Reciprocating_Compressors.pdf

Engineering Encyclopedia Saudi Aramco DeskTop Standards

RECIPROCATING COMPRESSORS

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 : General Engineering File Reference: AGE-102.04

For additional information on this subject, contact PEDD Coordinator on 874-6556

Engineering Encyclopedia

Rotating Equipment Reciprocating Compressors

Section

Page

INTRODUCTION............................................................................................................. 3 IDENTIFYING MECHANICAL COMPONENTS OF RECIPROCATING COMPRESSORS AND THEIR FUNCTIONS .................................................................. 4 Principle of Operation ................................................................................................ 4 Mechanical Components ........................................................................................... 4 Lubrication ................................................................................................................. 7 PERFORMANCE CALCULATIONS ................................................................................ 8 Intercoolers .............................................................................................................. 10 VOLUMETRIC EFFICIENCY ........................................................................................ 11 DESCRIBING BASIC CONTROL SYSTEMS FOR RECIPROCATING COMPRESSORS .......................................................................................................... 14 Suction Valve Lifters ................................................................................................ 14 Clearance Pockets................................................................................................... 14 Recycle .................................................................................................................... 16 Variable Speed ........................................................................................................ 16 PROBABLE CAUSES OF PROCESS PROBLEMS ...................................................... 17 Liquid in Suction....................................................................................................... 17 Vibration of Piping.................................................................................................... 17 Leakage of Valves and Piston Rings ....................................................................... 19 Detecting Valve Leakage ......................................................................................... 19 Mechanical Problems............................................................................................... 19 WORK AID 1: RECIPROCATING COMPRESSOR - CALCULATION FORM .............. 20 WORK AID 2: ISENTROPIC EFFICIENCY OF RECIPROCATING COMPRESSERS .......................................................................................................... 22 WORK AID 3: RECIPROCATING COMPRESSOR - VOLUMETRIC EFFICIENCY CALCULATION FORM ........................................................................... 23 WORK AID 4: VOLUMETRIC EFFICIENCY LOSS CORRECTION ............................. 26 GLOSSARY .................................................................................................................. 27 REFERENCES.............................................................................................................. 29

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LIST OF FIGURES

Figure 1. Reciprocating Compressor – Principle of Operation -------------------------------- 5 Figure 2. Reciprocating Compressors – Components and Functions----------------------- 6 Figure 3. Intercoolers---------------------------------------------------------------------------------- 10 Figure 4. Action of Suction and Discharge Valves --------------------------------------------- 12 Figure 5. Clearance Pockets ------------------------------------------------------------------------ 15 Figure 6. Recycle Control ---------------------------------------------------------------------------- 16 Figure 7. Reciprocating Compressor Piping----------------------------------------------------- 18 Figure 8. Reciprocating Compressor Piping----------------------------------------------------- 18 Figure 9. Typical Isentropic Efficiency of Reciprocating Compressors ------------------- 22 Figure 10. Loss Correction For Reciprocating Compressor Volumetric Efficiency Calculation ---------------------------------------------------------------- 26

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INTRODUCTION Reciprocating compressors are used in the following circumstances where centrifugal compressors--the most common type--cannot be used or are not appropriate: •

Low flow rates, where centrifugal compressors are impractical or not economic.

•

Very high discharge pressures, over 10,000 psig.

•

Gases with low molecular weight, below 10

•

Services where the molecular weight of the gas can vary greatly.

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IDENTIFYING MECHANICAL COMPONENTS OF RECIPROCATING COMPRESSORS AND THEIR FUNCTIONS Principle of Operation Figure 1 shows the principle of operation for a reciprocating compressor. It is very similar to a reciprocating pump. A piston moves back and forth within a cylinder. Valves on the suction and discharge sides of the cylinder open at the appropriate time to admit gas to the cylinder or to expel it through the discharge line. The valves are spring loaded. The springs help to make sure that the valves seat positively at the proper time. The valves open and close automatically as the gas pressures change.

Mechanical Components Figure 2 is a cross-sectional drawing of a reciprocating compressor showing the major mechanical components. The cylinder is the chamber in which the piston moves back and forth. The spring-loaded valves are mounted in the ends of the cylinder. Note that this cylinder is double acting. Compression takes place on both the forward stroke and the backstroke of the piston. The piston is fitted with piston rings that provide a close fit between the piston and the cylinder. The cylinder and cylinder heads contain cooling jackets. Cooling water circulates through these spaces. The piston rod moves back and forth to drive the piston. The piston rod is the mechanical part that has the most stress. Packing seals the point where the piston rod enters the cylinder, to prevent loss of gas to the atmosphere.

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Figure 1. Reciprocating Compressor – Principle of Operation

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Figure 2. Reciprocating Compressors – Components and Functions The connecting rod transmits motion from the crankshaft to the piston rod, through the crosshead. The connecting rod converts rotary motion to reciprocating motion. The crosshead absorbs the non-axial forces from the connecting rod and transmits only axial forces to the piston rod. The crankshaft is driven by a mechanical engine, which may be an electric motor, steam turbine, or a gas turbine. It may also be driven by a gas engine with reciprocating pistons. In this case, a single crankshaft is connected to the pistons of both the engine and the compressor. A single driver and crankshaft may be attached to several cylinders. The cylinders may be successive stages of the same process gas service or they may be separate services.

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Lubrication Compressor cylinders may be lubricated or non-lubricated. In the lubricated type, oil is injected: •

Between the piston and cylinder

•

Between the piston rod and packing

Lubrication reduces power requirements and temperature, which decreases the amount of maintenance required. However, some of the lubricating oil is always entrained in the gas stream. Oil separators at the discharge can remove most of this oil, but not all of it. Non-lubricated cylinders may be used in services where oil contamination of the gas is undesirable. In this case, the piston rings and packing are made of low-friction materials such as Teflon, and piston maximum speeds are less than lubricated applications (approximately 75%). Examples of services where non-lubricated compressors may be used are: •

Instrument air, because oil in instrument air supplies can clog instruments.

•

Oxygen, because mixtures of oil and oxygen are explosive.

•

Refrigeration, because oil in refrigerant can freeze in lowtemperature sections of the process.

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PERFORMANCE CALCULATIONS The manufacturer's performance predictions should always be used as a first choice. If they are not available, you can use the equations in the following sections to make reasonable approximations. The equations for calculating gas horsepower and brake horsepower (which are the same for any compressor -- positive displacement or centrifugal) are as follows: Gas Horsepower (ghp ) =

Brake Horsepower =

Head × Flow rate (lb min) Isentropic Eff . × 33,000

Gas Horsepower Mechanical Efficiency

Eqn. (1)

Eqn. (2)

Typical isentropic and mechanical efficiencies for reciprocating compressors are given in Work Aid 1 or Figure 9. The isentropic formula for head is used. In this form, the gas constant k is used directly rather than the exponent n, which is used in polytropic compression. Z1RT1 Head = (k − 1) MW k

 k −1  (r ) k − 1    

Eqn. (3)

P r= 2 P1

where: Z1

= Compressibility factor, suction.

R

= Gas constant = 1545

T1

= Suction temperature, ºR

ft − lb lb mol − °F

MW = Molecular weight

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k

= Cp/Cv, average of suction and discharge. (Determine k from GPSA Figure 13-8 or 13-6 and 13-7)

r

= Compression ratio

P1

= Suction pressure, psia

P2

= Discharge pressure, psia

The isentropic formula for head is used because reciprocating compression follows a path that is very close to a true isentropic path. There are two reasons for this: •

The efficiency of compression is very high, approximately 90%. Therefore, the excess heat resulting from inefficiency is small.

•

The jackets surrounding the cylinder are cooled with cooling water. This provides some cooling for the gas during the compression stroke. This cooling offsets the heat gain from inefficiency. The net effect is a temperature rise that is almost the same as in isentropic compression.

The equation for discharge temperature is also of the isentropic form. k −1

P  k T2 = T1 2   P1 

Eqn. (4)

This equation gives the correct discharge temperature for most situations. However, in some cases, the efficiency of a reciprocating compressor can be somewhat lower, and the discharge temperature will be higher than predicted by this equation. The conditions that might cause lower efficiency are: •

Very low compression ratios, below 2 to 1.

•

High speed machines, which have proportionately higher valve losses.

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Intercoolers An intercooler reduces the gas temperature between stages of compression. For most reciprocating compressors, allowable discharge temperatures are limited to approximately 350ºF. Above this temperature, degradation of lube oil can occur. In non-lubricated compressors, damage to the piston rings and packing can occur. The discharge temperature restriction places limits on the compression ratio that may be used; if higher compression ratios are required, two or more stages of compression are used. Figure 3 illustrates a common intercooler application, a utility air compressor. Utility air is typically compressed to about 100 psig. This requires a compression ratio of 8. Without intercooling, the discharge temperature would be 550ºF.

Figure 3. Intercoolers

When the air is cooled after the first compression stage, liquid water condenses from the gas and must be removed in a knockout drum. NOTE: When you calculate head and power for two-stage or multistage compressors, calculate each stage separately. Remember that the intercooler and its piping will take some pressure drop.

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VOLUMETRIC EFFICIENCY Figure 4 illustrates the action of the suction and discharge valves during one complete cycle of a reciprocating compressor. The volumetric efficiency of an operating compressor is calculated to determine whether the valves and the piston are operating properly. An actual volumetric efficiency significantly less than the theoretical value indicates that the valves or the piston rings are leaking and that maintenance is required. The method is as follows: (1) Calculate the actual volumetric efficiency from plant data. Divide the suction flow rate by the displacement volume. The displacement volume is obtained from Eqn. (5), (6), or (7). Single acting:

D=

Double acting (without tail rod)

D=

Double acting (with tail rod)

D=

A × m × Ls × n 1728

(2A − a ) m × Ls × n 1728 2(A − a ) m × Ls × n 1728

Eqn. (5)

Eqn. (6)

Eqn. (7)

where: D

= Displacement, ACFM (actual cubic feet/minute)

A

= Cross-sectional area of cylinder, sq. in.

a

= Cross-sectional area of piston rod, sq. in.

m

= Number of cylinders

Ls = Length of stroke, in. n

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= Speed, strokes/minute, or rpm of crankshaft.

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Figure 4. Action of Suction and Discharge Valves

A tail rod is an extension of the piston rod included in some compressors, on the side opposite the main piston rod. It helps to stabilize piston motion, and to reduce peak stress on the piston rod. (2) Obtain the theoretical value for volumetric efficiency from Eqn. (8). If the operating conditions are those originally specified, the theoretical volumetric efficiency may be available from the vendor specifications. Volumetric efficiency is the volume flow rate of suction gas divided by the displacement. For a reciprocating compressor, the theoretical volumetric efficiency is considerably less than 100% because of clearance. Clearance is that portion of the cylinder not swept by the piston. Clearance includes volume at the end of the cylinder and underneath the valve chambers. At the end of a discharge stroke, the clearance volume is filled with gas at discharge pressure. During the subsequent suction stroke, this gas begins to expand. The suction valve does not open until the gas in the clearance volume expands from discharge pressure to suction pressure. After that point, gas is admitted to the cylinder until the end of the suction stroke.

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However, gas is admitted during only 70 to 80% of the total suction stroke. The amount of lost suction volume depends on the compression ratio, the properties of the gas, and the amount of clearance volume. The equation for calculating theoretical volumetric efficiency is as follows: 1   Z s  k VE = 1.00 − L − C r − 1  Zd   

Eqn. (8)

where: C

=

Clearance Volume Displaceme nt Volume, Decimal Fraction

Zs = Compressibility factor, suction Zd = Compressibility factor, discharge L

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= Loss factor, for losses resulting from backflow through valves and around the piston. See Work Aid 2 or Figure 10.

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DESCRIBING BASIC CONTROL SYSTEMS FOR RECIPROCATING COMPRESSORS Suction Valve Lifters Suction valve lifters are one technique for controlling volume flow. If the suction valve is held open continuously, gas will pass back and forth through the suction valve without passing through to the discharge line. This technique is used commonly to reduce the starting torque of the machine and for capacity control. However, it has three drawbacks: •

Capacity control is limited to 0 or 100% for each cylinder end.

•

Valves may overheat because the same gas is continuously passing back and forth across them.

•

Forces on the crankshaft become unbalanced.

Clearance Pockets Clearance pockets are another way to control compressor capacity. A clearance pocket is a small volume just outside the cylinder. If this volume is open to the cylinder, it increases the clearance because this volume will not be swept by the piston. The increased clearance reduces throughput. Saudi Aramco permits the use of clearance pockets with fixed volume but not variable volume. There may be one clearance pocket per cylinder or as many as four. Finally, a flow controller may control clearance pockets manually or automatically. See Figure 5.

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Figure 5. Clearance Pockets

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Recycle The third method for controlling reciprocating compressors is to recycle some of the discharge gas back to the suction. See Figure 6. This is the least efficient control method because the compressor is always operating at full capacity and consuming full power. However, it is often the most reliable method from a mechanical point of view, because valve lifters and clearance pockets can be causes of frequent maintenance. They tend to result in unbalanced loads on the mechanical components. With gases containing dirty or fouling components, the mechanisms that operate lifters and pockets can become fouled. If recycled gas is employed, it is important to have a cooler in the loop, so that the same gas is not recycled continuously to the compressor without cooling.

Figure 6. Recycle Control

Variable Speed Variable speed is sometimes used for capacity control, but not often. Suction throttling, which is common for centrifugal compressors, is not used for reciprocating machines because it increases piston rod loads.

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PROBABLE CAUSES OF PROCESS PROBLEMS Liquid in Suction Reciprocating compressors cannot tolerate liquid in the suction gas. The process must be carefully designed to prevent this condition. Small amounts of liquid continually entering the cylinders will damage the valves. Liquid will also wash lubricant away from cylinder walls, increasing wear. Large slugs of liquid can cause serious damage. Pistons or piston rods can break. In extreme cases, the cylinder head will blow off the compressor, and a fire will result. To prevent liquid formation in suction lines (Figure 7 and 8): •

Locate the suction knockout drum close to the compressor.

•

Avoid low points in the line between the drum and the compressor.

•

Steam trace the suction line from the drum to the compressor to prevent condensation. Steam tracing is a small pipe carrying steam, located outside the gas pipe, under the insulation.

•

Control cooling water temperature to avoid condensation within the cylinders.

Vibration of Piping Vibration of the suction or discharge piping sympathetically with the movement of the pistons can also be a problem. Two solutions are possible: •

Larger pulsation bottles can be used. Every reciprocating compressor has large, cylindrical pieces of piping called pulsation bottles or pulsation dampeners, at the suction and the discharge. They smooth out the pulsations of the flow.

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•

The vibration may be due to a resonance between the piping and the compressors. In this case, the piping configuration can be changed or orifices added in order to change the natural frequency of the piping system. Such action requires a pulsation analysis to be performed to predict the action required.

Figure 7. Reciprocating Compressor Piping

Figure 8. Reciprocating Compressor Piping

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Leakage of Valves and Piston Rings The most common cause of low volumetric efficiency is leakage or back flow in the valves. Leaking piston rings are another cause. In addition, the cylinder may be worn to an oval shape. Valves and rings are normally replaced during scheduled maintenance periods. Six months is a typical interval between planned maintenance operations. Significant wear and leakage can occur in less than six months. The most common cause of rapid wear is the presence of liquid and/or solids in the gas.

Detecting Valve Leakage A leaking suction valve is usually hotter than normal. This is caused by the back flow of hot gas through the valve during the discharge stroke. Discharge valves are always hot. So this means of detection cannot be used. On two-stage compressors, the interstage pressure is a good indication of valve leakage. If the interstage pressure is higher than normal, a suction valve or piston ring in the second stage is leaking. If the interstage pressure is lower than normal, a suction/discharge valve or piston ring in the first stage is leaking.

Mechanical Problems For a checklist of mechanical troubles, see GPSA Engineering Data Book, Figure 13 - 27.

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WORK AID 1:

RECIPROCATING COMPRESSOR - CALCULATION FORM (Page 1 of 2)

Gas MW Suction Flow Rate: P1

psia

P2

psia

r =

P2/P1 =

SCFM,

lb/min,

ACFM

= ºF,

T1

ºR

Isentropic Efficiency: (From Manufacturer's Specification or from Work Aid 2)

T2, ºF/ºR assumed

lst Trial

2nd Trial

3rd Trial

/

/

/

T +T Tavg, °F 1 2 2

K at Tavg (GPSA Figure 13-8 or 13-6) (k − 1)/k T2 = T1(r)(k-1)/k ºR T2 ºF calculated

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(Page 2 of 2)

Z1 (GPSA 23-3) Isentropic Head:  k −1  Z1(1545)T1  k His = (r ) − 1  (k − 1)  MW    k

[

]

His =

( )(1545 )( ) ( )( ) − 1 ( )( )

His =

feet

Gas Horsepower: lb min ghp = Is. Eff . × 33,000 His ×

ghp =

(

(

)× ( ) ) × (33,000 )

ghp =

Mechanical Efficiency Brake Horsepower =

=

(Work Aid 2) ghp Mechanical Efficciency

( (

) )

= __________________

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WORK AID 2:

ISENTROPIC EFFICIENCY OF RECIPROCATING COMPRESSERS

BHP =

W His 33,000 ηis ηm

Figure 9. Typical Isentropic Efficiency of Reciprocating Compressors

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WORK AID 3:

RECIPROCATING COMPRESSOR - VOLUMETRIC EFFICIENCY CALCULATION FORM (Page 1 of 3)

EXPECTED VOLUMETRIC EFFICIENCY: P1

Suction pressure, psia

P2

Discharge pressure, psia

P r= 2 = P1

( (

) )

k, average Zs

Z at suction

Zd

Z at discharge

C

Clearance Volume Displaceme nt Volume

=

L

Loss Correction Factor

=

VE

=

=

(Work Aid 4) 1 Z  1.00 − L − C  s (r )k − 1  Zd 

1.00 − (

)− (

  ) ( ( 

)( )

)

 1    

  − 1  

=

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(Page 2 of 3) ACTUAL VOLUMETRIC EFFICIENCY Actual Suction Flow Rate: SCFM

Compressor Suction Flow:

lb/min

MW Zs T1

Temp.

ºF,

ºR

P1

Press.

psig

psia

Suction SCFM =

Suction ACFM

lb 379 x =( min MW

)x

= SCFM s

=

(

379 (

)

=

14.7 T1 × × Zs P1 520

)×

(

14.7

=

)

×

( 520

)×(

)

ACFM

Displacement: A,

Cross-sectional area of cylinder

sq. in.

a,

Cross-sectional area of piston rod

sq. in.

m,

Number of cylinders

Ls, Length of stroke n,

in.

strokes/minute (rpm)

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(Page 3 of 3) Single Acting: D=

D=

A × n × Ls × n 1728

(

)(

)(

)(

)=

1728

ACFM

Double Acting without tail rod D=

D=

(2A − a ) m × Ls × n 1728

[2(

)] (

)− (

)(

)(

)=

)(

)(

)=

1728

ACFM

Double Acting with Tail Rod D=

D=

2 (A − a ) m × L s × n 1728 2 [(

)− (

)] ( 1728

ACFM

Volumetric Efficiency: VE = =

Suction ACFM D

( (

) )

=

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WORK AID 4:

VOLUMETRIC EFFICIENCY LOSS CORRECTION

Figure 10. Loss Correction For Reciprocating Compressor Volumetric Efficiency Calculation

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GLOSSARY Clearance

A volume in a cylinder that is not swept by the piston.

Clearance Pocket

A chamber attached to a cylinder that may be open to the cylinder or closed off. It is used to control capacity of a reciprocating compressor.

Connecting Rod

The rod that connects the crankshaft to the crosshead. It changes circular motion to reciprocating motion.

Crankcase

The housing for the crankshaft.

Crankshaft

The rotating element that transmits power from the driver to the connecting rods.

Crosshead

The mechanical element between the connecting rod and the piston rod. It absorbs the non-axial forces from the connecting rod and transmits only axial forces to the piston rod.

Cylinder

The principal component of a reciprocating compressor. It contains the piston, suction and discharge valves, and packing around the piston rod.

Displacement

The volume of the space swept by the piston(s). The theoretical maximum capacity of a reciprocating compressor.

Double Acting Cylinder

A cylinder with compression chambers on both sides of the piston.

Intercooler

A gas cooler located between compressor stages.

Interstage Pressure

On a two-stage or multistage compressor, the pressure existing between stages.

Isentropic Efficiency

The ideal work for a compression service divided by the actual work applied to the gas. The extra, non-ideal work is converted to heat that raises the gas temperature or is removed by jacket cooling.

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Jacket

A chamber that surrounds the cylinder. Cooling water circulates through the jacket.

Mechanical Efficiency

The work applied to the gas divided by the driver brake horsepower. The difference, or loss, is due to mechanical friction.

Packing

A flexible material that seals the space between the moving piston rod and the cylinder.

Piston

The component that moves back and forth in the cylinder. It compresses the gas.

Piston Ring

A ring surrounding the piston providing a close fit with the cylinder. It minimizes gas leakage past the piston.

Piston Rod

A rod that transmits force to move the piston.

Pulsation Bottle or Dampener

A large chamber made of piping components immediately upstream or downstream of a cylinder. It smoothes out the pulsations caused by the piston.

Single Acting Cylinder

A cylinder with a compression chamber on only one side of the piston.

Tail Rod

A piston rod extension in some compressors on the side opposite the main piston rod. It helps to stabilize piston motion and reduces peak stress on the piston rod.

Valve Lifter

A device that holds a valve open, normally used on suction valves. When a valve is held open, gas does not flow through the cylinder. Used to reduce motor torque during starting. Also used for compressor capacity control. However, the open period is usually automatically controlled to avoid overheating and/or lubricant accumulation in the cylinder.

Volumetric Efficiency

The actual volume of suction gas compressed, divided by the theoretical displacement.

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REFERENCES Supplementary Text •

Gas Processors Suppliers Association, Engineering Data Book, Section 13

Industry Standard •

API 618, Reciprocating Compressors for General Refinery Service

Saudi Aramco Engineering Standards •

SAES-K-403, Reciprocating Compressors

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