ANSI-HI-9-6-1 Centrifugal and Vertical Pumps for NPSH Margin

ANSI/HI 9.6.1-1998

American National Standard for

Centrifugal and Vertical Pumps

ANSI/HI 9.6.1-1998

for NPSH Margin

9 Sylvan Way Parsippany, New Jersey 07054-3802 www.pumps.org

This page intentionally blank.

Copyright © 2000 By Hydraulic Institute, All Rights Reserved.

ANSI/HI 9.6.1-1998

American National Standard for

Centrifugal and Vertical Pumps for NPSH Margin

Secretariat

Hydraulic Institute www.pumps.org

Approved March 3, 1998

American National Standards Institute, Inc. Recycled paper


American National Standard

Approval of an American National Standard requires verification by ANSI that the requirements for due process, consensus and other criteria for approval have been met by the standards developer. Consensus is established when, in the judgement of the ANSI Board of Standards Review, substantial agreement has been reached by directly and materially affected interests. Substantial agreement means much more than a simple majority, but not necessarily unanimity. Consensus requires that all views and objections be considered, and that a concerted effort be made toward their resolution. The use of American National Standards is completely voluntary; their existence does not in any respect preclude anyone, whether he has approved the standards or not, from manufacturing, marketing, purchasing, or using products, processes, or procedures not conforming to the standards. The American National Standards Institute does not develop standards and will in no circumstances give an interpretation of any American National Standard. Moreover, no person shall have the right or authority to issue an interpretation of an American National Standard in the name of the American National Standards Institute. Requests for interpretations should be addressed to the secretariat or sponsor whose name appears on the title page of this standard. CAUTION NOTICE: This American National Standard may be revised or withdrawn at any time. The procedures of the American National Standards Institute require that action be taken periodically to reaffirm, revise, or withdraw this standard. Purchasers of American National Standards may receive current information on all standards by calling or writing the American National Standards Institute.

Published By

Hydraulic Institute 9 Sylvan Way, Parsippany, NJ 07054-3802 www.pumps.org

Copyright © 1998 by Hydraulic Institute All rights reserved. No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without prior written permission of the publisher.

Printed in the United States of America ISBN 1-880952-25-4


Contents Page Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v 9.6.1

Pump NPSH margin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

9.6.1.1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

9.6.1.2

Suction energy level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

9.6.1.2.1

Suction energy factors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

9.6.1.2.2

Suction energy determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

9.6.1.3

Cavitation damage factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

9.6.1.4

NPSH margin ratio recommendations . . . . . . . . . . . . . . . . . . . . . . . . 4

9.6.1.5

Application considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

9.6.1.5.1

Petroleum process pumps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

9.6.1.5.2

Chemical process pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

9.6.1.5.3

Electric power pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

9.6.1.5.4

Nuclear power/cooling tower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

9.6.1.5.5

Water/wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

9.6.1.5.6

General industrial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

9.6.1.5.7

Pulp and paper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

9.6.1.5.8

Building services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

9.6.1.5.9

Slurry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

9.6.1.5.10

Pipeline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

9.6.1.5.11

Waterflood (injection) pumps. . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

9.6.1.6

Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Appendix A

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

iii Copyright © 2000 By Hydraulic Institute, All Rights Reserved.



Foreword (Not part of Standard) Purpose and aims of the Hydraulic Institute The purpose and aims of the Institute are to promote the continued growth and well-being of pump manufacturers and further the interests of the public in such matters as are involved in manufacturing, engineering, distribution, safety, transportation and other problems of the industry, and to this end, among other things: a) To develop and publish standards for pumps; b) To collect and disseminate information of value to its members and to the public; c) To appear for its members before governmental departments and agencies and other bodies in regard to matters affecting the industry; d) To increase the amount and to improve the quality of pump service to the public; e) To support educational and research activities; f) To promote the business interests of its members but not to engage in business of the kind ordinarily carried on for profit or to perform particular services for its members or individual persons as distinguished from activities to improve the business conditions and lawful interests of all of its members.

Purpose of Standards 1) Hydraulic Institute Standards are adopted in the public interest and are designed to help eliminate misunderstandings between the manufacturer, the purchaser and/or the user and to assist the purchaser in selecting and obtaining the proper product for a particular need. 2) Use of Hydraulic Institute Standards is completely voluntary. Existence of Hydraulic Institute Standards does not in any respect preclude a member from manufacturing or selling products not conforming to the Standards.

Definition of a Standard of the Hydraulic Institute Quoting from Article XV, Standards, of the By-Laws of the Institute, Section B: “An Institute Standard defines the product, material, process or procedure with reference to one or more of the following: nomenclature, composition, construction, dimensions, tolerances, safety, operating characteristics, performance, quality, rating, testing and service for which designed.”

Comments from users Comments from users of this Standard will be appreciated, to help the Hydraulic Institute prepare even more useful future editions. Questions arising from the content of this Standard may be directed to the Hydraulic Institute. It will direct all such questions to the appropriate technical committee for provision of a suitable answer. If a dispute arises regarding the contents of an Institute publication or an answer provided by the Institute to a question such as indicated above, the point in question shall be referred to the Executive Committee of the Hydraulic Institute, which then shall act as a Board of Appeals.

v Copyright © 2000 By Hydraulic Institute, All Rights Reserved.

Revisions The Standards of the Hydraulic Institute are subject to constant review, and revisions are undertaken whenever it is found necessary because of new developments and progress in the art. If no revisions are made for five years, the standards are reaffirmed using the ANSI canvass procedure.

Scope This standard applies to centrifugal and vertical pump types. It describes the benefit to pump life when the NPSH available is greater than the NPSH required by a suitable margin, and suggests margins for various applications.

Units of Measurement Metric units of measurement are used; corresponding US units appear in brackets. Charts, graphs and sample calculations are also shown in both metric and US units. Since values given in metric units are not exact equivalents to values given in US units, it is important that the selected units of measure to be applied be stated in reference to this standard. If no such statement is provided, metric units shall govern.

Consensus for this standard was achieved by use of the Canvass Method The following organizations, recognized as having an interest in the standardization of centrifugal pumps were contacted prior to the approval of this revision of the standard. Inclusion in this list does not necessarily imply that the organization concurred with the submittal of the proposed standard to ANSI. A.W. Chesterton Company Agrico Chemical Corp. Ahlstrom Pumps, LLC Alden Research Lab Bechtel Corporation Black & Veatch Brown & Caldwell Camp Dresser & McKee CH2M Hill Chas S. Lewis & Co., Inc. Crane Pump & Systems DeWanti & Stowell Dow Chemical DuPont Engineering Electric Power Research Institute Engineering Devices Resource Group ENSR Consulting & Engineering Essco Pump Division Fairbanks Morse Pump Florida Power Corporation Floway Pumps Flowserve Corp. Fluor Daniel, Inc. Grundfos Pumps Corp. Ingersoll-Dresser Pump

ITT Industrial Pump Group ITT Flygt Corp. Iwaki Walchem Corp. J.P. Messina Pump & Hydraulics Consultant John Crane, Inc. Johnston Pump Co. Lawrence Pumps, Inc. M. W. Kellogg Co. Malcom Pirnie, Inc. Marine Machinery Association National Pump Co. Monsanto Co. Montana State University Montgomery Watson MWI, Moving Water Industries Oxy Chem National Pump Co. PACO Pumps Patterson Pump Co. PC Garvin & Associates Price Pump Co. Raytheon Engineering & Constructors Robert Bein, William Frost & Associates

vi Copyright © 2000 By Hydraulic Institute, All Rights Reserved.

Sewage & Water Board of New Orleans Skidmore South Florida Water Management Southern Company Services, Inc. Sta-Rite Industries Stone & Webster Engineering Sulzer Bingham Pumps, Inc.

Summers Engineering, Inc. Systecon, Inc. The Process Group, LLC Union Pump Co. US Bureau of Reclamation US Army Corp of Engineers

“Although this standard was processed and approved for submittal to ANSI by the Canvass Method, a working committee met many times to faciliate the development of this standard. At the time it was developed, the committee had the following members:”

CHAIRMAN - Allan Budris, ITT Industrial Pump Group

OTHER MEMBERS Ronald Brundage, ITT Flygt Fred Buse, Ingersoll-Dresser Pump Co. Greg Case, Price Pump R. Barry Erickson, ITT Industrial Pump Group Herman Greutink, Johnston Pump Al Iseppon, Sta-Rite Industries Ray Perriman, Sundstrand Fluid Handling Robert Stanbury, Flowserve Corporation

vii Copyright © 2000 By Hydraulic Institute, All Rights Reserved.



HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998 9.6.1 Pump NPSH margin 9.6.1.1

Introduction

HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin

The noise, the vibration and possibly the reliability of a centrifugal or vertical pump and mechanical seal may be significantly affected if an appropriate Net Positive Suction Head (NPSH) margin is not provided by the system above the published Net Positive Suction Head Required (NPSHR) by the pump. The NPSH Margin is defined as the NPSH Available (NPSHA) at the pump inlet, minus the NPSH Required by the pump. The NPSH Margin Ratio is the NPSHA divided by the NPSHR. The Net Positive Suction Head Available (NPSHA) is the total suction head available, over the vapor pressure of the liquid pumped corrected to the center line of the impeller (or impeller inlet vane tip datum if vertically mounted), and measured at the inlet to the pump.

the service life of the pump. The full published pump head will not, however, be achieved (by definition) when the NPSHA equals the NPSHR of the pump. The head will be 3% less than the fully developed head value (see Figure 9.6.1.1). It can take up to 2.5 times the NPSHR value just to achieve the 100 percent head value. Just because the definition uses the word Required, does not mean that providing that much NPSHA will necessarily give satisfactory pump life. It is also recognized that, as the suction energy of a centrifugal pump increases, so does the need for a larger NPSH margin above the 3% NPSHR of the pump, to avoid excessive noise, vibration, and possible cavitation erosion and seal damage.

NPSHA = hatm + hgs + hvs + Zs – hvp Where: hatm =

atmospheric pressure head

hgs =

suction gage head

hvs =

suction velocity head

zs = hvp =

Figure 9.6.1.1

suction elevation head liquid vapor pressure head

See the ANSI/HI 1.6 Centrifugal Pump Tests for further details on the definitions of NPSHA and NPSHR. By Hydraulic Institute definition, the NPSHR of a pump is the NPSH that will cause the total head (first stage head of multistage pumps) to be reduced by 3%, due to flow blockage from cavitation vapor in the impeller vanes. NPSHR is by no means the point at which cavitation starts; that level is referred to as incipient cavitation. The NPSH at incipient cavitation can be from 2 to 20 times the 3% NPSHR value, depending on pump design. The higher ratios are normally associated with high suction energy pumps or pumps with large impeller inlet areas. The 3% head drop criteria was selected for the NPSHR value based on the ease of determining the exact head drop off point. Most standard low suction energy pumps can operate with little or no margin above the NPSHR value, without seriously affecting

Most pump manufacturers use the industry standard 3% head drop for NPSHR values and provide the NPSH Margin recommendations separately. A few manufacturers do include the NPSH Margin in their pump NPSHR curves which then supersede the guidelines spelled out in this standard. Unless advised otherwise, however, the user must assume that there is no margin in the published NPSHR, and that it is based solely on the 3% head drop criteria. 9.6.1.2

Suction energy level

The suction energy level of a pump increases with the casing suction nozzle size, the pump speed, the suction specific speed and the specific gravity of the pumped liquid. Anything that increases the velocity in the pump impeller eye, the rate of flow of the pump, or the specific gravity, increases the suction energy of the pump. The suction nozzle size is used for simplicity because it approximates the impeller eye diameter and ties to the rate of flow of the pump. The rpm ties directly to the inlet tip speed of the impeller and relative inlet 1


HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998 velocities, and the suction specific speed is also dependant on rpm and rate of flow. The NPSHR in the suction specific speed is appropriate as a measure of suction energy in that larger impeller eye diameters are normally required for lower NPSHR values, which increases the impeller tip speed.

•

9.6.1.2.1 Suction energy factors

The overlap of the impeller vanes. Overlap values less than approximately 15 degrees, such as found on two or three vane impellers (see Figure 2), can allow the high discharge pressure (energy) to recirculate into the impeller suction at low rates of flow. Overlap is defined as the angular amount that the trailing edge of one vane (low pressure side) overlaps the inlet leading edge of the following adjacent vane (at the outer diameter).

Many factors are known to contribute to the suction energy level, and resulting NPSH margin requirements of a pump, more than used in the above definition. Those used in the above definition are factors which are typically available from standard pump manufacturer’s technical literature. Manufacturers of custom engineered pumps may use alternate evaluation methods to establish NPSH margin requirements and these would supersede the guidelines spelled out in this standard. For general information a list of suction energy factors is provided below: •

•

15°

VANE OVERLAP

The peripheral velocity at the O.D. of the impeller eye. Values below approximately 15 m/sec. (50 ft/sec) are generally considered low suction energy, while values above approximately 35 m/ sec. (120 ft/sec) are considered high suction energy. The suction specific speed of the pump (S = n × Q½/(NPSHR)¾). Suction specific speed values below approximately 8,000 metric (7,000 U.S. units) generally represent low suction energy, while above approximately 23,000 metric (20,000 U.S. units) are considered high suction energy. See Figure 9.6.1.3 for suction specific speeds between these values. (Note: Q is the BEP rate of flow entering the impeller eye. In double suction pumps, use one half total rate of flow. NPSHR is based on 3% head drop at BEP.)

•

The specific gravity of the liquid pumped. The higher the value the higher the suction energy.

•

Thermodynamic properties of the liquid. Cold water has one of the highest energy levels for imploding cavitation bubbles. See section on Electric Power pumps for more details.

•

ROTATION

The geometry of the pump inlet. The greater the variation in velocity across the impeller inlet and the higher the magnitude of velocities, the higher the energy level. For this reason, radial inlets, as found in split case pumps have higher suction energy levels due to the right angle turn in front of the impeller.

IMPELLER

VANE Figure 9.6.1.2

•

The incidence angle between the inlet impeller vanes and the approaching liquid. Typically an impeller is designed to have a “zero” incidence angle at design rate of flow. Higher or lower rates of flow cause a mismatch between the angle of the approaching liquid and the impeller vane inlet tips. The greater the incidence the greater the turbulence and suction energy.

•

The geometry of the inlet piping to the pump. The turbulence (added suction energy) that is generated at the pump inlet from piping turns and large changes in pipe diameter adds to the suction energy at the pump inlet.

•

Operation away from the best efficiency point (BEP) of the pump. At reduced rates of flow the pump may operate in its suction recirculation region. Operation off BEP rate of flow also increases the incidence angle to the impeller vanes, and suction recirculation adds to the suction energy level. See ANSI/HI 9.6.3-1997, Centrifugal and Vertical Pumps for Allowable Operating Region, for more information.

2 Copyright © 2000 By Hydraulic Institute, All Rights Reserved.

HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998 9.6.1.2.2 Suction energy determination This is a complex situation and a single equation or relationship has not been developed, which will accurately tie all of these factors together to predict pump noise, vibration, erosion, and reduced mechanical seal life from cavitation, and the NPSH margin level required to avoid these undesirable effects. Recommended margin ratios can typically range from one to five times the NPSHR value of the pump, with the higher values applying to high and very high suction energy pumps, and continuous operation outside the preferred operating region of the pump. The attached graph (Figure 9.6.1.3) is a simplified method for identifying high suction energy pumps. Pumps above the appropriate suction specific speed curve as shown in Figure 9.6.1.3, are considered high suction energy pumps. Very high suction energy pumps can be defined as pumps whose actual impeller operating speeds are in the range of 1.5 to 2.0 times the values shown in Figure 9.6.1.3, or higher. As an example, an end suction pump with a 10" suction nozzle size and 9,500 suction specific speed is shown to start high suction energy at 1,800 RPM. If this pump were to be operated at 3,600 RPM (2 times 1,800) the pump would be considered to have very high suction energy.

Figure 9.6.1.3A (metric)

It must be stressed that the impeller eye diameter is actually a better factor for identifying the suction energy level of a pump than the suction nozzle diameter. The nozzle size was chosen for Figure 9.6.1.3 because it is more often available to the pump user, and normally has a close relationship to the impeller eye. Therefore, reducing the suction nozzle size, without a corresponding reduction in the impeller eye diameter, will not reduce the true suction energy of a pump. It could even increase cavitation. Generally speaking, high suction energy pumps are susceptible to noise and increased vibration, but will not suffer significant erosion damage (especially with more erosion resistant impeller materials) when sufficient NPSH Margin is not provided. Very high suction energy pumps will more likely experience erosion damage from cavitation under inadequate NPSH margin conditions.

Figure 9.6.1.3B (US units) Notes for Figure 9.6.1.3: •

For two vane impellers and impeller trims with less than 15 degrees vane overlap, (see Figure 9.6.1.2) increase suction nozzle size by one or two sizes before using Figure 9.6.1.3.

•

Inducers, which are generally beyond the scope of this document, should have the suction nozzle decreased by at least one size before using Figure 9.6.1.3.

•

For axial split case (side Suction) pumps, decrease nozzle size by one size, before using Figure 9.6.1.3.


HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998 •

For pump speeds higher than 3600 rpm, the suction nozzle sizes should be increased, proportional to the increase in speed, and enter the graph at 3600 rpm. For example, increase the nozzle size by 2 times if the speed is doubled.

•

For vertical turbine (line shaft diffuser) type pumps, the Impeller Inlet eye diameter should be obtained from the supplier and used as the suction nozzle size, when using Figure 9.6.1.3.

gas may be to quiet the pump, since the cushioning may more than offset the added cavitation. However, with very high suction energy pumps, the force of the collapsing cavitation bubbles may be too great for any real cushioning, so the noise and damage will increase with increasing gas content. •

Additives in the liquid. Additives in the liquid which increase vapor pressure can increase cavitation damage. For example, cooling tower water treatment agents.

•

The corrosive properties of the liquid. This can accelerate the damage.

•

Solids/abrasives in the liquid. Adding abrasives to the high implosive velocities from the collapsing vapor bubbles increases the wear rate.

There are other factors which, although not affecting the suction energy of the pump, will affect the degree of cavitation erosion damage (and sometimes noise) within a pump when sufficient NPSH margin is not provided above the NPSHR of the pump. These nonsuction energy factors are:

•

The duty cycle of the pump. Cavitation damage is time related. The longer a pump runs under cavitation conditions, the greater the extent of damage. Fire pumps, which run intermittently, rarely have a problem with cavitation damage for this reason.

•

9.6.1.4

•

Multistage pumps, such as used for boiler feed and pipeline services, are excluded from this figure due to the typically large shaft diameters in the impeller eye, which distorts the relationship between the impeller eye diameter and the suction nozzle size.

9.6.1.3

•

•

Cavitation damage factors

The impeller material. Rigid plastics and composites are normally the least cavitation resistant materials. Cast iron and brass will experience the most damage of commonly used metals, while stainless steel, titanium and nickel aluminum bronze will have much less damage, under the same cavitation conditions. Pump size. Large pumps (impeller inlets over 450 mm (18 in) in diameter can be more prone to cavitation damage than smaller pumps. The gas content of the liquid. Small amounts of entrained gas (1 to 2%) cushion the forces from the collapsing cavitation bubbles, and can reduce the resulting noise, vibration and erosion damage. The lack of any entrained gas can have the opposite effect. Warmer liquids tend to release less dissolved gas, which increases the noise level of a pump. On the other hand gas can collect in the inlet of a pump which will block portions of the flow area, thus increasing the inlet velocity of the liquid and creating even more cavitation. This increases the apparent NPSHR of the pump. The net result of these two counter effects of gas content on pump noise and vibration will vary based on the suction energy level of the pump. In the case of low to high suction energy levels, the net effect of

NPSH margin ratio recommendations

Field experience is the most accurate predictor of future performance. Table 9.6.1.1 offers suggested minimum NPSH margin ratio guidelines (NPSHA/ NPSHR), within the allowable operating region of the pump (with standard materials of construction). The table is based on the experience of the many pump manufacturers with many different pump applications. Vertical turbine pumps often operate without NPSH margin without damage, but with slightly reduced discharge head. High and very high suction energy pumps that operate with only the minimum NPSH margin values recommended in Table 9.6.1.1 will normally have what is considered “acceptable” seal and bearing life. They may still be susceptible to elevated noise levels and erosion damage to the impeller. This can require more frequent impeller replacement than otherwise would be experienced had the cavitation been totally eliminated. It will typically take an NPSHA of 4 to 5 times the 3% NPSHR of the pump to totally eliminate cavitation. This ratio can reach 20 for very high suction energy pumps, and a low of 2 for some pumps with low suction energy levels. There are studies that show that the maximum cavitation damage can actually occur at


HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998 NPSHA values twice the NPSHR or more for very high suction energy pumps. In addition to the minimum NPSH Margins recommended in Table 9.6.1.1, extra margin may be required to account for changes in the pump geometry which can increase NPSHR, such as wear that can open impeller wearing ring clearances and increase the internal flow through the impeller eye. The NPSHR may also be affected by the gas content of the liquid pumped. Added NPSH Margin may be needed to cover uncertainties in the NPSH available or the actual operating rate of flow. If a pump runs further out on the curve than expected, the NPSHA of the system may be lower than expected and the NPSHR for the pump will be higher, thus giving a smaller (or possibly negative) NPSH Margin. (See ANSI/HI 9.6.3-1997, Centrifugal and Vertical Pumps for Allowable Operating Region). All pumping systems must be designed to have a positive margin throughout the full range of operation. Optimum pump performance also requires that proper suction/inlet piping practices are followed,

according to the Hydraulic Institute Standards (see ANSI/HI 9.8-1998, Pump Intake Design), to ensure a steady uniform flow to the pump suction at the required suction head. Poor suction piping can result in separation and turbulence at the pump inlet, which decreases the NPSHA to the pump and causes added cavitation. NPSHA Margins of two to five feet are normally required (above those shown in Table 9.6.1.1) to account for these uncertainties in the actual NPSHR and NPSHA values, and this added margin requirement could be even greater depending upon the severity of the conditions. If the application is critical, a factory NPSHR test should be requested. NPSH Margins are not normally a consideration for most standard vertical turbine pumps, since they generally have Low Suction Energy, and cavitation noise is normally not an issue. NPSHA must, however, be equal to or larger than the NPSHR over the allowable operating region of the pump, including at low water level. The determination of the minimum submergence required to avoid the formation of sump vortices

Table 9.6.1.1 Minimum NPSH margin ratio guidelines (NPSHA/NPSHR) Suction energy level Application

Low

High

Very high

Petroleum

1.1a

1.3c

Chemical

1.1a

1.3c

Electric power

1.1a

1.5c

2.0c

Nuclear power

1.5b

2.0c

2.5c

Cooling towers

1.3b

1.5c

2.0c

Water/waste water

1.1a

1.3c

2.0c

General industry

1.1a

1.2b

Pulp and paper

1.1a

1.3c

Building services

1.1a

1.3c

Slurry

1.1a

—

Pipeline

1.3b

1.7c

2.0c

Water flood

1.2b

1.5c

2.0c

a) Or 0.6m (2 feet), whichever is greater. b) Or 0.9m (3 feet), whichever is greater. c) Or 1.5m (5 feet), whichever is greater.


HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998 around the pump inlet must be considered independently from NPSHA, since they are a separate phenomena. (See ANSI/HI 9.8-1998, Pump Intake Design).

•

Low Suction Energy Single Stage Overhung, Vertical and Multistage Pumps: For all hydrocarbon liquids use an NPSH Margin Ratio of 1.1.

9.6.1.5

•

High and Very High Suction Energy Single Stage Overhung, Single Stage Double Suction Multistage Pumps: For all hydrocarbon liquids use a NPSH Margin Ratio of 1.3.

Application considerations

9.6.1.5.1 Petroleum process pumps Pumps used for petroleum (hydrocarbon) services can usually survive with relatively small NPSHA margins for several reasons: 1) Processes are typically steady, with few system upsets (transients) or quick flow change demands. 2) Process requirements are typically well known and demands can be planned and predicted. 3) Most hydrocarbon liquids have relatively low vapor volume to liquid volume ratios. This means that, if the liquid should vaporize at or near the pump suction (impeller inlet), the volume of the resulting vapor does not choke the impeller inlet passages as severely as does water vapor during cavitation. This results in a smaller drop in developed head for the same NPSH margin.

The majority of vertical turbine pumps in the petrochemical industry are normally installed in a barrel or can as shown in Figure 2.6 of the Hydraulic Institute standard ANSI/HI 2.1-2.2, Vertical Pumps for Nomenclature and Definitions. The NPSHA must exceed the NPSHR over the expected range of operation. Normally, the customers will give a margin value which will vary from 0 to approximately 1.5m (5 feet). The NPSHA is normally given at ground level or pump inlet level. The manufacturer then determines the length of the pump required to achieve sufficient NPSHA at the first stage impeller inlet to account for the NPSHR, pump inlet losses (inlet to eye of first impeller) and margin. 9.6.1.5.2 Chemical process pumps Pumps for these applications frequently share the following characteristics: 1) Operation frequently occurs at a wide variety of rates of flow.

4) Less energy is released when hydrocarbon vapor bubbles collapse (velocity from implosion is less), and this means less damage occurs as a result of cavitation. It is, therefore, not as critical that cavitation be avoided, as might be the case with other liquids.

2) Materials of construction are often stainless steel impellers. 3) They may operate with relatively low NPSHA.

Hydrocarbon liquids, especially mixtures of hydrocarbon liquids, because of their relatively low vapor volume, are sometimes associated with a “hydrocarbon correction factor.” This “correction factor” is applied to the water NPSHR values to “correct” for the fact that the vapor volume of “flashed” hydrocarbon liquid is substantially less than that of “flashed” water and, thus, has the effect of reducing the amount of NPSH required by the pump at a given rate of flow before cavitation results in a 3% drop in the developed head (first stage head) of the pump. This favorable vapor bubble size situation with hydrocarbons should be taken into account when determining the NPSHA Margin requirements for petroleum pumps. The margins can be lower than for other applications. Typical NPSH Margins for pumps on hydrocarbon services are as follows:

4) Operators are frequently located remotely from the pumps. These factors emphasize the need to apply large NPSH margins when selecting pumps. Taking these issues into consideration, the following NPSH Margin guidelines are proposed for Chemical Process pumps to account for the many uncertainties: •

For low suction energy pumps, the margin should be 10% of the NPSHR or 0.6m (2 ft), whichever is greater.

•

For high suction energy pumps the margin should be 30% of the NPSHR or 1.5m (5 ft), whichever is greater. NPSH tests are recommended if the


HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998 pump specific speed is above 2,300 metric (2,000 U.S. units). If a pump is applied to the right of BEP, careful consideration should be given to ensuring that, at the maximum flow rate permitted by the system, and its controls, the NPSHA is in excess of the NPSHR of the pump. If the above criteria cannot be met and there is no prior experience with the specific pump in the application, NPSH tests should be conducted on the pump. One test should be conducted at the rated conditions and must demonstrate that the NPSHR (3%) is equal to or less than the rated NPSHR. Tests should also be conducted at four additional rates of flow at approximately even intervals from the minimum to maximum anticipated rates of flow to fully define the NPSHR (3%) characteristic curve. 9.6.1.5.3 Electric power pumps Power plant pumps are water pumps. Cold water is one of the most difficult liquids to pump in that cavitation can cause severe damage. Unlike hydrocarbon liquids handled by petroleum pumps, water, when it vaporizes (flashes), expands tremendously. This results in higher impact velocities when the vapor bubbles implode, thus higher suction energy. One pound of water at room temperature which occupies 4.5×10–4 cubic meters (0.016 cubic feet), will flash to over 34 cubic meters (1200 cubic feet) of vapor. This is a volume ratio of 75,000 to 1. For typical hydrocarbon liquids, this volume ratio is one-half to one-tenth that of water. Hot water, on the other hand, can act similar to hydrocarbon liquids. When water is heated to 250-300° F, the vapor volume characteristics become similar to that of a typical hydrocarbon. This means that the effects of flashing are diminished; however, the opportunities for system transients increase significantly with temperature.

system upsets, or transients, for flashing to occur in the suction line to the pump, causing loss of suction flow and allowing the pump to “run dry”. A common side effect of a pump running dry is rapid mechanical seal face wear, general seal deterioration and premature, sometimes catastrophic failure. Other pumps in the power plant are not usually exposed to such severe transients as those in the boiler water system. Condensate pumps and heater drain pumps are usually isolated from severe system upsets. They too, however, have special demands or operating requirements which impact on NPSH and NPSH Margin requirements. Since they are typically required to operate with very low NPSHA, they are designed to function, and survive, with a certain amount of cavitation present. Some systems operate on what is termed “cavitation control,” i.e. the pumps operate with cavitation at all times. In such a system, the pump is constantly under some degree of cavitation which results in a reduced pump developed head. The quantity of flow through the pump, and system, is “controlled” by the intersection of the pump reduced head–rate-of-flow curve and the system curve. For such an application, there is no NPSH margin; and the pump must be designed to withstand constant cavitation. This means it must be of rugged construction to offset the detrimental effects of cavitation related vibration, and the materials of construction must be capable of withstanding the erosion associated with cavitation. Vertical turbine type pumps used as condensate pumps are normally installed in a barrel or can as shown in Figure 2.6 of ANSI/HI 2.1-2.2, Vertical Pumps for Nomenclature and Definitions. 9.6.1.5.4 Nuclear power/cooling tower Pumps in nuclear power plants share the following characteristics and requirements: a) Nuclear Reactor Duty:

In addition to possible severe vaporization effects, typical power plant operating cycles are not stable. Most pumps in these services do not remain at constant flow rates for extended periods of time. The pump flow demands vary widely with power demands. Because of varying power demands, system upsets may occur which result in rapid changes in pump flow demands and, many times, severe changes in pump suction pressure. This is especially true for pumps in the boiler water systems such as boiler feed pumps and boiler feed booster pumps. It is not unusual, during such

1) Users are more frequently requesting NPSHR curves based on a 1% head drop. 2) The NPSHA Margin, over NPSHR (3%), is often incorporated in the NPSH Required curve by the manufacturer. 3) High horsepower reactor cooling pumps, also called primary heat transport pumps, are of low to high suction energy levels.


HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998 4) Reactor cooling pumps normally operate at high temperatures and suction pressures, they will operate at ambient temperature and low suction pressures during transients and commissioning. b) Boiler Feed Duty: 1) NPSHR based on a 3% head drop is specified. 2) Suction energy levels are between low to very high. c) Cooling Tower Duty: Cooling tower water typically has modified chemistry due to water treating agents. These additives can increase the vapor pressure, which results in a lower NPSHA than calculated for pure water. This reduction can be as high as 1.8 meters (6.0 feet), although the exact number must be experimentally determined. 9.6.1.5.5 Water/wastewater The following considerations apply to pumps for this application: 1) During variable speed operation, all possible wet well levels, pump speeds, and rates of flow exist. It is important that the pump can function properly over the full operating range of the system curve. A flow duration diagram can be used to determine where the pump will operate most frequently. In the on/off mode of operation, the speed and rate of flow will be relatively constant, but the sump level will vary between a maximum water level and a minimum water level. The change of the water level in the sump will also change the pump’s total head, also slightly changing the speed and rate of flow of the pump, but the duty point will be nearly constant compared to variable speed operation. 2) Actual system head curves often differ from the calculated values. This will cause the NPSH Margin calculation to be incorrect. It is, therefore, very important to ensure that the calculated system head curves be as close as possible to the actual. For existing systems, it is possible to measure the head at a number of points to develop the system head curve. Two system curves should be calculated for new installations: one for the system as it will

be installed; and a second to represent the condition of the system after some increase in pipe roughness has occurred. 3) Many pumps are installed in wastewater applications with elbows mounted in front of the impeller eye. When suction elbows are necessary, it is best to use reducing or long radius elbows. 4) Materials of construction are typically cast iron (wastewater) or cast iron / bronze fitted (water). These materials are preferred for water/wastewater, but they do not stand up well under heavy cavitation. The protective layer that is built up under normal operation is destroyed by cavitation, causing abnormal material removal rates. It is advisable to change to tougher materials such as stainless steel or aluminum bronze alloys if the pump must withstand destructive cavitation levels, however, this will not help the seals or bearings. 5) Pump stations often operate unattended, and the malfunction of a pump must be avoided. A failed pump station processing water or wastewater will cause considerable inconvenience to the public, and should be designed to be as trouble-free as possible. 6) Single, two and three vane impeller designs are common in wastewater applications, with no or minimal vane overlap. Increase the suction nozzle sizes by one or two sizes for pumps with one to three vane impellers before using Figure 9.6.1.3. 7) Vertical Turbine barrel or can type pumps on water booster services are generally applied with little or no NPSH Margin, since they are mostly low suction energy applications. The above items are listed to illustrate the uncertainies related to the NPSHA calculations, and at the same time demonstrate the importance of accuracy when determining the required NPSHA. It seems as though the simple answer would be to over-compensate by adding margin on top of margin, guaranteeing that the pump would run far from the point of cavitation. Even though an excessive amount of NPSHA is often not detrimental to the pump, putting margin on top of margin would add to the cost of the pump stations. It is also important to note that there are a number of people involved in the supply chain from the specifier to the end user, and each one may add a margin of their


HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998 own. Some pump manufacturers include a margin in their published NPSHR curves. If everyone was to add a margin, the result of this excess margin would increase the cost of the pump stations dramatically.

NPSH Margins are suggested for stock consistencies up to 6%: •

For Low Suction Energy pumps use an NPSHA Margin Ratio (NPSHA/NPSHR) of 1.1 or a margin of 0.6m (2 ft), whichever is greater.

•

For High Suction Energy pumps, or pumps having Specific Speeds greater than 2300 metric (2000 US units), use an NPSHA Margin Ratio of 1.3 or a margin of 1.5m (5 ft), whichever is greater.

9.6.1.5.6 General industrial Pumps for this application are used to pump a great variety of liquids, ranging from water to concentrated chemicals. These pumps are often sold as standard catalog, pumps. They are generally low suction energy designs.

9.6.1.5.8 Building services Due to the variety of liquids pumped through an extreme range of temperatures, the specifier must carefully calculate the NPSHA in the system, taking into account the vapor pressure of the liquid at the extreme operating temperature. The use of hose connections and the associated piping bends must be accounted for. The use of hose or tubing connections with internal diameters smaller than the pump suction inlet should not be used on the suction side of the pump. NPSHA on tank draining applications should be calculated for the lowest possible level of the liquid in the tank during the pumping process. Another consideration in the NPSH Margin of catalog type pumps is the common changes in flow rates experienced during process changes, as well as the physical expansion of process systems to meet higher production rates. In general an NPSHR versus rate of flow curve has a parabolic shape. This may cause large changes in NPSHR especially if the pump is being run to the right of the best efficiency point. Due to the low suction energy of most general industrial pumps, operation of the pump without any NPSH margin does not normally cause substantial damage to the internal components of the pump. Typical problems are frequent replacement of the mechanical seal as well as the front motor bearings (on close coupled pumps) due to the intense vibration caused by the collapsing bubbles, when in fully developed cavitation. 9.6.1.5.7 Pulp and paper For horizontal end suction stock process pumps situated close to the suction chest, and operating in the continuous allowable operating region, it is normal to add sufficient NPSH Margin to account for the uncertainties in the actual NPSHR and NPSHA from poor suction piping and entrained air. The following minimum

Fluid systems for the building trades or HVAC Industry are comprised of both closed and open pumping systems. NPSH is generally not a concern when designing closed pumping systems. The typical closed system is filled and then pressurized to a “fill” pressure of 4 to 10 psig. If an inadequate NPSH available (NPSHA) condition should occur, it can usually be remedied by increasing the fill pressure. For open systems, NPSH margin is a very important consideration. As a guideline, the NPSHA for open systems should exceed the pump manufacturer’s stated NPSH-required (NPSHR) by a minimum of 0.6m, (2 ft) or 1.1 times the NPSHR for Low Suction Energy Pumps. For High Suction Energy pumps the margin ratio should be increased to at least 1.3, or a minimum of 1.5m (5 ft). Pumps operating at these established minimum NPSH margins may experience some degree of impeller erosion and/or noise but these effects should be minimal. System construction may contribute to the problem of noise, and cavitation. Increasing the NPSH margin will improve pump operation and reliability. 9.6.1.5.9 Slurry Pumps used in slurry service are frequently constructed of either hard metals or elastic materials. It is also common for the slurry concentration and flow rates to change rapidly, imposing significant loads on the impeller, shaft and bearings. Because of this, and the erosive nature of many slurries, slurry pumps are of an extremely rugged design, making them relatively insensitive to the mechanical effects of cavitation. Also, to minimize erosive effects, slurry pumps often operate at low speeds (less than 1200 RPM). As a result of this, they normally fall into the Low Suction Energy category, and have NPSHR values below 6m (20 ft).


HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998 vii)

Slurries are typically water based and at ambient temperatures. Suction flow is usually gravity fed. Consequently the NPSHA is normally in excess of 9m (30 ft), giving NPSHA/NPSHR ratios in excess of 1.5.

9.6.1.5.11 The recommended NPSH Margin Ratio for slurry pumps is 1.1 or a margin of 0.6m (2 ft) whichever is greater. For applications where the margin is less, characteristics of the slurry, and the NPSHR performance of the pump, should be reviewed to assure satisfactory performance. 9.6.1.5.10

Pipeline

For this paper, pipelines are defined as hundreds of miles in length for the transport of hydrocarbons or water. Pumps used for pipeline service normally share the following application criteria: 1) Customers more often request the NPSH “Required” values to be based on a 1% head drop. 2) The NPSHA Margin, over NPSHR (3%), is often incorporated in the NPSH “Required” curve by the manufacturer. 3) Some pipeline designers and operators request two NPSH Required curves. One being the conventional NPSHR curve based on a 3% head drop, and a second based on the NPSH required to guarantee a 40,000 hour impeller life. 4) Specifications frequently require that the NPSHA exceed the NPSH “Required” (40,000 hrs) over the full Allowable Operating Region for the pump (Minimum to Maximum Flow). 5) There is no standard method for determining the NPSH “Required” for 40,000 hours impeller life, however it is a function of: i)

Suction Energy Level.

ii)

Material of impeller.

iii)

Acidity of pumpage (pH).

iv)

Temperature of pumpage.

v)

Suction Specific Speed.

vi)

Operating rate of flow vs pump best efficiency point.

The NPSH “Required” (0%) vs NPSHR (3%) ratio throughout the Allowable Operating Region flow range. Waterflood (injection) pumps

Water injection pumps for flooding of oil wells typically operate against relatively constant systems. The system requirements vary with time, but normally these variations are gradual and do not impact on operating NPSH Margins. For sizing of the pumps initially, NPSHR considerations are based on a) expected flow rate requirements (changes) over the planned life of the injection project and b) the nature of the suction source for the pumps. Assuming that any changes in the nature of the suction source would also be gradual, the NPSH Margins required by the pumps are relatively small in order to ensure satisfactory, consistent pump performance. Typical NPSH Margins for injection pumps are set based on the following criteria, considering variations which could occur during the life of the injection project: 1) Pump NPSHR at maximum expected flow rate. 2) Minimum NPSHA expected at this maximum flow rate. The NPSH Requirement based on 40,000 hours minimum impeller life is being requested more frequently in this market. 9.6.1.6

Summary

In summary, the following key points should be understood about cavitation in a centrifugal pump, NPSH Margin requirements, and how they are affected by the Suction Energy level of the pump: •

Cavitation exists when NPSHA is at and substantially above the NPSHR of a pump.

•

The Suction Energy level of a pump (as installed in a system) determines if the cavitation that frequently exists in a pump will cause noise, vibration and/or damage to the pump.

•

Low Suction Energy pumps can normally operate at or near their NPSHR with little or no problems from cavitation, except for the 3% head drop.


HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998 •

High Suction Energy pumps are likely to be noisy with higher vibration and will possibly experience less than optimum pump life, if sufficient NPSH Margin is not provided.

•

High Suction Energy pumps are more susceptible to problems from poor suction inlet piping.

•

Entrained air, or dissolved air which comes out of solution in the impeller eye, can quiet the noise and vibration of High Suction Energy pumps at low NPSH Margins.

•

Very High Suction Energy pumps will be noisy, will have high vibration and are likely to experience reduced pump life if sufficient NPSH Margin is not provided. Very High Suction Energy pumps are very susceptible to problems from poor suction inlet piping.




HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin Index — 1998

Appendix A Index

This appendix is not part of this standard, but is presented to help the user in considering factors beyond this standard. Note: an f. indicates a figure, and a t. indicates a table. Additives in liquid, 4 BEP See Best efficiency point Best efficiency point, 2 Building services pumping systems, 9 Cavitation, 3, 6, 10 damage factors, 4 Chemical process pumps, 6 Cooling towers, 7 Corrosive properties of liquid, 4 Electric power pumps, 7 Gas content, 4 Impeller eye diameter, 3, 4 Impeller material, 4 Impeller vanes incidence angle, 2 overlap, 2f., 2 Industrial pumps, 9 Inlet geometry, 2 Inlet piping geometry, 2 Multistage pumps, 4 Net positive suction head available, 1, 1f. Net positive suction head margin See NPSH margin Net positive suction head required, 1, 1f. NPSH margin, 1, 10 building services pumping systems, 9 chemical process pumps, 6 cooling towers, 7 defined electric power pumps, 7 general industrial pumps, 9

guidelines, 4, 5t. nuclear power pumps, 7 petroleum process pumps, 6 pipeline pumps, 10 pulp and paper pumps, 9 ratio, 1 slurry service pumps, 9 and vertical turbine pumps, 6 water/wastewater pumps, 8 waterflood (injection) pumps, 10 NPSHA See also Net positive suction head available NPSHR See Net positive suction head required Nuclear power pumps, 7 Peripheral velocity, 2 Petroleum process pumps, 6 Pipeline pumps, 10 Pulp and paper applications, 9 Pump duty cycle, 4 Pump size, 4 Slurry service pumps, 9 Solids/abrasives in liquid, 4 Specific gravity, 2 Suction energy, 10 determination, 3, 3f. factors, 2 Suction energy level, 1 Suction specific speed, 1 Thermodynamic properties, 2 Vertical turbine pumps, 6 and inlet eye diameter, 4 and NPSH margin, 6 Water/wastewater pumps, 8 Waterflood (injection) pumps, 10


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ANSI-HI-9-6-1 Centrifugal and Vertical Pumps for NPSH Margin

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