operation of compressor control and protection systems

7/25/2019 Operation of Compressor Control and Protection Systems

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Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramcos employees.Any material contained in this document which is not already in the publicdomain may not be copied, reproduced, sold, given, or disclosed to thirdparties, or otherwise used in whole, or in part, without the written permissionof the Vice President, Engineering Services, Saudi Aramco.

Chapter : Mechanical For additional information on this subject, contactFile Reference: MEX-212.05 PEDD Coordinator on 874-6556

Engineering Encyclopedia

Saudi Aramco DeskTop Standards

OPERATION OF COMPRESSORCONTROL AND PROTECTION SYSTEMS


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Engineering Encyclopedia Compressors

Operation of CompressorControl and Protection Systems

Saudi Aramco DeskTop Standards i

Section Page

INFORMATION............................................................................................................... 4INTRODUCTION............................................................................................................. 4

DYNAMIC COMPRESSOR CONTROL SYSTEMS ........................................................ 5

Pressure Control................................................................................................... 6

Variable-Speed Constant Pressure Control............................................... 6

Adjustable Inlet Guide Vane Constant Pressure Control ........................... 8

Suction Throttling Constant Pressure Control.......................................... 11

Discharge Throttling Constant Pressure Control...................................... 14

Blow-Off (Recycle) Constant Pressure Control........................................ 17

Flow Control ....................................................................................................... 17

Variable-Speed Constant Flow Control.................................................... 17

Adjustable Inlet Guide Vane Constant Flow Control ................................ 19

Suction Throttling Constant Flow Control................................................. 21

Discharge Throttling Constant Flow Control ............................................ 22

Blow-Off Constant Flow Control............................................................... 22

DYNAMIC COMPRESSOR PROTECTION SYSTEMS ................................................ 23

Surge Protection................................................................................................. 24

Flow Systems .......................................................................................... 25

Surge Control on a Constant-Speed Compressor withSuction Throttling..................................................................................... 27

Variable-Speed Compressor Based on Delta Pressure and Flow ........... 33

Variable-Speed Multisection Compressors.............................................. 34

System Arrangements........................................................................................ 39

Series....................................................................................................... 40

Parallel..................................................................................................... 43

POSITIVE-DISPLACEMENT COMPRESSOR CONTROL SYSTEMS ......................... 47

Valve Unloading ................................................................................................. 47

Clearance Pockets ............................................................................................. 52

Bypass Operation ............................................................................................... 53


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Saudi Aramco DeskTop Standards ii

POSITIVE-DISPLACEMENT COMPRESSOR PROTECTION SYSTEMS ................... 55

Relief Valve (Stage)............................................................................................ 55

Startup Bypass ................................................................................................... 55

High Process Temperature................................................................................. 56

GLOSSARY .................................................................................................................. 57

LIST OF FIGURES

Figure 1. Variable-Speed Constant-Pressure Control System andCharacteristic Curves ................................................................................... 7

Figure 2. Adjustable Inlet Guide Vane Constant Pressure Control System andCharacteristic Curves ................................................................................. 10

Figure 3. Suction Throttling Constant-Pressure Control System andCharacteristic Curves ................................................................................. 12

Figure 4. Discharge Throttling, Constant-Pressure Control System andCharacteristic Curves ................................................................................. 16

Figure 5. Variable-Speed Constant-Flow Control System andCharacteristic Curve................................................................................... 18

Figure 6. Alternate Variable-Speed Constant-Flow ControlSystem Configuration ................................................................................. 19

Figure 7. Adjustable Inlet Guide Vane Constant-Flow Control System andCharacteristic Curves ................................................................................. 20

Figure 8. Suction Throttling Constant Flow Control System andCharacteristic Curve................................................................................... 21

Figure 9. Basic, Volume-Controlled, Anti-Surge System.............................................. 26

Figure 10. Typical Capacity and Surge Control System and the AssociatedPerformance Curve for a Constant-Speed Compressor with SuctionThrottling .................................................................................................... 28

Figure 11. Performance and Surge Lines with Changes in Ambient Conditions .......... 30

Figure 12. Pressure-Compensated Surge Control System .......................................... 31

Figure 13. Discharge Mass Flow Rate Measurement Compensated to InletVolumetric Flow Rate ................................................................................. 33


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Saudi Aramco DeskTop Standards iii

Figure 14. Surge Control System for a Variable-Speed Compressor Based onDifferential Pressure and Gas Flow Rate ................................................... 34

Figure 15. Multisection, Variable-Speed Compressor with Surge Control Valve that

Protects the Entire Compressor.................................................................. 35Figure 16. Performance Curves and Surge Line for Each Section of a Multisection

Compressor................................................................................................ 36

Figure 17. Multisection, Variable-Speed Compressor with a Surge ControlValve for Each Section ............................................................................... 37

Figure 18. Multisection, Variable-Speed Compressor with Remotely OperatedControl on the First Section ........................................................................ 39

Figure 19. Typical Surge Control System for Compressors in Series........................... 41

Figure 20. Integrated Surge Control System for Compressors in Series...................... 42

Figure 21. Discharge Pressure Control of Constant-Speed Parallel Compressorswith Dissimilar Operating Characteristics ................................................... 44

Figure 22. Control System that Uses the S-Criterion for Compressors in ParallelConfiguration .............................................................................................. 46

Figure 23. Suction Valve Unloader............................................................................... 48

Figure 24. Finger-Type Unloaders ............................................................................... 50

Figure 25. Clearance Pockets...................................................................................... 52


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Saudi Aramco DeskTop Standards 4

INFORMATION

INTRODUCTION

For the purposes of this module, a control system functions tomaintain process variables within their prescribed ranges. If aprocess variable approaches a value outside of its prescribedrange and that could result in damage to the monitoredequipment, a protection system will function either to restorethe variable to an acceptable value or to shut down theequipment.

The control and protection systems that are used on dynamicand positive-displacement compressors are different becausethe systems reflect the characteristics of the equipment. The

dynamic compressor control system must maintain thecompressor flow rate and the discharge pressure withinprescribed limits. The protection system must prevent thecompressor from operating under surge or stonewallconditions. Surge and stonewall are damaging conditions, andthey are discussed in more detail later in this module.

Unlike the control and protection systems of a dynamiccompressor, a positive-displacement compressor cannot self-regulate capacity against a given discharge pressure; thecompressor, because its characteristic is constant volume, will

simply continue to displace gas until it receives a signal not todo so. As a result, various methods of changing the volumeflow must be used. Because each rotation or stroke of thecompression elements will displace a given volume of flow inthe discharge system, protection of all positive-displacementcompressors requires a device to limit discharge pressure.Because the volume of the discharge system is fixed, thedischarge pressure will continue to rise.


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DYNAMIC COMPRESSOR CONTROL SYSTEMS

Dynamic compressor controls can vary from the very basicmanual recycle control to elaborate ratio controllers. Inaccordance with SAES-K-402, the control system must beadequate to control the compressor at all specified operatingconditions. The driver characteristics, the process response,and the compressor operating range must be determinedbefore the type of controls are chosen. Control systems fordynamic compressors that are used at Saudi Aramco facilitiesvary in their method of control. Control and protection systemsfor dynamic compressors have fundamentally only twofunctions to accomplish:

To provide stable control of the compressor at all of the

required operating conditions that are specified on the datasheet.

To provide protection against operation in the surge area ofthe performance curve.

Anti-surge control is part of the compressor protection systemthat is discussed later in this module.

Dynamic compressor control systems are designed to maintaina desired pressure to a process or a desired flow to a process.Where the process operation may result in variations in either

or both compressor flow and discharge pressure, manipulationof the compressor suction pressure may be required forupstream stability of the process. For example, on a variablespeed controlled compressor, the governor would receive acompressor suction pressure signal that would initiate a speedincrease upon an increase of suction pressure. A speedincrease would increase the compressor flow and probably thedischarge pressure. The reverse would occur if the suctionpressure decreased and the speed also decreased.

Multiple control systems may be applied to a system and

selected through the use of an auto-selector control. An auto-selector controller receives inputs, such as flow, suctionpressure and discharge pressure, from more than one sensor.The controller automatically selects, as the controlled variable,the input variable that is closest to its desired limit value.


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The head-capacity control methods (in order of decreasingefficiency) that are most commonly used for pressure controland flow control are as follows:

Speed control Adjustable inlet guide vanes (IGV) or adjustable diffuser

vanes

Suction throttling (STV)

Discharge throttling (DTV)

Blow-off

Recycle

Pressure Control

Pressure control is accomplished through modulation of aperformance control element. Process pressure is monitored,and a signal from a pressure transmitter is sent to the pressurecontroller. The pressure controller adjusts the control element,which might be a guide vane positioner, a suction or dischargecontrol valve, or a rotational speed governor. The controlelement would operate to maintain the process pressure at asetpoint value.

Variable-SpeedConstantPressure Control

The most efficient way to match the compressor characteristicto the required output is to change speed in accordance withthe fan laws. This variable-speed operation is most easilyaccomplished through use of steam turbines, gas turbines, orvariable-speed (frequency) electric motors as drivers forcompressors. With such drivers, the speed can be manually

controlled through adjustment of the speed controller by anoperator, or the speed adjustment can be made automaticallythrough use of a pneumatic or electric controller that changesthe speed in response to a pressure or flow signal. Becausethe only energy required by the process is provided by thecompressor without the use of throttling devices, variable-speed control is the most efficient method of control. Theoperating speed range of the driver must match or exceed the


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operating speed range of the compressor.

Figure 1 shows a typical variable-speed constant-pressure

control system for a steam turbine driven compressor, and italso shows the associated characteristic curves. Thecharacteristic curves shown in Figure 1 assume a constant inletpressure (P1), inlet temperature (T1), and gas composition.Each curve shows the pressure at which the compressorsupplies a certain volume rate of flow (Q) for a given speed. Ifthe compressor discharge pressure required by the processexceeds the maximum pressure the compressor can producefor a given speed, compressor surge will occur. The surge lineon the graph indicates the limit of minimum flow.

Figure 1. Variable-Speed Constant-Pressure Control Systemand Characteristic Curves


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If the compressor is operating at the constant pressure point, Y(flow rate Qy), and if the process requires a higher gas flow, thedischarge pressure immediately falls and the operating pointmoves to the right and downward along the characteristic curve

for the given speed. The pressure transmitter will sense thelowering pressure, and the pressure controller will send acontrol signal to the turbine governor. The governor willincrease the speed of the compressor (through an increase inturbine speed), which results in an increase in the systempressure back to the pressure setpoint. The new operatingpoint would be located on the desired pressure line but furtherto the right.

If the process required less gas flow, the discharge pressurewould begin to increase and the control system would

decrease the speed of the compressor (through a decrease inturbine speed) until the pressure setpoint is restored. The flowcould be reduced until point X was reached. Point X is set atthe minimum operating point before the surge line (surgecontrol line). Anti-surge controls, which are discussed later inthis module, will prevent the operating point from moving to theleft of point X on each speed curve. If the process required aflow rate of only point Z, the volume of gas (Qx- Qz) wouldhave to be blown off or recycled. The operating control wouldhave to be shifted from variable-speed control to blow-offcontrol, which is the only control that is available when the

process requires flows that are below the stable operatingrange. Blow-off control is discussed later in this module.

Adjustable InletGuide Vane ConstantPressure Control

Inlet guide vanes evenly distribute the inlet flow to thecompressor stage impellers. Adjustable inlet vanes are builtinto the inlet of the first stage, or succeeding stages of axialcompressors, and they can be automatically or manually

controlled through a linkage mechanism. Adjustable guidevanes are used for the control of axial and single-stagecentrifugal compressors. Single-stage compressors frequentlyincorporate an axial inlet, and they do not require fixed guidevanes.

Pre-rotation adjustable guide vanes pre-whirl the gas thatenters the compressor stage in the direction of rotation, which


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develops less head than at design. Between full open andmaximum pre-whirl position, adjustable guide vanes providesome degree of reduced horsepower over the suction throttlingvalve.

Counter-rotation adjustable guide vanes are used to extend theuseful operating range of any dynamic compressor. The rangeof operation is extended through a change of the angle ofattack and the inlet gas velocity to the impeller blade. For thehigh flow region, the angle of attack is increased to eliminateflow separation and to effect an increase in the produced headof the impeller or blade. The elimination of flow separation andan increase in the produced head will increase the capacityrange of the impeller.

Adjustable inlet guide vanes are expensive, limited ineffectiveness, and present many maintenance and operationalproblems. At Saudi Aramco, centrifugal compressor adjustableinlet guide vanes have proven to be mechanically unreliable ingeneral services; therefore, prior to control selection, theeconomics of inlet guide vanes must be considered because oftheir higher initial cost, complex mechanism, maintenance, andrequirement for frequent adjustment. Adjustable inlet guidevanes should not be used on process centrifugal compressors,and they should never be used in any sour gas service. Saudi

Aramco primarily uses adjustable inlet guide vanes for axial

and single-stage centrifugal air compressors.

Figure 2 shows a typical adjustable inlet guide vane constantpressure control system, and it also shows the associatedcharacteristic curves for a constant speed compressor.


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Figure 2. Adjustable Inlet Guide Vane Constant Pressure Control Systemand Characteristic Curves

The control element is the compressor guide vane mechanism.The guide vanes are adjusted through use of a positioningcylinder. This cylinder is operated by a servo-valve (SRV) thatreceives a signal from the pressure controller.

If the compressor is operating at flow rate Qy and if the processrequires an increase in flow, the discharge pressureimmediately falls, and the operating point moves to the right


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and downward along the characteristic curve for the given inletguide vane (IGV) position. The pressure transmitter (PT) willsense the lowering pressure, and the pressure controller (PC)will send a control signal to the SRV. The SRV will open the

inlet guide vanes, which increases both the gas flow throughthe compressor and the system pressure back to the pressuresetpoint. The new operating point would be located on thedesired pressure line but further to the right (point W).

If the process required less gas flow, the discharge pressurewould begin to increase and the control system would close theinlet guide vanes, which decreases the gas flow through thecompressor until the pressure setpoint is reached. The flowcould be reduced until point X was reached. Point X is set atthe minimum operating point before the surge line. Anti-surge

controls, which are discussed later in this module, will preventthe operating point from moving to the left on each inlet guidevane position curve similar to point X. Like the variable-speedconstant pressure control, if the process required a flow rate ofpoint Z, the volume of gas (Qx- Qz) would have to be blown offor recycled. The operating control would have to be shiftedfrom adjustable inlet guide vane control to blow-off control.

Suction ThrottlingConstant PressureControl

Suction throttling control, which is also known as intakethrottling or capacity modulation control, is usually used insituations in which the compressor is not equipped with inletguide vanes and is driven by a constant-speed drive. Suctionthrottling is more efficient than discharge throttling byapproximately 3 to 5%. This control is also applied in plant andinstrument air compressor systems when the demand for air isrelatively constant. The system usually includes a large airreceiver, which allows large volume draws to affect majorpressure changes in the receiver pressure so that the air

compressor can modulate the flow with relatively smallpressure variations.

Compressors with this type of control system have a singlepressure-volume characteristic curve. Figure 3 shows a typicalsuction throttling constant pressure control system, and it alsoshows the associated characteristic curves for a constant-speed compressor.


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Figure 3. Suction Throttling Constant-Pressure Control Systemand Characteristic Curves

The suction throttle valve (STV) is normally included as part ofthe compressor package. Starting the system, especially with

a motor driver, with the suction throttle closed and thedischarge anti-surge vent valve open, will develop a vacuum onthe inlet to the impellers. Although this type of startup reducesthe motor starting torque and the horsepower requirements, itmust be avoided. SAES-K-402 states that suction throttlingmust not result in subatmospheric pressure and risk of airingestion into the process streams. Starting torque is notcritical with a steam turbine driver, where the compressor


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package will be brought up to speed much slower. Thedischarge anti-surge vent valve will still be open, however, forturbine startup.

Suction throttling should be used when compressor flow and/ordischarge pressure may vary as required by the process. Thesuction throttling valve may receive actuation signals from theflow sensing device, from discharge pressure, or from suctionpressure upstream of the suction throttling valve.

If the compressor process is equipped with a recycle system,the suction throttle valve is located upstream of both thesuction knockout drum and the anti-surge recycle return line.The preferred location of the recycle return line is upstream ofthe knockout drum. Such a location ensures good mixing of

the recycle stream with the main suction stream prior toreaching the compressor. The preferred location of the suctionthrottle valve is close to the compressor suction.

When electric motors are used as constant speed drivers, thecentrifugal compressor is normally controlled through use of asuction throttling valve. Butterfly valves are typically used assuction throttling valves because they minimize flowdisturbance. Throttling the suction results in a slightly lowersuction pressure than the pressure for which the machine isdesigned and, therefore, a higher total head is required if the

discharge pressure must remain constant. The increase intotal head can be matched to the compressor head-capacitycurve, i.e., higher head at reduced flow. In throttling the inlet,the density of the gas is reduced, which results in a matching ofthe required weight flow to the compressor inlet-volumecapabilities at other points on the head/capacity curve.

In the control system that is shown in Figure 3, the value ofpressure is sensed by the pressure transmitter (PT). Thepressure transmitter converts this signal to a signal that isproportional to the process pressure, and it sends a signal to

the pressure controller (PC). The pressure controller amplifiesthe transmitter signal and sends a modified signal to the controlelement. Depending on system requirements, the controllermay require additional correction factors, which are called resetand rate. The control element is a suction throttle valve (STV)that reduces the flow of gas into the compressor.

If the compressor is operating at point W on its unthrottled


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characteristic curve and if there is a reduction in the processflow requirements, the pressure would increase to Y1 on theunthrottled characteristic curve. The increase in pressurewould be sensed by the pressure transmitter, and a control

signal would be sent by the pressure controller to the STV tomodulate the valve. By throttling across the STV, the inletpressure can be reduced, and, although the compressor isoperating at the pressure ratio and volume of point Y1, thedischarge pressure and volume flow to the process will beequivalent to point Y. To further explain the operation, thefollowing example, which assumes that the STV is fully open,should be considered:

Qw= 100%,P

P

3

1w

= 2 0.

Qy1= 80%,P

P

3

1

Y1

2 1= .

Inlet pressure P1= 14.7 psia = P2(No throttling)Desired P3= 29.4 psia

The compressor pressure ratio with 80% flow is 2.10. At(P3/P1)Y

1, the pressure ratio is 2.10. To maintain the dischargepressure of 29.4 psia, the inlet pressure (P2) must be reducedto 14.0 psia (29.4/2.10). The volume to the compressor Y

1(at

pressure P2) is 80%, but the equivalent volume Y (at pressureP1) is less than the ratio of 80% x (14.0/14.7), or 76.3%, whichis the actual volume at pressure P1that is delivered to theprocess.

Anti-surge controls, which are discussed later in this module,will prevent the operating point from moving to the left pastpoint X. Like the variable-speed constant-pressure control, ifthe process required a flow rate of point Z, the excess flowwould need to be blown off or recycled.

Discharge ThrottlingConstant PressureControl

Discharge pressure throttling for constant pressure is lessefficient than suction throttling; however, it may be moreeconomical from the standpoint of requiring a smaller throttlingvalve and flanges. The discharge throttling valve is located


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downstream of the anti-surge recycle supply line. If a maindischarge aftercooler is used to cool the recycle gas, then thethrottle valve is located downstream of this cooler, which maybe a considerable distance from the compressor. If the recycle

gas has a dedicated cooler, then the throttle valve can belocated upstream of the aftercooler.

As with suction throttling, only one pressure-volumecharacteristic curve is associated with discharge throttling for aconstant-speed compressor. Figure 4 shows a typicaldischarge throttling constant pressure control system and theassociated characteristic curves for a constant-speedcompressor. Pressure control is maintained by throttling theactual compressor discharge pressure to the desired setpointalong the characteristic curve. Discharge throttling requires

more power than suction throttling for the same flow. Forexample, if the process requires 80% flow with dischargethrottling, the compressor must operate at Y1, and the gas mustbe throttled to the desired pressure. A comparison of thisscenario with suction throttling shows that the compressorwould operate at W1with a lower pressure ratio. The actualinlet volume to the compressor would be higher with suctionthrottling, but the weight flow to the process is the same.Because the pressure ratio is lower with suction throttling thanwith the same conditions with discharge throttling, thehorsepower that is required for suction throttling would be

lower. The example shows that the advantage of suctionthrottling depends on the shape of the dynamic compressorcurve. The steeper the curve, the greater the advantage. If thecharacteristic curve is a flat, horizontal line, there is noadvantage to suction throttling.


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Figure 4. Discharge Throttling, Constant-Pressure Control Systemand Characteristic Curves


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Blow-Off (Recycle)Constant PressureControl

Blow-off constant-pressure control is the least efficient methodof control, and it is used to extend control range with only themore efficient control methods. As previously shown in Figure4, if only blow-off control is used, the compressor would alwaysoperate at point W, regardless of the process requirements.The difference in flow between the process requirements andQWwould have to be blown off, and all of the work expendedon the extra flow would be wasted. For flows that are less thanthe surge limit, blow-off (recycle) control must be used. Thistype of control is typically used as a protection device only,and, in particular, it is used for anti-surge control.

Flow Control

Flow control can be accomplished with the same head-capacitycontrol methods as pressure control. In a flow control system,a flow transmitter (FT) senses the process flow, converts thesignal to a signal proportional to the process flow, and sendsthe signal to the flow controller (FC). The flow controlleramplifies the transmitter signal and sends a modified signal tothe control element.

Variable-SpeedConstant FlowControl

Figure 5 shows a typical variable-speed constant-flow controlsystem, and it also shows the associated characteristic curve.The characteristic curve is highlighted with the constant flowrequirements. If the compressor is operating at point Y and thehead required increases, the operating point will move up andleft along the specific speed characteristic curve as the flow

decreases. The flow transmitter will sense the decrease inflow, and the flow controller will send a proportional signal tothe turbine governor. The governor will increase the speed ofthe compressor (through an increase in turbine speed), and itwill increase the system flow back to the flow setpoint at thehigher resistance. The new operating point, Y1, would belocated on the desired flow line but at a higher pressure. Theopposite reaction will occur if process resistance decreases


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with subsequent flow increase: the compressor speed will bereduced. Any desired flow may be chosen and controlledwithin the shaded area of the curve. If the compressor has aflow-oriented anti-surge control system, the flow transmitter and

controller that are used for system control can be the same aswhat is used in the anti-surge system. Once the systemoperating requirements fall within or to the left of the surge line,the anti-surge protection system takes over and flow control ofthe process is lost. If flow control were required in the area thatis located to the left of the surge line, separate flow transmittersand controllers would be required: one flow transmitter andcontroller to serve the process control and the other flowtransmitter and control to serve the anti-surge system.

Figure 5. Variable-Speed Constant-Flow Control System


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and Characteristic CurveAn alternate variable-speed constant-flow control systemconfiguration is shown in Figure 6. In this arrangement, theflow element (FE) is located at the compressor suction. The

operation of this system is identical to the operation on thecontrol system that was previously shown in Figure 5.

Figure 6. Alternate Variable-Speed Constant-Flow Control System Configuration

Adjustable InletGuide Vane ConstantFlow Control

Figure 7 shows two, typical, adjustable, inlet guide vane,constant-flow control systems and it also shows the associatedcharacteristic curves for a constant-speed compressor. Onecontrol system measures flow on the discharge of thecompressor, and the other control system measures flow onthe compressor inlet.

The control element is the compressor guide vane mechanism.The guide vanes are adjusted through the use of a positioningcylinder. This cylinder is operated by a servo-valve (SRV) that

receives a signal from the flow controller.

If the compressor is operating at point Y and if the processresistance decreases, the flow will begin to increase, and theoperating point moves to the right and downward along thecharacteristic curve for the given inlet guide vane position. Theflow transmitter will sense the increase in flow, and it will senda signal proportional to this increase to the controller. The flow


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controller will then send a control signal that is to the SRV. TheSRV will reposition the inlet guide vanes to a greater pre-rotation vane angle, which decreases gas flow through thecompressor back to the desired flow setpoint. The new

operating point, Y1, would be located on the desired flow linebut at a lower pressure. The desired flow setpoint can beanywhere to the right and below the surge line. Like the othercontrol systems that are discussed in this module, operation inthe surge region is controlled through the use of the anti-surgecontrol system.

Figure 7. Adjustable Inlet Guide Vane Constant-Flow Control Systemand Characteristic Curves


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Suction ThrottlingConstant FlowControl

Suction throttling constant flow control operates very similarlyto the suction throttling constant pressure control. Figure 8shows a typical suction throttling constant flow control system,and it also shows the associated characteristic curve.

If the compressor is operating at point W on its unthrottledcharacteristic curve and if there is a reduction in the headrequired, the flow would increase to Y

1on the unthrottled

characteristic curve. The increase in flow would be sensed bythe flow transmitter, which would send a corresponding signalto the flow controller, which would then send the required

control signal to the STV to modulate the valve. The STV willmodulate until the desired flow, Y, is reached. The pressureratio at the compressor flanges for points W and Y is equalbecause the compressor suction pressure (after the throttlevalve) is reduced to satisfy the flow setpoint.

Figure 8. Suction Throttling Constant Flow Control Systemand Characteristic Curve


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Discharge ThrottlingConstant FlowControl

Constant flow control can be accomplished with dischargethrottling; however, as with the discharge throttling constantpressure control system, it is less efficient and it requires morepower for the same flow than suction throttling. In Figure 8, ifthe compressor is operating at point W and if a reduction inprocess resistance occurs, the flow will increase toward Y1untilthe process resistance is matched. The control system sensesthe increase in flow, and it modulates the discharge valve toreduce flow and force the compressors operating point backup along the characteristic curve to point W. With dischargethrottling, the compressor will operate at a maximum power

level, regardless of the process resistance.

Blow-Off ConstantFlow Control

As with blow-off constant pressure control, blow-off constantflow control is only used to extend the operating range and asanti-surge protection for the more efficient control methods. InFigure 8, the compressor will always operate at point W withblow-off control. If the operating point for the required flow ispoint Z, the flow QW- QZwill be blown off, and all the work

done on the excess flow will thereby be wasted.


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DYNAMIC COMPRESSOR PROTECTION SYSTEMS

The purpose of a protection system, as it pertains to thismodule, is to prevent equipment from operating underdamaging conditions. When a monitored process variableapproaches a value that could cause damage to theequipment, the protection system takes action either to restorethe variable to an acceptable value or to shut down the affectedequipment. One of the main functions of a dynamiccompressor protection system is to provide protection againstoperation in the surge area of the performance curve.Compressor surge is a large pressure and flow fluctuation thatoccurs when the compressor is operated at a higher pressureratio than the design maximum. Surge typically occurs below50% to 70% of the rated flow through the compressor,however, the surge limit can be reached from a stableoperating point through a reduction in flow, a reduction in gasdensity, a decrease in suction pressure, or an increase indischarge pressure. An anti-surge system senses conditionsapproaching surge, and it maintains the compressor pressureratio below the surge limit by recycling some of the dischargeflow to the compressor suction. Because of the heat that isgenerated by compression, a method of cooling the recycledgas flow must be used to prevent overheating of thecompressor.

In addition to preventing compressor surge, dynamiccompressor protection systems may include controls to preventstonewall. For a constant speed compressor with fixed suctionconditions, a decrease in process resistance or an increase ingas density will cause the operating point to move along theperformance curve to the right, eventually reaching a point ofmaximum flow and minimum head. Beyond this point, a furtherreduction in the process resistance or an increase in gasdensity will not increase the flow rate. This point is referred toas the choke point or stonewall. Stonewall is not particularlydamaging to single-stage centrifugal compressors, but it can

affect the rotors and blades of multi-stage centrifugal and axialcompressors. To maintain a suitable process resistance and toprevent compressor stonewall, an anti-choke controller may beused to operate an anti-choke control valve. An anti-chokecontroller is not usually required because most processsystems provide sufficient resistance to prevent choke.


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Dynamic compressor protection systems vary subtly across thevariety of available compressor types. Anti-surge protectioncontrol systems may utilize a pressure control system, a flowcontrol system, or a combination of both pressure and flow.

Surge Protection

Every centrifugal or axial compressor has (at a given rotationalspeed and at given inlet conditions) a characteristiccombination of maximum head and minimum flow beyondwhich it will surge. Prevention of this damaging phenomenonis one of the most important tasks of a dynamic compressorcontrol and protection system.

The purpose of the surge system is to prevent the low velocitygas (low flow) from entering the compressor. Surging is anoperating condition that is caused by stall in the compressorsimpeller, stator, or diffusers. Stall is described as flowseparation that results from low gas velocities. When acompressor experiences stall, the energy that is produced bythe compressor (head) decreases. The result is backflowthrough the compressor from the process, which is known assurge. The following are some of the many harmful effects ofsurge that can damage the compressor:

Rapidly rising temperature Flow fluctuations

Pressure fluctuations

Speed fluctuations

Excessive thrust

Surge can be severely damaging to a compressor and caneven cause catastrophic failure. Protection systems areinstalled that will trip the compressor and cause an emergencyshutdown if any of these effects are detected. The function of

the surge system is to continuously monitor the compressoroperating point and to open the surge control valve before thecompressor surges. Surge control is effected through use ofthe following methods:

An increase in the throughput flow.

A decrease in the required head.

An increase in the compressor speed.


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All three methods will cause the operating point to move downand/or to the right of the operating curve, away from the surgecontrol line. Because surge conditions can be defined by inletpressure, discharge pressure, inlet temperature, speed,

compressibility, and molecular weights, surge control systemscan monitor a variety of variables to determine whether acompressor surge condition is imminent. Typical surge controlsystems use flow, pressure, differential pressure, density,differential temperature, and motor power, or combinations ofthese parameters.

The most dependable and widely used method of surge controlis an increase in the throughput of the compressor by openingthe surge control valve. The surge control valve is essentially abypass valve that either recycles gas around the compressor or

blows the excess gas off to the atmosphere. Opening thesurge control valve will reduce the process system resistanceand allow the compressor to operate at a flow rate high enoughto that will prevent surge; however, because bypassing orventing of the gas wastes power, surge flow should bedetermined as accurately as possible to avoid unnecessarybypassing or venting while maintaining safe compressoroperation. The surge control setpoint is usually 5 to 10% fromthe actual surge line.

Flow Systems

A basic, volume-controlled, anti-surge system for compressorswith constant speed drivers and constant inlet conditions isshown in Figure 9. The flow transmitter (FT) senses theprocess flow through use of an orifice or venturi that serves asthe primary flow element (FE). The FT produces a signal thatis proportional to the process flow, and it sends the signal tothe surge controller (SC).

The surge controller compares the transmitted signal to itssetpoint signal. If the setpoint signal is exceeded, the surge

controller sends a signal to the surge control valve (SCV). TheSCV releases the pressure buildup at the discharge of thecompressor in response to the demands of the surge controller.The discharge of the SCV is directed to a flare on an opensuction compressor, and back to the compressor suctionthrough a cooler, or, for air compressors, to the atmospherethrough a silencer.


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As flow decreases to less than the minimum volume setpoint, asignal from the surge controller will cause the surge controlvalve to modulate to keep a minimum volume flowing throughthe compressor.

Figure 9. Basic, Volume-Controlled, Anti-Surge System


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Surge Control on aConstant-SpeedCompressor withSuction Throttling

A typical compressor installation is a plant and instrument aircompressor that is driven by a constant-speed electric motor.The compressor is required to maintain a constant pressure inthe discharge piping header. Figure 10 shows a typicalcapacity and surge control system, and it also shows theassociated performance curve for a constant speedcompressor with suction-throttling. In this scenario, thepressure transmitter (PT) and the pressure controller (PC)maintain a constant discharge pressure by throttling the suctionvalve. A flow transmitter (FT) and a surge controller (SC) are

used to measure the gas flow through the compressor. Theoperation of the system is identical to the operation of thesuction-throttling, constant-pressure control system that waspreviously discussed. As system demand decreases, thesuction throttle valve will throttle close, which decreases flowthrough the compressor. The decrease in flow through thecompressor is sensed by means of the inlet flow transmitter.

As the compressor gas flow approaches the surge controlpoint, the surge controller will modulate the surge control valveand vent the excess gas flow. As a result of the venting (orrecycle), the discharge pressure will decrease due to the

throttling of the suction throttle valve. The PC will signal thesuction throttle valve to open slightly in order to maintain thesystem pressure. The pressure and the surge controller reactindependently from each other, but they will seek a balance ofmaintaining system pressure while maintaining minimumcompressor gas flow.


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Figure 10. Typical Capacity and Surge Control System and the AssociatedPerformance Curve for a Constant-Speed Compressor with Suction Throttling


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Constant speed dictates that the compressor will always followthe performance curve. A point can be selected on theperformance curve at a predetermined distance from the surgecurve at which the surge control valve will modulate and

prevent compressor flow from decreasing past that point. Thesurge control point will be the setpoint for the surge controller.

As discussed in previous modules, the performance and surgecurves are not single lines, but they will move with inlet(ambient) pressure, temperature, and molecular weight, asshown in Figure 11. The air compressor for the compressormap that is shown in Figure 11 must provide 220 psig to thedischarge header with the mass air flow delivered at thecontrolled pressure varying with the suction pressure andtemperature (assuming constant molecular weight). The

suction pressure will vary with the throttling of the inlet throttlevalve. Temperature will vary with the change from summer towinter or with changes in the installation facility ambienttemperature. The operation of the compressor will changefrom performance curve to performance curve with thepressure and temperature changes. Each performance curvewill have its own surge curve or surge point for a single-speedmachine. The point of convergence of the surge curves isshown in Figure 11 as the expected surge line.


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Figure 11. Performance and Surge Lines with Changes in Ambient Conditions


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Surge control systems can be designed to use compressordischarge pressure to compensate for the changes in the surgepoint. Figure 12 shows a pressure-compensated surge control

system.

Figure 12. Pressure-Compensated Surge Control System

This surge control system operates in the same manner as thesurge control system that was shown previously in Figure 10with the exception of the pressure compensation. The surge

control system summer () performs calculations through useof the inlet flow (which will vary with changes in inlet conditions)and the discharge pressure (which is a constant for a pressurecontrol system). A ratio relay (R) is used to set the pressuresignal gain and bias. The surge control system summerprovides a compensated measured variable (inlet flow) to thesurge controller to compensate for the different inlet conditions.

Another option to compensate for the changes in the surgepoint is through use of suction flow rate, temperature, andpressure sensors to provide the necessary values to calculatethe actual cubic feet per minute flow rate that enters thecompressor. Calculation of the actual cubic feet per minute istypically performed when the compressed gas molecular weightis fairly constant. If the compressed gas molecular weight


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varies significantly, a density transmitter, which is located onthe compressor inlet, is used to compensate the surge controlsystem for changes in inlet conditions. A density transmitter is

a pressure transmitter configured to translate a pressure signalfor density.

Because of the following reasons, flow measurement in thedischarge of the compressor is often preferable to flowmeasurement at the compressor suction:

The pressure gradient in the suction is too small to achievea reliable flow signal.

Flowmeter permanent pressure loss is not as objectionablein the discharge header.

The inlet pipe diameter is so large that the required straightrun of piping that is needed for accurate flow measurementwould not be practicable.

A discharge flow measurement is already required forprocess reasons.

Compressor discharge flow rate must be corrected for inletconditions for use in the surge control system, which is shownin Figure 13. The configuration in Figure 13 is based on themeasurement of mass flow in the discharge and the fact that

mass flow into the compressor equals mass flow out of thecompressor. The discharge flow is compensated to mass flow,and the mass flow is compensated to inlet volumetricconditions.


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Figure 13. Discharge Mass Flow Rate Measurement Compensatedto Inlet Volumetric Flow Rate

Variable-Speed

Compressor Basedon Delta Pressureand Flow

Figure 14 shows a typical configuration for a surge controlsystem on a variable-speed compressor that is based on thepressure difference across the compressor and the gas flowrate. This system can be designed for constant flow orconstant pressure control for normal operation. The speedtransmitter (XT) and the flow transmitter (FT) or the highpressure side (discharge) signal of the differential pressure

transmitter (DPT) can be used as setpoint to the variable-speedcontroller (XIC). For the surge system, the FT provides themeasured variable. The compensated compressor pressuresignal is provided by the DPT. Together, FT and DPT definethe operating point or the input to the surge controller. As theoperating point approaches the surge control point, the surgecontroller will open the surge control valve to maintain therequired protection flow through the compressor.


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Figure 14. Surge Control System for a Variable-Speed Compressor Based onDifferential Pressure and Gas Flow Rate

Variable-SpeedMultisectionCompressors

Centrifugal compressors that use multiple sections that areequipped with an interstage cooler capable of accommodatinggas removal or addition between the sections can be describedas two separate compressors that perform different duties butthat are driven by a single shaft. Each section of thecompressor has its own set of performance curves and its ownsurge line. Figure 15 shows a basic, multisection, variable-speed compressor with an anti-surge valve that protects the


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entire compressor. The system that is shown in Figure 15 isonly suitable if the process gas does not contain anyconstituents that will condense at compressor section 1discharge conditions. If the process gas contains constituents

that will condense at compressor section 1 dischargeconditions, condensate will be drained from the intercooler, andthe molecular weight to the second section will change. In thiscase, a separate surge protection system must be used foreach compressor section.

Figure 15. Multisection, Variable-Speed Compressor with Surge Control Valvethat Protects the Entire Compressor

Figure 16 shows the performance curve and surge line for eachsection of the compressor. The operation of this surge controlsystem is identical to the surge control for a variable-speed


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compressor based on delta pressure and flow, with theexception that the differential pressure is measured across thetwo sections.

Figure 16. Performance Curves and Surge Line for Each Section of aMultisection Compressor


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Multisection compressor anti-surge control systems aretypically designed for compatibility of the sections at 100%speed. Because the sections are different, manipulation of the

speed to accommodate changes in one section will affect theother section and possibly place that section in a surgecondition. Surge control on multisection compressors must bemore stringent than surge control of a single sectioncompressor. Frequently, a separate surge control system isrequired on each section to protect that section from surge, asshown in Figure 17.

The surge control system for each section of the compressoroperates like the surge control for a variable-speed compressorbased on delta pressure and flow, but each sections surge

control system operates independently to prevent a surgecondition in each section.

Figure 17. Multisection, Variable-Speed Compressor witha Surge Control Valve for Each Section


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The mismatch in pumping capacities among the compressorsections at lower speeds will occasionally result in a surgecondition in the first compressor section during compressorstartup. During startup, the first section will not have reached

the design pressure ratio, and the density of the gas that entersthe second section will be less than the density for which thesection is designed. Although the second stage is pumping theexpected volumetric flow, it is not pumping away the expectedmass flow rate. For this reason, volumetric flow through thefirst section is less than expected, which results in a surgecondition. If the surge in the first section is severe enough tocause damage, a remotely operated valve (H) can be installedin a recycle around the first section, as shown in Figure 18.During a startup, the remotely operated control valve is openedto recycle the first section gas back to the suction. A recycle

cooler cools the recycled gas to prevent overheating in the firstsection. When the compressor is up to speed, the remotelyoperated control valve is gradually closed, and it is then leftclosed during normal operation. In multistage compressorsthat have a surge control valve for each section, the surgecontroller for the first compressor section may have a manualfunction mode, which would eliminate the need for a separateremotely operated control valve.


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Figure 18. Multisection, Variable-Speed Compressor with Remotely OperatedControl on the First Section

It is good engineering practice to require that all automatic anti-surge control systems be equipped with a manual override.

System Arrangements

Multiple compressor systems are assembled with either series-or parallel-connected dynamic compressors. Compressors thatare connected in series provide the higher pressures that arerequired in some petrochemical applications. Compressorsthat are connected in parallel provide higher flow rates at thesame pressure, and the configuration provides greaterrangeability and reliability.


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The number of variables to be considered increasesdramatically with the number of compressors in theconfiguration. As previously mentioned in this module,

compressor control and protection systems for singlecompressors have two objectives: to provide stable control ofthe compressor at all required operating conditions and toprovide protection against operation in the surge area of theperformance curve. Multiple compressor systems have twomore objectives: to balance the load among dissimilarcompressors and to safely start up and shut down thecompressors.

Series

Several surge control system designs are available forcompressors that are installed in series. One design calls for acomplete surge control system for each compressor orcompressor section, as shown in Figure 19. This system usesa constant pressure controller that senses the final dischargeheader pressure and that controls the suction throttle valve tomaintain capacity control. The surge control system on the firstcompressor uses the discharge flow rate, the pressure, and thetemperature for a mass flow and discharge pressure system.The surge controller measurement is a compensated flowsignal, and the setpoint is a biased discharge pressure signal.

The first surge control valve releases to flare or recyclesthrough a cooler (not shown) back to the compressor suction.The second compressor surge control system uses an inletflow and the differential pressure across the compressor asvariables to the surge controller. When the second compressorsurge control system actuates, the discharge from the secondcompressor is recycled back to the compressor suction.Typical installations have the recycle line installed after the gascooler (not shown) and the return line connected upstream ofthe suction knockout drum (not shown). This arrangementallows the gas and compressor temperature to be maintained

by means of a single heat exchanger. In some cases, aseparate recycle heat exchanger (not shown) is used. Eachsurge controller operates independently of the other; however,the action of each surge controller will directly affect theoperation of the other surge control system because theparameters that are measured on the compressor systems areaffected by each compressors operation.


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Figure 19. Typical Surge Control System for Compressors in Series

To minimize conflicting control problems, an integrated surge

control system is used. Figure 20 shows a typical integratedsurge control system for two compressors in series. Like thecontrol system that was shown in Figure 19, both compressorshave an independent surge control system. Both compressorsthat are shown in Figure 20 use compressor differentialpressure and inlet flow as process variables for the surgecontrol system. Conflicting interaction between the two surgecontrol systems is minimized through transmission of the


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changes in output of any one surge controller to the othersurge controller. Each surge controller uses this information toprotect its own compressor (or section) from surge.

Figure 20. Integrated Surge Control System for Compressors in Series


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Parallel

When two or more centrifugal compressors operate in parallel

and discharge into a common header line, a method ofcontrolling each compressor independently must be provided.Parallel compressors should have identical characteristics, butthe compressors that are purchased with the samespecification data usually will not have identical characteristics.Individual compressor operating characteristics vary due tomanufacturing and assembly tolerances, which would havesome effect on their individual performance; therefore, paralleloperation should have a single discharge pressure sensor inthe common header and a flow sensing device for eachcompressor. A single controller would receive the common

pressure signal and the individual compressor flow signals, andit would provide an output signal that would actuate the specificcontrol element for each compressor. Figure 21 shows a basicpressure control system for constant-speed compressors thatare arranged in parallel.


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Figure 21. Discharge Pressure Control of Constant-Speed Parallel Compressorswith Dissimilar Operating Characteristics

The most effective parallel compressor operating strategy is tosimultaneously load and unload the compressors equally asrequired. Simultaneous, equal loading and unloading willimprove the efficiency and rangeability of the parallelcompressor configuration. In some installations, the controlsystem should be set up so that the compressors in the parallelsystem will sequentially load and unload. When the controlsystem is set up for sequential loading/unloading, the leastefficient compressor should be loaded last and unloaded first.The control system for compressors in a parallel configurationshould unload the compressors so that all compressors in theparallel system will reach their control lines simultaneously.


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The surge control system for compressors that operate inparallel is more complex than the system for compressors thatoperate in series. When two or more centrifugal compressorsoperate in parallel, the greatest surge protection and efficiency

results from all compressors operating equidistant from theirrespective surge control lines. The compressor andinstrumentation industries have adopted a criterion to measurethe angular distance between the operating point and the surgecontrol line. The criterion is known as the S-Criterion. The Svalue is a dimensionless number that is relative. The absolutevalue of the number has, therefore, no meaning; however,compressors that have the same S number will be operatingequidistantly from their respective surge lines. A surge controlsystem that causes all of the S numbers to be equal will,therefore, ensure that all compressors simultaneously

approach the surge control line.

The S-Criterion can be calculated through use of the followingequation:

Sp b

p

c

o

=+

Where:

pc = The differential pressure across the

compressor

b = The surge margin

po = The differential pressure across the flowelement

The S-Criterion will be less than 1 when the operating point issafely away from surge, and it will be equal to 1 when theoperating point is on the surge line control line. Figure 22shows a parallel compressor configuration with suction

pressure control, load sharing control based on the deviation ofthe compressor operating point from the surge control line (S -1), and anti-surge control. The anti-surge controllers calculatethe S value for the compressors, and they monitor thecompressor parameters to detect the approach to surgecondition. The two load-sharing controllers, one for eachcompressor, perform a calculation to ensure that allcompressors will simultaneously reach their surge lines.


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Figure 22. Control System that Uses the S-Criterion for Compressorsin Parallel Configuration


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POSITIVE-DISPLACEMENT COMPRESSOR CONTROL SYSTEMS

Unlike centrifugal compressors, positive-displacementcompressors cannot self-regulate capacity against a givendischarge pressure. A positive-displacement compressor willsimply keep displacing gas until indication is received to controlit otherwise. Capacity control for positive-displacementcompressors is usually accomplished in steps, eitherautomatically or manually, through the use of suction valveunloaders, clearance pockets or slider valves, or a bypassvalve. These basic control system options for positive-displacement compressors are used to maintain constantsuction pressure, constant discharge pressure, or a desiredflow rate through the compressor.

The control systems that are selected for use are dependentupon the operating requirements of the compressor. Suctionvalve unloading, which is the most commonly used, loads orunloads cylinders. Clearance pockets are commonly used forsmall capacity adjustment of cylinders with no power change;however, when clearance pockets are used, they reducecylinder efficiency. Bypass valves are useful in placing acompressor under load during a process system startup orshutdown; however, because the energy of compression iswasted, bypass valves do not provide an efficient method forloading or unloading a compressor. Bypass valves may be

used in conjunction with unloaders or clearance pockets toexactly obtain the desired capacity values. Variable-speedcontrol is not a preferred method for positive-displacementprocess compressors because the use of variable-speedcontrol may result in problems with valve design and rodreversal. Variable-speed control for positive-displacementcompressors will not be discussed in this module.

Valve Unloading

Suction valve unloaders, as shown in Figure 23, are the mostcommonly used capacity control device. An unloader holds thecylinder suction valve open during the suction and compressionpiston strokes; so, suction gas is only pushed back and forth inthe cylinder. The cylinder continues to take in gas normally;however, instead of completing the normal cycle ofcompression and discharge, the cylinder will simply pump thegas, still at suction pressure, back into the suction chamber via


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the open pathway. No gas is discharged to the process.Additionally, because there is no occurrence of compression,virtually no horsepower is consumed other than through

passageway losses.

Direct, manual operation of unloaders may be satisfactory forsimple one- or two- cylinder services in which the process doesnot require automatic control and in which sufficient time foroperation is available. When automatic control is required, theunloader is fitted with a piston or diaphragm. A signal from acontrol device (either the air or the gas being processed)depresses the diaphragm. The diaphragm is connected tofingers that open the suction valve.

Figure 23. Suction Valve Unloader


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All valve unloader types can be manually operated, or they canbe actuated through use of a pneumatic cylinder. Whenpneumatically actuated, these devices can be designed to loador unload upon either application on removal of air pressure.

The advantages of pneumatic operation are the ability toremotely control the capacity of the compressor or even toautomate the control.

In applications that use cylinder lubrication, the unloaders areusually timed to prevent excessive accumulation of lube oil inthe cylinder. On double-acting cylinders, for example, afterapproximately 30 minutes of operation, the head-end unloaderwill briefly close, and the crank-end unloader will briefly open todrain excessive oil.

Finger-type unloaders are shown in Figure 24. There are threetypes of pneumatically operated finger-type unloaders: (a)direct-acting (air-to-unload); (b) reverse-acting or fail-safe (air-to-load), which automatically unloads the compressor in theevent of control air failure; and (c) manual operation.

The finger-type unloaders consist of a series of small fingersthat are housed in the valve crab assembly and that areactuated through use of a push rod from an outside actuator.To unload the valve, the fingers are lowered so that theydepress the valve-sealing components and hold the valve in

the open position. The pathway between the cylinder bore andthe gas passage is through these open suction valves. Finger-type unloaders will typically be mounted on each suction valveso that the flow area of the unloaded pathway is maximized.

Also, because the fingers simply hold open the existing suctionvalves, no special valve design is required. Actuation of finger-type unloaders can be manual (through the use of a handwheeland screw or lever arrangement to lower the fingers) orautomatic (through the use of a small air cylinder on the top ofthe unloader stem).


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Figure 24. Finger-Type Unloaders


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One of the major problems that is associated with finger-typeunloaders is the potential for damaging the valve-sealingelements with the fingers. The force that is generated bypneumatically actuated fingers, as they are driven down

against the valve-sealing components, can contribute topremature valve failure.

In accordance with SAES-K-403 and in order to minimizemaintenance and to increase valve life, reduced unloaderpressure settings will be used whenever possible. Pressuresettings must be compatible with the ESD system set pressure,which is determined by the minimum acceptable systempressure that is required for safe plant operation. Pressuresettings must also provide sufficient receiver storage capacityto allow startup of the standby compressor. Unloader controls

must be set to maintain a 100 kPa (ga; 15 psig) pressuredifferential from loading to unloading. If air is used to operatethe unloader, external operators with a vent chamber betweenthe diaphragm and the vent packing are mandatory forflammable gas service.

Manufacturers standard automatic control may be either on/offor step unloading. On/off control is acceptable for smallprocess air or gas compressors in intermittent service, but thedriver must be sized for frequent on-load starting. Automatic ormanual step unloading may be accomplished through the use

of either suction valve unloaders, clearance pockets, or acombination of both. Five-step unloading must providecapacities of 100 percent, 75 percent, 50 percent, 25 percent,and 0 percent; three-step unloading must provide capacities of100 percent, 50 percent, and 0 percent; and two-stepunloading must provide capacities of 100 percent and 0percent. If a cylinder is unloaded to 0 percent, specialprecautions must be taken to prevent overheating in thecylinder.

In general, suction valve unloading is an excellent method to

control capacity. The devices are simple and easy to maintainand operate. Suction valve unloaders are efficient, and theyare very good for startup unloading so that starting torquerequirements are extremely low.


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Clearance Pockets

Clearance pockets, which are shown in Figure 25, are pocketsor reservoirs that are attached to the cylinders. For reducedcapacity operation, the clearance pocket valve is opened, andthe cylinder capacity is reduced by the effect of this addedclearance on the volumetric efficiency. The gas is compressedinto the pockets on the compression stroke, and the gasexpands into the cylinder on the suction stroke to reduce theintake of additional gas. Clearance pockets provide anadditional volume to the fixed clearance volume of a cylinder.This additional volume reduces the amount of gas that isintroduced during the suction stroke of the piston. Thereduction of the amount of gas that is introduced results in areduced capacity of the compressor.

Figure 25. Clearance Pockets


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The gas in the volume at the end of the compression stroke isat the elevated discharge pressure. During the suction strokeof the piston, the volume expands in the cylinder to a pressurethat is lower than the suction pressure to the particular cylinder.

The expansion volume is constant for any cylinder with aconstant stroke length; therefore, if additional volume of high-pressure clearance gas is available due to the clearancepocket, the pressure level of the expanded gas at the end ofthe suction stroke will be higher. Higher pressure results inless suction pressure gas being introduced into the cylinderand in a reduction in the compressor capacity.

The cylinder volume increased by the clearance pockets doesnot have an effect on the power that is required for thecompression stroke. The cylinder pressure at the beginning

and end of the compression stroke is the same, and the strokevolume remains unchanged. Clearance pockets are usually ofa fixed volume, and they are sized to reduce flow precisely to apredetermined level. Typically, the use of multiple fixed-volumeclearance pockets that allow for numerous reduced-capacitysteps of control are used.

In accordance with SAES-K-403, fixed-volume clearancepockets that allow the capacity to be reduced through anincrease of the clearance volume of the cylinders may bemanually or automatically operated. Variable volume pockets

must not be used.

Bypass Operation

A bypass valve system places a bypass valve in a line from thecompressor discharge back to the compressor suction to routesome or all of the compressor discharge to the suction. Abypass valve may be used as the sole means of control, but itis usually employed in combination with other control methods.The bypass valve controls capacity by directing thecompressed gas back to the compressors suction. Directing

the compressed gas back to the compressor suction isaccomplished by piping from the compressors discharge line,through a control valve, back to the compressors suction line.To reduce the flow to process, the bypass valve is opened, andthe excess flow is diverted back to the compressors suction.In addition to being simple, this system also has the advantageof being infinitely controllable (within the limitation of the size ofthe bypass line).


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Use of the bypass valve for continuous capacity controlrequires that the bypass gas stream be provided with a coolerto remove the heat of compression prior to returning to thesuction. The use of a bypass valve across the compressor is

not as power-efficient as is the use of cylinder unloading.

The most practical application for the bypass line is for smalldegrees of fine capacity control or for limited duration start-upunloading, where a simple loop around the compressor can beopened for a short period of time to relieve the initialcompression load.


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POSITIVE-DISPLACEMENT COMPRESSOR PROTECTION SYSTEMS

Typical positive-displacement compressor protection systemsconsist of relief valves, startup bypass valves, and high processtemperature indication and control. Individually or combined,these protection devices help to ensure safe and reliableoperation of the compressor system.

Relief Valve (Stage)

A relief valve (PZV) is an automatic pressure-relieving devicethat is actuated by the static pressure upstream of the valve.When the static pressure upstream of the PZV exceeds itsallowable value by a specified amount, the PZV actuates to

relieve the pressure. Conventional PZVs are the mostcommon, and they open fully when actuated. Pilot-operatedPZVs are less common, and they modulate when actuated.

In accordance with SAES-J-600, PZV(s) must be provided forpositive-displacement compressors where the pressure at aclosed discharge can exceed safe limits. For positive-displacement compressors, interstage PZVs, as well asdischarge PZVs, must be provided. The pressure setpointmust exceed the rated discharge pressure by 10 percent or175 kPa (ga), whichever is greater. For reciprocating

compressors, a greater differential than 10 percent may berequired due to pressure surges. Interstage PZVs must be setat or above the compressors settling-out pressure to avoidlifting at shutdown. In addition, the PZV capacity must equalcompressors capacity, and it must discharge to a safe area orflare and not to the compressor suction.

The relative setting of the relief valves in each stage of a typicalthree-stage reciprocating compressor is basically the same.The stage discharge piping and components are protectedfrom overpressurization by the PZV, which is set at

approximately 10% above the stage discharge pressure.

Startup Bypass

In most instances, a reciprocating compressor must beunloaded for startup. Practically all reciprocating compressorsmust be unloaded to some degree before starting so that the


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driver torque that is available during acceleration is notexceeded. The need for an unloaded startup is requiredbecause starting a positive-displacement compressor fullyloaded can have a 3:1 peak-to-mean torque ratio. This peak

torque requirement, coupled with breakaway friction, meansthat the driver now must have as much as a 350 percentstarting torque capability. Typical motors are designed to haveonly 40 to 60 percent starting torque capability.

Both manual and automatic compressor startup unloading isused. Common methods of unloading during startup includedischarge venting, discharge to suction bypass, and cylinderunloading.

High Process Temperature

In accordance with 31-SAMSS-002, the high temperatureshutdown device is required to safely shut down thecompressor. At a minimum, a high temperature shutdowndevice must be installed in the final stage discharge gasstream, and a high temperature shutdown device must beinstalled downstream of the aftercooler. Additional hightemperature shutdown devices may be installed for high lubeoil temperature. On some compressors, a temperature switchmay be unsuitable due to high vibration levels at or near the

cylinder head. Thermocouples or RTDs should be used as ameans of temperature measurement.

API-618 specifies that the maximum discharge temperature of

300F can be exceeded for compressors with non-lubricatedcylinders. Temperature control for non-lubricated compressorsmust comply with the requirements of 31-SAMSS-002. Notethat SAES-K-403 requires that compressors used in hydrogen

service limit discharge temperature to 275F (135C).


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GLOSSARY

aftercooler Heat exchanger for cooling air or gas discharged from

compressors. Aftercoolers also provide a means ofremoving moisture from compressed air and gases.

actual capacity Quantity of gas actually compressed and under actualpressure and temperature conditions.

clearance pocket An auxiliary volume that may be opened to the clearancespace to increase the clearance, usually temporarily, toreduce the volumetric efficiency and, therefore, actualcapacity of the compressor.

guide vane A stationary element, which may be adjustable, that directs

the gas to the inlet of a compressor impeller or blade.intercooler Heat exchanger for removing the heat of compression

between stages of a compressor.

modulation Manipulation of one variable (the manipulated variable) inorder to control another variable (the control variable).

performance curve A plot of expected operating characteristics, such as head ordischarge pressure versus inlet capacity.

stonewall A point of maximum flow and minimum head or dischargepressure on a dynamic compressor operating curve and

beyond which a reduction i

operation of compressor control and protection systems

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