CHE10709 Air Compressors

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Compressed Air Systems

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Process File Reference: CHE10709

For additional information on this subject, contact R. A. Al-Husseini on 874-2792

Engineering Encyclopedia

Process Compressed Air Systems

CONTENTS

PAGES

INTRODUCTION ....................................................................................................................... 1 EQUIPMENT TYPES AND APPLICATIONS........................................................................... 2 Air Compression Package.............................................................................................. 2 Inlet Air Filter and Silencer ........................................................................................... 3 Compressor Type........................................................................................................... 4 Compressor Type Characteristics .................................................................................. 5 Compressor Controls ..................................................................................................... 5 Aftercooler..................................................................................................................... 8 Air Receiver................................................................................................................... 8 Air Dryer Installation................................................................................................... 10 Distribution Systems ................................................................................................... 11 Air Balances ................................................................................................................ 12 SYSTEM DESIGN CONSIDERATIONS ................................................................................. 14 System Demand........................................................................................................... 14 Typical Air Requirements............................................................................................ 15 Number and Size of Compressors ............................................................................... 15 Compressor Driver Selection....................................................................................... 15 Monitoring System Operation ..................................................................................... 16 Existing System Optimization ..................................................................................... 16 WORK AID 1: RELATIVE COMPRESSOR COSTS .............................................................. 18 WORK AID 2: FACTORS AFFECTING COMPRESSOR SELECTION ................................ 19 WORK AID 3: COMPARISON OF COMPRESSOR TYPE FEATURES ............................... 20 WORK AID 4: MOISTURE CONTENT OF COMPRESSED AIR .......................................... 21 WORK AID 5: TYPICAL PLANT AIR REQUIREMENTS FOR PNEUMATIC TOOLS*.................................................................................................................................... 23 WORK AID 6: INSTRUMENT AIR CONSUMPTION ........................................................... 24 GLOSSARY.............................................................................................................................. 25 REFERENCES.......................................................................................................................... 26 Saudi Aramco Standards.............................................................................................. 26

Saudi Aramco DeskTop Standards



Saudi Aramco Design Practices ................................................................................... 26 Exxon Basic Practices.................................................................................................. 26




INTRODUCTION See Saudi Aramco Standard SAES-J-901. There are several uses for compressed air in a plant. Many valves and instruments are pneumatic. Maintenance and construction tools often require compressed air as a power source. Air is required in some processes. It is used for decoking furnaces and for regenerating some catalysts and desiccants. Figure 1 shows a typical compressed air system with two air compressors. Each compressor has inlet air filters and silencers and outlet air coolers. There is an air receiver where some moisture is removed. The receiver acts as a surge drum in the system to maintain system pressure during short outages.

Figure 1. Compressed Air System Simplified Flow Plan Downstream of the air receiver, the system splits into two systems. One is called the utility air system. This air is distributed without drying. The other is called the instrument air system. This air is filtered and dried before distribution.


1



EQUIPMENT TYPES AND APPLICATIONS The equipment included in a compressed air system consists of the air compression package, air coolers, air receivers, air dryers, and distribution piping.

Air Compression Package Components that are typically included in an air compression package are: • An air filter and silencer •

A compressor and gear if required

•

A driver with a coupling

•

Intercoolers with moisture separators and automatic water removal

•

A lube oil system

•

Suction throttle valve and discharge check valve

•

Discharge blowoff valve and silencer

•

Vibration monitoring system

•

Controls and instrumentation

Figure 2 shows how these components are connected. This illustration is for a centrifugal compressor, which requires a suction throttle valve and a discharge blowoff valve. The suction throttle and discharge blowoff valves are not required for a reciprocating compressor.


2



Figure 2. Compressed Air System Air Compression Package Inlet Air Filter and Silencer An inlet air filter removes dust, dirt, sand, and other abrasive or gritty particles. Filtering prevents damage to the compressor and minimizes maintenance and downtime. The silencer removes the objectionable air inrush noise.


3



Compressor Type There are two ways to increase the pressure of a gas. One is to reduce the volume of the gas. The other is to increase the velocity of the gas. Positive displacement compressors reduce the gas volume. There are several different types of positive displacement compressors. They include: • Reciprocating •

Rotary or helical screw, or rotary lobe

•

Sliding vane

•

Liquid piston

•

Diaphragm

Of these, reciprocating compressors and rotary screw or helical screw compressors are most often used in gas plant and refinery compressed air systems. Centrifugal compressors and axial compressors increase pressure primarily by increasing the gas velocity. Centrifugal compressors are more often used in compressed air systems. Capacity and discharge pressure are the key factors in selecting a compressor. Helical screw compressors can have a high capacity but are limited in discharge pressure to about 250 psig. Reciprocating compressors can have a high discharge pressure but are limited in capacity. A typical guideline is to consider reciprocating compressors up to 1,500 SCFM and centrifugal or screw compressors above 2,000 SCFM. Saudi Aramco prefers centrifugal compressors above 900 SCFM. All instrument-air compressors in the Saudi Aramco facilities must be of the oil-free type. Work Aid 1 gives an approximation of the relative cost of various types of air compressors versus the compressor capacity in SCFM. Other factors influence the selection of a type of compressor. These include: • Efficiency •

Noise

•

Sensitivity to fouling and solids

•

Availability or reliability

•

Maintenance requirements and costs


4



Compressor Type Characteristics Reciprocating compressors tend to have higher maintenance costs, lower availability, and the poorest sensitivity to fouling and solids. However, they are lower in cost in smaller sizes. Centrifugal compressors tend to have lower maintenance costs, higher availability, higher efficiency, and lower noise levels. They are also lower in cost in large sizes. Work Aid 2 summarizes the main factors to be considered in selecting a compressor type. Work Aid 3 compares the features of five types of compressors on a scale of 1 to 10. The final selection depends on the result of an over all economic analysis.

Compressor Controls Controls are provided in an air compressor package to: • Regulate the flow of air to match the demand •

Prevent surging of a centrifugal compressor

•

Regulate the discharge pressure

•

Automatically start spare compressors

•

Protect the compressor and driver from damage

Two methods are commonly used for regulating the compressor flow and discharge pressure. The conventional method with reciprocating compressors is a multistep load and unload method. With a centrifugal compressor, a two-step load and unload operation is also possible. However, capacity modulation with suction throttling has several advantages. Most variations in air demand can be accommodated within the throttling capacity of the control system. A constant discharge pressure can be maintained. Power demand is lower because no air is wasted down to the surge point, or about 70% of capacity. There are power savings down to zero air delivery.


5



Figure 3 illustrates a control system for two reciprocating air compressors, one motor-driven and one steamturbine-driven. A common discharge pressure controller loads or unloads the cylinders of the operating compressor to control the discharge pressure. A hand selector switch is included to indicate which compressor will normally run and which will be on standby. A low-pressure cut-in control is included to start the standby compressor if discharge pressure drops below a set point. In this figure, it is set at 110 psig.

Figure 3. Compressed Air System - Simplified Control System for Reciprocating Compressors Another pressure controller will start to shut off air to the utility air system if the discharge pressure drops to 100 psig. It will shut off utility air completely if the discharge pressure drips to 90 psig.


6



Figure 4 shows a different control scheme for two centrifugal compressors. One is motor-driven and the other is steam-turbine-driven. The common discharge pressure controller throttles the suction of the operating compressor to control the discharge pressure by controlling the air flow. On the discharge of each of the compressors, a flow recorder controller (FRC) is set 10% above the surge flow of the compressor.

Figure 4. Compressed Air System - Simplified Control System for Centrifugal Compressors If the suction throttle valve reduces the compressor air flow to the FRC setting, a discharge blowoff valve will start to open to ensure that flow stays above the surge point.


7



This control scheme also has a hand selector switch to select the normal and standby compressor and automatic cut-in of the standby compressor on low discharge pressure. The pressure cutoff for the utility air system is the same as for the reciprocating compressor.

Aftercooler The function of an aftercooler is to cool the compressed air after it has been compressed. Saudi Aramco specifies that compressed air must be cooled to 140°F or below. This cooling will condense up to 60% of the incoming water vapor. It will reduce the volumetric flow of air to downstream equipment and protect downstream equipment from overheating. Two types of aftercoolers are in common use. One is shell-and-tube exchangers using cooling water or air fins where ambient temperatures are low enough. Water-cooled exchangers are more common than air-cooled. They can be used in any climate, are usually less expensive, take less space, and are less noisy. Air cooled exchangers save water and generally require less maintenance. However, they are more expensive. Also they are limited in cooling to about 15°F above ambient temperature or higher.

Air Receiver An air receiver provides continuity of air flow during surges in demand, compressor trips, and loading and unloading. An air receiver provides a large volume to entrap and remove condensed water vapor and oil. It will also provide some time (usually a minimum of one minute and often two to three minutes) for operators to take corrective action following loss of all compressors. A typical standard for sizing an air receiver is to provide a minimum of one minute of base instrument air load while the air receiver pressure decays from 100 psig to 50 psig. This standard applies to systems that have standby compressors started automatically. For systems with manual-start compressors, this time can increase to as much as 15 minutes. It is common to have only one or two air receivers at an air compressor house where several compressors are installed. However, in some plants, air receivers are installed at other locations. For example, at Saudi Aramco, receivers are installed at most air-operated valves.


8



The following equations are used to determine the volume of an air receiver.

=

V

V = Volume of receiver, ft3 t = Time in minutes the receiver will supply air from upper to lower pressure limits C = Air flow rate, SCFM T = Air temperature, °Rankine P1 = Upper pressure limit, psig P2 = Lower pressure limit, psig For example, if the air demand for a Saudi Aramco plant is 3,000 SCFM, the receiver volume would be calculated as follows: t = 1 minute

Where

C

=

3,000 SCFM

T

=

140°F + 460 = 600°R

P1

=

100 psig

P2

=

50 psig

V

=

V

=

Work Aid 4 is a nomogram for determining the amount of moisture that enters an air compressor and the moisture remaining in air leaving the air receiver. The nomogram is based on isothermal compression; that is, the air leaving the air receiver is assumed to be the same temperature as the air entering the compressor.


9



For example, assume inlet air is 120°F at 40% humidity and air leaving the aftercooler is also at 120°F and 125 psig. To use the nomogram, first connect 120°F on the T scale with 40% on the R scale. Where the line crosses the M-1 scale, read the moisture per 1,000 SCF of inlet air. This is about 2.0 lb per 1,000 SCF. Next, align 2.0 on the M-1 scale with 125 psig on the P scale. Read the moisture in 1,000 SCF of 125 psig air at 120°F. This is about 0.2 lb per 1,000 SCF. That is, the air now contains only 10% of the water vapor originally taken into the compressor (0.2 divided by 2.0) x 100 = 10%. Therefore, 90% of the moisture was removed by intercooling and after cooling to 120°F.

Air Dryer Installation An air dryer installation usually includes a prefilter, a dryer, and an afterfilter. The total installation pressure drop should be less than 5 psi at maximum design flow rates. The prefilter removes liquid oil and water and solids to protect the dryer desiccant. The prefilter is usually designed to remove 98% of all oil droplets greater than 1 µm in diameter. Oil retention is usually 2 lb for each 100 SCFM of design capacity. The filter material typically is activated carbon or alumina. The filter should have an automatic drain. The afterfilter removes fragmented or pulverized desiccant from the dried air. It is designed to remove 100% of all particles larger than 1 µm in diameter. The afterfilter is usually a dual cartridge filter to allow cleaning without shutdown or bypassing. An air dryer keeps compressed air, particularly instrument air, free of water. This is necessary to avoid instrumentation malfunction and damage. Dry air keeps maintenance low, reduces pneumatic equipment downtime, and minimizes upsets in temperature controls, flow, and other process units. The four main types of air dryers are heat-regenerated absorption, non-heat-regenerated absorption, refrigeration, and rotary absorption dryers. The heat-regenerated and non-heat-regenerated absorption dryers are similar except for the regeneration system. Both types consist of two desiccant filled chambers connected in parallel. The desiccant in one of the chambers dries the air stream while the desiccant in the other chamber is being regenerated. In the heat-regenerated dryer, air is heated with medium-pressure steam or other heat medium. The hot air then is used to purge moisture from the spent desiccant. Silica gel or alumina is used as a desiccant. These dryers are relatively expensive to purchase and to operate.


10



In the non-heat-regenerated dryer, regeneration is carried out under vacuum using dry air for purging. Purging can require from 3 to 15% of the dryer capacity. Saudi Aramco prefers heatless regeneration, desiccant-type dryers. The rotary absorption dryers dry by the chemical reaction of desiccants to form hydrates or hydroxides. They are rarely if ever used to dry plant air. The design dewpoint for air from a dryer is often specified as 20°F below the minimum recorded temperature at a plant. At Saudi Aramco, the design dewpoint is –4°F. Dewpoint is one of the key factors in selecting the type of air dryer in many locations. The table below gives typical dewpoints that can be obtained by the various dryer types:

Dryer Type Refrigeration

Minimum Dewpoint (°F) at 125 psig 35

Heat-regenerated absorption

–40 to –60

Non-heat-regenerated absorption

–40 to –100

The number of compressors and dryers varies greatly from plant to plant. Some large industrial plants have only two air compressors and one air dryer. At Ras Tanura, there are two compressor houses, North and South, each with five compressors. Three compressors in each house are motor driven and two are 600-psig steam-turbine driven. In the North House, the compressors are 1,500 SCFM each, and there are three 1,500 SCFM dryers. In the South House, the compressors are 1,700 SCFM each, and there are two 1,700 SCFM dryers. In addition to these compressors, there is one special 900 SCFM emergency diesel-driven compressor for utility air.

Distribution Systems In most industrial plants with compressed air systems there are at least two separate systems. One supplies instrument air, which is dried. The other supplies plant or utility air, which is not dried.


11



At Ras Tanura, there are two instrument air systems; one is for utility instruments and the other is for refinery instruments. Instrument air is dried and oil free. In Saudi Aramco, the instrument air systems operate between 125 psig maximum and 75 psig minimum. The instrument air systems are looped so that sections of the distribution piping can be removed from service without cutting off instrument air. At Ras Tanura there are two additional air systems that are not dried: the plant air system and the process air system. Plant air is used for pneumatic tools, maintenance, and at utility stations. Process air is used for decoking, catalyst regeneration, and other process requirements. Normally, instrument and plant air are distributed at the same pressure. There are exceptions at some plants for different specific reasons. The distribution system usually is sized to provide a minimum of 75 psig at the most distant consumer from the compressed air source at maximum demand flow rate. The total pipeline pressure drop should not exceed 5 psi at this maximum demand flow rate.

Air Balances Plant air balances are very useful for designing, operating, and analyzing a plant air system. Balances should be prepared for all normal and extreme situations including: • Normal operation •

Peak demand

•

Upset and emergency situations

•

Seasonal variations, if any

•

Turnarounds and regenerations

An air balance summarizes the air production and air consumption unit by unit. The air balances are used for: • Establishing or verifying the capacity of various system components •

Developing operating techniques to handle upsets and emergencies

•

Establishing a basis for selecting a load-shedding scheme

•

Optimizing the use of air within a system

•

Checking the operating flexibility of a system


12



An example of an air balance is shown on here.

Consumer AF 1 AP 1 VP 1 NH 1 SL 1 PF 1 AP 1 Treating CC 1 CL 1 AL 1 AM 1 SU 1 CC 1 Treating OF 1 OM 1 & 2 UP

Instrument Air SCFM Maximum 40 150 100 35 70 175 15 290 100 160 60 5 60 59 70 270 1,659

Plant Air SCFM Normal Maximum

1,060 575 965 960

1,440

210

2,640

425 34 ____ 1,629

900 4,069

From this balance, the base system air demand would be the sum of the instrument air maximum and plant air normal loads or 1,659 + 1,629 = 3,288 SCFM. The maximum demand on the system would be the sum of the maximum instrument air load and the simultaneous maximum plant air load or 1,659 + 4,069 = 5,728 SCFM.


13



SYSTEM DESIGN CONSIDERATIONS System Demand The compressed air system demand includes air requirements for instruments, maintenance, and processes. The instrument air demand is usually a relatively steady load. It is estimated on a unit-by-unit basis. The maintenance air demand is sometimes called utility air, plant air, or yard air. It is a highly fluctuating load. Demand estimates are usually not precise. The maintenance air demand often peaks during a plant or unit turnaround. The process air demand may also be highly fluctuating. It is normally estimated by the designers of the individual unit. It can be included as part of the utility, plant, or yard air systems. Process air demands often peak during a decoking or regeneration operation. To establish the compressed air demand, you need to tabulate normal and maximum instrument and plant air loads. Add appropriate load growth factors to account for anticipated or unexpected future loads. Also add reserve or spare capacity factors to account for inaccuracies in estimates. Saudi Aramco standards call for a minimum reserve air space capacity of 50% for plant instrument air if 80% of a plant's control loops are electronic. This spare capacity drops to 20% if 80% of the control loops are pneumatic. Also, where heatless dryers are used, an additional 20% of the plant capacity is required to provide for dryer regeneration. In addition, Saudi Aramco requires additional compressor capacity to account for compressor wear. For centrifugal compressors, 10% additional capacity is required. For reciprocating compressors, 50% additional capacity is required. Nonessential or less critical loads can be shed as a means to protect more critical loads. For example, Ras Tanura has two instrument air systems, a plant air system and a process-air system. If a problem results in loss of compressed air pressure, a load-shedding sequence is started. At 90 psig the plant air system is shed. If pressure continues to fall, the process air system is shed at 80 psig. Next, the refinery instrument air system is shed at 75 psig. This keeps the utility instrument air system in service as long as possible.


14



Typical Air Requirements Work Aids 5 and 6 list some typical values for instrument air consumption and plant air consumption. You can use Work Aid 6 when better information is not available. Work Aid 5 gives typical air requirements for various pneumatic tools.

Number and Size of Compressors In general, economics favor using a small number of high-capacity compressors. This requires minimum space. Control and operations are easier. High capacity compressors are available at a lower per unit cost than smaller compressors. Many new industrial plants will have only two 100% or three 50% capacity compressors. This is not always true. For various reasons, such as staged expansion of a plant, a large geographical area, or for reliability, more compressors may be installed. The size of the compressed air system is very important. If a system is too small, it will not be able to maintain the required pressure at maximum demand. If it is too large, it will be inefficient at partial load. The system should be able to handle the peak air demand and the base air demand efficiently. The peak demand is the sum of the maximum instrument air demand and the maximum plant air demand. The base demand is the sum of the maximum instrument air demand and the normal plant air demand. It is customary to meet the peak demand with all installed compressors operating without load shedding. This peak demand is intermittent. It is often scheduled. If the peak demand can be scheduled enough in advance, a portion could be met by rental compressors or compressors brought in from other locations. It is also customary to design to meet the base demand with the largest compressor in the system out of operation for maintenance. If a peak demand should occur with the largest compressor out of service, load shedding of less critical loads would be necessary.

Compressor Driver Selection Common drivers used with a compressor are electric motors, steam turbines, and diesel or gas engines. The selection of driver types is based on system reliability, economics, compatibility with the plant steam balance, and the availability and reliability of an electric power supply. Typically, at least one motor driver and one steam turbine driver are selected to improve the overall system reliability. Often, a standby engine-driven compressor is also installed to provide air during a total steam and power outage.


15



Monitoring System Operation The significant parameters to monitor in a compressed air system are the moisture in the air, the air pressure and temperature, and the air compressor operation. Each air system will have a continuous moisture or dew point analyzer to monitor air dryers. It will indicate the moisture removal efficiency of a regenerated dryer and when a dryer should be regenerated. Spot checks should also be made of the moisture content of the inlet air as well as at key points throughout the instrument air system. Spot checks can indicate potential problems such as a collection of water in distribution system low points. Each main plant air receiver will have pressure instruments and a low pressure alarm to indicate whether the system pressure control is functioning properly. Spot checks of pressure at several critical points throughout the air distribution system should be made. These will indicate potential plugging of a line or excessive usage in an area of the plant. Temperatures should be monitored downstream of intercoolers and aftercoolers to make certain that the coolers are functioning properly. Each compressor should be checked frequently for satisfactory operation. For reciprocating compressors, cylinder head temperatures and lube oil pressures should be monitored. For centrifugal compressors, vibration, bearing temperatures, lube oil pressure and lube oil temperature should all be checked. Machinery specialists should be consulted if any readings differ from normal.

Existing System Optimization In an existing system, improvements may be possible. As systems age, many air leaks may develop. These should be repaired. The use of air for some services may no longer be required. Air for cleaning, cooling, or purging may be reduced or eliminated. Some consumers may have been unnecessarily connected to the instrument air system when plant air would be adequate. These can be reconnected. Also, air supplies to utility stations can be limited by restriction orifices to reduce unnecessary or excessive use. Compressor operation can be optimized. An economical driver selection can be made depending upon the plant steam balance. A compressor can be base loaded with variations taken on another compressor. If compressors are of different sizes, the compressors that fit the demand best can be selected. Unnecessary compressors can be shut down. Portable compressors can be used to supply scheduled peak demands.


16



Air filters should be kept clean. Steam releases near compressor suctions should be minimized. This will reduce the power requirement for the compressors since the suction air will be cooler and drier. The purge air for dryer regeneration should be controlled as well as the drying time, to avoid wasting of compressed air. Avoid excessive drying of air to very low dewpoints if not required.


17



WORK AID 1: RELATIVE COMPRESSOR COSTS The following graph shows the relative costs of five types of compressors in relation to required capacity. For example, at 1,500 SCFM, a lubricated reciprocating compressor would cost approximately 110% of base, whereas a centrifugal compressor would cost nearly 150% of base.

Figure 5. Relative Compressor Costs


18



WORK AID 2: FACTORS AFFECTING COMPRESSOR SELECTION

Common Mechanical Efficiency

Sensitivity to Fouling & Fine Solids

Relative Noise Generation Level

Relative Sensitivity to Liquid Mist CarryOver

Relative Maintenance Requirements

0.70-0.80

0.99

Medium

Medium

Medium

Low

0.997

Non-Lubricated Reciprocating

0.75-0.90

0.94

Very High

High

High

Very High

High Pressure Helical Screw

0.74-0.78

0.96

Low

Very High

Low

Medium

Compressor Type

Common Compression Efficiency Range

Conventional Centrifugal


Availability Factor Clean Fouling Service Service

Maintenance Costs Mat'l

Lab

Relative

0.995

20%

80%

Base

0.960

–

35%

65%

4 x Base

0.995

0.990

–

–

–

19



WORK AID 3: COMPARISON OF COMPRESSOR TYPE FEATURES

Compressor Characteristics

Lubricated Dry Reciprocating Reciprocating

Lubricated Screw

Dry Screw

Centrifugal

High Life Expectancy

7

2

7

3

10

Oil-Free Air

0

10

0

10

10

Low Installation Cost

5

5

10

9

10

Low Foundation Cost

4

4

10

10

10

High Long Term Efficiency

6

3

7

5

10

High Part Load Efficiency

7

7

2

2

9

Low Regular Maintenance

4

2

8

6

10

Ease of Inspection

6

5

5

3

10

Quietness of Operation

7

7

6

6

9

Freedom from Vibration

3

3

10

10

10

Safety

6

8

6

8

10

Capability of Uprate

0

0

0

0

10

NOTE: 1) Gradings against all items except the price are a guide to the capabilities of the 5 types of compressors. The scale is from 0 to 10 with 10 being the "best" grading.


20



WORK AID 4: MOISTURE CONTENT OF COMPRESSED AIR NOTE: Work Aid 4 is a two page reprint from Oil, Gas & Petrochem Equipment. The first page explains how to use the nomogram which is on the second page. Bill Sisson Pryor, OK This NOMOGRAM shows the effects of various ambient temperatures and relative humidities on the quantity of moisture in atmospheric air entering a compressor at 14.7 psia and on the moisture remaining in saturated air that is compressed isothermally to the pressures shown. This monogram may be used to find several different values. Example No. 1. What is the moisture content of 1,000 cu ft of incoming air at 80°F and 60% relative humidity? At what temperature can condensation be expected in a low pressure cylinder pumping this air to 25 psig? Solution (solid lines). Step 1, connect 80°ƒ on T scale with 60% relative humidity on R scale and read moisture content of incoming air as 0.95 lb/1,000 cu ft of air at intersection with M-1 scale. Also mark where line crosses M-2 scale. Step 1, align marked point on M-2 scale with 25 psig on P scale, extend line to T scale and read temperature at which condensation can be expected as 97°F to prevent condensation on cylinder walls. Example No. 2. Temperature of saturated air at compressor intake (0 psig) is 80°F. How much moisture is contained in 1,000 cu ft of this air? If compressed to 100 psig, and cooled to 80°F, how much moisture is contained in 1,000 cu ft of the air? Solution (dashed lines). Step 1, connect 80°F on T scale with 100% on R scale and where line crosses M-1 scale read moisture per 1,000 cu ft of inlet air as about 1.58 lbs. Step 2, align 1.58 on M-1 scale with 100 psig on P scale and read moisture in 1,000 cu ft of air as 0.20 lb where line crosses M-2 scale. That is, the air now holds only 12.66% of the water vapor originally taken into the compressor [(0.20/1.58) x 100 = 12.66%]. Or, 87.34% (100 – 12.66 = 87.34) of the original moisture has been removed by cooling the air to 80°F. With permission from Oil, Gas & Petrochem Equipment.


21



Figure 6. Moisture Content of Compressed Air With permission from Oil, Gas & Petrochem Equipment.


22



WORK AID 5: TYPICAL PLANT AIR REQUIREMENTS FOR PNEUMATIC TOOLS* Description of Tool Hand Grinders 1-1/2" Wheel 2" Wheel 4" Wheel 6" Wheel 8" Wheel Hand Riveters 4" Stroke 5" Stroke 6" Stroke 8" Stroke 10" Stroke Riveting Machines Rivet Busters Hand Sanders 7" 9" Chipping Hammers 1-1/2" to 4" Scaling Hammers Air Hoists 500-1,000 lb 2,000 lb 3,000 lb 4,000 lb 6,000 lb Geared Hoists 1 Ton 1-1/2/ Ton 2 3 4 5 6 8 10 Impact Wrenches 1/4" Size 3/8" 5/8" 3/4" 1-1/4" 1-3/4"


SCFM 12 16 28 to 35 40 to 45 50 to 60 32 34 37 38 40 50 55 to 60 35 45 20 to 25 15 0.5 2 3.2 6.3 8.4 Per ft Lift 3 5 6 8 10 15 20 25 30 8 to 9 12 22 28 38 50

Description of Tool Jacks Cylinder Diam. In. 9 10 12 14 16 20 24 Air Motors 2 hp 4 5 8 15 Rotary Steel Drills 1/4" Drill Size 1/2" 5/8" 7/8" 1" 1-1/4" 2" Screwdrivers Nut Runners 1/4" Bolt Size 3/8" 1/2" 3/4" 1" Sand Blasting Equipment Nozzle Size lb/h Sand 3/16" 500 1/4" 900 3/8" 1,700 1/2" 3,000 Tube Cleaners

SCFM 1.8 2.8 4.0 5.0 6.9 11.1 16 40 to 50 60 to 70 90 to 100 140 240 20 28 to 32 36 to 42 55 65 85 90 12 to 20 12 22 38 42 46 50 90 210 375 200

*Average SCFM of 90 psig air

23



WORK AID 6: INSTRUMENT AIR CONSUMPTION See SAES-J-901. • 0.5 SCFM for each consumer if better data not available –

Transmitters

–

Controllers

–

Manual load stations

–

Computing relays

•

0.75 SCFM for diaphragm control valve with positioner

•

5.00 SCFM for piston valve with positioner

•

Typical instrument loop includes transmitter, controller, and valve positioner

•

Positioners use more when stroking (7-10 SCFM) but average can be assumed to be 1 SCFM

•

Allow 10% for purges and leaks

•

In electro-pneumatic loops, consumers are transducers and valve positioners

•

Air motors –

Gate

SCFM

–

=

SCFM of 100 psig air

ÆP

=

Max valve pressure drop, psi

d

=

Nominal valve size, inches

t

=

Time to move valve between extremes, sec

 ∆Pd 2  × d Globe (control ) valves SCFM =  6 + 220  t


24



GLOSSARY aftercooler air receiver

axial compressor centrifugal compressor intercooler reciprocating compressor rotary compressor


Air cooler at the discharge of a compressor that cools the air and lowers the dewpoint as moisture in the air condenses. Drum in an air system that provides a calculated volume to absorb air surges and provide an air volume as pressure decays from an operating pressure to a lower minimum acceptable pressure. This helps to provide air flow continuity during compressor trips or loading and unloading. A dynamic machine that develops pressure by accelerating a gas in an axial direction and converts the resultant high velocity to pressure. A dynamic machine that develops velocity of a gas by centrifugal force, with flow in the radial direction, and converts the resultant high velocity to pressure. Air cooler located between stages of a compressor. A compressor with one or more reciprocating cylinders displacing a positive volume with each stroke. Positive displacement compressor that employs mainly rotary motion. Rotary compressors can be of lube, screw, cane and liquid-ring types. Each type has a casing and one or more rotating elements that either mesh with each other, such as lubes or screws, or that displace a fixed volume with each rotation.

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REFERENCES Saudi Aramco Standards •

SAES-J-901

Instrument Air Supply Systems

•

SAES-K-402

Centrifugal Compressors

•

SAES-K-403

Reciprocating Compressors

Saudi Aramco Design Practices •

SADP-K-402

Centrifugal Compressors

•

SADP-K-403

Reciprocating Compressors

•

SADP-K-605

Inlet Air Filtration for Rotating Equipment

Exxon Basic Practices •

BP10-3-3 •

•

Rotary Screw Compressors BP10-4-2

BP15-6-1

Reciprocating Compressors for Utility and Instrument Air Systems

Electronic and Pneumatic Instruments


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CHE10709 Air Compressors

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