air release properties and, if the turbine oil escapes into the turbine, there have been reports of the build u p of silicon on the turbine blades. As a result, the trend in mineral turbine oils is away from the use of silicones and towards other products such as polyglycolethers and polyvinylethers, but larger quantities of these are normally needed. The laboratory test procedure (ASTM D 892, Standard Test Method for the Foaming Characteristics of Lubricating Oils, or the ISO equivalent method, 6247) involves passing air at a fixed rate through either a spherical crystalline alumina diffuser or a cylindriccd sintered steel diffuser immersed in the oil or fluid contained in a measuring cylinder (Fig. 22). These
1 != pinion 2 = gear wheel 3 = drive gears 4 = load clutch
diffusers produce bubbles of a known and consistent size. After passing air through the fluid for 5 min at 24°C (TS^F), the volume of foam (tendency) is noted immediately and then also after 10 min (stability). The test is repeated at 93.5°C (200°F) and then again at 24°C to check if the foaming behavior has changed (possibly due to loss of cintifoam) as a result of the exposure to high temperature. The results are quoted in terms of the tendency/stability values obtained at each of the three temperatures. Values are normally much lower at the higher temperature and hence limits under this condition are more severe. Because of the importance of foaming and its related topic, air release, it is further discussed in the section on Performance Requirements, Their Significance and Evaluation. The performance of the oil is usually retained as long as the additives remain in solution a n d are not depleted owing to removal from the fluid as volatiles, by partitioning into any free water, by precipitation, or by natural usage. If the additive is removed by any of these means, the rate of degradation, for example, can suddenly accelerate. If the oil monitoring is not adequate it can result in the oil exceeding its operating limits
5 = locking pin 6 = lever arm iNWn weights 7 = torque measuring clutch 8 = temperature sensor
Schematic section of FZG Test Rig torque measuring clutch
shaft 2
test gears
drive motor
drive gears load clutch shaft 1 FIG. 21—Diagram of the FZG Test Rig.
CHAPTER 12: TURBINE LUBRICATING OILS AND HYDRAULIC FLUIDS 323 suggested a quantity of about 360000 tons of turbine oils, excluding aviation eind marine gas turbine requirements, were sold worldwide. Obviously, with a global market of this size there are a large n u m b e r of suppliers including both multinational companies and smaller, national oil companies. One of the "problems" associated with the supply of mineral turbine oil is that the are produced in different parts of the world from different crude sources. Although attempts have been made by the oil industry to minimize these differences by introducing "formula numbers" or product identification numbers that identify the product by certain physical characteristics, e.g., viscosity, flash point and pour point, etc., it is still possible for the base material to vary in its composition. As a result there may be variations in the performance, particularly stability, of finished products sold under the same trade name. This has caused some turbine builders to abandon formal approval lists in order to avoid liability claims and to rely on the local supplier to convince the end user that a product is available that meets the specification requirements.
FIG. 22—The development of foam in the ASTM D 892 test procedure. unnoticed. It is n o w widely accepted that with m o d e m turbine oils, condition monitoring is important; this aspect will be examined in more detail later.
THE CLASSIFICATION OF TURBINE LUBRICANTS AND HYDRAULIC FLUIDS In order to clarify the different applications for turbine lubricants and hydraulic fluids a n d hence ensure that the correct type of oil or fluid is used, ISO Standard 6743-5, Lubricants, Industrial Oils And Related Products (Class L)— Classification—Family T (Turbines), has been issued. This classifies the type of turbine, e.g., steam or gas, and then subdivides the application according to whether the requirement is for normal service or for a special application as, for example, in high temperature service or high load carrying ability. A further subdivision is according to viscosify. Table 14 shows the current classification and typical applications. At present the classification does not extend to lubricants for water, wind, or aviation gas turbines although the standard is currently under revision.
TURBINE OIL AND FLUID STANDARDS The turbine oil market is one of the largest segments of the industrial oil market. An estimate of consumption in 1998^ ^ Private communication, D. J. Whitby, Pathmaster Marketing Ltd., Woking Surrey, UK.
In order to ensure that the quality of the turbine oil supplied meets certain minimum standards, and to form the basis for purchasing agreements, performance specifications have been issued by international and national standards organizations as well as by the turbine builders and some end users. ASTM, for example, publishes D 4304, Specification for Mineral Lubricating Oil Used in Steam and Gas Turbines. As a result, the industry is now one of the most heavily specified. The major specifications currently used by industry are listed in Tables 15 and 16 for the main turbine applications. While past practice was for separate specifications on hydrocarbonbased steam and gas turbine oils, the current trend is to issue a combined document as, for example, in D 4304. At present there are n o separate specifications for water turbine oils, but requirements are sometimes included in standards for steam and gas turbine lubricants, e.g., ISO 8068, Petroleum Products and Lubricants-Petroleum Lubricating Oils for Turbines (categories ISO-L-TSA and ISO-L-TGA)-Specifications. Although there has recently been significant consolidation within the power generation industry, some of the specifications listed are still published under the n a m e of the previous builder. Some simplification of the list is therefore to be expected. When seeking approval, the oil or fluid supplier would first approach the turbine builder or end user with a request for their product to be examined against the specification. Tests would be carried out by the specifier (in-house or at an independent laboratory) to ensure that the product was technically acceptable. If successful, approval for field trials or commercial use would be given. In the former case it might be necessary to obtain several years of operating experience before full approval was given. Where no formal approval process existed, the supplier would have to convince the user that the product met the requirements, normally by providing the results of independent laboratory tests. Although most specifications are primarily concerned with the quality of new fluid as delivered, some manufacturers also specify the quality of any flushing fluid to be used and cilso the limits on fluid performance in use. The latter aspect will be referted to in more detail in the discussion on monitoring used fluid quality.
324 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 14—Classification of lubricants for turbines (ISO Standard 6743-5). Application Steam, direct coupled or geared to the load
Gas, direct coupled or geared to the load
Control system
Specific Application
Composition and Properties
Symbol ISO-L-
Typlcal Applications
Normal service
Highly refined petroleum oil with rust protection and oxidation stability
TSA
Special properties
Synthetic fluids with n o specific fire-resistant properties
TSC
Fire-resistant
Phosphate ester lubricant
TSD
High load-carrying ability
Highly refined petroleum oil with rust protection, oxidation stability, and enhanced load-carrying abilityoxidation stability, and enhanced load-carrying ability
TSE
Power generation and industrial drives and their associated control systems. Marine drives where improved load-carrying ability is not required for the gearing Power generation and industrial drives and their control systems where special properties of the fluid are advantageous, for example oxidation stability, low temperature properties. Power generation and industrial drives and their associated control systems with need for fire- resistance Power generation and industrial drives; marine geared drives and their associated control systems where the gearing requires improved load-canying ability.
Normal service
Highly refined petroleum oil with rust protection and oxidation stability
TGA
Higher temperature service
Highly refined petroleum oil with rust protection and improved oxidation stability
TGB
Special properties
Synthetic fluids with no specific fire-resistance properties"'*
TGC
Fire-resistant
Phosphate ester lubricant
TGD
High load-carrying ability
Highly refined petroleum oil with rust protection, oxidation stability and enhanced load-carrying ability
TGE
Fire-resistant
Phosphate ester control fluid
TCD
Aircraft^ Hydraulic"
Power generation and industrial drives and their associated control systems. Marine drives where improved load-carrying ability is not required for the gearing Power generation and industrial drives and their associated control systems where high temperature resistance is required due to hot spot temperatures Power generation and industrial drives and their control systems where special properties are advantageous, for example oxidation stability, low temperature properties. Power generation and industrial drives and their associated control systems with need for fire-resistance Power generation and industrial drives; marine geared drives and their associated control systems where the gearing requires improved load-carrying ability Steam, gas, hydraulic turbine control mechanisms where the fluid supply is separate from the lubricant and fire-resistance is needed
TA TH
"These products may not be compatible with petroleum-based products. 'This category Includes synthetic hydrocarbons as well as other chemical types. "Classifications for these categories have not been established. Reproduced with permission of the International Organization for Standardization, Geneva, Switzerland.
Specifications typically include r e q u i r e m e n t s o n b o t h the c h e m i c a l a n d p h y s i c a l c h a r a c t e r i s t i c s of t h e l u b r i c a t i n g oil o r h y d r a u l i c fluid. T h e y a l s o i n c l u d e ( b u t o f t e n fail t o differentiate b e t w e e n ) the so-called "type" tests t h a t identify a general level of p e r f o r m a n c e a n d r o u t i n e t e s t s u s e d for r e g u l a r q u a l ity c o n t r o l . T h e f o r m e r r e q u i r e m e n t c o u l d i n c l u d e l o n g t e r m
o x i d a t i o n t e s t s (for e x a m p l e t h e A S T M D 9 4 3 o x i d a t i o n t e s t referred to above) o r large-scale fire-resistance tests t h a t are clearly impossible and/or u n n e c e s s a r y to evaluate on a b a t c h p r o d u c t i o n b a s i s . A n e x c e p t i o n t o t h i s s i t u a t i o n is for m i l i t a r y a v i a t i o n s p e c i f i c a t i o n s , e.g., M I L - P R F - 2 3 6 9 9 F w h e r e t h e Ryd e r G e a r T e s t is p e r f o r m e d o n t h e first b a t c h of e a c h c o n t r a c t
CHAPTER 12: TURBINE LUBRICATING awarded and the bearing test performed on the first three full-scale production batches of any newly qualified oil. The co-existence of both international and national standards together with those of the individual turbine builders and end users is, perhaps, surprising. Although the publication of a European (EN) Standard requires the withdrawal of any competing national standcird, this is not the case for ISO (International Organization for Standardization) standards.
OILS AND HYDRAULIC FLUIDS
325
ISO/EN/national standards are also minimum requirements and, in some cases, may not be severe enough to meet the manufacturer's or user's requirements. In such cases the latter's specifications may still be used. National and international standards also require considerable time for revision, a process that may be inadequate for manufacturers seeking to make rapid changes in technical requirements to respond quickly to the concerns of industry.
TABLE 15—International and national specifications for turbine oils and hydraulic fluids. International or National Specifications International Standards Organisation International Electrotechnical Commission European Standard Canada China France Germany Japan India Russia UK USA
Steam Turbines
Industrial Gas Turbines
ISO 8068
Fire-Resistant Hydraulic Fluid
Aero-Derivative Gas
ISO 8068
CD 10050 lEC 61221 EN61221
3-GP-357Mb DL-571
DL/T 571-95 AIR 3514A AIR 3517
DIN 51515 nS-K-2213 IS 1012 Tp-22CTU 38.101821 Tp-22BTU 38.401-58 BS489 ASTM D 4304 ASTM D 4293
In preparation JISK-2213 IS 1012
DIN 51518 COST 12245-66 COST 13076-86
ASTM D 4304 ASTM D 4293
TABLE 16—Turbine builders and utility specifications for lubricating oils and hydraulic fluids. Turbine Builder/Utility Alstom Power (France/UK) Alstom Power Mannheim (Germany) Alstom Power (Switzerland) Alstom Power (Sweden)
Alstom Energy Lincoln, (UK) Alstom Power (Germany) Ansaldo BHEL EDF Fuji Electric General ElectricMedium & Large Steam Turbines General Electric-Industrial Steam Turbines Hitachi Kawasaki Mitsubishi Heavy Industries
Steam Turbine Oil Specification
Industrial Gas Turbine Oil Specification
Aviation Gas Turbine Oil Specification
NBAP50001A
SBVPRlOOl C
DIN 51515
HTGD 690149 VOOOIK
H G T D 9 0 117V001Q 812101 812102 812106 812107
HTGD 690 149 VOOOIK 81 23 00
812101 812102 812106 812107 65/0027
QM44-101/B IS 1012 HN 20-S-30 JISK-2213 GEK 46506D
QM44-100/B 602W917 ST 22007 HN20-S-41 Siemens TLV 9013.04 GEK 32568C GEK 101941 GEK-28136A*
D50TF1-S4
Solar Toshiba Siemens Westinghouse
GEK 46357E
165A974CE JISK-2213 GEK 4506D JISK2213 769 45192 JISK2213
GEK 32568 (mod) GEK 101941 (mod) H T G D 9 0 117(ABB) JISK2213
GEK 46357E
STM-1840
National Power (UK) Pratt and Whitney Siemens
Fire-resistant Hydraulic Fluid Specifications
Procurement Specification 207001, Part 9 521CTypell TLV9013/04 TLV 9016 03/02 (hydraulic oil) JISK-2213 55125Z3
TLV 9013/04
TLV:9012 01/05
ES 9-224 GEK 32568C 55125Z3
GEK 46357E 53740AL
326
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TABLE 17—Military specifications for turbine oils and hydraulic fluids. Country
Steam Turbine Oil
Belgium
BN-PO-175A
Canada France
3GP-357-Mb STM 7220B
Italy UK
MM-O-2001 Def Stan. 91-25/3
USA
MIL-L-17331H
Industrial Gas Turbine Oil
Aviation Gas Turbine Oil
BA-PO-lOeA" BA-PO-115A° BA-PO-103B'' BA-PO-109C AIR3514/A AIR3515/A-B'' AIR3516/A" Def. Stan 91-93 Def.Stan 91-94 Def.Stan 91-97/1" Def.Stan 91-98/1 Def.Stan 91-99/1" Def.Stan 91-100 Def.Stan 91-101 MIL-L-aOSlC MIL-PRF-7808L MIL-PRF-23699F
' Denotes a mineral oil-based product. The remainders are synthetic-based lubricants.
Table 17 lists some of the current military specifications for turbine oils. Where no standard is listed for a specific country it is probable that one of the standards developed by other countries (particularly the USA and the UK) is adopted for use. As can be seen, there are no military standards available for industrial gas turbines or fire-resistant turbine hydraulic fluids. The reason is that few industrial gas turbines are used in this application because of limitations on size and weight. There are military applications for fire-resistant fluids but these are not in turbines. One other complication in the chart is that there are several specifications available for aviation turbine lubricants. These are divided between mineral oil (extreme pressure and non-extreme pressure grades) and synthetic-based products. The latter are further subdivided according to the different fluid viscosities.
PERFORMANCE REQUIREMENTS, THEIR SIGNIFICANCE AND EVALUATION As might be expected in an application that may be using the latest technological advances, the technical requirements can be extensive and severe. This is also a result of providing a resource on which industry and the consumer totcdly depend and on which the safety of large numbers of the traveling public rely. Table 18 illustrates the technical requirements for: 1. A steam or industrial gas turbine oil without additional load carrying capacity (T5^e 1 oils listed in ASTM D 4304, Standard Specification for Mineral Lubricating Oil Used in Steam or Gas Turbines). 2. An aero-derivative gas t u r b i n e lubricant (MIL-PERF23699F, Lubricating Oil, Aircraft Turbine Engine Oil, Synthetic Base, NATO Code N u m b e r 0-156, standard grade). 3. A fire-resistant turbine control fluid (lEC 61221, Petroleum Products and Lubricants-Triaryl Phosphate Ester Turbine Control Fluids (Category ISO-L-TCD)—Specifications.
In each case the data in the table is given on one viscosity grade only as all these specifications contain multiple requirements. These would include, for example, a range of different viscosities; applications with additional load-carrying requirements; or, in the case of the aviation lubricant, grades with corrosion inhibiting or thermal stability requirements. For information on the grades not mentioned, reference should be made to the appropriate specification. As will be seen, there is commonality in some physiccd and chemical tests but considerable differences exist as far as "performance" or "type" tests (for example, stability a n d load-carrying requirements) are concerned. The following comments explain the background to the incorporation of the different performance requirements into the specifications and outline the procedures available for their determination. Table 19 lists the common ASTM, Federal Standard Method, or ISO standard tests used in turbine oil specifications; however, not all standards for the same property are identical. Viscosity The viscosity of a liquid can be regarded as its resistance to flow, but is more precisely a measure of the interned friction as the molecules move relative to one another. The property is important as it enables us to rank oils and fluids in terms of their relative "thickness" and to define the appropriate "level of thickness" necessary to ensure the presence of an adequate lubricating film in the p u m p , bearings, or gears throughout the operating temperature range of the equipment, including low temperature start-up capability. It was noted earlier, for example, that military requirements necessitate the use of a low viscosity lubricant for engine start-up in very cold climates. At the other extreme, when using mineral oils in turbines operating in high temperature environments, it may be necessary to use higher viscosity products to achieve adequate lubrication. However, if a lubricant has too high a viscosity, cavitation will occur while too low a viscosity can result in p u m p slippage and internal leakage (Fig. 23
CHAPTER 12: TURBINE LUBRICATING OILS AND HYDRAULIC FLUIDS 327 [59]). Also related to viscosity are volatility and, to a limited extent, flash point properties. The most c o m m o n procedure for measuring kinematic viscosity (ASTM D 445, Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids) involves measuring the time for a fixed volume of liquid to flow under gravity through a calibrated glass capillary viscometer. Although in the past, 100°F and 210°F were standard tempera-
tures for measuring viscosity, the most widely-used temperatures are now 40°C and 100°C, and these are cJso the basis for the determination of viscosity index and viscosity classifications (see below). The former temperatures, however, are still frequently quoted and care is needed to avoid confusing the different data. It is possible to measure viscosity over a wide temperature retnge using this method and Fig. 24 illustrates the viscosity/temperature relationships for the major fluids
TABLE 18—A comparison of the technical requirements for turbine oils and fire-resistant hydraulic fluids. Test Method Property
Viscosity Grade Appearance (20°C) Kinematic Viscosity (cSt) at 40X
Steam/Industrial Gas Turbine Oil
Aero-Derivative Gas Turbine Oil
Fire-Resistant Hydraulic Fluid
ASTM
32 Clear and bright
22
46
D2422
3448
D445
3104
28.8-35.2
41.4-50.6
-6 180
>23 4.9-5.4 13000 6 -54 246
D97 D92
3016
Report
1.0
100°C
-40X Vise. Change after 72 h at -40°C (%) Pour point (°C) Flash Point (X) Density at 15°C Total Acid No (mg KOH/g)
max mEix max min max max
-18 1.2 0.1
FED-STD-791 etc.
ISO etc.
3675 6619
D974 SAE ARP 5088
Foaming Tendency/Stability Sequence 1 Sequence 11 Sequence 111 Air Release at 50°C Chlorine Content (mg/kg) Water Content (%) Volume Resistivity at 20°C (Mfim) 60247 Emulsion Characteristics at 54°C min. to 3 ml emulsion Evaporation Loss (%) Rust Preventing Properties —Procedure A Rubber Compatibility (% swell) Corrosiveness to Copper Rating Fluid Compatibility-sediment (mg/L) Trace Metal Content (ppm) Shear Stability-Viscosity Loss at 100°F (%) Bearing Deposition-demerit rating Thermal Stability & Corrosivity at 274°C -Viscosity Change (%) -Acid Number Change (mg KOH/g) -Metal Weight Change (mg/cm^) Oxidation Stability h to acid number 2.0 Oxidation Stability-min. to 175kPa drop Oxidation Stability Viscosity change (%) Total Acid Number Change Sludge Content (mg/100 mL) Wt. change (mg/cm^)-Fe Wt. change (mg/cm^)-Cu Wt. change (mg/cm^)-Ag Wt. change (mg/cm^)-Mg Wt. change (mg/cm^)-Ti Wt. change (mg/cm^)-Al Oxidation Stability Acid number increase (mg KOH/g) Wt. loss-Fe (mg) Wt. loss-Cu (mg)
D892 max max max m£ix max max mm
6247
5
150/0 25/0 150/0 6 50 0.1 40
D3427
9120 15597 760 lEC
30
15
D 1401
6614
50/0
25/0 25/0 25/0
10
D972 D665
25-May
D6546 D 130
Pass
3604/3433
6072
3403 Atomic Emission Spectr.
max max
20 Various (1-11)
max
4
max
80
max max max min
5.0 6.0 4.0 2000
D943
min
350
D2272
D2603 3410 3411
5308 max max max max max max max max max max max max
- 5 to + 25 3.0 25/50 ±0.2 ±0.4 ±0.2 ±0.2 ±0.2 ±0.2 15595 1.5 1.0 2.0
328
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TABLE 19—A summary of the principal test methods for evaluating turbine oil properties. Property
ASTM Method
ISO Procedure
Color Kinematic Viscosity Low Temperature Viscosity and Viscosity Stability Viscosity Index Viscosity Classification Density-hydrometer method Density-digital density meter Acid Number-Colorimetric Potentiometric
D 1500 D445 D2532
3104
D2270 D2422 D1298 D4052 D974 D664
2909 3448 3675 12185 6618 6619
Water Content
D 1744
Pour Point Evaporation Loss Flash/Fire Points-Open Cup Autoignition Temperature Spray Ignition
D97 D972 D92 D2155/E659
760 12937 3016
Hot Manifold Ignition Wick Ignition Foaming Air Release Demulsification Shear Stability-Oxidation Stability
D5306 D892 D3427 D 1401 D2711 D943 D2272 D5846
2592 3988 15029 20823 14935 6247 9120 6614
Others (FED-STD-791 Unless Specified) 102 305 307 9111
5105 5106 SAE ARP 5088 3253 201 351 1103 1152 Factory Mutual Std. 6930 6053 3213
5307/5308 Hydrol5^ic Stability s h e a r Stability-diesel injector Shear Stability-sonic oscillator Shear Stability-tapered bearing Thermal Stability Particle Contamination-optical Particle Contamination-microscopic Particle Contamination-gravimetric Sizing particles Trace Sediment Copper Corrosion Rust Preventing Characteristics Corrosion prevention Seal Compatibility Fluid Compatibility Electrical Resistance 4-Ball Wear Test Vane P u m p Test Gear Test-Ryder Gear Test-FZG Bearing Test Deposition Test
D2619
15595 15596 CETOPRP 112H
D2603/D5621 D2070 F661 F313 D4898 D2273 D 130 D 665/D 3603 D 1748
CEC L-45-A-99 2508/3411 4402 4407/8 4405 4406 2160 4404 6072
D 1169 D4172 D2882 D 1947 D5182
used in turbine fluids and lubricants. Very low temperature viscosities (which are required for aviation gas turbine lubricants in view of their potential operation in very cold environments) are measured by a slightly different procedure. This involves not only a determination of viscosity at - 4 0 or -65°F but also a cold "soak" to check whether the fluid changes viscosity on prolonged exposure to low temperatures (ASTM D 2532, Standard Test Method for Viscosity and Viscosity Change After Standing at Low Temperatures of Aircraft Turbine Lubricants). This test is in addition to a requirement for stability at -18°C for six weeks without crystallization, separation, or gelling.
3009
3004/3010 5325 4011 3604/3432 3403 lEC 60247 CETOP RP 67H
20763 14635
6508 DIN 51354 3410 5003
Other ASTM standards that involve the measurement of viscosity include the determination of viscosity index (ASTM D 2270, Standard Practice for Calculating Viscosity Index from Kinematic Viscosity at 40 and 100°C). Viscosity Index is a way of representing the change in viscosity of an oil or fluid with temperature relative to two reference mineral oils arbitrarily assigned viscosity indices of 0 and 100. The lower the change of viscosity, the higher the Viscosity Index. Additionally, the classification scheme ASTM D 2422, Standard Classification of Industrial Fluid Lubricants by Viscosity System, (equiveJent to ISO 3448) enables fluids and lubricants to be classified according to their viscosity at 40°C (in centistokes)
CHAPTER
12: TURBINE
in a series of eighteen grades. These range iirom 2-1500 cSt and the width of each band is ± 1 0 % of the mid-point. This scheme is widely used to identify the level of viscosity required for different applications. The ASTM D 4304 turbine oil standard, for example, has limits on fluids u p to an ISO 150 viscosity grade while at least one major steam turbine builder specifies an ISO VG 100 hydraulic oil. However, the common viscosity grades for both hydraulic cind lubricating oils in industrial steam and gas turbines Eire 32 and 46 while products with ISO viscosity grades 10 and 22 are found in aero-engine oil applications. Pour Point/Low Temperature Storage Stability In addition to avoiding thickening at low temperatures, either in use or in storage, it is also important to ensure that the fluid in service does not solidify or precipitate. Such behavior could lead to pumping problems or operational failure. Conventional solvent-refined mineral oils tend to precipitate wax at low temperatures; the hydrocracked products m u c h less so and not at all with PAOs. In comparison, the synthetic esters may, if wet, become turbid at low temperatures and also more viscous on storage. Phosphates normally solidify without precipitation b u t may also become turbid in the presence of water. Additive incompatibility at low temperatures can also adversely influence the low temperature behavior of the oil or fluid and it is mainly for this reason that aviation gas turbine lubricants are tested for extended storage stability. The qualification sample for approval against MIL-PRF-23699F, for instance, is stored at temperatures ranging from - 4 0 ° C to +60°C for a period of three yeeirs. If at any time during this period the fluid fails to meet any of the technical criteria, the approved is withdrawn. The normal procedure for determining p o u r point (ASTM D 97, Standard Test Method for Pour Point of Petroleum
(mm2/sec) 10 000 5000 Cavitation
m
8
2000 4Jpp6^viscosity limit 1000 500 Slow respoRse-
X3
(0 • >
,g 'Jam
ro E
(D C
7)
o
100 50 20 10 5
Low efficieaiey=
a> 3 S 3 5' CD 3
Q. CD O.
i
3 CO (D
Reduced volumetRelfticiency -4i^3lv3Q0S
1 FIG. 23—Significance of viscosity for tlie operation of a typical hydrostatic system. Reproduced from "Hydraulic Fluids," with kind permission of Butterworth-Helnemann, Oxford, UK.
LUBRICATING
OILS AND HYDRAULIC
FLUIDS
329
Products) involves cooling a fixed volume in a cylindrical container with a thermometer inserted just below the surface. As the t e m p e r a t u r e is lowered, the flow behavior is noted by holding the container horizontal for 5 s and watching for movement in the meniscus. The pour point is that temperature w h e n the fluid ceases to flow under these conditions and to which three degrees centigrade has been added. Typical limits for Type 1 turbine oils (of all viscosities) given in ASTM D 4304 are - 6 ° C maximum. Acid N u m b e r The acidity present in the new oil or fluid arises from two sources: 1. Residual acidity from manufacture (in the case of the ester-based products), 2. From the presence of additives—usually acidic rust inhibitors and/or metal passivators, but also some antioxidants. A high level of acid in a n unused, inhibited, fluid is therefore not necessarily indicative of a poorly processed product or one that has aged significantly on storage. The latest version of MIL-PRF-23699, for example, has a limit of 1.0 mg KOH/g. On the other haind the presence of a high level of acidity cem be disadvantageous to surface active properties, for example foaming (ASTM D 892) and air release (ASTM D 3427). It may cause corrosion (ASTM D 665) and promote oxidation (ASTM D 2272). The level of acid in new, uninhibited, phosphate esters is tighfly controlled because this may contain strong acid, with a p H of < 4 , which can catalyze the hydrolytic degradation process. A low initicd acidity in esterbased products therefore ensures good storage stability, a satisfactory condition on filling into the system and a longer operating life. As will be shown later, the acidity level is a n important indicator of fluid "health." Monitoring this parameter is therefore essential to ensure the trouble-free operation of the system. Two techniques are used for the measurement of acidity or acid n u m b e r of turbine oils and fluids. Both involve neutralizing the acid by a base (potassium hydroxide) of known strength in a measured quantity of fluid. One procedure uses indicators that change color when the sample is neutralized (ASTM D 974, Standard Test Method for Acid and Base Number by Color Indicator Titration). The other method measures the acid n u m b e r by plotting the changes in the potential of a glass electrode as the alkali is added. The amount required to produce an inflexion point on the graph is taken as the neutralization point. Where inflexion points are not readily defined, a predetermined end point, for example a p H of 11, is frequently used (ASTM D 664, Standard Test Method for Acid N u m b e r of Petroleum Products by Potentiometric Titration). While b o t h procedures give similar results on fresh fluids, their accuracy, peirticularly reproducibility, is acceptable rather t h a n good. For example, in the colorimetric technique where the initial acidity lies below 0.1 m g KOH/g, the reproducibility is 0.04 m g KOH/g. More difficult still is the accurate determination of used fluid acidity where the fluid color has darkened or when the fluid has been dyed. This makes indicator end points more difficult to detect and precision may suffer with increasing acidity. The trend is therefore towards the use of the potentiometric method for
330 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK 5.0
(0
o .52 "c
ou CO • >
o
E
(U
c
200
0
10
20
40
55
65
75
85
degrees farenheit FIG. 24—Viscosity/temperature data for different turbine oil and fluid.
100
CHAPTER 12: TURBINE LUBRICATING fluids in service. For aviation lubricants a potentiometric method has been developed by the Society of Automotive Engineers (SAE-ARP 5088) which is now included in MIL-PRF23699F. In the past, determination of acidity was traditionally carried at the close of an oxidation test. However, because the major criterion for failure in the D 943 method was the time taken to reach a certain level of acidity, it was necessary to follow the development of this parameter throughout the test. This required regular sampling of up to 20 g of fluid per determination. For long-term tests this could significantly reduce the fluid volume in the test and would therefore disturb the relationship between catalyst surface area and the fluid and also between water and the fluid, making it difficult to compare test results. To reduce the effect of sampling on the oxidation test conditions, methods of measuring acidity on much smaller samples were developed. Currently ASTM D 3339, Standard Test Method for Acid Number of Petroleum Products by Semi-Micro Indicator Titration, and D 5770, Standctrd Test Method for Semi-quantitative Micro Determination of Acid Number of Lubricating Oils During Oxidation Testing, are used. The former method uses up to 5 g of fluid while the latter uses only "drops" of fluid. The D 5770 procedure is regarded as less precise than D 974 or D 664 and is not recommended for monitoring oils in service. Its principal use is therefore likely to be product development and the method advises that "each laboratory shall develop its own criteria for determining when to switch from this method to a more precise test method for acid number." Figure 25 shows typical automatic titration equipment used for determining total acid number on a series of samples. Specifications vary considerably in their requirements
OILS AND HYDRAULIC FLUIDS
331
on initial acidity. ASTM D 4304 has no limits but values are to be reported. Some turbine manufacturers also have no requirement while others call for a limit of 0.2 mg KOH/g maximum for non-geared units and 0.3 mg KOH/g for geeired turbines. Higher limits are found for the ester-based aviation lubricants. MIL-PRF-7808L, for example, specifies 0.3 or 0.5 mg KOH/g max. while at the other extreme, the specification limit on fire-resistant control fluids is typically 0.1 mg KOH/g maximum. Water Content Water is a problem contaminant for all tjrpes of fluid. It can be present in either dissolved or dispersed form and, in the case of major contamination, may form a completely separate layer. When the main reservoir contains mineral oil, free water normally falls to the bottom of the tank owing to the different density and requires removal by means of the vaJve on the tank base or via a sediment drain to the tank side. If the fluid has a density greater than that of water, as in the case of phosphate ester, the free water layer will be on top of the fluid in the tank and should be skimmed or siphoned off. Other methods used for free water removal include centrifugal separation and water absorbing filters. To remove dissolved water, vacuum dehydration is recommended. The latter technique may take the form of an extractor on the tank encouraging the flow of a stream of dry air across the surface of the oil or fluid. Alternatively, a vacuum dryer may be used on a by-pass to the main oil tank in which a thin film of the oil or fluid is exposed to a counter-current of dry air. This technique reduces the water level more rapidly. Although water contamination is more likely on steam turbines, it can
FIG. 25—Equipment for the measurement of acid number by automatic titration.
332
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also occur on gas turbines as these are occasionally washed to remove deposits forming on blades, etc. The solubility of water in turbine oils is very low—about 30-100 mg/kg at ambient temperatures, and excess water in new mineral oils can usually be detected visually as it causes turbidity. With esters, the solubility is m u c h higher, in the range of 2000-2500 mg/kg at ambient temperatures although levels in service are normally in the region of 500-1500 mg/kg. Small amounts of water can normally be tolerated, and are common in used steam turbine oils. While dissolved water can cause a reduction in viscosity, hydrolysis of additives (or even of the ester fluids themselves) and hence a loss of stability, etc., it is dispersed or free water that is the usual cause of emulsification, rusting, and possible extraction of additives. With ester fluids, hydrolysis is the normal form of degradation and the acid formed can promote further degradation. Monitoring the water content of oils or fluids is therefore a n essential part of any fluid maintenance program. The procedure used for detecting water has to be capable of accurately measuring very small amounts and the technique used is to titrate a measured a m o u n t of fluid or oil with standard Karl Fischer reagent to an electrometric end point. The most c o m m o n method is ASTM D 1744, Standard Test Method for the Determination of Water in Liquid Petroleum Products by Karl Fischer Reagent. Precision has been established for levels between 50 and 1000 mg/kg, but the method is frequently used for detecting m u c h higher values, peirticularly on used ester fluids. Limits are not frequently specified for new mineral turbine oils in view of the low solubility of w a t e r a n d the reliance on visual appearance (free w a t e r causes turbidity). Occasionally limits of 0.01-0.02% w/w (100-200 mg/kg) are quoted while, for fire-resistant control fluids and aviation esters, the values are significantly greater (0.1 % w/w or 1000 mg/kg), reflecting the higher solubility and the hygroscopic (water absorbing) nature of these fluids. Density Measurement of density or specific gravity is one way of identifying the use of the correct fluid and ensuring that the correct volume of fluid is purchased if the product is supplied by weight. It is also a n important factor for designers in determining the p o w e r required for p u m p i n g the oil or fluid around the system and in arranging for removal of free water. Phosphate esters, for example, have densities about 30% greater than mineral oils and require more power for circulation. Also, ciny free water in the tank will accumulate on top of the phosphate. This fluid t3rpe will hold more dirt in suspension than other fluids, which necessitates better filtration initiedly, but offers the possibility of a cleaner system in operation. Density or relative density (specific gravity) values for both transparent and opaque liquids Ccin be obtained either by the use of a hydrometer (ASTM D 1298, Standard Practice for Density, Relative Density (Specific Gravity) or API Gravity of Crude Petroleum and Petroleum Products by Hydrometer Method, or the ISO 3675 equivalent), or by a digital densitometer (ASTM D 5002, Standard Test Method for Density and Relative Density of Crude Oils by Digital Analyzer, for which the ISO equivalent method is 12985). Values are normally measured at 15 or 20°C, but can be calculated at other tem-
peratures from knowledge of the values at these or two other temperatures. API Gravity is a special function of relative density, and is obtained by hydrometer measurement carried out at 60°F (15.6°C). This test is not normally applied to non-hydrocarbon oils and gives values that are numerically significantly higher than the other procedures mentioned. Density limits featuring in specifications depend on the type of fluid being used. Mineral oils, for example, are typically quoted at 0.9 kg/1 maximum at 15°C while the API gravity for an ISO 32 gas turbine oil is 29-33.5. Triaryl phosphates have a substantially higher density and limits on this fluid would typically be 1.17 kg/1 maximum for an ISO VG 46 product. F o a m i n g a n d Air R e l e a s e Air is a "contaminant" of every turbine oil and fluid. Whether or not it causes problems it depends largely on whether it is soluble in the oil, or present as dispersed bubbles. System design, circulation rates, fluid cleanliness, etc., are also factors to be considered. Dissolved air is not normally a concern but if it comes out of solution when pressure is reduced locally it may cause p u m p cavitation. Dispersed air tends to be a problem if it is not readily released from the bulk of the fluid in the tank and compression of the bubbles by the p u m p does not cause their dissolution. Under such conditions dispersed air can cause a loss of control due to a change in the compressibility of the fluid, result in increased oxidation, and may have an adverse effect on lubrication. Foam is the accumulation of air bubbles surrounded by a thin film of oil and occurs at the surface of the liquid. Air release measurements, by contrast, are made in the bulk of the liquid where a relatively thick film of oil separates the air bubbles. There are a n u m b e r of factors relating to fluid properties, system design and use that can influence foam formation. These include tank design and circulation rates [60], fluid viscosity, surface tension, vapor pressure, contamination, bubble size [61], air leaks on suction lines, or simply too low an oil level in the tank. Some foam is inevitable as air is released from the oil or fluid when the pressure returns to ambient. In most systems the presence of a small a m o u n t of foam can be tolerated without any significant adverse effect on fluid/system performance. In extreme cases, excess foam may reduce lubrication performance, induce bearing vibration, cause fluid oxidation or loss of fluid, and create m u c h inconvenience to maintenance staff. In hydraulic systems operating at high pressures, the presence of dispersed air will depend on the system pressure, the size of the air bubbles, fluid temperature, and the compression time in the p u m p . Where the bubble is small, the compression time relatively slow, Eind the pressure Eind temperat u r e high, there is only a slight chance of bubbles being circulated that could affect the compressibility of the fluid euid hence the response time of the system. Conversely, if the bubbles do not readily dissolve under pressure, not only is there a risk of a loss of control but there may also be a n associated lubrication failure, fluid oxidation, and also the possibility of "dieseling." This is a phenomenon where compression/ignition takes place inside the bubble with resulting fluid degradation [62]. In turbine lubrication systems, pressures are normally too low for this phenomenon to occur but
CHAPTER 12: TURBINE LUBRICATING it is occasionally found in high pressure hydrauHc systems resuUing in a rapid darkening of the fluid, an increase in acidity, and the development of a "humt" odor. In addition to the D 892 foam test method discussed above, military aviation lubricant specifications also call for the use of a dynamic foam test, FED-STD 791, method 3214 in which the oil is heated and circulated around a loop consisting of a pump, oil heater, air injection orifice, foam test cell, and associated instrumentation. As the oil is circulated at a temperature of 80°C or 110°C, the sample is aerated for a period of 30 min at a fixed rate with measurements being taken of the foam, etc., every five minutes. At the end of the test, the foam level, oil pressure and oil volume are noted together with the time taken for the first patch of clear surface to appear or the foam remaining after 5 min. The test is then repeated with a different air-flow rate or at a higher temperature. The equipment is able to operate at low pressures thereby simulating the effect of high altitude flight. Typical foam limits for mineral turbine oils vary significantly and can also depend on whether the application is in a steam or gas turbine. Limits in the latter application are usually stricter in view of the high aeration of the gas turbine oil as it leaves the bearings. For both steam and gas turbines.
OILS AND HYDRAULIC FLUIDS
therefore, foaming tendency/stability values of 50/0 mL. for all conditions (as defined in the ASTM D 892 test described above) are required in ASTM D 4304 while, for steam turbines, the builder's requirements can be as high as 450/10, 50/10, and 450/10 mL for the three test sequences. In gas turbines, some equipment memufacturer's limits are 10/0, 20/0, and 10/0 mL. For fire-resistant control fluids, limits are also usually fairly severe at 50/0 ml. (all sequences) ranging up to 150/0, 50/0, and 150/0 mL. For aviation gas turbine oils, limits are also severe with the MIL-PERF-23699 requiring 25/0 mL metximum at all temperatures. Unfortunately, the laboratory test cannot simulate the conditions that are found in the turbine; such is the influence of system design. As a result, the test data should be treated as indicative of a trend in fluid behavior and any significant change, particularly the generation of stable foam, be initially confirmed by investigating the behavior in the tank itself. Air release properties (ASTM D 3427, Standard Test Method for the Air Release Properties of Petroleum Oils, or ISO 9120) are similarly measured by saturating the fluid (normally at 50°C, but other temperatures are also possible) with air bubbles and then measuring the time it takes for the fluid to return to an air content of 0.2%. Figure 26 shows the
c (D
Group 1 mineral oil
Groups mineral oil
333
ISOVG32 polyol ester
ISOVG46 PAO
ISOVG46 phosphate ester
FIG. 26—The effect of temperature on air release values for different turbine oils and fluids.
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air release properties over a range of operating temperatures for various turbine oils and fluids. Typical specification requirements for mineral oils, e.g., as required by ASTM D 4304, vary from 5 min (ISO VG 32), through 7 min (ISO VG 46) to 17 min (ISO VG 150) for nonEP oils and up to 25 minutes for products with improved load-carrying capacity. As would be expected, air release values increase with viscosity and this is reflected in these limits. Turbine manufacturers tend to have more severe requirements with 3-5 min m a x i m u m frequently specified for both steam and gas turbines. The air release properties of phosphate esters vary significantly with chemical type. The "natural" phosphates such as trixylyl phosphates tend to have very low air release values of about 1 min while the synthetic types (ISO VG 46 grade) are in the region of 5-8 min, hence specification limits for fire-resistant control fluids are frequently of the latter order. It is probable that m a n y fluid-related problems could be avoided if there was a greater awareness of the impact of system design on air entrainment.
Chlorine Content Limits on chlorine content are required with phosphates in order to avoid servo-valve erosion [63]. In fact it is the chloride ion (CI ) level that is critical and this can arise either as a trace contaminant from the manufacturing process (from the use of phosphorus oxychloride) or by air-bom contamination. Another source is the incorrect use of chlorinated solvents for cleaning system components. Although the chlorine in solvents is not initially present in ionic form it is thought that the solvents break down thermally in the system to release chloride ion. While limits are placed on new fluid levels (normally 50 ppm), the content of chlorine in fresh fluid is usually significantly lower (typically 20-30 p p m ) . In spite of the importance of the level of chloride ion in the fluid, it is rarely specified for new fluid and the limits that have appeared in the past are at much lower levels, e.g., 0.5-2 ppm. The accurate measurement of very low levels of chlorine is not easy and the standard b o m b calorimeter tests are not sufficiently accurate, as the combustion of phosphates by macro techniques is usually incomplete. ASTM D 808, Standard Test Method for Chlorine in New and Used Petroleum Products (Bomb Method), a typical bomb calorimeter process, for example, is only valid for chlorine levels of 0.1-50%. Instead, microcoulometry (e.g., IP proposed method AK99), a combustion process which uses very small amounts of sample is preferred, or possibly X-ray fluorescence (ISO 15597). The basis of all the calorimeter methods is the titration with silver nitrate of the chloride ion produced by combustion. Due to the purity of phosphates in commercial production this parameter is rarely a problem today and, as will be shown later, is controlled in use by special filtration techniques.
Fire-resistance Tests These properties are the most important requirements for the phosphate esters and for the small quantities of polyol esters that are supplied as hydraulic fluids into this application. The origins of fires in turbines are divided almost equally between the lubricant and hydraulic fluid [64] and are normally a re-
sult of contact with a hot surface. An obvious example of this would be a hot steam pipe, where temperatures are now reaching 610°C. However, there are other potential sources of ignition such as bearing housings and gas turbine exhausts that may be hot enough to ignite oil vapor, if not the liquid itself [65]. Unfortunately, there is no single test that can satisfactorily predict fire behavior and several tests are necessary to obtain a more complete assessment of performance. Much depends on the form in which the fuel (in this case the oil or fluid) is present. For example, it is easier to ignite the fluid in the vapor state as the access to oxygen is greater (combustion is an oxidation reaction with the release of heat in the form of a flame). The evaluation of fire resistance by tests that simulate the hazard is therefore of crucial importance. While many tests have been devised to assess this property, three types are widely used: 1) Spray ignition behavior, 2) Hot surface ignition, and 3) The flammability behavior when the fluid is absorbed onto a substrate [66]. Simple flash and fire points (ASTM D 92, Standard Test Method for Flash and Fire Points by Cleveland Open Cup) are used to identify very flammable (and volatile) materials but, for products of low volatility such as are used as fire-resistant fluids, flash and fire points do not necessarily relate to the performance of these fluids in other, more hazard-related, tests. A high fire point, for example, does not automatically mean good performance in spray or hot surface ignition tests (see Table 8 and [66]). Today, flash and fire point tests are used more for quality control purposes rather than for measuring fire resistance [66]. As indicated above, in turbines, the main hazard is a hot surface. Where the surface is lagged, the risk increases in view of exposure of thin films of oil or liquid to oxygen in the lagging. The wick test (below) does not properly simulate this condition as the oil film in the lagging (unless saturation is reached) is thinner and the access to oxygen is greater. However there is currently no standard method that accurately reproduces this condition. The test methods that are most widely used to assess fire resistance include the following: Spray Ignition
Tests
These are of two types: 1) Those that attempt to ignite the fluid by spraying at elevated t e m p e r a t u r e and pressure through a n open flame, noting whether ignition occurred and, if so, whether burning continued once it had moved away from the ignition source (ISO 15029, Parts 1 and 3, Petroleum and Related Products—Determination of Spray Ignition Characteristics of Fire-resistant Fluids). 2) Measuring the heat emitted by the burning fluid (Factory Mutual Test Standard 6930 for the Flammability of Industrial Fluids and ISO 15029-2). It might seem strange to assess the fire resistance of so-caJled fire-resistant fluids by deliberately igniting them, but "fire-resistant" in this context does not mean non-flammable. Most, if not cdl, fire-resistant fluids will combust if contacted by a high-energy source for a sufficient period of time. Past spray tests tended to be pass/fail methods with poor precision but the latest methods, such as ISO 15029-2 and the new Factory Mutual procedure, are able to rank fluid behavior with acceptable precision and generally in the same or-
CHAPTER
12: TURBINE
LUBRICATING
OILS AND HYDRAULIC
FLUIDS
335
exhaust channel anemometer exhaust gas thermocouple (Tp/Tex)
nozzle
\_
ambient air thermocouple
atomising air hydraulic fluid
FIG. 27—Schematic representation of the test chamber used for the ISO 15029-2 Spray Ignition Test. Reprinted with permission of the Health and Safety Laboratory, Sheffield, UK. der. Figures 27 and 28 are diagrammatic representations of the equipment used to carry out the above tests. Hot Surface Tests Although the latest spray ignition tests can indicate the relative fire resistance of fluids, the results tell us nothing about the temperature at which the fluid would ignite on contact with a hot surface. It is therefore important to obtain some idea of this aspect from appropriate procedures. Currently only one test is specified for turbine fluids, the determination of autoignition or s p o n t a n e o u s ignition t e m p e r a t u r e , by ASTM D 286 or D 2155 (both now obsolete) or by E 659, Standard Test Method for Autoignition Temperature of Liquid Chemicals. In each of these procedures a small, measured a m o u n t of the test fluid is inserted into a heated glass container (conical or round-bottomed flask). The temperature at which the fluid ignites with the production of a visible flame was the result recorded in the two earlier tests while the most recent variant also observes the production of "cool flames," which occur at lower temperatures than conventional hot flames. The recent procedures have resulted from attempts to obtain a more even temperature distribution throughout the container, while replacing the original molten lead bath, which was a health hazard. Major differences are found between the results of the three tests with the trend towards lower values with the later methods [66]. The reason for the differences in results is mainly due to the increased size of the container and therefore to the increased oxygen concentration present [67]. Unfortunately, the reduction in autoignition temperature values with recent methods has occasionally led the user to believe that an inferior product was being supplied and as a result data is still quoted on the earlier methods, sometimes together with the figures from the latest test. A further hot surface ignition method (The Hot Manifold Test), based on FED-STD-791 Method 6053, is currently being developed by ISO as Standard 20823 and may be preferred to autoignition temperature in the future as the conditions more obviously simulate a n industrial hazard. Wick Tests The flammability behavior of the fluid when it is absorbed into a substrate (or wick) can be assessed by ASTM D 5306,
S t a n d a r d Test Method for the Linear Flame Propagation Rate of Lubricating Oils and Hydraulic Fluids, which, as the title suggests, measures rate of flame travel along the wick. Alternatively, ISO 14935 records the time it takes for the burning fluid to self-extinguish after removal of the ignition source. I n the case of the ISO standard, the ignition source is an oxy-propane flame while in the ASTM method, matches are used to ignite the wick. Such a test might simulate the ignition of the fluid when soaked into cloth waste. In both tests, the unused fluid is usually evaluated whereas in reality the fluid may have been in contact with the absorbent over a period of time allowing volatile components, such as water, to escape. This could significantly alter the behavior of the fluid. Obviously where such tests are required it is expected that the fluid would meet the criteria for a "pass" irrespective of its condition. Figures 29 and 30 show the equipment required for the two tests. Water Separability/Demulsibility Tests As noted earlier, water is a common contaminant of steam turbine oils. It may arise as a result of the penetration of seals by steam, from cooler leaks, condensation in the tank or from steam valves dripping onto hydraulic actuators, etc. Although gas turbines are normally drier due to the absence of steam and the higher operating temperature, they also use waterfilled coolers and may be periodically washed with water to reduce the build-up of deposits within the turbine. The oil tanks for some gas turbines are also located out-of doors and may not be protected from the elements. Consequently, undesirable levels of water in turbine oils and hydraulic fluids can occasionally build u p . Free water is found more frequently with mineral oils because of its m u c h lower solubility in this medium. Water can affect turbine oil in m a n y ways: it promotes rusting and subsequent wear, causes a reduction in the oil stability—^possibly by partition of the additives into the water—and, in the event of the formation of an emulsion, has a n adverse effect on lubrication performance and may cause filter blockage [68]. The effects of water can be enhanced by the presence of other contaminants such as oxidation products, dirt, rust and other additive-treated mineral oils [69]. With both phosphate and polyol esters, the effect of water is
336
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
fire products collector
1.25 m
nozzle
®
®
^ y - / ^
flexible ^/connectors
T
1.4 m
propane burner floor
MdNn '
sample pressure vessel
propane
load cell
/7777Z FIG. 28—Factory Mutual Research spray ignition test equipment. Reprinted with permission of Factory IVIutual Research, Norwood, MA.
to cause hydrolysis and acidity generation with a potential adverse effect on stability, rusting and corrosion performance, and surface active properties like emulsification. A high water content in the long chain polyol ester hydraulic fluid has also been found to promote bacterial growth which, in severe cases, has resulted in fluid gelatification [70]. Water must therefore be removed from the fluid or oil as quickly as possible. In steam turbine systems free water can be removed using a centrifuge and by filters coated with water-adsorbing polymers. Dissolved water is removed using either chemiccd adsorbents, for example a molecular sieve, or
by applying a vacuum to the fluid. Physical methods for drying used oil and fluid are practiced widely but currently the use of chemical adsorbents is only found with fire-resistant fluids. Although the behavior of the fluid in use is more critical due to the likely presence of contaminants and fluid degradation products, new fluid is tested to ensure that emulsification does not initially occur as a result of the use of surfaceactive products such as corrosion inhibitors. The method most widely used to assess this aspect of performance is ASTM D 1401, Standard Test Method for Water Separability of Petroleum Oils and Synthetic Fluids, (ISO
CHAPTER 12: TURBINE LUBRICATING OILS AND HYDRA ULIC FLUIDS 337
ceramic fibre cord
chart recorder 50 g weight
1
50 g weight
FIG. 29—Diagram of apparatus for determining linear flame propagation.
reservoir
wick
stop holes for adjustment < •
TF5^ burner position for setting flame height r
|l |l |l burner may be raised and lowered in clamp
stop bar from propane regulating valve FIG. 30—Equipment used for the ISO 14395 wicl( flame persistence test.
338 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
6614) where the fluid at 40°C is stirred with an equal volume of water and the time taken for separation of the resulting emulsion (if formed), is noted. Typical limits for separation to a residue of 3 mL emulsion are 30 min maximum (mineral oils) and 15 min maximum for phosphate esters. Stability Characteristics Oxidative Stability All fluids are assessed for their oxidative stability as their service lives depend to a large extent on this property, and the effects of fluid degradation in terms of acidity generation, viscosity increase, and the production of deposits can have an important impact on the system performance. The principal methods used for evaluating oxidative stability were indicated earlier in the discussion on the testing of antioxidants. These methods are additionally used to assess the effects of other additives, e.g., rust inhibitors on oxidation stability and assessing the performance of complete formulations. In addition to the Universal Oxidation Test designed to overcome the long test duration associated with the ASTM D 943 test, other approaches have been or are being evaluated. These include a dry version of the TOST and, for research and development, quality control and residual life assessments, high pressure differential scanning calorimetry (ASTM D 6186, Standard Test Method for Oxidation Induction Time of Lubricating Oils by Pressure Differential Scan-
50
ning Calorimetry (PDSC), [71]) and voltammetric techniques [72,73]. The PDSC method involves heating a known (small) quantity of the oil or fluid under pressure in oxygen at a fixed temperature until an exotherm or release of heat occurs. The time to the onset of the exotherm is reported as the induction time at the specified temperature. Although it is an extremely rapid test, "no correlation has yet been established with service performance." The Remaining Useful Life of a Lubricant Evaduation Technique (RULLET) has recently been developed to assist the maintenance of military aircraft engine oils. In order to determine the remaining useful life, it is first necessary to thermally and oxidatively stress a fresh oil in the laboratory at 150-175°C and monitor the chemges in acidity and viscosity as the test proceeds. When the changes in these values start to deviate from a steady increase, this point is taken as the end of the useful life in service and intermediate data on the oil are used to predict the percentage remaining life in service (Fig. 31 [73]). This method has good repeatability and apparently correlates well with RBOT and PDSC tests. Cyclic voltammetric techniques can be used to quantify the remaining antioxidant concentration in used oil or fluid [73]. This technique involves the extraction of the antioxidants by a solvent containing a dissolved electrolj^te. The solvent/electrolyte system will vary depend on the type of fluid and antioxidant being analyzed. Using the solvent/electrolyte, a voltage is applied across an electrode system consisting of a
Hours of stressing time 100 150 I l__
200
250 1-22
21
--20
--19 (^ o 18 b To
178 ' '
in
--16
-Lis 200
150
100
IHours of remaining useful life FIG. 31—Percent remaining useful life (RUL), viscosity (40°C) and total acid number against h of stressing time and remaining useful life at 175°C for a typical aircraft engine oil. Reproduced with permission of STLE, Park Ridge, IL.
CHAPTER 12: TURBINE LUBRICATING "glassy" carbon electrode, a platinum reference electrode, and a platinum auxiliary electrode. The voltage of the auxiliary platinum electrode is scanned from 0 to 1.5 V at 0.2 V/sec. Data is produced by measuring the current through the cell as a function of the applied potential and the different antioxidants produce a typical "signature" from which the composition can be deduced. Comparison of the current produced for each antioxidant with reference data on solutions containing known amounts of the stabilizer will reveal the level of the antioxidant in the used oil. It was found possible to correlate the remaining oil life with the residual antioxidant concentration, and both techniques can be of benefit in applying predictive maintenance. The importance of avoiding deposits in aviation gas turbine lubricants is such that an additional full-scale bearing test that simulates engine behavior is specified for these oils. Details of the test used to assess this aspect of performance are given in FED-STD-791 method 3410 and involve lubricating a heated 100 mm diameter roller bearing with the test fluid at an elevated temperature (between 300-400°F or 149_204°C) for 100 h. At the end of the test the bearing is assessed for deposits against a "demerit rating scale" while the weight of filter deposits and changes in oil viscosity and acidity are also measured. Thermal Stability Tests for this parameter fall into two categories: 1. Those carried out in the absence of air, e.g., FED-STD-791 method 3411 (despite the fact that it is almost impossible to find a condition where the oil or fluid is heated in the complete absence of air). 2. Those where oxygen is present and may cause further degradation over and above the physical and chemical changes resulting from the thermal degradation of the oil. An example of this type of test is ASTM D 2070, Standard Test Method for Thermal Stability of Hydraulic Oils. The Federal standard method is currently only used in aviation turbine oil specifications, e.g., MIL-PRF-23699F, probably for historical reasons and to provide additional support for the oxidation stabihty data in applications where the temperatures are very high. This procedure involves sealing a portion of the fluid together with a steel specimen in a glass ampoule under vacuum, heating for 96 h at 274''C and then determining the changes in viscosity, acidity, and the weight of the metal catalyst. Limits of 5% on viscosity change and 6.0 mg KOH/g on acidity increase are called for in the above specification. The ASTM method involves heating the fluid (200 ml) in an open beaker together with iron and copper catalyst rods for 168 h at 135°C. At the completion of the test the color of the rods, which is the principal evaluation criterion, is assessed and the amount of sludge produced, determined. This procedure is used primarily to evaluate the stability of mineral hydraulic oils and is strongly influenced by the presence of additives. However, the standard notes, "No correlation of the test to field service has been made." By contrast the FEDSTD-791 method could be used to discriminate between ester types that display different levels of stability depending on their chemical structure. Diesters, for example, are able to find a decomposition path at lower temperatures due to their ability to more readily form a cyclic intermediate [74].
OILS AND HYDRAULIC FLUIDS
339
Hydrolytic Stability This property assesses the fluid stability in the presence of moisture. Normally the effect of water is to generate the acid and alcohol (or phenol) from which the ester was originally formed. All esters are sensitive in varying degrees to water and, since hydrolysis is the normal mode of breakdown, it is an important factor in determining the Hfe of both phosphates and the polyol ester fluids used in hydraulic and aero-engine lubricant applications. In steam turbine systems particularly, water contamination of the hydraulic fluid and lubricant is often found and requires regular monitoring. As noted earlier (Table 9) the structure of phosphates affects their hydrolytic stability and may influence the selection of product for use in steam (wet) or gas (dry) turbine applications. The test procedure most commonly used for assessing hydrolytic stability is ASTMD 2619, Standard Test Method for Hydrolytic Stability of Hydraulic Fluids (Beverage Bottle Method). In this test the fluid and water are mixed together in a rotating sealed container for 48 h at 93°C in the presence of a solid copper catalyst, followed by measurement of the acidity level in both the fluid and the water layers and the metal weight change. Procedures are available for fluids that are more or less dense than water and Fig. 32 shows the bottle with oil, water, and catalyst coupon.
n li
FIG. 32—The ASTM D 2619 beverage bottle container with oil sample (upper layer), water, and copper catalyst.
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For steam turbine control fluids, ISO 15596 is used. It is a static procedure that is uncatalyzed and the acidity increase of both fluid and water layers is measured. In this test, a limit on total acidity increase of 0.5 mg KOH/g is specified. Shear Stability Shear stability is an additional requirement that is part of some specifications for non-hydrocarbon oils. This is the resistance of the fluid to shear stress as the fluid is forced through a neirrow aperture and which can result in the breakdown of long-chain molecules into smaller units. It is of particular importance for hydraulic applications in view of the high pressures and very fine tolerances involved in pumps and valves, particularly when the fluid contains a high-molecularweight polymer, as is the case with polyol ester fire-resistant hydraulic fluids [66]. However, no specification requirements yet exist for the shear stability of these fluids in steam or industrial gas turbine applications. Limits do exist, however, for some aero-gas turbine oils, e.g., in the MIL-PRF-23699F specification. This requirement is thought to date back to the submission of thickened diester lubricants against early versions of the specifications. The instability of these fluids resulted in the addition of a limit on this parameter. Turbine oils (of all t5rpes) are themselves quite shear stable. Specifications for fire-resistant fluids based on phosphate esters have, for many years, stipulated that polymeric thickeners are not to be used as additives. This was due to their removal by the conditioning media (usually fullers earth) with the resulting blocking of filter cartridges and a marked reduction in fluid viscosity. Shear stability has traditionally been measured by two techniques viz resistance to high-frequency sound and circulation under pressure through an orifice. Sonic Shear The technique specified in MIL-PRF-23699F requires exposure under prescribed conditions to vibrations from a sonic oscillator followed by a measurement of the change in viscosity (ASTM D 2603, Standard Test Method for Sonic Shear Stability of Polymer-Containing Oils or ASTM D 5621, Standard Test Method for the Sonic Shear Stability of Hydraulic Fluid). The scope of the early version of this method, which was last reviewed in 1998, indicated that while useful for quality control purposes, it did not show good correlation with the service behavior of polymer-containing oils and may rate different types of thickener in a different order to the diesel injector test. The latest method however, states, "Evidence has been presented that a good correlation exists between the shear degradation that results from sonic oscillation and that obtained in a vane pump test procedure." The principle of the test is first to degrade a reference fluid with a sonic probe at 0°C using sufficient power for 12.5 min to produce a viscosity decrease at 40°C of about 15%. Then, using the same power setting that gave the above level of viscosity change, to irradiate the test fluid at 0°C for 40 min followed by a determination of viscosity. The initial and irradiated viscosities at 40°C are reported. Resistance to Shear in a Diesel-Injector Nozzle The second procedure involves measuring the viscosity loss at 100°C after the fluid has been circulated through a diesel
injector nozzle set at a predetermined lifting pressure, for example 17.5 MPa (175 bar) for 30 cycles, (ASTM D 3945, Standard Test Method for Shear Stability of Polymer-Containing Oils Using a Diesel Injector Nozzle). More recently, ASTM D 5275, Standard Test Method for Fuel Injector Shear Stability for Polymer-Containing Fluids has been published. The latter method was originally Procedure B of the D 3945 method but was separated after tests showed that the two procedures in the earlier method often gave different results. Although the principle of the two methods is the same, the equipment differs and the latest test uses conditions of 20.7 MPa (207 bcir) and 20 cycles. In both cases, reference fluids are required to calibrate the equipment prior to evaluating the test samples. A concern over the lack of field correlation with the results from the diesel injector rig has recently led to the proposal in Europe to use a tapered roller bearing as the means for shearing the fluid. This is Co-ordinating European Council (CEC) method L-45-A-99, "Viscosity Shear Stability of Transmission Lubricants" that promises improved precision and better correlation with field data. Rusting and Corrosion Behavior The requirement for an oil or fluid to prevent rusting (of ferrous metals) and corrosion (of non-ferrous metals) is an existing part of nearly all hydrocarbon turbine oil specifications. While most rust tests are carried out at typical bulk fluid temperatures of 35-60°C, high temperature corrosion performance requires evaluation either by oxidation/corrosion tests or by specific high temperature tests in non-oxidizing conditions such as those specified by the aviation turbine builders. Some tests, e.g., gas turbine "hot end corrosion" cannot be satisfactorily simulated in the laboratory and require evaluation in an engine test. The most widely-used test for assessing rusting characteristics was reported earlier as ASTM D 665, Standard Test Method for Rust Preventing Characteristics of Inhibited Mineral Oil in the Presence of Water. In spite of the title, the method is also used for non-hydrocarbon oils and there is provision for the testing of products such as phosphate esters, which are heavier than water. Although it may be difficult to distinguish between the ratings for "moderate" and "heavy" rusting in this test, this is largely academic, as specifications normally call for a "Pass," that is, the absence of rust. One of the limitations of the D 665 method is that it evaluates rusting on vertical surfaces only. In reality, horizontal surfaces that can retain droplets of water are more prone to rusting. A method has therefore been devised "to indicate the ability of oils to prevent rusting and corrosion of all ferrous surfaces in steam turbines." This is ASTM D 3603, Standard Test Method for Rust-Preventing Characteristics of Steam Turbine Oil in the Presence of Water (Horizontal Disk Method). The test involves immersing a horizontal steel disk and a vertical steel cylinder in a stirred mixture comprising 275 mL of the oil and 25 mL of distilled water for 6 h at 60°C. At the end of this period the specimens are washed and inspected for rust. The test is carried out in duplicate and no rust must be seen in either of the two tests for the oil to be considered a pass. The above procedures only consider rusting in fluid contact with the metal surface whereas vapor-phase rusting is
CHAPTER
12: TURBINE
also possible, for example above the liquid layer in the tank. This condition is simulated in ASTM D 5534, Standard Test Method for Vapor-Phase Rust-Preventing Characteristics of Hydraulic Fluids. With the exception of cin additional steel specimen, which is located in the vapor space above the liquid, the apparatus and test conditions are identical to those found in ASTM D 3603. The method is divided into two parts, the first relating to fluids where water is the continuous phase and the second to fluids where oil or other water-free fluids (except phosphates) are in the continuous phase. In the latter case the water necessary to cause corrosion is contained in a bcciker located below the specimen for assessing vapor-phase corrosion. Results are reported as a pass or fail. The introduction of a corrosion-inhibited grade of fluid in the MIL-PRF-23699 specification has also resulted in the incorporation of a humidity cabinet test (ASTM D 1748, Standard Test Method for Rust Protection by Metal Preservatives in the Humidity Cabinet.) This procedure involves immersing steel plates in the fluid, allowing them to drain and then exposing t h e m to water vapor in the cabinet. The time tciken for the production of rust is monitored. In use, metals of different t3rpes may be in contact with one another and the potential difference of the metal couple in these conditions can influence gcJvanic corrosion, particularly with water-based fluids. An attempt to simulate this condition for non-aqueous fluids is found in the CETOP Method RP 48H, which is also being published as ISO 4404 Part 2. In this test, selected metal pairs in electrical contact, after cleaning and pre-weighing, are half immersed in the fluid for 28 days at 35°C in a covered beaker. At the end of the test period the weight change is measured and examination made of the specimens for signs of corrosion, including the surface above the liquid layer, which will indicate whether vapor-phase corrosion has tciken place. Hydraulic or lubricating oil systems containing phosphate esters can be made from either mild or stainless steel and, while rusting can occur with these fluids, it is not a common occurrence and is normally limited to the area above the liquid layer in the tank where condensation occurs. If the tcink interior is coated with phosphate, this forms a protective layer and reduces the possibility of rusting. In cases where the mild steel surface does not come into contact with the fluid, condensation may be prevented by maintaining w a r m fluid in the tank and/or passing dry air over the surface of the liquid in the reservoir. As a result it is now standard practice to leave the interior surfaces of hydraulic systems unpainted when using phosphate esters. In the past, epoxy coatings were recommended, but unless the coating can be cured in position, the long-term resistance of this type of paint cannot be guaranteed. However, due to the reduced risk of rusting with these fluids in turbine applications, the frequent use of stainless steel systems and the possible removal of inhibitors by the adsorbent solids used in fluid conditioning to maintain low levels of acidity, there is normally no rusting requirement in the specification for this type of fluid. With regard to non-ferrous metals, the corrosion of copper/brass as is found, for example, in turbine oil coolers, can be a problem for all oil types, as the dissolution of this metal will catalyze oil oxidation. The potential for oils to cause corrosion of copper and its alloys and the use of ASTM D 130 for assessing this condition was mentioned above. While metal
LUBRICATING
OILS AND HYDRA ULIC FLUIDS
341
passivators can minimize this problem, use of stainless steel or titanium coolers—or even dry cooling—is now possible. Lead, silver, a n d magnesium, which are also prone to acidic corrosion, may need special protection when used. These metals have been used in aero-engine systems as lightweight casings for components (magnesium) and in solder (lead a n d silver). To prevent magnesium corrosion the surface is painted and propyl gallate has been used as an inhibitor for lead. A test for lead corrosion still features in MILPRF-7808L. This is FED-STD 791, method 5321, which involves immersing lead and copper specimens in the fluid for 1 h at 325°F (162°C) during which time air is passed through. At the conclusion of the test the weight change of the lead panel is measured. The same specification also requires a corrosion test to be carried out on silver and bronze using FED-STD-791, method 5305. This is a high temperature test carried out at 450°F (232°C) in which strips of each metal cire immersed in the oil for 50 h followed by an assessment of the weight change. In systems using phosphate esters, a l u m i n i u m surfaces need to be hard anodized to prevent attack by acidic degradation products. Lubrication The lubrication performance of a n oil or fluid can be regarded as its ability to reduce weeir on metal surfaces sliding relative to one another under load. It is a combination of the performance of the fluid base stock and the additive package. All the finished fluids discussed so far are acknowledged to have good lubrication performance and for most steam and gas turbine applications there are no additional lubrication performEince requirements. Occasionally there is a call for a 4-ball wear test—ASTM D 4172, Standard Test Method for Wear Preventative Characteristics of Lubricating Fluid. This involves measuring the wear produced on three ball bearings held stationary in a cup containing the test fluid. A fourth ball is rotated in contact with the stationeiry bccirings while a load (normally 40 kg) is applied for a period of 1 h (see Fig. 33). For oils needing extreme pressure performance, a gear-test failure load may be specified. As indicated above, this is now carried out according to ASTM D 5182, S t a n d a r d Test Method for Evaluating the Scuffing Load Capacity of Oils (FZG VisucJ method), or in aviation oils by the Ryder Gear Test.
Cleanliness The importance of clean oil, to ensure that small orifices in valves remain clear, that bcciring surfaces cire undamaged, and that p u m p s and motors r u n smoothly, is now well accepted. Dirt can EJSO play a part in catalyzing oil and fluid degradation by stabilizing foam and increasing oxidation. It may also promote emulsification in the presence of water and increase the conductivity of the fluid. Limits on cleanliness have therefore become steadily tighter over the years. At the same time the variety Eind efficiency of filters has improved [53]. Today, turbine lubrication systems use 6 fim (j8 = 200) filters while hydraulic systems generally have finer filtration, typically 3 jam (jS = 200) filters to remove particles as small as 3.0/im "absolute" in view of the use of fine toler-
342
MANUAL 37: FUELS AND LUBRICANTS
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Top ball turned by drive motor
3 lower balls clamped
Thermocouple
A/V\AAAA^
Heater
FIG. 33—Schematic of the four-ball wear test apparatus.
ance valves, such as servo-valves, where clearances can be of the order of 3-5 microns. Several methods are available for the determination of particulate levels. These fall into the categories of automatic counting by light interruption techniques, manual counting of particles deposited on a filter paper, and gravimetric procedures. Light interruption methods are probably the most widely used of the above and involve the passing of a fixed volume of the fluid across a light beam. When a particle interrupts the beam a record is made and the size is registered depending on the extent to which the beam has been blocked. The particle counters will automatically allocate the reading to one of several pre-programmed size ranges depending on the range required by the specification, for example, to a range of, >5)U,m, 5-15)am, 15-25jam, 25-50Atm, 50-100/j.m, etc. While it is a relatively quick way of counting particles, it is not the most precise. It presupposes that the sample is homogenous, i.e., the particles are evenly dispersed and that they are regularly shaped—assumptions that are entirely theoretical. Light interruption techniques can also count products dispersed in the fluid, e.g., antifoams. The ASTM method using this technique is F 661, Standard Practice for Particle Count and Size Distribution Measurement in Batch Samples Using an Optical Particle Counter, while the equivalent ISO standard is 4402. The membrane filtration methods are more accurate. In the manual counting method, e.g., ASTM F 312, Standard Test Method for Microscopical Sizing and Counting Particles from Aerospace Fluids on Membrane Filters, a known volume of fluid is filtered through a membrane filter, the surface of which is divided into squares of known and equal size. The number of particles in a representative selection (at least 10) of these squares is then counted manually and sized and scaled-up to produce an approximate total number on the filter pad. (ISO 4407/4408 are similar manual counting methods.) This procedure is very time-consuming and is also
based on the assumption that the deposit is distributed evenly over the filter. Potentially the most accurate procedure, and sometimes used as a referee method in cases of dispute, is the gravimetric method in which a known volume of fluid (usually 100 mL) is passed through a filter disk with a pore diameter of 0.45^lm or 0.80/Am and the weight of the deposit accurately determined (ASTM D 4898, Standard Test Method for Insoluble Contamination of Hydraulic Fluid by Gravimetric Analysis). ISO 4405 is based on the same technique. Again the method does not indicate the size of the particles, a factor that can be of use in determining the possibility of damage to equipment and may also suggest the source of the contamination. For example, the presence of sub-micron sized particles can be indicative of "dieseling" in the hydraulic system. The ability of the light interruption technique to size particles in bands or ranges allows the development of a classification system based on the maximum number of particles in each band. Original attempts to rank the contamination levels of hydraulic fluids were published as ASTM, SAE, and NAS standards. However the distributions used were largely geometric progressions that were unrelated to the actual pattern of particle distribution found in hydraulic fluids [76]. As an alternative, ISO subsequently published a standard, 4406, Hydraulic Fluid Power-Fluids-Method For Coding Level of Contamination by Solid Particles, the latest version of which (1999) classifies particles into ranges of >4 ^m, >6 ^m, and > 14 ^tm, respectively and, depending on the numbers of particles in each range, allocates a "scale number." This makes possible a numerical description for the particulate levels in both new and used fluids. For example the scale number 15 corresponds to 160-320 particles/mL and scale number 12 to between 20 and 40 particles/mL. Most turbine oil and fluid specifications still have to be updated and are using the previous version of the ISO classification based on the number of particles >5 /i,m and >15 yam in size. They typically call for scale numbers of 17/14 or 18/15
CHAPTER 12: TURBINE LUBRICATING for turbine lubricants and 15/12 for the control fluids. However, such is the pace of change that the SAE 749D classification procedure, which was disowned by SAE in 1971, is still occasionally specified. In order to obtain meaningful data from any of these tests it is essential that the correct sampling procedure, in terms of where and how to take a sample from the system or container, be used. ISO 4021, for example, indicates how samples should be taken from the lines of an operating system. ISO 3170 details suitable sampling procedures from containers while ISO 3722 details how containers should be cleaned before use, a point that is frequently overlooked and can invalidate particle count data. It is also important to ensure that the sample bottle is compatible with the fluid being sampled. Trace Metals The introduction of trace metal analysis into turbine oil specifications is now found quite widely, but for different reasons. In mineral turbine oils the trend to ashless products, and particularly away from the use of zinc dialkyldithiophosphate as an antioxidant/extreme pressure additive, has led to the introduction of limits on zinc content. Current specification limits can be very low (about 2 ppm meix.) or somewhat higher levels (100 ppm max.). In aviation gas turbine lubricants, limits of 2-10 ppm, depending on the metal concerned, have long been in place. This is to minimize contamination, to avoid a possible catal5^ic effect on oxidative breakdown, and because certain organo-metallic additives, used in the past as extreme pressure additives, precipitated on storage. Today, the data also functions as baseline or reference information for the spectrographic oil analysis programs (SOAP), which are used to monitor the condition of used military aviation gas-turbine lubricants. With phosphate esters, the final processing step in manufacture may involve contact with alkaline media to lower the acid number and the resulting presence of soluble metal salts can promote foaming and an increase in air release properties [77]. In use, phosphate esters are normally treated by an adsorbent medium to remove acidic degradation products. However, the most widely used adsorbents, fullers earth and activated alumina, contain compounds that react chemically with the acids to form soluble metal salts [78]. These salts can adversely the surface-active properties of the fluid and therefore need occasional monitoring. One method used for measuring low levels of metal contaminants in hydrocarbon oils is ASTM D 4951, Standard Test Method for Determination of Additive Elements in Lubricating Oils by Inductively-Coupled Plasma Atomic Emission Spectrometry. With phosphate esters, this method can give incorrect results on certain metals (e.g., sodium) and for this medium at least, atomic absorption spectrometry (AAS) is preferred. The ASTM procedure for AAS (D 4628) is currently targeted towards the analysis of barium, calcium, magnesium, and zinc but can easily be adapted to detect other metals. Volume Resistivity One requirement that is specific to phosphate ester control fluids is the measurement of volume resistivity. This param-
OILS AND HYDRAULIC FLUIDS
343
eter, which is the reciprocal of conductivity, has been found to relate to the tendency of the fluid to produce servo-valve erosion [63]. This is an electrochemical process caused by the development of a "streaming current" close to the valve surface arising from fluid flow across the valve. As well as being influenced by the chemical structure of the fluid, it also depends on the presence of contaminants such as water, acid, chloride ions, dirt, and metal soaps. Even polar additives can reduce the resistivity. The problem initially arose when polychlorinated biphenyls were used as fire-resistant fluids in turbine control systems and investigators linked the phenomenon with chlorine content and low resistivity. A minimum limit on resistivity of 5 or 10 by 10' ohm.cm. (50 or 100 Mi2m) was therefore introduced and is now part of all fire-resistant control fluid specifications. This property is measured by ASTM D 1169, Standard Test Method for Specific Resistance (Resistivity) of Electrical Insulating Liquids. Internationally, the method specified is lEC 60247, Measurement of Relative Permittivity, Dielectric Dissipation Factor, and d.c. Resistivity of Insulating Liquids. Both procedures involve measuring the resistance between the terminals of a test cell when a specified voltage is applied. Accurate measurement of the test temperature is important as resistivity is very sensitive to this property. While the lEC method precisely specifies the voltage gradient, the ASTM procedure allows considerable latitude in this key aspect. In order to improve the correlation between the methods, the voltage gradient along with other test variables such as electrode gap and test temperature should be the same. In view of the effect of small amounts of impurities on the results, it is important that no additional contamination of the sample takes place. Consequently, the sampling of the fluid and the cleanliness of the sample container are very important when evaluating this parameter. Figure 34 shows a general view of the test equipment used for determining volume resistivity, while Fig. 35 is a diagram of a typical test cell. Compatibility with System Materials In selecting system components for use with both mineral and synthetic turbine oils, it is important to ensure that the fluid is compatible with the constructional materials, that is contact with the fluid should not result in significant changes to the physical or chemical properties of the material. Small changes are to be expected in some materials and, indeed, may be beneficial. For example, the controlled swelling of a seal will help it fill the cavity and maintain its performance at high pressures. The main concerns in this area are the behavior of seals, paints, and gasket materials in the presence of the operating fluid. Inadequate compatibility with a seal, for example, may cause the swelling or softening of the rubber due to penetration by the oil or fluid, with subsequent fluid leakage. The use of unsuitable paints could cause softening, flaking and subsequent filter blockage, perhaps leading to pumping problems. Occasionally there are incompatibility problems with metals. Phosphate esters do not "wet" the surface of aluminium and therefore this metal is unsuitable as a bearing material for these fluids. Hydrocarbons are generally less searching than synthetic fluids in this area although the PAOs are problematic owing
344
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
to their lack of seal swelling and necessitate the use of a cocomponent (usually a carboxylate ester) as a seal-swelling agent. The high operating t e m p e r a t u r e s experienced in a n increasing n u m b e r of applications, for example aviation gas turbine lubricants, place severe stress on conventional elastomers. Consequently, the use of fluorocarbon (and, to lesser extent, perfluoroelastomer) seals is now widespread. Phosphates eire also very selective with regard to seals and paints, but fortunately a suitable range of materials is now available. Examples of suitable elastomers for turbine oils and fluids are given in Table 20. A general procedure for testing for the compatibility of seals in non-aerospace applications is detailed in ISO 6072, Hydraulic Fluid Power-Compatibility between fluids cind standard elastomeric materials, while the aero-gas turbine oil specifications require performance against FED-STD-791 methods 3604 and 3433 or procedures listed by the equipment manufacturers. An ASTM standard entitled "Standard Test Methods and Suggested Limits for Determining the Compatibility of Elastomer Seals for Industrial Hydraulic Fluid Applications" is currently in development. Table 21 illustrates the conditions recommended for testing different elastomers in the ISO 6072 standtird. The list indicates the principal types of elastomers used with the different fluids, but is not exhaustive. All the procedures involve suspending a rubber specimen of known volume in the oil or fluid u n d e r fixed conditions of temperature and test duration. This is followed at the end of the test by a second measurement of the volume to determine the percentage swell that has occurred. Additional measurements may be made of the chemges in elongation at break and
tensile strength. Depending on the rubber type and application, the test temperature may vary significantly. Fluorocarbon and perfluorocarbon rubbers are, for example, evaluated at higher temperatures than nitrile and neoprene. However, tests at high temperatures, e.g., 150°C as currently appear in the latest draft ISO 6072 standard, may not be representative of practice since, at this temperature, the basestocks and additive packages may not be stable and could give erroneous results. Until recently, all methods specified relatively short periods of immersion, usually u p to one week. It is now accepted that this period may be too short for the degree of swell to have stabilized, and in the latest version of ISO 6072, tests of 1000 h in length are now proposed in addition to the shorterterm exposure. No s t a n d a r d m e t h o d s in turbine oil specifications are known for evaluating paint compatibility and this aspect is generally left to the paint manufacturers w h o u n d e r t a k e long-term tests. The aspects of compatibility discussed above relate only to the fluid/constructional material interface. There is EJSO the aspect of the compatibility of different oils and fluids with each other. Should fluids from different manufacturers be mixed? This aspect is viewed differently by different industries. On the one h a n d it is a condition of militciry aviation oil specifications that all approved products should be compatible with one another. If this were not the case it could impair the combat readiness of aircraft. In the industrial power generation market some equipment builders specify that a new fluid must be compatible with the residue of the previous charge without any adverse effect on the new oil. However,
FIG. 34—A general view of the test equipment for measuring d.c. resistivity.
CHAPTER 12: TURBINE LUBRICATING
OILS AND HYDRAULIC FLUIDS
345
090 1
1
1
1 = inner elecrode 2 = outer etecrode 3 = guard ring 4 = inner elecrode 5 = silica washer 6 = silica washer 7 = minimum level of liquid
Volume of liquid is about 45 cm^ All surfaces in contact with liquid have a mirror finish Dimensions in millimetres FIG. 35—Example of a three-terminal resistivity cell. Reprinted with permission of the International Electrotechnical Commission, Geneva, Switzerland.
nothing is indicated regarding topping-up an existing fluid with a product from a different supplier, probably because such an occurrence is unusual. Obviously, mixing fluid tjrpes that are not completely miscible should be avoided. Some concern has also been expressed regarding the mixing of products that have the same base material but different additive packages. This is because of possible interaction of the different additives resulting in a reduction in performance. Very often, the attitude of the turbine builder is not to allow the use of a second oil (for top-up) during the warranty period of the turbine. Afterwards this would only be accepted if
the builder had investigated the compatibility of the two fluids cmd/or the user had accepted the risk associated with mixing. In some cases oils can be mixed without problems but laboratory tests are advisable to check the effects of physical and chemical (in)compatibility, e.g., the impact on stability and the surface active properties (e.g., foaming and air release). The test used by the aviation turbine oil industry, FED-STD-791 method 3403, Compatibility of Turbine Lubricating Oils, involves checking the miscibility with reference ester-based lubricants. Mixtures of the test oil and the reference lubricants at three concentrations are heated in an oven
346
MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
for 1 week at ZZl^F (105°C) followed by a determination of the sediment produced. Volatility In military aviation applications where a low viscosity oil is used in a high temperature environment, for example the 3 cSt oils specified in MIL-PRF-7808L, it is necessary to ensure that the oil is not too volatile. This is determined in ASTM D 972, Standard Test Method for Evaporation Loss of Lubricating Greases and Oils. This procedure involves passing air at 400°F (205°C) over the surface of a weighed quantity of oil heated to the same temperature and held in a test cell of specified dimensions. After 6'/2 h the weight loss of the sample is determined. In order to ensure that the equipment is providing the appropriate level of severity, a test is carried out with a reference liquid and, if necessary, a correction factor applied. The test limit is 10% maximum. Assuming that the new fluid meets all the requirements laid down in the specifications it may still be necessary to arrange trials in the turbine builder's equipment. These can require several years of satisfactory operation and be very costly to arrange and carry out. TABLE 20—Elastomer compatibilities of different turbine oils and fluids. Phosphate Mineral Oil Seal/Hose Material and PAOs Ester Neoprene Nitrile Chlor-sulphonated polyethylene Epichlorhydrin Styrene-butadiene Ethylene-propylene Butyl Fluorocarbon Perfluoroether PTFE
Polyol Ester
Yes Yes Yes
No No Yes''
No Yes" Yes
Yes No No No Yes Yes Yes
No No Yes" Yes Yes* Yes Yes
No No No No Yes Yes Yes
"Variations in the composition of these elastomers in particular can cause differences in fluid compatibility and the seal manufacturers should be consulted before use. ^Fluorocarbon rubbers are compatible with aryl phosphates, but not with alkyl phosphates.
THE IMPORTANCE OF SYSTEM CLEANLINESS One emphasis in this chapter has been on ensuring the cleanliness of the oil or fluid. However, there is no advantage in having a clean fluid only to fill it into a dirty system. The performance advantages of fresh fluid would soon disappear in these circumstances. There are two situations where attention to the system cleanliness is most important. The first is during commissioning of the new equipment or after major overhauls when the system is flushed. This procedure will remove dirt, preservative oils and greases, cleaning solvents, and other residual matter from cleaning and assembly operations, e.g., welding slag and cleaning rags. Effective flushing may involve removing or blanking-off system components and separately cleaning them while dividing the pipework into sections and circulating a flushing fluid through each section, preferably under turbulent flow conditions to remove as much debris as possible. Some builders remove rust by "pickling" the pipework by circulating a dilute solution of acid followed by a water wash and then dry with a current of warm air. In the past, chlorinated solvents have been used to remove oil films on components during assembly and residual product in the hydraulic system has subsequently caused servovalve erosion [63]. As a result, use of this tjrpe of solvent is now banned and has been replaced by hydrocarbons or, in some cases, by water-based products. Preservative oils containing inhibitors that prevent rust during the transportation and storage of the equipment can cause foaming of the operating charge if they are not adequately removed. The second important use of a flushing charge is when replacing badly degraded fluid, which, in addition to containing acid, may have resulted in deposit formation. In this case it is important to remove all but traces of the previous fluid and any associated deposits by a thorough flush and, if necessary on examination, a manual cleaning of components including the tarrk interior. More detailed guidance on the cleaning and flushing of lubrication systems is given in ASTM D 6439, Standard Guide for the Cleaning, Flushing and Purification of Steam, Gas
TABLE 21—Conditions recommended for testing the compatibility of elastomers with different turbine fluids. Suitable Temperature ISO Duration of Test Elastomer °C± 1 Test" h ± 2 Fluid Classification HH, HL HM, HR HV
NBRl, 2 HNBRl FKM2
100 130 150
Aryl phosphate esters
HFDR
FKM2 EPDMl
Polyol esters
HFDU
Mineral oils
Synthetic hydrocarbon
HEPR
168
1000
150 130
168
1000
NBRl, 2 HNBRl FKM2
100
168
1000
HNBRl FKM2
130 150
168
1000
"The test duration of 1000 h is recommended for evaluation of elastomer compatibility with highly critical fluids. NBR = nitrile (acrylonitrile butadiene) HNBR = hydrogenated nitrile EPDM = ethylene propylene FKM = fluorocarbon Reprinted courtesy of the International Organization for Standardization, Geneva, Switzerland.
CHAPTER 12: TURBINE LUBRICATING and Hydroelectric Turbine Lubrication Systems and, for hydraulic systems, in ASTM D 4174, Standard Practice for the Cleaning, Flushing and Purification of Petroleum Fluid Hydraulic Systems. Both documents relate specifically to hydrocarbon oils and discuss the types of contamination found in practice, how they can be controlled, guidance on suitable flushing procedures, oil sampling techniques, and condition monitoring. For non-hydrocarbon fluids, information is to be obtained from the fluid manufacturers or other appropriate standards, e.g., lEC 60978, Maintenance and Use Guide for Triaryl Phosphate Ester Turbine Control Fluids, which provides similar information except for recommendations on flushing. The fluid used for flushing is often the operating charge, but this is expensive if a second charge of fluid is used for normal operation. If only small amounts of the flush fluid are left in the system after flushing and draining, the use of a chemically similar material, usually the base stock for the operating charge may be considered. However, if significant quantities of the flush fluid remain in the system when the operating charge is filled-in, the use of a cheaper fluid could be counter-productive. The use of flush fluids that are a different chemical type to the operating charge is to be avoided in view of concerns with regard to their compatibility with system components and the fact that it is very difficult to remove residues from the system. Depending on the level of contamination acquired during the flush, it may be possible to re-use the flushing fluid several times.
TURBINE OIL AND FLUID MAINTENANCE EPRI Report CS-4555 [79] comments that "failures of steam turbine bearings and rotors cost the utility industry an estimated $150 million/year" and "one third of these failures involve contaminated lubricants or malfunctioning lubricant system supply." It is unfortunately the case that many of the operating problems found with turbine oils and hydraulic fluids are due to poor maintenance, occasionally compounded by inadequate system design. As operating conditions become more severe, the importance of regular maintenance (both system and fluid) increases. This comes at a time when utilities are achieving cost reductions by shedding jobs and trying to increase equipment availability. As a consequence, there is a distinct possibility that non-essential maintenance will suffer and the improvements in oil performance available as a result of recent technical developments will not be fully utilized. An essential part of all planned maintenance is the ability to carry out regular fluid monitoring. This involves taking representative samples from the system under strictly defined conditions as indicated, for example, in the above ASTM and ISO Standards and then evaluating critical parameters to determine the current condition of the material. Implicit in such a scheme is the availability of limits on these parameters and recommendations on appropriate action should they be reached. Also important is the frequency of sampling. This will vary depending on the particular parameter involved, the stress on the fluid in use, whether
OILS AND HYDRAULIC FLUIDS
347
or not the system is being commissioned, and if operational problems are being experienced. Using the data produced to establish trends in performance can assist in identifying the source of problems and when action may be needed [80]. Unlike the extensive range of specifications available on fresh oil or fluid, the situation with regard to the used oil is less complicated. Many, but not all, of the turbine builders incorporate used oil requirements into their new oil specifications. A few have separate standards and some of the oil or fluid suppliers offer guidance in the absence of information from the turbine manufacturers. Where no builder recommendations exist, guidance is also available from national/international standards. For mineral turbine oils there are ASTM D 4378, Standard Practice for In-service Monitoring of Mineral Turbine Oils for Steam and Gas Turbines; the lEC Standard 60962, Maintenance and Use Guide for Petroleum Lubricating Oils for Steam Turbines; and the very detailed EPRI report referred to earlier [79]. For fire-resistant hydraulic fluids, lEC 60978 and ISO 7745, Hydraulic Fluid Power-Fire-resistant Fluids-Guidelines for Use, are available, while for aero-engine oils there are recommended limits on acidity and viscosity increase from the turbine builders, but otherwise no published requirements. The rapid rate of top-up of oil in this application—even in industrial applications—means it is unlikely that the physical changes in fluid quality would normally reach values of concern. The above industry guides are comprehensive documents which typically examine the reasons for fluid degradation and its impact on fluid performance, identify fluid sampling techniques, give appropriate tests for monitoring both new and used oil behavior, as well as a proposed schedule for their use. Table 22, which is taken from ASTM D 4378, identifies the turbine properties of gas and steam turbine oils that are monitored in service, their warning limits and appropriate action to be taken in the event that these limits are exceeded. Quite a wide range of tests can be used to monitor fluid degradation, etc. These can be divided into primary tests that check for specific degradants or contaminants and secondary tests, the results of which are influenced by the degradant or contaminant. An example would be the measurement of water, which can be determined directly and in esters, its presence may be implied from a reduction in viscosity or a rapid increase in acid number. Acidity is probably the most important property monitored on a regular basis, but is the parameter most frequently disputed. As indicated earlier, of the two techniques used for determining the acid number of both new and aged fluid or lubricant, only the potentiometric method is really suitable for aged fluid [81]. This is because the fluid darkens on ageing and it becomes more difficult to estimate the endpoint in the colorimetric method. Some fire-resistant fluids are also dyed to ensure that they are not confused with mineral oils and for such fluids this technique is also unsuitable, particularly when new. Other important properties that are routinely measured for all Quids include viscosity, water content, and particulate levels. In addition, rapid changes in color and appearance can be indicative of developing problems. A measure of the residual life or stability of the product may also be advanta-
348 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK
Test
TABLE 22—Interpretation of test data and recommended action for mineral turbine oils. Steam (S) Oil Life Warning Limit or Gas (G) (Running Hours) Interpretation
Action Steps
s
up to 20 000 h up to 3000 h
This represents above normal deterioration Possible causes are: a) system very severe b) antioxidant depleted c) wrong oil used d) oil contaminated
Investigate cause. Increase frequency of testing—compare with RBOT data. Consult with oil supplier for possible reinhibition.
0.3-0.4 mg KOH/g
S, G
at any time during life of oil charge
Oil at or approaching end of service life, c) or d) above may apply
Look for signs of increased sediment on filters and centrifuge. Check RBOT. If less than 2 5 % of original, review status with oil supplier and consider oil change. Increase test frequency.
RBOT
less than hcJf value on original oil
S
up to 20 000 h
Above normal degradation
Investigate cause. Increase frequency of testing.
RBOT
less than half value on original oil
G
up to 3000 h
Above normal degradation
Investigate cause. Increase frequency of testing.
RBOT
less than 2 5 % of original oil
S, G
at any time
Together with high acid no. indicates oil at or approaching of service life.
Resample and retest. If same, consider oil change.
Water content
exceeds 0 . 1 %
at any time
Oil contaminated. Potential water leak.
Investigate and remedy cause. Clean system by suitable method.'' If still unsatisfactory, consider oil change or consult oil supplier.
Cleanliness
exceeds accepted limits"
at any time
Source of particulates may be: a) make-up oil; b) dust or ash entering the system; or c) wear condition in system.
Locate and eliminate source of particulates. Clean system oil by filtration or centrifuge or both.
Rust test D 665, procedure A*
light fail
up to 20 000 h
a) the system is wet or dirty or both b) the system is not maintained properly (e.g., water drainage neglected, centrifuge not operating.)
Investigate cause and make necessary maintenance changes. Check for rust. Consult oil supplier regarding reinhibition if test result unchanged.
Rust test D 665, procedure A*
light fail
S, G
after 20 000 h
Normal additive depletion in wet system. Maintenance
Consult oil supplier regarding reinhibition.
Appearance
hazy
S, G
at any time solids or both
Oil contains water, and remedy. Filter.
Investigate cause or centrifuge oil or both.
Color
unusual and rapid darkening
S, G
at any time
This is indicative of: a) contamination b) excessive degradation.
Determine cause and rectify.
Viscosity
5% from original oil viscosity
S, G
at any time
a) Oil is contaminated, b) oil is severely degraded, or c) higher or lower viscosity oil added.
Determine cause. If viscosity is low, determine flash point. Change oil if necessaiy.
Flash point
drop 30°F or more compared to new oil
S, G
at any time
Probably contamination.
Determine cause. Check other quality parameters. Consider oil change.
Foam test D 892
exceeds following limits tendency: 450 stability: 10
S, G
at any time
Possibly contamination or antifoam depletion. In new turbines residual rust preventatives absorbed by oil may cause problems
Rectify cause. Check with oil supplier regarding reinhibition. NOTE: plant problems often mechanical in origin.
Acid no. Increase over new oil
0.1-0.2 m g K O H / g
Acid no. Increase over new oil
Sequence 1
G
S, G
"Definition of suitable cleanliness levels depends on turbine and user requirements. ^Satisfactory for land turbines. '^Appropriate methods may include centrifuging, coalescence, or vacuum dehydration.
CHAPTER
12: TURBINE
geous. This can be obtained either by determining the RBOT value, an indication from analytical methods of the level of antioxidant concentration, or by the use of voltammetric techniques. For phosphate esters, volume resistivity may additionally need to be monitored in order to avoid the possibility of servo-valve erosion. One basic difference between the use of fire-resistant fluids/lubricants ajid hydrocarbon oils and synthetic esters is that, in order to remove the acid that is normally formed as a result of fluid hydrolysis or oxidation, the fire-resistant fluids are purified or reconditioned in situ. This involves passing the fluid on a by-pass loop from the m a i n reservoir through an adsorbent solid that removes acid and chloride from the fluid and also acts as a fine particle filter. For many years fullers earth has been used for this purpose but has often resulted in the formation of soluble calcium and magnesium soaps, which have h a d an adverse effect on fluid foaming a n d air release properties a n d w h i c h eventually precipitate in the form of gels in filters and in parts of the system where the fluid is cooled [80]. In an attempt to avoid this problem, other treatments have been used, notably activated alumina and blends of purified activated eJumina with alumino-silicates. The latter is a definite improvement but can still eventually result in the dissolution of sodium and aluminium in the fluid unless the acidity is kept very low [80]. More recently, the use of ion-exchange media h a s shown great promise a n d now enables the life of the fluid to be extended almost indefinitely [78] with a very positive effect on fluid maintenance costs and almost eliminating the need for fluid disposal.
FUTURE TRENDS Many of the developments discussed in the course of this chapter will continue to influence the development and availability of turbine lubricating oils. Some of t h e m were the subject of presentations at an ASTM Symposium entitled "Turbine Lubrication in the 21st Century." The papers presented are now published and serve as a useful reference to the subject [82]. Where possible, the turbine builders will continue in their sccirch for higher operating efficiencies that will result in even higher operating temperatures and greater thermal and oxidative stress on the lubricant. This, in turn, will accelerate the move from solvent refined oils towards severely hydrocracked oils and synthetic hydrocarbons. It may also accelerate a move toward fire-resistant lubricants particularly as the dcingerous "cocktail" of higher temperatures and reduced staffing could result in an increase in the frequency of oil fires. The d e m a n d for greater e q u i p m e n t availability and reduced outages in order to improve the financial returns to the IPPs will also result in a demand for longer oil/fluid lives. The increase in operational severity will place even greater emphasis on stability, but the advent of in-service monitoring of stabilizer levels, re-inhibition of the existing charge, the possibility of in situ reclamation by better filtration, and removal of developed acidity by by-pass treatment will extend oil life still further. As a result of the anticipated longer oil life there will be increased competition for the initial fill. This will substantially
LUBRICATING
OILS AND HYDRAULIC
FLUIDS
349
reduce the operating margins for new business cind encourage the suppliers to provide "cradle-to-the-grave" service agreements whereby they maintain the fluid throughout its life, supply top-up material as needed, and eventually dispose of the degraded product. The development of in situ condition-monitoring techniques will assist station staff in determining the level and timing of necessary fluid maintenance. The trend to environmentally friendly fluids, where required, may also develop—^particularly for water turbine applications, where some experience has been obtained with biodegradable fluids [83]. One recent development not previously mentioned that may have a future impact on the lubrication of gas turbines has been the successful evaluation in a static engine test of a vapor-phase lubricant based on a tertiarybutylphenyl phosphate [84,85]. Very small amounts of the lubricant are vaporized and react with the metal surface of a ceramic bearing to form a polymeric film that can sustain a load. The immediate interest appears to be for missiles and other unmanned aero-space vehicles because of the reduced weight penedty with the smaller volume of liquid required, b u t wider use in the longer term may be possible. Acknowledgments The author gratefully acknowledges the kind assistance (and patience) of R. Coombes and G. N. Kay of Alstom Power, M. Dennis of Rolls Royce, D. Irvine of Petro-Canada Lubricants, G. Jones of G.E. Energy, E. Letterman of General Electric Power Systems, H. Moeller of Elsam, M. Morris, Consultant, Dr. T. Okada of ExxonMobil, Dr. L. Quick of Siemens A. G., J. Pankowiecki of Siemens Westinghouse a n d B. Rayner, Consultant. Lastly, thanks are cJso due to Alan Holt of Great Lakes Chemical Corp. for the photographs and to Alan Watson for editorial assistance.
REFERENCES [1] Smith, D. J., "Private Ownership of Electric Power is More Efficient and Reliable than Public-Owned Plants," Power Engineering International, March/April 1995, pp. 25-28. [2] Kurtz, D., "Great Expectations," Power Engineering International, May 1999, pp. 33-37. [3] Lane, J., "IPPs Open Up New Markets," Power Engineering International, April 1998, pp. 27-30. [4] Council Directive 89/392/EEC, Official Journal, L183 of 29th June 1989, Office for Official Publications of the European Community, Luxembourg. [5] Dodman, K., "Efficiency Will be the Greatest Issue," International Power Generation, September 1997, pp. 51-53. [6] Collins, S., "Gas Turbine Power Plants," Power, June 1994, pp. 17-31. [7] Paterson, A. N., Simonin, G., and Neft, J. G., "Turbine Blading Materials Boost," International Power Generation, July 1996, pp. 65-67. [8] Curtis, T., "GE 2500+ Power Turbines," Turbomachinery International, May/June 1995, pp. 42-44. [9] Anon., "Micros-a New Gas Turbine Market," Turbomachinery International Handbook, 1997, pp. lli-lS. [10] Ashmore, C, "Power, Light and Cheap Heat," International Power Generation, September 1997, pp. 54-57. [11] Makansi, J., "Combined-Cycle Powerplants," Power, June 1990, pp. 91-126.
350 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK [12] Smith, D. J., "Combined-Cycle Gas Turbines: The Technology of Choice for New Power Plants," Power Engineering International, May/June 1995, pp. 21-26. [13] S w a n e k a m p , R., "Gas-Turbine/Combined Cycle Power Systems," Power, June 1995, pp. 15-26. [14] Swanekamp, R., "Single-Shaft Combined Cycle Packs Power in at Low Cost," Power, January 1996, pp. 24-28. [15] Anon., "Two Prototypes for Large-Scale Wind Turbines," International Power Generation, May 1995, pp. 37-38. [16] Anon., "Towards the 5 MW Turbine," Supplement to Modem Power Systems, October 2000, p. 36. [17] Lakkenborg, J. "Understanding the Mechanisms," International Power Generation, November 1996, p. 45. [18] Hall, D. T., "Turbine Generator Fire Protection Overview," presented sX American Power Conference, Chicago, IL, April 1986. [19] "Evaluation of Fire-Retardant Fluids for Turbine Bearing Lubricants," Electric Power Research Institute Report NP-6542, Palo Alto, CA, 1989, pp. 2-1 to 2-7. [20] Schober, J., "Fire-Resistant Hydraulic Fluids," The Brown Boveri Review, Vol. 53, No. 1/2, 1966, pp. 142-147. [21] Schenck, K., Hoxtermann, E., and Hartwig, J., "Operation of Turbines with Fire resistant Fluids, Including the Lubricating System," VGB Kraftwerkstechnik, Vol. 77, No. 6, 1977, pp. 412416. [22] Dufresne, P. D., "14 Million Hours of Operational Experience with Phosphate Ester Fluid in a Gas Turbine Main Bearing App.lication," Proceedings of the 11th International Tribology Colloquium, Esslingen, Germany, January 1998, pp. 1447-1452. [23] Smith, A. N., "Fire Resistant Lubricants," General Electric Publication GTU-5 78, 1978. [24] Knipple, R. and Thich, J., "The History of Aviation Turbine Lubricants," SAE Paper 810851, Society of Automotive Engineers, Warrendale, PA, 1981. [25] Anon., "Turbine Lubricants for Steam, Gas, Wind and Water Turbines," Industrial Lubrication and Tribology, September/October 1994, pp. 8-15. [26] Byford, D. C. sind Edgington, P. G., "The Development of Special Oils for the Power Plants of Supersonic Transports," Proceedings of the Eighth World Petroleum Congress, 1971, pp. 101-110. [27] Phillips, W. D., "Triaryl Phosphates-The Next Generation of Lubricants for Steam and Gas Turbines," ASME Paper, 94JPGC-PWR-64, American Society of Mechanical Engineers, NY, 1994. [28] Hoxtermann, E. and Richter, P., "Failures of and Damage to Gas Turbine Components," VGB PowerTech, Vol. 80, No. 10, 2000, pp. 51-54. [29] Kondo, H., " Recent Trends in Turbine Oils," Japanese Journal of Tribology, Vol. 35, No. 9, 1990, pp. 969-979. [30] DenHerder, M. J., "Control of Turbine Oil Degradation During Use," Lubrication Engineering, Vol. 37, No. 2, 1981, pp. 67-71. [31] Moeller, H., "Lubricants for Gas Turbines," Proceedings of the 11th International Tribology Colloquium, Technische Akademie Esslingen, Germany, January 1998, pp. 379-382. [32] Ashman, L. A., Vetrone, J., Curts, L., and Johnston, A., "Advantages of Turbine Fluids Blended with Hydro-treated Base Oils: Exceptional Oxidative Resistance, Filterability, Air and Water Separation," presentation at the 54"" Annual Meeting of the Society of Tribologists and Lubrication Engineers (STLE), Las Vegas, May, 1999. [33] McHugh, P., Stofey, W. D., and Totten, G. E., "Mineral Oil Hydraulic Fluids," Handbook of Hydraulic Fluid Technology, G. E. Totten, Ed., Marcel Dekker, NY, 1999, pp. 711-794. [34] Hoo, G. H. and Lewis, E., "Base Oil Effects on Additives Used to Formulate Lubricants," Adv. Prod. Appl. Lube., Proceedings of the International Symposium, H. Singh, P. Rao, and T. S. R. Tata, Eds., McGraw-Hill, New Delhi, 1994, pp. 326-33. [35] Henderson, H. E., "Base Oils for Engines and Drivetrains of the
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CHAPTER 12: TURBINE LUBRICATING OILS AND HYDRAULIC FLUIDS 351 [57] Forster, N. H., "High Temperature Lubrication of Rolling Contacts with Lubricants Delivered from the Vapor Phase and as Oil-Mists," WL-TR-97-2003, 1997. [58] Tourret, R. and White, N., "Aeration and Foaming in Lubricating Oil Systems," Aircraft Engineering, May 1952, pp. 122-130, 137. [59] Hodges, P. K. B., Hydraulic Fluids, Arnold, Great Britain, 1996, p. 46. [60] Staniewski, J. W. G., "The Influence of Mechanical Design of Electro Hydraulic Steam Turbine Control Systems on Fire-Resistant Fluid Condition," Lubrication Engineering, Vol. 52, No. 3, 1996, pp. 255-258. [61] Backe, W., and Lipphardt, P., "Influence of Dispersed Air on the Pressure Medium," Proceedings of the Institute of Mechanical Engineers, paper 091116, London, 1976, pp. 77-84. [62] Hatton, D. R., " Some Practical Aspects of Turbine Lubrication," Canadian Lubrication Journal, Shell Canada Products Co., Vol. 4, No. 1, 1984, pp. 3-8. [63] Phillips, W. D., "The Electrochemical Erosion of Servo Valves by Phosphate Ester Fire-Resistant Hydraulic Fluids," Lubrication Engineering, Vol. 44, No. 9, 1988, pp. 758-767. [64] Kaspar, K., "Einsatz von synthetischen schwerbrennbaren Hydraulikflilssigkeiten im Schmier- u n d Steuerkreislauf von Dampfturbosatzen," Der Maschinenschaden, Vol. 50, No. 3, p p . 87-92, 1977. [65] Schenck, K,, Hoxtermann, E., and Hartwig, J., "Operation of Turbines with Fire-Resistant Fluids, Including the Lubricating System," VGB Kraftwerkstechnik, Vol. 77, No. 6, 1997, pp. 412-416. [66] Phillips, W. D., "Fire Resistance Tests For Fluids and LubricantsTheir Limitations and Misapplication," Fire Resistance of Industrial Fluids, ASTM STP 1284, G. E. Totten, and J. Reichel, Eds., ASTM International, West Conshohocken, PA, 1996, pp. 78-101. [67] Howells, P., "Measurement of Autoignition Temperature," BP Technical Report 31 737/M, British Petroleum Company, Sunbury-on-Thames, UK, 31 January, 1976 [68] Reichardt, H. U., Fischer, R., a n d Schiilert, G., "Wasser im Turbinenol-Einfluss, Eigenschaften und Bestimmungsmoglichkeiten," Schmierungstechnik, Vol. 18, No. 11, 1987, pp. 335338. [69] Li, T-D. and Mansfield, J. M., "Effect of Contamination on the Water Separability of Steam Turbine Oils," Lubrication Engineering, Vol. 51, No. 1, 1995, pp. 81-85. [70] Rockwell, J., "The Slime Intermission," Herald Tribune, 27 May 1992. [71] Bowman, W. F. and Stachowiak, G. W., "New Criteria to Assess the Remaining Useful Life of Industrial Turbine Oils," STLE Pre-print 96-NP-4F-1, STLE Annual Meeting, Cincinnati, 1996. [72] Kaufmann, R. E., "Rapid, Portable Voltammetric Techniques for Performing Antioxidant, Total Acid Number (TAN) and Total Base N u m b e r (TBN) Measurements," Lubrication Engineering, January 1998, pp. 39-46.
[73] Kauffman, R. E., "Remaining Useful Life Measurements of Diesel Engine Oils, Automotive Engine Oils, Hydraulic Fluids, and Greases Using Cyclic Voltammetric Methods," Lubrication Engineering, Vol. 51, No. 3, 1995, pp. 223-229. [74] Critchley, S. W. and Miles, P., "Synthetic Lubricants-Selection of Ester Types for Different Temperature Environments," Proceedings of the Industrial Lubrication Symposium, London, March 1965. [75] Anon, "Oil Filters and C o n t a m i n a n t Control," Industrial Lubrication and Tribology, November/December 1993, p p . 14-19. [76] SAE J1165: "Reporting Cleanliness Levels of Hydraulic Fluids," SAE Recommended Practice, Society of Automotive Engineers, Wareendale, PA, July 1979. [77] Staniewski, J. W. G., "Operating Experience with Fire-Resistant Phosphate Esters in Steam Turbine Electro-Hydraulic Control Systems," presented at the EPRI/NMAC Lubrication Workshop, Cleveland, OH, June 1994. [78] Phillips, W. D. and Sutton, D. I., "Improved Maintenance and Life Extension of Phosphate Esters using Ion Exchange Treatment," Proceedings of the 10th International Tribology Colloquium, Technische Akademie, Esslingen, Germany, January 1996, pp. 405-432. [79] "Guidelines for Maintaining Steam Turbine Lubrication Systems," Electric Power Research Institute Report No. CS-4555, Palo Alto, CA, 1986. [80] Brown, K. J. and Staniewski, J. W. G., "Condition Monitoring and Maintenance of Steam Turbine-Generator Fire Resistant Triaryl Phosphate Control Fluids," STLE Special Publication SP27, Proceedings of the 1989 Condition Monitoring and Preventative Maintenance Conference, May 1989, pp. 91-96. [81] Christopher, S. a n d Marson, A. J., "Development of a Test Method for the Determination of the Total Acidity in Polyol Ester and Diester Gas Turbine Lubricants by Automatic Potentiometric Titration," Proceedings of the 11th International Tribology Colloquium, Technical Akademie, Esslingen, Germany, 1998, pp. 121-127. [82] Turbine Lubrication in the 21" Century, ASTM STP 1407, W. R. Herguth and T. M Warne, Eds., ASTM International, 2001. [83] Boehringer, R. H. and Ness, F., "Lubrication of Hydroelectric Turbine Thrust Bearings with a Diester-Based Synthetic Lubricant," Journal of Synthetic Lubrication, Vol. 6, No. 4, 1989, pp. 311-323. [84] Rao, A. M. N., "High Temperature Vapour Phase Lubrication," Proceedings of the 11th International Tribology Colloquium, Technische Akademie, Esslingen, Germany, January 1998. [85] Van Treuren, K. W., Barlow, D. N., Heiser, W. H., Wagner, M. J., and Forster, N. H., "Investigation of Vapor-Phase Lubrication in a Gas Turbine Engine," ASME Transactions, Journal of Engineering for Gas Turbines and Power, Vol. 120, April 1998, pp. 257-262.
MNL37-EB/Jun. 2003
Hydraulic Fluids W. A. Givens^ and Paul W. Michael^
T H E PRIMARY PURPOSE OF A HYDRAULIC FLUID is to
Where: K = Bulk modulus Vo = Original volume AP = Pressure change Ay = Change in volume
transfer
power. The concept of fluid power is based on a principle articulated by Blaise Pascal, which is usually given as follows: "Pressure applied to an enclosed fluid is transmitted undiminished to every portion of that fluid and the walls of the containing vessel" [1]. Within the context of fluid power, pressure is related to the force acting on a confined fluid as illustrated in Fig. 1 [2]. This principle has given rise to mode m hydraulics, which entails highly engineered systems for efficiently controlling fluid flow to transfer energy and accomplish work. The heart of any hydraulic system is the pump, which pulls in fluid from a reservoir by creating a vacuum at its inlet and then forces the fluid through its outlet, usually against pressure created by flow controllers and/or actuators downstream of the p u m p . Pumps, actuators, and other system components have surfaces that move relative to each other, often at high speeds, pressures, and temperatures. These components require cooling and lubrication for efficient performance and durability. Consequently, hydraulic fluids not only must transmit power, they serve critical functions as lubricant and heat transfer medium.
Heat Transfer Heat is generated as a by-product of normal operation of a hydraulic circuit. Friction between the moving parts of a p u m p or hydraulic motor, as well as friction between the fluid and surfaces of valves, pipes, and other circuit devices generates heat. In addition, heat is generated in a hydraulic system as a result of the dissipation of the potential energy of pressurized fluid [8]. As a hydraulic fluid is circulated through a system, heat is transferred from high temperature areas to coolers, reservoirs, and other regions of the circuit where it is dissipated. As can be seen in Table 1, typical specific heat and thermal conductivity values for hydraulic oils are a fraction of that of water [4]. These factors are an important consideration in sizing hydraulic system coolers because the inherent cooling efficiency of petroleum based hydraulic fluid is less than that of water. ASTM D 2717, Test Method for Thermal Conductivity of Liquids and ASTM D 2766, Test Method for Specific Heat of Liquids and Solids are used to determine these properties of fluids.
P o w e r Transfer To transfer power efficiently, a hydraulic fluid must exhibit minimal compressibility. Low compressibility allows all of the pressure applied to the fluid to be available for direct and effective transmission to system components such as motors, cylinders, or other actuators. The compressibility of a fluid is generally discussed in terms of its "bulk modulus," which describes the change in fluid volume as a result of applied pressure [3]. The bulk modulus of a fluid, which is the reciproccd of compressibility, is described by Eq 1. There are a n u m b e r of m e t h o d s available for estimating the isothermal secant bulk modulus of a fluid based upon its viscosity and density characteristics [4,5]. As depicted in Fig. 2, the bulk modulus for oil also varies with temperature [6]. For petroleum oils, compressibility is often assumed to be 0.5% for each 1000 psi pressure increase u p to 4000 psi [7]. Bulk modulus {K) = -Vo
(\PI\V)
Lubrication The durability of hydraulic equipment depends to a large extent upon the lubricating properties of the fluid. As a lubricant, the key function of the hydraulic fluid is to reduce friction between contact surfaces. A reduction in friction lowers contact t e m p e r a t u r e s a n d wear. This is accomplished through a combination of hydrodjoiamic and boundary lubrication mechanisms. The hydrodynamic lubricating properties of a fluid are governed by its physical properties while boundciry lubrication is a function of fluid chemistry. A discussion of hydraulic fluid wear testing is presented in the Wear Protection section of this chapter.
(1)
TRENDS
* Exxon Mobil Research & Engineering, Paulsboro Technical Center, 600 Billingsport Rd., Paulsboro, NJ 08066. ^ Benz Oil, 2724 West Hampton Avenue, Milwaukee, WI 53209.
A brief outline of major trends in the motion control industry, particularly with respect to hydraulic equipment design and fluid requirements, is presented as a backdrop for the discussion of hydraulic fluids test methods. As motion con-
353 Copyright'
2003 by A S I M International
www.astm.org
354 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK
Force
Area
F = force in pounds p = pressure in pounds / sq. incli (psi) A = sq. in.
FIG. 1—Relationship of force, pressure, and area in fluid power. Any one of the parameters equals the other two in the relationship depicted by the triangle. TABLE 1—Thermal conductivity and specific heat values for oil and water.
40
^
30
M Q.
Oil Water
z CO
Thermal Conductivity Btu/h/ft^/F/Ft @ 212°F
Thermal Conductivity W/m-K @373K
Specific Heat BTU/lb°F @68°F
Specific Heat J/kg • K @293K
0.08 0.39
0.14 0.67
0.47 1.0
1966 4184
20
I 10 3
m 100
200
300
FIG. 2- -Effect of temperature on the bulk modulus of petroleum fluid.
trol technology advances, there is a trend towards higher performance and efficiency. For hydraulic equipment, this translates into a concentration of horsepower in smaller components. There are a n u m b e r of reasons for such a trend. Equipment manufacturers are looking for ways to minimize raw material usage and cost. Users of the equipment demand smaller systems for better space utilization in industrial environments cind compact multifunctional capabilities in mobile equipment. These advancements in mechanical design along with e n c r o a c h m e n t of environmental, health, and safety regulations fuel the following trends: • Hydraulic equipment builders will continue to push comp o n e n t manufacturers to design parts to a c c o m m o d a t e high pressures a n d t e m p e r a t u r e s . F o r example, hoses, valves, and other fittings will continue to evolve in terms of materials used as well as actual functional design.
Smaller c o m p o n e n t s will m e a n smaller p u m p displacements [cubic inches or cc per p u m p revolution]. To maintain flow rates at present or higher levels, p u m p speeds will be increased [cubic inches/minute = displacement X speed (rpm)]. Smaller reservoir sizes will mean shorter fluid residence times and will therefore dictate use of hydraulic fluids with improved air release characteristics. Smaller dimensional clearances will be required. These smaller clearances will dictate more stringent fluid cleanliness requirements to prevent abrasive wear from particulate c o n t a m i n a n t s a n d failure of servo or proportional valves. Fluid cleanliness will increasingly be emphasized as an effective way of increasing equipment durability and controlling warranty costs. As a result, users will move to finer filtration and specify pre-filtered hydraulic fluids [9]. Consequently, the filterability of the hydraulic fluid will continue to grow in significance. (Filterability is described in section 4.6.) Quieter hydraulic systems will be required in order to meet workplace noise restrictions and compete with electric motors. Reduction of noise levels in hydraulic equipment has been attained by the insulation that absorbs the noise. This insulation results in higher system temperatures, as heat is not as readily dissipated. Components and actuators, such as cylinders, will be designed with tighter seals to increase efficiency and reduce
CHAPTER
13: HYDRAULIC
FLUIDS
355
leakage. The effects of this trend include increased stress on seal materials and cylinder chatter resulting from reduced lubrication between seal and cylinder wall. In addition, certain applications will require fill-for-life systems that translate into lower maintenance and disposal costs. Consequently, fluids will remain in a system for longer periods, since meike-up fluid is not required. • A growing awareness of the environmental impact of chemicals will lead to further restrictions on performance additives eind base stocks used in lubricants. As a result, lubricant p r o d u c e r s are required to address such issues through alternative (usually more costly) chemistry and the development of environmentally friendly (nontoxic/biodegradable) lubricants. • The hydraulic fluid industry has evolved from the use of plain water in hydraulic systems to the use of advanced fluid technologies that continue to evolve as performance requirements become more stringent and equipment designs become more sophisticated [10]. Due to environmental health and safety issues, hydraulic systems are once again being designed to employ pure water as hydraulic fluid [11].
Solvent refining yields base oils that fall into Group I while hydroisomerization and deep hydrogenation processes yield low sulfur, high paraffin content Group II a n d Group III base stocks. Because of their lower aromatic and sulfur content, hydraulic fluids formulated from Group II and Group III base stocks typically have superior oxidation stability. However, more highly refined stocks tend to be less effective at dissolving additives. Not only is additive solubility a concern, additive chemistries and their functional mechanisms may be b o t h synergistic a n d antagonistic. Thus, additive chemistry must be ceirefully balanced to achieve optimum performance. In the following section, test methods for evaluating key fluid properties such as oxidation stability, wear prevention, and corrosion inhibition are discussed. These methods have been developed to measure characteristics of hydraulic fluids that are thought to correlate to performance in "real-life" applications as well as gage additive response for the fluid formulator. In order to provide a link between fluid tests and additive chemistry, a description of the generally accepted functional mechanisms of additives is also included.
PETROLEUM BASE STOCKS
FLUID CHARACTERISTICS AND PERFORMANCE
Most hydraulic fluids consist of a base fluid and additives that are designed to i m p a r t chemical characteristics and functionality to the finished product. Operating conditions and equipment builder specifications generally dictate the type of fluid that is needed and thus, the kind of base stocks and additives employed. In petroleum based hydraulic fluids the typical concentration of additives is less than 3.0% by weight. Paraffinic oils are the primary base stock utilized in hydraulic fluids but other materials, from polyglycols to vegetable oil, serve as the basis for formulating hydraulic fluids. From a historical standpoint, solvent reflned paraffinic oils have been the most widely used base stock for hydraulic applications. In recent years alternative refining processes such as catalytic isomerization and deep hydrogenation have been developed to yield higher purity base oils that are better suited to withstand severe operating conditions [12]. These base stocks are categorized by the American Petroleum Institute (API) according to their composition and viscosity index [13]. Groups I through III consist of crude derived base oils while Group IV is reserved for synthetic polyalphaolefins. Low viscosity index naphthenic oils and other base stocks that do not meet Group I through IV criteria are classified as Group V. The API Base Oil classification is described in Table 2. TABLE 2—API base oil classifications. Category Group I Group II Group III Group rV Group V
Composition < 9 0 % Saturates or > 1 0 % aromatics £ 9 0 % Saturates or < 1 0 % aromatics > 9 0 % Saturates or < 1 0 % aromatics All polyalphaolefins (PAO) All others not included in Groups 1,11, m or IV
Suli^ir >0.03%
Viscosity Index 80-120
<0.03%
80-120
<0.03%
>120
Oxidation a n d Thermal Stability An important characteristic of a hydraulic fluid is its ability to withstand high temperatures. This is because horsepower losses in hydraulic systems directly result in transfer of heat to the fluid. Resulting high temperatures can cause hydraulic fluids to react with oxygen. The rate of this reaction accelerates exponentially with increasing temperatures and is further catalyzed by metals like copper and iron, especially at temperatures above 200°F [14]. Rate constants for the oxidation of saturated hydrocarbons at 125°C are as much as 40 times higher than rate constants at 60°C [15]. Thus, fluid oxidation is highly dependent upon hydraulic system operating temperatures. Lubricants expected to operate in high temperature environments are tjrpically fortified with additives known as antioxidants, which are discussed in the Antioxidants section. Oxidative stabilization of the fluid translates directly into extended oil service life. Failure to resist oxidation can result in thickening of the oil (viscosity increase), formation of acidic byproducts, and subsequent deposit formation. Not only can heat cause oxidation, fluids may thermally degrade upon exposure to high temperatures with litde or no oxygen present. The thermal stability of a hydraulic fluid is dependent mainly on the intrinsic ability of the base fluid or its components to resist decomposition at high temperatures. Unlike oxidation, controlled thermal degradation of certain types of additives [such as Zinc Dialkyldithiophosphate (ZDTP)] is desirable, because it is the very mechanism by which they react with the metal surfaces they are designed to protect [16]. Similar to oxidation however, the negative effects of thermal degradation may include increased acidity, thickening of the oil, and deposit formation. Therefore, good control of thermal degradation results in the retention of desired fluid properties.
356
MANUAL
37: FUELS AND LUBRICANTS
High Temperature/Oxidation
HANDBOOK
Tests
One of the most commonly sited methods for measuring the abihty of a fluid to resist oxidation is ASTM D 943, Standard Test Method for Oxidation Characteristics of Inhibited Mineral Oils (also known as the Turbine Oil Oxidation Stability Test - TOST). In this test, 300 ml of fluid a n d 60 ml of distilled water are placed in a large test tube together with coils of copper and iron wire (Fig. 3). The fluid is heated to 95°C (203 °F) and oxygen is bubbled through the fluid at a controlled rate. The test is complete when the Total Acid Number (AN) of the fluid reaches 2.0 mg KOH/g. As can be seen from the reaction scheme in Fig. 5, cJdehydes eire among the chemical by-products of hydrocarbon oxidation. These aldehyde compounds are readily converted to carboxylic acids in the hydraulic system [17,18]. Since carboxylic acids are corrosive to yellow metals and agglomerate to form deposits, they have a detrimental effect upon fluid performance when their concentration becomes excessive. The concentration of acidic oxidation debris present in a fluid can be determined by titration with potassium hydroxide. For the D 943 test, a variation on ASTM D 664 Acid N u m b e r of Petroleum Products by Potentiometric Titration is used. This method, ASTM D 3339, Test Method for Acid N u m b e r By Semi-Micro Color Indicator Titration is utilized because it permits a 0.2-1.0 g sample size for total acid numbers in the 0.5-3.0 mg KOH/g
OXYGEN DELIVERY TUBE
range. The hours to form 2.0 m g of KOH equivalents of acidic oxidation products per gram of some typical fluids are shown in Table 3. In general, turbine oils provide longer TOST oxidation life t h a n antiwear hydraulic fluids because turbine oils typically do not contain zinc dialkyldithiophosphate (ZDTP). Zinc dialkyldithiophosphate r e d u c e s the t i m e it takes for a fluid to reach 2.0 mg KOH/g because it is acidic and its mere presence raises the acid n u m b e r of the fluid. In addition ZDTP is subject to hydrolysis and forms acidic compounds as it degrades. Ester based fluids such as rapeseed oils are also subject to hydrolysis, which accounts for their poor performance in the D 943 test. When the D 943 test is run without water (dry method), the oxidation life of a synthetic ester can be extended by nearly a factor of 100. The a m o u n t of sludge produced in the TOST test may be measured by ASTM D 4310, Test Method for Determination of the Sludging Tendencies of Inhibited Mineral Oils. In this test, the fluid is subjected to D 943 test conditions for 1000 h. At the end of this time, the sludge produced is determined gravimetrically by filtration of the oxidation tube contents through 5-/u,m pore size cellulose acetate filter disks. To a certain extent the D 943 and D 4310 tests evaluate different mechanisms of high temperature degradation. In the D 943 test, acidity is measured and this acidity is predominantly due to formation of carboxylic acids by the conventional liquid phase oxidation mechanism shown in Fig. 4. In essence D 943 measures the stability of the base oils and the effectiveness of oxidation inhibitors. Sludge formation in hydraulic oils is to a greater extent due to theimal degradation of the ZDTP antiwear additive. Consequently, the result of a D 4310 test is an indication of the thermal stability of ZDTP. Figure 5 shows a model for the mechanism of sludge formation by zinc dialkyldithiophosphate [19]. Another method for measuring the sludging tendency of hydraulic fluids is the Cincinnati Machine Heat Test [20]. This test has been adopted as an ASTM procedure and is designated ASTM D 2070, Standard Test Method for Thermal Stability of Hydraulic Oils. In this test, polished pre-weighed copper and steel rods are placed in a beaker containing 200 cc oil and heated to 135°C (275°F) for 168 h. At the end of the test, the copper and steel rods are examined for discoloration due to corrosion caused by carboxylic acids and sulfur compounds formed by thermal degradation. Sludge content and viscosity increase are also measured (Table 4). Antioxidants
CATALYST COILS
Oxidation inhibitors, commonly referred to as antioxidants, are chemicals that reduce the rate at which oxidative degradation of a lubricant occurs. Degradation begins with the reaction of hydrocarbon molecules at elevated temperatures to form unstable reactive species known as free radicals. These TABLE 3—D 943 turbine oil oxidation test life of typical hydraulic fluids.
FIG. 3—Oxidation cell and sampling tube for ASTM D 943 apparatus.
Fluid Type
Hours to TAN of 2.0 by D 943 Method
Synthetic ester without antioxidant Mineral oil without additives Antiwear hydraulic oil. Group I base stock Antiwear hydraulic oil, Group 11 base stock Synthetic ester with antioxidant, dry method R & O hydraulic oil
65 300 2016 5040 5500 >10,000
CHAPTER 13: HYDRAULIC FLUIDS 357 Temperature Light, catalyst
_
Initiation
RH
Propagation
R • + O2
-•
ROO*
Peroxy radical
ROO• + RH
-•
ROOH + R*
Hydroperoxide
ROOH
-*•
RO • + • OH
Alkoxy radical
RO» + RH
->
ROH + R •
Alcohol
• OH + RH
->
H2O + R •
Water
Branching
Termination
Alkyl radical
R • + ROO •
Alcohols
RO • + ROO •
Aldehydes
ROO • + ROO •
Ketones
RO • + R •
Acids
R« + R«
Longer chain hydrocarbons
FIG. 4—Reaction scheme for liquid hydrocarbon oxidation.
Hydraulic Oil RO
S
S
RO
S—Zn—S
OR
OR
Base Oil (Paraffinic) and Additives
Machines and Outside Environment
T Reaction withi
Thermal P^°''^^ Deterioration Degradation witii Water ZnSq RO
Decomposition Oxidation Reaction with l\^etai Ions
Polyphosphates 0
0
OR
0- -Zn—0
OR
\ ^ RO
Oxidation Products and Metal Soaps
Wear Particles, Dust, Rust, Water andOtliers
T
Sludge FIG. 5—Mechanism of sludge formation by zinc dialkyldithiophosphate.
358 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 4—Cincinnati machine thermal stabihty test performance requirements. Property
Condition of steel rod Visual Deposits Corrosion Condition of copper rod Visual Corrosion Condition of fluid Viscosity Sludge Total acid number
Requirement
No discoloration 3.5 mg maximum 1.0 mg maximum 5 rating maximum 10.0 mg maximum 5% change maximum 25 mg/100 mL max ±50 % maximum
species react with oxygen and non-oxidized oil to form additional free radicals, which propagate the oxidation process. This generally accepted mechanism is described as free radical chain reaction and is illustrated by the steps shown in Fig. 4. Antioxidants interrupt this chain reaction and thus, reduce the rate of oxidation and the resulting viscosity increase and acid and deposit formation. There are two general mechanisms by which these additives inhibit oxidation. The antioxidants are therefore categorized as primary or secondary, depending u p o n the m e c h a n i s m of oxidation inhibition. Primary antioxidants, commonly referred to as "free radical scavengers," react with the peroxy radicals and hydroperoxides to form inactive compounds (Fig. 6) [21]. Examples of primary antioxidants include hindered phenols and aromatic amines. Secondary antioxidants, commonly referred to as "peroxide decomposers," react with hydroperoxides or peroxy radicals to form less reactive compounds. Examples of secondary antioxidants include sulfur a n d / o r p h o s p h o r u s c o m p o u n d s a n d metal dithiophosphates (Fig. 7). Antioxidants genereilly function in the bulk lubricant and are consumed as they do their job [22].
Detergents
IDispersants
Detergents and dispersants are used to delay formation and subsequent deposit of insoluble oil degradation species. The terms detergent and dispersant are often used interchangeably, but are generally differentiated by their composition and primary functionality. Detergents are metallo-organic compounds that neutralize acidic deposit precursors, while dispersants are predominantly organic chemicals that keep insoluble materials dispersed a n d suspended in the lubricant. The t e r m "ashless" dispersants, m e a n i n g non-metallic, is used to further differentiate dispersants from detergents. Some detergents have the ability to disperse and suspend insolubles, while some dispersants are capable of neutralizing precursors of deposits. Typical lubricant detergents include barium, calcium, and magnesium phenates, phosphates, salicylates a n d sulfonates. Ashless dispersants are typically alkyphenol-based or alkyl succinimides.
(R0)3P?-0—O3 (R0)3P + R'OOH H (R0)3P=0 + HOR' FIG. 7—Secondary antioxidants such as the phosphite compound depicted above inhibit oxidation by decomposing hydroperoxides. This prevents the oxidation process from progressing beyond the branching stage In the reaction mechanism.
o» + R00» ROO^^R FIG. 6—Reaction scheme for primary antioxidants. Primary or freeradical trapping antioxidants work by donating a hydrogen radical H* to the peroxy radical formed during mineral oil oxidation. Due to steric hindrance, the antioxidant radical does not attack mineral oil molecules, i.e., R-H bonds. Consequently, the radical chain is terminated.
CHAPTER 13: HYDRAULIC FLUIDS Wear Protection Reduction of friction and prevention of wear is the fundamental purpose of a lubricant. Lubricants reduce friction in machine components by producing a physical or chemical barrier between surfaces that slide or roll past each other. Depending on equipment design and function, lubricants function within three commonly recognized regimes: hydrodynamic, mixed-film, and boundary lubrication (Fig. 8) [23]. Hydrodynamic lubrication is often the dominant lubrication regime under conditions of moderate temperatures and loads. According to the ASM Handbook on Friction, Wear and Lubrication Technology, [24] hydrodynamic lubrication is "a system of lubrication in which the shape and relative motion of the sliding surfaces causes the formation of a fluid film that has sufficient pressure to separate the surfaces." In this regime, viscosity is the most important fluid characteristic because it, in combination with sliding speed, contact geometry and load, determines the thickness of the lubricating film, and determines whether or not the surfaces will contact each other. Fluid viscosity plays an important role in hydraulic applications. A hydraulic fluid that is too low in viscosity will cause low volumetric efficiency, fluid overheating, and increased pump wear. A hydraulic fluid that is too high in viscosity will cause poor mechanical efficiency, difficulty in starting, and wear due to insufficient fluid flow [25]. Since viscosity is a function of fluid temperature, the temperature operating window (TOW) for a particular viscosity grade of hydraulic fluid is a function of temperature. Figure 9 depicts
the TOW for straight grade mineral oil based hydraulic fluids. The viscosity grade indicated in the TOW corresponds to ASTM D 2422, Classification of Industrial Fluid Lubricants by Viscosity System. For example, ISO 32 hydraulic oil generally will provide satisfactory performance in a temperature window of - 8 to 64°C. There are several methods for measuring the viscosity of hydraulic fluid. The most widely utilized method is the ASTM D 445, Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids. In this test, the time is measured for a fixed volume of liquid to flow under gravity through the capillary of a calibrated viscometer at a closely controlled temperature. The kinematic viscosity is the product of the measured flow time and the calibration constant of the viscometer. Based upon D 2442 and ISO 3448, the standard temperature for measuring hydraulic fluid viscosity is 40°C [26]. Typically, the viscosity of a hydraulic fluid is 15-68 mm^/s (centistokes) at 40°C. ASTM D 446, Standard Specifications and Operating Instructions for Glass Capillary Kinematic Viscometers, describes more than 15 types of viscometers that may be employed in performing a D 445 viscosity test. With the exception of invert-emulsion type fluids, hydraulic fluids are generally transparent. Consequently, a tube suitable for transparent liquids such as the popular CannonFenske viscometer may be used. For opaque liquids, a reverse-flow tube is required because it is difficult to see the meniscus as the fluid flows by the timing marks on a standard viscometer. Cannon-Fenske tubes for viscosity measurement of transparent and opaque liquids are depicted in Fig. 10.
1 MIXFn FN M
LUBRICA-•|ON B OUNDARY LIJBRICATION 0.1
c o o c g> o
0.01
»^ O
o 0.001
0.001
0.01
359
0.1
hULL-hlLM LUBRICATION 1 1
10
Sommerfeld number, (rjA// P) x 10"^ FIG. 8—Stribeck Curve of coefficient of friction versus Sommerfeld Number (S), where S = r}N/P. N shaft speed; P, average pressure between shaft and bearing due to applied load; 7), lubricant viscosity.
360 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK 100
212
90
94 — 194
80
o
176
84
70
158 LL. 140
73
60
64
5 50 -I—•
55
122
104 CO CD 86 O.
44
I" 30
32
j5^20
CO
+10
10
+4 -2
0
-23
-4
-30 — 3 3
-22
-40 10
50
14
-15
-20
E .
32
-8
-10
CD V—
15
22
32
46
68
100
-40
ISO Viscosity Grade FIG. 9—^Temperature Operating Window (TOW) for 100 VI mineral oil based hydraulic fluids. Based upon survey of viscosity requirements for hydraulic pumps and motors, fluids will generally provide satisfactory performance at the temperature range that corresponds to 13 to 860 cSt.
FIG. 10—Cannon-Fenske standard and reverse flow kinematic viscosity tubes, respectively.
Frequently in high-pressure hydraulic applications, the loading conditions are sufficient to rupture hydrodynamic lubricating films. Consequently, boundary and mixed-film lubrication regimes play an important role in controlling wear in hydraulic applications. In boundary lubrication, friction cind wear between two surfaces in relative motion are de-
termined by the properties of the surfaces and by properties of the surfaces and lubricants other than viscosity [27 ]. In hydraulic equipment, these surfaces are typically composed of ferrous or yellow metals. Under magnification, tribologicsJ surfaces in hydraulic components reveal the presence of surface asperities. High load conditions cause these aperities to
CHAPTER make contact, resulting in friction and weeir. In most cases, mixed-film lubrication takes place and some hydrodynamic lubrication occurs, even as "asperity contact" creates boundary conditions. Depending upon the extent of asperity contact, scuffing or adhesive wear may occur. A schematic description of the various wear processes specific to hydraulic p u m p s is shown in Fig. 11. When cavitation, corrosion, or scuffing wear processes generate particles that are the same approximate size as p u m p clearances, synergistic wear may take place. Sjmergistic wear ultimately leads to failure that may appear to be abrasive in origin [28]. Wear protection under conditions of boundary lubrication may be enhanced through the use of additives that interact with surfaces to form protective chemical films. (See the Antiwear Performance Testing section for description of the boundary lubrication additives utilized in hydraulic applications.) These chemical films reduce friction by decreasing the shear strength of the surface relative to the underlying material. Thus, surface interaction under boundary lubrication conditions is primarily between the low-shear strength chemical films rather than the metal substrate. Good wear protection and friction reduction result in enhanced equipment durability, reduced heat generation, improved energy conservation, and many other operational advemtages. Antiwear
Performance
13: HYDRAULIC
FLUIDS
pressures, and entry angles in a functioning hydraulic system [29,30]. One of the more c o m m o n bench tests used for screening the antiwear performance of a hydraulic fluid is the FourBall Method. There are two versions of the test for liquid lubricants: ASTM D 2783, Standard Test Method for Measurement of Extreme-Pressure Properties of Lubricating Fluids (4-Ball Method) and ASTM D 4172, Standard Test Method for Wear Preventive Characteristics of Lubricating Fluids (4Ball Method). The former method is generally used for evalu a t i n g extreme pressure gear lubricants while the latter method is used for evEiluating antiwear hydraulic fluids. In the 4-Ball Wear Test (D 4172), three half-inch diameter steel balls are clamped together and covered with the lubricant to be evaluated. A fourth ball of equal diameter is pressed with a force of 1 5 ^ 0 kg into the cavity formed by the three stationary balls making a three-point contact (Fig. 12). Lubricants are evaluated by rotating the top ball under load at 1200 r p m for 60 min and measuring the average scar diameters worn in the three lower balls. In cooperative testing of fluids performed by members of ASTM D02.L on Industrial Lubricants, the addition of zinc dithiophosphate to 46 cSt mineral oil decreased the scar di-
Testing
The majority of hydraulic fluids are formulated with antiwear additives because surface loads associated with highpressure p u m p operation necessitate the use of fluids with enhanced wear protection. There are a variety of test methods available for assessing the antiwear performance of hydraulic fluids. These tests may either be bench-top or fullscale tests employing high-pressure piston and vane pumps. Bench tests are generally less expensive to perform t h a n p u m p tests. However, translating bench test data into realworld performance can be problematic because of the complexity involved in simulating all of the materials, velocities.
(a) FIG. 12—The four-ball test: (a) perspective view, (b) plan view.
EXTEERNAL PAR"riCLE INGREESSION \Air:Ap
CAVITATION JElr
ASPERITY CONTACT
FATIGUE WEAR
ADHESIVE WEAR
VVtArl
DEBRIS
WEAR DEBRIS1
>
ABRASIVE WEAR
TOTAL WEAR
— ^ •
ELECTROLYTE (WATER)
361
CORROSIVE WEAR WEAR
WEAR DEBRI S
DFRRI.q
FIG. 11—Synergistic view of pump wear process. Fatigue, adhesive, and corrosive wear can be triggered Independently. Resulting wear debris generation leads to abrasive wear.
362
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
ameter in 4-ball wear tests from 0.72 m m to 0.42 m m at 40kg [31]. These results are tjpical of a mineral oil based antiwear hydraulic fluid where average scar diameters of less than 0.50 m m are the norm (P. W. Michael, unpublished data). While four-ball tests are effective in screening antiwear additive response, they do not directly correlate with p u m p tests [32]. This is in part due to the fact that loads in the fourball tests are constant and do not pulsate in the same way that a hydraulic p u m p does as sliding surfaces transition from high pressure to low pressure regions of the pump. In an effort to enhance the correlation between the four-ball test and full-scale p u m p eveJuations Penn State University has performed investigations involving sequential four-ball wear tests. In the sequential four-ball test, wear scars are evaluated at 10 and 40 kg and 600 r p m and the diameter of the scar is measured after the fluid has been replaced by white oil in order to measure the durability of the antiwear film [33,34]. This method yields better correlation with vane p u m p tests. The FZG Test is another bench test used for screening hydraulic fluids. FZG test equipment consists of two gear sets arranged in a foursquare configuration (Fig. 13). The FZG p r o c e d u r e is described in ASTM D 5182 S t a n d a r d Test Method for Evaluating the Scuffing (Scoring) Load Capacity of Oils. In this test, pre-examined gears are immersed in 1600
mL of oil that is heated to 90C (194°F). The test gear set is r u n in the test fluid for 15 min at successively increasing loads until the failure criteria is reached. According to the ASTM procedure, failure criteria are reached when the summed total width of scuffing wear damage from all 16 teeth is estimated to equal or exceed one gear tooth width. In DIN 51524, Part 2, a m a x i m u m weight loss of 0.27 mg/kW h for antiwear hydraulic oil is specified as well as a m i n i m u m damage stage of 10. While Reichel reported a correlation between FZG Test results and hydraulic fluid performance in vane pumps, correlation with piston p u m p performance has proven difficult to establish [35]. The most widely referenced vane p u m p wear test for hydraulic fluids is ASTM D2882, Standard Test Method for Indicating the Wear Characteristics of Petroleum and NonPetroleum Hydraulic Fluids in a Constant Volume Vane Pump (Vickers 104C). In this test, a hydraulic fluid is circulated through a rotary vane p u m p for 100 h at a p u m p speed of 1200 r/min and a p u m p outlet pressure of 2000 psi. The fluid temperature is controlled to 150°F at the p u m p inlet for most fluids. Petroleum based fluids with a viscosity greater than 46 mm'^/s and some synthetic fluids must be evaluated at 175°F. At the end of the test, the total cam ring and vane weight losses are measured and reported. Based upon ASTM
Drive gear case
Test gears with long addenda
FIG. 13—The Neimann (FZG) Four-Square Gear Test Rig.
CHAPTER D 6158, Standard Specification for Mineral Hydraulic Oils, less than 50 mg of total wear is expected from properly formulated petroleum based antiwear hydraulic oil. For invertemulsion type fluids, higher wear rates in the 100-200 mg range are c o m m o n while water glycol fluids routinely generate less than 50 mg wear in the D 2882 test. While the D2882 test is a popular benchmark for evaluating hydraulic fluids, this method is not without its problems. First of all, Vickers has discontinued production of the V104C p u m p . This will ultimately necessitate the use of substitute hardware or abandonment of the test procedure. Second, rotor and bushing failures are common in the first few hours of the test. This may be due to the fact that the p u m p was originally designed for a m a x i m u m pressure of 1000 psig. Fluid performance in the V104C p u m p is evaluated at 1000 psi using the ASTM D 2271, Standard Test Method for Preliminary Examination of Hydraulic Fluids (Wear Test). In this procedure, the p u m p stand is operated for 1000 h, which provides an extended evaluation of p u m p wear behavior under normal operating conditions. Xie et al. provide a detailed discussion of the D 2882 Test Method in the Handbook of Hydraulic Fluid Technology [36]. For higher pressure a n d mobile applications Vickers prefers their 35VQ25 vane p u m p for screening hydraulic fluid wear performance (Table 5). In the 35VQ25 test, three 50-hour tests are conducted on the same charge of test oil. For each 50-hour test a new p u m p cartridge is used. The test rig is operated at 3000 psi and 200°F with a p u m p speed of 2400 rpm. Vickers limits the amount of wear on each test kit to 90 mg: 75 mg ring, 15 mg vanes. In addition there must be no sign of scuffing on the cam ring. The Denison T6C vane p u m p test is a variable pressure vane p u m p test. In this test, a Denison T6CSH 020 p u m p cycles between 7 b a r (—100 psi) and 250 bar (—3600 psi) at onesecond intervals for 300 h [37]. The p u m p speed is nominally 1700 r/min a n d fluid t e m p e r a t u r e is m a i n t a i n e d at 80°C (176°F) for mineral oil based fluids a n d 45°C (113°F) for those based on water. The test is r u n in two 305-hour sequences. Each 305-hour test consists of a 5-hour break-in period followed by 300 h of high pressure cycling. After the first 305-hour test, the p u m p cartridge is removed for inspection and a new cartridge is installed for the second sequence. The second 305-hour sequence is r u n with 1% distilled water added to the fluid. The first stage of the T6C test serves as an aging mechanism and increases the susceptibility of the fluid to the deleterious effects of water contamination. After the second 305-hour sequence the p u m p cartridge is again removed for inspection. As with the 35VQ25 test, weight loss of cam ring and vanes, vane tip profile, and visual appearance of all components are all reported. In addition, a wet filterability test is performed on the fluid to determine if water contamination will lead to filter blinding. (See the Filterability section for a discussion of filterability tests.) Although the V104C and 35VQ25 vane p u m p tests have served the industry well for many years, these tests are not sufficient to screen hydraulic fluids that will be used in highpressure piston p u m p s applications [38]. Thus, piston p u m p tests have been to qualify the antiwear capabilities of hydraulic fluids. Komatsu, Rexroth, and S u n d s t r a n d piston p u m p tests are described below. K o m a t s u developed a piston p u m p test to evaluate
13: HYDRAULIC
FLUIDS
363
biodegradable vegetable oil based hydraulic fluids [39]. This test is based on a Komatsu HPV35+35 twin-piston p u m p using cycled pressure test conditions. In this test p u m p efficiency change, wear and surface roughness, formation of lacquer and varnish, a n d hydraulic oil deterioration are evaluated. Rexroth has proposed a three-stage piston p u m p test based on the Brueninghaus A4VSO piston p u m p [40]. Stage one is conducted at the m a x i m u m operating pressure and temperature and at the m i n i m u m viscosity specified for the fluid being tested. The test duration is 250 h at which time the p u m p is dismantled and inspected. The second stage of the test is pulsed pressure test at the m a x i m u m displacement of the p u m p . This stage is operated for one million cycles. When this stage is complete, the p u m p is dismantled and inspected. The third stage is a variable displacement stage at maximum pressure, maximum temperature, and m i n i m u m fluid viscosity. The test duration is 280 h at which time the p u m p is dismantled and inspected again. The final pass/fail assessment is made with reference to a standard damage catalog. The Sundstrand Water Stability Test Procedure test originally employed a Sundstrand Series 22 piston p u m p at a constant pressure [41]. Currently, this test procedure is conducted using a Sundstrand Series 90 piston p u m p with a 55-cc displacement. The objective of the test is to determine the effect of water contamination on mineral oil hydraulic performance and yellow metal corrosion. However, other fluids, including water-containing fluids such as HFB and HFC fluids, may also be evaluated using this test. The test duration is 225 h, at which time it is disassembled and inspected for wear, corrosion, and cavitation. If the flow degradation is equal to or greater than 10%, the test is considered to be a "fail." Antiwear
and Extreme
Pressure
(EP)
Additives
Antiwear and EP additives prevent wear of metal surfaces by forming a protective chemical film between moving parts. These additives have traditionally been labeled as antiwear or extreme pressure (EP), depending on the mechanism of protection. Antiwear additives are generally considered to form protective films that adsorb on the metal surface and function effectively under relatively mild conditions of load and t e m p e r a t u r e . Extreme pressure additives form protective films by reacting with the metal surfaces at localized high temperatures to form low shear strength films that are relatively insoluble in the bulk oil. In either case, tribological contact is between the surface films rather than the metals. Various types of chemistry are employed in the prevention of wear in hydraulic applications. Typical compounds include zinc dialkyldithiophosphates (ZDTP), tricresylphosphates (TCP), sulfur compounds, amine phosphates, dithiocarbamates, and other chlorinated, phosphorus/sulfur, and molybdenum compounds. Water Content a n d Hydrolytic Stability In many hydraulic systems, the lubricant is susceptible to contamination with water. Contamination with water can lead to a host of problems including loss of lubricity, corrosion, additive degradation, and filter plugging. Consequently, machine builders and equipment users often attempt to limit the amount of water that enters their hydraulic systems. At the same time, fluid formulators endeavor to manufacture
364 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK TABLE 5—Machine builder specifications for antiwear hydraulic oil. Properties Method(s) ISOVG Kinematic Viscosity, cSt D445 0°C max., calc. D 5133 40°Cmax. 40°C min. 100°Cmin. Flash Point °C min. D92 Fire Point, C min. D 92 Pour Point, °C, max D97 Color, max D 1500 ISO Contam. Code, max ISO 4406 Density @15°C D1298 TAN, mg KOH/g, max D664/ D974 Rust Test A D665 A Rust Test B (Salt Water) 0665 B Cu Rating (3 hr, 100°C), max. D130 TOST Oxidation, Hours to 2.0 ,^^, TAN "*^ Air Release @ SOX, minutes Q » J ~ , (max) Foam tendency/statxiity D892 Seq 1 max Seq II max Seq III max DemulsilJility @ 54°C D1401 FZG Fail Stage D 5182 Change in Hardness NBR1,168hrs@100°C Change in Volume (%) NBR1,168hrs@100°C Viscosity Index, min D 2270 Aniline Point C min. D 611 CM Thermal Stabtity D 2070 A Viscosity Change, % max TAN Variation, % max * Comparative IR Scan Sludge, mg/100 ml max Cu metal removed, mg/200 ml, max. Copper rod appearance, rating (max.) Steel deposits, mg/200 ml, max Steel metal removed, mg/200 ml, max Steel rod appearance, rating (max) Oxidation (1000 h) D4310 AN, mgKOH/g max Total sludge, mg max. Copper, mg max Iron, mg max Hydrolytic Stability D 2619 Copper wt loss, mg/cm^ max Water layer TAN. mgKOH max V104 C Pump mg wear, max D 2882 Vickers 35 VQ 25 Pump Test Vane Wear, mg max Rir^ Wear, mg max Denison P-46 (100 h) DenisonTBC, vane wear TP-30283 Cam ling wear Denison Fnterability Test, sec TP 02100 Dry, max Wet, max
Denison HF-0
Vickers
Requirements
Requirements
6 M LS-2
Cincinnati iWachine .
P68 32
P70 46
P69 68
35.2 28.8
50.6 41.4
74.8 61.2
188 215
196 218
196 218
2
3
3
LH-02 22
LH-03 32
LH-04 46
LH-06 68
300 24.2 19.8 4.1 175
420 35.2 28.8 5 190
780 50.6 41.4 6.1 190
1400 74.8 61.2 7.8 195
-21
-18
-15
-12
10
10
19/16/13 0.84 - 0.90 <1.S> Pses
Pass Pass
Pass <1b> <1500> 5
5
<:50/0> <50/0> Timeto40/40«)(O/W/E) <30> <10>
-10 -/O -10
90 100
<90>
Oto-7
Oto-7
Oto-6
OtolS
0to12 <95>
0to12
OtolO
<5> <50> Record < 25 mg. /100ml > <10> <5>
<5> <50> 100 10 Report
Oto-8
< 25 mg. /100ml > <10> <5> <3.5> <1> <1.5>
<1.5>
2 200 50 SO (1) 0.2 4.0
<0.2> <4> I-2S6-S SO M-2950^ 15 75
Satisfactory Satisfactoiy
600 2xdry
(1) Rqmnt. Sut>ject to Denison discretion (based on other pump/fietd history) (2) D 2882 mn at 79,4C (higher temp.) for ISO 68 and higher grade.
< 50 >
(2) <10> <50> no smear, scratch, etc < 0.01 > No distress <600> < 2 X dry >
CHAPTER 13: HYDRAULIC FLUIDS hydraulic fluids that resist chemical degradation or hydrolysis in the presence of water and heat. Several ASTM methods are used to monitor water content of hydraulic fluids as well as their ability to resist hydrolytic degradation. Distillation, Centrifuge and Karl Fisher Titration Tests In ASTM D 95, Standard Test Method for Water in Petroleum Products and Bituminous Materials by Distillation, the material to be tested is diluted with a water-immiscible solvent such as toluene and heated under reflux conditions. The resulting distillate is condensed and separated in a trap. The amount of water present in the sample is determined by observing the volume of water settled in the graduated section of the trap. Centrifuge tests such as ASTM D 96, Standard Test Method for Water and Sediment in Crude Oil by Centrifuge Method, can also be used for un-emulsified or insoluble water contamination in fluids. While distillation and centrifuge methods provide reasonably accurate results for samples that contain free water contamination, these methods are generally not sensitive enough for hydraulic applications. A more accurate method for quantifying water in hydraulic fluid is the Karl Fischer test (ASTM D 1744, Standard Test Method for Determination of Water in Liquid Petroleum Products by Karl Fischer Reagent) [42]. In this test, the fluid is dispersed in a solvent such as methanol and titrated with standard Karl Fisher reagent to an electrometric endpoint (Fig. 14). The endpoint of the titration, at which free iodine is liberated.
365
may be registered either potentiometricly or by color indication. Although this method has the capability to be more accurate than distillation or centrifuge techniques, the Karl Fisher Test is susceptible to chemical interference. Calcium sulfonate, magnesium sulfonate, ZDTP and other oil additives react with iodine and have been known to interfere with the titration [43]. Hydrolytic Stability Testing Hydrolytic stability refers to the lubricant's resistance to chemical interactions with water that result in undesirable changes to fluid properties. Certain chemical components may react with water to decompose or form undesirable byproducts of hydrolysis. Heat and catalysts such as copper can accelerate the process of hydrolysis. Hydrolytically unstable oils form insoluble contaminants and acidic compounds that create hydraulic system malfunctions similar to those produced by oxidation and thermal degradation of fluids. Furthermore, antiwear additives and corrosion preventatives that are susceptible to hydrolysis are likely to lose their ability to perform their critical functions in the presence of heat and water. ASTM D 2619, Standard Test Method for Hydrolytic Stability of Hydraulic Fluids (Beverage Bottle Method) is used to measure this fluid property. In this test, 75 g of fluid and 25 g of water are sealed in a beverage bottle with a copper strip. The test bottie is rotated in an oven for 48 h at 93°C (200°F). At the end of the test, the oil and water layers are separated
FIG. 14—The Karl Fisher apparatus (a) titrant solution, (b) burette, (c) titration cell with electrode, (d) solvent, (e) waste.
366
MANUAL
3 7 ; FUELS AND LUBRICANTS
HANDBOOK
and insolubles are weighed. Viscosity a n d acid numbers are also determined. Based upon the Denison HF-0 specification (see Table 5 for details of this specification), t h e weight change of t h e copper specimen should b e less t h a n 0.20 mg/cm^ a n d the water layer acidity should be less than 4.0 mg KOH. Since exposure to water can be expected throughout the life of a fluid, hydrolytic stability is a n important design characteristic of hydraulic fluids. In genereJ, there are no additives specifically used to improve hydrolytic stability. Instead, hydrolytic stability is achieved by appropriate selection of stable components that maintain effectiveness even in t h e presence of water. Hydrolytic stability is also a key factor in the wet filterability behavior of hydraulic oils (see the Filterability section) [44]. Demulsibility Demulsibilty is the term used to describe a fluid's ability to separate from water. As discussed above in the Water Content and Hydrolytic Stability section, water contamination of the hydraulic oil may lead to various problems that adversely affect both fluid a n d equipment durability. Thus, it is desirable for hydraulic oil and water to separate as quickly as possible. In many industricJ applications, water is drained from the hydraulic oil reservoirs as it separates and settles on the bottom. For fluids with poor demulsibility, the separation is either very slow or unlikely to occur to any significant degree. Demulsibility
Testing
levels, entrained air is visible to the h u m a n eye as larger bubbles and can cause the oil to become cloudy. Uncontrolled air contamination results in a n u m b e r of undesirable consequences. Entrained air increases the compressibility of the fluid and can adversely affect its response to hydraulic control mechanisms or devices, especially in high-pressure systems. Dissolved or entrained air expands into larger bubbles as its solubility in the fluid decreases as a result of exposure to vacuum conditions at the p u m p inlet. This leads to noise and cavitation, which is the dynamic process of gas cavity growth a n d collapse in a liquid [47]. Several studies of this p h e n o m e n o n have suggested theoretical m e c h a n i s m s a n d documented experimental evidence of wear a n d increased oxidation due to cavitation [48]. Foaming is very much rooted in the fundamentcJ problem of air contamination and consequently, results in many of the same negative effects of air entrainment. It is characterized by the formation of a mass of relatively large bubbles on the surface of the fluid and is usually brought about by turbulent return of oil to the reservoir or migration of entrained air to the surface. It is desirable to have fluids with a low tendency to form foam in the first place a n d have the foam collapse quickly once formed. For effective foam control, the rate of foam collapse must be faster t h a n the rate at which entrained air migrates to the surface to form the foam. Otherwise, the foam layer will continue to increase and air may eventually be re-dispersed in the bulk fluid [49]. In severe cases, oil that produces a significant amount of foam may bubble out of hydraulic reservoir breathers, creating a fluid spill.
The speed at which water is separated from oil and the tendency of an oil to form a cuff of emulsified oil at the interface between the oil and water phases may be measured by ASTM D 1401, S t a n d a r d Test Method for W a t e r Separability of Petroleum Oils and Synthetic Fluids. In this test, a 40 ml sample of oil a n d 40 ml of distilled water are stirred for 5 min at 54°C (130°F) in a graduated cylinder. The time required for the emulsion to separate into water a n d oil phases is recorded. An oil with good demulsibility will completely separate in 30 m i n or less without a "cuff' of emulsified oil between the phases [45].
Air entrainment has increasingly become a concern due to a trend toward smaller reservoir sizes. Shorter fluid residence times therefore dictate use of hydraulic fluids with improved air release characteristics for the reasons discussed above. Several studies have shown that fluid viscosity is a critical factor influencing air release properties. Within a given class of fluids, higher viscosity and lower oil temperatures translate into slower air release characteristics. While different classes of base fluids have demonstrated unique air release advantages, there has been little success in identifying additives that improve air release properties of a base fluid.
Demulsifiers
Foam and Aeration
Demulsifiers are chemicals used to alter the surface tension at the oil/water interface to accelerate separation. T3rpical demulsifiers include alkylphenol ethers, low molecular weight synthetic sulfonates, and polyoxyalkylate resins.
Because of the importance of properly managing air contamination in hydraulic fluids, there are a n u m b e r of standardized test methods for evaluating this feature of fluid performance. The foaming tendency a n d stability of oil may be measured by ASTM D 892, Standard Test Method for Foaming Characteristics of Lubricating Oils. In this test, an oil sample is equilibrated at 24°C (75°F). Air is bubbled through oil for 5 min, and then the oil is allowed to settle for 10 minutes. The volume of foam is measured at the end of both periods. The test is repeated at 93.5°C (200°F) and again at 24°C (75°F) after the foam breaks. Various levels of foaming tendency are permitted by industry standards, but stable foam is generally not tolerated [50,51]. Not only must a hydraulic fluid resist the tendency to form stable foam, it also must allow air to rapidly rise and separate from the fluid. The Waring blender test is one test method that may be used to measure the air release properties of fluids [52]. In ASTM D 3519, Standard Test Method for Foam in Aqueous Media (Blender Test), 200 ml of the fluid is stirred
Aeration a n d Foam Under normal conditions there is always air present in a hydraulic fluid. By volume, it is present at about 7-9% at room temperature a n d atmospheric pressure [46]. In this state, it is not visible to the h u m a n eye and thus referred to as dissolved air. Higher temperatures and/or lower pressures (such as vacu u m conditions) lead to lower dissolved air levels. (See chapter on compressor lubricants for detailed discussion on gas solubility and methods of measuring gas solubility.) Fluid circulation through hydraulic systems and reservoirs may cause mecheinical introduction of air into hydraulic fluids, particularly if reservoir size or design does not allow sufficient residence time for air separation to occur. At elevated
Tests
CHAPTER at an agitation rate of 4000 to 13000 r p m for 30 s. The meixim u m total height at zero time, at 5 m i n a n d 10 m i n is recorded in order to assess the foaming and aeration tendency of a fluid under high shear conditions. Air release properties of a hydraulic fluid may also be quantified by IP 313, DIN 51381 or ASTM D 3427, Standard Test Method for Air Release Properties of Petroleum Oils. In these tests, the time in minutes for finely dispersed air in oil to decrease to 0.2% under standard test conditions is measured using a density balance. Air release times and specifications typically vary with oil viscosity. Defoamants Antifoam additives, generally referred to as defoamers or defoamants, are materials that destabilize the liquid film that surrounds air bubbles. The most commonly used defoamants are silicone polymers (particularly polydimethylsiloxanes), which function as finely dispersed marginally soluble liquid particles. Since silicon defoamants have very low surface tensions, they tend to accumulate at air/oil interfaces. When the larger bubbles rise to the surface and join other bubbles to form foam with only very thin films separating them, silicone defoamants cause these films to rupture, thus accelerating collapse of the foam. While silicone defoamants reduce the foaming tendency of a fluid, they may also tend to increase air entrainment (Fig. 15) [53]. Besides affecting air entrainment in hydraulic fluids, silicone defoamants tend to have poor filterability and storage stability due to their marginal solubility in oil. Non-silicone defoamants are increasingly used to address these disadvantages. Polyalkylacrylate additives are the most common class of non-silicone defoamants recognized in the industry. Although they do not possess the disadvantages of the silicone types, these polyalkylacrylates must be used at higher concentrations to deliver equivalent performance.
13: HYDRAULIC
It is widely recognized that beyond proper fluid selection, good fluid maintenance is the key to reliability and durability of hydraulic equipment. Fluid maintenance is closely linked to fluid cleanliness and filtration. Filtration devices, therefore, are critical tools for maintaining hydraulic fluids and system components. Hydraulic fluid "filterability" is concerned mainly with the appropriate flow characteristics of the fluid through filter media. For proper operation, the fluid should readily flow with m i n i m u m pressure drop across the filter and with negligable depletion of additives. The viscosity and chemistry of the lubricant will affect filterability. Therefore, filter size and materials should be compatible with the circulating fluid. The drive to increase hydraulic system reliability through the use of fine filtration magnifies the importance of this performance parameter. Filterability
Tests
Due to the likelihood of water contamination in many hydraulic systems and its potential impact on fluids, most of the filterability tests are designed to r u n dry and wet (with water added). Hydraulic fluid filterability tests generally consist of filtering a specified quantity of fluid t h r o u g h a standard medium while monitoring changes in flow rate (Table 6). The results are tj^pically reported in terms of a ratio between flow rates with and without water added to the fluid. This approach attempts to account for changes in filterability behavior independent of viscosity. In Denison TP 02100 the time required for complete flow of a standard volume of fluid through a specified filter is evaluated. In the Pall Filterability Test the differential pressure across a specified filter assembly is monitored over the duration of the test and cin appropriate limit is established to discriminate between fluids with good and poor filterability behavior. While key equipment
OIL WITH SILICONES
O O eg <
h-
SETTLING OR "TRANQUIL PHASE"
BLOWING OR TURBULENT PHASE"
367
Filterability
AIR RELEASED DURING BLOWING PHASE
VOLUME OF AIR BLOW IN
FLUIDS
>
»
TIME FIG. 15—Impact of silicone defoamer on foaming tendency and air release. Silicone defoamer decreases the tendency of the oil to generate foam while increasing the tendency of the fluid to retain air below its surface.
368
MANUAL
37: FUELS AND LUBRICANT
Method
TABLE 61—Filterability tests. AFNOR Pall" 0.8jU.M 0.2 70 h 70°C
Medium pore size Percent water added Aging time Temperature
HANDBOOK
Denison
3/LiM 1.0 24 h 70°C
1.2 ^M 2.0 None 25°C "Parkhurst, H., Pall Filterability Index Test for Paper Machine Oils, SLS Report No. 5669, April 1995.
builders and industrial manufacturers may require fluids to meet certain filterability criteria as measured by these tests, global hydraulic oil specifications (i.e. ASTM D 6158, ISO 11158, DIN 51524) have not yet incorporated these procedures. Filterablility
Additives
From a formulation standpoint, identifying and replacing additives with potential filterability problems (i.e., filter material incompatibility, gel-forming tendency, hydrolytic instability, etc.) has been the primary method of improving fluid filterability. Recently, dispersants have been identified that enhance filterability by preventing agglomeration of insoluble species present in the fluid. These dispersants are typically alkyphenol-based or alkyl succinimide polymers of varying molecular weights.
Corrosion Protection Chemical contaminants and corrosive by-products of fluid degradation can cause surface attack of metallic hydraulic system components. Ferrous metal corrosion in a hydraulic system is most often caused by water contamination, while copper and its alloys are susceptible to attack by the products of high temperature fluid degradation. Rusting of ferrous metal is an electrochemical reaction that occurs between the parent metal and the thin oxide layer on the metal surface formed as a result of exposure to the atmosphere [20]. Rust, which is hydrated iron oxide, compromises the integrity of the metaJ surface and adversely affects other important fluid properties w h e n it contaminates the bulk fluid. Ferrous metal corrosion protection in hydraulic systems is usually accomplished by incorporating surface-active additives such as rust inhibitors. There are several ASTM methods for evaluating the corrosion inhibition properties of hydraulic fluids. Corrosion
and Rust
Testing
The ability of fluids to prevent rusting of ferrous parts due to water c o n t a m i n a t i o n m a y be m e a s u r e d by ASTM D 665, Rust-Preventing Characteristics of Inhibited Mineral Oil in the Presence of Water. In Part A of this test, 10% distilled water is added to oil that has been heated to 60°C (140°F). Round steel rods are polished to remove their oxide coating and immersed in the oil. The oil-water mixture is continuously stirred to avoid separation while the temperature is maintained at 60°C. At the end of 24 h the specimens are inspected for rust (Fig. 16). In Part B of the method, the same procedure is used, except synthetic seawater is substituted for distilled water. As described in Part B, synthetic seawater is made by the addition
of sodium chloride, magnesium chloride, calcium chloride, and several other ionic compounds to distilled water. Part B is particularly pertinent to maritime hydraulic fluid applications where seawater, rather t h a n fresh water or condensation, is a likely source of contamination. The standard test method for measuring vapor phase corrosion inhibition of hydraulic fluids is ASTM D 5534, Test Method for Vapor-Phase Rust-Preventing Characteristics of Hydraulic Fluids. In this test, a steel specimen is attached to the cover of an ASTM D 3603 test apparatus that contains hydraulic fluid maintained at a temperature of 60°C (140°F). ASTM D 3603 is the Horizontal Disk Method for Rust-Preventing Characteristics of Steam Turbine Oils in the Presence of Water. The specimen is then exposed to water and hydraulic fluid vapors for a period of 6 h. At the end of this time, the specimen is inspected for evidence of corrosion and results are reported on a pass-fail basis. The ASTM D 5534 test is particularly relevant for water-glycol and invert-emulsion hydraulic fluids because corrosion of the underside of reservoir covers has been observed in systems that use these fluids. Accelerated corrosion can also occur when dissimilar metals are in electrical contact in the presence of an electrolyte (i.e., conductive solution). This corrosion mechanism, known as galvanic corrosion, has been found to be particularly relevant for certain biodegradable oils [54], The ability of a fluid to prevent galvanic corrosion may be measured by ASTM D 6547, Test Method for Corrosiveness of a Lubricating Fluid to a Bi-Metallic Couple. In this test, a brass clip is fitted to the oil coated surface of a steel disk. The bi-metallic (brass/steel) couple is then stored in 50% relative humidity for ten days. At the end of the ten-day period, the surfaces are inspected for evidence of staining like that depicted in Fig. 17. The steel disks are rated on a pass-fail basis. Sulfur containing additives such as zinc dithiophosphate, sulfurized olefins, organic polysulfides, and carbamates may be used as antiwear and extreme pressure additives in hydraulic fluids [55]. Depending u p o n the chemical activity of these sulfur compounds, hydraulic fluids exhibit varying degrees of corrosiveness to copper when activated by high temperatures. ASTM D 2070, Standard Test Method for Thermal Stability of Hydraulic Oils is one of the most effective methods for predicting the corrosiveness of a hydraulic fluid to copper and its alloys. The ASTM D-2070 test measures the aggressiveness of chemical constituents in the fluid toward yellow metals when aged under high temperature conditions. (See the section on High Temperature Oxidation Tests) In some cases, such as when a hydraulic fluid is contaminated with sulfur containing metalworking fluid, the fluid may exhibit corrosivity to copper without requiring thermal degradation. The standard test method for measuring the copper corrosion properties of oil is ASTM D 130, Standard Test Method for Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test. In this test, a polished copper strip is immersed in oil and heated for a predefined period of time. At the end of the test, the copper strip's appearance is compared to a standard. The rating system used for the D 130 test appears in Table 7. The rating system is on a scale of one to four. The higher the copper strip rating, the greater the degree of copper corrosion. Color standards are also available from ASTM for rating copper strips [56].
CHAPTER 13: HYDRAULIC FLUIDS
369
FIG. 16—ASTM D 665 passing vs. failing rod.
FIG. 17—Galvanic corrosion: staining on test specimen by vegetable oil.
Corrosion Inhibitors, Rust Inhibitors, and Metal Passivators Corrosion Inhibitors, Rust Inhibitors, and Metal Passivators are designed to prevent deterioration of metal surfaces that are in contact with the lubricant. Corrosion inhibitors are polar molecules that are surface active. They adsorb on the metal surface and inhibit the electrochemical reaction that produces rust. Some hydraulic fluids, particularly those used in applications that require enhanced fire resistance, are for-
mulated with water. Such fluids have entirely different corrosion inhibition requirements. For instance, water glycol hydraulic fluids must prevent corrosion in the vapor phase above the liquid due to evaporation. Thus they are formulated with vapor phase corrosion inhibitors such as morpholine. Typical classes of rust inhibitors include metallic sulfonates, amine phosphates, simple fatty acids, and succinic acid esters. Triazoles, or derivatives thereof, are commonly used metal passivators.
370
MANUAL
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HANDBOOK
TABLE 7—Copper strip classifications. Rating
Designation
la
Slight tarnish
lb 2a 2b 2c
Slight tarnish Moderate tarnish Moderate tarnish Moderate tarnish
2d 2e 3a 3b
Moderate tarnish Moderate tarnish Dark tarnish Dark tarnish
4a
Corrosion
4b 4c
Corrosion Corrosion
Light orange, almost the same as freshly polished strip Dark orange Claret red Lavender Multicolored with lavender blue or silver overlaid on claret red Silvery Brassy or gold Magenta overcast on brassy strip Multicolored with red and green showing (peacock), but no gray Transparent black, dark gray or brown with a trace of peacock Graphite or lusterless black Glossy or jet black
Seal Compatibility Very critical to the successful operation of a hydraulic system is the ability to prevent leakage and accidents that are a result of failed seals. Leaks can lead to contamination, loss of pressure, loss of lubricating fluid, and environmental damage depending on the severity of the spill. In extreme temperature and pressure operations, sudden failure of seeds may have life threatening consequences, considering the potential for explosions, fires, etc. [57]. Hydraulic fluids and elastomeric seals are composed of complex chemical components that can interact as they come into contact. Depending on the chemistries involved, time, t e m p e r a t u r e , a n d mechanical stresses cause fluid interactions with the seal material, resulting in swelling or shrinkage of the elastomer compound. It is desirable to select seal materials that exhibit minimal change in hardness, volume, tensile strength etc. in service. Slight swelling of seals is preferable to shrinkage as indicated in Table 8. This is because a reduction in seal volume may result in leakage of fluid due to failure of the seal to fill the gland that retains it in place. Seal Compatibility
TABLE 8—Recommended property change limits for determining compatibility of elastomer seals for industrial hydraulic fluid applications.
Description
Testing
In general, industry recognized seed compatibility tests entail exposure of the elastomer material to the test fluid for a specified duration and at a standard temperature u n d e r static conditions. Familiar industry seal compatibility tests include ISO 7619, ISO 6072, DIN 53 538, and ASTM D 6546-00, Standard Test Methods for and Suggested Limits for Determining Compatibility of Elastomer Seals for Industrial Hydraulic Fluid Applications. Other major organizations such as ASTM and SAE also have related specifications for sealing devices. Due to variations in elastomer chemistry, it is necessary to perform compatibility tests on the specific materials being used. While most standard tests measure changes in hardness, stress/strain properties, and volume changes after exposure to the test fluid, translation of these results to a practical application m a y be difficult, since geometry and mechanical conditions of the targeted application profoundly impact the elastomer. It is therefore recommended that seal materials be tested u n d e r conditions that closely simulate the actual application [58].
Maximum Volume Swell,
Time in Hours 24 70 100 250 500 1000
Seal Swell
Maximum Vol. Shrinkage,
%
%
Hardness Change, Shore A Points
15 15 15 15 20 20
-3 -3 -3 -4 -4 -5
±7 ±7 ±8 ±8 ±10 ±10
Maximum Tensile Strength Change, % -20 -20 -20 -20 -25 -30
Agents
These chemicals react with the elastomeric materials to cause slight swelling or softening to counteract the typical effects of temperature and mechanical stress. Seal swell agents are typically used with base fluids having very low aromatic content. Aromatic derivatives or phosphate esters are typically used to enhance the seal swell characteristics of a fluid. Coolant Separability Hydraulic systems used in machine tool operations are susceptible to contamination by aqueous cutting fluids, which contain components with poor oxidation resistance, high deposit forming tendency, and/or high corrosivity. In metcdworking applications, the hydraulic fluid may be considered a contaminant of the cutting fluid that alters its effectiveness in metal removal operations. Regardless of the perspective, a mix of these two categories of fluids is undesirable, especially if they have not been designed to be compatible. In this case, compatibility is defined as the ability of either fluid to complement, enhsince, or at least have no impact on the performance of the other w h e n mixed. The lubricant's ability to readily separate from coolants is highly desirable in most cases. However, the variety and complexity of coolant chemistries makes it difficult to ensure good separability of the hydraulic oil from all metalworking fluids [59]. There are generally n o additives specifically designed to improve coolant separability, since coolant chemistries vary so widely. The t3?pical approach is to formulate a lubricant to have good demulsibility (water separability) and then test its compatibility with specific coolants with which it is expected to come into contact. Coolant
Separability
Testing
A standard industry test method for assessing lubricant compatibility with coolants has not yet been established. However, some Icirge industrial manufacturers and lubricant suppliers do have in-house test procedures designed to simulate oil contamination by a low percentage of coolant, as well as coolant contamination by a low percentage of oil (typically referred to as tramp oil). In general, these procedures consist of mixing the lubricant with the coolant at a specified ratio a n d t e m p e r a t u r e for a s t a n d a r d duration. The fluid container, t5^ically a graduated cylinder, is then allowed to sit while the degree of separation between the coolant and the lubricant is observed at specific time intervals. Properties such as additive leaching and foam stability may also be ob-
CHAPTER served. Rapid separation, implying absence of a stable emulsion or cuff (the layer between way oil and coolant) at the interface, is very desirable (Fig. 18). Shear Stability Mobile hydraulic equipment such as excavators, farm tractors, cranes, and timber harvesters frequently are required to operate under extreme high and low temperature conditions. To accommodate wide-ranging environmental conditions, hydraulic fluids with enhanced viscosity - temperature properties are often employed. These fluids t3^ically contain viscosity index improving polymers that thicken oil at high temperatures, while having little impact u p o n their low temperature fluidity. Viscosity index (VI) is a common means for expressing the variation of viscosity with temperature. The viscosity index of an oil is calculated from the measured viscosity of the fluid at 40 and lOOX using ASTM Method D 2270, Standard Practice for Calculating Viscosity Index from Kinematic Viscosity at 40 and 100°C. A high VI indicates less relative change in viscosity for a given change in temperature. Vl-improved oils are commonly referred to as multigrade oils, because they meet both the low temperature requirements of low viscosity oils and the high temperature requirements of higher viscosity oils. Conceptually, an SAE
Good
13: HYDRAULIC
FLUIDS
371
lOW-30 multigrade oil consists of a lOW base oil and sufficient polymer to thicken the oil at 100°C to a viscosity equal to that of an SAE 30 weight oil (Fig. 19). Viscosity Index Improvers are typically subjected to mechanical degradation due to shearing of the molecules in high stress areas such as between gear teeth in gear pumps and vane-ring interface in vane p u m p s . High pressures generated in hydraulic systems subject fluids to shear rates up to 10^ s~' [60]. Not only does hydraulic shear cause fluid temperature to rise in a hydraulic system, but shear may bring about permanent viscosity loss in hydraulic fluids [61]. Permanent viscosity loss results from mechanical scission of polymer molecules in multigrade hydraulic fluids and often occurs after a relatively short period of time (<24 hours of operation). The polymer (as opposed to the base oil) is susceptible to mechanical shear because it has a higher molecular weight and therefore a larger molecular volume. As a result, with polymer-containing multigrade hydraulic fluids, the functional viscosity of an oil may differ from that predicted from kinematic viscosity measurements of new oil [62]. Shear Stability
Tests
It is desirable to formulate hydraulic fluids with shear-stable VI improvers so that the fluid retains its viscosity properties throughout its service life. Several laboratory test methods are
Fair
FIG. 18—Good vs. Bad coolant separability.
Poor
372 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
Multigrade Oils 100,000 10,000 1,000
of resistance to mechanical shear, as well as their chemical and thermal stability. For a given polymer type, shear stability decreases with an increase in molecular weight. Shear is indicated by a loss in fluid viscosity (Fig. 20). The "thickening efficiency" of the viscosity modifiers generally increases with an increase in molecular weight for a given polymer type. Selection of the best VI Improver must entail consideration of viscosity requirements, shear stability, and thermal and oxidative stability in actual equipment operation. Low Temperature Pumpability
2-i -40
:
. ^40 100 150 Temperature, ° C FIG. 19—Impact of VI improver on lubricant viscosity. -20
designed to stress multigrade oils so that they produce a permanent viscosity loss such as would take place in service. The two methods generally used are mechanical shearing with a Bosch diesel fiiel injection pump and sonic shearing with a high frequency sonic oscillator. In ASTM D 6278, Test Method for Shear Stability of Polymer Containing Fluids Using a European Diesel Injector Apparatus, the polymer-containing fluid is passed through a diesel injector nozzle at a shear rate that causes polymer molecules to degrade. Under standard test conditions, the kinematic viscosity of the fluid is measured after 30 to 250 cycles through the injector pump to determine the extent of permanent viscosity loss that has taken place. In ASTM D 5621, Standard Test Method for Sonic Shear Stability of Hydraulic Fluid, the polymer-containing oil is irradiated with a sonic oscillator for 40 min and changes in kinematic viscosity are measured. Based upon data from Kopko and Stambaugh, the Fuel Injector Shear Stability Test lacks the necessary severity to predict permanent viscosity loss produced by hydraulic equipment [62]. However, 40 min of irradiation with a high frequency sonic oscillator produced viscosity changes that closely correlate to that experienced in the ASTM D 2882 Vane Pump Test. Consequently, this test method has become the basis for ASTM D 6080, Practice for Defining the Viscosity Characteristics of Hydraulic Fluids. Viscosity Index Improvers Viscosity Index Improvers (also referred to as viscosity modifiers) are high molecular weight poljTuers that reduce the magnitude of viscosity change as a function of temperature. They function by enabling the oil to retain thickness at higher temperatures while having minimal impact on viscosity at lower temperatures. In general. Viscosity Index Improvers are oil-soluble organic polymers with molecular weights ranging from about 10000 to 1 million. The oil temperature controls coiling of the polymer molecules, which in turn controls the degree to which the polymer increases viscosity. The higher the temperature, the less the coiling and the greater the "thickening" effect of the polymer. Therefore, as temperature increases, there is less thinning of the lubricant compared to non-polymer-containing oils. The performance of VI Improvers is also described in terms
Paraffinic mineral oils, which comprise the bulk of hydraulic fluids, contain some amount of wax that forms crystalline structures at low temperatures. As these structures form, the oil becomes more viscous. At very low temperatures the fluid may become gel-like or even solid. For hydraulic systems, poor low temperature flow characteristics can result in catastrophic failures. During start-up at very low temperatures, significant pump cavitation can occur due to inadequate oil flow. Low Temperature Pumpability Tests A number of bench tests are commonly used to evaluate low temperature flow characteristics of lubricants. One of the most common tests specified for this purpose is ASTM D 97, Standard Test Method for Pour Point of Petroleum Products, which measures the lowest temperature at which a lubricant will flow. ASTM D 6351, Test Method for Determination of Low Temperature Fluidity and Appearance of Hydraulic Fluids is used for evaluating the pour characteristics of biodegradable oils. While this test gives an indication of low temperature flow characteristics of the fluid, it does not necessarily address fluid performance in many applications subjected to very low temperatures. Pumps, motors, engines, and many types of lubricated machinery require that the lubricant be pumped or circulated effectively at start-up. As a result, several tests have been developed to determine fluid viscosity at low temperature. For hydraulic systems, tests such as Brookfield (ASTM D2 983), Scanning Brookfield SBV (ASTM D 5133), and Mini Rotor Viscometer MRV (ASTM D
Multigrade Oils and Shear Stability Quiescent Polymer Coil in Oil Solution
Reversible/ Orientation of Coil Under Shear Forces
Temperature Viscosity Loss
sNonreversible Rapture of Coil and Subsequent Orientation Under Shear Forces
Permanent Viscosity Loss
FIG. 20—Impact of shear stress on VI improver molecule.
CHAPTER 13: HYDRAULIC FLUIDS 373 TABLE 9—Low temperature viscosity grades for hydraulic fluid classifications. Viscosity Grade
Min.
Max.
L5 L7 LIO L15 L22 L32 L46 L68
-49 -41 -32 -22 -14 -7 -1
-50 -42 -33 -23 -15 -8 -2 4
that will crystallize at low temperatures and cause the fluid to solidify. These additives do not entirely prevent wax from crystallizing in the oil. Rather, they lower the temperature at which large wax crystal structures are formed. By reducing the size of the crystal matrix, pour point depressants permit lubricants to flow at lower temperatures. Two widely used types of pour point depressants are alkylaromatic polymers, which adsorb on wax crystals to inhibit growth and adherence of crystals to each other, and polymethacrylates (PMA), which co-crystallize with wax to minimize growth of crystals. Depending mainly on the type of base fluid, the pour point of oil can be lowered typically by 20-30°F (I1-17°C).
4684)—all of which include specified cooling cycle and low shear rates simulating field conditions—can be used to assess fluid pumpability. Performance specifications that include low temperature pumpability requirements, such as ASTM D 6080, Standard Practice for Defining the Viscosity Characteristics of Hydraulic Fluids, typically specify a temperature range for different viscosity grades. In Table 9 (from Standard D 6080) the temperature range for a given L-grade is approximately equivalent to that for an ISO grade of the same numerical designation and having a viscosity index of 100. For instance, the temperature range for the L32 oil is approximately the same as an ISO VG 32 grade with a Viscosity Index of 100. Pour Point
Depressants
Pour point depressants are additives designed to reduce formation of rigid wax crystals in the lubricant at low temperatures. Conventionally refined mineral oils typically require the use of pour point depressants because they contain wax
TYPES OF HYDRAULIC FLUIDS The major compositional categories of hydraulic Quids are Petroleum Based, Synthetic Based, Water Based, Vegetable Oil Based, and Water (Fig. 21). As expected, these different categories have properties that make them especially desirable in particular applications. In this section, the types of hydraulic fluids will be discussed in terms of their defining functionality rather t h a n composition. For example, fire resistant fluids, which are typically water-based or ester-based fluids having high flash points, are used extensively in the basic metals industry where the risk of ignition is high, while "environmentally acceptable" fluids are used in environmentally sensitive areas. Hydraulic system hardware is usually designed and formally rated to work with mineral oils, since they are the predominant hydraulic fluid in use. Systems may have to
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FIG. 21—Schematic of hydraulic fluid types.
Mcra Emulalm
374
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37: FUELS AND LUBRICANTS
HANDBOOK
be modified to accominodate alternative types of hydraulic fluids.
precipitates that can cause hydraulic valve sticking and filter plugging [65]. HM
Mineral Hydraulic Oils The majority of hydraulic fluids in service are mineral oil based because they generally provide excellent performance at a relatively low cost. Within the mineral hydraulic oil category there is a wide range of viscosity grades and fluid types. The International Organization for Standardization (ISO) established a classification system for hydraulic fluids that is designated ISO 6743-4: 1999, Lubricants, Industrial Oils and Related Products (Class L)-Classification-Part 4: Family H (Hydraulic Systems). Using this classification system as a foundation, ASTM created ASTM D 6158, Standard Specification for Mineral Hydraulic Oils, which defines the physical properties and performance r e q u i r e m e n t s of mineral hydraulic fluids. Table 10 provides a list of mineral oil based fluids listed in ASTM D6158. HH Type H H fluids are straight base oils without any additives. They may be used in air-over-oil hydraulic systems such as is found in car lifts at automotive service centers. They are also used in manual hydraulic pumps, jacks, and other low-pressure hydraulic systems. While type H H fluids are able to perform the primary function of a hydraulic fluid, which is to transmit power, they are unable to withstand high temperatures and have limited lubricating capabilities. Thus these fluids find limited application in industry. HL Type HL fluids are also mineral oil based, but they contain rust and oxidation inhibitors to protect equipment from the detrimental effects of water contamination and chemical deterioration due to heat. These fluids are also known as R & O oils because they contain rust and oxidation inhibitors. Type HL fluids are often recommended for use in machine tool applications where system pressures are limited to 2000 psi or less. They are also recommended for some piston p u m p applications. For example, type HL fluids are the preferred fluid for Denison piston p u m p s [63]. This is because some ZDTP containing oils can be aggressive to yellow metal (brass and bronze) and silver alloyed components in piston pumps. R & O oils often are formulated using a rust inhibitor chemistry that contains succinic acid derivatives [64]. These additives may be incompatible with metallic sulfonate or phenate rust inhibitors or ZDTP antiwear additives used in many antiwear hydraulic fluids, resulting in formation of
TABLE 10—Mineral oil based hydraulic fluid classifications. Symbol
Classification
HH HL
Non-inhibited refined mineral oils Refined mineral oils with improved rust protection and oxidation stability Oils of the HL type with improved anti-wear properties Oils of the HM type with improved viscosity index properties
HM HV
Commercial Designation Straight base oils R&O oils Antiwear oils Multigrade oils
Type HM fluids contain antiwear additives in addition to the rust and oxidation inhibitors found in HL fluids. They are the most widely used mineral oil based hydraulic fluids because antiwear additives provide enhanced performance in highpressure hydraulic applications. The requirements for HM oils are listed in Table 11. While early versions of HM oils lacked the thermal stability necessary for satisfactory piston p u m p performance, modern fluids are able to perform quite well in piston p u m p applications. Zinc dialkyldithiophosphate is the most widely used antiwear additive for hydraulic applications. In recent years, concerns about the environmental effects of ZDTP have led to development of zinc-free or ashless antiwear hydraulic fluids. These products utilize sulfur and phosphorus compounds to achieve satisfactory antiwear performance. Thus, a type HM fluid may contain zinc or some other type of antiwear additive chemistry. HV Type HV fluids contain the same basic chemistry as HM fluids plus a viscosity index (VI) improver. (See the Shear Stability section.) Viscosity index improvers impart multigrade functionality to type HV fluids. While a wide range of polymers may be used for VI enhancement, these additives all function in the same basic manner. At low temperatures, VI improvers have a minimal effect upon fluid viscosity and at high temperatures they have a thickening effect. This enables the fluid to provide satisfactory performance at a wider operating temperature range [66]. T r a c t o r F l u i d s , ATF, a n d E n g i n e O i l s Tractor fluids are unique in that they are formulated to lubricate transmissions, final drives, wet brakes, clutches, and hydraulic systems from a c o m m o n fluid reservoir on the tractor [67]. Consequently, tractor fluids are often used in agricultural equipment, off-highway machinery, backhoes, and turf applications where a multifunctional hydraulic fluid is required. To lubricate gears, wet brakes, and hydraulic systems, tractor fluids typically utilize phosphate ester based friction modifiers and ZDTP. Automatic transmission fluids (ATFs) are similar to tractor fluids in that they are designed for multiple functionality, however, they generally utilize ashless antiwear additives r a t h e r t h a n ZDTP. ATFs tend to be used in applications where superior low temperature performance is desired because they are designed to remain fluid at temperatures as low as —40°C. Years ago it was common to use lOW engine oils in hydraulic applications. Until recently lOW diesel engine oil was the primary hydraulic fluid recommendation for Caterpillar equipment because lOW diesel engine oil contains ZDTP antiwear additives and is compatible with engine oil [68]. The disadvantage of using ATFs and engine oils in hydraulic applications is that they tend to have poor water separation properties, which reduces wet filterability performance due to hydrolysis of the metallic sulfonates and phenates. Consequentiy, fluids designed specifically for hydraulic performance are generally more desirable.
CHAPTER
13: HYDRAULIC
FLUIDS
375
TABLE 11—International specifications for antiwear hydraulic oil. DIN 51524 Part 2
ISO 11158 ASTM Properties ISOVG Kinematic Viscosity, cSt D445 40'C max. 40°C min. D92 Flash Point, °C min. Flash Point, °C min. 093 Brookfield Vis < 750 cP, Max D2983 D97 Pour Point, °C, max Visual Appearance D1744 Water Content, wt% 01298 Density @1S°C TAN, ma KOH/g, max 0664 or 0974 D665A Rust Test A Rust Test B (Salt Water) 06658 Cu Rating (3 hr, 100°C), max. 0130 TOST Oxidation, hours 0943 AN after 1000h, max. Air Release @ 50°C, minutes D3427 (max) Foam tendency/stability 0892 Seql max max SeqII max SeqIII Demulsibility @ 54°C 01401 Minutes to 37 mL water . 0 5182 FZG Fail Stage Ctiange in Hardness 0 471 NBR1,168 hrs @ 100°C ISO 7619 Change in Volume (%) NBR1,168hrs@100°C Viscosity Index, min 0 2270 CM Thermal Stab. 02070A Sludge, mg/100 ml max Copper rod appearwice, rating (max.) 0 4310 Oxidation (1000 h) AN, mgKOH/g max Total sludge, mg max. Total Metals in oil/water/sludge Vickers 104C mg. wear max. 0 2882
HM (Antiwear) 32 46
22
S8
22
68
24.2 19.8 165
35.2 28.8 175
50.6 41.4 185
74.8 61.2 195
24.2 19.8 165
35.2 28.8 175
50.6 41.4 185
74.8 61.2 195
-18
-15 < Report >
-12
-12
-21
-18
-15
-12
-15 -21
-8 -18
-2 -15
4 -12
2 1000
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2 1000
2 1000
5
5
10
13
30
30
Oto-7
Oto-6
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< Report > < Report > < Report >
< Report > < Report >
-
2
2
2
2
2
2
2
2
2
2
2
2
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2
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2
5
5
10
13
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10
10
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40
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40 10
60 10
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0to12
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2
HFAE
Oil-in-water emulsions containing typically >80% water Chemical solutions in water containing typically >80% water Water-in-oil emulsions containing approximately 45% water Water-polymer solutions containing approximately 45% of water Synthetic fluids containing no water and consisting of phosphate esters Synthetic fluids containing no water and of other compositions
< 150 in exterxled test>
Commercial Descriptions
Soluble oils High water based fluids Invert emulsions Water-glycols Phosphate esters Polyol esters
30
30
r288 hrs. f 000) Oto-8 Oto-7 Oto15
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25 5
25 5
25 5
200
200 < Report > 50
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Classification
HFDU
HM (Antiwear) 32 46
74.8 61.2 180 168
Symbol
HFDR
22
50.6 41.4 180 168
TABLE 12—ISO designations for fire resistant hydraulic fluids.
HFC
68
35.2 28.8 160 148
Fire resistant hydraulic fluids are used in the basic metals industry, die casting, military, and foundry applications. They may be found in any application where a ruptured hydraulic line presents a potential fire hazard. Fire resistant hydraulic fluids are formulated with materials that have a lower BTU content than mineral oils, such as polyol esters, phosphate esters, and water-glycol solutions. As a result, they b u m with less heat generation than mineral hydraulic oils. As with mineral hydraulic fluids, the International Organization for Standardization has established a classification system for fire resistant fluids based upon composition. Table 12 provides a list
HFB
ASTM D6158
24.2 19.8 140 128
Fire Resistant Fluids
HFAS
Requirements 32 46
50
of the ISO designations for fire resistant hydraulic fluids [69]. While power transmission, heat transfer, and lubrication are essential requirements for all types of hydraulic fluids, it is sometimes necessary to compromise these properties to accommodate a critical fluid characteristic. This is especially true of fire resistant hydraulic fluids. Fire resistant fluids differ from mineral hydraulic fluids in density, compatibility, and lubricating properties. As a result, hydraulic systems are often modified when utilizing a fire resistant fluid. To optimize the performance of fire resistant fluids, the National Fluid Power Association and ISO have published guides for their use [70,71]. These NFFA and ISO documents detail the operational characteristics of fire resistant fluids and provide suggestions for storage, use, and handling of these fluids. Table 13 provides a comparison of the properties of common fire resistant hydraulic fluids. HFA HFA fluids contain greater than 80% water. These products are sometimes referred to as 95:5 fluids, because 5% concentrations are commonly employed. The ISO 6743-4 classification divides HFA into two sub-categories: HFAE and HFAS. HFAE fluids are oil-in-water emulsions. HFAS fluids are chemical solutions or blends of selected additives in water. Typically these products are sold as concentrates and diluted prior to use in service. Because of the high vapor pressure of water, the m a x i m u m recommended bulk fluid temperature for HFA fluids is 50°C [72]. The antiwear properties of these fluids are inferior to mineral hydraulic fluids because the vis-
376
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TABLE 13—Comparison of c o m m o n fire resistant fluid properties. Property ISO Designation Heat of Combustion" Autoignition Temp, "F* Maximum" Temperature Vapor Pressure, m b a r Specific Gravity Viscosity @ 40°C, cSt Water Content Vane p u m p rating" Compatible Seals
Antiwear Hyd. Oil
Invert Emulsion
Water Glycol
Phosphate Ester
Polyol Ester
HM 29.1 kJ/g 650 150°F 0.001 @ 50°C 0.85-0.88 32-68 0.05%
HF-B 16.3 kJ/g 830 120°F NA 0.91-0.93 80-100 43%
HF-C 5.3 kJ/g 830 120°F 80 @ 50°C 1.05-1.10 40 43%
HF-DR 19.0 kJ/g 1100-H 150°F < 1 @ 150°C 1.02-1.16 22-100 0.05%
HF-DU 21.1 kJ/g 750" 150°F NA 0.91-0.96 46-68 0.1%
100% Buna-N, Viton
33% Nitroxyl, Buna-N
67% Buna-N
67% Butyl, EPR
100% Viton, Buna-N
"Roberts and Brooks Flammability Data, NFPA T2.13.8-1997, a calculated estimate was used for HFDU.
cosity of HFA fluids is comparable to water, approximately 1 cSt. Performance is satisfactory with HFA fluids when suitable components are used but is apt to be poor if used in conventional hydraulic systems. Special precautions also are required in the selection of filter construction materials and plumbing of p u m p inlets. Thus, it is necessary to work closely with fluid a n d c o m p o n e n t suppliers w h e n utilizing HFA fluids. HFB HFB fluids are water-in-oil emulsions consisting of p e t r o l e u m oil, emulsifiers, selected additives, and water. They are commonly referred to as invert emulsions. In an invert emulsion the oil phase, which provides lubricity and rust protection, encapsulates the water phase, which provides fire resistance. The water content of an HFB fluid is normally in the 4 3 - 4 5 % range (w/w). When water content of these fluids drops below 38% due to evaporation, the fire resistance of the invert-emulsion deteriorates. Maintenance of invert emulsions is complicated by the fact that when these fluids lose water through evaporation, a high-shear mixing device is normally necessary for proper addition of make-up. The viscosity properties of invert emulsions are u n u s u a l in that evaporation of water results in a viscosity decrease. Several ASTM methods have been developed specifically for invert emulsion hydraulic fluids. ASTM D 3709, Standard Test Method for Stability of Water-in-Oil Emulsions Under Low to Ambient Temperature Cycling Conditions, is used to evaluate the freeze-thaw stability of invert emulsions. ASTM D 3707, Standard Test Method for Storage Stability of Waterin-Oil Emulsions by the Oven Test Method is used to determine if the emulsion has a propensity to separate after 48 h at 85°C. As with HFA fluids, special precautions also are required in the selection of filter construction materials and plumbing of p u m p inlets. Thus, it is necessary to work closely with fluid and c o m p o n e n t suppliers w h e n utilizing HFB fluids. HFC HFC Fluids are solutions of water, glycols, additives, and thickening agents. They are commonly referred to as waterglycol hydraulic fluids. Typically, water-glycol fluids are formulated with diethylene glycol or propylene glycol and a polyalkylene glycol based thickening agent [73]. The low molecular weight glycol reduces the vapor pressure of tlje
fluid (relative to water) while high molecular weight polyalkylene glycol acts as a thickening agent, much like a viscosity index improver. This combination thickeners and glycols enhance the lubricating properties of a water-glycol and reduces the propensity of the fluid toward cavitation erosion. Nonetheless, operating temperatures for water-glycols are limited to a maximum of 50°C because of the effect of temperature on vapor pressure [74]. Water glycol fluids are highly alkaline due to the presence of amine based corrosion inhibitors. As a result, these fluids can attack zinc, cadmium, magnesium, and non-anodized aluminum, forming sticky or gummy residues. Consequently, these metals should be avoided when selecting system components. Special precautions also are required in the selection of filter construction materials and plumbing of p u m p inlets. Thus, it is necessary to work closely with fluid and component suppliers when utilizing HFC fluids. HFD HFD Fluids are non-water containing fire resistant fluids. The first edition of International Standard ISO 6743-4 classification (1982) divided HFD into four sub-categories: HFDR, HFDS, HFDT, and HFDU. In 1999 the standard was revised, deleting the HFDS and HFDT fluids from the classification system. HFDS and HFDT fluids are no longer commercially viable because they were based upon chlorinated materials such as polychlorinated biphenyls (PCBs) or other chlorinated aromatic compounds. Environmental concerns associated with chlorinated hydrocarbons led to withdrawal of these products from the market. On the other hand, HFDR and HFDU fluids continue to be widely used in a variety of commercial and military hydraulic applications. HFDR fluids are composed of phosphate esters. The majority of phosphate ester type hydraulic fluids used in industrial applications are based upon triaryl phosphate [75]. Trialkyl and mixed alkylaryl phosphate esters are used in aviation because of their lower density [76]. Phosphate esters are difficult to ignite because they are non-volatile and chemically stable. The stability of p h o s p h a t e esters is demonstrated by the fact that they do not propagate a flame in the Standard Test Method for Linear Flame Propagation Rate of Lubricating Oils and Hydraulic Fluids (ASTM D 5306-92). The principal reason they do not propagate a flame is that the chemical reactions that take place during
CHAPTER combustion of phosphate esters are endothermic. Thus, phosphate esters generate less heat when burned relative to other HFD fluids. In addition, because their Are resistance is not dependent upon the presence of water or mist suppressing additives, the fire resistance of HFDR fluids does not degrade in service. HFDR fluids have been used in hydraulic applications for more than forty years and are known for excellent inherent lubricating properties [77]. In fact, aryl phosphate esters serve as antiwear additives in mineral oil based hydraulic fluids [78]. However, phosphate esters have a steep viscosity temperature curve, which makes their temperature operating window rather narrow [79]. Hydrolysis is the most c o m m o n form of degradation in HFDR fluid, and can occur in the presence of a small amount of water and heat. When hydrolysis takes place, phosphate esters break down into their constituent acids and alcohols. Due to the frequent presence of water in hydraulic applications, the sensitivity of phosphate esters to water has limited their use and significantly reduced their service life. Phosphate esters are compatible with all common metals except aluminum. Phosphate esters do not "wet" the surface of aluminum and thus aluminum should not be used in tribological contacts such as bearings [80]. Phosphate esters should never be added to systems containing mineral oil or water-based fire resistant fluids. Not only are these materials chemically incompatible with each other, in all probability preexisting gaskets, seals, hoses, and coatings are also incompatible. Special precautions also are required in the selection of filter construction materials and plumbing of p u m p inlets. Thus, it is necessary to work closely with fluid and component suppliers when utilizing HFD fluids. HFDU fluids typically are composed of polyol esters although other materials such as polyalkylene glycols are included in the HFDU category. Trimethylol propane oleate, neopentyl glycol oleate, and pentaerythritol esters are the most c o m m o n of the synthetic polyol esters. Triglycerides derived from soybeans, sunflower, and rapeseed plants are naturally occurring polyol esters that also are used in HFDU fluids. Polyol esters derive their fire resistance from a combination of factors. First, polyol esters have a relatively high flash, fire, and autoignition point. Second, they b u m with less energy than oil because of the presence of oxygen in the molecule. And finally, polyol ester fire resistant fluids employ antimist additives that enhance their spray-flammability resistance [81]. Depending upon the shear stability of the polymer, the fire resistance of the fluid may deteriorate in service. Like phosphate esters, polyol esters have excellent lubricating properties but are prone to hydrolysis in the presence of water [82]. In addition, they are vulnerable to oxidation because of unsaturation irl the fatty acid portion of the ester. These factors tend to limit their service life relative to mineral oils. Most common metals used in hydraulic applications are compatible with polyol ester hydraulic fluids, with the exception of lead, zinc, and cadmium. Unlike other fire resistant fluids, polyol esters performance is satisfactory with comm o n filter construction materials and system designs. Thus it is relatively easy to convert a hydraulic system that operates on mineral oil based hydraulic fluids to HFDU fluids.
13: HYDRAULIC
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377
Environmentally Acceptable Hydraulic Fluids Environmentally acceptable hydraulic fluids have found their way into hydraulic applications where there is risk of fluid leaks and spills entering the environment (especially waterways) affecting aquatic and terrestrial life. Some examples of these niche markets include forestry, construction, locks a n d d a m s , heavy-duty lawn equipment, a m u s e m e n t parks/entertainment industry, offshore drilling, and maritime. Most environmentally acceptable hydraulic fluids exhibit two key environmental characteristics: virtual nontoxicity to aquatic life a n d aerobic biodegradability. Organizations such as the Organization for Economic Co-operation and Development (OECD), the Co-ordinating E u r o p e a n Council (CEC), and the U.S. Environmental Protection Agency (EPA) have developed standard test methods to determine the toxicity a n d biodegradability of substances. More recently ASTM has developed a Guide for Assessing Biodegradability of Hydraulic Fluids (ASTM D 6006) and a Classification of Hydraulic Fluids for Environmental Impact (ASTM D 6046) based on the above organizations' methods. Utilizing the methodology from these organizations, standard classifications and performance requirements for environmental fluids have also been established by the International Organization for Standardization (ISO) and regional environmental organizations t h a t a w a r d Eco Labels (i.e., German Blue Angel, Nordic Swan, Japanese EcoMark). ISO environmental hydraulic fluid classifications are described in Table 14. HETG Type HETG fluids are based on naturally occurring vegetable oils or triglyceride esters. Without the addition of a thickener, vegetable oils are limited to a narrow viscosity range between ISO 32 and 46. While HETG fluids biodegrade rapidly, have excellent natureil lubricity and have a natural VI in excess of 200, they are unsuitable for use at high and low temperature extremes. This is because they tend to gel at low temperatures and oxidize at high temperatures. The practical temperature limits for uses HETG fluids is —25°F to 165°F. HEES Type H E E S fluids are based on unsaturated to fully saturated synthetic esters. Common ester chemistries utilized for hydraulic fluids consist of TMP oleates, neopentylglycols, pentaerythritol esters, adipate esters, and complex esters. The synthetic esters provide better performance over HETG t5T3e hydraulic fluids with wider operating temperature ranges, broad range of ISO viscosity grades, and better oxidation stability while still maintaining biodegradability.
TABLE 14—ISO environmental hydraulic fluid classifications. Symbol
Classification
Commercial Designation
HETG HEES
Vegetable oil types Sjmthetic ester types
HEPG HEPR
Polyglycol types Polyalphaolefln types
Vegetable oils and natural esters Polyol esters, neopentylglycols, syntiietic adipate esters Polyglycols Polyalphaolefins (PAO) or synthetic hydrocarbons (SHC)
378 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK HEPG Type HEPG fluids are polyethyleneglycols (PEG), which possess good oxidation stability and low temperature flow characteristics. At molecular weights of up to 600-800, HEPG type fluids are ecotoxicologically harmless and readily biodegradable (>90% in 21 days) [83]. Some disadvantages of this class of fluids include miscibility with water, incompatibility with mineral oils, and aggressiveness toward some common t5^es of elastomer seal materials. HEPR HEPR type fluids are polyalphaolefins (PAO) or synthesized hydrocarbon (SHC) base fluids, which have significantly better viscometric properties over a wider range of temperatures than mineral base fluids with the same standard viscosity classification. Some low viscosity PAOs have shown acceptable primary biodegradability, though not as rapid as vegetable or synthetic ester base fluids (Fig. 22). Additional advantages claimed for synthetic lubricants over comparable petroleum-based fluids include improved thermal and oxidative stability, superior volatility characteristics, and preferred frictional properties.
Another challenge that comes with the various hydraulic applications is that of developing test methods that are truly representative of performance in actual systems. Bench-top tests are to be used as logical indicators of a fluid's response to expected conditions of temperature, pressure, contamination, etc. A significantly higher number of variables concurrently influence the fluid more than any single bench test can simulate. Therefore, standards and specifications consist of multiple bench tests as well as more realistic full-scale test stands that use actual pumps in typical hydraulic circuits. Test methods will continue to evolve as more sophisticated techniques are developed to predict field performance of hydraulic fluids.
ASTM STANDARDS No. D 92 D 95 D 96 D 97 D 130
CONCLUSIONS A well formulated hydraulic oil consists of a properly selected base fluid and the appropriate balance of additives, optimized to provide the best possible overall performance required for the targeted application. The versatility of hydraulics makes fluid power advantageous in a wide variety of industrial and mobile applications. With this versatility comes the challenge of developing fluids that function appropriately in a wide range of conditions, even as environmental health and safety requirements become more and more stringent. New fluid technologies continue to emerge to meet these challenges.
D 287 D 445 D 446 D 471 D 664
Title Test Method for Flash and Fire Points by Cleveland Open Cup Test Method for Water in Petroleum Products and Bituminous Materials by Distillation Test Method for Water and Sediment in Crude Oil by Centrifuge Method Test Method for Pour Point of Petroleum Products Test Method for Determination of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test Test Method for API Gravity of Crude Petroleum and Petroleum Products (Hydrometer Method) Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (the Calculation of Dynamic Viscosity) Specifications and Operating Instructions for Glass Capillary Kinematic Viscometers Test Method for Rubber Property-Effect of Liquids Test Method for Acid Number of Petroleum Products by Potentiometric Titration
I •
Polypropylene glycols
I Mininnum Maximum
Mineral oils
Hydro-treated mineral oils Polyethylene glycols —1 Vegetable oils
§
Synthetic esters 20
40
60
80
100%
FIG. 22—Chart comparing primary biodegradation of base fluids by CEC method.
CHAPTER D 665 D 892 D 943 D 974 D 1298
D 1401 D 1744 D 2070 D 2270 D 2271 D 2272 D 2422 D 2619 D 2717 D 2766 D 2783 D 2882
D 2983
D 3339 D 3427 D 3519 D 3603
D 3707 D 3709
D 4172 D 4310 D 4684
D 5133
D 5182
Test Method of Rust-Preventing Characteristics of Inhibited Mineral Oil in the Presence of Water Test Method for Foaming Characteristics of Lubricating Oils Test Method for Oxidation Characteristics of Inhibited Mineral Oils Test Method for Acid and Base Number by ColorIndicator Titration Test Method for Density, Relative Density (Specific Gravity), or API Gravity of Crude Petroleum Products by Hydrometer Method Test Method for Water Separability of Petroleum Oils and Synthetic Fluids Test Method for Determination of Water in Liquid Petroleum Products by Karl Fischer Reagent Test Method for Thermal Stability of Hydraulic Oils Practice for Calculating Viscosity Index from Kinematic Viscosity at 40°C and 100°C Test Method for Preliminary Examination of Hydraulic Fluids (Wear Test) Test Method for Oxidation Stability of Steam Turbine Oils by Rotating B o m b Classification of Industrial Fluid Lubricants by Viscosity System Test Method for Hydrolytic Stability of Hydraulic Fluids (Beverage Bottle Method) Test Method for Thermal Conductivity of Liquids Test Method for Specific Heat of Liquids and Solids Test Method for Measurement of Extreme-Pressure Properties of Lubricating Fluids (Four-Ball Method) Test Method for Indicating the Wear Characteristics of Petroleum a n d Non-Petroleum Hydraulic Fluids in a Constant Volume Vane Pump Test Method for Low-Temperature Viscosity of Automotive Fluid Lubricants Measured by Brookfield Viscometer Test Method for Acid N u m b e r of Petroleum Products by Semi-Micro Color Indicator Titration Test Method for Air Release Properties of Petroleum Oils Test Method for Foam in Aqueous Media (Blender Test) Test Method for Rust-Preventing Characteristics of Steam Turbine Oils in the Presence of Water (Horizontal Disk Method) Test Method for Storage StabiHty of Water-in-Oil Emulsions by the Oven Test Method Test Method for Stability of Water-in-Oil Emulsions Under Low to Ambient Temperature Cycling Conditions Test Method for Wear Preventive Characteristics of Lubricating Fluid (Four-Ball Method) Test method for Determination of the Sludging and Corrosion Tendencies of Inhibited Mineral Oils Test Method for Determination of Yield Stress and Apparent Viscosity of Engine Oils at Low Temperatures Test Method for Low temperature. Low Shear Rate, Viscosity/Temperature Dependence of Lubricating Oils Using a Temperature Scanning Technique Test Method for Evaluating the Scuffing Load Ca-
D 5306 D 5534 D 5621 D 6006 D 6046 D 6080 D 6158 D 6278
D 6351 D 6546
D 6547
13: HYDRAULIC
FLUIDS
379
pacity of Oils (FZG Visual Method) Standard Test Method for Linear Flame Propagation Rate of Lubricating Oils and Hydraulic Fluids Test Method for Vapor-Phase Rust-Preventing Characterisitics of Hydraulic Fluids Test method for Sonic Shear Stability of Hydraulic Fluid Guide for Assessing Biodegradability of Hydraulic Fluids Classification of Hydraulic Fluids for Environmental Impact Practice for Defining the Viscosity Characteristics of Hydraulic Fluids Specification for Mineral Hydraulic Oils Test Methods for Shear Stability of Polymer Containing Fluids Using a European Diesel Injector Apparatus Test Method for Determination of Low Temperature Fluidity and Appearance of Hydraulic Fluids Test Methods for and Suggested Limits for Determining Compatibility of Elastomer Seals for Industrial Hydraulic Fluid Applications Test Method for Corrosiveness of a Lubricating Fluid to a Bi-Metallic Couple
OTHER STANDARDS AFNOR NF E48-690: Hydraulic Fluid Power. Fluids. Measurement of Filtrability of Mineral Oils AFNOR NF E48-691: Hydraulic Fluid Power. Fluids. Measurement of Filtrability of Minerals Oils in the Presence of Water ANSI/(NFPA) S t a n d a r d T2.13.7R1-1996: Hydraulic Fluid Power - Petroleum Fluids - Prediction of Bulk Moduli ISO 6743/4 Part 4: Family H (Hydraulic Systems), Lubricants, Industrial Oils and Related Products (Class L ) : Classification Part 4: Family H (Hydraulic Systems) ISO 12922: Lubricants, Industrial Oils, and Related Products (Class L)—Family H (Hydraulic systems)—Specifications for categories HFAE, HFAS, HFB, HFC, HFDR and HFDU ISO/DIS 15380: Lubricants, Industrial Fluids and Related Procedures (Class L), Family H (Hydraulic Systems)-Specifications for Catagories HETG, HEPG, HEES and HEPR
REFERENCES [1] Halliday, D. and Resnick, R. Physics, 3rd ed., John Wiley & Sons, NY, 1978, p. 376. [2] Henke, R., Diesel Progress, Fluid Power Buyer's Guide, Diesel and Gas Turbine Publications, Waukesha, WI, 1994, p. 4. [3] Vickers Industrial Hydraulics Manual, Ch. 3, Vickers, Inc., Rochester Hills, MI, 1992, p. 3-1. [4] Booser, E. R., Handbook of Lubrication, Volume II Theory and Design, CRC Press, Boca Raton, FL, 1987, pp. 242-244. [5] NFPA Standard T2.13.7R1 - 1997: Hydraulic Fluid Power Petroleum Fluids - Prediction of Bulk Moduli, National Fluid Power Association, Milwaukee, WI, 1997. [6] Lightening Reference Handbook, 7th ed., Paul-Munroe Hydraulics, Whittier, CA, 1990, p.l22. [7] Totten, G. E., Webster, G. M., and Yeaple, F. D., "Physical Prop-
380 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK erties and Their Determination," Handbook of Hydraulic Fluid Technology, G. E. Totten, Ed., 2000, Marcel Dekker, NY, p. 253. [8] Esposito, A., Fluid Power with Applications, Prentice Hall, Englewood Cliffs, NJ, 1988, p. 478. [9] Michael, P. W. and Wanke, T. S, "Surgically Clean Hydraulic Fluid - A Case Study," Proceedings of the 47th National Conference on Fluid Power, National Fluid Power Association, Milwaukee, WI, 1996, pp. 129-136. [10] Esposito, A., Fluid Power with Applications, Prentice Hall, Englewood Cliffs, NJ, 1988, p.3. [11] Ruble, L. R., "The Expanded Focus, Use, and Future of Water Powered Rotary Actuators," Proceedings of the 48th National Conference on Fluid Power, National Fluid Power Association, Milwaukee, WI, 2000, pp. 567-574. [12] Perrier, R., "Lubricant Basestocks," Speech, STLE Chicago Lube School, Downers Grove, IL, 17 Mar. 1999. [13] Engine Oil Licensing and Certification System, 14th ed.. Publication 1509, American Petroleum Institute, Washington DC, 1996. [14] Wills, J. G., Lubrication Fundamentals, Mobil Oil Corporation, Fairfax, VA, 1980, p. 30. [15] Denisov, E., Handbook of Antioxidants, CRC Press, Boca Raton, FL, 1995, p. 19. [16] Lubricant and Fuel Additives, Advanced Lubrication Education Program, Report 1525, May 1992. [17] Rasberger, M., "Chemistry and Technology of Lubricants," Oxidative Degradation and Stabilisation (Sic) of Lubricants, Blackie and Academic Professional, London, 1992, pp. 83-123. [18] Igarashi, J., "Oxidative Degradation of Engine Oils," Japanese Journal of Tribology, Vol. 35, No. 10, 1990. p. 1098. [19] Saxena, D., Mookken, R. T., Srivastava, S. P., and Bhatnagar A. K., "An Accelerated Aging Test for AW Oils," Lubrication Engineering, Vol. 49, No. 10, 1993, pp. 801-809. [20] Approved Lubricants Manual, Cincinnati Machine P u b No. lO-SP-95046, Cincinnati Machine, Cincinnati, OH, 1995, pp. 2-40. [21] Reyes-Gavilan, J. L. and Odorisio, P., "A Review of the Mechanisms of Action of Antioxidants, Metal Deactivators and Corrosion Inhibitors," NLGI Spokesman, Vol. 64, No. 11, Feb. 2001, pp. 22-33. [22] Gatto, V. J. and Grina, M. A., "Effects of Base Oil Type, Oxidation Test Conditions and Phenolic Structure on the Detection and Magnitude of Hindered Phenol/Diphenylamine Synergism," Lubrication Engineering, Vol. 55, No. 1, Jan. 99, pp. 11-20. [23] Pike, R. and Conway-Jones, J. M., "Friction and Wear of Sliding Bearings," ASM Handbook, Volume 18, Friction, Lubrication, and Wear Technology, 1992, ASM International, Materials Park, OH, p. 519. [24] Blau, P. J., "Glossary pf Terms," ASM Handbook, Friction, Lubrication, and Wear Technology, Volume 18, 1992, ASM International, Materials Park, OH, p. 11. [25] Michael, P. W., Herzog, S. N., and Marougy, T. E., "Fluid Viscosity Selection Criteria for Hydraulic Pumps and Motors" Proceedings of the 48th National Conference on Fluid Power, Chicago, National Fluid Power Association, Milwaukee, WI, 2000, pp. 313-325. [26] ISO 3448: Industrial Liquid Lubricants-ISO Viscosity Classification, International Organization for Standardization, Geneva, 1992. [27] Michael, P.W., "Lubrication Basics," Plant Services, Vol. 14, No. 5, May 1993, pp. 19-22. [28] Silva, G., "Wear Generation in Hydraulic Pumps," presented at the SAE International Off Highway Congress, Milwaukee, WI, 1990, Paper 901679, Society for Automotive Engineers, Warrendale, PA. [29] Voitik, R. M., "Realizing Bench Test Solutions to Field Tribology Problems Utilizing Tribological Aspect Numbers," Tribol-
ogy - Wear Test Selection for Design and Application, ASTM STP 1199, A. W. Ruff and R. G. Bayers, Eds., ASTM International, West Conshohocken, PA, 1993, pp. 45-59. [30] Hogmak, S. and Jacobson, S., "Hints and Guidelines for Tribotesting and Evaluation," Lubrication Engineering, Vol. 48, No. 5, May 1992 p. 401. [31] ASTM Round Robin Test Program RR: D02-1152, ASTM International, West Conshohocken, PA, 1994. [32] Xie, L., Bishop, Jr., R. J., and Totten, G. E., "Bench and P u m p Testing of Hydraulic Fluids," H a n d b o o k of Hydraulic Fluid Technology, G. E. Totten, Ed., Marcel Dekker, NY, 2000, p. 526. [33] Perez, J. M., Klaus, E. E., and Hansen, R. C , "Comparative Evaluation of Several Hydraulic Fluids in Operational Equipment, A Full-scale Pump Stand Test and the Four-Ball Wear Tester. Part II - Phosphate Esters, Glycols and Mineral Oils," Lubrication Engineering, Vol. 46, No. 4, April 1990, p. 249. [34] Perez, J. M., Weller, D. E., and Duda, J. L., "Sequential Four-Ball Study of Some Lubricating Oils," Lubrication Engineering, Vol. 55, No. 9, 1994, pp. 28-32. [35] Reichel, J., "Importance of Mechanical Testing of Hydraulic Fluids," Tribology of HydrauUc P u m p Testing, ASTM STP 1310, George E. Totten, Gary H. Kling, and Donald J. Smolenski, Eds., ASTM International, West Conshohocken, PA, 1996. [36] Xie, L., Bishop, Jr., R. J., and Totten, G. E., "Bench and P u m p Testing of Hydraulic Fluids," H a n d b o o k of Hydraulic Fluid Technology, G. E. Totten, Ed., Marcel Dekker, NY, 2000, p. 526. [37] Denison Test E q u i p e m e n t and Instructions, Consigne TP30283, Denison Hydraulics, Marysville, OH, 1999. [38] Kling, G. H., Totten, G. E., and Ashraf, A., "Strategies for Developing Performance Standards for Alternative Hydraulic Fluids," in Lubricants for Off-Highway Applications, SAE SP-1553, Society for Automotive Engineers, Warrendale, PA, 2000, pp. 1-9. [39] Ohkawa, S., Konishi, A., Hatano, H., Ishihama, K., Tanaka, K., and Iwamura, M. "Oxidation and Corrosion Characteristics of Vegetable-Base Biodegradable Hydraulic Oil," SAE Technical Paper Series, Paper N u m b e r 951038, Society for Automotive Engineers, Warrendale, PA, 1995. [40] Melief, H. M., Totten, G. E., and Bishop, R. J., "Overview of the Proposed Rexroth High-Pressure Piston P u m p Testing Procedure for Hydraulic Fluid Qualification," Fluid Power for OffHighway Applications, SAE SP-1380, Society for Automotive Engineers, Warrendale, PA, 1998. [41] "Sundstrand Water Stability Test Procedure," Sundstrand Bulletin No. 9658, Sauer-Danfoss Corp., Ames, lA. [42] Hodges, P. K.B., Hydraulic Fluids, John Wiley, NY, 1996, p. 116. [43] Karl Fischer Applications, Mettler-Toledo AG., CH-8606 Griefensee, Switzerland, 1992. [44] Lloyd, B. J., "Water Water Everywhere," Lubes-N-Greases, Vol.11, No. 3, 1997, pp. 44-53. [45] Tsong-Dsu, L. and. Mansfield, J., "Effect of Contamination on the Water Separability of Steam Turbine Oils," Lubrication Engineering, Vol. 51, No. 1, 1995, pp. 81-85. [46] Barber, A. R. and Perez, R. J., Air Release Properties of Hydraulic Fluids, The Lubrizol Corporation, Wickliffe, OH. [47] Totten, G. E., Sun, Y. H., Bishop, R. J., and Lin, X., "Hydraulic System Cavitation: A Review," SAE 982036, Society for Automotive Engineers, Warrendale, PA, 1998. [48] Brew, D., Khonsari, M., and Ball, J. H.,"Current Research in Cavitating Fluid Films," STLE Special Publication SP-28, Society of Tribologists and Lubrication Engineers, Park Ridge, IL, 1990, pp. 63-65. [49] "Foaming and Air Entrainment in Lubrication and Hydraulic Systems," Mobil Technical Bulletin, Mobil Oil, Fairfax, VA, 1989. [50] HVLP HydrauUc Oils, Minimum Requirements, DIN 51524 Part II, Deutsche Norm, Beuth Verlag GmbH, BerUn, 1990. [51] Denison Fluid Standard HF-0: Hydraulic Fluid-For Use in Axial
CHAPTER 13: HYDRAULIC FLUIDS 381
[52] [53] [54]
[55]
[56] [57]
[58] [59]
[60]
[61]
[62]
[63] [64]
[65]
[66]
Piston Pumps and Vane Pumps in Severe Duty Applications, Denison Hydraulics, Marysville, OH, 1995. Claxon, P. D., "Aeration of Petroleum Based Steam Turbine Oils," Tribology, Februaiy 1972, pp. 8-13. Hatton, D. R., "Some Practical Aspects of Turbine Lubrication," The Canadian Lubrication Journal, Vol. 4, No. 1, 1984, p. 4. Rhee, I., Velz, C , and Von Bemewitz, K., Evaluation of Environmentally Acceptable Hydraulic Fluids, Technical Report No. 13640, U.S. Army Tank-Automotive Command, Research Development and Engineering Center, Warren, MI, March 1995. Rizvi, S. Q. A., "Lubricant Additives and Their Functions," ASM Handbook 10th ed., Vol. 18, Friction, Lubrication, and Wear Technology, ASM International, Materials Park, OH, 1992, pp. 98-112. Available from ASTM International Headquarters, 100 B a r r Harbor Drive, PO Box C700, West Conshohocken, PA. F a i n m a n , M. Z. and Hiltner, L. G., "Compatibility of Elastomeric Seals and Fluids in Hydraulic Systems," Lubrication Engineering, Vol. 37, May 1980, pp. 132-137. Ashby, D. M., "O-ring Specifications - Some Important Considerations," Hydraulics & Pneumatics, August 1999, pp. 53, 54, 90. Leslie, R. L. Sculthorpe, H. J., "Hydraulic Fluids Compatible with Metalworking Fluids," Lubrication Engineering, Vol. 28, May 1970, pp. 165-167. Carpsjo, C , Proceedings of International Symposium on Performance Testing of Hydraulic Fluids, Institute of Petroleum, London, 3-6 Oct 1978. Stambaugh, R. L. a n d K o p k o , R. J. "Behavior of Non-Newtonian Lubricants in High Shear Rate Applications," SAE Transactions 82, Paper 730487, Society of Automotive Engineers, Warrendale, PA, 1973. Stambaugh, R. L., Kopko, R. J., and Roland, T. F., "Hydraulic P u m p Performance - A Basis for Fluid Viscosity Classification," SAE International Off Highway Congress, Milwaukee, WI, Paper 901633, Society of Automotive Engineers, Warrendale, PA, 1990. Hydraulic Fluids Information, Denison Corp., Marysville, OH, 1995, p. 2. Rizvi, S. Q. A., "Lubricant Additives and Their Functions," ASM Handbook Volume 18 Friction, Lubrication, and Wear Technology, ASM International, Materials Park, OH, 1992, p. 106. Sharma, S. K., Snyder Jr., C. E., Gschwender, L. J., Lang J. C , and Schreiber, B. F., "Stuck Servovalves in Aircraft Hydraulic Systems," Lubrication Engineering, Vol. 55, No. 7, July 1999. Michael, P. W. and Webb, S., "Future Fluids - What's Coming Down the Hydraulic Line," OEM Off-Highway, January 1998, pp. 34-36.
[67] Lubrizol Ready Reference for Lubricant and Fuel Performance, The Lubrizol Corporation, Wickliffe, OH, 1998, p. 111. [68] Colver, R., Chek-Chart Publications, Simon & Schuster, NY, 1995, p. 45. [69] ISO 6743-4: Lubricants, Industrial Oils and Related Products (Class L) - Classification - Part 4: Family H (hydraulic systems). International Organization for Standardization, Geneva, 1999. [70] NFPA/T2.13.1: R3-1997, R e c o m m e n d e d Practice - Hydraulic Fluid Power - Use of Fire Resistant Fluids In Industrial Systems, NFPA, Milwaukee, WI, 1997. [71] ISO 7745: Hydraulic Fluid Power - Fire Resistant (FR) Fluids Guidelines for Use, International Organization for Standardization, Geneva, 1999. [72] Vickers Guide to Alternative Fluids, Eaton Corp., Southfield, MI, November 1992. [73] Totten, G. E. and Sun, Y., "Water-Glycol Hydraulic Fluids," Handbook of Hydraulic Fluid Technology, G. E. Totten, Ed., Marcel Dekker, NY, 2000, pp. 917-982. [74] NFPA/T2.13.1: R3-1997, Recommended Practice - Hydraulic Fluid Power - Use of Fire Resistant Fluids in Industrial Systems, NFPA, Milwaukee, WI, 1997. [75] Phillips, W. D., "Phosphate Ester Hydraulic Fluids," in Handbook of Hydraulic Fluid Technology G. E. Totten, Ed., Marcel Dekker, NY, 2000, pp. 1025-1027. [76] Parker O-Ring H a n d b o o k Y2000 Edition, Parker Hannifin Corp., Cleveland, OH, 1999, pp. 3-18. [77] O'Connor, J. and Boyd, J., Standard Handbook of Lubrication Engineering, McGraw-Hill, NY, 1968, pp. 2-16. [78] MIL-H-5606G Military Specification: Hydraulic Fluid, Petroleum Base, Aircraft, Missile and Ordinance, NATO code n u m b e r H-515, U.S. Department of Defense, Fort Belvoir, VA, 1994. [79] Parker O-Ring H a n d b o o k Y2000 Edition, Parker Hannifin Corp., Cleveland, OH, 1999, pp. 3-18. [80] Phillips, W. D., "Phosphate Ester Hydraulic Fluids," Handbook of Hydraulic Fluid Technology, G. E. Totten, Ed., Marcel Dekker, NY, 2000, pp. 1025-1027. [81] Gere, R. A. and Hazelton, R. A., "Polyol Ester Fluids," Handbook of Hydraulic Fluid Technology, G. E. Totten, Ed., Marcel Dekker, NY, 2000, pp. 983-1022. [82] Hohn, B. R., Michaelis, K., and Dobereiner, R., "Load Cjirrying Capacity of Fast Biodegradable Gear Lubricants," Lubrication Engineering, 1999, Vol. 55, p. 37. [83] Bartz, W. J., Environmentally Acceptable Hydraulic Fluids, Technische Akademie Esslingen, Ostfildern, Germany.
MNL37-EB/Jun. 2003
Compressor Lubricants Desh Garg, ^ George E. Totten, ^ and Glenn M. Webster^
Charle's Law states that at a constant pressure, the volume of a gas increases in proportion to the temperature:
COMPRESSORS ARE VITALLY IMPORTANT IN MANY INDUSTRIAL TECH-
NOLOGIES. For example, compressors are used in nearly every industry including steel, automotive, petroleum, mining, food, gas production, and storage and energy conversion [1]. The purpose of the compressor lubricant is to reduce friction and wear of the working parts of a compressor such as bearings, gears, and pistons; reduce internal leakage; and if the oil is compressed in the compression zone, to provide heat transfer to reduce the temperature of the gas being compressed [2]. In addition, a properly formulated compressor lubricant should provide corrosion protection and be sufficiently stable to minimize the potential for deposit formation on hot surfaces within the system [3]. The following topics will be discussed in this chapter: a basic tutorial on gas compression, the classification, operation and lubrication of typical gas compressors, lubricant types and classifications, solubility of common gases, and a review of recommended compressor lubricant testing. The chapter will not discuss refrigeration compressors applications. See Chapter 15.
II V2
T2
In addition, if the temperature of a gas increases as the pressure increases when the volume is held constant then Amonton's Law states that: Pi_ P2
T2
For these calculations, all temperatures are in reference to absolute zero. Therefore, if temperature in degrees Fahrenheit (°F) is used, absolute temperature in degrees Rankine (°R) is calculated from: "Rankine = °F 4- 460 Similarly, if the temperature is in degrees Celsius (°C), the absolute temperature in degrees Kelvin is calculated from: "Kelvin = "C + 273
DISCUSSION
Charle's Law and Boyle's Law are combined to form the wellknown Ideal Gas Law which states:
Gas Laws
PxV, P2V2 Ty T2 Avogadro's Law states that the equal volumes of gases at the same temperature and pressure contain the same number of molecules:
The objective of this section is to provide a basic background of the behavior of gases with respect to pressure, temperature, and volume. For example, Boyle's Law states that at a constant temperature, the product of a pressure and volume of a gas is constant:
PV = nRT
PiFi = P2V2 When performing these calculations, the reference value used to determine the pressure must be indicated. If the reference value is a vacuum, then the pressure is absolute {Pa) However, if the reference level is atmospheric pressure {Patm), then it is called gauge {Pg) pressure. They are related by:
Where n is the number of moles, R is the so-called gas constant that is selected to be consistent with the units of temperature, pressure, and volume used in the calculation (see Table 1). Dalton's Law states that the total pressure (Py) of a mixture is the sum of the partial pressures of the constituent gases (a,b,c, ) in the mixture: PT = Pa + Pb + Pc+
Absolute pressures must be used for the gas law relationships to be discussed here.
Similarly, the total volume of a gas is equal to the sum of the partial volumes of the constituent gases (Amagat's Law): VT=Va + Vb+V,+ If the temperature of a gas decreases or if the pressure increases sufficiently, the gas will undergo a change of state to a liquid. Further decreases in temperature or increases in pressure will convert the liquid into a solid. If the tempera-
' Desh Garg Consulting, 14 Carlson Terrace, Flshkill, NY 12524. ^G.E. Totten & Associates, LLC, P.O. Box 30108, Seattle, WA 98103. 3 63 Rockledge Rd., Hartsdale, NY 10530.
383 Copyright'
2003 by A S I M International
www.astm.org
384 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK ture is increased indefinitely, a point will be reached where the gas can no longer be liquified by increasing pressure. The highest temperature at which a gas can be liquified by increasing pressure is called the critical temperature of the gas. The pressure required to liquefy a gas at the critical temperature is called the critical pressure of the gas. A summary of physical constants for selected gases is provided in Table 2. When pressure, temperature, and volume variation of a gas follows the ideal gas law, it is referred to as an ideal gas. However, as the pressure increases, the behavior of a gas deviates from that predicted by the ideal gas law. This is due to the compressibility of the gas and is accounted for in the ideal gas calculation by using the compressibility factor (Z); PiV, TiZ,
PV curve as shown in Fig. 2 [4]. For isothermal compression, temperature is held constant during compression by removal of the heat of compression, and the work performed corresponds to: PrVr = P2V2
Adiahatic (isoentropic) compression occurs when there is no heat added or removed during compression: PiVf
Where k is the ratio of specific heats. Comparison of the work required for an isothermal process (ADEF) and an adiahatic process (ABEF) shows that less work is required for an isothermal process. However, an isothermal process is impossible to achieve, although compressors are designed for as much heat removal as possible. Similarly, adiahatic compression is also impossible since
P2V2 T2Z2
Values for the compressibility factor are obtained from reference charts called general compressibility charts such as those available in Ref. 4.
t
The Gas Compression Cycle PV Curves The product of pressure (P) X volume (V) is work. Work is equal to force X distance where pressure corresponds to force and volume corresponds to distance. The horizontal line shown in the PV curve of Fig. 1 corresponds to the distance, for example, the distance a piston moves and the vertical line corresponds to force on the cylinder of a piston, for example. The area under the curve is P X y and is equal to the work performed during the cycle.
Pd
I'".'"• .--' - V'. -'13
-J,-;
IJJ Isothermal versus Adiahatic Operation Theoretically, there are two ways that a positive displacement compressor can be operated; either isothermally or adiabatically. These modes of operation are illustrated using a
WORK
-h ;:v.
'"•^"/t-i-'i-r'
Ps
TABLE 1—Ideal gas law constants (R). 8.3143 8.3143 1.9872 82.054
P2V'2
I
VOLUME INCREASES
X l O V gs/deg mole joules/d eg mole cal/deg mole cc atm/deg mole
FIG. 1—Illustration of a PV diagram that d o e s not include clearance volume.
TABLE 2 --Physical constants of natural gas components.
Gas Methane Ethane Propane i-Butane n-Butane i-Pentane n-Pentane Hexane Carbon Dioxide Hydrogen Sulfide Nitrogen Oxygen
Chemical Formula CH4 C2H6 CjHg C4H10 C4H10 C5H12 C5H12
CeHn CO2 H2S N2 O2
Critical Temperature (°R) 344 550 666 735 766 830 846 915 548 673 227 278
Critical Pressure (psia) 673 708 617 529 531 483 489 440 1073 1306 492 732
Molecular Weight (g/mole) 16 30 44 58 58 72 72 86 44 34 28 32
Density @ 60°F, 14.7 psia Specific Gravity Air= 1.0 Lbs/ft^ 0.554 1.038 1.523 2.007 2.007 2.491 2.491 2.975 1.519 1.176 0.967 1.105
0.0424 0.0799 0.1180 0.1578 0.1581 0.190 0.190 0.227 0.1166 0.0897 0.0738 0.0843
Specific Heat @ 60°F, 14.7 psia Cp Cv BTU/lb/°F BTU/lb/°F 0.527 0.410 0.388 0.387 0.387 0.383 0.388 0.386 0.199 0.238 0.248 0.219
0.403 0.344 0.343 0.353 0.353 0.355 0.361 0.363 0.154 0.180 0.177 0.157
Cp/Cv 1.308 1.192 1.131 1.097 1.097 1.078 1.076 1.063 1.293 1.325 1.400 1.346
CHAPTER 14: COMPRESSOR ^E
AB - ADIABATIC AC - POLYTROPIC AD - ISOTHERMAL . \
[4]:
THEORETICAL NO CLEARANCE
Tjv = 100 - CiR^'") - 1)
0} W Ul DC
where R is the compression ratio which is defined as the absolute discharge pressure divided by the absolute inlet pressure of a compressor and C is the cylinder clearance (%). This is a theoretical value and equation must be modified to account for inefficiencies such as internal leakage, gas friction, pressure drops through valves, etc. This is done by introducing the factor "L."
Q.
F
..A
VOLUME
—•
FIG. 2—PV diagram illustrating theoretical compression cycles.
some heat is always added or emitted. Actual compression is referred to a polytropic cycle: Where n is experimentally determined for each type of compressor and usually not equal to k. Thermodynamically, isothermal and adiabatic processes are reversible but polytropic processes are irreversible, steady-state processes. The value of n may also be calculated from: (n-l)/n
Ti
385
Leeikage past valves and piston rings, Slight increase of gas volume due to heat rise from the warm cylinder. Theoretical volumetric efficiency (TJV) is calculated from
D CB
UJ
LUBRICANTS
7)^ = 100 - C(i?^* - 1) - L The value of L is variable depending on the compressor, lubricant and the gas. For a moderate pressure air compressor with a petroleum oil lubricant, the value of L may be approximately 5%. Power Requirement To properly size a compressor and its components, it is necessciry to determine the amount of power required to drive the system. To do this, it is necessary to determine the amount of brake horsepower required to compress a given volume of gas from the incoming inlet pressure to the desired discharge pressure [5]. Brake horsepower is defined as the ideal isoentropic (theoretical) horsepower plus any fluid (valve, fluid flow, and other leakage) or mechanical friction losses [5]. Theoretical horsepower may be calculated from:
Pi
Gas Compression Cycle Consider the situation where a gas is compressed in a piston cylinder from the inlet pressure, Ps, to the discharge pressure, Pd, along the lone 1-2 in Fig. 3a. Since it is impossible to discharge all of the gas due to the volume of space not covered by the piston stroke, there will be a residual value referred to as clearance volume. This is typically the area between the cylinder and the head of the piston illustrated in Fig. 3a. Typically, clearance volumes range from 4-20%. Figure 3fo illustrates the completion of the compression stroke along the path 2-3. When the piston reaches point 3, the discharge valve closes and the piston undergoes the expansion stroke 3-4 (Fig. 3c) until the pressure drops below the inlet pressure at point 4. At point 4, the inlet valve opens and the gas fills the cylinder as shown in Fig. 3>d and the process is repeated. Volumetric Efficiency Piston displacement represented by the line 5-1 on the PV curve shown in Fig, 4. The actual capacity is less than that represented by the piston displacement, line 4-1. The ratio of the actual capacity to the total displacement is referred to as the volumetric efficiency. The volumetric efficiency is always less than that that derived theoretically because: • The re-expansion of the gas trapped in the cylinder clearances, • Entrance losses due to the pressure drop at the inlet.
(B)
FIG. 3—The compression cycle. A. Compression Strol
386 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK Gas Compressors Compressors are used in many industrial operations to compress a variety of gases, such as natural gas, ethylene, air, and ammonia. There are several major compressor designs that may be selected to suit a variety of application needs. The selection of a particular type of compressor depends on such application requirements as the gas being compressed, flow rates, pressures involved, and downstream considerations. In this section, the classification of compressor types will be discussed. This will be followed by an overview of the most commonly encountered compressor types and their lubrication demands. Compressor types can be classified into two basic categories as illustrated in Fig. 6 [7 ] reciprocating and rotary. Reciprocating compressors are used for compressing natural gases and other process gases when desired pressures are high and gas flow rates are relatively low. They are also used for compressing air. Reciprocating
Piston Displacement-
TABLE 3—Effect of cylinder clearance and valve area on compression ratio.
FIG. 4—Illustration of the determination of volumetric efficiency from the PV diagram using piston displacement and actual capacity
Compression Ratio
Very high High Moderate Low 144 33000 \ k - 1
Compressors
Reciprocating compressors compress gas by physiccilly reducing the volume of gas contained in a cylinder using a piston. As the gas volume is decreased, there is a corresponding increasing in its pressure (as indicated by the gas laws discussed earlier). Therefore, this type of compressor is also referred to as a positive displacement type. An illustration of a reciprocating
More Important Factor
10-30 (vacuum pumps) 8-10 max. 5 max. 2 or less
Clearance Clearance principally Balanced Valving
(k-l/k)
R - 1
(PsVs)
Where: 144 is in'^ per ft.^, 33000 foot-pounds per minute = 1 horse power, k = k-value of the gas, Ps = inlet pressure in PSIA, Vs = inlet volume in ft^/min, and R is the compression ratio. The relative effect of cylinder clearance and valving (cylinder valve area) on the compression ratio is shown in Table 3. Compressors may also be characterized by specific power consumption (Pspec) [6]:
300 O
^spec = power consumption (kW)/volume flow (m^/min) For example, Pspec for a compressor lubricated with a poly a - olefin or synthetic ester may exhibit a 1-3% advantage relative to a petroleum oil [6]. Temperature As indicated by the gas laws, gas compression occurs with a rise in temperature. The greater the amount of gas compression, the higher the final temperature. If high discharge pressures are required, this process is conducted with cooling in two or more stages, which improves efficiency and reduces power consumption [9]. The temperatures actually encountered in these compression processes must be considered in lubricant selection since it affects fluid viscosity, oxidation, and potential deposit formation. An illustration of the temperature rise of air due to compression in a Broomwade 2050H reciprocating compressor is shown in Fig. 5 [8].
3
2 200 E
100 1 50
1 100
1 150
200
Air Delivery Pressure (psi) FIG. 5—Air temperature rise versus delivery pressure for a Broomwade2050H reciprocating compressor
CHAPTER 14: COMPRESSOR compressor is provided in Fig. 7 [7 ]. Reciprocating compressors are typically "once-through" processes. That is, gas compression and lubricant separation occurs in a single pass. Reciprocating compressors may be further classified as single acting or double acting. Single acting compressors, also classified as automotive compressors or trunk piston units [3], compress gas on one side of the piston, in one direction. Double acting compressors compress gas on both sides of the piston, bidirectional. Figure 7 is an illustration of a typical double-acting reciprocating compressor, which consists of a crankshaft, connecting rods, cross-heads, piston rods, pistons, cylinders and liners, and piston rings, which are all mounted onto a suitable frame. As the crankshaft turns, the connecting rod converts the rotation to a reciprocating linear motion via the cross-head. The cross-head transfers this motion to the piston via the piston rod. The gas contained in the cylinder is compressed and discharged through the discharge valve. The piston rod packing, also known as cylinder packing, seals the high-pressure gas from the low-pressure crankcase.
LUBRICANTS
To consider the lubrication process, it is convenient to divide the parts that need to be lubricated into two categories; namely cylinder parts and running parts. Cylinder parts include pistons, piston rings, cylinder liners, cylinder packing, and valves. All parts associated with the driving end (i.e., the crankcase end), cross-head guides, main bearing and wristpin, crankpin, and cross-head pin bearings are running parts. An equation recommended by Scales for estimating the amount of oil to inject into a cylinder for lubrication is [3]: Q
BXSXNX
62.8
10 000 000
Where: B is the bore diameter size (in.), S is the stroke (in.), N is the rotational speed (rpm), and Q is the usage rate in quarts per 24-hour day. The constant 62.8 is used to convert the numerator into the amount of cylinder bore area swept clean per day and is calculated from ITT X 10. The value TT (3.14) is multiplied by 2 (6.28) since the pump cycle is in two directions. The 6.28 value is multiplied by 10 because it is as-
COMPRESSOR TYPES RECIPROCATING
ROTATING
i
FIG. 6—Classification of compressor types. Reprinted with permission from Universal Compression, Inc.
Cylinder Liner
Crosshead Guide
Crosshead
Piston Rod pamnq
Water Jacketed Cylinder
\
Variable VOIURK
Clearance Pocket
FE 6501
Piston Rod Valve
Frame
Crankshaft
387
Connecting
Counter W«ght
FIG. 7—Cross section of a reciprocating compressor. Reprinted with permission from Universal Compression, Inc.
388 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK sumed that ten cycles are required for the total cylinder area to be swept clean. The 10000000 value converts an average assumed film thickness in microns to inches (in.) and also converts in^/day to quarts/day. The lubricant is then generally fed directly to the cylinders and packings using a mechanical p u m p and lubricator arrangement. Single-acting machines, which are usually open to the crankcase, usually utilize splash lubrication for cylinder lubrication. Compressor valves are lubricated from the atomized gas-lubricant in the system. Compared with cylinder part lubrication, the lubrication of running parts is typically m u c h simpler because there is no contact with the gas. The required viscosity grade is specified by the equipment manufacturer. Since gas temperature increases with increasing pressure, if heat is not removed, the lubricant will be exposed to high temperatures and undergo severe decomposition. Therefore, compressor cylinders are equipped with cooling jackets. The coolant is usually water or a water-glycol refrigerant. One of the most important roles of the compressor cylinder lubricant is as a coolant. Although the same lubricant can be used to cool both the cylinder and the running parts, there are many cases where different lubricants are used because the cylinder lubricant is exposed to compressed gas at high temperatures. Table 4 provides a comparison of illustrative compressor operation temperatures [15]. Therefore, the lubricant should also exhibit thermal and oxidative stability. Rotary
Compressors
Rotary compressors are further classified as positive displacement or djTiamic type compressors. A positive displacement type of compressor utilizes gas volume reduction to increase gas pressure. Examples of this type of compressor include: rotary screw, lobe, and vane compressors that are illustrated in Fig. 8 [10,12,14], Fig. 9 [9] and Fig. 10 [9]. The rotary screw compressor illustrated in Fig. 8 consists of two intermeshing "screws" or rotors that trap gas between the rotors and the compressor case [13]. One rotor is the male rotor and is driven by the motor. The outer rotor is the female rotor and is driven by the male rotor. Both rotors are encased in a housing provided with gas inlet and outlet ports. Gas is drawn through the inlet port into the voids between the rotors. As the rotors rotate, the volume of trapped gas is successively reduced and compressed by the rotors coming into mesh. These compressors are available as dry or wet (oilflooded) screw types. For the dry-screw type, the rotors run inside of a stator without a lubricant (or coolant). The heat of compression is removed outside of the compressor, limiting it to a single-stage operation. For oil-flooded screw type compressors, the lubricant is injected into the gas that is trapped inside of the stator. The functions of the lubricant include cooling, sealing, and lubrication. The gas is removed from the compressed gas-lubricant mixture in a separator. As opposed to reciprocating compressors, which are once through processes, rotary compressors, such as the screw compressor, continuously recirculate (1-8 times a min) the lubricant-gas mixture to facilitate gas cooling and separation [13]. In a rotary screw compressor, the lubricant is injected into the compressor housing. The rotors are therefore exposed to a mixture of the gas and lubricant. In addition to providing a
thin film on the rotors to prevent metal-to-metal contact, the lubricant also provides a sealing function to prevent gas recompression. It also serves as a coolant by removing heat that is generated during gas compression. For example, for rotary screw air compressors, the air discharge temperature may be 80-110°C (180-230°F), rapid oxidation is favored due to turbulent mixing of the air and lubricant [15]. In addition to these functions, the bearings at the inlet and outlet of the compressor must be lubricated. With rotary screw compressors, the lubricant is in contact with the gas being compressed at high temperatures and experiences high shearing forces between the intermeshing rotors. This creates very demanding use-conditions for the lubricant with respect to both lubrication and stability.
TABLE 4—Comparison of the operational temperature ranges for different compressor types. Operating Temperature Range Compressor Lubrication Problems Deposits block filter, Rotary screw 80-115°C(180-240°F) separator elements Varnish on bearings Deposits block filters Vane 80-150°C (180-300°F) Vane wear increases filter deposits and varnishing Single stage to: Varnish and carbon Reciprocating 270°C (500°F) deposits on exhaust (and inlet) valves Multi-stage to: Piston ring wear 160-210°C increases leakage and deposits (325^25°F)
FIG. 8—Cross section of a rotary screw compressor.
CHAPTER
.y Impeller^
Labyrinth Seal-, Oil Pump
14: COMPRESSOR
LUBRICANTS
389
Impeller
•'Roller Bearing
Timing Gears
(a)
(b) FIG. 9—Cross section of a rotary straight lobe compressor.
i INLET
DISCHARGE
-THRUST PIN SLOT VANE PACKING STRIP
ROTOR
FIG. 10—Cross section of a rotary sliding vane compressor.
A simplified diagram for lubricant flow in a typical rotary screw compressor is shown in Fig. 11 [12]. The lubricant and gas mixture from the compressor discharge line goes into a gas/lubricant separator where the compressed gas is separated from the lubricant. After separation, the lubricant is cooled and filtered, then p u m p e d back into the compressor housing and bearings. A schematic diagram for rotary lobe compressor is provided in Fig. 9 [9]. The principle of operation is analogous to the rotary screw compressor. As the lobe impellers rotate, gas is trapped between the lobe impellers and the compressor case where the gas is pressurized through the rotation of
lobes and then discharged. The bearings and timing gears are lubricated using a pressurized lubricating system. A rotary vane compressor schematically illustrated in Fig. 10 [9]. Rotary cane compressors consist of a rotor with multiple sliding vanes that are mounted eccentrically in a casing. As the rotor rotates, gas is drawn into areas of increasing volu m e (A) and discharged as compressed gas from areas of small volume (B). As with reciprocating compressors, lubrication of rotary vane compressors is also a once through operation. The lubricant is injected into the compressor casing and it exits with the compressed gas and is usually not recirculated. The lubricant provides a thin film between the compressor casing and the sliding vanes and also provides lubrication within the slots in the rotor for the vanes. The sliding motion of the vanes along the surface of the compressor housing requires a lubricant that can withstand the high pressures within the compressor system. A dynamic compressor, such as the centrifugal compressor shown in Fig. 12 [9], operates on a totally different principle. Energy from a set of blades rotating a high speed is transferred to a gas that is then discharged to a diffuser where the gas velocity is reduced and its kinetic energy converted to static pressure. One of the advantages of this type of compressor is the potential to handle large volumes of gases. In a centrifugal, the lubricant and gas do not come into contact with each other which is a major distinction from reciprocating, rotary screw and rotary vane compressors. The lubricant requirements are therefore simpler and usually a good rust and oxidation-inhibited oil will provide satisfactory lubrication of the bearings, gears and seals. Compressor Lubricants Performance
Demands
The choice of a compressor lubricant is dependent on: type a n d construction of the compressor, the gas being com-
390
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
LUBRICANT SCAVENCER LINE
-AIR DISCHARGE
FIG. 11—Oil-flooded rotary screw compressor lube system.
BACKPLATE DIFFUSER PACKING BOXLABRINTH SEAL SLEEVE BEARING
FIG. 12—Illustration of a centrifugal compressor.
pressed, the degree of compression and the final outlet temperature. Piston compressors provide the highest gas pressures and are among the most difficult from the standpoint of cylinder lubrication. Rotary compressors with final pressures below 1 MPa are less difficult to lubricate. Because of the potential for vane-on-ring contact, rotary vane compressors require the use of an antiwear oil [17], whereas an R & O oil is often sufficient for the crankcase splash lubrication
of a reciprocating compressor. Therefore, the selection of the proper compressor and application-dependent lubricant with the appropriate physical-chemical properties is vital to a successful process [2]. A classification procedure (ISO 6743-Part 3A) for compressor lubricants based on the type of equipment and operating conditions is provided in Table 5. Some of the most commonly reported oil-related service problems with compressors include [22]:
CHAPTER 14: COMPRESSOR LUBRICANTS
391
TABLE 5—ISO 6743-Part 3A: Family D-Compressor lubricant classification oil-lubricated air compressors. Particular Application Positive displacement air compressors with oil-lubricated compression chambers
More Specific Application Reciprocating (crosshead and trunk pistons or Rotary drip feed (vane)
Rotary oil-flooded (vane and screw compressors
Positive displacement air compressors with oil-free compression chambers
Dynamic compressors
Symbol ISO-L DAA
Typical Applications Light duty
Product Type and/or Performance Requirements Intermittent operation
Continuous operation
DAB
Medium duty
Intermittent operation
DAC
Heavy duty
Intermittent or continuous operation
DAG
Light duty
DAH
Medium duty
DAJ
Heavy duty
Liquid ring compressors and w a t e r flooded vane and screw compressors Reciprocating oilfree compressors Rotary oil-free compressors Radial and axial turbo compressors
'Under favorable conditions, light-duty oil may be used at discharge pressures higher than 800 kPa (8 bar).
Remarks Sufficient time to allow cooling between periods of operation Compressor stop and start Variable discharge capacity a. discharge pressure >10^kPa (lobar) discharge temperature >160°C stage pressure ratio >3:1 or b. discharge pressure >10^ kPa (lobar) discharge temperature >140°C stage pressure ratio >3:1 Sufficient time to allow cooling between periods of operation a. discharge pressure >10^kPa (lobar) discharge temperature >160°C or c. discharge pressure > 10^ kPa (lobar) discharge temperature >140''C or stage pressure ratio >3:1 As for "medium," when conditions a, b, or c above are fulfilled and where severe coke formation in a discharge system might be anticipated as a result of previous experience with a medium-duty oil. Air and air/oil discharge temperature <90°C Discharge pressure <800 k P a ' ( < 8 bar) Air and air/oil discharge temperature <100°C Discharge pressure 800-1500 kPa (8-15 bar) or Air and air/oil discharge temperature 100-110°C Discharge pressure <800 k P a ' ( < 8 bar) Air and air/oil discharge temperature > 100°C Discharge pressure <800 k P a ' ( < 8 bar) or Air and air/oil discharge temperature f l O O X Discharge pressure 800-1500 k P a ' ( 8 - 1 5 bar) or Discharge pressure > 1500 b a r (>15 bar) Lubricants suitable for gears, bearings and transmissions
Lubricants suitable for bearings and gears
392
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
• • • •
Increase in oil viscosity and total acidity Copper corrosion (oil turns green) Sludge deposits Substantial oil entrainment in discharge gas (air) due to decreased efficiency of the demister element • Oil strainer plugging • Bearing failure For successful operation, a compressor oil must exhibit: oxidation resistance, wide operating temperature range (high flash point, low pour point, high viscosity index), low volatility, superior jmti-wear performance, good demulsibility, adequate corrosion resistance, thermal/oxidative stability, rust and corrosion inhibition, hydrolytic stability, material compatibility, non-sludging performance, minimal oil loss to the system, non-foaming potential behavior, and m u s t be non-toxic [8,10,24]. The successful development of a compressor lubricant will depend on how well the oil meets these technical requirements. Testing procedures that may be used to develop and maintain compressor lubricants will be discussed here. Lubricant
Basestocks
Mineral Oils—Although it is more c o m m o n for naphthenic mineral oils to be used as a basestock for compressor fluid formulation, paraffinic oils may be used as well [15,23,26]. The composition of two illustrative examples is provided in Table 6 [15]. In either case, the basestock is formulated with additives to provide the desired stabilization to oxidation and sludge formation, rust and corrosion control, foam control, demulsibility, and antiwear behavior [23,26]. The response of the base oil to these additives is dependent on the composition of the oil [23]. Although the performance of these oils is generally good, there is a temperature and pressure limit, beyond which a mineral oil-based compressor lubricant should not be used. This is particularly critical for air compressors due to the relatively poor oxidation resistance and sludge-forming characteristics of mineral oils in the presence of air at high temperatures. Figure 13 illustrates the guidelines recommended by Matthews, et al. for the use of mineral oil lubricants for air compressors [23]. The oxidative stability and sludge (coke) forming properties exhibit a significant impact on both the operation and the explosion and fire h a z a r d of compressor oil in use [20,27,28]. As with other basestocks, mineral oil oxidation as represented by the autoignition temperature is further exacerbated by the presence of corrosion metals such as iron oxide as illustrated in Fig. 14 [1]. The poor fire-resistance, oxidative instability and other deficiencies of mineral oil compressor lubricants often limit their use. In these cases, a synthetic oil-based lubricant may be selected. However, one approach to this limitation of the use of mineral oil-based compressor lubricants is to further refine the mineral basestock using a two-stage hydrotreating process. In this process [15] and others that have been reported [12,14], polycycloparaffins are broken u p and aromatic compounds are converted into saturated cycloparaffins. In addition, sulfur, nitrogen, and oxygen-containing compounds are completely decomposed. Table 6 provides a comparative illustration of the chemical composition of a two-stage hydrotreating process to a conventionally refined paraffinic and a naphthenic oil [15]. Data reported by Cohen
TABLE 6—T3q5ical composition of mineral oil basestock used for a 32 centistoke compressor oil lubricant formulation. Solvent Refined Paraffinic ISO viscosity grade Ave. molecular weight Total saturates (%) Paraffins (iso+ normal) Cycloparaffins (total) Monocyclo Dicyclo Tricyclo Tetracyclo Pentacyclo Hexacyclo Total aromatics (%) Monoaromatics Diaromatics Triaromatics Tetraaromatics Pentaaromatics Total thiophenes (%) Total polar compounds (%) S, ppm N, ppm
32 395 84.14 17.74 66.40 (24.46) (15.24) (9.10) (10.58) (4.67) (2.35) 14.37 (10.49) (2.60) (0.48) (0.13) (0.67) 0.19 1.30 500 30
Solvent Refined Naphthenic 32 330 58.22 12.22 45.00 (15.69) (12.82) (8.21) (6.01) (2.45) (0.82) 31.06 (12.28) (12.58) (2.72) (2.28) (1.20) 9.09 1.64 13,400 160
Two-Stage Hydrotreated 32 405 99.97 32.60 67.37 (30.81) (19.52) (8.87) (4.75) (2.56) (0.86) 0.03 (0.03) Nil Nil Nil Nil Nil Nil 2 1
showed a significant improvement in oxidative stability when evaluated in an air compressor test relative to normally refined basestocks [15]. Synthetic Basestocks—In addition to exhibiting improved oxidative stability relative to mineral oil, synthetic basestocks also are used in compressing gases that may react with conventional mineral oils. These reactive gases include: methyl chloride, sulfur dioxide, hydrochloric acid, ammonia, and other process gases such as carbon dioxide (which may be c o n t a m i n a t e d with acids, hydrogen sulfide, or other oxidants), chlorosiloxanes, chlorinated hydrocarbons, and gases containing trace mineral acids. In many chemical plant processes, mineral oil based lubricants are not allowed due to potential catalyst contamination [14]. In these applications, an appropriate synthetic oil-based lubricant must be selected. In this section, a brief overview of the more commonly encountered synthetic lubricant basestocks will be provided. The basestocks to be discussed here include: polyalkylene glycol (PAG), ester, phosphate ester, polyalphaolefin (PAO), silicones, fluorocarbons, and alkylbenzenes. Compressor fluids formulated from these basestocks are the most commonly encountered in the industry [16]. Polyalkylene Glycol-PAG—Compressor lubricants formulated with polyalkylene glycols are being used increasingly as compressor lubricants, particularly in hydrocarbon gas compression applications because of their resistance to hydrocarbon dilution [12,19]. The poor solubility of hydrocarbon gases in PAGs also assists in preventing the washing away of the lubricant that would result in metal-to-metal contact (dry running) [16]. However, at high operating pressures, hydrocarbon gases may continue to condense, leading to a build-up of a hydrocarbon-rich phase in the lubricant separation tank. This problem is easily remedied even though the lubricanthydrocarbon phases do not readily separate by gravity. If such problems do occur, the hydrocarbon gas may be re-
CHAPTER 14: COMPRESSOR
LUBRICANTS
393
350r
Aryl Ester Based Product
..Operating Envelope for Many Reciprocating Air Compressors
100
200
300
PRESSURE (bar) FIG. 13—Recommended guideline for the use of mineral oil lubricants for air compressors.
Phosphate Ester — - — — » _ ^ Phosphate Ester _ w/ Iron Oxides Polyol Ester Diester, 100ISO Polyalphaolefin Mineral Oil #1 Mineral Oil #2 Mineral Oil #2 w/ Iron Oxides 90 120 150 180 PRESSURE (PSIG) FIG. 14—Autoignition of various synthetic and mineral oils compared to discharge pressures and temperatures. A. single stage, B. two stage, and C. three stage.
moved using a flash still unit such as the one shown schematically in Fig. 15 [19]. PAG polymers exhibit excellent viscosity-temperature behavior (high viscosity index) as shown in Fig. 16 [ 14] and they may usually be used up to compression temperatures of 200°C [16] However, they have been reported to degrade at compression temperatures of 230°C which would make them unsuitable for high temperature air compressor applications [23]. Other reported advantages include low pour point, high flash point, and low vapor pressure. PAG-based compressor lubricants have been used with success in natural gas, nitrogen, carbon dioxide, hydrogen, and helium compression [29]. Ethylene compression for low-density polyethylene (up to 50000 psig) has also been reported [29]. Blends of PAGs with other basestocks have been reported to offer properties superior to either basestock itself. For ex-
ample, it has been reported that a polypropylene glycol with a pentaerythritol ester can be used for air compression in a rotary screw compressor (where the oil is recirculated) with excellent long service life [11]. Polyalphaolefin (PAO)—Polyalphaolefins are also used extensively as a synthetic basestock for compressor lubricant formulation. In addition to good viscosity properties relative to mineral oil, as illustrated in Fig 16, PAOs exhibit excellent thermal and oxidative stability, and are hydrolytically stable, as shown in Fig. 17 [19], and are not attacked by reactive gases such as ammonia [30]. These factors have led to their growth in usage in rotary screw compressor applications [12]. In some cases, compressor oils are used in the food industry and may come in contact with food. They may also be used to compress gasses that are used in food applications, such as CO2 [2]. For these applications, a food-grade compressor lu-
394
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK COMPRESSED -GAS DISCHARGE
Gas Refrigerant
r0cCOOLER
n
MOTOR OMPRESSOR 1 I LUBE Gas + Lube + i r SEPARATOR H.C. Condensibles Z
Condensibles
_F;
fe 3
(fn
Lube + H.C. Condensibles
Recycle Stream^<^ 1 H.C. Vapors IFILTER
SUCTION TANK
^
-Or
^
HEATER
FLASH DISTILLER
LUBE PUMP COOLER
FIG. 15—Flooded rotary screw gas compressor system equipped with a flash still unit.
bricant must be used, which may be a white mineral oil [2] or a food-grade PAO [25]. Figure 18 illustrates the substanticJ improvement in fluid lifetime for the food grade PAO compared to the white mineral oil used as a reference [25]. Esters—Ester basestocks used in compressor oil formulations include: diesters, polyols, phthalates, and trimellitates [21] Esters are often used as low-cost basestocks for compressor fluids used in air compressors because of their improved oxidative resistance as shown in Fig. 14 [1]. Furthermore, esters have been used increasingly as cylinder lubricants for reciprocating compressors due to their high-temperature stability and solvency properties [12]. The use of ester basestocks, such as diesters and polyol esters, results in minimal deposit formation on hot pistons and discharge valves [12]. Silicones—Silicones, including dimethyl, alkylmethyl, and fluorinated polymers are EIISO used to formulate compressor fluids. These polymers are relatively inert to almost all chemicals except pure oxygen, and other strong oxidizing agents. They are tj^Dically used to formulate compressor fluids used
100,000 20,000 (0 u E CO O
1,000 300
75 40 15 PAG PAO Semi-Synfhetic Mineral Oil
>
-30
0
50
100
150 200
250 300 350
TEMPERATURE (°F) -17.8
37.7
93.9
148.9
TEMPERATURE (°C) FIG. 16—Comparative viscosity-temperature relationships for different compressor lubricants.
100 90 80
9 3 X UJ
> UJ
in
Polyalphaolefin "Fluid
70 60
/C
50
DIester Fluid
403020 10-
0
•
'
•
500
•
Jt-L
1000
- 1 ^iOO
• • • • ' • • ' • _ ' ' • • • ' • • • ' '
2000
2500
3000
3500
WATER CONCENTRATION (PPM) FIG. 17—Comparison of the hydrolytic stability of a compressor lubricant formulated using a PAO and an ester basestock.
CHAPTER
"0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
WEEKS FIG. 18—Comparison of the oxidative stability of a food grade Poly(Alphaolefin) (PAO) with a hydrocarbon white oil.
for HCl and chlorine compression [14]. Some silicones have exhibited outstanding corrosion protection on metal surfaces and have even been used as a corrosion inhibiting additive for the formulation of PAO compressor fluids [14]. Dimethylsilicones exhibit excellent viscosity-temperature characteristics and are hydrolytically stable and, if used with an effective antiwear additive, exhibit acceptable lubricating characteristics and exhibit excellent oxidative stability [19]. Fluorinated Basestocks—There are three classes of fluorinated fluids that are used for lubricant formulation. These include: chlorofluorocarbons, perfluoroalkylpolyethers, and fluorosilicones. All are very expensive a n d are only used where their outstanding chemical inertness, even to liquid oxygen and chlorine, is demanded [19]. Phosphate Ssters—Phosphate esters are used primarily for their fire-resistance properties, particularly in the mining industry [20], relative to mineral oil as illustrated in Fig. 14 [1]. In addition, they exhibit high flash points, excellent resistance to aging, and minimal coke formation. They eire used in high-pressure multistage compressors and other applications. However, if the lubricant degrades, by-products with poor material compatibility may be formed. In some cases, it has been reported that there may be problems with boundary lubrication between aluminum and cast iron cylinders. Alkylbenzenes—As illustrated in Fig. 13, the thermal-oxidative stability of mineral oils, particularly solvent refined naphthenic oils, is somewhat limited, particularly in air compression operations. In such applications, it has been shown that selected, well-refined alkylbenzenes do not exhibit carbonaceous sludge formation in reciprocating compressors with final compression temperatures of 220°C and 0.8-1.0 MPa [20] Alkylbenzenes m a y also be used to formulate a m m o n i a compressor fluids although they do exhibit slightly greater ammonia solubility than paraffinic oils and PAOs [30]. However, vapor pressure is lower than solvent refined naphthenic oils, but greater than PAOs or hydrotrated oils. Alkylbenzenes also exhibit relatively poor viscosity-temperature properties. Gas Solubility
in the
Lubricant
The solubility of natural gas and other hydrocarbons is m u c h higher in petroleum oils. That is expected because both hy-
14: COMPRESSOR
LUBRICANTS
395
drocarbon gas and petroleum oils are very similar molecules consisting of primarily C—H bonds. The solubility of hydrocarbons is m u c h less in some synthetic lubricants, especially PAGs. This is because, in a typical PAG molecule, each third atom in the polymer backbone, is an oxygen atom, which makes it quite polar. Therefore, hydrocarbons are less soluble in PAGs. In reciprocating and rotary screw compressors, the gas being compressed and the lubricant come into contact with each other. Hydrocarbon gases are infinitely soluble in mineral oil-based compressor lubricants, and the solubility of hydrocarbon gases increases with increasing pressure at a constant temperature in a less compatible fluid such as a ISO 220 polypropylene glycol, as illustrated in Fig. 19 [14]. Figure 20 illustrates that increasing the temperature at a constant pressure will result in lower gas solubility. [14]. At some point, if the viscosity reduction of the compressor lubricant is sufficient, lubrication failure may result because of loss of hydrodynamic lubrication [2]. The solubility of various gases in lubricants has been measured and reported [8,14,29]. The solubility was measured in a fixed volume apparatus. A known a m o u n t of gas and lubricant was allowed to reach equilibrium at a given temperature. Gas solubility was calculated using the gas laws. The lubricant was stirred to facilitate equilibrium. Solubility of methane at pressures u p to 5000 psig is compared at 50°C for three lubricants: PAG, PAO and a petroleum oil in Fig. 21. The m e t h a n e gas solubility in PAG is roughly one half of that for a PAO and petroleum oil and that the solubility was nearly as high in the PAO as in the petroleum oil. Gas solubility exhibits a significant effect on lubricant viscosity. The greater the solubility of the gas in the oil, the greater the viscosity loss (viscosity dilution). A lubricant viscosity-dilution chart is shown in Fig. 22 for methane at 50°. Similar gas solubility comparisons for nitrogen, hydrogen, ethylene, propane, c a r b o n dioxide, are provided in Figs. 23-27, respectively. [29]
300 • j f
<
Q^ 200 LU
DC i/>
Uj 100
a:
crfSs-
^5^=^
Q.
0
0
10
20
30
% GAS FIG. 19—Hydrocarbon gas solubility in an ISO 220 polypropylene oxide at constant temperature.
396
MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
o
o o >
2000 3000 4000 PRESSURE (PSIA)
500 1000 500i
5000
FIG. 23—Nitrogen gas solubility comparison in a PAG versus a PAO at 50°C.
0
20
40
60
80 100
TEMPERATURE (°C) FIG. 20—Effect of hydrocarbon gas dilution on viscosity with increasing temperature of a hydrocarbon gas in an ISO 220 polypropylene oxide fluid.
"0
1000
2000
3000
4000
Pressure (PSIA) FIG. 24—Hydrogen solubility comparison in a PAG. 2000
3000
PRESSURE (PSIA)
FIG. 21—Methane gas solubility comparisons for a polyalkylene glycol, Poly(Alphaolefin) and petroleum oil at 50°C.
Lubricant Solubility in the Gas The lubricant solubility in the gas should be minimized to reduce carryover by absorption of the lubricant in the gas. The absorption of the lubricant in natural gas was evaluated by Matthews using a constant pressure flow through a load (gravimetric) cell [8]. The results of this work, shown in Fig. 28 indicated that there was an appreciable absorption of the mineral oil in the gas. The PAG lubricant showed no appreciable loss. Test Procedures
2000
3000
PRESSURE (PSI)
FIG. 22—Viscosity dilution comparison for methane in a Poly(Alkyleneglycol) (PAG) and a petroleum oil at 50°C.
Kinematic Viscosity (ASTM D 445) Compressor fluid viscosity is important with respect to lubrication of bearings, gears, and other moving surfaces within the pump and for providing adequate sealing within the compressor. Typically, fluid viscosity at 40°C and 100°C is determined [8,25,32,34] and, if low-temperature use is expected, the viscosity at 0°C is determined [11,34]. Higher fluid viscosities result when the fluid is oxidized. Therefore, viscosity
CHAPTER 14: COMPRESSOR
LUBRICANTS
397
^ 0.5 (
I
I
I
lU 0.4t •
>•
CD 0.3
:
J
i
t
m
0.2
^ ^ s * ^ ^
0.1 —-{—"4—f —
1000
2000
(a)
3000 4000 5000 PRESSURE (PSIA)
6000
7000
(O
0
i
( l
i
l
l
1
1
(
1
i""-i 1
o
*
i S J PAG t
^
t
i
i
i
i
i
10
20
30
40
50
60
1
i
1
70
80
1
1
•
i
i
1
90 100 110 120
TEMPERATURE C O FIG. 27—Solubility of carbon dioxide in a PAG at SOX.
Specific Gravity (Test Methods D 287 and D 729SJ—Specific gravity depends on the chemical composition of the base stock used to formulate a compressor, such as the naphthenic/paraffinic ratio of a mineral oil. Test Methods D 287 and D 1298 for specific gravity require the use of both a hydrometer and an accurate thermometer or a thermohydrometer. Because specific gravity is temperature dependent, the temperature of the oil at the time of measurement, typically 25°C or 77°F, must be determined precisely [11,31,34]. 2000
1000
(b)
3000 4000 5000 PRESSURE (PSIA)
6000
7000
FIG. 25—Ethylene solubility comparison at 50°C of: A. a PAG versus a petroleum white oil and B. a PAG versus a polybutene.
-^ 0.5
1
1
1
I
r
i
j
X
o E 0.4 •.^.PJAG
^ 0.3 5 0.2 O
0.1
48
i ! 60 72 84 TEMPERATURE (°C)
i 96
108
FIG. 28—Solubility of propane in a PAG at SOX.
is also an important fluid maintenance procedure and is useful for comparison with the viscosity of a fresh fluid. Viscosity measurement by various tests, including Test Method D 445, is discussed in detail in Chapter 32—Flow Properties and Shear Stability, and will not be discussed further here. The viscosity-temperature relationship (viscosity index) of a compressor fluid is another important physical property that may be determined according to ASTM D 2270.
Pour Point (Test Method ASTM D 97) Pour point is used to characterize the suitability of a lubricant for use at low temperatures. Pour point is defined as the lowest temperature at which the movement of the fluid is observed. Water Content (Test Methods D 95, D 1744, D 4007) The presence of moisture in a compressor fluid, particularly with hydrocarbon gas compression, is important because hydrocarbon gases may form a "cold sludge" in the presence of moisture [6]. (If the compressor fluid contains trace amounts of water, it must be operated above the dew point of the fluid [10].) Also, water may condense in the system under conditions of low temperature and high humidity [11]. The presence of large slugs of water may lead to fatigue failure of bearings. Fluids that are susceptible to hydrolysis may then react with the residual water upon heating, thus reducing their lifetime and possibly leading to corrosive system damage. The presence of water in a compressor fluid may be determined by titration of the oil with Karl Fisher chemical reagent to an electrometric endpoint (Test Method D 1744). This test is recommended for water levels of 50-1000 ppm (less than 0.1%). Higher levels of water contamination may be quantified by distillation (Test Method D 95) or centrifugation of the sample and measurement of the volume of separated water according to Test Method D 4007. Demulsibility (Test Method D 1401 or D 2711) The ability of a fluid to separate from water is called "Demulsibility." Demulsibility properties of a compressor fluid may be determined using Test Method D 1401 where a 40 mL sample of the fluid and 40 mL of distilled water are
398
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
PRESSURE 340 bar TEMPERATURE 100 "C I
I MINERAL OIL (ISO 680)
^ ^ 2 0 0 est PAG
a. O 40 OT 30
o o W 20
hOGAS ATMOSPHER C PRE
URE
340 bar N,
SATURATED WITH METHANE
FIG. 28—Effect of pressure and gas dilution on the viscosity of a mineral oil and a PAG cylinder lubricant.
stirred for 5 min at 54.4°C (130°F) in a graduated cylinder [10,26,31,32,34,35]. The time for separation of the emulsion that is formed is determined. Test Method D 2711 is recomm e n d e d for evaluating the demulsibility properties of medium to high-viscosity lubricating oils. Hydrolytic
Stability
(Test Method
D
2619)
Some compressor lubricants hydrolyze in the presence of water at elevated temperature such as those based on ester basestocks [8]. Therefore, it may be important to determine the hydrolytic sensitivity of a compressor fluid to water. Conducting Test Method D 2619, the Beverage Bottle Method, can do this. Test Method D 2619 covers the determination of the hydrolytic stability of synthetic or petroleum-base hydraulic fluids b u t is as applicable to compressor fluids. In this method, 75 g of the compressor fluid and 25 g of water and a copper test specimen are sealed in a pressure-type bottle. The bottle is rotated, end for end, for 48 h in an oven at 93°C (200°F). Layers are separated, and insolubles are weighed. The weight change of copper is measured. Viscosity and acid numbers and the acidity of the water layer are determined. This method differentiates the relative stability of compressor lubricants in the presence of water under the conditions of the test. Hydrolytically unstable fluids form acidic and insoluble contaminants, which may cause compressor system malfunctions such as corrosion, valve sticking, or change of viscosity of the fluid. Foaming
Tendency
(Test Method
D 892)
The assessment of a wet or dry fluid to foam is important for various reasons. One is that foaming fluid may fill the separator and saturate the filter element, leading to severe fluid loss with the compressed gas. A "bubbly fluid" may cause lubrication failure, poor sealing properties and reduced efficiency [11]. As described in Test Method D 892, the foaming tendency of a fluid should be determined at 24°C and 93.5°C [26]. This test is conducted by equilibrating the test fluid at 24°C (75°F) at which point air is blown through the fluid at a constant rate for 5 min, then allowed to settle for 10 min. The foam volume is measured at the end of both periods. The test is repeated on a second sample at 93.5°C (200°F), and then after collapsing the foam at 24°C (75°F).
Air Release
(Test Method
D
3427)
In addition to foaming, it is important to monitor the ability of a fluid to release entrained air because excessive air entrainment may reduce fluid delivery rates to the compressor rotors. This has the same effect as operating the compressor below recommended fluid levels [25,32]. Air entrainment may be determined using Test Method D 3427 where compressed air is blown through the test fluid which has been heated to 25, 50, or 75°C. After the air flow is stopped, the time required for the air entrained in the fluid to reduce in volume by 0.2% under the conditions of the test and at the test temperature is measured. Corrosion
(Test Method
D
4310)
A well-formulated compressor fluid will contain additives to provide adequate copper and iron corrosion protection [10,11,15,22,32]. A build-up of corrosion metals in the separator could block the lubricating lines and impede fluid flow through the filter. Furthermore, if not properly inhibited, corrosion metals such as copper and iron will catalyze the oxidation of many fluids [11]. Copper Corrosion (Test Method D 130)~ASTM Test Method D 130 (Copper Strip Corrosion Test) will evaluate the corrosiveness of a compressor lubricant toward copper. In this test, a polished copper strip is immersed into the test fluid and heated at a temperature a n d for a length of time that is characteristic of the fluid being tested. At the conclusion of the test, the copper strip is removed, washed, and compared with ASTM Copper Strip Corrosion Standards. Iron Corrosion (Test Method D 665)—Iron corrosion, or "rust" potential of a compressor fluid is evaluated by Test Method D 665, which is also known as the Turbine Oil Rust Test. This test evaluates the ability of a compressor fluid to inhibit rusting of ferrous parts should water become mixed with the compressor fluid. The test is conducted by mixing 300 mL of distilled water (or synthetic sea water) at 60°C (140°F) with a cylindrical steel test rod completely immersed into the fluid/water mixture. The test is usually run for 24 h at which time the steel test rod is inspected for rusting. Flash and Fire Points (Test Methods D 92) The fire resistance and safety properties of a compressor fluid are t5rpically determined from its flash and fire point
CHAPTER [10,11,25,26,31,32,34,35]. The flash point is the lowest temperature, corrected to a barometric pressure of 101.3 kPa (760 m m Hg), at which the application of an ignition source causes the vapors of a fluid being tested to ignite under the specified conditions of the test. The test usually cited for compressor fluids is the so-called Cleveland Open-Cup flash point. This test procedure is described in detail in Chapter 25—Volatility. The fire point is the lowest temperature, corrected to a barometric pressure of 101.3 kPa (760 m m Hg), at which the application of an ignition source causes the vapors of a fluid being tested to ignite and continue to b u m for 5 s under the specified conditions of the test. Autoignition
(Test Method
E 659)
Compressor fluids should not be used above their autoignition temperature. Autoignition temperature is the temperature at which the fluid will ignite spontaneously in contact with air and may be determined by Test Method E 659. No sparks or open flame need be present. Hydrocarbon fluids absorbed on porous surfaces can ignite at temperatures more t h a n 50°C (approximately 100°F) lower t h a n indicated by Test Methods E 659. Evaporation Increasing volatility of the compressor fluid will result in increasing carbon build-up in the discharge system of a compressor [27]. This will occur if the oil fractions are sufficiently volatile to r e m a i n in the vapor state of the hot discharge zone where the oil can break down into carbon. Evaporation Loss (Test Method D 972)—Oil volatility can be measured by Test Method D 972. In this test, a weighed sample of the compressor oil is placed in a bath maintained at the desired test temperature, typically 100-150°C (210-300°F). Heated air is passed over its surface for 22 h. The evaporation loss is calculated from the loss of mass of the sample. NOACK Evaporation Loss (Test Methods D 5800 and D 6375)—The NOACK test method is a method of measuring evaporative loss of an engine oil during use, but can be related to any set of conditions. It was recently applied to measuring the evaporative loss of compressor fluids [35]. This test is conducted using the NOACK Evaporative Tester illustrated in Fig. 29. In this test, a measured quantity of the compressor fluid is placed in an evaporation crucible, which is then heated to the desired temperature with a constant flow of air drawn through it for 60 min. The loss in mass of the oil is determined. In this test, Wood's metal, which contains lead, bismuth, antimony, a n d cadmium, is used. These components have been found to be health hazards. Therefore, if it is possible. Test Method D 6375 should be used (see following discussion). An automated, smaller scale variant of Test Method D 5800 is Test Method D 6375, the "TGA NOACK Method." This method is applicable to lubrication oils that exhibit NOACK evaporative losses of 0-30%. In this test, a sample of the desired compressor fluid to be tested is placed in an appropriate Thermogravimetric Analyzer (TGA) specimen pan. The pan is placed on the TGA p a n holder and quickly heated to between 247-249°C under a stream of air and then held for an appropriate time. Throughout the process, the TGA monitors and records the mass loss experienced by the specimen
14: COMPRESSOR
LUBRICANTS
399
due to evaporation. The NOACK evaporation loss is subsequently determined from the specimen weight percent loss versus time curve (TG Curve) as the mass percent lost by the specimen at the NOACK reference time determined under the same TGA conditions. This procedure requires m u c h smaller test specimens and is m u c h faster when multiple samples are sequentially analyzed; it is also safer than the NOACK Test Method D 5800. Carbon
Residue
One of the greatest problems encountered when using a compressor fluid is the formation and accumulation of sludge, which can lead to a safety problem, possibly even an explosive situation if coupled with overheating. This will occur if sufficient quantities of c a r b o n a n d sludge are formed to cause the valves to stick open (Reference ISO 5388 part 6.6.1). Therefore, it is desirable to determine the sludgeforming potential of a compressor fluid. Conradson Carbon Residue (Test Method D189)—One method of quantifying the sludge-forming potential of a compressor fluid involves determination of the Conradson carbon residue (Test Method D189) [31 ], which measures the amount of polymeric material remaining in the oil after heating to elevated temperatures in the absence of sufficient oxygen to burn off all of the organic compounds present. The Conradson carbon residue is determined by placing a weighed sample in an iron crucible. The crucible is heated with a Meeker-type gas burner to a sufficiently high temperature to evaporate and b u m the oil. The sample is further heated until the bottom and sides of the crucible are cherry red and is held at this temperature for 30 min. The crucible is then cooled and weighed. The amount of tar remaining in the crucible relative to the original amount of the oil define the Conradson carbon residue value. Test Method D 189 may be affected by some additives used to formulate the compressor fluid. Ramsbottom Carbon Residue (Test Method D 524)—Another test that is used to determine the tendency for a compressor lubricant to form carbonaceous residues is Test Method D 524, the Ramsbottom Carbon Residue test [15]. In this test, the sample, after being weighed into a special glass bulb, is placed in a metal furnace and heated to 550°C. In this way, the sample is quickly heated to the point where all volatile material is evaporated out of the bulb, with or without decomposition, while the remaining residue undergoes cracking and coking reactions. After the test, the bulb is cooled and weighed. The residue remaining is reported as % of original sample and is called the Ramsbottom Carbon Residue. "Micro Method" for Carbon Residue (Test Method D 4530)—In the "micro method," a weighed sample of the fluid is heated in a 2 mL glass vial at 500°C under an inert atmosphere (nitrogen) in a controlled manner for a specific time. The sample undergoes coking reactions a n d the volatiles that are formed are swept away by the nitrogen. The carbonaceous residue formed is reported as % of original sample as "carbon residue (micro)." This test method is reported to be equivalent to the Conradson Carbon Residue. Precipitation
Number
(Test Method
D 91)
Sludge formation in a compressor oil is caused by oxidation of various components, leading to polymerization and crosslinking reactions. These cross-linked and polymerized byprod-
400
MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK ~N
Manometer
Thermometer
j ~ To Pump
FIG. 29—Schematic illustration of the NOACK Evaporative Tester (ASTM Test Method D 5800).
ucts are sufficiently fiigfi in molecular weight to cause them to be Insoluble in the oil. As indicated above, sludge can lead to valve sticking, creating an unsafe condition (ISO 5388). Increasing sludge formation often indicates increasing oil oxidation. The amount of sludge can be quantified by adding naphtha solvent to the compressor fluid sample and determining the volume of precipitate (sludge) after centrifuging. Acid Number (ASTM D 664 and D 974) The acid number (AN) measures the acidity of the fluid, acidity produced when the fluid is oxidized. Fresh, oxidation inhibited, compressor fluids tjrpically exhibit AN values of approximately 0.15 or less [22,26]. The AN value may be determined by Test Method D 664, which covers lubricants soluble or nearly soluble in mixtures of toluene and isopropanol. It is applicable for determination of acids whose dissociation constants in water are larger than 10"'. Extremely weak acids whose dissociation constants are smaller than 10~' do not interfere. Salts react if their hydrolysis constants are larger than 10~'. For Test Method D 664, the compressor fluid is dissolved in a mixture of toluene and isopropanol containing a small amount of water and titrated potentiometrically with alcoholic potassium hydroxide, using a glass electrode and a calomel reference electrode. Test Method D 974 is similar except the sample is titrated colorimetrically using a p-naphtholbenzein as the indicator. Infra-Red Spectroscopy An alternative method that is being used increasingly to identify and quantify oil oxidation, even in the presence of additives, is infra-red (IR) spectroscopy [22]. Figure 30 provides an illustration of the use of IR spectral analysis to identify oil oxidation [36]. Mang and Jiinemann monitored the IR
stretching vibrations ofC = O a t l 7 1 0 cm ', for carboxylic acids contained in oxidized oil. IR analysis has been used to detect and quantify other carbonyl-containing compounds [37]: Metal carboxylate salts-1600 and 1400 cm"' Carboxylic Acids-1710 cm"' Metal sulfates-1100 and 1600 cm"' Esters-1270 and 1735 cm "' Fluid Oxidation The severity thermal and oxidative stresses that a compressor fluid is subjected to are compressor dependent. For example, thermal and oxidative stresses on a fluid in a rotary vane and rotary screw compressor are considerably more moderate thcin a fluid would encounter in a reciprocating compressor. However, although the thermal and oxidative stress is greater/cycle in a reciprocating compressor, it is only a "once through process," whereas the expected lifetime of a fluid for a rotary vane or rotary screw compressor would be expected to last for several thousands of hours [23]. Various tests have been proposed to determine relative lifetimes of a compressor fluid under rigorous thermal and oxidative conditions. The tests that have gained greater acceptance as reasonably predicting actual production experience in a compressor will be reviewed here. Turbine Oil Stability Test - TOST (Test Method D 943)—Sugiura, et al. [23] has reported some success with the Test Method D 943 to determine the long term oxidative stability of a compressor oil. Test Method D 943 evaluates the oxidative stability of a lubricant with a specific gravity less than water, which contains rust and oxidation inhibitors in the presence of oxygen, water, copper, and iron at an elevated temperature. In this test, the sample is contacted with oxygen
CHAPTER in the presence of an iron-copper catalyst at 95°C. The test is continued until the measured acid n u m b e r is 2.0 mg KOH/g or above. The n u m b e r of hours required for the test fluid to reach 2.0 mg KOH/g is the "oxidation lifetime." Sugiura, however, utilized a m u c h lower critical AN value of 0.4 mg KOH/g since his work indicated that the oxidation rate of the compressor oils he examined increased dramatically above 0.4 mg KOH/g [22]. Rotating Bomb Oxidation Test-RBOT (Test Method D 2272)— Various workers have reported the use of RBOT as an accelerated test to evaluate compressor fluid lifetime [11,23,26]. This test m e t h o d utilizes an oxygen-pressurized vessel to evaluate oxidation stability of new and used compressor fluids in the presence of water and a copper catalyst at 150°C. For this test, the compressor fluid, water, and copper catalyst coil, contained in a covered glass container, are placed in a vessel equipped with a pressure gauge. The vessel is charged with oxygen to a gauge pressure of 620 kPa (90 psig, 6.2 bar), placed in a constant-temperature oil bath set at 150°C, and rotated axially at 100 r p m at an angle of 30° from horizontal. The n u m b e r of minutes required to reach a specific drop in gauge pressure is the oxidation stability of the fluid. Indiana Stirring Oxidation Test-ISOT (Test Method JIS K 2514)~The ISOT, although developed in the U.S., has only been standardized in Japan as JIS K 2514 and has been reported to provide superior correlation with actual results for actual industrial compressor use [22,26]. This test cell for this method is illustrated in Fig. 3 1 . This modified version of
14: COMPRESSOR
LUBRICANTS
401
the ISOT test reported by Sugiura [22] involves blowing dry air at 10 L/h through 300 mL of the test fluid held at 165.5°C (329.9°F) in the presence of a copper and iron catalyst. The n u m b e r of hours to reach a AN of 0.4 mg KOH/g is determined. Wolf Strip Oxidation Test [8]—Matthews has reported excellent correlations with carbon and sludge forming tendencies of a compressor fluid in an air compressor using the Wolf Strip Test [8,23]. This is a 12 h test that is conducted by pumping a fluid sample over an inclined steel plate test strip, which is heated to 250°C. The oil is removed from the plate by solvent washing at the conclusion of the test and the steel test strip is air dried. The weight of the residue adhering to the test strip is measured and the evaporative loss of the total volume of the oil after the test completion is recorded. This is very similar to the hot panel coking test which utilizes an alum i n u m test plate [38] and has been used in the automotive lubricants industry for many years [23]. Liquid Heptane Resistance f8]
Washing
Test for Oil Film
Wash-Off
Matthews has devised a test to indicate the resistance of an oil film to be removed by heavier hydrocarbons (C7 or higher), which may be present in natural gas [8]. This test is conducted by pre-weighing dry steel plates that are dipped into the test lubricant and then suspended for 1 h to allow the excess oil to drain off from the surface of the steel. The steel plates are reweighed to determine the weight gain due to the oil film and
Wave Number (cm'^) FIG. 30—Infra-red spectral identification of oxidation of a used compressor oil (mineral oil-derived fluid).
402
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
Dry Air 10 ±1 Mir.
fr Oil Bath Temperature = 170 "C
4-5Sample • 4J.44
^ ^ Dimensions in miilimeters (mm)
T
FIG. 31—Illustration of the test cell for the Indiana Stirring Oxidation Test (ISOT).
[15] or 1 h at 40 kgf, 1500 rpm [8]. In both cases, the wear scar at the completion of the test is measured. Matthews also reported the weld load, which is a determination of the load carrying (extreme pressure-EP) properties of the fluid using Test Method IP 239. The weld-load is the lowest applied load, in kilograms, under the conditions of the test at which the rotating ball welds to the three stationary balls, indicating that the extreme - pressure level of the lubricant-force has been exceeded. ASTM Test Method D 2783 can be used to determine the EP properties of a lubricant and it is conducted, after properly preparing the test machine, at the desired starting temperature: 18.33-35°C (65-95°F), 1760 ± 40 rpm, and a series of tests of 10 s duration are made at increasing loads until welding occurs. This is called the "weld point." FZG Visual Method (Test Method D 5182)—This test method, the Forschungstelle fiir Z a h n r a d e r u n d Getriebebau (Research Site for Gears and Transmissions) is commonly referred to as the FZG Visual Method. This method is intended to measure the scuffing load capacity of oils used to lubricate hardened steel gears. Scoring, is included as a failure criteria in this test method. The FZG Gear Test Machine is operated at a constant speed (1450 rpm) for a fixed period (21700 revolutions—approximately 15 min) at successively increasing loads until the failure criteria is reached. The initial oil tem100
Liquid Heptane Wasiiing Test
then the plates are dipped into a bath of heptane for 5 s. After ten dipping cycles, the steel plates are suspended in air to allow the h e p t a n e to evaporate and then the plates are reweighed to determine the a m o u n t of oil film removed. An illustration of the relative weight loss of two different compressor oils subjected to this test is provided in Fig. 32 [8]. Antiwear
Properties
If possible, it is desirable to determine the lubrication properties of every compressor oil in every compressor. However, in addition to being prohibitively expensive and requiring enormous amounts of time, industrial equipment is not designed with the precision required for ideal laboratory test instruments. Therefore smaller scale lubrication testing is performed as a laboratory fluid development tool and while very useful, these data must, at some point, be confirmed by infield testing in actual compressor units and processes where the use of the fluid is contemplated. Although any test could conceivably be used if properly correlated with compressor operation, there are two bench tests that are encountered most often in the published literature: FZG [23,32,35] and 4ball [8,15,25]. These tests will be discussed here. 4-BaU EP and Wear Test (Test Methods D 2783, D 4172 and IP 239)—Test Method D 4172 is used to measure the relative wear preventive properties of lubricating fluids under sliding contact conditions using a Four-Ball EP and Wear Test Machine. Although Test Method D 4172 calls for test loads of 15 or 40 kgf at 1200 r p m and 75°C, any load condition can be used, if more appropriate. Some test conditions that have been reported include: 1 h at 40 kgf, 1800 rpm and 54.4°C
Cycles FIG. 32—Comparison of the liquid heptane wash-off results for a 200 cSt PAG and a mineral oil compressor lubricant.
CHAPTER 14: COMPRESSOR LUBRICANTS perature is 90°C beginning at load stage 4. The test gears are examined initially and after the prescribed duration at each load stage for cumulative damage (scuffing) to the gear tooth flanks. Generally, it is expected that compressor fluids will pass at least ten load stages in this test [32]. Compressor Tests—^As with any machinery lubricant, the best test is to evaluate it in the machine of interest. However, just as bench tests do not necessarily correlate with machinery results, it is also true that performance in one machine does not necessarily predict performance in a different m a c h i n e . Matthews has reported the development of two compressor tests where tests with different compressor fluids do seem to correlate well with other compressors in the field. These two tests will be outlined here, although they are not national or international standard tests at this time. Broomwade 2050H Compressor Rig Test [8]—The Sebe (Compare) Broomwade 2050H is a single-stage, air-cooled, beltdriven, twin-piston air compressor that is mounted on an air receiver tank as shown in Fig. 33. A gear p u m p is used to splash-lubricate the bearings a n d cylinder walls using a rapid, 825 rpm, crankcase movement in the sump. The operating conditions are summarized in Table 7. The compressor rig was modified as follows: 1. The air discharge t e m p e r a t u r e is controlled by a miniether operating a magnetic on/off valve which regulates the delivery pressure in response to a thermocouple located in the air outlet stream. 2. The discharged air is passed through an air-cooled heat exchanger to reduce the temperature to an acceptable level during continuous operation. Before each test is performed, the compressor is fitted with new inlet and delivery valves, seals, gaskets, and filters and the cylinder head is ultrasonically cleaned and solvent washed between tests. The compressor is run continuously with visual inspections at 100 h intervals. At the conclusion of the test, the piston ring gap is measured in addition to the visual inspection. Reavell VHP 15 Compressor Rig Test [8]—The Reavell VHP 15 is a 4-stage, single acting, air compressor that is driven by an electric motor, illustrated in Fig. 34. The cylinder, V-form
403
compressor is mounted on the crankcase at 90°. Multi-stage water inter-coolers are filtered after each compression stage. Each compression valve combines both suction and delivery. The conventional trunk-type first and second stage pistons utilize splash lubrication generated by the rotating (1450 rpm) crankshaft in the sump. The crossheads and guides for the third and fourth stage pistons are lubricated separately using a mechanical lubricator. The operating conditions are summarized in Table 8 and Table 9. The test stand was fitted with transducers to monitor the inlet and outlet temperatures and pressures at each stage. A water-flow control valve operated by a thermocouple located in the air delivery stream at Stage 4 was used to control the air delivery temperature. Over-pressure relief valves were used on each stage to prevent overheating. New valves were used for each test and seals replaced as necessary. The compressor was run continuously with periodic inspection of the valves and cones. Valve removal should not be performed since this process would disturb the accumulated carbon deposits that would falsely indicate an apparent increase in the lifetime of the lubricant. Therefore, the valves should be left in position and the pressures and temperatures, which are continuously monitored, would be used to indicate compressor malfunction that would typically be: 1. Lifting of the inter-stage relief valves that would be indicative of a leaking inlet valve. 2. Unacceptably low inter-stage delivery pressures that would indicate a delivery valve failure. 3. Rapid rise in air delivery temperature indicating possible internal combustion. 4. Very high oil consumption indicating worn piston rings.
TABLE 7—Operating conditions for the Broomwade 2050H compressor. Electric Motor Output
7.5 k W
Speed Maximum oil deliveiy pressure Oil capacity Free air delivered Air discharge temperature
825 rpm 10bar(150psig) 4.5 L 15.2L/sat lOOpsig
H.P. AIR DELIVERY (t')
MANUAL REGULATING VALVE
— CONDENSATE DRAIN
FIG. 33—Schematic of the Broomwade 2050H compressor rig test.
300-325°C
404
MANUAL 37; FUELS AND LUBRICANTS
HANDBOOK
LP AIR T O ATMOSPHERE
C/W RETURN
"mERMOCOUPLE (STAGE 4)
Sr^ ^-©
© @
RECIRCULAtORY COOLING WATER - FLOW
H.P.AIR
•—'
^-^
&
REAVELL VHP15 4 STAGE AIR COMPRESSOR
10
LPAIR1. Non-retum valve 2. Pressure reHef 3. Manual blow-down 4. Over-pressure switch 5. Pressure-maintaining valve 6. Manual cooling water valve 7. Motorized cooling water valve 8. Temerature controller 9. Pressure storage vessel 10. Pressure storage vessel
DRIVE MOTOR 22 Kw CONDENSATE DRAINS
FIG. 34—Schematic of the Reavell VHP 15 compressor rig test.
Coalescer Blocking Tendency (CBT) Test [32]—As discussed earlier, compressor fluids are recycled through a rotary compressor, such as a rotary screw compressor. To this, the residual gas that is being compressed must be removed from the compressor fluid before it can be sent back to the reservoir. This is done using a coalescer for gas removal. During use, compressor fluids will contain degradation by-products that will adhere to or block the coalescer filter. This will create an increase in differential pressure across the filter that may lead to machine stoppage. Therefore, coalescer blocking tendency is an important parameter since it defines the useful life of a compressor fluid in a compressor. As indicated in Table 10 [32], although there are various bench tests to evaluate fluid oxidation, carbon and sludging tendencies, and filterability, no single test correlates well with actual compressor usage. The best test still is a compressor test. One test developed by Mills, et al. will be outlined here [8,32]. The CBT test is conducted using an oil-flooded 22 kW (30 Hp) rotary screw compressor with a free air delivery rate of 42 L/s (93 ft^/min) was used. The specifications for the compressor are summarized in Table 11 [32]. The following modifications were made to the compressor to construct the test stand illustrated in Fig. 35 1. Oil temperature was controlled by using a control system that maintained the inlet oil temperature at 40-170°C ± 2°C. 2. The compressor's 5-10 micron air filter was proceeded by two in-series foam air filters, each rated as 99.5 % efficient at 5 microns. The purpose of this modification was to minimize the day-to-day variation in air dust on the severity of the test. 3. An oil flow rate meter was placed in the oil feed circuit to monitor oil flow rate and an oil removal filter was fitted to the discharge air to give a maximum oil content of 0.01 ppm. The purpose of this modification was to minimize oil carryover.
TABLE 8—Operating conditions for the reavell VHP 15 air compressor. Electric Motor Output
22 kW
Speed Maximum delivery pressure Free air delivery rate Sump oil capacity 23.1 L
1450 r p m 350 b a r (5000 psig) 13.3 L/s
TABLE 9—Stage temperatures and pressures. Stage Delivery pressure (bar) Delivery temperature (°C)
1
2
3
4
3.1 140
19.6 179
84.5 146
310 180
4. A high-resolution Bourdon gauge was used to monitor the pressure differential across the coalescer. 5. The working discharge pressure of the compressor was established using a gate valve in the discharge main area. 6. The inlet oil temperature, air/oil outlet temperature, and air delivery temperature of the compressor was continuously monitored on a chart recorder. Before each test, the compressor was flushed using a flushing fluid of the same type being tested. (Mill called for using a mineral oil compressor fluid but this would not be appropriate if it we not compatible with the fluid type being tested.) The flushing conditions are; oil charge 22.5 L (6 gal), outlet air pressure 690 ± 1 5 kPa, inlet oil temperature 70 ± 2°C for 24 h. When the flushing is complete, the flushing fluid is drained hot from the reclaimer, compressor unit, and oil cooler for 15 min. A new 85% masked coalescer and new oil filter are installed, the unit is charged with 22.5 L (6 gal) of the test fluid and the soluble catalyst system. The catalyst system is 20 g each of copper and iron as a naphthenate in a hydrocarbon solvent. This will provide approximately 60 ppm of copper and iron in the test fluid.
CHAPTER 14: COMPRESSOR LUBRICANTS TABLE 10-—Performance of commercial mineral oil-based compressor fluids in various bench tests. Test l*" Test 3' Test 4'' % Viscosity Filtration Filtration % % Carbon Evaporation Increase TAN Increase Time Time % Residue Loss at 40°C (mg KOH/g) Sludge (min) (min) 0.48 3.0 0.44 7.4 0.11 17.6 Blocked 0.40 0.25 Blocked 0.5 2.0 0.83 Blocked Blocked 1.5 1.6 0.06 0.06 0.98 16 0.37 0.21 4.3 1.0 10.0 0.49 Blocked
405
Test 1°
Compressor Fluid A B C D
Field Rating (1 = Best) 3 4 1 2
"Test 1: DIN 51352 Fart 1 (PNEUROF Oxidation Test-Conradson carbon residue. ^Test 2: Modified DIN 51352-Same as Test 1 but 120°C, 100 h with 50 ppm Fe catalyst as iron naphthenate. ''Test 3: Beaker Filtration test-300 mL oil, 120°C, 168 h, filtered at ambient temperature through 47 mm diameter, 1.2 /xm Millipore filter under 88 kPa (26 in. Hg) pressure. ''Beaker Filtration test-100 mL oil, 120°C, 28 days,filteredas described for Test 3.
TABLE 11—Specifications for compressor used for the coalescer blocking tendency test. Minimum 70 psi (5 bar) Air Delivery Pressure Normal 100 psi (7 bar) Maximum 150 psi (10 bar) Free Air Delivery 42 L/s (93 hVmin) Unit 40 L (10.6 gal) Oil Capacity (nominal) Reclaimer 32 L (8.5 gal) Compressor Single stage, oil injected, asymmetric screw 22 kW (30 Hp) Motor Compressor-Air/oil cooled Cooling Air Cooler-Thermostatically controlled updraft fan Inlet air 5-10 micron paper cartridge Filters Oil 15 micron paper cartridge Coalescer 1-2 micron
Air Flow ON Flow Alr/OII Flow Water Flow
5M Air Filter
•
The compressor is then started under the test conditions: outlet air pressure 830 ± 1 5 kPa, inlet oil temperature 100 ± 2°C until a pressure differential across the coalescer of 207 kPa is obtained. At this point, the coalescer is defined as "blocked." Every hour the coalescer pressure differential is measured and recorded. Every 24 h, the compressor is shut down for 2-5 m i n for a 60 m L sample to be removed from the reclaimer. (A 100 m L presample is taken to flush the sampling system.) The oil carryover filter bowl contents are measured and discarded during the shut down period. No attempt is made to top-off the compressor fluid since losses should be very small. If the fluid level drops sufficiently for air entrainment to occur (as indicated by the oscillating outlet air pressure), or any other malfunction occurs in the
Differential Pressure Gauge
5-10 M Air Filter
Motor
Oil Cooler Drain Valve FIG. 35—Schematic illustration of compressor test circuit used for coalescer blocking tendency test.
406
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
system, then the test is invalidated. Gas Solubility into the Lubricant Knowledge of gas solubility is of vital importance when considering compressor lubrication. It is particularly important in b o u n d a r y lubrication, where sudden release of the dissolved gas may cause lubricating film collapse or even cavitation. There are various tests that can be conducted for gas solubility determination. These test procedures will be discussed here. Gas Solubility in Petroleum Oils at Atmospheric Pressure (Test Method D 2779)—Tesi Method D 2779 may be used to estimate the solubility of several common gases in hydrocarbon liquids. These include petroleum fractions with densities that range from 0.63-0.90 at 288 K (59°F). The solubihties can be estimated over the temperature range of 228 K(— 50°F) to 423 K(302°F). The test method is based on Henry's Gas Law and the Perfect Gas Law. This test is conducted as follows: • Step 1—Determine the density of the compressor fluid by Test Method D 1298 if it is not already available. • Step 2—Determine a value of "L" for the gas of interest at the desired temperature from Fig. 36 if it is shown. If it is not shown, then it may be calculated from the following equation and the value of Lo taken from Table 12.
100
-SO
90
L = 0.300 exp Where T = for a liquid 273 K for a ent density
o.639m;^
/H3.333LO
temperature in K, L = Ostwald coefficient at T of density 0.85 and LQ = Ostwald coefficient at liquid of density 0.85. For liquids with a differ(d), calculate Lc from: L, = 7.70L (0.980 - d)
Where Lc = the Otswald coefficient at T for a liquid of the specified density. • Step 4—Calculate the Bunsen Coefficient (B) from: B = 2697
(P-Pv)L
Where: P = pressure of the gas in Mpa, Py = vapor pressure of the liquid at the specified temperature (T). • Step 5—Calculate the solubility of the gas, which is sometimes expressed as part per million by weight, using the following equation: 1 - 0.000595
0.224
TEMPERATURE,-F 200 30O
100
tSO
200
eso
T E M P E R ATURE.'C
FIG. 36—Solubility of gases in petroleum liquids witii a density (d) = 0.85.
r - 288 - 2
CHAPTER TABLE 12—Ostwald coefficients at 0°C for petroleum liquids with d = 0.85. Gas
Ostwald Coefficient (LQ)
Helium Neon Hydrogen Nitrogen Air Carbon monoxide Oxygen Argon Methane Krypton Carbon dioxide Ammonia Xenon Ethylene Hydrogen sulfide
0.012 0.018 0.040 0.069 0.098 0.12 0.16 0.18 Use Fig. 36" 0.60 1.45" 1.7" 3.3 Use Fig. 36" 5.0"
// =
X
Estimation of Gas Solubility in Petroleum and Other Organic Liquids (Test Method D 3827)—This test method provides a procedure for estimating the equilibrium solubility of several c o m m o n gases in petroleum oil and synthetic lubricants at temperatures between 0 and 488 K. This method is limited to systems in which polarity and hydrogen bonding are not strong enough to cause serious deviations from ideal behavior. Specifically excluded are gases such as HCL, NH3 and SO2 and hydroxy liquids such as alcohols, glycols, and water. Also the estimation of CO2 in non-hydrocarbons is excluded. The calculation procedure is as follows: • Step 1—Determine the value of Si (solubility parameter of the liquid in (Mpa)''^ by one of the following procedures: a. Procedure 1—If the liquid is a non-hydrocarbon, determine 5i from Table 13. If the fluid is not listed in Table 13 and the structure is known, use the method of Fedors provided in Ref. 39. b. Procedure 2—If the liquid is a refined petroleum or a synthetic hydrocarbon, then determine the density of the fluid p by Test Method D 1218. If the fluid density p is 0.885 g/mL or less, calculate 5i as follows: Si = 12.03p + 7.36 c. Procedure 3—If the liquid is a refined hydrocarbon or a synthetic hydrocarbon with p = 0.886 or greater, or a nonhydrocarbon of unknown structure, determine the refractive index, nn, by Test Method D 1218 and calculate as follows:
IlD
^
B = 2697
T
Step 5—For hydrocarbon oils, calculate the density of the liquid at a specified temperature px as follows: 1 - 0.000595
r-288.2
Alternatively, the density may be determined experimentally by Test Method D 1298 or it may be obtained from the fluid supplier. Another method of density temperature correction is to use the value dp/dT from Table 13 and calculate as follows:
MG
Where ML = the moles of liquid and Mo = the moles of gas. Henry's Law constant (H) is calculated from:
407
Step 2—Obtain the value of the equivalent solubility parameter of the gas, S2, (MPa)''^ from Table 14. Step 3—Calculate the Ostwald coefficient (L) for a lubricant from: L = exp[(0.0395 (Si - 82)^ - 2.66) - 3.03Si - 0.0241 (17.60 - 82^ + 5.731] To calculate the Ostwald coefficient for a distillate fuel or a halogenated liquid, multiply by the fuel factor from Table 14. Step 4—Calculate the Bunsen coefficient (B) as follows:
PT-
X= 10"
LUBRICANTS
Si = 8.63 2 + 0.96
''Do not use this method for these gases in highly aromatic liquids
Where: 0.0224 is the molar volume at 273 K and 101.3 kPa in liters/mole, and 0.000595 and 1.21 are the empirical constants for correcting d to a specified temperature, T. Step 6—Calculate the gas solubility as mole fraction from:
14: COMPRESSOR
p^ = p-(T-
288.2) - ^
• Step 6—Calculate the gas solubility G in mg/kg using the value of M2, the molecular weight of the gas (g/mole), from Table 14 and the following equation: G = 44.6G^ Pi
• Step 7—Determine the value of the molecular weight of the liquid. Ml, by: a. For synthetic non-hydrocarbons, use the value of Mi from Table 13, or calculate it directly. b. The value of Mi for refined hydrocarbons or sjrnthetic hydrocarbons may be determined experimentally using Test Method D 2502.
TABLE 13—Constants for synthetic hydrocarbons. Compound
Di-2-ethylhexyl adipate Di-2-ethylhexyl sebacate Trimethylolpropane pelargonate Pentaerythritol caprate Di-2-ethylhexyl phthalate Diphenoxy diphenylene ether Diphenoxy triphenylene ether Polychlorotrifluroethylene Polychlorotrifluroethylene Polychlorotrifluroethylene Dimethyl silicone Methyl phenyl silicone Perfluoro polyglycol Tri-2-ethylhexyl phosphate Tricresyl phosphate
S,
dp/dT
M,
18.05 17.94 18.48
370 427 459
0.928 0.916 0.962
0.00075 0.00073 0.00070
18.95 18.97 23.24
540 390 440
1.002 0.986 1.178
0.00065 0.00075 0.00079
23.67
520
1.205
0.00076
15.47 15.55 15.71 15.14 18.41 14.30 18.27 18.82
600 700 1000 10000 5000 1000 467 368
1.925 1.942 1.998 0.969 1.063 1.914 0.923 1.158
0.00166 0.00154 0.00152 0.00093 0.00080 0.00180 0.00090 0.00090
408
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
TABLE 14—Solubility parameters of gaseous solutes. Gas
He Ne H2 N2
Air CO O2
Ar CH4
Kr CO2
M,
52 at 298 K
Fuel Factor
4 20 2 28 29 28 32 40 16 84 44
3.35 3.87 5.52 6.04 6.67 7.47 7.75 7.71 9.10 10.34 14.81
1.27 1.37 1.27 1.70 1.44 1.37 1.28 1.37 1.42 1.37 1.14
c. The value of Mi for non-hydrocarbons of u n k n o w n structure may be determined experimentally using Test Method D 2503. 7. Step 8—Calculate the gas solubility as a mole fraction from: X= 10"
M2
8. Step 9—Calculate Henry's Law constant (H) as: (P - Pv) H = X Experimental Determination of Gas Solubility in Liquids (Test Method 2780)—This procedure covers the experimental determination of the solubility of gases in liquids and is suitable for gas/liquid mixtures that do not react with each other. This method also permits the determination of the concentration of gases in solutions that are not saturated with the gas. In this test, the liquid is saturated with the gas of interest under specified temperature and pressure conditions. (The apparatus for ambient pressure is illustrated in Fig. 37 and for gas solubility experiments conducted at elevated pressures, the apparatus illustrated in Fig. 38 is used.) If the objective is to determine gas concentration, the saturation step is eliminated. A portion of the solution is then transferred to a gas extraction apparatus shown in Fig. 39 where the gas is quantitatively removed from the liquid. The separated gas is transferred to a gas burette in which its volume is determined. Viscosity of Gas/Liquid Mixtures Under Pressure—It is not only important to determine the solubility of a gas a compressor fluid, but it is also important to determine the impact of dissolved gases on fluid viscosity. This is of particular importance in determining the lubricating ability of the compressor fluid during gas compression. Measurement of viscosity of lubricants saturated with gas at elevated pressures is done in a fixed volume apparatus. Typically a Parr reactor (or other high-pressure autoclave) equipped with a stirrer, pressure transducer, thermocouples, and heating and cooling devices is used to conduct these experiments at desired temperatures. The vessel is fitted with a viscosity measurement probe suitable for measurements at high pressure. Viscosity probes that can operate u p to 5000 psia (—340 Bars) are readily available. Viscosity and gas solubility data at a given temperature and pressure can be obtained from the same experiment. The Parr reactor is charged with a known mass of lubricant
and gas at the desired temperature. Gas is typically transferred to the Parr reactor from a gas-charging vessel. The lubricant is stirred to obtain equilibrium as indicated by reaching a constant pressure. Viscosity of saturated lubricant at final pressure is measured with a viscosity probe and it's associated electronics. Gas solubility can be calculated from initial and final pressures using gas laws and compressibility factors. It can also be obtained by weighing the gas-charging vessel before and after equilibrium. Absorption of the Lubricant into the Gas Phase—This is the test procedure published previously by Matthews, which was performed to determine the amount of lubricant lost into the gas being compressed [8]. The test is conducted by weighing the test oil into a small aluminum container of about 25 m m diameter which was then placed into a 50 mL stainless steel, high pressure reactor which was then heated to 100°C. The test gas is then pressurized to 400 bar and fed through stainless steel tubing into the stainless steel reactor containing the test oil. The pressure is maintained by releasing the gas through a needle valve and the total volume of the gas being discharged is metered. At the end of the test, the aluminum container is re-weighed and the weight loss is recorded. Heat Transfer
Efficiency
Thermal Conductivity (Test Method D 2717) and Specific Heat (Test Method D 2766)—These thermal conductivity and specific heat tests are difficult to carry out, facilities for performing them are few, and the precision data is yet to be es-
Gos Inlet
g.
>v
Gas Outlet
1000-ml Seporotory Funnel (peor-shopedl Control ThecTnocouple
Test Liquid
Healing Mantle
Ts" \
, Cos Dispersion Element
PTFE Stopcock A
Boll Joint B i 12/2 FIG. 37—Schematic illustration of an ainblent pressure saturator.
CHAPTER
14: COMPRESSOR
LUBRICANTS
409
tablished. Values can be estimated for design use from the general chemical composition. The VEilues for thermal conductivity and specific heat may be available from the fluid supplier.
Pressure Goge
Tube Connection A"
Elastomeric
Seals Compatibility
(Test Method
D 471)
Seals may degrade in use. As an oil deteriorates in service, additional tests may be required to assure that seals remain compatible with the degraded oil. Mills et. al, have reported the determination of seal compatibility by a short term test by immersion of 50 m m X 25 m m X 2 m m test specimens of the elastomer (most often a nitrile rubber) into the compressor fluid of interest at 120''C for 168 h [32]. Volume swell was d e t e r m i n e d according to the procedures outlined in Test Method D 471 and the hardness was determined according to Test Method D 1415. The authors recommended setting the volume swell limits at —5 to -1-15% and the change in hardness to -t-20%. Although an accelerated test may be performed, it is preferable that the temperature ranges of the tests should correspond to temperatures to which seals will be exposed in service and the tests should be conducted for at least 1000 h.
Thermocouple
2500-ml Stainless Steel \fe$sel
Thermostotic Control
Jacket
Paint Compatibility
; 12/2 Stoinless Steel BaH Joint B FIG. 38—Schematic illustration of an elevated pressure saturator.
Test
[32]
Paints are used to protect metal surfaces of the compressor components and machinery. However, they may be attacked readily by the compressor fluid being used. The paint may then be removed and become a contaminant in the fluid, causing great damage. Therefore, it is important that the paints that are used be compatible with the compressor fluid. A simple test used to determine paint compatibility is to
To Condenser K 14/35
Solid Broce
stopcocks L,M,N, ore 2-woy
'^E-0.5 mm Cleoronce —60mn(i 0.0. •—SOmmO.D.
Lewting Bulb (Mercury)
Co pillory Monometer (Opin-Endl
(a)
Boiler-1
To Leveling BulbH-
J 12/2 to Boiler I
FIG. 39—Gas extraction system: A. schematic illustration of the entire apparatus and B: Illustration of the gas extraction chamber.
(b)
410
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
paint a small steel test specimen with the paint of interest and let it dry [32]. The painted surface is then scratched in the form of a cross and then immersed into the compressor fluid at 120°C for 168 h. If the paint is lifted or removed by the fluid, it fails the test.
D 445
D 471 D 524
Filter Bowl Compatibility
Test
[32]
A filter bowl is used to retain the compressor fluid after it is removed from the compressed gas by an absorbent filter. A commonly reported field problem is that compressor fluid attacks the filter bowl, whcih is often constructed from a plastic such as polycarbonate. A simple test that is used by many compressor manufacturers is to pressurize a filter bowl containing the compressor fluid being tested to 6900 kPa (100 psig). The test is considered to be a pass if the filter bowl does not fail within 100 h.
D 664 D 665 D 892 D 943 D 972 D 974
SUMMARY In this chapter, an overview of basic compressor fluid technology has been provided. This discussion included a description of the gas laws and how they applied to gas compression. Also provided was a short tutorial on how c o m m o n types of compressors operate. This was followed by a summary of the properties of the most common classes of compressor fluids in current use in the market place. Finally, there includes an extensive listing of various test procedures that have been applied to the analysis of the expected performance of compressor fluids such as water content, oxidative stability, carbon and sludge formation, gas solubility and lubrication properties. However, it is must be realized that there is no one single bench test that will best predict the performance of a particular compressor fluid, in a given compressor, in a given industrial process. This is illustrated in Table 10, which shows that no one single test predicts the actual relative performance of a compressor fluid in an industrial process. It is therefore recommended that groups of tests be performed and overall ranking be used. Of course, whatever the bench tests indicate, whether individually or collectively, final compressor machine performance validation is necessary.
ASTM STANDARDS No. D 91 D 92 D 95 D 97 D 130
D 189 D 287
Title Test Method for Precipitation Number of Lubricating Oils Test Method for Flash and Fire Points by Cleveland Open Cup Test Method for Water in Petroleum Products and Bituminous Materials by Distillation Test Method for Pour Point of Petroleum Products Test Method for Determination of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test Test Method for Conradson Carbon Residue of Petroleum Products Test Method for API Gravity of Crude Petroleum and Petroleum Products (Hydrometer Method)
D 1298
D 1401 A 1415 D 1744 D 2270 D 2272 D 2502
D 2503
D 2619 D 2711 D 2717 D 2766 D 2779 D 2780 D 2783
D 3427 D 3827 D 4007
D 4172 D 4530
Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (the Calculation of Dynamic Viscosity) Test Method for Rubber Property-Effect of Liquids Test Method for Ramsbottom Carbon Residue of Petroleum Products Test Method for Acid N u m b e r of Petroleum Products by Potentiometric Titration Test Method of Rust-Preventing Characteristics of Inhibited Mineral Oil in the Presence of Water Test Method for Foaming Characteristics of Lubricating Oils Test Method for Oxidation Characteristics of Inhibited Mineral Oils Test Method for Evaporation Loss of Lubricating Greases and Oils Test Method for Acid and Base Number by ColorIndicator Titration Test Method for Density, Relative Density (Specific Gravity), or API Gravity of Crude Petroleum Products by Hydrometer Method Test Method for Water Separability of Petroleum Oils and Synthetic Fuels Test Method for Rubber Property-International Hardness Test Method for Determination of Water in Liquid Petroleum Products by Karl Fischer Reagent Practice for Calculating Viscosity Index from Kinematic Viscosity at 40°C and 100°C Test Method for Oxidation Stability of Steam Turbine Oils by Rotating Pressure Vessel Test Method for Estimation of Molecular Weight (Relative Molecular Mass) of Petroleum Oils from Viscosity Measurements Test Method for Relative Molecular Mass (Molecular Weight) of Hydrocarbons by Thermoelectric Measurement of Vapor Pressure Test Method for Hydrolytic Stability of Hydraulic Fluids (Beverage Bottle Method) Test Method for Demulsibity Characteristics of Lubricating Oils Test Method for Thermal Conductivity of Liquids Test Method for Specific Heat of Liquids and Solids Test Method for Estimation of Solubility of Gases in Petroleum Liquids Test Method for Solubility of Fixed Gases in Liquids Test Method for Measurement of Extreme-Pressure Properties of Lubricating Fluids (Four-Ball Method) Test Method for Air Release of Petroleum Oils Test Method for Estimation of Solubility of Gases in Petroleum and Other Organic Liquids Test Method for Water and Sediment in Crude Oil by the Centrifuge Method (Laboratory Procedure) Test Method for Wear Preventive Cheiracteristics of Lubricating Fluid (Four-Ball Method) Test Method for Determination of Carbon Residue (Micro Method)
CHAPTER 14: COMPRESSOR LUBRICANTS D 5182 D 5800 D 6375
E 659
T e s t M e t h o d for E v a l u a t i n g t h e Scuffing L o a d Cap a c i t y of Oils ( F Z G V i s u a l M e t h o d ) T e s t M e t h o d for E v a p o r a t i o n L o s s of L u b r i c a t i n g Oils b y t h e N o a c k M e t h o d T e s t M e t h o d f o r E v a p o r a t i o n L o s s of L u b r i c a t i n g O i l s b y T h e r m o g r a v i m e t r i c A n a l y z e r (TGA) Noack Method T e s t M e t h o d f o r A u t o i g n i t i o n T e m p e r a t u r e of Liquid Chemicals
OTHER STANDARDS No. DIN 51352
DIN 51352
IP 239
ISO 5388 I S O 6743-3A
JIS K 2514
Title P a r t 1 D e t e r m i n a t i o n of A g i n g C h a r a c t e r i s t i c s of L u b r i c a t i n g O i l s . I n c r e a s e i n C o n r a d s o n c a r b o n residue after aging by passing air t h r o u g h t h e l u b r i c a t i n g oil. P a r t 2 D e t e r m i n a t i o n of A g i n g C h a r a c t e r i s t i c s of L u b r i c a t i n g O i l s . I n c r e a s e i n C o n r a d s o n c a r b o n residue after aging by passing air t h r o u g h t h e l u b r i c a t i n g oil i n t h e p r e s e n c e of iron(III) oxide. D e t e r m i n a t i o n of E x t r e m e P r e s s u r e a n d Antiw e a r P r o p e r t i e s of L u b r i c a n t s - F o u r Ball M a chine Method Stationary air compressors-Safety rules a n d c o d e of p r a c t i c e L u b r i c a n t s , i n d u s t r i a l oils, a n d r e l a t e d p r o d u c t s (Class L ) - C l a s s i f i c a t i o n - P a r t 3A: F a m i l y D (Compressors) T e s t i n g M e t h o d of O x i d a t i o n C h a r a c t e r i s t i c s of E n g i n e Oils
REFERENCES [1] Lilje, K. C , Short G. D., and Miller, J. W., "Compressors and Pumps," Ch. 4, Synthetic Lubricants and High-Performance Functional Fluids, 2"'' ed., L. R. Rudnick and R. L. Shubkin, Eds., Marcel Dekker Inc., NY, 1999, pp. 539-558. [2] Patzau S. and Szchawnicka, E., "Oils for Air and Technical Gas Compressors," Trybologia, Vol. 20, No. 4, 1989, pp. 18-21. [3] Scales, W., "Air Compressor Lubrication," Ch. 19, Tribology Data Handbook, E. R. Booser, Ed., CRC Press, Boca Raton, FL, 1997, pp. 242-247. [4] Bloch, H. P., A Practical Guide to Compressor Technology, McGraw-Hill Inc., NY, 1996. [5] "Unit Course 2-For Natural Gas Compressors-Considerations for Sizing a Reciprocating Gas Compressor Package," Weatherford Compression, Corpus Christi, TX. [6] Istvan, V., "Interplant Tests of Chemical Plant Lubricants: - Part 2-Compressor Oils and Heating System Oils and Their Application: Grease Lubrication," Magyar Kemikasok Lapja, Vol. 53, No. 3, 1998, pp. 139-144. [7] "Unit Course 1-For Natural Gas Compressors-An Introduction to the Basic Function and Components of a Gas Compressor Package," Weatherford Compression, Corpus Christi, TX. [8] Matthews, P. H. D., "Lubrication of Reciprocating Compressors," Journal of Synthetic Lubricants, Vol. 6, No. 4, 1990, pp. 292-317. [9] Wills, J. G., "Compressors," Ch. 14, Lubrication Fundamentals, Marcel Dekker, Inc., NY, 1980, pp. 365-394.
411
[10] Short, G. D., "Development of Synthetic Lubricants for Extended Life in Rotary-Screw Compressors," Lubrication Engineering, Vol. 40, No. 8, 1983, pp. 463-470. [11] Carswell, R., McGraw, P. W., and Ward, E. L., "A Blend of a Polyglycol and a Trtra Ester as a Lubricant for Rotary Screw Air Compressors," Lubrication Engineering, Vol. 39, No. 11, 1982, pp. 684-689. [12] Miller, J. W., "Synthetic and HVI Compressor Lubricants," Journal of Synthetic Lubricants, Vol. 6, No. 2, 1989, pp. 107-122. [13] Kist, K. R. and Doperalski, E . J . "Brief Introduction to the Screw Compressor," Paper 68E, AIChE 8 6 * National Meeting, 1979. [14] Tolfa, J. C , "Synthetic Lubricants Suitable for Use in Process and Hydrocarbon Gas Compressors," Lubrication Engineering, Vol. 47, No. 4, 1990, pp. 289-295. [15] Cohen, S. C , "Development of a "Synthetic" Compressor Oil Based on Two-Stage Hydrotreated Petroleum Basestocks," Lubrication Engineering, Vol. 44, No. 3, 1987, pp. 230-238. [16] Kajdas, C , "Industrial Lubricants," Ch. 8, Chemistry and Technology of Lubricants, 1992, pp. 196-222. [17] Arbocus, G., "Synthetic Compressor Lubricants-State of the Art," Lubrication Engineering, Vol. 34, No. 7, 1977, pp. 372-374. [18] Chen, C.-I., "Typical Lubricating Oil Properties," Tribology Data Book, E. R. Booser, Ed. CRC Press, Boca Raton, FL, 1997, pp. 333. [19] Miller, J. W., "Synthetic Lubricants and their Industrial Applications," Journal of Synthetic Lubricants, Vol. 1, No. 2, 1984, pp. 136-152. [20] Klamann, D., Ullmann's Encyclopedia of Industrial Chemistry, 5"" ed.. Vol. A15, VCH Verlagsges mbH, 1990, pp. 423-518. [21] Randies, S. J., "Esters," Ch. 3, Synthetic Lubricants and HighPerformance Functional Fluids, 2"^ ed., L. R. Rudnick and R. L. Shubkin, Eds., Marcel Dekker Inc., NY, 1999, pp. 63-102 [22] Sugiura, K., Miyagawa, T. and Nakano, H., "Laboratory Evaluation and Field Performance of Oil-Flooded Rotary Compressor Oils," Lubrication Engineering, Vol. 38, No. 8, 1982, pp. 510518. [23] Matthews, P. H. D., Walkden, R. H., and Williams, G. C , "Current and Future Developments of Lubricants for Industrial Air Compressors," Paper N u m b e r C477/023/94, ImechE Conference, Proceedings of the Institute of Mechanical Engineers, 1994, pp. 175-183. [24] Van Ormer, H. P., "Trim Compressed Air Cost with Synthetic Lubricants," Power, 1987, pp. 43-45. [25] Miller, J. W., "New Synthetic Food-Grade Rotary-Screw Compressor Lubricant," Lubrication Engineering, Vol. 40, No. 7, 1983, pp. 433-436. [26] Takashima, H., "Gas Compressor Oils," Toraiborojisuto, Vol. 38, 1993, pp. 135-139. [27] Hampson, D. F. G., "Reducing the Risk of Fires in Large Reciprocating Air Compressor Systems," Wear, Vol. 34, 1975, pp. 399407. [28] McCoy, C. S. and Hanly, F. J. "Fire-Resistant Lubes Reduce Danger of Refinery Explosions," The Oil and Gas Journal, November 10, 1975, pp. 191-197. [29] Garg, D. R., "Polyalkylene Glycol-Based Compressor Lubricants," presented at the Sixth Annual Reciprocating Compressor Conference, Salt Lake City, UT, 23-26 Sept. 1991. [30] Oberle J. and Rajewski, T., "The Development of Lubricants for Ammonia Refrigeration Systems," presented at the 11AR 19'*' Annual Meeting, 23-26 March 1997, New Orleans, LA. [31] Whiting, R., "Monitoring Energy Savings of Diester Compressor Oils," 6 * International Colloquium, Technische Akademie Essligen, 1998, pp. 14.41-14.43. [32] Mills, A. J., Tempest, M. A., and Thomas, A. S., "Performance Testing of Rotary Screw Air Compressor Fluids in Europe," Lubrication Engineering, Vol. 42, No. 5, 1986, pp. 278-286. [33] Blackwell, J. W., BuUen, J. V., and Shubkin, R. L., "Current and , Future Polyalphaolefins," Proceedings of the Conference on Syn-
412 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK thetic Lubricants, A. Zakar, Ed., Hungarian Hydrocarbon Institute, Szazhalombatta, Hungary, 1989, p p . 36-68. [34] Witts, J. J., "Diester Compressor Lubricants in Petroleum and Chemical Plant Service," Journal of Synthetic Lubricants, Vol. 5, No. 4, 1989, pp. 319-326. [35] Legeron J-P. and Beslin, L., "Rotary-Vane Compressors: Some Technical Aspects for Long-Life Lubricants," Journal of Synthetic Lubrication, Vol. 6, No. 4, 1990, pp. 299-309. [36] Totten, G. E., Bates, C. E., and Clinton, N. A., "Quench Bath Maintenance," Ch. 6, Handbook of Quenchants and Quenching
Technology, ASM International, Materials Park, OH, 1993, pp. 191-238. [37] Mang T. and Jiinemann, H., "Erdol kohle-Erdgas-Petrochem," vemeigt Brennstoff-Cheniie, Vol. 25, No. 8, 1972, pp. 459-464. [38] Rounds, F. G., "Coking Tendencies of Lubricating Oils," Paper 570097, Presented at the SAE National Fuels and Lubricants Meeting, Cleveland, OH, 1957. [39] Fedors, R. F. "A Method for Estimating Both the Solubility Parameter and Molar Volume of Liquids," Polymer Engineering and Science, Vol. 14, 1974, p. 147.
MNL37-EB/Jun. 2003
Refrigeration LubricantsProperties and Applications H. Harvey Michels^'^ and Tobias H. Sienel^
additives have been developed to improve the properties of a lubricant [7]. These include acid neutralizers, extreme pressure (EP) agents (for startup and compressor break-in operation), and anti-oxidants. The diminishing use of chlorinated refrigerants such as CCI3F, CCI2F2, and CHCIF2, mandated by the Montreal Protocol of 1987 to help reduce ozone depletion in the atmosphere, has put new requirements on refrigerant lubricants. The intrinsic lubricity characteristics of chlorinated refrigerant molecules are not found in the newer replacement refrigerants, such as the hydrofluorocarbons (HFCs). Further, many of the newer lubricants that have been formulated to be compatible with HFCs exhibit lubrication cheiracteristics that differ from the older mineral oils, especially in mixtures with HFC refrigerants. Such lubricants often depend heavily on additive properties to improve lubricity and anti-wear characteristics.
T H E PRIMARY FUNCTION OF A LUBRICANT IS TO REDUCE FRICTIONAL
LOSSES. This is achieved by interposing a film between the moving sohd surfaces that reduces or ehminates direct sohd to sohd contact [1]. In h y d r o d y n a m i c lubrication (HD), where contact surfaces conform geometrically, only the lubricant viscosity governs friction and lubricant film thickness [2]. In this region, which is typical of many bearing and piston surfaces, the contact surfaces are completely separated by the lubricant film and nominal contact pressures are low (<50 MPa). The lubricant film thickness varies with the same dependence on operating parameters, such as speed, shear rate, and applied force, as the viscosity for this region. This makes the correct choice of lubricant viscosity become an important issue in the HD region. Low viscosity may lead to solid contact, while too high viscosity leads to an increase in frictional losses resulting in increased motor loading a n d higher p o w e r c o n s u m p t i o n . In regions of elastohydrodyn a m i c lubrication (EHD), such as bearings or gears with high-pressure loadings, additional lubricant properties such as the pressure-viscosity coefficient and limiting shear stress come into play [3,4]. Temperature plays a critical role in both the HD and EHD regions because of its strong effect on the viscosity of a lubricant. In typical air conditioning (A/C) or refrigeration systems, both of these regions of lubrication may be encountered. The m a x i m u m pressure attained in a refrigeration system can be as high as 2.5 GPa, indicating operation well into fully developed elastohydrodynamic or boundary lubrication regions. Here, the thermophysical properties of the lubricant that become important are molecular conformability and resistance to physical degradation [5].
The proper choice of refrigerant lubricant also depends on the design of the A/C or refrigeration system and the choice of compressor. Since the compressor and evaporator are usually at the coldest region of the refrigeration system, migration of the refrigerant to these parts of the system during shutdown can cause dilution of the lubricant, resulting in a large decrease in solution viscosity. Several compressor designs have significant storage of lubricant in the sump region, thereby making the dilution of the lubricant from the condensed refrigerant a major start-up issue. This is often addressed by adding a strip heater and associated controls to boil off the more volatile refrigerant before starting the compressor cycle. Alternatively, additives are added to the lubricant to prevent direct metal to metal contact during start-up with a reduced viscosity lubricant/refrigerant mixture.
Secondary functions of a lubricant include: the transfer of heat from one region to another; to act as sealants, especially in pressurized systems; to protect surfaces from corrosion; and to capture and suspend system debris. Refrigerant lubricants often require compatibility with the system refrigerant in terms of solubility, miscibility, and chemical stability [6]. The solubility issue of lubricants with refrigerants is especially critical since dilution of the lubricant results is a dramatic reduction in the solution viscosity and a parallel reduction in the lubricant film thickness and lubrication properties. These characteristics have brought about a complex array of lubricants designed for specific system requirements, including the operating ranges of temperature and pressure, and the duty cycle. In addition, many chemical
A further consideration for proper choice of lubricant depends on whether the system is of the hermetically sealed type, such as that found in window rack or other portable A/C installations, or is designed to be serviced periodically, such as outdoor residential and commercial units. In hermetically sealed systems, the motor windings are exposed to the refrigerant/lubricant mixture. This mixture must have both good electric insulating properties a n d chemical compatibility with seals, materials of construction, and residual contaminants. A hermetically sealed system is charged with refrigerant and lubricant and sealed only once, with an expected maintenance-free life of fifteen years or more. The older lubricants, such as mineral oil or synthetic alkylbenzene formulations, were mainly non-polar and did not dissolve and hold significant amounts of residual materials of construction, assembly cleaning solvents, and the like in suspension. The solubility of such compounds is much higher in many of
^ United Teciinologies Research Center, East Hartford, CT 06108. ^ Present address: Physics Department, University of Connecticut, Storrs, CT 06269.
413 Copyright'
2003 by A S I M International
www.astm.org
414
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
the newer polar lubricants, such as the polyolesters. Further, the polar nature of several of the newer lubricants results in a hygroscopic characteristic whereby any residual water in the system is dissolved in the lubricant as well as carried about the system in the circulating vapor. This can permit insoluble residues to form, especially in additized lubricants, which then deposit in capillary tubes or other expansion devices, causing plugging and loss of performance [8]. In systems with service ports, a change of lubricant is possible after compressor break-in, which allows removal of insoluble system debris. In hermetically sealed systems, this residue problem must be addressed in the original system design. Complex lubricant additive packages and the use of filter driers to remove excess water and trap insoluble deposits are among current solutions being tested. A separate characteristic of refrigerant/lubricant mixtures that m u s t be considered is their foaming tendency. With most of the older chlorofluorocarbon/mineral oil mixtures, a dense, slowly collapsing foam is generated in the compressor, leading to good noise suppression and little carry-over to the condenser. The newer more polar refrigerant/lubricant mixtures exhibit rapidly collapsing foam characterized by initial large bubble formation. Such mixtures are less effective in lubrication of inlet ports in compressors and can result in excessive lubricant carry-over out of the compressor during start-up. Lubricant manufacturers often add small amounts of compounds, such as the polysiloxanes, that modify the mixture surface tension. The chemical stability and longterm behavior of these compounds are not well known. In addition to the chemical stability characteristics of refrigerant lubricants, there are equipment design and operational issues associated with the choice of lubricants for use with the newer hydrofluorocarbon refrigerant blends, such as R-410A, R-407C, and R-404A. The standard notation for single component refrigerants and refrigerant blends is given in ANSI/ASHRAE 34-2001. The designations are a function of the refrigerant atomic composition and, for blends, are chosen as ASHRAE industry standards. Miscible lubricants dissolve in the refrigerant and are circulated to a small extent throughout the system, returning to the compressor by either gravity or the drag force of the circulating refrigerant. Several of the new refrigerant blends, such as R-407C, exhibit fractionation owing to the different volatilities of the components of the blend [9,10]. The solubility characteristics of a lubricant may therefore be different in a compressor sump than in the evaporator or condenser. Regions of limited lubricant miscibility may therefore be encountered in the evaporator, for example, resulting in lubricant holdup and insufficient oil r e t u r n to the compressor. The required vapor velocity to insure good lubricant return can usually be obtained by proper sizing of the return lines [11]. For cooling mode operation, typical of an air conditioner (A/C), the worst lubricant return conditions are likely to be at the evaporator exit, where the vapor refrigerant line feeds into the accumulator. For heat p u m p operation, the worst lubricant circulation condition is usually found in the vapor line leaving the compressor, where the evaporator now serves as the system condenser. These equipment design issues associated with proper lubricant circulation and the elimination of excess holdup have been the subjects of several recent experimental studies.
Very recently, the use of immiscible lubricants, such as mineral oils, in systems using hydrofluorocarbon refrigerants, has been the subject of renewed discussion in the Heating, Ventilation, and Air Conditioning, (HVAC) and refrigeration industry. The initial decision of the HVAC industry to use polyalkylene glycol (FAG) and polyolester (POE) lubricants in conjunction with the new hydrofluorocarbon refrigerants was based on the assumption that high mutual solubility and miscibility between a lubricant and refrigerant were required for proper lubricant circulation and compressor operation. Thus, lubricants such as POEs were chosen because their polar characteristics are similar to hydrofluorocarbons, resulting in good mutual solubility and an assumed system behavior similar to the older hydrochlorocarbon and mineral oil mixtures. Recent lubricant circulation studies have shown, however, that except for low temperature refrigeration applications, immiscible lubricant (alkyl based)/refrigerant mixtures can be used in systems where return lines have been properly sized [12,13]. The potential for significant cost reductions due to the use of lower cost lubricants, such as mineral oils, rather than the more expensive synthetic lubricants, has generated renewed interest from HVAC system manufacturers. However, there are many issues associated with proper lubricant circulation, mixture chemical stability, and compressor lubrication that must be addressed before there will likely be a significant industry movement toward the use of immiscible lubricant/refrigerant mixtures. In the sections below, the properties of mineral oil based and synthetic refrigerant lubricants will be described. These lubricants differ in their chemical stability, solubility characteristics in different hydrohalocarbons, and viscosity and lubricity properties. There is extensive literature on the natural mineral oil based lubricants [14,15]. This review will focus more on the properties of the newer synthetic lubricant structures. The critical system issues such as oil holdup and compressor compatibility are also reviewed.
CHEMICAL STRUCTURE OF REFRIGERANT LUBRICANTS Mineral oil based lubricants used in refrigeration are generally of three types: paraffinic, which consist of both straight and branch chain saturated hydrocarbons; napthenic, which are saturated hydrocarbons that mainly consist of cyclic or ring structures; and aromatics, which are comprised mainly of unsaturated cyclic hydrocarbons. Typically, the crude oil sources from which these lubricants are distilled and refined contain a mixture of these mineral oil types. For this reason, it is c o m m o n to identify mineral oils in terms of the number of carbon atoms associated with each of the structures, paraffinic (Cp), napthenic (CN), and aromatic (NA): the carbon-type composition is usually expressed in terms of weight percent. Since this type of identification requires a time consuming laboratory analysis, more qualitative methods of analysis based on refractive index and photoabsorption have been introduced and standardized as ASTM D 2140, ASTM D 2549, and ASTM D 2008. In general, paraffinic oils have a higher percentage of paraffin wax (saturated straight chain molecules) than napthenic oils, which results in high pour points (ASTM D 97), the temperature at which the oil will flow
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AND APPLICATIONS
415
TABLE 1—Typical properties of refrigerant lubricants. Mineral Oil Property
Naphthenic
Parafflnic
0.92 33 12
Density @ 15°C (g/cc) Viscosity @ 40°C (cSt) Viscosity Index Pressure Viscosity Coefficient @ 6 0 X (GPa"') Average Molecular Weight Pour Point (°C) Flash Point (°C) Lower Critical Solution Temperature withR-134aorR-22'"(X)
0.86 34 95
25 300 -43 170
30 378 -18 200 52*
0*
Alkylbenzene
0.87 32 27 25 320 -46 178 -70*
Polyalkylene Glycol
0.99 30 210 14 750 -46 200
—
Polyolester
Polyvinylether
0.97 30 90
0.92 30 69
15-18 500-600 -48 234
20 500-600 -45 212
<34
S50
" Solutions witli mineral oil or alkylbezene.
CH3
CH3
I
I CH3 — CH — CH2 - C -
I CH3
I / ^
[CH—CH2]„—CH —CH3
' CH3 n = 2-5 branched alkyl groups
CH3 - CH - CH2 I [CHd^ I CH3
• CH - CH2 - CH2 I I [CHz]^ [CHj], I I CH3 CH3
FIG. 1—Structure of a commercial branched chain alkylbenzene.
FIG. 2—Structure of an oligomer of 1decene.
under gravity at prescribed conditions. Parafflnic oils, however, exhibit a m u c h higher viscosity index (ASTM D 2270) a measure of the viscosity change with temperature, t h a n napthenic oils. Owing to concerns about expansion valve and capillary line plugging, and oil trapping in evaporators, typical mineral oil formulations that have been in use in refrigeration have low wax content. Mineral oils are formulated in a fixed range of viscosity by adjusting the refining process. Specific system requirements, such as high resistance to oxidation, are mainly addressed by the use of additives. Synthetic lubricants are produced by chemically combining base stock hydrocarbon components in fixed proportions to form a mixture with the desired lubrication characteristics. Unlike mineral oils, which may consist of hundreds of different hydrocarbon molecules and whose composition varies with the source of the crude oil, synthetic lubricants are tailored from a small number of different molecular weight starting components that are reacted to yield a final mixture of compounds with the desired viscosity properties. These manmade lubricants can be classified by the type of base stock used in their manufacture. The following synthetic types are commonly used in A/C and refrigeration systems: alkylbenzenes, olefin oligomers (poly-a olefins), dibasic acid esters, polyolesters, and polyalkylene glycols [16,18]. Other synthetic lubricants such as the phosphate esters, silicones, monocarbonates, and fludrocarbon lubricants have not met with wide usage due either to their poor thermal stability, their viscosity and/or solubility characteristics with refrigerants, or their high cost of manufacture. Typical properties for several of the comm o n refrigerant lubricants are given in Table 1.
group can be either a straight chain or branched chain paraffin, usually containing 10-20 carbon atoms. A typical structure of a commonly used branched chain alkylbenzene is shown in Fig. 1. Alkylbenzenes have excellent low temperature fluidity and low pour points. Their viscosity indexes are somewhat lower than those of typical mineral oils. They are stable against oxidation, high temperature decomposition, and hydrolysis, and are completely compatible with mineral oils [19]. Alkylbenzenes have found widespread use with refrigerant systems using R-22 and R-502, but have had limited use to date with hydrofluorocarbon refrigerants owing to their immiscibility with these more polar molecules. However, since alkylbenzenes are man-made compounds, there have been several recent studies directed toward changing the typical balance of low, intermediate, and high molecular weight compounds that are formed in the synthesis process [12,13]. If the percentage of high molecular weight components could be kept low, the resulting alkylbenzene blend may have sufficient fluidity to be carried by the vapor stream back to the compressor with m i n i m u m holdup in the evaporator, for example. These newer alkylbenzene formulations are sometimes referred to as hard alkylbenzene lubricants (HAB). They are currently under evaluation for use in hydrofluorocarbon systems using rotary compressors, where vane wear is a serious issue with several of the synthetic lubricants.
Alkylbenzene (AB) These synthesized hydrocarbon molecules are formed by the alkylation of an aromatic compound, usually benzene, in the presence of a catalyst such as aluminum chloride. The alkyl
Poly-a Olefins (PAO) These compounds are formed by combining a low molecular weight unsaturated material, such as ethylene, into a specific olefin. The olefin structure is then polymerized to form a branched hydrocarbon structure, usually consisting of three or more repeating units. An example of this type of comp o u n d is shown in Fig. 2; it is formed by the polymerization of the olefin 1-decene.
416
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK thesized that exhibit similar physical properties [23]. These polybasic esters have tetrahedral character about a central carbon atom, similar to several of the polyolesters. They can be formed by the reaction of acrylate esters with malonate esters to form, for example, estrs of 1,3,3,5-pentanetetracarboxylic acid. Dibasic esters are used in the formulation of sjTithetic automotive lubricants and in jet engine oils. They are not highly miscible with the new hydrofluorocarbon refrigerants but their use in some refrigeration systems has been reported. However, this class of synthetic lubricants has not yet found widespread usage in new A/C and refrigeration systems.
Poly-a olefins are similar to paraffinic mineral oils but since they are m a n - m a d e , their c o m p o n e n t structure and composition are reproducible. By varying the pendant chain lengths and degree of branching, different viscosity formulations can be achieved. These compounds exhibit very high viscosity indexes and low pour points resulting in good low temperature fluidity [20]. They are often used in the formulation of synthetic automotive lubricants. Poly-a olefins have excellent thermal and oxidative stability and are compatible with mineral oils. They have found use in severe service refrigeration systems that employ screw compressors, using R22 as the refrigerant, and in ammonia systems. Like mineral oils, they exhibit good miscibility with several chlorofluorocarbons, such as R-12, but the use of this class of refrigerants is phasing out u n d e r the Montreal protocol. Poly-a olefins have limited miscibility in both R-22 and ammonia and typically are not used in light service A/C or refrigeration applications [21]. They are immiscible in hydrofluorocarbons due to their non-polar nature.
Polyolesters (POE) Polyolesters are the condensation product of an alcohol with two or more hydroxyl groups (polyhydric alcohol or polyol), and a monobasic (carboxylic) acid. In these structures, the carboxylic acid links to all available hydroxyls in the polyol, splitting off water molecules at each linkage. A typical polyolester is shown in Fig. 4. Similar to the dibasic esters, the polyolester structure can be varied by changing the carboxylic acid and by the use of one or more polyols [22]. Typically, neopentyl glycol, pentaerythritol, and dipentaerythritol are chosen as the polyols. Dipentaerythritol is used mainly to form polyolesters of higher viscosity but is typically found as a significant (—12%) by-product in commercial grades of pentaerythritol. Upon esterfication with a carboxylic acid, the result is a compound of high molecular weight and high viscosity. Such compounds are desired for high viscosity lubricant formulations but can lead to plugging and waxing in certain low temperature systems. Generally, pentaerythritol is reacted with two or more carboxylic acids to form an ester mixture with desired properties. Owing to the tetra-hydroxy structure of pentaerythritol, the use of two carboxylic acids results in the formation of five different esters. The number of possible esters increases rapidly if more carboxylic acids (or alcohols) are used resulting in complex chemical mixtures of the possible esters.
Dibasic Acid Esters (Diesters) This class of molecules is formed by combining a dibasic acid, such as adipic acid, with an alcohol that contains only one reactive hydroxyl group. The esterfication process joins the alcohols at the end of the dibasic acid, splitting off two water molecules to form the final ester compound [16,22]. An example of this type of ester molecule is shown in Fig. 3. Dibasic acid esters typically have very low pour points and excellent low temperature fluidity. Their viscosity indexes are tjTjically lower than poly-a olefins and they exhibit good thermal and oxidative stability. As with most ester structures, they exhibit a lower hydrolytic stability t h a n mineral oils since the double bonded oxygens have an affinity to hydrogen bond with water molecules. The viscosity of dibasic ester lubricants is determined by both the dibasic acid chain length and the type of alcohol employed in the esterfication process. A wide range of viscosities is possible by changes in the acid and alcohol structures. Polybasic esters have also been syn-
O CH3 — [CH2]3 — CH — CH2
O -
C -
O [CH2]4 -
C -
0 -
CH2 - C H
-
[CHzls - C H 3
I CH2
CH2
I
I
CH3
CH3
FIG. 3—dibasic acid ester of adipic acid and 2-ethyihexanoi.
O CH3 -
(CH2)4 -
C -
O O -
CH2 ^
^
CH2 -
O -
C -
(CH2)4 -
CH3
CH2 -
O -
C -
(CH2)4 -
CH3
c CH3 -
(CH2)4 -
C -
O -
CH2 /
II
11
o
o
FIG. 4—Polyolester of pentaerythritol and n-hexanoic acid.
CHAPTER
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LUBRICANTS—PROPERTIES
AND APPLICATIONS
417
Ri — O — [CH2 — CH — 0 ] n — R2
I CH3 n = # of branched — [CH2 — CH — O] —groups
I CH3 Rl ,R2 = alkyl end groups FIG. 5—Polyalkylene glycol derived from propylene oxide.
The viscosity a n d solubility properties of polyolesters can be modified by the use of different polyols and, more frequently, by the use of carboxylic acids t h a t are either branched or straight chain in character. Straight chain acids lead to esters with good lubricity, but these structures often suffer from limited miscibility and solubility characteristics with hydrofluorocarbon refrigerants. Similarly, b r a n c h e d chain acids lead to esters with good miscibility and solubility characteristics but poorer viscosity characteristics. Typically, a balance of acids (mixed acid POE) is used to formulate a polyolester blend with the specific properties required by the refrigeration equipment [1,24]. Ceu-boxylic acids that are in c o m m o n use in forming polyolester blends are n- and ipentanoic, n- and i-hexanoic, and 3,5,5-trimethylhexanoic. Blends using both straight and (n) branched (i) chain acids up to nonanoic (C9) structures have been foimulated [25]. Polyolesters have good high temperature stability and with proper acid (or polyol) choice, can be formulated with good low temperature fluidity and low pour point. They are chemically compatible with the hydrofluorocarbons that have been identified as replacement refrigerants for the chlorofluorocarbons. However, they exhibit several negative features in comparison with tj^ical mineral oils. Polyolesters have lower pressure-viscosity coefficients than mineral oils and alkylbenzenes. The pressure viscosity coefficient is defined as (3 In Tj/a p)x. This is a measure of how the viscosity of a lubricant increases with pressure at a constant temperature. See Table 1 for typical values for different lubricants. A lower pressure viscosity coefficient translates into reduced lubricant film thickness u n d e r high pressure loading such as would be encountered, for example, in rotary compressors. Polyolesters formulated with long, straight chain acids exhibit somewhat better pressure viscosity coefficients than those formulated with b r a n c h e d chain acids. Perhaps the most serious problem with polyolesters is their low hydrolytic stability. Hydrolysis of all polyolesters is thermodynamically similar with a small positive Gibbs energy change (—10 kJ/mol). Although this suggests that these compounds may hydrolyze at higher temperatures, the presence of a high energy activation barrier normally prevents their reaction back to form an organic acid. This activation barrier is larger (=» 40 kjol/mol) for polyolesters formed from a-branched carboxylic acids, where the branching nature of the alkyl chain hinders hydrogen bonding of water at the unsaturated 0 = C linkage in the ester. The carboxlyic acid, 2-ethyl (AG^ a 50 kv/mol) hexanoic, has been used extensively in Japan in polyolester formulations since it results in a very effective steric h i n d r a n c e against hydrolysis [26]. These polyolesters are generally referred to as hydrolytically stabilized polyolesters
(HSPOE), but their stability against hydrolysis is a function of all other acids used in their formulation [27]. This hydrolysis problem is of major concern since the resulting organic acids may initiate a chain of reactions with additives, metals, process fluids that are present in a system as contaminants, and even react with polyolester components in a process known as transesterfication that can result in ester modification. To minimize hydrolysis effects in hydrofluorocarbon systems, it is c o m m o n practice to incorporate a filter drier in the system to keep the water content to very low values (< 100 p p m ) . Despite the hydrolytic stability concern of polyolesters, they have emerged as the major lubricants in hydrofluorocarbon A/C and refrigeration systems. It is clear that their optimum formulation is dependent on the system application and compressor type. Polyalkylene Glycols (PAG) Polyalkylene glycols are sometimes referenced as polyglycols, polyethers, or polyglycol ethers. They are synthesized typically from ethylene or propylene oxides that are subjected to a polymerization process initiated using an alcohol or water [28]. The polymerization process can result in either monoether or diether types of structures, usually as a mixture with a range of molecular weight c o m p o u n d s . A typical polyalkylene glycol is represented in Fig. 5. Owing to their polar nature, polyalkylene glycols are completely miscible with most hydrofluorocarbon refrigerants. They have very good lubricity properties and exhibit large viscosity indexes. Polyalkylene glycols have good low temperature fluidity and low pour points, typical of naphthenic mineral oils [29]. They are not miscible with mineral oils and are somewhat less stable chemically than other synthetic lubricants, especially at high temperatures [30]. For many applications, thermal stability is not a major problem since the decomposition products are typically volatile, resulting in little sludge buildup. Thermal and chemical decomposition stabihty can be analyzed using the ANSI/ASHRAE 97-1989 test procedure. The major disadvantage is that these compounds, like all ethers, are very hygroscopic, due to hydrogen bonding of water at the ether linkages. This reduces the electrical resistivity of these lubricants to the point where they usually cannot be safely used in hermetically sealed or closed systems in which the motor windings are directly exposed to the refrigerant. Typical PAG formulations exhibit a volume resistivity of ~ 1 0 ' ^ fl cm in contrast to mineral oil and alkybenzene lubricants that typically exhibit values > lO^"* Cl cm. Volu m e resistivity can be measured using the ASTM D 1169 test procedure. This problem could be circumvented by care-
418 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK ful water control in the system, but their use in these applications has been replaced by polyolesters that are m u c h less hygroscopic. Polyalkylene glycols can be effectively used in systems using external motor drives for the compressor and have found significant acceptance in the automobile A/C market that has adopted R-134a as the replacement refrigerant for R-12. Polyvinylethers (PVE) Recently, a polyvinylether (PVE) structure has been synthesized at Idemitsu Kosan in Japan. This new ether-type structure is chemically similar to typical polyalkylene glycol structures with the exception that the ether linkages are in the branched hydrocarbon segments instead of being located in the backbone chain of the molecule [31]. A typical molecular structure for a poljrvinylether lubricant is given in Fig. 6. The groups indexed by n and m are adjusted to control lubricity and refrigerant solubility characteristics. Lubricity characteristics Eire discussed in ASTM F2161. A standard test procedure is available as ASTM D2670. Similar to polyalkylene glycol structures, and in contrast to polyolesters, polyvinylethers do not hydrolyze to form organic acids. However, because of the ether groups, they are hygroscopic. Tests indicate somewhat better stability against water pickup than is exhibited by many polyalkylene glycols. Polyvinylethers exhibit high pressure-viscosity coefficients that are nearly comparable to those measured for mineral oils. They are somewhat less thermally stable than the polyolesters. Poljrvinylethers are very miscibile in most hydrofluorocarbon refrigerants and exhibit a viscosity reduction in refrigerant/polyvinylether mixtures that is comparable to refrigerant/polyolester mixtures. Miscibility is usually described as the temperature at which a second liquid phase is formed, measured as a function of the refrigerant/lubricant composition. A lower temperature for phase separation indicates better miscibility. Their oxidation characteristics are somewhat poorer than mineral oils and alkybenzenes, and they are not as stable as polyolesters with regard to this property. The potential oxidation and hygroscopic characteristics exhibited by polyvinyl ethers have been addressed by the use of additives. This is a new class of refrigerant lubricants and will likely undergo extensive industry tests before decisions are made for its potential application in residential or commercial equipment [32]. In refrigerant system applications of these lubricants, some of the undesired properties can be modified by the addition of a small a m o u n t of an additive. The modification of the oxidation properties of a lubricant by the addition of an oxidation inhibitor is one of the oldest examples of this approach.
[Ri —CH2 — CHJn — CH2 — CH— R3 0
0-[R4-0]m-R5
R, Rx (x = 1-5) = alkyl groups FIG. 6—General polyvinylethers.
molecular
structure
for
REFRIGERANT LUBRICANT ADDITIVES Additives are chemical compounds added to lubricants to deliver specific properties such as moisture control, lubricity improvement, antioxidant character, etc. [7,33]. Standard industry test procedures are ASTM E 203 and D 6304 (moisture) and ASTM D 664 (total acid). Oxidation test include ASTM D 943 and ASTM D 5846. They can act chemically with system components or react with water and system debris to modify the break-in characteristics of an A/C or refrigeration system. Additives can also act to modify the physical properties of a lubricant such as its pour point or viscosity index. Most additives contain large hydrocarbon groups that often have a heteroatom, such as P, N, O, or S for functionality or to impart polar character. The solubility properties of additives in lubricants can be modified by changes in the hydrocarbon chain length or, in the case of additives used in synthetic lubricant formulations, by changes in the polarity of the molecule. Additives that are chemically active, in the sense that they can react with metals in the system or with contaminants in the lubricant, include dispersants, oxidation inhibitors, extreme pressure agents, corrosion inhibitors, and detergents. Additives that change physical property characteristics include pour point modifiers, anti-foam agents, viscosity improvers, and emulsifiers. Test procedures include ASTM D 97 (pour point), ASTM D 892 (foam tests), and ASTM D 445 (viscosity). The functionality of additives can be time dependent, such as those used for system break-in, or they can be expected to provide protection for the life of a system. In general, additives are more chemically reactive than the base stock lubricant or the refrigerant, and their formulation and dosage must be carefully examined. Excessive use of additives can lead to reactions that produce unwanted products that are insoluble or corrosive. Unfortunately, many lubricant manufacturers are reluctant to disclose additive formulations and it is often a hit or miss situation in formulating an additive package that provides the necessary protection for a system. It is often the case that an additive, such as an extreme pressure agent, can function properly in its role as added bearing protection during compressor break-in. However, it can also react with water and organic acids that may be present in the system to form insoluble compounds that can cause blockage in capillary tubes or expansion devices. The chemical formulation and required quantity of additives that are optimum for a given system design are difficult choices for both the lubricant supplier and the compressor or system manufacturer. The formulation of additives for use in synthetic lubricants is especially challenging since the possible chemistry with the more polar refrigerant and lubricant molecules is complex. Additive formulations that have been successfully used in chlorofluorocarbon/mineral oil systems have often proved to exhibit detrimental reactions in hydrofluorocarbon/synthetic lubricant systems (see [7] and references thereir). This has caused some HVAC equipment manufacturers to avoid the use of additive packages altogether, relying on system design changes for reliability. In general, less is better in the rule adopted for the use of additives in m o d e m equipment design. The functionality of lubricant additives include pour point depressants, viscosity index improvers, anti-foaming agents, detergents and dispersants, oxidation and corrosion inhibitors, rust inhibitors, and antiwear agents. There is a large literature
CHAPTER
15: REFRIGERATION
on the chemistry and properties of these additives (see [7] and references therein). This review will focus on problems that have been addressed by the use of additives in hydrofluorocarbon/s5mthetic lubricant systems. Anti-Oxidants All lubricants, since they are hydrocarbon in character, have some thermal limit against reaction with oxygen. Mineral oils have some natural anti-oxidant properties due to sulfur or nitrogen based components that are found in petroleum feedstocks. Synthetic lubricants that are used in high temperature applications, such as heat pumps, generally require the use of an anti-oxidant to modify their chemical reaction rate with oxygen [34]. Their chemical role is to modify the radical chain mechanism of the oxidation process, slowing it to a rate that is consistent with long term stability against significant decomposition [35]. Alkyl phenols, such as dibutylhydroxytoluene, are often used to protect polyolester lubricants against oxidation. Nitrogen containing inhibitors, such as the arylamines and phenols, that are often found in petroleum feedstocks and are effective anti-oxidants in mineral oil formulations, have been found to exhibit unwanted reactions in some synthetic lubricants. The typical amount of anti-oxidant that is used is 0.1-0.5 weight percent, depending on the potential exposure of the system to air in servicing operations.
Anti-Wear Additives Lubricants that are used in A/C and refrigeration systems suffer from dilution effects of the refrigerant with a corresponding decrease in viscosity. In systems that employ chlorofluorocarbon refrigerants, the refrigerant can react with the bearing metal under pressure, and the resulting frictional heat generated by sliding metal surfaces forms a metal chloride coating that protects against direct metal to metal contact. Hydrofluorocarbon refrigerants do not offer this protective feature and anti-wear additives are tj^ically added to the synthetic lubricants that are used in these newer systems. These additives can be classified into two categories: those that are formulated to provide protection under normal operating conditions (anti-wear agents) and those that are formulated for use during system break-in and extreme operating conditions (extreme pressure agents) [36]. Extreme pressure (EP) agents can be evaluated using the ASTM D 3233 test procedure. Both anti-wear and extreme pressure agents function by thermally activated chemical reactions, forming compounds that react with the metal surface to form a protective layer. Thiophosphates have t3TDically been used as anti-wear agents. Extreme pressure agents include aryl disulfides, thiocarbamates, and several classes of organic phosphorous compounds [37,38]. Extreme pressure agents react at higher temperatures than anti-wear agents. At higher temperatures (>200 C) and under heavy loads, these agents modify the structure of sliding surfaces by lowering the height of aspertites and forming a protective film. In effect, they act as polishing agents that reduce the high level of friction that may be encountered during system break-in. Experience with the use of anti-wear and extreme pressure agents in hydrofluorocarbon/synthetic lubricant systems has
LUBRICANTS—PROPERTIES
AND APPLICATIONS
419
uncovered several severe problems. Since most of these agents are designed to chemically react with metals, insoluble reaction products have been encountered that cause capillary and expansion device restrictions. Reaction of these agents with several types of synthetic lubricants has also been observed. Tricresyl phosphate, a particularly effective extreme pressure agent, is known to produce undesired reaction p r o d u c t s when used with some synthetic lubricants. Polyolester lubricants can absorb sufficient water to cause, under high temperature conditions, a series of reactions leading successively to diaryl phosphate, monoarylphosphate and phosphoric acid, which then reacts with iron bearing surfaces to form a protective iron phosphate surface coating. The problem is that these intermediate reaction products have limited solubility in hydrofluorocarbon/polyolester systems and typically deposit as metallic soaps rather than continuing in the reaction chain to form the desired ferrous phosphate coating. The use of oversized filter driers and careful control of moisture in a system are typical measures taken to circumvent this problem. Typically, 1.0 weight percent of extreme pressure agents are used in synthetic lubricant based systems, but values as high as 3.0 percent have been used in some polyolester based systems.
Acid Catchers The use of compounds to control acid levels is especially important in synthetic lubricant systems. In contrast to mineral oil lubricants, polyolester and polyalkylene glycols can adsorb (by hydrogen bonding), significant amounts of water. In some cases, the lubricant or system processing fluids can react with water, forming organic acids that can result in wear and corrosion. Tjrpical compounds that are used to control acid level are the alkanolamines, long chain amides and imines, carbomates, and epoxides. This latter class of compounds has found extensive use in polyolester based systems. T5rpical concentrations of 0.1-0.5 weight percent are used for water and acid control in hydrofluorocarbon systems. A potential disadvantage in the use of acid catchers is their reactivity with other additives such as anti-weeir agents. Control of water and acid by proper system design, cleanliness in system assembly, and proper maintenance are preferred strategies.
Anti-Foaming Agents Foaming in a refrigeration system is caused by mechanical mixing of the lubricant and the refrigerant, by the sudden release of refrigerant gas from the lubricant when pressures are reduced, and by outgassing that can occur at system start-up. The type of foam that is formed in chlorofluorocarbon/mineral oil systems is persistent and undergoes a slow collapse. In hydrofluorocarbon/synthetic lubricant systems, foam typically forms and collapses rapidly with the potential of lubricant transport out of the compressor sump zind ineffectiveness in its action to modify compressor noise. Compounds that have been used to control foaming characteristics include the poly dimethyl siloxanes, polyalkoxyamines, a n d polyacrylates. These compounds act to modify the surface tension of the lubricant/refrigerant mixture. They are typically employed at the 100-1000 p p m level. Their disadvan-
420 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
tage is that other chemical compounds in the system can often react with foaming agents rendering them ineffective. Their chemical stabihty under severe system operating conditions is also uncertain. Other additives that have been considered for use in A/C and refrigeration systems are copper deactivators to protect connecting lines and components, viscosity improvers, sealswell agents, detergents, and emulsifiers/demulsifiers. Such additives have the potential of creating more problems than are solved by their usage. Proper system design and choice of lubricant are more fundamental to long life system operation than the use of additives with their potential for adverse chemical reactions.
PURE LUBRICANT AND REFRIGERANT PROPERTIES The most important lubricant property from an A/C or refrigeration system application viewpoint is its viscosity. Viscosity is defined as the ratio of shear stress/shear rate and is a measure of the resistance of a fluid to flow. This property is often expressed in units such as Saybolt Seconds Universal viscosity (SSU), which are derived from a calibrated instrument used to measure flow resistance. The common metric unit for measuring absolute (djTiamic) viscosity is the poise or centipoise (cP). An alternate measure is called kinematic viscosity, which is measured in centistokes (cSt). These different viscosity measures are related by the density of the lubricant: McP) = p(g/cc) X T7(cSt)
(1)
The viscosity of a lubricant decreases as its temperature is increased. This viscosity decrease results in a thinning of the protective film that a lubricant forms on moving surfaces and is of critical concern in the choice of the viscosity grade of a lubricant. High viscosity lubricants can result in increased compressor requirements owing to fractional drag, while too low a viscosity can result in equipment failure. The temperature impact on the viscosity of a lubricant is often referred to as the viscosity index (see ASTM D 2270). An empirical relationship between kinematic viscosity and temperature is given by: ln[ln(T7 + 0.7] = a -I- b InT
(2)
where 77 is the kinematic viscosity given in cSt, T is the absolute temperature in Kelvin degrees and a and b are constants derived from experimental data. A more rigorous model for describing the temperature dependence of viscosity is the extended Andrade equation [39]: lnfi = A + YB + CT + DT^
(3)
where fji. is the absolute viscosity and A, B, C, and D are constants fitted to experimental data. This form is often more useful for modeling viscosity at the lower temperatures encountered in refrigeration. Eq 3 has been applied to model the temperature dependence for both refrigerant and lubricant viscosity. Pure refrigerant density is often represented by the equation: ft.(T) = a -I- bX -I- cX^ -t- dX^ + eX''
(4)
where T is absolute temperature, p in g/cc and X = (1 — T/Tc)^'^, where Tc is the refrigerant critical temperature, the temperature above which a substance cannot exist in the liquid state regardless of the pressure [40]. For lubricant density, the simpler polynomial form is often used: /»L(T)
a -F bT -(- cT^
(5)
where T is absolute temperature, p in g/cc. For refrigeration and other low temperature applications, a property known as the pour point is often important. The pour point of a lubricant is defined as the lowest temperature at which a lubricant will flow under gravity when tested by a prescribed method such as ASTM D97. The pour point is a complex function of viscosity, lubricant type, wax content (high molecular weight components) and additives [33]. It is of less value in A/C and refrigeration systems where the lubricant is diluted to some extent by the presence of the refrigerant but is of some value in the choice of lubricants that are immiscible with the refrigerant, such as carbon dioxide and ammonia systems. Other lubricant and refrigerant properties that are often measured are the flash and flame points. These are used to evaluate if a lubricant could decompose (coking) under extreme operating conditions. Vapor ignition points (lower flammability limits) have become of greater concern with the increased use of hydrofluorocarbon refrigerants. These data have been used to screen refrigerants and refrigerant mixtures to assure that they will not support combustion when exposed to air under normal system operating conditions. Flammability limits of refrigerants can be determined using ANSI/ASHRAE 34-1992 and ASTM G 125 test procedures. The models chosen to represent thermophysical properties of pure lubricants and refrigerants must be extended or modified to include the composition dependence of lubricant/refrigerant mixtures. The density of a lubricant/refrigerant mixture can be expressed from a rigorous thermodynamic analysis but requires mixture experimental data that are generally not available. The viscosity of lubricant/refrigerant mixtures is less well defined but several recent studies have resulted in useful (and accurate) models for A/C and refrigeration temperature and composition conditions [49,51]. The most difficult property of mixtures to model is the solubility of a refrigerant in a lubricant. Solubility effects are very important since the addition of a refrigerant to a lubricant lowers its viscosity [41,42]. This does not stand on its own but is described in detail in the following section.
SOLUBILITY AND VISCOSITY MODELING OF LUBRICANT/REFRIGERANT MIXTURES The role of refrigerant lubricants in A/C and refrigeration systems is complex. Their primary function is for compressor lubrication, but they also must function as coolants to remove heat from bearings and to transfer heat from the compressor shell to the exterior. Lubricants also minimize gas leakage at gaskets and fittings in the system and can act to reduce noise generated by the compressor. The principal differences between refrigerant lubricants and those used, for example, in automotive applications are the solubility ef-
CHAPTER
15: REFRIGERATION
LUBRICANTS—PROPERTIES
fects arising from a dilution of the lubricant with refrigerant. Under normal system operating conditions, the lubricant in the compressor crankcase may be diluted by a few percent of the refrigerant. Upon start-up, however, if the ambient temperature at the compressor location is cold, the lubricant in the compressor sump can be highly diluted (—30%) with refrigerant that has migrated to this cold point in the system. In most A/C and refrigeration applications, some lubricant is carried out of the compressor and circulates with the refrigerant. Typically, the percentage of circulating lubricant is small, especially for systems with scroll design compressors, but can be as high as 3% for systems with reciprocating compressors that have seen extensive service. This circulating lubricant-rich mixture can impact system performance in several ways. Refrigerant/lubricant miscibility limitations may result in a two-phase liquid region under certain conditions of temperature and pressure. The most serious effect is the possible development of a highly viscous phase in the evaporator section that may result in decreased heat transfer rates. In the case of refrigerant blends, such as R-407C, a new problem arises since the individual system components may exhibit different solubilities in the lubricant [9,10]. These different component solubilities can give rise to fractionation (distillation) effects in the evaporator, condenser, or compressor sump that result in different vapor-liquid equilibrium conditions and corresponding composition shifts. The effects of these refrigerant/lubricant interactions may impact both cycle analysis and system performance. If these chemical interactions are significant (non-ideal solution behavior), they may result in detrimental performance and lower system operating efficiency. Several models have been developed to analyze the solubility effects of lubricant/refrigerant mixtures. The thermophysical properties, density, viscosity and vapor pressure, must be modeled to include the effects of mixture composition.
Density The molar volume of a lubricant/refrigerant mixture can be written as: V„
U L V L + nRVR
(6)
where UL and n^ refer to the n u m b e r of moles of lubricant and refrigerant in the mixture, and VL = (3V/3nL)T,p,nR; VR = (c)V/c)nR)T,p,nL are the partial molar specific volumes of lubricant and refrigerant in the mixture, respectively [43]. Equation 6 can be cast into density form as: 1 Pmixture
Vmixture — ^ L V L + X R V R
(7)
where vmixture is the volume of one mole of the mixture and XL and XR represent the mole fractions of lubricant and refrigerant, respectively. It is often assumed that the specific volumes of the lubricant and refrigerant in the mixture are equal to their values as pure components: VL = VL and VR - VR. This simplification is useful if the lubricant and refrigerant components are chemically alike and are similar in size. Unfortunately, this is often not the case for synthetic refrigerant/lubricant mixtures, such as R-32/POE combinations, where the mixture exhibits significant non-ideal behavior.
AND APPLICATIONS
421
Assuming ideal behavior, Eq 7 becomes: 1 Pmixture
— Vmixture — X L V L +
XRVR
(8)
or recasting as mass fractions: 1 Pideal mixture
xf PR
(9)
where x*" now represents mass fraction and p"" represents mass density in g/cc. Empirical corrections to Eq 8 have been suggested in the form: Pmixture = pideai mixture/A, where A is a density correction factor that can be as large as 6% for high temperatures and high lubricant dilution. If the molecular weights of the refrigerant and lubricant are known (typical), Eq 6 can be used to extract partial molar volume data directly from measured mixture densities. This approach permits a cortelation of the deviations from ideal behavior with the individual component properties. The measurement of liquid densities to high accuracy is difficult (see ASTM D 891 for general methods).
Viscosity Viscosity models based on empirical polynomial expansions in temperature and composition have often been used to describe mixture behavior. Almost all models assume or require that values for the viscosity of the p u r e components be known. Thus, the primary feature for judging the validity of viscosity models for mixtures is their utility in interpolating data from the extreme composition limits of pure refrigerant and pure lubricant. It is known that the viscosity of liquid mixtures is not a simple function of composition. The viscosity of mixtures may exhibit a maximum, a minimum, or both over a specified composition range. For the pure components, the extended Andrade [39] equation, a polynomial in temperature, is often used to represent the temperature dependence of viscosity. Studies show that a four or five term expansion can accurately represent viscosity for pure comp o n e n t s over the n o r m a l operating range of interest, —20°C-80°C, for both refrigerant and lubricant. Difficulties can arise in developing reliable models to cover mixtures ranging from pure lubricant to high refrigerant dilution levels. Owing to the large size differences between refrigerant and lubricant molecules, a reliable model must account for both the difference in molar volumes of refrigerants and lubricants a n d for differences in molecular interaction strengths. Most studies of the viscosity of liquid mixtures have focused mainly on systems of molecules of nearly equal size and have been restricted mainly to near-ideal mixtures. An ideal mixture is one that undergoes zero volume change and zero heat exchange upon mixing. For such mixtures, Arrhenius [44] proposed that mixture viscosity should be an additive property, linearly connected to the viscosity of the pure components. Mixtures that exhibit this type of behavior are typiCcJly non-polar. Examples are hydrocarbons and halogenated hydrocarbons, such as CFCs. The introduction of HFCs as refrigerants, which are m u c h more polar, has led to the use of polar polyolester lubricants to improve solution solubility and miscibility. However, these new refrigerant/lubricant mixtures exhibit significant non-ideal character with very Icirge molecular size differences between refrigerant and lubricant.
422 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK If 4>i is taken as the volume fraction of component i, we can represent the viscosity of a binary lubricant/refrigerant mixture as: In rj„
Data Arrhenius: Kendall: Frenkel:
(10)
= (f>i In 171 + 4)2 In 172
Grunberg-Nissan: Yokozeki:
Equation 10 is commonly referred to as the Arrhenius [44] equation for ideal binary mixtures. It is useful for mixtures where there are no strong intermolecular forces (non-polar) and where there is n o volume change upon mixing. A variation of Eq 10 was proposed by Kendall [45], where composition is measured using mole fraction instead of volume fraction. Mixture equations of this additive type have b e e n extended by adding terms with one or more disposable constants. An example is the Grunberg-Nissan [46] equation, which for binary mixtures can be written as: lm7„
01 In TJi -I- <^2 In T?2 + 24>i(l>2Gi2
(11)
For binary polar mixtures, Irving [47] finds that the Grunberg-Nissan equation has the smallest average error when tested with nearly 1000 data points, taking 4>i as the mole fraction of component i and optimizing the value of the adjustable parameter, G12, using mixture data. A variation of Eq 11, based on a kinetic model involving binary pair activation energies, is due to Frenkel [48]: In T7„ix,ure = xf In Tji + xl In 172 + 2x1X2 In 1712
(12)
In Eq 12, Xi refers to the mole fraction of component i, and In TJ12 is a n interaction parameter between unlike molecules, usually determined experimentally from mixture data. Irving finds that Eqs 11 and 12 have comparable accuracy, comparing both polar and non-polar mixtures. Recently, Yokozeki [49] has proposed a mixture viscosity model in the form: mfxi 5^ ^i In Tji;
In rjn
6
McAllister 3-Body Model: McAllister 4-Body Model: —Q— Michels-Sienel:
0.3
0.4 0.5 0.6 Mass Fraction Refrigerant
0.7
0.8
FIG. 8—Viscosity fits for R134a/POE-68. (POE-68: polyolester of viscosit y 68 cP at 40°C.)
temperature. Yokozeki finds that k ~ 0.58 for many refrigerant mixtures but finds that k must be treated as an adjustable parameter for refrigerant/lubricant mixtures. For k = 0, the Yokozeki model reduces to the model proposed by Kendall. For k = 1, the concentration measure becomes weight fraction rather t h a n mole fraction. A genereilized definition of the concentration measure, ^i, can be shown to be related to the familiar q-ratio (the ratio of the van der Waals surface area for lubricant and refrigerant) in UNIFAC and other solution theory models [50,51]. In Figs. 7 and 8, the viscosities of R-22/AB150 and R-134a/FOE-68 mixtures are compared against several models that have been proposed [51]. In general, one- or two-parameter models are required to quantitatively represent experimental mixture data.
(13) Solubility
In Eq 13, Xj and m^ refer to the mole fraction and molecular weight of component I, respectively, and k is an empirical parameter, often modeled as a simple polynomial in the
Data -Airheimis: [44] -Kendall: [45] -Frenkel [48] -Grunberg-Nissan: [46] -Yoko2Eld: [49] - McAllister 3-Body Model: [47] - McAllister 4-Body Model: [47] -Michels-Sienel: [51]
I
The solubility of a refrigerant in a lubricant has been modeled using several different approaches. The older literature solubility data on mineral oil/CFC mixtures have mainly been analyzed using empirical correlations in temperature and composition [52]. Models that are thermodynamically more rigorous have been developed that are applicable to the more polar hydrofluorocarbon/synthetic lubricant mixtures [9,43,49,53]. These models have been based either on a n equation of state approach or on non-ideal solution theory. A comparison of the features of these different approaches is given in Table 2. Empirical correlations are usually polynomial fits of the mixture vapor pressure as a function of temperature (T) and composition (x). P = X aioT' + x X a i i P + x^ £ a i 2 r + . (14) P = £ bio/P + X X b„/T> + X2 X b i 2 / r +.
0
0.1
0.2
0.3
0.4 0.5 0.6 Mass Fraction Reffigerant
0.7
0.8
0.9
1
FIG. 7—Viscosity fits for R22/AB 150. (AB 150: allcylbenzene of viscosity of 150 cP at 40°C.)
InP = C(x)
D(x) T + constant
C(x) = Yi CiXi;
(Antoine form) [54] D(x) = 5^ diXi
CHAPTER 15: REFRIGERATION LUBRICANTS—PROPERTIES AND APPLICATIONS 423 TABLE 2—Approaches to modeling solubility of refrigerant/lubricant mixtures. Empirical correlations: • mathematically simple models (+) • often thermodynamically inconsistent (—) • applicable to limited composition ranges (—) • not useful for evaluating blend fractionation effects (—) • limited to bubble point predictions (-) • limited to two component (refrigerant/lubricant) composition analysis (-) Equation-of-state (EOS) approach: • mathematiccJly more complicated (—) • thermodynamically rigorous for pure components (+) • requires vapor phase properties of lubricant ( - ) • all thermodynamic properties derived from single EOS (+) • EOS mixture rules are often arbitrary and thermodynamically inconsistent (—) Non-ideal solution theory: • mathematically more complicated (—) • models are often thermodynamically inconsistent (—) • applicable to multi-component mixtures over entire compo-sition range {+) • obviates need for vapor properties of the lubricant (-I-) • applicable to both bubble and dew point predictions {+) • not useful in the vicinity of the critical point (—) • difficult to model temperature dependence (—) • most accurate modeling approach for mixtures, except in the vicinity of the critical point (-I-)
These forms are convenient for fitting to experimental data for mixtures. They often yield very inaccurate predictions if used outside of the fitting range and typically do not accurately represent the vapor pressure of the pure refrigerant. The non-ideal behavior of the liquid and vapor phase is lumped together in the expansion coefficients of these models. In general, there is n o thermodynamic basis for these functional forms. The Antoine form, for example, is developed from the Clausius-Clapeyron equation only for a pure fluid. In the equation-of-state (EOS) approach—models for mixtures are based on extension of the theory of corresponding states for pure substances using empirical mixing rules. Generally, cubic EOS are required for any quantitative treatment of mixtures: a) van der Waals model b) Soave modification of Redlich-Kwong equation (SRK) c) Peng-Robinson equation (PR) These EOS are described in detail by Prausnitz, et al. [43]. Yokozeki [49] and Morrison et al. [56] have independently developed EOS models that have been used to describe the solubility behavior of hydrofluorocarbon/polyolester lubricant mixtures. These models involve the difficult task of describing the pressure-volume-temperature (PVT) behavior for mixtures of two substances with vastly different vapor pressures. The usual mixture rules for combining the EOS fitting parameters for pure components are not applicable for refrigerant/lubricant mixtures without complicated modifications and inherent difficulties in fitting experimental data. The primary advantage of EOS models is their usefulness near the critical region of these mixtures. The most reliable solubility models are generally those that are developed from the thermodynamics of non-ideal solution theory [43]. The vapor-liquid equilibria of a mixture can
be formally described in terms of the component fugacities in the liquid and vapor phases. At equilibrium, we have fr = yiPT<^i' = f i = Xi7iPr
(15)
where yi = vapor phase molar composition of component i, PT = total system pressure at temperature T, 4>i = vapor phase fugacity coefficient which, for moderate pressure, can be estimated from second virial coefficient data, Xi = liquid phase molar composition of component i, •Yi = liquid phase activity coefficient, P,^ = vapor pressure of pure component / at temperature T, 4)* = fugacity coefficient for pure i at the system T and P, Fi = pointing factor for compressibility of the liquid phase. J , * ^= T7? (f)* F | = 1.0, this analysis reduces to an idea solution description and is representative of Raoult's Law. Combining the fugacity coefficients and Poynting factor into a correction term, F„ the vapor-liquid equilibria for component i in solution can be written as:
F o r 4>}'
^
iVc! yiPr = Xi7iPrFS
(16)
The correction term, Fu can be evaluated from liquid density and second virial coefficient data for pure refrigerants. One convenient source is the tabulation given in the NIST REFPROP [57] database. The difficult part of this modeling approach is the representation of the liquid phase activity coefficients, 7,, w h i c h are formally connected to the excess (non-ideal) Gibbs energy. These liquid activity coefficients may be extracted from experimental data or estimated using group additivity models such as UNIFAC [50]. The latter approach is difficult at present due to limited knowledge of the chemical formulations of the synthetic lubricants and the lack of reliable functional group interaction parameters. Preliminary evaluation of the non-ideal behavior of HFC32/POE, HFC-125/POE, and HFC-134a/POE binary mixtures has indicated b o t h positive and negative deviations from ideal solution behavior [9,58]. Many of the proposed forms for liquid phase activity coefficients cannot mathematically represent such behavior. The Wilson [59] model for the excess Gibbs energy, for example, is not applicable over the entire refrigerant/lubricant composition range. Various modifications of the Wilson model have been proposed [60] including those described in the literature as the Heil [61], NRTL [62], and T-K [63] equations. All of these equations represent local composition models in an attempt to incorporate effects of molecular size as well as mixture concentration. Their derivations, however, are mainly empirically based, and can lead to computed solution parameters that lack physical meaning. Recent studies have shown that the Wohl [3]-suffix expansion [64] for representing the excess Gibbs energy, written in terms of an effective volume measure of composition, yields reliable results for many lubricant/refrigerant mixtures. A comparison of experimental vap o r pressure data and calculations based on the Wohl [3]-suffix model is shown in Figs. 9 and 10 for R-134a/POE68 and R-410A/POE-68 refrigerant/lubricant mixtures [9]. The theory is somewhat better for modeling single component refrigerants, such as R-134a. Solution theory models, in
424 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK —
T=60C.--
• T-OfC
surements but can be formally located through an analysis of the Gibbs excess energy for the mixture [43].
-T=20fC
—
- T=40° C - T=60° C
c o
•a
0
0.2
0.4
0.6
0.8
A
T=0°Cdata
•
T= 20° C data
•
T= 40° C data
•
T=60°Cdata
1
R-134a liquid mass fraction
FIG. 9—Vapor pressure of R-134a/POE-68 mixture.
_ .
- T-OPC -T=2ff'C
—
- T=40° C - T=60° C
A
• •
• 0.2
0.4
0.6
T=0°Cdata T= 20° C data T- 40° C data T- 60° C data
0.8
R-410A liquid mass fraction
FIG. 10—Vapor pressure of R-410A/POE-68 mixture.
general, do not universally apply to liquid-liquid equilibria such as is required for studies of immiscibility. An advantage of a solution theory approach is that there is no need for vapor phase lubricant properties since it is a good assumption that the concentration of lubricant in the vapor phase is negligible. A disadvantage of this approach is that solution theory models are most useful for T < Tc. A very useful method to determine the kinematic viscosity of lubricant/refrigerant mixtures is based on combining viscosity and solubility data as shown by Daniel [20]. In this representation, the kinematic viscosity of refrigerant/lubricant mixtures is plotted against temperature with mixture isobars as parameters. These plots are now universally adopted by lubriccmt manufacturers to display the solubility characteristics of their lubricants. A typical Daniel plot for R-41OA and a 22 cSt mixed acid polyolester is shown in Fig. 11. The rapid decrease in mixture viscosity that results from refrigerant dilution of a lubricant is apparent for the lower temperature mixtures shown in Fig. 11. This is of critical concern for compressor design, especially for those conditions encountered at system start-up. Figure 11 also indicates the temperature/composition regions (gray shaded) where liquid phase immiscibility is encountered. This refrigerant/lubricant pair exhibits both a lower consolute temperature of —25 C and an upper consolute temperature of ~ 15 C. Within the shaded region, two liquid phases are present for this mixture. This immiscibility region is usually located from experimental mea-
LUBRICANT CIRCULATION The purpose of the lubricant in the HVAC system is to lubricate, cool, seal, and otherwise protect the compressor in the system from failure. In the course of executing these functions, some lubricant is captured in the compression process and carried by the refrigerant out of the compressor. Within the remainder of the system, the lubricant serves little to n o useful function, and acts as a detriment to system performance u n d e r most conditions. The challenge to the system designer is to ensure that the lubricant returns to the compressor satisfactorily u n d e r all operating conditions, as a prolonged migration of the lubricant from the compressor into the system will result in performance degradation and eventually compressor failure due to lack of lubricant. The design considerations influencing adequate lubricant return consist of line sizing, lubricant and refrigerant selection, system operational envelope, and transient effects, each of which will be described in further detail. Measurement of the circulation in these systems is also a challenge, particularly when transient phenomena occur, and this will be considered in further detail as well. HVAC Cycle Vapor compression systems, in basic form, consist of a compressor, a heat rejecting heat exchanger (typically referred to as a condenser), an expansion device, and a heat absorbing heat exchanger (t5^ically referred to as an evaporator). The cycle begins with the refrigerant as a high pressure vapor at the exit of the compressor. It next flows through the condenser, rejecting heat, cooling, and condensing to become a liquid at the discharge of the condenser. Next, the pressure of the refrigerant is dropped as it flows through the expansion device, which can be as simple as a kink in the refrigerant line or as sophisticated as a microprocessor-controlled, electronically-actuated stepper motor connected to a needle valve. At the exit of the expansion device, the refrigerant has typically entered into the two phase region again and enters the evaporator. The evaporator absorbs heat from the environment and uses it to evaporate the refrigerant back to vapor form. Finally, the vapor refrigerant enters the compressor, where it is compressed and heated to begin the cycle again. This process is shown schematically in Figs. 1 2 a n d l 3 , which depict the pressure-enthalpy and temperature-entropy diagrams of the cycle. This cycle is important to understand as it influences all of the design considerations impacting lubricant transport. In addition to the basic components, many systems are equipped with additional components serving a variety of functions. Charge storage devices, such as accumulators and receivers, are used in systems that have large operational envelopes and, due to the density differences of the refrigerant at the different operational temperatures, require significantly different amounts of refrigerant at different conditions. Receivers are employed where liquid flows through the lines, typically in the high pressure part of the system between the exit of the condenser and the expansion device. The liquid flowing into the expansion device is drawn from the receiver and, be-
CHAPTER 15: REFRIGERATION LUBRICANTS—PROPERTIES AND APPLICATIONS 425 400
10
20 Temperature (C)
60
30
FIG. 11—Daniel plot of R-410A/POE-22 mixture.
10000 Liquid «>
1000
i I
100
10 50
250 450 Enthalpy (kj/kg)
1.5
1 1.5 Entropy (kJ/kg-K)
2
FIG. 12—R-134a P-h diagram.
FIG. 13—R-134a T-s diagram.
cause there is a reservoir of liquid to draw from, a system equipped with a receiver is more tolerant to transients in the condenser. Accumulators, on the other hand, are applied where refrigerant vapor flows, typically between the exit of the evaporator and the inlet of the compressor. The purpose of the accumulator is twofold; it stores excess charge and also prevents liquid (which can damage the compressor) from entering the compressor. Accumulators act as effective oil separators, and m u s t b e equipped with a m e c h a n i s m to allow
lubricant to return to the compressor, as they would otherwise simply absorb all of the oil from the compressor. Because the refrigerant density is substantially higher in the high-pressure part of the system, receivers Eire preferred when the size of the equipment becomes a concern. All cheirge storage devices can trap substantial amounts of lubricant, particularly when the refrigerant-lubricant combination is not fuUy miscible. Other components utilized in HVAC systems include filter/dryers, reversing vtJves, and secondary heat exchangers
426
MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
such as liquid suction heat exchangers. Filter/driers, as the name imphes, serve the two functions. The fiUering of particulates, which can plug the expansion device or damage the compressor, is usually accomplished by screens, meshes, or felts. Moisture can react with lubricants in the system to decompose them back to acids and alcohols, and the dryer section of the filter/dryer is designed to remove any residual moisture from the system. Filter/dryers can be applied in the vapor or liquid portions of the system, but are usually found in the liquid flow due to pressure drop concerns. Reversing valves are used in heat pump systems, where the system operates part of the time as an air conditioner and part of the time as a heater. They simply reverse the flow through the heat exchangers, so that the condenser becomes the evaporator and visa-versa. The lubricant in the system aids the valve actuation process, but usually does not impact this component in any other way. Liquid suction heat exchangers act to further cool refrigerant exiting the condenser (to lower the entheJpy at the inlet of the expansion device) at the price of heating the suction vapor to the compressor. Finally, oil separators are sometimes employed to ensure adequate lubricant return to the compressor. These devices are tj'pically applied at the exit of the compressor, and use a variety of methods to separate and return the lubricant to the compressor. Equipment Line Sizing The system components are connected by lines, which cdlow the flow of refrigerant (and lubricant) from component to component. The pressure drop through these lines, and also through the heat exchangers, has a considerable impact on system performance and the tradeoff between system performance and cost is made in sizing the diameter of these lines [65,66]. For optimal performance, these lines would be sized to ensure minimum pressure drop, and this would be a valid philosophy if the lubricant were not present. In the liquid line, the lubricant flows in suspension (in the case of an immiscible lubricant-refrigerant combination) or as a fully soluble part of (when fully miscible) the refrigerant. There is typically no lubricant transport issue in liquid lines. In vapor lines, lubricant transport becomes much more of an issue, as the lubricant does not vaporize along with the refrigerant. Lubricant transport in vapor lines occurs by two primary mechanisms: mist flow and annular flow. In mist flow, the lubricant is atomized into small droplets that are carried in suspension with the vapor refrigerant. In annular flow, the lubricant builds up along the perimeter of the line and is dragged along by the velocity of the vapor flow. Mist flow dominates when the lubricant viscosity is low while annular flow is more prevalent when the lubricant viscosity is high (when the annular flow becomes laminar). As pressure drop is a function of Reynolds number, (Re = , where D = tube diameter, v = velocity, fi = viscosity) if the velocity of the vapor flow is decreased, the pressure drop will also decrease. However, as the velocity is reduced, the shear forces eventually become insufficient to drag the annular flow of lubricant, and the droplets in the mist flow succumb to gravity forces and drop out of suspension from the vapor flow. This condition essentially stops lubricant transport through the system, and is referred to as the minimum
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velocity condition. The relationship between the pressure drop, velocity, and flooding (or lubricant holdup) is shown in Fig. 14 for the combination of R-410A and POE-68. A number of correlations have been devised to predict when this condition is reached in practice [67,68]. Under this condition, the lubricant will build up in the section of the system experiencing minimum velocity, typically in traps or along the bottom of lines. This buildup of lubricant in the system is referred to as holdup, and can have severe consequences on system performance. As this buildup continues, the area available for flow will contract, and thus the velocity will increase again. Due to pressure and surface tension effects, the transition from a less than adequate velocity to a sufficient velocity is often dramatic. The pressure buildup behind the lubricant obstruction, coupled with the transition to adequate transport velocity, causes a slug of lubricant to burst from the trap and to be carried to another part of the system [69]. When the lubricant obstruction has been cleared, the flow velocity again drops and lubricant again becomes trapped. This cycle repeats on a frequency dictated by the line size, vapor velocity, and characteristics of the refrigerant-lubriccint pair. In some cases, the slug transport never occurs due to the amount of available surface area, such as in a horizontal line, or due to extremely high lubricant viscosity. Refrigerant/Lubricant Selection The selection of the refrigerant is based on the operational performance given the envelope of operating conditions, as well as, increasingly, the environmental effects of the refrigerant. The lubricant selection follows the refrigerant selection, as in most cases, it is based on the refrigerEint selection. The specific compressor type will dictate a minimum operational viscosity for adequate lubrication given the operational characteristics. The solubility of the lubricant with the chosen refrigerant, at the most severe operational conditions, determines the viscosity grade of the chosen lubricant. Other considerations for lubricant selection are related to the operational envelope of the system, particularly for refrigeration systems. If the refrigerant-lubricant combination exhibits regions of immisciblity, these must be carefully considered in system design, particularly for charge storage components. The re-
CHAPTER 80 60 40 20 0 E u -20 w -40 H -60 -80
I
15: REFRIGERATION
Immiscible Lower CST Miscible Upper CST Immiscible
20
40
60
80
Oil Concentration (%) FIG. 15—Critical solution temperatures for R410A/POE-68.
gions of immiscibility are usually defined by the critical (consolute) solution temperature, as shown in Fig. 15 for R-410A and POE-68. In the immiscible region, two liquid phases will form, one lubricant rich and one refrigerant rich. These two phases will have different densities, and in an actual system, stagnant liquid will tend to separate according to these density differences. If the lubricant rich phase is less dense than the refrigerant rich phase, it will float above the refrigerant dense phase. This is the preferred arrangement for most of the system lines, as the lubricant rich phase can still be entrained into the vapor flow. If the situation were reversed, the lubricant rich phase could become trapped below the refrigerant rich phase, leading to poor lubricant return to the compressor. On the other hand, for receivers the situation can be reversed, as liquid flow to the expansion valve is generally drawn from the bottom of the device. If the lubricant rich phase floats above the refrigerant rich phase in this case, it will not be drawn into the remainder of the cycle and will essentially become trapped. For accumulators, where vapor flows from the exit and only a small a m o u n t of liquid becomes entrained into this vapor flow through an oil return mechanism, an immiscible system in combination with system geometry can also produce a situation where lubricant does not flow back to the compressor. Experiences with this situation have led to accumulator designs where the contents are continuously agitated so as to reduce or eliminate stratification. Flooded heat exchangers are also a concern for lubricant holdup. These typically shell and tube heat exchanges are used in chiller or other liquid heat transfer systems where, if the lubricant rich phase is less dense than the refrigerant rich phase, oil holdup can occur in portions of the heat exchanger.
System Operational Envelope As the temperature of the lubricant is reduced, its viscosity will increase. The degree of solubility of the lubricant with the refrigerant, coupled with the viscosity index of the lubricant, combine to determine the mixture viscosity. This is shown in Daniel plots, such as the one in Fig. 11. For refrigeration conditions, the evaporation temperature can be very low, and the corresponding viscosity in the vapor phase can
LUBRICANTS—PROPERTIES
AND APPLICATIONS
427
be extremely high. Newer refrigerant-lubricant combinations, such as HFC-POEs, have tended to exhibit higher viscosities than the older combinations, such as CFC-MOs that have been used for these applications. When the lubricant viscosity becomes sufficiently high, it becomes similar to wax, and transport becomes difficult, if not impossible. As lubricant continues to build u p in the low temperature portion of the system (evaporator), this will have a noticeable effect on heat transfer and pressure drop in the heat exchanger and thus on performance. The saving grace for these conditions is often system transients, such as defrost cycles or slugs of relatively w a r m refrigerant, which sufficiently decrease the viscosity of the lubricant to allow transport back to the system. At the same time, due to the high pressure differences in the system, compressor discharge temperatures are higher than at low lift conditions. Both of these conditions lead to contradictory requirements: the desire for a low viscosity lubricant at low temperature, refrigerant dilute conditions; and a relatively high viscosity lubricant at high temperature, refrigerant rich conditions. This is, of course, the same lubricant, and must be chosen carefully.
Lubricant Circulation Rate The circulating lubricant concentration has an effect on performance and the design of system components; knowledge of this concentration (referred to as oil concentration rate, or OCR) can be quite important. The basic methods for determination of OCR can be broken down to invasive and non-invasive techniques. The easiest method for determination of OCR is by use of a sampling volume, as outlined by ASHRAE [70]. A sample of liquid refrigerant is c a p t u r e d in a volume (previously weighed), which is then weighed. The refrigerant in the volume is next slowly released with care taken not to allow lubricant to escape with the refrigerant. After refrigerant evacuation, the sample volume is weighed again to determine the residual oil weight. This system determines the instantaneous OCR in the liquid line of the system. This information is of limited vaiue and may be very misleading when significant transients, such as lubricant slugging, are occurring. This method, by its nature, is also quite invasive to the system and will result in a loss of charge every time a measurement is taken. A less invasive technique employs oil separators to ensure little or no residual CEirryover to the remainder of the system [71]. The separated oil flowrate is measured and compeired to the flowrate in the remainder of the system to determine OCR. This technique requires two flowmeters and will only work in the vapor region of the system (such as at the compressor discharge). It is still invasive as it disrupts the normal flow of lubricant through the system and may mask slugging trouble spots in the system. There have been a n u m b e r of minimally invasive or non-invasive OCR measurement systems developed recently, which are much better at identifying problems with system design. Coriolis flow meters can provide bulk flow density information. Knowledge of the pressure and temperature at the density measurement point coupled with refrigerant and lubricant properties, which are used to d e t e r m i n e p u r e fluid density at the measurement point, can lead to a determination of the OCR [72]. However, liquid refrigerant and lubri-
428
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
cant densities tend to be quite similar, requiring the density measurement accuracy to be high. In some cases, the error band associated with the instrumentation accuracy obscures the entire range of measured oil concentrations, although some authors have reported success with this technique. A commercially available unit for measurement of OCR by ultraviolet (UV) absorption has been on the market for some time, and some success has been reported in the use of this device [73]. Several researchers have been working on index of refraction type OCR measurement systems, although none have yet been commercialized [74]. Acoustic velocity and direct fluid viscosity measurement have also been proposed as the basis for determining OCR, once again by comparing the known pure fluid properties to the measured bulk property values [72]. Both the UV and index of refraction measurements cire optical techniques, and cire thus fully non-invasive, while the density m e a s u r e m e n t technique typically introduces a pressure drop in the liquid line. All of the above mentioned measurement systems work only in the liquid line of the system cind are severely influenced by system transients, which bring two-phase flow (bubbles) to the measurement points. In addition, the capability of these devices to measure OCR in immiscible mixtures is also in doubt.
formance cheiracteristics of the system. This is generally only of concern when the ratio of lubricant to refrigerant is high. Finally, careful consideration of edl of these system variables is the challenge given to designers of m o d e m HVAC and refrigeration equipment.
ASTM STANDARDS No. D 97 D 445 D 664 D 891 D 892 D 943 D 1169 D 2008
PERFORMANCE EFFECTS D 2140 The lubricant in the HVAC system has distinct performance impacts on the compressor, the heat exchangers, and working fluid, all of which combine to affect system performance. The compressor requires a certain lubricant viscosity for adequate protection. If the fluid viscosity drops below this limit, damage to the compressor may occur due to bearing failure. On the other hand, if the lubricant is too viscous, the drag on the bearings as weU as any sealing surfaces will increase, resulting in an increase in consumed power and a reduction in performance. For the heat exchangers, many of the lubricant impacts have already been discussed. Some studies have suggested that trace amounts of lubricant actually aid in heat transfer properties and thus system performance [75]. However, anything above trace amounts (typically 0.1% OCR) has generally been reported to have a negative performance impact. Lubricant in the vapor phase of the heat exchanger introduces a contact resistance, which detracts from the overall heat transfer coefficient. The degree to which this impacts performance is a function of the amount of OCR and lubricant hold up. Studies have suggested that the degree of performance degradation caused by the lubricant can be related to the lubricant viscosity at the exit of the evaporator [76]. Under refrigeration conditions, the holdup can have a severe performance impact as the holdup also influences pressure drop. Finally, the fluid performance is also impacted due to the displacement of refrigerant from the flow. The lubricant does not aid substantially in the transfer of heat from the heat absorbing heat exchanger to the heat rejecting heat exchanger. Thus, for every percent lubricant flowing in the bulk flow, a corresponding percent drop in performance can be expected. For multi-component refrigersint mixtures (ctny of the R-400 series of refrigerants), differential solubility effects can cause the circulating composition to fractionate, changing the per-
D 2270 D2549
D 2670
D 3233
D 5846
D 6304
E 203 F2161 G 125
Title S t a n d a r d Test Method for Pour Point of Petroleum Products Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids S t a n d a r d Test Method for Acid N u m b e r of Petroleum Products by Potentiometric Titration Standard Test Method for Specific Gravity, Apparent, of Liquid Industrial Chemicals Standard Test Method for Foaming Characteristics of Lubricating Oils Standard Test Method for Oxidation Characteristics of Inhibited Mineral Oils S t a n d a r d Test Method for Specific Resistance (Resistivity) of Electric Insulating Fluids Standcird Test Method for Ultraviolet Absorbance and Absorptivity of Petroleum Products Standard Test Method for Carbon-Type Composition of Insulating Oils of Petroleum Origin Standard Practice for Calculating Viscosity Index from Kinematic Viscosity S t a n d a r d Test Method for Separation of Representative Aromatic and Nonaromatic Fractions of High-Boiling Oils by Elution Chromatography Standard Test Method for Measuring WeEir Properties of Fluid Lubricants (Falex Pin a n d Vee Block Methods) Standzird Test Method for Measurement of Extreme Pressure Properties of Fluid Lubricants (FeJex Pin and Vee Block Methods) Standard Test Method for Universal Oxidation Test for Hydraulic a n d Turbine Oils Using the Universal Oxidation Test Apparatus S t a n d a r d Test Method for Determination of Water in Petroleum Products, Lubricating Oils, and Additives by Coulometric Karl Fischer Titration Standard Test Method for Water Using Volumetric Karl Fischer Titration S t a n d a r d Guide for I n s t r u m e n t and Precision Bearing Lubricants—Part 1 Oils S t a n d a r d Test Method for Measuring Liquid and Solid Material Fire Limits in Gaseous Oxidants
ANSI/ASHRAE STANDARDS 34-1992
Determination of Refrigerant Lower Flammability Limit in Compliance with Proposed Addend u m p to Standard 34.
CHAPTER 15: REFRIGERATION LUBRICANTS—PROPERTIES AND APPLICATIONS 429 97-1989
34-2001
Sealed Glass T u b e M e t h o d to Test the Chemical S t a b i l i t y of M a t e r i a l s for U s e w i t h i n R e f r i g e r a n t Systems D e s i g n a t i o n a n d S a f e t y C l a s s i f i c a t i o n of Refrigerants
REFERENCES [1] 'Wills, J.G.,LubricationFundamentals,MarcelDekker,NY, 1980. [2] Cameron, A., Basic Lubrication Theory, 3'''^ ed., Ellis Honvood, Ltd., NY, 1981. [3] Evans, C. R. and Johnson, K. L., "The Rheological Properties of Elastohydrodynamic Lubricants," Proceedings of the Institute of Mechanical Engineers, Vol. 200C, 1986, p. 303. [4] Gunsel, S., Korcek, S., Smeeth, M., and Spikes, H. A., "The Elastohydrodynamic Friction and Film Forming Properties of Lubricant Base Oils," Tribology Transactions, Vol. 42, 1999, p. 559. [5] Akei, M. and Mizuhara, K., "The Elastohydrodynamic Properties of Lubricants in Refrigerant Environments," Tribology Transactions, Vol. 40, 1997, p. 1. [6] Henderson, D. R., Rajewski, T. E., Speich, C. F., and Greig, B., "Lubricants in Refrigerant Systems," Chapter 7, ASHRAE Handbook-Fundamentals, ASHRAE Publications, Atlanta, 1997. [7] Rizvi, S. Q. A., "Lubricant Additives and Their Functions," ASM Handbook, ASM International, Materials Park, OH, Vol. 18, 1992, pp. 98-112. [8] DeVos, R., "R-134a Qualification-Industry Refrigerator Capillary Data," ASHRAE Transactions, BN-97-7-4, 1997, p. 1. [9] Michels, H. H. and Sienel, T. H., "Solubility Modeling of Refrigerant/Lubricant Mixtures," Proceedings of the 1995 International CHC and Halons Alternatives Conference, Alliance for Responsible CFC Policy, Frederick, MD, 1995. [10] Biancardi. F. R., Sienel, T. H., Pandy, D. R., and Michels, H. H., "Modeling and Testing of Fractionation Effects with Refrigeration Blends in an Actual Residential Heat P u m p System," ASHRAE Transactions, Vol. 103, Pt. 1, 1997, pp. 1-14. [11] Biancardi, F. R., Michels, H. H., Sienel, T. H., and Pandy, D. R., "Study of Lubricant Circulation in HVAC Systems," ASHRAE Proceedings, GRA19715, 1997, p. 940. [12] Sunami, M., Takigawa, K., Suda, S., and Sasoki, U., "New Immiscible Refrigeration Lubricant for HFCs," ASHRAE Transactions, SD-95-8-3, 1995, p. 940. [13] Sunami, M., Taldgawa, K., and Suda, S., "New Immersible Refrigerant Lubricant for HFCs," Proceedings of the ASHRAEPurdue CFC Conference, Purdue University, West Lafayette, IN, 1994, p . 129. [14] Godfrey, D. and Herguth, W. R., "Physical and Chemical Properties of Industrial Mineral Oils Affecting Lubrication," Lubrication Engineering, Vol. 51, No. 5, 1995, p . 397. [15] Van Nes, K. and Weston, H. A., Aspects of the Constitution of Mineral Oils, Elsevier, NY, 1951. [16] Gunderson, R. C. and Hart, A W., Synthetic Lubricants, Reinhold, NY, 1962. [17] Sanvordenker, K. S. and Larime, M. W., "A Review of Synthetic Oils for Refrigeration Use," ASHRAE Lubricants Symposium, ASHRAE Jandbook-Fundamentals, Atlanta, GA, 1972. [18] Komatsuzaki, S. and Honama, Y., "Lubricants of HFC Refrigerant Compressors," Sekiyu Gakkaishi, Vol. 37, No. 3,1994, p. 226. [19] Glova, D. J., "High Temperature Stability of Refrigerants in Lubricating Oils," ASHRAE Transactions, Vol. 90, No. 4, 1984, p. 806. [20] Daniel, G., Anderson, M. J., Schmid, W., and Tokumitsu, M., Performance of Selected Synthetic Lubricants in Heat Pumps, Heat Recovery Systems, Vol. 2, Pergamon Press, Oxford, 1982, p. 359.
[21] Shubkin, R. L., Polyalphaolefins in Synthetic Lubricants and High Performance Functional Fluids, Marcel Dekker, NY, 1993. [22] Short, G. D., "Synthetic Lubricants and their Refrigeration Applications," Lubricants Engineering, Vol. 46, No. 4, 1990, p. 239. [23] Lilji, K. C , Sabahi, M., and Hamid, S., "Polybasic Esters: Novel Synthetic Lubricants Designed for Use in HFC Compressors," ASHRAE Transactions, SD-95-8-2, 1995, p. 935. [24] Komatauzaki, S., H o m m a , Y., Itok, Y., Kawashima, K., and lizuka, T., "Polyolesters as HFC-134a Lubricants," Lubrication Engineering, Vol. 10, 1994, p. 801. [25] Sunami, M., Takigawa, K., and Suda, S., "Optimization of POE Type Refrigeration Lubricants," Proceedings of the Purdue Refrigeration Conference, Purdue University, West Lafayette, IN, 1994, p. 153. [26] Newan, M. S., "Some Observations Concerning Steric Factors," Journal of the American Chemical Society, Vol. 72, 1950, p. 4783. [27] Suda, S., Sasaki, U., Takigawa, K., Okada, M., and Sunami, M., "Optimization of Polyester Type Refrigeration Lubricants-Balance of Hydrolytic Stability and Wear Prevention," Proceedings of the International Tribology Conference, Japan Air Conditioning Society, Yokohama, Japan, 1995. [28] Geymayer, S., "Polyalkyllenglykote in Synthetic Lubricants and Operational Fluids," 4 * International Colloquium, Technische Akademie Esslingen, Vol. 3, No. 1, 1984. [29] Aderin, M. E., Johnson, G. L., Spikes, H. A., Babson, T. G., and Emery, M. G., "The Film-Forming Properties of Polyalkylene Glycols," Journal of Synthetic Lubricants, Vol. 10, 1993, p. 23. [30] Kussi, S., "Chemical, Physical and Technological Properties of Polyethers as Sjnithetic Lubricants," Journal of Synthetic Lubricants, Vol. 2, 1985, p. 63. [31] Takagi, M., "New Type Refrigeration Lubricants for Alternative Refrigerants," Energy Resources, Vol. 16, 1995, p. 497. [32] Hiodoski, S., Matsuura, H., Kanayama, T., Nishikawa, F., and Nomura, M., "Practical Evaluation of Polyvinylethers as a Lubricant for Alternative Refrigerant Compressor and Systems," ASHRAE Transactions, Vol. 103, 1997, p. 15. [33] Baczek, S. K., and Chamberlin, W. B., "Petroleum Additives," Encyclopedia of Polymer Science and Engineering, 2"^ ed.. Vol. 11, John Wiley NY, 1988. [34] Scott, G., "New Developments in the Mechanistic Understanding of Antioxidant Behavior," Journal of Applied Polymer Science-Applied Polymer Symposium, Vol. 35, 1979, p. 123. [35] Ingold, K. U., "Inhibition of Autoxidation of Organic Substances in the Liquid Phase," Chemical Reviews, Vol. 61, 1961, p. 563. [36] Feng, I. M., Perilstein, W. L. and Adarms, M. R., "SoUd Film Deposition a n d Non-Sacrificial B o u n d a r y Lubrication," ASLE Transactions, Vol. 6, 1963, p. 60. [37] Kharasch, N., Organic Sulfur Compounds, Vol. 1, Pergamon Press, NY, 1961. [38] Gerrard, W. and Hudson, H. R., "Organic Derivatives of Phosphorous Acid and Thiophosphorous Acid," Organic Phosphorous Compounds, Vol. 5, G. M. Kosolopoff and L. Maier, Eds., Wiley Interscience, NY, 1973, pp. 21-329. [39] Andrade, E. N. "DaC, Viscosity of Liquids," Proceedings of the Royal Society (London), Vol. 214A, 1952, p. 36. [40] Yen, L. C. and Woods, J. J., "A Generalized Equation for Computer Calculation of Liquid Densities," Journal of the American Institute of Chemical Engineers, Vol. 12, 1966, p. 95-99. [41] Parmelee, H. M., "Viscosity of Refrigerant-Oil Mixtures at Evaporator Conditions," ASHRAE Transacftons, Vol. 70, 1964,p. 173. [42] Spauschus, H. O. and Speaker, L. M., "A Review of Viscosity Data for Oil-Refrigerant Mixtures," ASHRAE Transactions, Vol. 93, 1987, p. 667. [43] Prausnitz, J. M., Lichtenthaler, R. N., a n d deAzevedo, E. G., Molecular Thermodynamics of Fluid Phase Equilibria, 2nd ed.. Prentice Hall, NJ, 1986.
430 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK [44] Arrhenius, S., "The Viscosity of Aqueous Solutions," Zeitschrift filr Physikalische Chemie, Vol. 1, 1887, p. 285. [45] Kendall, J., "Viscosity of Solutions," Meddelelser VentenskapsakadNobelinst, Vol. 2, 1933, p. 25. [46] Grunberg, L. and Nissan, A. H., "Mixture Law for Viscosities," Nature, Vol. 164, 1949, p. 799. [47] Irving, J. B., "Viscosities of Binary Liquid Mixtures: The Effectiveness of Mixture Equations," Report ISSN 03051439, National Engineering Laboratory, East Kilbride, Scotland, 1977. [48] Frenkel, Y. I., Kinetic Theory of Liquids, Oxford University Press, London, 1946. [49] Yokozeki, A. M., Solubility and Viscosity of Refrigerant-Oil Mixtures, Proceedings of the International Refrigeration Conference, Purdue University, West Lafayette, IN, July 1994. [50] Fredenslund, A., Gmehling, J., and Rasmussen, P., Vapor-Liquid Equilibrium Using UNIFAC, Elsevier, Amsterdam, 1977. [51] Michels, H. H. and Sienel, T. H., "Viscosity Modeling of Refrigerant/Lubricant Mixtures," Proceedings of the CFC and Halons Conference, Washington, DC, 1997. [52] Bambach, G., "Das Verhalten von Mineralol-F12 Gemischen in K a l t e m a c h i n a n Abhandlung," P u b . No. 9, des Deutschen Kaltetechnischen Vereins, C. F. MuUer, Karlsruhe, 1955. [53] Spauschus, H. O., "Thermodynamic Properties of RefrigerantOil Solutions, Part 1," p. 47; "Thermodynamic Properties of Refrigerant-Oil Solutions, Part 2," ASHRAE Journal, Vol. 5, 1963, p. 63. [54] Antoine, C , "Tensions des Vapeurs: Nouvelle Relation Entre les Tensions et les Temperatures," Comptes Rendus, Vol. 107, 1888, p. 681. [55] Sandler, S. I., Models for Thermodynamic and Phase Equilibria Calculations, Marcel Dekker, NY, 1993. [56] Morrison, J. D., Barley, M. H., Murphy, F. T., Parker, T. B., and Wheelhouse, R. W., "The Use of an MHV2 Equation of State for Modeling the Thermodynamic Properties of Refrigerant fixtures," International Journal of TherTnophysics,\o\. 16, 1994, p. 1165. [57] Gallagher, J., McLinden, M., Morrison, G., and Huber, M., "NIST Thermodynamic Properties of Refrigereints and Refrigerant Mixtures (REFPROP)," NIST, Standard Reference Database 23, Version 5.1, 1997. [58] Martz, W. L., Burton, C. M., and Jacobi, A. M., "Vapor-Liquid Equilibria for R-22, R-134a, R-123 and R-32/R-125 with a Polyolester Lubricant: Measurements and Departure from Ideality," ASHRAE Transactions, Vol. 102, 1996, p. 367. [59] Wilson, G. M., "A New Expression for the Excess Free Energy of Mixing," JACS, Vol. 86, 1964, p. 127. [60] Martz, W. L., Burton, C. M., and Jacobi, A. M., "Local Composition Modeling of the Thermodynamic Properties of Refrigerant and Oil Mixtures," International Journal of Refrigeration, Vol. 19, 1996, p. 25.
[61] Heil, J. F., and Prausnitz, J. M., "Phase Equilibria in Pol3'mer Solutions," AIChE Journal, Vol. 12, 1966, p. 678. [62] Renon, H. and Prausnitz, G. M., "LOCEJ Compositions in Therm o d y n a m i c Excess Functions for Liquid Mixtures," AIChE Journal, Vol. 14, 1968, p. 135. [63] Tsuboka, T. and Katayama, T., "Modified Wilson Equation for Vapor-Liquid and Liquid-Liquid Equilibria," Journal of Chemical Engineering of Japan, Vol. 8, 1975, p. 181. [64] Wohl, K., "Thermodynamic Evaluation of Binary and Ternary Liquid Systems," Transactions of the AIChE, Vol. 42, 1946 p. 215. [65] O'Donnell, T. K. and Ward, D. F., "Liquid Overfeed Systems," Ch. I, ASHRAE Handbook-Refrigeration, ASHRAE Publications Atlanta, GA, 1998, p. 1. [66] Jones, R. A., "System Practices for Halocarbon Refrigerants," Ch. 2, ASHRAE Handbook-Refrigeration, ASHRAE Publications Atlanta, GA, 1998, p. 1. [67] Wallis, G. B., One-Dimensional Two-Phase Flow, McGraw Hill, NY, 1969. [68] Macken, N. A., Scheideman, F. C , Jacobs, M. L., and Kazem, S. M., "An Experimented Study of Pressure Drop and Liquid Transport of Oil-Refrigerant Mixtures," Final Report, ASHRAE Project RP-109, Atlanta, GA, 1976. [69] Biancardi, F. R., Michels, H. H., Sienel, T. H., and Pandy, D. R., "Study of Lubricant Circulation in HVAC Systems," Final Report, ARTI MCLR 665-53100, Ariington, VA, 1996. [70] ANSI/ASHRAE Standard 41.4-1996: Standard Method for Meas u r e m e n t of Proportion of Lubricant in Liquid Refrigerant, ASHRAE Pubhcations, Atlanat, GA, 1996. [71] Hewitt, N. J., McMuUan, J. T., Mongey, B., and Evans, R. H., "From Pure Fluids and Azeotropic Mixtures: The Effects of Refrigerant-Oil Solubility on System Performance," International Journal of Energy Research, Vol. 20, 1996, pp. 57-67. [72] Baustian, J. J., Pate, M. B., and Bergles, A. E., "Properties of OilRefrigerant Liquid Mixtures with Applications to Oil Concentration Measurement," AS//i?AE TmniacfioMs, Vol. 92, lA, 1986, p. 55. [73] Katsuna, K., Inoue, I., Mizutsini, T., Sudo, E., and Araga, T., "Real Time Oil Concentration Measurement in Automotive Air Conditioning by Ultraviolet Light Absorption," SAE Paper 910222, Society for Automotive Engineers, Warrendale, PA, 1991. [74] Newell, T. A., "In Situ Refiractometry for Concentration Measurements in Refrigeration Systems," International Journal of HVAC&R Research, Vol. 2, No. 3, 1996, pp. 247-256. [75] Worsoe-Schmidt, P., "Some Characteristics of Flow Patterns and Heat Transfer of Freon-12 Evaporating In Horizontal Tubes," International Journal of Refrigeration, Vol. 40, 1960, pp. 40-44. [76] Sienel, T. H., and Michels, H. H., "Lubricant Effect on Performance," presented at the ASHRAE Seminar 12, Dallas, TX, 2000.
MNL37-EB/Jun. 2003
Gear Lubricants Vasudevan Bala^
FUNCTION OF GEAR LUBRICANTS
GEAR DESIGN AND THE LUBRICANTS USED FOR THEIR APPLICATION
have evolved continuously over the last 200 years [1]. These improvements were largely attributed to advances in gear design, metallurgy, lubricant additives and base oil refining [1,2]. Global trends in improved end user satisfaction, fuel economy, lower down time, cheaper maintenance, and improved equipment durability have provided the necessary motivation for increased competition among Original Equipm e n t Manufacturers (OEMs) [1-3]. As a result of these trends, gear lubricants currently are formulated to meet exacting standcirds required to lubricate gear boxes and automotive drivelines. The following sections cover the basics in gear lubrication and provide information in the selection of proper gear lubricants depending on their end application.
EARLY GEAR LUBRICANTS The history of gear lubrication dates back to Aristotle around 330 B.C. [4]. Since then application of gears in precursors to m o d e m timepieces, windmills, and p u m p s were quite common. Table 1 summarizes early gear applications and theorists. The early gears were predominantly made of wood that precluded the use of lubricants until cast iron was used to fabricate gears [5]. In examples recorded during the building of the p5Tamids, animal fats were used to lubricate wooden rollers that were used to move massive stones [5]. It was, however, with the introduction of spiral-bevel gears primarily for increased load-carrying capacities in the mid-1800s that gear lubrication become severe. This led to the development of extreme pressure (EP) additives and the first patent issued in 1869, describing the use of lead soap and active sulfur [6]. The first commercieil gear lubricants were mixtures of animal fats and mineral oils. F r o m the late 1800s to the 1970s, the demands of the gear industry were met with additives based on lead, chlorine, zinc, sulfur, and phosphorus. The use of lead oleates, napthenates, dithiocarbamates, and sulfurized olefins provided the necessary anti-score and corrosion protection. During the 1930s, these additives were found to be inadequate for jixles that operated under heavy loads and high temperatures. This led to the combined use of lead soaps, chlorinated paraffins, zinc dialkyldithiophosphates, and sulfurized olefins. By the 1960s, gear additives were predominantly based on zinc, chlorine, and sulfur that met Military, OEM, and industry standards. Table 2 illustrates EP gear chemistry used [7].
' Cognis Corporation, 4900 Este Ave., Cincinnati, OH, 45232.
The basic function of a gear lubricant is to reduce friction between the gear teeth under contact, which reduces tooth wear and prolongs its life. Another critical function of the gear lubricant is to dissipate heat generated in the contact zones. In automotive applications, the gear lubricant can reduce shock and noise emanating from reeir axles, hence providing s m o o t h a n d quiet operation during acceleration. Other important functions of the gear lubricant during service are removal, and prevention of corrosion and pitting, while prolonging gear life. These functions collectively define the properties of gear lubricants in automotive and industrial applications (described later). Gear lubricants meeting viscosity requirements, depending on their application, can assist in proper start-up and the normal operation of gear boxes and axles. Gear lubricants with very low viscosities can promote premature seizure of gear teeth, while those with high viscosities affect the efficiency and operating temperatures in gear boxes and axles. In automobiles, the combined effects of aesthetically pleasing aerod5Tiamics for improved fuel economy and installation of high performance engines, capable of achieving higher peak torques at lower revolutions per minute, collectively have increased sump operating temperatures and power densities transmitted by the gears used in transmissions and axles. As a result of these effects, gear lubricants must resist oxidation and be thermally stable under normal and severe service. Gear lubricants can contain dispersants that can disperse high molecular weight oxidation by-products or sludge, promoting clean operation of the gear boxes. In some instances, build-up of sludge can affect the performance of wheel-end and axle shaft lipseals, thus preventing loss of gear lubricant. The use of EP, antiwear additives (AW), and friction modifiers (FM) can protect the gear and bearing surfaces under boundary lubrication. The resulting drop in friction can reduce sump temperatures, t h u s prolonging lubricant and equipment (gears, bearings, seals) life. In high-speed operations (in paper and steel mills) or driving, gear lubricants are pumped and splashed incessantly. As a result, foam suppression by gccir lubricants is critical to prevent loss of lubricant delivery, cavitation, or implosion at the metal contact zones. In certain industrial operations, water separation for regular maintenance and lubricant top-off is desired. Other functions of gear lubricants include storage stability during transportation and fuel economy benefits under most driving conditions.
431 Copyright'
2003 by AS'I M International
www.astm.org
432
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
TABLE 1—Early gear inventors and theorists. Early Gear Information Approximate Date
Name Aristotle
Item Explained gear wheel drives in windlasses; pointed out that the direction of rotation is reversed when one gear wheel drives another gear wheel. Made water clocks and water organs using gears. Made rack and pinion devise to raise water. Made devices to multiply force or torque many times; studied spiral. Described the first real use of power gearing in mill drives. Described gears (cogwheels) for many uses; ceJculated actual gear ratios; devised distance (cyclometer) eind angle (dioptra) measurements using gears.
330 B.C. 250 230 220 27 60
Otesibius (Greek) Philo of Byzanitum Archimedes Vitruvlus Hero
B.C. B.C. B.C. B.C. A.D.
Gear Theorists Name
Approximate Date
Nicholas of Cusa Albert Durer Girolamo Cardano Philip de la Hire
1451 1525 1557 1694
French German Swiss French
Charles Camus
1733
French
Leonard Euler
1754
Swiss
Abraham Kaestner
1781
German
Robert Willis Edward Sang
1832 1852
English Scotch
Nationality
Contribution Studied cycloidal curve. Discovered epicycloids. Developed first mathematics of gears in print. Developed full mathematical analysis of epicycloids; recommended involute curve for gearing (involute not used in practice until about 150 years later. Expanded on work of la Hire; developed theories of mechanisms; studied lantern pinion and gear, crown gears, and beveled gears. Worked out design principles and rules for conjugate action some consider him the "father of involute gearing." Wrote practical methods for computing tooth shapes of epicycloid and involute gear teeth; considered 15 degrees to be a minimum pressure angle. Wrote and taught extensively in gear field—a pioneer in gear engineering. Developed generaJ theory of gear teeth; provided theoretical basis on which all gear tooth generating machines are based.
TABLE 2—Early EP gear oil additives. S:2 1943
S:3
S:4
Date
S:l 1935
1948
1957
Cliemistry Type
S, CI
S, CI, P
Pb, CI, S*
Zn, S, CI, P*
Chlorine, % Sulfur, % Phosphorus, % Lead, % Zinc, % Performance Level Dosage, Wt % Application
34 10
26 8 0.5
4.6 4.9
15 13 4
7.55 5-10 Automotive
GL-4'" 5-10 Automotive
GL-4* 28 Automotive
3.5 GL-4, GL-5* 5-10 Automotive
"Some versions of S-3 and S-4 contained limited slip additives. 'API gear oil classification system, Publication 1560.
GEAR TYPES Gears can be classified according to their tooth profile, shape, loading, and direction of torque transfer [8a]. The choice of these different gear types depends on these factors and is seen in automotive and industrial applications. The basic function of gears is to transmit power along with increasing or decreasing shaft speed. Gears can also be used to deliver the required torque and, depending on the gear design used, can be used to change the direction or axis of rotation. In specific automotive applications, gears are used to synchronize shaft speeds in transmissions and change torque transfer in front and rear axles. There are different gear types used in automotive and industrial applications. The c o m m o n gear types are spur, helical, herringbone, w o r m , straight, and spiral-bevel a n d hypoid. Spur gears come in straight, helical, and herringbone designs. The most c o m m o n type is the straight spur gear type that sees no axial forces a n d uses simple bearing design.
Helical gears are used in applications when higher peripheral speeds and quieter operations are needed. The resulting eixial forces require more complex bearing design. With herringbone gears, axial forces are eliminated, but the tooth width is generally twice as large compared to the helical design. Bevel gears have intersecting axes, line contact and low slide/roll ratios. Cylindrical w o r m gears also have crossed axes, line contact, but large slide/roll ratios. They permit the transmission of high ratios under quiet, smooth, and low-vibration operation. The hjrpoid gear design is predominantly used in automotive axles. They have a high center off-set with crossed axes, elliptical contact, and large slide to roll ratios. Figure 1 illustrates the different types of gears and provides information on their uniqueness and operation. These comm o n gears are used in automotive and industrial gear boxes. The proper selection of gear sets is dependent on gear size allowed, load bearing limit, desired gear ratio for torque and speed transfer. Table 3 provides additional information on the selection of gear sets based on their gear ratio,
CHAPTER 16: GEAR LUBRICANTS
433
HELICAL GEARS
WORM GEARS
• Teeth In Spiral Plane To The Axis • Smoother Than Spur Gears • Higher Load Capability • Multiple Tooth Contaot
• Extreme Case Of Hypoici @ear • • • •
Hardened Steel And Bronze Quiet Smooth Operation High Load Capability
SPUR GEARS • Teeth In Parallel Plane To The Axis • Oast Iron Or Steel • Noisy • Rough Operation • simple And Economical To ManufQeture • single Tooth Contaot • Rolling And Sliding Motion
HERRINGBONE GEARS • • • •
Expensive To Manufacture Quiet Smoom Operation Used For High Speed Gearing
HYPOID GEARS BEVEL GEARS
• Form Of Spiral Bevel Gear • Pinion Axis Does Not Intersect Ring Gear Axis • Smooth Running • Light Weight And Cempoet • Severe Sliding Action • Sleel Due To High Tooth Loading
• Axis Centers Intersect • • • •
Noisy Rough Operation Limited To Low Siseeds Greater Load Capabiltly With Spiral Bevel
FIG. 1—Different types of gear sets [8].
TABLE 3- Performance comparison of various types of gear sets. Position of Axes Parallel
Intersecting
Gear Type Spur Straight Helical
Bevel Straight Skew
Maximum Transmission Ratio
Maximum Peripheral Speed
Maximum Output (kN)
Maximum Torque (kNm)
10 10 10 10
5 25 200
2250 7500 22500 22500 22500
9000 2300 600
375 3750 3750
90 90 45
750 75 750
300 170
7 7 7
2.5 2.5 60
Crossing at 90°
Worm Crossed Helical H5^oid
50 50 50
60
Crossing at 80-100° (not 90°)
Worm Crossed Helical
50 50
50
115 170
434 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK peripheral speed, load, and torque [9]. The selection of the proper gear lubricant for these c o m m o n gear sets is dependent on the application and requirements set forth to meet certain gear box or axle performance criteria. This is further elaborated later in this chapter under Classification of Gear Lubricants for automotive and industrial applications.
METHODS OF GEAR LUBRICATION The c o m m o n modes of gear lubrications are [8a]: • manueJ application with brushes • drip feed from reservoir or tubes • intermittent or continuous lubrication under pressure or splash In the selection of gear lubrication, consideration has to be given to the end application and the prevention of gear failure. Manual application or drip feed is used for open gear sets where the temperature rises and load and wear on the gear teeth are all low due to the operation, and where the gears can be lubricated easily by a single application or repeated applications at long intervals [9]. Gears that demand longer service life operate continuously with high peripheral speeds (0.8-3 m/s) under moderate loads during operation re- quire intermittent lubrication. Continuous lubrication via splash, p u m p assisted or pressure fed is generally provided for power transmission with closed housings. Oil splash lubrication is by far the most c o m m o n method of lubricating gears. It combines the advantages of efficiency, continuous lubrication, and cooling over a wide range of speeds and loads. Effective splash lubrication where one of the gears dips into the reservoir and provides lubrication to the other gears is seen in industrial applications, automotive transmissions, and axles where closed gear housings are predominantly used [5].
nation within the lubricating system [8a]. Failures resulting from these effects cem lead to serious tooth surface damage such as pitting, spalling, wear, and scuffing. Table 4 provides the basic failure modes encountered in gear lubrication [10,11a]. Descriptions of common lubrication related failures in pitting, micropitting, adhesive wear, abrasive wear, polishing wear, and scuffing and their prevention follow. Pitting is a fatigue phenomenon and occurs when a surface initiated crack propagates into the metal before turning towards the surface [10,lla,12]. In some instances, pitting can also be initiated from a subsurface defect or inclusion, usually impurities (Fig. 2). When several pits grow together, the large pit is referred to as a spell. The recommended guidelines to prevent the onset of pitting and/or spalling in gear sets are [11a]: • reduce contact stresses by reducing loads • improve material strength of gear metal • increase specific film thickness (defined as film thickness/surface roughness) Micropitting, as the name suggests, is similar to pitting except that the pit sizes are several orders smaller [10,lla,12]. Other names used to describe micropitting are gray staining
GEAR FAILURE MODES C o m m o n gear failures usually can be traced to lack of lubrication, improper lubrication for the hardware, use of incompatible lubricant and/or additives, or possible contami-
FIG. 2—Example of common gear failure— pitting. Courtesy of AGMA from Standard ANSI/AGMA 1010-E95.
TABLE 4 -—Basic failure modes of gear teeth. Non-lubrication-related Failures Overload Bending Fatigue
Brittle fracture Ductile fracture Plastic deformation Cold flow Hot flow Indention Rolling Bruising Peening Brinelling Rippling (fish scaling) Ridging Bending (3aelding) Tip-to-root interference
Low-cycle fatigue (scycles to failure) High-cycle fatigue ( a 1000 cycles to failure)
Lubrication-related Failures Hertizan Fatigue
Wear
ScuiBng
Pitting Initial Superficial Destructive Spalling Micropitting Frosting Gray staining Peeling Subcase fatigue (case crushing)
Adhesion Running-in Mild Moderate Severe Excessive Abrasion Scoring Scratching Plowing Cutting Gouging Corrosion Fretting corrosion Cavitation Electrical dischsirge damage Polishing (burnishing)
Scoring Galling Seizing Welding Smearing Initial Moderate Destructive
CHAPTER and frosting [8a] (Fig. 3). The specific film thickness has been identified as a key parameter that influences micropitting [8]. The recommended guidehnes to prevent the onset of micropitting in gear sets are [11a]: • use smooth tooth surfaces produced by grinding or honing • use adequate amount of gear lubricant of the highest viscosity possible • use of high speed • use of carburized steel Adhesive wear can be classified as mild or severe depending on the cimount of wear seen on the gear tooth [10,11 a, 12]. Mild adhesive wear generally occurs during break-in when the tooth surface asperities are removed to improve the specific film thickness. Severe wear has occurred when there is a significant change in the tooth profile. The recommended guidelines for preventing the onset of adhesive wear in gear sets are [11a]: • use smooth tooth surfaces • run-in new gear sets at 1/2 the load before use • Use higher speeds when possible. Under slow-speed high loads, use nitrided gears combined with highest viscosity lubricants Abrasive wear is usually caused by foreign contamination or wear debris from run-in of the gear sets [8a, 10,11a, 12] (Fig. 4). Contamination can originate from inherently unclean gear boxes during build-up, ingressed through breather valves a n d seals. Proper m a i n t e n a n c e of the gear box by regular visual inspections and analyses of the lubricants can reduce abrasive wear. The recommended guidelines for preventing abrasive wear in gear sets are [11a]: 1. Remove contamination during build-up with proper flushing and draining of the lubricant. 2. Refill with fresh lubricant and filter where applicable. 3. Use smooth surface hardened gears in combination with high viscosity lubricants 4. Adopt regular maintenance schedules 5. Use filtrating systems with circulating oils 6. Follow OEM recommended drain intervals 7. Conduct used oil analyses to m o n i t o r condition of the lubricant Polishing wear is believed to be caused by continuous removal of the reactive boundary layer by fine abrasive material
FIG. 3—Example of common gear failure— micropitting. Courtesy of AGMA from Standard ANSI/AGMA 1010-E95.
16: GEAR LUBRICANTS
435
FIG. 4—Example of common gear failure—abrasion. Courtesy of AGMA from Standard ANSI/AGMA 1010-E95.
FIG. 5—Example of common gear failure— polishing. Courtesy of AGMA from Standard ANSI/AGMA 1010-E95.
in the lubricant [8a,10,lla,12] (Fig. 5). The use of EP and antiwear additives can alleviate this type of wear. The recommendations for reducing polishing wear in gear sets are [11a]: • Use of lubricants with the properly selected gear additives • Continuous filtration to remove fine abrasive debris present in lubricant Scuffing is defined as localized welding between mating metal surfaces. This genereJly occurs when the gears operate under boundary lubrication [8a, 10,11 a, 12] (Fig. 6). Gears are prone to scuffing, especially during start-up. It is not uncomm o n to use gears coated with phosphorus-maganese (lubrited), copper, or silver to prevent scuffing during run-in. Gear lubricants containing sulfur-phosphorus additives are formulated to reduce scuffing. The following guidelines are recommended to reduce scuffing [11a]: 1. Use gears with smooth surfaces that were honed. 2. Use gears with coatings to reduce propensity to scuff. 3. Use high viscosity lubricants containing sulfur-phosphorus additives. 4. Use heat exchanger to cool the lubricant sump. 5. Use nitrided gears. Several references [4,5,10,1 la,13] provide further details that elaborate on gear failure modes. The American Gear
436
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
Manufacturers Association Standard ANSI/AGMA 1010-E95 [12] provides a common language to describe gear weeir and failure and serves as a guide to uniformity and consistency in the use of its language. It describes the appearance of gear tooth failure modes and mechanisms to facilitate the identification of gear wear and failure.
GEAR LUBRICATION The conforming surfaces of gear teeth in mesh can operate under three different modes of lubrication: boundary, mixed, and elastohydrodynamic. Boundary lubrication occurs during start-up or stopping conditions. In certain conditions of heavy loads and slow speeds e.g., towing, boundary
FIG. 6—Example of common gear failure—scuffing. Courtesy of AGMA from Standard ANSI/AGMA 1010-E95.
condition persists between the pinion and ring gears in automotive hypoid axles. Under boundary conditions, the additive system present in the gear lubricant prevents scuffing, metal adhesion, and wear. Under higher speeds and/or lighter loads, the gears operate under mixed lubrication. Under mixed lubrication, gear surface asperities still come in contact and wear occurs at a slower rate than under boundary lubrication. Both the viscosity of the gear lubricant and the additive system can influence optimum gear performance under mix lubrication. ElastohydrodjTiamic (EHD) lubrication provides the optimum condition under which separation of gear teeth occurs, minimizing wear. The lubricant's viscosity is important under EHD lubrication and is affected by proper selection of the lubricant viscosity grade. Table 5 provides the necessary equations that describe the calculation of film thicknesses under EHD for spur and bevel gears with straight and helical teeth [9]. Figure 7 shows the typical contact between gear teeth for spur, helical, and bevel gears [14]. The contact starts with high sliding and some rolling. The sliding reduces to pure rolling at the center of the gear tooth or pitch line after which rolling decreases with increasing sliding. Generally, under these conditions, the gear teeth undergo boundary to EHD and back to boundary lubrication. Mixed and boundary lubrications generally persist for hypoid gear sets. EHD theory has shown that film thicknesses of ~1 micron can prevent surface asperities from coming in contact [15]. Under the conditions of high pressure, the lubricant can exhibit viscosity increases that maintain film thickness. Increases in the relative velocities of the gear tooth surfaces, gear lubricant viscosity, and in the radius of curvature at the point of contact, can lead to gear oil film thicknesses more dramatically than proportionate reductions in unit tooth load, or elastic modulus of the gear metal [9].
Table 5—Symbols used in elasto-hydrodynamic film thickness calculation [9]. Gap parameter P Load parameter Ep
^ = "4 G'=^aE
^mia
•^O
a « = ! ( « , + uj)
p^—
PI
X
Pi + E
PI
— PJ
X £,
^^> £2(1 - <^i) + £»(! - •'I)
«T„ ffj
w'^FIb
Velocity parameter Material parameter Minimum film thickness between surfaces Operating viscosity at atmospheric pressure Pressure coefficient of viscosity Effective peripheral velocity of two surfaces Effective radius of curvature of the two surfaces Effective modulus of elasticity of the two materials Poisson's ratio for the two materials Normal tooth force per unit tooth width
CHAPTER 16: GEAR LUBRICANTS
437
pinion pitch cSroie •
(to) • Pitch point
(•} • First point of contact (c) L » t point of contact
FIG. 7—Relative motion of meshing gear tooth surfaces. Reprinted with permission of CRC Press, Boca Raton, FL. [14]. Table 6—Calculation of the effective radius of curvature [9]. I l l 2 i - = —+ —= -r-r-x p />! P2 sin/J„ ' V"0. Spur gear Straight teeth
p =
Helical teeth
Pn =
1 = pinion;
1
"0. 2 /
Bevel gear (90 degrees)
ai sin p„
p = /!„ sin P,
(«• ± 1)' ai sin ^„ CDS'* Y{i ± 1)^
2 = gear;
'yAi' + iJ
4- = external teeth;
p — radius of curvature d^ = pitch diameter a — centre distance A - cone distance
Table 5 provides the description of the symbols used in m i n i m u m film thickness equation. The effective values for velocity, load, modulus of elasticity and curvature are determined first. Tables 6-8 contain information for calculating the effective radius of curvature, peripheral velocity, and specific normal tooth force, respectively. The viscosity-temperature and pressure-viscosity coefficient of the lubriccint at the operating temperature are determined. These are obtained by the following equations.
— = internal teeth
t = transmission ratio P = mesh angle Y = helix angle
The methodology for performing m i n i m u m film thickness calculations applicable to gear pairs (spur and bevel gears) is provided next [9]. The m i n i m u m film thickness equation is
log log (v + 0.7) = A - B log T
A„ ~ cos*
"
a=1.030+3.509(logvo)^ + 2.412*10-'*mgi'^(logvo)1.5976 -3.387 (logVo)^°'V"". where: a = mo = vo
=
p =
pressure-viscosity coefficient, [10~*Pa~'] viscosity-temperature property from the ASTM Walter equation and equal to (ASTM slope)/0.2 atmospheric kinematic viscosity at the temperature of interest, [mm^/S] atmospheric density at the temperature of interest, [lO^kg/m^]
These values are used to calculate the minimum film thickness. Table 9 provides the variation for the different paired gears of spur and bevel types.
where: log V
T AandB
logarithm to base 10 kinematic viscosity, [cSt] or [mm^/s] Temperature, [K] or [°R] dimensionless constants
TYPES OF GEAR LUBRICANTS Most commonly available gear lubricants are blended from petroleum basestocks. The following categories briefly de-
438
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
Table 7—Calculation of the effective peripheral velocity [9].
At pitch point: u = u, » KJ »«p,tOi = Pico^ Spur gear Straight teeth „ = ^ Helical teeth
Bevel gear (90 degrees)
( ——j sm ^.
„= - ^
u= ^
(^-A.) 3i„ ^„
>t„ sin ^„
„= __^^^sin^„
1 = pinion; 2 = gear; + = external teeth; —= internal teeth n = rotational speed i = transmission ratio A = cone distance p = mesh angle a = centre distance Y = helix angle u = peripheral velocity
Table 8—Calculation of the specific normal tooth force [9].
F w= F F = —^—; cos fit, P= ^ w F b Fu
straight teeth
F T-^ 77:; hehcal teeth (cos ^ i , cos y,) 2M
= specific normal tooth force = normal tooth force — tooth width = peripheral force
M = torque dy, = pitch circle diameter ^ = mesh angle 7 = helix angle
Table 9—Calculation of elasto-hydrodjmamic film thickness [9].
ft„,„ = 2.65 X a^'^lnowf >*•**£-*'*'*w-°»^ Spur gear Straight teeth ,
2.65 X,
'miB
„0.54 /
_„ \0.7
rO.*S
£0.03
Helical teeth 2.65 X a°-^* / nnj^y (a sin ^ J " ^ i°*' '»mi„- £0.03^0.13 ^^'^o 3 Q J X c o s " « y ( i ± l ) ' Bcpc/ gear Straight teeth 2.65 X « ° ' * /
nn,Y\
"mi- = £0.03^0.13 ( ''O -^j
^
. „
i^m Sm P„)
'•13
•0.27 '
(,•2 + j)0.43
Helical teeth 2.65 X a° '* / ''•»'"
£«"H''''M''*'
nnX'' 30;
i^m s«n ^ , 1.13 ) " ' «, 0 . 2 7 cos''*y (F + l ) ° * '
CHAPTER scribe the Vcirious types of gear lubricants and their general applications [14,15]. Gear lubricants can fall under any one of these categories and their fined application, whether industrial or automotive, is predicated on the meeting of specific performance requirements that will be discussed later in the chapter. R u s t a n d O x i d a t i o n I n h i b i t e d ( R & O) O i l s R & O oils are mineral oils containing rust and oxidation inhibitors, antifoamants, and/or antiwear additives. The rust inhibitors prevent corrosion of the metal surfaces by slowing the formation of acids and free radicals that are the by-products formed at the onset of oxidation. Without oxidation inhibitors, these acids and free radicals can form high moleculeir weight sludge and deposits manifesting as viscosity increase [26]. Inhibited oils are used in gear application under light to moderate loads. This type of gear lubricant can be used to lubricate bearings and gears operating between - 2 0 to 120°C. Inhibited oils can be reconditioned with filters to ensure cleanliness even with wear debris and contaminants are present.
16: GEAR LUBRICANTS
439
cation in w o r m gear drives where high sliding speeds between the gear teeth requires lubricity additives to reduce friction and improve torque efficiency. Compounded oils are required to operate in the temperature range of 5-120°C. Constant relubrication of the gear teeth is recommended due to the unique gear mesh that wipes off the oil from the teeth after contact. Reconditioning of the oil similar to inhibited oils is quite common. Open Gear Compounds These oils are used in large gear sets t h a t move at slow speeds. These lubricants contain tackifiers that adhere to the gear teeth that resist being thrown-off or squeezed out of the mesh. Solvents are sometimes used to help in the application; they then evaporate leaving behind a thick lubricant film. The normEd operating temperature range for these lubricants is 5-120°C. This type of lubricant does not offer any advantage that circulating lubricants do for cooling and heat removal. Greases
E x t r e m e P r e s s u r e Oils Extreme pressure oils contain additives predominantly of the sulfur & phosphorus type to reduce destructive wear and scoring. These additives combine with the metal surface under frictional heating and provide a sacrificial boundary film that is easily sheared. EP oils can also contain other surface active non-sulfur/phosphorus based additives. Examples include graphite, molybdenum disulfide, potassium triborate, and additives containing bismuth. EP oils are used to lubricate spur, helical, and spiral-bevel gears u n d e r loads too heavy for inhibited oils. EP oils with the addition of dispersants and detergents can b e used in hjrpoid axles that are exposed to heavy loads or shock loading. Shock loading occurs under sudden heavy accelerations and decelerations that can promote scoring at the gear surfaces. These automotive applications will be elaborated later in the chapter. The useful temperature range for EP oils is generally - 2 0 to 120°C. It is not u n c o m m o n to see automotive gear lubricants subjected to temperatures from —54 to 170°C.
C o m p o u n d e d Oils These oils contain fat or tallow as lubricity agents in heavy cylinder basestocks to reduce friction. These oils find appli-
Greases are usually mineral lubricants that are thickened with soaps to a gelatinous consistency. The thickener holds the liquid lubricant and releases it when in use. The advantage of greases is that they do not have to be added continuously. Greases can act as sealants to keep contaminants out. Packed wheel bearings and small gear boxes in household appliances are typical examples where greases are used. Greases are restricted to gears or bearings that operate at slow-speeds and more importantly, light loads. The useful operating temperatures for greases are —30 to 120°C, depending on the base oil and soap used. Table 10 summarizes the various types of gear lubricants and their application [15].
ADDITIVES USED IN GEAR LUBRICANTS As stated before, the purpose of gear lubricants is to reduce friction and wear between gear teeth, dissipating frictional heat (not referring to greases), removal and reduction of sludge and deposits, and prolonging gear tooth life. The additives used in gear lubricants impart and enhance the lubricating properties of mineral and/or synthetic base oils in which they are blended. The general properties of mineral and synthetic base oils are described later in the chapter.
TABLE 10—Types of gear lubricant used with various gear applications. Gear Types Lubricant R & O oil (non-EP)
Spur Normal loads
Helical Normal loads
EPoil
Heavy or shock loading Not normally used Slow-speed Open gearing Slow-speed Open gesiring
Compounded oil (ca. 5% tallow) Heavy-bodied open gear oils Grease
Bevel Normal loads
Hypoid Not recommended
Heavy or shock loading Not normally used Slow-speed Open gearing
Worm Light loads Slow speeds only Satisfactory for most applications Preferred by most gear manufacturers Slow-speeds only EP additive desirable
Heavy or shock loading Not normally used Slow-speeds Open gearing
Slow-speed Open gearing
Slow-speeds only EP additive desirable
Slow-speeds Open gearing
Required for most applications For light loading only Slow-speeds only EP additive required Not recommended
440
MANUAL
37: FUELS
AND LUBRICANTS
HANDBOOK
Gear lubricants can be referred to as nonengine lubricants used in noncombustible environments. The additives in gear lubricants can be classified as chemically active or inert. Examples of chemiccilly active additives are dispersants, detergents, antiwear, EP, oxidative inhibitors, friction modifiers, rust, and corrosion inhibitors. These additives generally interact, via adsorption and/or decomposition and/or rearrangement, with surfaces and with polar oxidation bi-products. Examples of chemically inert additives are emulsifiers, demulsifiers, p o u r point depressants (PPDs), seal swell agents, antifoamants, a n d polymeric thickeners. These additives alter the physiccJ properties of the finished gear lubricant. Most gear additive systems consist of all or most of the following additives components: • extreme pressure agents • detergents • dispersants • friction modifiers • oxidation and corrosion inhibitors • antiwear agents • demulsifiers and emulsifiers • foam inhibitors • pour point depressants These additives and their currently understood mechanism of activity are described in the following section. There, it will become apparent that some of these additives Eire multifunctional and understanding their S5Tiergistic and antagonistic properties towards each other collectively in the gear lubricant will allow formulators the ability to meet their performance targets. Extreme Pressure (EP) Agents The purpose of EP agents is to reduce metal wear from scoring, cold welding, and seizure under conditions of high loads. Typical c o m p o n e n t s t h a t provide E P properties are zinc dialkyldithiophosphates (ZDDP), alkyldithiophosphates, sulfurized olefins, alkylpolysulfides, sulfurized fats, potassium triborate, and chlorowaxes. Other examples are lead, bism u t h , and molybdenum dithiocarbamic and organic acid derivatives. Lead Etnd chlorine containing EP agents are becoming less popular due to their potential environmental and hazardous impact. Most sulfur and some phosphorus containing components can function as good EP agents. ZDDPs are formed by reacting organic alcohols with phosphorus pentasulfide to form dialkyldithiophosphoric acids, which are subsequently reacted with zinc oxide to form zinc salts. Organic dialkyldithiophosphates are made without the last reaction. Sulfurized olefins are m a d e by reacting elemental sulfur with olefins at high temperatures forming a mixture of sulfur containing compounds. Chlorowaxes are manufactured via a similar process using chlorine and fatty paraffins. Zinc, bismuth, emd molybdenum salts of dithiocarbamic acid are made by reacting the acid derived from reacting dialkylamine and carbon disulfide with the metal oxide. Polysulfides are generally prepared by oxidizing alkyl mercaptans. EP agents are relatively surface active and adsorb on metal surfaces. Frictional heat derived from sliding u n d e r high loads provides enough energy for bond scissation to occur, forming a sacrificial boundary film of lower yield stress
than the metal itself. The boundary film prevents scoring, colding, welding, and seizure of gear surfaces. Antiwear Agents The purpose of antiwear agents is also to reduce frictional wear, but under conditions of low loads. Typical components t h a t provide antiwear properties are zinc dialkyldithiophoshates, alkyl phosphates and phosphites, aryl phosphates and phosphites, p o t a s s i u m triborate, a n d chlorowaxes. Phosphates and phosphites are manufactured by reacting alcohols with phosphorus pentoxide, or trichloride, respectively. Generally, alkyl p h o s p h a t e s a n d phosphites are neutralized with amines for enhanced surface adsorption. Their function to reduce wear is similar to EP agents via surface adsorption and decomposition under frictional heating to form a sacrificial b o u n d a r y film. Generally, antiwear agents are more surface active than EP agents. Thus, their content in gear lubricants is generally lesser than EP agents by comparison.
Detergents Detergents keep metal surfaces clean of deposits. Tjrpical types of detergents are alkali earth metal (sodium, calcium, magnesium) sulfonates, phenates, salicylates, and carboxylates. These additives neutredize organic acids from lubricant oxidation and also associate with precursors that form sludge and varnish. This has the overall effect of keeping metal surfaces clean of deposits. Consequently, the m a n n e r in which detergents react with oxidation byproducts can result in prolonging the use of gear lubricants and maintaining clean working gear boxes. Neutral and basic metal sulfonates are formed by reacting alkylbenzenesulfonic acids with metal hydroxides or oxides (calcium, magnesium, sodium types) to form salts. Neutral salts are formed with correct stoichiometry and overbased salts are formed in the presence of carbon dioxide. Neutral and overbased metal phenates and carboxylates are prepared similarly to sulfonates except cJkylphenols and alkylsalicylic acids, respectively, are used instead of alkylbenzenesulfonic acids.
Dispersants Dispersants keep insoluble contaminants dispersed in the bulk lubricant. Typical examples of dispersants used in gear lubricants are of the alkenylsuccinimide, succinate ester, Mannich derived, and alkenylphosphonic acid derivatives. The latter is seldom used as the other three types see dominant use in crankcase engine lubricants. In manufacturing alkenylsuccinimides, succinic anhydride is prepared first by reacting polyalkenes with mzJeic anhydride. The most comm o n polyalkene used is polyisobutene. The resulting polyisobutene succinic a n h y d r i d e is then neutralized with aJkylenepolyamines to form polyisobutene succinimides. In some instances, further reaction of polyisobutene succinimides with m e t a b o r i c acid is commonly used to impEirt seal compatibility and enhance deposit and Vcimish control. Alkenylsuccininate esters are formed by reacting polyisobutene succinic anhydride with polyhydric alcohols. Mannich derived dispersants are formed by reacting
CHAPTER formaldehyde, pol}dsobutenyl, and alkylenepolyamines. Disp e r s a n t s associate with p r e c u r s o r s of insolubles, t h u s controUing deposit formation. The presence of a long hydrocarbon chain in polyisobutene helps in suspending the precursors of insolubles in the bulk lubricant.
16: GEAR LUBRICANTS
441
are edkylated naphthalene and phenolic polymers and polymethacrylates. Alkylated naphthalene is formed by the reaction of napthalene with chlorowax or olefins in the presence of aluminum chloride. Alkylphenolic polymers are also prepcired in similar fashion. These pour point depressants all act in similar fashion by hindering the formation of wax from the base lubricant by adsorption on wax crystals.
POLYMERIC THICKENERS Polymeric thickeners are also c o m m o n l y referred to as viscosity modifiers a n d viscosity index improvers. They function to reduce the lubricant viscosity's dependence on t e m p e r a t u r e . Typical examples of these include polyisobutene, polymethacrylates, and copolymers of st5Tene and butadiene. Isobutylene is polymerized to desired molecular weights in the presence of a Lewis acid. Polymethylacrylates are prepeired by free radical polymerization of alkyl acrylates and methacrylates. Styrene butadiene polymers are formed by free radical polymerization of styrene-butadiene mixtures in the presence of vanadium based catalysts. Similjir process is used in the polymerization of olefin block copolymers. This latter type of poljTner is not commonly used due to its high manufacturing cost. The molecular weight of these polymers vary greatly. Due to shear stability constraints imposed by viscosity classification for gear lubricants, the molecular weights (MW) generally range from 5000-50 000, depending on polym e r type. The most c o m m o n tj^e of viscosity modifier used for gear lubricants is polyisobutene, whose MW is around 5000. All viscosity modifiers act in similar fashion. At low temperatures, the polymers are tightly coiled cind their effects on the viscosity are low. As temperature increases, the polymer uncoils and occupies more space. This has the overall effect of not reducing the lubricant's viscosity at high temperatures.
F o a m Inhibitors Foam inhibitors act to deter the formation of foam in applications where there is substantial a m o u n t of churning and p u m p i n g , e.g., automotive axles. Typical c o m p o u n d s are polydimethylsiloxanes a n d polyalkylmethacrylates. These compounds effectively reduce the surface tension of the airlubricant interface, thereby facilitating their collapse. Polydimethylsiloxanes a n d alkyl methylacrylates are prep a r e d by polymerization of dimethylsiloxane a n d alkyl methylacrylates. Friction Modifiers Friction modifiers are c o m p o u n d s that act to reduce the coefficient of friction. Certain antiwear and EP agents such as long chain edkyl phosphates and molybdenum compounds can be used as friction modifiers. Other examples of friction modifiers are graphite, fatty alcohols, acids, and amides. Friction modifiers adsorb on the metal surfaces with their polar ends, thus forming a b o u n d a r y film with desired properties. Pour Point Depressants These compounds enable the gear lubricant to flow at low temperatures. Tjqjical examples of p o u r point depressants
Oxidation Inhibitors Oxidation inhibitors function to inhibit the oxidation and decomposition of the gesir additives and lubricant. Examples of oxidation inhibitors are ZDDPs, polysulfides, hindered and sulfurized phenols, and aromatic amines. The preparation of ZDDPs and polysulfides has been described. Hindered phenols are made by Lewis acid catalyzed alkylation of phenol or aJkylphenol with poljdsobutylene. Sulfurized phenols are formed by reacting alkylphenol with sulfur monochloride. Arylamines are prepared by alkylating diphenylamine with a n olefin in the presence of aluminum chloride. Oxidation inhibitors function to decompose hydroperoxides and radicals formed during oxidation. The reduction in hydroperoxides and radicals slows down the rate of oxidation of the gear lubricant.
Corrosion Inhibitors Corrosion inhibitors function to prevent corrosion and rusting of metal peirts in contact with the gear lubricant. Comm o n examples of these are ethoxylated phenols, sulfonate based detergents, triazoles, and thiadiazoles. Polyethoxylated phenols and amines are formed by reacting ethylene oxide, alkylphenols, a n d amines. Benzotriazole is m a d e from diazotization reaction with nitrous acid. Dimercaptothiadiazole (DMTD) derivatives are made of sodium salt of DMTD with Ein alkyl halide. Corrosion inhibitors adsorb on the metal surface via their polcir ends. Some of these, such as DMTD derivatives, can be used to prevent corrosion against yellow metals s u c h as synchronizers and brass washers.
Demulsifiers and Emulsifiers Demulsifiers enhance water separation from the gear lubricant c o n t a m i n a t e d with water. Emulsifiers promote the mixing of water and oil to form an emulsion. Examples of demulsifiers cire block polymers of ethylene oxide, or propylene oxide and glycerol, siloxanes, polyamines, and polyols. Common examples of emulsifiers are hydroxyalkylamines, amides, a n d ethers. Demulsifiers function oppositely of emulsifiers. They migrate to the water-lubricant interphase to create low viscosity zones, thus promoting the cocdescencing of water or lubricant. On standing, gravity drives the separation of water from oil. Emulsifiers reduce the surface tension of water, thus promoting its mixing with oil. Tables 11-18 [ 8 a , b , l l b ] provide a s u m m a r y of additive tjrpes and their function, with examples given for each type. These tables also provide details on their chemical structure and the general mode of sjmthesis. The additives described are collectively formulated into mineral cind/or synthetic base oils. Table 19 summarizes typical properties of base oils that
442 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 11—Chemically active additives and their function. Additive Class Antiwear and extreme presure (EP) agents
Purpose Reduce Mction and Wear, and prevent scoring and seizure
Typical Compounds Zinc dialkyl dithiophosphates, di or tri-alkyl and atyl phosphites, alkyl phosphoric acid esters and salts, organic sulfides and polysulfides, sulfurized fats, dithiocarbamic acid derivatives, and chlorowaxes."
Mechanism of Performance These additives react with the metal surface to form a sacrificisil chemical film with lower shear strength than the metal itself. This not only inhibits welding of two surfaces in contact but also minimized metal removal due to friction.
Corrosion and rust inhibitors
Prevent corrosion and rusting of metal parts in contact with the lubricant
Ethoxylated phenols, neutral and basic metal sulfonates, alkanolamines, alkenylsuccinic acids, and heterocyclic compounds, such as alkyltriazoles and dimercaptohiadiazole (DMTD and their derivatives.)*
These chemicals adsorb on the metal surfaces via their polar ends"^ and at the same time associating with the lubricant via their hydrocsirbon groups. This allows the formation of a durable lubricant film that acts as a barrier against corrosion-causing materials. Some of these are basic and have the ability to neutralize corrosive acids as well.
Detergents
Prevent metal attack by acidic byproducts of combustion and oxidation and keep metal surfaces free of deposits
Neutral and basic (contain base reserve, usueilly as metal carbonate sodium calcium and magnesium sulfonates, phenales, and carboxylates.)
These additives perform by two mechanisms. (1. They neutralize organic acids that result from lubricant oxidation. (2. They associate with sludge and varnish precursors and keep them dissolved in oil. Both these mechanisms prevent deposit formation, thereby keeping meteil surfaces clean.
Dispersants
Keep insoluble contaminants dispersed in the lubricant
Alkenylsuccinimides and succinate esters, Mannich products, and alkenylphosphonic acid derivatives
These chemicals, like detergents, associate with deposit precursors and prevent their agglomeration to insolubles. The presence of long hydrocarbon chains in dispersants help in suspending these precursors in the bulk lubricant.
"Tlieir use is being discontinued due to perceived negative impact of chlorine on the environment. ^Heterocyclic compounds are useful in controlling yellow metal corrosion. °A11 additives except some viscosity modifiers and pour point depressants have a polar end and a non-polar hydrocarbon chain. Polarity is due to the presence of a nitrogen, oxygen, sulfur, or phosphourus-containing functional group.
are used to blend automotive and industrial gear lubricants. The two categories of base stocks shown are mineral and synthetic. Mineral base stocks typically are solvent refined from crude and have viscosities ranging from approximately 4-30 cSt at 100°C. Synthetic base stocks are chemically different and cein range widely in viscosities. The selection of these various base stocks (mineral and/or synthetic) is dependent on the cost, application, and performance of the gear lubricant. There are no definite rules to follow except that any combination of base stocks can be used to meet the viscosity and performance targets, e.g., oxidation of the formulated gear lubricant.
well [16-18]. The purposes of defining service designations and classifications cire: • to promote a uniform practice for use by marketers of gear lubricants and by OEMs in identifying and recommending these lubricants by service designations [16-18]. • to assist the users of automotive and industrial equipment in the selection of gear lubricants for field use [16-18]. The following sections cover specifications to meet comm o n automotive and industrial gear lubricant requirements. A glossary of test methods covered is provided following this section. Automotive Gear Lubricants
CLASSIFICATION AND PERFORMANCE TESTING OF GEAR LUBRICANTS This section covers some commonly used requirements for qualifying automotive and industrial gear lubricants. Even though the purpose for developing SAE J308 was its application to automotive and manual transmission lubricants, the requirements apply to industrial gear lubricants as
The evolution of gear lubricants to their present composition and service requirements began with the introduction of hypoid gears by Gleason Works in 1925 [5]. This design was adopted because it provided automobile manufacturers the advemtages of a lower center of gravity, lower weight, quieter operation, and greater load carrying capabilities. However, the lubrication of h5rpoid geeirs proved challenging. They required gear lubricants with enhanced EP performance. At the
CHAPTER 16: GEAR LUBRICANTS
443
TABLE 12—Chemically inert additives and their function. Additive Cass
Purpose
Typical Compounds
Functions These materials have the ability to complex with metal ions to make them inactive. This inhibits the oxidation-promotion action of metal salts.
Metal deactivators
Reduce catal5'tic effect of metcils and their salts on the rate of oxidation
Complexing agents, such as ethylenediaminetetraacetic acid (EDTA and diseJicylidene-propanediamine)
Oxidation inhibitors
Inhibit oxidative decomposition of lubricant and additives
Zinc dialkyl dithiophosphates, organic sulfides and polysulfides, sulfur-coupled phenols (phenol sulfides and hindered phenols, and aromatic amines)
These additives have the ability to decompose hydroperoxides and to make radicals innocuous. Both these species promote oxidation. Their removal from the reaction sequence results in a slowdown of oxidation.
Emulsifiers
Promote mixing of water and oil to form an emulsion"
Nonionic surfactants, such as hydroxyalkyl amines, amides and ethers, and ionic surfactants, such as sodium carboxylate and trialkyl-ammonium halides and carboxylates
These materials facilitate emulsion formation by lowering surface tension of water and allowing its through mixing with oil. For oil-in-water emulsions, these chemicals encapsulate oil using their non-polar ends and keeping it in solution by associating with water via their polar ends. For water-in-oil emulsions, these additives perform in a reverse msinner. That is, they associate with water via their polar ends and keep it in solution by associating with oil via their non-polar ends.
Demulsifiers
Enhance water separation from oil contaminated with water
Block polymers of ethylene oxide or propylene oxide and glycerol, siloxanes, polyamines, or polyols
Demulsifiers have a function opposite to that of emulsifiers. They facilitate water separation from a water-contaminated lubricant. They concentrate at the water-oil interface to create low viscosity zones, thereby promoting drop coalescence and gravity-driven phase separation.
Foam inhibitors
Prevent lubricant from forming a persistent foam
Silicone polymers, such as polydimethylsiloxane, and organic polymers, such as poly (sJkyl methaciylates)
These additives reduce surface tension of air or gas bubbles, thereby facilitating their collapse.
Friction modifiers
Alter coefficient of friction
Organic fatty alcohols, acids, and amides, linear alkyl phosphites and phosphates esters and their salts, molybdenum compounds, and graphite
These materials associate with metal surfaces via their polar ends and with lubricant via their hydrocarbon chains. The result is the formation of a durable lubricant film of desired frictional characteristics.
Pour point depressants
Enable lubricant to flow at low temperatures
Alkylated naphthalene and phenolic polymers, polymethacrylates
These compounds adsorb themselves onto wax crystals and prevent the formation of undesirable crystalline networks that can adsorb leirge quantities of oil and hinder its flow.
Seal swell agents
Swell elastomer seals
Organic phosphates and aromatic hydrocarbons
These additives are adsorbed by the elastomer seal material causing it to swell. This not only assures the integrity of the seals but it also prevents them from cracking and deterioration.
Viscosity modifiers
Reduce the rate of viscosity change with temperature
Poly (alkyl methaciylates, polyolefins, and copolymers of styrene and butadiene
The key to the performance of these additives is their differential association* with lubriceint at different temperatures. At low temperatures, this association is low, and at high temperatures, it is high. The result is a lower loss in viscosity at high temperatures.
"Desired for some hydraulic and metalworking lubricants tiiat use such emulsions. ''The term solubility is often used to describe this phenomenon. However, the term association may be more appropriate.
444
MANUAL 37: FUELS AND LUBRICANTS
Table 13—^Antiwear and extreme pressure (EP) additives. Structure Methods of Synthesis
Compound Zinc dialkyl dithiophosphates
Phosphites and phosphates
HANDBOOK
These materials are made by reacting alcohols with phosphorus pentasulfide to form dialkyl dithiophosphoric acids. These acids react with zinc oxide to form zinc salts.
(RO),E^
Phosphites result when alcohols or phenols are reacted with phosphorus trichloride or a lower alcohol phosphite, such as trimethyl phosphite or dimethyl hydrogen phosphite.
O (RO)2P^ H Dialkyl Hydrogen Phosphite o
Triaryl Phosphite o
R O — P — a RNHj
+
RO—p—ff
OR
RNHj
OH
Amine Salts of Dialkyl and Mono-alkyI
Amine phosphates are obtained from amine salting of monoalkyl and dialkyl phosphoric acids. These acids are usually made by reacting alcohols with phosphorus pentoxide.
Phosphoric Acids
Organic sulfides and polysufides
CHj HjC-
I -c—ss- - C — C H , CH,
CHj
:—c—sss—c—CH,
GH,
CHJ
Dialkyl Disulfide
CHJ
Dialkyl Trisulfide
Disulfides can be made by oxidative coupling of mercaptans, which can react with sulfur to form higher polysulfides. Polysulfides can also be made by a one step process involving olefin-hydrogen sulfide-sulfur reaction.
R—CH=CH-CH;
Sulfiirized olefins
Sa
s/
s.
R-CH2-CH—CH2 RCH2'
R-CH2-CH—CHJ
These products are salts of dithiocarbamic acid. The acid, made from dialkylamine and carbon disulfide, is salted using zinc oxide.
R2N—c:^
Dithiocarbamates
Olefins can be sulfurized using sulfur at 125''C or higher. Usually, a mixture containing a variety of products is obtained.
Zinc Dithiocarbamates
N O T E : R and R2 represent alkyl groups.
Compound Neutral and basic metal sulfonates
Neutral and basic metal phenates
Table 14—Detergents. Structure Methods of Synthesis Alkybenzenesulfonic acids are reacted with stoichiometric S0j)»M^003 S05)2M amount of lime (calcium hydroxide) or magnesium oxide to form neutral salts. One can use excess of these reagents if Neutral Metal Basic Metal carbon dioxide is used as a co-reactant. The materials thus Sulfonate Sulfonate M = Ca or Mg obtained, called basic or overbased salts, contain excess base as metal carbonate.
•
'
oyn^ooj
.0J2M
Basic Metal Phenate
Neutral Metal Phenate
Neutral and basic metal carboxylates
M = Ca or Mg
-CCs - -ex,.. o o Neutralc Metal Salicylate ,QH
Basic Metal Salicylate
M=Ca or Mg
The process for making these materials is the same as that for metal sulfonates, except that the starting acid is an alkylphenol instead of an alkylbenzenesulfonic acid.
The process for making these compounds is the same as that for metal sulfonates and phenates. The starting material for these products is alkylsalicylic acid. This acid results when a phenol is reacted with a strong base, such as sodium or potassium hydroxide, and carbon dioxide (Kolbe's process).
CHAPTER
16: GEAR LUBRICANTS
Table 15—Dispersants. Compound
Methods of Synthesis These materials are made by reacting alkylenepolyamines with polyisobutenylsuccinic anhydride. Succinic anhydride is an ene reaction product of polyisobutylene and maleic anhydride.
Structure
Alkenylsuccinimides
./ \
NCHjCHjNHCHjCHiN
Polyisobutenvlsyccinimtde
Alkenylsuccinate esters L
Polyisobutenylsuccinic anhydride is reacted with polyhydric alcohols, such as neopentyl glycol and penterythritol, to form these esters.
(
Polvisobutenytsuccinate Ester
Mannic products
1^ "^1^5^
Alkenylphosphonic acid derivatives
These products result from the condensation of an alkylphenol, formaldehyde, and alkylenepolyamines. When these products are designed for use as dispersants, the alkyl group in alkylphenol is polyisobutenyl. These materials can be obtained by reacting olefinphosphorus pentasulfide adduct with polyhydric alcohols. Usually, the adduct is first hydrolyzed using steam and then reacted with an alkylene oxide, such as propylene oxide, to form these esters.
CHjNHCHjCHNHCHjCHzN Polyamtnomethylalkylphenol
PIB—P—OCHjCH-CW OCHiCH-OH CH, Bis-hydroxypropyl Alkenvlohosohonate
Table 16—Oxidation inhibitors. Compound
Methods of Synthesis These materials are made by reacting alcohols with phosphorus pentasulfide to form dialkyl dithibphosphoric acids. These acids are reacted with zinc oxide to form zinc salts.
Structure
Zinc dialkyl dithiophosphates
Organic sulfides and polysulfides
CH, H,C—C—SSCHj
f
HjC—C—SSS-
CHj
CH,
CH,
Dialkyl Trisulfide
Dialkyl Sulfide
Sulfur-coupled phenols (phenol sulfides) and hindered phenols
Monosulfides can be obtained by reacting a mercaptan with an olefin. Disulfides can be made by oxidative coupling of mercaptans, which can react with sulfur to form higher polysulfides. Polysulfides can also be made by a one step process involving olefm-hydrogen sulfidesulfur reaction. Sulfur-coupled phenols, or phenol sulfides, can be prepared by reacting an alkylphenol with sulfiir monochloride. Hindered phenols are made by Lewis acid-catalyzed alkylation of phenol or alkylphenol with an olefin, usually isobutylene. Diphenylamine is aklylated with an olefin in the presence of a Lewis acid, such as aluminum chloride, to yield these products.
Aromatic amines Arylamine
445
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Table 17—Viscosity index improvers. Compound
Methods of Synthesis
Structure CH,
Polyolefins
I
•CHj-C— CH,
^3 J n
Isobutylene polymerizes in the presence of Lewis acids to form these polymers. Higher olefins also polymerize to yield analogous materials.
Polyisobutylene
Olefin copolymers CH, -fCH2-CH2^
-CH—CHj-
Ethyiene-propylene Copolymer
Styrene diene copolymers
CH3 CH-CH2-
-CH2—CH=C-CH2-
Styrene-diene Polymer
Alkyl methacrylate polymers
CH, CH2-C-
COORJj, Alkyl Methacrylate Polymer
Styrene ester polymers
RO^
-C^o
-CH-CHj-CH-CH
/
0=0
RO
These polymers are prepared from mixtures by vanadiumbased Ziegler-Natta catalysis. In these polymers, the ratio of different olefins in the monomer mixture determines the polymer's low and high temperature properties. Two common polymers of this type are EPRs and EPDMs. EPRs are made fi^om ethylene-propylene mixtures. EPDMs, on the other hand, contain butadiene or isoprene as the third monomer. These materials result from free radical polymerization of styrene-butadiene mixtures. Because of the presence of unsaturation (double bond), these polymers are more susceptible to oxidation. Their oxidation resistance can be improved by removing the double bonds through hydrogenation. Free radical polymerization of alkyl acrylates and methacrylates results in these polymers. In addition to being good viscosity modifiers, these polymers have the ability to lower pour point. Hence, they are also used as pour point depressants. These esters are prepared by reacting styrene-maleic anhydride polymer with alcohols. Styrene reacts with maleic acid in the presence of fi-ee radical initiators to yield this polymer.
Styrene Ester Polymer
automobile manufacturers request, b o t h t h e additive a n d petroleum industries initiated the development of gear lubricants to meet hypoid gear performance. Gear lubricants based on sulfur-chlorine additives were then developed for passenger car rear axles for high speed a n d moderate loads. Chlorinated paraffins, sulfurized fatty acids, a n d olefins are examples of sulfur-chlorine additives used for this purpose. As the popularity of hypoid axles increased into t h e mid1930s, they were incorporated into trucks, which operated at low speeds a n d higher loads. This led to the development of sulfur-phosphorus-chlorine based gear lubricants for both passenger cars and light duty truck applications. This was evident during World War II, where gear lubricants designated by MIL-L-2105 were plagued with persistent problems associated with truck axles operating under high torques and low speeds. These problems were relieved with the use of phosphorus containing additives in zinc dithiophosphates, alkyl phosphates, a n d phosphates. After World W a r II, MIL-L2105 gear lubricants were becoming inadequate in meeting the requirements of new hypoid axles, which were subjected to higher torques. By the 1950s, the U.S. Military, working closely with the Coordination Research Council (CRC) a n d t h e American
Petroleum Institute (API), developed test methods a n d service designations for qualifying gear lubricants under low speed, high torque a n d high speed-shock load conditions. This led to the development of MIL-L-2105A. These tests and service designations were able to differentiate the loadcarrying capacities of automotive gear lubricants. The 1960s, with improved aerodynamics a n d demand for further increases in load-carrying requirements of hypoid axles, led to increasing thermal and oxidative problems in the field. This along with environmental concerns over lead and chlorine use prompted the use of advemced sulfur-phosphorus and later, borate based additives (in the 1970s), which are still in use today. The U.S. Military revised their specifications to incorporate thermal and oxidative requirements into MIL-L-2105B. Later, other tests for power divider, fatigue spalling, a n d corrosion were incorporated by the industry from the 1970s to 1980s. Table 20 highlights some of the important milestones affecting automotive gear lubricants. The corroborative efforts of API, ASTM, SAE, Military, and CRC led to the complete description of gear lubricants meeting viscometrics and performance requirements for various service designations. ASTM a n d CRC were responsible for defining the different service categories, while SAE defined
CHAPTER 16: GEAR LUBRICANTS
447
Table 18—Miscellaneous additives. Structure
Compound
Methods of synthesis CH2CH2OH
Corrosion and rust inhibitors
R—N^
Ca
\
Neutral Calcium Sulfonate
CH2CH2OH
Diethanolamine
Neutral and basic calcium sulfonates are made by reacting alkylbenzenesulfonic acids with lime in the absence or the presence of carbon dioxide. In basic detergents, the reserve base is usually in the form of calcium carbonate.
O SO3
R—CH—C—OH
^€f
I
CaxCaCOs
CH—C—OH
II
O Basic Calcium Sulfonate
Alkenylsuccinic Acid 0(CH2CH2)xCH2CH20H
^ ^ ^
Alkenylsuccinic acid is a hydrolysis product of alkenylsuccinic anhydride. Dimercaptothiadiazole (DMTD) derivatives are made either by reacting sodium salt of DMTD with alkyl halide or by reacting DMTD with mercaptans in the presence of an oxidizing agent. DMTD is manufactured by reacting hydrazine with carbon disulfide.
Polyethoxvlated Phenol
SR
RS.
Polyethoxylated phenols and amines result from the reaction of ethylene oxide with alkylphenols and amines.
N—N Oimercaptothiadiazole
Alkylbenzotriazole
Derivative
Benzotriazole is usually made from o-phenylenediamine via a diazotization reaction (reaction with nitrous acid).
CH2OOOH N=CHCH2CX30H
Metal deactivators
Ethylenediaminetefraacetic acid is made from base-catalyzed reaction of ethylenediamine and chloroacetic acid.
-CH2COOH W
\
HO
CH2COOH
Emulsifiers and demuisiflers
N, N-Disalicylidene-1,2Propanediamine
Ettivlenediaminetetraacetic Acid
\
R0(CH2CH20)xCH2CH20H
NH(CH2CH2);P Polymhoxylated Alcohol IHydraxyalkyI Etherl
N-Hydroxyalkvlamide CH2CH2OH
R—N'^
\
V
CH2CH2OH Diethanolamine R'
R' I R—N—H X
O—Si-
I CH;
CH,
\
R"
R" Trialkylammonium Salt CH3
Sodium Cartoxylate
R—N^-CHjCHjC
i
Foam inhibitors
ONa'*'
Tetraalkylammonium Carboxylate 1 CH,
CH3
Hydroxyalkyl and polyhydroxyalkyl derivatives are prepared by reacting amides, amines, and alcohols with alkylene oxides. Ethylene oxide is most often used for this purpose. Sodium carboxylate is prepared by reacting a carboxylic acid with sodium hydroxide. The reaction of amines with mineral acids yield alkylammonium salts. When amines are reacted with organic halides, quatemary ammonium salts are obtained. Tetraalkylammonium carboxylate is a product of chloroacetic acid with trialkylamines.
Siloxane polymers are products obtained from the reaction of dimethylsilyl chloride with water.
I
O—Si—O-j-SiCH,
Sahcylaldehyde reacts with 1,2-propanediamine to form the salicylidene.
•CHj-C— COORj n
CH,
Alkyl Methacrylate Polymer
Dimethylsiloxane Polymer
OH
Pour point depressants
Alkyl methacrylate polymers result from free radical polymerization of alkyl methacrylates.
Alkylnaphthalenes form when naphthalene is reacted with chlorowax and or olefins in the presence of a Lewis acid, such as aluminum chloride.
R'
Alkylnaphthalene
CH,
I
Alkylphenol
Alkylphenols are products of Lewis acid-catalyzed reaction of phenol with waxy olefins and or chlorowaxes.
-CH2-C
I
COORj AlkylMethacrylate Polymer R, R' Waxy Alkyl Groups
Alkyl methacrylate polymers result from free radical polymerization of alkyl methacrylates.
4 4 8 MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
TABLE 19—Physical properties: mineral oils versus synthetic base stocks [8b].
Type 90 100 200 350 650 150
Neutral Neutral Neutral Neutral Neutral Bright Stock
Kinematic 40°C 17,40 20,39 40,74 65,59 117,90 438,00
Alkylated aromatic Olefin oligomer Dibasic acid ester (Dioctyl sebacate) Trimethylolpropane Ester (C7 Acid) Polyglycol
Viscosity, cSt 100°C 3,68 4,11 6,23 8,39 12,43 29,46
(A) Mineral Oils Dynamic Viscosity, at -40°C (cP)
Solid Solid Solid Solid Solid Sohd (B) Synthetic Base Stocks
26,84 16,77 18,2
4,99 3,87
9047 2371 3450
13,94 45,69
3,4
2360
Viscosity Index
Pour Point (X)
92 101 99 97 96 95
-15 -13 -20 -18 -18 -18
190 192 226 252 272 302
119 126 176
-18,3 -43,0 -15,6
224 221 232
-18.3
232 177
150
TABLE 20—Chronological changes in automotive gear lubricants. Year
Performance Demands
Reasons and Consequences
1925
Hypoid Axle introduced
1940s 1950s
Service Classification MIL-L-2105A updated
1962
MIL-L-2105B revised
1975
Mack GO-G
1976
MIL-L-2105C
1987 1994
MIL-L-2105D MT-l/PG-1
1995
MIL-PRF-2105E
1996
Extended Service
1998 2001
SAE J306 MIL-PRF-2105E Modified
Lower center of gravity, weight, quieter operation By API and Military Wear and EP limits incorporated Oxidation & thermeJ stability limits incorporated Power divider & Spalling Tests introduced Low temperature, crossgrades, corrosion limits incorporated Quality issues: MSDS HD transmission specification introduced MIL-L-2105D + MTl/PG-1 introduced OEM specifications by Eaton, Mack, Meiitor Revised viscosity classification Extended field testing
TABLE 21—API gear oil service designations [5,8,17,21]. API Classification Type Applications GL-1
Straight mineral oil.
GL-2
Usually contains fatty materials. Contains mild EP Additives.
GL-3 GL-4
GL-5
GL-6 MT-I
viscosity grades and service designations for axle cind manual trEinsmission lubricants. Today's automotive gear lubricants are classified by several designations: • API Service Designation • SAE Viscosity Classification (SAE J306) • US Military MIL-PRF-2105E and SAE J2360 [19,20] • OEM Specifications These different classifications will be discussed next.
API SERVICE DESIGNATION There are six service designations defined by API, summarized in Table 21. API GL-1 designates the type of service t3rpical of manual trEmsmissions operating under mild conditions of low unit pressures and minimum sliding velocities. Mineral oils treated only with oxidation smd rust inhibitors.
COC Flash Point (°C)
Equivalent to obsolete MIL-L-2105; usually satisfied with 50% GL-5 additive level. Virtually equivalent to present MIL-L-2105D; primary field service recommendation of most passenger car a n d truck builders worldwide. Obsolete. Contains t h e m a l stability and EP additives
Truck manual transmissions. Worm gear drives. industrial gear oils. Manual transmissions and spiral bevel final drives. Manual transmissions and spiral bevel a n d hypoid gears in moderate service. Moderate and severe service in hypoid and other types of gears. May cdso be used in manual transmissions. Severe service involving high offset hypoid gears Nonsynchronized manual transmissions in heavy-duty service.
defoamants, and PPDs may be used under this service category. Use of EP, emtiwear, or friction modifiers are not permitted. API GL-2 designates the type of service typical of automotive worm gear axles operating under conditions of load, temperature, and sliding velocities, which lubricants satisfactory for API GL-1 service will not suffice. Use of antiwear and very mild EP additives are permitted under this service category. API GL-3 designates the type of service typical of manual transmissions and spiral-bevel axles operating under mild to moderate to severe conditions of speed cmd load. These conditions require the gear lubricant to have greater load carrying capabilities required for GL-1 and GL-2, but not meeting the requirements for GL-4. Generally, gear lubricants
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TABLE 22—Proposed light duty manual transmission fluid standard [22]. Test Method
Characteristic
Conditions/Modifications
Synchomesh SSP 180 Wear Thermal Oxidation
CEC L-66-X-96 ASTMD5182 GFC Procedure A
Under development 90°C, 3600 rpm 160°C, 192 hours
Viscosity after shear loss Pour Point Brookfield Viscosity Cold Crank Simulator Foam
CEC-L-45-T-93
20 hrs, 60°C, 5000 N
ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM
Rust Prevention Copper Corrosion Seal Compatibility
D D D D D D D D
97 2983 5293 892 6082 665 (Method A) 130 5662
TABLE 23—API GL-5 tests [21,23]. Test Designation
Type
L-33
Gear test using axle components.
L-37
Gear test using complete axle assembly.
L-42
Gear test using complete axle assembly.
L-60 ASTM D 892
Bench test using spur gears. Bench test.
ASTMD 130
Bench test.
Characteristics Measured
Resistance to corrosion in the presence of moisture. Resistance to gear distress under the conditions of low speed high torque. Resistance to gear distress (scoring) under conditions of high speed and shock loads. Thermal oxidation stability. Foaming tendencies. Stability in the presence of copper alloys.
meeting this service category 2U"e not intended for hypoid axles. API GL-4 designates the type of service typical of spiralbevel and hypoid gears in axles operating under moderate speeds and loads. Gear lubricants under this service designation m a y be used in selected m a n u a l transmissions and transaxles. While this service category is commercially used, some original equipment for qualifying against this service designation is n o longer available. ASTM is currently investigating the possibility of redefining this service designation using m o d e m test equipment. This service category is called Passenger Manual Transmission (PM-1). A summary of the proposed tests for PM-1 is given in Table 22. API GL-5 designates the tjrpe of service typical of hypoid gears, operated u n d e r high-speed and/or low speed with high-torque conditions (see Table 23). Lubricants formerly qualified u n d e r MIL-L-2105D satisfy the requirements for API GL-5 service category. Details of API GL-5 performance tests [21] are contained in the following publications: • ASTM STP-512A: Laboratory Performance Tests for Automotive Gear Lubricants intended for API GL-5 Service.
-40^ -30°C Seq I, II, III SeqIV 3 hrs, 1 2 r C l
Limits No clash at 100 000 cycles 10 load stage pass 50% vis inc. max 2.5 m g KOH/g 25% max 5.8 cSt m i n at lOOX Less t h a n - 3 9 ° C 50 000 cP max 5000 cP max 20,50,20/0 ml 100/0 ml Pass lb max Pass
• SAE J2360 Lubricating Oil, Gear Multipurpose Military Use • Or equivalent MIL-PRF-2105E API GL-6 designates the tjrpe of service characteristic of gears designed with very high pinion offset. Such designs require score protection in excess of that required by GL-5. This service category is almost obsolete, as equipment required for testing is no longer commercially available. The API GL-5 service category is currently the most popular worldwide. In 1994, a new category for service in non-synchronized manual transmissions used in buses and heavy-duty trucks was approved (API MT-1). Gear lubricants meeting API MT-1 were approved against performance tests for improved deposit control, oxidation and thermal stability, and seal compatibility beyond those required by lubricants meeting GL-1 through GL-5. Comparisons between API GL4, GL-5, and MT-1 are given in Table 24. SAE J306 Viscosity Classification Axle a n d m a n u a l transmission lubricants were originally classified by their viscosities described in SAE J306c - Recommended Practice, published in 1978. In this classification system the ' W designates 'Winter' operation and defines low temperature viscosity limits for 75W to 85W grades. This was to alleviate pinion bearing failures when axles operated at temperatures when viscosities were greater than 150 000 cP. From 1996 through 1998, the following changes were made to SAE J306: • Title change to "Automotive Gear Lubricant Viscosity Classification" • Labeling requirements similar to SAE J300 in format and content • Viscosity grade must be preceded by "SAE" • W grade precedes non-W grade • Only the lowest W grade satisfied to be used • Two additional grades added (SAE 80 and SAE 85) with viscosity limits at 100°C • Footnote regarding low temperature performance of manual transmission lubricants • Incorporation of stay-in-grade shear stability requirement using CEC-L45-T93 The requirement for shear stability was incorporated because m o d e m gear lubricants may contain high molecular weight polymers that may shear, resulting in a loss in viscos-
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TABLE 24—API GL-4, GL-5 and MT-1 performance comparisons (5,23). Performance
API GL-4 (MIL-L-2105)
Scoring resistance u n d e r high speed shock load conditions Resistance to gear distress under high torque, low speed conditions. Corrosion resistance in the presence of water.
CRC L-19'' test or FTM 6504T; equal to or better t h a n RGO-105. CRC L-20 test; no tooth disturbance such as rippling, ridging, pitting, or severe wear. 1. CRCL-13 or FRM 5313.1 2. CRCL-21 No evidence of rusting.
Thermal and oxidation stability/component cleanliness.
No requirement.
Antiwear
No requirement.
No requirement.
High temperature lubricant stability.
No requirement.
No requirement.
Antifoaming characteristics
CRC L-12 test: readings taken immediately after 5 min. aeration; Sequence 1 -23.9°C, 650 mL; Sequence 2 -93.3°C, 650 mL ASTMD130:3bmax. after 1 hr. at 1 2 r C . No requirement.
ATSM D892: readings taken immediately after 5 min. aeration; Sequence 1 —20 mL max; Sequence 2 - 5 0 mL max; Sequence 3 - 2 0 mL max.
Cooper corrosion Channeling characteristics
API MT-1 No requirement. No requirement.
L-33 test: n o evidence of rusting after 7 day's exposure on any working surface; m a x i m u m 0.5 in.^ rust on cover plate ( 1 % surface area). L-60-1 test; 100% max. viscosity increase; 3 % max. pentane insolubles; 2% max. toUuene insolubles.
No requirement.
L-60-1 test; 100% max. viscosity increase; 3 % max. pentane insolubles; 7.5 min. carbon/ varnish ration on large gear; 9.4 min. sludge rating on all gears. FZG (A/8.3/90); minimum 10 stage pass. Mack Transmission Test T-2180: equal to or better than reference ATSM D892: readings taken immediately after 5 min. aeration; Sequence 1—20 mL max; Sequence 2—50 mL; Sequence 3 - 2 0 mL max.
ASTM D130: 3 max. after 3 hr.
ASTM D130: 2A max. after 3 hr. at 1 2 r C . No requirement.
at 12rc.
FTM 3456.1 Modified: SAE 75, - 4 5 ^ max; SAE 80W90, - 3 5 ° C max; SAE 85W-140, - 4 0 ° C max. SS&CFED-STD-791
SS&CFED-STD-791
Compatibility with existing gear lubricants Solubility—measure separated material after centrifuging oil stored for 30 days at room temperature (29.4 ± 9.5°C). Compatibility—same solubility except mixed 50/50 with each of six reference oils.
API GL-5 (MIL-L-2105D) L-42 test: gear/pinion coast side scoring equal to or better thanRGO-110 L-37 test; no tooth disturbance such as rippling, ridging, pitting, or severe wear.
SS&CFED-STD-791
FTM 3430: 0.25 wt. % max of original nonpetroleum material in sample
FTM 3430: 0.25 wt. % max of original nonpetroleum material in sample
FTM 3430: 0.25 wt. % max of original nonpetroleum material in sample
FTM 3440: 0.50 wt. % max of original nonpetroleum material in SEimple.
FTM 3440: 0.50 wt. % max of original nonpetroleum material in sample.
FTM 3440: 0.50 wt. % max of original nonpetroleum material in sample.
"Equipment no longer available; impossible to conduct test per original procedure.
TABLE 25—Viscosity classifications for SAE J306 and MIL-PRF-2105E [18,19,23]. SAE Viscosity Classification Properties Vis at 100°C Min (cSt) Max (cSt) Vis of 150,000 cP Max Temp, °C
70W
75W
SOW
85W
80
85
90
140
250
4.1 None
4.1 None
7.0 None
11.0 None
7.0 <11.0
11.0 <13.5
13.5 <24.0
24.0 <41.0
41.0 None
-55
-40
None
None
None
None
Properties Viscosity at 100°C Min (cSt) Max (Cst) Vis of 150,000 cP Max Temp, °C Channel Point, min. "C Flash Point, min °C
-26
-12 None iUilitary Specification 75W 80W90 4.1
85W140
-40
13.5 <24.0 -26
24.0 <41.0 -12
-45 150
-35 165
-20 180
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TABLE 26—Chemical and physical property requirements of MIL-PRF-2105E [19]. Property
SAE 75W
SAE 80W-90
SAE 85W-140
As per SAE J306 -45 150 Report Report Report Report Report Report Report Report Report Report Report Report Report
As per SAE J306 -35 165 Report Report Report Report Report Report Report Report Report Report Report Report Report
As per SAE J306 -20 180 Report Report Report Report Report Report Report Report Report Report Report Report Report
ASTM Method
Viscosity Channel Point, °C, max Flash Point, "C, min API Gravity Viscosity Index Pour Point, "C Pentane Insolubles, wt% Sulfur, wt% Sulfur from Additive, wt% Phosphorus, p p m Chlorine, p p m Nitrogen, p p m Zinc, p p m Boron, p p m Potassium, p p m Organo-metals, ppm
D445 FTM 3456 D92 D287 D2270 D97 D893 D1552 D 1552 D 1091 D808 D3228 D4951 D4951 D4951 D4951
ity. In order to ensure that the designated high temperature viscosity grade is retained during use, gear lubricants must meet viscosity limits at 100°C after shear. The permanent loss in viscosity may be detrimental to the gears and bearings as film thicknesses may decrease. U.S. Military MIL-PRF-2105E In August 1995, MIL-L-2105D was superceded by MIL-PRF2105E. Most of the requirements needed for API MT-1 were incorporated into MIL-L-2105E. In 1998, at the U.S. Mihtary's request, SAE adopted the SAE J2360 Standard equivalent to MIL-PRF-2105E. The viscosity requirements are shown in Table 25 along with the SAE J306 Standard. The requirements for stay-in-grade also apply to the military viscosity grades. Tables 26-29 summarize key performance tests as required by MIL-PRF-2105E. Field tests using light duty trucks or passenger cars and heavy duty Class 8 trucks are also required to demonstrate no heirm performance in light and heavy duty axles and heavy duty non-synchronized manual transmissions. The gear lubricants covered by MIL-PRF-2105E or SAE J2360 are intended for automotive gear units, heavy-duty industrial type enclosed gear units, steering gear units, heavy duty non-s3rnchronized Class 7 and 8 manual transmissions, and fluid lubricated universal joints of automotive equipment. The lubricants covered by this specification are intended for use as defined by appropriate lubrication orders when ambient temperatures are above -54°C. In addition, API GL-5/MT1 tests, field tests in both light emd heavy-duty vehicles, are required for meeting MIL-PRF-2105E. Axles from the light and heavy-duty vehicles along with the transmissions from the heavy-duty vehicles are inspected for cleanliness, surface distress to gears, bearings, and shifting mechanisms. The gear lubricant's condition is also monitored at periodic intervals.
OEM Specifications Many OEMs have developed their own performance classifications for standard and extended drain services. These factory and service fill requirements generally meet API GL-5 performance requirements [1,24]. In addition, performance considerations for these lubricants meet specific performance targets in the areas of improved fuel economy, temperature reduction, improved fatigue performance, enhanced
TABLE 27—Bench and rig test requirements for MIL-PRF-2105E [19]. Passing Requirements
Test Method Copper Corrosion, ASTM D 130, 12rC, 3 h
Not to exceed 2a rating
Foam Tendency, ASTM D 892
Seq
Max Vol. at end of Blowing
I II III
20 ml 50 ml 20 ml
Oxidative and Thermal Stability, ASTM D 5704, 50 hrs, 163°C (L-60-1)
Viscosity Incr. at 100°C Pentane Insolubles Toluene Insolubles Carbon Varnish Rating Sludge Rating
Gear Wear Test, ASTM D 6121, 24 h, 275°F, Green and Lubrited Gears (L-37)
No severe surface distress to gears
Cyclic Durability Test, ASTM 5579
Better than reference oil
Seal Compatibility, ASTM D 5662
<100% < 3 wt% < 2 wt% >7.5 >9.4
Polyaciylate, 150°C, 240 h Parameters Elongation change, % Hardness change, points Volume change, %
Min No limit -35 -5
Max -60
+5 + 30
Fluoroelastomer, 150°C, 240 h
API L-33 Gear Corrosion API L-42 Gear Scoring Test
Parameters
Min
Max
Elongation change, % Hardness change, points Volume change, %
No limit -5 -5
-75 + 10 + 15
No rust on ages, < 1% on cover plate" Lower % scoring than reference on gears.
"Rating being modified for better consistency. yellow metal corrosion, frictional durability for limited slip axles, and power divider transaxles [25-47]. Other enhanced performance considerations include improved oxidative stability, reserve performance for extreme pressure [2], and wear. These requirements ensure that the axles and transmissions are protected under the increased
452 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 28—Light and heavy-duty field test requirements of MIL-PRF-2105E [19,20]. Field Test Type
Number of Vehicles Needed
Test Mileage
Sampling Interval
Heavy Duty Service (Axles and Transmissions) Light/Moderate Duty Service
Minimum of three Class 8 Trucks Minimum of five light trucks and/or sedans
100 000 miles (previous) 200 000 miles (current) 50 000 miles (previous) 100 000 miles (current)
4 oz, every 20 000 miles 4 oz, every 10 000 miles
TABLE 29—Field test oil analyses requirements of MIL-PRF-2105E [19]. Property
ASTM Method
Viscosity at 40 and 100°C, cST Phosphorus, p p m Sulfur, wt% Chlorine, ppm Nitrogen, p p m Zinc, p p m Boron, p p m Potassium, p p m Organo-metals, p p m Total Acid Number Total Base Number Insolubles, wt%
D445 D 1091 D1552 D808 D3228 D4951 D4951 D4951 D4951 D664 D4739 D983
TABLE 30—Current/future OEM axle specifications [1]." OEM Mack
SAE Viscosity Grades
Specification No. GO-J
80W-90, 85W-140
GO-J Plus
Performance Requirements
Recommended Drain Period (miles)
Mil-PRF-2105E Seal immersion test Mack spalling test(*) 500 000 mile field test 100 h o u r L-60-1 Extended Mack cycling testC)
250 000
500 000
500 000
Eaton/Dana
PS-163/ SHAES-256
75W, 80W-90, 85W-140
Mil-PRF-2105E Copper & bronze corrosion tests Dynamic seal tests Spalling test 500 000 mile field test Wet L-37 Test
ArvinMeritor
076-D & A
80W-90, 85W-140
MiL-PRF-2105E
100 000 miles without p u m p and filter to 500 000 miles with p u m p , filter a n d appropriate additives
Mercedes Benz
235
90, 85W90, 85W140
API GL-5 In-house axle tests
60 000 (long haul) for all without flexible service system (FSS) 220 000 for FSS
Mercedes Benz
235.6
80W90 85W90,90
API GL-5 In-house axle tests
60 000 (long haul) for all without flexible service system
Dynamic seal tests 50 p p m CI max.
Volvo
MAN
97 310 97 312
75W, SOW, 90, 85W140, API GL-5 80W140 API GL-5/Pitting test/field test 75W90
342N 342ML
90 80W90
342SL
75W90 80W90
Iveco ZF
TEML05
API GL-5 API GL-5 FZG PITS field test API GL-5 FZG PITS field test
(FSS) 220 000 for FSS 75 000 (long haul) 110 000 current (long haul) 250 000 target 50 000 100 000 (long haul) 200-250 000
90 85W140
API GL-5
75 000
90
API GL-5 In-house axle and FZG tests
50 000
"This oil is used for axles and transmissions. ^Test currently not available.
CHAPTER 16: GEAR LUBRICANTS driving durability they are currently being subjected to. Certain OEMs rely on extended versions of the L-60-1 test to define improved performance for oxidation [1]. Gear lubricants are also required to perform in power dividers. Interaxle and interwheel power dividers transfer torque between front-rear axles and wheels, respectively. Most conventional sulfurphosphorus and borate based gear lubricants are not capable of providing improved frictional durability required for performance in power dividers. These require the use of friction modifiers for enhancing the proper lubrication of gears, cams, and bearings found in power dividers. Similar frictional performance for light-duty vehicles is seen in limited-slip axles. These serve to transfer torque equally between the rear wheels. The friction plates of these limitedslip clutch packs are made of steel plates that are manganesephosphate, molybdenum, or paper coated. The frictional requirements for power dividers and limited slip axles allow for conventional sulfur-phosphorus and borate chemistries to be compatible with friction modifiers with no compromise to the gear lubricant's GL-5 performance. Table 30 summarizes several major U.S. and European heavy-duty OEM current and future specifications [1].
INDUSTRIAL GEAR LUBRICANTS In general, industrial gear lubricants are used under conditions of moderate loading. These lubricants are applied in enclosed and open gearboxes. Industrial gear sets are more diverse than automotive gear sets and are generally lubricated by manual application, drip feed, spray, and splash lubrication. Typical uses of industrial gear lubricants are in steel mills, construction and mining sites, kilns, and furnaces. The main function of industrial gear lubricants are to prevent corrosion of rust and yellow metcJs, EP, as antiwear agents, for foam suppression, to prolong oxidation, to demulsify, and to be able to lubricate under some level of contamination. Examples of specifications for industrial gear lubricants cire: • AISE 224 (formerly known as U.S. Steel 224) • American Gear Manufacturers Association 9005-D94 [16] • General Motors LS-2 • David Brown ET 33/80 • DIN 51517/3 • Flenders • Cincinnati Milacron P-35, P-59, P-63, P-74, P-77, P-78 The two most common industrial specifications among the ones hsted are AISE 224 and AGMA 9005-D94. AGMA formed the lubrication committee in 1938 to study gear lubrication problems. This committee was responsible for the adoption of the standard AGMA 250.01—Lubrication of Enclosed and Open Gearing in 1946. Several revisions culminated in the standard AGMA 250.04 pubhshed in 1981, which eliminated lead napthenate as an EP additive and modified the AGMA lubricant numbering to coincide with the viscosity ranges in ASTM 2422 and the British Standard Institute (BS.4231) and International Standards and Organization (ISO 3448). The elimination of open gearing where bearings were lubricated separately led to the adoption of AGMA Standcird 251.01 approved in 1963. This standard was revised again in 1974. The current AGMA standard 9005-D94 again
453
TABLE 31—^AISE 224 requirements lead free EP gear oil [16]. Test API Gravity, ASTM D 287 Viscosity Index, ASTM D 567 Precipitation Number, ASTM D 91 Pour Point, ASTM D 97 Flash Point, ASTM D 92 ISO Grade 150 and higher ISO Grade 68 and 100 Copper Corrosion, ASTM D 130 Rust Test (A & B), ASTM D 665 S-200 Oxidation—312 hrs, 121.1°C Viscosity Increase, 98.9°C Precipitation N u m b e r after test Demulsibility, ASTM D 2711 Free Water Emulsion Water in Oil 4 Ball EP, ASTM D 2783 Load Wear Index Weld Point 4 Ball Wear, ASTM D 2266 20 kg, 1800 rpm, 1 hr Timken Load Arm Test, ASTM D 2782 FZG Four Square Test
Limits 25 min 95 min Trace - 9 ° C max (based on viscosity) 232.2°C min 203.4°C min l b max Pass 6% max 0.1% max 80 ml min 1.0 ml max 2.0% max 45 kg min 250 kg m i n Scar Diameter of 0.35 m m max O.K. 60 lbs min 1 1 * Stage min
combines enclosed and open gearing superceding AGMA standards 250.04 and 251.02. It also covers the use of synthetic industrial gear lubricants. Specifications of EP oils have been upgraded to reflect advances in additive technology. Table 31 summarizes requirements for U.S. Steel 224 (AISE 224). Table 32 compares several industrial requirements for EP gear lubricants. Tables 33-35 are specifications for compounded, EP, and synthetic gear lubricants, respectively, from AGMA 9005-D94. Table 36 specifies the viscosity ranges for AGMA gear lubricants.'^ T E S T IMETHODS F O R EVALUATING G E A R LUBRICANTS Precipitation Number ASTM D 91 This test method covers the determination of precipitation number of lubricating oils including steam cylinder stocks and black oils. Fully refined petroleum oils normally contain no naphtha insoluble material. Semi-refined or black oils frequently contain some naphtha insoluble material, sometimes referred to as asphaltenes. This test measures the amount of naphtha insoluble material in oil. This quantity is reported as the precipitation number. The ASTM precipitation number, n, is the number of milliliters of precipitate formed when 10 mL of lubricating oil are mixed with 90 mL of naphtha and centrifuged between 600-700 rpm. ^ The original material is printed with permission of the copyright holder, the American Gear Manufacturer's Association, 1500 King Street, Suite 201, Alexandria, Virginia 22314. Statements presented are those of the author and may not represent the position of the American Gear Manufacturer's Association.
454 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 32—Comparison of industrial gear oil specifications [23]. Tests
AGMA 9005-D94 E P
DN 51517 Part 3 (CLP)
Timken OK Load, ASTM D 2782 FZG A/8.3/90, Pass Stage, min 4 Ball EP, ASTM D 2783 Weld Point Load Wear Index 4 Ball Wear, ASTM D 2266 120 kg, 1800 rpm, 1 hr Corrosion Protection ASTM D 130, 3 hrs/100°C ASTM D 665, min Oxidation Stability ASTM D 2893 (121°C), % Vis Incr S 200, % Vis Incr Precipitation Number Foam Suppression, ASTM D 892 Seq I Tendency/Stability, ml max Seq II Tendency/Stability, ml max Seq III Tendency/Stability, ml max
60 lbs min 12
11
AISE 224
60 lbs min 11 250 kg min 45 min Scar Diameter—0.35 max
lb max PassB
Pass A
lb max Pass A & B
6 max 6 max 0.1 max 75/10 75/10 75/10
Table 33—Minimum physical and performance specification for inhibited and compounded gear lubricants. Courtesy of AGMA from Standard ANSI/AGMA 9005-D94. Property
Viscosity Viscosity Index Oxidation Stability
Rust Protection Corrosion Protection Foam Suppression
Demulsibility
Cleanliness
Criteria for Acceptance
Test Procedure
ISO 3104 ASTM D 445 ISO 2909 ASTM D 2270 ISO 4263 ASTM D 943
ISO 7120 ASTM D 655B ISO 2160 ASTM D 130 ASTM D 892
ASTM D 2711
None
Must be as specified in Table 36 90 minimum Hours to reach a neutralization number of 2.0 ") AGMA Grade Hours (minimum) 0,1,2 1500 3,4 750 5,6 500 No rust after 24 h with synthetic sea water #lb strip after 3 h at 12rC (250°F) Must be within these limits: Max volume of foam (ml) after Temperature n. blow Sequence I 24°C (75°F) 75 Sequence II 9 3 . 5 ^ (200°F) 75 Sequence III 24°C (75''F) 75 *Must be within these limits: Max percent water in the oil after 5-hour test Max cuff after centrifuging Min total free water collected during entire test Must be free of visible suspended or settled contamination.
10 min. rest 10 10 30.0 ml 0.5% 2.0 ml 30.0 ml
"The criteria for acceptance indicated for oxidation stability and demulsibility is not applicable to compounded gear oils.
Flash a n d Fire Points b y Cleveland O p e n Cup ASTM D 92 This test method describes the determination of flash and fire point of petroleum products by a m a n u a l or a u t o m a t e d Cleveland open cup apparatus. Approximately 70 mL of the test specimen is filled into the test cup. The temperature of the specimen is increased rapidly at first and then at a slower rate as the flash point is approached. At specified intervals a test flame is passed across the cup. The flash point is the lowest liquid temperature at which the vapors from the specimen will ignite. To determine the fire point, the test is continued until the test flame causes the specimen to ignite a n d sustain burning for a m i n i m u m of 5 s.
Pour Point ASTM D 97 This test m e t h o d m e a s u r e s the lowest t e m p e r a t u r e of a petroleum specimen to determine its utility in certain low t e m p e r a t u r e applications. After preliminary heating, the sample is cooled at a specified rate and examined at intervals of 3°C for flow characteristics. The lowest temperature at which movement of the specimen is observed is recorded as the pour point. Copper Strip Tarnish Test ASTM D 130 The sulfur from the base oil and EP additives can have a corroding action on various metals but is not related to the total
CHAPTER 16: GEAR LUBRICANTS
455
Table 34—Minimum physical and performance specification for ep gear lubricants. Courtesy of AGMA from Standard ANSI/AGMA 9005-D94. Test Procedure
Property Viscosity Viscosity Index Oxidation Stability Rust Protection Corrosion Protection Foam Suppression
Demulsibility
Criteria for Acceptance
ISO 3104 ASTM D 445 ISO 2909 ASTM D 2270 ASTM D 2893
Must be as specified in Table 36 90 minimum (applies to viscosity grades 2 EP thru 8 EP only) Increase in kinematic viscosity of an oil sample at 121°C (250°F) should not exceed 6%°) No rust after 24 hours with synthetic sea water")
ISO 7120 ASTM D 655B ISO 2160 ASTMD 130 ASTM D 892
# l b strip after 3 hours at 100°C (212°F) Must be within these limits: Max volume of foam (ml) after: Temperature Sequence I 2 4 ^ (75°F) Sequence 11 93.5°C (200°F) Sequence III 2 4 X (75°F) *Must be within these limits'):
ASTMD 271IMOD
5 min. blow 75 75 75
10 min. rest 10 10 10
AGMA Grades 2EPto7EP 8 EP to 13 EP 2.0% 2.0%
Cleanliness Load carrying property
None ASTM D 2782 (Timken Test) and DIN 51 354 (FZG Test)
Filterability
None
Max percent water in the oil after 5-hour test Max cuff after centrifuging 1.0 ml 4.0 ml Min total free water collected 80.0 ml 50.0 ml during entire test (start with 90 ml of water) Must be free of visible suspended or settled contamination. An oil must meet both; a 60 pound Timken OK lad, and fail stage greater t h a n 12 on the FZG machine with A/8.3/90°C parameters for acceptance"). Must be filterable to 25u (microns) (wet or dry) without loss of EP additive (b25 = 200 fiher rating)
"Acceptance criteria has been upgraded.
Table 35—Minimum physical and performEince specification for synthetic gear lubricants. Courtesy of AGMA from Standard ANSI/AGMA 9005-D94. Property Viscosity Viscosity Index Oxidation Stability Rust Protection Corrosion Protection Foam Suppression
Demulsibility
Cleanliness Mciximum Pour Point
Test Procedure ISO 3104 ASTM D 445 ISO 2909 ASTM D 2270 ASTM D 2893 ISO 7120 ASTM D 665B ISO 2160 ASTMD 130 ASTM D 892
ASTMD 2711 MOD
Load Carrying Property
None ISO 7120 ASTM D 665 DIN 51 354
Filterability
None
Criteria for Acceptance Must be as specified in Table 36 120 minimum " Increase in kinematic viscosity of oil sample at 121°C (250°F) should not exceed 6% No rust after 24 h with synthetic sea water. # l b strip after 3 h at 1 2 r c (250°F) Must be within these limits: Max Volume of foam (ml) after: Temperature 5 min. blow Sequence I 24°C (75°F) 75 Sequence II 93.5°C (200°F) 75 Sequence III 2 4 X (75T) 75 Must be within these limits*). Max percent water in the oil after 5 hour test Max cuff after centrifuging Min. total free water collected during entire test (start with 90 ml of water) Must be free of visible suspended or settled contaminants. - 3 2 ° C (22°F)
10 min. test 10 10 10 1.0% 2.0 ml 60.0 ml
An oil must meet 11 stage fail (10 stage pass) on FZG machine with A/8.3/90°C parameters for acceptance. Must be filterable to 25/i (microns) wet or dry without loss of additives (/325 = 200 filter rating).
" Esters having a lower viscosity index and meeting all other requirements of this specification may be used in specific applications where proper viscosity at operating temperature has been verified. * Polyglycols which will not pass the demulsibility test, but meet all other requirements of this specification, may be used in specific applications where there is no danger of water contamination.
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Table 36—Viscosity ranges for AGMA gear lubricants. Courtesy of AGMA from Standard ANSI/AGMA 9005-D94. Rust and Oxidation Inhibited Gear Oils, AGMA Lubricant No.
0 1 2 3 4 5 6 7, 7 Comp'' 8, 8 Comp'' 8A Comp'' 9 10 11 12 13 Residual compounds'^ AGMA Lubricant No 14R 15R
Viscosity Range " mm'^/s (cSt) at 4000
Equivalent ISO Grade"
28.8-35.2 41.4-50.6 61.2-74.8 90-110 135-165 198-242 288-352 414-506 612-748 900-1100 1350-1650 2880-3520 4140-5060 6120-7480 190-220 cSt at 100°C (212°F) Viscosity ranges" cStatl00°C(212°F) 428.5-857.0 857.0-1714.0
32 46 68 100 150 220 320 460 680 1000 1500
Extreme Pressure Gear Lubricants * AGMA LubricEint No.
2EP 3EP 4EP 5EP 6EP 7EP 8EP 8AEP 9EP 10 EP 11 EP 12 EP 13 EP
Synthetic Gear Oils ° AGMA Lubricant No.
OS IS 2S 35 4S 5S 6S 7S 85 95 10 S US 12 5 13 S
"per ISO 3448, Industrial Liquid Lubricants - ISO Viscosity Classification, Also ASTM D 2422 and British Standards Institution B.S. 4231. ''Extreme pressure lubricants should b e used only w h e n recommended b y the gear manufacturer. "Synthetic gear oils 9S -13S are available but not yet in wide use. ''Oils marked Comp are compounded with 3 % to 10-% fatty or synthetic fatty oils. "Viscosities of AGMA Lubricant N u m b e r 13 and above are specified at 100°C (212°F) as measurement of viscosities of these heavy lubricants at 40°C (lOOT) would not be practical. l i e s i d u a l compounds - diluent type, commonly knows as solvent cutbacks, are heavy oils containing a volatile, no-flammable diluent for ease of applications. The diluent evaporates leaving a thick film of lubricant on the gear teeth. Viscosities listed are for the base compound with diluent. CAUTION: These lubricants may require special handling and storage procedures. Diluent can be toxic or irritating to the skin. Do not use these lubricants without proper ventilation. Consult lubricant supplier's instructions.
sulfur level in the gear lubricant. The effect varies pending on the activity of sulfur. This test is designed to assess the relative degree of corrosivity of lubricants. A polished copper strip is i m m e r s e d in a specified quantity of sample a n d heated at a temperature for a specified time. At the end of this period the copper strip is removed, washed, and compared with ASTM Copper Strip Corrosion Standards. Generally, industrizJ and automotive gear lubricants are tested at 100 and 121°C for 3 h, respectively. A P I Gravity A S T M D 2 8 7 This test covers the determination by means of a glass hydrometer of the API gravity of crude petroleum products. Gravities are m e a s u r e d at 60°F (15.56°C) or converted to values at 60°F, by means of standard tables. This test method relies on the principle that gravity of a liquid varies directly with the depth of immersion of the floating API hydrometer. Kinematic Viscosity ASTM D 445 This method specifies a procedure for determining the kinematic viscosity of lubricants by measuring the time of flow under gravity through a calibrated glass capillary viscometer. The viscometer is placed in a heated bath at typically 40 and 100°C. The kinematic viscosity is the product of the measured flow time and the calibration constant of the viscometer. The correct operation of equipment, such as gearboxes, axles and transmissions, depends on the appropriate viscosity of the gear lubricant.
Acid N u m b e r ASTM D 6 6 4 This method determines the acidic constituents in gear lubriceints that are present as additives or as degradation products during service. It is applicable for the determination of acids whose dissociation constants in water are Isirger than 10~^; extremely weaJs acids whose dissociation constants are smaller t h a n 10~^ do not interfere. Salts react if their hydrolysis constants are larger than 10"*. The sample is dissolved in a mixture of toluene a n d propan-2-ol containing a small amount of water and titrated potentiometrically with alcoholic potassium hydroxide (KOH). The acid n u m b e r is reported as mg KOH/g of sample. Since a variety of oxidation products contribute to the acid n u m b e r and the organic acids vary widely in corrosion properties, this test method cannot be used to predict corrosiveness of the geeir oil u n d e r service. Rust Inhibition ASTM D 6 6 5 This method evaluates the ability of oils to prevent rusting of ferrous parts if water mixed with the oil. A mixture of 300 mL of the oil under test is stirred with 30 mL of distilled water (Method A) or synthetic seawater (Method B) at 60°C in the presence of a completely immersed cylindrical steel rod. The duration of the test is 24 h. At the end of 24 h, the test rod is inspected for signs of rust. F o a m Characteristics ASTM D 892 This method covers the determination of foaming characteristics of oils at 24°C and 93.5°C. The tendency of oils to
CHAPTER foam can be serious in systems such as high-speed gearing, high volume pumping, and splash lubrication. Inadequate lubrication, cavitation, and overflow loss of oil can lead to m e c h a n i c a l failure. The s a m p l e is m a i n t a i n e d at 24°C while air is blown into it at a constant rate for 5 m i n a n d allowed to settle for 10 min. The volume of foam is measured at the end of both periods. The test is repeated on a second sample at 93.5°C a n d t h e n after collapsing the foam, at 24°C. Insolubles in U s e d Oils ASTM D 8 9 3 This m e t h o d covers the d e t e r m i n a t i o n of p e n t a n e a n d toluene insolubles in used oils. Procedure A covers the determination of insolubles without the use of coagulant in pentane. Procedure B determines insolubles of oils containing detergents and employs a coagulant for both the pentane and toluene. In both procedures, a representative amount of sample is mixed with pentane and centrifuged. The solution is decanted and washed with either pentane and/or toluene. The "wet" insolubles are dried and weighed.
16: GEAR LUBRICANTS
457
Water Content ASTM D 1744 This method covers the determination of water in the concentration from 50-1000 mg/kg in oil. The new or used oil is titrated with standard Karl Fischer reagent to an electrometric end point. Knowledge of the water level can be used to predict performance and quality of oils, especially used oils. High levels of water of > 2 wt% cem cause equipment failure and may necessitate an oil drain. F o u r BaU W e a r ASTM D 2 2 6 6 This method covers the determination of the weetr chciracteristics in sliding steel on steel applications only. This test Ccin be used to determine the antiwear properties of industrial gear lubricants relative to each other. No correlation has been established between this test and field service. A steel bcdl is rotated under load against three stationary balls. The diameters of the wear scars on the stationeiry balls are measured at the end of the test. Viscosity Index ASTM D 2270
Chlorine Level ASTM D 808 This method covers the determination of chlorine in new and used oils in the rzinge of 0.1-50 wt%. The procedure assumes that compounds containing halogens other than chlorine are not present. Knowledge of the level of chlorine in oils CEin be used to predict performance or handling of gear oils. The oil is oxidized by combustion in a b o m b containing oxygen under pressure. The liberated chlorine c o m p o u n d s are absorbed in a sodium carbonate solution. The level of chlorine present is determined gravimetrically by precipitation as silver chloride. Phosphorus Level ASTM D 1091 This test method determines the level of phosphorus in unused and used oils and is independent of the type of phosp h o r u s compounds. This is because of the conversion to a n aqueous solution of orthophosphate ion by oxidation of the sample during the course of the analysis. Knowledge of the phosphorus level can be used to predict performance a n d quality of oils. The organic material in oils is destroyed and the p h o s p h o r u s is converted to phosphate ion by oxidation with sulfuric acid, nitric acid, and hydrogen peroxide. The residual hydrogen peroxide is removed by diluting with water and evaporation. The level of phosphor u s is t h e n d e t e r m i n e d by p h o t o m e t r i c or gravimetric methods. Sulfur Level ASTM D 1552 This method covers three procedures for determining total sulfur content in oils and is applicable to samples boiling above 177°C and containing not less than 0.06 wt% sulfur. One procedure uses infrared detection following pyrolysis. The other two procedures use iodate detection followed by either a n induction furnace or resistance furnace following pyrolysis. Knowledge of the sulfur level can be used to predict performeince and qucdity of oils.
This method specifies the procedures for calculating the Viscosity Indices (VI) of oils from 40°C and 100°C. Described Procedures A and B determine the VI of oils u p to 100 and greater than 100, respectively. The VI is a widely accepted measure of the variation in kinematic viscosity due to the changes in temperature. The higher the VI of the oil is, the less the dependence of viscosity on temperature. JCnowledge of a gear lubricant's VI can be used to predict performance and quality. T i m k e n M e t h o d for E P ASTM D 2 7 8 2 This method covers the determination of the load-carrying capacity of oils by means of the Timken EP Tester. The tester is operated with a steel test cup rotating against a steel test block. The rotating speed is 123.71 ± 0.77 m/min or 800 ± rpm. Oil samples Eire preheated to 37.8°C before starting the test. Two determinations are made: the minimum load (score value) that will compromise the lubricant film between the rotating cup and stationary block and cause seizure or scoring; and the maximum load (OK value) at which the rotating cup will not compromise the lubricant film. Four Ball E P ASTM D 2 7 8 3 This method also determines the load carrying properties of lubricating oils. This method differentiates between lubricating oils having low, medium, emd high level of EP properties. The tester is operated with one steel ball under load rotating against stationary three balls. The rotating speed is 1760 ± 40 r p m . The test lubricant is b r o u g h t between 18.33-35°C (65-95°F) and a series of 10 s durations are made at increasing loads until welding occurs. DemulsibUity ASTM D 2711 This method determines the ability of oil and water to separate from each other. It is intended for oils with medium and high viscosity. The test provides a guide for determining the
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MANUAL 37: FUELS AND LUBRICANTS
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demulsibility of gear lubricants that are prone to water contamination and may encounter the turbulence of pumping and circulation capable of producing water-in-oil emulsions. The ability to separate water is desired in applications where it is relatively easy to perform maintenance without draining the oil sump completely. Oxidation of EP Oils ASTM D 2893 (S-200) This widely used method determines the oxidation characteristics of EP gear oils. The oil sample is subjected to a temperature of 95°C in the presence of dry air for 312 h. The oil is then tested for precipitation number and viscosity increase. Low Temperature Brookfield Viscosity ASTM D 2983 This test method describes the use of the Brookfield viscometer for the determination of the low-shear rate viscosity of gear lubricants in the temperature range from - 5 to -40°C. The viscosity range is 1000-1 000 000 cP. A lubricant sample is cooled in an air bath at test temperature for 16 h. It is carried in an insulated container to a nearby Brookfield viscometer where its viscosity is measured at the temperature of the bath. The low temperature properties of gear lubricants are critical for equipment starts that may be subjected to low temperatures. The lack of lubrication can cause severe catastrophic distress to the gears and bearings.
oils and additive packages. A small portion is weighed and diluted with solvent such as xylene. Internal and calibration standards are weighed into the test specimen. The mixture is then fed into an ICP instrument. By comparing the emission intensities of the elementals against the calibrated standards, the concentrations of elements in the sample oil are calculated. This test method can be used to confirm whether the additive packages or oil meet specifications with respect to elemental composition. FZG Scuffing Test ASTM D 5182 or DIN 51354 This test method, developed by the Forschungstelle fur Zahnrader und Getriebebau (Research Site for Gears and Transmissions), is commonly referred to as the FZG Visual Method. It is intended to measure the scuffing load capacity of oils used to lubricate hardened steel gears. High EP gear lubricants that meet API GL-4 and 5 easily exceed the capacity of the test rig. An FZG Gear Test Machine is operated at constant speed (140 rpm) for a fixed period (15 min.) at successively increasing loads (12 max) until the failure criteria is reached. The test gears are examined initially and after each load stage for cumulative damage (scuffing) to the gear tooth flanks. This test method can be used to screen the scuffing load capacity of oils used to lubricate spur and helical gear units. The FZG Four Square Test is required for AISE 224 that stipulates no greater than 10 mg between stages. Cold Crank Simulator ASTM D 5293
Nitrogen Level (Kjeldahl Method) ASTM D 3228 This test method determines the level of nitrogen for concentrations between 0.03 and 0.1 wt%. The sample is digested in a mixture of concentrated sulfuric acid, potassium sulfate, mercuric oxide, and copper sulfate. After digestion, sodium sulfate is added to the mixture to precipitate the mercury, and the mixture is made alkali with caustic. Nitrogen in the form of ammonia is then titrated to determine the level of nitrogen present. The concentration of nitrogen is a measure of the presence of nitrogen containing additives. Knowledge of nitrogen levels can be used to predict performance. Base Number ASTM D 4739 This method covers a procedure to determine the basic components in oils. This method resolves these components into groups having weaJi-base and strong-base ionization properties. Base numbers of up to 70 are covered by this method. The sample is dissolved in a mixture of toluene, isopropyl alcohol, chloroform, and water. The mixture is then titrated potentiometrically with alcoholic hydrochloric acid. The end point is selected from a titration curve and used to calculate a base number in units of mg KOH/g sample. The base number can provide information on the performance and quality of new and used oils. Elemental Content ASTM D 4951 This method quantifies the levels of barium, boron, calcium, copper, magnesium, phosphorus, sulfur, and zinc in unused
This test method provides the determination of the apparent viscosity of a lubricating oil at temperatures between —5 and -30°C at shear stresses between 50 000 to 100 000 Pa and shear rates of iC—10^ sec^ An electric motor drives a rotor that is closely fitted inside a stator. The space between the rotor and stator is filled with the test oil. The test temperature is measured near the stator inner wedl. The speed of the stator is calibrated against viscosity. The test oil's viscosity is determined from this calibration and measured motor speed. Thermal and Oxidation (L-60-1) ASTM D 5704 This test method determines the oil-thickening, insolubleformation and deposit-formation cheiracteristics of axle and transmission oils, especially when subjected to high temperature oxidation. A sample of the oil is placed in a heated gear case containing two spur gears, a test bearing, and a copper catalyst. The test oil is heated to 163°C for 50 h. Air is bubbled through the test oil at a constant rate. Parameters used to evaluate oil degradation after testing are: viscosity increase, insolubles, and gear cleanliness. The deterioration of the oil in the transmission or axles can lead to serious seal failures with deposit formation at the seal-shaft interface. Cyclic Durability Test ASTM D 5579 This test method evaluates the thermal stability of heavy-duty gear transmission lubricants when operated under high temperatures. The gear lubricant's performance is measured by the number of shifting cycles that can be performed with-
CHAPTER 16: GEAR LUBRICANTS out failure of synchronization when the transmission is operated while continuously cycling between high and low ranges. Field correlation with truck transmission service has not been established, however, this method has shown to discriminate between satisfactory and unsatisfactory transmission lubricants in the one petrticular transmission. After the transmission is rebuilt emd flushed with the test lubricant, it is started and operated at low range until the sump temperature reaches 250°F. This temperature is maintained throughout the duration of the test. The transmission is automatically cycled between low and high range until two unsynchronized shifts occur or the desired length of time is reached. At the conclusion of the test, parts are rated for wear. Seal Compatibility ASTM D 5662 This method evaluates the compatibility of nitrile, fluoroelastomer, and polyacrylate seals in lubricants. Seal failure in gearboxes, axles, and transmissions is caused by excessive seal hardening, elongation loss, and volume swelling. This method tests the likelihood of premature seal failure caused by the lubricant's effect on seals. The seal materials are placed in the test oil and subjected to the specified duration and temperature. At the end of the test, changes in elongation, volume, and hardness are determined. High Temperature Foam Inhibition ASTM D 6082 This method describes the procedure for determining the foaming characteristics of oils at 150°C. By comparison, ASTM D 892 determines foaming characteristics of up to 93.50C. A measured quantity of oil is heated to 49°C for 30 min and allowed to cool to room temperature. The oil is transferred to a 100 mL graduated cylinder and heated to 150°C and aerated at 200 mL/min with dry air for 5 min. The amount of static foam at the end of aeration and the stability after specific times are recorded. L-37 Low S p e e d High Torque Hypoid Test ASTM D6121 This test evaluates the load carrying capacity of gear lubricants under low speed and high torque conditions. The gears are conditions for 100 min at 440 wheel r/min and 394 Ibf-ft wheel torque, maintaining the sump at 297°F. The gears are inspected and the second phase is commenced. The duration of the second phase is 24 h and the sump temperature is controlled at 275°F. The wheel r/min and wheel torque are 80 and 1742 Ibf-ft, respectively. The gears are inspected for wear and various forms of other distress. This test measures the gear lubricant's ability to protect final drives from abrasive wear, adhesive wear, plastic deformation, and surface fatigue. Lack of protection can lead to premature gear or bearing failure, or both. This test procedure is followed for both coated and non-coated gears to evaluate gear lubricants against MILPRF-2105E requirements. The Wet L-37 test differs in the temperature during the second phase: a specified amount of water is added and the axle is motored for 12 h at temperatures below 175°F. The axle is then motored for another 12 h at a temperature of 325°F.
459
GFC Oxidation This test method measures the oxidative stability of gear lubricants under high temperatures. A quantity of sample placed in a flask is heated at a specific temperature with air bubbled throughout the duration of the test. At the end of the test, the viscosity increase, wt% insolubles, and acid number are reported. Synchronizer SSP 180 Test This test measures the ability of a transmission lubricant to provide proper synchronized shifts at a specified temperature and synchronizer friction material. The test rig uses cone t)rpe synchronizers of several friction types. The industry is currently conducting round robin testing to formulize a test procedure for PM-1. Shear Stability CEC-L45-T-93 The test procedure eveJuates the shear stability of lubricants containing polymeric additives. Polymeric additives are usually used for thickening and improving low temperature flow properties. A specified quantity of oil is placed in a tapered roller bearing fixture that is motored by the Four Ball Machine. The temperature is controlled at 60°C for 20 h with a load of 5000N. At the end of this duration, the percent change in viscosity at 100°C is determined. SAE J306 has shear stability requirements for gear lubricants to not shear out of grade. Loss in viscosity can caused compromise to lubricant films for proper bearing and gear performance. FZG (Forschungsstelle fur Zahnrader und Getriebebau) Pits C 180 TS This test procedure evaluates the influence of oil aging on the pitting life of gears under conditions of variable load, speed, and high temperature. The results are compared to a reference oil in the same test. The failure criterion is to report less than 4% of the active flank area of one single tooth at the end of the test. The pitting performance of gear lubricants is critical Channel Point FTM 3456 This test method is used to determine the channeling characteristics of oils at low temperature. The test consists of storing the oil sample for 18 h at the temperature required by the specification, cutting a channel in the oil with a metaJ strip, and determining whether the lubricant flows together to cover the bottom of the container within IDs. The ability of a lubricant to resist channeling at low temperatures provides adequate lubrication at startup conditions. Gear Scoring Test L-42 This test method evaluates the anti-scoring characteristics of a gear lubricant under high-speed and shock loading conditions using a hypoid axle. The axle is driven by a 5.7 liter V8 engine that drives the test axle, a 4-speed truck transmission, and two inertia dynamometers at a rate to simulate hard acceleration to approximately 100 mph. After break-in, the axle is ac-
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celerated through the gears to a speed of lOSOrpm, then decelerated to 530 rpm. This cycle is repeated five times. The highspeed sequence is followed by ten shock loadings. The gear lubricant is required to equal or better the scoring on the coast side of the pinion Eind ring gears of a passing reference oil. Gear Corrosion Test L-33 This test evaluates the rust and corrosion inhibiting properties of a gear lubricant when subjected to water contamination and elevated temperatures. The test utilizes an unloaded hypoid axle unit mounted on a test stand, which enables the unit to be motored over with heat lamps. The axle is filled with 1200 mL of oil and 30 mL of water and motored for 4 h at 82.2°C. On completion of this motoring phase, the axle is stored for 162 h for 51.7°C in an environmental oven. At the end of storage, the axle is disassembled and rated for corrosion, sludge, and other deposits. Dynamic Seals Test This test method evaluates the seal compatibility during operation at a specified temperature and high shaft speed of 2500 r p m for 1000 h. The lip shaft seals are inspected for wear and deviations from original dimensions at the end of test. Modifications to this test specific to OEM seal materials and shaft diameters can be made. This test attempts to simulate seal performance and the gear lubricant's effect in the field. Compatibility of Gear Lubricants FTM 3430 This test procedure is used to determine the compatibility of gear lubricant when blended with a reference gear lubricant by observing for precipitation of additive matericd after storage. The test consists of storing the blended oils for 30 days
at room temperature and centrifuging thereafter to quantify the level of incompatibility from the amount of insolubles. Storage Solubility of Gear Lubricants FTM 3440 This method is used for determining the storage stability characteristics of gear lubricants. The gear lubricant is heated at 120°C for 20 min and observed for separation of additive material after storage at room temperature for a period of 30 days. The level of incompatibility is determined from the amount of insolubles weighed at the end of storage. Air R e l e a s e A S T M D 3 4 2 7 o r D I N 5 1 3 8 1 This method covers the ability of the gear oil to separate entrained air. Compressed air is blown through the test oil, which has been heated to 25, 50, or 75°C. After the air flow is topped, the time required for the entrained air in the oil to reduce in volume to 0.2% is recorded as the air bubble separation time.
FUTURE TRENDS IN GEAR LUBRICANTS Automobile manufacturers are competing for consumers by claiming improved vehicle performance. Promoting vehicle performance in the areas of "Fill for Life" or extending drain intervals, seamless shifting, aerodynamic changes for aesthetics, and improved fuel economy are some examples. Tables 37 Eind 38 summarize the consequences of these trends to automotive gear lubricants. As a result of these trends, automotive gear lubricants are required t o have improved performance in fatigue, seal compatibility, oxidative and thermal stability, reduced chlorine, lower friction, lower cross grades in SAE 75W-90, EP,
Table 37—^Axle trends and consequences [1]. • Trend
' Consequences
• Extended Drain
Improved Component Durability
•> Reduced Airflow • Use of Retarders • Reduced Noise (Use of insulation) • Higher Unit Power/Wt Ratio
Higher Bulk Oil Temperatures
• Improved Efficiency • Improved Shiftability
Lower Oil Viscosity at Start and Lower Temperatures Under Normal Operation
• Lubricant Effect • Improved Pitting Performance • Improved Synchronizer Performance Improved Seal Compatibilty Better Thermal/Oxidative Stability
• Lower Viscosity Grades - Increased Use of 'Synthetics' • Lower Friction Additive Technology
CHAPTER 16: GEAR LUBRICANTS
461
Table 38—Transmission trends and consequences [1].
• Trend Extended Drain
• Lubricant Effect
• Consequences • Improved Component Durability • Use of Filters?
improved Pitting Performance Improved Seal Compatibilty
Z
I Higher Speeds < Reduced Airflow I increased Unit Loading
> Higher Bulk Oil Temperatures
I iVIore Environmentally Friendly
• Easier Disposal
Improved Thermal/Oxidation Stability
• Reduced Chlorine (Target 50 ppm Max)
< improved Fuel Economy
' Improved Axle Efficiency
• Lower Friction Additive Technology • Lower Viscosity Grades 80W-90 and 75W-90
• Component Cost Reductioi
I Simplified Technology
• IMore Robust Performance
< Hypoid Axle With Heavier Loadings < Reduced Noise
• Improved EP Performance
I Reduction in Use of Doubl< Reduction Axles
antiwear, and cost effectiveness. E n d users of industrial gear lubricants are requesting reduction in inventory and cost. It is not u n c o m m o n to see the use of advanced gear additive chemistries to rationalize or minimize inventory. Gear additive systems capable of meeting both industrial (low treats of 1.4 to 2.5 wt%) and automotive (treat levels of 4.0 to 15.0 wt%) performances are being used today. Other trends for industrial gear lubricants are improved oxidative stability, improved demulsibility, and lower additive treat cost. It is becoming more evident, as these trends for gear lubricants are materializing, that the additive and lubricant industries are meeting these challenges with improved technology in refinement, additive development, and formulation.
mance requirements. Last, a section on future trends for gear lubricants was dedicated to acknowledge the industry's intention to constantly upgrade gear lubrication specifications to meet its changing needs.
ASTM STANDARDS Unless otherwise indicated all of the following standards are current. No. D 91 D 92
CONCLUSIONS D 97 This chapter described gear lubricants and summarized their requirements for automotive a n d industrial gear applications. Lubricating gears in axles or gearboxes is challenging, as gear lubricants are required to provide efficient operation with minimized maintenance. Gear lubricants collectively provide various attributes such as: scuffing resistance, extreme pressure (EP), antiwear, corrosion resistance against ferrous and yellow metals, compatibility with various elastomeric seals, oxidative and thermal stability, possession of varying degrees of demulsibility depending on application, low and high temperature film strength, good low temperature fluidity, cost effectiveness, and environmental safety. Commonly used gear sets were described in this chapter along with some tj^ical gear failure modes. A list of typical additives used to formulate gear lubricants were also given. Current classifications for automotive and industrial gear lubricants were compared along with their testing and perfor-
D 130
D 287 D 445
D 664 D 665 D 808 D 892
Title Test Method for Precipitation Number for Lubricating Oils Test Method for Flash and Fire Points by Cleveland Open Cup Test Method for Flash Point by Pensky-Martens Closed Cup Tester Test Method for Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test Test Method for API Gravity of Crude Petroleum and Petroleum Products (Hydrometer Method) Test Method for JCinematic Viscosity of Transparent and Opaque Liquids (and the Calculation of Dynamic Viscosity) Test Method for Acid N u m b e r of Petroleum Products by Potentiometric Titration Test Method for Rust-Preventing Characteristics of Inhibited Mineral Oil in the Presence of Water Test Method for Chlorine in New a n d Used Petroleum Products (Bomb Method) Test Method for Foaming Characteristics of Lubricating Oils
462 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK D 893 D 1091
Test Method for Insolubles in Used Lubricating Oils Test Method for Phosphorus in Lubricating Oils and Additives D 1552 Test Method for Sulfur in Petroleum Products (High-Temperature Method) D 1744 Test Method for Determination of Water in Liquid Petroleum Products by Karl Fischer Reagent D 2266 Test Method for Wear Preventive Characteristics of Lubricating Grease (Four-Ball Method) D 2270 Practice for Calculating Viscosity Index from Kinematic Viscosity at 40 and 100°C D 2 7 1 1 Test Method for Demulsibility Characteristics of Lubricating Oils D 2893 Test Method for Oxidation Characteristics of Extreme-Pressure Lubricating Oils D 2983 Test Method for Low-Temperature Viscosity of Automotive Fluid Lubricants Measured by Brookfield Viscosmeter D 3228 Test Method for Total Nitrogen in Lubricating Oils and Fuel Oils by Modified Kjeldahl Method D 3427 Test Method for Air Release Properties of Petroleum Oils D 4739 Test Method for Base N u m b e r Determination by Potentiometric Titration D 4951 Test Method for Determination of Additive Elements in Lubricating Oils by Inductively Coupled Plasma Atomic Emission Spectrometry D 5182 Test Method for Evaluating the Scuffing Load Capacity of Oils (FZG Visual Method) D 5293 Test Method for Apparent Viscosity of Engine Oils Between-5 and-30°C Using the Cold-Cranking Simulator D 5579 Test Method for Evaluating the Thermal Stabihty of Manual Transmission Lubricants in a Cyclic Durability Test D 5662 Test Method for Determining Automotive Gear Oil Compatibility with Typical Oil Seal Elastomers D 5704 Test Method for Evaluation of the Thermal and Oxidative Stability of Lubricating Oils Used for Manual Transmissions and Final Drive Axles D 6082 Test Method for High Temperature Foaming Characteristics of Lubricating Oils D 6121 Test Method for Evaluation of the Load Carrying Capacity of Lubricants Under Conditions of Low Speed and High Torque Used for Final Hypoid Drive Axles
OTHER STANDARDS AGMA 9005-D94—Industrial Gear Lubrication Federal Test Method 3456—Channel Point of Federal Test Method Std. No. 791 FZG-PITS Test C 180 TS—Test Method for Evaluating the Influence of Oil Ageing on the Pitting Load Capacity of Lubricants GFC (Groupement Francais De Coordination) or CEC L-48-A00—Oxidative Stability of Lubricating Oils Used in Automotive Transmissions by Artificial Ageing (Laboratory Test) MIL-PRF-2105E or SAE J2360—Lubricating Oil, Gear Multipurpose (Metric) Military Use SAE J308—Axle and Manual Transmission Lubricants
SAE J306—Automotive Gear Lubricant Viscosity Classification SSP 180 or CEC L-66-T-99—Evaluation of the Sychromesh E n d u r a n c e Life using the FZG SSP 180 Synchromesh Test Rig
REFERENCES [1] Bala, v., MacPherson, I., and Walters, D. K., "Gear Additive Technology for Extended Drain Commercial Driveline Requirements," NLGI Paper No: 9719, Presented at The 64th Annual Meeting of National Lubricating Grease Institute, Carlsbad, Oct. 1997. [2] MacPherson, I., Kuhlman, R. E., Carlson, J. W., and Taylor, J. M., "Gear Additive Considerations for the Next Decade," NLGI Paper No: 9719, Presented at The 62"^ Annual Meeting of National Lubricating Grease Institute, Ponte Vedra Beach, Oct. 1995. [3] Norman, S., Kovitch, G. H., and Klein, R. M., "Industry Requirements for a New Generation of Automotive Gear Oils," NLGI Spokesman, Vol. 53, 1989, pp. 183-196. [4] Drago, R. J., Fundamentals of Gear Design, Butterworth Publishers, Boston, 1988. [5] NLGI Gear Education Course Material, Presented at The 65"' Annual Meeting of National Lubricating Grease Institute, Naples, 1998. [6] Hendrick, E. E., "Improved Compound for the Manufacture of Lubricating Oils," U.S. Patent No. 90,100, Washington D.C., May 1869. [7] Papay, A. G. and Dinsmore, D. W., "Advances in Gear Additive Technology," Presented at The 30th ASLE Annual Meeting, Atlanta, GA, May 1975. [8] a. Ethyl Petroleum Additives Specialties Gear Training Manual, Ethyl Petroleum Additives, Inc., Richmond, VA, 1996. b. Lubrizol Corporation's Additive Ready Reference for Lubricant and Fuel Performance. Lubrizol, Wickliff, OH, 1998. [9] Bartz, W. J., Lubrication of Gearing, Mechanical Engineering Publications, Ltd., London, 1993. [10] Benedict, G. H., "Gears," Ch. 20, Standard Handbook of Lubrication Engineering, McGraw-Hill, NY, 1968. [11] American Society of Metals Handbook on Friction, Lubrication and Wear Technology, Vol. 18, Oct. 1992. a) Errichello, R., "Friction, Lubrication, and Wear of Gears," pp. 535-545; b) Rizvi, S. Q. A., "Lubricant Additives and Their Functions," pp. 98-112. [12] ANSI/AGMA 1010-E95: Appearance of Gear Teeth—Terminology of Wear and Failure, AGMA, Alexandria, VA. [13] Ludema, K. C, Friction, Wear, Lubrication—A Textbook in Tribology, CRC Press Ltd., Boca Raton, FL, 1996. [14] Stucker, J. B., "Rear Axle and Gear Box Lubricants," CRC Handbook of Lubrication, Vol. I, CRC Press, Boca Raton, FL, 1992. [15] Radovich, J. L., "Gears," CRC Handbook of Lubrication, Vol. II, CRC Press, Boca Raton, FL, 1992. [16] AGMA 9005-D94: Industrial Gear Lubrication, AGMA, Alexandria, VA, August 1994. [17] SAE J308: Axle and Manual Transmission Lubricants, Society of Automotive Engineers, Warrendale, PA, 1989. [18] SAE J306: Automotive Gear Lubricant Viscosity Classification, Society of Automotive Engineers, Warrendale, PA, 1998. [19] MIL-PRF-2105E: Lubricating Oil, Gear Multipurpose (Metric) Military Use, U.S. Military, Aug. 1995. [20] SAE J2360: Lubrication Oil, Gear Multipurpose Military Use, Society of Automotive Engineers, Warrendale, PA, 1998. [21] Laboratory Performance Tests for Automotive Gear Lubricants intended for API GL-5 Service, ASTM STP 512A, ASTM International, West Conshohocken, PA, 1991. [22] Proposed Specification for Passenger Manual Transmission, Minutes of ASTM Committee D02.B0.003 Meeting, Nashville, TN, Nov. 2000, ASTM International, West Conshohocken, PA.
CHAPTER 16: GEAR LUBRICANTS [23] Ethyl Petroleum Additives Lubrication Specification Handbook, Ethyl Petroleum Additives, Inc., Richmond, VA, June 2000. [24] Goodhead, J. W. and Cook, S. P., "High-Temperature PerformEince Needs of Gear Oils: A Comparison of Current and Future Technologies," 4th CEC Symposium, Birmingham, UK, May 1993. [25] Bala, V. and Saathoff, L., "Oxidative Stability of Automotive Gear Lubricants," NLGI Paper No: 9821, Presented at The 65"' Annual Meeting of National Lubricating Grease Institute, Naples, Oct. 1998. [26] Bala, V., Hartley, R. J., and Lawrence, J. H., "The Influence of Chemical Structure on the Oxidative Stability of Organic Sulfides," Presented at The 1995 STLE Annual Meeting, May 1995. [27] Bala, V. and Pietras, J. M., "Enhanced Performance Considerations for Automotive Gear Lubricants," Presented at the 2"'' International Symposium on Fuels and Lubricants, New Delhi, 2000. [28] Pinkus, O. and Wilcock, B. K., "Strategy for Energy Conservation Through Tribology," Presented at the 33rd ASLE Annual Meeting, Dearborn, April 1978. [29] Bartz, W. J., "Fuel Economy Improvement by Engine and Gear Oils," 5th CEC International Symposium on the Performance Evaluation of Automotive Fuels and Lubricants, Goteborg, Sweden, May 1997. [30] Bartz, W. J., "Some Considerations Regarding Fuel Economy I m p r o v e m e n t s by Engine and Gear Oils," presented at t h e ASLE/ASME Conference, New Orleans, LA, Oct. 1981. [31] Chamberlin, W. B. and Sheahan, T. J., "Automotive Fuel Savings Through Selected Lubricants," SAE Paper No. 750377, Society of Automotive Engineers, Warrendale, PA, 1975. [32] Richardson, L. P., Schiemann, L. F., and O'Connor, B. M., "Economic and Energy Benefits through use of Multigraded Gear Oils," presented at the NPRA Fuels and Lubricant Meeting, Houston, Nov. 1977. [33] O'Connor, B. M., Graham, R., and Glover, I., "European Experience with Fuel Efficient Gear Oils," SAE Paper No. 790746, Society of Automotive Engineers, Warrendale, PA, 1979. [34] Naman, T. M., "Automotive Fuel Economy—Potential Improvement through Selected Engine and Gear Lubricants," SAE Paper No. 800438, Society of Automotive Engineers, Warrendale, PA, 1980. [35] Stambaugh, R. L., Galluccio, R. A., and KoUer, R. D., "Multigrade Gear Lubricants in Truck Fleet Testing—^Analysis for Fuel Economy Effects," SAE Paper No. 818178, Society of Automotive Engineers, Warrendale, PA, 1981.
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[36] Moat, N. W., "Canadian Experience with Multigrade Gear Oils," SAE Paper No. 811204, Society of Automotive Engineers, Warrendale, PA, 1981. [37] Watts, R. F. and Willette, G. L., "Newtonian Multigrade Gear Lubricants: Formulation and Performance Testing," SAE Paper No. 821180, Society of Automotive Engineers, Warrendale, PA, 1982. [38] Adams, J. H., Frost, K. A., Hartmann, L. M., and Painter, L. J., "The Effect of Gear Lubricant on Fuel Economy as Measured in a Line Haul Truck Fleet," SAE Paper No. 810179, Society of Automotive Engineers, Warrendale, PA, 1981. [39] Greene, A. B. and Risdon, T. J., "The Effect of Molybdenum Containing Oil Soluble Friction Modifiers on Engine Fuel Economy and Gear Oil Efficiency," SAE Paper No. 811187, Society of Automotive Engineers, Warrendale, PA, 1981. [40] Devlin, M. T., Naumann, J. K., Saathoff, L. D., and Walters, D. K., "Predicting the Fuel Economy Properties of Gear Oils Using Laboratory Friction Tests," GFC Conference, Paris, Nov. 1999. [41] Adamo, R. and Corso, S., "Frictional and Antiwear Behavior of Fuel Efficient Gear Lubricants," presented at the 36th ASLE Annual Meeting, Pittsburgh, May 1981. [42] Porrett, D., Miles, S., Werderits, E., and Powell, D., "Developm e n t of a Laboratory Axle Efficiency Test," SAE Paper No. 800804, Society of Automotive Engineers, Warrendale, PA, 1980. [43] Bala, V., Brandt, G., a n d Walters, D. K., "Fuel Economy of Multigrade Gear Lubricants," presented at the 12th International Colloquium Tribology, Esslingen, Germany, 2000. [44] Bala, V., RoUin, A. J., and Brandt, G., "Rheological Properties affecting the Fuel Economy of Multigrade Automotive Gear Lubricants," SAE Paper No. 2000-01-2051, Society of Automotive Engineers, Warrendale, PA, 2000. [45] Bala, V., Guidry, K. L., and Kuhlman, R. E., "Fuel Economy Considerations of Multigrade Automotive Gear Lubricants," NLGI Paper No: 0016, Presented at 67th Annual Meeting of National Lubricating Grease Institute, Asheville, NC, Oct. 2000. [46] Bala, V. and Pietras, J. M., "Fuel Economy Evaluation of Multigrade Automotive Gear Lubricants," Presented at the PetroTech Conference, New Delhi, January 2001. [47] Bala, V., Guidry, K. L., and Kuhlman, R. E., "Enhanced Performance Properties of Multigrade Automotive Gear Lubricants," NLGI Paper No: 0131, Presented at the 68'^ Annual Meeting of National Lubricating Grease Institute, Palm Beach, Oct. 2001.
MNL37-EB/Jun. 2003
Automotive Lubricants Shirley E. Schwartz, ^ Simon C. Tung,^ and Michael L. McMillan'
LUBRICANT TYPES MANY TYPES OF LUBRICANTS ARE USED IN A VEHICLE. Engine oil
and transmission fluid, on a volume basis, provide a major portion of the lubricants in a typical car or truck, and a great n u m b e r of standard tests are associated with these fluids. Greases provide a variety of highly specialized functions and are found in many locations within a vehicle (for example, in door locks, gears for seat adjustments and windshield wipers, bearings, electrical contacts, and numerous other places). For some automotive lubrication applications, standard tests are not available. An example is automotive air conditioning systems, which require a specialized lubricant that can be transported by the air conditioning medium. Those who use lubricants for which standard tests are not available must devise their own procedures to ensure that the lubricant performs acceptably. Functions of Automotive Lubricants Various issues influence the choice of lubricant composition for optimum performance in automotive applications. The lubricant must protect the automotive components that it lubricates. In some cases this protection is in the form of a fluid film that keeps opposing surfaces separated. In other cases, the lubricant provides weair protection by forming a wearresistant fllm on a surface, to generate boundary lubrication protection. During exposure of a lubricant to high temperature in the presence of oxygen, there is a risk that the lubricant will react with the oxygen to form organic acids, which can be corrosive and can cause varnish to form on engine components. Automotive lubricants eire protected against this oxidation by virtue of antioxidant additives, which are sacrificial agents that react with oxygen and thus reduce the probability of oxygen attacking the oil's base stock. In addition, alkaline additives (detergents) in the oil neutralize acids that may be formed. Detergents are chemicals consisting of a hydrocarbon tail and a polar head, such that the polar head will be attracted to the products of oil oxidation and nitration and will inhibit the formation of varnish or deposits, which otherwise could build u p on engine surfaces. ' Materials Engineer, General Motors Powertrain, 3003 Van Dyke Ave., Warren, MI 48090-9060. ^ Senior Staff Research Engineer and Principal Research Engineer, respectively. Chemical and Environmental Sciences Laboratory, General Motors Research and Development and Planning, 30500 Mound Road, Warren, MI 48090-9055.
Defoamers are agents that spread over a fluid surface and reduce surface tension, so that air bubbles can easily escape. Viscosity index (VI) improvers are long-chain chemical c o m p o u n d s t h a t change shape u n d e r various conditions. When the oil is hot, the VI improvers stretch out and, in the process of stretching out, they make the oil behave as if it was more viscous. When the oil is cold, the VI improvers fold in upon themselves, and as a consequence, they exhibit lower viscosity than would be the case if they didn't curl up. The lubricant base stock has the important function of transporting the various protective chemical additives to the sites in which they are needed and transporting waste products away from the sites in which the waste is generated. For example, engine oil t r a n s p o r t s anti-wear surface-active agents to the nose of the cam on a camshaft, a region in which chemical agents are essential for wear protection when a lifter slides over a cam nose. Protective anti-wear agents are not as critical when a roller follower is used, rather than a sliding contact. The wearing surfaces in a newly manufactured vehicle may have burrs or rough spots, which are smoothed during use. The lubricant transports these burrs or residues away from the rough spots in which they originated. The burrs can then be transported to a filter, or they may be dropped to the bottom of the oil reservoir. The diversity of function of automotive c o m p o n e n t s strongly influences the characteristics of the lubricant required for use with the component. That is, different vehicle components require different lubricant base stocks, fluid viscosities, and type and quantity of additive treatment for the component to function properly. Thus, a variety of specialized lubricants are required for automotive applications. Engine oil must remain effective despite the fact that fuel and combustion products can condense in the oil under conditions such as short trips and cold starts in a winter climate, during which fuel and water may accumulate in engine oil at concentrations greater than five percent [1,2]. During high temperature operation, engine oil must not evaporate or degrade excessively. Transmission fluid must withstand high temperatures and loads. Various types of bearings require the presence of a fluid film that separates a rotating shaft from its opposing bcciring surface. Brake fluid must continue to provide appropriate braking force, even if the vehicle drives through pouring rain or encounters puddles or slush on the road. During high load, high temperature conditions, such as the those experienced by piston rings at the top of the cylinder under the influence of the burning fuel, fluid film lubrication
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may not be sufficient to provide complete wear protection. In such cases, the lubricant must contain additives that interact with r u b b i n g surfaces to form chemical anti-wear films. These protective chemical films form on hot iron surfaces during vehicle operation as long as the lubricant has not degraded excessively. However, these chemical films may be partially removed during severe engine service [3]. Once the additives that provide this chemical film (typically, zinc dialkyl dithiophosphate, ZDP, and other anti-wear agents) are sufficiently degraded during use, the engine oil needs to be changed; otherwise engine wear can accelerate. In addition to providing a fluid film and a chemical surface film, automotive lubricants must also inhibit corrosion. For example, partially burned fuel can be acidic, and these fuelderived acids can condense in engine oil, especially during very short trips in exceedingly cold weather. An example of this effect is shown later in this chapter in the section on test methods for automotive lubricants. Engine oil itself, when exposed to hot spots in the presence of oxygen, can also form acids (due to oil oxidation) that promote varnish formation and engine corrosion, especially if the oil's alkaline anticorrosion additives have been degraded as a consequence of extended use without an oil change. A lubricant typically reduces friction, but in some cases an automotive lubricant is designed to provide a desired amount of friction, as is the case for proper clutch engagement in automatic transmissions. A desired amount of traction may be needed, as is the case for traction fluid in a continuously variable transmission. Traction fluids have a chemical structure that, under stress, forms a three-dimensional network that resists flow [4,5]. The lubricant associated with air conditioning fluid must be sufficiently compatible with the air conditioning medium that the air conditioning fluid can transport the lubricant to the wearing surfaces. In addition, the mixture of lubricant and air-conditioning fluid must have sufficient lubricating properties to protect the air-conditioning p u m p , b u t the fluid/lubricant combination must not become aggressive to hoses or to other materials in the air-conditioning system. These examples illustrate the complexity of the issues that must be addressed when attempting to provide adequate automotive lubrication. In some cases, standard test methods document whether a lubricant provides appropriate protection to the hardware. In other cases, vehicle, component, or lubricant manufacturers designate the lubricant that, in their estimation, best fits a given application.
COMPOSITION OF AUTOMOTIVE LUBRICANTS In general, automotive lubricants consist of a base stock and various additives. The choice of base stock depends on the function of the lubricant. In the case of engine oil, various types of organic oils may be used: paraffinic, naphthenic, and synthetic. Paraffinic hydrocarbons consist of hydrogen and carbon atoms that are chemically bonded together in the form of branched chains. Naphthenic hydrocarbons contain carbon atoms that are bonded together in the form of rings. Paraffinic and naphthenic hydrocarbons are typically derived by refining oil from oil wells. Synthetic hydrocarbons
are formed by combining small hydrocarbon building blocks to form longer chains of a desired composition that tends to be more resistant to chemiccd attack than is the case for oils produced by a refining process. Additives are blended into a base stock to provide desirable properties such as optimum friction characteristics for the desired application, anti-wear and antioxidant capability, defoaming capability, and corrosion inhibition. The chemical nature of the additives for a given application are chosen on the basis of their ability to perform their desired function, withstand the conditions u n d e r which they must operate, and be compatible with the base stock in which they are used. Test M e t h o d s for Automotive Lubricants A variety of tests are needed to guarantee that a given automotive lubricant performs as it should. These tests include: • Engine and component tests that mimic different driving or operating styles under a variety of test conditions, • Physical and chemical properties of the lubricant, • Corrosivity of a used lubricant, • Wear protection provided by the lubricant, • Remaining effectiveness of the lubricant's additive package. Additional desired information with regard to lubricant characterization includes: tendency to form deposits, rate and extent of oil oxidation, fuel efficiency, and emission system protection, including limits on phosphorus in engine oil, since phosphorus can adversely affect the performance of the catalyst that reduces automotive emissions [6-9]. For some lubrication functions, standard test methods are available to ensure that the various products in the marketplace provide the desired protection. For other lubrication functions, there are no standard tests, but a vehicle manufacturer may require that the lubricant of choice must pass the memufacturer's (or some other) designated tests. Each different type of automotive lubricant has its own unique test methods that address the issues relevant to the service needs of that particular lubricant. For example, engine tests are used to determine whether a new candidate engine oil formulation meets current certification requirements. These certification requirements must be passed before an oil container is allowed to show the "starburst" label that indicates the oil is acceptable for use in current vehicles. Bench tests can often be used to indicate whether certain fundamental lubricant properties are within a desired range (for example, tests for viscosity, pour point, volatility, shear stability, wear rate, wear depth, etc.). Chemical and physical tests measure the composition and properties of a lubricant, the changes that occur as the lubricant ages, and the characteristics of any contaminants or wear debris that may have entered the lubricant. The following sections describe issues relative to various types of automotive lubricants and test methods that have been developed to address those issues. As is t5?pically the case, older test methods are periodically upgraded, and new methods are developed. Thus, some of the tests described in any given list of standard methods may be out of date. An example of this evolutionary process is seen with regard to a lowtemperature engine test, ASTM D 5844, Test Method for Evaluation of Automotive Engine Oils for Inhibition of Rusting (ASTM Sequence IID). This engine test has recently been re-
CHAPTER placed by a bench test, ASTM D 6557, Standard Test Method for Evaluation of Rust Preventive Characteristics of Automotive Engine Oils, also known as the "BEJI Rust Test." During the creation of the Ball Rust Test, extensive testing was conducted to identify appropriate chemiccJ reagents and test conditions to ensure that the bench test would provide the same useful information as was gleaned from the engine test. AUTOMOTIVE E N G I N E OILS Introduction Identifying appropriate test methods for engine oils represents one of the most difficult challenges facing automobile and lubricant manufacturers. The reason for the difficulty is not because of any inherent characteristic of engine oil. The challenges in the development of test methods for engine oil arise because the environment in which engine oils must operate can be exceedingly harsh and difficult to control. Combustion gases, fuel and water in the oil, and outside contaminants (dirt, sand, other airborne materials) accelerate oil degradation, cause unique filtration problems, and result in m u c h shorter change intervals compared to other typical automotive lubricants. Engine oil properties and composition also affect vehicle fuel economy (as is the case for transmission fluids and rear axle lubricants). Engine oil influences vehicle emissions as a consequence of emission system contamination and catalyst poisoning. Engine oil can escape from the engine via mist exiting the exhaust system. In addition, small amounts of engine oil or oil mist can b u m when exposed to a flame front, which can create particulate emissions containing charred hydrocarbons and degraded oil additives. It is for these reasons that engine oil is the focus of much attention in the technical literature as well as in the standard test-development arena. The following sections describe standard test methods for evaluating engine oil performance (including both engine and bench tests) and the terminology used to describe engine oil performance, from both historical and current perspectives. The discussions are limited primarily to performance designations a n d associated test methods commonly used in North America. Oils described by other means and defined by different test methods are used in various other areas of the world, for example Europe and Japan. However, these international test methods are not addressed in detail in this chapter. E n g i n e Oil P e r f o r m a n c e S t a n d a r d s - - P a s t and Present Over the years, many organizations have contributed to the development of engine oil performance standards. Among these organizations are ASTM International (ASTM), the American Petroleum Institute (API), the Engine Manufacturers Association (EMA), the Coordinating European Council (CEC), the Japan Automobile Manufacturers Association (JAMA), and the International Lubricant Standardization and Approval Committee (ILSAC). ILSAC includes automotive and engine builders JAMA, EMA, and the former American Automobile Manufacturers Association (AAMA, whose members were General Motors, Ford, and Chrysler). When AAMA
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was dissolved in 1999, it was replaced by a new organization— the Alliance of Automobile Manufacturers (the Alliance), which includes the former members of AAMA plus six other manufacturers. The Alliance, however, is not actively involved in development of automotive lubricant standards. An early attempt at classifying engine oils was initiated in 1911, when the Miscellaneous Division of the Society of Automotive Engineers (SAE) introduced Specification No. 26, which specified that specific gravity, flash and fire points, carbon residue, and viscosity (resistance to flow at a given temperature) should be used to describe oil properties. In 1923 the specification was expanded to include specific ranges for the various measured properties of the engine oils. The designation of engine oils on the basis of performance began in 1947. API (American Petroleum Institute) adopted a classification system based on the intended use of the oil. Three types of oil were defined: Regulcir (termed ML), Prem i u m (MM), and Heavy Duty (MS). The performance categories for gasoline engine oils (ML, MM, and MS) imply motor oil ("M" in the designation) suitable for light (L in the designation), moderate (second M in the designation), and severe (S in the designation) service. Regular oils generally were straight mineral oils. Premium oils contained some oxidation inhibitor (but no other performance additives), and Heavy Duty oils were blended with oxidation inhibitors and detergents/dispersants. This system was modified in 1952 to create different performance requirements for diesel oils than those for gasoline engine oils and to rank the severity of the service. Performance categories for diesel engine oils were DL, DM, and DS, where D stands for diesel, and M, L, and S are the same as for gasoline engine-oil categories. Even though these designations (ML, MM, MS, DL, DM, and DS) indicated the quality of the engine oil, many vehicle and engine manufacturers found it necessary to add additional performance requirements to ensure adequate protection for their engines. This led to the development of lubricants tailored to a given manufacturer, and complicated the manner in which engine manufacturers informed their customers which oils were currently recommended. Finally, in 1969 three organizations, API (American Petroleum Institute), ASTM (was American Society for Testing and Materials, now called ASTM International), and SAE (Society of Automotive Engineers, now called SAE International), cooperated to establish a performance designation system, which is still being used. In this system, two series of engine oil performance and service categories were established. The "S" series, in which S designates service, is used with gasoline-fueled, 4-stroke-cycle, spark ignition engines. The "C" series denotes commercial, intended for use in compression-ignition diesel engines, both 2-stroke-cycle and 4-stroke-cycle). Table 1 shows the evolution of light-duty gasoline engine oil performance designations. The "~" symbol means that the terminology for API SA and API SB oils was created after those oils were no longer considered adequate for current vehicles. A similar evolution occurred for heavy-duty diesel engine oil performance. A detailed description of the history of both gasoline and diesel engine oil performance categories can be found in SAE (Society of Automotive Engineers) Standard J183, "Engine Oil Performance and Engine Service Classification."
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TABLE 1—Historical evolution of engine oil performance designations. Performance Designation Period of Use ML, MM, MS ~1950s and later API SA (designation created after this oil was already obsolete) -1900-1930 API SB (designation created after this oil was already obsolete) -1931-1964 APISC 1964-1967 APISD 1968-1971 APISE 1972-1979 APISF 1980-1988 APISG 1988-1992 API SH / ILSAC GF-1 1993-1996 API SJ / ILSAC GF-2 1996-2001 API SL / ILSAC GF-3 2001 and beyond
The API SA through API SD designations Hsted in Table 1 were adopted in 1971 to describe oils that had been available previously. API Category SA was adopted to describe oils generally available in North America between 1900 and 1930. These oils contained no performance additives. Such oils had previously been designated as "Regular" or "ML." API SA and SB oils can still be found in some retail establishments, where a n uninformed customer (who may need to add makeu p oil) may purchase an SA oil that is sadly deficient with regard to engine protection. API Category SB was adopted to describe oils for gasolinefueled engines that were generally available in North America between 1931 and 1963. These oils were comparable in performance to oils known as "Premium" or "MM" prior to 1971. They contained a minimal level of performance additives. API Category SC described oils typically available between 1964 and 1967 in North America. These oils were probably the first oils to contain a full range of performance additives. They had been designated "MS" prior to 1971. API Category SD described oils of the performance level generally available in 1971. Although these oils were also described as "MS" prior to 1971, they generally contained a higher additive treatment level than API SC oils. API "S" categories (API SE through API SL) were developed as needs arose for greater engine protection. API SE oils addressed high-temperature engine oil thickening problems in the field. API SF oils addressed customer concerns related to engine wear. API SG oils addressed oxidation and sludge formation concerns that had arisen in the field. Throughout this period (1972-1992), automotive engines had undergone tremendous changes to accommodate increasingly more stringent emission control requirements, both nationwide and in California beginning in 1972, and U.S. Corporate Average Fuel Economy (CAFE) requirements originating in 1978. The desire of vehicle manufacturers to specify oils which might help them address these requirements, as well as their desire to have a greater voice in establishing engine oil performance specifications, led to the formation of ILSAC (International Lubricant Standardization and Approval Committee) in 1987. Originally a u n i o n between the Motor Vehicle Manufacturers Association (MVMA) and the Japan Automobile Manufacturers Association (JAMA) for the purpose of establishing lubricant standards, ILSAC expanded to include the Engine Manufacturers Association (EMA). MVMA later became the American Automobile Manufacturers Association (AAMA), and subsequently became the Alliance of Automobile Manufacturers (the Alliance) in 1999. However, only the original three members of AAMA (General
Motors, Ford, and DaimlerChrysler) retained membership in ILSAC when AAMA was dissolved. The first ILSAC engine oil standard (ILSAC GF-1, where GF stands for gasoline-fueled engines) was published in 1993, as indicated in Table 1. ILSAC standards apply only to certain viscosity grades, namely SAE OW-XX, 5W-XX, and lOW-XX viscosity grades, in which "XX" represents a viscosity designation such as 20, 30, 40, etc. ILSAC standards also include m i n i m u m levels of fuel economy performance and contain other requirements (most notably a maximum phosphorus level) to address emission system compatibility and other issues. Even though API (American Petroleum Institute) engine-oil categories apply to all viscosity grades and do not include fuel economy requirements, an ILSAC GF-1 oil is roughly equivalent to an API SH quality oil, but in addition the GF-1 engine oil also meets requirements allowing the GF1 oil to be classified as energy conserving. The ILSAC GF-2 standard for engine oils is roughly equivalent to API SJ, but also includes fuel economy requirements. The ILSAC GF-3 standard is similar to API SL, but again, GF-3 oils must satisfy fuel economy requirements, which GF-2 and API SLquality engine oils are not required to meet. A well-stocked automotive supply store may have a dozen different brands of engine oil, many choices of viscosity grades, and several choices of oil quality from which a customer may select the oil of his choice. The ML, MM, a n d MS Categories, as well as API SA through API SG, have been declared technically obsolete by SAE (Society of Automotive Engineers). This was done because many of the engine tests necessary to demonstrate that oils meet the performance requirements of these categories cannot be conducted, since reference fuels, reference oils, and engine parts for these tests are no longer available. In addition, oils with recent designations (API SH, SJ, and SL) can be used in applications for which earlier oil designations (SA through SG, which are now considered technically obsolete) were required. API SH, while not technically obsolete, is generally not used except in combination with a "C" Category, to describe oils intended for mixed fleet (that is, diesel and gasoline) service. In such cases, the "C" Category designation usually precedes the "SH" designation on a label. E n g i n e Oil P e r f o r m a n c e T e s t s Beginning with oils designated as API SB, all of the oil performance categories described in Table 1 were or are defined by a series of engine dynamometer performance tests (usually termed engine sequence tests) and bench tests. The com-
CHAPTER plete description of each of these performance categories (both the current categories and those declared technically obsolete) is provided in SAE Standard J183, Engine Oil Performance and Engine Service Classification. Only the tests used to define the current performance categories (API SJ and ILSAC GF-2) and the tests used in the categories effective in 2001 (API SL and ILSAC GF-3) are discussed in this section. Some of these tests have been developed into ASTM Standard test methods, and others are in the process of being developed into ASTM Standards. It is not a requirement that all tests used to define a n engine performance category be ASTM Standards. However, those tests that are not ASTM Standards are usually advanced to ASTM Standards during the lifetime of a given category. ASTM Subcommittee D02.B0 on Automotive Lubricants maintains a section devoted exclusively to converting test methods developed for new lubricant performance categories into ASTM Standards.
Issues Addressed in Test Method Development Examples of the types of questions that need to be addressed to ensure that a given engine oil will protect an engine in various t5rpes of service are as follows: 1. If the engine is run at high engine speed and high oil temperature, • Will the engine oil thicken excessively? • Will there be sufficient wear protection? • Will harmful deposits form? 2. If the engine is driven in city service, will the oil provide sufficient sludge protection? 3. If the engine is driven in extreme short trips in a winter climate, will the oil provide adequate corrosion protection when water, fuel, and fuel oxidation products condense in the engine oil? Each numbered item in the previous paragraph represents a different driving style: high temperature high-load, city service with oil fully warm, and extreme short trip service in which the engine oil never warms completely. Each of these driving styles is associated with a specific ASTM standard test method to ensure that an engine oil provides sufficient protection for that particular type of service. An engine oil must pass all of these tests before its manufacturer is allowed to display a label indicating that the oil meets current specifications. Freeway driving (an additional driving style) is considered to be mild on engine oil and is not represented in a standard test, since it is assumed that the tests representing the other driving styles are sufficient to characterize lubricant properties. A test method must mimic the oil degradation and engine damage that occurs as a consequence of a given style of driving. That is, a test must exhibit the same fundamental mechanisms that occur during real-world operating conditions. A given engine test environment must be related to real-world driving conditions in at least three ways: • The chemical changes that talce place in the test should be comparable to those that occur in real-world driving. • The materials used in the test (for example, engine, piston rings, seals, bearing materials) should not create an environment that might cause a test to provide misleading information.
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• The temperatures and loads of the test should be appropriate for the tjrpe of service being emulated. For a given test method, a desired chemical environment is created by utilizing an appropriate fuel (including its degradation products) and generating oil degradation products that are similar to those generated in the driving style of interest. A desired materials environment is created by using engine and hardware materials that are representative of the engines that the test method is designed to protect. In addition, hardware should be chosen for the test such that interactions between hardware surfaces and chemicals used or generated in the tests (such as fuels, lubricants, and their degradation products) are similar to those in the real world. A desired t h e r m a l environment m e a n s that test temperatures, even if they are severe, are not so severe that fundamental mechanisms of damage have changed. Creating an appropriate test environment, but also finding test conditions such that the test does not require an inordinate a m o u n t of test time, is often difficult to achieve. An example of this difficulty is as follows. If a lubricant test method is designed to determine the corrosive effects of fuel degradation products on engine materials, the test must be conducted at a temperature such that those corrosive fuel oxidation products will not boil off, but will remain in the engine oil to create corrosive conditions. If a test temperature differs significantly from operating temperatures, an investigator must determine whether or not any apparent correlation is accidental. In addition, leaded fuel tjrpically contains chlorine and bromine compounds that can contribute to engine corrosion. If leaded fuel is used in an engine test that simulates a given type of vehicle operation, but current vehicle operation uses unleaded fuel, one m u s t determine whether any correlations observed between the leaded-fuel test and actual vehicle operation are real or accidental. That is, the investigator must determine whether the chemical environment in the engine test simulates an accelerated version of reality. Such correlation is often difficult to prove. Thus, chemical, thermal, and materials properties all influence the outcome of a test, and it is important to ensure that these properties, as utilized in a given test, are providing a realistic representation of actual operating conditions. In addition, when bench tests are used in place of engine tests to measure a given performance parameter, great care must be taken to ensure correlation between the bench test and engine effects. Gasoline Engine-Oil Performance Categories and Associated ASTM Standard Test M e t h o d s As of the year 2001, only four designations were widely used to describe gasoline engine oil performance: API SJ and SL, and ILSAC GF-2 and GF-3. As stated previously, the engine test and bench test performance requirements for API SJ and ILSAC GF-2 are similar with regard to engine test methods, but in addition ILSAC GF-2 oils must meet the stricter Energy Conserving fuel efficiency requirements. Similarly, API SL requirements as well as Energy Conserving requirements must be passed before an engine oil can be designated as ILSAC GF-3. The appropriate test methods for engine oils must be conducted in accordance with the requirements outlined in the
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American Chemistry Council (ACC) Product Approval Code of Practice. These requirements include test registration of all tests and use of only calibrated equipment and facilities. These requirements were implemented when API SH and ILSAC GF-1 designations for engine oil were adopted in 1993.
ILSAC GF-2 A N D API SJ STANDARD TESTS H i g h T e m p e r a t u r e Oil O x i d a t i o n a n d E n g i n e Wear, ASTM S e q u e n c e HIE During high speed, high load service, an engine oil becomes very hot. Engine deposits may form. The oil's viscosity may increase excessively as a consequence of any or all of the following: evaporation of the lighter ends of the oil, base stock polymerization, oxidation, and nitration of the oil. In addition, engine c o m p o n e n t s m a y exhibit unacceptable wear. These issues are addressed in ASTM D 5533, Standard Test Method for Evaluation of Automotive Engine Oils in the Sequence HIE Spark-Ignition Engine. In this test a 3.8L V-6 engine and a specified fuel are used, test preparation and operation are described in great detail, engine components are measured before and after a test, and oil samples are taken every 8 h and checked for viscosity increase. After a break-in period, the test is conducted for 64 h at an engine speed of 3000 revolutions per m i n and an oil temperature of 149°C. Additional operating parameters are also specified. At the end of the test, oil properties and engine components are rated for viscosity increase, sludge, varnish, deposits, lifter sticking, scuffing, ring sticking, and wear. Terminology used in ASTM D 5533 is defined in ASTM D 4175, Standard Terminology Relating to Petroleum, Petroleum Products, and Lubricants, in which the standard's test number and ASTM committee associated with the test are also listed. Limiting values for this and other tests are provided in ASTM D 4485, Standard Specification for Performance of Engine Oils. For SJ quality engine oil, limiting values indicate that kinematic viscosity increase at 40°C should not be more than 375% after 64 h of testing. There should be n o stuck lifters and no oil-related ring sticking. There should be n o c a m or lifter scuffing. Combined cam plus lifter wear should average less than 30 fim, with a maximum of no more than 64 |xm. Engine sludge is rated according to CRC (Coordinating Research Council) Sludge Rating Manual No. 12. Piston skirt varnish and oil ring land deposits are rated by comparing the engine components to CRC Varnish Rating Manual No. 14. These merit rating measurements should be higher t h a n a specified limiting value. In addition, a homogeneity and miscibility test. Federal Test Method Standard No. 79IC, Method 3470, is used to determine whether an engine oil is compatible with standard test oils. This test ensures that if a vehicle owner adds a liter (quart) of make-up oil to a different oil that is already in the oil pan, n o adverse effects will occur, assuming both oils meet current specifications. High Temperature CRC L-38
Copper-Lead
Bearing
Wear,
An additional high temperature engine test method is ASTM D 5119, Standard Test Method for Evaluation of Engine Oils
in the CRC L-38 Spark-Ignition Engine. This test evaluates bearing weight loss and oil deterioration under high-temperature, high-load service conditions. According to Section 1.4 of the test method, "correlation of test results with those obtained in automotive service has not been established." Test operating conditions include an engine speed of 3150 revolutions per min, fuel flow of 2.15 kg/hour, and gallery oil temperature of 143.5°C for SAE 20, 30, 40, 50, and multigrade oils. There is a 4-hour run-in period and a half-hour flush at the start of the test, followed by four 10-hour test segments. Any fuel that has entered the engine oil during the test is stripped (removed) before viscosity is determined. Stripping is accomplished by heating the oil at 120°C under vacu u m in a nitrogen a t m o s p h e r e for 1 h after the oil has warmed to the test temperature. ASTM D 445, Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (the Calculation of Dynamic Viscosity), is used to evaluate the viscosity stability of multiviscosity oils such as SAE lOW-40. The change in acidity of the engine oil is determined, since oil acidity increase is a measure of oil degradation. Acidity may be evaluated either by ASTM D 974, S t a n d a r d Test Method for Acid and Base N u m b e r by Color-Indicator Titration, or by ASTM D 664, S t a n d a r d Test Method for Acid Number of Petroleum Products by Potentiometric Titration. The extent of varnish a n d sludge deposits is determined. Weight loss of copper-lead bearings is determined. Bearing weight loss should be no greater than 40 mg. Additional specifications are listed in the test method. ASTM D 974 indicates that "the method does not measure an absolute acidic or basic property that can be used to predict performance of an oil under service conditions. No general relationship between bearing corrosion and acid or base n u m b e r s is known." ASTM D 664 indicates approximately the same thing. City Service Engine Sludge and Valve Train ASTM Sequence VE
Wear,
The Sequence VE test, ASTM D 5302, is titled Standard Test Method for Evaluation of Automotive Engine Oils for Inhibition of Deposit Formation a n d Wear in a Spark-Ignition Internal Combustion Engine Fueled with Gasoline and Operated Under Low-Temperature, Light-Duty Conditions. The purpose of the test is to ensure that a given engine oil protects an engine under city driving conditions. The test is r u n in three stages. Stage 1 lasts for 120 min at an engine speed of 2500 revolutions per min. The engine load is listed as 33.5 brake horsepower, at an engine oil inlet temperature of 155°F (note that English units are listed. 155°F is 68.3°C and 33.5 brake horsepower is 25 kW). Stage 2 lasts 75 m i n at 2500 revolutions per m i n at an oil temperature of 210°F (98.9''C) and an engine load of 33.5 brake horsepower (25 kW). Stage 3 lasts 45 min at an engine speed of 750 revolutions per min, a load of 1.0 brake horsepower (0.746 kW), and an engine oil inlet temperature of 115°F (46.1°C). Seventy two repetitions of this four-hour test cycle are conducted, to provide a total of 288 h of test time. The fuel used is Phillips J unleaded gasoline. Rocker arm cover, valve deck, cylinder block, camshaft baffle, front seal housing, and the oil pan are rated for sludge using CRC No. 12, Sludge Rating Manual. Varnish on the piston skirt is rated using CRC Man-
CHAPTER ual No. 14. Parts other than piston skirts are rated by comparing the engine parts to color chips. Piston undercrown deposits axid ring land deposits Etre EJSO evaluated. The extent of clogging of vEirious components is rated: oil screen, oil ring, PCV (positive crankcase ventilation) valve, and camshaft lobe orifice clogging. Oil screen clogging should be n o greater than 20%. The n u m b e r of stuck rings and hydraulic lifter plungers is recorded. There should be no hot-stuck compression rings. Various wear measurements are tciken, including: camshaft lobes, rocker arm, connecting rod bearing, and cylinder bore. Piston ring gap increase is also used as a measure of wear. Average cam wear should be no greater than 127 /xm. Maximum cam wear should be no greater t h a n 380 /am. The following oil analyses are conducted: iron, copper, and silicon in engine oil, kinematic viscosity (ASTM D 445), and pentane insolubles in oil (ASTM D 893 B). Short-Trip-Service
Engine
Rusting,
ASTM
Sequence
IID
ASTM D 5844, Standard Test Method for Evaluation of Automotive Engine Oils for Inhibition of Rusting, is an engine test conducted with a 1977 5.7L Oldsmobile engine. The test uses leaded gasoline, and a warning message is provided that indicates "The method may not be applicable for the evaluation of engine oils if unleaded gasoline is used." In Stage I (28 h) the engine speed is 1500 revolutions per min, engine load is 25 brake horsepower (18.6 kilowatts) at an oil temperature of 120°F (48.9 °C) in the filter block. In Stage II (2 h) coolant temperature is increased. In Stage III (2 h) the engine speed is 3600 revolutions per min, engine load is 100 brake horsepower (74.6 kW), and the oil temperature is 260°F (126.7°C). Additional operating parameters are listed in the test procedure. Various engine c o m p o n e n t s are evaluated for such measurements as rust formation on valve lifter bodies, valve lifter plungers, valve lifter balls, oil p u m p relief veJve, and pushrods. Lifter plunger sticking, oil p u m p relief valve sticking, and oil consumption are also measured. Energy Conserving Characteristics, ASTM Sequence VIA In addition to the engine performance r e q u i r e m e n t s described above, ILSAC GF-2 oils must meet fuel efficiency requirements according to the test procedure described in ASTM Standard D 6202 (Sequence VIA). In this engine dynamometer test, the fuel consumption associated with a test oil is measured and compared to the fuel consumption when using a standard reference oil. For the test oil to be called "Energy Conserving," it must exceed the fuel efficiency of the reference oil by a given percentage, based on its SAE viscosity grade. The m i n i m u m percent improvement for ILSAC GF2 oils is 1.4% for SAE OW-20 and 5W-20 viscosity grades, 1.1% for other OW and 5W multiviscosity grades, and 0.5% for SAE l o w multiviscosity and all other viscosity grades. API SJ oils are not required to meet fuel efficiency requirements, although oils satisfying the above Sequence VIA requirements can be designated as "Energy Conserving" in the bottom half of the API (American Petroleum Institute) service symbol, which appears on many oil containers. American Petroleum Institute publication API 1509 provides information on this subject.
Engine
Oil
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Volatility
Engine oil volatility (evaporation loss) has been correlated to engine oil consumption [10-12]. In ein effort to limit oil consumption, reduce its effects on emission systems, and improve customer satisfaction, volatility limits are incorporated in both API SJ and ILSAC GF-2 categories. Volatility can be measured by ASTM D 5800 (Noack Volatility method) or by ASTM D 5480 or ASTM D 6417. Both of these are gas chromatography methods in which the temperature of a sample of fluid is raised, the various components that Eire pEirt of the fluid mixture boil off, and the composition of the components that have volatilized off is determined. Foaming
Tendency
Oils that foam excessively can cause valve lifters in engines to collapse and oil pumps to lose pressure. Test methods ASTM D 892 (foaming characteristics) and ASTM D 6082 (hightemperature foaming characteristics) measure the tendency of an oil to foam and the stability of any foam created. In ASTM D 892 a lubricant at 24°C is blown with air for 5 min, then allowed to settle for 10 min. The volume of foam is measured at both the five-min point and the ten-min point. A second fluid sample is measured at 93.5''C, and again, after collapsing the foam, at 24°C. In ASTM D 6082, foam is generated with a diffuser. "Correlation between the amount of foam created or the time for foam to collapse, or both, and actual lubrication failure has not been established." Phosphorus
Content
Phosphorus in engine oil has been shown in numerous studies [6-8] to be related to poisoning of catalytic converters and other emissions system components. For this reason, phosphorus content of ILSAC GF-2 and certain viscosity grades of API SJ oils must remain below a designated value. Phosphorus content is measured either by ASTM D 4951, Standard Test Method for Determination of Additive Elements in Lubricating Oils by Inductively Coupled Plasma Atomic Emission Spectrometry or by ASTM D 5185, S t a n d a r d Test Method for Determination of Additive Elements, Wear Metals, and Contaminants in Used Lubricating Oils and Determination of Selected Elements in Base Oils by Inductively Coupled Plasma Atomic Emission Spectrometry.
Additional Requirements Several additional requirements must be satisfied before an oil can claim to be either ILSAC GF-2 or API SJ. These requirements include: • Flash Point either by ASTM D 93, Standard Test Methods for Flash-Point by Penske-Martens Closed Cup Tester, minimum flash point of 200°C or by ASTM D 92, Standard Test Method for Flash and Fire Points by Cleveland Open Cup, m i n i m u m flash point of 185°C, • Gelation Index (a measure of low-temperature oil gelling tendency) by ASTM D 5133, Standard Test Method for Low Temperature, Low Shear Rate, Viscosity/Temperature Dependence of Lubricating Oils Using a TemperatureScanning Technique,
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MANUAL 3 7: FUELS AND LUBRICANTS
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• TEOST-33 (Thermo-Oxidation Engine Oil Simulation Test, protocol 33, a deposit control test, with a maximum deposit weight of 60 mg), • Homogeneity and Miscibility by Federal Test Method 791c (Method 3470.1), • Engine Oil Filterability and Water Tolerance as determined by the Engine Oil Filterability Test (EOFT) • Engine Oil Water Tolerance Test (EOWTT). As of the year 2001, the latter two tests are in the process of being elevated to ASTM standards by ASTM Subcommittee D02.06 (Analysis of Lubricants). ILSAC GF-3 and API SL Standard Tests The five new engine performance tests that define ILSAC GF3 and API SL categories, at the time of this writing, are in the process of being advanced to ASTM Standards by ASTM D02.B0 Committee on Automotive Lubricants. These engine tests include the following. Sequence IIIF The Sequence IIIF Test replaces the Sequence HIE Test (ASTM D 5533) and measures high-temperature engine oil thickening and piston deposits. It also provides information on camshaft and valve lifter wear. Sequence VG The Sequence VG Test is a partial replacement for the Sequence VE Test (ASTM D 5302). Sequence VG measures the sludge and deposit control tendency of engine oils under engine conditions which simulate stop-and-go city service in vehicles. SequenceIVA Sequence IVA is also a partial replacement for the Sequence VE Test (ASTM D 5302). Sequence IVA measures the ability of an oil to provide overhead cam and slider-follower wear protection under stop-and-go engine conditions such as are experienced in city driving. Sequence VIII Sequence VIII replaces the CRC L-38 Test (ASTM D 5119). It measures the same parameters as the L-38 Test did: copperlead bearing weight loss under high-temperature operating conditions and permanent viscosity loss due to shearing. However, the Sequence VIII Test utilizes unleaded fuel, whereas the L-38 test utilizes leaded fuel. Sequence VIB Sequence VIB replaces the Sequence VIA Test (ASTM D 6202) for measuring the fuel-efficiency properties of an engine oil. Like its predecessor, the Sequence VIB Test measures the improvement in fuel efficiency of a test oil compared to an ASTM standard reference oil. Unlike its predecessor, however, the Sequence VIB Test not only measures the fuel efficiency of the oil when the oil is relatively new (after only 16 h of aging in the engine), but also the fuel efficiency after 96 h of aging, which corresponds to approximately 4000-5000 miles (6500-8000 km) of vehicle operation. Different levels of fuel efficiency improvement are required, depending upon the SAE viscosity grade of the en-
gine oil (the same groupings of viscosity grade as were defined in the ILSAC GF-2 requirements for the Sequence VIA Test, as described previously). Sequence VIB fuel efficiency requirements apply only to ILSAC GF-3 oils, not to API SL oils, although oils satisfying the Sequence VIB requirements for GF-2 can use the "Energy Conserving" label of the bottom half of the API service symbol, as was previously mentioned under the topic "Energy Conserving Characteristics," Sequence VIA. Bench and Other Test Requirements Most of the same bench and chemical property requirements that exist for ILSAC GF-2 and API SJ categories also exist for ILSAC GF-3 and API SL (although in some cases the limits associated with the tests for ILSAC GF-3 and API SL are different from those required for ILSAC GF-2 and API SJ). These requirements include: • Volatility loss as determined either by ASTM D 5800 or by ASTM D 6417, • Filterability criteria as measured by the Engine Oil Filterability Test and the Engine Oil Water Tolerance Test, • Foaming tendency by ASTM D 892 and D 6082, • Homogeneity and Miscibility by Federal Test Method 791c, Method 3470.1, • Gelation Index by ASTM D 5133, and • Maximum phosphorus content as measured either by ASTM D 4951 or by ASTM D 5185. There are, however, two new bench test requirements for ILSAC GF-3 and API SL oils: the Ball Rust Test and the TEOST MHT-4 test. Ball Rust Test ASTM D 6557, Standard Test Method for Evaluation of Rust Preventive Characteristics of Automotive Engine Oils, was developed to replace the Sequence IID Engine Test, which measures the ability of an oil to prevent rust and corrosion in engines operating on leaded gasoline under short-trip driving conditions. ASTM D 6557 is also known as the Ball Rust Test, since it uses a steel ball (lifter check ball) as a test specimen that is representative of a typical ferrous engine component. A ball and a measured sample of an engine oil that is to be evaluated are placed in a test tube. If more than one oil sample is to be tested, additional test tubes, each containing a ball and an oil sample are prepared. The test tubes are placed in a shaker in which shaker speed and temperature are controlled. Air and a corrosive fluid are fed into the test tube to create a corrosive environment. The corrosive fluid consists of representative samples of the type of chemicals that can be generated during the combustion process in short-trip winter driving. After 18 h, the steel test specimens (balls) are evaluated for the extent of rusting, using a computerized scanning system with optical imaging. Excellent correlation between results obtained in the Ball Rust Test and Sequence IID has been achieved with a wide variety of oil compositions. However, the method indicates that, in comparison with the results of ASTM D 5844 (Standard Test Method for Evaluation of Automotive Engine Oils for Inhibition of Rusting (Sequence IID), "correlation between these two test methods has been demonstrated for most, but not all, of the test oils evaluated."
CHAPTER TEOSTMHT-4 A new version of the Thermo-Oxidation Engine Oil Simulation Test (TEOST), the MHT-4, was developed for ILSAC GF3 and API SL specifications. The TEOST MHT-4 test is intended to provide additional high-temperature piston deposit control in m o d e m engines that are designed for improved driveability and in addition must meet federal emissions and fuel economy requirements. D i e s e l E n g i n e Oil P e r f o r m a n c e C a t e g o r i e s a n d Associated ASTM Standard Test M e t h o d s When diesel engines are operated at high temperature, the viscosity of the engine oil can increase due to oil oxidation and nitration, base stock polymerization, evaporation of the lighter ends of the oil, and accumulation of soot in the oil. As was the case for gasoline-fueled vehicles, diesel engine oil additives can degrade during long-term use. If the engine oil degrades to the point at which it becomes acidic, acidic reaction products that form on a metal surface can be scraped off by moving components, resulting in undesirable wear. In addition, soot accumulation can adversely affect engine wear by two fundamentally different mechanisms. Soot can interact with a n d sequester a n engine oil's antioxidant/anti-wear agent so that the anti-wear capability of the engine oil becomes diminished [13]. Soot can also abrade away a protective anti-wear film that has formed on heavily loaded regions of an iron surface [3]. These oil-related concerns are addressed using various standard tests. At the t i m e of this writing, there were five active API (American Petroleum Institute) diesel engine oil performance categories, designated as API CF, API CF-2, API CF-4, API CG-4 and API CH-4. Each of these categories has at least two engine performance tests t h a t m u s t be conducted to demonstrate compliance with category requirements. Various bench tests are also included. A complete description of each of these categories, including engine and bench tests, as well as associated test limits for each requirement, is documented in ASTM D 4485, Stcindard Specification for Performance of Engine Oils and in SAE J183, Engine Oil Performemce and Service Classification. It is anticipated that these five categories will be reduced to three: API CF-2, API CG-4, and API CH-4. Thus, API CF and API CF-4 will no longer be used. A description of each of these diesel engine-oil performance categories is as follows. APICF API CF oils are intended for use in off-road indirect-injection diesel engines using a broad range of fuel types, including those with high sulfur content. The only ASTM Standard test in the performance definition of this category is the CRC L-38 Test (ASTM D 5119), which measures bearing weight loss a n d oil deterioration, as described previously. NonASTM tests include the Caterpillar IM-PC test (PC stands for Pre-Chamber), which measures piston deposits and ring sticking in a single cylinder diesel engine. API CF-2 API CF-2 oils are intended for use in two-stroke-cycle engines, which require cylinder and ring-face scuffing resistance and deposit control. ASTM standard test requirements
17: AUTOMOTIVE
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473
include the CRC L-38 (ASTM D 5119) test, which has already been described, and the Detroit Diesel 6V92TA test (ASTM D 5862). ASTM D 5862, Standard Test Method for Evaluation of Engine Oils in Two-Stroke Cycle Turbo-Supercharged Detroit Diesel 6V92TA Engine, is a two-stroke cycle diesel engine test that measures such parameters as high-temperature stability of engine oil, cylinder liner scuffing, piston ring face distress, plugging of liner ports, and slipper bushing distress. Removal of tin from the cylinder liner is also eveduated. Engine components are measured before and after the test to determine the extent of wear or degradation. The test is conducted under two different modes of operation: torque mode and power mode. In the torque mode, engine speed is 1200 revolutions per m i n and the oil gallery temperature is 102°C. In the power mode, the engine speed is 2300 revolutions per min, at an oil gallery temperature of 111°C. Additional parameters are listed in the test procedure. A non-ASTM Standard test requirement for this category is the Caterpillar IMPC method described under the API CF category. API CF-4 The API CF-4 category describes the tests that are used for high-speed service in four-stroke cycle diesel engines. The category includes tests that m e a s u r e bearing weight loss, ASTM D 5119 (the L-38 test, which measures oil degradation and bearing corrosion—including a mEiximum limit of 50 mg bearing weight loss). Piston deposits, viscosity increase, and brake specific oil consumption are rated according to the Mack T-6 test. Average rate of kinematic viscosity increase in the last 50 h according to the Mack T-7 test and oil consumption and wear in the Cummins NTC-400 test ctre also measured. In addition, the Caterpillar IK engine test is used to measure piston and ring groove deposits, piston scuffing, ctnd oil consumption. API
CG-4
API CG-4 oils are intended for use in high-speed, four-strokecycle diesel engines operating in both highway and off-road conditions, with fuels in which the sulfur content may VEiry from less than 0.05% sulfur by weight to less than 0.5 weight % sulfur, as described in ASTM D 4485. These oils provide effective control of deposits and wear in engines designed to meet 1994 exhaust emission standards. ASTM Standard tests include the Mack T-8 engine test (ASTM D 5967), ASTM Sequence HIE engine test (ASTM D 5533), CRC L-38 bearing test (ASTM D 5119), the roller follower wear test (ASTM D 5966), and a corrosion bench test (ASTM D 5968). Foaming tendency according to ASTM D 892 is also included. ASTM D 5533, ASTM D 5119, and ASTM D 892 are described previously in this chapter. ASTM D 5966, Standard Test Method for Evaluation of Engine Oils for Roller Follower Wear in Light-Duty Diesel Engine, was developed to address soot accumulation in engine oil, which can contribute to engine wear [3,13]. This test is conducted at 1000 revolutions per min and a m a i n oil gallery temperature of 120°C. Roller follower shaft wear is evaluated. Kinematic viscosity at 100°C cind soot content are also monitored. ASTM D 5967, Standard Test Method for Evaluation of Diesel Engine Oils in T-8 Diesel Engine (the Mack T-8 Test), eveJuates both viscosity increase and soot accumulation in
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the engine oil of a Mack E7-350 diesel engine. Oil consumption is also monitored. The test is conducted at an engine speed of 1800 revolutions per min, a fuel rate of 63.3 kg/hour, an engine oil temperature of 100-107°C, and an inlet manifold temperature of 43.3°C. Additional test conditions are listed in the test procedure. The viscosity should not increase by more than 11.5 centistokes at a soot content of 3.8%. ASTM D 5968, Standard Test Method for Corrosiveness of Diesel Engine Oil, measures the extent to which non-ferrous metals are susceptible to corrosion as a consequence of inappropriate engine oil formulation (rather than oil degradation or contamination). Test coupons (lead, copper, tin, and phosphor bronze) are cleaned and placed in the test oil. Air is allowed to flow into the oil for 168hat 121°C. Test coupons are evaluated for corrosion and discoloration, and the oil is evaluated for the presence of metals that have entered the oil as a consequence of corrosion of the metcil samples. Non-ASTM tests used in defining the API (American Petroleum Institute) CG-4 specification include the Caterpillar IN engine test, which measures piston deposits in a fourstroke-cycle, direct injection diesel engine operating on fuel containing less than 0.05 percent sulfur by weight. API CG-4 also includes the Engine Oil Aeration Test (EOAT), which has been correlated with oil aeration in diesel engines equipped with hydraulically actuated, electronically controlled unit injectors. API CH-4 API CH-4 oils are intended for use in high-speed, four-strokecycle diesel engines designed to meet 1998 exhaust emissions standards. ASTM Standard tests include CRC L-38 (ASTM D 5119, Standard Test Method for Evaluation of Automotive Engine Oils in the CRC L-38 Spark-Ignition Engine), ASTM Sequence HIE test (ASTM D 5533), Mack T-8 (ASTM D 5967), and a high temperature corrosion bench test (ASTM 5968) were described previously. The Mack T-9 (ASTM D 6483) is described below. The tests use diesel fuels with sulfur content up to 0.5 weight %, as described in ASTM D 4485. Foaming tendency (ASTM D 892), volatility (ASTM D 5800 or ASTM D 6417), and sheared viscosity (ASTM D 6278) requirements are also included. ASTM D 6483 (Standard Test Method for Evaluation of Diesel Engine Oils in T-9 Diesel Engines, known as the Mack T-9 test) evaluates diesel engine oil for various performance characteristics, including resistance to lead corrosion and wear of piston rings and cylinder liners. The test uses a Mack E7-350 V-MAC II diesel engine. The test starts with a onehour break-in, followed by a two-phase test that includes 75 h at 1800 revolutions per min and 425 h at 1250 revolutions per min. Oil samples are taken during the testing and are analyzed for increase in viscosity and for weeir metals in the oil. The extent of engine-component wear is also assessed. For example, the maximum allowed cylinder liner wear is an average value of 25.4 \xxn at a soot content of 1.75%. The maximum allowed average-top-ring weight loss is 120 mg, and the maximum allowed increase of lead content in the engine oil is 25 parts per million. Non-ASTM standard tests used in defining the API CH-4 category include the following. The Caterpillar IP engine test measures piston and ring groove deposits in four-strokecycle, direct injection diesel engines calibrated to meet 1998
United States Federal Exhaust Emissions Standards with fuels containing less than 0.05% sulfur by weight. The Caterpillar IK engine test measures the same performance parameters as the Caterpillar IP engine test, but for engines in use prior to 1989, using aluminum pistons and fuel that has 0.4 weight % sulfur. The Cummins Mil High Soot Diesel Engine Test measures soot-related valve-train wear. Engine sludge and oil filter plugging tendency are measured. The Engine Oil Aeration Test (EOAT), mentioned in the API CG-4 specification, is also conducted. Additional Tests Used to Describe Engine Oil Performance and Properties The use of a bench test, if it can be shown to correlate to an engine test, can be considerably less labor intensive and less costly than an engine test. Bench tests for engine oil are of several types. Some, such as the Ball Rust Test, ASTM D 6557 (described previously), produce fundamental information about a specific aspect of the protection provided by an engine oil. Other bench tests measure various properties of the engine oil such as viscosity, alkalinity, acidity, extent of oil oxidation, presence of oil additives, and the presence of oil contaminants such as engine-wear debris or corrosion products. Many of these bench tests are included as part of the specified analysis procedure for an engine test. In other words, a bench test often provides necessary documentation to evaluate the results of a dynamometer test. Various tests that are used to ascertain the performance of automotive lubricants in general and engine oils in particular are described in the following sections. Engine Oil Acidity and Alkalinity Automotive lubricants typically contain oil-soluble alkaline agents that provide corrosion protection, since corrosion is less likely to occur in a moderately alkaline environment. Acids can be generated from fuel combustion or from oxidation of engine oil in hot spots. For example, if engine oil becomes acidic as a consequence of exposure to heat or if acidic combustion by-products from the fuel condense in the engine oil during short-trip, low temperature service, engine corrosion may occur. If oil alkalinity is too high, a lubricant or other fluid may also become aggressive to a metal surface. Several techniques are available to assess oil acidity and remaining alkalinity protection, for example: • ASTM D 664, Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration • ASTM D 974, Standard Test Method for Acid and Base Number by Color-Indicator Titration • ASTM D 1093, Standard Test Method for Acidity of Distillation Residues or Hydrocarbon Liquids • ASTM D 2896, Standard Test Method for Base Number of Petroleum Products by Potentiometric Perchloric Acid Titration • ASTM D 4739, Standard Test Method for Base Number Determination by Potentiometric Titration. In each of these tests, a sample of the oil or residue of interest is titrated (that is, reacted) with the desired test reagent. Different reagents used in the various tests yield different values for an acid number or base number. An investigator will have to determine which test best meets his needs.
CHAPTER 17: AUTOMOTIVE For Acid Number according to ASTM D 664, the following qualification is listed: "The test method may be used to indicate relative changes that occur in an oil during oxidizing conditions — The test method is not intended to measure an absolute acidic property that can be used to predict performance of an oil under service conditions. No general relationship between bearing corrosion and acid number is known." ASTM D 974 has a nearly identical notice. ASTM D 1093 indicates: "The results obtained by this method are qualitative expressions." Thus, an investigator would be wise to determine which type of analysis suits his needs. An example of transient engine-oil acidification due to condensation of partially burned fuel is shown in Figure 1 (derived from information available in Ref. 2), in which an oil sample was taken on a day when the outside temperature was -20°C. The test vehicles had been given 3-km trips three times a day. During this interval of bitter cold, the engine oil acidity spiked in all the vehicles that were on short-trip test that day. Two representative examples are shown in Fig. 1. The weather later warmed, so that the volatile fuel-derived acids that had condensed in the oil were removed.
Entry of Water into Engine Oil In addition to condensation of acids, water can condense in engine oil during short-trip, winter operation. ASTM D 1744, Standard Test Method for Determination of Water in Liquid Petroleum Products by Karl Fischer Reagent (Discontinued in January, 2000), has been used to determine the water content of engine oil. The amount of water that condenses in the engine oil during very cold short-trip operation is directly related to the amount of water-per-kilometer produced by a given fuel [2]. M85 fuel (85 percent methanol plus 15 percent gasoline) produces approximately four times as much waterper-kilometer as gasoline. This influence of fuel type on water in oil is shown in Fig. 2 (which was derived from information available in Ref. 2). Differences in oil analysis results due to differences in engine type and oil type are negligible in comparison to the fuel effect. With longer trips or warmer weather, the water is removed via evaporation. Viscosity Viscosity is a measure of the resistance to flow. Two types of viscosity measurements are typically used, kinematic and dynamic. Kinematic viscosity is measured by determining the time required for a fluid to flow past the marks on a specicd tube at a specified temperature, assuming that the flow is laminar, and not turbulent. Kinematic viscosity is described mathematically in terms of mm^/s. Dynamic viscosity is equal to kinematic viscosity multiplied by the density of the fluid of interest. Engine oils operate most effectively when their viscosity is within an optimum range for a given type of engine and a given set of driving conditions. Temperature influences viscosity. Higher oil temperature causes a reduction in oil viscosity, and lower oil temperature causes oil viscosity to increase.
3.1LV-6 engine
:i=^^ 200 300 400 Distance on oil, l(m
500
600
FIG. 1—Condensation of volatile acids into engine oil (per ASTM D 664), due to partial oxidation of fuel during exceedingly cold weather, and removal of the acids with warmer weather. Ml
25
Percent water In engine oil, Karl Fischer method
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I
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11
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475
Corrosiveness of Diesel Engine Oil ASTM D 5968, Standard Test Method for Coirosiveness of Diesel Engine Oil, was described previously, under section API CH-4.
Ambient temperature Near -20° C, 3-km trips
100
LUBRICANTS
5
4.5L gasoline B 4.5L gasoline B 1
'
200
400
600
800
1000
Distance traveled, km
FIG. 2—Influence of type of fuel on the amount of water that can accumulate in engine oil during short-trip, cold-start driving, according to ASTIM D 1744.
476
MANUAL 150
37: FUELS AND LUBRICANTS •
1
•
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1
HANDBOOK 1
85% methanol fuel, short trips, emulsion formation 1 Evaporation of water ^ — o and methanol during \ warm weather
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Viscosity of engine oil at 40°C, cSt 100
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Gasoline fuel in engine oil, short trips, solution formation
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If the viscosity vEiries significantly with sheeu", different results may be obtained from viscometers of different diameter. ASTM D 446 gives specifications and operating instructions for glass kinematic viscometers. ASTM D 4683 measures viscosity at high temperature and high shear a n d carries the warning, "applicability to petroleum products other than engine oils has not been determined." Engine oil at very low temperature can become highly viscous, per ASTM D 4684 (which carries the same warning as is found with ASTM D 4683). In ASTM D 4684, an engine oil is cooled from 80°C at a programmed rate to a fined low temperature (between -15°C and -35°C). Yield stress and apparent viscosity are measured.
SAE 10W-30, API SG-quality engine oil I
200
.
I
.
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800
^
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FIG. 3—Influence of solution-forming and emulsion-forming fuel contamination on engine oil viscosity (per ASTIVI D 445).
In addition to the changes in viscosity due solely t o temperature effects, operation at a given high temperature will cause the oil's viscosity to increase (in comparison to viscosity of the fresh oil at the same temperature) due to oil oxidation and to evaporation of the lighter ends of the engine oil. Various low-temperature viscosity cheinges can also be observed. Low engine-oil temperature causes an increase in viscosity. However, at low outside temperatures, hydrocarbon fuel can condense in engine oil, which causes a decrease in oil viscosity. Condensation of methanol (alcohol) fuel in engine oil formed an emulsion and caused a significant viscosity increase. An example of these opposite viscosity effects is shown in Fig. 3, which is derived from information available in Ref. 2. Thus, fuel-in-oil measurements can provide information about the nature of viscosity changes and the nature of contaminants in the engine oil. However, these effects only occur if the engine oil remains at low temperature during short-trip operation, otherwise once the oil t e m p e r a t u r e rises sufficiently, m u c h of the fuel will evaporate from the oil, and the oil viscosity will revert to a more typical value. Standard ASTM viscosity tests include: • ASTM D 445, Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (the Calculation of Djoiamic Viscosity), • ASTM D 446, Standard Specifications and Operating Instructions for Glass Capillary Kinematic Viscometers, • ASTM D 4683, Standard Test Method for Measuring Viscosity at High Temperature and High Shear Rate by Tapered Bearing Simulator, • ASTM D 4684, Standard Test Method for Determination of Yield Stress and AppcU^ent Viscosity of Engine Oils at Low Temperature. ASTM D 445 measures the time it takes for a liquid to flow through a standard capilleiry tube. This test method has the wzuning, "The result obtained from this test method is dependent u p o n the behavior of the sample cind is intended for application to liquids for which primarily the shear stress and shear rates are proportional (Newtonian flow behavior)."
EXTENT OF OIL OXIDATION As engine oil is used in a vehicle, the ability of the engine oil to resist oxidation becomes diminished as a consequence of exposure to heat and to contamination from fuel and fuel reaction products. This degradation can be monitored using test methods such as ASTM D 5483, Standard Test Method for Oxidation Induction Time of Lubricating Greases by Pressure Differential Scanning Calorimetry. The test method indicates that, "This test method can be used for research and development, quality control and specification purposes. However, n o correlation has been determined between the results of this test method and service performance." Information found in tests such as those illustrated in Fig. 4 suggest a very useful correlation between oil oxidation in road tests and DSC oil analysis results as described in Refs. 14 and 15. In this method, a small cimount of lubricant is placed in a cup, the cup is placed in a chamber which is filled with oxygen and heated rapidly to the desired test temperature, and held at that temperature for the duration of the test. The time from the start of the test until the onset of rapid oxidation is measured. An alternative procedure is to r a m p the temperature u p in a series of stages and observe the effects of elevated temperature on oil oxidation.
1(
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Oxidation induction time by 3 differential scanning calorimetry, natural 2 logarithm of DSC
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FIG. 4—Differences in loss of oxidative stability of engine oil (natural logarithm of the DSC value) as a consequence of type of service, determined using ASTM D 5483.
CHAPTER Even though this method was developed for greases, it can be used for other lubricants. An example is found in Figure 4, which shows differences in the rate of engine oil oxidation between similar vehicles, driving over identical terrain, but two of the vehicles were not pulling a trailer (dark circles), and one of the vehicles pulled a 1000-kg trailer (open circles) [14,15]. As can be seen, the constant-temperature version of the differential scanning calorimetry technique (abbreviated as "DSC") was able to document statistically significant differences in the chemical status of the engine oil from the vehicle that pulled the trailer. The natural logarithm of the DSC value was plotted. This figure indicates that oxidative stability can be a reliable measure of severity of service. The theoreticcil explanation for this effect is provided in Ref. 14. The various ASTM standard test procedures described in this section list a great n u m b e r of related test methods. Referenced Documents from these test methods are provided in Tables 2 and 3 at the end of this chapter, so that the reader will have a comprehensive list of the various tests associated with engine oil.
TRANSMISSION FLUIDS Composition and Performance of Transmission Fluids Transmission fluids lubricate a vehicle's transmission. They may be composed of synthetic or mineral oil. S5Tithetic oil typiccilly shows enhanced resistance to deterioration from exposure to heat. However, synthetic and mineral oil both contain additives that help maintain the desired friction properties, minimize wear, and provide corrosion inhibition to the transmission components. In addition, antioxidant additives are required to prolong the life of the transmission fluid. Maintaining stable friction properties is highly important, since any change in shift characteristics or generation of shudder (objectionable vibration) after prolonged or severe use is sure to be noticed by a vehicle owner. To provide the desired shift properties, one of the additives present in a transmission oil is typically a long-chain hydrocarbon that has a polar group on one end. The polar end group of the additive is attracted to the metal surface, and the long-chain hydrocarbon portion associates with the oil, thereby forming a durable, low-friction oil film on the metal surface. Too m u c h friction reduction will cause transmission components to slip, b u t too little friction reduction will not allow the transmission to shift smoothly cind will cause wear. In addition to appropriate friction characteristics, it is also essential to reduce wear a n d promote long transmission life. Wear protection is provided to a great extent by oil additives. If suitable anti-wear additives were not present, there would likely be excessive weeir of the transmission components under severe operating conditions. Thus, fluid degradation during prolonged service should not be extensive enough to comp r o m i s e transmission performance, a n d wear rates m u s t remain low so that excessive loss of material does not cause shift characteristics to deteriorate. Various standard test methods are available to ensure sustained transmission performance. Tests for automatic transmission fluids include measurement of physical eind chemical properties: viscosity, flash point, pour point, corrosion
17: AUTOMOTIVE
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resistance, wear resistance, friction characteristics, and resistance to foaming. Additional tests measure performance during selected duty cycles, as defined by the manufacturer. For example, major automotive producers in North America have their own specifications for transmission fluids. Thus, in retail establishments that sell transmission fluids, one can see labels on transmission fluid bottles indicating "meets the specifications for XXX a n d YYY", in which XXX and YYY represent the brand names of a specific automobile manufacturer. Transmission fluid test requirements published by a given automobile m a n u f a c t u r e r include GM-6417M for Dexron®-III (General Motors) a n d Mercon®-V (Ford). A n u m b e r of standard ASTM tests for transmission fluids are listed in the SAE International Surface Vehicle Information Report, SAE J311 (revised 2000-02): ASTM D 130 (Copper Strip Corrosion), ASTM D 665 (Rust Preventing Characteristics), ASTM D 892 (Foaming Characteristics, as described previously in the Engine Oil section), ASTM D 1275 (Corrosive Sulfur in ElectriccJ Insulating Oils, which is not listed in the 2001 ASTM a n n u a l book of standards), ASTM D 1748 (Rust Protection by Metal Preservatives in the Humidity Cabinet), and ASTM D 2882 (Wear Characteristics of Hydraulic Fluids in a Constant Volume Vane Pump). Additional ASTM tests are also provided in the Annual Book of ASTM Standards. The tests listed here are described below. Referenced d o c u m e n t s in these transmission-related test methods are provided in Tables 4 and 5 at the end of the chapter.
High Temperature, High Load Testing ASTM D 5579, S t a n d a r d Test Method for Evaluating the Thermal Stability of Manual Transmission Lubriccints in a Cyclic Durability Test, evaluates the thermal stability of fluids used in heavy duty manual transmissions during hightemperature (121°C) operation. This test measures the number of shifting cycles, between a high and a low gear, that can be performed without failure of synchronization in a manual transmission being operated at high temperatures. The test is terminated when the transmission experiences two shifts in which the clutch teeth produce a loud noise. According to Section 1.3 of the test method, "Correlation of test results with truck transmission service has not been established. However, t h e procedure has been shown to appropriately sepEirate two transmission lubricants, which have shown satisfactory a n d unsatisfactory field performance in the trucks of one manufacturer." ASTM D 5704, Standard Test Method for Evaluation of the Thermal a n d Oxidative Stability of Lubricating Oils Used for Manual Transmissions and Final Drive Axles (commonly referred to as the L-60-1 test), measures oil thickening and formation of insolubles and deposits in a manual transmission oil and final drive axle lubricating oil. ASTM D 5182, Standard Test Method for Evaluating the Scuffing Load Capacity of Oils (FZG Visual Method), measures the scuffing load capacity of oils used to lubricate hardened steel gears. Scoring (abrasive wear) is also assessed. An FZG Gear Test Machine is operated at a constant speed of 1450 revolutions p e r min. The test starts at 90°C at a predetermined load. The severity of the test is increased (using a designated test cycle) until failure occurs. The n u m b e r of
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shifting cycles to failure of synchronization is determined. Shifter fork wear is also measured. Examples of various types of wear patterns that may be observed during the testing (polishing, abrasive wear, moderate abrasive wear, and scuffing) are provided in the test method to help the investigator interpret test results. In the introduction, the following warning is listed: "High EP (extreme pressure) type oils, for example, those oils meeting the requirements of API GL-4 and GL-5, generally exceed the capacity of the test rig and, therefore, cannot be differentiated with this test method." "EP" in this statement stands for "Extreme Pressure," which typically represents extreme temperature. ASTM D 5760, Standard Specification for Performance of Manual Transmission Gear Lubricants, lists the methods and performance criteria for deciding whether a lubricant exhibits acceptable behavior in nonsynchronized heavy-duty manual transmissions. The limiting criteria are provided in ASTM D 5704, Standard Test Method for Evaluation of the Thermal and Oxidative Stability of Lubricating Oils Used for Manual Transmissions and Final Drive Axles. The limits include a viscosity increase of no more than 100% (per ASTM D 445) and a pentane insolubles content of no more than 3 % (per ASTM D 893). Limiting values for elastomer (seal) properties are also listed. For example, elastomer hardness should not decrease by more than 20% nor increase by more than 5%. B e n c h a n d Analytical Tests Related t o Transmission Fluids Various bench methods are used to help characterize transmission fluid properties a n d performance. ASTM D 130, Standard Test Method for Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test, is used to determine whether a fluid of interest will tarnish copper. In this test, a polished copper strip is immersed in the test fluid at a temperature and for a time appropriate for the test fluid. At the end of the test, the copper strip is cleaned and compared to a stemdard. ASTM D 1748, Standard Test Method for Rust Protection by Metal Preservatives in the Humidity Cabinet, uses steel panels which are dipped in the test lubricant, removed and allowed to drain, and are then suspended in a humidity cabinet at 48.9°C for a designated n u m b e r of hours. The size and n u m b e r of rust spots that form on the panel are determined. There should be n o more than three dots of rust, each of which should be smaller than 1 m m in diameter. ASTM D 2882, Standard Test Method for Indicating Wear Characteristics of Petroleum and Non-Petroleum Hydraulic Fluids in Constant Volume Vane Pump, measures the total mass loss from the twelve vanes used in the p u m p and from the cam ring on which the vanes ride. Good lubricants show very little wear in this test. Fluid cleanliness before and after the test, initial flow rate, and final flow rate are also measured to determine the extent of damage to hardware and degradation of the fluid. Additional tests pertinent to transmissions include measurement of the torque required to overcome the friction that occurs between clamped clutch plates at high and low slip speeds and assessing the tendency to shudder (which occurs as a consequence of stick-slip motion between mating surfaces). Measurement of "shift feel" aJso provides useful information to an investigator.
ASTM D 5662, Standard Test Method for Determining Automotive Gear Oil Compatibility with Typical Oil Seal Elastomers, measures whether the properties (hardness, volume, and elongation in a stretch test) of selected oil-seal elastomers (nitrile, polyacrylate, and fluoroelastomer) are changed on exposure to automotive gear oils. Additional transmission-related tests referenced in the standard methods described here are listed in Tables 4 and 5 at the end of this chapter.
STANDARD TEST METHODS FOR GREASES Introduction to Greases Greases provide a great n u m b e r of highly specialized functions in an automobile, such as lubrication of door locks, electrical connections, various t3rpes of bearings, gears, and levers. There may be more than 50 different types of grease specified by the manufacturer for use in a given vehicle. Greases typically are composed of oil (synthetic or mineral, representing 70 or more percent of the total composition of the grease), a thickening agent, and various additives to provide wear protection, corrosion protection, and stability. Thus, standard test methods for automotive greases focus on such parameters as physical and chemical properties, separation of the thickener from the oil, oxidation resistance, wear protection, water tolerance, and ability to retain the desired properties even after exposure to severe operating conditions such as high temperature. Physical and Chemical Properties of Greases Chemical
Properties,
Stability,
and
Contamination
A number of tests are available to ensure that a grease has the appropriate composition and that its properties have not deteriorated excessively during service. ASTM D 128, Standard Test Methods for Analysis of Lubricating Grease, describes techniques for analyzing the chemical constituents of a conventional grease that is composed primarily of petroleum oil and soap, a n d in addition may contain water, fatty acids, alkaline agents for corrosion inhibition, fat, glycerin, and insoluble c o m p o n e n t s . The method states, "Composition should not be considered as having any direct bearing on service performance unless such correlation is established." During hot operation, lighter components of a grease can evaporate. ASTM D 972, Standard Test Method for Evaporation Loss of Lubricating Greases and Oils, is a test method designed to provide a quantitative measurement of the mass loss that occurs during this evaporation process. A sample of grease or oil is weighed a n d then maintained at a desired temperature in the range of 100-150°C for 22 h. Heated air is passed over the sample. At the end of the test the mass loss as a consequence of exposure to heat is determined. The method indicates that "correlation between results from this test method and service performance has not been established." An additional test method for evaporative loss of a grease is also available. ASTM D 2595, Standard Test Method for Evaporation Loss of Lubricating Greases Over Wide-Temperature Range, can be conducted at a t e m p e r a t u r e anywhere from 93-316°C. Heated air is passed over a weighed sample of grease, emd the loss of weight of the grease is mea-
CHAPTER sured. The method indicates that "correlation between results from this test method and service performance has not been established." ASTM D 942 (Reapproved 1995), Standard Test Method for Oxidation Stability of Lubricating Greases by the Oxygen Bomb Method, measures the ability of a grease to resist oxidation in an oxidizing atmosphere at elevated temperature. A sample of grease is oxidized in a sealed b o m b that is filled with oxygen at 110 psi (758 kPa) at 99°C. Pressure is recorded during the test. The extent of oxidation is determined by measuring the decrease in oxygen pressure. The method carries the notification that this test method "predicts neither the stability of greases under dynamic service conditions, nor the stability of films of greases on bearings and motor-parts. It should not be used to estimate the relative oxidation resistance of different grease types." ASTM D 1743, S t a n d a r d Test Method for Determining Corrosion Preventive Properties of Lubricating Greases, measures the ability of a grease to protect a bearing against corrosion in the presence of water. A bearing is r u n for a short time and is then exposed to water. After 48 h at 52°C and 100% humidity the bearing is inspected for corrosion. In ASTM D 4048, Standard Test Method for Detection of Copper Corrosion from Lubricating Grease, a copper strip is immersed in a sample of grease and heated for a period of time. Typical time and temperature conditions are 100°C for 24 h. At the end of the test the copper strip is washed and compared to copper strip standards to provide a measure of the severity of corrosion. The method states, "no correlations with actual field service, most of which are under djmamic conditions, have been established. It does not measure either the ability of the lubricant to inhibit copper corrosion caused by factors other than the lubricant itself nor does it measure the stability of the grease in the presence of copper." ASTM D 1742, Standard Test method for Oil Separation from Lubricating Grease During Storage, measures the tendency of a grease to separate into distinct fractions during storage. A sample of grease is placed on a sieve and is subject to 1.72 kPa (0.25 psi) at 25°C for 24 h. The amount of matericJ that seeps into a beaker is weighed. The test represents separation during storage rather than separation under dynamic operating conditions. The following warning is provided: "This test method is not suitable for greases softer than NLGI No. 1 grade." In addition, the m e t h o d states, "this method is not intended to predict oil separation tendencies of the grease under dynamic conditions." The method does indicate oil separation that occurs during storage. ASTM D 1404, Standard Test Method for Estimation of Deleterious Particles in Lubricating Grease, measures the presence of abrasive or inappropriate particles in a grease. A small sample of grease is placed between two standard plastic plates, and pressure is applied to the plates. The presence of inappropriate abrasive peirticles produces a characteristic scratch pattern. The n u m b e r of such scratch patterns on the plates is determined. "The significance of the n u m b e r of scratches as far as correlation with field performance is concerned has not been established." If a grease comes into contact with an elastomer, several effects may occur. Components of the grease may be sufficiently compatible with the elastomer that grease components enter the elastomer structure and cause the elastomer
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to soften, swell, or to react adversely to the presence of the grease c o m p o n e n t s . Alternatively, the grease may extract elastomer additives such as plasticizers, which will result in changes in the stability of the elastomer. ASTM D 4289, Standard Test Method for Elastomer Compatibility of Lubricating Greases a n d Fluids, measures the extent to which a grease is compatible with various types of elastomeric or polymeric materials. The hardness of a standard-sized coupon of the elastomer to be tested is measured in the following way. Three standard sized coupons are cut out of a sheet of elastomer of specified thickness. The hardness of the stack of three coupons is measured with a standard indenter. Density and volume are also determined. The coupons are immersed in the grease of interest for 70 h at the test temperature that is designated for the elastomer of interest. After the immersion period, the elastomer is re-tested for changes in volume and hardness, to determine the extent of changes in these properties. The test method carries the notice that: "Some lubricant specifications m a y require different test conditions, such as longer d u r a t i o n s or lower or higher temperatures. In such instances, the repeatability and reproducibility values stated in Section 12 do not apply, and the user and supplier should agree on acceptable limits of precision." The method also states that "the volume and hardnesschange values determined by this test method do not duplicate similar changes that occur in elastomeric seals under actual service conditions." Viscosity
and Penetration
Effects
The viscosity of the base oil in a grease has a significant influence on the properties of a grease. ASTM D 445 (mentioned previously) describes the measurement of the base oil viscosity by determining the time required for the base oil to flow through a calibrated glass tube. The thickening agent in a grease uses surface tension and molecular attractions to help hold the grease's oil in the place in which it is supposed to provide protection. The thickening agent can consist of fibers, platelets, or spheres, which are insoluble or only slightly soluble in the grease. Thickeners can also be complex polymer-like chains such as complex polyureas. ASTM D 1092, Standard Test Method for Measuring Apparent Viscosity of Lubricating Greases, determines the viscosity of the grease over the range of —53 to 37.8°C. A sample of the grease is forced through a capillary, using a floating piston that is controlled by a hydraulic system. Viscosity is calculated from the flow rate and the force developed in the system. ASTM D 1263, Standard Test Method for Leakage Tendencies of Automotive Wheel Bearing Greases, measures the leakage of a grease from a modified front wheel h u b and spindle assembly. The wheel is rotated at a speed of 660 revolutions per min for 6 h at a temperature of 105°C. The extent of leakage is determined, and the condition of the bearing surface is noted at the end of the test. The method carries the notice that "it is not the equivalent of longtime service tests, n o r is it intended to distinguish between wheel bearing greases showing similar or borderline leakage." A related leakage test is ASTM D 4290, S t a n d a r d Test Method for Determining the Leakage Tendencies of Automotive Wheel Bearing Grease Under Accelerated Conditions. In this test, grease is placed in a modified version of an auto-
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mobile front wheel hub-spindle-bearings assembly. The hub is rotated at 1000 revolutions per min at the same time that the bearings are thrust-loaded to 111 N. The spindle temperature is maintained at 160°C for 20 h. Leakage of the grease, or the oil in the grease (or both), is measured. In addition, the condition of the bearing surface is noted. The method "is not the equivalent of long-term service tests." ASTM D 566, Standard Test Method for Dropping Point of Lubricating Grease, determines the temperature at which material falls through a hole in the bottom of a cup while a sample of grease is being heated. This type of test helps determine whether a particular grease is suitable for a given application. The following notice is given: "This test method is not recommended for use at bath temperatures above 288°C." ASTM D 2265, Standard Test Method for Dropping Point of Lubricating Grease Over Wide Temperature Range, measures the temperature at which the first drop of material falls from a test cup and reaches the bottom of a test tube (as corrected by a term related to the oven temperature). This test method can differentiate between different types of thickeners such as simple soaps, complex soaps, stearate soaps, and urea-based thickeners. The elapsed time and the temperature that the grease withstood in the dropping-point test indicate how well the thickener can hold the base oil within its matrix or within its complex lattice structure before the energy from the heat is able to weaken the internal energy of the lattice. The information derived from this test is used to maintain quality control of the grease and does not necessarily relate to performance of the grease. In ASTM D 217, Standard Test Method for Cone Penetration of Lubricating Grease, the consistency (or resistance to flow) of a grease is measured by observing the depth to which a cone penetrates the grease, using a device called a penetrometer. Properties of the grease may be influenced by the manipulation that the grease has experienced. Thus, the test method defines four test conditions: unworked penetration, worked penetration, prolonged worked penetration, and block penetration for grease that holds its shape. Unworked penetration means penetration is tested at 25°C without shearing the grease beforehand. The cone assembly of the penetrometer is released and is allowed to drop freely into the grease for 5 s. The worked condition represents properties after the grease has been sheared during 60 double strokes at 25°C in grease-working equipment. In the condition of penetration after prolonged work, the grease has experienced more than 60 double strokes in a grease worker at a temperature of 15-30°C, plus 60 additional strokes at 25°C. Penetration is then measured immediately. The block penetration test is used for a grease that is rigid enough to hold its shape. A cube of grease is cut from the bulk of the grease, and penetration is measured at a grease temperature of 25°C. In each of these ASTM D 217 methods, three determinations of penetration are conducted, and the results are averaged. This test method is not "considered suitable for the measurement of the consistency of petrolatums by penetration." ASTM D 1403, Standard Test method for Cone Penetration of Lubricating Grease Using One-Quarter and One-Half Scale Cone Equipment, determines cone penetration for small samples of grease, including both worked and unworked penetration. The test is conducted at 25°C, and a smaller cone than that described in ASTM D 217 is used in the test. The
test method indicates that "unworked penetrations do not generally represent the consistency of greases in use as effectively as do worked penetrations." Grease Performance Some of the issues related to grease performance include the ability of the grease to resist washout from water, effects of low-temperature operation on grease performance, wear protection provided, and determination of the useful life of the grease. ASTM D 1264, Standard Test Method for Determining the Water Washout Characteristics of Lubricating Greases, measures the resistance of a grease to being washed out of a bearing due to the presence of water. In the test, a grease is packed into a ball bearing. Water impinges on the bearing housing, and the amount of grease washed out is measured by weighing the bearing before and after the test. "No correlation with field service has been established." ASTM D 4049, Standard Test Method for Determining the Resistance of Lubricating Grease to Water Spray, also measures the influence of water on grease. In this test grease is applied within a scribed area on a panel. The panel is weighed, and water is sprayed onto the grease at a pressure of 276 kPa at 38°C. At the end of the test, any grease that is outside the scribed area is wiped off, and the panel is reweighed to determine the amount of grease that remains. ASTM D 1478 (reapproved 1997), Standard Test Method for Low-Temperature Torque of Ball Bearing Grease, determines the extent to which a grease retards the rotation of a slow-speed ball bearing by measuring starting and running torques at temperatures below —20°C. In this test a ball bearing is packed completely full with the test grease. The ambient temperature is lowered to the test temperature and held there for 2 h. At the end of this time the ball bearing is rotated, and the restraining force on the outer ring is measured (both at starting time and after one hour). The test procedure indicates that: "This test method has proved helpful in the selection of greases for low-powered mechanisms — . The suitability of this test method for other applications requiring different greases, speeds, and temperatures should be determined on an individual basis." ASTM D 4693, Standard Test Method for Low-Temperature Torque of Grease-Lubricated Wheel Bearings, measures the extent to which a grease retards the rotation of a specially-manufactured, spring-loaded, automotive-type wheel bearing assembly when subjected to low temperatures. In the test, a freshly stirred and worked sample of the grease is packed into the bearings of an automotive-type spindlebearings-hub assembly. The assembly is heated, then cooled and held at -40°C (or other desired temperature). The spindle is rotated at 1 revolution per min. The torque required to prevent rotation of the hub is measured. "It is the responsibility of the user to determine the correlation with other types of service." ASTM D 1831, Standard Test Method for Roll Stability of Lubricating Grease, is a low-speed, high-surface-area tumbling procedure at low shear. Cone penetration (ASTM D 1403) is first measured, then the grease is subjected to low shear for 2 h, and the cone penetration is again measured. This method is used to assess shear stability of the grease.
CHAPTER The following warning is listed, "Although this test method is widely used for specification purposes, the significance of the roll stability test has not been determined." ASTM D 2266, Standard Test Method for Wear Preventive Characteristics of Lubricating Grease (Four-Ball Method), measures the ability of a grease to resist wear in sliding steelon-steel contacts. It does not address wear in materials other than steel on steel, nor does it evaluate the extreme pressure characteristics of a grease. In this test a steel ball is rotated while in contact with three additional balls. The diameter of the wear scar on each of the non-rotating balls is measured. The test method indicates that "No correlation has been established between the four-ball wear test and field service." ASTM D 2509, Standard Test method for Measurement of Load-Carrying Capacity of Grease (Timken Method), uses a steel cup that rotates against a steel block at a rotation speed of 123.71 m per min. The m i n i m u m load needed to rupture the lubricant film that lies between the rotating cup and the block is determined. A determination is also made of the m a x i m u m safe load such that the lubricant film will not be ruptured by the rotating cup, and abrasion will not occur between the rotating cup and the block. The test procedure indicates that: "The test method is used widely for specification purposes and is used to differentiate between greases having low, medium, or high levels of extreme pressure characteristics. The results may not correlate with results from service." ASTM D 2596, Measurement of Extreme Pressure Properties of Lubricating Grease (Four-Ball Method), determines the load-carrying properties of lubricating grease. Two properties are measured: the diameter of the wear scar (for the case in which welding or seizure does not occur) and the load at which welding occurs. Two warnings are provided: 1) "The results do not necessarily correlate with results from service" and 2) "Lubricating greases that have as their fluid component a silicone, halogenated silicone, or a mixture comprising silicone fluid and petroleum oil, are not applicable to this method of test." ASTM D 3336, Standard Test Method for Life of Lubricating Greases in BeJl Bearings at Elevated Temperatures, evaluates the performance of lubricating greases operating at elevated temperatures (up to 371°C) in ball bearings, at a speed of 10 000 revolutions per min, but under light load. If the test temperature is 149°C or below, the test cycle is operated at test temperature for 21.5 h per day, with 2.5 h of shut-down per day in which heat is not applied. If the test temperature is r u n at a temperature greater than 149°C, the test is run for 20 h per day at the desired test temperature, with 4 h per day of shut-down in which n o heat is applied. Tests are conducted until failure or until a specified n u m b e r of hours is reached. The grease has failed the test when one of the following conditions has occurred: • Spindle output power increases to a value of three hundred percent above the steady state condition at test temperature. • An increase in temperature of 15°C over the test temperature occurs (but the daily start-up temperature rise is not to be construed as a test failure). • There is loading of the test bearing or belt slippage at startu p or during the test cycle. Hours to failure are plotted in terms of cumulative percentage of the tests conducted. Examples of these plots (known as WeibuU plots) are provided as part of the test-
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method description. F r o m such graphs, one can evaluate product life, for example, expected n u m b e r of hours before 10% of the greases will have failed, or hours to 50% failure. ASTM D 3337, Standard Test Method for Determining Life and Torque of Lubricating Greases in Small Ball Bearings, measures grease life and torque in a shorter time period than those tests that evaluate field performance. A single row of ball bearings, lubricated by the grease of interest, is rotated at high speed at the desired test temperature. The end of the test is determined when one of the following conditions occurs: • An instantaneous torque Ave times the minimum torque occurs. • A temperature 11 °C above the test temperature occurs at the outer race of the test bearing. • There is a noise level increase that persists for more than 1 min at start-up or when running at high speed. An example of the graphing procedure (WeibuU plot) for cumulative life of the grease is provided as part of the test procedure. ASTM D 3527, Standard Test Method for Life Performance of Automotive Wheel Bearing Grease, evaluates the life performance of a wheel-bearing grease at high temperature. A modified automobile front-wheel hub-spindle-bearing assembly is used in the test. Bearings are thrust-loaded to 111 Newtons, the h u b is rotated at a speed of 1000 revolutions per min, and the spindle temperature is held at 160°C for 20 h. The test is terminated when the pre-set torque limit is exceeded. The test "is not the equivalent of long-time service tests, nor is it intended to distinguish between the products having similar high-temperature performance properties." ASTM D 3704, Standard Test Method for Wear Preventive Properties of Lubricating Greases Using the (Falex) Block on Ring Test Machine in Oscillating Motion, uses a steel test ring oscillating against a steel test block. The test speed, load, angle of oscillation, time, hardness, and surface finish of the test specimen can be chosen to represent the field conditions of interest. The width of the wear scar on the test block is determined. The method evaluates the amount of wear protection provided by a given grease. "The user of this method should determine to his own satisfaction whether results of this test procedure correlate with field performance or other bench test machines." Fretting is surface damage that results from oscillation over a short distance. ASTM D 4170, Standard Test method for Fretting Wear Protection by Lubricating Greases, is a test method that documents the protection provided by a grease that is subjected to oscillation (30 Herz) between two ball bearings through a small arc (12°). The test lasts 22 h at a load of 2450 Newton. Mass loss of the bearings is determined. According to the method, "Test results do not necessarily correlate with results from other t5^es of service. It is the responsibility of the user to determine whether test results correlate with other types of service." Tables 6 and 7 show additional standard test methods related to greases.
GEAR AND AXLE LUBRICANTS Introduction to Gear Lubricants Gears must be designed to withstand high loads that are concentrated on gear surfaces. As a consequence, high tempera-
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tures may be generated at the point of contact. Despite the h a r s h environment, the gear lubricant must provide sufficient wear and extreme pressure protection to prevent material failure of the gears, and the gear lubricant must remain functional for extended periods of time. If the gear materials are not sufficiently robust and the lubricant is not sufficiently protective, irreversible damage may occur to the gear, which typically means that the equipment in which the gear is located will not perform as designed. Performance criteria for gear lubricants include such factors as ability to withstand the required loads and continuing ability to transmit power. Gear surface finish and gear tooth contour must not deteriorate. In addition, the properties of the gear lubricant must remain stable, including oxidative stability, anti-wear protection, maintenance of appropriate friction characteristics, low-temperature viscosity, corrosion protection, and resistance to foam formation. Various standard tests address these concerns. Standard Tests for Gear Lubricants ASTM D 5579, S t a n d a r d Test Method for Evaluating the Thermal Stability of Manual Transmission Lubricants in a Cyclic Durability Test, was described previously in the section on Transmission Lubricants. This test evaluates a gear lubricant's performance, since a transmission cycles through a continuing sequence of gear shifts. Shifter fork wear is measured, and deterioration of shift performance is the criterion for termination of the test. ASTM D 5760, Standard Specification for Performance of Manual Transmission Gear Lubricants, was also described in the section on manual transmissions. Performance requirements include a maximum of n o more than 100% viscosity increase and limits on the generation of insoluble contaminants in the lubricant. Elastomers must not deteriorate excessively, and scuffing should not occur at a specified high load. ASTM D 2670, Standard Test Method for Measuring Wear Properties of Fluid Lubricants (Falex Pin and Vee Block Method), evaluates the wear-resistant properties of fluid lubricants. In this test, a rod is rotated while being squeezed between two V-shaped blocks. Load is applied to the V blocks, and the amount of wear is determined. "The user of this test m e t h o d should determine to his or h e r own satisfaction whether results of this test procedure correlate with field performance or other bench test machines. If the test conditions are changed, wear values may change and relative ratings of fluids may be different." The load-carrying capacity of a lubricant is the amount of load a lubricant can withstand, without rupturing the oil film. ASTM D 2782, Standard Test Method for Measurement of Extreme-Pressure Properties of Lubricating Fluids (Timken Method), measures this property. In this test a steel test cup is rotated against a steel test block. The rotation speed is 123.71 m per min. Fluid samples are pre-heated to 37.8°C. Two measurements are obtained: the minimum load required to rupture the lubricant film and the maximum load at which the rotating cup will not rupture the fluid film and cause scoring or seizure. The u s e r is notified: "This test method is used widely for the determination of extreme pressure properties for specification purposes. Users are cautioned to carefully consider the precision and bias statements herein when establishing specification limits."
ASTM D 5182, Standard Test Method for Evaluating the Scuffing Load Capacity of Oils (FZG Visual Method), was described under transmission fluids. In this test, a gear test device is rotated at increasing loads until scuffing occurs. The following statement is provided, "The test method is limited by the capabilities of the equipment (test rig and gears), and the performance observed may not directly relate to scuffing performance observed with spiral bevel on hjrpoid gearing. It is also limited to discriminating between oils with mild EP additives or less." ASTM D 4172, Standard Test Method for Wear Preventive Characteristics of Lubricating Fluid (Four-BeJl Method), rotates one steel ball against three stationary steel balls that are immersed in the test lubricant. The test method measures the extent of the wear scar on the three stationairy balls. "The user of this test method should determine to his own satisfaction whether results of this test procedure correlate with field performance or other bench test machines." ASTM D 2783, S t a n d a r d Test Method for ExtremePressure Properties of Lubricating Fluids (Four-Ball Method), rotates one steel ball against three stationary steel balls. Lubricant covers the stationary balls. Increasing load is applied until welding occurs. The following notice is provided, "The user of this test method should determine to his own satisfaction whether results of this test procedure correlate with field performance or other bench test machines." An alternative test for measurement of extreme pressure properties is ASTM D 3233, Standard Test Methods for Measurement of Extreme Pressure Properties of Fluid Lubricants (Falex Pin and Vee Block Methods). A steel journal is rotated at 290 revolutions per min against two stationary V blocks that are immersed in the lubricant sample. Increasing load is applied, a n d the load at which failure occurs is noted. "The user should establish any correlation between results by either method and service performance." An additional topic of interest is whether a given gear oil is compatible with the elastomers it touches. ASTM D 5662, Standard Test Method for Determining Automotive Gear Oil Compatibility with Typical Oil Seal Elastomers, addresses this issue. In ASTM D 5662, an elastomer test specimen is exposed to the gear oil for 240 h. The test temperature depends on the composition of the elastomer to be tested. Nitrile is tested at 100°C. Polyacrylate and fluoroelastomers are tested at 150°C. Hardness, volume changes, and elongation are measured and compared to the start-of-test measurements of these same properties. The method states: "This test method addresses only those failures caused by excessive elastomer hardening, elongation loss, and volume swell and attempts to determine the likelihood that a n oil might cause premature sealing system failures in field use." Another important characteristic of a gear lubricant is the ability to release water rapidly, in case water should enter the gear lubricant. ASTM D 2711, Standard Test Method for Demulsibility Characteristics of Lubricating Oils, measures the ability of water and oil to separate from each other. A sample of the test oil and a designated amount of water are stirred together for 5 min at a temperature of 82°C. The extent of separation of water and oil is measured after 5 h. ASTM D 892, which measures the amount of foam that can form and the stability of the foam, was described previously, in the engine oil section. ASTM D 130, which measures the
CHAPTER extent of corrosion of a copper strip, was described in the section on transmission fluids. Test methods for gear oils are also available from CRC (Coordinating Research Council). The CRC L-37 method is a 24-hour dynamometer test that is conducted at high torque and low speed and that assesses gear distress. CRC L-42 is a 24-hour dynamometer test incorporating high speed shock loading, designed to measure resistance to gear scoring. CRC L-33 is a 7-day motored rust test in which geeirs are exposed to humidity in an oven. CRC L-60-1 is a test for oxidation and deposits, using motored gears over a period of 50 h. Viscosity characteristics of axle lubricants are measured according to ASTM D 445, Standard Test Method for Kinematic Viscosity of Transpsirent eind Opaque Liquids (the Calculation of Dynamic Viscosity), as described under the section on engine oils. Low temperature viscosity is measured using ASTM D 2983, Standard Test method for Low-Temperature Viscosity of Automotive Fluid Lubricants Measured by Brookfield Viscometer. In this test, viscosity is measured in the temperature range from 5 to —40°C, using a rotating spindle. Axle Lubricants An additional area of concern related to gear lubricants is the topic of axle lubricants. Requirements for axle lubricants are described in the American Petroleum Institute API-GL-5 requirements for gear lubricants. However, two of the tests listed in API-GL-5 (CRC L-37 and CRC L-42) are not required for axle lubricants. Desirable properties for axle lubricants include viscosity in an appropriate range (for example, near 14 or 15 centistokes at 100°C). Performance of an axle lubricant must not degrade excessively during service. The axle lubricant must not damage metal components, nor should the metal components accelerate an axle lubricant's degradation. In addition, axle lubricants must not damage seals. Desirable axle-lubricant attributes include fuel efficiency, thermal stability, long life, and appropriate limited-slip differential performance. Referenced documents related to geeir and axle lubricant test methods are provided at the end of this chapter. Tables 8 and 9.
BRAKE FLUIDS Brake fluid typically behaves like a hydraulic fluid, since fluid power is used to apply force to the brakes, therefore generating enough friction to slow and stop the forward motion of the vehicle. Fifty years ago, if water got into brakes during a heavy rain, the brakes might not work properly, and an accident might result. Today, with better technology, such occurrences are rare. However, someone who is servicing a brake system must make sure to instcill the appropriate brake fluid for that brake, otherwise there is risk that the brakes will not function as they should. Two types of braking systems are typically used in vehicles: drum brakes and disk brakes. In a d r u m brake, a drum rotates with the wheel. Curved brake shoes are inside the drum, separated by a slight air gap from the drum. When a driver steps on the brake, brake fluid, acting like a hydraulic fluid to apply force where it is needed, is sent to the braking system
17: A UTOMOTIVE
LUBRICANTS
483
such that the brake shoes are pressed onto the rotating drum. The friction of the shoe against the d r u m stops the wheel. A disk brake has a metal disk, and flat shoes or pads are pressed against the disk to provide the force to stop the vehicle, using the force supplied by the hydraulic action of the brake fluid. Thus, the motion of a braking mechanism is controlled by a hydraulic system that includes a pump, a fluid, and lines (tubes) that transmit the hydraulic force to the brake components. The brake fluid must not react with components of the brake system, and it must be able to withstand wide variations in temperature while still performing effectively. Requirements for the brake fluid are described in the 2001 SAE Handbook, Standards Development Program, Volume 2, Parts and Components and on-Highway Vehicles (Part I), "Motor Vehicle Brake Fluid-SAE J1703 Jan95", page 25.49, and "Borate Ester Based Brake Fluids-SAE J1704 Jan97", page 25.59. SAE J1703 and SAE J1704 specify such measurements as boiling point, fluid stability, viscosity, reserve alkalinity, corrosion resistance, low-temperature fluidity, evaporation, water tolerance, low temperature stability, resistance to oxidation, and effect on rubber materials such as seals. A number of standard ASTM methods related to these tests are listed at the end of this chapter.
SOLID LUBRICANTS Solid lubricants are used where it is important for the lubricant to stay in place. Solid lubriccints are typically composed of a solid, a binder, and additives such as corrosion inhibitors or solvents. Examples of c o m m o n solid lubricants include molybdenum disulfide, graphite, a n d polytetrafluoroethylene (PTFE) or other fluorine-containing polymers. Molybden u m disulfide and graphite have a chemical structure that is like flat plates such that an upper plate can easily slip eilong the surface of the next plate beneath it. PTFE has a chemical structure that typically does not attract or hold a variety of common fluids such as water or oil, as witnessed by the fact that the fluid tends to bead u p on the PTFE surface, rather than spreading over the surface. In contrast, if a drop of oil touches an iron surface, the oil spreads over the surface. Solid lubricants typically have a temperature range over which they are effective. Above the optimum reinge, they may degrade chemically or physically. For example, according to information in the Tribology Data Handbook, molybdenum disulfide has a n u p p e r t e m p e r a t u r e limit in the range of 400°C, but it can withstand high loads [16]. Graphite can withstand 650°C and moderate loads. If PTFE (polytetrafluoroethylene) is used in a high-temperature application, care must be taken to ensure that the temp e r a t u r e does not exceed the point at which PTFE is no longer stable. On the other hand, use of low-friction materials is an advantage with regard to fuel conservation in automotive vehicles. ASTM D 2714, Standard Test Method for Calibration and Operation of the Falex Block-on-Ring Friction and Wear Testing Machine, provides a test method for determination of friction coefficient of a solid lubricant. Friction coefficient is defined as the ratio of the friction force, F, that resists movement to the normal force, N, that presses the two bodies to-
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HANDBOOK
gether. A steel test ring rotates against a steel test block. The specimen assembly is immersed in the lubricant. A normEil load of 65 kg is applied. The velocity of the test ring is 7.9 m/min. The test method contains the warning that "The user of this test method should determine to his or her own satisfaction whether results of this test procedure correlate with field performance or other bench test machines. If the test conditions are changed, wear values can change and relative ratings can be different."
AUTOMOTIVE AIR CONDITIONING LUBRICANTS In the 1930s, chlorine-containing refrigerants were introduced, since they were considered to be safer than other refrigerant fluids that had been in use prior to 1930 [17]. However, in the late 1970s it was observed that, in some places, the protective ozone layer had been depleted as m u c h as 30 or 40% at an altitude of 40 km. As a consequence, the United States, Canada, Norway, Sweden, and others passed legislation prohibiting the use of these chlorofluorocarbon refrigerants in spray cans [17]. In 1985 investigators found that there had been a 40% loss of protective ozone during springtime in the antarctic. It was also found that a chlorofluorocarbon could be thousands of times worse than carbon dioxide in promoting global warming. Thus, a search was initiated to find a replacement for the chlorofluorocarbons used in automotive air-conditioning systems [17]. One of the authors of this chapter was involved in the effort to find a replacement for the refrigerants that had been used in automotive air-conditioning systems. Automotive air conditioning systems require a lubricant. However, n o standard tests h a d been created that were specific for this application. It was necessary to address several issues relative to the search for these chlorine-containing refrigerants. • Can a lubricant be identified that is compatible with the refrigerant? • Will the lubricant that is compatible with the refrigerant have enough lubricity to provide long life to the pump? • Will it be necessary to modify the composition of seals in the air conditioning unit to ensure that the seals are not degraded by the new refrigerant and its associated lubricant? • Which refrigeration-system materials should be used, such that they will not be degraded by the refrigerant or its lubricant? Extensive testing showed that, for an air conditioning compressor to work properly, the air conditioning fluid must be able to transport the air conditioning lubricant to the working components of the p u m p . In other words, there had to be a chemical compatibility between the lubricant and the air conditioning medium, in addition to the normal lubrication requirements such as providing a protective lubricating film and inhibiting corrosion. Thus, in order to have a viable airconditioning system, it was not sufficient to remove the original air-conditioning fluid and put another air-conditioning fluid into the system. This example indicates that whenever changes are made to any component of a lubrication system, testing must be performed to ensure that no incompatibilities have arisen.
ADDITIONAL FLUIDS THAT EXHIBIT LUBRICATING PROPERTIES Several additional fluids within a vehicle (coolant and fuel) must provide some lubrication, even though lubrication is not their primeiry function. Engine coolant must not allow excessive corrosion to occur in the coolant system. The coolant typically incorporates a mixture of ethylene glycol a n d water (to lower the freezing point, so that the coolant remains fluid) and corrosion inhibitors (to reduce corrosion of the coolant system). If engine coolant leaks from the coolant system and enters the engine oil, the oil properties can degrade, which can damage or destroy an engine if too much coolant has entered the engine oil. Thus, standard ASTM tests associated with effects of the coolant on the engine lubrication system are related to detection of coolant (ethylene glycol) in engine oil. ASTM D 4291, Standard Test Method for Trace Ethylene Glycol in Used Engine Oil, measures the amount ethylene glycol in the range of 5-200 parts per million by mass. A sample of oil is mixed with water, the ethylene glycol migrates to the water phase, and the amount of glycol present is determined by gas chromatography. ASTM D 2982, Standard Test Methods for Detecting Glycol-Based Antifreeze in Used Lubricating Oils, measures the glycol content of lubricating oils by extracting the glycol from the oil using an acid solution. The glycol is oxidized to form formaldehyde, which is then reacted to form a colored solution that can be analyzed colorimetrically. The fuel supply system must not wear out and must not be excessively corroded or eroded by the fuel. Corrosiveness, lubricity, and material compatibility properties of a fuel such as gasoline can differ significantly from those of an oxygenated fuel such as alcohol, as shown in Figs. 2 and 3. The lower volatility and higher viscosity of diesel fuel, in comparison to gasoline, can provide greater film thickness for lubrication of injectors. Standard tests that describe relationships between the fuel system and the engine lubrication system include measurement of fuel in oil and fuel economy. In S t a n d a r d Test Method for Gasoline Diluent in Used Gasoline Engine Oils by Distillation, ASTM D 322, used engine oil (which may be contaminated with fuel) is mixed with water. Several vaporizations are conducted to separate the fuel from the oil and from the water. The volume of fuel that was extracted is measured. In ASTM D 3525, Standard Test Method for Gasohne Diluent in Used Gasoline Engine Oils by Gas Chromatography, a known amount of internal standard is placed in the engine oil. The amount of fluid boiled off at a temperature below that of the internal standard is used to obtain a quantitative assessment of how much fuel was in the oil.
SUMMARY AND FUTURE DIRECTIONS As illustrated by these examples, automotive lubrication encompasses a wide range of fluid properties, fluid functions, and component composition and design within a vehicle. The great diversity of vehicle lubrication requirements mandates that many different kinds of lubricants, specifications, and standards Eire needed.
CHAPTER 17: AUTOMOTIVE LUBRICANTS Various issues arise on a continuing basis, with regard to automotive lubricants: • Can further improvements be m a d e in vehicle fuel economy by developing more efficient engine oils, transmission fluids, cixle lubricants, etc.? • Can changes in automotive design be implemented such that power eind performance are not compromised, but energy consumption during operation is reduced? • Can suitable alternative automotive power sources (fuel cells, hydrogen, batteries) be developed? • To w h a t extent can vehicle weight be reduced without compromising occupant safety? • Can lighter metals such as aluminum be used in engines to replace iron, and thus reduce energy consumption, since less energy will be required to keep the vehicle in motion? • What types of lubricants are effective for use with alternative materials? • Can the amount of lubriccint required for a given application be reduced, so that oil reserves will last longer? • Can the useful life of ciny given automotive lubricant be increased? • To what extent ccin automotive lubricants be recycled? • Are biodegradable automotive lubricants feasible?
485
Such issues will receive increased attention in the future. Displacing current power sources (gasoline and diesel engines), however, will be difficult to accomplish, since current systems have worked exceedingly effectively for m a n y decades. Incremented improvements have occurred, but revolutionary changes in design have not yet been sufficiently problem-free to reduce or eliminate use of the intemEd combustion engine. It will be left to future generations to find viable alternatives.
LIST OF REFERENCED DOCUMENTS A List of Referenced Documents is included with most of the standard test methods described previously. The referenced documents are listed below (Tables 2 through 10), so that the reader will have a comprehensive tabulation of standard test methods associated with each category of automotive lubricant. In addition, abbreviations a n d terms are defined in Table 11. Acknowledgment The a u t h o r s t h a n k Brent Calcut, Robert Olree, Jill Cummings, Robert Stockwell, Jim Linden, James Spearot, Colem a n Jones, John Flaherty, and Jeff Knight of General Motors for their contributions, suggestions, and corrections.
TABLE 2—Referenced documents in ASTM standard test methods related to engine oil. ASTM No.
C 1109 D 16 D56 D86 D91 D92 D93 D97 D117 D 129 D 130 D 140 D 156 D235 D240
D287 D323 D341 D381 D445 D446 D482 D524 D525 D613 D664 D850 D873 D892 D893 D974 D 1078 D1093 D1133 D1193 D 1217 D1250
Test Method or Document Title
Analysis of Aqueous Leachiates from Nuclear Waste Materials Using Inductively Coupled Plasma-Atomic Emission Spectrometry Definitions of Terms Relating to Paint, Varnish, Lacquer, and Related Products Flash Point by Tag Closed Tester Distillation of Petroleum Products Acidity of Hydrocarbon Liquids and Their Distillation Residues Flash and Fire Points by Cleveland Open Cup Flash Point by Penske-Martens Closed Tester Pour Point of Petroleum Products Guide to Sampling Test Methods, Standard Practices, and Guides for Electrical Insulating Oils of Petroleum Origin Sulfur in Petroleum Products (General Bomb Method) Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test Practice for Sampling Bituminous Materials Saybolt Color of Petroleum Products (Saybolt Chromometer Method) Specification for Mineral Spirits (Petroleum Spirits) (Hydrocarbon Dry Cleaning Solvent) Heat of Combustion of Liquid HydrocEirbon Fuels by Bomb Calorimeter API Gravity of Crude Petroleum and Petroleum Products (Hydrometer Method) Vapor Pressure of Petroleum Products (Reid Method) Viscosity-Temperature Charts for Liquid Petroleum Products Existent Gum in Fuels by Jet Evaporation BCinematic Viscosity of Transparent and Opaque Liquids (the Calculation of Dynamic Viscosity) Standard Specifications and Operating Instructions for Glass Capillary Kinematic Viscometers Ash from Petroleum Products Ramsbottom Carbon Residue of Petroleum Products Oxidation Stability of Gasoline (Induction Period Method) Cetane Number of Diesel Fuel Oil Acid Number of Petroleum Products by Potentiometric Titration Acidity of Hydrocarbon Liquids and Their Distillation Residues Oxidation Stability of Aviation Fuels (Potential Residue Method) Foaming Characteristics of Lubricating Oils Insolubles in Used Lubricating Oils Acid and Base Number by Color-Indicator Titration Distillation Range of Volatile Organic Liquids Acidity of Hydrocarbon Liquids and Their Distillation Residues Kauri-Butanol Value of Hydrocarbon Solvents Specification for Reagent Water Density and Relative Density (Specific Gravity) of Liquids by Bingham Pycnometer Standard Guide for Petroleum Measurement Tables (continues)
486 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK TABLE 2—(continued). ASTM No. D 1266 D 1298
D1310 D 1319 D 1353 D1480 D1481 D1552 D2170 D2171 D2422 D2500 D2509 D2622 D2699 D2700 D2709 D2782 D2887 D2896 D2982 D3120 D3231 D3237 D3244 D3338 D3339 D3525 D3606 D3829 D3941 D4052 D4057 D4175 D4177 D4206 D4294 D4307 D4485 D4626 D4628 D4636 D4683 D4684 D4737 D4739 D4741 D4863 D4927 D4951 D5119 D5133 D5134 D5185 D5186 D5290 D5302 D5480 D5481 D5533 D5800 D5844
Test Method or Document Title Sulfur in Petroleum Products (Lamp Method) Density, Relative Density (Specific Gravity), or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method Flash Point and Fire Point of Liquids by Tag Open-Cup Apparatus Hydrocarbon Types in Liquid Petroleum Products by Fluorescent Indicator Adsorption Nonvolatile Matter in Volatile Solvents for Use in Paint, Lacquer, and Related Products Density and Relative Density (Specific Gravity) of Viscous Materials by Bingham Pycnometer Density and Relative Density (Specific Gravity) of Viscous Materials by Lipkin Bicapillary Pycnometer Determination of Additive Elements in Lubricating Oils by Inductively Coupled Plasma Atomic Emission Spectrometry Kinematic Viscosity of Asphalts (Bitumens) Viscosity of Asphalts by Vacuum Capillary Viscometer Standard Classification of Industrial Fluid Lubricants by Viscosity System Cloud Point of Petroleum Products Measurement of Load-Carrying Capacity of Lubricating Grease (Timken Method) Sulfur in Petroleum Products by X-Ray Spectrometry Research Octane Number of Spark-Ignition Engine Fuel Motor Octane Number of Spark-Ignition Fuel Water and Sediment in Distillate Fuels by Centrifuge Measurement of Extreme-Pressure Properties of Lubricating Fluids (Timken Method) Boiling Range Distribution of Petroleum Fractions by Gas Chromatography Base Number of Petroleum Products by Potentiometric Perchloric Acid Titration Detecting Glycol-Base Antifreeze in Used Lubricating Oils Trace Quantities of Sulfur in Light Liquid Petroleum Hydrocarbons by Oxidative Microcoulometry Phosphorus in Gasoline Lead in Gasoline by Atomic Absorption Spectrometry Standard Practice for Utilization of Test Data to Determine Conformance with Specifications Estimation of Heat of Combustion of Aviation Fuels Acid Number of Petroleum Products by Semi-Micro Color Indicator Titration Gasoline Diluent in Used Gasoline Engine Oils by Gas Chromatography Determination of Benzene and Toluene in Finished Motor and Aviation Gasoline by Gas Chromatography Predicting the Borderline Pumping Temperature of Engine Oils Flash Point by the Equilibrium Method with a Closed-Cup Apparatus Density and Relative Density of Liquids by Digital Density Meter Standard Practice for Manual Sampling of Petroleum and Petroleum Products Standard Terminology Relating to Petroleum, Petroleum Products, and Lubricants Standard Practice for Automatic Sampling of Petroleum and Petroleum Products Sustained Burning of Liquid Mixtures by the Setaflash Tester (Open Cup) Sulfur in Petroleum Products by Energy-Dispersive X-Ray Fluorescence Spectroscopy Determination of Additive Elements in Lubricating Oils by Inductively Coupled Plasma Atomic Emission Spectrometry Standard Specification for Performance of Engine Oils Calculation of Gas Chromatographic Response Factors Analysis of Barium, Calcium, Magnesium, and Zinc in Unused Lubricating Oils by Atomic Absorption Spectrometry Corrosiveness and Oxidation Stability of Hydraulic Oils, Aircraft Turbine Engine Lubricants, and Other Highly Refined Oils Measuring Viscosity at High Shear Rate and High Temperature by Tapered Bearing Simulator Determination of Yield Stress and Apparent Viscosity of Engine Oils at Low Temperature Calculated Cetane Index by Four Variable Equation Base Number Determination by Potentiometric Titration Measuring Viscosity at High Temperature and High Shear Rate by Tapered-Plug Viscometer Determination of Lubricity of Two-Stroke-Cycle Gasoline Engine Lubricants Elemental Analysis of Lubricant and Additive Components-Bcirium, Calcium, Phosphorus, Sulfur, and Zinc by Wavelength-Dispersive X-Ray Fluorescence Spectroscopy Determination of Additive Elements in Lubricating Oils by Inductively Coupled Plasma Atomic Emission Spectrometry Test Method for Evaluation of Automotive Engine Oils in the CRC L-38 Spark-Ignition Engine Low Temperature, Low Shear Rate, Viscosity/Temperature Dependence of Lubricating Oils Using a Temperature Scanning Technique Detailed Anedysis of Petroleum Naphthas Through n-Nonane by Capillary Gas Chromatography Determination of Additive Elements, Wear Metals and Contaminants in Used Lubricating Oils by Inductively-Coupled Plasma Atomic Emission Spectrometry Determination of the Aromatic Content and polynuclear Content of Diesel Fuels and Aviation Turbine Fuels by Supercritical Fluid Chromatography Measurement of Oil Consumption, Piston Deposits, and Wear in a Heavy-Duty High-Speed Diesel Engine-NTC-400 Procedure Evaluation of Automotive Engine Oils for Inhibition of Deposit F o r m a t i o n and Wear in a Spark-Ignition Internal Combustion Engine Fueled with Gasoline and Operated Under Low-Temperature, Light-Duty Conditions Engine Oil Volatility by Gas Chromatography Measuring Apparent Viscosity at High-Temperature and High-Shear Rate by Multicell Capillary Viscometer Evaluation of Automotive Engine Oils in the Sequence HIE, Spark-Ignition Engine Evaporation Loss of Lubricating Oils by the Noack Method Evaluation of Automotive Engine Oils for Inhibition of Rusting (Sequence IID) (continues)
CHAPTER 17: A UTOMOTIVE LUBRICANTS ASTM No. D D D D D D D
5862 5966 5967 5968 6074 6082 6202
D D D D D D D E E E E E E E
6278 6299 6300 6335 6417 6483 6557 1 29 77 128 135 178 191
E 270 E 300 E 344 E 355 E 380 E 473 E 502 E 594 E 691 E 1119 E 1500 G 40
487
TABLE 2—(continued). Test Method or Document Title Evaluation of Engine Oils in Two-Stroke Cycle Turbo-Supercharged 6V92TA Diesel Engine Evaluation of Diesel Engine Oils for Roller Follower Wear in Light-Duty Diesel Engine EvcJuation of Diesel Engine Oils in the T-8 Diesel Engine Evaluation of Corrosiveness of Diesel Engine Oil Guide for Characterizing Hydrocarbon Lubricant Base Oils High Temperature Foaming Characteristics of Lubricating Oils Measurement of the Effects of Automotive Engine Oils on the Fuel Economy of Passenger Cars and Light-Duty Trucks in the Sequence VIA Spark Ignition Engine Shear Stability of Polymer Containing Fluids Using a European Diesel Injector Apparatus Applying Statistical Quality Assurance Techniques to Evaluate Anals^ical Measurement System Performance Determination of Precision and Bias Data for Use in Test Methods for Petroleum Products and Lubricants Determination of High Temperature Deposits by Thermo-Oxidation Engine Oil Simulation Test Estimation of Engine Oil Volatility by Capillary Gas Chromatography Evaluation of Diesel Engine Oils in the T-9 Diesel Engine Evaluation of Rust Preventative Characteristics of Automotive Engine Oils Specification for ASTM Thermometers Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications Inspection and Verification of Thermometers Maximum Pore Diameter and Permeability of Rigid, Porous Filters for Laboratory Use Terminology Relating to Analytical Chemistry for Metals, Ores, and Related Materials Practice for Dealing With Outlying Observations Specification for Apparatus for Microdetermination of Carbon a n d Hydrogen in Organic and Organo-Metallic Compounds Terminology Relating to Liquid Penetrant Examination Practice for Sampling Industrial Chemicals Terminology Relating to Thermometry and Hydrometry Gas Chromatographic Terms and Relationships Use of the International System of Units (SI) (The Modernized Metric System) Terminology Relating to Thermal Analysis Selection and Use of ASTM Standards for the Determination of Flash Point of Chemicals by the Closed Cup Method Testing Flame Ionization Detectors Used in Gas Chromatography Conducting an Inter-Laboratory Study to Determine the Precision of a Test Method Specification for Industrial Grade Ethylene Glycol Installing Fused Silica Open Tubular Capillary Columns in Gas Chromatographs Terminology Relating to Wear and Erosion
488 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK TABLE 3—Referenced documents to non-ASTM methods for engine oils. Organization ANSI (American National Standards Institute) API (American Petroleum Institute) Standard Chemical Manufacturers Association Code Coordinating Research Council (CRC) Motor Rating Method Manuals Deutsches Institut fiir Normunge Federal Test Method Standard General Motors Corporation Engineering Standard lEEE/ASTM (Institute of Electrical and Electronic Engineers) Institute of Petroleum ISO
Military Specification SAE (Society of Automotive Engineers) Standards
U.S. Federal Test Method Standards
Test Method or Document Title Standard MC96.1 Temperature Measurement—Thermocouples API 1509 Engine Service Classification and Guide to Crankcase Oil Selection CMA Petroleum Additives Product Approval Code of Practice CRC Rust Rating (CRC Manual No. 7) CRC Sludge Rating Manual (CRC Manual No. 12) CRC Varnish Rating Manual (CRC Manual No. 14) CRC Techniques for Valve Rating (CRC Manual No. 16) DIN 51.581 Noack Evaporative Test DIN 1725 Specification for Aluminum Alloys DIN 12785 Specification for Glass Thermometers No. 791b, Lubricants Liquid Fuels and Related Products, Methods of Testing No. 791C, Method 3470 No. 791, Method 5308.7 GM9099-P Engine Oil Filterability Test (EOFT) SI-10 Standard for Use of International System of Units (SI): The M o d e m Metric System IP 17 Colour by the Lovibond Tintometer IP 139 Neutralization Number by Color Indicator Titration IP 146 Test for Foaming Characteristics of Lubricating Oil Guide 25-General Requirements for the Calibration and Testing Laboratories Guide 34 Quality Systems Guidelines for the Production of Reference Materials Guide 35 Certification of Reference Material-General and Statistical Principles 3104, Petroleum Products—Transparent and Opaque Liquids-Determination of Kinematic Viscosity and Calculation of Dynamic Viscosity ISO 3105, Glass CapHlaiy Kinematic Viscometers—Specification and Operating Instructions ISO 3696 Water for AnalyticEil Laboratory Use—Specification and Test Methods ISO 9000 Quality Management and Quality Assurance Standards-Guidelines for Selection and Use MIL-L-2104, Lubricating Oil, Internal Combustion Engine, Combat/Tactical Services J183, Engine Oil Performance and Engine Service Classification (Other Than "Energy Conserving") J254, Instrumentation and Techniques for Exhaust Gas Emissions Measurement J300, Engine Oil Classification J304, Engine Oil Tests J726, Air Cleaner Test Code (Includes Piezometer Ring Specifications) J1423, Passenger Car and Light-Duty Truck Energy-Conserving Engine Oil Classification J1995, Engine Power Test Code - Spark Ignition and Compression Ignition—Gross Power Rating No. 791, Method 5308.7 Corrosiveness and Oxidative Stability of Light Oils (Metal Squares) No. 791b Lubricants Liquid Fuels and Related Products; Methods of Testing
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TABLE 4—Referenced documents in ASTM standard test methods for transmission fluids. ASTM No.
A108 A 109 A 240/A 240 M B224 D91 D130 D235 D396 D412 D445 D471 D512 D516 D664 D892 D893 D975 D 1193 D1217 D 1401 D1480 D 1481 D 1655 D 1838 D2170 D2171 D2240 D2422 D3603 D4057 D4175 D4177 D5182 D5579 D5662 D5704 D5760 D6074 El E77 E527 G40 ASTM Adjuncts
Test Method or Document Title
Specification for Steel Bars, Carbon, Cold-Finished, Standard Quality Specification for Steel, Strip, Carbon, Cold-Rolled Specification for Heat-Resisting Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels Classification of Coppers Precipitation N u m b e r of Lubricating Oils Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test Specification for Mineral Spirits (Petroleum Spirits) (Hydrocarbon Dry Cleaning Solvent) Specification for Fuel Oils Vulcanized Rubber and Thermoplastic Rubbers and Thermoplastic Elastomers—Tension Kinematic Viscosity of Transparent and Opaque Liquids (and the Calculation of Dynamic Viscosity) Test Method for Rubber Property-Effect of Liquids Chloride Ion in Water Sulfate Ion in Water Acid Number of Petroleum Products by Potentiometiic Titration Foaming Characteristics of Lubricating Oils Insolubles in Used Lubricating Oils Specification for Diesel Fuel Oils Specification for Reagent Water Density and Relative Density (Specific Gravity) of Liquids by Bingham Pycnometer Water Separability of Petroleum Oils and Synthetic Fluids Density and Relative Density (Specific Gravity) of Viscous Materials by Bingham Bicapillary Pycnometer Density and Relative Density (Specific Gravity) of Viscous Materials by Lipkin Bicapillary Pycnometer Specification for Aviation Turbine Fuels Copper Strip Corrosion by Liquefied Petroleum (LP) Gases Test Method for JCinematic Viscosity of Asphalts (Bitumens) Viscosity of Asphalts by Vacuum Capillary Viscometer Rubber Property-Durometer Hardness Classification of Industrial Fluid Lubricants by Viscosity System Rust-Preventing Characteristics of Steam Turbine Oil in the Presence of Water (Horizontal Disk Method) Manual Sampling of Petroleum and Petroleum Products Terminology Relating to Petroleum, Petroleum Products, and Lubricants Automatic Sampling of Petroleum and Petroleum Products Evaluating the Scuffing (Scoring) Load Capacity of Oils (FZG Visual Method) Evaluating the Thermal Stability of Manual Transmission Lubricants in a Cyclic Durability Test Determining Automotive Gear Oil Compatibility with Typical Oil Seal Elastomers Evaluation of the Thermal and Oxidative Stability of Lubricants Used for Manual Transmissions and Final Drive Axles Performance of Manual Transmission Gear Lubricants Guide for Characterizing Hydrocarbon Lubricant Base Oils Specification for ASTM Thermometers Inspection and Verification of Thermometers Practice for Numbering Metals and Alloys Terminology Relating to Erosion and Wear ASTM Copper Strip Corrosion Standard Engineering Drawings PCN ADJD5704
490 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 5—^Additional standards or specifications related to transmission fluids. Organization
Test Method or Document Title
ANSI/ISA (Instrument Society of America)-S7.3 DIN51354Teill Federal Specification
Quality Stsindard for Instrument Air FZG Zahnrad Verspannungs Priif Maschine-Allgemeine Arbeitsgrundlagen RRS-366 (Method 5329 of W-L-791e) Test Sieve Sizes QQ-S-698 Steel Sheet and Strip, Low Carbon PD-680 Standard Solvent JAN-H-792 Operations of Humidity Cabinet Method 3430.2 Compatibility Characteristics of Universal Gear Lubricants Method 3440.1 Storage Solubility Characteristics of Universal Gear Lubricants Storage Solubility Characteristics of Universal Gear Lubricants Specifications-IP Standard Thermometers, Appendix A Specifications for IP Standard Reference Liquids, Appendix B BS 871 Specification for abrasive papers and cloths BS 970: Part 1: Carbon and Carbon Manganese Steels Including Free Cutting Steels Guide 25—General Requirements for the Calibration and Testing Laboratories 3104 Petroleum Products-Transparent and Opaque Liquids-Determination of Kinematic and Calculation of Dynamic Viscosity 3105 Glass Capillary Kinematic Viscometers-Specification a n d Operating Instructions 3696 Water for Analytical Laboratory Use-Specification and Test Methods 4021 Hydraulic Fluid Power-Particulate Contamination Analysis-Extraction of Fluid Samples from Lines of an Operating System 4406 Hydraulic Fluid Power-Fluids-Method for Coding Level of Contamination by Solids Particles 9000 Quality Management a n d Quality Assurance Standards-Guidelines for Selection and Use Multi-Purpose (GO-H) MIL-L-2105 and 2105D, Lubricating Oil, Gear, Multipurpose MIL-C-15074C, Corrosion Preventive Compound Finger Print Remover MIL-C-16173D Corrosion Preventive Compound, Solvent Compound Cutback, Cold Application J308, Axle and Manual Transmission Lubricants J 405 Chemical Composition of SAE Wrought Stainless Steels 1009C Tee Reducer, Bulkhead on Side, Flareless Tube
Federal Specifications Standard No. 79IC Federal Standard No. 791C, Method 3440.1 Institute of Petroleum
ISO
Mack Trucks Oil, Gear Military Standard
SAE Standard
CHAPTER 17: AUTOMOTIVE LUBRICANTS
491
Table 6—Referenced documents in ASTM standard tests for greases. ASTM No.
Test Method or Document Title
D88 D95 D97 D130 D 156 D217 D235 A 240/A 240M
Saybolt Viscosity Water in Petroleum Products and Bituminous Materials by Distillation Pour Point for Petroleum Oils Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test Saybolt Color of Petroleum Products (Saybolt Chromometer Method) Cone Penetration of Lubricating Grease Specifications for Mineral Spirits (Petroleum Spirits) Hydrocarbon Dry Cleaning Solvent Specification for Heat-Resisting Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels Rubber Products—Chemical Analysis Specification for Acetone Vulcanized Rubber and Thermoplastic Elastomers-Tension BCinematic Viscosity of Transparent and Opaque Liquids (and the Calculation of Dynamic Viscosity) Specifications and Operating Instructions for Glass Capillary Kinematic Viscometers Rubber Property—Effect of Liquids Sediment in Crude Oils and Fuel Oils by the Extraction Method Dropping Point of Lubricating Grease Specification for Isopropyl Alcohol Rockwell Hardness of Plastics and Electrical Insulating Materials Cone Penetration of Petrolatum Evaporation Loss of Lubricating Greases and Oils Distillation Range of Volatile Organic Liquids Specification for Reagent Water Density and Relative Density (Specific Gravity) of Liquids by Bingham Pycnometer Flash Point and Fire Point of Liquids by Tag Open-Cup Apparatus Nonvolatile Matter in Volatile Solvents for Use in Paints, Varnish, Lacquer, and Related Products Cone Penetration of Lubricating Grease Using One-Quarter and One-Half Scale Cone Equipment Density and Relative Density (Specific Gravity) of Viscous Materials by Bingham Pycnometer Density and Relative Density (Specific Gravity) of Viscous Materieds by Lipkin Bicapillary Pycnometer BCinematic Viscosity of Asphalts (Bitumens) Viscosity of Asphalts by Vacuum Capillary Viscometer Rubber Property-Durometer Hardness Dropping Point of Lubricating Grease Over Wide Temperature Range Cloud Point of Petroleum Oils Evaporation Loss of Lubricating Greases Over Wide Temperature Range Calibration and Operation of the Falex Block-on-Ring Friction and Wear Testing Machine Practice for Rubber-Materials, Equipment, and Procedures for Mixing Standard C o m p o u n d s and Preparing Standardized Vulcanized Sheets Practice for Rubber-Preparation of Pieces for Test Purposes from Products Practice for Utilization of Test Data to Determine Conformance with Specifications Life Performance of Automotive Wheel Bearing Grease Terminology Relating to Petroleum, Petroleum Products, and Lubricants Elastomer Compatibility of Lubricating Greases and Fluids Determining the Leakage Tendencies of Automotive Wheel Bearing Grease under Accelerated Conditions Low-Temperature Torque of Grease-Lubricated Wheel Bearings Classification of and Specification for Automotive Service Greases Oxidation Induction Time of Lubricating Greases by Pressure Differential Scanning Calorimetry Determining Extreme Pressure Properties of Lubricating Greases Using a High-Frequency, Linear Oscillation (SRV) Test Machine Measuring Friction and Wear Properties of Lubricating Grease Using a High-Frequency, Linear Oscillation (SRV) Test Machine Corrosion-Preventive Properties of Lubricating Greases in Presence of Dilute Synthetic Sea-Water Environments Guide for Characterizing Hydrocarbon Lubricant Base Oils Corrosion-Preventive Properties of Lubricating Greases Under Dynamic Wet Conditions (Emcor Test) Oil Separation from Lubricating Grease (Conical Sieve Method) Practice for Evaluating Compatibility of Binary Mixtures of Lubricating Greases Specification for ASTM Thermometers Specification for Wire-Cloth Sieves for Testing Purposes Inspection and Verification of Thermometers Calibration of Thermocouples by Comparison Techniques Temperature-Electromotive Force (EMF) Tables for Standardized Thermocouples Practice for Preparation and Use of Freezing Point Reference Baths Specification for Sheathed Base-Metal Thermocouple Materials Specification for Metal-Sheathed Base-Metal Thermocouples Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method Terminology Related to Corrosion and Corrosion Testing ASTM Adjunct: Copper Strip Corrosion Standard, Adjunct PCN 12-401300-00 Test Methods for Rating Motor, Diesel, and Aviation Fuels; Motor Fuels (Section I), Reference Materials and Blending Accessories (Annex 2), Reference Materials (A2.7), and Table 32; (Specification for ASTM Knock Test Reference Fuel, n-heptane)
D297 D329 D412 D445 D446 D471 D473 D566 D770 D785 D937 D972 D 1078 D 1193 D 1217 D 1310 D 1353 D1403 D 1480 D 1481 D2170 D2171 D2240 D2265 D2500 D2595 D2714 D3182 D3183 D3244 D3527 D4175 D4289 D4290 D4693 D4950 D5483 D5706 D5707 D5969 D6074 D6138 D6184 D6185 E 1 E 11 E77 E220 E230 E563 E585 E608 E691 G 15 Adjunct ASTM
492 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK TABLE 7—^Additional standards or specifications related to greases. Organization
Test Method or Document Title
ABMA (American Bearing Manufacturers Association) AFBMA (Anti-Friction Bearing Manufacturers Association) ANSI (American National Standards Institute)
Standard 4. Tolerance Definitions and Gaging Practices for Ball and Roller Bearings
British Standards Institute Compressed Gas Association Federal Test Method IP (Institute of Petroleum)
ISO (American National Standards Institute)
Military Standard NLGI (National Lubricating Grease Institute) SAE (Soc. Automotive Engineers) Standard
U.S. Air Force
Standard 19 1974 (available from ANSI, American National Standards Institute, B.3.19-1975) Specification B3.12 for Metal Balls ANSI/AFBMA Standard 20-1987 Radial Bearings of Ball, Cyhndrical, Roller, and Spherical-Roller Type-Metric Designs (AFBMA Code 20BCO2JO) BS970: 1983 Part I, Section S Booklets G-4 and G-4-1 Standard 791C, Method 3603.5, Swelling of Synthetic Rubbers Specification for Standard IP Thermometers Institute of Petroleum 50, Method of Testing for Cone Penetration of Lubricating Grease Guide 25-General Requirements for the Calibration and Testing Laboratories 3104 Petroleum Products-Transparent and Opaque Liquids-Determination of Kinematic Viscosity and Calculation of Dynamic Viscosity 3105 Glass Capillary Kinematic Viscosmeters-Specification and Operating Instructions 3696 Water for Analytical Laboratory Use-Specification and Test Methods 9000 Quality M a n a g e m e n t and Quality Assurance Standards-Guidelines for Selection and Use MIL-G-10924F, Specification for Automotive and Artillery Grease Consistency Specification AMS (Aerospace Materials Specification) 3217A Standard Elastomer Stocks-Test Slabs AMS 3217/2A Test Slabs, Acrylonitrile AMS 3217/3A Test Slabs, Chloroprene (CR)-65-75 Butadiene (NBR-L)-Low Acrylonitrile, 65-75 Specification 539, Specification Bulletin for Standard Elastomer Stocks
CHAPTER 17: AUTOMOTIVE LUBRICANTS
493
TABLE 8—^Referenced documents in ASTM stzmdards related to gear lubricants. Test Method or Document Title
ASTM No.
B16 D96 D 130 D235 D329 D341 D396 D412 D446 D471 D484 D892 D975 D 1193 D 1217 D 1480 D 1481 D1655 D1796 D1838 D2170 D2171 D2240 D2266 D2509 D2670 D2783 D4175 D5182 D5579 D5662 D5704 D5760 D6074 E 1 E 128 G40 ASTM Adjuncts
Specification for Free-Cutting Brass Rod, Bar, and Shapes for Use in Screw Machines Water and Sediment in Crude Oil by Centrifuge Method (Field Procedure) Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test Specification for Mineral Spirits (Petroleum Spirits) (Hydrocarbon Drycleaning Solvents) Specification for Acetone Viscosity-Temperature Charts for Liquid Petroleum Products Specification for Fuel Oils Vulcanized Rubber and Thermoplastic Rubbers and Thermoplastic Elastomers—Tension Specifications and Operating Instructions for Glass Capillary Kinematic Viscometers Rubber Property-Effect of Liquids Specification for Hydrocarbon Drycleaning Solvents Foaming Characteristics of Lubricating Oils Specification for Diesel Fuel Oils Specification for Reagent Water Density and Relative Density (Specific Gravity) of Liquids by Bingham Pycnometer Density and Relative Density (Specific Gravity) of Viscous Materials by Bingham Pycnometer Density and Relative Density (Specific Gravity) of Viscous Materials by Lipkin Bicapillary Pycnometer Specification for Aviation Turbine Fuels Water and Sediment in Fuel Oils by Centrifuge Method (Laboratory Procedure) Copper Strip Corrosion by Liquefied Petroleum (LP) Gases Viscosity of Asphalts (Bitumens) Viscosity of Asphalts by Vacuum Capillary Viscometer Rubber Property-Durometer Hardness Wear Preventive Characteristics of Lubricating Fluid (Four-Ball Method) Measurement of Load-Carrying Capacity of Extreme Lubricating Grease (Timken Method) Wear Properties of Fluid Lubricants (Falex Pin and Vee Block Method) Measurement of Extreme Pressure Properties of Lubricating Fluids (Four-Ball Method) Terminology Relating to Petroleum, Petroleum Products, and Lubricants Evaluating the Scuffing (Scoring) Load Capacity of Oils Evaluating the Thermal Stability of Manual Transmission Lubricants In a Cyclic Durability Test Determining Automotive Gear Oil Compatibility with Typical Oil Seal Elastomers Evaluation of the Thermal and Oxidative Stability of Lubricating Oils Used for Manual Transmissions and Final Drive Axles Specification for Performance of Manual Transmission Gear Lubricants Guide for Characterizing Hydrocarbon Lubricant Base Oils Specification for ASTM Thermometers Maximum Pore Diameter and Permeability of Rigid, Porous Filters for Laboratory Use Terminology Relating to Erosion and Wesir Three Glossy Prints of Test Blocks Showing Scars (D 2780) ASTM Copper Strip Corrosion Protection
TABLE 9—Additional standards related to gear lubricants. Organization and Test No.
ANSI (American National Standards Institute) B3.12 DIN (Deutsch Industries Norm) 51 354 Teil (Part) 1 Federal Standard No 791 C Institute of Petroleum Standards IP 17 ISO
Mack Trucks, Inc. Military Standard SAE (Society of Automotive Engineers)
Test Method or Document Title
Specification for Metal Balls FZG Zahnrad Verspannungs Pruef Maschine—AUgemeine Arbeitsgrundlagen Method 3430.2 Compatibility Characteristics of Universal Gear Lubricants Method 3440.1 Storage Solubility Characteristics of Universal Gear Lubricants Color measured using the Lovibond Tintometer Guide 25-General Requirements for the Calibration and Testing Laboratories 3104 Petroleum Products-Transparent and Opaque Liquids-Determination of Kinematic Viscosity and Calculation of Dynamic Viscosity 3105 Glass JCinematic Capillary Viscometers-Specifications and Operating Instructions 3696 Water for Analyticeil Laboratory Use-Specification and Test Methods 9000 Quality Management and Quality Assurance Standards-Guidelines for Selection and Use GO-H, Oil, Gear: Multi Purpose MIL-L-2105, Lubricating Oil, Gear, Multipurpose J308 Axle and Manual Transmission Lubricants
494 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 10—ASTM standard test methods related to brake fluids. D91 D260 D344 D395 D412 D445 D664 D746 D 1120 D1209 D 1364 D 1415 D2240 D3182 D3185 El E 145 E 260-73
American Automobile Manufacturers Association Standard Recommended Practice for General Gas Chromatography Procedure Method of Test for Relative Dry Hiding Power of Paints Aerospace Materials Specification Rubber Properties in Tension K n e m a t i c Viscosity of Transparent and Opaque Liquids (and the Calculation of Dynamic Viscosity) Neutralization Number of Potentiometric Titration Brittleness Method of Test for Boiling Point of Engine Antifreezes Color of Clear Liquids (Platinum-Cobalt Pigments) Water in Volatile Solvents (Fischer Reagent Titration Method) Method of Test for International Hardness of Vulcanized Natural Rubber and Synthetic Rubbers Rubber Property—Durometer Hardness R e c o m m e n d e d Practice for Rubber-Materials, E q u i p m e n t and Procedures for Mixing S t a n d a r d Compounds a n d Preparing Standard Vulcanized Sheets Methods for Rubber-Evaluation of SBR (Styrene-Butadiene Rubber) including Mixtures with Oil Specification for ASTM Thermometers Specification for Gravity-Convection and Forced-Ventilation Ovens Standard Recommended Practice for General Gas Chromatography Procedure
TABLE 11—Definition of terms and abbreviations used in the text. AAMA AMS ANSI API ASTM CEC CF, CF-2, CF-4, CG, CG-4 CR CRC CRC Press CVT cSt DIN EMA EOAT EOFT EOWTT EP FZG GF-1, GF-2, GF-3, GF-4 ILSAC ISA ISO JAMA MIL NBR-L PC psi rpm SAE SF, SG, SH, SJ, SL TEOST ZDP or ZDDP
American Automobile Msinufacturers Association Aerospace Materials Specification American National Standards Institute American Petroleum Institute American Society for Testing and Materials Coordinating European Council "C" represents a category for engine oils that are used in compression ignition (diesel) engines. F, CF-2, CF-4, G, and G-4 represent a particular oil quality. Chloroprene Rubber Coordinating Research Council Chemical Rubber Company Press Continuously Variable Transmission Centistokes, a measure of viscosity Deutsch Industries Norm (German industry standard) Engine Manufacturers Association Engine Oil Aeration Test Engine Oil Filterability Test Engine Oil Water Tolerance Test Extreme Pressure (used for lubricants that experience high pressure or high temperature) Fahrzeug (German word for motor vehicle) Terms represent an engine oil quality that is defined by a series of tests that the oil must pass. GF indicates the oils are used with gasoline fuel. GF-1 corresponds to API-SH quality engine oil. International Lubricant Standardization and Approval Committee Instrument Society of America Represents a standard from the American National Standsirds Institute Japan Automobile Manufacturers Association Military specification Acrylonitrile-Butadiene Rubber with Low Acrylonitrile Pre chamber, associated with engine oil tests for diesel engines Pounds per square inch. 550 psi equals 3.8 million N/m^ Revolutions per min Society of Automotive Engineers "S" represents a category for engine oils that are used in spark ignition engines. Each successive final letter represents a more recent category. (SL is more recent than the others listed.) Thermo Oxidation Engine Oil Simulation Test Zinc dialkyl-dithio phosphate, an antioxidant and an anti-wear agent used as an additive in engine oil
CHAPTER 17: AUTOMOTIVE LUBRICANTS REFERENCES [1] Younggren, P. J. and Schwartz, S. E., "The Effects of Trip Length and Oil Type (Synthetic Versus Mineral Oil) on Engine Damage and Engine-Oil Degradation in a Driving Test of a Vehicle with a 5.7L Engine," SAE International Paper No. 932838, Society of Automotive Engineers, Warrendale, PA, 1993. [2] Schwartz, S. E., "A Comparison of Engine Oil Viscosity, Emulsion Formation, and Chemical Changes for M85 and GasolineFueled Vehicles in Short-Trip Service," SAE International Paper No. 922297, Society of Automotive Engineers, 1992. [3] McGeehan, J. A., Alexander, III, W., Ziemer, J. N., Roby, S. H., and Graham, J. P., "The Pivotal Role of Crankcase Oil in Preventing Soot Wear and Extending Filter Life in Low Emission Diesel Engines," SAE International Paper No. 1999-01-1525, Society of Automotive Engineers, 1999. [4] Machida, H. and Karachi, N., "Prototype Design and Testing of a Half Toroidal CVT," SAE International Paper No. 900552, Society of Automotive Engineers, WEirrendale, PA, 1990. [5] Hata, H. and Tsobuchi, T., "Molecular Structure of Traction Fluids in Relation to Traction Properties," Tribology Letters, Vol. 5, 1998, pp. 69-74. [6] CuUey, S. A. and McDonnell, T. F., "The Impact of Passenger Car Motor Oil Phosphorus Levels on Engine Durability, Oil Degradation, and Exhaust Emissions in a Field Trial," SAE International Paper No. 952344, Society of Automotive Engineers, Warrendale, PA, 1995. [7] CuUey, S. A., McDonnell, T. F., Ball, D. J., Kirby, C. W., and Hawes, S. W., "The Impact of Passenger Car Motor Oil Phosphorus Levels on Automotive Emissions Control Systems," SAE International Paper No. 961898, Society of Automotive Engineers, Wcirrendale, PA, 1996. [8] Drury, C. and Whitehouse, S., "The Effect of Lubricant Phosphorus Level on Exhaust Emissions in a Field Trial of Gasoline
495
Engine Vehicles," SAE International Paper No. 940745, Society of Automotive Engineers, Warrendale, PA, 1994. [9] Ueda, F., Sugiyama, S., Arimura, K., Hamaguchi, S., and Akiyama, K., "Engine Oil Additive Effects on Deactivation of Monolithic Three-Way Catalysts and Oxygen Sensors," SAE International Paper No. 940746, Society of Automotive Engineers, Warrendale, PA, 1994. [10] Deane, B. C, Choi, E., and Crosthwait, K., "The Relationship of Base Oil Volatility to Oil Loss in Automotive Application," NPRA Paper No. AM 97-40, National Petroleum Refiners Association, Washington, DC, 1997. [11] Choi, E. and Deane, B., "Base Stock Volatility and Oil Consumption," NPRA Paper No. AM-98-47, National Petroleum Refiners Association, Washington, DC, 1998. [12] Manni, M., Gommellini, C, and Sabbioni, G., "Effect of Physical Characteristics of Lubricating Oils on Emissions, Fuel Economy and Oil Consumption in a Light Duty Diesel Engine," SAE International Paper No. 952552, Society for Automotive Engineers, Warrendale, PA, 1995. [13] Rounds, F. G., "Effect of Lubricant Additives on the Prowear Characteristics of Synthetic Diesel Soots," Lubrication Engineering, Vol. 43, No. 4, 1987. [14] Schwartz, S. E., "A Model for the Loss of Oxidative Stability of Engine Oil During Long-Trip Service, Part I: Theoretical Considerations," STLE (Society of Tribologists and Lubrication Engineers) Tribology Transactions, Vol. 35, No. 2, 1992, pp. 235-244. [15] Schwartz, S. E., "A Model for the Loss of Oxidative Stability of Engine Oil During Long-Trip Service, Part II: Vehicle Measurements," STLE Tribology Transactions, Vol. 35, No. 2, 1992, pp. 307-315. [16] Gresham, R. M., Tribology Data Handbook, CRC (Chemical Rubber Company) Press, Boca Raton, FL, 1997, pp. 600-607. [17] Crutzen, P. J. and Ramanathan, V., "The Ascent of Atmospheric Sciences," Science, Vol. 290, pp. 299-304.
MNL37-EB/Jun. 2003
Metalworking and Machining Fluids Syed Q. A. Rizvi:\
METALWORKING IS THE PROCESS OF CONVERTING BULK METAL into
a component or apart and primarily involves two types of operations: those that produce metal debris and those that produce no debris. The former type is classified as metal removal operations and the latter type is classified as metal forming operations. Cutting and grinding are examples of the first type and drawing, stamping, and bending are examples of the second type. All metalworking operations involve bringing two solids, a tool and a work piece, together to create a new part or a shape. The process involves high friction, high temperatures, and tool wear; and it is the job of the lubricant, or the metalworking fluid, to control them. Metalworking fluids accomplish this by providing cooling, lubrication, and protection against corrosion. Therefore, they improve the efficiency of the operation, and hence increase productivity.
FUNDAMENTAL CONCEPTS Lubrication Friction is c o m m o n to all surfaces in contact and is usually represented by the friction coefficient, signified by the symbol /JL. The coefficient of friction (/x) is a unit-less ratio that equals 0/N, where 0 represents the frictional force experienced by the two contacting bodies in motion and N represents the n o r m a l force pressing the same two bodies together. Its value ranges from zero (0) to one (1). The higher the value, the higher the frictional force or the resistance of the contacting bodies towards motion. Under boundary lubrication conditions, ^l approaches 1. Minimizing friction is one of the fundamental functions of a lubricant. If friction is not controlled, it can lead to wear and surface damage, and ultimately to catastrophic failure of the equipment. Because of a generally direct correlation between friction a n d wear [1,2], p r o p e r lubrication of the equipment is important if its integrity is to be preserved over its designated lifetime. However, it is important to note that the correlation between friction and wear is a function of the system and is not always direct [3]. In lubricant-related applications, we are concerned with three types of friction, that is, the solid friction, fluid friction, and internal friction [12]. The main function of a lubricant is to minimize solid friction, which it achieves by forming a fluid film between two contacting (often metal) surfaces. Usually, a fluid's internal friction is not of any major concern ' Research and Development Manager, Lubricant Additives Division, King Industries, Inc., Science Road, Norwalk, CT 06852.
except at very low temperatures. At these temperatures, the lubricant gains viscosity, w h i c h can interfere with the smooth operation of the equipment. Internal friction is important when considering a lubricant's intrinsic properties, such as viscosity (ASTM D 445, D 2983, D 3829, D 4684, and D 5293) and pour point (ASTM D 97). All metal surfaces, irrespective of their finish, contain ridges, valleys, asperities, and depressions. When two metal surfaces come in contact, solid friction, sometimes called static or adhesive friction, ensues and the surfaces undergo adhesion and cold welding. The strength of such an association depends upon the hardness of the materials, the cleanliness of the surfaces, and the electronic structure of the metals as related to their tendency to form metal-metal solutions, or alloys [4,5]. As soon as the surfaces start to move, kinetic friction comes into play. Kinetic friction results from the plowing of the asperities of one surface across the other surface, plastic deformation or elastic hysteresis, and wear debris getting lodged between the moving surfaces [5]. Static friction is larger than kinetic friction. This is because of the extent and the duration of contact. Friction is also related to the tjTDe of motion of the bodies. Sliding motion, for instance, leads to higher friction than rolling motion and hence results in more wear. This is primarily a consequence of the larger contact zone of the sliding surfaces. Applications that encounter metal-to-metal contact involve either no lubrication (dry), solid lubrication, or liquid lubrication. Wear resistance in equipment designed to operate without lubrication is introduced by the use of low-wear metals and/or surface treatment, such as hardening or coating. Solid lubrication is c o m m o n where liquid lubrication is u n w a n t e d or is difficult because of equipment design or extremely high operating temperatures. Such lubricants, exemplified by graphite and molybdenum disulfide, have multilayered structures with low shear strength in some directions. Movement in such directions is therefore facilitated. These lubricants are applied to equipment in a number of ways such as b o n d e d dry films, sputtered films, or loose flakes. In metalworking applications, we are primarily interested in liquid lubrication. The purpose is either to separate contacting surfaces by introducing a liquid film or to minimize wear by delivering wear control additives to surfaces in contact. The relationship of coefficient of friction (/x) and the relationship of oil film thickness (p) to lubricant viscosity (Z), equipment speed (N), and equipment load, or pressure (P), are graphically represented in Fig. 1, Part A [6]. This graph is commonly referted to as the Streibeck Curve. As one can see, the
497 Copyright'
2003 by A S I M International
www.astm.org
498 MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
Lubricant Firm
W^Y^/.iA^^.i'V^A^lU*^
FLUID-FILIVI LUBRICATION Surfaces well separated by the bulk lubricartt film
LubricantFilm MIXED-FILM LUBRICATION Both the bulk lubricant and the boundary film play a role
BOUNDARY LUBRICATION Performance essentially depends upon the quality of the boundary flim (A)
RELATIONSHIP OF VISCOSITY {2), SPEED (W), AND LOAD {P) TO FRICTION AND FILM THICKNESS
{By GRAPHIC REPRESENTATION OF LUBRICANT FILM THICKNESS IN DIFFERENT LUBRICATION REGIMES
FIG. 1—Lubrication regimes in mechanical equipment.
ratio of ZN/P relates directly to oil film thickness (p), but inversely to coefficient of friction (/U,). This implies that high lubricant viscosity (Z), high equipment speed (N), and low equipment load (P) will allow the formation of a thick lubricant film, 2ind hence the equipment will encounter little or no friction. Conversely, low lubricant viscosity, low equipment speed, and high equipment load will create a situation where film thickness will be inappropriate and the equipment will encounter high friction. The initial drop in the coefficient of friction, shown in the figure, while moving from fluid-film to mixed-film lubrication, reflects a decrease in viscous drag due to a decrease in lubricant viscosity and or equipment speed. Mechanical equipment experiences three types of lubrication environments: fluid-film, boundary, and mixed-film. These are depicted in Fig. 1, Part B [6]. Fluid-film lubrication, also known as hydrodynamic lubrication, is of the most desirable type. This type of lubrication depends upon the viscosity of the lubricant and is effective only when the load in the contact zone is low. Under these circumstances, the sliding surfaces are separated by a lubricant film several times the thickness (2-100 ;nm) of the surface roughness. Another type of hydrodynamic lubrication, referred to as elasto-hydrodynamic lubrication or EHD [7-9], commonly occurs in parts with rolling motion. In this type of lubrication, the lubricant is exposed to high contact pressures and undergoes a large viscosity increase. See Viscosity section and Fig. 4. This results in an extremely rigid lubricant film (0.01-5.0 /xm thick), which causes elastic deformation of the surfaces in the lubricating zone. Elasto-hydrodynamic lubri-
cation is depicted in Fig. 2. The crosshatched area in the figure indicates solid lubricant film that causes elastic deformation of the metal surfaces. Boundary lubrication represents the other extreme of the lubricating environment. Under this kind of lubrication, high loads and very slow speeds produce extreme pressures that can lead to a lack of effective lubrication, and hence promote maximum metal-to-metal contact. The lubricant film thickness in this type of lubrication is in the order of 0.0-2.0 fjun. If not controlled, the resulting dry metallic friction will cause extensive wear and ultimately total seizure. Reactive chemicals, called antiwear and extreme pressure agents, provide protection in this kind of lubricating environment. Mixedfilm lubrication falls between the two extremes mentioned above and contains the characteristics of both the fluid-film lubrication and the boundary lubrication. There are regions of no metal-to-metal contact and of metal-to-metal contact. Metalworking operations are designed to have at least some metal-to-metal contact. This is because total separation of surfaces will make the process inefficient. For example, in metal removal operations, separation will lead to poor tool-work piece contact, which will interfere in chip formation. In metal forming operations, this will lead to low friction and hence loss of control. This implies that in metalworking, the lubricant must be designed to perform in mixed-film and boundary lubrication regimes. In general, metal removal operations require boundary lubrication and metal forming operations require mixed-film lubrication. Metal removal operations, therefore, involve little or no lu-
CHAPTER
Mutually Flattened Areas Under Load
18: METALWORKING
Metal Strip
AND MACHINING
FLUIDS
499
Elastlcally Deformed Areas
FIG. 2—Elasto-hydrodynamic lubrication.
bricant film and metal forming operations involve a fairly thin lubricant film, as shown in Fig. 1, Part B. Lubricant film in metal forming operations is of two types: wedge and squeeze. Wedge type films occur when two nonparallel surfaces in motion converge and squeeze type films occur when parallel surfaces in motion come together [10]. In forming operations, wedge type films are more common than the squeeze type. As the lubricant enters the converging zone, there is an increase in pressure that causes an increase in lubricant viscosity. This is a consequence of the lubricant molecules coming closer. The increase in viscosity changes the nature of lubrication in the contact zone from being hydrodynamic to elasto-hydrodynamic. As the lubricant exits, the gap widens and there is a decrease in viscosity cind an increase in velocity. This causes a reversal from elasto-hydrodynamic lubrication to hydrodynamic lubrication (Fig. 1, Part A). The result is separation of the surfaces and minimal metal-to-metal contact. A d r o p in viscosity is a consequence of the widening gap that permits lubricant molecules to move away from one another. The quality of the lubricant film determines the metal forming efficiency. The higher the operation speed, the higher the film thickness; similarly, the higher the viscosity, the higher the film thickness (Fig. 1). Both these conditions therefore force the converging surfaces to move away from each other (the gap widens), which improves the operation efficiency. For example, in rolling operations, the speed of strip determines the film thickness. Since temperature and pressure also impact viscosity, hence lubricant film thickness, these factors also affect the efficiency of the metal forming operations. Different base fluids have differing responses to temperature and pressure, and therefore their properties can influence the properties of the metal forming lubricants as well. Naphthenic basestocks are usually preferred over paraffinic basestocks. This is because they not only experience a faster viscosity decrease with an increase in temperat u r e (have a lower viscosity index), they also u n d e r g o a greater viscosity increase with a n increase in pressure. See Figs. 3 and 4. These factors make lubricants derived from these basestocks extremely effective in high-pressure applications, such as rolling. As mentioned earlier, squeeze films form when two parallel surfaces converge, a situation that occurs in operations
t (A M
E E
Naphthenic Oil
(0
o o (A
Paraffinic Oil m
E c
'2. Temperature ( C) FIG. 3—Viscosity-temperature relationship.
such as upsetting. The film thickness increases due to fast approaching surfaces and the lubricant viscosity increases because of an increase in pressure. Both factors hinder lubricant flow out of the contact zone, which results in a squeeze film. Deformation of the work piece during the operation results in a flatter surface, thereby making the squeeze film thinner. The thickness of the squeeze films depends on lubricant viscosity, speed, and load (force) in the same way as that of the wedge films, but for the reasons described. Lubrication regime defines a lubricant's ability to support load (modify friction and reduce metal transfer) and can be determined by taking into account film thickness and combined surface roughness. The ratio of the two, represented by A, equals film thickness (p) divided by the combined roughness of the surfaces (S), or p/5. The combined roughness is the roughness amplitude of two surfaces relative to their average levels. The value of A equal to 0.5 implies boundary regime; the value of A between 0.5 and 3.0 implies mixed film regime;
500 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
Solid
Paraffinic Oil Naphthenic Oil 0
20
40
60
80
100
120
Pressure (1,000 psi)
140 •
FIG. 4—Viscosity-pressure relationship.
and the value of A greater than 3 implies fluid-iilm or hydrodynamic regime [10]. Unlike metal removal operations that primarily involve boundary lubrication regime, metal forming operations start in the boundary regime but move into mixed-film and hydrodynamic regimes as the operation picks up speed. Lubricant properties, such as viscosity and the presence of film-forming additives play a role in boundary and mixed-film regimes. Film-forming additives include friction modifiers, antiwear agents, and extreme pressure agents. These additives protect surfaces against extensive friction and wear. They do so by forming low shear protective films either via physical adsorption, chemical adsorption, or through chemical reaction. See the Film-forming Agents section for further details. Viscosity The role of viscosity in forming effective lubricating films makes it one of the most important properties of a fluid [11]. Viscosity is defined as a fluid's resistance to flow and is primarily a consequence of the internal friction of the fluid [12]. The viscosity is of two tjfpes: absolute or dynamic viscosity and kinematic viscosity. Absolute viscosity is commonly expressed in poise (P) or Pascal-seconds (Pa-s) [1 Pa-s =10^ centipoise (cP)]. Kinematic viscosity is measured under the accelerating influence of gravity and equals absolute viscosity of the fluid divided by its density. It is expressed in centistokes (cSt), or square meters per second (m^/s) [1 m^/s =10* centistokes (cSt)]. Absolute viscosity is often not used for metalworking fluids, but kinematic viscosity is. Previously used units of kinematic viscosity, Saybolt Universal Seconds
(SUS), and Saybolt Furol Seconds (SFS) are now obsolete. The methodology to convert these units into Stokes is described in the ASTM Standard D 2161 [13]. A number of factors affect viscosity. These include temperature, pressure, time, and the structures of organic compounds present in the fluid and their response to shear forces [12]. The two variables whose effect primarily pertains to metalworking fluids eire temperature and pressure. Mineral oils and synthetic oils both experience a viscosity decrease with an increase in temperature. In general, the viscosity of naphthenic oils decreases at faster rate than that of the paraffinic oils (Fig. 3). The viscosity-temperature relationship of a fluid is expressed in terms of its viscosity index (VI) (DIN ISO 2909 [14], ASTM D 2270 [13]). Viscosity index, based upon kinematic viscosities at 40°C and 100°C, when originally introduced had a scale ranging from 0 (zero) to 100. The two oils that were used to establish this scale no longer exist, but their viscosities are still used to compute viscosity indexes. Many modem oils have viscosity indexes that extend well beyond 100. Such oils are made by isomerizing the low VI hydrocarbons in oil to high VI hydrocarbons, via a process called hydrocracking. The process involves treating the oil with hydrogen in the presence of a proprietary catalyst at high temperatures. Oils whose viscosities have a high sensitivity to temperature have a low VI, and oils whose viscosities have a low sensitivity to temperature have a high VI. Unlike temperature, which has an inverse relationship to viscosity, pressure bears a direct relationship. That is, when pressure increases viscosity increases (Fig. 4). However, the effect, which is a function of the chemical structure of the fluid, is not as dramatic as in the case of temperature [12].
CHAPTER 18: METALWORKING Lubricant viscosity is measured by the use of instruments, called viscometers. Of many types that are available for this purpose, capillary viscometers are used to measure kinematic viscosity. The lubricant is allowed to flow down a capillary and the viscosity is calculated by taking into account flow time and the viscometer constemt. This constant is supplied by the standardization laboratory along with the instrument constant. Dynamic viscosity is determined by multiplying kinematic viscosity with fluid density. This is described in the ASTM D 445 procedure [13]. Viscosity is critical to determining the quality of the lubricant film. In metal-forming applications, it determines the effectiveness of the film in separating the tool from the work piece and therefore controlling friction and wear. Metal removal operations, on the other hand, have diverse lubrication needs, and hence optimum lubricant viscosity must be estimated for each operation. This is accomplished by considering the ability of the lubricant to enter and remain in the contact zone, the durability of the lubricant film, the desired rate of spreading, and its cooling capability. These parameters are evaluated by the use of film-strength tests, such as the Timken Test (ASTM D 2782), the Four-Ball Tests (ASTM D 2783 and D 4172), and the SAE #1 Test.
FLUID CLASSIFICATION BASED ON BASE FLUID Metalworking and machining fluids fall under three general classes: oil-based, water-based, and solid suspensions [15]. Oil-based fluids are mineral oils with or without additives. Water-based fluids, on the other hand, are micellar solutions of oil and/or additives in water. These fluids are of three types: soluble oils, semisynthetic fluids, and synthetic fluids. The worldwide use of oil-based fluids is estimated at 45% and that of water-based fluids is estimated at 53%; with synthetics, semisynthetics, and soluble oils, having a share of 4%, 16%, and 33%, respectively. Solid suspensions make up the rest. The ASTM Standard D 2881 [15] and the European Standard DIN 51 385 [14] classify metalworking lubricants according to their nature and functions. Classes of metalworking fluids are described below. The essential difference between various classes is that as one moves from straight oil to semisynthetic fluid, the amount of water increases and the amount of oil decreases. See Table 9. • Straight Oil (not dilutable)—not supportive of microbial growth. • "Soluble" Oil (oil-based emulsifiable concentrate, diluted with water at the point of use)—more likely to support bacterial growth. It primarily contains oil, emulsifiers, and other additives. When diluted (typical dilution ratio is between 5 to 20 parts of water to 1 part of concentrate) it produces an emulsion. Oil provides lubrication and corrosion protection and water provides cooling. In some grinding applications, the dilution ratio can be as high as 200:1. • Synthetic Fluid (water-based solution concentrate; diluted prior to use)—more likely to support fungal growth. It contains no oil but is a solution of corrosion inhibitors and friction reducing additives in water. These fluids provide cooling and corrosion protection, but their lubrication properties arise from sjrnthetic lubricity components.
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501
• Semisynthetic Fluid (oil-in-water emulsion concentrate; diluted prior to use)—more likely to support bacterial growth. The fluids belonging to this class contain oil and additives emulsified in water. These preformed emulsions when diluted further are used to provide lubrication, cooling, and corrosion protection. Oil-based Fluids These lubricants, commonly referred to as straight oils, are mineral oil-based. These primarily utilize hydrotreated naphthenic basestocks and contain polyol esters and fatty oils as lubricity agents, and sulfur, chlorine, and phosphorus-derived extreme pressure additives. However, there is a growing trend towards the use of paraffinic basestocks because the derived fluids have a lower tendency to produce mist. The cooling ability of these fluids is not as good as that of the water-based systems, but it is respectable. Because of this and their excellent lubricating ability, these lubricants are often used in high temperature operations that involve high unit pressures and high spot temperatures. That is, those involving slow machine speeds. These oils are labeled active if they contain additives capable of releasing active sulfur (see discussion under extreme pressure agents). Otherwise, they are classified as inactive. Sulfur-releasing ability is high when sulfurized olefins and sulfurized fats used as additives are of high sulfur content (sulfur to olefin ratio of greater than 1). Active fluids are commonly used in machining steel. However, some metal removal operations, such as blanking, do employ inactive fluids. Inactive fluids contain fatty oils, and fatty oil - mineral oil mixtures and or inactive sulfur additives. Water-based Fluids These fluids are emulsions consisting of an organic phase and an aqueous phase. In emulsions, one phase is considered a dispersed phase and the other a continuous phase (usually the larger of the two). When the organic material (oil and/or additives) is the dispersed phase and water is the continuous phase, the emulsions are called normal, or oil-in-water emulsions. If, however, water is the dispersed phase and organic material is the continuous phase, the emulsions are called invert, or water-in-oil emulsions. The two types of emulsions are depicted in Fig. 5. Metalworking and machining fluids are usually oil-in-water tjrpe emulsions. The emulsions are of transient stability (ASTM D 3342, D 3707, and D 3709). Their stability depends upon a variety of factors. These include the nature of the oil phase, the amount and the type of emulsifier/s, the pH, the operating temperatures (ASTM D 3707), the nature and the amount of additives, and the impurities, either present or introduced into the system. All these factors can lead to coalescence of fine droplets into larger ones and lead to oil-water sepciration. In general, emulsions that are used as metalworking fluids are kinetically stable but thermodynamically unstable (10). This means that such emulsions maintain their integrity during use but have the tendency to phase separate when not in use. While phase separation in bulk is undesired, the emulsions must phase separate on surfaces to release oil/additives
502
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK HjO
HoO OIL
"^, .~N<00«~>
IL ./wOO'^^'^^
OIL
^^2*^
'VwO
0~«v
'^2*'
OIL
OIL OIL
HoO
(a)
(b)
FIG. 5—A representation of (A) water-in-oil emulsion (B) oll-in-water emulsion.
that improve lubrication. Oil availability depends u p o n its amount being present in the formulation and the emulsion stability. As a rule, the higher the oil content and the lower the emulsion stability, the greater the oil availability. The problem is that the two essential properties of emulsions, namely lubricating ability and cooling capacity, are inversely related. Emulsions of high oil content (lower water content) not only have poor cooling ability, but they are also of lower stability, which decreases their useful life. To further complicate matters, some metalworking operations, such as rolling, tend to destabilize these high oil-based emulsions. In other words, there is a lubrication-cooling tradeoff when emulsion tjrpe metalworking fluids are used. Emulsions are usually supplied as concentrates, which need to be diluted with water prior to use. In addition to cooling and lubricity (ASTM D 2782, D 2783, D 4172, and D 5619), these fluids possess inherent rust prevention properties (ASTM D 4627), and detergency. Another attribute of these fluids is the ability to incorporate additional performance additives. The main disadvantages are their sensitivity towards hard water, susceptibility to microbial attack, and skin sensitivity. Water-based fluids cire susceptible to microbial attack, primarily because of low organic (high water) content. Bacterial infestation of the fluid can lead to an objectionable odor due to the formation of hydrogen sulfide, a change of color to gray, the formation of black stains on the work piece, and foaming. Fungal growth can produce slime and is more difficult to control than bacterial growth. The major side effect of microbial attack is the emulsion's tendency to phase separate. Monitoring and control of emulsion quality are therefore important. The common techniques used for this purpose are summarized in Table 1. The quality of emulsions in metalworking fluids is maintained by: • the use of biocides. • maintaining p H (bacteria are d o r m a n t at pH of greater than 8.8; 8.8-9.2 is a good range). • controlling dissolved oxygen levels. • periodic removal of contaminants, such as tramp oil. • the use of filtration equipment.
TABLE 1—Metalworking fluid monitoring techniques. Appearance and Color PH Odor Reserve alkalinity (ASTM (ASTM D 1293) D974) Emulsion stability Low-shear foam (ASTM D 3601) (ASTM D 1479) Corrosion inhibition Microbial load (ASTM D 3946) (ASTM D 4627) Relative bio-resistance (ASTM D 3946)
It is important to periodically check the organic (oil) content of the emulsion by breaking it. An acid or a salt, such as sodium chloride, is used for this purpose. Alternatively, one can use a refractometer to determine the oil content. If the oil content is too low, it may be appropriate to top u p the fluid with fresh concentrate. Since contaminants, such as tramp oils, metal debris, and microbes, can destroy the integrity of emulsions, they should either be removed or controlled. Tramp oils are removed via skimming and metal debris is removed via filtration. As mentioned above, microbes are controlled by the periodic addition of the antimicrobial agents. Foaming is another problem (ASTM D 892) with emulsions that relates to the presence of emulsifiers. Foaming not only leads to poor lubrication, but it also impairs the fluid's heat transfer properties. While new emulsions are more stable than old emulsions, the lubricating ability (ASTM D 2782, D 2783, D 4172, and D 5619) of emulsions improves with use. This is because debris resulting from metalworking operations and the decomposition products from additives, especially EP agents, tend to destabilize emulsions via nucleation. Ultimately, a complete breakdown of emulsion occurs and a new batch must be used. The life of an emulsion during use is called its "batch life." In general, less stable emulsions have a shorter batch life. While emulsions contain a variety of chemicals (discussed in the Additive section), the chemical types that are critical to their formation and stability are emulsifiers and coupling agents. Emulsifiers are key to the formation of emulsions, and coupling agents help improve their stability. Emulsifiers are surfactants that reduce surface tension of the oil-water
CHAPTER interface, thereby promoting miscibility and leading to colloidal dispersions. In many cases, a combination of emulsifiers is used and its selection depends upon the type of base oil to be emulsified and the nature of other additives present in the formulation [10]. Emulsions are made by mechanically mixing the organic phase t h a t comprises additives and emulsifier/s, called a "concentrate," with water. Since the amount of water in water-based fluids can be as high as 95%, its quality not only determines initial stability of the emulsion but also its "batch life." The presence of greater than 200 p p m calcium or magnesium carbonate (ASTM D 511 and D 513) can lead to emulsion stability problems. And the presence of chloride and or sulfur levels (ASTM D 512 and D 516) of greater than 150 p p m will promote corrosion, instability, and rancidity. Soft water of less than 50 p p m hardness (ASTM D 1126), on the other hand, leads to foaming in many formulations. While regular water is usually acceptable, the use of distilled, deionized, or reverse osmosis water is recommended if problems are encountered. Emulsion appearance can vary from transparent to milky white, depending upon the droplet size. Table 2 shows emulsion appearance as a function of the droplet size. Oil-in-water emulsions with droplet sizes of approximately 0.1-0.2 um, sometimes referred to as microemulsions, are preferred in some metalworking applications. This is because their smaller droplet size makes them both kinetically and thermodynamically more stable. Macroemulsions with droplet sizes in the range of 0.2-10 u m are only kinetically stable. This means that microemulsions prefer the dispersed form and are therefore less likely to phase separate. Of course, the droplet size depends upon factors such as the nature of the oil and the amount and the type of emulsifier used. Despite greater kinetic and thermodynamic stability, pH (ASTM D 1293), oil-water ratio, and temperature have a profound effect on the stability of microemulsions. The larger droplet size and the tendency to coalesce are not always undesired. Hence, m a c r o e m u l s i o n s are often used in m a n y oncethrough applications, where immediate breakdown is desired to release oil for superior lubrication. The same properties are useful for effluent treatment. As mentioned earlier, emulsions for metalworking use are usually produced prior to use by mechanically mixing the additive concentrate with water. The concentrate comprises oil, emulsifier/s, and a variety of additives. The concentrate-towater ratio in such dilutions is between 1:10 (10%)-1:60 (1.5%). However, this ratio is changed if the operation requires a greater degree of cooling or more lubricity. Metalworking fluids for operations requiring more cooling contain a higher percentage of water than those requiring more lubrication. Microemulsions are stable (ASTM D 3342, D 3707, and D 3709), are resistant to microbial attack (ASTM D 3946), and are effective coolants. These attributes are primarily due to their low organic content and high emulsifier
18: METALWORKING
Droplet Size in Microns >1 0.1-1 0.005-0.1 < 0.005
Appearance Milky white Blue white Translucent to semi-transparent Transparent to translucent
FLUIDS
503
levels. Despite the listed advantages, microemulsions suffer from the disadvantages of being expensive, difficult to dispose of, and an extensive tendency to foam (ASTM D 892). Classification of Water-based Fluids Water-based fluids are classified as soluble oils, synthetics, a n d semisynthetics, depending u p o n their oil content and emulsion t3rpe. Soluble oils are macroemulsions containing about 2-10% naphthenic or paraffinic oil. Semisynthetic fluids are microemulsions containing less than 2% oil. Synthetic fluids contain no oil and are predominantly solutions of water-soluble organic compounds or polymers in water. The cooling ability of each tj^De is related to its water content. Therefore, synthetic fluids, with the highest water content, are extremely effective in operations, such as grinding, that generate a lot of heat. As mentioned earlier, the appeareince of an emulsion is related to its droplet size. When the droplet size is small, as in the case of microemulsions, the fluids appear transparent, and when it is relatively large, as in the case of macroemulsions, the fluids appear translucent. The form a t i o n of microemulsions generally requires a higher amount of emulsifier. The terms S5mthetic and semisynthetic, used to describe metalworking fluids, have a different meaning than the same terms used to describe automotive lubricants. In the case of metalworking fluids, these terms pertain to water-based systems, such as micellar solutions and emulsions. In the case of automotive lubricants, however, the terms synthetic and semi-synthetic pertain to the t3rpe of base fluid used to formulate these lubricants. Synthetic implies the use of synthetic basestocks, such as polyolefins, polyglycols, carboxylic esters, etc., and semi-S3mthetic implies a combination of synthetic basestocks and petroleum basestocks. Despite this difference, both types of fluids meet the general definition of synthetic as being man-made materials [20]. Soluble
Oils
(Macroemulsions)
These lubricants are emulsions of mineral and/or fatty oils in water. An emulsifying agent or a surfactant is necessary to form these emulsions. Fatty oil-based lubricants have excellent lubricating properties due to the friction reducing characteristics of such oils (ASTM D 2670, D 5183, and D 5619). Because of the high oil content ( > 2.0%), these emulsions are milky in appearance and are easily destabilized by salts, bacteria, and other materials that can deactivate emulsifiers. Sulfur, chlorine, and phosphorus-containing additives (Fig. 16) are used to make these oils suitable for more severe metalworking operations, such as broaching, tapping, and threading. They may also contain rust and foam inhibitors. In view of their excellent cooling and lubricating properties, soluble oils are used for operations that involve high speeds, low pressures, and generate high temperatures. Synthetic
TABLE 2—Droplet size versus emulsion appearance.
AND MACHINING
Fluids
(Micellar
Solutions)
Micelles are aggregates of emulsifier molecules that occur in water. These result from the tendency of the lipophilic portion of the molecule to avoid highly polar water solvent and of the hydrophilic portion of the molecule to associate with water, as is depicted in Fig. 6. Synthetic metalworking fluids do not contain any oil, petroleum or synthetic, and are sim-
504
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37: FUELS AND LUBRICANTS
HANDBOOK
Polar Group
Nonpolar Group
(Hydrophilic)
(Lipophilic)
Sodium Dodecylbenzenesulfonate - A Common Emulsifier
H,0
H,0
^im
H,0
Micelle FIG. 6—Micellar structure of emulsifier.
ply solutions of organic additives in water. Since most organic materials are hard to dissolve in water, high polarity of additives is necessary for solubility. Soaps and other surfactants are often added to facilitate solubilization. Because synthetic fluids are oil-free, they have poor lubricating ability but excellent cooling ability. These fluids are therefore ideal for high speed machining (metal removal) operations that generate a substemtial a m o u n t of heat. In these operations, these fluids are more effective than straight oils. Synthetics contain fatty amines, fatty amides, or fatty carboxylic acid salts for rust inhibition (ASTM D 665) and TEFLON® and poly(alkylene glycols) for lubricity. Since these fluids are solutions and not emulsions, they do not suffer from destabilization problems and are useful for high-speed machining applications. Higher stability is due to their small micelle size (which also makes them appear cleeir) and low organic content. The micelle size in these fluids typically ranges between 0.005 and 0.015 um. The fluids that do not contain any metal or amine carboxylates eire suitable for use in combination with h a r d water. Synthetics cem be formulated to shed t r a m p oil, i.e. undesirable contaminant oil, for easy skim-off. These fluids cire resistant to bacterial degradation (ASTM D 3946) because of low organic content and have good work piece visibility because of clarity. The disadvantages include reduced lubricity due to the absence of petroleum oils; tendency to leave hard crystalline residues; a n d high alkalinity, higher cost, a n d
greater tendency to foam. The higher cost and the greater tendency to foam relate to the higher amount of emulsifiers and coupling agents used to formulate them. F o a m is usuEilly controlled by the use of foam inhibitors. Sjfnthetics, in addition, are difficult to dispose of because it is not easy to separate organics from water without sophisticated sepciration techniques. Sodium nitrite (NaN02) corrosion inhibitor in water is a true solution. It was previously used for applications where effective cooling was the only consideration. The use of sodium nitrite is being discontinued because of toxicity concerns. Because additives used to formulate these fluids are synthetic in origin, these fluids are appropriately called sjTithetic lubricants, or chemical coolants. Synthetics, like emulsions a n d microemulsions, a r e obtained by diluting the additive concentrate with water, typically in 1:10 to 1:50 ratio. Semisynthetic
Fluids
(Microemulsions)
These fluids fall in between soluble oils and synthetic fluids in terms of their oil content. In every other aspect, they resemble soluble oils. Because of the low oil content, these fluids also appecir clear. However, some semisjrnthetic fluids of high oil content contain greater t h a n 3.0% oil and are translucent. As mentioned earlier, emulsion appearance is a function of the droplet size: the larger the droplet size, the milkier the appearance. These fluids consist of fine colloidal dispersions of organic and inorganic materials in water. The
CHAPTER 18: METALWORKING AND MACHINING FLUIDS 505 advantages of these fluids include the abihty to incorporate both the oil-soluble and the water-soluble chemicals, good lubricity, improved wettability, and easy waste treatment/disposal. Solid Dispersions These lubricants contain solids suspended in water or oil. Most solids are inorganic in origin, although at times orgEtnic polymers are also used. Dispersions are made by mechanically agitating finely divided solids in the presence of a high molecular weight dispersant. Common solids used to formulate these lubricants include graphite, molybdenum disulfide (M0S2), metal powders, metal oxides, metal halides, mica, and tetrafluoroethylene or TEFLON®. Dispersions are hard to maintain because solid particles are quite large and hence have an increased tendency to settle. During use, these lubricants form low-shear solid films (films that are easily removed when two contacting surfaces slide) at the tool-work piece interface, which protect surfaces against metal-tometal contact. The use of these fluids is limited to certain metal forming operations, such as extrusion a n d forging. They are reirely used in metal removal lubricants because of their propensity to settle in the presence of debris. Lubricants derived from the use of commercial passive EP (PEP) agents, which are colloidal dispersions, can also be considered to belong to this class.
FLUID CLASSIFICATION BASED ON END-USE Metalworking operations generate a lot of heat, which, if not controlled, will lead to tool damage and inefficiencies in metedworking operations. Metalworking fluids generally cool as well as lubricate; hence, they Eire often called metalworking coolants, or lubricant coolants. These fluids are formulated to fulfill cooling and weeir control needs of specific operations because each differs in these requirements. This meikes formulation or selection of a m o d e m metalworking lubricant, which meets all process and product requirements, a complicated task. In addition to cooling a n d lubrication, other properties sought in a metalworking fluid include appropriate viscosity (ASTM D 445), ability to wet and adhere to surfaces (ASTM D 2782 ASTM D 5183, D 2783, a n d D 4172), and non-corrosivity to ferrous and non-ferrous metals (ASTM D 665 and D 130). Metalworking fluids are sprayed or poured at the metal-tool interface to dissipate heat, lubricate, protect freshly exposed metal surfaces against corrosion, and remove debris away from the critical areas. Metalworking lubricants, based on the mode of their operation, can be classified as meted forming fluids, metal removal fluids, and miscellaneous others. Others include meted protecting fluids, meted treating fluids, and slideway lubricants. Because these fluids are not directly involved in metalworking, present discussion will largely deal w i t h metal forming and metal removal fluids, which are of the most predominant type. Metal removal fluids are lubricants that are used in applications where metal is removed from the work piece in order to obtain the desired shape. Such applications include drilling, broaching, turning, grinding, milling.
threading, reaming, boring, and sawing. The primary functions of the lubricant are to cool and to facilitate debris removal. Metal forming fluids Eire lubricants that are used in operations where metal in the work piece is plastically deformed to obtain the desired shape. Such operations involve molding of metal by the process of bending, stretching, Eind pounding. The primary function of a lubricant in these operations is to reduce friction. Lower friction helps in increasing tool life and in lowering energy usage. Metal forming operations include hot rolling, cold rolling, foil rolling, forging, wire drawing, tube drawing, deep drawing, ironing, extrusion, and spinning. Heat removed has an effect on surface finish and cold welding of the tool to the work piece, which affects tool life. The frictional heat generated during some metalworking operations, depending u p o n meted hardness, can reach temperatures of 1000°C or higher. In addition, extensive cutting pressures at points of contact can lead to specific surface loads of u p to 5000 N/mm'^. Both these factors can cause local welding during cutting. Shearing of welded spots will not only increase the roughness of surfaces, but it will also expose fresh surfaces that are more prone to welding. Consequently, additional frictional heat will be generated. The function of the fluid is to dissipate this heat and reduce the number and the size of the welding spots. Lubrication effectiveness depends upon the properties of the oil and the presence or absence of the friction reducing additives. Cooling ability, on the other hand, is a function of the amount of water present in the lubricant.
Metal Removal Fluids Metal removing operations are of two types: those where the work piece is moved against the stationary tool and those where the tool is moved against the stationary work piece. In both cases, the tool cuts into the work piece, resulting in chip formation. These fluids, eJso called metal cutting fluids, are utilized in operations that are used to remove excess metal on the way to manufacturing a new part. Both oil and waterbased fluids are used for these operations. Oil-based fluids can be of petroleum, synthetic, or biological (vegetable and animal) origin. These fluids are designed to perform four key functions: • cooling to prolong tool life • lubrication to minimize friction, and hence improve surface finish • facilitate removal of chips eind metal debris • protect freshly exposed surfaces against rust and corrosion. Heat produced during metal removal is primarily frictioneJ and the most is generated during chip formation. Additional heat results from deformation of the metal and during travel of the chip across the tool surface [10,16]. The primary function of the lubricant in metal removed operations is to reduce friction as well as remove heat quickly. It must eJso remove meted debris, resulting from cutting and grinding operations, away from the work piece. Otherwise, extensive tool wear will occur. While water is an excellent coolant, it lacks the ability to reduce friction and wear. Therefore, water-based fluids contain friction reducing and wear control additives. Friction reducing additives primarily generate protective sur-
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face films via physical interaction. Wear control additives, on the other hand, generate such films via chemical reaction. The former type includes fatty materials, such as vegetable oils and animal fats, and the latter type includes sulfur, chlorine, a n d p h o s p h o r u s derivatives. The presence of these additives also minimizes welding of the generated metal debris onto the tool edge, called the "built-up" edge. This not only reduces blemishing, but it also improves surface finish of the work piece. The material used for cutting tools is selected to facilitate metal (chip) removal. Vibration, metal feed rate, cutting speeds, and lubricant availability (in the cutting zone) also play a roll in this process. These factors must therefore be taken into consideration when selecting tool material. For high speed cutting operations, steel, special cutting alloys, ceramics, and cermets are commonly used. Cermets are composites that contain ceramics and metals bonded together. The material hardness increases in the order listed and so does the generated temperature, from ~ 600°C to 1200°C. The geometrical shape of the cutting edge also determines chip formation, which in turn affects the deformation and friction zones. That is where the cutting fluid plays a roll by providing lubrication and cooling, hence controlling wear. The selection of a proper lubricant is therefore important because it will affect cutting speed, tool life, surface finish, and precision of the work piece. Tool shape, the depth of cut, and temperature are also important considerations. A cutting fluid's ability to reduce friction can be tested by the use of traditional film-strength test machines. These include the Almen-Wieland rig, the Falex Pin-and-vee Block tester (ASTM D 2660 and D 3233), the Timken Wear and Lubricant Testing machine (ASTM D 2782), the Four-Ball Test rig (ASTM D 5183, D 2783, and D 4172), and the SAE #1 Test rig [13,17]. However, a correlation with field performance is lacking. Thread tapping tests (ASTM D 5619), used by some additive suppliers, may also be meaningful in assessing the efficiency of cutting oils. For cutting operations, one can use
either straight cutting oils; which are mineral oils containing active chemical ingredients, oil/water emulsions with or without these ingredients; or aqueous solutions. The test elements of the most commonly used film strength machines are shown in Fig. 7. Machines used for these tests measure the effect of load and temperature on the film-forming ability of the lubricant, as reflected by the friction coefficient and the wear. Results for the newly formulated oils Eire compared with those of the reference oils to measure their relative effectiveness. Because of the differences in test conditions, the results from different machines are not directly comparable. The Almen-Wieland, Falex, Timken, and Four-Ball test machines examine the effects of sliding friction only. The SAE #1 tester, on the other hand, evaluates a lubricant's performance both under sliding and under rolling conditions. The Almen-Wieland rig employs a soft steel pin that rotates in a soft steel split bushing at a constant speed. The load is increased in stages until either seizure occurs or the maximum load is reached. The Falex tester also uses a soft steel pin rotating at a constant speed b u t differs from the AlmenWieland machine in that the pin rotates between two hard Vshaped bearing blocks, instead of in a soft steel bushing. The test includes a run-in stage and two stages with increasing loads. The Timken machine consists of a rotating cylindrical steel ring pressed against a steel block. The machine is operated for 10 min to determine the highest pressure at which there is no scoring of the rotating ring or the block. In the Four-Ball tester, a one-half inch rotating steel ball is pressed against three stationary balls of the same size and quality. The system of balls is in a holder that contains enough lubricant to cover the three stationary balls. This setup provides three small jireas of initial point contact. Because of this, high specific pressures are created at low loads, which lead to the deformation of the contact surfaces. The load is increased until frictional heat welds the balls together.
Lubricant
Lubricant
Timken IVIachine
Four-ball EP Machine
Lubricant
SAE Machine FIG. 7—Film-forming test machines.
CHAPTER 18: METALWORKING
AND MACHINING
FLUIDS
507
Helicai Gears
Test Geais
Hydrostatic Pressure Hydrostatic Pressure Spur Gears
Ryder Machine
r
Test Gears
%
I X
Locl(ed-up Torque Coupling
Spur Gears
Gleason Machine FIG. 8—Test elements of Ryder and Gleason machines.
Weld load and wear scar diameter as a function of load are measured. Wear-reducing additives tend to form scars of smaller diameter, and welding occurs at higher pressures. The SAE #1 tester consists of two cylindrical rollers that are rotated against each other at different relative speeds. The test oil lubricates the lower roller, which is mechanically driven. The load is increased progressively until failure occurs. The SAE machine simulates both the rolling friction and the sliding friction. The ratio of the two types of friction can be changed by changing the rotating speeds of the cylinders. These tests are the only way to assess the suitability of lubricants for equipment where mixed and boundary lubrication conditions predominate. However, the data obtained from these tests are subjective. For slideway lubricants, the data from these tests can be substantiated by testing in FZG (ASTM D 5182), Gleason, and Ryder Gear Tests [17]. The test elements of Gleason and Ryder machines are shown in Fig. 8. Metal removal operations by their very nature (high pressures and temperatures) do not permit hydrodynamic lubrication. Hence, lubricants containing film-forming additives are usually required. Such additives include friction reducing agents and extreme pressure agents. The former tj?pe includes fatty acids, their metal salts (soaps), and their esters.
Extreme pressure agents used in metal removal fluids include chlorine, sulfur, and phosphorus compounds. Filmforming additives create physical and chemical protective films. For materials of moderate hardness and for operations involving high speeds, friction reducing agents provide the necessary performance. These chemicals react with metal either at ambient temperature or at high temperatures to form metal carboxylate films (see the Film-forming Agents section for a detailed discussion). However, for difficult-to-cut materials and operations involving high pressures and slow speeds, extreme pressure agents are needed. These additives form protective films via chemical reaction. The selection of a proper lubricant depends upon the nature of the metal and the severity of the cutting operation. In general, operations that employ low cutting speeds place a higher demand on the lubricant than those that employ high cutting speeds. Figure 9 shows the relative severity of different cutting operations and the demand they place on the lubricant [10,18]. Tool Wear Besides the severity of the machining operation, tool wear is also a function of the quality of the cutting fluid and the machinability of the metal. Cutting fluid is essential to mini-
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BROACHING
TABLE 3—Attributes of tool materials. Tool Material
Attribute
TAPPING
Carbon steel
THREADING
High-speed steel (HSS)
Short service, limited effectiveness, loses hardness at relatively low temperatures Maintains high cutting edge at high temperatures, contains tungsten (W), molybdenum (Mo), chromium (Cr), cobalt (Co), vanadium (V) Cobalt-based, more brittle than HSS, fills the gap between HSS and carbide tools Carbide-based, extremely hard, lack strength and toughness (need good support), not used where there is heavy vibration or shock Cemented metallic oxides, hardness close to diamond, do not pick up heat, are brittle and therefore chip easily Hardest of all abrasives, large stones are used in dressing tools, small stones are used in grinding wheels
GEAR SHAPING REAMING
Cast alloys
DRILLING
Cermets
MILLING TURNING FIG. 9—Relative severity of metal-removal operations.
mizing wesir and equipment damage. Common types of wear that can occur in cutting tools due to lubricant failure are as follows. • Adhesive wear—Welding between the tool surface and the work piece surface can occur due to high local temperatures. On shear, these spots Ccin either produce metal debris or result in metal transfer from the tool to the work piece. This kind of damage is more likely to occur at low cutting speeds. • Abrasive wear occurs when fragments of cutting or wear debris embedded in the chip abrade the rake face of the tool. • Diffusive wear occurs when metal atoms from one crystal lattice diffuse into the crystal lattice with lower atomic density (concentration). This type of wear is likely when tools m a d e of h a r d material are used to machine softer metals, for example during the use of tungsten carbide tools. • Corrosive wear results from the attack of atmospheric oxygen or of the decomposition products from antiwear and extreme pressure additives on the tool surface. Such additives include chlorine, sulfur, and phosphorus-containing chemicals. • Premature wear is the progressive loss of tool material at the tool-work piece interface and occurs due to plastic flow of metal at high temperatures. Machinability primarily relates to metal hardness, although other factors, such as cutting speed, tool strength, and p o w e r consumption, also play a role. In general, the harder the metal, the greater the tool wear. It is therefore important to select proper tool material for the job at hand. Commonly machined materiEils include steels, both alloy and stainless; cast iron; aluminum and its alloys; magnesium and its alloys; copper and its alloys; nickel and its alloys; heat resistance alloys that contain nickel, chromium, molybdenum, tungsten, and titanium; and plastics. Tool materials include carbon steel, high-speed steel, cast cJloys, ceramics, cermets, and diamonds. The attributes of various tool materials are provided in Table 3 and the characteristics of the metals machined are provided in Table 4.
Ceramics Diamonds
Of the metals listed, ferrous metals such as carbon steel, low alloy steel, and stainless steel, and eduminum are the largest volume metals machined. Copper, brass, and titan i u m are the next group, followed by nickel-based alloys, cobalt-based alloys, magnesium, zinc, tin, beryllium, zirconium, tungsten, molybdenum, tantalum, uranium, and vanadium. The order with respect to ease of machinability, from the easiest to the most difficult, is as follows: Magnesium
(Mg) and its alloys < zinc (Zn) and its alloys
< brass < aluminum
(At) and its alloys < cast iron (Fe)
< bronze < copper (Cu) < carbon steel < alloy steels < stainless steel < nickel (Ni) alloys < titanium
(Ti) and its alloys
Because metalworking operations involve both the p u r e metals and the alloys, selection and use of an appropriate lubricant is a complex process. Operation severity is a function of the cutting speed. In general, operations with low cutting speeds, such as broaching and tapping, are more severe than those that have high cutting speeds, such as turning and milling. Drilling a n d reaming fall in between the two extremes in terms of severity. High severity at low speeds is because at these speeds metal-to-metal contact is more extensive (boundary lubrication) than at high speeds, which promote hydrodynamic lubrication. Consequently, there is a greater need for extreme pressure agents a n d anti-weld additives in the former case than in the latter case, where either lubricant viscosity or friction-reducing additives suffice. Since good cooling and good lubrication are not easy to obtain in the same fluid, water or microemulsions (semisynthetic fluids) are used for operations that require more cooling. And, the use of straight oils is preferred for operations that require better lubrication. The majority of operations require both cooling and lubrication. Hence, macroemulsions (soluble oils) that contain substcinticJ amounts of both organic and aqueous portions are often employed. The operating range of these fluids is enhanced by the supplemental use of fatty additives. For a cutting fluid to perform effectively, a careful consideration of its
CHAPTER TABLE 4—Machining characteristics of machined metals. Machined Metal
Steels Free Machining Alloy steels
Stainless steel Cast Iron Gray cast iron Malleable cast iron Nodular iron White cast iron Wrought iron Aluminum and its alloys Magnesium and its alloys
Copper and its alloys Single phase alloys, such as bronze, coppernickel, copper-silicon Alloyed copper, leadbronze, tin-bronze, naval brass, aluminumbronze Alloys that contain substantial amounts of alloy materials Nickel and its alloys Pure nickel Incone (nickel-chromiumiron alloy) Hast alloy (nickel-copper alloy containing small amounts of iron, molybdenum and chromium) Monel (nickel-copper alloy) Nickel-silver aDoy Heat Resistant alloys Contain nickel, chromium, molybdenum, and titanium Plastics
Characteristics
Modified for machinability with added phosphorus or sulfur Contain chromium (Cr), cobalt (Co), nickel (Ni), molybdenum (Mo), tungsten (W), vanadium (V), singly or in combination; more difficult to machine Difficult to machine because of high chromium (Cr) and or nickel (Ni) that work harden Machines easily Machines easily Machines easily Hard to machine Hard to machine Machine easily relative to carbon steel; pure aluminum may be gummy Excellent machinability; chemically reactive hence use of water or acidic materials in the fluid must be avoided Hard to machine Medium machinability
Free machining
Good machinability
Draggy and difficult to machine Hard to machine and hence need heavily additized cutting fluids Thermoplastics melt and thermosett plastics char at low temperatures, hence cooling is most important. Soluble oils are often used.
cooling and lubricating requirements is essential. High-speed cutting operations, such as turning, milling, and drilling, result in higher temperatures, hence cooling ability of the fluid is critical. Water is an excellent coolant but has little or no lubricating ability. However, this deficiency is overcome by the use of friction reducers and EP agents, which are entrained into water by the use of wetting agents and emulsifiers. On the other hand, low-speed operations such as broaching and tapping experience high friction, and consequently lead to
18: METALWORKING
AND MACHINING
FLUIDS
509
heavy tool wear. Therefore the lubricants for these operations require the use of extreme pressure additives [19a]. A new technique, called dry machining, is presently being explored. The purpose is to eliminate purchase, handling, use, and disposal costs. Since this technique does not use any lubricant, it involves high temperatures. Thus far, the technique has been applied to boring operations. Efforts are underway to extend it to drilling operations [19b]. The depth of cut and surface finish are additional considerations for fluid selection. Low-speed operations where cuts are deep and good work piece finish is important; the lubricity of the fluid is critical. The use of straight oils is therefore appropriate. For high-speed operations that involve shallow cuts, quick heat dissipation is desired. This makes microemulsions or micellar solutions the lubricants of choice. In most cases, the fluid is applied under pressure into the cutting zone, which is to reduce friction, minimize metal transfer, and maximize cooling. Metal Forming Fluids These fluids are used for operations that depend on plastic flow of the metal. Such operations include rolling, extrusion, drawing (drawing tubes, tube bending, deep drawing, wire forming), forging, and sheet metal forming. Some of these operations involve both ambient temperatures (cold working) and high temperatures (hot working). Metal forming processes can be distinguished as being steady-state or nonsteady-state [16], each type with different lubrication requirements. In steady-state processes, such as rolling, it is possible to lubricate the surface of the work piece during its approach to the deformation zone. However, in non-steadystate processes, such as sheet metal forming, the lubrication is not usually possible because of the nature of the operation, and one must depend upon the pre-applied lubricant film. Some processes, such as long billet extrusion, have cheiracteristics of both; that is, n o lubricant application during certain parts of the operation and lubrication during other parts of the operation. The primary functions of a lubricant during metal forming operations are to reduce friction, which lowers energy consumption; and to minimize wear, which increases tool life. Friction reduction, in addition, facilitates the release of the forged part from the die and improves surface finish of the work piece. However, too m u c h friction reduction is undesired because it can lead to slippage and hence make the operation inefficient. Common metal forming lubricants include: • mineral oils—compounded oils (blends of mineral oils and fatty oils) • synthetic oils and esters—fatty acids and their derivatives • lanolin, tallow, and paraffin waxes—aqueous solutions and emulsions • polymer solutions—dispersions containing graphite, molybdenum disulfide (M0S2), salts, glass, bentonite, lime, mica, and talc Additives for these lubricants include fatty acids and fatty compounds, extreme pressure agents (sulfurized and sulfochlorinated fats and oils), emulsifiers, coupling agents, inorganic solids, and dispersants.
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Miscellaneous Fluids Metal Protecting
Fluids
These fluids, also called preservative oils, help protect freshly exposed metaJ surfaces against air, water, and corrosive materials. While most of these fluids cire oil-based, the use of water-based fluids is gaining popularity because of the lower cost, ease of disposal, environmental compatibility, and reduced volatile carbon content. Metal Treating
Fluids
These fluids Eire used for heat-treating operations, such as quenching and tempering. They act as heat transfer agents and can be oil-based or water-based. The base oils to formulate these fluids can be synthetic or petroleum in origin. The performance specifications of these fluids are established by OEMs Eind the end-users. Synthetic fluids, due to their superior fire resistcmce and fewer disposed concerns, Eire replacing oil-based formulations. Slideway
Lubricants
These lubricants are used to lubricate slideways and the accompanying pneumatic equipment. Slideways are guiding surfaces on the bed of a machine along which a table or a carriage moves. Since surfaces that slide over each other are flat, the area of contact is large. This leads to increased adhesive wear. These devices in addition experience motion of varying speeds, which causes sticking and slipping of the sliding surfaces. After wear-in, opposing surfaces form an even closer fit, which squeezes out and wipes away any lubricant that is in the path of motion. Essentially, a boundary lubrication condition exists. This type of lubrication to a degree is by design; otherwise, excess lubricant would form a hydrodynamic wedge that will interfere with the motion of the plane. BoundEiry lubricated surfaces will adhere to each other, especially during slow speed operations. Adhesion occurs when the static friction either equals or exceeds the force of motion. If adhesion is followed by movement due to the applied force, the p h e n o m e n o n is called stick-slip. Regular occurrence of stick-slip causes not only vibration Eind noise but also damage to the work piece, tool, and rider and way. Slideway lubricants perform at extreme t e m p e r a t u r e s , high loads, moisture, and poor ambient air quality. They must therefore possess both the EP activity EUid the rust and corrosion-inhibiting properties. These lubricants are formulated with friction reducers, primarily fatty carboxylic acid derivatives that minimize stick-slip, and extreme pressure agents that control wear damage resulting from boundary lubrication conditions. Operation at extreme t e m p e r a t u r e s leads to oxidative breakdown of the lubricant. This must be avoided; otherwise, poor lubrication and corrosion will result. This is accomplished by the use of oxidation inhibitors. It is important to note that stick-slip control requires combining proper machine design and superior lubricant quality. Since the oil is removed due to the wiping action of the slide, lubricant is supplied at different points along the slide route. Suspended metcJlic fines or chips in the lubricant can lead to scratching, gouging, or abrading of the ways. The use of a properly formulated lubricant can minimize this type of damage, as well as control friction, chatter (noise), and stickslip. Despite the fact that lubrication is often once through.
a n oil circulating system may also b e used to deliver the lubricant at different points along the slide.
FLUID COMPOSITION Metalworking fluids are composed of a base fluid and a collection of chemicals. While base fluid makes u p the bulk of the formulation, it lacks the properties necessary to act as an effective lubricant. Hence, chemical compounds, called additives, are used to either fortify the base fluid's existing properties or add new properties [20]. Because of the diversity of lubrication requirements in metalworking operations, the type and the quantity of additives differ from operation to operation. The quality and quantity of additives cdso depend upon the type and properties of the base fluid. Base Fluid The base fluid in metalworking lubricants can be biological (animal or vegetable) in origin, petroleum-based (mineral oil), synthetic-based, or water. Structures of some of the basestocks used to formulate metalworking fluids are shown in Fig. 10. The detailed description of these basestocks is provided in Chapters 7 and 10 of this manual. Straight oils use severely refined and hydrotreated mineral oils and synthetics, such as polyalphaolefins (PAOs), polybutenes, ethylene oxide-propylene oxide polymers (polyalkylene glycols or PAGs), and organic esters. For water-based fluids, naphthenic oils are preferred because of ease of emulsibility and because of their superior viscosity-pressure relationship (discussed earlier). For straight oils, however, paraffinics are the base oils of choice. Synthetic basestocks are less popular than mineral oils, primarily due to their higher cost. However, the use of these stocks is increasing in fluids where tailored properties are desired and petroleum basestocks are not effective. Polyalkyleneglycols, prepared by polymerizing ethylene oxide-propylene oxide (EtO/PrO) mixtures [21], are extensively used in metal removal lubricant formulations. Desirable properties Eire obtained by controlling molecular weight, altering terminal groups, and the EtO/PrO ratio. Higher molecular weight materials have higher viscosities a n d higher EtO/PrO ratio products have higher water solubility. One can even alter the EtO/PrO ratio to devise materiEils that are water-soluble at low temperatures but are water-insoluble at high temperatures. This property is useful in formulating fluids that are clear during circulation but separate polyalkylene glycol (and additives) at the hot tool-work piece interface during use. Polyisobutylenes, obtained by polymerizing isobutylene [21], have the tendency to depolymerize at high temperatures. This makes them useful in rolling and drawing oils for ferrous and nonferrous metals, where petroleum oil derived lubricants cause staining during subsequent annealing process. Polyalphaolefins, often not used in metalworking fluids, possess certain attributes that make t h e m desirable. These include effectiveness over a wider temperature range and lower hydrocarbon (HC) emissions t h a n petroleum oils of similar viscosities. Polyalphaolefins are also highly resistant to oxidative and thermal degradation.
CHAPTER 18: METALWORKING H2 ^'Cv^
H2 /C>^
H2 ^C^
H2 / ^ \
H,
H,
^C H,
FLUIDS
511
Paraffinics
CH,
CH H,C.
^CH,
^CH,
H,C
H3C'
H2 C^
H2
CH.
CH,
H2 ,C^
H2 /^"^
AND MACHINING
CH3 H2 H2 H2 I H2 ^ C ^ ^C^ ^O^ CH. ^C~^ CH CH CH CH CH CH3 Naphthenics CH3 HoCs. ^^''2 CH /
H,C\
H2C
CH \ H2C
Crl2 A HoC
CHo \ CH3
CH,
/ H,C
CH3 CH, — C H -(-CH2 — C H
I
-ca—c—
R
CH3 Polyisobutene
-^CHj
-CH,
R
R
R = C4H11; CgH-is; C8H17 Polyalphaolefins
O
O
//
CH20C^
II
RO —C—(CH2)n — C — O R R=C8 TO C-13
I
' ,
-^ CH3CH2C — CH2OC R CH2OC
Alkyl Carboxylate Ester
RO
(CH2 — C H — 0 ) n — H
R=C5 TO C10
// R
Polyol Ester
R = Linear or Branched Alkyl Group
R=C2 TO C3 Polyalkylene Glycol FIG. 10—Basestocks used in metalworking fluids.
Polyalkylated cyclopentanes are a new class of synthetic hydrocarbons that are promising in terms of future formulations [10]. Their properties can be varied over a wide range by varying the number euid the nature of the alkyl groups. They have low pour points, high viscosity indexes, and exceptionally low volatility. Other types of synthetic basestocks find limited use in metalworking applications.
Additives Additives used in metalworking fluids can be broadly classified into chemically inert and chemically active. Chemically inert additives primarily alter the physical properties of the fluid, such as lubricity and surface tension. Such additives include emulsifiers, coupling agents, friction reduc-
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MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
ers, rust and corrosion inhibitors, foam inhibitors, and antimicrobial agents. Chemically active additives, on the other hand, alter the chemical properties of the fluid, such as oxidation susceptibility. This group of additives includes some friction reducers, extreme pressure/antiwear agents, and oxidation inhibitors. Common types of additives that are typically used in metalworking fluids are provided in Table 5. Their chemistry and mode of their action are also summarized. Emulsion
Promoters
These additives facilitate the formation of emulsions and contribute to their stability. These include emulsifiers and coupling agents. Emulsifiers are chemicals that are used to emulsify (solubilize) organic additives and or mineral and synthetic oils in water. These are used to create soluble oils, synthetics, and semi-synthetics, all of which are water-based lubricants. Emulsifiers contain functional groups that have the capability to associate with water as well as oil. These additives tend to be situated at the boundary between oil and water (the oil-water interface), where they help reduce the interfacial tension and make the two phases miscible to form a stable emulsion. Emulsion formation requires some kind of mixing or stirring.
Emulsifiers are classified as nonionic or ionic, depending upon whether the polar part is uncharged or charged. Ionic c o m p o u n d s can be subdivided further into cationic if the charge is positive, and anionic if the charge is negative [22]. Nonionic emulsifiers that are often used in metalworking fluids include carboxylic acid a m i d e s and esters, polymeric ethers (polyglycol ethers), esters of polyhydric alcohols, and alkoxylated alkylphenols. Anionic emulsifiers include metal salts, primarily sodium, of carboxylic acids, alkyl phosphoric acids, and sulfonic acids (both natural and sjmthetic). Because of the lower cost, these emulsifiers aire used as generalpurpose additives. Cationic emulsifiers include mineral acid salts of amines and imidazolines. It is important to note that only the charge on the functional group attached to the carbon chain is used in this classification. The charge on the counterion, which is usually inorganic in origin, is ignored. The term amphoteric applies to a group of additives, which contain cationic and anionic groups of organic origin, preferably within the same molecule. They possess structural features and properties of both the cationic and the anionic materials grouped together. These surfactants are practically never utilized in metalworking fluid formulations. Compounds that find extensive use as emulsifiers include: • alkali metal (primarily sodium) sulfonates, both petroleum and synthetic
TABLE 5—Metalworking fluid additives Additive Type Water-based fluids Alkalinity buffers/pH Antifoam agents Antifungal agents Antimicrobials/preservatives
Control Prevent Control Control
Metal passivators Couplers Emulsifiers
Prevent staining and leaching of yellow metals Emulsion stability Emulsify organics in water
Extreme pressure/lubricity agents
Form heat-resistant chemical films that reduce friction
Odor masks Thickeners Corrosion and rust inhibitors
Suppress odor Improve viscosity Control oxidative and corrosive damage to metal surfaces and tools
Oil-based fluids Antifoam agents Anti-misting agents Corrosion and rust iniiibitors Demulsifiers DispersEints Extreme pressure/lubricity agents
Odor masks Thickeners Solid lubricants
Function bacteria-derived acidity foam fungal growth microbial growth
Prevent foam Suppress mist formation Control oxidative and corrosive damage to metal surfaces and tools Facilitate water separation and facilitate removal of tramp oil via skimming Prevent agglomeration of material particles that result during cutting and grinding Form heat-resistant chemical films that reduce friction Suppress odor Improve viscosity Improve film strength
Typical Compounds Amines and inorganic bases Silicones, silicates, and stearates Sodium omadine Triazines, omadine, phenol, oxazolidines, and imidazolines Triazoles Glycols, glycol esters, and alcohols Sodium and lithium sulfonates; poly-ethoxylated phenols, alcohols and acids; metal and amine carboxylates (soaps of fatty acids) Fatty acids, fatty amides, fatty esters, and fatty acid salts (soaps); phosphates; sulfur and chlorine containing compounds Natural and synthetic aromatic compounds Polyacrylate esters and glycol esters Carboxylic acid salts, amides, and amines Silicones, polymethacrylates, and stearates Polyisobutenes and other natural and synthetic polymers Metal sulfonates and phosphates; organic acids and esters; and triazoles Cationic and non-ionic polymeric surfactants Polyamides and metal sulfonates Fatty acids, fatty amides, fatty esters, and fatty acid salts (soaps); phosphates; sulfur and chlorine containing compounds; basic sulfonates Natural and synthetic aromatic compounds Polyacrylate esters Graphite, molybdenum disulfide, Poly(tetrafluoroethylene)-PTFE (TEFLON®)
CHAPTER 18: METALWORKING • alkali metal carboxylates • alkoxylated alcohols, phenols, fatty amines, fatty acids, and fatty amides • sulfated fatty oils Generalized structures of some of the common emulsifiers are given in Fig. 11. Emulsifiers reduce the interfacial tension of the water-organic (oil) interface therefore facilitating thorough mixing of oil and water to form an emulsion. The efficiency of an emulsifier depends upon its molecular weight (usually less than 2000), its HLB (hydrophile-lipophile balance) value, water pH and hardness, the nature of the oil, and the operating conditions, such as temperature. HLB scale spans zero (0) to greater than 30. The higher numbers mean oil compatibility decreasing and water compatibility increasing. Emulsifiers of HLB of greater than 13 generally lead to clear water solutions. Emulsifiers with an HLB of 3 to 6 are suitable for water-in-oil emulsions and those with an HLB of 8 to 18 are suitable for oil-in-water emulsions. The manner in which these additives form emulsions is shown in Fig. 5. Water-in-oil emulsions form when these additives associate with water via their polar ends and with oil and other additive molecules via their nonpolar ends. This is shown in part A of Fig. 5. The result is water miscibility in oil, or water-in-oil emulsion. The mechanism of oil-in-water emulsion is similar, except that the additive molecules associate in the reverse manner. This situation is shown in part B of Fig. 5. As a general rule, nonionic emulsifiers are used in metalworking fluids based on naphthenic stocks, and fatty acid carboxylates are used in those based on paraffinic stocks. Polyalkylene glycols (hydroxyalkyl ethers) are sometimes avoided because their enhanced solubility in water does not allow clean separation for disposal. As mentioned ecirlier, alkali metal salts (soaps) of carboxylic and sulfonic acids are among the most commonly used emulsifiers. However, metal ion exchange with calcium and magnesium, whose salts are present in hard water, degrade emulsions by removing these soaps as insoluble calcium and magnesium salts. Non-ionic emulsifiers lead to emulsions that are less sensitive to hard water.
NON-IONIC
-P R—Cs
R0{CH,CH,0),CH,CH20H
^CHjCHjOH R—N, CH2CH2OH
Polyethoxylated Alcohol (Hydroxyalkyl Ether)
Dialkanolamjde
,p R—C^ • 0(CH2CH20),CH2CH20H
CHjOH I CHOH
Fatty Diethanolamine
AND MACHINING
FLUIDS
Some emulsifiers show mild anti-rust performance (ASTM D 665). Since emulsification is a liquid phase phenomenon and the rust inhibition occurs at the liquid-solid interface, optimizing both these properties is not easy. One can obtain both the emulsification and the anti-rust performance by altering the hydrocarbon chain length in the emulsifier molecule. However, in many cases, this strategy is not very effective and it is necessary to improve the anti-rust performance by the use of supplemental additives. The common ones include sodium nitrite, borax, boric acid amides and esters, alkanolamines, and alkanolamides. The use of sodium nitrite is decreasing because it can form nitrosamines, which are suspected carcinogens (cancer-inducing agents). Coupling Agents (Couplers) Some additives that need to be emulsified have greater solubility in water than in oil. Consequently, they are not easy to formulate into emulsifiable oil concentrate (package) without the presence of a large amount of water (as much as 50%). Coupling agents facilitate emulsification of water into the base oil, emulsifier system, and other additives. In the long term, these additives maintain emulsion stability. Common coupling agents are low molecular weight alcohols, glycols, and triols, which become part of the non-oil portion of the package. This decreases the need for the large amount of water necessary to solubilize additives thereby improving emulsification. It is important to note that although these components are organic in nature, they impart little or no lubricity to the final fluid. Structures of some of these additives are given in Fig. 12. Also, see Fig. 11. Common examples of these additives include: • fatty alcohols, such as tridecyl alcohol • glycols, such as ethylene glycol, diethylene glycol, and propylene glycol • glycol ethers, such as propylene glycol monomethyl ether and hexylene glycol monomethyl ether • fatty acids, such as caprylic acid, isononanoic acid, and neodecanoic acid • nonionic surfactants, ethoxylated alcohol, nonylphenol ethoxylates, and polyethylene glycol esters Film-forming Agents This type of additives includes friction modifiers (also called film-strength additives, oiliness additives), antiwear and extreme pressure agents (also called boundary additives, loadbearing additives), and corrosion inhibitors.
0(CH2CH2O)xCH2CH2OH
CHjOH
CH20-C-0leyl
Polyglycol Ester of a Fatty Carboxylic Acid
Glycerol Monooleste (GMO)
/ H3C—CHI \ OH
CHjOH CH2OH
Alkylphenol Ethoxylate
/ HjC
Ethylene Glycol Monomethyl Ether
ANIONIC
00
OCH3
I .0.
R—ct©0 O NHECHjCHjOH], H,C^
Sodium Carboxylate
Trlethanolamlne Salt of a Fatty Acid
^^^^^ Alkyll^nzenesulfonate
R = Fatty Hydrocarbon Group
FIG. 11—Commonly used emulsifiers.
OH
I ,0
\ ^ C ^ H H,
H,C. OH,
\ CH,
Hexylene Glycol Monomethyl Ether
CH2OCH3
OH
Propylene Glycol
Ethylene Glycol
"^"^cPwa®
513
CH,
/
t?
OH
Isononanoic Acid
FIG. 12—Commonly used coupling agents.
514
MANUAL
37: FUELS
AND LUBRICANTS
HANDBOOK
Lubrication is necessary to facilitate the counter movement of two sliding surfaces. This function, which is usually performed by the base fluid, can be enhanced by the use of high viscosity oils, friction modifiers, or extreme pressure (EP) and antiwear (AW) agents. Since water is a poor lubricant, the lubricity in water-based fluids is achieved by the use of friction modifiers, also called the lubricity agents, and or EP agents. These additives improve lubrication ability of the metalworking fluids by forming m o n o m o l e c u l a r physical and chemiccJ films at the tool-work piece interface, which minimize metcJ-to-metal contact. Film-forming agents are extremely surface active because of a high polar-nonpolar ratio. The polar-nonpolar ratio in additives is discussed in detail in Chapter 9 on Additives and Additive Chemistry. These additives typically contain hydrocarbon groups of 10-18 carbon chain size and very polar functional groups. Therefore, they, have the tendency to separate on surfaces, where they interact by a physical or a chemical m e c h a n i s m . Modes of interaction of additive molecules with surfaces [23] are depicted in Fig. 13. Physical adsorption, or physisorption, is a weaker association of the additive with metal than chemical adsorption, or chemisorption, which in turn is weaker than chemicsd reaction. During adsorption, a n additive molecule generally keeps its structural integrity and involves no bond breakage or bond formation. This is t5rpical of additives, such as natural fats and oils, which have a low reactivity towards metals. However, when the temperature reaches beyond a certain threshold temperature, these additives desorb and the fluid loses its friction-reducing properties. The result is wear damage. Chemical adsorption, on the other hand, requires some
Additive (Molecule
Metal Surface
Physisorption
Chemisorption
o o o o o o o o o o o o Chemical Reaction FIG. 13—Modes of additive—surface interactions
reactivity of the additives towards metal and involves limited bond breakage and bond formation. This situation occurs in the case of fatty alcohols and fatty acids that react with the metal surface to form metal alkoxides and metal carboxylates, respectively. One c o m m o n feature of this class of additives is the presence of a fatty hydrocarbon chain (long linear chain). In addition to fatty alcohols and acids, this class includes fatty esters, natural and synthetic; fatty amides; fatty amines; and fatty alcohol derived alkyl acid phosphates. Those used most often include animal and vegetable oils (commonly called triglycerides), alkyl and polyol esters of fatty acids, ethylene oxide/propylene oxide polymers, TEFLON®, alkanolamides, and mono, di, and tri-ethanolamine salts of carboxylic acids. The structures of these additives are shown in Fig. 14. A greater degree of protection is needed in applications, such as metal removal operations, which generate high temperatures. This is because physically a n d chemically adsorbed additive films are easy to remove. That is where extreme pressure/antiwear agents become important. These additives chemically react with metal surfaces to form a more tenacious protective film. The process involves extensive bond breakage and bond formation via complete breakdown of the additive. These additives are primarily organic compounds of chlorine, sulfur, and phosphorus. The extent of EP protection in the equipment depends upon the conjunction temperature of the two metal surfaces in contact [2]. Figure 15 shows a direct correlation between the conjunction temperature and the degree of EP protection needed. Conjunction temperature is the temperature of the two surfaces in contact and determines the type of additives needed for protection. It is important for the activation temperature of the EP additives to match the conjunction temperature in order to provide the necessary protection. The effective t e m p e r a t u r e ranges of different tj^pes of additives when used in metalworking applications [16] are provided in Table 6. Equipment that operates at low speeds and high loads generally requires more EP protection than equipment that operates at high speeds and low loads. This is because the former t5rpe of operation involves more metal-to-metal contact, which generates higher temperatures as a consequence of increased friction. As mentioned before, extreme pressure agents are primarily organic compounds of chlorine, sulfur, and phosphorus, although sometimes boron and sulfur-nitrogen compounds are also used. Chlorine c o m p o u n d s include chlorinated olefins and paraffins, and chlorinated fatty acids and esters. Sulfur c o m p o u n d s include alkyl polysulfides, sulfurized olefins, and sulfurized fatty acids and esters of both natural and synthetic origin. Overbased sodium and calcium sulfonates, called PEP (passive extreme pressure) agents, and molybdenum disulfide (M0S2) are two other commonly used sulfur-containing EP additives. Phosphorus derivatives include phosphites and aliphatic and aromatic alkyl phosphoric acids (phosphate esters) and their salts. Some EP agents contain both sulfur and chlorine and others, such as dialkyl dithiophosphoric acid derivatives, contain both sulfur and phosphorus. Dithiocarbamates, on the other hand, contain both sulfur and nitrogen. However, sulfur derived fluids suffer from being odorous and supporting biological growth. Chlorine derived fluids, on the other hand, are unsuitable for
CHAPTER 18: METALWORKING AND MACHINING FLUIDS 515
>
,C^—O—CH,
R—c;
< .
\ ;c—o—cHa
OR'
y
Alkyl Carboxylate or Vegetable Oil R'« Methyl or Higher Alltyl G r o u p R ^ Fatty Hydrocarbon Chain
H,
H,
R
II C
O
C
C
O
C
R
R
C
O
CH2 H2C
O
C
R
/\
II HO-J-CHjCHj—o4-4-CH2CH—oj—H
O
O
Ethylene Oxide - Propylene Oxide Polymer
Pentaerythritol Carboxylate
J'" NECHjCHjOHJz
Dialkanolamide
O
HNECHjCHjOHh
0 © Triethanolamine Salt of a CarboxylJc Acid
FIG. 14—Commonly used friction modifiers.
400-
O S_-35C
£ 3 (U 30C
Q.
E
0) I-
msm
C 250
_o ^ o c
,3 'B20C
o O
IJI.hBdd
150
Temperature [2] FIG. 15—Extreme Pressure (EP) protection versus conjunction temperature. TABLE 6—Effective temperature range of lubricity agents and EP/AW agents. Additive Type
Carboxylic acids, esters, and metal salts (soaps) Chloropraffins and other chlorinated derivatives Organophosphorus compounds, such as phosphoric acids and their derivatives Organosulfur compounds, such as polysulfides
Effective Temperature Range
< 200°C 180^50°C 200-700°C 600-1000°C
use in working a l u m i n u m . Representative structures for some of the compounds that are used as EP agents are presented in Fig. 16. In all cases, active elements chlorine, sulfur, and phosphorus react with metal to form metal halide, meted sulfide, metal phosphite, metal phosphate, and metal phosphide protective films. These films are effective only below the eutectic point or the decomposition temperature of these salts. Chlorine compounds can lead to metal corrosion by hydrogen chloride, which results from their hydrolysis. Hence, this must be taken into account prior to their use in cutting oils. In view of their effective range, sulfur compounds are used in heavy-duty and extra heavy-duty cutting operations. Metal sulfide films app e a r to have a higher load-carrying capacity and shear strength t h a n carboxylic acid soaps and metal phosphates. Film formation by these additives occurs by a two-step mechEinism: adsorption on the metal surface and thermal decomposition and reaction with the metal surface because of frictional heat. The resulting metal Scilt films have low coefficients of friction and also demonstrate anti-weld properties, both of which minimize tool wear. Metal chloride films have a transition temperature of ~ 600°C and metal sulfide films have a transition temperature of ~ 1000°C. This makes organic sulfides better extreme pressure additives than organic chlorides, because sulfide films can endure higher temperatures before becoming soft and getting removed. Chlorinated fats, chlorinated esters, a n d chlorinated paraffins are the common organic chlorides used in formulating metalworking fluids. Sulfurized fats, sulfurized oils, sulfurized paraffins, dissolved sulfur, and sulfochlorinated products are the common organic sulfides used. The sulfur in these additives is of two types: active and reactive. Active sulfur is the dissolved or easily releasable form of sulfur, which has the tendency to corrode yellow metals—a low temperature reaction. Because of this, the presence of active sulfur in the metcJworking fluid formulation is of concern. On the other hand, reactive sulfur is the bound form of sulfur, for example as a sulfide or disulfide, and is released or reacts with metal only at high temperatures. Because of the reactivity of sulfurized hydrocarbons towards copper, bronze, and other non-ferrous metals, and the tendency of chloropareiffins to corrode metal via hydrolysis or thermolysis, the need for new extreme pressure agents exists. New extreme pressure technology based on overbased alkylbenzenesulfonates a n d carboxylates, which does not suffer from these drawbacks, has recently become available. Such additives, called passive EP agents, are believed to function by forming a metal carbonate film at the tool-work piece interface. The high effectiveness of these additives in cutting, tapping, a n d threading operations suggests a n alternative mechanism. That is, the high pressures at the tool-metal interface convert amorphous calcium carbonate, present in basic sulfonates and carboxylates, into crystalline salts that facilitate metal removal. The films formed are of low shear strengths and high melting points. These additives do not contain phosphorus, reactive sulfur, or chlorine but are synergistic with sulfur-containing E P additives. In addition, they are less corrosive, easier to dispose of after use, cause little or no foam, and are easily removed from the work piece surface. They can be used both for ferrous and nonferrous metals, which is an added benefit.
516
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
Dispersants
Corrosion
These additives are mainly used to suspend inorganic solids, such as graphite and molybdenum disulfide, in specialty lubricants and include high molecular weight alkenylsuccinimides, alkylsalicylates, and alkylbenzenesulfonates. The structures of these additives are shown in Fig. 17.
These additives are of two types: film-formers axid acid (corrosive component) neutralizers. Film-formers protect against corrosion by forming physical films on metal surfaces. Acid neutralizers control corrosion by neutralizing acidic species either present in a lubricant or produced during use. The selection of these additives depends upon the fluid type and the metal to be protected. For straight oils, overbased metal sulfonates are used to protect against iron corrosion (ASTM D 665 and D 4627).
Anti-misting
Agents
Some metalworking operations generate mist, which is not only of concern because of worker safety, but it also leaves an undesirable oily residue on parts and equipment. Anti-misting agents are polymeric thickeners that are used to minimize mist generation in mineral oil derived fluids. These additives control mist formation by modifying droplet size and include ethylene-propylene copolymers, polyacrylates and polymethacrylates, polyacrylamides, polybutenes and polyisobutenes, polystyrenes, and styrene-butadiene copolymers. Alkalinity
Inhibitors
P1B-X^^.._ / NCHjCHzNHCHjCHjtvr
Polyisobutenylsuccinimide
Agents
Metal Salicylate
.SOjJxM
These additives are used to impart reserve alkalinity (ASTM D 974 and D 4739) and include alkanolamines, complex amines, and metal hydroxides and carbonates. These not only stabilize emulsions through a buffering action, but they also control corrosion by neutralizing corrosive acids.
R R
(S),
M = Ca, Ba, Na X = 1 or2
> Alkylbenzenesulfonate
FIG. 17—Commonly used dispersants.
CH=CH
CH2—COOR" Sx
R
R — C H r — CH
Alkyl polysulfide R = C4 and higher; n = 2-4
CHzCOOR'
Sulfurized Cairboxylic Acids and Esters R' = H; Acid
M
R' = All
(ROJJR:
\
Dlalkyl Hydrogen Phosphite
Metal Dithiocarfoamate M = Zn (zinc), Sb (antimony)
.^' \.
(RO),P(OH)y- RNH2
(R0)2 P
Zn
Amine Phosphates X, y = 1 o r 2
Zinc Dlalkyl Dithiophosphate IVI = Na or Ca
SO, M . M (CO3)
*" ~^ '*"' sodium and 2 for calcium n =2 for sodium and 1 for calcium
Basic Alkylbenzenesulfonate FIG. 16—Structures of common EP agents.
CHAPTER 18: METALWORKING Additives used to control iron corrosion in metalworking fluids include fatty amines, neutral and basic barium and calcium alkylaromatic sulfonates, and metal and amine salts of carboxylic acids, boric acid, and organic acid phosphates. Sulfonates are either alkylbenzene derived or alkylnaphthalene derived. Both the fatty acids and the high molecular weight oxidates (oxidized hydrocarbons) are used to manufacture soaps. Alkanolamides, imidazolines, and sarcosines, often used as rust inhibitors, are also alkaline derivatives of carboxylic acids. Of these, amines and overbased sulfonates, which are basic, perform by acid neutralizing mechanism. Others perform via film formation. For yellow metal protection (ASTM D 130), benzotriazoles and dimercaptothiadiazole (DMTD) derivatives are used. Structures of these inhibitors are provided in Fig. 18. Antimicrobial Agents These additives, also known as biocides, are control specific. A fluid can therefore have a combination of these: one acts as an antibacterial and the other as an antifungal and yeast control agent. Microbial attack is undesired because it causes buildup of acidic materials, corrosion of machinery and tools, destruction of additives, objectionable odors, and it produces materials that destabilize emulsions. Fungal attack can lead to slimy material that can coat the machinery and tools as well as clog pumps and filters. This makes monitoring for microbes a necessity. Monitoring is carried out through commercial culture techniques. Standard practice is
AND MACHINING
R Neutral Calcium Sulfonate
O
II
SO,
R—CH —C—OH CH—-C—OH
II
Ca. xCaCOs 2
-LR
O Alkenylsuccinic Acid
Basic Calcium Sulfonate
O
II
RO—P—OH . R'NHj^CHjCHjOH OR
R—N, CH2CH2OH
O
II
RO—P—OH . R'NH,"
Dlethanolamine
I
OH
RS.^/*»\,^SR
\\
//
N—N
Dimercaptothiadiazole Derivative
517
to use two different biocides in an alternating fashion to guard against microbes developing immunity [10]. Commonly used biocides include formaldehyde-release agents and others. Others can be further divided into heterocyclics and organohalogen compounds. The structures of some of the biocides are shown in Fig. 19. l,3-Di(hydroxymethyl)-5,5-dimethyl-2,4-dioxoimidazole, 2-hydroxymethylaminoethanol, hexahydro-l,3,5-tris (2- hydroxyethyl)-s-triazine, and oxazolidine are examples of formaldehyde-release agents. These additives control bacterisJ growth by releasing formaldehyde, an antibacterial agent. Formaldehyde results from the hydrolysis of these additives in water-based fluids. Heterocyclics include isothiazolone, benzisothiazolinones, morpholine, sodium pyrithione (sodium omadine), benzotriazole, and dimercaptothiadiazole. Organohalogen compounds include 2,4,5-trichlorophenol, bis (2-chloroethyl) ether-tetramethylenediamine copolymer, and 2,2-dibromo-3nitrilopropionamide. A variety of other compounds that are outside these general classes are also used to control microbial infestation. These include o-phenylphenol, carbamates, dithiocarbamates, glutaraldehyde, and nitroalcohols. Most of these compounds destroy bacteria directly. Sodium omadine is an effective antifungal agent. Materials, such as 2,2-dibromo-3-nitrilopropionamide, are useful for controlling bacteria and fungi, including yeast. These additives, by virtue of protecting against infestation from all three, minimize worker exposure and prolong emulsion batch life. Boron containing formulations usually do not ex-
Ca
Polyethoxylated Phenol
FLUIDS
Alkylbenzotriazole
FIG. 18—Common corrosion inhibitors
AII
518
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
perience bacterial growth. While choosing an antimicrobial agent, one must consider its toxicity, its effect on emulsion stability, and regulations considering its discharge into waste streams. Foam Inhibitors (Antifoam Agents, Defoamers) These additives are used to control foam (ASTM D 892 and D 3601, IP312) in metalworking fluids. Foam in lubricants occurs because of air or gas entrainment and impairs their flow and lubricating ability. Air entrainment is c o m m o n during spraying and p u m p i n g of lubricants. F o a m is particularly c o m m o n in water-based fluids because they usually contain surfactant type water-soluble additives. F o a m is undesired because it interferes in lubrication, inhibits cooling, and creates a problem if it overflows sumps and tanks. Foam inhibitors are readily dispersible additives, which are added to the fluid in a low concentration, that is in parts per million or parts per billion. They perform by reducing the surface tension of bubbles (ASTM D 1590), thereby resulting in their coalescence. Common types include amide and ester waxes, silicones and modified silicones, long-chain saturated alcohols.
certain triglycerides, water-insoluble polyglycols, ethylene glycol-propylene glycol copoljaners, and polyacrylates. Silicone type inhibitors can adosrb on the metal surface and, in a subsequent operation, reduce the ability of paint or coating to adhere to the metaJ part. Their use is also undesired because of the environmental and waste treatment concerns, plating effect or "fisheyes," and effects on cleaning and finishing. They are h a r d to remove by washing, which may be necessary for finishing the part via painting, varnishing, and enameling. The structures of some of the foam inhibitors are shown in Fig. 20. Oxidation
Inhibitors
These additives are used in lubricants to minimize deterioration of the organic component due to oxidative attack. If not controlled, oxidation will lead to acidic products that tend to form sludge and corrode metal surfaces. For a more detailed discussion on oxidation, consult Chapter 30 of this treatise. Because many metalworking operations involve high temperatures a n d metal debris, the rate of oxidation is even faster. This makes the need for these additives even more
FORMALDEHYDE-RELEASE AGENTS eH,CH,OH Hj
I
H2
HOHjCHjC^ 2-{Hydroxymet)iylamino)ethanol
CH2CH2OH
Hexahydro-1,3,5-tris (2-hydroxyethyl)- s-triazine
CH3 H3C-C-NCH2OH
•:?;>
° ^ N ^ O CHjOH 1,3-dJ (hydroxymethyl)-5,5dimethyl-2,4-dioxoimidazole
Oxazolidine
OTHERS
HC
\\
NH
NH.
;
HC-C,
isothiazolone
BenzoisothiazoUnone
a:
/>-SH
N
^S
BenzotFiazole
_
0 Sodium pyrithione Sodium omadine
0
H
H^N-V*"^ Br Br
N 3
2,4,5-Trichlorophenol
2,2-Dibronio-3-nitrilopropionafnide
FIG. 19—Structures of some biocides.
CHAPTER 18: METALWORKING AND MACHINING FLUIDS 519 CH, I ^ -O-Si—O CH3
CH, I Si—O
CH, I ^ Si
CH3
™CH3
I
I
'/<
^ CH3
Dimethylsiloxane Polymer Arylamine
2,6-Di-t-butyl-4-niethylphenol
HO(CH2CH20)xCH2CH20[CH2CH(CH3)0]yCH2CH(CH3)OH .CHzCOOH
N.
Ethylene Oxide - Propylene Oxide Copolymer
CH2COOH ^CHzCOOH
FIG. 20—Common foam Inhibitors.
N,
'CH2COOH
Ethylenediaminetetraacetic Acid
p a r a m o u n t . Alkylated aromatic amines, alkylphenols, and quinolines are the c o m m o n oxidation inhibitors. Sulfur compounds that are extensively used in metalworking fluids to provide extreme pressure protection have the added advantage of acting in this capacity. Metal ions, such as those resulting from the attack of acidic species on metal are known oxidation promoters (see Chapter 30 on Oxidation). Metal passivators, or chelators, are used to control oxidation by complexing with the metal ions and making them innocuous. Chelators include triazoles, thiazoles, and organic diamines, such as ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid. Structures of c o m m o n l y used oxidation inhibitors are provided in Fig. 2 1 . Dyes These additives are used to impart color to the lubricant cind include a n t h r a q u i n o n e s , azo c o m p o u n d s , a n d triphenylmethane. Odor Control
Agents
These agents are used to control undesirable odors present in lubricants or produced during use. Odor control agents include sjrnthetic sassafras, pine oil, terpenes such as teipinol and d-limonene, and methyl salicylate. Inorganic/Organic
FIG. 21—Oxidation Inhibitors for metalworking fluids.
Lubricant Formulations E a c h metalworking operation places a different d e m a n d on the lubricant. A lubricant must therefore possess specific properties to perform effectively. The properties that fulfill the c o m m o n needs of most operations include cooling, lubrication, w e a r control, a n d protection against corrosion. Metcdworking fluids, like other lubricants, are obtained by blending base fluid and additives. These fluids must meet performance specifications, which for these fluids are primarily established by OEMs and end-users. Table 7 provides the additive composition of metal removal fluids and Table 8 provides the additive composition of metal forming fluids and miscellaneous others. In Tables 9 and 10, additives are listed in the form of formulations. Table 9 presents formulations based on the base fluid and Table 10 presents formulations based on the fluid function.
TABLE 7—General composition of metal removal fluids. Metal Removal Fluid Straight cutting oil
Solids
These include graphite, M0S2, metal powders, metal oxides, metal halides, mica, a n d tetrafluoroethylene polymer. Tetrafluoroethylene polymer, or TEFLON®, is one of the few organic materials that are used to formulate metalworking fluids. These solid additives are suspended in oil or waterbased lubricants by the use of high molecular weight dispersants, referred to as carriers. During use, the solid separates at the tool-work piece interface to form a low-shear film that provides lubrication, in a m a n n e r similar to that of the filmforming agents. These additives are generally not used in metal removal lubricants, although some passive EP agents that are used are colloidal dispersions. However, the use of inorganic solids is more c o m m o n in formulating fluids for certain metal forming operations, such as extrusion and forging. Suspension process generally involves mechanically agitating finely divided powders in oil or water, in the presence of an emulsifier or a dispersant.
Water-soluble cutting oil
Grinding oil Lapping oil Honing oil (precision grinding) Punching lubricants (emulsifiable and nonemulsifiable minereil, and solid dispersion type)
Composition" Mineral oil; friction reducer (fatty acids, their metal salts and esters); extreme pressure agent (chlorinated isobutylene, dibenzyl sulfide, organic phosphates). Water; corrosion inhibitor (sodium or potassium nitrite; mono-, di-, or tri-ethanolamine); emulsifier (non-ionic surfactants; natural and synthetic fatty acid soaps; polyalkylene glycols). Water; mineral oil; friction modifier; extreme pressure/antiwear agent; anti-misting additive. Mineral oil; dispersant; lapping agent. Mineral oil; friction reducer (fatty acids and triglycerides); extreme pressure agent (chlorinated isobutylene, dibenzyl sulfide, organic phosphates). Mineral oil; friction reducer (fatty oils, graphite, mos2, mica, or talc); extreme pressure agent (organophosphorus, organosulfur, and organohalgen compounds).
"Not all formulations contain all the listed additives.
520 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 8—General composition of metal forming and other fluids. Lubricant
Composition"
Metal forming fluids Hot rolling oil (emulsion type) Cold rolling oil (straight oil type) Cold rolling oil (emulsifiable type) Cold rolling oil (aqueous solution or synthetic type) Wire drawing lubricants (straight oils and emulsion types) Tube drawing fluids (straight oils and emulsion types) Deep drawing fluids (straight oils and emulsion types) Extrusion lubricants (powder, grease, solid, and straight oil types) Cold forging lubricants Metal treating fluids Quenching oils (straight oil type) Quenching oils (washable) Quenching oils (emulsifiable type) Metal protecting fluids Rust preventive oils straight oil type) Rust preventive oils (emulsifiable type) Slidewav lubricants
Water; mineral oil (primarily naphthenic); emulsifier; coupling agent; corrosion inhibitor (alkanolamides); antimicrobial agent (triazines); friction reducer (fatty acid derivatives); anti-misting additive (polyisobutylenes); and extreme pressure agent (organophosphorus compounds). Mineral oil (kerosine cut); friction reducer (dodecanol, fatty acids and esters); and extreme pressure agent (organophosphorus, organosulfur, and organohalgen compounds). Water; mineral or fatty oil; friction reducer (fatty alcohols, acids and esters); emulsifier; coupling agent; corrosion inhibitor (alkanolamides, sulfonates); foam inhibitor (ethoxylated fatty alcohols); antimicrobial agent (triazines); and extreme pressure agent (organophosphorus, organosulfur, and organohalgen compounds). Water; friction reducer/EP agent ( o r g a n o p h o s p h o r u s c o m p o u n d s , polyalkylene glycols); corrosion inhibitor (fatty amines). Mineral oil; friction reducer (animal or vegetable oils); lubricant carriers* (phosphoric acid, oxalic acid, lime); extreme pressure agent (organic chlorides) Mineral oil; friction reducer (animal or vegetable oil); lubricant carrier (phosphates, oxalates, lime); extreme pressure agent (organic chlorides); Mineral oil; friction reducer (animal or vegetable oil); corrosion inhibitor; sdhesion improver (polymethacrylates); extreme pressure agent (organophosphorus, organosulfur, a n d organohalgen compounds). Mineral oil; friction reducer (alksdi metal soaps of fatty acids, fatty animal and vegetable oils); lubricant carrier (phosphoric acid, oxalic acid, lime); extreme pressure agent (organophosphorus, organosulfur, and organohalgen compounds). Mineral oil; lubricant carrier (zinc phosphate); a n d extreme pressure agent (organosulfur a n d organohalgen compounds). Mineral Oil; oxidation inhibitor (alkylphenol); quenching p r o m o t e r (high molecular weight hydrocarbons). Mineral oil; emulsifier; oxidation inhibitor; quenching promoter (high molecular weight hydrocarbons). Mineral oil; emulsifier; oxidation inhibitor; quenching promoter (high molecular weight hydrocarbons); antimicrobial agent; corrosion inhibitor. Mineral oil; corrosion inhibitor Mineral oil; emulsifier; water; plasticizer (ethyl cellulose or cellulose acetate); micronized waxes; corrosion inhibitor (alkali and alkaline earth natural metal sulfonates; fatty carboxylates; naphthenates; and extensively oxidized hydrocarbons, both neutral and overbased, amino alcohols and triazoles) Mineral oil; friction modifier (fatty acid derivatives); antiwesir agent; oxidation inhibitor
"Not all formulations contain all the listed additives. ''These additives, appHed through surface bonding, increase surface roughness, which improves adhesion of the lubricant/EP additives.
TABLE 9—Fluid based formulations. Fluid Type Straight oil Soluble oil
Semisynthetic fluid
Synthetic fluid
Approximate Fluid Composition Mineral oil (60%), ususdly solvent-refined blends to achieve ISQ 10, 15, 22, 32, 46, 68 grades; lubricity agent (15.3%)—natural fats, synthetic esters; EP additives (20%), chlorinated paraffins, overbased sulfonates, phosphorus, sulfur, sulfurized fats, and sulfurized olefins; corrosion inhibitor (4.7%). Mineral oil of 40°C viscosity of 15-20 cSt (53.9%); emulsifler/corrosion inhibitor (30.3%)—anionic - sulfonates, alkanolamine salts—amphoteric - a m i n o acid soaps—nonionic - ethoxylates, alcohols, amines; other additives (10.3%)—foam inhibitor, bactericide/fungicide, extreme pressure agent/s; water (5.5%). This additive package is diluted with water in 1:10 to 1:50 ratio to yield the finished fluid, which appears opaque. Mineral oil (14.7%); emulsifier/corrosion inhibitor (50%)—anionic - sulfonates, alkanolamine soaps, boron amides; nonionic - ethoxylates, alcohols, amines; other additives (10.3%)—foam inhibitor, oiliness agent, extreme pressure agent/s; water (25%). This additive package is diluted with water in 1:20 to 1:60 ratio to give the finished fluid that appears almost clear. Corrosion inhibitor (39.7%)—alkanolamines, water soluble soaps, b o r o n soaps/salts, fatty acids/esters; wetting/lubricity agents (7.2%)—polyglycols and esters; other additives (3.1%)—bactericide/fungicide, foam inhibitor, extreme pressure agent; water (50%). This additive package is diluted with water in 1:10 to 1:100 ratio to give the finished fluid that appears completely clear.
CHAPTER 18: METALWORKING AND MACHINING FLUIDS 521 TABLE 10—^Application based formulation. Lubricant Mineral forming fluid Soluble-oil forming fluid
Straight removal fluid Soluble-oil removal fluid
Semi-synthetic removal fluid
Synthetic removal fluid
Mineral protecting fluid Synthetic protecting fluid Mineral treating fluid Synthetic treating fluid
Approximate Composition Extreme pressure agent (20.0-35%)-chlorinated wax a n d sulfurized fat mixture; corrosion inhibitor (1.0-2.0%)-alkanolamine; lubricity agent/friction modifier (10.0-20.0%)-soap or lard oil Extreme pressure agent (25.0-30.0%)-chlorinated wax a n d sulfurized fat mixture; corrosion inhibitor (4.0-6.0%)-alkanolamine; lubricity agent/friction modifier (10.0-20.0%)-soap o r lard oil; foam inhibitor (0.5-1.0%)-polyacrylate; buffer (4.0-6.0%)-diethanolamine o r triethanolamine; biocide (1.0-2.0%)-triazine; emulsifier (4.0-6.0%)-soap. Extreme pressure agent (10.0-20.0%)-chlorinated wax o r sulfurized olefins; corrosion inhibitor (0.5-1.0%)-alkanolamine or triazole; friction modifier (5.0-10.0%)-carboxylic acid derivative. Extreme pressure agent (10.0-15.0%)-chlorinated wax or sulfurized olefins; corrosion inhibitor/buffer (4.0-6.0%)-alkanolamine; lubricity agent/friction modifier (5.0-10.0%)-carboxylic acid derivative; foam inhibitor (1.0-2.0%)-polymethacrylate; biocide (2.0-3.0%)-triazine; emulsifier (5.0-10.0%)-soap. Extreme pressure agent (4.0-6.0%)-chlorinated wax or sulfurized olefins; corrosion inhibitor/buffer (5.0-10.0%)-alkanolamine; lubricity agent/friction modifier (5.0-8.0%)-carboxylic acid derivative; foam inhibitor (1.0-2.0%)-polymethacrylate; biocide (2.0-3.0%)-triazine; emulsifier (5.0-10.0%)-soap or petroleum sulfonate. Extreme pressure agent (2.0-3.0%)-sulfurized olefins or zinc dialkyl dithiophosphate; corrosion inhibitor/buffer (5.0-10.0%)-alkanolamine; lubricity agent/friction modifier (4.0-6.0%)-carboxylic acid derivative; foam inhibitor (1.0-2.0%)-polymethacrylate; biocide (2.0-3.0%)-triazine; emulsifier (5.0-10.0%)-soap or petroleum sulfonate. Corrosion inhibitor (2.0-3.0%)-neutral metal sulfonate mixture. Corrosion inhibitor (0.5-1.0%)-amine carboxylate and amine borate mixture; lubricity agent/friction modifier (0.5-1.0%)-phosphate ester; buffer (5.0-10.0%)-alkanolamine; biocide (1.0-5.0%)-triazine. Oxidation inhibitor (0.1-0.5%)-phenol and arylamine mixture; speed improver (1.0-10.0%)-calcium sulfonate. Corrosion inhibitor (1.0-2.5%)-alkanolamine; foam inhibitor (0.5-1.0%)-polyacrylate; buffer (5.0-10.0%)-alkanolamine; biocide (2.0-3.0%)-triazine.
TABLE 11—List of tests for metalworking fluids. Oil-Base Fluids Corrosion Tests
Water-Based Fluids Corrosion Tests
Ferrous metals (ASTM D 665)
Ferrous Metals (ASTM D 4627, IP 287, and IP 125 Cu and Cu alloys (ASTM D 130) Cu and Cu Alloys (ASTM D 130) EP-Load Carrying Tests EP-Load Carrying Tests Falex EP wear-Pin-on V-block(ASTM D 2670) and D 3233) 4-Ball Wear (ASTM D 4172) 4-Ball EP Weld (ASTM D 2783) Metal Removal
Falex E P wear-Pin-on-V-block(ASTM D2670 and D 3233) 4-Ball Wear (ASTM D 4172) 4-Ball EP Weld (ASTM D 2783) Meted Removal
Falex No. 8 Tapping Torque Lubrizol CNC tapping torque Lubrizol CNC drill wear/life Lubrizol CNC milling/surface finish Lubrizol pipe threading test Other Tests
Falex No. 8 Tapping Torque Lubrizol CNC tapping torque Lubrizol CNC drill wear/life Lubrizol CNC milling/surface finish Lubrizol pipe threading test Emulsion Stability
GM Quenchometer (ASTM D 3520) Cooling curve analysis (ASTM D 6200)
Emulsion stability characteristics-(ASTM D 1401, IP 263) Moderate Timken aquarium test Lubrizol centrifuge procedure Other Tests Cooling curve analysis (ASTM D 6482 and D 6549)
METALWORKING FLUID TESTS Test methods to evaluate the performance of these fluids are not well standardized. Some are standardized tests, such as ASTM, IP, and DIN Tests, while others are additive supplier or the end-user required tests. The standardized tests t h a t are either presently used or can be used to judge the suitability of metalworking fluids are listed in Table 11, and the p a r a m e ters they evaluate are briefly described in Table 12. Many
standards contain these tests, as summarized in Table 13. Details of these tests are available in books on ASTM, IP, and DIN Standards [13,14,24]. It is important to point out that the tests across standards do not always match. They may differ because of disparity in hardware, test method, or the way they are performed.
USED LUBRICANT RECYCLING Toxicity, safety, and environmented compatibility of chemicals are becoming a growing concern. Toxicity determines the ability of materials to h a r m life. While h a r m to h u m a n s is a major concern, the effect of chemicals on the environment as a whole cannot be ignored. Unused lubricants are generally considered less toxic t h a n used lubricants. Exposure to used oils primarily occurs through skin absorption. Over the shortterm, the toxic c o m p o n e n t s present in t h e used oil can lead to skin irritation; over the long-term, they can act as carcinogens. The Occupational Safety and Health Administration (OSHA) requires all lubricant manufacturers to provide material safety data sheets (MSDSs) on their products. While each MSDS contains a variety of information, its primary purpose is to provide physical and health hazard data on products, so as to facilitate safe handling. H u m a n exposure can be minimized by avoiding contact with the lubricant by using protective equipment, such as gloves, oil-impervious clothing a n d boots, and by adopting explosion and fire prevention measures. Environmental protection requires that neither new nor used lubricant be released to air, water, or land. This can be achieved only if proper procedures pertaining to storage and handling of lubricants, lubrication, equipment maintenance, and disposal of used lubricants are in place. Because lubricant disposal is costly and is subject to a n u m b e r of federal, state, and local regulations, minimizing the volume of the used or the leaked lubricant is highly desired.
522 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 12—Metalworking fluid tests. Test and Procedure Corrosion Copper (ASTM D 130) 3 h at 100°C Turbine oil rust (ASTM D 665) A-Distilled water B-Synthetic sea water Aqueous cutting fluid (IP 125) Filter paper chip breakpoint (IP287) Aluminum cup stain 20 min at 350°C Humidity cabinet rust (ASTM D 1748) Salt spray (MIL-B-117-64) Cleveland condensing humidity cabinet (ASTM D 2247) Extreme pressure 4-Ball wear (ASTM D 2266) 40 kg, 1200 rpm, 75°C,1 h average coefficient of friction; maximum scar diameter (mm) Timken (ASTM D 2782) OK Load (lb) 4-Ball EP (ASTM D 2783) seizure (kg); Weld (kg); LWI (kg) Falex EP (ASTM D 3233) Stability Foam (ASTM D 892, IP312) tendency/stability (ml) Panel coker 4 h at 260°C, continuous splash Demulsibility (ASTM D 1401) (mL oil/mL water/mL emulsion) Emulsion stability (IP263) Aquarium biostability Miscellaneous Color (ASTM D 1500) GM quenchometer (ASTM D 3520) R. C. index Thread tapping (Lubrizol test) Pipe threading (Lubrizol test) Stick-slip (Cincinnati milacron test) Bijur filtration Falex #8 SLT (Draw Bead Simulator) Reichert
Purpose Measures fluid's non-ferrous compatibility. Measures the ability of inhibited mineral oils to aid in preventing the rusting of ferrous metals in the presence of water. Measures corrosion protection of aqueous cutting fluids. Evjiluates rust inhibition properties of aqueous cutting fluids. Cup appearsince indicates non-ferrous corrosion properties. Measures ability of preservative oils to protect metal parts from rusting under conditions of high humidity. Steel part corrosion protection measured after exposure to 5% salt spray for 24 h. Measures anti-rust properties of metal preservative fluids on steel panels. Considered more severe than ASTM D1748 humidity test. Evaluates anti-wear and anti-weld properties of lubricants. Measures abrasion resistance and load carrying capacity of lubricants. Evaluates extreme pressure and anti-weld properties of lubricants. Measures load carrying capacity and wear properties of lubricants. Determines foaming characteristics of lubricating oils at specific temperatures. Determines relative stability of lubricants in contact with hot metal surfaces. Measures separation of oil and water from emulsion over time. Measures emulsion stability in water. Measures foam, bacteria, fungus, and odor over time in controlled aqueous environment. Visual determination of fluid color based on colorimetric readings. Determines heat removal speed of a quench oil. Evaluates fluid efficiency by measuring torque required during tapping operation in steel. Evaluates fluid efficiency by measuring torque required during threading operation on cast iron or stainless steel. Measures static and dyneimic coefficients of friction in slideway lubricants. Determines compatibility of lubricants with Bijur setup (specific to Bijur filter design) Evaluates fluid efficiency by measuring torque required during tapping operation in steel. Evaluates friction generated in a drawing process. Measures load-carrying properties of lubricants.
Recycling of the used lubricant is one option. At present, three methods are used for recycling: reprocessing, reclamation, and re-refining [25]. Reprocessing is used to make used oil suitable for combustion as burner fuel. The simplest in terms of treatment, this technique involves the removal of particulates, water, and contaminants that can cause scaling of the heat transfer surfaces and fouling of the burners and of the fuel lines. Particulates and water are removed by the use of settling, filtration, and or centrifugation. Sometimes demulsifiers and heat are employed to facilitate the removal of contaminants. Reclamation involves a higher degree of processing and is primarily used for industrial lubricants, such as hydraulic oils, gear oils, and metalworking fluids. It is more effective if the feed streams are unmixed. Reclamation involves: • settling, centrifuging, and filtering to remove solids • clay and eJkali treatment to remove acidic contaminants, followed by washing to remove the resulting soaps • mild heating or distillation to remove volatile components
• clay treatment to remove polar oxygenated materials or to improve color • aeration or biocide treatment to get rid of bacterial contamination • additive blending to make u p for the depleted additives Some feed streams need fewer steps than others do. A commercial lubricant reclaiming company uses a host of analytical techniques to determine the degree of treatment required to operate cost-effectively. The listed steps for reprocessing and reclamation are shown in Fig. 22. It is important to point out that the quality of the reclaimed lubricant is either the same or lower than that of the lubricant prior to use. Re-refining is the most complex of the three recycling processes and uses petroleum refining techniques, such as vacu u m distillation and hydrotreating, both of which require specialized equipment. Because of this, only a limited number of companies are involved in this process. Re-refining, primarily used to recycle engine oils, results in clean high quality basestocks.
CHAPTER 18: METALWORKING AND MACHINING FLUIDS 523 TABLE 13—Compsirative test standards. Test
Standard test method for pour point of petroleum products Standard test method for detection of copper corrosion from petroleum products by the copper strip tarnish test Standard test method for kinematic viscosity of transparent and opaque liquids (the calculation of dynamic viscosity) Standard test method for rustpreventing characteristics of inhibited mineral oil in the presence of water Standard test method for foaming characteristics of lubricating oils Standard test method for acid and base number by color-indicator titration Standard test method for water separability of petroleum oils and synthetic fluids Standard test method for emulsion stability of soluble cutting oils Standard test method for ASTM color of petroleum products (ASTM color scale) Standard test method for rust protection by metal preservatives in the humidity cabinet Standard test method for wear preventive characteristics of lubricating grease (four-ball method) Standard practice for csJculating viscosity index from kinematic viscosity at 40 and 100''C Standard test method for measurement of extreme-pressure properties of lubricating fluids (timken method) Standard test method for measurement of extreme-pressure properties of lubricating fluids (four-ball method) Standard test method for lowtemperature viscosity of automotive fluid lubricants measured by Brookfield viscometer Standard test method for foam in aqueous media (bottle test) Standard test method for wear preventive characteristics of lubricating fluid (four-ball method) Standard test method for iron chip Corrosion for water-dilutable metalworking fluids Standard test method for evaluating the scuffing (scoring) load capacity of oils Standard test method for apparent viscosity of engine oils between - 5 and -30°C
ASTM Designation
ISO Designation
IP Designation
DIN Designation
D 97-96a
3016-1994
15/95
D 130-94
2160-1998
154/95
DIN- ISO 3016:1982-10 51 759
D 445-97
3104-1994
71/97
D 665-98
7120-1987
135/93
51 585 A-HE
D 892-98
6247-1998
51 566 E Edition 12/83
D 974-97
6618-1997
146/82 312/74 139/98
D 1401-98
6614-1994
D 1479 (Discontinued) D 1500-98
51 550
51599 290/84
2049-1996
196/97
D 1748-83
366/84
51359
D 2266-99
239/97
51 350 Part 5
226/91
DIN-ISO 2909:1997-10
D 2270-93
2909-1981
D 2782-94
51 434 Part 2 51350 Part 2
D 2783-88
293/97
D 2983-87
267/84
D 3601-88
312/74
D 4172-94
293/97
D 4627-92
287/94 125/82
51 360 Part 2
D 5182-97
334/93
51 354 Part 2
D 5293-99-
383/94
51 395-1 Dynamic Test 51 350 Part 3
524 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK Filtration
Settling
Dehydration Reprocessed Oil
Satds and Water Settling
Fine Solids Fillratlon
Dehydration
/• \
^~^
V
I
Solids and Water
Fine Solids
Clay/Alkali Treatment
\
^
Water Washing /•
\
\
J
-Acidic Contaminants
Water
Heating/Distillation /•
\
-^ Volatiles (Solvents, Fuel, Water)
Additive Blending
Clay Treatment
Reclaimed Lubricant
FIG. 22—Flow schemes for lubricant reprocessing and reclamation.
REFERENCES [1] Bartenev, G. M. and Lavrentev, V. V., "Friction and Wear of Polymers," Elsevier Publishing, NY, 1981, p. 203. [2] Fein, R. S., "Boundary Lubrication," CRC Handbook of Lubrication, Theory and Practice in Tribology, Vol. 11, Theory and Design, Richard E. Booser, Ed., CRC Press, Boca Raton, FL, 1983, pp. 49-67. [3] Samuels, L. E., Doyle, E. D., and Turley, D. M., "Sliding Wear Mechanisms," Fundamentals of Friction and Wear of Materials, Papers presented at the 1980 ASM Materials Science Seminar, Pittsburgh, PA, October 4-5, 1980, David A. Rigney, Ed., ASM International, Materials Park, OH, 1981, pp. 214-216. [4] Sargent, L. B., Jr., "On the Fundamental Nature of Metal-Metal Adhesion," ASLE Transactions, Vol. 21, 1978, pp. 285-290. [5] Larsen-Basse, J., "Basic Theory of Solid Friction," American Society of Metals Handbook, Friction, Lubrication, and Wear Technology, Scott D. Henry, Ed., Vol. 18, ASM International, Materials Park, OH, 1992, pp. 27-38. [6] Schiemann, L. F. and Schwind, J. J., "Fundamentals of Automotive Gear Lubrication," SAE Paper 841213, Fuels and Lubricants Technology: An Overview, SP603, Society of Automotive Engineers, Warrendale, PA, October 1984, pp. 107-115. [7] Fundamentals of Tribology, N. P. Suh and N. Saka, Eds., the MIT Press, Cambridge, MA, 1980. [8] Wedeven, L. D., "What is EHD?" Journal of the American Society of Lubrication Engineers, Vol. 31, June 1975, pp. 291-296. [9] Dowson, D. and Higginson, G. R., Elasto-hydrodynamic Lubrication: The Fundamentals of Roller and Gear Lubrication, Pergamon Press, Oxford, England, 1966. [10] Laemmle, J. T., "Metalworking Fluids," American Society of Metals Handbook, Friction, Lubrication, and Wear Technology, Scott D. Henry, Ed., Vol. 18, ASM International, Materials Park, OH, 1992, pp. 139-149. [11] Rein, S. W., 'Viscosity-I," Lubrication, Vol. 64, No. 1,1978, pp. 1-12. [12] Klamann, D., "Tribology and Tribotechnology," Chapter 2, Lubricants and Related Products—Synthesis. Properties. Applications. International Standards, Verlag Chemie, Hamburg, 1984, pp. 4-25. [13] "Petroleum Products and Lubricants," Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA.
[14] DIN Handbuch—Mineralole und Brennstoffe, Beuth Verlag GmbH, Berlin, 1998. [15] ASTM D 2881: Standard Classification of Metalworking Fluids and Related Materials, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA. [16] Klamann, D., "Metal Working Fluids," Section 11.13, Lubricants and Related Products—Synthesis. Properties. Applications. International Standards, Verlag Chemie, Hamburg, 1984, pp. 351-383. [17] Sargent, L. B., Jr., "Lubricant Performance," Chapter 27, Standard Handbook of Lubrication Engineering, J. J. O'Connor, Ed., McGraw-Hill Book Company, NY, 1968, pp. 27-1 to 27-21. [18] Kajdas, C , "Industrial Lubricants," Chemistry and Technology of Lubricants, Chapter 8, R. M. Mortier and S. T. Orszulik, Eds., VCH Publishers, Inc., NY, 1992, pp. 196-222. [19] (a) Kajdas, C , "Additives for Metalworking Lubricants. A Review," Lubrication Science, Vol. 1, pp. 385-409. (b) Sutherlsmd, J. W., Olson, W. W., and Bergstrom, A., "Dry Machining," Summary of Manufacturing Research 1996-97, Department of Mechanical Engineering and Engineering Mechsmics, Michigan Technological University, Houghton, MI. [20] Rizvi, S. Q. A., "Lubricant Additives and Their Fimctions," American Society of Metals Handbook, Friction, Lubrication, and Wear Technology, Scott D. Henry, Ed., Vol. 18, ASM IntemationEil, Materials Park, OH, 1992, pp. 98-112. [21] Synthetic Lubricants and High-performance Functional Fluids, Ronald L. Shubkin, Ed., Marcel Dekker, Inc., NY, 1993. [22] Becher, P., Emulsions: Theory and Practice, American Chemical Society Monograph Series, Ch. 6, Reinhold Publishing Corporation, NY, 1957, pp. 209-231. [23] Bhushan, B. and Gupta, B. K., "Physics of Tribologicsil Materials," Chapter 3, Handbook of Tribology; Materials, Coatings, and Surface Treatments, McGraw-HiU, Inc., NY, 1991. [24] Standard Methods for Analysis and Testing of Petroleum and Related Products and British Standard 2000 Parts 1997, John Wiley and Sons, NY, 1997. [25] Becker, D. A. and Brinkman, D. W., "Recycling (Oil)," KirkOthmer Encyclopedia of Chemical Technology, Fourth Edition, Vol. 21, Martin Grayson, Ed., John Wiley and Sons, NY, 1996, pp. 1-10.
MNL37-EB/Jun. 2003
Petroleum Waxes G. All Mansoori, ^ H. Lindsay Barnes,^ and Glenn M. Webster^
WAXES ARE USUALLY SOLID AT ROOM TEMPERATURE because they
contain linear paraffinic hydrocarbons with carbon chains of various lengths. Waxes can vary in consistency from easily kneadable t o brittle. They exhibit relatively low viscosity at t e m p e r a t u r e s slightly above their melting point. The a p pearance of waxes can vary from translucent to opaque, but they are not glassy. The consistency (i.e., hardness) and solubility of waxes depends on the temperature at which they are observed. The use of waxes dates back more than 5000 years. As early as 4200 B.C. the Egyptians extracted a waxy substance from the honeycomb of bees and used it to saturate linen wrappings of m u m m i e s [1]. The sculptured portrait of the deceased decorating a coffin cover w a s often modeled in beeswax and painted with pigmented beeswax. Another use of wax was in the preparation of erasable writing tablets. Fastening together several tablets with fiber produced forerunners of the m o d e m book [2]. Waxes are classified by the matter from which they are derived: insect, vegetable, synthetic, and mineral [3]. Beeswax is a n example of insect wax. The chemical composition of beeswax is u n i q u e a n d its characteristics vary with t h e species of the honeybee. Apis mellifera is the most common cultured bee in the world and will provide a chemical generalization of composition of wax for this species [4]. Beeswax is secreted in eight glands on the underside of the worker bee. Bees are believed to secrete one p o u n d of wax for every eight pounds of honey they produce. Since secreted beeswax readily absorbs color, the final color of the beeswax is influenced by the source of the pollen. A typical composition analysis of beeswax is provided in Table 1. Beeswax is extracted by melting or boiling the honeycomb in water and has applications in pharmaceuticals and cosmetics, and is the primary component of religious candles. Vegetable waxes are extracted from the leaves, bark, and berries (seeds) of plants and trees. Almost all multi-cellular plants are covered by a layer of wax [5]. Only a few species grown in semiarid climates produce enough wax to be commercially viable for recovery. C a m a u b a and csmdelilla wax are two of the most c o m m o n vegetable waxes that are commercially marketed [6]. C a m a u b a wax is removed from the dried leaves (fronds) of palm trees grown in the northeast region of Brazil. C a m a u b a is utilized in the polish paste indus-
try as a gelling agent for organic solvents and as a raw material used in lipstick formulations for t h e cosmetic market. C a m a u b a wax is recognized generally a s safe by the United States Food and Drug Administration. Candelilla wax is harvested from the shrubs Eurphorbiea antisiphilitica, E. cerifera, and Pedilanthus pavonis in Mexico and southwest Texas. The candelilla wax is recovered after the entire mature plant is uprooted and immersed in acidified boiling water. During the immersion, the candelilla wax floats to the surface and is skimmed off. The primary market for candelilla wax is cosmetics where it is a component in lipstick formulations. The chemical composition of c a m a u b a and candelilla wax is listed in Table 2. Synthetic waxes are derived from either the Fischer-Tropsch process [7] or by ethylene based polymerization processes [8]. The Fischer-Tropsch (F-T) process originated in Germany in the 1920s and is illustrated schematically in E q 1. The F-T process was developed to synthesize hydrocarbons and oxygenated compounds from a mixture of hydrogen and carbon monoxide. During World War II, the F-T process was used by Germany to produce fuels from coal-derived gas. The first commercial plant in South Africa started in 1955 at Sasolburg, using coal as a feedstock. The so-called Sasol process is illustrated in Fig. 1 [9]. This plant produces waxes, fuels, pipeline gases (i.e., ethylene, methane), and other products using a fixed bed catalyst F-T process. During the F-T process, carbon monoxide, which is generated from coal gasification, is reacted u n d e r fixed-bed conditions using high-pressure at approximately 220°C in the presence of a n iron catalyst to p r o d u c e synthetic hydrocarbon waxes, as shown in Eq 1. Typical reaction products that may be derived from the F-T process are listed in Table 3. 2nH2 + nCO -> C„ Hzn + nH20
Poly(ethylene) waxes may be produced by the industrial polymerization of ethylene using high or low pressure ethylene polymerization technology [10], or as thermal decomposition products of the polyethylene polymers. The molecular weights Emd melting points of the synthetic waxes as compared with the Fischer-Tropsch waxes are listed in Table 4. The market stability of pricing a n d availability of insect and vegetable waxes is affected by climate conditions a n d natural disasters. With the advent of the petroleum industry, the waxes from mineral and synthetic sources surpassed the annual production of the combined total of the other two wax categories. Waxes from insect and vegetable sources are mixtures of long chain fatty acids, esters of aliphatic alcohols, and hydrocarbons. Waxes from mineral origins are chemi-
' Department of Chemical Engineering, University of Illinois at Chicago, 810 S. Clinton Street, Chicago, IL 60607-7000. ^ CITGO Petroleum Corporation, Highway 108 South, P.O. Box 1578, Lake Charles, LA 70602. ^ 63 Rocklege Rd., Hartsdale, NY 10530.
525 Copyright'
2003 by A S I M International
(1)
www.astm.org
526
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
TABLE 1—Compositional analysis of beeswax. Component
TABLE 3—Products derived from t h e Fischer-Tropsch process. Approx. Typical Yield (wt. %)
Amount in wt. %
Monoesters, CisHjiCOOCsoHsi; C25H51COOC30H61 55-65 Diesters, triesters, hydroxy diesters 8-12 Free fatty acids, C23COOH-C31COOH 9.5-10.5 Free fatty alcohols, C34OH-C36OH 1-2 Hydroxy-monoesters, Ci4H29CH(OH)COOC26H6i 8-10 Hydrocarbons", C25H52-C31H64 12-15 Moisture and mineral impurities 1-2 "Hydrocarbons most commonly found in beeswax include nonacosane (C29H60) and nentriacontane (C31H64).
Product Paraffins (i.e., methane, ethane, propane, and butane) Olefins (i.e., methylene, ethylene, propylene, and butylene) Gasoline (Cs-Cn) Diesel (C12-C18) Ci9 to C23 Medium Wax (C24-C35) Hard Wax (>C35) Water soluble non-acid chemicals Water soluble acids
7.2 5.6 18.0 14.0 7.0 20.0 25.0 3.0 0.2
TABLE 2—Chemical composition of carnauba and candelilla wax. Carnauba (wt.
Component
Monoesters Fatty alcohols Free fatty acids Hydrocarbons" Resins Moisture and inorganic residue
%)
Candelilla (wt. %)
83-88% 2-3 3-4 1.5-3.0 4-6 0.5-1
28-30% 2-3 7-9 49-57 4-6 2-3
"Hydrocarbons commonly found in carnauba and candelilla wax are principally hentriacontane (C31H64) and tritriacontane (C33H68).
TABLE 4—Comparison of Fischer-Tropsch waxes with other synthetic waxes. Type of Wax Fischer-Tropsch wax Low Pressure polyethylene wax High Pressure polyethylene wax Pyrolysis" waxes
iWolecular Weight
Melting Points, °C
500-1200 900-3000
85-110 90-125
500-4000
85-130
1000-3000
90-130
"Pyrolysis waxes are derived from thermo-cracking of polyethylene.
cally inert and are primarily composed of straight chain (paraffinic) hydrocarbons. Petroleum wax may vary compositionally over a wide range of molecular weight, up to hydrocarbon chain lengths of approximately C50-C60. It is typically a solid at room temperature and is derived from relatively high boiling petroleum fractions during the refining process. Petroleum waxes are a class of mineral waxes that are naturally occurring in various fractions of crude petroleum. They have a wide range of applications that include: coating of drinking cups; an adhesives additive; production of candles and rubber; as components of hot melts, inks, and coatings for paper; and they can be used in asphalt, caulks, and binders. This chapter will provide a review of petroleum waxes including history, production, types, chemical composition, molecular structure, and property testing.
POWER PLANT
COAL
SIEAM 1
GASmCAHOM 4—
''
OXYGEN PLANT
AIR
AE GON N, nrr^f
RAW CAS FDBinCAIION
CO, + H,S-*
OSOTE PflTAR NAPHTHA
PURE GAS
FT PROCESS AI.COHOLS KErroms
LA.
F T WATER WORK-UP
WATER
SEPARATION
FHACnOHAIION
GASES NAI>^HA
CRYOGENIC SEPARATION
WATER
praancATioN
WAX
DISCUSSION Classification of Crude Oils and Chemical Structure of Ingredients
Cj/Ci
CH« EIHYL0IE PLANT
REFORMING
on.
rt:o,
PHACnONAnON OUCOMEBISAHON
••OILS
STEAM O2
CHj
CjHj
FIG. 1—Generalized Sasol Plant for hydrocarbon synthesis by the Fischer-Tropsch Process.
Petroleum crude oil, commonly referred to as crude oil, is a complex mixture of hundreds of compounds including solids, liquids, and gases that are separated by the refining process. Solid components at room temperature include asphalt / bitumen and inorganics. Liquids of increasing viscosity vary from gasoline, kerosene, diesel oil, and light and heavy lubricating stock oils. Also included are the major components of natural gas, which include methane, ethane, propane, and butane [11]. An elemental einalysis of crude oil shows that it consists of primarily two elements: hydrogen (11-14%) and carbon
CHAPTER
19: PETROLEUM
WAXES
527
TABLE 5 --Crude oil content. Crude Type
Solvent Neutral Oil
Base Oil
Wax Content
Sulfur and Nitrogen
Asphalt
API Gravity"
ASTM Test Method
Paraffinic base Naphthenic base Intermediate base Asphaltic base
Yes No No No
Yes Yes Yes Yes
<10% No <6% 0%
Low Low Low High
No No Yes Yes
>40 <33 33-40 <10
E-1519 D-2864 D-8 D-1079
"American Petroleum Institute gravity is an arbitrary scale expressing the density of liquid petroleum products. The measuring scale is calibrated in terms of degree API ("API) and can be calculated in terms of the formula: "API = 141.5/(SG'-[60''F]) — 131.5 where SG'' stands for liquid specific gravity with respect to water. The higher the value of API gravity, the more fluid the liquid.
(83-87%). Crude oil hydrocarbons contain long hydrocarbon chains (saturated and unsaturated), branch structures, and ring structures, with each having specific physical and chemical properties. Small quantities of other compounds containing sulfur, oxygen, nitrogen, carbon, and hydrogen are frequently present in crude oils. Crude oils are generally classified based on their predominant hydrocarbon structure type, as shown in Table 5. The t5rpes are referred to as paraffinic, naphthenic, intermediate (mixture of paraffinic and naphthenic crude), and asphaltic base crude [12]. Paraffinic hydroccirbon fractions are saturated linear or branched alkanes. Naphthenic fractions contain five and six carbon cyclic alkane (alicyclic) structures. Naphthenes cire monocyclic in the lower-boiling fractions (i.e., gasoline) and polycyclic in the higher-boiling fractions (i.e., lubricating oils) [13]. The asphaltic crudes contain unsaturated aromatic structures containing rings of five and six m e m b e r carbon atoms. Aromatics are defined as those classes of organic compounds that behave chemically like benzene. They are cyclic, unsaturated organic compounds that C£m sustain an induced electronic ring current due to delocalization of electrons around the ring. Aromatic base oils contain 20-25% aromatic compounds. A constituent of asphaltic crudes is asphaltene. Asphaltenes are defined as the high molecular weight nonhydrocarbon fraction of crude oil precipitated by a designated paraffinic naphtha solvent at a specified temperature and solvent-oil ratio [14]. Like the naphthenic crude, the aromatic rings are monocyclic in the lower boiling fractions and polycyclic in the higher boiling fractions. Various ASTM test methods listed in Table 6 are used for sampling, separation, and characterization of petroleum fractions. Petroleum waxes are derived from both paraffinic and intermediate crude oils and are composed of three basic carbon structures (i.e., linear, branched, and ring) that are characteristic of the crude oil. Production, Transportation, and Refining of Waxy Petroleum Crudes] The majority of crude oils produced around the world contain substantial a m o u n t s of paraffin weix. These c o m p o u n d s , sparingly soluble in solution components of the crude oils, crystallize at lower temperatures and are the major contributors to petroleum Wcix deposits [15]. The wax present in petroleum crudes primarily consists of paraffin hydrocarbons (C18-C36), known as paraffin wax, and naphtenic hydrocarbons (C30-C60). Hydrocarbon components of WEIX can exist in various states of matter (gas, liquid, or solid) depending on their temperature and pressure. When these hydrocarbons freeze, they form crystals, which are known as macrocrys-
TABLE 6—ASTM test methods used for sampling, separation, and classification of various oil samples and the procedures used. Test Method D 4057 D 270 D 4007 D 86 D 2007
D 2425 D 2549
D 2786 D 2887 D 3239 D 3279
Procedure and Application Practice for manual sampling of petroleum and petroleum products Sampling of petroleum and petroleum products Centrifuge method for determination of water and sediment in crude oil Distillation of petroleum products Clay-gel absorption chromatography for oil samples of initial boiling point of at least 260°C (500°F) into the hydrocarbon types of polar compounds, aromatics and saturates, and recovery of representative fractions of these types Mass spectrometry for classification of hydrocarbon types in middle-distillate Elution chromatography for separation of representative aromatics and non-aromatics fractions of high-boiUng oils, between 232 and 538°C (450 and 1000°F) High ionizing voltage mass spectrometry for hydrocarbon types analysis of gas-oil saturate fractions Gas chromatography for boiling range distribution of petroleum fractions High ionizing voltage mass spectrometry for aromatic types analysis of gas-oil aromatic fractions Titration for determination of the weight percent of asphaltenes as defined by insolubility in normal heptane solvent
talline wax. Those formed from naphtenes are known as microcrystalline wax. A hydrocarbon in pure state has definite boiling and freezing (or melting) points, which can be measured in the laboratory [16]. Knowing the intermolecular energy parameters or critical properties cuid acentric factor and/or refractive index of hydrocarbons, one can predict their boiling point using vapor pressure correlations or equations of state as discussed in Section I of this report. However, such methods are not capable of predicting pure hydrocarbon freezing points. There are other methods that can be used to predict hydrocarbon and wax freezing (melting) point, which include but are not limited to variational statistical mechanical theory [17] and cell-lattice theories [18]. Waxy Crude
Oil
A waxy crude usually consists of: (a) a variety of light and intermediate hydrocarbons (paraffins, aromatics, naphtenic, etc.); (b) wax as defined above; and (c) a variety of other heavy organic (non-hydrocarbon) compounds, even though at very low concentrations they include resins, asphaltenes, diamondoids, organometcillics, etc. When the temperature of a waxy crude oil is lowered to its cloud point, first the heavier fractions of its weix content start to freeze out. Upon lowering of the temperature of a crude oil to its pour point al-
528 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK most all the fractions of its wax content will freeze out. A waxy crude is characterized by its cloud point and pour point, which are measured according to ASTM Test Methods D 2500 and D 97, respectively, as they are discussed later in this report. A clean waxy crude is defined as a crude oil in which there exists only hydrocarbons and wax as its only heavy organic constituent. As the clean waxy crude flows through a cold pipe or conduit (with a wall temperature below the cloud point of the crude) wax crystals may be formed on the wall, which could then grow until possibly the whole inner wall is covered with the encapsulating oil inside the weix layers. As the wax thickness increases, pressure drop across the pipe needs to be increased to maintain a constant flow rate. As a result, the power requirement for the crude transport will increase. The cirterial blockage problems of clean waxy crude can be efficiently controlled by insulation and heating of the pipe to a temperature above its cloud point. Most of the existing wax deposition problems of the clean waxy crudes are due to the lack of proper insulation and heating systems. As a result, application of chemical anti-foulants and frequent use of pigging operations have become necessary [15]. Regular paraffinic or waxy crudes are widespread. The major complex systems problems related to the production, processing, and transportation of these medium-gravity fluids is not just crystallization of their wax content at low temperatures, but the formation of deposits that do not disappear upon heating, and will not be completely removed by pigging. Regular waxy crudes are not clean and, in addition to wax, they contain other heavy organics such as asphaltene, resin, etc. [15]. Asphaltenes do not generjJly crystallize upon cooling and, for the most part, they may not have definite freezing points. Depending on their natures, these other heavy organics will have different interactions with wax, which could either prevent WEIX crystal formation or enhance it. Existence of branched paraffins, aromatics, naphtenes, and resins in petroleum, however, contribute less to these deposits, but modify their crystallization behavior. However, asphaltene presence in the crude oil could prevent or enhance wax deposition depending in the microscopic nature of asphaltene [19,20]. The precipitation of wax from petroleum fluids during production and transportation may give rise to a veiriety of problems [17]. One of the main problems observed is deposition of solid material on well and pipe walls as demonstrated in Fig. 2. This happens if (a) the temperature of the wall is below the cloud point of the oil, (b) a negative radial temperature gradient is present in the flow, (c) the wall friction is high enough for wax crystals to stick to the wall, and (d) asphaltene present in the crude oil has already deposited and has increased the friction of the wall (changed of wettability) and acting as mortar for the sticking together of wax crystals. Wax crystallization may cause three problems: (a) higher viscosity, which leads to pressure losses, (b) high yield stress for restartability of flow, and (c) fouling of petroleum flow arteries [15]. To predict wax deposition tendency of a crude oil it is important to know its composition for paraffin wax and the other components present in, or added to, the crude oil; their composition distributions; and the pressure ctnd temperature of the system. Thermodynamics and statistical mechanics of phase transitions in polydisperse mixtures can be utilized to develop predictive models for wax deposition in petroleum fluids [17].
y^'.-'
FIG. 2—Pipeline petroleum transport plugging due to wax and other heavy organics depositions (Courtesy of Phillips Petroleum Company).
To predict the deposition as a function of time, principles of energy and mass conservation, the laws of diffusion, and the principles of phase transitions need to be considered [21,22]. In order to prevent or remediate arterial blockage/fouling and facilitate the production of regular waxy crudes, many issues must be underteiken: (a) detailed fluid properties characterization, (b) production scheme alternatives, (c) retrograde condensation and deposition behavior prediction, (d) onsets of deposition studies, (e) equipment and facility options, (f) design and use of chemical anti-foulants and/or pour-point depressants and blending alternatives, (g) performance specification and maintenance planning, and (h) transportation, storage, and blending studies [23,24]. Petroleum Refining Crude oil is first desalted if salty, deasphalted if asphaltenic, and dewaxed if highly waxy, before it is distilled in an atmospheric distillation unit to separate light ends (gases), naphtha, gasoline, jet, kerosene, gas oil distillate, and residuum (resid) (see Fig. 3). The residuum (resid) remaining after the atmospheric distillation is then further fractionated in a vacuum distillation unit into fractions that are distinguishable by viscosity for further processing into lubricating oil base stocks. Wax is concentrated in the distillate stream and the residuum fraction is used to produce the base oils for lubricant formulation. Both the distillate and residual lube fractions (stock) contain undesirable constituents such as aromatics that must be removed by extraction to jdeld base oils that are thermally stable with a sufficiently high viscosity index'' product. The distillate fraction is extracted with a sol* Viscosity Index is defined as V.I. = (/XL ~ hiodKlJ'i. ~ MH)- wiiere /AL is the viscosity at 100°F of the zero-V.I. oil, /U-H is tiie viscosity at 100°F of the 100 V.I. oil, and /itx is the viscosity at 100°F of the unknown (test) oil. See ASTM D 567 and D 2270 for further detail. A measure of the magnitude of viscosity changes in lubricating oils with changes in temperature. The higher the viscosity index number, the more resistant the oil is to change in viscosity.
CHAPTER 19: PETROLEUM WAXES 529 vent (such as furfural) that has a greater solubihty (selective solvent) for the components having a low viscosity index. The residuum fraction is extracted with propane to remove bitum e n (asphalt) and resinous material. The desirable oil and wax component is solubilized for further processing. The nonsoluble portion of the distillate extraction and the soluble portion of the residuum fraction are referred to as the raffinate phase and b o t h contain the m o r e paraffinic oil. Wax, which typically exhibits a high viscosity index, remains in the raffinate phase for further processing. Because the raffinate produced from the extraction process contains wax, which crystallizes at relatively high temperatures (>15°F = -9.4°C), the fluidity of the base oil that exhibits a high pour
point (i.e., the temperature where the oil ceases to be fluid) is reduced.
Solvent Dewaxing Process The solvent dewaxing process can be divided into three distinct sections: (a) crystallization of the wax components by dilution and chilling, (b) filtration of the wax from the solution of dewaxed oil and solvent, and (c) recovery of the solvent from the dewaxed oil and wax products [25]. To overcome the high pour point, a solvent dewaxing process has been developed to remove the wax from lubricating oil basestocks, as shown in Fig. 4. The most widely used solvent dewaxing pro-
s' X Dewaxed
iDeasphaltingp Asphalt FIG. 3—Schematic illustration of various possible locations of wax production in petroleum refining.
OflyWax Receiving
Product Shipped ik
ik
Blended Product Oily Wax Storage
f .Ciystallization
Sulvent
Blending & Packaging
I
Mditives
Decoloiid]^
&
Deodortdng
FntratiDn Snlinnt Recnwer;
Stop Tlbxnr Fnnti Oil
Wax & Solvrait StmagB
PrnductllAx
FIG. 4—Solvent dewaxing process for tine removal of wax from lubricating oil slacltwax basestocks.
530 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK gogo H 70
«o
i
50 — 4030 20 — 100 -10—1 -20 -30 -40—1 -50 30
OaPbaaie S«panition
\
\
\
\
\
I
40 50 60 70 80 90 % MEK Content in Dewax Solvent
100
FIG. 5—Illustration of the effect of the ratio of MEK to the petroleum fraction being dewaxed on the resulting cloud point of the mixture.
cesses are based o n solvent mixtures of methyl ethyl ketone and toluene, methyl ethyl ketone and methyl isobutyl ketone, or methyl isobutyl ketone itself. Figure 5 illustrates the effect of the ratio of MEK to the petroleum fraction being dewaxed on the resulting cloud point of the mixture. In the dewaxing process, the raffinate (feedstock) is diluted with solvent and heated 15-20°F ( - 8 - 1 1 °C), above the cloud point of the raffinate/solvent mixture (or slurry) and chilled at controlled rates in double-pipe scraped-surface heat exchangers and chillers. The slurry is chilled to 5-20°F ( ~ 3 - l l ° C ) below the desired pour point of the oil. When the wEix/solvent solution is cooled, wax crystals precipitate from the solution, which are then removed by filtration using a rotary vacuum filter. The crystallized wax forms as a layer (cake) on the surface of the rotary vacuum filter. The wax cake (filtrate) is washed with a spray of a cold solvent to remove any residual oil before being discharged from the primary filters. At this point, the wax contains 10-40% oil and is referred to as "slack wax" if it is derived from the distillate lube fraction, or "petrolatum" if it is derived from the residual lube fraction. Figure 6 illustrates the effect of solvent dilution ratio on the a m o u n t of residual oil content in the slack wax. To produce waxes with lower oil contents (<5%), an additional dewaxing process is performed. The wax cake from the primary filter is diluted with additioneil solvent and filtered in a second (repulp) rotary vacuum filter using the same operating conditions as the primary filters to obtain the desired wax oil content. The solvent is recovered from the dewzixed oil filtrate by flash vaporization and distillation. The solvent is recycled for future use in the dewaxing process. ResiducJ solvent in the wax is recovered by flash vaporization and is recycled for future process use. Dewaxing Process Variables Wax production yield, oil content of the wax, and the pour point of base oil are directly affected by variables of the
deweixing process. The major process variables include: 1) solvent composition, 2) feedstock composition, 3) solvent dilution procedure, 4) filtration temperature, 5) filtration procedure, and 6) solvent recovery method. 1. Ketone based solvents Etre excellent solvents for oils at low temperatures necessary to remove the wax by filtering. Diluting the raffinate with too much ketone-based solvent can cause the oil to separate into a distinct layer. Oil phase separation will adversely affect the yield of wax and result in the w£ix portion having a n undesirable higher oil content. The likelihood of oil phase separation can be determined experimentally by maintaining a constant solvent dilution ratio and changing the percentage of ketone content. 2. If the raffinate feedstock contains a high proportion of paralBnic content, it will have a high viscosity or viscosity index. An oil phase sepeiration can occur when a ketonebased dilution solvent is mixed with the raffinate. 3. The amount of dilution with the solvent can affect the oil content of the wax. Using a solvent dilution greater thcin 2 parts solvent to 1 part raffinate will result in a reduction of the oil content of the wax. 4. The cooling temperature used to crystallize the wax during the filtration process can affect the oil content of the wjix and the desired physical properties such as melting point and hardness. If the dilution solvent is too cold or low cooling temperatures are used, the crystal size of the wax formed on the surface of the rotary filter will be small and will retain more oil. As the dewEixing temperature is reduced, softer and lower melting point wax fractions will increase the overall production yield. As illustrated in Table 7, as a
FIG. 6—Illustration of the effect of solvent dilution ratio on the amount of residual oil content in the slack wax.
TABLE 7—Effect of dewax temperature on wax. Dewax Temperature ("F) (°C)
60 55 50
15.6 12.8 10.0
Wax Yield
(%)
Wax Melt.Point (ASTM D 127) (°F) CC)
62 67 72
141 139 137
60.6 59.4 58.3
Wax Needle Penetration @ yy-F (25°c) (ASTM D 1321)
11 13 16
CHAPTER 19: PETROLEUM WAXES 531 lower dewEixing temperature is used, wax yield increases and the melting point and softness change. 5. The dewaxing process is performed to maximize the recovery of the wax with the desired oil content and physical properties such as melting point a n d hardness. This requires maintaining a uniform thickness (less than 2.5 cm) of the wax cake on the rotary filter by controlling the process temperatures and rotational speed of the filter. Applying wash solvents (for reducing oil content) uniformly prevents cracking of the wax cake. Diluting with adequate repulp solvent is necessary to provide a sufficiently fluid raffinate. The Wax Finishing Process The last step in producing petroleum waxes is the finishing process. This process involves the removal of odor and questionable color. In addition, the finishing process may involve steps to reduce the polycyclic hydrocarbons to a level that meets the Food and Drug Administration regulations for food contact.^ Wax color removal m a y b e performed by flowing wax through a static bed of activated clay or bauxite. There is a production loss in the amount of wax after completing the clay or bauxite contact process. This loss is attributed to absorption of the wax on the clay or bauxite medium and the production loss is greater for darker colored weixes. Newer finishing process technology is based o n hydrofinishing (fixed bed catalj^ic process using hydrogen) and doesn't require any filtering medium. Hydrofinishing has the advantage of processing waxes with negligible product loss [26]. If the wEtx exhibits a questionable odor (such as extraction or deweixing solvent odor), the wax may be steam stripped (distilled) to remove traces of processing solvent. Hydrofinishing may also be used to produce odor free wjixes. After the wax has completed the finishing process step, it can be shipped to consumers; either in solid form (i.e., 22 kg cartons) or as a molten liquid (in specialized tanks with electrical heaters or steam coils). Types of Petroleiun Waxes There are two general types of petroleum waxes that are produced during the dewaxing process. WEIX that is obtained from the distillate lubricating oil fractions is k n o w n as macrocrystalline wax (paraffin wax), and wax derived from the residual distillate lubricating oil fraction is referred to as microcrystalline wax (microwax). This n o m e n c l a t u r e is based on the crystal structure of the wax as seen through a microscope (microstructure). A peiraffin wax can be distinguished from a microwax by its larger crystal structure. Paraffin waxes usually exhibit plate-like crystal structures while microwEixes exhibit needle-like crystal structures. The composition, nomenclature, and physical properties of petroleum are related to the refinery process used in their production. Slack wax is a refinery term for distillate-derived waxes that have oil contents ranging from 3-40% by weight oil. Scale wax is a distillate wax that has an oil content be-
' FDA regulations for waxes, 21 CFR 172.886 and 21 CFR 178.3710.
tween 1 and 3 % . Petrolatums are derived from the residual lubricant fractions with oil contents between 10 and 30% Compositional a n d Molecular Characteristics of Petroleum Wax Paraffin waxes consist predominately of a mixture of straight chain saturated hydrocarbon molecules (normal-alkanes) with the chemical formula CnH2n+2 with n > 16 [27,28]. In order to demonstrate the physical properties of straight chain saturated h y d r o c a r b o n molecules. Table 8 is reported as taken from Ref 28. In this table the molecular weights, melting points, latent heats of fusion, densities (at 20°C), specific heats in solid and liquid states, and boiling points of the normal cdkanes from Ci to Cioo, all at atmospheric conditions, are reported. According to this table, the first four alkanes of the series, (from methane, CH4, u p to butane, C4H10) are gaseous at room temperature and atmospheric pressure. The alkanes between C5 and C17 are liquids a n d alkanes with more than 17 carbon atoms are waxy solids at room temperature. The melting points and heats of fusion of alkanes increase with their n u m b e r of Ccirbon atoms. In addition to the n-alkanes, paraffin waxes may contain varying amounts of iso- and cyclo-alkanes (i.e., branched chains and aliphatic rings). Typiccdly, peiraffin weixes contain carbon atom chains of Cis to C44. Their macrocrystalline structure is illustrated in Fig. 7. Their plate-like crystal structures are illustrated by an atomic force microscope image given in Fig. 8. Their molecular weights are usually less than 450 and their kinematic viscosity at 100°C (212°F) will usually be less than six centistokes. Being derived from distillate fractions, paraffin waxes have distinct boiling point curves that consist of a minimum and maximum value. Microcrystcilline waxes contain higher proportions of isoand cyclo-alkanes (naphthenic) t h a n paraffin wEixes. MicrorystalUine waxes exhibit molecular weights between 500 and 700 with carbon atom chains ranging typically from C23 to C85 in length. Their microcrystalline structure is illustrated in Fig. 9. Microcrystalline waxes (microwaxes) exhibit kinematic viscosities greater t h a n 10 centistokes a t 100°C (212°F). Because microcrystalline waxes are derived from residual fractions, they do not have a distinct boiling range. Physical properties of microcrystalline waxes vary with the t5^e of crude oil and processing conditions used to produce the wax. Tj^pically, microcrystalline, naphthenic waxes exhibit needle-like microstructures. Intermediate wax properties are intermediate between those exhibited by paraffin and microcrystalline waxes. They generally exhibit viscosities between 6 and 10 centistokes at 100°C (212°F) a n d a melting point between 155-165°F (~68-74°C). Intermediate waxes are derived from the highest boiling distillate lubricating oil fraction and like paraffin WEixes, they exhibit a distinct boiling point range. Petrolatums are soft, unctuous products having a melting point between 100-149°F (~38-65°C). The term "unctuous" means "smooth and greasy" in texture. Petrolatums are genercJly produced from the same residual oil fraction as microcrystcJline waxes and can be prepared by controlled blending of microcrystalline wax with mineral oil. Petrolatums generally exhibit oil contents greater than 10% and are meirketed with colors that vary from dcirk brown to white. Table 9 lists the general physical properties of the different petroleum waxes.
532 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 8—Physical properties of n-Alkanes [28].
Alkanes
No. of C Atoms
Mol. Wt
Melting Ft (K)
Latent Heat of Fusion (kJ/kg)
Methane Ethane Propane Butane Pentane Hexane Heptane Octane Nonane Decane Undecane Dodecane Tridecane Tetradecane Pentadecane Hexadecane Heptadecane Octadecane Nonadecane Eicosane Heneicosane Docosane Tricosane Tetracosane Pentacosane Hexacosane Heptacosane Octacosane Nonacosane Triacontane Hentriacontane Dotriacontane Tri triacontane Tetratriacontane Pentatriacontane Hexatriacontane Heptatriacontane Octatriacontane Nonatriacontane Tetracontane Dotetracontane Tritetracontane Tetrateracontane Hextetracontane Octatetracontane Pentacontane Hexacontane Heptacontane Hectane
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 42 43 44 46 48 50 60 70 100
16 30 44 58 72 86 100 114 128 142 156 170 184 198 212 226 240 254 268 282 296 310 324 338 352 366 380 394 408 422 436 450 464 478 492 506 520 534 548 562 590 604 618 646 674 702 842 982 1402
90.68 90.38 85.47 134.79 143.45 177.83 182.55 216.37 219.65 243.50 247.55 263.55 267.75 278.95 283.05 291.25 295.05 301.25 305.15 309.75 313.35 317.15 320.65 323.75 326.65 329.45 331.95 334.35 336.35 338.55 341.05 342.85 344.55 346.25 347.85 349.35 350.85 352.15 353.45 354.65 357.32 358.65 359.55 361.45 363.45 365.15 372.15 378.65 388.40
58 95 80 105 117 152 141 181 170 202 177 216 196 227 207 236 214 244 222 248 213 252 234 255 238 250 235 254 239 252 242 266 256 268 257 269 259 271 271* 272 273 273* 274 276 276 276 279 281* 285*
Specific Heat (/mol K) Density at lO'C (kg/m3) 0.658 (g) 0.124 (g) 1.834(g) 2.455 (g) 621 (1) 655 (1) 679 (1) 699 (1) 714 (1) 726 (1) 737 (1) 745 (1) 753 (1) 759 (1) 765 (1) 770 (1) 775 (s) 779 (s) 782 (s) 785 (s) 788 (s) 791 (s) 793 (s) 796 (s) 798 (s) 800 (s) 802 (s) 803 (s) 805 (s) 806 (s) 808 (s) 809 (s) 810 (s) 811 (s) 812 (s) 814 (s) 815 (s) 815 (s) 816 (s) 817 (s) 817 (s) 819* (s) 820* (s) 822* (s) 823 (s) 825* (s) 831* (s) 836* (s) 846* (s)
Solid at 298 K
485.4 514.6* 544.3 570.7* 598.1* 625.0* 651.4* 670.4* 677.8 728.1* 752.8* 777.2* 801.2* 824.5* 867.4 871.0* 887.4 916.0 937.5* 959.1* 980.4* 1001* 1022* 1062* 1085* 1102* 1140* 1177 1213* 1380* 1526* 1869*
Liquid at 353 K
Boiling Pt (K)
0167.2 195.4 225.0 254.2 284.5 314.5 345.0 376.0 406.9 438.5 470.0 501.5 534.3 564.4 618* 658* 698* 739.0 772.0 805.0 815.9 870.0 928* 937.0 1001* 1037* 1073* 1095 1113 1149 1210* 1206 1276* 1305* 1341* 1411 1435 1465* 1495* 1553* 1595 1665* 1916* 2131* 2598*
116.6 184.6 231.1 272.7 309.0 341.9 371.6 398.8 424.0 447.3 469.1 489.5 508.6 526.7 543.8 560.0 575.2 589.5 603.1 617.0 629.7 641.8 653.4 664.5 675.1 685.4 695.3 704.8 714.0 722.9 731.2 740.2 748.2 755.2 763.2 770.2 777.2 784.2 791.2 795.2 804.2 813.2 818.2 829.2 838.2 848.2 888.2 919.2 935.2
(*)Predicted value.
(a) X 200
X 1000
FIG. 7—A scanning electron microscopic illustration of a macrocrystalline structure wax (a = 200X;b = 1000X).
(b)
CHAPTER 19: PETROLEUM WAXES
533
FIG. 8—An atomic force microscope image of the spiral growth of paraffin crystal (measuring approximately 15 microns across). Inset shows orthorhombic arrangement (0.49 nm x 0.84 nm) of chain ends of one of the crystal terraces (courtesy of Professor M.J. Miles).
X 1000
(a) X200
(b)
FIG. 9—A scanning electron microscopic illustration of a microstructural characterization of a refined paraffin wax (a. = 200 X; b = 1000 X).
TABLE 9 --Physical properties of petroleum waxes. Property
Paraffin Wax
Intermediate
Microcrystalline
Petrolatum
Melt. Point (°F) Molecular Wt. Crystal Structure Color
110-155 320^50 Plates White
150-165 450-550 Needles White—Yellow
140-195 450-700 Needles White—Dark Brown
110-180 450-700 Needles White—Dark Brown
Crystal Structure ParafBn waxes exhibit several crystalline structures depending on their carbon chain length. Odd n u m b e r carbon chains between C19 and C29 exhibit an orthorhombic type crystal structure. Even numbered carbon chains between Cjg and C26 exhibit a triclinic structure. Even n u m b e r e d carbon chains between Cjg and C36 exhibit a monoclinic structure. All paraffins with carbon chains between C20 and C36 have a distinct transition point (change in crystal form) lower than the temperature at which they solidify. The transition point
occurs when the wax crystal structure rotates from a hexagonal to o r t h o r h o m b i c form as the wax solidifies from a molten state. Paraffins with carbon atom chains above C37 do not exhibit a transition point due to the wax solidifying directly into a n o r t h o r h o m b i c crystal structure. Microcrystalline and intermediate type waxes do not exhibit any transition point because they contain higher a m o u n t s of branched alkanes. Because of the steric effects caused by the arrangement of atoms in the molecule there is a difference between alkanes with odd and even numbers of carbon atoms. The even-num-
534 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
bered homologs have higher latent heat than the odd-numbered homologs. Humphries [29] showed that alkanes with an even number of carbon atoms (between 20 and 32) and alkanes with odd number of carbon atoms (higher than 7) exhibit a lattice transition in the solid state. The even-numbered carbon atom alkanes exhibit this transition closer to their melting point than the odd-numbered alkanes, as demonstrated in Fig. 10. The boiling point of normal-alkanes for the temperature range on the figure are sJso shown in Fig. 10. The lattice transition in alkanes is accompanied by the release of heat of transition. Generally, lattice transition occurs
in the solid state at about 2-5°K below the melting point. The difference between the transition temperature and melting temperature becomes smaller with increasing molecular weight and finally disappears for alkanes with more than 36 carbon atoms [25,28] as demonstrated in Fig. 10. The heat associated with this solid-solid transition is subtracted from the lattice heat of melting. Figures 11 and 12 show variations of the latent heat of melting, melting point, and density of normal alkanes versus increasing number of carbon atoms in their structure. According to these figures, while the melting point and density versus the number of carbon atoms have
500
I
'
I
25 30 35 Number of carbon atoms
40
FIG. 10—Variation of meiting point (MP), transition temperature (TrT), and boiling point (BP) of normal alltanes with their number of carbon atoms [28].
1 . 3
_^-
non
cS
•g250 o> I
Si •S200
O o9?fm AAI&OO"
.
"O
r,
"
^
QRRF"^^^ )»00
9NM
rtAl i t " 1
wX* *
sw-
-
_i 150 -
f 6
100 -
20
40
60
80 100 Number of carbon atoms
FIG. 11—Variation of the latent heat of melting of normal alkanes with the number of carbon atoms in alkanes and exhibition of the steric effect [28].
CHAPTER 19: PETROLEUM
1000
400
a u 800 •11
3 •§>
600
400
I
200
0
20
40
60
80
^0 .100
Number of carbon atoms FIG. 12—Variation of tlie melting point and density (@ 20°C) of normal alkanes with the number of carbon atoms in alkanes [28].
smooth variations, the latent heat of meUing goes through fluctuations. Because of the steric effects (the solid-solid phase transitions mentioned above) the latent heats of melting of two consecutive alkanes do not always increase with increasing number of carbon atoms, as demonstrated in Fig. 11. Each even-numbered alkane (with eight carbon atoms or more) exhibit a lower latent heat than the odd numbered alkane having one carbon atom less than it. This fluctuation of the latent heat of melting vanishes as the number of carbon atoms approaches 40, and after that the latent heat increases smoothly with increase of the number of carbon atoms. As an example, the composition and thermophysical data of a paraffin wax sample (Suntech PI 16) [30], which contains almost 100% normal alkanes, is reported in Table 10. According to this table, the hydrocarbons with 20-32 carbon atoms constitute 99% of the mixture and the ones with more than 32 carbon atoms constitute the remaining 1%. Paraffin waxes are generally polydisperse compounds for which polydisperse solution (continuous mixture) theories may be used for characterization [31]. Figure 13 is the graphic representation of the composition data of Suntech PI 16 paraffin wax reported in Table 10. Wax can be crystallized out of a solution by lowering its temperature. Varying the temperature gradient causes a transition between the growth of wax plates and growth of a treelike structure with regular branches as it is shown on Fig. 14. Also shown on Fig. 14 is the banded growth of wax due to addition of a crystallization inhibitor. Equations of State In order to characterize the petroleum wax and perform various operations on weix mixtures, such as wax fractionation,
WAXES
535
it is necessary to be able to predict thermodynamic properties of wax. In this section we present five equations of state, which are used for prediction of molar volumes, vapor pressures, and supercritical solubilities of alkanes [32]. The simplest and one of the most widely known equations of state is that of van der Waals. However, this equation of state is not accurate enough to predict thermodynamic properties of most fluids. Inspired by the van der Waals model, investigators have proposed several equations of state through the years. Almost every equation of state has been claimed to Table 10—Chemical composition and thermophysical properties of Suntech PI 16 Paraffin Wax [30]. Hydrocarbon Weight -% n-C-20 n-C-21 n-C-22 n-C-23 n-C-24 n-C-25 n-C-26 n-C-27 n-C-28 n-C-29 n-C-30 n-C-31 n-C-32 Melting range Heat of fusion Liquid specific heat Solid specific heat Liquid thermal conductivity Solid thermal conductivity Liquid density Solid density Liquid viscosity Molecular weight
2.0 5.5 14.0 23.0 22.0 14.0 6.5 3.0 2.5 2.0 1.7 1.5 1.3 316-329K 266 kJ/kg 2.51kJ/kgK 2.95 kJ/kgK 0.24 W/mK 0.24 W/mK 760 kg/m^ 818kg/m^ 1.90kg/ms 332 g/mol
20 21.22 23 24 25 26 27 28 29 30 31 32 Number of carbon atoms FIG. 13—The distribution of n-alkanes in Suntech P116 paraffin wax as a function of the number of carbon-atoms [28].
536
MANUAL
3 7 ; FUELS AND LUBRICANTS
A-. . " . • - V
• •
•:
' -'i*.- T i ; • • • • : -
' >
HANDBOOK
fr^, - . . ••
PR, and SRK are three-constant-parameter equations. All the above-mentioned five equations of state can be written in the following generalized form [32]:
2 =
'ivX
v-V yb
(2)
T%v + Tjc)(w + Ac)
where a=naaR^ Tj^-^'^ / Pc and 6 = c = rij, ^ i? T,. / P^. Parameters {1^, ^b, y, V> and ^ ^re component-independent constants, while a a n d j3 are component-dependent constants, and their numerical values for various equations of states are given in Table 11. In extending the equations of state to mixtures, parameters a, b, and c are replaced with Um, b„, and Cm with the following expressions (mixing rules):
RK. PR, SRK, RM-1: a„ = 5 2 :>'«>'/^y i i
i
(4)
bm = 1352y«3'Ay + X>''^« ) r ^rri =
'^yAi
For the RM equation there is another alternative in extending it to mixtures by replacing Tc and Pc with Tcm and Pcm as given below:
(b)
.
•
».
..
FIG. 14—(a) An atomic force microscope image of wax "trees" growth in a lowering temperature solidification of wax from solution. Varying the temperature gradient causes a transition between the growth of wax plates and growth of a tree-like structure with regular branches; (b) An atomic force microscope image of banded growth of wax due to addition of crystallization inhibitors (courtesy of Prof. J.L. Hutter).
RM-2: T,m =
{^yiyiTl^pMy^yiyiT,i,pX
Rfn = YI,yiyiR% i J
be superior in some respects to the earlier ones. The RedhchKwong (RK) equation that is a modification of the van der Waals equation, was a considerable improvement over other equations of relatively simple forms at the time of its introduction. In the Soave-Redlich-Kwong (SRK) equation, the temperature-dependent term of a/T"-^ of the RK equation is replaced by a function denoted by a that depends on the acentric factor of the compound and temperature. The PengRobinson (PR) equation is another cubic equation of state involving acentric factor. Riazi and Mansoori [33] modified the parameter h of the RK equation by introducing a function, denoted by '?, that depends o n the refractive index of the compound. They showed that the resulting equation is quite accurate in the prediction of hydrocarbon densities. MohsenNia et al. [34] proposed that the 3M equation in which the repulsive part of the RK equation is modified based on the statistical mechanics improved the thermodynamic predictions appreciably. This equation is shown to be more accurate for heavy hydrocarbon phase behavior calculation than most of the other equations of state. RK and 3M equations are two-constant-parameter equations of state, while the RM,
These equations of state can be used to calculate properties of Wcix, its components, i.e., vapor pressure, and molar volumes of liquid at saturated-, sub-cooled and supercriticEd-conditions as well as the solubility of wax in supercritical solvents. To perform phase equilibrium and other saturated property calculations for WEIX in liquid and vapor states, we need to perform equality of pressures and fugacity calculations [32]. The fugacity coefficient of a component of the wax in a mixture (<^r)?derived from the generalized Eq 1 is in the folTABLE 11—Parameters of the generalized equation of state. Eq. of State —» Parameters 4y iq
RK
MMM
RM
PR
SRK
0 1 0 0.5 0.42748 0.08664 1 1
1.3191 1 0 0.5 0.487480 0.064462 1 1
0 1 0 0.5 0.42748 0.08664 1
0 1 -H V2
0 0 0 0 0.42748 0.08664
1 -V2 A 0 e 0.45724 Oa 0.07780 ftfa a «PR CSRK 1 1 PRM /3 apR = [1 + (0.37464 + 1.524226M - 0.26992m^)(l - T?')]^ CSRK = [1 + (0.48508 + 1.55171a) - 0.15613«2)(1 - T°-^)T (fe«)-' = 1+ [0.02[1 - 0.92 exp(-l,000 \T, - I|)] - 0.035 (T, - 1)]{R*-1)
CHAPTER lowing form [32]: In
dinbmVdtii
CnRT'^^^"^
1
Uffj
dtli
+ XCm)
K = y ° [ ( l + 2fv)l(l
1 d{nc„)
1 d(rp-a„)
ir, - \)c„RT^'^^^
WAXES 5 3 7
erence system, SG = specific gravity, Th = normal boiling point, in degrees Rankine, a = \ - Th I T°, ASG = specific gravity correction and f = correction factor. The critical volume (in cubic feet per pound mole) is given by the following expressions [37]
ln(i; - bm/v)
{v + TprnXv
19: PETROLEUM
Cm
-
2fv)r
(8)
(6) where
drii
fv = ^SGv [0.466590/ri'2 + (-0.182421 + 3.0172/r^'^)ASGv] ASGv = exp [4(SG°2 - SG^)] - 1
where for RK, PR, SRK, RM-1: 1 din^Um)
^^
V? = [(0.419869 - 0.505839a - 1.5643a3 - 9481.70a'4)]-»
d{nbm)
a(nc,n) L
The critical pressure (in psia) is given by the following expression [37]:
for 3M:
P, = P° (r,/7^)(V?/yj[(l + 2fp)/(l - 2fp)f
1 din^dm) n anj
-V =
2 >
^(nb„,) an;
(9)
where
V.77.V —
fp = ASGp [(2.53262 - 4 6 . 1 9 5 5 / 7 ^ - 0.00127885 n)
M^l^yih - YLyiythj + ^«
+ (-11.4277 + 252.140ITl'^ + 0.00230535 Tb)ASGp]
bu
ASGp = exp [0.5(SG° - SG)] - 1 P° = (3.83354 + 1.19629ai'2 + 34.8888a
and for RM-2:
+ 36.1952a2 + 104.193a^)2 J_ d{n a^) J-
-J
^ ^ RTO-SI^Y
V T^ IP - - T
ii^xvj cfn\ -^/^^jj]^ CI}'^ CI}
T vT
IP
and where V = molar volume, in f?^/lbmole, P = pressure, in psia and subscripts V and P refer to the volume and pressure. The acentric factor can b e estimated with the use of the generalized Edalat et al. vapor pressure equation [38]:
^ cm / , Yi^ cii'^ ci
'
\ / / d{nbm) d(ncm) / o, 9/3 V v v T- /D ^ — = (-li + n - ^ j \ l l yffi TclPciij drii dm
In PR = (ar + br^'^ + cr^ + dr^)l{\ - T)
(10)
T=l-T/Tc,
(11)
where To calculate liquid molar volume and vapor pressure using equations of state, t h e data of critical temperature a n d pressure, acentric factor, a n d molar refraction are needed. The experimental critical properties of n-alkanes u p to C24 are available in the literature [35,36], while those of n-alkanes higher t h a n C24 c a n b e estimated using correlations. The critical temperature (in degrees Rankine) can be written as [37]: r , = r ° [ ( l + 2fr)/(l - 2 / r ) ] '
(7)
where JT = ASGr [-0.362456/7^'^ + (0.0398285 - 0.948125/4'2)ASG7-] ASGr = exp [5(SG° - SG)] - 1 Tt = Tb (0.533272 -t- 0.191017 X 10"^ Tb + 0.77968 X 10"^ Ti -0.284376 X l O " ' " Tl + 0.959468 X 10^^ T^ " ) SG" = 0.843593 - 0.128624a - 3.3615a^ - 13749.5a'^ and where subscript T refers to t e m p e r a t u r e , subscript c refers to the critical conditions, superscript o refers to the ref-
W= (-logP«)7-^=o.7 - 1 and a{ca) = - 6 . 1 5 5 9 - 4 . 0 8 5 5 & ; fo(w) = 1.5737 - 1.0540(0 - 4.4365^:10^^^(0)) c(w) = - 0 . 8 7 4 7 - 7.8874W d{(,}) = (-0.4893 - 0.9912W + 3.15510^)'' The above equation is quite accurate for calculation of vapor pressure provided the acentric factor and critical properties of a fluid a r e available. The molar refractions of wax compounds needed in the RM equation of state are available in Ref. 36. The accuracy of molar volumes of saturated liquid wax components, molar volumes in sub-cooled and supercritical conditions a n d vapor pressures calculated using various equations of state are reported in Tables 12-14, respectively. According to these tables the three-constant RM equation of state is quite satisfactory for molar volume prediction while the SRK is accurate for vapor pressure prediction of wax. In Fig. 15 the solubility of n-tritriacontane (n-CaaHgg) in supercritical carbon dioxide is depicted along with the predictions obtained from various equations of state. According to
538 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK TABLE 12—The average deviations of various equations of state in predicting saturated liquid molar volumes of pure compounds compared with those csilculated using the hankinson and thomson (1979) correlation. AAD% Compound CO2 CH4 C2H6 C3H8 H-CAHIO M-C5H12 n-C(,}ii4 n-CjHif,
n-CsHis M-C9H20 n-CioH22 n-CiiH24 n-Ci2H26 "-C13H28 n-Ci4H3o W-C15H32 "-C16H34 M-Ci7H36 H-C18H38 M-C19H40 ra-C2oH42 M-C22H46 ra-C24H5o n-C28H58
Tr Range
RK
3M
RM
PR
SRK
0.71-1.00 0.48-0.99 0.33-0.99 0.35-0.98 0.36-0.96 0.47-0.99 0.39-0.99 0.41-0.99 0.41-0.99 0.42-0.98 0.43-0.98 0.55-0.78 0.54-0.89 0.56-0.80 0.54-0.85 0.58-0.82 0.56-0.81 0.59-0.83 0.55-0.84 0.56-0.85 0.57-0.85 0.55-0.81 0.56-0.82 0.58-0.84
19.5 4.5 10.3 11.2 13.3 16.8 19.9 22.3 24.7 26.8 29.9 31.1 35.3 37.6 42.8 45.8 49.7 56.2 59.5 63.5 67.9 57.6 66.2 62.9 36.5
8.8 13.9 11.8 10.8 9.0 7.5 6.9 7.4 6.9 7.8 10.5 10.5 14.7 16.5 21.4 24.1 26.9 32.9 35.5 39.0 42.9 31.8 39.2 36.4 19.7
19.5 4.5 6.4 3.9 3.0 2.4 2.2 1.4 1.2 0.8 0.7 0.3 0.4 0.2 1.2 1.2 1.8 3.9 4.0 4.4 4.8 4.5 2.9 11.2 3.6
4.7 8.6 6.0 5.3 3.6 3.4 2.2 2.7 4.2 5.1 7.0 7.4 10.1 11.4 15.0 16.7 19.7 24.3 26.9 29.6 32.2 25.1 31.1 27.7 13.8
14.7 4.5 9.2 9.2 10.3 12.5 14.8 16.0 17.7 18.7 20.8 21.3 24.3 25.8 29.9 31.8 35.1 40.3 43.1 46.1 49.0 40.6 47.2 43.1 26.1
Overall
TABLE 13—The average deviations of various equation of state in predicting molar volumes of liquids in sub-cooled and supercritical conditions compared with experimental data. AAD%
Experimental Data
Compound
Tr Range
Pr Range
RK
3M
RM
PR
SRK
No. of Data Pts.
Ref
CO2 CH4 C2H6 C3H8 M-C4H10 W-C5H12 n-C6Hi4 n-C7Hi6 n-C9H2o W-C11H24 M-C13H28 n-CnH36 W-C20H42 M-C30H62 Overall
0.7-2.2 0.5-2.6 0.3-2.3 0.2-1.9 0.3-1.7 0.4-0.7 0.4-0.7 0.6-1.1 0.5-1.0 0.5-0.9 0.4-0.9 0.4-0.8 0.4-0.8 0.4-0.8
1.0-13.6 0.0-15.2 0.0-14.3 0.0-16.5 0.0-18.5 0.0-71.3 0.0-332 1.8-183 2.2-218 2.6-259 3.0-303 4.1^10 5.0-500 6.8-682
5.0 2.0 3.8 5.9 8.4 11.2 14.7 14.5 18.8 25.1 32.3 48.5 59.9 62.7 22.3
4.8 11.1 11.2 9.6 8.4 7.6 5.8 5.6 3.1 3.1 7.9 19.8 28.6 28.9 11.1
5.0 2.0 1.7 1.8 1.6 0.8 2.0 2.3 2.2 2.6 3.2 7.2 9.9 6.8 3.5
2.9 7.4 5.6 4.2 3.8 2.5 2.5 3.3 5.1 10.4 16.7 31.0 41.1 43.5 12.8
6.1 2.4 4.1 5.9 7.9 9.4 12.6 13.5 17.1 23.1 30.1 45.9 57.1 59.8 21.0
447 459 474 533 638 880 510 70 66 70 70 60 50 50 4377
a b c d e f f f.g S S
" Angus et al., 1976. ''Goodwin, 1974. ' Goodwin and Roder, 1976. '^ Goodwin and Haynes, 1982. ' Haynes and Goodwin, 1976. f Frenkel et al., 1997a. i^Doolittle, 1964.
s s s s
CHAPTER 19: PETROLEUM
WAXES
539
TABLE 14—The average deviations of various equations of state in predicting vapor pressures of pure compounds compared with the experimenteJ data. AAD% Compound CO2 CH4 C2H6 C3H8 n-C4Hio
n-CsHn n-C6Hi4 n-CyHie n-CgHis n-C9H2o f3-CioH22 n-CiiH24 n-Ci2H26 n-Ci3H28 n-Ci4H3o W-C15H32 «-Cl6H34 n-Ci7H36 n-CisHss ?2-Ci9H40 n-C2oH42 ra-C22H46 n-C24H5o n-C28H58 M-C29H60 n-C3oH62 M-C32H66 n-C33H68
Experimental Data
Tf Range
RK
3M
RM
PR
SRK
No. of Data Pts.
Ref
0.71-1.00 0.48-0.99 0.33-0.99 0.35-0.98 0.36-0.96 0.47-0.99 0.39-0.99 0.41-0.99 0.41-0.99 0.42-0.98 0.43-0.98 0.55-0.78 0.54-0.89 0.56-0.80 0.54-0.85 0.58-0.82 0.56-0.81 0.59-0.83 0.55-0.84 0.56-0.85 0.57-0.85 0.55-0.81 0.56-0.82 0.58-0.84 0.54-0.84 0.54-0.84 0.55-0.85 0.55-0.85
19.1 17.2 11.5 19.8 43.2 63.4 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 -2100
4.0 50.0 36.3 39.5 32.4 19.2 15.8 11.0 18.5 33.2 51.4 81.7 89.9 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 -650
19.1 17.2 11.9 8.3 7.0 9.9 16.5 20.7 28.9 33.3 37.4 37.7 31.7 28.8 30.3 23.4 29.3 23.9 30.2 30.4 33.0 69.1 79.4 57.9 82.5 75.8 61.5 56.9 35.4
0.8 0.7 3.0 3.0 5.6 0.8 3.1 2.4 2.7 2.1 3.1 6.6 2.9 3.3 5.8 4.0 5.6 6.2 9.2 10.4 8.8 18.6 24.7 33.9 42.5 44.5 48.9 51.8 12.7
0.5 2.9 2.6 1.9 1.9 1.5 1.9 1.2 1.2 1.5 1.2 4.4 0.4 0.5 2.5 0.6 0.9 1.6 3.0 3.6 2.4 1.2 1.5 1.7 2.7 2.7 3.2 3.9 2.0
47 84 114 101 130 91 88 80 87 82 86 27 40 27 42 27 45 43 44 42 42 21 22 23 12 12 12 12 1483
a b,c b,d b,e b,f b,h b,h b,h b,h c,h b,g,h b b,g,h b b,g,h b b,g,h b b,g b,g b,g b,g b,g b,g b b b b
Overall " Angus et al.. 1979. '' Frenkel et al. , 1997a. •= Goodwin, 1974. '' Goodwin and Roder, 1976. " Goodwin and Haynes, 1982. ' H a y n e s and Goodwin, 1976. ^ Morgan and Kobayashi, 1994. '• Salerno et al. , 1986.
this figure, the 3M and RM equations aire capable of predicting supercritical solubilities accurately. In all these cases the unlike-interaction parameter,fcy,is best fitted to experimental data. Table 15 shows the interaction parameters of various equations of state for a number of systems at various temperatures along with the AAD%. According to this table, the 3M equation of state gives the least value of AAD%. Differential Scanning Calorimetry When a solid is heated, it may absorb heat resulting in a temperature increase or a structural change (phase transition) such as a solid to liquid or a transition from one crystalline form to another. These transitions may be endothermic (absorb heat) or exothermic (emit heat) depending on the thermal process that is occurring. These thermal processes may be quantitatively measured by differential scanning calorimetry (DSC). DSC analysis is performed by heating two small sample pans, one containing the material being analyzed and the other empty and used as a reference. The analysis concept is that the two sample pans are maintained at a very small temperature difference (± 0.01°C). Each pan is heated with two heaters; a main heater and an auxiliary heater. After begin-
ning the experiment by supplying heat with the main heaters, while heating the temperature difference (AT) between the sample and reference pans is sensed using a thermopile (set of thermocouples) which produces a small (0-5/xV) off-setting voltage. The auxiliary heater is then used to heat the sample pan to keep the off-balance voltage close to zero. The instrument displays the differential power (AP) between the two pans as a function of temperature. The area under the peak of differential power (AP) versus temperature (T) provides an experimental measure of the energy or total enthalpy change (AH) of the entire process [39,40]. As described in ASTM Test Method D 4419, the melting point can readily be determined by DSC analysis, as can heat of fusion, which is also an important characterization parameter for waxes. Heat of fusion is defined as the increase in enthalpy accompanying the conversion of one mole, or a unit mass, of a solid to a liquid at its melting point at constant pressure and temperature [43]. The heat of fusion (AATf) is obtained from the melting transition peak illustrated in Fig. 16, by measuring the total area under the peak that is proportional to the heat flow per mass of material. Heat flow is the heat emitted per second, therefore the area under the peak is given in units of (heat • temperature • time"*) for the mass of the sample used. As a result the area per unit mass (APUM)
540
MANUAL 37: FUELS AND LUBRICANTS o
/—tr-s-S^*-" °
-5 -6 ^-7
j 1
0-9 -11 -12
HANDBOOK
/ / RK •
Pr •4
o
/--rrQjQ.oo
-5 -6
Ss -n -12 -13
•
PR
•
f
„M-~-.
Pr
2
Pr
FIG. 15—Solubility of n-tritriacontane (n-CssHes) in supercritical carbon dioxide at 308 K as predicted by various equations of state and compared with the experimental solubility data [32].
of the sample will be APUM
Heat X Temperature Ttime X Mass
Q J^ ~0M
(12)
Typically, the actual units of \Hf&re (joules • Kelvin • seco n d s " ' • g r a m s " ' ) . Typically, the APUM is divided by the heating rate (K/s) of the DCS experiment used to collect the data. This will simplify the expression to yield the specific heat of melting: Q.T APUM Heating Rate
e.M
X e
Q_
M
(13)
Since the mass of the sample that was analyzed is known, it is then multiplied by the heat emitted/gram of sample to 5deld the amount of heat given off (Q) during the melting process. (14) %XM =M M Figure 16 illustrates the DSC traces for three different petroleum waxes; one for each wax type - paraffin (Fig. 16a), intermediate (Fig. 16^), and microwax (Fig. 16c). The DSC
trace shown by Fig. 17 demonstrates the decrease in crystallinity as the melting point of the wax increases. The thermal analysis procedure for this work was started at - 50°C for o p t i m u m crystallization of the wax. The wax sample was heated at a controlled rate to +150°C. The point at which there is a deflection in the base line is the temperature that the wax begins to melt. The point at which the peak scan returned to the base line is the temperature the wax sample is completely melted. The peak area represents the amount of energy used to melt the wax sample and is calculated as described above. In addition, an estimate on the expected melting point can be distinguished. The experienced technologist could tell by looking at the shape of a DSC trace if the wax is a paraffin, intermediate, or microwax. Paraffin waxes typically exhibit sharp peaks as shown in Fig. 16a, DSC peak shapes for intermediate waxes are less sheirp as shown in Fig. \6b, and microwaxes exhibit even less sharp peaks, typically like the peak shown in Fig. 16c. It should be noted that there is a characteristic small transition peak in the DSC trace for a macrocrystalline paraffinic wax as illustrated in Fig. 16fl. The transition that is indicated is a solid-solid phase change (orthorhombic to hexagonal
TABLE 15 —Interaction parameter (ki2) of some systems. System C2H6 - M-C28H58 C2H6 - n-C29H6o C2H6 - M-C30H62 C2H6 - K-C32H66
C2H6 - ra-C33H68
CO2 - n-C28H58
C 0 2 - M-C29H60 CO2 - «-C3oH62 CO2 - n-C32H66
CO2 - n-CjsHfts
AAD%
kyi
T [K]
P [bar]
RK
3M
RM-2
308.2 308.2 308.2 313.2 308.2 313.2 318.2 319.2 308.2 313.2 318.2 307.2 308.2 313.2 318.2 318.6 323.4 325.2 308.2 318.2 308.2 318.2 308.2 318.2 328.2 308.2 318.2 328.2
56-240 65-240 66-200 66-136 66-240 66-200 80-240 80-136 65-240 65-202 65-240 123-181 80-240 90-275 100-250 119-284 125-327 121-284 100-240 100-240 90-250 105-250 120-240 140-240 140-240 120-240 140-240 140-240
-0.4638 -0.4146 -0.4777 -0.4738 -0.5124 -0.5011 -0.5248 -0.4872 -0.4632 -0.4459 -0.4506 -0.3458 -0.3161 -0.2910 -0.2915 -0.3067 -0.2973 -0.2946 -0.2751 -0.1961 -0.3254 -0.3125 -0.4140 -0.3913 -0.3777 -0.3461 -0.3384 -0.3057
-0.2099 -0.1618 -0.2066 -0.2137 -0.2259 -0.2264 -0.2438 -0.2241 -0.1933 -0.1845 -0.1918 -0.0936 -0.0901 -0.0835 -0.0867 -0.0859 -0.0869 -0.0867 -0.0530 -0.0540 -0.1141 -0.1197 -0.1500 -0.1462 -0.1345 -0.1051 -0.1043 -0.0990
0.0807 0.1215 0.1025 0.0901 0.1020 0.1050 0.0966 0.1087 0.1100 0.1433 0.1433 0.2487 0.2477 0.2532 0.2504 0.2531 0.2552 0.2540 0.2782 0.2789 0.2481 0.2439 0.2448 0.2483 0.2596 0.2825 0.2832 0.2878
PR
SRK
RM-1
RK
3M
RM-2
PR
-0.0553 -0.0131 -0.0571 -0.0517 -0.0707 -0.0658 -0.0762 -0.0565 -0.0286 -0.0240 -0.0203 0.0110 0.0296 0.0365 0.0359 0.0347 0.0385 0.0321 0.0645 0.0818 0.0327 0.0273 -0.0162 -0.0035 0.0044 0.0262 0.0280 0.0428
-0.0189 0.0260 -0.0206 -0.0139 -0.0297 -0.0263 -0.0347 -0.0157 0.0137 0.0193 0.0228 0.0507 0.0708 0.0765 0.0746 0.0736 0.0764 0.0690 0.1075 0.1451 0.0779 0.0700 0.0316 0.0426 0.0477 0.0754 0.0872 0.0876
-0.1283 -0.0701 -0.1121 -0.1233 -0.1214 -0.1131 -0.1142 -0.1219 -0.0779 -0.0580 -0.0530 0.0211 0.0194 0.0286 0.0232 0.0278 0.0314 0.0287 0.0670 0.0672 0.0084 -0.0005 -0.0139 -0.0092 0.0104 0.0496 0.0494 0.0557
46.2 53.1 25.0 13.9 46. 24.4 45.2 23.6 50.2 39.6 45.7 51.0 52.3 46.7 64.7 53.7 62.5 64.2 71.8 81.2 69.6 67.5 59.5 67.3 57.5 68.4 65.0 67.4 60.0
27.0 29.9 22.8 30.8 43.0 34.5 17.7 37.4 34.4 21.5 24.2 7.5 18.3 25.0 13.4 8.2 5.8 7.3 21.5 22.2 17.1 8.3 8,1 6.7 9.2 25.0 22.9 18.5 20.3
14.4 23.9 57.2 29.7 56.2 40.2 22.8 39.3 50.7 28.9 28.0 34.4 35.0 44.5 31.0 33.2 28,9 22,6 12,9 6,9 28,8 28.7 24.1 22.3 27.1 11,3 4,8 3,5 28,3
38, 48. 22. 18. 41. 20. 32. 28. 43. 25. 42. 45. 49. 39. 47. 45. 53. 45. 67. 76. 66. 57. 54. 55. 40. 64. 61. 60. 46.
Overall " Kalaga and Trebble, 1997. ' Moradinia and Teja, 1986. " Suleiman and Eckert, 1995 •' Moradinia and Teja, 1988. " McHugh et al., 1984 ^Reverchon et a ., 1993. ^ Chandler et al., 1996.
S5.0
sao 7SJ>
fata ^
TOO
2\S»C 55.syc SISJ 1316.99 2IB.99
sg s ^*' &o1 *"35,BJM Z5JI
"T—
I soo
-2SJ)
TSJ»
IOOLO
12M
1500
Tenq>a-^iire(<0
XI
^xsrc
TS3yC PCA 6ssn Am I13&63 rtH 17172 H e i ^ 43Jf7
«
4M-
^
S, 4 M -
* s
-
1-:
..
3W1-
-1S.0
1
1
1
25,0
5IU
1 7U
TemperatnTO (°Q
lOOJ)
1 125,0
151
Teiiii)eratiire('C)
FIG. 16—The heat of fusion (AHr) calculation from the DSC melting transition peak by measuring the total area under the peak, (a) paraffin, (b) intermediate, (c) microwax.
542 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK
1 -50.0
-25.0
r ao
25.0
50.0
75.0
1 lOaO
r 125.0 150.0
Temperature (°Q FIG. 17—The heat of fusion (AH) calculation from the DSC melting transition peak by measuring the total area under the peak of several paraffins demonstrating the decrease in crystallinity as the melting point of the wax increases. crystal structure). As the wax crystal continues to absorb energy, a larger peak is recorded and then actucil melting occurs with a return to the base line as the temperature continues to increase. In addition, there is a bimodality indicated in the DSC trace peak shapes for intermediate (Fig. 16b) and microciystalline (Fig. 16c) waxes. Bi-modal shape is related to the breadth of the wax composition. Bimodal peak shape is not related to transition. The apparent bimodality indicates that the wax has not been made as a narrow distillation cut. The melting point of wax is in the DSC area that the curve begins to return to the base line (downward slope) as the temperature increases. The squat DSC peak shape of the microwax shown in Fig. 16c indicates that it is less crystalline and has a broader melting. The apparent bimodality of the microwax is related to the different melting fractions that appear in this particular wax. Determination of the heat of fusion of a wax is of practical significance for a n u m b e r of reasons. For example, the changes of shape of a DSC trace to that of known waxes may indicate that a wax has been contaminated or altered. This may be confirmed by comparing the heat of fusion of a previously purchased paraffin wax with the suspect wax. For example, a historical value for heat of fusion of a wax may be 200 J/g and a newly purchased paraffin wax may have a heat of fusion of 180 J/g. This variation confirms that the two waxes exhibit different properties. Another application of the heat of fusion could be for the comparison of properties of nominally similar waxes offered by two different suppliers. The higher the heat of fusion, the more crystalline the wax is. For some applications, like candles, high crystallinity is desirable to aid in the mold release properties due to shrinkage upon cooling. Low crystalline waxes do not shrink as m u c h as high crystalline products. ASTM Test Method D 4419 has been developed to characterize petroleum waxes and measurement of their transition t e m p e r a t u r e s by Differential Scanning Calorimetry (DSC). Figures 16a, 16b, and 16c are DSC endothermic scans of a paraffin, intermediate, and microcrystalline wax.
respectively. Referring again to Fig. 16a (paraffin wax), the endotherm is started at — 50°C for optimum crystallization of the wax. The wax sample is heated at a controlled rate to -l-150°C. The point at which there is a deflection in the base line is the temperature that the wax begins to melt. The point at which the peaik scan returned to the base line is the temperature the wax sample is completely melted. The peak area represents the a m o u n t of energy used to melt the wax sample as discussed above. Figure 17 illustrates that as the melting point of paraffin wax increases, the heat of fusion decreases because of the higher content of less crystalline branched alkane structures. Microcrystalline waxes have a lower heat of fusion than paraffin wax that is directly related to the greater amount of branched alkanes (less crystalline microstructure). Listed in Table 16 are the typical heats of fusion data for both paraffin and microcrystalline waxes.
Effect of Additives In the petroleum wax industry, it is often necessary to use additives to improve the processability of wax or wax mixtures by modifying their physical properties. This may be accomplished by the addition of additives that may include stearic acid, polyethylene, ethylene-vinyl acetate copol5'mer or a Fischer-Tropsch wax. For example, stearic acid may be added to a paraffin weix to increase firmness, reduce melting point, aid in mold release, prevent candles from losing their shape in warm weather, etc. Polyethylene is another additive that may be used. Polyethylene m a y b e added to a paraffin wax to harden the wax structure, modify burning rate, and improve strength and gloss. In addition to physical property modification, additives also could alter the microstructure of waxes as demonstrated by Fig. 14.
Test Procedures for P e t r o l e u m Wax Characterization There are three properties used to characterize petroleum waxes: (1) physical properties, (2) chemical properties, and (3) functional properties. Physical
Property
Determination
Melting Point - Test Methods ASTM D-87,D 3944, and D127— Melting point is a wax property that is of interest to the consumer and can be an indication of the performance properties of the wax being tested. The melting point of a wax is defined as the temperature at which the melted petroleum wax first shows a m i n i m u m rate of temperature change when allowed to cool under prescribed conditions. Test Method D 87 is one of the most commonly utilized tests for melting point determination for petroleum waxes. Paraffin waxes are often marketed based on melting point data produced by D 87. This test method is performed by placing a specimen of molten wax in a test tube equipped with a thermometer as illustrated in Fig. 18a. The test tube is TABLE 16—Typical heats of fusion (J/g). Fully Refined Paraffin wax 180-210 Microcrystalline wax 140-190
CHAPTER 19: PETROLEUM WAXES
Dimensions in inches (millimeters)
'MtanaxfabMaa Wax.
PuraRn WiuE
t
I I Tnutskkai Point-^ \ p
(b)
Time-*
. Tim*-*
FIG. 18—(a) Apparatus for ASTM Test Method D 87. Cooling curve for: (b) a paraffin wax, (c) for intermediate wax, microwax, petrolatum, or waxes containing a higfi percentage (>50%) of branched alkanes
(c)
543
544
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
then placed in an air bath that is immersed in a water bath and held at 16-28°C (~61-82°F). Temperature readings are taken periodically until the wax solidifies u n d e r specified conditions. During solidification, the rate of temperature decreases and produces a plateau in the cooling curve, which is obtained by plotting the temperature versus elapsed time as, illustrated in Fig. I8b. The temperature where the plateau occurs is defined as the melting point. (Note: The thermometer used for this work shall conform to ASTM Specification E-1.) Test Method D 87 is not applicable for microcrystalline wax, intermediate wax, petrolatum, or waxes containing a high percentage (>50%) of branched alkanes, because a temperature plateau will not occur with such type of waxes as illustrated in Fig. 18c and because these type of waxes have a much broader melting distribution (characterized by DSC) t h a n paraffin waxes. For non-paraffin type waxes. Test
Method D 3944, which is a "solidification point" method (Fig. 19a), can be used for melting point determination. The solidification point of a petroleum wax is: the temperature in the cooling curve of the wax where the slope of the curve first changes significantly as the wax sample changes from a liquid to a solid state. This is illustrated in Fig. I9b, which is a typical cooling curve for solidification point measurement of a petroleum wax. Test Method D 3944 is performed by heating 50 mg of sample in a test tube above the solidification temperature. Once the wax is melted. Fig. 20a, a thermocouple (connected to a recorder) is placed in the sample, as illustrated in Fig. 20b, and allowed to cool to ambient temperature. As the sample cools, a plot of temperature versus time (Fig. 18a) is obtained. This test method is based on the same methodology as D 87 with the exception that automated test equipment is
Iticxmo couple Lead-
Ifi-Iluorocarbon Boldsr Adapter exSO nun Test Tiibe ,
Aluminum Heating Bloek 50x50x50 nm (2x2x2 in).
0.020 i n (H) Metal Sheathed Tbexnocouple Probe
r~^~^ Au£otrans£otner TO 110 AC Outlet
IFE-Sluorocarbon Disk Centering Guide
(a)
50 mg Wax Sample'
Temperature
solidification Point
llOV 30-Hatt Cartridge Heater
Heat turned off
First significant change in slope
(b) FIG. 19—(a) Apparatus for determination of melting (solidification) point (cooling curve) of non-paraffin type waxes used in ASTM Test Method D 3944; (d) A typical cooling curve for melting (solidification) point measurement of non-paraffin type waxes.
CHAPTER 19: PETROLEUM WAXES 545
DmKnsionsmmches(mS&r,
(a)
To Sccorder
^ 6 jt SO vm T e s t Tubs 0 . 0 2 0 I n c h O.D. Metal Sheathed Thermocouple 1 / 8 " Thick I?£-n.iwr»c«"bcm Diak Cant«-tnc Quid*
t
(b)
35 X 52 ran. Vial—
FIG. 20—(a) Apparatus for determination of melting point of paraffin wax. The same methodology as Test Method D 87 with the exception that automated test equipment shown in Fig. 20b is used; (b) Once the wax is melted (Fig. 20a), a thermocouple (connected to a recorder) is placed in the sample, as illustrated here, and allowed to cool to ambient temperature.
Test Method D 127 is performed by depositing a sample on two thermometer bulbs by dipping chilled thermometers into the sample. The thermometers are then placed into test tubes and heated in a water bath until the specimens melt and the first drop of wax falls from the thermometers. The average of the temperatures at which these drops fall is recorded as the drop melting point. Other ASTM test methods for melting and freezing point determination eire listed in Table 17. Pour Point - Test Method D 97—The pour point of a fluid is that temperature where the fluid ceases to flow and it may be determined by Test Method D 97 (Test Methods ISO 3016). This test method is conducted by preheating the sample until it is sufficiently fluid to pour into a test jar. At this point the sample is cooled at a specified rate and examined at intervals of 3°C for flow characteristics. The lowest temperature at which the movement of the test specimen is observed is recorded as the pour point. Test Method D 97 is intended for use on any petroleum product. Congealing Point - Test Methods D 938 and D 3944— Another important physical property for wax characterization is congealing temperature (set point), which is defined as that temperature at which molten petroleum wax, when allowed to cool under prescribed conditions, ceases to flow or that temperature where molten wax begins to solidify. Test Methods D 938 and D 3944 (see above discussion for Test Method D 3944) are used for determining the congealing temperature. (ISO 2207 may also be used.) These tests are more applicable for waxes with broader melting ranges, such as microwaxes, petrolatums, and wax blends containing additives. The congealing a n d solidification point is always lower than the recorded drop melting point. Test Method D 938 is performed by dipping the bulb of a thermometer to cause a droplet of wax to adhere on the therm o m e t e r bulb. The t h e r m o m e t e r is then placed in a prewarmed air jacket and cooled at a fixed rate while the thermometer is rotated in a horizontal position until the wax sample ceases to flow (congeals). When congealing occurs, the wax may be at the solid state or it may be at the semisolid state, depending on the composition of the sample. Cloud Point - Test Method D-2500—The cloud point of a petroleum product is the temperature at which a cloud of solid crystcJs appears in a liquid when it is cooled under prescribed conditions. The cloud point is an index of the lowest
TABLE 17—Other test methods for melting and freezing point determination. Method
used. (ISO 3841 or BS 4695 are also used for cooling curve determination.) Test Method D 127 may also be used to determine the "drop melting point" of a wax. The drop melting point is defined as that temperature where the Wcix becomes sufficiently fluid to drop from the thermometer being used for making the determination under prescribed test conditions. Test Method D 127 is commonly used for non-paraffin type waxes and paraffin wax blends. These include paraffin/microcrystalline wax blends and paraffin/additive blends that may contain polyethylene, ethylene-vinyl acetate copolymer, or a Fischer-Tropsch wax. Test Method D 127 is used to determine the melting characteristics of petrolatums and other high viscosity petroleum waxes.
Capillary/liquid bath Capillary/metal block Kofler hot bar Differential thermal analysis & differential scanning calorimetry Freezing temperature
Pour point
Test Procedure
ASTM E 1 DIN 53181 DIN 53736 ASTM D 3451 ASTM E 473 ASTM E 537 BS 4633 BS 4695 DIN 51421 DIN 53175 ISO 1392 JIS K 00-65 NFT60-114 NFT20-051 ASTM D 97 ISO 3016
546
MANUAL
37: FUELS
AND LUBRICANTS
HANDBOOK
temperature of its utility for certain applications. The cloud point is determined by cooling a petroleum sample in a cloud point test apparatus illustrated in Fig. 2 1 . At a specified rate a n d periodic examination, the t e m p e r a t u r e at which a "cloud" is first observed at the b o t t o m of the test j a r is recorded as the cloud point. This test method covers only petroleum products that are transparent in layers 40 m m thick and with a cloud point below 49°C. Oil Content - Test Method ASTM D 721—Test Method D 721 describes the determination of oil in petroleum weixes having a congealing point of 30°C (86°F) or higher as determined in accordance with Test Method D 938, and containing not more than 15% of oil. The test is conducted by dissolving the weix in a solvent (methyl ethyl ketone—MEK) and chilling to a low temperature to precipitate the wax. The wax is filtered from the solvent and the solvent soluble residue is evaporated to determine the oil content. The oil content of paraffin waxes is indicative of the degree of refining or processing that has been performed. Fully refined wax generally does not have an oil content exceeding 0.5%. Paraffin waxes having oil contents exceeding the 0.5 limit are marketed as scale ( < 3 % oil) and slack paraffin waxes (<40% oil). Microwaxes a n d petrolatums have a higher affinity for oil than the paraffin waxes and are marketed with oil contents as high as 20%. With some t5fpes of waxes with oil contents greater than 5%, there may be an incompatibility with MEK resulting in
the formation of two liquid phases. If this occurs, the method is not applicable to the material being tested. Solvent Solubility - Test Method D 3235—Test Method D 3235 is a solvent extraction method used to determine the total amount of wax components that are soluble in the extraction solvent. The test is performed in the same manner as Test Method D 721 except that a different solvent composition is used. Viscosity - Test Methods D 445, D 2669, and D 3236— Molten wax viscosity is important for determining the proper melt flow properties for applying waxes by end use machinery under low shear conditions. Test Method D 445 for the m e a s u r e m e n t of kinematic viscosity of t r a n s p a r e n t and opaque liquids is used for petroleum waxes with viscosities less than 25 centistokes at 99°C. Test Methods D 2669 and D 3236 are used for waxes that have been c o m p o u n d e d with wax additives resulting in a high viscosity, u p to 2000 poise at 175°C (347°F). Fluid viscosity is determined by placing a representative sample of the molten wax into a thermally controlled sample chamber. Apparent viscosity is determined u n d e r thermal equilibrium conditions using a precision rotating spindle type viscometer as illustrated in Fig. 22. Data for several temperatures can be plotted on appropriate semi-logarithmic graph paper and apparent viscosity at intermediate temperatures can be estimated. Although precision has not been studied. Test Method D 3236 may be adaptable to measure fluids with viscosities
4*^-Mk»Uk 3U-3iJ0lIlu aiiLO- aiAU).
CORK '
OOOUWr LEVEL-
JMnr I
9
MMMC •
-rP ini
All dimensions in millimeters FIG. 21—Apparatus for Standard Test Method D 2500 to measure the cloud point of petroleum products.
CHAPTER 19: PETROLEUM WAXES
547
VISCOMETER
TMEKMOMmn auui IN HomzoNDU. PLMC WITH C i N I U OF SPINOLC
aoncM or luiwii «M> HTTOKOrtUMO
MCK
x_x
^
:
^
"
FIG. 22—Viscosity measurement apparatus, side view, used in ASTIM Test Method D 2669.
greater than present 2000 Poise limit and temperatures above 175°C (347°F). Equipment described in this procedure permits testing of materials having viscosities as high as 160 000 Poise and provides temperatures up to 260°C (500°F). Hardness - Test Method D1321—The hardness of a wax is a measure of its consistency. Waxes can show variation in hardness and the differences are more pronounced at higher temperatures. For example, two waxes may have the same hardness as measured at one temperature, but are dissimilar at a different temperature. Hardness is determined by penetration and is defined as the depth in tenths of a millimeter to
FiG. 23—Needie penetration apparatus of Standard Test Method D 1321 used for measurement of hardness of petroleum waxes.
which a standard needle penetrates into the wax under defined conditions using a penetrometer such as the one illustrated schematically in Fig. 23. To measure the hardness of a wax, Test Method D 1321, the Needle Penetration Method is commonly used. This method is based on a standardized needle penetrating into the wax surface at a specific temperature. The smaller the numerical value obtained from this test, the harder the wax is. The hardness of the wax can be indicative of undesirable blocking tendency for wax-coated paper. (Note: Test Method D 1321 is similar to Test Method D 937, which is a cone penetrometer
548 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
Sample Reference Resistance Thermometer Heater
(a)
Resistance Thermometer Heater
AT-Signal
Nitrogen
First-Order Transition: Apex
Second Order-Transition: Apex - Tjj^ Onset End
(b)
T.-^,,*..,. -r-
FIG. 24—(a) Differential scanning calorimetry (DSC) experimental set up; (b) Schematic of petroleum wax DSC curve (heating cycle) sample determined to have soiid-liquid and solid-solid transitions. This figure is similar to Fig. 16a.
method for petrolatum, which may also be used for softer waxes. The cone penetration value is more of a measure of firmness or consistency rather than hardness.) Transition Temperatures by Differential Scanning Calorimetry (DSC)-Test Method D4419—Test Method D 4419 is a rapid and convenient method for determining the temperature limits governing the change a wax undergoes from solid to liquid or as a solid-solid transition. This test method measures the transition temperatures of petroleum waxes, including microcrystalline waxes, by differential scanning calorimetry (DSC) as shown in Fig. 24a. The normal operating temperature range extends from 15-150°C. DSC is a technique that measures the difference in energy inputs into a substance and a reference material using a controUed-temperature pro-
gram. DSC can differentiate the tj^je of petroleum wax being evaluated by its melting and crystallization property. Figure 2Ab (which is similar to Fig. 16a) is the schematic of a petroleum wax DSC curve exhibiting solid-liquid and solidsolid transitions (Heating Cycle) and the calculation of such temperatures. Paraffin waxes being derived from the distillation process have sharp peak shapes, while microcrystalline waxes being derived from residual fractions have broader peak shapes. This is shown in Fig. 16 (Note: Refer to Standard Terminology E 473 for additional information). Chemical Property Determination Petroleum waxes being composed of hydrocarbons Eire relatively inert but they can undergo compositional chemical
CHAPTER changes when exposed to elevated temperatures in the presence of oxygen due to oxidation. Waxes can degrade in the presence of heat and oxygen. The degradation process involves breaking a bond between a carbon and a hydrogen atom to make a free radical. The free radicals quickly form peroxides initially and further react to form acids. The changes in composition can possibly be detected by testing for color a n d odor. Antioxidants are added to p e t r o l e u m waxes to chemically stabilize them from the heat degradation process [41]. Color - Test Methods D 156 and D 1500—The color of petroleum waxes can indicate the degree of refinement or possible contamination. Color is not always a reliable parameter for determining quality and should be used judiciously as a specification. There are two methods for determining the color of petroleum waxes: Test Methods D 156 and D 1500, and both are subjective and measure the empirical value based on visual observation of the wax in the molten state. Test Method D 156 is the Saybolt Chronometer Method for quantifying the color of petroleum products such as a petroleum wax. Saybolt color is an empirical definition of the color of a clear petroleum liquid based on a scale of —16 (darkest) to +30 (lightest). The number is derived by finding the height of a column that visually matches the appropriate one of three glass standards and referring to Table 1 of Test Method D 156. This is done using a Saybolt chronometer (see Fig. 25), which consists of a sample and standard tubes, optical system, light source, and ASTM color standards. While Test Method D 156 is used to determine the degree of whiteness of a wax, Test Method D 1500 is used to measure the color of waxes that have a tint darker than off-white. Test Method D 1500 is conducted using a standard light source, with liquid sample placed into a standard glass container (sample jar) (see Fig. 26) and compared with colored glass disks ranging in value from 0.5-8.0 with 0.5 increments. Carbonizable Substances - Test Method D 612—Test Method D 612 is applicable to paraffin waxes for pharmaceutical use as defined in the United States National Formulary. Molten wax is treated with sulfuric acid and the acidic layer is compared visually with a colorimetric reference standard to determine if it passes the conformance criteria for refined wax using the color comparator shown in Fig. 27. Peroxide Number - Test Method D 1832—Waxes are heat sensitive and they are susceptible to the action of the oxidation process. The detection of peroxides is the first indication that a wax has begun to deteriorate in terms of oxidation stability. Petroleum waxes should not have any measurable peroxide values. Deterioration of petroleum wax results in the formation of peroxides a n d other oxygen-carrying compounds that will oxidize potassium iodide. Peroxide content is reflected by the peroxide n u m b e r that is defined as the milliequivalents of constituents per 1000 g of wax that will oxidize potassium iodide. Odor - Test Method D 1833—In some end-use applications, such as food packaging, the intensity of the odor is an important characteristic. Odors can be an indication of the degree of refining, contamination, or oxidation. Test Method D 1833 describes how to rate the odor intensity based on a subjective evaluation using a multiple-member test panel. This test is conducted by preparing odor test specimens from petroleum
19: PETROLEUM
WAXES
549
wax and placing approximately 10 g of thin shavings on odorfree paper or glassine. Individual test specimens are then evaluated for odor by each panel member and assigned a number according to the odor scale shown in Table 18 that best fits the intensity of the odor. As an alternative procedure, the wax shavings are placed in bottles with each panel member making an odor determination between 10 and 60 min after the shavings are placed into the bottles. The average of the panel ranking is reported as the odor rating of the sample. Composition by Gas Chromatography-Test Method D 5442— Test Method D 5442 is applicable to petroleum derived waxes, including blends of waxes. This test method covers the quantitative determination of the carbon n u m b e r distribution of petroleum wEixes in the range of n-C17 through n-C44 by gas chromatography using internal standardization. In addition, the content of normal and non-normal hydrocarbons for each carbon n u m b e r is also determined. Material with a carbon number above n-C44 is determined by difference from 100 mass % and reported as C 45 +. (Note: Standard Practice E 260 provides further information on gas chromatography and Standard Practice E 355 provides information relating to gas chromatography terms and relationships.) Test Method D 5442 is not applicable to oxygenated waxes, such as synthetic polyethylene glycols (i.e., Carbowax), or natural products such as beeswax or c a r n a u b a . This test method is not directly applicable to waxes with oil content greater t h a n 10% as determined by Test Method D 721. Functional
Property
Determination
The following methods are for the evaluation of wax base coatings intended for paper and paperboard. The methods were developed in concert with the Technical Association of Pulp and Paper Industries. Specular Gloss - Test Method D 1834—Specular gloss is defined as the degree to which a surface simulates a mirror in its capacity to reflect incident light. Test Method D 1834 is a method designed to determine the capacity of a wax coated surface to simulate a mirror in its ability to reflect an incident light beam using a glossmeter such as that illustrated in Fig. 28. Surface gloss is desirable for some waxed paper applications. For determining the gloss of book paper, reference should be made to Test Method D 1223. For very high gloss paper refer to Wink et al. [42]. Gloss Retention - Test Method D 2895—This test is intended to correlate with the conditions that are likely to occur in the storage and handling of wax-coated paper and paperboard. Test Method D 2895 is intended primarily to measure the gloss retention, which is defined as the percent of original gloss retained by a test specimen after aging under specified conditions. It is calculated as the final gloss divided by the initial gloss multiplied by 100. The initial gloss of waxed paper or p a p e r b o a r d is m e a s u r e d in accordance with Test Method D 1834, then remeasured after aging the sample in an oven at 40°C (104°F) for 1 and 7 days. The 1-day test is conducted to observe trends while the 7-day test is the standard test duration. Surface Wax - Test Methods D 2423, D 3521, and D 3522— Wax coatings are applied to provide a better moisture barrier, appearance, and abrasion resistance. These performance features are influenced by the amount of wax present on the surface. Test Method D 2423 is used to determine the
550 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK
1
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0 Suae m j
0 0 0 (i) © 0 0
oouw couM oovn mm «uiw nacMw (iMm «T mm suun oi*x
0
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UTHADML «•» (LTCntlfK
MUUM seM»ovia
0 UtlX X) t n i m * LS « 0 WATT soil. MMTHI
SCOTIOH "A-A"
Saybolt Chromometer Tube Heater
Adapter
FIG. 25—ASTM Standard Test Method D156 Saybolt Chromometer and artificial daylight lamp.
CHAPTER 19: PETROLEUM WAXES 551 amount of wax present on the surface of the substrate, but not the absorbed wax. Test methods that determine the applied wax by solvent extraction (such as Test Method D 3344) do not differentiate between the wax present at the surface and to that which penetrated the substrate.
Test Method D 3 521 also determines the a m o u n t of wax that is present on the surface of corrugated paperboard. This method is applicable to a board on which wax has been applied by curtain coating, roll coating, or other methods. The substrate board may or may not contain impregnating (saturation) wax within its structure. If it is known that the specimen has coating wax only, with no internal saturating wax, then Test Method D 3344 may determine the total coating wax applied. Determination of the total amount of wax present by ASTM D 3521 involves delamination of the coated facing to obtain a ripple-free sheet, then scraping off the wax using a razor blade and calculating the amount of wax removed. Test Method D 3522 is used to determine the amount of wax that has been applied to the individual layers of the corrugated paperboard and the a m o u n t of the impregnating (saturation) wax in the same facing. This is accomplished by peeling the coated facing from the medium and then splitting it into two layers; one bearing the coating on waxed fibers only and the other containing waxed fibers only. The layers are extracted separately, collecting both fibers and wax. This will permit the calculation of the applied surface coating and the amount of impregnating wax.
TABLE 18—Odor intensity scale. Numerical Rating Odor Description None Slight Moderate Strong Very strong
FIG. 26—Standard glass sample jar used in Test Method D 1500 to measure the color of waxes.
e-i'*Mo/es
Section
Plan
A-A. T l 3 STOPPER
SiaTlSNECK
m A
-i-n-j. T Elevation
i
I Hi I h !
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i li
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A
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I
End
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ISLi I40±2MM Appnox. CAPACITY TO STOPPER late TO {SA ml 145H5.0 MM 0.0. WALL" I.ZOIMM AVER
TEST TUBE
FIG. 27—Color comparator used in Standard Test Method D 612 for measurement of carbonizable substances in paraffin waxes for pharmaceutical use.
552
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
Vacuum
' J J t j
J J
^
l^L '^-—
Specimen Holder Porou* Plate f^\\
>.Specimen
ViU.:.v.M>'.'-.-..!.-.vA-.X!-'.J-; L
disruption occurs across 50% of the waxed p a p e r surface when the test strips are separated. The temperature at which the first film disruption occurs on the waxed paper when the test strips are separated is the wax picking point. Test Method D 1465 is used to d e t e r m i n e the t e m p e r a t u r e at which two strips of wax-coated p a p e r will adhere to each other. Surface disruption of wax coatings at relatively low a m b i e n t t e m p e r a t u r e s is a performance problem for low melting point waxes. If the surface of a waxed p a p e r is blocked together, then surface gloss and barrier properties will be altered. Two strips of wax-coated paper are placed on a calibrated t e m p e r a t u r e gradient plate for 17 h and removed, cooled, and peeled apart to determine the block point temperature. Figure 29 illustrates a Type A and a Type B blocking plate used for these measurements. Coefficient of Kinetic Friction - Test Method D 2534—A coated surface under load is pulled at a uniform rate over a second coated surface. This is d o n e experimentally by preparing a "sled" with a weight and then pulling it over the surface to be tested using a horizontal plane and pulley assembly. The force required to move the load is measured, and the coefficient of kinetic friction (/j-k) is calculated as follows: fH, = A/B
rf
J ^JJ
^^*/J
FIG. 28—Diagram of relative positions of essential elements of Glossmeter used in Standard Test Method D 1834.
Total Wax Content - Test Method D 3344—Many of the functional properties of a wax-treated paperboEird are dependent on the amount of wax that is present. Test Method D 3344 determines the total amount of wax in a sample of wax-treated corrugated paperboard by extraction. It is applicable to specimens that have been waxed by either impregnation (saturation) operations or coating operations, or combinations of the two. Weight of Wax Applied During Coating - Test Method D 3708—Test Method D 3708 is used to determine the weight of a hot melt coating applied to corrugated board by curtain coating. This method is intended for use as a routine process control in the plant. The a m o u n t of wax applied is determined by attaching a folded sheet of paper to production corrugated board, running the combination through the curtain coater, and subsequently determining the applied weight of wax on the sheet of paper. Blocking Point - Test Method D 1465—The blocking point of a wax is defined as the lowest temperature at which a film
(15)
Where A = the average scale reading from the electronic load cell-type tension tester for 150 m m (6 in) of uniform sliding and B = sled weight (g). The value obtained is related to the slip property of the wax coating. High slip property values may not be desirable for many commercial articles that have been coated with petroleum wax. Abrasion Resistance - Test Method D 3234—This test method is designed to help predict the resistance in change of gloss that coatings may be subject to during the normal handling of coated paper and paperboard products. Abrasion resistance is the resistance to change in gloss when that coating has been subjected to an abrading action by an external object. Test Method D 3234 is conducted by dropping 60 g of sand on a very small area of a coating under fixed conditions. The abrasion resistance test apparatus is illustrated in Fig. 30. Gloss is measured with a 20° specular glossmeter illustrated in Fig. 28 before and after the abrading action by the falling sand. Hot Tack - Test Method D 3706—Hot tack is defined as the cohesive strength during the cooling stage before solidification of a heat seal bond formed by a wax-polymer blend. Flexible packaging materials are formed into finished packages by joining surfaces with heat sealed bonds. The bonding process is performed o n high-speed packaging lines and the application pressure used to hold the surfaces together is released before the bond has completely solidified. The wax-polymer blend must have enough hot tack while still in a molten stage to hold the sealed areas together until the blend has cooled. In Test Method D 3706, flexible packaging specimens are heat-sealed together over a series of temperatures and dwell times. Immediately after each seal is formed and before it has started to cool, a force tending to separate the specimens is applied by a calibrated spring. If the hot tack of the blend is strong enough, the seal remains closed until it has solidified; if not, the seal separates. Thus each spring force and test condition either passes or fails. The pattern of pass/fail results is plotted to the blend characteristics.
CHAPTER 19: PETROLEUM WAXES 553 $1 nun K 30S KHX Itey ttDck mtil Medkt 2S mm » 2B RHR (1" s 1").
K S I S MDI
( r « i s r K »*i
7C2 MR iVty Iwit.
13 MM ^ l / r i h (AM. T«m4l lor 13 mm d / T }
Type A Blocking Plates
Into (Mb tWRi tmiiiimd
Type B Blocking Plate FIG. 29—The two types of blocking plates used In Standard Test Method D-1465 to measure the blocking point of wax.
554 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK 500 ml Separatory Funnel
60 g of Sand Stopcock
Size I H
Stem Cut Off from Separator/ Funnel U.S. Standard Sieve No. 12
^ " ^ 2 B mm (I") I.D.
Specimen
For tscating Exact Position for Mailing Glossmeter Readings
For Dropping 60 g of Sand
FIG. 30—Apparatus for measuring abrasion resistance of wax coatings in Standard Test lUlethod D 3234.
Acknowledgments The authors thank Dr. George Totten for his helpful advice and guidance in the preparation of this chapter and Dr. Sony Oyekan, Dr. Chen-Hwa Chiu, Dr. Sang J. Park, and Mr. Adrian D'Sousa for their technical assistance.
D 721 D 937 D 938 D 1168 D 1298
ASTM STANDARDS D 1223 No. D 87 D 97 D 127 D 156 D 287 D 445 D 612
Title Test Method for Melting Point of Petroleum Wax Test Method for Pour Point of Petroleum Products Test Method for Drop Melting Point of Petroleum Wax Including Petrolatum Test Method for Color, Saybolt, of Petroleum Products Test Method for Gravity, API, of Crude Petroleum and Petroleum Products (Hydrometer Method) Test Method for Kinematic Viscosity of Transparent and Opaque Liquids Test Method for Carbonizable Substances in Paraffin Wax
D 1321 D 1465 D 1500 D 1832 D 1833 D 1834 D 2423
Test Method for Oil Content of Petroleum Waxes Test Method for Cone Penetration of Petrolatum Test Method for Congealing Point of Petroleum Wcixes, including Petrolatum Test Method for Hydrocarbon Waxes Used for Electrical Insulation Test Method for Density, Relative (Specific Gravity) or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method Test Method for Specular Gloss of Paper and Paperboard Test Method for Needle Penetration of Petroleum Waxes Test Method for Blocking and Picking Points of Petroleum Wax Test Method for Color, ASTM, of Petroleum Products (ASTM Color Scale) Test Method for Peroxide Number of Petroleum Wax Test Method for Odor of Petroleum Wax Test Method for 20° Specular Gloss of Wax Paper Test Method for Surface Wax on Waxed Coated Paper
CHAPTER 19: PETROLEUM WAXES 555 D 2500 D 2534 D 2669 D 2895 D 3234 D 3235 D 3236 D 3 344 D 3451 D 3521 D 3522
D 3 706 D 3708 D4419
D 5442 E 1 E 260 E 355 E 473 E 537
Test Method for Cloud Point of Petroleum Products Test Method for Coefficient of Kinetic Friction for Wax Coating Test Method for Apparent Viscosity of Petroleum Waxes Compounded with Additives (Hot Melts) Test Method for Gloss Retention of Waxed Paper and Paperboard after Storage at 40° C (104° F) Test Method for Abrasion Resistance of Petroleum Wax Coatings Test Method for Solvent Extractables in Petroleum Waxes Test Method for Apparent Viscosity of Hot Melt Adhesives and Coatings Materials Test Method for Total Wax Content of Corrugated Paperbocird Standard Practices for Testing Polymeric Powders and Powder Coatings Test Method for Surface Wax Coating on Corrugated Board Test Method for Applied Wax Coating and Impregnating (Saturating) Wax in Corrugated Board Facing Test Method for Hot Tack of Wax-Polymer Blends by Flat Spring Test Test Method for Weight of Wax Applied During Curtain Coating Operation Test Method for Transition Temperatures of Petroleum Waxes by Differential Scanning CaJorimetry Test Method for Analysis of Petroleum Waxes by Gas Chromatography Specification for ASTM Thermometers Practice for Packed Column Gas Chromatography Practice for Gas Chromatography Terms a n d Relationships S t a n d a r d Terminology Relating to Thermal Analysis Test Method for Assessing the Thermal Stability of Chemicals by Methods of Thermal Analysis
OTHER STANDARDS No. BS 4633 & 4634
BS 4695 DIN 53175
DIN 53181
Title Method for the determination of crystallizing point. Method for the determination of melting point and/or melting range Method for d e t e r m i n a t i o n of melting point of petroleum wax (cooling curve) Binders for paints, varnishes and similar coating materials; determination of the solidification point (titer) of fatty acids (method according to Dalican) Binders for paints, varnishes and similar coating materials; determination of the melting interval of resins by the capillary method
ISO 1392 ISO 2207 ISO 3016 ISO 3841 JIS K 00-64 JIS K 00-65 NFT60-114 NFT20-051
Determination of crystallizing point— General method Petroleum waxes—Determination of congealing point Petroleum products—Determination of pour point Method for determination of melting point of petroleum wax (cooling curve) Testing methods for melting points of chemical products Test methods for freezing point of chemical products Petroleum products—Melting point of paraffins Chemical products for industrial use. Determination of melting point. Method for the determination of crystallizing point (freezing point).
REFERENCES 1] Hackett, W. J., Maintenance Chemical Specialties, Chemical Publishing Co., Inc., NY, 1972. 2] Warth, A. H., Chemistry and Technology of Waxes, Reinhold Publishing Corp., NY, 1956. 3] Bennet, H., Industrial Waxes, Vol. 1, Chemical Publishing Company, Inc., NY, 1963. 4] Puleo, S. L., "Beeswax," Cosmetics and Toiletries, Vol. 102, Allured Publishing CompEiny, Inc., Chicago, 1987. 5] Warth, A. H., Chemistry and Technology of Waxes, Reinhold Publishing Corp., NY, 1956. 6] Letcher, C. S., "Waxes," Kirk-Othmer: Encyclopedia of Chemical Technology, Vol. 24, 3'''' ed., 1984, pp. 466-481. 7] Dry, M. E., "Sasol's Fischer-Tropsch Experience," Hydrocarbon Processing, August, 1982, pp. 121-124. 8] Erchak, Jr., M., "Process for the Oxidation of High Molecular Weight Aliphatic Waxes and Product 880kb, U. S. Patent 2,504,400, Washington DC, April 18, 1950. 9] Haggin, J., "Fischer-Tropsch: New Life for Old Technology," Chemical and Engineering News, October 1981, pp. 22-32. 0] Caraculacu, A., Vasile, C, Caraculacu, G., "Polyethylene Waxes, Structure, and Thermal Characteristics," Acta Polymerica, Vol. 35, No. 2, 1984, pp. 130-134. 1] Brooks, B. T., Boord, C. E., Kurtz, S. S., and Schmerling, L., The Chemistry of Petroleum Hydrocarbons, Vol. 1, Reinhold Publishing Corp., NY, 1954. 2] Gruse, W. A., Chemical Technology of Petroleum, 2°'' ed., McGraw-Hill Company, NY, 1942. 3] Mazee, W. M., "Petroleum Waxes," Modem Petroleum Technology, 4"" ed., 1973, pp. 782-803. 4] Vasquez, D. and Mansoori, G. A., "Identification and Measurement of Petroleum Precipitates," Journal of Petroleum Science and Engineering, Vol. 26, Nos. 1-4, 2000, pp. 49-56. 5] Misra. S., Baruah, S., and Singh, K., Paraffin Problems in Crude Oil Production and Transportation: A Review, SPE Production and Facilities, Society of Petroleum Engineers, Richardson, TX, Feb. 1995, pp. 50-54. 6] Holder, G. A. and Winkler, J., "Wax Crystsillization from Distillate Fuels," Journal of the Institute of Petroleum, Vol. 51, No. 499, 1965, pp. 228-243. 7] Mansoori, G. A. and Canfield, F. B., "Variational Approach to Melting," Journal of Chemical Physics, Vol. 51, No. 11, 1969, pp. 4967-4972.
556 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK [18] Pourgheysar, P., Mansoori, G. A., and Modarress, H., "A SingleTheory Approach to the Prediction of Solid-Liquid and Liquid-Vapor Phase Transitions," Journal of Chemical Physics, Vol. 105, No. 2 1 , 1996, pp. 9580-9587. [19] Park, S. J. and Mansoori, G. A., "Aggregation and Deposition of Heavy Organics in Petroleum Crudes," International Journal of Energy Sources, Vol. 10, 1988, pp. 109-125. [20] Branco, V. A. M., Mansoori, G. A., De Almeida Xavier, L. C , Park, S. J., and Manafi, H., "Asphaltene Flocculation and Collapse from Petroleum Fluids," Journal of Petroleum Science and Engineering, Vol. 32, 2001, pp. 217-230. [21] Svendsen, J. A., "Mathematical Modeling of Wax Deposition in Oil Pipeline Systems," AIChE Journal, Vol. 39, No. 8, 1993, pp. 1377-1388. [22] Brown, T. S., Nielsen, V. G., and Erickson, D. D., "Measurement and Prediction of the Kinetics of Paraffin Deposition," Journal of Petroleum Technology, April 1995, p p . 328-329. [23] Noll, L., "Treating Paraffin Deposits in Producing Oil Wells," Topical Report NIPPER-551 (DE92001010), Bartelsville Project Office, U.S. Department of Energy, Bartelsville, OK, 1992. [24] Sanchez, J. H. P. and Mansoori, G. A., "In Situ Remediation of Heavy Organic Deposits Using Aromatic Solvents," Paper # 38966, Proceedings of the 68th Annual SPE Western Regional Meeting, Bakersfield, CA, May 1998. [25] Paraffin Products: Properties, Technology, Applications, G. Y. Mozes, Ed., Elsevier, NY, 1982. [26] Murad, K. M., Lai, M., Agarwal, R. K., and Bhattachaiyya, K. K., "Improve Quality of Wax by Hydrofinishing," Petroleum Hydrocarbons, Vol. 7, No. 2, 1972, pp. 144-7. [27] Ferris, S. W., "Characterization of Petroleum Waxes Tappi," TAPPI Special Technical Association Publication No. 2, 1963, pp. 1-19. [28] Himran, S., Suwono, A., smd Mansoori, G. A., "Characterization of Alkanes And Paraffin Wcixes for Application as Phase Change Energy Storage Medium," £nergySoMrce5, Vol. 16,1994, pp. 117-128. [29] Humphries, W. F., Performance of Finned Thermal Capacitors, NASA TND-7690, Washington, D.C., 1974. [30] Haji-Sheikh, A., Eftekhar, J. and Lou, D. Y. S., "Some Thermophysical Properties Of Paraffin Wax as a Thermal Storage Medium," Progress in Astronatics and Aeronautics 86, 1983, pp. 241-253. [31] Du, P. C. and Mansoori, G. A., "Phase Equilibrium of Multicomponent Mixtures: Continuous Mixture Gibbs Free Energy
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40] [41] [42]
[43]
Minimization and Phase Rule," Chemical Engineering Communication, Vol. 54, 1987, pp. 139-148. Hartono, R., Mansoori, G. A., and Suwono, A., "Prediction of Molar Volumes, Vapor Pressures and Supercritical Solubilities of Alkanes by Equations of State," Chemical Engineering Communications, Vol. 173, 1999. pp. 23-42. Riazi, M. R. and Mansoori, G. A. "Simple Equation of State Accurately Predicts Hydrocarbon Densities," Oil & Gas Journal, 1993, pp. 108-111. Mohsen-Nia, M., Modarress, H., and Mansoori, G. A., "A Simple Cubic Equation of State for Hydrocarbons and Other Compounds," SPE Paper No. 26667, Proceedings of the 1993 Annual SPE Meeting, Society of Petroleum Engineers, Richardson, TX, 1993. Nikitin E. D., Pavlov, P. A., and Bessanova, N. V., "Critical Constants of n-Alkanes with from 17 to 24 Carbon Atoms," Journal of Chemical Thermodynamics, Vol. 26, 1994, p p . 177-182. Frenkel, M., Gadalla, N. M., Hall, K. R., Hong, X., and Marsh, K. N., TRC Thermodynamic Tables-Hydrocarbon; Non-Hydrocarbon, R. C. Wilhoit, Ed., Thermodynamic Research Center, The Texas A & M University System, College Station, TX, 1997. Twu, C. H., "An Internally Consistent Correlation for Predicting the Critical Properties and Molecular Weights of Petroleum and Coal-Tar Liquids," Fluid Phase Equlibrium, Vol. 16, 1984, pp. 137-150. Edalat, M., Mansoori, G. A., and Bozar-Jomehri, R. B., "Vapor Pressure of Hydrocarbons, Generalized Equation," Encyclopedia of Chemical Processing and Design - 61, Marcel Dekker, Inc., NY, 1997, pp. 362-365. Letoffe, J. M., Claudy, P., Garcin, M., and Voile, J. L., "Evaluation of Crystallized Fractions of Crude Oils by Differential Scanning Calorimetry, Correlation With Gas Chromatography," Fuel, Vol. 74, No. 1, 1995, pp. 92-5. Braun, R., "Limits in Differential Thermoanalysis of Wsixes," Fette Seifen Anstrichm, Vol. 82, No. 2, 1980, pp. 76-81. Handbook on Antioxidants and Antiozonants, Goodyear Chemicals, Akron, OH, 1977. Wink, W. A., Delevanti, C. H., and Van den Akker, J. A., Instrumentation Studies LXXVII, Study on Gloss I, A Goniophotometric Study of High Gloss Papers, TAPPI, Technical Association of the Pulp and Paper Industry, Vol. 35, December 1953, p. 163A. Tomsic, J., Dictionary of Materials and Testing, 2"^ ed., SAE International, Warrendale, PA, 2000, p. 205.
MNL37-EB/Jun. 2003
Lubricating Greases Thomas M. Verdura, ^ Glen Brunette, ^ and Rajesh Shah~
tremely small, uniformly dispersed, and capable of forming a relatively stable, gel-like structure with the liquid lubricant. Greases are distinct from lubricating pastes that can appear grease-like. Pastes are mostly solids; generally about 70-95% solids, but sometimes merely wetted solids. The solid thickener concentrations of greases range from about 3-30%, tjrpically about 10%. Also, for pastes, affinity between the solid and liquid phases is not essential; neither is it necessary that a stable, gel-like structure be formed. The manufacturing methods also differ. Pastes are simply solid-liquid mixtures formed using low-shear mixing. Grease manufacturing requires considerably m o r e processing, usually including synthesizing the thickener in the fluid. A strict time, temperature, a n d mixing profile must be followed to properly synthesize the thickener. Next, thorough mixing and blending in the desired additives at the proper time and temperature has to be done prior to the final finishing a n d processing. Included in the finishing a n d process steps is homogenization, where t h e grease is passed through a mill to disperse the thickener and additives, or deaerating, to remove entrained air or both where needed.
T H E ESSENTIAL FUNCTION OF ANY LUBRICANT is to prolong the life
and increase the efficiency of mechanical devices by reducing friction and wear. Secondary functions include heat dissipation, corrosion protection, power transmission, and contaminant removal. Generally, fluid lubricants are difficult to retain at t h e point of application a n d m u s t be replenished frequently. If, however, a fluid lubricant is thickened, its retention is improved, a n d lubrication intervals c a n b e extended. A lubricating grease is simply a lubricating fluid which has been gelled with a thickening agent so that the lubricant can be retained more readily in the required area. This is not to say that the thickener does not play a part in the lubrication. Depending on the type of thickener being used and the lubricating regime, some thickeners will contribute in the lubrication. Lubricating greases have a n u m b e r of advantages over lubricating fluids. Some of these are: • Dripping and spattering are nearly eliminated • Less frequent applications are required • Greases are easier to handle • Less expensive seals can be used • Greases can form a seal in many cases and keep out contaminants • They adhere better to surfaces • They reduce noise and vibration • Some grease remains even when relubrication is neglected Grease was previously defined as a gelled lubricating fluid. Although this simplistic definition conveys the general concept of a grease, a more extensive discussion is required to provide a fuller understanding of just what constitutes a lubricating grease. A lubricating grease is a semi-fluid to solid product of a dispersion of a thickener in a liquid lubricant. Additives, either liquid or solid, Eire usually included to improve grease properties or performance. By definition, grease is a lubricant. It is also essentially a two-phase system—a liquid-phase lubricant into which a solid-phase finely divided thickener is uniformly dispersed. The liquid is immobilized by the thickener dispersion that must remain relatively stable with respect to time and usage. At operating temperatures, thickeners are insoluble or, at most, only slightly soluble in the liquid lubricant. There must be some affinity between the solid thickener and the liquid lubricant in order to form a stable, gel-like structure. The thickener can be constituted of fibers (such as various metallic soaps), or plates or spheres (such as certain non-soap thickeners). The essential requirements are that the particles be ex-
' Retired, General Motors NAO Research and Development, Warren, MI. ^ CITGO Petroleum Corporation, Oklahoma City, OK. ^ Koehler Instrument Company Inc., Bohemia, NY.
COMPOSITION Greases are composed of a lubricating fluid, a thickener, and usually performance enhancing additives. A complete discussion of the many variations of grease formulations and manufacturing process would require far more space than allotted for this work. Extensive discussion on grease chemistry and manufacturing process has been published [1-4]. Therefore, only a n overview of the most common grease types and methods has been presented. The lubricating fluids t h a t c a n b e thickened to form greases vary widely in composition and properties and are a n extremely important component of the grease. Lubricating fluids can account for as m u c h as 95% of a grease. By iar, the largest volume of greases in use today consists of those made with petroleum oils thickened with soaps. Many types of petroleum oils are used, including naphthenic, paraffinic, hydrocracked, and hydrogenated. The viscosity of the oil used also varies. Oil blends with a viscosity between an ISO 100 and 220 are most common, however, for specialized greases, the oil viscosity can be lighter or much heavier. In addition to petroleum oils, other lubricating fluids such as vegetable oils, silicones, synthetic hydrocarbons, and others can be used. Of the synthetic fluids used in grease manufacturing the most common type is poly(alpha)olefin (PAO). Because it is more expensive, its usage is small compared t o petroleum oil. Greases made with PAO as the lubricating fluid can provide good performance over a wide-temperature range. Many products have been used as thickeners for grease. Soaps were the first thickeners used and still have the widest
557 Copyright'
2003 by A S I M International
www.astm.org
558 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK
S Lithium soaps 0 Calcium soaps H Aluminum soaps H Sodium and other soaps g^o m Clay & other non-soaps B Polyurea FIG. 1—Worldwide grease production by thickener type (1999 NLGI Survey). application (see Fig. 1). Some of the other thickeners that have been employed include polymers, clays, silica gel, and pigments. Soaps are present in greases in the form of fibers. The structure and size of these fibers, i.e., thickness and length, depend on the metallic moiety and the conditions u n d e r which they are formed. In general, soap fibers can vary from about l-100^lm in length with about a 10 t o l length-to-diameter ratio. Large, coarse fibers do not absorb fluids as well as fine, closely knit fibers. Thus, a higher percentage of thickener is required for coarse fiber soaps to make greases having the same consistency as those made with fine fiber soaps. Non-soap thickeners are generally smaller, even colloidal, and have either a spheroidal or plate like structure. Soap thickeners are formed by neutralization reaction of an acid and base to form a salt and water as a byproduct. When the acid is a fatty acid, its salt is called a soap, and the reaction is Ccilled saponification. If the acidic component has a narrow range of moleculcir weight, as in fatty acids, a simple soap is made, e.g., lithium stearate. Reacting the metallic base with two dissimilar acids of widely different molecular weight will form a complex soap, e.g., calcium stearate acetate. Mixed-base greases consist of a mixture of two or more different thickener systems, such as sodium-calcium or alum i n u m complex-clay. The natural fatty materials used for soap formation can be of animed or vegetable origin. Beef tallow and its derivatives are the major source of animal fatty materials. The reaction product with beef tallow derivatives Eind lithium hydroxide is lithium stearate soap. Castor oil derivatives are the m a i n source of vegetable oil fatty materials. Because the acids derived from castor oil have a hydroxyl group (OH) on the 12''' carbon of the acid molecule, the reaction product with castor oil derivatives and lithium hydroxide is lithium 12-hydroxystearate soap. Although the resulting soap is named according to the acid most prevalent, the natural occurring fatty acids are actually mixtures of similar acids (in the form of a glyceride) with slightly different molecular weights. The type of fatty materials used affects the properties of the soap and the grease. Greases made from castor oil derivatives Eire more work stable and have higher dropping points than greases made from beef tallow derivatives due to the hydrogen bonding at the hydroxyl group. The use of mixtures or blends of fatty materials can obtain improved cost, oil separation, worked stability, or other properties.
The most common basic components used to neutralize the fatty acids are hydroxides of lithium, calcium, and sodium. The resulting greases are named according to the basic component and the acidic component used to form the soap, e.g., calcium stearate grease, lithium 12-hydroxystearate grease, etc. Grease Selection When selecting a grease for a particular application, one should first determine the type and viscosity of the lubricating fluid required because the fluid has a major influence in grease lubrication. The properties of greases are determined by the characteristics of the thickener system, by the viscosity and type of the fluid component, and by the performance additives used. Similar criteria as is used for selecting industrials oils can be used when determining the type and viscosity of the lubricating fluid needed in a grease for a particular application. In general, heavily loaded and slow speed applications require higher oil viscosity Eind lightly loaded high speed applications require lower oil viscosity. E a c h thickener system has its own unique characteristics. Additional discussion of the properties of the various soap type greases will be provided in subsequent paragraphs.
GENERAL CHARACTERISTICS Describing the general characteristics of a grease based on the thickener type is difficult because the base oil components and the performance additives used effect the characteristics of the grease as m u c h or more than the thickener type. Therefore, assigning characteristics to a grease based on thickener t5^e has limited value, however, some characteristics inherent to the thickener tjfpe are discussed below. The reader should be aware that some of the weaknesses of a thickener type can be improved and some of the strengths of a thickener type can be degraded with the inclusion of performance additives. Aluminum Soap Greases Aluminum soap greases are usually made with preformed soap unlike most other thickeners that Eire made in situ. Alum i n u m distearate is the m o s t c o m m o n conventional alum i n u m soap used to make grease. It is dissolved in hot oil in a mixing grease kettle, and the hot mixture is poured into pans to gel and cool. The cooling rate affects the final consistency. The final product is a smooth, transparent grease with a tendency to thin when worked but with excellent water resistance. Aluminum soap greases are used as thread and way lubricants. The poor work stability of aluminum-soap greases is used to advantage in electrically conducting conveyers of electroplating systems; the grease thins down during use, allowing good electrical contact in the track rollers while still providing good lubrication.
Calcium Soap Greases The Ccirliest known greases were made with calcium soaps. Greases thickened with hydrated calcium soaps (usually cal-
CHAPTER 20: LUBRICATING cium stearate) are water resistant, work stable, and inexpensive. The name hydrated calcium comes from the fact that a small amount of water (about 0.1% wt.) is required to stabilize the grease. The water associates with the calcium soap and provides the molecular cohesion needed for efficient thickening. These calcium greases are also known as conventioneJ calcium, calcium cup, or lime soap greases. The greatest shortcoming of a hydrated calcium soap grease is its low dropping point, typically about 95°C (200°F). This limits conventioneJ calcium soaps to use in only low temperature applications. Anhydrous calcium soaps (usually calcium 12-hydroxystearate) are not as temperature limited because they do not need water to hold the soap together due to the stabilizing effect of the hydroxyl group (OH). With a dropping point of about 150°C (300°F), they are somewhat more temperature resistant. Anhydrous calcium greases have the same advantages as hydrated calcium greases, but are slightly more expensive cind do not run the risk of drying out and softening. They can be used as multi-purpose greases within their temperature limitations. Historically, the greatest usage for Einhydrous calcium grease has been in a low temperature grease made with a low-viscosity base oil designed to meet earlier versions of the military specification MIL-G-10924 for applications where operation over a wide range of climatic conditions is essential.
GREASES
559
thermal limitations of the oils and additives they contain. Aluminum complex greases have excellent water tolerance characteristics as well as high-temperature resistance, consequently, they are widely used in steel mills and other wet applications. The soap used for aluminum complex grease is usually eJuminum benzoyl stearoyl hydroxide. This soap can be formed by reacting aluminum isopropylate with stearic acid, benzoic acid, and water. To limit or avoid the isopropyl alcohol byproduct, other aluminum compounds can be used. Compatibility with other greases can be a problem with aluminum complex greases. Calcium Complex Soap Greases Calcium complex greases Eire usually made by reacting acetic acid and a fatty acid with lime. The resulting grease has inherent EP (extreme pressure) properties and provides good friction and wear performance. Thickening efficiency is not very good with this soap, therefore, a relative high soap concentration is required. As a result, the low-temperature performance is not as good as other complex soap greases, and they tend to harden in long-term storage or under pressure in lubrication systems. Used within its limitations, calcium complex greases are cost effective but are not compatible with most other grease types. Lithimn Complex Soap Greases
Sodium Soap Greases Sodium (soda) soap greases have higher dropping points [about 175''C (350°F)] than calcium greases and form a very fibrous grease. They Eire not water resistant and emulsify in the presence of water, yet, they have inherent rust protection properties. The soap can be made by reacting sodium hydroxide cind tallow, therefore, is a very inexpensive thickener. In earlier times, when calcium and sodium greases made up most of the market, these greases were the most temperature resistant. They are still used in moderately high temperature applications, such as electric motor bearings. Sodium soap greases are not normally compatible with other greases. Lithium Soap Greases Lithium greases were the first so-called multipurpose greases. They provide both good water resistance, similar to calcium soap greases, and even higher-temperature properties than sodium soap greases. The soap is usually manufactured in a portion of the lubricating oil using lithium hydroxide and a fatty material of either animal or vegetable origin. Lithium 12-hydroxystearate greases have dropping points of about 190°C (375°F). They also have good work stability; that is, they do not soften much when worked. Lithium greases have good over-all performance and are cost effective. In North America more lithium grease is made them all other tj^es combined. Aluminum Complex Soap Greases Complex soap greases are noted for their high-dropping points [230°C (450°F) and higher], although most are not recommended for use up to the dropping point due to
Lithium complex grease performance is generally like that of lithium greases except dropping points are about 50°C higher. Lithium complex is a misnomer used to describe high dropping point lithium greases. Because, lithium is mono-valent (mccining it can only react with one acid per ion) it cannot form a traditional complex soap where two or more acids are reacted with one basic ion. However, there are severed components that can be used to enhance the molecular interactions of the soap molecules and increase the dropping point enough to call the resulting grease a "lithium complex." The most common method is by forming a lithium salt of a dibasic acid (usually azelaic or sebacic) in situ with lithium 12-hydroxystearate soap. Lithium complex greases provide good low-temperature performance and excellent high-temperature life performance in tapered roller bearings. It is the most populcir of the complex greases and has wide-spread application. Polyurea Greases Polyurea greases are similar to the complex soap greases with respect to high-temperature performance; dropping points are about 245°C (475°F). The reactants (isocyanates and amines) are hazardous but the resulting thickener is considered quite safe. Polyurea greases can be made by various methods resulting in various strengths and weakness. Most have good oxidation resistance, water resistance, pumpability, and high temperature performemce. Wesiknesses generally associated with poljoirea greases include poor shear stability, poor storage stability, and incompatibility with other greases, however, some of these weaknesses have been overcome with more recent formulations [5]. Although used in all types of bearings, they have proved especially useful for the
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lubrication of sealed-for-life, ball bearings used in electric motor bearings where superior oxidation resistance and high-temperature life is essential. Organo-Clay Greases Clay thickened greases are made with a modified clay design to have some affinity for the lubricating oil. The process can be as easy as mixing the modified clay thickener with the lubricating oil and providing enough shear to disperse the thickener. They have been referred to as non-melting greases because they tend to decompose before reaching their dropping point. They usually have poor work stability properties as compared to soap greases. Because many of the common additives used in grease will soften clay thickened greases, they are more difficult to inhibit with extreme pressure additives. Although water resistant, they can be susceptible to severe degradation from other contaminants, such as brine. Low-temperature performance could be considered satisfactory, but many clay greases are formulated with high-viscosity base oils for high-temperature applications; such greases will have poor low-temperature properties. Clay greases are not normally compatible with other greases.
CLASSIFICATION AND SPECIFICATIONS Over the past six decades, efforts to develop a grease classification system were frustrated by seemingly insurmountable difficulties. Until recently, the American grease industry had not developed grease specifications for industry-wide applications. However, in 1989, after about 15 years of development, ASTM D4950, Standard Classification and Specification for Automotive Service Grease, was published. Development of this standard was a cooperative effort by ASTM International, NLGI (National Lubricating Grease Institute), and SAE (Society of Automotive Engineers). It is the first and only cooperatively-developed grease specification accepted by American industry. Development of this standard was made possible only because several grease performance tests were developed specifically for automotive applications. These performance tests (D 3527, D 4170, D 4289, D 4290, and D 4693) are discussed subsequently. D 4950 classifies automotive service greases into two chassis grease and three wheel bearing grease categories based on the performance needs of several service conditions. A guide to the requirements of these categories is given in Table 1. The NLGI has established a licensing procedure for greases qualifying for ASTM D 4950, categories GC and LB. In conjunction with licensing, three certification symbols can be used only with the highest-performance categories of chassis and wheel bearing greases. NLGI specifically prohibits the use of the symbols with lesser-performance grease categories. Industry response to the licensing system has grown over the last decade and containers of grease bearing the certification symbols are commonly available in the aftermarket, and their availability is expected to expand. Beginning with the 1992 model year, most U.S. automakers began recommending the use of NLGI Service Greases GC, LB, and GCLB for scheduled maintenance of chassis and wheel bearings of passenger cars and light-duty trucks.
TABLE 1—Guide to requirements for grease categories (ASTM D 4950). Test
Description
LA
LB
GA
GB
GC
D-217 0-566" D-1264 D-1742 D-1743 D-2266 D-2596 D-3527 D-4170 D-4289 D-4290 D-4693
Penetration Dropping Point Water Washout Oil Separation Rust Protection 4 Ball Wear 4 Ball EP High Temperature; Life Fretting Wear Elastomer Compatibility Leakage Low Temperature Torque
/ /
/ /
/ •
• / • / • / /
/ / / / / / / •
• / •
/ / /
•
/
/ / / / / / /
/
'^D-2265 may be substituted.
TEST METHODS AND THEIR SIGNIFICANCE Greases are used for particular lubrication applications because of their intrinsic properties. Users and producers alike need a common means to describe the properties required for grease performance for particular applications. Test methods are devised to describe the requisite properties. When the usefulness of some test becomes known throughout the industry, it is developed, through cooperative effort, into an industry standard such as an ASTM standard. The standard tests used to determine the properties of petroleum oils Eire commonly applied to grease base stocks, as well. Few among these are kinematic viscosity (D 445), flash and fire points (D 92), pour point (D 97), and aniline point (D 611), etc. The significance of these tests are discussed elsewhere throughout this book. There are no standard tests for the evaluation of grease thickeners, per se, because most thickeners (soaps) are formed in situ and are not used independently of the grease. Consequently, there is little need for standard tests to evaluate neat thickeners. However, work is currently in progress, including an ASTM Round Robin using the Penn State Microoxidation test, to study oxidation characteristics of greases, which helps lend limited insight to the evaluation of the thicker system. ASTM and IP (Institute of Petroleum) have developed and standardized a number of tests to describe the properties and performance characteristics of lubricating greases. Because these tests are conducted in laboratories under well-defined conditions, they are used primarily as screening tests. Some of the grease tests do indicate how a grease might perform in service, but direct correlation between laboratory and field performance is often unattainable because the tests never precisely duplicate service conditions. Consistency Consistency can be defined as the degree to which a plastic material, such as a lubricating grease, resists deformation under an applied force. It is a measure of the firmness or rigidity of the thickener structure of the grease. The standard method for measuring grease consistency is the penetration test. Consistency is reported in terms of ASTM cone penetration, NLGI Consistency Number, or apparent viscosity. Cone
CHAPTER 20: LUBRICATING penetrations and NLGI number are discussed in the following paragraphs. Apparent viscosity is included in the Shear Stability section. Cone Penetration ASTM D 217 (IPSO) Test Methods for Cone Penetration of Lubricating Grease is the universal standard for the determination of the penetration of a normal grease sample. (In this context, normal greases are those that are neither too soft nor too hard to be measured by this method.) In this method, a double-tapered cone of prescribed construction sinks under its own weight into a sample of grease at 25°C (77°F) for 5 s. The depth of penetration, measured in tenths of a millimeter, is the penetration value. (It is common practice to omit the units when reporting or specifying penetration vEilues.) Firm greases have low-penetration values, whereas soft greases have high-penetration values. Full-scale penetration tests require about 500 g (1 lb) of sample. The penetration number for smaller samples of grease can be determined by using ASTM D 1403 (IP310), Test Methods for Cone Penetration of Lubrication Grease Using One-Quarter and One-Half Scale Cone Equipment. The one-quarter and one-half scale tests require about 5 and 50 g samples, respectively. As might be expected, the reducedscale tests are not as precise as the full-scale tests. Reducedscale penetration values are not normally used as such; instead, equations are given to convert the reduced-scale penetrations to equivalent full-scale values. Recent advances in instrumentation, such as automatic and digital Penetrometers, have helped improve the precision of this test method. The following paragraphs describe the several types of penetration measurements. Unworked Penetration—This value is obtained when a penetration measurement is made on a grease transferred from the original container to the standard grease worker cup, with only a minimum amount of disturbance. This result is not always reliable, because the amount of disturbance cannot be controlled or repeated exactly. This measurement may be significant to indicate consistency variances in transferring grease from container to equipment. Worked Penetration—Worked penetration is the standard penetration measurement for a grease. It is measured after a grease has been worked for 60 double strokes in the standard grease worker. This method is more reliable than unworked penetration, because the disturbance of the grease is standardized by the prescribed working process. A significant difference between unworked and worked penetration can indicate poor shear stability—and indicate a need for further evaluation by prolonged working or roll test (see the Shear Stability section). Prolonged Worked Penetration—This value is obtained after a sample has been worked for a prolonged period in the grease worker, i.e., 10000, 50000, 100000, etc., double strokes. After prolonged working, the sample and worker are brought back to penetration test temperature 25°C (77°F) in 1.5 h. It is then worked for 60 double strokes and the penetration is measured. This test is significant because it can indicate the degree of shear stability of a grease. Block Penetration—If a grease is firm enough to hold its shape, it is not transferred to a worker cup container for test.
GREASES
561
Instead, the penetration is determined on three faces of a freshly-cut, 50-mm (2-in.) cube of grease. Undisturbed Penetration—Undisturbed penetration is that measured on a grease sample in a container as received, without any disturbance. This measurement was formerly a requirement in D 217, but because of the uncertainty of repeatable sample handling, it now is included merely as an information item. Such measurements can be used for consistency control in grease manufacture, and to assess the degree to which a grease develops false body or set with prolonged storage. NLGI Consistency Numbers—On the basis of worked penetrations, the NLGI has standardized a numerical scale as a means of classifying greases in accordance with their worked consistency. This scale is shown in Table 2 in order of increasing hardness. The majority of greases used in automotive and industrial applications fall in the range of NLGI No. 1 to NLGI No. 3. Consistency Stability The consistency of a grease may change with its history. Some greases may harden with age; others may change due to wide fluctuations in temperature. Evaluation of these changes needs to be on an individual basis. That is, the test grease needs to be subjected to controlled aging or temperature fluctuations, with penetration measurements taken periodically. The consistency of greases may also change in service due to changes in the size and dispersion of thickener particles resulting from mechanical shearing. The ability of a grease to resist changes in consistency during mechanical working is referred to as consistency stability, shear stability, work stability, or mechanical stability. Two test methods have been standardized to evaluate the stability of a grease resulting from two degrees of low-shear working. Prolonged Worked Penetration & Low Temperature Penetration ASTM D 217 (IP50), described previously, is used before and after prolonged working in a grease worker to determine the change in grease consistency. Because shear rates are low, evaluation of shear stability by prolonged working is time consuming (working 100000 strokes takes nearly 28 h). Mechanical grease workers with cut off timers help make this a little more manageable task, however, this test is not extensively used anymore.
TABLE 2—NLGI consistency classification. NLGI Consistency No.
ASTM Worked Penetration at 25°C
000 00 0 1 2 3 4 5 6
445-475 400-430 355-385 310-340 265-295 220-250 175-205 130-160 085-115
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MANUAL
Roll
Stability
37: FUELS AND LUBRICANTS
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Results can be obtained more quickly with the roll stability test, which operates at somewhat higher shear conditions. Roll stability is determined by ASTM D 1831, Test Method for Roll Stability of Lubricating Grease. It is used in conjunction with the reduced-scale penetration test [D1403 (IP310)]. After a worked penetration has been measured on a grease sample, 50g of the worked grease is transferred into a horizontally-mounted cylinder containing a 5-kg (11-lb) steel roller. The cylinder is rotated at 165 r p m for 2 h. The inner roller rolls over the grease, working it during the test. After the test, the penetration of the grease is once again measured by D 1403, and the change between before and after penetrations gives a n indication of shear stability. Recent trend (last few years) has been to run the roll stability tests at elevated temperatures, often even above 100°C, instead of the conventional room temperature tests, to help evaluate the roll stability of greases at a higher temperature. Roll stability tests are widely used in specifications. Test results are significant insofar as they can show a directional change in consistency that could occur during service. No accurate correlation between roll-test results and actual service performance has been established. In both shear stability tests, the change in consistency is reported as either the absolute change in penetration values or the percent change as outlined in ASTM D 1831. If absolute values are to be reported, the quarter-scale values are first converted to full-scale values.
Flow Properties The consistency of a grease is a critical p a r a m e t e r which helps define the ability of a grease to perform under given operating conditions. Consistency, as measured by penetration, is affected by temperature. But the penetration test is not suitable for determining the minor, yet sometimes significant, changes in consistency as the grease approaches temperatures at which phase changes in the thickener occur. Penetration is basically a flow measurement; in addition, there are other flow-measurement tests that can be utilized to evaluate this property at other conditions. Flow Properties
at High
Temperatures
ASTM Test Method D 3232, Measurement of Flow Properties of Lubricating Greases at High Temperatures, can be used to evaluate flow properties of lubricating greases under hight e m p e r a t u r e , low-shear conditions. Using this method, a grease sample is packed into a n annular channel in an alum i n u m block. The packed block is placed on a hot plate capable of attciining temperatures in excess of 315°C (600°F) at a heating rate of 5 ± r C (10±2°F)/min. A special trident probe spindle, attached to a Brookfield-RVF viscometer, is lowered into the grease sample, and the hot plate is turned on. Simultaneously, the spindle is rotated at a constant 20 rpm, and torque measurements are read from the viscometer every minute. Readings are continued until the reading drops below 0.5 on the viscometer scale or until the maximum sample temperature of interest is attained. With these data and a n appropriate conversion equation, a plot of apparent viscosity versus temperature is prepared.
The trident probe test gives information not obtained with the dropping point test. The dropping point test (to be described subsequently) determines the temperature at which the first drop of material drops from the orifice of the standard cup. That first drop may be "melted" grease or leeiked fluid. For example, if a grease becomes so fluid that it drops from the cup, the trident probe test will show a low viscosity at the same temperature. In another case at the same temperature, where it is only oil that drops from the cup, the trident probe will show a higher viscosity than the first case, indicating that the grease is still firm. The results of this test provide a n indication of the flow properties of a grease between room a n d elevated temperatures. Although the test does not give actual flow rates, as in a pipeline, it provides a means for obtaining some indication of this property. This test is used more for grease development, rather than for specifications. Apparent
Viscosity
Grease is by nature a non-Newtonian material. It is characterized by the fact that flow is not initiated until stress is applied. Increases in shear stress or pressure produce disproportionate increases in flow. The term apparent viscosity is used to describe the observed viscosity of greases; it is measured in Poises in D 1092, Test for AppEirent Viscosity of Lubricating Greases. Since apparent viscosity varies with both temperature and shear rate, the specific temperature and shear rate must be reported along with the measured viscosity. In this test, a sample of grease is forced through a capillary tube by a floating piston actuated by a hydraulic system using a two-speed gear p u m p . From the predetermined flow rate cind the force developed in the system, the apparent viscosity is CcJculated using the classic Poiseuille equation. A series of eight capillaries and two p u m p speeds provide 16 shear rates for the determination of apparent viscosities. The results are expressed in a log-log graph of apparent viscosity as a function of shear rate at a constant temperature, or apparent viscosity at a constant shear rate as a function of temperature. This apparatus also has been used to measure the pressure drops of greases u n d e r steady-flow conditions at constant temperature. Such information can be used to estimate the pressure drop or required pipe diameters in distribution systems. Also, apparent viscosity data are useful for evaluating the ease of handling or pumping at specified temperatures of dispensing systems; it is often used to evaluate pumpability at low temperatures. (The NLGI can provide a group of charts that relate pressure drop, apparent viscosity, shear rate, and pipe-flow data.) Apparent viscosity also is used to provide an indication of the directional value of starting and running torques of grease-lubricated mechanisms. Specifications may include limiting values of apparent viscosity for greases to be used at low temperature. Currently an ASTM Round Robin is being conducted to rewrite the U.S. Steel Grease Mobility test method under the aegis of ASTM. The Low Temperature grease mobility test method utilizes only one capillary tube, maintained at various temperatures. The amount of grease collected in a specific time (gms/minute) is measured and used as an indication of the mobility of the grease. The test is easier to conduct
CHAPTER t h a n ASTM D 1092, and uses axi apparatus similEir to the apparent viscosity test method. Low-Temperature
Torque
(Ball
Bearings)
Greases designed for low-temperature applications must not stiffen or offer excessive resistance to rotation. However, greases harden emd become more viscous as the temperature is lowered. Sometimes the grease can become so rigid in the bearing t h a t excessive torque is required for rotation. In extreme cases, the grease can solidify to the point of bearing lock-up. Two standard test methods are available to measure bearing torque at low temperatures. Both operate on the same principle—the restraining force or torque is measured while grease-lubricated test bearings are r u n at low speed. The tests differ in the size and types of bearings, the intended applications, and types of greases (i.e., in viscosity characteristics). ASTM D 1478, Test Method for Low-Temperature Torque for Ball Bearing Greases, measures the starting and running torque of lubricating greases packed in small ball bearings at temperatures as low as —54°C (—65°F). In this procedure, fully packed bearings are installed on a spindle that can be rotated at 1 rpm. The assembly is inserted in a cold box. The outer race is connected by a cord assembly to a spring scale, which measures the restraining force. When the m o t o r is started, the initial, peak restraining force is recorded. After running for 10 min, the restraining force is recorded again. The force values are multiplied by the length of the lever arm, and the products are reported as the starting and running torque in gram-centimeters (g-cm). [Because the cgs (centimeter-gram-second) metric unit, g-cm, is nearly universally used in grease specifications, it is the standard unit of torque measurement for this test; some newer specifications require the SI torque unit, N-m (Newton-meter).] In the past several years, advances in data acquisition techniques have helped in making these test methods easier to r u n and have improved precision and reliability. Because this method was developed using greases with extremely low-torque cheiracteristics at - 5 4 ° C (-65°F) it may not be applicable to other greases, speeds, or temperatures. If a machine has significantly more power available than is actually required, torque is not an important consideration. On the other hand, it can be very i m p o r t a n t in low-powered equipment. This test is significant because it provides a means of comparing the low-temperature torque effects of widely different greases. It is useful in the selection of greases for low-powered mechanisms, such as instrument bearings used in aerospace applications. The suitability of this method for other applications requiring different loads, speeds, and temperatures should be determined on an individual basis. Usually, test conditions are substantially different from those found in service, so test results may not correlate with actUcJ service performance. Low-Temperature
Torque
(Tapered Roller
Bearings)
For applications using larger bearings or greater loads, D 4693 Test Method for Low-temperature Torque of GreaseLubricated Wheel Bearings, is better suited than D 1478. D 4693 can be used to predict the performance of greases in automotive wheel bearings operating at low temperatures. It will differentiate among greases having distinctly
20: LUBRICATING
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563
different low-temperature characteristics. It is used for specification purposes, a n d it is one of the automotive grease performance tests required by D 4950. It is not known to correlate with other types of service. It should be noted that greases having such characteristics that permit torque evaluations by either D 1478 or D 4693 will not give the same values in the two test methods (even when converted to the same torque units) because the apparatus and test bearings are different. D 4693 determines the extent to which a grease retards the rotation of a specially-manufactured, tapered roller bearing assembly. The test unit is a model device that closely duplicates an automotive wheel bearing assembly. Additionally, it employs a spring-loading m e c h a n i s m to improve test repeatability; and it is not intended to simulate any service-load condition. Although the test assembly cind torque-measuring transducer are necessarily inside the cold-test chamber, the drive mechanism can be either inside or outside. In this test, a sample of test grease is stirred and worked, and a specified a m o u n t is packed into the two test bearings. The test assembly is heated to mitigate the effects of grease history; it is then cooled at a specified rate to —40°C (—40°F). A drive m e c h a n i s m rotates the spindle at 1 rpm, a n d the torque required to prevent rotation of the h u b is measured by a strain gage load cell at precisely 60 s after start of rotation. The results are recorded on a strip chart or using a data acquisition system that can be displayed and stored via computer. (Starting torque was found to be less repeatable, as well as a redundant measurement, and is not determined in this test.) Heat Resistance Heat affects greases in several ways. As the temperature increases, greases soften and flow more readily (see description of D 3232); oxidation rate increases (see description of D 942); oil evaporation increases (see descriptions of D 972 and D 2595); the thickener melts or loses its ability to retain oil (see descriptions of D 566 and D 2595). Sometimes these phen o m e n a are all involved simultaneously (see descriptions of D 3336, D 3337, and D 3527). Dropping
Point
The dropping point of a grease is the temperature at which it passes from a semi-solid to a liquid. Although a thickener can have a definite melting point, the resulting grease does not. Rather, the thickener loses its ability to function as a grease thickener as the temperature is increased. As the temperature is raised, the grease softens to the extent that it loses its selfsupporting characteristic, the structure collapses, and the grease flows under its own weight. When this phenomenon takes place in a standard cup under standard conditions, it is called the dropping point. Two similar procedures are used to determine the dropping point of grease. In both methods, a prescribed layer of grease is coated on the inner surface of a small cup whose sides slope toward a hole in the bottom. With ASTM D 566 (IP132), Test Method for Dropping Point of Lubricating Grease, the sample is heated at a prescribed rate until a liquid drop falls from the cup. In ASTM D 2265, Test Method for Dropping Point of Lubricating Grease Over Wide Tempera-
564 MANUAL 37: FUELS AND LUBRICANTS
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ture Range, the sample is introduced into a preheated environment so that the heating rate is controlled more uniformly. In both tests, the difference in temperature between the grease in the cup and the environment are taken into account when calculating the dropping point of the grease. Some greases containing non-soap thickeners may not separate oil or melt. Cooperative testing indicates that dropping points by Methods D 566 and D 2265 generally agree up to about 260°C (500°F). In cases where results differ, there is no known significance. The dropping point is useful (1) in establishing bench marks for quedity control, (2) as an aid in identifying the type of thickener used in a grease, and (3) as an indication of the maximum temperature to which a grease can be exposed without complete liquefaction or excessive oil separation. (For simple soap greases, the maximum usable temperature is about 20-30°C lower than the dropping point. For complex soap grease, the maximum usable temperature is limited by the base oil characteristics, and under dynamic conditions seldom exceeds 175°C. Some complex soap greases will tolerate intermittent operation at higher temperatures, and although they may liquefy somewhat, their structure will reform as the temperature is reduced.) Greases normally do not perform satisfactorily at temperatures near or above the dropping point; other factors are involved. High-temperature performance can depend on the application method and frequency, whether a softened grease is retained at the point of application by proper seals, and whether the high temperature is continuous or intermittent. High temperature stability and evaporation properties of the grease also can affect performance. Dropping point is most useful as a quality-control tool. Unless correlation has been established, dropping point has no direct bearing on service performance. Performance at high temperature would be better evaluated with one of the performance-type tests or by actual experience. Evaporation Loss Exposure of a grease to high temperatures can cause evaporation of some of the liquid lubricant, thus causing the remaining grease to become drier and stiffer or leading to other undesirable changes in the grease structure. Greases containing low-viscosity oils for good low-temperature performance may be susceptible to evaporation losses at higher temperatures. Evaporation also can cause problems where vapors may be hazardous or combustible, or interfere with operations. In most applications, even high-temperature applications, evaporation is not a serious problem because of effective sealing. However, when it is necessary to evaluate evaporation loss, two ASTM test methods are available. Evaporation Loss of Greases and Oils D 972, Test Method for Evaporation Loss of Lubricating Greases and Oils, determines mass evaporative losses from greases or oils at any temperature in the range of 100-150°C (210-300°F). A weighed sample of lubricant is placed in an evaporation cell in an oil bath at the desired test temperature. Heated air at a specified flow rate is passed over the sample surface for 22 h, after which, the loss in sample mass is determined.
Evaporation Loss Over Wide Temperature Range D 2595, Test Method for Evaporation Loss of Lubricating Greases over Wide Temperature Range, augments D 972, which is limited to 150°C (300°F), and was developed because of higher service temperatures. D 2595 can be used to determine the loss of volatile materials from a grease over a temperature range of 93-316°C (200-600°F). This test uses the same sample cup as D 972, but the rest of the apparatus is markedly different. It uses an aluminum block heater, instead of an oil bath, to achieve much higher temperatures. The other test conditions, i.e., air flow rate and test duration, remain the same. Laboratories equipped with D 2595 do not need D 972; the results will be similar, but not necessarily identical. Within their respective temperature constraints, both tests can be used to compare evaporation losses of greases intended for similar service. All other factors being equal, greases having the least evaporative losses will probably perform longer in high-temperature service. Results of these evaporation tests may not be representative of volatilization that can occur in service. Oil Separation (Static Bleed Test) Nearly all greases will separate some oil during storage, but they differ markedly in the amounts that are liberated. If a grease separates too much oil, the grease could harden to the extent that lubrication will be affected. Opinions differ on whether or not lubrication depends on oil bleeding; greases that do not separate some oil during operation can be noisy in service. However, excessive liberation of free oil during storage is to be avoided. Oil can be released from a grease at varying rates depending on the gel structure, the nature and viscosity of the lubricating fluid, and the applied pressure and temperature. ASTM D 1742, Test Method for Oil Separation from Lubricating Greases During Storage, is used to determine the tendency of lubricating greases to separate oil when stored at 25°C (77°F) at an applied air pressure of 1.72kN/m^ (0.25psi). It gives an indication of the oil retention characteristics of lubricating greases stored in both normally-filled and partiallyfilled containers. This test is not suitable for use with greases softer than NLGI No. 1 consistency because of a tendency for the grease to seep through the screen. The test is useful because the results correlate directly with oil separation, which occurs in 16-kg (35-lb) containers of grease stored at room temperature. Storage in other containers gives similar results. This test should not be used to predict the oil separation of grease under dynamic service conditions. (See Standards Under Development, Cone Test, for a description of an elevated-temperature, static bleed test.) Oil Separation (Centrifuge Test) ASTM D 4425 describes a procedure for determining the tendency of lubricating grease to separate oil when subjected to high centrifugal forces. The results can be related to grease performance in shaft couplings, universal joints, and rolling element thrust bearings subjected to large or prolonged centrifugal forces. Results correlate well with actual service performance. In this test, pairs of centrifuge tubes are charged with test grease and placed in a high-speed centrifuge. The grease sam-
CHAPTER 20: LUBRICATING pies are subjected to a centrifugal force equivalent to a relative centrifugal acceleration, G value, of 36000 at 50°C. (The units for the G value are awkward and not used, but they have acceleration dimensions of length/time'^.) The normal test duration is 24 h, but it can be extended to 48 or 96 h. At these specified time intervals, the centrifuge is stopped, and the amount of separated oil is measured and the volume percent calculated. The resistance-to-separation index, called the K36 value, is reported as the volume percent of separated oil/total test hours (both actual values are reported as a fraction; the fraction is not to be reduced). Leakage from Wheel Bearings There are two tests to evaluate leakage of grease from wheel bearings at high temperatures. The older test, D 1263, Test Method for Leakage Tendencies of Automotive Wheel Bearing Greases, utilizes a modified automotive front hub assembly (1940s vintage design and bearings). The two bearings are packed with a specified amount of test grease, and an additional 85g is distributed in the hub. The assembly is run at 660 rpm for 6 h at 104°C (220°F). After the test, the amount of grease that leaked into the hubcap and collector is weighed. The bearings are washed and examined for varnish, gum, and lacquer-like materieJ. ASTM Test D 1263 provides a means to differentiate among grease products with distinctly different leakage characteristics. In addition, skilled operators can observe significant changes in other grease characteristics that may have occurred during the test. However, these observations are subject to differences in personal judgment and cannot be used for quantitative ratings. The test does not distinguish between wheel bearing greases having similar or borderline leakage. Accelerated Leakage from Wheel Bearings ASTM D 4290, Test Method for Determining Leakage Tendencies of Automotive Wheel Bearing Grease Under Accelerated Conditions, uses the same principle of operation as does D 1263. It is, however, a more modem test that uses a model front wheel-hub-spindle assembly employing current production, tapered roller bearings. The test apparatus is identical to that used in D 3527 (however, the test conditions are markedly different). In D 4290, the test temperature, 160°C (320°F), is significantly higher, and the duration, 20 h, is much longer than that of D 1263. D 4290 was developed to evaluate wheel bearing leakage of greases intended for use in vehicles equipped with disk brakes, which involve much higher operating temperatures. It is used in grease specifications for such applications, and it is one of the performance tests required by D 4950. This test is not known to correlate with any other type of service. Oxidation Stability Bomb Oxidation Test The Standard Test Method for Oxidation Stability of Lubricating Greases by the Oxygen Bomb Method, D 942 (IP142), determines the resistance of lubricating greases to oxidation when stored statically in an oxygen atmosphere in a sealed system at an elevated temperature. In this test, five glass dishes are filled with 4g of grease, each, for a total of 20g. These dishes are then racked and sealed in a bomb, which is
GREASES
565
then pressurized to 758kPa/cm^ (llOpsi) with oxygen. The bomb is heated in a bath at 99°C (210°F) to accelerate oxidation. The amount of oxygen absorbed by the grease is recorded in terms of pressure drop over a period of 100 h and, in some cases, 500 or 1000 h. The pressure drop is a net result of the consumption of oxygen by oxidation of the grease and the gain in pressure due to any gases or volatile by-products released from the grease. Care must be exercised in the interpretation of data derived from the oxidation bomb test. Additives incorporated into the grease can produce misleading results because they can also react with oxygen. As an example, sodium nitrite is sometimes added to grease to serve as a rust inhibitor. In the oxidation bomb test, this material consumes oxygen to form sodium nitrate. In this instance, the drop in pressure is not indicative of the amount of oxidation of the grease alone. Also, greases containing excess carbonate can release CEirbon dioxide gas whose vapor pressure will tend to offset the pressure decrease due to oxygen absorption. The bomb oxidation test was originally designed to predict shelf storage life of greases in prepacked bearings. Whatever its original intent, experience has shown little correlation with the stability of grease films in bearings or on other parts. It predicts neither the stability of greases stored in containers for long periods nor those used under dynamic conditions. Its primary usefulness is for quality control to indicate batchto-batch uniformity. It can be used to estimate relative oxidation resistance of greases of the same type, but it should not be used to compare greases of different types. Although widely used for specification purposes, it is important to note that D 942 has been severely criticized for its potential for misleading results and for having no relation to oxidation in service. There are no standard, dynamic oxidation tests. For dynamic tests that are influenced to a greater or lesser extent by the oxidation resistance of the grease, see descriptions of D 3336, D 3337, and D 3527. PDSC Oxidation Test A number of grease test methods have recently been developed by the ASTM grease committee, one of them being the PDSC (pressure differential scanning calorimetry) technique for evaluating oxidation stability. In this test, a few mg of grease is placed in a sample pan in a bomb, which is pressurized to 3.5MPa (500psi) with oxygen and regulated at that pressure until an exothermic reaction occurs. From a plot of heat as a function of test time, the oxidation induction time (called extrapolated onset time) is determined. This method of evaluating oxidation stability has two significant advantages over D 942: 1) It is considerably faster, generally less than an hour, vs. 100 h or more, and 2) unlike D942, this method is not subject to false values from greases that give off carbon dioxide or other gases during heating. The disadvantages are that the apparatus is expensive, and like D942 the results are not known to correlate with service performance. Another test being developed is the thin film microoxidation test, which has been shown in literature to provide a good indication of the oxidation stability of a grease sample. Greases in Ball Bearings at Elevated Temperatures ASTM D 3336, Test Method for Performance Characteristics of Lubricating Greases in Ball Bearings at Elevated Temper-
566
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atures, is used to evaluate the performance characteristics of lubricating greases in ball bearings operating u n d e r light loads at high speeds cind elevated temperatures for extended periods. Correlation with actual field service cannot be assumed. This method has been criticized for having two test spindles qualified that purportedly do not necessarily give the same results. In this test, the lubricating grease is evaluated in a 20-mm, SAE No.204, heat resistant, steel ball bearing rotated at 10000 r p m under light loads of 22-67N (5-151bf) at a specified elevated temperature u p to 370°C (700°F). The test is r u n on a specified, test-temperature-dependent, operating cycle until lubrication failure or completion of a specified time. (Unless automatic controls Eire used, a 72-h weekend shutdown is required.) With superior greases, tests can last u p to several thousand hours. Multiple tests need to be r u n because the results follow Weibull, rather than normal, distributions. Greases in Small
Bearings
The computer and aircraft industries have used miniature bearings for many years. As the trend toward miniaturization increased in other industries, a suitable test was needed to evaluate lubricating greases in small bearings. ASTM D 3337, Test Method for Evaluation of Greases in Small Bearings, was developed to serve this purpose. Although this test is not the equivalent of long-term service-performance tests, it can be used to predict relative grease life at high temperature in a reasonable test period. Also, this test can measure running torque at both one r p m and 12000 r p m if this property is significcmt for the intended application. The method will not differentiate among greases of closely related characteristics. ASTM D 3337 determines grease life and torque in a small (6.35mm bore) R-4 ball bearing. In this test the bearing is run at 12000 r p m with a 2.2N (1/2-lbf) radial and a 22N (51bf) axicJ load. While a test temperature of 250°C (or 500°F) may be typiccJly specified, the equipment is capable of testing u p to 315°C (600°F) if high-temperature bearings are used.
Extreme Pressure and Wear A lubricant functions by separating bearing surfaces. If the separation was always complete, parts would never wear. However, the integrity of the lubricant film cannot be maintained u n d e r all conditions, and contact occurs to varying degrees. Such contact depends on operating conditions (such as load and speed), lubricant properties (such as fluid viscosity and grease consistency), and lubricant chemistry (such as the presence of wear inhibitors and extreme pressure additives). Several tests are available for evaluating the antiwear and load-carrying properties of greases. Extreme
Grease
Life
With the advent of automotive disk brakes in the 1960s, then c u r r e n t test m e t h o d s proved inadequate for evaluating greases for this high-temperature application. A specific, correlating test method was needed, and after several yeeirs of development, one was standardized. ASTM D 3527, Standard Test Method for Life Performance of Automotive Wheel Bearing Greases, evaluates grease life in a tapered roller, wheel bccirings in a model, front wheel assembly r u n at 1000 rpm, under a thrust load of 11 IN, at 160°C, using a cycle of 20 h on and 4 h off. The test apparatus is the same as that of D 4290, b u t the operating conditions a n d measured peirameters are quite different. Motor torque is monitored, and the test is terminated at a calculated, preset torque vEilue. Grease life is indicated by the n u m b e r of "on" hours (or n u m b e r of cycles) to failure. This is a severe test; the results are influenced by a combination of grease properties, such as oxidation stability, shear stability, and volatility. As with D 3336, multiple tests need to be r u n because the results follow Weibull distributions. This test method is used for specification purposes and is required by D 4950.
Timken
Method
ASTM D 2509, Test Method for Measurement of Extreme Pressure Properties of Lubricating Grease (Timken Method), can be used to determine the load carrying capacity of a grease at high loads. Non-stemdard techniques have been devised to measure wear at lighter loads, but they are not discussed. In the Timken test, a tapered roller bearing cup is rotated against a stationary, hardened steel block. Both parts are lubricated with the test grease prior to starting the test. During the test, the p a r t s are continuously lubricated with fresh grease by means of a feed mechanism. Using a lever system with a ten-fold mechanical advantage, fixed weights apply a force to the block in line contact with the rotating cup. Loads are applied step-wise until lubrication failure occurs, as evidenced by inspection for scoring or welding. The "OK Value" is the m a x i m u m load the lubricant film will withstand without rupturing and causing scoring in the contact zone after a 10-min. run. This test is a rapid method that can be used to differentiate between greases having low, medium, or high levels of extreme pressure properties. It is widely used for specification purposes; however, the results may not correlate with service performance. Extreme
Wheel Bearing
Pressure
Pressure
Four-Ball
Test
ASTM D 2596, Test Method for Measurement of Extreme Pressure Properties of Lubricating Grease (Four-Ball Method), is another test used to determine the load-carrying properties of lubricating greases. With this procedure two evaluations are made: (1) the Load-Wear Index (formerly called Mean-Hertz Load), and (2) the Weld Point. This test was developed to evaluate the extreme pressure and antiweld properties of a lubricant. The tester is operated with one steel bcdl rotated under load against three like bsJls held stationary in the form of a cradle. The grease under test covers the contact area of the four balls. Loads u p to 800 kgf (7845N, 17601bf) can be applied to the balls to achieve unit pressures u p to 6.9 X lO^kPa (1 000 000 psi). The procedure involves the running of a series of 10-second tests over a range of increasing loads until welding occurs. During a test, scars are formed in the surfaces of the three stationary balls. The diameter of the scar depends on the load, speed, test duration, and lubricant. The scars are measured under a microscope having a calibrated grid. From the scetr measurements, the Load-Wear Index is calculated. The lowest load at which the rotating ball seizes and then welds to the stationary balls is called the weld point; it indicates
CHAPTER that the load carrying capacity of the grease has been exceeded. The significance of this test is that it is a rapid method that can be used to differentiate a m o n g greases having low, medium, or high levels of extreme pressure properties. It is widely used for specification purposes, b u t the results may not correlate with service performance. Because of their poor lubricity, some lubricating greases containing a silicone or a halogenated silicone fluid component are not suitable for testing by this test method. Wear Preventive
Characteristics
of
Grease
ASTM D 2266, Test Method for Wear Preventive Characteristics of Lubricating Grease (Four-Ball Method) is used to determine the wear-preventive characteristics of greases in sliding steel-on-steel applications. This test does not distinguish between EP and non-EP greases. As in D 2509, a four-ball configuration is used, but there are few other similarities as the apparatus and operating conditions are conspicuously different. Wear prevention qualities are evaluated from the diameters of the wear scars that occur on the stationary balls during the test. The test is significant because it can be used to determine the relative wear-preventing properties under the test conditions, so it is useful for grease development. D 2266 is widely used in grease specifications, but its actual usefulness is suspect because of the following limitations. If test conditions are changed, the relative ratings may change. 1. Wecir characteristics are not predicted for metal combinations other than AISI (American Iron and Steel Institute) E52100 steel unless non-stcindard balls of other materials are used. 2. No differentiation can be made between extreme pressure and non-extreme pressure greases. 3. No correlation can be inferred between the results of the test and field service unless such correlation has been established. Fretting
Wear
Fretting wear is a form of attritive wear caused by vibratory or oscillatory motion of small amplitude. It is characterized by the removal of finely-divided particles from the rubbing surfaces. Air can cause immediate loccil oxidation of the wear particles produced by fretting wear, and moisture can hydrate the oxidation product. In the case of ferrous metals, the oxidized wear debris is abrasive iron oxide (Fe203) having the appearance of rust, which gives rise to the nearlysynonymous terms, fretting corrosion and friction oxidation. Fretting is a serious problem in industry. If severe enough, it can cause destructive vibrations, premature failures, and pcirts seizure. No grease can give total protection if fretting conditions exist, but greases vary significcintly in their ability to mitigate fretting wear. ASTM D 4170, Test Method for Fretting Wear Protection by Lubricating Grease, evaluates grease performance in a proprietary test machine (Fafnir Friction Oxidation Tester) which oscillates two grease-lubricated, ball thrust bearings, u n d e r specified conditions of load, speed, and angle. Fretting wear is determined by measuring the mass loss of the bearing races (the balls Eind retainers are not included).
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A related, but somewhat different, phenomenon often accompanies fretting wear. False brinelling is localized fretting wear that occurs when the rolling elements of a bearing vibrate or oscillate with small amplitude while pressed against the bearing race. The mechanism proceeds in stages: 1) asperities weld, are torn apart, and the wear debris which is subsequently formed is oxidized; 2) due to the small-amplitude motion, the oxidized detritus cannot readily escape, and being abrasive, the oxidized wear debris accelerates the wear. As a result, wear depressions are formed in the bearing race. These depressions are often polished and appear similar to the Brinell depressions obtained with static overloading, hence the term, false brinelling. D 4170 cannot distinguish between false brinelling and fretting wear. If false brinelling does occur, it is included in the determination of fretting wear when bearing race mass losses are measured. This test correlates with the fretting performance of greases in wheel bearings of passenger cars shipped long distances. D 4170 also has been used to predict grease performance in automobile drivelines. It is used for specification purposes and is one of the performance tests required by D 4950. Oscillating
Motion
There is another wear test involving oscillatory motion, namely, ASTM D 3704, Test Method for Wear Preventive Properties of Lubricating Greases Using the Block on Ring Test Machine in Oscillating Motion. Ring and block parts, similar to those of the Timken Tester (D 2509), are operated u n d e r varying conditions of load, speed, oscillation angle, time, temperature, and specimen surface finish and hardness to simulate service conditions. This test can distinguish among greases of low, medium, and high wear preventive properties and can be used for grease development. The user should determine whether test results correlate with service performance or results from other bench test machines. Oscillating
Wear (SRV)
Test
A high load, high frequency, low amplitude, high speed reversal test, using the SRV (Schwingung, Reibung, Verschleiss) a p p a r a t u s simulates high-speed vibrational or start-stop motions that occur in many mechanisms. Two procedures, using a ball-on-disk configuration, have been developed: [ASTM D 5706 (EP Test) and ASTM D 5707 (Wear)] one to measure wear-protection qualities and coefficient of friction, and the other to measure the ability of a grease to carry loads under extreme pressure. (This apparatus can use other configurations to suit different applications.) Both procedures have been correlated with grease performance in automotive driveline mechanisms and are used in grease specifications for these applications. These procedures can be used to evaluate lubricants and materials for other applications of similar motion. Corrosion Copper
Corrosion
Lubricated parts that contain copper alloys, such as copper or brass electrical components or bronze gears and bearings, m a y be susceptible to the corrosive effects of formulated greases. For example, such corrosion can cause high resis-
568 MANUAL 37: FUELS AND LUBRICANTS
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tance in electrical contacts, or premature bearing failure from chemical attack. D 4048, Test Method for Detection of Copper Corrosion from Lubricating Grease by the Copper Strip Tarnish Test, is the grease analog of the more familiar D 130 used to evaluate oils. In this test, a prepared copper strip is totally immersed in test grease and heated at specified conditions, usually 100°C (210°F) for 24 h. At the end of the test period, the strip is removed, washed, and compared with the ASTM Copper Strip Corrosion Standards. Although this method is used in specifications, the user must establish correlation between test results cind actual service performance. This test does not determine the ability of a grease to inhibit copper corrosion caused by factors other than the grease itself; neither does it determine the stability of grease in the presence of copper. The sole determination is the chemical staining of copper by lubricating grease. Rust Prevention Greases must not be corrosive to metals they contact and should not develop corrosion tendencies with aging or oxidation. A method for assessing rust prevention by greases is ASTM D 1743, Test Method for Corrosion Preventive Properties of Lubricating Greases. In this method, a tapered roller bearing is packed with grease, and following a short run-in period, dipped into distilled water and stored above the water in 100% relative humidity at 52°C (125°C) for 48 h. The bearing is then cleaned and examined for corrosion. Either a Pass or Fail result is reported. The significance of this test is that it indicates those greases capable of preventing rust and corrosion in static or storage conditions. This test is widely used in grease specifications. The correlation with service conditions, particularly under static conditions, is considered to be quite good. Accelerated Corrosion Tests Two test methods were recently worked on to evaluate the corrosion protection properties of greases under severe conditions. These are: 1) a version of D 1743 using synthetic sea water now called ASTM D 5969, and 2) two procedures of the IP220/DIN51802 dynamic rust test [commonly known as the EMCOR test (ASTM D 6138)], one using distilled water and another using salt water. Effect of Water Contamination by water can affect greases and grease performance in several ways. Corrosion or rust protection, previously discussed, is one. Other effects include change in consistency, texture, or adhesiveness. An emulsion can be formed, which will probably be an inferior lubricant, or it could be washed away. Attempts to standardize means of evaluating these effects have had mixed success. Two standard tests do exist, however: the water washout test and the water spray-off test. Water Washout The ability of a grease to resist washout under conditions where water may splash or impinge directly on a bearing is an important property in the maintenance of a satisfactory lu-
bricating film. ASTM D 1264, Test for Water Washout Characteristics of Lubricating Greases, evaluates the resistance of a lubricating grease in a bearing to washout by water. This test method uses a standardized bearing, available from ASTM. It is a 204K Conrad-type, ball bearing equipped with shields but without seals. The bearing is packed with 4g of test grease then rotated at 600 rpm while a jet of water, at either 38°C (100°F) or 79°C (175°F), impinges on the bearing housing for 1 h. The bearing is then dried, and the percent grease loss by weight is determined. This test method serves only as a relative measure of the resistance of a grease to water washout. It should not be considered the equivalent of a service evaluation unless such correlation has been established. Test results are affected by grease texture and consistency. Test precision is poor, especially with soft greases. Although widely used, this test can give misleading results. Even comparative results between similar greases may not predict the relative performance of the two greases in actual service. Water Spray-Off ASTM D 4049, Test Method for Resistance of Lubricating Grease to Water Spray, is used to evaluate the ability of a grease to adhere to a metal panel when subjected to direct water spray. Test results correlate directly with operations involving direct water impingement, such as steel mill roll neck bearing service and certain automotive body hardware applications. In this test, a 0.79 mm (1/32 in.) film of test grease is uniformly coated onto a stainless steel panel; then water, at 38°C (100°F), is sprayed directly on the panel for 5 min. The spray is controlled by specified spray nozzle, pump, and plumbing. After the spraying period, the panel is dried, weighed, and the percentage of grease spray-off is determined. Miscellaneous Contamination ASTM D 1404, Test Method for Estimation of Deleterious Pcirticles in Lubricating Grease, defines a deleterious particle as one which will scratch a polished plastic surface. The test is applicable to all greases, even those containing fillers. In fact, it can be used to test fillers, such as graphite, if they are dispersed into a grease (or petrolatum) that is known to be free of deleterious particles. It can be used also to test other semi-solid or viscous-liquid substances. With this method, the test material is placed between two clean, highly polished acrylic plastic plates held rigidly and parallel to each other in metal holders. The assembly is pressed together by squeezing the grease into a thin layer between the plastic plates. Any solid particles in the grease larger than the distance of separation of the plates and harder than the plastic will become embedded in the opposing plastic surfaces. The apparatus is so constructed that one of the plates can be rotated about 30° with respect to the other while the whole assembly is under pressure. This will cause the embedded particles to form characteristic arc-shaped scratches in one or both plates. The relative number of such solid particles is estimated by counting the total number of arc-shaped scratches on the two plates.
CHAPTER The test has significcince because it is a rapid means for estimating the n u m b e r of deleterious particles in a lubricating grease. However, a particle that is abrasive to acrylic plastic m a y not be abrasive to steel or other bearing materials. Therefore, the results of this test do not imply performance in field service. Elastomer
Compatibility
Nearly all grease-lubricated mechanisms have elastomeric seeds to retain lubricant zmd exclude contaminants. In order for these seals to function properly, the grease must be compatible with the rubber-like elastomer seal. ASTM D 4289, Test Method for Compatibility of Lubricating Grease with Elastomers, is a simple total immersion test designed to evaluate the compatibility of grease with elastomer specimens cut from standard sheets. It also can be used as a guide to evaluate compatibility of greases with rubber products not in standard sheet form. Unlike other standard compatibility tests, which are designed to evaluate elastomers in standard fluids, the emphasis of D 4289 is the evaluation of the greases. Elastomer specimens are cut from s t a n d a r d ASTM sheets (D 3182) a n d immersed in test grease for 70 h at either 100 or ISO'C. Compatibility is evaluated by determining the changes in volume and DurometerA hardness (D 2240). (Volume is determined by the water displacement method, D 471.) The volume and hardness change values determined in this test do not duplicate the changes that occur in rubber seals in actual service conditions. However, they can be correlated in many instances. For example, the volume-change values correlated very well (r^ = 0.99) with those that occurred in a vehicle test. Because of wide variations in grease and elastomer formulations and service conditions, correlations between this test and particular applications should be determined on an individual basis. This method provides for optional testing with two Reference Elastomers to evaluate relative compatibility. The results CcUi be used to judge a service characteristic of lubricating greases; in this respect, the test m e t h o d is useful for specification purposes. ASTM D 4950 requires testing with Reference Elastomer CR (polychloroprene) cind Reference Elastomer NBR-L (acrylonitrile-butadiene). Compatibility Mixing of two different grease types often occurs when a mechanism is service lubricated with a type of grease different from that already in the bearing. If the two greases are incompatible, the likelihood is that lubrication will be inadequate and/or the lubrication life will be greatly shortened. The problem of incompatible grease mixtures has long been known. Foreknowledge of the chemistry of the greases is not often reliable in predicting compatibility. Compatibility needs to be judged on a case-by-case basis. In light of this need, a standard practice was developed Eind is now our new ASTM method ASTM D 6185. There are several non-standard means worthy of consideration, however. One such practice involves the preparation of three binary mixtures in concentrations of 10:90, 50:50, and 90:10 mass ratios. These three mixtures and the two neat greases are then tested for dropping point (D 566 or D 2265),
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569
shear stability (either by prolonged working or by the roll test, D 1831), oil separation (D 1742), and storage stability (cheinge in consistency after prolonged storage, such as one to six m o n t h s ) . Other tests, s u c h as D 3527 (life performance), D 4290 (leakage), water spray-off (D 4049), etc. EJSO should be r u n if the application warremts such. Incompatibility is indicated if any mixture tests worse t h a n the poorer of the neat greases. The repeatability of the test methods must be tciken into consideration when making such determinations. Static Bleed (Cone)
Test
Federal Test Method (FTM) 791C, Method 321.3, static bleed test (also known as the cone test), was developed as ASTM Standard D 6184. In this method, a 10-g sample of grease is placed in a 60-mesh wire cone, which is then suspended in a covered bejiker and placed in a n oven at 100°C (212°F) for 30 h (some specifications may require different temperatures or durations). The amount of oil that bleeds from the grease collects in the bejiker is weighed, and the percentage of separated oil is calculated. Some specifications require the cone and residual grease to be weighed also, and the additional weight loss is reported as evaporation.
Chemical Analysis D 128, Test Methods for Analysis of Lubricating Grease, is the premier standard for grease analysis. This procedure gives flow diagrams and details for the analysis of conventioneJ greases, i.e., those made of soap-thickened petroleum oils. The constituents that can be determined are soap, unsaponifiable matter (petroleum oil, etc), water, free alkalinity, free fatty acid, fat, glycerin, a n d insolubles. A supplementary test method is provided for application to greases that cannot be analyzed by conventional methods because of the presence of nonpetroleum oils or nonsoap thickeners. These test procedures can be used to identify and estimate the a m o u n t of some of the constituents of lubricating greases. The methods are applicable to many, but not all, greases. Composition should not be considered as having a direct bearing on service performance unless such correlation has been established. D 128 references several useful, but nonstandard, methods that can be used for grease analysis. Infrzired Spectroscopic (IR) analysis is commonly used for research purposes, quality control, and specification purposes. But, a n IR method has not been standardized because the technique frequently must be adapted to the specific grease being analyzed. Although D 128 is the general analytical method for greases, t h e r e are other test m e t h o d s for specific constituents. These include the following: • D 95—Test Method for Water in Petroleum Products and Bituminous Materials by Distillation • D 129—Test Method for Sulfur in Petroleum Products (General B o m b Method) • D 808—Test Method for Chlorine in New a n d Used Petroleum Products (Bomb Method) • D 1317—Test Method for Chlorine in New and Used Lubricants (Sodium Alcoholate Method) • D 3340—^Test Method for Lithium and Sodium in Lubricating Greases by Flame Photometer.
570
MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
DISCONTINUED STANDARDS Severed standards have been discontinued since the 5th edition of the Manual on Significance of Tests for Petroleum Products. Unless otherwise noted, the reason for discontinuance of the following test methods was lack of interest, i.e., the standcirds were no longer being used or supported by the industry: • D 1262—Test Method for Lead in New and Used Greases (Discontinued 1991) • D 1402—Test Method for Effect of Copper on Oxidation Stability of Lubricating Greases by the Oxygen Bomb Method (Discontinued 1985) • D 1741—Test Method for Functional Life of Ball Bearing Greases (Discontinued 1991) • D 3428—Test Method for Torque Stability, Wear, and Brine Sensitivity Evaluation of Ball Joint Greases (Discontinued 1990). (D 3428 was withdrawn because the ball joint used as the test piece was no longer available and a suitable substitute was not found.)
STANDARDS UNDER DEVELOPMENT Ignition Test Because grease fires in steel mills are a common occurrence, there is a need for a standard method for evaluating the ignition and flammability characteristics of greases used in such applications. Techniques for evaluating these cheiracteristics are being developed. The procedure most likely to be evaluated in round robin testing consists of placing a shaped sample of grease on a metal ramp; a methenamine tablet is inserted in the grease and ignited. If the grease burns, the characteristics of the fire (time to grease or oil ignition, bum time, fire size, self-extinguishing, etc.) are compared with referenced descriptions and assigned a rating.
ISO STANDARDS In 1987, ISO (International Standards Organization) published ISO 6743-9, which is an international classification system for grease. About 75% of commercial greases can be described by this standard. This standard classifies lubricating greases according to the operating conditions of the use— unlike most other ISO product standards, which are classified according to specific end-use. The nature of greases allows a specific grease to be used in many applications. This makes it impractical to classify greases by end-use, and a properties description is a reasoned alternative. Consequently, users are advised to use ISO 6743-9 to define the requisite grease properties, but are they cautioned not to rely solely on the standard for grease selection for a particular application. Rather, users are advised to consult with the grease supplier, as well. In this classification system, each grease will have one symbol only. This symbol should correspond to the most severe conditions of temperature, water contamination, and load in which the grease can be used. Grease products are designated in a uniform manner, with each character having its own sig-
nificance. The line call-out code will have the following format: ISO-L-XSiS2S3S4N where: ISO L X Si 52 53 54 N
= = = = = = = =
identification of the standards organization designator for class (lubricants) designator for family (grease) symbol for Lower Operating Temperature symbol for Upper Operating Temperature symbol for Water Contamination symbol for EP NLGI consistency number
Symbols (letters) are used to designate the four operating conditions. The Lower Operating Temperature is defined by symbols representing requirements at 0, -10, -20, —30, —40, and <—40°C; the Upper Operating Temperatures is defined by symbols representing seven temperatures from 60 to >180°C. Nine symbols are used to define the effects of water, which include both water contamination and antirust requirements. Only two symbols are used for the EP or load carrying requirement. Limits have been tentatively established for the operating conditions. But, full implementation of IS06743-9 depends on the development of ISO standard test methods. National standards must be converted to ISO format and approved by the ISO community. The national standards proposed to ISO are as follows: Lower Operating Temperature Upper Operating Temperature Water Effect
EP (load carrying capacity)
NFT60-171 (France), a lowtemperature penetration test. ASTM D 566 and D 3336 ASTM D 1264 for Water Contamination and DIN 51802 (Germany)/ IP220 (United Kingdom) for Rust Protection ASTM D 2596
ASTM STANDARDS* No. D 217 D 566 D 942 D 972 D 1092
Title Test Methods for Cone Penetration of Lubricating Grease Test Method for Dropping Point of Lubricating Grease Test Method for Oxidation Stability of Lubricating Greases by the Oxygen Bomb Method Test Method for Evaporation Loss of Lubricating Greases and Oils Test Method for Apparent Viscosity of Lubricating Greases
' Analytical standards not included.
CHAPTER D 1263 D 1264 D 1403
D 1404 D 1478 D 1742 D 1743 D 1831 D 2265 D 2266 D 2509
D 2595 D 2596
D 3232 D 3336
D 3337 D 3527 D 3704
Test Method for Leakage Tendencies of Automotive Wheel Bearing Greases Test Method for Water Washout Characteristics of Lubricating Greases Test Methods for Cone Penetration of Lubricating Grease Using One-Quarter and One-Half Scale Cone Equipment Test Method for Estimation of Deleterious Particles in Lubricating Grease Test Method for Low-Temperature Torque of Ball Bearing Greases Test Method for Oil Separation from Lubricating Grease During Storage Test Method for Corrosion Preventive Properties of Lubricating Greases Test Method for Roll Stability of Lubricating Grease Test Method for Dropping Point of Lubricating Grease Over Wide Temperature Range Test Method for Wear Preventive Characteristics of Lubricating Grease (Four-Ball Method) Test Method for Measurement of Extreme Pressure Properties of Lubricating Grease (Timken Method) Test Method for Evaporation Loss of Lubricating Greases Over Wide-Temperature Range Test Method for Measurement of Extreme-Pressure Properties of Lubricating Greases (Four-Ball Method) Test Method for Flow Properties of Lubricating Greases at High Temperatures Test Method for Performance Characteristics of Lubricating Greases in Ball Bearings at Elevated Temperatures Test Method for Evaluation of Greases in Small Bearings Test Method for Life Performance of Automotive Wheel Bearing Grease Test Method for Wear Preventive Properties of Lu-
D 4048
D 4049 D 4170 D 4289 D 4290
D 4425 D 4693 D 4950 D 5483
D 5706
D 5707
D 5969
D 6138
D 6184 D 6185
20: LUBRICATING
GREASES
bricating Grease Using the (Falex) Block on Ring Test Machine in Oscillating Motion Test Method for Detection of Copper Corrosion from Lubricating Grease by the Copper Strip Teirnish Test Test Method for the Resistance of Lubricating Grease to Water Spray Test Method for Fretting Wear Protection by Lubricating Greases Test Method for Compatibility of Lubricating Grease with Elastomers Test Method for Determining the Leakage Tendencies of Automotive Wheel Bearing Grease Under Accelerated Conditions Test Method for Oil Separation from Lubricating Grease by Centrifuging (Koppers Method) Test Method for Low-Temperature Torque of Grease-Lubricated Wheel Bearings Classification and Specification for Automotive Service Greases Test Method for Oxidation Induction Time of Lubricating Greases by Pressure Differential Scanning Calorimetry Test Method for Determining Extreme Pressure Properties of Lubricating Greases Using a HighFrequency, Linear-Oscillation (SRC) Test Machine. Test Method for Measuring Friction and Wear Properties of Lubricating Grease Using a High-Frequency, Linear-Oscillation (SRV) Test Machine. Test Method for Corrosion Preventive Properties of Lubricating Greases in the Presence of Dilute SjTithetic Sea Water Environments Test Method for Determination of Corrosion Prevention Properties of Lubricating Greases Under Dynamic Wet Conditions (EMCOR Test) Test Method for Oil Separation from Lubricating Grease (Conical Sieve Method) Standard Practice for Evaluating Compatibility of Binary Mixtures of Lubricating Greases
OTHER STANDARDS^ Standard No. ISO
DIN
2176 2137
51801 51804/1-2
132 50
51805 51802 51811 51803
220 112 5
51809/1-2 51813
37/137/139 134
51807/2 51807/1 51808 51817
215
2160
51814 51815
IP
FTM791b
COST
Characteristic/Property
102 132
1421 311
6793 5346
135
40001.2 5309.4
5757
Determination of Dropping Point Determination of Cone Penetration 1/1 - 1/2 - 1/4 Cone Determination of Flow Pressure Corrosion Preventing Properties Corrosive Effects on Copper Determination of Ash of Greases (Incl. Sulfate) Neutralization Number Content of Solid Foreign Matters Effect of Water Water Washout Test Static Test Oxidation Stability Oil Separation
NF-T 60
M 07-037
1461 6474
133/112
142 121 M 07-38
571
3005.3
6370
3453 321/2
5734 7142 1631
Content of Base Oil and Soap Content of Li/Na/Ca by Atomic
Absorption Spectroscopy (continues)
572 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK Standard No. ISO
DIN
IP
FTM 791 b
NF-T 60
COST
Characteristic/Property
9270
Determination of Li/Na by Flame Photometer Determination of Solids (Graphite or M0S2) Determination of Particle Size of Solid Lubricants Density Evaporation Loss Content of Water
199 51831
3720/22
51832 59 183 74
3733
9566 1044 2077
113
51816/1 51816/2
139 51350-4/5
6503.2
239 326
331.2/333.1
51806 168 266 (51821/2) (51821/1) 186 R868 1817
53505 53521
3104 3016 2592 2977
51562 51597 51376 51775/51787 51820 E
3603.3 71 15 36 2
Pumpability Properties SheU-DeLimon Rheometer Decompression Characteristics Apparent Viscosity Roll Stability Extreme Pressure Properties Shell-Four-Ball Test Timken Test Mechanical Dynamic Testing SKF-R2F Roller Bearing Performance Churning FAG-FE 8 (EP Greases) FAG-FE 9 Wheel Bearing Leakage Low Temperature Torque Elastomer Compatibility Hardness Change (Shore A) Volume Change Tests on Base Oil Viscosity Pour Point Flash Point Aniline Point Infrared-Analysis
^ Reprinted with permission from The Lubrizol Corporation, Wickliffe, OH.
REFERENCES [6] [1] Polishuk, A. T., A Brief History of Lubricating Grease, Llewellyn & McKane, Inc., Wilkes-Barre, PA, 1998. [2] Boner, C. J., Manufacture and Application of Lubricating Grease, Reinhold PubUshing Corp., NY, 1954. [3] Boner, C. J., Modem Lubricating Greases, Scientific Publications (G.B.) Ltd, Broseley, Shropshire, England, 1976. [4] Lubricating Grease Guide, 4 * ed.. National Lubricating Grease Institute, Kansas City, MO, 1996. [5] Ward, C. E., "Polyurea Greases," National Lubricating Grease
[8]
[9] [10]
Institute (NLGI) Grease Education Advanced Course, Kansas City, MO, 1998. "Lubricating Greases," Ch. 9, Manual on Significance of Tests for Petroleum Products, 5th ed., G. V. Dyroff, Ed., ASTM International, West Conshohocken, PA, 1989. Federal Test Method Standard No. 79IC: Lubricants, Liquid Fuels, and Related Products: Methods of Testing, Available from Global Engineering Documents, Irvine, CA, Sept. 30, 1986. "Index of the NLGI Spokesman," Compact Disk, National Lubricating Grease Institute, Kansas City, MO, 2000. Annual Book of ASTM Standards, Volumes 05.01, 05.02, 05.03, ASTM International, West Conshohocken, PA.
MNL37-EB/Jun. 2003
Mineral Oil Heat Transfer Fluids John Fuhr, ^ Jim Oetinger, ^ George E. Totten,^ and Glenn M. Webster^
The focus of this chapter will be on mineral oil derived heat tremsfer fluids. This discussion will include a basic overview of the heat transfer coefficient as a fluid chciracterization parameter followed by a discussion of fluid chemistry and the impact on properties. An overview of various test procedures used in the selection and maintenance of mineral oil heat transfer fluids will be provided. System maintenance, operation, and design will be discussed toward the closing.
BECAUSE OF COST AND POTENTIAL FOULING PROBLEMS, indirect
process heating designs are usually preferred over direct, fuel-fired heating or individual electrically heated units [2]. For indirect process heating, heat transfer is usually accomplished by steam or by using a h e a t transfer fluid (HTF). Examples of HTFs include: petroleum based mineral oils, glycols, silicones a n d various S3Tithetic fluids such as alkyleated aromatics, terphenyls, a n d mixtures of bi and diphenyls and their oxides. Selected physical properties for a n u m b e r of illustrative tj^pes of heat transfer fluids are provided in Table 1. These data show that there are notable differences in the physical properties of different classes of different fluids within a class.
DISCUSSION Heat Transfer Coefficient
Steam is one of the most economical, since it is generated easily and it possesses excellent heat transfer properties, due to its relatively high heat of vaporization and high thermal conductivity. However, steam suffers frora a n u m b e r of disadvantages such as corrosion and the fact that it must be used in high-pressure equipment. It is often most suitable for temperatures of <300°F [3]. In some cases, reactors must not only be used for heating, but also for cooling. In these cases, a secondary coolcint may be used such as a n aqueous glycol coolant like those illustrated in Table 2 [3]. In these situations, the use of steam and cooling water in separate loops reduces the available heat treinsfer cirea and increases the amount of piping required. It also increases the thermal stresses arising from the alternating flow of chilled water and steam. In general, glycol water solutions are most applicable for applications requiring moderate heating and/or temperatures to 0°F. Above 500°F, mineral oil and synthetic eiromatic based heat transfer fluids are often used because of the cost of using steam at the corresponding temperatures. Furthermore, heat transfer fluids are often the better choice for intermediate temperatures above 300°F, particularly for the single process user who cannot justify a steam boiler and also for systems requiring both heating and cooling. Some mineral oil heat transfer fluids may be used above 600°F although the most c o m m o n use range for these fluids is 300-570°F. The ISO 6743-12 fluid classifications for mineral oil based heat transfer fluids is shown in Table 3. For higher temperatures u p to 700°F, fluids such as modified terphenyls may be used cind biphenyl emd other fluids such diphenyl oxides may be used u p to 750°F [3].
For heat transfer fluid selection, a n u m b e r of fluid-related factors m u s t be considered including fluid performance, thermal stability, and safety. Several physiccd properties of a heat transfer fluid directly affect fluid flow, a n d thus heat transfer performance, which are quantitatively illustrated using the classic Seider and Tate equation for the fluid-film heat transfer coefficient (h) for fully turbulent flow in a tube [1,4,20]: hdA = 0.027 (Jvp/M)°* (.Cpfi/kf-^^ (fc)"-*^ (M/MW)"'^ Where: h is the fluid-film heat transfer coefficient, d is the inside diameter of the tube, k is thermal conductivity, v is velocity, p is fluid density, Cp is the specific heat capacity, fi is the absolute viscosity at the bulk fluid temperature, and /AW is the absolute fluid viscosity at the wall temperature. Seider and Tates correlation analysis showed that there was little variation between /x and /xw in the region of turbulent flow fluids seldom exhibit large temperature coefficients of viscosity and because heat transfer rates are high, there are less high temperature differences. W h e n the exponent of the viscosity gradient (;U//AW) was 0.14 for fully turbulent flow, (see above equation for hd/k), the viscosity gradient was equal to 1. Therefore, the simplified equation for h, the fluid-film heat transfer coefficient is obtained [1]. h = 0.027 id)'"-^ (v)«« (p)0« (Cp)0" ikf"
The Seider and Tate equation shows that higher heat transfer is favored by: increasing fluid density, increasing heat capacity, increasing conductivity, increasing fluid velocity and turbulence, and decreasing fluid viscosity [2]. Rates of heat transfer are also affected by system heat sources and heat sinks, geometry, and temperature gradients [2]. Heat transfer properties of two fluids can be compared by assuming they are flowing through the same pipe. Therefore,
' Paratherm Corporation, Consholiocken, PA 19428. ^ G.E. Totten & Associates, Inc., LLC, PO Box 30108, Seattle, WA 98103. ^ 63 Rockledge Rd., Hartsdale, NY 10530. 573 Copyright'
2003 by A S I M International
(/x)-o-47
www.astm.org
574
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
TABLE 1—Comparison of typical heat transfer fluid properties.
Type of Fluid
Fluid Caloria HT43 Dowtherm A Dowtherm G Therminol 66 Dowtherm J Dowtherm Q Mobiltherm 603 Paratherm NF® Syltherm 800 Syltherm XLT
Comparison Inside Film Coefficients (BTU/Hr) (ft^) (°F) 2" Sched 40 Pipe @ 87Sec 400^ 500°F eoo-F
Max" Bulk Temp °F
Max" Film Temp °F
Flash Point (coc) °F
Fire Point (coc) °F
Auto Ignition Temp °F
Coefficient of Thermal Expansion %rF
Pour Point "F
Vapor Pressure @600°F psia
Vise 100°C cSt
236
267
302
600
660
406
453
632
0.0483
10
15
5.0
492
547
590
750
800
236
245
1139
0.0642
53
45.34
0.13
350
404
449
685
750
266
275
1083
0.0525
-40
22
0.29
332
397
449
650
705
363
414
705
0.046
-25
449
428
384
600
650
136
140
788
0.105
-100
Diphenyl
417
451
465
625
675
249
255
773
0.0569
-30
Paraffinic mineral oil Napthenic hydrocarbon Polydimethylsiloxane Polydimethylsiloxane
304
354
399
550
625
380
375
670
0.0531
25
317
388
464
600
650
345
385
690
0.0304
-45
239
265
279
750
800
320
380
725
0.0813
-76
87
0.46
329
298
242
500
550
116
N/A
662
0.1143
-168
163.4
0.09
Paraffinic hydrocarbon Diphenyl oxide/ biphenyl blend Di- a n d triaryl ether Modified terphenyl Diethyl benzene
3.8
7.47
0.07
174 37.74
0.14
2.9
4.0
4.72
3.686
' Manufacturer's recommended maximum temperatures.
TABLE 2—Comparison of selected secondary coolants. Coolant Aqueous Glycol Solutions Propylene Glycol: 30% Ethylene Glycol: 30% Propylene Glycol: 50% Ethylene Glycol: 50% Other Coolants Synthetic Hydrocarbon'' Silicone Oil'' Alkylated Benzene'' Water
Pumping Limit ("F)"
Temperature for 10 cP Viscosity'' (°F)
Corrosion Inhibitor Required"
Normal Boiling Point (°F)
5(FP) 6(FP) 141 -31
17 < 6 (FP) 51 20
Yes Yes Yes Yes
212 212 223 224
<-120 <-120 - 1 0 0 (FP) 32 (FP)
-52 -85 -100 32
No No No Yes
378 347 358 212
"Pumping (fluidity) limit is defined as the point where coolant is at the freezing point (FP) or 2000 cP viscosity [3]. ^Possible limit to efficient heat exchange, below which turbulent flow is not effectively obtained in economic heat exchangers. 'When in contact with metal equipment. ''Data shown tor typical commercial grades in coolant service.
velocity and tube diameter are equal and the ratio of the two heat transfer coefficients is dependent only on the ratio of the fluid densities, specific heat capacities, thermal conductivities, and absolute viscosities raised to the appropriate power [1]. From this equation and the above expressions for heat transfer coefficient, it is evident that h is a good summary parameter that incorporates all of the important fluid properties as they relate to their effect on heat transfer.
Pressure Drop An i m p o r t a n t p a r a m e t e r used by process designers in the analysis of heat transfer systems is pressure drop (/IP). The pressure drop of a system is related to the energy required to move the heat transfer fluid through a stem at a given pressure and it is related to the physical properties of the fluid.
This is illustrated with the following equation for pressure drop in r o u n d pipe that is expressed in lbs/100 ft of pipe: AP =
(62.3292)(p)(V)^
0.0001906 +
0.01171 (927.4777)(fi,)(p)(V)\o-381
where: di = I.D. of pipe in inches p = Fluid density in lb/gal K = Thermal conductivity in BTU/hr-ft2 °F/ft fi = Viscosity in cPs V = Fluid velocity in feet per second Cp = Specific heat in BTU/lb °F NOTE: p, K, u, and Cp at average fluid temperature
(2)
CHAPTER Many manufacturers supply computer programs that are m o r e easily used to perform such calculations. These programs are available upon request, or many fluid and hardware suppliers make such softweire available on the Internet. Selected examples of such p r o g r a m s that are currently available at a web site are provided in Table 4. M i n e r a l Oil F l u i d C o m p o s i t i o n Crude petroleum oil is a complex mixture of petroleum hydrocarbons as well as nitrogen, oxygen, and s u l f u r containing compounds, which are separated into fractions by refining. Solid components include petroleum coke, asphalt bitumen, and inorganics. Liquids of increasing viscosity vary from gasoline, kerosene (paraffin oil), diesel oil, engine crankcase oil, light a n d heavy m a c h i n e oil, a n d cylinder oil. Also included are methane, ethane, propane and butane, the major components of natural gas, which are gases at room temperature [6]. Aromatic aspheilt com-
21: MINERAL
OIL HEAT TRANSFER
FLUIDS
575
pounds are classified as resins, asphaltines, or carbenes, depending on their solubility [6]: • Resins—pentane or heptane soluble • Asphaltenes—pentane or heptane soluble • Carbenes—^benzene insoluble, carbon disulfide soluble Crude oils eire classified as paraffinic, naphthenic, intermediate (mixed), or asphaltic as shown in Table 5. Paraffinic fractions are saturated linear or b r a n c h e d alkanes. Naphthenic fractions contain cyclic alkane fractions. Also included are aromatic (pol5ainsaturated cyclic) chemical structures derived from benzene. Aromatic components exhibit a greater tendency to form sludge and varnish than paraffinic or naphthenic derivatives [7]. Generic examples of typiceil structures are illustrated in Fig. 1 [8,9] In addition to paraffinic, n a p h t h e n i c , a n d aromatic derivatives, petroleum oils also contain heterocyclic (cyclic derivatives of nitrogen, sulfur and oxygen), and other polar c o m p o u n d s such as carboxylic acids, usually saturated aliphatic or cycloaliphatic (naphthenic) acids, and aldehy-
TABLE 3—ISO 6743 classification of fluids for lieat transfer. More Specific Application
Code Letter
General Application
Particular Application"
Q
Heat Transfer
Maximum Measured Temperature <250C
Open circuit
Maximum Measured Temperature <300C
Product Type and Performance Requirements
Symbol ISO-L
Typical Application
Refined mineral oil or synthetic fluid mechanical or with stability To oxidation
QA
Open oil containers For heating of electrical components
Closed circuit with/or without forced circulation
Refined mineral oil or synthetic fluid with thermal stability
OB
-Heat transfer fluid heating system -Closed-circuit water bath
Maximum Measured Temperature >300 C and <320C
Closed circuit with forced circulation
Refined mineral oil or synthetic fluid with thermal stability
QC
Maximum Measured Temperature <320C Maximum Measured Temperature >-30Cand <200C
Closed circuit with forced circulation
Synthetic fluid with particularly high thermal stability Refined mineral oil or synthetic fluid with low viscosity at low temperatures and with thermal stability
QD
Heat transfer fluid heating systems
QE
Unit with hot flow and/or cold flow
Cooling circuit
Remarks
The fire risk for the particular application shall be a consideration. including the system, operating environment and the fluid itself 1) Units with heat transfer fluid heating systems should be fitted with efficient expansion tank, vent and filtrationsystems. 2) For units with heat Exchangers for heating of foodstuffs the heattransfer fluid shall comply with applicable national hygiene and safety regulations.
"Temperatures indicated in the column are of the bulk of the oil, measured in the discharge line from the heater. They are not the temperatures of the film of oil contact with the heater, which may reach higher values.
TABLE 4—Sources and web site addresses for computer programs to calculate heat transfer coefficients. Company
Program Name
Web Site Address
Dow Chemical Company Paratherm Solutia Inc. Engineering Process Tools K & K Thermal Connection
FLUIDFILE™ PARACALC™
http://www.dow.com/heattrans/index.html http;//www.paratherm.com http://www.therminol.com http://www.processassociates.com/process/tools.htm http://www.tak2000.com
576
MANUAL 37: FUELS AND LUBRICANTS
Crude Type
Solvent Neutral Base Oil
Pciraffinic base Naphthenic base Mixed base Asphaltic base
Yes No No No
HANDBOOK
TABLE 5—Crude oil content and suitability. Base Specialty Wax Oil Oil Content Yes Yes Yes Yes
No Yes No No
Yes No Yes No
S&N Content"
Asphalt
Low Low Low High
No No Yes Yes
API Gravity'' >40 <33 33-40
"S is sulfur and N is nitrogen. ''API gravity is scale in degrees adopted by the American Petroleum Institute where: "API = [141.5/specific gravity] — 131.5.
NmmaiclatnTg
Viscosity Index
n-Panffins
Very high
iso-Paraffins
m^
Low
Moderate
Low
Low
Low
Naphthenes
Anunatics
-^
T^Y^
^ I
Pour Point High
FIG. 1—Typical organic structures present in mineral oil.
Dibenzothiophene
1,7-Phenandirone
Furan
Pynole
Thitqihene FIG. 2—Examples of heterocyclic derivatives typically present in a petroleum oil.
des. Selected examples of polar derivatives that may be found in petroleum oils are illustrated in Fig. 2 [6-8]. Traces of phenolic and furan derivatives may also be present. A summary of the overall effect of these different derivatives on physical and chemical properties is provided in Table 6 [7]. Yoshida has shown that the pro-oxidant effect of nitrogen heterocyclic compounds is a function of the basicity of the nitrogen in the compound. Generally, only the more basic nitrogen heterocyclic compounds, pyridine and quinoline
derivatives, exhibit a pro-oxidant effect; this effect is accelerated in the presence of copper [10]. ParafHnic base oils contain 45-60% paraffinic compounds. Naphthenic base oils contain 65-75% of naphthenic compounds and aromatic base oils contain 20-25% aromatic compounds [8]. Table 7 provides a summary of the effect of paraffinic, naphthenic, and aromatic content of base oils on fluid density [6]. Base oil composition affects physical and chemical properties of the oil. Base oil properties may be estimated from aromaticity (aromatic carbon content), which is primarily dependent on the degree of refining [11,12]. The degree of aromatic compound content of a base oil may be determined from the aniline point and is determined by ASTM D 611. The aniline point is determined by mixing specified volumes of the base oil, aniline, and heptane and then the mixture is heated at a controlled rate until it becomes miscible. The mixture is then cooled at a controlled rate and the temperature where two phases separate is recorded as the aniline point. Aromatic hydrocarbons exhibit the lowest value and paraffinic hydrocarbons the highest value. Cycloparaffins and olefins exhibit intermediate aniline points. Another parameter that is reflective of base-oil quality is sulfur content since sulfur-containing polar compounds may exhibit significant antioxidant activity. Therefore, a determination of both the aniline point and sulfur content are necessary to determine base oil type, which is reflective of its resistance to oxidation. API has developed a classification system for petroleum oil base, which can be used as an aid in selecting basestocks that exhibit improved oxidative stability. These classifications are [27]: • Group I—Base oils with a viscosity index of 80-120, and containing less than 90% saturated hydrocarbons and/or more than 0.03% elemental sulfur. • Group II—Base oils with a viscosity index of 80-120, and containing > 90% saturated hydrocarbons and/or ^ 0.03% elemental sulfur. • Group III—Base oils with a viscosity index of s 120, and containing a 90% saturated hydrocarbons and/or £ 0.03% sulfur. • Group IV—^All polyalphaolefins • Group V—All other base stocks not included in Groups I, II, III, or IV. A reduction in the overall composition of aromatic derivatives and polar compounds of a petroleum base stock may be accomplished by high-temperature catalytic hydrogenation. This will produce a base stock with improved color and oxidative stability [13,14],
CHAPTER 21: MINERAL OIL HEAT TRANSFER FLUIDS There are various methods for the determination of the composition of petroleum oil derived heat transfer fluids. These methodologies, many of which have not been developed into ASTM or other national standards, have been reviewed previously. One such review is provided in Ref. 28. ASTM methods for petroleum oil compositional analyses are classified by Drews into: correlative, chromatography, a n d spectrometric methods [21]. Correlative methods are those that are derived by statistical analysis of a measured composition-dependent property. It is important to note that correlative carbon-type data are not directly comparable to data obtained by molecular structural analysis. However, correlative analysis data are often an effective means of monitoring the effect of a process change, such as oxidative stability, with variations in the composition of the oil. Examples of ASTM correlative methods are provided in Table 8. Test Method D 2140 utilizes a correlation between
577
the viscosity-gravity constant (VGC) and refractivity to determine oil composition in terms of aromatic carbon (%CA), naphthenic carbon (%CN), and paraffinic carbon (%CP). The VGC parameter is derived from kinematic viscosity and density and is dependent on the saturated/aromatic hydrocarbon ratio. The equation to calculate VGC from kinematic viscosity at 100°C (V') and density at 15°C (G) is: VGC-
G - 0.108 - 0.1255 Log ( V ' - 0.8) 0.90 - 0.097 Log (V - 0.8)
Aromatic (RA) and naphthenic (RN) ring distribution of the so-called "average molecule" is determined using ASTM Test Method D 3238. Naphthenic content of a petroleum oil can be determined by ASTM D 2159 using the refractivity intercept which is defined as: Refractivity Intercept = n - {d/2) where n is the
TABLE 6—Effect of composition on base stock properties. Property
n-Alltane
Napthene
Aromatic
Polar Compounds
Viscosity Index Pour point (high/low) Oxidative Stability
Very high High Good
High Low Good
iso-Alkane
Low Low Average
Low Low Average/Poor
Response to Antioxidants Volatillity (high = poor) (low = good)
Good
Good
Good
Some poor
Low Low S is antioxidant; N and 0 are pro-oxidant Poor
Good
Good/Average
Average
Poor
Poor
TABLE 7—Relationship between molecular carbon type and density. Base Oil Type
Paraffinic Carbon
Naphthenic Carbon
Aromatic Carbon
Density (kg/L)
Paraffinic Naphthenic Mixed Aromatic
65-70 50-60 48-57 21-35
25-30 30-40 24-33 20-30
3-8 8-13 17-22 40-50
0.800-0.812 0.834-0.844 0.850-0.872 0.943-1.005
ASTM No.
D 1319 D2007 D2140 D2425 D2501 D2502 D2549 D2786 D3238 D3239 D3524 D3525 D4291 D4808 D5186 D5291 D5292
TABLE 8--ASTM test methods for determining petroleum oil composition used for heat transfer fluid formulation. Standard Test Method for Separation Type
Hydrocarbon Types in Liquid Petroleum Products by Fluorescent Indicator Adsorption Characteristic Groups in Rubber Extender and Processing Oils and Other PetroleumDerived Oils by the Clay-Gel Absorption Chromatographic Method Carbon-Type Composition of Insulating Oils of Petroleum Origin Hydrocarbon Types in Middle Distillates by Mass Spectrometry Calculation of Viscosity-Gravity Constant (VGC) of Petroleum Oils Estimation of Molecular Weight (Relative Molecular Mass) of Petroleum Oils From Viscosity Measurements Separation of Representative Aromatics and Nonaromatics Fractions of High-Boiling Oils by Elution Chromatography Hydrocarbon Types Analysis of Gas-Oil Saturates Fractions by High Ionizing Voltage Mass Spectrometry Calculation of Carbon Distribution and Structural Group Analysis of Petroleum Oils by the n-d-M Method Aromatic Types Analysis of Gas-Oil Aromatic Fractions by High Ionizing Voltage Mass Spectrometry Diesel Fuel Diluent in Used Diesel Engine Oils by Gas Chromatography Gasoline Diluent in Used Gasoline Engine Oils by Gas Chromatography Trace Ethylene Glycol in Used Engine Oil Hydrogen Content of Light Distillates, Middle Distillates, Gas Oils, and Residua by Low-Resolution Nuclear Magnetic Resonance Spectroscopy Determination of Aromatic Content and Polynuclear Aromatic Content of Diesel Fuels and Aviation Turbine Fuels by Supercritical Fluid Chromatography Instrumental Determination of Carbon, Hydrogen, and Nitrogen in Petroleum Products and Lubricants Aromatic Carbon Contents of Hydrocarbon Oils by High Resolution Nuclear Magnetic Resonance Spectroscopy
Chromatography Chromatography Correlation Spectrometry Correlation Correlation Chromatography Spectrometry Chromatography Spectrometry Chromatography Chromatography Chromatography Spectrometry Chromatography Spectrometry Spectrometry
578
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
refractive index at 20°C and d is the density at 20°C. The naphthene content in percent by volume is determined from a plot of refractivity intercept and density as shown in Fig. 3. Polycyclic naphthenes, if present, will cause a high value for total naphthenes to be indicated. Although basic data for this correlation compensate, in part for the presence of dicyclic naphthenes, results may be high by as m u c h as 1% for each 1 volume % dicyclic naphthenes present. Thus, the n a p h t h e n e content indicated for stocks with end points greater than 163°C (325°F) should be considered "equivalent naphthenes." The chemical composition of a petroleum base oil may also be calculated from physical properties such as refractive index {ni°) at 20°C, density at 20°C (df), percentage sulfur (S) concentration, and average molecular weight (M) by ASTM D 3238, which describes the "n-d-M Method" (refractive index, density and molecular weight method). Chromatographic methods for determining the composition of p e t r o l e u m oil are s u m m a r i z e d in Table 8. These methods include gas chromatography and open-column separations on silica gel or clay-silica gel columns. Moleculartype analyses are conducted using open-column separations such as those outlined in ASTM D 2007. ASTM Test Method D 2007 may be used to classify petroleum oil samples with an initial boiling point of at least 260°C (500°F) into the hydrocarbon types of polar compounds, aromatics, and saturates and recovery of representative fractions of these types. ASTM D 2007 is conducted by diluting the sample with solvent and then charging it to a glass-percolating column containing clay in the upper section and silica gel plus clay in the lower section. n-Pentane is then charged to the double-colu m n until a definite quantity of eluent has been collected. The upper (clay) section is removed from the lower section and washed with n-pentane. A toluene-acetone mixture is t h e n charged to the clay section for desorption a n d a specified volume of eluent is collected. The lower (silical) gel column may be desorbed by recirculation of toluene. After solvent removal, the aromatics are collected by difference. This is a useful procedure for classifying petroleum oils into structural groups that may be related to their inherent ther-
n
1.047
—' ^^ - -^
mal/oxidative stability properties. This test method is not directly applicable to petroleum oils of greater than 0.1 mass % pentane insolubles. The hydrocarbon types and structural groups measured by this method include: • Asphaltenes, or n-pentane insolubles that are defined as insoluble matter that precipitates from a solution of oil in npentane under specified conditions. • Polar compounds that are material retained ion adsorbant clay after percolation of the same in n-pentane eluent under specified conditions. • Polar aromatics, which is a synonym for polar compounds. • Aromatics that are materials that, o n percolation, pass through a column of adsorbant clay in a pentane eluent but adsorb on silica gel u n d e r the conditions specified. • Saturates which are materials that, on percolation in an npentane eluent, is not adsorbed on either clay or silica gel under the conditions specified. In addition to ASTM D 5186, which utilizes supercriticcJ fluid chromatography (SFC), a n u m b e r of additional methods have been reported for use with petroleum oils including thin layer chromatography (TLC) and high pressure liquid chromatography (HPLC) [21,22-25]. There are various spectrometric methods for petroleum compositional analysis, which are summarized in Table 8. Non-ASTM methods for the use of infra-red (IR) spectroscopy for quantitative analysis of hydrocarbon t5^es have been reported. Fourier trzmsform (FT) IR methods have also been reported for the analysis of degradation by-products present in used oils [21,26]. Nuclear magnetic resonance (NMR) spectroscopy methods have been developed including ASTM D 4808 and D 5292. There are three spectrometric test methods for petroleum analysis based on mass spectroscopy (MS): ASTM D 2549, D 2786, and D 3239. ASTM D 2786 yields information on the relative amounts of paraffins, 1-through 6-ring naphthenes and alkyl naphthenes. ASTM D 3239 determines eighteen aromatic t3^es and three aromatic thiophene structures [21]. Both methods require the use of Test Method D 2549 for prior separation of the paraffinic a n d aromatic fractions. ASTM D 2425 may be applicable for petroleum oils in the lower molecular weight range [21]. M i n e r a l Oil F l u i d P r o p e r t i e s
\ ^ ~>-N
•
0.B2
It has been shown that heat transfer fluid performEuice is dependent on the physical properties of the fluid including: thermal conductivity, density, specific heat capacity, and viscosity. It has also been shown that the physical and chemical properties of a mineral oil are dependent on its composition a n d degree of refining. However, Table 1 showed that the physiced properties of heat transfer fluids of a given type, including mineral oil-derived fluids, varied with respect to each other. This is illustrated in Table 9 where the physical properties of two commercially available mineral oil heat transfer fluids are compared. One fluid is a hydrotreated pEiraffinic base and the other is a napthenic base. It is essential that the engineer obtain the actual data he requires for the fluid being considered. This data is available from the fluid supplier.
FIG. 3—Naphthene content by volume percent using the refractivity intercept.
A s u m m a r y of ASTM test procedures r e c o m m e n d e d in ASTM D 5372 for mineral oil-based heat treinsfer fluids is provided in Table 10.
0^
^^ .o\
o
-—•
IM
> N
e j»
i-
1
'^
—' ^ ^ ,\ ^ y "^ y >< y % > fP ^ \ \A "'•>,
ec
/
sMN
/'
iO<]
\ '•
/
*• rf)N
/
jPN
f
,^
1.033
0.64
/ > , / -" / / ' ^' /'
0.66
0.6B
0.70
OTZ 0.74 0cflsityaf2OC
0.76
07B
0.60
CHAPTER 21: MINERAL
OIL HEAT TRANSFER FLUIDS
TABLE 9—Comparison of the physical properties of two different commercially available heat transfer fluids. ASTM Test Procedure Fluid 1 Physical Property Napthenic Feedstock Base Viscosity (cSt) 300°F 400°F 500°F
Density (lb/gallon) 300°F 400°F 500°F Specific Heat (BTU/lb-°F) 300°F 400°F 500°F Thermal Conductivity (BTU/hr-ft^-°F 300°F 400°F 500°F Vapor Pressure, psia @ 300°F 400''F 500°F Coefficient of Thermal Expansion
Viscosity TABLE 10—Summaiy of ASTM D 5372 recommended test procedures for mineral oil heat transfer fluids. Recommended Test Mineral Oil Fluid Properties Flash Point, Closed Cup Pour Point Viscosity Specific Viscosity Water Content Safety Autoignition Temperature Flash Point, Open Cup Flash Point, Closed Cup Effect on Equipment Rubber and Elastomeric Seals Corrosion Efficiency Thermal Conductivity Specific Heat Service Life Thermal Stability Laboratory Tests" Precipitation Number Insolubles Flash Point, Open Cup Flash Point, Closed Cup Carbon Residue
" " Viscosity Distillation
" "
Neutralization Number Water Content
ASTM Test Procedure D93 D97 D445 D1298 D95 E659 D92 D93 D471 G4 D2717 D2766
D91 D893 D92 D93 D189 D524 D4530 D445 D86 D 1160 D2887 D664 D95
"At present no suitable method has been approved for heat transfer fluids. Test method D 2160 is of limited value due to limitation in testing small noncirculatory samples in glass containers.
Fluid 2 Paraffinic
1.96 1.16 0.74
2.79 1.52 0.98
6.82 6.64 6.45
6.51 6.22 5.92
0.56 0.61 0.66
0.56 0.61 0.66
0.072 0.069 0.067
0.071 0.069 0.066
0.0003 0.1470 0.9670 0.000304/°F
579
0.00039 0.00970 0.13500 0.000592/°F
(ASTM D 445)
Viscosity is important in determining Reynolds and Prsindtl numbers for heat transfer systems, to estimate fluid turbulence, h e a t transfer coefficients, and heat flow. Viscosity measurement by various tests, including ASTM D 445, is discussed in detail in Chapter 32, Flow Properties and Shear Stability, and will not be discussed here.
Pumpability Pumpability refers to the ease of pumping a fluid in a heat transfer system. According to ASTM D 5372, if the fluid viscosity is greater than 200 cS, it is difficult to p u m p . If a heat transfer fluid is subjected to temperatures below its minim u m p u m p i n g t e m p e r a t u r e w h e n not in use, the system should be heat traced to w a r m the fluid start-up. The p u m p and system design will determine the viscosity limit required for pumping, and therefore the minimum pumping temperature. The construction of a viscosity/temperature curve using measured viscosities in the temperature range of interest can be used to estimate m i n i m u m pumping temperature. It should be noted that the "pumpability" concept of ASTM D 5372 is not the same concept as "pumping limit" used by Green and Morris [3] and cited in Table 2. The Green and Morris concept refers to those conditions defined by freezing point of the fluid or a viscosity of 2000 cP as conditions where a fluid is n o longer p u m p a b l e . In the ASTM approach, "pumpability" refers to that fluid viscosity (200 cS) above which the fluid would be difficult to pump—which is a consensus value.
Thermal
Stability
Thermeil stability (cracking) is defined as the resistance of a heat transfer fluid to permanent physical changes caused by exposure to heat (themiEil cracking). Depending on the composition of the oil, these changes can become noticeable at
580
MANUAL
37: FUELS
AND LUBRICANTS
HANDBOOK
temperatures as low as 250°C while rapid changes in composition (thermal cracking or pjrolysis) begin around 350°C. These changes include evolution of solid carbon particles (coke) and lower boiling point components (light ends). If there are substantial a m o u n t s of asphaltenes and polar residues in the fluid, the coke may adhere to the heater surfaces, causing fouling, which in t u r n reduces heat transfer rates. The lower boiling point components will cause the overall system pressure to increase and m a y result in vaporous cavitation at the p u m p section. These components will also tend to depress the Flash Point of the fluid. Thermal cracking is often accompanied by color change [19] however, color change is more c o m m o n when oxidation is present. While higher viscosity oils are generally more susceptible to cracking than lower viscosity, the main cause is due to overheating. Overheating typically results from flame impingement and/or low velocity through the tubes of fired heaters. In forced circulation electric immersion heating systems, satisfactory fluid life has been obtained with heat flux of 15-22 watts per square in., and in free convection systems (no circulating pumps), 5-12 watts per square in. Thermal Under
Stability (New ASTM Development)
Test
Method
Although it is true that there are few tests that directly assess the thermal stability of a heat transfer fluid, there is one test currently being balloted within ASTM, Standard Test Method for Thermal Stability of Organic Heat Transfer Fluids [ 1 ]. This test is conducted by adding the heat transfer fluid to be analyzed into a test cell that is purged with nitrogen and tightly sealed to remove and preclude the introduction of oxygen and water from the atmosphere. The fluid is then heated in the test cell for a prescribed time and temperature. The boiling range of the fluid after themicJ stressing is then determined by gas chromatography (using Test Method D 2887) and compared to the unused fluid. This portion of the thermally stressed fluid that has a boiling point outside of the boiling range of new, unused fluid is defined as "degradation." Guffey has introduced the concept of "overall degradation rate" [1], where components exhibiting boiling points above or below the original boiling point range are assumed to be degradation products. To determine the overall degradation rate, it is assumed that 99% of the fluid volume is equally distributed at the heater-outlet, heater-inlet, and the heater-inlet temperature minus a selected amount such as 50°F (which accounts for the fluid in the system expansion tank). The remaining 1% is assumed to be at the fluid-film temperature. Degradation rates are then taken from the test at various temperatures. This data is then used to calculate the "overall degradation rate" [1]. Oxidative
increase in the amount of air contact (foaming air entrainment, fluid turbulence) will also affect the rate of oxidation. Oxidative stability is a function of the base stock selected. For example, paraffinic oils exhibit better oxidative stability than naphthenic oils, but they exhibit poorer "solvency"— poorer ability to dissolve byproducts [17]. The presence of alkylaromatic derivatives a n d some polar components further increases the susceptibility of a petroleum oil to oxidation, and other derivatives such as some sulfur-containing compounds may exhibit a stabilizing effect [18]. Carbon Residue
(ASTM D 189, D 524, and D
4530)
"Carbon residue" is defined as the residue formed by evaporation and thermal degradation of a carbon containing material, which provides an indication of the fluid's tendency to form carbonaceous deposits. One test that is used to determine carbonaceous residues is Test Method D 189 the Conradson Carbon Residue. In this test, a sample is added to a crucible and subjected to destructive distillation. The residue formed undergoes cracking and coking during the fixed period of severe heating. At the conclusion of the test, the crucible is cooled and weighed. The Conradson Carbon Residue is the % of residue remaining at the conclusion of the test. This test provides a rough approximation of the tendency of the heat transfer fluid to form deposits when heated and the formation of volatile byproducts is possible. Another test that is used to determine the tendency for a heat transfer fluid to form carbonaceous residues is Test Method D 524, the Ramsbottom Carbon Residue test. In this test, the sample is weighed into a special glass bulb and placed in a metal furnace where it is heated quickly to 550°C. All volatile material is evaporated out of the bulb, with or without decomposition, while the remaining residue undergoes cracking and coking reactions. After the test, the bulb is cooled and weighed. The residue remaining is reported as % of original sample and is called the R a m s b o t t o m Carbon Residue. Test Method D 4530 is a "Micro Method" for determination of carbon residue. In this test, a weighed sample of the fluid is heated in a 2 mL glass vial at 500°C under an inert atmosphere (nitrogen) in a controlled manner for a specific time. The sample undergoes coking reactions and the volatiles that are formed are swept away by the nitrogen. The carbonaceous residue formed is reported as % of original sample as "carbon residue (micro)." This test method is reported to be equivalent to the Conradson Carbon Residue (ASTM D 189). Specific
Gravity
(ASTM D
1298)
Hydraulic shock during p u m p i n g has been predicted via the use of a combination of density and compressibility data.
Stability
Oxidation occurs when the hot fluid comes into contact with air (oxygen). Oxidation rates double every 10°C (18°F) increase temperature. The principal oxidation byproducts are organic acids which can undergo further reaction to form extremely high molecular weight components that are soluble in the fluid and increase the viscosity of the fluid [15]. Above 200°C these components will precipitate out on hot surfaces and become insoluble. Insoluble byproducts will lead to fouling of heater surfaces, filter plugging, and sticking valves. Any
Metal
Content
Transition metals (0.1-50 ppm), capable of a one electron transfer process, will act as a catalyst for hydrocarbon oxidation. Homolytic peroxide decomposition occurs by:
RCOOH + M"+ -> RO • + M<"+"+ + H O " RCOOH + M<"+'>+ ^ RCOO • + M"+ + H^ 2 RCOOH M"^/M<-^"^ ^ j^QQ ,+ R o • + H2O
CHAPTER These free radical processes, a n d others not shown, are strongly affected by temperature. This is illustrated in Table 11 where it is shown that if both water and the metal are present, the oil degradation rate was 10-fold (iron) to 30 fold (copper) that of the uncontaminated oil [16]. There are numerous test procedures that are discussed in detail in Chapter 26, Elemental Analysis, cind therefore will not be discussed further here. Thermal Properties—Thermal Conductivity 2717) and Specific Heat (ASTM D 2766)
(ASTM
D
Thermal conductivity a n d specific heat tests are difficult to carry out, facilities for performing them are few, Eind the precision data is yet to be established. Since thermal conductivity and heat capacity are temperature dependent, to be of use to the designer, they m u s t be determined at different temperatures. Values can be estimated for design use from the general chemical composition. The values for thermjd conductivity and specific heat may be available from the fluid supplier. Autoignition
(ASTM E
659)
It is important to note that while heat transfer fluids are routinely used above their flash a n d fire points, they should never b e used above their autoignition t e m p e r a t u r e . Autoignition is the temperature at which the fluid will ignite spontaneously in contact with air. It is recommended that the fluid supplier be consulted for standard recommended safe practices. The autoignition temperature of a heat transfer fluid is determined according to ASTM D 659. Effect on Rubber
or Elastomeric
21: MINERAL
OIL HEAT
TRANSFER
FLUIDS
581
by the fluid supplier or contract laboratory. The following are some simple tests that should be performed in the plant weekly or biweekly. Listen to the
System
Minered oil heat transfer fluids exhibit various characteristic sounds that may indicate potential problems. For example: snapping, crackling, Eind popping noises indicate that water may have entered the system. Loud noises in the circulation p u m p may indicate potential cavitation, which is a significant problem and should be dealt with immediately. Sources of excessive vibration of any p a r t of the system should be identified and eliminated. Visual System
Inspection
When the system is operating, smoke ciround piping indicates that fluid is oozing through gaskets or valve or p u m p seals. Eventually, a black carbon crust will form at the leak. Unless there is an external source of ignition, these leaks are not normally a safety hazard, but more of a housekeeping issue and should be corrected as soon as possible. During each inspection, examine the expansion tank, vent and overflow catchcontainer. Steam coming from the vent may indicate water in the system. The presence of a n oil mist may indicate that the heat transfer fluid is oxidizing. Both situations require immediate attention. The catch-container should be closely inspected. If there is heat trzinsfer fluid in the container, the cause must be identified. Whether the fluid in the container is water, oil, solvent or another liquid, it is a safety hazard and must be removed. Always keep the catch container clean and dry.
Seals (ASTM D 471)
Heat exchange equipment typically utilizes mechanical seals fabricated from steel or other metal. If elastomeric seals are present, it is desirable that they exhibit rubber swelling in the range of 1-5% to prevent leakage because of poor seal contact. Seals may degrade in some fluids. As an oil deteriorates in service, additiontJ tests may be required to assure that seeds remain compatible with the altered oil. The temperature ranges of the tests should correspond to temperatures to which seals will be exposed in service and the tests should be conducted for at least 1000 h. Fluid Maintenance—In-plant Tests Well-maintained, unabused mineral oil heat transfer systems may be smooth running and trouble-free over many yccirs of use. To assure long-term and trouble-free operation, it is recommended that periodic maintenance be performed. Some of these tests are simple and may be performed by the user. Other tests may need to be performed
TABLE 11—Effect of water and catalysts on oil oxidation. Catalyst
Water
Hours
Final Acid Number
None None Iron Iron Copper Copper
No Yes No Yes No Yes
3500 3500 3500 400 3000 100
0.17 0.9 0.65 8.1 0.89 11.2
Unusual
Odors
A burning oil odor is usually indicative of a leak. Identify the source of the leak and repair, if necessary. Fluid
Appearance
With the heater off, start the circulating pump(s) and wait ten or fifteen minutes. Open a low-point valve, edlowing a small amount of fluid to drain into a clear glass container. Put the container on a bench and let it sit. Color may range from light yellow to dark brown or black. Darker colors do not necessarily indicate that the fluid has become unusable. A hazy fluid suggests water contamination or oxidation. If there are particles on the b o t t o m (or floating in the liquid), fluid "coking" may be indicated or mill scale or other hard contaminants may be present. Either condition should be investigated before problems escalate. Fluid Sampling Recommended locations from which to remove a fluid sample include a low-point drain near the p u m p or heater or the blow-down valve mounted on the Y-strainer. The Y-strainer is usually located in the return line just upstream of the p u m p suction. Fluid samples should b e taken when the p u m p has been running so that the sample is representative of the fluid in the system. Ideally, the fluid should be removed from the system between 180 and 280°F and put directly into the sample container. If the fluid sampled is too hot, it will not only smoke, but the acid n u m b e r may indicate artificially high
582
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
acid levels due to oxidation during the sampling process. The sample container must be sealed tightly as quickly as possible after filling to minimize contact with air (oxygen). Samples taJcen from the expansion tank, or from a "dead leg," are not representative of fluid in the system. The fluid supplier should supply a suitable sample container. Contract laboratories typically require approximately one quart (750 mL) of sample to properly conduct the analysis.
. . fion o
Fluid Maintenance—Laboratory Tests
m
(1)
550
High Boilers-
3
•I-*
2
500
E
450
a) CL
10)-
c 400 o m
4-*
350
n
Laboratory analysis of the fluid should be performed at set intervals to ensure that abnormal degradation is not occurring. Abnormal degradation can result from contamination that catalyzes the breakdown, oxidation, or overheating. The condition of the fluid can be accurately established by three basic tests, which determine the amount of thermal cracking and/or oxidation that has occurred. Additional tests may be required to identify specific contaminants such as water and solids. Acid Number (ASTM D 664 and D 974) Acid number (AN) measures the acidity produced when the fluid is oxidized. Unless the expansion tank is maintained under inert gas blanket, some increases in AN are unavoidable over time. The higher the operating temperature, the more critical the AN becomes since the ensuing fluid degradation is temperature dependant. In general an increase of 0.5 should be viewed as a warning sign, with immediate fluid replacement at an increase of 1.0. The increase in AN with increasing time is illustrated in Fig. 4 [19]. Test Method D 664 is conducted by dissolving a sample of the HTF to be analyzed into a solution of toluene and isopropanol containing a small amount of water and titrated potentiometrically with alcoholic potassium hydroxide and a saturated calomel reference electrode. The meter readings are plotted manually or automatically against the respective volumes of KOH titrant and endpoints are taken only at welldefined inflections of the resulting curve. This titration may be performed colorimetrically according to Test Method D 974. Alternatively, infrared analyses may be used, which provides a more reliable method of detecting oxidation.
1.4 #—«n
5> 1.2 X
•
Oxidized Fluid
§1.0 o>
•
/
-
e 3 0.6
•D B 0.4 -
• •
|0 0.2 0
...U ....
1. n
1
Increasing Fluid On-Stream Time —>• FIG. 4—Illustration of the increasing solids content present in a mineral oil based heat transfer fluid with increasing time after the onset of degradation.
•o 300 a>
_ Low
Boilers
<
Fresh Fluid
- • - Thennally Degraded Fluid
JO 250 3
E
CO 200
10 20 30 40 50 60 70
80
_i_
90 100
Amount Distilled (%) FIG. 5—Gas chromatographic distillation (GCD) comparison of a new and used mineral oil based heat transfer fluid.
Distillation (ASTM D 1160, and D 2887) The distillation test compares the boiling temperatures of standard fractions of new fluid with those for a used fluid. This is illustrated graphically in Fig. 5 [19]. When a heat transfer fluid undergoes thermal or oxidative degradation (overheated) it generates lower boiling and/or higher boiling components, which in turn causes a change in the boiling temperatures of the various fractions. The percentage shift in temperatures versus new fluid collectively determines how much degradation had occurred. Typically, a shift of 10% indicates that the fluid has been severely damaged. In Test Method D 1160, 100 ml of fluid sample is distilled in a single stage equivalent apparatus under vacuum. The temperature readings and volumes of condensate are recorded along with the volume of residue and any vapor losses. The data is converted to atmospheric equivalent temperatures in a table. Test Method D 2887 is a gas chromatographic procedure that simulates distillation. It is conducted by injecting a quantity of the fluid sample on to a nonpolar packed or open (capillary) gas chromatographic column. The hydrocarbon components are eluted in order of increasing boiling point. The column temperature is raised at a reproducible linear rate and the area under the chromatogram is recorded throughout the analysis. Boiling points are assigned from a calibration curve obtained under the same conditions of a known mixture of hydrocarbons covering the boiling range expected for the sample. Viscosity (ASTM D 445) While viscosity is not the most critical test for fluid maintenance, it does provide useful information and also confirmation of other test results. Viscosity is of import in determination of Reynolds and Prandtl numbers for heat transfer systems, to estimate fluid turbulence, heat transfer coefficients and heat flow. In general, viscosity increases as the result of oxidation and decreases due to thermal cracking. Changes in viscosity may also be the result of contamination by a different fluid. Fluid viscosity is compared with that of new fluid in Table 12. Viscosity measurement by various
CHAPTER 21: MINERAL OIL HEAT TRANSFER FLUIDS TABLE 12—Comparison of a new and used mineral oil fluid. Physical Property Flash Point, Open Cup (°F) Viscosity (cS) AN (mg KOH/g) Solids Content, (%) Iron (ppm) GCD Low Boilers, (%)
ASTM Procedure
Fresh Fluid
D92
450
D445 D664
25.0 0.05 0.0 0.0
D2887
Used Fluid 392 94.5 2.12 7.6 700 4.7
TABLE 13—Correlation of base stock aromatic content and color generation during oxidation. Fluid
N (ppm)
S (ppm)
Wt. % aromatics
ASTM Color After Heating in Air
A B C
27 1 <0.2
6200
26" 4 1
>8 5.5 2.5
92 1
tests, including Test Method D 445, is discussed in detail in Chapter 32, Flow Properties and Shear Stability and will not be discussed further here. Open Cup and Closed Cup Flash Points (ASTM D 92 and D 93) Because the majority of systems operate safely above the Flash Point of the fluid, periodic determination is usually unnecessary. A reduction in flash point does indicate the presence of low boiling components, which result from thermal cracking. A rapid decrease in flash point may indicate that severe overheating has occurred. The presence of volatile components can be determined by ASTM D 92, Cleveland Open Cup Flash Point and/or ASTM D 93 Pensky-Martens Closed Cup Flash Point Test. As determined by these methods, the flash point is the lowest temperature, corrected to barometric pressure of 101.3kPa (760 mm Hg), where the application of an ignition source causes the vapors of the fluid being tested to ignite, either in an open cup (D 92) or closed cup (D 93). These test procedures are described in detail in Chapter 25, Volatility. Water Content (ASTM D 95 and D 1744) Heat trsinsfer systems operating at temperatures of 120°C or greater must, for reasons of safety, be dry, because destructive high pressures are generated when water enters the high temperature sections of the system. If water is not removed, the vapor will cause pump cavitation and possible fluid discharge through the expansion tank vent Test Method D 95 is a classic distillation procedure for water content. It is conducted by heating the fluid under reflux with a water immiscible solvent that codistills with water in the fluid. Condensed water and solvent are continuously separated in a trap, the water settles in the graduated section of the trap. The amount of water is reported as % by volume of the original sample. However, distillation is only suitable for relative high levels of water content. In most well-maintained heat transfer systems, the actual water content is lower than that necessary for distillation. A more suitable procedure for determination of water content is Test Method D 1744, a Karl Fischer method where the sample is titrated to an electrometric end point [19].
583
Safety Mineral oil heat transfer fluids are hydrocarbons, and therefore are combustible with the associated safety risks. However, mineral oil based heat transfer systems have been in safe operation for over 100 years. System Leakage Unlike hydraulic systems, mineral oil heat transfer systems are not generally pressurized. The leaks that do occur are found mostly in threaded fittings, joints, valves, and pumps— the fluid will slowly weep rather than gush or spray. Leaking heat transfer fluid will tjrpically smoke rather than bum—even at temperatures in excess of their flash and fire points. However, hot mineral oil fluid vapors can also be highly flammable if allowed to accumulate in a poorly ventilated area. Insulation Fires All organic heat transfer fluids are capable of spontaneous combustion when leaks occur into porous insulation (such as fiberglass or calcium silicate). The exact mechanism of the autoignition has not been established. One possibility is that partial oxidative decomposition occurs in the insulation (similar to the way heat is generated in a pile of oily rags or wood chips). The heat produced adds to the system heat and ultimately exceeds the autoignition temperature of the fluid. An alternate mechanism is that the autoignition temperature of the fluid decreases as it degrades in the insulation. In either case, if air enters the insulation at this point and contacts the degraded fluid, spontaneous combustion will occur. Automatic Shutdown Controls Although rare, fires have occurred in heaters when fluid circulation is interrupted (due to pump failure or blockage) and the high temperature shutdown fails. In this situation, the fluid temperature rapidly increases above the autoignition temperature while the increasing thermal stress eventually causes either the heater tube or housing to fail. All systems should incorporate automatic shutdown controls for high fluid outlet temperature and loss or reduction of fluid flow through heater. Design and Construction Like any other industrial system, the proper design and installation of heat transfer fluid systems is critical to their smooth functioning and extended operating life. In this section, an overview of recommended practices to minimize the potential of system fires will be provided. Component Selection In designing and constructing a thermal oil system, attention must be paid to the selection of appropriate components. If care is not taken, poor operation, system failure, and fires can result. In this section, an overview of various system design considerations for mineral oil heat transfer fluids will be provided. Piping Schedule 40 seamless carbon steel piping is recommended. Schedule 80 piping is recommended for threaded installations up to 1 in. Threaded fittings are not recommended for
584
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
piping greater than 2 in. Pipe should be free of mill scale, wedding flux, quench oils, and lacquers. The use of copper should be minimized. Flanges and
Fittings
Flanges and fittings must be 300 lb forged steel, Yu in. raised face. Studs and nuts should be continuous threaded, alloy steel with heavy hex nuts. Valves should be 300 lb cast or forged steel, or nodular (ductile) iron with steel or stainless steel trim. For o p t i m u m service, bellows may be considered. The use of cast iron in thermal oil systems is not recommended. Suggested gasket and packing materials include: • Flange gaskets—Spiral wound graphite filled or filled PTFE (maximum temperature 450°F) • Valve stem packing—Rings of die formed graphite foil • Pump packing—End (nonextrusion) rings of braided carbon yeim • Mechanical Seals—Carbon versus silicon or tungsten carbide for continuous service. Silicon Ccirbide versus tungsten carbide for intermittent service. Elastomers Fluoroelastomer r e c o m m e n d e d to 450°F, perfluoroelastomers recommended to 600°F. Compatibility of other elastomers should be evaluated using tests such as ASTM D 471. Insulation Heat loss should not exceed 80 btu/ft at operating temperature. Nonporous (closed cell foam glass type) insulation is recommended. Porous insulation(such as glass fiber and calcium silicate) can be used on straight piping runs where leakage is unlikely. In such installations, nonporous insulation should be installed around leak prone areas such as valve and instrument taps and extended 18 in. m i n i m u m on either side. Weep holes should be drilled in the b o t t o m of insulation around veJves. If possible, flanges should be left uninsulated. Metal covers with weep holes can be installed for personnel protection. Pumps P u m p s should be cast carbon steel. Positive displacement p u m p s should be of the "gear within a gear" design. Canned or magnetic drive p u m p s are typically not required since fugitive emissions regulations do not apply for mineral oils. It is recommended that the alignment be rechecked after the system is operating at temperature. Flex hose should be installed on the inlet and outlet. Pressure
Gauges
Y-strainers One-quarter to Ys in. opening is recommended. A 60 mesh element should be instcJled for start-up only. Flow
Protection
Many systems utilize a pressure differential switch to provide a method of shutting the system down when fluid flows drop below set limits. Some systems are equipped with flowmeters in addition to the pressure differential switches. However, since flowmeters can fail in the open position, they are typically not recommended for use as a flow switch. Installation During installation or construction, four areas should be addressed: • system cleanliness • component orientation • system tightness • allowance for thermal expansion and contraction System
Cleanliness
Care must be taken to assure that the system is clean and dry. Both the "hard" and "soft" contamination must be removed as the system is being assembled. Hard contamination such as mill scale, weld spatter/slag, and dirt can cause premature failure of p u m p and vzJve seals. A startup strainer should be monitored continuously during initial system circulation. Soft contamination such as quench oil, welding flux, and protective lacquer coatings can potentially dissolve in the fluid. Although this would be expected to exhibit only a minimal effect in most cases, such contaminants may be carried through the heater where they may degrade at much lower temperatures than the fluid itself, and can cause fouling of heater surfaces. Component
Orientation
Expzinsion tanks should be located far enough away and piped so that the temperature is n o greater than 1 SOT for vented systems. Design the expansion tank for twice the expansion volu m e of the system where the tcink is % full cold and M full when the system is at operating temperature. Valves should be m o u n t e d system sideward so that leakage from the stem or from bonnet gasketing drips away from the piping. System
Tightness
The system should be chcirged with an inert gas once construction is completed. Not only will corrosion be prevented, but the system can be pressure-tested using simple soap-bubble detection methods at potential leak points.
Pressure gauges should be rated to 100 psi, 650°F (343°C) at a temperature range of 300-600°F; Thermometers should be calibrated according to ASTM E l to provide accurate readings in this range [6].
ASTM STANDARDS
Expansion
No. D 91
Joints
For expansion joints, it is recommended that the HTF system be designed for an expansion growth of 4 in. per 100 ft, mini m u m . Both loops and joint expansion devices are acceptable. Either m u s t be high-temperature rated and must be considered part of the piping system.
D 92 D 93
Title Test Method for Precipitation Number of Lubricating Oils Test Method for Flash and Fire Points by Cleveland Open Cup Test Method for Flash Point by Pensky-Martens Closed Cup Tester
CHAPTER D 97 D 130
D 189 D 445 D 471 D 524 D 611
D 664 D 877 D 893 D 1160 D 1169 D 1298
D 1319 D 1500 D 1744 D 1747 D 2007
D 2140 D 2425 D 2501 D 2502
D 2549
D 2717 D 2766 D 2786
D 2887 D 3238
D 3239
Test Method for Pour Point of Petroleum Products Test Method for Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test Test Method for Conradson Carbon Residue of Petroleum Products Kinematic Viscosity of T r a n s p a r e n t a n d Opaque Liquids (The Calculation of Dynamic Viscosity) Test Method for Rubber Property-Effect of Liquids Test Method for Ramsbottom Carbon Residue of Petroleum Products Test Method for Aniline Point and Mixed Aniline Point of Petroleum Products a n d H y d r o c a r b o n Solvents Test Method for Acid N u m b e r of Petroleum Products by Potentiometric Titration Test Method for Dielectric Breakdown Voltage of Insulating Liquids Using Disk Electrodes Test Method for Insolubles in Lubricating Oils Test Method for Distillation of Petroleum Products at Reduced Pressure Test Method for Specific Resistance (Resistivity) of Electrical Insulating Liquids Test Method for Density, Relative Density (Specific Gravity), or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method Hydrocarbon Types in Liquid Petroleum Products by Fluorescent Indicator Adsorption Test Method for ASTM Color of Petroleum Products (ASTM Color Scale) Test Method for Determination of Water in Liquid Petroleum Products by ICarl Fischer Reagent Test Method for Refractive Index of Viscous Materials Characteristic Groups in Rubber Extender and Processing Oils and Other Petroleum-Derived Oils by the Clay-Gel Absorption Chromatographic Method Carbon-Type Composition of Insulating Oils of Petroleum Origin Hydrocarbon Types in Middle Distillates by Mass Spectrometry Calculation of Viscosity-Gravity Constant (VGC) of Petroleum Oils Estimation of Molecular Weight (Relative Molecular Mass) of Petroleum Oils F r o m Viscosity Measurements Separation of Representative Aromatics a n d N o n a r o m a t i c s Fractions of High-Boiling Oils by Elution Chromatography Test Method for Thermal Conductivity of Liquids Test Method for Specific Heat of Liquids and Solids Hydrocarbon T5rpes Analysis of Gas-Oil Saturates Fractions by High Ionizing Voltage Mass Spectrometry Test Method for Boiling Range Distribution of Petroleum Fractions by Gas Chromatography Test Method for Calculation of Carbon Distribution and Structured Group Analysis of Petroleum Oils by the n-d-m Method Aromatic Types Anedysis of Gas-Oil Aromatic Fractions by High Ionizing Voltage Mass Spectrometry
21: MINERAL D 3524 D 3525 D 4291 D 4530 D 4808
D 5186
D 5291 D 5292
D 5372 D 6546
D 6743 E 659 G4
OIL HEAT TRANSFER
FLUIDS
585
Diesel Fuel Diluent in Used Diesel Engine Oils by Gas Chromatography Gasoline Diluent in Used Gasoline Engine Oils by Gas Chromatography Trace Ethylene Glycol in Used Engine Oil Test Method for Determination of Carbon Residue (Micro Method) Hydrogen Content of Light Distillates, Middle Distillates, Gas Oils, and Residua by Low-Resolution Nuclear Magnetic Resonance Spectroscopy Determination of Aromatic Content and Polynuclear Aromatic Content of Diesel Fuels and Aviation Turbine Fuels by Supercritical Fluid Chromatography Instrumental Determination of Carbon, Hydrogen, and Nitrogen in Petroleum Products and Lubricants Aromatic Ccirbon Contents of Hydrocarbon Oils by High Resolution Nuclear Magnetic Resonance Spectroscopy Guide for Evaluation of Hydrocarbon Heat Transfer Fluids Stcindard Test Methods for a n d Suggested Limits for Determining Compatibility of Elastomer Seals for Industrial Hydraulic Fluid Applications Test Method for Thermal Stability of Organic Heat TrEinsfer Fluids Test Method for Autoignition Temperature of liquid Chemicals Method for Conducting Corrosion Coupon Tests in Plant Equipment
OTHER STANDARDS ISO 6743-12: 1989 (E) International Standard, "Lubricants; Industrial Oils and Related Products (Class L)"-Classi£ication-Part 12: Family Q (Heat Transfer Fluids)
REFERENCES [1] Guffey, G. E., "Sizing Up Heat Transfer Fluids and Heaters," Chemical Engineenng, Vol. 104, No. 10, 1997, pp. 126-131. [2] Green, R. L., Larsen, A. H., and Pauls, A. C, "The Heat Transfer Fluid Spectrum," Chemical Engineering, Vol. 96, No. 2, 1989, pp. 90-98. [3] Green, R. L. and Morris, R. C, "Heat Transfer Fluids-Too Easy to Overlook," Chemical Engineering, Vol. 102, No. 4, 1995, pp. 88-92. [4] Seider, E. N. and Tate, G. E., "Heat Transfer and Pressure Drop," Industrial Engineering and Chemistry, Vol. 28, 1936, pp. 1429-1436; b. Kern, D. Q., "Chapter 6-Counterflow: Double Pipe Exchangers," Process Heat Transfer, McGraw Hill Inc., NY, 1950, p. 103. [5] ASTM E 1, "Specification for ASTM Thermometers," Annual Book of ASTM Standards, Vol. 14.03. [6] Anon. "Product Review: Oil Refining and Lubricant Base Stocks," Industrial Lubricatoin and Tribology, 1997, Vol. 49, No. 4, pp. 181-188. [7] Singh, H., "Characterization of Lube Oil Base Stock—Approach and Significance," Advances Production & Application of Lubricant Base Stocks, Proceedings, International Symposium, H. Singh, P. Rao, and T. S. R. Tata, Eds., McGraw-Hill, New Delhi, 1994, pp. 303-310.
586 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK [8] Hoo, G. H. and Lewis, E., "Base Oil Effects on Additives Used to Formulate Lubricants," Adv. Prod. Appl. Lube Base Stocks, Proceedings, International Symposium, H. Singh, P. Rao, and T. S. R. Tata, Eds., McGraw-Hill, New Delhi, 1994, pp. 326-333. [9] Prince, R. J., "Base Oils from Petroleum," Chemistry and Technology Lubricants, R. M. Mortimer a n d S. T. Orszulik, Eds., Blackie, Glasgow, 1992, pp. 1-31. [10] Yoshida, T., Watanabe, H., and Igarashi, J., "Pro-Oxidant Properties of Basic Nitrogen Components in Base Oil," Proceedings of the 11''' International Colloquium of Industrial and Automotive Luhrication-Vol. 1, W. J. Bartz, Ed., Technische Academie Esslingen, Esslingen, 1998, pp. 433-444. [11] Adhvatyu, A. and Singh, I. D., "FT-NMR and FT-IR Applications in Lubricant Distillation and Base Stock Characterization," Tribotest Journal, Vol. 3, No. 1, 1996, pp. 89-95. [12] Singh, H. and Singh, I. D., "Use of Aromaticity to Estimate Base Oil Properties," Advances Production & Application of Lubricant Base Stocks, Proceedings International Symposium, H. Singh, P. Rao, and T. S. R. Tata, Eds., McGraw-Hill, New Delhi, 1994, pp. 288-294. [13] Al-Bamwan, M., "Base Stocks Properties/Characteristics, Additive Response and Their Interrelationship," Advances Production & Application of Lubricant Base Stocks, Proceedings International Symposium, Proceedings, International Symposium, H. Singh, P. Rao, and T. S. R. Tata, Eds., McGraw-Hill, New Delhi, 1994, pp. 303-310. [14] Al-Sammerrai, D., "Study of Thermal Stabilities of Some Heat Transfer Oils," Journal of Thermal Analysis, 1985, Vol. 30, No. 4, pp. 163-110. [15] Jones, C , "Properties of Hydraulic Fluids," Mechanical World Engineering, January 1964, pp. 3-5. [16] Farris, J. A., "Extending Hydraulic Fluid Life by Water and Silt Removal," Field Service Report 52, Industrial Hydraulics Division, Pall Corporation, Glen Cove, NY. [17] Godfrey, D. a n d Herguth, W. R., "Physical and Chemical Properties of Industricil Mineral Oils Affecting Lubrication-Part 4," Lubrication Engineering, Vol. 51, No. 12, 1995, pp. 977-979. [18] Adhvaryu, A., Pandey, D. C , and Singh, L. D., "Effect of Composition on the Degradation Behavior on the Decomposition of
[19] [20] [21]
[22]
[23]
[24]
[25]
[26]
[27]
Base Oil," Symposium on Worldwide Prospectives on the Manufacture, Characterization, and Application of Lubricant Base Oils, Division of Petroleum Chemistry, Inc., 213 National Meeting of the American Chem. Society, 1997, pp. 227-228. Fuchs, H. C. G., "Understand Thermal Ansdysis Techniques," Chemical Engineering Progress, Vol. 93, No. 12, 1997, pp. 39-44. Kern, D. Q., Process Heat Transfer, McGraw-Hill Book Company, NY, 1950, pp. 99, 103. "Chapter 3-Viscous Oils," Manual on Hydrocarbon Analysis, 6'*^ Edition, A. W. Drews, Ed., 1998, ASTM International, West Conshohocken, PA, p p . 25-30. B a r m a n , B. N., "Hydrocarbon-Type Analysis of Base Oils and Other Heavy Distillates by Thin Layer Chromatography with Flame-Ionization Deection and by the Clay-Gel Method," Journal of Chromatograpic Science, Vol. 34, No. 5, 1996, pp. 219-225. Sassiat, P., Machtalere, G., Hui, F., Kolodziejczyk, H., and Rosset, R., "Liquid Chromatographic Determination of Base Oil Composition and Content in Lubricating Oils Containing Dispersants of the Polybuteneylsuccinimide Type," Analytical Chimica Acta, Vol. 306, No. 1, 1995, pp. 73-79. Kagdiyal, V., Joseph, M., Sastry, M. I. S., Satapathy, S., Basu, B., Jain, S. K., et al., "Estimation of Polycyclic Aromatic Hydrocarbons of Base Oils by HPLC and by UV Spectroscopic Technique: A Comparison," Proceedings of the Advances Production & Application of Lubricant Base Stocks, Proceedings International Symposium, H. Singh and T. S. R. Prasada Rao, Eds., Tata McGraw-Hill, New Delhi, India, 1994, pp. 295-302. Jain, M. C , Bansal, V., Jain, S. K., Srivastava, S. P., and Bhatnagar, A. K., "The Role of Thermal and High Temperature Gas Chromatographic Techniques in the Characterization of Base Oils Blends," Proceedings Adv. Prod. Lube Base Stocks, H. Singh and T. S. R. Prasada Rao, Eds., Tata McGraw-Hill, New Delhi, India, 1994, pp. 272-279. Powell, J. R. and Compton, D. A. C. "Automated FT-IR Spectrometry for Hydrocarbon-Based Engine Oils," Lube Engineering, Vol. 49, No. 3, 1993, pp. 233-239. Anon., "Characterizing Base Oils," Lubrizol Newsline, Lubrizol Corporation, Wickliffe, OH, December 1996, pp. 5.
MNL37-EB/Jun. 2003
Non-Lubricating Process Fluids: Steel Quenching Technology Bozidar Liscic, ^ Hans M. Tensi, ^ George E. Totten, ^ and Glenn M. Webster^
THIS CHAPTER WILL FOCUS ON QUENCHING TECHNOLOGY FOR STEEL
HEAT TREATING APPLICATIONS. Quenching is the process of cooling metal parts to achieve the desired microstructure, hardness, strength or toughness. Quenching can produce both desirable and undesirable residual stresses and distortion in addition to cracking. Steel, for example, is heated to the austenitizing t e m p e r a t u r e , that t e m p e r a t u r e where the austenite m i c r o s t r u c t u r e is formed. To obtain o p t i m u m hardness, strength, and toughness, the maximum amount of martensite transformation microstructure is desired. The primary function of the quenching medium is to control the rate of heat transfer from the surface to optimize the microstructure while minimizing undesirable features such as cracking and distortion [1]. The selection of a quenching medium is dependent on the composition of the alloy, the desired final microstructure and the surface to volume ratio of the part. The most c o m m o n quenchant media are usually liquids or gasses. Liquid quenchants include: mineral oils, water, water that may contain salt or caustic additives, and aqueous polymer solutions. The most c o m m o n gasses, which may or may not be pressurized, include nitrogen, helium, and argon. Various aspects of quenching technology will be discussed in this chapter. This includes: hardenability, fundamentals of quench processing, description of c o m m o n quench media, cooling curve analysis, and quench bath maintenance. Included in this discussion is the current status of international standards development for quenchant characterization. This discussion will be limited to steel quenching technology. Quenching of non-ferrous alloys are not discussed here although the basic surface cooling principles involved are the same.
DISCUSSION Steel Transformation Properties such as hardness, strength, ductility, and toughness are dependent on the microstructure and grain size. The first step in the process is to heat the steel to its austenitizing temperature. The steel is then cooled rapidly to avoid the formation of ferrite and maximize the formation of martensite, which is a relatively hard transformation product, to achieve the desired as-quenched hardness. ' University of Zagreb, Zagreb, Croatia. ^ Technical University of Munich, Munich, Germany. ^ G. E. Totten & Associates, Inc., LLC, Seattle, WA.
The most common transformation products that may be formed in quench-hardenable steels from austenite in order of formation with decreasing cooling rate: martensite, bainite, peeirlite (which is a mixture of ferrite and cementite), and pearlite/ferrite. Each of these microstructures provides a unique combination of properties, and especially the relationship between ferrite and cementite in pearlite—depending on the carbon content and the cooling velocity—strongly influences the mechanical properties. These transformation products have been described by J.R. Davis (in order of their formation with increasing cooling velocity) [2]: 1. Austenite—A microstructural phase characterized by a face-centered cubic iron (gamma iron) crystallographic structure. It is the desired solid solution microstructure produced prior to hardening. An austenite microstructure is illustrated in Fig. lA [2]. 2. Ferrite—^A near carbon-free solid solution of one or more elements in a body-centered, cubic arrangement in which alpha iron is the solvent. Fully ferritic steels are only obtained when the carbon content is very low. Ferritic grain boundaries, as illustrated in Fig. IB [2] is the most obvious microstructural feature 3a. Pearlite—A microstructural phase characterized by its body-centered crystallographic structure which is a metastable lamellcir aggregate of ferrite and cementite (or with an extremely low cooling rate, a mixture of globular cementite in ferrite) whose microstructure is shown in Fig. 1C [2] resulting from the transformation of austenite at temperatures above the bainite range. 3b. Cementite—Brittle compound of iron and carbon, which is known as iron carbide with the approximate chemical formula FesC and is characterized by an orthorombic crystal structure. When it occurs as a phase in steel, the chemical composition will be affected by the presence of m a n g a n e s e a n d other carbide-forming elements. The highest cementite contents are observed in white cast irons (Fig. ID) [2]. 4. Bainite—^A metastable aggregate of ferrite and cementite resulting from the transformation of austenite at temperatures below the pearlite transformation temperature, but above the start of martensite transformation (Ms). Upper bainite is an aggregate that contains parallel lathshape units of ferrite and carbides and produces a feathery appearance in optical microscopy, as shown in Fig. IE [2]. It is formed above approximately 350°C (660°F). Lower bainite exhibits an acicular appearance similar to t e m p e r e d martensite, as shown in Fig. I F [2] a n d is formed below approximately 350°C (660°F).
587 Copyright'
2003 by A S I M International
www.astm.org
588 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK
i>6-
(a),(b)
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(g),(h) FIG. 1—Illustrations of microstructural transformation products: a) Equiaxed austenite grains and annealing twins in an austenitic stainless steel. 250X; ASM Materials and Engineering Dictionary, ASM International, Materials Park, OH 44073-0002, Fig. 20, p. 26. b) Low-carbon ferritic sheet steel etched to reveal ferrite grain boundaries-. 100X; ASM Materials and Engineering Dictionary, ASM International, Materials Park, OH 44073-0002, Fig. 25, p. 30. c) Pearlite structure in high-carbon steel (Fe-0.5C). The cementite lamellae are white; the ferrite is dark: 500X; ASM Materials and Engineering Dictionary, ASM International, Materials Park, OH 44073-0002, Fig. 26, p. 30. d) White cast iron contains massive amounts of cementite (white) and pearlite (dark): 500X; ASM Materials and Engineering Dictionary, ASM International, Materials Park, OH 44073-0002, Fig. 64, p. 66. e) Upper bainite in 4360 steel; ASM Materials and Engineering Dictionary, ASM International, Materials Park, OH 44073-0002, Fig. 173, p.156. f) Lower bainite (dark plates) in 4150 steel; Reprinted from Ref. 3, p. 67 by courtesy of Marcel Dekker, Inc. g) Microstructure of lath martensite; 500X; Reprinted from Ref. 3, p. 68 by courtesy of Marcel Dekker, Inc. h) Microstructure of plate martensite; light shading is retained austenite; 500X; Reprinted from Ref. 3, p. 69 by courtesy of Marcel Dekker, Inc. i) Microstructure of tempered martensite; 500X; ASM Materials and Engineering Dictionary, ASM international. Materials Park, OH 44073-0002, Fig. 365, p. 308.
(f)
CHAPTER
22: NON-LUBRICATING
PROCESS
5. Martensite—A generic term for microstructures formed by a diffusionless phase transformation in which the parent and product phases have a specific crystallographic relationship. Martensite in steel is characterized by its body-centered tetragonal crysteJlographic structure. The a m o u n t transformation from austenite to martensite depends on the cooling rate and on the lowest temperature attained since there is a distinct t e m p e r a t u r e where martensitic transformation begins (Ms) and ends (Mf). Three microstructural forms of martensite are: lath (Fig. IG [3]), plate (Fig. I H [3]) a n d t e m p e r e d (Fig. 11[3]) martensite. The formation of these products and the proportions of each are dependent on the austenitization time (because of increasing solution of elements in austenite with increasing time), the time and temperature cooling history of the particulcir alloy, and composition of the alloy. The transformation products formed are typicEilly illustrated with the use of transformation diagrams, which show the temperature-time dependence of the microstructure formation process for the alloy being studied. Two of the most commonly used transformation diagrams are TTT (time-temperature-transformation) a n d CCT (continuous cooling transformation) diagrams. TTT
Diagrams
TTT diagrams, which are also called isothermal transformation (IT) diagrams, are developed by heating small samples of steel to the t e m p e r a t u r e where austenite transformation structure is completely formed i.e., austenitizing temperature, and then rapidly cooling to a temperature intermediate between the austenitizing and the Ms temperature, and then holding for a fixed period of time until the transformation is complete, at which point the trcinsformation products are determined. This is done repeatedly until a TTT diagram is constructed such as that shown for a n unalloyed steel (AISI 1045) in Fig. 2A. TTT diagrams can only be read along the isotherms.
FLUIDS: CCT
STEEL
QUENCHING
TECHNOLOGY
589
Diagrams
Alternatively, seimples of a given steel may be continuously cooled at different specified rates and the p r o p o r t i o n of tremsformation products formed after cooling to various temperatures intermediate between the austenitizing temperature and the Ms determined to construct a CCT diagram, such as the one shown for an unalloyed carbon steel (AISI 1045) in Fig. 2B. CCT curves provide data on the temperatures for each phase trzmsformation, the amount of transformation product obtained for a given cooling rate with time, and the cooling rate necessary to obtain mEirtensite. The critical cooling rate is dictated by the time required to avoid formation of pearlite for the particular steel being quenched. As a general rule, a q u e n c h a n t m u s t produce a cooling rate equivalent to, or faster than, that indicated by the "nose" of the pearlite transformation curve in order to maximize mEirtensite transformation product. CCT diagrams can only be read along the curves of different cooling rates. Caution: Although it is becoming increasingly common to see cooling curves (temperature-time profiles) for different cooling media (quenchants) such as oil, water, air, and others superimposed on either TTT or CCT diagrams, this is not a rigorously correct practice and various errors are introduced into such analysis due to the inherently different kinetics of cooling used to obtain the TTT or CCT diagrams versus the quenchants being represented. A continuous cooling curve can be superimposed on a CCT, but not on a TTT diagram. Hardenability Hardenahility has been defined as the ability of a ferrous material to develop hardness to a given depth after being austenitized and quenched. This general definition comprises two subdefinitions, the first of which is the ability to achieve a certain hardness [4]. The ability to achieve a certain hcirdness level is associated with the highest attainable hardness, which depends on the carbon content of the steel and more
Austenite Temperature = 880 °C
ill m
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(a)
-Time-
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FIG. 2—a) Time-Temperature-Transformation (TTT) diagram of an unalloyed steel containing 0.5% carbon; b) ContinuousCooling-Transformation diagram of an unalloyed steel containing 0.6% carbon.
(b)
590
MANUAL
3 7: FUELS
AND LUBRICANTS
HANDBOOK
specifically on the amount of carbon dissolved in the austenite after austenitizing. This is illustrated by considering the problem of hardening of high-strength, high-carbon steels. The higher the concentration of dissolved c a r b o n in the austenitic phase, the greater the increase in mechanical strength after rapid cooling and transformation of the austenite in the metastable martensite phase. Martensitic steels typically exhibit increasing hardness and strength with increasing carbon content, as shown in Fig. 3, but they also exhibit relatively low ductility. However, with increasing carbon concentration, martensitic transformation from austenite becomes more difficult, resulting in a greater tendency for retained austenite and correspondingly lower strength. The second subdefinition of hardenability refers to the hardness distribution within a cross section from the surface to the core under specified quenching conditions. It depends on the carbon content, which is interstitially dissolved in austenite and the a m o u n t of alloying elements substitutionally dissolved in the austenite during austenitization. Therefore, as Fig. 3 shows, carbon concentrations in excess of 0.6% do not yield correspondingly greater strength [7]. Also, increasing carbon content influences the Mf temperature relative to Ms during rapid cooling as shown in Fig. 4 [8]. In this figure, it is evident that for steels with carbon content above 0.6%, the transformation of austenite to martensite will be incomplete if the cooling process is stopped at 0°C or higher. The depth of hardening depends on the following factors: • Size cmd shape of the cross section • Heirdenability of the materiEd • Quenching conditions The cross section shape exhibits a significant influence on heat extraction during quenching and therefore, on the hardening depth. Heat extraction is dependent on the surface area exposed to the quenchant. Bars of rectangular shape achieve
^600
E
S.400 E
\M,
C
o ro 200 CO
I 0
0.5 1.0 Carbon Content (wt. %)
1.5
FIG. 4—Influence of the carbon content in steels on the temperature of the start of martensite formation (Ms) and the end of martensite formation (Mf).
Equivalent bar diameter. In. 1 175 E E 150 m
2
3
4 5 6 1 Round Dars-
7
Squ are bi jrs —
in
w
= 125
_o
E. t 100
1 Plates
0)
c o
*0) -»
I 75
't
(0 TJ
o
3 T3 O
150
"^N^
\-
e
50
*-^ u
25
•D O
2
f
0 25 50 75 100 125 150 175 Equivalent bar diarneter, mm 100
FIG. 5—Correlation between rectangular cross sections and their equivalent round bar and plate sections.
D) C
£
w •^
>-
50
0.2 0.4 0.6 0.8 Carbon Content (wt. %) I
30
25 20 Nickel Content (wt. %)
FIG. 3—Influence of the carbon content in steel on the yield strength {ao.e) after quench hardening. The yield strength values were obtained from compression tests; the additional variation of nicltel content causes a negligible solid-solution-hardening and was selected to obtain a constant Ms temperature for the start of martensitic transformation.
less depth of hardening than round bars of the same cross section size. Figure 5 can be used to convert square and rectangular cross sections to equivalent circular cross section sizes [6]. The effect of steel composition on hardenability may be calculated in t e r m s of the "ideal critical diameter" or Di, which is defined as the largest b a r diameter t h a t can be quenched to p r o d u c e 50% martensite at the center after quenching in an "ideal" quench, i.e., under "infinite" quenching severity. The ideal quench is one that reduces surface temperature of a n austenitized steel to the bath temperature instctntaneously. Under these conditions, the cooling rate at the center of the bar depends only on the thermal diffusivity of the steel.
CHAPTER 22: NON-LUBRICATING
PROCESS FLUIDS: STEEL QUENCHING
The ideal critical diameter may be calculated from: Di = Di Base (carbon concentration and grain size) X fmn X f s i ^ f c r ^ fiVIo X /V X f c u X /"NI X f^
Where fx is a multiplicative factor for the particular substitutionally dissolved alloying element. The base Dj Base value and one set of alloying factors are provided in Table 1 [6] (Note: This is not an exhaustive listing of alloying factors but these are commonly encountered and they permit calculations to illustrate the effect of steel chemistry variation on hardenability.) Grain size refers to the dimensions of grains or crystals in a polycrystalline metal exclusive of twinned regions and subgrains when present. Grain size is usually estimated or measured on the cross section of an aggregate of grains. Common units are: (1) average diameter, (2) average area, (3) number of grains per linear unit, (4) number of grains per unit area, and (5) number of grains per unit volume. Grain size may be determined according to Test Method E 112. The test methods covered in Test Method E 112 describe the measurement of average grain size and include the comparison procedure, the planimetric (or Jeffries) procedure, and the intercept procedures. Standard comparison charts are provided. These test methods apply chiefly to single phase grain structures but they can be applied to determine the average size of a particular type of grain structure in a multiphase or multiconstituent specimen. In addition, the test methods provided in E 112 are used to determine the average grain size of specimens with a unimodal distribution of grain areas, diameters, or intercept lengths. These distributions £ire approximately log normal. These test methods do not cover methods to characterize the nature of these distributions. Characterization of grain size in specimens with duplex grain size distributions is described in Test Method E 1181. Measurement of individual, very coarse grains in a fine-grained matrix is described in Test Method E 930. These test methods deal only with determination of planar grain size, that is, characterization of the twodimensional grain sections revealed by the sectioning plane.
TECHNOLOGY
591
Determination of spatial grain size, that is, measurement of the size of the three-dimensional grains in the specimen volume, is beyond the scope of these test methods. The test methods described in E 112 are techniques performed manually using either a standard series of graded cheirt images for the comparison method or simple templates for the manual counting methods. Utilization of semiautomatic digitizing tablets or automatic image analyzers to measure grain size is described in Test Method E 1382. The ASTM grain size number (G), referred to in Table 1, is a grain size designation bearing a relationship to average intercept distance at 100 diameters magnification according to the equation: G = 10.00 - 21og2L Where L = the average intercept distance at 100 diameters magnification. The smaller the ASTM grain size, the larger the diameter of the grains. The effects of quenching conditions on the depth of hardening are not only dependent on the quenchant being used and its physical and chemical properties, but also on process parameters such as bath temperature and agitation. Hardenability Measurement There are numerous methods to estimate steel hardenability. However, perhaps the two most common are: Jominy curve determination and Grossmann hardenability. These two procedures will be discussed here. Jominy Bar End-Quench Test The most familiar and commonly used procedure for measuring steel hardenability is the Jominy bar end-quench test. This test has been standardized and is described in ASTM A 255, SAE J406, DIN 50191, and ISO 642. For this test, a 100 mm (4 in.) long by 25 mm (1 in.) diameter round bar is austenitized to the proper temperature, dropped into a fixture, and one end rapidly quenched with 24°C (75°F) water from a 13 mm (0.5 in.) orifice under specified conditions, as
TABLE 1—Hardenability factors for carbon content, grain size and selected alloying elements in steel. Carbon Content (%) 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00
Carbon Grain Size No.
Alloying Element
6
7
8
Mn
Si
Ni
Cr
Mo
0.0814 0.1153 0.1413 0.1623 0.1820 0.1991 0.2154 0.2300 0.2440 0.2580 0.273 0.284 0.295 0.306 0.316 0.326 0.336 0.346
0.0750 0.1065 0.1315 0.1509 0.1678 0.1849 0.2000 0.2130 0.2259 0.2380 0.251 0.262 0.273 0.283 0.293 0.303 0.312 0.321
0.0697 0.0995 0.1212 0.1400 0.1560 0.1700 0.1842 0.1976 0.2090 0.2200 0.231 0.241 0.251 0.260 0.270 0.278 0.287 0.296
1.167 1.333 1.500 1.667 1.833 2.000 2.167 2.333 2.500 2.667 2.833 3.000 3.167 3.333 3.500 3.667 3.833 4.000 4.167 4.333
1.035 1.070 1.105 1.140 1.175 1.210 1.245 1.280 1.315 1.350 1.385 1.420 1.455 1.490 1.525 1.560 1.595 1.630 1.665 1.700
1.018 1.036 1.055 1.073 1.091 1.109 1.128 1.146 1.164 1.182 1.201 1.219 1.237 1.255 1.273 1.291 1.309 1.321 1.345 1.364
1.1080 1.2160 1.3240 1.4320 1.54 1.6480 1.7560 1.8640 1.9720 2.0800 2.1880 2.2960 2.4040 2.5120 2.62 2.7280 2.8360 2.9440 3.0520 3.1600
1.15 1.30 1.45 1.60 1.75 1.90 2.05 2.20 2.35 2.50 2.65 2.80 2.95 3.10 3.25 3.40 3.55 3.70
592 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
illustrated in Fig. 6 [3]. The austenitizing temperature is selected according to the specific steel alloy being studied, however, most steels are heated in the range of 870-900°C (1600-1650OF). In the Jominy end-quench test, a cylindrical specimen is heated to the desired austenitizing temperature and welldefined time, which is material composition dependent, and then it is quenched on one end with water. The cooling velocity decreases with increasing distance from the quenched end. After quenching, parallel flats are ground on opposite sides of the bar and hardness measurements are made at 1/16 in. (1.6 mm) intervals along the bar, as illustrated in Fig. 7 [6]. The hardness as a function of distance from the quenched end is measured and plotted and, together with measurement of the relative areas of the martensite, bainite, and pearlite that is formed, it is possible to compare the hardenability of different steels using Jominy curves. As the slope of the Jominy curve increases, the abiUty to harden the steel (hardenabihty) decreases. Conversely, decreasing slopes (or increasing flatness) of the Jominy curve indicates increasing hardenabihty (ease of hardening). The Jominy end-quench is used to define the hardenabihty of carbon steels with different alloying elements like chromium (Cr), manganese (Mn), molybdenum (Mo) and has different critical coohng velocities. Jominy curves for different alloy steels are provided in Fig. 8 [9]. These curves illustrate that the unalloyed, 0.4% carbon steel exhibits a relatively small distance for martensite (high hardness) formation. The 1% Cr and 0.2% Mn steel, however, can be hardened up to a distance of 40 mm. Figure 8 illustrates that steel hardenabihty is dependent on the steel chemistry, that unalloyed steels exhibit poor hardenabihty, and that Jominy curves provide an excellent indicator of relative steel hardenabihty. The Jominy test provides valid data for steels having an ideal diameter from about 25-150 mm (1-6 in.). This test can be used for Di values less than 25 mm (1 in.), but Vickers or microhardness tests must be used to obtain readings that are closer to the quenched end of the bar and closer together than generally possible using the standard Rockwell "C" hardness test method [4]. The austenitizing time and temperature, extent of special carbide solution in the austenite and extent of oxidation or surface decarburization during austenitizing, care and consistency of surface flat preparation, and bar positioning prior
„ ... iwith increasing Pearlite fdistance ofttie
I Cemenlite Lamella Probi WaterNozzle—i
200 400 600 800 Hardness (H,)
FIG. 6—Schematic illustration of the Jominy end-quench test and microstructural variation with increasing distance from the quenched end.
270 70 18 5-6 4B9-\Z4'-32.3"1
-rr- 1
r,i}cooling rates Distance from quenched end., in.
-
1
^ "a \U
60 50
•
S 40 I
230 •o 0
^ M
'
to »
'
'
1.0 « '
25
'
2.0 -^—I 50
3.0 in. I
t
I— 75 mm
Distance from quenched end FIG. 7—Measuring hardness on the Jominy test specimen and plotting hardenability curves. to making hardness measurements are important factors that influence test results. Therefore, all tests should be conducted in compliance with the standard being followed [4]. Grossmann
Hardenability
Grossmann's method of measuring hardenability uses a number of cylindrical steel bars of different diameters hardened in a given quenching medium [6]. Aftier sectioning each bar at mid-length and examining it metallographically, the bar that has 50% martensite at its center is selected, and the diameter of this bar is designated as the critical diameter D^^f Other bars with diameters smaller than Dent will have more martensite and correspondingly higher hardness values and bars with diameters larger than D^^it will attain 50% martensite only up to a certain depth as shovro in Fig. 9 [4]. The D^^^ value is valid only for the quenching medium and conditioiTs used to determine this value. To determine the hardenability of a steel independently of the quenching medium, Grossmann introduced the term ideal critical diameter, Di, which as discussed previously, is the diameter of a given steel bar that would produce 50% martensite at the center when quenched in a bath of quenching intensity H = oo. Here H = co indicates a hypothetical quenching intensity that reduces the temperature of heated steel to the bath temperature in zero time.
CHAPTER
22: NON-LUBRICATING
PROCESS
To identify a quenching medium and its condition, Grossm a n n introduced the Quenching Intensity (Severity) factor "H". Table 2 provides a summary of Grossmann H-Factors for different quench media and different quenching conditions [1]. Ahhough this data has been pubhshed in numerous reference texts for many years, it is of relatively limited quantitative value. One of the most obvious reasons is that quenchant agitation is not adequately defined and is often unknown, yet it exhibits enormous effects on quench severity. The Grossmann value "H" is based on the Biot (Bi) number, which interrelates the interfacial heat transfer coeffi-
FLUIDS:
STEEL
QUENCHING
TECHNOLOGY
593
TABLE 2—Effect of agitation on quench severity as indicated by Grossmann quench severity factors (H-factors). Grossmann H Factor Agitation
Oil
Water
Caustic Soda or Brine
None Mild Moderate Good Strong Violent
0.25-0.3 0.30-0.35 0.35-0.4 0.4-0.5 0.5-0.8 0.8-1.1
0.9-1.0 1.0-1.1 1.2-1.3 1.4-1.5 1.6-2.0 4
2 2-2.2
5
cient (a) thermal conductivity (A) and the radius (R) of the round bar being hardened: Bi = a/\-R H = al{2-
0 10 20 30 40 50 Distance from the Quenched Front Side (mm) FIG. 8—Jominy curve comparison of the hardenability of one unalloyed and a number of other different alloy steels. All alloy concentrations are weight %.
M
i 40
r-i-
-HRC .,s50%M
i/i
a>
Cfll
"I 20
-?c•^t^
o I
1
'm» FIG. 9—Determination of critical diameter Dcrit according to Grossmann.
H-D X)
Since the Biot number is dimensionless, this expression means that the Grossmann value, H, is inversely proportional to the b a r diameter. This method of numerically analyzing the quenching process presumes that heat transfer is a steady state, linear (Newtonian) cooling process. However, this is seldom the case and almost never the case in vaporizable quenchants such as oil, water, and aqueous polymers. Therefore, a significant error exists in the basic assumption of the method. Another difficulty is the determination of the //-value for a cross section size other than one experimentally measured. In fact, //-values depend on cross section size. Values of H do not account for specific quenching characteristics such as composition, oil viscosity, or temperature of the quenching bath. Tables of //-values do not specify the agitation rate of the quenchant either uniformly or precisely (see Table 2). Therefore, although //-values are commonly used, more current and improved procedures ought to be used when possible. For example, cooling curve analyses a n d the various methods of cooling curve interpretation that have been reported [1,5] are all significant improvements over the use of Grossmann Hardenability factors.
Q u e n c h i n g F i m d a m e n t a l s a n d C o o l i n g Curve Analysis Steel Wetting
GO
=
Kinetics
Hardening of steels (so cedled Martensitic- or Bainitic- Hardening) requires preheating (austenitizing) of the steel to temperatures in the range of 750-1100°C, from which the steel is quenched (cooled) in a defined way to obtain the desired mechanical properties such as hardness a n d yield strength. Most liquid vaporizable quenchants used for this process exhibit boiling temperatures between 100 and 300°C at atmospheric pressure. When parts are quenched in these fluids, the wetting of the surface is usually time dependant, which influences the cooling process and the achievable hardness. G. J. Leidenfrost described the wetting process about 250 years ago [10]. The Leidenfrost Temperature is defined as the surface temperature where the vapor film collapses and the surface is wetted by the liquid. Literature describes temperature-values for this event for water at atmospheric pressure between 150 and 300°C [11-14]. It is apparent that the Leidenfrost Temperature is influenced by a variety of factors, part of which cannot be quantified precisely even today.
594 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK For a nonsteady state cooling process, the surface temperature at all parts of the workpiece is not equal to the Leidenfrost Temperature. When the vapor blanket (or film boiling) collapses, wetting begins by nucleate boiling due to the influence of lateral heat conduction (relative to the surface) [15]. This is due to the simultaneous presence of various heat transfer conditions during vapor blanket cooling (or film boiling [FB]), nucleate boiling [NB], and convective heat transfer [CONV] with significantly varying heat transfer coefficients apB (100 to 250 W/irn^K)); aNB (10 to 20 kW/{m^K)y, and ttcoNv (ca. 700 W/(m'^K)). Figure 10 schematically illustrates the different cooling phases on a metcJ surface during an immersion cooling process with the so-called "wetting front," w, (separating the "film boiling phase" and the "nucleate boiling phase") and the change of the heat transfer coefficients, a, along the surface coordinate, z, (mantle line). In most cases during immersion cooling, the wetting front ascends the cooling surface with a significant velocity, v, whereas during film cooling the wetting front descends in the fluid direction [13,16]. An example of wetting heated cylindrical and prismatic specimens which are submerged in water is shown in Fig. 1 l a and fo [13,17]. Because of the different wetting phases on the metal surface (and the enormous differences of their values of apB,ttNB,and acoNv). the time dependant temperature distribution within the metal specimens will also be influenced by the velocity and geometry of the wetting front (for example, circle or parabolic-like) as well as geometry of the quenched part. Figure 12 illustrates different types of wetting behavior under different conditions [17]. By changing a quenching parameter, for example the chemical composition of the fluid, the period of wetting (tw) can be reduced over more than one order of magnitude. The time interval, t^^, when the wetting front appears—usually at the lower end of the specimen—^up to the time the wetting front has moved across the entire specimen surface; sometimes two wetting fronts appear as illustrated in Fig. l i d . In addition to explosive-like wetting (fw ~ 0), a foam may appear in the fluid neeir the specimen
Immersion cooling Wetting front»
surface, which will depress the heat flux from the specimen into the fluid. Factors Influencing
Film
Boiling
To quantitatively define the change of the wetting behavior, for example, to ascertain the cooling process or to develop or aneJyze quenching fluids, the measurement of the electrical conductance between the submerged sample and a counter electrode is helpful [13]. During the film boiling phase, the hot metal is largely insulated by the vapor film surrounding the metal and conductance between the metal a n d the counter electrode is low. When the vapor blanket (film boiling) ruptures on the metal surface, localized wetting begins and conductance increases. The increase in conductance of the wetted metal is proportional to the amount of the metal surface wetted by the quenchant. When the metal surface is completely wetted, conductance is at its highest value. Figure 13a schematically illustrates a normal electrical conductance, (G) increase corresponding to the percentage of the wetted surface (compare Fig. 11A or 1 IB). Three other possibilities of wetting are also shown. Figure I3b shows a rapid rewetting process (or "explosive" wetting) similar to that shown in Fig. 12. Figure 13c illustrates rapid wetting followed by insulation by bubbles adhering to the metal surface and Fig. I3d illustrates rapid wetting with repeated new formation of film boiling, a process which occurs with many aqueous polymer quenchant solutions. In all four diagrams, the temperature, Tc, is shown, which is measured in the center of the probes. The time, ts, characterizes the time (and the corresponding temperature value from the Tc (t) slope, the temperature Ts) when wetting begins. This shows that temperature measurements in the center of probes provide poor information about the real quenching process that is insufficient to adequately characterize the hardening process. This is also illustrated in Fig. 14 [19] by T(t) slopes, measured in the center and the surface of cylindrical probes, quenched in water with t^v + 0, and in an aqueous poljTner solution with a short ty^ time. To obtain a better definition of the wetting kinematic, the starting time, ts, of wetting, the finishing time, tf, of wetting and the difference between tf and ts as the wetting time fi^ should be used. The effect of vEiriation of these parameters on the quenching process is summarized in Table 3.
Film boiling
Impact of the Wetting Process o n Cooling Behavior
Nucleate boiling Convective beat transfer Heal transfer coefficient at
Film cooling Film of liquid Convection boiling
Fluid drops Wetting front ir
FIG. 10—Wetting behavior and cliange of heat transfer coefficient a along the surface [13,16].
An illustration of the influence of a wetting process occurring over a long time (a so-called "non-NEWTONIAN wetting") on the temperature - time cooling curves, measured near the surface at different distances from the lower end of the probe, is illustrated in Fig. 15 [17]. The wetting front requires about 18 s to arrive at a height of Z = 80 m m taken from the discontinuities of the curve "0" (ca. Is) and curve "80" (ca. 19s). If there is an explosion-like wetting of the probe (a so called "NEWTONIAN wetting"), these five t e m p e r a t u r e slopes cire congruent. If the temperature was measured (like usucJ) in the center of the probe, the large differences in wetting behavior would not be observed. The temperature distribution within the probe during quenching (indicated by isotherms at different times after
CHAPTER 22: NON-LUBRICATING
(a)
(b)
(c)
4.0 s
3.8 s
4.3 s
7.0 s
5.7 s
8.3 s
PROCESS FLUIDS: STEEL QUENCHING
TECHNOLOGY
595
10 s
6.9 s
12,3
3^2 s
4,92 s
5,97 s
7,38 s
(d)
FIG. 11—Process of transition between tlie three cooling phases—film boiling (FB), nucleate boiling (NB) and convective cooling (CONV) during immersion cooling of CrNi -steel specimens with a cylindrical geometry 25 mm dia x 100 mm), a. Wetting process of a cylindrical CrNi-steel specimen being quenched from 850°C into water at 30°C with an agitation rate of 0.3 m/s b. prismatic geometry (15 x 15 x 45 mm) in water of 60°C without forced convection; Immersion temperature of 860°C, c. CrNi-steel probe (25 mm dia. x 100 mm) in oil at 60°C without agitation, d. hollow cylinder (60 od X 30 id X 60 mm long) quenched into oil at 60° and no agitation. Note: there are two wetting fronts.
beginning the quenching process), having a "non-NEWTONIAN wetting" is shown in Fig. 16a. where there is a great difference relative to that of the "NEWTONIAN wetting" (Fig. 166). In the second case, the temperature gradient, T, is radial whereas in the case 'a it is axial. Of course the hardness distribution in the probes must be extremely different. In the case of "a," a strong axial hardness distribution (accompanied by a very low radial hardness distribution); in the case of "b," a very strong radial hardness gradient is observed.
Cooling Curve Data Acquistion and Analysis Data Acquistion and Analysis The need to acquire sufficient data to adequately define a cooling curve for subsequent analysis has long been recognized. Special data acquisition devices, including hardware such as oscillographs, were used for work reported by Jominy [20], French [21], and others [21,22]. However, this equipment is difficult to calibrate, which has inhibited widespread use of cooling curve analysis. Currently, sufficient data ac-
596
MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
quisition rates can be achieved with personal computers equipped with analog-to-digital (A/D) converter boards. Although computer hardware is available, there are no published guidelines for selecting the proper data acquisition rate, which varies with probe alloy, size, and quench severity. Perhaps the best method for selecting the required acquisition rate is to determine it experimentally. This can be done by repeatedly quenching a probe in cold (25°C, or 77°F) water, one of the more severe quenchants, and collecting data at
1000 O o
! *
750
S
(0 ki. ID Q.
E
; a
c_
i «
\
JO
£
r",
(b)
(a)
500
2 \ "* \
1 /Water
AK
250 polymer ^
9
10
/3
X.
20
30
3 2 1
1-3
^
40 0
10
15
20
Time in s
Time in s
FIG. 14—Comparison of cooling curves measured at different positions in a cylindrical CrNi-steel probe (25 dia. x 100 mm) during slow wetting (water) and sudden wetting (aqueous polymer solution) at (a) center and (b) close to the probe surface at three indicated heights (1,2, and 3).
TABLE 3"—Effect of fluid and metal property variation on quench severity.
6.35 s
Effect on Property Variation Increasing, 4- =Decreasing)
7.65 s
(t ==
FIG. 12—Transition from film boiling (FB) to nucleate boiling (NB) during immersion of cylindrical silver specimen (15 mm dia x 45 mm) quenched from 850°C into a 10% aqueous polymer quenchant solution at 25°C without agitation [18] In comparison with the tw-values of Fig. 13 a and b, the wetting time is extremely short [18].
iIk (a) i^
G
0) 1
ts.tf
Atw
a
n Ti i
ti Ti i
ti ti
T
T
i
T
t
t
T
T
i
i
i
t
i
i
i
t
".Parameters: ts'. time when wetting starts; tf. time when wetting is finished; Atw- time interval of wetting [s]; a: heat transfer coefficient.
m
E 0)
Fluid Property Type of quenchant Addition of additives Increasing agitation (v) Increasing bath Temperature (Tt) Metal Property Increasing thermal Diffusivity (a) Increasing cross section Size Increasing surface Roughness Increasing surface Oxidation
^ ^ timet
ts = tf
timet
timet FIG. 13—a-d): Temperature decrease, Tc, and increasing electrical conductance, G, (proportional to the wetted surface) during quenching in different quenchants; a) slow wetting, b) rapid (explosive-like) wetting; c) rapid wetting followed by isolation of bubbles adhering to the metal surface; d) rapid wetting with repeated formation of film boiling after this process (see also Figs. 14 and 15) [13,18].
various acquisition rates. For example, a 13 mm X 100 mm (1/2 in. X 4 in.) cylindrical Inconel 600 probe with a tjqje K thermocouple inserted at the geometric center was quenched into 25°C (77°F) water with data acquisition rates of 1, 2.5, and 5 Hz. (A "hertz" value is equivalent to a data point per second.) The corresponding cooling time and rate curves are shown in Fig. 17 [5]. These results show that for this particular probe, a data acquisition rate of at least 5 Hz was required to obtain a smooth cooling rate curve in the maximum cooling rate region. Smooth curves are required to minimize errors in determining the maximum cooling rate in this critical region of the quenching process. In data acquisition, the proper A/D converter card must be selected to match the thermocouple being used. The thermocouple output is nonlinear with respect to temperature. If software for data analysis is being written for this use, this nonlinearity must be taken into account. One method is to create a data file that will convert thermocouple electromotive force (emf) values to the appropriate temperature.
CHAPTER 22: NON-LUBRICATING
PROCESS FLUIDS: STEEL QUENCHING TECHNOLOGY
597
Tj^ical data acquisition errors were found to be ±0.5°C (±0.9°F) within the temperature range of 0-900°C (32-1650°F). Another source of error is the algorithm used to calculate cooling rates. In general, however, it was found that the error obtained was primarily a result of an error in the data. Most data have some background noise; consequently, most data acquisition techniques employ data averaging or smoothing to minimize the noise. One method is a five-point "running" average [24]:
r„ = {-%5Tn^2) + (%r„_i) + (%rj + (%rn+i)-e/35j„+2)
15 20 Quenching time FIG. 15—^Temperature drop on the surface of a cylindrical steel probe (25 mm dia x 100 mm) at various distances zfrom the lower end [16].
where T is the temperature at the n,n - \,n + l,n - 2, and n + 2 groups of points being averaged. A running average means that after T^ is calculated, the average of the next (n + 1) point is then averaged. This process is continued until all of the data points have been averaged. Another feature of the A/D converter card that must be considered is ice-junction compensation. Because thermocouples are only able to measure temperature differences, to obtain accurate temperature measurements, it is important to calibrate to a reference temperature, typically ice water. Cooling Time and Rate Parameters
/=5s
-825'C
-775"C t=5s
/=10s
/=11s
/=15s
FIG. 16—a and b) Time-dependant temperature distribution during cooling of cylindrical steel probes (25 mm dia x 100 mm) in the case of slow non-Newtonian wetting and explosive Newtonian wetting.
Zhu [23] analyzed computer data acquisition for cooling curve analysis and reported the following sources of error: • Thermocouple voltage to temperature conversion • A/D signed conversion • Preamplifying circuit and signal transmission circuits • Electronic component instability • Error caused by signal interference
In recent years, several rate- and time-dependent parameters have been suggested to quantify the severity of various quenchant media. Some of these parameters include the cooling rates at 705 and 205°C (1300 and 400°F), maximum cooling rate, and time to the cooling rate at 730-260°C (1350-500°F) [48]. It is generally desirable to maximize the cooling rate at 705°C (1300°F) to avoid the pearlite transformation region. It is desirable to minimize the cooling rate at 205°C (400°F), which is in the region of the Ms transformation temperature for many steels, to minimize cracking. It is also desirable to minimize the time to cool from 730-260°C (1350-500°F) in order to optimize the potential hardness by avoiding pearlite formation. Tensi and Steffen [25] have prepared a set of critical cooling parameters for cooling curve characterization as shown in Fig. 18: • Full-film boiling (vapor blanket cooling) to nucleate boiling transition time, temperature, and rate,i?DHmin- These parameters characterize the transition from A- to B-stage cooling. • Maximize the rate of cooling, i?max. and the temperature where this occurs, TRmax- GenereJly, it would be desirable to maximize i?max and minimize ?Rmax• Rate of cooling at temperatures such as 200°C (i?2oo) and 300°C, (i?3oo)- In order to minimize cracking and distortion, it is desirable to minimize cooling rates in this region. • Although the dimensions for cooling rate most typically used throughout the heat treating industry are [°C/s], this is physically incorrect. The correct units for cooling rates are [K/s]. One problem with this approach is the potential misinterpretation of low cooling rates in the Ms transformation region. Traditionally, it has been thought that minimizing the cooling rates in the Ms transformation region is essential for
598 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK Dana collected at 1 HZ
60 Time (sec.) 40
20
80
60
100
120
140
Cooling rate (C/sec.,
Data collected at 2.5 HZ 800 • _^^ ^ 600 w 3
13 w 400 Q. E V
1-
200
w
n 0
i
« • • • • -• * « -• •
1
11
10
2G
20
40
t
30
60
80
•
40
100
1
»
50 120
60 Time (sec.) 140
Cooling rate rc/sec.j
Daita collected at 5 HZ
60 Time (sec.) 0
20
40
60
80
100
120
140
Cooling rate fCsec.)
FIG. 17—Effect of data acquisition rate on cooling rate curve smoothness.
CHAPTER 22: NON-LUBRICATING
PROCESS FLUIDS: STEEL QUENCHING
minimizing cracking and distortion. This is desirable, but not necessarily a sufficient, condition for reduction of quench cracking. Zhelokhovtseva [26] has shown that uniformity of cooling in the Ms region is also critical parameter. It seems reasonable, therefore, that minimal and uniform cooling rates to minimize stresses that occur due to thermal gradients are probably the necessary and sufficient conditions for optimizing quenchants in the Ms temperature region.
TECHNOLOGY
599
Hardening Capability (Power) Segerberg [27] has developed an empirical approach for determining of hardening power (HP) of quench oils without agitation. Segerberg's hardening power approach utilized cooling curves obtained with the Wolfson probe shown in Fig. 19 (see ASTM D 6200). After the cooling curve was determined according to ASTM D 6200 (or ISO 9950), statistical correlation of hardness and cooling rate data provided the following regression equation: HP = 3 . 5 4 C R F -1- 12.30 CRM -
168
Where: CR^ is the cooling rate at 550°C (990°F) which is the at, or at least near, the nose of the pearlitic transformation for many steels and CRM is the cooling rate at 330°C (595°F), which is near the Ms temperature for many steels [5]. Segerberg [27] used this procedure to predict the asquenched hardness achievable with a commercial quench system, by strategically placing a series of instrumented steel probes throughout a typical quench load. The hardening power at these various positions was calculated from the above regression equation. Using the correlation curve for the steel of interest (e.g., the curve for the AISI 1045 steel shown in Fig. 20, it is possible to predict the hardness for steel bar with a 16 mm Dia. (0.6 in.) cross section. A similar procedure can be used for other alloys and different cross section sizes. Shock Film Boiling Time FIG. 18—Critical cooling curve parameters.
-30mm
30mm-
Full-film boiling, nucleate boiling, and convective cooling heat transfer modes have been studied by various authors
M—6mm
9.5mm 13mm (12.5mm after - f finish grinding) 1.5mm tight push fit "nominar
Not to be center drilled Probe details
Probe body Details Infig.3(a^
Finish grind
Support tube Material - Inconel alloy 600 Tl£^t fit on probe end iwith 30° angle weld allowance Dimensions as (a)
12.5 ± 0 . 0 1 mm
Mineral Insulated thermocouple Type'K-CNiCr/NIAI) Sheath materlaNlncone) alloy 600 Diametei=1.5mm; Route length190mm min; Talls-25mm Hot iunction~lnsulated; cold seal= epoxy resin
End support -tube Material» Stainless steel
Termination Standard tttermocouple typeV(NlCf/NlAI)
General assembly FIG. 19—^Wolfson Inconel 600 probe.
600 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK Acoustical
i I
60 •
50 DC
-o 40 (0 X
30 »
HP == 3.54 CRp + 12.30 CR|y| - 168
20
— •
0
200
400 600 800 1000 Hardening power (HP)
FIG. 20—Quenchant classification by hardening power.
[31-34]. A m u c h less studied and understood heat transfer process is shock-film boiling. Shock-film boiling refers t o the initial formation of the vapor film around the hot metal upon initial immersion. In most experiments, this cooling process is not detected unless surface temperature-time measurements Eire made or if alternative experimental methods cire used to detect this unstable b u t important heat transfer mode. The cooling capacity of quenchants during full-film boiling, nucleate boiling, a n d in one-phase convection is characterized by critical heat flux density a n d by the heat transfer coefficient (or specific heat flux) [28-29]. Nonstationary heat processes: shock boiling, full-film boiling, transition boiling, nucleate boiling, a n d convection heat transfer have been readily detected by conventional cooling curve analysis [28,30]. Except for shock-film boiling, all of these cooling processes may occur simultaneously o n the hot metal surface [30,31]. Determination of the first a n d second critical heat flux densities are necessary to properly explain these cooling modes, [35,36]. Heat trzinsfer in these boiling regimes is characterized by two values. One is the first critical heat flux density (?cri), which typically occurs within approximately 0.1 s after initial immersion of the h o t metal (shock-film boiling). The next step is full-film boiling. The second critical heat flux ilcri) is the m i n i m u m heat flux at which the transition from full-film boiling to nucleate boiling will occur. BCruzhilin has shown that the ratio of qci2 I ^cri is a constant [37]. Kobasko et. al. have determined that this constant is 0.204-0.207 for water [35], If the constamt is known, the difficult to observe qcri c a n b e calculated from t h e m o r e easily observed a n d measured g'cr2-
Measurements
A schematic of the experimental device is shown in Fig. 21 [38]. The working firequency of the acoustical detector is 0-20 kc, which is further divided into 200 channels. The width of each channel is 100 c/s. E a c h heat transfer mode shockboiling, full-film boiling, a n d nucleate boiling is observed acoustically at characteristic frequencies. These are summarized in Table 4. Note: ASTM E 131 defines frequency as "the number of cycles (c) per unit time." ASTM E 349 states "When the independent variable is time, t h e unit of frequency is the hertz. Symbol: Hz (1 H z = P ' ) . (This unit is also called "cycle per second c/s".) The term "kc" means kilicycles and as used in the above discussion, 0-20 kc means 0-20 000 cycles. Sound amplitude is proportional to the intensity of the boiling process. High frequencies can be used to detect the initiation of shock boiling. The time difference corresponds to the duration of t h e full-film boiling. Knowing t h e r m o physical properties of the standard sample and duration of full-film boiling, the second critical heat flux density can be determined [35,36]. From the duration of the fuU-film boiling and nucleate boiling phases a n d the second critical heat flux density it is possible to characterize quenching capacity. A 20 m m silver probe has been utilized, which had a soldered 0.1 m m chromel-alumel thermocouple inserted t o in the center of the probe, as shown in Fig. 22. The thermocouple was in a 1.5 m m metal case filled with insulating materisJ. The thickness of the case wall is 0.1 m m . The piezoceramic sensor for acoustic noise detection was
FIG. 21—Schematic illustration of acoustical noise analysis measurement device: 1. Furnace, 2. Quenchant, 3. Probe, 4. Thermocouple, 5. Transducer, 6. Amplifier, 7. Acoustic analyzer, 8. Glass fiber system, 9. Computer,10. Printer, 11. Time analyzer.
TABLE 4—Characteristic acoustic frequencies for various heat transfer modes. Heat Transfer Mode
Frequency Mode (kc)
Shock-film boiling Full-film boiling Nucleate boiling
9-20 0.05-1.5 0.5-4.0
CHAPTER 22: NON-LUBRICATING PROCESS FLUIDS: STEEL QUENCHING TECHNOLOGY 601 located at equal distances between probe and wall of the container with the liquid to be investigated as shown in Fig. 21. The quenchant was placed in a container of 150 mm diameter and height of 270 mm. The signal from piezoceramic sensor is transmitted to amplifier (6) and then to analyzer (7)—see Fig. 21—of acoustic signals connected to a digital voltmeter and computer. With this apparatus, in addition to acoustic analysis, cooling curve temperature-time analyses are also conducted. With this data, the duration of the full-film boiling, second critical heat flux density, heat transfer coefficient, and the duration of non-stationary nucleate boiling are determined. These values are sufficient for the characterization of quenching capacity. The results obtained by this method are presented on Fig. 23. The silver probe was cooled from 800°C in 20% aqueous solution of a poly(alkylene glycol) quenchant. The start of full-film boiling and transition to the nucleate boiling can be detected by conventional time-temperature cooling curve analysis. However, acoustical analysis
1.S mm
FIG. 22—Illustration of cast spherical silver probe.
9,IW
lot, 4r
i5o:
lOS
A136 100
sn
I A 2
jxi.m iot,i
FIG. 23—Comparison of quenching properties of aqueous PAG polymer quenchant (concentration 20% at 20(C). A5 and A136 correspond to 0.5 and 13.6 kc.
602
MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK was required to observe the transition from shock- film boiling via full-film boiling to nucleate boiling. Quench Oils Quenching oils are analogous to other petroleum-derived products including engine oils, spindle oils, and industrial lubricating oils such as gear lubricants [39]. Although petroleum oils are usually refined for specific applications, they remain complex mixtures with a variety of possible compositions, which may vary even when produced by a single refinery. The compositional complexity of quench oils affects their quenching performance. Segerberg [40] compared a series of mineral-oil-based quenchants, which were tested by cooling curve analysis according to ISO 9950 (ASTM D 6200) and obtained a wide variety of cooling rates, as shown in Fig. 24. Formulated oils can produce an even wider range of cooling rates. Quenching oils that contain substantial quantities of naphthenic derivatives usually exhibit inferior cooling characteristics, greater deposit-forming tendency, and lower flash points than paraffinic oils. Tensi [41] has shown that the quench severity of a particular quench oil is directly related to its ability to wet a metal surface. The wettability of a quench oil is quantified by measuring "rewetting" times, as shown in Fig. 25.
O o 3
«
a E
Quench Oil Classification For commercial acceptance, a quench oil must exhibit a number of important properties including: • Acceptable flash and fire points, • Low sludge formation • Nonstaining of parts, • Appropriate heat removal properties. • Oxidation Resistance
Cooling R a t e . ' C FIG. 24—Potential variation of the severity of different quench oils.
10
30
50
70
90
Distance from end (mm) FIG. 25—Wetting time and surface hardness of 1045 steel as a function of distance from the end of the specimen.
CHAPTER 22: NON-LUBRICATING PROCESS FLUIDS: STEEL QUENCHING TECHNOLOGY Most quench oils are derived from refined petroleum-base stocks. Higher naphthenic fractions usually result in lower flash points and greater sludge formation. Sludge formation reduces heat transfer efficiency, which may result in inadequately hardened parts. Increased sludging cJso reduces oil flow t h r o u g h heat exchangers used to cool the oil during quenching. Ideally, a quench oil should not stain parts. High-quality paraffinic oils impart a light gray color to quenched parts. Sulfur-containing derivatives in the oil cause unacceptable black stains. Some quench rate enhancing additives may also cause staining. However, the color of a quench oil is not indicative of its staining tendency. Since quench oils are flammable, there is always the potential to catch fire in use. The flash point of an oil is used as an indicator of its tendency to ignite. Maximizing the flash point minimizes the fire hazard. Increasing the naphthenic content of an oil usually decreases the flash point. Quench oils are selected on the basis of their ability to mediate heat transfer during the quench. Oils are classified on the basis of quenching speed and temperature of use. ISO
Code Letter
General Application Heat Treatment
6743 provides a classification code for veirious quench media including quench oils, which is shown in Table 5. The three major quench oil classifications are [ 4 2 ^ 5 ] : • Conventional (nonaccelerated) oils, or cold oils, • Accelerated oils, • Marquenching oils, or hot oils. Various characterization criteria for these oils are summarized in Table 6 [46]. The differences in cooling curve profiles for typical m e m b e r s of these oils classifications are illustrated in Fig. 26 [46]. Conventional quenching oils are typically mineral oils, which m a y contain antioxidants to reduce oxidation a n d thermal degradation. Most of these oils have viscosities in the range of 100-110 SUS (Saybolt Universal Seconds) at 40°C (100°F), although some have viscosities u p to 200 SUS at 40°C (100°F). Accelerated quenching oils are usually formulated from a mineral oil and contain one or more additives to increase cooling rates during the high temperature portion of the cooling process. This is accomplished by using an additive that
TABLE 5—ISO 6743 Quenchant classification system. Product Type More Specific Particular and/or Application" Application Performance Requirements Oil for heat Cold Hardening Oil for normal hardening Treatment e < 8o°c Oil for quick hardening Semi-hot hardening 8O°C<0< ISCC Hot hardening 130°C
Oil for Oil for Oil for Oil for Oil for Oil for
normal hardening quick hardening normjd hardening quick hardening normal hardening quick hardening
Vacuum hardening Other cases Aqueous Fluids for Heat treatment
Surface hardening
Mass hardening
Molten Salts for Heat
Gas for heat treatment
Other cases 150°C<700°C Other cases
603
Water Aqueous fluid Aqueous fluid Water Aqueous fluid Aqueous fluid
for slow hardening for quick hardening for slow hardening for quick hardening
Molten salts 150°C<6»<500°C Molten salts 500°C
Symbol ISO-L UHA UHB UHC UHD UHE UHF UHG UHH UHV UHK
Remarks Certain oils may be easily eliminated by washing with water. This characteristic is brought about by the presence of emulsifiers in the oil formulation. Such oils are then known as "washable." It is up to the supplier at the request of the ^^'^ user, to provide this characteristic*
UAA UAB UAC UAA UAD UAE UAK USA USB USK UGA UGB UGC UGD UF UK
Fluidized Bed Other cases "8 indicates the iluid temperature at the time of hardening; ''The washing solution used to remove the emulsified oil may be either detergent (soap) and water or water-borne alkaline cleaners. The cleaning conditions shall be provided by the oil supplier.
604
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
TABLE 6—Quench oil physical property characteristics. Bath Temperature
Typical GM
°F
Typical Viscosity at 40°C (100°F) SUS
340 355 570
105 94 700
16.0 10 30
Flash Point
Type of Oil
°C
op
Conventional Accelerated Marquenching
<65 <120 <200
<150 <250 <400
°C 170 180 300
Time (s)
1600 1400 1200 G?
N ^^Conventional V quench oil \ 150°F \ \ \ Martempering oil 500 °F
Fused salt 500 °F
«
2 1000 2
1
800 600
High speedquench oil 150'F
400 • 0.7
1.0
1.5
2
3
4
5
7
10
"^.^ ^ > ^ '^Molten "^.^ N. metal '^O'^ 500 °F 15
20
30
40
60
Time (sec) FIG. 26—Comparison of cooling curves for various quench media. facilitates surface wetting, which destabilizes the film boiling process. Examples of additives that may be used to accomplish this include: calcium naphthenate [47], alkenyl succinate [47], and sodium sulfonate [48]. Viscosities of these oils may vary from 50-100 SUS at 40° (100°F). For some steels, quenching at an elevated temperature is necessary to reduce thermal and transformational stresses that may lead to cracking and increased distortion. One process that may be used to accomplish this is martempering (or marquenching). In meirtempering, metal is usually quenched from the austenitization temperature to a temperature just above (see Fig. 27B) the stcirt of martensite transformation (Ms) just long enough for the temperature to equalize between the surface and the center of the steel. The metal is then removed from the quench bath and air-cooled. The thermal cycle for both conventional quenching Eind martempering is illustrated in Fig. 27 [1]. Martempering or hot quenching oils are used at temperatures between 95-230°C (200 a n d 450°F). They are usually formulated from solvent-refined mineral oils with a very high parafBnic fraction to optimize oxidative and thermal stability. Stability is enhanced by the addition of antioxidants. Nonaccelerated and accelerated martempering oils are available Typical t e m p e r a t u r e ranges for m a r t e m p e r i n g oils are shown in Table 7 [46]. Because martempering oils are used at relatively high temperatures, a protective, nonoxidizing atmosphere over the oil is typically used, which allows operation closer to the flash point of the oil t h a n if open-air conditions are used [49]. A protective atmosphere is any atmo-
sphere that will inhibit oxidation of the meted surface during austenitization, prior to quenching. A protective atmosphere may be an inert gas such as nitrogen or argon or it may be a gas used for a heat treating furnace. These same atmospheres Eire also used to protect the quenching oil, especially martempering oils, from the oxygen in air during use, thus reducing the rate of sludge formation and increasing lifetime of the quenching oil. Classification
by GM Quenchometer
Cooling
Times
Cooling rates produced by quench oils have been classified on the basis of the General Motors (GM) quenchometer test, also known as the "nickel ball" test [50,51]. Details of this test Eire provided in Test Method ASTM D 3520. This test involves heating a 22 m m (7/8 in.) d i a m e t e r nickel ball to 885°C (1625°F) and then dropping it into a wire basket suspended in a beaker containing 200 m L of the quenching oil at 21-27°C (70-80°F) [52]. A timer is activated as the glowing nickel bEJl passes a photoelectric sensor. A horseshoe magnet is located outside the beaker as close as possible to the nickel ball. As the ball cools, it passes t h r o u g h its Curie point (354°C, or 670°F), the temperature at which nickel becomes magnetic. When the ball becomes magnetic, it is attracted to the magnet, activating a sensor that stops the timer as illustrated in Fig. 28 [46]. The cooling time required to reach the Curie temperature is sometimes referred to as the "heat extraction rate" or "quenchometer time." Table 8 provides t } ^ ical GM quenchometer times for the different quench oil classifications. [46,53,54]
CHAPTER 22: NON-LUBRICATING
PROCESS FLUIDS: STEEL QUENCHING
Although the GM quenchometer test is commonly used for evaluating quench oils, the quenchometer times are based on a relatively limited portion of the total cooling process and therefore are not related to as-quenched hardness or the propensity for cracking. In fact, this test only defines a heat removal rate, regardless of variation of cooling pathway, over the high-temperature portion of the cooling curve. Aqueous Polymer Quenchants A number of polymers have been used as polymer quenchants for heat treating applications [55]. These include: poly(vinyl alcohol) [56], poly(alkylene glycol) [56], poly (acrylamide) [58], cellulosic derivatives [59], polyvinylpyrrolidone [60], poly(sodium aciylate) [61], and poly(ethyl oxazoline)
Surface
TECHNOLOGY
[62]. Of these, the most commonly used quenchants are poly(alkylene glycol)—PAG, poly(vinyl pyrrolidone)—PVP, and poly(ethyI oxazoline)—PEOX. However, the most commonly encountered quenchant for induction heat treating applications worldwide are PAG-type quenchants. Some water soluble polymers that are used as quenchants undergo a phase separation as the solution temperature is increased; the hydrogen bonds break and the polymer chain coils upon itself and separates into two phases [63]. The temperature where this occurs is the separation temperature or cloud point. PAG pol3Tners separate as hydrates and the degree of hydration is dependent on the temperature of the solution, as shown in Fig. 29 [63]. In addition to solution temperature, the degree of polymer hydration for PAG polymers is dependent on the ethylene oxide/propylene ratio in the
Surface
Center
Center
Time
(b)
!
I o o. E
FIG. 27—Types of quenching cycles: (a) conventional quenching and tempering, (b) martempering, (c) isothermal quenching and tempering, and (d) austempering. TABLE 7 --Typical use temperatures for martempering oils. Use Temperature
Minimum Flash Point
605
Viscosity at 40°C (100°F) SUS
"C
°F
°C
°F
250-550 700-1500 2000-2800
220 250 290
430 480 550
95-150 120-175 150-205
200-300 250-350 300-400
Open Air
Protective Atmosphere »F °C 95-175 120-205 150-230
200-350 250-400 300^50
606 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK polymer chain. Generally, increasing propylene oxide content in the copolymer decreases the separation temperature. Poly(ethyl oxazoline)—PEOX, a n o t h e r polymer currently used in the heat treating industry—also exhibits a sepeu-ation temperature [63]. Cloud point behavior of polymer quenchants can be used as an inexpensive way to purify contaminated (by salt, or hard metal ions from h a r d water, for example) quench bath. Raising the temperature of the aqueous solution above its separation temperature as shown in Fig. 29, causes it to separate into two layers. The upper layer, usually the contaminated aqueous s u p e r n a t e n t solution, is removed a n d the lower layer of the purified polymer is then redissolved to the required concentration [64]. Nonionic polymers are usually used since ionic polymers, such as poly (sodium acrylate), typically precipitate in the presence of hard metal ions. The most c o m m o n examples of nonionic quenchant polymers are PAG, PVP and PEOX [64]. C o o l i n g Curve A n a l y s i s - I n t e r n a t i o n a l S t a n d a r d s Cooling Curve Analysis Test oil Nickel ball Quench (heated)
Curie point
FIG. 28—Schematic illustration of a GM Quenchometer and the principle of operation.
TABLE 8—GM Quenchometer time classification of quench oils. GM Quenchometer Time (s) Classification Fast Oil 8-10 Medium Oil 11-14 Slow Oil 15-20 Martempering Oil 18-25
Separation temperature "C 80 90
Water iri separated PAG layer, %
°F (175) (195)
70 46
Heat 100 0(212°?)
Water with contaminants Separated polymer quenchant
Homogeneous auenchant solution FIG. 29—Thermal separation of an aqueous solution of a polyalkylene glycol quenchant.
Standards
Although numerous sizes, shapes, and metals have been, and continue to be, instrumented for cooling curve analysis, most national and international standards have utilized a nickel alloy such as Inconel 600 or silver as probe materials. One of the earliest national standards that utilized a silver probe with a surface thermocouple was JIS K 2242, schematically illustrated in Fig. 30 [65]. Since this probe is very sensitive, it can sometimes be very difficult to obtain reproducible data. To obtain greater data stability and to provide a material with heat transfer properties much more similar to steel, a reusable probe based on an Inconel 600 alloy that does not undergo phase transformation was used [66,67]. This probe has the additional advantage of being resistEint to oxidation and was used to develop the widely used Wolfson Heat Treating Centre Engineering Group Specification [66]. It was subsequently used as the probe for ISO 9950 and ASTM Methods D 6200, D 6482, and D 6549 is shown in Fig. 19. A n u m b e r of groups have traditionally not accepted the use of either an Inconel or a stainless steel probe for cooling curve analysis. They prefer the use of a high thermal conductivity material that does not exhibit phase transformations such as silver. There are currently at least two national standards that utilize a silver probe. One is the French standard AFNOR NFT - 60778 and the other is the Chinese standard ZB E 45003-88. These two probes differ from each other primarily in size. The French probe is 16 m m . dia. X 48 m m and the Chinese probe is 10 m m dia. X 30 m m . They are illustrated in Figs. 31 and 32, respectively [65]. Table 9 provides a comparison of the experimental requirements for relevant standards [65]. To maintain probe CcJibration and to assure interlaboratory reproducibility, it is essential that the probe be periodically calibrated during use. This process requires the use of a calibration fluid for which there are specified cooling properties. The JIS K 2242 and the Chinese standard ZB E 4500388 utilize a JIS K6753 "standard liquor" which is: di (2-ethylhexyl) phthalate. Although available in > 9 9 % purity, the toxicity properties of this fluid m a k e it undesirable for widespread and routine use. The Wolfson Standard [66], ISO 9950, and the AFNOR NFT 60778 utilize a "standard" min-
CHAPTER
22: NON-LUBRICATING
PROCESS
FLUIDS:
STEEL
QUENCHING
TECHNOLOGY
Insulating lube
607
il
s
S i l v e r wire
Pipe made of silver Heat resistanl insulator
M6 X pitch 1
Alumel wire
M> X pitch 0 . 5
Dead
M5 X pitch 0 . 5
> <
«o
Insulating tube
FIG. 30—Japanese JIS K 2242 silver probe (All dimensions are in mm). eral oil whose physical properties are summarized in Table 10 [65]. The ASTM Test Method D 6200 is based on the Wolfson standeird [65] and ISO 9950, except the specification limits are controlled more tightly and were based on various statistically designed experimentation as required by ASTM [68-70]. Development of Cooling with Agitation
Curve Analyses
Methods
Many quenchants, a n d nearly all aqueous polymer quenchants, cannot be evaluated reproducibly without the use of agitation. Such quenchants always require the use of agitation w h e n used industrially in various heat-treating processes. All of the cooling curve methods discussed d o not employ agitation and are essentially limited to analysis of minercd oil derived quenchants. There are two ASTM stcindards for cooling curve analysis with agitation. One method is ASTM D 6482 and the other is ASTM D 6549. Both test methods utilize the Wolfson probe illustrated in Fig. 19. Test Method D 6482 uses an agitation
device, which is shown in Figs 33. [65,71] and 34A. This device is constructed from a transparent material such as Plexiglas or glass and holds approximately 1.5 L of quenchant. Agitation is provided by a plastic impeller of 50 m m dia. and pitch of 42 m m [72]. One of the advantages of this agitation system is the uniformity of turbulence and flow throughout the quench zone as shown in Fig. 35 [71]. This means that the sensitivity to error with respect to precise placement of the probe in the quench zone will be minimal. Test Method ASTM D 6549 is significantly different from D 6482 in that p u m p agitation is used. This is illustrated in Fig. 34B. This p u m p agitation system is based on earlier work performed by N.A. Hilder at Aston University [73]. Probe placement in the fluid reservoir during quenching is illustrated in Fig. 36. Quench Bath Mamtenance Sampling Flow is never uniform in agitated quench tanks. There is always variation of flow rate and turbulence from top to bot-
608
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
torn and across the tank. This means that there may be significant variations of particulate contaminants including sludge from oil oxidation and metal scale. For uniform sampling, a number of sampling recommendations have been developed: 1. Minimum Sampling Time—The circulation pump shall be in operation for at least 1 h prior to taking a sample from a quench system.
pef^etofv Ofobecamg.
Thermocoupfe0l
2. Sampling Position—For each system, the sample shall be taken from the same position each time that system is sampled. The sample shall be taken at the most practical point of maximum flow turbulence. The position in the tank where the sample is taken shall be recorded. 3. Sampling Valves—If a sample is taken from a sampling valve, then sufficient quenching oil should be taken and discarded to ensure that the sampling valve and associated piping has been flushed, before the sample is taken. 4. Sampling From Tanks With No Agitation—If samples are to be taken from bulk storage tank or a quench tank with no agitation, then samples shall be taken from the top and bottom of the bulk system or quench tank. If this is not possible and the sample can only be taken from the top, then the Laboratory Report shall state that the results represent a sample taken from the top of the bulk system or quench tank and may not be representative of the total system. 5. Effect of Quenching Oil Addition as Make-Up Due to DragOut—It is important to determine the quantity and frequency of new quenchant additions, as large additions of new quench oil will have an effect on the test results, in particular the cooling curve. If a sample was taken just after a large addition of new quench oil, this shall be taken into consideration when interpreting the cooling curve of this oil sample. 6. Sampling Containers—Samples shall be collected in new containers. Under no circumstances sheJl beverage or food containers be used. R e c o m m e n d e d Test Procedures-Quench Oils (ASTM D 6710) Physical and Chemical Properties
FIG. 31—French AFNOR silver probe (All dimensions are in mm).
• m* • •
'•• < l i
iJ
Kinematic Viscosity, (Test Method D 445)—The performance of a quench oil is dependent on its viscosity, which varies with temperature and oil deterioration during continued use. Increased oil viscosity typically results in decreased heat transfer rates [74]. Oil viscosity varies with temperature, which affects heat transfer rates throughout the process. The flow velocity of a quench oil depends on both viscosity and temperature. Some quench oils are used at higher temperatures, such as martempering oils, also known as hot-oils.
1B.m.4>W»i
m
' "• "^ 'J!^ " ^ ""^ ••p* 4 « • « • • . « • *fc w w * . nMk •mmt mm* mmtmmm^m
s\\\\ 30ia.26
mm mm i » . »
•!
v'<' < ^ " ^ ^ < V '^••"''•••"r
mmm
MiO.fi
I a FIG. 32—Chinese silver probe (All dimensions are in mm).
CHAPTER 22: NON-LUBRICATING PROCESS FLUIDS: STEEL QUENCHING TECHNOLOGY 609 TABLE 9—Comparison of cooling curve standards for unagitated quench oils. AFNOR (NFT 60778)
Variable
ISO 9950
Probe Alloy
Inconel 600
Probe Dimensions Standard Reference Oil Vessel Dimensions Oil Volume Oil Temperature
12.5 m m Dia. X 60 m m See Table 10
Silver 99.999% pure 16 m m Dia X 48 m m See Table 10
115 ± 5 m m diameter 2000 mL 40°C ± 2°C
height 138 m m X diameter 99 m m 800 mL 50°C ± 2°C
850°C ± 5°C
800°C ± 5 X
Probe Temperature
JIS (IS 2242)
GB (ZB E 45003)
Silver 99.99% pure 10 m m Dia X 30 m m Dioctyl phthalate 300 mL beaker
Silver 99.96% pure 10 m m Dia X 30 m m Dioctyl phthalate 300 mL beaker
250 mL 80''C,120°C, 160°C 810°C ± 5°C
250 mL 80°C ± 2°C
See Table 10 115 ± 5 m m diameter 2000 mL 40°C ± 2°C
810°C ± 5°C
850''C ± 5°C
ASTM D 6200 Inconel 600 12.5 m m Dia X 60 m m
TABLE 10—Physical properties of the so-called "Wolfson Oil" used as a reference for ASTM D 6200, ISO 9950, and AFNOR NRT 60778 standards. Institute of Petroleum Test Minimum Maximum Method/ASTM Method Value Physical Characteristics Value Kinematic Viscosity at 40°C, cS" 23.0 19.0 IP 71/D 445 Kinematic Viscosity at 100°C, cS" 4.4 3.9 IP 71/D 445 Viscosity Index 105 IP 226/D 2270 95 Density at 15°C, kg/L IP 160/D 1298 0.855 0.870 Flash Point PMCC, "C IP 34/D 93 210 190 5% Distillation, °C IP 123 360 330 50% Distillation, °C IP 123 420 400 ' 1 cS = lO"** m^/s.
Although the viscosity of a martempering oil may not fluctuate substantially at elevated t e m p e r a t u r e s , the oil may become almost solid u p o n cooling. Thus, the viscositytemperature relationship of a quench oil may be critically important from the dual standpoint of quench severity and flow velocity. Typically kinematic viscosity determination by Test Method D 445 is used. Viscosity measurements are made at 40°C (100°F) for conventional or accelerated oils and also at 100°C (212°F) for martempering oils. Flash Point and Fire Point (Test Methods D92, D93, D13I0)— Use of a quench oil at a temperature near its flash point in an unprotected environment, or near its fire point, may result in an oil fire. General guidelines have been developed for usetemperatures of a quench oil relative to its flash point. N o t e 2: There are various manufacturer-dependent guidelines for relating the suitability for use of a used quenching oil with respect to its flash point and they shall be followed. In the absence of such guidelines, it is recommended that the use temperature of a quenching oil in an open system with n o protective a t m o s p h e r e shall be 60-65°C (108-117°F) lower than the actual open-cup flash point. In closed systems where a protective atmosphere is used, the use temperature of the used quenching oil shall be at least 35°C (60°F) lower than its actual open-cup flash point. Specific Gravity (Test Method D287)—Specific gravity depends on the chemical composition of the base stock used to formulate a quenching oil. The oxidative stability of the quenching oil is dependent on the chemical composition, such as naphthenic/paraffinic ratio. Therefore, the specific
gravity of a quench oil is an indirect measure of its inherent oxidative stability. N o t e 3: Specific gravity is of limited value in monitoring the quality of a formulated or used quenching oil. The Test Method D 287 for specific gravity requires the use of both a hydrometer and an accurate thermometer or a thermohydrometer. Because specific gravity is temperature dependent, the temperature of the oil at the time of measurement must be determined precisely. Aged Fluid Properties-Quench
Oils
Acid Number (Test Methods D 664 and D 974)—Quench oil oxidation results in the formation of carboxylic acids and esters. These b5fproducts are similar to compounds that may be used as rate accelerating additives. These acids and esters significantly affect the viscosity and viscosity-temperature relationship of the oil, which in turn affect quench severity. Carboxylic acids may also act as wetting agents and increase the quench rate by increasing the wettability of the quench oil on the metal surface [77]. Oxidation of the oil may be monitored by tracking changes in the neutralization number. Because the fresh oil may be either alkaline or acidic, depending on the additives present, the absolute value of the neutralization number itself is not indicative of qucJity. However, changes in the neutralization n u m b e r from the initial condition may be used to indicate the degree of oxidation. Increasing neutralization numbers generally indicate increasing amounts of aforementioned byproducts. The acid n u m b e r (AN) is determined by titrating the acidity of a sample of known size with a known amount of stan-
610
MANUAL
3 7: FUELS
AND LUBRICANTS
HANDBOOK absence of such a value, it is recommended that the AN not exceed 2.00 mg/g for a used quenching oil. Infra-Red Spectroscopy—^An alternative method that is being used increasingly to identify and quantify oil oxidation, even in the presence of additives, is infra-red (IR) spectroscopy. Figure 37 provides an illustration of the use of IR spectral cmcJysis to identify oil oxidation [77]. Mang zmd Jiinemann monitored the IR stretching vibrations o f C B O a t l 7 1 0 c m ~ \ for carboxylic acids contained in oxidized oil. IR analysis has been used to detect and qucintify other carbonyl-containing compounds [78]: Metal carboxylate salts-1600 and 1400 c m " ' Carboxylic Acids-1710 c m " ' Metal sulfates-1100 and 1600 cm ' Esters-1270 and 1735 cm ' Saponification Number (Test Method D 94)—Oil degradation may produce both acids and ester by-products. The acid number quantifies the amount of acidic degradation by-products in the oil, whereas the saponification number is a measure of the presence of esters or fatty esters in the oil. The saponification number of an oil is determined (Test Method D 94) by heating a sample of the oil with a known amount of basic reagent and measuring the amount of reagent consumed. Because some quench oils are formulated with components that also have saponification numbers, it is necessciry to monitor trends over time than to rely on an absolute value [81]. An increase in the saponification number indicates an increased propensity to sludge formation. It has been suggested that if the results of other tests are satisfactory, that saponification numbers below 3 mg KOH/g oil may be acceptable [75,82]. Contamination-Oil
FIG. 33—Schematic illustration of the Tensi agitation system (all dimensions are in mm).
dard base (Test Methods D 664 or D 974). The test is performed by dissolving the oil in a mixture of toluene and isopropanol, then titrating it with a standard solution of potassium hydroxide (KOH). The end-point may be determined colorimetrically with a pH-sensitive indicator. The acid n u m ber (AN) is reported in units of milligrams of KOH per g r a m of sample (mg/g). Note 4: The quenching oil supplier will provide a maxim u m TAN value for the quenching oil being used. In the
Quenchants
Water Content (Test Methods D 95, D 1744, D 4007)—TYie presence of water in a quench oil, which may be present due to condensation or a leaking heat exchanger, presents a potentially serious problem. Water concentrations as low as 0.1% may cause the bath to foam during the quenching process, greatly increasing the risk of fire. Overflowing oil from the foaming bath may result in a more serious fire than if the flames were contained by the bath, as the oil may contact nearby furnaces or other ignition sources. If a sufficient amount of water accumulates in a hot bath, an explosion caused by steam generation m a y result [53]. Note 5: The problem of water contamination in the quench bath, with respect to foaming is illustrated in Fig. 38 where it is shown that 1 mL of water becomes 1700 mL of vapor when evaporated (near instantaneously). Note 5: The amount of foaming that does occur is often dependent on the degree of agitation. Some baths may be agitated to the point where the quenching oil is ncEirly splashing on the floor. In such baths, the water vapor is released even faster causing a greater potential foEiming problem. The presence of water in a quench oil may also produce variable cooling properties depending o n the nature a n d amounts of cooling rate-accelerating additives present in the oil. The magnitude and direction of these effects depend on the particular quench oil and the amount of water present in the oil. Water contamination may also result in staining of the part being quenched, uneven hardness, and soft spots.
CHAPTER
22: NON-LUBRICATING
PROCESS
FLUIDS:
STEEL
QUENCHING
TECHNOLOGY
611
(b) FIG. 34—Illustration of commercially available quenchant agitation systems: (A) Tensi agitation system, (B) Drayton agitation system.
FIG. 35—The flow strings illustrate that the flow in the quench zone is without bubbles or twist. A. 0.25 m/s and B is 0.6 m/s.
A quantitative test for water contamination involves titration of the oil with Karl Fisher chemical reagent to an electrometric endpoint (Test Method D 1744). This test is recommended for water levels of 50-1000 p p m (less than 0.1%). Higher levels of water contamination may be quantified by distillation (Test Method D 95) or centrifugation of the sample and measurement of the volume of separated water according to Test Method D 4007. Note 6: A c o m m o n qualitative field test for water contamination is the so-called "crackle test," which is conducted by heating a sample of the quenching oil and listening for an audible crackling sound [77]. If the oil is contaminated with water, a crackling sound will be heard before the quenching oil has reached its smoke-point. Carbon Residue (Test Method D189)—One of the greatest problems encountered when using a quenching oil is the formation and accumulation of sludge. Although the various analyses procedures including viscosity, neutralization number, and saponification number may indicate that a quench oil is adequate for continued use, the amount of sludge build-up in the tank may demand that the system be drained and cleEtned. Clccining and sludge disposal cire growing problems for the
612
MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
heat treating industry. Therefore, determination of the sludgeforming potential of a quench oil prior to use is important. One method of quantifying sludge-forming potential involves determination of the Conradson carbon residue (Test Method D 189) measures the polymeric material remaining in the oil after heating to elevated temperatures in the absence of sufficient oxygen to bum off all of the organic com-
pounds present. The Conradson carbon residue is determined by placing a weighed sample in an iron crucible. The crucible is heated with a Meeker-type gas burner to a sufficiently high temperature to evaporate and bum the oil. The sample is forther heated until the bottom and sides of the crucible are cherry red and is held at this temperature for 30 min. The crucible is then cooled and weighed. The amount of
PLAN VIEW FIG. 36—Recommended probe placement in the Drayton centrifugal pump apparatus shown in Fig. 34b (all dimensions are in mm).
CHAPTER 22: NON-LUBRICATING
PROCESS FLUIDS: STEEL QUENCHING
TECHNOLOGY
613
ou c
jQ
<
1600
1700
1800
Wave number, cm - ^ FIG. 37—Infra-red spectral identification of oxidation of a used quenching oil.
waterquantity FIG. 38—Illustration of the volumetric expansion of 1 mL of liquid water to 1700 mL of water vapor.
tar remaining in the crucible relative to the original amount of the oil defines the Conradson carbon residue value. Note 7: Test Method D 189 may be affected by some additives used to formulate quenching oils. Note 8: In some heat treating operations, steel is austenitized in air, which causes the increased formation of metal
oxide scale, which will act as a contaminant in the oil. If this occurs, the Conradson carbon residue number may be abnormally high and misleading. Precipitation Number (Test Method D 91)—Sludge formation in a quenching oil is caused by oxidation of various components, leading to pol3THerization and cross linking reactions. These cross linked and polymerized by-products are sufficiently high in molecular weight to cause them to be insoluble in the oil. Other sources that contribute to sludge are dirt, carbon formation, and soot from the use of a furnace atmosphere with a high carbon potential. Sludge can plug filters and foul heat-exchanger surfaces. The loss of heat-exchanger efficiency may result in overheating of the quenchant and possibly a fire [83]. Increasing sludge formation often indicates increasing oxidation of the oil. In addition, sludge may adsorb on a part, causing nonuniform heat transfer during the quenching process. It is important to maintain particulate contamination to < 1 micron to optimize quenching performance [84]. Precipitation numbers as low as 0.2% may produce staining of normally bright surfaces [46]. However, staining is more commonly observed with precipitation numbers of > 0.5%. Note 9: These sludge levels correspond to acid numbers of > 0.5 mg KOH/g by Test Method D 974. Sludge formation may be accompanied by increased volatile oxidation by-product formation that may cause a simultaneous increase in fire potential. The viscosity of a quench bath also changes with the formation of sludge, affecting both heat transfer and quench severity. The amount
614
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
of sludge can be quantified by adding n a p h t h a solvent to the oil sample a n d determining the volume of precipitate (sludge) after centrifuging (Test Method D 91). Ash Content (Test Method D 482)—^Although mineral oil basestocks possess very low ash values, many formulated quench oils contain metallic components that contribute to ash. If the ash content in a bath filled with a formulated quenching oil is decreasing, it is likely that an ash-containing additive is being removed by drag-out or some other process. If the ash content is increasing, the additive is either accumulating in the bath or metallic contamination is increasing, perhaps in the form of scale accumulation. Ash contents are determined by Test Method D 482, which involves heating a quenching oil in a muffle furnace at 775°C (1425°F) under conditions that b u m off organic compounds but leave metallic species such as metal oxides or hydroxides. Quenching
Properties
of Oil
Magnetic Quenchometer Method (Test Method D 3520)—Cooling rates produced by quenching oils are often classified on the basis of Test Method D 3520, the magnetic quenchometer test illustrated in Fig. 28 [46]. Table 8 provides a summary for quench oil classification by magnetic quenchometer times [46]. Although this is a widely used test, the quenchometer times do not relate to metallurgical properties. This is because only a single cooling time is determined; therefore, there is insufficient information to indicate the actual temperature-time cooling pathway that is required to determine if a specific steel may be successfully hardened. This is illustrated in Fig. 39 [46] Therefore, this test is of limited value, with the possible exception as a quench oil classification test. Guisbert and Moore conducted an extensive study in an at-
tempt to identify some method of correlating a portion of a cooling curve with GM Quenchometer times [69,70]. The results of this work showed that although some correlations were obtained, they were not particularly good. Bates and Totten have also been unable to find a correlation [84]. Cooling Curve Analysis (Test Method D6200 and ISO 9950)— The most c o m m o n method in use throughout the world to evaluate the cooling properties of a quenching oil is cooling curve analysis. Cooling curve analysis provides a cooling time versus temperature pathway, which is directly proportional to physical properties such as hardness, obtainable u p o n quenching of metal. The results obtained by this test may be used as a guide in heat treating oil selection or comparison of quench severities of different heat treating oils, new or used. Cooling curve analysis of a quenching oil according to Test Method D 6200 and ISO 9950 is conducted by placing the probe assembly illustrated in Fig. 19. into a furnace and heating to 850°C (1562°F). The heated probe is then immersed in a 2000 mL volume of the quenching oil, typically at 40°C, or other preferred use temperature. Lower quenchant volumes, as low as 700 mL, have been used for compeirative analysis. The t e m p e r a t u r e inside the probe assembly and cooling times are recorded at selected time intervals to establish a cooling temperature versus time curve. From the temperature-time curve, the cooling rate is derived. A series of cooling rate curve comparisons illustrating the effect of oil oxidation on a conventional quench oil and an accelerated quenching oil are illustrated in Figs. 40A and 40B, respectively. The effect of water contamination on a conventional quenching oil and an accelerated quenching oil are illustrated in Figs. 41A and 41B, respectively. The maximum cooling rate will shift in proportion to the water content of the oil.
The GM Quenchometer time is the time required (f- f ) to cool " nicicei tiail from 885-354 Xi
Time (Sec.) FIG. 39—Illustration of the Inability of a single cooling time value to successfully predict the outcome of a steel hardening process where a cooling temperature-time curve is required.
CHAPTER 22: NON-LUBRICATING PROCESS FLUIDS: STEEL QUENCHING TECHNOLOGY 615 SOO
4months' oxidation
Smonttu' oxidation '
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100
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(b)
140
Coolingrate(°C/s)
(a)
FIG. 40—Illustration of tlie effect of oil oxidation on tfie cooling rate curve for (A) a conventional quenching oil and (B) an accelerated quencliing oil.
Accelerated-speed oil
' • 1 " -
...|...j....
No contamination •\— 0.01%
water { 2%
No water 0.5% water 1 % water water
• 0.05% water 0.10% water
i 0.20% water -;
J 40
(a)
60
i
80
^ I 100
i 120
i 140
Cooling rate (^C/s)
Cooling rate (°C/s)
FIG. 41—Illustration of the effect of water contamination on the cooling rate curve for (A) a conventional quenching oil and (B) an accelerated quenching oil.
(b)
616
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
Performance-Related Physical and Chemical Properties-Polymer Quenchants (ASTM D 6666) Appearance—Contamination of aqueous polymer quenchants by such fluids as hydrauhc or quench oils may result in a nonuniform quench with thermal gradients sufficient to cause cracking or increased distortion, or possible staining, of the metal being quenched. The simplest (and an excellent) test is to examine the visual appearance of an aqueous polymer quenchant in a clear glass container such as a bottle [86]. However, if the oil readily separates from the aqueous polymer quenchant solution, it may often be removed by skimming. On the other hand, oil may form a milky-white emulsion that is not readily reclaimed by heat treaters. Other problems that are easy to visually identify include carbon and sludge contamination, which often results in cracking problems. Metal scale contamination is often identifiable by its magnetic properties by placing a magnet on the outside of the bottle next to the scale and determining if the scale exhibits any attraction for the magnet. Carbon, sludge, and scale may be removed from the quenchant by filtration or centrifugation.
1.341
10.0 15.0 QUENCHANT CONCENTRATION (%)
FIG. 42—Illustration of the linear relationship between refractive index and concentration of an aqueous solution of a polymer quenchant.
Refractive Index, (Test Method 1747)—One of the most common methods of monitoring the concentration of aqueous polymer quenchants formulated using poly(alkylene glycol) copolymers is refractive index. As Fig. 42 [86] shows, there is a linear relationship between quenchant concentration and refractive index. The refractive index of the quenchant solution is determined using an Abb6 refractometer (Test Method 1747) equipped with a constant temperature bath. Although the refractive index could potentially be used at any temperature with the control limits of the constant temperature bath, typically either 40°C or 100°F is selected. Although refractive index is a relatively simple and rapid method for determination of polymer quenchant concentration, it is not sensitive to low levels of polymer degradation and it is often significantly affected by solution contamination. Note 10: Refractive index is typically unsuitable for aqueous polymer quenchants formulated with polymers with molecular weights greater than 50000-60 000 because: 1) The total amount of water-soluble polymer in solution is not sufficiently different from the water diluent to provide adequate sensitivity and 2) because the total concentration is relatively low, small changes in polymer concentration may result even from normal use, which imparts significant process effects. However, the corresponding variation in refractive index may not be detectable. Note 11: Although it is most desirable to use an Abbe refractometer because of its sensitivity, this is only practical in a laboratory environment. In the heat treating industry, for tankside monitoring and control, a temperature-compensated handheld refractometer similar to the one illustrated in Fig. 43a and b should be used. The hand-held refractometer is self-compensated for temperatures of 60° and 100°F. Although there are various models available, the most common models provide refractive index readings in Brix degrees over a 0-30° range. Typically, the smallest scale that can be read directly is in divisions of 0.2° as shown in Fig. 44. A concentration-refractive index curve obtained by a hand-held refractometer is shown in Fig. 45 [86].
FIG. 43—Typical hand-held refractometer; (A) application of quenchant solution to refractometer, (B) observation of refractive index.
CHAPTER
22: NON-LUBRICATING
PROCESS
Viscosity, (Test Method D 445)—^Aqueous polymer quenchant viscosity depends on the quenchant concentration and temperature, as shown in Fig. 46 [86]. It is readily determined using a Cannon-Fenske tube (see Fig. 47), stopwatch, and constant temperature bath as described in Test Method D 445. N o t e 12: Although viscosity of most water-soluble polymers is significantly affected by contaminants, particuleirly ionic salts, the viscosity of quenchants formulated from poly(alkylene glycol)s are not particularly affected by the presence of contaminants. However, solution viscosity is strongly affected by degradation.
DEGREES BRIX T/C FIG. 44—Illustration of the Brix refractive index scale used for a hand-held refractometer.
£3MU
FLUIDS:
STEEL
QUENCHING
TECHNOLOGY
Comparison of Concentration by Refractive Index and Viscosity—^A useful procedure for monitoring variations in aqueous polymer quenchants, particularly poly(alkylene glycol) quenchants, is to compare the difference (delta) in the quenchant concentration values obtained by refractive index (C«) and viscosity {Cv) [87]. ^ =
CR-CV
Differences in A of greater t h a n 6-8 are significant and the source of this difference (contamination or degradation), should be determined. Procedures for doing this follow. Water Content (Test Methods D 95 and D 1744)—Aqueous polymer quenchants are composed of water, a water soluble polymer, and a n additive package to provide corrosion inhibition, foam control, etc. Therefore, determination of water content is necessary to establish the concentration of the quenchant in a way that is relatively insensitive to poljTner degradation. Water content may be determined by Karl Fisher analysis (Test Method ASTM D 1744). The advantage of Karl Fisher analysis is that it is a direct m e a s u r e of water content, whereas refractive index and viscosity are both indirect measurements that are substantially affected by either contamination (refractive index) or degradation (viscosity). pH Determination, (Test Method E 70)—The performance of an aqueous polymer quench bath may be criticeJly dependent on its pH. The p H of a quenchant solution may be determined by Test Method E 70. There are many excellent commercially available sources of p H meters and glass electrodes. The choice of the instrument will be primarily affected by the desired precision of measurement. Electrodes used for pH measurement are designed for specific pH ranges and temperature; therefore the solution p H and temperature shall be considered when the elecrodes are selected for use.
^
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FIG. 45—Typical refractive index (Degrees Brix) versus quenchant concentration relationship.
I
618 MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
250 -
<0
a z o o u
200 (75 °F, 23.9°C)yC^
-
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30 10
15
20
25
30
35
40
45
QUENCHANT CONCENTRATION (%) FIG. 46—Ouenchant viscosity as a function of concentration and temperature for an aqueous solution of a poiymer quenchant.
FIG. 47—CannonFenslce viscometer tube.
For example, solution pH values of >10 for quenchants used in aluminum heat treating applications may be disastrous in view of potential caustic corrosion processes that may occur. The relatively simple determination of pH of an aqueous polymer quenchant may provide significant insight into potential polymer degradation, corrosion inhibitor depletion, and contamination.
Polymer degradation is t5^ically accompanied by the formation of acidic by-products, which will decrease pH. Some polymer quenchants, particularly when containing sodium nitrite as a corrosion inhibitor, cannot be used below pH 7.0 without increasing the polymer degradation rate. Some aqueous polymer quenchants contain amine or amine/fatty acid mixtures as corrosion inhibitors. If substantial decrease in the concentration of these inhibitors occurs, a decrease in pH will result. Thus, pH determination may be a useful indicator of corrosion protection of some quenchants. In some cases, the quench bath may be contaminated by ammonia, which is used in some heat treatment processes. Clearly, pH is an excellent indicator of potential ammonia contamination. Quench baths may be contaminated by various hydraulic and metalworking fluids, which may significantly affect the resulting pH of the aqueous polymer quenchant. Conductance, Test Method D 2624—One of the most common and most deleterious contaminants of an aqueous polymer quenchant is hard metal ions (Ca"^^, Mg"^^, Mn"^^, and Fe"^^). The presence of hard metal ions will lead to corresponding increases in cooling rates. To extend the lifetime of the quenchant, it is often recommended that either distilled or deionized water be used for initial polymer quenchant dilution or
CHAPTER 22: NON-LUBRICATING
PROCESS FLUIDS: STEEL QUENCHING
for make-up water, which must be added periodically due to normal evaporation processes. Typically, it is recommended that the conductivity of the water used for this purpose not exceed 15 ^iS/cm [86]. Note 13: ASTM D 653 defines "conductance (specific)" as: "a measure of the ability of the water to conduct an electric current at 77°F (25°C). It is related to the total concentration of ionizable solids in the water. It is inversely proportional to electrical resistance." ASTM D 616 defines "conductivity" as: "the property of a substance's (in this case water and dissolved ions) ability to transmit electricity. The inverse of resistivity. Measured by a conductivity meter, and described in microsiemens/cm or micromhos/cm, /LtS/cm." Another source of ionic contamination that will result in cooling rate increases is salts from molten salt furnaces (baths), which may contaminate an aqueous polymer by dragout on the part upon removal from the salt pot and subsequent immersion in the aqueous polymer quenchant. Increased ionic contamination may also result if excessive corrosion inhibitor is added to the quench system. Since metal ions may result in increased cooling rates, which may potentially result in cracking of the metal due to increased thermal and transformational stresses, it is important to monitor the variation in the ionic content of an aqueous polymer quenchant. This is easily done because increasing ionic content results in increasing electrical conductance. Any equipment capable of giving a conductivity reading almost instantaneously with the application of voltage across the two electrodes comprising the conductivity cell as de-
FIG. 48—Commercially available portable conductivity meter.
TECHNOLOGY
619
Thermal Separation Process For PAG Polymer Quenchant
INmAL 20% SOLUTION (WITH AGITATION)
SOtOTION IMMEDIATELY AFTER SEPARATION (WITH AGITATION)
REDISSOLUTION OF SOLUTION AFTER LAYER FORMATION
SOLUTION AFTER LAYER FORMATION (NO AGITATION) (LOWER LAYER IS CONCENTRATED POLYMER)
FIG. 49—Illustration of the reversible thermal separation process of an aqueous solution of a polyalkylene glycol quenchant.
scribed in Test Method D 2624 is acceptable. A typical portable conductivity meter is illustrated in Fig. 48. The procedure followed is the same as that described in Test Method D 2624 except that the aqueous polymer quenchant being analyzed is used. Separation Temperature (Cloud Point)—Some aqueous polymer quenchants exhibit a characteristic temperature above which the water soluble polymer becomes mostly insoluble in the aqueous medium [87]. This reversible process, which is shown in Fig. 49, [87] is sometimes called the separation temperature or cloud point. The separation temperature is determined by heating a solution of the aqueous polymer quenchant and noting the temperature where the fluid becomes sufficiently cloudy that the thermometer is no longer visible. Although some salts may affect the separation temperature, oxidative degradation of the polymer is the most common problem. Degradation that causes the separation temperature to rise 2-4°C (4-7°F) over the lifetime of the quenchant bath is not unusual. A larger increase or sudden change in separation temperature is cause for concern. Corrosion Inhibitor—Because poljrmer quenchants are water based, they must be formulated with a corrosion inhibitor(s). Corrosion inhibitors protect the tank, fixtures, and parts being quenched by either surface passivation or protective film formation. Depletion of the inhibitor during use is to be expected, and periodic replenishment of the corrosion inhibitor is required to maintain adequate protection of the quenching bath, parts being heat treated, and fixtures. Many pol5TTier quenchants use sodium nitrite as the corrosion inhibitor. The concentration of sodium nitrite may be
620
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
determined according to Test Method D 3867, a cadmium reduction method that may be performed manually or by an a u t o m a t e d procedure. Alternatively, nitrite anion m a y be quantitatively determined by ion chromatography according to Test Method D 4327. Note 14: The concentration of nitrite in a quenchant may also be determined by a relatively simple color test using a commercially available test kit. A tablet, furnished with the test kit, is dissolved in a specific volume of the solution, and the resulting color of the quenchant solution is compared to a standard color chart of known concentrations of sodium nitrite, as shown in Fig. 50. Although sodium nitrite is a commonly used corrosion inhibitor used in the formulation of aqueous polymer quenchants, there is an increasing trend in the marketplace for socalled non-nitrite quenchants. Many of non-nitrite inhibitor systems are based on various amines or amine-fatty acid combinations. Due to the specificity of the required analytical procedures for determination of each non-nitrite inhibitor package that may be used, it is most common to submit samples periodically to the quenchant manufacturer for analysis. Foam Testing, Test Methods D 892, D 3519, and D 3601—A commonly encountered problem in production quench tanks is excessive foaming. Excessive foaming is bad because it may potentially lead to cracking and/or increased distortion. Relative foaming propensity for one fresh quenchant compEired to another or a used quencheint compared to a fresh quenchant or two used quenchant samples may be readily determined by various tests including Test Methods D 892 (using a gas diffusion tube), D 3519 (blender test), or D 3601 (bottle test). Polymer Molecular Weight Analysis, Test Method D 5296—^As a test for pol5rmer degradation, gel permeation chromatogra-
200
^
150;
z LxJ I— Z 100 SEC trace for a polymer from an unused aqueous polymer quenchant solution y 50H CO
40
50
55
60
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RETENTION TIME (sec.) FIG. 51—Size-exclusion chromatography data for a fresh and severely degraded poly(alkylene glycol) quenchant.
phy (GPC), also k n o w n as size exclusion chromatography (SEC), provides the most unambiguous results. This chromatography technique, described in Test Method D 5296, separates poljrmers on the basis of their molecular size. Note 15: Test Method D 5296 may be modified for use with various nonionic polymers such as those based on poly(alkylene glycol). However, many water-soluble polymers are not readily soluble in solvents other than water. In this case, a n a q u e o u s GPC p r o c e d u r e , a procedure where water is used as the solvent for GPC analysis, will need to be developed. Illustrative chromatograms for a fresh undegraded polymer from a polymer quenchant and a badlydegraded polymer are superimposed in Fig. 51 [86]. Note 16: Another method of pol5rmer degradation analysis is to compare, by superimposition, the GPC chromatogram for the fresh and used polymer for the specific quenchant of interest as shown in Fig. 51 [86]. The total area under the pecik for the degraded polymer is normeJized to that of the area u n d e r the fresh polymer peak. The resulting "area shift" of the broader-peaked degraded polymer curve is a quantitative measure of degradation. Although the area shifts may vary widely, typically values greater than 10% are usually cause for concern. Biological
FIG. 50—Portable color test to determine nitrite concentration.
SEC trace for a polymer from a used aqueous polymer quenchant solution
Stability
Fluid Biodeterioration Processes—Fluid biodeterioration involves the reaction of water with a substrate, such as the water-soluble polymer used to formulate a n aqueous polymer quenchant, in the presence of b a c t e r i a or fungi to yield biomass [89]. If this degradation process is not inhibited, enormous queintities of biomass may be present in the system in the form of sludge or microbial scum, which are composed of dead cells, gelatinous slimes, and fungal threads. It has been reported that a bacterial cell may double in size cind divide into two new cells every 15 min until a limiting condition is encountered [89,90].
CHAPTER
22: NON-LUBRICATING
PROCESS
In addition to solid by-products, obnoxious gases may be formed from the biodeterioration of certain additives acting as microbial nutrients, such as nitrites and nitrates, which are converted to ammonia and sulfur or sulfate, which is converted to hydrogen sulfide (H2S), which exhibits a characteristic "rotten egg" odor [91]. Biodeterioration processes that occur in the presence of air (oxygen) are enhanced by system agitation and are designated as "aerobic" processes. However, biodeterioration processes may also occur without air (oxygen) being present. These are called "anaerobic" processes that are inhibited by system agitation. Most aqueous polymer q u e n c h a n t s undergo biodeterioration by an anaerobic process. Biodeterioration Monitoring Procedures (Test Methods D 3946, E 686, and E 979)—There are four strategies for monitoring microbial contamination: 1. gross, 2. physical, 3. chemical, and 4. microbiological. Gross detection procedures include visucd observation of slimes or detection of foul odors. Physical detection procedures include the observation of haze and visible, nonmetsdlic particulate matter in the fluid. Chemical tests that are often used include pH. Sudden decreases in p H indicate a strong potential for microbial contamination [91]. The fourth procedure is to conduct a microbial test. One test is to directly observe the microbial species on a glass slide under a microscope [91]. Currently, there are three standard bench test procedures that m a y be used for monitoring resistance to microbial growth: D 3946, E 686, and E 979. Alternatively, a commercial dip-slide test that is coated with a microbial growth media is often used in the heat treat shop. This is CcJled a "viable titer method," in which the population densities of the microbial species are estimated after incubation for 24-72 h as illustrated in Fig. 52. Viable titer proce-
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dures m a y not detect microbial species that do not form colonies and, therefore, may not correlate with biodeterioration processes. Cooling Curve Analysis—Polymer Quenchants (Test Methods D 6482 and D 6549)—Cooling curve analysis provides a cooling time versus temperature pathway that is directly proportional to physical properties, such as hardness obtainable upon quenching of metal. Aqueous polymer quenchants are typically used with agitation and it is r e c o m m e n d e d that cooling curve analysis of this class of quenchants be performed according to Test Method D 6482 (Tensi Method, see agitation device s h o w n in Fig. 34A) or D 6549 (Drayton Method, see agitation device shown in Fig. 34B). The results obtained by these tests may be used as a guide in quenchant selection or compairison of different quenchants or dilutions of the same quenchant, whether new or used. Cooling curve analysis of an aqueous polymer quenchant by Test Methods D 6482 or D 6549 is conducted by placing the probe assembly illustrated in Fig. 19 into a furnace a n d heating to 850°C (1562°F). The heated p r o b e is then immersed into the agitated quenchant solution at a known agitation rate and desired temperature. The temperature inside the probe assembly and cooling times are recorded at selected time intervals to establish a cooling temperature versus time curve. A series of cooling rate curve compeirisons illustrating the effect of aqueous polymer quenchant concentration and bath temperature is illustrated in Figs. 53 and 54, respectively. The effect of agitation is illustrated in Fig. 55. Cooling curve comparison of a fresh and severely degraded polymer quenchant (compared to water) is shown in Fig. 56. Advanced Cooling Curve Analysis—^All of the cooling curve analysis methods described thus far are essentially laboratory quenchant cinalysis procedure. Although they are invaluable for this use, they are of relatively limited direct value to the heat treater, who needs to know if he can through-heirden a part being produced in production and if he will encounter distortion or even cracking. Therefore, there are various efforts underway at the present time to advance cooling curve anedysis methodology to permit modeling smd simulation of the production quenching process. In this section, one cool-
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FIG. 53—Illustration of the effect of quenchant concentration on cooling curve and cooling rate curve performance for an aqueous polyalkylene glycol quenchant at SOX and 0.5 m/s.
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CHAPTER
22: NON-LUBRICATING
PROCESS
ing curve analysis methodology that has been successfully used in the heat treating shop will be described. In addition to illustrating the overall utility of modeling and simulation of production quenching conditions, this example also illustrated advanced techniques for investigating quenching behavior. Temperature
Gradient
Quenchant
Analysis
Background—Liscic designed a system for practical measurement, recording, and evaluation of quenching (cooling) intensity u n d e r w o r k s h o p conditions, which expresses quenching intensity by a continuous change in relevant therm o d j n a m i c functions during the entire quenching process. This approach should be contrasted with the Grossmann Hfactor concept, which expresses quenching intensity with a single value and which was shown earlier to be of limited value in quantitatively represented quench severity when agitation is used. All of the different cooling curve analysis methods discussed thus far utilize relatively small, usually 12.5 m m or 0.5 in diameter, round bar probes with a single thermocouple placed at the geometric center. Such probes, while excellent for quality control purposes, are of limited value for use under workshop conditions. The reasons include: • Because of its relatively small mass and low heat capacity, these small probes will cool in about 10-30 s whereas an actual workpiece of 50 m m (2.0 in) diameter will require 500-600 s to cool below 200°C (392°F) in the center of the workpiece when quenched into an unagitated quench oil. Therefore, to adequately model actual quench processes under production conditions, a probe of similar mass and dimensions is necessary. • The actual heat transfer coefficient during quenching of actual production parts may be simulated using a small cylindrical probe. However, the heat transfer coefficient during nucleate boiling is heavily dependent on bar diameter [92]. The magnitude of this dependence increases with decreasing bar diameter below 50 m m (2.0 in). The dependence is less pronounced for bar diameters greater than 50 m m (2.0 in). Therefore, for the same quenching conditions, the heat transfer coefficient on the surface of a small diameter cylinder is quite different than that expected on the surface of most production parts with diameters > 5 0 m m (2.0 in). Important criteria for a cooling curve analysis system to be utilized to model quench processes under workshop conditions should be applicable to: 1) a wide variety of quenchant media including: water, brine, aqueous polymer solutions, salt baths, quench oils, fluidized beds, and gas quenchants; 2) a wide variety of quenching conditions including: different bath temperatures, agitation rates, and fluid pressures; and 3) all quenching techniques including: direct i m m e r s i o n quenching, interrupted quenching, martempering, austempering, and spray quenching. The method to be reported here provides for recording of thermodynamic functions during each test to enable the user to ancdyze the peirticular quenching process of interest and quenching conditions, to evaluate quenching intensity, and to compare it with previously performed tests in other facilities under different conditions. To do this, the user will establish a database of quenching intensities of different systems within the production facility. This database will provide the user with input data for subsequent computer simulation of
FLUIDS:
STEEL
QUENCHING
TECHNOLOGY
623
the quenching process to determine optimal quenchant and quenching conditions for every part being produced. In addition, for optimal simulation, it is important that the m e a s u r e m e n t m e t h o d be sufficiently sensitive to reflect changes in each of the important quenching peirameters (specific character of the quenchant, quenchant bath temperature, in addition to mode and degree of agitation). This criterion is addressed by recording the temperature-time history (cooling curve) at particular points within the probe used for analysis. This requires the measurement of transient temperatures within a solid body when high thermal gradients are involved, which requires that the following inherent effects be considered: • Damping Effect—Changes in surface t e m p e r a t u r e are damped in magnitude when sensed inside the body compared to their magnitude at the surface. • Lagging Effect—Changes in the surface temperature are sensed within a finite time after they occur at the surface. The greater the distance of the temperature measurement point from the surface, the greater the damping and lagging effects. • Response Time—When working with thermocouples, it is important to consider another effect that is inherent with every thermocouple-response time, which is the time necessary to reach 63.2% of its total signal output when the thermal junction is subjected to a step change in temperature [93]. The time constant of a sheathed, grounded thermocouple of 1.5 m m (0.062 in) outer diameter is 1.5 s. To reach 99% of its full signal output when subjected to a step change in temperature, the time constant must be multiplied by 5, which makes a 7.5 s delay. Thermocouples with an even greater diameter will exhibit an even greater time constant and delay in response time. Theoretical Principles—Because of all of the above described requirements, effects, and limitations, instead of recording only one cooling curve at the center of a small cylindrical test probe (as in laboratory tests) the heat flux density at the surface of the quench probe has been selected as the main feature in measuring, recording, and eveduating quenching intensity. This is because changes of the h e a t flux density during the quenching process best represent the dynamics of heat extraction. The method itself, known in the literature as the Temperature Gradient Method, is based on the known physical rule that heat flux at the surface of a body is directly proportional to the temperature gradient at the surface multiplied by the thermal conductivity of the material of the body being cooled: q = \
dT dx
Where: q is the heat flux density (W/m^), i.e., the quantity of heat transferred through a surface unit per unit time, A is thermal conductivity of the body material (W/mK), and dT/dx is the temperature gradient inside the body at the body surface, perpendicular to it (K/m). Hardware—The essential feature of the method being described here is the LISCIC/NANMAC quench probe." It is "* The Llscic-Nanmac probe is manufactured by the NANMAC Corporation, Framingham, MA.
624
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
constructed from AISI Type 304 stainless steel, which is 50 mm (2.0 in) diameter X 200 mm ( 4.0 in ) length, instrumented with three thermocouples placed at the half-length cross section as shown in Fig. 57. One thermocouple measures the actual surface temperature of the probe {T„), another measures the temperature at a point 1.5 mm (0.06 in.) below the surface (TO, and the third one measures the temperature at the center of the cross section. The thermocouples inside the body are standard sheath-tjrpe thermocouples. The thermocouple at the surface is of special design (U.S. Patent 2,829185), which allows continuous measurement of the true surface temperature of a solid body without any damping or lagging effects, in real time, because of its extremely fast response time of about 10 /AS
/ixm (0.0036 in), is placed between a split-tapered insert and pressed into the thermowell (body). The thermal junction is formed by grinding and polishing across the sensing tip. The mica insulation between the two dissimilar ribbons is so thin that metallic whiskers of one ribbon element bridges across the mica to the other ribbon element, and makes hundreds of microscopic friction welded junctions, which are parallel to one another, thus forming one composite measuring junction. The microscopic burrs of the metal from the thermowell (housing) bridge the thin layers of mica, thus electrically grounding the thermal junction to the thermowell at the sensing tip. The metal of the thermowell becomes the third intermediate element, and since the temperature is the same on both sides of the thermal junction, the EMF produced by the secondary junctions on both sides of the main thermal junction cancel each other out, leaving the EMF of the main thermal junction as the only observed EMF (Law of Intermediate Metals in Thermocouples). Any subsequent erosion of the surface of the thermowell (body) simply forms new junctions while removing the old junctions, hence its name "Self Renewing Thermocouple." Because of its unique characteristics, this is the best type of thermal junction to be used for heat transfer calculations because it registers all of the phenomena occurring at the surface in real time. The temperatures recorded at the surface iT„) and at 1.5 mm below the surface of the quench probe (Ti) permit the
(10-5 s)
There are two important requirements for a thermocouple used for measuring surface temperature: 1. Its thermal junction should be two-dimensional (instead of the usually encountered three dimensional geometry) 2. It should be flush with the surface The unique details of the sensing tip of this thermocouple are as follows. In the vicinity of the hot measuring junction, the round thermocouple wires are flattened into ribbons of about 38 /xm (0.0015 in) thickness. These ribbon elements are electrically insulated from each other and from the thermowell by sheets of mica insulation of about 5 /xm (0.0002 in) thickness. This "sandwich" of ribbon elements and mica insulations, having a total thickness of about 91
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FIG. 57—The LISCIC/NANMAC quench probe.
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CHAPTER
22: NON-LUBRICATING
PROCESS
calculation of the temperature gradient within this surface layer at each m o m e n t throughout the quenching process. The role of the center thermocouple (Tc) is to indicate the timing for heat extration from the core of the probe and to provide a continuous measurement of the temperature difference between the surface and the core, which is essential for the calculation of thermal stresses. Specific features of the LISCIC/NANMAC quench probe are: 1. The size of the probe and its mass ensures sufficient heat capacity and radially symmetric heat flow in the cross sectioned plane where the thermocouples are located. 2. The probe is constructed from austenitic stainless steel and does not undergo microstructural change upon heating or quenching, n o r does it evolve or absorb heat because of microstructural transformations. 3. The surface condition of the probe can be maintained for each test by polishing the sensing tip of the surface thermocouple before each measurement. 4. Extremely fast response times of the surface thermocouple (10~^ s), and the absence of any damping or lagging effects allow any transient temperature at the surface to be measured and recorded exactly and in real time. 5. The heat transfer coefficient at the probe's surface, because of its sufficiently large diameter, may be used in computer simulation of actual production parts. W h e n the quenching intensity is to be determined, the probe is heated to 850°C (1562°F) in a suitable furnace, then transferred quickly to the quenching bath and immersed vertically. The probe is connected to a data acquisition system including a personal computer. The data acquisition card contains three A/D converters and amplifiers enabling digital recording of all three thermocouple signal outputs. Software—In addition to cooling curve data, the cooling curve analysis p r o g r a m (TGQAS - T e m p e r a t u r e Gradient Quenching Analysis System) described here permits advanced computational modeling of production quenching systems. The TGQAS consists of three modules: 1. Module I: zTEMP-GRAD (Temperature Gradient Method)— In each test, three cooling curves are obtained: r „ for the surface of the probe, Ti for the point 1.5 m m (0.06 in) below the surface, and Tc for the center of the probe. TypiccJ cooling temperature-time profiles for each of these points is illustrated in Fig. 58. The temperature gradient between Ti and Tn is calculated from these cooling curves by multiplying the corresponding data by the temperature-dependent thermal conductivity, and the heat flux density versus time, q = fit) (see Fig. 58b) and the heat flux density versus surface temperature, q = f{T„) (see Fig. 58c) are calculated. Calculation of the differences between each thermocouple location versus time, AT — f(t), provides the functions illustrated in Fig. 59d. Calculation of the integral under the heat flux density curve (which represent the a m o u n t of heat extracted) from the beginning of immersion until a predetermined time, provides the functions shown in Fig. 59e. For heat extracted from the probe, the curve designated by "(" (i.e., for the surface layer of 1.5 m m thickness) is valid. In each point where thermocouples are located, the cooling rate curves versus surface temperature: dT/dt = fiT„) are calculated as shown in Fig. 59f.
FLUIDS:
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QUENCHING
TECHNOLOGY
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Calculated functions, graphically represented in Fig 58 and Fig. 59 permit comparison of the actual quenching intensity among different quenchants, quenching conditions, and techniques. Based on these thermodynamic functions, each quenching test m a y be evaluated with respect to: depth of hardening, (when comparing two quenching processes), thermal stresses, a n d possible superposition of structural transformation stresses that will occur during a particular quenching process and delayed quenching, i.e., whether continuous or discontinuous cooling rates are occurring (with consequences on hardness distribution on the cross section after hardening). These thermodynamic functions also provide the basis for automatic control of quenching intensity during the quenching process. 2. Module II: HEAT-TRANSF (Calculation of Heat Transfer Coefficient and Cooling Curves)—The function of this module is the calculation of the temperature distribution in the cross section of round bars. It is based on the numerical method of control volumes where the heat conduction in the radial direction is solved as a one-dimensional problem. The software consists of two m a i n subroutines. The first subroutine utilizes the measured surface temperature as an input parameter to calculate the temperature distribution over the probe's cross section versus time and the heat transfer coefficient between the surface and the quenchant versus time and versus the surface temperature as illustrated in Fig. 60a and Fig. (sOb. The second subroutine utilizes the calculated heat transfer coefficient for a particular quenching test as input parameter, which permits the simulation of quenching cylindrical workpieces of varying diameters u n d e r the conditions of each quenching test that are stored in the users database of quenching intensities. Physical properties of the workpiece can be selected for the desired steel grade. The cooling curve at any point within the cross section can be calculated as illustrated in Fig. 60c along with corresponding heat transfer coefficient versus time and versus the surface temperature. 3. Module III: CCT-DIAGR (Prediction of Microstructure and Hardness after Quenching)—This module is used to predict the microstructure and hardness after quenching of cylindrical workpieces of different diameters. It contains a n open data file of CCT (Continuous Cooling Transformation) diagrams in which the user may store u p to 60 CCT diagrams of his own choice. This program enables the user to superimpose every calculated cooling curve on the CCT diagram of the desired steel. From the superimposed cooling curves (shown on the CRT monitor during analysis) the u s e r can select the percentage of microstructural phases transformed and the hardness value at the selected point within the cross section after hardening as illustrated in Fig. 61. For a cross section of the selected diameter, cooling curves are calculated at three or five characteristic points (surface, 3/4R, 1/2R, 1/4R and center), using the HEAT-TRANSF module. The CCT-DIAGR module enables the user to d e t e r m i n e hardness values at these points, which will permit prediction of the hardness distribution curve. Note 17: In the case of delayed quenching, where a discontinuous change of cooling rates occur, the prediction of microstructural transformations and hardness
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FIG. 60—Graphical display from Module 2 (HEAT-TRANSF), when quenching the LISCIC/NANMAC probe into a 20°C mineral oil bath without agitation (left row, vertically) and a 25% aqueous polyalkylene glycol quenchant solution at 40°C bath temperature and 0.8 m/s agitation rate (right row, vertically), (a) Heat transfer coefficient versus time, a = f(t); (b) Heat transfer coefficient versus surface temperature, a = f(r„); and (c) Comparison of measured and simulated cooling curves for the center of the quench probe (50 mm diameter).
CHAPTER
22: NON-LUBRICATING
C Chemicot Composition 0.38
Si 0.23
PROCESS
FLUIDS:
STEEL
Mn P Cr S 0.64 0.019 0.013 0.99
Cu 0,17
QUENCHING Mo 0.16
TECHNOLOGY
629
V Ni 0,08 «0.01
1000
Times FIG. 61—CCT diagram of AISI4140 steel with superimposed calculated cooling curves for surface (S), three-quarter radius (3/4 R), and center (C) of a round bar of 50 mm diameter.
values after hardening ft-om an ordinary CCT-diagram is not correct, because the incubation time consumed (at any point on the cross section) before the coohng rate was abruptly changed, has not been taken into account. For a detailed explanation see Ref 94. Hardware Requirements—For the system being described here, the hardware requirements are relatively modest and include: IBM compatible PC (286/386/486/Pentium), 640 Kbytes RAM, a color graphic system, 2 MB free hard disk space plus additional space for files to be stored, printer port, cind disk drive. Example of a Quenching Process Analysis Two different quenching processes are illustrated in Fig. 58 and Fig. 59; Case A and Case B. Case A is a quenching process in a mineral oil at 20°C (68°F) without agitation. (All associated diagrams are on the left side of the figure.) Case B is a quenching process in a 2 5 % aqueous polyalkylene glycol (PAG) solution at 40°C (104°F) and 0.8 m/s agitation rate. (All of the diagrams for this process are on the right side of the figure.) By comparing the diagrams of the heat flux density versus time in Fig. 58b, it is shown that Case B exhibits delayed quenching because in Case A, the time required for meixim u m heat flux to occur (f^max) is only 15 s, whereas in Case B it is 72 s (due to the thick PAG poljmer film). In Case A (oil quenching). Fig. 59e shows that by 20 s after immersion, 34 MJ/m^ has already been extracted and by 50 s, 50 MJ/m'^ heat has been extracted. In Case B, (quenching in a high concentration of a PAG quenchant solution), by 20 s after immersion, only 5 MJ/m^ and by 50 s, only 20 MJ/m^ has been ex-
tracted. However, immediately after that period, between 50 and 100 s, the heat extracted in Case A has increased only from 50 to 55 MJ/m^, whereas in Case B, the heat extracted has increased from 20-86 MJ/m^. This shows that in Case B, the thick polymer film has prevented the heat extraction for a relatively long time at which point the insulating polymer film surrounding the cooling probe burst and a sudden increase of heat extraction occurred. By comparing the time required to decrease the heat flux density from its m a x i m u m to a low value, e.g., 100 KW/m^ as shown in Fig. 58b, it is observed that in Case A, 45 s is necessary, whereas in Case B, only 28 s is necessary. This illustrates that Case B is a quenching process in which after the s u d d e n burst of the thick polymer film surrounding the probe, there is practically n o boiling, as observed in oil quenching, but an abrupt change in the convection cooling stage. A discontinuous cooling change is inherent to such a quenching regime. It is interesting to analyze cooling rate versus surface temperature as shown in Fig. 59f. While in Case A (oil quenching), the cooling rate at the surface of the probe "o" exhibits a greater m a x i m u m cooling rate t h a n the cooling rate at 1.5 m m below the surface ( ). In Case B, the maximum cooling rate at 1.5 m m below the surface (during the period of 350°C to 300°C of surface temperature) is higher than the m a x i m u m cooling rate at the surface itself! This is also evident from the cooling curves illustrated in Fig. 58a for Case B where at 570°C, a discontinuous change of cooling curve slope of Ti occurs, and between 500°C a n d 300°C, this slope is greater t h a n the slope of the cooling curve for the surface (T^). This is an experimental proof of the theoretical calculation described in Ref 95, that in delayed
630 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
quenching, cooling rates below the workpiece surface can be higher than at the surface itself. Another analysis, with respect to thermal stresses during quenching (on which residual stresses and possible distortion depend), is possible by comparing Fig. 59d. This comparison shows that quenching in an aqueous PAG quenchant solution of high concentration (Case B) compared to oil quenching (Case A), resulted in 27% lower maximum temperature difference between the center and the surface of the probe (o). Whereas in Case A, the maximum temperature difference between the center and 1.5 mm below the surface (A) is higher than the maximum temperature difference between the point 1.5 mm below the surface and the surface itself ( ); in Case B, the maximum temperature difference between the point 1.5 mm below the surface and the surface itself ( ) is slightly higher than the maximum temperature difference between the center and the point 1.5 mm below the surface (A), which is reached about 20 seconds later. This analysis also shows an abrupt heat extraction when the polymer film bursts. However, Figure 59d shows that in oil quenching, the maximum temperature difference between the center and the surface (o) occurs 20 s after immersion when the surface temperature is 450°C (See Fig. 58a for Case A), i.e., above the martensite start temperature (Ms)- In an aqueous solution of a PAG polymer quenchant at high concentration, the maximum difference between the center and the surface (o) occurs much later, i.e., after 80 s when the surface temperature has already decreased to 350°C (see Fig. 58a for Case B). Because of this, when working with steels with high Ms temperatures, there is a greater possibility of overlapping thermal stresses with those created due to austenite - to martensite transformation. The probability of crack formation can be observed by comparing the surface temperature of the probe at the moment when maximum heat flux density occurs (Tqmax,)- As seen in Fig. 58c for oil quenching, Tqmax is 515°C, while for the aqueous PAG quenchant solution at high concentration (Case B) this occurs at 380°C. The lower the value of Tq^ax, the greater the risk of crack formation, especially with steels with high Ms temperature. When comparing two different processes with continuous cooling (for the same workpiece and the same steel grade), to determine which will provide the greater depth of hardening, the following analysis may be used (see Fig. 58b). The larger the value of g^ax, and the shorter the time (fqmax), the greater will be the depth of hardening. In the case of a quenching process with discontinuous (delayed) quenching such reasoning is not valid. The sensitivity of the method to variation of quenching parameters is demonstrated in Fig. 62, which shows the calculated heat flux densities versus time for the LISCIC/NANMAC quench probe, all experiments performed in the same aqueous PAG polymer quenchant solution of 40°C bath temperature and 0.8 m/s agitation rate. The only parameter that was changed was the polymer concentration from: a) 5% to b) 15% to c) 25%. This figure illustrates how increasing polymer quenchant concentration decreases the heat flux density, i.e., quenching intensity. Shifting the heat flux density maximum to a longer time (because of increasing thickness of the insulating polymer film surrounding the probe with increasing
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FIG. 62—Calculated heat flux densities versus time, when quenching the LISCIC/NANMAC probe into an aqueous polyalkylene polymer solution at 40°C and 0.8 m/s agitation rate, but having concentrations: (a) 5%, (b) 15%, and (c) 25%.
quenchant concentration) changes a normal quenching process to a delayed one. This example illustrates the possibiUties that are available for simulation of production quenching processes using advanced data acquisition and analysis techniques. ASTM STANDARDS No. A 255 D 91 D 92 D 93
Title Standard Method for Determination of Hardenability of Steel Test Method for Precipitation Number of Lubricating Oils Standard Test Method for Flash and Fire Points by Cleveland Open Cup Standard Test Method for Flash-Point by PenskeMartens Closed Cup Tester
(c)
CHAPTER D 94
22: NON-LUBRICATING
PROCESS
Standard Test Method for Saponification Number of Petroleum Products D 95 S t a n d a r d Test Method for Water in Petroleum Products and Bituminous Materials by Distillation D 189 S t a n d a r d Test Method for Conradson Carbon Residue of Petroleum Products D 287 S t a n d a r d Test Method for API Gravity of Crude Petroleum and Petroleum Products (Hydrometer Method) D 445 Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (the Calculation of Dynamic Viscosity) D 482 S t a n d a r d Test Method for Ash from Petroleum Products D 653 Standard Terminology Relating to Soil, Rock, and Contained Fluids D 664 S t a n d a r d Test Method for Acid N u m b e r of Petroleum Products by Potentiometric Titration D 892 Standard Test Method for Foaming Characteristics of Lubricating Oils D 974 Standard Test Method for Acid and Base Number by Color-Indicator Titration D 1298 Test Method for Density Relative Density (Specific Gravity) or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method D 1310 S t a n d a r d Method for Determination of Flash Point and Fire Point in Liquids by Tag Open-Cup Apparatus D 1744 Standard Test Method for Determination of Water in Liquid Petroleum Products by Karl Fischer Reagent D 1747 Standard Test Method for Refractive Index of Viscous Materials D 2624 Standard Test Method for Electrical Conductivity of Aviation and Distillate Fuels D 3519 Standard Test Method for Foam in Aqueous Media (Blender Test) D 3 5 20 Standard Test Method for Quenching Time of HeatTreating Fluids (Magnetic Quenchometer Method) D 3601 Standard Test Method for Foam in Aqueous Media (Bottle Test) D 3867 Standard Test Method for Determination of NitriteNitrate in Water D 3946 Standard Test Method for Evaluating the Bacteria Resistcince of Water-Dilutable Metalworking Fluids D 4007 Standard Test Method for Water and Sediment in Crude Oil by Centrifuge Method (Laboratory Procedure) D 4327 S t a n d a r d Method for Determination of Anions in Water by Chemically Suppressed Ion Chromatography D 5296 Standard Method for Determination of Molecular Weight Averages and Molecular Weight Determination of Polystyrene by High Performance Size Exclusion Chromatography D 6161 Standard Terminology Used for Crossflow Microfiltration, Ultrafiltration, Nanofiltration, and Reverse Osmosis Membrane Processes D 6200 S t a n d a r d Method for Determination of Cooling Characteristics of Quench Oils by Cooling Curve Analysis
FLUIDS: D 6482
D 6549
D 6666 D 6710 E 112 E 131 E 349 E 686 E 930
E 979
E 1181 E 1382
STEEL
QUENCHING
TECHNOLOGY
631
S t a n d a r d Method for Cooling Curve Analysis of Aqueous Polymer Q u e n c h a n t s by Cooling Curve Analysis with Agitation (Tensi Method) S t a n d a r d Method for Cooling Curve Analysis of Aqueous Polymer Q u e n c h a n t s by Cooling Curve Analysis with Agitation (Drayton Method) Stcindard Guide for Evaluation of Aqueous Polymer Quenchants Standard Guide for Evaluation of HydrocarbonBased Quench Oil Standard Test Methods for Determining Average Grain Size Standard Terminology Relating to Molecular Spectroscopy Standard Terminology Relating to Space Simulation Standard Method for Determination of Antimicrobial Agents in Aqueous Metalworking Fluids Standard Test Methods for Estimating the Largest Grain Observed in a Metallographic Section (ALA Grain Size) Standard Method for Determination of Antimicrobial Agents as Preservatives for Invert Emulsions and Other Water Containing Hydraulic Fluids Standard Test Methods for Characterizing Duplex Grain Sizes Standard Test Methods for Determining Average Grain Size Using S e m i a u t o m a t i c a n d Automatic Image Analysis
OTHER STANDARDS SAE J406 DIN 50191 IP 34 IP 71
IP 123
IP 160
IP 226
ISO 642 ISO 6743
ISO 9950
JIS K2242
Methods of Determining Hardenability of Steel Hardenability Testing of Steel by EndQuenching Determination of Flash Point - PenskeMartens closed cup method Petroleum products - Transparent Eind opaque liquids - Determination of kinematic viscosity and calculation of dynamic viscosity Petroleum products - Determination of distillation characteristics at a t m o spheric pressure Crude petroleum and liquid petroleum products - Laboratory determination of density - Hydrometer method Petroleum p r o d u c t s - Calculation of viscosity index from kinematic viscosity Steel Hardenability Test by E n d Quenching (Jominy Test) Part 14: Family U (Heat Treatment), Lubricants, Industrial Oils & Related Products (Class U) Industrial Quenching Oils-Determination of Cooling Characteristics-NickelAlloy Probe Heat Treating Oil
632 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK JIS K6753 ZBE 45003-88 AFNOR NFT 60778
Standard Liquor H e a t T r e a t i n g O i l s - D e t e r m i n a t i o n of C o o l i n g Ability Industrial Quenching Oils-Determinat i o n of C o o l i n g C h e i r a c t e r i s t i c s - S i l v e r Probe Test Method
REFERENCES [1] Bates, C. E., Totten, G. E., and Brennan, R. L., "Quenching of Steel," ASM Handbook-Volume 4 Heat Treating, ASM International, Materials Park, OH, 1991, pp. 67-120. [2] Davis, J. R., ASM Materials Engineering Dictionary, ASM International, Materials Park, OH, 1992, [3] Sverdlin, A. V. and Ness, A. R., "The Effects of Alloying Elements on the Heat Treatment of Steel: Chapter 2," Steel Heat Treatment Handbook, G. E. Totten a n d M. A. H. Howes, Eds., Marcel Dekker, Inc., NY, 1997, pp. 45-91. [4] Liscic, B., "Chapter 3-Hardenability," Heat Treatment of Steel Handbook, G. E. Totten a n d M. A. H. Howes, Eds., Marcel Dekker, NY, 1997, pp. 93-156. [5] Totten, G. E., Bates, C. E., and Clinton, N. A., "Chapter 3-Cooling Curve Analysis," Quenchants and Quenching Technology, ASM International, Materials Park, OH, 1993, pp. 69-128. [6] Totten, G. E., Bates, C. E., and Clinton, N. A., "Chapter 2-Measuring Hardenability and Ouench Severity," Quenchants and Quenching Technology, ASM International, Materials Park, OH, 1993, pp. 35-68. [7] Winchell, P. G. and Cohen, M., "Strength of Martensite," ASM Transactions, Vol. 55, No. 2, June 1962, pp. 347-361. [8] Houdremont, E., "Strength of Martensite," ASM Transactions, Vol. 55, No. 2, June 1962, pp. 347-361. [9] Hornbogen, E., "Strength of Martensite," ASM Transactions, Vol. 55, No. 2, June 1962, pp. 347-361. [10] Leidenfrost, G. J., "De Aqua Communis Nonnullis Tractus," Original from 1756, in C. Waves, Inemational Journal of Mass Transfer, Vol. 9, 1966, pp. 1153-1166 (trans.). [11] Yamanouchi, A., "Effect of Core Spray Cooling in Transient State after Loss of Cooling Accident," Journal of Nuclear Science and Technology, No. 5, 1968, pp. 547-558. [12] Duffly, R. B. and Porthouse, D. T. C , "The Physics of Rewetting in Water Reactor Engineering Core Cooling," Nuclear Engineering and Design, Vol. 3 1 , 1973, pp. 234-245. [13] Kunzel, T., "Einfluss der Wiederbenetzung auf die allotrope Modifikationsanderung tauchgekiihlter Metallkorper," Dissertation, Faculty for Mechanical Engineering of the Technical University of Munich, 1986, 138 pp. [14] Hein, D., "Modellvorstellung zur Wiederbenetzung d u r c h Fluten," Doctoral Thesis, Technical University of Hannover, 1980, 182 pp. [15] Ladish, R., "Untersuchung der minimalen Filmsiedetemperaturen auf keramischen u n d metallischen Leitem," Report of "Kemforschungsstelle Karlsruhe," KfK-2970, 1980, 96 pp. [16] Stitzelberger-Jakob, P., "Hartevorherbestimmung mit Hilfe des Benetzungsablaufes beim Tauchkiihlen von Stahlen," Dissertation, Faculty for Mechanical Engineering of the Technical University Munich, 1991, 160 p p . [17] Tensi, H. M. and Lainer, K., "Wiederbenetzung und Warmeilbergang beim Tauchkiihlen in Hochleistungsolen," HTM, 1997, 52, pp. 298-303. [18] Tensi, H. M., "Wetting Kinematics," Theory and Technology of Quenching, B. Liscic, H. M. Tensi, and W. Luty, Eds., SpringerVerlag, Berlin, 1991. [19] Tensi, H. M., Stich, A., and Totten, G. E., "Fundamentals of Quenching," Metal Heat Treating, Mar/Apr 1995, pp. 20-28.
[20] Jominy, W. E., Hardenability of Alloy Steels, ASM IntemationeJ, Materials Park, OH, 1939, p. 73. [21] French, H. J., "A Study of the Quenching of Steels," Transactions ofASST, May 1930, pp. 646-727. [22] Paschkis, V. and Stolz, G., "How Measurements Lead to Effective Quenching," Iron Age, Vol. 22, 1956, pp. 95-97. [23] Zhu, H., "Analysis of the Algorithms for Measuring the Cooling Rates of Quenching Media by Computer," Heat Treatment of Metals (China), Vol. 11, 1986, pp. 26-31. [24] Zhang, S., Liu, H., and Zhou, X., "Measurement of the Characteristic Curves of Quenching Media with a Microprocessor," Heat Treatment of Metals (China), Vol. 11, 1986, pp. 35-39. [25] Tensi, H. M. and Steffen, E., "Measuring of the Quenching Effect of Liquid Hardening Agents on the Basis of Synthetics," Steel Research, Vol. 56, 1985, p p . 489-495. [26] Zhelokhovtseva, R. K., "Elimination of Quenching Cracks by Means of Optimization of Cooling Conditions," Steel USSR, Vol. 15, 1985, p p . 238-239. [27] Segerberg, S., Oral Presentation at ASM Heat Treating Conference, Chicago, March 1990. [28] Kovalenko, G. V. and Kobasko, N. I., "Modeling of the Unsteady State Boiling Process in Water Quenching of Metal," Heat Transfer-Soviet Research, Vol. 20, No 1, 1988, p p . 69-78. [29] Kobasko, N. I., "Itogi nauki I tehniki (Results of Science and Technique)," Section: Metal Study and the Heat Treatment, Quenchants, VINITI, Moscow, Vol. 23, 1989, p p . 127-166. [30] Kobasko, N. I., "Technological Aspects of Cooling in Quenching: Review," Metallovedenie Termicheskaya Obrabotka Metallov, No. 4, 1991, pp. 2-8. [31] Tensi, H. M. and Stitzelberger-Jacob, P., "Influence of Wetting Kinetics on Quenching and Hardening in Water-Based Polymers With Forced Convection," Proceedings of the 6'^ International Heat Treatment of Metals, ASM International, Materials Park, OH, 28-30 Sept. 1988, Chicago, p p . 171-176. [32] Tensi, H. M. a n d Stitzelberger-Jacob, P., "Influence of Repeated Submerging on Quenching," Promyshlennaya Teplotehnika, Vol. 11, No. 4, 1989, pp. 57-66. [33] Tensi, H. M., Stich, A., and Totten, G. E., "Fundamentals About Quenching by Submerging," Proceedings of the International Heat Treating Conference: Equipment and Processes, ASM International, Materials Park, OH, 18-20 April 1994, Shaumberg, IL, pp. 243-251. [34] Kobasko, N. I. and Timchenko, N. P., "Filming of Sample Quenching in Aqueous Solution of Polymer," Metallovedenie Termicheskaya Obrabotka Metallov, No. 10, 1986, pp. 25-29. [35] Kobasko, N. I., Moskalenko, A. A., Totten, G. E., and Webster, G. M., "The Determination of t h e Second Critical Heat Flux Density on the Basis of Tests of Stsindard Samples," Promyshlennaya Teplotehnika, Vol. 17, No. 5, 1995, pp. 83-87. [36] Kobasko, N. I., Moskalenko, A. A., Totten, G. E., and Webster, G. M., "The Investigation of the Influence of Different Technological Factors on the Second Critical Heat Flux Density," Promyshlennaya Teplotehnika, Vol. 17, No. 6, 1995, pp. 57-64. [37] Kruzhilin, G. N., "Teplootdacha ot Horizontalnoi Plity k Kipyaschey Zhidkosti pri Svobodnoi Konvekstii," Doklady Akademie Nauk, Vol. 58, No. 8, 1947, pp. 1657-1660. [38] Moskalenko, A. A., Kobasko, N. I., Tolmachova, O. V., Totten, G. E., a n d Webster, G. M., "Quenchants Characterization by Acoustical Noise Analysis Cooling Properties of Aqueous Poly(Alkylene Glycol) Polymer Quenchants," Conference Proceedings of the 2nd International Conference on Quenching and Control of Distortion, G. E. Totten, K. Funatani, M. A. H. Howes, and S. Sjostrom, Eds., ASM International, Materials Park, OH, 1996, pp. 117-122. [39] Zhou, R. L., "The Improvement of Quenching Techniques and the Development of New Quenching Media," Proceedings of the 4'^ Annual Conference on Heat Treatment, Institute of Chinese
CHAPTER 22: NON-LUBRICATING PROCESS FLUIDS: STEEL QUENCHING TECHNOLOGY 633 Mechanical Engineering Society, Nanjing, 25-31 May 1987, Chinese Mechanical Engineering Society, Beijing, pp. 348-353. [40] Segerberg, S. O., "Classification of Quench Oils: A Method of Comparison," Heat Treating, Dec. 1988, pp. 30-33. [41] Tensi, H. M. a n d Stitzelberger-Jacob, P., "Bedeutung des HWertes fiir die Bestimmung der Harteverteilung," HTM, Vol. 44, 1989, pp. 99-105. [42] Furman, G. R., "Quenching I—Metallurgical Aspects," Lubrication, Vol. 57, 1971, pp. 13-24. [43] Han, S-W and Yul'Chev, R., "Conservation of Quenching Oils," Kong Hak Hoe lie. Vol. 2, 1989, pp. 59-65. [44] Dicken, T. W., "Modern Quenching Oils: an Overview," Heat Treatment of Metals, Vol. 1, 1986, pp. 6-8. [45] Lasday, S. B., "Metal Quenching with Oils and Synthetic Media," Industrial Heating, Oct. 1976, pp. 8-19. [46] Totten, G. E., Bates, C. E., and Clinton, N. A., "Chapter 4 - Q u e n c h i n g Oils," Quenchants and Quenching Technology, ASM International, Materials Park, OH, 1993, pp. 129-159. [47] Tkachuk, T. I., Rudakova, N. Y., Sheremeta, B. K., and Novoded, R. D., Metallovedenie Termicheskaya Obrabotka Metallov, Oct. 1986, pp. 4 2 ^ 5 . [48] Allen, F. S., Fletcher, A. J., and Mills, A., "The Characteristics of Certain Experimental Quenching Oils," Steel Research, Vol. 60, 1989, pp. 522-530. [49] Boyer, H. E. and Cary, P. R., Quenching and Control of Distortion, ASM International, Materials Park, OH, 1988. [50] Gilliland, H.-J., "Measuring Quenching Rates with the Electronic Quenchometer," Metals Progress, Oct. 1960, pp. 111-114. [51] Bender, E. A. a n d Gilliland, H. J., "New Way t o Measure Quenching Speed," Steel, Dec. 1957, pp. 56-59. [52] Barley, C. A. and Aarons, J. S., The Lubrication Engineers Manual, U.S. Steel Corp., Pittsburgh, PA, 1971, pp. 56-57. [53] F u r m a n , G. R., "Quenching 11—The Oil's Role," Lubrication, Vol.57, 1971, pp. 25-36. [54] Blackwood, R. R., CEO, Texanol, Inc., Milwaukee, WI, personal communication, 1987. [55] Totten, G. E., "Polymer Quenchants: The Basics," Advanced Materials and Processes, Vol. 137, No. 3, 1990, pp. 51-53. [56] Cornell, E. R., "Process for Quench H a r d e n i n g Steel," U.S. Patent 2,600,290, Washington D.C., 8 Aug. 1950. [57] Blackwood, R. R. a n d Cheesman, W. D., "Metal Quenching Medium," U.S. Patent 3,220,893, Washington D.C., 30 November 1965. [58] Terekhova, N. F., Tanicheva, O. N., Tiunova, N. M., a n d Lobanova, N. M., "Quenching Medium," U.S.S.R. Patent 3,745,271, 15 Aug. 1986. [59] Gordon, M., "Method of Quenching Metals," U.S. Patent, 2,770,564, Washington DC, 13 Nov. 1956. [60] Meszaros, A. G., "Water-Based Quenching Composition Comprising Polyvinylpyrrolidone and Method of Quenching," U.S. Patent 3,902,929, Washington DC, 2 Sept. 1975. [61] Kopietz, K.-H., "Process for the Controlled Cooling of Ferrous Metal," U.S. Patent, 4,087,290, Washington DC, 2 May 1978. [62] Warchol, J. F., "Polyoxazolines in Aqueous Quenchants," U.S. Patent 4,486,246, Washington DC, 4 Dec. 1984. [63] Jarvis, L. M., Blackwood, R. R., and Totten, G. E., "Thermal Separation of Polymer Quenchants for More Efficient Heat Treatments," Industrial Heating, November 1989, pp. 23-24. [64] Totten, G. E., Bates, C. E., and Clinton, N. A., "Chapter 5-Polym e r Quenchants," Handbook of Quenchants and Quenching Technology, ASM International, Materials Park, OH, 1993, pp. 161-190. [65] Totten, G. E., Webster, G. M., Tensi H. M., and Liscic, B., "Standards for Cooling Curve Analysis of Quenchants," Heat Treatment of Metals, 1997, No. 4, pp. 92-94. [66] "Laboratory Test for Assessing the Cooling Curve Characteristics of Industrial Quenching Media," Wolfson Heat Treatment
[67]
[68]
[69]
[70]
[71] [72] [73] [74] [75] [76]
[77]
[78] [79] [80] [81] [82]
[83]
[84]
[85]
[86] [87]
[88]
[89]
Centre Engineering Group Specification, Wolfson Heat Treatment Centre, Birmingham, UK, 1982. Tensi, H. M., "Methods and Standards for Laboratory Tests," Theory and Technology of Quenching: A Handbook, B. Liscic, H. M. Tensi, and W. Luty, Eds, Springer-Verlag, Berlin, Germany, 1992, p p . 208-219. Guisbert, D. A., "Precision and Accuracy of the Cooling Curve Analysis Test Method," Proceedings of the 16th ASM Heat Treating Society Conference and Exposition, Cincinnati, OH, 19-21 M a r c h 1996, ASM International, Materials Park, OH, p p . 435^42. Guisbert, D. A., "Correlation with Magnetic Quenchometer to Cooling Curve Analysis Technique," Proceedings of the 16th ASM Heat Treating Society Conference and Exposition, Cincinnati, OH, 19-21 March 1996, pp. 4 5 1 ^ 6 0 . Guisbert, D. A. and Moore, D. L., "Influence of Test Conditions on Cooling Curve Test Results," The 1st International Automotive Heat Treating Conference, R. Colas, K. Funatani, and C. A. Stickels, Eds., ASM International, Materials Park, OH, 1998, pp. 449-455. Tensi, H. M. and Stich, A., "Characterization of Polymer Quenchants," Heat Treating, May 1993, pp. 25-29. One source of the "dynamic boat propeller" Part #1472, is Fa. Robbe. D-36355, Grebenhain, Germany. Hilder, N. A., "The Behavior of Polymer Quenchant," PhD Thesis, University of Birmingham, UK, 1988. Tagaya, M. and Tamura, I., Technol. Rep., Osaka University, Vol. 7, 1957, pp. 4 0 3 ^ 2 4 . Boyer, H. E. and Cary, P. R., Quenching and Control of Distortion, ASM International, Materials Park, OH, 1988, p. 169. Totten, G. E., Bates, C. E., and Clinton, N. A., "Chapter 6-Quench Bath Maintenance," Handbook of Quenchants and Quenching Technology, ASM International, Materials Park, OH, 1993, pp. 191-238. Totten, G. E., Bates, C. E., and Clinton, N. A., "Chapter 6-Quench Bath Maintenance," Handbook of Quenchants and Quenching Technology, ASM International, Materials Park, OH, 1993, pp. 191-238. Horton, B. R. and Weetman, R., "Quench Oil Recovery," Heat Treatment of Metals, Vol. 2, 1984, pp. 49-51. Watanabe, H. and Kobayashi, C , Lubrication Engineering, Vol. 38, No. 8, 1978, pp. 421-428. Hasson, J. A., "Preventative Maintenance for Quenching Oils," Industrial Heating, Sept. 1981, pp. 21-23. Boyer, H. E. and Cary, P. R., Quenching and Control of Distortion, ASM International, Materials Park, OH, 1988, pp. 4 4 ^ 5 . von Bergen, R. T., "The Control and Monitoring of Polymer Quenchant Systems," Heat Treatment of Metals Vol. 2, 1991, pp.37^2. Srimongkolkul, V., "Is There a Need for Really Clean Oil in Quenching Operations?," Heat Treating, December 1990, p p . 27-28. Bates, C. E. and Totten, G. E., "Quantifying Quench-Oil Cooling Characteristics," Advanced Materials & Processes, 1991, No. 3, pp. 25-28. Totten, G. E. and Webster, G. M., "Quenching Fundamentals: Maintaining Polymer Quenchants," Advanced Materials and Processes, Vol. 149, No. 6, 1996, pp. 64AA-64DD. Mueller, E. R., "Polyglycol Quenchant Cleanliness: Are There Benefits," Heat Treating, October 1993, pp. 24-27. Jarvis, L. M., Blackwood, R. R., and Totten, G. E., "Thermal Separation of Polymer Quenchants for More Efficient Heat Treatments,"/nrfwiina/Heafrng, November, 1989, pp. 23-24. HiU, E. C. and Hill, G. C , "Biodegradable After Use But Not In Use," Industrial Lubrication & Tribology, Vol. 46, No. 3, 1994, pp. 7-9. Hill, E. C , "The Significance and Control of Microorganisms in
634 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK Rolling Mill Oils and Emulsions," Metals Materials, No, 9, 1967, pp. 294-297. [90] Anon., "Microbiology of Lubricating Oils," Process Biochemistry, May 1967, pp. 54-56. [91] Passman, F. J., "Biocide Strategies for Lubricant Rancidity and Biofouling Prevention," Proceedings of the AISE Annual Convention, Vol. 1, Association of Iron and Steel Engineers, 1996, pp. 413-428. [92] Kobasko, N. I., "Teplovie Procesi Pri Zakalke Stall," Metalovedenie I Termiceskaja Obrabotka Metalov, 1968, Vol. 3, pp. 2-6. [93] Liscic, B. and Dowding, K., "Calculation and Measurement of the Workpiece's Surface Temperature During Quenching," Heat
Treating-Proceedings of the 20th Conference Vol. 2, K. Funatani and G. E. Totten, Eds., 9-12 October 2000, ASM International, Materials Park, OH, pp. 688-695. [94] Shimizu N. and Tamura, L, "Effect of Discontinuous Change in Cooling Rate During Continuous Cooling on Pearlite Transformation Behavior of Steel," Transactions ISIJ, Vol. 17, 1977, pp. 469-476. [95] Chen, M. and Zhou, H., "Numerical Heat Transfer Ansdysis on the Effect of Enhancing the Thickness of the Hardened Layer by Delayed Quenching," Jinshu Rechuli Xuebao, (Transactions of Metal Heat Treatment), Vol. 14, No. 4, 1993, pp. 1-6. (in Chinese).
Section IV: Performance/Property Testing Procedures Steven R. Westbrook and Rajesh J. Shah, Section Editors
MNL37-EB/Jun. 2003
Static Petroleum Measurement Lee Oppenheim^
STATIC PETROLEUM MEASUREMENT CAN BE BEST SUMMED UP as
termine the quantity of. The measure is linear (meters, feet). Along the way, it is necessary to determine the a m o u n t of sediment and/or water on the bottom of the tank and depending on the product, the amount of entrained material within the liquid to be measured. While doing so, a sample is obtained to determine its density. A temperature of the product is also tctken at the same time. When the basic steps are completed, calculations side of static petroleum measurements are performed. Linear measures are converted to volumes, sediment and water found at the bottom of a tank is eliminated from our quantity, entrained sediment and water (S & W) mixed in the product is subtracted, and then the product density and temperature of the product at time of measurement is used to produce a net volume at the specified reference temperature (60°F in the U.S.). A small error may cost either the buyer or the seller to gain or lose much with a significcuit effect on a company's bottom line. In a small way, this chapter will help unravel the mystery of determining the quantity of petroleum in a bulk container, and also provide a n understanding of the importance of each step in the process.
the
procedures and methods used to determine the quantity of liquid in a storage tank, marine vessel, road transport vehicle, or a rail tank car. In determining the quantity of the liquid in the container, there may be other things in the container that detract from determining the quantity of useable petroleum or petroleum products. They do take u p space in the container and therefore must be desilt with as peirt of any measurement procedure. These can include water, sediment, sludge, rust scale, and sand to n a m e just a few. As used here, petroleum refers to crude oil, therefore the continuous reference to petroleum and petroleum products. Petroleum products as used here are the liquid stocks (e.g., naphtha, kerosene, fuel oils) derived from the refinery processes from crude oil. It is important to know the total of the liquids and solids in a tank to prevent a tank from overflowing during a receipt. However, the more exacting science of static petroleum measurement is concerned with determining a n accurate measure of the quantity of petroleum Eind petroleum products used in the purchase, sale, or inventory control of the commodity. This section on static petroleum measurement will deal primary with manual gauging techniques and the related sampling and quality tests needed to obtain accurate petroleum and petroleum product quantity measurements. Automatic tank gauging, not discussed in this chapter, relies on the manued gauging methods for calibration purposes. Newer techniques for leak detection combine a form of static petroleum measureraent with inventory control to assure that tanks and lines storing and moving the products are tight. Leak detection procedures primarily combine automatic gauging methods with computer programming. This chapter is designed to explain h o w to accurately m e a s u r e the petroleum and petroleum products, why certain procedures are necessary, sampling and testing processes, and the basics of converting these measures into quantity m e a s u r e m e n t s used in the industry.
Petroleum and petroleum products expand and contract as the temperature changes. The rate of this expansion may vary depending on the type of product encountered. An underlying principle used in static petroleum measurenaent is the volume of petroleum at a specified standard temperature. This standard temperature is a means of common reference and helps to avoid the misunderstcuidings that occur when comparing unlike items. It is important to understand what temperature is being used as the reference temperature. In using the System Internationale, sometimes referred to as the metric system, the standard temperature is usually 15°C and sometimes 20°C. Using the American measurement system, 60°F is used. This does not imply that volumes at ambient temperatures cannot be used for the sale of a product. Small quantities such as those sold using meters at an automotive service station or at a single user fleet dispensing location are usually determined at the ambient (surrounding air) temperature.
The primary reference for static petroleum measurements is the American Petroleum Institute (API) Manual of Petroleum Measurement Standards (MPMS). Increasingly, the s t a n d a r d s and practices are standardized with other Standards bodies such as ASTM International (ASTM), Institute of Petroleum (IP), and the International Organization of Standards (ISO) as well as other national standards organizations. Yet, static petroleum measurements normally do not even measure the volume of the product we are trying to de' Chief, Quality Operations Division, Defense Energy Support Center, Ft. Belvoir, VA 22060-6222.
Storage and transport tanks have several openings from which measurements may be taken. However, only specially designated locations on the tank are to be used. This is because calibration of the tank volume has been made from this point or a combination of several designated points. Smaller transport tanks may have volume measurements scribed on an approved measurement rod, commonly referred to as a dipstick. These calibrated dipsticks are only for use on the specified tanks conforming to the originjd construction of the tanks. The majority of manual measurements are taken using
635 Copyright'
2003 by A S I M International
www.astm.org
636
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
a linear measure from a properly designated gauging point on the tank. The linear measure is converted to a volume measure by use of the certified calibration table (or tank capacity tables or strapping table/chart as it may be referred to) for that tank. A strapping chart contains the official conversion of the linear measure to a volume measure within that tank. Tank calibrations are performed using API MPMS Chapter 2 criteria. Tajik bottoms may slope in various ways, changing the shape of the bottom. Taking measurements from a point not designated as the gauging point will most likely mean a change to the linear measurement needed for accurate quantity measurements. Tank calibrations take into account internal structures such as supports and piping, change in tank diameters, as well as deductions for floating roofs. Each tank requires a certified calibration table or strapping chart when performing static petroleum measurements. It will contain much needed information for the performance of accurate measurement and validation should the need arise. To begin the process of understanding how to measure and quantify a large tank or marine vessel compartment, it is necessary to start with some basic terminology. Beginning at the tank where the product resides, use Fig. 1 to describe the terms. Each tank to be measured has designated a place where gauging is to occur, or reference gauge point. Normally a gauge hatch is designated for this purpose. Check the strapping chart for the point or points so designated. Some gauge hatches, primarily those in stationary shore tanks, have a slot or mark where the measuring tape can be inserted. These slots or marks designate which side of the gauging tube is to be used. If there is no indication on the strapping chart designating a specific location, gauging should be performed opposite the hinge on the gauging hatch for uniformity. Beneath the gauging point, there may be a datum plate, preferably on the floor of the tank. Occasionally a datum
REFERENCE POINT READING
GAGING TAPE GAGING HATCH
REFERENCE POINT READING
GAGING TAPE / GAGING HATCH
OUTAGE TAPE CUT TANK SHELL
^
LIQUID LEVEL
BOB CUT TANK SHELL
LIQUID LEVEL
INNAQE
plate may be attached to the side of the tank or the bottom of the stillpipe. The datum plate is fixed in a level position on the floor of the tank directly underneath the reference gauge point. It is between these two locations that we take the linear measurement. The datum plate may be raised off the floor. In such cases a reading of zero does not indicate that the tank is empty as product may be below the measuring point. When this occurs, the strapping chart will have an indicated volume for a measurement of zero. It takes that much product just to reach the zero point. If a tank does not have a datum plate, the reference height is from the reference gauge point to the floor of the tank. Again, this may not be the low point of the tcink and a zero reading may account for a certain fill quantity. This is particular true of cone down type tank bottom construction with side gauging points, but exists in other tank styles cdso. Between the reference gauge point and the datum plate, the distance is called the reference gauge height. This distance will be indicated on the strapping chart and may also be marked on the top of the tank by the gauging point. During the measurement process, knowing whether the sediment, water, and product heights are accurate requires knowledge of the reference gauge height. This will be especially true for tanks that may contain sufficient sediment or sludge, preventing the measuring device from hitting the datum plate or bottom of the tank. A tape and bob combination is used to make the linear measurement. There are several different styles of tapes and bobs (Fig. 2). It is important that the correct combination is used. We must therefore understand the difference between the two measurement styles: innage and outage. Innage is the measurement of the liquid and solids from the bottom of the tank to the upper level of the product. Most refined products in shore tanks, road transport vehicles, and rail tank cars are measured in this manner. If the measurement device can get through the sediment, sludge, and water in the tank (the reference gauging height is obtained at the reference gauge point), then the innage method ccin be used to determine the nonproduct or nonmerchantable part of the tank. Outage, sometimes referred to as ullage, especially on marine vessels, is the measurement of the void space in the tank. Most marine vessel compartments are strapped using the ullage method. The outage method is also preferred to determine the amount of fuel oil or heavy, viscous products. These products may prevent the tape and bob from getting to the bottom in a straight line, increasing the total linear distance. It is also very messy having to clean the tape and bob after each measurement. The innage and outage methods may be used together for different purposes. An outage measurement may be taken for the product, but an innage taken for sediment, sludge, and water measurements. Whether an innage or outage gauge is taken, always remember to check the strapping chart to determine which method to use for converting from a linear measurement to a volume measure. You can compensate taking an innage when an outage is required (or vice versa) by remembering the simple relationship between innage and outage. innage height + outage height = reference gauge height
INNAGE
FIG. 1—Gauging diagram.
OUTAGE
For example, take an innage gauge with a total height of 7.500 m. The Reference gauge height is 19.670 m. An outage gauge is required when the strapping chart is used. This
CHAPTER 23: STATIC PETROLEUM MEASUREMENT
637
S t a l l T a p « i tob«^^« or *^t in. w l o , O D O S lo 0.012 In. thick Bobs l o b * mode of coriDsion-rislstQnt m e t a l
A r-B
II
r-ll
•»
IS
'OUTAGE TAPE-
-H 0 U
r-ll
i—n
\f
\ /
10"
r-J
X
Z
INNAGEtOa
SCALE ZERO
EXTCNSION OUTACe — SOB
PIAIM OUTASE— SOB
DEEP GROOVED— OUTAGE
BOS
bC^
FIG. 2—Tapes and bobs.
shows that the pregauging review was not performed. Using the formula reference gauge height — innage height = outage height or 19.670m - 7.500m = 12.170m for the outage height. A bob is a heavy metal (usually brass) weight with linear measurements on it. The heavy weight is designed to keep the tape stretched in a straight line below the gauging point. Each bob is specifically designed for various tapes. An innage bob begins at the bottom with zero and provides the linear measure upwards towards the tape. It will have some means for the bob to be attached to the tape. The latching area is normally without linear gradients so measurement should not occur in this area. The tape continues the linear measurement up to the reference gauge point. It is important to use the corresponding bob cind tape to provide for a continuous linear measurement. Some bobs are 6 in. with a dead area of 2 in. between the latching area and the beginning of the linear tape gradients. Other bobs may be 10 in. Check and validate that the measurement from the tip of the bob up past the beginnings of the tape linear gradients are continuous. Outage bobs can be designed to work with either an innage tape or and outage tape. An outage bob with an outage tape is designed so that the zero point is at
the top of the outage bob where the tape latches onto the bob. The bob will then be graduated from the top of the bob downwards. An extension outage bob is designed to be attached to an innage tape. Below where the zero point would be if an innage bob was attached is a metal extension. Readings begin at the zero point and increase below the zero point. When using an outage tape, readings on the bob below the zero point are added to the reading tciken at the reference gauge point. Treat the tape properly as the linear measure is a critical step in getting the quantity accurate. Kinks in the tape are to be avoided. Tapes that have been spliced together shall not be used when gauging for custody transfer purposes as it may change the measurement above the splice point. A bob showing wear at the tip may affect gauging accuracy. Where primarily water and small amounts of soft sediment may be the contaminant, a water gauging bar may be used to measure the water that has settled to the bottom of the tank. One drawback is if the bar is not attached to a corresponding tape, it is not possible to obtain a reading of the reference height gauge. Therefore if the bar did not hit the datum plate or floor of the tank, the gauger will not know if the bar actually reached the datum plate or the floor. A new tape and bob should be checked before use. Inspect
638
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3 7: FUELS AND LUBRICANTS
HANDBOOK
the entire length to assure that the n u m b e r s are clcEirly indicated in the correct order and the graduation meirks between the numbers are present. Tapes should be made of steel or corrosion resistant material. Meike sure the snaps and eyes between the tape and the bob afford for proper fit and accuracy for measurement purposes. Tapes and bobs should be checked daily w h e n in use. Check the tape for kinks and splices, which would make the apparatus unsuitable for accurate gauging. The tape and bob fittings should provide for an accurate measure. Also ensure that the bob tip has not been worn down or damaged, which would create another measurement error. Lastly, make sure the tape and bob Eire fit for the purpose to be used. A tape and bob shall not be introduced into another product where the residue on the tape and bob would contaminate the product to be measured. A s t a n d a r d operating procedure to r e m e m b e r is t h a t crude oil/residual fuel oil tapes should not be used to measure distillate products. Many specialty petrochemical products are very sensitive to extremely small levels of contamination. Either separate tape and bobs may be necessary or the tape and bob may require special cleaning before use. There are still several items needed before a linear measurement is m a d e at the tank. Remember several other factors m u s t be accounted for to obtain accurate petroleum measures. The first to be discussed is what is needed in determining the temperature of the product to be measured. A thermometer or temperature measuring device is required. The industry standard for reference is API MPMS Chapter 7, Method of Measuring the Temperature of Petroleum and Petroleum Products. The temperature of the product is critical to correct the measured quantity to the standcird temperature. The temperature in a tank may vary greatly, especially for crude oil and residual fuel oils that are heated to higher temperatures to maintain fluidity and preventing gelling of the product. The heating coils are near the floor and heat convection helps swirl the product in the tank. It may not, however, provide for a uniform t e m p e r a t u r e within the tank. Even distillate products in tanks have t e m p e r a t u r e variations. After distillation and condensation of the product, it enters a storage tank. If measurements are made soon after that, there may be a temperature gradient within the tank. The equilibration of the product has not occurred. Another factor affecting the temperature within a tank is the external environmental conditions. If gauging is performed on a side of the tank were the sun is shining, it may indicate a higher temperature than if the gauge was taken on the shady side of the tank. Rapid ambient temperature cheinges may also increase the t e m p e r a t u r e variation within a tank. Product within the tank may not have h a d a chance to equilibrate. Increased temperature readings are advisable where temperature stratification is found within a tank. The temperature within the tank is measured using a thermometer or an electronic temperature probe (thermoprobe). Thermometers must meet ASTM E l , ASTM Thermometers specifications for the range of temperatures cinticipated in the product to be measured. Thermometers shall be calibrated at least yearly against a thermometer traceable to a certified national standard thermometer. In the U.S., the Standard body is the National Institute of Standards and Technology (NIST). Thermoprobes rely on electronic instrumentation to provide
a readout of the temperature. These must be calibrated o n a yearly basis or as recommended by the manufacturer. Calibration should cdso be accomplished whenever there is reason to doubt the accuracy of the measurement. Other temperature measuring devices may be temperature dials placed in the wall of the tank. These are placed strategically at various heights, normally along the stairs going u p to the tank roof. If a tank is not full, care must be taken only to read those thermometers on the tank side weJl that are immersed in product. One disadvantage to the dial type is that the probe may not extend sufficiently into the tank. They would then be very sensitive to the environmental factors at the wetll surface. For example, with the sun on the tank wall, it may have warmed up the product within 6 in. of the wall, yet the remainder of the tank remains cooler. Using these readings would obtain a higher temperature t h a n if a thermometer was used within the middle of the tank. These differences could have a significant effect on the volume correction factor, resulting in greater than anticipated quantity variations at the agreed temperature. Other temperature devices may also be used in the tank with electronic readouts at the tank itself, or more likely a control room. When performing manual temperature readings using a mercury in glass thermometer, a thermometer assembly or holder is also required. These assemblies protect the thermometer from being broken in the course of being lowered and raised. More importantly, they provide for a reservoir of product to surround the thermometer bulb. Thermometers have a tendency to react quickly to temperature changes. Without a product reservoir, by the time the thermometer is raised and turned to determine the reading, it would have changed temperatures first with the variation within the tank, but more importantly with the ambient air temperature. Several types of assemblies are in use. Several basic t5?pes are the cup case, flushing cup, and armored case (Figs. 3-5). Flushing cups are not referenced in the current MPMS Chapter 7, but occasionally may be found in use. As the assembly is lowered into the product to be measured, it quickly fills with product. The assembly is lowered to the height required for the temperature reading and it is allowed to equilibrate for at least 5 min at that level. Products having viscosities greater thein 100 Saybolt Universal Seconds may require 15 or 30 min before equilibration occurs. By raising the assembly rapidly, the p r o d u c t in the reservoir is more likely to maintain the temperature at the level to be measured. Temperature readings therefore need to be taken as soon as possible after the assembly is raised. Conversely, temperature probes with electronic readouts may not require any time for equilibration. Readings may be tciken shortly after reaching the desired level, reading when the read-out is stable. Steps needed before going to the teink. Check that the thermometer is not broken. Make sure that the liquid in the colu m n has not separated. Do not use thermometers with these defects. Ensure that the thermometer is within the temperature range of the product to be measured or obtain a correct thermometer. Lastly, ensure that the assembly adequately holds the thermometer in place and that the bulb of the thermometer is placed within the reservoir. The reservoir should not have holes or openings that would prevent the retention of the product in the reservoir. Replace the assembly if defective.
CHAPTER 23: STATIC PETROLEUM MEASUREMENT
639
CLAMP.
ASTMTHERMOMETER
CORROStONRCSBTANT METAL
CD CUP CASE FIG. 3—Cup case Thermometer.
MAX.p FIG. 5—Armored cup case. ASTM THERMOMETER
FLUSHING CASE
FIG. 4—Flushing cup case.
Free water is defined as the water present in a tank that is not in suspension or dissolved in the petroleum. To measure free water, something is needed so that the water level after drawing the bob or bar through the product can be observed. To detect the free water, a water finding paste is used to distinguish between the petroleum and petroleum product layer and the water level. For crude oils and many residual fuel oils, the bob or bar may retain a significant coating of the crude or product, hindering our ability to see the watermark to be read. In these cases, a petroleum solvent may be needed to rinse the bob or bar. Care must be taken not to disturb the water finding paste when rinsing the bob or bar with the petroleum solvent. This procedure also requires advance planning so that the proper solvent can be taken to the tank and equipment needed to preclude environmentcJ and safety problems with the rinse are available. It is also necessary to have a water finding paste that is suitable for the product to be measured. Some products have water soluble additives that leach out of the product into the water layer. The water paste must therefore be able to register the water/additive mixture as well as free water. An example of this interaction is fuel system icing inhibitor (FSII) (sometimes referred to as anti-icing additive). It is only slightly sol-
640
MANUAL
37: FUELS
AND LUBRICANTS
HANDBOOK
uble in aviation turbine fuel but very soluble in water. The water finding paste for these applications must be sensitive to a 50% water/50% FSII mixture. One way to test whether the water finding paste is still working is to place a drop of water on a bob with the paste. This will show a color change. Pastes are different, some changing from pink to blue, others one shade of pink to another, etc. By testing the paste before going u p to the tank, a new ganger will understand what is about to be seen when gauging for water. Remember, it will not tell if the water paste is working in water/other product mixtures. These must be tested in a laboratory at the prescribed mixture ratios. The measurement of sediment uses the same bob or bar used for the detection of free water. Water should sit on top of the sediment. While both are subtracted from the calculations of product quantity, it is necessary to know the level of sediment in the tank. Distillate product tanks have less sediment layering them do crude oil and residual products. Therefore, noticing sediment building u p on the floor may indicate it is time to clean the tank before a quality problem occurs with the product. Crude oil and residual fuel tanks may have larger quantities of sediment and sludge. Should the level become too high, it could affect the ability of the p u m p s to draw product out of the tank. The other problem it creates is that shipment of the contaminates decreases the value of the product. For most distillate products, sediment and water quickly falls to the bottom of the tank. A visual appearance of the fuel determines that the water and sediment have settled to the bottom of the tank and we do not have to worry about it for calculation purposes. However, for crude oils and residual fuel oils, sediment and water may also be suspended or otherwise in the product to be measured. The buyer of the products may not want to pay for these contaminants and therefore require an accurate measure of them. For crude oil and residual fuel oils, there is an anticipation that these contaminates will be received and dealing with them requires advance knowledge of the levels of each. If not already determined, a sample for the density measurement is required. The density is necessary as product expansion and contraction is affected not only by the temperature, but the product type and its density. This relationship will be explained later in this chapter. Crude oil and residual fuel oils are more likely to be non-homogeneous and proper sampling is more critical so that an accurate picture of the product quality can be determined through the testing process. Automatic sampling is the preferred method for sampling crude oils cind residual fuel oils because of this problem. In some cases, the density, water, a n d sediment for official custody transfer purposes will be determined based on in-line sampling at the time of shipment. By agreement, or to protect one of the parties, a manual sample may still be taken in case of a failure of the automatic in-line sampler. Automatic in-line samplers are recommended for custody transfer purposes as they may better represent petroleum and petroleum products that would normally contain suspended (entrained) S & W. Manual sampling may not be as accurate in sampling for these contaminants, which are present in the product in a non-uniform manner. Manual sampling of the tanks is covered by API MPMS Chapter 8.1 (ASTM D 4057), Manual Sampling of Petroleum and Petroleum Products. The object of sampling is to take a
representative sample of the product to be sampled. Distillate products and finished lubricating oils from a single batch are normally considered to be homogenous. That is, a sample from anywhere in the product should be uniform and, in theory, represent the product. In reality, slight changes in the refining or manufacturing process may cause some variation in the product. A permitted testing variation on specified parameters determined by prior agreement would provide for agreement that the batch tank is deemed homogeneous for quality and quantity procedures. Not discussed in this chapter is the automatic in-line sampling procedures covered by API MPMS Chapter 8.2 (ASTM D 4177), Automatic Sampling of Petroleum and Petroleum Products. While previously used to determine the quality of crude oil and residual fuel oils in the past, they are increasingly being used for automotive gasolines, aviation fuels and diesels both for quality control purposes and environmental regulation compliance. The tools needed for manual sampling include a sample bottle(s) and sampling cage or a weighted beaker (Figs. 6 and 7). A sample receiver or receptacle is a container in which all sample bites are collected during the sampling operation. The n u m b e r of bottles and sample receivers needed must be determined before going out to sample. Bottles and weighted beakers must be properly cleaned so as not to contaminate the sample before taking the samples. Sample bottles and containers may be clear or brown glass. Ccins used must have their seams soldered on the exterior surfaces to preclude contamination with flux used in their manufacture. Polyethylene unpigmented linear plastic bottles can be used for gas oil, diesel oil, fuel oil, and lubricating oils. These plastic bottles should not be used for gasoline, aviation jet fuel, kerosene, crude oil, white spirit, medicinal white oil, and special boiling-point products unless testing indicates there is no problem with solubility, contamination, or loss of light ends. The container closure must be clean and ensure a good tight seal. Loss of light ends have an effect on the density of petroleum and petroleum products, which is an important element for petroleum mea-
1-Lllre (1 qt.) Sample Weighted Cage (can be (abricated to HI any size bottle)
FIG. 6—Weighted bottle sampler.
CHAPTER
Copper handle
hitah
Copper wire lugs
Sheet lead
Cork arrangements
Beaker
1-Litre {1 qt) Weighted Beaker FIG. 7—Weighted beaker sampier.
surement. A sampling cage with a bottle is preferred for volatile products to preclude the loss of light ends created by other techniques during product transfers. Poorly taken samples provide inaccurate sediment and water results. Taking and protecting the samples is critical both for proper sampling technique, to provide accurate data for petroleum calculations, and to prevent disputes. What samples should be taken? An official sample is taken at the point of custody transfer and is used for custody transfer properties. The location where this sample is taken is by mutual agreement between the buyer and seller. The object for obtaining accurate samples is to have a sample representing the entire contents of the tank. A representative sample is a sample representing a small portion of the tank volu m e (that is, for tanks, ships' c o m p a r t m e n t s , containers, pipeline tenders, a n d so forth) obtained with a precision equal to or better t h a n that of the laboratory m e t h o d by which the sample is to be analyzed. Other samples are taken for Vcirious process, custody, inventory, a n d quality reasons. These samples can be taken as a single sample or as various spot samples as agreed upon. Some of the various types of samples that can be taken as shown in Fig. 8. A spot sample is one taken at a single location in a tank. It represents only material at that location. Examples of spot samples are: top, upper, middle, lower, and outlet samples. • Top sample is one taken at spot six inches below the top surface of the liquid. • Upper sample is one taken from the middle of the upper third of the tank contents. • Middle sample is one taken from the middle of the tank contents and should be half the distance between the upper and lower samples. • Lower sample is one taken at the middle of the lower third of the tank contents. • Outlet sample is one taken at the level of the bottom of the tank outlet, but not more than one meter from the bottom of the tank.
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When any sample is taken where the loss of volatile components would affect the sample result, the procedures in API MPMS Chapter 8.4, ASTM D 5842, Standard Practice for Manual Sampling and Handling of Fuels for Volatility Measurement, should be used. The object is to take and handle a sample without losing the light hydrocarbons. For instance, when a density is to be run, the loss of the light ends will increase the density (lower the API Gravity) of the tested sample. This is t u r n m a y affect the volume correction factor (VCF) used in determining the net quantity of fuel at the standard temperature. Basic technique requires the sample container to be filled to the 80% fill level, capped quickly, kept cool, and delivered to the laboratory for testing promptly. Volatile samples taken from taps should use an extension that can reach close to the bottle of the sample container. This will preclude splashing and the generation of vapors leading to light end loss. Brown or a m b e r bottles or sample cans must be used when taking samples of petroleum products that are light sensitive. Failure to use the appropriate container could cause changes to the quality of the product before testing of the product is accomplished. This could lead to inappropriate results that may affect the quantity determination also. When taking spot samples to be composited for a representative sample, a standard way to take samples is as follows: For tanks over 1000 barrels that contain more than 15 ft. of oil, an upper, middle and lower sample are taken and combined in equal volume to make a composite sample. For tanks over 1000 barrels which contain between 10 and 15 ft of oil, a middle sample from the top half and a middle sample from the bottom half are taken and composited. A single sample is taken normally when the level of the oil is less than ten feet. For tanks smaller than 1000 barrels, either a single sample or multiple spots can be used. Again make sure the parties involved agree on the t3rpes of samples in advance of the sampling process. Other spot samples can be designated or taken upon mutucd agreement for custody transfer purposes. A bottom sample is one that obtains material from the bottom surface of the tank or container at its lowest point. A
Hatch
r rj TH Tank contents
Outlet
•15 cm (6") Top sample Upper sample
Upper third
- Middle sample
Middle third
Lower sample Outlet sample
Lower third
Bottom sample
NOTE 1—TTie location shown tor the outlet sample appfes only to tanks with side Qutltta. It doea not apply whaa ttn outiat comas from the tkxx of the tani( or turns down into a sump. Bottom sample location must tie specified. NOTE 2—Samples shoiid t » otitained from within solid stand pipes as the materials normaiiy not representative of the material in the tanit at that point. FiG. 8—Sampiing terms.
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bottom water sample is a spot sample of free water taken beneath the petroleum contained in a tank or marine vessel tank. These last two samples are used as indicators of the contamination accumulating on the bottom of a tank, to determine where water came from (process water or sea water), or to detect bacteriological or fungal growth. Combining agreed upon spot samples is one acceptable method as is testing individual spot samples and averaging the results. A composite spot sample is a blend of spot samples mixed volumetrically proportionally for testing. For instance a multiple-tank composite tciken from marine vessel compartment tanks is a mixture of individual samples from several compartments, each of which contains the same grade of petroleum material. The mixture is blended in proportion to the volume of material in each compartment (weighted average method). API MPMS Chapter 8.3, ASTM D 5854, Standard Practice for Mixing and Handling of Liquid Samples of Petroleum and Petroleum Products, should be used for this process. This blending should be performed in a laboratory. Some tests may be made on the spot samples before blending and the results averaged. Two types of samples can be taken that represent the column of petroleum or petroleum product in the tank. Each has drawbacks as to its being a representative sample. They are used primarily for homogeneous product. First is an alllevel sample. An all-level sample is taken by submerging a stoppered bottle or beaker as near as possible to the draw-off level, and then opening the sampler, and raising the sampler at a uniform rate such that it is about three-fourths full as it emerges from the liquid. A running sample is one obtained by lowering an unstoppered beaker or bottle from the top of the oil level to the level of the bottom of the outlet connection and returning it to the top of the oil at a uniform rate of speed so that the beaker or bottle is about three-fourths full when it breaks the surface of the oil. Both a running sample and an all-level sample may not be a true representative sample, because the tank volume may not be proportional to the depth or because the operator may not be able to maintain the variable rate required for proportional filling. The rate of filling is proportional to the square root of the depth of immersion also affecting sample representativeness. The lower the beciker or bottle, the faster liquid flows into the beeiker or bottle. Pipeline samples can be of several t3rpes. Automatic samplers are covered by API MPMS Chapter 8.2 (ASTM D 4177), Automatic Sampling of Petroleum and Petroleum Products. Automatic samplers are designed to take a representative sample from liquid flowing through a pipeline. It consists generally of a probe, an extracting mechanism, a controller that determines when to extract the sample bites, and a sample receiver. Flow proportioning samplers adjust to product volume changes resulting from changes in the flow rate. This is the best way to get a representative sample from a pipeline. Another tj^e is a time controlled automatic sampler. It takes a sample bite at preset time intervals. Manual samples from pipelines, sometimes called drip samples, can be taken. A valve is cracked leading to a sample receiver so that over the duration of the movement, product slowly fills the sample receiver so that it fills the container over the determined period. This does not provide for a truly representative sample. Pipeline spot samples are another tj^e of manual samples.
They represent the product at a particular moment of sampling. Several pipeline spot samples can be combined to make a pipeline composite sample to represent the product in question. The number of samples required should be agreed between buyer and seller when used for custody transfer purposes. Changes in flow rates have little effect on the amount of sample being taken. Care must be taken to preclude overfilling samples taken from a pipeline. To get a truly representative sample from a pipeline, the product must be flowing at a turbulent flow rate. Turbulent flow promotes product and contaminants distributed equally throughout the pipeline cross section. When product has a laminar flow, there is a faster section moving above a slower section. Contaminants such as sediment and water can settle to the lower section of the pipeline or at the low points. Samples taken at laminar flow tend not to be representative. Before climbing the tanks, determine if control of the volumes in the pipeline are required. For custody transfer purposes, the condition of fill of the pipelines to be used must be known, both before and after the movement. If the pipelines are stated as full, validate by pressure or other means that the line is full. One reference for this operation can be found in API MPMS Chapter 17.6, Guidelines for Determining the Fullness of Pipelines Between Vessels and Shore Tanks. If empty, confirm as best as possible. If there is a difference in the condition of the pipeline fill before and after a movement, the differences must be accounted for in the product calculations. It's time to climb that tank or go to the marine vessel to take samples and to gauge the tanks. What is the proper order to perform the water cut, product gauge, temperature, and sampling? If water cut is done first, the oil column above the water layer may become contaminated. This would lead to erroneous results coming from the contaminated samples. Therefore the order should be to take the samples first. Which samples should be taken? For the same reason, taking the bottom water cut measurement, samples are taken starting from the highest in the tank. If a top sample is needed, take that first. Then the upper, middle, lower, outlet, and bottom samples. If after these samples an all-level or running sample is required, take these after the spot samples. The order of taking the samples is the same from compartments on a marine vessel. When homogeneous product is on board a vessel, a single middle spot sample may only be required. At other times the individual compartment may only require an all-level sample. However, when crude and heavy fuel oils that contain water and sediment in a non-homogeneous manner are sampled, several spot samples may be required and/or a running or all-level sample. Samples are taken and properly labeled with a marker that will not be affected should the oil splash or be introduced onto the label. The label must be affixed to the sample bottle or containers immediately after the sample is taken. This is extremely important when several different samples are taken from one tank. Always check that any labels, especicilly preprinted ones, are put on the correct sample. Ensure that all relevant details are on the sample label. No one wants to climb a tank a second time because the results from the testing indicate a problem. Side tap samples along the staircase can be taken either going up or going down. However, if samples are taken on the way up, it will make the carrier weigh more as you ascend the staircase. Recommend tEiking the samples on the way down
CHAPTER 23: STATIC PETROLEUM MEASUREMENT after gauging is complete. When using the side taps, know how high the product is in the tank and make a determination on which taps are needed from that information. On a full tank, the upper, middle, and lower samples are easily judged. When the tank is not full, the requirement for two sets of more than five taps on a tank can be understood. By knowing the height of the product in the tank, appropriate level samples can be taken. Some sample taps include extensions that enable sampling from the base of the tank. When these are encountered, make sure the tubing is flushed completely before taking samples. Otherwise, a prior batch left in the extension tubing will be sampled. At the top, the gauging and sampling tubes should be clearly marked. In most cases they are the same point. Sampling and gauging tubes must be appropriately slotted in order to obtain accurate samples and gauges. The gauging hatch or hatches should have the reference height marked on or near the hatch. While this must be validated from the certified capacity tables, it is a good reference while on top of the tank. Take the required number of samples in accordance with API MPMS Chapter 8.1 and Chapter 8.4 when taking volatile samples. After the samples are taken, go to the gauging. When using thermometers, place them at the appropriate levels now if they will not be in the way for the gauging operation. This will give them time to equilibrate to the surrounding temperatures. Table 3 of API MPMS Chapter 7.1, Static Temperature Determination Using Mercury-in Glass Tank Thermometer, provides for the minimum number of readings required based on product height. Where the depth is greater than 10 feet, three temperatures are required, one each from the middle of the upper, middle, and lower sections of the tank. For depths less than 10 feet, a single temperature taken at the middle of the liquid is required. Tables 4A and 4B of Chapter 7.1 provide for the minimum immersion time for these thermometers. The immersion time is based on the density of the product and whether the thermometer is stationary at the required level or in-motion (raised cind lower one foot above and below the designated level). Table 2 of API MPMS Chapter 7.3, Static Temperature Determination Using Portable Electronic Thermometers, provides recommended immersion times for electronic thermometers. The times again vary based on the density of the product and whether the thermometer is held stationary or kept in-motion as described above. Check these publications for the appropriate immersion times. When gauging for custody transfer purposes, there is, by necessity, an opening gauge and a closing gauge. When standing on a roof, your weight and the weight of those watching or helping you may depresses the roof. It is important that the same number of persons, hopefully of the same approximate weight, accompany the gauger for each measurement. Encourage everyone to stand in the same relative positions (at the side) to keep these factors constant. If a water gauge and product gauge will be taken at the same time, apply the water finding paste to the bob and a thin coating of product finding paste along the anticipated levels. When gauging crude oils or heavy fuel oils, it may help to coat the water finding paste and the bob with a light lubricating oil in order to help shed the oil after its return to the top for reading. Product finding paste is not needed for heavy fuel oils.
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crude oils, and other dark viscous products as they do not evaporate or rise up the tape. Talc and chalk may not be used if the product has a tendency to creep up the powdered tape from the true reading giving erroneous readings. Place the tape against the reference point on the gauge hatch. This releases static charge, which may build up. Lower the bob slowly through the product. While lowering the tape and bob, try not to disturb the liquid level greatly. Motion at the top of the liquid level causes an inaccurate reading. The waves caused by the disturbance or the wind can result in an increase in the total liquid height measured. For an innage gauge, lower the tape until the length in the tank is close, but not over, the reference height. Remember it is most likely stenciled or written by the gauge hatch. Just compare it to the tape reading and when they are close (that's the point to pay close attention) be very careful, lowering the tape until you feel the bob hit the datum plate or floor of the tank. It is very important to come to a full stop when this is felt. Lowering the tape any further results in the bob and the tape arching in the tank. The bob leans to one side and more tape goes into the product. Both measurements will have increased by some amount. If you go far enough, the bob will actually be resting horizontally (flat on the floor or datum plate). Maintain the tape in the stop position. Record the total height that the tape is out. This should be equal to the reference height. If it is not, then when performing the calculations, some adjustment may be needed to the product height. The length of time the stop position must be held is dependent on the product in the tank and the amount of water. Heavy products tend to coat the bob and water finding paste before the bob enters the water layer. This type of product requires the bob to rest longer in position for accurate water readings. It is appropriate for clean light products to maintain this position for 10 s. For heavy crude oils and fuel oils, it is appropriate to maintain this position for 1-5 min. If there is a large body of sediment and sludge on the bottom, using the innage method may be impractical as the bob will never reach the bottom of the tank or datum plate. The bob is unable to cut through it. More than likely, an experienced gauger will be able to discern the problem by the way the tape feels as it hits the sediment and sludge. But more practically, the reference height will be greater than the tape reading at the reference point when this occurs. When there is sediment and sludge precluding the bob from reaching the bottom, the outage method of gauging is required. The outage method may be required for both the product and water cuts. However, there are occasions when a bob is unable to cut through sediment and sludge, but a water gauging bar or rod can. As long as the bar or rod hits the bottom, gauging using an innage method of the total sediment and water is appropriate. One other problem occurs with tanks that have a large bottom sediment and sludge layer. The solid material may not be layered in the tank. Rather, pejiks and valleys may have developed due to the dynamics in the tank. Close to the inlet, the sediment may have been pushed up forming peaks away from the inlet. Conversely, d5mamics at the suction point results in washing away of the sediment building up. Again a valley may be formed here. Several measurements for sediment at various levels may be necessary to discern these anomalies. There are several schools of thought on gauging total height of the liquid at the same time as the free water mea-
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surement. First, gauge the free water, first getting the reference height estabhshed once. This is only done once. Then proceed on with gauging for the total height of the liquid in the tank. Second—Perform b o t h the free water and total height of the liquid simultaneously. Assure that an agreement has been reached on this point between the buyer and seller or as a company policy and then advise the ganger and/or independent surveyor of the agreed procedure. Continuing on with the innage method using the second school of thought, raise the tape until the product cut can be seen. Read the location of the product, referred to as a cut, to the nearest millimeter (or 1/8 in). Record the product cut. Raise the tape and bring the bob to be viewed. Be careful when bringing the bob u p and out so as not to disturb the water finding paste. Read the water cut and record it also. For tanks with crude oil and heavy fuel oils, it may be necessary to rinse the oil off the bob to view the water cut. Use an appropriate solvent to clean the bob. Take care in rinsing with a solvent so as not to push the water line u p or down. Be environmentally friendly when performing these actions. Additionally, do not blow or wipe the petroleum or petroleum products off the bob as this too may distort the water cut and thereby give incorrect readings. Perform the gauging actions again. They should be identical. If not try once more. If all three are within 3 m m (1/8 in) of each other and two are identical, then the two identical readings are reported as the official gauge. If not within 3 m m (1/8 in), continue gauging until three consecutive gauges are within 3 m m (1/8 in) of each other. If none are identical, averaging of the gauges m a y be permitted. Averages are to the nearest millimeter or 1/8 in, depending on the unit being used. Other a r r a n g e m e n t s are permitted by agreement. Smcdler tanks or tanks gauged in windy conditions may preclude the level of accuracy desired as the product level in the tank is unstable. Small waves are created by the wind and the differences in the wave heights between each measurement create the inaccuracy in measuring the product height. Once completed it is time to return to the office to perform the calculations. However, before coming down off the tanks, if several people are gauging or witnessing the gauging, agree on the gauges before coming down. This precludes disagreements after the fact. Most marine measurements are not used for custody transfer purposes, but primarily for inventory control purposes. Let us look at performing a n outage or ullage gauge. This is used for most heavy dark oils and is the preferred method for marine vessel gauging. The basic difference is that the bob does not reach the bottom of the tank. When using an outage tape and bob, ground the tape and bob to the tank. Lower the tape and bob into the product. Lower the bob into the product only slightly. Remember, the object of a n outage tape and bob is that the product reading is on the bob between the zero cmd the plus side (lower end of the bob). At the reference point, have the tape come to a whole increment, either centimeter or inch. Note the distance the tape has been lowered into the tank. Bring the tape back to the surface emd read the bob. If within the correct reading zone, the bob reading is added to the distance the tape was measured as being lowered into the tank. Unfortunately, there are occasions when the gauger for any n u m b e r of reasons lowers the tape too far. When you bring the tape and bob back up, you can read a measurement
on the u p side of the tape. In this case, subtract the product reading from total distance the tape was lowered into the tank. Of course, if you used an extension outage bob designed for an innage tape there is a section where n o linear measurements are encountered on the bob. Just clean the bob and redo the measurements. As in the innage method, two consecutive readings are required to be identical. Otherwise use the same innage rule for obtaining gauged measurements. Taking an outage gauge with an innage gauge and bob follows the same procedures as an outage gauge. Ground the tape and bob to the tank by placing the tape against the tank rim. Lower the tape and bob just barely into the product. Lower the tape to the next whole increment on the tape. Read and record the total tape height let out. Bring the tape and bob back u p to the top and read the product cut on the tape or bob. If the reading is within the eye or eye hook area where no linear markings are, clean the tape and bob and redo the measurement using a different lowered distance. For innage tapes and bobs, the outage is figured by subtracting the lower tape or bob reading from the total height the tape and bob were lowered into the tank. E x a m p l e 1: O u t a g e T a p e a n d B o b a) Total height of tape p u t into the tank read at the tank gauging point = 7.450 m b) Measurement found o n the bob when raised = 0.090 meters. Add (a + b). c) Outage Gauge for total liquids in the tank = 7.540 m E x a m p l e 2: I n n a g e T a p e a n d O u t a g e B o b a) Total height of tape p u t into the tank read at the tank gauging point = 7.450 m b) Measurement found on the bob when raised = 0.090 meters. Add (a + b). c) Outage Gauge for total liquids in the tank = 7.540 m E x a m p l e 3: Innage T a p e a n d B o b or a n Outage Tape a n d B o b Reading above the Zero Mark or an Innage Tape with an Extension Outage Bob Reading above the Zero Mark o n the B o b a) Total height of tape p u t into the tank read at the tank gauging point = 7.450 m b) Measurement found on the bob when raised = 0.090 m. Subtract (a — b). c) Outage Gauge for total liquids in the tank = 7.360 m After performing the total liquid measure in the outage method, a measurement for water and sediment is still required. This can be done by the innage method as described above provided the bob or rod can reach the bottom of the tank. For tanks that may contain sediment and sludge, make sure the bottom is reached. When the reading is tciken, estimate the level of sediment and sludge by looking for its adherence to the water finding paste. There will then most likely be a section where the water finding paste turned the appropriate color without any sediment. Both measurements are important information, be it for unpumpable bottoms, or levels that would indicate a possibility that the water and sediment would be pumped from a tank.
CHAPTER 23: STATIC PETROLEUM MEASUREMENT However an outage measure can be performed to obtain the height of the water cind sediment. This is a little tricky as some trial and error may be necessary. Apply the water finding paste to the bob and if the water layer is believed to be greater than 15 cm (6 in.), apply the water finding paste to the tape for an appropriate distance. Ground the tape and bob to the tank top and then lower until you believe you are in the water layer. Remember the problems that occur if the tape and bob hit sediment or sludge cire identical to going too far in the innage gauge procedure. If you hit a solid layer of sediment and sludge, the bob could lean. If this happens and the gauger shows that the tape and bob have leaned or arched, bring the tape and bob back up, clean the equipment and lower the tape and bob a little shorter. Lower the tape to the next whole number on the tape and record the reading. Bring the tape and bob up. It may be necessary to use an appropriate rinse to clear heavy oils off of the section to be read. Use the same precautions as discussed under the innage method. Read the water cut before the calculations as above. Depending on the tank strapping chart, this reading may have to be converted to an innage gauge. Remember that the reference height of the tank minus the outage gauge equals the innage gauge. A quick mention of automatic tank gauging is needed. API MPMS Chapter 3.IB covers the Standard Practice for Level Measurement of Liquid Hydrocarbons in Stationary Tanks by Automatic Tank Gauging. These instruments require calibration before use. Manual gauging in accordance with API MPMS Chapter 3.1 A is recommended for most calibration procedures. Additionally, some may not have the accuracy required for custody transfer purposes. Use of an automated tank gauge (ATG) requires agreement by the parties prior to their use. ATGs can have remote readouts, gauge readers at the bottom or the top of a tank and sometimes an external marker on the side of a tank. Even with ATGs, slotted standpipes when used are recommended. Solid stilling wells ("still" pipes) used for environmentetl reasons are not recommended for custody transfer purposes as they can lead to inaccuracies of the level and temperature measurements. There are several types of ATGs in use currently. A float operated ATG contains a float, which travels along one or two wires. The floats are designed to be buoyant at certain densities. They will therefore remain on top of the product intended. Turbulence in a tank can put the float out of calibration. When used in floating roof tanks, wind can stretch the wire holding the float cind provide inaccurate readings. A servo-operated automated tank gauge is another type that uses a small displacer into the liquid level from a flexible cable or tape. These are normally located at the top of a tank in a still pipe. Hydrostatic tank gauges use precision pressure measurement devices located at several levels of the tank. These gauges measure the hydrostatic head pressure at each point and use strapping chart data to calculate the mass, density, volume, and level within the tank. Temperatures are tcJcen and fed into the automated calculation and readout system. This system is prone to inaccuracies caused by product stratification, especially density stratification. Another type of ATG is the radar tank level gauge. Radar waves are used to determine the distance remaining in the tank. It measures the time it tcikes for a radar wave to go to the liquid surface and return. Then there are sonic and ultrasonic tank level gauges. These operate
645
using an acousticcJ sound. The device measures the elapsed time of the sound and the return of its echo. These are a few of the automated gauging types. Some ATGs are also able to measure the water/ product interface. To be truly accurate, at least one more measurement is needed. The ambient air temperature just on the outside of the tank is needed for use in the correction for steel of the tank shell calculations. The ambient air measurement is made at least 3 ft. off the ground on the shady side of a tank. It should be taken by the gauger either just before or just after the tank gauges. Allow enough time for the thermometer to equilibrate with the ambient air before reading. Measurements on marine vessels are taken identically as on land. There are two additional factors that are required. On marine vessels, the vessel may not be level in two directions. The front to back slope on a marine vessel is called trim while the right to left tilt is called list. These are needed because the strapping charts are prepared for vessels that are level. When the vessels become out of level, the liquid level remains parallel to the pull of gravity, changing the gauge readings at the point where it is normally taken. To understand this concept, take a glass of water filled only half way. Tilt the glass in any direction. The liquid level remains parallel to the ground. The steeper the tilt either forward or backwards or side to side can place a liquid surface close to the edge of at the top while the other side gets close to the bottom of the glass. Marine vessels have calculated values contained on the strapping cheirt for trim and list. It is therefore necessary to determine the trim and list. Marine vessels have markings on the side of the vessel near the bow, middle, and stem to indicate how far into the water they are. These draft markings are used to determine the trim. Read the draft markings as best as possible. If the bow is higher than the stem, the vessel is down by the stem. Should the bow be lower than the stem, the vessel is down by the head. Note that the higher the draft, the lower the vessel is sitting in the water at that point. The middle draft is used to determine whether the vessel is arching or bowing in the water. Weight is not always equal at every point within the vessel and some hog or sag is expected. Additionally, the vessel may lean to the right (starboard) or left (port). A vessel's inclinometer, usually on the bridge, is used to measure the vessel's list. If the inclinometer is not available, the list can be calculated from the midpoint port and starboard draft readings. The measurement of the trim and list must be made at the time the vessel gauging is performed. Otherwise the corrections applied to the actual vessel gaugings will be inaccurate. That is one reason why gauging should occur after completion of loading or discharge. During the transfer operations, the trim and list may change. The corrections are added or subtracted to the observed gauges based on the instructions contained in the strapping chart. In a marine vessel, on-board quantity (OBQ) and remaining on board (ROB) quantity must be measured. As long as these volumes cover the entire bottom surface and all four walls, the strapping chart applies. There are times when this does not occur, but measurements must be tsiken. A wedge formula is used to calculate the OBQ or ROB. Some linear measurements of the cargo compartment are needed. For use of the wedge formula, use API MPMS Chapter 17.4 Method of Quantification of Small Volumes on Marine Vessels (OBQ/ROB). Where a solid or nearly immovable substance is
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layered on the bottom of a marine vessel, trim and list corrections are not applied. Rather for this layer, it can be considered as layering the bottom of the vessel as a solid sheet. Use the strapping chart to obtain this quantity as if the vessel were on even keel. Nov*? for the really easy pEirt, the calculations back in the shore office or the ship's office. The calculation procedures are governed by API MPMS Chapter 12.1, Calculation of Static Petroleum Quantities. We will be taking the linear measurements and converting them to volumes now. In doing so, we need to define some terms. • Total Observed Volume (TOV) is the volume found from the measured total liquid height. It includes all petroleum liquids, water, and sediment in the product, free water, and bottom sediment. • Free Water (FW) means all free water and bottom sediment. This is the water cut measurement converted to a liquid volume. It includes the bottom sediment a n d sludge, if present, as the measurement includes it. Additionally, the free water and the sediment and sludge have different volume correction factors from petroleum products. • Gross Observed Volume (GOV). This is the total volume of petroleum liquids and sediment and water in the product less the free water volume. (GOV = TOV — FW). • The Volume Correction Factor (VCF) is correction for temperature of the liquid to a standard temperature. It may also be referred to as the Correction for Temperature of the Liquid or CTL. • Gross Standard Volume (GSV) is the GOV times the VCF (GOV X VCF). • Net Standard Volume (NSV) is the total volume of all petroleum liquids, excluding S & W and free water, corrected by the appropriate VCF. (G.S.V. - S & W = N.S.V.) For sales, this is the value required. However for inventory purposes and storage, shipping, and handling, we need to know and understand the other definitions. A tank may be able to contain the NSV, but not the TOV. Remember that petroleum products expand when the temperature of the product increases. Leave enough room in the tank for said expansion. Volumes in pipelines also expand and contract. Static lines may not be full if several expansion/contraction cycles caused by day/night t e m p e r a t u r e variations have occurred. Some product may have moved to another tank through the pressure relief system. From the laboratory tests we receive the density at 15°C (API Gravity at 60°F) and the tested S & W contained in the product as a percent reading. Performing a density or API Gravity test, follow the procedures in API MPMS Chapter 9 Density Determination. Several ASTM test methods are D 5002 (Crude Oil by Digital Density AnjJyzer), D 4052 (Liquids by Digital Density Meter), and D 1298 (Hydrometer Method). The basic principle for the manual method is to take the sample and place it in a hydrometer cylinder. Make sure all air bubbles Eire eliminated from the top of the liquid. Use a hydrometer with an appropriate scale reading. Some hydrometers have a thermometer as p a r t of the hydrometer. Otherwise, a separate thermometer is required. It is best to have the product being measured equilibrate with the ambient temperature as it precludes temperature drift during the measurement. Especially in the field, this may not also be possible. Lower the hydrometer into the liquid to a position close to where the hydrometer
would float. Getting liquid above this position on the hydrometer stem can result in slightly inaccurate readings as the liquid may be retained above the level of the product depressing the hydrometer slightly. Spin the hydrometer to disperse air bubbles on the bottom of the hydrometer. Allow the hydrometer to come to rest. Read the hydrometer to get the observed density. As quickly as possible read the thermometer to obtain the temperature that the observed density was taken. When using separate hydrometer and thermometer, remember to allow the thermometer time to equilibrate in the liquid before reading it. To convert density at observed temperature to density (API Gravity) at standard temperature, follow the procedures in API MPMS Chapter 11.1 (ASTM D1250 or IP 200) and the appropriate table designations. There are tables for specific range of products. Table A is used for crude oil and naphtha based aviation turbine fuels. Table B is used for most other petroleum fuels. Table D is used for lubricating oils. For those that wish to perform the coefficient of expansion tests each time a VCF is needed for a specified product, use table C. Tests for S & W include centrifuge method, water by distillation, water by Karl Fischer and Sediment by extraction. Round values to the precision required as stated in API MPMS Chapter 12.1.1, paragraph 6.2 and Table 1. You can do the calculations or a computer may be able to do them if programmed appropriately Eind the correct numbers are entered into it. For shore tanks, take the total liquids gauged measurement and go to the strapping chart. The volume obtained is the TOV. For marine vessels, the trim and list factors must be applied to t h e total liquids gauged. This will b e contained within the strapping chart. Based on the trim and list, the strapping chart will advise you to add or subtract some linear measurement to the observed reading. From this correct linear measurement, obtain the TOV from the strapping chart. Whether a shore or marine tank, free water is normally obtained as an innage value. Look u p the total volume of free water (remember this includes the b o t t o m sediment) and record this volume as FW. There may be occasions when the level or type of free water and sediment necessitated an outage gauge. If the chart is in innage measurements, convert the outage gauge to a n innage gauge and look u p the FW volume. If the volume of free water and b o t t o m sediment is great, using the outage gauge table may be appropriate. On marine vessels, remember trim and list corrections when the bottom layer is fluid. For true corrections, the correction for temperature of the shell (CTsh) is needed. This compensates for the expansion and contraction of the tank shell caused by the temperatures of the product and the ambient air. Tanks are strapped assuming a specified temperature range. The CTsh is used to bring the liquid volume back into calibration. In accordance with API MPMS Chapter 12.1.1, Section 9.1.3 the adjustment used is CTsh = 1 - 1 - 2aA T + a^\^. The symbol a equals a linear coefficient of expansion of the tank shell, with Appendix B-2 containing several coefficients for various metals. The AT equals the Tank Shell Temperature — Base Temperature. The base temperature is the temperature that the strapping chart was calibrated at. The tank shell temperature for an insulated tank is assumed to be the temperature of the product. For noninsulated tanks, use the formula in Appendix B of API MPMS Chapter 12.1.1. Calculations for a floating roof tank include a floating roof
CHAPTER adjustment (FRA). Remember that a floating roof displaces product due to its weight. Some strapping charts contain the formula for this adjustment. Others just provide the weight of the roof and the FRA must be calculated. Check this carefully w h e n performing the CcJculations. The basic principle is that the roof correction = weight of the roof / (density X VCF). The displacement a m o u n t was also determined at a specified density and part of the correction includes an adjustment for the difference between the observed density and the density used in the calibration, most likely 15°C or 60°F. Now we can determine the GOV. For shore tanks, the GOV is the [(TOV - FW) X CTsh] ± FRA. The FW is deducted before the FRA because water and s e d i m e n t compress differently t h a n petroleum a n d petroleum products. For most purposes, it is assumed not to have a compression factor and therefore is ignored in the calculations. For marine vessels the GOV = (TOV ± Trim/List Corrections) — FW. This now gives the total liquid volume to be sold or moved. The next step is to convert this volume at an observed temperature to a volume at the standcird temperature, referred to as the Gross Standard Volume (GSV). The GSV = GOV X VCF. For distillate fuels and lubricating oils, the trace amounts of entrained water and sediment are accepted without adjustments to the quantity. Therefore for distillates and lubricating oils, we also have the Net Standard Volume (NSV). In fact for these products, we normally do not refer to a GSV, only the NSV in the CcJculations. However, in the case of crude oil and some heavy heating oils, a deduction is made for the amount of S & W contained in the product. From the laboratory, the water and sediment value is provided as a percentage. The NSV is therefore the GSV times a correction for the sediment and water (CSW). To obtain the CSW, convert the sediment and water percentage into a decimal and then subtract that value from one (1.0000). For example, if the sediment and water contained in a crude oil is 4.25%, converting to a decimal gives 0.0425. Then (1.0000 - 0.0425) = 0.9575 as the CSW. Then the NSV = GSV X CSW. Conversely, use the converted sediment and water decimal and multiply it times the GSV. This will give a volume of sediment and water contained in the GSV. Then the NSV = GSV minus the volume of sediment and water CcJculated. This concludes volume calculation. Lastly, many users of this information also require the apparent mass or weight in air of the product. For this we use a Weight Conversion Factor (WCF), which is a relationship between the density of the product and its weight in air. There are m a n y tables contained in Volume XI a n d XII of API MPMS Chapter 11.1 that give these relationships. There are also numerous methods to convert from volume to weight. It is preferable to approach the ceJculation process from the simplest, most direct direction. Using different approaches results in slightly different weights. It is best to obtain agreement on the conversion process as part of the agreement between buyer and seller or as a company policy for internal use. This will preclude problems by those performing the calculations or inventory control reviews. The order of calculation is TOV to GOV to GSV to NSV to Weight in Air. The exception is when the tank measurement is by a direct mass method, for example a hydrostatic tank gauge. The reading is already in mass units and the calculations are performed within the little black box. All that is
23: STATIC
PETROLEUM
MEASUREMENT
647
needed is to confirm all the adjustments have been made for FRA, CTsh, FW, and sediment and water. As noted earlier, the calculation psirt of static petroleum measurement is really the easy part. Let u s perform a sample calculation here. Information from the tank and lab are provided: Product Crude Oil TOV = 10,250,500 USG FW = 765 USG Sediment and water = 2.25% Product Temperature == 67°F LT/BBL TOV FW minus equals times equals FRA times
GOV CTsh
API Gravity = 28.5 FRA = - 7 5 . 5 5 USG CTsh = 1.00009 VCF (Table 6A) = 0.9969 WCF (Table 11) = 0.13808 10,250,500 USG 765 10,249,735 USG X 1.00009 10,250,657 USG - 76 USG
10,250,581 USG VCF table 6A x 0.9969
GSV 10,218,804 USG Note: (S = Standard temperature, in this example it is 60°F) CSW (1-0.0225) 0.9775 times
equals NSV 9,988,881 USG Divided by 42 USG/barrel equals 237,830.50 barrels times WCF (table 11) 0.13 808 long tons/barrel Long Tons
32,839.64 long tons
Using this information requires an understanding of the purpose of the measurement and its relationship to other measurements. In the petroleum industry, careful control of quantities is expected. First, the profitability of a company depends on accurate measures for sales and purchases. Losses affect the company's bottom line. If you obtain a gain, someone else has taken the loss. Second, inventory control procedures use these quantities to determine theft during movements or spills and leaks during storage or movement. The accuracy of these determinations affects the time spent in investigating the loss or gain. Losses may require an expenditure of funds in investigating the loss, some internally m a n d a t e d , b u t m a n y regulatory mandated. Within the industry, it is recognized that the measurements between two points contain some vjiriation. The principle causes of these differences are contained in the measurement precision of the methods used in determining the linear gauge, temperature, automated gauging precision, density measures, evaporation loss, and others. These slight variations may give slightly different volumes between two different measurements. One source of error is slowly being displaced by the use of precision equipment. A temperature obtained using a thermometer will have an error caused by our ability to discern between the graduation marks. The electronic versions provide the temperature more accurately. These changes reduce the variation expected. Agree on the variation due to measurement errors in adveince or have company policy clearly explain that some variation is expected. However, losses that can be determined, such as spills, leaks, and thefts are not contained in the variation.
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These need to be identified and explained, and procedures should be developed to eliminate them. Volatile hydrocarbon losses can be calculated. With increased attention on the environment, environmental groups are mandating reductions in the release of volatile organic hydrocarbons. These normally require some sort of engineering changes to the tanks and facilities. The true measure of successful static petroleum measurement is the care and practice of the techniques described in obtaining the required data. The calculations are easy and straightforward. However, with incorrect data, all that is obtained is incorrect volume eind weight measurements. Take the time to train the samplers and gaugers to perform the required steps in accordance with the defined procedures. Make sure they understand how important they are to the health and financial success of your operation.
ASTM STANDARDS No. D 1250 D 1298 D 4052 D 4057 D 4177 D 5002 D 5842
Title Petroleum Measurement Tables Hydrometer Method Liquids by Digital Density Meter Manual Sampling of Petroleum and Petroleum Products Automatic Sampling of Petroleum and Petroleum Products Crude Oil by Digital Density Analyzer Sampling and Handling of Fuels for Volatility Measurement
D5854 El
Mixing and Handling of Liquid Samples of Petroleum and Petroleum Products ASTM Thermometers
OTHER STANDARDS American Petroleum Institute (API) Manual of Petroleum Measurement Standards (MPMS), Chapter 3.1 API MPMS Chapter 7, Method of Measuring the Temperature of Petroleum and Petroleum Products API MPMS Chapter 8.1, Manual Sampling of Petroleum and Petroleum Products API MPMS Chapter 8.2, Automatic Sampling of Petroleum and Petroleum Products API MPMS Chapter 8.3, Standard Practice for Mixing and Handling of Liquid Samples of Petroleum and Petroleum Products API MPMS Chapter 8.4, Standard Practice for Manual Sampling and Handling of Fuels for Volatility Measurements API MPMS Chapter 10, Density Determinations API MPMS Chapter 11.1, Physical Properties Data API MPMS Chapter 12.1, Calculation of Static Petroleum Quantities API MPMS Chapter 17.4, Method of Quantification of Small Volumes on Marine Vessels (OBQ/RBOB) API MPMS Chapter 17.6, Guidelines for Determining the Fullness of Pipelines Between Vessels and Shore Tanks International Petroleum (IP) 200, Petroleum Measurement Tables
MNL37-EB/Jun. 2003
Hydrocarbon Analysis James C. Fitch^ and Mark Barnes^'•^
Basic Principles of NMR Spectrometry
THE AIM OF THIS CHAPTER IS TO BRIEFLY PRESENT SIX ANALYTICAL
METHODS for characterizing hydrocarbon compounds found in fuels and lubricants. The methods presented are NMR spectrometry, gas chromatography, liquid chromatography, ultraviolet spectroscopy, mass spectrometry, and infrared spectroscopy. The analysis of hydrocarbons deploying these methods is well-founded in scientific laboratories and is the basis of numerous ASTM standards. Because of the large body of published work on these methods, it is not the intention of the authors to attempt complete coverage of all methods, but rather to provide an overview of their use in the analysis of fuels and lubricants. Following each section are summaries specific to ASTM D 02 standards where the analytical method is deployed in the areas of fuels and lubricants.
Absorption spectrometry works because applied electromagnetic radiation of an appropriate frequency induces a transition from one (lower) energy level to another (higher) energy level within the molecule, causing the radiation to be absorbed by the sample. In IR and UV spectrometry, these energy levels are the vibrational and electronic energy levels of the molecule, whereas in NMR the energy levels are associated with the nuclear spin of the nuclei that make up the molecule. For the purpose of detailing how NMR works, we will focus on the most commonly studied nucleus, the hydrogen nucleus 'H, which has a nuclear spin of li. A nucleus such as 'H with a spin of 'A possesses two nuclear spin states, +'A and —'A. Under normal circumstances, these spin states have the same energy and are said to be degenerate. However, in the presence of a strong magnetic field (typically 2.33 tesla (T) for the ^H nucleus), the two nuclear spin states split into two energy levels, their separation being proNUCLEAR MAGNETIC RESONANCE (NMR) portional to the applied magnetic field as shown in Fig. 1 [1]. SPECTROSCOPY Applying a source of electromagnetic radiation and slowly Introduction scanning the frequency can induce absorption when the radiation frequency matches or is in resonance with the energy Nuclear magnetic resonance (NMR) spectroscopy is one of separation of the two nuclear spin states under the applied the most widely used analytical tools in chemical analysis. magnetic field. The frequency of radiation that must be apJust like infrared (IR) and ultraviolet (UV) spectroscopy, plied to induce such a transition is typically in the radio freNMR is a form of absorption spectrometry, whereby the quency (rf) range. For a ^H nucleus under an applied magamount of absorbed electromagnetic radiation at a given frenetic field of 2.33 T, the resonance frequency is around 100 quency can be related to the concentration of certain chemiMHz [2]. For practical reasons, NMR spectrometers usually cal species that absorb at that frequency. However, unlike IR work in the opposite sense, with a fixed electromagnetic raspectroscopy, which looks at functional groups within a diation frequency (commonly 100 MHz for ^H NMR) and a molecule, or UV spectroscopy, which looks at the molecule as scanning magnetic field [3]. However, the same principle apa whole, NMR is used to determine the concentration of speplies: the magnetic field is scanned until the energy separacific atoms within a sample. tion of nuclear spin states induced by the field comes into resonance with the applied electromagnetic radiation. It is not the mandate of this chapter to provide a detailed quantum physical explanation of NMR or any other analytiIn reality, the magnetic field that must be applied to bring cal method described herein. However, the basic theory bea nuclear spin transition into resonance with the applied hind NMR spectroscopy will be outlined in this section, so electromagnetic radiation varies slightly with the chemical that the reader can obtain an understanding of NMR and environment of the nuclei in question. This phenomenon is how it can be applied to the analysis of large hydrocarbon known as shielding and can be attributed to the effects of the molecules typically found in fuels and lubricants. electrons surrounding a particular nucleus, shielding the nucleus from the effects of the magnetic field [2]. Because the electron density surrounding a particular nucleus is directly related to the chemical structure of the molecule, the electron distribution within the molecule gives rise to shielding ef1 Noria Corporation, 1328 E. 43''' Court, Tulsa, OK 74105. ^ Technical Editor, Practicing Oil Analysis Magazine, Tulsa, OK fects, which are typically called the chemical shift, which can be used to differentiate between the same nuclei (for example 74105. 649 Copyright'
2003 by A S I M International
www.astm.org
650 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK will thus be split into two groupings, one associated with the methyl hydrogen nuclei, t h e second with the aromatic hydrogen nuclei. The relative intensities of the absorption pesiks for the two groups will be in the ratio 3:5 corresponding to the relative ratios of the nuclei in the two environments. The ' H NMR spectrum for toluene is shown in Fig. 2 [4]. The chemical shifts reported in NMR spectrometry are usually expressed as the pEtrts-per-million (ppm) shift from the r e s o n a n c e frequency of a reference standard, usually tetramethylsilane (IMS). The chemical shift c a n be calculated using the following formula [5]:
' H ) in different chemical environments. Take for example the molecule toluene: H
H
C _ H H
H
Chemical shift (in p p m ) = (RFS-RFTMS)/RFTMS
H
H
Toluene h a s t w o types of protons, either as part of t h e methyl group, or attached t o a n aromatic carbon. Because the shielding experienced by these two different types of protons will depend on both electron density and ring current effects, the resonance frequency for interaction with the applied field will be different. The NMR spectrum for toluene
Where RFs is t h e resonance frequency of t h e sample a n d RFTMS is t h e resonance frequency of t h e TMS reference. Some typical chemical shifts for ' H in hydrocarbon molecules are shown in Table 1 [5]. The key to using NMR spectrometry in t h e analysis of hydrocarbon molecules is to recognize the chemical shifts observed in the NMR spectra £ind relate the relative concentrations of nuclei in these different environments to help elucidate the bulk properties of a hydrocarbon sample, for example the concentration of aromatic molecules in a hydrocarbon oil. " C NMR Spectrometry NMR spectrometry is not limited to just hydrogen nuclei. In fact, any nucleus with a n odd s u m of protons a n d neutrons will possess a magnetic m o m e n t and can thus be analyzed using NMR. Aside from 'H, the second most commonly studied nucleus is '^C, which like ' H has a nuclear spin of'/. Carbon exists in two naturally occurring isotopes ^^C a n d '^C, with relative natural abundances of 98.9% and 1.1%, respectively. Although '^C has a n even s u m of protons a n d neutrons a n d thus does not possess a magnetic moment, with sufficient ex-
Magnetic Field of Strength
Relative Energy
TABLE 1—Typical chemical shifts for 'H nuclei in hydrocarbon molecules. Chemical Shift/ppm ' H Nucleus Environment CH2 and CH3 H atoms far from 0.5-2.0 aromatic rings 2.0-4.0 CH2 and CH3 H atoms in the a position relative to an aromatic rings 6.0-7.2 H atoms in monocyclic aromatic rings H atoms in Polycyclic aromatic rings 7.2-9.0
Absorbed Energy • Increasing Frequency FIG. 1—The separation of proton nuclear spin states under an applied magnetic field.
aromatic hydrogen resonance
methyl hydrogen resonance
7
X 10*
6
5
4
3
2
FIG. 2—^H NMR spectrum for toluene.
CHAPTER 24: HYDROCARBON TABLE 2—Typical chemical shifts for '^C nuclei in hydrocarbon molecules. " C Nucleus Environment
Chemical Shift/ppm
Aliphatic and naphthenic carbon atoms Carbon atoms in terminal CHj groups Carbon atoms in CH2 groups in the middle of a chain Carbon atoms in aromatic rings
10.0-60.0 14.1 29.2 110-160
perimental resolution, the NMR spectrum of samples containing '•'C nuclei in natural abundance can be recorded and analyzed, in much the same way as 'H NMR spectra can be analyzed to determine key structural properties of the molecules that comprise the bulk sample. Just like ' H NMR, the resonant frequency of the ^•'C nuclei are influenced by the chemical environment in which they are surrounded and again, chemical shifts can be calculated in ppm relative to a calibration standard such as TMS using the same formula used to calculate 'H chemical shifts. Chemical shifts for '^C nuclei tend to be somewhat larger than 'H shifts in hydrocarbon molecules. Some typical chemical shifts for '^C nuclei in hydrocarbon molecules are shown in Table 2 [5]. Because the natural abundance of ^^C is so low, the experimental measurement of '^C NMR spectra is slightly more difficult than 'H NMR spectra because the spectra tend to be much weaker. To overcome this difficulty, the spectra are recorded not by scanning the frequency as for *H NMR (a technique often called continuous wave or CW) but by pulsing a series of short pulses of broad band radiation (of the order of 0.5-50 fjis) and monitoring the free induction decay of the signal from the '^C nuclei. This time domain single is then converted into the frequency domain using Fourier Transformation [2]. This, coupled with other techniques too advanced to discuss here, have made the recording and interpretation of '^C NMR spectra commonplace [5]. The Use of ^H and " C NMR in the Analysis of Hydrocarbons Because 'H and '^C are the two nuclei most commonly analyzed by NMR spectrometry, it stands to reason that NMR has proven useful in hydrocarbon analysis. There are two main areas where NMR has been used for hydrocarbon fuel and lubricant aneJysis, specifically the identification of hydrogen content in aviation fuels (ASTM D 3701) and petroleum distillates (ASTM D 4808), and in determining the aromatic hydrogen and aromatic carbon content using 'H and " C NMR spectrometry (ASTM D 5292). The determination of hydrogen content is an important property of a fuel or lubricant because it is closely related to key performance characteristics and can be used as a measure of quality control both during and after production. Both test methods (ASTM D 3701 and ASTM 4808) work by taking the ratio of the total integrated hydrogen signal from the sample NMR spectrum to a known n-dodecane standard run on the same NMR instrument. In each case, the total hydrogen content by mass (H%) is then calculated using the formula, H(%)
= S/R X (WR/WS)
X
15.39
ANALYSIS
651
where S is the total integrated hydrogen signal from the sample, R is the integrated signal from the n-dodecane reference, Wn is the weight of n-dodecane used, Ws is the weight of sample used, and 15.39 simply reflects the percentage by mass of hydrogen in the n-dodecane reference standard. The accuracy of this test method is around 0.2-0.4% for a typical hydrogen content of 14% [5]. Other variations on this method have also been used, which involve pulsed FT NMR, which are capable of errors as low as 0.05% [6, 7]. Hydrocarbon oils made by the refining of crude petroleum are generally classified parafHnic or naphthenic depending on the predominant class of hydrocarbon compounds found in the oil [8]. Despite this, most oils of this type contain a significant proportion of aromatic hydrocarbons. The aromatic content of an oil can affect a number of physical and chemical properties such as boiling range, viscosity, stability, and compatibility. For this reason, it is important to be able to determine to amount of aromatic hydrocarbon content in a hydrocarbon oil [9]. One way of characterizing the aromatic content is to count the number of aromatic carbon and hydrogen atoms in a bulk sample. Because NMR is capable of separating signals from different atoms within molecules based on their chemical structure and environment using the concept of chemical shifts, NMR is a natural for this type of determination: this is the basis behind ASTM D 5292. In this case, the test method is even simpler than measuring total hydrogen content. The sample is analyzed using pulsed FT NMR and the total aromatic ' H and "C content determined by taking the ratio of the aromatic carbon or hydrogen nuclei integrated signal to the corresponding aliphatic signal. For the purpose of assigning ' H nuclear environments, peaks in the chemical shift range —0.5 to 5.0 ppm are considered aliphatic hydrogen, while those in the 5.0-10.0 ppm range are considered aromatic. A t5^ical NMR spectrum of this kind is shown in Fig. 3. For " C NMR, peaks in the —10 to 70 ppm range are considered aliphatic, while those in the 100-170 ppm range are considered aromatic carbons [9]. The biggest drawback with NMR is the expense of purchasing and running an NMR instrument. However, in circumstances where the determination of the relative proportions of various constituents of a bulk sample in different chemical environments at high precision are required, NMR offers a quick simple means of obtaining quick, accurate data. For this reason, the use of NMR as a tool for hydrocarbon analysis will continue to grow. ASTM Petroleum Products and Lubricants NMR Test Standards Under Subcommittee D 02.04 Aromatics in Hydrocarbon Oils by High Resolution Nuclear Magnetic Resonance (HR-NMR) (ASTM D 5292) This method is applicable to a wide range of hydrocarbon oils that are completely soluble in chloroform and Cctrbon tetrachloride at ambient temperature. The data obtained by this method can be used to evaluate changes in aromatic contents of hydrocarbon oils due to process changes. Hydrogen ('H) NMR spectra are obtained on sample solutions in chloroform-d using a continuous wave or pulsed FT high resolution NMR spectrometer. Carbon ('^C) NMR spectra are obtained
652
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1HFTNMR
PPM FIG. 3—^H NMR spectrum of gas oil showing thie aliphatic (-0.5 to 5.0 ppm) and aromatic (5.0 to 10.0 ppm) hydrogen peaks. Reprinted from ASTM D 5292-93.
on the sample solution in chloroforni-
GAS CHROMATOGRAPHY (GC) Introduction GC is one of the most widely used analytical tools in the evaluation of petroleum products, as witnessed by the n u m b e r of relevant standcirds summarized at the end of this section that use GC. Its principal value lies in the fact that the technique can b e used t o separate complex mixtures of different molecules or groups of molecules based on their physical properties, which is precisely w h y it is such a valuable method for fuel a n d lubricant analysis. Because GC involves first getting the sample into the gas phase, it is used in the analysis of the more volatile components of fuels and lubricants. Less volatile fractions are typically analyzed using the related technique of Liquid Chromatography, which will be discussed in the following section. GC is also used to simplify several spectrometric techniques such as Mass Spectrometry (MS) a n d Fourier Transform Infrared Spectroscopy (FTIR) by first separating the sample into different molecular component types. The use of GC in GC-MS and GC-FTIR is covered under the relevant spectroscopic technique. The direct application of Gas Chromatography (GC) to the evaluation and analysis of hydrocarbons will be discussed in this section.
Gas Chromatography Gas Chromatography can be subdivided into two categories, gas-liquid chromatography a n d gas-solid chromatography [10]. In each case, the technique involves the separation of components of a gaseous sample using a stationary phase, either a liquid in the case of gas-liquid chromatography, or a solid in the case of gas-solid chromatography. Because the overwhelming majority of test standards used for hydrocar-
bon analysis rely on gas-liquid chromatography, we will focus exclusively on this method, although the same basic principals apply to both methods. In gas-liquid chromatography, it is the interaction between the gaseous sample being carried t h r o u g h the column by means of an inert gas (the mobile phase) and a standard liquid (the stationary phase), which causes the separation of different molecular constituents. The stationary phase is either a polar or nonpolar liquid, which either coats the inside of the column, in the case of a capillary column, or is impregnated onto an inert solid, which is then packed into the GC column [ 11 ]. A schematic layout of a GC instrument is shown in Fig. 4. The basic components are a n inert carrier gas, most commonly helium, nitrogen or hydrogen, a GC column, which will be described below, held inside an oven that allows for precise temperature control, and some type of detector capable of detecting the sample as it elutes from the column. Gas-liquid GC works because the molecules in the samples are carried along the column in the carrier gas and partition between the gas phase a n d the liquid phase. Because this partitioning is critically dependent on the solubility of the sample in the liquid phase, different molecular species travel along the column a n d exit or elute at different times. Those molecules that have a greater solubility in the liquid phase take a longer time to elute and thus are measured at a longer time interval. Because solubility is dependent on the physical and chemical properties of the solute, separation between different components of the sample occurs based on molecular properties such as relative polarity (for example, oxygenated molecules, aromatics and nonaromatics) and boiling point. A tjTDical gas chromatogram is shown in Fig. 5. In order to create this separation, a n u m b e r of different liquid phase materials are used that can be broadly classified as either polar or nonpolar. For hydrocarbon analysis the two most commonly used liquid phase materials are TCEP (1,2,3tris(2-cyanoyethoxy)propane) for polar columns, and methyl silicone for nonpolar columns, although a multitude of different polar and nonpolar columns suitable for hydrocarbon analysis are commercially available. I n general, polar columns are used to separate aromatics from nonaromatics, while nonpolar columns are used to sepeirate hydrocarbon components by their boiling point. There are two basic ways in which GCs are used for hydrocarbon analysis, either to separate different components of the sample based on their differing chemical characteristics (for example, aromatics versus nonaromatics), or to separate different chemically similar fractions based on their boiling point. To separate molecules based on their chemical properties, a GC is run in the method described above, with a polar column used to provide longer retention times for compounds such as aromatics relative to paraffins and olefins. For accuracy, it is vital that the oven temperature in which the GC column is housed is precisely controlled. This method is called isothermal GC analysis [10]. GCs can also be used in another mode called temperature programming. In temperature programming, the oven temperature is slowly swept from an initially low temperature ( s e e ) to a much higher temperature (typically 350-400°C) at a CcirefuUy controlled rate [12]. The effect is to cause an increased separation of chemically similar species based on
CHAPTER
24: HYDROCARBON
ANALYSIS
653
VENT
OVEN
CARRIER GAS •
••^ RECORDER
INJECTION PORT
COLUMN R s REFERENCE S s SENSING
FIG. 4—GC Instrument with a thermal detector.
their boiling point; those low boiling fractions eluting at a faster rate than fractions with higher boiling points. Temperature programming is used in hydrocarbon GC ancJysis to determine boiling point ranges for various crude and refined petroleum fractions. Whether isothermal or temperature programmed GC analysis is used, a detector capable of measuring molecules as they elute is required. For hydrocarbon analysis, the two most c o m m o n types of detectors are flame ionization and thermal conductivity [13]. In flame ionization type detectors, the column effluent is mixed with hydrogen and air and ignited. This flame b u m s any organic material (such as hydrocarbons) when they elute from the column producing, among other things, ions and electrons. This increase in charged particle concentration causes an increase in current between the tip of the burner, which is held at a high electrical potential, and a collector electrode above the flame. It is this increased current that is used to measure the elution of sample molecules from the column. An illustration of a flame ionization type detector is shown in Fig. 6. Thermal conductivity detectors contain a tungsten filament that is heated using a constant current [10]. The elution of pure carrier gas (helium) has a cooling effect on the filament, which controls the temperature and hence the resistance of the filament to the applied constant current. As hydrocarbon molecules elute, they have less of a cooling effect than the carrier gas, resulting in an increased temperature. Sample molecule elution is determined based on a change in resistant of the filament, c o m m o n l y m e a s u r e d using a Wheatstone bridge electrical circuit [10]. For specialized applications where element specific detection is required (for example, the determination of sulfur content in hght petroleum liquids by GC (ASTM D 5623)), other detection systems are used such as atomic emission (AED) and chemi-luminescence. These will not be covered in detail in this chapter. T h e U s e o f GC i n H y d r o c a r b o n Analysis The use of GC in the analysis of petroleum products falls into three genereJ categories: the eveJuation of the relative concentrations of different types of molecules (for example, aromatics, olefins, naphthenes etc.), the determination of boiling point ranges and carbon numbers in fuel and lubricants
u_ TIME. MINUTES
FIG. 5—Gas chromatogram of C^ to C^ species in gasoline. Reprinted from ASTM D 2427.
— 1 | ^
e ^ 1—
-Collector electrode
Flame ignition coil
+300V Polarising voltage
Hydrogen Column FIG. 6—Flame ionization detector.
using temperature programming, and the detection of certain contaminants such as diesel fuel or ethylene glycol in engine oil or oxygenated additives such as MTBE. The determination of the aromatic content of hydrocarbon fuels is a n important application of GC because benzene, toluene, and other aromatics pose a serious heedth threat and
654
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
is carefully regulated in gasolines and other petroleum products. There are a n u m b e r of test methods that use GC for this purpose, all of which follow the same basic principle (see ASTM D 3606, D 4420, and D 5580). In each case, a sample containing an internal calibration standard is injected into a GC instrument equipped with two columns, one containing a nonpolar liquid phase such as methyl silicone, the second using a polar liquid phase such as TCEP. The polar liquid phase separates aromatics and nonaromatics because aromatics take longer to elute from a polar column, while the nonpolar column separates the aromatic components by their boiling point, allowing the concentrations of benzene, toluene, and heavier aromatics to be determined. In each case, the measurements are carried out isothermally (that is at a fixed colu m n temperature). A tjrpical gas chromatogram of the aromatic components of gasoline using this method is shown in Fig. 7. Another i m p o r t a n t application of GC to h y d r o c a r b o n analysis is the simulated distillation of crude petroleum [13]. Determining the boiling point range of petroleum fractions has a variety of uses, from determining the composition of feed stocks to evaluating the presence of volatile components, which may have implications on the performance or safety cheiracteristics of finished hydrocarbon fuels and lubricants. Historically, the boiling point range of petroleum fractions has been determined by trial distillations in the laboratory. While this method still has some validity, and in some circumstances may be advantageous, the use of gas chromatography offers a quick, simple alternative for controlling refining processes and product evaluations. Boiling point range determination of petroleum products using GC involves injecting a sample into a nonpolar column, which is housed in a temperature programmable oven. The GC is first calibrated using a calibration solution containing known concentrations of normal paraffins. The temperature of the column is increased at a known rate and the retention time of the n-paraffins is plotted against their known boiling points, as shown in Fig. 8. The crude sample is then analyzed in the same way, and the retention times of the eluting sam-
Toluene
0}
c m N
<-8 aromatics
Switch to forward flow (Time T2)
Non-aromatics
• aromatics
t—^r^
10 15 Time, minutes
25
FIG. 7—Typical gas chromatogram of the aromatic fractions of gasoline. Reprinted from ASTM D 4420.
1100
600
10
15 20 25 30 Retention Time, Minutes
35
40
FIG. 8—^Typical boiling point ranges of n-paraffins as a function of column retention time. Reprinted from ASTM D 5307.
pie fractions converted to boiling points using the calibration curve shown in Fig. 8. An example of the simulated distillation of a crude oil s a m p l e using gas c h r o m a t o g r a p h y is shown in Fig. 9. The third area where GC has an increasingly important role to play is in the determination of certain contaminants in used oil samples, such as ethylene glycol, used as a coolant in many combustions engine, and fuel dilution in diesel engine samples. While used oil analysis is a subject unto itself, it is instructive to review how GC is applied in this area. The determination of fuel dilution in engine oil samples is of prime importance because it can impair the performance of the lubricating oil [14]. Because diesel fuel is chemically very similar to the oil itself, it is almost impossible to quantify fuel dilution by conventional wet chemistry tests. While other physical tests such as changes in viscosity, FTIR, and flash point offer means of determining the presence of fuel in a lubricating oil, GC is generally a more precise and reliable means of determining fuel dilution based o n the temperature programmed elution of fuel relative to the larger (higher boiling point) fractions of the lubricating oil. The same can be said for ethylene glycol coolant detection. Ethylene glycol is an insidious c o n t a m i n a n t in engine oil samples that can result in catastrophic engine failure if left unchecked [15]. Again a n u m b e r of wet chemistry tests (ASTM D 2982) and physical tests (FTIR) are available, but, again, GC offers superior detection limits. Because ethylene glycol is a polar molecule, GC analysis is performed by first extracting the glycol from the used oil sample with water. The water extract is then run directly on a n isothermal GC column against calibration standards containing known concentrations of ethylene glycol in water. The sensitivity of GC
CHAPTER 24: HYDROCARBON
ANALYSIS
655
AIS = AREA OF CRUDE PLUS INTERNAL STANDARD
23500-
21500 19500 17500V) Z 15500 O 0. tU 13500 OC
538°C (1000°F)
11500 BASELINE
9500 7500
JIMMAAAAMA^ AIS •
5500 3500
•o
1
1
r
TIME
1
1
O
00
CO
CM
P4
M
A. CRUDE + INTERNALSTANDARD B = TOTAL ELUTED AREA UPTO538*C(1000°F) B' = AREA CORRESPONDING TO NON-ELUTED SAMPLE BIS " AREA OF SEGMENT WHERE INTERNAL STANDARD ELUTES IN FIGURE 9A
19500
538''C(1000''F)
BASELINE
yUwiiUAAAz/AAx^ I BIS
TIME
B. CRUDE ONLY
FIG. 9—Gas chromatogram of calibration mixture and crude oil sample used to determine boiling point range. Reprinted with permission from ASTM D 5307.
to detecting contaminants in used oil samples makes it an ideal test where high precision test data is required. Whether it is used for evaluating different molecular components of gasoline, determining boiling point ranges for crude oil fractions, or detecting contaminants, one of the major drawbacks with GC is that while its is excellent at quantifying the relative concentrations of various eluted fractions, it cannot determine empirically what those fractions actually are [11]. Without the application of suitable cetlibration standards, which requires at least some knowledge of the sample constituents, this problem could severely limit the use of GC
with complex mixtures such as fuels and lubricants. However, one way of addressing this issue is to couple GC with some other applicable method that is capable of not only detecting eluting fractions, but can also identify their chemical composition. For this reason, the dual combination of GC with mass spectrometry (GC-MS), infra-red spectrometry (GC-FTIR), and atomic emission (GC-AES) will continue to grow in importance in the quantitative analysis of hydrocarbon fuels and lubricants [16,17]. The application of GC to MS and FTIR is covered in the appropriate spectrometric analysis sections of this review.
656
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK time axis from a calibration curve, obtained under the same conditions by running a known mixture of hydrocarbons covering the boiling range expected in the sample.
A S T M P e t r o l e u m P r o d u c t s a n d L u b r i c a n t s GC Test Standards Aromatics in Finished Gasoline by Gas Chromatography (ASTM D 4420) This test method determines benzene, toluene, Cs, C9, and heavier aromatics, and total aromatics in finished m o t o r gasoline and gasoline blending components. A two column chromatographic system connected to a dual filament thermal conductivity detector (or two single filament detectors) is used. Aromatics
in Finished
Gasoline
by GC (ASTM D
5580)
Aromatics in the following concentration reinge can be determined by this test method; benzene 0.1-5; toluene 1-15; individual Cg aromatics 0.5-10; total C9 and heavier aromatics 5-30; and total aromatics 10-80 liquid volume percent. The aromatic hydrocarbons are separated without interference from other hydrocarbons in the finished gasoline. A two colu m n c h r o m a t o g r a p h i c system equipped with a columnswitching valve and a flame ionization detector is used. Benzene/Toluene in Gasoline (ASTM D 3606)
by Gas
Chromatography
This test method determines benzene and toluene in finished m o t o r and aviation gasoline by gas chromatography. The sample is doped with an internal standard, methyl ethyl ketone (MEK), and is then injected into a gas chromatograph equipped with two columns connected in series. Boiling Range Distribution of Crude Petroleum Chromatography (ASTM D 5307)
by Gas
The method covers boiling range distribution of water-free crude petroleum through 538°C (100°F). A solution of crude oil in carbon disulfide is injected into a gas chromatographic column that separates hydrocarbons by their boiling point order. Boiling Range Distribution of Gasoline Fractions Gas Chromatography (ASTM D 3710)
by
This test method is designed to measure the entire boiling range of gasoline with either high or low Raid vapor pressure. The sample is injected into a gas chromatographic column, which separates hydrocarbons in boiling point order. Boiling Range Distribution of Petroleum Gas Chromatography (ASTM D 6352)
Distillates
by
The gas c h r o m a t o g r a p h i c (GC) determination of boiling point ranges is used to replace conventional distillation methods for control of refining operations. A nonpoleir open tubular capillary GC column is used to elute the hydrocarbon c o m p o n e n t s of the sample in order of increasing boiling point. Boiling Range Distribution of Petroleum Gas Chromatography (ASTM D 2887)
Fractions
by
The d e t e r m i n a t i o n of the boiling range distribution of petroleum fractions by gas chromatography is effective as a rapid anal5^ical tool. The sample is introduced into a gas c h r o m a t o g r a p h i c column that separates hydrocarbons in boiling point order. Boiling temperatures are assigned to the
Diesel Fuel Diluent in Used Diesel Engine Chromatography (ASTM D 3524)
Oils by Gas
This test method covers the determination of diesel fuel as a contaminant in used lubricating oil. A mixture of n-decane and used lubricating oil is introduced into a gas chromatographic column that separates hydrocarbons in the order of their boiling points. Estimation of Engine Oil Volatility Chromatography (ASTM D 6417)
by Capillary
Gas
The determination of engine oil volatility at 371°C is a requirement in some lubricant specifications. A sample aliquot diluted with a viscosity reducing solvent is introduced into the gas chromatographic system, which uses a nonpolar open tubular capillary gas chromatographic column for eluting the hydrocarbon components of the sample in the order of increasing boiling point. Ethanol Content In Denatured (ASTM D 5501)
Fuel Ethanol
by GC
This test method covers the determination of ethanol content of d e n a t u r e d fuel ethanol by gas chromatography. A fuel ethanol sample is injected into a gas c h r o m a t o g r a p h equipped with a methyl silicone bonded phase fused silica capillary column. Ethylene
Glycol in Used Engine
Oil (ASTM D
4291)
This test m e t h o d provides for early detection to prevent coolant from accumulating and seriously damaging the engine. The sample of oil is extracted with water and the analysis is performed on the water extract, which is injected into a gas chromatograph using on-column injection. The eluting compounds are detected by a flame ionization detector. Engine
Oil Volatility
by GC ASTM
(D
5480)
This test method provides the determination of the amount of engine oil volatilized at 700°F (371°C). The sample is mixed with an internal standard and a dilute tetracosane solution, and injected into a gas chromatograph. Gasoline Diluent Chromatography
in Used Engine Oils Gas Method (ASTM D 3525)
This test method uses gas chromatograph equipped with a flame ionization detector and a programmable oven. Hydrocarbon Types in Gasoline by Gas Chromatography (ASTM D 2427) This test method provides information on C2 through C5 ceirbon paraffins and mono-olefins in gasolines. The sample is injected into a gas-liquid partition column. The components are separated as they pass through the column with an inert carrier gas and their presence in the effluent is detected and recorded on a chromatograph. Methyl Tert-Butyl (ASTM D 4815)
Ether in Gasoline
by GC
This test method determines ethers and edcohols in gasoline by gas chromatography, a n d is applicable to both quality
CHAPTER control in the production of gasoline and the determination of deliberate or extraneous oxygenate additions or contamination. A gasoline sample is doped with an internal standard such as 1,2-dimethoxyethane, and is injected into a gas chromatograph equipped with two columns and a column switching valve. The eluted components are detected by a flame ionization or a thermal conductivity detector. Olefins In Engine
Fuels by GC (ASTM D
6296)
This test method can determine olefins in the C4 to Cio range in spark ignition engine fuels or related hydrocarbon streams such as naphthas and cracked naphthas. A sample fuel is injected into a computer controlled gas chromatographic system, which consists of a series of columns, traps, and switching valves operating at various temperatures. The final eluted olefins are detected by flame ionization detector. Oxygenates in Gasoline (ASTM D 5599)
by Gas
Chromatography
This test method provides sufficient oxygen-to-hydrocarbon selectivity a n d sensitivity to allow d e t e r m i n a t i o n of oxygenates in gasoline samples without interference from the bulk hydrocarbon matrix. An internal standard of a noninterfering oxygenate (for example, 1,2-dimethoxyethane) is added in a quantitative proportion to the gasoline sample. An aliquot of this mixture is injected into a gas chromatograph equipped with a capillary column operated to ensure separation of the oxygenates. Oxygenates are detected with the oxygen-selective flame ionization detector. Oxygenates O-PONA Hydrocarbons GC (ASTM D 6293)
in Fuels by
This test method provides for the quantitative determination of oxygenates, paraffins, olefins, naphthenes, and aromatics in low-olefin speirk-ignition engine fuels by multidimensional gas chromatography. A fuel sample is injected into a computer-controlled gas chromatographic system consisting of switching valves, columns, and a n olefin hydrogenation catalyst, all operating at various temperatures. The eluted hydroccirbons are detected by flame ionization detector. Sulfur Determination (ASTM D 5623)
by GC-Sulfur
Detector
This test method covers the determination of volatile sulfur compounds in light petroleum liquids. The sample is analyzed by gas chromatography with an appropriate sulfur selective detector.
24: HYDROCARBON
ANALYSIS
657
hydrocarbon petroleum products, specifically, absorption chromatography, high performance liquid chromatography (HPLC), fluorescent indicator absorption (FIA) and super critical fluid chromatography (SFC). L i q u i d C h r o m a t o g r a p h y (LC) In classical LC, a liquid sample is introduced either neat or diluted with a n a p p r o p r i a t e solvent into a glass column, which has been prepacked with an appropriate solid material such as silica, alumina, or some other solid medium. The sample is then washed down the column using a flowing stream of solvent, starting with a relatively low strength solvent and progressing to stronger and stronger solvents until the sample has been washed out or eluted from the column [18]. Depending on the nature of the interaction between the sample (the mobile phase) and the column material (the stationary phase), mixtures can be separated by retention time in the column; those molecular species present in the mobile phase which have a greater affinity for the stationEiry phase material take a longer time to elute t h a n those that have little or n o affinity for the stationary phase. This is called absorption chromatography and the columns used for this kind of methodology are absorption columns. An illustration of absorption liquid c h r o m a t o g r a p h y is shown in Fig. 10. In this example, a mixture of two compounds, X and o, Eire added to an LC column packed with a stationary phase that has a greater affinity for compound x than compound o. The mixture is washed down the column with a suitable solvent, however, because compound x has a greater affinity for the stationary phase than compound o, it will take a longer time to exit or elute from the column. Sepciration is thus affected between the two compounds, which can then be monitored as a function of their elution times by a suitable detector. This methodology is called absorption chromatography Eind is the most commonly used method for hydrocarbon emalysis. In certain circumstances, absorption by the stationary phase is so effective that the absorbed components of the sample can only be removed by washing with a solvent that has a stronger affinity for the absorbed sample component than the stationary phase. Two other types of LC columns exist, specifically partition columns and exclusion columns. Pcirtition columns work in m u c h the same way as GC columns. In peirtition chromatography, instead of being filled with a solid stationary phase, the LC column contains a liquid, which is coated or chemically bonded onto a solid medium contained within the col-
LIQUID C H R O M A T O G R A P H Y (LC) TIME
Introduction The technique of liquid chromatography (LC) is closely allied with gas chromatography (GC) and other chromatographic methods. Just like GC, liquid chromatography separates different molecules or molecular groups based on their physical and chemical properties. However, unlike gas chromatography, LC as the name implies the use of a liquid mobile phase to analyze samples. The field of liquid chromatography is very broad ranging and is used in many different areas of analj^ical chemistry. In this section, we will cover just those areas of liquid chromatography that Eire used in the analysis of
XOXOI
oxox xoxo oxox
X X
oxox xoxo oxox
o 0
X X X X
xoxo oxox 0 0 0 0
X X X X X X
oxox 0 0 |0 O 0 0
X X XX X X X X 0 0 0 0 0 0 O 0
•U 1/ W U \J FIG. 10—The chromatographic process.
X X X X X X X X O 0 O 0 0 0 0 0
u
658
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
umn. Separation is affected by the interaction between the Hquid mobile phase and the liquid stationary phase (often called the partitioning phase), which separates components of the sample based on their relative solubilities in the partitioning liquid [19]. This method is often referred to as liquidliquid chromatography. It is less commonly used for hydrocarbon analysis, but can be used to separate hydrocarbons of the same family, which differ only by their substituents [20]. Exclusion column chromatography is another form of solid-liquid chromatography, like absorption chromatography. However, instead of separation being affected by the rate of absorption (and desorption) by the solid stationary phase, exclusion columns are packed with porous materials with carefully controlled pore sizes. In this case, the rate of elution is controlled by molecular size, and the ability of the sample molecules to enter the pores of the stationary phase material. In general, very small molecules, which are smaller on average than the pore of the stationary phase material become trapped in the column and take longer to elute, while larger molecules cannot fit inside and thus elute faster than smaller molecules. This method is typically used to separate large polymeric additives, such as viscosity index (VI) improvers from base oils [21,22]. Liquid chromatography can be further subdivided into two categories: open column liquid chromatography (OCLC) and high performance liquid chromatography (HPLC). With OCLC, the mobile phase (sample and solvent) is forced through the column either under gravity, or a small head pressure of gas, typically 25 psig or less [19]. Most modem analytical instruments now use HPLC. Originally standing for "high-pressure liquid chromatography," HPLC or high performance liquid chromatography uses significantly higher pressures than OCLC to force the mobile phase through the column, resulting in shorter analysis times. A schematic illustration of the basic components of an HPLC instrument is shown in Fig. 11. A detailed review of HPLC instrumentation can be found in Ref 23. Whether an absorption column, partition column or exclusion column is used, or whether gravity (OCLC) or high pressure (HPLC) is used to push the mobile phase through, a detection system is required to measure the sample fractions as they elute from the column. The most common methods include UV spectrometers, used to measure the characteristic
SOLVENT RESERVOIR
5=
• INJECTION POINT
DETECTOR
RECORDER
COLUMN-
\J
THERMOSTAT
FIG. 11—An HPLC instrument.
UV absorption of organic molecules as they elute from the column; refractive index (RI) detectors, which record a change in RI as sample molecules elute relative to the solvent being used; and flame ionization (FID) detectors, which measure a change in conductivity through a hydrogen flame as the eluting molecules are ionized in the flame. The Use of LC in Hydrocarbon AniJysis Like gas chromatography (GC), LC is used for the analysis of petroleum fuels and lubricants because of its ability to separate complex mixtures based some physical or chemical property. Although lower in sensitivity than GC, LC has several distinct advantages, which make it more widely applicable to hydrocarbon analysis. Some of these advantages include [21]: • Shorter analysis times • Higher precision for identifying and quantifying components • Analysis in series using the same column • Selectivity for certain classes of hydrocarbons • Ability to measure high molecular weight and high boiling point compounds The main application of chromatographic techniques to hydrocarbon analysis is the separation of different fractions that possess wildly different physical and chemical properties such as polar compounds, aromatics, and saturates. This is the principle behind ASTM D 2007, which is used to define API base stock categories and engine oil interchangeability rules [24]. In this test method, a dual column apparatus is used containing two columns in series. The first column contains clay, which retains any polar compounds allowing both saturates and aromatics to pass through using n-pentane as a solvent. The second column contains clay and silica gel, which retains aromatics but again allows saturates to pass through. Collecting the total effluent through both columns allows the percentage by mass of saturates to be calculated as a function of total sample mass, because both polar compounds and aromatics are stripped from the sample by the two columns. The clay column is then removed and washed with a toluene acetone mixture allowing the absorbed polar compounds to elute. The percentage by mass of polar compounds can then be calculated as the total polar effluent from the toluene-acetone washed column, again as a function of the total sample mass. Finally, the silica gel/clay column is rinsed with toluene, which eJlows the aromatics to elute, again allowing their percentage by mass to be calculated. The precision of determining the saturate and aromatic content using this method (as per API 1509) is around 2% [25]. A similar method is used in ASTM D 2549, this time to separate aromatic and nonaromatic components of high boiling oils. In this method, a single absorption column containing activated bauxite and silica gel is used. Initially, n-pentane is used as a solvent, allowing nonaromatics to pass through the column. These are then collected and expressed as a percentage by mass of the initial sample, after evaporation of the npentane solvent. The column is then washed with diethyl ether, chloroform, and ethyl alcohol which allows the elution of the aromatics compounds, which are then weighed and expressed as a percentage by mass of the initial sample mass. A number of related techniques have been used to achieve sim-
CHAPTER 24: HYDROCARBON
240
_
ANALYSIS
659
A = Start integration of IMAIHs
i
B = End integration of IMAHs/Start integration of DAHs
1 1
210
180
C = End integration of DAils
1
> E ISO
1
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c o
^ 1
sQ \
j!l20
c 90
o o ^ X o 13 E 8
_ "
60
30
>• 11 "° to
1
>
1
1
\
1
1
E
o I
i\^i
1
I
X- 1 I
<= -
3i-Aromatic Hydrocarbons
c1
CO
1
•^ 6 1 £ "
1
1 I I
M '
1
\
C •
1
1
1
1
1
1
1
1
1
I
l
l
Time (minutes) FIG. 12—^The use of liquid chromatographic separation to differentiate mono and di-aromatics in aviation fuel. Reprinted with permission from ASTM D 6379.
ilar separations, which differ simply by the column material and solvents used [21]. These simple absorption techniques under atmospheric pressure have, to a large part, been superceded by high performance liquid chromatography (HPLC), principally because of the much shorter analysis times afforded with HPLC [21]. However, just like absorption chromatography, HPLC is used mainly as a tool for separating samples based on physical and chemical properties. Just like absorption chromatography, separation of complex mixtures into saturates and aromatics is possible with HPLC, as well as differentiating between similar molecules such as mono-ciromatics (MAHs) and diaromatics (DAHs). This method is used in ASTM D 6379 to determine MAH and DAH content in petroleum products. In this case, an amino bonded stationary phase is used, together with n-heptane, as the solvent to resolve different compounds based on elution times. An example of a chromatogram of this type is shown in Fig. 12. Aside from absorption chromatography and the related technique of HPLC, there are two other LC methods used for hydrocarbon analysis that are worth mentioning, specificedly fluorescent indicator adsorption (FIA) and supercritical fluid chromatography (SFC). Fluorescent Indicator Adsorption (FIA) Despite its rather grandiose name, FIA is a relatively simple technique. First developed in the early 1940s, it is used to determine the relative amounts of ciromatics, olefins, and saturates in petroleum fractions [18]. The technique involves standard OCLC using a silica gel column. However, in addition to being packed with silica gel, the column contains a small portion of the gel towards the top that is doped with a
mixture of dyes. The dye mixture is carried along the column with the petroleum sample, which stcirts to separate because the saturates have less of an affinity for the silica gel than the olefins, which in turn have less of an affinity than aromatics. The dye mixture contains three different components, one which has an affinity for silica gel, which is greater than that of the saturates, but less than that of the olefins. This dye component thus marks the interface between saturate molecules and olefins as they move through the column because the dye's retention time will be between the saturates and olefins. A second dye component has an affinity for silica somewhere between that of olefins and aromatics and thus marks the olefin-aromatic boundary, while a third dye component falls between aromatics and the isopropyl alcohol used as the solvent, marking the final boundary. By measuring the separation of each dye, the relative proportions of saturates, olefins, and aromatics can be determined to within 1-2% depending on conditions. This test is described under ASTMD 1319. Super Critical Fluid Chromatography (SFC) Although not strictly a liquid chromatography technique, SFC shares many of the same basic principles of HPLC. The principal difference between SFC and HPLC is that the mobile phase containing the sample is neither a liquid, as used in HPLC, or a gas, as used in gas chromatography, but is a supercritical fluid, most commonly CO2 [23]. A supercritical fluid is one that has been heated above its critical temperature and pressure such that it exists in a new state of matter, with properties between that of a gas and a liquid. In particular, supercritical fluids have greater mobility (are less viscous) than liquids, but are denser than gases. This property of supercritical fluids has great advantages as the mobile
660 MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
phase Ccuxier over conventional HPLC or gas chromatography (GC). For hydrocarbon analysis, SFC has an advantage over GC for fuel samples because it uses much lower temperatures than GC and is thus less likely to cause thermal decomposition. SFC also typically offers high resolution than HPLC, making it a more accurate test for determining different moleculcir components in hydrocarbon fuels [26].
umn and eluted using supercritical carbon dioxide mobile phase. The detector response to hydrocarbons is recorded throughout the analysis time corresponding to the mono-, polynuclear, and nonaromatic components determining the mass percent content of these groups.
ASTM Petroleum Products and Lubricants LC Test Standards
Introduction
ULTRAVIOLET S P E C T R O S C O P Y
Ultraviolet (UV) spectroscopy, also known as UV spectrometry, was one of the first spectroscopic techniques used to quantify chemical composition [27]. Like other spectroscopic techniques, it is used to identify characteristic moleculEir fingerprints enabling the quantitative detection of specific molecules or molecular species in complex mixtures. UV spectroscopy is most commonly used in hydrocarbon analysis as a detection tool after high performance liquid chromatography (HPLC).
Hydrocarbon Types by Fluorescent Indicator Adsorption (ASTM D 1319) This test method determines hydrocarbon types in the range of 5-99 volume percent aromatics, 0.3-55 volume percent olefins, and 1-95 volume percent saturates in petroleum fractions that distill below 315°C. Hydrocarbon Types Aromatic Hydrocarbon Types in Aviation Fuels and Petroleum Distillates (ASTM D 6379) This test method is intended for use as one of several possible aJtemative instrumental test methods that are used for quantitative determination of hydrocarbon t5rpes in fuels. This test method determines mono- and di-aromatic hydrocarbon contents in aviation kerosenes and petroleum distillates boiling in the range from 50-300°C, such as Jet A or Jet A-1 fuels. The sample is diluted 1:1 with a solvent such as heptane, cind a fixed volume of this solution is injected into a high performance liquid chromatograph fitted with a polar column.
Basic Principles of UV Spectroscopy In order to understand UV spectroscopy, we need to first understand the electronic structure of molecules. While a detailed quantum mechanical explanation of molecular electronic structure is beyond the scope of this book, there are a few simple concepts that will allow us to understand the basic theory behind UV spectroscopy. The total energy of a molecule is given by the sum of the electronic energy, vibrational energy, and rotational energy, with the electronic energy being significantly greater than the vibrationed energy, which in turn is significcuitly greater than rotationed energy. While rotational energy levels are only of significance in gas phase spectroscopy, the electronic and vibrational energy levels will be important in solution phase spectroscopy such as FTIR and UV spectroscopy. An illustration of the electronic and vibrational energy levels of a molecule is shown in Fig. 13. Under normal conditions, most of the molecules in a sample will be in the ground (lowest energy) electronic state. By
Aromatics and Polynuclear Aromatics in Diesel and Aviation Turbine Fuels By SFC (ASTM D 5186) The aromatic hydrocarbon content of motor diesel fuels affects their cetane number and exhaust emissions. The aromatic hydrocarbon and the naphthcdene content of aviation turbine fuels affects their combustion characteristics and smoke forming tendencies. In the test, a small aliquot of the fuel sample is injected onto a packed silica adsorption col-
i k
excited electronic state
^ i k
k
i L
^
i k
>
increasing energy
electronic energy levels, measured using UV spectroscopy
J ground electronic state
A
4
vibrational energy levels, measured using FTIR spectroscopy
A
FIG. 13—^The electronic and vibrational energy levels of a molecule.
CHAPTER 24: HYDROCARBON passing infrared light through the sample at the exact frequency corresponding to the difference in energy of the vibrational levels, transitions within the vibrational levels of the ground state can be induced, which is the principle behind FTIR spectroscopy. For the same reason, passing light through the sample at an appropriate frequency corresponding to the separation of the electronic energy levels can induce a transition between these levels, resulting in absorption of the light. For some molecules, the frequency of light required to induce electronic transitions is in the visible region of the electromagnetic spectrum, however, for most hydrocarbon molecules, the energy that is required is much higher and requires ultraviolet radiation. This is the principle behind UV spectroscopy. For hydrocarbon analysis, the most important factor determining the energy separation of the electronic energy levels is what is often referred to as the molecular chromophore [28]. A chromophore is simply a covalently unsaturated bond or group of bonds that is responsible for electronic absorption. In tjrpical hydrocarbon fuels and lubricants, the most common types of molecules to possess a chromophore will be aromatic molecules, which have characteristic absorption bands in the 185^00 nm wavelength range [28]. This range of the electromagnetic spectrum is typically called the near-UV.
ANALYSIS
661
lamp
entrance slit
diffraction grating photo cell detector
FIG. 14—UV spectrometer.
max
Beer's Law In order to use UV spectrometry as a quantitative tool, we need to understand the relationship between the concentration of specific molecules and the amount of light they absorb. This fundamental property is determined by Beer's Law: A = d)c where A is the absorbance, e is the molar absorptivity, b is the cell path length containing the sample, through which the UV light passes, and c is the concentration in mol/L [27]. Because the molar absorptivities of most molecules of interest have been measured, and i> is a known quantity based solely on instrument design, measuring the amount of light absorbed by the sample (A in the above equation) allows the concentration c to be determined simply by rearranging the Beer's law equation. UV Spectrometers A schematic illustration of a UV spectrometer is shown in Fig. 14. The main components are a white light source, which is focused onto the entrance slits of a spectrometer, a cuvette or cell containing the sample to be analyzed, and a photocell detector to measure light intensity. The spectrometer is equipped with a diffraction grating, which acts much like a prism to split the white light into discrete wavelengths. Because the angle of diffraction is defined by the wavelength of light, slowly changing the angle of the grating relative to the incoming light allows the wavelength of light that exits the spectrometer through the exit slits to be slowly scanned, allowing an absorption spectrum to be recorded, as shown in Fig. 15. The amount of light absorbed at the maximum excitation wavelength (Amax). will be dependent on the concentration c, path length b, and molar absorptivity Smax as per Beer's Law.
'max FIG. 15—Typical UV absorption spectrum.
The Use of UV Spectrometers in the Analysis of Hydrocarbons UV spectrometers are used both directly and in conjunction with high performance liquid chromatography (HPLC) instruments, principally to detect aromatics because they typically have strong UV absorption bands in the near UV region (200-400 nm). The specific maximum absorption wavelength and absorptivity is determined by a molecule's electronic structure, however, in general it is found that mono-aromatics (those with a single aromatic ring such as benzene and toluene) have an absorption maximum around 197 nm, diaromatics have absorption maxima around 230 nm, while piclyaromatics absorb around 260 nm [29]. One application of this method, employed by several oil companies, follows the work of Burdett [29, 30]. A typical UV absorption spectrum of mono-, di-, and tri-aromatics is shown in Fig. 16. The method of Burdett employs UV spectrometry to record the absorption spectrum in the range 175-270 nm. The molar absorptions is then recorded at the
662 MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
Molar Absorption 1
100,000
1 Mono
|Di
1
1
1
[ Tricycli 1
/ / N j ™ /^(~" \ 10,000 ^—-^
1 1 11 1 1 1 I 1 1 1
)1 1
\.
Dicyclics
\
^ ^— ^
\
1 1
Monocyclics 1,,^
/ yf 1 \ / 1 \ / 1 \ ^ / 1 1 1 1 1
/
/
1 t
•
180
•
190
'
I
200
1
210 220
230 240
\ ^
1 1
11
100
10
\
VMono
1,000
1
-/^
'iTriX/
—^ ' \'
|Di
\
1 1 1 1 1 1 1 1 1
—1
250
'
k.
260 A(nm)
FIG. 16—UV absorption spectrum for monocyclic, dicyclic and tricyclic aromatic hydrocarbons.
three characteristic absorption maxima, 190, 230, and 260 nm corresponding to mono-, di-, and poly-aromatics and can be converted to a percent by weight of each type of molecular species using a simple formula based on moleculcir mass, and the average molar absorptivity of each species at the three wavelengths. Details of this procedure can be found in Ref. 29. One of the most common applications of UV spectrometry to the analysis of hydrocarbon fuel and lubricants is its use in determining the amount of toxic impurities, particularly PAHs (polyaromatic hydrocarbons). The term PAH covers a multitude of different hydrocarbon molecules, with one common theme: the presence of multiple aromatic rings. A number of different PAHs, which are known to be present in hydrocarbon fuels and lubricants, have been found to be carcinogenic. For this reason, the EPA and other regulatory bodies have established limits for PAH concentrations in fuels, lubricants, and other media. Because UV spectrometry is capable of detecting polyaromatic species such as PAHs in hydrocarbons by their characteristic absorption at 260 nm, it is only natural that it has been used in this manor for determining PAH concentrations and carcinogen content in hydrocarbon oils [31]. While UV absorption is incapable of differentiating between two different molecules that may have absorption bands in the same region (for example, two different PAH molecules, both of which will absorb at 260 mm), the use of separation techniques such as liquid chromatography (LC) or gas chromatography (GC) can be employed to differentiate between different molecular species. In this case, the UV spectrometer is fixed at a known absorption wavelength (for example, 197 nm for mono-aromatic species), and is used to measure different molecular species as they elute from the GC or LC column; this provides a more molecule specific means of detection than flame ionization or thermal detectors tj^ically used with GC and LC instruments. One other technique closely related to UV absorption spectrometry that is used in hydrocarbon analysis is spec-
trofiuorometry [29]. The principle behind spectrofluorometry is illustrated in Fig. 17, which shows theoretical absorption spectra for two different polyaromatic molecules (molecules A and B), together with the corresponding fluorescence spectra. Because both molecules are polyaromatic, the absorption spectrum for each will show a meiximum around 260 nm. Recording the absorption spectrum for a mixture of molecule A and molecule B will not allow the relative concentrations of each to be determined because both absorb simultaneously at 260 nm. However, after excitation at 260 nm, both molecules are in an excited electronic state etnd will emit radiation to return to a lower lying electronic state, typically in the form of light. Because molecule A and molecule B possess different electronic structures, the energy of the lower lying electronic states will not necessarily be the same. Because the emitted radiation is quantized, measuring the emission (fluorescence) spectrum as opposed to the absorption spectrum allows the concentration of two molecules to be determined independently of each other. Used in conjunction with LC, spectrofluorometry is an extremely sensitive technique capable of detecting concentrations of polycyclic aromatics and other species as low as picograms [32]. ASTM Petroleum Products and Lubricants UV Test Standards Naphthalene Hydrocarbons in Aviation Turbine Fuels by Ultraviolet (UV) Spectrophotometry (ASTM D 1840) This test method covers the determination of the total concentration of naphthalene, acenapthene, and alkylated derivatives of these hydrocarbons in straight-run jet fuels containing no more than 5% of such components and having end points below 600°F. The total concentration of naphthalenes in jet fuels is measured by absorbance at 285 nm of a solution of the fuel at known concentration.
MASS SPECTROMETERY Introduction Mass spectrometry is used to detect the presence of different molecules in bulk samples. The technique involves the gas phase cinalysis of samples that are either already gases, or are liquids or solids that can be vaporized prior to mass spectrometric analysis. In conventional mass spectrometry, the gas phase sample is ionized to form various primary and fragmentation ions, which are then analyzed according to their mass-to-charge ratio. The mass spectrum thus generated can then be used as a cheiracteristic fingerprint to detect the presence of certain molecules with the relative intensity of the mass spectra allowing an estimate of the concentration of the molecule or molecular species within a bulk sample. The application of Mass Spectrometry (MS) to the analysis of hydrocarbon fuels emd lubricants is outlined in this section. MS Theory As previously mentioned, MS works on the principal of separating primary and fragmentation ions according to their
CHAPTER mass-to-charge (m/z) ratios. Take for example the w a t e r molecule H2O. Bombarding water vapor with a high-energy electron source will create both primary ions and fragmentation ions as follows: HOH
>HOH+
+ OH+
I primary ion m/z = 18
+ 0+
• m/z = 17
+
H+-F3e~
. secondary ions m/z = 16
m/z = 1
24: HYDROCARBON
ANALYSIS
663
of hydrocarbons typicedly found in petroleum fuels and lubricants. With many thousands of different components including paraffins, branched paraffins, naphthenic molecules, and aromatics, the m a s s spectrum of a typical petroleum fraction is extremely complex. Nevertheless, mass spectrometry can and has been successfully applied to the analysis of complex petroleum mixtures as will be discussed later. MS Instrumentation
Recording the mass spectrum of a bulk sample and measuring the relative intensities of the m/z = 1, 16, 17, and 18 peaks will allow the presence of water to be determined, and by inference its concentration, provided a suitable calibration standard has been determined. The biggest drawback with MS is that the complexity of the m a s s s p e c t r u m increases exponentially with increasing molecular size (molecular mass). Take for example benzene, a relatively simple hydrocarbon. The electron ionization mass spectrum of benzene (Fig. 18) has 17 different peaks corresponding to m/z values ranging from 27-79, which include primary and fragmentation ion mass peaks, as well as peaks associated with the naturally occurring isotope '^C [35]. Now consider applying mass spectrometry to a mixture
The basic components of a mass spectrometer are shown in Fig. 19. The system comprises an inlet source, which is designed to introduce the sample in the vapor phase to the spectrometer, a n ionization source that serves to ionize the sample, a mass analyzer which separates the ions by their m/z ratio, and an ion detector for detecting the ions once they have been mass selected. The inlet system for most basic mass spectrometers is called a batch inlet and comprises a means of injecting a gas, or more commonly a liquid sample under reduced pressure and elevated temperatures such that the sample can be vaporized for presentation to the spectrometer [34]. In m o d e m instruments, the liquid is commonly injected into the vacuum chamber using a hypodermic syringe arrangement called a probe [35].
-*• energy
UV absorption (A&B) Amaxs260nm absorption
molecule A emission
emission spectrum (A)
absorption
molecule B emission
emission spectrum (B) FiG. 17—Tlieoretical fluorescence emission spectrum of two different poiyaromatic moiecuies, A and B.
664
MANUAL 3 7: FUELS AND LUBRICANTS
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U 40
a: 20
1111 111 111 1111111 1111111 11 M 111 111 111 11
10
15
20
25
30
35
40
45
50
"i""|i il'i""i"'lMl'h 55
60
65
70
75
80
m/z FIG. 18—Mass spectrum of benzene.
Sample Inlet
Ionization Source
Mass Analyzer
—•
Ion Detector
FIG. 19—Basic components of a mass spectrometer.
Once in the gas phase, the sample is drawn into the ionization section of the instrument where it is intersected at right angles by a high-energy electron beam created by a hot filament. This is called an electron ionization mass spectrometer. The electron beam ionizes the sample into primary and fragmentation ions, which are accelerated into the mass analyzer section using an electrostatic field formed by a series of electrodes, which serve to focus and accelerate the ions into the mass analyzer. There are many different types of commercial mass spectrometers available. They all differ slightly in their design, but all use one of three basic methods to mass analyze the ionized sample, namely magnetic field deflection, quadrupole mass spectrometry, or time-of-flight [35]. A detailed explanation of these different types of mass analyzers is beyond the scope of this chapter, but an excellent description of these different methods can be found in Ref. 35. No matter what type of mass analyzer is used, they all separate ions according to their m/z ratio, with the ion signal being detected, amplified, and recorded as a function of their massto-charge (m/z) ratio. Gas Chromatography-Mass Spectrometry (GC-MS) One adaptation, which has served to simplify the interpretation of MS data, is to use a standard Gas Chromatograph (GC) as the sample inlet source for the mass spectrometer. As explained earlier in this chapter, a gas chromatograph is an instrument that separates complex mixtures of chemicals into single molecular components, or groups of components with similar physical properties. By combining MS with GC, the analysis of complex mixtures of chemicals such as hydrocarbon fuels and lubricants has become greatly simplified.
The usual way in which GC-MS is used for the analysis of petroleum products is to fix the mass spectrometer to detect one specific ion peak, for example m/z = 78, corresponding to the primary ion peak for benzene. The ion intensity at this fixed m/z ratio is then measured for different retention time in the GC. The retention time is simply the time taken for a specific molecular component of a complex mixture to emerge or elute from the GC column, which in turn is related to the physical and chemical properties of the molecule. By separating the signal in the m/z = 78 into different elution times, the true signal due to benzene can be differentiated from other fragmentation ions from different molecular species that may have an m/z ratio of 78, but have different retention times in the GC column. This makes the difficult task of interpreting the MS data significantly less complicated. The Application of MS t o Hydrocarbon Analysis Mass spectrometry is an important tool in the analysis of petroleum products because it can give information that cannot be obtained by other means. Just like other spectrometric techniques, it gives important information on the chemical composition of complex mixtures of hydrocarbons commonly found in fuels and lubricants, particularly when coupled with other techniques such as gas chromatography. The most common application of mass spectrometry to hydrocarbon analysis is in determining the composition of different process stream and boiling fractions during the refining process. There are a number of ASTM test procedures that cover the use of MS in this area, which are listed in Table 3. In each case, chemical composition is determined by calculating mass groupings, which correspond to the summation of characteristic mass peaks for different classes of molecules that are likely to be present. The m/z peaks that are used to calculate the different mass groupings in the ASTM procedures listed in Table 3 are simply based on the known primary and fragmentation ions patterns using mass spectrometry for different molecules under controlled laboratory conditions. For example, in ASTM D
CHAPTER TABLE 3—ASTM test procedures that use mass spectrometry for determining the composition of different fractions. ASTM Test Procedure D 2425 D 2786 D 2789 D 3239
Significance Saturates and aromatics in middle distillates Saturates in gas oil fractions Hydrocarbon types in low olefinic gasoline Aromatics in gas oil fractions
2786 that is based on the Hood and O'Neal method, the mass grouping E71 is defined as the sum of peaks at m/z = 71,85, 99, and 113 and is used to determine alkane content [37,38]. The difference in masses between each peak in the summation is simply the extension of the aJkane chain by one CH2 unit, with a corresponding increase in molecular mass of 14 atomic mass units. The biggest drawback with this method, and indeed MS in general, for the analysis of hydrocarbons is that the n u m b e r of mass peaks that must be considered increases significantly with molecular size. To illustrate this complexity, consider the complete list of mass groupings, which must be considered under ASTM D 2786, and the corresponding molecular types they represent; S71 = 71 + 85 + 99 -F 113 alkanes 1 6 9 = 69 -I- 83 + 97 + 111 -F 125 + 139 1 ring naphthenes E109 = 109 -I- 123 + 137 -I- 151 + 165 -I- 179 -I- 193 2 ring naphthenes X149 = 149 + 163 + 177 + 191 -I- 205 + 219-I-233-(-249 S189 = 189 + 203 + 217 + 231 + 245 + 259 + 273 + 287 + 301 X229 = 229 + 243 + 257 + 271 -I- 285 + 299 + 313 + 111 + 341 + 355 1269 = 269 + 283 4- 297 -F 311 + 325 -I- 339 + 353 -1-367-1-381 -F 395 + 409 S91 = 91 -F- 105 + 117 -I- 119 + 129 + 131 4- 133 -I- 143 + 145 -I- 147 + 159-1- 171
3 ring naphthenes 4 ring naphthenes 5 ring naphthenes 6 ring naphthenes
Mono-aromatics
Because of this, it is not surprising given the complex nature of different hydrocarbon fractions, each of which may contain several thousand different types of molecules, that the precision of using MS in this mode for different samples types is at best 10% [36]. To circumvent these difficulties, MS is often used in conjunction with other techniques such as gas chromatography (GC) or liquid c h r o m a t o g r a p h y (LC). In this case, chrom a t o g r a p h y is used to reduce the n u m b e r of different molecule types that enter the mass spectrometer, reducing the n u m b e r of mass peaks observed, and hence the complexity of interpreting the data accurately. In fact, separation of the sample into aromatic and nonsiromatic fractions using liquid chromatography (ASTM D 2549) prior to MS analysis is a prerequisite for ASTM D 2425 [39]. One way in which MS Ccin be combined with GC is in fact not to record a complete mass spectrum of the sample, but rather to fix the mass (or more strictly the m/z ratio) of the spectrometer to a known ion peak, and monitor this mass as
24: HYDROCARBON
ANALYSIS
665
a function of elution time. This can either be done directly by recording the signal in a fixed mass channel, or by scanning the spectrometer across cJl mass ranges and reconstructing the signal in a single mass channel using softweire. The latter is the method used u n d e r ASTM D 5769. In this method, separation of molecular species according to their characteristic p r i m a r y a n d fragmentation ions allows a more complete analysis to be performed. This is illustrated in Fig. 20. In this case, by combining the selectivity of GC in separating different molecular species with the mass specificity of MS, the different molecular concentrations of specific molecules such as benzene, toluene and other aromatics can be determined. Without this mass selectivity, the gas chromatogram would be extremely complex, with no guarantee that the elution peak corresponds to a specific molecule such as benzene or toluene. Although limited by its ability to resolve information from complex mixtures of chemicals typically found in petroleum products, mass spectrometer, particularly when combined with other separation techniques such as LC and GC is an important tool in the analysis of hydrocarbons. ASTM Petroleum Products and Lubricants M S Test Standards Under Subcommittee D02.04 Hydrocarbon Types in Gasoline (ASTM D 2789)
by Mass
Spectrometry
This test method covers the determination by mass spect r o m e t r y of total paraffins, monocycloparaffins, dicycloparaffins, alkylbenzenes, indans, or tetralins or both, and n a p h t h a l e n e s in gasoline. Samples are analyzed by m a s s spectrometry, based o n the s u m m a t i o n of characteristic mass fragments, to determine the concentration of the hydrocarbon types. Aromatics in Gasoline by Gas Spectrometry (GC-MS) (ASTM D
Chromatography-Mass 5769)
This test method can be used for gasolines that contain oxygenates such as alcohols and ethers as additives. They do not interfere with the analysis of benzene and other aromatics by this test method. The sample is injected either through the capillary splitter port or a cool-on-column injector into a gas c h r o m a t o g r a p h equipped with a dimethylpolysiloxane WCOT column interfaced to a fast scanning mass spectrometer. The mass analyzer processes the signal at specific m/z values corresponding to the principal ion masses for various components allowing benzene, toluene, and total aromatic content to be measured in gasolines.
INFRARED SPECTROSCOPY Infrared spectroscopy is a widely applied, nondestructive test method for assessing a variety of molecular physical qualities of a lubricant or fuel sample. The spectrum analyzed by the method is considered a distinct physical property of the sample and, as such, is unique from other physical and chemical properties, such as viscosity, specific gravity, flash point, etc. From the spectrum, in most cases, a positive identification of the sample and its greater molecular constituents can be obtained [40].
666
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Most p e t r o l e u m products, a n d more specifically, lubriCcints, fuels, additives, and contaminants are mixtures, which results in the spectrum being a composite of m a n y different spectral components that are additive. Nevertheless, for most fluids, the spectrum is sufficiently characteristic to enable the identification of the unique spectred features of individual components. Still, in certain cases, a physical or chemical separation of the tcirget components from the rest of the sample, such as gas chromatography, m a y be desirable to improve detection limits and resolving power of the technique. The infreired spectrum of a substance is produced when a portion of a beam of infrared energy passing through a fixed path length of the sample is absorbed by a specific functional group within a specific molecule present in the sample. This absorption has unique wavelength dependencies relating to the sample's molecules. Most molecules are in constant modes of rotation, stretching, and bending at unique and often multiple frequencies. In addition, these vibrational frequencies will vary as the bonded atoms, functional groups, a n d bond strengths are changed. These differences in absorption at certain wavelengths in the mid-infr£ired spectrum collectively describe the quality and quantity of molecular makeup of the sample [41]. Infrared spectroscopy is especially well suited for the analysis of organic c o m p o u n d s comprised of characteristic molecular structures. The various groups, called functional
groups, which make u p part of a molecule's structure, give rise to absorption at specific infrared frequencies but more typically spectral b a n d s comprising a broader range of wavelengths. These cire commonly referred to as group frequencies. For example, a well-defined group frequency is associated with C = 0 at around 1700 wavenumbers (cm~'). Wavelength, Wavenumber and Frequency The frequency or energy at which unique infrared absorptions occur is usually expressed as wavelength in micrometers (fjim) or wavenumbers in reciprocal centimeters (cm~'). Studies have shown that a tjrpical organic molecule requires an average energy of 1.6 X lO"'^" joules (J) for vibrational excitation, which corresponds to 12 fji,m of infrared radiation. The reciprocal of the wavelength is referred to as wavenumbers and represents the n u m b e r of cycles passing a fixed point per unit of time. The scale in Fig. 21 shows the conversion of wavelength to w a v e n u m b e r s [42]. The use of wavenumbers (cm~^) in infrared spectroscopy is preferred to a scale of wavelengths (jj-m) because wavenumbers Eire linear a n d proportional to the energy a n d frequency being absorbed. Wavenumbers are sometimes incorrectly referred to as frequencies, a n error because the w a v e n u m b e r is expressed in reciprocal centimeters (cm~^) and frequency is expressed in units of reciprocal time (s~') [42].
Abundance.,
ION 128
80000: 70000. 60000. SCOOO. NAPHTHALENE
40000: 30000' 2000010000 0 Time » 18 00
^ 18.50
Abundance 50000
I
19.00
19.50
20.00
20.50
21.00
21.50
22.00
ION 142
4S000
2-METHYmAPHTHALENE
40000 39000: 1-METHYL-NAPHTHALENE,
30000
""-*
2SOO0' 20000 15000100005000; 0^ Time -• 18 00
, 18.50
19.00
19.50
20.00
20.50
21.00
21.50
22.00
MINUTES
FIG. 20—^Typical mass selected gas chromatogram showing the ability of GC-MS to separate molecular species by their elution time and characteristic ion mass peak. Reprinted with permission from ASTIVI D 5769-98.
CHAPTER 24: HYDROCARBON Wavenumber
Wavelength
4000- - 2 . 5 - 1 3500-
-3
3000;
•
2500- ^ 4
Functional group region
2000- ^ 5 1800^ : L-e /Am (micrometei) 1600 — L f-7 1400|-8 1200^9 1 0 0 0 - |-10 8 0 0 - |-12 tl5 — elength goo — P-20 conversion scale for efor infrared data. 1. 400 - F 3 0 2 0 0 - ^50 Frequency •Wavele ngth -1 cm
Fingerprint region
FIG. 21—Conversion scale for infrared data.
ANALYSIS
667
The reference and sample beams are then passed ahematively by a rotating half mirror to a dispersive device, which separates the radiation into specific frequencies (wavelengths). A prism is one type of dispersive device that separates radiation in different wavelengths based on differences in refractive indexes at different wavelengths. Since glass will absorb infrared radiation sodium chloride, other materials cire required. Alternatively, a grating can be used as a dispersive device using a series of closely spaced parallel grooves etched into a flat surface. The grating simply reflects the diffracted light from its surface and absorbs very little radiation. Many infrared spectrometers require special temperature cind/or desiccants to prevent moisture damage to the optics in the monochromator [42]. The monochromator directs the radiation reflecting from the grating through a series of narrow slits, which luminate onto a detector, for example, a thermocouple. This quantifies the intensity of the energy at only the desired band of frequencies. Many dispersive-type infrared spectrometers are of the double-beam optical null type. In such cases, the radiation passing through the reference beam is reduced or attenuated to match the intensity of the sample beam [42].
Beer-Lambert Law
Fourier Transform Infrared Spectroscopy
A good place to begin the discussion of using infrared spectroscopy for quantitative analysis of fuels and lubricants is the Beer-Lambert Law. It relates the amount of infrared light absorbed by a given sample to the concentration of the target compound and path length. More to the point, the law states that concentration is directly proportional to absorbance at a given wavelength and path length at a specified temperature and pressure.
Compared to the dispersive spectrometer, the Fourier transform spectrometer provides improved speed and range of spectral sensitivity in making infrared measurements. The Michelson interferometer is a basic component of the Fourier transform instrument. Unlike the dispersive spectrometer, the interferometer has no slits or grating. Instead, it consists of two mirrors and a beam splitter. The beam splitter transmits half of all the incident radiation from a source to a moving mirror and reflects half to a stationary mirror. Each component reflected by the two mirrors returns to the beam splitter where the amplitudes of the waves eire combined to form an inteferogram, which is Fourier-trEinsformed into the frequency spectrum. The interferometer scans the infrared spectrum in just fractions of a second at moderate resolution. The resolution is uniform across the optical range. Multiple scans can be co-added to reduce background noise [42].
A = abc = logio I°/I
(1)
A = absorbance / = IR power-reaching detector with sample in beam 7° IR power-reaching detector with no sample in beam a = absorption coefficient of pure component of interest at anal54ical wavelength; the units depend on those chosen for b and c b = sample path length c = concentration of sample component This linearity permits simple calibration plots of known samples between absorbance (A) and concentration (C) for analyzing the concentration in unknown samples. In addition, the Law is helpful in choosing the optimum sample path length for accurate eincJysis, as will be further discussed [43]. Dispersive-Type Infrared Spectrometers In this design, zin infrared source of energy is usually provided by a Nemst glower, which is composed of rare earth oxides (zirconium, cerium, etc.) formed into a cylinder. On electrically heating to approximately 1500°C, this filament material produces the needed infrared radiation. The beam of radiation is split into a reference and sample beam by mirrors. The reference beam is passed through air or a beam attenuator and the sample beam is passed through the sample where selective radiation absorption occurs [42].
Analysis of Midticomponent Solutions For solutions, samples should be diluted and placed in cells of appropriate path length (typically 0.2-1.0 mm). It is preferred to use lower concentrations in longer path length cell rather than higher concentrations in shorter path length cell. The desired absorbance is in the reinge of 0.3-0.8. Lower concentrations will minimize nonlinear effects due to dispersion (that is, change of refractive index with wavenumber). Where freedom from intermolecular effects is uncertcdn or where intermolecular effects are known to be present, calibration must be based on measurements taken from S5rnthetic mixtures of all components. The procedure is described in ASTM E 168 [44]. Calibration is achieved by dissolving a known weight of a pure component in a suitable infrared solvent. Next, the absprbcmce is measured at all analj^iceJ wavenumbers and cor-
668
MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
rected for baselines as further discussed. The procedure is repeated for a range of concentrations covering the expected concentrations. From this, absorbance is plotted against concentration. Analytical curves can also be constructed for each of the wavenumbers or bands where the target component is represented. So too, the procedure is repeated for each of the components to be analyzed in the solution [44]. Analysis of Gases For gases, all calibration measurements for a given analysis must be made at a fixed total pressure. This pressure must be equal to the total pressure employed in the analysis. Low molecular weight gases frequently produce very strong, sharp absorption features. Addition of a diluent gas and use of pressure below atmospheric may be necessary. Absorbances cire measured for each of the standard wavenumbers selected for analysis. Where possible, integrated absorbances are preferred to offset the effect of small pressure vetriations. The absorbances Eire plotted against the partial pressures (or mole fractions) to produce analytical curves [44]. Achieving Reproducible Baselines Any quantitative method depends on the choice of a reproducible baseline. The correction of raw data for baseline absorbance is important in some methods. The guiding factor in baseline selection is the reproducibility of the results. Methods used for drawing baselines with computerized instruments are similar in most ways to those for data recorded on chart paper [44]. A technique known as the cell-in-cell-out method is often used in single-beam infrared work. In this method, a blank (that is, solvent in cell, potassium bromide (KBr) pellet, or other substrate) is measured at a fixed wavenumber and then the analyte readings are recorded. One variation involves the subtraction of the absorbance minimum from the absorbance maximum at the chosen baseline point. The point of minimum absorbance is adjacent to or at least near the band under evaluation. Alternatively, two points may be needed if the band of interest is superimposed on a sloping background. In such case, a line is drawn from one side to the other and absorbance is calculated as the value at the peak maximum minus the baseline absorbance minimum. An inappropriate choice of baseline in this situation may have deleterious effects on the accuracy of the final calculation [44]. The baseline correction described above should be performed only if the spectrum is plotted in absorbance units. When the spectrum is plotted in transmittance, the two-baseline transmittances and the transmittance at the analytical wavenumber Eire converted to absorbance. Conversion to absorbance is required because a sloping linear baseline in transmittance becomes curved in absorbance [44]. Employing the Difference Method Spectral subtraction using a computer is a common practice in qualitative infrared analysis. This technique is also used to perform quantitative infrared analyses. The advantage of spectral subtraction (the difference method) is that small concentration differences can be measured with greater ac-
curacy than is possible on superimposed bands. The general procedure involves the removal of spectral contributions of specific and known components from a spectrum containing multiple components in order to assess the unique spectral characteristics of the remaining target component [44]. Band Area Methods Band shape changes can cause peak-height data to be nonlinear. Band area, however, may remain essentially unaffected by the changes in shape of the band because band area is a function of the total number of absorbing centers in the sample. If the shape change is caused by changes in intermolecular forces, even band area may be linear [44]. Band area is calculated by integrating across bandwidth. Band area is advantageous when band shape undergoes change as a function of increasing concentration. Frequently, band area is found to be more accurate than peak-height measurements because one is, in effect, averaging multipoint data. When integrated area is used for quantitative analyses, the reliability of the results frequently depends on the baseline treatment selected. The accuracy by band area is often improved by limiting the range of absorbances. The wings contribute very little signal while contributing substantial uncertainties to the totEil Eirea. A useful guideline is to limit the integration limits to absorbance values, which are no smaller than 20-30% of the peak absorbance [44].
Hydrocarbon Analysis Hydrocarbon analysis using infrared spectroscopy must begin at the parent backbone structure. The simple aliphatic hydrocarbon is the root of most Ediphatic compounds and consists of simple linear chains, branched chains, and cyclic structures. Aliphatic compounds can consist of one or more of these structures. The infrared spectrum is useful in providing specific information on the existence of most of these structures, by inference or directly. The spectrEd contributions are characterized by C—H and C—C stretching and bending vibrations, which are generally unique for each molecule. For aromatic compounds, ring C=C-—C stretching and bending vibrations are distinctly characteristic as are carbon-carbon multiple bonding in alkene and alkjrne structures [45]. In terms of recognizing a compound as having an organic structure with aliphatic constituents, the C—H stretch vibrations for methyl and methylene are the most characteristic. In Fig. 22, methylene/methyl bands can be seen at 1470 and 720 cm"^ An important methylene rocking vibration occurs at 725-720 cm~'. As C—H stretching absorptions all occur below 3000 cm"', bands between 3150 and 3000 cm~' and are almost always associated with unsaturation, for example, C=C—H and/or aromatic rings. Figure 23 shows examples of either single or pair absorbancies of unsaturated hydrocarbons featuring C=C with attached hydrocarbons. The number of bands and their associated positions point to the double bond location Emd spacing around the double bond [45]. One or more aromatic rings can tj^ically be recognized by the C=C—C and C—-H ring-related vibrations/bending. Typically, the C—H stretching occurs about 3000 cm~\ revealing a number of weak-moderate bands compared with the aliphatic C—H stretch. The number and locations of the C—H
CHAPTER
Saturated Aliphatic (ajlcane/allcyl) Group Frequencies Group frequency (cm') 2970-2950/2880-2860 1470-1430/1380-1370 1385-1380/1370-1365 1395-1385/1365 2935-2915/2865-2845 1485-1445 750-720 1055-1000/1005-925 2900-2880 1350-1330 1300-700 2850-2815 2820-2780
Functional group/assignment Methyl (-CHj> Methyl C-H asym./sym. stretch Methyl C-H asym./sym. bend gem-Dimethyl or "iso" - (doublet) Trimethyl or "tert-butyl" (mlltiplet) Methylene (>CH2) Methylene C-H asym./gym. stretch Methylene C-H bend Methylene - (CH2)-rocklng (n>3) Cyclohexane ring vibrations Methyne (>CH-) Methyne C-H stretch Methyne C-H bend Skeletal C-C vibrations Special methyl (-CHj) frequencies Methoxy methyl ether O-CH3 C-H stretch Methylamino, N-CH3, C-H stretch
FIG. 22—Saturated aliphatic (alkane/alkyl) group frequencies. Courtesy Coates Consulting.
Olefinic (alkene) Group Frequencies Origin 0=0
C-H
Group frequency wavenumber (cm') 1680-1620 1625 1600 3095-3075 +3040-3010 3095-3075 3040-3010
0-H
1420-1410 1310-1290
C-H
995-985+915-890 895-885
C-H
970-960 700 (broad)
Assignment Alkenyl 0 = 0 stretch Aryl-substituted C=C Conjugated 0 = 0 Tenninal (vinyl) C-H stretch Pendant (vinylldene) C-H stretch Medial, cis- or frans-C-H stretch Vinyl C-H in-plane bend Vinylidene C-H in plane bend Vinyl C-H out-of-plane bend Vinylidene 0-H out-of-plane bend trans 0-H out-of-plane bend cis C-H out-of-plane bend
bonds around the aromatic ring is defined by the structure of the bands in the spectrum. Other important bcuids for aromatic ring vibrations are positioned around 1600 and 1500 c m ~ \ which are exhibited as pairs with some splitting. The nature and structure of these two bands are largely influenced by the position and nature of substituents on the ring [45]. On the surface, the interpretation of halogenated comp o u n d s contained in infrared spectra is functionally very simple. While not always true, the polar nature of the group consisting of a single atom linked to carbon produces a dis-
ANALYSIS
669
tinctive spectral contribution. T5T3ically, a unique group frequency associated with halogen-carbon stretching is assigned to the C—^X bond (Fig. 24). If more than one halogen is present, the identification of the group frequency is somewhat more complex. It is largely influenced by whether the halogens are on the same or different CcU"bon atoms, and if different, their relative proximity is important. Relating to alcohols and hydroxy c o m p o u n d s , the 0—H stretch is probably one of the most pronounced and characteristic of all the infrared group frequencies. There is typiCcdly a high degree of association coming from hydrogen bonding with other hydroxy groups. And, in cases, these may come from hydroxy groups from within the same molecule (intramolecular bonding). Alternatively, they may associate with nearby molecules (intermolecular bonding). Collectively, the effects of hydrogen bonding result in the production of a well-defined but broad band cuid the lowering of mean absorption frequency. This is exhibited in compounds such as carboxylic acids, which produce strong hydrogen bonding. See Fig. 25 for alcohol a n d hydroxy c o m p o u n d group frequencies [45]. Because alcohols exist as three distinct classes, primary, secondcury, and tertiary, they are identified by the extent of carbon substitution on the central hydroxy-substituted carbon. The infrared characterization of these alcohols is reflected in the position of the OH stretch absorption but also by other absorptions including the C—O— stretching frequency. These can be observed in the primary and secondary alcohols shown in the spectra shown in Fig. 26. Ethers are somewhat related to edcohol and hydroxy compounds where the hydrogen of the hydroxy group is substituted by an aromatic (aryl) or aliphatic (alkyl) molecular fragment. Otherwise, the overall appearance of an ether spectrum is sharply different from any associated alcohol due to the impact of the hydrogen bonding on the hydroxy group [45]. In amines, the terms primary, secondary, and tertisiry are used to describe the substituted nitrogen as opposed to carbon as with alcohols. As with alcohols, these structural differences are significant and distinctly influence the infrared
Aliphatic Organohalogen Compound Group Frequencies Origin
FIG. 23—Olefinic (alkene) group frequencies. Courtesy Coates Consulting.
24: HYDROCARBON
Group frequency wavenumber (cm"')
C-F
1150-1000
C-CI
800-700
C-Br
700-600
0-1
600-500
Assignment
Aliphatic fluoro compounds, C-F stretch Aliphatic chtoro compounds, C-CI stretch Aliphatic bromo compounds, C-Br stretch Aliphatic iodo compounds, C-l stretch
Note that the ranges quoted serve as a guide only; the actual ranges are influenced by carbon chain length, the actual number of halogen substituents, and the molecular conformations present.
FIG. 24—Aliphatic organohalogen compound group frequencies. Courtesy Coates Consulting.
670
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
Alcohol and Hydroxy Compound Group Frequencies Group frequency Assignment Origin wavenumber (cm-1) 0-H
0-H
0-H C-0
3570-3200 (broad) 3400-3200 3550-3450 3570-3600 3645-3630 3635-3620 3620-3540 3640-3530' 1350-1260 1410-1310 720-590 -1050' -1100° -1150' -1200'
Hydroxy group, H-bonded OH stretcti Normal "polymeric" OH stretch Dimeric OH stretch Internally bonded OH stretch Nonbonded hydroxy group, OH stretch Primary alcohol, OH stretch Secondary alcohol, OH stretch Tertiary alcohol, OH stretch Phenols, OH stretch Primary or secondary, OH in-plane bend Phenol or tertiary alcohol, OH bend Alcohol, OH out-of-piane bend Primary alcohol, C-0 stretch Secondary alcohol, C-0 stretch Tertiary alcohol, C-0 stretch Phenol, C-0 stretch
'Frequency influenced by nature and position of other ring substituents. "Approximate center of range for tlie group frequency. Courtesy Coates Consulting.
FIG. 25—Alcohol and hydroxy compound group frequencies. Courtesy Coates Consulting.
I I I I I I I I I I I I I o o o o o o o
o
o
Wavenumber (cm') 100
o
e M n
o eo CM
. M
Wavenumber (cm'') ATR spectra of (a) primary (1-octanol) and (b) secondary (2-octanol) alcohols. Courtesy poates Consulting. FIG. 26—ATR spectra of (a) primary (1-octanol) and (b) secondary (2-octanol) alcohols. Courtesy Coates Consulting.
spectra. It is possible to readily describe functional group structures, especially for the primary and secondary amino compounds. Although somewhat weaker than hydroxy compounds, the hydrogen bonds play an important role in the selective identification of the various amino compounds. In the case of ammonium and amino salts, strong hydrogen bonding is observed. Primary and secondary amines are largely characterized by the N—H bond group frequencies. The tertiary amines, like esters, produce vibrations associated with the C—^N bond, see Fig. 27 for group frequencies for Amino Compounds [45]. The carbonyl group, C = 0 absorption, is often the most characteristic and intense of the entire infrared spectrum. Common group frequencies for carbonyl compounds can be seen in Figure 28. With the exception of the aldehyde, all other carbonyl compounds can be generedly considered to be derived from the base structure of ketone. This occurs as one or both alky (or aryl) substituents are replaced by a single hydroxy group (carboxylic acids) or two ether groups (organic Cctrbonate). Commonly, the group frequencies for the various classes of carbonyl compounds overlap and the carbonyl band alone is insufficient to identify the specific functional group. Common examples include carboxylic acids where C—O, C—O—H, and O—H vibrations are characteristic, as are esters C^O—C and amides, C—N and N—H. So too, a characteristic broad feature in the range 3300-2500 cm"' that overlaps the C—H stretching region is observed for hydrogen-bonded O—H in most carboxylic acids [45]. Characterization of Inorganics Infrared spectroscopy can be used very effectively to characterize inorganic molecular species that often co-occupy space with various hydrocarbon compounds found in fuels and lu-
CHAPTER 24: HYDROCARBON Amine and Amino Compound Group Frequencies
Origin
N-H
Group frequency wavenumber (cm"^)
N-H C-N
3400-3380 +3345-3325 3510-3460 +3415-3380 1650-1590 1090-1020
>N-H
3360-3310
>N-H
-3450
>N-H
3490-3430
=N-H
3350-3320
>N-H C-N
1650-1550 1190-1130
C-N
1210-1150
C-N
1340-1250
C-N
1350-1280
C-N
1360-1310
N-H
Assignment
Primary amino Aliphatic primary amine, NH stretch Aromatic primary amine, NH stretch Primary amine, NH bend Primary amine, CN stretch Secondary amino Aliphatic secondary amine, NH stretch Aromatic secondary amine, NH stretch Heterocyclic amine, NH stretch Imino compounds, NH stretch Secondary amino, NH bend Secondary amine, CN stretch Tertiary amino Tertiary amine, CN stretch Aromatic amino Aromatic primary amine, CN stretch Aromatic secondary amine, CN stretch Aromatic tertiary amine, CN stretch
FIG. 27—Amine and amino compound group frequencies. Courtesy Coates Consulting.
Example Carbonyl Compound Group Frequencies Group frequency (cm-1) 1610-1550/1420-1300 1680-1630 1690-1675/(1650-1600)' 1725-1700 1725-1705 1740-1725/(2800-2700)' 1750-1725 1735 1760-1740 1815-1770 1820-1775 1850-1800/1790-1740 1870-1820/1800-1775 2100-1800
Functional group Carboxylate (carboxyllc acid salt) Amide Quinone or conjugated ketone Carboxyllc add Ketone Aldehydle Ester SIx-membered ring lactone Alkyl carbonate Add (acyl) hallde Aryl carbonate Open-chain add anhydride Five-membered ring anhydride Transition metal carbonyls
' Lower frequency band is from the conjugated double bond. "Higher frequency band characteristic of aldehydes. associated with the termlhal aldehydic C-H stretch.
FIG. 28—Carbonyl compound group frequencies. Courtesy Coates Consulting.
ANALYSIS
671
bricating oils. Essentially any compound that forms covalent bonds within a molecular ion fragment will produce a characteristic absorption spectrum with unique group frequencies. The metal complexes and chemical fragments associated with heteroxy groups such as nitrates, sulfates, phosphates, silicates, etc. and transition metal carbonyl compounds have already been generally discussed as related to the salts of carboxyllc acids, amino, and ammonium compounds [45]. ASTM Petroleum Products and Lubricants IR Test Standards w i d e r Subcommittee D02.04 Aromatics in Finished Gasoline by GC-FTIR: ASTM D 5986 This method can be used for determining aromatic content in gasolines that contain oxygenates such as alcohols emd ethers as additives. It can be used for both, and does not interfere with benzene and other aromatics by this method. The sample is injected through a cool on-column injector into a gas chromatograph equipped with a methylsilicone WCOT column interfaced to a FT-IR instrument. Benzene/Toluene in Gasoline by Infrared (Ir) Spectroscopy: ASTM D 4053 A gasoline sample is examined by infrared spectroscopy and following a correction for interference is compared with calibration blends of known benzene concentration. Benzene/Toluene in Engine Fuels using Mid-IR Spectroscopy: ASTM D 6277 A beam of infrared light is imaged through a liquid sample cell onto a detector, and the detector response is determined. Wavelengths of the spectrum that correlate highly with benzene or interferences are selected for analysis using selective bandpass filters or mathematically by selecting areas of the whole spectrum. Methyl Tert-Butyl Ether in Gasoline by Infrared Spectroscopy: ASTM D 5845 This infrared method measures MTBE and other oxygenates in the concentration ranges from about 0.1 to about 20 mass percent. A sample of gasoline is analyzed by infrared spectroscopy. ASTM STANDARDS No. Title D 1319 Hydrocarbon Types by Fluorescent Indicator Adsorption D 1840 Naphthalene Hydrocarbons in Aviation Turbine Fuels by Ultraviolet (UV) Spectrophotometry D 2427 Hydrocarbon Types in Gasoline by Gas Chromatography D 2789 Hydrocarbon Types in Gasoline by Mass Spectrometry D 2887 Boiling Range Distribution of Petroleum Fractions by Gas Chromatography D 3524 Diesel Fuel Diluent in Used Diesel Engine Oils by Gas Chromatography
672 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK D 3525
Gasoline Diluent in Used Engine Oils Gas Chromatography Method D 3606 Benzene/Toluene in Gasoline by Gas Chromatography D 3 710 Boiling Range Distribution of Gasoline Fractions by Gas Chromatography D 4053 Benzene/Toluene in Gasoline by Infrared (IR) Spectroscopy D 4291 Ethylene Glycol in Used Engine Oil D 4420 Aromatics in Finished Gasoline by Gas Chromatography D 4815 Methyl Tert-Butyl Ether in Gasoline by Gas Chromatography D 5186 Aromatics and Polynuclear Aromatics in Diesel and Aviation Turbine Fuels by SFC D 5480 Engine Oil Volatility by Gas Chromatography D 5501 Ethanol Content in Denatured Fuel Ethanol by Gas Chromatography D 5580 Aromatics in Finished Gasoline by Gas Chromatography D 5599 Oxygenates in Gasoline by Gas Chromatography D 5623 Sulfrir Determination by GC-Sulfrir Detector D 5769 Aromatics in Gasoline by Gas ChromatographyMass Spectrometry (GC-MS) D 5845 Methyl Tert-Butyl Ether in Gasoline by Infrared Spectroscopy D 5986 Aromatics in Finished Gasoline by GC-FTIR D 6277 Benzene/Toluene in Engine Fuels Using Mid-IR Spectroscopy D 6293 Oxygenates O-PONA Hydrocarbons in Fuels by Gas Chromatography D 6296 Olefins in Engine Fuels by Gas Chromatography D 6352 Boiling Range Distribution of Petroleum Distillates by Gas Chromatography D 6379 Hydrocarbon Types Aromatic Hydrocarbon Types in Aviation Fuels and Petroleum Distillates D 6417 Estimation of Engine Oil Volatility by Capillary Gas Chromatography ASTM Petroleum Products and Lubricants N M R Test Standards under Subcommittee D02.04 Aromatics in Hydrocarbon Oils by High Resolution Nuclear Magnetic Resonance (HR-NMR)
OTHER STANDARDS IP 156
Hydrocarbon Types in Liquid Petroleum Products by Fluorescent Indicator Adsorption
REFERENCES [1] Oehler, U., NMR-A Short Course, University of Guelph, Ontario, Canada, http://www.chembio.uogueIph.ca/driguana/NMR/TOC. HTM, Jan. 2001. [2] Silverstein, R., Bassler, G., and Morrill, T., Spectrometric Identification of Organic Compounds, Fifth Edition, John WUey and Sons, Inc., NY, 1991, pp. 166-201. [3] Shugar, G. and BaUinger, J., Chemical Technicians' Ready Reference Handbook, Fourth Edition, McGraw-Hill, Inc., NY, 1996, pp. 802-808.
[4] NMR Spectral Archive, National Institute of Advanced Industrial Science and Technology. Tsukuba, Ibaraki, Japsin. SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/. [5] Denis, J., Briant, J., and Hipeaux, J., Lubricant Properties Analysis and Testing, Editions Technlp, Paris, 2000, p. 21. [6] Bouquet, M., "Determination of Hydrogen Content of Petroleum Products Using Low Resolution Pulsed NMR Spectrometry," Fuel, Vol. 64, 1985, pp. 226-228. [7] Gauthier, S. and Quignard, A., "Accurate Determination of Hydrogen Content in Petroleum Products by Low Resolution," '// NMR Revue IFF. Vol. 50, No. 2, 1995, pp. 249-282. [8] Willis, J., Lubrication Fundamentals, Marcel-Dekker, Inc., NY, 1980, pp. 1-25. [9] ASTM D 5292-93: Aromatic Carbon Contents of Hydrocarbon Oils by High Resolution Nuclear Magnetic Resonance Spectroscopy, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, 1993. [10] Shugar, G. and BaUinger, J., Chemical Technicians' Ready Reference Handbook, Fourth Edition. McGraw-Hill, Inc., NY, 1996, pp. 831-864. [11] Norris, T. A., "Chromatography II," Lubrication, Vol. 65, No. 2, 1979, pp. 13-24. [12] ASTM D 5307: Determination of Boiling Range Distribution of Crude Petroleum by Gas Chromatography, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA. [13] Altgelt, K. and Gouw, T., Chromatography in Petroleum Analysis, Marcel-Dekker, Inc., NY, 1979, pp. 41-73. [14] Troyer, D. and Fitch, J., Oil Analysis Basics, Nona Corporation, Tulsa, OK, 1999. [15] Fitch, J., "Oil and Water Don't Mix," Practicing Oil Analysis Magazine, July/August 2001, p. 20. [16] Denis, J., Briant, J., and Hipeaux, J., Lubricant Properties Analysis and Testing Editions Technip, Paris, p. 53. [17] "Gas Chromatography with Atomic Emission Detector for Turbine Engine Lubricant Analysis," JOAP Conference Proceedings, University of Dayton Research Institute, Dayton, OH, 1994, pp. 485^96. [18] Norris, T. A., "Chromatography II," Lubrication, Vol. 65, No. 2, 1979, pp. 13-24. [19] Li, Z., "Separation Techniques with Liquid Chromatography," The FRH Journal, 1984, pp. 69-76. [20] Stevenson, R., "Rapid Separation of Petroleum Fuels by Hydrocarbon Type," Journal of Chromatographic Science, Vol. 9, 1971, pp. 257-262. [21] Denis, J., Briant, J., and Hipeaux, J., Lubricant Properties Analysis and Testing, Editions Technip, Paris, 2000, pp. 31. [22] "La Chromatographic d'Exclusion sur Gel," Journal of Dubois Analysis 16, Vol. 3, No. LWVI-LXXIII, 1988. [23] Shugar, G. and BaUinger, J., Chemical Technicians' Ready Reference Handbook, Fourth Edition, McGraw-HUl, Inc., NY, 1996, pp. 865-896. [24] API 1509, Engine Oil Licensing and Certification System, American Petroleum Institute, Washington, DC, 1996. [25] ASTM Test Standard D 2007: Characteristic Groups in Rubber Extender and Processing Oils and Other Petroleum-Derived Oils by the Clay-Gel Absorption Chromatographic Method, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA. [26] RR: D02-1388, ASTM International, West Conshohocken, PA, 1996. [27] Shugar, G. and BaUinger, J., Chemical Technicians' Ready Reference Handbook, Fourth Edition, McGraw-Hill, Inc., NY, 1996, pp. 753-759. [28] Silverstein, R., Bassler, G., and MorriU, T., Spectrometric Identification of Organic Compounds, Fifth Edition, John Wiley and Sons, Inc., NY, 1991, pp. 289-314. [29] Denis, J., Briant, J., and Hipeaux, J., Lubricant Properties Analysis and Testing, Editions Technip, Paris, 2000, pp. 41-58.
CHAPTER 24: HYDROCARBON ANALYSIS [30] Determination of Aromatic Hydrocarbons in Lubricating Oil Fractions by Far Ultraviolet Absorption Spectroscopy, R. A. Burdett Molecular Spectroscopy, Institute of Petroleum, George Sell, London, 1954, pp. 30-41. [31] Haas, J., et al., "A Simple Analytical Test and a Formula to Predict the Potential for Dermal Carcinogenicity From Petroleum Oils," American Industrial Hygiene Association Journal, Vol. 48, No. 11, 1987, pp. 935-940. [32] Ogan, K., BCatz, E., and Slavin, W., "Determination of Polycyclic Aromatic Hydroceirbons in Aqueous Samples by Reverse Phase Liquid Chromatography," Analytical Chemistry, Vol. 51, No. 8, 1979, pp. 1315-1320. [33] MS Spectral Archive, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/. [34] Norris, T. A., "Chromatography II," Lubrication, Vol. 68, No. 3, 1979, pp. 25-40. [35] Silverstein, R., Bassler, G., and Morrill, T., Spectrometric Identification of Organic Compounds, Fifth Edition, John Wiley and Sons, Inc., NY, 1991, pp. 3-39. [36] Denis, J., Briant, J., and Hipeaux, J., Lubricant Properties Analysis and Testing, Editions Technip, Paris, 2000, pp. 24-28. [37] ASTM D 2786-91: S t a n d a r d Test Method for H y d r o c a r b o n Types Analysis of Gas-Oil S a t u r a t e s Fractions by High
[38]
[39]
[40]
[41] [42]
[43] [44]
[45]
673
Ionizing Voltage Mass Spectrometry, Annual Book of ASTM Standards, ASTM I n t e r n a t i o n a l , West Conshohocken, PA, 1991. Status of Application of Mass Spectrometry to Heavy Oil Analysis, Advances in Mass Spectrometry, AMSPA, Waldron, 1986, pp. 175-191. ASTM D 2425-93: S t a n d a r d Test Method for Hydrocarbon Types in Middle Distillates by Mass Spectrometry, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, 1993. Coates, P. and Setti, L., Oils, Lubricants and Petroleum Products, Characterization by Infrared Spectra, Marcel-Dekker, Inc., NY, 1985. Archer, E. D., "Infrared Analysis I," Lubrication, Vol. 55, 1969, pp. 13-32. Shugar, G. and Ballinger, J., Chemical Technicians' Ready Reference Handbook, Fourth Edition, McGraw-Hill, Inc., NY, 1996, pp. 761-775. Anonymous, Chapter 10.9, Infrared Analyzers. ASTM E 168: Standard Practices for General Techniques of Infrared Quantitative Analysis, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, 1999. Coates, J., Infrared Spectra, A Practical Approach to the Interpretation of Coates Consulting, Marcel Dekker, NY, 1985.
MNL37-EB/Jun. 2003
Volatility Rey G. Montemayor^
DISCUSSION
VOLATILITY, IN ITS SIMPLEST DEFINITION, IS THE TENDENCY OF A
LIQUID TO CHANGE INTO VAPOR. For fuels, lubricants, and other petroleum products, this tendency is measured in a variety of ways. Volatility parameters are related to the perform a n c e characteristics and/or safety of these materials. Among the various ways of determining the volatility properties of materials are: distillation, rate of evaporation measurement, flash point test, and vapor pressure determination. Distillation determines the temperatures required to evaporate known portions of the material, as well as the temperatures at which distillation begins and ends. Distillation also determines the boiling range of the materials. A volatility property particularly important in solvents and coating materials is the rate of evaporation. The flash point of a liquid is the lowest temperature, corrected for bcirometric pressure, at which application of an ignition source causes the vapor above the specimen to ignite. Vapor pressure is the force per unit area exerted on the walls of a closed container by the vaporized portion of the liquid material in the container. The significance of the different volatility properties varies from one material to another. For crude oil, distillation data are critical assay information. For solvents distillation, flash point and evaporation rate are important parameters. For motor and aviation gasoline, distillation and vapor pressure data are paramount since these properties are related to the performance characteristics of these materials. Distillation characteristics of diesel and other nonaviation fuels exert a great influence on their performance. Flash points and fire points for distillates and residual fuels, as well as lubricants, are significant from the perspective of safety in handling, storage, and transportation of these materials. This chapter will deal with the different test methods dealing with the volatility characteristics of fuels and lubricants. A summary of the test methods, scope, significance, precision, and results will be given. Other internationcd standards dealing with volatility properties determination and their corresponding ASTM standards will be referenced. Distillation for materials other than crude oils is discussed under the section on Distillation. Flash point eind fire point determinations are discussed in the Flammability section. Distillation test methods applicable to crude oils are dealt with in the Crude Distillation section. Vapor pressure measurements are covered in the Vapor Pressure section.
' Imperial Oil Ltd., Products and Chemicals Division, Sarnia, Ontario Canada
Distillation Distillation parameters are importcuit volatility characteristics of motor and other automotive spark-ignition fuels, aviation gasoline, aviation turbine fuels, diesel and other nonaviation gas turbine fuels, solvents, and other petroleum products. Gasolines and gasoline blends are used in a variety of engines operating u n d e r various atmospheric and mechanical conditions. In order to provide satisfactory performance, gasoline must have the optimum distillation characteristics. Gasoline that vaporizes too readily in pumps, fuel lines, and carburetors will cause decreased fuel flow to the engine resulting in rough engine operation or stoppage. If the gasoline does not vaporize easily, difficulty in start-up, poor warm-up and acceleration, as well as unequal distribution of fuel to the combustion cylinders may result. The distillation temperatures of various petroleum products can be determined at atmospheric pressure using ASTM D 86, Standard Test Method for Distillation of Petroleum Products [1] or at reduced pressure using ASTM D 1160, Standard Test Method for Distillation of Petroleum Products at Reduced Pressure [2]. The 10%, 50%, and 90% (recovered) distillation points are important control points in the production of fuel blends. In the mid-1980s the use of automatic distillation equipment gained popularity because of increased ability to control the rate of distillation, emd efficiency in operation. Figure 1 [3] shows the classical manucJ D86 distillation set-up using a gas burner. Figure 2 [4] shows the manual D86 distillation apparatus assembly using electric heater. The use of an electric heater improved the ability to control the rate of distillation. However, it was still difficult to m a i n t a i n the 4 - 5 mL/min distillation rate required by the method. The advent of automatic distillation apparatus allowed the distillation parameters specified in the method to be controlled accurately without operator intervention. The distillation flask and the receiving cylinder in an automatic D86 distillation unit are essentially the same as the manual unit. Although electric heaters eire used, the use of microprocessors to control the rate of distillation provided precise temperature control and allowed conformance to the method requirements. Platinum resistance temperature probes or thermocouples replaced the mercuiy-in-glass thermometers to allow unattended operation. Automatic level followers using optical sensors removed the necessity of manually observing and measuring recovered distillation volumes. Data obtained by automatic distillation units are very similar to those obtained by manual instruments. However, the data are not statisti-
675 Copyright'
2003 by A S I M International
www.astm.org
676
MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
Thermometer Distilling Flask
Bath Cover
Heat Resistant Boards
Air Vents SupportFIG. 1—Manual D 86 distillation unit with gas burner.
cally equivalent. Data are available for users to compare manual and automatic D 86 results (Tables lA and IB) [5,6]. Similar improvements have occurred with D 1160.The relative bias between manual and automatic D 1160 are given in Table 2 [7]. ASTM D 86, Standard Test Method for Distillation of Petroleum Products at Atmospheric Pressure This test method is under the jurisdiction of ASTM Committee D-02 on Petroleum and Lubricants, and is the direct responsibility of Subcommittee D02.08 on Volatility. This standard was originally published in 1921. The latest edition is D 86-OOa and incorporates a major rewrite of the standard, which was started in 1996. This test method covers the distillation at atmospheric pressure of natureil gasolines, motor gasolines, aviation gasolines, aviation turbine fuels, special boiling point spirits, naphthas, white spirit, kerosines, gas oils, distillate fuel oils, and similar petroleum products, utilizing either manual or automatic equipment. In a D86 distillation, a 100 mL specimen is distilled under prescribed conditions that are appropriate to its nature. Table 3 [8] shows the distillation group characteristics of various materials tested to provide D86 distillation data. Table 4 [9] and Table 5 [10] indicate the various test parameters required for each distillation group. Systematic observation of temperature readings and volumes of condensate are made, and from these data, the results of the test are calculated and reported. The basic method of determining the boiling range of a petroleum product by performing a simple batch distillation has been in use as long as the petroleum industry has existed. It is one of the oldest test methods under the jurisdiction of ASTM Committee D 02, dating from the time when it was still referred to as the Engler distillation. Since the test method
has been in use for such an extended period, a tremendous amount of historical databases exist for estimating end-use sensitivity on products and processes. The distillation characteristics of hydrocarbons often have an important effect on their safety and performance, especially in the case of fuels and solvents. Volatility is the major determinant of the tendency of a hydrocarbon to produce potentially explosive vapors. It is also critically important for both automotive and aviation gasolines, affecting starting, warm-up, and tendency to vapor lock at high operating temperature or at high altitude, or both. The presence of high boiling point components in these and other fuels can significantly affect the degree of solid combustion deposit formation. Distillation limits are often included in petroleum product specifications, in commercial contract agreements, process refinery/control applications, and for compliance to regulatory requirements. The precision of manual and automatic D86 distillation for group 1 materials is given in Table 6 [11]; Table 7 [12] for groups 2,3,and 4 using manual distillation; and Table 8 [13] for groups 2,3, and 4 using automatic distillation. Generally, the distillation temperatures corrected for barometric pressure, corresponding to IBP, 5%, 10%, 20%, 30%, 40%, 50%), 60%), 70%), 80%, 90%, 95%, and FBP are reported. The initial boiling point (IBP) is the corrected temperature reading that is observed at the instant the first drop of condensate falls from the lower end of the condenser tube. The 5-95% distillation temperature is the corrected temperature reading corresponding to each 5-95% volume distilled or recovered. The final boiling point (FBP) or end point (EP) is the maximum corrected temperature reading obtained during the test. Sometimes, the dry point (DP) is required. DP is the corrected temperature reading that is observed or detected at the instant the last drop of liquid evaporates from the lowest point in the distillation flask. The corrections to be applied to the observed temperature readings are obtained by means of the Sydney Young equation given in Eqs 1,2, and 3, as appropriate, or by the use of Table 9 [14]: Cc = 0.0009 (101.3 - Pk) (273 -I- t^)
(1)
G = 0.00012 (760 - P) (273 + tc)
(2)
Cf = 0.00012 (760 - P) (460 + tf)
(3)
where: Cc and Cf = corrections to be added algebraically to the temperature reading in Celsius or Fahrenheit tc = observed temperature reading in Celsius tf = observed temperature reading in Fahrenheit Pk — barometric pressure prevailing at the time of the test in kPa P = barometric pressure prevailing at the time of the test in mm Hg. After applying the corrections, the corrected temperature readings are rounded and reported to the nearest 0.5°C (1.0°F) or 0.1 °C (0.2°F), as appropriate to the apparatus used. However, other calculated values can be reported from D86 distillation data, such as percent recovery and percentages evaporated at prescribed temperature readings. Typical D86 results for motor gasoline, reformulated gasoline, diesel, and jet fuel are given in Table 10 [15], Table 11 [16], Table 12 [17], and Table 13 [18].
CHAPTER 25: VOLATILITY
25 min.
Length of part ' bath approx. 390
Front View
NOTE 1—^Legend: l-Condenser bath 2-Bath cover 3-Bath temperature sensor 4-Bath overflow 5-Bath drain 6-Condenser tube 7-Shield 8-Viewing window 9a-Voifage regulator 9b-Voltmeter or ammeter 9o-Power switch 9d-Power light indicator 10-Vent
11-Distillation flask 12-Temperature sensor 13-Flasl< support board 14-Flasl« support platform 15-GrDund connection 16-Electric heater 17-Knob for adjusting level of support platform 18-Power source cord 19-Reoeiver cylinder 20-Receiver cooling bath 21-Receiver cover
FIG. 2—Manual D 86 distillation unit with electric heater.
677
TABLE lA—Relative bias between manual and automatic D86 distillation. Sample
IBP
5%
10%
20%
30%
40%
50%
60%
70%
80%
90%
95%
FBP
Sample
1 2 3 4 5 6 7 8 9 10 11 12 13* 14*
+ 1.1 (+0.9) +0.7 +0.3 +0.5 + 1.2 +0.3 +0.3 + 1.7 + 1.5 +0.9 + 1.0 +0.3 +0.5
+ 1.9 (0.0) + 1.4 +0.6 + 1.3 + 1.2 +0.8 +0.5 +2.0 + 11.5 + 1.1 (+2.4)" +0.3 +0.4
+2.2 +0.8 + 1.6 +0.8 + 1.3 + 1.6 +0.8 +0.7 + 1.8 + 1.2 + 1.2 +2.3 +0.4 +0.7
+ 1.6 +0.5 + 1.0 +0.8 + 1.3 + 1.2 +0.7 +0.6 + 1.5 +0.7 +0.8 + 1.2 +0.3 +0.5
+ 1.4 +0.4 +0.8 0.3 + 1.2 + 1.2 +0.8 +0.7 + 1.5 +0.4 +0.7 + 1.2 +0.2 +0.8
+0.7 +0.6 +0.6 0.7 + 1.0 + 1.1 +0.8 + 1.2 + 1.5 +0.6 +0.6 + 1.2 +0.9 + 1.1
+0.8 +0.2 +0.3 +0.6 +0.9 +0.8 + 1.0 + 1.2 + 1.2 +0.9 + 1.1 + 1.2 + 1.4 + 1.7
+0.7 +0.1 +0.1 +0.8 +0.6 + 1.1 + 1.5 + 1.1 +0.9 + 1.0 + 1.0 +0.9 + 1.0 + 1.7
+0.7 +0.1 +0.2 + 1.1 +0.8 + 1.2 + 1.6 + 1.3 + 1.3 + 1.4 +0.4 + 1.1 +0.1 + 1.0
+0.1 +0.4 +0.9 + 1.2 + 1.0 +0.2 + 1.6 + 1.9 +0.6 + 1.9 +0.5 +0.2 + 1.1 +0.5
+0.4 (+4.7)" +0.5 +0.8 +0.4 -0.1 + 1.5 + 1.1 -0.4 +0.9 -0.4 -0.7 + 1.2 +0.3
0.7 ( + 1.3)" +0.1 +0.5 +0.4 +0.2 + 1.7 + 1.2 +0.4 +0.1 +0.1 (-0.8) + 1.0 0.0
-0.4 (-1.2)° -0.8 -0.9 -0.9 -0.3 -0.7 -0.8 -1.2 -2.1 -0.8 -0.9 -1.2 -0.8
1 2 3 4 5 6 7 8 9 10 11 12 13* 14*
''Points between parentheses have not been included in the precision analysis. 'Gasoline-alcohol blends. NOTE—1. Data reported are based on averages of ASTM and IP data. 2. Fourteen samples of gasoline were analyzed in 26 laboratories. 3. The bias reported below is (average of automated results)—(average of manual results). TABLE IB—Relative bias between manual and automatic d 86 distillation, various samples [6] (currently under review in ASTM Subcommittee D02.08). Summary of Average Relative Bias in °C (auto—manual) ASTM Interlaboratory Crosscheck Data from 1994 to 1998 Sample Jet A Diesel Mogas Refor
12 14 13 36
#Lab A
#Lab M
99 129 76 86
26 43 17 8
IBP -0.7 -1.9 -1.1 -1.1
5% 1.9 2.7 0.2 0.7
10%
20%
30%
40%
50%
60%
70%
80%
90%
1.1 2.1 -0.1 0.3
0.8 1.2 -0.5 0.2
0.6 1.2 0.1 -0.1
0.7 1.1 -0.7 -0.1
0.7 1.0 -0.3 -0.2
0.8 0.7 -0.3 0.4
0.9 0.7 -0.4 0.5
0.8 0.6 -0.5 0.1
0.6 0.5 -1.9 -0.5
60%
70%
80%
1.4 1.2 -0.5 0.8
1.6 1.2 -0.7 0.9
1.5 1.1 -0.9 0.2
90% 1.0 0.8 -3.5 -0.9
95% 0.3 0.3 -3.4 -2.1
FBP -0.6 0.4 -1.0 0.4
95%
FBP -1.1 0.8 -1.7 -0.7
Summary of Average Relative Bias in °F (auto—manual) ASTM Interlaboratory Crosscheck Data from 1994 to 1998 Sample
N
#Lab A
#Lab M
IBP
5%
Jet A Diesel Mogas Refor
12 14 13 36
99 129 76 86
26 43 17 8
-1.3 -3.4 -1.9 -2.0
3.3 4.9 0.3 1.4
10% 2.1 3.8 -0.2 0.6
20%
30%
40%
1.3 2.2 -0.9 0.3
1,1 2.1 0.1 -0.2
1.2 2.0 -1.2 -0.2
50%) 1.3 1.9 -0.5 -0.4
0.5 0.5 -5.2 -3.8
TABLE 2—Relative bias between m a n u a l and automatic D 1160 [7]. 1 mm Hg Pressure (All AET Values in °C) Sample 2
Sample 1 Boiling Point IBP 5% 10% 20% 30% 40% 50% 60% 70% 80% 90% FBP
Manual
Automatic 225.1 268.2 288.9 321.0 347.1 370.1 392.1 416.0 440.8 472.1 518.5 547.5
239.7 270.1 290.1 324.0 349.5 373.4 395.7 420.0 443.5 472.5 514.1 544.8
Manual
Automatic 342.6 371.3 380.7 388.7 394.4 400.9 407.2 414.0 422.6 433.1 452.6 493.9
338.4 373.5 380.7 388.4 394.3 400.0 405.9 412.8 421.7 433.1 452.0 488.1
Sample 3 Manual
Automatic 321.3 363.7 378.7 397.5 412.1 426.6 439.9 453.2 467.7 486.0 511.5 547.4
330.8 364.4 379.0 397.2 411.8 425.9 439.6 452.4 467.8 485.6 514.1 538.7
10 m m Hg Pressure (All AET Values in °C) Sample 1
Sample 2
Sample 3
Boiling Point
Automatic
Manual
Automatic
Manual
Automatic
Manual
IBP 5% 10% 20% 30% 40% 50% 60% 70% 80% 90% FBP
203.2 252.3 274.7 313.5 340.6 363.1 385.6 408.4 433.0 461.3 507.5 538.5
199.6 254.5 280.7 316.5 342.2 366.4 388.9 411.5 436.5 465.5 506.9 536.3
343.0 370.2 376.9 383.3 391.0 397.0 402.9 409.1 419.4 430.3 450.5 492.8
342.7 370.3 378.1 384.8 391.1 396.9 403.3 410.2 419.0 430.9 451.2 482.7
319.0 360.7 374.6 392.7 408.0 422.4 436.4 450.7 465.3 483.3 509.1 544.0
319.0 359.9 374.2 392.5 407.4 421.2 434.1 448.1 462.9 480.3 504.8 536.3
CHAPTER 25: VOLATILITY
679
TABLE 3—Group characteristics. Group 0 Sample characteristics Distillate tj^pe
Group 1
Group 2
Group 3
Group 4
a65.5 >9.5
<65.5 <9.5
<65.5 <9.5
<65.5 <9.5
S250 <482
slOO <212 >250 >482
>100 >212 >250 >482
natural gasoline
Vapor pressure at 37.8°C, kPa 100°F, psi (Test Methods D 323, D 4953, D 5190, D 5191, D 5482, IP 69 or IP 394) Distillation, IBP "C "F EP°C °F
£250 £482
TABLE 4—Preparation of apparatus. Flask, mL ASTM distillation thermometer IP distillation thermometer range Flask support board diameter of hole, m m Temperature at start of test Flask °C °F Flask support and shield
Group 0
Group 1
Group 2
Group 3
Group 4
100 7C (7F) low A 32
125 7C (7F) low B 38
125 7C (7F) low B 38
125 7C (7F) low C 50
125 8C (8F) high C 50
0-5 32-^0 not above ambient
13-18 55-65 not above ambient
13-18 55-65 not above ambient
13-18 55-65 not above ambient
0-5 32^0
13-18 55-65
13-18 55-65
Receiving cylinder and 100 ml charge °C °F
13-18" 55-65"
not above ambient
13-ambient" 55-ambient''
"See 10.3.1.1 for exceptions.
TABLE 5—Conditions during test procedure. Temperature of cooling bath"
cF
Temperature of bath around receiving cylinder
C 'F
Time from first application of heat to initial boiling point, m i n Time from initial boiling point to 6% recovered, s to 10% recovered, min Uniform average rate of condensation from 5% recovered to 5 mL in flask, mL/min Time recorded from 5 mL residue to end point, m i n
Group 0
Group 1
Group 2
Group 3
Group 4
0-1 32-34 0-4 32-40
0-1 32-34 13-18 55-65
0-5 32-40 13-18 55-65
0-5 32-40 13-18 55-65
2-5
5-10
5-10
5-10
0-60 32-140 ±3 ±5 of charge temperature 5-15
60-100
60-100 4-5
4-5
3^ 4-5
5 max
4-5
4-5
5 max
5 max
5 max
5 max
''The proper condenser bath temperature will depend upon the wax content of the sample and its distillation fractions. The test is generally performed using one single condenser temperature. Wax formation in the condenser can be deduced from (a) the presence of wax particles in the distillate coming off the drip tip, (b) a higher distillation loss than what would be expected base on the initial boiling point of the specimen, (c) an erratic recovery rate and (d) the presence of wax particles during the removal of residual liquid by swabbing with a lint-free cloth (see 8.3). The minimum temperature that permits satisfactory operation shall be used. In general, a bath temperature in the 0 to 4°C range is suitable for kerosine, Grade No 1 fuel oil and Grade No. 1-D diesel fuel oil. In some cases involving Grade No. 2 fuel oil. Grade No. 2-D diesel fuel oil; gas oils and similar distillates, it may be necessary to hold the condenser bath temperature in the 38 to 60°C range.
680
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
TABLE 6—Repeatability and reproducibility for group. Manual Repeatability'^ °C
Point, % IBP 5 10 20 30-70 80 90 95 FBP
1.9 1.2 1.2 1.2 1.2 1.2 1.2
3.3 + 0.86Sc + 0.86Sc + 0.86Sc + 0.86Sc + 0.86Sc + 0.86Sc + 0.86Sc 3.9
3.4 2.2 2.2 2.2 2.2 2.2 2.2
Manual Reproducibility"
Automated Repeatability"
op
°C
°F
6
5.6 3.1 + 1.74Sc 2.0 + 1.74Sc 2 . 0 + 1.74Sc 2.0 + 1.74Sc 2.0 + 1.74Sc 0.8 + 1.74Sc 1.1 + 1.74Sc 7.2
10
+ + + + + + +
0.86SF 0.86SF 0.86SF 0.86SF 0.86SF 0.86SF 0.86SF
7
5.6 3.6 3.6 3.6 3.6 1.4 1.9
+ + + + + + +
°C
1.74SF 1.74SF 1.74SF 1.74SF 1.74SF 1.74SF 1.74SF
13
2.1 1.7 1.1 1.1 1.1 1.1 2.5
Automated Reproducibility" °C
-F
3.9 + 0.67Sc + 0.67Sc +0.67Sc +0.67Sc +0.67Sc +0.67Sc + 0.67Sc 4.4
7 3.8 3.0 2.0 2.0 2.0 2.0 4.5
+ + + + + + +
0.67SF 0.67SF 0.67SF 0.67SF 0.67SF 0.67SF 0.67SF
4.4 3.3 3.3 2.6 1.7 0.7 2.6
8
-F
7.2 + 2.0Sc + 2.0Sc + 2.0Sc + 2.0Sc + 2.0Sc + 2.0Sc + 2.0Sc 8.9
13 7.9 6.0 6.0 4.7 3.0 1.2 4.7
+ + + + + + +
2.0SF 2.0SF 2.0SF 2.0SF 2.0SF 2.0SF 2.0SF
16
"Sc or SF is the average slope (or rate of change) calculated in accordance with 13.2.
TABLE 7—Repeatability and reproducibility for groups 2, 3, and 4 (manual method). Repeatability"
Reproducibility" "F
IBP 5-95% FBP % volume at temperature reading "Calculate Sc or Sp from 13.2.
1.0 + 0.35Sc l.O + 0.41Sc 0.7 + 0.36Sc
1.9 1.8 1.3 0.7
0.7 + 1.92/SC
2.8 1.8 3.1 1.5
+ 0.35SF + 0.41SF +0.36SF + 1.66/SF
+ 0.93Sc + 1.33Sc +0.42Sc + 1.78/Sc
5.0 3.3 5.7 1.53
+ + + +
0.93SF 1.33SF 0.42SF 3.20/SF
TABLE 8—Repeatability and reproducibility for groups 2, 3 and 4 (automated). Repeatability" Collected, %
Reproducibility" op
"C
IBP 2% 5% 10% 20-70% 80% 90-95% FBP
1.1 1.2 1.2 1.2 1.1
3.5 3.5 + 1.08Sc + 1.42Sc + 1.42Sc + 1.42Sc + 1.08Sc 3.5
"C
6.3 6.3 2.0 2.2 2.2 2.2 2.0
+ + + + +
I.OSSF 1.42SF 1.42SF 1.42SF 1.08SF
2.6 2.0 3.0 2.9 3.0 2.0
6.3
8.5 + 1.92Sc + 2.53Sc + 2.64Sc + 3.97Sc + 2.64Sc + 2.53Sc 10.5
°F
15.3 4.7 3.6 5.4 5.2 5.4 3.6
+ + + + + +
1.92SF 2.53SF 2.64SF 3.97SF 2.64SF 2.53SF
18.9
"Sc or SF is the average slope (or rate of change) calculated in accordance with 13.5. TABLE 9—Approximate thermometer reading correction. Temperature Range °C
°F
10-30 30-50 50-70 70-90 90-110 110-130 130-150 150-170 170-190 190-210 210-230 230-250 250-270 270-290 290-310 310-330 330-350 350-370 370-390 390-410
50-86 86-122 122-158 158-194 194-230 230-266 266-302 302-338 338-374 374^10 410-446 446-482 482-518 518-554 554-590 590-626 626-662 662-698 698-734 734-770
Correction per 1.3kPa(10mmHg) Difference in Pressure °F °C 0.35 0.38 0.40 0.42 0.45 0.47 0.50 0.52 0.54 0.57 0.59 0.62 0.64 0.66 0.69 0.71 0.74 0.76 0.78 0.81
0.63 0.68 0.72 0.78 0.81 0.85 0.89 0.94 0.98 1.02 1.07 1.11 1.15 1.20 1.24 1.28 1.33 1.37 1.41 1.46
"Values to be added when barometric pressure is below 101.3 kPa (760 mm Hg) and to be subtracted when barometric pressure is above 101.3 kPa.
ASTM D 1160, Standard Test Method for Distillation Petroleum Products at Reduced Pressure
of
This test method is under the jurisdiction of ASTM Committee D-02 and is the direct responsibility of Subcommittee D2.08 on VolatiHty. This standeird was originally published in 1951. The latest edition is D 1160-99. This test method covers the determination, at reduced pressures, of the boiling range of petroleum products that can be partially or completely vaporized at a m a x i m u m liquid temperature of 400°C. The reduced pressure inside the apparatus during the test is measured by a Mcleod vacuum gage or a n appropriately calibrated electronic gage such as the Baratron. Both manual and automatic methods are specified. The schematic diagram of a vacuum distillation apparatus is indicated in Fig. 3 [19]. Both manual and automatic D 1160 distillation units use platinum resistance temperature probes or thermocouples. The specimen is distilled at an accurately controlled pressure between 0.13 and 6.7 kPa (1 and 50 m m Hg) under conditions that are designed to provide approximately one theoretical plate fractionation. The distillation pressure is controlled by a pressure regulating system consisting of a low
CHAPTER
25: VOLATILITY
681
TABLE 10—ASTM D-2 interlaboratory cross check program—auto D 86 distillation for motor gasoline. Lab Code
IBP
5%
10%
20%
30%
40%
50%
60%
70%
80%
90%
95%
FBP
%REC
1 2 3 4 5 6 7 8 10
34.6 30.6 38 34.0 33.8 31.7 34.8 37.1 35.4
51.7 52.2 54 44.5 53.0 49.7 55.1 56.4 47.0
62.0 62.8 64 56.1 62.7 60.0 64.1 65.8 57.8
79.1 81.7 83 75.7 79.7 78.9 82.3 83.9 76.5
94.8 96.1 98 92.8 95.1 95.0 96.8 99.1 94.1
105.7 106.1 107 104.6 105.7 106.2 106.6 108.8 105.7
112.9 112.7 114 112.3 112.5 112.7 113.1 115.6 113.1
119.4 119.4 121 118.7 118.7 119.2 120.2 122.4 119.6
127.4 128.9 131 127.3 126.5 127.4 129.4 131.8 127.7
141.7 146.1 149 141.4 142.1 142.0 146.5 149.0 142.2
167.2 172.2 176 166.7 166.8 164.8 171.1 174.5 166.8
184.2 198.4 217 183.8 185.5 182.2 200.6 208.3 183.4
215.8 214.4 223 223.1 222.2 218.8 214.1 225.0 225.5
97.5 96.6 95.8 96.5 97.9 97.4 96.2 96.9 97.0
TABLE 11—ASTM D-2 interlaboratory cross check program—auto D 86 distillation for reformulated gasoline. Lab Code
IBP
5%
10%
20%
30%
40%
50%
60%
70%
80%
90%
95%
FBP
%REC
%LOSS
E200
E300
1 2 4 5 6 7 8 10
103.1 108.8 100.2 104 105.3 94.1 107.6 109.8
132.5 124.7 130.5 134 120.1 129.2 124.5 132.5
141.6 134.7 140.2 137 133.5 138.0 135.5 142.5
153.3 147.7 151.9 150 147.6 150.0 148.1 159.1
165.6 160.8 164.1 162 160.1 161.0 160.3 170.1
180.5 175.1 178.3 177 175.2 175.3 175.1 183.8
199.2 195.0 198.0 196 195.2 193.5 193.6 202.6
222.5 218.1 220.7 220 219.5 217.4 217.7 222.4
250.2 246.5 247.8 248 247.6 245.5 244.7 244.3
279.1 276.6 276.4 278 395.3 274.7 273.7 278.2
318.1 316.9 313.5 317 319.1 311.5 311.7 319.0
351.8 347.7 346.2 349 353.5 341.4 340.3 353.3
391.1 395.6 396.9 400 395.3 393.2 394.7 394.2
97.4 97.3 97.8 96.9 96.9 99.1 97.7 98.1
1.3 0.9 1.2 3.1 1.9 0.5
50.4 52.8 50.9 51.8 53 52.9 52.9 48.7
85.7 86.8 86.4 86.1 87 87.3 87.2 85.7
0.9
TABLE 12—^ASTM D-2 interlaboratory cross check program—^Auto D 86 distillation for diesel fuel. Lab Code
IBP
5%
10%
20%
30%
40%
50%
60%
70%
80%
90%
95%
FBP
%REC
1 3 4 5 8 9 10 11 13 14
177.4 182.8 176.3 177.1 183.7 179.7 178.8 179.5 182.0 169.5
203.2 205.2 201.7 202.0
228.8 230.7 226.5 228.6 228.7 227.5 227.8 232.6 231.4 229.0
243.2 244.4 240.1 243.4 243.9 241.7 242.2 247 245.9 244.0
253.8 255.2 251.3 254.9 254.9 253.4 253.3 257.4 256.6 255.0
263.3 264.6 260.6 264.1 264.7 261.9 262.8 266.9 **268.3 265.0
271.2 273.4 269.1 273.1 274.1 270.8 271.1 276.3 274.9 273.5
281.6 282.8 278.2 282.0 283.3 280.4 280.6 285.4 283.9 283.0
293.0 294.3 289.2 293.8 294.8 291.9 291.7 297.1 296.1 293.5
309.0 311.8 305.2 310.4 311.9 308.0 308.9 315.0 313.4 309.5
323.9 330.4 319.3
341.3 343.5 336.0 335.2 340.0 339.5 340.6 343.5 341.1
98.3 97.5 98.7
203.8 204.4 203.7 200.5 201.0
209.4 214.1 210.5 210.0 210.5 212.9 212.8 216.4 211.9 212.5
98.3 98.3 97.8 97.6
Lab Code
IBP
5%
10%
20%
30%
40%
50%
60%
70%
80%
90%
95%
FBP
%REC
1 2 3 4 5 6 8 9 10
158.6 159.7 163.0 160.0 161 159.9 161.4 160.1 161.8
175.0 175.2 178.6 174.1 175 172.4 175.0 176.1 175.9
180.3 179.2 183.0 180.1 179 177.3 180.2 180.4 180.1
187.0 186.1 190.7 186.6 187 184.7 187.3 186.8 186.8
194.4 192.8 197.6 193.0 193 191.3 193.9 194.1 192.5
202.0 199.5 205.1 200.3 200 198.9 200.8 201.3 199.8
210.0 207.7 213.0 207.9 208 206.1 208.0 209.1 207.6
219.3 216.3 221.8 216.6 216 214.3 210.6 217.7 216.2
230.0 226.2 232.5 227.0 227 224.1 226.9 227.9 225.9
239.4 238 244.4 239.0 238 235.4 238.8 239.8 237.3
254.7 253.3 259.6 254.1 253 249.5 253.3 255.6 251.5
261.6 266.1 269.9 265.0 264 259.8 263.8 267.3 261.8
273.3 274 290.8 275.2 277 272.6 277.3 281.2 277.3
99.0 97.5 98.9 98.1 98 99.7 98.4 98.0 98.4
329.9 323.1 325.6 332.9 333.4
TABLE 13—ASTM interlaboratory cross check program—auto D 86 distillation for jet fuel.
efficiency, high capacity vacuum p u m p connected to one of two surge tanks, each having a capacity of 10-20 L and arranged in series. A solenoid vsJve or other type of regulator is installed in the connection between the tanks so that the first tank is maintained at p u m p pressure and the second one at the pressure of the distillation apparatus. Data are obtained during the distillation, from which the initieJ boiling point, the final boiling point, and a distillation curve relating volume percent distilled and Atmospheric Equivalent Temperature (AET) can be obtained. Atmospheric Equivalent Temperature is the temperature converted from the observed temperature during the test, to the expected distillate temperature if the distillation was performed at atmospheric pressure and there
was n o decomposition. The equations [20] used to convert the observed distillation temperature to AET are: AET =
748.1 X A - 273.1 [1/(VT,K] + (0.3861 X A) - 0.00051606
(4)
5.143222 - (0.972546 X logp) 2 5 7 9 . 3 2 9 - ( 9 5 . 7 6 X log p)
(5)
5.994295 - (0.972546 X log P) 2 6 6 3 . 1 2 9 - ( 9 5 . 7 6 X log P)
(6)
or A =
682
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
Digital Temperature Indicator [•
J_
PRT Sensor
IvjcimoiGHel Option # 2
Pressure
Reguiating System
.L (ColdTrap
Repressi/ring tft^Connec
Option # 1
Surge Tanks Vacuum Pump
Vacuum Source
Thermowell Circulating
•
^—^ ._S2?'' I XTCooiant F i o w ^ ^ = > ~ * / U Thermo-RegulaW Immersion Heater
Coolant Circulating System
FIG. 3—D 1160 vacuum distillation apparatus.
where: AET = A= VT,K = p=
atmospheric equivalent temperature in °C value obtained from Eqs 5 and 6 observed vapor temperature, °K pressure of the system in pKa, observed when the vapor temperature was read P = pressure of the system in m m Hg, observed when the vapor temperature was read.
Alternatively, the AET corresponding to the observed vapor temperature can be obtained from a set of Tables given in the method (Tables 14-19) [21]. This test method is used for the determination of the distillation characteristics of petroleum products and fractions that may decompose if distilled at atmospheric pressure. The boiling range, obtained at conditions designed to obtain approximately one theoretical plate fractionation, can be used in engineering calculations to design distillation equipment, to prepare appropriate blends for industrial purposes, to determine compliance with regulatory rules, to determine the suitability of the material as feed to a refining process, or for a host of other purposes. The boiling range is directly related to viscosity, vapor pressure, heating value, average molecular weight, and many other chemical, physical, and mechanical properties. Petroleum product specifications often include distillation limits based on data by this test method. Many engineering design correlations have been developed on data by this test method. These correlative methods are used extensively in current engineering practice. The precision for manual D 1160 distillation is given in Table 20 [22]. Table 21 gives the precision for automatic D 1160 as obtained from Research Report RR:D2-1206 [7]. The observed vapor temperature readings are converted to Atmospheric Equivalent Temperature (AET) using the tables and/or equations mentioned earlier. The AET is reported to
the nearest degree Celsius corresponding to the volumetric percentages of liquid recovered in the receiver. The sample identity, density, and total amount of distillate recovered in the receiver and the cold trap are also reported. Other International Distillation
Standard
Test Methods
for
All the aforementioned distillation test methods are ASTM standard test methods. Although ASTM standard test methods are used extensively t h r o u g h o u t the world, there are other international distillation standards that are used and are applicable to specific geographiceJ eireas. Table 22 gives a cross reference for a n u m b e r of international distillation standards and their corresponding ASTM designation. Flaitimability From a safety point of view, a critical volatility property of fuels and lubricants is flash point. The flash point of a liquid is the lowest temperature corrected to a pressure of 101.3 kPa (760 m m Hg) at w h i c h application of an ignition source causes the vapors of a specimen of the sample to ignite u n d e r specified test conditions. Flash point measures the tendency of the specimen to form a flammable mixture with air under controlled laboratory conditions. It is only one of a n u m b e r of properties that must be considered in assessing the overall flammability hazard of a material. Flash point is used in shipping and safety regulations to define flammable and combustible materials. Flash point can indicate the possible presence of highly volatile and flammable materials in a relatively nonvolatile Eind nonflammable material. Various regulatory organizations use flash point to define flammable and combustible materieds as it relates to transporting, shipping, and safety regulations. Prior to 1990, the Federal Register of the U.S. Department of Transport defined
CHAPTER 25: VOLATILITY 683 TABLE 14—AET conversion table at 0.13 kPa (1 mm Hg) pressure °C. P (mm Hg) 1
T 35 40 45 50 55 60 G5 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200 205 210 215 220 225 230 235 240 245 250 255 260 265 270 275 280 285 290 295 300 305 310 315 320 325 330 335 340 345 350
0 195.2 202.1 208.9 215.7 222.5 229.2 235.9 242.7 249.3 256.0 262.7 269.3 275.9 282.5 289.1 295.6 302.1 308.6 315.1 321.6 328.0 334.5 340.9 347.3 353.6 360.0 366.3 372.6 378.9 385.2 391.4 397.6 403.9 410.0 416.2 422.4 428.5 434.6 440.7 446.8 452.9 458.9 465.0 471.0 477.0 482.9 488.» 494.8 500.7 506.6 512.5 518.4 524.3 530.1 535.9 541.7 547.5 553.3 559.0 564.7 570.5 576.1 581.8 587.5
0.5 195.9 202.7 209.6 216.4 223.1 229.9 236.6 243.3 250.0 256.7 263.3 270.0 276.6 283.2 289.7 296.3 302.8 309.3 315.8 322.2 328.7 335.1 341.5 347.9 354.3 360.6 366.9 373.2 379.5 385.8 392.0 398.3 404.5 410.7 416.8 423.0 429.1 435.2 441.3 447.4 453.5 459.5 465.6 471.6 477.6 483.5 4^.5 495.4 501.3 507.2 513.1 519.0 524.8 530.7 536.5 542.3 548.1 553.8 559.6 565.3 571.0 576.7 582.4 588.1
1 196.6 203.4 210.2 217.0 223.8 230.6 237.3 244.0 250.7 257.4 264.0 270.6 277.2 283.8 290.4 296.9 303.4 309.9 316.4 322.9 329.3 335.7 342.1 348.5 354.9 361.2 367.6 373.9 380.1 386.4 392.7 398.9 405.1 411.3 417.5 423.6 429.7 435.9 442.0 448.0 454.1 460.1 466.2 472.2 478.2 484.1 490.1 496.0 501.9 507.8 513.7 519.6^ 525.4 531.3 537.1 542.9 548.6 554.4 560.2 565.9 571.6 577.3 583.0 588.6
1.5 197.3 204.1 210.9 217.7 224.5 231.2 238.0 244.7 251.4 258.0 264.7 271.3 277.9 284.5 291.0 297.6 304.1 310.6 317.1 323.5 330.0 336.4 342.8 349.2 355.5 361.9 368.2 374.5 380.8 387.0 393.3 399.5 405.7 411.9 418.1 424.2 430.4 436.5 442.6 448.6 454.7 460.7 466.8 472.8 478.7 484.7 490.7 496.6 502.5 508.4 514.3 520.2 526.0 531.8 537.6 543.4 549.2 555.0 560.7 566.5 572.2 577.9 583.5 589.2
2 198.0 204.8 211.6 218.4 225.2 231.9 238.6 245.3 252.0 258.7 265.3 271.9 278.5 285.1 291.7 298.2 304.7 311.2 317.7 324.2 330.6 337.0 343.4 349.8 356.2 3625 368.8 375.1 381.4 387.7 393.9 400.1 406.3 412.5 418.7 424.8 431.0 437.1 443.2 449.2 455.3 461.3 467.4 473.4 479.3 485.3 491.3 497.2 503.1 509.0 514.9 520.7 526.6 532.4 538.2 544.0 549.8 555.6 561.3 567.0 572.7 578.4 584.1 589.8
2.5 198.6 205.5 212.3 219.1 225.8 232.6 239.3 246.0 252.7 259.3 266.0 272.6 279.2 285.8 292.3 298.9 305.4 311.9 318.4 324.8 331.2 337.7 344.1 350.4 356.8 363.1 369.4 375.7 382.0 388.3 394.5 400.7 406.9 413.1 419.3 425.4 431.6 437.7 443.8 449.8 455.9 461.9 468.0 474.0 479.9 485.9 491.9 497.8 503.7 509.6 515.5 521.3 527.2 533.0 538.8 544.6 550.4 556.1 561.9 567.6 573.3 579.0 584.7 590.3
3 199.3 206.2 213.0 219.7 226.5 233.3 240.0 246.7 253.4 260.0 266.6 273.3 279.9 286.4 293.0 299.5 306.0 312.5 319.0 325.5 331.9 338.3 344.7 351.1 ^7.4 363.8 370.1 376.4 382.7 388.9 395; 1 401.4 407.6 413.8 419.9 426.1 432.2 438.3 444.4 450.5 456.5 462.5 468.6 474.6 480.5 486.5 492.4 498.4 504.3 510.2 516.1 521.9 527.8 533.6 539.4 545.2 550.9 556.7 562.4 568.2 573.9 579.6 585.2 590.9
3.5 200.0 206.8 213.6 220.4 227.2 233.9 240.6 247.3 254.0 260.7 267.3 273.9 280.5 287.1 293.6 300.2 306.7 313.2 319.7 326.1 332.5 338.9 345.3 351.7 358.1 364.4' 370.7 377.0 383.3 389.5 395.8 402.0 408.2 414.4 420.5 426.7 432.8 438.9 445.0 451.1 457.1 463.1 469.2 475.2 481.1 487.1 493.0 AS&.Q
504.9 510.8 516.6 522.5 528.3 534.2 540.0 545.8 551.5 557.3 563.0 568.7 574.4 580.1 585.8 591.5
4 200.7 207.5 214.3 221.1 227.9 234.6 241.3 248.0 254.7 261.3 268.0 274.6 281.2 287.7 294.3 300.8 307.3 313.8 320.3 326.7 333.2 339.6 346.0 K2.3 358.7 3^.0 371.3 377.6 383.9 390.2 396.4 402.6 408.8 415.0 421.1 427.3 433.4 439.5 445.6 451.7 457.7 463.7 469.8 475.8 481.7 487.7 493.6 499.6 505.5 511.4 517.2 523.1 528.9 534.7 540.5 546.3 552.1 557.9 563.6 569.3 575.0 580.7 586.4 592.0
4.5 201.4 208.2 215.0 221.8 228.5 235.3 242.0 248.7 255.4 262.0 268.6 275.2 281.8 288.4 295.0 301.5 308.0 314.5 320.9 327.4 333.8 340.2 346.6 353.0 359.3 365.7 372.0 378.3 384.5 390.8 397.0 403.2 409.4 415.6 421.8 427.9 434.0 440.1 446.2 452.3 458.3 464.4 470.4 476.4 482.3 488.3 494.2 500.2 506.1 511.9 517.8 523.7 529.5 535.3 541.1 546.9 552.7 558.4 564.2 569.9 575.6 581.3 586.9 %2.6
684 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK TABLE 15—AET conversion table at 0.27 kPa (2 mm Hg) pressure °C. (mm Hg) 2
T 36 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 1^ 140 145 150 155 160 165 170 175 180 185 190 195 200 205 210 215 220 225 230 235 240 245 250 255 260 265 270 275 280 285 290 295 300 305 310 315 320 325 330 335 340 345 350
0 180.9 187.6 194.3 201.0 207.6 214.2 220.8 227.4 234.0 240.5 247.1 253.6 260.1 266.5 273.0 279.4 285.8 292.2 298.6 305.0 311.3 317.6 323.9 330.2 336.5 342.8 349.0 355.2 361.4 367.6 373.8 379.9 386.1 392.2 398.3 404.4 410.4 416.5 422.5 428.5 434.5 440.5 446.5 452.4 458.4 464.3 470.2 476.1 481.9 487.8 493.6 499.4 505.3 511.0 516.8 522.6 528.3 534.1 539.8 545.5 551.1 556.8 562.5 568.1
0.5 181.6 188.3 195.0 201.6 208.3 214.9 221.5 228.1 234.6 241.2 247.7 254.2 260.7 267.2 273.6 280.1 286.5 292.9 299.2 30S.6 311.9 318.3 324.6 330.9 337.1 343.4 349.6 355.8 362.0 368.2 374.4 380.5 386.7 392.8 398.9 405.0 411.0 417.1 423.1 429.1 435.1 441.1 447.1 453.0 459.0 464.9 470.8 476.7 482.5 488.4 494.2 500.0 505.8 511.6 517.4 523.2 528.9 534.6 540.3 546.0 551.7 557.4 563.0 568.7
1 < 182.3 189.0 195.6 202.3 208.9 215.6 222.2 228.7 235.3 241.8 248.4 254.9 261.4 267.8 274.3 280.7 287.1 293.5 299.9 306.2 312.6 318.9 325.2 331.5 337.8 344.0 350.2 356.5 362.7 368.8 375.0 381.2 387.3 393.4 399.5 405.6 411.6 417.7 423.7 429.7 435.7 441.7 447.7 453.6 459.5 465.5 471.4 477.2 483.1 489.0 494.8 500.6 506.4 512.2 518.0 5?3.7 529.5 535.2 540.9 546.6 552.3 557.9 563.6 569.2
1.5 182.9 189.6 196.3 203.0 209.6 216.2 222.8 229.4 236.0 242.5 249.0 255.5 262.0 268.5 274.9 281.3 287.8 294.1 300.5 306.9 313.2 319.5 325.8 332.1 338.4 344.6 350.9 357.1 363.3 369.5 375.6 381.8 387.9 394.0 400.1 406.2 412.2 418.3 424.3 430.3 436.3 442.3 448.3 454.2 460.1 466.0 471.9 477.8 483.7 489.5 495.4 501.2 507.0 512.8 518.5 524.3 530.0 535.8 541.5 547.2 552.8 558.5 564.2 569.8
2 183.6 190.3 197.0 203.6 210.3 216.9 223.5 230.0 236.6 243.1 249.7 256.2 262.6 269.1 275.6 282.0 288.4 294.8 301.2 307.5 313.8 320.2 326.5 332.7 339.0 345.3 351.5 357.7 363.9 370.1 376.2 382.4 388.5 394.6 400.7 406.8 412.8 418.9 424.9 430.9 436.9 442.9 448.9 454.8 460.7 466.6 472.5 478.4 484.3 490.1 496.0 501.8 507.6 513.4 519.1 524.9 530.6 536.3 542.0 547.7 553.4 559.1 564.7 570.4
2.5 184.3 191.0 197.6 204.3 210.9 217.5 224.1 230.7 237.3 243.8 250.3 256.8 263.3 269.8 276.2 282.6 289.0 295.4 301.8 308.1 314.5 320.8 327.1 333.4 339.6 345.9 352.1 358.3 364.5 370.7 376.9 383.0 389.1 395.2 401.3 407.4 413.5 419.5 425.5 431.5 437.5 443.5 449.5 455.4 461.3 467.2 473.1 479.0 484.9 490.7 496.5 502.4 508.2 513.9 519.7 525.5 531.2 536.9 542.6 548.3 554.0 559.6 565.3 570.9
3 185.0 191.6 198.3 205.0 211.6 218.2 224.8 231.4 237.9 244.5 251.0 257.5 263.9 270.4 276.8 283.3 289.7 296.1 302.4 308.8 315.1 321.4 327.7 334.0 340.3 346.5 ;K2.7
358.9 3^.1 371.3 377.5 383.6 389.7 395.8 401.9 408.0 414.1 420.1 426.1 432.1 438.1 444.1 450.0 456.0 461.9 467.8 473.7 479.6 485.4 491.3 497.1 502.9 508.7 514.5 520.3 526.0 531.8 537.5 543.2 548.9 554.5 560.2 565.8 571.5
3.5 185.6 192.3 199.0 205.6 212.3 218.9 225.4 232.0 238.6 245.1 251.6 258.1 264.6 271.0 277.5 283.9 290.3 296.7 303.1 309.4 315.7 322.1 328.4 334.6 340.9 347.1 353.4 359.6 3K.8 371.9 378.1 384.2 390.3 396.4 402.5 408.6 414.7 420.7 426.7 432.7 438.7 444.7 450.6 456.6 462.5 468.4 474.3 480.2 486.0 491.9 497.7 503.5 509.3 515.1 520.9 526.6 532.3 538.1 543.8 549.4 555.1 560.8 566.4 572.0
4 186.3 193.0 199.6 206.3 212.9 219.5 226.1 232.7 239.2 245.8 252.3 258.8 2K.2 271.7 278.1 284.5 290.9 297.3 303.7 310.0 316.4 322.7 329.0 335.3 341.5 347.8 354.0 360.2 366.4 372.5 378.7 384.8 391.0 397.1 403.1 409.2 415.3 421.3 427.3 433.3 439.3 445.3 451.2 457.2 463.1 469.0 474.9 480.8 486.6 492.5 498.3 504.1 509.9 515.7 521.4 527.2 532.9 538.6 544.3 550.0 555.7 5S^.3 567.0 572.6
4.5 187.0 193.6 200.3 207.0 213.6 220.2 226.8 233.3 239.9 246.4 252.9 259.4 26S.9 272.3 278.8 285.2 291.6 298.0 304.3 310.7 317.0 323.3 329.6 335.9 342.1 348.4 354.6 360.8 367.0 373.2 379.3 385.4 391.6 397.7 403.8 409.8 415.9 421.9 427.9 433.9 439.9 445.9 451.8 457.8 463.7 469.6 475.5 481.3 487.2 493.0 498.9 504.7 510.5 516.2 522.0 527.7 533.5 539.2 544.9 550.6 556.2 561.9 567.5 573.2
CHAPTER 25: VOLATILITY 685 TABLE 16—AET conversion table at 0.67 kPa (5 mm Hg) pressure °C. P (mm Hg) 5
T 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200 205 210 215 220 225 230 235 240 245 250 255 260 265 270 275 280 285 290 295 300 305 310 315 320 325 330 335 340 345 350
0 162.0 168.5 174.9 181.4 187.9 194.3 200.7 207.1 213.5 219.9 226.2 232.6 238.9 245.2 251.5 257.8 264.0 270.3 276.5 282.7 288.9 295.1 301.3 307.5 313.6 319.7 325.8 331.9 338.0 344.1 350.1 356.2 362.2 368.2 374.2 380.1 386.1 392.1 398.0 403.9 409.8 415.7 421.6 427.5 433.3 439.1 445.0 450.8 456.6 462.3 468.1 473.9 479.6 485.3 491.0 496.7 502.4 508.1 513.7 519.4 525.0 530.6 536.2 541.8
0.5 1^.6 169.1 175.6 182.1 188.5 194.9 201.4 207.8 214.2 220.5 226.9 233.2 239.5 245.8 252.1 258.4 264.7 270.9 277.1 283.4 289.6 295.7 301.9 308.1 314.2 320.3 326.4 332.5 338.6 344.7 350.7 356.8 362.8 368.8 374.8 380.7 386.7 392.7 398.6 404.5 410.4 416.3 422.2 428.0 433.9 439.7 445.5 451.3 457.1 462.9 "468.7 474.4 480.2 485.9 491.6 497.3 503.0 508.6 514.3 519.9 525.6 531.2 536.8 542.4
1 163.3 169.8 176.2 182.7 189.2 195.6 202.0 208.4 214.8 221.2 227.5 233.8 240.2 246.5 252.8 259.0 265.3 271.5 277.8 284.0 290.2 296.4 302.5 308.7 314.8 320.9 327.0 333.1 339.2 345.3 351.3 357.4 363.4 369.4 375.4 381.3 387.3 393.2 399.2 405.1 411.0 416.9 422.8 428.6 434.5 440.3 446.1 451.9 457.7 463.5 469.3 475.0 480.7 486.5 492.2 497.9 503.5 509.2 514.9 520.5 526.1 531.8 537.4 542.9
1.5 163.9 170.4 176.9 183.3 189.8 196.2 202.6 209.0 215.4 221.8 228.1 234.5 240.8 247.1 253.4 259.7 265.9 272.2 278.4 284.6 290.8 297.0 303.1 309.3 315.4 321.6 327.7 333.7 ^9.8 345.9 351.9 358.0 364.0 370.0 376.0 381.9 387.9 ^3.8 399.8 405.7 411.6 417.5 423.3 429.2 435.1 440.9 446.7 452.5 458.3 464.1 469.8 475.6 481.3 487.0 492.7 498.4 504.1 509.i3 515.4 521.1 526.7 532.3 537.9 543.5
2 164.6 171.1 177.5 184.0 190.4 196.9 203.3 209.7 216.1 222.4 228.8 235.1 241.4 247.7 254.0 260.3 266.5 272.8 279.0 285.2 291.4 297.6 »}3.8 309.9 3t'6.0 322.2 328.3 334.4 340.4 346.5 352.5 358.6 .364.6 370.6 376.6 382.5 388.5 394.4 400.4 406.3 412.2 418.1 423.9 429.8 435.6 441.5 447.3 453.1 458.9 464.6 470.4 476.1 481.9 487.6 493.3 499.0 504.7 510.3 516.0 521.6 527.3 532.9 538.5 544.1
2.5 165.2 171.7 178.2 184.6 191.1 197.5 203.9 210.3 216.7 223.1 229.4 235.7 242.1 248.4 254.7 160.9 267.2 273.4 279.6 285.8 292.0 298.2 304.4 310.5 316.7 322.8 328.9 335.0 341.0 347.1 353.1 359.2 365.2 371.2 377.2 383.1 389.1 395,0 401.0 406.9 412.8 418.7 424.5 430.4 436.2 442.0 447.9 453.7 459.4 465.2 471.0 476.7 482.5 488.2 493.9 499.6 505.2 510.9 516.6 522.2 527.8 533.4 539.0 544.6
3 165.9 172.3 178.8 185.3 191.7 198.2 204.6 211.0 217.3 223.7 230.0 236.4 242.7 249.0 255.3 261.5 267.8 274.0 280.3 286.5 292.7 298.8 305.0 311.1 317.3 323.4 329.5 335.6 341.6 347.7 353.7 359.8 365.8 371.8 377.8 383.7 389.7 395.6 401.5 407.5 413.4 419.2 425.1 431.0 436.8 442.6 448.4 454.2 460.0 465.8 471.6 477.3 483.0 488.7 494.4 500.1 505.8 511.5 517.1 522.8 528.4 534.0 539.6 545.2
3.5 166.5 173.0 179.5 185.9 192.4 '198.8 205.2 211.6 218.0 224.3 230.7 237.0 243.3 249.6 255.9 262.2 268.4 274.7 280.9 287.1 293.3 299.4 305.6 311.8 317.9 324.0 330.1 336.2 342.2 348.3 354.3 360.4 366.4 372.4 378.4 384.3 390.3 396.2 402.1 408.0 413.9 419.8 425.7 431.5 437.4 443.2 449.0 454.8 460.6 466.4 472.1 477.9 483.6 4%.3 495.0 500.7 506.4 512.0 517.7 523.3 528.9 534.6 540.1 545.7
4 167.2 173.6 180.1 186.6 193.0 199.4 205.8 212.2 218.6 225.0 231.3 237.6 244.0 250.3 256.5 262.8 269.0 275.3 281.5 287.7 293.9 300.1 306.2 312.4 318.5 324.6 330.7 336.8 342.9 348.9 354.9 361.0 367.0 373.0
37ao 384.9 390.9 396.8 402.7 408.6 414.5 420.4 426.3 432.1 438.0 443.8 449.6 455.4 461.2 466.9 472.7 478.4 484.2 489.9 495.6 501.3 506.9 512.6 518.3 523.9 529.5 535.1 540.7 546.3
4.5 , 167.8 174.3 180.8 187.2 193.7 200.1 206.5 212.9 219.2 225.6 231.9 238.3 244.6 250.9 257.2 263.4 269.7 275.9 282.1 288.3 294.5 300.7 306.8 313.0 319.1 325.2 331.3 337.4 343.5 349.5 355.6 361.6 367.6 373.6 379.6 385.5 391.5 397.4 403.3 409.2 415.1 421.0 426.9 432.7 438.6 444.4 450.2 456.0 461.8 467.5 473.3 479.0 484.7 490.5 496.2 501.8 507.5 513.2 518.8 524.4 530.1 535.7 541.3 546.8
686 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 17—AET conversion table at 1.33 kPa (10 mm Hg) pressure °C. P (mm Hg) 10
T 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200 205 210 215 220 225 230 235 240 245 250 255 260 265 270 275 280 285 290 295 300 305 310 315 320 325 330 335 340 345 350
0 146.8 153.1 159.5 165.8 172.1 178.4 184.6 190.9 197.1 203.3 209.6 215.8 222.0 228.1 234.3 240.4 246.6 252.7 258.8 264.9 271.0 277.0 283.1 289.1 295.2 301.2 307.2 313.2 319.1 325.1 331.0 337.0 342.9 348.8 354.7 360.6 366.5 372.3 378:2 384.0 389.8 395.6 401.4 407.2 413.0 418.8 424.5 430.3 436.0 441.7 447.4 453.1 458.8 464.4 470.1 475.7 481.3 487.0 492.6 498.2 503.7 509.3 514.9 520.4
0.5 147.4 153.8 160.1 166.4 172.7 179.0 185.3 191.5 197.7 204.0 210.2 216.4 222.6 228.7 234.9 241.0 247.2 253.3 259.4 265.5 271.6 277.6 203.7 289.7 295.8 301.8 307.8 313.8 319.7 325.7 331.6 337.6 343.5 349.4 355.3 361.2 367.1 372.9 378.8 384.6 390.4 396.2 402.0 407.8 413.6 419.3 425.1 430.8 436.5 442.3 448.0 4S3.6 459.3 465.0 470.6 476.3 481.9 487.5 493.1 498.7 504.3 509.9 515.4 521.0
1 148.1 154.4 160.7 167.0 173.3 179.6 185.9 192.1 198.4 204.6 210.8 217.0 223.2 229.4 235.5 241.7 247.8 253.9 260.0 266.1 272.2 278.2 284.3 290.3 296.4 302.4 308.4 314.4 320.3 326.3 332.2 338.2 344.1 350.0 355.9 361.8 367.6 373.5 379.3 385.2 391.0 396.8 402.6 408.4 414.2 419.9 425.7 431.4 437.1 442.8 448.5 454.2 459.9 465.6 471.2 476.8 482.5 488.1 433.7 499.3 504.9 510.4 516.0 521.5
1.5 148.7 155.0 161.4 167.7 174.0 180.2 186.5 192.8 199.0 205.2 211.4 217.6 223.8 230.0 236.1 242.3 248.4 254.5 260.6 266.7 272.8 278.8 284.9 290.9 297.0 303.0 309.0 314.9 320.9 326.9 332.8 338.8 344.7 350.6 356.5 362.4 368.2 374.1 379.9 385.8 391.6 397.4 403.2 409.0 414.7 420.5 426.2 432.0 437.7 443.4 449.1 454.8 460.5 466.1 471.8 477.4 483.0 488.6 ^4.2 499.8 505.4 511.0 516.5 522.1
2 149.3 155.7 162.0 168.3 174.6 180.9 187.1 193.4 199.6 205.8 212.0 218.2 224.4 230.6 236.7 242.9 249.0 255.1 261.2 267.3 273.4 279.5 285.5 291.5 297.6 303.6 309.6 315.5 321.5 327.5 333.4 3^.3 345.3 351.2 357.1 362.9 368.8 374.7 380.5 386.3 392.2 398.0 403.8 409.5 415.3 421.1 426.8 432.5 438.3 444.0 449.7 455.4 461.0 466.7 472.3 478.0 483.6 489.2 494.8 500.4 506.0 511.5 517.1 522.6
2.5 150.0 156.3 162.6 168.9 175.2 181.5 187.8 194.0 200.2 206.5 212.7 218.9 225.0 231.2 237.4 243.5 249.6 255.7 261.8 267.9 274.0 280.1 286.1 292.1 298.2 304.2 310.2 316.1 322.1 328.1 334.0 339.9 345.9 351.8 357.7 363.5 369.4 375.3 381.1 386.9 392.7 398.5 404.3 410.1 415.9 421.6 427.4 433.1 438.8 444.5 450.2 455.9 461.6 467.3 472.9 478.5 484.2 489.8 495.4 501.0 506.5 512.1 517.7 523.2
3 150.6 156.9 163.3 169.6 175.8 182.1 188.4 194.6 200.9 207.1 213.3 219.5 225.7 231.8 238.0 244.1 250.2 256.4 262.4 268.5 274.6 280.7 286.7 292.7 298.8 304.8 310.8 316.7 322.7 328.7 334.6 340.5 346.4 352.4 358.2 364.1 370.0 375.8 381.7 387.5 393.3 399.1 404.9 410.7 416.5 422.2 428.0 433.7 439.4 445.1 450.8 456.5 462.2 467.8 473.5 479.1 484.7 490.3 495.9 501.5 507.1 512.7 518.2 523.7
3.5 151.2 157.6 163.9 170.2 176.5 182.7 189.0 195.3 201.5 207.7 213.9 220.1 226.3 232.4 238.6 244.7 250.9 257.0 263.1 269.1 275.2 281.3 287.3 293.3 299.4 305.4 311.4 317.3 323.3 329.3 335.2 341.1 347.0 352.9 358.8 364.7 370.6 376.4 382.3 388.1 393.9 399.7 405.5 411.3 417.0 422.8 428.5 434.3 440.0 445.7 451.4 457.1 462.7 468.4 474.0 479.7 485.3 490.9 496.5 502.1 507.6 513.2 518.8 524.3
4 151.9 158.2 164.5 170.8 177.1 183.4 189.6 195.9 202.1 208.3 214.5 220.7 226.9 233.1 239.2 245.3 251.5 257.6 263.7 269.7 275.8 281.9 287.9 293.9 300.0 306.0 312.0 317.9 323.9 329.9 335.8 341.7 347.6 353.5 359.4 3^.3 371.2 377.0 382.8 388.7 394.5 400.3 406.1 411.9 417.6 423.4 429.1 434.8 440.5 446.3 451.9 457.6 463.3 468.9 474.6 480.2 485.8 491.4 497.0 502.6 508.2 513.8 519.3 524.9
4.5 152.5 158.8 165.1 171.4 177.7 184.0 190.3 196.5 202.7 208.9 215.1 221.3 227.5 233.7 239.8 246.0 252.1 258.2 264.3 270.4 276.4 282.5 288.5 294.5 300.6 306.6 312.6 318.5 324.5 330.4 336.4 342.3 348.2 354.1 360.0 365.9 371.7 377.6 3^.4 389.3 395.1 400.9 406.7 412.4 418.2 423.9 429,7 435.4 441.1 446^8 452.5 458.2 463.9 469.5 475.2 480.8 486.4 492.0 497.6 503.2 508.8 514.3 519.9 525.4
CHAPTER 25: VOLATILITY 687 TABLE 18—AEl conversion table at 2.67 kPa (20 mm Hg) pressure °C. P (mm Hg) 20
T 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200 205 210 215 220 225 230 235 240 245 250 255 260 265 270 275 280 285 290 295 300 305 310 315 320 325 330 335 340 345 350
0 131.0 137.1 143.3 149.4 155.6 161.7 167.8 173.9 180.0 186.0 192.1 198.2 204.2 210.2 216.2 222.2 228.2 234.2 240.2 246.1 252.1 258.0 263.9 269.9 275.8 281.7 287.5 293.4 299.3 305.1 310.9 316.8 322.6 328.4 334.2 340.0 345.7 351.5 357.3 363.0 368.7 374.5 380.2 385.9 391.5 397.2 402.9 408.5 414.2 419.8 425.5 431.1 436.7 442.3 447.9 453.4 459.0 464.6 470.1 475.6 481.2 486.7 492.2 497.7
0.5 131.6 137.8 143.9 150.1 156.2 162.3 168.4 174.5 180.6 186.6 192.7 198.8 204.8 210.8 216.8 222.8 228.8 234.8 240.8 246.7 252.7 258.6 264.5 270.5 276.4 282.2 288.1 294.0 299.8 305.7 311.5 317.4 323.2 329.0 334.8 340.6 346.3 352.1 357.8 363.6 369.3 375.0 380.7 386.4 392.1 397.8 403.5 409.1 414.8 420.4 426.0 431.6 437.2 442.8 448.4 454.0 459.6 465.1 470.7 476.2 481.7 487.2 492.7 498.2
1 132.2 138.4 144.5 150.7 156.8 162.9 169.0 175.1 181.2 187.3 193.3 199.4 205.4 211.4 217.4 223.4 229.4 235.4 241.4 247.3 253.3 259.2 265.1 271.0 276.9 282.8 288.7 294.6 300.4 306.3 312.1 317.9 323.8 329.6 335.3 341.1 346.9 352.7 aSb.A 364.1 369.9 375.6 381.3 387.0 392.7 398.4 404.0 409.7 415.3 421.0 426.6 432.2 437.8 443.4 449.0 454.5 460.1 AIS5.7
471.2 476.7 482.3 487.8 493.3 498.8
1.5 132.8 139.0 145.1 151.3 157.4 163.5 169.6 175.7 181.8 187.9 193.9 200.0 206.0 212.0 218.0 224.0 230.0 236.0 242.0 247.9 253.9 259.8 265.7 271.6 277.5 283.4 289.3 295.2 301.0 306.9 312.7 318.5 324.3 330.1 335.9 341.7 347.5 353.2 359.0 364.7 370.4 376.2 381.9 387.6 393.3 %8.9 404.6 410.2 415.9 421.5 427.1 432.8 438.4 444.0 449.5 455.1 460.7 466.2 471.8 477.3 482.8 488.3 493.8 499.3
2 133.4 139.6 145.8 151.9 158.0 164.1 170.2 176.3 182.4 188.5 194.5 200.6 206.6 212.6 218.6 224.6 230.6 236.6 242.6 248.5 254.5 260.4 266.3 272.2 278.1 284.0 289.9 295.8 301.6 307.4 313.3 319.1 324.9 330.7 336.5 342.3 348.1 353.8 359.6 365.3 371.0 376.7 382.4 388.1 393.8 399.5 405.2 410.8 416.5 422.1 427.7 433.3 438.9 444.5 450.1 455.7 461.2 466.8 472.3 477.8 483.4 488.9 494.4 499.9
2.5 134.1 140.2 146.4 152.5 158.6 164.7 170.8 176.9 183.0 189.1 195.1 201.2 207.2 213.2 219.2 225.2 231.2 237.2 243.2 249.1 255.1 261.0 266.9 272.8 278.7 284.6 290.5 296.3 302.2 308.0 313.9 319.7 325.5 331.3 337.1 342.9 348.6 354.4 360.1 365.9 371.6 377.3 383.0 388.7 394.4 400.1 405.7 411.4 417.0 422.6 428.3 433.9 439.5 445.1 450.6 456.2 461.8 467.3 472.9 478.4 483.9 489.4 494.9 500.4
3 134.7 140.8 147.0 153.1 159.2 165.4 171.5 177.5 183.6 189.7 195.7 201.8 207.8 213.8 219.8 225.8 231.8 237.8 243.8 249.7 255.6 261.6 267.5 273.4 279.3 285.2 291.1 296.9 302.8 308.6 314.4 320.3 326.1 331.9 337.7 343.4 349.2 355.0 360.7 366.4 372.2 377.9 383.6 389.3 395.0 400.6 406.3 411.9 417.6 423.2 428.8 434.4 440.0 445.6 451.2 456.8 462.3 467.9 473.4 479.0 484.5 490.0 495.5 501.0
3.5 135.3 141.5 147.6 153.7 159.9 166.0 172.1 178.1 184.2 190.3 196.3 202.4 208.4 214.4 220.4 226.4 232.4 238.4 244^4 250.3 256.2 262.2 268.1 274.0 279.9 285.8 291.6 297.5 303.4 309.2 315.0 320.8 326.7 332.5 338.2 344.0 349.8 355.5 361.3 367.0 372.7 378.4 384.2 389.8 395.5 401.2 406.9 412.5 418.1 423.8 429.4 435.0 440.6 446.2 451.8 457.3 462.9 468.4 474.0 479.5 485.0 490.5 496.0 501.5
4 135.9 142.1 148.2 154.3 160.5 166.6 172.7 178.8 184.8 190.9 196.9 203.0 209.0 215.0 221.0 227.0 233.0 239.0 244.9 250.9 256.8 262.8 268.7 274.6 280.5 286.4 292.2 298.1 303.9 309.8 315.6 321.4 327.2 333.0 338.8 344.6 350.4 356.1 361.9 367.6 373.3 379.0 384.7 %0.4 396.1 401.8 407.4 413.1 418.7 424.3 430.0 435.6 441.2 446.7 452.3 457.9 463.4 469.0 474.5 480.1 485.6 491.1 496.6 502.1
4.5 136.5 142.7 148.8 155.0 161.1 167.2 173.3 179.4 185.4 191.5 197.5 203.6 209.6 215.6 221.6 227.6 233.6 239.6 245.5 251.5 257.4 263.4 269.3 275.2 281.1 286.9 292.8 298.7 304.5 310.4 316.2 322.0 327.8 333.6 3^.4 345.2 350.9 356.7 362.4 368.2 373.9 379.6 385.3 391.0 396.7 402.3 Afy&.Q
413.6 419.3 424.9 430.5 436.1 441.7 447.3 452.9 458.4 464.0 469.5 475.1 480.6 486.1 491.6 497.1 502.6
688 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 19—AET conversion table at 6.7 kPa (50 mm Hg) pressure °C. => (mm Hg) T 35 50 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 IK 160 165 170 175 180 185 190 195 200 205 210 215 220 225 230 235 240 245 250 255 260 2^ 270 275 280 285 290 295 300 305 310 315 320 325 330 335 340 345 350
0 109.0 114.9 120.8 126.7 132.6 138.5 144.3 150.2 156.0 161.9 167.7 173.6 179.4 185.2 191.0 196.8 202.6 208.3 214.1 219.9 225.6 231.4 237.1 242.8 248.5 254.3 260.0 2K.7 271.3 277.0 282.7 288.4 294.0 299.7 305.3 310.9 316.5 322.2 327.8 333.4 338.9 344.5 350.1 355.7 361.2 366.8 372.3 377.8 383.4 388.9 394.4 399.9 405.4 410.9 416.4 421.8 427.3 432.7 438.2 443.6 449.1 454.5 459.9 465.3
0.5 109.6 115.5 121.4 127.3 133.2 139.0 144.9 150.8 156.6 162.5 168.3 174.1 180.0 185.8 191.6 197.4 203.1 208.9 214.7 220.4 226.2 231.9 237.7 243.4 249.1 254.8 260.5 266.2 271.9 277.6 283.3 288.9 294.6 300.2 305.9 311.5 317.1 322.7 328.3 333.9 339.5 345.1 350.7 356.2 361.8 367.3 372.9 378.4 383.9 389.4 394.9 400.4 405.9 411.4 416.9 422.4 427.8 433.3 438.7 444.2 449.6 455.0 460.4 4^.8
1 110.2 116.1 122.0 127.9 133.7 139.6 145.5 151.4 157.2 163.1 168:9 174.7 180.5 186.3 192.1 197.9 203.7 209.5 215.3 221.0 226.8 232.5 238.2 244.0 249.7 255.4 261.1 266.8 272.5 278.2 283.8 289.5 295.1 300.8 306.4 312.0 317.7 323.3 328.9 334.5 340.1 345.6 351.2 356.8 362.3 367.9 373.4 378.9 384.5 390.0 395.5 401.0 406.5 412.0 417.4 422.9 428.4 433.8 439.3 444.7 450.1 «5.6 461.0 466.4
1.5 110.7 116.7 122.6 128.5 134.3 140.2 146.1 151.9 157.8 163.6 169.5 175.3 181.1 186.9 192.7 198.5 204.3 210.1 215.8 221.6 227.3 233.1 238.8 244.5 250.3 256.0 261.7 267.4 273.0 278.7 284.4 290.0 295.7 301.3 307.0 312.6 318.2 323.8 329.4 335.0 340.6 346.2 351.8 357.3 362.9 368.4 374.0 379.5 385.0 390.5 396.0 401.5 407.0 412.5 418.0 423.5 428.9 434.4 439.8 445.3 450.7 A56A 461.5 466.9
2 111.3 117.2 123.1 129.0 134.9 140.8 146.7 152.5 158.4 164.2 1701 175.9 181.7 187.5 193.3 199.1 204.9 210.7 216.4 222.2 227.9 233.7 239.4 245.1 250.8 256.5 262.2 267.9 273.6 279.3 285.0 290.6 296.3 301.9^ 307.5 313.2 318.8 324.4 330.0 335.6 341.2 346.8 352.3 357.9 363.4 369.0 374.5 380.0 385.6 391.1 396.6 402.1 407.6 413.1 418.5 424.0 429.5 434.9 440.4 445.8 451.2 456.7 462.1 467.5
2.5 111.9 117.8 123.7 129.6 135.5 141.4 147.3 153.1 159.0 164.8 170.6 176.5 182.3 188.1 193.9 199.7 205.5 211.2 217.0 222.7 228.5 234.2 240.0 245.7 251.4 257.1 262.8 268.5 274.2 279.9 285.5 291.2 296.8 302.5 308.1 313.7 319.3 325.0 330.6 336.1 341.7 347.3 352.9 358.4 364.0 369.5 375.1 380.6 386.1 391.6 397.1 4(^.6 408.1 413.6 419.1 424.6 430.0 435.5 440.9 446.3 451.8 457.2 482.6 468.0
3 112.5 118.4 124.3 130.2 136.1 142,0 147.8 153,7 159,5 165,4 171,2 177.0 182.9 188,7 194,5 200,3 206,0 211.8 217.6 223.3 229.1 234.8 240.5 246.3 252.0 257.7 263.4 269.1 274.8 280.4 286.1 291.7 297.4 303.0 308.7 314.3 319.9 325.5 331.1 336.7 342.3 347.9 353.4 359.0 364.5 370.1 375.6 381.2 386.7 392.2 397.7 403.2 408.7 414.2 419.6 425.1 430.6 436.0 441.5 446.9 452.3 457.7 463.1 468.6
3.5 113.1 119.0 124.9 130.8 136.7 142.6 148.4 154.3 160.1 166.0 17f.8 177.6 183.4 189.2 195.0 200.8 206.6 212.4 218.1 223.9 229.6 235.4 241.1 246,8 252.5 258.3 263.d 269.6 275.3 281.0 286.7 292.3 298.0 303.6 309.2 314.9 320.5 326.1 331.7 337.3 342,8 348,4 354,0 359,5 365.1 370.6 376.2 381.7 387.2 392.7 398.2 403.7 409.2 414.7 420.2 425.6 431.1 436.6 442.0 447.4 452.9 458.3 463.7 469.1
4 113.7 119.6 125.5 131.4 137.3 143.1 149.0 154.9 160.7 166.6 172.4 178.2 184.0 189.8 195.6 201.4 207.2 213.0 218.7 224.5 230.2 236.0 241.7 247.4 253.1 258.8 264.5 270.2 275.9 281.6 287.2 292.9 298.5 304.2 309.8 315.4 321.0 326.6 332.2 337.8 343.4 349.0 354.5 360.1 3K.7 371.2 376.7 382.3 387.8 393.3 398.8 404.3 409.8 415.3 420.7 426.2 431.6 437.1 442.5 448.0 453.4 458.8 464.2 4^.6
4.5 114.3 1202 126.1 132.0 137.9 143.7 149.6 155.5 161.3 167.1 173.0 178.8 184.6 190.4 196.2 202.0 207.8 213.5 219.3 225.0 230.8 236.5 242.3 248.0 253.7 259.4 265.1 270.8 276.5 282.1 287.8 293.4 299.1 304.7 310.4 316.0 321.6 327.2 332.8 338.4 344.0 349.5 355.1 360.7 366.2 371.8 377.3 382.8 388.3 393.8 399.3 404.8 410.3 415.8 421.3 426,7 432,2 437,6 443,1 448,5 453,9 459,4 464.8 470,2
CHAPTER 25: VOLATILITY TABLE 20—Precision of manual D 1160. Repeatability Pressure
689
Reproducibility
0.13kPa(l mmHg)
1.3 kPa (10 mmHg)
0.13 kPa(l mmHg)
1.3 kPa (10 mmHg)
17 3.3
15 7.1
56 31
49 27
IBP FBP Volume Recovered
5-50%
60-90%
5-50%
60-90%
5-50%
60-90%.
5-50%
60-90%
C/V% 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5 14.0 14.5 15.0
2.4 2.9 3.2 3.4 3.6 3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.8 4.9 5.0 5.0 5.1 5.1 5.2 5.2 5.3 5.3 5.4 5.4 5.5 5.5
2.5 3.0 3.3 3.5 3.7 3.9 4.0 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.8 4.9 5.0 5.1 5.1 5.2 5.2 5.3 5.4 5.4 5.5 5.5 5.6 5.6 5.7 5.7
1.9 2.4 2.8 3.1 3.3 3.6 3.8 3.9 4.1 4.3 4.4 4.5 4.7 4.8 4.9 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 6.0 6.0 6.1 6.2 6.3
2.0 2.5 2.9 3.2 3.5 3.7 3.9 4.1 4.3 4.4 4.6 4.7 4.8 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 6.0 6.1 6.2 5.3 6.3 6.4 6.5
6.5 10 13 16 16 21 23 25 27 29 30 32 34 35 37 38 40 41 43 44 46 47 48 50 51 52 54 55 56 57
3.9 6.0 7.6 9.4 11 12 13 15 16 17 18 19 20 23 22 23 24 25 25 26 27 28 29 30 30 31 32 33 33 34
7.0 9.3 11 12 14 15 16 16 17 18 19 19 20 21 21 22 22 23 23 24 24 25 25 26 26 27 27 27 28 28
5.4 7.2 8.5 9.8 11 11 12 13 13 14 15 15 16 16 16 17 17 18 18 19 19 19 20 20 20 21 21 21 22 22
TABLE 21—Precision of automatic D 1160 distillation in °C [7]. 1 mm Hg Pressure Boiling Point IBP 5% 10% 20% 30% 40% 50% 60% 70% 80% 90% FBP
Repeatability 15.9 8.0 5.5 3.8 3.8 5.2 4.7 6.5 4.4 7.3 6.5 9.5
10 mm Hg Pressure
Reproducibility 29.9 17.5 16 12.2 12 9.6 8.8 9.5 11.1 13.1 24.8 37.4
Repeatability
Reproducibility 35.4 11.1 8.0 10.4 7.1 8.6 10.2 12.8 11.3 14.1 25.5 19.7
16.7 6.8 3.7 4.3 3.2 2.9 3.1 3.7 3.4 3.8 5.0 6.4
TABLE 22—Cross reference of international distillation standards relative to ASTM distillation test methods. ASTM Designation U.S. D86 D 1160 D2892 D5236
Distillation Pressure Atmospheric Vacuum Vacuum Vacuum
ISO
IP
BS
AFNOR
DIN
Europe 3405 6616 8708
U.K. 123
U.K. 7392
France M07-002
Germany 51 751 51 356 51567 51 567
FTM
JIS
791-1001
Japan K2254 K2258
690
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
flammable liquids as those having flash points below 37.8°C and combustible liquids as those having flash points between 37.8°C and 93.3°C using specified ASTM flash point test methods. In 1990, the U.S. decided to align its definition of flammable and combustible material with the United Nation's definition, and now defines [23] flammable liquid as having a flash point below 60.5°C and a combustible liquid with flash point between 60.5°C and 93.3°C. Gasoline and aviation fuels are obviously flammable liquids. Flash point specifications have been established for aviation turbine fuels, kerosines, diesel fuels, fuel oils, hydrocarbon solvents, lubricants, and other petroleum products. There are a number of ASTM flash point test methods: D 56, Flash Point by Tag Closed Tester [24]; D 92, Flash and Fire Points by Cleveland Open Cup [25]; D 93, Flash Point by Pensky-Martens Closed Cup Tester [26]; D 1310, Flash Point and Fire Points of Liquids by Tag Open Cup Apparatus [27]; D 3278, Flash Point by Setaflash Closed Cup Apparatus [28]; D 3828, Flash Point by Small Scale Closed Tester [29]; and D 3941, Flash Point by the Equilibrium Method With a Closed Cup Apparatus [30]. A new flash point method, D 6450, Flash Point by Continuously Closed Cup (CCCFP) Tester [31] that was recently approved as an ASTM standard is included in this work. Table 23 gives the comparison of the different ASTM test methods of determining the flash point of a material. These flash point test methods all require a given specimen size, a prescribed rate of heating, temperature measuring device, introduction of a heating source at specific stage during the test, some mechanism of detecting the flash point, and barometric pressure correction. In manual flash point equipment, the ignition source is a gas test flame, and the mechanism of detecting the flash point is the visual observation of a flame that instantaneously propagates itself over the entire surface of the fluid. When the ignition source is a gas flame, the application of the test flame may cause a blue halo or an enlarged flame prior to the actual flash point. Such a phenomenon is not a flash and should be ignored. In the automated equipment, the specimen size and equipment dimensions are the same as those of the manual apparatus. However, the rate of heating, temperature measurements, and detection of the flash point are done automatically according to the meinual procedure requirements. Resistance
temperature probes or thermocouples are used to measure the temperature of the specimen during the test, and changes in the ionization current, thermal conductivity, or pressure are used to detect the occurrence of the flash point. Table 24 gives a summary of the applicable scope, temperature range, repeatability, and reproducibility for these various flash point methods. All reported flash point values are corrected for the ambient barometric pressure at the time of the test. The observed flash point is corrected for barometric pressure by using the equations [32]: Corrected flash point = C + 0.25 (101.3 - p)
(7)
= C + 0.033 (760 - P)
(8)
where: p = ambient barometric pressure in kPa P - ambient barometric pressure in mm Hg. Historically, the proper operation of flash point testers was verified by determining the flash point of 1,4-dimethyl benzene (p-xylene). However, due to its toxicity and its relatively low flash point (27°C), eiltemative flash point verification fluids were studied. The results of an interlaboratory study conducted in 1993 by the ASTM S-15 Coordinating Committee on Flash Point in cooperation with the NationsJ Institute of Standards and Technology led to the establishment of consensus reference flash point values for n-decane, n-undecane, n-tetradecane, and n-hexadecane to be used as flash point verification fluids for the different flash point test methods. Table 25 summarizes the method specific flash points of these reference standards [33,34]. The study also attempted to determine the relative bias of the different flash point test method as shown in Table 26 [33]. However, the report of the study stressed that the observed relative bias among the different flash point test methods are only applicable to the pure hydrocctrbon liquids used in the study, and may not be applicable to mixtures. The observed D 56/D 93 of 0.97 for n-decane and 0.98 for n-undecane was not unexpected since the rate of heating for D 56 is much slower than D 93 thus allowing thermal equilibrium between the bulk of the specimen and the vapors above it. Open cup flash point methods like D 92 are expected to give higher flash points than closed cup flash point methods. This is because sufficient volatile vapor
TABLE 23—Comparison of the test parameters of different ASTMflashpoint test methods. ASTM Designation D56 D92 D 93 Free. A
Cleveland Pensky-Martens
Open Closed
70 mL 75 mL
slice
Stirring Rate N/A N/A N/A 90-120 r p m
D 93 Proc. B
Pensky-Martens
Closed
75 mL
>110X
250 r p m
D 1310 D3278 D3828 D3941
Tag Setaflash Small Scale Equilibrium Method Continuously Closed Cup
Open Closed Closed Closed Closed Closed
50 mL 2 mL 2mL 50 mL 75 m L ImL
All All All All All All
D6450
Apparatus Tag
Cup Type Closed
Sample Size
Expected Flash Point
50 mL
<60°C
aeox N/A
ranges ranges ranges ranges ranges ranges
N/A N/A N/A N/A 90-120 r p m
Heating Rate rC/min 3°C/mln 14-17°C/min 5-6°C/min l-1.5°C/min rC/min N/A N/A N/A N/A 5.5 ± 0.5° C/min
Flash Pt. (FP) Test Initiation 5°C below FP 5°C below FF 28°C below FF 23 ± 5 ^ below FF 23 ± 5 ^ below FF 10°C below FF Target FF Target FF Target FP Target FF 18°C below FF
Repeat FP Test Every
Report to Nearest
0 . 5 X rise 1.0°Crise 1.0°C rise 1.0°C rise
0.5''C 0.5°C 1.0°C 0.5°C
2.0°C rise
0.5°C
1.0°C rise N/A N/A N/A N/A l.O-Crise
0.5^ 0.5°C 0.5°C 0.5°C 0.5°C 0.5-0
CHAPTER 25: VOLATILITY
691
TABLE 24—Comparison of the scope and precision of the different ASTM flash point test methods. ASTM Designation D56
Apparatus Tag
Scope
D92 D 93 Proc. A
Cleveland Pensky-Martens
D 93 Proc. B
Pensky-Martens
D1310
Tag
D3278
Setaflash
D3828
Small Scale
D3941
Equilibrium method
Liquids with specimen and vapor temperature approximately in equilibrium
D6450
Continuously Closed Cup
Fuel oils, lubricating oils, other solvents, and liquids
Liquids, viscosity <5.5 mm^/s at 40°C or 9.5 mm^/s at 25°C. Petroleum products Distillate fuels (diesel, kerosene, heating oil, turbine fuels, new lubricating oils, and other homogeneous petroleum liquids not covered by Proc. B
Temperature Range Below 93°C
79-400''C 40-360°C
Repeatability (95% Confidence)
Expected Flash Point
Reproducibility (95% Confidence)
<60°C >60-C
1.2''C 1.6°C
4.3°C 5.8°C
All ranges
8°C 0.035 X °C X = reported result in°C
18°C 0.078 X °C X = reported result in °C
Residual fuel oils. cutback residua. used lube oils. mixtures of petroleum liquids with solids, liquids that form surface films, viscous liquids. Liquids
40-360''C
Residual fuels Others
2°C 5°C
6°C 10°C
- 1 8 to 165°C 0-110°C
2°C 5°C 1.7°C 3.3''C
4°C 7°C
Paints, enamels, lacquers, varnishes. and related products with viscosity, 150 St at 25°C Petroleum products
-18to93°C 93 to 165°C Solvents Resins/paints
20-70°C above TCC
O.S'C 0.022 M"-^ °C M = mean of two results
0.03(M + 29) "C 0.083 M"^ °C M = mean of two results
0-1 l O X
All ranges
2°C
3°C
10-250°C
All ranges
1.9°C
3.rc
Ambient to 300°C
3.3X 5.0°C
TABLE 25—Flash point verification fluids. Reference Material
Test Method
Reference Value °C
Expanded Uncertainty (°C)
# of Independent Observations
17 ±0.8 ± 1.0 21 ± 1.2 6 ±0.7 17 RM8518 n-Undecane 14 ±2.5 ± 1.4 21 6 ± 1.6 RM8519 ±2.6 13 n-Tetradecane 17 ±2.7 RM 8520 ±2.4 13 n- Hexadecane 16 ±2.8 Uncertainty: The uncertainty of each value in this Report is the numerical value of an expanded uncertainty V = kuc, with U determined from a combined standard uncertainty Uc, and a coverage factor k equal to a t-factor from the ^distribution with degrees of freedom equal to the number of independent observations minus 1. This expanded uncertainty defines a range of values for the certified value within which the true value is believed to lie, at a confidence level of 95%. RM8517 n-Decane
D56 D93 D 3278/D 3828 D56 D92 D93 D 3278/D 3828 D92 D93 D92 D93
50.9 52.8 49.7 67.1 73.2 68.7 65.9 115.5 109.3 138.8 133.9
692
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
TABLE 26—Relative bias among various flash point test methods. Verification Fluid
D 56/ D 93
n-Decane n-Undecane n-Tetradecane n-Hexadecane
0.964 0.977
1.065 1.057 1.036
Flome Size Bead
D 3278/D 93
D 3828/D 93
0.941 0.982
the oldest D-2 standard test method in publication. The latest edition is D 56-OOa. The term "Tag" in this standard is a contraction of Tagliabue, who invented the original small closed tester for flash point determination in 1861 [35]. The Tag closed tester is a modification of the Tagliabue closed tester and was first tried in 1916. The modified closed cup tester was the subject of full comparison series of tests conducted by ASTM in 1917. Figure 4 shows a schematic diagram of a manual Tag Closed Cup tester [24].
Both ThermomeJer
y \ Thermometer
\
D 92/D 93
Flome Tip Oil Chomber
ASTM D 92, Standard Test Method Points by Cleveland Open Cup
for Flash and Fire
This test method is under the jurisdiction of ASTM Committee D-02 and is the direct responsibility of Subcommittee D2.08 on Volatility. It was originally published in 1921. The latest edition is D 92-98b.This open cup tester has been used in the United States since 1908. In early models, testing was conducted by passing a small test flame across the top of the cup by hand. Later on, this was converted to the use of a swinging arm, which controlled the movement of the test flame at a defined height above the top of the cup. Figure 5 [25] shows the schematic diagram of a manual Cleveland Open Cup Apparatus. In addition to determining the flash point of a liquid material, D 92 EJSO determines the fire point. Fire point is defined as the lowest temperature corrected to a barometric pressure of 101.3 kPa (760 m m Hg) at which application of an ignition source causes the vapors of the test specimen to ignite and sustain burning for a m i n i m u m of 5 s under specified conditions of test. The fire point determination is an extension of the flash point test. After the flash point is determined, heating of the specimen is continued at a rate of 5-6°C/min. The test flame is applied at 2°C intervals until the test specimen ignites and sustains burning for a minimum of 5 s. Just like flash point, the fire point is also corrected for barometric pressure using the same equation utilized for flash point. ASTM D 93, Standard Test Method for Flash Points Pensky-Martens Closed Cup Tester FIG. 4—D 56 tag closed cup tester (manual).
concentration above the liquid will tcike longer in an open test cup relative to a closed test cup. D 92 flash point results were indeed higher than D 93. D 3278 and D 3828, which also allow thermeJ equilibrium between specimen and vapor temperature, gave lower flash points when compared to D 93 for n-decane a n d n-undecane. ASTM D56, Standard Tag Closed Tester
Test Method
for Flash Point
by
This test method is under the jurisdiction of ASTM Committee D-02 and is the direct responsibility of Subcommittee D2.08 on Volatility. It was originally published in 1918 and is
by
This test method is under the jurisdiction of ASTM Committee D-02 and is the direct responsibility of Subcommittee D2.08 on Volatility. It was originally published in 1921. The latest edition is D 93-00. It was developed in Germany in 1870 by Meirtens, and was based on the original tester by Pensky. Its development in 1870 was for flash points well above 100°C to test lubricating oils and other similar materieds such as bitumen products. A schematic diagram of a manual PenskyMartens Closed Cup Flash Tester is given in Fig. 6 [26]. Test Method D 93 Procedure A covers the determination of flash points of relatively non-viscous, homogeneous materials. Test Method D 93 Procedure B, on the other hand, is meant to measure viscous, nonhomogeneous materials like residual oils, materials that tend to form surface film when heated, and liquids that contain suspended solids. A 1994 in-
CHAPTER 25: VOLATILITY
693
THCRMOMETEII A8TM NO. l i e IP > • C
TEST FLAMC APPLICATOR
TEST CUP HEATIN* PLATE
A
ORIFICE
MCTAL aCAO
TO «AS SUPPLY HEATER PLANE TVPE OR ELECTRIC RESISTANCE TVPE inches
millimetres A—Diameter B—Radius C—Diameter D E F—Diameter
min
max
min
max
3.2 152 1.6
4.8 nominal nominal 2 7 nominal
0.126 6 0.063
0.189 nominal nominal 0.078 0.276 nominal
6 0.8
0.236 0.031
FIG. 5—D 92 Cleveland open cup apparatus (manual).
terlaboratory study [36,37] indicated that samples with kinematic viscosity greater than 13 mm^ls at 40°C gave a lower observed flash point when tested by Procedure A compcired to the flash point determined by Procedure B. A possible explanation is a lag in the temperature detected by the temperature measuring device as the kinematic viscosity of the sample increases, especially when equilibration of the vapor and the bulk of the sample is not established before the flash point is detected. Therefore, viscosity effects must be taken into account when deciding on the appropriate procedure for D 93 flash point determination. ASTM D 1310, Standard Test Method for Flash Point and Fire Points of Liquids by Tag Open Cup Apparatus This test method is under the jurisdiction of ASTM Committee D-01 on Paints and Related Coatings, Materials, and Applications, and is the direct responsibility of Subcommittee D1.22 on Health and Safety. It was originally published in 1952. The latest edition is D 1310-01. The modem Tag open cup tester was derived from the Tagliabue Open cup Tester patented in 1862. ASTM D 3278, Standard Test Method for Flash Point by Setaflash Closed Cup Apparatus This test method is under the jurisdiction of ASTM Committee D-1 and is the direct responsibility of Subcommittee D1.22. It was originally published in 1973. The latest edition is D 3278-96. The original closed cup miniflash point tester was invented by T. Kidd in the early 1960s. It was commer-
cialized by Stanhope-Seta Ltd. during 1967 to 1969, and became known as the Setaflash flash point tester. The Setaflash flash test method differs from the other flash point methods because the temperature equilibrium between the specimen and the vapor above it is allowed to be established prior to testing for a flash point. A fresh specimen is used for each test temperature, unlike D 56, D92, and D 93, where multiple flash point testing is done on a given specimen at various test temperatures. A schematic diagram of the Setaflash apparatus is shown in Fig. 7 [28]. ASTM D 3828, Standard Test Method for Flash Point by Small Scale Closed Cup Tester This test method is under the jurisdiction of ASTM Committee D-02 and is the direct responsibility of Subcommittee D 2.08. It was originally published in 1979. The latest edition is D 3828-98. The apparatus used in this test method is the same as that described in D 3278. Just like the Setaflash test method, this method allows temperature equilibrium between the specimen and the vapor above it to be established prior to testing for a flash point. This test method is applicable to petroleum products not covered within the scope of D 3278. ASTM D 3941, Standard Test Method for Flash Point by the Equilibrium Method with a Closed Cup Apparatus This test method is under the jurisdiction of ASTM Committee D-01 and is the direct responsibility of Subcommittee D 1.22. It was originEdly published in 1980. The latest
694
MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
SHUTTER
HANDLE (OPTIONAL) (MUST NOT TIP EMPTY CUP)
STIRRER DRIVE FLEXIBLE SHAFT (PULLEY DRIVE OPTIONAL) FLAME EXPOSURE DEVICE
SHUTTER OPERATING KNOB
THERMOMETER
DISTANCE PIECE
D TEST CUP
mm n*
F MIN THICKNESS OVER CUP AREA lE.METAL SURROUNDING THE CUP
A B C 0
e F
max
in. min
max
4.37 5.16 0.172 0.203 41,94 42.06 1.651 1.6S6 1.58 3.18 0.062 0.125 9.52 0.375 Sr.23 57.86 2.253 2.278 6.35 0.25
HEATER FLAME-TYPE OR ELECTRIC RESISTANCE TYPE (FLAME TYPE SHOWN)
NOTE 1—Lid assembly can be positioned either right or left-handed.
FIG. 6—D 93 Pensky-Martens closed cup apparatus (manual).
edition is D 3941-96. The apparatus used in this test method is the same as that described in D 56 and D 93. However, similar to the Setaflash or Small Scale test method, this method allows temperature equilibrium between the specimen and the vapor above to be established prior to testing for a flash point. ASTM D 6450, New Standard Test Method for Flash Point by the Continuously Closed Cup (CCCFP) Tester This test method is under the jurisdiction of ASTM Committee D-02 and is the direct responsibility of Subcommittee D 2.08. The first pubUshed edition will be the 1999 edition. The apparatus is continuously closed during the test but a predetermined amount of air is injected into the test chamber after each introduction of the ignition source. This test method is different than the other flash point methods because a preset change in the pressure inside the test chamber is used to detect when the flash point has been reached. In addition, an electric arc is used as the source of ignition when testing for a flash point occurrence. This flash point method gives simi-
lar results to those obtained by D 93. However, the results are not equivalent [38]. Other International Flash Point Test Methods All the aforementioned flash point test methods Eire ASTM standard test methods. Although ASTM standard test methods are used extensively throughout the world, there are other international flash point standards that are used and are applicable to specific geographical jireas. Table 27 gives a cross reference for a number of international flash point standards and their corresponding ASTM designations. There are flash point test methods used mainly in Europe, which do not have an equivalent ASTM standard. The Abel flash point tester is a closed cup tester developed originally in 1876. It is currently used in parts of the United Kingdom. It is similar to the D 56 Tag closed-cup tester but with provision for stirring the specimen. The Abel-Pensky closed cup tester was developed in Germany in 1879 and adopted as the German standard for testing petroleum product in 1882. It is still used widely use in Europe.
A—Hinge B—Ud C—Pilot flame jet D—Test flame jet E—Filing orifice F—Test flane gas control screw G—Shutter guide H—Shutter knob J—Shutter K—Udlock L—Ud sealing 0-ring M—Thermometer N—Sample cup P—^Thermometer well R—Test flame gage
BORE OF TEST JET 12.65 12.60
30.45
7.65 7.60
30/W
m^ 12.47 12.42
5.10 5.05
Li|:W
SHUTTER (1.22 THICK NOM.) 48.42
BORE OF FILLtNO ORIFICE 1.60/1.85 DIA.
46.37 4.00
10.00
7.65 7.60
IT^iZZZ^^ 16.00
T
10.18 IO.I3
p 7.00 OIA. NOM.
42.00 41.00
49.70 49.40
5.10 TYP. 5.05
12.47 12.42
SPECIMEN CUP AND LID
LID
( 2 . 0 0 THICK NOM.)
NOTE 1—All dimensions are in millimetres.
FIG. 7—D 3278 and D 3828 small scale (Setaflash) apparatus.
TABLE 27—Cross reference of international flash point methods versus ASTM flash point test methods. ASTM
u.s D56 D92 D93 D 1310 D3278 D3828 D3941 D6450
ISO
IP
DIN
AFNOR
Europe
U.K. 304 36 34
Germany
France
303 303
55 680 55 680
2592 2719 3679 3679 1523
170 (Abel) 304
51 376 51 758
51 755 (Abel-Pensky)
FTM
EN
SIS
791-1101 791-1103 M07-019
K2265-1996 K2265-1996 K2265-1996
456
M07-036
ns Japan K2265-1996 K2265-1996 K2265-1996
Europe
150 223
696
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
Crude Distillation Crude oil is a complex mixture of mostly organic molecules with minute quantities of inorganic materials. The organic molecules are mostly hydrocarbons, with minor sulphur, nitrogen, and oxygen components, and trace quantities of metals such as vanadium and nickel. In the assay or preliminary characterization of crude oil, atmospheric and reduce pressure distillations are performed in order to determine the yields of various boiling ranges. Two test methods are used in the distillation of crude oils, ASTM D 2892, Standard Test Method for Distillation of Crude Petroleum (15-Theroetical Plate Column) [39], and ASTM D 5236, Standard Test Method for Distillation of Heavy Hydrocarbon Mixtures (Vacuum Potstill Method) [40]. Crude oil distillation is one of a number of tests conducted on a crude oil to determine its market value. The fractions produced during crude oil distillation can be used alone or in combination with other fractions to produce samples for analytical studies, engineering, and product quEility evaluations. The precision values for these two methods are summarized in Table 28. ASTM D 2892, Standard Test Method for Distillation of Crude Petroleum (15-Theoretical Plate Column) This test method is under the jurisdiction of ASTM Committee D-02 and is the direct responsibility of Subcommittee D02.08. It was originally published in 1970. The latest edition is D 2892-99a. It describes the procedure for the distillation of stabilized crude petroleum to a final cut temperature of 400°C AET (Atmospheric Equivalent Temperature). AET is the atmospheric equivalent temperature converted from the observed vapor temperatures and was previously discussed in the ASTM D1160 section. Equations 4, 5, and 6, as well as Tables 14-19, are applicable for D 2892 also. This test method employs a fractionating column having an efficiency of 14-18 theoretical plates operated at a reflux ratio of 5:1., thus corresponding to the standard laboratory distillation efficiency referred to as 15/5. Figure 8 [37] shows the schematic diagram of the apparatus used for this test method. Distillation yields by mass or volume are calculated from the data and reported to the nearest 0.1%. Vapor temperatures are reported to the neeirest 0.5°C. TypiceJ D2892 distillation results for two different crude oils are given in Tables 29 [41] and 30 [42].
ASTM D 5236, Standard Test Method for Distillation of Heavy Hydrocarbon Mixtures (Vacuum Potstill Method) This test method is under the jurisdiction of ASTM Committee D-02 and is the direct responsibility of Subcommittee D02.08. It was originally published in 1992. The latest edition is D 5236-99. It describes the procedure for the distillation of
CONOENSOR
SOI.ENOID
li
COLUMN.WIT MANTLE
VACUUM CONNECTION (WHEN USED) A P SENSOR PRODUCT : COOLER
RECEIVER TEMPERATURE PROBE
w
stTirri--'
I-
N 2 BUBBLER
DISTILLING FLASK WITH MANTL'ES
STIRRING MOTOR
FIG. 8—D 2892 test apparatus.
TABLE 28—Precision values for crude distillation methods. ASTM Designation
iWethod
Procedure or Cuts
Repeatability
D2892
15/5 Distillation
Under determination Under determination
D5236
Vacuum Potstill
Atmospheric Pressure Reduced Pressure 10 vol % 20 vol % 30 vol % 40 vol % 50 vol % 60 vol % 70 vol % 80 vol % 90 vol %
6.rc 6.rc 4.9°C 4.5°C
5.7°C
4.rc 4.8°C 4.9°C 4.4°C
Reproducibility 1.2 mass % 1.4 mass % 16.9°C 12.8°C 13.5X 11.2''C 14.2°C 8.4°C 11.4°C
5.rc
4.4''C
1.2 vol % 1.5 vol %
CHAPTER 25: VOLATILITY
697
TABLE 29—D 2892 crude distillation results for a new grade oil. Boiling Range (AET, °C)
Weight in Grams
Init. to 20 21-50 93 163 200 250 280 316 330 343 343 + Recov Loss
88 91 212 452 199 260 188 278 208 90 1266 3332 6
Charge Density == 0.8406 API = 36.7
Wt%
Volume (mL)
2.6 2.7 6.4 13.6 6.0 7.8 5.6 8.3 6.2 2.7 38.0
146.6 144.1 300.9 593.3 247.5 312.6 221.3 323.0 237.0 102.0 1358.4 3987
No Loss Charge
LV % Crude
Cum LV%
Mid LV%
API Gravity
Density at 15°C
3.7 3.6 7.5 14.9 6.2 7.8 5.5 8.1 6.0 2.6 34.1
3.7 3.6 7.5 14.9 6.2 7.8 5.5 8.1 6.0 2.6 34.1
3.7 7.3 14.8 29.7 35.9 43.8 49.3 57.4 63.4 65.9 100.0
1.8 5.5 11.1 22.3 32.8 39.8 46.5 53.4 60.4 64.7 82.0
104.3 92.5 69.3 54.2 44.4 38.5 34.9 32.8 29.8 28.8 20.2 37.7
0.6001 0.6315 0.7046 0.7618 0.8039 0.8317 0.8479 0.8608 0.8765 0.8820 0.9320 0.8357
0.2 Charge Weight = 3345 Weight of Water = 7
T A B L E 3 0 — D 2 8 9 2 C r u d e d i s t i l l a t i o n r e s u l t s for l i g h t s o u r b l e n d oil Boiling Range (AET, ° C)
Weight in Grams
Init. to 20 21-50 100 125 150 175 205 220 235 265 295 319 343 343+ Recov Loss
530 1709 2063 1077 1559 1289 1896 815 945 1940 1948 1870 1538 15105 34333 68
Charge Density = 0.8508 API = 34.7
Wt%
Volume (mL)
1.5 5.0 6.0 3.1 4.5 3.8 5.5 2.4 2.9 5.7 5.7 5.4 4.5 44.0
937.1 2605.2 2812.9 1443.3 2035.2 1647.7 2383.1 1009.0 1216.8 2330.6 2286.9 2157.1 1753.3 15921.8 40540.1
No Loss Charge
LV % Crude
Cum LV%
Mid LV%
API Gravity
Density at 15°C
2.3 6.4 6.9 3.6 5.0 4.1 5.9 2.5 3.0 5.7 5.6 5.3 4.3 39.3
2.3 6.4 6.9 3.6 5.0 4.1 5.9 2.5 3.0 5.7 5.6 5.3 4.3 39.3
2.3 8.7 15.7 19.2 24.3 28.3 34.2 36.7 39.7 45.4 51.1 56.4 60.7 100.0
1.2 5.5 12.2 17.5 21.7 26.3 31.3 35.4 38.2 42.6 48.3 53.7 58.6 80.4
118.7 84.2 61.4 58.1 53.1 49.3 46.3 43.6 41.6 38.4 34.5 31.6 29.7 17.6 35.4
0.5656 0.6560 0.7334 0.7462 0.7660 0.7823 0.7956 0.8077 0.8169 0.8324 0.8518 0.8669 0.8772 0.9487 0.8469
0.2
Charge Weight = 33448 Weight of Water = 47
heavy hydrocarbon mixtures having an initial boihng point greater than 150°C such as heavy crude oil, petroleum distillates, residues, and synthetic mixtures. It employs a potstill with a low pressure drop entrainment separator operated under total takeoff conditions. The maximum achievable AET (see discussion on D 1160 under the distillation section) of 565°C is dependent on the heat tolerance of the chcirge. A weighed volume of sample is distilled at reduced pressures between 6.6 and 0.013 kPa. at specified distillation rates. Cuts are taken at preselected temperatures. Records of vapor temperature, operating pressure, and other variables are made at intervals and each cut point. Figure 9 shows the schematic diagram of the apparatus used for this test method [40]. Distillation yields by mass or volume are calculated from the data and reported to the nearest 0.1%. Vapor temperatures are reported to the nearest 0.5°C. T3fpical D 5236 distillation results for two different crude oils are given in Tables 31 [43] and 32 [44]. Other International Test Methods for Crude Distillation All the aforementioned test methods are ASTM standard test methods for crude oil distillation. Although ASTM standard test methods are used extensively throughout the world.
there are other international crude distillation standards that are used and are applicable to specific geographical areas. Table 22 gives a cross reference for a number of international crude distillation standards and their corresponding ASTM designation. Vapor Pressure In addition to distillation characteristics, vapor pressure is a critical volatility parameter of gasoline and other petroleum fuels. Vapor pressure is the force per unit area exerted by the vapors of the liquid contained in a closed container. Vapor pressure affects the performance chsiracteristics of gasoline and other hydrocarbon fuels, especially during cold starting and vapor-lock conditions. Vapor- liquid ratio at specified temperatures is also an important parameter. For gasoline and simileir fuels, the vapor pressure is dependent on the vapor to liquid ratio in the container, and the temperature. The vapor pressure of a fuel measured at 37.8°C (100°F) with a 4:1 vapor to liquid ratio in a designated container, is known as the Reid Vapor Pressure or RVP. This important fuel characteristic is measured by ASTM D 323, Vapor Pressure of Petroleum Products (Reid Method) [45].
698
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
There are other ASTM test methods related to the measurement of vapor pressure. Among these are: ASTM D 4953, Standard Test Method for the Vapor Pressure of GasoHne and Gasohne-Oxygenate Blends (Dry Method) [46]; ASTM D 5190, Standard Test Method for the Vapor Pressure of Petroleum Products (Automatic Method) [47]; ASTM D 5191, Standard Test Method for the Vapor Pressure of Petroleum Products (Mini Method) [48]; and ASTM D 5482, Standard Test Method for the Vapor Pressure of Petroleum Products (Mini MethodTO VACUUM PUKPINO LIME
RE*0 TKAP TO VACUUM GAUGE C080EN8ER—>
VAPOR THERMOCOUPLE
HEAD COHPENSATIMG MANTLE (not ahown)
VACUUM ADAPTER
Atmospheric) [49]. Vapor-liquid ratio measurements can be done by: ASTM D 5188, Standard Test Method for the VaporLiquid Ratio Temperature Determination of Fuels (Evacuated Chamber Method) [50] and ASTM D 2533, Standard Test Method for the Vapor - Liquid Ratio of Spark-Ignition Engine Fuels [51]. Two new vapor pressure test methods have been approved recently: ASTM D 6377, Standard Test Method for Vapor Pressure of Crude Oil: VPCRx (Expansion Method) [52] and ASTM D 6378, Standard Test Method for Determination of Vapor Pressure VPx of Petroleum Products, Hydrocarbons, and Hydrocarbon-Oxygenate Mixtures (Triple Expansion Method) [53]. The vapor pressure of liquefied petroleum gases is determined using ASTM D 1267, Standcird Test Method for the Gage Vapor Pressure of Liquefied Petroleum- (LP) Gases (LP-Gas Method) [54]. Vapor pressure is critically important for both automotive and aviation gasolines. It affects starting, warm-up, and tendency to vapor lock at high operating temperatures or eiltitudes. M a x i m u m vapor p r e s s u r e limits for gasoline are legeJly mandated in some etreas as a measure of air pollution control. Vapor pressure of crude oils is important to the crude producer and refiner for general heindling jind initial refinery treatment. Vapor pressure is also used as an indirect measure of the evaporation rate of volatile petroleum solvents, with higher vapor pressure being associated with a faster rate of evaporation. Table 33 summarizes the salient features of these various vapor pressure and vapor/liquid ratio test methods. The precision values for the different vapor pressure and V/L test methods are shown in Table 34.
FLASK THERMOCOUPLE
ASTM D 323, Standard Test Method for the Vapor Pressure of Petroleum Products (Reid Method)
PLASK MAHTLES
OISTILLATIOH FLASK
MAGHETIC STIRRER
FIG. 9—D 5236 test apparatus.
This test method is under the jurisdiction of ASTM Committee D-02 and is the direct responsibility of Subcommittee D02.08. It was originally published in 1930. The latest edition is D 323-99. This test method covers procedures for the determination of vapor pressure of gasoline, volatile crude oil, and other volatile p e t r o l e u m products. Because the external atmospheric pressure is counteracted by the atmospheric pressure initially present in the vapor chamber, the Reid vapor pressure is an absolute pressure at 37.8°C. The Reid vapor pressure differs from the true vapor pressure of the sample due to some small sample vaporization and the presence of water
TABLE 31—D 5236 crude distillation results for a new grade oil bottoms. Boiling Range (AET, °C)
Weight Grams
Wt%
Volume (mL)
Init. to 375 399 450 482 525 550 565 WETT 565 + Recov Loss
28 261 247 130 144 66 36 5 338 1255 2
2.2 20.8 19.7 10.4 11.5 5.3 2.9 0.4 26.9
31.4 292.2 273.2 141.7 155.5 70.7 38.4 5.3 337.5 1345.9
in
Charge Density = 0.9320 Weight of Water = 0
0.2 Charge Weight = 1253 API = 20.2
No Loss Charge
LV% Crude
Cum LV%
Mid LV%
API Gravity
Density at 15°C
2.3 21.7 20.3 10.5 11.6 5.3 2.9 0.4 25.1
0.8 7.4 6.9 3.6 3.9 1.8 1.0 0.1 8.6
66.7 74.1 81.0 84.6 88.6 90.3 91.3 91.4 100.0
66.3 70.4 77.6 82.8 86.6 89.4 90.8 91.4 95.7
27.0 26.9 24.9 22.6 21.2 20.0 19.3 18.9 9.7 20.1
0.8921 0.8931 0.9041 0.9175 0.9260 0.9336 0.9375 0.9402 1.0015 0.9324
CHAPTER 25: VOLATILITY
699
TABLE 32—D 5236 crude distillation results for a light sour blend bottoms. Boiling Range (AET, °C)
Weight in Grams
Init. to 399 427 454 482 510 538 565 WETT 565 + Recov Loss
363 635 363 355 305 314 245 10 1150 3740 5
Wt%
Volume (mL)
9.7 17.0 9.7 9.5 8.2 8.4 6.6 0.3 30.7
403.3 702.4 397.0 385.6 328.6 335.1 258.1 10.5 1121.8 3942.4
No Loss Charge
LV% Crude
Cum LV%
Mid LV%
API Gravity
Density at 15°C
10.2 17.8 10.1 9.8 8.3 8.5 6.5 0.3 28.5
4.1 7.1 4.0 3.9 3.3 3.4 3.6 0.1 11.4
64.2 71.3 75.3 79.2 82.5 85.9 88.5 88.6 100.0
62.1 67.7 73.3 77.3 80.9 84.2 87.2 88.6 94.3
25.6 24.9 23.2 22.1 20.9 19.4 17.5 17.0 6.4 17.5
0.9000 0.9041 0.9144 0.9207 0.9282 0.9369 0.9492 0.9521 1.0251 0.9486
0.1
Charge Density == 0.9484 API = 17.6
Charge Weight = 3745 Weight of Water = 0
TABLE 33—Comparison of ASTM vapor pressure and vapor/liquid ratio test methods. ASTM Designation
Reid Method
D 323 Proc. B D 323 Proc. C D 323 Proc. D
Reid Method Reid Method Reid Method
D 4953 Proc. A D 4953 Proc. B
Dry Reid Method Dry Reid SemiAutomatic Automatic Method Mini-Method
D5190 D5191 D5482 D6377 D6378
Scope
Test Method
D 323 Proc. A
Mini-Method, Atmospheric Expansion Method Triple Expansion Method
Vapor Pressure Range
Gasoline, volatile crude oil, and other volatile petroleum products rr
Test Temperature
Vapor to Liquid Ratio
All at 37.8°C
4:1
Vapor Pressure
Resuh
<180kPa <180kPa > 8 0 kPa @ 50 kPa
Aviation gasoline Gasoline and gasolineoxygenate blends, other aircontaining volatile petroleum products
35-100 kPa
37.8°C
4:1
Vapor Pressure
//
7-172 kPa
37.8°C
4:1
DVPE*
" "
7-130 kPa 7-llOkPa
37.8°C 37.8^
4:1 4:1
DVPE* DVPE*
5-80°C
4:1 to 0.021:1 4:1
Crude Oils Volatile petroleum products, hydrocarbons, and hydrocEirbonoxygenate mixtures
7-150 kPa
37.8°C
Vapor Pressure Vapor Pressure
D5188
Evacuated Chamber Method
Gasoline and gasoline oxygenate blends.
101.3 kPa
36-80°C
8:1 to 75:1
V/L Temperature
D2533
Evacuated Chamber Method
Gasoline
101.3 kPa
36-80°C
20:1
V/L and Temperature
D 1267
LP-Gas Method
Liquified Petroleum Products
37.8-70°C
Vapor Pressure
*DVPE = dry vapor pressure equivalent (see discussion under individual test method)
vapor £ind air in the confined space. The true vapor pressure of the sample is the pressure exerted only by the molecular species in the sample on the walls of the container. The vapor pressure obtained by this method includes the vapor pressure due to water vapor and air. Figure 10 [45] shows the schematic diagram of the vapor pressure apparatus for Procedure A, C, and D. Figure 11 [45] illustrates the apparatus for Procedure B. The liquid chamber of the vapor pressure apparatus is filled with the chilled sample and connected to the vapor chamber that has been heated to 37.8°C in a bath. The assembled apparatus is immersed in a bath maintained at 37.8°C until a constant pressure is ob-
served. The pressure reading, suitably corrected for any difference between the vapor pressure gage and calibration manometer reading, is reported to the nearest 0.25 kPa (0.05 psi) as the Reid vapor pressure. ASTM D 4953, Standard Test Method for the Vapor Pressure of Gasoline and Gasoline-Oxygenate Blends (Dry Method) This test method is under the jurisdiction of ASTM Committee D-02 and is the direct responsibility of Subcommittee D02.08. It was originally published in 1989. The latest edition is D 4953-99.
700 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK TABLE 34—Precision of various vapor pressure and V/L ratio test methods. ASTM Designation
Reid Method
D4953
Dry Reid Method
D5190
Procedure/Model
Metliod Name
D323
A B A A C D A B B B
(Gasohne) (Gasoline)
Repeatability
Range 35-100 kPa (5-15 psi) 35-100 kPa (5-15 psi) 0-35 kPa (0-5 psi) 110-180 kPa (16-26 psi) 180kPa(>26psi) 50 kPa (7 psi)
Automatic Method Mini-Method
D5191
psi) psi) psi) psi)
2.48 kPa (0.36 psi)
3.45 kPa (0.50 psi)
0.00807(DVPE + B) B = 124 kPa (18.0 psi) 1.31 kPa (0.19 psi) 1.79 kPa (0.26 psi)
0.0161(DVPE + B) B = 124 kPa (18.0 psi) 2.69 kPa (0.39 psi) 4.14 kPa (0.60 psi) To be determined
kPa kPa kPa kPa kPa kPa
3.65 4.00 2.14 3.58
(Gage) (Herzog) (Precision)
Reproducibility 5.2 kPa (0.75 psi) 4.5 kPa (0.66 psi) 2.4 kPa (0.35 psi) 2.8 kPa (0.40 psi) 4.9 kPa (0.70 psi) 1.0 kPa (0.15 psi) 5.52 kPa (0.80 psi) 5.38 kPa (0.78 psi) 2.90 kPa (0.42 psi) 4.27 kPa (0.62 psi)
3.2 1.2 0.7 2.1 2.8 0.7
(0.46 (0.17 (0.10 (0.30 (0.40 (0.10
kPa kPa kPa kPa
psi) psi) psi) psi) psi) psi)
(0.53 (0.58 (0.31 (0.52
Mini-Method, Atmospheric VPCRx, Expansion Method
Herzog SC970 ABB 4100
D6378
VPx, Expansion Method
Petroleum Products
0.50 kPa (0.07 psi)
1.63 kPa (0.22 psi)
D5188
Evacuated Chamber Evacuated Chamber LP-Gas Method
V/L
0.6° C
0.9° C
1.0° C 1.4° C 12kPa(1.8psi)
1.3° C 1.6° C 18kPa(2.8psi)
D5482 D6377
D2533 D 1267
B
Vapor Chambar
Key A B, C, D E F, G H I J
Glycerol Mercury
V/L
'Coupling C
0.015 VPCR4 0.055 VPCRo 1 0.065 VPCRo 02
V/L = 4 and 37.8°C V/L = 0.1 and 37.8° C V/L = 0.02 and 37.8° C
Crude
Coupling
\
MUU)G 6ABE VERSION
F
tiquid Choinbtf (Oni Opening)
DIMENSIONS OF VAPOR PRESSURE APPARATUS Description mm Vapor chamber, length 254 ± 3 Vapor and gasoline chambers, 51 ± 3 Liquid ID Coupling, ID min 4.^ Coupling, OD 12.7 Coupling, ID 12.7 \felve 12.7 Valve 6.35
in. 10 ± Ve 2 ± Vs
FIG. 10—D 323 - Procedure A, C, and D apparatus.
¥16 Vi % % Vt
non na
"CI
—~—or
FIG. 11—D 323 - Procedure B apparatus.
CHAPTER This test method is a modification of D 323, providing two procedures to determine the vapor pressure of gasoline and gasohne-oxygenate blends. Procedure A utilizes the same apparatus and essentially the same procedure as D 323 with the exception that the interior surfaces of the liquid and vapor chambers are maintained completely free of water. Procedure B utilizes a semi-automatic apparatus as shown in Figure 11 [46] with the liquid and vapor chambers identical in volume to those in Procedure A. The apparatus is suspended in a horizontal bath and rotated while attaining equilibrium. Either a Bourdon gage or pressure transducer can be used with this procedure. Just like Procedure A, the interior surfaces of the liquid and vapor chambers are maintained free of water. The vapor pressure determined by this method differs from the true vapor pressure of the sample due to some small sample vaporization and includes the vapor pressure of the air in the confined space. The liquid c h a m b e r of the vapor pressure a p p a r a t u s is filled with a chilled sample and connected to the vapor chamber that has been heated to 37.8°C in a bath. The assembled apparatus is immersed in a bath maintained at 37.8°C until constant pressure is observed. The pressure reading, suitably corrected for any difference between the gage or pressure transducer and manometer reading, is reported to the nearest 0.25 kPa (0.05 psi) as the Dry Reid vapor pressure. Data from a 1991 interlaboratory cooperative program [55] indicated a statistically significant bias between Procedure A and Procedure B. The relative bias between procedures can be corrected by applying the following equation: For Procedure B, Gage:
25: VOLATILITY
701
FLOW SYSTEM
J*-f^*-VENTED
""^ 40-80 RSI
L
PRESSURE-VAC FOR CAUBRATION Z1
13' RING INSAMPLE INLET CONNECTOR
FIG. 12—D 5190 apparatus. DVPE, Procedure A = 1.029 X
(9)
For Procedure B, Transducer (Herzog): DVPE, Procedure A = 0.984 X
(10)
where X = observed total vapor pressure using Procedure B. ASTM D 5190, Standard Test Method for the Vapor Pressure of Petroleum Products (Automatic Method) This test method is u n d e r the jurisdiction of ASTM Committee D-02 and is the direct responsibility of Subcommittee D02.08. It was originally published in 1991. The latest edition is D 5190-99. This test method is suitable for gasoline samples that contain oxygenate. This method is suitable for calculation of a dry vapor pressure equivalent (DVPE), i.e., the vapor pressure corresponding to that obtained by D 4953, using a n equation derived from a 1991 interlaboratory cooperative study [55]. DVPE, D 5190 = (0.954 X ) - ^ A
(11)
where: X = m e a s u r e d total vapor pressure, in units consistent with A, ctnd A = 1.984 kPa (0.281 psi). The chilled sample cup of the automatic vapor pressure instrument (Fig. 12 [47]) is filled with the chilled sample and coupled to the instrument inlet fitting. The sample is automatically forced from the sample chamber to the expansion
chamber where it is held until thermal equilibrium at 37.8°C is reached. The total vapor pressure is measure by a pressure trcinsducer. The measured total vapor pressure is converted by the instrument to the DVPE value using equation 11. The DVPE is reported to the nearest 0.1 kPa (0.01 psi) without reference to the temperature. ASTM D 5191, Standard Test Method for the Vapor Pressure of Petroleum Products (Mini-Method) This test method is under the jurisdiction of ASTM Committee D-02 a n d is the direct responsibility of Subcommittee D02.08. It was originally published in 1991. The latest edition is D 5191-99. This test method is suitable for gasoline and gasoline samples that contain oxygenates. This method is suitable for CEJculation of a dry vapor pressure equivalent (DVPE) using an equation derived from a 1988 interlaboratory cooperative study [56]. DVPE, D 5191 = (0.965 X) - A
(12)
where: X = m e a s u r e d total vapor pressure, in units consistent with A, and A = 3.78 kPa (0.548 psi). A similar correlation equation was developed by the U.S. Environmental Protection Agency (EPA) using its own data. The equation they use to correlate D 5191 results to D 4953,
702
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
Procedure B is: D V P E , D5191 = (0.956 X ) - A
(13)
where: X = measured total vapor pressure, in units consistent with A, and A = 2.39 kPa (0.347 psi). Another correlation equation to correlate D 5191 results to D 323 was developed by the California Air Resources Board (CARB). It is given by the equation: DVPE, D 5191 = (0.972 X ) - A
(14)
where: X = measured total vapor pressure, in units consistent with A, and A = 4.93 kPa (0.715 psi) A known volume of chilled, air-saturated sample is introduced into an evacuated, thermostatically controlled test chamber, the internal volume of which is five times that of the total specimen introduced into the chamber. After injection into the test chamber, the test specimen is allowed to reach thermal equilibrium at 37.8°C. The resulting rise in pressure in the chamber is measured using a pressure transducer sensor and indicator. The total pressure measured is converted to a DVPE using Eq 12. The DVPE is reported to the neeirest 0.1 kPa (0.01 psi) without reference to the temperature. ASTM D 5482, Standard Test Method for the Vapor Pressure of Petroleum Products (Mini-Method-Atmospheric) This test method is under the jurisdiction of ASTM Committee D-02 and is the direct responsibility of Subcommittee D02.08. It was originally published in 1993. The latest edition is D 5482-99. This test method is a modification of D 5191. In this test method, the test chamber is not evacuated but rather at atmospheric pressure at the start of the test. This test method is suitable for gasoline samples that contain oxygenates. This method is suitable for calculation of a dry vapor pressure equivalent (DVPE) using an equation derived from a 1991 interlaboratory cooperative study [55]. DVPE, D 5482) = (0.965 X ) + A
(15)
where: X = measured total vapor pressure, in units consistent with A, and A = 0.538 kPa (0.078 psi) for Herzog Model SC970 A = 1.937 kPa (0.281 psi) for ABB Model 4100 A known volume of chilled, air-saturated sample is introduced into a thermostatically controlled test chamber at atmospheric pressure, the internal volume of which is five times that of the total specimen introduced into the chamber. After injection into the test chamber, the test specimen is allowed to reach thermal equilibrium at 37.8° C. The resulting rise in pressure in the chamber is measured using a pressure transducer sensor and indicator. The total pressure mea-
sured is converted to a DVPE using Eq 15. The DVPE is reported to the nearest 0.1 kPa (0.01 psi) without reference to the temperature. ASTM D 6377, Standard Test Method for the Vapor Pressure of Crude Oil, VPCRx (Expansion Method) This test method is under the jurisdiction of ASTM Committee D-02 and is the direct responsibility of Subcommittee D02.08. It was originally published in 1998. The latest edition is D 6377-98. This test method covers the use of automated vapor pressure instruments to determine the vapor pressure exerted by crude oils at temperatures between 5 and 80°C. Employing a measuring chamber with a built-in piston, a sample of known volume is drawn from a pressurized sampling system (floating piston cylinder) into the temperature controlled chamber at 20°C or higher. After sealing the chamber, the volume is expanded by moving the piston until the final volume produces the desired V/L value. The temperature of the chamber is then regulated to the measuring temperature. After temperature and pressure equilibrium, the measured pressure is recorded as the VPCRx of the sample. The results are reported to the nearest 0.1 kPa with the test temperature and vapor-liquid ratio. For results related to ASTM D 323, the final volume of the measuring chamber shall be five times the test specimen volume and the measuring temperature shall be 37.8°C. The relative bias between the Reid vapor pressure (RVP) obtained by D 323 and the value obtained by this method can be corrected by using the correlation equation: RVPE, D323 = A X VPCR4(37.8-(C) + B
(16)
where: A = 0.752 B = 6.07 kPa (0.88 psi) ASTM D 6378, Standard Test Method for the Vapor Pressure, VPx, of Petroleum Products, Hydrocarbons, and Hydrocarbon-Oxygenate Mixtures (Triple-Expansion Method) This test method is under the jurisdiction of ASTM Committee D-02 and is the direct responsibility of Subcommittee D02.08. It was originally pubhshed in 1998. The latest edition is D 6378-98. This test method is similar to D 6377 with the exception that expansion is done in three steps to a final volume of (x -I-1) times that of the test specimen. After each expansion, the total pressure is determined. The partial pressure of the dissolved air and the solubility of air in the specimen are calculated from the three resulting pressures. The temperature of the chamber is then increased to a specified value and total pressure is determined. The vapor pressure (VPx) is calculated by subtracting the partial pressure of the dissolved air in the liquid, which has been gas-corrected for temperature, from the total pressure. In this test method, air saturation prior to the measurement is not required. Results are reported to the nearest 0.1 kPa with the test temperature and vapor-liquid ratio. The vapor pressure determined by this method at a vapor-liquid ratio of 4:1 of gasoline and gasolineoxygenate blends at 37.8°C can be correlated to the DVPE value determined by ASTM D 5191. The bias of the results ob-
CHAPTER 25: VOLATILITY tained by this test method relative to D 5191 can be corrected by using the correlation equation: D V P E , D 5191 — VP4(37.goc) — A
(17)
where: A = 1.027 kPa (0.15 psi). ASTM D 5188, Standard Test Method for the Vapor-Liquid Ratio Temperature Determination of Fuels (Evacuated Chamber Method) This test method is under the jurisdiction of ASTM Committee D-02 and is the direct responsibility of Subcommittee D02.08. It was originally published in 1991. The latest edition is D 5188-99. This test method covers the determination of the temperature at which the vapor formed from a selected volume of volatile petroleum products saturated with air at 0-1 °C produces a pressure of one atmosphere in an evacuated chamber of fixed volume. This test method is suitable for gasoline samples that contain oxygenate. A known volume of chilled, air-saturated sample is introduced into an evacuated, thermostatically controlled test chamber of known volume. The sample volume is ceJculated to give the desired vapor-liquid ratio for the chamber in use. After injection, the chamber temperature is adjusted until a stable chamber pressure of 101.3 kPa is achieved. The tendency of a fuel to vaporize in automotive engine fuel systems is indicated by the vapor-liquid ratio of that fuel. Automotive fuel specifications generally include T(V/L =20) limits to ensure products of suitable volatility performance. For high ambient temperature, a fuel with a high value of T(v/L =20) indicating a fuel with a low tendency to vaporize, is generally specified. Conversely, for low ambient temperatures, a fuel with a low T(V/L =20) value is specified. ASTM D 2533, Standard Test Method for the Vapor-Liquid Ratio of Spark-Ignition Engine Fuels (Evacuated Chamber Method) This test method is under the jurisdiction of ASTM Committee D-02 and is the direct responsibility of Subcommittee D02.08. It was originally published in 1966. The latest edition is D 2533-99. This test method covers a procedure for measuring the volume of vapor formed at atmospheric pressure from a given volume of gasoline. The ratio of these volumes is expressed as the vapor-liquid (V/L) ratio of the gasoline at the temperature of the test. A measured volume of liquid fuel at 34-40°F (0-4°C) is introduced through a rubber septum into a glycerol or mercury filled burette. The charged burette is placed in a temperature-controlled water bath. The volume of the vapor in equilibrium with liquid fuel is measured at the desired temperature or temperatures and the specified pressure (usually 760 mm Hg). The vapor-liquid (V/L) is then calculated. If it is desired to know the temperature corresponding to a given V/L, the vapor-liquid ratio is determined at several temperatures, and the selected pressure. The results are plotted and the temperature read at the given V/L. The vapor-liquid ratio is reported to the nearest 0.1 unit and the corresponding temperature reading to the nearest 0.1 °C (0.2°F). If other than 760 mm Hg the pressure in millimeters of mer-
703
cury is also reported. ASTM D 1267, Standard Test Method for Gage Vapor Pressure of Liquefied Petroleum (LP) Gases (LP-GAS Method) This test method is under the jurisdiction of ASTM Committee D-02 and is the direct responsibility of Subcommittee D02.H on Liquefied Petroleum Gas. It was originally published in 1953. The latest edition is D 1267-95. The test apparatus (see Fig. 13 [54]) consisting of two interconnected chambers and equipped with a suitable pressure gage, is purged with a portion of the sample, which is then discarded. The apparatus is then filled completely with the portion of the sample to be tested. Thirty-three and one third to forty volume percent of the sample content of the apparatus is immediately withdrawn to provide adequate free space for product expansion. The apparatus is then immersed in a water bath at the standard test temperature of 37.8°C or, optionally, at some higher temperature up to and including 70°C. The observed gage pressure after temperature equilibration, corrected for gage error and barometric pressure, is reported to the nearest 5 kPa (0.5 psi) as the LPG Vapor Pressure at the selected test temperature. The observed gage vapor pressure is corrected by adding any gage correction to obtain the corrected vapor pressure. The corrected vapor pressure is then corrected to a barometric pressure of 760 mm (29.9 in) Hg by use of the following equations: LPGVPcorr = observed gage pressure + gage correction LPGVPeorr to 760 mm Hg
LPGVPcorr, kPa - ( 7 6 0 - P i ) 0.1333
(CAGE)
C
(STRAIGHT-THROUGH
VALVE)
B (LOWER CHAMBER)
(BLEEDER VALVE COUPLING) F (INLET
VALVE)
C (UPPER C H A M B E R )
FIG. 13—D 1267 apparatus.
(19) (20)
LPGVPcorr, psi - (760 - Pi) 0.0193
A
(18)
704
MANUAL
3 7: FUELS
AND LUBRICANTS
HANDBOOK
TABLE 35—Cross reference of international vapor pressure standards relative to ASTM distillation test methods. ASTM Designation U.S. D323 D4953 D5190 D5191 D5482 D6377 D6378 D5188 D2533 D 1267
ISO
IP
BS
AFNOR
DIN
Europe 3007
U.K. 69
U.K. 2000
France M07-007
Germany 51754
4256
161
L P G V P c o r r to 750 mm Hg = L P G V P c o r r , k P a
(21) - (29.92 - Pz) 3.3864 = LPGVPcorr, psi - (29.92 - P2) 0.4912
(22)
where P2 = observed barometric pressure, in. Hg. Information on the vapor pressure of hquefied petroleum gas (LPG) products under temperature conditions of 37.8-70° C is pertinent to the selection of properly designed storage vessels, shipping containers, and customer utilization equipment to ensure safe handling of these products. Determination of the vapor pressure of LPG is important for safety reasons to ensure that the maximum operating design pressures of storage, handling, and fuel systems will not be exceeded under normal operating temperature conditions. For LPG, vapor pressure is an indirect measure of the most extreme low temperature conditions under which initial vaporization can be expected to occur. It can be considered a semi-quantitative measure of the amount of the most volatile material present in the product. Test Methods
for Vapor
JIS
791-1201
Japan K2258
51 616
where P] = observed barometric pressure in m m Hg.
Other International Determination
FTM
Pressure
All the aforementioned test methods are ASTM standcird test methods for vapor pressure determination. Although ASTM standard test methods are used extensively throughout the world, there are other international vapor pressure standards that are used and are applicable to specific geographical area. Table 35 gives a cross reference for a n u m b e r of international vapor pressure s t a n d a r d s a n d their corresponding ASTM designations.
por/Liquid ratios cem be determined using a variety of techniques a n d instrumentation. For safety in handling and transporting veirious fuels cmd lubiicemts, a variety of flash point test methods are available. For the most part, this chapter on volatility characteristics of fuels and lubricants has dealt mainly with ASTM standard test methods that are used worldwide. However, there are other international standards that correspond to the ASTM standards described herein that are used and are applicable in other parts of the world. The discussions on the different ASTM test methods mentioned in this chapter are fairly brief, by design a n d space constraints. For m o r e details, it is strongly recommended that the readers refer to the actual ASTM test methods themselves.
ASTM STANDARDS No. D 0056 D 0086 D 0092 D 0093 D 0323 D 1160 D 1310 D 2533 D 2892
CONCLUSION D 3278 Volatility parameters of petroleum products and lubricants are important parameters that are related to the performance characteristics and safety in handling and transporting these materials. Optimum distillation and vapor pressure veJues are paramount in the proper and efficient operation of various engines fueled by different petroleum products u n d e r different conditions. Distillation can be carried out either at atmospheric or reduced pressures. Vapor pressure and Va-
D 3828 D 3941 D 4953
Title Standard Test Method for Flash Point by Tag Closed Cup Tester Standard Test Method for Distillation of Petroleum Products Standard Test Method for Flash and Fire Points by Cleveland Open Cup Tester Standard Test Method for Flash Point by PenskyMartens Closed Cup Tester S t a n d a r d Test Method for Vapor Pressure of Petroleum Products (Reid Method) Standard Test Method for Distillation of Petroleum Products at Reduced Pressure S t a n d a r d Test Method for Flash Point and Fire Points by Tag Open Cup Apparatus Standard Test Method for the Vapor-Liquid Ratio of Spark-Ignition Engine Fuels S t a n d a r d Test Method for Distillation of Crude Petroleum (15-Theoretical Plate Column) Standard Test Method for Flash Point by SetaFlash Closed Cup Apparatus S t a n d a r d Test M e t h o d for Flash Point by Small Scale Closed Cup Tester Standard Test Method for Flash Point by the Equilibrium Method with a Closed Cup Apparatus S t a n d a r d Test Method for the Vapor Pressure of Gasoline and Gasoline-Oxygenate Blends (Dry Method)
CHAPTER 25: VOLATILITY D 5188
D 5190 D 5191 D 5236 D 5482 D 6377 D 6378
D 6450
Standard Test Method for the Vapor-Liquid Ratio Determination of Fuels (Evacuated C h a m b e r Method) Standard Test Method for the Vapor Pressure of Petroleum Product (Automatic Method) Standard Test Method for the Vapor Pressure of Petroleum Product (Mini Method) Standard Test Method for Distillation of Heavy Hydrocarbon Mixtures (Vacuum Potstill Method) Standard Test Method for the Vapor Pressure of Petroleum Product (Mini Method-Atmostpheric) Standard Test Method for Vapor Pressure of Cude Oil: VPCRx (Expansion Method) Standard Test Method Determination of Vapor Pressure VPx of Petroleum Products (Triple Expansion) Standard Test Method for Flash Point by Continuously Closed Cup (CCCFP) Tester
OTHER STANDARDS No. ISO 1523 ISO 2592 ISO 2719 ISO 3007 ISO 3405 ISO 3679 ISO 4256 ISO 6616 ISO 8708 AFNOR M07-002 AFNOR M07-007 AFNOR M07-019 AFNOR M07-036 DIN 51 356 DIN 51 376 DIN 51 567 DIN 51 616 DIN 51 680 DIN 51 751 DIN 51 754 DIN 51 755 DIN 51 758 JIS K2254 JIS K2258 JIS K2265 SIS 150 223 BIS 2000 BS 7392 IP 034 IP 036 IP 123 IP 303 IP 304 IP 069 IP 161 IP 170 (Abel) FTM 791-1001
Title Flash Point, Equilibrium Method, Close Cup Flash Point, Cleveland Open Cup Flash Point, Pensky-Martens, Closed Cup Reid Vapor Pressure Distillation, Atmospheric Pressure Flash Point, SetaFlash, Closed Cup Vapor Pressure, LPG Distillation, Reduced Pressure Crude Distillation, 15/5 Distillation, Atmospheric Pressure Reid Vapor Pressure Flash Point, Pensky-Martens, Closed Cup Flash Point, Abel-Pensky Distillation, Reduced Pressure Flash Point, Cleveland Open Cup Crude Distillation, 15/5 Vapor Pressure, LPG Flash Point, SetaFlash, Closed Cup Distillation, Atmospheric Pressure Reid Vapor Pressure Flash Point, Abel-Pensky Flash Point, Pensky-Martens, Closed Cup Distillation, Atmospheric Pressure Distillation, Reduced Pressure Flash Point, Tag, Closed Cup Flash Point, Abel-Pensky Reid Vapor Pressure Distillation, Atmospheric Pressure Flash Point, Pensky-Martens, Closed Cup Flash Point, Cleveland Open Cup Distillation, Atmospheric Pressure Flash Point, SetaFlash, Closed Cup Flash Point, Tag, Closed Cup Reid Vapor Pressure Vapor Pressure, LPG Flash Point, Abel Distillation, Atmospheric Pressure
FTM 791-1101 FTM 791-1103 FTM 791-1201
705
Flash Point, Tag, Closed Cup Flash Point, Cleveland Open Cup Reid Vapor Pressure
REFERENCES [1] Annual Book of ASTM Standards, Vol. 5.01, ASTM International, West Conshohocken, PA, 1999, pp. 16-37 [2] Annual Book ofASTM Standards, Vol. 5.01, ASTM International, West Conshohocken, PA, 1999, pp. 416^33 [3] Annual Book of ASTM Standards, Vol. 5.01, ASTM International, West Conshohocken, PA, 1999, p. 18 [4] Annual Book ofASTM Standards, Vol. 5.01, ASTM International, West Conshohocken, PA, 1999, p. 19 [5] Annual Book ofASTM Standards, Vol. 5.01, ASTM International, West Conshohocken, PA, 1999, p. 34 [6] ASTM Research Report RR: D2-xxxx "Comparison of ASTM Crosscheck D 86 Resuhs (Automatic versus Manual) for Various Samples," ASTM International, West Conshohocken, PA, to be submitted. [7] ASTM Research Report RR: D2-1362 "Interlaboratoty Study to Determine Precision of Automatic D 1160 and Comparison with Manual D 1160 Results," ASTM International, West Conshohocken, PA, 1995. [8] Annual Book ofASTM Standards, Vol. 5.01, ASTM International, West Conshohocken, PA, 1999, p. 20 [9] Annual Book of ASTM Standards,Vol. 5.01, ASTM International, West Conshohocken, PA, 1999, p. 21. [10] Annual Book ofASTM Standards,Vol. 5.01, ASTM International, West Conshohocken, PA, 1999, p. 22. [11] Annual Book of ASTM Standards, Vol. 5.01, ASTM International, West Conshohocken, PA, 1999, p. 26. [12] Annual Book ofASTM Standards, Vol. 5.01, ASTM International, West Conshohocken, PA, 1999, p. 27. [13] Annual Book ofASTM Standards,Vol. 5.01, ASTM International, West Conshohocken, PA, 1999, p. 28. [14] Annual Book ofASTM Standards, Vol. 5.01, ASTM International, West Conshohocken, PA, 1999, p. 24. [15] ASTM D-2 Interlaboratory Crosscheck Program, Motor Gasoline, ASTM International, West Conshohocken, PA, June 1999, p. 35. [16] ASTM D-2 Interlaboratory Crosscheck Program, Reformulated Gasoline, ASTM InternationeJ, West Conshohocken, PA, June 1999, p. 57. [17] ASTM D-2 Interlaboratory Crosscheck Program, Diesel Fuel, ASTM International, West Conshohocken, PA, June 1999, p. 57. [18] ASTM D-2 Interlaboratory Crosscheck Program, Jet Fuel, ASTM International, West Conshohocken, PA, June 1999, p. 37. [19] Annual Book of ASTM Standards, Vol. 5.01, ASTM International, West Conshohocken, PA, 1999, p. 423. [20] Annual Book ofASTM Standards, Vol. 5.01, ASTM International, West Conshohocken, PA, 1999, p. 431. [21] Annual Book of ASTM Standards,Vol. 5.01, ASTMlntemational, West Conshohocken, PA, 1999, p. 417^22. [22] Annual Book ofASTM Standards, Vol. 5.01, ASTM International, West Conshohocken, PA, 1999, p. 427. [23] Section 173.120, Ch. 1, 49 CFR, (10-1-97 edition). Codes of Federal Register, Washington DC, p. 446. [24] Annual Book of ASTM Standards, Vol. 5.01, West Conshohocken, PA, 1999, pp. 1-10. [25] Annual Book of ASTM Standards, Vol. 5.01, West Conshohocken, PA, 1999, pp. 44-51. [26] Annual Book of ASTM Standards, Vol. 5.01, West Conshohocken, PA, 1999, pp. 52-61. [27] Annual Book of ASTM Standards, Vol. 6.01, West Conshohocken, PA, 1997, pp. 126-132.
706 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK [28] Annual Book of ASTM Standards, 1997, Vol. 6.01, West Conshohocken, PA, 1997, pp. 345-351. [29] Annual Book of ASTM Standards, Vol. 5.02, West Conshohocken, PA, 1999, pp. 596-597. [30] Annual Book of ASTM Standards, Vol. 6.01, West Conshohocken, PA, 1997, pp. 418-420. [31] Annual Book of ASTM Standards, Vol. 5.03, West Conshohocken, PA, 2000. [32] Annual Book of ASTM Standards, Vol. 5.01, West Conshohocken, PA, 1999, p. 5. [33] ASTM Research Report RR: S15-1010 "Interlaboratory Study on Flash Point Calibration Fluids," ASTM International, West Conshohocken, PA, 1995. [34] Montemayor, R. G., Rogerson, J. E., Colbert, J., and Schiller, S., Journal of Testing and Evaluation, November 1999, pp Ali-AIl. [35] White, E. W., "Manual on Flash Point," unpublished material. [36] ASTM Research Report RR: D2-1350, ASTM International, West Conshohocken, PA, 1995. [37] Montemayor, R. G., "The EiSect of Kinematic Viscosity on the Flash Point of Liquids Determined by ASTM 93 Procedure A, ASTM D 93 Procedure B, and ASTM D 56," Journal of Testing and Evaluation, November 1999, pp. 388-395. [38] ASTM Research Report RR: D2-1464, ASTM International, West Conshohocken, PA, 1999. [39] AnnualBook ofASTM Standards, Vol. 5.02, ASTM International, West Conshohocken, PA, 1999, pp. 220-246. [40] AnnualBook ofASTM Standards, Vol. 5.03, ASTM International, West Conshohocken, PA, 1999, pp. 197-212.
[41] Hunter, A., Technologist, Imperial Oil Ltd., Ontario, Canada. [42] Hunter, A., Technologist, Imperial Oil Ltd., Ontario, Canada. [43] Hunter, A., Technologist, Imperial Oil Ltd., Ontario, Canada. [44] Hunter, A., Technologist, Imperial Oil Ltd., Ontario, Canada. [45] Annual Book ofASTM Standards, Vol. 5.01, ASTM International, West Conshohocken, PA, 1999, pp. 159-166. [46] AnnualBook of ASTM Standards, Vol. 5.03, ASTM International, West Conshohocken, PA, 1999, pp. 32-38. [47] AnnualBook ofASTM Standards, Vol. 5.03, ASTM International, West Conshohocken, PA, 1999, pp. 185-189. [48] Annual Book ofASTM Standards, Vol. 5.03, ASTM International, West Conshohocken, PA, 1999, pp. 190-94. [49] AnnualBook of ASTM Standards, Vol. 5.03, ASTM International, West Conshohocken, PA, 1999, pp. 444-447. [50] Annual Book of ASTM Standards, Vol. 5.03, ASTM International, West Conshohocken, PA, 1999, pp. 181-184. [51] AnnualBook ofASTM Standards, Vol. 5.01, ASTM International, West Conshohocken, PA, 1999, pp. 889-896 [52] Annual book of ASTM Standards, Vol. 5.03, ASTM International, West Conshohocken, PA, 2000, pp. xxx [53] AnnualBook ofASTM Standards, Vol. 5.03, ASTM International, West Conshohocken, PA, 2000, pp. yyy [54] AnnualBook ofASTM Standards, Vol. 5.01, ASTM International, West Conshohocken, PA, 1999, pp. 457^75 [55] ASTM Research Report RR: D2-1286, ASTM International, West Conshohocken, PA, 1991. [56] ASTM Research Report RR: D2-1260, ASTM International, West Conshohocken, PA, 1988.
MNL37-EB/Jun. 2003
Elemental Analysis R. Kishore Nadkami ^
A MAJORITY OF ELEMENTS IN THE PERIODIC TABLE ARE PRESENT IN PETROLEUM PRODUCTS Tcinging in concentration from percent levels of C—H—O—S to parts per billion levels of trace elements. The metals in crude oils originate from marine animals and plants. Additives and lubricating oils have metals purposely added to them to affect their performance in internal combustion engines. Except for carbon and hydrogen, sulfur, oxygen, and to a lesser extent nitrogen, are the most abundant elements typically found in crude oils. Some specific crude oils may also contain significant amounts of vanadium and nickel, such as those from Venezuela. Generally, it is agreed that vanadium, nickel, and iron occur as metallo-porphyrins; mercury, antimony, and arsenic as organometallic compounds; mercury can also occur as dissolved element mercury; molybdenum and germanium as carboxylic acid salts, and silica and salt as colloidal minerals [1]. Extensive data on crude oil composition are available. The so-called crude assay analysis of oils carried out by many oil companies includes elemental analysis of sulfur, nitrogen, vanadium, nickel, and iron among other parameters. There are probably millions of such data points in the oil companies' proprietary databases. Significance of Metals in Petroleum Products The reason for interest in elemental analysis of petroleum products is two-fold. First, many metals and nonmetals present in crude oils have a deleterious effect on the refinery and processing operations, generally acting as catalyst poisons. The sulfur and nitrogen compounds generated during processing are also potential environmental concerns. Second, the lube additives and oils contain deliberately added organometallic compounds to enhance their performance. Though m u c h of this chemistry is proprietary, certain aspects are universally known. Generally, the presence of sulfur, phosphorus, alkaline earth metal, zinc, copper, etc. compounds define the composition of the additives and lubricating oils. A summary of additive elements generally used and their effects on engine performance is given in Table 1. Two documents published by ASTM are useful in understanding the significance of tests carried out on petroleum products [3,4]. In this chapter, the elemental analysis of petroleum products will be discussed. Particulcir focus will be on the most commonly used methods, although many other methods may also be available.
Analysis of Petroleum Products All well-established elemental analytical techniques have been used for the analysis of petroleum products. Several m o n o g r a p h s are available on this subject although all of them are at least two decades old. A more recent monograph has been published by ASTM, which contains papers from several authors covering a wide variety of techniques [5]. Most prominent among these techniques are atomic spectroscopy (atomic absorption spectroscopy, AAS, and inductively coupled plasma atomic emission spectroscopy, ICPAES), X-ray fluorescence (XRF), a n d micro-elemental techniques. Sample
Wear Metals in Used Oils Trace metals in used lubricating oils come from the mechanical wear from oil-wetted components of an engine or as a contaminant from air, fuel, and liquid coolant. Generally, the metals are present as particulate materials rather than as true solutions. The presence of specific metals in used lubricating oils can be associated with specific metal components of an engine. In a normally running engine, weeir metal content of the oil slowly increases due to normal wear. However, a sudden increase in one or more metal concentrations in oil
' Millennium Analytics, Inc., 47 Helena Street, East Brunswick, NJ 08816. 707 Copyright'
2003 by A S I M International
Preparation
Sample preparation techniques can vary from none to quite elaborate depending on the final measurement step. Microelemental analysis and, for some matrices, atomic spectroscopy and XRF techniques need n o sample preparation at all. For others, it can be as simple as dilution in an organic solvent for m e a s u r e m e n t by AAS, ICP-AES, or XRF. For some specific analyses, a sample may be ashed (D 482 and D 874) and then brought into an aqueous acidic solution before being analyzed by a n atomic spectroscopic technique. In such cases, dry ashing (D 482) may not be preferred due to the potential for the loss of volatile elements. Wet ashing using various acids or their mixtures can minimize volatilization losses. The D 874 procedure uses sulfuric acid for this work. This method is well-suited for bringing the organic samples into inorganic solutions, however, as a stand-alone method for estimating the total meted oxides/sulfates in the sample, it has several drawbacks due to the formation of intermetallic compounds during oxidation and sulfation. Generally, this results in significantly underestimating the amount of total metal oxides using the residual weight from D 874 compared to the ICP-AES or XRF sum of oxide results. This means that the residual mass is less than the ICP-AES or XRF sum of oxides [6].
www.astm.org
708
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
TABLE 1—Presence and role of specific elements in lubricants [2]. Element
Compounds
B
Borax and esters
Ba
Sulfonates, phenates, Diorganodiphosphates, phosphonates, Thiophosphonates Sulfonates, phenates Dithiophosphates Salts Organic Compounds Sulfonates, phenates M0S2, dibutyldithicarbamate, phosphate Cyclopentadienyl complexes
Ca Cd Cr Hg Mg Mo Ni P Pb Sb Se Si Sn Zn
Metaldialkyldithiophosphates Naphthenate Dialkyldithiocarbamates. Dialkylphosphorodithionates Selenides Silicone Polymers Organo Compounds Dialkyldithiophosphates, dithiocarbamates, phenolates
TABLE 2—Wear metals (elements) in used lubricating oils [2]. Elements
Wear Indication
Ag
Wrist pin bearings in railroad and auto engines, silver plotted spline lubricating pump Piston and bearings wear, push rods, air cooler, pump hosings, oil pumps, gear castings, box castings Coolant leakage in system Bearings Ring wear, cooling system leakage, Cr-plated parts in aircraft engines, cylinder liners, seal rings Wear in bushings, injector shields, coolant core tubes, thrust washers, valve guides, connecting rods, piston rings, bearings, sleeves, bearing cages Wear from engine block, cylinder, gears, cylinder liners, valuve guides, wrist pins, rings, camshaft, oil pump, crankshaft, ball and roller bearings, rust Cylinder liner, gear box housings in aircraft engines Wear in bearing alloys and in oil coolers; various Moalloyed components in aircraft engines, piston rings Antifreeze leakage Bearings, valves, gear platings Bearings, fuel blowby, thrust bearings, bearing cages, bearing retainers Crankshaft and camshaft bearings Dirt intrusion from improper air cleaner, seal materials Bearings and coatings of connecting rods and iron pistons Various Ti-alloyed components in aircraft engines Bearings Neoprene seals, galvanized piping
Al B Cd Cr Cu Fe Mg Mo Na Ni Pb Sb Si Sn Ti W Zn
indicates failure or excessive wear of a specific engine component. Table 2 summarizes the typical wear metals and the t3rpe of engine wear they may indicate, since in some cases, more than one source may exist for certain elements. Elemental analysis of used oils is generally performed using atomic spectroscopy techniques, particularly ICP-AES in recent years, because of its capability of simultaneous determination of multielements. The U.S. Air Force for a n u m b e r
Performance
Antiwear agents, antioxidant, deodorant cutting oils, greases, brake fluids Detergent inhibitors, corrosion inhibitors, detergents, rust inhibitors, ATF, greases Detergent inhibitors, dispersants Stream turbine oils Grease additive Bactericide in cutting oil emulsions Detergent inhibitors Greases, extreme pressure additives Antiwear agents, carbon deposit reduction, improved lubrication and combustion Detergents, antirusting agents Extreme pressure additive, greases, gear oils Antiwear, extreme pressure, antioxidant Oxidation and bear corrosion inhibitors Foam inhibitors Antiscuffing additives, metal deactivators Antioxidant, corrosion inhibitors, antiwear additives, detergents, extreme pressure additives, crankcase oils, hypoid gear lubricants, greases, aircraft piston engine oils, turbine oils, ATF, railroad diesel engine oils, brake lubricants
of years conducted this kind of analysis on their jet engine oils under the acronym "SOAP" (Spectrometric Oil Analysis Program) [7]. As a result of the success of this program, the U.S. Air Force, Army, and Navy established a Joint Oil Analysis Program (JOAP) in 1976. Over 100 laboratories throughout the world are involved in these analyses using AAS, ICPAES, and rotating disk electrode emission spectrometry. To expedite in-field analysis, a portable graphite furnace multielement AAS instrument has been developed that is capable of analyzing nine wear metals in undiluted oil samples [8]. By use of a rapid sequential analysis mode, u p to 24 elements in a single aspiration have been determined using a multielement AAS spectrometer [9]. Rotating disk electrode emission spectrometry either by a single spark technique or ashing rotrode technique has been shown to rapidly provide multielement analysis of used oils. By anailyzing two aliquots, one directly and one after acid dissolution, differentiation can be m a d e between large and small wear particles [10]. A correlation has been shown to exist between ICP-AES and rotrode emission techniques. Results suggest that rotrode is somewhat more effective in sampling particulates than ICP-AES [11]. ICP-mass spectrometry has been proposed as a sensitive tool for the determination of environmentally important elements in used oils. The technique has detection limits of parts per billion and needs no sample preparation other than solvent dilution. However, because of the matrix interference and formation of polyatomic species, it lacks the precision and accuracy of other atomic spectroscopy techniques [12], Inhomogeneous sampling, due to the effect of particle size of wear metals, continues to be a big concern in atomic spectroscopic analysis. Severely worn engines produce large particulates. It is more important to euialyze large particles t h a n small ones in such cases. An appreciable fraction of susp e n d e d metallic particles, especially large ones, may not reach the atomization source; r a t h e r they collect on the chamber walls and are washed down the drain, resulting in
CHAPTER low biased results. Also, particulates that reach the source may not be "totally" atomized. To overcome this particle size effect, the so-called particle size independent methods have been developed, which consist of heating the oil in a small amount of minerad acids and then diluting with an appropriate organic solvent [13]. At present, in the D2 committee, the only available method for the determination of wear metals in used oils is D 5185, which is a n ICP-AES method. Two methods based on the rotrode technology have also been recently published: D 6595 and D 6728. D02 Test Methods There are about 80 test methods under the jurisdiction of Subcommittee 3 on Elemental Analysis (see Table 3). These m e t h o d s could be roughly classified according to their principle chemical/physical technique that is utilized in the analysis of samples. Classical
Wet Chemistry
Methods
These methods are based on the gravimetric-titrimetric finish for the final determination of the species of interest. Although at one time widely used, they have been largely displaced by spectrometric techniques particularly for metals in most m o d e m laboratories. Many times these methods are considered as the referee methods; however, the expertise in conducting these tests is hard to find today. These methods include: Analysis Phosphorus
Vanadium Nitrogen Lithium/Sodium Chlorine Lead Sulfur, active Sulfur, mercaptan
Bomb
Combustion
ASTM
Technique
D1091 D3231 D4047 D 1548 D3228 D3340 D 4929A D5384 D3341 D3348 D4952 D3227
Photometry and Gravimetry Photometry Titrimetry Photometry Kjeldahl Titration Flame Photometry Potentiometry Titrimetry Titrimetry Colorimetry Colorimetry Potentiometry
Methods
These methods utilize combustion in a b o m b (a sealed metal container pressurized with oxygen and an ignition source) or an enclosed flame to concentrate the species of interest— generally a gaseous element—in a n aqueous solution followed by a final determination such as gravimetry or titrimetry. Similar to the classical wet chemistry methods cited above, many of these methods are also considered as referee methods, but have been increasingly replaced by m o d e m instrumental methods of analyses. These methods include: Analysis
ASTM
Detection Technique
Chlorine Hydrogen Sulfur
D808 D 1018 D 129 D 1266 D1552 D2784
Gravimetry Gravimetry Gravimetry Gravimetry or Titrimetry Titrimetry or IR Detection Titrimetry or Turbidimetry
26: ELEMENTAL
ANALYSIS
709
Atomic Spectroscopic Methods The most widely used techniques for metal analysis in petroleum products are atomic absorption (AAS) and inductively coupled p l a s m a atomic emission (ICP-AES) spectroscopy methods. Other related techniques such as flame emission spectroscopy, graphite furnace AAS, and hydride generation AAS, although still viable, do not appear to be popular at least in the petrochemical analysis area, perhaps because of the adequate sensitivity offered by AAS and ICP-AES. Atomic
Absorption
Spectrometry
(AAS)
This widely used metal determination technique, not only in petrochemicals, but in all industrial application areas, has evolved into a "cookbook" type of analysis. Many m o d e m instruments are fully automated. The technique is applicable to nearly 70 elements with precision and accuracy of 1-3% achievable u n d e r optimal conditions. Once the material is in solution, the analysis time is less than a minute per sample although time depends on n u m b e r of elements/sample. Other than refractory oxide-forming elements, detectability by AAS is less than p p m for most metals. Although mostly free from atomic spectral interferences, AAS does suffer from molecular spectral, ionization, chemical, and matrix interferences. However, most of these are well documented and are taken care of during routine analyses. In AAS, a sample solution is vaporized and the elements atomized at high temperatures. The element concentration is determined by absorption of light of a characteristic wavelength emitted by a primary source. The light source is typically a hollow cathode lamp, which consists of a tungsten anode and a cylindrical hollow cathode enclosed in a gas-tight chamber. The cathode contains the analyte metal; usually individual lamps are needed for each element, although some multielement lamps are available. The detector is usually a photomultiplier tube. A monochromator is used to isolate the spectral line, and the light source is modulated to discriminate against continuum radiation from atomization source. Usually the AAS instruments use a flame as the atomization source. The fuel/oxidant composition and ratio determine the temperature of the flame. An air-acetylene flame is used for most elements. The NaO-acetylene flame is used for elements that form stable monoxides in air-acetylene flame. There are literally hundreds of papers published using AAS for industrial products including petrochemicals. An exhaustive review on this subject is available, although somewhat dated [14]. There are several standard test methods based on this technique in the D2 committee; these include: Analysis Aluminum/silicon in fuel oils by ashing, fusion and AAS Lead in gasoline by complexation and AAS Manganese in gasoline by AAS Trace metals in gas turbine fuels Additive metals in oils by AAS Trace metals in coke by ashing, fusion, and AAS Ni, V, Fe, and Na in crude oils and residual fuels by AAS direct or after ashing
Graphite Furnace (GFAAS)
Atomic
Absorption
ASTM D 5184B D 3237 D 3831 D 3605 D 4628 D 5056 D 5863
Spectrometry
This technique uses graphite furnace rather than a flame as an atomization source, a n d it improves the sensitivity of
710 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 3—D02.SC3 Elemental analysis subcommittee test methods. Method Designation D 129 D482 D808 D874 D 1018 D 1091 D1266 D 1317 D 1368 D 1548 D1552 D 1839 D2622 D2784 D3116 D3120 D3227 D3228 D3229 D3230 D3231 D3237 D3340 D3341 D3348 D3431 D3605 D3701 D3831 D4045 D4046 D4047 D4294 D4628 D4629 D4808 D4927 D4929 D4951 D4952 D5056 D5059 D5184 D5185 D5291 D5384 D5453 D5600 D5622 D5708 D5761 D5762 D5863 D6334 D6443 D6445 D6470 D6481 D6595 D6667 D6728 D6732
Current Year of Publication 2000 2000 2000 2000 2000 2000 1998 1989 (WD) 1989 (WD) 1992 (WD) 2001 1991 (00) 1998 1998 1989 (WD) 1996 2000 2001 1988 (WD) 1999 1999 1997 1998 2000 1998 1987 (WD) 2000 2001 1998 1999 1991 (00) 2000 1998 1997 1996 2001 2001 1999 2000 1997 1996 1998 2000 1997 2001 1995 2000 (00) 1998 1995 2000 (00) 1996 2001 2000 1998 1999 1999 1999 1999 2000 2001 2001 2001
Subject
Sulfur in Petroleum Products (Bomb Method) Ash from Petroleum Products Chlorine in Petroleum Products (Bomb Method) Sulfated Ash Hydrogen in Petroleum Fractions Phosphorus in Oils and Additives Sulfur in Petroleum Products (Lamp Method) Chlorine Na-Alcoholate Method Lead in Reference Fuels Vanadium in Heavy Fuel Oil Sulfur in Petroleum Products (High Temp. Method) Amyl Nitrate in Diesel Fuels Sulfur by X-ray Spectroscopy Sulfur in LPG Trace Lead in Gasoline Sulfur in LPG by |x coulometry Mercaptan Sulfur by Potentiometry Modified Kjeldahl Method for Nitrogen Low Level Lead in Gasoline Salts in Crude Oil Phosphorus in Gasoline Lead in Gasoline by AAS Li and Na in Lubricating Greases Lead in Gasoline (I CI method) Field Test Method for Pb in gasoline Nitrogen by Microcoulometiy Metals in Gas Turbine Fuels Hydrogen by NMR Mn in Gasoline by AAS Sulfur by Hydrogenolysis Alkyl Nitrate in Diesel Fuels Phosphorus by Wet Chemistry Sulfur by EDXRF Additive Metals by AAS Nitrogen by Chemiluminescence Hydrogen by NMR Additive Metals by XRF Organic Chloride in Crude Oil Additive Metals by ICPAES Active Sulfur Species Metals in Petroleum Coke by AAS Lead in Gasoline by XRS Al and Si by ICPAES Metals by ICPAES Instrumental CHN Field Test Kit for Chlorine Sulfur by UV Fluorescence Metals in Petroleum Coke by ICPAES Oxygen by Reductive Pyrolysis Ni, V, Fe in Oils by ICPAES Practice for Emulsification Nitrogen by Boat Chemiluminescence Ni, V, Fe, Na in Oils and Fuels Sulfurs in GasoUne by WDXRF Metals in Lubes by WDXRF Sulfur in Gasoline by EDXRF Salt in Crude Metals in Lubes by EDXRF Wear Metals in Lubes by Rotrode Sulfur in LPG by UV-Fluorescence Contaminants in Fuels by Rotrode Copper in Jet Fuels by GFAAS
(WD): Standard is withdrawn from publication. flame AAS by several orders of magnitude. Even though GFAAS covers all elements which can be determined by AAS, the strength of GFAAS lies in the determination of elements at sub-ppm levels, and those that are somewhat difficult to do by AAS, e.g., V, Ni, Pb, Sb, As, and Cd. It is not an ideal technique for additive metals in lube oils since the high concen-
tration of the metals in this matrix necessitates very large dilutions. The technique uses m u c h smaller sample than that required for flame AAS, but it takes longer time for analysis. Matrix interferences are also m o r e significant t h a n with flame AAS, and this translates into poorer precision (and accuracy).
CHAPTER Most applications of GFAAS in the petroleum products area have been in research. Recently, a method to measure copper in jet fiiels by GFAAS has been published (D 6732). Inductively Coupled Plasma-Atomic Spectrometry (ICP-AES)
Emission
The most significant development in atomic spectroscopy in the last two decades has been the development and wide popularity of the ICP-AES technique. It is increasingly replacing AAS instruments in the laboratories worldwide as a reliable, accurate, precise, and rapid technique that is sensitive to ppb levels for multielement aneJysis of petroleum products. It is second only to graphite furnace AAS in its low detection limits for most elements. However, its wide dynamic range (10''-10*) makes it unique among other atomic spectroscopic techniques. This permits the measurement of percent levels of some elements Eind p p m levels of others in the same solution with one aspiration without multiple dilutions. Because of the high temperature of the plasma, chemical and matrix interferences c o m m o n in flame AAS are not seen in ICP-AES. In ICP-AES, the liquid sample sprayed into the argon plasma through a nebulizer is vaporized, atomized, ionized, and electronically excited. The excited state ions and atoms, in returning to their ground states, emit photons of wavelengths characteristic of an element. The emitted light is measured by a polychromator in a simultaneous spectrometer or by a single photomultiplier in a scanning or sequential spectrometer. Nebulizers are a critical pEirt of the instrument and a n u m b e r of models have been devised to overcome the physical interferences encountered in nebulizers of earlier yecirs. Commonly used nebulizers are glass concentric pneumatic, cross flow or grid pneumatic, more advanced ultrasonic, and Babington high solids nebulizers. See Ref. 15 for a good review of ICP-AES instrumentation. A major drawback of the ICP-AES technique is the spectral interference that makes accurate trace element determination difficult in complex matrices. High-resolution spectrometers are used to separate the overlapping spectral lines and correct for background interferences. However, usually the atomic emission lines and their associated interfering wavelengths are well characterized, and normEdly interference-free lines can be selected for the elements of interest. Many sensitive lines are "ion" lines, also. Some typically used wavelengths a n d their detection limits for oil aneJysis are given in Table 4. The sample preparation is usually simple in ICP-AES involving 10- to 100-fold dilution in an organic solvent such as kerosene, xylene, methyl ethyl ketone, methyl iso-butyl ketone, etc. The organometallic standards ctre used for instrument calibration. The viscosity of oil samples plays em important role in accurate ICP-AES measurements. Usually, base oil is added to samples and calibration standards, to approximately equate the viscosity. In an important piece of research, Banscd and McElroy [16] showed that viscosity modifiers can produce severe biases in ICP-AES measurements due to the changes in the nebulizer aerosol formation affecting the ICP-AES torch temperatures. The biases are largest for low excitation potential elements. This problem can be solved by using both an internal standard (e.g., Cobalt) and adequate sample dilution (e.g. 100-fold). This work was independently confirmed by Jansen et al [17]. In light of this work, both D 4951 and D
26: ELEMENTAL
ANALYSIS
711
TABLE 4—ICPAES Analytics for oil analysis [2]. Element Ag Al B Ba Ca Cd Co Cr Cu Fe K Mg Mn Mo Na Ni P Pb S Sb Si Sn Ti V Zn
Wavelength, nm 328.07 308.22; 249.77 455.40; 393.37; 214.44; 228.62 276.65; 324.75; 259.94 766.49 279.55; 257.61; 202.03; 589.00 341.47; 214.91; 220.35; 190.00; 231.15; 288.16; 283.99; 334.94; 292.40; 206.20;
309.27; 396.15 233.53; 493.41 317.93; 315.88 226.50; 228.80 283.56; 267.71 327.40
285.21; 279.08 357.60; 293.30 204.60; 281.62 231.60; 178.29; 283.31 180.73; 206.84 251.61 189.99; 337.28; 309.31; 213.86;
216.56 177.51 182.04
242.95 350.50 311.07 202.55
Detection and Upper Limits, ppm 0.005-100 0.02-100 0.05-100 0.0003-50 0.001-20 0.007-100 0.007-100 0.001-100 0.002-50 0.004-100 0.2-50 0.0004-20 0.001-50 0.014-100 0.015-100 0.02-100 0.10-200 0.05-100 0.05-100 0.05-100 0.02-100 0.05-100 0.001-100 0.005-100 0.005-20
NOTE: Other suitable interference-iree wavelengths may also be used. 5185 test methods for the determination of metals in additive, lube oils, base oils, and used oils have been modified to include the mandatory addition of an internal standard and increasing the dilution factor. Numerous applications of ICP-AES in the petroleum products area have been published. Memy of these were reviewed in Ref. 2. Some of the work published since then is reviewed here. Robotics can be used for sample dilutions in ICP-AES a n d XRF. Generally, the precision of XRF was found to be slighdy better than that of ICP-AES [18]. Use of an internal standard and kerosene as a preferred solvent has been shown to offer superior precision for a n u m b e r of elements in crude oils [19]. Hausler and Carlson have reviewed vEirious sample preparation techniques used for petroleum products before ICP-AES measurements were made at the Phillips Petroleum Company [20]. Similar work performed in Exxon has been reviewed by Botto [21]. Applications of more advanced ultrasonic nebulizers in ICP-AES have been described by Botto [22,23]. Although at present, there are only five test methods utilizing this technique in D2 committee, it is expected that this will increase in the future. In many laboratories, ICP-AES instruments are replacing AAS instruments because of the former's superior capabilities discussed earlier in this chapter. Generally, these methods are used in the petrochemical laboratories for the determination of a n u m b e r of elements in a wide concentration range in a variety of petrochemiccJ matrices. These methods include: Analysis Additive elements in lubricating oils Additive and trace elements in fresh and used lubricating oils and base oils Aluminum and silicon in fuel oils after ashing £ind fusion Trace metals in petroleum coke after ashing and fusion Ni, V, and Fe in crude oils and residual fuels sifter ashing or dissolution in a solvent
ASTM D 4951 D 5185 D 5184A D 5600 D 5708
712
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
Although not ICP-AES, two methods based on atomic emission spectroscopy (D 6595 and D 6728) have been pubHshed using rotrode to determine contaminants and wear metals in lube oils and fuels, respectively.
reviewed in Ref. 2. There are, however, no test methods available in Subcommittee 3 jurisdiction at present that utilize ion chromatography for the elemental analysis. MicroElemental Analysis Methods
Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) This technique combines the power of ICP for atomizing and ionizing the samples with the sensitivity and the selectivity of mass spectrometric detection. By combining ICP-MS with solution separation techniques such as high-pressure liquid chromatography (HPLC) or ion chromatography (IC), the technique can be used for metal speciation studies. The technique can simultaneously determine nearly 70 elements in a sample with detection limits down to parts per trillion for most elements. However, often these low detection limits cannot be achieved in complex matrices. The matrix effects are common to ICP-MS just as they are in ICP-AES. Many internal standards are required to cover the entire mass range because of mass bias. A serious concern in ICP-MS measurement is the sample preparation step. Extreme care must be exercised in controlling the blank contamination because of the generally ultra-low levels of trace elements being determined. In spite of the versatility of this technique, the applications in the petrochemicals field are limited. Al-Swaidan [24] used dilution of samples in xylene followed by nitric acid extraction to determine low levels of several elements in Saudi Arabian crude oil and gasoline samples. Williams [12] has used ICP-MS to analyze diluted waste oils for the determination of several trace elements as a screening tool to decide whether the used oils can be recycled or must be disposed of as hazardous wastes. McElroy et al. [25] have demonstrated the use of this technique, using cobalt as the internal standard, in determining vanadium, phosphorus, nickel, and calcium in a number of crude oils. More interestingly, they have combined HPLC with ICP-MS to identify selenium, nickel, and vanadium species in crude oils. The D02.SC3 subcommittee has not published any ICP-MS test methods so far. An informal survey did not indicate that ICP-MS was being widely used in the petrochemical laboratories, perhaps because of the high cost of the instrumentation, as well as ICP-AES offering adequate sensitivity in most cases. Ion Chromatrography This technique has revolutionized the determination of anionic species from individual wet chemistry techniques to one simultaneous multi-anion instrumental method. The technique is a combination of ion exchange chromatography, eluent suppression, and conductometric, amperometric, or UV-visible detection. It needs only microliters of a sample and can rapidly determine several anions in a large dynamic range in a single sample. The analysis, however, is aqueous based which means petrochemical samples must be brought into aqueous solution generally by combustion in an oxygen atmosphere in a closed vessel. The technique has been used to determine halogens, sulfur, phosphorus, and nitrogen in samples such as fuel oil, gasoline, diesel fuel, waste oil, coke, etc. The literature has been
The micro- in this category of methods usually refers to the presence of these non-metals in minor quantities in the petroleum products. It may also reflect on the fact that usually a very small quantity of sample is used in the analysis. In all of these cases, the sample is combusted and the element of interest in the gases produced is detected by a variety of techniques. These methods are widely used in the petrochemical industry for the analyses of a variety of different products. Essentially all these methods are based on the commercially available instruments. This list continues to grow as more commercial instruments become available. These methods include: Analysis Carbon-HydrogenNitrogen Chlorine, Organic Nitrogen Oxygen Sulfur
ASTM D5291 D 4929B D4629 D5762 D5622 D3120 D4045 D5453 D6667
Technique IR or GC Microcoulometry Chemiluminescence Chemiluminescence Reductive Pyrolysis Oxidative Microcoulometry Hydrogenolysis and Rateometric Colorimetry UV Fluorescence UV Fluorescence
Generally, these methods eire free from interference from other constituents of the sample, are capable of rapid and many time automated analyses, and have precisions of about 2-5% dependent on the concentration level of the analyte (Table 5). Neutron Activation Analysis (NAA) A large body of data exists on the applications of this technique to crude oil and petroleum products. However, because of the difficulty of access to a nuclear reactor and associated radiation hazards, it has found little use in petrochemical or oil company laboratories. Since the petroleum products contain essentially a C—H—O matrix that does not get activated during thermal neutron irradiation, there is very little background interference. However, there could be interference from other trace element nuclei present in the sample. Generally, these isoTABLE 5—Precision of MicroElemental test methods. Analysis
Test Method
Repeatability
Reproducibility
Carbon Chlorine Hydrogen Nitrogen
D5291 D 4929B D5291 D4629 D5291 D5762 D5622 D3120 D4045 D5453
(X + 48.48) 0.0072 0.7 (X)O-* (X''-^) 0.1162 0.15 (X)''-^'' 0.1670 0.009 (X) 0.06 - 0.81% 28% 0.16 V ^ 0.1867(X)°"^
(X +48.48) 0.018 1.0 (X)"-^' (X°-5) 0.2314 0.85 iXf-^'* 0.4456 0.291 (X) 0.26 - 0.81% 38%
Oxygen Sulfur
X: Average value of two results.
0.26 VJ 0.2217(X)0-^
CHAPTER topic interferences are well-known and suitable alternate interference-free g a m m a rays can be used for quantitative analysis. The literature until the late '80s has been reviewed earlier [2].
X-Ray Fluorescence Spectrometry (XRF) Similar to AAS and ICP-AES, XRF is a technique widely used in the petroleum products testing laboratories mainly for the analyses of sulfur, chlorine, and additive metals. The technique is non-destructive, has multi-element capability, and high level of precision combined with low (ppm level) detection limits. The cost of the instrumentation, particularly a wavelength dispersive XRF, is quite high, typically greater than that of ICP-AES. However, many dedicated inexpensive single element instruments are available. In the XRF technique, a sample is bombarded with X-rays resulting in ejection of electrons from the inner shells of the target atoms. In the process, X-rays of discrete characteristic energy are emitted as electrons from the outer shells replace the ejected electrons. Every element produces a unique secondary X-ray spectrum whose intensity is proportional to the element concentration in the sample. XRF has been typically applied to all elements above the atomic n u m b e r 10 (neon). The basic components of X-ray spectrometers are a source of excitation consisting of a high p o w e r X-ray tube (usually comprised of a tungsten, molybdenum, chromium, rhodium, or scandium target), a wavelength or an energy dispersive spectrometer for selecting the characteristic fluorescent signals, and integrating or counting circuits for measurements. Due to n u m e r o u s interelement matrix effects, standards with compositions similar to the samples need to be used and corrections m a d e for matrix element absorption and enhancement of the analyte element. For solids, particle size and shape can affect accuracy. Many of these interferences and matrix effects can be compensated by corrective techniques such as standard addition, internal standards, matrix dilution, thin film method, and mathematical corrections, provided interference and matrix effects are accounted for. Speed, convenience, minimal sample preparation, and its nondestructive nature are the main advantages of the XRF technique. Inherently very precise, XRF rivals the accuracy of wet chemistry techniques in the analysis of major constituents, provided interferences and matrix effects are accounted for. The two main types of XRF instrumentation available can be divided into wavelength (WDXRF) and energy dispersive (EDXRF) instruments. In WDXRF, primary X-rays irradiate the sample and generate fluorescent X-rays. These are then diffracted by a crystal. A goniometer selects the geometry between the crystal and detector. The geometry controls the detection of X-rays from the element of interest. Different crystals can have different sensitivities a n d cover different wavelength ranges, and are used depending on the X-ray wavelength of the analyte element. Many commercial WDXRF instruments have two detectors and u p to six crystals to optimize the instrumental conditions for each element. The EDXRF instrument generally uses a lower power X-ray tube. The emitted X-ray radiation from the sample impinges directly on a detector, typically a Si(Li), which generates a
26: ELEMENTAL
ANALYSIS
713
change proportional in magnitude to energy of the incident X-ray. The series of changes are sorted and counted by a multi-channel analyzer. Simultaneous determination of all elements (with atomic numbers greater than Mg) is possible. Optimization of specific elements is accomplished through the use of secondary targets and filters. Radioisotope sources can be used in place of X-ray tubes in instruments designed for limited element applications. Sensitivity in X-ray fluorescence analysis is highly dependent on the sample matrix. Detection limits are typically in the p p m range, with the highest sensitivity observed for the transition metals. Sensitivity declines significantly for the lower atomic n u m b e r elements. Resolution and sensitivity of EDXRF is typically a n order of magnitude worse t h a n for WDXRF. However, the cost of an EDXRF spectrometer, especially the single-element models, can be ten times less than for a WDXRF spectrometer. XRF is a well-established technique with numerous applications in most petrochemical testing laboratories. The pre1990 literature in this area was reviewed in Ref. 2. See Ref. 26 for a review of currently available instrumentation in both small and large WD- and ED-XRF instruments. Sieber et al. have described the use of XRF in complementary fashion to other spectroscopy techniques for the determination of lubricant additive elements [27]. EDXRF has been applied to the determination of vanadium, nickel, and iron in petroleum and petroleum residua [28], and Mg, P, CI, K, Zn, Ca, and Mo in crude oils and lubricating oils [29]. Both qualitative and quantitative analysis of low atomic n u m b e r elements (e.g., magnesium) have recently become feasible due to the use of synthetic multilayer crystals and improvement in detector windows and associated electronics [30]. Several ASTM m e t h o d s use a n XRF technique a n d are complementary to other methods using AAS or ICP-AES listed earlier. All three techniques should produce equivalent results when properly followed. The XRF methods include: Analysis Lead in gasoline with Bi internal standard or direct comparison Metals in lubricant and additive components Sulfur in gasoline and methanol fuels Sulfur in petroleum products Metals in lube oils Metals in lube oils Sulfer in gasoline Sulfer in gasoline
ASTM D 5059 D 4927 D 2622 D 4294 D 6443 D 6481 D 6334 D 6445
In general, AAS, ICP-AES, and XRF have similar precision for the most commonly determined elements in petroleum products. Table 6 illustrates this by comparing the repeatability and reproducibility of D 4628 AAS, D 4927 XRF, and D 4951/D 5185 ICP-AES methods for selected additive elements in lube oils. Miscellaneous
Methods
There are two methods available for the determination of hydrogen content of aviation turbine fuels, and of light and middle distillates, gas oils and residua (D 3701 and D 4808, respectively), based on nuclear magnetic resonance. Perhaps the oldest methods used in the elemental analysis of petroleum products are those for the determination of ash
714
MANUAL
37: FUELS
AND LUBRICANTS
HANDBOOK
TABLE 6—Precision of alternate methods for elemental analysis of lubricating oils. Repeatability Element*^
D 4628 (AAS)
Barium Calcium Magnesium Phosphorus Sulfur Zinc
0.011 0.0050 0.0037 0.0054
D 4927 (XRF) A B 0.0027 0.0047
0.0015 0.0034
0.0065 0.0047 0.002
0.0042 0.0042 0.0019
Reproducibility D 4927 (XRF)
ICP-AES D4951
D5185
D 4628 (AAS)
0.011 0.0058 0.0032 0.0026 0.016 0.0022
0.0023 0.0032 0.0061 0.0071 0.0132 0.0066
0.04 0.017 0.015 0.012
A
ICP-AES
B
D4951
D5185
0.006 0.018
0.016 0.016
0.011 0.020 0.0051
0.018 0.019 0.012
0.019 0.0083 0.012 0.010 0.061 0.0072
0.034 0.0119 0.0147 0.0136 0.021 0.0166
[ mass percent elemental concentration.
and sulfated ash (D 482 and D 874, respectively). The methods need a minimal a m o u n t of apparatus, perhaps available in any laboratory. Because of their apparent simplicity, the tests, however, may give an erroneous impression about the composition of complex products. The sulfated ash method (D 874), in particular, is fraught with drawbacks resulting in extremely erroneous d a t a inconsistent with the chemical composition of the additive products. This happens because of the formation of a n u m b e r of nonstoichiometric compounds rather than only sulfates during sulfation and combustion [6]. It has been suggested that this empirical test be replaced by m o d e m instrumented analysis, such as AAS, ICPAES, or XRF, which give m u c h more precise information on the metal content of the petroleum additive products.
TABLE 7—Standard reference materials for elemental analysis of petroleum products available from NIST. Sample Type Crude oil Diesel fuel oil Fuel oil, Distillate Fuel oil. Residual
Kerosene Lube additive Lubricating oil
Petroleum coke Reference Fuel
NIST Designation 8505 2723; 2724a 1624c 1618 1634c 1619a; 1620b; 162Ie; 1622e; 1623c; 2717 1616a; 1617a 1848 1818a 1819a 1836 1083; 1084a; 1085a 2718; 2719 2712-2715
Certification Vanadium Sulfur Sulfur Vanadium and Nickel Trace elements Sulfur Sulfur Additive Elements Chlorine Sulfur Nitrogen Wear Metals Trace Elements Lead
Standard Reference Materials Standard reference materials (SRM) play a vital role in evaluating new anal3?tical test methods, assessing the laboratory capability for performing the tests with required precision and accuracy, and for routine quality assurance of the data generated in the laboratories. Often SRMs are also used as the calibrating materieds. National Institute of Standards and Technology (NIST) is perhaps the leading source of extremely reliable SRMs in a variety of matrices. NIST certifies each SRM based on replicate analysis values obtained by at least two independently based methods. There are other national bodies in the U.K., Germany, etc. which also produce similar SRMs. There are also some commercial companies that supply reference materials, although their certified values do not undergo the rigorous scrutiny that NIST undertakes. The NIST SRMs useful for elementzJ aneJysis Eire listed in Table 7. As can clearly be seen, it would be highly desirable to have additioneJ SRMs available for other analyses, particularly for additive elements in adpacks and lubricating oils. NIST is currently engaged in preparing several other materials for certification. Another source for reference materials is from the industry round robins such as the Inter-Laboratory Cross-Check Programs (ILCP) conducted by Committee D2. About 1800 laboratories worldwide take part in these thrice-a-year crosschecks. At present, 16 materials are used in these cross-checks: engine oil lubricants, #2 diesel fuel, aviation turbine fuel, motor gasoline, #6 fuel oil, automatic transmission fluid, gear oil, automotive lubricants additives, industrial gear oil, turbine oil, reformulated gcisoline, base oil, petroleum wcix, crude oil, inservice diesel oil, and lubricating grccise. However, the use of
these aneJyzed samples as reference materials need to be considered with caution. These are consensus values and not certified values. Experience in these programs has shown that a large degree of uncertainty is associated with the calculated mccin vjJues. Hence, such materials are better suited EIS quality control materials rather than as primary reference standards. ASTM Proficiency Testing A n u m b e r of cross-checks Eire conducted by the coordinating Subcommittee 92 on a four-month cycle. This program has been extremely popular worldwide providing the participating laboratories with a measure as to how well they are performing against their counterparts in the oil industry as well as against the precision stated in the ASTM test methods. A large n u m b e r of Subcommittee 3 test methods are utilized in these exercises (Table 8). The reproducibilities obtained in these programs have genercdly been poorer than those specified in the ASTM test methods. With recent changes in the statistical calculations to delete the outliers, the precisions have improved and are somewhat consistent with the published reproducibilities. To help the laboratories improve their test precision, Subcommittee 3 is adding non-mandatory "helpful hints to the analyst" sections a n d mandatory quality control sections to the test methods under its jurisdiction. International Test M e t h o d s Harmonization With the increasing globalization of commerce in the petrochemicals industry, many national standeirds bodies are heir-
CHAPTER 26: ELEMENTAL ANALYSIS monizing the test methods usage in different countries. Many companies use alternate test methods in their product specifications to ensure that the best methods quoted are viable in the countries where the products are sold. In such instances, it is vital that the a h e m a t e method designations quoted produce equivalent results. Table 9 lists the test methods for elemental analysis issued by six leading national standards organizations: ASTM for U.S.A., IP for U.K., DIN for Germany, ISO, JIS for Japan, and AFNOR for France. This table is excerpted from a m u c h larger table of equivalent test methods available in Ref. 32 by Nadkami. Clearly, m u c h more work on test method harmonization is necessary when Tables 3 emd 9 Eire compared.
715
Future Developments A n u m b e r of new test methods for elemental analyses are being developed in the D02 Subcommittee 3. Many are for the determination of trace elements and some for specific analysis in a specific matrix. Some of the work underway at the industry request includes: • lead, m a n g a n e s e , p h o s p h o r u s , and silicon in n a p h t h a , gasoline, and light petroleum products by ICP-AES; • phosphorus in reformulated gasoline by WDXRF; • sulfur and trace metals in petroleum coke by XRF; • chlorine in gasohol; • hydrogen by pulsed NMR;
TABLE 8—Subcommittee 3 test methods used in ASTM D2 Interlaboratory Cross-check programs. Product Engine Oil Lubricants #2 Diesel Fuel Aviation Turbine Jet A Motor Gasoline #6 Fuel Oil ATF Automotive Lubricant Additives Reformulated Gasoline Gear Oil Base Oil Crude Oil In-service Diesel Engine Lubricating Oil
Analysis (Test Method) Additive elements (D 4628; D 4927; D 4951; D 5185); Ash (D 482); Ash, sulfated (D 874); nitrogen (D 3228, D 4629; D 5291, D 5762); sulfur (D 129, D 1552, D 5453). Ash (D 482); sulfiir (D 129, D 2622, D 4294, D 5453); C-H-N (D 5291) Mercaptan sulfur (D 3227); sulfur (D 1266, D 1552, D 2622, D 4045, D 4294, D 5453) Lead (D 3237); phosphorus (D 3231); sulfur (D 1266, D 2622, D 3120, D 4294, D 5453) Al and Si (D 5184); Ash (D 482); Nitrogen (D 3228, D 4629, D 5291, D 5762); sulfur (D 1552, D 2622, D 4294, D 5453); V, Ni, Fe, Na (D 1548, D 5708, D 5863) Additive Elements (D 4628, D 4927, D 4951, D 5185); Nitrogen (D3228, D 4629, D 5291, D 5762); Sulhxr (D 1552) Additive Elements (D 4628, D 4927, D 4951, D 5185); Ash (D 482); Ash sulfated (D 874); Nitrogen (D 3228, D 4629, D 5291, D 5762); sulhir(D1552) Sulhir (D 2622, D 3120, D 4294, D 5453) Additive Elements (D 4628, D 4951, D5185); Ash sulfated (D 874); sulfur (D 4294). Nitrogen (D 4629, D 5291); Sulfur (D 2622, D 4294). Ni-V-Fe (D 5708, D 5863); Nitrogen (D 4629, D 5762); Sulfur (D2622, D 4294) Additive Elements (D 5185, D 6595)
TABLE 9 --International equivalent test methods for elemental analysis of petroleum products." Analysis
ASTM (D)
IP
DIN (51-)
Ash Lead-AAS ICl Volumetric WDX Metals-AAS ICP XRF Nitrogen-Kjeldahl Chemilum Phosphorus in Lubes Si/Al in Fuels Sulfated Ash Sulfur-Bomb EDX High Temp. Lamp Mercaptan Microculometry Oxidative microculometry WDX Wickbold Vanadium V, Ni by AAS
482 3237 3341 2547 2599 4628 4951 4927 3228 4629 4047 5184 874 129 4294 1552 1266 3227 3120 3246 2622 2785 1548 3605
4 428 270 248 228 308 437 407
575
6245
EN 237 2083 769T2 391
EN 237
" Excerpted from Ref. 32.
379 149 377 163 61 336
ISO
3830
JIS (K-)
AFNOR
2272
M07-045 EN 237
M07-014 2255
M07-082
2609 2609
M07-058
391
790 575
4265 10478 3987
2272
577
107 342
8754 3012 16591
2276
373 400T6
14596 4260
790T3
8691
243
M07-025 M07-031 M07-022 M07-052 T060-142
2541 413
T60-143 T60-109
M07-027
716 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK • • • •
mercury in crude oil; silicon on gasoline-naptha by by ICP-AES; metals in middle distillate fuels by ICP-AES; on-line sulfur determination in gasoline and diesel by XRF.
Quality
Control
A good quality control or quality assurance program is essential in obtaining reliable data. It has, however, been seen that many laboratories do not incorporate this step in their analytical sequence. To help the laboratories improve their performance, a nonmandatory section on quality control is being added to most of the applicable Subcommittee 3 test methods. It is hoped that this will encourage the laboratories to institute quality control practices as an integral and pivotal part of their testing protocols. Concluding Remarks Most of the test methods used for the elemental analysis of petroleum products are based on m a t u r e analytical techniques a n d i n s t r u m e n t a t i o n . No one technique can be a panacea for analyzing all elements in all materials. An ideal analytical technique should have high sensitivity, broad linear dynamic range, high precision, no matrix interferences, m i n i m u m sample preparation, and be inexpensive to acquire and operate, useful for all elements, simple to operate, rapid, nondestructive, etc. [31]. Few, if any, of the methods can meet all these criteria. The analyst has to make a wise choice as to the best available technique for a specific analysis in a particular matrix.
REFERENCES [1] Role of Trace Metals in Petroleum, T. F. Yen, Ed., Ann Arbor Science Publishers, Ann Arbor, MI, 1975. [2] Nadkami, R. A., "Overview," Modem Instrumental Methods of Elemental Analysis of Petroleum Products and Lubricants, ASTM STP 1109, R. A. Nadkami, Ed., ASTM International, West Consiiohocken, PA, 1991, p. 1. [3] Significance of Tests for Petroleum Products, ASTM STP 7C, K. Boldt, and B. R. Hall, Eds. ASTM International, West Conshohocken, PA, 1977. [4] Manual on Significance of Tests for Petroleum Products, ASTM MNLl, G. V. Dyroff, Ed. ASTM International, West Conshohocken, PA, 1989. [5] Modern Instrumental Methods ofElemental Analysis of Petroleum Products and Lubricants, ASTM STP 1109, R. A. Nadkami, Ed., ASTM International, West Conshohocken, PA, 1991, p. 19. [6] Nadkami, R. A., Ledesma, R. R., and Via, G. H., "Sulfated Ash Test Method: Limitations of Reliability and Reproducibility," SAE Technical Paper No. 952548, Society of Automotive Engineers, Warrendale, PA, 1995. [7] Eisentraut, K. J., Newman, R. W., Saba, C. S., Kauffman, R. E., and Rhine, W. E., Analytical Chemistry, Vol. 56, 1984, p. 1086A. [8] Niu, W., Haring, R., and Newman, R., American Laboratory, Vol. 19, No. 11, 1987, pp. 40. [9] Carter, J. M., Batie, W., and Bemhard, A. E., Modem Instrumental Methods of Elemental Analysis of Petroleum Products and
Lubricants, ASTM STP 1109, R. A. Nadkami, Ed., ASTM International, West Conshohocken, PA, 1991, p. 70. [10] Lukas, M., and Anderson, D. P., Modem Instrumental Methods of Elemental Analysis of Petroleum. Products and Lubricants, ASTM STP 1109, R. A. Nadkarni, Ed., ASTM Intemational, West Conshohocken, PA, 1991, p. 83. [11] Nygaard, D., Bulman, P., and Alavosus, T., Modem Instrumental Methods of Elemental Analysis of Petroleum Products and Lubricants, ASTM STP 1109, R. A. Nadkarni, Ed., ASTM International, West Conshohocken, PA, 1991, p. 77. [12] Williams, M. C, Modem Instrumental Methods of Elemental Analysis of Petroleum Products and Lubricants, ASTM STP 1109, R. A. Nadkami, Ed., ASTM Intemational, West Conshohocken, PA, 1991, p. 96. [13] Kauffman, R. E., Saba, C. S., Rhine, W. E., and Eisentraut, K. J., Analytical Chemistry, Vol. 54, 1982, p. 975. [14] Sychra, V., Lang, I., and Sebor, G., Progress in Analytical Atomic Spectroscopy, Vol. 4, 1981, p. 341. [15] Noble, D., Analytical Chemistry, Vol. 66, No. 2, 1994, p. 105A. [16] Bansal, J. G. and McElroy, F. C, "Accurate Elemental Analysis of Multigrade Lubricating Oils by ICP Method: Effect of Viscosity Modifiers," SAE Technical Series Paper 932694, Society for Automotive Engineers, Warrendale, PA, 1993. [17] Jansen, E. B. M., Knipscheer, J. H., and Nagtegaal, M., Journal of Analytical Atomic Spectroscopy, Vol. 7, 1992, p. 127. [18] Mackey, J. R., Watt, S. T., Cardy, C. A., Smith, S. I., and Meunier, C. A., Modem Instrumental Methods of Elemental Analysis of Petroleum Products and Lubricants, ASTM STP 1109, R. A. Nadkami, Ed., ASTM Intemational, West Conshohocken, PA, 1991, p. 62. [19] Gonzales, M., and Lynch, A. W., Modem Instrumental Methods of Elemental Analysis of Petroleum Products and Lubricants, ASTM STP 1109, R. A. Nadkami, Ed., ASTM Intemational, West Conshohocken, PA, 1991, pp. 62. [20] Hausler, D. and Carlson, R., Spectrochemica Acta Reviews, Vol. 14, 1991, p. 125. [21] Botto, R. I., Spectrochimica Acta Reviews, Vol. 14, 1991, p. 141. [22] Botto, R. I., Journal of Analytical Atomic Spectrometry, Vol. 8, 1993, p. 51. [23] Botto, R. I. and Zhu, J. J., Journal of Analytical Atomic Spectrometry, Vol. 11, 1996, p. 675. [24] Al-Swaidan, H. M., Analytical Letters. Vol. 21, 1988, p. 1487. [25] McElroy, F. C, Mennito, A., Debrah, E., and Thomas, R., Spectroscopy, Vol. 13, No. 2, 1998, p. 42. [26] Newman, A., Analytical Chemistry, Vol. 69, 1997, p. 493A. [27] Sieber, J. R., Salmon, S. G., and Williams, M. C, Modem Instrumental Methods of Elemental Analysis of Petroleum Products and Lubricants, ASTM STP 1109, R. A. Nadkami, Ed., ASTM Intemational, West Conshohocken, PA, 1991, p. 118. [28] Shay, J. Y. and Woodward, P. W., Modem Instrumental Methods of Elemental Analysis of Petroleum Products and Lubricants, ASTM STP 1109, R. A. Nadkami, Ed., ASTM Intemational, West Conshohocken, PA, 1991, pp. 128. [29] Wheeler, B. D., Modem Instrumental Methods of Elemental Analysis of Petroleum Products and Lubricants, ASTM STP 1109, R. A. Nadkami, Ed., ASTM Intemational, West Conshohocken, PA, 1991, p. 136. [30] Vrebos, B., Spectroscopy, Vol. 12, No. 6, 1997, p. 54. [31] Hieftje, G. M., Journal of Analytical and Atomic Spectrometry, Vol.4, 1989, p. 117. [32] Nadkami, R. A., Guide to ASTM Test Methods for the Analysis of Petroleum Products and Lubricants, Manual 44, ASTM Intemational, West Conshohocken, PA, 2000.
MNL37-EB/Jun. 2003
Diesel Fuel Combustion Characteristics Thomas W. Ryan III^
the current procedures are needed. An alternative to the current method for rating the ignition quality of diesel fuel is presented and described.
HISTORICALLY, FUELS HAVE BEEN MADE TO MEET THE REQUIRE-
MENTS of specific engine tj^es. This tailoring of the fuels has been accomplished in an iterative process, in which problems are uncovered in the field and efforts are made to resolve the problems through a process of specification and qualification. An example of this process is the development of the currently used "cetane number"(CN). Cetane n u m b e r is the measured parameter defined to provide a rating of the ignition quality of fuels for diesel engines. As refinery processes became more complex and focused on the production of gasoline in the 1920s, and as the diesel engine gained acceptance, the quantities of high quality, straight run diesel fuel became limited. The fuels that were available in that time period demonstrated problems with cold starting and low temperature white smoke. As will be described in more detail in the next section, industry groups were formed to address these and other fuel related issues. The mechanism for defining specifications for fuels has evolved through the efforts of organizations such as the Coordinating Research Council (CRC), the American Petroleum Institute (API), the Society of Automotive Engineers (SAE), the American Society of MechaniccJ Engineers (ASME), and ASTM International (ASTM). In more recent times, government environmental regulatory agencies have taken a very active role in defining limits on the fuel properties that affect the exhaust emissions. The basic mechanism consists of the definition of standard fuel rating test procedures and the compilation of fuel specifications, which define the allowable limits of the variety of fuel properties controlled by the specifications. The development of specifications has been based on a wealth of empirical data and, as indicated above, usually in response to specific problems. The specifications of the fuels for the various types of heat engines caji generally be considered in terms of those properties, which affect, or reflect, fuel stability and handling, contamination, c o m b u s t i o n characteristics, and emissions. ASTM D 975 is an example of a standard established for diesel fuel. This specification is summarized in Table 1 and generally reflects concerns for fuel stability, and engine durability and performance. Those properties that impact the engine emissions are discussed in another section of this manual. The focus of this chapter is identification and discussion of the issues related to the fuel properties that affect the ignition and combustion characteristics of fuels for diesel engines. It will become clear from the discussion that modifications of ' Institute Engineer, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78228.
ENGINE COMBUSTION AND EMISSIONS Combustion in any heat engine can be divided into three primary phases: (1) Formation of the appropriate mixture of fuel and air, (2) ignition, and (3) completion of combustion. In a diesel engine, the fuel is injected into air that has been compressed to high pressure and temperature. Some of the fuel from the incoming spray jet vaporizes and mixes prior to ignition, forming a pre-mixed fraction of the fuel that ignites and b u m s as a premixed flame. The remaining fuel is ignited in the jet and b u m s as a diffusion flame, ideally at the same rate as it is injected. The three processes are described in more detail in the following subsections.
FUEL INJECTION Combustion in a diesel engine occurs when fuel is injected at very high pressure (up to 200 MPa) through a small orifice or multiple orifices in the injection nozzle into the combustion chamber, that contains air that has been compressed to high pressure and temperature. The high-pressure injection process results in a break-up of the fuel injection jets into small droplets due to the shear forces induced between the high velocity jets and the relatively quiescent air in the combustion chamber. The fuel droplets, consisting of drops in sizes ranging from approximately ten micrometers to several hundred micrometers, traverse the combustion chamber at high velocity. The droplets go through a process of heating and evaporation due to heat transfer from the hot air, and deceleration due to aerodynamic drag. The evaporation process leads to a preferential disappearance of the small droplets and rapid mixing of the vaporized fuel with the air resulting in the formation of a very fuel-rich mixture in the tip of the fuel jets. The challenge for diesel engine designers is to match the combustion chamber size and shape with the characteristics of the fuel injection spray jets. The fuel jets must traverse the combustion chamber in order to reach the air in all parts of the combustion chamber. At the same time, the fuel must vaporize, mix with the air, and start to react. If the fuel jet penetrates too far, the fuel interacts with the wall, resulting in degraded mixing, low temperature combustion on the walls, and high u n b u m e d hydrocarbon and smoke emissions. If the
717 Copyright'
2003 by A S I M International
www.astm.org
718 MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
TABLE 1—Specification properties for various fuel oils. Property
Flash, C, min Water, v% T90, C m a x K. Vis.,(min) mm^/s@40C K.Vis.,(max) mm^/s@40C Ash, %inass,inax Copper Corrosion Cetane Number, m i n Cetane Index Aromatics, % vol, max Cloud Pt, C, max Sulfur, % mass, max Ramsbottom Carbon on 10% residue, mass %
Method
Low S No. 1
SNo.2
No. 1
No. 2
No. 4
D93 D1796 D86 D445 D445 D482 D130 D613 D976 D1319 D2500 D2622 D524
38 0.05 288 1.3 2.4 0.01 3 40 40 35 6 0.05 0.15
52 0.05 338 1.9 4.1 0.01 3 40 40 35 6 0.05 0.35
38 0.05 288 1.3 2.4 0.01 3 40
52 0.05 338 1.9 4.1 0.01 3 40
55 0.05
6
6
0.15
0.35
fuel vaporizes and mixes too close to the nozzle, the mixture will be overly rich, leading to high unbumed hydrocarbons and smoke emissions. The fuel properties that have the greatest effect on injection include viscosity, density, and surface tension. Viscosity, a measure of the fuel's resistance to flow shear, impacts the fuel spray characteristics through flow resistance inside the injection system and in the nozzle holes. Higher viscosity generally results in reduced flow rates for equal injection pressure and degraded atomization. Diesel fuel injection systems are designed to precisely meter the required volume of fuel into the combustion chamber during each appropriate part of the compression portion of the combustion cycle of the engine operation. All diesel injection systems meter the fuel on a volume basis, so that fuel density affects the mass of fuel injected. Increased density beyond specification results in higher than designed fuel injection rates due to the direct relationship between mass, volume, and density. Surface tension, or the tendency of the fuel to adhere to itself at the fuel-air interface, affects the tendency of the fuel to form drops at the jet-air interface. Increased surface tension tends to degrade atomization rates. It should be noted, however, that the surface tension of most hydrocarbons is very similar one to another. Based on this fact, surface tension does not play a first order role in the jet break-up and atomization process, at least on a comparative basis.
Low
5.5 24.0 0.01 30 6
ing to ignition. In reality, these processes are occurring simultaneously. Likewise, a very simplistic view of chemical aspects of ignition is based on achievement of the fuel's autoignition temperature. This can be measured following ASTM E 659, as described in Chapter 37. In reality, ignition is a very complex chemical process that involves pyrolysis and oxidation reactions that are kinetically controlled, with an exponential dependence on the local temperatures and a nonlinear relationship with pressure. Diesel fuel ignition characteristics are very important for several reasons. First, they affect the ability of the fuel to ignite during cold-start. In the limiting case, if the fuel resists autoignition at the highest achievable compression temperature it is impossible to start the engine. Second, the ignition characteristics are important because they affect the quantity of fuel injected during the ignition delay time. If the ignition delay time is long, a large quantity of injected fuel becomes premixed prior to the start of combustion. When combustion does occur, however, most of the premixed fuel ignites at the same time, leading to very high rates of combustion, high rates of pressure rise, and "diesel knock." Third, if the ignition delay time is long, the resulting premixed burning leads to higher combustion temperatures and increased oxides of nitrogen (NOx) exhaust emissions. The mechanism for this NOx formation process is described below.
COMBUSTION IGNITION Dec [1] theorizes that ignition occurs in the rich region at the tip of the fuel jet, when the fuel's vapors achieve temperatures sufficient for decomposition and oxidation. In a diesel engine, the fuel does not ignite instantaneously. The time elapsed from injection to ignition is known as the ignition delay time. The ignition delay time occurs because a certain amount of time is required for the fuel to vaporize and ignite. A very simplistic view of the ignition process separates the physical and chemical aspects of the ignition process. The time necessary for the fuel to vaporize and mix is known as the physical delay, while the chemical delay is the time required for the onset of chemical reactions lead-
The combustion process in diesel engines can be monitored by measuring the pressure in the combustion chamber during the compression and combustion processes. It is possible to apply the First Law of Thermodynamics to the diesel engine in order to obtain an estimate of the instantaneous rates of heat release during combustion. The First Law for a closed, constant mass system is dQ/dt + dW/dt = dU/dt where. Q = heat transfer W = work U = internal energy
(1)
CHAPTER
27: DIESEL
(2)
dU/dt = m Cv dT/dt
(3)
and,
where, m = mass trapped in the cyUnder (air + fuel mass) Cv = specific heat at constant volume of mixture T = temperature in the combustion chamber. Also it is assumed that, (4)
dW/dt = P dV
CHARACTERISTICS
719
Figure 1 is a heat release rate diagram for an engine condition in which there is a fairly large premixed b u m fraction. Also plotted in Fig. 1 is the injection pressure as it was measured simultaneously with the cylinder pressure. The inflection in the injection pressure at 178° indicates the point in time when the injection nozzle valve opens and fuel starts to be injected into the cylinder. This is noted as the start of injection. Also indicated are the premixed and the diffusion b u r n phases, or fractions. The time from the start of injection to the start of the premixed b u m phase is called the ignition delay time. Note that during this delay period a significant quantity of fuel is injected, vaporized, and mixed with the hot air in the cylinder. When ignition does occur, this premixed fuel ignites spontaneously.
If it is assumed that the combustion process can be treated as a heat transfer into the system, then Eq 1 can be rearranged dQ/dt = dU/dt - dW/dt
FUEL COMBUSTION
As shown in Fig. 1, the premixed b u m fraction typically has a very high heat release rate. As indicated above, this rapid release of energy results in an increase in the engine noise called "diesel knock." It is this almost instantaneous release of energy that raises the temperature in the cylinder early in the combustion process and leads to high combustion temperature throughout the rest of the combustion process. While this increase in the combustion t e m p e r a t u r e might be assumed to increase the overall cycle efficiency due to Camot consideration, it can also increase the heat transfer and it almost always increases the NOx emissions.
where. P = instantaneous pressure in the cylinder V = instantaneous volume in the cylinder. Assuming Ideal Gas behavior, PV=mRT or, dT/dt = (1/mR) (p dV/dt + V dp/dt) where. R = Gas Constant.
EMISSIONS
With appropriate substitution, Eq 2 becomes, dQ/dt = (Cv/R - 1) P dV/dt + (C^/R) V dP/dt
Combustion temperatures are important because they dominate the rate of formation of NOx emissions. NOx is important because it reacts with the unburned hydrocarbons in the atmosphere to form smog. Most of the NOx emissions result from a chemical kinetic process, known as the Zeldovich [3] Thermal NOx emissions mechanism, where,
(5)
It can be seen that the apparent heat release rate during combustion, dQ/dt, can be computed using Eq 5, the measured cylinder pressure, and using the geometric relationship between cylinder volume and crankangle. The computation is performed using cylinder pressure data that is resolved based on crankangle [3]. The pressure is measured using a high speed piezoelectric pressure transducer installed in one of the engine's cylinders. The pressure data is recorded simultaneously with a record of the crankangle position.
N2 + O = NO -h N N + O2 = NO -h O
The rate constants for the two reactions are exponential functions of the temperatures in the combustion chamber. These
20000
0.20
Heat release rate Injection pressure
u>
a
I<
tu
s X
0.15 -
15000 .S lU 10000
0.10
UJ
a
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z < Iz <
oc v>
0) 3 lU
- 5000
0.05
0.
o 111
"N .
0.00
Premixed •^-Diffusion-^
Start of Injection -0.05 1
90
1
n-
r
1
1
(6)
*T*—*—i
1
r
105 120 135 150 165 180 195 210 225 240 255 270 285
CRANK ANGLE [degrees; TDC=180] FIG. 1—Heat release rate diagram.
-5000
720
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
temperatures depend on the thermodjrnamic processes that occur during compression and combustion, as well as on the adiabatic flame temperature of the fuel-air mixture. The adiabatic flame temperature is a fundamental property of the fuel air mixture, depending on the initial pressure and temperature, the air-fuel ratio, and the fuel properties, including hydrogen, carbon, and oxygen content, and heat of combustion. The highest adiabatic flame temperature occurs in stoichiometric, or chemically balanced mixtures, using fuels with high carbon contents. The adiabatic flame temperature can be computed based on the above parameters using a n equilibrium calculation [3]. In a diesel engine, the pressure and temperature at the start of reaction are defined by the engine design and the operating condition. If the temperatures are high, the ignition delay times tend to be short, leading to a smaller premixed b u m phase and lower initial temperatures for the diffusion b u m phase. If the ignition delay times are long, a large quantity of fuel is injected during the delay time and the premixed b u m phase is larger, leading to higher temperatures at the start of the diffusion b u m phase. In a diffusion flame, the fuel actually b u m s at the location where the fuel and air mix to the chemically correct (stoichiometric) fuel-air ratio. The fuel b u m s at the stoichiometric adiabatic flame temperature. If there is a large quantity of fuel in the premixed b u m phase, the initial temperature for the diffusion b u m phase is high, leading to higher temperatures during the diffusion b u r n phase. M o d e m practice in diesel engine design is to minimize the premixed b u m fraction by controlling the initial rate of fuel injection and thereby minimizing the a m o u n t of fuel injected during the ignition delay time. Figure 2 is a plot of the heat release rate and injection pressure for an engine and associated operating condition in which the initial injection rate is controlled in order to minimize the amount of fuel injected during the ignition delay time. This is done by designing the injection system to provide a slow initial rate of injection, followed by a rapidly increasing rate once ignition occurs. This is demonstrated by the increasing injection pressure in Fig. 2,
which was measured in a 1994 Caterpillar 3176 engine. Note the very small premixed b u m phase for this engine and the fact that the injection pressure and the heat release rates have similar shapes. The correspondence of the injection and heat release rates indicates that the fuel is being burned as it is injected. In addition to NOx, the u n b u m e d hydrocarbons (HC) and particulate emissions (PM) from diesel engines are important, not only for their role in reducing efficiency and contributing to smog formation, but also from the direct health risk associated with these pollutants. The U.S. Environmental Protection Agency (EPA) and the California Air Resources Board [4] have defined diesel PM as an air toxin, recognizing the potential health risks associated with h u m a n exposure to diesel PM. Diesel PM is defined by regulatory bodies as anything collected on a specified filter that is maintained at 125°F. The collection procedure involves dilution of the exhaust sample with fresh air. The method has been defined by EPA to simulate what occurs when diesel exhaust is emitted into the atmosphere. Diesel PM is a complex mixture of soot, u n b u m e d fuel, and partially reacted fuel. The formation of u n b u m e d HC and PM is a result of the heterogeneous n a t u r e of the diesel c o m b u s t i o n process where fuel injection, vaporization, mixing, ignition, and combustion are all occurring nearly simultaneously in the engine. The HC emissions result from a combination of sources including fuel that is deposited on the combustion chamber walls and does not react, or only partially reacts. It also includes fuel that has mixed and is either too lean or too cold to react. It should be noted that the mass of HC emissions per BHP/hr from diesel engines are typically an order of magnitude lower than the engine out HC emissions from spark ignition engines. Soot is the product of diffusion burning where some of the fuel partially reacts, forming compounds that ultimately result in the growth of high molecular weight compounds that are very rich in carbon. Soot formation is a very complex chemical process that is affected by the physical processes
0.30 0) 0) •a
0.25
ID
0.20
B
25000
Heat release rate Injection pressure
0.10
z < 1-
0.05
< 1(/z>
0.00
z
a. 3 OT V) 10000 Ui
0.15
O
UJ
V
"5 15000
1X If) 3
20000
a. z 5000
_-^\-
0
-0.05
-5000 90
105 120 135 150 165 180 195 210 225 240 255 270 285
CRANK ANGLE [degrees; TDC=180] FIG. 2—Heat release rate diagram-small premixed burn phase.
O
H U lU -5 Z
CHAPTER 27: DIESEL FUEL COMBUSTION that occur during the fuel mixing and combustion processes. A simplistic view of the diesel diffusion burning process involves a relatively thin flame front formed at the interface between the fuel and air. In this view, the fuel is on one side of the flame and air on the other. The initial soot formation reactions include thermal break down (pyrolysis) of the fuel before it enters the flame. Under these conditions, the soot precursor reactions (leading to greater soot formation potential) favor high temperature and long residence times. More soot precursors are formed if the mixing rates are slow and the temperatures are high. Higher temperatures also lead to higher soot oxidation rates. The level of PM emissions from a given diesel engine is the result of the difference between the formation rate and the oxidation rate. In general, higher temperatures tend to lead to lower soot and PM emissions, due primarily to the effect of the increased temperature on the PM oxidation rate. The diesel engine designer faces a conflict between PM and NOx emissions. High temperatures are desirable for PM emission control, but they also lead to high NOx emissions. It is this trade-off that constitutes one of the primary concerns of modem diesel engine design. In general, current practice is to reduce the combustion temperatures to the highest level possible consistent with achievement of the NOx emissions standard. The PM emissions are controlled through engine combustion chamber and injection system design to prevent interactions of the fuel and combustion chamber wall. In addition, higher injection pressures are used to produce higher mixing rates that lead to lower residence times in the pyrolysis zone and thus lower soot formation rates. The effects of fuel properties and composition on soot formation were studied extensively by Naegeli and Moses [5]. They concluded that the primary controUing fuel property is the hydrogen content. This is demonstrated in Fig. 3 where the soot emissions from a diffusion flame are plotted versus the fuel hydrogen to carbon ratio. Fuel hydrogen and carbon contents are determined in the laboratory using ASTM D 5291. It is interesting to note that the fuel hydrogen content is the primary fuel variable affecting both NOx and PM. High hydrogen content (as H atoms) fuels have lower adiabatic flame temperatures and thus lower NOx formation tendencies. The NOx relationship to fuel hydrogen to carbon ratio is presented in Fig. 4, where the adiabatic flame temperatures and the predicted NOx emissions (Zeldovich Mechanism) are plotted versus fuel hydrogen to carbon ratio [6]. Higher hydrogen content fuels also tend to form less soot, and thus lower PM emissions.
1.5
1.75
1.85
Fuel H/C Ratio
FIG. 3—Soot concentration versus fuel H/C ratio.
2
CHARACTERISTICS
3
4
721
5
H/C (Atomic Ratio)
FIG. 4—Adiabatic flame temperature and NOx versus fuel hydrogen to carbon ratio.
D I E S E L F U E L I G N I T I O N QUALITY Current Practice (Cetane Number) Diesel fuel specifications currently include only two specified properties that are related directly to combustion. Heat of Combustion, and cetane number. Heat of Combustion is a fundamental property of the fuel, based on the Heat of Formation of the individual molecules. Its determination is fairly straightforward following ASTM D 240, which employs a bomb calorimeter. The procedure consists of completely burning a weighed sample of the unknown fuel in a combustion bomb that is contained in a controlled temperature bath. The Heat of Combustion is determined by measuring the temperature increase of the bath. Cetane number, on the other hand, as determined following ASTM D 613, is not a fundamental property of the fuel. Cetane number is a defined parameter designed to provide an indication of the ignition quality of diesel engine fuels. Higher cetane number means that the fuel has better ignition qUcJity than fuels with lower cetane numbers. Cetane number is determined in an engine test in which the ignition quality is rated versus those of blends of two reference fuels. The history and method of ASTM D 613 and cetane numbers are described in the following paragraphs. In 1932, Boerlage and Broeze [7] proposed that the ignition quality of a fuel be based on a comparison of its ignition delay time in a diesel engine to that of a blend of two reference fuels. They developed the "cetene scale" in which a fuel was assigned a "cetene number." The reference fuels were two pure hydrocarbons, cetene (C16H32) and mesitylene. Cetene burned readily in conventional engine, while mesitylene did not bum at all.
722 MANUAL 37: FUELS AND LUBRICANTS
nil
HANDBOOK
11 u
7//////1 1
PRE-CHAMBER
w/rr/rrz/rm
FIG. 5—Cross section of the CFR engine.
In 1935, ASTM adopted this form of diesel fuel rating system using hexadecane (C16H34) and alpha-methylnaphthalene ( d iH]o) as the reference fuels. The former was assigned a cetane number of 100, while the latter was given a cetane number of 0 [8]. In 1962, ASTM added heptamethylnonane, (C16H34) to the cetane scale as an intermediate, low ignition quality fuel with a defined cetane number of 15 [9]. The standard apparatus for determining and comparing ignition delay times is a Coordinated Fuels Research (CFR) diesel engine developed by the Waukesha Motor Company. It is a one-cylinder, four-stroke cycle engine with a cylindrical prechamber chamber design (see Fig. 5) and a compression ratio capability ranging from 6:1 to 28:1. The ASTM D 613 test procedure consists of running the test fuel at specified conditions of speed, load, and intake temperature. The injection timing is adjusted so that the start of injection is 13° Before Top Dead Center (BTDC). The compression ratio is adjusted until ignition occurs at TDC. The test fuel is then replaced with blends of the reference fuels until one is determined to have a slightly higher compression ratio and one with a slightly lower compression ratio than the test fuel. The cetane number of the test fuel is determined by linear interpolation of the cetane numbers of the reference fuels. As an example, typical U.S. diesel fuel has a cetane number of 45. The 45 cetane number reference fuel is a blend consisting of 64.7 vol.% hexadecane and 35.3 vol.% heptamethylnonane . Unfortunately, a number of problems are associated with using the CFR engine for evaluating ignition delay time and thus cetane number. It has been criticized for a variety of significant shortcomings. The primary complaint is a failure of cetane number to consistently provide an accurate measurement of ignition quality. The specific shortcomings of the current cetane procedure were extensively discussed during a CRC-hosted workshop, Diesel Fuel Combustion Performance, held in Atlanta, Georgia in 1984 [10]. It appears that the main
problem with the cetane procedure is related to the fact that neither the engine nor the test conditions are representative of current engine design or typical operating conditions. The current procedure, developed over 50 years ago, involves the use of the Waukesha engine. The prechamber has a movable end plate, which is used to change the volume of the prechamber and, thus, the compression ratio, as shown in Fig. 5. The specified operating conditions of the test are equivalent to a high-speed idle test, with the speed set at 900 rpm and the fuel flow set at 13 ml/min (equivalent to an air/fuel ratio of approximately 30:1). One of the more frequent complaints is the fact that the CFR engine design is not representative of current diesel engines. The engine design includes a cylindrical precombustion chamber. The fuel injector is located in one end of the prechamber and the other end is moveable and is used to vary the compression ratio. The motion of the prechamber end plate not only changes the compression ratio, but also changes the distance from the injection nozzle to the combustion chamber wall. Modern diesel engines, both lightduty and heavy-duty, utilize direct in-cylinder fuel injection. These engines have mixture preparation, surface condition, combustion chamber geometry, and thermodynamic condition effects that are significantly different than the prechamber CFR engine. This means the current cetane test engine and test method expose the fuel to temperature history, airfuel ratio, and surface effects that are different than those encountered in most actual engines in the field. Another complaint is that the test condition used in the test method is effectively a moderate load at very low speed. The test speed is 900 rpm and the air-fuel ratio is 30:1. This speed is much higher than cranking speed and thus does not represent cold start conditions. The speed is also much lower than normal operating speeds in most high-speed diesel engines, so that it is not representative of hot running conditions, and thus it is not a good indicator of the hot running characteristics of the fuel. The moderate load, represented by the 30:1 air/fuel ratio, does not incorporate intake pressures above ambient (not boosted) so that compression pressure histories are different than in modem turbocharged diesel engines. Some of the other reported problems are related to the cost and time required for the measurement [11]. Other problems are associated with the repeatability and reproducibility [11-13]. Accurate and repeatable determination of the cetane number of alternative and ignition-improved fuels, and fuels at the lower end of the cetane scale, are all problematic [12-15]. Many researchers are even questioning the validity of the cetane scale as an indicator of ignition quality [9,12,14,16-18]. The current test method has poor repeatability and reproducibility. LeBreton [12] conducted tests on the repeatability of the CFR engine results and found the standard deviation for the data to be 0.8 cetane numbers. However, the sample only contained fuels with cetane numbers between 45 and 50. Glavinceveski et al. [13] reported repeatability results for a set of 48 fuels as being 1.57 cetane numbers. The test resuks for these same fuels, performed in a number of engines, was calculated to be 4 cetane numbers. Once again, the majority of the fuels had cetane number ratings between 40 and 50, and none had cetane number below 37. Glavinceveski et al. [13] showed that the scatter of the cetane number rating
CHAPTER
2 7: DIESEL
method increases dramatically as the rating drops below 40 cetane number. The standard test method (ASTM D 613) is reported to have a repeatability of ± 1 CN. Indritz [19] examined data for a large n u m b e r of reference and test fuels, tested in a n u m b e r of test engines. The results of his comparisons are presented in Fig. 6, where the cetane numbers are plotted versus compression ratio. Two observations can be made regarding the results. First, it is clear that there is not a universal relationship between the cetctne n u m b e r and the compression ratio. It is likely that the lack of a universal relationship between cetcine n u m b e r a n d compression ratio results from day-to-day and engine-to-engine differences in the combustion. Injection nozzle performance, combustion chamber deposits, variation in compression ratio calibration, a n d variations in the injection p u m p pressure and calibration all contribute to the differences. Second, the variation in the measurements is on the order of +/— 5 CN. An error of 5 CN Ccin mean the difference between starting or not starting in cold weather, depending u p o n the specifics of the engine design. Much of the research into ignition quality has centered on how the ignition delay time a n d cetane n u m b e r relate to the conditions under which the tests are conducted. Yu et al. [20], in experiments performed in a specifically designed research engine, conducted duplicate tests in which combustion occurred every other cycle. They found that the peak of the difference in the pressure traces of the fired a n d unfired cycles increased with decreasing ignition delay time. Tsao et cd. [21] found that delay times increased with decreasing air temperature and engine speed. Hardenberg and Hase [22] reported the delay times decreased with increasing compression pressure a n d air temperature. Finally, Parker et al. [23] and Walsh and Cheng [24] showed the ignition delay time can be decreased by increasing the fuel temperature at injection. Other studies of the ignition delay time have focused on differentiating the physical and chemical aspects of the ignition delay time. The physical aspects include the atomization, mixing rates, a n d vaporization rates of the fuel. These physical aspects are related to the fuel properties of viscosity, density, and distillation characteristics. The chemical aspects relate mostly to the chemical composition of the fuel and the corresponding impacts on the chemical kinetics of the thermal decomposition and free radical generation mechanisms. 820
•80
880
WOO
t040
1080
CAtCULATB) ISENTROPIC TEMPERATURE - K
fS
FUEL COMBUSTION
CHARACTERISTICS
723
Elliott [25] noticed the ignition delay time decreased with lower fuel/air ratio. In a more comprehensive study of physiCcil delay time, Wakil et al. [26] drew the same conclusion and showed that it was caused by the relative spacing of the fuel droplets. They also found that physical delay times increased with droplet size a n d fuel boiling point due to vaporization characteristics. Rao a n d Lefebvre [27] also determined that the physical delay time is always a significant pEirt of ignition delay. In a study of chemiccd delay times, Chang et al. [28] found that the rate of reaction of high-cetane-number fuels increases faster after ignition t h a n t h a t of low-cetanen u m b e r fuels. Cox a n d Cole [29] studied the chemical kinetics involved in chemical delay time and demonstrated a large increase in delay time with decreasing oxygen concentration of the air/fuel mixture. There have been a n u m b e r of studies concerning the effects on cetane number caused by blending alternative fuels with diesel fuel ASTM #2 (DF-2). Saeed a n d Henein [30] found that the addition of 10 vol% ethanol in diesel fuel caused only a slight decrease in cetane number. However, they noted a drastic decrease in cetane n u m b e r as the amount of ethanol in t h e blend increased from 20-70 vol%. Henein a n d Fragoulis [9] studied the effects of blending a number of alternative fuels with DF-2. They found that blends containing indolene, unleaded gasoline, and No. 6 fuel oil each produced a drastic d r o p i n cetane n u m b e r . Blends of DF-2 with m e d i u m n a p h t h a a n d Jet A fuel p r o d u c e d very small decreases in cetane number, while a blend with No. 4 fuel oil caused the cetcine n u m b e r to increase slightly. Dabovisek and Savery [31] concluded that the ignition delay a n d therefore cetane n u m b e r of a blend of two fuels is controlled by the component with the greatest autoignition resistance, or lowest cetane number. Needham a n d Doyle [14] determined cetane number for synthetic and alternative fuels. They conducted studies of the cetane number of blends containing naphtha, sunflower oil, sunflower oil ester, shale oil, coal synthetic liquids, and tar sands. They found that ASTM D 613 for the vegetable oil Eind the blends of naphtha and methanol with DF-2 did not accurately predict the ignition delay. Finally, Siebers [18] conducted constant volume combustion b o m b tests on blends of naphtha Etnd coal derived liquids with DF-2, and on a degummed sunflower oil, a sunflower oil monoester, and methanol to determine how the delay times varied with temperature in the bomb. All fuels behaved similarly t o reference fuels w i t h the exception of methanol, for which the delay time increased dramaticcdly as the temperature decreased, presumably due to the unique chemical structure of methanol.
•
P r o p o s e d Alternatives to ASTM D 6 1 3
I
Due to the issues discussed above, including the time Eind expense required to conduct D 613 cetane n u m b e r determinations, a n u m b e r of edtemative methods have been proposed. The proposed methods have included numerous correlations with other physical a n d chemical properties, constantvolume combustion b o m b based methods, and correlative techniques based on NMR and FTIR analysis of the test fuel. Hardenberg and Hase [22] tried to use activation energy as an indicator of cetane number. Collins a n d Unzelman [32] used API gravity a n d mid-point temperature. Klopfenstein
R • Rvtaranca FtMl 0 " Unknown Furt
"^n^^ -1HU_^, •I 12
13
M
W
It
17
COMPRESSION RATIO
FIG. 6—Scatter plot of Cetane measurements.
724
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
[33] correlated density and mid-boiling point temperature with cetane number, while Murphy [34] included percent hydrogen. Glavincevski et al. [13] tried to use aromatics to predict cetane number, and Steere [35] developed a correlation between cetane number, aniline point, viscosity, density, and D 86 distillation. Cetane index (ASTM D 976) involves correlations that are modified periodically, and that incorporate a number of different properties. While all of the correlations provide reasonable predictions of the CN of fuels that are similcir to those used to develop the correlations, they are not reliable for prediction of cetane numbers of fuels that are different than the fuels used to develop the coirelations. Typically, these correlations must be modified periodically to accommodate changes in feedstock source and refinery processing technology. In addition, the accuracy of the correlation is limited to the precision of ASTM D 613 test method and the number of engine tests used to obtain an average cetane number for an individual fuel sample. Nuclear magnetic resonance (NMR) has been used to predict cetane number [10,36,37]. Bailey et al. [37] used the relative quantities of methine and methylene hydrogen to predict the cetane numbers of non-aromatics. They also developed a model for aromatic hydrocarbon fuels, based on the relative quantity of alpha hydrogen and relative squared sum of alkyl hydrogen along with the methine and methylene hydrogen. It was reported that the deviations from perfect correlation with D 613 cetane number were most likely due to scatter in the D 613 results. Fodor [38] reported some promising results using Fourier Transform Infrared (FTIR) analysis of the fuel. In these techniques the fuel sample is analyzed using FTIR and the resulting spectra are used to develop correlations with the cetane number. While the techniques are generally very fast and require very small fuel samples, the methods are based on correlations and are unstable when used outside the range of properties/specifications of the fuels used to develop the correlation. Again, as stated above, the accuracy of the correlation is related to the accuracy and number of ASTM D 613 engine tests. Another significant issue with the current cetane rating scale is related to a concern over whether cetane number provides an accurate and representative indication of the ignition quality of all diesel fuels. Ignition quality in a diesel engine is important for cold start, for engine noise, and can be related to both NOx and PM. Tavacha and Cliffe [17] noted that cetane number is a measure of the ignition temperature, not ignition quality. Needham and Doyle [14] found that ignition delay is not the controlling factor in determining overall performance. They recommended development of a new rating method. Sieber's [18] results showed that cetane number does not provide an accurate measure of ignition quality of fuels whose ignition delay dependence on temperature, or compression ratio, or the type of ignition (single two stage) differs from the reference fuels. Oilman et al. [39-40] studied the effects of fuel properties on engine emissions. The tests, performed in a 1988 Detroit Diesel Series 60 engine calibrated to near 1991 emissions levels, indicated that both the NOx and the PM were related to the cetane number. The Series 60 is a direct injection, turbocharged, intercooled diesel engine. The CFR engine is an indirect injection, naturally aspirated engine. Recent results
by Ryan et al. [41], however, indicate that cetane number is not related to emissions from modem heavy-duty diesel engines operating at standard ambient conditions. Analysis of the heat release rates indicate that the combustion process in these modem diesel engines is dominated by diffusion burning, with the combustion rate controlled by the rate of fuel injection. Under these circumstances, cetane number, as a measure of the ignition delay time, is not important for typical diesel fuels because the start of combustion is controlled by the conditions in the engine and the design of the injection system. Injection system designs that allow for control of the rate of fuel injection can reduce the amount of fuel injected during the ignition delay time, and thus reduce the dependence on the ignition quality of the fuel, as discussed previously. It is clear that the current procedure for rating the ignition quality of diesel fuel is no longer adequate. First, it probably does not represent the sensitivity of current and future engines as indicated by the work of Ryan et al. [41], and second, because it is not as repeatable or reproducible as it needs to be for use in current emd older engines. If the results of Indritz [19] are accepted, a variation of +/— 5 cetane numbers is possible. Using the results of Ullman et al. [39,40] a 5 cetane number variation can produce as much as a 4% difference in the NOx emissions and as much as a 13% variation in the PM emissions from older and current engines. Ryan et al. [42-45] developed a constant volume combustion bomb based technique (CVCA) for rating the ignition quality of fuels for diesel engines. The primary objective of these efforts was to develop a technique that is based on measurement of the ignition delay time, a fundamental combustion property of the fuel. The goal was to eliminate the engine sensitivity inherent in the ASTM D 613 technique. A great deal of effort was devoted to developing a technique based on a calibration using the defining reference fuel rather than a correlation. Several hundred fuels were examined using this technique. The test fuels included petroleum based diesel fuels, various refinery products, coal slurries, various oxygenated fuels, blends of various fuels and fuel components, a wide range of cetane improver additives, and a large number of alternative fuels. The apparatus and technique, now called the IQT (Ignition Quality Tester), has been further refined and developed by Allard et al. (45-47) and is currently being examined for approval as an ASTM test method. Based upon the above discussion, it appears that the engine-based technique for diesel-fuel ignition quality rating is not acceptable for the numerous reasons listed. It is also clear that the other noncombustion-based techniques (FTIR, NMR, Property Correlation) are also not acceptable because they involve correlations that are valid only for the type of fuels used to develop the correlations. It appears that the best approach is one in which a fundamental combustion property, such as ignition delay measured in a well-defined experiment, is used as the basis for the rating. It appeeirs that it is also desirable to use a technique that is based on a calibration rather than one based on a correlation. CVCA/IQT Method As indicated above, the IQT method is based on the use of the CVCA technique developed by Ryan et al. [42-45]. This tech-
CHAPTER 27: DIESEL FUEL COMBUSTION nique was originally suggested by Hum et al. [ 15] and Yu et al. [14]. In this method, the fuel sample is injected into a constantvolume combustion chamber, which contains air at a pre-selected elevated temperature and pressure. The constant-volume combustion chamber is equipped with a pressure transducer at one end and an inward opening pintle nozzle at the other end. A proximeter is installed on the injection nozzle for recording the start of injection. The injection nozzle is supplied with fuel from a pneumatically driven, single-plunger fuel injection pump. The initial conditions in the bomb, the quantity of fuel injected, and the fuel injection characteristics are all precisely controlled. Figure 7 is a schematic showing the internal geometry of the constant-volume combustion chamber. Figure 8 is a photograph of the IQT.
CHARACTERISTICS
725
The test method consists of setting the initial conditions in the bomb, followed by injection of the fuel sample and recording of the pressure history during injection, ignition, and combustion. The needle lift trace, using the output of the proximeter on the injection nozzle, is used to document the start of injection. The needle lift data and the pressure variation in the vessel are recorded and used to determine the time from the start of injection to the start of combustion. This time has been defined as the ignition delay time. Figure 9 is a pressure trace showing actual data generated during a test in the IQT. As can be seen in the figure, the pressure in the bomb initially drops due to evaporative cooling of the injected fuel. The pressure then rises rapidly due to combustion. The igni-
Inlet Tc Chamber Surface
Tc High Temperature Policeman
Injection Nozzle Body
Tc Nozzle Tip
Exhaust
FIG. 7—Schematic of the bomb geometry used in the IQT.
3
4
5
6
7
9
10
Ignition Delay (mS) FIG. 8—Photograph of the IQT.
FIG. 9—Pressure versus time in the IQT during ignition and combustion.
726
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
tion delay time is precisely defined by the initial rise of the needle lift trace and the point at which the pressure recovers back to the initial pressure. The IQT is a calibrated technique, meaning that the cetane number-ignition delay time relationship is defined by calibration with a large n u m b e r of reference fuel blends. Initially, the calibration curves were developed using blends of the secondctry reference fuels, but recent results [48] indicate that the use of ASTM D 613 National Exchange Group (NEG) checks fuels provide for a more consistent and cost effective calibration. Figure 10 is a plot showing the calibration curve
for the IQT, where the cetane n u m b e r is plotted versus the ignition delay time. The test method consists of charging the fuel reservoir on the injection p u m p with the unknown fuel. This requires approximately 50 mL sample of fuel to accomplish both the system flush as well as fuel charge for testing. The system is then set for initiation a n d the test sequence is automatically started. The sequence consists of venting the vessel and pressurizing to the initial pressure. After a short stabilization time the test fuel is injected and the pressure and needle lift histories are recorded and used to define the ignition delay
N-cetane: 100 CN
E 3 O
c
ffi
O
Heptamethylnonane: 15 CN 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Ignition Delay (mS) FIG. 10—IQT calibration curve [48].
70 \ r • A
65 0)
iqtvscl-613 IQTCNvsColH
^
Col22vsd613
• — ^
Col17vsCol16 Col 25 vs Col 24
•
Ji 60
E 3
^•k.
Z c
55
0)
50
O
•
•
•
w^
CO
(D 45
a ^"^^
40 35 35
40
45
50
55
60
IQT Predicted Cetane Number FIG. 11—Results of recent round robin testing of the IQT.
65
70
CHAPTER 27: DIESEL FUEL COMBUSTION CHARACTERISTICS time. The vessel is then vented and the process repeated. This sequence is repeated 32 times for each fuel test. The entire test is accomplished in approximately 15 min. Upon completion of the 32 injections, the average ignition delay time is used to automatically determine the derived cetane n u m b e r from a software based ignition delay/derived cetane n u m b e r model. All IQT systems used the same model for determination of the cetane number. The software averaging and the resulting statistical data provide a very good indication of the quality of the data a n d the health of the system. Routine operation of the system also involves periodic, repeated tests using a check fuel to determine the validity of the calibration and the long-term health of the system. Figure 11 is a plot showing the results of a small-scale round robin test in which three different IQT units were compared to each other and to well-documented ASTM D 613 data. The results indicate that the IQT provides a very repeatable and reliable rating of the fuels. Similar comparisons with a wide rcuige of fuels indicate that the IQT also works very well for additized fuels and alternative fuels.
ASTM STANDARDS No. D86 D93 D130
D240 D445
D482 D524 D613 D975 D976 D1319
D1796
D2500 D2622
D5291
E659
Title Standard Test Method for Distillation of Petroleum Products at Atmospheric Pressure Standard Test Method for Flash-Pont by PenskyMartens Closed Cup Tester Standard Test Method for Detection of Copper Corrosion from Petroleum Products by the Copper Strip tarnish Test Standard Test Method for Heat of Combustion of Liquid Hydrocarbon Fuels by Bomb Calorimeter Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (the Calculation of Dynamic Viscosity) Standard Test Method for Ash from Petroleum Products S t a n d a r d Test Method for R a m s b o t t o m Carbon Residue of Petroleum Products Standard Test Method for Cetane Number of Diesel Fuel Qil Standard Specification for Diesel Fuel Oils Standard Test Methods for Calculated Cetane Index of Distillate Fuels Standard Test Method for Hydrocarbon Types in Liquid Petroleum Products by Fluorescent Indicator Adsorption Standard Test Method for Water and Sediment in Fuel Oils by the Centrifuge Method (Laboratory Procedure) Standard Test Method for Cloud point of Petroleum Products Standard Test Method for Sulfur in Petroleum Products by Wavelength Dispersive X-ray Fluorescence Spectrometry Standard Test Methods for Instrumental Determination of Carbon, Hydrogen, and Nitrogen in Petroleum Products and Lubricants Standard Test Method for Autoignition Temperature of Liquid Chemicals
727
REFERENCES [1] Dec, J. and Espey, C, "Chemlluminescence Imaging of Autoignition in a DI Diesel Engine," SAE Paper 982685, Society of Automotive Engineers, Warrendale, PA, 1998. [2] Bowman, C. T., "Kinetics of Pollutant Formation and Destruction in Combustion," Progress in Energy Combustion Science, Vol. 1, 1975, pp. 3 3 ^ 5 . [3] Heywood, J. B., Internal Combustion Engine Fundamentals, McGraw-Hill, NY, 1988. [4] "Air Quality Criteria for Particulate Matter," U.S. EPA, EPA 600/p-99/002aB, March 2001. [5] Naegeli, D. W. and Moses, C. A., "Effects of Fuel Properties on soot Formation in Turbine Combustion," SAE Paper 781026, Society of Automotive Engineers, March 1978. [6] Ryan, T. W., Ill, "Emissions Performance of Fischer Tropsch Diesel Fuel," presented at the Intertech Gas to Liquids Conference, 17-19 May 1999, San Antonio, TX. [7] Boerlage, G. D. and Broeze, J. J., "Ignition Quality of Diesel Fuels as Expressed in Cetane Numbers," SAE Journal, Vol. 27, 1932, pp. 283-293. [8] Schweitzer, P. H., "Methods of Rating Diesel Fuels," Chemical Reviews, Vol. 22, 1938. [9] Henein, N. A., Fragoulis, A. N., and Luo, L., "Correlations Between Physical Properties and Autoignition Parameters of Alternate Fuels," SAE Paper 850266, Society of Automotive Engineers, Warrendale, PA, 1985. [10] Diesel Fuel Combustion Performance Workshop, CRC, Atlanta, GA, 1984. [11] Gulder, O. L., Glavincevski, B., and Burton, G. F., "Ignition Quality Rating Methods for Diesel Fuels-A Critical Appraisal," SAE SP, Qct. 1985. [12] LeBreton, M. D., "Repeatability Test on the CFR Cetane Engine," SAE 841340, Society of Automotive Engineers, Warrendale, PA, 1984. [13] Glavincevski, B., Gulder, O. L., and Gardner, L., "Cetane Number Estimation of Diesel Fuels from Carbon Type Structural Composition," SAE Paper 841341, Society of Automotive Engineers, Warrendale, PA, 1984. [14] Needham, J. R. and Doyle, D. M., "The Combustion and Ignition Quality of Alternative Fuels in Light Duty Diesels," SAE Paper 852102, Society of Automotive Engineers, Warrendale, PA, 1985. [15] Hum, R. W. and Hughes, K. J., "Combustion Characteristics of Diesel Fuels as Measured in a Constant-Volume Bomb," SAE Transactions, Vol. 6, No. 1, p. 24. [16] Hardenberg, H. O. and Ehnert, E. R., "Ignition Quality Determination Problems with Alternative Fuels for Compression Ignition Engines," SAE Paper 811212, Society of Automotive Engineers, Wartendale, PA, 1981. [17] Tavacha, J. W. and Cliffe, J. O., "The Effects of Cetane Quality on the Performance of Diesel Engines," SAE Paper 821232, Society of Automotive Engineers, Warrendale, PA, 1982. [18] Siebers, D. L., "Ignition Delay Characteristics of Alternative Diesel Fuels: Implications on Cetane Number," SAE Paper 852102, Society of Automotive Engineers, Wartendale, PA, 1986. [19] Indritz, D., "What is Cetane Number," Symposium on the Chemistry of Cetane Number Improvement, ACS, Miami, FL, April, 1985. [20] Yu, T. C, Uyehara, Q. A., Meyers, P. S., Collins, R. N., and Mahadevan, K., "Physical and Chemical Ignition Delay in an Operating Diesel Engine Using the Hot-Motored Technique," SAE Transactions, Vol. 64, 1962, p. 690. [21] Tsao, K. C, Myers, P. S., and Uyehara, O. A., "Gas Temperatures During Compression in Motored and Fired Diesel Engines," SAE Transactions, Vol. 70, 1962, p. 136.
728 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK [22] Hardenberg, H. O. and Hase, F. W., "An Empirical Formula for Computing the Pressure Rise Delay of a Fuel from Its Cetane Number and from the Relevant Parameters of Direct-Injection Diesel Engines," SAE 790493, Society of Automotive Engineers, Warrendale, PA, 1979. [23] Parker, T. E., Forsha, M. D., Stewart, H. E., Horn, K., Sawyer, R. F., and Oppenheim, A., "Induction Period for Ignition of Fuel Sprays at High T e m p e r a t u r e s and Pressures," SAE Paper 850087, Society of Automotive Engineers, Wsirrendale, PA, 1985. [24] Walsh, G. J. and Cheng, W. K., "Effects of Highly Heated Fuel on Diesel Combustion," SAE Paper 850088, Society of Automotive Engineers, Warrendale, PA, 1985. [25] Elliot, M. A., "Combustion of Diesel Fuel," SAE Transactions, Vol. 3, No. 3, p. 490. [26] El Wakil, M. M., Myers, P. S., and Uyehara, O. A., "Fuel Vaporization and Ignition Lag in Diesel Combustion," SAE Transactions, Vol. 64, 1956, p . 712. [27] Rao, K. V. L. and Lefebvre, A. H., "Spontaneous Ignition Delay Times of Hydrocarbon Fuel/Air Mixtures," ASME Transactions, Vol. 70, 1985. [28] Chiang, C. W., Myers, P. S., and Uyehara, O. E., "Physical and Chemical Ignition Delay in an Operating Diesel Engine Using the Hot-Motored Technique-Part II," SAS Transactions, Vol. 68, 1960, p. 562. [29] Cox, R. A. and Cole, J. A., "Chemical Aspects of the Autoignition of Hydrocarbon/Air Mixtures," Combustion and Flame, Vol. 60, 1985, p. 109. [30] Saeed, M. N. and Henein, N. A., "Ignition Delay Correlations for Neat Ethanol DF-2 Blends in a DI Diesel Engine," SAE Paper 841343,Societyof Automotive Engineers, Warrendale, PA, 1884. [31] Dobovisek, Z. and Savery, C. W., "Ignition Delay of Selected Alternative Fuels in IC Engines," SAE Paper 859225, Society of Automotive Engineers, Warrendale, PA, 1985. [32] Collins, J. M. and Unzelman, G. H., "Diesel Trends Emphasize Cetane Economics, Quality, and Prediction," presented at the API 47th Midyear Refining Meeting, May 1982. [33] Kloptenstein, W. E., "Estimation of Cetane Index for Esters of Fatty Acids," JAOCS Vol. 59, No. 12, Dec. 1982, p. 531. [34] Murphy, M. J., "An Improved Cetane Number Predictor for Alternative Fuels," SAE Paper 831746, Society of Automotive Engineers, Warrendale, PA, 1983. [35] Steere, D. E., "Development of the Canadian General Standards Board (CGSB) Cetane Index," SAE Paper 841344, Society of Automotive Engineers, Warrendale, PA, 1984.
[36] Bowden, J. N. and Frame, E. A., "Effect of Orgsinic Sulfur Compounds on Cetane Number," ACS, April, 1985. [37] Bailey, B. K., Russell, J. A., Wimer, W. W., and Buckingham, J. P., "Cetane N u m b e r Prediction Modeling," SwRI Report No. SwRI 9435, Southwest Research Institute, San Antonio, TX, 1986. [38] Fodor, G. E., "Analysis of Petroleum Products by Midband Infrared Spectroscopy," SAE Paper 941019, Society of Automotive Engineers, Warrendale, PA, 1994. [39] UUman, T., "Investigation of the Effects of Fuel Composition o n Heavy-Duty Diesel Engine Emissions," SAE Paper 892072, Society of Automotive Engineers, Warrendale, PA, 1989. [40] UUman, T., Mason, R. L., and Montalvo, D. A., "Effects of Fuel Aromatics, Cetane Number, and Cetane Improver on Emissions from a 1991 Prototype Heavy-Duty Diesel Engine," SAE Paper 9072171, Society of Automotive Engineers, Warrendale, PA, 1990. [41] Ryan, T. W., Olikara, C , Buckingham, J., and Dodge, L. G., "The Effects of Fuel Properties on Emissions from a 2.5 gm NOx Heavy-Duty Diesel Engine," SAE Paper 982491, Society of Automotive Engineers, Warrendale, PA, Oct., 1998. [42] Ryan, T. W., "Correlation of Physical and Chemical Ignition Delay to Cetane Number," SAE Paper 852103, Society of Automotive Engineers, Warrendale, PA, 1985. [43] Ryan, T. W. and Stapper, B., "Diesel Fuel Ignition Quality as Determined in a Constant Volume Combustion Bomb," SAE Paper 870586, Society of Automotive Engineers, Warrendale, PA, 1987. [44] Ryan, T. W. and Callahan, T. J., "Engine and Constant Volume B o m b Studies of Diesel Ignition and Combustion," SAE Paper 881626, Society of Automotive Engineers, Warrendale, PA, 1988. [45] Ryan, T. W., "Development of a Portable Fuel Cetane Quality Monitor," Belvoir Fuels and Lubricants Research Report No. 277, Southwest Research Institute, San Antonio, TX, 1992. [46] AUard, L. N., et al., "Diesel Fuel Ignition Quality as Determined in the Ignition Quality Tester (IQT)," SAE Paper 961182, Society of Automotive Engineers, Warrendale, PA, 1996. [47] AUard, L. N., et al., "Diesel Fuel Ignition Quality as Determined in the Ignition Quality Tester (IQT)-Part II," SAE Paper 971636, Society of Automotive Engineers, Warrendale, PA, 1997. [48] AUard, L. N., et al., "Diesel Fuel Ignition Quality as Determined in the Ignition Quality Tester (IQT)-Part III," SAE Paper 1999010, Society of Automotive Engineers, Warrendale, PA, 1999.
MNL37-EB/Jun. 2003
Engineering Sciences of Aerospace Fuels Eric M. Goodger^
RFNA red fuming nitric acid RP-1 rocket propellant narrow-cut kerosine SAE SOCIETY OF AUTOMOTIVE ENGINEERS SIT spontaneous ignition temperature S entropy Sa air specific impulse Sf fuel specific impulse s.f.e.e steady flow energy equation syngas synthetic, or synthesis, gas, (CO -I- H2) AT flame temperature rise in luminometer J"* m a x i m u m adiabatic reaction temperature at constant pressure TAFLE Thornton aviation fuel lubricity evaluation TLV threshold limit value U,u internal energy, specific internal energy UDMH unsymmetrical dimethylhydrazine UHC u n b u m t hydrocarbons V,v volume, specific volume VM molar volume VP vapour pressure W , w work transfer, specific work transfer WSIM water separation index modified P fuel density r ratio of specific heat capacities {cp/c^
NOMENCLATURE A/F air-fuel ratio o n volume basis a/f air-fuel ratio on mass basis ARP Aerospace Recommended Practice (SAE) BOCLE ball-on-cylinder lubricity evaluator C velocity of air CI/LIA corrosion inhibitor/lubricity improving additive CNG compressed natural gas CU conductivity unit (microsiemens/meter) D(X-Y) dissociation enthalpy between a particular X—Y bond E overall energy E activation energy E(X-Y) m e a n empirical dissociation enthalpy between many X-Y bonds FBP final boiling point FSII fuel system icing inhibitor H, h enthalpy, specific enthalpy \Ha standard enthalpy of atomization AHf standard enthalpy of formation AH° standard enthalpy of reaction Hi total thermochemical enthalpy at temperature T based on standard initial temperature of 298.15 K HFRR high frequency reciprocating wear rig HiTTS high temperature thermally stable lATA INTERNATIONAL AIR TRANSPORT ASSOCIATION IBP initial boiling point Id density impulse Is specific impulse JFTOT jet fuel thermal oxidation tester K partial pressure equilibrium constant K' concentration equilibrium constant LNG liquefied natural gas M molar mass, g/mol MTBE methyl tertiary butyl ether m mass, kg NG natural gas NIR near infrared spectroscopy n.f.e.e. non-flow energy equation p pressure Q, q heat transfer, specific heat transfer R gas constant Ro universal gas constant ' Managing Editor, Landfall Press, 28E Jessopp Road, Norwich, Norfolk, NR2 3QB, UK.
IN AEROSPACE APPLICATIONS, PROPULSION SETS THE MOST STRIN-
GENT requirements in terms of the levels of energy to be provided for the purpose, subject to the constraints of mass and volume available for carriage of the fuel within the vehicle. In this study, the following definitions are employed, with the maximum levels of net specific energy shown in parentheses: Conventional fuels - aviation fuel mixtures of a hydrocarbon nature invariably derived from petroleum (44 MJ/kg) High-performance fuels—hydrogen, and individual hydrocarbon materials of particularly high energy content (120 MJ/kg) Substitute high-performance fuels—materials based on non-cryogenic compounds of C, H, O, N, boron, etc. (68 MJ/kg) The performance of a bulk fuel in practice is a function of both the properties of the fuel in question and the conditions
729 Copyright'
2003 by A S I M International
www.astm.org
730 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK under which it is used. The former in turn are dependent on the nature and properties of the fuel components, whereas the latter influence the extent of intrinsic energy released and its conversion to produce vehicular thrust. This is particularly true with ramjet and rocket engine fuels since the chemiccJ behavior of the combustion products within the duct between the combustion chamber and the thrust nozzle outlet can exert an overriding influence on the level of resultant thrust. This chapter provides a concise overview of the heat input requirements of high performance engines together with the heat release available from candidate fuels. It also includes brief comment on the handling characteristics of these fuels in order to ensure that the most attractive candidates are not precluded from use by insurmountable problems within the distribution, storage, and vehicular fuel systems.
ENGINE THERMODYNAMICS The strength of the bonds between individual atoms comprising a fuel molecule represents stored chemical energy, and this energy is required to be released and transformed in some way to produce propulsive thrust, the customary chain of conversions following the energy route from chemical to heat to mechanical. These events take place in a heat engine, and the b r a n c h of engineering science that includes such heat-to-work conversions is known as Thermodynamics. Although the actual processes involved throughout a practical heat engine are quite complex, a simplified overview is adopted by providing a thermodynamic basis of gases associated with idealized conditions and processes, then incorporating the necessary effects of reality since reeil gases do not follow exactly these idealized processes. The science of thermodynamics is built on the concept of a perfect gas that follows certain laws absolutely during changes in its major properties of pressure (p), temperature (T), and volume (V) [Appendix 1]. Fortuitously, the complete range of properties of such a gas at any given state can be fixed by specifying two (unrelated) properties only. This makes it possible to represent the complete state of the gas by means of a unique point on a two-dimensional graph of one property plotted against another (e.g., p against V). Furthermore, when the state of the gas changes in etn ideal (reversible) manner, this process of change can be represented by a unique line on the graph. Even further, when a n u m b e r of different changes follow each other in such a m a n n e r as to return to the original state, they form a closed loop, which represents a cycle that could be repeated indefinitely. Careful selection of the cyclic loop processes can therefore provide a heat transfer into the gas together with a work transfer outwards, thus giving the basis of an ideal cyclic heat engine [Appendix 2]. Such a cycle, even though idccJ, is subject to the fundamental laws of thermodjTiamics which stipulate that, although energy cannot be created or destroyed, not all the heat input can be converted to work because part of the heat must be rejected at a lower temperature. Hence, the net heat input must equal the work output, but the word net must be included or implied. Ideal gas cycles conceived by sequencing various processes include the following: 1. Camot—Meiximum efficiency between given temperature limits, but insufficiently practical since work output low.
2. Stirling, & Ericsson—Camot efficiency; approximated by small-scale non-flow units employing continuous external combustion. 3. Otto—Efficiency lower t h a n C a m o t ; broadly similar to property changes in spark-ignition reciprocating piston engine. 4. Diesel—Efficiency lower than Camot; broadly similar to property changes in compression- ignition reciprocating piston engine. 5. Bray ton—Efficiency lower than Camot; broadly similar to property changes in continuous-flow gas turbine, ramjet and rocket engines. A major advantage of continuous-flow over reciprocating engines is that power is being generated continuously rather t h a n intermittently, hence the power-volume ratios are higher. The particular theoretical cycle of interest as a yardstick for assessing high-performance jet propulsion is therefore the Bra5?ton Cycle (known in Europe as the Joule cycle). This comprises em initial compression without heat rejection through the walls (i.e., "adiabatic," and in fact "isentropic" since it is also reversible), followed respectively by heat addition at constcint pressure (isobaric), work output by isentropic expansion, and finally an isobciric heat rejection returning to the initial state (Fig. 1). Thermodynamic expressions are available for the heat and work transfers applying to the various components of this cycle and, on completing an overall energy book-keeping exercise, the thermcil efficiency of the Brayton cycle appears as follows: I Ur-D/r
work output = 1 heat input
''Brayton
where rp is the pressure ratio of the cycle, equal to p^max'/'mm; x/Pl and the index y is the ratio of / specific heat capacity of gas at constant pressure \ I specific heat capacity of gas at constant volume j and equals 1.4 approximately for air.
E}4}ansion
Compression
Heat rejection
Heat addition
in Z
rejection
-•
3to4
• 4to1
FIG. 1—^The Brayton cycle shown on a non-flow basis.
CHAPTER 28: ENGINEERING SCIENCES OF AEROSPACE FUELS 731 Hence, the higher the pressure ratio, the better the efficiency of heat-work conversion. AppHcations of the Braj^on cycle to gas turbine engines are shown schematically in Fig. 2. The Brayton and other ideal cycles were conceived as having the heat transfers effected across the boundary wall separating the working gas (usually air, hence the expression "air standard cycle") and the surrounding atmosphere. This is a practical possibility and is used, in fact, in Stirling and Ericsson type engines; however such heat transfers tend to take time. Since the chemical energy of a fuel cem be released readily by reaction with oxygen (as shown later under Oxidation Heat Release) combustion with atmospheric air is the most convenient and effective approach to oxidation (although not necessarily the most efficient or desirable, but electrochemical oxidation as in a fuel cell does not appear likely to suit the high-performance r e q u i r e m e n t s of aerospace propulsion). The alternative oxidants required in the case of rocket engines operating beyond the Earth's atmosphere are included in Table 1.
Heat addition
2» •
^
^
Turbine
Compressor
Heat rejection a) Closed circuit (industrial)
Heat addition
^3 •
^
^
/|
1/ Propulsive gases 1 b) Open circuit (propulsion)
FIG. 2—Schematic of gas turbine engines utilizing the Brayton cycle on a steady-flow basis.
Fortuitously, under most conditions the behavior of real gases, such as air upstream of the combustor and burnt products downstream, approximates very closely to that of perfect gases, and ideal cycles can thus be used for comparative purposes with the actual changes of events in practical engines. In practice, also, it is customary to fit a diffuser (a duct of increasing cross sectional area) upstream of the compressor so that the reduced air velocity promotes an initial part of the compression. The corresponding fitment of a nozzle (a duct of decreasing cross sectional area) downstream of the turbine reduces the pressure with a corresponding increase in exit gas velocity and thus in thrust. Once the exit velocity reaches the sonic level, (= V ( g RT) = 331.45 m/s in dry air at 0°C), the exit area chokes, with n o possibility of the effects of chEmges in conditions being transferred back to the nozzle inlet. Consequently, further acceleration of the gas requires expansion of the nozzle, as shown in Fig. 3. Thus, the key thermodjTiamic requirement of high performance fuels is an ability to b u m readily and completely within high-pressure air or other oxidant flowing at high speed, with minimal radiation so that the bulk of the released heat remains within the working gas. The next step, therefore, is to
TABLE 1—Relative performance of rocket fuels and oxidants. Merit order Formula Fuels Liquid hydrogen Hydrazine UDMH Hydyne RP-1 Liquid a m m o n i a Ethanol Liquid diborane Oxidants Liquid fluorine Liquid oxygen Nitrogen tetroxide HTP RFNA
Chlorine trifluoride
Id
LH2
N2H4 (CH3)2N2H4
Unsymmetrical dimethylhydrazine 60/40 mass mixture of UDMH and diethylenetriamine (NH2CH2CH2)2NH rocket propellant narrow-cut kerosine
LNH3 C2H50H LB2H6 LF2 L02
N204 H202 HN03
CIF4
High test peroxide with cone. H2O2 > 80% Red fuming nitric acid containing 7% or more of dissolved oxides of nitrogen
732
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
identify the candidate fuels available, examining their properties with a view to their meeting this requirement.
FUEL MOLECULAR STRUCTURE The majority of conventional fuels, and also some alternative high performance variants, are based on compounds of hydrogen and carbon (hydrocarbons). The structure of these molecules is determined largely by the valency (chemical combining power) of the two basic elements. Since hydrogen is monovalent, and carbon tetravalent, the simplest hydrocarbon s t r u c t u r e possible is clearly CH4 (Fig. 4). This is named methane, and forms the main constituent of the many natural gases (NG) found around the world, invariably incorporating small amounts of heavier hydrocarbons, nitrogen, carbon dioxide, etc. depending on the location and the geological history. As the standard domestic/industrial fuel, this is one of the exceptions of using an individual hydrocarbon fuel commercially. As a gas, it does not lend itself easily to t r a n s p o r t applications unless stored u n d e r compression (CNG) or cryogenically liquefied (LNG). Many other hydroFuel Nozzle
Diffuser
Air-
Open circuit gas turbine with Internal heat release plus diffuser and subsonic nozzle Fuel
t-—Exhaust jet
Air-
f
(1)
Note that "mole" is the n a m e of the physical quantity, whereas "mol" is the symbol. As a n example, the molar mass of methane, CH4, is derived as follows: -—Exhaust jet
Oxidant
Since the atom of hydrogen is the lightest of all the elements, its mass was taken as unity, cind the masses of all other atoms assessed relative to it. With the subsequent adoption of the C''^ isotope as the mass basis with a value of 12 exactly, adjustment to other elemental values gives a value just exceeding unity for naturally-occurring hydrogen, but approximate rounded integers generally give sufficient accuracy in practice. Hence, the molar mass (M) of a hydrocarbon CaHb for example would be given by: M = 12 a + b g/mol (not kg/mol)
Fuel Ramjet with supersonic nozzle Fuel Oxidant
t-
^
Fuel b)
The saturated hydrocarbons contain their maximum content of hydrogen and are, therefore, stable since they do not need to react to seek further hydrogen. On the other hand, the unsaturated hydrocarbons incorporate multi-bonding between adjacent carbon atoms, which imparts instability as these bonds open relatively easily to admit additional hydrogen or to permit inter-bonding of like molecules (polymerization), leading to self-contamination by the formation of gums and other deposits. Nevertheless, these unsaturated hydrocarbons do exist because although they are energetically unstable to elemental decomposition, they are reasonably stable kinetically in that the rates of reaction are very slow below SOO^C at low pressure in the absence of catalysts—comparable to a stone remaining stationary resting on a hillside rather than sliding down the slope. B u o y a n c y i n Air
Fuel
b)
Storage Stability
— > Exhaust jet
#
a)
carbon structures are feasible, most of them categorized into a n u m b e r of "series" depending on their general formulae, as shown in Table 2. The main properties of the first members of these hydrocarbon series are shown in Table 3. ICnowledge of the molecular structure of a hydrocarbon in terms of size, shape, and carbon-carbon bonding together permit prediction of a number of physical a n d chemical characteristics that transfer across to fuel in bulk, as shown in the following analysis:
Rocl
FIG. 3—Schematic of continuous flow jet engines.
CH4
Condensed molecular
H-c-H
Plane structural
(XTJD:)
12.01115 + 4 (1.00797) = 16.04303 g/mol exactly, or 12 + 4 ( 1 ) = 1 6 g/mol approximately. Comparison of the molar mass levels with the value of 28.96 g/mol for atmospheric air shows that any leaks of methane or ethene (plus, of course, hydrogen) would disperse upwards into the atmosphere rather than sink and accumulate as potentially explosive mixtures with air.
S^-
Electron bond Spatial structural Ball and stick Stuart plane structural (Tetrahedral) model model
FIG. 4—Representations of the structure of methane [1].
CHAPTER 28: ENGINEERING
SCIENCES
OF AEROSPACE FUELS
733
TABLE 2—General features of the major hydrocarbon series. Series name Petroleum Organic technology chemistry
Series molecular formula
Description
Cg example Condensed formula
General properties
Plane structural formula HHHHHH
Paraffins
Alkanes
I I I I II
CjjH2n+2 Open-chain saturated
C5H14 H-C-C-C-C-C-C-H Stable I I I I I I Hydrogen-rich HHHHHH (high specific energy) Spontaneously ignitable (imless isomerized) Hexane HHHHHH
Olefins
Alkenes
CjjH2u
Open-chain unsaturated CgH22
I I I I II
C=C-C-C-C-C-H Unstable
I
I I II
H
HHHH Hex-1-ene
Naphthenes Cyclanes
Aromatics
(CH2)jj
Aromatics CjjH2ii.6 etc.
Closed-chain saturated
C5HJ2
Closed-chain resonance stabilized
CgHg
H2
H2
Benzene
Physical State The variations in boiling and freezing points show that increase in molecular size incurs a progressive change from gas to liquid to solid. Although hydrocarbon molecules are generally not polar, in that they do not tend to have their centers of negative and positive charges so displaced as to give a permanent electrostatic field around them, they do exhibit weak forces of inter-molecular attraction because of the orbital movements of the myriad of electrons surrounding the host of atomic nuclei. With small molecules (e.g., the paraffins CH4 to CaHg) these attractions are so small that the molecules are free to move independently, and the bulk material exists as a gas at normal temperature and pressure. With larger molecules (e.g., C4H10 to C16H34), the attractions are sufficient to hold the molecules together to give bulk liquids. The even larger molecules (C17H36 plus) are so strongly attracted as to be firmly locked together as solids. Similar arguments
Stable
Stable but attacks elastomers Carbon-rich (smoke, deposits, radiant flame)
apply to the molecules of the other hydrocarbon series. Liquid density also increases progressively with molecular size. Spontaneous Ignitability Molecules will commence oxidation only when sufficient energy is supplied to rupture one of the internal bonds, permitting a combination with an oxygen atom (see the section on Oxidation Heat Release and its Appendix). This preliminary product is liable to be unstable, so initiate a chain of reactions until eventually full combustion occurs with the formation of CO2 and H2O. This initial energy may be supplied as heat, leading to thermal agitation of the molecules and consequent bond rupture. The minimum temperature at which this chain process leads to ignition is known as the spontaneous ignition temperature, SIT (sometimes described as autoignition temperature, AIT). The larger molecules, being more unwieldy, are less able to withstand thermal agitation
734 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK TABLE 3—Properties of first members of the major hydrocarbon series. Series name
Member name
Formula
H/C molar
Molar mass g/raol
Liquid density
Paraffins
methane
CH4
4
16.04
0.415''
-161.5 -184.0
bp °C
SIT Stoic a/f °C mass kj/mol
fp
20°C 540
17.19
Net calorific values Energy kJ/mol Specific energy density MJ/kg MJ/L
-74.90
-0.80
Maximum combution temperature °C
50.05
20.77
1974
ethane
C2H6
3
30.07
0.561''
-88.3
-172.0
515
16.05
-84.72
-1.43
47.52
26.66
2009
propane
C3H8
2.67
44.10
0.585''
-44.5
-190.0
450
15.63
-102.92
-2.05
46.39
27.14
2016
butane
C4H10
2.5
58.12
O.6OO''
-0.6
-135.0
405
15.42
-124.82
-2.66
45.77
27.46
2020
pentane
C5H12
2.4
72.15
0.626
36.2
-131.5
260
15.29
-146.54
-3.27
45.38
28.41
2022
hexane
^6^14
2.3
86.18
0.660
69.0
-94.3
255
15.14
-167.30
-3.89
45.13
29.79
2023
ethene
C2H4
2.0
28.05
0.384''
-103.8 -169.4
490
14.75
52.32
-1.32
47.19
18.20
2147
Olefins
propene
C3H6
2.0
42.08
0.610''
-47.0
-185.2
460
14.75
20.43
-1.93
45.81
27.94
2089
but-1-ene
C4H8
2.0
56.11
0.631''
-6.3
-185.4
385
14.75
1.17
-2.54
45.35
28.62
2075
pent-1-ene
275
14.75
-20.93
-3.16
45.03
28.86
2066
14.75
-77.29
-3.10
44.23
23.22
2037
C5H10
2.0
70.14
0.641
34.0
-139.0
Naphthenes cyclopentane
C5H10
2.0
70.14
0.751
49.5
-93.9
cyclohexane
C6H12
2.0
84.16
0.778
80.7
6.6
280
14.75
-123.22
-3.69
43.86
34.12
2032
benzene
C6H6
1.0
78.11.
0.878
80.1
5.5
577
13.24
82.98
-3.17
40.61
35.66
2093
toluene
C7H8
1.14
92.14
0.866
110.5
-95.0
592
13.47
50.03
-3.77
40.97
35.48
2075
xylenes (avg) C9H10
1.25
106.17
0.868
140.2
-31.3
563
13.64
18.06
-4.38
41.25
35.82
2069
-259
574
8.42
2171
72.4S
2036
Aromatics
Parent
hydrogen
H2
00
2.02
0.070''
carbon
C(gr)
0
12.01
2.2IS
elements
-252:7
3667 sublimes
34.19
0
-0.24
120.24
11.48
0
-0.3
32.76
b = at boiling point s = solid
without rupture, and thus exhibit lower levels of SIT. Furthermore, molecules that are m a d e more compact by isomerization (rearrangement geometrically with n o loss or gain of any atoms) show higher levels. These two p h e n o m e n a are illustrated by the following examples: Methane CH4
Normal Octane n-CgHig H
H
H H H H 1 1 1 1 1 11 H—C—C—C—C—C-C—C—H 1 1 H H H H H H H H
540
230
H H—C—H
SIT
H H H
Isooctane i-CgHig (2,2,4-trimethylpentcine) H H—C—H H—C
C
H
CaHb + ms (O2 + 3.76 N2) = nj CO2 + n2 H2O + n^ N2
H
where nis = stoichiometric moles of O2 per mole of fuel. Molar balances of the four elements individually give the vEilues of the unknown molar quantities, and permit expansion of E q 2 to the following:
stoichiometric air-fuel molar ratio = (A/F)s
C
C
C—H
H
H
H
= 4.76 m J l = 4.76 a + 1.19 b
(4)
and.
H—C—H
stoichiometric air-fuel mass ratio = (ci/f)s
H 467
(2)
Hence,
H—C—H H
The stoichiometric mixture is chemically correct in that the proportions of fuel and oxidant leave no excess or deficiency of either material on reaction. With atmospheric air adopted as the standard oxidant, it is noted that nitrogen and oxygen exist in ratios of 3.76 by volume, aind 3.31 by mass. Consequently m moles of oxygen are contained in 4.76 m moles of air, representing 32 m grams of oxygen and (4.31 X 32 m) or 137.9 m grams of air. The stoichiometric equation with air for a hydrocarbon fuel of known formula, CaHb, becomes:
CaHb + (a + 0.25 b) (O2 + 3.76 N2) (3) a CO2 + 0.5b H2O + (3.76 a + 0.94 b) N2
H
H
Stoichiometry
"C
(A/F)s
28.96 _ 137.85 a + 34.46 b 12a+ b 12a+ b
(5)
CHAPTER
28: ENGINEERING
Since hydrogen, in comparison with carbon, needs more air for combustion, the stoichiometric air-fuel ratios are higher with the hydrogen-rich fuels, as in Table 3. Oxidation Heat Release Since the events of interest here relate to flowing, rather than non-flowing, mixtures, changes of energy are expressed in terms of enthalpy rather than internal energy. Thermodynamically, the standard enthalpy of formation (AHf) of a compound is the net enthalpy change occurring when the component elements are first raised to a gaseous atomic state and then permitted to release the 'dissociation enthalpy' in forming the compound in question. The resulting vEilue is commonly negative, indicating an enthalpy loss of the system material due to a net outwards flow, and showing the resultant compound to be more stable than its parent elements. Similarly, when both the compound and its stoichiometric proportion of O2 are energised to gaseous atoms by provision of the dissociation enthalpy, and then permitted to react, the net enthalpy output is known as the standard enthalpy of reaction (Af/°) and, again, is negative when flowing outwards. (In petroleum technology terms, on the other hand, heat released by combustion is shown as positive since this is the desired objective.) These standard enthalpies involve equal initial and final temperatures, the standard value being 25°C (298.15 K) and denoted by the superscript o [Appendix 3]. The molar enthalpies of reaction in Table 3 are seen to increase with molecular size. Maximum Combustion Temperature The maximum temperature realized by combustion occurs when no heat is lost through the combustor walls (adiabatic). Since the conditions of interest in this study relate to mixtures flowing at constant pressure (isobaric), the relevant m a x i m u m temperature is described here as the isobaric adiabatic reaction temperature, and denoted by T*. The method of computation is based on the concept of equating the enthalpy released by the reactants, in generating the dissociated products at the initial temperature, with that which would have been required to heat these products from the initial temperature to the final temperature T*. Hence, AHr = physical enthalpy absorbed by products heated from 25°C (298.15 K) to T* The first step in computation, therefore, is to derive the stoichiometric combustion equation, taking into account that at a level of about 1800°C the products of reaction become so agitated thermally as to start dissociating back to their reactant form. The extent of this dissociation is dependent on the level of temperature, and expressed in terms of an equilibrium constant, which is the ratio of the partial pressures of the products and reactants. Since carbon burns in a twostage process through CO to CO2, the combustion reactions at high temperature are written reversibly as follows: H2 + 0.5 O2 ^ H2O, and CO + 0.5 O2
CO2
The stoichiometric equation at high temperature therefore appears in the following form: CaHb + m, (O2 + 3.76 N2) ni CO2 + n2 H2O + ns CO -F n4 H2 + nj O2 + n^ N2
(6)
SCIENCES
OF AEROSPACE
FUELS
735
[Appendix 4]. By trial and error, a value of T* is found where the total thermochemical enthalpy absorbed by the products arising at that temperature is equal to the value of AH" [Appendix 5]. The combustion temperatures shown in Table 3 do not indicate any marked variation. This follows because any higher hydrogen content, despite giving a higher specific energy, would also generate larger quantities of H2O product. The particularly high heat capacity of this product absorbs the additional heat release and so permits little change in the resulting temperature. As a general rule, hydrogen is preferable to carbon in view of its high specific energy, its clean burning capability with minimsJ emissions, its wide range of flammability, and its relatively high flame velocity. Consequently, the hydrogenrich paraffins tend to be preferable to the carbon-rich aromatics. However, other requirements sometimes pertain.
COMMERCIAL FUELS PROPERTIES AND TESTS To date, the most common source of liquid fuels for high performance engines is crude oil, which evolves from decayed marine matter and collects within the strata of porous rocks (hence rock oil, or "petr"-"oleum"). Crude oil consists mainly of hydrocarbons with traces of other materials depending on the nature of the source and reservoir rocks. Individual fractions are derived by continuous distillation (fractionation) followed by various methods of treatment to ensure the specified quality. Commercial fuels invariably comprise mixtures of many different compounds with properties that either represent a m e a n of the c o m p o n e n t values (e.g., density) or range over a m i n i m u m to a m a x i m u m (e.g., distillation temperatures). Exceptions occur in some cases where perform a n c e is overriding the making of individual materials, rather than mixtures, essential (e.g., hydrogen). The quality of aviation fuels is maintained by detailed specifications devised under the auspices of national authoritative bodies such as ASTM International in the U.S.A., the Ministry of Defence in the U.K., and other organizations elsewhere. As well as the properties, the test methods by which they are assessed are also specified, as with ASTM (civil) and Department of Defense (military) in the U.S.A., the Institute of Petroleum in the U.K. and other national bodies elsewhere. Typical properties of representative fuels are shown in Table 4. ASTM and IP now adopt an increasing number of tests jointiy. A convenient technique for presenting an overview of the major properties of petroleum-derived fuels is to select density as a basis for comparison, and to plot all other major properties against it, as shown for petroleum fuels in Fig. 5. The following discussion deals with the properties in an order comparable to that presented in Table 4, including values for substitute high performance fuels and the composite plot against density. Density The standard ASTM method of measuring the density (p) of a liquid fuel is by means of a hydrometer floating in the fuel
736
MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
TABLE 4—Typical properties of conventional and substitute high-performance fuels.
Density Boiling range fp Kinematic Flash SIT kg/L@ °C °C viscosity point °C 15°C cSt@ °C 15°C
Fuel
Stoic a/f Flamm. Net calorific values Maximum Energy combustion limits Specific mass density temperature a/f mass energy MJ/kg MJ/L °C
Aviation gasoline
0.72
46 - 145
-70
0.5
-40 470
14.9
26-4
44.2
31.8
Wide cut ftiel
0.78
73 - 235
-62
1.5
-10 270
14.5
24-4
43.6
34.0
Aviation kerosine
0.80
152 - 256 -52
1.8
43
254
14.8
22-4
43.4
34.7
High flash fuel
0.82
185-244 -51
2.3
63
250
14.7
22-4
43.2
35.4
Gas oil
0.84
180-360 -30
6.0
70
247
14.6
21-3
42.9
36.0
2027
Methanol CH3OH
0.79
64.8
-95.5
0.8
11
385
6.5
12.6- 1.6 19.9
16.0
1969
MTBE CH3OC4H9 0.75
55.0
-109
?
435
11.7 20.2 - 3.6 35.1
26.0
2002
Nitfomethane 1.14 CH3NO2
101
-17
0.6
35
419
1.7
10.9
12.5
2412
1.01
113.5
2
1.0
52
270
4.3
18.3 - 0
16.7
16.9
2219
Pentaborane B5H9 0.63
58.4
-46.6
0.6
13.1
45-?
67.8
42.7
2527
95
-134
0.5
14.8
47.1
32.0
2025
2022
Hydrazine N9H4
Triethyl borane (^2^5)36
0.68
NOTE: Lamina flame speeds: most hydrocarbon fuels = 0.5 m/s approximately, hydrocarbon = 35 m/s boranes = 45 m/s
sample, correcting the results to the standard temperature of 15°C by meajis of international standard tables. Details are given in ASTM D 1298, Density, Relative Density (Specific Gravity) or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method. The data in Table 4 show that the densities of the hydrocarbon oxygenates (methanol and MTBE) differ little from those of petroleum fuels, whereas nitrogen compounds are heavier. Liquid hydrogen, of course, has been seen to be very much lighter, although a 50% liquid/solid slush mode increases density by about 18%. The droplet size resulting in fuel sprays bears a direct relationship to density, typically showing an increase of about 20% with a doubling of density. Volatility The volatility characteristics of a commercial fuel mixture can be assessed by standard methods of measuring distilla-
tion temperature/recoveries, vapor pressure, and flash point. These data can be supported by experimental examination of equilibrium air distillation (usually confined to piston-engine gasolines), vapor-liquid ratio, and the direct laboratory measurement of the evaporation rates of individual droplets. In the standard test ASTM D 86, Distillation of Petroleum Products at Atmospheric Pressure, the apparatus comprises a controlled heat source, a flask equipped with a thermometer, and a side arm leading into a cooled condenser tube, plus a measuring cylinder. The 100 mL test sample is progressively vaporized in the flask, and the condensed droplets collected in the cylinder. The corresponding readings of temperature and (percentage) volume recovery are plotted as a distillation curve. The initieJ boiling point (IBP) is taken as the temperature at the fall of the first condensed droplet, and the final boiling point (FBP) as the highest vapor temperature reached during the test, both temperatures being corrected for barometric pressure. The data in Table 4 show that
CHAPTER
28: ENGINEERING
the boiling points of the substitute fuels generally lie within the range of gasoline and light kerosine. Volatility can also be assessed by measurement of vapor pressure, but this test applies mainly to wide-cut and the more volatile fuels. Similarly, the vapor-liquid ratios at given temperatures indicate volatility. For gasoline, the standard test ASTM D 2533, Vapor-Liquid Ratio of Spark-Ignition Engine Fuels is undertciken under static conditions, whereas the SAE ARP492, Aircraft Engine Fuel P u m p Cavitation E n durance Test relating to pumping applications with wide-cut fuels is undertaken dynamically. In the latter test, the "vapor" volume measured incorporates the air released during the pumping process, and is dependent on the air solubility of the fuel. Another s t a n d a r d test concerns flash point. For the petroleum fuel range, this is an assessment of volatility rather than flammability since the m i n i m u m fuel concentration to sustain combustion (the weak mixture limit), which exhibits a value of about 1% by volume in the mixture with air, is c o m m o n over the whole range of petroleum fuels from gasolines to residual fuel oils. ASTM D 93, Flash Point By PenskyMartens Closed Cup Tester is based on heating the fuel sample progressively, and repeatedly inserting a standard-sized flame into the vapor-air mixture above the liquid free surface until a temperature is reached where the flame promotes momentary combustion. The flash point (the weak temperature limit), in contrast to the weak mixture limit, varies directly with density, reflecting the change in volatility. The substitute fuels in Table 4 show slightly lower values of flash point than the conventionals. (Note that the fire point relates to the slightly higher temperature where combustion is continuous rather than momentary.) Viscosity Viscosity arises in a bulk fuel from the weak electrostatic forces acting between the molecules. These permit many hydrocarbon fuels to exist in the liquid phase under ambient conditions but, at the same time, promote resistance to the internal displacement involved in flow. This resistance is termed dynamic viscosity, and is defined as the tangential force on unit Eirea of either of two peirallel planes within the fuel sample located at unit distance apart when one plane moves with unit velocity in its own plane relative to the other plane. Thus, dynamic viscosity is the ratio of the applied shear stress and the rate of shear, such that: force Dynamic viscosity = 17 = •^
area
length . . .^ . X —^—r— in units of Poise velocity
The smaller unit, centipoise (cP), is more convenient, where 1 cP = 0.01 P = 1 mN s/m^ Division with density provides the kinematic viscosity, as follows Kinematic viscosity = v = ri/p in units of Stokes Here again, it is more convenient to use the smaller unit, centistokes (cSt), where 1 cSt = 0.01 St = 1 mm^/s Dynamic viscosity can be determined by measuring the terminal velocity of a small sphere falling under gravity in the
SCIENCES
OF AEROSPACE
FUELS
737
sample, based on the inverse relationship between viscosity and sphere terminal velocity. Kinematic viscosity is generally determined from the time taken for a given volume of sample to flow under gravity through a U-shaped capillary-tube viscometer, usually vertically disposed, at a specified temperature, as in ASTM D445, Kinematic Viscosity of Transparent and Opaque Liquids (The Calculation of Dynamic Viscosity). In this test, the hydraulic head of the sample, which is density dependent, promotes the flow by gravity, whereas the dynamic viscosity resists the flow, hence the result is dependent on the quotient of these two effects. The test involves a low rate of shear, the kinematic viscosity of the sample being determined by multiplying the measured flow time with the calibration constant for the instrument used. An increase in fuel density is associated with increases in molecular size and inter-molecular forces, and also in kinematic viscosity. Raising the temperature of a fuel promotes expansion, and the increase in molecular spacing weakens the molecular attraction. Typical of action-at-a-distance forces, this attraction follows an inverse power law, and plots as a hyperbolic curve against temperature. For convenience, these curved relationships have been converted to straight lines by plotting on the following empirical logarithmic basis: log log (u + a) = n log r -I- b where a, n, and b are constants. On such axes printed on specially designed charts (e.g. ASTM D341, Standard ViscosityTemperature Charts for Liquid Petroleum Products), two points only need be determined by experiment, the remainder being found graphically on the straight line passing through them, limited at the high-temperature end by vaporization and/or ignition, and at the low temperature end by freezing. In fuel technology, dynamic viscosity is of interest in relation to the setding rate of contaminants during storage, but the m o r e generally-used kinematic viscosity provides an indication of the power required for p u m p i n g , filtration, and spraying, bearing a direct relationship to spray droplet size. Such data available in Table 4 show that the viscosities of the substitute fuels lie between those of gasoline and wide cut fuel.
Solidification Comparable with the range of boiling points resulting from progressive heating of a commercial fuel mixture as the several components distill in turn, a range of solidification temperatures can be expected from progressive cooling as each component crystallizes as wax. Eventually a temperature is reached where the viscosity approaches infinity as the fuel solidifies completely. The relevant solidification temperatures can be determined and expressed in several ways. Generally, the sample is cooled until the first appearance of wax crystals then, in order to avoid spurious results from supercooling, it is allowed to warm until the wax just disappears. (Thermodynamically, the temperatures of appearance and disappearance of wax must be different.) In ASTM D 2386, Freezing Point of Aviation Fuels, the sample is stirred and the result designated as the freezing point. D3rnamic methods are also available, as in ASTM D 4305, Filter Flow of Aviation Fuels at Low Temperatures, in which the sample is cooled under pre-
738
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
scribed conditions and repeatedly subjected to a low pressure pulse directing it through a fine wire mesh filter until a temperature is reached at which the filter plugs. Other suggested methods for assessing fiiel fi-eezing include a system for optically sensing the presence of wax crystals in the sample, and a chemical analysis method based on the relationship of freezing to the concentrations of aromatics and saturates as determined by near-infrared (NIR) spectroscopy. An alternative "Phase Technology" method incorporates freezing, cloud, and pour points based on a light scattering technique [10]. The freezing points of most of the substitute fuels shown are relatively low, with the exception of hydrazine. Surface Tension This property helps to determine the size of fuel droplets in a spray. It can be assessed in a laboratory by measuring the force required to free some flat object (e.g., a glass slide or wire ring) vertically upwards from the surface of the fuel, and then dividing this force by the length of periphery of the object. Values for both conventional and substitute fuels reduce with increasing temperature but vary little from a range of 20-40 mN/m, the exception being hydrazine with values ranging from 60-80 mN/m. ASTM D 3825 can be used, but for liquid gases only. Flammability The flammable nature of a fuel can be assessed in various ways. In the case of the petroleum fuels, the weak mixture limit was seen to be approximately c o m m o n (at about 1% vapor in the mixture with air), consequently the weak temperature limit at which this mixture pertains, that is, the flash point, depends on the volatility of the fuel. With different types of fuel, however, the weak mixture limits differ, hence flash point then gives a direct indication of fire safety. The flash points for the substitute fuels in Table 5 are seen to be slightly lower than those of the conventionals. Mixture limits of flammability can be determined in a laboratory tube apparatus by seeking the particular air-fuel ratios at which the flame fails to propagate following ignition. A similar rig can be employed to determine the rate of flame propagation during the uniform movement over the first few centimeters after ignition. When the flame progresses further cdong the tube, it becomes subjected to the pressure waves
reflected from the ends of the tube, and in extreme cases may even experience detonation involving a combination of shock wave a n d flame. These tests are not standardized, b u t adopted in combustion laboratories and on chamber rigs by engine manufacturers. The data in Table 4 show certain differences in flammable mixture limits, with those for hydrogen and pentaborane being particularly wide. Also, the maximum flame velocity for hydrogen is relatively high. S p o n t a n e o u s Ignitability In the standard ASTM E 659, Autoignition Temperature of Liquid Chemicals, a small charge of liquid fuel is delivered as a droplet by means of a syringe into a heated open flask, and the delay noted before the onset of ignition. The method comprises repeated tests at progressively lower temperatures until the m i n i m u m level is found to promote ignition (the spontaneous ignition temperature, or SIT). The associated delay period is a maximum, and usually comprises several seconds under the test condition of atmospheric pressure. The levels of SIT for the conventional hydrocarbons vary inversely with density but tend to be higher for the substitute fuels. Stoichiometry As an extension of the treatment of hydrocarbons in the section on Stoichiometry, the stoichiometric combustion equation for the substitute fuels provides the following general expression: CaHbOcNd + ms (O2 + 3.76 N2) = nj CO2 + Uz H2O + Us N2 From molar balances of the four elements, m . = a + 0.25 b - 0.5 c Hence, stoichiometric air-fuel molar ratio = (A/F), = 4.76 a
1.19 b - 2 . 3 8 c
and, stoichiometric air-fuel mass ratio 137.85 a -I- 34.46 b 68.92 c (a/f)s = 12 a-H b + 16 c + 14 d The values in Table 4 show, in comparison with the hydrocarbons, the reductions in stoichiometric air-fuel ratios expected in view of the presence of oxygen and/or nitrogen in the fuel. Hydrogen, of course, is the exception on a mass basis.
TABLE 5—Relative stoichiometric calorific and specific impulse values of elements, non-carbon hydrides and organometsdlics [5].
Sp. En.
En. Dens.
Rel. Fuel Density Impulse (Sfd)
100 (43.4 MJ/kg) 276 153 133 57 72 43 38 168 156 98 125
100 (34.7 MJ/L) 24 353 400 124 241 33 48 90 123 103 86
100 (l.OSkNs/L) 19 119 193 48 96 63 67 58 73 69 60
Rel. Fuel Material
Symbol
Av. kero. Hydrogen Beryllium Boron Magnesium Aluminum Ammonia Hydrazine Dlborane Pentaborane TEA Aluminum borohydrlde
H2
Be B Mg Al NH3 N2H4 B2H6 B5H9 A1(C2H5)3 A1(BH4)3
Rel. Fuel Specific Impulse (Sf)
Rel. Air Specific Impulse (So)
100 (25.5 kN s/kg) 254 54 65 25 29 83 53 108 92 66 89
100 (1.76 kN s/kg) 105 116 105 122 122 88 93 107 106 108 107
CHAPTER 28: ENGINEERING SCIENCES OF AEROSPACE FUELS 739 When boron is present in the fuel, the resuking product of combustion is B2O3, and the same principles of molar balance apply to derive the stoichiometric air-fuel ratios.
Calorific Values Whereas the oxidation heat release was treated thermochemically in the Oxidation Heat Release section leading to the derivation of AHr°, the thermodynamic approach permits an application to practical test methods in the laboratory. The thermodynamic laws of gases (Appendix 6) give rise to the following energy equation: Q — W = At/ Q - W = AH
in non flow case, and in steady flow case with negligible changes in potential and kinetic energies.
where Q = heat transfer W = work transfer At/ = change in internal energy AH = change in enthalpy, where H = U +pV, andp V = flow energy. Since it is far easier to measure heat transfer than work transfer, conditions are selected where work transfer is zero so that these expressions simplify to Q = AC/ in non flow case at constant volume, and Q = AH in steady flow case at constant pressure. With gaseous fuels, the enthalpy released on combustion is determined as the energy density (MJ/L) by burning at constant pressure through a steady flow calorimeter. The heat so released is measured from the rise in temperature of the flowing cooling water. The test result is corrected for the standardized volume of fuel gas burnt, together with the expansion of both the cooling water and the flue gases. With liquid fuels, specific energy is determined by burning a known mass of fuel in a stainless steel b o m b located in a water-filled calorimeter u n d e r non-flow constant volume conditions, as in ASTM D 4809, Heat of Combustion of Liquid H y d r o c a r b o n Fuels by B o m b Calorimeter (Precision Method). The b o m b is pressurized to 3.0 MPa (30 atm) with water-saturated oxygen gas to ensure complete combustion. The combustion heat so produced raises the temperature of the caJorimeter water by about 3K, as measured by a platinum resistance or thermistor instrument. The mathematical product of this temperature rise and the thermal capacity of the calorimeter—determined previously using benzoic acid of known specific energy—provides a value of the quantity of heat absorbed. Division by the initial mass of the sample thus gives the specific energy of the sample fuel at constant volume. Because the final temperature of this test is only marginally above ambient, the water produced by combustion of the hydrogen content of the fuel condenses to the liquid phase, adding its latent enthalpy of vaporization to the combustion energy. Consequently, the resulting specific energy is described as the "gross" value. In contrast, the combustion products leaving the working section of any heat engine must, of
necessity, still be hot, following the second law of thermodynamics that permits partial conversion only of heat to work. Hence the combustion water leaves in the vapor phase, and the "net" value of specific energy is more meaningful in engine practice. This is routinely derived by subtracting the latent enthalpy of the combustion water using the following expression, which also converts the non-flow constant-volume result to steady-flow constant-pressure: Net specific energy @ 25°C and constant pressure, in units of MJ/kg = gross specific energy @ 25°C & constant volume — 0.2122 (mass % hydrogen in sample) These experimental results, of course, will not tally exactly with values of AHr° because initial and final temperatures differ in the former case, and are c o m m o n in the latter (Table 3). In order to improve accuracy, n u m e r o u s corrections are made to allow for the enthalpies of formation of nitric and sulfuric acids produced on combustion, and for the heat input from the burnt firing wire. Further refinements to correct for heat transfers into and out of the CcJorimeter during the test are represented by the adiabatic method and the isoperibol method, both of which incorporate an outer water bath. In the former method, the temperature of the outer bath automatically matches that of the calorimeter water, so minimizing any heat interchange. In the latter method, the outer bath is maintained at some selected constant temperature so that heat interchange may be c o m p u t e d accurately, the whole instrument being heavily insulated. In this latter case, the local environment (the outer bath) is isothermal but the external environment (the laboratory) is not, hence the use of the term isoperibol rather than isothermal. Because of the requirements of test time and practiced expertise in the above methods, attention has turned towards methods of estimating specific energy based on statistical correlation between accurate values of related properties. ASTM D 3338, Estimation of Net Heat of Combustion of Aviation Fuels, involves correlation between aromatic content, density, and averages of distillation temperatures at stated recoveries, whereas alternative methods are based on density, aniline point, and/or sulfur content. Such methods are dependent on the combined accuracies of the test results involved, and are therefore less precise than those obtained by direct calorimetric determination. The results presented in Table 4 for the petroleum fuels reflect the density effect of reducing specific energy and increasing energy density shown for the individual hydrocarbons in Table 3. The former effect, again, is understandable from the reducing level of hydrogen content, whereas the latter is underscored by the fact that energy density is the direct arithmetical product of specific energy and density, and whereas the former falls by a mere 20% over the density range, the latter rises by about 50% and is therefore the dominant term in the product, as seen in Fig. 5. A special requirement arises in the case of ramjet powered aircraft. These are essentially high-speed vehicles, hence fuel storage volume is at a p r e m i u m to minimize drag, and energy density becomes the criterion. The customary rise with increasing density is shown in Fig. 6 for some specially selected high-performance fuels indicating the expected need for high density. Substitute fuels of higher energy density can be achieved by replacing the c a r b o n in the h y d r o c a r b o n with some
740 MANUAL 37: FUELS AND LUBRICANTS
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Hydrogen 15 content 14 %Mass IS 1= - 45 Specific - 44 energy 43 MJ/kgnet Energy 35 density density MJ/Lnet ^^ •=
=-^
- 4 . 0
Viscosity est @ 40°C
Temperature °C
Density, Icg/L @ 15°C FiG. 5—Basic properties of jet fueis [1]. Properties in braclcets represent limits to distiiiation ranges.
>i
^ 40-
H-cot d
Shelldyne M Adamantines _ Pertiydrofluoranthene l-Methyipertiyfluorene icalin 4 ^ '* Cyciododecarres
Energy-dense cyclics
Normal and monocyclic hydrocarbons C^
An interesting conclusion concerns the level of specific energy available from unit masses of stoichiometric fuel-air mixtures. Although hydrogen enrichment of a fuel tends to a higher specific energy on a fuel mass basis, the stoichiometric air-fuel mass ratio also rises, consequently more air is available to share the energy output. Hence, the stoichiometric mixture values of specific energy vary little from about 2.9 MJ/kg (net). This means that, in general, whereas the fuel carrying capacity of a vehicle is largely affected by specific energy of the fuel, the performance of the engine is not.
7^
XT
Conventional petroleum fuels
Maximum Comibustion Temperature HaW
0.4
T
1
I
0.8 Density kg/L at 15°C
1.2
"7 1.6
FiG. 6—Energy densities of candidate high-performance fuels [1,2].
element contributing more calorific value to the resultant liquid hydride. Examples of such fuels include hydrides of boron, beryllium, lithium, magnesium, aluminium, and titanium, plus compounds of hydrogen, carbon, and boron as included in Table 4.
The isobaric adiabatic reaction temperature for a commercial fuel mixture cannot be calculated without knowledge of the individual chemical constituents and their proportions. However, a broad assessment can be made on the assumption of an average molecular formula in each case. The combustion temperature values included in Table 4 have been computed on the following basis: Fuel
Assumed Average Formula
Aviation gasoline Aviation kerosine Gas oil
Cl2.5H24.4 C15H27.3
C7.3H15.3
The calculated vEilues for the petroleum fuels are seen to vary little from 2025°C. On the other hand, methanol is lower
CHAPTER 28: ENGINEERING SCIENCES OF AEROSPACE FUELS 741 at 1969°C, while nitromethane and pentaborane are higher at 2412 and 2527°C, respectively.
FUEL COMBUSTION PERFORMANCE IN CONTINUOUS FLOW
Smoke Tendency
Fuel Preparation and Mixing
Within fuel-rich regions of a flame, hot fuel molecules can become so agitated thermally that they crack into portions, involving the release of free atoms of carbon in the form of smoke. This is undesirable in a heat engine because: • Combustion efficiency is reduced through the loss of the calorific value of the free carbon. • Glowing particles of carbon radiate heat at a relatively high rate and thus overheat the combustor liner so reducing its life. • Tendencies to soot deposition within the combustor are increased, with the possibilities of mechanical damage due to t e m p e r a t u r e gradients a n d differential expansion in the combustor liner. • Tendencies to exhaust smoke emissions to the environment are increased. For jet fuels, ASTM D 1322, Smoke Point of Kerosine and Aviation Turbine Fuel, is based on determination of the maxi m u m height to which a diffusion flame, wick-fed from the sample, can be adjusted without smoking (Fig. 7). Being based on an early design of an illuminating oil lamp bearing no resemblance physically to an aero gas-turbine combustor, this method may appear primitive. Nevertheless, it does test the fuel u n d e r diffusion conditions where smoke is m o r e likely to be generated in the actual combustor operating at high pressure. In ASTM D 1740, Luminometer Numbers of Aviation Turbine Fuels, the luminometer n u m b e r is determined with a smoke-point type lamp operating at a fixed level of flame radiation by comparing the flame temperature rise above the inlet air level for the sample with those for two reference fuels, tetralin (an aromatic) and zsooctane (a paraffin), as follows:
Fuels b u m only in the vapor/gas phase, consequently the first stage in satisfactory combustion is to vaporize the fuel and mix it thoroughly with the oxidant. In an aero gas turbine engine, the low volatility of kerosine is such that carburation at ambient air temperatures would be too slow for a satisfactory rate of vapor generation. Vaporization at flame temperature is practicable, however, and the "walking stick" (or T-shaped double walking stick) type of vaporizer tube fed with air under relatively low pressure receives narrow streams of liquid fuel that are vaporized to an extent depending upon the fuel flow rate, and introduced directly into the flame [1].
f AT sample - \T tetralin \ Luminometer Number = 100 U j f-octane - AT tetralin I Correspondence has been found between smoke point cUid luminometer n u m b e r over a wide range of kerosines.
©
Inclined mirror for ~ viewing smoke in flame centre
^ Photocell
In most o t h e r applications to gas-turbine engines, the volatility of the fuel is increased substantially to an effective level roughly equivalent to that of a gasoline by subdividing into a large n u m b e r of very small droplets. This process of spraying, loosely termed "atomization," gives a massive increase in surface area per unit volume of fuel, and so augments the vaporization rate by several orders of magnitude. The fuel is also distributed in space so that mixing with air is effected rapidly. Immediately after a fuel droplet is injected into the flame zone, vapor will form and b u m as a premixed flame. As the velocity of the droplet falls, it will become surrounded by a diffusion flame. The proportion of premixed burning may be raised by increasing the relative velocity between the fuel and air, consequently combustion in the gas turbine chamber is dictated by the characteristics of the fuel injector as well as the geometry of the combustor liner itself. This subdivision of a film of fuel into filaments and droplets is achieved by the shearing action arising from the velocity differences between the spray elements and the air. A number of different pressure-jet injection designs are available. These practices are not standardized but are adopted in engine manufacturer's development laboratories. An alternative approach is to achieve the required shear from velocity difference by accelerating the air rather than the fuel. This gives rise to the air-assist a n d air-blast injectors where a stream of high-velocity air meets a stream of low-velocity fuel. Droplet sizes tend to be small and, being controlled by airflow, spatial distribution of fuel is largely unaffected by fuel flow rate [1].
/ Chimney
Ignition
"niemiocouple Reflecting scale
Luminometer Configuration ^®''*''"
Flame-height adjuster
,
Sample teo-ootane
J ^standard 1 1 1 U 1
i
7
i
/\ rs'A
\
\
Flame temperature rise, iT Smoke Lamp
Luminometer No = 100
/Ars-AT|.\ ^ATU-ATL;
FIG. 7—Smoke point and luminometer number. Schematics of apparatus [1].
Ignition is initiated by some form of high-energy spark or torch igniter, and the flame then sustains continuous ignition of the entering mixture, whereas in rocketry, certain fueloxidant pairs aire selected because they are hjrpergolic, i.e., ignitable on contact, which eliminates the need for separate ignition equipment. In this case, however, it is essential for spontaneous ignition to tak;e place on start-up with the minim u m of delay, otherwise a "heird start" will ensue, comparable to the mechanism of diesel knock but with much more damaging consequences. As there is no standard laboratory method, this refers to the method adopted in continuousflow energy practice to effect ignition.
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Flame Stabilization In the aero gas turbine combustor, the flame is required to be stabiHzed in a defined location against the entry stream of air over a wide range of conditions, consequently a balcince must be maintcuned at all times between the velocities of the flame and the approaching mixture. The flame velocities of most fuels in laminar flow eire in the order of 0.5 m/s only and, although increased markedly by temperature and turbulence, the velocity of the incoming mixture, emd thus the eiir, must be reduced by some form of betffle in order to achieve velocity balance. In the gas turbine chamber, the velocity of the entering air is reduced initially by diffusion. The air then goes through a process of flow reversal by being fed around the outside of the flame tube before entering through side apertures and encountering a low-pressure region in the core of an air swirl. The result is a stabilized toroidal vortex of swirling air, the inner surface of which flows upstream. Introduction of the fuel into this surface promotes a region of flame-air velocity balcince. In advanced aero gas turbine engines, a technique of premixing and prevaporization is used in order to control emissions more effectively. In rocketry, sprays of the liquid fuel Eind oxidant may be ctrranged to impinge for thorough mixing.
Air D i l u t i o n In contrast to piston engines where the working surfaces (piston crowns) are subjected to combustion temperatures for only short periods of time, the blades on a gas-turbine disk operate continuously in a high-temperature environment, and are also subjected to centrifugeJ stresses. Consequentiy, the products from the flame must be cooled sufficiently to suit the metallurgy of the turbine blades. Customarily, some 28% of the chamber air is used for combustion in the primary zone, which gives a flame temperature of about 2050°C. The remaining air is introduced progressively downstream, first as secondary Eiir to reduce the temperature of the combustion products to about 1450°C in order to offset the effects of dissociation, and then as dilution air to bring the turbine entry temperature (TET) down further to the m a x i m u m level of about 1000°C. Higher TET values of 1350°C and above are acceptable by intemeJ cooling of the nozzle guide vjines and turbine blades with air bled from the compressor. Despite fuel residence times within the chamber of a few milliseconds only, combustion efficiencies under design conditions are high, typically 99.5% or above. Even exhaust smoke that is just visible represents a combustion inefficiency of no more than 0.01%.
Turbine Entry Temperatiu-e Distribution The distribution of entry temperature over the turbine disk should be reasonably uniform in order to avoid localized
peak temperatures that would limit blade life and engine performance. The temperature traverse qucJity at the outlet of the chamber is known as the pattern factor, defined as follows: ^ mean.exit ' mean.inlet
Pattern factor =
The m e a n exit temperature values are generally obtained by taking measurements at test points located at the centroids of a n u m b e r of equal sub-cireas comprising the total exit area. Customarily, velocities are also measured at these test points, and each temperature weighted on a mass flow basis. Specific Impulse In ramjet engines, a higher flame temperature is permitted in the absence of turbine blades located within the hot propelling gas stream, whereas in rocket engines, even higher temperatures pertain in the absence of atmospheric diluents. In both cases, therefore, the extent of dissociation of the combustion products, and of any subsequent part recombination within the propelling nozzle, are usucJly sufficient to influence substantially the thrust level based on energy density alone. The performance of high speed jet engines is therefore rated in terms of the stream thrust exerted at the nozzle throat exit plcine where the velocity is sonic, using a parameter termed specific impulse since it is the impulse (thrust) based on unit flow rate of the propelling products. For ramjets, this parameter may be based on the mass flow rate of the air alone, or on either the mass of volume flow rates of the fuel alone. Thus ,. .r. . , „ Air specific impulse = bn = f t " F ma _ , .p. • 1 o Fuel specific impulse = Sf= f f T rrif
Stream thrust ~ra :~ Air mass flow rate N s/kg (relates to engine thrust) Steam thrust -=—5 5 ;— Yuei mass flow rate
N s/kg (relates to mass limitation)
„ , J .^ . , cSteam thrust Fuel density impulse = Sf^ = Fuel volume flow rate p = -y- ~ Sfpf N s /L (relates to volume limitation) where py = fuel density, kg/L. Relative vsJues of the specific impulse parameters are included in Table 5 for selected elements, non-carbon hydrides, and organometallies. These show a broad interrelationship with Ccdorific values, but eJso some significant differences. Data for representative monoreactants are shown in Table 6.
TABLE 6—Properties and performance of representative monoreactants [6-8]. Density Monoreactant UDMH Hydrazine Nitromethane HTP Tetranitromethane
' Expanding from 68 to 1 atm.
Formula
(kg/L)
Id* (N s/L)
(CH3)2N2H2 N2H4 CH3NO2 H2O2 C(N02)4
0.78 1.008 1.12 1.44 1.638
1530 1958 3126 2330 2907
(N s/kg)
Reaction Temp. *(K)
1961 1942 2491 1618 1775
1154 905 264 1278 2170
CHAPTER 28: ENGINEERING SCIENCES OF AEROSPACE FUELS 743 For the rocket engine, the performcince is simiWly based on variants of the specific impulse, as follows: p Specific impulse = Is = N s/kg, and Density impulse = /<; = 4 PR N s/L where
rup = mass flow rate of propellants = m a s s flow rate of reactant mixture in the chemiCcJ rocket PR = density of reactant mixture at storage conditions
Since the expression for specific impulse [1] contains the term V'(Tc/M), where Tc = combustion temperature, and M = m e a n molar m a s s of combustion products comprising propulsive fluid, it follows that hydrogen offers the double benefits of high Tc burning to H2O of low molar mass (18) in comparison with a carbon-bearing fuel burning to CO2 of high molar mass (44). Since combustion characteristics, and resulting thrust levels, vary markedly with fuel-oxidant mixture ratio, the "peak" values of specific impulse and density impulse are used for comparative purposes. Figure 8 shows the variations of peak impulse values with densities of the corresponding liquid reactant mixtures. The familiar inverse and direct relationships are seen, reflecting earlier variations of specific energy and energy density with fuel density. Hence, heavy reactants are favored for atmospheric operation, and light reactants for space flight. The values plotted are for "shifting" equilibrium, which allows for changes in product composition along the length of the propelling duct, a n d are approximately 5% above corresponding values for "frozen" equilibrium where n o further chemical changes are assumed downstream of the combustor.
Combustion Emissions Aero gas turbine emissions are of concern for their direct effects on airport environments, and their cumulative effects at altitude. The m a i n emissions are CO and UHC at idle/taxiing conditions, NOx (= NO + NO2) and smoke at high power during take-off and climb, and CO2 a n d H2O throughout. Some ccirbon monoxide forms in the primary zone of the gas turbine c h a m b e r but then tends to be oxidized by the secondary air. However, u n b u m t hydrocarbons may comprise u n b u m t fuel and also partially reacted fuel in the form of methane and related light hydrocarbons. They normally arise through over-large spray droplet sizes and/or the chilling effects of mixing with air at low power output. Nitric oxide occurs in atmospheric air flame-heated to above 1800 K, a n d also following the three different sources of thermal NO at high temperatures, prompt NO at fuel-rich low temperatures, and fuel NO from fuel-bound nitrogen at fuel-lean high temperature. Some of the NO oxidizes to NO2 at full load, and more at low-temperature idle. The effects of NOx from subsonic aircraft operating in the troposphere (approximately 6-14 km altitude) are to increase the level of ozone, which acts as a powerful greenhouse gas. With supersonic aircraft operating in the tropopause/stratosphere (approximately 18-23 k m altitude), o n the other hand, the ozone shield from harmful ultraviolet solar radiation is destroyed by the regeneration reactions of NO2 back to NO. Smoke comprises finely divided peirticles of carbon-rich soot, and forms within fuel-rich regions of the flame such as the core of the fuel spray, but is largely consumed downstream. ASTM D 1322 and D 1740 are used to measure combustion emissions. The fuel parameters likely to improve emissions summarize as follows: • PhysiccJ: Low viscosity to generate small droplets in fuel sprays. High volatility to assist rapid vaporization. • Chemical: Low carbon content a n d low aromatics to reduce smoke. Addition of organobarium or mcinganese to reduce smoke. The most effective hardware methods of reducing emissions appear to center on improvements in fuel prepeiration a n d aeration in the combustor, plus the use of fuel staging, variable chamber geometry, and lean premix vaporization.
FUEL HANDLING CHARACTERISTICS E
-2
0.2
0.6
—1— 1.0
i
— I —
1.4
1.8
Density of peak reactant mixture kg/L
FIG. 8—Values of peak specific impulse and density impulse for liquid rocket fuels and oxidants, with comparative values for mono and solid reactants [3,4].
For purposes of delivering fuel through the supply chain, loading into aircraft tanks, and then transferring fuel to the engine, the liquid phase is the most convenient. Fuels that are normally gaseous at ambient temperature and pressure can be liquefied, but this entails either the weight of high-pressure containers or the bulk of a cryogenic system. As indicated earlier, the density of a cryogenic liquid can be increased moderately by adopting the slush mode in which a significant proportion is frozen. Similarly, particles of solid fuel may be dispersed throughout a liquid fuel carrier to promote a high-density slurry. Solid fuels tend to apply to "oneshot" rocket engines only. In aerospace, the major requirement of calorific value applies on a mass basis, i.e., specific energy, in cases of both
744
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subsonic and extra-atmospheric operation. Supersonic and hypersonic (Mach No. > 5 ) flight within the atmosphere, however, is subject to such high levels of drag as to be volume-limited, thus requiring optimal energy density. The next key requirements for handling properties relate to the following modes of operation although, of course, some requirements apply to more than one mode: 1. Preflight fueling—static charge explosion resistance 2. During flight—^volatility, viscosity, lubricity, thermal stability 3. After flight—freedom from deposition 4. Between flights—absence of solidification (ice and wax) 5. Throughout—freedom from corrosion, insensitivity to fuel-borne water Since all these requirements are unlikely to be met at once, it becomes necessary to employ additives. The characteristics and methods of test for volatility and viscosity, both of which influence the droplet formation in sprays and subsequent vaporization in the combustor, have been dealt with above, plus the appearance of wax. The following properties relate to the overall system of fuel production, distribution, and storage right u p to entry to the engine bay in the aircraft. F u e l i n g Fire Safety During the flow of fuel through pipelines, the presence of trace polar contaminants leads to charge separation through the interchange of electrons, giving rise to ionization. Charges of one sign will tend to drift towetrds and accumulate on the walls of the container, whereas those of the opposite sign will remain in the fuel itself, and thus fuel flow will also constitute a streaming electrical current. This effect is augmented by the presence of droplets of water or particles of dirt, and very m u c h so by the extensive contact area of a filter medium. Owing to the high state of cleanness of transport fuels, the electrical conductivity is very low (e.g., 1-3 CU^ for aviation kerosine cf. 10 X 10* CU for demineralized water). Consequently, high electrostatic potentials are generated, which cannot be dissipated easily through such an inert fuel medium. Within the filter-separator or receiver tank, therefore, an electrostatic gradient develops between fuel and wall surfaces that can r e a c h a level of breakdown leading to sparks, and to subsequent explosions, if the vapor-air mixture lies within the flammable range. This dissipation process is known as charge relaxation, and the time taken for the charge to fall by one half from a given value (the half-value time, ti/2) is a n importcint factor indicating whether static charges generated in the fuel are hazardous or not. In practical units. 12
tl/2 = Conductivity seconds
Typical values of ti/2 are 12 s for aviation turbine fuel of 1 CU, and 1.2 /as for demineralized water of 10 X 10* CU. Charges can accumulate to an appreciable extent only if ti/2 is high. The generation of static electrical charges in the handling of aviation kerosine has long been recognized as a potential hazard during aircraft fueling. A n u m b e r of rules covering ^ 1 CU = 1 Conductivity Unit = IpS/m (picosiemen/meter)
pumping rates, line velocities, bottom loading (no splash filling), and settling times have been generally adopted by the major fuel suppliers as standard practice for the safe handling of such products. The electrical conductivity of a fuel is determined by applying a voltage across two electrodes immersed in the sample, and recording the resulting current, as in ASTM D 4308, Electrical Conductivity of Liquid Hydrocarbons by Precision Meter. An alternative method, ASTM D 2624, Electrical Conductivity of Aviation a n d Distillate Fuels, applies to fuels containing a static dissipator additive. Civil and military requirements differ because of the different additives used. For example, experience shows a m i n i m u m conductivity of 50 units to be safe for civil jet fuels, whereas military fuels require higher conductivity, and this offsets the slight synergism possible between the additives FSII and CI/LIA. A m a x i m u m limit is also set for jet kerosines (450 CU), all limits applying at the point, time, and temperature of delivery to the user. The conductivity of a fuel in a storage tank may be measured directly by portable meters manufactured commercially. The Maihak instrument has a conductivity cell on the end of a cable wound onto a drum. The cell is lowered into the tank through the sampling hatch, and the direct-reading meter attached to the drum. With the pocket-sized Emcee instrument, the conductivity cell may be detached from the cable and fitted directly to the meter to give a laboratory model for use with a fuel sample in a beaker. Alternatively, the cell may be attached to the end of a probe and inserted into the tank for measurements at a series of levels (e.g., top, middle, and bottom) to check for homogeneity of the contents. An additional measure of safety is the use of static dissipator additives (e.g., Stadis 450 for jet fuels). Complex polymeric materials containing nitrogen and sulphur are used to render the bulk fuel sufficiently conducting to provide rapid dissipation of the charge to an appropriate earthing surface [1]. Incorporating this additive into an aviation kerosine would raise the conductivity from 1 to about 150 CU, lowering the half-value time from 12-0.08 s. Lubricity Lubricity, sometimes described as film strength, can be defined as the ability to lubricate with a low tendency to generate friction, wear and/or scuffing. Lubrication takes the three basic forms: • hydrodynamic (with n o surface contact, as controlled by viscosity) • mixed (with limited surface contact) • boundary (with predominantly surface contact). In the latter instance, lubricity applies particularly to fuel pumping equipment operating at high pressure, where potential surface contact predominates. Although not included in current specifications for jet fuels, the ability to maintain boundary lubrication in the close cleEirances between reciprocating elements or highly-loaded gears in fuel pumps is imperative to avoid adhesive wear (surface scuffing or welding) and oxidative wear (corrosion and abrasion). This was manifested indirectly in aviation under the following circumstances: • sticking and hang-up of fuel control systems using widecut fuel
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• use of corrosion inhibitors (e.g., dilinoleic acid derivatives) to keep pipelines clean indicated surfactant action which prevented water from coagulating and made removal difficult. Discontinuation of the inhibitor resulted in problems of fuel p u m p seizure, which were solved on replacement of the inhibitor, indicating its action as a lubricity enhancer • chemical treatment to convert objectionable mercaptans RSH to innocuous disulphides RSSR' does not appear to affect lubricity, but severe hydrogenation to reduce mercaptans to paraffins and hydrogen sulphide removes the polar boundary lubricants and promotes p u m p seizure. Boundary lubrication is, in fact, dependent more on the trace concentrations of polar compounds than on the nature of the m a i n h y d r o c a r b o n c o m p o n e n t s of the bulk fuel. These polar compounds most probably comprise high molar mass polycyclic aromatics containing sulfur, oxygen, and/or nitrogen. Lubricity is customarily assessed in practice using a moving specimen rubbing on a stationary specimen under an applied load in the presence of the test fuel as, for example, in the lightly-loaded ASTM D 5001, Measurement of Lubricity of Aviation Turbine Fuels by Ball-On-Cylinder Evaluator, BOCLE. This rig is used to measure lubricated mild wear rate of the ball but the results m a y be influenced adversely by corrosive wear. Wear rates that are high are usually caused by scuffing, and a new development is the BOCLE scuffing test. The correlation between mild wear and scuffing depends on the composition of the fuel, consequently comparing the scuffing performance of fuels from their mild wear behavior is valid only for fuels of similar composition. The BOCLE is therefore satisfactory for • p u m p development using a low-lubricity fuel • monitoring supplies of low-sulfur additive-free fuel • monitoring changes in additive concentration. The Dwell test measures friction whereas the more complex Thornton Aviation Fuel Lubricity Evaluation (TAFLE) rig measures seizure load, friction, and wesir as well as scuffing. In the High Frequency Reciprocating Wear Rig (HFRR), a hardened steel ball oscillates under load across a hardened steel plate in the presence of the fuel sample. As before, lubricity rating is based on comparing the wear scar diameter with that of a reference fuel, in this case at two test temperatures. A fuel with high lubricity is described as SOFT, and with a low lubricity as HARD. At this time, these are experimental tests derived through industrial research. (For Lubricity Improving Additives, see the Water Contamination section.) Stability Chemical stability in storage is characterized by hydrocarbons that are hydrogen-saturated, whereas instability is endemic in the presence of double carbon-carbon bonding as in the olefins (and even more so in the treble carbon-carbon bonding in the acetylenes, which renders them unusable for aviation). Reactions of the unsaturated hydrocarbons occur as polymerization of like-to-like molecules, and/or oxidation to H.C.O compounds, the products of both types of reaction taking the form of gums, acids, fuel-borne solid particulates, and/or surface deposits. Olefins do not occur in crude oils or their straight-run fractions, of course, because of the exten-
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sive time available to achieve stability during formation, but cracking or other refinery processes can be severe enough to promote multi-bonding. With jet fuels, for example, a maxim u m limit of 5% volume of olefins was set in order to prevent the incorporation of non-hydrotreated cracked stocks into the finished product. The combined effects o n a fuel of heat, dissolved oxygen and nitrogen, trace sulfur content (particularly disulfides and thiols), and some initiator such as a catalj^ic metal result in the formation of small quantities of insoluble products. These comprise two forms depending on the conditions of flow and temperature, as follows: • Intermediate temperature (<200°C) particulate deposits from bulk heating accumulating on filter screens causing blockages. • High temperature (350°C) films of lacquer (varnish) from localized hot surfaces accumulating in fuel-fed oil coolers, hydromechanical devices, injector feed arms and the fuel injectors themselves. These promote loss of heat-transfer efficiency, sluggishness and hysteresis with moving parts involving fine tolerances leading to sticking and eventual seizure o n attempts at engine starting. Furthermore, the higher temperatures within the injector feed arms can lead to thermal cracking, the resulting carbonaceous material either depositing and reducing the fuel flow, or breaking away and blocking the injectors, distorting the spray patterns with consequent thermal damage to combustor liners and turbine blades. During flight, the key issues for fuel stability under the action of heat are the peak temperature experienced, and the overall thermal history u p to the point of combustion. For subsonic civil aircraft, for example, the bulk tanks have little effect, b u t in some supersonic aircraft, bulk tanks may make some contribution through kinetic heating. In the latter case, the main source of the problem is the use of fuel as a heat sink for cooling engine lubricating oil and various items of avionic equipment. The overall result is that fuel approaches the injection manifold with a much higher content of sensible heat t h a n previously. This situation is exacerbated by further heating in the injector feed arms because of their location within the air stream, which is compression-heated to t e m p e r a t u r e s approaching 600°C. A m a x i m u m operating temperature of 163°C is set for the fuel reaching the inlet of the injectors, but the objective of a currently active research program is to raise this limit by 100°F (56°C) to 219°C, hence increasing the heat capacity of the fuel by 50%. Even if the injector fuel should eventually reach 254°C, the SIT level for aviation kerosine, spontaneous ignition cannot occur in the absence of adequate oxygen. Nevertheless, hydrocarbon fuels invariably contain u p to 14% by volume of dissolved atmospheric gases. Furthermore, the concentration of oxygen in these dissolved gases is raised from the standard atmospheric level of 2 1 % to about 40%. As a result, the application of heat to such fuels can give rise to slow processes of autoxidation. In ASTM D 381, Gum Content in Fuels by Jet Evaporation for Aviation Fuels, the fuel sample is evaporated under controlled conditions of temperature and flow of steam (now air), and the resulting residue weighed. In Western countries, stability at higher soak temperatures is assessed by ASTM D 3241, Thermal Oxidation Stability of Aviation Turbine Fuels (JFTOT Procedure), comprising an aluminium tube resis-
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HANDBOOK
tance-heated to 260°C that experiences a skewed normal temperature distribution along its length, and downstream a heated stainless-steel cloth precision filter of 17/Am nominal porosity fitted with a differential gauge. The fuel sample flows at 3 mL/min for 2.5 h along the outer surface of the tube: in aircraft practice, the fuel flows within the tubing but external flow is used in the test so that any deposits can be examined visually. Any pressure drop across the filter indicates the presence of solid particulates, b u t examination of the upstream heater tube was found to be necessary since tube deposits were sometimes evident in the absence of filter blockage. The sample is reported as either a pass or fail, determined jointly by the intensity of tube deposition against set color standards (ranging over a ten-step basis from 0, < 1 , 1, < 2 , 2, < 3 , 3, < 4 , 4, >4) and/or level of pressure drop across the filter u p to a maximum of 25 m m Hg. Among abnormal surface colors is blue/gray, usually associated with copper contamination. The JFTOT has also been used at t e m p e r a t u r e s above 260°C in order to find the breakpoint, that is, the temperature at which the fuel fails both the specified tube rating and mEixi m u m pressure drop (25 m m Hg) criteria. The currently determined brccikpoint for a typical UK aviation turbine fuel ranges from about 280-310°C, and hydrotreated fuels have been found more stable t h a n sweetened fuels in which trace contaminants are converted rather than removed. Generally, breakpoint has also been found to correlate directly with hydrogen content a n d smoke point. In order to provide a database, the US DOD is collecting information on thermal stability results at a test temperature raised from 260-275°C. In the near future, USAF is expected to develop a "JP-8 + 100" fuel, being a JP-8 fuel with a 100°F (56 K) improvement in break point by means of a package of anti-oxidant, metcJ deactivator, detergent, and dispersant additives. These especially stable products are now described as High Temperature Thermally Stable (HiTTS) fuels. Thermal stability is degraded in the presence of olefins, copper, and other metcJs, and of sulfur. Copper is a particularly potent oxidation catalyst and, even at the parts per billion level of concentration, has given rise to a n u m b e r of problems through carryover when jet fuels were subjected to copper sweetening. However, with the adoption of alternative methods of treatment, a limit on copper content is no longer specified, reliance being placed on the results of the JFTOT. Should a jet fuel contain copper, it can be chelated (neutralized by axi isolating coating) by means of a metal deactivating additive such as a propanediamine. However, excessive use of MDA can passivate the tube surface of the JFTOT and give rise to misleading results.
Water Contamination Even with the strictest housekeeping discipline, traces of water invariably collect in liquid fuels during storage smd transfer. Apart from accidental ingress through carry-over from water-washing in the refinery, or defective tank vents or seams (pEirticularly in sea-going vessels), the main source is the humidity of the atmosphere above the fuel surface. Water dissolves in hydrocarbons to a n extent dependent upon temperature, any additional water existing freely as dispersed droplets. The agitation of fuels by pumping through valves and pipeline systems tends to disintegrate the water droplets.
hindering their coagulation and settlement. Certain soap-like surface-active agents (surfactants) may occur naturally in fuel, or be introduced either by contamination or in the form of otherwise acceptable additives. These inhibit small droplets of free water from coalescing with each other and settling rapidly. The settlement rates of water droplets can be determined from Stokes' law. terminEj velocity of spherical peirticle = where
18ij
d = diameter of particle (cm), a = density of particle (g/cm^), p = density of fuel (g/cm^), and 17 = dynamic viscosity of fuel (g/cm s).
Thus, settling rates are proportional to the square of the diameter of the contaminating particle, emd the density difference between the peirticle and the fuel. As well as increasing the concentration of free water, cooling a distillate fuel from 0°C to about —30°C gives rise to the progressive formation of ice crystals that tend to remain in suspension. Supercooling may occur, but impact or contamination promotes instant freezing of both water and some hydrocarbon hydrates. A loose network of ice particles builds u p on filter surfaces, resulting in an increased pressure differential and, eventually, complete blockage. In general, the icing problem in civil aviation is met by the use of fuel-filter heating, whereas military practice is to additize the fuel with a fuel system icing inhibitor, FSII (diethylene glycolmonomethylether) u p to 0.15% by volume. Since water, carbon, and nitrogen are essential ingredients to life, microbiological activity is possible at the water-hydrocarbon interface. The most c o m m o n aerobic micro-organism to flourish under these conditions is Cladosporium resinae (Lindau) de Vries. This is a fungus comprising long threads that produce spores too small (approximately 3 ^^m) to be filtered out. The threads branch heavily to form a visible tangled greenish-black mat or "mycelium," the mechanical strength of which is sufficient to cause filter blockage cind malfunction of fuel-contents gauges. The fungus is able to extract the carbon from a hydrocarbon fuel such as kerosine, and to generate products which, in association with water, corrode through the WEJIS of aluminium fuel systems, promoting leakage. These tanks are now designed to give free access of all internal water to a drain plug, Eind/or with probes into the low points to pick up collected water so that it CEin be injected into the m a i n fuel delivery system to the engine. Biostats are materials that inhibit the growth of bacteria and fungi, whereas biocides are materials that kill them. In military aviation practice, the fuel system icing inhibitor to prevent water freezing problems is also relied upon as a biostat. In civil aviation, on the other hand, where filter heating is used instead, biocidal shock treatment is given every few months when the aircraft is out of service, by the addition of u p to 270 p p m of a boron compound, which is left to act for three days. In addition to the corrosive action of the products of microorganisms discussed above, direct contact of metaJ with water and dissolved air leads to the formation of rust. Materials employed as corrosion inhibitors are polcir in nature with hydrophilic heads and oleophilic/hydrophobic tails that accu-
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m u l a t e as an oil m o n o m o l e c u l a r layer preventing watermetal contact. As indicated in the earlier section on lubricity, experience showed these corrosion inhibitors to serve a second purpose in improving lubricity by the same molecular action, consequently such materials have become known as corrosion inhibitor/lubricity improving additives, CI/LIA. Other problems associated with water during the pumping of fuels include the augmentation of static charging, as discussed ecirlier, and also foaming in the presence of heat. With aviation kerosines, free water concentrations existing as dispersed droplets become visible as a cloud (the specification calling for the fuel to be "clear") at concentrations varying from 30-50 p p m mass depending inversely on droplet size. The former figure is recognized by lATA as the m a x i m u m permissible at the time a n d temperature of delivery, whereas a maximum of 15 p p m is assured by the use of filter/separators and monitors in the supply system. The concern with jet fuels is the potential for certain surfactants to disarm water separation equipment resulting in the unexpected delivery of water to aircraft. A test was needed initially to guard against the carry-over of powerful surfactants from the refinery, particularly when sulfonation techniques were much in use. ASTM D 2550, Water Separation Characteristics of Aviation Turbine Fuels, employs a separometer to determine the efficiency by which a fuel-water emulsion can be separated by a coalescer, assessment being based on passing a prepared water-fuel emulsion through a standard glass-fiber coalescer and measuring the turbidity of the effluent by light transmission. An arbitrary scale described as WSIM (i.e. Water Separation Index Modified) indicates 100 for clear fuel, and less for any remaining water droplets. A small-scale version of this apparatus, D 3948, "Determining Water-Separation Characteristics of Aviation Turbine Fuels by Portable Separometer," comprises a portable micro-separometer with a high-speed mechanical stirrer, giving a numericaJ rating designated MSEP. However, the situation is complicated by the fact that certain mandatory fuel additives, either singly or in combination, depress WSIM number without, in reality, interfering with the water separator. Tanks for kerosine usually have fixed roofs, coned to shed rainwater and snow, with free-vents to assist drying. A coned o w n b o t t o m of m i n i m u m slope 1 in 30 leads t o a water s u m p a n d drain valve, and a floating suction line ensures freedom from water entrainment during offtake. After filling, a period of 1 h is usually considered sufficient for water droplet settlement, irrespective of tank depth. Health Issues in Handling The major physiological reactions on exposure to the highperformance transport fuels may have localized and/or systemic (remote) effects, a n d are tabulated under the following headings: • Ingestion—swallowing of liquid into the digestive tract • Inhalation—in-breathing of vapor into the lungs • Aspiration—introduction of liquid into the lungs either through inhalation or by vomiting • External—contact with skin and, particularly, eyes. Furthermore, some materials are slow t o eliminate (for example, methanol), whereas others are cumulative (for example, decaborane).
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The evaluation of atmospheric quality with regard to the presence of contaminants is commonly expressed in terms of the threshold limit value (TLV) defined as the acceptable airborne concentration of the given c o n t a m i n a n t in occupational exposures for u p to 8 h daily without adverse effects. However, since the volatility of the contaminant determines its vapor concentration at ambient temperature, it is also customary to use the hazard index, defined by (VP/TLV), where VP is the vapor pressure. In broad terms, internal effects comprise nausea, dizziness, h e a d a c h e , and irritation of the m u c o u s m e m b r a n e , leading to loss of consciousness with the aromatics, a n d blindness with methanol. External contact can cause cold b u m s , dermatitis, eye damage, and skin cancers. Reference to the fuel manufacturers' handbooks is recommended in all
FUTURE TRENDS Extrapolation of past experience into the future suggests continuing use of conventional fuels for as long as is practicable, supplemented by virtually identical fuels derived from sources other than petroleum. Subsequently, recourse will need to be made to substitute materials, as discussed below: Reformulation (Short Term) In the automotive world, the fuel approach to emission reduction is a process of reformulation. This entails tighter controls on vapor pressure, and on the m a x i m u m permitted concentrations of aromatics, olefins, and sulfur. With jet fuels on the other hand, component concentrations are already very closely controlled, but the "product giveaway" (the difference between the actual property and its specified limit) has h a d to be reduced progressively for reasons of economics. Nevertheless, recent reviews show modest improvement in aromatic content a n d smoke point despite small adverse changes in sulfur, freeze point, emd flash point. As the sources of crude oil change because of shifting markets a n d availabilities, the product processes will have to be continually adjusted t o achieve the necessary specification, and these will no doubt involve some form or reformulation to a greater or lesser extent. Supplemental Fuels (Medium Term) Both natural gas and coal can be converted to "syngas" (CO + H2), which in turn can be synthesized to distillate-type liquids either through WEIX, which is then hydroconverted catalytically, or via a methanol stage. Alternatively, coal can be de-ashed a n d its molecular components reduced in size sufficiently to give liquids directly without the additional energy required for the more extensive reduction to gaseous dimensions. Hydrogenation of the aromatic components then gives the saturates—^paraffins a n d naphthenes. More exotic alternative sources of hydrocarbons include shale oil, tar, coal, peat, and biomatter generally. The first two have not yet emerged as conventioned fuel sources with established production rates, and therefore cannot be assigned meaningful (reserves-production) ratios, but large deposits are known to exist worldwide.
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HANDBOOK ing either pre-vaporization within the hot flame zone or relatively fine atomization. They are burnt with minimal radiation at virtually constant pressure, with flame stability and temperature distribution across the combustor section controlled by engine design. Combustion efficiency is at a premium, despite the very short residence time of the fuel molecules within the combustor, not only for fuel economy, but also for minimal emissions. In addition, handling properties are required to meet the wide range of temperatures and pressures pertaining throughout the supply system a n d within the aircraft itself during service. All these requirements combine to comprise a relatively extensive specification list of some 28 items or more, as summarized in Table 7, yet without precluding either availability or economics.
Substitute Fuels (Long Term) The longer-term approach towards future replacements is to use alternative fuels that may bear very little similarity to the c o n v e n t i o n a l . This class may be described as "Substitutes," and includes the compounds of carbon with hydrogen, oxygen, and nitrogen. For aviation, the most promising candidate substitutes appear to be liquid methane and liquid hydrogen, but the handling problems with cryogenic fuels are substantial.
SUMMARY AND CONCLUSIONS Current fuels for jet engines are of the petroleum-derived mid-distillate kerosine t3'pe with moderate volatility requir-
TABLE 7—Major features of representative U.S. jet fuel specifications (certain variations apply depending on type and concentrations of additives, etc.).
Property Color, Saybolt, min Total acid no. mg KOH /g, max Aromatics, % vol., max Olefins, % vol., max Sulfur, total, % mass, max Sulfur, mercaptan, % mass. max Initial boiling point, °C 10% vol. recovered, °C 20% vol. recovered, °C 50% vol. recovered, °C 90% vol. recovered, °C E n d point °C, max Flash point °C, min Reid vapour pressure @ 38.7°C, kPa Density® 15°C, kg/L Freezing point, °C, max Viscosity @ -20°C, mm^/s, max Specific energy, net, MJ/kg, mm Hydrogen content, % mass. min Smoke point, m m , m i n Copper strip corrosion, max Thermal stability (JFTOT) Change in pres. drop, m m Hg, max Tube deposit code TDR spun mcix Existent gum;, mg/100 mL,
Wide-cut Gasoline MIL-DTL-5624T JP-4 NATO F-40
Kerosine MIL-T-813133D ASTM D-1655 JetA-l" NATO F-35
High-flash Kerosine MIL-DTL-5624 JP-5 NATO F-44
Report 0.015
Report 0.015
Report 0.015
25.0
25.0 5.0 0.30 0.002
0.40 0.002 Report Report m i n 100 min 125 Report 270
Report max 205 Report Report Report 300 38
Thermally stable fuel TMIL-DTL-25524E JPTS
Low volatility fuel MIL-DTL-38219D JP-7
-1-24 0.015
-1-25
23.0-27.0
5.0-20.0
5
0.40 0.002
0.3 0.001
0.1 0.001
Report max 206 Report Report Report 300 60
min. 157 max 193
0.751-0.802 -58
0.775-0.840 -47 8.0
0.788-0.845 -46 8.5
0.767-0.797 -53 12 @ - 4 0
min 182 min 196 m i n 206 Report max 260 288 60 max 20.7 @ 149°C & 3 3 1 @260°C 0.779-0.806 -43.3 8.0
42.8
42.8
42.6
42.8
43.5
13.5
13.4
13.3-13.5
14.0
14.40
20.0 1
25.0 1
18.0-21.0 1
25.0 lb
lb
25.0
25.0
25.0
25
25.0
<3
<3
<3 12 5.0
12 5.0
max 204 max 238 260 43
14-21
7.0
7.0
7.0
High density synthetic MIL-P-87107B JP-10
54.4 0.935-0.943 -79 40@-54, 10@-18 42.1 (E.D. 39.4 MJ/L)
10 2 5.0
max 1.0 1.0 1.0 0.3/0.5 0.3/0.5 1.0 Particulate matter, mg/L, max 10 15 15 Filtration time, minutes, max lb lb lb lb lb Water reaction interface rating, max Microscparator rating, min 70/90 70/85 70/90 85 Report Electrical conductivity. 150-600 50-450 pS/m, "Known as F-34 (JP-8) when additized with FSII and CI /LIA. The Jet Fueling System Check List embodies the most stringent requirements of Jet A-1, the British DEF STAN 91-91, and the lATA Guidance Material for Aviation Turbine Fuels. NOTE—1. Distillation tests incorporate limits of 1.5% for both residue and loss. 2. Types and concentrations of additives are also specified.
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Conventional and some special aviation kerosines are likely to be available from petroleum sources for the next few decades, after which they are likely to be supplemented by virtually identical materials from other natural sources. For the higher performance applications, attention is likely to turn to the C.H.O.N compounds, and possibly boron. From emission considerations, molecules with the higher proportions of hydrogen are attractive, with an eventual elimination of carbon by the adoption of liquid hydrogen. Nevertheless, the handling problems with cryogenic fuels are substantial, and the presence of large quantities of water vapor in the stratosphere may incur problems.
ASTM STANDARDS No. D 86 D 93 D 341 D 381 D 445 D 1298
D D D D D
1322 1740 2386 2533 2550
D 2624 D 3241 D 3338 D 3825 D 3948 D 4305 D 4308 D 4809 D 5001
E 659
Title Distillation of Petroleum Products at Atmospheric Pressure Flash-Point by Pensky-Martens Closed Cup Tester Standard Viscosity-Temperature Charts for Liquid Petroleum Products Gum Content in Fuels by Jet Evaporation Kinematic Viscosity of Transparent a n d Opaque Liquids (The Calculation of Dynamic Viscosity) Density, Relative Density (Specific Gravity) or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method Smoke Point of Kerosine and Aviation Turbine Fuel Luminometer Numbers of Aviation Turbine Fuels Freezing Point of Aviation Fuels Vapor-Liquid Ratio of Spark-Ignition Engine Fuels Water Separation Characteristics of Aviation Turbine Fuels Electrical Conductivity of Aviation and Distillate Fuels Containing a Static Dissipator Additive Thermal Oxidation Stability of Aviation Turbine Fuels (JFTOT Procedure) Estimation of Net Heat of Combustion of Aviation Fuels Dynamic Surface Tension by the Fast-Bubble Technique Determining Water-Separating Characteristics of Aviation Turbine Fuels by Portable Separometer Filter Flow of Aviation Fuels at Low Temperatures Electrical Conductivity of Liquid Hydrocarbons by Precision Meter Heat of Combustion of Liquid Hydrocarbon Fuels by B o m b Calorimeter (Precision Method) Measurement of Lubricity of Aviation Turbine Fuels by the Ball-On-Cylinder Lubricity Evaluator (BOCLE) Autoignition Temperature of Liquid Chemicals
SAE STANDARDS ARP492
Aircraft Engine Fuel P u m p Cavitation Endurance Test
REFERENCES [1] Goodger, E. M., Transport Fuels Technology, Landfall Press, Norwich, UK, October 2000.
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2] Goodger, E. M. and Vera, R. A., Aviation Fuels Technology, Macmillan Publishers Ltd., Basingstoke, UK, 1985. 3] Anonymous, "Theoretical Performance of Rocket Propellant Combinations," Rocketdyne, CA, January 1959. 4] Goodger, E. M., "Property Requirements of Liquid Rocket Propellants," Journal of the Institute of Petroleum, London, Vol. 46, No. 442, October 1960, pp. 314-27. 5] Wilson, W. E. and Berl, W. G., "Fuels," Ch. 6, Ramjet Technology, TG 610-6, The Johns Hopkins University, Baltimore, MD, July 1965. 6] Glassman, I. and Sawyer, R. F., The Performance of Chemical Propellants, AGARDOgraph 129, NATO, January 1970. 7] Samer, S. F., Propellant Chemistry, Reinhold Publishing Corporation, NY, 1966. 8] Sutton, G. P., Rocket Propulsion Elements, John Wiley, NY, 1963. 9] StuU, D. R. and Prophet, H., JANAF Thermochemical Tables, National Bureau of Standards, Washington, DC, 1971. 10] Broadbank, R., "New Technology Revolutionises Fuels Testing," Petroleum Review, June 1999, pp. 30-31.
BIBLIOGRAPHY 1] Schobert, H. H., The Chemistry of Hydrocarbon Fuels, Butterworths, London, UK, 1990. 2] Handbook of Chemistry and Physics, R. C. Weast, Ed., CRC Press, Cleveland, OH, republished periodically. 3] Rogers, G. F. C. and Mayhew, Y. R., Engineering Thermodynamics Work and Heat Transfer, Longmans Group Ltd., Harlow, UK, latest edition. 4] Rose, J. W. and Cooper, J. R., Technical Data on Fuel, The British National Committee, World Energy Conference, London, 1977. 5] Datschefski, G., Lewis, C, and Walters, M. B., Jet Fuel Specification Requirements, Defense Evaluation and Research Agency, London, 1997. 6] Fielding, D. and Topps, J. E. C, Thermodynamic Data for the Calculation of Gas Turbine Performance, Aeronautical Reseach Council R & M 3099, 1959. 7] Breitwieser, B., Gordon, S., and Gammon, B., Summary Report on Analytical Evaluation of Air and Fuel Specific-Impulse Characteristics of Several Non-hydrocarbon Jet Engine Fuels, NACA R M E52L08, 1952. 8] Dukek, W. G., Selected Hydrocarbons as High Performance Fuels, American Chemistry Society, Symposium on High Energy Fuels, Philadelphia, 25 Feb. 1960. 9] Brewer, G. D., Hydrogen Aircraft Technology, CRC Press, London, 1991. APPENDIX 1 T h e r m o d y n a m i c Properties a n d their Interrelationship in Static Gases A collection of hot gases suited to act as a working fluid in a heat-work conversion is known as a thermodynamic system. At any instant, this system exists in a particular thermodynamic state as defined by its set of properties, which are consequently described as state functions. Such a property is recognizable by the fact that it exhibits no change when the system goes through a complete cycle of events (as discussed in Appendix 2). In a thermodynamic system: • an extensive property is dependent on the mass of the system, e.g., volume V, internal energy U, enthalpy//, energy£, entropy S, etc. • a specific property is an extensive property expressed on a basis of unit mass of the system, e.g., specific volume v.
750 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK specific internal energy u, specific enthalpy h, specific energy e, specific entropy s, etc. • and an intensive property is independent of the mass of the system, e.g., pressure p, temperature T, height z, etc. In the above list, the three properties that can be measured physically by means of instrumentation are known as prime properties. These comprise pressure, volume, and temperature, the last-named being expressed in absolute terms in units of kelvin. These three properties can be related by combining the following two laws of gases: 1. Boyle's law—^At constant temperature, p a 1/pv, hence v = a function of T 2. Charles' law—^At constant pressure, v a T, hence v/T = a function of p Consider a mass m of system material at condition 1 going through a constant temperature process to condition 3, followed by a constant pressure process to condition 2. Then, piVi = P2V3, and V3/T1 = V2IT2 Combination leads to the prime properties being related to give the equation of state Pi V/ =P2V2TilTz,
• S, the entropy, is a measure of molecular disorder, and is equal to the ratio of the heat transfer between the system and its surroundings to the temperature, hence S = Q/T. In engineering thermodynamics, interest generally centers on changes in the above derived properties, hence At/, A//, AS, etc. APPENDIX 2 The Unique Nature of Thermodynamic Parameters Fortuitously, the complete set of properties of a given mass of a system at any given state is fixed by defining two properties only, provided that these properties are independent of each other, and that the system itself comprises a pure substance, that is, its chemical composition is both homogeneous and constant. This feature is known as the two-property rule. Fortunately, also, mixtures of air and gaseous fuel prior to combustion, and of exhaust gases subsequently, can both be classified as pure substances.
hence Pi vilTi = P2 V2IT2 = p v/T = constant = R The Unique Point One very valuable consequence of the two-property rule is that and it enables the state of a pure-substance system to be pV/T = mR represented by a unique state point on conventional graph paper, which is itself two-dimensional. As an example, the equawhere R is the gas constant for the particular material tion of state shows that a system comprising 1.23 kg of air oc(= 0.287 kJ/kgK for air). cupying a volume of 1 m^ at a pressure of 1 atm (101.325 kPa) A material that follows this equation exactly is known as an would have to exist at 288 K, all other properties being set by ideal gas. (Note that a perfect gas is an ideal gas with values these state conditions. The two-property rule provides a most of specific heat capacity constant at all temperatures.) The valuable tool in that the complete thermodynamic state of this molar volume, VM, of a gas is the volume occupied by one air system can be represented by a unique point, X, on the twomole of molar mass M, and from Avogadro's law, this is comdimensional p-V graph shown in Fig. A2.1. mon for all ideal gases at the same temperature and pressure. Hence, since VM = M R T/p and is common, (M R) also is common, and is known as the Universal Gas Constant, Ro, equal to 8.3143 J/mol K at 1 atm and 0°C. The remaining properties are discussed further below: • U, the internal energy, is the sum of all forms of energy contained within the internal structure of the system material itself. It therefore comprises: the internal potential energy stored within the atomic and molecular particles from work done in moving them apart against binding forces of mutual attraction, and the internal kinetic energy of the particles moving randomly in translation, rotation, vibration, and spin. • H, the enthalpy, is the sum of the internal energy and the flow energy involved in penetrating the surface of the imaginary space of interest (the control region) where conditions and processes Eire being examined. This latter term is derived simply from the following expression: Flow energy = work done in penetrating surface = Force X distance moved = (force/area) X (distance X area) = pV Hence, H =U+pV • E, the overall energy of the system, incorporates H and some other parameters (see Appendix 6).
The Unique Process Line If the initial thermodynamic state is changed progressively through different states, for example by external transfers of energy, the various unique points can be connected by a
0.5
1
1.5
Volume, Vm3
FIG. A2.1—Representative reversible {p-V) paths for ideal gas. (Point X represents 1.23 kg air at 288 K) [1].
CHAPTER 28: ENGINEERING SCIENCES OF AEROSPACE FUELS 751 unique line on the graph representing the process of change. Such a hne follows a relationship of the form p y " = constant. An infinite n u m b e r of processes is possible depending on the value of the index n, which can vary from 0 to <». The following values of n, and the names of the related processes, are shown in the figure, and are listed below: n
Process Name
0 1 >1 to<-)' y
Isobaric (constant pressure) Isothermal (constant temperature) Polytropic Adiabatic" (zero heat transfer) Isochoric* (constant volume)
"Also described as isentropic in the ideal case of a reversible process since entropy remains constant throughout. Sometimes described as isometric.
Cyclic
TABLE A2.2—Derivation of efficiency of steady-flow Bra5^on cycle [1]. Process (1-2) Isentropic compression (2-3) Isobaric heating (3-4) Isentropic expansion (94-1) Isobaric cooling Tn =
The transfers of heat and work associated with the above processes are shown in Table A2.1 The Unique
ciency of the Brayton cycle, operating on a steady-flow basis, is shown in Table A2.2, and calculated values are shown plotted in Fig. A2.2. It is interesting to note that, although temperature (T) is a property of a system, whereas an external transfer of heat (Q)
i -
-Cp{T, -To)' CpiTs --T2)
.
Heat Transfer
Work Transfer
0
-CpiTj - T2)
CpiTs - T,)
0
0
Cp(T3 - T4)
-Cp{T, - T4)
0
{T4 - T,) (T3 - T2)
From isentropes 1-2 and 3 ^ T4/T3 = (P4IP3)'-^ -'^"'= U/rpP - '>'^ = (pi/pzf'' ''^'->= T1/T2 Hence-n = 1 -(;/(>)<•>'"-"'^
Process
It is evident that reversible processes can be arranged sequentially in a variety of ways so that they return to the initial point and comprise complete cycles. Thus, as outlined earlier, the properties do not change once the cycle is completed since the initial and final states coincide. Clearly this cyclic process encompasses a fixed area (which represents work transfer in the case of pressure-volume axes), and can be repeated indefinitely. Each individual component process is related to external transfers of energy (as heat and/or work), which are all calculable from thermodynamic theory. Consequently, suitable selection of component processes, and of their relationship within the cycle, can result in overall transfers of heat and work. Traversing the cycle diagram clockwise gives rise to the potential of the continuous conversion of heat input to work output (as in a heat engine), provided the second law of thermodynamics is followed in that such a conversion can never be complete, i.e., some of the heat input must be rejected as heat output at a level of temperature lower than the initial. The derivation of the effi-
Compression ratio r^ 10 20 L Typical S-l Typical C-l may r
maX i j .
60 OTTO
2 'o 40 ic (U
. •" -
maxrn Cut-off ratio Spark ignition piston engine Compression ignition piston engine Gas turtine engine
—r 10
"T—
20
Isothermal
r=k
Non Flow
Steady Flow °
Cp (T2 - Ti)
R (T2 - Ti)
0
R T In (p,/p2) = RTln(v2/v;)
RT\n{p,/p2) = « r i n (V2 /V,)
^ T ^ (Ti - T2)
—I
n= 1 Pol3^ropic pv° = k n = n
(7 - n) , ^ Cv j^ _ J {T,
^ , T2)
dD
SI
30
FIG. A2.2—Variation of engine cycle thermal efficiencies.
Heat Transfer
RT\n(p,/p2) RT\n(v2/vt)
20
Pressure ratio Xp
Work Transfer Isobaric p = k n = 0
75 E )_
Typical GT a S-l C-l GT
TABLE A2.1-- E n e r g y distribution in thermodynamic processes \_p v" = constant, a n d p V = i? r ] . Process
30
(T, - T2)
Isentropic p v"* = k n = -y
0
Cv {Ti - T2)
Cp {T, - T2)
Isochoric v = k
Cv {T2 - T,)
0
R {T, - T2)
n = 00
' Assuming no changes in potential (height) or kinetic (velocity) energies.
752 MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
is not, the quotient (Q/T) - which is the entropy (S) - does meet the definition of a property since it does not change over the complete cycle. A fuller discussion of these thermodynamic issues is given in Ref. 1.
APPENDIX 3
TABLE A3.1—Thermochemlcal bond enthalpies, kj/mol [1]. Bond
Enthalpy
Bond
Enthalpy
AHa.H2(g) AHa.02(g) AH,.N2(g) AHa.C2(gr) D (H—O) E (H—0)
435.4 498.2 946.2 717.2 428.7 463.1
D(H—OH) E(C—H) E(C—C) E(C=C) E(C=C) E(C—0)
497.5 414.5 347.5 615.5 812.2 351.7
Oxidation Heat Release The study of heat transfers associated with chemical reactions is termed Thermochemistry. Since the events of interest here relate to flowing, rather than non-flowing, fluids at constant pressure, changes of energy Eire expressed in terms of enthalpy r a t h e r t h a n internal energy (see the section on Calorific Values). The formation of a molecule (for example, a fuel) from its component elements can be imagined as the following twostage process: 1. Absorption of sufficient enthalpy to release the individual atoms of the c o m p o n e n t molecule into freely gaseous form, as for example; C(gr) ^ C(g), and H2(g) ^ 2 H(g)
which is the mean value for a n u m b e r of such bonds in different molecules, and in different locations within them. A simple physical analogy of the complete formation process is given by a mass descending from a depression in a hilltop; the overall change in potential energy is the net result of the climb to the lip of the depression, and the subsequent descent from it (Fig. A3.1). Representative values of AHa, D(X—^Y) and E(X—Y) are given in Table A3.1 Hence, in the methane example above, the net enthalpy released, which is described as the standcird enthalpy of formation, AHf, is given by: A//f .CH4 = lAHa - l E ( X - Y ) approximately
where (gr) represents carbon in its standard condition of graphite, and (g) represents gas. 2. Release of excess enthalpy on the combination of these free atoms on the formation of the combined molecule, as for example; C(g) + 4 H(g) -> CH4(g) methane The energy involved in the first stage of the process is designated the "enthalpy of atomization," AHa, and is invariably directed inwards. In thermochemistry, therefore, this is classed as positive since it adds to the total stock of energy of the elemental material. The second stage involves an energy release which, although considered negative on the above basis, is customarily described in the opposite sense as the "bond dissociation enthalpy," D(X—Y), representing the atomization enthalpy required to dissociate the X—^Y bonds of the completed molecule back to free atoms. For convenience, use is often made of the empirical bond enthalpy, E(X—Y),
= [AHa-Cigr) + 2 AHa-H2{g)] - 4 [ E ( C - H ) ] [717.2 + 870.8] - 1658.0
70 kJ/mol
(cf. — 74.90 kJ/mol by measurement) The term "standard" (superscript "o") indicates that the initial and final temperatures are c o m m o n at 25°C. The negative value above indicates that a net release of entheJpy occurs during formation, hence the resulting methane molecule is more stable than its parent elements. Values of AHf for other hydrocarbon molecules are included in Table 3. If now the dissociation enthalpy (but see activation energy below) is applied to a fuel molecule, together with the atomization enthalpy of its stoichiometric oxygen molecules, the total number of free atoms may rearrange themselves as oxide products, and so fall into a m u c h deeper enthalpy trough as they release their new quantities of bond dissociation enthalpy. Hence, Standard enthalpy of oxidation reaction = AH? = 2:(A//;)p -
= E(n AHf\
I(A//;?)R
- Km
AHf\
where subscripts P cmd R refer to products and reactants respectively, mi = moles of reactcmt i, and
XD (X-Y)p
(AHpP AH?
Uj = moles of product j . As an example, consider the stoichiometric oxidation of methane to gaseous CO2 and H2O given the following vaJues; A///-CH4(g) = - 7 4 . 9 0 kJ/mol
Product molecules
FIG. A3.1—Schematic of standard molar enthalpies of formation (A H,°) and of reaction (AHr°) [1].
AHf •C02(g) = - 3 9 3 . 5 2 kJ/mol, and A//°-H20(g) = - 2 4 1 . 8 3 kJ/mol Since CH4(g) + 2 02(g) = C02(g) + 2 H20(g),
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it follows that A/:/^°.CH4(g) = [1 (-393.52) + 2 (-241.83)] - [1 (-74.90) + 0]
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753
The Arrhenius plot also gives: Gradient = E/R^ = 73 100/8.314 3 = 8792 K
= - 3 9 3 , 5 2 - 483.66 + 74.90 APPENDIX 4
= - 8 0 2 . 2 8 kJ/mol It is noteworthy that the enthalpy of formation of O2, N2, C(gr) and other elemental molecules at their standard conditions is zero since AHa and D(X—^Y) are equal and opposite in sign. It is also of interest to note that the above value is a net quemtity since the product water is in the vapor phase. (See the section on Calorific Values.) From the above discussion on enthalpies of formation, it might appear that the full complement of the dissociation bond energies would be required in order to dissociate a fuel molecule into its component free gaseous atoms of carbon and hydrogen. However, the chain nature of the ignition process implies that only one bond need be broken to start the chain. Furthermore, some molecules will have more than the average level of energy, and also both bond breaking and remaking are occurring together. All these factors result in an "activation energy" of a considerably lower level t h a n D(C—C) or D(C—H). The actual value can be determined experimentally by plotting ignition temperature (T) against delay (t) on the Arrhenius basis as follows:
Proportions of Dissociated Combustion Products At temperatures above about 1800 K, the thermal agitation of combustion products is such that they begin to dissociate back towards their reactant materials, giving reversibility to the combustion reaction, thus A + B^
combustion. dissociation
At a sustained high temperature, therefore, a condition of dynamic equilibrium exists with rates of combustion and dissociation exactly equal, so t h a t the reactant a n d product materials coexist in proportions that remain constant. These proportions can be determined by using values of parameters known as "partial pressure equilibrium constants" at the given temperature, as shown below. The rates of such reactions at selected temperatures can be determined by experiment, and are found to be proportional to the instantaneous concentration of each material raised to some power, that is, forward reaction rate = kp [A]^ [B]'', and
Reaction rate where
q t
e E Ro T
dt " ^
quantity of heat released per unit mass reaction time = Naperian base = 2.7183 = activation energy under the given conditions = universal gas constant = 8.3143 J/mol K = absolute temperature
reverse reaction rate
kR [C]'^
where
instantaneous molar concentration of material X,
[X]
••
dq Since --T- a t -\tae'^''°^:
constant e
k = rate constant for the reaction, and a, b, and c are experimentally-determined powers. Since the two rates are equal at d5Tiamic equilibrium.
[Cf
It follows that Int
In (ti/t2) = Hence, E
E
_\_
J_
Rn
Ti
T2
Ro lniti/t2) Ti
J_
T2 For n-octcine the calculations appear as follows: 8.3143 In (20/1.2)
It follows from Avogadro's law (equal volumes of all gases at the same t e m p e r a t u r e a n d pressure contain the same n u m b e r of molecules) that
^
^
598/ 8.3143 In 16.67 0.001992 - 0.001672
8.3143 X 2.8135 0.00032
= 73 100 J/mol = 73.1 kJ/mol This compares with 347.5 and 414.5 kJ/mol for initially dissociating the C—C and C—H bonds respectively, and with 208.59 for AHf.
t, = K = partial pressure equilibrium constant for the reversible reaction,
Furthermore,
—^ = -—n r. where p = total pressure. p total mol ^ ^ Thus in each case, p^ can be expressed in the following manner: Px = P, (
L\
^^502
= concentration equilibrium constant for the reversible reaction
Y + constant + constant
which is the equation of a straight line of In t against 1/T. The value of £ can then be determined by taking two points on the straight line, as follows:
U
^ = K' kR
[A]^ [Bf
1, where Uj = total moles of product present.
Since carbon oxidizes in two stages, first to CO and then to CO2, it is the final stage at the high temperature that experiences dissociation, that is CO + O.5O2 ^ CO2 Hence,
PCO2 pCO {pOzT
,, _ Kco, —
"CO2 nr>_/n+^0.5 "C0(p"02/"t)'
754 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK Similarly for the oxidation of hydrogen,
Solution for the six unknown values of n requires six equations, four of which are provided by molar balances of C, H, O, and N as for the simple non-dissociated case. The remaining two equations are then derived from the published values of Kco2 and KHJO for the temperature in question, where
H2 + 0.5 O2 ^ H2O PH2O PH2 ip02)°'
Hence,
"H2O = K, '' " " ' " "H2 (p"O2/"t)0-5
(Note; These two equilibrium constants are used for solving the dissociated general mixture case below). Since both reactions are occurring together, the half mole of oxygen produced by dissociation of the mole of carbon dioxide may be considered as the oxygen required by the mole of hydrogen. Combination of the two combustion equations then gives: CO2 + H2 ^ CO + H2O This is known as the water-gas shift reaction (not to be confused with the water-gas reaction, which is: CO -I- H2 ^ C + H2O) and leads to: Water-gas shift equilibrium constant = KWGS K,H20
PCOPHIO
KcOj Pco2 Pw.2 (Note: This equilibrium constant is used for solving the imaginary non-dissociated rich mixture case since, although n o dissociation complications arise, the distribution of the limited oxygen available to the CO and H2 has to be determined). Selected values of the partial pressure equilibrium constants are given in Table A4.1. Although at the higher temperatures dissociation proceeds further to promote radicals such as O, H, OH and NOx (that is, NO + NO2), a first approximation to realistic conditions can be made by considering dissociation restricted to CO, H2 and O2 only, giving the following combustion equation for any general mixture strength; CaHb + m (O2 + 3.76 N2)
where m = moles of 02/mole fuel, as determined from the given air-fuel ratio. Solution follows by iteration on values of ns/nt. It might be assumed that, as the products cool, the effects of dissociation would vanish, and the combustion equation then revert to the complete form in Eq 2 of Stoichiometry. But in fact, the partially-burnt products endure on cooling due to freezing of the reactions when energy is lost through contact with the walls of the containing vessel and any inert molecules present. TABLE A4.1—Partial pressure equilibrium constants for oxidation reaction, atm~°'^ [9]. 298.15
300 500 1000 1500 2000 2100 2200 2300 2400 2500 2700 3000
KcOj
1.1641 X 10"^ 575.44 X lO''^ 10.593 X lO^'' 16.634 X 10^ 207.01 X 10^ 765.60 345.94 168.27 87.097 47.753 27.543 10.351 3.0549
K,H20 •
ni n j {p Us/Ut)"
and
n2
n4 {p ns/nt)"
Computer programs exist for the determination of product concentrations, but the method of solution shown below permits derivation from first principles, typical of those o n which the computer software itself is based. 1. At given temperature, read Kco2 and KHJO from Table A4.1 1. Assume value for ns/nt, and evaluate (p ns/nt)"-^ 3. Evaluate ni/ua and n2/n4 from Kco2 and KHJO expressions respectively, using Step 2 value. 4. Evaluate Ui and ns from carbon balance, and n2 and n4 from hydrogen balance using Step 3 value. 5. Evaluate ns from oxygen balance. 6. Evaluate nj from nitrogen balance. 7. Evaluate nt = n5/(assumed ns/ut), and compare with Enj, with j = 1 to 6. 8. Repeat from Step 2 until Ut = Xnj. Example Allowing for dissociation to CO, H2, and O2 only, the stoichiometric combustion equation for methane-air at its isobaric adiabatic combustion temperature of 2247K (Appendix 5) appears as follows: CH4 + 2 (O2 + 3.76 N2) = 0.903 CO2 + 1.961 H2O + 0.097 CO + 0.038 H2 + 0.068 O2 + 7.52 N2
= ni CO2 + n2 H2O + na CO -I- n4 H2 + Uj O2 + ug N2
Temperature, K
Kco2
KH20
KWGS
11.169 X 10^^ 6.1094 X 10^' 76.913 X 10^' 11.535 X 10' 530.88 X 10^ 3.4670 X 10^ 1.6866 X 10' 874.98 480.84 277.43 167.49 68.077 22.029
9.5945 X 10"^ 10.617 X 10"^ 7.2607 X 10"' 0.6935 2.5645 4.5285 4.8754 5.1999 5.5207 5.8097 6.0810 6.5769 7.2110
Comparison of these product quantities with those for the non-dissociated case below shows clearly the influence of dissociation. CH4 + 2 (O2 + 3.76 N2) = CO2 + 2 H2O + 7.52 N2
APPENDIX 5 Calculation of M a x i m u m Reaction Temperature The method of solution is based on the concept of equating the enthalpy released by the reactants, in generating the dissociated products at the initial temperature, with that which would have been required to heat those products from the initial temperature to the final temperature J*. As in most thermochemical work, the standard initial temperature is taken as 25°C (298.15 K). Hence, [Chemical enthalpy released (negative) with reactants at 298.15 K oxidized to products at 298.15 K, that is, the standard enthalpy of reaction] is equal to [Physical enthalpy absorbed (positive) by products in heating from 298.15 K t o r * ] that is, - MIr° = (//^*jp algebraically But \Hr° = [{^H^)p - (^H^)B.'\ Thus, (H^* + A//;)p - (AHf )R = 0
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28: ENGINEERING
Values of AHf for the products are listed in the literature [9], together with values of the physical enthalpies absorbed when heating from 298.15 K to the different levels of T. The author has found it helpful to sum the two values shown on the left-hand side above and designate it as total thermochemical enthalpy at temperature T, ( H i ) , tabulating these sums for direct use at the temperature levels of interest. The above equation then becomes
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OF AEROSPACE
FUELS
755
E. Repeat from step A at new level of T (say 2300 K). F. Find T* by interpolation (or minor extrapolation) to give zero in equation of step D. (All values of n at T* can be found by similar extrapolation). Example All gaseous reaction of stoichiometric methane-air at 1 atmosphere with dissociation pattern as shown;
(AH/)R = 0
where H[* = total thermochemical enthalpy at temperature T* based on initial 298.15 K
CH4(g) + 2 (O2 + 3.76 N2) = ni CO2
Expansion gives
For methane, a = 1, and b 4. A. Select T = 2200 K B 1 Kco2 = 168.27, KHJO = 874.98 2 Assume nj/ut = 0.00519, thus (p ns/ut)"-^ = 0.07204 3 ni/nj = 0.07204 (168.27) = 11.936 n2/n4 = 0.07204 (874.98) = 61.387 4 n3 = a/(l + ni/ns) = 0.07730 ni = 1 - nj = 0.9227 n4 = (b/2)/(l + n2/n4) = 0.03206 n2 = 2 - n4 = 1.9679
Euj (Hr*)j - Emi (A//;)i = 0,
for reactants i and products j
Since AHf-Oz = Mlf-Hz = 0 due to the self-canceling of the atomization and bond dissociation enthalpies, (for example 02—i^Ha) -^ 2 O—[D(0—O)] -^ 02), it follows that the previous expression can be simplified further to Euj (fff*)j - mfuei (AH/)fuei = 0, and then to EUj ( / / r * ) j -
+ n2 H2O -t- n3 CO + n4 H2 -I- ns O2 + 7.52 N2
(A///°)fuel
= 0, since only one mole of fuel is usually considered. Since both linear and non-linear equations require solution simultaneously here, iteration is necessary on T in the non-dissociated case, and on both n and T in the dissociated case. As with product concentrations, computer programs exist for the determination of maximum reaction temperature, but derivation from first principles using the total thermochemical enthalpy concept is as shown below. A. Select some appropriate level of t e m p e r a t u r e T (say, 2200 K). B. Determine all molar product "n" values; For the non-dissociated case, from Eq 3 For the dissociated case, from the method shown in Appendix 4 C. Weight each value of n by its appropriate H[ at the selected T, from Table A5.1 D. Sum these weighted values, subtract {8Hf)iue\, and check for zero.
5 ns
2m - X - (ni + n2) _ 3 - 2.8906
= 0.05468
6 ng = 7.52 ^ 0.05468 ns 10.536 (assumed nj/ut) 0.00519 Suj = (10.52 + ns) = 10.575
7 n,
Since slight difference between Ut and Xuj, repeat from step B. 2 using ng/ut = 0.00518 This gives reasonable equality of 10.57, and all appropriate values of n, as follows: ni = 0.923; uj = 1.968; ns = 0.774; n4 = 0.0321; ns = 0.0547 C. Weight each n value with its appropriate Ht^ value at 2200 K e.g., for CO2, ni {Hf) = 0.923 (-289.95) = - 267.6 D. Summation of these weighted values, less formation enthalpy for fuel, gives: Snj (Hj)i - (A//;)f,ei = - 1 0 2 . 4 4 - (-74.85) = [-27.59] Since this is not zero, repeat from Step A for new temperature of 2300 K.
TABLE AS. 1—Standard-based total thermochemical enthalpy levels for• gases, kj/mol [9] [1], H[= (H^ + AH}) for Compounds
Hi = H"for Elements
Temperature, K
CO2
H2O
CO
H2
O2
N2
298.15 300 500 1000 1500 2000 2100 2200 2300 2400 2500 2700 3000
-393.52 -393.46 -385.21 -360.12 -331.81 -302.07 -296.02 -289.95 -283.85 -277.73 -271.60 -259.27 -240.66
-241.83 -241.76 -234.91 -215.85 -193.73 -169.14 -164.00 -158.79 -153.53 -148.22 -142.86 -132.01 -115.47
-110.53 -110.47 -104.60 -88.843 -71.680 -53.790 -50.154 -46.509 -42.853 -39.183 -35.505 -28.121 -16.987
0 0.054 5.883 20.686 36.267 52.932 56.379 59.860 63.371 66.915 70.492 77.718 88.743
0 0.054 6.088 22.707 40.610 59.199 62.986 66.802 70.634 74.492 78.375 86.199 98.098
0 0.054 5.912 21.460 38.405 56.141 59.748 63.371 67.007 70.651 74.312 81.659 92.738
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Similar calculations give ns/iit = 0.00785, hence; n , = 0.881; na = 1.953; na = 0.119; n4 = 0.047; nj = 0.083 Summation of weighted values gives [ + 3 1 . 6 3 ] This also is not zero, but interpolation with value - 27.59 from Step D gives r * = 2247 K Similar interpolation gives the related vEilues of the product moles: ni = 0.903; nz = 1.961; nj = 0.097; n4 = 0.038; ns = 0.068 Note For stoichiometric methane-air, T* = 2327 K, nondissociated = 2247 K, dissociated to CO, H2, and O2 only (as shown above) = 2223 K, dissociated to CO, H2, O2, H, O, OH, and NO For stoichiometric methane-oxygen, r * = 3045 K, dissociated to CO, H2, O2, H, O, OH, and NO
In the absence of the effects of gravity or motion, this energy change is located entirely within the internal energy of the system material, hence: n.f. (q — w) = and
The Energy Equation From the laws of thermodynamics, it is evident that, for a system operating in a cycle, the initial and final states are identical, and the net heat input is equal to the work output, with some of the heat input being rejected to a lower temperature sink. On a specific basis, therefore: qnet - W = ( q i n -
Qout) " W = 0
This is usucJly expressed simply as: q - w = 0 When the process is not cyclic, the initial and final states differ, and the energy equation then becomes: q — w = Ae
(A6.1)
n.f. d q z - 1W2) = M2 - Ml
This is known as the non-flow energy equation (n.f.e.e.), any change in internal energy leading to changes in temperature or phase. In the presence of effects of gravity or motion, as in a system flowing steadily through a control region, balaxices of mass cind of energy appear as shown below: Mass balance: mi = m2 = p A C = Ai Cj/vi = A2 C2/V2 where p = density of fluid A = cross-sectionzJ area of flow C = velocity of flow relative to control region, and V = specific volume of fluid = 1/p Energy balance: s.f.(iq2 — 1W2) = (hi -hi)
APPENDIX 6
AM
+ g (z2
^2
£2^
where z is the height above the Earth's surface, or some other datum. This is known as the steady-flow energy equation (s.f.e.e.), which can also be expressed in various reduced forms depending on the constancy of certain terms. One useful form concerns negligible changes in both potential and kinetic energies, applicable to horizontal flow with approximately similcir entry and exit velocities, as shown below: Reduced s.f.e.e. = s.f. dqz — 1W2) = h2 — hi
(A6. 2)
This compares directly with the non-flow case in Eq A6.1. Since this present study concerns steady flow applications to jet engines, most t h e r m o d y n a m i c considerations deal in terms of enthalpy rather thein internal energy.
MNL37-EB/Jun. 2003
Properties of Fuels, Petroleum Pitch, Petroleum Coke, and Carbon Materials Semih Eser^ and John M. Andresen^
their important properties, including standard methods of measurement and the significance of individual properties in industrial applications.
THIS CHAPTER PROVIDES AN OVERVIEW ON THE ORIGIN, PROPERTIES,
AND APPLICATIONS of fuel oils, petroleum pitch, petroleum coke, and some related carbon materials. A c o m m o n thread among these materials is the line of p r o d u c t i o n that connects petroleum refining to the manufacture of carbon materials. Bottom fractions from catalytic cracking of gas oil (FCC decant oil) or from thermal cracking of naphtha (ethylene tar) can be used as residual fuel oil, or as feedstocks for producing carbon black, petroleum pitch, and p r e m i u m petroleum coke for graphite electrodes. Petroleum pitch is, in turn, used for producing carbon fibers and carbon-carbon composite materials, and for densification of graphite electrodes used in electric-arc furnaces to recycle scrap iron and steel. The residua from vacuum distillation in petroleum refining are subjected to severe thermal cracking in coking processes to produce light and medium distillates and petroleum cokes that are used as solid fuel, or as filler for manufacturing carbon anodes for electrochemical production of aluminum. It is important to recognize that ferrous metals and light metals industries, as well as manufacturing a n u m b e r of carbon materials, are closely linked with the products of petroleum refining, including decant oils, petroleum pitch, and petroleum coke.
S p e c i f i c a t i o n s a n d A p p l i c a t i o n s o f F u e l Oils Fuel oils are burned to generate heat for different purposes ranging from home heating to raising steam in utility boilers to generate electricity. Different types of burners used in these applications under various climatic and operating conditions dictate the need for different grades of fuel oils. A standard specification by the ASTM International has divided fuel oils into five basic grades, designated as Nos. 1, 2, 4, 5, and 6 (ASTM D 396). Based on the production methods used in petroleum refining, fuel oils fall into two broad classifications: distillates and residuals. The distillates comprise overhead or distilled fractions, whereas the residuals consist of bottoms remaining from the distillation, or blends of distillates with the bottoms from distillation, visbreaking, and catalytic cracking processes [2-4]. The six grades of oils can consist of different t5^es according to the refining processes used in their production, as described in the section on Petroleum Refining [5]:
For each material, production processes, product properties, and specifications are discussed in the context of respective industrial applications. Standard methods used for characterization of these materials are identified with a focused discussion on the interpretation of the results obtained from the standard tests. Literature references are provided for further information on each topic.
1. 2. 3. 4.
FUEL OILS Petroleum refining processes generate many product streams that can be used as fuel oil either in single streams or in blends to adjust the desired properties for specific applications. A broad definition of fuel oils does occasionally include diesel fuels since they are closely related to distillate and heavy fuel oils. In this section, however, diesel fuels are not included for discussion, since they are covered separately in this manual [1]. Industrial use of fuel oil for generating heat is the principal focus in this section. Specifications and applications of fuel oils are introduced with an overview of
' Department of Energy and Geo-Environmental Engineering and The Energy Institute, respectively. College of Earth and Mineral Sciences, Pennsylvania State University, 110 Hosier Building, University Park, PA 16802.
straight-run distillate; straight-run residual (i.e., reduced crude); catalytically cracked distillate; cracked residuals from thermal or catalytic cracking, or hydrocracking; 5. blends of any of the streams listed above. Cracked oils have rather different composition and properties from those of the straight-run oils, as discussed later. In the ASTM specification. Grades No.l and No.2 are distillates and Grades No. 4-6 are usually residuals. Some heavy distillates may, however, be sold as Grade No. 4 fuel oil. Grade Nos. 4 and 5 are subdivided into light and heavy categories. Table 1 summarizes the common uses and some significant properties of the different grades of fuel oils [6]. Grades 1 and 2 are used in domestic and small industrial burners. Grade 1 is a particularly light distillate for use in the vaporizing type burners and under storage conditions that require low pour points. Grades 4-6 are used in pressure atomizing-type commercial/industrial b u r n e r s that can handle high viscosity fuels. The viscosity of these residual fuels increase with the increasing number in the grade scale such that Grade 6 fuel, also called Bunker C, requires preheating for handling and burning [6]. In general, all grades of fuel oil should be homogeneous hydrocarbon oils, free from inorganic acid, and free from ex-
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2003 by A S I M International
www.astm.org
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TABLE 1—Common uses and some significant properties of different grades of fuel oils. Grade
Classification
No. 1
Distillate
No. 2
Distillate
No. 4 (Light)
Heavy Distillate
No.4
Heavy Distillate, Distillate/Residual blends
No. 5 (Light) & No. 5 (Heavy)
Residual
No. 6 or Bunker C
Residual
cessive amounts of solid or fibrous foreign matter. All grades containing residual components should remain uniform in normal storage and not separate by gravity into light and heavy oil components outside viscosity limits for the grades. Table 2 lists the required specifications for all fuel oil grades according to the standard designation ASTM D 396. A representative sample should be taken for testing in accordance with ASTM Practice D 4057. Modifications of limiting requirements agreed upon between the seller and the buyer should fall within limits specified for each grade, except as stated in supplementary footnotes for Table 2. The most important considerations in selecting a burner and a particular grade of oil include the volume of the oil consumed, and a match between the capabilities of the b u r n e r systems and properties of the fuel oils. The heavier grades are less expensive, but they must be handled and burned efficiently to take advantage of the lower fuel cost, especially in large volume applications. Sufficient heating in the storage tank and insulated transfer pipelines are usually necessary to ensure steady flow of heavy oil to the burner. High-viscosity oils require additional preheating at the burner for proper atomization.
I m p o r t a n t F u e l Oil P r o p e r t i e s Fuel oils are burned to generate heat. The a m o u n t of heat captured in combustion systems is the principal concern for the fuel user. Important properties of fuel oils must, therefore, relate to their combustion characteristics and performance in different types of burners and fuel handling systems. Understanding these properties helps the user to select a fuel best suited for a specific application. It is also useful to discern the relationships between different properties of fuel oils. All the properties of fuel oils identified in Table 2 are discussed below, with reference to standard measurement methods, and the significance of the measured properties. Gravity The density of petroleum oils is often expressed in terms of API gravity, a scale devised by the American Petroleum Institute and National Bureau of Standards (continued as National Institute of Standards and Technology). The API
Application Domestic and small industrial burners of the vaporizing type Atomizing type domestic and small industrial burners Pressure-atomizing type commercial/industrial burners Pressure-atomizing type commercial/industrial burners; preheating not required for handling or burning Industrial burners; preheating may be required for handling and burning Industrial burners; preheating required for handling and burning
Significant Properties Volatility and pour point Volatility and viscosity Viscosity and pour point Viscosity, flash point, and sulfur content
Viscosity, flash point, and sulfur content Viscosity, flash point, and sulfur content
gravity is inversely proportional to specific gravity—the ratio of the density of oil at 60°F to the density of water at 60°F— according to the following equation: °API = (141.5/spgr60°F/60°F) - 131.5 The API gravity is the most commonly used property to classify crude oil and refined petroleum products since the early days of petroleum industry. It has also been used to predict many other characteristics of petroleum oils. Recent variations in crude oil composition coupled with new processes in petroleum refineries have, however, diminished the usefulness of API gravity as a descriptor for other properties of petroleum oils. Two oils with the same API gravity, for example, can have many different characteristics because of large differences in composition (e.g., different combinations of paraffinic, naphthenic, and aromatic hydrocarbons). The API gravity still remains to be an important property, since it can be measured easily, and used in a number of empirical correlations for approximate estimation of other properties of petroleum fractions [7]. The API gravity of fuel oils is measured by using a standard hydrometer according to ASTM Test Method D 287 or D 1298. Because of the significant volume expansion of oils upon heating, the API gravity varies strongly with temperature. An API gravity of 12 at 60°F, for example, would correspond to an API gravity of 18 at 180°F (82°C) [4]. Therefore, the volumetric heating value of oils (Btu/gal) decreases significantly with the increasing API gravity. The reasons for this trend are discussed in the next subsection. Table 2 shows the gravity limits for the three grades of oil (>35 for No. 1; > 3 0 for No. 2; < 3 0 for No. 4 light). Although the differences in gravity between different grades may vary depending on the composition of the oils and the refining processes, typical API gravity ranges for fuel oils are as follows: No. 2: 26-39°; No. 4: 24-32°; No. 5: 16-22°; No. 6: 10-15°. As expected, API gravity decreases with the increasing grade number. The API gravity also changes according to the fuel types within a given grade. For No.2 fuel, for example, the API gravity of straight-run fuel oils range between 36 and 39°, while that of thermally, and catalytically cracked oils would fall in the range 24-28°, and 29-32°, respectively. For
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OF FUELS, PETROLEUM PITCH. PETROLEUM
COKE, AND CARBON MATERIALS
759
TABLE 2—Detailed requirements for fuel oils (ASTM D 396). Properties
Specific Gravity, 60/60°F deg API Flash point, °C (°F) min Pour point, °C (°F) msix Kinematic viscosity, mm'^/s (cSt)'' At38°C(100°F)min max At40''C(104''F)min max At 50°C (100°F) min max Saybolt Viscosity Universal at 38°C (100°F) min max Furol at 50°C (122°F) min max Distillation temperature, °C (°F) 10% point max 90% point min max Sulfur content, mass, mcix Corrosion copper strip, max Sulfated ash, % mass, max Carbon residue, 10%*; %m, max Water and sediment, % vol, max
No. 1
0.8499 35 min 38 (100) 18''(0) 1.4 2.2 1.3 2.1
No. 2
0.8762 30 min 38 (100) - 6 " (20) 2.0" 3.6 1.9 3.4
(32.6) (37.9)
No. 4 (Light)
No. 4
No. 5 (Light)
No. 5
No. 6
55 (130)
55 (130)
60 (140)
0.8762" 38 (100) -6" (20) 2.0 5.8
(32.6) (45)
55 (130) - 6 " (20) 5.8 26.4^" 5.5 24.0'
(45) (125)
>26.4 65^
>24 58^
(>125) (300)
>65 194^^ >58 168^ (42) (81)
>92 638^^
(>300) (900)
(>900) (9000)
(23) (40)
(>45) (300)
215(420)
0.5 3
282'^ (540) 338 (640) 0.5* 3
0.15
0.35
0.05
0.05
288 (550)
0.05
0.10
0.15
0.15
(0.50)"
(0.50)'
(1.00)'
(1.00)"
(2.00)"
"It is the intent of these classifications that failure to meet any requirement of a given grade does not automatically place an oil in the next grade unless in fact it meets all the requirements of the lower grade. ''In countries outside the United States other sulfur limits may apply. "Lower or higher pour points may be specified whenever required by conditions of storage or use. When pour point less than - 18°C (0°F) is specified, the mini m u m viscosity for grade No. 2 shall be L7 cSt (31 SUS) and the m i n i m u m 90% point shall be waived. '^Viscosity values in parentheses are for information only and not necessarily limiting. "The amount of water by distillation plus the sediment by extraction shall not exceed the value shown in the table. For Grade No. 6 fuel oil, the amount of sediment by extraction shall not exceed 0.5 weight %, and a deduction in quantity shall be made for all water and sediment in excess of I.O weight %. ^Where low su Ifur fuel oil is required, fuel oil falling in the viscosity range of a lowered numbered grade down to and including No. 4 may be supplied by agreement between purchaser and supplier. The viscosity range of the initial shipment shall be identified and advance notice shall be required when changing from one viscosity range to another. This notice shall be in sufficient time to permit the user to make the necessary adjustments. *This limit guarantees a m i n i m u m heating value and also prevents misrepresentation and misapplication of this product as Grade No. 2. ''Where low sulfur fuel is. Grade 6 fuel oil will be classified as low pour -H5°C (60°F) max or high pour (no max). Low pour fuel should be used unless all tanks and lines are heated.
oils from a single refinery stream, the API gravity can indicate w h e t h e r they are straight-run, or cracked oils. For blends, e.g., No. 4 oil, the concentration of different types of oils present, would, therefore, determine the API gravity of the blend in the corresponding range. In general, the API gravity of fuel oils can be qualitatively related to other properties of the oils as shown below [7]: • The higher the API gravity, the lower the viscosity and carbon residue. • The higher the API gravity, the lower the volumetric heating value (Btu/gal), and the higher the gravimetric heating value (Btu/lb). • The higher the API gravity, the lower the C/H ratio. • The higher the API gravity, the higher the rate of combustion, and the shorter the flame length. Heating
Value
Heating value is broadly defined as the amount of heat released by complete combustion of a unit quantity of fuel. Experimental measurements can be reported as total (or high).
or net (or low) heating values in Btu per gallon. The total heating value includes the latent heat of evaporation of the water vapor produced during the combustion. For determining the net heating value, the water from combustion is considered to remain in the gaseous state, and, therefore, the latent heat of evaporation is not recovered. Although no direct reference is made to heating value measurement in the ASTM classification of fuel oils (there is indirect reference through limiting API gravity. See footnote g in Table 2), heating value is a n important specification requirement. The heating value, or the heat of combustion of fuel oils, can be measured by the ASTM D 240. The net heating values of fuel oil samples can also be measured by the ASTM D 4529, D 3338, or D 4809. Depending on the specific gravity and composition of fuel oils, total heating value ranges typically between 130 000 and 160 000 Btu per gallon (36 400-44 800 kJ/liter). The net hea-ting value is usually 8400-8500 Btu per gallon (2350-2380 kJ/liter) lower t h a n the total heating value, depending on the hydrogen content of the fuel. Figure 1 shows
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.5' 160000 ..... ...-r-:. 5 155000 J^^^/^,'::™ :• .:v.;-::=:;.:.;V " 150000 oT 145000 -^sPiRiet^/.f:^*** *^ii_ ••••&• I 140000 •^'-'iftKi^> 135000 ^•[^••-T.':--^^^^ o) 130000 ~ 125000 ••••; ri^7^:;:\ S 120000 "\r^ ^ ^i ' ^ I r' f H I". ^ ' i.":—' '1 — r
"^^^^feu'
••
^
•
'
*
'
'
^
*
^
;
^
HANDBOOK
-Total Heating Value -Net Heating Value
'
0 4 8 1216202428323640 API Gravity
FIG. 1—Total and net heating values of fuel oils versus API gravity.
a plot total heating value and net heating value of selected fuel oils as a function of API gravity from the data given in reference [8] (1 Btu/gallon = 0.28 kJ/liter). It can be seen that the volumetric heating value increases with the decreasing API gravity, because of the decreasing hydrogen content of the fuels with the decreasing API gravity. In addition to measurement by standard methods and prediction from the API gravity, there are a number of empirical correlations used to calculate the heating value. Two of the most commonly used correlations are given below: Total Heating Value (Btu/lb) = 14 600C + 62 000(H-O/8) + 4000S where C, H, O, and S are the weight percentages of these elements in the fuel [8]. Net Heating Value (kJ/kg) = 55 500 - 14 400d - 320S where d is the density (kg/liter), and S is the sulfur content in wt% [9]. It should be noted that on a weight basis (e.g., Btu/lb, or kJ/kg), the heating value of a fuel oil decreases with the increasing density (or decreasing API gravity). However, this trend is reversed when reporting the heating value on a volu m e basis (e.g., Btu/gal, or kJ/liter) since the decreasing heating value is more than compensated by the decreasing volume as the density increases. For example, an oil with 14 API gravity have 290 Btu less per pound (676 kJ/kg) than a 20 API gravity oil, but one a volume basis the 14 gravity oil will have 3840 Btu more per gallon (1075 kJ/liter) than the 20 gravity oil [8]. Therefore, oils with a lower API gravity will provide more heat on a volume basis, but other properties of the fuel oil, such as viscosity, pour point, and carbon residue, may be limiting factors in selecting a given oil, as discussed below.
ture. The kinematic viscosity, expressed in mm^/s (formerly called centistoke, cSt), is determined as the product of the m e a s u r e d flow time a n d the calibration constant of the viscometer. Although the kinematic viscometers are the approved instruments, Saybolt viscometers (Universal and Furol) are also widely used for measuring viscosity. In Saybolt viscometers, the time for the flow of a given volume of liquid is measured under variable head (as opposed to the constant head in kinematic viscometers), which decreases as the volume of the oil in the tube decreases. The measured viscosity is expressed as Saybolt Seconds. The only difference between Universal and Furol viscometers is the larger orifice of the Furol tube (3.15 mm) t h a n that of the Universal (1.77 mm). The Universal is used for testing low-viscosity oils, while the Furol is used for high-viscosity oils to obtain reliable m e a s u r e m e n t s by avoiding the exceedingly long, or short, test times depending on the viscosity of oil. Most frequently used temperatures for viscosity measurements are 38°C (100°F) for Saybolt Universal and Kinematic viscometers, and 50°C (122°F). Conversion tables can be used for viscosity conversions between different values [11]. Table 3 gives the Saybolt and kinematic viscosity limits for different grades of fuel oils. The grade Nos. 5 and 6 have very broad ranges of viscosity because of the use of many different types of burners that can handle a range of viscosities. In applications, where viscosity needs to be closely controlled, oils can be purchased on a viscosity basis. Some suppliers specify their oil as, for example. No. 5-300, indicating No. 5 fuel oil with a viscosity of 300 SSU (Saybolt Seconds Universal) at 100°F (38°C). The viscosity of fuels depends strongly on temperature; the viscosity decreases sharply with the increasing temperature [11]. This strong temperature dependence is used to control the viscosity of residual fuel oils by preheating. When the viscosity of heavy oils is reduced, pumping becomes easier and better atomization is achieved for combustion. Conversely, heavy oils become viscous and extremely difficult to handle at low temperatures. In cold weather applications, the lowest operating temperatures must be considered for selecting the oil with the adequate viscosity for easy pumping and handling. In some applications, the viscosity of heavy fuel oils is controlled by blending with stocks of lower viscosity. Mixing of fuel oils with different chemical characteristics may, however, cause incompatibility problems leading to deposit and sludge formation in fuel handling systems [12]. The high viscosity of fuel oil causes the following problems [11,13]: • Difficulty in pumping from storage tank to burner; loss of p u m p suction
Viscosity The viscosity of oil measures its resistance to flow [10]. It is one of the most important properties of especially the residual fuel oils that affect handling, heating, pumping, and atomization of heavy residual fuels in combustion. The most commonly used viscosity term, kinematic viscosity, is determined by measuring the time for a fixed volume of liquid to flow under gravity through the capillary of a calibrated viscometer (ASTM D 445 with specifications given in ASTM D 446). The measurements must be m a d e under a reproducible driving head and at a closely controlled and known tempera-
TABLE 3—Summary of ASTM methods for the characterization of petroleum pitch properties. Property
Softening point Viscosity Solvent Fractionation Coking Value Density Ash Sulfur Content
ASTM Designation
D 3461, D 2319, D 36, D 61, D 3104 D5018 D 4746, D 2764, D 2318, D 4072, D 4312 D 4715, D 2416
D 2320, D 70, D 4892, D 71, D 2962 D2415 D 1266, D 4045, D 2622
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OF FUELS, PETROLEUM PITCH, PETROLEUM COKE, AND CARBON MATERIALS
• Insufficient oil flow to the burner causing problems with starting and erratic combustion • Poor atomization causing inefficient combustion and dribbling of oil at the burner nozzle The low viscosity of the fuel oil causes, in turn, the following problems [11,12]: • Too much oil pumped to the burner causing incomplete combustion, resulting in smoke, carbonization of burner nozzle, and soot formation in the combustion chamber • Low heat generation because of the low heating value of low-viscosity, or high API gravity of the fuel oil • P u m p slippage, too much oil can slip the cup with rotary cup burners, resulting in poor atomization. Pour
Point
Pour point represents the temperature at which a fuel oil stops flowing. This property, measured by ASTM D 97, indicates the lowest temperature of the utility of a fuel in flow systems, and it is particularly relevant to waxy heavy oils and heavy oils that require preheating for pumping. Straight-run oils usually have higher p o u r points than cracked oils because of the higher concentrations of normal paraffins in straight-run oils. The usefulness of the pour point test in relation to residual fuel flow properties is questionable. The pour point test does not indicate what happens when an oil has a considerable head of pressure behind it w h e n it is pumped along a pipeline, or gravitated from a storage tank. Among the tests devised to assess the low-temperature flow characteristics of heavy residual fuel oils is ASTM D 3245. ASTM D 3245 is, however, a time-consuming test, and not suitable for routine control testing, along with its limitations in application to very waxy fuels. Flash
Point
As fuel oils are heated, they evaporate, and the vapors flash at a certain temperature when ignited by an external flame. This temperature is called the flash point. There are two standard ASTM tests to measure the flash point: ASTM D 92 (Cleveland), a n d ASTM D 93 (Pensky-Martens), using an open-cup, and a closed-cup flash tester, respectively. The closed-cup tester gives lower flash points because of the more effective retention of very light vapors that are blown away by the air flow in the open-cup tester. Therefore, the closed-cup test is more sensitive in detecting the small amounts of light vapors in the fuel oil samples. As shown in Table 3, desirable flash points of fuel oils range from 38°C (Grades N o . l , 2) to 60°C (Grade No.6) measured by closed-cup testers. Flash points lower than the desired values may cause fire hazard, whereas high flash points can lead to difficulties with starting, especially in a cold furnace. Distillation ASTM D 86 is used to determine the 10% and 90% points of the distillate fuel oils (Grade No. 1 and No. 2) as specified in Table 3. The distillation characteristics (volatility) of these fuels are related to their ignition and combustion properties and the tendency to form solid combustion deposits. The lower and upper limits to volatility are set to ensure safety and smooth operation in the use of these fuels. Water and Sediment (BSW)—The presence of water and sediment, also called bottom sediment and water (BSW), in
761
fuel oils can cause a n u m b e r of operational problems, including [14], • Plugging of burner tips • Erratic and unsteady combustion • Flame instabilities • Erosion of burner tips and mechaniccJ parts. The origin of BSW can be traced back to the original crude oil, contamination during refinery processes, or thermal degradation a n d oxidation reactions during storage. Usually heavy oils (No. 5 and No. 6) contain greater amounts of BSW than light oils because of the concentration of BSW of crude oil in the residual fractions due to the high specific gravity and high viscosity of the residual fuels. Light oils, such as No. 2 and No. 4, are usually clean, except for some water and small amounts of fine sediment. Table 1 shows that m a x i m u m BSW is limited to 0.05%vol for oils No. 1 and No. 2, 0.5% vol for No. 4, 1.0% vol for No. 5, and 2% vol for No. 6. Three s t a n d a r d m e t h o d s are used to determine water and bottom sediment either together (ASTM 1796 -by centrifugation with added toluene), or separately (ASTM D 95, Azeotropic distillation for water and ASTM D 473, Extraction with toluene and weighing the residue). Specific rules apply to reporting BSW, if water and sediment measurements are carried out separately (see footnote in Table 3). The occurrence of BSW in fuel oils can be reduced by careful storage, blending, and transportation practices that reduce the formation and/or dispersion of the sediments in the fuel oils [14]. Carbon
Residue
The term carbon residue is used in several different connotations related to the use of fossil fuels, including carbonaceous particles present in fuel, carbon formed on the burner tips and furnace walls because of incomplete combustion, and carbonaceous residue remaining after pjTolysis of fuels in standard tests. Only the results from standard carbon residue tests are used in fuel specifications. It is important to distinguish between the carbon deposition due to the high carbon residue of the fuel oil, and coke or soot formation as a result of poor combustion [15]. Understanding the difference between these two types of deposition helps identify the root cause of any deposition problem: fuel composition or combustion conditions [16]. Three standard methods used to determine carbon residue are ASTM D 189, Conradson Carbon Residue (CCR), ASTM D 524, Ramsbottom Carbon Residue (RCR), and ASTM D 4530, Micro Method. Most specifications are based on CCR, but correlations exist to convert between CCR and RCR test results [ASTM D 189]. Table 3 shows that carbon residue is specified only for light fuel oils, No. 1 and No. 2, because small vaporizing-pot and sleeve-t3^e burners used in domestic applications have less tolerance for carbon deposition. The carbon tests are conducted on 10% bottom residue of the fuel samples remaining after distillation. Ash Ash results from the noncombustible organic and inorganic species found in fuel oils. Most of the ash can be traced back to the constitution of the crude oil from which the fuel oils
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were derived. To a lesser extent, contamination during refining and handling may be responsible for the ash content. Most of the ash-producing materials present in crude oil (e.g., water soluble sodium and calcium chlorides, and oil soluble organometallic compounds of nickel and vanadium) tend to concentrate in the heavy products, such as residual fuel oils. Ash contents of fuel oils can be determined by weighing the n o n c o m b u s t i b l e residue after c o m b u s t i o n using ASTM D 482. Table 3 shows the permissible ash contents specified for Grades No. 4 (0.05%, 0.10% for light and heavy, respectively), and No.5 (0.15%). No specifications are given for No. 1 and 2 because ash is seldom found in distillate oils. Also, there is n o ash specification for No. 6 oils (which may contain u p to 0.2% ash [17]) since the combustion equipment designed for burning heavy residual fuels can handle relatively high ash contents. Problems encountered with high ash contents depend on the application and type of combustion operation, including contamination of products in direct firing applications (e.g., glass and ceramic industry), erosion of p u m p p a r t s and burner tips, and accumulation of ash on boiler tubes. High metal contents of heavy oils would also be responsible for producing particulate emissions from combustion with potentially significant toxicity [18]. The removal of ash from fuel oils is not practical and very costly because most ash-forming compounds are soluble in the oil. Problems with high ash contents are usually addressed by blending with low ash oils and/or by reducing the impact of ash in combustion operations with fuel additives and combustion system treatments [17]. Sulfur
Content
Sulfur is one of the most troublesome elements found in fuel oils. Sulfur is p r e s e n t in a variety of complex chemical compounds in crude oils. Since the molecular chemistry of sulfur compounds is extremely complex, sulfur specifications depend on the measurement of the total sulfur content of fuels. Because of the high-boiling points of sulfur-containing compounds, sulfur tends to concentrate in heavy residual fractions obtained from petroleum refining. The sulfur content of fuel oils generally increases with the increasing grade number. The sulfur range in each grade shows large variations because of differences in sulfur contents of crude oils, refining processes, and blending. There are several ASTM tests that are used to determine the sulfur contents of fuel oils, inclu-ding ASTM D 1266, Lamp Method (for No. 1 only), ASTM D 1552, High-Temperature Combustion, ASTM D 129, Oxygen Bomb, and ASTM D 4294, Non-Dispersive X-ray Fluorescence. The sulfur limits in fuel oil specifications shown in Table 3 designate the m a x i m u m total sulfur contents for No. 1 and No. 2 oils as 0.5%. No sulfur specification is listed for grades No. 4, 5, and 6. For many applications, however, consumers specify the maximum allowable sulfur limit to comply with the environmental regulations (e.g., Clean Air Act a n d Amendments in the U.S.) or to limit damage to materials in combustion systems. Problems associated with high sulfur contents in fuels are related principally to combustion products, namely sulfur dioxide (SO2) and sulfur trioxide (SO3), which further react
in the presence of oxygen and moisture to produce sulfurous, or sulfuric acid. These acids will cause corrosion of any exposed metal surfaces, such as boiler shell and tubes. The presence of sulfur compounds may also cause problems in various materials applications, such as glass, ceramic, and metals production [19]. Total emissions of sulfur oxides from combustion systems depend almost entirely on the sulfur content of the fuel without any significant effect of the boiler size, combustor design, or the fuel grade. Sulfur contents of fuel oils can be reduced by hydro treatment in petroleum refining [4,5] or by blending with oils having lower sulfur contents. Flue gas scrubbers are used in large installations to remove sulfur from stack gases to comply with SO2 emission limits. A particular corrosion test, ASTM D 130, Copper Strip Corrosion, is used to characterize the corrosive properties of fuels related mainly to sulfur compounds, such as dissolved hydrogen sulfide, mercaptans, active elemental sulfur, along with inorganic acids and ammonia. The presence of these compounds in fuels lead to the corrosion of copper heating lines, cooling coils, and nonferrous metal fittings. The copper strip corrosion test measures the extent of discoloration of copper strips that came into contact with fuel samples under specified conditions. Reference strips used to measure the extent of discoloration are rated from No. 1 (light-orange) to No. 4 (jet black). The No. 1 and No. 2 grades have a m a x i m u m Corrosion Strip specification of No. 3, dark tarnish. Instability
and
Incompatibility
The instability of residual fuel oils refers to the tendency of a fuel to produce deposit by itself, while incompatibility is the tendency of a fuel to produce a deposit when blended with other fuels [20]. Two oils that are each stable as single fuels may become incompatible when they are mixed. Major incompatibility problems occur when an oil with an asphaltene content of greater t h a n 3 - 5 % is blended with paraffinic oils. The incompatibility results in the formation of tar-like precipitates that cause problems in handling and combustion systems. ASTM D 4740 can be used to predict the compatibility between a residual fuel oil and a specific distillate fuel oils, such as a No. 1 or No. 2 oil. This test is also used for predicting the compatibility of residual fuels, although it may not be reliable for residual oil mixtures. The asphaltene content of residual oil is believed to affect its stability and compatibility [3,12]. ASTM D 3279 can be used to determine the asphaltene content of residual fuel oils.
Conclusions A standard classification of fuel oils according to selected specifications facilitates the selection of the right fuel oil for wide ranging applications in different combustion systems that show significant variations in size, combustor design, and process needs. A large number of ASTM methods exist to characterize many properties of fuel oils that are of interest for a particular application. Understanding the significance of these tests and careful interpretation of the test results will help troubleshoot m a n y performance problems in fuel oil handling and combustion systems.
CHAPTER 29: PROPERTIES
OF FUELS, PETROLEUM PITCH, PETROLEUM COKE, AND CARBON MATERIALS
PETROLEUM PITCH Petroleum pitch has become an important material for a n u m b e r of industrial applications, in particular, for manufacturing high-performance c a r b o n fibers and carboncarbon composites. This section provides an overview of p r o d u c t i o n processes, applications, and properties of petroleum pitch. Production Processes and Applications Petroleum pitch is a term used for certain petroleum residues due to their resemblance to other pitch materials, such as coal tar pitch, that are thick, dark colored bituminous substances obtained from destructive distillation processes. Generally, petroleum pitch is the nonvolatile product obtained from thermal or catalytic cracking of heavy petroleum residua [4,21]. It may also be defined as the high boiling point fraction (420-520°C) obtained by vacuum distillation (0.5-1.0 m m Hg) of catalytic cracking bottoms [22], or as solvent deasphalted bottoms [23]. Due to its resemblance to coal tar pitch, the ASTM methods that are summarized in Table 3 are often used to test both materials. However, petroleum and coal tar pitches are substantially different in origin, structure, and behavior, and, therefore, a ccireful interpretation of the test results is necessary. CoaJ tar pitch is obtained from the distillation of volatile by-products, or tar, from coke ovens during the manufacture of metallurgical coke from coal. The differences in the origin and processes that lead to the production of petroleum and coal tar pitch are apparent in their chemical composition and respective industrial applications [24,25]. For instance, the carbon content of petroleum pitch is around 85-90%, which is somewhat lower than that of coal tar pitch (94-96%). This is linked to the higher hydrogen content of 4-6% for petroleum pitches, versus 2 - 3 % for coal tar pitches. Hence, petroleum pitches contain a relatively high proportion of aliphatic carbons compared to coal tar pitches. Both petroleum and coal tar pitches are predominantly aromatic, with the majority of alkyl groups being methyl. However, while a tj^ical coal tar pitch has an aromaticity of 98-99%, petroleum pitches have around 10-20% aliphatic carbon. Coal tar pitches contain relatively more condensed aromatic ring structures with a bridgehead aromatic carbon content (i.e., aromatic carbon only bound to other aromatic carbons) of 0.45-0.50. Therefore, the volatile compounds in coal tar pitches, as detected by GC-MS, are mainly three to six aromatic ring compounds, such as phenanthrene, fluoranthene, pyrene, benzo[a]pyrene, benzo[b]fluroanthene, and a n t h a n t h r e n e , while the nonvolatile compounds have a much higher condensed structure. In contrast, petroleum pitches have a rather open structure, with a bridgehead aromatic carbon content around 0.35-0.40 [26]. The volatile compounds are two-to four-ring compounds that are heavily alkylated. The non-volatile compounds are non-defined entities of larger a r o m a t i c ring structures substituted with long-chain alkyl groups and possibly some saturated rings. These differences in structure and composition explain the differences in softening points and viscosity behavior, solubility, density, and coking yields from the coal tar and petroleum pitches, as described in Properties of Petroleum Pitch.
763
The applications of different p e t r o l e u m pitches are governed by their properties, d e p e n d i n g largely on their chemical composition. ASTM D 2569 is used to determine the distillate contents of pitches. A soft pitch, which appears semi-viscous at ambient temperatures due to its low glass transition temperature (see Softening Point), will produce a relatively large amount of distillate even at low temperatures and weak vacuum (if applied). On the other hand, hard pitch will yield little distillates even at temperatures up to 360°C. Accordingly, industrial distillation of pitches is frequently used to produce carbon-precursors with high softening point and coking yields [27]. An alternative approach utilizes airblowing to alter the chemical composition of the pitches to meet the required specifications [23]. Generally, petroleum pitch is used in the production of prem i u m petroleum coke or as an impregnation pitch in the manufacture of carbon artifacts for the aluminum and steel industry [28]. The high carbon content and low mineral matter content of petroleum pitch makes it an excellent precursor for the production of high performance carbon fibers and carbon-carbon composites (see Carbon Fibers from Petroleum Pitch). Molded in sheets, carbon fibers have higher strength than steel (227 and 200 GPa, respectively) but only 1/5 of the density, 1720 a n d 7830 kg m~^, respectively [29]. These impressive properties result from the formation of a discotic liquid crystal phase, carbonaceous mesophase that produces high tensile strength and modulus. Carbonaceous mesophase is formed by alignment of disk-like molecules (see Formation of Coke Microtexture in Coking Processes for a description of carbonaceous mesophase). The a m o u n t of mesophase in a p e t r o l e u m pitch can be established following ASTM D 4616. Although most of the terminology of this method concerns coal tar pitches, the central concept about isotropic (non-mesophase) and anisotropic (mesophase) is valid for petroleum pitches. The influence of primary quinoline insolubles, described in detail in Solvent Fractionation, is minimaJ or nonexistent in petroleum pitches. Hence, mesophase is both readily developed and detected in petroleum pitches, with clear identification of mesophase spheres or anisotropic texture domains. The a m o u n t of mesophase, as found through ASTM D 4616, is generally a function of pitch condensation, heat-treatment temperature, and time. Recently, an in-situ ^H NMR technique has been developed to follow the mesophase development directly during heat treatment [30]. High performance carbon products from mesophase pitches have a wide range of applications, including lightweight components for vehicles, boats, planes and the space industry; high strength and wear parts in brakes and engine pistons, and medical artifacts (e.g., in artificial heart, bone plates and ligaments) due to their bio-compatibility with blood, soft tissue, and bones [31,32].
Properties of Petroleum Pitch The determination of various properties of petroleum pitch is crucial for the establishment of pitch consistency, which can be utilized as a strong marketing tool. Pitch buyers often dem a n d that the properties described in the Softening Point through Ash sections meet their specifications, to assure that the product they are buying fits their requirements and process line. Currently, there are two m a i n practices used for
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pitch transportation. The most frequent practice involves transporting pitch as a hquid around 180°C, but some users prefer to receive the pitch as a soHd (often referred to as pencil pitch from its extrusion into small rods). Due to the various ways of handling, some localized variation in pitch composition can occur. Hence, an important tool for establishing sample uniformity is the use of ASTM D 4296, Stand a r d Practice for Sampling Pitch. The strength of this method lies in its compatibility with both solid and liquid pitch. A proper sampling lays the foundation for valuable data collection from the following standard test methods. Softening
TABLE 4—Comparison of the five standard test methods used for determining softening point in pitches. Method
Description
D 3461
Softening point of asphaltene and pitch (Mettler cupand-ball method)—The pitch is loaded in a small metal cup with a bottom orifice and a ball is placed on top of the solid pitch. The assembly is placed in a furnace and a light beam is used to detect the temperature when the ball is penetrating the pitch (the softening point). Softening point of bitumen (ring-and-bsdl apparatus)— The pitch is loaded in a small metal ring and a ball placed on top of the pitch. The assembly is placed in a bath and the temperature when the ball is penetrating the pitch is manually detected (the softening point). Softening point of pitch (cube-in-air method): Molded cubes of pitch with center holes are placed on hooks and suspended in an oven. The temperature of the hooks when the pitch is flowing is manually detected (the softening point). Softening point of pitch (cube-in-water method): Molded cubes of pitch with center holes are placed on hooks and suspended in water. The temperature of the hooks when the pitch is flowing is manually detected (the softening point). Softening point of pitches (Mettler softening point method): The pitch is loaded in a small metal cup with a bottom orifice. The assembly is placed in a furnace and a light beam is used to detect the temperature when the pitch is flowing through the orifice (the softening point).
D 36
Point
Petroleum pitches are mixtures of condensed aromatic compounds. As a result, petroleum pitches are eutectic, i.e., soften at a lower temperature than the individual melting points of the different compounds that make u p the pitch. At ambient temperatures, the pitch has an isotropic structure and is generally characterized as a glassy solid (hardpitch). When the pitch is heated in an inert atmosphere, it has no defined melting point, but will pass through a glass transition region before it becomes an isotropic liquid. While a pure compound, e.g., anthracene, goes from a solid to liquid at 217.5°C, a mixture of similar aromatic compounds and their alkylated derivatives will exhibit a wide temperature interval (4Tg can range from 10' to over 10^°C) from the end of the solid phase to a fully liquid state. The transition from a glassy solid to a viscous substance gives rise to a physico-chemical characteristic CcJled the glass transition temperature, Tg [33]. Although there is no ASTM method yet developed, both the Tg and the temperature interval, ZlTg can be determined by different methods [34] such as ' H N M R , electron-nuclear double resonance (ENDOR) or differential scanning calorimetry (DSC). The enthalpy, /IH, and activation energy, Ea, for pitches can then be derived from the above techniques, ft has been shown that the Ea can be considered to be proportional to the mean molecular size in pitch, and can give an estimate of the degree of condensation [35]. During the transition from a glass to a liquid, the viscosity of the pitch changes drastically (see Viscosity). However, it is relatively tedious and labor intensive to determine the viscosity behavior for every single pitch. Therefore, several standard test methods, ASTM D 3461, D 36, D 2319, D 61, and D 3104, have been developed to establish when a pitch reaches a viscosity of about 10^ Pa s, generally referred to as the softening point of the pitch [36]. Table 4 lists the differences between the five standard test methods given by ASTM. D 3104, Mettler Softening Point Method or D 61, Ring and Ball Softening Point, are the most commonly used. D 3104, Mettler Softening Point Method, provides fast and accurate determination of the softening point but requires the purchase of an instrument. D 61, Ring and Ball Softening Point, is fast and inexpensive, and, therefore, it is also frequently used. The softening points can be given both in °C or °F, and sometimes they give rise to pitch nomenclature, where Ashland petroleum pitches A-170 and A-240 have softening points of 170 a n d 240°F, respectively. Furthermore, it has been reported that pitches having similar softening point can have different Tg, which can be associated to differences in viscosity as described below.
D 2319
D 61
D3104
Viscosity The determination of changes in viscosity and other rheological properties during and after the glass transition region is crucial for applications of pitch as a binder for electrodes and road aggregate. ASTM D 5018, Standard Test Method for Shear Viscosity of Coal-Tar and Petroleum Pitches, is a good tool to follow the rheological properties of pitches at temperatures of 40-100°C over its softening point, since it requires a relatively simple setup. The setup consists of a hotplate, a temperature controller, a thermometer, and a rotational viscometer. The pitch is melted into a cup on the hotplate where the rotor from the viscometer is inserted, and at different temperatures the viscosity is measured. The method is limited to 230°C and 15 000 cps (15 Pa s). Viscosity, 17, is defined as the ratio of the shear stress, T, to the rate of change of shear strain, ySR, at constant temperature and pressure, TJ = r/ySR [37]. When the viscosity of a system is only a function of temperature and pressure, and independent of the shear rate at constant temperature and pressure, the fluid can be classified as Newtonian. For nonNewtonian systems, such as polymers and most liquid crystals, the viscosity is dependent on the shear rate as well [34]. Isotropic pitches before thermal decomposition are mainly Newtonian [38]. However, non-Newtonian flow, such as Bingham behavior, has been detected where there is no flow until the shear stress exceeds a critical value called the yield stress [39]. The variations in viscosity with temperature for the petroleum pitch Ashland A-240 is compared to a coal tar pitch and its solvent fractions (see Solvent Fractionation in Fig. 2 [40]. With increasing temperature, there is a rapid decrease in viscosity during softening of the pitch. In the tem-
CHAPTER 29: PROPERTIES
OF FUELS, PETROLEUM PITCH, PETROLEUM COKE, AND CARBON MATERIALS
765
FIG. 2—Changes in viscosity with temperature for Ashland A-240 petroleum pitch compared with a coal tar pitch, its toluene soluble (TS) and insoluble (Tl) fraction together with various TS:TI mixtures [40]. Reprinted with permission from Elsevier Science.
TABLE 5—Nomenclature and solvents used historically in the petroleum and coal tar pitch industries. Petroleum Pitch
Coal Tar Pitch
Carboids Carbenes
Insoluble in CS2 Insoluble in CCI4, but soluble in CS2
a-resin /3-resin
Asphaltenes
Insoluble in n-pentane, but soluble in CCI4 or CeHft
y-resin Resinoid Crystalloid
perature region prior to the pitch becoming a hquid, there is a tail off in the viscosity reading, e.g., from 150-2008C for A240 petroleum pitch. ASTM D 5018 deals with the change in viscosity in this region. From the rheological measurements, the suitability of a pitch for certain applications can be established, e.g., as a binder or for impregnation purposes. In Fig. 2, the increase in the viscosity after 4508C is due to t h e r m a l induced chemical reaction in the pitch, such as coking (Coking Value), or the development of mesophase (further explained in F o r m a t i o n of Coke Microtexture in Coking Processes). In addition, rheology studies give information about elastic properties important for the thermo-forming process of fibers and composite impregnation [41]. An example of the effects of the elastic properties is the die-swell during the extrusion process in fiber spinning, which often is undesirable [42]. Solvent
Fractionation
A drawback in pitch characterization is that the nomenclature used for solvent fractionation of petroleum pitch and coal tar pitch has been different in the past [36]. The
Insoluble in quinoline (pyridine) Insoluble in toluene (benzene, dimethylformamide), but soluble in quinoline Soluble in toluene Insoluble in n-hexane (petroleum ether), but soluble in toluene Soluble in n-hexane
petroleum industry used CS2, CCI4, CsHe, and n-pentane, while the coal tar pitch producers used quinoline or pjridine, benzene or toluene, and petroleum ether or n-hexane. Table 5 lists the nomenclature associated with each solvent for the two industries. The standard test methods developed by ASTM Eire largely based on the nomenclature and solvent fractionation scheme originally developed for cocJ tar pitch that has become the common terminology in pitch fractionation [43]. The a-resin, or QI (quinoline-insoluble) content, is determined using either ASTM D 4746 or D 2318. Table 6 compares the two methods for determining the QI content. Generally, petroleum pitches contain very little or n o QI unless they have been heat-treated (see below). The 7-resin, or TS (toluene-soluble), content can be established using ASTM D 4072, D 4312, or D 2764. Table 7 compares the three methods for obtaining the TS content. From the two previous measurements the ;8-resin, or TI - QS (toluene-insoluble and quinoline-soluble), can be calculated by difference. The different resin fractions above consist of c o m p o u n d s with different molecular masses and heteroatom contents. A sim-
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HANDBOOK
TABLE 6—Comparison of the two ASTM methods for determining the QI content in pitches. Method D 4746
D 2318
Description Determination of quinoline insolubles (QI) in tar and pitch by pressure fihration; The pitch is dissolved in quinoHne at 75°C and fihered through a porcelain filtration crucible with a medium-porosity bottom at a pressure in the range of 10-30 psig using nitrogen. The sample is washed in hot quinoline until clear followed by acetone and dried. The portion of the pitch remaining in the crucible is defined as the QI fraction. Quinoline-insoluble (QI) content of tar smd pitch; The pitch is dissolved in quinoline at 75°C and digested for 20 min. The solution is filtered through a porcelain filtration crucible with a fine-porosity (7 yum) bottom using a suction filter apparatus. The sample is washed in hot quinoline until clear, followed by toluene, then acetone and dried. The portion of the pitch remaining in the crucible is defined as the QI fraction.
TABLE 7—Comparison of the three ASTM methods used for determining the TS content in pitches. Method D 4072
D 4312
D 2764
Description Toluene-insoluble (TI) content of tar and pitch: The pitch is dissolved in toluene at 95°C for 25 min and filtered through an extraction thimble using a gravimetric filtration tube. The extraction thimble is transferred to an extraction apparatus where it is further extracted for 18 h using a toluene reflux rate of 1 to 2 drops/s. The extraction thimble is dried at 105°C for 30 min and the portion of the pitch remaining in the thimble is defined as the TI fraction. Toluene-insoluble (TI) content of tar and pitch (short method): The pitch is dissolved in toluene at 95°C for 25 min and filtered through an extraction thimble using a gravimetric filtration tube. The extraction thimble is transferred to an extraction apparatus where it is further extracted for 3 h using a toluene reflux rate of 120-150 drops/min. The extraction thimble is dried at 110°C for 30 min and the portion of the pitch remaining in the thimble is defined as the TI fraction. Dimethylformamide-insoluble (DMF-I) content of tar and pitch: The pitch is dissolved in dimethylformamide at 95-100°C and digested for 30 min. The solution is filtered through a porcelain filtration crucible with a fine-porosity (7 /xm) bottom using a suction filter apparatus. The sample is washed in hot dimethylformamide until clear followed by acetone and dried at 105-110°C. The portion of the pitch remaining in the crucible is defined as the DMF-I fraction (mainly used as a rapid TI determination).
plified comparison between the three fractions could be that the y-resin (TS) is the lowest molecular weight fraction, the ;8-resin is an intermediate, while the a-resin (QI) is very high molecular weight material. These differences result in the TS having high H/C atomic ratio and low softening point, while the QI has low H/C atomic ratio and generally does not soften. The a-resin (QI) is a primary specification of pitches in the carbon industry, and Table 8 summarizes the desired content for some applications. A further sub-division of the a-resins can be made into primary, secondary QI, and extraneous im-
TABLE 8—Pitch apphcations depending on QI content [43]. Application Binder pitch Impregnating pitch Needle coke manufacture (highly anisotropic)
'-Resin (QI) content of pitcli, % 9-18 2-5 <1
purities. The primary fraction consists of material that partially retards coalescence during carbonization of pitch. The secondary fraction is in itself able to coalesce and is therefore referred to as the mesophase QI. The impurities consist of dirt and metal particles, and in the case of coal tar pitch, c a n y over coke from the by-product furnace. Both the primary and the secondary QI consist of high molecular mass species as reflected in low atomic H/C ratios (tjrpically 0.25 and 0.35, respectively). The effect of the QI on pitch carbonization is heavily dependent on the size and the distribution of primary and secondary QI in the a-resin. Petroleum pitches are generally free of primary QI. On the other hand, the j3-resin can be a substantial component of the pitches produced from petroleum (ranging from 0-80%), while coal tar pitches for binder purposes contain around 10-20% ;S-resin. Even though the H/C ratio of around 0.40 of the 13-resin fraction is close to that of the secondary QI, the first will undoubtedly have a lower molecular mass since thermal treatment of the /3-resin could transform it partially into secondary QI. This suggests that the structures of secondary QI and /3-resin are relatively similar and only differ by some degree of condensation [26]. The y-resin, i.e., the toluene-soluble fraction, has the highest H/C ratio and, due to its high solubility, it can be readily investigated by chromatographic a n d other analytical techniques. Such techniques include high-pressure liquid chromatography (HPLC) where different aromatic compounds can be identified by their UV spectra following a separation by elution through a packed column with different solvents, gel permeation chromatography (GPC) to identify the molecular mass distributions, and nuclear magnetic resonance (NMR) for the determination of the average hydrogen and carbon skeletal parameters [26,27]. The influence of primary QI on pitch carbonization has been the subject of intensive research. Primary QI has sometimes been referred to as "carbon black-like" due to its smeJl particle size. However, there are several differences between the two forms. First of all, primary QI can reach a particle size of about 1 /xm and secondary QI much higher, while carbon black particles have an average size of 20-30 n m [44]. Scanning electron microscopy (SEM) studies have shown that primary QI particles are generally spherical, reaching an average particle size of 0.1-1 fim. Figure 3 illustrates the particle size distribution of (a) typical primary QI of a coal tar pitch and compares those to (b) the particle size of secondary QI particle from a heat-treated petroleum pitch. The differences are clearly observed in Positions A, B, and C in Fig. 3 (a) and Position N in (b). The coal tar pitch has a range of particle sizes between 0.1-1 /xm in diameter, while the petroleum pitch contains agglomerated secondary QI particles in the range of 2-10 /xm in diameter. Secondly, carbon black particles have the ability to form chains, where hundreds of units are aggregated together, with some degree of
CHAPTER 29: PROPERTIES
OF FUELS, PETROLEUM PITCH, PETROLEUM COKE, AND CARBON MATERIALS
an ordered microstructure and smooth particle surfaces. In contrast, for primary QI, high-resolution transmission elect r o n microscopy (HRTEM) studies have described the microstructure to be generally disordered, without agglomeration of the particulates [45]. Figure 4 shows HRTEM micrographs c o m p a r i n g the m i c r o s t r u c t u r e of (a) a c a r b o n black, and (b) primary QI particle from a coal tar pitch. The c a r b o n black has highly ordered layer planes that form concentric spheres (Fig. 4a), while the QI shows a generally disordered microstructure (Fig. 4b). However, it should be mentioned that the term QI is only useful as a guide to particulate matter in pitch, since isotropic matter that is insoluble in quinoline can be soluble in the pitch [46]. This has been shown through heating QI where some of the material becomes fusible. As mentioned earlier, secondary QI is less condensed with higher H/C ratios than the primary fraction [47]. Secondary QI in binder pitch is unwanted due to its fusibility, which makes the pitch more viscous than that of pitches solely containing primary QI. For a potential binder pitch, it is therefore important to estimate the secondary QI content. Secondary QI particles are in general larger than
(a)
(b)
FIG. 3—SEM micrographs comparing the particle size distribution of (a) typical primary QI of a coal tar pitch and (b) the particle size of secondary QI particle from a petroleum pitch. Position A, B and C: primary QI spheres ~ 0.1 /xm in dia., agglomerated, and ~1 fim in dia., respectively. Position N: Agglomerated secondary QI particles. Reprinted with permission from Elsevier Science.
(a)
767
1 iMui, and an attempt has been made to quantify the secondary QI content by the difference in particle size. However, secondary QI particles are frequently coating the infusible particles of primary QI, which makes this technique inaccurate [45]. Coking
Value
During thermal treatment of petroleum pitch in an inert or oxygen deficient atmosphere, volatile matter is released as a result of distillation of smaller constituents and the thermal decomposition of the pitch [48]. The weight loss as a function of temperature is normally measured by thermogravimetric analysis (TGA). In general, there is no or little loss before 200-3 00°C followed by a rapid release, which goes through a m a x i m u m weight loss rate at a fixed temperature, Tmax. This Tmax is found by taking the derivative of the TGA slope (DTG). The rate of weight loss decreases after Tmax, reaching a m i n i m u m at 500-600°C. The standard test methods of ASTM do not consider the weight loss profiles of pitches, but rather the remaining carbon content, or coke, after heating to temperatures well over 500°C, which is a general coking temperature of pitch (Fig. 2). After the pitch has formed coke, the weight loss is minimal. Hence, the standard test method ASTM D 4715, or "Alcan" coking value, requires 2 h heat-treatment at 550°C, while the "Modified Conradson" coking value, ASTM D 2416, is established after only 30 min at 900°C. The thermal weight loss, as determined by TGA, for a coal tar pitch is shown in Fig. 5 and the weight loss curve for a petroleum pitch is similar. The loss is initiated around 200°C, with the highest loss rate between 300-400°C, then levels out at 500-550°C, and decreases very slowly at higher temperatures. Hence, the "Alcan" coking value (D 4715) must be established using sufficient time for the coke to form—2 h holding time at 550°C. Increasing the temperature to 900°C, as for the "Modified Conradson" (D 2416), shortens the coking time, but there is also some loss of gaseous products that does not occur during D 4715. Therefore, the "Alcan" coking value (D 4715) is generally 2 wt% higher than the "Modified Conradson" coking value (D 2416). For comparison with the whole pitch, the weight-loss profiles for its three solvent fractions, TS (y-resin), beta-resin ()3-resin), a n d QI (a-resin), weighed on their extraction yield are also stack-plotted in
(b)
FIG. 4—HRTEIVI micrographs comparing the microstructure of (a) a carbon black, and (b) primary QI particle from a coal tar pitch. Reprinted with permission from Elsevier Science.
768
MANUAL
3 7: FUELS AND LUBRICANTS
100%-
•1VI1..1
;^
^'^^
NX^
T3
1
^^; X
75%-
H
1
HANDBOOK
50%-
n
^ • ^ ^
s
F o r Sulfur I n Petroleum P r o d u c t s (High-Temperature Method, can be used for petroleum pitches. The petroleum pitch is combusted either in an induction or resistance type furnace. The sulfur is released as SO2 that is retained in a KIO3 solution and quantified by titration. ASTM D 2492, Standard Test Method for Forms of Sulfur in Coal, is often specified for coal tar pitches, where sulfate sulfur is extracted from the pitch using dilute HCl and pyritic sulfur is calculated as a stoichiometric c o m b i n a t i o n with iron. However, as for petroleum pitch, a Leco sulfur analyzer is generally used.
> TT'
1^ Ivl.i-l -1II1
' --',
a 25%-
'^ 0%-
'
1
1
.r'J
}*j*j
r
\}*J Temperature / °C
1
o'ju
1
"Jo
I'OO
FIG. 5—Comparison of TGA traces for a coal tar pitch and its TS, beta-resin and Ql fractions. Fig. 5. There is a good agreement between the whole pitch and the added loss profile from its solvent fractions. From the individual solvent fractions, it is clear that the QI loses very little weight, which is also the case for the )3-resin. The main weight loss occurs in the TS fraction, which indicates that the weight loss is particularly dependent on the y-resin content. Hence, a particular concern for the utilization of petroleum pitch as a binder for carbon artifacts is its low content of insolubles, in particuljir QI and to some extent also )3-resin. Density The density of pitch is a property closely watched by consumers, since low pitch density is generally related to low coking value, increased costs, and reduced quality of the resultcint carbon products. The ASTM standard methods differentiate between specific gravity (density) and relative density, however, specific gravity is usually reported for pitches. Due to the different appearance of pitch, three standard methods for specific gravity have been developed by ASTM: D 2320 deals with sohd fragmented hard pitch, D 70 concerns soft pitch, and D 4892 applies to pulverized pitch. All three methods involve the use of pycnometer. Since the pitch density is dependent on temperature, ASTM has developed a standard test method, D 2962, for calculating a volume-temperature correction. However, this test method is only developed for coal tar pitch and there is no equivalent m e t h o d for p e t r o l e u m pitch. The d e p e n d e n c e of specific gravity upon pressure is normally ignored. The standard test method for relative density, D 71, utilizes water displacement, a n d is therefore a quick method. However, relative density of pitch is rarely reported. Petroleum pitches have densities spanning from 1.10-1.30 gcm^^ cind are somewhat lower than that of coal tar pitches, which have specific gravity in the range 1.30-1.40 gcm~^. Hence, increasing the density of petroleum pitches is a key issue in their competition with coal tcir pitches. Sulfur
Content
The sulfur content in some oil crudes may be very high (3-5 wt%) and correspondingly can result in high sulfur levels in the resultant petroleum pitch, where levels u p to 4 wt% have been reported [23]. Since there is no specific ASTM method for sulfur determination in pitch, it is typical to use a Leco sulfur analyzer. However, ASTM D 1552, Standard Test Method
Ash Petroleum pitches have generally low ash contents below 0.1%. The standard test method for ash in pitch is described in ASTM D 2415 and applies to both hard and soft petroleum pitches. The m a i n elements in ash from petroleum pitch are iron, nickel, and vanadium [23], and these heavy metals are known to m a r the carbon products used in the aluminum and steel industries [49]. Conclusions The ASTM standard methods for pitch chetracterization Eire i m p o r t a n t tools for the establishment of petroleum pitch properties such as softening point a n d viscosity behavior, solubility, density and coking yield, and ash and sulfur levels, as described in the above sections. The monitoring of pitch quality through standardized methods is crucial for the future of the carbon industry, due to its dependence on high performance pitches for purposes such as binders and for impregnation of carbon materials, and precursors for carbon fibers and composites. Although coal tar pitch is presently dominating the market, a decline in its availability is forecasted, especially in the U.S., due to the reduced numbers of by-product coke ovens. Hence, petroleum pitch cam gain access to these markets through innovative manufacturing processes for tailored properties. The ASTM standard methods will be important reference points for the development of high performance petroleum pitches.
PETROLEUJVI COKE Petroleum coke is a generic term used to describe a variety of carbonaceous solid products intentionally produced by severe thermal cracking of p e t r o l e u m heavy fractions. The composition, microstructure, and properties of petroleum cokes depends on the nature of the feedstock and the coking process. Specific chemical a n d physical properties a n d microtextural characteristics of petroleum cokes are critically important for their industrial/commercial uses. Petroleum coke production processes, the composition and properties of coker feedstocks, and the formation of coke microtexture in coking processes are described a n d discussed in this section along with classification, properties, and uses of petroleum cokes. Petroleum Coke Production Processes Low-temperature carbonization (coking) at temperatures below 500°C converts high-boiling petroleum fractions or
CHAPTER 29: PROPERTIES
OF FUELS, PETROLEUM PITCH, PETROLEUM
residua into mixtures of gases, distillate liquids (naphtha and gas oils), and carbonaceous solids (petroleum cokes). In the general petroleum refining scheme, coking is a severe thermal cracking process that is used primarily for producing distillate liquids from heavy ends, such as vacuum distillation residua (VDR). The resulting petroleum coke is considered a by-product that may have some commercial value depending on the nature of the coke. More specialized coking processes also exist to produce a high-value premium petroleum coke as the primary product from more aromatic feedstcoks, such as Fluid Catalytic Cracking Unit Decant Oil (FCCU DO). Two principal coking processes used in petroleum refining are Delayed Coking and Fluid Coking [50]. Figure 6 shows simple flow diagrams of Delayed Coking and Fluid Coking Processes. Delayed Coking is the most commonly used coking process in petroleum refining. It is a semi-continuous process. The coker feed is preheated in externally fired tube furnaces and continuously charged into a large insulated d r u m where coke formation takes place at approximately 475°C as the drum is filled [51]. The pressure in the coke d r u m varies in the range 40-50 psig (0.35-0.45 Mpa). Gases and distillate liquid products from severe thermal cracking reactions are collected from the top of the d r u m . Once the coke d r u m is filled, the feed is switched to another d r u m for continuous charging. Upon completion of the coking cycle, the coke product is mechanically removed from the d r u m using high-pressure water jets to cut the coke. Thus, the production of coke is intermittent, although the feed is charged continuously. The resulting delayed coke is usually regarded as a low-value byproduct, generally used as fuel coke. Higher quality delayed cokes with lower sulfur and metal contents, called sponge cokes because of their porous nature (see Fig. 7), are further
COKE, AND CARBON MATERIALS
APPEARANSi
< -
769
OPTICAL TEXTURE
t
NEEDLE COKE
OPTICAL MICROGRAPH OF NEEDLE COKE
SPONGE COKE
OPTICAL MICROGRAPH OF SPONGE COKE
SHOT COKE
OPTICAL MICROGRAPH OF SHOT COKE
FIG. 7—Appearance and optical textures of needle, sponge, and shot cokes. (TvTSs
/"^^"N
NAPHTHA
SEPARATOR
FIG. 6—Schematic flow diagrams for Delayed Coking and Fluid Coking.
processed to manufacture carbon anodes used mainly for aluminum production [52]. An occasioned and usually undesired co-product from delayed coking of VDR is called shot coke because of its morphology that resembles the clusters of buckshot [53] (see Fig. 7). Shot coke is extremely hard to grind; it does not have any significant commercial use except for some small-scale niche applications, such as the production of TiOa [52]. An important property of fuel coke is its grindability, which can be measured by ASTM D 5003, HardgroveGrindability Index (HGI) of Petroleum Coke. Fuel grade cokes should have an HGI of greater than 80. Slurry oils (or decant oils), highly aromatic bottoms product from Fluid Catalytic Cracking Unit (FCCU DO), are also used as feedstocks for delayed coking. Coking takes place u n d e r slightly different conditions, e.g., higher pressures (60-80 psig) and lower temperatures (450°C), to produce a premium petroleum coke, needle coke. The term needle coke derives from the splintery appearance of this coke because of its highly anisotropic microtexture (see Fig. 7). After calcination, needle coke is used as a filler for manufacturing graphite electrodes for electric-arc furnaces to mEmufacture iron and steel [54]. In addition to FCCU DO, thennal tars from naphtha and gas oil cracking are also used as feedstocks for needle coke production. Compared to decant oils, however, thermcJ tars usually produce lower quality needle cokes under comparable coking conditions.
770
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
Very different in design and operation from Delayed Coking, Fluid Coking is another commercial coking process commonly used in petroleum refining to convert VDR and other heavy fractions into distillate liquids, gases, and fluid coke. In contrast to the semi-continuous nature of Delayed Coking, Fluid Coking is a continuous process (see Fig. 6). In Fluid Coking, the feed is sprayed onto fine size hot coke particles in the coker where coke is deposited on these seed particles by thermal cracking of the feed at temperatures between 480-525°C. Some of the coke product is introduced into a combustor where partial combustion of the coke particles with air takes place. Partially combusted hot coke particles are returned to the coker to provide the heat necessary for thermal cracking/coking to complete the cycle. The resulting coke product, fluid coke, has a different structure and different properties compared to those of the delayed cokes, as discussed in the following sections. Fluid coke is used principally as fuel with some limited use in the manufacture of carbon materials. Flexicoking process is a variation of Fluid Coking where the fluid coke product is partially or completely gasified with air and steam to produce fuel gas in the refinery [50,52]. Table 9 s u m m a r i z e s the coking processes, coker feedstocks, different kinds of petroleum cokes and their principal industrial applications. Petroleum cokes produced by delayed or fluid coking are generically called green cokes because of their relatively high volatile matter content (5-15%). Fluid cokes have generally lower volatile matter contents than delayed cokes (typically around 5%) because of the relatively high temperatures used in fluid coking. Formation of Coke Microtexture in Coking Processes Delayed cokes produced from VDR (sponge, fuel, and shot coke) show substantial differences in microtexture, including variations in the degree of structural anisotropy, the size of anisotropic domains, and porosity. Sponge cokes are more anisotropic, i.e., with a higher degree of microstructural order, and more porous than shot cokes. Fuel cokes have an intermediate degree of anisotropy and porosity between those of sponge and shot cokes. Differences in the degree of structural anisotropy of delayed cokes (illustrated in Fig. 7) can be traced to the development of an intermediate liquidcrystalline phase, carbonaceous mesophase, during liquidphase carbonization in the delayed coker. Carbonaceous mesophase is a unique, ordered fluid consisting of essentially planar molecules. The prevailing molecular order, which exhibits an intermediate state between isotropic liquids and
Process Delayed Coking
Fluid Coking Flexi-coking
anisotropic solid crystals, is similar to that of the nematic (thread-like) liquid crystals [55]. The basic molecular units of mesophase are, however, disc-shaped alkyl substituted polyaromatic hydrocarbons and range widely in size [56]. Polarized-light microscopy makes use of an incident planepolarized light that is reflected from polished specimen surfaces to reveal the extent of microstructural order present in the sample [57]. In plane polarized-light, the waves of light are confined to a single plane, in contrast to random orientations of waves in ordinary light. Using polarized-light microscopy, uninhibited formation a n d development of m e s o p h a s e can be observed to occur in three sequential stages: 1) nucleation of anisotropic spheres (0.5-10 m m in size) from an isotropic liquid; 2) growth of anisotropic spheres; and 3) their coalescence to form anisotropic microtextures in cokes upon solidification. The polarized-light micrographs in Fig. 8 show the three stages of mesophase development as a function of time during carbonization of a decant oil (CCB) and an ethylene tar residue (ETR) sample in a laboratory batch reactor at 4758C. The extent of microtextural anisotropy in the resulting cokes is described and classified with respect to the size and shape of the anisotropic d o m a i n s produced by mesophase development. Under a polarized-light light microscope with a phase-sensitive retardation plate, the anisotropic domains are seen on polished specimen surfaces as the same color areas—isochromatic regions, depending on the orientation of ordered regions. The overall appearance of the polished sections of the cokes under a polarized-light microscope is referred to as the optical texture. Table 10 describes a commonly used classification scheme to describe the optical texture of green cokes, or other carbonaceous solids [57]. When a phase retardation plate is used in a polarized-light microscope, isotropic regions seen as light purple areas represent the lack of any structural order in units greater than 0.5 /am (the resolution of light microscope). Anisotropic structures display dark purple, blue, and yellow areas of varying shape and size. In the absence of a phase retcirdation plate, the anisotropic regions appear as different gray levels. The term mosaics describes small structures of anisotropic units that are 0.5-10 ;u,m in diameter. Small domains and domains refer to isometric areas with diameters of 10-60 ix.m, and >60 ^im, respectively. Elongated domains that are greater than 60 ^(,m in length and greater than 10 /am in width are called flow domains. Flow domains are formed by the deformation, or shearing of anisotropic domains by the evolution of volatiles right before the solidification of mesophase to produce coke. The polarized-light micrographs in Fig. 9 show flow domains and mosaics textures of cokes
TABLE 9—Coking proceses, feedstocks, and commercial uses of petroleum coke. Feedstock Product Commercial Uses of Coke Vacuum Distillation Residue Sponge coke Carbon anodes for aluminum industry pr Vacuum Distillation Residue Fuel for cement kilns, industrial heat and Fuel coke steam raising, fuel for utility industry Vacuum Distillation Residue Shot coke No significant commercial use, some niche applications, e.g., Ti02 production, packing material Clarified Slurry Oil from FCCU Needle coke Graphite electrodes for electric-arc furnaces for iron and steel industry Vacuum Distillation Residue Industrial heat and steam raising Fluid coke Vacuum Distillation Residue Industrial heat and steam raising Fluid coke, or fuel gas
CHAPTER 29: PROPERTIES
OF FUELS, PETROLEUM PITCH, PETROLEUM COKE, AND CARBON MATERIALS
CCB
CCB
771
CCB
ETR FIG. 8—Stages of mesophase formation during carbonization of a decant oil (CCB) and ethylene tar residue (ETR) sample, nucleation, growth, and coalescence of mesophase spheres to form anisotropic coke texture.
TABLE 10—Classification and description of the optical texture of green cokes as observed by polarized-light microscopy using a retardation plate [57]. Classification
Isotropic Anisotropic Fine mosaics Medium mosaics Coarse mosaics Small domains Domains Flow domains
Description
Uniform, light purple color Dark purple, yellow, and blue areas of varying shape and size Isochromatic areas (lA), 0.5-1.5 fim in diameter lA, 1.5-5.0 fjLTn in diameter lA, 5.0-10.0 ^tm in diameter lA, 10-60 fim in diameter lA, >60 ixin in diameter lA, >60 (u.m in length, >10 fitn in width FLOW DOMAINS
produced by carbonization of a decant oil and a vacuum distillation residue in a laboratory batch reactor. Figure 7 shows the optical textures of the samples of needle, sponge, and shot coke particles, in addition to their external morphologies, as described before. Commonly, the optical texture of delayed cokes is heterogeneous, comprising a mixture of structures with different levels of anisotropy from mosaics to flow domains. A preponderance of a given texture, or a mixture of textures, characterizes the principal optical texture of delayed cokes. The optical texture of needle cokes, for example, consists mostly of straight flow domains. Sponge cokes, on the other hand, display a predominance of acicular, or twisted flow domains and large domains, while shot coke texture consists mostly of mosaics and some small domains. In other words, on the two extremes of optical texture scale, needle cokes result from a high degree of mesophase development, whereas shot cokes display an inhibited mesophase development. Distinct differences in the optical texture of delayed cokes explain many differences in physical and chemical properties of needle, sponge, and shot cokes, as further discussed in the Classification of Petroleum Cokes section. Fluid cokes have very different microtextures from those of the delayed cokes because of the substantial differences in the operation and conditions of the two coking processes, as
MOSAICS FIG. 9—Flow domains and mosaics textures as viewed by polarized-light microscopy.
described in Petroleum Coke Production Processes. Characteristically, fluid cokes have layered structures resembling onions, because coke deposition tcikes place in layers on the seed coke particles. Figure 10 shows the optical texture of a fluid coke sample. In contrast to liquid-phase carbonization in delayed coking, vapor phase cracking and polymerization
772
MANUAL
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HANDBOOK
FIG. 10—Optical texture of fluid coke (Petrographic Atlas, Prepared by Ralph Gray and Jack Crelling, http://mccoy.lib.slu.edu/cgi-bin/projects/crelling/pageBuilder.pl?find=P36). reactions on coke surfaces is primarily responsible for coke formation in fluid coking. The opticcil texture of fluid cokes is, therefore, very different from those of the delayed cokes. Parallel orientation of concentric layers of coke creates more isotropic structures in contrast to large anisotropic domains present in sponge and needle cokes. Coker Feedstocks Commonly used coker feedstocks are vacuum distillation residua (VDR), FCC U decant oils (DO), and thermal tars (TT). Table 11 summarizes coker feedstock properties, presented either as average values, or ranges of variation for typical properties. The important differences between VDR and DO include typically lower °API gravity and higher aromaticity of DO. VDR contains a large fraction of sulfur and metals (particularly Ni and V) found in crude oil because of their close association with asphaltene structures. VDR can contain u p to 40% wt asphaltenes (defined here as toluene solubles and pentane insolubles), whereas the maximum asphaltene content of DO is usually not greater than 10% wt [54]. Further, the aromatic molecules in the asphaltenes of VDR, consisting of polycyclic aromatic ring systems connected with alkyl bridges, are much larger than those in DO. Heteroatoms (S, N, and metals) tend to concentrate in these large molecules of VDR asphaltenes. The aromatic compounds in DO consist of much simpler structures, containing mainly two- to six-ring fused polycyclic aromatic hydrocarbons (PAH) [54,58] Table 11 shows that despite their high °API gravity (or low specific gravity), VDR has a m u c h higher Conradson Carbon value than DO, mainly because of the high asphaltene contents of VDR. In general, Conradson Carbon value is directly proportional to coke yield from the delayed coking, or fluid coking processes [50]. Generally higher sulfur contents of VDR compared to those of DO should also be noted. Thermal tars are similar in overall properties to DO; although TT tends to be more aromatic than DO, and contain much less naphthenes. These differences in feedstock properties combined with differences in coking conditions control the structure and important properties of petroleum cokes, such as the optical texture, heteroatom contents, strength, and reactivity. Chemical composition of coker feedstocks strongly affects the mesophase development through the carbonization chem-
istry and the rate of coke formation [59]. In general, feedstocks with high aromaticity produce a high degree of mesophase development, as explained by the planarity and lower thermal reactivitity of aromatic c o m p o u n d s . Coker feedstocks, particularly VDR, with a high asphaltene content produce inferior cokes with a low degree of anisotropy, and high heteroatom a n d metals contents. It has been shown, however, that the molecular nature of the asphaltene fractions is as important as, if not more important than, the asphaltene contents of the feedstocks for controlling the mesophase development [60]. Hydrogen aromaticity, rather than carbon aromaticity of the asphaltene fractions, appears to be an important structural parameter that relates to the carbonization behavior of the coker feedstocks, such as carbonization reactivity, and hydrogen shuttling ability of the molecular constituents during carbonization. High hydrogen aromaticity, which indicates small aromatic ring systems and low degree of alkyl substitution on the aromatic rings, gives rise to a high degree of mesophase development through controlling the rate of molecular growth and fluidity (or viscosity) during carbonization. Heavily alkyl substituted, large aromatic ring systems, as found in the asphaltene fractions of VDR, lead to rapid growth of n o n p l a n a r intermediates, and a rapid increase in the viscosity of the carbonization medium, thus impairing the mesophase development. In extreme cases, these conditions lead to severe inhibition of mesophase development and to the formation of shot coke in delayed coking [53]. A good understanding of the chemistry of mesophase formation from complex petroleum feedstocks still remains a current challenge, especially in terms of predicting the quality of needle cokes produced by delayed coking. Differences in molecular constitution of DO, in terms of the distribution of two- to four-ring aromatic compounds, have been shown to affect the mesophase development in laboratory reactors. An abundance of pyrene and alkylpyrenes in the DO, for example, leads to a high degree mesophase development. In contrast, high concentrations of biphenyl, fluorene, and alkylated phenanthrenes produce inferior optical textures in the resulting cokes [58]. Figure 11 shows a two-dimensional high-pressure liquid chromatography trace of a decant oil sample obtained with a photodiode array detector from normal phase separation using n-hexane and methylene chloride as solvents. The HPLC chromatogram (x-axis:time, y-axis:ulraviolet light wavelength, and z-axis: intensity of ultraviolet light absorption) with some indicates the complexity of the molecular structure of the decant oils with a few peaks labeled (pyrene, methylpyrenes, and methylbenzopyrenes) for demonstration purposes. However, individual compounds in
TABLE 11—Typical coker feedstock properties [4,50]. Feedstock Property
Vacuum Distillation Residue
Decant Oil
Thermal Tar
°API Gravity Sulfur, wt% Conradson Carbon, wt% Molecular Weight C/H Ratio, Atomic Aromatics, % wt Paraffins, % wt Naphthenes, %wt
7-15 0.5-3.0 10-20 850 0.65 35 10 55
1-8 0.5-0.8 5-10 300 0.8 62 0 38
-1-4 0.5 8-9 380 0.9 68 7 25
CHAPTER 29: PROPERTIES
OF FUELS, PETROLEUM PITCH, PETROLEUM
the decant oils can be identified, based on their UV spectra, if a good chromatographic separation can be achieved. Classification of Petroleum Cokes Coke is defined as "a soHd high in content of the element carbon and structurally in the non-graphitic state" [61]. It is a carbonaceous product from carbonization of organic material, which has passed, at least in part, through a liquid or liquid crystalline state. Petroleum coke is generally defined as a carbonization product of high-boiling fractions obtained in petroleum processing. The overall classification of green petroleum cokes (sponge, fuel, shot, needle, and fluid coke) as shown in Table 9, is based on the morphology, microstructure, elemental composition, and selected physical and chemical properties of the cokes. As discussed in the previous sections, these important characteristics of the cokes, which determine their commercial applications, depend on a combination of factors including feedstock properties, the coking process, and coking conditions. Definitions of some technical terms relevant to petroleum coke classification are reproduced below from lUPAC's "Recommended Terminology for the Description of Carbon as a Solid" [61]. • Calcined Coke is a petroleum coke obtained by heat treatment of green coke to about 1600 K. It will normally have a hydrogen content of less than 0.1 wt%. • Filler (also called grist) is a petroleum-coke fraction of a green carbon mix or formulation. • Graphitic Carbon represents all varieties of substances consisting of the element carbon in the allotropic form of graphite. • Graphitizable Carbon is a nongraphitic carbon, which upon graphitization heat treatment, converts into graphitic carbon.
COKE, AND CARBON MATERIALS
• Graphitization Heat Treatment is a process of heat treatm e n t of nongraphitic carbon, industrially performed at t e m p e r a t u r e s in the range between 2500-3300 K, to achieve transformation into graphitic carbon. • Green Coke (raw coke) is the primary solid carbonization product from high boiling hydrocarbon fractions obtained at temperatures below 900 K. It contains a fraction of matter that can be released as volatiles during subsequent heat treatment to approximately 1600 K. This mass fraction, the so-called volatile matter constitutes 4 and 15wt% of green cokes. • Needle Coke is a special t5rpe of coke with extremely high graphitizability resulting from a strong preferred parallel orientation of its layered structure and a particular physical shape of the grains. • Premium Coke is a n extremely well graphitizing carbon with a high degree of optical anisotropy and is characterized by the combination of the following properties: high real density, low reversible thermal expansion, and low ash content combined, in most cases, with low sulfur content. • Puffing is an irreversible expansion of some carbon artifacts during graphitization heat treatment between 1650 K and 2700 K. Petroleum Coke Properties Green cokes with relatively low sulfur, metal, emd ash contents are calcined-heat treated to 1600 K-to remove volatile matter. Polarized-light m i c r o g r a p h s in Fig. 12 show the shrinkage cracks and pores developed upon calcination of a needle coke and a sponge coke. Calcined coke has a mass fraction of hydrogen less than 0.1%. Calcined cokes are used as fillers for carbon anodes, graphite electrodes, or specialty carbons. Table 12 lists the specifications of calcinable green
0.030
0.025
0.020
'immnm^KMSm
^375.00
0.00
5.00
10.00
15.00 20.00 Hinutes
773
25.00
30.00
35.00
FIG. 11—A two-dimensional HPLC chromatogram for a decant oli sample.
774
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK sis, p r o v i d e s u s e f u l g u i d e l i n e s f o r s a m p l e p r e p a r a t i o n f o r a v a r i e t y of l a b o r a t o r y a n a l y s e s . Volatile
Matter
V o l a t i l e m a t t e r c o n t e n t of g r e e n c o k e s is d e t e r m i n e d b y m e a s u r i n g t h e loss i n w e i g h t r e s u l t i n g f r o m h e a t i n g u n d e r r i g i d l y controlled c o n d i t i o n s (ASTM D 4 4 2 1 , Volatile M a t t e r in P e t r o l e u m Coke a n d ASTM D 6374, Volatile Matter in Green P e t r o l e u m Coke, Q u a r t z Crucible P r o c e d u r e ) . T h e volatile m a t t e r c o n t e n t is p a r t i c u l a r l y i m p o r t a n t for t h e i g n i t i o n c h a r a c t e r i s t i c s of t h e fuel c o k e s ; t h e h i g h e r t h e v o l a t i l e m a t t e r
Calcined needle coke
TABLE 13—Calcined coke properties [52]. Properties
Calcined sponge coke
FIG. 12—Polarized-light micrographs of calcined needle coke and sponge coke samples, indicating anisotropic texture, and porosity developed by devolatilization. TABLE 12—Typical green and calcined coke specifications [50-52]. Green Coke Ash Calcium Fixed carbon Hydrogen Iron Manganese Magnesium Moisture Nickel Nitrogen Real Density Silicon Sulfur Titanium Vanadium Volatile Matter
0.1-1.0% 25-500 p p m 87-97% 3.0^.5% 50-2000 p p m 2-100 p p m 10-250 0.5-2.0% 10-100 p p m 0.1-0.5% 1.6-1.8 g/cc 50-300 p p m 0.2-2.5% 2-60 p p m 5-500 p p m 5-15%
Calcined Coke 0.2-1.5% 25-500 p p m 97-99% <0.1% 50-2000 p p m 2-100 p p m 10-250 p p m Negligible 10-100 p p m <0.1% 2.08-2.13 g/cc 50-300 p p m 0.2-2.5% 2-60 p p m 5-500 p p m <0.5%
cokes a n d calcined cokes used in c a r b o n a n o d e a n d graphite m a n u f a c t u r e . N o t all g r e e n c o k e s p r o d u c e d b y d e l a y e d coki n g of v a c u u m d i s t i l l a t i o n r e s i d u a a r e s u i t a b l e f o r c a l c i n a t i o n . O n l y o n e - t h i r d of a p p r o x i m a t e l y 2 5 m i l l i o n t o n s of g r e e n c o k e p r o d u c e d i n t h e U . S . a r e Ccdcined for f u r t h e r u s e [ 5 2 ] . S o m e i m p o r t a n t p r o p e r t i e s of t h e g r e e n c o k e s t h a t d e t e r m i n e t h e i r s u i t a b i l i t y for c a l c i n e d c o k e p r o d u c t i o n a r e d e fined a n d discussed below w i t h reference to t h e industrial s i g n i f i c a n c e of t h e k e y p r o p e r t i e s . T a b l e 13 lists a d d i t i o n a l p r o p e r t i e s of c a l c i n e d c o k e s t h a t a r e r o u t i n e l y m e a s u r e d , a n d T a b l e 14 l i s t s s o m e s t a n d a r d m e t h o d s u s e d f o r m e a s u r i n g t h e s e p r o p e r t i e s . A S T M D 3 4 6 , S t a n d a r d P r a c t i c e for Collect i o n a n d P r e p c i r a t i o n of C o k e S a m p l e s for L a b o r a t o r y Analy-
Water Content Oil Content Granulometry >8 mm 8-4 4-2 2-1 1-0.5 0.5-0.25 <0.25 Vibrated Bulk Density 8-4 m m 4-2 2-1 1-0.5 0.5-0.25 Grain Stability 8 ^ mm Reactivity in CO2 at lOOOX Ignition Temperature Reactivity in Air at 600°C Elements S V Ni Si Fe Al Na Ca K Mg CI Crystallite Size, Lc Density in Xylene Specific Electrical Resistance Total Porosity
Typical Value 0-0.2% 0.1-0.3% 10-20% 15-25% 15-25% 10-20% 5-15% 5-15% 0 0.64-0.70 kg/dm^ 0.73-0.79
0.80-0.86 0.86-0.92 0.88-0.93 75-85% 5-10% 615-630°C 0.1-0.20%/min 1-3% 80-300 p p m 80-160 50-250 50-250 50-250 30-120 20-100 5-15 10-30 0.1-0.2% 26-30 A° 2.05-2.09 kg/dm^ 480-520 Micro O h m m 15-20%
TABLE 14—Standard test methods for determining calcined coke properties. Measurement Granulometry Bulk Density Grain Stability Oil Content (Dedusting Agent) Xylene Density Ash Content Specific Electrical Resistance Reactivity in CO2 Reactivity in Air Chemical Elements
Method ISO 2325 DIN 51 916 ASTM 5003-89 ISO 8723 ASTM 5004-89, ISO 8004 ASTM D 4422-89, DIN 51 903 DIN 51 911 Thermal Gravimetric Analysis (TGA) TGA Elemental Analysis, X-Ray Methods, Atomic Absorption Spectroscopy
CHAPTER 29: PROPERTIES
OF FUELS, PETROLEUM PITCH, PETROLEUM
content, the easier the ignition. Emission of PAH from combustion of fuel grade cokes is an important environmental concern that requires strict monitoring and control of emissions from the combustors. Fluid cokes have lower volatile matter contents than the cokes produced by delayed coking, because of higher coking temperatures used in Fluid Coking. Among the cokes produced by delayed coking, needle cokes have lower volatile matter contents (5-8%) than sponge and shot cokes (10-15%). Green cokes with high volatile matter content are not suitable for calcination. High volatile matter content result from i m p r o p e r coking conditions, such as short residence time and low coking temperatures, or feedstock properties. Calcination reduces the volatile matter content of the green cokes to less than 0.5% in the calcined cokes used for carbon anode, or graphite electrode manufacture. Ash Ash content of petroleum coke can be determined by reducing the sample to an ash by heating in a muffle furnace according to the standard method ASTM D4422, Standard Test Method for Ash in Analysis of Petroleum Coke. For fuel cokes, high ash contents create operational a n d disposal problems. Green cokes with high ash contents are not suitable for calcination mainly because of the detrimental effects of inorganic impurities on calcined coke properties, e.g., reactivity, as explained below. Ash contents of less than 0.5 wt% are required for sponge cokes used for carbon anode manufacture [52]. Sulfur Sulfur content of petroleum cokes can be measured using several ASTM standard methods including D 1552, Sulfur in Petroleum Products (High-Temperature Method), D 3177, Total Sulfur in the Analysis Sample of Coal and Coke, and D 4239, Sulfur in the Analysis Sample of Coal and Coke Using High-Temperature Tube Furnace Combustion Methods. Sulfur content of petroleum cokes is important for both fuel cokes, a n d green cokes calcined for c a r b o n anode and graphite electrode applications. For fuel coke and sponge coke applications, concerns related to SO2 emission and equipment corrosion are the main considerations for limiting the sulfur content. Sulfur content in fuel grade petroleum cokes and sponge cokes vary in the range 3-6 wt%, and < 3 wt%, respectively. Needle coke has a more strict specification on the sulfur content (<0.5wt%), because sulfur causes puffing during the graphitization heat t r e a t m e n t resulting in lower density and lower strength of graphite electrodes. Nickel,
Vanadium,
and Other
Metals
Nickel, vanadium, a n d other trace metals contents of petroleum cokes can be measured using different ASTM standard methods, including D 5056, Trace Metals in Petroleum Coke by Atomic Absorption; D 5600, Trace Metals in Petroleum Coke by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES); and D 6376, Determination of Trace Metals in Petroleum Coke by Wavelength Dispersive X-Ray Fluorescence Spectroscopy. For fuel grade cokes, nickel and vanadium contents are specified as 300-600 ppm. For sponge cokes, in addition to limits on nickel and vanadium contents (<500 ppm), iron content needs to be less t h a n < 2 5 0 p p m . Principal concerns with trace metals in
COKE, AND CARBON MATERIALS
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sponge cokes used for anodes are related to the contamination of aluminum, or other produced metals with the heavy metal impurities. Heavy metal contamination requires further treatment for purification, or limits the use of the produced metal in certain applications. High alkali metal contents, e.g., Ca and Na, are also not desired in sponge cokes, since these metals act as strong oxidation catalysts a n d increase anode c o n s u m p t i o n during electrolysis. Density Several ASTM standard methods can be used to measure the apparent and real densities of petroleum cokes, including D 167, Apparent and True Specific Gravity and Porosity of Lump Coke; D 2638, Standard Test Method for Real Density of Calcined Petroleum Coke by Helium Pycnometer; D 5004, Real Density of Calcined Petroleum Coke by Xylene Displacement; and D 4292, Determination of Vibrated Bulk Density of Calcined Petroleum Coke. Both apparent and real densities of petroleum cokes depend on their thermal history and operating variables in the production processes. For many applications of petroleum cokes, apparent, or real density of the cokes are closely monitored, since many industrial properties of cokes, such as strength, thermal and electrical conductivity, and reactivity of cokes can be related to coke density. Porosity of the petroleum cokes can be calculated from the apparent and true density measurements by the following formula: Porosity = 100 - 100(apparent density/true density) For true density measurements, helium pycnometry is commonly used to obtain meiximum penetration of the pores by helium gas. For petroleum cokes, helium density is usually 5% below the theoretical density. This difference is attributed to "closed porosity" in cokes that is not accessible to helium. Porosity of cokes is particularly important for applications where petroleum coke is used as filler in mixed formulations with a binder (e.g., carbon anode, and graphite electrode manufacture), usually coal tar pitch. Desired pitch/coke ratios and the resulting properties of the baked, and graphitized carbons, therefore, depend strongly on the porosity, or apparent density of the cokes. Strong correlations were reported between the bulk density of the filler cokes and the electrical resistivity, strength, and coefficient of thermal expansion of the resulting graphite electrodes [62]. The real density of petroleum cokes increases with the increasing heat treatment temperature, and depending on the graphitizability of the cokes, approaches the real density of graphite u p o n graphitization heat treatment. Apparent density, on the other hand, can go through a m i n i m u m with the increasing heat treatment temperature during calcination of green cokes, because of porosity formation by devolatilization. F u r t h e r increase in h e a t t r e a t m e n t t e m p e r a t u r e increases the apparent density because of collapsing porosity upon increasing the microstructural order. Optical Texture
and Crystalline
Structure
Polarized-light microscopy can be used for characterization of cokes and assessing their graphitizability. The appearance of a polished coke sample viewed under a polarized-light microscope is described as the optical texture, which refers to the anisotropy/isotropy of the solid phase. The isochromatic regions seen u n d e r a polarized-light microscope represent
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sections of the coke with a particular structural orientation. Generally, the examination of green coke texture proves to be useful to predict the calcined or graphitized coke quality. Optical texture varies greatly depending on the microstructure of a given coke and can be broadly defined as isotropic or anisotropic. When viewed under a polarized-light microscope, a material is considered to be isotropic if the light reflecting from the surface does not change with the direction of observation. Non-graphitizable, or poorly graphitizing cokes appear to be isotropic because their range of order is smaller than the resolution of the light microscope (<0.5 fjLin). Anisotropic cokes, on the other hand, have a much longer range order, 0.5-500 /xm, resulting from a high degree of mesophase development. These cokes show considerable change in reflected light intensity with direction of observation. In general, the larger the size of anisotropic, or isochromatic regions, the higher is the graphitizability of the coke. This is particularly important for needle cokes used for manufacturing graphite electrodes. Nomenclatures have been developed for a more precise description of different optical textures in solid carbons [63,64]. According to the proposed nomenclatures, an optical texture index can be determined by assigning different weight factors to different anisotropic units. Table 15 lists assigned texture weight factors, or optical texture index (OTI) values, proposed by Oya et al. [57] for different anisotropic structures. The larger the size of the anisotropic unit, the higher the optical texture index. As seen in Table 15, mosaics are assigned relatively small OTI ranging from 1 to 7, depending on the size of the mosaics, while domains and flow domains are assigned a large OTI, 30. The optical texture of solid carbons is usually very heterogeneous, consisting of a variety of different anisotropic structures, such as flow domains, domains, and mosaics. Consequently, a point counting method is widely used for characterizing the texture of solid carbons [57,65,66]. The fraction of points counted for each component is multiplied by the corresponding optical texture index (OTI) value to calculate a factor for each component. The OTI of a sample is, then, calculated by summing the factors for all the texture categories. Using the optical texture classification scheme given by Oya et al. [57], shown in Table 15, in conjunction with a point counting procedure one can differentiate between anisotropic textures of solid carbons over a broad range of texture quality. This general texture classification, however, fails to distinguish between similcir textures of coke samples that fall into a more specific category, e.g., sponge cokes, or needle cokes. Needle cokes, for example, consist predominantly of domains and flow domains. Small differences in texture qual-
ity, i.e., the degree of anisotropy, cause substantial differences in thermal expansion behavior (or in coefficient of thermal expansion (CTE)) of needle coke samples. For example, different proportions of flow domains and domains in a needle coke sample, that are assigned the same OTI value, 30, would give very different CTE in a given direction. Low CTE of needle cokes along their long axis is attributed to the dominant flow domain anisotropy parallel to the long axis. On the other hand, large isometric domains in needle cokes would give rise to relatively high CTE. The same optical texture index value assigned for both domains and flow domains would, therefore, not capture this critical difference in thermal expansion anisotropy of needle coke particles. Different optical texture index assignments were proposed by Eser [58], as shown in Table 15, to account for the differences in thermal expansion anisotropy of different texture units. Based on point counting data, the optical texture index for a coke sample can be calculated using the following formula: OTI = 100*(FD) + 50*(D) + 5*(SD) +1*(M) where the letters represent the fractions of the flow domains (FD), domains (D), small domains (SD), and mosaics (M), in the order of decreasing degree of anisotropy. The higher the OTI, the higher the degree of anisotropy (or graphitizability), and the lower the CTE edong the long aixis of the coke particles. Calcined needle cokes tend to have larger anisotropic units than green cokes. Calcination usually increases the size of optical domains, but it does not change their shape. Upon calcination, flow domain structures keep their shape to form needle-like grains along with large isometric domains. For calcined needle cokes used as filler for graphite electrode manufacture, the size of optical domains becomes less important compared to their shape and orientation. This is often demonstrated when coke is over calcined and display larger anisotropic domains, but poor thermal expansion behavior [67]. Figure 12 illustrates optical textures of calcined sponge and needle coke samples. Using scanning electron microscopy (SEM), Pysz et al. [68] proposed a different nomenclature for classification of calcined needle cokes, shown in Table 16, to include larger texture domains. Analogous to green coke texture characterization, using similar OTI designations for the texture components of increasing anisotropy as shown in the last column of Table 16, Qiao [69] showed good correlations between OTI of calcined needle coke particles and their CTE measured on the same particles. There is no standard method for texture characterization of petroleum cokes. ASTM D 5061, Microscopical Determination of Volume Percent of Textural Components in Metallurgical Coke describes procedures to identify the tex-
TABLE 15—Optical texture index (OTI) assignments to distinguish between different anisotropic texture components [57,58]. Classification
Description
OTI [57]
OTI [58]
Isotropic Anisotropic Fine mosaics Medium mosaics Coarse mosaics Small domains Domains Flow domains
No optical activity Isochromatic areas (lA) lA, 0.5-1.5;u,ni in diameter lA, 1.5-5.O^m in diameter lA, 5.0-lO.Ojixm in diameter lA, 10-60/!Am in diameter lA, > 6 0 ^ m in diameter lA, >60/Ltm in length, >10/xm in width
0
0
1 3 7 20 30 30
1 1 1 5 50 100
CHAPTER 29: PROPERTIES
OF FUELS, PETROLEUM PITCH, PETROLEUM
TABLE 16—Nomenclature for and description of microtexture in calcined needle cokes by scanning electron microscopy [68]. Classification
Description
I A S F
Isometrics Acicular Stringy Fibrous
Size <100 ij.m 100-300 ixm 300-1000 fj.m > 1000 length, > 6 0 /iim width
OTI [58] 1 5 50 100
t u r a l c o m p o n e n t s of coke according to their degree of anisotropism, c a r b o n form d o m a i n sizes, b o u n d a r y size, color of individual isochromatic domains, their morphology, relative reflectance, and other optical properties. Representative crushed particulate coke samples are prepared using ASTM D 3997, Preparing Coke Samples for Microscopical Analysis by Reflected Light. Gray and Devanney [70] provided a detailed description of microscopic classification and industrial applications of coke carbon forms. Texture characterization by optical microscopy and SEM provides information on the degree of mesophase development that leads to the anisotropic microtexture formation in petroleum cokes. This information is particularly useful for studying and controlling mesophase development in delayed coking [54,58,66] and predicting the important industrial properties of calcined cokes such as strength, reactivity, and thermal expansion. The microtextural properties of cokes are related to the prearrangement of carbon atoms into three-dimensional crystalline structures, which determine the graphitizability of cokes. X-ray diffraction is used to determ i n e the degree of crystalline alignment in cokes. For calcined cokes, a p p a r e n t crystallite size (Lc), a n average stacking height of graphene layers (see Fig. 13), is a good indication of pregraphitic order, and, therefore, their graphitizability upon graphitization heat treatment. The apparent crystallite size of calcined cokes can be determined according to ASTM D 5187, Determination of Crystallite Size (Lc) of Calcined Petroleum Coke by X-Ray Diffraction. Thermal
Expansion
The coefficient of linear thermal expansion (CTE) is a critically important property for many applications of graphitic carbons, particularly graphite electrodes used in electric-arc furnaces. CTE is defined as the increase in length per unit length per degree rise in t e m p e r a t u r e [71]. A low CTE is essential for high thermal shock resistance, one of the most important properties of graphite electrodes, defined as the ability to resist weakening, or fracture when subjected to sudden heating, or cooling [72,73]. Compared to other materials, graphite stands out in resistance to thermal stress and high t e m p e r a t u r e s above 1300 K. As the m a i n c o m p o n e n t s in graphite electrodes, the needle coke fillers exert the most important influence on thermal expeuision behavior of graphite electrodes. Strongly anisotropic crystals, such as graphite, have layered crystalline structure, in which the bonding is highly directional. Graphite has a m u c h lower thermal expansion in the layer planes (a-direction) that contain covalently bonded carbon atoms, than in the direction perpendicular to the layer planes (c-direction) that are held together by van der Waals forces [74], as shown in Fig. 13. The major factors that influence CTE of cokes and graphites are texture and the orientation of crystallites. Pores and cracks
COKE, AND CARBON MATERIALS
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in the structure of cokes and graphites can also play a role in accommodating thermal expansion, especially in the c-direction [75-77]. Many researchers have investigated how temperature affects thermal expansion of cokes and graphites [78-80]. It is known that, for anisotropic cokes, as the heat treatment temperature increases, CTE decreases because of increased structural ordering. After heat treatment at 600°C, cokes still have high CTE values in both a- and c-directions. There is a significant decrease in CTE upon heat treatment between 600 and 1300°C. Although there is not much decrease in CTE upon heat treatment at temperatures above 1300°C, it has been suggested that the CTE values in the c-direction decreases with the increasing stack height of crystallites (Lc) [81]. Thermal expansion measurements can be made using a dilatometer to measure linear expansion as a function of temperature in a given direction. A linear variable differential transformer (LVDT) mechanism is used to measure a dimensional expansion. Thermal expansion plots (expansion vs temperature) are seldom linear. Therefore, the mean CTE, defined as the slope of the line joining the two defined points, depends on the temperature range used for calculating its value, as follows [82]: Mean CTE = (4L/L)/ (4T) where zlL = observed changed in sample length over the temperature range AT, in AT = temperature range of measurement, °C L = sample length at room temperature, in The CTE values are reported using the unit 10~* in/in °C. The CTE measurements are made on calcined, or graphitized test bars prepared by extrusion of calcined coke particles with a standcird binder pitch. Highly graphitizable calcined needle cokes would give mean CTEs lower than I x 10"* in/in °C. Typical temperature ranges used for measuring the m e a n CTEs of calcined cokes are 100-600°C, or 300-700°C [83,84]. The CTE values measured at low temperatures are more sensitive to structural differences in cokes t h a n those measured at high temperatures [85].
a-direction
Stacking height:
c-direction FIG. 13—A diagram of section of microcrystals of pregrahipitic order indicating covalent bonding in basal planes (a-direction) and planes held together by van der Walls forces (c-direction) responsible for anisotropic structure and anisotropic properties.
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Reactivity Reactivity of cokes in oxidizing atmospheres constitutes another important industrial property of cokes. For fuel cokes, high reactivity is desired for ease of ignition and combustion. Relatively high volatile m a t t e r contents are, therefore, desirable for fuel cokes. For calcined cokes used in material applications, either intermediate or low reactivity is desired depending on whether carbon needs to be consumed, or not in the particular application. Carbon anodes used for electrochemical reduction of AI2O3 are, for example, consumed in the reaction to liberate alum i n u m metal. Four major carbon-consuming reactions are identified during electrolysis for aluminum production [25]: 1) 2Al203(diss) +2C(anode) —> 4Al + SCOa - desired reduction reaction 2) Al203(diss) +2C(anode) ^ 2Al + 3CO - generation of primary CO, low C efficiency 3) CO2 + 2C(anode) —> 2CO - uudesired consumption of carbon 4) O2 + C(anode) ^ 2C02 ; O2 + 2C(anode) ^ 2 C 0 - uudesircd consumption of carbon by a i r b u m Obviously, the first reaction is the desired reaction, whereas third and fourth reactions lead to undesired consumption of anode carbon by Boudouard and a i r b u m reactions, respectively [25]. The second reaction also produces Al but only with half of the yield obtained from the first reaction. An intermediate reactivity of carbon anodes (sponge coke and pitch coke) is, therefore, desirable to promote the desired reduction reaction and to inhibit the undesired anode consumption. In contrast to the electrochemical production of aluminum by the consumption of carbon anodes, the consumption of graphite electrodes in electrical-eirc production of steel takes place only due to the uncontrolled b u m - u p of the electrodes in air [88]. Consequently, extremely low reactivities of graphite electrodes (made u p of needle coke filler and coal tar pitch binder derived carbons subjected to graphitization heat treatment) are desired to minimize electrode consumption by a i r b u m . The reactivity of cokes in oxidizing atmospheres depends primarily on the degree of microcrystalline order (i.e., optical texture), accessible surface eirea (related to the porosity), and catalysis by inorganic impurities, such as Ni, V, Fe, K, Na, and Ca. Oxidation, or gasification of cokes is a surface reaction that occurs at active sites at elevated temperatures. Carbon atoms at the edges of graphene layers (or basal planes) or at defects on graphene layers such as vacancies and dislocations, are m u c h more reactive than the carbon atoms in basal planes. The high reactivities of edge atoms result from the dangling bonds, or unpaired sp^ electrons, at these sites that readily chemisorb oxygen. Highly anisotropic cokes, such as needle cokes, with relatively large microcrystallites found in well-developed flow domains or domain textures have low concentrations of these active edge sites, and, therefore, exhibit low reactivities. Sponge cokes, in comparison, have a lower degree of anisotropy, and higher porosity, and, therefore, have higher reactivities than needle cokes. High porosity, or high accessible surface area, increases the ease of diffusion of oxidant molecules to the active sites. Inorganic impurities, on the other hand, promote the dissociation of molecular oxygen to produce more active oxygen species that readily react with carbon atoms even in the basal planes,
producing pits by removal of carbon atoms as CO, or CO2 [86]. Increasing the heat treatment temperatures decreases the concentration of structural defects [87] and at sufficiently high temperatures (e.g., during graphitization heat treatment u p to t e m p e r a t u r e s 3300K), helps remove inorganic impurities by evaporation. The graphite electrodes have low reactivities because of low defect, or active concentration, low porosity, and low concentrations of inorganic impurities [88,89]. Thermal gravimetric analysis (TGA) can be used to measure the reactivity of coke samples in different oxidizing atmospheres (e.g., air, O2, or CO2). In these experiments, the weight of a sample placed in a microbalance is monitored and recorded continuously as a function of increasing temperature, or as a function of time at a given temperature. ASTM D 5341 Measuring Coke Reactivity Index (CRI) and Coke Strength after Reaction (CSR), can be adapted to measure the reactivity of petroleum cokes.
Conclusions Petroleum cokes have many industrial applications, ranging from generating heat (fuel coke) to manufacturing carbon products such as carbon anodes and graphite electrodes. Produced from various petroleum heavy residua or fractions from petroleum refining operations, petroleum cokes are classified into different types on the basis of feedstocks (e.g., vacuum distillation residua or FCC decant oil), coke production processes (e.g., delayed coking or fluid coking), or their appearance (e.g., sponge coke, shot coke, or needle coke). Feedstock composition, the coking process used, and coking conditions determine the structure a n d properties of petroleum cokes that are important in various applications. Among the important properties of petroleum cokes are volatile matter content, density, sulfur and metal contents, optical texture, coefficient of thermal expansion, and reactivity. Standard methods have been developed to quantitatively determine many of these properties for selecting, or producing the most desirable cokes for a given application. A fundamental understanding of the relationships between feedstock constitution and the structure and composition of petroleum cokes, and those between coke properties and coke structure is critically important for controlling the coking processes and subsequent thermal treatment operations to manufacture carbon materials with the desired properties.
CARBON MATERIALS FROM RESIDUAL FUEL OIL, PETROLEUM PITCH, AND PETROLEUM COKE As introduced previously in this chapter, residual fuel oil (mainly FCC decant oils), petroleum pitch, and petroleum coke are used as precursors for manufacturing a variety of carbon materials with very different properties and applications. In this section, four different carbon materials will be further discussed as examples to illustrate a glimpse of the diversity in microstructure and properties of the carbon materials. Four carbon materials are: carbon blacks, carbon fibers, carbon anodes, and graphite electrodes that are produced from decant oil, petroleum pitch, sponge coke, and
CHAPTER 29: PROPERTIES
OF FUELS, PETROLEUM PITCH, PETROLEUM
needle coke, respectively. Manufacturing processes, properties, and applications of these four carbon materials will be reviewed briefly with reference to their respective precursors as discussed before in this chapter. Carbon Blacks Classification, Manufacturing and Applications
Processes,
Carbon black is defined as "an industrially manufactured colloidal carbon material in the form of spheres and of their fused aggregates with sizes below 1 /u-m" [61]. They are produced by gas phase thermal decomposition of hydrocarbons using various feedstocks and processes [90-93]. Of the five major processes, Lampblack Process, The Cheinnel Process, Acetylene Process, Thermal Process, and Oil Furnace Process, the last one is the most widely used process to produce most of the carbon blacks available today [90]. These processes yield products t h a t are identified by the process names, such as lamp black, channel black, acetylene black, themiEj black, and furnace black. Each carbon black has a unique structure and a set of properties that are determined by the different manufacturing processes. Except for the Channel (or Impingement) and Acetylene processes that convert natural gas and acetylene to carbon blacks, respectively, a r o m a t i c oils are used as principal feedstocks for carbon black production. Most commonly used oils are residual fuel oils, or FCC decant oils. Carbon black oils, a distillate fraction of coal tar produced in by-product coke ovens, are also used as feedstocks in some processes. Thermal Black and Furnace Black processes that use aromatic oils as feedstocks are described below, along with the major applications of carbon blacks manufactured by these processes. Thermal blacks are produced by thermal decomposition of oils in the absence of air using a cyclic process that consists of heating and production cycles that rotate in a pair of furnaces (generators) in 2.5 min intervals [90]. The furnaces are lined with open checker brickwork that is preheated before the introduction of oil feed. Production of carbon black takes place in a heated furnace followed by a steam purge to remove the products (carbon black and byproducts including hydrogen gas). The products are sprayed with water for cooling and passed through a collection filter to separate the carbon black particles. Following the steam purge, air is passed through the furnace to bum-off the carbon black remaining in the furnace (supplemented by burning oil, if necessary) to produce heat for the next production cycle. Pairs of furnaces are used for continuous operation using S3rnchronous heating and production cycles in separate furnaces. Thermal blacks consist of larger particles (—250 to 500 n m average particle diameter) with lower degree of aggregation compared to other types of carbon black. They are used in applications that require very high volume fractions of fillers, including the production of r u b b e r a n d cross-linked polyethylene, as well as some specialty pol5rmers [90]. As opposed to intermittent production in the Thermal Black process, carbon black is produced continuously from highly aromatic oils in a combustion gas environment at high temperatures in a Furnace Process [90]. Production of furnace black takes place in a fraction of a second when the feed is injected into a flame that is established in the reactor with oil or
COKE, AND CARBON MATERIALS
779
natural gas and excess air. Immediately after carbon black production, the products are cooled with a water spray and further cooled as they pass through a heat exchanger before carbon black particles cire collected in a bag filter. Because of high gas flow rates, the carbon black particles reach the bag collector in less than a second after the feedstock oil is injected into the reactor. The size distribution of furnace black particles is controlled by the rate of cooling with water spray. For production of particles with a very small size (for high color applications), large quantities of water are needed for cooling, u p to 40:1 process water to carbon black ratio [90]. For use in rubber and plastic applications, a large fraction of furnace product is pelletized (beaded) using water to provide easy handling and less dust formation. For applications that require dry beads, such as in inks and coatings, powdered ceirbon black particles are beaded in a rotating drum where agglomerated carbon black act as nuclei to grow beads [90]. For carbon black production in the oil furnace process, the feedstock oil must be completely vaporized and pyrolyzed. If the conversion of oil to a carbonaceous solid takes place in a liquid phase, large particles of "coke" are produced. Coke particles, undesired contaminants in carbon black product, are much larger in size and very different in microstructure compared to ceirbon black particles. The formation of coke in Oil Furnace Process probably results from the presence of high-molecular-weight, aJkyl substituted PAH that contain more than five rings in polycondensed aromatic systems [58]. These compounds would tend to polymerize and go through liquid-phase carbonization to form coke. Figure 14 shows polarized-light-micrographs of some particles from a sample of "coke" produced in carbon black manufacturing. As opposed to the isotropic texture of small particles of carbon black, the "coke" particles showed anisotropic texture of pyrolytic carbon microstructures produced by gas to solid transformation taken place during liquid-phase carbonization (bottom micrograph in Fig. 14).
FIG. 14—Polarized-light micrographs of "coke" produced in carbon blacl< manufacturing.
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Many industrial applications of c a r b o n blacks rely on strength reinforcement (in elastomers), color (in inks and coatings), UV protection, and electrical conductivity (in polymers). The uses of carbon blacks in these applications derive from a n u m b e r of properties that are related to the size, morphology, and surface chemistry of carbon blacks. The meas u r e m e n t and significance of these properties for specific applications are described in the following section. Properties
of Carbon
Blacks
Four fundamental properties of carbon blacks that are important for industrial applications are particle size, structure (aggregate size and shape), porosity, and surface activity. These fundamental properties determine, to varying extents, a n u m b e r of functional properties, such as surface area, tint strength, and oil absorption, which are measured by well-established ASTM m e t h o d s to characterize carbon blacks. Table 17 lists ASTM methods used for characterization of carbon blacks. The particle d i a m e t e r (fineness) is the most i m p o r t a n t property of carbon blacks that relates to the level of elastomer reinforcement and color. Both the level of elastomer reinforcement and the degree of blackness increases with the decreasing particle size. The measurement of carbon particle size is very difficult and highly dependent on the measurement technique used. The most dependable method involves direct m e a s u r e m e n t of particle sizes a n d aggregate sizes using standard transmission electron microscopy (TEM) procedures (ASTM D 3849). Surface area measurements are also used (ASTM D 6556, D 3765, and D 1510) as indications of average particle size. All these surface area measurements are also affected by the pore size distribution, and in some instances, by surface properties of the particles. The results obtained by a commonly used standard method (ASTM D 1765) adopted in 1965 for characterization of rubber grade carbons are often inconsistent with those obtained by TEM procedures (ASTM D 3849). Because of difficulties in replicating particle size measurements even using ASTM D 3849 in different laboratories, particle size ranges reported by ASTM D 1765 have been abandoned in the ASTM designation of carbon blacks since 1996. Average nitrogen surface areas are re-
ported in ASTM D 1756 instead of particle size ranges. Using ASTM D 1765, carbon blacks are classified into different grades based on the average N2 surface area using a letter (N or S - referring to Normal, or Slow cure) and a three-digit number (e.g., N121 through N990 for rubber grades). First n u m b e r of the designation roughly corresponds to the particle size. The average N2 surface area, and the mean particle size of grade N121 are 121-150 m^lg, and 19 nm, respectively. For grade N990, the same parameters are 0-10 rr?lg, and 285 nm, respectively [90]. The term structure designates the irregular shapes of the chain- or grape-like aggregation of carbon black particles and is m e a s u r e d by absorption of dibutylphthalate (DBTA) (ASTM D 2414). The packing of chain- or grape-like particles creates internal voids t h a t absorb DBTA. Therefore, the higher the measured DBTA absorption, the higher the total level of structure, or more irregular shaped particles in the sample. The structure is a very important property of carbon blacks that affects the dispersion of carbon blacks in different mixtures. Porosity in carbon blacks is produced by steam gasification of carbon during cooling of particles after formation. A large fraction of porosity is found at the interior of the carbon black particles, because the surface layers in the concentric (onionlike) structure of carbons are more stable than the disordered internal layers. Porosity is an important property of carbon blacks used in conductivity and color applications. Carbon blacks used in elastomer reinforcement have low porosity. Surface activity is defined as the tendency of a carbon black to interact with its surroundings. Specific interactions depend on the physical and chemical characteristics of the surface a n d on the properties of the surrounding matrix. Surface composition, e.g., surface functional groups, particularly oxygen functional groups, strongly affects surface activity. Surface activity, the most difficult fundamental property to characterize, is very important in understanding and controlling the elastomer reinforcing properties and the dispersion of carbon blacks, including the rheological properties of polymer and carbon black mixtures. Other important properties of carbon blacks include sulfur content, extractable organics, ash, and sieve residue that axe.
TABLE 17—Standard test methods used for characterization of carbon blacks. Fundamental Property
Particle Size
Structure
Measured Property
ASTM Method
N2 surface area CTAB adsorption Iodine number Particle and aggregate sizes Particle size range, N2 surface area
D4820 D3765 D1510 D3849
Tint strength Dibutylphthalate (DBTA) absorption Sulfur content Extractable organics Ash
D3265 D2414
Sieve residue
D1765
D1619 D4527 D1506 D1514
Remarks
Surface area is considered as a measure of particle size, but measurements by different techniques are also sensitive to pore size distribution and surface chemistry. Particle and aggregate sizes are determined directly by transmission electron microscopy (TEM) Used for rubber grade carbons; usually not consistent with the results from D3849 especially for larger particle sizes; reporting of particles size ranges was abandoned in 1996. The smaller the particle size, the higher is the degree of blackness Higher DPTA absorption, higher the total level of structurechain-or grape-like aggregation Mostly non-reactive sulfur in aromatic heterocyles Thermally stable compounds produced in the flame extracted with toluene and analyzed by UV transmission at 325 nm Mostly Inorganics irom water, or trace metals and catalyst particles in the feedstock (FCC decant oil) Particulate contamination reported as 35 and 325 mesh residues
CHAPTER 29: PROPERTIES
OF FUELS, PETROLEUM PITCH, PETROLEUM COKE, AND CARBON MATERIALS
measured by the standard methods hsted in Table 17. Figure 14 shows that the sieve residue may consist of coke particles produced by liquid-phase carbonization, and by pjrolytic deposition processes. Carbon Fibers from Petroleum Pitch Carbon fibers are produced from polyacrylonitrile (PAN), and pitch by melt spinning, stabilization, carbonization processes, and from hydrocarbons by vapor phase decomposition reactions catalyzed by metal catalysts [93]. Carbon fibers are used to fabricate C—C composites, or other composite materials such as ccirbon fiber reinforced plastics (CFRP) [94,95]. Petroleum pitch is also used to produce the carbonaceous matrix in the C—C composites. Unique properties of C—C composites, such as high specific strength and stiffness, high temperature strength, high corrosion resistance, good friction and wear properties, and low thermal expansion make them very desirable materials. Some important applications of C—C composites include manufacturing aircraft brakes, rocket nozzles, nose cones, and materials in the aerospace industry [95]. Currently, approximately 90% of all commercial carbon fibers are produced from PAN [94], but pitch-based fibers offer significant improvement in some fiber properties and, in some cases, reduced cost. Compared to PAN, petroleum pitch costs less and gives a higher carbon yield. Both PAN and petroleum pitch can be used to produce isotropic fibers. Inferior mechanical properties of isotropic pitch-based fibers limit their use to thermal insulation and some friction material applications. The principal advantage of petroleum pitch-based fibers is in the p r o d u c t i o n of anisotropic fibers from mesophase pitch. Mesophase pitch fibers offer m u c h more ordered structures, and, thus, much higher modulus, but lower strength properties compared to those of PAN fibers. Table 18 lists the tensile modulus, tensile strength, and density of selected PAN- and pitch-based fibers. Mesophase pitch is produced by thermal or catalytic processing of isotropic petroleum pitch. Thermal processing involves heating petroleum pitch from room temperature to 400-500°C. The solid pitch melts at temperatures between 100 and 200°C and its viscosity decreases. At temperatures greater t h a n 400°C, the viscosity of pitch melt starts to
TABLE 18—Properties of carbon fibers [94]. Fiber Pitch-based P-25 P-55 P-75 P-100 E-35 E-75 E-105 PAN-based T-300 T-2 AS-4 T-^0 HMS
Tensile Modulus, GPa
Tensile Strength, GPa
Density, g/cm^
159 379 724 724 241 517 724
1.38 1.72 2.24 2.24 2.83 3.10 3.31
1.90 2.00 2.15 2.15 2.10 2.16 2.17
231 172 231 290 345
3.24 2.24 3.64 3.25 2.21
1.79 1.80 1.78 1.83
781
increase upon pyrolysis/polymerization reactions with the attendant formation of mesophase (see Formation of Coke Microtexture in Coking Process). Strictly controlled heating is necessary to control the kinetics of mesophase formation to produce a mesophase pitch that is suitable for melt spinning process. A complete conversion of isotropic pitch to mesophase is necessary, but the resulting mesophase pitch must have a sufficiently low melting point and low viscosity to allow melt spinning. This becomes a challenge for treating pitches with a complex composition containing a wide distribution of molecular constituents with different reactivities towards mesophase formation. A pre-fractionation of such pitches may be necessary to obtain a more homogenous mixture of molecular species with comparable propensities for mesophase formation [96]. These pretreatment processes increase the cost of producing mesophase pitches suitable for carbon fiber production. Mesophase pitches produced from single compounds such as acenaphthylene, naphthalene, or methylnaphthalene by thermal, or catalytic procesess [97-99] are ideal, but expensive precursors to anisotropic carbon fibers. Continuous c a r b o n fibers are p r o d u c e d in a three-step process: spinning, stabilization (oxidation), and carbonization. To produce anisotropic carbon fibers using a melt-spin process, powdered mesophase is heated above its melting point and forced through a spinneret and wound onto a rotating reel. The m i c r o d o m a i n s of mesophase are aligned along the fiber axis as the melt passes through the spinneret. The fibers are drawn to approximately 10-/xm diameter from 100 /Am diameter at the exit of the spinneret, producing a fiber with a high degree of molecular orientation. The spun fibers need to be stabilized by oxidation to prevent melting and loss of structure during carbonization. For stabilization, fibers are oxidized at temperatures below their melting point, typically at 275-325°C, depending on the composition and dia m e t e r of the fibers [100]. The stabilized fibers are carbonized to increase their carbon content by heating to 1000-1600°C in an inert atmosphere. Carbonized fibers can be subjected to graphitization heat t r e a t m e n t to produce graphitic fibers. Mesophase carbon fibers have high modulus (>520 Gpa), high thermal and electric conductivity, and low thermal expansion coefficients. These properties cannot be achieved by PAN-based isotropic carbon fibers. In addition to being a precursor to mesophase carbon fibers, petroleum pitch is also used to produce a carbonaceous matrix in C-C composites with PAN fibers and as impregnating pitch for densification of C-C composites [100,101].
Carbon Anodes for Aluminum Production The principal function of carbon anodes in aluminum production is to provide reactant carbon for electrochemical reduction of AI2O3 to produce aluminum metal (see Reactivity). As introduced in Section 3, calcinable sponge cokes produced by delayed coking are used as fillers with coal-tar pitch binders to manufacture ceirbon anodes for aluminum production. There are two types of anodes used for industrial aluminum production: the prebaked anode and the Soderberg anode [25]. The principal difference between the manufacture of the two anodes is found in the carboniza-
782
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
tion/baking stage. The "green" Soderberg anode paste is carbonized/baked by the heat generated by the electrolysis cell, whereas the prebaked anode, as the n a m e suggests, is carbonized and baked in a separate furnace to produce the finished anode. Manufacture of prebaked anode involves three stages: • green paste production • paste compaction • anode baking For grain paste production, grains of anode grade calcined sponge coke (55-65%) is mixed with butts (material from used anodes) a n d coke fines (15-30%) and coal-tar pitch binder (14-17%). The mixing is carried out at a temperature 50-60°C higher than the melting point of the binder pitch to ensure a sufficiently low viscosity of the binder to flow into pores and voids of the aggregates. Particle size distribution of the aggregates plays an i m p o r t a n t role in controlling the properties and the performance of the anode. Paste compaction, i.e., forming of the green paste into anode blocks, is achieved either by hot pressing, or vibratory compacting. Vibratory compacting is a preferred process for large anode blocks heavier than 700 kg [27]. The final stage of anode production is carbonization/baking to convert the thermoplastic binder pitch into coke by heat treatment. In this stage both expansion and shrinkage take place, respectively, during carbonization of the pitch binder at temperatures between 200 and 600°C, and upon subsequent heating to temperatures of 100-1250°C over several weeks. Cores (usually 2-inch diameter) taken from commercial anodes are tested for some or all of the following properties: green apparent density, baked apparent density, conversion of pitch to coke, volume change during baking, electrical resistivity, carbon dioxide/air reactivity, consumption during electrolysis, thermal expansion, thermal conductivity, compressive and flexural strength. Young's modulus, gas permeability, porosity, microstructure, and cracking resistance. Among these properties, baked apparent density and electrical resistivity are the primary characterization parameters that correlate well with many of the other variables. ASTM Section D2.5E is developing standard methods for testing laboratory anodes and cores taken from commercial anodes. The following methods are in development: sampling, electrical resistivity, C02/air reactivity, thermal conductivity, cind thermal expansion. Graphite E l e c t r o d e s for Electric-Arc F u r n a c e s High-performance graphite electrodes are required for electric-arc steel production because of extremely high temperatures, u p to 4000°C, and large temperature gradients involved in the process [102]. The function of the graphite electrodes in electric-arc furnaces is to provide high electrical current density to produce an arc between two- or three-electrodes to generate sufficient heat to melt the furnace charge of scrap iron and steel. In contrast to the use of carbon anodes in the aluminum industry, carbon consumption is not necessary in the operation, but is unavoidable under the extreme conditions present in the furnace. As in carbon anode production, graphite electrodes for the steel industry are manufactured from a green mix of petroleum coke filler and coal-tar pitch binder. There are, however, substantial differences in preparation of the green mix and subsequent processing steps com-
pared to those employed in the carbon anode production, as described in Carbon Anodes for Aluminum Production. A highly graphitizable needle coke (see Petroleum Coke) is used as filler for the graphite electrodes. The amount of binder pitch used is increased from about 15 %wt for anodes to 20 %wt for graphite electrodes. Processing steps for manufacturing graphite electrodes include mixing of needle coke with the binder pitch, extrusion, baJsing, impregnation, rebaJiing, and graphitization. After preparation of the green mix with the carefully sized needle coke particles and the binder pitch, the electrode is formed by extrusion to the desired diameter and length (up to 70 cm in diameter and 270 cm in length). An extrusion aid, such as an aliphatic, or fatty acid, is added to the mix to provide lubrication on the adapter walls during extrusion. The critical effect in this step is the alignment of the needle coke particles parallel to the adapter wall, or the working direction of the electrode [54]. Following extrusion, the green electrode is baked by heating slowly to 800-1000°C and baked for several days depending on the electrode formulation. This step converts binder pitch into solid coke and helps maintain the shape of the electrode. As the temperature increases, the pitch softens, melts, devolatilizes and goes through carbonization to form pitch coke. Rapid baking causes problems, such as expansion, distortion, and formation of pits, which would lead to quality faults in the final product [71]. Pitch impregnation steps are employed to increase the density of the electrodes by filling the open pores in the baked electrode, created by devolatilization and carbonization of the binder pitch during the baking step. Usually, petroleum pitch (see Petroleum Pitch) is used for impregnation because it is essentially free of solids (Ql-quinoline insolubles) that can form a cake on the surface of the electrode, slowing down or stopping the impregnation process. After impregnation, the electrode is rebaked to convert the impregnating pitch into coke. Depending on the desired density of the electrodes, the impregnation and rebaking cycle may be repeated several times. In the final step, the rebaked electrode is graphitized by heating to approximately 3300 K to convert the filler and pitch cokes into synthetic graphite. For graphitization heat treatment, an electric current is passed directly through the electrode to generate the heat necessary to reach the graphitization temperature. The manufactured graphite electrodes must have high mechanical strength, high electrical conductivity, low chemical reactivity, and low thermal expansion. Methods and procedures used to measure the important properties of graphite electrodes were described by Heinz [103]. Most standard methods described for characterization of needle cokes can also be used for evaluation of commercial graphite electrodes or laboratory test beirs. It should be recognized that because of the high structural anisotropy of graphite electrodes, these properties show extremely large variations depending on the direction in which the measurements are made, i.e., withgrain, parallel to the working direction, or across-grain, perpendicular to the working direction. In graphite, for example, the thermal expansion normal to the basal planes is 30 times higher than that parallel to the planes; the electriccJ conductivity is 10 000 times higher along the basal planes than that across the planes.
CHAPTER 29: PROPERTIES OF FUELS, PETROLEUM PITCH, PETROLEUM COKE, AND CARBON MATERIALS 783 Conclusions A glimpse of the fascinating diversity and versatility in structure and properties of different materials made up, essentially, of a single element, carbon, should be apparent in the four examples discussed in this section: carbon black, carbon fibers, carbon anodes, and graphite electrodes. Carbon blacks, particulate aggregates of carbon produced by vapor phase decomposition of aromatic oils, provides elastomer reinforcement and color pigments used in many industrial operations. Pitch-based carbon fibers offer very high modulus and impressive strengths that find applications in the manufacture of Carbon-Carbon composites. Carbon Carbon anodes are used for electrochemical reduction of cJuminum oxide to produce metallic aluminum. Graphite electrodes, with their excellent thermal properties along with high electrical conductivity, high strength, and low chemical reactivity, find applications in electric-arc furnaces for recycling scrap iron and steel. These materials are manufactured from petroleum- and coal-based feedstocks in well-established processes under carefully controlled conditions. Although the detailed mechanisms of how the feedstocks are converted into the final products are not well known, critical properties of these carbon materials are effectively controlled by the right selection of feedstocks and the operating conditions. Many established standard methods are used for the characterization of feedstocks and the finaJ products, and several ASTM standard methods are currently under development.
D 70 D 71 D 86 D 92 D 93 D 95 D 97 D 129 D 130 D 167 D 189 D 240 D 287 D 346
CONCLUSION Covering a vast area of petroleum-derived products ranging from fuel oils, highly aromatic feedstocks from catalytic and thermal cracking, petroleum pitch, and petroleum coke to industrial carbons and graphites, within the bounds of one chapter presents a great intellectual challenge. The coverage in this chapter has been necessarily limited to highlighting the interconnectivity between the end products in this wide array of materials in reference to production processes, property characterization using standard methods, and the significance of measured properties in relation to specific industrial applications. It is important to recognize that despite the great diversity in the nature and applications of the materials covered in this chapter, there are common threads that constitute the complex web of converting petroleum feedstocks to fuels and materials. A careful use of standard methods for classification of materieJs and quantification of important properties has contributed to the technology of converting petroleum feedstocks into desired fuels, hydrocarbons, and carbon materials, as discussed in this chapter and others.
D 396 D 445 D 446 D 473 D 482 D 524 D 1266 D 1298
D 1510 D1552 D 1756
ASTM STANDARDS
D 1765
No. D 36
D 1796
D 61
Title Standard Test Method for Softening Point of Bitumen (Ring-and-Ball Apparatus) Standard Test Method for Softening Point of Pitches (Cube-in-Water Method)
D2318
Standard Test Method for Specific Gravity and Density of Semi-Solid Bituminous Materials (Pycnometer Method) Standard Test Method for Relative Density of Solid Pitch and Asphalt (Displacement Method) Standard Test Method for Distillation of Petroleum Products at Atmospheric Pressure Standard Test Method for Flash and Fire Points by Cleveland Open Cup Standard Test Methods for Flash-Point by Pensky-Martens Closed Cup Tester Standard Test Method for Water in Petroleum Products and Bituminous Materials by Distillation Standard Test Method for Penetration of Bituminous Materials Standard Test Method for Sulfur in Petroleum Products (General Bomb Method) Standard Test Method for Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test Standard Test Method for Apparent and True Specific Gravity and Porosity of Lump Coke Standard Test Method for Conradson Carbon Residue of Petroleum Products Standard Test Method for Heat of Combustion of Liquid Hydrocarbon Fuels by Bomb Calorimeter Standard Test Method for API Gravity of Crude Petroleum and Petroleum Products (Hydrometer Method) Standard Practice for Collection and Preparation of Coke Samples for Laboratory Analysis Standard Specification for Fuel Oils Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (the Calculation of Dynamic Viscosity) Standard Specifications and Operating Instructions for Glass Capillary Kinematic Viscometers Standard Test Method for Sediment in Crude Oils and Fuel Oils by the Extraction Method Standard Test Method for Ash from Petroleum Products Standard Test Method for Ramsbottom Carbon Residue of Petroleum Products Standard Test Method for Sulfur in Petroleum Products (Lamp Method) Standard Practice for Density, Relative Density (Specific Gravity), or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method Standard Test Method for Carbon Black—Iodine Adsorption Number Standard Test Method for Sulfur in Petroleum Products (High-Temperature Method) Standard Test Method for Rubber ChemicalsSolubility Standard Classification System for Carbon Blacks Used in Rubber Products Standard Test Method for Water and Sediment in Fuel Oils by the Centrifuge Method (Laboratory Procedure) Standard Test Method for Quinoline-Insoluble (QI) Content of Tar and Pitch
784
MANUAL
D2319 D 2320 D 2414 D 2415 D 2416 D 2492 D 2569 D 2622
D 2638 D 2764 D 2962 D 3104 D 3177 D 3245 D 3279 D 3338 D 3461 D 3765 D 3849
D 3997 D 4045
D 4057 D 4072 D 4239
D 4292 D 4294
D 4296 D 4312 D4421 D 4422
37: FUELS AND LUBRICANTS
HANDBOOK
S t a n d a r d Test Method for Softening Point of Pitch (Cube-in-Air Method) Standard Test Method for Density (Relative Density) of Solid Pitch (Pycnometer Method) Standard Test Method for Carbon Black—Oil Absorption Number Standard Test Method for Ash in Coal Tar and Pitch Standard Test Method for Coking Value of Tar and Pitch (Modified Conradson) S t a n d a r d Test Method for F o r m s of Sulfur in Coal Standard Test Method for Distillation of Pitch Standard Test Method for Sulfur in Petroleum Products by Wavelength Dispersive X-ray Fluorescence Spectrometry Standard Test Method for Real Density of Calcined Petroleum Coke by Helium Pycnometer Standard Test Method for DimethylformamideInsoluble (DMF-I) Content of Tar and Pitch Standard Test Method for Calculating VolumeTemperature Correction For Coal-Tar Pitches S t a n d a r d Test Method for Softening Point of Pitches (Mettler Softening Point Method) Standard Test Methods for Total Sulfur in the Analysis Sample of Coal and Coke Standard Test Method for Pumpability of Industrial Fuel Oils Standard Test Method for n-Heptane Insolubles Standard Test Method for Estimation of Net Heat of Combustion of Aviation Fuels Standard Test Method for Softening Point of Asphalt and Pitch (Mettler Cup-and-Ball Method) Standard Test Method for Carbon Black—CTAB (Cetyltrimethylammonium Bromide) Surface Area S t a n d a r d Test Method for Carbon Black—Primary Aggregate Dimensions from Electron Microscope Image Analysis Standard Practice for Preparing Coke Samples for Microscopical Analysis by Reflected Light Standard Test Method for Sulfur in Petroleum Products by Hydrogenolysis and Rateometric Colorimetry S t a n d a r d Practice for Manual Sampling of Petroleum and Petroleum Products Standard Test Method for Toluene-Insoluble (TI) Content of Tar and Pitch Standard Test Methods for Sulfur in the Analysis Sample of Coal and Coke Using High Temperature Tube Furnace Combustion Methods Standard Test Method for Determination of Vibrated Bulk Density of Calcined Petroleum Coke Standard Test Method for Sulfur in Petroleum Products by Energy-Dispersive X-Ray Fluorescence Spectroscopy Standard Practice for Sampling Pitch Standard Test Method for Toluene-Insoluble (TI) Content of Tar and Pitch (Short Method) S t a n d a r d Test Method for Volatile Matter in Petroleum Coke S t a n d a r d Test Method for Ash In Analysis of Petroleum Coke
D 4529 D 4530 D 4616
D 4715 D 4740 D 4746
D 4809
D 4892 D 5003 D 5004 D 5061
D 5056 D 5018 D 5600
D 5341
D 5187
D 6374 D 6376
D 6556
Standard Test Method for Estimation of Net Heat of Combustion of Aviation Fuels Standard Test Method for Determination of Carbon Residue (Micro Method) S t a n d a r d Test Method for Microscopical Analysis by Reflected Light a n d Determination of Mesophase in a Pitch Standard Test Method for Coking Value of Tar and Pitch (Alcan) Standard Test Method for Cleanliness and Compatibility of Residual Fuels by Spot Test S t a n d a r d Test Method for Determination of Quinoline Insolubles (QI) in Tar and Pitch by Pressure Filtration Standard Test Method for Heat of Combustion of Liquid Hydrocarbon Fuels by Bomb Calorimeter (Precision Method) Standard Test Method for Density of Solid Pitch (Helium Pycnometer Method) Standard Test Method for The Hardgrove Grindability Index (HGI) of Petroleum Coke Standard Test Method for Real Density of Calcined Petroleum Coke by Xylene Displacement Standard Test Method for Microscopical Determination of Volume Percent of Textural Components in Metallurgical Coke S t a n d a r d Test Method for Trace Metals in Petroleum Coke by Atomic Absorption S t a n d a r d Test Method for Shear Viscosity of Coal-Tar and Petroleum Pitches S t a n d a r d Test Method for Trace Metals in Petroleum Coke by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) Standard Test Method for Measuring Coke Reactivity Index (CRI) and Coke Strength After Reaction (CSR) Standard Test Method for Determination of Crystallite Size (Lc) of Calcined Petroleum Coke by XRay Diffraction S t a n d a r d Test Method for Volatile Matter in Green Petroleum Coke Quartz Crucible Procedure S t a n d a r d Test Method for Determination of Trace Metals in Petroleum Coke by Wavelength Dispersive X-Ray Fluorescence Spectroscopy Standard Test Method for Carbon Black—Total and External Surface Area by Nitrogen Adsorption
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OF FUELS,
PETROLEUM
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PITCH, PETROLEUM
COKE, AND CARBON
MATERIALS
785
[29] Beetz, Jr., C. P., Schmueser, D. W., and Hansen, W., "Summary of Panel Discussion, Challenges to the Researchers of Carbon-Fibers and Composites from the Automotive and Boatbuilding Industries," Carbon, Vol. 27, 1989, pp. 767-771. [30] Andresen, J. M., Garcia, R., Maroto-Valer, M. M., Moinelo, S. R., and Snape, C. E., "Characterization of Mesophase Develo p m e n t in Pitch by High T e m p e r a t u r e In Situ H NMR," Preprints., American Chemical Society, Division of Petroleum Chemistry, Vol. 4 1 , No. 3, 1996, pp. 621-624. [31] Fitzer, E., "The Future of Carbon-Carbon Composites," Carbon, Vol. 25, 1987, pp. 163-190. [32] Burchell, T. D., "Carbon Material for Advanced Energy Applications," International Conference on Carbon, Newcastle, UK, 1996, p p . 185-188. [33] Ehrburger, P., "Glassy Properties of Coal Tar Pitch Materials," Energeia, Vol. 5, No. 3, 1994, pp. 1-3. [34] Marsh, H., Introduction to Carbon Science, Butterworth, London, 1989. [35] Lahaye, L., Ehrburger, P., Saint-Romain, J. L., and Couderc, P., "Physicochemical Characterization of Pitches by Differential Scanning Calorimetry," Fuel, Vol. 66,1987, pp. 1467-1471. [36] Rand, B., Ch. 8, Handbook of Composites, Vol. 1, W. Watt and B. V. Perov, Eds., Elsevier, NY, 1989. [37] Dealy, J. M., Rheometers for Molten Plastics, Van Nostrand Reinhold Company, NY, 1982. [38] Cheung, T., Turpin, M., and Rand, B., "Controlled Stress, Oscillatory Rheometry of Mesophase-Pitches," Carbon, Vol. 33, 1985, pp. 1673-1679. [39] Bhatia, G., Aggarwal, R. K., Chari, S. S., and Jain, G. C , "Rheological Characteristics of coal Tar and Petroleum Pitches With a n d Without Additives," Carbon, Vol. 15, 1977, pp. 219-223. [40] Bhatia, G., Fitzer, E., and Kompalik, D., "Mesophase Formation in Defined Mixtures of Coal Tar Pitch Fractions," Carbon, Vol. 24, 1986, pp. 4 8 9 ^ 9 4 . [41] Nazem, F. F., "Flow of Molten Mesophase Pitch," Carbon, Vol. 20, 1982, p p . 345-354. [42] Turpin, M., Cheung, T., and Rand, B., "Controlled Stress, Oscillatory Rheometry of a Petroleum Pitch," Carbon, Vol. 32, 1994, pp. 225-230. [43] ICremer, H. A., "Recent Development in Electrode Pitch and Coal Tar Technology," Chemistry and Industry, Vol. 27, 1982, pp. 702-713. [44] Kuo, K., Marsh, H., Eind Broughton, D., "Influence of Primary QI and Particulate Matter on Pitch Carbonizations," Fuel, Vol. 66, 1987, pp. 1544-1551. [45] Marsh, H., Latham, C. S., and Gray, E. M., "The Structure and Behaviour of QI Material in Pitch," Carbon, Vol. 23, 1985, pp. 555-570. [46] Taylor, G. H., Pennock, G. M., Fitz G. J. D., and Brunckhorst, L. F., "Influence of QI on Mesophase Structure," Carbon, Vol. 31, 1993, p p . 341-354. [47] Romovacek, G. R., "Estimating the Concentration of Secondary Quinoline Insolubles," Carbon, Vol. 24, 1986, pp. 4 1 7 ^ 2 1 . [48] Menendez, R., Granda, M, and Bermejo, J., Ch. 9, Introduction to Carbon Technologies, H. Marsh, E. A. Heintz, and F. Rodriguez-Reinoso, Eds., Universidad de Alicante, Secretariado de Publicaciones, Spain, 1997. [49] Grjotheim, K., Krohn, C , Malinovsky, M., Matiasovsky, K., and Thonstad, J., Aluminium Electrolysis: Fundamentals of the Hall-Heroult Process, Aluminium-Verlag, Diisseldorf, 1982. [50] Gary, J. H. and Handwerk, G. E., Petroleum Refining: Technology and Economics, Marcel Dekker, NY, 1994, pp. 71-99. [51] Ellis, P. T. £ind Hardin, E. E., "How Petroleum Delayed Coke Performs in a Drum," Light Metals, 1993, pp. 509-513. [52] Adams, H. A., Ch. 10, Introduction to Carbon Technologies, H. Marsh, E. A. Heintz, and F. Rodriguez-Reinoso, Eds., Universidad de Alicante, Secretariado de Publicaciones, Spain, 1997.
786 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK [53] Eser, S., Jenkins, R. G., Malladi, M., and Derbyshire, F. J., "Carbonization of Coker Feedstocks and Their Fractions," Carbon, Vol. 24, 1986, pp. 77-82. [54] Mochida, I., Fujimoto, K., a n d Oyama, T., Chemistry and Physics of Carbon, Vol. 24, P. A. Thrower, Ed., Marcel Dekker, NY, 1994, pp. 111-212. [56] White, J. L., "Petroleum Derived Carbons," ACS Symposium Series No. 21, M. L. Deviney, and T. M. O'Grady, Eds., American Chemical Society, Washington, DC, 1976, p. 282. [57] Oya, A., Qian, Q. Z., and Marsh, H., Fuel, Vol. 62, 1983, p. 274. [58] Eser, S., Supercarbon: Synthesis, Properties, and Applications, Vol. 147, S. Yoshimura and R. P. H. Chang, Eds., SpringerVerlag, BerUn, 1998. [59] Eser, S. and Jenkins, R. G, "Carbonization of Petroleum Feedstocks. 1. Relationships Between Chemical Constitution of the Feedstocks and Mesophase Development," Carbon 27, 1989, pp. 877-887. [60] Eser, S. and Jenkins, R. G, "Carbonization of Petroleum Feedstocks. 2. Chemical Constitution of Feedstocks Asphaltanes and Mesophase Development," Carbon 27, 1989, pp. 889-897. [61] Fitzer, E., Kochling, K.-H., and Marsh, H., "Recommended Terminology for the Description of Carbon as a Solid-(IUPAC Recommendations 1995), Pure and Applied Chemistry, Vol. 67, 1995, pp. 473-506. [62] Rumsey, J. C. V. and Pitt, G. J., "Some Techniques for the Characterization of Cokes and Graphites," Fuel, Vol. 57,1978, p. 155. [63] Patrick, J. W., Reynolds, M. J., and Shaw, F. H., "Development of Optical Anisotropy in Vitrains during Carbonization," Fuel, Vol. 52, 1973, p. 198. [64] Sanada, Y., Furuta, T., Kimura, H., and Honda, H., "Formation of Anisotropic Mesophase from Various Carbonaceous Materials in Early Stage of Carbonization," Fuel, Vol. 52, 1973, p. 143. [65] Ragan, S. and Marsh, H., "The Influence of Oxidation upon Strength and Structure of a Needle-Coke and a Coal Extract Coke," Carbon, 1983, Vol. 2 1 , No. 2, p. 157. [66] Mochida, I., Korai, Y., Nesumi, Y., and Oyama, T., 'Carbonization in a Tube Bomb. 1. Carbonization of Petroleum Residue into a Lump of Needle Coke," Industrial & Engineering Chemistry Product Development,Yo\. 25, No. 2, 1986, p. 201. [67] Martin, S. W., Petroleum Products Handbook, V. B. Guthrie, Ed., McGraw-Hill, NY, 1960, pp. 14. [68] Pysz, R. W., Hoff, S. L., and Heitz, E. A., "Terminology for the Structural Evaluation of Coke via Scanning Electron-Microscopy," Carbon, Vol. 27, 1989, p. 935. [69] Qiao, G., "Digital Image Analysis of Needle Cokes and Other Solid Carbons and Their Thermal Expansion Behavior," Ph. D. Thesis, The Pennsylvania State University, May 2000. [70] Gray, R. J. and Devanney, K. F., "Coke Carbon Forms—Microscopic Classification and Industrial Applications," International loumal of Coal Geology, Vol. 6, 1986, pp. 277-297. [71] Mantell, C. L., Carbon and Graphite Handbook, John Wiley & Sons, NY, 1968, p . 520. [72] Page, D. J., The Industrial Graphite Engineering Handbook, UCAR Carbon Company Inc., 1991. [73] Reynolds, W. N., Physical Properties of Graphite, Elsevier, London, 1968, p . 80. [74] Kingeiy, W. D., Bowen, H. K., and Uhlmann, D. R., Introduction to Ceramics, John Wiley & Sons, NY, 1976, p. 591. [75] Hutcheson, J. M. and Price, M. S. T., Proceedings of the 4 * Carbon Conference, 1960, p. 645. [76] Sutton, A. L. and Howard, V. C , Journal of Nuclear Materials, Vol. 7, 1962, p. 58. [77] Price, R., Bokros, J. C , and Koyama, K., "Thermal Expansivities and Preferred Orientation of Pyrolytic Carbon," Carbon, Vol.5, 1967, p . 423. [78] Zimmer, J. E. and White J. L., 1 3 * Biennial Conference on Carbon, Extended Abstracts, 1977, p. 318.
[79] Mochida, I., Ogawa, M., and Takeshita, K., Bulletin of the Chemical Society of Japan, Vol. 49, No. 2, 1976, p. 514. [80] Collins, F. M., "Dimensional Changes During Heat-Treatment a n d T h e r m a l Expansion of Polycrystalline Carbons a n d Graphite," Proceedings, 3''''Conference on Carbon, 1957, p. 659. [81] Matsuo, H. and Sasaki, Y., "Relation between Anisotropy Ratio of Thermal Expansion Coefficient and Bacon Anisotropy Factor," Carbon, Vol. 12, 1974, p . 351. [82] Widmann, G. and Riesen, R., Thermal Analysis, Terms, Methods, Application, Heidelberg, Germany, 1987, p. 17. [83] Hole, M., Foosnaes, T., and 0 y e , H. A., "Relationship between Thermal Expansion and Optical Texture of Petrol Coke," Light Metals, 1991, p. 575. [84] Eilertsen, J. L., Rrvik, S., Foosnaes, T., and 0ye, H. A., "An Automatic Image Analysis of Coke Texture," Carbon, Vol. 34, 1996, p. 375. [85] Heintz, E. A., "Influence of Coke Structure on the Properties of the Carbon-Graphite Artefact," Fuel, Vol. 64, 1985, p. 1192. [86] Blyholder, G. and Eyring, H., Journal of Physical Chemistry, Vol. 61, 1957, p. 682. [87] Marsh, H., Taylor, D. A., and Lander, J. R., "Kinetic Study of Gasification by Oxygen a n d Carbon Dioxide of Pure and Doped Graphitizable Carbons of Increasing Heat Treatment Temperatures," Carbon, Vol. 19, 1981, p. 375. [88] Marsh, H. and Kuo, K, Introduction to Carbon Science, H. Marsh, Ed., Butterworths, London, 1989, p. 108. [89] Gregg, S. J. and Tyson, R. F. S., "The Kinetics of Oxidation of Carbon and Graphite by Oxygen at 500°-600°," Carbon, Vol. 3, 1965, p. 39. [90] Taylor, R., Ch. 4, Introduction to Carbon Technologies, H. Marsh, E. A. Heintz, and F. Rodriguez-Reinoso, Eds., Universidad de Alicante, Secretariado de Publicaciones, Spain, 1997. [91] Donnet, J. B., Bansal, R. C , and Wang, M.-J., Carbon Black Science and Technology, Second Edition, Marcel Dekker, NY, 1993. [92] Schwob, Y., "Acetylene Black: Manufacture, Properties, and Applications," Chemistry and Physics of Carbon, P. L. Walker and P. A. Thrower, Eds., 1982, p. 109. [93] Donnet, J. B. and Bansal, R. C , Carbon Fibers, Marcel Dekker, NY, 1984. [94] Murdie, N., Ch. 14, Introduction to Carbon Technologies, H. Marsh, E. A. Heintz, and F. Rodriguez-Reinoso, Eds., Universidad de Alicante, Secretariado de Publicaciones, Spain, 1997. [95] Johnson, D. J., Introduction to Carbon Science, H. Marsh, Ed., Butterworths, London, 1989, p. 197. [96] Edie, D. D. and Diefendorf, R. J., Ch. 2, Carbon-Carbon Materials and Composites, NASA 1254, Park Ridge, NJ, 1992. [97] Singer, L. S., "The Mesophase a n d High Modulus Carbon Fibers from Pitch," Carbon, Vol. 16, 1978, pp. 409-415. [98] Yoon, S.-H., Korai, Y., Mochida, I, and Kato, I., "The Flow Properties of Mesophase Pitches Derived from Methylnapthalene and Napthalene in the Temperature-Range of Their Spinning," Carbon, Vol. 32, 1994, pp. 273-280. [99] Mochida, I., Yoon, S.-H., Korai, Y., Kanno, K., Sakai, Y., and Komatsu, M., 'Carbon-Fibers from Aromatic-Hydrocarbons," Chemtech, Vol. 25, No. 2, 1995, p p . 29-36. [100] Cranmer, J. H., Plotzken, I. G., Peebles, L. H., and Uhlmann, D. R., "Carbon Mesophase-Substrate Interactions," Carbon, Vol. 21, 1983, pp. 201-207. [101] Meyer, R. A. and Gyetvay, S. R., Petroleum Derived Carbons, ACS Symposium Series No. 303, J. D. Bacha, J. W. Newman, and J. L. White, Eds., American Chemical Society, Washington, D.C., 1986, pp. 380-394. [102] Vohler, O., von Sturm, F., and Wege, E., Vlhnann's Encyclopedia of Industrial Chemistry, A5, VCH, Heidelberg, 1986, p . 98. [103] Heinz, E. A., "The Characterization of Petroleum Coke," Carbon, Vol. 34, 1996, p. 699.
MNL37-EB/Jun. 2003
Oxidation of Lubricants and Fuels Gerald J. Cochrac^ and Syed Q. A. Rizvi^
OXIDATION OF LUBRICANTS AND OXIDATION INHIBITORS
viscosity index for Group III Oils is >120. In general. Group II, Group III, and Group IV Oils are low in aromatic structures and structures with unsaturation. Hence, they oxidize at a slower rate than Group I Oils that are high in such structures. This is because such structures more readily form hydroperoxides and peroxy radicals that are essential to the oxidation process. Synthetic basestocks (Group V Oils) have oxidation rates that vary because of varying structures. Alkylaromatics, for example, contain aromatic rings, and hence oxidize faster than ester basestocks, which in turn oxidize faster than olefin oligomers (PAOs) that belong to Group IV. Vegetable oils oxidize at a fast rate as well because of the presence of unsaturation. The oxidatively susceptible structures in various basestocks are shown in Fig. 1. The hydrogens in the immediate vicinity of a r o m a t i c rings, double bonds, and oxygen atoms are most vulnerable to oxidative attack. Such hydrogens are shown in Fig. 2 and discussed in detail below.
MODERN LUBRICANTS PRIMARILY COMPRISE PERFORMANCE ADDI-
TIVES such as dispersants, detergents, oxidation inhibitors, viscosity modifiers, a n d others, blended in a base fluid (see chapter on Additives and Additive Chemistry). Many properties of the lubricant such as viscosity, viscosity index, slipperiness (reduced friction), film-strength, p o u r point, oxidation stability, volatility, and flammability, therefore depend on the properties of both the base fluid and the additive package. However, since the base fluid in a lubricant is present in excess of 70%, its contribution towards the listed properties predominates that of the additive package. Oils used to lubricate today's equipment, by virtue of being hydrocarbon based, are susceptible to oxidation. Oxidation results in the formation of polar compounds such as aldehydes, ketones, carboxylic acids, and oxygenated polymers. These products of oxidation are either corrosive or lead to the formation of resin, deposits, and sludge, all of which can impair the proper functioning of the equipment. The resin derived from lubricant oxidation consists of oxygenated oligomeric or polymeric molecules of approximately 500-1000 molecular weight. Deposits, on the other hand, comprise materials of a m u c h higher molecular weight. Sludge is a mixture of these materials with water, oil, and other contaminants. Oxidation resistance of a base oil leirgely depends upon the structure of the hydrocarbons present. Base oils are of mineral (petroleum) origin, synthetic chemical origin, or biological origin. "While mineral oil basestocks are obtained directly from petroleum fractionation, synthetic basestocks are manufactured through transformations of primarily petroleum derived organic chemicals. Base stocks of biological origin include vegetable oils and animal fats. API (American Petroleum Institute) has established five base oil categories on the basis of percent sulfur and percent saturates. Group I contains oils that have >0.03% sulfur and < 9 0 % saturates by mass. These oils have a viscosity index range of 80-120. Group II and III Oils, on the other hand, have <0.03% sulfur and s 9 0 % saturates by mass. However, they differ from each other in their viscosity index. The viscosity index for Group II Oils ranges from 80-120 and the
Each type of basestock, mineral, biological, or synthetic, has a stable threshold beyond which oxidation inhibitors are needed to retard oxidation. Refined base oils once contained sulfur and nitrogen-based "natural" inhibitors, which under mild conditions were sufficient to protect lubricants against oxidation. However, new refining technologies used to enhance other desirable properties of base oils either partly or fully remove these beneficial compounds. This is particularly true for base oils belonging to API Group II and higher Groups, where the limit on sulfur is 0.03 percent by mass or less. Most lubricants therefore require oxidation inhibitors to protect them under increasingly severe high temperature service environments.
' Product Manager, The Lubrizol Corporation, 29400 Lakeland Blvd., Wickliffe, OH 44092. ^ Research & Development Manager, Lubricant Additives Division, King Industries, Inc., Science Road, Norwalk, CT 06852.
Most lubrication applications expose lubricants to oxygen in some manner. Oxygen readily reacts with labile hydrogens of the hydrocarbon structures that make u p the lubricant. Such sites, in order of decreasing ease of reaction, are benzylic, allylic, tertiary alkyl, secondary alkyl, and primary alkyl hydrogens. Benzylic hydrogens are hydrogens on carbons next to an aromatic ring. Allylic hydrogens are on carbons next to a double bond. Tertiary hydrogens are on carbons attached to three other carbon atoms. Secondary hydrogens are on carbons attached to two other carbon atoms and primary hydrogens are on carbons attached to only one other carbon atom. Figure 2 identifies these hydrogens in model structures. The oxidation of these hydrogens results in the formation of peroxy and other radicals. The mechanism of oxidation is illustrated in Fig. 3 [1]. The process of oxidation proceeds in three stages: initiation, propagation, and termination. During the initiation
787 Copyright'
2003 by A S I M International
www.astm.org
788 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK UNSATURATES
Alkenes AROMATICS
VEGETABLE OIL
)c-0—CHj
R
.R
I
R=
HC—O—C
R.
I
O
X-0—CH,
ESTERS CH20C^
O O II II RO — C ~ ( C H 2 ) n — C — O R
R
R=C5TOCio
CH3CH2C—CH2OC ^ O CH2OC //
R=C8 TO Ci3
Alkyl Carboxylate Ester
Polyol Ester
FIG. 1—Base oil structures with high susceptibility to oxidation.
Allylic Benzylic-
AJIylic
I I H
H
C'
I
H
Aflylic
I l"x„/H
H
H
/\ H
H
- Allylic
Y"-
• Allylic
I H
H
BenzylicAriylic
Secondary
I
I
Tertiary
U
LJ
'
/\ H H
tt
'*"
\ / HjC—CH,
Primary
H, HjC
CH,
H2 "2 H H2 -•2 HI, H2 . . 2 HI, Hj H CH3H2 A_ HsC" I CH3 H
t t Tertiary
1
I
'
Tertiary irtiarv
W
tt
Secondary I I
Tertiary
FIG. 2—Classification of hydrogens in organic molecules.
CHAPTER 30: OXIDATION OF LUBRICANTS AND FUELS
789
INITIATION activation RH + O2 + RH
*-
R- + H'
(1)
R- + H2O + R'
(2)
PROPAGATION (3)
R' + O, -^
RO,' + RH
ROOH + R'
(4)
RO- + OHRO- + RO2- + H2O
(5) (6)
Various Products
(7)
Nonradical Products
(8)
Decomposition of Peroxide: ROOH 2 ROOH RO- + ROOH ROz' ROOH
Induced Decomposition of Peroxide: X Y Z M
+ + + +
ROOH ROOH ROOH ROOH
Free Radicals
(9)
ROH + YO
(10)
Inactive Prodcts + Z
(11)
Free Radicals
(12)
TERMINATION Self Termination: ROa' + RO2-
Inactive Products
(13)
Chain-Breaking Termination: ROz + IH
ROOH + r
(14)
RO- + IH
ROH + |-
(15)
RH + |-
(16)
R-
+ IH
RH = Organic substrate
ROa' = Peroxy radical
ROOH = Hydroperoxide
X, Y, Z = Decomposers of Radicals RO' = Alkoxy radical R- = Alkyl radical M = IVIetal IH = Chain-breaking inhibitor or inhibitor of free radicals 1" = Inhibitor radical
FIG. 3—Mechanism of oxidation.
stage, oxygen reacts with the fuel and the lubricant to form alkyl radicals (Eqs 1 and 2; Fig. 3). During the propagation stage, these radicals react with oxygen and the lubricant to form peroxy radicals and hydroperoxides (Eqs 3 and 4). As indicated by the oxygen uptake in Fig. 4 [2], hydroperoxides are accumulated during the induction period, after which the autoacceleration of oxidation occurs. Hydroperoxides, either thermally or in the presence of metal, decompose to a variety of additional radicals and oxygen-containing compounds (Eqs 5-12, Fig. 3). The oxygen-containing compounds include alcohols, aldehydes, ketones, and carboxylic acids. The detailed mechanism of thermal decomposition of
hydroperoxides to form these species is shown in Fig. 5, Eqs 18 and 19 [3]. Aldehydes and ketones can form polymers (Eq 20 in Fig. 5 and Fig. 6) in the presence of acids, such as nitric and sulfuric acids. These acids result from the interaction of nitrogen oxides and sulfur oxides, products of combustion, with water (Fig. 7). Carboxylic acids can attack iron metal, and copper and lead bearings to form metal carboxylates (Eq 21 in Fig. 5 and Fig. 8), which can further increase the rate of oxidation. Figure 9 [2] depicts the catalj^ic effect of iron carboxylate on hydroperoxide formation. Please note that certain metal salts, when present in a low concentration, can act as oxidation inhibitors. Oxida-
790
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
tion inhibition by copper salts is described in the latter part of the chapter. An increase in temperature affects the oxidation process profoundly, with the rate of oxidation approximately doubling with every ten-degree centigrade rise in temperature. Wear metals can also enhance the rate of oxidation [4]. If oxidation is not controlled, lubricant decomposition will lead to oil thickening, sludge formation, and the formation of varnish, resin, deposits, and corrosive acids (Fig. 10) [5,6].
@
J ^
"1 OH
"^
=^
OH
HO
/© HO
R2
.^
Ri
O x y g e n a t e d Polymer HO
R2
FIG. 6—Oxygenated polymer formation.
-*-
N2 + O2 2NO2 Induction Period
+
F I G . 4 — E f f e c t of h y d r o p e r o x i d e c o n c e n t r a t i o n o n t h e rate of
4 - ^
HN03 +
H,0
s + o.
Time •
2 NO
-»-
SO2
-1°^
2N02
(22)
HN02
(23)
S03
(24) (25)
12S04
H,0
oxidation. F I G . 7 — F o r m a t i o n of a c i d s .
R—0-0
RO' + HjO + ROO'
H-O-O-R
(17)
H
6 R C O O H + 2 Fe
*•
2 (RCOO)3Fe + 3 H2
(26)
4 R C O O H + 2 Cu
*-
2 ( R C 0 0 ) 2 C u + 2 H2
(27)
2 ( R C 0 0 ) 2 P b + 2 H2
(28)
R,. CHOO-
/^ 4 R C O O H + 2 Pb
F I G . 8 — F o r m a t i o n of metal c a r b o x y l a t e s .
C=0
,CHOH +
+
O2
(18)
Carbonyl Compound
RiCH2\
c=o
Carbonyl Compound
_
-fe
r
3 u
Hydrocarbon Containing 500 ppm Iron Octanoate
2 6 X
o u u s Q. O
X O 3-
Rl-Cs
(19) OH
H
Aldehyde
£
Carboxylic Acid
Polymeric Materials
(20)
/
/
/
/ — Pure Hydro£arfeop
y
<1
y
/L
RCHO
/
c 3 2
/
'
°
_t> cr-
^
1
4
1
6
1
10
Time (days) RCOOH + Metal
-»-
Metal Salts
F I G . 5 — M e c h a n i s m of h y d r o p e r o x i d e d e c o m p o s i t i o n .
(21)
F I G . 9 — C a t a l y t i c e f f e c t of Iron c a r b o x y l a t e o n t h e rate of oxidation.
CHAPTER Oil thickening occurs mainly due to polymerization or association of certain oxidation products. A model showing oxidative and thermal degradation of lubricants is shown in Fig. 11 [2]. During the t e r m i n a t i o n stage, t h e radicals either selft e r m i n a t e or t e r m i n a t e by reacting with oxidation inhibitors (Eqs 13-16, Fig. 3). The m e c h a n i s m involving self-termination leading to the formation of harmful carbonyl compounds is shown in Eq 18 of Figs 5. Oxidation inhibitors circumvent the radical chain mechanism of the oxidation process (Eqs 9-11 and 14-16, Fig. 3). Oxidation inhibitors can be Sulfur
F u e L ^ H2SO4
(29) (30)
R,C=0
-*-
Resins
Resins + Soot + Oil + H2O Resin + Soot
(31) —*"
Sludge
Resin Coated Soot
(32) »- Deposits
FIG. 10—Formation of harmful products.
(33)
30: OXIDATION
OF LUBRICANTS
AND FUELS
791
classified as hydroperoxide decomposers and radical scavengers, depending upon the mode of their controlling action. Sulfur and phosphorus-containing inhibitors, such as sulfides, dithiocarbamates, phosphites, and dithiophosphates, structures shown in Fig. 12, act as hydroperoxide decomposers. Nitrogen and oxygen-containing inhibitors, such as arylamines and phenols, structures shown in Fig. 13, act as radical scavengers. Hydroperoxide decomposers convert chain-propagating hydroperoxides to alcohols while they themselves become oxidized to higher oxidation levels. Sulfur compounds, represented by alkylsulfides, react with hydroperoxides and are converted into sulfoxides or sulfones. Sulfoxides can decompose thermally to form other sulfurcontaining products, such as sulfonic and sulfuric acids, which themselves are hydroperoxide decomposers. Nonhindered phenols also act as hydroperoxide decomposers and, in the process, are converted to polyhydroxy compounds [1]. See Parts 1 and 2 of Fig. 14 for the mechanism. Phosphorus compounds also act as hydroperoxide decomposers. Phosphines, rarely used as inhibitors because of their toxicity, are not as effective as other phosphorus derivatives. This is because they react stoichiometrically with hydroperoxides to form phosphine oxides that lack further oxidationinhibiting ability. Alkyl phosphites are better because during
FIG. 11—High-temperature lubricant degradation model.
792 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
Sulfides
R-S-(s)-S-R
R-S-R Monosulfide
R = Olefin or fatty ester derived
Polysulfide
DIthlocarbamates
R.
-N-C,
R = Alkyl
Zn
CH,
R
S -12
Ashless Dithiocarbamate
Zinc Dithiocarbamate
Phosphites and DIthiophosphates
RO^ /^S
R0-. RO
R0"%
H
Diall
R = Alkyl
Zn
Zinc DialkyI Dithlophosphate
FIG. 12—Hydroperoxide decomposers.
N-Phenyl-1 -naphthylamine (PANA)
Diall^yldiphenyiamine
2,6-DI-f-butyl-4-niethylphenol (BHT)
Arylamines FIG. 13—Radical scavengers.
hydroperoxide decomposition they are converted into phosphates, which continue inhibiting oxidation by forming acidic materials via thermal decomposition [3]. The hydroperoxide decomposing mechanism by phosphorus additives is shown in Part 3 of Fig. 14. Dithiophosphoric acid derivatives are the most potent of the phosphorus compounds, primarily because they can inhibit oxidation both by hydroperoxide decomposition and by radical scavenging [7]. They decompose hydroperoxides catalytically as well as by reacting with them to form compounds that are peroxide decomposers in their own right [3]. Initially, dithiophosphoric acid derivatives are converted into dialkyl dithiophosphoryl disulfide, which is the key intermediate in the oxidation-inhibiting process. The disulfide oxidizes to thiosulfenate, followed by the sulfur-sulfur bond cleavage to form sulfenyl and dithiophosphoryl radicals. These radicals react with hydroperoxides and other radiccJs
to form intermediates that result in strongly acidic species, which are also responsible for oxidation inhibition. This sequence of events is depicted in Fig. 15. Dithiocarbamic acid derivatives, a related class of oxidation inhibitors, also act as both hydroperoxide decomposers and radical scavengers [8]. Radical scavengers render radicals innocuous either by transferring a hydrogen atom to them or by an oxidationreduction mechanism [1,3]. Hydrogen transfer from the inhibitor to the radical generates a new inhibitor-derived radical. See Eqs 14-16 of Fig. 3. However, unlike the radicals from the oxidation process, such radicals are incapable of propagating oxidation. This is because these new radicals are either too sterically hindered or resonance stabilized (see Fig. 16) to take part in the oxidation process. The oxidationreduction mechanism involves electron transfer to or from the peroxy radical, thereby converting it into an ion and removing it from the oxidation process. The formation of dialkyl dithio-
CHAPTER 30: OXIDATION OF LUBRICANTS phosphoiyl disulfide from zinc dialkyl dithiophosphoric acid is an example of such a mechanism. See Fig. 15. Hindered phenols and arylamines are two prominent classes of inhibitors that act as radical scavengers through hydrogen transfer [3]. The most commonly used hindered phenol inhibitor, 2, 6 di-t-butyl-4-methylphenol (BHT), controls oxidation by transferring its phenolic hydrogen to the alkoxy radical and converts it into alcohol. The phenoxy radical that results from this process is incapable of propagating oxidation because of steric hindrance. However, this radical either rearranges to form the benzylic radical, which dimerizes to form the diphenoxyethane, or reacts with an alkylperoxy radical to form the quinone methide. This is shown in Fig. 17. The diphenoxyethane still contains the hindered phenol functionality and the benzylic hydrogens; therefore, it can act as an oxidation inhibitor. Oxidation inhibition by arylamines, such as dialkyldiphenylamine and N-phenyl-a-naphthylamine, also involves radical scavenging. The transfer of hydrogen from the NH group of the amine to peroxide radicals results in the
O
O
II
3R0H
+
Ri—S—R2
^
II
•
O
Dialkyl Sulfide
Dialkyl Sulfone
Dialkyl Sulfoxide O II
Rl—S—R2
-*~
-^
H2SO4
Dialkyl Sulfoxide 2. Phenols
ROOH
+
+
ROH
3. Phosphorus C o m p o u n d s
ROOH
+
(Ri)3P
-
ROH
+
Trialkylphosphlne
ROOH
+
(RiO)3P
(R0)2P\
{Ri)3P0 Trialkylphosphlne Oxide
ROH
+
(RiOJaPO
H3PO4
TrialkyI phosphate
TrialkyI phosphite
R—C-OOH +
793
formation of the diphenylamino radical [3]. This radical is resonance-stabilized and, hence, cannot start new chains. However, it does react with hydroperoxides and peroxy radicals to form the nitroxy radical, which is also a very potent inhibitor. This is because it has the ability to terminate a large number of oxidation chains through a catalytic action. The mechanism of oxidation inhibition by diphenylamine is depicted in Fig. 17. Transition metals can act both as oxidation promoters and as oxidation inhibitors, depending upon their oxidation state. They act as promoters if they facilitate the formation of free radicals; and they act as inhibitors if they remove free radicals from the oxidation process. For example, heavy metals, such as iron and lead and their salts, are well known as oxidation promoters [4,9]. Figure 18 shows how ionic iron promotes hydroperoxide decomposition to oxidation causing alkoxy and peroxy radicals. Metal deactivators, another class of oxidation inhibitors, are usually used to control oxidation due to metal ions. These inhibitors, extremely effective at low concentrations, perform by forming com-
1. Sulfur C o m p o u n d s
3 ROOM + R 1 S R 2 -
AND FUELS
Zn
ROH Alcohol
Zinc Dithiophosphate
.
+
;c=o ^Ketone
Zinc Dithiophosphate
FIG. 14—Mechanism of hydroperoxide decomposition.
794 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK //
Zn
(RO)2P^
+ 2R00-
{R0)2P\
/P(0R)2 S—S
DiakyI Dithiophosphoryl Disulfide
Zinc Dithiophosphate
/
\
(RO)2P\
I! o
/P(0R)2
Dialcyi Dithiophosplioryl Disulfide
Thiosulfenate
/
\ S-
2 ROOH
//
//o
a
^SH
II
Thiosulfonic Acid
Thiosulfenyl Radical
/o
{ R 0 ) 2 P C „ ^ + SO2
"^QQ"*.
On
S03(H2S04) Oxidation-inhibiting Species
FIG. 15—Oxidation inhibition by zinc diallcyl dithiophosphate.
•
RO-
^
+
ROH + •^1
Arylamine
~^
ROO- +
t^;
Diphenylamino Radical
•
^
+
ROOH
^ R-i
R2 Arylamine
ROOH or ROO-
?• ^
f^
ROH Nitroxy Radical
•••^J Resonance Stabilized/ Delocalized Radical
OH I N
OR I N
R-
P-elimination or liydrolysis
i^ Ri
ROH + Olefin FIG. 16—Oxidation inhibition by hindered phenols.
CHAPTER
30: OXIDATION
OH
RO-
+
OF LUBRICANTS
AND FUELS
795
O-
f-Bu r T ^ t-Bii II I
t-Bu fT " ^ t-Bii •
ROH +
2,6-Di-t-butyl-4methylphenol
CHj-
CH2 Quinone Methide
Benzylic Radical
HO
t-Bu
t-Bu
t-Bu
OH t-Bu
Cn^^n2
Diphenoxyethane FIG. 17—Oxidation inhibition by arylamine. Fe"^** + ROOH
++ Fe +
+ H + ROO
Hydroperoxide
Fe
+
ROOH -
Peroxy Radical
+++ Fe + OH
+ RO' Alkoxy Radical
FIG. 18—The role of metal ions in oxidation.
plexes with the metal ions and taking t h e m out of the chain reaction. These additives have a synergistic effect with oxidation inhibitor types discussed above. Ethylenediaminetetraacetic acid derivatives, N-ScJicyhdene-ethylamine, and N, N-disahcyhdene-1, 2-propanediamine represent the most popular members of this class. However, their use is limited to fuels. Other derivatives, which are used in this application, include lecithin, derivatives of heterocycles, such as thiadiazole, imidazole, and pyrazole, and citric and gluconic acid derivatives [10]. Figure 19 shows the structures of some of these materials. Copper ions promote oxidation just like other transition metal ions. They do this by forming radicals both directly and by delivering molecular oxygen in a more reactive state. In addition, they catalyze hydroperoxide decomposition to free radicals. Copper ions also exhibit excellent oxidation-inhibiting ability [11]. They remove radicals by converting them into
ions, which do not have the ability to take part in the oxidation process. Oxidation promoting and the oxidation inhibiting reactions by copper ions are presented in Fig. 20. Their oxidation-inhibiting activity is concentration dependent. They act as inhibitors when used at low levels, i.e. at 250 p p m or below, above which they become oxidation promoters. In some instances, compounds that intercept oxidation by different m e c h a n i s m s reflect synergism w h e n present together. Synergism is an effect greater than the additive effect of the two or more compounds [12,13]. A combination of a sulfur compound with an arylamine or a hindered phenol is a c o m m o n way to benefit from this phenomenon. Oxidation inhibitors are used in almost all lubricants, with gasoline and diesel engine oils, and with automatic transmission fluids, which account for 60% of the total use. High operating temperature and high air exposure applications require a high level of oxidation protection. Zinc dialkyl dithiophosphates are the primary inhibitor type, followed by aromatic amines, sulfurized olefins, and phenols. This order is based on effectiveness, cost considerations, and supplemental benefits. A n u m b e r of tests are used to assess a lubricant's oxidation stability u n d e r conditions of accelerated oxidation. A reasonably comprehensive list of such tests is provided at the conclusion of the chapter. Some of the tests are described in detail in the following section.
796 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK
a
^CHzCOOH CH=N-C,H=
H3C.
^ N
CH2COOH ^CHjCOOH
OH
CH2COOH
N-Salicylidene'ethylamine Ethylenediaminetetraacetic Acid
N,N-DrsalJcyfidene-1,2propanediamine
CH,—C-OH
H2C-O-C-R
HO-C-
R=C16-18
Hc-o-c;
I
OH CH,—C-OH
HoC-O
COOH I H-C-OH HO-C-H H-C-OH
op
II
P\ + 1^ OCH2CH2N(CH3)3
O
Citric Acid
HS
Lecithin
S
Gluconic Acid
H
SH
A // N-N Dlmercaptotliiadiazole
R
H-C-OH I CH2OH
H N
,N N
N Imidazole
Pyrazole
Benzotriazole
FIG. 19—Metal deactivators
Oxidation Promoting Reactions RH + Cu** Cu* + O2 Cu**+ROOH Cu* + ROOH
^ Cu* + R + H * *- Cu**(02)- + RH -*- Cu* + R 0 2 + H * -*- Cu** + R O + O H -
Purpose of Testing Cu**+RO+OH-
Oxidation Inliibiting Reactions R02- + Cu* ROj- + Cu** RO- + Cu*
RO2+CU** R*+02+Cu* RO" + Cu**
FIG. 20—Role of copper ions in oxidation.
OXIDATION TESTING OF LUBRICANTS AND FUELS This compilation of oxidation bench tests does not claim to address every oxidation and thermal stability test method used in the lubricants industry. Many companies have proprietciry methods they use in-house for evaluating and assessing the performance of their additives and lubricants and as such are not included in this chapter.
The purpose of oxidation testing is to study and evaluate the oxidation and thermal performance of lubricant additives, lubricant systems and fuels under simulated operating conditions in order to predict the performance of those lubricant and fuel systems in real world applications. There are two essential ways to test lubricant systems, bench testing, and field testing. Bench testing studies performance of lubricants and fuels under simulated conditions and field testing predicts lubricant performance in actual equipment. Because of the high costs that are often associated with field testing, bench testing becomes the choice to study and evaluate lubricants. Bench testing can be used as a cost-effective way to evaluate the performance of experimental or new additives and lubricant systems. It can be used to compare the relative performance of different commercially available lubriccints, and finally it can be used to assess the remaining, useful oxidation life of lubricants and fuels in service. Field testing can be used to prove the performance of lubricants that have been evaluated in bench tests and have successfully met the bench test criteria. Description of Oxidation B e n c h Tests A number of oxidation and thermal stability tests have been developed by ASTM: ASTM International; IP: The Institute of
CHAPTER Petroleum; DIN: Deutsches Institute for N o r m u r s ; ISO: International Standards Organization, and other organizations. These tests are designed to evaluate performance of additives and lubricant systems including industrial lubricants, greases, gear oils, engine oils, and fuels. Outlines of these various methods and procedures follow. Antiwear
Hydraulic,
R&O, and Turbine
Oils
ASTM D 943: Oxidation Characteristics of Iniiibited Mineral Oils—This method was developed for and is used to determine the oxidation life of inhibited turbine oils. It is now widely used for predicting the oxidation life of anti-wear hydraulic oils, and R&O oils, as well as turbine oils. The test is designed to simulate the conditions found in a typical steam turbine system. The test oil is heated in the presence of copper and iron catEilysts, which are tjrpical of the metallurgy found in a steam turbine. Water is added to simulate steam condensate and finally, oxygen is introduced to accelerate the oxidation process. The degree of oxidation is determined by an increase in the acid n u m b e r of the lubricant oil. The test is conducted in the following mzinner: 300 mL of test oil, along with catalyst coils of copper and steel are placed into a large glass test tube and placed into a heated oil bath, maintained at 95°C. Sixty milliliters of distilled water is introduced into the test tube. A water-cooled condenser is used to prevent the loss of water vapor during the test. Oxygen is bubbled through the oil sample at a rate of 3 1/h. Periodic samples of the oil are taken and the acid number is determined per ASTM D 974. The test is usually con-
30: OXIDATION
OF LUBRICANTS
AND FUELS
797
cluded when the total acid n u m b e r (TAN) reaches 2.0 mg KOH/g. The n u m b e r of hours needed to reach 2.0 is considered to be the "oxidation lifetime" of that oil. This test method is widely used for specification purposes, and is considered useful in estimating the oxidation stability of lubricants. Uninhibited oils will usually fail within 200 h, while high quality oils can exceed 5000 h. However, it should be recognized that the correlation between this test and actual field performance can vary markedly. Even though the test data indicate that one oil has a n oxidation life twice as long as another oil, this does not m e a n that the first oil will provide twice the service life of the second oil. It is assumed t h a t the longer the oxidation life is in the D 943 test, the longer the lubricant will perform in the field. Table 1 shows t5rpical results for antiwear hydraulic oils and R&O oils when using different base oils. The additives used were the same for each group, that is, AWH all contained the same additives and the R&Os used another additive package. It should be noted that the D 943 has an upper life limit of 10 000 h. Values higher than 10 000 h are considered to be nonstandard extensions of the method (Figs. 21 and 22).
TABLE 1—Typical ASTM D 943 oxidation life of various base oil types and same additives. AWH R&O
Rere fined
Group I
Group II
Group III
Synthetic
2000+" 2,000+"
2500+" 8000+ "
6000+" 12 000+"
10 000+ 12 000+
10 0 0 0 + " 15 000+ "
"(Hours to NNA of 2.0 mg/KOH).
I I
tOWM^K SftMMe » M l
oun&ricM ciu
M « * « « M Cki
FIG. 21—Schematic drawing of ASTM D 943, Oxidation Test.
798
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK The test apparatus consists of a pressurized vessel axially rotating at 100 rpm, at an angle of 30° from the horizontal, in a bath maintained at 150°C (302°F). Fifty grams of test oil, 5 g of distilled water, and a freshly polished copper coil are placed into a glass liner, and inserted into the vessel. The vessel is initieJly pressurized to 90 psi (600 kPa) at room temperature. The 150°C bath temperature causes the pressure in the vessel to increase to approximately 200 psi (1400 k Pa). As oxidation occurs, the pressure drops, and the usual failure point is taken at 25 psi (175 k Pa) from the maximum pressure obtained at 150°C. The results are reported as the number of minutes to 25 psi (175 k Pa) loss.
FIG. 22—ASTM D 943, Oxidation Test, progression of test.
TABLE 2—Typical sludge and copper requirements. Insoluble Sludge (mg.) Total Copper (mg.)
AWH
R&O
100 max. 200 max.
100 max.
D 4310: Determination of the Sludging and Corrosion Tendencies of Inhibited Mineral Oils—This method is a modified alternate to the ASTM D 943 test method, and is used to determine the tendencies of inhibited mineral oils, especially turbine oils, to form sludge during oxidation (see Table 2), The test conditions described under ASTM D 943 are used. After 1000 h, the test is stopped. The oil and water layers are separated and filtered. The weight of insoluble material is determined gravimetrically by filtration of the contents of the oxidation test tube through a 5 micron pore size filter. The a m o u n t of copper in the oil, water, and sludge phases is also determined according to any suitable method, such as atomic absorption (AA), direct current plasma (DCP), inductively coupled plasma (ICP), or x-ray fluorescence (XRF). It should be noted that there is n o requirement for the amount of iron present. This method is used primarily for specification purposes. It is widely accepted that the presence of sludge and insoluble material is due to oxidation. Formation of oil insolubles or metal corrosion products during this test may indicate that oil will form insolubles or corrode metals, or both, during field service. However, correlation with field service has not been established. D 2272: Oxidation Stability of Steam Turbine Oils by Rotating Pressure Vessel (formerly Rotary Bomb)—The Rotaiy Pressure Vessel Oxidation Test (RPVOT) is a rapid method of comparing the oxidation life of lubricants in similar formulations, in the presence of water and a copper catalyst. This method can be used to evaluate the oxidation characteristics of turbine oils, hydraulic oils, and transformer oils.
The RPVOT is favored as a quality control test because it is rapid. Results are usually available within 2-5 h, as opposed to 1000 h or more in the ASTM D 943 test. The RPVOT is not intended as a replacement for the commonly used ASTM D 943. The RPVOT result is useful in controlling the continuity of this property for batch-to-batch acceptance of production lots, having the same composition. No correlation has been established between RPVOT and ASTM D 943 methods. The RPVOT is also useful in determining the remaining oxidation life of in-service systems by charting the original RPVOT value versus subsequent samples of that system. It should be noted that the D 2272 test method is dependent on additive chemistry. Comparisons of oils having different formulations are of limited value. As with the D 943, there is no established correlation between the D 2272 method and actual field service (Figs. 23 and 24). In Service Monitoring of a Turbine Oil Using the ASTM D 2272 (RPVOT) Warning Limits: Interpretation: Action: Warning Limits: Interpretation: Action:
System
Less than 50% of original value, u p to 20 000 h. Indicates above normal degradation. Investigate cause; Increase frequency of testing; Consider reinhibition. Less thzm 25% of original value. Together with high TAN indicates oil nearing end of its service life. Resample, if value is the same, consider changing oil.
IP-280: Determination of Oxidation Stability of Inhibited Mineral Turbine Oils—This method comes u n d e r the jurisdiction of the Institute of Petroleum (IP) and is commonly used for E u r o p e a n specifications relating to turbine oils and other hydraulic fluids. This method is technically identical to the CIGRE method "Turbine Oil Oxidation Stability Test." The test apparatus consists of a suitable size test tube containing 30 g of test oil, plus copper naphthenate and iron n a p h t h e n a t e as soluble catalysts. The sample test tube is placed in a heated bath, 120°C for 164 h. During the test period, oxygen is bubbled through the oil sample at a rate of 1.0 1/h. Both the test temperature and the oxygen flow rate must be carefully m a i n t a i n e d t h r o u g h o u t the test period. The volatile acids are trapped in a tube containing distilled water and alkali blue solution. At the conclusion of the test period the oxidized oil sample is filtered and the sludge is determined. The water in the absorption tube is titrated with alcoholic potassium hydroxide (KOH) to determine the volatile
CHAPTER 30: OXIDATION OF LUBRICANTS QXVOCN
AND FUELS
799
—C.
••-624 JOIMT
;S5-2fr5 Qg WAUI-2-W
4<-5
-OIL SAMPLE
r-J
FIG. 23—Schematic drawing of D 2272, Rotary Vessel Test (RBOT).
1
I
WSTiaEO WATER + INDICATOR
m^ 100-200
OXIDATION TUBE
ABSORPTION TUBE
FIG. 25—IP-280, Cigre Oxidation. Oxidation and absorption tubes.
FIG. 24—ASTM D 2272, Rotary Vessel Test (RBOT), rotary vessel oxidation charts.
acids. The oxidized oil is mixed with heptane, and this solution is titrated with alcoholic KOH to determine the soluble acids. The volatile acids, soluble acids, and the sludge are used to calculate the "Total Oxidation Products" (TOP). Because of the relatively short test time, this method is sometimes used as a replacement for the longer running ASTM D 943. However, no correlation between this test and the D 943 exists. Formation of acids and oxidation products during this test may indicate that the oil will form sludges and acids that might corrode metals during field service (Figs. 25 and 26). IP-306: Determination of Oxidation Stability of Straight Mineral Oils—This method is designed to give an indication of the oxidation stability of straight, unadditized, mineral oil based lubricants under specific conditions. This test method employs the same apparatus and a similar procedure to those used in IP-280. IP-306 differs from the IP-280 procedure in (a) the time test time is reduced to 48 h, (b) two conditions, namely no catalyst and solid copper catalyst, are used and (c) the degree of oxidation is expressed as "total oxidation products" (TOP) percent. IP-307: Determination of Oxidation Stability of Mineral Insulating Oils—This method is designed to give an indication of the oxidation stability of insulating oils under specific conditions. Insulating oils are nonelectric conducting oils used in various electrical applications, for example, electric transformers.
FIG. 26—IP-280, Cigre Oxidation. Typical metal heating bath.
This test method employs the same apparatus and a similar procedure to those used in IP-280. IP-307 differs from the IP-280 procedure in (a) the test temperature is reduced to 100°C (212°F), (b) a solid copper catalyst is used, and (c) the degree of oxidation is expressed as sludge and acidity. D 2070: Standard Test Method for Thermal Stability of Hydraulic Oils—At elevated temperatures, the long hydrocarbon chains in mineral oils may break apart into shorter
800
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
hydro-carbon chain lengths (thermal decomposition). While some of the chains may vaporize and escape into the atmosphere, others tend to combine with other chains (pol5rmerize) to form hard, sticky byproducts known as gums, varnish, and other deposits. Thermal stability is a lubricant's ability to resist breakd o w n u n d e r conditions of high t e m p e r a t u r e s . The zinc dialkyl dithiophosphates (ZDP) anti-wear agents used in the early 1970s lacked thermal stability and would decompose u n d e r high t e m p e r a t u r e s (approximately 135-140°C) a n d form acids of phosphorus and sulfur. These acids would then attack the yellow metals, copper, bronze, and brass that were commonly used in hydraulic systems. Of particular sensitivity were the bronze piston shoes, used in piston p u m p s . The lack of thermal stability of these ZDPs led to the premature failure of many piston p u m p s because of corrosion problems. Cincinnati Machine (formerly Cincinnati Milacron), a leading manufacturer of machine tools, originally developed this test method to assess the thermal stability of the zinc dialkyl dithiophosphates containing oils that were being used in their equipment. The motivation behind this test was the high cost of warranty claims this manufacturer experienced. The test apparatus consists of a beaker, a copper and steel test rod, and an electric convection oven capable of maintaining 135°C (275°F) for 168 h. The copper and steel test rods are polished, weighed, and placed into a beeiker of test oil. The rods are arranged in an "X" pattern with a single contact point. The assembled apparatus is placed in the test oven. This method does not involve the use of air or oxygen blowing, nor is any agitation involved. At the end of the test period, the test rods are compared to a reference chart to determine the degree of chemical attack. Ideally, the rods should show little evidence of any discoloration. The oil is evaluated to determine any changes in viscosity, to measure ciny increase in acid n u m ber, and to determine the amount of sludging. This method is widely used for approval purposes and is useful in evaluating the thermal stability of lubricants. It is used primarily for hydraulic oils, but it can also be used to evaluate other industrial fluids. A lower temperature is used to evaluate industrial gear oils. This ASTM method differs from the Cincinnati Machine version as follows: Cincinnati requires a potassium cyanide (KCN) wash of the test rods in order to determine metal weight loss. Because of the hazards related to the use of potassium cyanide, ASTM does not endorse this step. See Cincinnati Machine test method 'Procedure A,' 'Procedure B' (see Figs. 27 and 28). D 4636: Standard Test Method for Corrosiveness and Oxidation Stability of Hydraulic Oils, Aircraft Turbine Engine Lubricants, and Other Highly Refined Oils—This method is the result of combining Federal test methods 5307.2 and 5308.7. Currently, this method is of interest primarily to the military who use it to determine the resistance to oxidation and corrosion tendencies of hydraulic oils, aircraft turbine engine lubricants, and other highly refined oils used in military aircraft and equipment. This method is no longer a mandatory requirement for any major steam turbine manufacturer. The test method can b ^ used to evaluate mineral oils as well as synthetic fluids. It Ccin be r u n using dry or moist air, as well as with or without the metal test specimens. There are two basic versions of this test method. Procedure 1 uses "washer" type metal specimens, which include
Acceptable
Unacceptable
FIG. 27—ASTM D 2070, Thermal Stability Test. Results, Cincinnati Procedure A. - 1
r
•
I
^
Unacceptable
Acceptable
FIG. 28—ASTM D 2070, Thermal Stability Test. Results, Cincinnati Procedure A. titanium, magnesium, steel, bronze, silver, and aluminum. Procedure 2 uses "square-shaped" metal specimens, which include copper, steel, aluminum, magnesium, and cadmium. The test is conducted in the following manner: The specified a m o u n t of test oil, 100 m L or 200 mL, is placed into a large test t u b e along with the polished and weighed test specimens. The assembled apparatus is weighed and placed into the constant temperature bath. Test temperature can range from 100°C (212°F) to 360°C (680°F), for a specified amount of time. At the end of the test period the tube assembly is removed from the bath Euid the meted specimens are reweighed to determine weight loss, which is an indication of the oil's corrosiveness. The final viscosity and TAN of the oil is determined. Any sludge remaining in the test tube is determined gravimetrically. This method simulates the environment encountered by fully formulated lubricants in actual service, and uses an accelerated oxidation rate to permit measurable results in a timely manner. Interpretation of results should be done by comparison with data from oils of known field performance (see Fig. 29).
CHAPTER 30: OXIDATION OF LUBRICANTS
(0.2Acm)
AND FUELS
801
DRILL 2 HOLES j ^ WAMETER (0.16cin)
MAGNESIUM STEEL
ALUMINUM CADMIUM COPPER FIG. 29—ASTM D 4636, Oxidation of Hydraulic Fluids. Metal square dimensions and arrangement.
D 5846: Universal Oxidation Test for Hydraulic Fluids and Turbine Oils—This test method was developed to evaluate the oxidation stability of petroleum base, rust, and oxidation inhibited hydraulic oils, antiwear hydraulic oils, and oils for steam and gas turbine. This test method covers a procedure for evaluating the oxidation stability of inhibited mineral oils in the presence of air, copper and iron metals, and utilizes the Universal Oxidation test apparatus. The test is conducted in the following manner: One hundred grams of the test oil is weighed into the test cell, along with freshly polished coils of copper and iron wire. The assembled test cell is placed into a heated block capable of maintaining the test oil at a temperature of 135°C (275°F). Air is bubbled through the test oil at a rate of 3 1/h. The acid number and spot forming tendencies of the oil are measured daily. The oil is considered to oxidize or degrade when either the acid number has increased by 0.5 mg KOH/g over the fresh oil, or when the oils begin to form insoluble solids so that when a drop of oil is placed onto filter paper it shows a clearly defined dark spot surrounded by a ring of clear oil. Degradation of hydraulic fluids and turbine oils caused by oxidation and thermal breakdown can result in the formation of acids or insoluble solids, and render the oil unfit for further use (see Figs. 30 and 31). D 6514: High Temperature Universal Oxidation Test for Turbine Oils—This test method is designed to compliment test method D 5846 and is intended for evaluation of fluids that do not degrade significantly within a reasonable period of time at 135°C. This test uses the universal oxidation test equipment and was developed for, and is used to evaluate the high temperature oxidation stability and deposit forming tendencies of oils for steam and gas turbines. A summary of the test method is as follows: One hundred grams of test oil is weighed into the test cell along with freshly polished coils of copper and iron wire. The
FIG. 30—D 5846, Universal Oxidation Test. Test apparatus.
FIG. 31—D 5846, Universal Oxidation Test. Universal Oxidation glassware.
assembled test cell is placed into a heated block capable of maintaining the test oil at a temperature of 155°C (310°F). Air is bubbled through the test oil at a rate of 3 1/h for 96 h. At the end of the oxidation time, the sample is cooled and the oil is filtered through a preweighed No. 41 Whatman filter and a preweighed 8 micron filter membrane and reweighed. Total weight of insoluble material, change in acid number, percent increase in viscosity at 40°C, and the veirnish adhering to the test cell is reported.
802 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
Degradation of fluid lubricants because of oxidation or thermal breakdown can result in fluid thickening or in the formation of acids or insoluble acids and render the fluid unfit for further use as a lubricant. DIN 51352: Pneurop Oxidation—This test method is designed to evaluate the oxidation stability of compressor oils, and is used to qualify compressor oils in European manufactured equipment. The test is conducted in the following manner: Soluble ferric (III) oxide is used as a catalyst and is measured into the glass test tube. Forty mL of test oil is then measured into the tube. The assembled apparatus is placed in an oil bath at 200°C (392°F), and air is bubbled through the oil mixture at a rate of 15 1/h. The test is conducted for 24 h. At the end of the test period, the apparatus is allowed to cool and reweighed to determine any evaporation loss. The oxidized oil is then submitted for determination of carbon residue using either the Conradson Carbon Method ASTM D 189, or the Ramsbottom Carbon Method ASTM D 524. The amount of carbon residue formed in the oxidized oil is used to predict field performance. The carbon residue should not exceed 2.5% for and ISO 46 oil (see Fig. 32).
CSimensions in m m
Vent lube
Insert
NS 29/32 conical ground joint a$ specified in DIM 122^2 Part I
Ageing vessel
Gear Oils D 2893: Oxidation Characteristics of Extreme Pressure Lubricating Oils—This test was developed to measure the high temperature oxidation stability of industrial gear oils. It is important to note that D 2893 was designed to run at 95°C. However, to meet the requirements of U.S. Steel requirement, #224, and AGMA 9005 (American Gear Manufactures Association), the temperature is elevated to 121°C (250°F). The other test conditions remain unchanged. The test method is designed to measure resistance to oxidation by determining the change in viscosity. In this test, the same glassware required by the D 943 T.O. Oxidation test is used. Three hundred milliliters of the test lubricant is measured into the tube. The glass inlet tube from D 943 is used to introduce the air into the test lubricant. The entire assembly is placed in an oil bath at 95°C, (121°C for the USX S-200 procedure (U.S. Steel)). Dry air is bubbled through the sample at 10 1/h for a period of 312 h. At the end of the test the final viscosity and precipitation number are determined. The appearance of the test tube and oil may be inspected for evidence of oxidation. Qualifying industrial gear oils give 0-6% viscosity increase in test. Though unspecified, cleanliness of the glassware is very important for today's industrial gear oils. It is felt that glassware cleanliness is an indication of the lubricant's thermal stability (see Figs. 33 and 34). D 5763: Oxidation and Thermal Stability Characteristics of Gear Oils Using Universal Glassware—This test method is a modification of D 2893. Of significant value is the determination of sludge in this method, which is not done in D 2893. This method can be used to evaluate both extreme pressure and nonextreme pressure gear oils. This test method is run using universal glassware apparatus. A 100 g sample of gear oil is weighed into the test tube. The test assembly is placed in a heated bath at 120°C for 312 h. Dry air is bubbled through the sample at a rate of 3 1/h. At the end of the test period the apparatus is cooled to room temperature and reweighed to determine any oil loss. The
Ring mark at 4 0 ml
*25,S--o.s
FIG. 32—DIN 51352, Pneurop Oxidation. Test apparatus for determining the aging characteristics of lubricating oils.
oxidized oil is filtered through at 2.8 micron filter. Any sludge adhering to the glassware is rinsed down and filtered through the filter. The total sludge is then determined. Kinematic viscosity is determined on the filtered oil and the percent change in viscosity is reported. This is the only test method that employs glassware to measure the amount of sludge produced during oxidation and thermal degradation of industrial gear oils. Sludge build up from oxidation and thermal degradation can render the oil unsuitable for further use as a lubricant. The correlation between this test and actual field performance is unknown. D 5704: Evaluation of the Thermal and Oxidative Stability of Lubricating Oils Used for Manual Transmissions and Final Drive Axles—This test method is commonly referred to as the L-60-1 test. It measures the oil-thickening, insolublesformation, and deposit formation characteristics of automotive manual transmissions and final drive axle lubricating oils when subjected to high temperature oxidizing conditions.
CHAPTER 30: OXIDATION OF LUBRICANTS
AND FUELS
803
Thermometer
Flowmeter Measures Air Liters per Hour
Air Inlet
FIG. 33—ASTM D 2893, Gear Oil Oxidation. Apparatus schematic.
KJ
Q
Thermally Stable Oil
i fi Borderline Thermally Stable Oil
Non-Thermally Stable Oil
FIG. 34—ASTM D 2893, Gear Oil Oxidation. Examples of typical results.
A 120 mL sample of the lubricant to be evaluated is placed in a heated gear case containing two spur gears, a test bearing, and a copper catalyst. The lubricant is heated to a temperature of 163°C (325°F) and the gears are operated at 175 rpm for 50 h at a predetermined load of 128W, which is provided by an automotive alternator. Air is bubbled through the lubricant at a rate of 1.11/h. Upon completion of the 50-hour test period, the oil is evaluated for change in viscosity and the presence or amount of insoluble materials. The test gears are evaluated and rated for their cleanliness. The copper catalyst is weighed and any weight change of the copper is reported (see Figs. 35 and 36). JIS-K2514: Oxidation Stability of Automotive Gear Oils—This test method is a part of the Japanese standard JASO M33991, and is basically identical to the Indiana Stirrer Oxidation Test (ISOT). Because of the trend of increasing operating temperatures encountered during normal service, geeir oils having superior oxidation stability are required to meet the demands of GL-3 and GL-5 performance. The test is outlined as follows:
FIG. 35—ASTM D 5704 (L-60-1), Gear Oil Test. Test stand.
Mineral
Mineral
Mineral
Mineral
o o o O O C#
o o o o Minimum
Standard
Premium
Top Tier
FIG. 36—ASTM D 5704 L-60, Gear Oil Test. Thermal stability of industrial gear oils.
804 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK Polished steel and copper catalysts are placed in the bottom of a suitable sized beaker (see Table 3). Two hundred and fifty mL of test gear oil is measured into the beaker. A phenolic resin cover, with holes provided to allow a glass varnish rod, and a glass stirrer to be immersed in the test lubricant is placed on the beaker. The assembled apparatus is placed in a heated bath capable of maintaining the test lubricant at a temperature of 150°C (302°F) for 96 h. Air is drawn and mixed into the test lubricant through the vortex generated by the stirrer. At the end of the test period the oxidized lubricant is evaluated for change in viscosity increase, acid n u m b e r (AN) and the amount of pentane in solubles. The glass varnish rod is inspected for lacquer deposits. This method is primarily used for specification purposes, and shall be applied to gear oils designed to meet GL-3 to GL5 as specified by API service classification, which are used for manual transmission gears and final reduction gears in passenger cars, trucks, and buses. CEC-L-48-08: Test Method for Oxidation Stability of Lubricating Oils Used in Automotive Transmissions to Artificial Aging (DKA Method)—This m e t h o d indicates the tendency of transmission lubricants to deteriorate by oxidation u n d e r specified conditions. The methods apply to fully formulated transmission lubricants, ATF, and gear oils. The tests are conducted in the following m a n n e r (see Table 3): • Apparatus A—Three hundred mL of test fluid is measured into a preweighed, suitably sized Erlenmyer flask. A glass air delivery tube is placed into the test oil and is connected to an airflow meter. The assembled apparatus is then placed into a heated bath, maintained at 150°C (302°F) during the test duration. Airflow is adjusted and maintained at 10 1/h. After 192 h the test is ended, the flask is removed from the heated bath, and the test fluid is measured for % change in viscosity, heptane insolubles, and change in AN. Sludge is determined and carbon/varnish deposits on the Erlenmyer flask are noted. (See Table 4 for test limits.) • Apparatus B—One hundred mL of test fluid is measured into a suitably sized glass test tube. The test tube is fitted with a water cooled condenser, a n d a glass air delivery tube. The assembly is placed into a heated bath and the temperature of the test fluid is maintained at 160°C for the duration of the test. The air flow is adjusted and maintained at 5.01/h. After 192 h, the test is ended, the glass test
tube is removed from the heated b a t h and the test fluid is measured for % change in viscosity, for change in AN, and sludging is determined by using a blotter spot test (see Table 4 for limits and Table 5 for oxidation stability of test fluids) (Figs. 3 7 ^ 0 ) . Grease D 942: Oxidation Stability of Lubricating Greases by the Oxygen Vessel Method—The D 942 Vessel Oxidation Stability Test was specifically developed to evaluate the oxidation stability of lubricating greases when stored under static conditions for extended periods of time. This test is included in most of the industrial and automotive grease specifications. The test is performed by subjecting a sample of grease to pure oxygen under a pressure of 110 psi (758 kPa) at 99°C (210°F) for 100 h or longer. The degree of oxidation is then related to the amount of oxygen reacted and is determined by pressure decrease. The pressure is recorded every 24 h over the duration of the test. ASTM D 942 Grease Vessel Aluminum Complex Grease + Additive Additive A
Psi loss @ 100 hours
Additive B C
10.0
Additive
Additive D
No Additive
1.8
1.9
14.2
0.6
Base Grease (No Additive) Grease Type
Aluminum Complex
Calcium Complex
Lithium
Lithium Complex
Psi loss @ 100 h o u r s
14.2
2.6
1.4
3.6
TABLE 4—Equivalence and differences between the results with Apparatus A and Apparatus B. Viscous Oils ( > 0 mm^/s at 100°C) Low Viscosity Oils, ATFs (
s i SOX 160°C 170°C
Equivalent A more severe thanB Only B is evaluated
Current Type E x : R L 181
High Temperature Resistant Gear Box OilEx:RL 184
Equivalent B more severe than A
Equivalent Equivalent
TABLE 3—Gear oil oxidation test comparison. L-60-1
Test Apparatus Duration, h Temperature, (°C) Oil Volume (1) Air Flow, (1/h) Agitation Metal Catalysts Air/Oil Volume Ratio
Steel Case 50 163 0.12 1.1 Gears at 1725 min~* Iron and Copper 458
Viscosity A% A AN Insolubles Carbon/Varnish Sludge Blotter Test
100% max
CEC L-48-08 (DKA)
Glass Tube 192 160 0.1 5.0 Air None 9600 Evaluation Criteria < 2 cSt. <2.0 cSt. mg KOH/gm
2% max. 7.5 large gear 9.4 100% (no sludge)
ISOT JIS-K2514
Glass Beaker 96 135 0.25 None Impeller at 1300 m i n . " ' Iron and Copper
CHAPTER 30: OXIDATION OF LUBRICANTS AND FUELS 805 TABLE 5—Oxidation stability of the test fluids. CAT-1 CAT-2 PAT-1
IT-1
Fluids DKA Oxidation Test (192 h, 170°C) AN Increase % Viscosity Increase at 40°C % Viscosity Increase at 100°C Tube Rating Blotter Spot Rating
Industrial Traction Fluid
Commercial Automotive Traction Fluid 1
PAT-2
PAT-3
PAT-4
Commercial Automotive Traction Fluid 2
Prototype Automotive Traction Fluid 1
Prototype Automotive Traction Fluid 2
Prototype Automotive Traction Fluid 3
Prototype Automotive Traction Fluid 4
4.0 10.5
1.1 7.2
7.6 53.4
0.9 10.6
1.3 13.7
0.7 8.8
0.2 5.0
4.7
3.4
26.2
5.9
4.8
0.2
1.3
light 100
light 100
light 100
light 100
light 100
light 100
heavy 48.5
04.2.1 04.2.2 04.2.3 04.2.5 04.2.10
Special Erlenmeyer Oil bath Flowmeter enabling to control 101/h + 0.5 1/h Clean and dry air supply Silicone hosepipe
\\\\\\\\v\\\\A\\\\\\\\\^:\T FIG. 37—CEC L-48-08, Gear Oil Oxidation. Sample storage container.
806 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
A«fe^^'^ ^ ^ . . : i ^ ^
te:iJ Aspects
Aspect 2
Aspect 1
Several factors, including type and quantity of the oxidation inhibitor, influence the performance of a grease in this test. Most grease specifications require no more than a 5 psi loss after 100 h, or 10 psi after 500 h (see Figs. 41 and 42). D 5483: Oxidation Induction Time of Lubricating Greases by Pressure Differential Scanning Calorimetry (PDSC)—This method covers the determination of oxidation induction time of lubricating greases subjected to oxygen at 500 psig (3.45 mPa) and temperatures between 155°C (SITF) and 210°C (410°F). Oxidation Induction Time (OIT) is the period of time from the first exposure to an oxidizing atmosphere until the extrapolated onset time. A small quantity of grease is weighed into a sample pan and placed in a test cell. The cell is heated to a specified temperature and then pressurized with oxygen. The cell is held at a regulated temperature and pressure until an exothermic reaction occurs. The extrapolated onset time is measured and
FIG. 38—CEC L-48-08 Gear Oil Oxidation. Apparatus or glassware for Part A.
Dimensions in mm Air inlet tube
DIN 1 2 2 4 2 - V 2 9 / 3 2 conical ground joint
Air outlet tube
DIIM 1 2 2 1 5 - D 1 1 adapter
DIN 1 2 2 1 5 - D T I adapter
^
FIG. 40—CEC L-48-08 Gear Oil Oxidation. Assembled apparatus for Part B.
-DIN 1 2 2 4 2 - V 2 9 / 3 2 conical ground joint
Lead Gasket
0 = diameter
Oxygen Atmosphere «s
Supplier «»0'i
W.O. Schmidt OHG Eschenbunjstr. 7 33106 Bnaunschweig Tel.: 0531-331572 Fax: 0531-338032
FIG. 39—CEC L-48-08 Gear Oil Oxidation. Dimensions of glassware for Part B.
Grease Sample
Oxygen Bomb Oil Bath 210°F-rF FIG. 41—ASTM D 942, Grease Vessel Oxidation. Oxygen vessel apparatus.
CHAPTER 30: OXIDATION OF LUBRICANTS AND FUELS
FIG. 42—ASTM D 942, Grease Bomb Oxidation. Test end.
reported as the oxidation induction time for the grease under the specified test temperature. Oxidation induction time can be used as an indication of the oxidation stabiHty. This method is used for research and development, q u a h t y control, a n d specification purposes. However, no correlation has been determined between the results of this method and actual service performance (see Fig. 43fl and b). Evaluating the Oxidation Stability of Greases Using the Penn State Micro-Oxidation Test Apparatus (PSMO)—This method is not an ASTM standard; it is under consideration within an ASTM technical committee, but has not received all of the approvals required to become an ASTM standard. This method is a modification of the Penn State MicroOxidation Test designed to evaluate heavy-duty diesel lubricants. The PSMO test is of interest because of the micro-sized sample used for the test evaluation and the relatively short time required to perform the oxidation test. Using the PSMO apparatus, a 100 mg sample of test grease is oxidized at the following temperatures and time periods; 6 h at 150°C (302°F), 3 h at 160°C (320°F), or 1.5 h at 170°C (338°F). The grease sample is blown with air during the test period to accelerate the oxidation. At the end of the oxidation period the oxidized grease sample is reweighed and the weight loss of the grease is determined. This weight loss is related to the oxidative volatiles lost during oxidation. The oxidized grease sample is solubilized in hexane or tetrahydrofuran (THF) and is subjected to analysis by GPC to determine the molecular weight distribution of the oxidation byproducts. The formation of the polymeric byproducts is an indication of the oxidation stability of the test grease. This method is a useful bench test for studying the thermooxidative stability of greases. The PMSO test, when coupled with GPC, a chromatographic test m e t h o d that separates molecules based on molecular weight, is useful for studying high molecular weight product formation that leads to increased viscosity during oxidation. D 3527: Life Performance of Automotive Wheel Bearing Grease—This test method covers a laboratory procedure for evaluating the high t e m p e r a t u r e life, (oxidation stability)
807
performance of wheel bearing greases. A summary of the test method follows: A 5-gram sample of test grease is distributed in the bearings of a modified, automobile front wheel hub-spindlebearings assembly. While the bearings are thrust-loaded to 11 IN, the hub is rotated at 1000 rpm and the spindle temperature maintained at 160°C (320°F) for 20 h, 4 h off operating cycle. The test is terminated when grease deterioration (oxidation) causes the drive motor torque to exceed a calculated motor cut off value. The grease life is expressed as accumulated on-cycle hours, or hours to failure. This method differentiates among wheel bearing greases having distinctly different high temperature (oxidation) characteristics. It is not equivalent of long-time service tests nor is it intended to distinguish between the products having similar high temperature performance properties. This test method has proven helpful in screening greases with respect to life performance for automotive wheel bearing applications (see Fig. 44). Engine
Oils
D 4742: Thin Film Oxygen Uptake Test (TFOUT)—This method evaluates the oxidation stability of engine oils for
Thermal Analyzer PDSC Cell
rc^ 0
4>*\A I 8 B
Flowmeter
mm
0000000 i 00000001
ill
ooooooo j
>-/ ® « ®
' r '. %^''m^
B
#
'fgJMvDSC Oxygen Cylinder FIG. 43a—ASTM D 5483, Grease Oxidation by PDSC. Test unit.
200
Typical PDSC Thermal
180 120 80 40
Sample: Grease A Size: 2.00 mg Temperature: 210°C Oxygen Flow: 100 mUminutes Induction Time: 42.4 minutes
EXO
0
ENDO 42.4
-40
209.8°C
-80 J 0.0 4.0 8.0 12.0 16.0 20.0 24.0 28.0 32.0 38.0 40.0 44.0 Time (min) FIG. 43b—ASTM D 5483, Grease Oxidation by PDSC. Onset temperature.
808
MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
Wheel Bearing Lubricant Tester (Elevation View)
• tdclfontc pick up
Inboard 8«arlng
LM&704e Cone LM&7CI0 Cup
OulboTd Baannq-
LM11949 Cone LMII9I0 Cup
FIG. 44—ASTM D 3527, Wheel Bearing Life. Test apparatus.
gasoline automotive engines. This test is run at 160°C (320°F) and utilizes a high-pressure reactor pressurized with oxygen, along with a soluble metal catalyst package, a fuel catalyst, and water. These conditions partially simulate the environment to which an oil may be subjected in a gasoline combustion engine. The test oil is mixed with an oxidized/nitrated fuel component; a mixture of soluble metal naphthenates (lead, copper, iron, manganese and tin naphthenates), and distilled water. The oil mixture is placed into an oxidation vessel charged with oxygen to a pressure of 620 kPa (90 psig). The assembled pressure reactor is placed in an oil bath held at 1608C, and is rotated axially at a speed of 100 rpm. A thin film of oil is formed within the glass container by vessel rotation, result-
ing in effective oil-oxygen contact. The test is terminated when a rapid decrease of the reactor pressure is observed. The time when the pressure begins to decrease rapidly is called the Oxygen Induction Time, and is used as a measure of the oil's oxidation stability. This method is used to evaluate oxidation stability of lubricating base oils with additives in the presence of chemistries similar to those found in gasoline engine service. This test does not constitute a substitute for engine testing, which measures wear, oxidation stability, volatility, and deposit control characteristics of engine oil lubricants (see Figs. 45 and 46). Thermo Oxidation Engine Oil Simulation Test (TEOST) (MHT-4 Protocol)—This method is not an ASTM standard; it
CHAPTER is under consideration within an ASTM technical committee, but has not received all of the approvals required to become an ASTM standard. Savant Laboratories originally developed this test in the 1980s. In 1989, Savant, with cooperation of the Chrysler Corporation, developed a procedure that correlated with turbocharger deposits. The e q u i p m e n t b e c a m e k n o w n as the Chrysler/Tannas TEOST® apparatus. This test method describes the general oxidation and depositforming characteristics of engine oils at moderately high temperatures (MHT) of 285°C, using the TEOST apparatus. Using a TEOST test apparatus, a 10-gram sample of the engine oil containing an organo-metallic catalyst (lead, iron, manganese, tin and copper naphthenates) is forced to flow past a tarred, wire-wound depositor rod held in a glass mantled casing. The rod is resistively heated to obtain a constant
30: OXIDATION
OF LUBRICANTS
Drive Unit
FIG. 45—ASTM D 4742, Thin Film Oxygen Uptake Test (TFOUT). Test apparatus.
To Pressure Recorder Ctosure
0.64 cm
^'-^"^ ti Sample Container
O-Ring Seal TFE Cover Sample Container
Aluminum Insert
7.46 cm
6.03 cm Diameter Aluminum Insert
809
temperature of 285°C (545°F) for 24 h. During this time, dry air is forced to flow through the mantle chamber at a specific rate of 10 mL/minute. At the end of the test, the depositor rod and the components of the chamber are carefully rinsed of oil residue using a volatile hydrocarbon solvent. After drying the rod, the mass of the deposits is determined. The hydrocarbon solvent rinse is filtered and weighed, and the mass of the accumulated filter deposits is determined. The mass of deposits on the rod plus the mass of deposits on the filter is the total deposit mass. The mass of deposits that have accumulated on the inside surface of the mantle are also weighed. The maximum deposit requirements for this method Eire: ILSAC GF-2/Factor Fill = 60 mg; Chrysler Service Fill Only = 30 mg; API SJ = 60 mg. Table 6 shows typical test results for six oils, plus additional GF-3 prototype, used in round robin testing.
Bomb Stem
r
AND FUELS
(Bomb I.D. Is 6.03 cm)
FIG. 46—ASTIM D 4742, Thin Film Oxygen Uptal
810
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
TABLE 6—Relative oxidative stability of various biodegradable fluids. Formulation 1) Sunflower oil (no additive) 2) Rapeseed oil (no additive) 3) Trioleate ester (no additive) 4) Sunflower oil + additive 5) Rapeseed oil + additive 6) Sunflower + ester (70/30) + additive 7) Rapeseed + ester (70/30) + additive
Orig cSt at 40°C
Final cSt at 40"'C
% Vis increase
39.6
56.2
41.9
33.3
63.1
89.5
46.5
79.6
71.2
39.3
40.7
3.5
33.4
38.7
15.8
46.1
46.8
1.5
41.5
42.0
1.2
Average percent viscosity increase indicates that degree of viscosity change does not correlate with level of deposits formed. This method is designed to predict the moderately high temperature deposit forming tendencies of engine oils, especially in the piston ring belt area (see Fig. 47). D 6335: Determination of High Temperature Deposits by Thermo-Oxidation Engine Oil Simulation Test (TEOST)—This method covers the procedure to determine the amount of deposits formed by automotive engine oils at high temperatures (480 C) utilizing the thermo-oxidation engine oil simulation test (TEOST). A sample of the engine oil that contains ferric naphthenate is heated to a temperature of 100°C (212°F). The mixture is in contact with nitrous oxide and moist air, and is pumped at a set flow rate past a tcirred depositor rod. The nitrous oxide simulates the byproducts of combustion Eind acts as a precursor to oxidation. The rod is resistively heated through twelve, 9.5 m i n t e m p e r a t u r e cycles t h a t go from 200°C (392°F) to 480°C (896°F). When the twelve-cycle program is complete, the depositor rod is rinsed with hexane to remove the oil residue, dried, and the gross rod mass is obtained. The oil sample is flushed from the system and filtered through a tcirred polypropylene filter. The mass deposits on the rod, plus the mass deposits on the filter, are reported as the total deposit mass. The test method is designed to predict the high temperature deposit forming tendencies of an engine oil, particularly in the piston/bare areas. This method can be used to screen engine oil samples or as a quality assurance tool (see Fig. 48a and b). Ford, Mercon V; Aluminum Beaker Oxidation Test (ABOT)— Since 1987, the Aluminum Beaker Oxidation Test (ABOT) has replaced full transmission testing (i.e., the Mercomatic Transmission Oxidation Test) needed for Ford Mercon® product approval. This procedure is used to determine the oxidative stability of automatic transmission fluids (ATFs) in a thermal, shear, and air stressed environment typical of an automatic transmission. The ABOT evaluates the oxidative stability of a n ATF in the presence of some of the materials currently used
in the construction of automatic tremsmissions. The test is conducted as follows: Air is introduced into the sample at a flow rate of 5 mL/min. The sample fluid is held at a specific temperature, 155°C (310°F), for a specified time of 300 h. To assure intimate mixing of the air and sample fluid, the air is introduced directly into a rotating gear p u m p immersed in the oil. This gear p u m p also serves to circulate the oil at approximately 5 mL/min and also subjecting the test fluid to a shearing action. A clccin, freshly etched aluminum beaker is used as the oxidation vessel. This beaker is equipped with a cover from which test strips of veirious metals, copper, aluminum, and steel, for example, can be suspended into the fluid and then withdrawn after different lengths of exposure (see Table 7). Deterioration of the fluid during the test is determined by analysis of aliquot samples withdrawn periodically and by examination of the test strips a n d beaker components for deposits. For ATFs a copper strip is withdrawn and rated for corrosion at specified times during the test, and re-inserted for the duration of the test. An aluminum strip is rated for varnish deposits at the end of the test. Also at the end of the test, the beaker is disassembled and the top surface of the p u m p housing is rated for sludge deposits. The control of this test is dependent upon control of the test variables, motor rpm, temperature, airflow rate, and others that have been determined to affect the precision of the test results (see Figs. 49 and 50). Caterpillar Micro-Oxidation Test (CMOT)—This method describes an experimental procedure to measure the thermal and oxidative stability of fully formulated diesel engine oils. Caterpillar indicates that this test method shows good correlation to performance in Caterpillar 3600 series engines. The results generated give an Induction Time of Percent Insoluble deposits versus time. A small sample of the oil to be evaluated (»0.02g) is placed on a polished metal coupon and is held at a temperature of 230°C (446°F) for a m i n i m u m of 40 min in a special glass test cell while being air blown at a rate of 20 mL/min.
FIG. 47—Thermal Oxidation Engine Oil Simulation Test (TEOST®). Chrysler/Tannas apparatus.
CHAPTER 30: OXIDATION OF LUBRICANTS AND FUELS 811
Gas Outlet ControIUnj Tharmocaupla L
N,0-
Oil now Path
Vdve
Th«rTnocoupl«s
P0I-I11.S
(a) FIG. 48—a) Thermal Oxidation Engine Oil Simulation Test (TEOST®). Configuration of TEOST layout for Test Protocol #33; b) Depositor rod ratings.
Eight test cells are used concurrently and the test is a representation of insoluble development in 10-minute intervals; i.e., 40, 50, 60, 70, 80, 90, 100, and 110 min. If the induction time is not reached in the first test series, a second series stcirting at 80 min and continuing u p to 150 min is conducted. The air blowing is continued for a one-minute mini m u m after completion of the test period. The test coupon is rinsed in tetrahydrofuran (THF) and the total weight gain of coupon plus the weight of the THF insolubles is converted to a percentage of the initial sample weight versus the test time and are plotted to determine the sample induction time. The results of the CMOT have been correlated to piston deposit levels measured on Caterpilleir oil evaluation tests. Engine oils having an induction time less than 90 min have generally produced excessive levels of piston deposits in laboratory test engines and in actual field operations. The CMOT is only one part of the oil selection criteria. A high.CMOT induction time does not assure adequate oil performance in the 3600 series diesel engine. Caterpillar will only recognize CMOT results from tests performed at certified laboratories. Presently only Caterpillar offers this service^ (see Figs. 51a and b and 52a and b) BT-10: International Harvester Oxidation Corrosion—International Harvester developed this m e t h o d during the early
^ To obtain this Caterpillar approval, fluids must be submitted to: Caterpillar Inc., Anal3^ical Chemistry Laboratory, Component Development, Division 876, Technical Center, Bldg. E, Peoria, IL 616561875, Ph: 309-578-4858, Fax: 309-578-4496.
1960s. This method is used to evaluate the oxidation stability of lubricants and their corrosive attack on metals. The test is conducted as follows: Polished and weighed metal strips of steel and bronze are suspended in the sample lubricant at temperatures of 121°C (250°F), for either 100 h (Version A) or 190 h (Version B). Oxygen is bubbled through the sample lubricant at a flow rate of 50 mL/min. After the specified test period, each metal strip is eveJuated by determining the weight loss of the test strip, and the weight of residue on the test strip. The test lubricant is submitted for elemental analysis by inductively coupled plasma (ICP). The oxidation tube is edlowed to drain for 24 h, after which time a visual inspection is made to rate the sludge build-up remaining in the oxidation tube. Gasolines
and
Fuels
D 381: Existent Gums in Fuels by Jet Evaporation—This test method covers the determination of the existent gum content of aviation fuels, the g u m content of m o t o r gasolines, or other volatile distillates in their finished forms, and at the time of the test. The gums that occur in gasolines and fuels are varnish like oxidation products formed in the gasoline/fuel during the conditions of the test. The test is conducted in the following manner: 50 mL of test sample is measured into a clean, weighed glass beaker cind placed into a heated evaporation bath maintained at 162°C (325°F). Dry steam heated to 232-246°C (450-475°F) is introduced into the sample at a rate of 1000 mL/s for 30 min. At the end of the evaporation time, the
812 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK
TEOST MHT4-GF3 NEW
•'
.'
-•
/
' . ' • ' .
VERY GOOD
PASS
BORDERLINE PASS
FAIL
TEOST D6335-GF2 NEW
VERY GOOD
n
PASS
««.•••
BORDERLINE PASS
lUK^l
FAIL (b) FIG. 16—(Continued)
CHAPTER
30: OXIDATION
OF LUBRICANTS
AND FUELS
813
TABLE 7—TEOST MHT-4 total deposits. TEOST W t/M g
33 R e f e r e n c e O i l
T E O S T 33 W t/M g
-TotalDeposits
80 -
60 40 20
CRO-l
m CRO-2
CRO-3
CRO-4
hn o
^ ^
^^
^^ • N O IR O N • WITH
IRON
Races and Bearing Set Screw
Shaft Bearing
Pump Housing Teflon Gasl
Cu and Pb Plates
FIG. 49—Aluminum Beaker Oxidation Test (ABOT). Assembly drawing.
beakers are cooled a n d reweighed to determine the unwashed gum, for aviation gasohne and aircraft turbine fuel. For motor gasolines, the residue is washed with n-heptane. The heptane washed residue is reported as existent g u m content. The gums contained in gasolines and fuels can be harmful, causing sticking of the intake valves, and plug carburetor jets. It may interfere with the action of moving parts and, in general, obstruct the flow of fuel through the lines and p u m p . Existent gum is related to the quantity of gum already present in gasoline and fuel (see Fig. 53).
FIG. 50—Aluminum Beal
D 525: Oxidation Stability of Gasoline (Induction Period Method)—This test m e t h o d covers the determination of the oxidation stability of finished gasolines under accelerated conditions. The test is conducted in the following manner: A 50 mL sample of gasoline is measured into a glass sample container and is placed into the pressure vessel. The as-
5/8" Dia Hole Drilied in Insulation 1-3/8" Dia Hole in Insulation Typical for 8
Marinite Board Insulation Cover Plate
Test Block
Fiber Frax Loose Insulatiai Case
1-5/32" Dia Hole Typical 2" Dia Hole Marinite Board Insulation
(a)
•
•y
(b) FIG. 51—a) Caterpillar Micro-Oxidation Test (CiVIOT). Test apparatus side view of cover plate, b) Blotter spots showing acceptable versus unacceptable sludge deposits. Inlet Tube Inlet/Outlet Cap 27 cm
Outlet Tube
4 mm ID 24/40 Ground Glass Joint
Glass Vessel
Metal Catalyst Specimen
(a)
(b) FIG. 52—a) CMOT glass test vessel; b) Test coupon.
814
CHAPTER
30: OXIDATION
OF LUBRICANTS
AND FUELS
815
This thermometer ond thermometer well ore optional Cotton or Gloss Wool Filter Air Supply
Removoble Adopter
if-in, 0.0.
FIG. 53—ASTM D 381, Existent Gums in Fuels. Test apparatus.
sembled pressure vessel is charged with oxygen to a pressure of 690-705 kPa. The pressurized vessel is placed in a heated oil bath, maintained at 98-102°C (208 to 215°F). The pressure is observed and read at stated time intervals until the breakpoint is reached. The breakpoint is described as the point when the pressure drops 14 kPa in 15 min, followed by a drop of not less than 14 kPa in 15 min. The minutes to this break point are reported. The induction period may be used as an indication of the tendencies of the motor gasoline to form gums in storage. It should be noted that its correlation with the formation of g u m in storage may vary under different storage conditions and with different gasolines (see Figs. 54 and 55). D 873: Oxidation Stability of Aviation Fuels (Potential Residue Method)—This test method covers the determination of the tendency of aviation reciprocating, turbine, and jet engine fuels to form gum and deposits under accelerated aging conditions. A summary of the test procedure follows: A weighed glass sample beaker containing 100 mL of test sample is placed into a pressure vessel. The vessel is assembled and charged with oxygen to a pressure of 690-705 kPa. The cheirged pressure vessel is then placed into a heated bath maintained at 100°C (212°F). At the end of the oxidation period, the aged sample is transferred to a graduated cylinder. The glass sample container is washed with "gum solvent," which is a mixture of equal pairts of toluene and acetone. This wash is added to the oxidized fuel, and is filtered through a weighed sintered glass crucible. The crucible is dried and
weighed. The increase in weight will be used as a part of the calculation to determine the gum content. The filtered oxidized fuel gum solvent mixture is divided into two equal portions and submitted to testing under ASTM D 381, Existent Gum in Fuels by Jet Evaporation. The final "soluble gum" is calculated in accordance with the test method. The results of these tests can be used to indicate the storage stability of these fuels. The tendency of fuels to form gum deposits in these tests has not been correlated with actual field performance and varies widely under different storage conditions. D 3241: Thermal Oxidation Stability of Aviation Turbine Fuels (JFTOT Procedure)—This test method covers the procedure for rating the tendencies of gas turbine fuels to deposit decomposition products within the fuel system. The test is conducted in the following manner: This test method for measuring the high temperature stability of gas turbine fuels uses the Jet Fuel Thermal Oxidation Tester (JFTOT). The JFTOT subjects the test fuel to conditions that can be related to those occurring in gas turbine engine fuel systems. The JFTOT can be run to a maximum tube t e m p e r a t u r e of a b o u t 350°C (662°F). The t e m p e r a t u r e at which the test should be run and criteria for judging results are normally embodied in the fuel specification. The fuel is pumped at a fixed volumetric flow rate through a heater after which it enters a precision stainless steel filter where fuel degradation products may become trapped. The apparatus uses 450 mL of test fuel ideally during a 2.5-hour test. The es-
816
MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
^m±n
•I®
II
Hi
NOTE 1—Toterance ± 0.25 mm unless otherwise specified. NOTE 2—Key. A-Bomb B « Bomb closure C - B o m b jtem . 0«Connectioo E - Btitstdisc assembly F-RHerrod
FIG. 54—ASTM 0 525, Potential Gums in Fuels. Test apparatus.
sential data derived are the amount of deposits on the aJuminum heater tube and the rate of plugging of a 17-micron nominal porosity precision filter located just downstream of the heater tube. The test results are indicative of fuel performance during gas turbine operation and can be used to assess the level of deposits that form when liquid fuel contacts a heated surface. The test is rejected if the amount of spent fluid pumped during the test is less than 405 mL. The final result from this test method is a tube color rating based on an arbitrary scale, established for the method, plus two additional yes/no crite-
ria that indicate the presence of an apparent large excess of deposit or unusual deposit, or both. D 2274: Oxidation Stability of Distillate Fuel Oil (Accelerated Method)—This method covers the measurement of inherent stability of middle distillate petroleum fuel under accelerated oxidizing conditions. It is not applicable to fuels containing residual oil, or any significant component derived from a nonpetroleum source. A summary of the method is as follows: A 350 mL volume of filtered middle distillate is aged at 95°C (203°F) in ASTM D 943 glassware for 16 h while oxygen
CHAPTER
COVER PORCeUMN OR OLASS
-Z BUSTS OR DEPRESSIONS
SAMPLE
30: OXIDATION
OF LUBRICANTS
AND FUELS
817
Because the storage periods are long (4-24 weeks), the test method is not suitable for quality control testing, b u t does provide a tool for research on storage properties of fuels, ft should be noted that the results obtained by this test are not necessarily the same as those obtained during storage in a specific storage situation (see Figs. 58 and 59). D 6468: High Temperature Stability of Distillate Fuels—This test method covers relative stability of middle distillate fuels
CONTAINER OLASS CLASS CONDENSER
FIG. 55—ASTM D 525, Potential Gums In Fuels. Glass sample container and cover (glass or porcelain). is bubbled through the fuel sample at a rate of 3L/h. After aging, the sample is cooled to approximately room temperature before filtering the aged fuel through a 0.8-micron filter, to obtain the filterable insolubles quantity. Adherent insolubles are removed from the oxidation cell and associated glassware with tri-solvent (equal volumes of toluene, acetone, a n d methanol). The tri-solvent is evaporated t o obtain t h e quantity of adherent insolubles. The sum of the filterable and inherent insolubles, expressed as milligrams per 100 mL, is reported. This test method provides a basis for the estimation of the storage stability of middle distillate fuels with a n initial boiling point above approximately 175°C (347°F) and a 90% recovery point below 370°C (698°F) such as No. 2 fuel oil. It is not applicable to fuels containing residual oil, or any significant component derived from a nonpetroleum source. The test method may not provide a prediction of the quantity of insolubles that will form in field storage over any given period of time. The amount of insolubles formed in such field storage is subject to the specific conditions, which are too variable for this test method t o predict accurately (see Figs. 56 and 57). D 4625: Distillate Fuel Storage Stability at 43°C (110°F)—This test method covers a method for evaluating the inherent storage stability of distillate fuels having flash points above 38''C (100°F) and 90% distilled points below 340°C (644°F). This method is not suitable for quality control testing, but rather it is intended for reseairch use to shorten storage time relative to that required at ambient storage temperatures. A summary of the method is as follows: Four 100-mL volumes of filtered fuels are aged by storage in borosilicate glass containers at 43°C (110°F) for periods of 0, 4, 8, 12, 18, and 24 weeks. After aging for a selected time period, a sample is removed from storage, cooled to room temperature, and analyzed for filterable insolubles and adherent insolubles. Fuel oxidation and other degredative reactions leading to formation of sediment (and color) are mildly accelerated by the test conditions, compared t o typical storage conditions. Test results have been shown to predict storage stability more reliably thaji other more accelerated tests.
p
«
MM O.D.
P
FIG. 56—ASTM D 2274, Oxidation Stability of Distillate Fuel Oil. Test apparatus.
FILTER
FUNNEL
MEMBRANE
FILTER
SUPPORT
FIG. 57—ASTM D 2274, Oxidation Stability of Distillate Fuel Oil. Filter apparatus.
818
MANUAL 37: FUELS AND LUBRICANTS '^
HANDBOOK
GLASS S T O P P E R
tive relationship exists between the pad rating and the gravametric mass of filterable insolubles. Lubricants
500 ML S E P A R A T C FUNMEL
*4 PORCELAIN CRUCIBLE
OOOCH HOLDER
• TO VACUUM
500 M L P V R E X SUCTION FLASK
FIG. 58—ASTM D 4625, Distillate Fuel Storage Stability at 43°C (110°F). Self feeding filtering assembly.
under high temperature aging conditions with limited air exposure. A summary of the method is as follows: Two 50-mL volumes of filtered middle distillate fuel are aged for 90 or 180 min at 150°C in open glass tubes with air exposure. After aging and cooling, the fuel samples are filtered through a Whatman No. 1 filter. The average amount of filterable insolubles is estimated by measuring the light reflectance of the filter pads. The 100 and 0% extremes of the reflectance rating range are defined by an unused filter pad and a commercial black standard, respectively. This test method provides an indication of thermal oxidative stability of distillate fuels when heated to high temperatures that simulate those that may occur in some types of recirculating engine or burner fuel delivery systems. Results have not been substantially correlated to engine or burner operation. The test method can be useful for investigation of operational problems related to fuel thermal stability. When this method is used to monitor manufacture or storage of fuels, changes in filter rating values can indicate a relative change in inherent stability. The test method uses a filter paper with a nominal porosity of 11 microns, which will not capture all of the sediment formed during aging, but allows differentiation over a broad range. Reflectance ratings eire also affected by the color of filterable insolubles, which may not correlate to the mass of the material filtered from the aged fuel. Therefore, no quantita-
D 6186: Oxidation Induction Time of Lubricating Oik by Pressure Differential Scanning Calorimetry (PDSC)—This method covers the determination of oxidation induction time of lubricating oils subjected to oxygen at 3.5 MPa (500 psig) and temperatures between 130°C and 210°C (265^10°F). Oxidation Induction Time (OIT) is the period of time from the first exposure to an oxidizing atmosphere until the extrapolated on-set time. A summary of the test procedure follows: A small quantity of oil is weighted into a sample pan and placed into a test cell. The cell is heated to a specified temperature and at a regulated temperature and pressure until an exothermic reaction occurs. The extrapolated on-set time is measured and reported as the oxidation induction time for the lubricating oil at the specified test temperature. Oxidation Induction Time (OIT) may be used as an indication of oxidation stability. This test method is faster than other oil oxidation tests and requires a very small amount of sample. This method may be used for research and development, as a quality control, and for specification purposes. However, no correlation has been established between the results of this method and actual service performance. Panel Coker Test—The Panel Coker Test is a method for determining the relative stability of lubricants in contact with hot metal surfaces. The test apparatus consists of a rectangular stainless steel reservoir, inclined 25° from horizontal. The reservoir is fitted with a machined steel piece fitting integrally into the top of the reservoir, and framing a 95 mm by 45 mm aluminum panel. The test panel is held in place by a heating element, which is fitted with thermocouple probes to control the temperature of the aluminum test panel. A horizontal shaft, BOROSILICATE GLASS BEND
CAP WITH PTFE INSERT
500 ml. BOROSILICATE ' GLASS BOTTLE
FIG. 59—ASTM D 4625, Distillate Fuel Storage Stability at 43°C (110°F). Sample storage container.
CHAPTER fitted with a series of tines, is positioned above tlie oil and is rotated at 1000 rpm. During rotating of tlie shaft, the tines sweep through the test lubricant and lubricant droplets are thrown onto the heated aluminum test panel. A summary of the procedure follows: The apparatus is assembled forming a closed system and the test panel is heated to the specified test temperature, usually 260-325°C. The shaft is rotated at 1000 r p m and the test lubricant is splashed onto the heated, weighted test panel. The splashing heating cycle is continued for normally 2-6 h, from the time the splasher is started. Some other variables in the p r o c e d u r e w o u l d b e the i n t r o d u c t i o n of air, a n d the splash/bake cycle. At the end of the specified time period, the heat is removed, the splasher is stopped, and the test panel is allowed to cool. The test panel is reweighed and the a m o u n t of deposit is determined. Weight gain of the aluminum test panel and the amount of test lubricant consumed during the test are an indication of the lubricant's performance under high temperature conditions. Examples of accepted performance are as follows: • Gear Oils: leaded type give deposits in the range of 700-1000 mg, in contrast to sulfur/phosphorus t5T3es which give 150-220 mg.
S/P Lub*
174 mg Wt Cain
30: OXIDATION
OF LUBRICANTS
AND FUELS
819
• Quench Oils: Non-thermally stable chemistries give 20-25 m g deposits, with contrast to thermally stable chemistries that give 1-2 mg deposits. • Diesel Lubricants are normally rated per the universal rating system where 100% is best and 0% worst. Better than 65% is considered acceptable in this test. (See Fig. 60.) ft is important to note that the temperature, splash rate, time cycle, and metallurgy of the test panel can vary depending on the tjrpe of lubricant being evaluated. D 37H: Deposition Tendencies of Liquids in Thin Films and Vapors—This test m e t h o d covers the determination of the tendency of liquids in thin films and of vapors to form deposits on heated metal surfaces. This m e t h o d applies to petroleum based and synthetic lubricants, engine oils, hydraulic fluids, heat-transfer fluids, and related materials. The test is conducted in the following manner: One hundred milliliters of test lubricant is allowed to flow slowly (50 mL/hour) in a thin film over a steel test specimen in a constant temperature chamber (furnace liner). A test temperature of 533 K has been found useful for evaluating the thin film deposit characteristics of engine oils, and similar petroleum based lubricants. Higher temperatures may be
L**d«
FIG. 60—Panel Coker Test. Typical samples.
820
MANUAL 3 7; FUELS AND LUBRICANTS
HANDBOOK
used for sjTithetic materials and highly refined petroleum fluids if desired. Circulation of the sample from the sump to the heated surface and back to the pump is accomplished by means of a peristaltic pump. After the prescribed test period, normally 5 h, the steel test specimen is removed from the apparatus and evaluated. The masses of deposits remaining after washing with pentane, after washing with chloroform, and after wiping with a tissue, are reported. An optional procedure provides a method for the determination of the tendency of sample vapors to form deposits on heated surfaces. A second test specimen is placed in the vapor space over a thin flowing film of the liquid in the constant temperature chamber. After circulation of the test liquid, the deposits on the test specimen exposed to the liquid and vapor phases, cire measured as above. The test method shall measure the deposit formation tendencies of liquid petroleum products on heated steel surfaces, in air, at 101.3 kPa (10 atm) pressure. Other surfaces and other atmospheric media may be substituted for steel and air at 1 atm, provided the substitution is noted in the test report. RULER (Remaining Useful Life Evaluation Rig)—This method is not an ASTM standard. It is under evaluation within an ASTM technical committee, but it has not received the approvals required to become an ASTM standard. The RULER was originally designed for use by the U.S. Air Force to determine the remaining useful life of lubricants. The remaining useful life of a lubricant is the length of time from the original lubricant sampling, until large changes in the lubricant's physico-chemical properties occur. This test can be performed with any lubricant containing at least one antioxidant species. The test is conducted as follows: The test vial (electrolytic cell) is prepared by mixing an oil sample with a solvent and a solid substrate. The solvent separates the antioxidant from the oil as the vial is shaken. When
C/) Q.
the substrate settles to the bottom of the vial, the oil and other debris common to used oil adheres to the substrate particles. An electrode is inserted into the vial and a controlled voltage ramp is applied to the electrode inserted into the diluted oil sample. As the voltage potential increases, the antioxidants become more chemically active, causing the increasing oxidation current to reach a peak, and then decrease as voltage potential continues to increase. Through time-series testing, the remaining useful life of a lubricant can be tracked from test to test, enabling the user to identify "normal" trends for a given lubricant. Variations from this trend can be indicative of changes in operating conditions causing accelerated oxidation. Determinations can be made to predict when rapid changes in the lubricant are likely to occur and decisions can be made regarding oil changes of additive reinforcement. This method can be used to determine the remaining useful life of automotive engine oils, diesel oils, hydraulic fluids, turbine engine lubricants, transmission fluids, and greases (See Figs. 61 and 62). Biodegradable
Lubricants
AOCS Official Method Cd 12-57: Fat Stability, Active Oxygen Method (AOM)—This method measures the time required for a sample of fat or oil to attain a predetermined peroxide value under the specific conditions of the test. The test is conducted as follows: A 20 mL of sample is measured into a cleaned glass test tube. An aeration tube assembly is placed into the test tube, and the assembled apparatus is placed into a bath of boiling water for 5 min. The assembly is removed from the boiling water bath, wiped dry, and placed into a heated bath maintained at 97.8°C (208.8°F). Air is blown into the sample at a rate of 2.33 mL per second. This is continued until the sam-
Compound "B" Phenyl - a - naphylamine
Linear Ramp (Voltage)
E CO
CD
o Voltage increases with time FIG. 61—RULER test. Oxidation curve for the RULER test.
CHAPTER
30: OXIDATION
OF LUBRICANTS
AND FUELS
821
Hours: 25 Sample ID: GO 1 Sample Date: 5/06/98 Standard: 60&
/Sample
/Additive
5
6
7
8
9
10
11 12 13 14 15 16
17
Seconds (V lUlode) Ruler Numbers Standard: 552 Sample: 441 RUL: 80%
Ruler Areas Standard: 7451 Sample: 6294 RUL: 84%
Additive RULs
#1:85% #2: 77%
FIG. 62—RULER test.
pie reaches a peroxide value of 100 milliequivalents. The peroxide value is determined by method AOCS official method Cd 8-53. The length of this period of time is assumed to be an index of resistance to rancidity. The exact relationship between peroxide value and such queJities as shelf-life, actual rancidity, and oxidative stability has not been firmly established. AOCS Official Method Cd 126-92: Oil Stability Index (OSI)— This method measures the oxidation resistance of oils and fats. Initially the oxidation proceeds slowly until the oxidation resistance is overcome, at which point the oxidation rate accelerates a n d becomes very rapid. The length of time before this rapid oxidation occurs is commonly referred to as the "induction period." The test is conducted as follows: A sample of oil or fat is heated at the specified test temperature, and a stream of purified air is bubbled through the sample at a rate of 2.5 mL/s. The effluent air from the sample is then bubbled through a test tube containing 50 mL of deionized water. The conductivity of the water is continually monitored. The effluent air contains volatile organic acids that increase the conductivity of the water as the oxidation proceeds. The conductivity of the water is monitored by computer or strip chart recorder. The Oil Stability Index (OSI) is defined as the point of m a x i m u m change of the rate of oxidation. The test time is normally a m i n i m u m of 4 h or a maximum of 15 h. The OSI may be r u n at temperatures of 100, 110, 120, 130, and 140°C.
This method is applicable in general to all fats and oils and has been subjected to collaborative studies covering a wide range of sample tj^es. DIN 51554: Test of Susceptibility to Ageing According to Baader—The Baader ageing test is an accelerated oxidation test enabling the probable in-service behavior of various lubricants to be predicted. The Baader test was developed to evaluate mineral oil based hydraulic fluids. However, today it has found wide acceptance in predicting the performance of biodegradable hydraulic fluids. Both vegetable oil (triglyceride) and synthetic ester based fluids are evaluated. The test is conducted in the following manner. Three hundred millimeters of freshly cleaned copper wire is wound into a coil per the procedure. The coil is then placed into the glass test tube containing 60 mL of test fluid. A Liebig condenser is fitted to the glass test tube. The assembly is then placed into the heated oil bath. The catalyst coil is attached to a mechanical lifting device that raises and lowers the catalyst coils into and out of the test fluid at a rate of 25 cycles/min. A cycle is understood to be one full upward and downward movement, during which the catalyst coil must be alternately totally positioned for identical periods of time in the test fluid and in the air. The test conditions are 140 h at 110°C for insulating oils and synthetic ester hydraulic fluids, and 72 h at 95°C for mineral oil hydraulic fluids and vegetable based hydraulic fluids. At the end of the ageing period, the viscosity of the aged fluid is determined and compared to the original fluid viscosity. Percent viscosity increase at 40°C is reported.
822 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK TABLE 8—Oxidation stability of the test fluids.
Fluids ABOT Test (300 h, 155°C) (requirement*) TAN Increase (3.5 m£ix) IR Carbonyl Increase (30 max) % Viscosity Increase at 40°C (25 max) % Pentane Insolubles Increase (0.35 max)
IT-1
CAT-1
CAT-2
PAT-1
PAT-2
Industrial Traction Fluid
Commercial Automotive Traction Fluid 1
Commercial Automotive Traction Fluid 2
Prototype Automotive Traction Fluid 1
Prototype Automotive Traction Fluid 2
0.50
0.9
8
20
14.4
9.2
0.20
1.4
0.6
13
7.5
13
22
16.4
13.1
0.09
4C, flaking
Copper Strip
2.4
3B
Prototype Automotive Traction Fluid 4
1.2
2.4
12 8.7 0.07
3A
3B
3B
PAT-4
PAT-3 Prototype Automotive Traction Fluid 3
3B
35 19.7 1.73
3B
are for biodegradable fluids and require less than a 20% increase in viscosity to be considered acceptable. Table 8 represents the relative oxidative stability of various biodegradable fluids as measured by their change in viscosity before and after being subjected to the Baader test (see Fig. 63).
TEST METHODS Hydraulics, R & O, and Turbine Oils D943 Olive DIN
12215-08
D4310 D2272
Ground taper NS 45127 in accordance witfi DIN 12242
IP-280 IP-306 IP-307 D2070 D4636 D5846 D6514
Tolerances for ground taper corresponding to NS 45/40 in accordance with DIN 12 242
DIN 51352
Gear Oils D2893
Test apparatus according to Baader FIG. 63—Baader Oxidation Test apparatus. Dimensions of tine glassware. Though not required by any of the current specifications, saponification number, % insolubles (sludge), and the dissipartion factor are sometimes determined. There are currently two European specifications requiring the Baader test. They are the Swedish ECO standard, SS 155434, and the ISO DIS 15380. Both of these specifications
Oxidation Characteristics of Inhibited Mineral Oils Determination of Sludging and Corrosion Tendencies of Inhibited Mineral Oil Oxidation Stability of Steam Turbine Oils by Rotating Pressure Vessel (formerly Rotary Vessel) Oxidation Stability of Inhibited Mineral Turbine Oils Oxidation Stability of Straight Mineral Oils Oxidation Stability of Mineral Insulating Oils Thermal Stability of Hydraulic Oils Corrosiveness and Oxidation Stability of Hydraulic Oils, Aircraft and Turbine Engine Lubricants and Other Highly Refined Oils Universal Oxidation Test for Hydraulic Fluids and Turbine Oils High Temperature Universal Oxidation Test For Turbine Oils Pneurop Oxidation Test
D5763 D5704
nS-K2514 L-48-08
Oxidation Characteristics of Extreme Pressure Lubricating Oils Oxidation and Thermal Stability Characteristics of Gear Oils Using Universal Glassware Evaluation of the Thermal and Oxidative Stability of Lubricating Oils Used for Manual Transmissions and Final Drive Axles L-60 and L-60-1 Tests Oxidation Stability of Automotive Gear Oils (ISOT Method) Test Method for the Oxidation Stability of Lubricating Oils Used in Automotive Transmissions by Artificial Aging (DKA Method).
CHAPTER 30: OXIDATION OF LUBRICANTS AND FUELS 823 Grease D942 D5483
D3527
Oxidation Stability of Lubricating Greases by the Oxygen Vessel Method Oxidation Induction Time of Lubricating Greases by Pressure Differential Scanning Calorimetiy (PDSC) Penn State Micro-Oxidation Test (PSMO) Life Performance of Automotive Wheel Bearinjg Grease
Engine Oils D4742 D6335
BT-10
Thin Film Oxygen Uptake Test (TFOUT) Thermo Oxidation Engine Oil Simulation Test (TEOST) (MHT-4 Protocol) Determination of High Temperature Deposits by Thermo-Oxidation Engine Oil Simulation Test (TEOST) Ford, Mercon V Aluminum Beaker Oxidation Test (ABOT) Caterpillar Micro-Oxidation Test (CMOT) International Harvester Oxidation Corrosion Test
Gasolines and Fuels D381 D525 D873 D3241 D2274 D4625 D6468
Existent Gums in Fuels by Jet Evaporation Oxidation Stability of Gasoline (Induction Period Method) Oxidation Stability of Aviation Fuels (Potential Residue Method) Thermal Oxidation Stability of Aviation Fuels (JFTOT Procedure) Oxidation Stability of Distillate Fuel Oil (accelerated method) Distillate Fuel Storage Stability at 43°C (110°F) High Temperature Stability of Distillate Fuels
General Lubricants D6186 D3711
Oxidation Induction Time of Lubricating Oils by Pressure Differenticil Scanning Calorimetry (PDSC) Panel Coker Test Deposition Tendencies of Liquids in Thin Films and Vapors RULER (Remaining Useful Life Evaluation Routine)
VW1302 Test VW Diesel intercooler test VW DI engine test Peugeot XUDl lATE and XUDUBTE M-B OM 364A and M-B OM 441LA MAN 5305 tests
Engine Tests For Japanese Oils Toyota IG-FE Nissan VG 20E Nissan SD22 Nissan TD25 OMC 40-HP and 70-HP for two-stroke cycle engine oils
CROSS REFERENCE OF TEST METHODS ASTM D381 D525 D873 D942 D943 D2070 D2272 D2274 D2893 D3241 D3527 D3711 D4310 D4625 D4636 D4742 D5483 D5704 D5763 D5846 D6186 D6335 D6468
IP 131 40 138 142 157
DIN 51780 51575 51808 51587
ISO 6246
4263
388 51586
280 306 307 51352 51554
15380
Biodegradable Lubricants AOCS Cd 12-57 AOCS Cd 12b-92 DIN 51554
Fat Stability, Active Oxygen Method (AOM) Oil Stability Index (OSI) Baader Oxidation Test
The following listing of engine tests is included for reference only and outlines of these methods are not included in this chapter.
Engine Tests For North American Engine Oils ASTM Sequence IIIE/F ASTM Sequence VE/G CRC L-38 Caterpillar IK/IN/IM-PC/IP Cummins M-11 Mack T-6/T-7/T-8/T-9
Engine Tests for European Engine Oils Peugeot TU-3M high temperature test M-B M i l l black sludge test
REFERENCES [1] Ingold, K. U., "Inhibition of Autoxidation of Organic Substances in Liquid Phase," Chemical Reviews, Vol. 61, 1961, pp. 563-589. [2] Rasberger, M., "Oxidative Degradation and Stabilization of Mineral Oil Based Lubricants," Ch. 4, Chemistry and Technology of Lubricants, R. M. Mortier and S. T. Orszulik, Eds., VCH Publishers, Inc., NY, 1992, pp. 83-123. [3] Rizvi, S. 0. A., "Lubricant Additives and Their Functions," ASM Handbook, Friction, Lubrication and Wear Technology, Vol. 18, 1992, pp. 98-112. [4] Abou El Naga, H. H. and Salem, A. E. M., "Effect of Worn Metals on the Oxidation of Lubricating Oils," Wear, Vol. 96, 1984, pp. 267-283. [5] Lachowicz, D. R. and Kreuz, K. L., "Peroxynitrates. The Unstable Products of Olefin Nitration with Dinitrogen Tetroxide in the Presence of Oxygen. A New Route to a-Nitroketones," Journal of Organic Chemistry, Vol. 32, 1967, pp. 3885-3888. [6] Kreuz, K. L., "Gasoline Engine Chemistry as Applied to Lubricant Problems," Lubrication, Vol. 55,1969, pp. 53-64. (b) Kreuz,
824 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK K. L., "Diesel Engine Chemistry as Applied to Lubricant Problems," Lubrication, Vol. 56, 1970, pp. 77-88. [7] Johnson, M. D., Korcek, S., and Zinbo, M., "Inhibition of Oxidation by ZDTP and Ashless Antioxidants in the Presence of Hydroperoxides at 160°C," Lubricant and Additive Effects on Engine Wear, SP-558, pp. 71-81, Fuels and Lubricants Meeting, San Francisco, 31 Oct.-3 Nov. 1983. [8] Al-Malaika, S., Marogi, A., and Scott, G., Journal of Applied Polymer Science, Vol. 33, 1987, pp. 1455-71. [9] Vijh, A. K., "Electrochemical Mechanisms of the Dissolution of Metals and the Contaminants Oxidation of Lubricating Oils Under High-temperature Friction Conditions," Wear, Vol. 104, 1985, pp. 151-158. [10] Klamann, D., "Additives," Ch. 9, Lubricants and Related Products: Synthesis, Properties, Applications, and International Standards, Verlag Chemie, Weinheim, Germany, 1984. [11] Scott, G., Developments in Polymer Stabilization, G. Scott, Ed., Elsevier Applied Science Publishers, NY, 1984, pp. 65-104. [12] Scott, G., "New Developments in the Mechanistic Understanding of Antioxidant Behavior," Journal of Applied Polymer Science, Applied Polymer Symposium, Vol. 35, 1979, pp. 123-149. [13] Hamblin, P. C , Kristen, U., and Chasan, D., "A Review: Ashless Antioxidants, Copper Deactivators, and Corrosion Inhibitors, Their Use in Lubricating Oils," Lubrication Science, Vol. 2, 1990, pp. 287-318.
BIBLIOGRAPHY [1] Aspects of Lubricant Oxidation, ASTM STP 916, Stadmiller and Smith, Eds., ASTM International, West Conshohocken, PA, 1986. [2] Significance of Tests for Petroleum Products, ASTM STP 7C (now ASTM Manual 1), Boldt and Hall, Eds., ASTM International, West Conshohocken, PA, 1934. [3] American Society of Metals (ASM) Handbook, Volume 18, P. J. Blau, Volume Chairman, American Society of Metals, Materials Park, OH, 1992. [4] Lubrication Engineers Manual, Association of Iron and Steel Engineers (AISE), Pittsburgh, PA, 1996.
[5] Standard Handbook of Lubrication Engineering, J. J. O'Connor and J. Boyd, Eds., McGraw Hill, NY, 1968. [6] Annual Book of ASTM Standards, Section 5, Petroleum Products, Lubricants, and Fossil Fuels, Vols. 05.01, 05.02 and 05.03, ASTM International, West Conshohocken, PA, 2000. [7] Standard Methods for Analysis and Testing of Petroleum and Related Products Institute of Petroleum, Vols. 1 and 2, Institute of Petroleum, London, 2002. [8] Ingold, K. U., "Inhibition of Autoxidation of Organic Substances in Liquid Phase," Chemical Reviews, Vol. 61, 1961, pp. 563-589. [9] Rasberger, M., "Oxidative Degradation and Stabilization of Mineral Oil Based Lubricants," Ch. 4, Chemistry and Technology of Lubricants, R. M. Mortier and S. T. Orszulik, Eds., VCH Publishers, Inc., NY, 1992, pp. 83-123. [10] Rizvi, S. Q. A., "Lubricant Additives and Their Functions," ASM Handbook, Friction, Lubrication and Wear Technology, Vol. 18, American Society of Metals, Materials Park, OH, 1992, pp. 98-112. [11] Rasberger, M., Stroud, P. M., Mortier, R. M., Orszulik, S. T., Hoyes, T. J., a n d Brown, M., "Oxidative Degradation and Stabilization of Mineral Oil Based Lubricants," Chemistry and Technology of Lubricants, R. M. Mortier and S. T. Orszulik, Eds., VCH Publishers, Inc., NY, 1992. [12] Lansdown, A. R., Lubrication and Lubricant Selection, Mechanical Engineering Publications, Anthony Rowe, Ltd., Chippenham, Wihshire, 1996. [13] Heinz, P. and Block, P. E., Practical Lubrication for Industrial Facilities, Fairmont Press, Inc., Lilbum, GA, 2000. [14] Cincinnati Machine Book of Lubricant Purchase Specifications, Pub. # 10-SP- 00038, Part # 9703432212A, CM Worldwide Support Services Group, Cincinnati, OH, 2000. [15] Florkowski, D. W. and Selby, T. W., "The Development of a Thermo-Oxidation Engine Oil Simulation Test (TEOST)," SAE # 932-837, Society of Automotive Engineers, Warrendale, PA, 1993. [16] Tipton, C. D., Qureshi, F., and Huston, M., "Automotive Traction Fluids: A Shift in Direction for Transmission Fluid Technology," SAE #2000-01-2906, Society of Automotive Engineers, Warrendale, PA, 2000.
MNL37-EB/Jun. 2003
Corrosion Maureen E. Hunter^ and Robert F. Baker^
P E R A S T M G 15, CORROSION IS "THE CHEMICAL OR ELECTRO-
CHEMICAL REACTION between a material, usually a metal, and its environment that produces a deterioration of the material and its properties." Corrosion is one of the causes of premature failure of a lubricated system. Fuels and lubricants will be exposed to water, one of the necessary components for electrochemical corrosion to occur. This chapter will outline the nature of ferrous and non-ferrous corrosion and the comm o n additive approach to inhibiting corrosion and will summarize the c o m m o n tests employed to evaluate the effects and prevention of corrosion. There are fluid performance properties sometimes referred to as "corrosion," such as fretting and fatigue failure, which are not electrochemical. They are better characterized as a form of wear and are not covered in this chapter. These properties and the tests for them are discussed in Chapters 35, 36, and 37 on Lubrication and Wear.
BACKGROUND Corrosion becomes a problem when the appearance or functionality of metal is impaired. To some extent, corrosion is inevitable and may not be prevented, only delayed or retarded. Chemical "inhibitors" are added to fuels and lubricants to protect metal surfaces. The metcJs requiring protection are typically iron and its alloys (steel) and copper and its alloys (primarily brass and bronze). Other metals, including aluminum, lead, tin, zinc and cadmium, come in contact with fuels and lubricants and are subject to veirious forms of corrosion, but most fuel and lubricant testing focuses on the corrosion of copper and the oxidative corrosion of iron, commonly called "rust."
• Electrolyte—a wet p a t h connecting the anode and cathode (generally aqueous). • Conductor—a metal bridge that electrically connects the anode and cathode to complete the circuit. The rust corrosion cell is very similar to a battery-energized electrical circuit. Figure 1 shows the flow of electrons in an external circuit between an anode ( - ) and a cathode (-I-) and the departure of ions from the anode in a simple wet battery cell. The mechanism of oxidative corrosion of iron is depicted in Fig. 2 where electrons flow from the anode to the cathode and ions (positively charged iron atoms) leave the anode surface and enter the electrolyte as ferrous ions. This degradation of the anode surface coupled with hydration and further oxidation of the ferrous ions is the mechanism known as rusting. Chemistry
of
Rust
Even small differences in electrical potential can cause the formation of reactive cinodic sites. This can be the result of surface imperfections, grain boundaries in alloys, temperature gradients, physical stress, and electrochemical polarization [1]. The formation of these sites causes electrons to leave the anode at the electrical contact with the cathode. The departing electrons turn the iron atoms into ferrous ions at the anode-electrolyte interface resulting in disintegration of the anode: F e ^ F e + + + 2e
The ferrous ions react with water to form ferrous hydroxide: Fe++ -H 2H2O -> Fe(0H)2 + 2H+
(3)
The combined anode equation is obtained by adding the above equations:
Electrochemical Nature of Rust Formation As indicated above, the rusting of iron and its alloys is an electrochemical process. There are four essential elements required for the electrochemical process to occur. The elimination of any one of the four prevents the formation of the electrical cell and inhibits corrosion. • Anode—a site where oxidation of a metal generates positively charged metal ions and electrons. • Cathode—a site where reduction of oxygen and hydrogen in water consumes electrons.
4Fe + 4H2O + 0 2 - ^ 2Fe203 -I- 8H+ + 8e
(4)
The electrons migrate from the anode towards the cathode and at the cathode-electrolyte interface they react with oxygen and water forming hydroxide ions: O2 + 2H2O + 4e ^ 4 0 H ~
(5)
The result of combining Eqs 4 and 5 is 4Fe + 8H2O + 3O2 -^ IFCiOi + 8H2O
(6)
Note that in Eq 6 water is conserved and only oxygen and iron are consumed. It is important to recognize, therefore.
' Technical Service Manager and National Accounts Manager, respectively. King Industries, Inc., Norwalk, CT 06852. 825 2003 by A S I M International
(2)
The ferrous hydroxide reacts with oxygen to produce ferric oxide (the commonly recognized form of rust) and water: 4Fe(OH)2 + 0 2 ^ 2Fe203 + 4H2O
Copyright'
(1)
www.astm.org
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->• electron flow
catalyst metals for signs of corrosion, deposits, discoloration, and/or weight changes.
External Circuit
Surface MX+ N
MX+ MX+ _ electron flow
Cathode
Anode
FIG. 1—Typical electrochemical cell.
H,0
H,o
Cathodic Site
Cathodic Site 0,
F
O
«^o
«^« X 0
o OH
-
HjO
Fe++ electrons
Fe
®^
1
Fe/
^ Fe++ Fe++ Fe
Fe Fe
Activity
Inhibitors
electrons
•<
*•
Anodic Site Iron oxide layer is porous and non-passivating FIG. 2—Mechanism of rust formation. that once a corrosion cell has been established, the formation of rust is maintained without the further addition of water. Only oxygen is required for rust to propagate. The rusting of iron and steel is destructive to engines, machinery, equipment, and containers contacting fuels and lubricants. Additionally, in a lubricated system, ferrous ions are recognized to have a catalytic pro-oxidant effect on the degradation of the lubricant [2], so the rust inhibitor also assists in prolonging the life of a lubricant by the inhibition of oxidation. Copper
of Corrosion
There are numerous strategies that can be used to inhibit the formation of a corrosion cell, including material selection and design [3]. All involve eliminating at least one of the four elements required for the electrochemical process to occur. Tjrpically, additives are incorporated into fuels a n d lubricants to prevent water a n d oxygen from interacting with the metal surface. These additives consist of polar compounds commonly known as rust inhibitors (for iron and iron alloys only) or corrosion inhibitors (for other metals and/or iron). The chemistries that are effective rust inhibitors are genercdly hydrophobic polar compounds that function as surfaceactive agents. The structure of these polar compounds, as represented in Figs. 3 a n d 4, consists of a "head," which h a s a strong affinity for the metal surface a n d a hydrocarbon "tail," which orients itself away from the surface in the bulk of the fuel or lubricant. Figure 3 illustrates how these molecules m a y form a tightly adsorbed monolayer t h a t inhibits water and oxygen from reaching the metal surface. When water (the electrolyte) and oxygen are excluded, the corrosion cell is incomplete, a n d the formation of rust is prevented. Figure 4 shows the c o m m o n chemistries for the polar "head" part of the molecules for ferrous corrosion (rust) in-
HjO O,
O,
O,
FIG. 3—Polar mechanism for rust inhibition.
Corrosion
Copper and its alloys will also experience corrosion similar to the oxidation process of iron described above. Copper corrosion tends not to be as destructive, cJthough pitting can occur, but discoloration occurs easily. Any sulfur present in the fuel or lubricant will also participate in the copper corrosion chemistry, forming copper sulfate on the surface. In addition, dissolved copper ions, especially at concentrations between 100 and 300 ppm, are known to have a catalytic pro-oxidant effect on the degradation of lubricants [2]. This can also be recognized by the review of the oxidation test procedures where iron a n d copper are routinely added as catalysts to accelerate oxidation of a fluid. Since oxidation and corrosion are significantly linked, the criteria for several of the oxidation tests mentioned in the previous chapter (ASTM D 2070, D 4310, and D 4636) include the examination of the
Carboxylic Acid
-CO2-H+
Carboxylic Acid Ester
-CO2R
Carboxylate
-CO2M+
Sulfonate
-SO3-M+
Imidazoline Amine
,N ->__/
-NH2
FIG. 4—Common rust inhibitor chemistries.
CHAPTER 31: CORROSION hibitors. Figure 5 shows c o m m o n chemistries for inhibiting copper corrosion, generally different from the ferrous inhibitor chemistries. The ferrous and non-ferrous chemistries cire frequently added together to achieve a combined effect. Depending upon the application and performance tests required, typiccd rust and corrosion inhibitor additive levels in liquid lubricants and functionzil fluids range from 0.05-0.5%. Additive levels in greases typically range from 0.05-2.0%. See Chapter 9 for further discussion of additive chemistries.
HS^S SH \\ //
HS^^S SSR \\ // N-N
N-N Dimercaptothiadiazole (DMTD)
DMTD Derivative
827
CORROSION TESTS The test methods and practices most commonly used to measure performance and/or specify the requirements for lubriCcints to resist rust emd corrosion will be covered in this section. The significemce of each test will be discussed and a brief summary of the test procedure will be given. The rust and corrosion methods and procedures eire outlined in Tables 1 and 2. Generally the procedures fall into four categories: fuels, automotive lubricants (including engine and gear oils), industrial (non-automotive) lubricants, and greases. Greases are separated because they have unique tests and they do not distinguish between automotive and non-automotive applications. The ability to resist rust and corrosion is an important lubricant characteristic; however, fuels are generally not evaluated for anti-rust. In some fuel specifications, copper corrosion testing is required. Fuels and Lubricants
H,C
ASTM D 130, Standard Test Method for Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test
H,C Tolyitriazoie Tolyitriazoie Derivative
FIG. 5—Common yellow metal deactivator chemistries
The ASTM D 130 test method is used to detect the corrosivity of petroleum products to yellow metals. This test is widely used for R&O, turbine, hydraulic and gear oils where non-
TABLE 1—Corrosion specifications and test methods for fuels and lubricants. ASTM D 130
Common Name Copper Strip Corrosion
Fuels D910
Description Copper corrosion test widely used for fuels and lubricants
Related Procedures" DIN 51 759 IP 154
Standard specification for aviation gasolines Standard specification for diesel fuel oils
D975 Automotive Engine OUs D4485 D5844
Sequence IID
D6557
Ball Rust Test
Heavy Duty Diesel Engine OUs D 5968 Cummins Bench Test
Standard specification for performance of engine oils Stationary engine rust test for automotive engine oils Bench rust test for automotive engine oils Corrosion test for copper, lead, tin and bronze components
Gear Oils CRC L-33 Gear Oil Corrosion Test Industrial Fluid Lubricants
Rust test for gear oils subjected to water
FTMS791B(M5326.1)
B 117
Salt Spray (Fog)
DIN 50 021
D665
Spindle Test
D 1748
Humidity Cabinet
D2070 D3603 D4310
Cincinnati Milacron Thermal Stability Test Horizontal Disk Test 1000 Hours Sludge Test
D4627
Iron Chip Test
D4636
Oxidation-Corrosion Test
ASTM practice defines conditions, not evaluation Standard rust test for most industrial fluids Rust test for temporary coatings ThermEil stability test of oils in presence of copper and steel Rust test for steam turbine oils Sludging and corrosion test for oils (variation of D 943 oxidation test) Rust test for water dilutable metalworking fluids Corrosion test for various metals
' Test procedures may not be identical but evaluate similar characteristics.
DIN 51 585 IP 135 DIN 51 359 IP 366
DIN 51 360 IP 287 DIN 51 394 FTM 5307.2
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TABLE 2—Corrosion test methods for greases. ASTM Greases D 1743
Bearing Rust Test
D4048
Copper Strip Corrosion
D5969
Bearing Rust Test with Sea water Emcor Rust Test
D6138
Description
Related Procedures"
Tapered roller bearing test with distilled water Copper corrosion test (D 130 for greases) D 1743 modified to use synthetic sea water Double row ball bearing test with various types of water
DIN 51 802 IP 220
Common Name
" Test procedures may not be identical but evaluate similar characteristics. TABLE 3 --Copper strip Classification
Appearance Designation
Color Descripti on
Slight tamish Moderate tamish
Dark tamish Corrosion
ferrous bushings, gears, and bearings are employed as well as for cutting fluids used in the machining of non-ferrous metals. A three-inch copper strip is prepared by cleaning and polishing all sides so that n o discoloration or blemishes are visible. The strip is then placed in the test oil and held for a specified time at a specified temperature. Three hours at 100°C is the typical starting point. At the end of the exposure period, the strip is removed, wiped clean and matched with colored reproduction strips characteristic of the descriptions provided in Table 3. Classification results are reported along with the duration of the test and the test temperature. Fuels ASTM D 910 and D 975, Standard Specification Aviation Gasolines and Standard Specification Diesel Fuel Oils
for for
These two standards define limits of acceptability for two fuel types. They are employed primarily for purchasing specifications to allow agreement between a buyer and a seller. The specifications do not provide for any rust inhibiting requirements, but both use ASTM D 130 to define acceptable levels of copper corrosion. A u t o m o t i v e E n g i n e Oils ASTM D 4485, Standard Specification Performance of Engine Oils
classifications.
of
ASTM D 4485 is not a procedure, but the "specification covers engine oils for light-duty and heavy-duty internal com-
a = Light orange, almost the same as the freshly polished strip b = Dark orange a = Claret red b = Lavender c = Multicolored with lavender blue or silver, or both, overlaid on claret red d = Silvery a = Brassy or gold a = Magenta overcast on brassy strip b = Multicolored with red and green (peacock), but no gray a = Transparent black, dark gray or brown with peacock green barely showing b = Graphite or lusterless black c = Glossy or jet black bustion engines used under a variety of operating conditions in automobiles, trucks, vans, buses and off-highway farm, industrial, and construction equipment." If refers to the actual test methods, both engine tests and bench tests, used to define the various American Petroleum Institute (API) performance categories. Rust and corrosion requirements are written into the API performance categories to be run according to the standard test methods ASTM D 130, D 5844, D 5968, and D 6557. ASTM D 5844, Standard Test Method for Evaluation of Automotive Engine Oils for Inhibition of Rusting (Sequence IID) This test was designed to evaluate the rust inhibiting characteristics of engine oils, particularly under service conditions of short trips in winter conditions. The formation of corrosion products in engines will interfere with any closetolerance parts and contribute to the overall engine wear. This procedure has been correlated with vehicles using leaded gasoline prior to 1978. It is, however, still employed in the current engine oil specifications, including API Category SJ. This test uses a 350-in^ (5.7-L displacement) Oldsmobile V-8 engine that is mounted on a test stand and connected to a dynamometer. After the engine is completely disassembled, cleaned, and reassembled prior to each test, it is run under specified operating conditions for a total of 32 h. The engine is disassembled a n d specified p a r t s (including the valve lifters, plungers, balls, and pushrods) are examined for evidence of rust and corrosion. Results are reported according to the rating criteria enumerated in the test method.
CHAPTER ASTM D 6557, Standard Test Method of Rust Preventive Characteristics of Engine Oils
for Evaluation Automotive
This bench test was designed to replace ASTM D 5844, both because the engine test was costly and because it was only correlated with vehicles using leaded fuel in a less-than-modem engine. However, this test has not replaced D 5844 in the API service categories, although it provides a less costly screening test. ASTM D 6557 uses a Ball Rust Test (BRT) procedure that subjects metal test specimens to an acidic solution in test tubes. The results have been shown to correlate reasonably well with D 5844 and therefore indicate the ability of an oil to prevent or resist the formation of rust under short-trip conditions where the engine tends to build u p corrosive acids. Multiple test tubes, each containing test oil and a carbon steel ball, are placed in a mechanical shaker. Air and an acid solution (consisting of HCl, HBr and acetic acid) are continuously fed into each test tube for 18 h. The balls are then rinsed and analyzed by an opticcil imaging system designed to qucmtify the amount of rusting. The anti-rust capability of the test oil is generally compared to the results of a reference oil r u n simultaneously. H e a v y Duty D i e s e l E n g i n e Oils ASTM D 5968, Standard Test Method for of Corrosiveness of Diesel Engine Oil
Evaluation
The ASTM D 5968 test method is used to determine the tendency of diesel engine lubricants to corrode various metals, specifically alloys of lead and copper commonly used in cam followers and bearings. This test method is similar to the Cummins Bench Corrosion Test and is based on Federal Test Method Standard 791, Method 5308. Four clean, polished, pre-weighed metal coupons of copper, lead, tin, and phosphor bronze are tied together and placed in a test tube containing 100 ml of test oil. The test tube is heated in a bath to 121°C and air is delivered to the system at a rate of 5 L/h. The test is r u n for 168 h. An industrial reference oil is evaluated with each group of tests to verify test acceptability. Upon completion of the test, the following results are reported: • the raw data of the calibration and the analysis of the reference oil • concentrations of copper, lead and tin in the oil before and after adjustment based on the internal standard • corrected change in lead concentration • the tarnish rating of the copper coupon • the change in weight of each of the coupons
31: CORROSION
motored for 4 h at 82.2°C. At the end of 4 h of operation, the whole differential unit is stored in an environmental storage box for 162 h at 51.7°C. At the end of the storage time, the unit is disassembled and rated for corrosion, sludge, and other deposit formation. Industrial Fluid Lubricants ASTM B 117, Standard Practice for Salt Spray (Fog) Apparatus
Operating
The ASTM B 117 Salt Spray is commonly referred to as Salt Fog. Salt spray and salt fog are frequently thought of as different but are in fact the same, as noted in the title. It may be easier to think of this apparatus as producing a mist. In any case, it offers an accelerated method for evaluating the rust prevention characteristics afforded by a coating on a metal surface. For example, failure (rusting) can occur in a few hours for a thin, oily coating, or in thousands of hours for a thick, hard coating. Often used as a screening test because of the speed at which results can be obtained, an aqueous solution of 5% sodium chloride is continuously atomized in the cabinet engulfing the test panels, creating an environment conducive to accelerated corrosion. The test apparatus consists of a cabinet capable of maintaining a temperature of 35°C where pressure and the introduction of the salt containing vapors can be controlled. Test panels are set on internal racks, as shown in Fig. 6, and are subjected to the salt a t m o s p h e r e for variable amounts of time. Results are tjrpically reported as the number of hours to failure (onset of rust). ASTM D 665, Standard Test Method for Rust-Preventing Characteristics of Inhibited Mineral Oil in the Presence of Water Originally designed to determine the rust prevention characteristics of steam-turbine oils when contaminated with water, the ASTM D 665 test method now is used to indicate the rust preventive properties of almost all types of industrial lubricants, particularly hydraulic fluids and circulating oils where water contamination can be expected. Rusting of steel is, of course, destructive to the hydraulic or circulating system itself. Also, rust particles in the fluid can cause plugging of close-clearance passages and/or filters and, being abrasive
Gear Oils CRC L-33, Gear Oil Corrosion
Test
This test method evaluates the rust and corrosion inhibiting properties of gear oils subjected to water contamination and elevated temperature. It duplicates service conditions where water condenses on metal parts. (This test method is based on Federal Test Method 5326.1.) This test utilizes a Dana Model 30 hypoid differential unit. This unit is cleaned (including shotblasting), assembled, and then filled with 1200 ml of oil and 30 ml of water. The unit is
829
FIG. 6—ASTM B 117, Salt Spray (Fog) Panel.
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in nature, can contribute to wear, particularly of p u m p s . Additionally, particles of rust in the oil can act as catalysts to increase the rate of oil oxidation. A polished steel rod (spindle) is suspended in a 400 ml beaker containing 300 ml of the test oil, which is stirred at 60°C. After 30 min, 30 ml of water are added to the oil. It is customary to run the test for 24 h; however, other times may be specified. The 10% water addition to the oil can be either distilled water (Procedure A) or a defined synthetic sea water (Procedure B). The test is normally r u n in duplicate and any visible rust on both rods indicates failure. Results are reported as a pass or fail. Because of the greater severity, the synthetic sea water (Procedure B) is frequently used as a screening test [salt makes water a better electrolyte and accelerates the corrosion process]. Results should always specify which procedure (A or B) was used a n d the amount of time the test was run. Figure 7 shows a passing (rust-free) spindle on the left and a failing spindle on the right. ASTM D 1748, Standard Test Method Rust Protection by Metal Preservatives Humidity Cabinet
for in the
The ASTM D 1748 test method is used to evaluate the relative abilities of metal preservatives to prevent the rusting of steel panels under conditions of 100% relative humidity at 50°C. This test is not as severe as the Salt Fog Test (ASTM B 117). It is not u n c o m m o n for test panels to r u n well over 1000 hours before the onset of rust. Steel panels are prepared to a prescribed surface finish, dipped in the test fluid, allowed to drain and then suspended in the humidity cabinet, as shown in Fig. 8. A continuous supply of air is delivered to the cabinet, which is held at 50°C. Water is m a i n t a i n e d in the b o t t o m of the cabinet as the source of humidity. Panels are periodically checked for signs of rust. A failure occurs at the point in time when either a rust spot Icirger than 1 m m in diameter appears or four rust spots of any size are observed. Results are reported as hours to failure as described above. ASTM D 2070, Standard Test Method Thermal Stability of Hydraulic Oils
for
The ASTM D 2070 test m e t h o d is widely known as the "Cincinnati Milacron" test. This test method is designed to
FIG. 8—ASTM D 1748, Humidity Cabinet Interior.
evaluate the thermal stability of h y d r o c a r b o n based hydraulic oils in the presence of copper a n d steel at 135°C. However, oxidation of the oil may also occur during the test. Clean, polished, pre-weighed copper a n d steel rods are placed in a 250 ml beaker, which contains 200 ml of the test oil. The bcciker is placed in an aluminum block in an oven for 168 h at a test temperature of 135°C. At the completion of the test, the copper and steel rods are rated visually for discoloration and the oil is analyzed for the quantity of sludge formation. ASTM D 3603, Standard Test Method for Rust-Preventing Characteristics of Steam Turbine Oil in the Presence of Water (Horizontal Disk Method) The ASTM D 3603 test method is used to eveJuate the ability of steam-turbine oils (under full flow and quasi-static conditions) to prevent the rusting of horizontjil and vertical ferrous surfaces w h e n water becomes mixed with oil. Horizontal metal surfaces, on which water droplets tend to be retained, are more prone to corrosion than vertical or sloping surfaces. This test m e t h o d is therefore more discriminating t h a n method ASTM D 665 Procedure A, since it gives a separate evaluation of the oil on a horizontal emd a vertical surface. A horizontal steel disk and vertical steel cylinder assembly is placed in a bath of 275 ml of oil, which is stirted at 60°C. After 30 min, 25 ml of distilled water are added to the oil. The test is r u n for 6 h. The test is r u n in duplicate and any visible rust on b o t h assemblies indicates failure. Results are reported as a pass or fail. ASTM D 4310, Standard Test Method for of the Sludging and Corrosion Tendencies Inhibited Mineral Oils
FIG. 7—ASTM D 665, Passing and Failing Spindles.
Determination of
The ASTM D 4310 test procedure is a modification of the test method D 943. It is used to determine the tendencies of inhibited mineral oil based steam turbine lubricants and mineral oil based anti-wear hydraulic oils to corrode copper catalyst metcil and to form sludge during oxidation. The test is conducted in the presence of oxygen, water, and copper and iron metals at an elevated temperature. This test method is also used for testing circulating oils having a specific gravity less t h a n that of water and containing rust and oxidation inhibitors.
CHAPTER This test utilizes a special piece of glassware known as a n oxidation cell. A 300 ml sample of the test oil, 60 ml of water, and a catalyst (a 225 m m braided low carbon steel-copper coil) are placed in the oxidation cell, which is heated in a bath to 95°C. Oxygen is delivered to the system at a rate of 3 L/h. The test is run for 1000 h. Upon completion of the test, the weight of the insoluble material (sludge) that is formed and the total a m o u n t of copper in the oil, water, a n d sludge phases is reported. ASTM D 4627, Standard Test Method for Iron Chip Corrosion for Water-Dilutable Metalworking Fluids Also known as the Cast Iron Chip Test, the ASTM D 4627 test method evaluates the ferrous corrosion control characteristics of water-dilutable metalworking fluids. The results obtained by this test are useful in determining the ability of water-diluted metalworking fluids to prevent or minimize rust under specific conditions. This procedure is typically used for screening and comparative purposes. Cast iron chips are placed in a petri dish containing a filter paper and a water-diluted metalworking fluid. The dish is covered and allowed to stand for 20-24 h. At the end of the test period, the filter paper is rinsed with water and the percent of the filter paper area that was stained by the rusting chips is estimated. The amount of rust stain on the filter paper is an indication of the corrosion control provided by the fluid. The "breakpoint" is defined as the weakest concentration tested that left no stain on the filter paper. ASTM D 4636, Standard Test Method for Corrosiveness and Oxidation Stability of Hydraulic Oils, Aircraft Turbine Engine Lubricants, and Other Highly Refined Oils The ASTM D 4636 test method is used to test hydraulic oils, aircraft turbine engine lubricants, and other highly refined oils to determine their resistance to oxidation and corrosion degradation and their tendency to corrode various metaJs. Petroleum and synthetic fluids may be evaluated using moist or dry air with or without metal test specimens. (This test method is basically Federal Test Method 5307.2 expanded to include Federal Test Method 5308.7.) This test method consists of a standard test procedure and two alternative procedures. In the test procedure, a large glass tube containing the test oil and pre-weighed metal specimens is placed in a constant temperature bath and heated for a specified n u m b e r of hours while air is passed through the oil. The different test procedures allow for various oil sample sizes (200 ml, 165 ml and 100 ml), various bath temperatures (typically 100°C to 360°C) and different shaped (washer-shaped and square) specimens of various metals (titanium, magnesium, steel, bronze, silver, aluminum, copper, cadmium and others). At desired test times, oil samples are withdrawn from the test oil and checked for changes in viscosity cind acid number. At the end of the test, the amount of sludge present in the oil remaining in the tube and the quantity of oil lost (evaporated) during the test are determined. The corrosiveness of the oil is determined by loss of mass of the metal specimens and microscopic examination of the metal surfaces. Also, the presence of liquid or solid material o n the inside of the condenser and the appearance of deposit on the sample tube are reported.
31: CORROSION
831
Greases ASTM D 1743, Standard Test Method for Determining Corrosion Preventive Properties of Lubricating Greases Since the early 1960s, the most popular grease rust test in the United States has been ASTM D 1743. This method determines the rust preventive properties of greases using tapered roller bearings stored under static conditions in the presence of distilled water. Tapered roller bearings, as shown in Fig. 9, are packed with grease and r u n under a light load to distribute the grease evenly. The bearings are exposed to distilled water and then stored at 52°C and 100% relative humidity for 48 h. After cleaning, the bearing races are examined for rust. Since 1987, the ASTM D 1743 procedure has specified a pass or fail rating on the basis of a single corrosion spot of 1.0 m m or larger in the longest dimension on two of three bearings tested simultaneously. ASTM D 4048, Standard Test Method for Detection Copper Corrosion from Lubricating Grease
of
The ASTM D 4048 test method is similar to the ASTM D 130 test used for industrial oils but is designed to evaluate the copper corrosion prevention properties of greases. A polished and cleaned 3-in. copper strip as prescribed in ASTM D 130 is placed in a jar in which the copper strip is totally i m m e r s e d in the test grease. The jar is capped and heated to a specified temperature for a defined period of time. Commonly used conditions are 100°C for 24 h. At the end of the exposure period, the strip is removed, wiped clean and matched with colored reproduction strips characteristic of the descriptions provided in Table 3. ASTM D 5969, Standard Test Method for CorrosionPreventive Properties of Lubricating Greases in Presence of Dilute Synthetic Sea Water Environments The ASTM D 5969 test method is the synthetic seawater version of ASTM D 1743, which specifies the use of only distilled water. The test methods are identical except for the following changes, which are incorporated into the ASTM D 5969 method: • The tapered roller bearings are exposed to desired concentrations of synthetic seawater (prepared as specified in test
FIG. 9—ASTM D 1743 and ASTM D 5969, Tapered Rolling Bearing.
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method ASTM D 665, Procedure B) diluted with distilled water. • The test time is reduced from 48 h to 24 h. Tapered roller bearings are packed with grease and r u n under a light load to distribute the grease evenly. The bearings are exposed to desired concentrations of synthetic seawater, then stored at 52°C and 100% relative humidity for 24 h. After cleaning, the bearing races are examined for corrosion. This procedure specifies a pass or fail rating on the basis of a single corrosion spot of 1.0 m m or larger in the longest dimension on two of three bearings tested simultaneously. ASTM D 6138, Standard Test Method for CorrosionPreventive Properties of Lubricating Greases Under Wet Conditions (Emcor Test)
FIG. 11—ASTM D 6138, Emcor Bearing Race (showing rust spots).
The ASTM D 6138 test method is commonly referred to as the "Emcor Test" and is an adopted version of the IP 220 European procedure. This test is a dynamic procedure used to determine the corrosion protection of a grease in the presence of water approximating typical service conditions. Double r o w self-aligning ball bearings are packed with grease and r u n to distribute the grease evenly. The bearings are exposed to distilled water, synthetic seawater, or sodium TABLE 4—Emcor corrosion classifications. Ranking
Description No evidence of corrosion No more than three spots of a size just sufficient to be visible Up to 1% surface corrosion Between 1% and 5% surface corrosion Between 5% and 10% surface corrosion More than 10% surface corrosion
FIG. 12—ASTM D 6138, Emcor Apparatus. chloride solution, and the test rig is operated under alternating r u n n i n g a n d standing conditions for one week. After cleaning, the bearing races are examined for corrosion and rated "0" to "5" as detailed in Table 4. Figure 10 shows a rustfiree Emcor bearing race. Figure 11 shows an Emcor bearing race with rust. The test rig, as shown in Fig. 12, will simultaneously run eight bearings. Duplicate determinations are required, so the rig is normally used to evaluate four different formulations at a time.
REFERENCES
FIG. 10—ASTM D 6138, Emcor Bearing Race (rust-free).
[1] Basic Corrosion Technology for Scientists and Engineers, The Institute of Materials, London, 1996, pp. 37-61. [2] Clark, D. B., Klaus, E. E. and Hsu, S. M., "The Role of Iron and Copper in the Oxidation Degradation of Lubricating Oils," Lubrication Engineering, May, 1985, pp. 280-286. [3] Basic Corrosion Technology for Scientists and Engineers, The Institute of Materials, London, 1996, pp. 95-130.
MNL37-EB/Jun. 2003
Flow Properties and Shear Stability Robert E. Manning^ and M. Richard Hoover^
T H E IMPORTANCE OF FLOW PROPERTIES IN THE MANUFACTURE AND
USE OF MATERIALS Cannot be over emphasized. The flow properties of materials ranging from gasses, such as oxygen, to hquids, such as water, paints and lubricants, to semi-solid and solid materials such as asphalt cements, glass, and steel aire important in both the manufacture and use of these materials. In the manufacturing of materials, flow properties can affect the manner in which materials are handled and packaged. For example, p u m p s , mixing equipment, and packaging equipment must be correctly selected and sized to handle the material being processed. Likewise, flow properties are important in the use of materials. Paints must be designed to flow when brushed, but remain on the painted surface, even when the surface is vertical. Lubricants must have flow properties allowing them to be distributed to the required location and then remain to provide appropriate lubrication. As can be seen from these few examples, m e a s u r e m e n t and characterization of flow properties are essential. There is a long history of the measurement of flow properties of materials. Isaac Newton laid the groundwork for the m a t h e m a t i c a l t r e a t m e n t of the flow of materials in his Philosophiae Naturalis Principia Mathematica, 1687, Book 2, Section IX, HYPOTHESIS: The resistance arising from the want of lubricity in the parts of a fluid is, other things being equal, proportional to the velocity with which the parts of the fluid are separated from o n e another. In 1823 Navier stated the general equations for fluids in motion [1]. Hagen [2] and Poiseuille [3] both studied the viscosity of water flowing in capillaries in the mid 1840s, and they determined the relationship between the quantity of flow and the pressure drop, diameter, and length of the capillary tube. In 1890 Couette developed the rotating concentric cylinder apparatus [4]. In the early 1900s A. Pochettino noted a n u m b e r of types of apparatus for the measurement of viscosity of semi-solid materials such as asphalts and tars [5]. Bingham and Jackson [6] reviewed viscosity data attributed to water, and selected a value (1.005 mPa-s at 20°C) for the primary viscosity standard, a value that was unchanged for nearly 40 years. Ubbelohde [7] and Cannon and Fenske [8] developed glass capillary viscometers in the mid-1930s, which were responsible for the adoption and wide use of kinematic viscosity measurements in the petroleum industry.
' Consultant, 225 Harris Drive, State College, PA 1680L ^ President, Cannon Instrument Company, State College, PA 168040016.
Research by Appeldoorn [9], Klaus [10], Wright [11], and many others in the 1950-1960s and later led to many of the instruments and specifications for m o d e m - d a y lubricants. Very active interest in viscosity and flow properties continues today within the scientific and engineering communities. International organizations such as ISO (International Standards Organization) and OIML (International Organization of Legal Metrology) along with national organizations such as the Institute of Petroleum (United Kingdom), DIN (Germany), JIS (Japan), and ASTM International (International), actively develop and maintain test methods and specifications for viscosity and fluid flow properties. Although there are differences between each of the several test methods, all of the test methods relating to similar properties are interrelated and, in some cases, derived from a corresponding ASTM test method. In cases where more than one test method is available for a given measurement, the selection of the method to use is based on factors such as the specifications to be met, the sample size, equipment already available in the laboratory, budget, etc. Within ASTM International, various committees have responsibility for viscosity-related test methods to support the development and specification of materials falling under the jurisdiction of the individual committees. Some of these committees include Petroleum Products a n d Lubricants (D02), Road and Paving Materials (D04), Paints and Related Coatings (DOl), Plastics (D20), Rubber ( D l l ) , and Adhesives (D14). This chapter focuses primarily on the ASTM methods for the measurement and characterization of flow properties of petroleum products, although reference to other types of material is m a d e as appropriate. Various characterization techniques are discussed along with their historical perspectives. Specific references to ASTM test methods are included to provide the reader with a better understanding of both the m e a s u r e m e n t technique and the application of the test method. Note that all referenced ASTM methods are included in a table at the conclusion of the chapter.
NOMENCLATURE AND RHEOLOGY CONSIDERATIONS The science of the deformation and flow of matter including both the flow and elasticity of material is defined as rheology. The material may be a solid such as tar or bitumen, or a liquid such as molasses or water, or a gas such as air. In addition, there are many materials that do not deform when a small stress is applied to the material, but flow readily at a
833 Copyright'
2003 by A S I M International
www.astm.org
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higher stress. Rheology is central to the measurement of flow properties of materials. All measurements of viscosity involve imparting motion to a fluid and observing the resulting deformation of that fluid. Often a material shows both flow and elastic c o m p o n e n t s , such as the example of Silly Putty*, which can be formed into a ball that bounces when dropped, but flows as a viscous liquid u p o n standing. Materials that have the ability to absorb some of the applied stress (and when the stress is relieved, giving back the energy) are said to be viscoelastic, because they exhibit both viscous and elastic properties. Some materials exhibit a decreased viscosity as the applied stress is increased (shear thinning), while other materials exhibit an increased viscosity with an increase in the applied stress (dilatant material).
• Dynamic Viscosity—TJ, the ratio of the shear stress, T, to the shear rate, y. Another term for viscosity, dynamic viscosity is used, especially in Europe. It should be noted that the term dynamic viscosity is also used for the determination of viscosity when the stress is applied in an oscillatory mode. Hopefully this dual use of dynamic viscosity will not be confusing. In this chapter, unless otherwise stated, the term viscosity is equivalent to dynamic viscosity and does not imply stress applied in an oscillatory mode. • Kinematic Viscosity—the ratio of viscosity, TJ, to the density of the fluid, p, both at the same temperature. It is a measure of the resistance to flow of a liquid under gravity.
To assist the r e a d e r in u n d e r s t a n d i n g the details of viscosity measurement, terminology related to viscosity is presented in Table 1. Throughout this chapter, the System International (SI) units are primarily used; these units are based on the meterkilometer-second (mks) system of units, and should not be confused with the centimeter-gram-second (cgs) system of units. Because the current usage of terms includes both the SI and the cgs systems of units, the conversions in Table 2 may be helpful. Some definitions of c o m m o n terms (mostly adapted from ASTM methods) that will be used in this chapter are:
• Viscoelastic Materials—materials that exhibit both elastic and viscous properties. Under an oscillatory mode of applying the stress to Newtonian liquids, stress a n d the resulting strain are in p h a s e with each other. For viscoelastic liquids, the stress and strain are out of phase. The phase angle between stress and strain is a measure of the elasticity of the liquid. Certain automotive engine oils contain polymers that cause the oil to be viscoelastic. • Shear Stress—the motivating force per unit area for fluid flow. • Shear Rate—the velocity gradient in fluid flow. • Newtonian Liquid—a fluid that exhibits a constant viscosity at all shear rates. If the viscosity is not constant, the liquid is non-Newtonian. Simple liquids such as water, hexane, and refined oils without polymeric additives are Newtonian. • Fluidity—the reciprocal of viscosity. • Consistency—the resistance of a non-Newtonian material to deformation or flow. Consistency is not a fundamental property but is m a d e u p of viscosity, plasticity, and other rheological phenomena. These are not normally found in petroleum jargon. • Shear Thinning (pseudoplastic)—the property of a nonNewtonian material exhibiting r e d u c e d viscosity at increasing shear rates. Lubricating Oils with high molecular weight polymeric additives are t3^ically shear thinning. • Shear Thickening (dilatancy)—the property of a nonNewtonian material exhibiting increased viscosity at increasing shear rates. Suspended particles emd slurries can be examples of dilatant materiEils. • Yield Stress—the stress required to initiate flow. For all Newtonian fluids and some non-Newtonian fluids, yield stress is zero. Automotive oils when cooled slowly to low temperature can form a partial, interconnecting wax structure; this structure can cause a yield stress sufficient to prevent pumping of the oil. • Plastic (Bingham) Body—a liquid having a yield stress, TQ, to be overcome before flow results, and thereafter the flow is Newtonian.
• Rheology—the science of t h e deformation a n d flow of materials • Viscosity—the resistance experienced by one portion of a material moving over another portion of the material. It is the ratio of the shear stress existing between laminae of moving fluid and the rate of shear between these laminae. Viscosity is often called the coefficient of viscosity. Viscosity, rj, is defined as the ratio of the shear stress, T, to the shear rate, y. Tj =
riy
(1)
TABLE 1—Terminology & units. Quantity
Symbol
SI Units
cgs Units
Shear stress Shear rate Viscosity JCinematic viscosity Viscosity of Solution Relative viscosity, % = {riJ^i) Specific viscosity, (% — 1) Intrinsic viscosity, [rj] = lim(rjsp/c ), c ^ 0 First normal stress. Weissenberg effect
T or
Pa (Pascal) s~' Pa-s m'^/s Pas
dyn/cm^ s-> P (Poise) St (Stokes) P
7)r %p
[ij]
•••
m^/kg
mL/g
Ni(y)
V = (T-
TABLE 2 --Conversion factors. Viscosity:
Kinematic Viscosity:
1 Pa-s 1 mPa-s 1 Pas 1 m^/s 1 mm^/s 1 m^/s
= = = = = =
V = Tj/p = r / p y
10 P IcP 1000 m P a s lO^'St IcSt IC* mm^/s
To)ly
(2)
(3)
Thixotropy—that p r o p e r t y of a material to thin u p o n isothermal agitation, and to thicken upon subsequent rest. Some paints are m a d e to be thixotropic, such that the energy needed to b r u s h the paint at high shear is reduced, but when the brushing is stopped the structure reforms and the paint stays in place.
CHAPTER 32: FLOW PROPERTIES Apparent Viscosity—The viscosity determined for a nonNewtonian material using a test method for which Newtonian standards are used for cahbration, for example, the viscosity determined according to test method D 5293, Apparent Viscosity of Engine Oils Between —5 and —30°C Using the Cold-Cranking Simulator. The apparent viscosity so determined is thus also a function of the test apparatus. Softening Point—the temperature at which a material becomes soft enough to flow, as determined by an arbitrary, narrowly defined method. Power Law Model—a mathematical model used to describe the behavior of shear-thinning liquids. Experimental results for many shear thinning liquids fit closely the power law model over a limited shear rate range - often up to three decades of shear rates r] = Ky'( n - l )
(4)
when n = 1 Newtonian liquid n< 1 Shear thinning liquid • First Normal Stress, Weissenberg Effect—The vector normal to the direction of the applied shear stress. The first normal stress is a measure of the elastic component of viscoelastic materials. This elastic component can cause viscoelastic materials to "climb" a rotating shaft or push the cone away from the plate in a cone-plate instrument. Automotive engine oils with some polymeric additive packages can exhibit the Weissenberg Effect. See the Annex to ASTM D 5293 for a discussion of the Weissenberg Effect as related to the Cold-Cranking Simulator. Note that all referenced ASTM methods are included in a table at the conclusion of the chapter. • Shear Strain—The quotient of shear deflection divided by the thickness of the test piece. • Modulus—The ratio of stress to strain; that property of a material which, together with the geometry of a specimen, determines the stiffness of the specimen; may be static or dynamic, and if dynamic, is mathematically a vector quantity, the phase of which is determined by the phase of the complex force relative to that of deflection (ASTM D 5992). • Elastic—as a modifier of dynamic force, descriptive of that component of complex force in phase with dynamic deflection, that does not convert mechanical energy to heat, and that can return energy to an oscillating mass-spring system; denoted by the single prime (') as a superscript symbol, as F' (ASTM D 5992).
AND SHEAR STABILITY
For two planes of a unit area A separated by a unit distance Y, the viscosity TJ of the material between the planes can be defined [12] as V = (F/A)/(V/Y)
(5)
Many of the general measurements of viscosity incorporate a rotational geometry. For example, the fluid may be between a flat plate and a cone (see e.g., ASTM D 4287, D 3205) (Fig. 2) or between two flat plates (Fig. 3). Two additional rotating viscometers include the concentric cylinder incorporating the annular space between a cylindrical rotor inside a cylinder (Couette geometry), Fig. 4, (e.g., D 4864, D 5133), and a paddle in a cup (e.g., D 562), Fig. 5. In these rotational geometry viscometers, either the inner or outer member may rotate while the other member is held static. The rotational speed may be constant while the torque or stress is measured, or the torque or stress may be held constant while the speed is measured. The speed may increase from zero to a maximum and back to zero, or it may follow a sinusoidal speed variation.
drive
mechanism
torque
spring
cone ''.:.",] t e s t sample plate FIG. 2—Cone-and-plate viscometer.
drive
mechanism
Viscometer Geometries and General Techniques There are a number of geometries and techniques commonly used to determine viscosity. To visualize viscosity, we first refer to Fig. 1. torque
spring
MOVING PLANE, AREA
THICKNESS OF FLUID FILM = h
STATIONARY PLANE AREA = A
FIG. 1—Visualization of viscosity.
835
"7~|^ upper plate test sample ] lower plate FIG. 3—Plate-to-plate geometry.
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Another c o m m o n geometry involves the flow of a fluid through a capillary tube (Fig. 6). If the motion of the fluid through the capillary is driven by its hydrostatic head under the influence of gravity, kinematic viscosity will be the measured property. If the fluid is forced t h r o u g h the capillary by a n applied pressure (or d r a w n through the capillary by a vacuum), viscosity is measured. As noted above for the various rotational geometries, there are also many special geometries used for the measurement of kinematic viscosity. Other less c o m m o n techniques for the measurement of viscosity exist. An object such as a ball (e.g., D 1343) or a needle (e.g. D 5478) can be moved through a liquid under the influence of gravity at a rate influenced by the difference in density between the ball and fluid in a vessel. Alternatively, the ball m a y merely roll down hill through a cylinder, or a
drive
flow through capillary
Newtonian Liquid
driving head
piow Patte
Bingham Liquid
FIG. 6—Flow through capillary tube profiles.
mechanism
torque
spring Falling Ball viscometer
Rolling Ball visconneter
FIG. 7—Falling and rolling ball viscometers.
T
rotor cup
FIG. 4—Concentric cylinder viscometer.
drive
mecinanism
torque spring
paddle cup FIG. 5—Paddle type rotational viscometer.
bubble of air can rise in a cylinder containing the test sample (e.g., see D 1545) (Fig. 7). Other variations include needle and crystal viscometers. A needle can be caused to descend through a fluid, wherein its velocity is proportional to the ratio of its velocity to the density difference between the liquid and the needle. A crystal or a tuning fork can be caused to oscillate in a liquid. The restrictive force of the fluid along the surface of the crystal is a measure of the viscosity, the elasticity, or both. A vibrating crystal can cause a wave to be generated through the fluid. The reflection of the wave back to the crystal allows the measurement of the viscosity of the fluid by the attenuation of the wave, and a measure of the elasticity of the fluid by the phase change of the wave. There are numerous ASTM methods used to determine the viscosity of petroleum products. As shown in Table 3 the type of viscometer used in each method differs.
FLOW PROPERTIES OF PETROLEUM MATERIALS Measurement and control of flow properties are extremely important to the petroleum a n d lubricants industries. The ASTM subcommittee on Flow Properties (D02.07) is prominent in the development of test procedures for petroleum
CHAPTER 32: FLOW PROPERTIES AND SHEAR STABILITY
837
TABLE 3—Table of petroleum related viscometer methods. Title of Method Saybolt Viscosity Kinematic Viscosity of Transparent and Opaque Liquids (and the Calculation of Dynamic Viscosity) Measuring Apparent Viscosity of Lubricating Greases Basic Calibration of Master Viscometers and Viscosity Viscosity and Viscosity Change After Standing at Low Temperature of Aircraft Turbine Lubricants Apparent Viscosity of Petroleum Waxes Compounded with Additives (Hot Mehs) Low-Temperature Viscosity of Automotive Fluid Lubricants Measured by Brookfield Viscometer Apparent Viscosity of Hot Melts Adhesives and Coating Materials Pumpability of Industrial Fuel Oils Predicting the Borderline Pumping Temperature of Engine Oil Kinematic Viscosity of Volatile and Reactive Liquids Measuring Apparent Viscosity by Capillary Viscometer At High-Temperature and High-Shear Rates Measuring Viscosity at High Shear Rate and High Temperature by Tapered Bearing Simulator Determination of Yield Stress and Apparent Viscosity of Engine Oils at Low Temperature Measuring Viscosity at High Temperature and High Shear Rate by TaperedPlug Viscometer Shear Viscosity of Coal-Tar and Petroleum Pitches Low Temperature, Low Shear Rate, Viscosity/Temperature Dependence of Lubricating Oils Using a Temperature- Scanning Technique Apparent Viscosity of Engine Oils Between - 5 and -30°C Using the ColdCranking Simulator Measuring Apparent Viscosity at High-Temperature and High-Shear Rate by Multicell Capillary Viscometer Evaluation of Diesel Engine Oils in the T8 Diesel Engine Defining the Viscosity Characteristics of Hydraulic Fluids
Method No. D88 D445 D 1092 D2162 D2532 D2669 D2983 D3236 D3245 D3829 D4486 D4624 D4683 D4684 D4741 D5018 D5133 D5293 D5481 D5967 D6080
Type of Viscometer/Geometry short capillary capillary low shear capillary high shear capillary low shear capillary low shear rotational coaxial cylinder rotational coaxial cylinder rotational coaxial cylinder rotational coaxial cylinder rotational coaxial cylinder capillary low shear capillary high shear rotational tapered cylinder rotational coaxial cylinder rotational tapered cylinder rotational coaxial cylinder rotational tapered cylinder rotational coaxial cylinder high shear capillary capillary low shesir capillary low shear
products from high temperature (<150°C) and high shear rates (>10* s~') to low temperatures (< —SOX) and to low shear rates approaching zero s~'. Their flow-related test procedures are widely used in product specifications, within both the ASTM and the Society for Automotive Engineers (SAE).
The factor, p-H, is the hydrostatic pressure that forces the liquid through the capillary. By combining the density, p, with the viscosity, TJ, neither density nor viscosity have to be separately measured to determine kinematic viscosity (ASTM D 445, ASTM D 446).
Kinematic Viscosity
Most generally the equation for the glass capillary kinematic viscometers combines into a constant, C, all of the factors that are not normally individually measured except the flow time, t, resulting in the equation:
Kinematic viscosity, v, is the ratio of viscosity, rj, to density, p. It is very widely used in petroleum specifications and in specifications for many other materials. V = Tj/p
(6)
While the measurement of viscosity and density allows the calculation of kinematic viscosity, the direct measurement of kinematic viscosity is most frequently done with simple, lowcost glass capillary viscometers. For glass capillary viscometers, viscosity, -rj, is defined by: Tj = ( i 7 g D 4 H p t ) / ( 1 2 8 V L ) where
g D L H
= = = =
(7)
acceleration due to gravity diameter of the capillary length of the capillary average distance between the upper and lower menisci V = the timed volume of liquid passing through the capillary t = flow time
V = Tj/p = (TTg D" H t) / (128 V L)
V = Tj/p = C t where
(8)
(9)
C = viscometer constant t = measured flow time between two timing meirks
The constant, C, is determined by using a certified reference viscosity standard, paying careful attention to the filling of the viscometer, mounting the viscometer in the same orientation to ensure a constant driving head, temperature measurement and control, etc., as is required in the measurement of kinematic viscosity. For short flow times (less than 200 s) and for liquids having a kinematic viscosity of less than 10 mui^/s, a "kinetic energy" correction should be applied. The equation then becomes V = C t - E/t^
(10)
The kinetic energy correction, E, is represented by the equation E = 52.5 V^'^/L (C D)"2
(n)
838 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK where
L V C D E
= = = = =
working capillary length timed volume viscometer constant working capillary diameter kinetic energy correction factor
The equation for the calculation of E allows the estimation of the kinetic energy correction. If possible, it is preferable to determine the value of E using several viscosity standards [13]. The three general types of glass capillary, kinematic viscometers that are most frequently used include the: • modified Ostwald tj^es for transparent liquids, • suspended level types for transparent liquids, and • reverse-flow types for transparent and opaque liquids. The m i n i m u m and maximum kinematic viscosity ranges for each instrument type are found in Tables 4—6. The viscometers are shown in Figs. 8-10. While there are precise instructions for running each of the above kinematic viscometers, generally all procedures follow the same set of basic steps. The test sample is inserted into the viscometer and allowed to reach bath temperature. The b a t h temperature is normally controlled to ±0.01°C. After reaching test temperature the test sample is allowed to flow under gravity past the two timing marks, and the flow time is measured to the nearest 0.1 s or more recently, 0.01 s. Kinematic viscosity is calculated as the product of the flow time and the calibration constant, C. All of the modified Ostwald
TABLE 4—Modified Ostwald types for transparent liquids. Kinematic Viscosity Range, mm^/s Type Comments Cannon-Fenske routine 0.5 to 20 000 widely used Zeitfuchs 0.6 to 3000 rarely used BS/U-tube 0.9 to 10 000 used in Europe BS/U/M miniature 0.2 to 100 <2 mL sample SIL 0.6 to 10 000 rarely used Cannon-Manning semi-micro 0.4 to 20,000 < l m L sample Pinkevitch 0.6 to 17 000 rarely used
TABLE 5—Suspended-level Types for Transparent Liquids Kinematic Viscosity Range, mm^/s Type Comments used in Europe BS/IP/SL 3.5 to 100 000 rarely used BS/IP/SL(S) 1.05 to 10 000 <2 mL sample BS/IP/MSL 0.6 to 3000 widely used Ubbelohde 0.3 to 100 000 rarely used 0.6 to 1200 FitzSimons rarely used 0.75 to 5000 Atlantic widely used 0.5 to 100 000 Cannon-Ubbelohde dilute solution 0.5 to 100 000 Cannon-Ubbelohde dilution 0.4 to 20 000 <2 mL sample Cannon-Ubbelohde semi-micro
TABLE 6—Reverse-flow types for transparent and opaque liquids. Kinematic Viscosity TypeRange, mm^'sComments 0.4 to 20 000 widely used Cannon-Fenske opaque 0.6 to 100 000 asphalt solutions Zeitfuchs cross-arm 0.6 to 300 000 widely used BS/IP/RF U-tube reverse flow 60 to 100 000 rarely used Lantz-Zeitfuchs reverse-flow
Euid suspended-level viscometers are used for liquids that are transparent—that is, the meniscus can be observed through the sample at the timing marks. Choosing which of these several viscometers is to be used is generally a m a n n e r of choice of the operator, since the precision of the measurement is nearly the same. While all of the listed glass-capillary, kinematic viscometers can be used for accurate kinematic viscosity measurements, some of t h e m have special advantages or regional uses. Reverse-flow
Viscometers
The severed reverse-flow viscometers can be used for liquids so dark (opaque) that the timing of the test liquid cannot be observed after the test sample has previously wet the glass capillary as is required in the modified Ostwald a n d suspended-level types. Tj^jically, the precision of the reverseflow viscometers is slightly poorer than the precision of the modified Ostwald and suspended level viscometers; however, the dark liquids, such as marine fuels, liquid asphalts, etc. are often somewhat non-Newtonian. In North America, the Zeitfuchs Cross-arm viscometers are generally used for asphalts (ASTM D 2170) while the Cannon-Fenske Opaque viscometers are typically used for lubricants that are so dark that the meniscus cannot be seen through the test sample. In Europe, the BS/IP/RF U-tube reverse flow viscometers are m o r e popular. British
Standard
(BS)
Types
The British Standard (BS) types are only infrequently used in North America, while a n u m b e r of the other tj^es of viscometers commonly used in North America are less often found in Europe. Small
Volume
Types
For requirements where only a small amount of test sample is available, the Cannon-Manning semi-micro, Zeitfuchs cross-arm, and miniature suspended-level (BS/IP/MSL) are preferred. All of the suspended-level viscometers are distinguished by having the bottom of the working capillary open to the atmosphere; thus, the driving head (the distance between the upper and the lower menisci) is independent of the precise volu m e of sample in the viscometer, and the calibration constant is independent of temperature. Modified
Ostwald
Viscometers
Certain modified Ostwald viscometer designs have the precise volume of charge at test temperature adjusted so as to have the calibration constant independent of the test temperature. Several of the Modified Ostwald designs have a precise volume of chcirge (generally at ambient temperature) initially introduced, with a small temperature correction required if the test temperature is not the same as that of the initial charge temperature (Figs. 8-10). Some liquids are either very reactive to the surrounding atmosphere (they could explode in the presence of oxygen) or too volatile (at the temperature of test, the pressure is near or above that of the atmosphere) to be measured in the normeil kinematic viscometers. Such liquids may be able to be run in specially designed "tilting" viscometers, in which the test
CHAPTER 32: FLOW PROPERTIES AND SHEAR STABILITY o o
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CHAPTER 32: FLOW PROPERTIES
Caimon-Fenske Opaque
Zeitftichs Cross-Arm
AND SHEAR STABILITY
BS/IP/RF U-tube Reverse-flow
841
Lantz-Zeitfiichs Reverse-Flow
FIG. 10—Reverse-flow kinematic viscometers.
sample is sealed in the viscometer. See Fig. 11. By tilting the viscometer, the test sample is allowed to fill the upper timing bulb. Then the viscometer is restored to the normal upright position, and run in the manner of other kinematic viscometers (ASTM D 4486). To measure kinematic viscosity, the typical practice is to assume that the net hydrostatic pressure used to force the test sample through the capillary is p-H, where p is the density of the test sample. However, the test sample is always buoyed up by the density of the gas on the other side of the viscometer. Thus, the true hydraulic force is H-(pi — pg), where pi and pg are the density of the test liquid on the one side of the viscometer and the gas on the other side (ASTM D 2162). Normally the density of the gas is insignificant in comparison to that of the liquid; operation of kinematic viscometers at high static pressures requires correction of the density of the gas. Other special short capillary viscometers (such as the Saybolt, Redwood, Engler, Ford cup, ISO cup, Zahn cup, Shell cup, and bubble rise tubes ) are widely used to determine a measure of kinematic viscosity, although the results are often expressed as "instrument seconds", (e.g., Saybolt Universal Seconds, SUS) instead of kinematic viscosity units. See Figs. 12 and 13. Kinematic Viscosity History The kinematic viscosity of a liquid has long been recognized as a very important property. In 1840, Poiseuille, a professor of physics in the Paris medical schools, studied the viscosity
CONNECTOR
KOVAfl TO GLASS SEAL
I
I8ml VOL.
o o
CAPILLARY TUBING
e (O
E lOmm 0.0. ARM ROTATED 9 0 * TO REAR
HEAVY WALL CONSTRUCTION THROUGHOUT
C
12mm O.D. 8 4SmfflO.O.
FIG. 11—Tilting viscometer for vulnerable liquids.
842
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK of water flowing in glass capillaries of 0.013-0.65 m m in diameter and u p to 1 m in length so as to better understand the flow of blood through h u m a n capillaries [14,15]. The following equation was developed: Q = k P D^'/L
(12)
where: Q L D P k
ZAHN
GARDCO EZ
GARDCO Fisher
= = = = =
rate of flow through the capillary length of the capillary diameter of the capillary applied pressure drop across the capillary constant for water at a given temperature
Thus, the foundation of the flow properties of materials was laid. Fortunately, the study of water flowing through capillaries instead of blood was pursued. Blood is a non-Newtonian liquid, whose apparent viscosity varies with the shear rate, and it does not obey the above equation. Books by Barr [15] and Bingham [16] give an excellent history of early measurements of viscosity, the equations used, cind the equipment. The term kinematic viscosity appears to have been used only sparingly in the early 1920s, as Barr (loc. cit., p. 4) implies in 1931: The quotient of viscosity by density (TJ/P) assumes much importance in hydraulics and aerodynamics, and has received the name "kinematic viscosity," usually abbreviated as Greek letter nu (v). It has been suggested that the concept of kinematic viscosity should be regarded as the more
GARDCO FORD
FIG. 12—Dip-cup viscometers. Viscosity cup drawings provided by the Paul N. Gardner Company, Inc.
32.5 ±0.5
LEVEL OF LIQUID IN BATH-
OVERFLOW RIM
^
1.765±0.015 (0.0695t 0.0006)
1
12.25tO.10 (0.48210.004)
IO±l
^ 3.0 + 0.2 (0.12t 0.01) UNIVERSAL TIP 12511.0 (4.92t0.04)
3 MIN 3 MIN. _,;. . 3 . 1 5 0 ±0.020
'^.i24otaoooe I 12.25*0.1 0 r(0.482t0jl 0X)04)
BOTTOM OF BATH
CORK STOPPER
;^ 4.3+0.2 1(0.17 10,01) FUROL TIP
FIG. 13—Saybolt viscometer tube and receiver.
I.D. AT GRADUATION MARK
CHAPTER 32: FLOW PROPERTIES fundamental, that of the ordinary or dynamical viscosity being derived from it, instead of vice versa. The importance of technical viscometers even into the later part of the 20th century perhaps is best illustrated by Bingh a m [16] in 1922, for these viscometers continued to play a prominent position in the history of the petroleum industry, as described by Bingham: Instruments very different from, those employed in scientific work are much in vogue both in this country (United States) and abroad for industrial purposes, particularly in the oil industry. Thus we have the Engler Viskosimeter in Germany, the Redwood Viscometer in Great Britain, the Saybolt Viscosimeter in the United States, the Barbey Ixometre in France, and a host of others. Most of them seem to have been devised with the idea in mind that the time of flow of a given quantity of various liquids through an opening is approximately proportional to the viscosity, without much regard to the character of the opening. There is usually a container that is filled to a certain level and a short efflux tube opening into the air. The number of seconds required for a given quantity of liquid to flow out under gravity is taken as an indication of the viscosity. As it was gradually realized that these times of flow were not even proportional to the true viscosities, efforts have not been wanting to reduce the times of flow to true viscosities. Since the pressure is due to an average head of liquid h, the pressure is hgp and the viscosity formula may be written 7]/p=At-B/t
(13)
Having obtained the values of the constants A and B by calibrating the viscometer with liquids of known viscosity it appears possible to calculate the kinematic viscosity -q/p, but if absolute viscosities are desired it is necessary to make a supplementary determination of the density p. Thus, elaborate tables and charts have been devised for converting Engler "Degrees" (cf. Ubbelohde (1907)), and Redwood (cf. Higgins (1913), Herschel (1918) o r Saybolt "Seconds") into true viscosities. ASTM D 88 - 21 T, Standard Test Method for SayboU Viscosity was originally published in 1921, and it is still cited in a few specifications, although none for petroleum products. Glass capillary viscometers appear to have been used little if at all in ASTM specifications until the late 1930s, when ASTM D 445 made its appearance in 1937. This delay is probably due to the overwhelming use of the viscometers (in particular the Saybolt viscometer), as described by Bingham. However, the Saybolt, Engler, and Redwood viscometers all have a problem in the measurement of kinematic viscosities less than about 2.0 mm^/s. This problem may not have been realized in the petroleum industry until the late 1930s. The problem is illustrated by the Saybolt Universal kinematic viscosity scale at very low kinematic viscosities, where the efflux time becomes essentially constant regardless of the kinematic viscosity of the liquid. Thus, the Saybolt Universal scale is limited to a m i n i m u m kinematic viscosity of 32.0 SUS (about 1.8 mm^/s).
The lubrication of bearings and cylinders in severe service in automotive engines is influenced by the high-temperature.
843
high-shear (HTHS) properties of the lubricant. Lubricant temperatures in the bearings of engines can reach 150°C and higher and shear rates can exceed 10* s" ^ under such extreme conditions. A lubricant of low viscosity may not provide sufficient protection to ensure boundary lubrication at critical bearing surfaces, resulting in unacceptable bearing wear. Additionally, an engine lubricant having a high viscosity can result in excessive energy requirements and poor fuel efficiency. A comprehensive review of literature is available [17,18]. For lubricants such as engine oils, which have a base stock and a high-molecular-weight polymer-containing additive package, the log of viscosity as a function of the log of shear rate can exhibit a complex curve as shown in Fig. 14. Such liquids can be Newtonian at the lower shear rates, since there is not sufficient energy t o change the alignment of the molecules of the high molecular weights of the polymer (A). At somewhat higher shear rates, the polymeric additive becomes partially aligned with the axis of flow, showing decreasing viscosity with increasing shear rate (shear thinning behavior) and the relationship between viscosity and shear rate can approximate a power-law segment of the curve (B). At the highest shear rates, the polymer can become fully aligned with the axis of flow, and the liquid can again approach Newtonian behavior, the second Newtonian region (C). In the second Newtonian region, the lubricant viscosity can approach that of the base stock. Severe operating conditions in an automotive engine can subject the lubricant to extremely high shear rates, causing the oil to approach the second Newtonian region. The resulting reduction in viscosity can have an advantage in the reduction in absorption of energy within the engine, but it has a disadvantage in that the lower viscosity Ccin reduce the ability of the lubricant to provide adequate boundary lubrication [19]. The types of viscometers used to study the high-temperature, high-shear rheology of lubricants include both rotational (ASTM D 4683, ASTM D 4741) and capillary (ASTM D 4624, ASTM D 5481) geometries, and test results using these viscometers are in SAE specifications J300. The viscosity of gasoline engine lubricants is generally about 2- to 7-mPa-s at 150°C and a shear rate of 1 X 10* s^'. At these low viscosities and high shear rates, the state of the art of measurement instrumentation is pushed to its limits. Precise calculations of the shear rates for both types of instruments are complicated by difficulties in measuring the tight
C
high
- shear
Newtonian
A \ ^ B F (n o u
\
^
C
Base Stock 3 10
High-Temperature High-Shear
AND SHEAR STABILITY
!
4 10
1
5 10 1
6 10 1
7 10 1
Rote of shear y
FIG. 14—Log viscosity versus log shear rate.
844
MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
gap between the rotor and stator in the rotational instruments and in measuring the precise diameter of the capillary in the capillary instrument. Because of the very close tolerances required to achieve the very high shear rates in the rotational viscometers, the designs have used a slight, matching taper to allow precise adjustment of the gap between the stator and rotor (Figs. 15 and 16). This adjustment ensures that the proper shear rate will be maintained during the running of the test. As described in the test method, A motor drives a tapered rotor that is closely fitted inside a matched stator. The rotor exhibits a reactive torque re-
FILTER SCREEN
w (
)
>
^-
—PLUG VALVE PRESS./EXH, " CDNNECTDR
• ALUMINUM SHELL
HEATED -ALUMINUM CYLINDER
^CAPILLARY FIXTURE
FiSei- HoWi
FIG. 15—Tapered bearing simulator viscometer tapered bearing simulator viscometer.
FIG. 16—Rotor, stator and stator housing of the tapered bearing simulator viscometer.
Viscometer Cell FIG. 17—Capillary high-temperature highshear viscometer.
sponse when it encounters a viscous resistance from an oil that fills the gap between the rotor and stator. Two oils, a calibration oil and a non-Newtonian reference oil, are used to determine the gap distance between the rotor and stator so that a shear rate of 1 X 10'' s^^ is maintained. Additional calibration oils are used to establish the viscosity/torque relationship, which is required for the determination of the apparent viscosity of test oils at 150°C (ASTM D 4683). The HTHS multicell capillary viscometer (see Fig. 17) uses capillaries of very short lengths (about 15 mm) and very small capillary diameters (about 0.15 mm). Unlike the rotational designs, the rate of shear in the capillary design varies from zero at the center of the capillary to a maximum at the capillary wall. Because of the variation in the shear rate across the diameter of the capillary, the equivalent shear rate to that of the rotational design apparatus is 1.4 X 10* s~' at the wall of the capillary (ASTM D 5481). Like the rotational viscometer, the capillary viscometer design also requires calibration with Newtonian oils for each of the five test cells. The group of test cells is held at 150°C with a controUedtemperature aluminum block. A 10-mL test sample is
CHAPTER inserted into the test cell and allowed time to reach test temperature. The excess sample is withdrawn, and a controlled pressure applied to the sample, forcing it through the working capillary. With automatic timing (approximately 10-30 s), the viscosity and shear rate are ceJculated. If necessary, a second measurement is made with a revised applied pressure to ensure a shear rate at the wall of the working capillary of 1.4 X 10* s " ^ This apparatus can also be used from 35-175°C [20,21] (see Fig. 17). For automotive gasoline engines, the m i n i m u m HTHS SAE specifications at 150°C are 2.6 mPa-s (SAE 20 grade), 2.9 mPa-s (SAE 30, OW-40, 5W-40 and lOW-40 grades), and 3.7 mPa-s (SAE 15W-40, 20W-40, 25W-40, 40, 50 and 60 grades) as of December 1999 [22]. However, as the SAE Fuels and Lubricants Committee continues their studies of the best specifications, the HTHS specifications can be expected to be continuously u p d a t e d in the future. There is significant interest for measurements at other temperatures (100-200°C) and shear rates u p to at least 1 X 10^ s~'. Much research in automotive lubricants also is related to rheological studies, even though there may not be a n ASTM test or SAE specification. One example of this research relates to the study of dynamically loaded journal bearings, the m i n i m u m oil film thickness (MOFT) and the load bearing capacity of engine oil lubricants. Many engine lubricants are polymeric solutions, and as such many are also viscoelastic. The research requires a n u m b e r of rheological studies, such as the slit die rheometer (Lodge Stressmeter [23]) to study the first normal stress difference [24], studies of the MOFT [25,26], and cavitation [27].
LOW TEMPERATURE REQUIREIVIENTS OF ENGINE LUBRICANTS In the 1950s and 1960s the nature of the light-duty engine oil lubricants changed with the addition of polymers to blended oils. The changes in both the lubricants and the automotive engines caused three major cooperative, industry-wide research programs to be undertaken in order to improve the engine lubricants, and these programs have had a profound influence in the improvement of engine lubricants. Prior to the changes in the engine oils, the kinematic viscosity at low temperature had been calculated using the measured kinematic viscosities at 100°F (37.8°C) and 210°F (98.9°C). The kinematic viscosity at 0°F (-17.8°C) was calculated by extrapolation using the ASTM viscosity-temperature charts (ASTM D 341), or calculated tables using the MacCouU equation, or calculations using the MacCoull (Walther) equation: logio (logio (v -f 0.7)) = A - B logio (t + 460) where:
(14)
v = kinematic viscosity, mm'^/s (or cSt) t = temperature, °F A and B = constants
The low-temperature lubricant specification was based on the extrapolated kinematic viscosity at 0°F. However, it was found that with the wax components of the base stock and the polymeric additives, the correlation between the extrapolated kinematic viscosity of the lubricant at low temperature a n d the ability to start the automotive engine no longer
32: FLOW PROPERTIES
AND SHEAR
STABILITY
845
existed. In June 1954, C. W. Georgi [28] reported. It appears motor oils become non-Newtonian fluids at subzero temperatures and may have actual viscosities ranging up to ten times higher than indicated by the customary ASTM Viscosity-Temperature extrapolations. Some caution seems necessary in interpreting ASTM charts in the low temperature region, as well as in formulating motor oils for low temperature service. In the 1960s a very large research program was undertaken with the cooperation of the Society of Automotive Engineers (SAE), the Coordination Research Council (CRC) and the petroleum division of ASTM Subcommittee D02.07 on Flow Properties [29,30,31,32]. Two groups of special oils were prepared to study the effect on cranking current engines at low temperature. Oils in the first group were essentially Newtonian viscosity calibration standards at 0°F over the viscosity range of 1390-8400 mPa-s. They were used to calibrate full-size, motored engines as if the engines were large viscometers. The second series of oils were multigrade engine lubricants blended from paraffinic base stocks with polyisobutylene, vinyl copolymer, and polymethacrylate viscosity index improver-polymer types. These were intended to be non-Newtonian oils at low temperature and typical of commercial engine oils. The viscosity for the second series of oils was determined in the calibrated, motored engines. These data were also used to develop bench scale viscometers. This program was further extended to include additional reference oils to develop data at - 2 0 ° F . The results of this program are discussed in the Low-Temperature High-Shear Tests of Engine Lubricants in a later section. In 1973-4, ASTM and SAE sponsored a second, similar large program that used motored engines at low temperatures to determine the lowest temperature that each special oil could be p u m p e d to the bearing surfaces in the engines without loss of pressure [33]. These data were used to develop b e n c h scale i n s t r u m e n t s to correlate with the pumpability data. The results of this program are discussed in the Low-Temperature Pumpability of Engine Lubricants in a later section. Since the 1970s, the design of the automotive engines has undergone many improvements both in cold starting and in pumping ability of lubricants. There has been concern as to whether the design data of the 1970s r e m a i n s adequate. ASTM and SAE sponsored a third large low-temperature study in May 1992, using the then current engine designs. The ASTM Low Temperature Engine Performance (LTEP) program within ASTM subcommittee D02.07.C issued a summary research report in November 1998 [34]: Cold starting and pumpability studies have been carried out in the 1993/1994 model year light duty gasoline engines under controlled climatic conditions and correlated to rheological properties of test oils. It was found that, within the engines, oils and cooling profiles tested in these protocols: 1. Modem design engines start, on average, at lower temperatures than earlier engine designs (approximately 5V). 2. An operational safety margin of 5-9 tT exists starting and pumping of these test oils.
between
846
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
3. The cranking and pumping characteristics of single grade (non VI improved) engine oils can be predicted from their low temperature rheological properties. 4. These modern engine designs have limiting average pumpability viscosities of approxim.ately 93 Pa -s as measured by the MRV [Mini-Rotary Viscometer] test, which is outside the measurement range of the current Scanning Brookfield technique. Air-binding was observed when significant structure was detected in the oils (>40 gelation index or MRV Yield Stress ±70 Pa) TABLE 7—Viscosity data for analyses of low-temperature high-shear studies at 0°F. Average Viscosity, mPa-s, at 0°F CRC* 8-engine Average 210/100°FExtrap.
Oils REO-174-63 REO-151-61 REO-152-61 REO-153-61 REO-154-61 REO-155-63 REO-156-63 REO-157-63 REO-15 8-61 REO-159-61 REO-160-61 REO-161-63 REO-162-63 REO-171-63 REO-172-63 REO-173-63 REO-175-63
(730) (1390) (2340) (3500) (8400) 1470 1830 2700 1800 2590 3110 1000 2760 1700 600 890 5040
670 1320 2320 3410 8050 1970 1710 1900 1910 1990 1900 1080 3190 1710 790 730 5850
NOTE: Items in parentheses are Newtonian calibration oils. TABLE 8—Viscosity data for analyses of low-temperature high-shear studies at - 2 0 ° F . Average Viscosity, mPa-ii, at -20°F Oils
aiO/lOO-FExtrap.
CRC* 10-engine Average
REO-174-65 REO-151-65 REO-152-65 REO-153-65 REO-155-63 REO-156-63 REO-158-63 REO-159-63 REO-172-65 REO-173-65 REO-183-65 REO-184-65
(3000) (6320) (11500) (19800) 6920 11400 6480 14360 2260 4040 3600 9440
2700 5760 10600 18900 6590 5280 5630 5880 2180 1800 2570 6750
NOTE: Items in parentheses are Newtonian calibration oils.
Low-Temperature High-Shear Tests of Engine Lubricants With the changes to engine lubricant formulations in the 1970s, it was anticipated that a problem might develop in low temperature starting. It had been known that some engines, when cranked by the starting motor, could start at cranking speeds as low as 6 rpm. Other lubricants were so high in viscosity that even though the engine could be cranked, the initial firing of the engine would not allow acceleration and starting of the engine. Prior to the 1970s, the normal way of estimating the kinematic viscosity at 0°F (-17.8°C) was by extrapolation, using the MacCoull-Wright equation from kinematic viscosity data at 100 and 210°F. Problems with this method became obvious. The new formulations sometimes were semi-solid at 0°F, even though the prediction by equation indicated the lubricant to be sufficiently fluid to allow flow at low temperatures, thus allowing the automotive engine to readily stcirt at low temperatures. Because there was a general lack of accepted information on both automotive engines and their lubricants at starting conditions, a large program [29,35,36,37] was undertaken with the cooperation of the Society for Automotive Engineers (SAE) Fuels and Lubricants committee, the ASTM Committee on Petroleum Products and Lubricants (Subcommittee 7 on flow properties), and the Coordinating Research Council (CRC). A group of five "Newtonian" Reference Engine Oils (REO) was prepared to cover the viscosity range of 730-8400 mPa-s at 0°F (— 17.8°C). These were used to calibrate full-size, motored engines in cold rooms at 0°F as if the engines were super-large viscometers. In addition, a group of twelve, multigrade REOs was prepared. This group of oils was typical of the variety of non-Newtonian, commercial engine oils. The viscosity of this second group of special oils was determined in the calibrated, full-scale engines also at 0°F. As part of this program, the same set of oils with the addition of several additional oils was also studied at —20°F (—28.9°C). The results, at 0°F and -20°F, respectively, are found in Tables 7 and 8. With this information, various bench-scale test instruments were tested to determine which would correlate with the engine-viscosity data. See Table 9. Data analysis was done on the 0°F data using linear regression [38]. Those viscometers in the above table generally did not undergo a full round robin program, but this is an excellent analysis of their various capabilities. On the initial analysis of the correlation data, the Brookfield rotational viscometer.
TABLE 9—Results of linear regression analysis. Intercept (mPa-s) Std Error of Est. (R)
Instrument
Slope
Extrap Kin Vis x density Ferranti-Shiriey (CAP) GMR Forced Ball GMR Forced Ball Brookfield Haake Rotovisco (CAP) PRL (16 cm capillary) PRL (8 cm capillary) SOD Mason Torsion Crystal High Shear-rate Rotational Cannon-Manning Pressure
0.817 0.908 0.999 0.979 0.572 0.962 0.881 1.08 0.824 0.923 1.65 0.985
447 -105 -54 -284 442 -86 79 -284 -23 139 -1340 -300
534 138 147 355 1400 153 257 358 381 193 390 582
Corr Coef 0.908 0.994 0.997 (gel-corrected) 0.984 (Uncorrected) 0.705 0.992 0.991 0.983 0.981 0.989 0.984 0.958
CHAPTER
32: FLOW PROPERTIES
AND SHEAR
STABILITY
847
the SOD viscometer, and the Cannon-Manning Pressure viscometers were ehminated iirom additional consideration, as the shear rate was too low. The Mason Torsion Crystal and Texaco High Rate of Shear viscometers were both very complex and of a limited availability. However, all of the data from laboratory instruments indicated that a higher shear would correlate better with the engine cranking data. About the time the above data became available, there was a progress report on the development of low temperature viscometric techniques by ASTM Section B on Flow Properties of Non-Newtonian Fluids. The GMR Forced Ball viscometer consisted of a thermostatted, closed-end cylinder filled with the test sample. A ball with a heavy weight was forced through the test sample, and the time of fall automatically measured (Fig. 18). Manning reported that the GRM Forced Ball viscometer, even with the Gel correction, would give inadequate correlation with the engine data [39]. At this same symposium two specially designed instruments showed promise. The reciprocating action of a single cylinder model airplane engine [40] (later made m u c h more sophisticated) by Stewart and Spohn gave much promise. At the same time, a single Couette rotational viscometric cell
FIG. 19—Cannon cold-cranking simulator. Reprinted with permission from the Cannon Instrument Company, State College, PA.
AIR BEARINGS PUTFORM REUASE CABLE
WEIGHTS
COUNTERWEIGHT
CONTACT SWITCHES AIR BEARING
BALL & SHAFT
OIL CYLINDER
FIG. 1 8 — G M R f o r c e d ball viscometer. Reprinted with permission f r o m t h e C a n n o n Instrument C o m p a n y , State College, PA.
turned by an AC/DC series-wound motor was suggested by Kim [41]. The AC voltage to the motor was plotted against the acquired speed of the rotor on an x-y chart recorder. Both sets of special oils were run at a range of voltages. The optim u m voltage was determined for best fit to the data from the motored-engine average. While b o t h instruments showed good correlation with the 7-engine data from motored engines, the Couette geometry seemed to be a more straightforward principle to develop into an instrument. Using the concept of Kim, Lloyd and Manning designed an improved bench-top Cold-Cranking Simulator (CCS), that has become the basis of engine oil viscosity measurements at high shear (10"* to 10^ s e c ^ ^ at low temperatures (Fig. 19). The CCS consists of a direct current electric motor that drives a rotor inside a copper stator; a tachometer to measure rotor speed; a controlled direct current to drive the motor; a stator temperature control system; and a coolant circulator compatible with the temperature control system. When the test oil is in the annular space between the rotor and stator and at test temperature, current is applied to the motor. The speed that the motor attains is a measure of the viscosity at high shear rate. The shear rate is in the range of lO^-lO^ s " ^ The use of this instrument and its later improvements serve as the partial basis for engine oil viscosity specifications [22]. Initially, the SAE use of the ASTM D 2602 test procedure required all test oils to be measured at 0°F ( - 17.8°C). The test
848 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 10—CCS requirements SAE Low-Temperature Viscosity Grade Cranking Viscosity (mPa-s)
ow 5W
low
15W 20W 25W
6200 at - 3 5 ° C 6600at-30°C 7000 at - 2 5 ° C 7000 at - 2 0 ° C 9500at-15°C 13000 a t - 1 0 ° C
NOTE: Reprinted with permission from SAE J300_19912 1999 SAE International.
and specification temperature was later lowered to — 18°C. The current test temperatures and specifications for each low temperature grade in SAE J300-99, and in ASTM D 5293 are found in Table 10.
LOW TEMPERATURE PUMPABILITY OF ENGINE LUBRICANTS Lubricant pumpability is critically i m p o r t a n t to engines. Catastrophic failure can occur if there is a lack of a full-flow of lubricant to the bearings (bearing starvation) of m o d e m engines and, thus, the rheology of oils at low temperatures is of highest importance. Thus, the term pumpability has been widely used in the lubricant industry to indicate the ability of a lubricant to be pumped and/or to flow to the surfaces being lubricated, especially at low temperature. Several papers appeared in the 1940s concerning the pour point stability of lubricating oils [42,43,44, 45]. In the early 1960s a study by Moyer [46] indicated that there may be a problem with the pumpability of engine lubricants at low temperatures. He observed that on pumping in a test apparatus, as the temperature is lowered in the oil pan, a hole can be drawn in the oil in the reservoir, causing the p u m p assembly to be air bound. Selby [47] measured rheological characteristics of reblends of oils previously studied by Moyer, showing that cavitation occurred at sheeir rates less than 50 s^^ and above a critical viscosity of 24,000 mPa-s. McMillan and Murphy noted that: An analysis of oil pumpability reveals that engine oil pumping failures may occur because either the oil cannot flow under its own head to the oil screen inlet, or the oil is too viscous to flow through the screen and inlet tube fast enough to satisfy pump demands [48]. They constructed a laboratory apparatus (Fig. 20), "to identify the rheological factors involved in low-temperature engine oil pumpability problems" over the temperature range of - 1 7 . 8 to - 4 0 ° C . Their work included Brookfield-type, cone-and-plate and concentric cylinder rotational viscometers. Soak-time, low-temperature studies for the oils were also included. Cavitation due to oil starvation and oil soak time contributing to pumping failures were important conclusions. A model relating to pumping failures was proposed, which suggests an oil with a Brookfield viscosity near 10 000 mPa-s will fail to p u m p due to oil starvation at the screen inlet if either it exhibits a yield stress or it is found to have a low-shear (0.2 s " ' ) to high shear (6.5 s~') viscosity ratio greater t h a n 2. If a hole from the surface of the oil to the inlet of the p u m p is formed and the surrounding oil cannot col-
lapse fast enough into the hole, the p u m p will become "airbound" and not continue to p u m p oil to the bearings of the engine (Fig. 6). Moyer [49] reported that ASTM pour points, channel tests and the — 17.8°C extrapolated kinematic viscosities were poor predictors of oil pumpability. Stewart and Smith [50] and Stewart and Spohn [51] reported that high shear ColdCranking Simulator viscosities, Brookfield viscosities, GM pour times, and ASTM pour points all failed to predict the oil gallery pump-up time in their test engine. "One test oil, which had a pour point of — 15°F and appeared to be solid at —20°F, p u m p e d satisfactorily in a test engine at — 20°F." It was quite evident t h a t a cooperative study of the factors regarding pumpability of engine oils at low temperatures must be performed, a n d a reliable bench test m u s t be developed for predicting this low-temperature pumpability. Many benchtest procedures were proposed, including (a) pour point (see ASTM D 97), vacuum viscometer (along the lines of ASTM D 2171), (c) Brookfield viscometer (see ASTM D 2983), (d) GM Pour Time, (e) pressure glass capillary viscometer, (f) cylinder with a hole in the bottom, (g) and several specially modified low-temperature, high-shear instruments including the Cold-Cranking Simulator (see ASTM D 2602), and Haake Rotovisco® viscometers. Engine gallery pump-up times gave a temperature at which the oil failed to p u m p . The ability of each of the several bench tests to predict the temperature at which full scale engines noted pumpability failure served as the criteria for a successful bench test. As a result of these works and those of many other investigators, the SAE Fuels and Lubricants Subcommittee 2 and the ASTM Subcommittee D02.07 on Flow Properties defined pumpability characteristics of reference oils in engines and developed a bench test method to predict satisfactory pumpability for use in lubricant specifications. In 1973, a large cooperative p r o g r a m was initiated by ASTM subcommittee D02/07, as described by Shaub, Smith and Murphy [55] by the following: A joint ASTM/SAE symposium, the first of several, was held in 1973 to better define the mechanisms of oil pumpability failures. A number of studies [48,50,52,53] were beneficial in this respect and provided a basis for formulating an ASTM program. The ASTM program established an engine test procedure and determined the pumpability performance of thirteen Pumpability Reference Oils (PROs) in seven full-scale engines. This program was begun in 1973,
PUMP OUTtET
'PUMP
AVAILABtE HEAD
J Olt INLET SCREEN
PUMP INLET TUBE
FIG. 20—McMillan/Murphy pumpability apparatus. Reprinted with permission from SAE Paper #730478 © 1973.
CHAPTER completed in 1974, and summarized in an ASTM report and an SAE Paper in 1975 [33,54]. The engine pumpability program included a degree of complexity that earlier engine cranking programs did not: in the engine pumpability program, oils were run at successively lower temperatures until the oil actually failed to pump. The data and mechanisms from the ASTM Engine Oil Pumpability Program have served as a basis to develop bench tests and to assess their ability to predict engine pumpability. There were 13 fully formulated engine oils r u n in 7 engines, according to the prescribed procedures, at successively lower cold room temperatures until pumping failure occurred. The temperature at which failure occurred was defined as the Borderline Pumping Temperature (BPT) and the mechanism of p u m p i n g failure recorded—air binding (AB) or flow limited (FL). See Table 11 [55]. Note that the several engines show as m u c h as 17°C differences in the FL BPT temperatures. Thus the design of the pumping assembly in the engine as well as the lubricant is very important. With the results of the ASTM Pumpability Program as a guide, a n u m b e r of instruments were evaluated to assess their ability to predict pumpability. These i n s t r u m e n t s included the Philippoff Rotary Viscometer (Fig. 21), a vacu u m capillary viscometer, the Haake rotary viscometer, the Texaco orifice viscometer, the Brookfield rotciry viscometer, and others. In order to narrow down the selection relative to possible round-robin studies, the Pumpability Data Analysis Panel (acting on a motion passed at the December 1975 ASTM meeting) visited the sites of the various pumpability test methods. Based upon the (1) degree of correlation with pumpability engine data, (2) cost, (3) ease of operation, and (4) number of samples which could be run at one time, the Data Analysis Panel recommended (at the June, 1976 ASTM meeting) that two tests—the mini-rotary viscometer (a miniaturized version of the (Phillippoffj rotary viscometer) and the vacuum pipette—be considered for round-robin studies. The panel also suggested that other tests be considered if their sponsors could provide a set of data showing correlation with the engine data [55]. The bench test resulting from this work was based on a simplified Couette-type rotary viscometer (mini-rotary viscometer or MRV) (Fig. 22).
32: FLOW PROPERTIES
AND SHEAR STABILITY
The test procedure included a two-hour soak period at 80°C to allow solution of any material not in solution at ambient temperatures. This was followed by a slow, carefully controlled cool down cycle similar to that experienced in engines over a 10-hour period and a final 6-hour soak at test temperature. The "Yield-Stress," if any, is first measured with static weights to cause rotation. Any yield stress detected is cause for suspicion that the lubricant may not be satisfactory for use at low temperature. Then using a higher stress (325 Pa), the viscosity of the lubricant is measured. This test procedure was formalized as ASTM D 3829. The MRV viscometer showed good correlation with the engine pumpability data [54], and ASTM D 3829 was adopted. Later work showed that the original 16-hour cooling cycle should be revised, and thus the current 48-hour slow cooling cycle (TP-1) has been adopted in ASTM D 4684 [54,56,57,58]. Through this study, much has been learned about the lowtemperature pumpability of engine lubricants and the gel-
'/////////////
AIR BEARING
OIL WEIGHT
STATOR
FIG. 21—Philippoff rotary viscometer. Reprinted with permission from SAE Paper #730481 ©1973.
TABLE 1 l—Engine BPT ("C). Individual Engine Averages Number of Engines PRO No
Total
AB
FL
01 03 05 06 07 08 09 10 11 12 13 15 16
7 4 7 7 4 4 4 7 7 7 7 7 7
3 0 5 0 0 0 3 5 0 0 2 0 0
4 4 2 7 4 4 1 2 7 7 5 7 7
849
Flow Limited Avg (Min/Max)
Avg (Min/Max)
- 3 5 (-38.5/-30) - 2 9 (-33.5/-24.5) -25(-32.5/-17) -23.5(-33/-15.5) -28(-31/-24) -28.5 (-31.5/-24.5)
- 3 7 (-39.5/-35.5)
-24
-28.5 (-30/-27) - 3 3 (-33.5/-32.5)
- 3 1 (-31.5/-30.5) -30.5 (-34.5/-25) -28.5 (-34.5/-23.5) - 3 1 (-33.5/-29.5) -33 (-38.5/-24.5) -23 (-29.8/-15)
-14.5(-22/-12)
-34.5
850 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK made about the behavior of fully formulated engine oils at low temperature with regard to pumpability problems: • Two modes of pumpability failure have been identified. The first and generally the more important is "air binding," wherein the oil does not slump adequately on pumping when the engine is started at a low temperature. As the oil immediately above the p u m p inlet is moved through the pump, air is drawn through a hole in the oil in the oil pan and into the oil p u m p . Thus further oil is not pumped to the bearings and overhead cam. The second failure mode is identified as "flow limited." In this failure mode, the oil is too highly viscous to flow adequately to the engine bearings. An oil exhibiting flow limited failure will most likely also show too high a viscosity for starting the engine as tested by the Cold-Cranking Simulator (CCS).
FIG. 22—Mini-Rotary Viscometer (l\1RV). TABLE 12—SAE J300 maximum low-temperature viscosity specification. SAE Viscosity Grade OW 5W
low
Low-Temperature Viscosity, Maximum" 60 60 60 60 60 60
000 m P a s at - 4 0 ° C 000 mPa-s at - 3 5 ° C 000 mPa-s at - S C C 000 mPa-s at - 2 5 ° C 000 m P a s at - 2 0 ° C 000mPa-sat-15°C
15W 20W 25W " Tlie presence of any yield stress detectable by this method constitutes a failure regardless of viscosity. NOTE: Reprinted with permission from SAE J300_19912 © 1999 SAE International.
structure that can form when engine lubricants are cooled slowly. The Engine Oil Classification Specification J300 [59] sets limits for both yield stress and viscosity as indicated in Table 12. In the meantime, other cool-down cycles and rotational instrumentation have been proposed and considered by the appropriate ASTM subcommittee. In particular, a slowly rotating, continuously recording viscometer cdso can show a sudden increase in viscosity whenever a gel-structure occurs at a narrow reinge of temperatures during a slow-cool cycle. One such instrument is the Scanning Brookfield design. (See Fig. 23) After preheating at 90°C, the test cell is immersed in a liquid cold bath and cooled at l°C/h over the temperature range of —5 to —40°C. Gelation Index, Gelation Index t e m p e r a t u r e a n d Critical Pumpability (in the range of 30 000-40 000 mPa-s) are processed in the computer from the viscometric data. This test has been given the general name "Temperature-Scanning Technique" ASTM D 5133 [60,61]. With a great a m o u n t of work by many investigators, in particular Henderson [62] and many others on ASTM subcommittee D02.07.C, a n u m b e r of generalizations have been
• As formulated oils cool to low temperatures, the wax can crystallize out of solution. If the wax crystallizes into small, unconnected particles within the continuous phase of liquid, it is unlikely that any structure will form that would prevent the oil slumping to the inlet of the oil p u m p . Conversely, if it crystallizes onto a relatively small number of wax crystals, long, interconnected chains of wax fibers can occur throughout the liquid oil solution. This forms a "gelstructure" which can have a yield-stress sufficient to cause an air-binding t j ^ e failure of the oil at low temperature. • Some formulated oils are designed to have base stock and additive packages containing such a small amount of wax that no significant gel-structure will form at low temperature. • A pour-point depressant is often a part of the formulation of engine oil. This depressant can form crystal sites at a predetermined low temperature, thus providing many sites onto which wax will crystallize on cooling to lower temperatures. The temperature at which these sites form must be higher than the temperature at which the wax will crystallize by itself. Otherwise, significant wax can crystal-
Operating " fluid level leve
Glass Stator (Inner dianrefer 22.07 mm)
Rotor (Length: 65.5 mm; Diameter 18.42 mm)
Fill line
(marked on Sfator)
\
^
FIG. 23—Scanning Brookfield cell diagram.
CHAPTER 32: FLOW PROPERTIES AND SHEAR STABILITY lize before crystal sites are formed by the pour-point depressant crystal, and the conditions may be right for forming a gel-structure, thus causing air-binding pumpability failure. • The rate of cool-down through a temperature range can be very important. A very rapid cool-down through a temperature range m a y not allow sufficient time for the formulation of a gel-structure. As the viscosity of the engine oil increases at lower temperatures, the mobility of the wax molecules in the solution is greatly diminished and this reduced mobility can prevent formation of a gel-structure. • It has been difficult to formulate a test oil that demonstrates only marginal pumpability problems in engines. Fortunately, there have been only a few commercial oils that have shown failures in the field, although these oils have actively been sought for testing. The design of the engine pumpability hardware is very important. It has been demonstrated that the temperature for pumping failure can vary by more than 10°C between engines due to the differences in the p u m p and inlet designs. By the early 1990s, the design of the engines had changed such that the earlier studies of pumpability might no longer be valid for m o d e m engines. Thus, the Society of Automotive Engineers's Fuels and Lubricants Division requested a new study on pumpability of engine oils, with the following goals: • to determine if there exists a "safety margin" between limiting cranking and p u m p i n g viscosities in m o d e r n engines, • to determine the cranking and p u m p i n g limitations of single-grade (non Vl-improved) engine oils, and • to assess the benefits or limitations of current methods for identifying oils which could result in pumpability failures in engines [63]. This Low Temperature Engine Performance (LTEP ) study included preparation of test oils for study in both modern engines and test apparatus, and has served as a basis for revision of the SAE low temperature specifications for engine oils (SAE J300) [22].
a still lower temperature and through a critical temperature range rapidly, will flow in a normal manner. As the temperature of a petroleum product is lowered, wax contained in the product can crystallize into long crystals, thus forming a gel structure. Frequently, chemicals such as "pour-point depressants" are added to fluids to lower the t e m p e r a t u r e at w h i c h gels can form. S u c h pour-point depressants serve to lower the pour-point of the fluid by forming many, m a n y crystals at a temperature somewhat higher t h a n the t e m p e r a t u r e at which wax crystals form. With cooling to still lower temperatures, wax will crystallize onto the unconnected crystal sites of the pour-point depressant instead of forming a gel structure. Without the yield stress provided by an interlocking gel-structure, the material will show a much lower pour point. Chapter 33, entitled Cold Flow Properties, discusses in detail p o u r point tests, aviation fuels freezing tests, and the cloud point and wax appearance point tests.
SHEAR STABILITY OF POLYMER-CONTAINING OILS With the advent of polymer-containing lubricating oils in the 1960s, there was concern that the polymeric additives may degrade under the influence of the high-shear and high-temp e r a t u r e environment found in a n operating engine. A n u m b e r of devices, including mechanical shear devices, a sonic oscillator and nuclear irradiation, were examined as possible bench-test apparatus. This research resulted in the development of a sonic oscillator device (Fig. 24) specified in
OTHER LOW TEMPERATURE TESTS The viscosity of a liquid consisting of only one type of molecule will increase when the liquid is cooled. If the liquid is cooled sufficiently, the viscosity will increase until the liquid solidifies (freezes). However, most petroleum products are composed of mixtures of m a n y different types of molecules, each of which has its own freezing point. When these liquids are cooled to low temperatures, unanticipated results can occur. The diesel fuel for cars and trucks can form crystals that will plug the fuel filter while the vehicle is being driven, causing the vehicle to stop. Heating oil can be loaded into a delivery vehicle, but at very low temperatures the oil may not flow through the delivery hose to the business or h o m e receiving the oil. Automotive engine p r o d u c t s in conventional quart, liter, or gallon containers may, when left outside the filling station in the winter, solidify so they will not p o u r out of the container. Some petroleum products will form a "gel" structure at a certain low temperature. However, the same material when cooled from a warm temperature to
851
FIG. 24—Sonic shear stability cell.
852
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
V) Motor (2) (3) (4) (5) (6) (7) (8) (9) (TO) (11) (12) (73) (14)
Pump Filter Nozzle holder Nozzle Upper reservoir Solenoid drain valve Lower reservoir Solenoid drain valve Sample-collection bottle Pump control rack Rack adjustment screw Rack spring Filling funnel
FIG. 25—Fuel Injector Shear Stability Test (FISST). A S T M D 2 6 0 3 m 1967: This test method permits the evaluation of shear stability with minimum interference from thermal and oxidative factors that may be present in some applications. Within the limitations expressed in the scope of this test method, it has been successfully applied to hydraulic fluids, transmission fluids, tractor fluids and other fluids of similar applications. It has been found applicable to fluids containing both readily sheared and shear-resistant polymers. Correlation with performance in the case of automotive engine applications has, to date, not been established (ASTM D 2603) (Fig. 24). Correlation of the shear stability found in mechanical devices with the shear stability found in bench tests is limited to within a given family of polymers. Such correlation has not been found to be very successful when comparisons are made between polymer families. Nevertheless, correlation between members of the same polymer family has been established. A 30-mL sample of a poljoner-containing oil is irradiated in a sonic oscillator for a preset time, and the changes in kinematic viscosity are determined according to ASTM D 445. This device has been established to evaluate the shear stability of a hydraulic fluid in terms of the final viscosity that results from irradiating a sample of the hydraulic fluid in a sonic oscillator (ASTM D 5621). Considerable development work on alternate shear stability devices has been undertaken both in Europe and in North America. The use of a diesel injector nozzle to study the degradation of polymer-containing blends has resulted in the use of the European diesel injector test equipment and the Fuel Injector Shear Stability (FISST) equipment (ASTM
D 3945, ASTM D 5275, ASTM D 5621) [64]. See Fig. 25. Both procedures: . . . evaluate the percent viscosity loss for polymer-containing fluids resulting from polymer degradation in the high shear nozzle device. Minimum interference from thermal or oxidative effects are anticipated. . . . This test method is not intended to predict viscosity loss in field service for different polymer classes or for different field equipment. Some correlation for a specific polymer type in specific field equipment can be possible. The a p p a r a t u s consists of two fluid reservoirs, a singleplunger diesel fuel injection p u m p with a n electric motor drive, a pintle-tjqDe fuel injection nozzle and control instrumentation. The shearing severity of the apparatus is verified by a standard reference oil and measurements of the kinematic viscosity at 100°C. Test specifications in Europe also refer to the Coordination European Council (CEC) standards (ASTM D 6278) [65-67]. According to test method D 3945: [T]he polymer-containing fluid is passed through a diesel injector nozzle at a shear rate that causes the less shear stable molecules to degrade. The resultant degradation reduces the kinematic viscosity of the fluid under test. This loss of kinematic viscosity is a measure of the shear stability of the fluid. It is important in the use of these polymer-containing lubricating oils that the viscosity of the oils remain stable and not decrease as a result of degradation of the polymer, especially in the high shear and high t e m p e r a t u r e conditions of m o d e r n gasoline and diesel engines.
CHAPTER The permanent shear stabUity index (PSSI, see ASTM D 6022) is a m e a s u r e of the change due to shearing in an additive's contribution to the lubricant's viscosity. PSSI= 100X(Vo-Vs)/(Vo-Vb) where:
PSSI Vo Vs Vb
= = = =
(15)
Permanent Shear Stabihty Index, viscosity of the unsheared oil, viscosity of the sheared oil, and viscosity of the base fluid.
From the notes to ASTM D 6022, Some methods, especially engine tests and field service, may include conditions where other effects (e.g., evaporative loss, oxidation, fuel dilution, soot accumulation, etc.) contribute to viscosity changes. The PSSI calculated from these types of services may not be representative of pure shearing.... PSSI may depend more strongly on base fluid, additive concentration, additive chemistry, and the presence of other additives for base fluids of unusual composition (e.g., esters) or if additives outside the common range of chem^istries and concentrations are used. Caution should be exercised when interpreting results from, different sources.
USE OF ASTM FLOW PROPERTY PROCEDURES While there are many rheological test instruments, the ASTM flow-related methods of test are developed to satisfy a need for a specific test related to the performance of the material in the use to which the material is to be put. The particular rheological device(s) in each test is selected because it is satisfactory for the test or in some cases, it may have been developed specifically for the needs of the test. The wide n u m b e r of rheologicsd devices available is of little consideration in the writing and use of the flow-related test methods, except that those chosen represent the specific needs for testing the material. In general they will allow the test method to be used in a specification for the material. There are a great many specifications for petroleum materials, most of which refer to one or more of the flow-related methods. An outstanding example of such specifications is for the performance of engine oils, as described in the introduction to ASTM D 4485. This document covers all the currently active American Petroleum Institute (API) engine oil performance categories that have been defined in accordance with the ASTM
32: FLOW PROPERTIES
AND SHEAR STABILITY
consensus process In the ASTM system, a specific API designation is assigned to each category. The system is open-ended, that is, new designations are assigned for use with new categories as each new set of oil performance characteristics are defined.... Other service categories not shown in this document have historically been used to describe engine oil performance (SA, SB, SC, SD, SE, SF, SG, CA, CB, CC, CD, CD-II, CE). The Society for Automotive Engineers, Inc. (SAE) has specified engine oil performance categories and classifications for many years. The SAE defines engine oil performance categories for gasoline and diesel engines. SAE J183, Engine Oil Performance and Engine Service Classifications [68], outlines the engine oil performance categories and classifications developed t h r o u g h the efforts of the American Petroleum Institute (API), ASTM International (ASTM), the American Automobile Manufacturers Association (AAMA), the Engine Manufacturers Association (EMA), the International Lubricant Standardization and Approval Committee (ILSAC), and SAE. These specifications for engine oils incorporate a great many ASTM methods such as sequence tests, volatility, chemicals, flash point, foaming tendency, etc. in addition to those for flow-properties. Included in this specification is a discussion as to the background for each category, the dates the category was in use and the reasons why the category became technically obsolete. The several categories are included in Table 13. The reader is encouraged to refer to SAE J183. Current flow-property specifications for gasoline engines are included in SAE J300 [22], making reference to II ASTM flow-property methods. This specification states that "the limits specified are intended for use by engine manufacturers in determining the engine oil viscosity grades to be used in their engines, and by oil marketers in formulating, manufacturing, and labeling their products." The OW, 5W, lOW, 15W, 20W a n d 25W SAE viscosity grades for engine oils specifications at low-temperatures from —35 to — 10°C refer to the high shear Cold-Cranking Simulator (CCS ASTM D 5293) tests. Pumpability specifications include temperatures from —40 to — 15°C and reference the Cannon Mini-Rotary Viscometer (MRV ASTM D 4684). Each of the "W" grade oils has a m i n i m u m kinematic viscosity limit at 100°C. From SAE J300, . . . because engine pumping, cranking, and starting are all important at low temperatures, the selection of an oil for winter operation should consider both the viscosity required for successful oil flow, as well as that for cranking and starting, at the lowest ambient temperature expected.
TABLE 13—Lubricant category chronology. API Category API Category SA (1900-1930) API Category SB (1931-1963) API Category SC (1964-1967) API Category SD (1968-1971) API Category SE (1972-1979) API Category SF (1980-1988) API Category SG (1989-1992) API Category SH (1993-1996) API Category SJ (1996-«»«)
853
Description No additives. Not suitable for use in gasoline automotive engines built after 1930. Minimum level of performance additives. Oil considered non-detergent. Additives improved to meet multicylinder engines sequence tests. Enhanced additive treatment to meet performance requirements for current engines. Emission control, high speed driving. CAFE advances required improved oils. Reduced oil consumption, oxygenated fuels, longer drain intervals requuired new oils Sequence HIE and VE to evaluate oxidation and sludge, CAFE and fuel injection. Phosphorus and volatility control tests added. Reduced oil consumption. Continuous improvements in motor oils as required for modem engines.
NOTE: reprinted with permission from the Society of Automotive Engineers, Warrendale, PA. The reader is encouraged to refer to SAE J183.
854
MANUAL
37: FUELS
AND LUBRICANTS
HANDBOOK
Each of the five SAE grades 20, 30, 40, 50, cind 60 reference the low-shear-rate kinematic viscosity test range (ASTM D 445). The same grades also reference high shear (10* s"')> 150°C tests (ASTM D 4683, or D 4741, or D 5481) to ensure a m i n i m u m high-shear viscosity for each of the five SAE grades. KINEMATIC VISCOSITY/TEMPERATURE RELATIONSHIP In 1921, Neil MacCoulI created a viscosity - t e m p e r a t u r e chart (Fig. 26) for sale by The Texas Company [69]. The ordinate of the chart was the double logio of the kinematic viscosity plus a constant and the abscissa was the logio of the absolute temperature. This chart was used to graphically show the relationship between viscosity a n d t e m p e r a t u r e for mineral oils. It was stated that A close study of the various values of viscosity at different temperatures reveals the fact that temperature-viscosity relations can be so expressed by a simple formula that when plotted to the proper coordinates a straight line relationship is shown ... It has been found that there is no curvature to the line for any mineral oil so far tested, until the temperature has been reduced to the point where paraffine begins to precipitate . . . [69]. The equation used in the graph was logio (logio {v + constant)) = A - B logio T where,
pears that Walther may not have been aware of the prior work by MacCouU until 1929 when advised by Herschel [73]. The constant in the above equation is required for the hightemperature, low-kinematic-viscosity part of the chart. The constant was the subject of study in the 1930s, and various values from 0.6 to 0.8 have been used. However, as reported by W. Andrew Wright [74], . . . a constant of 0.6 was used down to viscosities of 1.5 cSt. Below this viscosity, the constant was changed to 0.65 for the range from 1.5 to l.OcSt, 0.70for l.Oto 0.7 cSt, and 0.75 for 0.7 to 0.4 cSt. This problem was the subject of an intensive study by Wright resulting in the development of an improved chart at very low kinematic viscosities and high temperatures. The equation for the new charts is listed in the appendix to ASTM D 341 93. In 1974, Manning suggested a modification of the equations by Wright. These equations, shown below, are also listed in the appendix to ASTM D 341 - 93, and have been widely used, especially in the computational fitting of kinematic viscosity-temperature data by computers [75]. This relationship is expressed as follows: logio logio Z = A - B logio T
(17)
Z = V -I- 0.7 + exp(-1.47 - 1.84v - 0.51 v^)
(18)
V = [Z - 0.7] - exp(-0.7487 - 3.295 [Z - 0.7] + 0.6119 [Z - O.lf - 0.3193 [Z - 0.7]^)
^^^^
(16)
v = kinematic viscosity, mm'^/s or cSt T = absolute t e m p e r a t u r e , degrees Rankine or Kelvin A, B = coefficients determined for the liquid.
In addition to charts prepared with kinematic viscosity in units of centistokes versus temperature in degrees Fahrenheit and degrees Centigrade (Celsius), charts were also prepared with the ordinate in Saybolt Universal seconds (SUS) and Saybolt Furol seconds (SFS). The abscissa of these charts is the logarithm of the absolute temperature and could be in degrees Fahrenheit or in degrees Celsius. The "constant" in the above equation is necessary if the kinematic viscosity is less than 1. Otherwise, the term logio (logio W)) cannot be evaluated because it would require the taking of a logarithm of a negative number. According to an article in Lubrication [70], the early charts had a constant varying from 0.4 to 1.0, and 0.7 was "considered to give the best overall results." The chart (see Fig. 26) was later published in the 1927 International Critical Tables [71], along with the following paragraph: Variation of Viscosity with Temperature—If the viscosity of an oil is known at two temperatures, its viscosity at a third temperature may be obtained graphically with the aid of [Fig. 26]. When the viscosity temperature values for any oil are graphed on this chart, a straight line will be obtained for all portions of the temperature range within which the oil remains a homogeneous liquid of constant composition [69]. Copies of this chart may be obtained by addressing The Texas Company, New York City. In the meantime, C. Walther published other viscosity-temperature charts in the late 1920s. After experimenting with several different equations, Walther adapted the equations of MacCouU, using a constant of 0.8 instead of 0.7 [72]. It ap-
V = kinematic viscosity, xnm^ls (or cSt) T = temperature, K (or °R) A and B = constants A review of the MacCoull viscosity-temperature charts is given in the May and June 1950 issues oi Lubrication. Various alternative charts have been proposed by others [76,77,78,79]. Andrew Wright was chairman of the ASTM D02 Subcommittee 7 on Flow Properties for a n u m b e r of years, and his paper [74] contains information on the development of the ASTM Viscosity-Temperature charts and their use within ASTM. The MacCoull chart has been published by ASTM since 1932, starting with the standard D 341 - 32T. Some liquids show a good relationship with the equation given in the Andrade [80] paper of 1930 (and also those of S. E. Sheppard, Ejring and others). This is of the form: 77 = A e^'T
(20)
The MacCoull equation is essentially of a similar form: 77 + 0.7 = A e'^'T'^
(21)
The SUS/°F charts were abandoned many years ago with the worldwide emphasis on the use of the System Internationale (SI) units in the specifications for petroleum products. For many years a segment of these viscosity-temperature charts has been used for blending two-component stocks. The constant-temperature method of blending requires the kinematic viscosity of each component at a given temperature. Using the chart from 0-100°F as if these temperatures represented 0 and 100% of the two components respectively, the kinematic viscosity is plotted on the charts, drawing a straight line between 0 and 100% of the components. Thus, the approximate kinematic viscosity of blends could be estimated from the straight line connecting the two components
CHAPTER 32: FLOW PROPERTIES AND SHEAR STABILITY
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VISCOSITY-TEMPERATURE CHART FIG. 26—Neil MacCoull 1927 chart. Reprinted with permission from Texaco, Inc., White Plains, NY.
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ISO
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856 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK Using the equations of the viscosity-temperature charts (see Figs. 27, 14, a n d 15), this m e t h o d can be calculated by computer. It can be adapted to multi-component blends. Table 14 shows the seven charts available from ASTM [82] as described in ASTM D 341 (see Figs. 28 and 29).
at blends from 0-100% of the one component. This constanttemperature blending method is not very accurate, and mention of this process (as well as a special chart) was withdrawn from ASTM D 341 many years ago. The constant-viscosity method reported by W. A. Wright [81] is more accurate and is described in the appendix to ASTM D 341-93 as follows: Plot the known data for each component on an ASTM Viscosity-Temperature Chart and carefully draw straight lines through the points. The lines should extend beyond the blend kinematic viscosity required. Locate, or draw, the desired blend kinematic viscosity horizontal line on the chart through both of the component oil lines. Lay a centimeter scale along this line and carefully measure the distance between the lines for the two oils where they cross the line of the desired blend kinematic viscosity. Without moving the scale, on the same horizontal kinematic viscosity line read the distance from the low viscosity oil line to the temperature desired. Dividing the latter by the first measurement between the two oils gives the volume fraction needed for the high viscosity oil.
VISCOSITY INDEX As described in the Significance and Use section of ASTM D 2270 (originally published by ASTM as D 567 - 40T, and revised and renumbered in 1964), The viscosity index is a widely used and accepted measure of the variation in kinematic viscosity due to changes in the temperature of a petroleum product between 40 and 100°C. A higher viscosity index indicates a smaller decrease in kinematic viscosity with increasing temperature of the lubricant. The viscosity index is used in practice as a single number indicating temperature dependence of kinematic viscosity.
ui O
o z
hi O I;; U>
in O
O
o
U
< w z
z
40
50
eo
ro
80
90
100
120
TEMPERATURE. DEGREES. CELSIUS F I G . 2 7 — O i l b l e n d i n g calculations,
TABLE 14—ASTM viscosity-temperature chart descriptions. Kinematic Viscosity Chart #
Description
Range (cSt)
Temperature
I II III IV V VI VII
High Range Low Range High Range Low Range High Range Low Range Middle Range
0.3 to 20 000 000 0.18 to 6.5 0.3 to 20 000 000 0.18 to 6.5 0.3 to 20 000 000 0.18 to 3.0 3 to 200 000
-70to+370°C -70to+370°C - 7 0 to -l-370°C -70to+370''C -100to-l-700°F -100to+700°C -40to+150°C
NOTE: Additional information is given in ASTM D 341-93.
Size (mm) 680 by 680 by 217 by 217 by 680 by 520 by 217 by
820 820 280 280 820 820 280
Facsimile Reference Fig. 28 Fig. 29 not included not included not included not included not included
CHAPTER
32: FLOW PROPERTIES
AND SHEAR
STABILITY
857
i U « . tHOHH. MLStuS
ittttrMnjt. onma. ceiaiua
FIG. 28—Facsimile of Kinematic Viscosity-Temperature Chart 1 High Range (temperature in degrees Celsius).
Viscosity index (VI) numbers have been used for many years in the marketing of petroleum products. It was well known t h a t lubrication fractions of oils from various sources of crude oils showed large differences in their variation of viscosity with a change in temperature, and a way to characterize this change was needed. As described on page 69 of the June, 1950 edition oi Lubrication [83], The original viscosity index system was set up by an arbitrary selection of two series of oils derived from opposite extreme types of crude oil: The "H" series having the less change in viscosity with temperature, and the "L" series the greater change. Seven blends were made up in each series, with viscosities varying from about 200 to 1200 seconds Saybolt at WOT. Each oil in the "H"series was arbitrarily assigned a V.I. value of 100, whereas each oil in the "L" series was assigned a V.I. value of 0. In practice, a reference oil is selected from each series with the same viscosity at 210 as the oil to be evaluated. The numerical difference in viscosities of these two oils at WOT will always, by defini-
tion, represent 100 V. I. divisions. Thus the V. I. of an oil may he computed by the equation below: VI = ((L - U)/(L - H)) X 100 Where
(22)
L = kinematic viscosity at 100°F of an oil of 0 viscosity index having the same kinematic viscosity, Y, at 210°F as the oil whose viscosity index is to be calculated, mm^/s (cSt) H = kinematic viscosity at 100°F of an oil of 100 viscosity index having the same kinematic viscosity, Y, at 210°F as the oil whose viscosity index is to be calculated, mm^/s (cSt), U = kinematic viscosity at 100°F of the oil whose viscosity index is to be calculated, mm^/s (cSt). The original scale was proposed by Dean and Davis in 1929 [84] and revised by Davis, Lapeyrouse, and Dean in 1932 [85] and in ASTM D 567. The original scale was based on data for L and H in the range of 7.29 to 75 cSt at 210°F. Later, light oils were used to extend the table between 2.0 and 3.99 cSt.
858
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
FIG. 29—Facsimile of Kinematic Viscosity-Temperature Chart 11 Low Range (temperature in degrees Celsius). Values for L and H were obtained for the range of 4.00 to 7.29 mm^/s (cSt)
developed in the use of Method D 567 - 53. W. A. Wright, September 1965.
. . . by a special method of calculation which was equivalent to graphic interpolation on a greatly magnified scale. In this range of viscosity the constants are not defined by equations [86].
With the world using the System International (SI) metric units, it was agreed at a London conference [92] in November 1972 that the traditional measurements of kinematic viscosity at 100 and 210°F would be replaced by measurements at 40 and 100°C. The Viscosity Index tables were redone using the MacCouU equation of kinematic viscosity versus temperature for recalculation of the new tables of L and H. The change in these tables results in only minimal changes in the VI of oils as previously based on the kinematic viscosities at 100 and 210°F [93]. With the adoption of the 40 and 100°C temperatures for kinematic viscosity measurements, a book of viscosity index tables [91] has replaced the former degree Fahrenheit tables.
It had been assumed that all oils would have Vis between 0 and 100. However, many m o d e m oils, especially those with the additive packages, have a VI above 100. Extrapolation of Vis u p to 120 seemed to fit the viscosity index scale without a problem. However, there was a serious problem above 130. Accordingly, in the late 1950s W. Andrew Wright [87,88] devised a special calculation to extend the VI calculation above 100. This was adopted into ASTM D 2270 in 1964, and for several years the viscosity index so calculated used the symbol Vlg to differentiate VI between the new extended VI equation and that by extrapolation from the equation for use u p to a VI of 100. ASTM had published a book of viscosity index tables based on 100 and 210°F temperatures as a Special Technical Publication [89,90]. This book was replaced by another book [91] based on D 2270 - 64, but for temperatures in degrees Celsius. The forward states in part The studies leading to the change in calculating the viscosity index are the result of recommendations by the Third and Fourth World Petroleum Congress that a method be selected which would avoid the problems which ultimately
Calculation of Viscosity Index The equations for the calculation of viscosity index are given in ASTM D 2270, along with the tables of L and H, and equations for the values of L and H above 70 mm^/s. For values of viscosity index from less t h a n 0 u p to and including 100, the equation (Procedure A) VI = ((L - U)/(L - H)) X 100
(23)
is used. Viscosity index of 100 and above use (Procedure B) VI = [((antilogio N) - 1)/0.00715] X 100
(24)
CHAPTER where N = (logio H - logio U)/logio Y
(25)
The values for L and H are shown in Table 15. Table X2.2 from ASTM D 2270 (Table 16) provides the values for determination of L and H at different viscosities. For kinematic viscosities above 70 mm'^/s at 100°C, the following values are used:
L = 0.8353 Y^ + 14.67 Y - 216
(26)
H = 0.1684 Y 2 + 11.85 Y - 9 7
(27)
and
The following example demonstrates the calculation of viscosity index both for values of VI below 100 and above 100 (see Table 17). Calculation example A: Measured kinematic viscosity, U, a t 4 0 ° C = 171.8 Measured kinematic viscosity, Y, at 100°C = 15.69 by interpolation, L = 320.74 and H = 159.95 VI = 100*(320.74 171.8)/(320.74 - 159.95) = 92.63 and rounding to the nearest whole number, VI = 93 Calculation example B: Measured kinematic viscosity, U, a t 4 0 ° C = 120.1 Measured kinematic viscosity at, Y, 100°C = 15.78 by interpolation, H = 158.28 N = [(log 158.28 - log 120.1)/log 15.78] = 0.100060 VI = [((antilog 0.100060) - 1)/ 0.00715] + 100 = 136.24 and rounding to the nearest whole number, VI = 136 There are some highly waxy base stocks that can be determined at 100°C but not at 40°C due to wax formation. The kinematic viscosity of such materials can often be determ i n e d at a higher t e m p e r a t u r e , e.g., SOX. Using the MacCouU equation the kinematic viscosity at 40°C can be estimated. This allows the VI to be estimated, but this value is not to be considered the true VI of the material. The precision attributed to the viscosity index is based entirely on the precision of the kinematic viscosity measurement. For base and formulated oils, information in Table 18 is calculated from the repeatability and reproducibility of ASTM D 445. Tak:e note that the precision of the kinematic viscosity measurement is different for each of the types of materials, and thus the precision attributed to viscosity index is a function of the product. Table 18 indicates the precision to be attributed to viscosity index for base and formulated oils [94]. Because of the importance of VI in the metrketing of petroleum products, it is important to keep in mind that the reproducibility attributed to base and formulated oils may vary from 2-9 VI units. Thus, the rounding of the VI to the nearest whole n u m b e r is required. Other proposals have been made for characterization of the vatriation of kinematic viscosity with temperature [95,96], especially by E. E. Klaus et al. from the Petroleum Refining
32: FLOW PROPERTIES
AND SHEAR STABILITY
859
Laboratory of The Pennsylvania State University. While the use of the calculated VI serves as a measure of the rate of change of kinematic viscosity with temperature, the slope of the kinematic viscosity data plotted on the ASTM Viscosity Charts also serves this purpose. This slope, when plotted on the ASTM Viscosity Charts, is known as the "ASTM Slope." From the introduction of a report [97], "ASTM Slope" has become a well-known term and has attained rather wide usage as a measure of the viscositytemperature characteristics of oik. Plots of the viscosity temperature data for petroleum oils on the ASTM Charts for Liquid Petroleum Products (D 341-39) result in straight lines. . . ASTM Slope of an oil may be defined as the negative "measured" slope, or the measured linear distance on the ordinate of ASTM Charts C and D (D 341-39) between viscosity values at 100°and 210°F divided by the measured linear distance between 100°and 210°F on the abscissa. For base stocks of mineral oils without polymeric additives, the VI may also serve as an indication of the chemical nature of the liquid. For example, a material with a VI = 10 likely has a high naphthalenic component, while a paraffinic base stock may have a VI = 95. A lubricant with a VI = 150 is likely a blend of mineral oil and a polymeric additive, or a synthetic fluid (Fig. 30). The use of the ASTM Slope has diminished over the years, probably due to the wider use of Viscosity Index in lubricant specifications. Pressure Considerations It is well known that the viscosity of lubricants increases with an increase in isotropic pressure at a constant temperature. Indeed, the increase in viscosity can be several orders of magnitude, as illustrated in Fig. 31 [98,99]. An empirical model for the effects of both temperature and pressure is •»7 = K e'^'T e"P where
TJ = T = P = a = K, b =
(28)
viscosity, the absolute temperature, the pressure pressure-viscosity coefficient, Euid constants for the material.
Typically, this equation is valid over a 50 Kelvin temperature range and 1 Kbar pressure range for polymers [100,101]. Measurements of the viscosities of liquids at high pressures have been made by a n u m b e r of investigators. The works of So and Klaus [102] and Wu, Klaus, and Duda [103] show correlations of the pressure-viscosity coefficient developed for various types of liquids, including hydrogenated, naphthenic, ciromatic, and paraffinic mineral stocks, a number of pure hydrocarbons, etc. The correlations of the pressureviscosity coefficient of Wu are of the form a = mo(0.1657 + 0.2332 logvo) where
(29)
mo: Viscosity-temperature property from the ASTMMacCoull equation and equal to (ASTM slope)/0.2 VQ: Atmospheric kinematic viscosity at the temperature of interest, mm^ls
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CHAPTER 32: FLOW PROPERTIES AND SHEAR STABILITY vnO'OfNOO-^O'OrriON'OfNO'OfNOS^O'^'-tQO'Or'^'.-^OO
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861
862 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
TABLE 16—Coefficients of quadratic equations. ASTM D 2270. Ymin
Ymax
a
b
c
d
e
f
2.0 3.8 4.4 5.0 6.4 7.0 7.7 9.0 12 15 18 22 28 40 55 70
3.8 4.4 5.0 6.4 7.0 7.7 9.0 12 15 18 22 28 40 55 70 Up
1.14673 3.38095 2.5000 0.10100 3.35714 0.01191 0.41858 0.88779 0.76720 0.97305 0.97256 0.91413 0.87031 0.84703 0.85921 0.83531
1.7576 -15.4952 -7.2143 16.6350 -23.5643 21.4750 16.1558 7.5527 10.7972 5.3135 5.2500 7.4759 9.7157 12.6752 11.1009 14.6731
-0.109 33.196 13.812 -45.469 78.466 -72.870 -56.040 -16.600 -38.180 -2.200 -0.980 -21.820 -50.770 -133.310 -83.19 -216.246
0.84155 0.78571 0.82143 0.04985 0.22619 0.79762 0.05794 0.26665 0.20073 0.28889 0.24504 0.20323 0.18411 0.17029 0.17130 0.16841
1.5521 1.7929 1.5679 9.1613 7.7369 -0.7321 10.5156 6.7015 8.4658 5.9741 7.4160 9.1267 10.1015 11.4866 11.3680 11.8493
-0.077 -0.183 0.119 -18.557 -16.656 14.610 -28.240 -10.810 -22.490 -4.930 -16.730 -34.230 -46.750 -80.620 -76.940 -96.947
L = aY' +bY+ c H = dY' + cY+f
TABLE 17—Segment of table from ASTM D 2270. Kinematic Viscosity at 100°C, mm^/s 15.5 15.6 15.7 15.8 15.9
L
H
313.9 317.5 321.1 324.6 328.3
157.0 158.6 160.1 161.6 163.1
TABLE 18—Precision attributable to VI calculations. Procedure A VI == 100
VI = 0 Kinematic Viscosity at100°C (mm^/s) 4 6 8 15 30 50
Reproducibility, R
Repeatability, r
Repeatability, r
Reproducibility, R
Base Oil
Formulated
Base Oil
Formulated
Base Oil
Formulated
Base Oil
Formulated
0.98 0.71 0.57 0.45 0.39 0.36
2.31 1.68 1.35 1.06 0.92 0.85
5.77 4.20 3.38 2.56 2.29 2.11
6.75 4.91 3.95 3.11 2.68 2.47
0.73 0.40 0.30 0.20 0.14 0.11
1.73 0.94 0.70 0.48 0.33 0.26
4.32 2.35 1.75 1.19 0.82 0.65
5.05 2.75 2.05 1.39 0.96 0.76
Procedure B VI == 200
Vl= 100 Kinematic Viscosity at 100°C (mm^/s) 4 6 8 15 30 50
Repeatability, r
Reproducibility, R
Repeatability, r
Reproducibility, R
Base Oil
Formulated
Base Oil
Formulated
Base Oil
Formulated
Base Oil
Formulated
0.50 0.37 0.31 0.23 0.19 0.17
1.18 0.87 0.74 0.55 0.44 0.40
2.94 2.18 1.84 1.37 1.11 0.99
3.44 2.55 2.15 1.61 1.30 1.16
0.77 0.57 0.48 0.36 0.29 0.26
1.82 1.34 1.13 0.84 0.68 0.61
4.54 3.35 2.82 2.11 1.71 1.52
5.31 3.92 3.30 2.46 2.00 1.78
Wu, Bflaus and Duda [103] note the following: A simple and accurate method is presented for the prediction of the pressure-viscosity coefficients of various kinds of lubricating oils. For a given oil, the correlation only required the viscosity-temperature relationship and the viscosity at the temperature of interest of the oil. When it is applied to a polymer blend, the viscosity-temperature relationship of the base oil is also needed. The failure of this method to predict the pressure behavior of some synthetic hydrocarbons and nonhydrocarbons appears to be related to the unusual high, or low, bulk moduli of these fluids.... The proposed method has the simplest equation form of all
the methods published to date and is the only one related to free-volume theory. It is of importance to ensure that the lubricant will have the ability to function under pressure to lubricate bearings and gears. There are a number of tests related to the ability of the lubricant to resist adhesive wear, or scuffing under pressure. This is illustrated (ASTM D 5182) as follows: The transmission of pcnver in many automotive and industrial applications is accomplished through the use of geared systems. At higher operating speeds it is well known that the lubricant/additive system can be a significant factor in preventing scuffing (adhesive wear) damage to gears.
CHAPTER 32: FLOW PROPERTIES AND SHEAR STABILITY This test method (ASTM D 5182) is used to screen the scuffing load capacity of oils used to lubricate spur and helical (parallel axis) gear units. There are several extreme-pressure tests for lubricating fluids to differentiate the ability of the lubricant/additive systems to prevent scuffing damage: • ASTM D 5182 Test Method for Evaluating the Scuffing (Scoring) Load Capacity of Oils • ASTM D 2782 Test Method for Measurement of ExtremePressure Properties of Lubricating Fluids (Timken Method) • ASTM D 2783 Test Method for Measurement of ExtremePressure Properties of Lubricating Fluids (Four-Ball Method) • ASTM D 5183 Test Method for Determination of the Coefficient of Friction of Lubricants Using the Four-Ball Wear Test Machine
ABSOLUTE VISCOSITY OF WATER The primary viscosity standard is distilled water at 20°C. The measurement of the absolute viscosity of the primary viscosity standard from the absolute dimensions of the apparatus has sdways been a challenge. The establishment of the primary viscosity standard is made by measurements carried out in special instruments of known physical dimensions. The severEil geometries of these instruments include capillary tubes [104,105], the damping of oscillations of a water-filled glass sphere [106], and the damping of an oscillating disk or cylinder [107,108]. The measurement of physical dimensions of the several apparatus and the calculations of the viscosities of water by the several methods is very complex, and it is difficult to ensure that all uncertainties have been properly evaluated. The data from these several sources have been reviewed by Marvin [109] and Bauer [110]. Of especial note
A.S.T.M. STANDARD VISCOSITY-TEMPERATURE CHARTS FOR LIQUID PETROLEUM PRODUCTS (D 341-43)
soo 400
CHART C: KINEMATIC VISCOSITY, HIGH RANGE
:
300
\
200
^
ISO S
I/)
100
\, >>=s^
—i> s
S,
^' S
I—I
H
v ^ V-
5
•
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so
t^ TT V ^.* \ /o s;
40
—
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s,
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—
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S
30
s
V
"^
u ^^ H
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N k. v^
20
s^
V S •v
'•>
S
H
IS
SOs .
N*^
^^ ^ ^
^
10 9.0 8.0 7.0 6.0
90
100
110
120
130
140
863
150
160
170
180
190
200
TEMPERATURE, DEGREES FAHRENHEIT FIG. 30—Illustration of VI for VI = 10, 95, and 150.
210
220
230
240
864 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK VISCOSITY STANDARDS
10,000
1000 — o
a.
E >-"
100 -^
h; 00
O O
00
>
1000
2000
3000
PRESSURE, ATMOSPHERES FIG. 31—Viscosity as a function of pressure.
For almost all of the tests and procedures for the measurement of kinematic viscosity and viscosity, viscosity standards are necessary to calibrate the equipment and to ensure that the test is being carried out correctly. The kinematic viscosity range of standards commercially available is from about 0.6- to 100 000-mm^/s, over a temperature range from about —40 to 150°C. These standards cover the full range of the instruments used for the measurement of kinematic viscosity. The viscosity range of standards commercially available is from about 0.6- to 5 000 000-mPa-s (5000 Pa-s) over a t e m p e r a t u r e range from about —40 to 150°C. From the late 1930s to about 1959, ASTM Subcommittee D02.07 Section A on Newtonian Viscometry made available three API viscosity standards. These standards were sponsored by the American Petroleum Institute, and certification was done by m e m b e r s of ASTM D02.07.A. The standards were API Standard
Alpha Beta Gamma
T A B L E 1 9 — V i s c o s i t y of w a t e r . Temperature (°C)
Viscosity (mPa-s)
Density (kg/m')
Kinematic Viscosity (mm^/s)
15 20 23 25 30 40
1.1378 1.0016 0.9321 0.8899 0.7971 0.6524
999.10 998.20 997.54 997.04 995.65 992.21
1.1388 1.0034 0.9344 0.8925 0.7995 0.6575
is t h a t the several m e a s u r e m e n t s , while close, vary from 1.00035-1.00334 mPa-s, a variation of nearly 0.3%. For many years the viscosity value of 1.005 mPa-s (cP) at 20°C was accepted throughout the world. This value was derived from the evaluation by Bingham and Jackson [111], and based on their evaluation of all the available measurements [112]. New measurements of the value assigned to the viscosity of water began in the late 1930s and were completed in the early 1950s by Swindells, Coe, and Godfrey [113]. In 1953 these data resulted in a recommended revision to 1.002 mPa-s at 20°C based on the international temperature scale of 1948. It must be noted that there have been a number of temperature scales (e.g., in 1927, 1948, 1968, and 1990), each of which have small differences in the precise definition of t e m p e r a t u r e at 20°C. The data by Swindells et al. has been updated by Bauer [110] to the International Temperature Scale of 1990 (ITS90). The currently accepted values for the viscosity of water at atmospheric pressure are found in Table 19. This is published in ISO 3666 [114], which contains additional information on the variation of the viscosity of water at other pressures. The overeJl uncertainty of the viscosity and kinematic viscosity of water at 20°C is estimated to be 0.17%.
Kinematic Viscosity and Temperature
60 mm^/s at 100°?, and 8 mm^/s at 210°F 400 mm^/s at 100°F, 190 mm^/s at 120°F and 27 mm^/s at 210°F 3 mm^/s at 100°F, 86 mm^/s at -40''F and 350 mm^/s at -65°F
By the late 1950s, these limited kinematic viscosity ranges did not adequately meet the needs of the petroleum industry. Accordingly, a new set of seven viscosity standards (later expanded to nine standards and including density measurements) covering t h e range of 3- to 27 OOO-mm'^/s w a s sponsored by ASTM, and made available from Cannon Instrument Company (P. O. Box 16 State College, Pennsylvania 16804-0016). The standards from ASTM D 445 are found in Table 20. At the same time, the National Bureau of Standards had a separate series of viscosity standards widely used in the chemical industry, while the ASTM series was widely used by the petroleum industry. This series was available from the late 1930s, covering the range of 2- to 45 000-mPa-s [115]. After the expanded ASTM series became available, these two series duplicated each other for the most part, and the two series were merged [116] into the series now listed in ASTM D 445 (Table 20). Viscosity standards are generally available throughout the world, both from semi-official laboratories such as Cannon Instrument Company, and the various official laboratories in Germany, France, Russia, Japan, China, Australia, and South America, as well as many commercial laboratories. A n u m b e r of viscosity standards have been developed for specific instruments and applications such as the special low temperature requirements of gear oils, the cold cranking simulator, the mini-rotary viscometer, the high-temperature high-shear viscometers, etc. S a y b o l t , S h o r t C a p i l l a r y a n d Orifice V i s c o m e t e r s Early in the development of the measurement of flow as one of the important properties of petroleum lubricants, a number of "technical viscometers" were developed and widely
CHAPTER
32: FLOW PROPERTIES
AND SHEAR STABILITY
865
TABLE 20—Table A1.2 from ASTM D 445, Viscosity Oil Standards. Designation" 20°C
...40°C
S3 S6 S20 S60
4.6 11 44 170 640
80
S200 S600 S2 000 S8 000 S30 000
Approximate Kinematic Viscosity, mm /s 40°C'' 25°C
2 400 8 700 37 000
50°C
2.9 5.7 18 54 180 520
4.0 8.9 34 120 450 1 600 5 600 23 000 81000
100°C' 1.2 1.8 3.9 7.2 17 32 75
280
1 700 6 700 23 000
11 000 " The actual values for these standards are established and annually reaffirmed by cooperative tests. In 1991, tests were made using 15 different types of viscometers in 28 laboratories located in 14 countries. * Kinematic viscosities may also be supplied at 100°F. ^ Kinematic viscosities may also be supplied at 210°F.
used. Bingham [16] in 1922 describes t h e m in Appendix C as follows: Instruments very different from those employed in scientific work are much in vogue in this country [England] and abroad for industrial purposes, particularly in the oil industry. Thus, we have the Engler Viskosimeter in Germany, the Redwood Viscometer in Great Britain, the Sayholt Viscosimeter in the United States, the Barbey Ixometre in France and a host of others. Most of them seem to have been devised with the idea in mind that the time of flow of a given quantity of various liquids through an opening is approximately proportional to the viscosity, without much regard to the character of the opening. There is usually a container which is filled to a certain level and a short efflux tube opening into the air. The number of seconds required for a given quantity of liquid to flow out under gravity is taken as an indication of the viscosity. Kinematic viscosity calculations for these viscometers follow the equation V = rj/p = A t - B/t where
2000 •
1000-
SAYBOLT UNIVERSAL, SECONDS as 0 function of KINEMATIC VISCOSITY
600
200-
30 6
10
20
30
60
100
200 300
600
KINEMATIC VISCOSITY mm V s
FIG. 32—Saybolt viscosity vs. kinematic viscosity.
(30)
v = kinematic viscosity 77 = viscosity p = density A, B empirical constants
Tables a n d charts have been constructed for converting Engler Degrees (cf. Ubbelohde 1907), Redwood seconds (cf. Higgins 1913), Saybolt Universal seconds (SUS), and Saybolt Furol seconds (SFS) into kinematic viscosity units. An example is found in Fig. 32. The Saybolt Universal seconds (SUS) units are defined by the following equation. Ut = [1.0 +0.00006 l(t
100)] • [1.0 +0.03264 v)] 4.6324 v + (3930.2 + 262.7 v + 23.97 v^ + 1.646 v^) X 10"
Where: V = kinematic viscosity, mm^s (cSt) at f°F Ut = Saybolt Universal Viscosity at ?°F Until recently, charts with SUS units as a function of temperature in degrees Fahrenheit were published by ASTM. These charts have been replaced by kinematic viscosity in mm^/s as a function of temperature in degrees Celsius (ASTM D 341 - 32 T, ASTM D 341-93), as it is the custom to use the International Systems of Units (SI).
Over the years, there has been relatively little change in these instruments. Until quite recently, the units SUS, Redwood and Engler degrees were extensively used, and there is still widespread use of SUS units in technical publications, advertisements etc. in the United States. Related expressions such as "100 Neutral" are widely used in the petroleum industry, where it is understood the "100" refers to the Saybolt Universal Seconds kinematic viscosity at 100°F. While the use of the short capillary viscometers has largely faded from the petroleum industry, short capillary or orifice viscometers are widely used in the paint and coatings industry. Examples of these instruments are the Zahn and the Shell dip cups (ASTM D 4212), the Ford Cup (ASTM D 1200), the ISO Cup (ASTM D 5125), and a host of other cup viscometers used in the United States and throughout the world. The following equation is generally used to represent the correlation of the sample efflux time with the kinematic viscosity of the sample: V = 7j/p = A t - B/t
where
v= t] = p = A, B =
kinematic viscosity viscosity density. empirical constants
(31)
866
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
However, often there is no attempt to convert the efflux time for a particular device with kinematic viscosity. Instead, the cups are more often used for control purposes, and kinematic viscosity is reported as "seconds" for whatever cups and size are used. This equation limits the low-kinematic-viscosity (and also short efflux-time) use of each of these viscometers. Under these conditions, the kinematic viscosity becomes almost independent of the efflux time. Temperature Measurement Considerations The importance of accurate temperature measurement cannot be overemphasized. Because of the extreme sensitivity of viscosities of various materials to changes in temperature, calibration of thermometers to a national standard is required. To a large extent, both toluene- (or other non-mercury-liquids) and mercury-filled thermometers are used throughout flow-related ASTM methods. The continued use of mercury-in-glass thermometers in the future may be in question. Environmental regulations in some countries may prohibit the use of mercury in any device. There is considerable help to ensure understanding of thermometry in the writings of ASTM Committee E20. However, other thermometric sensors such as resistance elements, thermocouples, thermistors, diodes etc. are also widely used. The practical temperature scale [117] is based on two fixed temperatures. Absolute zero and the triple point of water are assigned, respectively, 0 K and 273.16 K in the Kelvin Thermodynamic Temperature Scale (KTTS). The KTTS references are assigned the symbol T (ASTM E 1594), without a degree symbol. The unit of temperature in the KTTS is the Kelvin, symbol K (without the degree sign). The unit of Celsius (centigrade) temperature is the degree Celsius (°C). By definition one degree Celsius is equal to a temperature incre-
O 00 CD
0.02 O (D O C CD
^._ CD
CD s^
D CD CL
E CD
•100
0
100
200
t, X FIG. 33—Temperature correction between IPTS-68 and ITS—90 temperature scales.
TABLE 21—Standard platinum reference thermometer calibrations. Temperature Material
Equilibrium State
T90 (K)
tso {°C)
02 Ar Hg H2O Ga In Sn Zn
Triple point Triple point Triple point Triple point Melting point Freezing point Freezing point Freezing point
54.3584 83.8058 234.3156 273.16 302.9146 429.7485 505.078 692.677
-218.7916 -189.3442 -38.8344 0.01 29.7646 156.5985 231.928 419.527
ment of one Kelvin, and t = 0°C is the same as the thermodynamic temperature T = 273.15 K [118]. The practical temperature scale now in use is the International Temperature Scale of 1990 (ITS-90). Formerly used temperature scales include the International Practical Temperature Scale of 1968 (IPTS-68), the International Practical Temperature Scale of 1948 (IPTS-48), and the International Temperature Scale of 1927. There are small differences in temperatures between the several temperature scales, and for accurate experimental work, the temperature scale used should be specified. See Fig. 33 (ASTM E 1594), which describes the differences between the ITS-90, adopted on January 1, 1990, and the IPTS-68 temperature scale. Liquid-in-glass thermometers are fundamental to accurate viscosity measurements. ASTM methods El and E77 give considerable guidance in the construction and use of thermometers. These thermometers must be calibrated against Standard Platinum Resistance Thermometers (SPRT) for the most highly accurate temperature measurements. Once calibrated, liquid-in-glass thermometers need only be recalibrated at one of the temperatures in the future—most often the ice point temperature, as follows: [119] High quality liquid-in-glass thermometers require only one complete calibration in their lifetime and it is possible to avoid the usual requirement for complete recalibration of the instrument by the recalibration of a single previously calibrated temperature. The need for recalibration of properly manufactured liquid-in-glass thermometers is due to the gradual relaxation of residual mechanical strains in the glass that have a significant effect on the volume of the bulb. The recalibration of a single point provides a reliable indication of the effect of this change in volum^e and provides a means for the accurate adjustment of the remainder of the scale. Those calibrated, mercury-in-glass thermometers which have an ice-point scale allow verification of the complete calibration by measurement of the ice point. A change in the ice point reading, if any, from the original calibration at the ice point will apply to all temperatures in addition to any corrections given in the original calibration. The certificate of calibration for the thermometer must state whether the thermometer is to be used at total immersion, at partial immersion, or at complete immersion. Total immersion thermometers require immersing those parts of the thermometer's liquid at test temperature, while keeping the emergent stem exposed to ambient temperatures. Complete immersion thermometers are to be completely immersed
CHAPTER 32: FLOW PROPERTIES AND SHEAR STABILITY including the emergent stem, and they are rarely so used or calibrated. Replacement thermometric devices include highly accurate resistance thermometer devices (RTDs, ASTM E 644) a n d industrial p l a t i n u m resistance t h e r m o m e t e r s (PRTs, ASTM E 1137). For S t a n d a r d Platinum Resistance Thermometers (SPRT), which are used to define the temperature scale, the resistance at the triple point of water (ASTM E 1750) is a very important reference temperature. The SPRT t h e r m o m e t e r s are calibrated at the triple point of water and one or more of several "fixed point" temperatures. See Table 2 1 . Various thermistor thermometers are often used (ASTM E 879). A thermistor is a thermally sensitive resistor, the primary function of which is to exhibit a large change in electrical resistance w i t h a change in sensor t e m p e r a t u r e . The equation relating temperature and resistance is t = [ao -t- ai (InR) -I- a2 (InR)^ -I- as (lnR)3]-i - 273.15.
(32)
Elsewhere, the equations for thermistors sometimes omit the second degree, aa, term. A t h e r m i s t o r can be calibrated against a SPRT at a n u m b e r of temperatures using a good five or six place digital ohmmeter allowing the measurement of temperature to a precision close to ±0.01°C. Another temperature measuring devices is a thermocouple. Thermocouples are used over the temperature range of - 1 8 0 to 1700°C. The eight types of thermocouples (Types B, E, J, K, N, R, S, and T) are most commonly used (ASTM D 230). Depending on the temperature remge and the type of thermocouple, the uncertainty in the interpolated temperature using thermocouples is 0.1 to 5°C (ASTM E 220).
867
the rheological tests are used in c o m m o n to the various materials, but the preparation of each material for testing is often specific to the particular material. Paints and Coatings Flow-Related Instruments The paint and coatings industry has many rheological measurements made on its products. Adaptations of a n u m b e r of these measurements are of use to petroleum products. The viscosities of printing inks, solvents (vehicles), and similar materieils are measured by the Leiray or Duke fallingrod viscometers (ASTM D 4040). These apparatuses are based on measurements of the time required for a weighted rod to fall through a n aperture containing the specimen at 25°C having a viscosity in the range of 1 to 30 Pa-s at a shccir rate of 2500 s ^ ^ Calibration of the apparatus is by the use of multiple viscosity standards at 25°C, and includes the determination of the fall time equivalent to a shear rate of 2500 s ^ Using the power-law relationships between shear stress and shear rate, the apparent viscosity at a shear rate of 2500 s " ' , the degree of non-Newtonian behavior (shear-thinning or shear-thickening) and reference temperature are determined, although the complete rheological properties are not defined. Low shear rate properties are not measured. Such information is useful in the practical control and use of ink. The viscosities of paints, varnishes, lacquers, coil coatings, and related liquid materials are measured by dip-type and short capillary cups such as Zahn dip cup. Shell, Ford, and ISO cups (ASTM D 4212, ASTM D 1200, ASTM D 5125). See Fig. 34. Dip Cups
OTHER NON-PETROLEUM MATERIALS USING ASTM FLOW PROPERTIES While we think of petroleum-related products especially in connection with lubricants and fuels, the chemistry of derived petroleum products includes such materials as paints, coatings, adhesives, rubber, bitumen, plastics, etc. The flow properties of these materials are of the highest importance in their manufacture and use. In addition, non-petroleum materieds such as tar, medical products, glass, ceramics, metals, etc. have flow properties that are important in their manufacture and use. There is a wealth of rheologiccJ information about these materials in various ASTM documents. Many of
Zahn Cup
ISO Cup
Dip cups are tjrpically used for kinematic viscosities in the range of 5-1800 mm^/s at 25°C and flow times of 20-100 s. There are a n u m b e r of such cups, such as the Zahn, Ford, Fisher, etc. dip cups. Flow time measurement begins with the start of flow as the cup breaks the surface of the liquid into which it had been lowered and ends with the first definite break in the stream at the base of the cup. The relation between kinematic viscosity and flow times is generally given by the equation Kinematic Viscosity = K t — B/t where
K, B = constants t = flow time
Ford Cup
FIG. 34—Dip and paint cup instruments.
Shell Cup
(33)
868
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
K and B are cup constants determined through the use of kinematic viscosity standards. Other equations are somet i m e s used to approximate the relationship. The Unear equation Kinematic Viscosity = K (t - c) where
(34)
K, c = cup constants t = flow time
is sometimes used over a part of the total range of the dip-cup (ASTM D 4212). It is very important to observe the m i n i m u m flow-time restrictions for these short-capillary instruments, since the kinematic viscosity relationships cannot be extrapolated to lower flow-times. In a similar manner, the maxim u m flow time specification m u s t also be observed, as a m u c h longer flow-time results in a premature break of the efflux stream and a shortened measurement of the flow time without a fully emptied cup. Bubble Tubes A measure of the kinematic viscosity at 25°C of polymers and resins is approximated by the rise time of an air bubble inside a clear glass tube (10.65 m m in diameter by 108 m m long) filled with the liquid (ASTM D 1545). The result of the comparative measurement is reported as the Gardner-Holt letter (A5-A1, A-Z, and Zl-ZlO) grade of the bubble tube standard having a rise time closest to that of the test sample. Over a limited range, the bubble rise time approximates the kinematic viscosity in stokes (1 stokes = 100 mm^/s). But generally these tubes are used to compare the bubble rise times to one or more calibrated tubes, such that, for example, a material has a bubble rise time between R and S tubes. The result may also be reported as the rise time in "bubbleseconds." Falling Needle The viscosity of Newtonian and non-Newtonian varnishes and paints can be measured by allowing various thin needles to fall through the liquid, measuring the velocity of the nee-
dle (ASTM D 5478). By varying the density of the needle, shear rates can be adjusted. The Brush Drag of Latex Paints is said to generally give good rank order between the brush test (ASTM D 4958) and the high-shear test (ASTM D 4287). In the brush test a 50-mm polyester brush is used to apply the test paint on a lOO-cm'^ (1.076-ft^) area at a spreading rate of 400 ft^/gal and is to be completed in 30-35 s. From the Note 1 to the ASTM D 4958 reference. The brush drag of paints is directly related to their highshear viscosity. There is generally good rank order agreement between results obtained by this method and Test Method D 4287. The sensitivity of this brushout method has been found sufficient to distinguish betv^een brushabilities corresponding to high-shear viscosity differences not lower than 0.3 poise (0.03 Pa-s). Cellulose derivatives are measured by a variety of viscometers. Commonly used glass viscometers include the CannonFenske Routine, the Ubbelohde, the Wagner, and other modified capillary instruments (ASTM D 871, ASTM D 914, ASTM D 1347, ASTM D 1439, ASTM D 1795, ASTM D 2363). The ball-drop method (ASTM D 871) and the low shear rotational viscometers (ASTM D 1439, ASTM D 5400) are also used. The viscosities of rosin and tall oils (ASTM D 803, ASTM D 1131, ASTM D 1725) m a y be m e a s u r e d by bubble tubes (ASTM D 4958), the Saybolt viscometer (ASTM D 88), and rotational viscometers (ASTM D 2196). The low shear Stormer viscometer (ASTM D 562) (Fig. 35) has been in use since at least the year 1909 [120], and still is widely used. A m o d e r n electronic version of the Stormer/Krebs viscometer is also available. Other low and moderate shear rate viscometers (ASTM D 2196) along with high shear-rate viscometers (ASTM D 4287) are also widely used. Rotational viscometers play a large part in the measurements of the viscosities of paints and coatings (Fig. 36). The viscosity of molten powder coatings at baking temperatures is determined using a cone and plate Weissenberg rheogoniometer (cone-plate geometry, ASTM D 3451).
^ g ( N o . 22 US. Ooge} All Oimwsions Subject to o ,. . • , ... . , ^olewnce of ±0.004" Material: Sloinless Steel
FIG. 35—Stormer paddle-type rotor with stroboscope timer.
CHAPTER
32: FLOW PROPERTIES
AND SHEAR
STABILITY
869
lUPAC is the International Union of Pure and Applied Chemistry. Intrinsic viscosity: [TJ] = lim (7] — tjoVv c (36) c —> o Intrinsic viscosity can be used to calculate the molecular weight [TJ]
There is a series of mechanical tests for determining and reporting the viscoelastic properties of thermoplastic, thermosetting resin, a n d composite systems. These polymeric materials may be in solution, or melts, or solids. A compilation of definitions and descriptions of technical terms used in all of these dynamic mechanical properties is given in ASTM D 4092. From the "significance and use" of these methods: Dynamic mechanical testing provides a sensitive method for determining thermomechanical characteristics by measuring the elastic and loss moduli as a function of frequency, temperature, or time. Plots of moduli and tan delta of material versus temperature provide graphical representations indicative of functional properties, effectiveness of cure (thermosetting-resin systems), and damping behavior under specified conditions (ASTM D 5023,
rotational
Plastics Flow-related tests are fundamental in the making of plastic products. There are many "dilute solution" methods of tests for plastics (ASTM D 789, ASTM D 1243, ASTM D 1601, ASTM D 2857, ASTM D 3591, ASTM D 4603, ASTM D 4878, ASTM D 5225, ASTM D 1823); typically there is a specific procedure for each polymer. These methods allow the meas u r e m e n t of the viscosity ratio, which is the ratio of the viscosity of a very dilute polymeric solution to that of the pure solvent. Generally, a glass kinematic viscometer is used to measure both the kinematic viscosity of the solution and of the solvent. Because the density of the dilute solution is nearly the same as the density of the solvent, the relative viscosity is merely the ratio of the flow time, t, of the solution to that of the solvent, to (assuming there is no significant kinetic energy correction to be applied to the flow time). Viscosity Ratio: Tjr = TJ/TJO = t/to
(37)
M''
M = molecular weight a, b = constants determined for each type of polymer.
where
FIG. 36—Electronic paint tester.
= a
Inherent Vticoilty _(I-og«rtthn.lc Vl.coalty Number)
Incrinilc Vlscotlty (Limiting Vlicoaity Number)
(35)
where
T]^ = viscosity ratio 17 = viscosity of the solution Tjo = viscosity of the solution t = flow time of the solution to = flow time of the solvent With the known concentration of the solution, the reduced viscosity and limiting viscosity n u m b e r (intrinsic viscosity) are calculated. A sample plot to determine intrinsic viscosity is found on Fig. 37. From the viscosity n u m b e r the intrinsic viscosity is determined (often extrapolated to zero concentration). It is used to determine the molecular weight of polymer molecules. The reader of these methods is alerted that there is a double set of nomenclature, as indicated in Table 22. Many tests are done at 30°C, with some tests performed at 135X.
Polymer Concentration, g / c m '
FIG. 37—Example of plot to determine intrinsic viscosity (limiting viscosity number). TABLE 22—Polymer viscosity measurement nomenclature. Conventional Term
lUPAC Alternative Term
Relative Viscosity Reduced Viscosity Inherent Viscosity
Viscosity Ratio Viscosity Number Logarithmic Viscosity Number Limiting Viscosity Number
Intrinsic Viscosity "where
Definition"
Vr = V'no %ed = (Vr - 1)/C ijinh = In r)r /c [TJ]
rf = viscosity of the dilute polymeric solution r)o = viscosity of the solvent c = concentration of the solution in units of g/mL
870
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
ASTM D 5024, ASTM D 5026, ASTM D 5279, ASTM D 5418, ASTM D 5422). Each of these tests is characterized by the different designs of the apparatus and the required special test procedures. Pol3aneric powders are appHed to surfaces by electrostatic spray or fluidized bed procedures. The Weissenberg Rheogoniometer (cone and plate geometry, ASTM D 3451) is used to study the use of such powders at baking temperatures. A separate test consists of making a small pellet of the powder and measuring the flow at 150°C on ein inclined plane. Bituminous Materials The use of rheological properties is highly important for the specifications and uses of asphalt cements, liquid asphalts, emulsified bitumens, and tar materials. Since the early 1900s, b i t u m i n o u s materials have been graded according to the results of a "rheological test" using the penetration of a needle (Fig. 38) into a specially prepared asphalt test sample. The needle is allowed to vertically descend into semi-solid and solid materials u n d e r known conditions of loading (typically 100 g), time (5 s) and temperature (25°C), although other conditions may be specified (ASTM D 5). The results are expressed in tenths of a millimeter, and specifications for paving grade asphalts include penetration ranges, e.g., 40-50, 85-100, 200-300, etc. (ASTM D 946). In the early 1960s, several new rheologicEilly-based specifications and related tests for bituminous materials [121] at three defined temperature zones were created: • 135°C, a temperature approximating the temperature in the hot-mix towers for the mixing of aggregate with the bituminous material, glass capillary kinematic viscometers (ASTM D 2170), • 60°C, approximating the temperature of a road surface in the hot s u m m e r sun, glass capillary vacuum viscometers (ASTM D 2171, ASTM D 4957) [122], and • 4°C, approximating the temperature of a pavement at a temperature just above the freezing point of water, test methods and specifications never implemented. Where the viscosity of the material is higher than can be tested with kinematic viscometers, force is applied by means of a regulated vacuum to glass capillary viscometers, thus extending the viscosity range u p to 580 000 Pa-s (ASTM D 2171). Some examples of vacuum viscometers may be found in Fig. 39. Other tests include the Saybolt viscometer (ASTM E 102, ASTM D 88, ASTM D 244) and the Engler viscometer (ASTM D 1665) especially for specifications of roofing, waterproofing, and bituminous materials for liquid and emulsified asphalts and tars. Rotational viscosity tests are also utilized for
0.14 k) 0.16 mm ./.GO h 1.02mm 8°40'io 9°40'^ I
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>
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k
as required
_=j opprox. ^6.35 mm
FIG. 38—Needle for penetration test.
^
these products both at high temperatures (ASTM D 2994) and very high viscosities (ASTM D 3205) using special coneplate and Rheogoniometer viscometers. In the early 1990s the Federal Highway Administration (FHWS) sponsored a large program for the improvement of the bituminous materials comprising much of our highwaypaving program. The Strategic Highway Research Program (SHRP) resulted in a n u m b e r of rheological recommendations for the specifications for bituminous materials. The Bending-Beam Rheometer (BBR, Fig. 40) performs low-temperature flexural creep stiffness measurements on asphalt binders as specified in ASTM D 6648, American Association of State Highway a n d Transportation Officials (AASHTO), and SHRP methodology over the temperature range of —36 to 0°C and creep stiffness range from 20 MPa to 1 GPa and creep compliance values in the range of 50 n P a ~ ' to 1 n P a " ' [123]. (Note that 1 MPa = 10* Pa, and 1 GPa = 10^ Pa). A "beam" (6.35 m m by 12.7 m m by 127mm) of asphalt is cast using a mold. At the specified low temperature (such as — 18°C), the beam is supported at each end. Under computer control, a very low load is applied, 100 g, to the center of the beam and rate of deflection is determined. From the collected data, the stiffness modulus is calculated and reported. The Dynamic Shear Rheometer (DSR) [124] with parallel plate geometry is used to measure the complex shear modulus (G*) and phase angle (5) of asphalt binders over the temperature range of 5-85°C and a dynamic shear modulus between 100 Pa to 10 MPa. Test specimens 1-mm thick by 25-mm in diameter are tested at a n oscillatory loading frequency of 10 rad/s in a temperature controlled environm e n t a l c h a m b e r (some have liquid baths). The complex shear modulus and the phase angle are calculated automatically. The test t e m p e r a t u r e for this test is related to the t e m p e r a t u r e experienced by the p a v e m e n t in the geographical area for which the asphalt binder is intended. The complex shccir modulus is a n indicator of the stiffness or resistance of the binder to deformation under load. The complex shear modulus and the phase angle define the resistance to shear deformation of the asphalt binder in the linear viscoelastic region. For certain tests, the asphalt binder must be processed using several aging methods. These practices simulate the effects of a long-term environment with oxidation and high temperature and pressures, which include a rolling thin-film oven and a pressure-aging vessel (AASHTO PPl). Binder so tested is a part of the SHRP program. Adhesives Adhesives present special p r o b l e m s to the rheologist, although the m e t h o d s used for the analysis of adhesives are very similar to those used for other materials. For freeflowing adhesives, the Saybolt, Zahn dip cup. Ford cup, bubble t i m e tubes, S t o r m e r low shear rotational, and other rotational viscometers are a m o n g the viscometer methods cited (ASTM D 1084). The apparent viscosity of shearrate-dependent adhesives is measured . . . based upon a reversible isothermal change in apparent viscosity with change in rate of shear. Measurement is performed with a spindle, disk, T-bar, or coaxial cylinder rotational viscometer under standardized conditions with
CHAPTER 32: FLOW PROPERTIES AND SHEAR STABILITY
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HANDBOOK
FIG. 40—Cannon bending beam rheometer. rigid control of the time intervals of measurement. Readings are obtained on the viscometer dial scale at the end of a minute for each rotational speed. Changes from the lowest speed to the highest speed, and return to the lowest speed, are made without stopping the instrument (ASTM D 2556). The apparent viscosity of hot m e h adhesives and coating materials at temperatures u p to 175°C and apparent viscosities u p to 200 Pa-s are m e a s u r e d using low-shear rotational viscometers (ASTM D 3236, ASTM D 4499).
Rubber Rheology is the science of deformation and flow of materials. Deformation can be divided into reversible deformation (elasticity) and irreversible deformation (flow). Many materials exhibit b o t h reversible a n d irreversible deformation, exhibiting what is called viscoelasticity. This property is of critical importance in the manufacture of rubber products, although it is exhibited in many other products including lubricants, paints, and coatings, etc. The variety of terms used relating to the rheology of rubber illustrate the complexity of this subject: viscosity, real dynamic viscosity, relaxation, shear, strain, stress, modulus, loss modulus, storage modulus, complex shear modulus, dynamic complex viscosity, elastic recovery, elastic fatigue, and many others. Types of instruments used to measure the rheological behavior of unvulcanized raw rubbers and rubber compounds include: • • • •
rotating disk within a cylindrical cavity (ASTM D 1646) capillary rheometry-piston type (ASTM D 5099) capillary rheometry-screw extrusion type (ASTM D 5099) dynamic testing using servohydraulic, mechanical a n d electrodynamic exciters (ASTM D 5992) • compression between parallel plates (ASTM D 6049) • rotorless shear rheometers (ASTM D 6204) The widely used Mooney viscosity is defined at the shearing torque resisting rotation of a cylindrical metal disk embed-
ded in r u b b e r within a cylindrical cavity. The Mooney viscometer is: . . . an instrument consisting of a motor-driven rotating disk within a cylindrical die cavity formed by two dies maintained at specified conditions of temperature and die closure force. The Mooney viscometer measures the effect of temperature and time on the viscosity of rubbers. If the stress relaxation test is to be performed, the instrument must be capable of stopping the rotation of the disk and monitoring the relaxation of stress versus time (ASTM D 1646, ASTM D 3346). Piston, or screw extrusion-type capillary rheometers (ASTM D 5099, ASTM D 3835) are widely used for characterizing plastics and rubber products. There is an excellent, detailed description of rheological terms in the terminology of ASTM D 5099 including terms such as the Rabinowitsch shear rate correction for capillary geometry, the Bagley "end-effect" correction, and power law fluids, as well as the significance and use of this these apparatus as related to rubber products. Additionally, there is an extended technical description of the stress relaxation testing of raw rubber, unvulcanized rubber compounds and thermoplastic elastomers (ASTM D 6049, ASTM D 6048). Unvulcanized rheological properties using rotorless shear rheometers measure viscoelastic properties of raw r u b b e r as well as unvulcanized r u b b e r c o m p o u n d s (ASTM D 6204). In a m a n n e r similar to the plastics industry, dilute solutions of synthetic rubber in a suitable solvent (e.g., methyl ethyl ketone or toluene) are m e a s u r e d using kinematic viscometers (ASTM D 445). In low-gel or gel-free rubbers, the dilute solution viscosity correlates directly with the molecular weight (ASTM D 3616).
ASTM STANDARDS
No. ASTM D 5 ASTM D 88 ASTM D 88-21 T ASTM D 97 ASTM D 244 ASTM D 341 ASTM D 341-32 T ASTM D 341-93 ASTM D 445
ASTM D 446
ASTM D 562
Title Test Method for Penetration of Bituminous Materials Test Method for Saybolt Viscosity. Standard Test Method for Saybolt Viscosity, 1921 Test Method for Pour Point of Petroleum Products Test Method for Emulsified Asphalts Viscosity-Temperature Charts for Liquid Petroleum Products, Appendix XI Viscometer-Temperature Charts for Liquid Petroleum Products Viscometer-Temperature Charts for Liquid Petroleum Products Standard Test Method for Kinematic Viscosity of T r a n s p a r e n t a n d Opaque Liquids (the Calculation of Dynamic Viscosity) Standard Specifications and Operating Instruction for Glass Capillary Kinematic Viscometers Test Method for Consistency of Paints using the Stormer Viscometer
CHAPTER Test Methods for Determination of Relative Viscosity, Melting Point and Moisture Content of Polyamide (PA) Test Methods for Tall Oil ASTM D 803 Test Methods of Testing Cellulose ASTM D 871 Acetate Test Methods of Ethylcellulose ASTM D 914 Specification for Penetration-Graded ASTM D 946 Asphalt Cement for Use in Pavement Construction ASTM D 1084 Test Methods for Viscosity of Adhesives Methods of Testing Rosin Oils ASTMD 1131 Test Method for Viscosity by Ford VisASTM D 1200 cosity Cup Test Method for Dilute Solution ViscosASTM D 1243 ity of Vinyl Chloride Polymers Test Methods for Methylcellulose ASTM D 1347 Test Methods for Sodium CarboxymethylASTM D 1439 cellulose Test Method for Viscosity of TransparASTMD 1545 ent Liquids by Bubble Time Method ASTMD 1601 Test Method for Dilute Solution Viscosity of Ethylene Polymers Test Methods for Rubber—Viscosity, ASTM D 1646 Stress Relaxation, and Pre-Vulcanization Characteristics (Mooney Viscometer) Test Method for Engler Specific ViscosASTM D 1665 ity of Tar Products Test Method for Viscosity of Resin ASTMD 1725 Solutions Test Method for Intrinsic Viscosity of ASTMD 1795 Cellulose ASTM D 1823 Test Method for Apparent Viscosity of Plastisols and Organosols at High Shear Rates by Extrusion Viscometer ASTM D 2162 Standard Test Method for Basic Calibration of Master Viscometers and Viscosity Oil Standards Test Methods for Kinematic Viscosity of ASTMD 2170 Asphalts Test Methods for Viscosity of Asphalts ASTMD 2171 by Vacuum Capillary Viscometer Test Methods for Rheological Properties ASTMD 2196 of Non-Newtonian Materials by Rotational (Brookfield) Viscometer ASTM D 2270 - 93 Standard Practice for Calculating Viscosity Index From Kinematic Viscosity at 40 and 100°C. (Originally pubUshed by ASTM as D 567 - 40T, and revised and renumbered in 1964.) Test Methods for Hydroxypropyl ASTM D 2363 Methylcellulose ASTM D 2556 Test Method for Apparent Viscosity of Adhesives Having Shear-Rate-Dependent Flow Properties Test Method for Apparent Viscosity of ASTM D 2602 Engine Oils at Low Temperature Using the Cold-Cranking Simulator, Note: Replaced by D 5293 Sonic Shear Stability of PolymerASTM D 2603 Containing Oils ASTM D 789
32: FLOW PROPERTIES ASTM D 2857 ASTM D 2983
ASTM D 2994 ASTM D 3205 ASTM D 3236 ASTM D 3346
ASTM D 3451 ASTM D 3591
ASTM D 3616
ASTM D 3829
ASTM D 3835
ASTM D 3945 ASTM D 4040
ASTM D 4092
ASTM D 4212 ASTM D 4287 ASTM D 4485 ASTM D 4486 ASTM D 4499 ASTM D 4603
ASTM D 4624
ASTM D 4683
ASTM D 4741
ASTM D 4878
AND SHEAR
STABILITY
873
Test Method for Dilute Solution Viscosity of Polymers Test Method for Low-Temperature Viscosity of Automotive Fluid Lubricants Measured by Brookfield Viscometer Test Methods for Rubberized Tar Test Method for Viscosity of Asphalt with Cone and Plate Viscometer Test Method for Apparent Viscosity of Hot Melt Adhesives and Coating Materials Test Methods for Rubber PropertyProcessability of SBR (Styrene-Butadiene Rubber) with the Mooney Viscometer Practice for Testing Polymeric Powders and Powder Coatings Practice for Determining Logarithmic Viscosity N u m b e r of Poly (Vinyl Chloride) (PVC) in Formulated Compounds Test Method for Rubber, Raw—Determination of Gel, Swelling Index, and Dilute Solution Viscosity Test Method for Predicting the Borderline Pumping Temperature of Engine Oil Test Method for Determination of Properties of Polymeric Materials by Means of a Capillary Rheometer Shear Stability of Polymer-Containing Fluids Using a Diesel Injector Nozzle Test Method for Viscosity of Printing Inks and Vehicles by the Falling Rod Viscometer) Standard Terminology Relating to Dyn a m i c Mechanical Measurements on Plastics Test Method for Viscosity by Dip-Type Viscosity Cups Test Method for High-Shear Viscosity Using the ICI Cone/Plate Viscometer Specification for Performance of Engine Oils Standard Test Method for Kinematic Viscosity of Volatile and Reactive Liquids Test Method for Heat Stability of HotMelt Adhesives Test Method for Determining Inherent Viscosity of Poly (Ethylene Terephthalate) (PET) Test Method for Measuring Apparent Viscosity by Capillary Viscometer at High-Temperature a n d High Shear Rates Test Method for Measuring Viscosity at High Shear Rate and High Temperature by Tapered Bearing Simulator Test Method for Measuring Viscosity at High Temperature and High Sheeir Rate by Tapered-Plug Viscometer Test Methods for Polyurethane Raw Materials: Determination of Viscosity of Polyols
874
MANUAL
ASTM D 4957
ASTM D 4958 ASTM D 5023
ASTM D 5024
ASTM D 5026
ASTM D 5099
ASTM D 5125 ASTM D 5133
ASTM D 5182 ASTM D 5225
ASTM D 5275 ASTM D 5279
ASTM D 5293
ASTM D 5400 ASTM D 5418
ASTM D 5422
ASTM D 5478 ASTMD 5481
ASTM D 5621 ASTM D 5992
ASTM D 6022 ASTM D 6048
37: FUELS AND LUBRICANTS
HANDBOOK
Test Method for Apparent Viscosity of Asphalt E m u l s i o n Residues and NonNewtonian Bitumens by Vacuum Capillary Viscometer Test Method for Comparison of the Brush Drag of Latex Paints Test Method for Measuring the Dynamic Mechanical Properties of Plastics Using Three Point Bending Test Method for Measuring the Dynamic Mechanical Properties of Plastics in Compression Test Method for Measuring the Dynamic Mechanical Properties of Plastics in Tension Test Methods for Rubber—Measurem e n t of Processing Properties Using Capillary Rheometry Test Method for Viscosity of Paints and Related Materials by ISO Flow Cups Test Method for Low Temperature, Low Shear Rate, Viscosity/Temperature Dependence of Lubricating Oils Using a Temperature-Scanning Technique Test Method for Evaluating the Scuffing (Scoring) Load Capacity of Oils Test Method for Measuring Solution Viscosity of Polymers with a Differential Viscometer Fuel Injector Shear Stability Test (FISST) for Polymer Containing Fluids Test Method for Measuring the Dynamic Mechanical Properties of Plastics in Torsion Apparent Viscosity of Engine Oils Between - 5 a n d - 3 0 ° C Using the ColdCranking Simulator Test Methods for Hydroxypropylcellulose Test Method for Measuring the Dynamic Mechanical Properties of Plastics Using a Dual Cantilever Beam Test Method for M e a s u r e m e n t of Properties of Thermoplastic Materials by Screw-Extrusion Capillary Rheometer Test Method for Viscosity of Materials by a Falling Needle Viscometer Test Method for Measuring Apparent Viscosity at High-Temperature a n d High-Shear Rate by Multicell Capillary Viscometer Sonic Shear Stability of Hydraulic Fluid Guide for Dynamic Testing of Vulcanized Rubber and Rubber-Like Materials Using Vibratory Methods Calculation of Permanent Shear Stability Index Practice for Stress Relaxation Testing of Raw Rubber, Unvulcanized R u b b e r Compounds, a n d Thermoplastic Elastomers
ASTM D 6049
ASTM D 6204
ASTM D 6278
ASTM D 6648
ASTM E 1 ASTM E 77 ASTM E 102
ASTM E 220 ASTM E 230
ASTM E 644 ASTM E 879
ASTM E 1137 ASTM E 1594 ASTM E 1750
Test Methods for Rubber Property—Mea s u r e m e n t of the Viscous and Elastic Behavior of Unvulcanized Raw Rubbers and Rubber Test Method for Rubber—Measurement of Unvulcanized Rheological Properties Using Rotorless Shear Rheometers Shear Stability of Polymer Containing Fluids Using a European Diesel Injector Apparatus Test Method for Determining the Flexural Creep Stiffness of Asphalt Binder Using the Bending B e a m Rheometer (BBR) Specification for ASTM Thermometers Test Method for the Inspection and Verification of Thermometers. Test Method for Saybolt Furol Viscosity of Bituminous Materials at High Temperatures Test Method for Calibration of Thermocouples by Comparison Techniques Specification and Temperature-Electromotive Force (EMF) Tables for Standcirdized Thermocouples. Test Methods for Testing Industrial Resistance Thermometers Specification of Thermistor Sensors for Clinical Laboratory Temperature Measurement Specification for Industrial Platinum Resistance Thermometers Guide for Expression of Temperature Guide for Use of Water Triple Point Cells
OTHER STANDARDS Saybolt
Viscosity
AASHTO T 72 Pour Point of Petroleum
Products
AFNOR T60-105: Determination of Pour Point JIS K 2269: Pour Point and Cloud Point of Crude Oils and Petroleum Products IP 15: Pour Point of Petroleum Products ISO 3016: Determination of Pour Point Viscosity
Measurement
and
Calculation
AFNOR T60-100: Measurement of Viscosity DIN 51562: Viscometry—Measurement of Kinematic Viscosity by Mecins of the Ubbelohde Viscometer - Part 1: Viscometer Specification and Measurement Procedure IP 71, Section 1: Kinematics Viscosity of Transparent and Opaque Liquids IP 71, Section 2: Specifications and Operating Instruction for Glass Capillary Kinematic Viscometers ISO 3104: Viscosity of Petroleum Products ISO 3105: Viscometers, Glass Specs and Operating Instructions JIS K 2283: Kinematic Viscosity at 40°C T60-100: Viscosity Measurement.
CHAPTER 32: FLOW PROPERTIES AND SHEAR STABILITY Kinematic Viscosity A F N O R T 4 2 - 0 1 1 : Latex A F N O R T60-136: K i n e m a t i c Viscosity D I N 5 1 3 7 7 : T e s t i n g of L u b r i c a n t s - D e t e r m i n a t i o n of t h e A p p a r e n t V i s c o s i t y of M o t o r Oils a t L o w T e m p e r a t u r e F r o m —5 °C t o - 3 5 °C - U s i n g t h e C o l d - C r a n k i n g S i m u l a t o r I P 2 2 2 : V i s c o s i t y of Asphalts b y V a c u u m C a p i l l a r y V i s c o m e t e r IP 226: Calculating Viscosity Index from K i n e m a t i c Viscosity at40andl00°C I P 2 6 7 : D e t e r m i n a t i o n of L o w T e m p e r a t u r e V i s c o s i t y of Automotive Fluids-Brookfield Viscometer M e t h o d I P 3 1 9 : K i n e m a t i c V i s c o s i t y of A s p h a l t s ISO 2909: Viscosity Index Fuel Injector
Shear
Stability
Test
D I N 5 1 3 8 2 : T e s t i n g of L u b r i c a n t s - D e t e r m i n a t i o n of S h e a r S t a b i l i t y of L u b r i c a t i n g Oils C o n t a i n i n g P o l y m e r s - M e t h o d W i t h Diesel Injection Nozzle, Relative Viscosity Loss D u e to Shear I P 2 9 4 : D e t e r m i n a t i o n o f S h e a r Stability of P o l y m e r - c o n t a i n i n g Oils-Diesel I n j e c t o r R i g M e t h o d
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875
[16] Bingham, E. C , Fluidity and Plasticity, McGraw-Hill, NY, 1922. [17] The Relationship Between High-Temperature Oil Rheology and Engine Operation, ASTM Data Series 62. Also parts II, III, IV and V, ASTM International, West Conshohocken, PA, 1985. [18] Rheotribology of Automotive Lubricants and Fluids, Society of Automotive Engineers, Warrendale, PA, 1994. [19] Sorab, J., Holdeman, H. A., and Chui, G. K., "Viscosity Prediction for Multigrade Oils," SAE #932833, Tribological Insights & Performance Characteristics of Modem Engine Lubricants, SAE Sp-996, Society of Automotive Engineers, Warrendale, PA, 1993. [20] Bala, V., Klaus, E. E., Duda, J. L., and Palekar, V., "Extension of the Temperature and Shear Rate Range for Polymer Containing Lubricants Using the C a n n o n H T H S Capillary Viscometer," SAE #932695, Tribological Insights & Performance Characteristics of Modem Engine Lubricants, (SP-996), 1993, pp. 69-85. [21] Bala, V., Klaus, E. E., and Duda, J. L., "Improving the Precision of High T e m p e r a t u r e High S h e a r Rate Viscosity Measurements," SAE #932688, Tribological Insights & Performance Characteristics of Modem Engine Lubricants, (SP-996), 1993, pp. 1-8. [22] SAE J300: Engine Oil Viscosity Classification, Society of Automotive Engineers, Warrendale, PA, 1999. [23] Lodge, A. S., A New Method of Measuring Multigrade Oil Shear Elasticity and Viscosity at High Shear Rates, SAE #872043, Society of Automotive Engineers, Warrendale, PA, 1987. [24] Williamson, B. P. and Milton, A., "Characterisation of the Viscoelasticity of Engine Lubricants at Elevated Temperatures and S h e a r Rates," SAE #951032, Society of Automotive Engineers, Warrendale, PA, 1995. [25] Bates, T. W., Williamson, B . P., Spearot, J. A., and Murphy, C. K., "A Correlation Between Engine Oil Rheology and Oil Film Thickness in Engine Journal Bearings," SAE #860376, Society of Automotive Engineers, Warrendale, PA, 1986. [26] Bates, T. W., Roberts, G. W., Oliver, D. R., and Milton, A. L., "A Journal Bearing Simulator Bench Test for Ranking Lubricant Load-Bearing Capacity; A Theoretical Analysis," SAE #922352, Society of Automotive Engineers, Warrendale, PA, 1992. [27] Li, X. K., Scales, L. E., Phillips, T. N., Davies, A. R., Gwynilyw, D. Rh., and Williamson, B. P., "The Effect of Lubricant Rheology on the Performance of Dynamically Loaded Journal Bearings," SAE #973002, Society of Automotive Engineers, Warrendale, PA, 1997. [28] Georgi, G. W., A Few Technical Problems Introduced by the New Trend in Motor Oils, The Role of Engine-Oil Viscosity in LowTemperature Cranking and Starting, SAE, Pergamon Press, NY, 1966. [29] "Development of Research Technique for Determining the LowTemperature Cranking Characteristics of Engine Oils," CRC Report No. 374, Coordinating Research Council, Inc., NY, 1964. [30] "Determination of Low-Temperature Cranking Characteristics of Engine Oils at —20°F Using CRC L-49 Research Technique," CRC Report No. 405, Coordinating Research Council, Inc., NY, 1967. [31] "Prediction of Low-Temperature Cranking Characteristics of Engine Oils by Use of Laboratory Viscometers," CRC Report No. 381, Coordinating Research Council, NY, 1965. [32] "Evaluation of Laboratory Viscometers for Predicting Cranking Characteristics of Engine Oils at 0°F and —20°F," CRC Report No. 409, Coordinating Research Council, Inc., NY, 1968. [33] "Low-Temperature Pumpability Characteristics of Engine Oils in Full-Scale Engines," ASTM Report DS-57, ASTM International, West Conshohocken, PA, 1975. [34] "Cold Starting and Pumpability Studies in M o d e m Engines," ASTM Research Report RR-D02-1442, ASTM International, West Conshohocken, PA, November 1998. [35] "Extrapolated Oil Viscosities," SAE Handbook, SAE J305a, Society of Automotive Engineers, Warrendale, PA, 1966, p. 299.
876 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK [36] Lowther, H. V., Meyer, W. A. P., Selby, T. W., and Vick, G. K., "Development of Research Technique for Determining t h e Low-Temperature Cranking Characteristics of Engine Oils," Presented at t h e SAE Automotive Engineering Congress, Detroit, January 1964. [37] "Prediction of Low-Temperature Cranking Characteristics of Engine Oils by Use of Laboratory Viscometers," CRC Report No. 381, Coordinating Research Council, NY, March 1965. [38] Vick, G. K., Meyer, W. A. P., and Selby, T. W., "Prediction of the Low-Temperature Cranking Characteristics of Engine Oils by Use of Laboratory Viscometers, The Role of Engine-Oil Viscosity in Low-Temperature Cranking and Starting," Progress in Technology, Volume 10, Society of Automotive Engineers, Warrendale, PA, 1966. [39] Selby, T. W. and StafHn, G. D., "The Development of ASTM Low Temperature Viscometric Techniques, The Role of Engine-Oil Viscosity in Low-Temperature Cranking and Starting," Progress in Technology, Volume 10, Society of Automotive Engineers, Warrendale, PA, 1966. [40] Stewart, R. M., Lion, W. D., and Meyer, W. A. P., "A Reciprocating Viscometer for Predicting the Low Temperature Cranking Characteristics of Engine Oils, The Role of Engine-Oil Viscosity in Low-Temperature Cranking and Starting," Progress in Technology, Volume 10, Society of Automotive Engineers, Warrendale, PA, 1966. [41] Kim, D. S., commentary on Stewart, R. M., Lion, W. R., Meyer, W. A. P., "A Reciprocating Viscometer for Predicting the Low Temperature Cranking Characteristics of Engine Oils," SAE Paper #650443, SAE Progres in Technology, Vol. 10, 1965, pp. 205-225. [42] Henderson, L. M. and Annable, W. G., "Pour Point Stability of Lubricating Oils," Oil and Gas Journal, 9 Sept. 1943, p p . 54-9. [43] Hodges, C. E. a n d Boehm, A. B., "Pour Point Stability of Treated Oils Under Winter Storage Conditions," Oil and Gas Journal, 24 June 1943, pp. 103-6. [44] Hodges, C. E. and Rogers, D. T., "Some New Aspects of PourDepressant Treated Oils," Oil and Gas Journal, 4 Oct. 1947, pp. 89-93, 99. [45] McNab, J. G., Rogers, D. T., Michaels, A. E., and Hodges, C. E., "The Pour-Point-Stability Characteristics of W i n t e r - G r a d e Motor Oils," SAE Quarterly Transactions, Vol. 2, No. 1, Jan. 1948, pp. 34-44. [46] Moyer, R. G., "Low Temperature Pumpability of Oils," Journal of the American Society of Lubrication Engineers, April 1962, pp. 165-8. [47] Selby, T. W., "Viscosity and the Cranking Resistance of Engines at Low Temperature", Sixth World Petroleum Congress Proceedings, Section VI, Frankfort, June 1963, pp. 241-258. [48] McMillan, M. L. and Murphy, C. K., "The Relationship of LowTemperature Rheology to Engine Oil Pumpability," SAE National Automobile Engineering Meeting, Detroit, 14-18 May 1973, Paper #730478, Society of Automotive Engineers, Warrendale, PA. [49] Moyer, R. G., "Low Temperature Pumpability of Oils," Journal of the American Society of Lubrication Engineers, April 1962, p p . 165-8. [50] Stewart, R. M. and Smith, M. F., Jr., "Proposed Laboratory Methods for Predicting the Low-Temperature Pumpability Properties of Crankcase Oils," SAE National Automobile Engineering Meeting, Detroit, 14-18 May 1973, Paper #730479, Society of Automotive Engineers, Warrendale, PA. [51] Stewart, R. M. and Spohn, C. R., "Some Factors Affecting the Cold Pumpability of Crankcase Oils," SAE Transactions, Vol. 81, 1972, Paper #720150. [52] Nolf, A. R., "Engine Oil Pumpability and Related Properties," SAE SP-382, Viscometry and Its Application to Automotive Lubricants, Paper #730480, Society of Automotive Engineers, Warrendale, PA, 1973.
[53] Tao, F. F. and Waddey, W. E., "Low-Shear Viscometry and Cold Flow Mechanism-Engine Oils," SAE SP-382, Viscometry and Its Application to Automotive Lubricants, Paper #74038, Society of Automotive Engineers,Warrendale, PA, 1973. [54] McMillan, M. L., Stewart, R. M., and Smith, M. F., Jr., "LowTemperature Pumpability Characteristics of Engine Oils in FuU-Scale Engines," Paper #750691, 3-5 June 1975, Society of Automotive Engineers, Warrendale, PA. [55] Shaub, H., Smith, M. F., and Murphy, C. K., "Predicting Low Temperature Engine Oil Pumpability with the Mini-Rotary Viscometer," SAE Congress and Exposition, Detroit, 25-29 Feb. 1980, Paper #790732, Society of Automotive Engineers, Warrendale, PA. [56] Stambaugh, R. L. a n d O'Mara, J. H., "Low Temperature Flow Properties of Engine Oils," SAE Paper #821247 or 820509, Society of Automotive Engineers, Warrendale, PA. [57] Stewart, R. M., Shaub, H., Smith, M. F., Jr., and Selby, T. W., Summary of ASTM Activities on Low Temperature Engine Oil Pumpability, SAE Paper #821206, Society of Automotive Engineers, Warrendale, PA. [58] Smith, M. F., Jr., "Better Prediction of Engine Oil Pumpability Through a More Effective MRV Cooling Cycle," SAE Paper #831714, Society of Automotive Engineers, Warrendale, PA. [59] SAE J300: Engine Oil Viscosity Classification, Society of Automotive Engineers, Warrendale, PA, December 1999. [60] Selby, T. W., "The Scanning Brookfield Technique of LowTemperature, Low-Shear Rheology—Its Inception, Development, Eind Applications," Low-Temperature Lubricant Rheology Measurement and Relevance to Engine Operation, ASTM STP 1143,R.K Rhodes, Ed., ASTM International, West Conshohocken, PA, 1992, pp. 33-64. [61] Selby, T. W., "The Use of the Scanning Brookfield Technique to Study the Critical Degree of Gelation of Lubricants at Low Temperatures," Paper #910746, Society of Automotive Engineers, Warrendale, PA, 1991. [62] "Cold Starting and Pumpability Studies in Modem Engines," ASTM Research Report D02-1442, ASTM International, West Conshohocken, PA, November 1998 and March 1999. [63] Henderson, K. O., Manning, R. E., May, C. J., and Rhodes, R. B., "New Mini-Rotary Viscometer Temperature Profiles That Predict Engine Oil Pumpability," Paper #850443, Society of Automotive Engineers, Wartendale, PA. [64] CEC L-14-A-93: Evaluation of the Mechanical Shear Stability of Lubricating Oils Containing Polymers, Coordinating European Council Secretariat, B-1210 Brussels, Belgium, 1993. [65] CEC L 37 T 85: Shear Stability of Polymer-Containing Oils (FZG), Coordinating European Council, Leicester, UK, 1985. [66] CEC L 45 T 93: Viscosity Shear Stability of Transmission Lubricants (KRL), Coordinating E u r o p e a n Council, Leicester, UK, 1993. [67] CEC L-14-A-93: Evaluation of the Mechanical Shear Stability of Lubricating Oils Containing Polymers, Coordinating European Council Secretariat, Brussels, Belgium, 1993. [68] SAE J183: Engine Oil Performance and Engine Service Classification, Society of Automotive Engineers, Warrendale, PA, 1999. [69] "A New Chart for Viscosity Temperature Relations," Lubrication, May 1921, pp. 5-8. [70] "VISCOSITY, Part IV—Effects of Temperature and Pressure," Lubrication, June 1950, pp. 63-64. [71] "International Critical Tables of Numerical Data," Physics, Chemistry, and Technology, McGraw-Hill, NY, 1927, p p . 146-147. [72] Walther, C , "The Variation of Viscosity with Temperature—I, II, III," Erdol und Teer, Vol. 4, 1928, pp. 510, 526, 614. [73] Walther, C , "The Variation of Viscosity with Temperature— IV," Erdol und Teer, Vol. 5, 1929, p p . 34, 619.
CHAPTER 32: FLOW PROPERTIES AND SHEAR STABILITY [74] Wright, W. A., "An Improved Viscosity-Temperature Chart for Ky diocsLrhons," Journal of Materials, Vol. 4, No. 1, March 1969, pp. 19-27. [75] Manning, R. E., "Computational Aids for Kinematic Viscosity Conversions from 100 and 210°F to 40 and 100°C," Journal of Testing and Evaluation, Vol. 2, No. 6, 1974, pp. 522-8. [76] Ubbelohde, L., Zur Viskosimetrie, 5th ed., S. Hirzel Verlay, Leipzig, Germany, 1941. [77] Umstatter, H., "Einfuhrung in die Vikosimetrie und Rheometrie," Viskogramm nach Dr.-Ing. H. Umstatter, Springer-Verlag O. H. G., Berlin/Gottingen Heidelberg Germany, 1952, pp. 138-139. [78] Roelands, C. J. A., Bloc, H., and Vlugter, V. C , "A New Viscosity -Temperature Criterion for Lubricating Oils," ASME-ASLE International Lubrication Conference, Washington, DC, Oct. 1964. [79] Roelands, C. J. A., "Correlational Aspects of the Viscosity-Temperature-Pressure Relationship of Lubricating Oils," Druk. V.R.B., Kleine der A 3 - 4 Groningen, 1966. [80] Andrade, E. N. da C , Viscosity and Plasticity, Chemical Publishing, NY, 1951. [81] Wright, W. A., Prediction of Oil Viscosity Blending, American Chemical Society, Atlantic City Meeting, 8-12 April 1946. [82] "Standard Viscosity-Temperature Charts for Liquid Petroleum Products," ADJD 034101-ADJD 034107, ASTM International, West Conshohocken, PA, 1974. [83] "part V-Viscosity Index," Lubrication, Vol. 36, 1950, p. 69. [84] Dean, E. W. and Davis, G. H. B., "Viscosity Variations of Oils with Temperature," Chemical and Metallurgical Engineering, V o l 36, 1929, p. 618. [85] Davis, E. W., Lapeyrouse, M., and Dean, G. H. B., "Applying Viscosity Index to Solution of Lubricating Problems," Oil and Gas Journal, Vol. 30, March 1932, p. 92. [86] Dean, E. W., Bauer, A. D., and Berglund, J. H., "Viscosity Index of Lubricating Oils," Industrial and Engineering Chemistry, Vol. 32, 1941, pp. 102-107. [87] D 2270: S t a n d a r d Practice for Calculating Viscosity Index From Kinematic Viscosity at 40 and 100°C, RR: D02-1009, ASTM International, West Conshohocken, PA, 1998. [88] Wright, W. A., "A Proposed Modification of the ASTM Viscosity Index," Proceedings of the American Petroleum Institute, Vol. 44, Sec. 3, Refining, 1964, pp. 535-541. [89] ASTM Viscosity Index, Calculated from Kinematic Viscosity, ASTM DS 39a, ASTM International, West Conshohocken, PA, 1965. [90] Watt, J. J., et al., ASTM Viscosity Index, Calculated from Kinematic Viscosity, ASTM STP 168, ASTM International, West Conshohocken, PA, 1965. [91] Wright, W. A., Talbot, A. F„ and Manning, R. E., Viscosity Index Tables for Celsius Temperatures, ASTM DS 39b, ASTM International, West Conshohocken, PA, 1975. [92] "Changes in ISO Industrial Oil Viscosity Classification," ASTM Standardization News, April 1973, pp. 36-7. [93] D 2270: Metrification of Viscosity Index System Method, RR: D02-1009, ASTM International, West Conshohocken, PA, 1998. [94] ISO 2909: Petroleum Products—Calculation of Viscosity Index from Kinematic Viscosity, International Organization for Standardization, Geneva, 2002. [95] Klaus, E. E. and Fenske, M. R., "The Use of ASTM Slope for Predicting Viscosities," ASTM Bulletin, July 1956. [96] Geniesse, J. C , "A Comparison of Viscosity-Index Proposals," ASTM Bulletin, July 1956. [97] Eby, L. T., Tables for Determination of ASTM Slope and Prediction of Viscosities, Chemical Division, S t a n d a r d Oil Development Co, 21 July 1946. [98] Schiessler, R. W. and Sutherland, H., "The Synthesis and Properties of High Molecular-Weight Hydrocarbons," API Re-
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877
search Project 42, Proceedings of the American Petroleum Institute, Section III, Vol. 32, 1952. Lowitz, D. A., Spencer, J. W., Webb, W., and Schiessler, R. W., "Temperature-Pressure-Structure Effects on the Viscosity of Several Higher Hydrocarbons," Journal of Chemical Physics, Vol. 30, 1959, pp. 73-82. Tanner, R. I., Engineering Rheology, Oxford University Press, London, 1985. Macosko, C. W., Rheology Principles, Measurements, and Applications, VCH PubUshers, NY, 1994. So, B. Y. C. and Klaus, E. E., "Viscosity-Pressure Correlation of Liquids," ASZ,£: Transactions, Vol. 23, No. 4, 1980. pp. 4 0 9 ^ 2 1 . Wu, C. S., Klaus. E. E., and Duda, J. L., "Development of a Method for the Prediction of Pressure-Viscosity Coefficients of Lubricating Oils Based on Free-Volume Theory," Journal of Tribology, Vol. I l l , 1989, pp. 121-128. Swindells, E. C , Coe, J. R., Jr., and Godfrey, T. B., "Absolute Viscosity of Water at 20°C," Journal of Research of the National Bureau of Standards, Vol. 48, 1962, pp. 1-31. Mailiarov, G. A., "Determination of the Viscosity of Water at 20 °C," Proceedings of the AU-Union Science Research Institute of Metrology, Vol. 97, No. 37, 1959, pp. 125-140 (in Russian). Roscoe, R. and Bainbridge, W., "Viscosity Determination by the Oscillating Vessel Method. II. The Viscosity of Water at 20 °C," Proceedings of the Physical Society, Vol. 72, 1958, pp. 585-595. Kestin, J. and Shankland, J. R., "The Free Disk as an Absolute Viscometer and the Viscosity of Water in the Range 25 °C-150 °C," Journal of Non-Equilibrium Thermodynamics, Vol. 6, 1981, p p . 241-256. Berstad, D. A., Knapstad, B., Lamvik, M., Skjorklep, K., and Oye, H. A., "Accurate Measurements of the Viscosity of Water in the Temperature Range 19.5 °C - 25.5 °C," Physica A, Vol. 151, 1988, pp. 246-280. Marvin, R. S., "The Accuracy of Measurements of Viscosity of Liquids," Journal of Research of the National Bureau of Standards, Vol. 75A, Nov-Dec 1971, pp. 535-540. Bauer, H., Binas, E., Broeke, H., and Volkel, L., 'New Recommended Viscosity Value for Water as the MetrologicEil Basis of Viscometry," PTB-Mitteilungen, Vol. 105, Feb. 1995, p p . 99-105. Bingham, E. C. and Jackson, R. F., "Standard Substances for the Calibration of Viscometers," National Bureau Standards (U.S.) Bulletin, Vol. 14, 1919, pp. 59-86. Marvin, R. S., "The Accuracy of Measurements of Viscosity of Liquids," Journal of Research of the National Bureau of Standards, Vol. 75A, Nov-Dec 1971, pp. 535-540. Swindells, E. C , Coe, J. R., Jr., and Godfrey, T. B., "Absolute Viscosity of Water at 20 "C," Journal of Research of the National Bureau of Standards, Vol. 48, 1962, pp. 1-31. ISO/TR 3666: Viscosity of Water, International Organization for Standardization, Case postale 56, CH-1211, Geneva, Switzerland, 1998. Hardy, R. C , "NBS Viscometer Calibrating Liquids and Capillary Tube Viscometers," National Bureau of Standards Monograph 55, U.S. Department of Commerce, Washington DC, 26 Dec. 1962. "Viscometer Calibrating Liquids, Notice of Discontinuance," Federal Register, Doc. 67-1245, U.S. Department of Commerce, Washington DC, Filed Feb. 2, 1967. "The International Temperature Scale of 1990 (ITS-90)," Metrologia, Vol. 27, 1990, pp. 3-10. "The International Temperature Scale of 1990 (ITS-90)," Metrologia, Vol. 27, 1990, pp. 3-10. Wise, J. A., "A Procedure for the Effective Recalibration of Liquid-in-Glass Thermometer," NIST Special Publication 819, National Institute of Standards and Technology, Gaithersburg, MD, 1991.
878 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK [120] Stormer, E., Transactions of the American Ceramic Society, Vol. 11, 1909, p. 597. [121] Griffith, J. M. and Puzinauskas, V. P., "Relation of Empirical Tests to Fundamental Viscosity of Asphalt Cement," Symposium on Fundamental Viscosity of Bituminous Materials, ASTM STP 328, ASTM International, West Conshohocken, PA, 1962, pp. 20-47. [122] Pochettino, A., Nuovo Cimento, Serie 6, Vol. 8, No. 2, 1914, pp. 77-108.
[123] AASHTO TPl-98: Method for Determining the Flexural Creep Stiffness of Asphalt Binder Using the Bending Beam Rheometer (BBR), American Association of State Highway and Transportation Officials, Washington DC, 1998. [124] AASHTO TP5-98: Method for Determining the Rheological Properties of Asphalt Binder Using a Dynamic Shear Rheometer (DSR), American Association of State Highway and Transportation Officials, Washington DC.
MNL37-EB/Jun. 2003
Cold Flow Properties Robert E. Manning^ and M. Richard Hoover^
POUR POINT
T H E VISCOSITY OF A LIQUID CONSISTING OF ONLY ONE TYPE OF
MOLECULE will increase when the liquid is cooled. If the liquid is cooled sufficiently, the viscosity will increase until the liquid solidifies (freezes). However, most petroleum products are composed of mixtures of many different types of molecules, each of which has its own freezing point. When these liquids are cooled to low temperatures, unanticipated results can occur. The diesel fuel for cars and trucks can form crystals that will plug the fuel filter while the vehicle is being driven, causing the vehicle to stop. Heating oil can be loaded into a delivery vehicle, but at very low temperatures the oil may not flow through the delivery hose to the business or home receiving the oil. Automotive engine products in conventional quart, liter, or gallon containers may, when left outside the filling station in the winter, solidify so they will not pour out of the container. Some petroleum products will form a "gel" structure at a certain low temperature. However, the same material, when cooled rapidly from a w a r m temperature to a still lower temperature and through a critical temperature range, will flow in a normal manner. As the temperature of a petroleum product is lowered, wax contained in the product can crystallize into long crystals thus forming a gel structure. Frequently, chemicals such as "pour-point depressants" (wax-crystal modifiers) are added to fluids to lower the temperature at which gels can form. Such pour point depressants serve to lower the pour point of the fluid by forming many smaller crystcJs at a temperature somewhat higher t h a n the temperature at which wax crystals form. With cooling to still lower temperatures, wax will crystallize onto the unconnected crystal sites of the pour-point depressant instead of forming a gel structure. Without the yield stress provided by a n interlocking gel-structure, the material will show a much lower pour point.
In the 1910s, it was realized that some petroleum products failed to flow when subjected to low temperatures. Thus, a simple test apparatus (Fig. 1) was devised to characterize the fluidity of many petroleum products and to establish a temperature below which the material does not readily flow—^the "pour point" temperature. Such materials include: • • • • • •
Lubricating oils Distillate and residual fuels Black oils used in heavy-duty equipment Cylinder stock Heat transfer fluids Crude oils Originally published as ASTM D 4 7 i n l 9 1 7 and revised and renumbered D 97 in 1927, the Pour Point of Petroleum Products is among the oldest of petroleum test methods still in use. A test sample is placed in a jar and heated without stirring to 9°C above its expected pour point (but to a minimum of 45°C) by placing the jar in a bath at a m i n i m u m of 48°C. The jar is cooled in a non-uniform manner by cooling in sequence inside a group of successively lower temperature baths from 0° to -69°C. These sequences include: • • • • • •
Specimen is at 45°C (or above), place in a bath at 24°C Specimen is at -l-27°C, move to 0°C bath Specimen is at -l-9°C, move to - 1 8 ° C bath Specimen is at —6°C, move to —33°C b a t h Specimen is at -24°C, move to - S T C bath Specimen is at —42°C, move to —69°C bath At temperature intervals of 3°C, the test sample jar is removed and tilted: . . .just enough to ascertain whether there is a movement of the specimen in the test jar. The complete operation of removal, wiping, and replacement shall require not more than 3 s . . . . Continue in this manner until a point is reached at which the specimen shows no movement when the test jar is held in a horizontal position for 5 s. Record the observed reading of the test thermometer (ASTM D 97). A similar method of test is written specifically for crude oils (ASTM D 5853). With the desire to use automatic or automated pour point apparatus, several methods have been developed and accepted that cire less labor intensive (ASTM D 5949, ASTM D 5950, ASTM D 5985, ASTM D 6749) than the manual procedure of ASTM D 97. Each of these methods uses its own geometry and special techniques, but essentially the same cool-down sequence. Although the specific tech-
This chapter discusses p o u r point tests, aviation fuels freezing tests, and the cloud point and wax appearance point tests used for a variety of petroleum products. The behavior and performance of engine oil products at low temperature is of very high importance and is incorporated into the general Society of Automotive Engineers (SAE) flow properties specifications. Engine oil lubricants at low temperatures are discussed in detail in Chapter 32, Flow Properties and Shear Stability. Note that all referenced ASTM methods are included at the conclusion of the chapter.
' Consultant, 225 Harris Drive, State College, PA 16801. ^ President, Cannon Instrument, State College, PA 16804.
879 Copyright'
2003 by A S I M International
www.astm.org
880 MANVAL 37: FUELS AND LUBRICANTS HANDBOOK THERMOMETER ' t l . 2 - -15.8 ID. 3 0 - 32.4 ID. 33.2 - 3 4 . 8 OD.
CORK
25 MAX. - L
COOLANT LEVEL
DISK NOTE 1—Dimensions are in miUimelres (not to scale).
FIG. 1—Pour point cell (ASTM D 97).
niques of sensing pour point differ from one instrument to another, the determined p o u r points from each method have shown correlation to the resuUs from ASTM D 97 [1]. These automatic test instruments allow the determination of pour point at temperature intervals of 1, 2, or 2.5°C in addition to the 3 X interval specified in ASTM D 97. Novel changes introduced a m o n g t h e several devices include: • heating a n d cooling accomplished by a Peltier device (ASTM D 5949), • a pressurized pulse of nitrogen gas imparted onto the surface of the specimen, a n d multiple optical detectors observing the response of the surface (ASTM D 5949), • automatic tilting of the cell and observation by optical detector of the tilt of the surface of the specimen (ASTM D 5950), • rotation of the test specimen cup at approximately 0.1 r p m against a stationary, counter balanced, spherical b o b (ASTM D 5985), a n d • a slightly positive air pressure gently applied to the surface of the specimen causes upward movement of the specimen in a communication tube (ASTM D 6749). While each of these methods has been extensively tested and all have been compared against ASTM D 97, various test specimens can be expected to show smeJl differences in each of the several pour point instruments. Although the use of the automated/automatic instruments improves the precision of the determination of pour point temperature, ASTM D 97 remains as the referee method. It should also be noted that a free-flowing (i.e., without gel structure) liquid can become so highly viscous at low temperature so as to also have a pour point, a so called viscous pour point.
AVIATION FUELS FREEZING POINT The freezing point of aviation fuels is defined in ASTM D 2386 as . . .the fuel temperature at which solid hydrocarbon crystals, formed on cooling, disappear when the temperature of the fuel is allowed to rise The freezing point of an aviation fuel is the lowest temperature at which the fuel remains free of solid hydrocarbon crystals that can restrict the flow of fuel through filters, if present, in the fuel system of the aircraft. The temperature of the fuel in the aircraft tank normally falls during flight depending on aircraft speed, altitude, and flight duration. The freezing point of the fuel must always be lower than the minimum operational tank temperature (ASTM t) 2386). To measure the freezing point of an aviation fuel, a 25-mL specimen is transferred into a clean, dry, jacketed sample tube. The tube with stirrer, thermometer, and moisture-proof collar is placed in a vacuum flask, and allowed to cool. The cooling medium may be solid carbon dioxide in acetone or methyl, ethyl o r isopropyl alcohol, or liquid nitrogen, or mechanical refrigeration maintained at —70 to — 80°C. The fuel is stirred continuously while moving the stirrer u p and down at a rate of 1-1.5 cycles/s. The temperature at which crystals of hydrocarbon appear is recorded (ignoring a water haze which may form at — 10°C). Upon the formation of hydrocarbon crystals, the jacketed sample tube is removed from the coolant a n d allowed to warm, with continued stirring. The temperature at which the crystals completely disappear is reported to the nearest 0.5°C as the freezing point. The manual method of test has been automated (ASTM D 5901, ASTM D 5972) using apparatus similEir to the traditional a p p a r a t u s with mechanical refrigeration o r Peltier
CHAPTER 33: COLD FLOW PROPERTIES CLOUD POINT AND WAX APPEARANCE POINT
Stirring Rod
Moistureproof Collar
881
Thermometer Cork Stopper
Refrigerant Carbon Dioxide Vacuum Flask NOTE 1—^AIl dimensions are in mm and ±0.1 mm glass wall thickness isl mm.
FIG. 2—Freezing point apparatus (ASTiVI D 2386).
cooling and optical detection of the appearance a n d disappearance of the hydrocarbon crystals. The automated apparatus allows reporting the freezing point to the nearest 0.1 °C, and somewhat improved repeatability and reproducibility as compared to ASTM D 2386 (Fig. 2). The lowest temperature at which aviation fuels remain free of solid hydrocarbon crystals is a key safety parameter in the specification and use of fuels, because the hydrocjirbon crystals can restrict the flow of fuel through filters in an aircraft fuel system. A cooling and warming cycle of the fuel through a specified filter can also be used to determine the temperature, called the flow point, at which the filter becomes unblocked on weirming (ASTM D 4305). There is a small bias between the flow point t e m p e r a t u r e a n d the freezing point temperature. This procedure can also be used to investigate the formation of wax crystals or cold flow properties of other products. As low temperatures can materially affect the aircraft fuels, low temperatures can also affect the aircraft turbine lubricants (ASTM D 2532). On prolonged standing at low temperature, a structure can form in the lubricant, causing an increase in the kinematic viscosity of the lubricant. This viscosity increase can cause lubrication problems in aircraft engines. Thus, kinematic viscosity is measured typically at - 6 5 ° F (-53.9°C) by allowing the test sample to soak in the viscometer at test temperature for periods of 35 min, 3 h, a n d 72 h. The percent increase in kinematic viscosity over the initial m e a s u r e d value at 3 h or/and 72 h is also reported.
The presence of wax crystals in burner fuels, diesel fuels, and turbine engine fuels is an indication of the lowest temperature of the material's utility for certain applications. In sufficient quantity, such crystals can plug filters in fuel systems. This property may cause limitations on the use of the fuel at low temperatures, or it can require reblending of the fuel. The cloud point of a petroleum product is the temperature at which a cloud of wax crystals first appears in a liquid when it is cooled under the specific conditions prescribed in the test method. The wax appearance point is the temperature at which wax first begins to separate from the fluid when it is cooled under the conditions prescribed in the test method. An apparatus used to measure cloud point is illustrated in Fig. 3. The sample is placed in a clear, cylindrical glass jar, which is cooled in a series of b a t h s as shown in Table 1 (ASTM D 2500). This cooling sequence allows a slow, controlled cooling of the test vessel. As the sample temperature drops, the test jar is removed quickly b u t without disturbing the oil, inspected for the presence of cloudiness or a cloud formation at the bottom of the test cell, a n d replaced, all within 3 s. The cooling bath materials used include water and ice (at 0°C), crushed ice and sodium chloride crystals (at — 18°C), crushed ice and calcium chloride crystals (at —33°C), acetone, or methyl or ethyl alcohol, or petroleum naphtha and solid carbon dioxide (at - 5 1 and -69°C). A note from ASTM D 2500 states the following: A wax cloud or haze is always noted first at the bottom of the test jar where the temperature is lowest. A slight haze throughout the entire sample, which slowly becomes more apparent as the temperature is lowered, is usually due to traces of water in the oil. Generally this water haze will not interfere with the determination of the wax cloud point. In most cases of interference, filtration through dry lintless filter papers is sufficient. The cloud point is reported to the nearest 1 °C Automated Cloud Point apparatus and specific test methods have been developed. These include cooling in steppedcooling temperature baths (ASTM D 5771), linear cooling rate (ASTM D 5772), a n d constant cooling rate (ASTM D 5773) of the test sample, with optical detection of the cloud point. When the specification requires Test Method D 2500, an automated cloud point method may not be substituted. The test method to determine the Wax Appearance Point of Distillate Fuels (ASTM D3117) is similar to the cloud point test, b u t the test sample is stirred while being cooled. The "wax appearance point" is that temperature at which wax p h a s e first begins to separate from the liquid when it is cooled under the prescribed conditions.
Bath 1 2 3 4 5
TABLE 1—Bath and sample temperature ranges. Bath Temperature Setting, °C Sample Temperature Range, °C - 1 to2 -18to-15 -35 to-32 - 5 2 to - 4 9 - 6 9 to - 6 6
Start to 10 10 t o - 7 -7 to-24 - 2 4 to - 4 1 - 4 1 to - 5 8
882 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK
3ta- a4LB.
T
NOTE — All dimensions in millimeters FIG. 3—Apparatus for cloud point test (ASTM D 2500).
LOW-TEMPERATURE HIGH-SHEAR TESTS OF ENGINE LUBRICANTS
LOW TEMPERATURE PUMPABILITY OF ENGINE LUBRICANTS
With the changes to engine lubricant formulations in the 1970s, it was anticipated that a problem might develop in low temperature stcirting. It had been known that some engines, when cranked by the starting motor, could start at cranking speeds as low as six rpm. Other lubricants were so high in viscosity that even though the engine could be cranked, the initial firing of the engine would not allow acceleration and starting of the engine. Prior to the 1970s, the normal way of estimating the kinematic viscosity at O^F (—17.8°C) was by extrapolation using the MacCouU-Wright equation from kinematic viscosity data at 100 and 210°F. Problems with this method became obvious. The new formulations sometimes were semi-solid at 0°F, even though the prediction by equation indicated the lubricant to be sufficiently fluid to allow flow at low temperatures, thus allowing the automotive engine to readily start at low temperatures. Because there was a general lack of accepted information on both automotive engines and their lubricants at starting conditions, a large program [2,3,4,5] was undertaken with the cooperation of the Society for Automotive Engineers (SAE) Fuels and Lubricants committee, the ASTM Committee on Petroleum Products and Lubricants (Subcommittee 7 on flow properties), and the Coordinating Research Council (CRC). A group of five "Newtonian" Reference Engine Oils (REO) was prepared to cover the viscosity range of 730-8400 mPa-satO°F(-17.8°C). The low-temperature high-shear tests of engine lubricants are discussed in Chapter 32, Flow Properties and Shear Stability.
Lubricant pumpability is critically important to engines. Catastrophic failure can occur if there is a lack of a full flow of lubricant to the bearings (bearing starvation) of modern engines. Consequently, the rheology of oils at low t e m p e r a t u r e s is of highest importance. Thus, the t e r m pumpability has been widely used in the lubricant industry to indicate the ability of a lubricant to be p u m p e d and/or to flow to the surfaces being lubricated, especially at low temperature. Several papers appeared in the 1940s concerned with the pour point stability of lubricating oils [6,7, 8, 9]. In the early 1960s a study by Moyer [10] indicated that there might be a problem with the pumpability of engine lubricants at low temperatures. He observed that on pumping in a test apparatus, as the temperature is lowered in the oil pan a hole can be drawn in the oil reservoir, causing the p u m p assembly to be air bound. Selby [11] measured rheological characteristics of reblends of oils previously studied by Moyer showing that cavitation occurred at shear rates less than 50 s ' ' and above a critical viscosity of 24 000 mPa-s. McMillan and Murphy noted that: An analysis of oil pumpability reveals that engine oil pumping failures may occur because either the oil cannot flow under its own head to the oil screen inlet, or the oil is too viscous to flow through the screen and inlet tube fast enough to satisfy pump demands [12]. The low-temperature pumpability of engine lubricants is discussed in Chapter 32, entitled Flow Properties and Shear Stability.
CHAPTER 33: COLD FLOW PROPERTIES ASTM METHODS
IP 444/99
No. D 97 D 2386 D 2500 D 2532
IP 445/99
D 3117 D 4305 D 5771 D 5772 D 5773 D 5853 D 5901 D 5949 D 5950 D 5972 D 5985 D 6749
Title Test Method for Pour Point of Petroleum Products Test Method for Freezing Point of Aviation Fuels Test Method for Cloud Point of Petroleum Products Test Method for Viscosity and Viscosity Change After Standing at Low Temperature of Aircraft Turbine Lubricants Test Method of Wax Appearance Point of Distillate Fuels Test Method for Filter Flow of Aviation Fuels at Low Temperature Test Method for Cloud Point of Petroleum Products (Optical Detection Stepped Cooling Method) Test Method for Cloud Point of Petroleum Products (Linear Cooling Rate Method) Test Method for Cloud Point of Petroleum Products (Constant Cooling Rate Method) Test Method for Pour Point of Crude Oils Test Method for Freezing Point of Aviation Fuels (Automated Optical Method). Test Method for Pour Point of Petroleum Products (Automatic Pressure Pulsing Method) Test Method for Pour Point of Petroleum Products (Automatic Tilt Method) Test Method for Freezing Point of Aviation Fuels (Automatic Phase Transition Method Test Method for Pour Point of Petroleum Products (Rotational Method) Test Method for Pour Point of Petroleum (Automatic Air Pressure Method)
OTHER STANDARDS No. AFNOR M07-048
AFNORT60-105 DIN 51421
IP IP IP IP
15/95 16 219/94 441/99
Title Petroleum Products: Determination Point of the Dissipation Point of Aviation Motor Fuel CrystcJs Petroleum Products: Determination Point of Discharge Testing of Mineral Oils; Determination of Freezing Point of Aviation Fuels, Gasolines and Motor Benzols Pour Point of Petroleum Products. Freezing Point of Aviation Fuels. Cloud Point of Petroleum Products Pour Point of Crude Oils.
IP 446/99 ISO 3013 ISO 3015 ISO 3016 JIS K 2269 JIS K 2276
883
Cloud Point of Petroleum Products (Optical Detection Stepped Cooling Method) Cloud Point of Petroleum Products (Linear Cooling Rate Method). Cloud Point of Petroleum Products (Constant Cooling Rate Method) Determination of Freezing Point of Aviation Fuels Determination of Cloud Point Determination of Pour Point Pour Point and Cloud Point of Crude Oils a n d Petroleum Products Testing Method for Aviation Fuels
REFERENCES [1] ASTM Research Report D02-1312, ASTM International, West Conshoshocken, PA, 1993. [2] SAE J305a: Extrapolated Oil Viscosities, SAE Handbook, Society of Automotive Engineers, Warrendale, PA, 1966, p. 299. [3] "Development of Research Technique for Determining the LowTemperature Cranking Characteristics of Engine Oils," CRC Report No. 374, January 1964. [4] Lowther, H. V., Meyer, W. A. P., Selby, T. W., and Vick, G. K., "Development of Research Technique for Determining the Low-Temperature Cranking Characteristics of Engine Oils," presented at SAE Automotive Engineering Congress, Detroit, January 1964. [5] "Prediction of Low-Temperature Cranking Characteristics of Engine Oils by Use of Laboratory Viscometers," CRC Report No. 381, March 1965. [6] Henderson, L. M. and Annable, W. G., "Pour Point Stability of Lubricating Oils," Oil and Gas Journal, 9 Sept. 1943, pp. 54-59. [7] Hodges, C. E. and Boehm, A. B., "Pour Point Stability of Treated Oils Under Winter Storage Conditions," Oil and Gas Journal, 24 June 1943, pp. 103-6. [8] Hodges, C. E. and Rogers, D. T., "Some New Aspects of Pour-Depressant Treated Oils," Oil and Gas Journal, 4 Oct. 1947, pp. 89-93, 99. [9] McNab, J. G., Rogers, D. T., Michaels, A. E., and Hodges, C. E., The Pour-Point-Stability Characteristics of Winter-Grade Motor Oils, SAE Quarterly Transactions, Vol. 2 No. 1, Jan. 1948, pp. 34-44. [10] Moyer, R. G., "Low Temperature Pumpability of Oils," Journal of the American Society of Lubrication Engineers, April 1962, pp. 165-168. [11] Selby, T. W., "Viscosity and the Cranking Resistance of Engines at Low Temperature," Sixth World Petroleum Congress Proceedings, Section VI, Frankfort, June 1963, pp. 241-258. [12] McMillan, M. L. and Murphy, C. K., "The Relationship of LowTemperature Rheology to Engine Oil Pumpability," SAE National Automobile Engineering Meeting, Paper #730478, Detroit, ML 14-18 May 1973.
MNL37-EB/Jun. 2003
Environmental Characteristics of Fuels and Lubricants Mark L. Hinman^
sure concentration is continuously changing due to: • Diffusion away form the point of release • Movement to other media (i.e., transport between soil, water, sediment, and air) • Degradation (biological and abiotic) of the material The spatial and temporal variability present in the natural populations results in an increased range of responses to the exposure. Other materieJs and chemicals in the background could contribute to any effects observed. Further, the need for data on the fate and effects of materials for regulatory purposes exceeds the data available from both intentional and unintentional releases. A more reliable approach to evaluate the inherent hazard of a material to the environment is by testing the individual chemical properties in the environmental components of concern in separate, carefully controlled laboratory test systems:
FUELS AND LUBRICANTS RANGE FROM MODERATELY SIMPLE MIX-
TURES OF SIMILAR MOLECULES to very complex mixtures of very different molecules. As the performance of the product becomes more demanding, the composition of the product generally becomes more complex. There are few products that do not have additives to modify the inherent performance characteristics of the base material (or materials) and enhance its application for a specific use. Fuels are generally a blend of specific hydrocarbon distillation fractions with additives to facilitate performance. The fuel use defines the type and range of hydrocarbon fractions and any additives, whereas lubricants reduce friction, heat, and wear when applied to the surface of moving parts. Frequently, the lubricity or other key characteristic of the base material is enhanced by additive treatment to meet a specific need. The base material may be of various origins (petroleum, synthetic, or vegetable oil). For this discussion, various specialized industrial fluids that do not fall into the fuels category are included in with the lubricants. These would include a range of products, such as hydraulic fluids, metal cutting fluids, turbine oils, greases, etc.
• The media exchange rates of a material may be deduced from measurements of key physical properties • The degradation rates under specific conditions may be determined in controlled tests of biodegradability, hydrolysis rate, photolysis rate, etc. • The toxicity to organisms may be determined in single species tests using organisms of k n o w n n u m b e r , age, health and u n d e r carefully controlled, constant environmental conditions. Controlled laboratory tests of physical properties, degradation rates, and toxicity may be used to indicate the potential fate and effects of the material in the environment under various conditions and exposure possibilities. Conventional wisdom suggests that transport properties and degradation rates may be combined in mathematical models to calculate the exposure concentrations and the exposure-response relationship is used to estimate the environmental effects. Controlled laboratory test results could be extrapolated to the environment when the exposure regimes are similar. This is possible because the standardization of tests has permitted the development of data for the necessary predictive models. Most of these methods have been developed for simple, water-soluble, non-volatile substances and have been shown to be useful for these materials.
Fuels are seldom released into the environment intentionally. More often, the release is the result of spillage at transfer, during storage, disposal, or some unexpected event (e.g., transport collision, pipeline break, etc.). Some lubricating fluids are intentionally introduced into the environment as a function of their use (rail-flange grease, chainsaw oils, 2stroke oil passthrough residuals, etc.). For the remaining lubricants, between 13% (EU Countries) and 32% (USA) of t h e m cire released to the environment after use. This results in an annual worldwide loss to the environment of approximately 12 million tons [1]. Rationale for Standardized Testing The potential for materials to cause harmful effects in the environment needs to be evaluated. The most straightforward way to accomplish this might seem to be to evaluate organism populations in areas where the material has been released or spilled into the environment of interest. Although field studies are the most realistic way of evaluating all possible chemical effects, they are usually impossible to quantitatively evaluate in terms of the level of the h a r m associated with the magnitude of the release. This is because the expo-
In contrast, most fuels and lubricant products are mixtures (simple and complex) of organic molecules, often relatively insoluble in water, and/or volatile. Further, additives can introduce metals and other inorganic compounds into the mixture. If the testing and interpretation account for the solubility and volatility properties of the test material, the media
' Environmental Scientist, ExxonMobil Biomedical Sciences, Inc., 1545 Route 22 East, Annandale, NJ, 08801-0971.
885 Copyright'
2003 by A S I M International
www.astm.org
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exchange and degradation rates may be combined in mathematical models to calculate the exposure concentrations and the exposure-response relationship is used to estimate the environmental effects. Controlled laboratory test results can be extrapolated to the environment because of the development of predictive models ([2,3,4], E 978-92, Standard Practice for Evaluating Mathematical Models for the Environmental Fate of Chemicals). Another benefit of standardized testing is that different materials m a y be c o m p a r e d on a c o m m o n basis. For instance, testing toxicity with the same species of fish in water of the same quality allows one to compare the relative toxicity of the substances. Such comparisons have led to the development of scientific relationships between the chemical structures and their physico-chemical and biological properties. For example, when a homologous series of chemicals is tested, such as alcohols of increasing carbon chain length, there is a fairly continuous change in the physical properties and toxicity. The predictable change may be used to extend the result (through interpolation) to untested members of the series. This process is often called "read-across." Correlations developed between chemical structure, physical properties, and biological activity have led to validated relationships referred to as Quantitative Structure Activity Relationships (QSAR). The most widely accepted QSARs are the relationship between the lipophilicity of a material and the water solubility, soil sorption, bioconcentration, and toxicity. The precision of the QSAR estimates for some classes of chemicals has led to the acceptance of QSAR generated data for some regulatory activities and substitution for missing data [4].
PHYSICO-CHEMICAL PROPERTIES A number of physical properties are especially relevant to the environmental behavior of substances. Some of these characteristics (e.g., water solubility, vapor pressure, etc.) are measured on a routine basis for products. However, complex mixtures often require more detailed information about the range of the property than pure substances (i.e., for a product that is a mixture of components with greatly differing boiling points). T5rpically, the boiling range is reported as a n average. When the boiling range is broad, there is also a broad range in vapor pressures of the components. This leads to differences in the environmental compartmentcilization of the components. The more volatile components will partition into the air phase to a greater extent and so will be present to a lesser extent in the soil and aquatic environments than the less volatile components. Consequently, for substances with multiple components, specific test methods need to be applied eind reported which measure the range rather than the average of a property. Chemical Structure The properties of chemicals depend u p o n their chemical structures. For fuels, this may involve a n u m b e r of hydrocarb o n structures having a range of molecular weights and chemical types, as well as a variety of organic and inorganic compounds included in additive treatments. Knowledge of the relative hydrocarbon and specific additive composition is important (see Table 1). For lubricants, the structure types range widely. Mineral hydroccirbon basestocks, synthetic basestocks, or plant oil
TABLE 1—Physical properties that are of environmental importance (Adapted from Ref. 7). Definition
Physical Property
Environmental Importance
Molecular weight (range) Formula Functional groups branching Equilibrium concentration ratio between octanol and water
Determines other physical and chemical properties
Vapor Pressure" (VP)
Partial pressure of vapor
Water Solubility"
Aqueous concentration at saturation
Characterizes tendency to leave liquid state cind become a gas which can enter the air Allows estimation of evaporation and volatilization rates and extent Characterizes extent to which it will dissolve in water Allow estimation of water concentration and volatility
Soil (Organic Carbon) Partition Coefficient (Ka or Koc)
Equilibrium concentration between soil and water
Dissociation Constant (Ka)
Ionization of acids or bases
Melting" and Boiling Point" (MP, BP)
Critical temperature for phase changes: solid—liquid—gas
Chemical Structure
OctanolAVater Partition Coefficient" (Kow)
"A range of values is appropriate for complex mixtures.
Characterizes partitioning to lipids in organisms or the organic carbon of sludges and soils QSAR estimates of toxicity, partitioning, and water solubility
Chciracterizes the tendency to sorb to particles in the soil Allows estimation of mobility through soil and sediment Ionization extent at environmental pH ranges influences other properties Characterizes physical form in the environment Related to vapor pressure and solubility
CHAPTER
34: ENVIRONMENTAL
CHARACTERISTICS
basestocks may be involved as well as a variety of additives. The m a i n classes of additives are succinimide ashless dispersants, calcium sulphonates, calcium phenates, zinc dialkyldithiophosphates, oxidation inhibitors, and antiwear inhibitors [5]. In addition to providing one or more specific characteristics to the final product, additives increase the complexity of the mixture and difficulty of predicting the environmental impact of the material on the environment. O c t a n o l / W a t e r P a r t i t i o n C o e f f i c i e n t (Kow) The octanol / water partition coefficient (Kow or Pow) is the ratio of a chemical's solubility in n-octanol and water at steady state. The Kow is a characteristic of significant importance in environmental studies. It is a c o m m o n surrogate for bioconcentratition for classification determinations and is a key parameter in QSAR calculations. There are three basic approaches to the determination of Kow: • The original a n d reference m e t h o d is the "shake flask method." An aqueous solution of the material is equilibrated with n-octanol. After equilibrium is reached, the water and the n-octanol phases are separated and analyzed for the material. The ratio of the material concentration in n-octanol to water is the Kow[6]. • The HPLC (high-pressure liquid chromatography) method m e a s u r e s the retention t i m e of the material on a hyd r o p h o b i c column a n d compares it with the retention times of a n u m b e r of standards with known Kow values (E 1147-92 (1997), Standard Test Method for Partition Coefficient (N-OctanolA/Vater) Estimation by Liquid Chromatography). The method is based on the correlation of the retention time with the Kow. • The QSAR estimation method involves the calculation of Kow based upon the known, empirical contribution of the various structural fragments of the molecule to the Kow value. This gives excellent results for many hydrocarbons [4]. Due to the wide range of measured values, Kow is usually expressed as the logarithm (Log Kow or Log Pow). The ranges are a direct outcome of both the carbon n u m b e r range and
in ifl
OF FUELS AND LUBRICANTS
the chemical types present in the mixture. I n general within a chemicEil class, as carbon numbers increase, so does the Kow. However, for the same carbon number, aromatic compounds have a lower Kow than the aliphatic counterpart. The Kow values for classes of aliphatic structures do not differ very much, while the linear ("normal") paraffins have slightly higher values than the corresponding branched ("iso") paraffins or cyclic paraffins ("Naphthenes"). A m e a n Kow value for a mixture is less meaningful than the range of values for all components in the mixture [7].
Water Solubility The persistence, biodegradation, and toxicity of a material in the aquatic environment are very dependent upon its water solubility. Generally, the solubility of hydrocarbons decreases as the carbon n u m b e r increases [8]. However, this parameter is cdso affected by the structure. Water solubility will be different for linear, branched, and cyclic paraffins with t h e s a m e c a r b o n n u m b e r . The m e a s u r e m e n t of this characteristic Ccin be very straightforwEird: • Increasing additions of the pure material are made to water and the water analyzed until saturation is reached. The analysis must be specific to the material. • An QSAR method is available for hydrocarbons that provide good water solubility estimates. For complex mixtures, the individual components in the mixture have different water solubility limits. Once the least soluble component saturates the water, an undissolved phase begins to form (typically at the water's surface). The remaining c o m p o n e n t s will partition between the water and the undissolved phase and therefore never reach their individual water solubility limit. The composition of the water extract will be different than the composition of the complete mixture as it also depends on the ratio of material volume to the water volume (Fig. 1). This solubility behavior is very complex. It should be remembered that the definition of solubility as it relates to pure substances does not readily apply to complex mixtures [7].
100
SS o
>• C
o re u o •a
>> X Pure Gasoline
0.01
887
100
Loading (Gasoline: Water, mg/L) FIG. 1—Water solubility of gasoline components at different gasoline:water ratios (personal communication [83]).
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Vapor Pressure The vapor pressure of a substance is a measure of its tendency to enter the gas phase. This is important for determining both the rate of evaporation and the relative amount of the substance that will be in the air phase. Vapor pressure tends to decrease with increasing molecular weight, thus the materials that are the most water soluble tend to have high vapor pressures and move from the water to the air fairly quickly [7,8] For pure materials, direct pressure measurements may be used. For materials with very low vapor pressure values or for a mixture, a vapor saturation method is used. The stationeiry medium in a generator column is coated with the material and air is cycled over the medium until it is saturated [7]. Henry's Law Constant Henry's Law Constant (HLC or H) is essentially the partition coefficient of a material between air and water. It is important in estimating the degree and rate of volatilization of a material from water into air. It may be measured directly by analytical means within a test system or taken as the ratio of the vapor pressure to the molar water solubility. Henry's Law Constant is generally expressed as Pa-m^ / mole. For complex mixtures, each component has a different water solubility and vapor pressure, so the HLC may cover a wide range. The low water solubility and relatively high vapor pressure of hydrocarbons result in high HLC values. Generally, an HLC value of 10 Pa-m^/mole or higher indicates that volatilization is a significant fate process for the material. Hydrocarbons are particularly likely to volatilize from water. This is more likely for paraffins than for aromatics, which have somewhat greater water solubilities. The solubility of most substances decreases faster then the vapor pressure with increasing molecular weight, so that the HLC values increase with increasing molecular weight. For large molecules, the vapor pressure becomes too low for volatilization to be an important fate process [7]. Adsorption / Desorption Adsorption is the taking u p of a gas, vapor, or dissolved material on the surface of a solid. Desorption is the release of a gas, vapor, or dissolved material from a solid under the same conditions. The adsorption of organic chemicals from water into organisms, sediment, or soil plays an important role in the fate of materials. This process is also important in accumulation of materials by aquatic, sediment dwelling, and soil organisms. The soil/water equilibrium also has a determining influence on the rate at which chemicals will leach into ground water. For simple un-ionized organic chemicals, the adsorption/desorption behavior is strongly affected by the organic matter contained in the soil or sediment. This concentrating behavior is not always a simple linear relationship and is usually described as an "isotherm." However, for most materials, the assumption of a constant distribution coefficient between soil and water (Ka) is adequate. For organic substances, the important determinant of adsorption is the organic matter contents of the soil or sediment. As a result, the values for various soils/sediments are "normalized" by calculating the partitioning coefficient be-
tween water and soil organic matter (KQC) ( E 1195-87 (1993), Standard Test Method for Determining a Sorption Constant (Koc) for an Organic Chemical in Soil and Sediments). The Koc is often calculated directly from the Kow [7]. Dissociation Constant Some components of both fuels and lubricants dissociate into ionic form in the aquatic environment. Dissociation constants (pKa) are only moderately affected by changes in temperature a n d ionic strength. In general, dissociation decreases with increasing temperature, while increasing the environmental ionic strength tends to favor the ionic form. Dissociation greatly influences all other environmental processes and properties such as sorption, bioconcentration, and toxicity [2]. Ions cannot partition into the air or nonpolar environments. Additionally, the ionic species may have very different aquatic toxicity from the unionized form [7] as a result of different mechanisms to transport the molecule across cell membranes.
FATE IN THE ENVIRONMENT Upon entering the environment, a material may be transported within and between environmental media (also called compartments or phases) and respond to ambient factors in the environment (Fig. 2). The primary factors that affect the fate of materials are as follows: • Physical transport (advection, dispersion, partitioning) • Transformation (biotic and abiotic) • Accumulation (bioaccumulation, bioconcentration) The final equilibrium distribution of a material between the air, water, sediment, soil, and biota significantly influences the potential impact of the material within each media. The characteristics of the material and the environmental media determine which of a variety of transformations that the material may undergo. Abiotic transformations include hydrolysis, photolysis, and oxidation. Biodegradation is not a physical property of a material, such as water solubility. Microbial organisms may modify a material in a variety of ways, all of which could be considered biodegradation. The different types of biodegradation have been classified according to the extent of the modification. These are simple structural changes to complete degradation to CO2. Transport Processes Environmental transport can be divided into movement within and between media. For example, a material released into a river will be transported by the river flow. If this material is highly volatile, it will move from the aquatic comp a r t m e n t to the air c o m p a r t m e n t . Both types of transfer processes involve advective and diffusive mechanisms. Advection causes the material to move in the same directions the media (e.g., air or w a t e r currents). Diffusion causes movement in response to concentration gradients. Table 2 lists some of the important transport processes. Environmental media are comprised of different phases. For example, surface water consists not only of an aqueous component, but also contains suspended solids. Similarly,
CHAPTER
34: ENVIRONMENTAL
CHARACTERISTICS
UV/Visible Light
OF FUELS AND LUBRICANTS
889
Volatilization
Hydrolysis \ Biotransformation - ^
- - Adsorption Biological \ \ * Uptake \ \ Dissolved Organic \ \ Materials
Biodegradation
/ Food Web Transfer
Suspended Particles
Sediment FIG. 2—Possible degradation pathways for a substance in the aquatic environment. TABLE 2—Examples of important environmental transport processes (adapted from [7]). Movement within Media
Movement Between Media
Air advection (winds) Air diffusion Surface water advection (currents) Surface water diffusion Ground water advection (currents) Ground water diffusion
Wet and dry atmospheric deposition Volatilization Gas absorption Soil leaching/run-off Sediment burial Sediment—water exchange
the atmosphere is composed of air and particulates. An important driving force for diffusive transport is the tendency of a material to reach equilibrium between these phases. Partition coefficients quantify the equilibrium distribution of a material between these different phases. The use of equilibrium partitioning theory is an important element of current environmental fate assessments, since the form and compartmentalization of the material influences subsequent transformations and accumulation processes in the environment [7].
tions convert the chemical into degradation products, but usually do not totally convert it to the inorganic forms (e.g., CO2, H2O). Transformation of a material may result in a change in toxicity and physical characteristics. Toxicity of the product(s) may change, but do not necessarily result in a reduction. Thus, it is useful to understand the degradation products, if possible, to be able to evaluate their impact on the environment. Biodegradation is m u c h more complicated than abiotic degradation. Only certain microorganisms can accomplish it and not all microorganisms can degrade all materials in all environments. In some cases, a microorganism may be able to degrade only the products of a previous biodegradation process. The multitude of organisms and the highly variable genetic make-up of the microbial communities usually result in at least some microorganisms that can degrade an organic material to some extent. The outcome of this degradation may range from a simple chemical modification to a complete utilization by the organism for growth of new cells or for energy. The type and number of organisms are as important to the outcome as the structure of the material. Abiotic Degradation
Transformation Processes
Hydrolysis
Standardized tests have been developed to allow some prediction of the degradability of materials in the environment. These are generally divided into chemical (abiotic) degradation and biodegradation tests. Standardized tests for determining the degradation rates are available from many Standards Developing Organizations (e.g., ASTM, ISO, etc.). International Consortia (e.g., OECD, CONCAWE, etc.), and Regulatory Bodies (e.g., U.S. EPA, Environment Canada, European Union, etc). Abiotic degradation of fuels and lubricants is primarily due to photolysis, hydrolysis, and photooxidation. These reac-
Hydrolysis is a chemical transformation process in which an organic molecule, R-X, reacts with water to form a new carbon-oxygen bond and cleaving a carbon-X bond in the original molecule. It is a well-understood chemical process and the types of chemicals that hydrolyze and the catalysis of the process has been studied extensively [2]. The standard test for hydrolysis involves preparing aqueous solutions of the material at pH 4, 7, and 9, incubating at 50°C in the dark, and analyzing the remaining concentration at various time intervals (E 895-89 (1995) Standard Practice for Determination of Hydrolysis Rate Constants of Organic Chemicals in Aqueous
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HANDBOOK
Solutions). Stability for more than a week indicates n o significant hydrolysis has occurred. The bonds in most hydrocarbons do not tend to hydrolyze. In general, only those materials with ester bonds are expected to be affected by this process. Hydrolysis half-lives can range from days to years for these materials, with the rate of hydrolysis of organic compounds increasing with temperature in an exponential relationship [2]. Hydrolysis in not expected to be a major fate pathway for hydrocarbon-based fuels and lubricants, while synthetic and vegetable oil based lubricants may be more susceptible. Photolysis Photolysis is a light-initiated transformation reaction that is a function of the incident light energy (typically at wavelengths between 290 and 400 nm) and the structure of the material. Different classes of molecules absorb sunlight at different wavelengths a n d with different efficiencies. Molecules need to have a strong adsorption band in this region to undergo significant photolysis in the environment, so the UVA'isible spectrum of chemicals is used to screen for the likehhood of photolysis (E 896-92 (1997) S t a n d a r d Test Method for Conducting Aqueous Direct Photolysis Tests). It is not necessary that the material directly absorb the light to undergo photolysis. With indirect (or sensitized) photolysis, an intermediary chemical becomes energized and then energizes the material of interest. Generally, phenols, furans, aromatic amines, sulfides, and nitro-aromatics are susceptible to indirect photolysis [2], Some polynuclear aromatic hydrocarbons (PAHs) can undergo a light-mediated increase in toxicity. Some intermediate weight PAHs, such as anthracene, fluoranthene, a n d pyrene, have demonstrated photo-induced toxicity (greatly enhance toxicity when compared to the inherent toxicity of the material). When the PAH is present in the tissues of a n organism, phototoxicity can occur when UV radiation is absorbed by the molecule. The current theory is that oxygen free radicals are produced that are highly oxidizing and can destroy the molecules in the tissue [9]. Photooxidation Atmospheric photooxidation is the degradation of chemicals in air due to reaction with ozone or hydroxyl radicals and is dependent upon the structure of the chemical, its concentration, and the ozone/hydroxyl radical concentration. Hydroxyl radical attack is the predominant degradation mechanism for alkanes, olefins, alcohols, and simple aromatics, which are rapidly oxidized in air [2]. Hydroxyl radicals are produced through the interaction of sunlight with oxygen and other substances in the atmosphere. The volatility of hydrocarbons and the rapid oxidation in air promotes photooxidation as a significant fate process for most hydrocarbons. There is a significant amount of test data on the rate of hydroxyl radical reaction for chemicals that has allowed for the development of effective QSAR models that ceJculate atmospheric half-lives. Biodegradation As indicated by the name, biodegradation is the biologically mediated transformation of a material. The microorganisms
that accomplish the biodegradation are very important to the n a t u r e of the result. Generally, the interest in studying biodegradation is to use the results to predict the environmental fate of a chemical. Thus, the microorganisms used in laboratory tests are mixtures of species collected from particular environments. For a variety of reasons, sewage sludge has become the standard source of mixed inoculum for introduction into laboratory biodegradation test systems. In the aquatic environment, bacterial numbers are lower than in sewage. They are also exposed to low concentrations of a variety of chemicals and nutrients. I n the standardized tests, the organisms are constrained within a small volume and presented with a high concentration of the test material as the sole source of carbon. Those organisms capable of degrading the chemical will use it for a source of carbon for new cells and energy. Pre-exposure to petroleum products cam result in an increase of petroleum-degrading microorganisms from 1-10% of the total population [10]. The rate of this biodegradation reflects the growth rate of those particular organisms u n d e r the conditions of the test. Although the kinetics are completely defined by the test and are not easily related to a rate in the environment, the test results are used to classify the persistence of the material and estimate its halflife in the environment. Certain environmental conditions (e.g., availability of oxygen) greatly influence the types of o r g a n i s m s and their metabolic pathways. Standardized tests exist for aerobic and anaerobic conditions in the major environmental compartments (freshwater, marine water, sediments, and soils). In some cases, simulation tests are intended to replicate the natural conditions of the organisms, their environment, a n d their exposure to the chemical [11]. Certain standardized tests have been developed for regulatory p u r p o s e s to evaluate b o t h the rate a n d extent of biodegradation (Table 3). The primary terms used to describe biodegradation (primciry and ultimate) distinguish between two extents of biodegradation. Primary refers to the initial transformation from the parent material and ultimate refers to mineralization of the material. Rate is described by the t e r m s inherent a n d Ready. To b e classified as inherently biodegradable, there m u s t be unequivocal evidence of biodegradation by any test method. The OECD method regulates degradation of the material for this classification. Ready Biodegradability is a regulatory classification originating in the European Union that has very specific criteria. The tests are characterized by low initial biomass ( < 3 0 mg/L) of sewage organisms, which have not been previously exposed (adapted) to the material. The substance must meet the criterion of 60% degradation to CO2 or O2 (or 70% removal of DOC) within 28 days. Further, the pass criteria must be met within 10 days after the biodegradation exceeds 10% of the mass loaded [12,13]. The Ready biodegradation tests are considered so stringent that if a substance passes the test, it will also rapidly degrade under most environmental conditions. If a material does not pass a ready test, it does not meeui that it will not degrade in the environment A n u m b e r of ready biodegradability tests are available and differ mainly in the method of analysis. The different assays give different results. The analytical methods dictate details of the procedures and result in different applicabilities for each test. Generally, the disappearance of dissolved organic
CHAPTER 34: ENVIRONMENTAL CHARACTERISTICS OF FUELS AND LUBRICANTS
891
TABLE 3—Types and characteristics of biodegradation tests (adapted from [7]). Attribute xtent of Biodegradation
Categories Ultimate
Ease of Biodegradation
Measurement
Properties • Measures total conversion to inorganic forms (e.g., CO2, etc.) • Mineralization Based on analysis of specific chemical or chemical or chemical class abiotic losses controlled
O2 CO2 CH3
Primary
Specific Analysis
Removal
Specific Analysis
Ready
O2 CO2
Primary biodegradation Both abiotic and biodegradation Regulatory definition of "rapid" biodegradation Uses <30 mg/L non-adapted inoculum reaches minimum 60% degradation to O2 or CO2 or 70% removal of DOC in 28 days must go from 10% to the pass level (60 or 70%) in 10 days
DOC
Inherent
O2 CO2 CH3
Enhanced conditions to show possibility of eventually biodegrading Generally a high biomass adapted inoculum
DOC Specific Ansilysis Specific Analysis
Simulation
1
2
3
4
5
6
7
8
Reflects actual environmental behavior Difficult to simulate most actual exposure situations
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Day of Test FIG. 3—Typical biodegradation curves for the same material when analyzed for different endpoints (adapted from [7]). carbon (DOC) is more rapid than the utilization of O2 or the production of CO2 (Fig. 3). Table 4 provides a list of commonly used standardized tests, the general characteristics of the test, Emd general applicability. Comparing the results from different test methods for the same or similar materials can be problematic. The "percent biodegradation" is very dependent u p o n what type of biodegradability was tested. A result of 100% p r i m a r y degradation in an inherent test cannot be compared to the results of a Ready test, where 100% mineralization is not possible emd where conditions are much more stringent. Even within similar test methods, the inoculum makes a
great deal of difference in the outcome of the test. The number of competent organisms greatly affects the duration of the lag phases. If the n u m b e r of these organisms in sewage sludge is low, different tests could show either good biodegradation or none at all. The microorganisms in the inoculum may vary significantly over time. The tests for biodegradation, even using a "standard" inoculum, will give much more variable results than the tests for physical properties or toxicity. As a result of this and various complications resulting from use of inappropriate tests for the physical characteristics of the material, the comparison of biodegradability between materials is only quantitative if
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HANDBOOK
they are tested in the same test systems using the same ino c u l u m p r e p a r e d at the same time. The major use of biodegradation tests is to provide an estimate of the potential of a material to degrade in the environment. In general, hydrocarbons are biodegradable to varying degrees [14,15]. Fuels and lubricants are mixtures with some components having low water solubility. This low water solubility often limits the concentration in solution and thus their availability to the microorganisms. The aromatics tend to be slightly more water-soluble and give "better" results in standardized tests than the paraffins. Among the paraffins, the linear hydrocarbons are considered more biodegradable t h a n the branched hydrocarbons. This results from microbial oxidation of one end of the linear molecule to a carboxylic acid. The metabolic processes in nearly all organisms can easily degrade the resulting fatty acid. The presence of branching hinders degradation to some extent. Much of the older data for volatile hydrocarbons used the Closed Bottle test (OECD 301D)[12]. This is an un-stirred test system with a low inoculum. Using this method resulted in most of these hydrocarbons being considered as non-Readily biodegradable. Recent testing of the same materials using the Manometric Respirometry Test system (OECD 30IF)[12] has identified many of them to be readily biodegradable (Fig. 4)[ 16].
1022-94 Standard Guide for Conducting Bioconcentration Tests with Fishes and Saltwater Bivalve MoUusks; E 1688-00 Standard Guide for Determination of the Bioaccumulation of Sediment-Associated Contaminants by Benthic Invertebrates; E 1676-97 Standard Guide for Conducting Laboratory Soil Toxicity or Bioaccumulation Tests With the Lumbricid Earthworm Eisenia fetida). Past research has generally focused on accumulation in the aquatic environment. Terms commonly used to quantify accumulation potential by aquatic organisms are the bioconcentration factor (BCF) [17], the bioaccumulation factor (BAF) [18], and biota-sediment accumulation factor (BSAF) 100 Manometric Respirometry Metliod
•S 8 0 n 2 g" 60 •o .2 " 40 c «
20 Closed Bottle IVIethod '
Accumulation Processes Accumulation processes result in the transfer of a substance from a primary environmental m e d i u m (e.g., air, water, soil, and sediment) to a plant or animal. Materials that pose the greatest concern for accumulation potential enter the environment from diffuse sources, are poorly degraded in the primary medium, exhibit a high affinity for lipids, and are resistant to the metabolic breakdown by plants and animals (E
0
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' 13
20
24
Day Of Test
^^J^ Termination
FIG. 4—Comparison of hexadecene biodegradation curves for data from tfie Closed Bottle test method and the Manometric Respirometry Method (personal communication [16]).
TABLE 5—Comparison of acute and ciironic environmental toxicology tests (adapted from [7]). Test
Onset of effect Concentration of test material Exposure frequency
Test duration
Effects typically measured
Chronic
Acute
Determine test material concentration that causes effect on organism during siiort term exposure Evaluates relative toxicity of different materials to same organism Evaluates relative sensitivity of different species to same material
Objective
28
Designed to determine the lowest observable effect and the no observable effect concentrations of a material
Sudden or delayed
Delayed
Relatively high
Relatively low
• • • •
Static Static renewal Continuous (Flow-through) Pulsed
• Static renewal • Continuous (Flow-through) • Pulsed
• Typically < 4 days • Exposure period small when compared to total life-cycle
• Weeks to months to years • May include sensitive part of life-cycle, entire life-cycle, multiple life-cycles
Mortality Immobilization Changes in growth rates (algae)
Organism development and growth Reproductive success Based on most sensitive endpoint
CHAPTER
34: ENVIRONMENTAL
[19]. The BCF is a laboratory derived measurement that simply defines the partition coefficient of the substances between aquatic organism tissue (primarily the lipid component) a n d the surrounding water. The BAF is generally a field-derived m e a s u r e m e n t determined from monitoring data. The BAF is calculated by dividing the observed field tissue concentration of the material by the corresponding (dissolved) concentration of the material in the water (soil, sediment, or food), although the value can also be determined from microcosm and mesocosm experiments. The BAF differs from the BCF in that the BAF represents the accumulation via all possible exposure routes that may occur in the field (i.e., water, sediment, soil surface, and diet), whereas the BCF only reflects the accumulation due to aqueous exposure. The BSAF expresses the steady-state difference between the concentration of a bioaccumulating nonpolar organic chemical normalized to the organic carbon content of a sediment and the concentration measured in the total extractable lipids of an organism for which that sediment represents the source of the contamination in its habitat. Although the BAF and BSAF values provide a more realistic appraisal of the accumulation potential of a material, there is a high degree of uncertainty associated with field data due to variability in the exposure values from various environmental sources a n d associated contaminates. Further, field derived data are not available for most materials. Consequently, BCF provides a valuable surrogate parameter for assessing the relative bioaccumulation potential of materials under controlled laboratory conditions. The BCF test methods generally include both uptake and depuration (loss) of the material in different segments of the test. During the uptake segment, the organism is exposed to a constant aqueous concentration of the substance. At periodic intervals, organisms are sampled and analyzed to determine the tissue concentrations. Once steady-state tissue concentrations are approached, the test organisms are transferred to clean water to determine how rapidly the material is depurated. Based on the results of both segments, a simple model is used to estimate the BCF and the half-life for the test material. BCF measurements are applicable to test materials that are well-defined single components, b u t not to complex mixtures. Based o n the results of numerous BCF experiments for different substances, QSARs have been developed that relate the BCF value to the KQW. In the absence of BCF data, the Log Kow values are used to predict the bioaccumulation potential for materials. These predictions are often used for hazard classification and risk assessment. However, this approach may be overly conservative for two reasons: 1. QSARs are generally based on measured BCF values for substances t h a t are poorly metabolized. Many hydrocarbons and industrial chemicals can undergo significant enzymatic degradation once absorbed into an organism. This process is referred to as biotransformation (or metabolism) and serves to lower the BCF value when compared to the QSAR prediction. 2. QSARs are limited to materials with Log KQW values generally lower than 6.0. For materials with higher KQW values, research indicates that the BCF declines as the Log Kow increases even for substances that are poorly metabolized.
CHARACTERISTICS
OF FUELS
AND LUBRICANTS
895
EFFECTS IN THE ENVIRONMENT Standardized Tests Standardization of toxicity testing is important. Within a population of organisms, there may be a wide variation in age, size, vitality, and health among individuals. These variations lead t o different sensitivities within t h e population. Further, there can be significant variation in sensitivity between different species. Reducing sources of variability improves the reproducibility of the test results. In standardized tests, biological variability is reduced by using organisms of selected species that are the same age range, similar in size, and in good health with no observable abnormalities. When the goal of the test is to measure toxicity parameters such as mortality or effects on reproduction, a single species test is used. Concerns about different species sensitivities are addressed in environmental toxicology by testing representative or indicator species from various levels in the food chain (tropic levels). Considerable research and test method development has gone into the selection of test species that are representative of the more sensitive species (within the same grouping of organisms) in the environment (Fig. 5). One advantage of u s i n g such representative species is that substances tested at different times and in different laboratories can be compared for relative toxicity (Table 6). The good predictability of QSARs that have been developed for specific species and classes of organisms has demonstrated the utility of this standardization approach. These QSARs allow prediction of toxicity t o a q u a t i c organisms based on structural or physical properties of the substance (E 1242-97 Standard Practice for Using Octanol-Water Partition Coefficient to Estimate Median Lethal Concentrations for Fish Due to Narcosis). As with the biodegradation test methods, each specific test provides data that is representative of a specific aspect of the natural environment. The dominating factor relating the toxicity test to the environment is the mode of exposure. Exposures As with the variability associated with the test organisms, each test material has unique characteristics that must be considered when conducting a standardized toxicity test. The volatility, water solubility, and complexity/variability of composition of the material can significantly affect the exposure of the test organism to the material and must be considered in the design of the test. Appropriate test design considerations allow for reproducibility of results within the same laboratory and comparison between laboratories for the same test material. In order t o relate effects t o exposure, one needs to have a defined, quantified exposure test material concentration for a specific period of time. Since the effect is related to the internal dose received by the organisms (a value not generally known), exposure duration must be adequately long to insure maximum uptake of the material by the test organism. For low solubility materials, such uptake may be quite slow. As a result of these considerations, the exposure duration of aquatic toxicity tests has been standardized. For acute toxicity tests, these periods range from two days for Daphnia tests to four days for most fish tests.
896 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
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0.1
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Copper Concentration, |ig / L FIG. 5—Species Sensitivity to Copper Sulfate (adapted from [84]).
For stable, water soluble, pure materials, maintaining a constant exposure concentration is not difficult. T5rpically, an arithmetic or geometric series of exposure concentrations Eire tested. The test method and preliminary testing usually determines the specific range and number of concentrations. This facilitates having a range in which the extremes provide no mortality and complete mortality. Test guidelines usually require that the actual concentration be analytically confirmed over the course of the exposure period. The resulting data allow the use of statistical techniques to develop a concentration-response curve and to find the midpoint or LC50 (E 1847-96 Standard Practice for Statistical Analysis of Toxicity Tests Conducted under ASTM Guidelines; E 1023-84 (1996) Standard Guide for Assessing the Hazard of a Material to Aquatic Organisms and Their Uses; E 729-96 Standard Guide for Conducting Acute Toxicity Tests on Test Materials with Fishes, Macroinvertebrates, and Amphibians). The same approach is used to establish other effect endpoints such as impact on growth or reproduction for chronic testing. For endpoints not involving lethality, EC is often used to indicate the "effect concentration" rather than the "lethal concentration." Discussing the concentration-response can be confusing. The smaller the concentration of the material that causes an effect, the greater the toxicity of the material. Typically, lubricant and fuel products are considered "difficult materials" for aquatic toxicity testing. Difficult materials may have low water solubility, be volatile, or be complex
mixtures. Much of the historical information on the toxicity of these materials can be misleading or confusing. Often, the toxicity values are not reasonable (e.g., reported LC50 exceeds the water solubility of the material) and the value does not accurately represent the toxicity of the material. All that can be said is that the LC50 is greater than the water solubility. Generally, the material must be retested to develop scientifically defensible values. Designing, conducting, and interpreting aquatic toxicity studies on such materials involves special consideration [20]. Volatility For volatile or unstable materials, it is difficult to maintain a consistent concentration for the exposure period. The traditional approach is to replace the exposure matrix concentrations with frequent renewals of the exposure medium from freshly prepared test solution (static renewal test), or even with a continuous flow of test solution through the exposure chambers (flow-through tests). An approach that has been successfully used for volatile substances is to prevent loss by testing in tightly closed containers with no headspace. Precautions that organisms get sufficient oxygen must be taken. It should be re-emphasized that the purpose of these tests is to produce a quantitative concentration—response relationship, not to simulate the fate and effects of the substance in the environment. So the expected (nominal) concentration must be confirmed with periodic analyses and, if necessary.
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899
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MANUAL
37: FUELS
AND LUBRICANTS
HANDBOOK
the LCso calculation may be based upon the measured concentrations [20]. Low
Solubility
There are a n u m b e r of issues related to low solubility materials. The acute LC50 values for a homologous series of chemicals may sometimes seem confusing. In general, the members of the series with lower water solubility will have greater toxicity. However, as the n u m b e r of carbon atoms in the series increases, the water solubility decreases. The decrease in solubility is usucJly greater than the increase in toxicity. So a point will be reached when no acute mortality is evident. This is sometimes referred as the "solubility cut-off." Larger members of the series are too insoluble to cause acute mortality (Fig. 6). Although it is possible that the small amount of the larger members in solution may cause chronic toxicity and may contribute to the acute toxicity caused by other dissolved substances, they contribute little or no acute toxicity unto themselves. Further additions of a single material above its solubility limit will not increase the dissolved portion that is absorbable by the test organisms, a n d therefore is not bioavailable. The conclusion of these tests should be "no acute toxicity at the m a x i m u m water solubility of the substance." Sometimes tests are conducted as limit tests. These are tests in which the substance is added in an a m o u n t greater than its solubility to meet arbitrary toxicity limits (usually regulatory), such as 100 mg/L. In this example, if less than 50% mortality is observed, the LC50 is reported as >100 mg/L and is considered to have minimal toxicity. There are two potential sources of error inherent in this approach: • The presence of minor components that are water-soluble may affect the results. At additions above the water solubility of the main substance, the contaminants may continue to dissolve and may constitute a higher percentage of the water solution than the substance itself. In these cases, the toxicity observed is generally erroneously ascribed to the main substance.
Solubility is greater than LC50
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FIG. 6—Relationship of water solubility and aquatic toxicity of linear paraffins (adapted from [7]).
• The occurrence of physical effects caused by the undissolved test material. Such effects may become confused with the chemiccJ toxicity of the bioavailable portion. Examples of such problems cire the fouling of fish gills by oil droplets, which causes suffocation, or surface entrapment of Daphnia in a surface film of a chemical, causing immobility (an endpoint in the Daphnia acute toxicity test). In some cases, emulsifiers or solvents are used to attain solution of poorly soluble substances. Generally, these aids only speed dissolution to equilibrium and do not significantly influence solubility. If there is n o associated toxicity with these dissolution aids, there is n o problem with this procedure. If large amounts of such materials are used and the water properties are significantly affected, material a m o u n t s may be put into solution above its water solubility. In this case, there are concerns with this approach. It may distort the environmental conditions prevailing in the test system a n d may significantly alter the test results, which further complicates its interpretation [20]. Complex
Mixture
Toxicity
Fuels and lubricants do not consist of a single chemiccJ compound. Some products consist of isomers with a similar number of carbons that differ primarily in branching pattern. As a result, the toxicity of these materieds is similar between individual components, since isomeric differences tj^sically have little effect on aquatic toxicity. In contrast, other materials are composed of components that vary significantly in carbon n u m b e r and chemical characteristics. Consequently, evaluating the toxicity of these materials is more complicated and involves both the amount and toxicity of individual components. Most hydrocarbons are believed to be toxic by the same mode of action and so are considered to have additive toxicity. Thus, the toxic unit (TU) approach can be used to predict the acute toxicity of such products [21]. This approach involves the following steps: 1. A TU for each component in the product is determined. A TU is defined as the aqueous concentration of the component divided by the component's corresponding LC50. 2. The TUs for each component in the mixture are summed. 3. If the s u m of TUs > 1 , toxicity is expected; whereas, if the sum of TUs is < 1, toxicity is not expected. For complex mixtures of poorly water-soluble substances, another challenge becomes apparent in testing and evaluating the results. For complex mixtures there is n o exact definition of water solubility. Each component may have a different water solubility so that with increasing additions of the substance to water, some components reach their solubility limit, become insoluble and float on the surface, while other components continue to dissolve. In fact, a complex equilibrium is established between the insoluble chemical component phase (usually at the surface) and the water, never quite reaching true saturation for any but the least soluble component (Fig. 1). Additions of a complex material to water at levels beyond the water solubility of the components results in a solution and a toxicity measurement which does not accurately reflect the composition or toxicity of the complete material. Some scientific and regulatory organizations have suggested using the "Lethal Loading" approach to overcome these difficulties
CHAPTER 34: ENVIRONMENTAL CHARACTERISTICS OF FUELS AND LUBRICANTS [20]. This test methodology is specifically for poorly watersoluble complex mixtures and bases the toxicity measurement on the a m o u n t of substance added to water (not the concentration of the dissolved components that the organism is exposed to). The "amount of substance added" has been dubbed the "loading" as a simpler term. The toxicity value derived from this testing approach is described as LL50 (ELso) for "lethal loading, 50%" ("effect loading 50%") to distinguish the results from tests on soluble, simple substances (LC50 or EC50). For fuels a n d lubricants, the "Lethal Loading" approach is preferred for developing information on the quantity of released material that might be required to initiate significant ecologiccd risk and classification/labeling schemes. The lethal loading approach is not a reliable surrogate for quantified exposure data. An uninformed reader may confuse the loading values for the exposure values (which are usueJly much smaller than the loading values) and underestimate the inherent toxicity of a material. Nor is this approach the same as the nominal concentration since: • entire substance is generally not all dissolved • dissolved fraction is different in composition from the substance • composition of the dissolved fraction changes with the loading and often different components predominate at different loadings The information derived from the different exposure approaches is of great value if applied to appropriate environmental situations. An additional reason for the use of a separate (LL50) terminology is that when conducting lethal loading tests, it is generally unknown whether the water phase is a true solution. The test substcince is brought to equilibrium with water at each concentration (loading) to be tested. The portion of substance which separates after equilibration is generally removed and the aqueous phase referred to as the "water accommodated fraction" (WAF). The WAF may be a solution or a n emulsion. Unlike previous methodologies, dilutions of the WAF are not tested but rather WAFs are prepared at each loading tested. This allows the toxicity result to be related to the entire substance, giving a result that is relevant to realistic environmental situations. For example, if a product spill occurs, it will not be of a concentrated water extract of the substance but of the whole substance (generally) at rather low substance to water ratios. LL50 data allows an immediate quantitative assessment of how m u c h substance in a given volume of water is likely to result in toxicity [20,22]. Beyond the problems of low water solubility, some materials also pose the additional problem of being quite volatile from water (high Henry's Law Constant). The same discussion given above for volatile substances applies here, since such substances need to have the water concentrations held constant. The difficulty is more operationcil than conceptual, since the composition of the WAF is usucdly unknown and varies with the loading. It is necessary to test in closed systems with very close attention to minimizing any air spaces or losses to air of volatile components either in performing the test or in preparing the WAF. Generally, water concentrations of one or more volatile components are evaluated analytically, not to measure exposure concentration, but rather to confirm that equilibrium is achieved in WAF preparation, and that losses to air are minimized during exposures.
901
Standardized Toxicity Test Procedures—Acute The general approach to all toxicity tests is to expose the organisms to a series of concentrations of the test substance. The concentrations are chosen, often based on some preliminary tests, so that the biological effect (e.g., mortality) occurs over the range of concentrations. At the lowest concentration, little to n o effect is observed, while at the highest concentration the effect is maximal. See Table 6 for a listing and description of standardized environmental toxicology test methods. Aquatic
Toxicity
Tests
Tests to determine the aquatic toxicity of substances have been in place longer than tests in other media. The intention is to provide testing at different levels within the aquatic food web. Algae, invertebrates (which graze on the algae), and fish (which feed on the invertebrates), are included in the usual aquatic testing scheme. A variety of species have been identified which are used as standard test organisms in these studies (Fig. 5). The intent is to have representative or indicator species that have the following characteristics: • life histories are well understood and can be cultured in the laboratory • survive well in the laboratory • are relatively sensitive to a broad range of toxins • are well studied so that their responses may be readily interpreted These species are intended to be a conservative representation of the entire aquatic environment because of the preference for selecting sensitive species and usually sensitive life stages for testing. The species frequently selected for freshwater toxicity tests are the fish—rainbow trout {Oncorhynchus mykiss) or fathead minnow (Pitnephales promelas), invertebrate—^waterflea (Daphnia magna and Ceriodaphnia dubia), and green algae {Selenastrum capricomutuni). (The scientific n a m e of this green alga has recently been changed to Pseudokirchneriella subcapitata (Korshikov) Hindak [23]. In that this n a m e is not yet in common use, the name Selenastrum capricomutum will be used in this document.) Data resulting from toxicity tests are generally used to calculate the concentration of material that will adversely affect 50% of the test orgcinisms (i.e., LC50, LL50, a n d EC50).
Tests With
Algae
Cultures of algae in their exponential growth phase are exposed to various concentrations of a substEince over several generations under defined conditions. Cell density is determined by microscopic counting of cells or by a spectrophotometric m e a s u r e m e n t of chlorophyll. The inhibition of growth in relation to a control (non-exposed) culture over a fixed period of time is determined. The algal cell density in each flask is determined at 24, 48, and 72 h (and 96 h for some test methods) after the start of the test. The mean cell density for each concentration is plotted against time to produce growth curves. The growth endpoint is determined by comparing the area under the growth curves for each concentration against the control. The average specific growth rate (the increase in cell density over time) is also evaluated (D 3978-80 (1998) Standard Practice for Algal Growth Potential Testing with Selenastrum capricomutum;
902
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
E 1218-97a Standard Guide for Conducting Static 96-h Toxicity Tests with Microalgae). Examples of growth curves and exposure-response curves (for both endpoints) for the effects of Light Naphtha on Selenastrum capricomutum are depicted in Fig. 7. Because several generations of algal cells are exposed in this test system, it is sometimes referred to as a chronic test. Tests With Invertebrates Acute toxicity tests with invertebrates generally expose organisms to a potentially toxic substance for 2-4 days. This period of exposure encompasses at least one sensitive period of growth during the juvenile life-stage. In the acute immobilization test with Daphnia, effects on the swimming capability are investigated. The number of immobile organisms for each concentration is compared to the control to calculate the ECso value. An example of the exposure-response curve
for the effect of Light Naphtha on Daphnia magna is depicted in Fig. 7. Tests With Fish A variety of fish are routinely used for acute toxicity tests. One of the most widely utilized species is the rainbow trout, Oncorhynchus mykiss (formerly Salmo gairdneri). Fish are exposed to the test substance for a 96-h period. Mortality observations are performed daily and the concentration that kills 50% of the fish (LC50) is calculated for each observation period. An example of the exposure-response curve for the effect of Light Naphtha on Rainbow Trout is depicted in Fig. 7. The test may be performed under flow-through, semi-static (solution is renewed at regular intervals), or static (no replacement of solution) conditions as long as constant environmental conditions and test material concentrations are maintained.
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FIG. 7—Exposure-Response curves for acute toxicity tests with algae, Daphnia, and Rainbow Trout for exposure to light naphtha (adapted from [7]).
CHAPTER
34: ENVIRONMENTAL
Standardized Toxicity Test Procedures—Chronic and Subchronic There are a variety of chronic toxicity test exposures: • Short term chronic, where orgainisms are exposed for a relatively short duration (often seven days) during a critical life-stage • Prolonged toxicity tests (usually two or more weeks) • Full-life cycle test (usually two to three generations). The chronic tests all have in c o m m o n the measurement of sub-lethal effects, e.g., growth (weight or length), development, r e p r o d u c t i o n (fecundity, n u m b e r of offspring produced), or survival of offspring (Table 6). These endpoints are important to the survival of the population and are thus considered m o r e sensitive endpoints t h a n those of the acute tests. Endpoints for chronic toxicity tests are usually expressed as a calculated concentration with a low level of toxicity (e.g., ECio) or as a concentration that is not statistically different than the control. The no observable effect concentration (NOEC) is the effect value corresponding to greatest concentration of the test substance that is not statistically different when compared to the control. The lowest observable effect concentration (LOEC) is the effect value corresponding to the lowest concentration of the test substance that is statistically different when compared to the control. See Table 6 for a listing and description of standardized environmental toxicity test methods. Tests With
Invertebrates
A c o m m o n chronic toxicity test with invertebrates is the Daphnia Reproduction Test in which less t h a n 24-hr old daphnia are exposed to a range of test substance concentrations for at least 14 days. Within this time period, each healthy daphnid can produce three or more broods of young. The n u m b e r of broods and the n u m b e r of young per brood are recorded. This test may be extended to three or four weeks, in which six to nine broods of young should be produced. This test is performed u n d e r semi-static or flowthrough conditions. Both survival of the peirent generation and the n u m b e r of young produced are evaluated. See Table 6 for other test methods using invertebrates. Tests With
CHARACTERISTICS
OF FUELS AND LUBRICANTS
903
• Length and weight data • Any abnormalities observed. The exposure period for the ejirly life-stage test varies for each species; for example, tests with the warm, marine water fish (Sheepshead m i n n o w ) t e r m i n a t e after 28-days posthatch, whereas tests with the cold, freshwater fish (Rainbow trout) terminate after 60-days post-hatch. Non-Aquatic Toxicity Tests Sediment
Toxicity
Tests
Sediment tests are primarily performed on invertebrates, which will either burrow into or ingest the sediment, using standard test protocols developed for freshwater amphipods {Hyalella azteca), midges (Chironomus tentans, C. riparius), mayflies (Hexagenia sp.), worms (Tubifex tubifex), saltwater amphipods (Rhepoxynius abronius, Eohaustorius sp.), and polychaetes {Neanthes arenacoedentata, N. virens). A control sediment can be spiked with a test chemical or a contaminated natural sediment can be tested. Exposure ranges from 10-30 days, with renewal of the overlying water during extended periods. Endpoints evaluated during short-term acute tests can consist of survival (LC50), growth and development (EC50), and reproduction (egg production and n u m b e r of young produced). Terrestrial
Toxicity
Tests
As with aquatic toxicity tests, a variety of species have been identified which are used as standard test organisms in terrestrial studies. These studies are focused on plants and animals that live in soil. For some applications, birds are tested [24]. See Table 6 for c o m m o n invertebrate and plant test methods. The plant toxicity studies allow the use of a very long list of species, most of which are grains or vegetables of commercial or native value. The acute studies relate the concentration of the chemical or diet to the mortality of the test organism or to germination (in the case of plants). For many tests with soil, the test material is dissolved, or emulsified in de-ionized water and then thoroughly mixed with artificial or reference soil. Diluting a contaminated soil with an artificial or reference soil, then exposing the organisms, can allow testing of a field sample.
Fish
There eire two c o m m o n chronic toxicity tests for fish. The first is a 14-day prolonged test, which is performed under similar conditions as the acute toxicity test, previously discussed. A representative weight and length measurement is taken for the fish, at the start of the test. The test is performed using either semi-static or flow-through exposure conditions for 14 days. At termination, all surviving fish Eire weighed and measured. The NOEC a n d LOEC values are determined based on survival, weight or length, or other abnormal effects observed throughout the test. The second is an early life-stage test that begins exposure on newly fertilized eggs. This test encompasses the embryo, larval, and juvenile stages of various fish species (Table 6). A flow-through test design is preferred to semi-static. This test design allows for evaluation of the following endpoints: • Mortality during the embryonic stage, larval stage, and juvenile stage • Days to hatch, zmd n u m b e r hatched
Tests With
Plants
GenerEilly, a m i n i m u m of three species is exposed to the test chemical. One of the three species is usually a monocotyledon, and the others dicotyledons (a legume and a root crop). The test is t e r m i n a t e d 14 days after 50% of the control seedlings have emerged. Several endpoints are usually evaluated. The n u m b e r of plants that emerge (LC50) and the meem dry weight at termination (ECso) as compared to the controls are c o m m o n endpoints. Shoot and/or root length or mass may also be evaluated for this test. Tests With
Worms
There are m a n y m e t h o d s for testing material toxicity to earthworms, including spot application and immersion tests. Often, a simple paper contact toxicity test cem be used as an initial screen test to identify those substances that should be tested in artificial soil. For this test, artificial soil is spiked with the test chemical, diluted into a concentration series.
904
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
and added to the exposure chambers. Earthworms are rinsed and placed on top of the soil in each chamber. Since worms are negatively phototropic (they do not like light), the test is performed u n d e r continuous lighting to encourage the worms to borrow and maximize exposure to the soil. Mortality observations are performed after 7 and 14-day exposures for subsequent LC50 determinations.
HAZARD AND RISK Ecotoxicity assessments are used to define environmental h a z a r d s a n d concentration-response relationships which, w h e n used together with m e a s u r e d or predicted environmental exposure concentrations, enable environmental risk to be quantified. The distinction between the terms hazard and risk is important. Hazard refers to the inherent capacity of a substance to cause an adverse effect. By contrast, risk is the likelihood that the hazard will occur in the real world, based on likely exposures. Thus, a product can pose an environmental hazard by causing the death of fish under exposure conditions achieved in laboratory toxicity tests but not constitute a significant environmental risk if such exposures have a very low probability of occurring in the aquatic environment.
SUMMARY Over the past decade or more, n u m e r o u s standard testing methods and computer-based predictive models have been developed to help assess the environmental persistence, biodegradation, and toxicity of materials. Most of these m e t h o d s have been developed for simple, soluble, nonvolatile substances. In contrast, most fuels and lubricant products are not defined as such, but rather are mixtures (simple and complex) of organic molecules, often relatively insoluble in water, and/or volatile. Further, additives can introduce metals and other inorganic compounds into the mixture. Nevertheless, the same standard methods were and are used to test these products. This often produces misleading results and interpretations. In a mixture, each c o m p o n e n t has properties that are unique to its structure and composition. As such, a range of values for a specific characteristic is often more meaningful than the average value when describing mixtures. Care must be taken when choosing the test method for producing these values. Characteristics such as water solubility, vapor pressure, dissociation constant, and sorption can render a specific test method inappropriate for many of these materials. In many cases, the results of different test methods for the same basic characteristic (e.g., biodegradation) cannot be directly compared. Toxicity testing of these materials requires that specicJ care and consideration be given to the characteristics of the material. The key to any toxicity test is ensuring that the test organism is properly exposed to the material being tested, thus the preparation and maintenance (or appropriate quantification) of the exposure matrix is critical to the success of the test. For fuels and lubricants, the "Lethal Loading" approach is preferred when developing informa-
tion on the quantity of released material that might be required to initiate significant ecological risk or for labeling. This approach is not a reliable surrogate for quantified exposure data. An uninformed reader may confuse the loading values for the exposure values (which are usually much smaller than the loading values) and underestimate the inherent toxicity of a material. Various standards, practices, and guidelines have recently been published that address the different approaches that are successful and those that aren't for difficult to test materials. Terminology Acute
Aerobic Anaerobic BAF BCF Bioaccumulation
Bioaccumulation Factor
Bioconcentration Factor
Biodegradation
Biota-Sediment Accumulation Factor
A brief exposure to a stressor or the effects associated with such an exposure. It can refer to an instantaneous exposure or continuous exposure of minutes to a few days [25]. Taking place in the presence of oxygen (D 6006-97a). Taking place in the absence of oxygen (D 6006-97a). See Bioaccumulation Factor. See Bioconcentration Factor. The net accumulation of a substance by an organism as a result of uptake from all environmental sources (E 943-97b). A value that is the ratio of tissue chemical residue to chemical concentration in an external environmental phase (i.e., sediment or food). BAF is measured at steady state in situations where organisms are exposed from multiple sources (i.e., water, sediment, food), unless noted otherwise [26]. A term describing the degree to which a chemical can be concentrated in the tissues of an organism in the aquatic environment as a result of exposure to waterborne chemical. At steady state during the uptake phase of a bioconcentration test, the BCF is a value that is equal to the concentration of a chemical in one or more tissues of the exposed aquatic organisms divided by the average exposure water concentration of the chemical in the test [26]. The transformation of a material resulting from the complex enzymatic action of microorganisms (e.g., bacteria, fungi). It usually leads to the disappearance of the parent structure and to the formation of smaller chemical species, some of which are used for cell metabolism. Although tjTDically used with reference to microbial activity, it may also refer to general metabolic breakdown of a substance by any living organism [26]. The steady-state difference between the concentration of a bioaccumulating non-polar organic chemical normalized
CHAPTER
BSAF Chronic
Dissolved Orgcinic Carbon
DOC EC50 or EC50 Effect
Henry's Law Constant
HLC Incipient LC50
Indicator Species
Inherent BiodegradabiHty
Inhibition Concentration xx
34: ENVIRONMENTAL
to the organic carbon content of a sediment and the concentration measured in the total extractable lipids of an organism for which that sediment represents the source of the contamination in its habitat [19]. See Biota-Sediment Accumulation Factor. An extended exposure to a stressor (conventionally taken to include at least a tenth of the life span of a species) or the effects resulting from such an exposure. [25]. The fraction of the organic carbon pool that is dissolved in water and that passes through a 0.45 /xm glass fiber filter. DOC quantifies the chemically reactive organic fraction a n d is an accurate measure of the simple and complex organic molecules maJsing u p the dissolved organic load. The majority of the DOC is humic substances [26]. See Dissolved Organic Carbon. See Median Effective Concentration. A change in the state or dynamics of an organism or ecological system resulting from exposure to a chemical or other stressor (equivalent to response but used with the emphasis on the chemical) [25]. A partition coefficient defined as the ratio of a chemical concentration in air to its concentration in water at steady state. The constant can be dimensionless or with units [26]. See Henry's Law Constant. See Inhibition Concentration xx. The concentration of a chemical that is lethal to 50% of the test organisms as a result of exposure for periods sufficiently long (time independent) t h a t acute lethal action has essentially ceased. The asymptote (part of the toxicity curve parallel to the time axis) of the toxicity curve indicates that value of the incipient LC50, approximately [26]. A species that is surveyed or sampled for analysis because it is believed to represent the biotic community, some functional or taxonomic group, or some population that cannot be readily sampled or surveyed [25]. Classification of chemicals for which there is unequivocal evidence of biodegradation (primary or ultimate) in any test of biodegradability [13]. A point estimate of the chemical concentration that would cause a given percent reduction (e.g., IC25) in a (typically) nonlethal biological measurement of the test organisms, such as reproduction or growth [26].
CHARACTERISTICS Inoculum
LC50 or LC50 Lethal Load xx
Loading Rate
LOEC Lowest Observed Effect Concentration
Median Effective Concentration
Median Lethal Concentration
OF FUELS
AND LUBRICANTS
905
Living spores, bacteria single celled organisms, or other live materials that are introduced into a test medium (D 638499a). See Octanol-water Partition Coefficient. See Median Lethal Concentration. A statistically or graphically estimated loading rate of test material that is expected to be lethal to xx% of a subpopulation of organisms under specified conditions. Discussion—This terminology should be used for lubricants instead of the standard LCxx to designate that the material is not completely soluble at the test treatment rates (D 6384-99a). The ratio of test material to water (in mg/L) used in the preparation of a WAF [20]. See Lowest Observed Effect Concentration. The lowest concentration of a material used in a toxicity test that has a statisti cally significant adverse effect on the exposed population of test organisms compared with the controls. Also called the lowest observed adverse effect level (LOAEL) [26]. The concentration of a material in water to which test organisms are exposed that is estimated to be effective in producing some sublethal response in 50% of the test organisms. The EC50 is usually expressed as a time-dependent value (e.g., 24 h or 96 hr EC50). The sublethal response elicited form the test organisms as a result of exposure to the test material must be clearly defined. For example, test organisms may be immobilized, lose equilibrium, or undergo physiological or behavioral changes [26]. The concentration of material in water to which test organisms are exposed that is estimated to be lethal to 50% of the test organisms. The LC50 is often expressed as a time-dependent value (e.g., 24 h or 96 h LC50; the concentration estimated to be lethal to 50% of the test organisms after 24 or 96 h of exposure). The LC50 may be derived by observation (i.e., 50% of the test organisms may be observed to be dead in one test material), by interpolation (i.e., mortality of more than 50% of the test organisms occurred at one test concentration and mortality of fewer than 50% of the test organisms died at a lower test concentration, and the LC50 is estimated by interpolation between the two data points), or by calculation (i.e., the LC50 is statically de-
906 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK
NOEC No Observed Effect Concentration
Octanol-Water Partition Coefficient
Primary Biodegration QSAR Quantitative Structure Activity Relationship Readily Biodegradable
SAR Sorption
Structure-Activity Relationship
Toxic Unit
rived by analysis of mortality data from all test concentrations) [26]. See No Observed Effect Concentration. The highest concentration of a material in a toxicity test that has no statistically significant adverse effect on the exposed population of test organisms compared with the controls. Also called the n o observed adverse effect level (NOAEL) or the no observed effect level (NOEL) [26]. The ratio of a chemical's solubility in n-octanol and water at steady state; also expressed as P. The logarithm of P or Kow (i-C-, log P or log Kovv) is used as an indication of a chemical's propensity for bioconcentration by aquatic organisms [26]. Degradation of the test substance resulting in a change in its physical or chemical properties, or both (D 6006-97a). See Structure-Activity Relationship. See Structure-Activity Relationship.
An arbitrary classification of chemicals that have passed certain specified screening tests for ultimate biodegradability; these tests are so stringent that it is a s s u m e d t h a t such c o m p o u n d s will rapidly a n d completely biodegrade in aquatic environments u n d e r aerobic conditions [13]. See Structure-Activity Relationship. The process by which molecules of a chemical dissolve into or are retained on the surface of a material. Inclusive of absorption and adsorption processes [27]. Studies relating the structure (and related properties) of chemicals to their activities in biological systems. SARs are used to assist in explaining (and predicting) the occurrence and mechanisms of biological responses to chemicals and to aid in prediction of incidence and magnitude. Although lethal and sublethal toxicity test results are often employed, other biological activities, such as bioaccumulation, biodegradation, a n d biotransformation, may also be used. SARs can also be used to explain or predict physiochemical characteristics of chemicals. Also called quantitative structureactivity relationships (QSARs) [26]. The strength of a chemical (measured in some unit) expressed as a fraction or proportion of its threshold acute effect concentration (measured in the same unit). The strength may be calculated as follows: toxic unit = actual concentration of chemical in solution/threshold ef-
Toxicity
TU Ultimate Biodegradation
WAF Water Accommodated Fraction
Water Soluble Fraction
WSF
feet concentration. For the case where 1.0 toxic unit equal the incipient LC50, a TU greater t h a n 1.0 should cause the death, during long exposures, of m o r e t h a n half of a group of aquatic organisms. If the TU is less than 1.0, less than half of the organisms should be killed [26]. The inherent potential or capacity of an agent or material to cause adverse effects in a living organism when the organism is exposed to it [26]. See Toxic Unit. Degradation achieved when a substance is totally utilized by microorganisms resulting in the production of carbon dioxide (and possibly methane in the case of anaerobic biodegradation), water, inorganic c o m p o u n d s , a n d new microbial cellular constituents (biomass or secretions, or both) (D 6006-97a). See Water Accommodated Fraction. The predominately aqueous portion of a mixture of water a n d a poorly watersoluble material that sepEirates in a specified period of time after the mixture has undergone a specified degree of mixing a n d includes water, dissolved components, a n d dispersed droplets of the poorly water soluble material. Discussion—The composition of t h e WAF depends on the ratio of poorly soluble material to w a t e r in the original mixture as well as on the details of the mixing procedure (D 6384-99a). The filtrate or centrifugate of the water accommodated fraction that includes all parts of the WAF except the dispersed droplets of the poorly soluble material (D 6384-99a). See Water Soluble Fraction.
REFERENCES [1] Bartz, W. J., "Lubricants and the Environment," Tribology International, Vol. 31, Nos. 1-2, 1998, pp. 3 5 ^ 7 . [2] Lyman, W. J., "Transport and Transformation Processes, in Fundamentals of Aquatic Toxicology: Effects," Environmental Fate and Risk Assessment, G. M. Rand, Ed., Taylor & Francis, Washington, DC, 1995, pp. 449-492. [3] Mackay, D., Bums, L. A., sind Rand, G. M., "Fate Modeling, in Fundamentals of Aquatic Toxicology: Effects," Environmental Fate and Risk Assessment, G. M. Rand, Ed., Taylor & Francis, Washington, DC, 1995, pp. 563-588. [4] Lipnick, R. L., "Structure-Activity Relationships, in Fundamentals of Aquatic Toxicology: Effects," Environmental Fate and Risk Assessment, G. M. Rand, Ed., Taylor & Francis, Washington, DC, 1995, pp. 609-656. [5] Cisson, C. M., Rausina, G. A., and Stonebraker, P. M. "Human Health and Environmental Hazard Characterization of Lubricating Oil Additives," Lubrication Science, Vol. 8, No. 2, 1996, pp. 145-177.
CHAPTER 34: ENVIRONMENTAL CHARACTERISTICS OF FUELS AND LUBRICANTS [6] Lyman, W. J., "Octanol/Water Partition Coefficient," Handbook of Chemical Property Estimation Methods: Environmental Behavior of Organic Compounds, W. J. Ljraan, W. F. Reehl, and D. H. Rosenblatt, Eds., McGraw-Hill, NY, 1982, pp. 1-54. [7] A Practical Guide to Protecting Man and the Environment, Machelen, Exxon Chemical Europe, Inc., Machelen, Belgium, 1999. [8] A Critical Review of Toxicity Values and an Evaluation of the Persistence of Petroleum Products for Use in Natural Resource Damage Assessments, American Petroleum Institute, Washington, DC, 1995, p. 196. [9] Pelletier, M. C , Burges, R. M., Ho, K. T., Huhn, A., McKinney, R. A., and Ryba, S. A., "Phototoxicity of Individual Polycyclic Aromatic Hydrocarbons and Petroleum to Marine Invertebrate Larvae and Juveniles," Environmental Toxicology and Chemistry, Vol. 16, No. 10, 1997, pp. 2190-2199. [10] Wrabel, M. L. and Peckol, P., "Effects of Bioremediation on Toxicity and Chemical Composition of No. 2 Fuel Oil: Growth Responses of the Brown Alga Fucus Vesiculosus," Marine Pollution Bulletin, Vol. 40, No. 2, 2000, pp. 135-139. [11] OECD Guideline for Testing of Chemicals: 303A—Simulation Test—Aerobic Sewage Treatment: Coupled Units Test, Organization for E c o n o m i c Cooperation a n d Development, Paris, 1981, p . 14. [12] OECD Guideline for Testing of Chemicals: 301—Ready Biodegradability, Organization for Economic Cooperation and Development, Paris, 1992, p. 62. [13] Harmonized Test Guidelines: 835—Fate, Transport and Transformation Test Guidelines—OPPTS 835.3110 Ready Biodegradability, U.S. Environmental Protection Agency, Office of Prevention, Pesticides, and Toxic Substances, Washington, DC, 1998, p. 45. [14] Petroleum Microbiology, R. M. Atlas, Ed., Macmillan Publishing Company, NY, 1984, p. 692. [15] Microbial Degradation of Organic Compounds, Isted., D. T. Gibson, Ed., Microbiology Series, Vol. 13, A. I. Laskin and R. I. Mateles. Eds., Marcel Dekker, NY, 1984, p. 535. [16] Parkerton, T. F., Hexadecene Biodegradation Data for Closed Bottle Test and Manometric Respirometry Test Methods, ExxonMobil Biomedical Sciences, Inc., 2001. [17] Bysshe, S. E., "Bioconcentration Factor in Aquatic Organisms," Handbook of Chemical Property Estimation Methods: Environmental Behavior of Organic Compounds, W. J. Lyman, W. F. Reehl, and D. H. Rosenblatt, Eds., McGraw-Hill, NY, 1982, p. 30. [18] Spacie, A., McCarty, L. S., and Rand, G. M., "Bioaccumulation and Bioavailability in Multiphase Systems," Fundamentals of Aquatic Toxicology: Effects, Environmental Fate, and Risk Assessment, G. M. Rand, Ed., Taylor & Francis, Washington, DC, 1995, pp. 493-521. [19] McFarland, V. A., Evaluation of Field-Generated Accumulation Factors for Predicting the Bioaccumulation Potential of SedimentAssociated PAH Compounds, U.S. Army Corps of Engineers, Washington, DC, 1995, p. 158. [20] Guidance Document on Aquatic Toxicity Testing of Difficult Substances and Mixtures, OECD, Paris, 2000, p. 53. [21] Guidance Document on Application and Interpretation of Singlespecies Tests in Environmental Toxicology, Environment Canada, E n v i r o n m e n t Technology Centre, Ottawa, Ontario, Canada, 1999, p. 203. [22] Singer, M. M., Aurand, D., Bragin, G. E., Clark, J. R., Coelho, G. M., Sowby, M. L., and Tjeerdema, R. S., "Standardization of the Preparation and Quantitation of Water-Accommodated Fractions of Petroleum for Toxicity Testing," Marine Pollution Bulletin, Vol. 40, No. 11, 2000, pp. 1007-1016. [23] Nomenclature for "Selenastrum capricomutum," International Organization for Standardization, Geneva, 1999, p. 4.
907
[24] OECD Guideline for Testing of Chemicals: 205—Avian Dietary Toxicity Tests, Organization for Economic Cooperation and Development, Paris, 1984, p. 9. [25] Ecological Risk Assessment, G. W. Suter, Ed., Lewis Publishers, Boca Raton, FL, 1993, p. 538. [26] Fundamentals of Aquatic Toxicology: Effects, Environmental Fate, and Risk Assessment, 2nd ed. G. M. Rand, Ed., Taylor & Francis, Washington, DC, 1995, p. 1125. [27] Bioaccumulation: How Chemicals Move from the Water into Fish and Other Aquatic Organisms, American Petroleum Institute, Washington, DC, 1997, p. 54. [28] "C-4 Biodegradation: Determination of the "Ready" Biodegradability," Official Journal of the European Communities, L383A, 29 Dec. 1992, pp. 187-226. [29] ISO 7827: Water Quality, Evaluation in an Aqueous Medium of the "Ultimate" Aerobic Biodegradability of Organic Comp o u n d s — M e t h o d by Analysis of Dissolved Organic Carbon (DOC), ISO, Geneva, 1994, p. 7. [30] ISO 9439: W a t e r Quality, Evaluation of Ultimate Aerobic Biodegradability of Organic Compounds in Aqueous Medium— Carbon Dioxide Evolution Test, ISO, Geneva, 1999, p. 17. [31] Harmonized Test Guidelines: 835—Fate, Transport and Transformation Test Guidelines, U.S. EPA, Washington, DC, 1996. [32] OECD Guideline for Testing of Chemicals: 302C—Inherent Biodegradability: Modified MITI Test (II), Organization for Economic Cooperation a n d Development, Paris, 1981, p. 21. [33] ISO 10707: Water Quality—Evaluation in an Aqueous Medium of the "Ultimate" Aerobic Biodegradability of Organic Compounds—Method by Analysis of Biochemical Oxygen Demand (Closed Bottle Test), ISO, Geneva, 1994, p. 9. [34] ISO 10708: Water Quality—Evaluation in an Aqueous Medium of the Ultimate Aerobic Biodegradability of Organic Compounds—Determination of Biochemical Oxygen Demand in a Two-Phase Closed Bottle Test, ISO, Geneva, 1997, p. 17. [35] ISO 9408: Water Quality—Evaluation of Ultimate Aerobic Biodegradability of Organic Compounds in Aqueous Medium by Determination of Oxygen Demand in a Closed Respirometer, ISO, Geneva, 1999, p. 17. [36] OECD Guideline for Testing of Chemicals: 306—Biodegradability in Seawater, Organization for Economic Cooperation and Development, Paris, 1992, p. 27. [37] OECD Guideline for Testing of Chemicals: 302B—ZahnWellens/EMPA Test, Organization for Economic Cooperation and Development, Paris, 1992, p. 8. [38] ISO 9888: Water Quality—Evaluation of Ultimate Aerobic Biodegradability of Organic Compounds in Aqueous Medium— Static Test (Zahn-Wellens Method), ISO, Geneva, 1999, p. 11. [39] OECD Guideline for Testing of Chemicals: 302A—Inherent Biodegradability: Modified SCAS Test, Organization for Economic Cooperation and Development, Paris, 1981, p. 7. [40] ISO 9887: Water Quality—Evaluation of the Aerobic Biodegradability of Organic Compounds in an Aqueous Medium—Semicontinuous Activated Sludge Method (SCAS), ISO, Geneva, 1992, p. 9. [41] ISO 11733: Water Quality—Evaluation of the Elimination and Biodegradability of Organic C o m p o u n d s in an Aqueous Medium—Activated Sludge Simulation Test, ISO, Geneva, 1995, p. 15. [42] OECD Guideline for Testing of Chemicals: 304a—Inherent Biodegradability in Soil, Organization for Economic Cooperation and Development, Paris, 1981, p. 11. [43] ISO 14239: Soil Quality—Laboratory Incubation Systems for Measuring the Mineralization of Organic Chemicals in Soil Under Aerobic Conditions, ISO, Geneva, 1997, p. 17. [44] ECETOC, "Evaluation of Anaerobic Biodegradation," Technical Report No. 28, ECETOC: Brussels, 1988.
908 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK [45] ISO 11734: Water Quality—Evaluation of the "Ultimate" Anaerobic Biodegradability of Organic C o m p o u n d s in Digested Sludge—Method by Measurement of the Biogas Production, ISO, Geneva, 1995, p. 16. [46] CEC L-33-A-93 Test Method, Coordinating European Council, Brussels, 1995, p. 18. [47] ISO 14593: Water Quality—Evaluation of Ultimate Aerobic Biodegradability of Organic Compounds in Aqueous Medium— Method by Analysis of Inorganic Carbon in Sealed Vessels (C02 Headspace Test), ISO, Geneva, 1999, p. 16. [48] A Test Method to Assess the 'Inherent' Biodegradability of Oil Products, CONCAWE, Brussels, 1999, p. 33. [49] Battersby, N. S., "The ISO Headspace C 0 2 Biodegradation Test," Chemosphere, Vol. 34, No. 8, 1997, pp. 1813-1822. [50] Biological Test Method: Toxicity Test Using Luminescent Bacteria (Photobacterium phosphoreum), Environment Canada, Environment Technology Centre, Ottawa, Ontario, Canada. 1992, p. 61. [51] OECD Guideline for Testing of Chemicals: 201—Alga, Growth Inhibition Test, Organization for Economic Cooperation and Development, Paris, 1984, p. 14. [52] "C-3 Algal Inhibition Test," Official Journal of the European Communities, 1992, pp. 179-187. [53] H a r m o n i z e d Test Guidelines: 850—Ecological Effects Test Guidelines, U.S. EPA, Washington, DC, 1997. [54] ISO 8692: Water Quality—Fresh Water Algal Growth Inhibition Test with Scenedesmus subspicatus and Selenastrum capricornutum, ISO, Geneva, 1989, p. 6. [55] ISO 10253: Water Quality—Marine Algal Growth Inhibition Test with Skeletonema costatum and Phaeodactylum tricornutum, ISO, Geneva, 1995, p. 9. [56] Biological Test Method: Growth Inhibition Test Using the Freshwater Alga Selenastrum c a p r i c o r n u t u m . Environment Canada, E n v i r o n m e n t Technology Centre, Ottawa, Ontario, Canada. 1992, p. 41. [57] Biological Test Method: Test for Measuring the Inhibition of Growth Using the Freshwater Macrophyte, Lemna minor. Environment Ccinada, Environment Technology Centre, Ottawa, Ontario, Canada. 1999, p. 98. [58] OECD Guideline for Testing of Chemicals: 202—Daphnia sp.. Acute Immobilisation Test and Reproduction Test, Organization for Economic Cooperation and Development, Paris, 1984, p. 16. [59] "C-2 Acute Toxicity to Daphnia," Official Journal of the European Communities, 1992, pp. 172-179. [60] ISO 6341: Water Quality—Evaluation of the Inhibition of the Mobility of Daphnia m a g n a Straus (Cladocera, Crustacea)— Acute Toxicity Test, ISO, Geneva, 1998, p. 9. [61] Biological Test Method: Acute Lethality Test Using Daphnia spp.. Environment Canada, Environment Technology Centre, Ottawa, Ontario, Canada, 1990, p. 78. [62] OECD Guideline for Testing of Chemicals: 207—Earthworm, Acute Toxicity Tests, Organization for Economic Cooperation and Development, Paris, 1984, p. 9. [63] ISO 11268-1: Soil Quality—Effects of Pollutants on Earthworms (Eisenia fetida)—Part 1: Determination of Acute Toxicity Using Artificial Soil Substrate, ISO, Geneva, 1993, p. 6. [64] Biological Test Method: Acute Test for Sediment Toxicity Using Marine o r Estuarine Amphipods, Environment Canada, Envir o n m e n t Technology Centre, Ottawa, Ontario, Canada. 1992, p. 83. [65] ISO 14669: Water Quality—Determination of Acute Lethal Toxicity to Marine Copepods (Copepoda, Crustacea), ISO, Geneva, 1999, p. 16. [66] Biological Test Method: Test of Reproduction a n d Survival
[67]
[68]
[69]
[70]
[71]
[72]
[73]
[74]
[75]
[76]
[77]
[78]
[79]
[80]
[81]
[82]
[83]
[84]
Using the Cladoceran Ceriodaphnia dubia. Environment Canada, E n v i r o n m e n t Technology Centre, Ottawa, Ontario, Canada, 1992, p. 72. ISO 10706: Water Quality—Determination of Long Term Toxicity of Substances to Daphnia m a g n a Straus (Cladocera, Crustacea), ISO, Geneva, 2000, p. 17. Biological Test Method: Fertilization Assay Using Echinoids (Sea Urchins and Sand Dollars), Environment Canada, Environment Technology Centre, Ottawa, Ontario, Canada, 1992, p. 97. ISO 11268-2: Soil Quality—Effects of Pollutants on Earthworms (Eisenia fetida)—Part 2: Determination of Effects on Reproduction, ISO, Geneva, 1998, p. 16. ISO 11268-3: Soil Quality—Effects of Pollutants on Earthworms—Part 3: Guidance on the Determination of Effects in Field Situations, ISO, Geneva, 1999, p. 8. OECD Guideline for Testing of Chemicals: 203—Fish, Acute Toxicity Test, Organization for Economic Cooperation and Development, Paris, 1992, p. 9. Biological Test Method: Reference Method for Determining Acute Lethality of Effluents to Rainbow Trout, Environment Canada, E n v i r o n m e n t Technology Centre, Ottawa, Ontario, Canada, 1990, p. 18. Biological Test Method: Acute Lethality Test Using Rainbow Trout, Environment Canada, Environment Technology Centre, Ottawa, Ontario, Canada, 1990, p. 51. Biological Test Method: Test of Larval Growth and Survival Using Fathead Minnows, E n v i r o n m e n t Canada, Environment Technology Centre, Ottawa, Ontario, Canada, 1992, p. 70. ISO 7346-1: Water Quality—Determination of the Acute Lethal Toxicity of Substances to a Freshwater Fish (Brachydanio rerio H a m i l t o n - B u c h a n a n (Teleostei, Cyprinidae))—Part 1: Static Method, ISO, Geneva, 1996, p. 11. ISO 7346-2: Water Quality—Determination of the Acute Lethal Toxicity of Substances to a Freshwater Fish (Brachydanio rerio Hamilton-Buchanan (Teleostei, Cyprinidae ))—Part 2: SemiStatic Method, ISO, Geneva, 1996, p. 11. ISO 7346-3: Water Quality—Determination of the Acute Lethal Toxicity of Substances to a Freshwater Fish (Brachydanio rerio Hamilton-Buchanan (Teleostei, Cyprinidae ))—Part 3: FlowThrough Method, ISO, Geneva, 1996, p. 11. ISO 12890: Water Quality—Determination of Toxicity to Embryos and Larvae of Freshwater Fish - Semi-Static Method, ISO, Geneva, 1999, p. 14. OECD Guideline for Testing of Chemicals: 204—Fish, Prolonged Toxicity Test: 14-Day Study,. Organization for Economic Cooperation and Development, Paris, 1984, p. 9. OECD Guideline for Testing of Chemicals: 210—Fish, EarlyLife Stage Toxicity Test, Organization for Economic Cooperation and Development, Paris, 1992, p. 18. Biological Test Method: Toxicity Tests Using Early Life Stages of Salmonid Fish (Rainbow Trout, Coho Salmon, or Atlantic Salmon), Environment Canada, Environment Technology Centre, Ottawa, Ontario, Canada, 1992, p. 81. ISO 10229: Water Quality—Determination of the Prolonged Toxicity of Substances to Freshwater Fish—Method for Evsduating the Effects of Substances on the Growth Rate of Rainbow Trout (Onchorhynchus mykiss Walbaum (Teleostei, Salmonidae)), ISO, Geneva, 1994, p. 12. Parkerton, T. F., Gasoline WAF Composition Data, personal communication, ExxonMobil Biomedical Sciences, Inc., Clinton, NJ, 2001. 1995 Updates: Water Quality Criteria Documents for the Protection of Aquatic Life in Ambient Water, U.S. EPA, Washington, DC, 1996, p. 114.
MNL37-EB/Jun. 2003
Lubrication and Tribology Fundamentals Hong Liang, ^ George E. Totten,^ and Glenn M. Webster-^
LUBRICANTS PLAY A VITAL ROLE IN EVERY INDUSTRY INCLUDING:
ELECTRONIC, automotive, aerospace, forestry, naval, and numerous others. Lubrication failure m a y result in thousands of dollars of p r o d u c t i o n losses including downtime a n d equipment failure. Therefore, the general area of lubrication is one of the most important in industrialized societies. A lubricant may be a gas, a liquid or a solid and operates by preventing direct interfaciaJ contact of surfaces in relative motion to each other and/or minimizing damage when interfacial contact does occur. In this chapter, the fundamental concept of lubrication a n d w e a r will be discussed. Wear surfaces, contact geometry and stress, wear mechanisms, materials properties, and lubrication mechanisms will be addressed. Wear mechanisms will include corrosive, abrasive, adhesive, fretting, ploughing, and rolling contact fatigue failure. Identification a n d troubleshooting of these w e a r regimes, particuleirly with respect to bearing and gear wear under varying lubrication regimes, will be included. This is the first in a series of three chapters; Chapter 35 (Lubrication Fundamentals), Chapter 36 (Bench Test Modeling), and Chapter 37 (Lubrication Friction and Wear Testing). In this chapter, the fundamental principles involved in lubrication and wear processes and how these basic principles are applied to lubrication testing will be discussed. In Chapter 36, the most commonly encountered bench tests will be discussed in detail. In addition to describing all of the pertinent engineering details of the standardized tests such as wear contact conformity, loading, speed, reciprocating or linear motion, and testing principle being measured, this chapter will provide guidelines for actual testing such as test selection, cleaning, material pair chemistry, and other factors related to currently existing bench tests and those in development. In Chapter 37, the elements and application tribological design as it relates to new test development will be outlined in detail. ASTM G 40 defines tribology as "the science and technology concerned with interacting surfaces in relative motion, including friction, lubrication, wear and erosion." Since this chapter addresses basic design principles of tribological processes involved in lubrication and weeir testing, various equations are utilized. The applicability of these relationships to tribological testing is an important consideration. The structure of this chapter is to first address basic principles involved in friction and wear. After friction and wear, principles of fluid lubrication will be discussed. The last sec-
tion of this chapter will provide a brief overview of additive chemistry involved in the formulation of lubricants and related testing protocols.
DISCUSSION Friction The ASTM D 996 definition of friction is "resistance to relative motion of two bodies in contact." The force (Fp) that must be applied to an object to initiate and maintain relative motion is proportional to the applied load (L). The proportionality constant is the coefficient of friction (/A). F = liL There are two values reported for the coefficient of friction. The static co-efficient of friction is used in reference to the initial movement of the object from the rest position and is defined by ASTM D 996 as "the ratio of force required to move one surface over another, to the total force applied normal to those surfaces, at the instant motion starts." The kinetic coefficient of friction is used for two surfaces in relative motion and is defined by ASTM D 996 as "the ratio of force required to move one body over another, to the total force applied normal to those surfaces, once that motion is in progress." Representative of diy static and kinetic coefficients of friction for various material pairs are provided in Table 1[1]. It has been suggested that the best material pair candidates for non-lubricated contacts should exhibit ;u, < 2 [2]. Factors that affect dry sliding friction include [3]: 1. True area of contact between the sliding surface 2. Bond strength between the two bodies at the contact interface 3. Mechzinism of material shear and rupture processes in the contacting region The coefficient of friction is often used as a measure of the transition between boundary lubrication and elastohydrodynamic (EHD) lubrication. The transition between these two regimes is designated as either "mixed" lubrication or "mixed EHD." Variation of the coefficient of friction with increasing film thickness between two metal surfaces is shown schematically in Fig. 1 [4]. Area of True
' Department of Mechanical Engineering, University of Alaska Fairbanks, P.O. Box 755905, Fairbanks, AK 99775-5905. ^ G. E. Totten & Associates, LLC, P.O. Box 30108, 514 N. 86* St., Seattle, WA 98103.
Surfaces are are covered with asperities, meaning they are not perfectly smooth. An asperity is defined by ASTM G 40 as a "protuberance in the small-scale topographical irregulari-
909 Copyright'
2003 by A S I M International
Contact
www.astm.org
910 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK ties of a solid surface." When two surfaces are in contact, the "area of true contact" or "real contact area" (Ar) is dependent on the total area of asperity contact (asperity contact area) as illustrated in Fig. 2. The area of friction is assumed to be dependent on Ar and shear strength (Ss) [1,3]:
TABLE 1—Coefficients of dry static and kinetic sliding friction. Material Pair
/x (Static)
/A (Kinetic)
Hard steel on Hard Steel Mild Steel on Mild Steel Hard Steel on Graphite Hard Steel on Babbitt(ASTM 1) Hard Steel on Babbitt (ASTM 8) Mild Steel on Phosphor Bronze Mild Steel on Lead Aluminum on Mild Steel Magnesium on Magnesium Teflon on Teflon Teflon on Steel Cadmium on Mild Steel Copper on Mild Steel Nickel on Nickel Brass on Mild Steel Brass on Cast Iron Zinc on Cast Iron Copper on Cast Iron Tin on Cast Iron Lead on Cast Iron Aluminum on Aluminum Cast Iron on Cast Iron Bronze on Cast Iron
0.78 0.74 0.21 0.70 0.42
0.42 0.57
0.95 0.61 0.6 0.04 0.04 0.53 1.10 0.51 0.85 1.05 1.05 1.10
0.33 0.35 0.34 0.95 0.47
0.46 0.53 0.53 0.44 0.30 0.21 0.29 0.32 0.43 1.40 0.15 0.22
Area of real contact FIG. 2—Illustration of two surfaces in contact, "area of true contact" or "real contact area" (Ar) is dependent on the total area of asperity contact. Ui = velocity of surface 1, U2 = velocity of surface 2,di= asperity hieight of surface 1, §2 = asperity fieight of surface 2, and h = fluid film thicicness.
O •-I-'
o
LL
d ^ O
0.020.01
o O Adsorption! Multiple Multiple • Multiple iLayer : Reaction Reaction 1 Reaction i Layers Layers ! Layers + + Liquid Liquid Film Layers FIG. 1—Dependence of coefficient of friction (/u) on the layer thickness between two metal sliding contact pairs.
CHAPTER 35: LUBRICATION If the load (L) is carried by asperity contact area (Ar) creating the contact pressure P^ then [1]: M =
Although conceptually illustrative, in practice the calculation of the coefficient of friction is much more complex requiring the integration of other interfacial bonding and failure modes including [5]: 1. area of true contact 2. friction due to adhesion 3. friction due to ploughing 4. friction due to deformation Adhesion, ploughing, and deformation are wear processes that will be described in detail subsequently. Another wear mode is scuffing, which ASTM G 40 defines as "a form of weeir occurring in inadequately lubricated tribosystems that is characterized by macroscopically observable changes in surface texture, with features related to the direction of relative force." Alternatively, ASTM D 4175 defines scuffing as "surface damage resulting from localized welding at the interface of rubbing surfaces with subsequent fracture in the proximity of the weld area." An acceptable scuffing criterion for metals may be estimated from the plasticity index (i/*), which is dependent on the area of true contact [5]: ^=— — where : E is the elastic modulus, Py is the yield pressure, cr is the root mean square average asperity height and P is the radius of contact. It ^< 0.6, the contact is classified as elastic and if ^ > 1, the plastic deformation will occur within the contact. From this relationship, the area of true contact (Ar) is estimated [5]: For elastic contact: Ar-
wY
where: Vi < n < 1, W is the normal load and, E is the elastic modulus. For plastic contact; ^'-
^ Py
H
where: C is the proportionality constant and H is the hardness. Park and Ludema have shown that this approach should be used with considerable caution. [6]. Friction Due to Adhesion Adhesive wear is defined by ASTM G 40 as "wear due to localized bonding between contacting solid surfaces leading to material transfer between two surfaces or loss from either surface." In some cases, these moving surfaces, such as each of the four balls in a 4-ball wear testing machine, can actually weld together and the complete ball assembly may undergo seizure. Fracture during the adhesive wear process is caused by the rupture of interfacial adhesive bonds, which are the
AND TRIBOLOGY FUNDAMENTALS
911
summation of all of the component bond strengths within the contact including, van der Waals, metallic, ionic, and metallic bonds. The adhesive component of the coefficient of friction ifia) has traditionally been determined from:
where: Fa is the interfacial shear strength and L is the load. Friction Due to Ploughing An object may undergo a change in its dimensions after applied stress is removed. This is referred to as "plastic deformation." Plasticity is the susceptibility at a certain temperature and loading condition to permanent deformation after application of sufficient stress to exceed the yield point of the material. Ploughing is defined by ASTM G 40 as "the formation of grooves by plastic deformation of the softer of the two surfaces in relative motion." The formation of a groove in the material surface may be caused by either wear debris or by asperities within the wear contact. The contribution due to ploughing may dominate the friction force. Currently, there is no reliable model to estimate the contribution to friction force by ploughing [5]. Figure 3 illustrates typical damage due to abrasion, scuffing, and ploughing [7]. Friction Due to Deformation Deformation is a stress-induced change of form or shape. A material may undergo deformation either plastically, as defined above, or mechanically. Mechanical deformation is the dimensional change of a material that occurs from its initial dimensions due to continued cyclic loading over a specific time. Friction is the most fundamental factor in tribologicaJ processes and therefore, experimental determination of friction is an important lubrication and wear parameter. ASTM D 4999 describes the evaluation of friction effects on oil cooled brakes with bronze friction material in combination with steel disks. Often, surface deformation failure occurs with ploughing or adhesion when the braking materials are insufficiently lubricated. Wear Surfaces Surface Roughness Real surfaces are covered with asperities, as shown in Fig. 4. Characterization of surface roughness is fundamentally important. One way to represent surface roughness effects on lubrication, particularly fatigue life failure, is to use the Lambda {A) factor (specific film thickness) correlation, which is defined as [8]: A = ho/cr where: ho is the lubricating film thickness and cr is the average surface roughness for the two surfaces coming into asperity contact and is defined as: If the film parameter A is equal to 1, boundary lubrication and asperity contact occurs, mixed lubrication occurs when A = 1-3, and EHD lubrication occurs when A = 3-10 [8].
912 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK
FIG. 3—Examples of: a) three-body abrasion occurring using one 10 mm dia. round SUJ 2 steel disc standing (axially) on another 25 mm dia. round SUJ S45C steel disc lying down flat (radially). The abrasive is SiC #400 suspended in paraffin oil. The load on the wear contact is SON, sliding amplitude is 10 mm, sliding speed is 10 mm/s and the sliding distance is 36 m (source: Prof. T. Kawazoe, Nagasake University, Nagasaki City, Japan); b) ploughing using the same SUJ 2 round disk on a 60 mm round tin (Sn) disc. The contact load is 2.479 N, sliding speed is 0.3 mm/s, and sliding distance is 50 mm. (source: Prof. T. Kawazoe, Nagasake University, Nagasaki City, Japan); c) ploughing for a medium carbon steel pair block-on-shaft, load = 110-184 N, 156-220 rpm and the picture has 250X magnification (source: Dr. Sergey Tarasov, Institute of Strength Physics and IVIaterials Sciences, Tomsk, Russia); and d) scuffing as illustrated by the cross-section of an AISI 1045 disk at 2000X showing the highly deformed zone after scuffing (source: Prof. Liu Jiajun, Tsinghua University, Beijing, PRC).
Table 2 provides a correlation of A factors with fatigue cracking of roller bearings [8]. T3^ically, A factors of 1.2-1.5 are used for bearing life calculations [9]. Representative surface roughness values for different types of bearing applications are summarized in Table 3 [3,8]. The Lambda ratio provides Ein indication of the lubricant film thickness relative to the surface roughness (asperity height
TABLE 2—Correlation of lambda (A) and fatigue cracking of roller bearings. A Value
Wear Observation
< 1.0 1-1.5 1.5-3.0 a3.0
Surface smearing or deformation Surface distress accompanied by surface pitting Surface glazing accompanied by subsurface failure Minimal wear, long life, eventual subsurface fatigue failure
TABLE 3—Representative values of composite surface finish for different bearing applications. Composite Surface Finish Bearing Application
/im (rms)
Large Industrial Industrial Off-the-Shelf Aerospace
0.25 0.12 0.65
Height (^m) Depth (|i,m)
Width (|im) FIG. 4—Illustration of surface roughness. The peaks covering this sample area are called "asperities."
(U.in (rms)
10 5 25
CHAPTER 35: LUBRICATION Lambda factor. Therefore, achieving a fundamental understanding of surface roughness is crucial for tribological design, testing, and failure analysis. Although A factors provide a useful approximation of the interdependence of fluid film thickness and combined surface roughness of bearings, it is an over simplification of the lubrication problem. For example, thick films may result in high power losses (efficiency loss) due to oil churning, or excessive operational temperatures may prohibit the required fluid viscosity, preventing the desired film thickness. In addition, non-newtonian behavior of the oil, often due to the addition of various additives such as viscosifiers, etc., may lead to oil film collapse. Therefore, the use of A factors for characterizations for thin film lubrication should be used with caution, particularly for a film thickness that is less than the combined surface roughness parameter [9].
AND TRIBOLOGY FUNDAMENTALS
913
^
^
(a) M-system
(b) Ten-point average
Analysis of Surface Roughness Data Experimental surface roughness data, typiccdly obtained by profilometry, is numerically analyzed [12] to obtain an average roughness value such as those provided in Table 4. Dur-
5 I
(c) Least squares FIG. 6—illustration of numerical methods used to calculate surface roughness values.
I &
Specific Film Thicicness Ratio FIG. 5—Increasing Lambda values (Specific Film Ratio) provide greater fatigue life. TABLE 4—Surface roughness nits and interconverslons. Roughness Values (Ra) /Am
/lin
Grade
50 25 12,5 8.3 3.2 1.6 0.8 0.4 0.2 0.1 0.05 0.025
2000 1000 500 250 125 63 32 16 8 4 2 1
N12 Nil NIO N9 N8 N7 N6 N5 N4 N3 N2 Nl
ing numerical analyses of experimental data it is necessary to establish a roughness line through the topographical peaks and valleys. Computerized data analyses are usually performed by one of the three methods, as illustrated in Fig. 6 [13]. 1. M-System—^A line is selected that passes through the topographical profile such that the areas above and below the line are equal, as shown in Fig. 6a. 2. Ten Point Average—^A line is drawn through the center of the five highest peaks and the five lowest valleys as shown in Fig. 6b. 3. Least Squares Reference—^A least squares average through the topographical peaks and the valleys is determined as shown in Fig. 6c. The texture of a surface is complicated and there are many parameters used to quantify the various surface characteristics. Three methods often used to numerically represent are surface roughness, centerline average (Re), root mean square (Rq), and maximum peak-to-valley height (Rt). These parameters are defined as: 1. Centerline average (CLA)—^This is the most common designation of surface roughness and is cilso denoted as arithmetic average (Ra). Here Ra is calculated from: Ra^
1 '^
\Zi\
where: A^ is the total number of measurements, Z is the absolute peak to valley height with respect to a reference line.
914 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK 2. Root-Mean-Square (RMS)—The RMS representation of a specific height (Rq) is: " 1
Ra = N
N
"ll/2
^ ^ '
3. Maximum Peak-to-Valley—The m a x i m u m peeik-to-valley asperity height (Rt) is: R, — R„
Kn
Typically, the m a g n i t u d e of these peak asperity height measurement methods follow the relative order: Ra^Rg^
Rt
Surface roughness may also be represented by a surface roughness grade based on the Ra value [14]. Surface roughness is also referred to as Ra ("average roughness") or it may be referred to as "roughness grade." Typical values for tribological surface roughness are provided in Table 4 (ISO 1302). There are various ASME and ISO standards for instrument calibration, data processing, and data analysis, which are listed at the end of this chapter. Surface roughness values typically encountered for various components are provided in Table 5 [13]. Surface
Conformity
To understand the wear process, it is necessary to determine the load acting on the contacting surfaces, the type of motion, e.g., rolling vs. sliding or linear vs. oscillating, and the geoemetry of the wear contact. The geometry of the contacting surfaces may be classified as conformal or nonconformal (or counter-formal), as shown in Fig. 7. Conformal surfaces, such as a journal - sleeve bearing and hydrodyn a m i c t h r u s t bearings, fit together geometrically. The loading of conformal surfaces is typically carried over a wide area of peak asperities and the load-bearing surface area remains nearly constant with increasing loads. The lubrication contact area is relatively large. Non-conformal surfaces, such as two mating gear teeth, vane-on-ring, and rolling elements in their raceways, do not fit together geometrically and the load is concentrated on a relatively small contact area that increases significantly with increasing load. Classification of the non-conformal contact depends on the scale. A seemingly non-conformal pair of surfaces might be conformal at an atomic level. It could also be true that a pair of worn non-conformal surfaces could become conformed as a result of the wear process. The contacting area is called the Hertzian Contact Area, as illustrated in Fig. 8 and is defined by ASTM G 40 as "the app a r e n t area of contact between two nonconforming solid bodies pressed against each other, as calculated from Hertz's
TABLE 5—Typical component surface roughness values. Component
Arithmetic Average (Ra) /iM
ju.m.
Gears Plain Bearing-Journal Plain Bearing-Bearing Rolling Elements Rolling Bearing Tracks - Roller Bearing
0.25-10 0.12-0.5 0.25-1.2 0.025 0.12 0.1-0.3
10-400 5-20 10-50 1-5 5 4-12
FIG. 7—The geometry of the contacting surfaces are classified as conformal or non-conformal (or counter-formal).
Pressure-, (a)
Inlet Region ^——Hertzian Region
I Outlet Region
(b) FIG. 8—a) Hertzian contact region, b) Hertzian contact area.
equations of elastic deformation." The pressure at this contact is called the Hertzian Contact Pressure and is defined by ASTM G 40 as: "the magnitude of the pressure at any specified location in a Hertzian contact area, as calculated from Heitz's equations of elastic deformation." The geometry of a tribocontact is usually defined as a point contact (ball or pin-on-disk), line contact (vane-on-ring, roUer-on-disk), emd area contact (flat surface-on-disk). Tribo-
CHAPTER
35: LUBRICATION
contact classifications are illustrated in Fig. 9. However, contact geometry may veiry. Contact geometry affects the load actually applied, surface area, and conformation. Contact geometry, taken together with other variables such as sliding speed, material chemistry, and the physical and chemical properties of the lubricant presents a specific tribological problem. This is important because a bench test is selected to evaluate certain lubricating properties of a fluid/additive system with a specific material pair of interest or to model real wear systems of interest in industrial equipment. Therefore, the specific contact geometry selected for wear testing is dependent on the tribological system being modeled (or generated) by the test. Figure lOa-h illustrates eight different wear contact variations that may be used with a single test machine, including the well-known 4-ball test [15]. (Other test geometries are also possible.) Bench tests, such as the 4-ball test, are often used to examine lubricant additive reactivity [15]. However, ranking of lubrication properties depends on the test conditions and performance criteria.
Line
Area
Point
FIG. 9—Examples contact geometries of various common bench test configurations.
^
H FIG. 10—Illustration of various tribocontact geometries that were used by Matveesky with a 4-ball machine: a) four ball, 8 mm dia. balls made from chromium steel Re = 61-62; b) four roller, 5 mm dia. rollers and 45(contact; c) ball-ring, ball from chromium steel of Re = 61-62, dia. 12.7 mm, ring from different materials with geometry-20 x 18 x 4 mm; d) diskball; ball from chromium steel, Re = 61-62, d = 8 mm, disk from different materials with geometry 60 x 18 x 4 mm; e) ball-three-roller; ball from chromium steel. Re = 61-62, d = 12.7 mm, 5 mm dia. rollers made from different materials; f) ball-three plates, ball and materials the same as: "e"; g) conical-ring, cone (110°), geometry of ring 20 x 8 x 4 mm; h) ballplate, plate from different materials with geometry 40 x 40 x 5 mm, ball from chromium steel Re = 61-62 and d = 8 mm.
AND TRIBOLOGY
FUNDAMENTALS
915
One test machine was developed to evaluate the kinetics of the tribochemiccd reaction of additives to a metal substrate during lubrication [16]. From a kinetic correlation, tribochemical mechanisms could be esterblended. This test machine has been constructed where varying loads are applied to a rotating disk that is brought into contact with a stationary disk [16]. Both disks were constructed from S45 plain carbon steel (without heat treatment). The test machine may be configured to provide line, whole-plane, Eind partial plane contact [16]. This test apparatus is used to determine the value (ki), which is the rate constant of the reaction between the additive, in this case radioactive dibenzyl disulfide, DBDS, eind the metcJ (Fe) friction surface in ni^ mol'^s"'. The value 1 is the effective concentration of the DBDS in mol. m"'^ in the oil film adsorbed on the friction surface. The value k2 is the rate constant of chemical wear of iron sulfide in s"'. Okabe et al. also used the reaction rate constant ki in analyzing the corresponding wear data. In this case, kj increases with sliding velocity, which will also increase the contact temperature, thus increasing reaction rates. The value 1 will increase with the hydrodynamic effect of the lubricating oil. The data could not be explained by considering ki eJone since it cannot decrease with increasing sliding speed. However, excellent correlations were obtained with the product kil [16]. This experimental approach illustrated how tribochemical and tribophysical prospects can be extracted from experimental wear data. Figure 1 l a shows the significant effect of contact pressure on tribochemical reactions, which is partially determined by the conformity of the wear contact [16]. This is illustrated in the typically poor correlation between the 4-ball test and the Timken test in which both exhibit different surface contact conformity, as well as test conditions [17]. Figure lib shows the effect of sliding velocity [16]. These data show: • The tribochemical reaction is affected by base oil viscosity that affects not only hydrodynamic lubrication but also additive diffusion from the bulk oil to the metal-oil interface. • Wear is not totally controlled by tribochemical reactions but also mechanical processes, since totcil wear depends on the sliding distance—not reaction time. This is because friction processes may remove (detach) the additive from the surface, which then must be reformed (readsorption). • For line contact, both kil and ka are independent of contact pressure. However, for whole -plane contact kjl and ka vary with contact pressure since the value 1 is reduced with increasing pressure. • For line contact, increasing sliding velocity produces a convex curve for kjl Emd ka is independent of sliding velocity. However, ka increases with sliding velocity for whole-plane contact. Taken together, it is very important to note that additive behavior is not only controlled by base-oil interactions, but edso by contact pressure, sliding speed, and contact geometry. For example, increasing contact pressure increases wear, as illustrated by a palm oil lubricated contact in Fig. I2a-c.
916
MANUAL 37: FUELS AND LUBRICANTS
xlO 5
T^
*
o'—
HANDBOOK
whole plane contact
\
A '«
3
2" 2 1
T^
0 0.1
1
(a)
10
too
Contact pressure ,
woo
integral component of the overall wear contact and must be treated as such. Figure 14 illustrates physically adsorbed films, and tribochemical reaction films, which are discussed in more detail in the Surface Films section. The micro-EHL thin film is the first hne of protecting sliding failure. Its thickness depends on the fluid viscosity and therefore is enhanced by increasing pressure on the lubricating contact. Fluid viscosity, however, is significantly increased by increasing pressure, particularly at the pressures encountered both on the inlet region to the Hertzian contact and within the Hertzian contact itself. The exponentially
M Pa
xlO,
xlO
XlO 3
3 'in
1 0 XlO
,X
T^°"
partial plane 7 contact 6 S 4 3 ("whole , plane contact
0.01
(b)
0.1 S l i d i n g velocity ,
1 m-s'
FIG. 11—The quality ranking of lubricants strongly depends on test conditions and performance criteria, a) The effect of contact geometry and pressure on tribochemical reactions and wear; b) the effect of contact geometry and sliding speed on tribochemical reactions and wear.
Wear Contact Material Structure Four structural elements involved in wear mechanisms are shown in Fig. 13 [18]. These are: surface films, which are typically present at < 1 ^nm; near-surface structures, which are typically present within 1-150 ^im from the surface; subsurface structures, which are observed from 50-1000 fjum from the surface; and bulk material properties. One type of surface film is an oxide film that is formed by surface oxidation with oxygen that may be present in the atmosphere [18], dissolved in the lubricant, or both. Pressure-Viscosity Coefficient Surface asperities and lubricating thin films determine tribological behavior and types of lubrication modes, as illustrated in Fig. 14 [19]. It is important to note that the lubricant is an
FIG. 12—Effect of load on wear of AISI 52100 steel ball on cast iron plate where contact lubricated with a 5% palm oil methyl ester. Test conditions: sliding speed 0.34 m/s, 1 hour test time, ambient temperature at the start of the test, a = SOON, b = 700N and c = 1100N. As expected, increasing the load increases the wear. (Source: Prof. M. Maleque, Oept. of Mechanical Eng., University of Malasia, Kuala Lumpur, Maylasia.)
CHAPTER
35: LUBRICATION
AND TRIBOLOGY
FUNDAMENTALS
917
EHD/Mlcro-EHD Films (<1nm)
Surface Films (<1nm)
Lubricant
Near Surface (< 50 urn) Residual Stress (Bulk)
Hardness (case)
Principal Shearing Stress ( X ) Subsurface (<50-1000^m)
/
Core
FIG. 13—Structural elements of a wear contact.
Surface Films
lubrication analysis. The value of aoT is defined as [21]: _dlnri\ n . ^ 1 diq —• T,P = I atm aor = = — ; — \T,P = 1 atm = dt, \ n dP However aox, which is obtained by graphical differentiation, is dependent on relatively few low-pressure data points, which contribute substantially to overall error. A more favorable alternative solution, w h i c h provides a m o r e reliable viscosity-pressure response, is to solve for a* as shown in Fig. 15b by graphically integrating the following equation for a* [22]:
Adsorbed,
Film
li
Oxide
Metal
Metal
FIG. 14—Surface asperities and lubricating thin films determine tribological behavior and types of lubrication modes.
increasing viscosity with increasing pressure relationship has been classically modeled by the Barus Equation [20]. The classic Barus equation describes the effects of pressure on viscosity [20]: VP
where:
= Voe
TTP is the centipoise viscosity at pressure (P), T/o is the viscosity at atmospheric pressure, a is the pressure-viscosity coefficient.
Jones et. al. have shown by plotting log rj as a function of pressure (P) that a linear graphical solution to the Barus equation is not obtained except at low pressures (Fig. 15a) [21]. The slope of the tangent to log 17 as a function of pressure (P) isotherm (aox) at atmospheric pressure is often used for the calculation of the pressure-viscosity coefficient used in the Reynolds equation for elastohydrodynamic (EHD)
T? (r, P = 1 atm) dP V iP, T) The advantage of a* is that all of the variations of viscosity with pressure over the entire pressure range are included in the calculation as illustrated in Fig. \5b [21], In many cases, the value of a is not available. One early attempt to predict a for petroleum oils at pressures u p to 10 000 psi was reported by Fresco et al. [23]. Fresco's a- prediction chart is shown in Fig. 16 and is based on the viscosity-temperature properties of a fluid, which are represented by the so-called "ASTM Slope" [24]. The ASTM slope is the slope of the viscosity temperature line of the McCouU-Walther equation that is plotted as described in ASTM D 341. In addition to a graphical solution, it can also be calculated from: ASTM Slope =
log log {V1IV2) l o g (Tz/Ti)
where: v is the kinematic viscosity and T is the absolute temperature. The ASTM slope is the slope of the viscosity-temperature line of the Walther equation that is described in ASTM D 341. Note: The ASTM slope is n o t equivalent to the value B in the Walther equation [25]: log log {v + 0.7) = A - 5 log r
918
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
"OSIOK*"'
1
J
10 (a)
(b)
2
3x10'
Pressure (N/nf ) i I 30 X 10' 20
Pressure (PSI)
Pressure FIG. 15—a) Viscosity-pressure beliavior witli aor and uos (Barus Equation); b) Illustration of a viscosity-pressure isotherm and the calculation of a*.
CHAPTER
35: LUBRICATION
AND TRIBOLOGY
FUNDAMENTALS
919
The pressure-viscosity relationship for a paraffinic and a n a p h t h e n i c mineral oil at different temperatures is illustrated by Fig. 17 [28], Table 7 tabulates pressure-viscosity coefficients for different temperatures of different oils [25].
0.90
Surface
Films
There are numerous t5rpes of surface films that are used to reduce wear. Physical or chemical adsorption also provides a protecting film for lubrication. A thin surface film is formed by adsorption of polar lubricant molecules onto the surface, providing an effective barrier against metal-to-metal contact. Physical adsorption (vein der Waals adsorption or physisorption), as defined by ASTM D 2652, refers to "the binding of an adsorbate to the surface of a solid by forces whose energy levels approximate those of condensation." Chemical adsorption (chemisorption) is defined by ASTM D 2652 as: "the binding of an adsorbate to a surface of a solid by forces whose energy levels approximate those of a chemical bond without the formation of a new chemical bond." Chemisorption may be irreversible.
2 3 45
10
20 30 50
100 200
500
1000
Atmospheric Viscosity (cSt) FIG. 16—Fresco's nomogram for predicting viscosity-pressure coefficients of a mineral oil hydraulic fluid.
Both A and B are fluid dependent constants. Since fluid viscosity usually decreases with increasing temperature, the slope of B will be negative. The viscosity-temperature coefficient is obtained from the first derivative of the McCouU-Walther equation and is equal to [25]: Slope B =
1
dr
Other methods that have been used to predict the value of a are summarized in Table 6 [26]. The Appledoom equation has been proposed for the calculation of the effects of both temperature and pressure on viscosity simultaneously [27]: log (V/VA) = a log (F/Fo) + aP
Where: v is the viscosity at temperature F, expressed in °F, and pressure P, VA is a reference viscosity at atmospheric pressure and temperature FQ, a is the viscosity—temperature (Appledoom) co-efficient and a is the pressure—^viscosity coefficient. Both temperature amd pressure exhibit potentially large effects on viscosity and the simultaneous inclusion of both in viscosity calculations is desirable. However, there are n o broadly accepted methods for conducting these calculations at the present time.
The effectiveness of the adsorbed film is dependent on temperature. A thicker film provides better surface protection. Two types of protective films, adsorbed films and reaction films, are illustrated in Fig. 14, respectively [18]. Examples of the use of adsorbed films that will reduce interfacial friction include the ionically adsorbed (chemisorption) fatty acids on steel (and other metal) surface layers as shown in Fig. 18 [ 18], or the physical adsorption of hydrocarbons as illustrated in Fig. 19 [18]. Metal surfaces may also be modified by the formation of reaction films. Some reaction films are formed during heat treating processes such as carburizing, carbonitriding and nitriding. Others are formed in situ by surface chemical reactions between a n additive such as ZDDP (zinc dialkydithophosphates) as shown in Fig. 20 [18]. The formation of a chemical film produced by a paraffinic base oil containing ZDDP as an additive is illustrated in Fig. 20b. An illustration of the ability of a n oil additive to reduce wear is provided by comparing Fig. 3c and Fig. 21. Some surface films are formed by tribochemical processes such as tribopolymerization [37-40] during the lubrication process. Surface topographical structure is shown in Fig. 22 [18]. Lubrication failure in this region will produce plastic flow and failure mecheinisms such as ploughing caused by a hard asperity sliding in the softer plastically deformed metal. Various illustrations of ploughing wear are provided in Fig. 3c. Near Surface
Structure
As indicated in Fig. 23, near-surface structure occurs u p to approximately 50 /i and is affected by surface hardening or reactions film formation in this region [18]. Near-surface structure, as indicated in Fig. 23, is particularly susceptible to severe deformation a n d is i m p o r t a n t since the residual stresses in this region will affect crack propagation during lubrication failure. Sub-Surface
Structure
Sub-surface structure, shown in Fig. 23, predominates at depths of 50-1000 yam [18]. This region is susceptible to gross deformation a n d is affected by the bulk properties of the
920 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 6—Various methods used in the comparisons of the effectiveness in predicting the pressure effects on viscosity Developer
Correlation
Kouzel
log ^
Roelands, Vlugter
for non-polymer fluids
and Wateman
=( P V i ° g ^ = i5ooo; [ ( 0 . 0 0 2 C A
Reference
= 3 ^ ( 0 . 0 2 3 9 + 0.01638^;
[29] [30]
3L
+ 0.003Cw
0.055) log r,o + 0.228]
l o g {y - 0 . 8 9 0 ) = 0 . 0 0 9 5 5 (CA + l.SCjv) - 1.930for polymer fluids
log (log r,p + 1.200) = ZBI log f l + 2 8 ^ ) + ^°^ ^^°^ '^'> + ^-^OO) Zfii = O.eZgr + OAZBI Fresco
log — = Pa'" (lO-'') a = ^
[31]
a' = A + B log (Va) + C(log V„)^
A, B, and C are function of ASTM slope Kim
[32]
log — = Pia' + /3)(10-'') a' is calculated from Fresco's Method p = A'logVr + B'(logK)^ A' and B ' are function of temperature graphical method is also available
So and Klaus
log
Voj
[33]
a(98.1 XP)
a = 2 ^ ^ [1.216 + 4.143 [log (Vjf •'>'•• s = 2.848 X 10-" m^'^o^ [log (VJl'-^'^' t = 3 . 9 9 9 [ l o g (yJ]3.0975p0.0Tl62
Chu and cameron
log ( ^ ) = 16 log 1 +
0.062 (10+") PIO^o.o«
[34]
+ a = [0.183 + 0.2951 log ( r j j ]
for naphthenics log (^]
Worster
= aKlO"'*) + a = [0.183 + 0.2951 log(T)o)] for naphthenics
Johnston
PEr a, ICRT^
Shibada
a = 0 . 6 2 0 4 4 ^d"^'^
[35] [36]
^2.04746 ^0.29292
[26]
TABLE 7—Selected pressure-viscosity coefficients. aoxCNm^)"' Dynamic Viscosity (mPa.s) at:
^ 100,000 O, 10,000 "^ (0
o o (A
1000
Fluid
38°C
99°C
149°C
2-Ethylhexyl Sebacate Paraffinic Oil Naphthenic Oil A Naphthenic Oil A + VI Improver' Naphthenic Oil B Polybutene (Mn = 409) a. VI Improver = 4% PMA (Mv = 560,000)
11 29 22 60
1.39 2.18 2.15 1.83
1.19 1.78 1.44 1.24
68 90
3.07 3.18
1.81 2.22
100 10 1 0.1 0
Paraffinic OH Napthenic Oil
20 40 60 80 100 120 140
Pressure (xlO^ psi) FIG. 17—Pressure-viscosity relationship for a paraffinic and a napfithenic mineral oil at different temperatures.
m a t e r i a l . S p a l l i n g failure t y p i c a l l y o c c u r s i n t h i s r e g i o n . Figu r e 2 4 p r o v i d e s v a r i o u s i l l u s t r a t i o n s of s p a l l i n g . W e a r p r o c e s s e s s u c h a s s p a l l i n g l e a d t o f a t i g u e failure, o n e of t h e m o s t c o m m o n g e a r f a i l u r e m e c h a n i s m s . V a r i o u s i l l u s t r a t i o n s of g e a r f a i l u r e m o d e s a r e s h o w n i n F i g . 15a-i. T h e s e s u b - s u r f a c e w e a r m o d e s i n c l u d e : p i t t i n g (Fig. 25a,b,d); m i c r o p i t t i n g (Fig. 2 5 c ) ; scuffing (Fig. 25f,h,j); " t e a r i n g " (Fig. 2 5 i ) ; r i p p l i n g (Fig. 25e); a n d s e v e r e w e a r (Fig. 25g). S p a l l i n g a n d f a t i g u e failure p r o c e s s e s will b e d e s c r i b e d i n m o r e d e t a i l s u b s e q u e n t l y .
CHAPTER 35: LUBRICATION Cohesion
FIG. 18—Adsorbed films which will reduce interfaclal friction include the ionic adsorbed fatty acids to the steel (and other metal) surface layer.
AND TRIBOLOGY
FUNDAMENTALS
921
Bulk Material Properties Some of the most common bulk material properties that influence wear are: melting point, Young's modulus, yield strength, hardness, and surface energy. Table 8 provides a summary of these values for selected pure metals [41]. Young's modulus (elastic modulus) is the ratio of change of stress to change in strain within the elastic region of a material. This is limited to materials exhibiting a linear stressstrain relationship over the range of loading being tested. ASTM E 6 defines yield strength as the "engineering stress at which, by convention, it is considered that plastic elongation of the material has commenced. This stress may be specified in terms of (a) a specified deviation from a linear stress relationship, (b) a specified total extension attained, or (c) maximum or minimum engineering stresses measured during discontinuous yielding." Yield stress as defined by ASTM D 653 is "the stress beyond which the induced deformation is not fully annulled after complete distressing." Hardness is defined by ASTM E 6 as "resistance of a material to deformation, particularly permanent deformation, indentation, or scratching." The magnitude of hardness is often simply considered to be the softer of two materials with respect to the material pair of the wear contact. However, hardness is a much more complex variable. Rigney [42] has shown that hardness can vary with position and time and is dependent on temperature, sliding speed, and the chemical environment. The sign of the hardness gradient adjacent to the sliding surface affects sliding behavior. Furthermore, hardness variations may affect the transition from friction to wear. Therefore, potential variations in hardness in friction and wear testing must also be considered.
n-Hexadecane (Cetane) (C16H34)
FIG. 19—Physical adsorption of hydrocarbons. Note the weaker van der Waals interactions and related orientational effects between the hydrocarbon and the metallic substrate relative to those illustrated for chemisorbed additives illustrated in Fig. 18.
922
MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
FIG. 21—Illustration of the ability of an additive to reduce wear on a copper substrate. IVIedium carbon steel pair block-on-shaft lubricated with a mineral oil with a copper nanometal additive, load = 110-184 N, 156-220 rpm and the picture has 750X magnification. (Source: Dr. Sergey Tarasov, Institute of Strength Physics and Materials Sciences, Tomsk, Russia). Compare to Fig. 3c.
FIG. 20—Surface additive reactions are tribochemical processes which are driven by the conditions within the wear contact. This is illustrated by the following examples: a) The in situ reaction film is formed by surface chemical reactions between an additive such as ZDDP (zinc dialkydithophosphates). b) Illustration of cracking of a chemical reaction film produced on a gray cast iron surface lubricated with a paraffininc base oil containing ZDDP as an additive under a contact pressure of 16.7 MPa and at 135°C for 17 h. The sliding velocity was 1.4 m/s and the base oil viscosity was 0.053 Pa at 38°C. (Source: Prof. Hon So, Dept. of Mechanical Engineering, iviational Taiwan University, Taipei, Taiwan.)
Surface energy is the energy (work) required to expand the surface area of a hquid and is quantified as energy per surface area. Surface energy is related to the surface tension of a Hquid, which is defined by ASTM B 374 as "the property due to molecular forces, that exists in the surface film of aJl liquids and tends to prevent the liquid from spreading." Surface energy defines the ability of a fluid to "wet out" a solid surface. Surface "wet out" refers to how well a liquid or viscoelastic solid flows and intimately covers a surface. The greater the wet out, the better the surface coverage and the greater the attractive forces between the fluid and the solid surfaces. Surfaces with high surface energy bond more readily because they are easier to wet than low-energy surfaces. Surface free energy is time consuming to measure. It would be helpful if structure could be correlated with surface free energy to enable prediction of the surface energy. This correlation is done using a Zisman plot, developed at the Naval Research Laboratory. The plot is made by plotting the
cosine of the contact angle versus the surface free energy of various wetting liquids on a given solid. The resulting plot is a straight line. Thus, there exists some unique value for each solid where the cosine of the contact angle is unity. This value is termed the critical surface free energy. A liquid with surface free energy below the critical value will wet and spread over the solid surface, whereas a liquid with surface energy above the critical value might wet but will not spread. To provide optimal antiwear properties, the material should: 1. Possess high strength to resist plastic flow. 2. Have high ductility to withstand repeated plastic strain without cracking. Ductility is the ability of a material to deform plastically before fracturing. 3. Be homogeneous and free from discontinuities such as impurities and soft transformation products. Smooth Eind homogeneous material generally has better anti-wear properties since failure takes place at higher energy areas such as boundaries and rough area sites. Two general classes of steels are typically used as materials for wear contacts. Based on the phase diagram shown in Fig. 26 [18], a eutectoid steel contains 0.8% carbon (see dashed line). Hyper-eutectoid steel contains greater than 0.8% carbon and hypo-eutectoid steel contains less than 0.8% carbon. Hyper-eutectoid steel is a deep-hardening steel typically containing 0.8-1.15% carbon. Hypo-eutectoid steel is used for various surface-hardening processes such as carburizing, carbonitriding and nitriding. These steels typically contain 0.1-0.25% carbon. A summary of the effect of various steel microstructures on wear is provided in Fig. 27 [18]. Table 9 shows the effect of metallurgical microstructure on wear [43].
CHAPTER
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AND TRIBOLOGY
FUNDAMENTALS
923
FIG. 22—Lubrication performance depends on surface topographical structure. Lubrication failure in this region will produce plastic flow and failure mechanisms such as ploughing caused by a hard asperity siding in the softer plastically deformed metal. See Fig. 3b for an illustration of ploughing.
Severe Deformation Gross Deformation Bulk
FIG. 23—Sub-surface structure predominates at depths of 50-1000 fim. This region is susceptible to gross deformation and is affected by the bulk properties of the material.
Wear Mechanisms The primetiy wear mechaxiisras that occur in non-lubricated contacts are: adhesive wear, abrasive wear, tribochemical wear, and fatigue wear. Adhesive
Wear
Adhesive wear occurs when surface asperities come into sUding contact under a load. If sufficient heat is generated, microwelding of the two contacting asperities with subsequent shearing and material transfer will be observed, as illustrated schematically in Fig. 28a. When two interacting surfaces are not sufficiently lubricated, adhesive transfer or removal of near-surface material may be observed. Surface adhesion is dependent on the nature of the contacting surface. Wear material transfer in this region h a s been called "solid-phase welding" (also known as the micro and macro-welding pro-
cess), which occurs at asperity contacts [7,44]. Adhesive wear can be reduced by the proper selection of material pairs and sufficient lubrication to provide a protective surface a n d EHD/micro-EHD film. (EHD films will be described subsequently.) Examples of adhesive wear are shown in Fig. 28. One form of adhesive wear is "smearing." If the contact temperature is sufficiently high, metal flow and smearing will be observed. If the material is constructed from steel, tempering colors may be observed, and if the temperature is high enough, plastic deformation and fracture may result. An example of smearing wear is illustrated in Fig. 29a. Polishing wear is a special case of smearing. One method by which polishing wear may occur is during a break-in process when surface asperities undergo an adhesive wear process until a very fine, polished surface results. This is caused by insufficient EHD lubrication to prevent such asperity contact. An example of polishing weeir is provided in Fig. 29b. (Polishing weeir may also be caused by EP additives that may be too aggressive, leaving a bright, mirror-like surface. This is an undesirable process since this process typically leaves the metal surface less resistant to wear.) This series of events is partially observed with Fig. 30. (Provided by D. Drees, Falex Tribology NV, Belgium.) In this case, a series of pin-on-v-block tests was conducted in a lubricant containing extreme pressure additives. In Fig. 30a, the machine was sufficiently loaded to cause a change of color (bluing) due to the high frictional temperature of the material pair. When the lubricant failed, welding of the metal test specimens occurred, resulting in adhesive wear as shown in Fig. 30b. Another term relating to adhesive failure is "galling," which is defined by ASTM G 40 as "a form of surface damage arising between sliding solids, distinguished by macroscopic, usually localized, roughening and creation of protrusions above the originEd surface; it often includes plastic flow and material transfer, or both." An example of galling wear is illustrated in Fig. 31, which shows the results of surface wear
924 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK
FIG. 24—Illustrations of wear by spalling. a) Spalling wear of a AISI52100 steel ball on a nitrlded AISI 1045 steel disk under lubricated conditions loaded at 150N and sliding speed of 2.0 m/s; b) spalling wear is also illustrated by this example of nitrided AISI 52100 after testing with a No. 20 engine oil at 1500 N and 650 rpm. The spalling occurred in the nitrlded layer; c) Close up of the spalling pit shown in b. (Source: Prof. Liu Jiajun, Tsinghua University, Beijing, PRC.)
(a,b) FIG. 25—Gears may wear by several modes and the FZG test may be used to evaluate wear resistance by Identifying failure modes. Illustration of various potential gear wear modes observed in the FZG gear test are shown here: a) pitting/spalling on a case carburized test gear; b) pitting/spalling from a micropitted area on a case carburized test gear; c) severe micropitting on a case carburized test gear; d) pitting on a through-hardened gear in practice; e) rippling on a case-carburized test gear; f) scuffing on a case-carburized test gear; g) severe wear on a case carburized test gear; h) scuffing on a case carburized test gear (compare to Fig. 24 e on the same gear geometry); i. "tears" on a nitrlded test gear; j) severe scuffing on a case carburized test gear. (Source: Dr. K. Michaelis, Lehrstuhl fur Maschine Elemente, Technical University of Munich, Munich, Germany.)
CHAPTER 35: LUBRICATION AND TRIBOLOGY FUNDAMENTALS 925
(g,h)
(i) FIG. 25—(continued) T A B L E 8 — P r o p e r t i e s of m e t a l l i c e l e m e n t s .
Metals
Melting Temp. (°C)
Young's Modulus (E) Dyne/cm2
Aluminum Cadmium Copper Iron Lead Magnesium Manganese Nickel Tin Zinc
660 270 1083 1534 325 650 1245 2.08 232 420
0.63 0.32 1.20 2.04 0.16 0.44
Q
Yield Strength (<7y) 109 Dyne/cm2
Hardness (P) K=/mm^ erg/cm2
1.0
27 7 80 82 4 46 300 210 5.3 38
3.2 2.5 0.09 1.5 2.5 3.2 0.15 1.3
0.44 0.91
900
y+Fe,C
I '
600 ^
Cementite ( FesC ) + Pearlite
Ferrite + Pearlite Pearlite -1 0.0
1—1—I—1—I— 0.2 0.4 0.6
-•
1 1.0
'
1— 1.2
Weight percentage carbon FIG. 26—^The phase diagram of hyper-eutectoid and hypoeutectoid steel.
Surface Energy
900 390 1100 1500 450 560 1700 570 790
926 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK
Sliding Velocity V(m/s) STEEL Wear-Mechanism Map Pin-On-Disk Configuration
2 tn 0) T3 (U _N ID
E
. Ultra' Mild Wear
Normalized Velocity V FIG. 27—Effect of steel martensitic structural transition on wear as shown by a wear mechanism map for a non-lubricated sliding steel pinon-desk pair. Contours of wear rates (which are normalized) are superimposed on regions with different wear mechanisms. Wear rates in parentheses represent mild wear. The transition between mild and severe wear are indicated by the shaded area.
TABLE 9—Effect of metallurgical microstructure on wear [24]. Microstructure
Ferrite
Comment
Poor wear, low hardness and high toughness Good firecrack resistance Lamellar Good strength Emd firecrack resistance Pearlite Fine pearlite structure improves strength and wear but reduces ductility and firecrack resistance Spheroidized High toughness and good wear resistance, Pearlite Soft steel with good firecrack resistance Bainite High strength a n d hardness with good wear resistance a n d good firecrack resistance Martensite Very high hardness, very good wear resistance poor toughness and firecrack resistance carbide Carbide Extremely good wear resistance, high content carbridge causes embrittlement Graphite Improves firecrack a n d spall resistance, reduces strength
CHAPTER 35: LUBRICATION AND TRIBOLOGY FUNDAMENTALS
a) Surfaces being pressed together
(b)
(a)
(d) FIG. 28—Mechanism and illustration of adhesive wear, a) A schematic illustration of the mechanism of adhesive wear, b) Illustration of adhesive wear with an Fe/IVIg wear contact under the following conditions: 5 N/cm^, 1 m/s, > 10 l
927
928 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK
BS (3/8" brass ball)
((|)18x*30Nidisk)
Ni - BS (1.5 kgf)
(f)
200^ I
I
x50
SUS 304 (({)18 X* 30 stainless steel disk) (g)
SUJ2 (3/8" stainless steel ball)
SUS - SUJ2 (5.0 kgf) FIG. 28—(continued)
CHAPTER 35: LUBRICATION AND TRIBOLOGY FUNDAMENTALS
(b) FIG. 29—a) Cam ring smearing damage and discoloration. (Courtesy of Caterpillar Inc, Peoria, IL.) b. Hydraulic pump piston with polishing wear by a smearing mechanism. (Courtesy of Caterpillar Inc., Peoria, IL)
(a)
(b) FIG. 30—P!n-on-V-block test results of an lubricant showing: a) when the machine was sufficiently loaded, a change of color (blueing) was observed due to the high frictional temperature of the material pair; b) when the lubricant failed, welding of the metal test specimens occurred resulting in adhesive wear. (Courtesy of D. Drees, Falex Triboiogy NV, Heverlee, Belgium.)
FIG. 31—Surface wear obtained with two copper cylinders tested in a crossedcyllnder geometric configuration under dry sliding conditions. This figure is an SEM photograph, which shows typical signs of galling or seizure, with grooves, a heavily deformed surface layer and material transfer. (Source: S. Hodgmark, Uppsala University, Sweden.)
929
930
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
obtained with two copper cylinders tested in a crossedcylinder geometric configuration u n d e r dry sliding conditions. This figure is an SEM photograph, which shows typical signs of galling or seizure, with grooves, a heavily deformed surface layer, and material transfer. The classic expression to quantitatively describe the volu m e (V) of adhesive wear debris removed for a given load (L), on a material over the sliding distance (x) for a dry wear contact is known as Archard's equation. V
k Lx H
where: k is the Archard's wear constant and H is the hardness of the softer material of the material pair. Typical values of Archard's wear constant (for non-lubricated wear) are provided in Table 10 [45]. F o r non-lubricated wear contacts (solid friction and wear), it has been recommended that the wear constant should be < 10"* mm^ N"^ m ~ ' [2]. Archard's wear constant "k" is also defined in ASTM G 40 as the "wear co-efficient," "wear factor," or "specific wear rate." However, to avoid confusion, it is recommended that these terms be fully defined within the context that they are used. Figure 32 provides a schematic illustration of the Rabinowitz abrasive wear model [45]. dV _L dl ~
tan e TTH
where dV/dl is the change in volume (dV) due to the wear that is removed after sliding a distance dl, L is the load being applied, 0 is the angle of the hypothetical cone indenting the horizontal surface, and H is the hardness of the softer surface. Archard's equation shows that material hardness is an important variable in the wear process. However, hardness is a complex value that is dependent on: position and time, temperature, sliding speed, and environment. Localized hardness
TABLE 10—Wear constant (k) for various sliding combinations [25]. Material Pair
Zinc on Zinc Low Carbon Steel on Low Carbon Steel Copper on Copper Stainless Steel on Stainless Steel Copper on Low Carbon Steel Carbon Steel on Copper
Wear Constant (k 3 103)
160 45 32 21 L5 0.5
variation can affect the transition between friction and wear [46]. Meng and Ludema have reviewed various wear models that have been reported to date and have found that there is no single set of general equations that adequately describe a wide range of practical wear prediction problems [47]. Abrasive
Wear
Abrasive wear refers to the cutting of a metal by a hard particle or a rough surface by a ploughing or microcutting (scratching) mechanism [10,48]. ASTM G 40 defines spalling as: "the separation of macroscopic particles from a surface in the form of flakes, or chips, usually associated with rolling element bearings and gear teeth, but also resulting from impact events." Abrasive wear is dependent on particle size distribution, shape, toughness (capacity of a material to absorb energy), and hardness. Giltrow has demonstrated that an excellent, although non-linear, correlation exists between abrasive wear and cohesive energy for both thermoplastic polymers and metals [49]. Cohesive energy is that energy, composed of physical and chemical forces, that holds the constituents of a mass of material together. It is desirable to use a value such as cohesive energy since it can be calculated based on chemical composition of the material and thus provides the opportunity to interrelate wear with chemical composition. Giltrow reported that abrasive wear of many metals was inversely proportional to the cube of the cohesive energy and inversely proportional to the square root of the cohesive energy of thermoplastic polymers [49]. Note: cohesive energy of metals may be related to the latent heat of sublimation (at 25°C) minus the work (RT) required to expand atoms beyond their sphere of influence. Latent heats of metals are readily available from thermodynamic tables [49]. The abrasive wear mechanism involves the penetration of a surface by a hard particle that is subsequently embedded into one of the wear surfaces. Abrasive wear occurs when one of the contacting surfaces is harder than the other. The observed wear behavior involves plastic deformation and material displacement during ploughing or smearing. Ploughing was described in the Friction Due to Ploughing section. This results in surface-initiated fatigue spalling. EHD and microEHD films reduce or eliminate local surface plastic flow. Abrasive wear m a y also be measured by determining the wear volume, as discussed previously. When abrasive wear is caused by a hard particle between two surfaces, it is called three-body wear, as illustrated in Fig. 33a [50]. Hard particles causing three-body wear may be introduced into a system from the manufacturing process, gen-
V Bearing surface
FIG. 32—Rabinowitz simplified abrasive wear model with material removal by a cone tip.
CHAPTER
35: LUBRICATION
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FUNDAMENTALS
931
"Hard" Surface
toothed, tapered cutting tool across a workpiece surface or through a pilot hole [51].
"Soft" Surface
Cavitation Wear Cavitation refers to the formation and rapid collapse, by implosion, of cavities or bubbles that contain vapor or gas or both (ASTM G 15). The collection of a large n u m b e r of cavitation bubbles is called a cavitation cloud. The bubbles in a cloud are small, typically less than 1 m m in cross section. A surface that is being eroded by cavitation is typically obscured by a cavitation cloud (ASTM G 40). Different studies have shown that every bubble in the cloud does not cause damage upon implosion and that the damage frequency is reported to vary from 1 in 16 000 to 1 in 30 000. A cavitation bubble m a y undergo multiple collapse a n d reformation processes. A single bubble may undergo as many as seven collapse and reformation processes. A wide range of bubblecollapse pressures, u p to > 7 0 Mpa, have been reported [52]. Cavitation damage is exemplified by pitting and the formation of holes and craters on the wear surface, as illustrated in Fig. 34.
(a)
'Soft" Surface Hard" Surface (b)
(d)
(c) FIG. 33—Mechanisms and examples of abrasive wear, a) Schematic illustration of a threebody abrasive wear model; b) schematic illustration of a two-body abrasive wear process; c) illustration of abrasive wear: Fe/AI material pair, 2.3 N/cm^, 1 m/s, ? 10 km, 5 x 10~® Torr, and fji varies from 0.61-1.3 (Source: E. Santner, BAM, Berlin Germany); d) abrasive wear obtained with AISI52100 steel ball on an AtSI Type 304 stainless steel disk, load = 150 N, sliding speed = 1.5 m/s. (Source: Prof. Liu Jiajun, Tsinghua University, Beijing, PRC.)
Erosion
Wear
Erosion wear is the "progressive loss of original material from a solid surface due to mechanical interaction between that surface and a fluid, a multicomponent fluid, or impinging liquid or solid particles" (ASTM G 40). Such wear may result in scratching, surface indentation, chipping, and gouging as shown in Fig. 35. Erosion wear occurs where there is a change in fluid-flow direction such as in orifices, line restrictions turns in fluid passageways, and the leading edge of rotating parts. Corrosion
erated internally as wear debris, introduced as a contaminant with the lubricant being tested, or possibly by all three introduction mechanisms. Two-body wear is caused by a harder surface with asperity dimensions sufficiently large enough to penetrate the lubricating oil film, causing a ploughing or microcutting (scratching) action on the softer metal surface of the wear contact, which is in relative motion [50]. The mechanism of two-body wear is shown in Fig. 33b. Examples of abrasive wear, which may involve cutting, ploughing, lapping, gouging, grinding or broaching, are provided in Fig. 33. Lapping is defined by ASTM B 374 as "rubbing two surfaces together, with or without abrasives, for the purpose of obtaining extreme dimensional accuracy or superior surface finish." "Gouging" refers to the formation of a groove in an object by mechanically or manually removing material, possibly using the sharp edges of a hard particulate abrasive material. ASTM E 7 defines grinding as "the removal of material from a surface of a specimen by abrasion through the use of randomly oriented hard-abrasive particles bonded to a suitable substrate," such as paper or cloth, where the abrasive particle size is generally in the range of 60-600 grit (approximately 15-150 jam), but may be finer. Broaching refers to cutting a finished hole or contour in a solid material by axially pulling or pushing a bar-shaped.
Wear
Corrosion wear is surface damage related to chemical electrochemical attack of a metal surface, resulting in significant surface damage as illustrated in Figs. 36 and 37. Common causes of corrosive wear include: 1. Water from humid air ingression and subsequent condensation, liquid water from spray or splash contamination, water ingression from defective enclosure, cooling coil leakage and water-contaminated lubricant.
FIG. 34—Surface damage by pitting and cratering due to cavitation. (Reprinted with permission of IFAS, Aachen, Germany.)
932
MANUAL
37: FUELS
AND LUBRICANTS
HANDBOOK plate in seawater. In this case, electrons wiU flow from steel to copper, resulting in a deposit of iron oxide on copper. This is gcilvanic corrosion. Combinations
FIG. 35—Illustration of erosion surface wear. (Reprinted with permission of IFAS, Aachen, Germany.)
of Cavitation/Erosion/Corrosion
(a)
FIG. 36—Illustration of the porous nature of corroded surfaces. (Reprinted with permission of IFAS, Aachen, Germany.)
2. Corrosive chemical vapors in the atmosphere. 3. Corrosive processing fluids such as coolants and cleaning fluids. 4. Lubricant chemistry: poor lubricant formulations using aggressive extreme pressure (EP) additives, presence of acids from decomposition or exposure to active metals. One example is provided in Fig. 37 where increasing additions of palm oil as an additive in a petroleum oil resulted in increased corrosion of the wear surface. Shi et al. have shown that there is a synergistic effect between corrosion and wear a n d that wear rates of steels under corrosive conditions were m u c h higher than in the dry wear condition [53]. (Ion implantation could reduce but not eliminate this effect.) Galvanic Corrosion—Gcilvanic corrosion is recognizable by the appearance of increased corrosion near the junction of dissimilar metals, caused by electrochemical transfer of one metal to another. The propensity for galvanic corrosion is dependent on the position of two metals in the galvanic series. Any metal will have a greater tendency to corrode when it is in contact with another metal in a lower position in the series and when it is in the presence of an electrolyte. The further apart the two metals in the series, the greater the potential for galvanic attack. For example, consider steel rivets in a copper
Wear
There are various combinations of cavitation/erosion/corrosion wear. These will be briefly summarized as defined by ASTM specifications. Cavitation Corrosion—This is a form of localized, accelerated corrosion characterized by deep pitting and caused by high mechanical forces resulting from coolant vapor bubble collapse at the surface of a metal. (ASTM D 4725) Cavitation Erosion—Defined as the loss of material from a solid surface from exposure to cavitation. This may include
(b)
(c) FIG. 37—Corrosive wear obtained with an AISI 52100 steel ball on cast iron with 0%, 5%, and 10% palm oil methyl ester (POME) used added to an oil lubricant. The test was conducted at 80°C, 0.34 m/s for 50 h. (Source: Dr. M. Maleque, Univ. of Malasia, Kuala Lumpur, Maylasia.)
CHAPTER 35: LUBRICATION loss of material, surface deformation, or changes in properties or appearance. (ASTM G 40) Cavitation Erosion Corrosion—The mechanical removal of protective films on metal by the formation and collapse of vapor bubbles in a liquid and the abrasive action of a liquid, which may contain suspended solids moving at high velocity. (ASTM D 4725) Erosion Corrosion—This is a synergistic process involving both erosion and corrosion, in which each of these processes is affected by the simultaneous action of the other and in many cases is thereby accelerated (ASTM G 40). Meyer et al. has briefly reviewed various testing procedures to evaluate erosion corrosion processes [54a]. Oxidational Wear Archard and Hirst originally classified the sliding wear of metals in the presence of air as "severe" during the initial stages of the sliding wear process and "mild" in later stages [54b]. Therefore, the term "mild wear" is often used to indicate "oxidation wear" or "oxidative wear." Quinn's model of oxidation wear involves oxidation of the metal asperities while in contact during the sliding process [55a]. The extent of oxidation depends on the surface temperatures developed at the asperity contacts during the wear process. The oxidation wear process involves removal of oxide films after a critical thickness is obtained. The formation of porous oxide films as a result of the tribooxidation process are provided in Fig. 38. Tribochemical Wear Different from chemical reactions, tribological reactions are initiated by friction causing processes resulting in wear. Tribochemical wear may be viewed as one type of corrosive wear. When corrosion is activated by mechanical interactions between the contacting surfaces, it produces an activated surface site and localized high temperatures sufficient for chemical reaction. Tribochemical wear involves surface charging of electrons, surface passivation, and surface film removal processes. The mechanisms of tribochemical wear
AND TRIBOLOGY FUNDAMENTALS
are complicated and they Eire still under investigation. Tribochemical wear can be controlled by the use of appropriate inhibiting additives. Fatigue Wear ASTM G 40 defines fatigue wear as "wear of a solid surface caused by fracture arising from material fatigue." Material fatigue, as defined by ASTM E 1823, refers to "the process of progressive locedized permanent structural change occurring in a material subjected to conditions that produce fluctuating stresses and strains at some point or points that may culminate in cracks or complete fracture after a sufficient number of fluctuations." Fatigue wear prior to cracking is indicated microscopically by surface pitting and spalling, which are caused by induced subsurface shear stresses that exceed the critical shear stress of the material. Pitting is defined by ASTM G 40 as "a form of wear characterized by the presence of surface cavities the formation of which is attributed to processes such as fatigue, localized adhesion, or cavitation." Spalling is defined as "the separation of macroscopic particles from a surface in the form of flakes or chips, usually associated with rolling element bearings and gear teeth but also resulting from impact events." Various examples of pitting and spalling due to fatigue wear processes are illustrated in Figs. 39-41. Severe types of wear often involve scuffing, as shown in Fig. 42. Fatigue life is influenced by EHD lubricant film/surface roughness ratio [10,11]. High localized stresses at surface defect sites, asperities, dents, and material inhomogeneity, will reduce fatigue life [55b]. Figure 39 is an example of the fatigue wear in a diesel engine lifter. The effect of surface roughness and lubricant film thickness on fatigue wear was provided previously. Bench tests may be used for fatigue evaluation. Figures 40 and 41 provide examples of fatigue wear evaluation using bench test equipment. It should be noted that the fatigue lives of nominally identical rolling contact bearings tested under identical conditions could be widely different. This results in the generation of a fatigue pit (see Fig. 41), which typifies incipient fatigue failure as dependent upon the action of cyclic stresses and the motion of the rolling elements that randomly contacted at asperities. The fatigue strength at N cycles, SN (FL •^) of a material may be determined by ASTM D 1823.
(a) FIG. 38—Illustrations of oxidative wear, a) Oxidative wear obtained witli a medium carbon steel block-on-shaft lubricated with a mineral oil in the presence of a copper nanometal additive, load = 110-184 N, 156-220 rpm and the picture has 750X magnification. (Source: Dr. Sergey Tarasov, Institute of Strength Physics and IVIaterials Sciences, Tomsit, Russia); b) tribooxidation (oxidative wear) occurring om a Fe pin run on a Co disk run under the following conditions: 14 N/cm^, 1 m/s, > 10 km, 760 Torr and ft, = 0.25-0.28 (Source: E. Santner, BAM, Berlin, Germany).
933
FIG. 39—Fatigue wear in a diesel engine lifter. Shows subsurface cracks at 500X magnification. (Source: J. Zackarian, Chevron Research Company, Richmond, CA.)
934
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
.->"- ..„„i.=^=,.--.
^pn
(a) (b) FIG. 40—Examples of fatigue wear: a) Co pin/Fe disk material pair at 14 N/ cm^, 1 m/s, 710 km, 1 Torr, and fi = 0.33-0.34. (Source: E. Santner, BAM, Berlin Germany.), b) An AISI52100 pin on an AIS11015 steel disk, load = 15 kg, sliding speed = 500 rpm and lubricated with a liquid paraffin. (Source: Prof. Liu Jiajun, Tsinghua University, Beijing, PRC.)
Direction of Rotation
^ FIG. 41—Pitting fatigue wear obtained with twin 60 mm rotating S25C carbon steel disks The /u value for the driver is 1.0 and the ft value for the follower is 0.40. The load is 1.6 x 10^ ''Hertz = 950 MPa, and the rotating pair was lubricated with a paraffin oil at 2 L/min. (Source: Prof. T. Kawazoe, Nagasake University, Nagasaki City, Japan.)
The SN value is a value of stress at exactly N cycles as determined by an S-N diagram. The value of SN, thus determined, is subject to the same conditions as those that apply for the S-N diagram [50]. Fretting Wear—Fretting is small amplitude oscillatory motion, usucdly tangential, between two surfaces in contact and fretting wecir is wear that results from fretting motion (ASTM G 40). Fretting is a form of adhesive wear that may occur when the tips of the asperities of adjacent moving surfaces come into contact and microwelding occurs. Continued surface movement causes the tip of microwelded asperities to
FIG. 42—Scuffing of a surface of a 7075-T6 aluminum disk sliding on a gearing steel ball of 6.36 mm dia. at a sliding velocity of 0.33 m/s with a normal load of 4 kg. The sliding pair was lubricated with a paraff inic oil with a viscosity of 0.11 Pa s at 40°C containing a colloidal PFTE (Source: Dr. Sergey Tarasov, Institute of Strength Physics and Materials Sciences, Tomsk, Russia.)
pull off, producing a pitting effect. The heat of the microwelding process enhances oxidation of the fresh metal surfaces. If water is present, corrosion will result (fretting corrosion). Waterhouse has reported that the fretting process of steel is dependent on amplitude of slip; frequency and normal force of the oscillatory process; condition of the steel, particularly hardness; and temperature and humidity of the environment [56,57]. Fretting wear will occur in locations where there are oscillatory deflections of clamped joints, spline and gear couplings, and connecting rod joints in internal combustion engines. Fretting wear is often accompanied by the production of finely powdered and oxidized wear debris [58]. The four regimes of wear that are dependent on the displacement amplitude have been observed by Baker and Olver
CHAPTER
35: LUBRICATION
[59]. These four regimes, which are shown in Fig. 43, are designated as: • Regime 1—"Stick" occurs at very small amplitudes producing very little damage, • Regime 2—"Stick-slip" occurs with increasing displacement and wear rates increase slowly, • Regime 3—Gross slip produces severe damage by oxidation assisted wear which is the "fretting wear" regime • Regime 4—In this region, reciprocating sliding wear occurs eind if the amplitude is sufficient, the wear rate becomes approximately constant which is characteristic of unidirectional sliding wecir. i 1 1 = Slip
2 = Stick-Slip 3 = Gross Stick-Slip (Fretting) 4 = Oscillating Wear
• ^
c 0) o !^
^__4____ /
1
(D O 3 /
o(D <4mJ
CO
,1
on L_
CD
— — ^ — -t ^
1 y"^
^
Amplitude of Displacement (urn) FIG. 43—The four wear regimes including fretting and reciprocating wear.
AND TRIBOLOGY
^•»»«^
935
The transition from Regime 3 to Regime 4 is the transition from fretting wear to reciprocating sliding wear. This transition is controlled by the instantaneous quantity of wear debris remaining within the contact [59]. Although fretting and cracking are not related, cracking of the metal at the wear contact may occur as shown by Zhou and Vincent for fretting weeir of various aerospace aluminum alloys such as that shown in Fig. 44 for 2091 and 7075 alum i n u m alloys [60]. In this case, a fretting fatigue crack was nucleated due to the contact traction stress in the fretting wear regime and then propagated. The cracking direction was perpendicular to the fretting direction. No cracking was observed until after 5 X 10^ for 2019 and 10* cycles for 7075, at which time cracking occurred to a depth of 200 /j,m and 3500 /xm for 2019 and 7075 respectively, in this experiment. Kalin et al. have examined fretting wear and the role of tribochemical reactions in the wear process under lubricated conditions in detail [61-63]. It is shown that the very high flash t e m p e r a t u r e s within the wear contact affect microstructural changes a n d phase transformations of steel. However, one of the most important factors under lubricated conditions was the effect of flash temperature on the tribochemical processes that may occur, even u n d e r relatively low-speed, low-temperature conditions [61,62]. These studies showed that oil is a n important and chemically active component of the wear contact. For example, carbon and other elements that may be present may undergo thermally induced surface reactions. One such reaction that was
eOOtim
lOOtom (a)
FUNDAMENTALS
I
1
«.e?^
Z-> '- •^^-~«^^-:^
(c)
(b) FIG. 44—Fretting wear test results obtained with aluminum alloys: a) Al-Li (2091) alloy, N = 5 x 10^ cycles, Fn = 500 N, f = 1 Hz, cracl( depth is 200 fjitn; b) Al-Zn (7075) alloy, N = 10^ cycles, Fn = 1000 N, f = 5 Hz, crack depth is 3500 /tm (Source: Z.R. Zhou, Southwest Jiaotong University, Chengdu, China.)
936
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
observed was the introduction of carbon into the surface structure of steel (carburizing) if the flash temperature at the wear contact is sufficiently high. This is important because increasing carbon content typically increases hardness, thus affecting wear [61,62]. Recently, Kalin et al. examined these processes (along with fretting wear of different material combinations) using a commercially available ball on plate high-frequency fretting machine [63]. The ball was composed of silicon nitride and the plate was manufactured from AISI 52100 bearing steel. The load was 88 N and the oscillation frequency was 210 Hz. The lubricant was a petroleum oil. The wear surfaces were examined microscopically, wear scars were measured and examined by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Selected wear test results are shown in Fig. 45. Figure 45a shows cracks perpendicular to the sliding direction on the steel plate after 360 min, and at a 5 /am amplitude [63]. The size of the cracks increased with time. Figure 45b illustrates that a tribochemical layer was formed on the steel plate after 10 min of testing at a frequency of 25 /im [63]. This layer was brittle and it is evident that spalling has occurred, which was consistent with the high wear loss that was observed over this time. SEM and EDS analysis of the brittle layer shown in Fig. 45b suggested that a tribochemical reaction between the silicon nitride and the steel had occurred. Figure 45c shows the results of SEM analysis of the wear scar on the steel plate after 360 min and a 50 /xm amplitude [63]. EDS analysis of this tribolayer indicated a high amount of silicon from the silicon nitride ball. The cracks occurred during cooling.
Scuffing Scuffing is defined as "a form of wear occurring in inadequately lubricated tribosystems that is characterized by macroscopically-observable changes in surface texture, with features related to the direction of relative motion" (ASTM G 40). Surface texture is a term relating to surface finish, which is described by the roughness (peak) height in microinches and the n u m b e r of peaks per inch or it may be described as a surface pattern relative to a smooth finish. In lubrication, ASTM D 4175 defines scuffing as "surface damage resulting from localized welding at the interface of rubbing surfaces with subsequent fracture in the proximity of the weld area." An example of scuffing wear is provided in Fig. 42. The "Borsoff effect," which was originally published in 1959 [64], assumes that surfaces will scuff when adsorbed (polar) molecules, which are used to provide extreme pressure (EP) activity, become critically depleted or disoriented. A four-ball machine test was conducted to determine the scuffing loads for hexadecane containing various concentrations of hexadecanoic acid. (The EN31 steel balls contained 1% chromium.) The rotational speed of the moving ball was 200 rpm. A plot of the scuffing load as a function of hexadecanoic acid is shown in Fig. 46 [65]. It was assumed that the rotational speeds were sufficiently slow to permit molecular response to the adsorption/desorption equilibrium process and that scuffing occurred at the rear of the contact where the contact temperature is maximum and surface coverage is reduced to a constant, although unknown, value. To explain this wear process. Spikes a n d Cameron assumed that scuffing failure will occur by an additive desorption process. At the point where this occurs, there is n o EP
FIG. 45—Illustration of fretting wear under petroleum oil lubricated conditions. The wear contact is a SiN4 ball on an AISI 52100 disk, a) SEM micrograph of the wear scar of the steel plate after 360 min. at 5 fivn amplitude; b) SEM micrograph of the steel plate after 10 min at a 25 /jivn amplitude; and c) SEM micrograph of the lubricating layer on the steel plate after 360 min. at 50 fivn aplitude. (The cracks occurred during cooling) (Source: M. Kalin, Center of Triboiogy and Technical Diagnostics, University of Ljubljana, Ljubljana, Slovenia.)
activity that would be analogous to a base oil with no EP additives. This effect may be shown thermodynamically using the Langmuir adsorption equation for adsorbed (physisorption) polar molecules [66,67]. 1 - 0
• kC exp
-AH RT
CHAPTER
35: LUBRICATION
AND TRIBOLOGY
FUNDAMENTALS
dislocations, and that the low speed sliding wear of metals is caused by the subsurface crack nucleation and propagation nearly parallel to the surface" [69,70]. Figure Ala shows subsurface deformation and strain gradients of AISI 52100 steel [70]. Figure 47fe shows crack formation due to delamination wear in cold-worked 1020 steel [58]. Lim and Ashby proposed a mechanism for delamination wear, which is illustrated in Fig. 48 [71]. In this model, it is proposed that under conditions of plastic strain, voids nucleate around inclusions in the metal at a critical depth. Continual shearing action and compression cause the shape of the
600
0.002
0.004
Concentration (molar) FIG. 46—Scuffing load as a function of hexadecanolc acid in hexadecane obtained using a four-ball machine at 200 rpm.
where: 6 is the proportion of surface covered by adsorbed polar molecules, k is a temperature-independent adsorption constant, AH is the enthalpy of adsorption, T is absolute temperature, R is the gas constant, and C is a proportionality constant. AH for reversible adsorption of additives is a constant and a value of —40 cal/mol has been reported by Spikes and Cameron [67] and a value of —46 cal/mol was reported by Grew and Cameron [68] for a dodecanoic acid/dodecane system. This work provides a reasonable explanation of the generally accepted view that a thick oil film is "safer" than a thin film, even w h e n there is continual intermittent contact. When there is a thick film, the tips of the asperities alone will touch and touch only momentarily. This means that the time when there is metal-to-metal contact is very short. Due to the short time duration of the contact, a considerably higher temperature is needed to desorb the polar materials than the equilibrium value. As the film gets thinner, the areas in contact increase so each asperity is in contact longer and the molecules have a longer time to respond to the asperity tip temperature. For constant friction load, the load that can be carried safely is also lowered. This means that a large film thickness requires a higher surface temperature for scuffing than does a thin film, therefore, thick films may carry greater loads. Recent studies show that the scuffing is based on the residence time of molecules physically or chemically adsorbed [66]. Scuffing wear is illustrated in Figs. 25a,b,c and Fig. 42. Delamination
937
(a)
Wear
Steel, and other metals, may undergo more traditional heat treating processes such as carburizing, nitriding, and carbonitriting, in addition to newer methods such as PVD, CVD, ion implantation, etc. to achieve more wear resistant surfaces, which are partially due to increased hardness and also to increases in compressive stresses. When these materials are subjected to a wear process they may produce "delamination," which is defined by ASTM A 902 as "the separation of a coating (either full or partial thickness) from underlying layers: the separation can occur in smaJl localized areas or in large areas of a surface." This process is called "delamination wear." The mechanism of the delamination wear process is described by Suh et al. as "a non-work-hardening soft layer surface which deforms continuously due to the instability of
(b) FIG. 47—a) Subsurface deformation of AISI 1020 steel; load = 2.25 kh after sliding 54 m in argon atmosphere; b) three segments of a subsurface crack in cold-worked AIS11020 steel, a and c show the ends of the crack and b shows the mid-section of the crack. (Source: S. Janhanmir, NIST, Gaithersburg, MD.)
938
MANUAL
37: FUELS
AND LUBRICANTS
HANDBOOK
Surfaca^.
voids to change, leading to crack formation as shown in Fig. 48 [71]. Wear
In actual practice, there is typicEilly more than one mechanism in a wear process. Wear volume, for example, depends on the contact geometry, test conditions, lubricating additives, and materiEds. Wear volume or weight loss can be measured continuously during the test or at the conclusion of a test. T3'pes of wear, however, must be analyzed through surface characterization. Micrographs of different types of wear have been shown in previous sections. Examples of wear as a function of some of these variables are shown in Fig. 49 [72]. These figures illustrate the experimental evidence of load, sliding distance, speed, and thermal conductivity. The combination of basic wear processes is summarized in Fig. 50 [72]. In summary, it is difficult to simulate a specific wear pattern by using a particular test machine with standardized test conditions from a standard test. Instead, it is necessary to
FIG. 48—Photomicrographs of void formation around inclusions and crack propagation from these voids near the surface in annealed Fe-1.3% Mo alloy. (Source: S.C. LIm, National University of Singapore, Singapore.)
O)
CO
Testing
10"
5r
10-' o
B
10"
E
"TO (0 0)
o
> 10
J_
10" 10
J-
10^
10
10^
800
c •55
o
600
CO
0)
/c a
/ /
i
c 1—
(b)
/
i
SAE 1095 Steel Mineral Oil Lubricant
25
Q
~20
CO o
400 o ^
a. CO
30
o
^ // o)
0)
- O (/)
.ii
E -a
o S
2
Sliding Distance (cm X 10" ) • Load 0
- c c
^ri-2kg , •
0
• 223 BHN Hardness X 430 BHN Hardness
5^1 iilO
200
n
(c)
1
Load (g)
(a)
0.2
1
3 kg 1
0.4
Thermal Conductivity (caL/cm-sec^C)
> SO
J
-r'9-7 0.1
0.2
0.3
J 0.4
i_j. 0.5
Normal Pressure / Hardness
FIG. 49—Wear is dependent on several factors, a) load and material; b) sliding distance and material; c) thermal conductivity, load and speed on the conditions to produce a photographic hot spot; d) wear dependence on load and hardness.
(d)
CHAPTER 35: LUBRICATION
Metallic Bearing Surfaces
Transferred
7
Metallic Wear Debris
Corroded Surfaces
Corrosion Product Debris
1 Plowing or Fatiguo SAdhosion 3 Corrotioii
FIG. 50—Summary of basic wear mechanisms.
C FIG. 51—Pin-on-V-block tests conducted by progressively increasing the load on the rotating pin to evaluate the lubricating properties of a petroleum oil loaded with EP additives. The test results indicated that the steel pin would undergo extrusion without wear indicating excellent lubricating properties. (Source: D. Drees, Falex Tribology NV, Heverlee, Belgium.)
systematically consider materials, contact geometry, lubricants, applied pressure, and surface speeds in order to appropriately model the expected wear in a particular lubrication process. It is also necessary to develop test conditions that simulate actual use conditions and failure modes that are typically encountered during the use of the lubricant through a tribological wear test design process. There are numerous bench testing machines and standardized testing protocol. Current testing machines and procedures will be discussed in the next chapter and will not be discussed further here. However, two illustrations of the value of lubricant testing will be provided here from the standpoint of wear analysis. In one test, the lubricant properties of a petroleum oil containing extreme pressure (EP) additives was evaluated using a pin-on-V-block machine. In this case, the rotating pin was
AND TRIBOLOGY
FUNDAMENTALS
939
loaded at very high loads, which caused extrusion of the steel pin, instead of wear, as shown in Fig. 51. These results indicated excellent lubricating properties. More extensive, but similar, tests were conducted using a block-on-ring test configuration to classify the antiwear properties of a lubricant and are shown in Fig. 52. While wear test machines (tribometers) are commonly used to determine relative lubrication and wear performance of a lubricant and additive system, these results do not directly indicate how the test lubricant will perform in any particular application. Instead, opposite results may be obtained in industrial equipment, rendering such bench test results as misleading. Therefore, before such an assessment can be made, some correlation of test results obtained under certain conditions using a tribometer with actual performance in industrial equipment must first be performed. Wear Debris Analysis—Ferrography, which is an optical (visual) anal3rtical procedure, is one of the principle methods of wear debris analysis. Ferrography is conducted using a ferrograph where a sample is prepared by depositing wear particles on a glass slide placed over a strong magnetic field. This analysis procedure provides a measure of the concentration and distribution of wear particles. Generally "benign" wear occurs when the particles are less than 15 fim. Wear particles producing catastrophic failure are typically > 200 jtim. Ferrography can be used to detect and characterize particles in this range [73]. Examination of wear debris may be a useful indicator of wear life of industrial machinery, which can be obtained without shutting the equipment down and incurring often immense production cost losses. Wear debris analysis may also be considered to be a validation of wear tests if the debris analysis indicates the same wear is obtained in the industrial machine and in the bench test [74]. Cho has suggested a phenomenological approach to wear debris analysis, which reportedly provides a concurrent correlation of material surface, wear mechanism, wear debris analysis, and subsurface deformation [74]. Considerably more complex wear maps have been developed for lubricated contacts like those developed for steel as shown in Fig. 53 [75,76]. The map developed by Beerbower is based on a large body of literature data for the correlation of Specific Wear Rate, which was defined in the Adhesive Wear section, and Specific Film Thickness (A), which is calculated from the oil film thickness (ho) divided by the composite surface roughness ((aj + aiy^) as follows: A='
ho
The importance of such an approach is illustrated by the work reported by Leng and Davies who used wear debris analysis to identify characteristic types of wear resulting in scuffing, which were obtained using the four-ball test machine [77]. The conclusions of this study showed: 1. Scuffing generates large particles of wear debris showing evidence of shear fracture on one surface. However, such debris can be generated by a single overload. 2. Both carburized steel and AISI 52100 bearing steel, two common steels used in tribological applications, exhibit a featureless white layer during scuffing. It was proposed that this white layer resulted from a friction weld.
940
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
(a,b)
(c)
FIG. 52—Illustration of lubricant evaluation using the block-on ring tests by wear mode identification. The tests conducted include: a) an example of adhesive wear on the test block caused by a bad lubricant; b) micrograph of the test block indicating mostly uniform wear, but in some parts of the wear track there is adhesive wear. This behavior is often called "scoring" but it signifies the limiting load that a lubricant can carry, c) General adhesive wear, also called seizure, indicated by excessive scoring of the test block; d) some parts of the ring (of the block-on-ring pair) show that adhesive failure occurred; e) in this case, there is uniform wear on the test block which, in this case, indicates that the lubricant prevented adhesive wear. The extent of the wear scar (width) can be used to characterize "antiwear" properties. (Courtesy of D. Drees, Falex Tribology NV, Heverlee, Belgium.)
10'-
/Pseudo - Scuffing ^ scumng Corrosive IVIod^
Severe Scuffing •^
y'^
<,#^/Mixed ^"S^- /Mixed
\<^
/
^'X^
_cp' C!^
/ ^ /•
\,
„
Corrosive I
\ .
Corrosive
and Transfer I 1 Modes L.
IVIodes
Violently Corrosive
\
\
\ \ \
\
10-*
) | / "S ' / ^/ ro I Transfer / v. / %«>^ Mode f/ r|.5l 11 ^ a j 1 ^ w:
\
X
Vv
x
\
Sliding (Contact) Fatigue
Mixed Corrosive Fatigue
X.
\ \
o
\
x
I
Ideal System ^
I "Molecular Wear"
\
I I I
0.5
\
1.0
1.5
2.0
Specific Film Thickness
2.5
3.0
Mode _L 3.5
4.0
1/2
A=ho/ (oi+ai)'
FIG. 53—Wear map for lubricated steel wear variation with changes in specific oil film thickness.
CHAPTER 35: LUBRICATION AND TRIBOLOGY FUNDAMENTALS 3. Adhesion wear debris on the contact surfaces is very rare in this system. 4. This wear debris cannot be used to characterize scuffing wear generated using a four-ball machine. This work shows that like all well-performed laboratory work, proper correlations must be performed to assure the applicability of the test data to the problem being analyzed.
941
where: U is the sliding speed in ft/min, and Tm is the flash temperature in °F. In performing lubrication studies, there are occasions when it is necessary to estimate the contact temperature, such as when performing lubricant scuffing studies with a 4ball lubricant testing machine. To calculate the TCR, it is necessary to estimate the flash temperature (AT). The ATmax temp e r a t u r e for a "standard" 4-ball wear contact m a y be calculated from a Blok-type equation [79]:
Flash Temperature Flash
^Tn
Temperatures
As the wear surfaces come into contact, there is cin increase in temperature due to a n increase in the heat of friction. The contact temperature at the wear interface is dependent on the applied load at the wear contact, time within the contact zone, and sliding speed [78]. Work by Okabe et al. [16] and Matveevsky [15] have shown that the effectiveness of additive chemistry is also dependent on the geometry of the wear contact. The contact temperature is therefore a critically important controlling factor in controlling the effectiveness of lubricant additive chemistry and is a n important psirameter to be measured [16,79]. Within a wear contact, there is localized instantaneous rise in surface temperature due to friction. In most cases the temperature of the two frictionally heated surfaces is assumed to be approximately equal and is defined as [80]: Tc = Ts + AT where: Tc is the m a x i m u m instantaneous surface temperature of the wear contact, Ts is the quasi-steady-state surface temperature which is, although incorrectly, assumed to be the equivalent to the oil bulk temperature of the lubricating oil, and AT is the maximum rise of the instantaneous surface temperature above the quasi-steady-state oil temperature. The value of AT is also called the "flash temperature" [80]. When the value of Tc is sufficient to cause scuffing, this is called the "critical contact temperature for scuffing" (TCR) and is equal to: TCR = TS +
AT
This is k n o w n as Blok's Critical Contact t e m p e r a t u r e model. The value of TCR is a constant for a given oil-metcd surface and (oil-metal surface-atmosphere) c o m b i n a t i o n [80]. Interestingly, the value of TCR is independent of the oil viscosity. However, variation of the metal and/or surface treatment will significantly affect the value of TCR. The interrelationship between steel chemistry and antiwear protection offered by various additives containing sulfur or phosphorous was described in detail by Huang et al. [81]. Rabinowicz has used the following general rule to estimate the flash temperature of pure dry sliding motion [45]:
1.61nW^'V k[l + 0.627(W/a)]0
where: /M is the coefficient of friction, W is the top ball load (kg), n is the rotational speed in revolutions/second of the top ball, k is the thermal conductivity (2.604 kg/s °C), a is the thermal diffusivity of steel (6.045 mm^/s), and r is the Hertz contact in mm. Experimentally, the flash temperature (AT) has been estimated directly from wear data using a 4-ball wear tester using the equation [82,83]: AT = ^ W 1.4 where: W is the applied load in newtons (N), and d is the m e a n wear sccir diameter (mm). NOTE: The test balls used for the 4-ball wear tester Eire constructed from 0.5 in diameter AISI 52100 Steel. The nominal composition of these balls is listed in Table 11 [82] and the physical properties of AISI 52100 steel are summarized in Table 12 [82]: Sethuramiah reported that the relationship between load and Hertzian (elastic) contact radius of the tetrahedreJ test bcdl geometry was: Load (kg) = W (applied load) X 0.408 Ball Radius of elastic contact (a) = 5.9 X lO""* W°-^ Other equations for the calculation of flash temperature have been reported by Cowan a n d Winer [84] a n d Kuhlmann-Wilsdorf and will not be discussed in detail here [85,86]. The effect of sliding versus rolling motion on flash temperature at the wear contact is illustrated in Fig. 54 [87]. Sliding m o t i o n generates significantly higher flash temperatures
Tm - y ( ± factor of 3)
TABLE 12—Physical properties of MSI 52100 steel. Property Hardness Thermal Conductivity Specific Heat Density Thermal Diffusivity
Value 760 kg/mm^ 0.109 cal/cm C-s 0.11cayg°C 7.78 g/cm^ 0.127 cm^/s
TABLE 11—Composition of AISI 52100 used for test balls for 4-ball machine. Composition C
Mn
P
S
Si
Cr
0.95-1.10
0.25-0.45
0.025 max
0.025 max
0.20-0.35
1.30-1.60
942
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
90°C Pure Sliding
U
Pure Rolling
FIG. 54—Illustration of the effect of rolling versus sliding motion on flash temperature. As the relative amount of sliding motion increases, there is a corresponding increase in flash temperature.
than rolling motion within the wear contact. This suggests that wear in a rolling contact is mostly dependent on the rheology of the lubricant, whereas lubrication of sliding contacts is m u c h more dependent on the additive chemistry since the tribochemistry of some additives is "activated" by the temperature within the wear contact. Wear
Maps
There has always been great interest in being able to predict not only failure but also the failure mode. For example, Lim and Ashby have reported a wear mechanism m a p for a nonlubricated sliding steel pin-on-disk pair, which is shown in Fig. 27 [87]. In this particular study, four major wear mechanisms were identified including: seizures, melt-dominated wear, oxidation-dominated wear, and plasticity-dominated wear. (It should be noted that the term seizure in lubrication has been defined in ASTM D 4176 as "welding between surfaces in relative motion that results in immobilization of the parts,") Lim and Ashby defined the following wear mechanisms, which are indicated on the m a p in Fig. 27 [87]: 1. The seizure process involves large localized pressures, which can form metallic junctions if the load is sufficient to cause plastic flow. During sliding, these welded junctions are sheared and welded areas grow. If the loading is sufficient, the real contact area will grow until it reaches the nominal area at which point seizure occurs. 2. Melt wear occurs when the friction-induced localized temperatures are sufficient to melt the metal. 3. At sliding speeds below 1 m/s, the wear debris is mostly metallic. However, at sliding speeds of 1 m/s, the flash temperatures (approx. 700°C) are sufficient to cause mild oxidation wear. This is a localized oxidation but the oxide that is formed is hard and brittle. However, at higher sliding speeds, e.g., 10^ m/s, a thicker oxide film is formed, which is hotter and more plastic and an even lower wear rate is obtained. This is called sever-oxidation wear. The terms "mild" and "severe" refer to the extent of oxidation, not wear rate. 4. Plasticity-dominated wear is adhesive and delamination wear. These processes were detailed in the Adhesive Wear section above.
5. Ultra-mild wear occurs in a regime where the loads and sliding velocities are sufficiently low so that very little wear occurs. Frictionally-induced temperature increases occur within the wear contact, and the associated thermal stresses are induced, which should be accommodated in the analysis of sliding wear. This was performed by Cowan and Winer using a t h e r m o m e c h a n i c a l wear model to generate micro-and macro-contact wear maps for designer use (Fig. 55) [84]. This was performed by superimposing isothermal stresses induced from the surface traction on the thermal stresses from frictional heating. These results were integrated with material yield criteria required for plastic surface deformation to occur in the form of wear. The wear m a p in Fig. 55 was generated for AISI 52100 bearing steel. The dimensionless Hertzian contact pressure is (Po/
yfVa
Eac kCl-r
where: E is elastic modulus of the material, at is the thermal expansion coefficient, k is thermal conductivity, y is the heatpartition factor, V is Poisson's ratio, f is the coefficient of friction, and V is the relative sliding velocity of two bodies. Winer uses the value of Gt as a thermomechanical wear control parameter. With respect to Fig. 55, a micro-contact possesses a contact radius of 0.1 ^tm-1 m m . A macro-contact exhibits a contact radius of 1-10 mm. There are also four curves in Fig. 55 representing a thermal steady state (FQ), 0, 0.1, 1, and 10. The scuffing wear region is above the FQ line. The n o wear region is below the FQ line for the material of interest. For AISI
10'
a. 3 (A (0
Wear Region 10° Conditional Wear Region
10 y
9> c
10
CQ
•e
(D
X
10'
CO M
c o 'm c (D
E b
No Wear Region
lO"! 10 10^
\ 1 lUiK^—I 1 luiiH—\ 1 mill—\ \ Uiiit—I 'siuii|—
10 '
10' 10° 1_ micro-contact (G,<1.7X10'')
10' TI
10' G,
10' J
macro-contact (10 < G t < 1.7x10^)
FIG. 55—The Cowan and Winer thermomechanical wear map for AISI 52100 steel.
CHAPTER
35: LUBRICATION
0.5 0.4-
0 0.2 0.4 0.6 0.8 1.0 Shear Strength at the Contact Surface (f) FIG. 56—Wear mapping procedures have also been developed to elucidate the classification of specific types of wear such as the model for abrasive wear.
52100 steel, the Fo line is approximately 10. The conditional wear region is located between the FQ = 0 and Fo = 10. No wear will occur in the "no wear" region because the applied stress (represented by the dimensionless Hertzian pressure) is less than that required to cause failure. Wear will occur in the wear region because the dimensionless Hertzian pressure exceeds the strength of the material. In the conditional wear region, the possibility of wear depends on the Fo value, which is a dimensionless measure of the contact time. Wear mapping has also been performed to elucidate the mechanism of abrasive wear [88,89]. The wear m a p shown in Fig. 56 [88] provides a correlation of the degree of penetration (Dp) and the shear strength of the material. The depth of penetration is calculated from: D
= ^
^ -
„2
AND TRIBOLOGY
Dp = R
UH, 2W
UH rR^ 2W
The lubrication mechanism for the transition from Region I to Region II is that as the load increases, the asperities within the contact are separated (/JL = 0.2) until the load is sufficient to bring the asperities close enough to begin to make contact. The higher asperities undergo deformation and oxidation and tribochemical reaction films are formed if suitable additives are present. At this point, the coefficient of friction decreases (/a < 0.1). With further increases in the load, there is an exponential increase in the n u m b e r of asperity contacts. However, until F ^ = FNm, the deformation, oxidation and additive tribochemical surface modification process is sufficient to maintain a low coefficient of friction. At loads greater than F^^, the n u m b e r of asperities within
III "Scuffing Regime"
\ 1
Where: a is the contact radius, h is the depth of the groove, Hv is the Vickers hardness of the flat specimen, R is the radius of the cutting pin tip, and W is the load. The shear strength (f) at the contact surface is calculated from:
II "Boundary"
f= 0.3-0.5
\ \
p
E o "5.
Where: r is the contact shear stress and k is bulk shear stress. The abrasion mechanism, ploughing, wedge formation or cutting, was identified by determining the wear volume/groove volume by SEM analysis [75]. On the basis of this work, the following conclusions were drawn: • There are three mechanisms of abrasive wear; • The degree of penetration provided a wear severity index that was dependent on normal load, cutting pin radius, and material hardness.
943
This figure illustrates the complexity of steel lubrication. Clearly, given the information here, it is inappropriate to take a single value indicating fluid lubricating properties and attempt to apply t h e m in any application where the fluid may be used. De Gee et al. developed a load-velocity failure m a p representing "lubricated wear," shown in Fig. 57 to classify the mode of lubrication as a function of applied force (L) and speed (V) for AISI 52100 steel ball on a cylinder using paraffin oil as the lubricant [90]. In this approach, three wear regions were observed: Region I, which is partial elastohydrodynamic (EHD) lubrication; Region II where b o u n d a r y lubrication occurs; and Region III which is representative of wear for an unlubricated contact—even though the contact was fully submerged in the paraffin oil lubricant. The lower curve, Ai-S represents the transition from Region I to Region 2 and S- A3 is the transition from Region I to Region III. The curve A2-S is the transition from Region II to Region III. Because the position of curve Ai-S-As is viscosity dependent, it is assumed that it represents the transition from EHD lubrication. The point FNIH is the maximum load carrying capacity, which occurs at the velocity of V^. The increase of load carrying capacity at V > Vm was assumed to be due to a loss of lubricating film viscosity because of frictional heating.
1/2
D„ =
FUNDAMENTALS
Q.
<
f= 0.2-0.4
\
\
^I
\
Al
Region I Negligible Wear Region II Mild Wear Region III . Severe Wear and Scuffing
I ^ \ . "(Partial) EHD" f= 0.04-0.1,
"^ 1 1 1 1
- — _ ^
A3 — —
v Velocity (V) — » FIG. 57—de Gee, Begelinger and Solomon wear map (transition diagram) for n AISI 52100 ball-cylinder wear contact in a super refined paraffin oil at 30°C and a viscosity of 1.4 x 10~^ Pa.s in dry air.
944
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
©Micro—attrition # Micro-abrasion -Mechanical fatigue * Thermal fatigue + Chipping
.500
H.275
•T3.200
60
cutting speed m/min. J I L 120 180 240
360
Wear Mechanism Map, C5 Milling Inserts, 4140 Preheated Steel
FIG. 58—Wear map illustrating the wear mechanisms involved in cutting of preheat treated AISI4140 steel using an uncoated C5 carbide insert.
the contact exceed the oxidation and tribochemical modification rate and incipient scuffing (Region II) or full-scale scuffing (Region III) begins (Vs). De Gee and coworkers have shown that the contact temperatures in this system may be as high as 500°C at the A2-S transition, which would be sufficient to lead to metallurgiccJ transformations [90]. It is therefore proposed that the onset of scuffing may be "triggered" by a metallurgical transformation. The general subject of metallurgical transformations that may occur on the wear surface will not be discussed further here, however, the reader is referred to Refs. 91-97 for a more detailed discussion of these processes. As a result of this work de Gee cited that the often widely varying wear rates reported in the literature may be more due to differences in the wear regime during testing than differences in additive performance. Furthermore, meaningful additive evaluation could only b e achieved with identical conditions of boundary lubrication. De Gee et al. have studied the wear mechanisms involved in the cutting of preheat treated AISI 4140 steel [98]. The mechanisms of flank wear of a n uncoated C5 carbide insert (as well as various nitride coated inserts) were determined and mapped as a function of cutting speed and feed rate. The predominant wear mechanisms were determined to be adhesion wear (also referred to as attrition wear) abrasion wear, mechanical fatigue, cind thermal fracture. The wear map for the use of a C5 carbide insert to cut preheat treated AISI 4140 steel is provided in Fig. 58 [98]. Figure 59 is SEM photographs of flank face wear, which illustrates the trend of mi-
0.275 mm/rev.
0.200 mm/rev.
3" (D Q.
3J 0.125 mm/rev.
59.9 m/min
119.7 m/min
179.6 m/min
239.4 m/min
359.1 m/min
7m/s
2m/s
3m/s
4m/s
6m/s
Speed FIG. 59—Wear of the flank face using a scanning electron microscope (SEIU) at 1000X magnification to illustrate the trend of microattrition wear, microabrasion wear and thermal pitting for cutting of preheat treated AISI 4140 steel using a C5 carbide insert under "wet cutting" conditions. Increasing cutting speed is shown going from left to right and increasing feed rate is shown going from bottom to top of the photo array. (Source: S. Tung, GM Research and Development Center, USA.)
CHAPTER
35: LUBRICATION
croattrition wear, microabrasion wear, and thermal pitting for cutting of preheat treated AISI 4140 steel using a C5 carbide insert under "wet cutting" conditions. A single load capacity n u m b e r does not adequately define the multi-dimensional character of a lubricant and provides little help to the designer. A multi-dimensional characterization of an oil must also include such attributes as "antiwear," EHD film-forming ability, and traction coefficient. Wedeven has demonstrated a more comprehensive approach to evaluate the lubricating characteristics of an oil by mapping the performance of a contact system over a range of rolling amd sliding velocities. This is illustrated in Fig. 60 for the oil HercoA [18]. These diagrams are called "Performance Maps." Performance maps Eire developed in terms of rolling (entraining) velocity versus sliding velocity. The generation of a n EHD film is primarily a function of the entraining velocity in the inlet region of the contact upstreemi of the Hertzian contact region. In this region, the lubricating film generation is primarily a function of the physical properties of the lubricant (viscosity and pressure-viscosity coefficient). The sliding speed determines the shear strain in the Hertzian contact region. This region is important with respect to heat generation, surface film formation, and wear and scuffing within the tribo-contact. The magnitude of the sliding velocity component within the Hertzian contact region, along with the degree of surface interactions, invoke the chemical properties of the fluid, e.g., adsorbed films, chemical reaction films, tribochemical reactions, and thermal/oxidative stability. A "performEmce m a p " is used to characterize the scuffing boundaries for a lubricant. These scuffing boundaries are
AND TRIBOLOGY
FUNDAMENTALS
identified by plotting the onset of scuffing after testing the lubricant on a precision ball-on-disk machine at various rotational and sliding speeds at a given load as shown in Fig. 60 [18]. These examples show that development of a wear m a p permits a more global illustration of various lubrication and wear mechanisms that may potentially be involved in a wear process and provide a much greater and more useful overview of lubricant performance than a single bench test [99]. Lubrication Mechanisms Lubricants are delivered to two surfaces undergoing relative motion to reduce friction. Although lubricants may be in liquid, gaseous, and solid states, this chapter will focus on liquid lubricants, primarily those derived from petroleum oil. It is genercdly accepted that liquids exhibit three lubrication regimes: boundsiry lubrication, elastohydrodynamic lubrication, and hydrodynamic lubrication. The Stribeck curve is a plot of the friction coefficient (/i) as a function of the product of absolute viscosity (17) and the rotation speed in revolutions per unit second (N) divided by the load per unit projected bearing area (P), as illustrated in Fig. 61 [100]. Because the Stribeck curve exhibits a characteristic minimum, this suggested to McKee and McKee as early as 1929 that more than one lubrication mechanism is involved [101]: M At least four lubrication regimes [18] are indicated in Fig. 62, which is a plot of the coefficient of friction versus the
Performance Map Herco-A, 300 KSI Stress "HG" M50 Ball (Ra=10), M50 Disc (Ra=3) 6S0
600 -
NA276-NA306
sso 1^
500
1,30 •IS
400
o > (SO
MO
g j3
250
200 -
XTl
ISO 100 -
945
Scufr
50
^ so
250
R. Entraining Velocity (in/s) FIG. 60—Wedeven performance map correlating sliding and rolling velocity at a constant load to identify the scuffing region for a lubricant. (A precision ball-on-disk test.)
946
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK substantially greater t h a n surface asperity heights. EHD lubrication is characterized by thin film lubrication with film thickness of approximately 0.025-2.5 fim. Although these are thin films, they are greater than the asperity contact heights. Boundary lubrication is characterized by film thickness of ^ 0.025 /u,m, which is less than the height of the asperity contacts. Mixed film, or EHD/boundary lubrication, occurs at the transition from boundary to EHD lubrication. For comparison, the relative sizes of various components of a wear contact are provided in Table 13 [102]. Hertzian
Contact
If one body is pressed against another with sufficient pressure, an elastic deformation will result, as shown in Fig. 8 [103]. The pressure generated in this contact is the Hertzian 200
Z = centipoise N" = rev/min
400
600
stress (CTH):
ZN' P
(TH
load/length x dia p.s.i. = clearance ratio thou/inch
1.5 LE^ TT^r^dv'Y
where: L is the contact loading, v is Poisson's ratio, for most materials it is 0.3, r is the contact radius, and E is the modulus of elasticity. Table 14 provides typical values for the elastic modulus for various materials including: aluminum, copper, and stainless steel [104]. Hertz's equation for plastic deformation permits the calculation of the "true area of contact" (A^) in the Hertzian Contact region is [45]:
FIG. 61—The Stribeck curve.
rBoundary rMixed
=
rElastohydrodynamic
Ar = 2.9 [L rf^
c o
assuming that Poisson's ratio (j') for both surfaces is 0.3. Lubricated Hertzian Contact—It was shown previously that a ball will elastically deform in the Hertzian contact region, which is much smaller than the radius of curvature [105]. The "Hertzian pressure" that produces the elastic deforma-
Hydrodynamic
c o
TABLE 13—Size ranges of various components of a wear contact.
e
(D O
o 0
5
10
Specific Film Parameter
20 A=.
JL^
(af+ai)
0.5
FIG. 62—Characterization of lubrication regimes as a function of the coefficient of friction and the dimensionless film parameter.
"Typical" Component
Approximate Size Range (lira)
Monomolecular Layer Sliding Wear Debris Boundary Film EHD Film Asperity Height Rolling Wear Debris Asperity Contact Hydrodynamic Film Asperity Tip Radius Concentrated Contact Width Engineered Counterformal Radius Engineered Conformal Radius
0.2-2 X 10^^ 0.002-0.1 0.002-3 0.01-5 0.01-5 0.7-10 0.7-10 2-100 10-1000 30-500 1-100 X 10^ 12-2500 X 0*
TABLE 14—Elastic modulus of selected materials
lambda VcJue described earlier and which is also known as the dimensionless film p a r a m e t e r . The four lubrication regimes are: hydrod3mamic, elastohydrodynamic (EHD or EHL), mixed EHD (or EHL) and boundary lubrication, and dry-film boundary lubrication. Hydrodynamic lubrication is characterized by relatively large film thickness, tj^jically greater than 0.25 /tm, which is
Materials
Elastic Modulus (Gpa)
Aluminum Copper Stainless steal Alumina Silicon nitride Polyethylene-high density PTFE (25% glass fiber)
70.6 12.98 180-220 300-400 280-310 0.5-1.2 1.7
CHAPTER 35: LUBRICATION tion exhibits a parabolic distribution, as shown in Fig. 8a and Fig. 13, which is high in the center and low at the edges of contact. Typical Hertzian pressures found in bearing and gear contacts are very high, approximately 1.4 X 10' N/m^ (200 000 psi). The Hertzian contact is a dominating feature of elastohydrodynamic (EHD) lubrication. It establishes the overall shape of the contacting surfaces. The fluid will first enter an "inlet region" and then pass through the converging surfaces and finally exit through a diverging region. The hydrodynamic pressure in this region must be sufficient to separate the surfaces that are forced together by the approximately 1.4 X 10' N/m^ pressure within the Hertzian contact. Inlet Region—The surfaces shown in Fig. 63 are in pure rolling motion, each moving with the same velocity (U). Each surface carries with it a certain quantity of fluid that joins to-
AND TRIBOLOGY FUNDAMENTALS
gether at the same location to fill the gap between the surfaces. Due to surface convergence, fluid in the interior must slow down, as shown in Fig. 63, or even flow in the reverse direction. The reduction in velocity causes an increase in hydrodjmamic pressure of the fluid. Because of the viscositypressure relationship of the fluid, as illustrated in Fig. 64, fluid viscosity increases as the pressure increases within the inlet region [18]. (See the Pressure-Viscosity Coefficient section for more discussion on the viscosity-pressure relationship of fluids.) EHD Film Thickness Measurements by Interferometry—EHD film thickness measurements may be experimentally determined using the optical fringe color in the center of the contact as a function of rolling velocity. Optical film thickness is measured at the "center" of each fringe and at the transition between each fringe. The optical film thickness data is con-
Rlm
FIG. 63—Illustration of fluid flow contours with the Inlet region. Pure rolling motion at relative velocities Ui and U2 is assumed for this illustration.
O Fluorinated Polyether • Linear Polyperfluoroalkylether A Super-Refined Naphthenic Mineral Oil V Synthetic Hydrocarbon (traction fluid) V- Advanced Ester
50
947
100
Temperature °C
FIG. 64—Fluid viscosity increases with increasing pressure as the fluid enters the inlet region and Hertzian contact region. Typical viscosity pressure relationships of different base fluids is provided.
948 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 15—Conversion of optical fringe colors to film thickness. Optical Fringe Color
Optical Film Thickness (M X 10"')
First yellow Red Blue Green Second Yellow Red Blue Green Third Yellow Red Blue Green
2.0 2.7 3.4 4.3 4.7 5.6 6.1 6.7 7.6 8.6 9.0 9.6
verted to actual film thickness by making corrections for the refractive index of the test fluid, including the effect of pressure on density under the Hertzian contact. Fringe colors and their relation to film thickness are shown in Table 15. The Lorenz equation [106] is used to correct the refractive index for density emd the Hartung's empirical formula for hydrocarbons is used to correct the density for the Hertzian pressure used in the test. Reynolds Equation—The practiced importance of the EHD lubrication mechanism lies in the thickness of the oil film between the surfaces, which is controlled by operating conditions such as surface velocity, load, cind fluid viscosity. The influence of various operating parameters on film thickness (hm) is illustrated by the following Reynolds equation for a line contact formed by a cylinder on a plane [107]:
h„ Where:
0.7 „,0.54 DO.43 2.65 (rjo f/r^ a' R"
hm = film thickness at the rear constriction of the Hertzian contact rjo = viscosity at atmospheric pressure a = pressure - viscosity co-efficient U = velocity defined as 1/2 (Ui + U2) where Ui and U2 are the individual velocities of the moving surfaces R = radius of equivalent cylinder L = load per unit width E = elastic m o d u l u s of equivalent cylinder (flat surface assumed completely rigid)
An important feature of EHD lubrication is that the influence of load on film thickness is relatively small since increasing the contact load would result in an increase in the maximum Hertzian pressure, which increases the Hertzian contact region. Therefore, there is little effect on the inlet region where hydrodynamic pressure is generated. Although the effect of pressure on lubricant viscosity is an i m p o r t a n t p a r a m e t e r in lubricant fluid-film analysis, the pressure-viscosity coefficient is often unavailable. However, this data can be obtained by calculation using the Reynolds equation if film thickness can be measured in the contact region [108]. This is obtained from a plot of the dimensionless film thickness parameter (ho/R) where: ho = film thickness in the center of the contact, R = combined radius of curvature of the Hertzian contact
and the speed parameter which is defined as: E^R where:
Ue = E' = R = T/o =
entraining velocity = 'A (Ui + U2) combined elastic modulus combined radius of curvature viscosity at atmospheric pressure and at the test temperature.
The only EHD lubricant characteristic missing from the two dimensionless parameters is the pressure viscosity coefficient (a) and load (w). The "effective" pressure-viscosity coefficient is calculated from the slope of the line for the fluid being eveduated as shown in Fig. 65 [108]. For the work illustrated, the film thickness measurements were conducted under the same load. If the a-value for each fluid was the same, the film thickness data would fall on the same line. Higher values of ho/R for the same (rjoUe/E'R) reflect higher pressure-viscosity coefficient. Non-Newtonian EHD and Micro-EHD Lubrication—Fluid viscosity within the inlet region of the Hertzian contact is not only dependent on temperature and pressure but also on the non-Newtonian rheological properties of the fluid [109]. Hirst and Moore demonstrated that the critical factor is not shear rate but shear stress within this region [110]. The critical stress is dependent on the pressure and molecular size. When the critical stress is exceeded in EHD lubrication, traction coefficients, heat generation, and film thickness will be affected. The impact of non-Newtonian viscosity behavior on effective film thickness has been successfully modeled with an equation developed by Bair and KhansEiri, which incorporates the "second Newtonian" obtained from the Carreau viscosity equation [109]:
"l+A(f^^*"""^ Dimensionless Film Thickness vs Speed Parameter
Nominal Test Temperature 23 °C
E'R FIG. 65—Dimensionless film thickness versus speed for different fluids at 23°C.
CHAPTER
35: LUBRICATION
where A is a time constant, n is the well-known power law coefficient, ryo is the viscosity at zero shesir rate, TJOO is the viscosity at very high shear rates, 17 is the non-Newtonian viscosity, and dV/dz is the change of viscosity with cheinge of shccir rate. The Carreau model provides a continuous transition between the first and second Newtonian regions of the power law viscosity curve (Log viscosity versus Log shear rate). Chang a n d Zhao have c o m p a r e d Newtonian a n d nonNewtonian analysis of micro-EHD contacts. Based on numerical analysis, it was concluded that the shear thinning effects of the lubricant would produce significant errors if the non-Newtonian behavior is not properly accounted for [111]. Effective pressure-viscosity coefficients have been reported to be dependent on the non-Newtonian behavior of lubriczints within the inlet region of the Hertzian contact [112]. Mixed EHD Film Lubrication—Mixed-EHD film lubrication is encountered during the transition from fuU-EHD film to boundary lubrication. In this region, surface roughness (texture) effects are particularly critical. A c o m m o n method of accounting for surface roughness effect is through the use of the specific film parameter (A): h
•
h
RMS
+Ritr'
(^q,a
where: Rms is the RMS composite asperity amplitude [R^,a + R^b]"'^ for surfaces a and b of the wear contact. Figure 57 illustrates the effect of the magnitude of the film pairaxneter on the lubrication mechanism [3]. The effect of both oil viscosity and surface roughness has been studied [113,114]. The influence of surface roughness is m u c h greater than the effect of fluid viscosity. The loading capacity before scoring, galling, scuffing or seizure was highly dependent on surface smoothness. The presence of a single scratch may cause total failure by seizure. Hydrodynamic
Lubrication
Hydrodynamic lubrication is characterized by conforming surface so that the load is carried over a large area. Since relatively thick films characterize this region, lubrication is primarily dependent on bulk fluid viscosity at the operating temperature of the system as predicted by the Stribeck curve parameters (ZN/P). Petroff's Law—Petroff, in his classic aneJyses of lubrication, assumed that the lubricant sticks to the surfaces of the journal bearing, the shaft was concentric, and that the velocity gradient was equal to U/c where U is the surface speed of the shaft jind c is the film thickness. The frictional torque (T) is: ^ vRU T =— c
, A
Where : rj is viscosity, R is the radius of the bearing and A is the "wetted" area [115]. The coefficient of friction (/LA) is: jLt =
2
vn
1000
where: n is the rotational speed in 1000 revolutions per second, P is the load per unit area, and 8 is the radial cleareince ratio 1000 c/R in thouscindths of inch per inch of shaft diameter.
AND TRIBOLOGY
FUNDAMENTALS
949
This may be rewritten in the more familiar form of the Stribeck curve shown in Fig. 6 1 . Zn Where: Z is Zahigkeit, the German term for viscosity, n is the surface speed, P is the applied pressure, and ho is the average film thickness. It is important to note that although the most c o m m o n form of this equation, the so-called Stribeck curve, only provides a plot of the coefficient of friction (^tt) versus Zn/P. However, this is not rigorously correct. The actual value of ho is not 1.0, although it rarely exceeds 1.5 [115]. The nearly linear right hand portion of the Stribeck curve representing hydrodynamic lubrication is called the "Petroff Line." In Fig. 61, both the experimental relationship for the coefficient of friction and ZN/P cind the Petroff line are shown. The experimental data differs from Petroff s line at low values of ZN/Pho because Petroff did not account for the temperature dependence of viscosity, especially at higher temperatures or the effects of surface asperity vziriation at low cleareinces. Boundary
Lubrication
Tribochemistry of the Lubricated Wear Contact Wear Material Surfaces—As the Stribeck curve in Fig. 61 shows, if a fluid film with sufficient thickness and viscosity separates two surfaces, lubrication is hydrodynamic with no asperity contact. However, as the load is increased or speed decreases, the two surfaces will converge, and if they are sufficiently close, asperity contact will just begin to occur. Of course, thicker fluid films are required as the surface roughness increases as indicated by the Lambda {A) factor (see previous discussion). This region is designated as "Mixed-film" lubrication as shown in Fig. 62. As the two surfaces come even closer in proximity to each other there will be sufficient asperity contact to enter the "boundciry lubrication" regime. Under conditions of asperity contact, increased friction, asperity welding, and adhesive weeir will result [116]. To prevent excessive w e a r and m a c h i n e c o m p o n e n t failure u n d e r these conditions, the use of boundary additives is necessary. Although nascent metal surfaces may be formed during the wear process and may be involved in some lubrication mechanisms, the surfaces that are most commonly encountered are typically in an oxide form. The surface chemistry and the lubricant films that are subsequently formed through additive interactions are dependent on the presence of oxygen and water [117,118]. In addition to the identification of the failure mode, it is often desirable to examine the mechanism of the surface chemical reactions that have occurred. This may be accomplished by conducting the additive reaction during the lubrication process using bench testing laboratory equipment. After the test, the surface chemistry is studied by various surface chemical characterization methods. It is beyond the scope of this chapter to review the characterization methods such as those summarized in Table 16 [119a]. Friction
Modifiers
Friction modifiers are polar compounds, usually derived from animal or vegetable oils, and they operate by adsorbing
950 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 16—Summary of various methods of surface analysis. Acronym
Name
Probe
Detection Signal
Information
Space Resolution
Applications to Tribology
Elemental analysis of the surface
= 50 nm. depth 1.5 m m
Illumination spectrum, composition Electronic status of very small portion
= 5 fan
Depth direction profile of each element at friction surface (+ ion gun) Lattice defects and compression analysis {+ EPMA) Elemental analysis, electron state analysis (4-TEM) Stress due to slip. film thickness changes, etc. 50 n m Analysis of slip products
AED
Auger electron spectroscopy
Electron
CL
Cathode luminescence
Electron
Photon
EELS
Electron energy loss spectroscopy
Electron
Electron (Scattered)
ELL
EUipsometry
Light (laser)
Light (polarized)
Optical constant, film thickness
EPMA
Electron probe microanalysis
Electron
Characteristic xray
EDX, WDX quantitative analysis
ESR
Electron spin resonance
Magnetic field
Electromagnetic wave
Unpaired electron (radicals, etc.)
XAFS
X-ray absorption fine structure
X-ray
X-ray absorption
0.2 n m
FIM
Field ion microscopy
Neutal molecule + electric field
Cation
EXAFS (distcince to surrounding atoms, number): XANES (atmic value configuration) Atomic configuration
NMR
Nuclear magnetic resonance
Magnetic field
Electromagnetic wave
Molecular bonding condition
> 10 fim
IR
Infrared absorption spectroscopy
Light (infrared)
Light (infrared)
Vibration condition
Thin film acceptable
LEED
Low energy electron diffraction
Electron
Electron (diffraction)
Molecular bonding condition
RBS
Rutherford backscattering spectroscopy Optical profilometry
Ion beam
Scattered ion
Vibration condition
Several atom layers below the surface > 100 fjLm
Light (laser)
Light (interference)
Surface atomic structure
RS
Raman scattering
Light (laser)
Light (Ramsin scattering)
SEM
Scanning electron microscope
Electron
Electron (secondary scattering)
Trace elements, atomic configuration Surface topography
OPM
Auger electron
= 10 n m Depth direction 50 fjon Several 10 /Lim, depth direction = 0.5 /xm. depth direction 0.3 to several ^ m 10'° spins
Sub - /jan. depth direction 0.5several n m 1 fj,m
=» 1 nm. depth direction 0,3several /nm
Lubricant deterioration and dangling bond analysis Composition of adsorbed substances
Three dimension structure at atomic level resolution, (+ electric field evaporation) Adsorption condition of lubricant, molecular mobility, surface functioned radicals Analysis of adsorption performance of gases and lubricants (FT-IP, RAS, ATR, polarization) Gas adsorption at surface (+AES, RHEED) Alalysis of slip products, thin film density) Imaging to slip surface
Electronic properties of thin films, etc Damage forms (In-situ as well. + EPMA)
(continues)
CHAPTER 35: LUBRICATION AND TRIBOLOGY FUNDAMENTALS
951
TABLE 16—(continued). Acronym
Name
SIMS
Secondary ion mass spectroscopy
SPA
Surface potential analysis Scanning probe microscopy
SPM
STEM
TA
TEM
TOFSIMS
TDS
TXRF
Probe Ion beam
Detection Signal
Information
Secondary ion
Combination condition
Electric field
Topography
Space Resolution
Applications to Tribology
Several fan, depth direction 0.5several n m Several 10s
Exrtremely small quantity elements detection including hydrogen Slip charge
fjm
Electric field
Tunnel current
Sixrface and subsurface formation
Scanning transmission microscopy Thermal analysis
Electron
Electron (transmission)
Polarization
Thermal energy
Heat dissipation
Transmission electron microscopy Time of flight SIMS
Electron
Ion beam
Electron (transmission. diffraction) Secondary ion
Thermal desorption spectroscopy Total-relection x-ray flourescence spectroscopy
Thermal energy
Desorbed atoms, molecules
Microscopic surface structure Shape, structure elements Heat of adsorption, heat of transition Imaging
X-ray
Fluorescent x-ray
Composition distribution
« 0.1 nm. depth direction = 0.1 n m «< 1 n m
10 n m 0.1 n m 100 fjim. depth direction 1 nm
10^
atoms/cm^ depth direction several n m 1 fim
UPS
Ultraviolet photoelectron spectroscopy
Light (tdtraviolet)
Photoelectron
Adsorption, material decomposition
XRD
X-ray diffraction
X-ray
X-ray diffraction
Trace element cmalysis
Several 100s fjan, depth direction
XPS
X-ray Photoelectron spectroscopy
X-ray
Photoelectron
Chemical composition
100 /xm. depth direction several n m
on to the wear surface by a physisorption or chemisorption process from the fluid lubricant. Other terms used to refer to this class of additives include: oiliness additives, lubricity improvers a n d film strength enhancers [119b]. Although the thickness of these adsorbed boundary lubricating molecular films is typically 2 X 1 0 ^ to 3 X 10~^m, they reduce friction and wear under sliding conditions that are too severe for the base lubricating oil [120]. Friction modifiers are typically used under low to medium load conditions. Under more severe conditions where the load £ind contact temperature increases at the wear contact, the protective adsorption film ruptures, resulting in increased wear. Although investigation of additive reactions are commonly studied by tribological experiments, it is also important to consider the effect of not only contaminated surfaces but also solvent cleaning procedures since either may exhibit enor-
Various applications including STM and AFM Crystallization in very fine regions (+ EDX, + EELS) Adsorption energy. lubricant deterioration, etc. Transition cell structure, reaction products, etc. Lubrication distribution, deterioration Identification of adsorbed and stored materials Surface contaminants
Surface oriented tribological anedysisbonding state Thin film structure, internal pressure defects, etc. (including neutron diffraction) Surface oriented tribological analysis, bonding state, lubricant film thickness (+ ion gun)
m o u s and often a n o m a l o u s affects on fluid lubrication [121,122]. Therefore, it is essential that the various surface cleaning procedures as well as lubrication conditions be reported. Additive Classification As indicated above, as the wear contact surfaces approach each other with decreasing film thickness, surface asperities come into contact. To prevent reduced friction and wear, boundary lubrication additives are used. Table 17 provides a summary of the c o m m o n classes of boundary lubricant additives used in lubricant formulation along with their purpose, function, and chemical type [123]. The selection of appropriate friction modifiers, and antiwear and extreme-pressure additives is dependent on a number of factors including: activation temperatures, load effects and reaction mechanisms between the additive and the sub-
952 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 17—Common additive types and function in lubricating oil formulations. Additive Friction Modifier (FM) or Oiliness Additive
Purpose Reduce friction under near-boundary conditions
Antiwear Additive
Reduce wear
Extreme Pressure (EP) Additive
Prevent, galling, scoring and seizure
666666666666
Typical Compounds High molecular weight compounds such as fatty oils, oxides, waxes, fatty acids, fatty alcohols Organic phosphates, phosphites, zinc dithiophosphates Sulfur, phosphorous, chlorinecontaining materials
It
Van Der Waal Forces
©EsSSJ^ (a)
Function Adherence of polar materials to metal surfaces Forms a film on metallic contacting surfaces Formation of low-shear films on metal surfaces at the wear contact
R
1
C-ssO 1 ; 0 .'
IV
H t
OCu 0 Cu 0
OCuO CuO
FIG. 67—A hydrogen exchange interaction for fatty acids on a copper substrate.
iJjggJjgffivjSJi (b) FIG. 66—Adsorption models for: a) physisorptlon; b) chemisorption.
strata. These effects will be discussed in detail in the subsequent discussion. Friction Modifiers—Friction modifiers are adsorbed from the lubricant on to the wear surface, either physically (physisorption—see Fig. 66a) or chemically (chemisorption—see Fig. 66b). Physisorption is a thermodynamically reversible, relatively weak adsorption processes (no chemical bond formation) based on dipole or van der Waal's forces [124]. Chemisorption is thermodynamically irreversible and is based on considerably stronger ionic bond formation with the metallic oxide wear surface. The adsorption forces between a carboxylic acid and a metal substrate due to hydrogen bonding and debye orientation forces have been reported in the range of 13-15 kcal/mol [125]. Figure 66 illustrates the difference between the two bonding mechanisms [126]. A hydrogen exchange interaction for fatty acids on a metal substrate such as copper has been postulated, as shown in Fig. 67 [127]. Additive adsorption on to the metal substrate of interest is an important method of characterization of friction modifiers. The role of surface adsorption of various carboxylic acids on boundary lubrication was studied as early as 1922 by Hardy and Doubleday [128]. More recent work by Studt has illustrated that increasing fluid temperature decreases the efficiency of adsorption of stearic acid from cetane, as shown in Fig. 68 [129,130,150]. Steric hindercince on the adsorption efficiency of the adsorbing group with respect to the remainder of the molecule signiflcantly affects adsorption of 1-, 2- and 3-octadecanol in
-or
1
1
3
«l
5
6
7
8 x 1 0
Mole Fraction of Stearic Acid in Cetane FIG. 68—Increasing fluid temperature has decreased adsorption of stearic acid from cetane.
heptane, as shown in Fig. 69 [102,130]. In general, adsorption efficiency decreases as the polar group is moved from the end (1-octadecanol) to the middle (5-acdecanol), which is due to increasing stearic hinderance of the adsorbing group. Hironaka illustrated the dramatic differences in the heat of adsorption of stearic acid as a function of the surface condition of the metal substrate, as shown by the heat of adsorption of stearic acid on FeS, Fe304 and Fe203 in Fig. 70 [120]. Experimental determination of adsorption isotherms, such as those shown in Figs. 68 and 69, m a y b e performed through procedures described earlier by Forbes and Reid and others [130] where a tumbling a p p a r a t u s described by Clunie is used [131]. For these experiments, 5 mL borosilicate glass adsorption tubes are used into which 1.0 g of the iron powder
CHAPTER 35: LUBRICATION
AND TRIBOLOGY FUNDAMENTALS
953
Glass Tube
1-Octadecanol •Solution
FIG. 71—The horizontally clamped tube allows the Iron powder and solution to equilibrate separately.
2-Octaclecanol 5-Octadecanol
J_ O.S
1.0
1.S
2.0
2.S
3 . 0 x 1 0- 1
Mole Fraction of Octadecanols in Heptane FIG. 69—The position of the adsorbing group significantiy affects adsorption.
and a mL of the solution containing the additive of interest is placed. The tube is sealed and rotated in an oil bath until the iron powder and the additive solution is thoroughly mixed. At the end of the experiment, the iron powder is separated at the test temperature from the solution with a strong magnet. The tube is then clamped horizontally, as shown in Fig. 71, to allow the iron powder and solution to equilibrate separately. The iron powder is separated as before, the tube was cooled in liquid nitrogen and the end containing the iron powder is sealed off. The solution concentration of the additive is determined by gas chromatography and the heat of adsorption is determined by the microcalorimetric procedure described previously [131-133] Fowkes [134] adapted the Yamins and Zisman method [135] to determine the adsorption isotherm of various carboxylic acids and amines from a naphthenic white oil. In this method, a vibrating gold reference electrode (100-500 c.p.s.) was mounted approximately 1 mm from a parallel test electrode. The signal was detected with an oscilloscope after amplification. The potential between the two electrodes was compensated with a potentiometer until a null point was observed. The change of the null potential (AV) of the potentiometer for a given metal due to the adsorption of the additive was measured by determination of the potential of the metal plate with a thin layer of the oil (0.1 mm) and then measuring the potential of a similar layer of the oil containing the additive. Similar procedures have been described by Sewig and Zisman [136], Frumkin [137] and Jahanmir [133]. Using this procedure, adsorption isotherms for different additives on different metals can be determined in addition to adsorption time and the "strength of adsorption." Adsorption isotherms can also be derived from coefficient of friction data using standard bench testing machines such as a four-baJl machine [133,138] and a ball-on-cylinder machine [139]. Jahanmir determined the adsorption isotherms from [133]: f = fb(i - e) + f^e where: 0 = the fractional additive surface coverage, f = friction coefficient for boundary lubrication, fa = friction coefficient for the additive at S = 1.0, fb = friction coefficient of the base fluid at S = 0. The fractional additive surface coverage (ff) is calculated from: e=(fb-f)/(fb-fa)
0 0.5 10.0 Heat of Adsorption, (mJ/m^) Adsorbent FIG. 70—The dramatic differences in the heat of adsorption of stearic acid as a function of the surface condition of the metal substrate.
The values of fa and fb are determined from two bench tests; one without any additive and one with a high concentration of the additive. The value of 0 is determined by measuring the coefficient of friction at various intermediate additive concentrations. Jahanmir, et. al. found that the data was fit better using a Temkin adsorption isotherm which states that the exother-
954
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
m i c a d s o r p t i o n free e n e r g y d e c r e a s e s l i n e a r l y w i t h s u r f a c e coverage [140]: AGads = AGo + aO w h e r e AGQ = t h e p r i m a r y a d s o r p t i o n free e n e r g y a t fl = 0 a n d a = p o s i t i v e c o n s t a n t t h a t is d e p e n d e n t o n t h e i n t e r a c t i o n of t h e additive with t h e surface. At l o w a d s o r b a t e c o n c e n t r a t i o n s w h e r e 0 a fl a 0.2, t h e n : e « ( R T / a ) (C/Ko) w h e r e : C i s t h e a d d i t i v e c o n c e n t r a t i o n i n m o l e fraction Ko is t h e p r i m a r y a d s o r p t i o n c o n s t a n t d e f i n e d a s :
and
Ko = e x p ( d G o / R T ) At i n t e r m e d i a t e a d d i t i v e s u r f a c e c o v e r a g e w h e r e : 0.2 < 6 s 0.8, t h e f o l l o w i n g i s o t h e r m m a y b e u s e d [ 1 3 3 ] : e = ( R T / a ) In (C/Ko) At h i g h levels of c o v e r a g e , 0.8 ^ fl s 1.0, t h e T e m k i n i s o t h e r m becomes: 9 « 1 - ( R T / a ) [ e x p ( a / R T ) - 1] (Ko/C) Using this a p p r o a c h , J a h a n m i r d e t e r m i n e d the friction coefficients a n d free e n e r g y of a d s o r p t i o n for a s e r i e s of c a r boxylic acids in a solvent-refined, dewaxed, hydrorefined n e u t r a l l u b r i c a n t b a s e s t o c k w i t h a v i s c o s i t y of 32 c S a t 40°C. T h e d a t a o b t a i n e d is s u m m a r i z e d i n T a b l e 18. T h i s d a t a s h o w s t h a t t h e f r i c t i o n coefficient d e c r e a s e s w i t h i n c r e a s i n g h y d r o c a r b o n c h a i n l e n g t h £ind w i t h i n c r e a s i n g free e n e r g y of adsorption [133,141,142].
T h e a n t i w e a r a c t i v i t y i s d u e t o t h e f o r m a t i o n of a m o n o l a y e r of s t e a r a t e a s t h e l u b r i c a t i n g film. As s h o w n b y J a h a n m i r , I n c r e a s i n g c h a i n l e n g t h s of c a r b o x y l i c a c i d s g e n e r a l l y d e c r e a s e coefficient of f r i c t i o n [ 1 2 7 , 1 2 8 , 1 3 3 ] . H o w e v e r , it h a s b e e n p r o p o s e d t h a t a l t h o u g h at lower sliding speeds the coefficient of friction is c o n t r o l l e d b y m o n o l a y e r f o r m a t i o n , a t higher sliding speeds, o r d e r e d multilayers control friction [125,127]. G r o s z e k , d e s c r i b e d t h e u s e of a p u l s e flow m i c r o c a l o r i m e t e r t o s t u d y t h e a d s o r b a n c e of m o l e c u l e s of v a r y i n g polctrity o n t o different s u b s t r a t e s including: graphite, cast iron, i r o n a n d steel p o w d e r s , i r o n oxides, nickel oxide a n d lithium s t e a r a t e [ 1 4 3 a ] . Also, v a r i o u s s o l v e n t m e d i a w e r e e v a l u a t e d . This w o r k showed t h a t certain metal oxides a n d graphite preferentially adsorb molecules with long hydrocarbon c h a i n s from l o w m o l e c u l a r w e i g h t s o l v e n t s . H o w e v e r , i r o n , steel a n d F e 2 0 3 d o n o t e x h i b i t a n y t e n d e n c y t o p r e f e r e n t i a l l y a d s o r b l o n g c h a i n h y d r o c a r b o n s . I n a d d i t i o n , it w a s s h o w n that detergent additives are strongly adsorbed o n a l u m i n a a n d engine deposits. H i r o n a k a a n d o t h e r s h a v e a l s o s h o w n t h a t t h e effectiveness of v a r i o u s friction m o d i f i e r s o n f r i c t i o n a n d w e a r r e d u c t i o n is r e l a t e d t o t h e h e a t of a d s o r p t i o n . T h i s is i l l u s t r a t e d b y t h e d a t a s h o w n i n T a b l e 19 [ 1 2 4 ] a n d T a b l e 2 0 [ 1 2 0 ] . F r o m t h e s e r e s u l t s a n d o t h e r s , t h e f o l l o w i n g r u l e of t h u m b is u s e d : " t h e g r e a t e r t h e h e a t of a d s o r p t i o n , t h e b e t t e r t h e l u b r i c i t y " [ 1 2 0 ] . Antiwear a n d E P additives are classified by t h e t e m p e r a t u r e s r e q u i r e d t o activate t h e m . Antiwear additives typically act by a d s o r p t i o n a n d E P additives r e q u i r e a t h e r m a l activat i o n as illustrated schematically in Fig. 72 a n d Fig. 7 3 .
TABLE 18—Effects of hydrocarbon chain length on friction coefficient and free energy of adsorption. Carboxylic Acid
Acid Structure
Friction Coefficient
AGo (kcal/mole)
Capiic Acid Laurie Acid Myiistic Acid Palmitic Acid Stearic Acid Oleic Acid
CH3(CH2)sCOOH CH3(CH2)ioCOOH CH3(CH2)i2COOH CH3(CH2)i4C00H CH3(CH2)i6COOH CH3(CH2)7 CHBCH (CHj)? COOH
0.111 0.104 0.099 0.086 0.077 0.099
-5.5 -5.4 -4.9 -5.2 -5.0 -4.8
TABLE 19—Friction and wear metal transfer for cadmium surfaces at r o o m temperature. Lubricant None Cetane-C 15H31CH3 Cetyl Alcohol-Ci5H3iCH20H Palmitic Acid-C,5H3iCOOH
Coefficient of Friction 0.8 0.6 0.4 0.07
Metal Pick-Up X lO^g/cmof Track 50 000 500 100 1
TABLE 20—Relationship between heat of adsorption and wear reduction. Wear Volume Heat of Adsorption ( X lO""* cm^)^ (cal/g)' Mineral Oil 3.5 8.5 Aromatic Distillate 4.5 3.5 Castor Oil 3.9 4.3 8.0 2.4 Cetyl Alcohol 0.8 Stearic Acid 36.0 0.4 38.5 Cetyl Amine 1.7 Oleic Acid 9.6 1.9 BUcinoleic Acid 12.3 Adsorption from benzene solution on to iron. Pin-on-disk friction testing machine, contact pressure 16,800 kgl^cm^ Lubricant
Initial Galling Load (kgf) 40 40 70 40 40 60 40 40
CHAPTER
35: LUBRICATION
Paraffin Oil c o
o
^
0)
o O
l_ EP and Polar Film Formers
Temperature FIG. 72—Increasing temperatures will lead to failure of lubricating films formed by friction modifiers.
S
S
j 1
-t> 1> s S
S
1> S
II
S +2R-
II
Alkane, Olefin, etc.
Fe Surface I 11 Surface Extrusion [Adsorption Bond Formationll Into Steel Surface AW Region EP Region I > > >^)
11 >'i I > I > 11 > } \ > > I > 11 >} > > > ) \ > I > >\ > > I > >
Temperature — • FIG. 73—Illustration of the difference of surface reactions for antiwear and EP additives. Increasing temperatures will lead to failure of lubricating films formed by friction modifiers t h a t are related to: [32,124] • Adsorption films undergo two-dimensional melting at temperatures close to the bulk melting point, disorienting temperature, of the additive. This disorientation temperature distinguishes physisorbed additives from chemiadsorbed additives. In some cases, it has been reported that if the temperatures are sufficiently high, the adsorbed films will "bum" forming carbonaceous residues [143b] • While physisorbed films become detached from their surface at the melting point of the additive, chemisorbed additives are effective above their bulk melting point although they do ultimately fail with further increases in temperature [32]. Therefore, the relatively limited film strength and thermal instability of the friction reducing films formed by friction modifiers necessitates the use of additives that Eire more effective in this region. Antiwear Additives—Antiwear additives often contain phosphorous such as those compounds illustrated in Fig. 74 [144]. Generally, eintiwear additives are most effective under mixed lubrication conditions where low-medium loads or high temperature conditions Eire encountered. In this region, there is intermittent asperity contact. The antiwear additive reacts with the metal asperities at the wear contact temperatures to
AND TRIBOLOGY
FUNDAMENTALS
955
form films which are not intended to prevent scoring or seizure but to reduce wear that may occur during times of insufficient full-film lubrication. Under high loads, the lubricating film breEiks down due to shear and/or friction heat and galling results [120]. There is not a clear trEinsition from friction modifiers to antiwear additives and friction modifiers. In fact, there are conditions where friction modifiers may exhibit both friction reduction a n d a n t i w e a r properties [120]. It is proposed, however, that the m e c h a n i s m of the phosphorous derivatives, such as ZDDP (zinc dialkyldithiophosphate) or tiicresyl p h o s p h a t e passes t h r o u g h all three stages: adsorption, chemisorption, and chemical reaction [145]. These multiple reaction pathways, which occur tribochemicEdly within the wear contact, Eire evident when considering the complexity of a recently determined structure of zinc dithiophosphate reaction film which is illustrated in Fig. 75 [146]. Extreme-Pressure (EP) Additives—^As the load on a wear contact is increased still further, the contact temperature increases still further until the fluid film collapses a n d the metal asperities come into contact. This will be accompanied by a still further sudden rise in temperature and adhesive failure with subsequent welding of the surfaces [120,143] However, when extreme pressure additives are present, such as organic halides, organic phosphates and organosulfur compounds, the wear surface will react u n d e r conditions of high load and temperature forming iron halides, iron phosphorous or iron sulfides (see Fig. 76 ) respectively which exhibit greater film strength t h a n those formed with friction modifiers or antiwear additives [147]. The lubricity exhibited by EP additives is dependent on the contact temperatures and the reactivity of the EP additive with the metal surface under these conditions Eind shear strength of the reaction films that are formed [120]. Generally, the load carrying capacity of the reaction films increase with chemical reactivity. The following generalizations can be made: • Iron chloride films exhibit relatively low melting points (approx. 350°C) a n d therefore relatively low wear resistEince. • Organophosphorous-based EP additives react at somewhat lower temperatures than organic halides thus exhibiting an improvement in wear resistance. HO, i--3M
Metal dialkyl dithiophosphate
RO
\ )
Tricresyl phosphate (R = cresyl) I.e.
Dialkyl phosphite
XM
(S
CH,
Phosphate
FIG. 74—Antiwear additives often contain phosphorous such as those compounds.
956
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK representative thickness (nm) - 9 0 0
alkylphosphate precipitates
(poly)idiospbate sulphide/oxide ferrous substrate FIG. 75—Multiple reaction pathways, which occur tribochemically within the wear contact, are evident when considering the complexity of a recently determined structure of zinc dithiophosphate reaction film.
4-
1 Sutfur-type / EP agents
#3.5.
a •
1 (0
o c
O)
/
1 1 • / • / /
•g
S o
2-
1• 1.5- J/ / r 7 1 //
DC
// 1 .
•
/ 1 1 j
3
f§2.5. — 0 .2
/ /
^,,,^
fi--<7.
1
Phosphorus-type EP agents / /
^A--
^
^^-"^'^ Chlorine-type EP agents
~ ,, . -
1.1 1.2 1.3 1.4 Relative chemical reactivity
1.5
FIG. 76—Correlation of relative reactivity and load carrying capacity of EP additives with respect to the functionality class.
The iron sulfide surfaces formed from organosulfur based EP additives exhibit excellent heat resistance (750°C) and superior wear resistance. For dithiophosphate derivatives, although the antiwear activity increases with decreasing thermal stability, the EP activity decreases [143] All EP additives are corrosive to the surface for which they are used to protect. Since the reacted surface has lower
shear strength, friction is reduced, as is the tendency for severe adhesive failure. [144] There is often no clear division between the mechanism of activity between antiwear and EP additives. In some cases, a given additive may exhibit activity by either mechanism, depending on the use conditions. This is illustrated in Fig. 73 where the disulfide additive is adsorbed on wear surface with subsequent disulfide bond cleavage to form an iron sulfide [148]. As the load increases and the lubrication regime undergoes a transition from mixed film to boundary lubrication, the contact temperatures increase causing a cleavage of the carbon sulfur bond resulting in an iron sulfide layer similar to that shown in Fig. 73. Thus a given additive may exhibit both antiwear and EP activity. It has been shown that EP activity is dependent on the reaction or wear contact temperature. Therefore, it is important to determine the reaction temperature of an additive if an assessment of its ability to continuously form, or regenerate, boundary lubrication films in the wear contact [79]. Figure 77 provides a summary of the additive EP requirements as a function of the contact temperature [103] which provides a useful guideline for additive classification. One of the problems in applying Fein's EP additive selection criteria is that additive activation temperatures are often unknown. One method that may be used for studying the influence of contact temperature on the generation of EP films is to use the "hot-wire method", originally developed by Barcroft and Rossett et al. [148,149] This method permits the fast generation of surface temperatures of several hundred °C with significantly increasing the bulk fluid temperatures. Surface analysis is then used to characterize the reaction products formed under these conditions.
CHAPTER 35: LUBRICATION
5 350
Strong EP
s a E 300 0) H
I 200 o 150"-
I
NonEP
FIG. 77—EP Requirements with respect to contact temperatures for additized hydrocarbon oils with a steel material pair.
CONCLUSIONS An overview of fundamental principles involved in lubrication has been provided. This included fundamental material and surface properties along with an overview of friction. Also provided was an extensive overview of weeir mechanisms and their identification. This discussion was followed by a review of fluid film lubrication and the role and criteria of additive chemistry in friction reduction. The fluid was treated as a component of the tribocontact and methodologies of examining and classifying -wear mechanisms was provided. Finally, the role of additives in lubrication was discussed. This information can be applied to the test procedures and experimental designs proposed in the following chapters to provide a greater depth of understanding of the data obtained. This information, in combination with the discussion provided in Chapters 25 and 26, will provide the reader with a reasonably in-depth overview of the basic principles of lubrication and wear: its measurement and test methodologies and design.
ASTM STANDARDS
D 996 D 2652 D4175 D 4725 E 6 E 7 E 1823 G 15 G 40
ASME ASME Y14.36M: Surface Texture Symbols. ASME B46.1: Surface Texture (Surface Roughness, Waviness, and Lay). ISO
O
B 374 D 653
957
Mild EP
.2 250
No. A 902
FUNDAMENTALS
OTHER STANDARDS
O 400
u c
AND TRIBOLOGY
Title Standard Terminology Relating to Metallic Coated Steel Products Standard Terminology Relating to Electroplating Standard Terminology Relating to Soil, Rock, and Contained Fluids Standard Terminology of Packaging a n d Distribution Environments Standard Terminology Relating to Activated Carbon S t a n d a r d Terminology Relating to Petroleum, Petroleum Products, and Lubricants Standard Terminology for Engine Coolants Standard Terminology Relating to Methods of Mechanical Testing Standard Terminology Relating to Metallography Standard Terminology Relating to Fatigue and Fracture Testing Standard Terminology Relating to Corrosion and Corrosion Testing Standard Terminology Relating to Wear and Erosion
ISO 1302: Technical Drawings—Method of indicating surface texture. ISO 4288: Geometrical product specifications (GPS)—Surface texture: Profile method - Rules and procedures for the assessment of surface texture. ISO 12085: Geometrical Product Specifications (GPS)—Surface texture: Profile method - Motif parameters ISO 3274: Geometrical Product Specifications (GPS)—Surface texture: Profile method - Nominal characteristics of contact stylus instruments. ISO 11562: Geometrical Product Specifications (GPS)— Surface texture: Profile method - Metrologiccd characteristics of phase correct filters. ISO 13565-1: Geometrical Product Specifications (GPS)— Surface texture: Profile method; Surfaces having stratified functional properties - Part 1: Filtering and general measurement conditions ISO 13565-2: Geometrical Product Specifications (GPS)— Surface texture: Profile method; Surfaces having stratified functional properties - Part 2: Height Characterization using the linear material ratio curve. ISO 4287: Geometrical Product Specifications (GPS)—Surface texture: Profile method - Terms, definitions and surface texture psirameters. ISO 5436: Calibration specimens—Stylus instruments— Types, calibration and use of specimens. ISO 1302: Technical Drawings - Method of indicating surface texture. ISO/TR 14638: Geometrical product specification (GPS)— Masterplan (This is an ISO Technical Report (TR), not an ISO Standard.)
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[119] a. Yanagisawa, M., "Molecular S t r u c t u r e of Thin Organic Films," Japanese Journal of Tribology, 1994, Vol. 36, No. 5, pp. 549-559. b. Schilhng, G. J. and Bright, G. S., "Fuel and Lubricant Additives," Lubrication, Vol. 63, No. 2, 1977, pp. 13-24. [120] Hironaka, S., "Working Mechanisms of Additives in Lubricating Oils," Sosei to Kako, Vol. 36, No. 413, 1995-1996, pp. 579-585. [121] Bemett, M. K., Kinzig, B. J., Murday, J. S., and Ravner, H., "Surface Analysis of Bearing Steels After Solvent Treatment," ASLE Transactions, Vol. 26, No. 1, 1981, pp. 98-106. [122] Bemett, M. K. a n d Raven, H., "Surface Analysis of Bearing Steels After Solvent Treatment II: Lubricant-Coated Bearing Surfaces," ASLE Transactions, Vol. 25, No. 1, 1982, pp. 55-63. [123] Chu, J. and Tessmann, R. K., "Additive Packages for Hydraulic Fluids," The BFPR Journal, Vol. 12, No. 2, 1979, pp. 111-117. [124] Anon., "Fundamentals of Wear," Lubrication, Vol. XLII, No. 12, 1956, pp. 149-160. [125] Crawford, J. and Psaila, P., "Miscellaneous Additives," Chemistry and Technology of Lubricants, R. M. Mortier and S. T. Orszulik, Ed., Blackie, VCH, Glasgow, New York, 1992, pp. 160-173. [126] Makowska, M., Gradowski, M., and Molenda, J., "Interfacisil Interactions in Tribological Contact," Tribologia, Vol. 3, 1998, pp. 254-264. [127] Hu, Z.-S., Hsu, S. M., and Wang, P. S., "Tribochemical and Thermochemical Reactions of Stearic Acid on Copper Surfaces Studied by Infrared Microspectroscopy," Tribology Transactions, Vol. 35, No. 1, 1992, pp. 189-193. [128] Hardy, W. B. and Doubleday, I., "Boundary Lubrication-The Paraffin Series," Proceedings of the Royal Society of London, Vol. AlOO, 1921, pp. 550-574. [129] Studt, P., "Boundary Lubrication: Adsorption of Oil Additives on Steel and Ceramic Surfaces smd its Influence on Friction a n d Wear," Tribology International, Vol. 22, No. 2, 1989, pp. 111-119. [130] Forbes, S. and Reid, A. J. D., "Liquid Phase Adsorption/Reaction Studies of Organo-Sulfur Compounds and Their LoadCarrying Mechanism," ASLE Transactions, Vol. 16, 1973, pp. 50-60. [131] Clunie, A. and Giles, C. H,, "TumbUng Apparatus for Liquid Phase Adsorption Experiments," Chemical Industry, April 1957, pp. 481-482. [132] Allum, K. G. and Forbes, E. S., "The Load-Carrying Properties of Organo-Sulfur Compounds. The Influence of Chemical Structure on Anti-Wear Properties of Organic Disulfides," Journal of the Institute of Petroleum, Vol. 53, 1967, p. 174. [133] Jahanmir, S. and Beltzer, M., "Effect of Additive Molecular Structure on Friction Coefficient and Adsorption," Journal of Tribology, Vol. 108, 1986, pp. 109-116. [134] Fowkes, F. W., "Orientation Potentials of Monolayers Adsorbed at the Metal-Oil Interface," Journal of Physical Chemistry, Vol. 64, 1960, pp. 726-728. [135] Yamins, H. G. and Zisman, W. G., "A New Method for Studying the ElectriccJ Properties of Monomolecular Films on Liquids," Journal of Chemical Physics, Vol, 1, No. 9, 1933, p. 656-661. [136] Sewig, K. W. and Zisman, W. A., "Low Energy Reference Electrodes for Investigating Adsorption by Contact Potential Measurement," Advances in Chemistry Series, Vol. 33, 1961, pp. 100-113. [137] Frumkin, A., "Influence of Electric Field on the Adsorption of Neutral Molecules," Journal of Physics, Vol. 35, 1926, pp. 792-802. [138] Askwith, T. C , Cameron, A., and Crouch, R. F., "Chain Length of Additives in Relation to Lubricants in Thin Films and Boundary Lubrication," Proceedings Royal Society of London, Vol. 291A, 1966, p p . 500-519.
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Mortier and S. T. Orszulik, Eds., Blackie, VCH, Glasgow, New York, 1992. [146] Bee, S., Tonck, A., Georges, J. M., Coy, R. C, Bell, J. C. and Roper, G. W., "Relationship Between Mechanical Properties and Structures of Zinc Dithiophasphate Anti-Wear Films," Proceedings of the Royal Society of London, Vol. 455, 1999, pp. 4181^203. [147] McFadden, C, Soto, C, £ind Spencer, N. D., "Adsorption and Surface Chemistry in Tribology," Tribology International, Vol. 30, No. 12, 1997, pp. 881-888. [148] Barcroft, F. T., "A Technique for Investigating Reactions Between E.P. Additives and Metal at High Temperatures," Wear, Vol.3, 1960, pp. 440-453. [149] Rosset, E., Matthieu, H. J., and Landolt, D., "A New Experimental Technique for the Study of Surface Reactions of Extreme Pressure Additives at Elevated Temperatures," Wear, Vol. 94, 1984, pp. 125-133. [150] Studt, P., "Adsorption of Lubricating Oil Additives. The adsorption of Lubricating Oil Additives on Steel Surfaces and its Tribological Significance," Tribologie + Schmierungstechnik, Vol. 34, No. 1, 1987, pp. 2-9.
MNL37-EB/Jun. 2003
Bench Test Modeling Lavem D. Wedeven^
THIS IS ONE IN A SERIES OF THREE CHAPTERS DEALING WITH Lu-
brication a n d Wear (Chapter 35), Bench Test Modeling (Chapter 36), and Wear and Lubricant Testing (Chapter 37). In this chapter, the lubrication a n d wear mechanisms discussed in Chapter 35 are used to extend the selection and application of the tribology tests discussed in Chapter 37. The challenge in tribology testing is to make bench testing meaningful. Meaningful bench testing is critical for performance prediction in service. In addition, meaningful bench testing is an essential part of design and the development of materials and lubricants. Without the development of more meaningful tests, major advances in complex mechanical systems will continue to be h a m p e r e d by developmental risks. The root cause of the problem lies in the ability to capture lubrication m e c h a n i s m s in design a n d to assure the working of these mechanisms in service. It is essential that tribology mechanisms be assured prior to the execution of fabrication and performance testing of the final system. As part of the design process, tribology issues must be addressed and validated u p front. Compared to other engineering and design disciplines, the level of capability for tribology design a n d testing is primitive. There is' a general lack of appreciation for the multitude of lubrication a n d failure mechanisms that control durability at the contact interface of major mechaniccil systems. In the aerospace bearing industry, material enhancements for surface fatigue are frequently accompanied by a n offset in abrasive or adhesive wear. High thermal stability oils frequently come at the expense of wear and scuffing performance. Unknowingly, when one tribology attribute is sacrificed for a gain in another, the net result can be disastrous. It is recognized at the outset that when chemical and material interactions are in the equation for life and durability, engineering design for bearings, gCctrs, and other mechanical components is not a rigorous or high precision process. It is further recognized that, while tribology mechanisms operate on a micro-scale, we m u s t cope with the r e q u i r e m e n t of translating micro-scale processes into engineering scale parameters. The path to success is through mechanistic testing that captures an empirical link to design and performance testing. This process is called "Systematic Tribology." Systematic Tribology treats the tribo-contact as a system of technologies, which can be systematically developed to capture tribology mechanisms that are assured to provide durability in the application system.. The goal of Systematic Tribology is to leave no attribute behind.
In this chapter, the Systematic Tribology theme is illustrated by highly stressed Hertzian contacts commonly represented by rolling element bearings, gears, and other contacts with non-conformal geometry. The sections from Dynamic Mechanisms of the Tribo-System through Generalized Performance Map identify the key tribology parameters that control lubrication and failure mechanisms and allow linkage to service performance. The sections from Simulation of Gear Lubrication with Systematic Tribology Testing through Load Capacity Database for Qualified Aviation Gas Turbine Oils illustrate the bench test modeling process with the simulation of lubrication and failure mechanisms in a gear mesh.
THE DYNAMIC MECHANISMS OF THE TRIBO-SYSTEM The performance of a tribo-system contact is derived from the integrity of four general regions shown in Fig. 1. Each region is required to perform specific functions with respect to lubrication and failure mechanisms. The output of the tribosystem depends upon h o w well lubrication mechanisms handle the normal stress a n d the accommodation of tangential shear within these regions. The general features of the structured elements of a contact are given below. Hydrodynamic
Surface
Film
Region
The surface film region contains the thin outer layers of the surface. They may consist of surface oxides, adsorbed films, ctnd chemical reaction films derived from the lubricant and
963 2003 by A S I M International
Region
The formation of an oil film between bodies in contact is a structural element that is dynamically generated. Its creation is a function of motion, which generates a pressure within a viscous fluid. Hydrodynamic films generated within conformal contacts may be tens of microns (/Am) thick. Hydrodynamic or elastohydrodynamic (EHD) films generated in nonconforming contacts m a y be on the order of 1 fim thick. On a global scale, the EHD film is derived from the hydrodynamic pressure generated in the inlet region of the contact. On a local or asperity scale, hydrodynamic pressure is derived from the micro-EHD lubrication action associated with the local topographic features of the surfaces. These microEHD films are typically m u c h less than 1 /am thick. Since the performance benefit of EHD film generation is enormous, the philosophy of the tribo-system design is to assure operation of hydrodynamic a n d EHD mechanisms.
' Wedeven Associates, Inc., 5072 West Chester Pike, Edgmont, PA 19028-0646.
Copyright'
Film
www.astm.org
964 MANUAL 3 7; FUELS AND LUBRICANTS HANDBOOK Dynamic Interactions of a Tribo-system
I
TritwttYStem Hydrodi/nainio fflm
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Trtbo-a«gtein m^chpnisma atemm^chpnisma Viscous film generation SurfacA filiti formation Hsap«urbce accommodation AdheslDii', chemical reaction PMSlie fldvi^ Fatigue
Output
I
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Tribology Induced changes In structural elements
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FIG. 1—Structural elements and dynamic interactions of a tribo-system.
its additives. These surface films Eire Ejmost always less than 1 /u-m thick. While the surface film region is extremely thin, its formation by mechanisms of surface forces and chemical reactions has a profound effect on surface lubrication. In highly stressed long-life mechanical systems, surface films maintciin the integrity of the surface topography to promote hydrodjTiamic jtnd EHD mechanisms. Near-Surface
Region
The near-surface region contains the inner layers of the surface. During operation this region may include a finely structured and highly worked or mechanicEilly mixed layer. It may also include compacted wear debris or transferred material from a mating surface. The deformed layers, which are of a different microstructure than the matericJ below them, may arise from surface preparation techniques such as grinding and honing. They may EJSO be induced during operation; for example, during run-in. Hardness and residual stress may vary significantly in this region. They may also be substantially different from the bulk material below. The nearsurface region may be on the order of 50 ^im below the surface. For life and durability, the mechanistic processes in this region are designed to provide near-surface accommodation of tangenticJ stress without loss of surface integrity. Subsurface
Region
The subsurface region is particuleirly important for highly stressed concentrated contacts. The subsurface region can be on the order of 50-1000 /tm below the surface. This region is not significantly affected by mechanical processes that produce the surface or by the asperity-induced changes that occur during operation. Material microstructure and hardness may still be different from the bulk material below it, and sig-
nificant residual stresses may still be present. These stresses and microstructures, however, are the result of macro processes such as heat treatment, surface hardening, and forging. For typical Hertzian contact pressures a n d neglecting asperity pressures, the m a x i m u m shear stress is located within the subsurface region (see Fig. 1). In other words, the detrimental global contact stresses are c o m m u n i c a t e d to t h e subsurface region w h e r e subsurface-initiated fatigue commences. The above structured regions define the operating environment for lubrication and failure mechanisms. They EJSO form the ingredients by which surface life and durability is determined. Since some of the structural regions are self-generated within the contact, it is clear that the structural regions constitute a dynamic system, as shown in Fig. 1. When one considers the practical construction of this dynamic system, we find the wide-ranging businesses of mechanical and chemical technology intimately linked. The lubricated contact is a "tribo-system" where everything is joined together and operated in a dynamic fashion. The tribo-system is a melting pot for many commercial contributions involving materials, surface treatments, lubricating fluids, additives, equipment designs, and manufacturing. What is created in the tribo-system during operation can be m u c h different from the technologies assembled in the structural regions at the outset. The dynamics of this interdisciplinary tribo-system are generally not adequately known. This state of affairs makes it difficult to design for enhanced performance a n d to assure success in service. Tribo-system development is t5^ically a disjointed effort in developing mechanical system technology. This is a c o m m o n problem among non-integrated businesses and disciplines, which supply matericds and lubricants
CHAPTER 36: BENCH TEST MODELING into the interdisciplinary field of tribology. Clearly the assembly of new tribology technology has not teiken advantage of the potential synergism that can be obtained by constructing load-carrying materials, fluids, and designs as a system, which operates on amazingly powerful mechEinisms. In view of the complexity of these mechanisms, particularly the interactions between surface films and the near-surface region, the Systematic Tribology process relies on simulation testing of the mechanisms that control performance in service.
TRIBO-SYSTEM SIMULATION Tribo-system tests generally have two practical purposes: (1) they provide fundamental material property data as well as mechanistic understanding and (2) they are used to predict performance in service. For any test to have practical value, there must be a clear link between testing and hardware performance. The challenge associated with this task is due to the complexity of p e r f o r m a n c e m e c h a n i s m s in a tribosystem. An effective linkage between testing and hardware performance can only be obtained with suitable simulation of the mechanistic processes that control performance. In most cases, it is more important to simulate lubrication and failure mechanisms than it is to simulate the mechanical parameters of loads and speeds of component hardware. An important ingredient in test simulation is the pathway to failure. Performance limits of bearings, gears, a n d other component hardware are defined as much by the lubrication and deterioration mechanisms on the way to failure as by the final failure mode itself.
Key Parameters Controlling Lubrication and Failure M e c h a n i s m s The key to unlocking the mysteries behind lubrication and failure mechanisms is the identification of the functional regions of a lubricated contact, as shown in Fig. 2. The EHD contact can be divided into three regions. The inlet region is the convergent inlet section upstream of the high pressure Hertzian contact region. The inlet region is the functioned region for EHD pressure generation, which has rigorous math-
ematical foundations for predicting film thickness within the Hertzian region. The dynamic motions within the inlet region p u m p s the film up. The Hertzian region rides the film, and the exit region discharges the film. The decoupling of these functional regions is the key feature behind simulation testing machines and testing methods [1]. Oil, with its viscosity and pressure-viscosity properties within the convergent inlet region, controls EHD film thickness in a reliable cind predictable fashion. The shear and traction of the pressurized film within the Hertzian contact region gives rise to heat generation, which is dissipated within the contacting bodies. For surface life and durability, the preservation of the EHD or micro-EHD mechanism is essential. The primary purpose of oil chemical attributes is to preserve the EHD m e c h a n i s m s as m u c h as possible. Loss of "surface integrity" to accommodate the EHD mechanism is the first step toward wear and scuffing failure mechanisms. While EHD mechanisms are essential for life and durability, chemical attributes for boundary lubrication mechanisms are equeJly essential. Chemical attributes play a critical role in preserving surface integrity. The processes of physiceil adsorption and chemical reaction heal and protect the surfaces against local adhesion and disruption of the surface topography. Tribo-contact systems, which have oil properties for EHD film-forming capability and chemical attributes for surface film-forming ability working together, cein achieve remarkable levels of durability. The decoupling of the inlet region EHD lubrication functions from the Hertzian region boundary and micro-EHD mechEinisms provides a means to identify the key parameters associated with lubrication mechanisms with failure mechanisms. Failure mechanisms of wear, scuffing, and surface initiated fatigue, are the result of micro-scale events associated with roughness features, as illustrated in Fig. 3. The level of EHD film thickness relative to surface roughness height determines the normal stress at asperity sites. If roughness features do not "run-in" or accommodate local stress by plastic flow, high normal stresses with repeated contact cycles can result in surface-initiated fatigue or "frosting." Since EHD film thickness is a function of viscous properties in the inlet region and the entraining velocity (Ug), the entraining velocity becomes a key test parameter.
^PRESSURE
0.71 „,0.57 i-)0.40
(^oUe)""a"^^R' hm= 3.07
ijfh-
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965
£,0.03 ^ 0 . 1 1
hm = min. film thickness Ho = viscosity at atm press Ue = entraining velocity, /4(ui + U2) a = pressure-viscosity coefficient R = combined radius of curvature E' = combined elastic modulus w = applied load
FIG. 2—Functional regions of an EHD contact.
966 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
EHD pressure
Entraining
Lubr mechanisms Surface film EHD Micro-EHD Failure mechanisms Wear Scuffing Fatigue
Sliding FIG. 3—Entraining veiocity affects asperity normal stress; sliding velocity affects tangential shear.
The sliding velocity within the Hertzian region determines the tangential strain at asperity sites. The tangential stress at these sites is directly connected to the frictional conditions created at asperity sites. The chemical or physical processes that occur at the local interface between roughness features determine the frictional conditions. These processes can be highly sensitive to local temperatures. Since sliding velocity controls the strain at asperity sites, as well as frictional heating, sliding velocity becomes a key parameter. The material response to normal and tangential stress and its reaction with oil chemistry will determine the outcome on asperity encounters. High normal stresses at asperity sites can lead to a distressed or frosted surface deterioration. Tangential stress and strain at asperity sites can lead to polishing wear, "corrosive" wear, or adhesive wear. Tangential stress and strain under high-speed conditions, and without sufficient chemical response at the interface, can lead to scuffing failure. In some contact situations, wear, scuffing, and fatigue may occur simultaneously. The dominant failure mode will depend upon the key parameters. With respect to lubrication and failure mechanisms for a given tribo-contact system, the key performance parameters are the entraining velocity, Ue, the sliding velocity. Us, and temperature. Temperature can be viewed as the bulk temperature (Tb) of the contacting bodies and contact temperature (Tc) within the contact itself. The bulk temperature controls the viscous properties of the fluid in the inlet region for EHD film thickness. The contact temperature controls the bulk fluid traction coefficient in the Hertzian region. The contact temperature is also a major contributor to oil and material chemical reactivity within the Hertzian region.
It is recognized that contact load or stress is an important engineering parameter. Contact stress is certainly important with respect to subsurface initiated fatigue, where material crack initiation is directly related to shear stresses below the surface. From a tribological perspective of processes at the surface interface, particularly under highspeed lubricated conditions, load is translated into contact size, heat generation, and to some degree, asperity stress and strain. While engineers like to work in terms of load and stress, the critical phenomena within the contact is really seen to a greater degree as temperature or tangential stress and strain. Theories have gone a long way in prediction of EHD film thickness including micro-EHD mechanisms associated with interaction of roughness features. These modeling activities support theories of rolling contact fatigue, where stresses and strains within the material or at the surfaces are used to predict fatigue initiation. What is missing from these theories is the chemical and physical boundary lubricating mechanisms that control surface topography, friction phenomena and the strength properties of the surface and near-surface material. While surface analytical tools can probe the chemical elements of the surface, little is known about the shear strength of the interfacial material under stress. The only way forward is to conduct tests for surface durability under service-like simulated conditions. These tests are essential for oil and material development as well as for oil qualification. To make tests relevant to service performance, the key parameters and their domain of operation must be understood. The lubrication and failure mechanisms that these key parameters invoke must then be properly simulated.
CHAPTER 36: BENCH TEST MODELING WAM Test Machine Technology Wedeven Associates, Inc. machine (WAM) technology provides a highly flexible tribology testing environment. Machines like that shown in Fig. 4 are used to conduct tribology research in Europe and the U.S. by major bearing companies and other research organizations. These machines are capable of controlling the entraining velocity, sliding velocity, and bulk temperature of the specimens independently. Ball and disk test specimens are used to create a simulated contact system. Custom designed crowned rollers are used in place of ball specimens when special materials or surface finishes are required. The relative position of the specimens can be arranged to change the orientation of the surface velocity vectors of the two specimens at the contact point. This feature provides in-
dependent control of the entraining velocity and sliding velocity. The spindles are said to be aligned when the velocity vector of the ball is coUinear with the velocity vector of the disk at the point of contact. To obtain pure rolling across the contact interface, the axis of the ball is tilted. The test machine allows the direction of the velocity vectors to be changed by moving away from the aligned position. The angle Z between the ball and disk velocity vectors can be continuously varied between 0 and 180°. The entraining velocity is defined as one-hsdf the sum of the ball and disk velocity vectors. The sliding velocity is defined as the vector difference between the ball and disk velocity vectors. The ability to vary surface velocities in direction and magnitude provides a large range of entraining velocities and sliding velocities. The independent control of entraining velocity and sliding velocity allows the formation of EHD film separation
PLATE
VERTICa.L--' LOAD CELL
967
Ball and disc velocity
A:R BEARING LOAD CELL BALL SPINDLE SALi. DISC
Ball angle
0m , 1
I i f ^ i^'^\K\f"'
>
(c) Disc velocity
Entraining velocity
_X
Sliding velocity
FIG. 4—Performance mapping of lubrication attributes and life-limiting surface modes.
968
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
between the surfaces to be m a d e independent of the tangential strain within the contact. The decoupUng of film thickness from teingential strain provides the opportunity to control EHD and boundary lubrication mechanisms along with their failure pathways. The EHD mechanism is a remarkably powerful lubrication feature that is fundamentally linked to oil properties. As long as a minute quantity of fluid is captured in the inlet region of a moving contact, EHD pressure and film thickness is generated. With proper oil supply, EHD film generation is a highly stable process. The separating film has enormous "stiffness." The thickness of the film is precisely determined from the entraining motion of the surfaces, geometry of the contacting bodies, Eind temperature of the fluid in the confined space upstream of the contact. Other t h a n temperature, most of the controlling parameters are readily available. These features make EHD film thickness an obvious tool to control surface separation a n d the failure processes associated with the degree of asperity penetration between surfaces. The following sections describe the use of EHD film thickness with lubricating oils for controlling wear and scuffing modes of failure. M u l t i - d i m e n s i o n a l Oil C h a r a c t e r i z a t i o n (Performance Mapping) The performance of oils and materials can be mapped out by conducting tests over a range of entraining velocities, Ue, (i.e., EHD film thicknesses) and sliding velocities Us. In addition, the mapping of performcince with these parameters can be directly connected to gear and bearing hardware. This linkage is made possible by specific relationships between Ue and Us that exist in bearing emd gear contacts. With this linkage, the characterization of oils and materials can potentially be translated into design and performance prediction. In addition, oil and material characterization tests can b e linked to service hardweire. Performance mapping is illustrated below with two DODL-85734 aviation oils. These oils, which have "load-carrying" or "EP" additives, are used in helicopter transmissions and supersonic aircraft (Concorde) [2]. The tests were carried out u n d e r the following conditions:
The performance m a p s for the DOD-L-85734 oils are shown in Figs. 5 and 6. Each data point marks the Ue and Us operating conditions for a ten-minute test. A solid circle indicates a test that encountered a scuffing event. The two solid circles shown in the performance m a p for oil PE-5-L1705 at R = 100 in./s indicate the scuffing point for two test series. A second test series was r u n because of an incorrect operating position during the first test series, which would be expected to cause an early scuff. The second test series gave a slightly higher scuff point. Each performemce m a p can be divided into three regions: £in EHD region, where the oil separates the surfaces with an EHD film; a mixed-film lubrication region where the interaction of surface features between the surfaces takes place in conjunction with a partial EHD film; and finally, a scuffing or severe wear region, where neither EHD films nor boundary films are sufficient to preserve the integrity of the surfaces. The transition between the EHD region and the mixed-film region in this case was judged by visual evidence of polishing wear of the surface features when observed in a microscope at lOOx magnification. Actually, the unaided eye is more sensitive to local surface polishing because only slight modifications in surface topography are easily detected by the reflection of light from the surface. Since both DODL-85734 oils have essentially the same viscosity, the transition boundary for each oil should b e the SEime. The performance maps show that the transition from the EHD region is not exactly the same for the two oils. This may be due to the chemistry of oil PE-5-L1761, which tends to preserve the topographical features from polishing wear better t h a n oil PE-5-L1705. The most striking feature of the performance maps is the shape and location of the scuffing boundary, which is quite different for each oil. At Ue = 100 in./s, the scuff performance of oil PE-5-L1761 is significantly greater than oil PE-5-L1705. Yet, the latter oil does as well, or better than, the former oil at Ue = 170 in./s. This behavior is investigated below in the sections that focus on traction, wear, and specimen temperature within the performance maps.
Wear and Traction Within Performance Maps Ball specimen Disk specimen
AISI 9310, 13/16 in. dia, Ra = 10 ^lin AISI 9310, 4 in. dia X 0.5 in. thick, Ra = 6 /xin Contact stress 300 Ksi (2.07 GPa), 93 lb load Rolling velocity 30, 100, a n d l 7 0 i n . / s Sliding velocity Increasing from 14 in./s, until scuff Test time 10 min for each set of conditions Oil flow rate Drip feed, 2.7 ml/min Temperature Ball and disk temperatures allowed to increase with frictional heating; specimen temperatures measured with trailing thermocouples Two types of test series: (1) constant U^, deProcedure creasing Ue and (2) constant Ue, increasing Us until scuff for three values of Ue. New specimens are used for each test series. DOD-L-85734 oils PE-5-L1705 and PE-5Test Oils L1761
The performance maps outline the operating conditions that define the boundaries of vsirious lubrication and failure regions. The scuffing boundaries of the two DOD-L-85734 oils are found to b e quite different. If an oil is to be judged by its scuffing performance, we see that its performance cannot be adequately defined by a single test. In addition, the relative performance or load capacity ranking of the two oils varies considerably depending on the selected operating conditions. The ranking of these oils is relative to how they are tested. Likewise, their performance in service will depend on the key operating parameters encountered in service. AdditionaJ insight into the performeince of these oils can be obtained by investigating the wear and traction behavior as the contact transitions into and through the various lubrication and wear regions. The initial traction coefficient of the two oils as they transition from the EHD region into the mixed-film region is on the order of 0.08. This is higher than the maximum traction coefficient of 0.057 found for full-film EHD conditions with smooth surfaces. The higher traction is
300 KSI Stress "HG" 9310 Ball (Ra=10Min), 9310 Disc (Ra=6Min) 600 -
Scuff DOD-L-85734 Oil PE-5-L1761
O
Severe wear and scuffing region
o 400 c
3
' Measured ball temperature
200 Scuff
EHD_regioD - soi50
100
200
150
Ue, Entraining Velocity (in/sec) FIG. 5—Performance Map for DOD-L-85734 oil PE-5-L1761. 700 650 600 O O -W
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550
400
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Bulk temp, of test ball
DOD-L-85734 Oil PE-5-L1705
Severe wear and scuffing
Testing direction
=> 200 150 100
50 0
EHD region 75
100
125
150
Ue, Entraining velocity, in/sec FIG. 6—Performance Map for DOD-L-85734 oil PE-5-L1705
175
200
225
970
MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
due to surface finish, which seems to influence traction even when there is no noticeable wear of the surface features. As the entraining velocity and EHD film decreases, the traction coefficient for oil PE-5-L1761 increases at a more rapid rate than oil PE-5-L1705. The greater increase in traction is due to better preservation of the surface features from wear by the former oil—a behavior found repeatedly, at least when the surface temperatures are not very high. The traction behavior of the DOD-L-85734 oils from the performance map tests decreases with sliding velocity. The traction coefficient decreases due to the increased frictional heating with sliding velocity. The initial traction coefficient at the lowest sliding velocity of 14 in./s is on the order of 0.08, and it is approximately the same for each oil. As discussed above, the maximum traction coefficient for a full EHD film at the same stress, but with smooth surfaces, is 0.057. The boost in traction is due to the contribution from the surface features. The initial traction coefficient is noticeably less for high entraining velocities, which generate thicker EHD films. The initial traction coefficients for each oil are listed below: Initial Traction Coefficient (Ug = 1 4 in./s) Entraining Vel (in./s)
Traction Coefficient PE-5-L1705
PE-5-L1761
30 100 170
0.0845 0.0747 0.0739
0.0857 0.0776 0.0744
These results show that the traction coefficient decreases with entraining velocity because of its direct connection with EHD film thickness. Yet, the traction coefficient at Ue = 170 in./s, which is on the order of 0.074, is still greater than that measured for smooth surfaces, which is on the order of 0.057. It is quite remarkable that surface features can have a significant impact on traction coefficient, even when there is no visible evidence of wear on the surface features. The boost in traction is assumed to be due to micro-EHD pressure generation at loccJ asperity sites when there is relative motion between the surfaces due to the sliding component. It is also evident that the traction coefficients for oil PE-5-L1761 are greater than for oil PE-5-L1705. This is due to more polishing wear of the surface features with the latter oil. These results show that traction is sensitive to the tribological interaction of surface features at various locations within a performance map. Traction is a sensitive measure of the interfacial conditions within the lubricated contact. From a testing perspective, traction coefficient is the "heartbeat" of the contact. As the sliding velocity increases, the traction coefficient decreases due to frictional heating of the oil film, which lowers the limiting shear strength of the pseudo-solid oil in the contact. The impact of this thermal phenomenon on traction does not seem to be offset by the increase in traction that one would anticipate as more asperity interaction occurs with decreased surface separation. The onset of a scuffing event frequently occurs when the traction coefficient is no longer reduced by the sliding velocity. This may indicate that a smaller portion of the contact is being sheared by an oil film, and a greater portion is being sheared by surface films or the plastic flow of near-surface material. In all tests, the traction
coefficient for oil PE-5-L1705 is less than oil PE-5-L1761. The two oils show consistently different traction behavior, although their differences may be diminished if the topographical run-in of the surfaces proceeds to a similar level for each oil. One could argue that with sufficiently long running time, the two oils may eventually reach identical traction coefficients. The difference in traction behavior and heat generation between these oils in field hardware may only have significance during the initial operating life or during an event where the surfaces have not completed the run-in process. This may explain why difficulties during green-run of new propulsion hardware, particularly gearboxes, are sensitive to the particular oil selected. In any case, the data suggest a strong association between traction coefficient and surface finish. This association can be illustrated with the help of Figs. 7-9, which are plots of the visible wear track width on the test ball following each 10min test at a selected sliding velocity. Wear is a qualitative measure, judged by the width (measured in mm) of the visible polishing wear of the surface topographical features on the ball. In all cases, oil PE-5-L-1705 shows more polishing wear on the ball than oil PE-5-L1761. If the wear data is compared with the traction data, the greatest traction differences between the oils tend to occur when they have the greatest difference in polishing wear. Lubrication Regions and Thermal Considerations The above results with Performance Mapping show that precision control of the EHD film separation between surfaces with the entraining velocity Ue can be used to map scuffing performance over a range of conditions. A Performance Map goes beyond the identification of a scuffing boundary. Performance Maps identify regions of safe EHD lubrication, mixed-film lubrication region and a region of severe wear or scuffing. These regions are characterized by traction coefficient, a key parameter for monitoring the health and welfare of the contact. While the entraining velocity creates the EHD separation between the surfaces, the response to this separation when asperity interactions occur is decided by oil chemistry. Oil chemistry can significantly affect wear and scuffing within the domain of a Performance Map, as demonstrated by the two DOD-L-85734 oils PE-5-L1705 and PE-5-L1761. These differences are reflected in traction coefficient, which is a function of the shear strength of the bulk oil within the contact, the degree of asperity interaction and the shear strength of surface films. Since traction coefficient is directly connected to heat generation, the temperature within the contact and surrounding surfaces is linked to the tribological phenomena created within the contact. Temperature isotherms are shown in the Performance Maps in Figs. 5 and 6. Oil PE-5-L1761 generally runs hotter than PE-5-L1705. Since temperature affects chemiccJ reaction rates, the preservation or wear of surface features and how they affect traction are intimately linked to how the oil performs. This dynamic action among surface asperity interaction, wear, traction coefficient and temperature leads one to conclude that single point testing to characterize oil for service is not likely to be sufficient in order to assure performance. Performance Mapping, covering the domain of operating conditions in service, is a more reliable approach.
CHAPTER 36: BENCH TEST MODELING
Rolling Velocity = 30 in/sec Ball: 9310, Ra = 10 (j-inch Disc: 9310, Ra = 6 jj-inch Stress = 300 KSI
0.5 0.4 0.3 50
100
150
200
250
300
350
400
450
500
550
600
650
700
Sliding Velocity, in/sec FIG. 7—Wear track width versus sliding velocity for two DOD-L-85734 oils (Ue = 30 inVs).
-i
1
1
1
1
^
I
I
1.3 1.2
Scuff
Ball: 9310, Ra = 10 M-inch Disc: 9310, Ra = 6 M-inch Stress = 300 KSI
Rolling Velocity = 100 In/sec
1.1
0.5 0.4 0.3 0
50
100
150
200
250
300
350
400
450
500
550
600
650
700
Sliding Velocity, in/sec FIG. 8—Wear track width versus sliding velocity for two DOD-L-85734 oils (Ue = 100 ln./s).
971
972 MANUAL 37: FUELS AND LUBRICANTS 1.3
HANDBOOK
Rolling Velocity = 170 in/sec
1.2
Ball: 9310, Ra = 10 M-inch Disc: 9310, Ra = 6 M-inch Stress = 300 KSI
1.1
0.5 0.4 0.3 0
50
100
150
200
250
300
350
400
450
500
550
600
650
700
Sliding Velocity, in/sec FIG. 9—Track width versus sliding velocity for two DOD-L-85734 oils (Ue = 170 inJs).
Linkage B e t w e e n Bearing and Gear Contacts and Performance Maps The importance of the entraining velocity with respect to its control over EHD oil film thickness and the degree of asperity interaction has been shown. Once some asperity interaction is initiated, the above sections show the importance of the relative sliding velocity, U^, across the interface. Low sliding velocities result in wear of roughness features according to how the chemistry of the oil responds to the severity of the interaction. High sliding velocities are required to invoke scuffing failures. The parameters Ue and Us, along with temperature, control the lubrication and failure mechanisms within a testing environment. Also, Ue and Us control lubrication and failure mechanisms in service hardware. With proper replication of thermal and surface conditions, a linkage can be made between performance testing and lubricated contacts in service. Every contact in service can be characterized with respect to the distribution of entraining velocities and sliding velocities within the contact. The range and magnitude of sliding velocities within a gear contact can be high. By contrast, rolling element bearings operate under micro-slip conditions. In either case, the kinematic conditions of service hardware can be linked to Performance Maps as illustrated in Figs. 10 and 11. High sliding velocities near the root or tip of gear teeth are vulnerable to scuffing failure, as illustrated in the Performance Map in Fig. 10. The low slip operation in rolling element bearings is more vulnerable to wear and surface initiated fatigue failures. The initiation of surface-initiated fatigue under low slip conditions is discussed in the next section.
Impact of Oil Attributes on Surface Fatigue Following initial engine or gearbox operation, the long-term durability of rolling element bearings and gears is genercJly a surface distress mode of failure. Surface distress is used as a generic term. It can include several surface deterioration modes of failure. The surface features produced by surface distress are usually characterized with descriptive terms such as: peeling, frosting, micro-cracking, micro-pitting, pitting, gray staining and wear. The gear and bearing communities will frequently use different descriptive terms to characterize the same surface phenomena. The surface distress features formed are precursors to major life limiting failures, such as spalling or excessive wear. Failure modes by surface initiated fatigue can be interactive and competitive with failure modes of wear and scuffing. One of the characteristic features of surface distress for bearings and gears is that it is primarily controlled by a surface fatigue process. As such, the normal stress and tangential (friction) stress cycles that a surface must encounter are important. At least part of the tangential stress that initiates surface distress is due to the interaction of surface features under low film thickness (h) operation, where the average surface roughness height (a) is of the same order of magnitude as h. Low hJcr operation also causes wear, especially under non pure-rolling contact conditions. The process of initiating micro-cracks at a surface is frequently in competition, or at least interactive, with wear processes. If the surfaces wear at a faster rate than micro-cracks can initiate and propagate, no micro-pitting will occur. For this reason, it is wise to consider the fatigue processes asso-
CHAPTER 36: BENCH TEST MODELING 12
1
r
OU: 4 cSt, Herco-A 10
Ball specimen: M50, Ri Disc specimen: M50, Ri
Sliding Velocity, e Us (m/sec)
cS.
^
, ^ » : #
Entraining velocity, Ue (m/sec) FIG. 10—Linkage between Performance Map and gear contacts.
600 500 400
Oil: 4 cSt, Herco-A Stress: 300 ksi (2.07 GPa) Ball: M50, Ra = 10 pin (0.25 fun) Disc: M50, Ra = 3 jUn (0.075 mn)
Sliding °° Velocity, ^^ Us (in/sec) 100
-100
-200
Entraining velocity, Ue (in/sec) FIG. 11—Linkage between Performance Map and roliing element bearing contact.
#
973
974
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
ciated with surface distress under low h/cr operation along with wear processes. Because surface interaction under low h/cr operation can produce nascent surfaces and high local temperatures, it is not unexpected to find oil chemistry playing an important role in surface distress as well as bearing and gear life. There is sound evidence that good oil chemistry for scuffing can sometimes be detrimental to surface distress modes of failure. The user of an oil product would feel most comfortable if he could be assured of its performance with respect to three failure modes: (1) scuffing, (2) wear, and (3) surface distress (fatigue). A meaningful screening test for all three is an important goal. Exploratory tests have been conducted to show how the utilization of entraining velocity and sliding velocity can be used to introduce surface-initiated fatigue damage [3]. The test m e t h o d includes surface deterioration for b o t h mild wear and micro-pitting. The results give support to a testing approach that can screen oils for surface distress performance in a relatively short period of test time.
must exist a sufficient local surface stress to initiate cyclic fatigue. The formation of micro-cracks may be a low cycle or high cycle fatigue process, depending on the severity of the local stresses produced. The severity of loccJ stresses is controlled by the ratio of h/cr. However, the h/cr ratio is not a sufficient parameter to control the details of the local stresses within a lubricated contact. The average roughness height (a) does not reflect the shape of surface features that control asperity pressure. Peak asperity pressures depend on the slope of asperities near their summits. One also has to consider asperity size relative to the Hertzian contact width and the size of the inlet region, which generates the EHD film thickness. A stylus trace of the surface finish generated for surface distress tests is shown in Fig. 12. The disk surface has a roughness of 24 ^lin, Ra as measured by a stylus instrument. Figure 12 also includes a calculation of the EHD film thickness (h) relative to the combined surface roughness height (cr). A 5 cSt oil operating at 180°C under the prescribed test conditions gives a calculated h/cr of 0.10. The relatively rough surfaces of the specimens are designed to accelerate the surface distress process so that micro-pitting can be achieved within a relatively short testing period.
Testing for Surface Distress Under Incipient Sliding WAM tests for surface distress were run with a re-circulating oil flow within a heated test chamber. To engage a surface distress failure mode, the contact is operated in a low h/cr regime. Operating conditions were selected to invoke oil chemistry during the process of generating surface distress features. The tests include the following features: 1. M50 steel test specimens to focus oil evaluation toward bearing surface-initiated failure 2. Simulation of turbine engine bearing temperatures at approximately 180°C 3. Operation n e a r pure-rolling to simulate rolling element bearing contacts and pitch line conditions for gears 4. Operation at high rolling velocities to introduce as many cycles as possible 5. Introduction of surface distress by high "asperity" contact pressures instead of high Hertzian contact stress 6. Initiation of each surface distress test with a run-in period to promote repeatability Tests are run under low h/cr conditions with a disk surface texture that causes high local stresses at grinding ridges to induce micro-cracking and micro-pitting. A run-in test stage is used with a new M50 ball. When the run-in is completed, the micro-pitting test is initiated with the following conditions: Mat'ls: Load: Temp: Spd: Time: Analysis:
M50 ball with "hard grind" finish vs M50 disk (Ra = 24 fiin) 120 lbs. (apply load slowly, from 60 lbs to 120 lbs; 10 lbs every min) Initiate test at 100°C and work u p to 180°C 300 in./s (selected to give an h/cr < 0.3), 2% slip 120 min. Traction and temperature plots, measure wear, p h o t o m i c r o g r a p h s of surfaces and count of micro-pits
To create surface distress in the form of micro-cracking and micro-pitting within a reasonable period of time, there
Features of Surface Distress Testing With the above test protocol, typical surface distress in the form of micro-pitting after approximately 4 h of testing is shown in Fig. 13. Longer running times increase the n u m b e r of micro-pits as well as their size. The micro-pits are on the order of 30-60 /xm in diameter. The ball surface in Fig. 13a is from a test conducted with a 4 cSt polyol ester oil, Herco-A, with n o additives. The surface contains gray areas in addition to micro-pits. The gray appearance of the surface may be the result of oxide formation. We have noticed that areas with gray staining seem to have less wear, but more micro-cracking and micro-pitting. This may be due to the frictional behavior of the oxide film or its susceptibility to micro-cracks. The ball surface in Fig. I3b is from a test conducted with a 9 cSt gearbox oil (PE-9-G0041) with a load-carrying additive package. The gearbox oil results in less wear but approximately the same amount of micro-pitting as Herco-A. A phen o m e n o n of "gray staining" is a surface distress feature sometimes observed in field hardweire. The test results can be examined in terms of wear and micro-pitting. Wear and micro-pitting results cire plotted in Fig. 14 for three oils. They include the 4 cSt oil, Herco-A, the 9 cSt gearbox oil PE-9-0041 and a 5 cSt corrosion inhibited oil PE5-L1785. Wear is a qualitative m e a s u r e m e n t obtained by measuring the polished track width on the ball and dividing the track width by the Hertzian contact width. Micro-pitting is measured by estimating the n u m b e r of pits on the running track of the ball. Some of the data points in Fig. 14 are from tests conducted for 2 h (Condition 3.2), and some of the tests have been run for 4 h. The data in Fig. 14 show that it is possible to have a 9 cSt gearbox oil with a load-carrying additive result in more micro-pitting than a 5 cSt corrosion inhibited oil. On the other hand, the gearbox oil shows less wear. The non-formulated 4 cSt oil, Herco-A, shows higher wear and micro-pitting compared to the formulated oils.
CHAPTER 36: BENCH TEST MODELING
975
M50 ball, Ra = 0.25nm (10 nin) }4-100.0 itin +50.0 |iin
^ ^ j ^ ^
^J«^ V H I * ^ ' * . * ^
r-
0.0 Itin -50.0 iiin
I-
•I Hertzian contact width
M50 disc, Ra = 0.60nm (24 ^iln)
/^VM^Ww' Contact stress: Entraining velocity: Contact slip: Temperature: h/a (5 cSt oil):
2.25 GPa (327 ksl) 7.62 m/sec (300 in/sec) 2% 180°C(356°F) 0.10
FIG. 12—Surface roughness for surface initiated fatigue tests.
Testing for Surface Distress Under Gross Sliding
FIG. 13—Wear and micro-pitting with surface initiated fatigue protocol, a) 2hour fatigue initiation test with 4.5 cSt non-formulated oil, Herco-A; b) 2-hour fatigue initiation test with 9 cSt gearbox oil, PE-9-G0041.
The micro-pitting tests discussed above were conducted under conditions with 2% micro-sUp. Surface distress, particularly for gear contacts, encounter a range of sliding that goes well beyond incipient sliding found in rolling element besirings. An increase in the sliding component of velocity promotes wear and generates more frictional heat. An accelerated test protocol for surface distress with gross sliding is shown in Fig. 15a and 15b. The test protocol continuously decreases the entraining velocity and increases the applied load. The entraining velocity provides a continued reduction in EHD film thickness. As the surface features lose their geometric sharpness due to wear, the load is increased to continue the application of local stress at each roughness feature. With independent control of the entraining and sliding velocities, the test can be run with a near-constant or decreasing sliding velocity so that the frictional heat generation within the contact can be maintained at a desired level. The test protocol can be completed in approximately one hour. With properly selected test conditions, this approach causes the surfaces to wear without initiation of scuffing. The degree of contact severity can be continually increased by running multiple tests at progressively increasing temperatures. The final operating temperature can be selected to correspond to the operating temperature of the component hardware of interest. The traction data for three tests run at ambient temperature, 50°C and 90°C, are shown in Fig. I5b. The traction coefficient progressively increases at each incremental change
976
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
Surface Distress Tests Wear and Micropitting: Conditions 5000 Each ball was run for a total 4 hrs under severe surface distress test conditions. Open symbols are results after 2 hrs..
PE-9-G0041 (Load additive) Condition 3.2 + 2 hrs EX482
4500 4000
1 o 6
3500 3000
1
2500
1
1500
Heroo-A (Basestocit) y^ Condition 3.2 \ / EX429
PE-9-G0041 (Load additive) Condition 3.2 + 2 hrs EX480
2000
A
1000
500
Condition 3.2 EX470 ^
PE-5-L1785 (Anti-wear additive) Condition 3.2 + 2 lirs EX483 PE-6-L1785 (Anti-wear additive) Condition 3.2 + 2 tire EX479
Condition 3.2 EX472
I
1.0
1.1
1.2
1.3
1.4
1.5
1.6
Wear (Tracic widtii/Hertz width) FIG. 14—Relationship between micro-pitting and wear for three types of oils
of entraining velocity and load. Repeating the test protocol at a higher temperature increases the surface traction further. If the operating conditions are properly selected, the final set of conditions can introduce micro-pitting within a few hours of testing. The photomicrograph in Fig. 15a shows transverse micro-cracks and micro-pits on the ball specimen following 1 h of testing. The test protocol above allows at least one surface to lose its original surface features by a mild wear process before the onset of micro-pitting. This is a common scenario found in the areas of high sliding velocity on gear teeth. The high sliding eu"eas are vulnerable to wear and scuffing. This is a more difficult simulation than the initiation of micro-pitting under incipient sliding, where surface roughness features are much less affected by wear. The latter situation is more often found near the pitch line of spur gears, where the sliding velocities are low. Because of the close interaction between wear and micro-pitting, simulation testing must consider wear and micro-pitting together. Since the rates of wear and micropitting processes frequently vary, both of these pjirameters should be monitored over time. Other Types of Surface-Initiated Distress The above tests demonstrate surface fatigue-initiated distress due to two types of contact conditions: (1) high local asperity stresses under incipient sliding conditions and (2) high as-
perity stress accompanied by tangential stress under gross sliding conditions. While the latter promotes wear mechanisms, they both invoke a material fatigue process. Tribology mechanisms, which alter asperity stress, wear and nearsurface properties, certainly enter the equation of fatigue life. At least two other surface-initiated fatigue mechanisms can also be operative. Surface-initiated distress can be influenced by corrosive reactions under high stress contact conditions. In addition, micro-pitting can be initiated at local sites that have accumulated micro-scuffing damage. This is frequently found in gears following initial operation with relatively rough surfaces. Figure 16 illustrates at least four mechanisms that can cause surface distress involving fatigue. One is due to high normal stresses at asperity sites caused by low h/cr operation under incipient sliding conditions. Oil performance with respect to EHD film thickness and wear resistance is expected to be a significant factor. A second mechanism is due to high local traction under low h/cr conditions. Oil attributes associated with surface film protection with low friction are expected to be significant. A third mechanism is associated with chemical attack. Chemical attack can create corrosion pits. It also influences wear and crack initiation or propagation. Finally, a fourth possible mechanism is associated with bre£ik-in damage, where superficial micro-cracks or microscuffs during initial running create initiation sites for subsequent fatigue propagation.
CHAPTER 36: BENCH TEST MODELING
primary parameter for load capacity tests, sometimes plays a secondary role for tests to invoke surface-initiated distress. Intrinsic in the test parameters that control surface separation are the oil properties that control EHD film generation. These are viscosity and pressure-viscosity coefficient. These fundamental properties of the oil are controlled within the inlet region by the bulk temperature of the contacting surfaces. Depending upon the particular operating parameters within the contact, the above tests show that a number of surface distress features can be initiated. Inherently, the nature of
Discussion of Micro-pitting Results Surface distress, which covers a variety of surface damage— including fatigue processes, has been demonstrated by controUing the normal stresses and tangential strain within the contact. These stresses and strains tend to be concentrated locally on the upper-most surface features of the surfaces. For asperity interaction, surface separation is precisely controlled with the entraining velocity. Tangential strain is controlled with the sliding velocity. Contact load, which is a
lOOx
(a) Traction During WAM3 Wear/Micro-pitting Tests
500
1000
1S0O
977
2000
2S00
3000
3SO0
4000
Run Time (seconds)
(b) FIG. 15—Test protocol for micro-pitting and micro-cracking, a) Transverse cracks and micro-pitting; b) Traction results from wear and micro-pitting test protocol.
978 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK
CHAPTER surface distress and the stochastic processes of fatigue mzike surface-initiated failure processes complex. A flexible test machine can be programmed to provide a controlled decrease in h/crin order to cause micro-pitting and wear within a few hours of testing. The degree of test acceleration, as well as the sequence of wecir and micro-pitting events along the p a t h w a y t o w a r d failure, can be easily changed. In most cases, due to the mystery behind what actucdly tremspires in real hardware, the practical relevEuice of simulation testing for micro-pitting may be questionable. To obtain a short-term screening test with meaningful connection to specific hardware, a good understanding of the wear and surface fatigue process in service hardware is needed. Until controlled data from the field is obtained, there is no assurance that these test methods for surface distress are giving meaningful information for oil formulation, material construction, design or operating condition.
Oil a n d M a t e r i a l T e s t i n g f o r A d h e s i v e W e a r The above sections focused on testing for oil or material attributes with respect to wear, scuffing and surface-initiated fatigue. Traditional testing of oil attributes for weeir, scuffing, and fatigue resistance is less than adequate to fully characterize an oil and to predict performance in service. The same can be said about bearing and gear materials. Traditional bearing material test methods, which generally focus on subsurface-initiated fatigue, do not assure success in component hardware. The whole notion of oil and material testing and development is rccilly intimately linked in the real world. One cannot address oil eveiluation without immediately crossing the boundary into material evaluation. Load, speed, temperature and corrosion protection are driving bearing and gesir materials toward greater rolling contact fatigue life, wear/scuffing resistance, and corrosion resistance. The introduction of hybrid ceramic/metal materials presents new surface chemistry that may be incompatible with conventional lubricating oils. Corrosion resistant bearing materials with different surface chemistry, as the result of chrome oxides instead of iron oxides on the surface, can significantly affect the reaction rates between material and oil chemistry to form surface films. Surface distress tests with some corrosion resistant materials, conducted with low EHD film thickness relative to surface roughness, show that surface fatigue mechanisms ccin be preempted by local adhesion events. The term "adhesive wear" is commonly used when failed surfaces appear to have undergone plastic flow due to local "adhesion" at the interface. Surface failure by adhesive wear can be initiated at microscopic sites of insufficient surface film lubrication or at sites of debris encounters. With limited chemical reactivity between lubricating oil and some corrosion resistant materials, locEil adhesion events, which are not able to recover, propagate into broad patches of adhesive wear damage. In addition, material microstructure and properties, like hardness, that affect plastic flow seem to influence the onset of adhesive wear. With sufficient sliding velocity and contact stress, adhesive wear can transition into a major scuffing event. A scuffing event is characterized by a rapid rise in friction and temperature. These tribological features, as measured with Ein adhesive wear test method, correlate with experience in full-scale bearing tests.
36: BENCH
TEST MODELING
979
At the heart of surface durability is material compatibility with lubricating oil chemistry to form surface films, which prevents local adhesion. The adhesive wear test method described below progressively increases the degree of asperity encounter at the interface u n d e r rolling/sliding conditions. The test method invokes tribological interactions, which are measured in terms of friction (traction), gentle polishing wear of surface features, adhesive wear events, and scuffing. Adhesive
Wear Test
Method
Adhesive wear tests, run with the WAM test machines, use ball and disk specimens representing bearing rolling element and raceway surfaces. Ball specimens are supplied by beeiring vendors. The disk specimens are heat treated and finished to simulate bearing ring specifications. Surface preparation of the disks consists of fine grinding, followed by abrasive lapping to a surface finish of 2 /A-inch, Ra. The lapping operation is done with specimen rotation about its centerline to create a circumferentieJ lay in the direction of rolling motion. The finishing method consists of several lapping stages to obtain a consistent surface texture. Care is taken to avoid microscopic bends and folds ("leafing") in the surface texture. The final finish is similar to a finely honed bearing raceway of aircraft quality. Ball sizes may range from 0.5 in. (12.7 m m ) to 1.125-in. (28.58 m m ) diameter. The selection of ball size and quality is determined by availability. The test ball specimens, which are production quality, have m u c h better surface finish than the disk specimen. Highlights of the test conditions are given below. Maximum Hertzian contact stress: Entraining (rolling) velocity, Ug: Contact slip:
1.95 GPa (282 ksi)
1 0 . 1 6 - 1.27m/sec (400 - 50 in/s) Ue reduced in four stages of 180 s each 15%, subsequent tests 30% and 50% until failure If adhesive wear occurs at 15% slip, 8% slip is run % slip = (Ub - Ud)/ l/2(Ub + Ud) X 100 where Ub = surface velocity of ball Ud = surface velocity of disk MIL-PRF-23699 (Mobil Jet II), reference Test oil; oil Test temperature: 200°C (392°F) An adhesive wear test series for a materieJ generally consists of three tests: one each at 15% slip, 30% slip, and 50% slip. If an adhesive wccir event occurs at 15% slip, the test is repeated at 8% slip. Generally, two test series are run for each material pair. At the end of each test, the running tracks on the ball and disk specimens are photographed at lOOX. Traction coefficient is plotted over time for each test. The surfaces of ball and disk specimens are documented with photomicrographs, which are stored as digital images. At 200°C the disk surface can become lightly discolored with oxide or surface films from the oil. To avoid the influence of accumulated surface chemistry on the disk, the test track on the disk specimen is refinished after each test.
980
MANUAL
Traction
Test
37: FUELS AND LUBRICANTS
HANDBOOK
Plots
Tj^ical traction test plots are shown in Fig. 17. Initial operation at an entraining velocity of 10.16 m/s gives an EHD film thickness to surface roughness ratio (h/cr) close to 1.0 and a traction coefficient on the order of 0.02. Incremental reductions in entraining velocity increase the traction coefficient due to thinner EHD films and greater surface interaction. A gradual decrease in traction coefficient is attributed to polishing wear of surface features. An adhesive wear event is identified by a rapid excursion in traction coefficient. While the traction may recover after a few seconds, the local surface damage that occurs during the adhesive wear event is permanent. The local adhesive wear damage becomes a vulnerable site for surface-initiated fatigue. Ranking
Adhesive
Wear
Resistance
A system of ranking adhesive wear performance has been developed to characterize the overall lubricating performance of various material pairs and lubricants. Performance is based on measured behavior reflecting the ability of the material pair to accommodate low h/cr and high slip operation without adhesive wear and with traction behavior reflecting good lubrication at the interface. A point system is used to give weighting factors for tribology attributes that we feel are important for high-speed rolling element bearings. The criteria and weighting factors for performance ranking are shown in Table 1.
0
Perfoimance is judged relative to a baseline material pair and lubricant of M50/M50 and Mobil Jet II. Mobil Jet II is classified as a Standard (STD) MIL-PRF-23699 oil. The lubricating ability of Mobil Jet II ranks high among typical jet engine oils. The material pair of M50/M50 gives excellent performance for adhesive wear resistance. These baseline materials, which survive the test protocol with no significant surface damage, achieve a score of 50 with the performance criteria discussed above. Other materials and lubricants can be ranked relative to the baseline materials, as shown in Fig. 18.
TABLE 1—Criteria for tribology attributes. Weighting Factor Measured Tribology Attribute No adhesive wear events at 15% slip. 10 No large step increases in traction @ low A 6 (15% slip). No rapid decline in traction coefficient reflecting advanced polishing wear. Survives 8% slip without adhesive wear. Survives 8% slip without major increase in traction. Survives high slip (>15%) without adhesive wear events: 30% slip = 8; 50 % slip = 17 "Steady" traction behavior (no fluctuations due to a struggling contact). Survives without a catastrophic scuff.
100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 Run Time (seconds) FIG. 17—Adhesive wear test protocol and traction results.
CHAPTER
36: BENCH
TEST MODELING
981
M50/M50 M50/M50 MSO/Stainless M50/Stainless M50/Stainless M50/Stainless M50/Stainless High qual. STD Low qual. STD High qual. STD Run-in with Low qual. High qual. HTS NTS brand of IMIL-PRF-23699IHIL-PRF-23699 MIL-PRF-23699 DOD-L-85734 MIL-PRF-23699 MIL-PRF-23699 IMIL-PRF-23699 Operation with High qual. STD
Mafis tested in bearing
/
MIL-PRF-23699 rig
g^u ^lafi/Disc mat'l
Ball diameter: 12.7 mm (G.5 inch)
FIG. 18—Summai^ bar chart of adhesive wear attributes.
Discussion
of Adhesive
Wear
Testing
Some material pairs and lubricants m a y experience adhesive wear problems with this test protocol under low h/a conditions, particularly in the presence of high slide-to-roll ratios. A deficiency in adhesive wesir compared to the baseline materials does not preclude the successful utilization of certain materials and lubricants. The vulnerability of a tribo-system to adhesive wear can be at least partially offset by careful runin, where the presence of chemically active additives and improvement in surface topography condition the surface for durability. However, sudden "events" like skidding or debris rollover, may again make the surfaces vulnerable to adhesive wear. If start/stop cycles are not severe with respect to microslip and contact stress, and if near full-film EHD conditions are prevalent during operation, a material pair deficient in adhesive wecir may operate satisfactorily. On the other hand, a good performance ranking in this test does not assure perform a n c e in service. Successful performance u n d e r the selected test conditions of the test protocol does not guEirantee success u n d e r other conditions. Assessment of the trlbological parameters for the peirticular application, along with simulation testing, is recommended for applications with challenging operating conditions. As a minimum, the test protocol provides a good screening test for materials and lubricants.
Oil Evaluation
with Adhesive
Wear
Testing
Adhesive wear testing can be used to evaluate jet engine oils as well as bearing materials. Bearing materials were evaluated with a reference oil, MIL-PRF-23699 (Mobil Jet II). The data in Fig. 18 shows that improved durability against adhesive wear can be achieved with an initial run-in with a high load-Ccirrying DOD-L-85734 oil. The more reactive chemistry in the DOD-L-85734 oil conditions the surface with protective surface films to avoid adhesion. If follow-on operation after run-in is conducted with MIL-PRF-23699 oil, the run-in surface films may be a temporary benefit. Perhaps the most interesting finding with oil evaluation with the adhesive wear test protocol is that high performing oils in load capacity tests are not always reflected in adhesive wear tests, psirticulzirly when corrosion resisteint materiEils are used. In load capacity tests, standard (STD) oils generally have higher scuff resistance than high thermal stability (HTS) oils. Figure 18 shows that this is not the case for adhesive wear tests. In addition. Fig. 18 shows that the "Low Quality" MIL-PRF-23699 oil, according to scuffing load capacity tests, does better than a "High Quality" MILPRF-23699 oil. This appears to be the case when at least one surface is chkracterized with chromium oxide for corrosion resistance.
982 MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
More recent adhesive wear testing confirms the finding that some low performing oils for scuffing resistance can do quite well in adhesive wear tests. This seems to lead to two conclusions: (1) the effectiveness oil chemistry is dependent upon material surface chemistry and (2) oil attributes for scuffing resistance are not necessarily translated into adhesive wear resistance. The reason for this may be associated contact conditions that affect chemical reactivity. Contact temperature can have a significant effect. Oil-off Testing The adhesive wear testing described above increases the contact severity by reducing the entraining velocity to decrease EHD film thickness. A concern in service hardware is momentary oil interruption or starvation. A "starved" contact is actually a reduction of fluid within the inlet region of the contact to the degree that it affects EHD pressure generation. A starved contact operates with a reduced EHD film thickness according to the degree of starvation. Oil-off testing is a method to evaluate oil attributes for their ability to make surfaces survive or recover with marginal quantities of lubricant. Starvation prevents the entrainment of fresh chemistry into the contact. The sections below describe an oil-off test protocol and test results with the three aviation oils. Oil-off Test Procedure An oil-off test protocol evaluates bearing materials and lubricants for durability and operational life under marginal lubrication conditions sometimes experienced in service. The commonly used aircraft bearing steel M50 is used as a baseline for evaluation purposes. Oil-off tests generally support other tribology tests for material and lubricant performance. Oil-off test conditions are derived from adhesive wear tests discussed above. The test specimens consist of a .5 in. ball specimen (or alternate ball sizes), which rides against a flat disk specimen. The disk specimen represents bearing ring materials. Surface preparation of the disk specimen is the same as adhesive wear tests discussed in the Adhesive Wear Test Method section. An oil-off test protocol for bearing operation was developed with the following test conditions: Ball Disk Contact stress Entraining velocity Contact slip Temperature (disk) Lube supply
M50, .5 in. dia., Grade 5 (or alternate size or Grade) M50, finished to 2 /tin Ra (baseline material and finish) 2.49 GPa (360 ksi) 10.16 m/s (400 in./s) 5% 200 °C (392°F) drip feed until oil-off at 600 s
The test is initiated after the disk specimen temperature stabilizes at 200°C. The first 60 s of the test is operated at or near pure rolling. The remainder of the test is run at 5% slip. A computer-controlled peristaltic pump is shut off after 600 s. To avoid release of residual oil on the bcJl and disk surfaces, tests are conducted with the ball thermocouple, oil shielding material and parts of the heating equipment re-
moved from the test area immediately following oil shut off. Oil-off tests can be conducted with various criteria for test termination. These include: (1) oil-off operation until a rapid rise in traction coefficient to an arbitrary value; (2) oil-off operation until a rise in traction coefficient to 0.15; and (3) oiloff operation for 220 s, which is beyond the time typically required to reach a maximum traction coefficient (—0.4) and the onset of a high wear rate. Except where noted, the test oil is Mobil Jet II. From testing with qualified jet engine oil products, Mobil Jet II has excellent lubricating ability compjired to most oil brands on the qualified products list for MIL-PRF-23699. Typical Oil-off Test Results To establish the role of oil lubricating characteristics during oil-off conditions, two tests were run with a 4 cSt MIL-PRF7808 oil (TEL-0004), which has known difficulties in certain applications. Oil-off tests were also conducted with high load-carrying DOD-L-85734 oil, which is used in aircraft gearboxes and demanding engine applications. Typical oil-off traction behavior for M50/M50 materials and the three oil types are shown in Fig. 19. With continuous oil supply and a test temperature of 200°C, the traction coefficient for a full EHD oil film is on the order of 0.025. A t3rpical traction coefficient for jet engine oil at ambient temperature (~23°C) is on the order of 0.07. The rise in traction coefficient after oil-off is associated with local adhesion and transfer of matericJ from the ridges of the finishing marks on the disk specimen. The local material transfer, smearing, and oxidation creates material pileup and a loss of surface integrity. At this stage the surfaces encounter high local friction events due to material pileup and high friction oxides. The amount of wear is not significant. While the surface disturbance appears to be minor at the time of test termination, the situation is in a run-away condition heading for gross surface failure and a traction coefficient on the order of 0.4. The maximum traction coefficient of 0.4 is attributed to massive adhesion and material transfer between the surfaces. The rise in traction coefficient is associated with the spread of adhesive events across the operating track on the specimens. A momentary departure from an increase in traction is believed to be the result of lower tangenticil shear caused by the onset of wear. These tests show that oil starvation leads to local adhesion and pileup of material on the surfaces with a corresponding rise in traction coefficient. Once this process starts (and without subsequent oil replenishment) adhesion and oxidation of transferred material become a run-away process. The runaway process seems to be well on its way by the time the traction coefficient has increased from 0.025-0.15—a six-fold increase in traction coefficient. Effect of Material or Oil Lubricating
Quality
The above results show that the initiation of run-away traction is due to adhesion and oxide growth at local sites. Resistcmce against adhesion depends upon the material pair, along with protection against removal of surface films that prevent atomic bonding between the surfaces. If this is the case, the important attributes of the material for oil-off performance cire associated with the material's response to chemistry or "tribo-chemistry" of the oil.
CHAPTER 36: BENCH TEST MODELING 983 300
0.40
Ball: M50 (0.5 In. dia) Disc: M50 Entraining Velocity: 400 In/sec. Slip: 5% Temperature: 200°C Stress: 360 ksl
0.35 -
- 250
0.30 1 of 2 heat sources removed
Disc Temperature
200
8 0.25 ^ S 0.20 c .g
150 I
MIL-PRF-7808K (TEL-0004) Gra.de 4
o
Q.
DOD-L-85734
MIL-PRF-23699
S 0.15
O
e 100 •"
PWM53
0.10
PWM55
Lube supply removed @ 600 sec
0.05
50
Traction Coefficient
0.00 1
1
1
1
1
500
r
—I
J
1
j _
550
1
600
1
1
1
1
1
650
1
r
700
Run Time (seconds) FIG. 19—Oil-off test protocol and effect of oil quality on oil-off performance.
The oil-off test results with the three test oils in Fig. 19 are summarized below with respect to the average time to a traction coefficient of 0.15. Test Oil
Average time to traction coefficient of 0.15
TEL-0004 (4 cSt) MIL-PRF-7808 Mobil Jet II (5 cSt) MIL-PRF-23699 DOD-L-85734
10.5 s (2 tests) 42.0 s (3 tests) 81.0 s ( l test)
Oil additive chemistry and other factors affecting the lubricating ability of the oil are significant contributors to oiloff capability. An eight-fold difference between the time-tofailure is evident between a low queility oil for lubrication compared to a high quality oil. The traction data in Fig. 19 show that the onset of failure for oil TEL-0004 is a sudden event. The additive chemistry in the DOD-L-85734 oil results in a gradual rise in traction coefficient. The gradual rise in traction coefficient is associated with a controlled tribological process, which results in less severe local damage to the surfaces. There are noticeably fewer areas of material transfer or islands of oxidized material on the surfaces with the DOD-L-85734 oil. Discussion
of Oil-off Test
Results
These results strongly suggest that a high quality oil for lubrication can provide greater tolerance against surface damage during oil-off operation with better probability for recovery after a high traction event due to momentary oil
interruption conditions. The significant feature that these tests bring out is that testing for bearing or material performance for oil-off capability can be significantly affected by oil type. The oil-off test protocol can also be used to evaluate bearing materials, surface finishing processes (roughness a n d texture) and effects of surface defects. The test protocol can be expanded to include material a n d oil attributes a n d their potential for recovery following a high traction and material transfer event. The recovery is accomplished by repair of the surfaces through a wear process. However, incomplete surface repair and accumulated damage below the surface are likely to reduce fatigue life. In any case, recovery, even with a sacrificial wear process, has the potential to achieve survivability in the near term.
EHD FILM-FORMING CAPABILITY OF OIL The above simulation tests have been developed using key performance parameters including entraining velocity, sliding velocity, contact temperature, a n d surface topography. These parameters should be viewed with respect to fundamental properties of the oil, or at least characteristic oil "attributes." The simulation approach is to link test (or service) parameters with fundamental oil properties as much as possible. This can be done with physical properties like viscosity, pressure-viscosity coefficient, and even traction coefficient.
984
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
The direct and sensitive connection between entraining velocity and EHD film thickness holds the key to linking practical engineering peirameters to tribology phenomena within a lubricated contact. This linkage makes the task of deeding with chemical attributes of oils and materieds m u c h easier. EHD theory provides the only useful design guide among all the complex mechanisms in tribology. EHD film generation mechanisms give a n opportunity to establish a c o m m o n denominator between testing and service performance. The importance of entraining velocity is its association with the entrainment of oil into the contact to generate a hydrodynamic or EHD film. Since the ratio (h/cr) representing EHD film thickness (h) and surface roughness (cr) is important with respect to the degree of asperity penetration within the confines of the contact, fundamental fluid properties of viscosity and pressure-viscosity coefficient are essenticJ for oil characterization. Viscosity and pressure-viscosity coefficient give rise to an inherent capability of oil to generate Ein EHD film for surface separation. The sections below describe a test method for determining EHD film-forming capability and the determination of pressure-viscosity coefficient. If EHD film thickness can be measured, it can be used to calculate pressure-viscosity coefficient. If the EHD filmforming capability of cin oil can be determined directly from oil film thickness m e a s u r e m e n t s , the characterization of a n oil can go beyond the standard viscosity properties used in oil specifications. These viscosity features include nonNewtonian effects and enhanced viscosity due to molecular forces at the surface. The sections below s u m m a r i z e the method used to measure EHD film thickness. Discussions include the role of pressure-viscosity coefficient on EHD filmforming capability and its impact on oil specification. WAM Configuration for Optical E H D Film Thickness Measurement The oil film thickness of test oils can be m e a s u r e d between ball and disk test specimens under dynamic EHD con-
tact conditions with the WAM test machine configured for optical measurements. The configuration for opticcJ interference is shown in Fig. 20. The contact load, surface speed and temperature of the test specimens are computer-controlled. Typical film thickness tests are conducted with the test specimens and contact conditions identified below. Ball
13/16 in. dia. M50 steel (grade 5), Ra < 1 /tin. Disk 4 in. dia X 0.5 inch, Pyrex 7740 with optical coatings MEIX. Hertz Stress 0.59 GPa (86 000 psi) Entraining Velocity Variable, defined as 1/2 sum of surface velocities Sliding velocity 0, pure rolling Nominal Temp Ambient to 130°C The control and measurement of temperature is a critical factor, especially for the precision required to obtain the pressure-viscosity coefficients from film thickness measurements. Tests are conducted within a heated chamber with a recirculating oil supply, which uses approximately 100 ml of test oil. A computer-controlled oil heater, resistance heater and heat lamp EJIOW for a disk temperature that varies between 1 and 2°C during the course of a test. As the test progresses, temperature fluctuations for optical film thickness measurements are due to veirying amounts of windage created by the rotating test specimens. Surface speeds may rcinge from 0.1-10 m/sec. For optical measurements, a Pyrex disk is used as a mating specimen. The bcdl is mounted below, rather than above the disk, as shown in Fig. 20. The Young's Modulus of the Pjrex disk is 9.1 X 10—6 psi and the Poisson's ratio is 0.20. The t r a n s p a r e n t disk is coated with a partially reflecting chromium coating to allow interference fringes to form between the ball and disk at the contact point. The contact is illuminated with white light filtered with a special d u o c h r o m a t i c filter that transmits red a n d green
FIG. 20—WAM machine with optical configuration to measure EHD film thickness with interferometry.
CHAPTER
TEST MODELING
985
EHD Film Thickness at 2 m/s Hertz Stress: 0.59 GPa Bali Diameter: 2.06 cm.
CO
c o i
36: BENCH
1
CO Q.
(0
PE-9-G0041
O
PE-5-L1876
O) c
H/IIL-L7808 Herco-A
0.1
O I
LU
20
30
40
50
60
70
80
90 100
Temperature (°C) FIG. 21—EHD film-forming capability versus temperature at 2 m/s.
wavelength bcinds. This optical system forms a sequence of colored fringes in and around the contact (see Fig. 20). Each colored fringe corresponds to a particular optical thickness between the surfaces. These optical fringes allow the wavelength of light to be used as a unit of measure to determine the thickness of a n EHD generated oil film. The colored fringes are calibrated to determine the optical film thickness corresponding to each color. The average oil film thickness between each colored fringe is 0.076 /xm (3 X 10"* in.). If one includes the thickness determinations corresponding to the transitions between each colored fringe, the average thickness between each fringe is 0.038 /xm (1.5 X 10 * in.). The resolution of oil film thickness is on the order of 0.025 /xm (l.OX 10"* in). An ultra-thin film technique for oil film thicknesses significantly improves resolution. It also allows oil film thicknesses to be measured down to ± 3 nanometers. The entraining velocity (rolling velocity) is used to control the thickness of the EHD film. A film thickness test is run from low speed to high speed for a given test temperature. Film thickness measurements Eire obtained by determining the entraining velocity (rolling velocity) corresponding to each fringe color. Since the wavelength of light varies with the optical properties of the media of transmission, the optical film thickness data are converted to actual film thickness by making corrections for the refractive index of the test oil. The corrections include the effect of pressure on density. E H D Film-forming Capability of Test Oils The EHD film-forming capability of an oil can be defined as the ability of the oil to generate a n EHD oil film. The oil is judged by the thickness of the oil film, which is a function of the viscosity and pressure-viscosity properties of the oil. The EHD film-forming capability of an oil is a function of several rheological factors that go beyond the simple viscosity used in oil specifications. The raw oil film thickness data are plotted on log-log plots. The data on the log-log plots show that the relationship be-
tween film thickness and speed is exponential. From EHD theory, the film thickness varies with entraining velocity, Ue, to the power of 0.67 as seen below. 1.9
(^oUe)"
3RO
(1)
where film thickness, center of contact combined radius of curvature 1/R = 1/Ri -II/R2 vis at atm. press and test t e m p ^o a pressure-viscosity coefficient Ue entraining vel., Ue = l/2(Ui +• U2) E' combined elastic modulus, 1/E' = 1/2[(1 — o^/Ei) + (1 - o^/Ez)] Film thickness decreases with temperature due to the sensitivity of viscosity with temperature. The pressure-viscosity coefficient (a) can also decrease with temperature. Because the thickness of the EHD film is sensitive to temperature and to the difficulty in controlling each test at exactly the required temperature, the test results are plotted in terms of film-forming capability versus temperature. To do this we have selected a reference entraining velocity of 2 m/s. Using a reference entraining velocity of 2 m/s, the film thickness for test oils is determined from the test plots of film thickness versus speed for each test temperature. These EHD film thickness values are then plotted as a function of temperature. The EHD film-forming capability for a number of polyolester t5^e aviation oils supplied by the U.S. Navy is shown in Fig. 2 1 . These aviation oils cover a range of viscosities from 3-9 cSt. The property data provided for these oils are given in Table 2. ho
R
Pressure-Viscosity Coefficients The film thickness results of the test oils can b e plotted in dimensionless form where a film thickness parameter (ho/R) is plotted against a dimensionless speed parameter (T7oUe/E'R).
986
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
TABLE 2—Properties
of test oils.
Code
Oil Type
Viscosity @100°C
Viscosity @ 40°C
SpecGr@15°C
SpecGr@40°C
SpecGr@70°C
Refractive Index
PE-5-L1557 PE-5-L1818 PE-5-L1876 PE-5-G0041 MIL-L-7808J PE-5-L1274 Krytox 143AB 16350, lot 12
5 cSt basestock 5 cSt basestock 5 cSt basestock 9 cSt formulated 3 cSt formulated 4.5 cSt Herco-A lOcStPFPE 80 cSt PFPE
4.84 cSt 4.93 cSt 5.06 cSt 8.79 cSt 3.15 cSt 4.5 cSt 9.8 cSt 80cSt
23.47 cSt 22.98 cSt 24.61 cSt 50.99 cSt 12.22 cSt 20.0 cSt 75cSt 900 cSt
0.9954 0.9683 0.9961 0.9858 0.955 1.003 1.902 1.925
0.9829 0.9574 0.9836 0.9739 0.938 0.9900 1.861 1.882
0.9743 0.9491 0.9736 0.9663 0.910 0.9808 1.808 1.830
1.4513 1.4526 1.4511 1.4610 1.56 1.56 1.36 1.3
1
1
1
1
r-
Values calculated from reference fluid Krytox 143AB
r
Krytox 16350
it
0)
o ;r2?
o
s.
Referance fluid (Kiytox 143AB)
8 -5 =? fl) 3 CO
10
^ ro
(1) r n LL <
2-
a)
>
/
o (1) 11=
m
20
110
30 Temperature (°C)
FIG. 22—Pressure-viscosity coefficients determined from EHD film thickness measurements.
The only EHD lubricating parameter missing from these two dimensionless parameters is the pressure-viscosity coefficient (a). Since all the tests were run under the same load, the applied load (w), which is also missing from these dimensionless parameters, is neglected. If the a-value for each oil were the same, the data on a plot of these dimensionless parameters would fall on a single line. The test results, which depart from a single line, reflect a different pressure-viscosity coefficient. A "relative" pressureviscosity coefficient for a test oil can be obtained from test data for a reference fluid with known pressure-viscosity versus temperature behavior. A reference fluid, Krytox 143AB, is selected for this purpose. The pressure-viscosity coefficient for this fluid was determined by a high-pressure capillary viscometer. The pressure-viscosity coefficient used for the reference fluid is defined as the reciprocal asymptotic isoviscous pressure (a*). Pressure-viscosity data seldom follow a simple mathematiccJ relation. The definition of a* takes into account variations of viscosity with pressure over a pressure range. Using a reference fluid, it is possible to calculate "relative" a-values for each test oil at a given temperature, with the following equation derived from EHD theory: ho/R' 01= a?efoil
(ho/R)refoil
According to EHD theory, the above equation assumes that ho is proportional to o:"'^^. The relative pressure-viscosity coefficients for the test oils are determined at a selected speed parameter of 1 X 10"^". This value is selected because it has corresponding film thickness data for essentially all the tests. The value of ho/R for the test oil at each temperature is determined from log-log plots of (ho/R) vs (rjoUe/ E'R), where (TJOUE/ E'R) = 1 X 10~^°. These values are used in the above equation to calculate an a-value, which is determined relative to the reference fluid data. With the above procedures, the pressure-viscosity coefficients of all the oils are determined for each test temperature. The calculated values, which were used to determine the pressure-viscosity coefficient, are plotted in Fig. 22. The plot in Fig. 22 includes the reference oil, Krytox 143AB, as well as the high molecular weight PFPE oil 16350. Over the temperature range in Fig. 22 the aviation oils have a-values that range between 0.5 and 2.0 X 10~* Pa~'. Pressure-viscosity coefficients tend to decrease with temperature. Discussion of Pressure-Viscosity Data The most important feature of the data in Fig. 22 is that the pressure-viscosity coefficients of the polyolester gas turbine and gearbox oils are noticeably different. The average pres-
CHAPTER 36: BENCH TEST MODELING sure-viscosity coefficient between 30°C and 70°C for each oil is given in Table 3. From the data in Table 3, which includes oil viscosity at 100°C, it can be seen that the pressure-viscosity coefficients increase with base oil viscosity. The EHD film-forming quality of an oil is associated with the pressure-viscosity coefficient as well as its viscosity. Higher viscosity oils benefit from an increase in pressure-viscosity coefficient as well as its viscosity. Oils with similar molecular structures and molecular weights seem to have similar pressure-viscosity coefficients. The measurement of EHD film thickness and the determination of the pressure-viscosity coefficients are essential for c o m p o n e n t design, performance prediction, a n d testing simulations.
GENERALIZED PERFORMANCE MAP The availability of viscosity and pressure-viscosity coefficients allows calculations to be made for lubrication regimes of operation in component hardware and test simulations. For first order calculations, EHD film thickness (h) can be calculated and related to the combined surface roughness (a) of the contacting surfaces. Calculated values of hJa greater than three are generally considered fuU-EHD film lubrication where surface roughness has no influence on friction or surface deterioration. When h/cr is less than 3, and particularly less than 1, the lubrication regime is considered "mixed film," where lubrication is a mixture of EHD film and surface film (or boundary) lubrication. The use of the ratio h/cr normalizes tests r u n over a range of entraining velocities and surface roughness. The ratio h/cr also reflects the degree of asperity interaction with the con-
TABLE 3—Measured pressure-viscosity coefficients. Viscosity, cSt Avg. Press-Vis Coef for 30, 40 &70 "C at100°C Oil Code Description High mw PFPE 80.0 5.11. X 10' Pa PFPE, 16350 10.5 3.99 Krytox 143AB PFPE ref. oil 9 cSt formulated 8.79 1.87 PE-9-G0041 5 cSt basestock 5.06 1.75 PE-5-L1876 5 cSt basestock 4.84 1.68 PE-5-L1557 5 cSt basestock 4.93 1.65 PE-5-L1818 4.5 cSt ref. oil 4.50 1.31 Herco-A 3 cSt formulated 3.15 1.26 MIL-L-7808J
tact region. In a similar fashion, tests cem be normalized with the slide-to-roll ratio, which is the ratio of the sliding velocity (Us) to the rolling (or entraining) velocity (Ue). The slide-toroll ratio is sometimes expressed as S/R. The S/R ratio reflects the tangential strain of asperity interaction within the contact region. These two dimensionless p a r a m e t e r s describe a simple first order connection between micro-scale tribology p h e n o m e n a in a lubricated contact and macro engineering parameters of bearing or gear hardware. The dimensionless parameters, h/cr and S/R can be used in place of the entraining and sliding velocity parameters (Us a n d Ue) used in the Performance Maps discussed in the Multi-dimensional Oil Characterization (Performance Mapping) section. This substitution creates a more generalized Performance Map reflecting life-limiting failure modes, as shown in Fig. 23. The mapping of the surface failure modes was determined from thousands of tribo-system simulation tests with typical bearing and gear materials. When h/cr is larger than three, we can expect minimal surface interaction and long life. Small values of h/cr, along with low sliding velocities compared to rolling velocities, will result in mild polishing wear or perhaps micro-pitting. High sliding-to-roll values can introduce severe wear and scuffing failures, particularly for high-speed operation. These scuffing failure modes can be sudden events. Low speed applications can operate at extremely low h/cr without scuffing. Here the life-limiting failure mode is accumulation of wear over a period of time. While this two-dimensional m a p does not illustrate the role of temperature, the methodology provides an approach to link lubrication and failure mechanisms to engineering parameters. Flexible tribo-system testing provides an opportunity to connect micro-scale tribological mechanisms with macroscale engineering parameters. Micro-scale tribology mechanisms leading to selected surface deterioration modes of wear, scuffing, or fatigue are invoked by precision control of normal stress and tangential shear at topographiccd features. The degree of normal stress and tangential shear at topographical features can be crudely linked to engineering parameters of slide-to-roll ratio S/R (or Us/Ue) and the ratio h/cr, which gives the relative thickness (h) of the oil film compared to the surface roughness dimension (cr). Because Ug/Ue and h/cr can be related to bearing and gear tribo-contact systems, performance testing can be directly connected to specific service hardware. Performance tests
(Low speed)
(High speed)
Slide to
Roll Ratio (S/R) L ong I rt«- •—- I EHD nim-tomiinB capablll^
1
987
^ --^^"^
I
Lub film thickness / surface roughness ratio {hh} FIG. 23—Schematic Performance Map of lubrication attributes and life-limiting surface failure modes (boundaries are postulated).
988 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK can EJSO be mapped over a range of conditions to locate performance limits and the effectiveness of the tribo-system attributes. From thousands of tribo-system simulation tests, life-limiting surface failure modes can be mapped as illustrated in Fig. 23. While this two-dimensional map does not illustrate the role of temperature, the methodology provides an approach to link lubrication and failure mechanisms to engineering parameters.
SIMULATION OF GEAR LUBRICATION WITH SYSTEMATIC TRIBOLOGY TESTING The above sections develop the key tribology parameters required to test a lubricated contact as a system with connection to bearing or gear contacts in service. The ability to independently control key parameters, such as entraining velocity, sliding velocity, and temperature, gives the opportunity to simulate a multitude of lubrication and failure mechanisms. Meaningful bench test modeling for specific performance limits like scuffing, various types of wear, and surface initiated fatigue can then be accomplished. This section demonstrates the application of Systematic Tribology. The application selected is the development of a simulation test for an historic gear test method used for jet engine oil qualification. Similar simulation steps can be used for the simulation of rolling element bearings and other highly loaded lubricated contacts. Overview of Gear Testing for Aviation Gas Turbine Oil Qualification Since the early days of jet engines, gear tests have been used to develop and qualify aviation gas turbine engine oils. The most widely accepted tests for lubricating performance of engine oils are the Ryder Gear Test and lAE Gear Test. Highspeed gear tests eveiluate the ability of candidate oils to prevent gear tooth scuffing. Gear tests have served the industry for five decades, since the early formulations of synthetic aviation oils. Virtually every jet engine oil in use today has been subjected to Ryder Gear testing. The Ryder Gear Test Method, which was developed by Earle Ryder [4], was derived from a gear test rig when the need for an oil test was required for gas turbine engines. Because of operating cost, test gear availability and test reliability, gear tests are in limited use. Today, there is only one operational Ryder Gear Test, which is operated by the U.S. Navy, Patuxent River, Maryland. Alternative bench tests have been developed over the last decade [5]. The development of alternative test methods, particularly the test efforts supported by the U.S. Navy [6-8], have provided a better understanding of the complexities and mechanisms associated with oil lubricating attributes. Alternative test methods show that scuffing performance is a function of test paramieters. Test parameters are artificvEilly selected in order to rank oils according to Ryder Gear test data. Activation of oil chemistry is so closely linked to conditions of local contact temperature and micro-scale tribological stress that simple qualification tests have difficulty characterizing oil performance in a meaningful way. Since the Ryder Gear Test has amassed a large database, alternate test methods have focused on replicating Ryder
Gear testing with scuffing criteria. While the huge Ryder Gear database should not be ignored, replication of gear test results do not provide the critical link to bearing and gear hardware needs in service. Yet the tribological phenomena that occur in a gear test have value with respect to potential lubrication and failure mechanisms that may operate in service. It is now recognized that if industry is to be properly served, performance tests for oils must be integrated with performance tests for materials. Advanced testing, which is needed for oil qualification, not only requires inclusion of material considerations, but requires a paradigm shift in the whole process of technology development of lubricants, materials, and components. While this paradigm shift has not yet occurred in many industries, some companies are now endorsing new strategies toward a systems approach [9] for development and testing. In the meantime, improvements in test methods are being made using concepts of Systematic Tribology. Some of these concepts are illustrated in the sections below.
Ryder Gear Test Conditions The Ryder Gear Test Method defines the maximum load at which a test gear pair experiences gross surface distress, as evidenced by excessive gear "scuff." The Ryder gear test head, which is shown in Fig. 24, hydraulically loads a pair of spur gears. The test geeir, which has a narrow face, drives a wide gear with equal number of teeth. Ryder gears have the following specifications: Pitch diameter: 88.9 mm (3.5 in.) Pressure angle: 22.5 degrees Material: AISI 9310
Number of teeth: 28 Surface finish: 0.45-0.63 ;u,m (18-25 fiin.), Ra
Hardness: 63 HRC minimum
The test method uses an oil jet feed which is stabilized at a bulk oil temperature of 74°C (165°F). The test gears are operated at 10 000 rpm in a series of 10-min intervals with increasing load prior to each interval. The test gear is visually inspected after each load interval to assess the percent scuff. Failure is defined when the 28 teeth on the test gear have an average scuffing area of 22.5% across the tooth face. In the following sections, Ryder test gear surfaces are examined to identify the tribological features and the progression of events that occur as the operator counts the "scuffed" area that eventually reaches the arbitrary criteria of 22.5% of the gear teeth surfaces scuffed. The detailed tribological features of the Ryder test gears are documented to understand the lubrication and failure mechanisms required for simulation tests and to show how they may or may not be representative of service hardware. Of particular interest are: the initiation of scuffing, the propagation of the scuffing front along the tooth face, the thermal response of the gear tooth surface, the stress profile across the tooth face, and the rolling/sliding velocity profile across the tooth face. The U.S. Navy conducted the Ryder gear tests described below. Because of limited availability of "good" Ryder gears, these tests were run with Allison gears, which have a gener-
CHAPTER 36: BENCH TEST MODELING 989 Maj^eticPidcUp Coiqiling to Drive TUitine (10,000 rpm)
Load Chamber
Slave Oear (Spur)
Ryder Gear Test (Top View)
FIG. 24—Cross section of Ryder Gear machine test head and gear mesh.
990
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
ous tip relief. Tip relief reduces the stress at the tip of the tooth where the sliding velocity is high. This results in higher than average load capacity with these geeirs. Because of this, tests r u n with high load carrying DOD-L-85734 oils, suspended prior to the 22.5% scuffing point. Nevertheless, the tests are very useful for establishing the onset of scuffing and the progression of the scuffing phenomena. Scuff Initiation Features o n Ryder Gears Figure 25 shows side 'A' of the Ryder tests gears (Allison T630603R) used to evaluate a DOD-L-85734 oil with Navy reference n u m b e r PE-5-L1761. This test was suspended at an 80 psi load pressure (8th load stage) which gives a tooth load of 3696 ppi (pounds per inch). The Allison geeirs with generous tip relief resulted in only 4.21% scuffed area. The n u m b e r 1 tooth of the narrow test gear is shown in the upper photo of Fig. 25 and its mating tooth on the wide gear is shown in the lower photo. The Ryder operator counted 5% scuff on the n u m b e r 1 tooth of the test gear surface. The lighting used on the tooth face for the photograph in Fig. 25 is similar to what is used in the Ryder test to illuminate the test gear surface for inspection with a TV camera. The small dark patches on the dedendum of the test gear are areas where the grinding fea-
FIG. 26—No. 1 tooth pair from Ryder gears in FIG. 2.2 used for microscopic examination. Light scuff (5%) on dedendum of narrow test gear (a). Polishing wear footprint on wide gear (b) shows axial movement with load stage.
(b)
FIG. 25—Allison gear set (T630603R) with generous tip relief; oil PE-5 1761. Run to 80 psi load, avg. scuff 4.21%. No. 1 tooth has 5% scuff. Dark areas on test gear (a) and wide gear (b) caused by polishing wear.
tures have been polished. The polished areas appear dark because the oblique lighting reflects the light away from the camera, which is mounted normal to the face of the gear tooth. These dark areas of polishing wear are also seen by the Ryder operator on the TV monitor, but they are ignored by the Ryder operator with regard to its contribution to the scuffed area. The wide gear in Fig. 25 shows similar polishing areas, but they appear at the addendum rather than the dedendum. The narrow test gear drives the wide gear. As the gear teeth mesh, the initial contact occurs at the dedendum of the test gear and the a d d e n d u m of the wide gear. There is much less polishing wear on the opposite ends to the tooth face where the teeth go out of mesh, at least for this gear set. The polishing wear footprint on the wide gear clearly shows the axial movement of the wide gear that occurs as the torque is increased for each load stage. The amount of axial movement seems to decrease with loading stage. The axial movement of the wide gear causes the right hand edge of the test gear to run against fresh material on the wide gear for each load stage. This explains why there is frequently more scuffing on the right side of the test gear.
CHAPTER 36: BENCH TEST MODELING
(a) 100 nm
991
damage in these areas seem to contribute very little to the total traction across the contact. In the Ryder test, polishing wear seems to precede scuff initiation. The first few % scuff that is counted may be nothing more than polishing wear of the grinding ridges with abrasive scratches in the direction of the sliding velocity. It seems that the scuff criteria for the Ryder operator is a disruption of the cixial grinding features by perpendicular smearing of material, even to the relatively minor level of abrasive scratches. Further evidence of scuff initiation is shown in Figs. 28 and 29, which aire taken from Ryder gears supplied by the U.S. Navy from tests conducted with the reference fluid Herco-A and a MIL-PRF-23699 oil (PE-5-L1790), respectively. These tests were conducted with Pratt & Whitney gears. The two teeth represent only a minor degree of scuffing compared to the other teeth on the test gears. In both cases, scuff initiation occurs at the dedendum of the test gear, where the meshing teeth initiate contact and where the sliding velocity is maximum. The scuffed Eirea grows in the direction of the pitch line. The scuffing damage is more advanced on the right side of the tooth, which, as discussed above, is due to the presence of fresh material on the wide gear introduced into the contact
100 nm
FIG. 27—Photomicrographs of 5% "scuffed" area on dedendum shows that abrasive scratches are counted as part of the % area scuffed. Location of photomicrographs (a) and (b) identified in FIG. 26
Microscopic Examination of Tribological Features To conduct a microscopic examination of the gear teeth surfaces, the gear teeth were cut off as shown in Fig. 26. These photographs were taken with shghtly different obhque Hghting, which causes some of the pohshed areas to appear highly reflective and other polished areas to appear dark as before. Examination of the test gear reveals that the 5% scuffed area is located on the bottom right hand side of the tooth face. Photomicrographs of this area are shown in Fig. 27. The dominant tribological features are extensive polishing wear, abrasive scratches, ajid small areas of micro or superficial scuffing. The length of the abrasive scratches are an indicator that the observed area is at the dedendum, where the sliding velocity is high. The surfaces also show brown and blue surface films in the most disturbed eireas. In some areas the blue film appears to have been removed. From simulation test experience, surfaces that have encountered polishing wear with abrasive scratches, and even superficial scuffing, do not produce any significant change in traction. The surface damage is very local and relatively minor. The local frictional perturbations that accompany the
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FIG. 28—Tooth from Pratt & Whitney test gear (a) having 7% scuff with oil PE-5-L1274. Photomicrograph of scuffed area (b) showing polishing wear and superficial scuffing confined to the grinding features only.
992 MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK Herco-A. From the severity of the scuffed area, one would judge that a notable increase in traction would result as the tooth contact traverses this area of the tooth face. The increase in traction will accelerate the frictional heating of the tooth surface. Propagation of the Scuffing Front In the above sections we determined that scuff initiation, where the scuffed area is less than 10%, consists of superficial scuffing associated with the grinding features only. The Ryder operator will even include narrow abrasive scratches as part of the scuffed area. Polishing wear, even when quite extensive, is not included. Scuff initiation, at least in this case, occurs at the dedendum of the test gear where the sliding velocity is maximum. But the final judgment of load capacity is not made until, on average, 22.5% of the tooth face surfaces are scuffed. The percent area scuffed counted for each load stage as the scuffing progresses to, or just beyond, 22.5% is used to extrapolate, or interpolate, the load at which 22.5% is reached. This is accomplished by plotting the log of the percent scuff versus the load pressure. The progression of scuffing is frequently, but not always, an extension of the scuffed area that has initiated at the dedendum, or possibly the addendum. The scuffed area grows in the direction of the pitch line, even though the sliding velocity decreases in that direction. The "scuffing front" is able to move in that direction with increasing load.
(a)
100 ^m
FIG. 29—Tooth from Pratt & Whitney test gear (a) having 10% scuff with oil PE-5-L1790. Photomicrograph of scuffed area (b) which is likely to cause an increase in traction coefficient.
by the axial movement of the wide gear as the load is increased. The photomicrographs in Figs. 28 and 29 show that polishing wear precedes scuff initiation. In Fig. 28, where 7% scuff has accumulated with Herco-A, the scuffed area is composed of smeared surface material. Some original grinding furrows are still present indicating that the smeared materiEd is quite shallow, involving primarily the grinding features of the surface. Some of the protruding material in the scuffed area shows polishing wear, indicating that the scuffing event is followed by some degree of recovery with continued running. The degree of surface disturbance in the scuffed area appears to be similar to what we call a "micro-scuff," which is surface damage confined to the roughness features. From this, we would judge that the 7% scuff damage on this tooth has occurred with a less than significant perturbation in traction. The Ryder tooth in Fig. 29 has accumulated 10% scuff with oil PE-5-L1790. The scuffed area appears more severe than the 7% scuff in Fig. 28. This oil, which includes an anti-wear additive (TCP), shows more brown and blue surface films in the unscuffed area than the test run with Herco-A, which has no anti-wear additive. The scuffed area shows much wider paths of smeared material than the 7% scuffed material with
The progression of a scuffing front past the 22.5% criteria is shown in Fig. 30. This gear tooth pair has accumulated 32% scuff with oil DOD-L-85734 (PE-5-L1761). These gear teeth are the side 'B' of the gear teeth shown in Fig. 25. The test was terminated at a load pressure of 90 psi with em average scuff of 18.71%. The axial movement of the wide geeir can easily be seen from the polishing wear footprint. The polishing wear path also shows that contact is not made at the root and tip of the gear face until about the seventh load stage. This is consistent with the generous tip relief and relatively high load capacity with these particular Allison gears. The tribological features at the scuffing front are shown in Fig. 31. Colored surface films have formed in both the unscuffed and scuffed areas as shown in Fig. 31. Surface films are not as apparent in the lower photomicrograph in Fig. 31, where the scuffing is more advanced. The formation of these protective surface films is expected to inhibit the propagation of the scuffing front. We find that the presence of a reaction film to a thickness that causes a blue interference film gives very good wear protection. The reaction film forms on the grinding ridges that encounter the most severe surface interaction and localized temperature. The reaction film seems to protect the surfaces from polishing wear as well as scuff initiation—a feature consistent with simulation testing with this DOD-L-85734 oil. Colored surface films can also be seen in the scuffed area, as shown in the upper photomicrograph of Fig. 31. This implies that the load additive may be functioning in the scuffed region by helping the surface to recover topographically and chemically after a scuff has occurred. Effect of OU Chemistry on Ryder Scuffing Criteria The benefit of the load additive in oil PE-5-L1761 can be seen in the Ryder test data plotted in Fig. 32. Following scuff ini-
CHAPTER 36: BENCH TEST MODELING
'••tew
993
face films. Surface films become visible on the unscuffed areas as well as the scuffed areas. It can be postulated that if the test conditions are not appropriate to invoke the load additive, the ranking of the oils could be quite different. Also, it is clear that oil ranking may vary if the selected scuff criteria were different from the arbitrary selection of 22.5%. Tribological Processes During Scuff Progression For oils with the same viscosity, the tribological processes that control the progression of the scuffing front will determine the scuffing load capacity and the relative ranking of test oils. An understanding of the tribological progression of events is important with respect to how the oil functions as well as how a Ryder load capacity test should be simulated. While scuffing on the narrow test gear is used to determine load capacity, it is the wide gear that provides more useful information on the progression of events that lead to the scuff criteria. Figure 33 shows the #15 gear tooth pair that has encountered 33% scuff with DOD-L-85734 oil FE-5-L1705. The axial movement of the wide gear with load preserves a history of the surfaces for each load stage. The upper photomicro-
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FIG. 30—Tooth pair #1, side 'B' (bacic side of gear teeth in FIG. 2.2) having 32% scuff, oil PE-5-L1761. Scuff propagates from dedendum toward pitch line on test gear (a), and from addendum to pitch line on wide gear (b).
tiation, which may only be minor abrasive scratches, the progression of the scuff for this oil with the Sier Bath gears is much less than the reference oil PE-5-L1274 (Herco-A) as well as the MIL-L-23699 oil, PE-5-L1664. The results are not plotted in semi-log fashion, as is customary for Ryder data, so that the real progression of scuffing versus load can be illustrated. It is interesting that in the early days of the Ryder test, Earle Ryder acknowledged that the slope of the data on a semi-log plot vsiried from test to test. He attributed this to "a random effect that can never be tamed" [4]. Actually, the slope has a tribological explanation. The slope is a measure of the rate of propagation of the scuffing front on the tooth face from the dedendum or addendum toward the pitch line. The slope is lower for oils with a load additive because once a scuff is initiated the additive inhibits its propagation. From the data in Fig. 32, it seems that the presence of a load additive does not influence the first evidence of a scuff. This could be due to insufficient surface temperature or asperity interaction to cause the additive to react. Once the scuff progresses beyond 5 or 10%, the tooth temperature increases, and the additive chemistry is activated to form protective sur-
(a)
I-
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FIG. 31—Photomicrographs of the scuffing front shown in FIG. 2.7. Colored surface films inhibit propagation of scuffing front (a). Area of more advanced scuffing (b).
994 MANUAL 3 7: FUELS AND LUBRICANTS
50 45 40
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Each plot is the average of two tests - side 'A' and side 'B' of the same gear set
DOD-L-85734 PE-5-L1304(A) PE-5-L1761 (B) Sler Bath #6 gears
Herco-A PE-5-L1274 SierBath#3 gears
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40
50
60
70
100
LOAD OIL PRESSURE, psi FIG. 32—Effect of load stage on Ryder Gear scuffing.
graph in Fig. 34 shows the left edge of the load path during the first load stage where polishing wear of the grinding ridges is clearly evident. Calculation of the ratio of elastohydrodynamic (EHD) film thickness to surface roughness (h/cr) for a surface temperature equal to the inlet oil temperature of 74°C (165°F) is less than one. If this is correct, some sort of polishing wear or surface damage is expected. In fact, with such a small h/a, more surface interaction would be expected than shown in Fig. 34. This leads one to question the assumption that the tooth surface temperature for the first load stage is as high as the inlet oil temperature of 74°C (165°F). Prior to the start of test, the gear mesh exit, which is located at the bottom of the gear pair, is heated with the oil supply at a flow rate of 270 ml/min for a period of 1/2 h. The gears are not rotated during this heat-up period. The oil supply is the only source to control gear temperature. The actual gear tooth temperature will depend on the rate of heat dissipation through the gear and shaft support system. The inlet oil temperature cam be viewed as an upper bound for the tooth surface temperature. The actual temperature, at least when the test is initiated, will be less than the inlet oil temperature. The lower photomicrograph in Figure 34 shows the left edge of the scuffed area as well as the surface areas adjacent to the scuff, which have only seen contact at previous load stages. As one views the photomicrograph from left to right, the tribological history of the surfaces is revealed. When the load (and surface temperature) increases, the surfaces en-
counter: (1) polishing wear of the tips of the grinding ridges, (2) more extensive polishing wear with abrasive scratches, (3) superficial scuffing, or smearing of the grinding features and (4) macro-scuffing where the topographical features are completely removed. From simulation testing experience, only the latter event will cause a significant increase in traction coefficient. In this particular case, the test was terminated because of excessive smoke 9 min. into an 81 psi load stage. The average scuffed cirea at the end of the test is 53.6%. These surfaces show very little evidence of colored reaction films like that with oil PE-5-L1761 in Fig. 31. This is consistent with simulation tests where DOD-L85734 oil PE-5-L1761 more readily forms surface films than DOD-L85734 oil PE-5L1705. Simulation tests show that the latter oil tends to show more polishing wear which, incidentally, is ignored by the Ryder operator as a contribution to scuffing. The scuffed area in Figs. 33 and 34 show all the features of what we would call a macro-scuff, where there is a complete loss of surface integrity and a sudden rise in traction coefficient. A surface that has suffered a macro-scuff event is not likely to recover. Continued running results in severe wear, high traction coefficient, and high surface temperature. These features are evident in the #1 tooth pair of the same gear set, as shown in Fig. 35. This tooth pair has accumulated 98% scuff. Both dedendum and addendum areas of the teeth are scuffed. The macro-scuff has caused several teeth in the vicinity of the #1 tooth to become thermally discolored, as
CHAPTER 36: BENCH TEST MODELING
995
From the scuffing data in Fig. 32, it seems that the selected Ryder operating conditions cause typical MIL-L23699 oils and DOD-L-85734 oils to transition into a macroscuff failure mode about the time that the 22.5% scuff criteria is reached. This is a tentative statement because most of the gear teeth examined were from Allison gears with generous tip relief that fail at relatively high loads where a macro-scuff event is more likely (see scuff data for Allison gears in Fig. 32). Since the oils tested have the same base oil viscosity, the scuffing differentiation between the oils is a result of the way oil chemistry handles the roughness features on the gear tooth face. To activate oil chemistry, gear tooth temperature must be an important factor. Another important factor is the "tribologiccd" interaction at asperity (or grinding ridge) sites. Tribologicai interaction involves normal load and tangential strain at asperity sites. Asperity pressure and tangential strain (sliding velocity) control the local temperature that activates oil chemistry. Oil chemistry is also influenced by activated iron surfaces, where the rubbing process removes surface films to expose material at a high energy state. What makes the Ryder Geetr Test work then are two important fea-
FIG. 33—No. 15 tooth pair, side 'B' of Allison gears having 33% scuff with oil PE-5-L1705. Wide gear (b) used to identify progression of tribologicai events.
shown in Fig. 35. The only unscuffed area is a small band at the pitch line, where the sliding velocity component is small and reverses direction. The tooth surface has turned gray, indicating a highly thermally distressed material that may have undergone significant changes in metallurgical structure. The high temperature generated at these gear teeth must have produced the excessive smoke that caused the termination of the test before the end of the 10-minute load stage. Discussion of Ryder Gear Scuffing Criteria The above description of the tribologicai progression of events that lead to the selected scuff criteria of 22.5% scuff shows that the scuffing phenomenon is very broad in scope. The scuffed area that is counted ranges from minor abrasive scratches and superficial scuffing, which are accompanied by only small perturbations in traction coefficient, to macroscuffing events, which are accompanied by a very rapid rise in traction coefficient with little chance of surface recovery with continued running. A single point on the gear surface near the dedendum may encounter a progression of events including polishing wear, abrasive scratches, superficial scuffing (or smearing of the grinding ridges only) and finally a macro-scuff event.
100fun
FIG. 34—Photomicrographs of wide gear from FIG. 2.10. Polishing wear during first load stage (a) progression of polishing wear, abrasive scratches, superficial scuffing and macro scuff (b).
996 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK
FIG. 35—No. 1 tooth pair, side 'B' of Allison gears having 98% scuff with oil PE-5-L1705. Scuffing on both addendum and dedendum for narrow test gear (a) and wide gear (b).
tures: (1) bulk and asperity temperatures of the contacting bodies and (2) the degree of asperity interaction.
CONDITIONS FOR RYDER GEAR TEST SIMULATION A basic premise for the value of simulation of field hardware in a test machine is to capture the same lubrication and failure mechanisms that the field hardware experiences. The gear pair in the Ryder test can be viewed as an example of field hardware that is worthy of tribological simulation. The focus of the simulation is to address the important tribological features that control the performance of an oil in a gear pair. It is clccir that an oil does not have an inherent "load capacity" property. Its performance can only be judged by the tribological system in which it must operate. The sections below examine some of the parameters that are relevant to the simulation of a gear pair, specifically the Ryder test gears. Estimated Ryder Tooth Temperatures The shapes (or slopes) of the Ryder scuffing curves in Fig. 32 are influenced by oil chemistry. This is due to the formation
of surface films that can control the initiation and progression of the scuffing front toward the pitch line. Temperature is a key parameter that controls surface film formation. Surface temperature is also an important parameter for controlling the EHD film thickness. Two temperatures seem to be important: (1) the tooth surface temperature, which controls the inlet oil viscosity and possibly the out-of-contact surface film formation, and (2) the total temperature within the contact (flash temperature plus out-of-contact surface temperature). The total temperature is frequently used as a scuff prediction criteria (flash temperature theory). The flash temperature is the thermal response to frictional heating within the contact. It is directly related to the traction coefficient, applied load, and sliding velocity. Flash temperature is a calculated quantity with quite limited precision because of heat partitioning and dissipation assumptions that have to be made. Flash temperature in the Ryder test is difficult to estimate because the traction coefficient is not measured. It is now possible to improve the calculated flash temperature in the Ryder test by using the traction coefficients measured in simulation tests for load capacity. The flash temperature that is calculated is the global flash temperature associated with a single heat source represented by the Hertzian contact. Since we found much of the tribological action to occur at the grinding ridges, it is likely that local asperity flash temperatures may be a more relevant quantity in the control of surface film formation. In this case, the precision of temperature prediction depends on surface topography and local traction coefficients at asperity sites—both of which are not steady state phenomenon. With regard to Ryder Gear simulation tests, it is possible, and desirable, to be able to refine the calculation of the relevant temperatures due to frictional heating. Several temperatures are relevant. They range from the bulk specimen temperatures to the asperity temperatures within the contact. For now, we confine our thinking to the two temperatures identified above. From a practical viewpoint, with regard to Ryder test simulation, it is more important to focus on the out-of-contact surface temperature. If the surface temperature is correct, then the flash temperatures will take care of themselves provided the other contact parameters are appropriately simulated, such as: contact stress, sliding velocity, surface finish, h/cr, and contact dimensions. There does not seem to have been a detailed study of the Ryder tooth surface temperatures. At the start of the first load stage one can assume that the tooth surface temperature is similar to the inlet oil temperature of 74''C (165°F). Actually, from the above discussion on polishing wear and h/oduring the first load stage, the initial tooth temperature is likely to be less than the oil supply temperature. A second data point on the temperature profile for Ryder Gear simulation can be derived from the "smoke point." The smoke point is the temperature at which the surface immediately outside the contact area becomes hot enough to vaporize the oil to produce "smoke." In our Ryder Gear simulation tests we found ball temperatures in a few cases to be sufficiently high to produce smoke. The measured ball temperatures that produce smoke are on the order of 200°C (392°F) for typical MIL-L-23699 and DOD-L-85734 oils. The Ryder tests frequently produce smoke, especially when the percent scuff goes beyond 22.5%. It is assumed that the tooth
CHAPTER 36: BENCH TEST MODELING surface temperature increases rapidly when a high friction macro-scuff occurs. In the absence of a macro-scuff, where the traction coefficient is not significantly perturbed by polishing wear or even superficial scuffing, the tooth surface temperature must increase with load, despite the constant oil supply temperature. The Ryder tests with the Allison gears having generous tip relief and high load capacity show that the smoke point can be reached without any significant scuffing. The Ryder test for DOD-L-85734 oil PE-5-L1705 shown in Fig. 32 reached a load oil pressure of 80 psi with only 0.61% scuff. Ryder "smoke" was observed 8.5 min into the load stage. This means that somewhere on the tooth surface, outside the contact, the surface temperature is on the order of 200°C (392°F). From WAM ball and disk temperature measurements in simulation tests, the relation between temperature and load is not linear. The temperature may vary with load to a power less than one. We have selected, as a starting point, a relationship where the temperature varies with load to the power 1/3. This is based on the assumption that the rate of heat input is proportional to the nominal contact pressure. For a line contact, the pressure varies with the load to the power 1/3. It is assumed that the Ryder tooth temperature will have a similar relationship. A simulated Ryder tooth surface temperature profile might look something like that in Fig. 36. In simulation tests for load capacity, the measured bcdl temperature increases more rapidly than the disk temperature because of the different thermal mass of the specimens. This
must also be the case for the narrow and wide gears used in the Ryder test. A visual comparison of the simulation test specimens and Ryder specimens is shown in Fig. 37. The Wedeven Associates, Inc. WAM load capacity tests to date have not addressed a precise simulation of the Ryder tooth surface temperatures, which are still unknown quantities. If our assessment of the Ryder tooth temperatures is correct, simulation tests may have ball temperatures higher than the narrow test gear temperatures and disk temperatures lower than the wide gear of the Ryder. Rolling and Sliding Velocities Across the Tooth Face The rolling and sliding velocities across the tooth face are directly linked to oil performance because they are key parameters that control lubrication and failure mechanisms. The rolling (R) and sliding (S) velocities are defined as R = 1/2 (Ui + U2) S = (Ui - U2) where Ui and U2 are the velocities of surfaces 1 and 2. Note: Rolling velocity, R, is sometimes referred to as the entraining velocity Ue, and sliding velocity, S, is sometimes designated as Us. The rolling, or entraining, velocity generates an EHD pressure in the inlet region that separates the surfaces in the Hertzian region with an EHD film. The sliding velocity con-
SIMUUTED RYDER TEMPERATURE PROFILE
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Loadt (lbs) FIG. 36—Simulated surface temperature profile for WAM3 tests based on estimated Ryder tootli surface temperatures.
998
MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK trols the rate of shear and heat generation within the Hertzian region. A necessary condition for scuffing is that the EHD film is sufficiently thin to cause the surfaces to interact and that the shear and heat generation within the contact region is sufficiently great to cause the breakdown of microEHD and boundary film lubrication mechanisms. The rolling and sliding velocities across the Ryder tooth surface can be calculated from the geeir geometry and rotating speed. The calculated rolling and sliding velocities, shown in Fig. 38, are plotted as a function of roll angle. For reference, specific points are identified along the tooth face using the following notations: LPC LPSTC PD HPSTC OD
low point of contact low point single tooth contact pitch diameter high point single tooth contact outside diameter
At the Ryder gear speed of 10 000 rpm, the rolling velocity across the tooth face is nearly constant at about 17.8 m/s (700 in./s). The sliding velocity is zero at the pitch line. It increases to a maximum of 13.1 m/s (517 in./s) at the low point of contact (LPC) and the tip of the tooth (OD). A typical Ryder scuff initiation occurs near the root or tip of the tooth where the sliding velocity is maximum; but the final judgment of load capacity is not made until scuffing has propagated to 22.5% of the tooth face. If the scuff is initiated at the lower point of contact (LPC) and propagates toward the pitch line, the location at which the scuff reaches 22.5% is almost up to the low point single tooth contact (LPSTC). This is illustrated on the FIG. 37—WAM and Ryder test specimens. LPC
24
LPSTC
PD
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OD
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14
16
18
20
22
24
26
28
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34
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Roll Angle, Degrees FIG. 38—Rolling and sliding velocities across the Ryder tooth face expressed in terms of roll angle.
CHAPTER 36: BENCH TEST MODELING 24
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Anticipated pragzeBsioa t o 22,5% scuff.
yvc FIG. 39—Geometric positions across Ryder tooth face Including anticipated progression to 22.5% scuff.
silhouette of a Ryder tooth in Fig. 39. The sliding velocity at 22.5% scuff is about 7.3 m/sec (290 in./s)—see Fig. 38. It is interesting to note that if the scuffing front progresses past the 22.5% criteria, it transitions into the area on the tooth face where the load rises very rapidly because there is now only a single tooth pair that is carrying the load. This may be one reason why the scuff propagation rate increases so rapidly beyond the 22.5% criteria, as shown in Fig. 32. In simulation tests for load capacity, a sliding velocity of 8.78 m/s (346 in./s) is used. This sliding velocity is midway between the maximum sliding velocity at the lower point of contact and the sliding velocity where hypothetically 22.5%
scuff occurs. One could argue that a more appropriate sliding velocity would be 7.3 m/s (290 in./s) so that the shear strain and heat generation within the contact would more precisely simulate the conditions at the position where the 22.5% scuff criteria is typically reached. For Ryder simulation tests a rolling velocity of 5.72 m/s (225 in/s) is used. This rolling velocity is less than 1/3 of the Ryder gear mesh rolling velocity. This rolling velocity was selected to generate an EHD film thicknesses (h) on the order of the combined surface roughness (cr). The selected rolling velocity allows only minor polishing wear at the first load stage, but causes a macro-scuff at loads within typical limits
1000
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
of the Ryder contact stresses. A trade-off in rolling velocity is m a d e to a c c o m m o d a t e differences in surface finish. The combined surface roughness (a) of the ball and disk specimens used in simulation tests is 0.29 /Am (11.7 /tin). The Ryder gear surfaces have a combined roughness on the order of 0.8 /im (32 /tin). It is believed that the difference in the absolute values of surface finish is a sacrifice that should not be made for good simulation. This is based on the fact that m u c h of the tribological interaction is associated with surface features. If there is a significant difference in the absolute value of surface roughness, a duplication of hJa alone is not likely to provide the appropriate simulation. The size and shape of surface features are important for micro-EHD mechanisms, local asperity temperatures and local deformation or plastic flow associated with run-in and scuff initiation. However, practical simulation tests require some c o m p r o m i s e . In this case, rougher surfaces, as found in the Ryder gears, would require higher surface speeds or larger test specimens to achieve an EHD film thickness in keeping with the heights surface roughness features. Satisf3ring these requirements requires a test machine of impractical size. A compromise on surface roughness not only brings the test machine size down to reality, but cJso makes the simulation closer to roughness features found on rolling element bearings.
Contact Stress Simulation During the mid portion of the Ryder gear mesh cycle, the load is carried by a single tooth. The load is sheired between two teeth when the geeirs are coming into mesh and when they are going out of mesh. The high eind low points of the single tooth contact are identified in Figs. 38 and 39. As noted above, the typical progression of a scuffing front to the criteria of 22.5% arrives very close to the low point single tooth contact (LP-
1000
2000
3000
STC). For the most part, one would expect the load on the area being scuffed to be a portion of the total load that is shared between two pairs of teeth. Only under ideal conditions would we find equal load sharing between two pairs of meshing teeth. Because of practical geometric tolerances and dynamic loads, the contact stresses that actually exist on the tooth face can be m u c h higher thein that predicted under ideal conditions. For this reason, the contact stress profile at the low point single tooth contact (LPSTC) is used as a reference for simulation. The contact stress profile on the tooth face at the LPSTC is plotted in Fig. 40 as a function of tooth load in pounds per inch (ppi). The relationship between the Ryder load oil pressure (psi) and the tooth load (ppi) is approximately ppi = 46 X (psi). The contact stress from the first to the ninth load stage for the Ryder remges from 0.79 GPa (115 ksi) to 2.07 GPa (300 ksi). The load and stress profiles used in simulation load capacity tests are similar. Thirty load stages are used from 17.9 N (4 lbs) to 623 N (140 lbs) in order to provide a contact stress from 0.73 GPa (106 ksi) to 2.4 GPa (348 ksi). With a proper selection of sliding velocity, contact stress protocol and degree of asperity interaction, as represented by hJcr, the tribological mechanisms experienced in the Ryder Gear Test Method (or any other gear operation), can be simulated. All that is needed is a test machine to cover these conditions and test specimens that replicate the material qualities of Ryder gears. Simulation tests with these conditions cire described in the next section to illustrate how a tribology test machine can simulate a high-speed gear machine.
GEAR TEST SIMULATION FOR OIL EVALUATION The above sections described the Ryder Gear Test Method and identified the key operating peirameters for simulation.
4000
5000
6000
7000
8000
Tooth Load - PPI FIG. 40—Hertzian contact stress (KSI) at low point single tooth contact (LPSTC) versus Ryder tooth load pounds per inch (PPI).
CHAPTER The purpose of this section is to show h o w the Ryder Gear Test Method can be simulated with respect to the lubrication and failure mechanisms that control scuffing performance. Simulation tests are different from correlation tests. The aim of correlation tests is to rank oils like the Ryder Gear Test Method. Simulation tests invoke lubrication a n d failure mechanisms with test parameters that can be linked to the gear test method. The development of a simulation test is given in detail to illustrate the process of linking tribology testing with a full-scale component, like a gear pair. This process is foundational to linking oil testing to field service performance. B a c k g r o u n d of Load Capacity Simulation Testing The U.S. Navy has supported efforts [3,7,10] to provide Ryder-like load capacity data of gas turbine and gearbox oils. These efforts EJSO expand the scope of oil characterization beyond the perspective of a pass/fail or ranking of oils, with scuffing performance being the only criteria. To provide continuity between Ryder Gear load capacity data and future oil characterization methods, a "WAM Economical Load Capacity Screening Test" was developed to rank a wide range of engine and geEirbox oils similar to the Ryder Gear Test Method [5]. This test method ranks oils with respect to a scuffing failure event. It also characterizes oils with respect to traction (friction) behavior. The introduction of high thermal stability (HTS) oils, and particularly corrosion inhibited (CI) oils, has highlighted the need for greater testing sensitivity for oils exhibiting lower than average lubricating performance. Low lubricating performance, as evidenced in the Ryder test, reveals itself in the form of a superficial form of scuffing ("micro-scuffing"). The test conditions selected highlight the load capacity performance features of oils that are submitted for qualification under the MIL-PRF-23699 specification. Load capacity tests are conducted with ball and disk specimens, which are operated u n d e r tribological contact conditions similar to the U.S. Navy Ryder Gear Test Method. The WAM High Speed Load Capacity Test Method has been approved by SAE E-34C for incorporation into AIR4978 "Temporary Method for Assessing the Load Carrying Capacity of Aircraft Propulsion System Lubricating Oils." Failure Criteria in T e r m s o f Scuffing a n d Micro-scuffing The purpose of this test method is to evaluate oils according to the Ryder Gear Test Method, with enhanced sensitivity for lower than average lubricating performance. It is importcint to recognize that the Ryder Gear performance criteria are based upon the visucJ observations of "scuffing" damage on Ryder geeir teeth. Since some scuffing features found on Ryder geEir teeth are superficial, a Ryder-like test method must also invoke the same t5^e of surface deterioration mechanism. Micro-scuffing is a superficial form of scuffing, which is confined to the surface topographical features of the gear teeth. Micro-scuffing is generally associated with surface damage at low load stages where contact stresses are too low to cause "macro" scuffing. Scuffing, or "macro-scuffing," is associated with the complete loss of surface integrity. Scuff-
36: BENCH
TEST MODELING
1001
ing involves gross failure of near-surface material, in addition to surface roughness features. When traction (friction) is measured, micro-scuffing is generally detected by a rapid decline in traction coefficient. The decline in traction coefficient is associated with the removal of surface roughness features. While this action actually restores some of the EHD fluid film separation between the surfaces, the rapid removal of surface features by plastic flow and rapid polishing wear reflects a failure of the oil to provide adequate surface films for boundary lubrication. In contrast, macro-scuffing is associated with a sudden increase in traction coefficient resulting from massive adhesion and plastic flow of near surface material. A sudden and massive scuffing failure requires high contact stresses in the presence of high sliding velocities. The observation of traction coefficient during a load capacity test is quite informative. High precision measurements of traction coefficient clearly identify "events" like scuffing and micro-scuffing, as discussed above. Traction behavior also reflects the continual interactive process between oil chemistry and the mating material pair within the contact. Subtle chatnges in topographical features due to wesir are reflected in traction behavior. Simulation Approach The load capacity test protocol is conducted with a WAM test facility shown in Fig. 4 1 . The test machine controls specimen position, contact load, and motions of a single contact in space. A computerized r u n file controls load and contact kinematics between the specimens. Specimen temperatures are recorded with trailing thermocouples. The high-speed test protocol uses AISI 9310 bedl and disk specimens with tight specifications for surface finish and hardness. To capture Ryder-like oil performance features, the following test specimen specifications ctnd test conditions have evolved. BaU
Disk
Ball vel. Disk vel. Orientation Entraining vel. Sliding vel. Load Test duration Failure criteria
Performance
2.0638 cm (13/16 in.) dia., AISI 9310, "hard grind" surface roughness, Ra = 0.25 /am (10 fjuin), hardness HRC 62.5-63.5. 10.16 cm (4 in.) dia., AISI 9310, surface finish Ra = 0.15 /xm (6 /tin), hardness, HRC 62-64. Ub = 7.21 m/s (284 in./s) Ud = 7.21 m/s (284 in./s) Non-coUinear velocity vectors (angle between velocity vectors = 75°) 5.72 m/s (225 in./s) 8.78 m/s (346 in./s) Exponential increase from 1.8 kg (4 lbs) to 63.6 kg (140 lbs) in 30 stages Until scuff, or suspension (30 stages = 30 min) Scuff defined by loss of surface integrity and s u d d e n increase in traction. Microscuff defined by rapid decline in traction coefficient. Oil performance is judged by load stages causing micro/macro scuffing event(s) and traction behavior, which reflects wear of surface topography.
1002
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
TOP PLATE A m BEAKING BALL SPINDLE BALL DISC SmE MOTION
TC INPUT AC OUTPUT
FIG. 41—WAM4 Test Facility. Temperature
Oil supply
Specimen temperatures controlled by frictional heating. Surface t e m p e r a t u r e s increase with load stage from a m b i e n t to - 2 0 0 °C. Computer controlled peristaltic p u m p , approximately 1 drop/s. Oil flow rate is selected for adequate lubrication without significant cooling.
The entraining velocity (Ue) and sliding velocity (Us) are defined below: Ue = l/2(Ub + Ud) Us = (Ub - Ud) where
Ub = surface velocity vector of the ball at the contact point Ud = surface velocity vector of the disk at the contact point
The entraining velocity (Ue) and sliding velocity (Us) are key parameters that control the degree of surface separation and the rate of surface tangential shear that the oil must accommodate. With the parameters selected, the initiation of a load capacity test is similar to the Ryder Gear Test in that there is generally little or no evidence of surface damage during the first load stage.
The test parameters recorded include the following: Ball and disk temperatures Traction coefficient Ball and disk surface velocities Contact load Time Option: video recording of running track on disk specimen The test method utilizes the following features: 1. Slow application of load to avoid surface damage during test startup 2. Exponential rather than linear increase in load so that a final scuffing event is reached, rather than a transition into a wear mode without scuffing. 3. Prominent surface finishing features to highlight surface film formation and wear protection through the use of traction coefficient behavior. 4. Use of frictional heating to control specimen temperature and to cover a wide range of temperatures. 5. Continuous specimen contact rather than cyclic contact to avoid load/unload damage. 6. Small incremental load stages to increase resolution. 7. Non-coUinear velocity vectors to capture Ryder-like sliding velocities and film thickness-to-surface roughness ratio. The test protocol parameters focus on creating tribological conditions that activate the same type of chemical response • • • • • •
CHAPTER 36: BENCH TEST MODELING as the Ryder gear test. The key parameters controUing these conditions are: (1) entraining velocity to control EHD film thickness, (2) sliding velocity, (3) surface topography, and (4) specimen temperatures (including effects of frictional heating). If the ranking of oils by a scuffing event falls in line with the Ryder Gear Test, it is assumed that the key tribological conditions invoked must be similar to the Ryder. The progression of surface features (like abrasive scratches, polishing of grinding ridges, and surface film formation) formed prior to a scuffing event eJso follows the same sequence generated in the Ryder test. Test Procedure Prior to each test series, the ball and disk specimens are cleaned in an ultrasonic bath with petroleum ether, followed by acetone. The AISI 9310 "hard grind" ball specimens are processed through the hard grind stage of a ball manufacturing process. The "hard grind" ball specimens tend to have a consistent surface finish of Ra = 0.25 - 0.33 (Ra = 1 0 - 13 ^in) for good repeatability. The test balls are obtained from a single manufacturing batch consisting of approximately 8000 balls. The disk specimens are carburized to a hardness of HRC 62.5-64.5 Following machine calibration, checkout tests are conducted with reference oil, Herco-A. Load capacity tests conducted with Herco-A encounter micro-scuff events. Continued testing beyond a micro-scuff event eventually results in a scuffing event. A scuffing event is not always clccirly defined for Herco-A when it is preceded by multiple micro-scuffing events. Exploratory testing, conducted under Navy PO No. N00421-98-M-6001, shows that specimen hardness influences both micro-scuff and scuffing events. Disk specimens are heat treated in large batches to maintain consistency. The test protocol gives an exponential rise in load with load stage. The exponential rise is to partially offset a cube root relationship between load and contact stress. The exponential rise in load also balances an increase in chemical activity with temperature so a scuffing event can be reached before the end of the test protocol with typiccd jet engine oils. A minimum of four test determinations is made for each test oil. Test Description Figure 42 shows a typical load capacity test plot. A test plot includes the contact load, ball and disk temperatures, and traction coefficient. Typical traction coefficients during the first few load stages are on the order of 0.03. The test conditions during the first few load stages provide nearly full-film EHD lubrication. Ball and disk temperature increase with load stage due to frictional heating. As load and temperature increase, the ratio of EHD film thickness to surface roughness decreases. An increasing traction coefficient reflects a greater degree of asperity interaction within the contact. The rate of rise in traction coefficient reflects the ability of the oil to form surface films at asperity sites for wear resistance. A decreasing traction coefficient reflects polishing wear. A sudden drop in traction is associated with a rapid loss of surface topographical features. This is generally preceded by a micro-scuffing event caused by adhesion and plastic flow of surface features. The surface features following a micro-scuffing
1003
event are shown in Fig. 43. Micro-scuffing events, represented by momentary reductions in traction coefficient, reflect marginal oil chemistry to sustain surface films for protection against local adhesion and wear of surface features. Some oils show multiple micro-scuffing events. Multiple micro-scuffs are characteristic of the non-formulated 4 cSt oil, Herco-A. A macro-scuffing event is easily detected by a sudden increase in traction coefficient.
Data Processing and Traction Behavior The way oil chemistry reacts with the surface to control wear of topographical features is reflected in traction coefficient. Oil chemistry is activated by contact temperature and exposure of "clean" metal surfaces caused by shear at asperity sites. For the selected test conditions, oils show characteristic traction behavior. The characteristic traction behavior of two formulated oils is shown in Figs. 44 and 45. Since traction behavior reflects oil chemistry for wear resistance, the traction data for each test is processed to obtain an average traction coefficient for each load stage. The average traction coefficient versus load stage is then plotted to compare the traction behavior of the test oil with other oils, as shown in Fig. 46. The vertical arrows on the test plots identify the average load stage at which micro-scuffing or scuffing events occur. Figure 46 includes the traction behavior for Herco-A, which is the reference oil used for Ryder Gear testing. The test oils show different traction and scuffing behavior compared to Herco-A. During the first few load stages, the high traction of Herco-A may be associated with two factors: (1) lower base oil viscosity (4.5 cSt at 100°C) and EHD films thinner than the 5 cSt test oils, and (2) the formation of wear protective, and perhaps high friction, oxides. Once the oxides and organo-metallic films are removed from the surface by wear with Herco-A, there is little boundary lubricating chemistry available to allow continued running without local adhesion and plastic flow of asperities. Load capacity tests with Herco-A show multiple micro-scuff and scuffing events between load stages 8 and 14. The traction and scuffing behavior of Herco-A is used as a reference for low lubricating ability. Figure 46 also includes the traction behavior of a test conducted with polished surfaces. To quantify the effect of surface finish on the measured traction during a load capacity test, experiments were conducted with smooth surfaces to establish a baseline traction behavior where the effect of surface topographical features are minimized. This was accomplished with a standard MIL-PRF-23699 oil and the use of smooth M50 balls cind a polished M50 disk. The results show that with smooth surfaces the traction coefficient is very low (0.02). The traction coefficient under ambient conditions with low sliding velocity was found to be about 0.057. The low traction coefficient of 0.02 under the conditions for a load capacity test is due to the frictioncil heating of the oil film caused by the high sliding velocity used to simulate the Ryder Gear Test Method. The data in Fig. 46 show that with smooth surfaces the traction is nearly constant for the entire loading profile. Actually, under typical EHD conditions the traction coefficient increases with load and decreases with temperature. These
1004 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK
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CHAPTER 36: BENCH TEST MODELING 220 Test: NA1292 Lube: PE-5-L-1836 Ball: HG1455-9a, 9310, Ra=10 (Jin. Disc: 9-14a, 9310, Ra=6 (Jin. Entraining Velocity: 225 in/sec. Sliding Velocity: 346 in/sec. Ambient Temperature Velocity Vector Angle (Z): 75°
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FIG. 43—Test terminated after tenth load stage to show micro-scuffing features.
220 Test: NA1333,NA1334,NA1335,NA1336 Lube: PE-5-L2053 Ball: 9310, Ra=10 |jin Disc: 9-49b, 9310, Ra=6 pin. Entraining Velocity: 225 in/sec. Sliding Velocity: 346 in/sec. Temperature: Ambient Velocity Vector Angle (Z): 75°
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Run Time (seconds) FIG. 44—Traction characteristics of formulated oil (four test determinations).
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Run Time (secxjnds) FIG. 45—Traction characteristics of formulated oil.
0.10
Run File: naa.run Ball: 9310, Ra = .25 ^m (10 ^in) Disc: 9310, Ra = .15 urn (6 pin) Entraining Velocity: 5.72 m/s (225 in/sec) Sliding Velocity: 8.78 m/s (346 in/sec) Velocity Vector Angle (Z): 75° HTS oil with
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0.04 STD oil high scuff resistance (3 of 4 tests suspended)
0.03 0.02 ^Cl oil with unacceptable wear resistance and scuff resistance (2 tests)
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Load Stage FIG. 46—Scuffing and traction behavior of MIL-PRF-23699 type oils.
CHAPTER 36: BENCH TEST MODELING two effects seem to offset each other to result in an almost constant traction coefficient with load. During the loading sequence, the EHD film thickness decreases due to the rise in specimen temperature with frictional heating. The EHD film thickness also decreases with stress. If one assumes a combined surface roughness of 0.012 (tm (0.5 fJLin), the calculated h/o-at the first load stage is 15. At the last load stage of 569 N (128 lbs) h/cr is 1.4. Examination of the specimens after the test, shown in Fig. 47, reveals only a few minor surface scratches and the initiation of a light brown and blue surface film. An additional load capacity test with smooth surfaces was conducted with a DOD-L-85734 oil. The results for a DOD-L85734 oil are essentially the same as a MIL-PRF-23699 oil. The traction characteristics under full film EHD conditions are very similsir among oils so long as their basestocks do not differ in molecular structure. Since a DOD-L-85734 oil has a load-carrying additive package, it appears that the loadcarrying additive in the DOD-L85734 oil does not make any significant difference in the traction behavior under full film EHD conditions. There is also evidence that the formation of a surface film does not significantly affect the measured traction. As shown in Fig. 47, a visible brown and blue reaction film has formed on the ball. The blue surface film that has formed on the ball is thicker than the slightly brown film on the disk. The thicker film on the ball may be due to the higher temperature (200°C)
(b) disk track
100 |»m
FIG. 47—Smooth M50 specimens from test NA912: a) ball track with brown and blue surface films; b) minor scratches and light brown film on disc surface.
1007
that the ball has reached compared to the disk (130°C). The reaction films are likely to have formed near the end of the load capacity test where the specimen temperatures are high and h/cr is small. Even with the presence of a significcint reaction film on the ball, there seems to be no perturbation in traction coefficient. The above tests show that the variation in traction behavior during a load capacity test does not seem to be affected by bulk oil shear of an EHD film or perhaps even the presence of boundary films. The traction behavior appears to be a function of how oil chemistry affects the topographical features on the surfaces by way of polishing wear and scuffing. The traction data with smooth surfaces and a standard (STD) MIL-PRF-23699 oil provide a lower bound traction, which is essentially unaffected by surface roughness features and boundary lubrication. The lower bound traction is attributed to the shear behavior (traction) of the bulk oil. Interactions between surface topographical features do not occur until late in the test protocol, when the contact temperatures are high and the EHD film is thin. Performance Criteria From all the load capacity traction data collected over time, there seems to be a strong connection between traction coefficient and wear of surface finishing features. While fluid temperature within the contact zilso affects traction, the rise and fall of traction coefficient still reflects the process associated with how the physical and chemical properties of the oil handle the intimate collisions of surface features within the contact during a load capacity test. Since the WAM High Speed Load Capacity Test protocol covers a large temperature range, we assume that the lubricating ability of the oil, as reflected in traction, is also being tested over a large temperature range. If this is the case, the lubricating ability of the test oils can be differentiated with respect to preservation of surface topographical features, at least over a limited range of temperature or contact severity. Additional investigations are required to determine if subtle differences in traction truly reflect variations in chemical activity for wear resistance in service hardware. It can be postulated that the desired lubricating attributes of oils are good wear resistance and scuffing resistance (and surface fatigue resistance) "across-the-board" of temperature and stress. The WAM High Speed Load Capacity Test protocol may be covering at least some of the desired performance features and test conditions. For gear or other surfaces with prominent roughness features one could argue that some mild polishing wear is desired to topographically condition the surfaces for low asperity stress to prevent early micro-pitting. If this were the case, good performance would be associated with relatively low traction coefficient and high scuffing load stages. Further tribology studies of service hardware are needed to clarify the desired oil attributes and testing conditions. Until this is done, we have to live with a tenuous link between qualification testing and field performance. For now, the traction behavior and scuffing resistance of an oil, as determined with the present set of Ryderlike test conditions, can serve as an initial step toward full characterization and clarification of performance criteria. In the meantime, the collection of a database for test oils, along
1008
MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
with field experience, should provide greater confidence in the test method and a near-term tool for evaluation of oil formulations.
attributes include various types of wear (adhesive, abrasive, chemical) and various types of surface initiated fatigue. While a comprehensive picture of oil performance attributes is unfinished business, there is a way to utilize the current database with field experience in order to establish a preliminary connection between load capacity testing and field service performance. Thanks to the unfortunate experience in military hardware with two oils, there is now a link between WAM High Speed Load Capacity testing and known deficiencies in the field. The average traction data for these two test oils are plotted in Fig. 48. Oil DLA 522 resulted in high iron content in engine oil systems with specific engine hardware (T53). Oil TEL-0004 caused wear problems in the Canadian Air Force. Both oils in Fig. 48 show a rapid rise in traction coefficient reflecting resistance to polishing wear of surface finish protrusions. At early load stages, these oils consistently transition into micro-scuffing and a rapid wear mode. Micro-scufHng is a superficial form of scuffing where loss of surface film lubrication causes local adhesion and wear of surface features. The plastic flow and rapid wear of surface roughness features is amply revealed by a sudden reduction in traction coefficient. The test data from these two oils are presented in Fig. 49, along with other U.S. Navy oils that are qualified products. The other oils are currently used in the field with no appar-
Preliminary Link to Service Performance The WAM High Speed Load Capacity Test Method provides traction and scuffing evaluation of jet engine oils with no specific connection to service performance. The test method is designed to simulate Ryder Gear ranking of selected reference oils supplied by the U.S. Navy. Qualification of MIL-PRF23699 oils with the Ryder Gesir Test Method requires oils to pass a gear scuffing load a few percentage points above the reference oil, Herco-A. The WAM High Speed Load Capacity Test Method provides greater differentiation between formulated oils and Herco-A than the Ryder Gear. Although there is greater differentiation, the alternative test method(s) for the Ryder Gear are no more useful with respect to judging the level of performance required for reliable performance in service. While the gear mesh contact conditions of the Ryder Gear do come close to engineering parameters of some aeropropulsion systems, extrapolation of results to other thermal, kinematic and material systems is not reliable. It is further recognized that scuffing performance is only one of several other tribological attributes that control performance. Other
0.10 0.09 C 0
0.08
Run File: naa.run Ball: 9310, Ra = .25 jam (10 pin) Disc: 9310, Ra = .15 ^m (6 |jin) Entraining Velocity: 5.72 m/s (225 in/sec) Sliding Velocity: 8.78 m/s (346 in/sec) Velocity Vector Angle (Z): 75°
Failure criteria (avg. of all tests)
iE 0.07 0
o O 0.06 c o 0.05 o
\ , \ I
CO
0.04
MIL-L-7808 Grade 4 TEL-0004 (4 tests) (NA1368-NA1371)
0 CO
I
MIL-PRF-23699 CI DLA 522 (4 tests)
0.03 0.02 -\
- Q-'
O'
e~^-o
o
Lower bound reference, polished surfaces, S I D oil
0.01 0.00
-T—1—1—1—1—I—I—I—r-'—I—1—1—1—1—1—r
0
2
4
6
8
-i—i—1—1—r-i—1—1—1—1—I—1—r-
10 12 14 16 18 20 22 24 26 28 30 32
Load stage FIG. 48—Traction and scuffing failure characteristics of oils with known performance difficulties is specific military aircraft hardware.
CHAPTER 36: BENCH TEST MODELING
0.10
0.09
Run File: naa.run Ball: 9310, Ra = .25 ^m (10 |jin) Disc: 9310, Ra = .15 urn (6 |Jin) Entraining Velocity: 5.72 m/s (225 in/sec) Sliding Velocity: 8.78 m/s (346 in/sec) Velocity Vector Angle (Z): 75°
I Lower bound { I failure stage | I based on militaryi hardware I experience I IProposed OEM (scuffing limit
1009
I \ Scuffing or micro-scuffing failure criteria (avg, of all tests) Oils DLA 522 and TEL-0004 have known deficiencies in field service. Oils DLA 562 and PE-5-L1859 failed the Ryder Gear test.
PE-5-L1874 4 tests)
DOD DLA 511 (4 tests) 4 of 4 tests suspended DOD PE-5-L2040 5> (4 tests)
3 of 4 tests suspended
0
6
8
10 12 14 16 18 20 22 24 26 28 30 32
Load stage FIG. 49—^Traction and scuffing characteristics of qualified oils and oils with known performance deficiencies.
ent difficulty. The family of oils in Fig. 49 includes STD, HTS, CI and DOD oils. The oils with known deficiencies are a CI oil (DLA 522) and a MIL-PRF-7808 Grade 4 oil (TEL0004). The traction data show that both of these oils encounter microscuffing at low load stages between 11 and 14. Early microscuffing and a rapid wear mode are consistent with high iron content detected in service with the engine oil DLA 522. Figure 49 also shows two CI oils (DLA 562 and PE-5L1859) that have failed the Ryder Gear test. These oils are plotted in bold. While these oils are not directly connected with field experience, the low Ryder performance is sufficient to raise questions about their qualification. The scuffing load stages for these oils are relatively low (15 and 18). A unique feature of these oils is that the traction behavior is substantially different from the previous two oils with known deficiencies in the field. The traction coefficient rises rapidly with load stage indicating a resistance to polishing action of roughness features. Apparently, the surface film chemistry that provides early resistance to polishing wear is not sufficient to prevent early scuffing. The connection between high traction and low scuffing performance is not understood. Anti-wear performance (high traction) does not translate into good scuffing performance, at least for these oils. Oil PE-5L1874, which also has high traction, shows good scuffing performance. The traction behavior of this oil implies good anti-wear and anti-scuffing attributes.
The oils plotted in gray in Fig. 49 are qualified products currently being used in service by the U.S. Navy. Many of the oils, except for the CI oils, are also used in commercial service. The family of oils in Fig. 49 shows significantly different traction behavior due to varying degrees of polishing wear. It is not known how variations in polishing wear translate into service performance. On the other hand, the scuffing performance criteria do have connection with service experience. The two oils with low scuffing (i.e., micro-scuffing) performance have known deficiencies from service experience. The two DOD oils, which are suspended at load stage 30 without a scuffing event, are known to be high performing oils for power transmissions. Based on somewhat limited field experience, one can establish a lower bound for load capacity scuffing performance. If military hardware has experienced difficulty with oils showing scuffing failures at load stages around 12 and 14, then a lower bound (without margin) could be established at a load stage 15. OEM Proposed Criteria The scuffing and traction performance data in Fig. 49 identifies the lower bound performance based on military hardwEire experience. A qualification limit of load stage 19 has also been proposed. The proposed scuffing limit is also identified in Fig. 49. With a proposed load stage limit of 19, some
1010 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
of the oils in Fig. 49 would be just short of passing. It is tempting to say that the difference between the lower bound at load stage 15 cind the OEM proposed limit at load stage 19 is a performance meirgin. Since performance margin is engine and application specific, any naargin above the lower bound is generic at best. The data in Fig. 49 reflect the scuffing or micro-scuffing performance of engine oils under specific test conditions. Extrapolation of this data to other operating conditions is not reliable. Since wear and fatigue attributes are not included in load capacity testing, the oil evaluation results should be considered incomplete. Some additional value to the scuffing results can be obtained by considering the traction behavior throughout the test. While the linkage between traction behavior and service performance is not clear, traction data and its connection with polishing wear can at least identify which oils are in-family or out-of-family. From preliminary examination of surface conditions of engine hardware at overhaul shops, we believe that the tribological surface conditions generated during the early load stages of the test protocol represent many of the features found on bearing and gear hardware under normal operation. If this is true, the wide variation in traction coefficient during the early load stages indicates that the performance of these oils may be noticeably different in service. Detailed tracking of test engine hardware and field service hardware with specific oils is a highly recom-
0.10 0.09 0.08
mended activity to sort out the significance of oil performance characteristics. This action is also an essential step for the development of testing methods, which are truly linked to service performance. Applicability of WAM High Speed Load Capacity Test Method The WAM High Speed Load Capacity Test Method is a simulation of Ryder Gear Test ranking. The test conditions are carefully selected to mtike the results correlate with the Ryder Gear Test. While the Ryder Gear Test operating conditions, in terms of rolling/sliding speeds, temperatures and contact kinematics, are representative of helicopter gearbox hardware, slight operational changes are likely to cause different rjinking. This is based on WAM load capacity tests conducted over a range of test conditions, which affect EHD film generation and contact temperature. Load capacity tests over a range of conditions are recommended. The conditions selected here are specific to Ryder ranking using a set of five reference oils supplied by the U.S. Navy. In addition, there is no confirmation that scuffing load capacity performance is in any way connected with other prominent life-limiting performance criteria, expressed as surface distress (wear and micro-pitting). Additional tests for surface distress, or a complete simulation of specific hardware, are recommended to supplement scuffing load capacity results.
260
Run File: naa.run Ball: 9310, Ra=.25|jm(10)Jin) Disc: 9310, Ra= .15 |jm (6 iim) Entraining Velocity: 5.72 m/s (225 in/sec) Sliding Velocity: 8.78 m/s (346 in/sec) Velocity Vector Angle (Z): 75°
Failure criteria (avg. of all tests)
r 240 7 220 - 200
0.07
180
c 0)
160
PE-5-L2000 (4 tests)
;o 0.06
140
O 0.05 c o ••g 0.04
PE-5-L1823 (3 of 4 tests suspended)
OilB (4 tests)
2
0) CO
100 80 60
0.03 ..Q^.-e—e—e--^*~^—49-
40
Lower bound reference, polished surfaces, STD oil
20
0.01 0.00
o
a. 120 E
0)
0.02
O
0 6
8
10
12
14 16 18 20 22 Load Stage Qualified STD MIL-PRF-23699 Oils
24
26
28
30
FIG. 50—Average traction and scuffing stages for qualified Standard (STD) IVIIL-PRF-23699 oils.
32
1
CHAPTER 36: BENCH TEST MODELING LOAD CAPACITY D A T A B A S E F O R Q U A L I F I E D AVIATION G A S T U R B I N E O I L S Since the initiation of the WAM High Speed Load Capacity Test Method a large database of jet engine oils has been established. A summary of the qualified oils in this database reflects the general performance of gas turbine oils available today, at least with respect to the current test method. Most of the commonly used qualified oil products under the specifications MIL-PRF-23699 and DOD-L-85734 are included in the database. The oil brands, which are coded, are presented according to oil type, such as standard (STD), high thermal stability (HTS) and corrosion inhibited (CI). Standard MIL-PRF-23699 Oils Five STD qualified oils are presented in Fig. 50. Oil PE-5L1823, which has been evaluated many times during test method development, consistently shows high scuffing load capacity. Three of four tests represented in the traction curve in Fig. 50 were suspended without a scuffing failure. Oil B, and particularly oil A, show relatively low scuffing load capacity compared to the other STD oils. The traction behavior for oil A reflects somewhat different chemical response than the other oils.
0.10 0.09 0.08
1011
High Thermal Stability MIL-PRF-23699 Oils Figure 51 shows four HTS oils with widely varying traction behavior. The steep rise in traction coefficient, along with the high scuffing failure stage, for oil B reflects highly responsive chemiccd film forming capability that protects surface features from wear and scuffing. The relative low traction coefficient for oil A reflects more polishing or chemical wear. Except for oil B, the scuffing failure stage for HTS oils is clustered around load stage 20. Corrosion Inhibited MIL-PRF-23699 Oils Except for one oil, the CI oils in Fig. 52 fail by a micro-scuffing event. The relatively low scuffing performance of CI oils is attributed to the competition between anti-wear additive (TCP) and the corrosion inhibitor. Advanced formulations, such as oil NAVAIR.00072, have been able to overcome this problem. High Load-Carrying DOD-L-85734 Oils The two DOD-L-85734 oils in Fig. 53 operate through the test protocol without a scuffing event. The good scuffing load capacity of these oils is attributed to a load-carrying
260
Run File: naa.run Ball: 9310, Ra=.25|jm(10Min) Disc: 9310, Ra= .15 •jm (6 pin) Entraining Velocity: 5.72 m/s (225 in/sec) Sliding Velocity: 8.78 m/s (346 in/sec) Velocity Vector Angle (Z): 75°
Failure criteria (avg. of all tests)
I- 240
220 200 O h 180
0.07 -_ c PE-5-L1990 (4 tests)
0.06 -_
c o
•SI
o CD
0.05 -_
160 OilB (4 tests)
Oil A (4 tests) (PWL14-PWL17)
0.04 ^
140
Q.
120
.0)
100
I I
80
0.03 z
h 60
0.02 ^
-e—&—o-
~*G"^"'
40
Lower bound reference, polished surteces, STD oil
0.01 -
20
0.00
T
4
5
8
10
12
14 16 18 Load Stage
20
22
24
26
28
30
32
Qualified HTS l\/IIL-PRF-23699 Oils FIG. 51—Average traction and scuffing stages for qualified High Thermal Stability (HTS) MIL-PRF-23699 oils.
E
1012 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
0.10 Run File: naa.run Ball: 9310, Ra = .25 urn (10 |jin) Disc: 9310, Ra = .15 ^im (6 iJin) Entraining Velocity: 5.72 m/s (225 in/sec) Sliding Velocity: 8.78 m/s (346 in/sec) Velocity Vector Angle (Z): 75°
0.09
c
0.08
it 0.07 o
o
PE-5-L1997 (4 tests) NAVAIR.00072 (4 tests)
0.06
c o 0.05 o 03
•NI
0.04
PE-5-L2001 tests)
O)
0.03
5
PE-5-L2002 (4 tests)
0.02 Lower bound reference, polished surfaces, STD oil
0.01 0.00
0
6
8
10 12 14 16 18 20 22 24 26 28 30 32
Load Stage Qualified CI MIL-PRF-23699 Oils FIG. 52—Average traction and scuffing stages for qualified Corrosion Inhibited (CI) MIL-PRF-23699 oils.
additive package. While both oils have good scuffing performance, their traction behavior reflects quite different wear performance. Extensive testing with these oils reveals significantly more chemical polishing wear with oil A compared to oil DLA 511. The lower traction coefficient with oil A is consistent with lower oil out temperature, as measured in Ryder Gear tests. While both oils have extensive field service, the noticeable difference in wear behavior, as reflected in these tests, has not been investigated in field service hardware.
Basestocks
S e l e c t e d U . S . Air F o r c e O i l s M I L - P R F - 7 8 0 8 Grade 3 and Grade 4
Master Chart S h o w i n g Lubricating P e r f o r m a n c e of Qualified Oils
The U.S. Air Forces oils in Fig. 54 show the effect of oil viscosity during the initial load stages where temperatures are relatively low. The markedly different traction behavior of the two Grade 4 (4 cSt at 100°C) oils indicates the strong role oil chemistry has on wear and scuffing capacity. The Grade 3 MIL-PRF-7808 has higher load capacity t h a n the Grade 4 oils. Since viscosity differences at elevated temperatures are insignificant, oil chemistry becomes the dominating factor in scuffing and wear performance.
Figure 56 shows a master chart of lubricating ability for the various classes of oils. This chart was created by averaging the traction and failure load stages of all the oils in a particular class. The Grade 4 MIL-PRF-7808 oils and the CI MILPRF-23699 oils show quite different traction and scuffing performance than the other oil categories. It is significant to note t h a t the more recent and "advanced" oils, such as HTS, CI and the U.S. Air Force Grade 4 have lower scuffing capacity t h a n the vintage STD oils. En-
The load Ccurying performemce of six basestocks is shown in Fig. 55. The higher traction and lower scuffing failure stages for oils PE-5-L2039 and PE-5-L1836 reflect m u c h different lubricating ability t h a n the other basestocks. At least for some oils, basestock performance affects the wear and scuffing capacity of the formulated oil [11]. In other cases, it appears that the additive chemistry can overwhelm the influencing chemistry of the basestock.
0.10 0.09 0.08
260
Run File: naa.mn Ball: 9310, Ra=.25|jm(10Min) Disc: 9310, Ra=.15Mm(6|jin) Entraining Velocity: 5.72 m/s (225 in/sec) Sliding Velocity: 8.78 m/s (346 in/sec) Velocity Vector Angle (Z): 75°
*^ 0.07 c
Failure criteria (avg. of all tests)
b 240
220 200
180 160 I
DLA511 (4 tests) (Na1251-Na1254)
1 0.06 a> O 0.05 c •g 0.04
0)
140 Q120 ^
100 «
CD I-
80 0.03
•o
CD O
60
0.02
40
Lower bound reference, polished surfeces, STO oil
20
0.01 0.00
o
T"
2
4
6
"T
T"
"T
"T
T
0
T
8
10 12 14 16 18 20 22 24 26 28 30 32 Load Stage Qualified DOD Oils, DOD-L-85734
FIG. 53—Average traction and scuffing stages for qualified DOD-L-85734 oils.
0.10 0.09 0.08 c 0) 0.07 o
Scuffing or micro-scuffing failure criteria (avg. of all tests)
Run File: naa.run Ball: 9310, Ra = .25 urn (lOnin) Disc: 9310, Ra = .15 urn (6 jjin) Entraining Velocity: 5.72 m/s (225 in/sec) Sliding Velocity: 8.78 m/s (346 in/sec) Velocity Vector Angle (Z): 75° Grade 4 MIL-PRF-7808 TEL-0001 A (4 tests)
O
O 0.06 : c o 0.05 o
Grade 3 MiL-PRF-7808L (4 tests) . Original /^ 27502 Prototype (4 tests)
CO
0.04 O) CO 1-
0.03
5
0.02
Grade 4 H/IIL-PRF-7808 TEL-0004 (4 tests)
CD
6.5 cSt
—«
e
e——&— o
Lower bound reference, polished surfaces, STD MiL-PRF-23699 oil
0.01 0.00
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 32
Load Stage U.S. Air Force Oils FIG. 54—Average traction and scuffing stages for U.S. Air Force oils.
1013
0.10 0.09
c
0.08
Run File: naa.run Ball: 9310, Ra = .25 urn (10 |jin) Disc: 9310, Ra = .15 urn (6 pin) Entraining Velocity: 5.72 m/s (225 in/sec) Sliding Velocity: 8.78 m/s (346 in/sec) Velocity Vector Angle (Z): 75°
Failure criteria (avg. of all tests)
PE-5-L1876 (4 tests) (NA1306-NA1309)
0
0.07
PE-5-L1887 (4 tests) (NA1300-NA1303)
(D O
O 0.06 c o 0.05 o
PE-5-L2039 (4 tests) (NA1293-NA1296)
PE-5-L2056 (6 tests) (NA1360-NA1385)
CO
0.04 (D CD
I
0.03
PE-5-L2057 (4 tests) (NA1364-NA1367)
PE-5-L1836 (4 tests) (NA1287-NA1296)
0.02 Lower bound reference, polished surfaces, STD oil
0.01 0.00 J 0
4
6
8
10 12 14 16 18 20 22 24 26 28 30 32
Load stage Basestocks FIG. 55—Average traction and scuffing stages for basestocic oils.
0.10 0.09 0.08 c o o 0.07 JE CD O
o
Run File: naa.run Ball: 9310, Ra = .25 urn (10 |jin) Disc: 9310, Ra = .15 urn (6 jjin) Entraining Velocity: 5.72 m/s (225 in/sec) Sliding Velocity: 8.78 m/s (346 in/sec) Velocity Vector Angle (Z): 75°
If' Failure criteria (avg. of all tests)
CI Oils MIL-PRF-23699
(4 oils)
Grade 4 MIL-PRF-7808 (2 oils)
HTS Oils MIL-PRF-23699 (4 oils)
0.06 ] STD Oils MIL-PRF-23699 (5 oils)
c o 0.05 o CO
0.04 ]
0
D) CO
I
0.03
DOD Oils DOD-L-85734 (2 oils) all tests suspended
0.02
Lower bound reference, polished surfaces, STD oil
0.01 0.00 )
2
4
6
8
10
12
14
16
18
20
22
24
26
28
Load Stage Qualified product types and basestocks FIG. 56—IVIaster chart with average traction and scuffing stages for various oii types.
1014
30
32
CHAPTER 36: BENCH TEST MODELING gine temperature demands for HTS properties and component corrosion problems requiring CI properties are being satisfied at the expense of lubricating performance. Advanced formulations have been able to avoid this problem with the aid of simulation test methods. The above experience d e m o n s t r a t e s h o w a quick turnaround performance test can be used by the oil formulator. While the qualified CI oils appear to be functioning in service, their development for b o t h corrosion inhibition attributes and lubricating performance has been difficult. Surface chemistry for corrosion protection easily interferes with the activation of surface films for boundary lubrication. Much can be accomplished with the availability of a reliable test.
SUMMARY AND CONCLUSIONS To reduce the risk in development of major mechanical systems, it is possible to make tribology simulation tests meaningful to service hardware. The secret to linking bench testing with performance prediction is the identification and quantification of key tribology parameters. These parameters include entraining velocity, sliding velocity, contact temperature, and degree of asperity contact. Effective simulation of the controlling lubrication and failure mechanisms requires highly flexible testing equipment. The independent control of entraining velocity and sliding velocity provides a means to development tribology Performance Maps. Performance Maps, which identify regimes of lubrication and failure modes, can be linked to lubricated contacts in service. If the operating parameters of service h a r d w a r e are sufficiently known, specific tests for scuffing, wear and fatigue can be conducted. The process of bench test modeling for material and lubricant development (Systematic Tribology) was illustrated with the simulation of a gear test (Ryder) used for jet engine oil qualification. Traction behavior and the documentation of surfaces reveals the controlling EHD and surface film lubrication mechanisms, as well as competitive failure mechanisms of wear and scuffing. To reduce the risks involved in the development of major mechanical systems, bench test modeling, involving multiple tribology attributes, aire needed to assure success.
1015
Acknowledgments The author gratefully acknowledges support from the U.S. Navy, U.S. Air Force, QinetiQ, and Pratt and Whitney, along with several lubricant suppliers and bearing companies.
REFERENCES [1] Wedeven, L. D., "Method and Apparatus for Comprehensive Evaluation of Tribologlcal Materials," United States Patent Number 5,679,883, Washington DC, 1997. [2] Wedeven, L. D., Goodell, A. J., Black, W. F., Stimler, M. J., and Ajayi, O. O., "Assessment of the Friction Characteristics and Scuffing Performance of DOD-L-85734 Oils," University of Dayton Subcontract No. RI-21494X, U.S. Air Force Contract No. F33615-92-C-2218, Feb. 11, 1994. [3] "WAM3 Economical Load Capacity Test," U.S. Naval Research Laboratory PO No. N00173-95-P-9981, September 11, 1995; U.S. Navy PO No. N00421-95-M-0037, September 28, 1995. [4] Ryder, E. A., "The Gear Rig as an Oil Tester," presented at the ASLE Gear Symposium, Chicago, IL, 26-27 Jan. 1959. [5] SAB AIR4978: Temporary Methods for Assessing the Load Carrying Capacity of Aircraft Propulsion System Lubricating Oils, Society of Automotive Engineers, Warrendale, PA, 1997. [6] Wedeven, L. D. and Hille, E., "Tribology Testing for Load Carrying Capacity of Aircraft Propulsion System Lubricating Oils," Bench Testing of Industrial Fluid Lubrication and Wear Properites Used in Machinery Applications, ASTM STP1404, G. E. Totten, J. R. Dickey, L. D. Wedeven, and M. Anderson, Eds., ASTM International, West Conshohocken, PA, 2001, pp. 318-332. [7] Wedeven, L. D., "Tribologlcal Performance Measurement Research," U.S. Navy Contract No. N00140-92-C-BD32, August 2000. [8] Wedeven, L. D. and D'Orazio A. J., "Technical Approach to Oil Testing and Field Service Performance," Presented at the SAEE34 Propulsion Lubricants Technical Symposium, 22-23 Sept. 1999, Cardiff, Wales. [9] Brown, S. C, Chin, H. A., Haluck, D. A., and Wedeven, L. D., "Linking Lubricants, Materials, Design and Tribology," Lubrication and Fluid Power, Vol. 2, No. 4, November 2001, pp. 7-21. [10] "R&D Technique to Determine Scoring Load Capacity of Lubricants," U.S. Navy Contract No. N00140-88-C-1717, August 1988. [11] Wedeven, L. D. and Hille, E., "Tribology Testing for Load Carrying Capacity of Aircraft Propulsion System Lubricating Oils," Bench Testing of Industrial Fluid Lubrication and Wear Properties Used in Machinery Applications, ASTM STP 1404, G. E. Totten, L. D. Wedeven, J. R. Dickey, £md M. Anderson, Eds., ASTM International, West Conshohocken, PA, 2001, pp. 118-332.
MNL37-EB/Jun. 2003
Lubricant Friction and Wear Testing Michael Anderson^ and Frederick E. Schmidt^
THIS CHAPTER PROVIDES VARIOUS METHODS TO EVALUATE THE FRIC-
TION AND WEAR PROPERTIES of lubricants and materials. If friction and wear can be controlled then the engineer can select materials and lubricants with a high degree of confidence. Many laboratory tests are used to evaluate the interaction of materials under a broad range of test conditions and controlled environments. In this chapter, the following topics will be discussed: • History of tribology testing • Basic types of tribology test systems and reasons for their use • Fundamentals in designing tribology tests • How to select a test device to simulate a field condition • Contact geometry used in bench tests • Standard and commonly used test devices • Designing special application bench tests • Common terminology relating to friction and wear testing
HISTORY OF TRIBOLOGY [1] From the beginning of time, m a n has tried to overcome friction and wear. The earliest application of friction is its use for building fires. To early man, fire offered many advantages including safety, light, warmth, and cooked food. Man also needed weapons to kill animals for food. Primitive techniques were used to sharpen sticks and stones. As simple as these would appear, this use of friction greatly enhanced man's quality of life during this primitive period. Later, as m a n began to cultivate the land to provide food to supplement his diet of animal meat and fish, agricultural tools became a necessity. Not only must they be durable, but they also had to be shaped. Simple manufacturing techniques were employed such as grinding. More durable materials were more difficult to make. As time went on, m a n used new techniques to help in this manufacturing stage. Simple engineering methods were employed such as pottery wheels. At the time the great pyramids and monuments in ancient Egypt were being built, m a n was beginning to use not only engineering techniques, such as rolling elements (logs) to reduce friction, but he was also introducing liquid media between the surfaces. Sometimes, these liquids were simply hydrated earth (clays, soaps, or other materials). Nevertheless, lubrication was becoming a part of life. ' Vice President, Falex Corporation, 1020 Airpark Drive, Sugar Grove, IL 60554. ^ Manager Services for Industry, Engineering Systems Inc., 3851 Exchange Avenue, Aurora, IL 60504.
Even though m a n was employing simple engineering principles and lubrication for manufacturing, it wasn't until the late 15th century, when Leonardo DiVinci first deduced laws governing the motion of a block over a flat surface, that the science of friction and lubrication was developed. During this time, primitive testing devices were developed to measure the force of one object moving against another. Scientist during this time also realized that measured forces were less when a material such as pig fat was introduced between sliding or moving surfaces; hence, the study of lubrication had begun. During the years that followed, friction, wear, and lubrication studies increased. As the industrial revolution brought more advanced machines for transportation and power generation, engineering became part of the curriculum at universities. These studies included the fundamentals of friction, lubrication, and wear. With new extraction techniques for obtaining crude oil and the ability to refine this oil, lubricants became m o r e commonplace. As lubricants became more widely used, technology was needed to eveduate the differences in properties and in various applications. In 1927, the first commercial tribomoter was introduced to blenders and manufacturers of finished lubricants. This tester "Pin and Vee Block test machine" provided suppliers with a method of measuring anti-wear and extreme pressure properties of the lubricants they were selling. Subsequently, tribometers such as the Timken®^ tester. Four Ball Wear and Four Ball EP, Block-on-Ring, and others were introduced to evaluate lubricants and materials under a variety of test conditions. These machines are described in this chapter. Further developments in transportation, medicine, and space exploration have provided impetus for the development of new lubricants and materials. With these technologies has come the development of test machine designs and test methods to meet the challenges of these new applications. Today, over 225 commercial and independent testing devices [2] have been developed.
BASIC TYPES OF TRIBOLOGY TEST SYSTEMS Laboratory testing of lubricants used in a tribology system involves different levels of sophistication. A tribological system consists of all relevant test parameters, materials in contact including the lubricant, if present, and any external, environmental conditions [3]. Each level of test sophistication ^ Timken Corporation, Canton, OH.
1017 Copyright'
2003 by A S I M International
www.astm.org
1018 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK has benefits and drawbacks. The three levels of test sophistication Eire as follows [4]: • Laboratory bench test devices (simple geometric contacts) • Component bench test devices (use of actual parts and assemblies in a laboratory test rig) • Field tests (materials and lubricants tested in actual systems) The most representative test program is one that uses the proposed material combination in the actual field situation [4,5]. Generally, this approach is not practical for several reasons. Costs can be prohibitive; the time to develop a meaningful program may be too long; and the environmental or ambient conditions are difficult if not impossible to control. These limitations leave many uncontrollable variables and possibly, a wide scatter of test data. Even component testing, which is a laboratory test rig that uses the particular section of the machine (or field application) that is of interest and in which the parts are made of the materials under evaluation, is more cost effective but rarely used as a first approach [3,6]. These instrumented laboratory test devices possess the same limitations as field tests, except that the ambient conditions are more controllable. However, laboratory bench tests are designed to move test pieces with simplified geometry under a variety of test loads, speeds, and environmental conditions. Although these simplified devices cannot produce exact operating conditions, they have the potential to produce results that provide meaningful data for a range of similar applications [5]. The wide use of these test devices, such as the Four Ball and Timken machines, for determination of extreme pressure properties reflects the low cost and ease of such measurements and the belief that test results correlate to some extent with performance [7]. The use of simple bench testing reduces the test evaluation to a single, specific tribological condition simulating, as close as possible, the operating conditions for the material and lubricants. Data generated from these tests are compared and those materials and lubricants are selected that jaeld the best wear life or performance for further testing under more specific test designs. Repeatability of the obtained test results can be better when the test is kept as simple as possible. Figure 1 gives a relative economic comparison versus repeatability for bench tests, component tests, and field tests. • • • •
The ability to use a bench test offers many benefits including: Simplicity of operation Lowest testing cost Accelerated test results Real time presentation of data to facilitate recognition of changing conditions.
BENCH TESTS Test tvDe
Relative cost
Repeatabiltv
bench
$
*
component
$$$
***
field FIG. 1—Economic comparison of test types.
• Accurate and precise indication of wear rates and performance properties given the test parameters under which the test is conducted • Inexpensive and uniform consumable test pieces • Test pieces from a wide range of materials and conditions • Small volumes of test fluid • Controlled test environments and ambient conditions • Convenience of operation. Commercial test devices offer significant benefits over test equipment made in-house. Because commercial test machines are made in quantity to the same manufacturing specifications, they can offer better test result comparisons between the laboratories using them. Commercial testers are often used when developing standardized test methods because of the availability of users willing to cooperate in the development of precision statements. In most cases, the test parameters are listed in standardized test methods. However, they may not provide the user all the necessciry information for evaluating his materials. Therefore, standardized test methods can be suitable starting points, but the user may need to modify the test parameters to achieve meaningful test results [3,8]. Usually more data can be obtained throughout the test rather than just the final specified endpoint or reported test result. Many data occur during the course of a test, including but not limited to, changes in lubricating mechanisms, changes in surface areas giving different contact pressures, development of lubricating films and surfaces, and so on. Therefore, the operator must identify these changes and develop test methods that facilitate obtaining as much pertinent information as is possible or required. Commercial test devices provide the following benefits: • Established and known precision • Simplicity of test operation • Many meet ASTM, SAE, ISO and other standard test methods • Flexible test procedures • Ability to compare results worldwide • Correlation with previously published field results • Support and assistance in operation and method development by the manufacturer
FUNDAMENTALS IN DESIGNING THE TRIBOLOGY TEST One of the most important concepts in understanding tribological testing is that it is a system. As a system, each test parameter affects the test result. Changing any test parameter can effect differences in wear rates and/or frictionaJ properties. One must identify the components and possible conditions that exist during the test as part of the test program and try to match these as closely as possible with those that are occurring in the field. The objective of testing is to produce, on the test rig, similar surface damage to that which occurs after failure in service [5,6]. The first step is to obtain a complete understanding of the field condition. The second step is the selection of the most representative bench test available and the development of the test procedure to be used. The third step is to conduct the test. And the fourth and final step is the review of the test data and development of conclusions [3]. Formulation
CHAPTER of conclusions at the end of the test requires the user to analyze the data obtained with respect to actual field conditions. The user then draws the appropriate conclusions and then may develop models for predicting wear of future applications involving similar materials and operating conditions [9]. When designing a laboratory test program, the basic steps for successful testing are as follows: Field Problem 1. Identify the location of the wear problem in the test system 2. Determine the failure mechanism 3. Identify the tribological conditions • Motion Sliding, unidirectional Sliding, bidirectional or reversing Rolling Fretting Speed Linear velocity Rotational velocity Contact Geometry Point Line Area Pressure/Load Normal loads Surface area of contact Temperature Bulk lubricant Contact temperature Type of Lubricant Fluid Solid Semi-solid (grease) Dry film None Lubricant Performance Properties Anti-wear Extreme pressure Chemistry • Lubricating Mechanism Enclosed chamber (flooded sump) Constant circulation Spray Coated Contacting Materials External Operating Conditions Ambient temperature Atmosphere Humidity Vibration Contaminants B e n c h Test Selection 1. 2. 3. 4.
Experience Standardized test methods Available test equipment Analytical methods such as Tribological Aspect Number (TAN) [3]
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Test Parameter Selection 1. Motion • Sliding • Rolling • Combination roll/slide • Unidirectional • Reciprocating 2. Speed • Constant • Ramping • Changing 3. Load • Constant • Ramping (to failure) • Cycling 4. Contact Pressure (Normal Load Over Area of Contact) • Hertzian (constant) • Hertzian (changing) • Area (constant) 5. Materials • Composition • Hardness • Surface finish • Micro-structure • Coatings • Surface treatments 6. Duration Fixed (compare amount of wear) • Time • Number of cycles • Total linear distance Time to failure (compare life of test) • Excessive friction/torque • Excessive temperature • Excessive wear rate • Excessive total wear Test Results 1. Wear • Volume of material lost • Weight loss • Dimension of wear scar • Dimension change of material 2. Friction Force or Torque 3. Coefficient of Friction • Static Coefficient of Friction • Dynamic Coefficient of Friction • Maximum • Minimum • Average 4. Correlation with Field Results • Type of wear mechanism • Type of failure mechanism • Relative a m o u n t of wear • Comparative ranking of different material's performance with field results • Meets specification 5. Lubricant Analysis • Additive depletion • Wear particle count • Viscosity change
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All controlled laboratory testing yields test data that reflect what is happening in the test device under the selected test conditions. If the test results do not correlate with the field results, then either the test device is not representative of the field application or an incorrect test procedure was chosen. Therefore, it is of the utmost importance that the user develops a meaningful test procedure and utilizes the type of test equipment that best represents the field condition. There Eire many approaches that are used to select the best test machine and the test parameters. This chapter will reference and discuss a practical, analytical approach to selecting the best bench test for simulating a particular field condition.
SELECTING AND DESIGNING THE TEST The first and most challenging step in conducting a successful laboratory test program is the selection of a test that will accurately simulate the field condition being investigated. As mentioned previously, there are three basic types of tests: bench, component, and field. Field tests are time consuming, and generally very expensive. They tend to have poor repeatability as they are subject to many complications in the control of their test parameters and ambient conditions. For these reasons this chapter is limiting the discussion to laboratory tests. Materials selected using laboratory tests must be qualified in a field test before actually being p u t into service. This is ultimately where the tested materials must work, but considerable testing can be done in the laboratory far more economically. Two types of tests that will be discussed in detail are bench tests and component tests. A good rule is to attempt the simplest test possible that accurately simulates what occurs in service. To that end, test devices have been developed that are used to simulate service conditions in the laboratory [2]. Selection of a laboratory test rig should be based on the ability to simulate as closely as possible the conditions of the field application. There are many ways to select a bench test, including experience, standardized test m e t h o d s , a n d p u b lished analjrtical approaches. One such approach is a paper written by R. Voitik, "Realizing Bench Test Solutions to Field Tribology Problems by Utilizing Tribological Aspect Numbers," incorporates an analytical approach to selecting the best bench test [3]. Laboratory tests are used in two basic functions: specification testing (or quality assurance) and research testing. Specification testing uses standardized laboratory bench tests, such as those based on the Pin and Vee Block, Timken, FourBall Wear, and Four-Ball EP test machines. Considerable testing has been performed on these test machines in the evaluation of materials for specific applications. Industrial laboratories have a sufficiently high degree of confidence that test results obtained on these test machines under prescribed conditions will correlate with performance in the field. These tests work well for screening formulations during lubricant development, for monitoring the quality of lubricant production and for lubricant specification [7]. Other uses for specification testing include evsiluation of a new formulation to meet an existing application, competitive performance properties evaluation, and approving new batches of
materials. Laboratory tests for specifications are generally accepted for verifying material performance. A widely used test procedure is generally submitted to a standardization organization to develop a test method for determining certain properties. A standardized test method is selected for use in specifications because the user determined that standardized test results correlate with field experience. Qualifying materials to a specification allows further development and improvement of materials for field use, and provides a means for assuring performance quedity. A critical part of the standardization process is the development of the precision of the test method. Precision is developed through the efforts of many testing laboratories participating in a cooperative testing program. In such a program, participants test selected materials according to a prescribed sequence. The laboratories obtain back-to-back duplicate test results. These test results form the basis for determining both repeatability and reproducibility of the test procedure. Precision gives the user of the test method the confidence to determine whether the test results obtained are significant, in other words, whether the results obtained are merely within the repeatability of the test method and are not actually different. Additionally, uncertainty could be reduced by a duplicate test or comparison with reference oil [7]. When there is no established or agreed upon test procedure, test method development can be more involved and create significant delays. The test method developer must select the test device, design the test method, run the tests and then determine the validity of the test results [3,6]. This process of developing a laboratory test method to predict field conditions can be difficult due to lack of information about the field conditions, availability of the certain test equipment and desired acceleration of the test. The volume of field performance data available for various test materials will determine the level of difficulty in developing base case information for test validation. Sometimes the designer chooses a component test as being the preferred approach for materials evaluation. Frequently, the customer requires data to be produced on the actual components. Or quite possibly, preliminary test data was generated on a simple bench test and subsequent developmental testing must be performed at the component level. Nevertheless, the fundamental criteria for selecting test conditions and parameters Eire similar whether one selects a component test or a bench test.
B e n c h Test Bench Test, as used in this chapter, is the term used to describe laboratory test devices that are simple in design, yet complex enough to rank a materiEil's performance for a specific property or to simulate an actual field condition. Unlike component test stands, bench tests are designed to isolate specific contacts, motions, loads, and geometric contacts. These physical characteristics combined with the selected test parameters give the researcher a means of easily evaluating materials for their effects in field applications or further component testing. In a properly designed test, operating under a lubrication regime similar to the field condition, the asperities at the point of contact will react the same as they would in the field condition that the bench test is represent-
CHAPTER ing. Correlation has been shown by Faville [8] when using the Pin and Vee Block for evaluating transmission and other lubricants. Component Test Design Component testing, which includes, but is not limited to, p u m p tests and engine test beds, uses actual parts or components from manufactured equipment. The test stand designer must carefully select these parts to be within well-defined manufacturing tolerances for dimensions, surface finish, and hardness. The component fixturing is typically designed to hold the components and test them in a manner that represents, as close as possible, the actual field conditions and environment while maintaining the desired test parameters. The test device design should be sufficiently flexible to permit a wide range of operating conditions. To achieve reasonable repeatability and reproducibility with the test results obtained from the test stand, close control of the test parameters must be maintained. In the actual field condition it is virtually impossible to control all of the a m b i e n t conditions, due to the wide fluctuations in surrounding conditions. These test parameters for a component test stand should be selected to represent conditions that might occur in actual applications. The selected parameters must be closely controlled, and the test stand must be designed to provide provisions for monitoring the selected test parameters. Monitoring the test parameters will provide the operator with a recorded history indicating whether these parameters were maintained during the test. Often times controlled atmospheres, large sumps for test fluids, special air and fluid filtration, temperature control, load control, and other more specific systems must be designed into the test stand to maintain test parameters or for better simulation of field conditions. These test devices are designed to simulate a particular aspect of actual operation and are valuable for the development of additives and lubricant formulations [10]. In engine test stands, for example, proper simulation of actual driving conditions includes cycling of load, speed, and temperatures according to a designed test program. It is essential to integrate the cyclic characteristics to simulate the actual driving conditions on the laboratory test rig. The use of a computer to control test parameters provides more consistent test operation and facilitates data acquisition Monitoring test variables during the test sequence is critical. The computer, with its capability of acquiring and storing the test data, has offered considerable benefit to the test operator by recording data throughout the test. This data will advise the operator that the test has run within the selected controlled parameters and that the test is operating as desired. It will also alert the operator to a change or failure in any of the measured properties. Because ambient conditions can also influence test results, it is important that these conditions such as t e m p e r a t u r e and relative humidity be recorded. The computer may provide more rapid data collection when one or more of the variables exceed an alarm condition. This rapid data collection will give more detailed information of the test results when unusual conditions are present. The computer can also activate the control function of the test stand, eliminating the need for the operator to make the control adjustments after the test has initiated. The
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computer can also cause the test to terminate if a predetermined set-point is exceeded or if a dangerous condition exists. And finally the computer can organize, calculate, and present the data in tabulated or graphic format for ease of interpretation. This valuable tool is used in most component test formats and for an increasing number of standard bench tests stands. With component tests, the parts chosen for the test fixturing must have some sensitivity to the materials under evaluation [11]. For example, p u m p s that are used to evaluate wear of hydraulic fluids should be sensitive to formulated fluids that have sufficient anti-wear properties and those that do not [12]. There should always be some check that the test device and selected test parameters and sequences are robust enough to discriminate between materials with known poor field performance and those with acceptable field performance. The user should prove that the component test has a n acceptable degree of precision, both in repeatability and in reproducibility. Repeatability is the closeness of the agreement of test data on back-to-back testing on the same test stand, in the same laboratory, with the same operator, on the same test materials or fluids, within a short time span. Reproducibility is the agreement of test results using the same test materials or fluids but on different test stands, with different operators, in different laboratories, and run at different times. When designing any laboratory test rig, the design should provide for wide latitude of test parameters. This will assist in discriminating between materials or components tested with materials of varying performance properties. When a series of test parameters has been chosen that demonstrates differences in materials of known field performances, the test stand should successfully rank materials with unknown field performance. Extreme care should be exercised when selecting existing manufactured parts for use as testing components, as these parts must have some guarantee of consistency of manufacture and known tolerances for dimensions and materials. Nevertheless, successful testing has been developed and has given the test engineer considerable information on the performance of materials predicted from the results of component testing under actual or near field conditions. Properly designed test stands should mimic the field conditions as closely as possible so that the contacting materials will exhibit properties exactly as if they were in the actual field condition.
TEST PARAMETER SELECTION The basic test parameters to be considered when developing any type of test method are discussed in this section. Temperature Temperature control can be effected by the use of heating systems if elevated temperatures are required and/or cooling systems if lower than ambient test temperature is required. The test fixturing can be designed to contain heaters or cooling tubes to maintain the bulk temperature of the lubricant or the test pieces. It is virtually impossible to measure the temperature at the point of asperity contact, but by
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HANDBOOK
locating the temperature sensor in close proximity to the contact surface, one can get an idea of the contact temperature versus the bulk lubricant temperature. In some test designs, the atmosphere surrounding the test area must have a controlled temperature. This is usually achieved through some type of air circulation if lower than ambient temperature is required, or through the use of heaters external to the test component but within the enclosed testing chamber. Large fluid reservoirs can help in maintaining the stability of test temperature by providing a large volume for heat absorption. A large reservoir also permits the use of the fluid in extended duration testing with a minimal amount of additive depletion.
Load Load control must be considered when selecting a test. Test load can be applied by a static or dead weight system, a pneumatic system, hydraulic system, or by fluid pressure. Consideration should be given to including an initial "break-in" or "wear-in" at a lighter load representing partial EHD [13]. Inclusion of a break-in can form a more uniform surface rather t h a n testing under machined conditions. This more uniform surface can add to the consistency of test results. It can be difficult to select test loads that will be representative of the loads encountered in the field, yet of a magnitude large enough to challenge the test system without introducing any complex or other type wear mechanisms. Selecting loads that are excessive could produce results that are not representative of what is occurring in the actual field condition. If the operator chooses a test load that is low, he may encounter very long test duration before failures occur. Another way to determine an appropriate test load is to conduct a step load test and look for an erratic change in the torque or friction force as load is increased [8]. This variability of the torque indicates a breakdown of the lubricant film, allowing for metal to metal contact. The load immediately prior to the load that corresponds to the erratic torque or friction force can be looked at in terms of the threshold limit of the test system load. The operator can choose a test load at or below the threshold load that should give a controlled amount of wear in a reasonable time. Note that too light a test load may not give enough weeir to discriminate between samples and too heavy a load may yield too m u c h wear to discriminate. At this point, it is basically an educated guess as to the best test load. Several loads in this area may have to be tried in order to select the test load that gives the best discrimination with regards to the other test variable. Ideally, if materials with known field data are being used to set u p the test procedure, discrimination should be the focus. If discrimination is obtained, then this test load should be used as the starting point for the tests, that is to say that at some point in the testing with other materials, these test parameters may have to be reevaluated. Often times the load must be controlled to allow for cycling of test loads to simulate in-field conditions. Cycling versus static load can better simulate the stresses encountered in the test system and is used to better maintain the temperature of the test system in long-term endurance or life tests.
Speed If possible, select the linear test speed to be the same as the field condition. This is done in rotational tests by taking the speed of the field condition and dividing by diameter of the point of contact or wear track, and by pi, msiking the appropriate conversions for distance units results in the corresponding rpm. Certain applications require very slow or very fast test speeds. Test speeds have a profound effect on lubrication regime in the test system. The operator should consult the Stribeck curve for general effects of the change in load or speed on the coefficient of firiction and lubricating regime. The Stribeck curve (Fig. 2) shows the relation of coefficient of friction to the ratio of viscosity, speed, and the inverse of load, known as the Sommerfeld N u m b e r [14] with respect to coefficient of friction. Sommerfeld Number = speed X viscosity/load This fundamental curve gives an illustration of the effect of changing viscosity, speed or load on the coefficient of friction in the various lubrication regimes of boundary, elastohydrodynamic, mixed and hydrodynamic [15]. 1. Hydrodynamic lubrication: the surfaces are separated by the lubricant film resulting in low friction. 2. Mixed lubrication: the load is carried by the lubricant and the interacting asperities 3. Boundary lubrication: the load is solely carried by the interacting asperities, resulting in high friction. Although it may be impractical to construct the entire classic Stribeck curve, specific portions of the curve for vEirious test systems can be determined as illustrated in the curves listed. It should also be noted that different materials, geometries, and test systems can yield curves that are different in shape than the classic Stribeck Curve. For example, certain systems may not have a boundary lubrication region and may rise directly to a very high friction value indicating severe metal-to-metal contact. The curve in Fig. 3 depicts Stribeck curves in the mixed lubrication region as a function of lubricant thickness [15]. By changing speed and/or load in the Sommerfeld number, the resulting change in coefficient of friction is determined and plotted for each lubricant thickness. In the self-lubricated
speed X viscosity/ioad FIG. 2—Theoretical Stribeck Curve. Reprinted with permission of STLE.
CHAPTER 37: LUBRICANT FRICTION AND WEAR TESTING
tion and the wear of each material combination is compared. Test duration in this case is either by time or by number of cycles. If the materials are being severely challenged, tests can be run to a predetermined failure point such as a friction, wear, or temperature limit. In this type of test, the result is the time to failure or in the case of increasing load or speed, the load or speed at which failure occurred. When selecting test parameters to represent a field condition, it may not be practical in the laboratory to have failure occur in the same time frame or duration as it would in the field, or it may be desired to accumulate data in a relatively short period of time. This is referred to as an "accelerated test." Therefore, the test parameters must be selected to enhance the desired wear condition or to produce a desired wear condition in a shorter period of time. Careful consideration of accelerating test results must be given since it is important not to introduce any wear mechcinisms into the test system that are not occurring in the field. Verification must be made at the end of test to insure that the wecir mechcuiism or failure is the same as that which occurs in the field. Otherwise, it is possible to obtain test results that do not correlate with field results.
. . — ... 1.00E-04 1.00E-05 1.00E-06 5.00E-07 3.00E-07
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3.60E-08
FIG. 3—Stribeck curves for starved lubricated line contacts as a function of the applied lubricant layer thickness in )j.m [15]. Reprinted with permission of Prof. D. J. Schipper, University of Twente, The Netherlands.
Materials 0.20
Standardized test methods generally specify the materials cuid conditions of the test pieces consumed in the test. Simulation tests should utilize consumable test pieces from the same materials and conditions that are present in the system being modeled. The use of different materials than present on the system being modeled, can result in poor correlation of the test with the system [11].
•U 0.15 U-
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o O
0.05
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N=0.2 N/cm2
Special Atmospheres
N=0.4 N/cm N=1.0N/cm
N=0.8 N/cm I
10"
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10"
10"
10"
Sommerfeld Number FIG. 4—Coefficient of friction as a function of Sommerfeld number for a plastic on a PMMA disk lubricated by a saline solution [14]. Reprinted with permission from the Society of Plastics Engineers.
condition, as illustrated in Fig. 4, changes in speed with constant loads are plotted against coefficient of friction determined by testing. Similarly, tests could be conducted by varying the load maintaining constant viscosity and speed. Although it is possible to vary more than one variable in determining Sommerfeld numbers and their relation to coefficient of friction, it is more typically studied by varying just one of the variable's affect on the coefficient of friction, as illustrated in the above examples. Duration Test duration is also important. Often tests are conducted on many different samples at the same predetermined test dura-
If special atmospheres are present in the field condition, the test designer should consider these when designing the laboratory test procedure. Special atmospheres would include humidity, pressurization, and inert or reactive gases. Gases can be introduced into any closed test chamber. Inert gases can be introduced in an attempt to exclude the formation of oxide layers at the surface. Specialty gases, for example Freon, can be introduced into the lubricant, often under pressure, to understcuid the effect of these gases when solublized in the test lubricants. This approach is used for eveduating lubricants for use in compressors or refrigeration systems [16]. Test Fluids Test design of lubricated systems will always include the presence of test fluids and their influence on the prevention of wear or on load carr5dng ability. The method of lubricant introduction into the contact zone will affect test results. The most commonly used system is the test fluid bath. This lubricated system uses a contained quantity of test fluid, into which the moving test pieces are placed and remain as the test is performed. The contact may be submerged in the test fluid or a portion of the test piece enters the test fluid and carries the lubricant into the contact zone. Another lubricated system involves the use of an external reservoir, which intro-
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HANDBOOK
duces the test fluid into the contact zone either by continuously changing the fluid in a flooded test chamber or by injecting or depositing a controlled volume of test fluid directly into the contact zone. Special Testing Special testing is the addition of a test variable that is unique to a situation to better understand certain phenomena. These would include the introduction of abrasives or solid particles, the testing of used oils or test fluids with known history, or the addition of contaminants such as water or some other solid or liquid. Special testing is generally used in the laboratory test program when the field condition typically contains these special materials. Anything that can be done to better simulate field results will improve the simulated laboratory test results.
TEST SELECTION AND USE OF THE TRIBOLOGICAL ASPECT NUMBERS Selecting the best bench test is the first part of a test development program. Prudently selecting the bench test will provide for a more efficient development of test data. There are many ways used to select the best bench test to evaluate materieJs for a specific application. A common and important method is to draw on experience. Although experience is critical in developing any test method, it is also a limitation when evaluating materials and processes for new applications. Trial and error is another method that can be successful, but can also be very tedious and frustrating. Others have expanded geometry, speed, load, etc. and studied their effect on friction and wear [8,11,12]. In 1993, a paper was presented at the ASTM symposium on Wear Test Selection for Design and Application, entitled, "Realizing Bench Test Solutions to Field Tribology Problems by Utilizing Tribological Aspect Numbers" [3]. This paper and a subsequent review paper by Anderson [17] describe an analytical approach to selecting the best bench test for simulating a particular field application. This procedure identifies the tribological condition and characterizes it in a four-digit number based on motion, geometry, load (pressure), and entry angle of the test fluid. This approach can be used to select laboratory bench tests for simulating field conditions. Field applications can involve many different tribological conditions in the same system. It is the premise of using the Tribological Aspect Number (TAN) approach to isolate the tribological condition under consideration and then select the bench test that has the same TAN. The TAN is a four digit number that characterizes a tribological condition. The first digit identifies the velocity or motion characteristic. The second digit identifies the contact area. The third digit identifies the load or pressure. And finally, the fourth digit identifies the entry angle of the lubricant. The first step in the process of using the TAN system is to identify the TAN of the field condition. Second, the user selects the bench test that has the same TAN number. Note that a testing device may have been designed to provide for more than one TAN and there may be more than one tester that can
yield the same TAN. After identifying the bench tester to be used, the researcher must select the test parameters under which the test will be performed. These conditions should be selected to mimic or reflect the actual field conditions. These parameters are discussed earlier in this chapter. With the exact match of the TAN for the field and the tester, and selecting the same test parameters, the proposed test program is identified as a simulation. A simulation is a set of conditions that exactly represents the field condition. It is under this condition that the contact asperities should behave in an identical manner as those contact asperities in the field condition. First Digit—Speed The contact velocity or motion characteristic is subdivided into four types. Unidirectional, identified as 1, is a motion that does not change directions. It is unidirectional and is the sliding motion encountered in most typical wear testers. Cyclic motion, identified as 2, is motion that changes direction. It is also known as reciprocating motion and is more specific to select applications involving this type of motion. Roll/Slip, identified as 3, is motion that includes partial or complete rolling. Complete rolling occurs in ball or roller bearings; partial rolling, or those conditions that only have a percentage of rolling, is typical of gear simulations. Finally, fretting, identified as 4, is given its own, unique condition. Second Digit—Contact Area The contact area describes the geometry of the areas in which the moving pieces contact each other. The descriptions simply and accurately describe the type of geometric contacts that are present in the system. The open nature of the TAN 7 and 8 is unique in that one of the contact surfaces is being contacted in a previously untouched area. The fixed refers to non-changing contact geometry; the variable refers to changing contact geometry. Third Digit—Contact Pressure or Loading Characteristic Unidirectional, third digit of 1, identifies a condition of constant load or increasing load. High frequency, third digit of 2, is a quickly changing test load. Cyclic Loading, third digit of 3, refers to a condition of cycling the test load. Fourth Digit—Entry Angle The entry angle identifies the geometric angle of the leading edge of the tribological contact. It indicates the facility at which the lubricant can enter the contact zone. An angle of 0°-0°, fourth digit of 1, describes a contiguous contact. That is to say, there is no provision for the lubricant to enter the contact zone. This TAN is typical of thrust washer or seal applications. Because no lubricant can enter into the contact zone, these applications would do well using solid bonded lubricants, self-lubricating plastics, or high wear resistant ceramics. An angle of 90° to 75°, TAN of 2, is a very steep angle and would typically allow very little lubricant into the contact zone acting like a plow. As one moves from a fourth digit of
CHAPTER 3 7: LUBRICANT FRICTION AND WEAR TESTING 2 through Tan of 7, the angle of entry gets increasingly smaller. This smaller entry angle permits entry of the lubricant with ever increasing ease. An entry angle of < 10° - >2°, TAN of 8, has a very small angle of entry. This small angle of entry allows for the lubricant to come into the contact zone and reveals the effectiveness of the ability of a journal bearing, with its small entry angle, to easily carry the lubricant into the contact zone. And finally, >2°->0°, fourth digit of 9, illustrates the very, very small entry angle as is t5T3ical of rolling elements. Once the TAN for a field condition has been identified, matching the bench test's TAN to the field condition's TAN can easily affect selecting the bench test. However, this is not always possible. Many times there is no exact match available to the researcher. This occurs because of the complexity on one or more of the TAN digits or in the fact that the best test device just is not available to the researcher. In such cases, the researcher should select a bench test with a TAN as close as possible to the field condition. He may then have to make some test procedural compromises to permit as close a representation of the selected laboratory tester with the field application. This condition represents a ranking rather than a simulation. A ranking test basically will provide performance test data for a particular property based on the value obtained from running the test. Additional considerations when testing in the laboratory are to select the material for the consumable test pieces to be representative of the field condition material. This includes selecting not only the type of material, but the condition of the material such as hardness, case depth, surface finish, coatings, surface treatments, and microstructure. In the laboratory environment, if one were to simulate the test parameter conditions exactly as in the field, the test would take about the same time to fail in the laboratory as in the field. This just is not feasible; therefore, it is always desirable to acquire the data under accelerated conditions. Care should be taken not to introduce any complex or catastrophic wear mechanisms that are not present in the field and may adversely affect representative test results. This chapter has discussed some of the techniques available for selecting bench tests, choosing test parameters, acquiring test data and evaluating the results with respect to the test procedure. The following sections will contain some of the more common test devices available and their respective standardized test methods.
The basic design of the Pin and Vee Block consists of two opposing Vee blocks loaded against a rotating journal pin (Fig. 6). It conducts tests in four-line contact, unless optional C-Blocks are used. C-Blocks give a conformal area contact. The test is run with the pins and blocks submerged in the test lubricant (D 2670, D 3233) (Fig. 7), with the test pieces coated with a bonded film lubricant (D 2625), or with the lubricant coating the test pieces (D 5620). Load is applied via a ratchet wheel and eccentric pawl. Each turn of the motor will advance the ratchet wheel one tooth when the pawl is engaged. Tests can be run at constant load (D 2670, D 2625 procedure A) or at increasing load until failure (D 3233, D 2625 procedure B). In the increasing load test, failure is indicated by a break in either the shear pin or test pin or in the inability of
FIG. 5—Pin and Vee Block Tester (Falex Lubricant Tester).
FIG. 6—Pin and Vee Block test pieces.
COMMON TYPES OF B E N C H TEST DEVICES Pin and Vee Block This Pin and Vee Block (Fig. 5) is the most widely used commercialized wear tester for evaluating lubricants [18] Also known as the Falex®'* Lubricant Tester, the Pin and Vee Block test machine has been successfully used for evaluating lubricating and wear preventing properties of lubricants, both fluid and solid, for over 75 yccirs. This tester is used for evaluating metalworking fluids, automotive and industrial lubricants, and bonded solid film lubricants [8]. '' Falex Corporation, Sugar Grove, IL
1025
FIG. 7—Pin and Vee Block test piece configuration.
1026
MANUAL
3 7: FUELS AND LUBRICANTS
HANDBOOK
FIG. 8—Falex Test Pieces [8]. A: Unused (new); B: After ASTM D 2670 wear test; C: After ASTM D 3233 EP test, torque failure; D: After ASTM D 3233 EP test, weld failure. the test system to maintain test load. Careful monitoring of the torque with respect to the test load can yield valuable information as to the lubricating properties of the test fluid as it interacts with the selected test piece materials [8,19] Changes in the slope of the torque curve can reveal changes in the lubricating regime of the test system. Careful examination of the torque, load, and wear values gives information on the anti-wear and extreme pressure properties of the tribology system. Tests can be r u n at constant load for evaluating anti-wear properties and also u n d e r increasing load conditions to evaluate lubricating effects at different load conditions. Although the ASTM test m e t h o d s for evaluating extreme pressure properties of lubricants directs the user to increase load u p to the point where either the test or shear pin breaks, the information obtained during the entire test can provide important data as to the performance of the lubricating properties. An important p h e n o m e n o n described by Faville [8] and later elaborated by Helmetag [19] is the occurrence of a sudden increase in the torque, also referred to as the torque "pop-up." Anti-weld evaluations can be made only in the load range immediately following this initial seizure. Products lacking anti-weld properties tear out metal, resulting in weld type seizure (Fig. 8D), while some products develop high torque, which result in twisting off the shear pin without any occurrence of scoring (Fig. 8C). The latter failures cire referred to as torque seizures [20]. The ASTM test methods that relate to the Pin and Vee Block test machine and their typical test results (Fig. 8) Eire: • ASTM D 2625, Endurance (Wear) Life and Load-Carrying Capacity of Solid Film Lubricants (Falex® Pin and Vee Method) • ASTM D 2670, Measuring Wear Properties of Fluid Lubricants (Falex® Pin and Vee Block Method) • ASTM D 3233, Measurement of Extreme Pressure Properties of Fluid Lubricants (Falex® Pin a n d Vee Block Methods) • ASTM D 5620, Evaluating Thin Film Fluid Lubricants in a Drain and Dry Mode Using a Pin and V-Block Test Machine Pin on Disk Pin-on-Disk test is the simplest and most c o m m o n wear test device [21]. It consists of a rotating disk upon which is loaded a pin or ball (Figs. 9 and 10) In the simplest versions, the ball rotates on the Scune wear scar. Other mechanisms can be incorporated such that the pin or ball comes in contact with a n untouched portion of the rotating disk. This results in a spiral type of wear track. Pin-on-disk testing is used most widely
FIG. 9—Pin-on-Disk.
FIG. 10—Pin on disk test configuration.
for determining wear rates and endurance life of coatings and bonded film lubricants, but can be used with liquid lubricants for determining wear rates and coefficients of friction (G 99). As wear progresses, material can be removed from the pin or ball or from the disk or transferred from one piece to the other. Therefore, as with any wear test, both pieces should be examined for wear or material transfer. The pin or ball can be eveJuated by measuring the diameter of the wear scar and observing any material transfer. The wear scar on the disk can best be characterized by a profilometric trace across the surface to determine depth juid width of the wear scar or by another surface characterization method (Chapter 35). Pin on Disk tests will give relative wear rates only, as few actual field components match a pin-on-disk configuration. The test methods that relate to the Pin on Disk test machine are: • ASTM G 99, Wear Testing with a Pin-on-Disk Apparatus • ASTM G 132, Pin Abrasion Testing Four Ball Four Ball tests include the Four Ball Wear Test, the Four Ball Extreme Pressure (EP), Rolling Four Ball, and Ball on Three Disks. Both the Four Ball Wear and the Four Ball EP test machine have three lower balls that are either held stationary (Fig. 11) or allowed to roll in a race. The upper ball is held in
CHAPTER 3 7: LUBRICANT FRICTION AND WEAR TESTING a chuck and rotated. The tests in which the three balls are held stationary are the most common of the Four Ball tests. It provides for pure sliding wear in initial three-point contact. After initial motion begins, the point contact develops into a load bearing, area contact. At the end of the test run, the width of the wear scar on each ball is measured using a microscope designed for this purpose (Fig. 12). Measurements are taken, once with the striations of wear and again at 90° (Figs. 13 and 14). The six scar diameter measurements are averaged for the reported wear scar diameter. The main advantage of testing with balls is that they are very consistent in shape and properties and that if they Eire available, they are very cost effective. The main disadvantage is that if they are not available, they can be very expensive to manufacture and perhaps impossible to manufacture depending on the shape of the bulk material. For instance, if the materiEd is available only in sheet stock, a ball cannot be made. In this case an adapter called the Ball-Three-Disks can be used to effect sim-
1027
FIG. 14—Ball scar measurement.
FIG. 11—Four Ball.
FIG. 15—Four Ball Wear test machine.
ilar geometrical contact. It allows geometry similar to the Standard Four-Ball configuration while permitting the testing of additional materials. This test has found application for testing for Einti-wear properties of diesel fuels containing lubricity additives [22,23,24]. FIG. 12—Typical scar measurement system (microscope).
FIG. 13—Typical wear scars.
Four Ball Wear In the Four Ball Wear tests (D 2266, D 4172), the upper ball is allowed to rotate under load for an extended, predetermined period of time, tj^pically 60 min. The wesir scar average is reported Eind compared against a specification or other wear scars obtained by testing comparative fluids under the same conditions. The Four Ball Wecir test machine (Fig. 15) has a very precise loading system Eind because of this, has a limited range of test loads. Coefficient of friction is often a desired result of the test. ASTM D 5183, using the Four Ball
1028 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
Wear test machine, was standardized because the friction trace in standard wear tests (D 2266, D 4172) can be erratic under normal operating conditions, making it difficult to determine a reportable value. This alternate test provides for a break-in run using a mineral oil to intentionally create a consistent load bearing area contact prior to the introduction of the test lubricant. The minereJ oil is drained from the ball cup; the balls aire cleaned without their removal from the ball cup; and the test is run in a series of 10 kg increment loads for 10 min each to determine the coefficient of friction at each load tested. Incipient seizure, which is the localized fusion of metal between the rubbing surfaces of the test pieces, can also be observed. Incipient seizure indicates that the film is breaking down and allowing metal-to-metal contact. One can report the coefficient of friction at any desired test load. This test will generally give a steady, coefficient of friction value that will be easy to determine. The standardized test methods that use the Four-Ball "Wear test machine are as follows: • ASTM D 2266: Wear Preventive Characteristics of Lubricating Greases (Four Ball Method) • ASTM D 4172: Wear Preventive Characteristics of Lubricating Fluids (Four Ball Method) • ASTM D 5183: Determination of Coefficient of Friction of Lubricants Using the Four-Ball Wear Test Machine
APPLIED LOAD, kgf ABE—compensation line. B—Point or last nonseizure load. EC—Region of incipient seizure. CD—Region or immediate neizure. D—Weld point.
FIG. 16—Schematic plot of scar diameter versus applied load.
Four Ball EP In the Four Ball EP test (D 2596, D 2783), the upper ball is allowed to rotate under load for 10 s, after which the resultant wear scars are measured and averaged. In this test, a series of test runs is performed at logarithmically increasing loads up to the weld point. The weld point is the load at which the lubricant film breaks down and the temperature at the point of contact is so high that it melts the metal, causing the test balls to weld together, indicating that the extreme pressure level of the lubricant has been exceeded. When the lubricant is performing as designed, the wear scars will be very smzdl, only slightly larger than the corresponding theoretical Hertzian scar diameters for the given materials, load, and radii of the test balls. The Hertzian scar diameter is the average diameter of an indentation caused by the deformation of the balls under static load (prior to test). The line that parallels the Hertzian line is referred to as the compensation line. When the lubricating film breaks down, metal-to-metal contact occurs and mild to severe incipient seizure occurs. This seizure is evidenced by the disproportionate increase in the average scar diameter. When incipient seizure is present, the test ball scar diameter is no longer on the compensation line. The highest test load that yields a scar diameter within 5% of the compensation line value for the corresponding load is the last non-seizure load. The ASTM D 2596 and D 2783 test methods provide an index of the relative wear performance with respect to load for the lubricant under evaluation., which is shown in a graph of the wear scar versus test load (Fig. 16). This term is called the Load Wear Index (LWI). Because of the wide range of test loads (8-1000 Kg) 2ind the severe conditions that occur when a weld point is reached, the Four Ball EP test machine (Fig. 17) is designed to be very robust in construction. ASTM D 2266, D 4172, and D 5183 warn against using the Four Ball EP test machine for running
FIG. 17—Four Ball EP test machine.
wear tests, as it lacks the necessary precision. When running wear tests, the Four Ball Wear Test Machine should be used. The Four Ball EP test machine is also used for testing rolling elements (Fig. 18). This is because of the high test loads that are available. These tests are referred to as contact fatigue tests and are used to predict the life of a lubricant when used in ball or roller bearings. A vibration detection device is required to identify the onset of surface fatigue. Another test of interest is a lubricant shear test. Also known as the KRL test (CEC L-45-T-93), this test uses a tapered roller bearing under high test load to shear polymer containing lubricants. The viscosity is measured before and after shearing to determine viscosity loss. Results of this test have correlated well with the shear losses experienced with multiviscosity gear oils used in manual transmissions.
CHAPTER
37: LUBRICANT
FRICTION
AND WEAR
TESTING
1029
The standardized test methods that use the Four Ball EP test machine are as follows: • ASTM D 2596, Measurement of Extreme Pressure Properties of Lubricating Greases (Four Ball Method) • ASTM D 2783, Measurement of Extreme Pressure Properties of Lubricating Fluids (Four Ball Method)
Block on Ring The Block-on-Ring test machine (Fig. 19) is more of a research tool. It is primarily used to determine wear rates of materials and to rank materials in pure sliding motion. The tester is designed to accommodate different test fixtures to effect point, line, ellipsoid, and area contact. The standard block-on-ring test uses a rectangular block on a rotating ring and starts as Hertzian line contact. As motion begins, a load carrying bearing surface forms, allowing the formation of anti-wear and/or EP films to form on the surface. The wear scar width is measured and reported at the end of the test (Fig. 20). The preferred method of reporting is volume loss; however, if the same metals are being used, simply reporting the wear scar diameter for comparative wear is acceptable. A table in ASTM G 77 gives block scar volumes for measured wccir scar widths. Oscillating drive mechanisms can be installed to effect reciprocating (back and forth) motion. This motion is used in test methods for evaluating greases and bonded film lubricants. Testing with a ball on ring combination results in initial high Hertzian point contact. After motion begins, the Hertzian point contact area develops into a load carrying bearing surface. With increasing loads, moni-
FIG. 20—Block on ring test piece configuration. TImken size (left); Falex size (right).
RING a. BLDCK
BOTATIDN
ROTATION
FIG. 21—High pressure (rectangular) block (left); Low pressure (conformal) block (right).
FIG. 18—Rolling four ball.
FIG. 19—Block-on-ring test chamber.
toring the friction force of lubricants containing EP additives has shown good correlation with predicting load limits obtained from more sophisticated component tests [11]. Area tests that simulate journal bearing applications are effected on this test machine using the conforming or curved test block (Fig. 21). This configuration is most effective when testing polymeric or plastic materials. When testing under area contact, it is advisable to perform an initial break-in to achieve complete contact between the mating surfaces. If complete contact is not achieved, limited contact will give higher t h a n desired pressures on the contacting aireas, resulting in premature failure. Recently, a new adapter has shown promise in predicting anti-wear properties and coefficient of friction values of thin film lubricants s u c h as automatic transmission fluids. The canted cylinder adapter holds a cylinder against the test ring with initial Hertzicin point contact. An additional benefit is the ease of alignment during setup. As wear develops, an elliptical wear scar develops. The resultant wear scar provides for easy measurement eind determination of wear and friction properties. The standardized test methods that use the Block-on-Ring test machine sire as follows: • ASTM D 2714, Calibration and Operation of the Falex Block-on-Ring Friction a n d Wear Testing Machine • ASTM D 2781, Wear Life of Solid Film Lubricants in Oscillating Motion
1030 MANUAL 3 7: FUELS AND LUBRICANTS
HANDBOOK
• ASTM D 3704, Wear Preventive Properties of Lubrication Greases Using the (Falex) Block on Ring Test Machine in Oscillating Motion • ASTM G 77, Ranking Resistance of Materials to Sliding Wear Using the Block-on-Ring Wear Test • ASTM G 137, Ranking Resistance of Plastic Materials to Sliding Wear Using a Block-on-Ring Configuration Timken The Timken Extreme Pressure Test Machine (Fig. 22) was developed in 1932 to measure load carrying capacity of EP lubricants for use in steel production. It too, is a block-on-ring type test (Fig. 20, Fig. 23) and is CcJled out in many specifications for oils and greases requiring extreme pressure
properties or high levels of load carrying ability. This tester is designed to evaluate lubricants for low, medium, and high levels of extreme pressure for lubricating greases (D 2509) and fluids (D 2782). The test is carried out by running a series of 10 min duration test runs at increasing test loads to the point where scoring or seizure is indicated on the wear scar. This scoring is evident as lines that extend past the edge of the wear scar or as a scar in which scuffing is evidenced by jagged irregularities (Fig. 24) The tester is also designed to provide friction data, but is seldom used for this function [25]. The standardized test methods that use the Timken Extreme Pressure test machine are as follows: • ASTM D 2509, Measurement of Load Carrying Capacity of Lubricating Grease • ASTM D 2682, Measurement of Extreme Pressure Properties of Lubricating Fluids Tapping Torque
FIG. 22—Timken test machine.
The Tapping Torque test machine (Fig. 25) is designed to perform actual metalworking applications in a laboratory environment. Originally designed to perform thread cutting and thread forming, the Tapping Torque test machine can perform additional metalworking functions. There are two basic metalworking applications: metal removal and metal deformation. Metal removal techniques remove material to achieve the desired final shape, while metal deformation techniques reshape or form the existing material into the desired shape. Under metal removal, there are thread cutting (tapping), drilling, and reaming. Under metal deformation, there are thread forming (tapping), roll forming, drawing, and rolling mill simulation. The tool rotates and descends at a rate determined by the rotational speed and pitch of the lead screw of the tapping head. The test machine measures the torque as the tool descends and enters the material to be machined (Fig. 26). The piece to be machined will have various forms depending on the test selected. It is most important to observe the tight tolerances required for the consumable test pieces. Even slight variances can have an affect on the precision of the test results. The ASTM standardized test method that uses the Tapping Torque test machine is as follows: • ASTM D 5619, Comparing Metal Removal Fluids Using the Tapping Torque Test Machine Multi-Specimen/Multi-Purpose (Thrust Washer Tester)
FIG. 23—Timl
The Multi-Specimen test machine (Fig. 27) is designed to be a versatile tribology test apparatus. It consists of two opposed, vertical test shafts. One rotates; the other is stationary. It is called Multi-Specimen because of the use of different adapters that can be placed between the opposing vertical shafts. These adapters affect many different tribological configurations. The Multi-Specimen can measure friction and wear under point, line, and area contacts, in pure sliding, pure rolling or combination roll/slide motion in unidirectioneJ or oscillation. Because a lubricant is another parameter for the tribological system, virtually any of the adapters
CHAPTER 3 7: LUBRICANT FRICTION AND WEAR TESTING
T>pic4l OK No Storing
1031
Improper Sclup
Scoring (failure) FIG. 24—Typical Timken test wear scars.
• Pin-on-Disk (G 99), (Figs. 9,10) • Oscillating Roll Slide for evaluating greases used in constant velocity joints of front wheel drive automobiles (Fig. 29) [26] • Stick/Slip for determining static coefficients of friction of way lubricants • Gear Cam Contact and Hypoid Gear with combination rolling and sliding motion for evaluating wear and friction of lubricants used in gear applications (Fig. 30) [27] • Ball Bearing tests for evaluating a lubricant's effect on wear in a ball bearing assembly • Sliding Bottle test for evaluating lubricants used on conveyors in beverage bottle filling machines • Sheet Metal Forming for evaluating the friction obtained during the forming of a flat piece of sheet stock into a grooved surface Linear Reciprocation PILOT PLUG SEALS WITI O'RINC
FIG. 25—Tapping Torque - exploded view of test area.
can accept a lubricant to determine its effect on friction and wear of the selected tribological system. Some of the adapters that are most specific to the evaluation of lubricants are as follows: • Vane on Disk test for evaluating friction and wear of hydraulic fluids under cycling stressed pressures (Fig. 28) • Thrust Washer test for area contact wear of plastic and ceramic materials (D 3702)
Linear reciprocation test machines are popular because they add the element of reciprocation or back and forth motion. This type of motion has proven effective in studying the friction, wear, and lubricating films occurring in applications such as a piston ring on a cylinder wall [28-30]. High-speed linear reciprocation has also shown good correlation in testing greases used in constant velocity joints of front wheel drive automobiles. Linear reciprocation can be achieved in point, line, or area contacts. Load is applied vertically. The speed is controlled by the rate of reciprocation, stroke length and test machine design and is not constant, but rather a sinusoidal wave shape, due to the start/stop reversal of the reciprocation function. The rate of reciprocation is measured in Hertz, or cycles per second. Therefore, the user must specify the rate of re-
1032 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK
T I M E (RCJTATION)
•
FIG. 2B—Tapping Torque trace of tap entering test piece (insets show position of tap in specimen blank).
FIG. 29—Oscillating roll/slide. GEAR/CAM CONTACT
R0TAT1C3N
FIG. 27—Multi-Specimen type tester.
TORQUE
FIG. 28—Vaneon-disk.
t
LOAD
FIG. 30—Gear cam contact.
CHAPTER 37: LUBRICANT FRICTION AND WEAR TESTING ciprocation (Hertz), stroke length, load, and temperature. The wear scar on the ball and disk can be evaluated as a measurement for wear. In EP tests, the load at which incipient seizure occurs is typically identified as the load carrying capacity of the lubricant with this test apparatus. The ball on flat reciprocating test is used for evaluating the lubricity of diesel fuels (D 6079) [31-33]. Depending on the design of the test machine, longer stroke lengths may compromise the rate of reciprocation. The user should verify desired requirements with the capabilities of the test equipment. The commercially available linear reciprocating test machines are the Flint and Partners TE77, High Frequency Friction Machine [34] (Figs. 31 and 32) and the SRV®5 [35] (Figs. 33 and 34). The TE77 test machine also provides for contact resistance measurements for evaluating lubricating surface films in elastohydrodynamic and boundary lubrication conditions. The ASTM standardized test methods that use linear reciprocating test machines are as follows: • ASTM D 5706, Determining Extreme Pressure Properties of Lubricating Greases Using a High-Frequency, LinearOscillation (SRV®) Test Machine • ASTM D 5707, Measuring Friction and Wear Properties of Lubricating Grease Using a High-Frequency, LinearOscillation (SRV®) Test Machine • ASTM D 6425, Measuring Friction and Wear Properties of Extreme Pressure (EP) Lubricating Oils Using SRV® Test Machine • ASTM G 133, Linearly Reciprocating Ball-on-Flat Sliding Wear ' Optimol Insturments, Munich, Germany.
1033
COMMON TYPES OF WIDELY U S E D COMPONENT TEST DEVICES FZG Gear The FZG Gear Test Rig [36], (Fig. 35) is a standard laboratory test machine, designed to test wear and load carrying capacity of fluid lubricants. The tester uses a matched set of spur gear (Fig. 36), referred to as Type A test gears, as the consumable test pieces. These are used for evaluating fluids for wear (scuffing) [37] and load carrying properties. Alternatively, a gear set of slightly different geometry having greater surface area is available, referred to as Type C gears.
FIG. 33—SRV® test machine. Reprinted with permission of Optimol Instruments Priiftechnik GmbH.
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I
•
t-^^i^ -^Bjfijl^j point FIG. 31—High speed linear reciprocating test rig (TE77). Reprinted with permission from Phoenix Tribology.
twm
am»
FIG. 34—Schematic of SRV® test pieces. Reprinted with permission of Optimol Instruments Pruftechnilc GmbH.
FIG. 32—Schematic of the test area geometries for TE77. Reprinted with permission from Phoenix Tribology.
1034
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
Type C gears are used for evaluating pitting and micro-pitting tendencies of industrial gear oils [38]. As with most component tests, this gear tester uses actual parts, in this case, gear sets. A constant load test is used for evaluating anti-wear properties of tractor hydraulic oils (D 4998). The test gears are weighed before and after the test. A load stage test of increasing test loads is used for evaluating industrial gear oils for their ability to carry a load. After each test load, the gear teeth are evaluated for signs of scoring (D 5182) (Figs. 37 and
® PlBion
®
LodUng Pin
®
GMrWiietl
® Uver Ann with WelgM Kk
®
Oriva QitMm
® TOKpi* Mtaswing Ciuteh
® UMd Clutch
38). Lubrication can be either dip lubrication in which the test gears are submerged in a known quantity of test oil, or in jet spray lubrication. Jet spray lubrication uses a nozzle to deliver the test lubricant directly onto the test gears. The ASTM standardized test methods using the FZG Gear Test Rig are as follows: • ASTM D 4998, Evaluating Wear Characteristics of Tractor Hydraulic Fluids • ASTM D 5182, Evaluating the Scuffing Load Capacity of Oils (FZG Visual Method)
® TtmptnMm Stnser
FIG. 36—FZG test gears on test rig. Reprinted with permission from StramaIV1PS IVIaschinenbau GmblH & Co. KG.
FIG. 35—Diagram of FZG test rig.
Polishing
Scoring
Scoring and Scuffing FIG. 37—Examples of FZG gear distress.
Scuffing
CHAPTER
37: LUBRICANT
FRICTION
AND WEAR
TESTING
1035
CONCLUSION
•
3 |
•1
Distress;
Pass
Scoring (15mm)
Scorinq (Smm)
Rating :
See 10.4.2
Scratches & Scoring
Rating
See 10.4.2
In this chapter, a systematic method to evaluate how to select appropriate bench or laboratory tests has been described using the TAN approach. Test equipment, methods, and their advantages have been detailed. Using the tools available in the market today, researchers can develop meaningful tests, acquire test data easily and rapidly, and then draw appropriate conclusions for improving the life and performance of industrial equipment. In summary, extensive progress has been made to formalize the procedures, practices, and test methods required to obtain predictive information on the performance of materials and lubricants. Additional information on the use of test e q u i p m e n t and the significance of test results can be obtained from the meuiufacturer and can be found in the Significance of Test section in the respective ASTM test methods.
eBj
5 m
ASTM STANDARDS Friction and Wear Properties Distress:
Scoring (20mm) Fail
7 M
Scratches & Scuffing (2mm) R8tlns
See 10.4.2
No. D1367
8 |M D2266
Oistmss: Rating :
Scoring & Scuffing (Smm) See 10.4.2
[)istress:
Scuffing (20mm)
Rating :
Fail
FIG. 38—Examples of FZG gear distress.
D2271
D2625
Vickers P u m p Stand The Vickers P u m p Stand is a controversial yet widely used component bench test. Although u n d e r current scrutiny for i m p r o v e m e n t of its precision, this test stand evaluates hydraulic fluids for wear using an actual p u m p . The p u m p parts are inspected metrologically a n d corrected, cleaned, weighed, and assembled prior to beginning each test. The test load is the fluid pressure, which can be either 1000 psi or 2000 psi depending on the test method. In some tests the loaded pressure exceeds that pressure recommended for normal operation. This pressure is chosen to challenge the test system in order to screen lubricants. At the end of the test, the p u m p cartridge pieces are again inspected for damage, cleaned, and weighed. This test is undergoing considerable modifications. It is recommended to review the latest draft prior to beginning ciny test program using this tester. The ASTM standardized test methods using the Vickers P u m p Stand are as follows: • ASTM D 2271, Preliminary Examination of Hydraulic Fluids (Wear Test) • ASTM D 2882, Indicating the W e a r Characteristics of Petroleum and Non-Petroleum Hydraulic Fluids in a Constant Volume Vane P u m p
D2670
D2714
D2882
D2981
D3336
D3704
Title Standard Test Method for Lubricating Qualities of Graphites (general laboratory test) Standard Test Method for Wear Preventive Characteristics of Lubricating Grease (Four Ball Method) (general laboratory test for wear of greases in sliding contact) Standard Test Method for Preliminziry Examination of Hydraulic Fluids (Wear Test) (general laboratory test for wear of hydraulic fluids under low pressures in p u m p test) Standard Test Method for Endurance (Wear) Life and Load Carrying Capacity of Solid Film Lubricants (Falex Pin and Vee Method) (genercil laboratory tests for load carrying and wear properties of solid lubricants) Standard Test Method for Measuring Wear Properties of Fluid Lubricants (Fcdex Pin and Vee Block Method) (general laboratory test for sliding wear) Standard Test Method for Calibration and Operation of the Falex Block- on-Biing Friction and Wear Testing Machine (generzJ laboratory test in sliding motion) S t a n d a r d Test Method for Indicating the Wear ChEiracteristics of Petroleum a n d Non-Petroleum Hydraulic Fluids in a Constant Volume Vane Pump (general laboratory test, known as the Vickers p u m p stand test) Standard Test Method for Wear Life of Solid Film Lubricants in Oscillating Motion (general laboratory test) Standard Test Method for Life of Lubricating Grease in Ball Bearings at Elevated Temperatures (general laboratory test) Standard Test Method for Wear Preventive Properties of Lubricating Greases Using the (Falex) Block-on-Ring Test Machine in Oscillating Motion (general laboratory test)
1036 MANUAL 3 7: FUELS AND LUBRICANTS D 4172 D 4173 D 4998 D 5001
D 5183
D 5619 D 5620
D 5707
D 6078
D 6079 D 6425 G 77 G 83 G 99 G 115 G 118 G 133
HANDBOOK
Standard Test Method for Wear Preventive Characteristics of Lubricating Fluid (Four Ball Method) (general laboratory test) Standard Practice for Sheet Metal Forming Lubricant Evaluation (general methodology for testing) Standard Test Method for Evaluating Wear Characteristics of Tractor Hydraulic Fluids (general laboratory test) Standard Test Method for Measurement of Lubricity of Aviation Turbine Fuels by the Ball-onCylinder Lubricity Evaluator (BOCLE) (general laboratory test) Standard Test Method for Determination of the Coefficient of Friction of Lubricants Using the Four-Ball Wear Test Machine (general laboratory test) Standard Test Method for Comparing Metal Removal Fluids Using the Tapping Torque Test Machine (general laboratory test) Standard Test Method for Evaluating Thin Film Fluid Lubricants in a Drain and Dry Mode Using a Pin and V-Block Test Machine (general laboratory test) Standard Test Method for Measuring Fiction and Wear Properties of Lubricating Grease Using a High-Frequency, Linear-Oscillation (SRV) Test Machine (general laboratory test) Standard Test Method for Evaluating Lubricity of Diesel Fuels by the Scuffing Load Ball-on-Cylinder Lubricity Evaluator (SLBOCLE) (general laboratory test) Standard Test Method for Evaluating Lubricity of Diesel Fuels by the High-Frequency Reciprocating Rig (HFRR) (general laboratory test) Standard Test Method for Measuring Friction and Wear Properties of EP Lubricating Oils Using the SRV® Test Machine (general laboratory test) Standard Test Method for Ranking Resistance of Materials to Sliding Wear Using Block-on-Ring Wear Test (general laboratory test) Standard Test Method for Wear Testing with a Crossed Cylinder Apparatus (general laboratory test) Standard Test Method for Wear Testing With a Pin-on-Disk Apparatus (general laboratory test) Standard Guide for Measuring and Reporting Friction Coefficients (guide for methodology) Standard Guide for Recommended Data Format for Sliding Wear Test (guide for methodology) Standard Test Method for Linearly Reciprocating Ball-on-Flat Sliding Wear (general laboratory test)
Extreme Pressure Properties D 1947
D 2509
Standard Test Method for Load-Carrying Capacity of Petroleum Oil and Synthetic Fluid Gear Lubricants (general laboratory test for testing load carrying capacity of oils in sliding and rolling contact) Standard Test Method for Measurement of Load Carrying Capacity of Lubricating Grease (Timken Method) (general laboratory test for extreme pres-
D 2596
D 2782 D 2783 D 3233
D 5182 D 5706
sure using block-on-ring type tester for seizure, galling and scuffing) Standard Test Method for Measurement of Extreme-Pressure Properties of Lubricating Grease (Four Ball Method) (general laboratory test for extreme pressure properties in sliding motion) Standard Test Method for Measurement of Extreme Pressure Properties of Lubricating Fluids (Timken Method) (general laboratory test) Standard Test Method for Measurement of Extreme Pressure Properties of Lubricating Fluids (Four-Ball Method) (general laboratory test) Standard Test Method for Measurement of Extreme Pressure Properties of Fluid Lubricants (Falex Pin and Vee Block Method) (general laboratory test) Standard Test Method for Evaluating the Scuffing Load Capacity of Oils (FZG Visual Method) (general laboratory test) Standard Test Method for Determining Extreme Pressure Properties of Lubricating Greases Using a High-Frequency, Linear Oscillation (SRV) Test Machine (general laboratory test)
OTHER STANDARDS Friction and Wear Properties DIN-Deutsches No. 50280
Institut fur
Normung
Title Running Test on Radial Plain Bearings; General Plain Bearings; Testing of the Tribological Behavior of Plain Bearings with Hydrostatic and Mixed Lubrication in Bearing Testing (General Laboratory Test for Sliding Wear of Bearings) 50281 Friction in Bearings; Definitions; Tjrpes; Conditions; Physical Quantities (definitions) 50320 Wear; Terms; System Analysis of Wear Processes; Classification of Wear Phenomena (definitions of wear terms and classifications) 5032l.G Wear Quantities (definitions of various wear types) 50322 Wear; Wear Testing Categories (definitions of scales of testing of all tjrpes of wear) 50323 Tribology; Terms (4 parts) (terms and definitions of wear t5fpes) 50324 Tribology; Testing of Friction and Wear Model Test for Sliding of Solids (Ball on Disc System) 51350 Testing in the Shell Four-Ball Tester (lubricant characterization) • Determination of the Wearing Characteristics of Liquids (Part 3); • Determination of the Wearing Characteristics for Consistent Lubricants (Part 5); • Determination of Shear Stability of Lubricating Oils Containing Polymers (Part 6) 51354 Mechanical Testing of Lubricants in the FZG Gear Rig Test (gear lubricant classification in sliding/rolling contact)
CHAPTER 37: LUBRICANT FRICTION AND WEAR TESTING
51389
51509
51834
• General Working Principles, (Part 1); • Method 1/8.3/90 for Lubricating Oils, (Part 2) • Shear Stability of Polymer Containing Oils (Parts) Mechanical Testing of Hydraulic Fluids in the Vane-Cell P u m p (lubricant characterization of hydraulic fluids) • General Working Principles (Part 1) • Method A for Anhydrous Hydraulic Fluids (Part 2) • Method B for Aqueous Not Easily Inflammable Fluids (Part 3) Selection of Lubricants for Gears (recommended use of hydraulic fluids in sliding/rolling contact) • Gear Lubricating Oils (Part 1) • Semi-Fluid Lubricants (Part 2) Testing of Lubricants—Tribological Test in Translatory Oscillation Apparatus • General Working Principles (Part 1) • Determination of Friction and Wear Data for Lubricating Oils (Part 2) • Determination of Tribological Behavior of Materials (Part 3)
ISO-International 7148/1
TR6281
Standards
Organization
Testing of the Friction and Wear Behavior of Bearing Material / Mating Material / Oil Combinations under Conditions of Boundary Lubrication (bearings, friction and wear tests) Testing Under Conditions of Hydrodynamic and Mixed Lubrication in Test Rigs-Guidelines (guidelines for bearings in sliding wear)
JIS - Japanese Agency of International Technology Standards Department K2519
334
166 239
240
326
and
Testing Methods for Load-Carrying Capacity of Lubricating Oil (note: Japanese Timken Test) (lubricant testing in sliding or rolling contact)
IP - Institute 281
Science
of
Petroleum-UK
Determination of the Anti-Wear Properties of Hydraulic Fluids—Vane P u m p Method (lubricant characterization of hydraulic fluids in a vane pump) Determination of Load Carrying Capacity of Lubricants-FZG Gear Machine Method (lubricant characterization of gear oils in sliding and rolling contact) Load Carrying Capacity for Oils-IE A Gear Machine (testing of geeir oils in gear test rig) Extreme Pressure Properties: Friction and Wear Tests for Lubricants: Four Ball Machine (seizure and welding characteristics using Four Ball EP) Standard Method for Measurement of Extreme Pressure Properties of Lubricating Fluids (Timken Method) (general laboratory test for extreme pressure properties of oils) S t a n d a r d Method for Measurement of Extreme Pressure Properties of Lubricating Grease (Timken Method) (general laboratory test for extreme pressure properties of greases)
1037
ASTM RELATED STANDARDS D4175 G40
S t a n d a r d Terminology Relating to Petroleum, Petroleum Products, and Lubricants (terminology compilation) Standard Terminology Relating to Wear and Erosion (terminology compilation)
REFERENCES [1] Dowson, D., History ofTribology, Professional Engineering Publishing Limited, London, UK, 1998. [2] Benzing, R., Goldblatt, I., Hopkins, V., Jamison, W., Mecklenburg, K., and Peterson, M., Friction and Wear Devices, STLE, Park Ridge, IL, 1976. [3] Voitik, R. M., "Realizing Bench Test Solutions to Field Tribology Problems," Tribology: Wear Test Selection for Design and Application, ASTM STP 1199, A. W. Ruff, and Raymond G. Bayer, Eds., ASTM International, West Conshohocken, PA, 1993. [4] Sture, H. and Staffan, J., "Hints and Guidelines for Tribotesting and Evaluation," Lubrication Engineering, Vol. 48, No. 5, 1991, pp. 401-409. [5] Neale, M. J. and Gee, M., Guide to Wear Problems and Testing for Industry, y^ ed., Williams Andrew Publishing, Norwich, NY, 2001. [6] Calabrese, S. J. and Muray, S. F., "Methods of Evaluating Materials for Icebreaker Hull Coatings," Selection and Use of Wear Tests for Coatings, ASTM STP 769, R. G. Bayer, Ed., ASTM International, West Conshohocken, PA, 1982, pp. 157-173. [7] Banniak E. A. and Fein R. S., "Precision of Four Ball and Timken Tests and Their Relation to Service Performance," NLGI Spokesman, January 1973. [8] Faville, F. and Faville, W., "Falex Procedures for Evaluating Lubricants," Journal of the American Society of Lubrication Engineers, STLE, Park Ridge, IL, August 1968. [9] Bayer R. G., Shalkey A. T., and Wayson A. R., "Designing for Zero," Machine Design, IBM Corporation, Endicott, NY, 1969. [10] Wei D.-P., "Future Directions of Fundamental Research in Additive Tribochemistry," Lubrication Sciences, Vol. 7, April 1995. [11] Mizuhara K. and Tsuya Y., "Investigation of a Method for Evaluating Fire-Resistant Hydraulic Fluids by Means of an Oil-Testing Machine," JSLE International Tribology Conference, Tokyo, Japan, 8-10 July 1985. [12] Feldman, D. G. and Kessler, M., "Development of a New Application-RelatedTest Procedure for Mechanical Testing of Hydraulic Fluids," Hydraulic Failure Analysis: Fluids, Components and System Effects, STP 1339, G. E. Totten, D. K. Wills, and D. G. Feldman, Eds., ASTM International, West Conshohocken, PA, 2001, pp. 75-89. [13] De Gee, A. W. J., "Characterization of Five High-Performance Lubricants in Terms of IRG Transition Diagram Data," Proceedings of the IMechE, International Conference on Tribology-Friction. Lubrication, and Wear, Fifty Years On, London, Mechsinical Engineering Publications Lmtd., Bury St. Edmunds, 1987, Vol. 1, pp. 427-436. [14] Vaim, J. A. and Jising, T.-B, "Measurement of the Friction and Lubricity Properties of Contact Lenses," Proceedings of the ANTEC1995, Boston, MA, May 7-11, 1995. [15] Schipper, D. J. and Faraon, I. C, "Stribeck Curves for Starved Line Contacts," University of Twente, The Netherlands, Report number TROl-2227, 2001. [16] Sanvordenker, K. S., "Lubrication by Oil-RefrigerEint Mixtures: Behavior in the Falex Tester," ASHRAE Transactions, KC-84-14, No. 3, pp. 799-805. [17] Anderson, M., "The Use of Tribological Aspect Numbers in Bench Test Selection-A Review Update," Bench Testing of In-
1038 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK dustrial Fluid Lubrication and Wear Properties Used in Machinery Applications, ASTM STP 1404, G. E. Totten, L. D. Wedeven, J. R. Dickey, and M. Anderson, Eds., ASTM International, West Conshohocken, PA, 2001. [18] Condensed Catalog ofFalex Test Equipment and Custom Services, QG-18, Falex Corporation, Sugar Grove, IL, 1999. [19] Helmetag, K., "A New Look at an Old Idea: The Torque Curve Revisited," Bench Testing of Industrial Fluid Lubrication and Wear Properties Used in Machinery Applications, ASTM STP 1404, G. E. Totten, L. D. Wedeven, J. R. Dickey, and M. Anderson, Eds., ASTM International, West Conshohocken, PA, 2001. [20] "Extreme Pressure Lubricant Test Machines," Mobil Technical Bulletin, Mobil Oil, Roswell, GA, 1970. [21] "A Compilation of International Standards for Friction and Wear Testing of Materials," VAMAS Technical Working Area 1, Peter J. Blau, Ed., Report No. 14, NIST, Gaithersburg, MD, 1993. [22] Voitik, R., and Ren, N., "Diesel Fuel Lubricity by Standard Four Ball Apparatus Utilizing Ball on Three Disks, BOTD," SAE Paper #950247, Society of Automotive Engineers, Warrendale, PA, 1995. [23] Voitik, R., "Diesel Fuel Lubricity BOTD Status-1995," Presented at the Fuels & Lubricants Meeting & Exposition, Toronto, Ontario, 16-19 Oct. 1995, SAE Paper #952371, Society of Automotive Engineers, Warrendale, PA, 1995. [24] Nikeinjam, M., " Diesel Fuel Lubricity: On the Path to Specifications," presented at the International Spring Fuels & Lubricants Meeting & Exposition, Dearborn, MI, 3-6 May 1999, SAE Paper #1999-01-1479, Society of Automotive Engineers, Warrendale, PA, 1999. [25] The Timken Company Model 1750 Lubricant and Wear Tester, The Timken Company, Canton, OH, 1977. [26] Anderson, M., "Oscillating Roll/Slide Test Machine for Screening Properties of Greases Used in Constant Velocity Joints," Journal of the National Lubricating Grease Institute: NLGI Spokesman, Vol. 54, No. 3, June 1990. [27] Voitik, R. M., and Heerdt, L. R., "Wear and Friction Evaluation of Gear Lubricants by Bench Test," Journal of the American Society of Lubrication Engineers: Lubrication Engineering, Vol. 40, No. 12, pp. 719-724. [28] Bell, J. C. and Delargy, K. M., "Lubrication Influences on the Wear of Piston-Ring Coatings," Proceedings of the 16''' Lees-Lyon Symposium, Mechanics of Coatings, 1989, pp. 371-377. [29] Patterson, D. J., Hill, S. H., and Tung, S. C, "Bench Wear Testing of Engine Power Cylinder Components," Lubrication Engineering, Vol. 49, No. 2, 1993, pp. 89-95 [30] Hartfield-Wunsch, S. E., Tung, S. C, and Rivard, C. J., "Development of a Bench Wear Test for the Evaluation of Engine Cylinder Components and the Correlation with Engine Test Results," Tribological Insights and Performance Characteristics of Modem Engine Lubricants, SAE SP-996, SAE Paper #932693, 1993. [31] Lacey, P. I. and Lestz, S. J., "Fuel Lubricity Requirements for Diesel Injection Systems," SWRI Report No. BFLRF 270, Southwest Research Institute, San Antonio, TX, 1991. [32] Spikes, H. A., Meyer, K., Bovington, C, Caprotti, R., and Krieger, K., "Development of a Laboratory Test to Predict Lubricity Properties of Diesel Fuels and its Application to the Development of Highly Refined Diesel Fuels," presented at the 9*^ International Colloquium, Ecological and Economical Aspects of Tribology, Paper 3.22, Technische Akademie Esslingen, Germany, 1994, pp. 1-16. [33] Hadley, J. W., Owen, G. C, and Mills, B., "Evaluation of a High Frequency Reciprocating Wear Test for Measuring Diesel Fuel Lubricity," SAE Paper #932692, Society of Automotive Engineers, Warrendale, PA, 1993. [34] Plint and Partners Catalog of Tribology Test Equipment, Flint and Psutners, Newbury, Bershire, England, 2001.
[35] SRV® Test System for Evaluating the Tribological Properties of Lubricants and Materials, Optimol Instruments Priiftechnik GmbH, Miinchen, Germany, 1996. [36] D-94303: FZG Zahnrad-Verspannungs-Prufstand FZG Gear Test Rig, Strama GmbH & Co. KG, Straubing, Germany, 1995. [37] "Method to Assess the Scuffing Load Capacity of Lubricants with High EP Performance Using an FZG Gear Test Rig," FVA Information Sheet, Research Project No. 243, Forschungsvereinigung Antriebstechnick E.V., 60528 Frankfurt/Main, 1995. [38] "Test Procedure for the Investigation of the Micro-Pitting Capacity of Gear Lubricants," FVA Information Sheet, Research Project No. 54/1 - IV, Forschungsvereinigung Antriebstechnick E.V., 60528 Frankfurt/Main, 1993.
APPENDIX Terminology Related to Fuels and Lubricants Testing abrasive wear, n—wear due to hard peirticles or hard protuberances forced against and moving cdong a solid surface. additive, n—a material added to another, usueJly in small amounts, to impart or enhance desirable properties or to suppress undesirable properties. adhesive wear, n—wear due to localized bonding between contacting solid surfaces leading to material transfer between the two surfaces or loss from either surface. apparent area of contact, n—in tribology, the area of contact between two solid surfaces defined by the boundaries of their macroscopic interface. (Contrast with real a r e a of contact.) asperity, n—in tribology, a protuberance in the small-scale topographical irregularities of a solid surface. break-in, n—in tribology, an initicJ transition process occurring in newly established weEuing contacts, often accompanied by transients in coefficient of friction or wear rate, or both, which are uncharacteristic of the given tribological system's long-term behavior. catastrophic vwear, n—rapidly occurring or accelerating surface damage, deterioration, or change of shape caused by wear to such a degree that the service life of a part is appreciably shortened or its function is destroyed. coefficient of friction, fi or f, n—in tribology, the dimensionless ratio of the friction force (F) between two bodies to the normal force {N) pressing these two bodies together. /x or f = (F/N) Discussion—a distinction is often made between static coefficient of friction and kinetic coefficient of friction. corrosive wear, n—^wear in which chemical or electrochemical reaction with the environment is significEuit. debris, n—in tribology, pjirticles that have become detached in a wear or erosion process. dry solid film lubricants, n—dry coatings consisting of lubricating powders in a solid matrix bonded to one or both surfaces to be lubricated. extreme pressure (EP) additive, n—in a lubricant, a substcince that minimizes damage to metcil surfaces in contact under high stress rubbing conditions (D 4175). fatigue wear, n—^wear of a solid surface caused by fracture arising from material fatigue. fretting wear, n—a form of attiitive wear caused by vibratory or oscillatory motion of limited amplitude characterized
CHAPTER 37: LUBRICANT FRICTION AND WEAR TESTING by the removal of finely-divided particles from the rubbing surfaces. Discussion—^Air can cause immediate local oxidation of the wear particles produced by fretting wear. In addition, environmental moisture or humidity can hydrate the oxidation product. In the case of ferrous metals, the oxidized wear debris is abrasive iron oxide (FeaOs) having the appearance of rust, which gives rise to the nearly synonymous terms fretting corrosion and friction oxidation. A related, but somew h a t different, p h e n o m e n o n often accompanies fretting wccir. Fedse brinelling is localized fretting weeir that occurs when the rolling elements of a bearing vibrate or oscillate with small amplitude while pressed against the bearing race. The mechanism proceeds in stages: (1) asperities weld, are torn apart, and form wear debris that is subsequently oxidized; (2) due to the small-amplitude motion, the oxidized detritus cannot readily escape, and being abrasive, the oxidized wear debris accelerates the wear. As a result, wear depressions are formed in the bearing race. These depressions appear similar to the Brinell depressions obtained with static overloading. Although false brinelling can occur in this test, it is not chcu-acterized as such, and instead, it is included in the determination of fretting wear. fretting corrosion, n—a form of fretting wear in which corrosion plays a significEuit role. fretting wear, n—wear arising as a result of fretting (see fretting). friction, n—the resistance to sliding exhibited by two surfaces in contact with each other. Basically, there are two frictional properties exhibited by any surface: static friction EUid kinetic friction. friction force, n—^the resisting force tangential to the interface between two bodies when, under the action of an external force, one body moves or tends to move relative to the other. (See also coefficient of friction.) Hertzian c o n t a c t area, n—the a p p a r e n t area of contact between two nonconforming solid bodies pressed against each other, as calculated from Hertz's equations of elastic deformation. Hertzian c o n t a c t pressure, n—the m a g n i t u d e of the pressure at any specified location in a Hertzian contact Eirea, as calculated from Hertz's equations of elastic deformation. kinetic coefficient of friction, n—the coefficient of friction u n d e r conditions of macroscopic relative motion between two bodies. kinetic friction, n—the force that resists motion when a surface is moving with a uniform velocity; it is, therefore, equal and opposite to the force required to maintain sliding of the surface with uniform velocity. lubricant, n—any substance interposed between two surfaces for the purpose of reducing the friction or wear between them. lubricating grease, n—a semi-fluid to solid product of a dispersion of a thickener in a liquid lubricant. Discussion—the qualifying term, lubricating, should always be used. The term, grease, used without the qualifier refers to a different product, namely certain natural or processed animal fats, such as tallow, lard, cind so forth (D 128). Discussion—the dispersion of the thickener forms a twophase system a n d immobilizes the liquid lubricant by surface tension eind other physical forces. Other ingredients cire commonly included to impart specied properties (D 217).
1039
lubricating oil, n—a liquid lubricant, usually comprising several ingredients, including a major portion of base oil and minor portions of various additives (D 5966). lubricity, n—a qualitative term describing the ability of a lubricant to minimize friction between and damage to surfaces in relative motion u n d e r load (D 4857, D 4863). precision, n—the degree of agreement between two or more results on the same property of identical test material. In this practice, precision statements are framed in terms of the repeatability and reproducibility of the test method (D 3244). pitting, n—in tribology, a form of wear characterized by the presence of surface cavities, the formation of which is attributed to processes such as fatigue, local adhesion, or cavitation. repeatability, n—the quantitative expression of the random error associated with a single operator in a given laboratory obtaining repetitive results with the same apparatus under constant operating conditions on identiced test material. It is defined as the difference between two such results at the 95% confidence level. Discussion—interpret as the value equal to or below which the absolute difference between two single test results obtained in the above conditions may expect to lie with a probabihty of 95%. Discussion—the difference is related to repeatability standard deviation but is not the standard deviation or its estimate. reproducibility R, n—quEintitative expression of the r a n d o m error associated with operators working in different laboratories, each obtaining single results on identical test material when applying the same method. result, n—the value obtained by following the complete set of instructions of a test method. rolling contact fatigue, n—a damage process in a triboelement subjected to repeated rolling contact loads, involving the initiation and propagation of fatigue cracks in or under the contact surface, eventually culminating in surface pits or spalls. rolling wear, n—wear due to the relative motion between two nonconforming solid bodies whose surface velocities in the n o m i n a l contact location are identical in magnitude, direction, and sense. Discussion—Rolling wear is not a synonym for rolling contact fatigue, although the latter can be considered one form of rolling wear. run-in, n—in tribology, an initial transition process occurring in newly established wearing contacts, often accompanied by transients in coefficient of friction, or wear rate, or both, which ju-e uncharacteristic of the given tribological system's long term behavior. {Synonym: break-in, wear-in.) run-in, v—in tribology, to apply a specified set of initial operating conditions to a tribological system to improve its long term frictional or wecir behavior, or both. {Synonym: break in, V. and wear in, v. See also run-in, n.) scoring, n—in tribology, a severe form of wear characterized by the formation of extensive grooves and scratches in the direction of sliding. scratching, n—^the formation of fine lines in the direction of sliding that may be due to asperities on the harder slider or to h a r d particles between the surfaces or embedded in one of them. Discussion—Scratching is considered less damaging than scoring or scuffing.
1040 MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
scuff, scuffing, n—in lubrication, damage caused by instantaneous localized welding between surfaces in relative motion, which does not result in immobilization of the parts. scuffing—n, a form of wear occurring in inadequately lubricated tribosystems that is characterized by macroscopiccJlyobservable changes in surface texture, with features related to the direction of relative motion. Discussion—features characteristic of scuffing include scratches, plastic deformation, and transferred material. (Related terms: galling, scoring.) seizure, n—in lubrication, welding between surfaces in relative motion that results in immobilization of the parts. Localized fusion of metal between the rubbing surfaces of the test pieces (D 5707). Discussion—Seizure is usually indicated by an increase in coefficient of friction, wear, or unusual noise and vibration. In this test method, increase in coefficient of friction is displayed on the chart recorder as rise in the coefficient of friction from a steady state value. sliding wear, n—wear due to the relative motion in the tangential plane of contact between two solid bodies. speilling, n—in tribology, the separation of macroscopic particles from a surface in the form of flakes or chips, usually associated with rolling element bearings and gear teeth, but also resulting from impact events. standard test, n—a test on a calibrated test stand, using the prescribed equipment according to the requirements in the test method, and conducted according to the specified operating conditions. Discussion—the specified operating conditions in some test methods include requirements for determining a test's operational validity. These requirements are applied after a test is completed, and can include (1) midlimit ranges for the average values of primary and secondary parameters that are narrower than the specified control ranges for the individual values, (2) allowable deviations for individual primary and secondary parameters from the specified control ranges, (3) downtime limitations, and (4) special parameter limitations. static coefficient of friction, n—the coefficient of friction corresponding to the maximum friction force that must be overcome to initiate macroscopic motion between two bodies. stick-slip, n—in tribology, a cyclic fluctuation in the magnitudes of friction force and relative velocity between two elements in sliding contact, usually associated with a relaxation oscillation dependent on elasticity in the tribosystem and on a decrease of the coefficient of friction with onset of sliding or with increase of sliding velocity. Discussion—Classical or true stick-slip, in which each cycle consists of a stage of actual stick followed by a stage of overshoot "slip," requires that the kinetic coefficient of friction is lower than the static coefficient. A modified form of relaxation oscillation, with near-harmonic fluctuation in motion, can occur when the kinetic coefficient of friction decreases gradually with increasing velocity within a certain velocity range. A third t5^e of stick-slip can be due to spatial periodicity of the friction coefficient along the path of contact. Random variations in friction force measurement do not constitute stick-slip. test oil, n—any oil subjected to evaluation in an established procedure.
test sample, n—a portion of the product taken at the place where the product is exchanged, that is, where the responsibility for the product quality passes from the supplier to the receiver. Actually, this is rarely possible and a suitable sampling location should be mutually agreed on. triboelement, n—one of two or more solid bodies that comprise a sliding, rolling, or abrasive contact, or a body subjected to impingement or cavitation. (Each triboelement contains one or more tribosurfaces.) Discussion—Contacting triboelements may be in direct contact or may be separated by an intervening lubricant, oxide, or other film that affects tribological interactions between them. tribology, n—the science and technology concerned with interacting surfaces in relative motion, including friction, lubrication, wear, and erosion. tribosurface, n—any surface (of a solid body) that is in moving contact with another surface or is subjected to impingement or cavitation. tribosystem, n—any system that contains one or more triboelements, including all mechanical, chemical, and environmental factors relevant to tribological behavior, (See also triboelement.) thin film Quid lubricant, n—fluid lubricants consisting of a primary liquid with or without additives or lubricating powders and without binders or adhesives, which form a film on one or both surfaces to be lubricated and perform their function after application and after excess material has drained from the application area, and without additional material being supplied by either a continuous or intermittent method. wear, n—damage to a solid surface, generally involving progressive loss of material, due to relative motion between that surface and a contacting substance or substances. wear, n—the removal of metal from the test pieces by a mechanical or chemical action, or by a combination of mechanical and chemical actions. wear rate, n—the rate of material removal or dimensional change due to wear per unit of exposure parameter; for example, quantity of material removed (mass, volume, thickness) in unit distance of sliding or unit time. wear coefficient, n—in tribology, a wear parameter that relates sliding wear measurements to tribosystem parameters. Most commonly, but not invariably, it is defined as the dimensionless coefficient k in the equation wear volume = k (load X sliding distance/hardness of the softer material) (1) The equation given above is frequently referred to in the literature as "Archard's equation" or "Archard's law." (2) Sometimes the term wear coefficient has been used as a sjrnonym for wear factor. While this usage is discouraged, the term should always be fully defined in context to prevent confusion. wear rate, n—the rate of material removal or dimensional change due to wear per unit of exposure parameter, for example, quantity of material removed (mass, volume, thickness) in unit distance of sliding or unit time. Discussion—Because of the possibility of confusion, the manner of computing wear rate should always be carefully specified.
CHAPTER 3 7: LUBRICANT FRICTION AND WEAR TESTING welding, n—in tribology, the bonding between metallic surfaces in direct contact, at any temperature. Terminology Specific to Standards load-canying capacity, n—as determined by D 2782, the maximum load or pressure that can be sustained by the lubricant (when used in the given system under specific conditions) without failure of the sliding contact surfaces as evidenced by scoring or seizure or asperity welding (D 2509, D 2782). OK value, n—as determined by D 2782, the maximum mass (weight) added to the load lever weight pan at which no scoring or seizure occurs (D 2509, D 2782). score value, n—as determined by D 2782, the minimum mass (weight) added to the load lever weight pan, at which scoring or seizure occurs (D 2509, D 2782). Discussion—When the lubricant film is substantially maintained, a smooth scar is obtained on the test block, but when there is a breakdown of the lubricant film, scoring or surface failure of the test block takes place, see Fig. 24. In its simplest and most recognized form, scoring is characterized by the furrowed appearance of a wide scar on the test block and excessive pick-up of metal on the surface of the test cup. The form of surface failure more usually encountered, however, consists of a comparatively smooth scar, which shows local damage that usually extends beyond the width of the scar. Scratches or striations that occur in an otherwise smooth scar and that do not extend beyond the width of the scar are not considered as evidence of scoring. seizure or asperity welding, n—localized fusion of metal between the rubbing surfaces of the test pieces. Seizure is usually indicated by streaks appeciring on the surface of the test cup, an increase in friction and wear, or unusual noise and vibration. Throughout D 2782, the term seizure is understood to mean seizure or asperity welding (D 2509, D 2782). load-wear index, n—(or the load-carrying property of a lubricant)—an index of the ability of a lubricant to minimize wear at applied loads. Under the conditions of this test, specific loadings in kilograms-force (or Newtons) having intervals of approximately 0.1 logcirithmic units, are applied to the three stationary balls for ten runs prior to welding. The loadwear index is the average of the sum of the corrected loads determined for the ten applied loads immediately preceding the weld pair (D 2596, D2783). weld point, n—under the conditions of this test, the lowest applied load in kilograms at which the rotating ball welds to the three stationary balls, indicating the extreme-pressure level of the lubricants-force (or Newtons) has been exceeded. Discussion—Some lubricants do not allow true welding, and extreme scoring of the three stationary balls results. In such cases, the applied load that produces a maximum scar diameter of 4 mm is reported as the weld point (D 2596, D 2783).
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corrected load, n—the load in kilograms-force (or Newtons) for each run obtained by multiplying the applied load by the ratio of the Hertz scar diameter to the measured scar diameter at that load (D 2596, D 2783). Hertz scar diameter, n—the average diameter, in millimeters, of an indentation caused by the deformation of the balls under static load (prior to test). It may be calculated from the equation Dh = 8.73 X \Q-^ (P)''3 Where: Dh = Hertz diameter of the contact area, and P = the static applied load. (D 2596, D 2783) compensation scar diameter, n—the average diameter, in millimeters, of the wear scar on the stationary balls caused by the rotating ball under an applied load in the presence of a lubricant, but without causing either seizure or welding (D 2596, D 2783). Discussion—The wear scar obtained shall be within 5% of the values noted in Table 1, Column 3 of ASTM D 2596/D 2783. Hertz line, n—a line of plot on logarithmic paper, as shown in Fig. 16, where the coordinates are scar diameter in millimeters and applied load in kilograms-force (or Newtons), obtained under static conditions. compensation line, n—a line of plot on logarithmic paper, as shown in Fig. 1, where the coordinates are scar diameter in millimeters and applied load in kilograms-force (or Newtons), obtained under dynamic conditions. (D 2596, D 2783) Discussion—Coordinates for the compensation line are found in Table 1, Columns 1 and 3 of ASTM D 2596/D 2783. Discussion—Some lubricants give coordinates which are above the compensation line. Known examples of such fluids are methyl phenyl silicone, chlorinated methyl phenyl silicone, silphenylene, phenyl ether, and some mixtures of petroleum oil and chlorinated paraffins. last nonseizure load, n—the last load at which the measured scar diameter is not more than 5% above the compensation line at the load (D 2596, D 2783). incipient seizure or initial seizure region, n—that region at which, with an applied load, there is a momentary breakdown of the lubricating film. This breakdown is noted by a sudden increase in the measured scar diameter and a momentary deflection of the indicating pen of the optional friction-measuring device (D2596, D2783). immediate seizure region, n—that region of the scar-load curve characterized by seizure or welding at the startup or by large wear scars. Initial deflection of indicating pen on the optional friction-measuring device is larger than with nonseizure loads (D 2596, D 2783).
MNL37-EB/Jun. 2003
Statistical Quality Assurance of Measurement Processes for Petroleum and Petroleum Products Alex T. C. Lau^
I N A QUALITY CONSCIOUS AND COST COMPETITIVE ENVIRONMENT,
proficiency and statistical control of measurement processes used for product certification are expected between suppliers and customers. Statistical control occurs when the measurement process is operated with stable and quantifiable accuracy and precision. Likewise, proficiency occurs when the measurement process accuracy and precision performance metrics are reasonable when compared to similar processes that are operated with an industry-agreed upon standard protocol. ASTM test methods are examples of such standardized protocols. This chapter reviews the measurement process used in the petroleum industry, and measurement process performance metrics that are adopted and promoted by ASTM Committee D02. The main body deals with the application of statistical quality assurance techniques to design and implement measurement process quality assurance (QA) programs to assess measurement process performance. The chapter then summarizes discussion on various importcint aspects of QA program implementation. Discussion of theory for the referenced statisticcd tools and control charts are beyond the scope of this chapter. Interested readers should see textbooks on statistics and statistical quality assurance, such as An Introduction to Statistical Quality Assurance by D.C. Montgomery. Portions of the ASTM D 6299 Annex on the computational mechanics with worked examples are reproduced, with min o r modifications, at the end of this chapter. They are included to assist practitioners in the proper use of the described tools.
USE OF MEASUREMENT PROCESS IN THE PETROLEUM INDUSTRY The m e a s u r e m e n t process is a sub-process integral to all manufacturing processes. Like any process, it has input(s) and output(s). Input(s) to the measurement process are usually sample(s) taken at various stages of the manufacturing process; the output(s) are usually numerical values. In the petroleum refining industry, two types of measurement processes are commonly encountered. One is the traditional laboratory-based process where a small sample is extracted from the main process streams and analyzed off-line using
standard test methods, most of which are those developed a n d m a i n t a i n e d b y ASTM D02. The other, c o m m o n l y referred to as a continuous process analyzer system, is a field-deployed, fully automated instrumentation system designed to provide analytical information on representative samples continuously extracted from the m a i n process streams, or on the actueil process stream using in-line probes. The use of measurement process outputs (numerical values), can be categorized into three c o m m o n applications: 1. manufacturing process control: to support decisions on appropriate manipulations (known as control actions) to key manufacturing process variables in order to meet required process performance criteria 2. product property conformance to specification: to test the hypothesis t h a t the batch of product from which the test sample is taken meets required quality criteria (specifications) 3. measurement process self-monitoring: to test the hypothesis that the results generated by the measurement process are fit-for-use. Manufacturing P r o c e s s Control In this application, the measurement process is used to provide a window to the refining (raw materiEd manufacturing) and blending (product manufacturing) processes. The outputs from the measurement process £tre compEired to target values, and appropriate manipulations (known as control action) are made to key operating variables (pressure, temperature, flow, level) in these processes based on evaluation of the current and past deviations of actual versus target values. Both laboratory-based and continuous process analyzer systems are used for this application. Due to the high cost of installation and maintenance of continuous process analyzer systems, the latter are usually deployed only on critical manufacturing process streams suitable for feedback control. Control action frequency and magnitude are tailored to the processes' gain and dynamics, and calculated using process models and cJgorithms derived from classical process control theories. These process models, in general, are quite different than the steady-state Gaussian model commonly employed in the discrete components manufacturing open-loop processes.
Product Property Conformance to Specification ' Blending Performance Optimization Specialist, Exxon Mobil Research and Engineering, Fairfax, VA 22037.
This is the most c o m m o n application for ASTM D02 test methods. The measurement process is used to inspect small
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quantities of material that are representative of the finished product. The results, or outputs, from the measurement process are used to ascertain the specification conformance status of the product. Depending on governing stcindards and regulations, either laboratory-based processes or continuous process analyzer systems, or both, can be used. Measurement Process Self-Monitoring This application is the focus of this chapter. In order to be able to use results generated from a measurement process with confidence, the fitness-for-use of these results must be demonstrated continuously. This can be achieved by reguleir application of the measurement process to samples of special status, known as quality control samples, as well as engaging the measurement process in regular interlaboratory collaborative testing programs (round robins). Through application of statistical control chart techniques to the quality control samples and round robin results, the performance of the measurement process over time can be monitored and assessed against required standards. The set of activities used to achieve this objective is known as a measurement process quality assurance program. MEASUREMENT PROCESS PERFORMANCE METRICS Precision Some inherent sources of the measurement process cause random variation. A generic term commonly used to describe this type of random variation is common-cause variation. The specific term for common-cause variation is precision. Outlined below are the ASTM E 456 [1] definitions of precision, and conditions under which precision is measured. These definitions are internationally accepted, hence similar definitions can be found in ISO 4529, and ISO 5725: • precision, n—the closeness of agreement between test results obtained under prescribed conditions (E 456). • repeatability conditions, n—conditions where mutually independent test results are obtained with the same test method in the same laboratory by the same operator with the same equipment within short intervals of time using test specimens taken at random from a single sample of material (E 456/E 177). • reproducibility conditions, n—conditions where test results are obtained in different laboratories with the same test method using test specimens taken at random from the same sample of material (E 456/E 177). ASTM Form and Style (Blue Book) requires that all test methods have published estimates of precision under both repeatability and reproducibility conditions. Furthermore, these estimates must be calculated from data collected in accordance with interlaboratory study protocol meeting the technical requirements as prescribed in ASTM E 691 [2] or other equivalent standards. For ASTM D02 test methods, the equivalent protocol that had been in use for decades predates E 691. It is described in the Manual on Determining Precision Data for ASTM Test Methods on Petroleum Products and Lubricants (RR: D02-1007), published in December, 1972. In 1998, an updated version of this protocol was published as
ASTM D 6300 [3], under the jurisdiction of ASTM D02.CS94 Coordinating Subcommittee on Quality Assurance and Statistics. For ASTM D02 test methods, the repeatability and reproducibility figures published are obtained in accordance with their definitions in D 6300: • repeatability, n—the quantitative expression of the random error associated with a single operator in a given laboratory obtaining repetitive results with the same apparatus under constant operating conditions on identical test material. It is defined as the difference (see 3.1.10.1) between two such results at the 95% confidence level. (RR D02:1007) • reproducibility, n—is a quantitative expression of the random error associated with different operators using different apparatus, etc., each obtaining a single result on an identical test sample when applying the same method. It is defined as the 95% confidence limit for the difference between two such single and independent results (RR: D021007). These figures are interpreted as the maximum value expected for the absolute difference between two single test results obtained in the prescribed conditions for about 95% of the time. They are obtained by multiplying the variation estimates for the prescribed conditions by a factor based on an assumed statistical distribution for these absolute differences. In addition to the above, ASTM D 6299 [4] defines an additional set of conditions under which precision can be estimated as follows: • site precision conditions, n—conditions under which test results are obtained by one or more operators in a single site location practicing the same test method on a single measurement system using test specimens taken at random from the same sample of material over an extended period of time spanning at least a 15 day interved. Repeatability and reproducibility conditions as described above are intended to provide precision estimates under the "most controlled" to "least controlled" scenarios, while site precision condition prescribes a scenario common to most individual laboratories. In general, the magnitude of common cause variation measured under site precision conditions lies somewhere between that for repeatability and reproducibility conditions, due to the difference in controlled factors as illustrated in Fig. 1. Site precision conditions should include all sources of variation that are typically encountered during normal, longterm operation of the measurement system. Thus, all operators involved in the routine use of the measurement system should contribute results. Since the objective is to estimate long-term common cause variation, multiple results obtained within a 24 h period are discouraged. Bias The following are definitions for bias and two key related terms quoted from ASTM E 456 and D 3244[5]: • bias, n—a systematic error that contributes to the difference between a population mean of the measurements or test results in an accepted reference or true value (E456).
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B l not allowed to vaiy by design
measurement process environment
operator
equipment
not altowed to vary by design
measurement process antftfonment
[" operator 1
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based on collaborative expeiimental work under the auspices of a scientific or engineering group (E456). • true value, n—for practical purposes, the value towards which the average of single results obtained by N laboratories tends, when N becomes veiy large. Consequently, such a true value is associated with the particular test method employed (D 3244). The term true value as defined above, is also referred to as a consensus value as described by item (3) under the definition of accepted reference value (ARV). Hence, if an average was calculated from N results obtained for the same material, from a single measurement process, under site precision conditions, AND, if the material has an accepted reference value, then, for large values of N (greater than 30), any statistically significant difference between this average and the ARV of the material is referred to as bias. Bias provides a quantitative measure of how well the actual process outcome agrees with an expected value. • accuracy, n—the closeness of agreement between a test result and an accepted reference value. Note: The term accuracy, when applied to a set of test results, involves a combination of a random component and of a common systematic error or bias component (E 456). Hence, precision and bias are quantitative measures of different attributes of the measurement process. The combined effect of both attributes affects the accuracy of the results. M E A S U R E M E N T P R O C E S S QUALITY A S S U R A N C E (QA) P R O G R A M
pj^^^i^. B not allowed to vary by design
measurement process May or may not vary
environment
operator
material
FIG. 1—a) ASTM reproducibility conditions; b) ASTM repeatability conditions; c) site precision conditions.
accepted reference value, n—a value that serves as an agreed-upon reference for comparison, and which is derived as: (1) a theoretical or established value, based on scientific principles, (2) an assigned value, based on experimental work of some national or international organization such as the U.S. National Institute of Standards and Technology (NIST), or (3) a consensus value,
The primary objectives of a measurement process quality assurance program are: 1. to obtain quantitative estimate of precision and bias achievable by the measurement process under site precision conditions; 2. to continue monitor capability to sustain the above values; 3. to ensure compliance of precision and bias performance measures with values mutually agreed upon between supplier and customer. These objectives can be achieved through the following activities: • monitoring stability and precision through quality control sample testing; • monitoring bias through testing of materials with ARV; • periodically evaluating system performance in terms of precision and/or bias • periodically and independently validating the system • regularly participating in interlaboratory exchange programs Monitoring Stability and Precision Through Quality Control Sample Testing Summary of Activity: For the purpose of establishing the instatistical-control status of the measurement process since the last valid calibration, quality control samples are regularly analyzed as if they are production samples. Results are immediately plotted on control charts, followed by chart
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Einalysis in accordcince with industry-accepted rules to ascertain the statistical control status of the measurement process. Any out-of-control data should trigger a n investigation for root causes in a timely manner. Pending on outcome of this investigation, remedial action may be required.
ance is due to c o m m o n cause variation associated with the combination of time/operator/reagents/instrument calibration factors, which will not b e observable u n d e r repeatability conditions. Data
Quality Control Material and Storage
Selection,
Preparation,
Selected materials should be a stable, finished material with quality values and composition similar to those regulcirly tested by the measurement process. For off-line (lab-based) processes, a batch of smaller aliquot portions can be prepared from a large volume of sample, usually, but not necessarily, extracted from the manufacturing process. The large volume of sample should be mixed thoroughly prior to aliquot preparation to ensure between-aliquot homogeneity. These smaller portions are then stored in a controlled environment to preserve sample integrity over time. For c o n t i n u o u s process analyzer systems, customdesigned sub-systems and procedures are usually employed to carry out the batch extraction, mixing, storage, cind delivery functions. Sufficient volume should be extracted per batch at one time to support as mjiny quality control sample tests as practical (depending on the planned testing frequency, the number of samples could range from 30 to 100). This would EJIOW performance statistics to be calculated with reasonable n u m ber of data points, using each batch as a subgroup. Quality
Control Material
Testing
The testing should b e CEurried out under site precision conditions, with participation of as many operators as practical, and test time during the day should be randomized to avoid any unknown systematic effects. Test frequency should be designed in accordance with the business needs, taking into considerations the following factors: • frequency of use of measurement process • criticality of the parameter being measured • measurement process stability and precision performance based on historical data • business economics • regulatory, contractual, or test method requirements. The sample treatment and testing of quality control materials should emulate the processing of day-to-day samples. Application of special treatment to the quality control sample in order to get a better result is discouraged, as this would undermine the integrity of the data in providing a valid estimate of the long-term measurement process precision under normal production conditions. TypicEilly, for automated large batch analyses commonly practiced in testing using autosamplers, it is good practice to include a quality control sample in the beginning, middle, emd end of batch run. Periodic re-submission of quality control samples, disguised as normal samples, provides data to test the authenticity of precision established using known quality control samples as truly representative of the test m e t h o d precision u n d e r n o r m a l testing conditions. Testing multiple aliquots of quality control samples as a batch, under repeatability conditions, is not appropriate for estimating the m e a s u r e m e n t process long t e r m precision. The bulk of the total long term measurement process vari-
Treatment
The most practical and technically applicable tool for recording and assessing quality control sjimple data is the control chart. Quality control test results should be plotted on control charts as soon as they cire generated and the chart should be interpreted immediately to assess the acceptability of process performance. For measurement processes where there is n o issue with quality control sample homogeneity among aliquots prepeired from one batch of materieJ, the Individual (I) cind Moving Range of 2 (MR2) are the appropriate charts. These charts have been the basic workhorses in the statistical process control (SPC) discipline for over half a century, primarily deployed in the discrete component manufacturing sector. They are also known as "Shewart" charts, a tribute to Dr. Walter Shewhart, one of the early quality pioneers w h o preached the use of these charts for manufacturing process monitoring and control. Designed with simplicity in mind, the I chart is not as effective in detecting small process shifts as it is for large shifts. To address this shortcoming, its use is often augmented with a set of r u n rules, often referred to as the "Western Electric" r u n rules, a tribute to the company that pioneered their use. More advanced statistical techniques such as the Exponentially Weighted Moving Average (EWMA), or Cumulative Sum (CUSUM) can be deployed in lieu of the r u n rules with equivalent effectiveness. The EWMA is recommended due to ease of deployment, a n d its close emulation to detection power of the r u n rules [6]. An abridged summeiry description of these cheirts and the r u n rules can be found in the Annex under the section on Control Charts. Hence, a combination of the I chart with the run rules, or augmented with EWMA, should be used for the detection of both large and small shifts of the location value simultcineously. The MR2 chart tracks the total process variation (precision) over multiple operators cind time. In order to deploy control chcirts for regular use, control limits and center values need to be CEilculated. These values can be obtained initially after a veJid test equipment calibration from a m i n i m u m of 15 valid quality control test results (for the same batch of qucJity control samples), then periodically updated as new data arrive. Prior to calculating the initial values, the following screening steps should be CEirried out: 1. Screen for suspicious results—Results should first be visually screened for unusual values, such as those that could have been caused by transcription errors. Results flagged as suspicious should be investigated. Discard of any data at this stage must be supported by evidence that the discarding is due to special causes that Eire not part of the normal testing process. 2. Screen for unusual patterns—^The next step is t o plot the results in chronological order. In SPC terminology, this is known as a r u n chart. The chart should be examined for non-random patterns such as continuous trending in either direction, unusual clustering, eind cycles. Several of the non-random patterns described in control chart literature
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can be used as guides at this step [8]. Detection of any nonrandom pattern should trigger investigation for causes. 3. Test distribution assumptions—If, after removal of appropriate results, there is a minimum of 15 observations remaining, the normal distribution assumption for the control chart needs to be validated, as this assumption is used to predict the behavior of the measurement process under statistical control. For a small number of observations (15-25), the tool of choice is the normal probability, or quantile-quantile plot, since it tends to provide better information regarding the "goodness-of-fit" of the data to the assumed model than the more commonly known histogram, which generally requires 40 or more data points to be effective. A plot indicating a departure from the assumed model may reveal abnormal conditions not discovered by previous screening steps. 4. Assess process stability—If no obvious non-random patterns are detected from the run charts, and the normal probability plot reveals no unusual pattern, proceed with detail calculations of control limits. Use the control chart rules (or EWMA), in conjunction with the control limits, to determine if the measurement process is under the influence of common causes variation only (in statistical control). Out-of-control points should be investigated in detail. For those associated with special causes, their exclusion from further data analysis is warranted only if the special causes are deemed not part of the normal process. These control limits should be viewed as starting values only, with appropriate reassessment and/or updating as more data become available. The initial control chart limit established above should be re-assessed based on a minimum number (15) of new data points. Update calculations should be preceded by an appropriate statistical test to assess if the precision of the new data is essentially the same as previous data. If the test is not significant, a new estimate of the total process precision should be re-estimated using data from both periods. A significant test outcome should trigger an investigation for root causes. When the total number of data points used to estimate process precision exceeds 100, a re-estimate of the site precision when additional data becomes available reaches the point of diminishing return. The value based on 100 data points is a fairly accurate representation of the long term site precision. Batch Change of Quality Control Material Due to inherent variation of the manufacturing process and different operation targets, differences may exist between batches of materials extracted at different times for quality control use. Since control limit calculations for the I chart require a center value usually established by the process itself, a special procedure is required to ensure that the center line for a new batch of QC material is established with a process that is in statistical control. This procedure can be designed as follows: • Collect and prepare a new batch of QC material as the current supply is close to depletion. • Start a new set of I, MR2 charts for the new material. Commence collection of data for the new material each time a current quality control sample is tested. The new data is deemed valid if the process in-control status is established by the current QC material.
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• Use the control limits for the current MR2 chart or a previous MR2 chart where the matericil is at nominally similar property level, for the new MR2 chart. • After a minimum of 5 in-control data points are collected on the new material, calculate a temporary center line for the new I chart. • Calculate the control limits for the new I chart using the new center and MR-BAR value from previous MR2 chart. • Assess subsequent data points for new material against the new control limits. Temporatrily suspend the run rules. • After 15 data points have been accrued, re-calculate the new I-chart center line. Assess the precision of the new dataset against the previously established precision for equivalency. Combine and re-estimate the precision if necessary. • Deploy the new control chart and activate all run rules. The reason for the temporary center line is to minimize the commissioning period for the new QC material. The suspension of run rules is to avoid false alarms due to a less precise estimate of the center line using five data points. With an effective QC material inventory management program, the calculation and transfer of control chart limits between batches of quality control materials should be seamless. When the System is in statistical-control, i.e. no assignable cause is present, the expected outcome when switching QC material are: • fluctuation between I-chart center line values for different QC batches; it represents material difference only; • no statisticcdly significant difference is expected for the MR chart control limits for different batches of QC material at similar nominal levels. For detailed instructions on how to execute the aforementioned calculations and techniques, refer to the Appendix at the end of this chapter.
Monitoring Bias Through Testing of Quality Control Materials with Accepted Reference Values (ARV) Summary of activities: For the purpose of ascertaining whether the measurement process is biased, the testing activities described above for quality control material are periodically carried out on material(s) with ARV(s). ASTM D 6299 refers to this type of material as "check standards." Deviations between the test result and the check steindard ARV are immediately plotted on control charts and analyzed similarly to those for quality control materials. Acceptable deviations from ARV (or, control limits for this chart) should be based on the site precision estimates obtained from quality control samples at similar levels described above. The factors considered for determining the frequency of check standard testing are similar to those described above for quality control material testing. T5^ically, due to the higher costs and/or availability of check standards, bias monitoring with check standards are conducted less frequently, and testing of locally produced quality control material is used to demonstrate the in statisticctl control status of the measurement process between check standard testing.
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HANDBOOK
ARV
In an ideal world, a check standard should have a specified composition that is representative of routinely produced materieJ, and have an ARV determined with zero error. Unfortunately, in the real world, ARV with zero error is virtually unattainable. However, ARV with error that is small relative to the test method precision can be achieved by averaging the results from multiple testing organizations. As well, for ASTM test methods that are designed to measure performance properties, it is not u n c o m m o n to find products with similar performance properties but significantly different compositions (e.g., octane rating, cloud, flash). For these types of test methods, only the equipment, equipment set-up and operating procedures are standardized, with very limited testing and compositional requirements on check standards. Even for those test methods that do have limited check standard testing requirements, the check standards specified are typically pure compounds, or a blend of pure compounds, which are typically not compositionally representative of the products tested. Therefore, unknown, or unknowable composition effects preclude the extrapolation of the performance statistics obtained on these specified check standards to that for routine products testing. For these test methods, check standards are usually compositionally specific to the site that produced the material. Typically, a manufacturing site extracts a large quantity of a check standard candidate material from the production facilities (e.g., gasoline or diesel fuel blender). Aliquots prepared from this material are then round robin tested. After statistical examination and outlier treatment has been applied, the average of test results is assigned as the ARV for this material. (Refer to ASTM D 6300[3] and E-178[7] for protocol in statistical and outlier treatment of round robin data.) Hence, acquisition of check standard materials with composition representative of the current production for an individual producer can be a time-consuming and expensive undertaking. Materials circulated as part of an interlaboratory exchange program can offer a cost-effective alternative to site-specific check standards. This is provided the measurement process is not adversely affected by the composition difference between the exchange sample and the composition of material that the measurement process is normally set u p for, and the timeliness of receiving exchange results is not an issue. In order for an exchange sample to be usable as a check standard, the stand a r d deviation of the interlaboratory exchange p r o g r a m should not be statistically greater than the reproducibility standard deviation for the test method, even though the uncertainty in the ARV can be acceptable as the latter is inversely proportional to the square root of the n u m b e r of values used to calculate the average. In general, it is recommended that a m i n i m u m of 16 non-outlier results be used in cedculating the ARV to reduce the uncertainty of the ARV by a factor of 4 relative to the measurement system single value precision. Examples of exchanges that may be used to generate ARVs are: ASTM-D02.CS92 Inter-laboratory Crosscheck Program (I.L.C.P); ASTM-D02.01 National Exchange Group(N.E.G.); ASTM D02.01.A Regional Exchange Programs; Alberta Research Council -International Cooperative Exchange Testing Programs
Test results from ARV testing can be treated in a similar fashion as those from QC material testing. Typically, the results are pre-treated by subtracting the ARV in order to highlight the sign and magnitude of any bias. For test methods with precision that changes with level, the bias can also be expressed in units of the steindard deviation to facilitate performance comparison over check standards with different values. Readers are referred to the Appendic for examples of Check Standard results treatment. Test Equipment
Calibration
It is a well-known fact (Deming Funnel Experiment) that frequent and unnecessary adjustment to any process will result in the inflation of the c o m m o n cause variation of the process. Therefore, equipment calibration should only be done when there is evidence, supported by quality control testing data, suggesting that the process requires adjustment as the calibration process itself is subject to c o m m o n cause variation. A c o m m o n and frequently overlooked source of calibrationinduced variation is the calibration s t a n d a r d preparation process, a n d / o r c o m m o n cause composition variation of commercial standards themselves.
Estimate Process Precision and Bias Periodically, results from control charts, excluding those with assignable causes, should be analyzed to achieve the following: 1. Test for Bias—If the quality control material has an ARV for the measured property, a one-sample t-test should be used to determine if a statistically significant bias exists for the measurement process versus the ARV. If the test outcome is significant, and, if the magnitude is of practical concern in the application of the test method, all efforts should be made to establish the root causes for bias. The reason for assessing the bias magnitude for practical concern is that as the n u m b e r of data increases, the power of detecting bias also increases. Hence, there may be situations where the statistical test is significant, but the magnitude of bias detected is fairly small relative to the business application need. In these instances, the bias is deemed of statistical but not practical significance. The decision on practical significance is a purely subjective call based on the business application and domain knowledge. The best estimate of the bias is the difference between the average of all the results obtained on the quality control material and its ARV. 2. Precision estimate—Either the average of MR chart, or the more traditional root-mean-square calculation technique using the results from the I-chart can be used to estimate the process variability u n d e r site-precision conditions. This should be statistically assessed against that published for ASTM-reproducibility condition to see if it is larger. If so, follow up investigation should b e conducted since comm o n cause veiriation under site-precision conditions is not expected to be larger than that for ASTM-reproducibility conditions. Readers can refer to the Appendix for detailed instructions on h o w to perform the aforementioned statistical assessments of system precision and bias.
CHAPTER 38: STATISTICAL
QUALITY ASSURANCE
Periodic and Independent System Validation If practicable, quality control samples should be periodically introduced blindly without revealing their identity, among regular production samples. This is to verify that the precision calculated using quality control sample results is representative of what's achieved during routine operation when there is no priori knowledge of sample identity and expected value. D 6299 refers to materials used for this purpose as Validation Audit (VA) samples. For measurement systems susceptible to human influence, the precision and bias estimates calculated from data where the analyst is aware of the sample status (QC or check standard) and/or expected values, may underestimate the precision and bias achievable under routine operation. At the discretion of the users, and, depending on the criticality of these measurement systems, the QA program may include blind testing of VA samples where the information regarding the sample VA status and expected value are withheld from the analyst. Precision and bias estimates calculated using VA samples test data can be used as an independent VEilidation of the routine QA program performance statistics The specific design and approach to the VA testing program will depend on features specific to the measurement system and its associated quality management process, and is beyond the intended scope of this chapter. Two possible approaches are noted below: • QC samples and/or check standards may be submitted as unknown samples at a specific frequency. Such submissions should not be so regular as to compromise their blind status. • Retains of previously analyzed samples may be re-submitted as unknown samples under site-precision conditions. Generally, data from this approach can only yield precision estimates as retain samples do not have ARVs. Regular Pari:icipation in Interiaboratory (Roundrobin) Exchange Testing Participation in regularly conducted interiaboratory exchanges where tjrpical production samples are tested by multiple production test facilities, using a specified (ASTM) test protocol, is a cost-effective supplement to in-house check standard testing, provided the timeliness of receiving the results is not critical. Participants can control chart their signed deviations from the consensus values established by the exchange averages to ascertain if their measurement processes Eire in statistical control and non-biased. Use of interiaboratory exchange statistics (consensus value and precision) requires scrutiny of how these statistics are generated. The latter should be generated with tools that provide adequate safeguard against outliers. Bamett and Lewis defined outliers as a subset of observations that appears to be inconsistent with the remainder of that set of data. These outliers usually have non-random assignable cause(s) associated with them, such as transcription error, incorrectly executed procedure, out-of-calibration instrumentation, or sample contamination/degradation. Without the proper safeguards, these outliers can incorrectly skew the consensus value and inflate the sample standard deviation value to beyond that deemed reasonable by experienced practitioners. The exchanges quoted above (ASTM-D02.CS92 Inter-laboratory
OF MEASUREMENT
PROCESSES
FOR
PETROLEUM
Crosscheck Program (I.L.C.P); ASTM-D02.01 National Exchange Group(N.E.G.); ASTM D02.01.A Regional Exchange Programs; Alberta Research Council -International Cooperative Exchange Testing Programs) all use outliers treatment to generate exchange statistics. With regular and meaningful participation in these exchanges, an industry benefit is the availability of an on-going estimate of the achievable precision in executing the standard test methods. The test method custodian (e.g., ASTM subcommittee member) can regularly compare this information to the published precision to identify the need for method improvement, and/or revisions to published precision figures. For example, the cetane method (ASTM D 613) precision was revised based on historical exchange data, primary from the NEG (National Exchange Group), with data from exchanges such as IP in the UK also considered.
IMPORTANT ASPECTS OF QA PROGRAM IMPLEMENTATION Successful implementation of measurement process QA programs requires the recognition that the efforts are at least 80% organizational and 20% technical: Organizational • Workers have to pEirticipate in the complete control charting process from data generation to chart intrepretation. They must be proficient with and own the QA tools if they're expected to maintain their processes in statistical control. The person generating the QA data must have the capability and the authority to interpret the control chart and make sound technical decisions as to the "fitness-for-use" status of the measurement process. Then and only then would the workers feel a sense of ownership of their control charts. • Management must "walk the talk" by: • providing clear priority to workers that control charting is PART of the toteJ work process, not done in their spare time, • accounting for the control charting efforts in resource planning. • Management must address incapable processes by providing the necesscity training, equipment and/or procedure upgrade as identified by the control chart data. • Management must foster an environment of trust and open communication
Technical • Resist the temptation of being seduced by computer automation technology: Computers can't think. People do. • Make the paper system work first before making any attempt at automation. When designing for automation, do not settle for partial charts or text entries without the pictorial information and the capability to directly annotate on the chart. Insist on the fuU features and functionalities to include as a minimum: • instant access to a pictorial representation of at least 16 previous points • direct emnotation features on cheirt
1050
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
CONCLUSION In summary, proficiency in executing standardized measurement protocol for product quality testing is a key mutual expectation between suppliers and customers. Proficiency assurance requires data to demonstrate: • m e a n a n d precision of measurement process in statistical control, i.e., the process is stable, (hence predictable), and, the total magnitude of c o m m o n cause variation is quantifiable • precision metrics obtained on samples compositionally representative of product routinely tested cire acceptable relative to standards • measurement process is non-biased when testing materials that have ARVs (check standards) assigned by the standard measurement protocol. An effective quality management system that ensures the timely and proper collection of the required data, documentation, and review of performance statistics will provide the assurance of measurement proficiency, as well as continued improvement of the measurement processes.
ASTM STANDARDS No. D 3244 D 6299
D 6300
E 456 E 69
E 178
Title Standard Practice for Utilization of Test Data to Determine Conformance with Specifications Standard Practice for Applying Statistical Quality Assurance Techniques to Evaluate Analytical Measurement System Performance Standard Practice for Determination of Precision and Bias Data for Use in Test Methods for Petroleum Products a n d Lubricants Standard Terminology for Relating to Quality and Statistics Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method Standard Practice for Dealing with Outlying Observations
[3] ASTM D 6300: Standard Practice for Determination of Precision and Bias Data for Use in Test Methods for Petroleum Products and Lubricants, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, 2002. [4] ASTM D 6299: Standard Practice for Applying Statistical Quality AssurEince Techniques to Evaluate Analytical Measurement System Performance, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, 2002. [5] ASTM D 3244: Standard Practice for Utilization of Test Data to Determine Conformance with Specifications, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, 1998. [6] Hunter, J. S., "A One-Point Equivalent to the Shewhart Chart with Western Electric Rules," Quality Engineering, Vol. 2, No. 1, 1989-90, pp. 13-19. [7] ASTM E 178: Standard Practice for Dealing with Outlying Observations, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, 2002.
APPENDIX Statistical Quality Control Tools Purpose
of this
Appendix
This appendix provides guidance to practitioners, including worked examples, for the proper execution of the procedures described in this chapter. Pretreatment
of Test
ISO 5725
Title Determination a n d Application of Precision Data in Relation to Methods of Test Accuracy (Trueness and Precision) of Measurement Methods and Results
REFERENCES [1] ASTM E 456: Standard Terminology for Relating to Quality and Statistics, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, 1997. [2] ASTM E 69: Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, 2000.
[Al]
/; = Yu
with no pretreatment being required. An example of a sequence of results, Y/, from a single QC sample is given in Column 2 of Table A l . If [Yi'J = 1. . .M] is a sequence of results from a single check standard, from multiple check standards having nominally the same ARV, or from multiple check standards having different ARVs where the precision of the measurement system does not vary with level, and if [X,:7 = 1. . .n) is the sequence of corresponding ARVs, then
OTHER STANDARDS No. ISO 4259
Results
Throughout this appendix, [Yi:! = 1 . . .n] denotes a sequence of measured test results. [/;•:/= 1. . .n] will signify a sequence of test results after pretreatment, if necessary. If [Y,:/ = 1 . . .M] is a sequence of results from a single QC sample, then
/,- = Yi-
[A2]
Xi
TABLE Al Sequence Number I
QC/Check Standard Result
1 2 3 4 5 6 7 8 9 10 11 12 13
55.3 55.8 56.3 56.1 55.8 55.5 55.3 55.4 56.6 56.1 55.0 55.5 55.5
Yi = Ii
Sequence Number I
QC/Check Standard Result
14 15 16 17 18 19 20 21 22 23 24 25
55.2 56.5 55.7 55.6 55.2 55.7 56.1 56.3 55.2 55.4 55.4 55.6
Yi = Ii
CHAPTER 38: STATISTICAL
QUALITY ASSURANCE
TABLE A2 Sequence Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Difference Result - ARV
(Yi)
Accepted Reference Value (ARV = Xi)
55.3 55.8 56.3 56.1 55.8 55.5 55.3 55.4 56.6 56.1 55.0 55.5 55.5 55.2 56.5 55.7 55.6 55.2 55.7 56.1 56.3 55.2 55.4 55.4 55.6
55.88 55.88 55.88 55.88 55.88 55.88 55.88 55.88 55.88 55.88 55.88 55.88 55.88 55.88 55.88 55.88 55.88 55.88 55.88 55.88 55.88 55.88 55.88 55.88 55.88
-0.58 -0.08 0.42 0.22 -0.08 -0.38 -0.58 -0.48 0.72 0.22 -0.88 -0.38 -0.38 -0.68
Check Standard Result
li
0.62 -0.18 -0.28 -0.68 -0.18 0.22 0.42 -0.68 -0.48 -0.48 -0.28
The reproducibility standard deviation of the measurement process must be essentially the same for EIII values [Xi]. An example of a sequence of results from a single check standard is given in Table A2. The preprocessed result, /,-, is given in Column 4 of Table A2. If [Yi ] is a sequence of results from different check standards, and if the reproducibility varies with the level of the accepted reference values, [Xi], then li = (Yi - XiVOi
[A3]
where D, are estimates of the reproducibility standard deviation of the measurement process at levels [Xi]. Table A3 shows an example of results for multiple check standards where the precision of the measurement system is level dependent.
OF MEASUREMENT
PROCESSES
FOR
PETROLEUM
rection to accommodate the expected minimum and maximum of the data. Example of a run chart for QC results—The first 15 results from Column 2 of Table Al are plotted in sequence as they are collected, as shown in Fig. Al. The data would be examined for unusual patterns. Example of a run chart for multiple results from a single check standard—The first 15 preprocessed results (differences) from Column 4 of Table A2 are plotted in sequence as they are collected as shown in Fig. A2. The data would be examined for unusual patterns. Example of a run chart for results from multiple check standards—The first 15 preprocessed results (differences scaled by
TABLE A3 Result Sequence Number, i
Raw Result Yi
XI
Raw Difference
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
71.0 65.8 70.3 66.2 93.8 102.9 102.2 103.2
71.4 64.9 70.2 67.7 93.4 104.0 101.8 103.9 99.8 71.5 76.4 61.8 43.9 69.7 59.19 98.87 95.21 103.94 96.7 100.65 84.15 83.75 65.93 68.0
-0.40 0.90 0.10 -1.50 0.40 -1.10 0.40 -0.70 0.20 0.10 0.30 -0.60 0.20 0.01 0.31 0.76 -1.51 -0.17 -0.52 -0.95 0,17 -0.46 -0.77 0.19
100 71.6 76.7 61.2 44.1 69.71 59.5 99.63 93.7 103.77 96.18 99.7 84.32 83.29 65.16 68.19
ARV
Preprocessed Result li
1.14 1.10 1.13 1.11 1.26 1.33 1.31 1.32 1.30 1.14 1.16 1.08 0.98 1.13 1.06 1.30 1.27 1.32 1.28 1.31 1.21 1.21 1.10 1.12
-0.35 0.82 0.09 -1.35 0.32 -0.83 0.30 -0.53 0.15 0.09 0.26 -0.56 0.20 0.01 0.29 0.59 -1.19 -0.13 -0.41 -0.73 0.14 -0.38 -0.70 0.17
The Run Chart A run chart is a plot of results in chronological order, which can be used to screen data for unusual patterns. Preferably, pretreated results are plotted. Use a run chart to screen data for unusual patterns such as continuous trending in either direction, unusual clustering, and cycles. Several nonrandom patterns are described in control chart literature [3,4]. When control parameters have been added to a run chart, it becomes a control chart of individual values (/-chart). Plot results on the chart. Plot the first result at the left, and plot each subsequent point one increment to the right of its predecessor. The points may be connected in sequence to facilitate interpretation of the run-chart. Allow sufficient space in the x-axis direction to accommodate as many results as you expect to obtain from a consistent batch of material. Allow enough space in the y-axis di-
10
15
20
Result Sequence Number FIG. A1—Example of a run chart for QC results.
30
1052 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK Normality Checks
0
5
10
A normal probability plot (a special case of a q-q plot) is used to test the assumption that the observations are normally distributed. Since the control chart and limits prescribed in this practice are based on the assumption that the data behavior is adequately modeled by the Normal distribution, it is recommended that a test of this Normality assumption be conducted. To construct a normal probability plot: 1. Create a column of the observations sorted in ascending order. 2. Select the appropriate column from Table Al based on the number of observations (n). 3. Plot each observation in the sorted column (y-value) against its corresponding value from Table A4 (z-value). Visually inspect the plot for an approximately linear relationship. If the results are normally distributed, the plot should be approximately linear. Major deviations from linecirity are an indication of non-normal distributions of the differences. Note Al: The assessment methodology of the Normal probability plot advocated in this practice is strictly visual, due to its simplicity. For statistically more rigorous assessment techniques, users are advised to consult a statistician.
15
Result Sequence Number FIG. A2—Run chart for multiple results from a single check standard.
10
Anderson-Darling Statistic The Anderson-Darling Statistic is used to test for normality. The test involves the following steps: Order the non-outlying results such that Xi £ X2 ^ . . . . Xn Obtain standardized variate from the Xj's as follows:
15
Result Sequence Number
FIG. A3—Run chart for results from multiple check standards.
Wi = {xi -
[A4]
x)/s
TABLE A4—z-values. n
Order
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
-1.83 -1.28 -0.97 -0.73 -0.52 -0.34 -0.17 0.00 0.17 0.34 0.52 0.73 0.97 1.28 1.83
-1.86 -1.32 -1.01 -0.78 -0.58 -0.40 -0.24 -0.08 0.08 0.24 0.40 0.58 0.78 1.01 1.32 1.86
-1.89 -1.35 -1.05 -0.82 -0.63 -0.46 -0.30 -0.15 0.00 0.15 0.30 0.46 0.63 0.82 1.05 1.35 1.89
-1.91 -1.38 -1.09 -0.86 -0.67 -0.51 -0.36 -0.21 -0.07 0.07 0.21 0.36 0.51 0.67 0.86 1.09 1.38 1.91
-1.94 -1.41 -1.12 -0.90 -0.72 -0.55 -0.41 -0.27 -0.13 0.00 0.13 0.27 0.41 0.55 0.72 0.90 1.12 1.41 1.94
-1.96 -1.44 -1.15 -0.93 -0.76 -0.60 -0.45 -0.32 -0.19 -0.06 0.06 0.19 0.32 0.45 0.60 0.76 0.93 1.15 1.44 1.96
-1.98 -1.47 -1.18 -0.97 -0.79 -0.64 -0.50 -0.37 -0.24 -0.12 0.00 0.12 0.24 0.37 0.50 0.64 0.79 0.97 1.18 1.47 1.98
-2.00 -1.49 -1.21 -1.00 -0.83 -0.67 -0.54 -0.41 -0.29 -0.17 -0.06 0.06 0.17 0.29 0.41 0.54 0.67 0.83 1.00 1.21 1.49 2.00
-2.02 -1.51 -1.23 -1.03 -0.86 -0.71 -0.58 -0.45 -0.33 -0.22 -0.11 0.00 0.11 0.22 0.33 0.45 0.58 0.71 0.86 1.03 1.23 1.51 2.02
-2.04 -1.53 -1.26 -1.05 -0.89 -0.74 -0.61 -0.49 -0.37 -0.26 -0.16 -0.05 0.05 0.16 0.26 0.37 0.49 0.61 0.74 0.89 1.05 1.26 1.53 2.04
-2.05 -1.55 -1.28 -1.08 -0.92 -0.77 -0.64 -0.52 -0.41 -0.31 -0,20 -0.10 0.00 0.10 0.20 0.31 0.41 0.52 0.64 0.77 0.92 1.08 1.28 1.55 2.05
-2.07 -1.57 -1.30 -1.10 -0.94 -0.80 -0.67 -0.56 -0.45 -0.34 -0.24 -0.15 -0.05 0.05 0.15 0.24 0.34 0.45 0.56 0.67 0.80 0.94 1.10 1.30 1.57 2.07
-2.09 -1.59 -1.32 -1.13 -0.97 -0.83 -0.70 -0.59 -0.48 -0.38 -0.28 -0.19 -0.09 0.00 0.09 0.19 0.28 0.38 0.48 0.59 0.70 0.83 0.97 1.13 1.32 1.59 2.09
-2.10 -1.61 -1.35 -1.15 -0.99 -0.85 -0.73 -0.62 -0.51 -0.41 -0.32 -0.23 -0.13 -0.04 0.04 0.13 0.23 0.32 0.41 0.51 0.62 0.73 0.85 0.99 1.15 1.35 1.61 2.10
-2.11 -1.63 -1.36 -1.17 -1.01 -0.88 -0.76 -0.65 -0.54 -0.45 -0.35 -0.26 -0.17 -0.09 0.00 0.09 0.17 0.26 0.35 0.45 0.54 0.65 0.76 0.88 1.01 1.17 1.36 1.63 2.11
-2.13 -1.64 -1.38 -1.19 -1.04 -0.90 -0.78 -0.67 -0.57 -0.48 -0.39 -0.30 -0.21 -0.13 -0.04 0.04 0.13 0.21 0.30 0.39 0.48 0.57 0.67 0.78 0.90 1.04 1.19 1.38 1.64 2.13
-2.14 -1.66 -1.40 -1.21 -1.06 -0.93 -0.81 -0.70 -0.60 -0.51 -0.42 -0.33 -0.25 -0.16 -0.08 0.00 0.08 0.16 0.25 0.33 0.42 0.51 0.60 0.70 0.81 0.93 1.06 1.21 1.40 1.66 2.14
-2.15 -1.68 -1.42 -1.23 -1.08 -0.95 -0.83 -0.72 -0.63 -0.53 -0.45 -0.36 -0.28 -0.20 -0.12 -0.04 0.04 0.12 0.20 0.28 0.36 0.45 0.53 0.63 0.72 0.83 0.95 1.08 1.23 1.42 1.68 2.15
CHAPTER 38: STATISTICAL
QUALITY ASSURANCE
OF MEASUREMENT
PROCESSES
FOR
PETROLEUM
Table A4 (cont.) n
Order
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
-2.17 -1.69 -1.43 -1.25 -1.10 -0.97 -0.85 -0.75 -0.65 -0.56 -0.47 -0.39 -0.31 -0.23 -0.15 -0.08 0.00 0.08 0.15 0.23 0.31 0.39 0.47 0.56 0.65 0.75 0.85 0.97 1.10 1.25 1.43 1.69 2.17
-2.18 -1.70 -1.45 -1.26 -1.12 -0.99 -0.87 -0.77 -0.67 -0.58 -0.50 -0.42 -0.34 -0.26 -0.19 -0.11 -0.04 0.04 0.11 0.19 0.26 0.34 0.42 0.50 0.58 0.67 0.77 0.87 0.99 1.12 1.26 1.45 1.70 2.18
-2.19 -1.72 -1.47 -1.28 -1.13 -1.01 -0.89 -0.79 -0.70 -0.61 -0.52 -0.44 -0.37 -0.29 -0.22 -0.14 -0.07 0.00 0.07 0.14 0.22 0.29 0.37 0.44 0.52 0.61 0.70 0.79 0.89 1.01 1.13 1.28 1.47 1.72 2.19
-2.20 -1.73 -1.48 -1.30 -1.15 -1.02 -0.91 -0.81 -0.72 -0.63 -0.55 -0.47 -0.39 -0.32 -0.25 -0.17 -0.10 -0.03 0.03 0.10 0.17 0.25 0.32 0.39 0.47 0.55 0.63 0.72 0.81 0.91 1.02 1.15 1.30 1.48 1.73 2.20
-2.21 -1.74 -1.49 -1.31 -1.17 -1.04 -0.93 -0.83 -0.74 -0.65 -0.57 -0.49 -0.42 -0.35 -0.27 -0.20 -0.14 -0.07 0.00 0.07 0.14 0.20 0.27 0.35 0.42 0.49 0.57 0.65 0.74 0.83 0.93 1.04 1.17 1.31 1.49 1.74 2.21
-2.22 -1.76 -1.51 -1.33 -1.18 -1.06 -0.95 -0.85 -0.76 -0.67 -0.59 -0.52 -0.44 -0.37 -0.30 -0.23 -0.17 -0.10 -0.03 0.03 0.10 0.17 0.23 0.30 0.37 0.44 0.52 0.59 0.67 0.76 0.85 0.95 1.06 1.18 1.33 1.51 1.76 2.22
-2.23 -1.77 -1.52 -1.34 -1.20 -1.08 -0.97 -0.87 -0.78 -0.69 -0.62 -0.54 -0.47 -0.40 -0.33 -0.26 -0.19 -0.13 -0.06 0.00 0.06 0.13 0.19 0.26 0.33 0.40 0.47 0.54 0.62 0.69 0.78 0.87 0.97 1.08 1.20 1.34 1.52 1.77 2.23
-2.24 -1.78 -1.53 -1.36 -1.21 -1.09 -0.98 -0.89 -0.80 -0.71 -0.64 -0.56 -0.49 -0.42 -0.35 -0.29 -0.22 -0.16 -0.09 -0.03 0.03 0.09 0.16 0.22 0.29 0.35 0.42 0.49 0.56 0.64 0.71 0.80 0.89 0.98 1,09 1.21 1.36 1.53 1.78 2.24
-2.25 -1.79 -1.55 -1.37 -1.23 -1.11 -1.00 -0.90 -0.82 -0.73 -0.66 -0.58 -0.51 -0.44 -0.38 -0.31 -0.25 -0.18 -0.12 -0.06 0.00 0.06 0.12 0.18 0.25 0.31 0.38 0.44 0.51 0.58 0.66 0.73 0.82 0.90 1.00 1.11 1.23 1.37 1.55 1.79 2.25
-2.26 -1.80 -1.56 -1.38 -1.24 -1.12 -1.02 -0.92 -0.83 -0.75 -0.67 -0.60 -0.53 -0.46 -0.40 -0.33 -0.27 -0.21 -0.15 -0.09 -0.03 0.03 0.09 0.15 0.21 0.27 0.33 0.40 0.46 0.53 0.60 0.67 0.75 0.83 0.92 1.02 1.12 1.24 1.38 1.56 1.80 2.26
-2.27 -1.81 -1.57 -1.40 -1.26 -1.14 -1.03 -0.94 -0.85 -0.77 -0.69 -0.62 -0.55 -0.48 -0.42 -0.36 -0.30 -0.24 -0.18 -0.12 -0.06 0.00 0.06 0.12 0.18 0.24 0.30 0.36 0.42 0.48 0.55 0.62 0.69 0.77 0.85 0.94 1.03 1.14 1.26 1.40 1.57 1.81 2.27
-2.28 -1.82 -1.58 -1.41 -1.27 -1.15 -1.05 -0.95 -0.87 -0.79 -0.71 -0.64 -0.57 -0.50 -0.44 -0.38 -0.32 -0.26 -0.20 -0.14 -0.09 -0.03 0.03 0.09 0.14 0.20 0.26 0.32 0.38 0.44 0.50 0.57 0.64 0.71 0.79 0.87 0.95 1.05 1.15 1.27 1.41 1.58 1.82 2.28
-2.29 -1.83 -1.59 -1.42 -1.28 -1.16 -1.06 -0.97 -0.88 -0.80 -0.73 -0.66 -0.59 -0.52 -0.46 -0.40 -0.34 -0.28 -0.22 -0.17 -0.11 -0.06 0.00 0.06 0.11 0.17 0.22 0.28 0.34 0.40 0.46 0.52 0.59 0.66 0.73 0.80 0.88 0.97 1.06 1.16 1.28 1.42 1.59 1.83 2.29
-2.29 -1.84 -1.60 -1.43 -1.29 -1.18 -1.07 -0.98 -0.90 -0.82 -0.74 -0.67 -0.61 -0.54 -0.48 -0.42 -0.36 -0.30 -0.25 -0.19 -0.14 -0.08 -0.03 0.03 0.08 0.14 0.19 0.25 0.30 0.36 0.42 0.48 0.54 0.61 0.67 0.74 0.82 0.90 0.98 1.07 1.18 1.29 1.43 1.60 1.84 2.29
-2.30 -1.85 -1.61 -1.44 -1.31 -1.19 -1.09 -1.00 -0.91 -0.83 -0.76 -0.69 -0.63 -0.56 -0.50 -0.44 -0.38 -0.33 -0.27 -0.21 -0.16 -0.11 -0.05 0.00 0.05 0.11 0.16 0.21 0.27 0.33 0.38 0.44 0.50 0.56 0.63 0.69 0.76 0.83 0.91 1.00 1.09 1.19 1.31 1.44 1.61 1.85 2.30
-2.31 -1.86 -1.62 -1.45 -1.32 -1.20 -1.10 -1.01 -0.93 -0.85 -0.78 -0.71 -0.64 -0.58 -0.52 -0.46 -0.40 -0.35 -0.29 -0.24 -0.18 -0.13 -0.08 -0.03 0.03 0.08 0.13 0.18 0.24 0.29 0.35 0.40 0.46 0.52 0.58 0.64 0.71 0.78 0.85 0.93 1.01 1.10 1.20 1.32 1.45 1.62 1.86 2.31
-2.32 -1.87 -1.64 -1.47 -1.33 -1.21 -1.11 -1.02 -0.94 -0.86 -0.79 -0.72 -0.66 -0.60 -0.54 -0.48 -0.42 -0.37 -0.31 -0.26 -0.21 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 0.21 0.26 0.31 0.37 0,42 0.48 0.54 0.60 0.66 0.72 0.79 0.86 0.94 1.02 1.11 1.21 1.33 1.47 1.64 1.87 2.32
-2.33 -1.88 -1.64 -1.48 -1.34 -1.23 -1.13 -1.04 -0.95 -0.88 -0.81 -0.74 -0.67 -0.61 -0.55 -0.50 -0.44 -0.39 -0.33 -0.28 -0.23 -0.18 -0.13 -0.08 -0.03 0.03 0.08 0.13 0.18 0.23 0.28 0.33 0.39 0.44 0.50 0.55 0.61 0.67 0.74 0.81 0.88 0.95 1.04 1.13 1.23 1.34 1.48 1.64 1.88 2.33
for (I = 1 . . . n), where s is sample standard deviation of the results, and x is the average of the results. Convert the Wi's to standard normal cumulative probabilities Pi's using the cumulative probability table for the standardized normal variate z (Table A5): Pi = Probability (z < Wj)
[A5]
Compute A as £ ( 2 i - l)[ln(p;) + ln(l - p „ + i - 0 12 _ i s l _
- n
[A6]
Compute A^* as A^ I +
0.75 I 2.25 n n^
[A7]
If the computed value of A^* exceeds 0.752, then the hypothesis of normality is rejected for a 5% level test. Example of Normal Probability Plot for QC Results—Once 15 results have been obtained (Table Al), they are sorted in
ascending order and paired with the corresponding z-values from Table A4. The paired results (Table A6) are plotted as (x,y) points (Figs. A3 and A4). A line can be added to the plot to facilitate examination of the data for deviations from linearity. For the above example, the w^ emd Pi values used in the calculation of the Anderson-Darling statistic are shown in Table A6, as is the individual terms in the summation for A^. The value for A^ is 0.415, and the value for A^* is 0.440. Since this value is less than 0.752, the hypothesis of normality is accepted at the 95% confidence level. Example of Normal Probability Plot for Multiple Results From a Single Check Standard—The first 15 preprocessed results (Table A2, Column 4) are sorted in ascending order and paired with the corresponding z-values from Table A4. The paired results (Table A7) are plotted as x,y points (Fig. A5). A line can be added to the plot to facilitate examination of the data for deviations from lineeuity. For this example, the Wi, and pi values used in the calculation of the Anderson-Darling statistic are shown in Table A7,
TABLE A5—Pi values. Probability ( z < wj) where w, is the sum of the number in the left column and top row. -0.09
-0.08
-0.07
-0.06
-0.05
-0.04
-0.03
-0.02
-0.01
0.00
0.0
0.0002 0.0002 0.0003 0.0005 0.0007 0.0010 0.0014 0.0019 0.0026 0.0036 0.0048 0.0064 0.0084 0.0110 0.0143 0.0183 0.0233 0.0294 0.0367 0.0455 0.0559 0.0681 0.0823 0.0985 0.1170 0.1379 0.1611 0.1867 0.2148 0.2451 0.2776 0.3121 0.3483 0.3859 0.4247 0.4641
0.0002 0.0003 0.0004 0.0005 0.0007 0.0010 0.0014 0.0020 0.0027 0.0037 0.0049 0.0066 0.0087 0.0113 0.0146 0.0188 0.0239 0.0301 0.0375 0.0465 0.0571 0.0694 0.0838 0.1003 0.1190 0.1401 0.1635 0.1894 0.2177 0.2483 0.2810 0.3156 0.3520 0.3897 0.4286 0.4681
0.0002 0.0003 0.0004 0.0005 0.0008 0.0011 0.0015 0.0021 0.0028 0.0038 0.0051 0.0068 0.0089 0.0116 0.0150 0.0192 0.0244 0.0307 0.0384 0.0475 0.0582 0.0708 0.0853 0.1020 0.1210 0.1423 0.1660 0.1922 0.2206 0.2514 0.2843 0.3192 0.3557 0.3936 0.4325 0.4721
0.0002 0.0003 0.0004 0.0006 0.0008 0.0011 0.0015 0.0021 0.0029 0.0039 0.0052 0.0069 0.0091 0.0119 0.0154 0.0197 0.0250 0.0314 0.0392 0.0485 0.0594 0.0721 0.0869 0.1038 0.1230 0.1446 0.1685 0.1949 0.2236 0.2546 0.2877 0.3228 0.3594 0.3974 0.4364 0.4761
0.0002 0.0003 0.0004 0.0006 0.0008 0.0011 0.0016 0.0022 0.0030 0.0040 0.0054 0.0071 0.0094 0.0122 0.0158 0.0202 0.0256 0.0322 0.0401 0.0495 0.0606 0.0735 0.0885 0.1056 0.1251 0.1469 0.1711 0.1977 0.2266 0.2578 0.2912 0.3264 0.3632 0.4013 0.4404 0.4801
0.0002 0.0003 0.0004 0.0006 0.0008 0.0012 0.0016 0.0023 0.0031 0.0041 0.0055 0.0073 0.0096 0.0125 0.0162 0.0207 0.0262 0.0329 0.0409 0.0505 0.0618 0.0749 0.0901 0.1075 0.1271 0.1492 0.1736 0.2005 0.2296 0.2611 0.2946 0.3300 0.3669 0.4052 0.4443 0.4840
0.0002 0.0003 0.0004 0.0006 0.0009 0.0012 0.0017 0.0023 0.0032 0.0043 0.0057 0.0075 0.0099 0.0129 0.0166 0.0212 0.0268 0.0336 0.0418 0.0516 0.0630 0.0764 0.0918 0.1093 0.1292 0.1515 0.1762 0.2033 0.2327 0.2643 0.2981 0.3336 0.3707 0.4090 0.4483 0.4880
0.0002 0.0003 0.0005 0.0006 0.0009 0.0013 0.0018 0.0024 0.0033 0.0044 0.0059 0.0078 0.0102 0.0132 0.0170 0.0217 0.0274 0.0344 0.0427 0.0526 0.0643 0.0778 0.0934 0.1112 0.1314 0.1539 0.1788 0.2061 0.2358 0.2676 0.3015 0.3372 0.3745 0.4129 0.4522 0.4920
0.0002 0.0003 0.0005 0.0007 0.0009 0.0013 0.0018 0.0025 0.0034 0.0045 0.0060 0.0080 0.0104 0.0136 0.0174 0.0222 0.0281 0.0351 0.0436 0.0537 0.0655 0.0793 0.0951 0.1131 0.1335 0.1562 0.1814 0.2090 0.2389 0.2709 0.3050 0.3409 0.3783 0.4168 0.4562 0.4960
0.0002 0.0003 0.0005 0.0007 0.0010 0.0013 0.0019 0.0026 0.0035 0.0047 0.0062 0.0082 0.0107 0.0139 0.0179 0.0228 0.0287 0.0359 0.0446 0.0548 0.0668 0.0808 0.0968 0.1151 0.1357 0.1587 0.1841 0.2119 0.2420 0.2743 0.3085 0.3446 0.3821 0.4207 0.4602 0.5000
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5
0.5000 0.5398 0.5793 0.6179 0.6554 0.6915 0.7257 0.7580 0.7881 0.8159 0.8413 0.8643 0.8849 0.9032 0.9192 0.9332 0.9452 0.9554 0.9641 0.9713 0.9772 0.9821 0.9861 0.9893 0.9918 0.9938 0.9953 0.9965 0.9974 0.9981 0.9987 0.9990 0.9993 0.9995 0.9997 0.9998
0.5040 0.5438 0.5832 0.6217 0.6591 0.6950 0.7291 0.7611 0.7910 0.8186 0.8438 0.8665 0.8869 0.9049 0.9207 0.9345 0.9463 0.9564 0.9649 0.9719 0.9778 0.9826 0.9864 0.9896 0.9920 0.9940 0.9955 0.9966 0.9975 0.9982 0.9987 0.9991 0.9993 0.9995 0.9997 0.9998
0.5080 0.5478 0.5871 0.6255 0.6628 0.6985 0.7324 0.7642 0.7939 0.8212 0.8461 0.8686 0.8888 0.9066 0.9222 0.9357 0.9474 0.9573 0.9656 0.9726 0.9783 0.9830 0.9868 0.9898 0.9922 0.9941 0.9956 0.9967 0.9976 0.9982 0.9987 0.9991 0.9994 0.9995 0.9997 0.9998
0.5120 0.5517 0.5910 0.6293 0.6664 0.7019 0.7357 0.7673 0.7967 0.8238 0.8485 0.8708 0.8907 0.9082 0.9236 0.9370 0.9484 0.9582 0.9664 0.9732 0.9788 0.9834 0.9871 0.9901 0.9925 0.9943 0.9957 0.9968 0.9977 0.9983 0.9988 0.9991 0.9994 0.9996 0.9997 0.9998
0.5160 0.5557 0.5948 0.6331 0.6700 0.7054 0.7389 0.7704 0.7995 0.8264 0.8508 0.8729 0.8925 0.9099 0.9251 0.9382 0.9495 0.9591 0.9671 0.9738 0.9793 0.9838 0.9875 0.9904 0.9927 0.9945 0.9959 0.9969 0.9977 0.9984 0.9988 0.9992 0.9994 0.9996 0.9997 0.9998
0.5199 0.5596 0.5987 0.6368 0.6736 0.7088 0.7422 0.7734 0.8023 0.8289 0.8531 0.8749 0.8944 0.9115 0.9265 0.9394 0.9505 0.9599 0.9678 0.9744 0.9798 0.9842 0.9878 0.9906 0.9929 0.9946 0.9960 0.9970 0.9978 0.9984 0.9989 0.9992 0.9994 0.9996 0.9997 0.9998
0.5239 0.5636 0.6026 0.6406 0.6772 0.7123 0.7454 0.7764 0.8051 0.8315 0.8554 0.8770 0.8962 0.9131 0.9279 0.9406 0.9515 0.9608 0.9686 0.9750 0.9803 0.9846 0.9881 0.9909 0.9931 0.9948 0.9961 0.9971 0.9979 0.9985 0.9989 0.9992 0.9994 0.9996 0.9997 0.9998
0.5279 0.5675 0.6064 0.6443 0.6808 0.7157 0.7486 0.7794 0.8078 0.8340 0.8577 0.8790 0.8980 0.9147 0.9292 0.9418 0.9525 0.9616 0.9693 0.9756 0.9808 0.9850 0.9884 0.9911 0.9932 0.9949 0.9962 0.9972 0.9979 0.9985 0.9989 0.9992 0.9995 0.9996 0.9997 0.9998
0.5319 0.5714 0.6103 0.6480 0.6844 0.7190 0.7517 0.7823 0.8106 0.8365 0.8599 0.8810 0.8997 0.9162 0.9306 0.9429 0.9535 0.9625 0.9699 0.9761 0.9812 0.9854 0.9887 0.9913 0.9934 0.9951 0.9963 0.9973 0.9980 0.9986 0.9990 0.9993 0.9995 0.9996 0.9997 0.9998
0.5359 0.5753 0.6141 0.6517 0.6879 0.7224 0.7549 0.7852 0.8133 0.8389 0.8621 0.8830 0.9015 0.9177 0.9319 0.9441 0.9545 0.9633 0.9706 0.9767 0.9817 0.9857 0.9890 0.9916 0.9936 0.9952 0.9964 0.9974 0.9981 0.9986 0.9990 0.9993 0.9995 0.9997 0.9998 0.9998
-3.5 -3.4 -3.3 -3.2 -3.1 -3.0 -2.9 -2.8 -2.7 -2.6 -2.5 -2.4 -2.3 -2.2 -2.1 -2.0 -1.9 -1.8 -1.7 -1.6 -1.5 -1.4 -1.3 -1.2 -1.1
-1 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1
CHAPTER 38: STATISTICAL QUALITY ASSURANCE OF MEASUREMENT PROCESSES FOR PETROLEUM as is the individual terms in the summation for A^. The value for A^ is 0.415, and the value for A^* is 0.440. Since this value is less than 0.752, the hypothesis of normality is accepted at the 9 5 % confidence level. Example of Normal Probability Plot for Results from^ Multiple Check Standards—The first 15 preprocessed results (Table A3, Column 6) are sorted in ascending order and paired with the corresponding z-values from Table A4. The paired results (Table A8) are plotted as x,y points (Fig. A6). A line can be added to the plot to facilitate examination of the data for deviations from linearity.
Original Sequence #, I 11 14 1 7 8 6 12 13 2 5 10 4 3 15 9
z—value 1.83 -1.28 -0.97 -0.73 -0.52 -0.34 -0.17 0.00 0.17 0.34 0.52 0.73 0.97 1.28 1.83
TABLE A6 Sorted Result 55.0 55.2 55.3 55.3 55.4 55.5 55.5 55.5 55.8 55.8 56.1 56.1 56.3 56.5 56.6
-1.47 -1.07 -0.86 -0.86 -0.66 -0.46 -0.46 -0.46 0.15 0.15 0.76 0.76 1.16 1.57 1.77
0.07 0.14 0.19 0.19 0.25 0.32 0.32 0.32 0.56 0.56 0.78 0.78 0.88 0.94 0.96
i^Term in [A6] -5.91 -14.35 -18.70 -21.94 -25.77 -21.44 -25.34 -22.80 -16.52 -18.46 -11.50 -10.80 -8.65 -5.79 -3.25
FIG. A5—Example of a normal probability plot for multiple results from a single check standard.
Sort* 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Original Sequence # 11 14 1 7 8 6 12 13 2 5 10 4 3 15 9
TABLE A8 Sorted Result z-value 1.83 1.28 0.97 0.73 0.52 0.34 0.17 0 0.17 0.34 0.52 0.73 0.97 1.28 1.83
-1.35 -0.83 -0.56 -0.53 -0.35 0.01 0.09 0.09 0.15 0.2 0.26 0.29 0.3 0.32 0.82
i* Temi in [A6] -3.12 -1.94 -1.32 -1.25 -0.84 -0.02 0.16 0.16 0.30 0.41 0.55 0.62 0.64 0.69 1.83
0.00 0.03 0.09 0.11 0.20 0.49 0.56 0.56 0.62 0.66 0.71 0.73 0.74 0.75 0.97
-10.41 -15.11 -18.58 -24.98 -25.59 -19.68 -19.93 -21.05 -22.33 -20.75 -11.91 -9.73 -9.99 -8.34 -1.02
z-values
FIG. A4—Example of a normal probability plot for QC results. TABLE A7 Sort# 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Original Sequence * 11 14 1 7 8 6 12 13 2 5 10 4 3 15 9
Sorted Result -0.88 -0.68 -0.58 -0.58 -0.48 -0.38 -0.38 -0.38 -0.08 -0.08 0.22 0.22 0.42 0.62 0.72
z—value -1.83 -1.28 -0.97 -0.73 -0.52 -0.34 -0.17 0 0.17 0.34 0.52 0.73 0.97 1.28 1.83
Pi
-1.47 -1.07 -0.86 -0.86 -0.66 -0.46 -0.46 -0.46 0.15 0.15 0.76 0.76 1.16 1.57 1.77
0.07 0.14 0.19 0.19 0.25 0.32 0.32 0.32 0.56 0.56 0.78 0.78 0.88 0.94 0.96
i* Tenn in [A6] -5.91 -14.35 -18.70 -21.94 -25.77 -21.44 -25.34 -22.80 -16.52 -18.46 -11.50 -10.80 -8.65 -5.79 -3.25
FIG. A6—Example of a normal probability plot for results from multiple check standards.
The Control Chart I-Chart The Control Chart of Individual Values (/-Chart) is a run chart to which control limits and center line have been added. In order to establish placement positions of the control limits for the I chart, an estimate of the process variability will need to be obtained from the data. While there are
1056
MANUAL
37: FUELS
AND LUBRICANTS
HANDBOOK
several statistical techniques that can be used for this purpose, this practice advocates use of an MR (Moving Range of Two) chart for its simplicity axid robustness to outliers. Produce an /-Chart only after a m i n i m u m of 15 preprocessed results have been obtained from the measurement system, and the data have been screened (8.3.1 and 8.3.2) and tested for normality (A3). A horizontal center line is added at the level of the mean of eJl the results, / :
lli (A8) Upper and lower control limits are added, also, computed from the average moving range of two:
equation: EWMAi = h EWMAi = (1
n
A)£WMA;-i + \Ii
(A15)
where X is the exponential weighting factor. For application of this practice, a A value of 0.4 is recommended. Note A2: For the EWMA trend, a A value of 0.4 emulates closely t h e run rule effects of conventional control charts, w h i l e a value of 0.2 h a s o p t i m a l prediction properties for the next e x p e c t e d value. In addition, these A values also conveniently place t h e control limits (3-sigma) for the EWMA trend at the 1 (for A = 0.2) to 1.5-sigma (for A = 0.4) values for I chart. The control limits for the EWMA chart are calculated using a weight (A) as follows:
S|/.-.. MR =
(A14)
UCLx = 1 + 2.66MR
(A16)
LCLx = / -
(A17)
1
UCLi = 1 + 2.66 MR,
(AlO)
LCL, = 7 - 2.66 MR.
(All)
Individual values that are outside the upper or lower control limits are indications of a n unstable system, a n d efforts should be made to determine the cause. Optionally, any one of the following occurrences should be considered as potential signals of instability: 1. Two out of three consecutive results on the /-Chart that are more than 1.77 MR distant from the center line in the same direction; 2. Five consecutive results on the /-Chart that are more than 0.89 MR distant from the center line in the same direction; 3. Eight or more consecutive points in the /-Chart that fall on the same side of the center line.
2.66MR
E x a m p l e s o f C o n t r o l C h a r t s f o r QC and Check Standard Results Example of an MR Chart for QC Results—MRi values for the data from Table Al are calculated and plotted in sequence. After 15 results are obtained, the MR = 0.500 value is calculated and added to the plot. Computations are shown in Table A9. A UCLMR= 1.64 is a d d e d to produce the MR Chart (Fig. A7). Example of I Chart and EWMA Overlay for QC Results—The average of the first 15 QC results (Table A9, Column 2) is calculated and plotted on the r u n chart as 7 = 55.73. The upper
TABLE A9
MR-Chart
Sequence Number, I
A Moving Range of Two {MR) Chart is obtained by plotting the sequential range of two values given by: MRi = \Ii - /;-,!
(A12)
and connecting each point. The upper control limit for the MR chart is given by UCLMR
= 3.27
AIR.
There is no lower control limit for an
(A13)
MR-chait.
EWMA-Overlay A EWMA overlay is a trend line constructed from Exponentially Weighted Moving Average {EWMA) values calculated using the I-values. The EWMA trend line is typically overlaid on the /-chart to enhance its sensitivity in detecting m e a n shifts that are small relative to the measurement system precision. Each EWMA value is a weighted average of the current result and previous results, with the weights decreasing exponentially with the age of the reading. A sequence of values, EWMA,, are calculated, and overlaid on the /-chart and connected. Use the following recursion
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Average 16 17 18 19 20 21 22 23 24 25
QC Result (Y, = li) 55.3 55.8 56.3 56.1 55.8 55.5 55.3 55.4 56.6 56.1 55 55.5 55.5 55.2 56.5
Moving Range MRi 0.5 0.5 0.2 0.3 0.3 0.2 0.1 1.2 0.5 1.1 0.5 0.0 0.3 1.3
55.73
0.500
55.7 55.6 55.2 55.7 56.1 56.3 55.2 55.4 55.4 55.6
0.8 0.1 0.4 0.5 0.4 0.2 1.1 0.2 0.0 0.2
EWMAi 55.3 55.50 55.82 55.93 55.88 55.73 55.56 55.49 55.94 56.00 55.60 55.56 55.54 55.40 55.84 55.78 55.71 55.51 55.58 55.79 55.99 55.68 55.57 55.50 55.54
CHAPTER 38: STATISTICAL QUALITY ASSURANCE OF MEASUREMENT PROCESSES FOR PETROLEUM
3
5
10
15
10
20
Result Sequence Number FIG. A7—Example of an MR chart for QC results.
57.5
20
FIG. A9—Example of a MR chart for multiple results from a single check standard. o EWMA Value
o Result Value
• EWMA Value
acL,
57 56.5 ^
15
Result Sequence Number
UCL, VCL,
acLEtna
56 LCL,
(0 55.5
LCL'EtmA
55 54.5
LCL-, 10
LCL,
15
20
30
Result Sequence Number 54
10
15
20
25
30
Result Sequence Number
FIG. AlO—Example of an l-Chart with EWMA overlay for multiple results from a single check standard.
FIG. A8—Example of an i-Chart with EWMA overlay for QC results.
TABLE AlO Sequence Number
and lower control limits are calculated from Eqs A6 and A7 as 54.40 and 57.06 and added to the r u n chart to produce the / chart (Fig. A8). EWMA values (Table A9, Column 4) and EWMA control limits, 55.06 and 56.39, are overlaid on the / chart. Additional results and calculated EWMA values are added as they are determined. Example of an MR Chart for Multiple Results From a Single Check Standard—MRi values are calculated and plotted in sequence. After 15 results are obtained (Table A2), the MR value is calculated and added to the plot. A UCLMR is added to produce the MR Chart (Fig. A9). Example of I Chart and EWMA Overlay for Multiple Results From a Single Check Standard—The average of the first 15 QC results (Table^A2, Column 4) is calculated and plotted on the r u n chart as / . The upper and lower control limits cU"e calculated from Eqs A6 and A7 and added to the r u n chart to produce the I chart. EWMA values and EWMA control limits may be overlaid on the I chart (Fig. AlO). Additional results and calculated EWMA values are added as they are determined. The MR values for this example are shown in Table AlO, Column 3.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Average 16 17 18 19 20 21 22 23 24 25
Check Standard Result (li)
Moving Range MRi
-0.58 -0.08 0.42 0.22 -0.08 -0.38 -0.58 -0.48 0.72 0.22 -0.88 -0.38 -0.38 -0.68 0.62
0.5 0.5 0.2 0.3 0.3 0.2 0.1 1.2 0.5 1.1 0.5 0.0 0.3 1.3
-0.153
0.500
-0.18 -0.28 -0.68 -0.18 0.22 0.42 -0.68 -0.48 -0.48 -0.28
0.8 0.1 0.4 0.5 0.4 0.2 1.1 0.2 0.0 0.2
EWMAi -0.58 -0.38 -0.06 0.05 -0.00 -0.15 -0.32 -0.39 0.06 0.12 -0.28 -0.32 -0.34 -0.48 -0.04 -0.10 -0.17 -0.37 -0.30 -0.09 0.11 -0.20 -0.31 -0.38 -0.34
1058
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
Example of an MR Chart for Results From Multiple Check Standards—MRi values are calculated a n d plotted in sequence. After 15 results are obtained (Table A3, Column 6, displayed again in Table A l l ) , the MR value is calculated a n d added to the plot. A UCLMR is added to produce the MR Chart (Fig. A l l ) . Example of I Chart and EWMA Overlay for Results From Multiple Check Standards—The average of the first 15 QC results (Table A3, column 6) is calculated and plotted on the r u n chart as I. The upper and lower control limits are Ccdculated from equations A6 a n d A7 and added to the r u n chzirt to produce the I chart. EWMA values emd EWMA control limits may be overlaid on the I chart (Fig. A12). AdditioncJ results and calculated EWMA values are added as they are determined.
Result Value
. EWMA Value
2
DCL,
CC 1
PCi,
*(D
-LCL,
10
15
20
25
30
Result Sequence Number
FIG. A12—Example of an l-Chart with EWMA overlay for results from multiple check standards.
TABLEAU Result Sequence Number, i
Preprocessed Result
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
T-Test Moving Range MRi
EWMAi
-0.35 0.82 0.09 -1.35 0.32 -0.83 0.30 -0.53 0.15 0.09 0.26 -0.56 0.20 0.01 0.29
1.17 0.73 1.44 1.67 1.15 1.13 0.83 0.68 0.06 0.17 0.82 0.76 0.19 0.28
-0.35 0.12 0.11 -0.48 -0.16 -0.43 -0.14 -0.29 -0.12 -0.03 0.08 -0.17 -0.02 -0.01 0.11
erage
-0.073
0.791
16 17 18 19 20 21 22 23 24
0.59 -1.19 -0.13 -0.41 -0.73 0.14 -0.38 -0.7 0.17
0.3
li
1.78 1.06 0.28 0.32 0.87 0.52 0.32 0.87
0.30 -0.29 -0.23 -0.30 -0.47 -0.23 -0.29 -0.45 -0.20
A two sided t-test is used to check if a sample of veilues comes from a population with a mean different from ein hypothesized value, Ho. In this practice, a t-test may be performed on pretreated check standard test results to check for bias relative to the ARVs. Since during pretreatment, accepted reference value(s) have been subtracted from the raw results, the hypothesized mean value is zero. For the purpose of perfoiming the t-test, two methods for calculating the t-value are presented: 1. By the Root-Mean Square Method, the standcU"d deviation of the pretreated results is calculated as
[A18] 2. The t vEilue is calculated as: t = V ^ | 7 , - iJLoVs,,
[A19]
where /AQ is the hypothesized mean which is zero (A6.1). 3. Alternatively, by the MR approach, compute the alternate t value as: tMR = V ^ | / , -
M'(MR/l.n8),
[A20]
30
where /no is the hypothesized mean which is zero (A6.1). Compare the computed t value from [A19] with the critical t vEilues in Table Al2 for (n—l) degrees of freedom. If IMR from [A20] is used, the appropriate degrees of freedom is (n —1)/2. If the absolute veJue of the calculated t (or IMR) value is less than or equcil to the criticcd t vzJue, then (IQ is statisticcJly indistinguishable from the mecin of the distribution. For the case of check standard testing, this would indicate that there is n o statistically identifiable bias. If the absolute value of t is greater t h a n the critical t value, then /Ao is statisticcJly distinguishable from the mean of the distribution, with 9 5 % confidence. For the case of check standard testing, this would indicate a statistically identifiable bias in the measurement system.
FIG. A11—Example of an MR chart for results from a multiple check standards.
Example of t test applied to multiple results from a single check standard - F o r t h e first 15 preprocessed results in colu m n 4 of Table A2, / i s —0.153. Since the results being ana-
10
15
20
Result Sequence Number
CHAPTER 38: STATISTICAL
QUALITY ASSURANCE
OF MEASUREMENT
PROCESSES
FOR
PETROLEUM
TABLE A12—95th percentile of student's \t\ distribution.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Degrees of Freedom
Degrees of Freedom
Degrees of Freedom
21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
12.7062 4.3027 3.1824 2.7764 2.5706 2.4469 2.3646 2.3060 2.2622 2.2281 2.2010 2.1788 2.1604 2.1448 2.1314 2.1199 2.1098 2.1009 2.0930 2.0860
2.0796 2.0739 2.0687 2.0639 2.0595 2.0555 2.0518 2.0484 2.0452 2.0423 2.0395 2.0369 2.0345 2.0322 2.0301 2.0281 2.0262 2.0244 2.0227 2.0211
lyzed are the difference relative to the ARV, (IQ is 0. The standard deviation of the first 15 preprocessed resuUs is 0.493, and the t vaJue is 1.2034. The t value is less than the critical value for 14 degrees of freedom (t^ = 2.1448), so the average difference between the check standard results and the accepted reference value is statistically indistinguishable from zero. Example oft test applied to results from multiple check standards - For the first 15 preprocessed results in column 6 of Table A3, 7 is —0.0719. Since the results being analyzed are the difference relative to the ARV, ^o is 0. The standard deviation of the first 15 preprocessed results is 0.550, and the t value is 0.506. The t value is less than the critical value for 14 degrees of freedom (fis = 2.1448), so the average difference between the check standard results and the accepted reference value is statistically indistinguishable from zero. Approximate Chi-Square Test The chi-square (x^) test is used to compare the estimated site reproducibility to a published reproducibility value, as instructed in section 9.1.2. Compute the Chi-Square
Statistic
.2_{n-\)R' X-
1
n2
2R^
[A21]
where R' is the estimated site reproducibility (MR) and R is the published reproducibility of the method. Compare the computed x^ value to the critical x^ value in Table A13, with (n-l)/2 degrees of freedom. If n is even, interpolate. If the computed x^ value exceeds the tabled value, then the site reproducibility exceeds the published reproducibility of the method, with 95% confidence. If the computed x^ value is less than or equal to the tabled value, then the site reproducibility is either less than or statistically indistinguishable from the published reproducibility of the method. Example:The site reproducibility calculated from R' = 2.46 AIR for the first 15 QC results in Table Al is 1.23. The pub-
Degrees of Freedom
41 42 43 44 45 46 47 48 49 50 55 60 65 70 75 80 85 90 95 100
105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200
2.0195 2.0181 2.0167 2.0154 2.0141 2.0129 2.0117 2.0106 2.0096 2.0086 2.0040 2.0003 1.9971 1.9944 1.9921 1.99006 1.98827 1.98667 1.98525 1.98397
1.98282 1.98177 1.98081 1.97993 1.97912 1.97838 1.97769 1.97705 1.97646 1.97591 1.97539 1.97490 1.97445 1.97402 1.97361 1.97323 1.97287 1.97253 1.97220 1.97190
TABLE A13—95th percentiles of the chi square distribution. Degrees Freedom
X
Degrees Freedom
X
Degrees Freedom
X
7 8 9 10 11 12 13 14 15 16
14.1 15.5 16.9 18.3 19.7 21.0 22.4 23.7 25.0 26.3
17 18 19 20 21 22 23 24 25 26
27.6 28.9 30.1 31.4 32.7 33.9 35.2 36.4 37.7 38.9
27 28 30 35 40 45 50 60 70 80
40.1 41.3 43.8 49.8 55.8 61.7 67.5 79.1 90.5 101.9
lished reproducibility for the measurement method at the 58.88 level is 1.05. x^ is therefore 14 • 1.23^/2 • 1.232/2 1.05^ = 11.57. This value is less than the critical x^ value of 14.1 for 7 degrees of freedom, so the site reproducibility is not statistically greater than the published reproducibility of the method. Approximate F-Test In this practice, an approximate F-test is used to compare the variation exhibited by a measurement system over two different time periods. It can also be used to compare the site reproducibility estimated from a series of result from one QC sample with that estimated using a different QC sample (section 8.6.1). Compute the F value as F = MRllMRl,
[A22]
where MRi is the larger of the two average moving ranges, and Mi?2 is the smsdler. Compare the computed F value to the critical F value read from Table A14, with (ni — l)/2 degrees of freedom for the numerator and (M2 ~ l)/2 degrees of freedom for the denominator. If the computed F value exceeds the tabled value, then the two precisions are statistically distinguishable. We can be 95% confident that the process that gave rise to the moving
1060
MANUAL
37: FUELS AND LUBRICANTS
HANDBOOK
TABLE A14—97.5 Percentiles of the F Statistic. Numerator Degrees of Freedom
Denom
d.f.
7 8 9 10 11 12 13 14 15 16 17 18 19 20 25 30 35 40 45 50 60 70 80 90 100
7
8
9
10
12
14
16
18
20
25
30
40
50
100
4.99 4.53 4.20 3.95 3.76 3.61 3.48 3.38 3.29 3.22 3.16 3.10 3.05 3.01 2.85 2.75 2.68 2.62 2.58 2.55 2.51 2.47 2.45 2.43 2.42
4.90 4.43 4.10 3.85 3.66 3.51 3.39 3.29 3.20 3.12 3.06 3.01 2.96 2.91 2.75 2.65 2.58 2.53 2.49 2.46 2.41 2.38 2.35 2.34 2.32
4.82 4.36 4.03 3.78 3.59 3.44 3.31 3.21 3.12 3.05 2.98 2.93 2.88 2.84 2.68 2.57 2.50 2.45 2.41 2.38 2.33 2.30 2.28 2.26 2.24
4.76 4.30 3.96 3.72 3.53 3.37 3.25 3.15 3.06 2.99 2.92 2.87 2.82 2.77 2.61 2.51 2.44 2.39 2.35 2.32 2.27 2.24 2.21 2.19 2.18
4.67 4.20 3.87 3.62 3.43 3.28 3.15 3.05 2.96 2.89 2.82 2.77 2.72 2.68 2.51 2.41 2.34 2.29 2.25 2.22 2.17 2.14 2.11 2.09 2.08
4.60 4.13 3.80 3.55 3.36 3.21 3.08 2.98 2.89 2.82 2.75 2.70 2.65 2.60 2.44 2.34 2.27 2.21 2.17 2.14 2.09 2.06 2.03 2.02 2.00
4.54 4.08 3.74 3.50 3.30 3.15 3.03 2.92 2.84 2.76 2.70 2.64 2.59 2.55 2.38 2.28 2.21 2.15 2.11 2.08 2.03 2.00 1.97 1.95 1.94
4.50 4.03 3.70 3.45 3.26 3.11 2.98 2.88 2.79 2.72 2.65 2.60 2.55 2.50 2.34 2.23 2.16 2.11 2.07 2.03 1.98 1.95 1.92 1.91 1.89
4.47 4.00 3.67 3.42 3.23 3.07 2.95 2.84 2.76 2.68 2.62 2.56 2.51 2.46 2.30 2.20 2.12 2.07 2.03 1.99 1.94 1.91 1.88 1.86 1.85
4.40 3.94 3.60 3.35 3.16 3.01 2.88 2.78 2.69 2.61 2.55 2.49 2.44 2.40 2.23 2.12 2.05 1.99 1.95 1.92 1.87 1.83 1.81 1.79 1.77
4.36 3.89 3.56 3.31 3.12 2.96 2.84 2.73 2.64 2.57 2.50 2.44 2.39 2.35 2.18 2.07 2.00 1.94 1.90 1.87 1.82 1.78 1.75 1.73 1.71
4.31 3.84 3.51 3.26 3.06 2.91 2.78 2.67 2.59 2.51 2.44 2.38 2.33 2.29 2.12 2.01 1.93 1.88 1.83 1.80 1.74 1.71 1.68 1.66 1.64
4.28 3.81 3.47 3.22 3.03 2.87 2.74 2.64 2.55 2.47 2.41 2.35 2.30 2.25 2.08 1.97 1.89 1.83 1.79 1.75 1.70 1.66 1.63 1.61 1.59
4.21 3.74 3.40 3.15 2.96 2.80 2.67 2.56 2.47 2.40 2.33 2.27 2.22 2.17 2.00 1.88 1.80 1.74 1.69 1.66 1.60 1.56 1.53 1.50 1.48
range MRy is less precise (has larger site-reproducibility) than the process that produced MR2. Note : Although the approximate F-test is conducted at the 95% probability level, the critical F values against w h i c h the calculated F is compared c o m e from the 97.5 percentiles of the F-statistic. If the ratio MR|/MRb is calculated without requiring that the larger variance is in the numerator, the ratio would have to be compared against both the lower 2.5 percentile point and the upper 97.5 percentile point of the F-distribution to determ i n e if the t w o variances w e r e statistically distinguishable. Because of the nature of the F-distribution, comparing MRa/MRb to the 2.5 percentile point w h e n M R | < MRg is equivalent to comparing MRb/MR| to the 97.5 percentile point. Requiring that larger varieuice is always in the numerator allows the "two-tailed" test to be accomplished in one step. If the variance of the two populations were equal, t h e n there w o u l d b e only a 2.5% chance that MR? > MRi by more than the tabulated amount, and a 2.5% chance that MRi > MRf by more than the tabulated amount with degrees of freed o m reversed. If the computed F value is smaller than the tabled value, then the precisions of the two samplings of the measurement process are statistically indistinguishable. If two precision estimates are statistically indistinguishable, they may be pooled into a single estimate. For example, if MRi was obtained from measurements on a single lot of QC sample material, while MR2 was obtained from measurements on a different lot of material, and, if they are not statistically distinguishable, they may be pooled. The appropriate pooled precision estimate is MR,pooled
(fij - 1) MRi +
(M2
- 1) MR2
Ml + W2 — 2
[A23]
TABLE A15 Sequence Number
QC Result
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
54,2 56.1 55.2 54.1 53.7
54 54.3 54.8 53.9 53.2 52.5 52.8 54.3 52.7 53.4 53.1
54 53.2 52.8 53.2 53.1 53.3 52.8
MR
1.9 0.9 1.1 0.4 0.3 0.3 0.5 0.9 0.7 0.7 0.3 1.5 1.6 0.7 0.3 0.9 0.8 0.4 0.4 0.1 0.2 0.5
55.15 55.17 54.90 54.66 54.55 54.51 54.55 54.48 54.35 54.18 54.07 54.08 53.99 53.95 53.89 53.90 53.86 53.81 53.78 53.74 53.72 53.68
LCL
UCL
54.21 54.08 53.75 53.47 53.34 53.28 53.31 53.22 53.09 52.91 52.79 52.81 52.70 52.66 52.61 52.61 52.57 52.51 52.48 52.44 52.42 52.38
56.09 56.25 56.05 55.85 55.76 55.75 55.79 55.73 55.61 55.45 55.34 55.36 55.27 55.23 55.18 55.19 55.15 55.10 55.07 55.04 55.02 54.98
Example: Table A15 contains QC results for a second QC sample measured by the same measurement system as was used to generate the results in Table Al. The MR value for the 25 results from the original QC sample (Table Al) was 0.454. The MR value for the 23 results for the new QC sample is 0.700. The F value is 2.38 which is less than the critical value of 3.33 for 11 and 12 degrees of freedom in the numerator and denominator, respectively. The precision of the measurements for the two QC batches is statistically indistinguishable.
MNL37-EB/Jun. 2003
Index olefin-based polymers, 224-226 performance package, 200-201 performance testing, 235-242 engine oils, 237-239 finished formulations, 235-236 gear oils, 239-240 greases, 242 hydraulic fluids, 240-241 individual additives, 235 machine tests, 236 metalworking fluids, 241 miscellaneous industrial oils, 241-242 physical tests, 236 ttansmission fluids, 239 viscosity tests, 237 polymeric, 223-233 pour point depressants, 230-232, 373 primary fimctions, 200 refrigeration lubricants, 418-420 rust and corrosion inhibitors, 218, 220-223, 318-320 stabilizers/deposit control agents, 201-211 standard, 243-246 for turbines, 312-323 antifoams, 321-323 antioxidants, 312-318 antiwear and extreme-pressure, 319-321 rust and corrosion inhibitors, 318-320 viscosity modifiers, 223-224 see also Antifoam additives; Antiwear additives; Antioxidants; Corrosion inhibitors Adhesion, friction due to, 911 Adhesives, viscosity, 870-872 Adhesive wear, 435, 923, 927-930, 979-982 resistance ranking, 980-981 testing, oil evaluation, 981-982 test method, 979 traction test plots, 980 Adiabatic flame temperature, gasoline, 84 Adiabatic operation, 384 Adipic acids, synthesis, 276, 279 Adsorption, environmental characteristics, 888 Adsorption efficiency, steric hinderance, 952-953 Adsorption isotherms, 953-954 Aeration, hydraulic fluids, 366-367 Aero-derivative gas turbines, 301-302 Aerospace, engine thermodynamics, 730-732 Aerospace fuels, 729-756 air dilution, 742 bouyancy in air, 732 calorific values, 739-740 combustion emissions, 743 density, 735-736 flame stabilization, 742
Abiotic degradation, environmental characteristics, 889-890 Abrasive wear, 435, 930-931 Accepted Reference Values, 1047-1048 Accumulation processes, environmental characteristics, 894-895 Acid catchers, refrigeration lubricants, 419 Acidity, automotive lubricants, 474-475 Acid neutralizing agents, 221 Acid number compressor lubricants, 400 gear lubricants, 456 mineral oil heat transfer fluids, 582 quench oils, 609-610 turbine lubricating oils and hydraulic fluids, 329, 331 Acids formation, 790 for lubricants, 272 Acoustical measurements, steel quenching, 600-602 Active oxygen method, 820-821 Additives, 199-246 antiseize, 218 aviation fuels, 104-105 classification, 951-957 demulsifiers, 232-233, 366 detergents, 208-211 dispersants, 203-208 dispersant viscosity modifiers, 228-229, 231 dyes, 234 emulsifiers, 232-233 environmental impact, 234 ester polymers, 225-228 film-forming agents, 211-223 filterability, 368 friction modifiers, 211-213, 441, 949, 951-952 gear lubricants, 439-443, 447 hydraulic fluids, 363 introduction of new, 234 metalworking and machining fluids, 511-519 alkalinity agents, 516 antimicrobial agents, 517-518 anti-misting agents, 516 corrosion inhibitors, 516-517 coupling agents, 513 dispersants, 516 dyes, 519 emulsion promoters, 512-513 film-forming agents, 513-515 foam inhibitor, 518 inorganic/organic solids, 519 odor control agents, 519 oxidation inhibitors, 518-519 multifunctional nature, 234 1061 Copyright'
2003 by A S I M International
www.astm.org
1062
MANUAL 3 7: FUELS AND L UBRICANTS
flammability, 738 fueling fire safety, 744 future trends, 747-748 handling characteristics, 743-747 health issues in handling, 747 ignition, 741 lubricity, 744-745 maximum combustion temperature, 735, 740-741 molecular structure, 732-734 oxidation heat release, 735 performance of rocket fuels and oxidants, 731 physical state, 733 preparation and mixing, 741 properties, 735-736 reformation, 747 smoke tendency, 741 solidification, 737-738 specifications, 748 specific impulse, 742-743 spontaneous ignitability, 733-734, 738 standards, 749 stoichiometry, 734-735, 738-739 storage stability, 732, 745 substitute fuels, 748 supplemental fuels, 747 surface tension, 738 thermal stability, 745-746 turbine entry temperature distribution, 742 viscosity, 737 volatility, 736-737 water content, 746-747 Aging, Baader aging test, 821-822 Air conditioning lubricants, 484 Aircraft, sumpingprocedure. 111 Aircraft range, aviation fuels, 100-101 Air-fuel ratio, octane number requirement, 76 Airport fuel systems, 106 quality control, 110-111 Air release compressor lubricants, 398 gear lubricants, 460 turbine lubricating oils and hydraulic fluids, 332-334 Alcohol bonds, environmentally friendly oils, 271-272 Alcohols hydrogen bond, 270 for lubricants, 271-273 Algae, acute toxicity tests, 901-902 Alkalinity agents automotive lubricants, 474-475 metalworking and machining fluids, 516 Alkanes lattice transition, 534 physical properties, 532 Alkenylsuccinates, synthesis, 206 Alkylated cyclopentanes, 260 Alkylation, 15-17
HANDBOOK Alkylbenzene compressor lubricants, 395 refrigeration lubricants, 415 Alkyl lead history, 62 phasedown, 63 Alkyl lead content, 68 ASTMD4814, 82 Alkylphenols, 209 Alkyl phosphates, synthesis, 216, 218 Alkyl phosphites, synthesis, 216-217 Alternative fuels, diesel engines, 138-139 Aluminum, production, carbon anodes, 781-782 Aluminum beaker oxidation test, 810, 813 Aluminum complex soap greases, 559 Aluminum soap greases, 558 Ammonia, in LPG, 50 Amonton's Law, 383 Aniline point, hydrocarbon base oil, 181 "Anti-dumping" provisions, 65 Antifoam additives, 233-234 gear lubricants, 441 hydraulic fluids, 367 metalworking and machining fluids, 518 refrigeration lubricants, 419-420 for turbines, 321-323 Anti Knock Index, 73-74 ASTMD4814, 78 Antimicrobial agents, metalworking and machining fluids, 517-518 Anti-misting agents, metalworking and machining fluids, 516 Antioxidants, 201-202, 793-796 arylamine, 795 gasoline, 69, 82-83 gear lubricants, 441,445 hindered phenols, 794 hydraulic fluids, 356-358 metalworking and machining fluids, 518-519 refrigeration lubricants, 419 structures, 314 synergistic combination, 315 turbine lubricating oils and hydraulic fluids, 312-318 zinc dialkyl dithiophosphate, 794 Antiseize additives, 218 Antiwear additives, 69, 213-216, 955-956 gear lubricants, 440,444 hydraulic fluids, 363 refrigeration lubricants, 419 surface reactions, 954-955 turbines, 319-321 Antiwear performance testing, hydraulic fluids, 361-364 Antiwear properties, compressor lubricants, 402-406 API CF oils, 473 API CG-4 oils, 473-474 API CH-4 oils, 474 API gravity
INDEX
diesel fuel, 117-118 fuel oils, 758-759 gear lubricants, 456 API service designation, gear lubricants, 448-453 Appledoom equation, 919 Approximate F-test, 1059-1060 Aquatic toxicity tests, acute, 901 Aqueous polymer quenchants, 605-606 Archard's equation, 930 Aromatic esters, 249-250 Aromatics, 28 determination, 195 diesel fuel, 131-132 saturation, 17-20 Arrhenius equation, 127-128 Arylamine, oxidation inhibition, 795 Aryl phosphates, synthesis, 216, 218 Aryl phosphites, synthesis, 216-217 Ash content diesel fuel, 135 fuel oils, 761-762 marine petroleum fuels, 150 petroleum coke, 775 petroleum pitch, 768 quench oils, 614 Ash modifiers, marine petroleum fiiels, 154 Asphaltene content, marine fuels, 151 Association of Natural Gasoline Manufacturers, 36 ASTMD 4814, 77-83 alkyl lead content, 82 anti-knock index, 78 copper corrosion test, 82 driveability index, 78-79 gasoline and gasohol blending, 80, 82 oxidation stability, 82-83 phosphorus content, 82 soluble gum, 83 sulfur content, 82 vapor pressure specifications, 78- 81 volatility, 78-80 class specifications, 82 water tolerance, 83 ASTM proficiency testing, elemental analysis, 714 Atmospheric gas oil, 3 Atomic absorption spectrometry, elemental analysis, 709 Austenite, 587 Autoignition compressor lubricants, 399 mineral oil heat transfer fluids, 581 temperature, gasoline, 83-84 Automatic tank gauging, 645 Automatic transmission fluids aluminum beaker oxidation test, 810, 813 hydraulic fluids, 374 performance testing, 239
1063
Automotive wheel bearing grease, 807 Automotive engine oils, 467-470 ASTM standard test methods, 469-470 corrosion tests, 828-829 issues in test method development, 469 performance standards, 467-468 performance tests, 468-469 Automotive gear lubricants, 442, 446, 448 oxidation stability, 803-804 performance testing, 239-240 Automotive lubricants, 465-494 additional fluids, 484 air conditioning lubricants, 484 axle lubricants, 483 brake fluids, 483 composition, 466-467 extent of oxidation, 476-477 functions, 465-466 greases, 478-481 ILSAC GF-2 and API SJ standard tests, 470-476 acidity and alkalinity, 474-475 ball rust test, 472-473 city service engine sludge and valve train wear, 470-471 corrosiveness, 475 diesel engine oil, 473-474 energy conserving characteristics, 471 entry of water, 475 foaming tendency, 471 high temperature copper-lead bearing water, 470 phosphorus content, 471 short-trip-service engine rusting, 471 standard tests, 472 viscosity, 475-476 volatility, 471 solid lubricants, 483-484 standards, 485-494 test methods, 466-467 Automotive transmission fluids, 477-478 oxidation stability, 804 Auto-Oil Air Quality Improvement Research Program, 65 Auto propane, 51-52 Auto-refrigeration, 46 Autoxidation, ester based fluids, 277, 279 Aviation fuels, 89-113 additives, 104-105 aircraft range, 100-101 combustion, 97-98 corrosion tests, 828 corrosivity, 101-102 freezing point, 880-881 gasoline, 89-92 high temperature, 95-97 history, 89 jetfuel, 89, 91-93 low temperature fuel related, 93-95
1064
MANUAL 3 7: FUELS AND LUBRICANTS
water related, 95 lubricity, 102-103 manufacturing, 103-104 metering, 99-100 oxidation stability, 813 standards, 823 quality control, 106-111 aviation gasoline, 107 contaminants, 106 contamination detection equipment, 109-110 contamination removal equipment, 107-109 jet fuel, 107 procedures, 110-111 standards, 112-113 static electricity, 103 storage stability, 102 transportation, 105-106 volatility and flammability, 98-99 Aviation gas turbine oils basestocks, 1012, 1014 corrosion inhibited MIL-PRF-23699 oils, 1011 gear testing, 988 high load-carrying DOD-L-85734 oils, 1011- 1012 high thermal stability MIL-PRF-23699 oils, 1011 load capacity database, 1011-1015 master chart, 1012, 1014 selected MIL-PRF-7808 grade 3 and grade 4 oils, 10121013 standard MIL-PRF-23699 oils, 1011 Aviation turbine fuels, thermal oxidation stability, 815-816 Avogadro's Law, 383 Axle lubricants, 483 B Baader aging test, 821-822 Backpressure control, 164-165 Bainite, 587 Ball bearings, greases, at elevated temperatures, 565-566 Ball on Cylinder Lubrication Evaluator, 102 Ball rust test, 472-473 Band area methods, infrared spectroscopy, 668 Barus equation, 917-918 Base number, gear lubricants, 458 Base oil, see Hydrocarbon base oil Bearing contacts, performance maps and, 972-973 Beer-Lambert law, 667 Beer's law, 661 Beeswax, 525-526 Bench test, 1020-1021 Block-on-Ring test, 1029-1030 four ball extreme pressure test, 1028-1029 four ball tests, 1026-1027 four ball wear test, 1027-1028 linear reciprocation, 1031 -1033 multi-specimen test machine, 1030-1031 pin and vee block tester, 1025-1026
HANDBOOK Pin-on-Disk test, 1026 selection, 1019 tapping torque test, 1030 Timken extreme pressure test machine, 1030-1031 Bench test modeling, see Ryder gear; Systematic tribology Bending-beam rheometer, 870, 872 Benzene, 28 Benzene Emission Number, 66 Bessemer Gas Engine Company, 37 Bias measurement process, 1044-1045 monitoring, 1047-1048 Bioaccumulation factor, 894-895 Biocides, marine petroleum fuels, 154 Bioconcentration factor, 894-895 Biodegradability diesters, 252 environmental characteristics, 890-894 hydrocarbon base oil, 182 polyolesters, 252 synthetic lubricants, 263 Biodegradable lubricants, oxidation standards, 823 testing, 820-822 Biodiesel, 138-139 Biological properties, hydrocarbon base oil, 181-182 Biological stability, polymer quenchants, 620-623 Biota-sediment accumulation factor, 894-895 Bituminous materials, viscosity, 870-872 Black deposits, LPG, 49-50 Black penetration, lubricating greases, 561 "Blaugas", 35-36 Blending marine fuel oils, 147-148 petroleum oil, 28 Blocking point, petroleum waxes, 552-553 Block-on-Ring test, 1029-1030 Boeing 747-400 fuel tankage, 93 Boiling point, 28 Boiling range, synthetic hydrocarbons, 195 Bomb combustion methods, elemental analysis, 709 Bomb oxidation test, lubricating greases, 565 Bonded fuel, aviation fuels, 106 Boost systems, diesel fuel system, 164 Borsoff effect, 936 Boundary lubrication, 214, 498, 949 Bouyancy, aerospace fuels, 732 Brake fluids, 483 standards, 493 Brayton cycle, 730-731 Broomwade 2050H Compressor Rig Test, 403 Brush drag of latex paints, 868 Btx recovery, 14-15 Bubble tubes, 868 Bulk material properties, flow properties, 921-922, 925-926 Bunsen Coefficient, 406 Bureau of Mines Correlation Index, 186
INDEX
Butadiene analysis, 193 production, 190-192 purity, 193 Butane, 46 Butane-butene mixtures, chlorides in, 194 Butylene analysis, 193 production, 190-192
Cadmium corrosivity, aviation fuels, 102 Calcium complex soap greases, 559 Calcium soap greases, 558-559 Calorific values, aerospace fuels, 739-740 Canadian Environmental Protection Act, 66 Candelilla wax, 525-526 Carbon acids, hydrogen bond, 270 Carbon anodes, for aluminum production, 781-782 Carbon blacks, 779-781 Carbon fibers, petroleum pitch, 781 Carbonizable substances, petroleum waxes, 549 Carbon residue compressor lubricants, 399 dieselfuel, 135-136 fuel oils, 761 marine petroleum fuels, 151-152 mineral oil heat transfer fluids, 580 quench oils, 611-613 standards, 783-784 Carbonyls, in C4 hydrocarbons, 194 Carbonyl sulfide, in propylene, 193 Carboxylic acids environmentally friendly oils, 272-273 isomers, 270 Camauba, 525-526 Carreau viscosity equation, 948-949 Casinghead gasoline, 36-37 Catalytic reforming, 12-15 yield/octane relationship, 14 Caterpillar micro-oxidation test, 810-811, 813-814 Cat fines, marine petroleum fuels, 150 "Cat" gasoline, 11 Cavitation corrosion, 932 Cavitation erosion, 932-933 Cavitation erosion corrosion, 933 Cavitation wear, 931-941 corrosion, 931-932 delamination, 937-938 erosion, 931-932 fatigue, 933-936 oxidational, 933 scuffing, 936-937 tribochemical, 933 CCT diagrams, 589 Cementite, 587 Centrifugal compressor, 389-390
1065
Centrifuges, diesel fuel system, 161, 163 Centrifuge tests, 365 lubricating greases, 564-565 Cetane index, 119-120 Cetane number, 119 diesel fuel oils, 721-723 Channel point FTM, gear lubricants, 459 Chemical analysis, lubricating greases, 569 Chemical corrosion, 220 Chi-square test, 1059 Chlorine content butane-butene mixtures, 194 gear lubricants, 457 turbine lubricating oils and hydraulic fluids, 334 City service engine sludge and valve train wear, 470-471 Clay treatment vessels, 108-109 Clean Air Act of 1963,63 Clean Air Act of 1970, 64 Clean Air Act Amendments of 1977, 64 Clean Air Act Amendments of 1990, 65-66, 135 Cleanliness dieselfuel, 124-125 turbine lubricating oils and hydraulic fluids, 341-343 Cleveland Open Cup, 692 Cloud point, 881-882 dieselfuel, 121-122 petroleum waxes, 545-546 polymer quenchants, 619 '^C NMR spectrometry, hydrocarbon analysis, 650-651 CNS effects, LPG, 54-55 Coalescer Blocking Tendency Test, compressor lubricants, 404-406 Coatings, flow-related instruments, 867 CoBr test, 47-48 Coefficient of kinetic friction, petroleum waxes, 552, 554 Cogeneration, 28 Coking, delayed, 21-23 Coking value, petroleum pitch, 767-768 Cold crank simulator, 850 gear lubricants, 458 Cold Filter Plugging Point, diesel fuel, 123-124 Cold flow properties, 879-883 aviation ftiels freezing point, 880-881 cloud point, 881-882 high-shear tests of lubricants, 882 pour point, 879-880 pumpability of lubricants, 882 standards, 883 wax appearance point, 881-882 Color, petroleum waxes, 549-551 Combustion, diesel fiiel, 118-119 Combustion products, dissociated, 753-754 Combustion temperature, aerospace fuels, 735, 740-741 Compatibility gear lubricants, 460 marine pefroleum fiiels, 152 turbine lubricating oils and hydraulic fluids, 343-346
1066
MANUAL 37: FUELS AND LUBRICANTS
see also Seal compatibility Compressibility factor, 384 Compression, power requirement, 385-386 Compression ignition engines, 115-116 Compression ratio, octane number requirement, 76 Compressor lubricants, 383-411 acid number, 400 air release, 398 antiwear properties, 402-406 autoignition, 399 basestocks, 392-395 carbon residue, 399 classification, 391 copper corrosion, 398 corrosion, 398-399 demulsibility, 397-398 elastomeric seals compatibility, 409 evaporation, 399 Filter Bowl Compatibility Test, 410 fire point, 398-399 flash point, 398-399 fluid oxidation, 400-401 foaming tendency, 398 gas solubility, 395-396, 406-409 heat transfer efficiency, 408-409 hydrolytic stability, 398 infra-red spectroscopy, 400-401 iron corrosion, 398 kinematic viscosity, 396-397 Liquid Heptane Washing Test, 401-402 oxidation stability, 802 Paint Compatibility Test, 409-410 performance demands, 389-392 pour point, 397 precipitation number, 399-400 solubility in the gas, 396, 398 specific gravity, 397 specific heat, 408-409 standards, 410-411 thermal conductivity, 408-409 viscosity of gas/liquid mixtures under pressure, 408 water content, 397 Compressor tests, 403 Concentric cylinder viscometer, 835-836 Conductance, polymer quenchants, 618-619 Conductivity, gasoline, 85 Cone-and-plate viscometer, 835 Cone penetration, lubricating greases, 561 Congealing point, petroleum waxes, 545 Conradson carbon residue, compressor lubricants, 399 Consistency, lubricating greases, 560-561 Consistency stability, lubricating greases, 561-562 Construction materials, gas turbine fiael system, 166-167 Contact stress, 966 simulation, Ryder gear, 997-1000 Contamination aviation fuels, 106
HANDBOOK detection equipment, aviation fuels, 109-110 gasoline, 69 lubricating greases, 568-569 quench oils, 610-614 removal equipment, aviation fuels, 107-109 Continuous cooling transformation diagrams, 589 Control chart, 1055-1056 Conventional Oxidation Catalyst, 63-64 Coolants, secondary, 574 Coolant separability, hydraulic fluids, 370-371 Cooling curve analysis, polymer quenchants, 621-623 Copper corrosion, 826 aviation fuels, 101-102 compressor lubricants, 398 lubricating greases, 567-568 Copper corrosion inhibitors, 69 Copper corrosion test, ASTM D 4814, 82 Copper deactivation, 221 Copper passivators, 221-222 Copper strip corrosion, diesel fuel, 134-135 Copper strip tarnish test, 827-828 gear lubricants, 454, 456 Corrosion, 825-832 compressor lubricants, 398-399 in LPG, 49 specifications, 827 Corrosion behavior, turbine lubricating oils and hydraulic fluids, 340-341 Corrosion inhibitors, 218, 220-223 gear lubricants, 441 hydraulic fluids, 369-370 metalworking and machining fluids, 516-517 polymer quenchants, 619-620 surface activity, 826-827 for turbines, 318-320 Corrosion protection, hydraulic fluids, 368-370 Corrosion tests, 827-832 automotive engine oils, 828-829 aviation gasolines, 828 diesel fiiel oils, 828 gear oils, 829 greases, 831-832 lubricants, 827-828, 829-831 Corrosion wear, 931-932 Corrosivity aviation fuels, 101-102 diesel engine oil, 475 Coupling agents, metalworking and machining fluids, 513 Cracking, crude oil, 146-148 Critical pressure, 384 Critical temperature, 384 Crude oil gases, 3 disposition, 5 Crude oils, 6-7, 145-148 classification, 526-527, 575-576 cracking, 146-148 distillation, 146-147
INDEX
distillation, 696-698 preparation and separation, 9-10 refinery blending and storage, 147-148 refining, 528-529 vapor pressure, 702 waxy, 527-528 Crystalline structure, petroleum coke, 775-776 Crystallization, esters, 289-291 Cyclic durability test, gear lubricants, 458 Cyclohexane derivatives, 260 D Dalton's Law, 383 Defoamers, see Antifoam additives Deformation, friction due to, 911 Degree of overbasing, 208 De-icers, 68 Delamination wear, 937-938 Delayed coking, 21-23 Demulsibility compressor lubricants, 397-398 gear lubricants, 457-458 hydraulic fluids, 366 Demulsifiers, 232-233 gear lubricants, 441 marine petroleum fuels, 154 Density aerospace fuels, 735-736 crude oil, 6 dieselfuel, 117-118 marine petroleum fuels, 149 petroleum coke, 775 petroleum pitch, 768 refrigeration lubricants, 421 turbine lubricating oils and hydraulic fluids, 332 Deposit control additives, 69-70 Desorption, environmental characteristics, 888 Detergents, 208-211 gear lubricants, 440, 444 hydraulic fluids, 358 micellar structure, 210-211 synthesis, 210 Dewaxing, hydrocarbon base oil, 171-172 Dewaxing process variables, 530-531 Dew Point test, 47-48 Dialkyl dithiophosphoric acid derivatives, synthesis, 215 Dibasic acid esters, refrigeration lubricants, 416 Diesel engines alternative fuels, 138-139 history, 115-116 Diesel fuel oils, 9 API gravity, 117-118 aromatics, 131-132 ash, 135 carbon residue, 135-136 cetane index, 119-120 cetane number, 119
cleanliness, 124-125 Cloud Point, 121-122 Cold Filter Plugging Point, 123-124 composition/performance correlations, 178-179 copper strip corrosion, 134-135 corrosion tests, 829 corrosiveness, 475 density, 117-118 distillation, 120-121 dyed, 135-137 Flashpoint, 128-129 formation of insolubles, 125 grades, 116-117 heat content, 132-134 heat of combustion, 132-134 ignition and combustion characteristics, 118-119 low-sulfur, 135-137 Low-Temperature Flow Test, 123 lubricity, 129-131 oxidation stability, 126-127 performance categories, 473-474 Pour Point, 122-123 premium, 137-138 requirements, 118 specifications, 116-117 stability, 125-126 storage stability, 127-128 thermal stability, 126 total sulfur, 134 viscosity, 121 volatility, 120-121 World Wide Fuel Charter, 137 Diesel fuel combustion, 717-727 combustion process, 718-719 emissions, 719-721 fuel injection, 717-718 ignition, 718 Diesel fuel oils cetane number, 721-723 corrosion tests, 828 CVCA/IQT method, 724-727 ignition quality, 721-727 proposed alternatives to ASTM D 613, 723-724 specifications, 718 standards, 727 Diesel fuel system, 161, 163-165 back pressure control, 164-165 boost systems and deaeration, 164 centrifuges, 161, 163 emulsifiers, 165 filters and strainers, 163-164 fuel final filter, 164 fuel service heaters, 164 fuel supply flowmeter, 164 homogenizer, 165 viscosity control, 164 waste heat economizers, 165
1067
1068
MANUAL 3 7: FUELS AND L UBRICANTS
Diesel index, 118 Diesel-injector nozzle, shear resistance, turbine lubricating oils and hydraulic fluids, 340 Diesel plant fuel system, 161-162 Diesters, 249-252 application and performance characteristics, 252 biodegradability, 252 chemical characteristics, 251 chemistry and manufacturing, 249-251 manufacturing technology, 250-251 physical properties, 251 Difference method, infrared spectroscopy, 668 Differential scanning calorimetry petroleum waxes, 539-542 transition temperatures, 548 Dip cups, 867-868 Dispersants, 203-208 bridged structure, 206-207 gear lubricants, 440-441,445 hydraulic fluids, 358 in marine petroleum fuels, 153 metalworking and machining fluids, 516 performance, 204-205 structural features, 205-206 uses, 206, 208 Dispersant viscosity modifiers, 206, 228-229, 231 Dispersive-type infrared spectrometers, 667 Disposition tendencies, liquids in thin films and vapors, 819-820 Dissociation constant, environmental characteristics, 888 Distillate fuels high temperature stability, 817-818 oxidation stability, 816-817 storage stability, 817-818 Distillation, 675-682 atmospheric pressure, 676-680 crude oils, 146-147, 696-698 dieselfiiel, 120-121 fiiel oils, 761 hydrauhc fluids, 365 hydrocarbon base oil, 171 international standards, 689 mineral oil heat transfer fluids, 582 reduced pressure, 680-690 Distillation fractions, 3-4 2,6-Ditertiarybutyl-4-methylphenol, 314-315 DOD-L-85734 oils, 1011-1012 Driveability index, 78-79 Dropping point, lubricating greases, 563-564 Dyes, 234, 519 Dynamic seals test, gear lubricants, 460 Dynamic shear rheometer, 870 E Ecological tests, environmentally friendly oils, 289, 291292 Elastohydrodynamic contact, functional regions, 965
HANDBOOK Elastohydrodynamic film-forming capability, engine oil, 983-987 optical thickness measurement, 984-985 pressure-viscosity coefficients, 985-987 Elastohydrodynamic lubrication, 413, 968 film thickness, 1007 measurements by interferometry, 947-948 mixed film lubrication, 949 non-Newtonian, 948-949 Reynolds equation, 948 Elastomeric seals, see Seal compatibility Elastomers, turbine lubricating oils and hydraulic fluids compatibility, 344 Electric-arc furnaces, graphic electrodes, 782 Electrochemical corrosion, 218, 220 Electrostatic stabilization, 204-205 Elemental analysis, 707-716 ASTM proficiency testing, 714 atomic absorption specttometry, 709 bomb combustion methods, 709 classical wet chemistry methods, 709 D02 test methods, 709-710 future developments, 715-716 graphite furnace atomic absorption spectrometry, 709-711 inductively coupled plasma-atomic emission spectrometry, 711-712 inductively coupled plasma-mass spectrometry, 712 international test methods harmonization, 714-715 ion chromatography, 712 microelemental analysis methods, 712 neutron activation analysis, 712-713 sample preparation, 707 standard reference materials, 714 wear metals in used oils, 707-709 X-ray fluorescence spectrometry, 713 Elemental content, gear lubricants, 458 Emissions aerospace fuels, 743 diesel fuel combustion, 719-721 Emulsifiers, 232-233 diesel fuel system, 165 gear lubricants, 441-442 Emulsion promoters, metalworking and machining fluids, 512-513 Energy conserving characteristics, 471 Energy content, gasoline, 72 Energy equation, 756 Engine deposits, effect on octane number requirement, 77 Engine management systems, 71 Engine oils adhesive wear testing, 981-982 elastohydrodynamic film-forming capability, 983-987 hydraulic fluids, 374 oxidation standards, 823 testing, 807-811 performance tests, 237-239
INDEX
standards, 485-488 thermo oxidation engine oil simulation test, 808-812 traction and scuffing failure characteristics, 1008-1009 traction characteristics, 1003, 1005 WAM high speed load capacity test method, 1010 Engine spark timing, octane number requirement, 76-77 Engine tests, engine oils, 823 Entraining velocity, 966-967 Environmental acceptability, synthetic lubricants, 262-263 Environmental characteristics, 885-906 abiotic degradation, 889-890 accumulation processes, 894-895 acute toxicity tests, 894, 901-902 adsorption, 888 biodegradation, 890-894 chemical structure and, 886-887 chronic and subchronic tests, 894, 903-904 complex mixture toxicity, 900-901 desorption, 888 dissociation constant, 888 exposures, 895-896, 900-901 hazard and risk, 904 Henry's Law Constant, 888 low solubility materials, 900 noo-aquatic toxicity tests, 903-904 octanol/water partition coefficient, 887 rationale for standardized testing, 885-886 standardized tests, 895-899 terminology, 904-906 transformation processes, 889 transport processes, 888-889 vapor pressure, 888 volatility, 896, 900 water solubility, 887 Environmental considerations, marine petroleum fuels, 156 Environmental impact, 234 Environmentally acceptable hydraulic fluids, 377-378 Environmentally friendly oils, 267-294 alcohol bonds, 271-272 carboxylic acids, 272-273 chemical properties, 273-276 crystallizing, 289-291 ecologically toxic properties, 289, 291-292 esterification, 270-271 functional groups and elementary compound, 269-270 functions and requirements, 267-268 future developments, 291-293 hydrolysis, 274 low temperature properties, 289-291 native esters, 275, 277-278 oxidation stability, 274-275 standards, 293-294 synthetic esters, 276, 278-279 viscosity, 273, 289-290 see also Pressure media Equation-of-state approach, 423, 535-539 Equipment line sizing, refrigeration lubricants, 426
1069
Erosion corrosion, 933 Erosion wear, 931-932 Ester fluids, viscosity vs. temperature, 289-290 Esterification, 249-250 catalysis, 251 mechanism, 270-271 Ester polymers, 225-228 Esters compressor lubricants, 394 native, environmentally friendly oils, 275, 277-278 native and synthetic chemical bases, 269-273 chemical properties, 273-276 pour points, 273 synthetic environmentally friendly oils, 276, 278-279 turbines, 309-311 Ethanol, in propane, 47 Ethylene characterization, 192 producdon, 186-190 commercial production, 188-190 feedstock, 186-188 thermal cracking reaction, 188 Ethyl mercaptan, as odourant, 52, 55 Evaporation, compressor lubricants, 399 Evaporation loss, lubricating greases, 564 EWMA overlay, 1056 Exhaust emissions, marine petroleum fuels impact, 154 Exponentially Weighted Moving Average values, 1056 Exposures, environmental characteristics, 895-896, 900-901 Exfreme pressure additives, 213-214, 216-219, 955-957 gear lubricants, 440,444 hydraulic fluids, 363 metalworking and machining fluids, 513-515 standards, 1036 surface reactions, 954-955 turbines, 319-321, 457, 1030-1031 Extreme pressure Timken method, lubricating greases, 566
Failure, parameters controlling, 965-966 Falling needle, 868-869 Falling viscometer, 836 Fatigue wear, 933-936 Fat solubility, 820-821 Ferrite, 587 Ferrous corrosion inhibitors, 69 Film boiling, shock, steel, 599-602 Film-forming agents, 211-223 metalworking and machining fluids, 513-515 Filterability, hydraulic fluids, 367-368 Filter Bowl Compatibility Test, compressor lubricants, 410 Filters, diesel fuel system, 163-164 Filter-separators, 107-108 Fire point Cleveland Open Cup, 692
1070
MANUAL 3 7: FUELS AND L UBRICANTS
compressor lubricants, 398-399 gear lubricants, 454 quench oils, 609 Tag Open cup, 693 Fire-resistance tests, turbine lubricating oils and hydraulic fluids, 334-337 Fire resistant fluids, hydraulic fluids, 375-377 Fischer-Tropsch diesel fuels, 138 Fischer-Tropsch process, 525-526 Fish, toxicity tests, 902-903 Fixed bed hydrocracking, 24 Flame ionization detector, 653 Flammability, 682, 690-696 aerospace fuels, 738 aviation fuels, 98-101 gasoline, 84 LPG, 52 see also Fire point; Flash point Flash point bias among test methods, 692 Cleveland Open Cup, 692 compressor lubricants, 398-399 continuously closed cup, 694 diesel fuel, 128-129 equilibrium method with closed cup, 693-694 fuel oils, 761 gear lubricants, 454 international test methods, 694-695 marine petroleum fuels, 150 Pensky-Martens Closed Cup Tester, 692-694 quench oils, 609 Setaflash Closed Cup, 693 small scale closed cup, 693 Tag Open cup, 693 test methods, 690-691 verification fluids, 691 Flash temperatures, 941-942 Fluorescent indicator adsorption, hydrocarbon analysis, 659 Fluorinated basestocks, compressor lubricants, 395 Flow properties, 833-875 ASTM procedures, 853-854 bulk material properties, 921-922, 925-926 flash temperatures, 941-942 high-temperature high-shear, 843- 845 kinematic viscosity, 837-841 history, 841-843 temperature relationship, 854-856 low temperature tests, 851 lubricating greases, at high temperatures, 562 near surface structure, 919, 923 nomenclature, 833-835 non-petroleum materials, 867-872 pressure-viscosity coefficient, 916-920 standards, 072-875 sub-surface structure, 919-920, 924 surface conformity, 914-916 surface films, 919, 921-923
HANDBOOK viscometer geometries, 835-836 wear maps, 942-945 wear surfaces, 911-913 see also Cold flow properties; Viscosity Fluid biodeterioration, polymer quenchants, 620-621 Fluid catalytic cracking, 9-12 yields, 11 Fluid coking, 770 Fluidized bed hydrocracking, 24-25 Fluid oxidation, compressor lubricants, 400-401 Foaming automotive lubricants, 471 compressor lubricants, 398 gear lubricants, 456-457 hydraulic fluids, 366-367 turbine lubricating oils and hydraulic fluids, 332-334 Foam inhibitors, see Antifoam additives Foam testing, polymer quenchants, 620 Four ball extreme pressure test, 457, 1028-1029 compressor lubricants, 402 lubricating greases, 566-567 Four ball tests, 1026-1027 Four ball wear test, 457, 1027-1028 Fourier transform infrared spectroscopy, 667 Free water, 639-640, 646 in aviation fuels, 109-110 Fretting wear, 567, 934-936 Friction, 909-911 area of true contact, 909-911 due to adhesion, 911 due to deformation, 911 due to ploughing, 911-912 standards, 1035-1037 types of, 497 Friction coefficient, 945 effect of hydrocarbon chain length, 954 Friction modifiers, 211-213, 949, 951-952 gear lubricants, 441 F-test, 1059-1060 Fuel additives, marine petroleum fuels, 153-154 Fuel-injection system, 115-116 Fuel injector shear stability test, 852 standards, 875 Fuel oils, 8, 757-762 API gravity, 758-759 ash, 761-762 carbon residue, 761 compatibility of blended, 762 distillation, 761 flashpoint, 761 heating value, 759-760 instability, 762 pour point, 761 specifications and applications, 757-759 sulfiir content, 762 viscosity, 760-761 Fuels, oxidation testing, 811, 813, 815-818
INDEX
Fuel service heaters, diesel fuel system, 164 Fuel treatment, gas turbine fuel system, 165-166 Fuses, aviation fuels, 108 FZG Gear Test Machine, 320, 322, 1033-1035 compressor lubricants, 402-403 FZG Pits C 180 TS, gear lubricants, 459 FZG scuffing test, gear lubricants, 458 FZG test, 362
Galling wear, 923, 929-930 Galvanic corrosion, 932 Gas chromatography hydrocarbon analysis, 652-657 liquified petroleum gas, 40 petroleum waxes composition, 549 principles, 652-653 Gas chromatography-mass spectrometry, hydrocarbon analysis, 664 Gas compression cycle, 384-386 Gas compressors, 386-389 Gas hydrates, 44-46 Gas laws, 383-384 Gas line anti-freeze, 68 Gasohol, blending with gasoline, 80, 82 Gasoline, 7-8, 61-86 additives, 68-70 adiabatic flame temperature, 84 autoignition temperature, 83-84 aviation, 89, 91-92 additives, 105 combustion, 97 history, 89-90 knock resistance, 97 low temperature, 94-95 manufacturing, 103 quality control, 107 volatility and flammability, 98 combustion, 70-77 air-fuel ratio and stoichiometry, 71-72 engine management systems, 71 gasoline energy content, 72-73 spark ignition engines, 70-71 composition, 66-68 conductivity, 85 flammability, 84 heat capacity, thermal conductivity, and heat of vaporization, 84 heat of combustion, 84 history, 61-66 2004+ vehicle and fuel regulation, 66 alkyl lead, 62 Clean Air Act Of 1963, 63 Clean Air Act of 1970 and 1977 amendment, 64 Clean Air Act Amendments of 1990, 65-66 leaded gasoline, 61-62 nonleaded gasoline, 63-64
octane ratings, 62-63 vapor controls, 64-65 nonleaded, 63-64 octane performance properties, 73-77 anti-knock ratings, 73-74 combustion and knock, 73 motor octane number, 73-74 octane distribution through fuel boiling range, 75 octane number requirement, 75-78 on-line analyzer octane rating, 74-75 research octane rating, 74 oxidation stability, 813, 815-817 standards, 823 testing, 811,813, 815-818 oxygenates in, 68 performance additives, 69-70 reformulated, 65-66 specifications, see ASTM D 4814 standards, 85-86 static electricity, 85 trace constituents and contaminants, 69 vapor pressure, 699, 701 viscosity and lubricity, 84-85 Gasoline-oxygenate blends, vapor pressure, 699, 701 Gas solubility compressor lubricants, 395-396, 406-408 petroleum oils, 406-407 Gas turbine engines, 115, 298-299 development, 302-303 lubrication system, 301-302 Gas turbine fuels, 139-142 requirements, 140-141 standards, 142-143 trace metal limits, 141-142 see also Ryder gear Gas turbine fuel system, 165-166 construction materials, 166-167 Gearbox oils, see Ryder gear Gear contacts, performance maps and, 972-973 Gear corrosion test, gear lubricants, 460 Gear lubricants, 431-462, 481-483 acid number, 456 additives, 439-443, 447 air release, 460 antiwear agents, 440, 444 API gravity, 456 API service designation, 448-453 base number, 458 channel point FTM, 459 chlorine level, 457 cold crank simulator, 458 compatibility, 460 compounded, 439 Copper Strip Tarnish Test, 454, 456 corrosion inhibitors, 441 cyclic durability test, 458
1071
1072
MANUAL 3 7: FUELS AND L UBRICANTS
demulsibility, 457-458 demulsifiers, 441 detergents, 440, 444 dispersants, 440-441, 445 dynamic seals test, 460 early, 431-432 elemental content, 458 emulsifiers, 441-442 extreme pressure, 439 oxidation, 458 extreme pressure agents, 440, 444 fire point, 454 flash point, 454 foam characteristics, 456-457 foam inhibitors, 441 four ball EP, 457 four ball wear, 457 friction modifiers, 441 function, 431 future trends, 460-461 FZG Pits C 180 TS, 459 FZG scuffing test, 458 gear corrosion test, 460 gear scoring test, 459-460 GFC oxidation, 459 greases, 439 high temperature foam inhibition, 459 insolubles in used oils, 457 kinematic viscosity, 456 low speed high torque hypoid test, 459 low temperature Brookfield viscosity, 458 MIL-PRF-2105E, 451-452 nitrogen level, 458 OEM specifications, 451, 453 open gear compounds, 439 oxidation inhibitors, 441, 445 phosphorus level, 457 polymeric thickeners, 441-442 pour point, 454 pour point depressants, 441 precipitation number, 453 rust and oxidation inhibited, 439 rust inhibition, 456 Ryder gear test conditions, 988-990 SAE 3306 viscosity classification, 449-451 scuffing front, propagation, 992-993 initiation, 990-992 seal compatibility, 459 shear stability, 459 standards, 461-462, 493 standard tests, 482-483 storage solubility, 460 sulfur level, 457 synchronizer SSP 180 test, 459 systematic tribology, 988-996 thermal and oxidation test, 458
HANDBOOK Timken EP Tester, 457 fribological features, microscopic examination, 991-992 fribological processes, 993-996 types, 437,439 viscosity index, 457 water content, 457 Gear oils corrosion tests, 829 oxidation, standards, 822 oxidation testing, 802-804 performance testing, 239-240 Gears failure modes, 434-436 lubrication, 434, 436-438 types, 432-434 Gear scoring test, gear lubricants, 459-460 Gear testing, aviation gas turbine oil, 988 GFC oxidation, gear lubricants, 459 Glass capillary vacuum viscometers, 870-871 Gloss retention, petroleum waxes, 549 GM quenchometer cooling times, 604-606 Graphic electrodes, for elecfric-arc furnaces, 782 Graphite furnace atomic absorption spectrometry, elemental analysis, 709-711 Greases, see Lubricating greases Grossmann hardenability, 592-593 Gum content, gasolines and fuels, 811, 813, 815 H Halocarbons, in LPG, 50 Hardenability, steel, 589-591 measurement, 591-593 Hardness, petroleum waxes, 547-548 Hazard, environmental characteristics, 904 Health effects, LPG exposure, 54-55 Health issues, aerospace fuels handling, 747 Heat capacity, gasoline, 84 Heat content, diesel fuel, 132-134 Heating value fiiel oils, 759-760 marine petroleum fuels, 149-150 Heat of adsorption, wear reduction and, 954 Heat of combustion, 72 diesel fuel, 132-134 gasoline, 84 Heat of vaporization, gasoline, 84 Heat resistance, lubricating greases, 563-565 Heat transfer efficiency, compressor lubricants, 408-409 hydraulic fluids, 353-354 Heat fransfer coefficient, 573-575 Heat transfer fluids, see Mineral oil heat transfer fluids Heat transfer fluid system, design and construction, 583-584 HEES fluids, 377 Henry's Law Constant,; HEPG fluids, 378 HEPR type fluids, 378
INDEX
Hertzian contact, 914, 946-949, 965-966 Heteroatom, removal, 17-20 HETG fluids, 377 HFA fluids, 375-376 HFB fluids, 376 HFC fluids, 376 HFD fluids, 376-377 High frequency reciprocating rig, 129, 131 High Pressure Difl^erential Scanning Calorimetry, 258 High-shear high-temperature, 843-845 low-temperature, lubricants, 846-848 High temperature copper-lead bearing wear, 470 High temperature foam inhibition, gear lubricants, 459 High temperature stability, distillate fuels, 817-818 Hindered phenols, oxidation inhibition, 794 ' H N M R spectroscopy, hydrocarbon analysis, 651 Homogenizer, diesel fuel system, 165 Hot surface tests, turbine lubricating oils and hydraulic fluids, 335 Hot tack, petroleum waxes, 552 HSR, 3 HVAC cycle, refrigeration lubricants, 424-426 Hydraulic fluids, 353-379 aeration and foam, 366-367 antifoam additives, 367 antioxidants, 356-358 antiwear and extreme pressure additives, 363 antiwear performance testing, 361-364 automatic transmission fluids, 374 boundary conditions, 360-361 coolant separability, 370-371 corrosion inhibitors, 369-370 corrosion protection, 368-370 demulsibility, 366 detergents, 358 dispersants, 358 engine oils, 374 environmentally acceptable, 377-378 fiherability, 367-368 fire-resistant, 301-302, 375-377 frictional properties, 212-213 heat transfer, 353-354 hydrolytic stability testing, 365-366 low temperature pumpability, 372-373 lubrication, 353 machine builder specifications, 364 metal passivators, 369-370 mineral oils, 374-375 oxidation stability, 355-359, 801 standards, 822 tests, 356-357 performance testing, 240-241 petroleum base stocks, 355 pour point depressants, 373 power transfer, 353-354
1073
rust inhibitors, 369-370 seal compatibility, 370 shear stability, 371-372 standards, 378-379 thermal stability, 355-359 tractor fluids, 374 trends, 353-355 types, 268-269 viscosity, 359-360 viscosity index improvers, 371-372 water content, 363, 365 wear protection, 359-364 see also Pressure media Hydraulic oils oxidation characteristics, 797-798 thermal stability, 799-800, 830 Hydrocarbon analysis, 649-672 "C NMR spectrometry, 650-651 fluorescent indicator adsorption, 659 gas chromatography, 652-657 gas chromatography-mass spectrometry, 664 infrared spectroscopy, 668-670 liquid chromatography, 657-660 mass spectrometry, 664-665 NMR spectroscopy, 649-652 standards, 671-672 super critical fluid chromatography, 659-660 ultraviolet spectroscopy, 661-662 Hydrocarbon base oil, 169-183 biodegradability, 182 biological properties, 181-182 composition/performance correlations, 178-179 effects in industrial lubricants, 179-180 hydrocarbon type analysis, 172-173 impact of sulfur compounds on chemistry, 176-177 market, 169-170 molecular structures, 174-175 naphthenic vs. paraffmic, 172 nitrogen compound reactivity, 177-178 NMR spectroscopy, 174-175 olefins, 178 oxidation, 175-176 refining and processing technology, 170-172 refining capacity. United States, 169-170 rheology and composition, 178 rubber compatibility, 181 solvency effects, 181 spectrometric identification components, 173-174 standards, 182-183 structures with high susceptibility to oxidation, 788 viscosity-pressure coefficient composition effect, 180-181 Hydrocarbon oils, turbines, 305-306 Hydrocarbon-oxygenate mixtures, vapor pressure, 702-703 Hydrocarbons crude oil composition, 6 in gasoline, 66-67 physical adsorption, 921
1074
MANUAL 37: FUELS AND LUBRLCANTS
HANDBOOK
terminology, 5-6 see also Synthetic hydrocarbons Hydrocracked basestocks, turbines, 307-308 Hydrocracking fixed bed, 24 fluidized bed, 24-25 hydrocarbon base oil, 171 Hydrodynamic film region, 963 Hydrodynamic lubrication, 214, 413,498, 949 Hydrofinishing hydrocarbon base oil, 172 Hydrogen in petroleum fractions, 195 classification in organic molecules, 788 Hydrogen production, 25-26 Hydrogen refining, hydrocarbon base oil, 171 Hydrolysis environmental characteristics, 889-890 environmentally friendly oils, 274 pressure media, 279-281 Hydrolytic stability compressor lubricants, 398 diesters, 251-252 polyolesters, 251-252 pressure media laboratory aging, 285-287 test stand aging, 287-289 turbine lubricating oils and hydraulic fluids, 339-340 Hydrolytic stability testing, hydraulic fluids, 365-366 Hydroperoxide decomposers, 791-792 decomposition, 790, 793 oxidation rate and, 790 Hydroprocessing options, 19 terminology, 18 Hydrostatic tank gauges, 645 Hydrotteated basestocks, turbines, 307-308 Hyfinishing, hydrocarbon base oil, 172 I /-Chart, 1055-1056 Icing, in LPG systems, 45 Ideal Gas Law, 383 Ignition quality, marine petroleum ftiels, 149-150 Indiana Stirring Oxidation Test, compressor lubricants, 401 Induction Period Method, 813, 815-817 Inductively coupled plasma-atomic emission spectrometry, elemental analysis, 711-712 Inductively coupled plasma-mass spectrometry, elemental analysis, 712 Industrial gear lubricants, 453-456 perfr)rmance testing, 240 Indusfrial hydraulic fluids, performance testing, 241 Industrial lubricants, base oil effects, 179-180 Infrared specfroscopy, 665-671 achieving reproducible baselines, 668 analysis of multicomponent solutions, 667-668
band area methods, 668 Beer-Lambert law, 667 compressor lubricants, 400-401 difference method, 668 dispersive-type infrared specttometers, 667 Fourier fransform, 667 gas analysis, 668 hydrocarbon analysis, 668-670 inorganic characterization, 670-671 quench oils, 610 standards, 671 wavelength, wavenumber and frequency, 666-667 Inhibitor content, determination, 194 Inorganic/organic solids, metalworking and machining fluids, 519 Insolubles, in used oils, 457 Instability, fiiel oils, 762 Insulation fires, mineral oil heat fransfer fluids, 583 International Harvester oxidation corrosion, 811 Invertebrates acute toxicity tests, 902 chronic and subchronic toxicity tests, 903 Ion chromatography, elemental analysis, 712 Iron based organo-metallics, in gasoline, 69 Iron carboxylate, oxidation rate and, 790 Iron chip corrosion, metalworking fluids, 831 Iron corrosion, compressor lubricants, 398 Isomerization, 14, 16 Iso-propanol, in propane, 47 Isothermal operation, 384 Isothermal transformation diagrams, 589
Jet fiiel, 8, 92-93 additives, 104-105 combustion, 97-98 history, 89, 91 low temperature, 93-94 manufacturing, 103-104 product color, 109 quality control, 107 Smoke Point, 98 visual appearance, 109 volatility and flammability, 98-101 Jet Fuel Thermal Oxidation Tester, 96, 815-816 Jominy bar end-quench test, 591-592 Joule-Thomson cooling, 46 K Karl Fisher Titration Test, 365 Kerosine, 3 Kinematic viscosity, 837-841 compressor lubricants, 396-397 gear lubricants, 456 history, 841-843 quench oils, 608-609 standards, 875
INDEX
temperature relationship, 854-856 Kinetic coefficient of Iriction, 909
Laboratory aging hydrolysis stability, 285-287 oxidation stability, 281-283 Lambda values, 912-913 Lattice transition, alkanes, 534 Lead corrosion, 221 Leakage, lubricating greases, from wheel bearings, 565 Leidenfrost temperature, 593 "Liedenfrost effect", 53 Light hydrocarbons high temperature vapor pressure, 32 low-temperature vapor pressures, 33 Linear reciprocation, 1031-1033 Liquid chromatography fluorescent indicator adsorption, 659 hydrocarbon analysis, 657-660 principles, 657-658 standards, 660 super critical fluid chromatography, 659-660 Liquid Heptane Washing Test, compressor lubricants, 401402 Liquified petroleum gas, 31-56 auto propane, 51-52 exposure, 54-55 flammability, 52 gas hydrates, 44-46 handling, 52-54 history, 31-32, 35-38 industry, 32, 35 use, 35-37 odorization, 55 properties and thermodynamics, history, 37-38 sample cylinder approvals, 54 specifications, 38-50 composition, 40 contaminants, 49-50 density, 40 dryness of propane, 42-49 history of ASTM standards, 38-39 naturally occurring radioactive materials, 50 octane, 40 olefins, 40-41 residual matter, 49 sampling, 39-40 sulfiir content, 49 vapor pressure, 40-42 volatility residue, 42 uses, 56 vapor, 53 vapor pressure, 703-704 volume correction factors, 50-51 water in, 47-49 LISCIC/NANMAC quench probe, 623-625
1075
Lithium soap greases, 559 Low speed high torque hypoid test, gear lubricants, 459 Low temperature Brookfield viscosity, gear lubricants, 458 Low-temperature flow test, diesel fuel, 123 Low-temperature high-shear tests, lubricants, 882 Low temperature properties diesters and polyolesters, 251 environmentally friendly oils, 289-291 Low temperature pumpability hydraulic fluids, 372-373 lubricants, 882 Low temperature storage stability, turbine lubricating oils and hydraulic fluids, 329 Low-temperature torque, lubricating greases, 563 LPG injector systems, 52 LSR, 3 Lube oil base stock production, 25 Lubricant base oil, market, 169-170 Lubricants basestocks, 199-200 category chronology, 853 composition, 199 corrosion tests, 827-831 function, 413 high-temperature degradation model, 791 low-temperature high-shear tests, 846-848, 882 low temperature pumpability, 848-851, 882 low temperature requirements, 845-848 oxidation standards, 823 testing, 818-821 quality ranking, 915-916 Lubricating greases, 439, 478-481, 557-572 accelerated corrosion tests, 568 aluminum soap, 558 apparent viscosity, 562-563 in ball bearings at elevated temperatures, 565-566 black penefration, 561 bomb oxidation test, 565 calcium soap, 558-559 chemical analysis, 569 chemical properties, 478-479 classification, 560 compatibility of blends, 569 complex soap, 559 composition, 557-558 cone penefration, 561 consistency, 560-561 consistency stability, 561-562 contamination, 568-569 copper corrosion, 567-568 corrosion tests, 831-832 dropping point, 563-564 elastomer compatibility, 569 evaporation loss, 564 extreme pressure four-ball test, 566-567 extreme pressure Timken method, 566
1076
MANUAL 3 7: FUELS AND L UBRLCANTS
flow properties, at high temperatures, 562 fretting wear, 567 heat resistance, 563-565 leakage from steel bearings, 565 lithium soap, 559 low-temperature torque, 563 NLGI consistency numbers, 561 oil separation, 564-565 organo-clay, 560 oscillating motion, 567 oscillating wear test, 567 oxidation induction time, 806-807 stability, 565-566 standards, 823 testing, 804-807 PDSC oxidation test, 565 performance, 480-481 performance testing, 242 polyurea, 559-560 roll stability, 562 rust prevention, 568 selection, 558 in small bearings, 566 sodium soap, 559 specifications, 560 standards, 570-572 discontinued, 570 under development, 570 static bleed test, 569 viscosity, 479-480 water spray-off, 568 water washout, 568 wear preventive characteristics, 567 wheel bearing grease life, 566 Lubricating oil additives, 952 extreme pressure, oxidation testing, 802 oxidation induction time, 818 Lubrication, 497-500, 909-957 boimdary, 949 elastohydrodynamic, 947-949 friction, 909-911 friction modifiers, 949, 951-952 Hertzian contact, 946-949 hydraulic fluids, 353 hydrodynamic, 949 parameters controlling, 965-966 performance, turbine lubricating oils and hydraulic fluids, 341-342 Petroff s law, 949 regions, thermal considerations, 970 standards, 957 synthetic lubricants, 261-262 types, 214 Lubricity aerospace fuels, 744-745
HANDBOOK aviation fuels, 102-103 dieselfiiel, 129-131 gasoline, 85 Lubricity additives, 69 Luminometer, 98 M MacCoull equation, 854 MacCoull-Walther equation, 919 Macroemulsions, metalworking and machining fluids, 503 Magnetic quenchometer method, quench oils, 614-615 Magnetic residues, LPG, 49-50 Manifold, air temperature and pressure, octane number requirement and, 76 Mannich products, synthesis, 206-207 Marine petroleum fuels, 145-167 ash, 150 ash modifiers, 154 biocides, 154 carbon residue, 151-152 cat fines, 150 compatibility, 152 demulsiflers, 154 density, 149 diesel plant fuel system, 161-162 environmental considerations, 156 flashpoint, 150 fuel additives, 153-154 heating value, 149-150 ignition quality, 149-150 impact on exhaust emissions, 154 on board testing, 157 oxidation, 152 pour point, 150 sampling, 156 sediment, 152-153 shore side analysis, 156 sodium in, 152-153 specifications, 154-156 stabilizers and dispersants, 153 standards, 167 steam plant fuel service system, 157, 161 storage systems, 157-160 sulfiir, 151 toxicity, 154 transfer systems, 157 vanadium and nickel in, 152 viscosity, 148-149 water in, 151 Marine transport, aviation fuels, 105-106 Mass spectrometry, 662-665 hydrocarbon analysis, 664-665 instrumentation, 663-664 standards, 665 theory, 662-663 Master chart, aviation gas turbine oils, 1012, 1014 McMillan/Murphy pumpability apparatus, 848
INDEX
Measurement process bias, 1044-1045 manufacturing process control, 1043 precision, 1044-1045 product property conformance to specification, 10431044 self-monitoring, 1044 see also Quality assurance Melting point, petroleum waxes, 542-545 Mercaptan odorants, 55 Metal content, mineral oil heat transfer fluids, 580-581 Metal deactivators, 795-796 Metal forming fluids, 509, 520 Metal passivators, 319-320, 369-370 Metal preservatives, 830 Metal protecting fluids, 510 Metal ratio, 208 Metal removal fluids, 505-509, 519 Metals crude oil, 6 oxidation potentials, 220 significance in petroleum products, 707-708 Metal treating fluids, 510 Metalworking and machining fluids, 234, 497-524 additives, 511-519 alkalinity agents, 516 antimicrobial agents, 517-518 anti-misting agents, 516 corrosion inhibitors, 516-517 coupling agents, 513 dispersants, 516 dyes, 519 emulsion promoters, 512-513 film-forming agents, 513-515 foam inhibitors, 518-519 inorganic/organic solids, 519 odor control agents, 519 base fluid, 510-511 classification based on end-use, 505-510 formulations, 519-521 metal forming fluids, 509 metal protecting fluids, 510 metal treating fluids, 510 oil-based, 501 performance testing, 241 semisynthetic fluids, 504-505 slideway lubricants, 510 solid dispersions, 505 soluble oils, 503 synthetic fluids, 503-504 tests, 521-523 used, recycling, 521-522, 524 water-based, 501-503 classification, 503-505 Metering, aviation fiiels, 99-100 Methane, structure, 732 Methanol
Kill
in propane, phase behavior, 46-47 in propylene, 193 Methylcyclopentadienyl manganese tricarbonyl, in gasoline, 68-69 Micellar solutions, metalworking and machining fluids, 501-502 Micro-elastohydrodynamic lubrication, 948-949 Microelemental analysis methods, 712 Microemulsions, metalworking and machining fluids, 504505 Microfihers, 109 "Micro Method" for carbon residue, compressor lubricants, 399 Micronic filters prefilters, 109 Microorganisms, in aviation fuels, 110 Micropitting, 434-435, 977, 979 Micro-scuffing engine oil performance, 1009-1010 failure criteria, 1001 Military fuel specifications, marine petroleum fuels, 155156 M1L-PRF-2105E, gear lubricants, 451-452 MIL-PRF-7808 oils, 1012-1013 MIL-PRF-23699 oils, 1011 Mineral gas turbine oils, 317-318 Mineral oil heat transfer fluids, 573-585 acid number, 582 autoignition, 581 carbon residue, 580 classification, 575 composition, 575-578 distillation, 582 elastomeric seal compatibility, 581 fluid maintenance, 581 metal content, 580-581 oxidative stability, 580 physical properties, 578-581 pumpability, 579 safety, 583 sampling, 581-583 specific gravity, 580 specific heat, 581 standards, 584-585 thermal conductivity, 581 thermal stability, 579-580 viscosity, 579, 582-583 water content, 583 Mineral oils aero-engine lubricants, 301 compressor lubricants, 392 hydraulic fluids, 374-375 inhibited oxidation characteristics, 797-798 oxidation stability, 798-799 rust-preventing characteristics, 829-830 sludging and corrosion tendencies, 798, 830-831 metalworking and machining fluids, 501
1078
MANUAL 37: FUELS AND LUBRICANTS
HANDBOOK
organic structures, 576 refrigeration lubricants, 414-415 Mini-rotary viscometer, 849-850 Mixed-film lubrication, 214 Modified Ostwald viscometers, 838-841 Monitors, aviation ftiels, 108 Motor gasoline, see Gasoline Motor octane number, 73-74 MR-Chart, 1056 MTBE, 16-17, 66 Multi-specimen test machine, 1030-1031 N Naphthenic base oils, 172 Naphthenic hydrocarbon fractions, 527 Naphthene, determination, 195 Natural gas liquids volume correlation factors, 50-51 Natural Gasoline Association of America, 37 Naturally occurring radioactive materials, in LPG, 50 Near-surface region, 964 Neutron activation analysis, elemental analysis, 712-713 Nickel content, 152,775 Nitrogen, crude oil, 6 Nitrogen compound reactivity, hydrocarbon base oil, 177178 Nitrogen level, gear lubricants, 458 NLGI consistency numbers, 561 Nonleaded gasoline, 63-64 Non-lubricating process fluids, 587-632 aqueous polymer quenchants, 605-606 standards, 630-632 temperature gradient quenchant analysis, 623-629 see also Quench oils Non-petroleum materials using ASTM flow properties, 867872 Normality checks, 1052-1055 NOK,Clean Air Act, 64 Nuclear magnetic resonance spectroscopy hydrocarbon analysis, 649-652 hydrocarbon base oil, 174-175 principles, 649-650 standards, 651-652 O Occupational exposure limits, LPG, 54 Octane distribution throughout fuel boiling range, 75 liquified petroleum gas, 40 Octane number requirement, 75-78 air-fuel ratio, 76 compression ratio, 76 effect of engine deposits, 77 engine design parameters, 76 engine spark timing, 76-77 manifold air temperature and pressure, 76 Octane rating, history, 62-63 Octane rating analyzers, on-line, 74-75
Octanol/water partition coefficient, 887 Odor, petroleum waxes, 549, 551 Odor control agents, metalworking and machining fluids, 519 Odorization, LPG, 55 OEM specifications, gear lubricants, 451, 453 Oil blending calculations, 856 Oil content, petroleum waxes, 546 Oil film wash-oil resistance, compressor lubricants, 401-402 Oil-off testing, 982-983 Oil separation, lubricating greases, 564-565 Oil-soluble dye, in gasoline, 69 Oil stability index, 821 Oil stain, LPG, 49 Olefin saturation, 17-20 sulfurization, 216 Olefin-based polymers, 224-226 Olefin content, 28 hydrocarbon base oil, 178 liquified petroleum gas, 40-41 Optical texture, petroleum coke, 775-776 Organo-clay greases, 560 Organo-metallics, iron based, in gasoline, 69 Orifice viscometers, 864-865 Oscillating motion, lubricating greases, 567 Oscillating wear test, lubricating greases, 567 Oxidation, 787-823 automotive lubricants, 476-477 aviation fuels, standards, 823 biodegradable lubricants, standards, 823 engine oils, standards, 823 extreme pressure oils, 458 gasolines, standards, 823 gear oils, standards, 822 grease, standards, 823 hydraulic fluids, standards, 822 hydrocarbon base oil, 175-176 hydroperoxide decomposers, 791-792 lubricants, 787-796 standards, 823 marine petroleum fuels, 152 mechanism, 789 pressure media, 277, 279 radical scavengers, 791-792 role of metal ions, 793-794 turbine oils, standards, 822 Oxidational wear, 933 Oxidation heat release, 752-753 aerospace fuels, 735 Oxidation induction time, lubricating oils, 818 Oxidation inhibitors, see Antioxidants Oxidation potentials, metals, 220 Oxidation stability ASTM D 4814, 82-83 automotive gear oils, 803-804 automotive transmission fluids, 804
INDEX
aviation fuels, 813 diesel&el, 126-127 distillate fuel oil, 816-817 environmentally friendly oils, 274-275 gasoline, 813, 815-817 lubricating greases, 565-566 mineral oil heat transfer fluids, 580 pressure media, 281-284 laboratory aging, 281-283 test stand aging, 282-284 synthetic lubricants, 258. 260-261 turbine lubricating oils and hydraulic fluids, 338-339 Oxidation Stability test, aviation fuels, 102 Oxidation testing, 315, 317, 796-822 antiwear, 797-798 biodegradable lubricants, 820-822 engine oils, 807-811 fuels, 811,813, 815-818 gasolines, 811,813, 815-818 gear oils, 802-804 grease, 804-807 hydraulic fluids, 356-357 lubricants, 818-821 purpose of testing, 796 using universal glassware, 802-803 Oxygen, crude oil, 6-7 Oxygenated polymer, formation, 790 Oxygenates, 7, 29 addition to gasoline, 65-66 in gasoline, 68
Paddle type rotational viscometer, 835-836 Paint Compatibility Test, compressor lubricants, 409-410 Paints, flow-related instruments, 867 Panel Coker Test, 818-819 Paraffin, determination, 195 Paraffinic base oils, 172 Paraffinic hydrocarbon fractions, 527 Partial least squares modeling, 179 Partial oxidation, 26 Particulate emissions, diesel engines, 720 Particulates, in LPG, 49-50 PDSC oxidation test, lubricating greases, 565 Pearlite, 587 Peng-Robinson Equation of State, 81 Penn State micro-oxidation test apparatus, 807 Pensky-Martens Closed Cup Tester, 692-694 Perfluoroalkyl ethers, 259 Performance additives, gasoline, 69-70 Performance maps, 945, 968-969 bearing and gear contacts and, 972-973 generalized, 987-988 lubrication atfributes, 967 wear and fraction within, 968, 970 Performance package, 235 Performance testing, see Additives
1079
Permanent shear stability index, 853 Peroxide number, pefroleum waxes, 549 Peroxides, in butadiene, 193-194 Petroff s law, 949 Pefroleum coke, 768-778 ash, 775 classification, 773 crystalline structure, 775-776 density, 775 feedstocks, 772 microtexture formation, 770-772 optical texture, 775-776 production processes, 768-770 properties, 773-774 reactivity, 778 standards, 783-784 sulfijr content, 775 thermal expansion, 777 frace metals contents, 775 volatile matter, 774-775 Petroleum fractions, hydrogen in, 195 Petroleum gas, sulfur content, 194-195 Pefroleum industry, history, 145 Petroleum oil, 3-29 classification, 576 alkylation, 15-17 capacity as percent of crude, 5 catalytic reforming, 12-15 cogeneration, 28 crude oil, 6-7 preparation and separation, 9-10 fluid catalytic cracking, 9-12 fufrire, 28-29 gas solubility in, 406-407 heteroatom removal and aromatic/olefin saturation, 17-20 hydrogen production, 25-26 isomerization, 14, 16 lube oil base stock production, 25 MTBE, 16-17 pollution control, 27-28 product blending, 28 products, 7-9 refinery flow plan, 3-5 resid conversion and upgrading, 20-25 sulfur recovery, 26-27 Petroleum pitch, 763-768 ash, 768 carbon fibers, 781 coking value, 767-768 density, 768 production processes and applications, 763 properties, 763-764 softening point, 764 solvent fractionation, 765-767 standards, 783-784 sulfur content, 768 viscosity, 764-765
1080
MANUAL 37: FUELS AND LUBRICANTS
Petroleum waxes, 525-555 additive effect, 542 blocking point, 552-553 carbonizable substances, 549 cloud point, 545-546 coefficient of kinetic friction, 552, 554 color, 549-551 composition, gas chromatography, 549 compositional and molecular characteristics, 531-533 congealing point, 545 crystal structure, 533-535 dewaxing process variables, 530-531 differential scanning calorimetry, 539-542 equations of state, 535-539 gloss retention, 549 hardness, 547-548 hot tack, 552 melting point, 542-545 odor, 549, 551 oil content, 546 peroxide number, 549 physical properties, 533 pour point, 545 solvent dewaxing process, 529-530 solvent solubility, 546 specular gloss, 549, 552 standards, 554-555 surface wax, 549, 551 total wax content, 552 transition temperatures, differential scanning calorimetry, 548 types of, 531 viscosity, 546-547 wax finishing process, 531 weight of wax applied during coating, 552 pH determination, polymer quenchants, 617-618 Philippoff rotary viscometer, 849 Phosphate esters, compressor lubricants, 395 Phosphonic acid dispersants, synthesis, 206-207 Phosphorus content ASTMD4814, 82 automotive lubricants, 471 gear lubricants, 457 Photolysis, environmental characteristics, 890 Photooxidation, environmental characteristics, 890 Pin and Vee Block tester, 1025-1026 Pin-on-Disk test, 1026 Pitting, 434 Pipelines, aviation fuels, 105 Plants, toxicity tests, 903 Plastics, viscosity, 869-870 Plate-to-plate geometry, 835 Ploughing, friction due to, 911-912 Polar additives, adsorption on metal surface, 211-212 Polishing wear, 923, 929 Pollution control, petroleum oil refining, 27-28 Polyalkylene glycols, 252-256
HANDBOOK application and performance characteristics, 255-256 chemical characteristics, 254-255 chemistry, 253 compressor lubricants, 392-393 physical properties, 254 refrigeration lubricants, 417-418 synthesis, 253-254 Polyalphaolefins, 257-258 compressor lubricants, 393-394 refrigeration lubricants, 415-416 for turbines, 308-309 Poly(ethylene) waxes, 525 Polymer, viscosity, 869 loss due to degradation, 228-230 Polymer-containing oils, shear stability, 851-853 Polymeric additives, 223-233 Polymeric thickeners, gear lubricants, 441-442 Polymer molecular weight analysis, polymer quenchants, 620 Polymer quenchants appearance, 616 biological stability, 620-623 cloud point, 619 conductance, 618-619 cooling curve analysis, 621-623 corrosion inhibitor, 619-620 fluid biodeterioration, 620-621 foam testing, 620 pH determination, 617-618 polymer molecular weight analysis, 620 refractive index, 616-617 viscosity, 617-618 water content, 617 Polyolesters, 249-252 application and performance characteristics, 252 biodegradability, 252 chemical characteristics, 251 chemistry and manufacturing, 249-251 manufacturing technology, 250-251 physical properties, 251 refrigeration lubricants, 416-417 Polyphenyl ethers, 259-260 Polyurea greases, 559-560 Polyvinylethers, refrigeration lubricants, 418 Potential Residue Method, 815 Pour point, 879-880 compressor lubricants, 397 diesel fuel, 122-123 effect of double bonds, 273 esters, 273 fuel oils, 761 gear lubricants, 454 marine pefroleum fuels, 150 petroleum waxes, 545 standards, 874 turbine lubricating oils and hydraulic fluids, 329 Pour point depressants, 205, 230-232
INDEX
gear lubricants, 441 hydraulic fluids, 373 Power transfer, hydraulic fluids, 353-354 Power transmission fluids, performance testing, 239 Precipitation number compressor lubricants, 399-400 gear lubricants, 453 quench oils, 613-614 Precision measurement process, 1044-1045 monitoring, 1045-1047 Pressure differential scanning calorimetry, 806, 818 Pressure drop, 574-576 Pressure media, 267 hydrolysis, 279-281 hydrolysis stability, aging, 285-289 oxidation, 277, 279 oxidation stability, aging, 281-284 type, 268-269 see also Hydraulic fluids Pressure-viscosity coefficient, 916- 920, 985-987 Propane auto, 51-52 depressurized, 46 dryness, 42-49 auto-refrigeration, 46 dissolved water, phase of behavior, 43-44 gas hydrates, 44-46 methanol, phase behavior, 46-47 water measurement, 47-49 water sources, 47 liquid, 53 phase behavior of dissolved water in, 43-44 Propane deasphalting, hydrocarbon base oil, 171 Propane hydrates, 45 Propane P-H diagram, 34 Propylene characterization, 192-193 production, 190 Pumpability low temperature, lubricants, 848- 851 mineral oil heat transfer fluids, 579 PV curves, 384
Quality assurance, 1043-1060 approximate F-test, 1059-1060 ARV assignment, 1048 batch change of material, 1047 check standard composition, 1048 chi-square test, 1059 control chart, 1055-1056 data treatment, 1046-1047 estimate process precision and bias, 1048 EWMA overlay, 1056 implementation aspects, 1049 interlaboratory exchange testing, 1049
1081
material selection, preparation and storage, 1046 material testing, 1046 monitoring bias, 1047-1048 monitoring stability and precision, 1045-1047 MR-Chart, 1056 normality checks, 1052-1055 run chart, 1051-1052 standards, 1050 statistical tools, 1050-1051 system validation, 1049 Mest, 1058-1059 Quality control aviation fuels, see Aviation fuels airport, 110-111 elemental analysis, 716 Quench oils, 602-605 acid number, 609-610 ash content, 614 carbon residue, 611-613 contamination, 610-614 fire and flash points, 609 infra-red spectroscopy, 610 kinematic viscosity, 608-609 magnetic quenchometer method, 614-615 precipitation number, 613-614 properties, 604 saponification number, 610 specific gravity, 609 water content, 610-611 R Rabinowitz abrasive wear model, 930 Radical scavengers, 791-792 Rail transport, aviation fuels, 106 Ramsbottom carbon residue, compressor lubricants, 399 RBOT test, 315-316 compressor lubricants, 401 Reaction temperature, maximum, 754-756 Reactivity, petroleum coke, 778 Reavell VHP 15 Compressor Rig Test, 403 Reciprocating compressors, 386-388 Recycling, used lubricants, 521-522, 524 Redlich-Kwong equation, 536 Reference gauge point, 636 Reformulated gasoline, 65-66 Refractive index, polymer quenchants, 616-617 Refrigeration lubricants, 413-429 acid catchers, 419 additives, 418-420 alkylbenzene, 415 antifoaming agents, 419-420 antioxidants, 419 antiwear additives, 419 chemical structure, 414-418 choice, 413-414 circulation, 424-428 density, 421
1082
MANUAL 3 7: FUELS AND LUBRLCANTS
dibasic acid esters, 416 equipment line sizing, 426 HVAC cycle, 424-426 mineral oils, 414-415 performance effects, 428 polyalkylene glycols, 417-418 polyalphaolefms, 415-416 polyolesters, 416-417 polyvinylethers, 418 properties, 415, 420 selection, 426-427 solubility, 422-424 standards, 428-429 system operational envelope, 427 viscosity, 420-422 Reid method, 698-700 Reid vapor pressure, 99 Remaining useful life evaluation rig, 820-821 Repressed oil, refining process, 278 Re-refining, hydrocarbon base oil, 172 Research Octane Rating, 62, 74 Resid, 20-25 delayed coking, 21-23 fixed bed hydrocracking, 24 fluid coking/flexicoking, 24 fluidized bed hydrocracking, 24-25 solvent deasphalting, 21-22 visbreaking, 20-21 Residual matter, LPG, 49 Reverse-flow viscometers, 838 Reynolds equation, 948 Rheology, 833-835 hydrocarbon base oil, 178 Risk, environmental characteristics, 904 Road transport, aviation fuels, 106 Rolling ball viscometer, 836 Rolling velocity, Ryder gear, 997-1000 Roll stability, lubricating greases, 562 R & O oils, performance testing, 241-242 Rotary compressors, 388-389 Rotary lobe compressor, 389 Rotary Pressure Vessel Test, 281, 798 Rotary screw compressor, 388-390 Rubber, base oil compatibility, 181 RULER, 820-821 Run chart, 1051-1052 Rust, chemistry, 825-826 Rust formation, electrochemical nature, 825-827 Rusting behavior, turbine lubricating oils and hydraulic fluids, 340-341 Rust inhibitors, 218, 220-223 gear lubricants, 456 hydraulic fluids, 369-370 lubricating greases, 568 for turbines, 318-320 Ryder gear
HANDBOOK applicability of WAM high speed load capacity test method, 1010 backgroimd of load capacity testing, 1001 contact stress simulation, 997-1000 data processing and traction behavior, 1003, 1004-1007 failure criteria in terms of scuffing and micro-scuffing, 1001 link to service performance, 1008-1009 OEM proposed criteria, 1009-1010 oil evaluation, 1000-1010 performance criteria, 1007-1008 procedure, 1003 rolling and sliding velocities across tooth face, 997-1000 simulation approach, 1001-1003 surface film formation, 1007 test description, 1003-1005 tooth temperatures, 996-997 Ryder gear scuffing criteria, 995-996 effect of oil chemistry, 992-994 effect of load stage, 993-994 initiation, 990-992 Ryder Gear Test, 320-321 simulation conditions, 996-1000 test conditions, 988-990
SAE 3306 viscosity classification, gear lubricants, 449-451 Safety aerospace fuels fueling, 744 mineral oil heat transfer fluids, 583 Salt spray, 829 Sampling, LPG, 39-40, 48 Saponification, esters, 280 Saponification number, quench oils, 610 Sasol distillate fuels, 138 Saybolt patent, 37 Saybolt viscometer, 842, 865, 870 Saybolt viscosity, standards, 874 Scuffing, 435, 458, 936-937 boundary, 968-969 engine oil performance, 1009-1010 failure criteria, 1001 front, propagation, 992-993 initiation features on Ryder gears, 990-992 oil chemistry effect, 992-994 progression, tribological processes, 993-996 Scuffing load ball-on-cylinder lubricity evaluator, 129-130 Seal compatibility compressor lubricants, 409 gear lubricants, 459 hydrauhc fluids, 370 lubricating greases, 569 Seals, functions, 233-234 Seal swell agents, hydraulic fluids, 370 Sediment, marine petroleum fuels, 152-153 Sediment toxicity tests, 903
INDEX
Semisynthetic fluids, metalworking and machining fluids, 504-505 Servo-operated automated tank gauge, 645 Setaflash Closed Cup, 693 Shear stability gear lubricants, 459 hydraulic fluids, 371-372 polymer-containing oils, 851-853 requirements, 228-229 turbine lubricating oils and hydraulic fluids, 340 Short capillary viscometers, 865 Short-trip-service engine rusting, 471 Silicones, 258-259 compressor lubricants, 394-395 Slideway lubricants, 510 Sliding gross, surface distress testing, 975-977 incipient, surface distress testing, 974-975 velocity, 966, 997-1000 Sludge, formation by zinc dialkyldithiophosphate, 356-357 Small scale closed cup, 693 Smearing wear, 923, 929 Smoke Point, 98 Smoke tendency, aerospace fuels, 741 Snelling, Walter, 36 Soap content, detergents, 208 Soave-Redlich-Kwong equation, 536 Sodium, in marine petroleum fuels, 152 Sodium soap greases, 559 Softening point, petroleum pitch, 764 Solid dispersions, metalworking and machining fluids, 505 Solidification, aerospace fuels, 737-738 Solid lubricants, 483-484 Solid particles, in aviation fiaels, 109 Solubility, refrigeration lubricants, 422-424 Soluble gum, ASTM D 4814, 83 Soluble oils, metalworking and machining fluids, 503 Solvent deasphalting, 21-22 dewaxing process, 529-530 fractionation, pefroleum pitch, 765-767 refining hydrocarbon base oil, 171 process, 306-307 solubility, pefroleum waxes, 546 Sommerfeld number, 1022-1023 Sonic shear, turbine lubricating oils and hydrauHc fluids, 340 Soot, 203-2-4, 720-721 "Space velocity", 18 Spark-ignition engine fiiels specifications, see ASTM D 4814 vapor-liquid ratio, 703 Spark ignition engines, 70-71 Specific gravity compressor lubricants, 397 mineral oil heat transfer fluids, 580
1083
quench oils, 609 Specific heat compressor lubricants, 408-409 mineral oil heat fransfer fluids, 581 Specific impulse, aerospace fuels, 742-743 Specfromettic identification, base oil components, 172-174 Specular gloss, pefroleum waxes, 549, 552 Sponge coke, removal, 23 Spontaneous ignitability, aerospace fiiels, 733-734, 738 Spray ignition tests, turbine lubricating oils and hydraulic fluids, 334-336 Squeeze films, 499 Stability, diesel fiiel, 125-126 Stabilization patent, 37 Stabilizers, in marine petroleum fuels, 153 Stabilizers/deposit control agents, 201-211 Standard platinum resistance thermometers, 866-867 Standard reference materials, elemental analysis, 714 Surfactants, in aviation fiiels, 110 Static bleed test, lubricating greases, 564, 569 Static elecfricity, 85, 103 Static gases, thermodynamic properties, 749-750 Static petroleum measurement, 635-648 automatic tank gauging, 645 bottom sample, 641-642 examples, 644-645 free water, 639-640 gauging diagram, 636 gross observed volume, 646-647 innage gauge, 643-644 official sample, 641, 643 on-board quantity/remaining on board, 645 other standards, 648 pipeline samples, 642 representative sample, 641 spot samples, 641-642 standards, 656-657 tapes and bobs, 636-638 temperature in tank, 638, 643 weighted bottle sampler, 640-641 Statistical quality conttol tools, 1050-1051 Steam cracking, ethylene, 188-190 Steam plant fuel service system, 157, 161 Steam reforming, 25-26 Steam turbine, 297-298 conttol fluid system, 301 shaft driven lubrication system, 300 Steam turbine oil oxidation stability, 798 rust-preventing characteristics, 830 Steel cooling behavior, wetting process impact, 594-595, 597 cooling curve analysis, standards, 606-607 cooling curve data acquisition and analysis, 595-598 cooling time and rate parameters, 597, 599 corrosivity, aviation fuels, 102 film boiling, factors influencing, 594, 596
1084
MANUAL 37: FUELS AND LUBRICANTS
hardenability, 589-591 measurement, 591-593 hardening capability, 599 shock film boiling, 599-602 transformation, 587-589 wetting kinetics, 593-596 Steel quenching acoustical measurements, 600-602 bath maintenance, 607-608 process analysis example, 626-629 Steric stabilization, 204-205 Stoichiometry, aerospace fuels, 734-735, 738-739 Storage stability aerospace fuels, 732 aviation fuels, 102 diesel fuel, 127-128 distillate fuel, 817-818 gear lubricants, 460 low temperature, 329 Stormer viscometer, 868 Strainers, diesel fuel system, 163-164 Stribeck curve, 945-946, 949, 1022-1023 Styrene-diene polymers, 225 Subsurface region, 964-965 Sulfur content, 19, 28-29 ASTMD4814, 82 diesel fuel, 134 crude oil, 6 fuel oils, 762 gear lubricants, 457 impact on base oil chemistry, 176-177 LPG, 49 marine petroleum fuels, 151 petroleum coke, 775 petroleum gas, 194-195 petroleum pitch, 768 Sulfur recovery, 26-27 Super critical fluid chromatography, hydrocarbon analysis, 659-660 Surface analysis, methods, 950-951 Surface conformity, 914-916 Surface distress testing other types, 976-978 imder incipient sliding, 974-975 Surface energy, 922 Surface fatigue, impact of oil attributes, 972, 974 Surface film region, 963-964 Surface films, 919, 921-923 Surface roughness, 911-913 data analysis, 913-916 Surface tension, aerospace fuels, 738 Surfacewax, 549, 551 Synchronizer SSP 180 test, gear lubricants, 459 Synthetic basestocks, compressor lubricants, 392 Synthetic ester fluids, for turbines, 309-311 Synthetic fluids, metalworking and machining fluids, 503504
HANDBOOK Synthetic hydrocarbons, 185-196 analytical test methods, 192 basestocks, 185-186 boiling range, 195 butylene and butadiene production, 190-192 C-4 product, characterization, 193-194 ethylene, 186-190, 192 propylene, 190, 192-193 standards, 195-196 Synthetic lubricants, 249-264 alkylated cyclopentanes, 260 biodegradability, 263 classes, 249 cyclohexane derivatives, 260 environmental acceptability, 262-263 FDA incidental food contact approval, 263 lubrication, 261-262 perfluoroalkyl ethers, 259 polyalkylene glycols, 252-256 polyalphaolefins, 257-258 polyphenyl ethers, 259-260 raw materials, 249-250 relative cost, 263 silicones, 258-259 standards, 263-264 Synthetic waxes, 525 System operational envelope, refrigeration lubricants, 427
Tag Open cup, 693 TAME, 17 Tankage, gas turbine fuel system, 165 Tapping torque test, 1030 Temkin isotherm, 953-954 Temperature, kinematic viscosity relationship, 854-856 Temperature gradient method, 623, 625 Temperature gradient quenchant analysis, 623-629 Terminology, 1038-1041 Terrestrial toxicity tests, 903 Test stand aging hydrolysis stability, 287-289 oxidation stability, 282-284 Thermal and oxidation test, gear lubricants, 458 Thermal conductivity compressor lubricants, 408-409 gasoline, 84 mineral oil heat transfer fluids, 581 Thermal cracking reaction mechanism, ethylene, 188 Thermal expansion, petroleum coke, 777 Thermal oxidation stability, aviation turbine fuels, 815-816 Thermal stability aerospace fuels, 745-746 diesel fuel, 126 diesters, 251 hydraulic oils, 799-800, 830 mineral oil heat transfer fluids, 579-580 polyolesters, 251
INDEX
turbine lubricating oils and hydraulic fluids, 339 Thermodynamic properties, 750 static gases, 749-750 Thermo oxidation engine oil simulation test, 808-812 Thickening efficiency, 225-226 Thin film oxygen uptake test, 807-809 Three-terminal resistivity cell, 343 Thrust washer tester, 1030-1031 Timken extreme pressure test machine, 1030-1031 lubricating greases, 457, 566 Tool wear, metal removal fluids, 507-509 TOST test, 315 Total base number, detergents, 208-209 Total wax content, petroleum waxes, 552 Toxicity acute tests, 894,901-902 chronic tests, 894 complex mixture, environmental characteristics, 900-901 marine petroleum fuels, 154 Trace metal contents limits, gas turbine fuels, 141-142 petroleum coke, 775 turbine lubricating oils and hydraulic fluids, 343 Traction, within performance maps, 968, 970 Traction coefficient, 970-972 Traction test plots, 980 Tractor fiuids, 374 Tractor hydraulic fluids, performance testing, 240-241 Transformation processes, environmental characteristics, 889 Transition metals, as both oxidation promoters and inhibitors, 793, 795 Transmission fluids, 477-478 irictional properties, 212-213 performance tests, 239 standards, 489-490 Transport processes, environmental characteristics, 888-889 Triaryl phosphates, for turbines, 310, 312 Tribochemical wear, 933 Tribochemistry, lubricated wear contact, 949 Tribological aspect number, 1024-1025 Tribological features, microscopic examination, 991-992 Tribological processes, scuff progression, 993-996 Tribology definition, 909 history, 1017 systematic dynamic mechanisms, 963-965 gear lubrication, 988-996 hydrodynamic film region, 963 impact of oil attributes on surface fatigue, 972, 974 lubrication regions, thermal considerations, 970 micro-pitting results, 977, 979 multi-dimensional oil characterization, 968-969 near-surface region, 964 oil-off testing, 982-983
1085
parameters controlling lubrication and failure mechanisms, 965-966 performance maps, generalized, 987-988 subsurface region, 964-965 surface distress testing, 974-976 under gross sliding, 975-977 under incipient sliding, 974-975 surface film region, 963-964 testing for adhesive wear, 979-982 traction coefficient, 970-972 WAM test machine technology, 967-968 Tribology test systems, 1017-1018 component test design, 1021 designing, 1018-1020 duration, 1023 economic comparison, 1018 load, 1022 materials, 1023 parameter selection, 1021-1024 selecting, 1020-1021, 1024-1025 special atmospheres, 1023 special testing, 1024 speed, 1022-1023 temperature, 1021-1022 test fluids, 1023-1024 Tribo-system, see Tribology, systematic ?-test, 1058-1059 TTT diagrams, 589 Turbine lubricating oils and hydraulic fluids, 297-349 acid number, 329, 331 additives, 312-323 antifoams, 321-323 antioxidants, 312-318 antiwear and extreme-pressure, 319-321 rust and corrosion inhibitors, 318-320 air release, 332-334 basestocks, 305-312 hydrocarbon oils, 305-306 hydrocracked/hydrotreated, 307- 308 polyalphaolefins, 308-309 solvent-refined types, 306-307 synthetic ester fluids, 309-311 triaryl phosphates, 310, 312 chlorine content, 334 classification, 323-324 cleanliness, 341-343 compatibility with system materials, 343-346 density, 332 duty cycle, 304 effect of maintenance, 304 fire-resistance tests, 334-337 fire-resistant, 301, 311 foaming, 332-334 function, 300 future trends, 349 hot surface tests, 335 hydrolytic stability, 339-340 importance of system cleanliness, 346-347
1086
MANUAL 3 7: FUELS AND L UBRICANTS
low temperature storage stability, 329 lubrication performance, 341-342 maintenance, 347-348 oil types, 305 operating environment, 300-305 oxidation characteristics, 797-798 stability, 801 standards, 822 oxidative stability, 338-339 performance requirements, 326-328 performance testing, 241-242 pour point, 329 resistance to shear in a diesel-injector nozzle, 340 rusting and corrosion behavior, 340-341 shear stability, 340 sonic shear, 340 spray ignition tests, 334-336 stability testing, 317, 400-401 standards, 323-326 thermal/oxidative stress, 303 thermal stability, 339 top-up rates, 304 trace metals, 343 viscosity, 326-330 volatility, 346 volume resistivity, 343-345 water content, 331-332 water separability, 335-336, 338 wick tests, 335, 337 Turbine oil system, in service monitoring, 798-802 Turbines gas, 298-299 steam, 297-298 system cleanliness, 346-347 water, 298-299 wind, 299 Turbofan combustion system, 97 U Ultraviolet spectroscopy Beer's law, 661 hydrocarbon analysis, 661-662 principles, 660-661 standards, 662 Unique process line, 750-752 Used oils, wear metals, 707-709
Vacuum gas oils, 4 Vacuum potstill method, 696-697 Vacuum resid, 4 Vanadium content, 152-153, 775 Vane pump wear test, 362-363 Vapor control systems, 64-65 Vapor-liquid ratio, 78, 703 Vapor lock temperature, 79
HANDBOOK Vapor pressure, 697-704 automatic method, 701 crude oils, 702 environmental characteristics, 888 evacuated chamber method, 703 gasoline, 699, 701 gasoline-oxygenate blends, 699, 701 liquified petroleum gas, 40-42, 703-704 mini-method, 701-702 mini-method-atmospheric, 702 Reid method, 698-700 triple-expansion method, 702-703 vapor-liquid ratio Vegetable waxes, 525 Vehicle emission standards, history, 63 Vickers pump stand, 1035 Visbreaking, 20-21 Viscometer geometries, 835-836 Viscosity, 500-501 absolute, water, 863-864 adhesives, 870-872 aerospace fuels, 737 apparent, lubricating greases, 562-563 automotive lubricants, 475-476 bituminous materials, 870-872 bubble tubes, 868 coatings, 867 diesel fuel, 121 diesters, 251 dip cups, 867-868 environmentally friendly oils, 273, 289-290 falling needle, 868-869 fuel oils, 760-761 gas/liquid mixtures under pressure, 408 gasoline, 84-85 greases, 479-480 hydraulic fluids, 359-360 jet fuel, 94 loss due to polymer degradation, 228-230 shear-related, 225, 227-228 marine petroleum fuels, 148-149 mineral oil heat transfer fluids, 579, 582-583 paints, 867 petroleum pitch, 764-765 petroleum waxes, 546-547 plastics, 869-870 polymer quenchants, 617-618 polyolesters, 251 refrigeration lubricants, 420-422 rubber, 872 standards, 864-867, 872-875 temperature measurement considerations, 866-867 tests, 237 turbine lubricating oils and hydraulic fluids, 326-330 visualization, 835 see also Kinematic viscosity
INDEX
Viscosity control, diesel fuel system, 164 Viscosity index, 856-863 calculation, 858-863 gear lubricants, 457 pressure considerations, 859, 862- 863 Viscosity index improvers, hydraulic fluids, 371-372 Viscosity modifiers, 223-224, 235 Viscosity-pressure coefficient, effect of base oil composition, 180-181 Volatile matter, petroleum coke, 774-775 Volatility, 675-705 aerospace fiiels, 736-737 automotive lubricants, 471 aviation fuels, 98-101 class specifications, ASTM D 4814, 82 crude oil distillation, 696-698 diesel fuel, 120-121 diesters, 251 distillation, 675-682 environmental characteristics, 896, 900 flammability, 682, 690-696 gasoline, 78-80 polyolesters, 251 standards, 704-705 turbine lubricating oils and hydraulic fluids, 346 vapor pressure, 697-704 Volatility residue, LPG, 42 Volume correction factors, liquefied petroleum gas, 50-51 Volume resistivity, turbine lubricating oils and hydraulic fluids, 343-345 W Walther equation, 917 WAM configuration, optical EHD film thickness measurement, 984-985 WAM high speed load capacity test method, engine oils, 1010 WAM test machine technology, 967-968 Waste heat economizers, diesel fuel system, 165 Water, absolute viscosity, 863-864 Water-absorbing cartridges, 108 Water-based, metalworking and machining fluids, 501-503 Water content aerospace fuels, 746-747 compressor lubricants, 397 dissolved in propane, phase behavior, 43-44 gear lubricants, 457 hydraulic fluids, 363, 365 LPG, 42-49 marine petroleum fuels, 151
1087
mineral oil heat transfer fluids, 583 polymer quenchants, 617 quench oils, 610-611 turbine lubricating oils and hydraulic fluids, 331-332 Water separability, turbine lubricating oils and hydraulic fluids, 335-336, 338 Water solubility, environmental characteristics, 887 Water spray-off, lubricating greases, 568 Water tolerance, ASTM D 4814, 83 Water turbines, 298-299 Water washout, lubricating greases, 568 Wax appearance point, 881-882 Waxy crude oil, 527-528 Wear combinations of cavitation/erosion/corrosion wear, 932933 debris analysis, 939-941 mechanisms, 923, 927-931 standards, 1035-1037 testing, 938-941 within performance maps, 968, 970 Wear contact, material structure, 916-925 Wear maps, 942-945 Wear material, surfaces, 949 Wear metals, used oils, 707-709 Wear preventive characteristics, lubricating greases, 567 Wear protection, hydraulic fluids, 359-364 Wear reduction, heat of adsorption, 954 Wear surfaces, 911-913 Weathering, liquified petroleum gas, 42 Weighted bottle sampler, 640-641 Wheel bearing grease life, 566 Wheel bearings, grease leakage, 565 Wick tests, turbine lubricating oils and hydrauhc fluids, 335, 337 Wind turbines, 299 Wolf Strip Oxidation Test, compressor lubricants, 401 World Wide Fuel Charter, 137 Worms, toxicity tests, 903-904 X X-ray fluorescence spectrometry, elemental analysis, 713
Yellow metal corrosion, 221
Zinc dialkyldithiophosphate, 314 oxidation inhibition, 794 sludge formation, 356-357
