Operation of Compressor Control and Protection Systems

Engineering Encyclopedia Saudi Aramco DeskTop Standards

OPERATION OF COMPRESSOR CONTROL AND PROTECTION SYSTEMS

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

Chapter : Mechanical File Reference: MEX-212.05

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

Engineering Encyclopedia

Compressors Operation of Compressor Control and Protection Systems

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INFORMATION ............................................................................................................... 4 INTRODUCTION............................................................................................................. 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 with Suction 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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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 and Characteristic Curves ................................................................................... 7 Figure 2. Adjustable Inlet Guide Vane Constant Pressure Control System and Characteristic Curves ................................................................................. 10 Figure 3. Suction Throttling Constant-Pressure Control System and Characteristic Curves ................................................................................. 12 Figure 4. Discharge Throttling, Constant-Pressure Control System and Characteristic Curves ................................................................................. 16 Figure 5. Variable-Speed Constant-Flow Control System and Characteristic Curve ................................................................................... 18 Figure 6. Alternate Variable-Speed Constant-Flow Control System Configuration ................................................................................. 19 Figure 7. Adjustable Inlet Guide Vane Constant-Flow Control System and Characteristic Curves ................................................................................. 20 Figure 8. Suction Throttling Constant Flow Control System and Characteristic Curve ................................................................................... 21 Figure 9. Basic, Volume-Controlled, Anti-Surge System .............................................. 26 Figure 10. Typical Capacity and Surge Control System and the Associated Performance Curve for a Constant-Speed Compressor with Suction Throttling .................................................................................................... 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 Inlet Volumetric Flow Rate ................................................................................. 33 Saudi Aramco DeskTop Standards

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Figure 14. Surge Control System for a Variable-Speed Compressor Based on Differential Pressure and Gas Flow Rate ................................................... 34 Figure 15. Multisection, Variable-Speed Compressor with Surge Control Valve that Protects the Entire Compressor.................................................................. 35 Figure 16. Performance Curves and Surge Line for Each Section of a Multisection Compressor ................................................................................................ 36 Figure 17. Multisection, Variable-Speed Compressor with a Surge Control Valve for Each Section ............................................................................... 37 Figure 18. Multisection, Variable-Speed Compressor with Remotely Operated Control 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 Compressors with Dissimilar Operating Characteristics ................................................... 44 Figure 22. Control System that Uses the S-Criterion for Compressors in Parallel Configuration .............................................................................................. 46 Figure 23. Suction Valve Unloader............................................................................... 48 Figure 24. Finger-Type Unloaders ............................................................................... 50 Figure 25. Clearance Pockets ...................................................................................... 52

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INFORMATION INTRODUCTION For the purposes of this module, a control system functions to maintain process variables within their prescribed ranges. If a process variable approaches a value outside of its prescribed range and that could result in damage to the monitored equipment, a protection system will function either to restore the variable to an acceptable value or to shut down the equipment. The control and protection systems that are used on dynamic and positive-displacement compressors are different because the systems reflect the characteristics of the equipment. The dynamic compressor control system must maintain the compressor flow rate and the discharge pressure within prescribed limits. The protection system must prevent the compressor from operating under surge or stonewall conditions. Surge and stonewall are damaging conditions, and they are discussed in more detail later in this module. Unlike the control and protection systems of a dynamic compressor, a positive-displacement compressor cannot selfregulate capacity against a given discharge pressure; the compressor, because its characteristic is constant volume, will simply continue to displace gas until it receives a signal not to do so. As a result, various methods of changing the volume flow must be used. Because each rotation or stroke of the compression elements will displace a given volume of flow in the discharge system, protection of all positive-displacement compressors requires a device to limit discharge pressure. Because the volume of the discharge system is fixed, the discharge pressure will continue to rise.

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DYNAMIC COMPRESSOR CONTROL SYSTEMS Dynamic compressor controls can vary from the very basic manual recycle control to elaborate ratio controllers. In accordance with SAES-K-402, the control system must be adequate to control the compressor at all specified operating conditions. The driver characteristics, the process response, and the compressor operating range must be determined before the type of controls are chosen. Control systems for dynamic compressors that are used at Saudi Aramco facilities vary in their method of control. Control and protection systems for dynamic compressors have fundamentally only two functions to accomplish: • To provide stable control of the compressor at all of the required operating conditions that are specified on the data sheet. • To provide protection against operation in the surge area of the performance curve. Anti-surge control is part of the compressor protection system that is discussed later in this module. Dynamic compressor control systems are designed to maintain a 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, manipulation of the compressor suction pressure may be required for upstream stability of the process. For example, on a variable speed controlled compressor, the governor would receive a compressor suction pressure signal that would initiate a speed increase upon an increase of suction pressure. A speed increase would increase the compressor flow and probably the discharge pressure. The reverse would occur if the suction pressure 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 autoselector controller receives inputs, such as flow, suction pressure 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 decreasing efficiency) that are most commonly used for pressure control and 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 a performance control element. Process pressure is monitored, and a signal from a pressure transmitter is sent to the pressure controller. The pressure controller adjusts the control element, which might be a guide vane positioner, a suction or discharge control valve, or a rotational speed governor. The control element would operate to maintain the process pressure at a setpoint value. Variable-Speed Constant Pressure Control The most efficient way to match the compressor characteristic to the required output is to change speed in accordance with the fan laws. This variable-speed operation is most easily accomplished through use of steam turbines, gas turbines, or variable-speed (frequency) electric motors as drivers for compressors. With such drivers, the speed can be manually controlled through adjustment of the speed controller by an operator, or the speed adjustment can be made automatically through use of a pneumatic or electric controller that changes the speed in response to a pressure or flow signal. Because the only energy required by the process is provided by the compressor without the use of throttling devices, variablespeed control is the most efficient method of control. The operating 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 it also shows the associated characteristic curves. The characteristic curves shown in Figure 1 assume a constant inlet pressure (P1), inlet temperature (T1), and gas composition. Each curve shows the pressure at which the compressor supplies a certain volume rate of flow (Q) for a given speed. If the compressor discharge pressure required by the process exceeds the maximum pressure the compressor can produce for a given speed, compressor surge will occur. The surge line on the graph indicates the limit of minimum flow.

Figure 1. Variable-Speed Constant-Pressure Control System and Characteristic Curves Saudi Aramco DeskTop Standards

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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, the discharge pressure immediately falls and the operating point moves to the right and downward along the characteristic curve for the given speed. The pressure transmitter will sense the lowering pressure, and the pressure controller will send a control signal to the turbine governor. The governor will increase the speed of the compressor (through an increase in turbine speed), which results in an increase in the system pressure back to the pressure setpoint. The new operating point would be located on the desired pressure line but further to the right. If the process required less gas flow, the discharge pressure would begin to increase and the control system would decrease the speed of the compressor (through a decrease in turbine speed) until the pressure setpoint is restored. The flow could be reduced until point X was reached. Point X is set at the minimum operating point before the surge line (surge control line). Anti-surge controls, which are discussed later in this module, will prevent the operating point from moving to the left of point X on each speed curve. If the process required a flow rate of only point Z, the volume of gas (Qx - Qz) would have to be blown off or recycled. The operating control would have to be shifted from variable-speed control to blow-off control, which is the only control that is available when the process requires flows that are below the stable operating range. Blow-off control is discussed later in this module. Adjustable Inlet Guide Vane Constant Pressure Control Inlet guide vanes evenly distribute the inlet flow to the compressor stage impellers. Adjustable inlet vanes are built into the inlet of the first stage, or succeeding stages of axial compressors, and they can be automatically or manually controlled through a linkage mechanism. Adjustable guide vanes are used for the control of axial and single-stage centrifugal compressors. Single-stage compressors frequently incorporate an axial inlet, and they do not require fixed guide vanes. Pre-rotation adjustable guide vanes pre-whirl the gas that enters the compressor stage in the direction of rotation, which

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develops less head than at design. Between full open and maximum pre-whirl position, adjustable guide vanes provide some degree of reduced horsepower over the suction throttling valve. Counter-rotation adjustable guide vanes are used to extend the useful operating range of any dynamic compressor. The range of operation is extended through a change of the angle of attack and the inlet gas velocity to the impeller blade. For the high flow region, the angle of attack is increased to eliminate flow separation and to effect an increase in the produced head of the impeller or blade. The elimination of flow separation and an increase in the produced head will increase the capacity range of the impeller. Adjustable inlet guide vanes are expensive, limited in effectiveness, and present many maintenance and operational problems. At Saudi Aramco, centrifugal compressor adjustable inlet guide vanes have proven to be mechanically unreliable in general services; therefore, prior to control selection, the economics of inlet guide vanes must be considered because of their higher initial cost, complex mechanism, maintenance, and requirement for frequent adjustment. Adjustable inlet guide vanes 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 constant pressure control system, and it also shows the associated characteristic curves for a constant speed compressor.

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Figure 2. Adjustable Inlet Guide Vane Constant Pressure Control System and Characteristic Curves The control element is the compressor guide vane mechanism. The guide vanes are adjusted through use of a positioning cylinder. This cylinder is operated by a servo-valve (SRV) that receives a signal from the pressure controller. If the compressor is operating at flow rate Qy and if the process requires an increase in flow, the discharge pressure immediately falls, and the operating point moves to the right Saudi Aramco DeskTop Standards

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and downward along the characteristic curve for the given inlet guide vane (IGV) position. The pressure transmitter (PT) will sense 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 through the compressor and the system pressure back to the pressure setpoint. The new operating point would be located on the desired pressure line but further to the right (point W). If the process required less gas flow, the discharge pressure would begin to increase and the control system would close the inlet guide vanes, which decreases the gas flow through the compressor until the pressure setpoint is reached. The flow could be reduced until point X was reached. Point X is set at the minimum operating point before the surge line. Anti-surge controls, which are discussed later in this module, will prevent the operating point from moving to the left on each inlet guide vane position curve similar to point X. Like the variable-speed constant pressure control, if the process required a flow rate of point Z, the volume of gas (Qx - Qz) would have to be blown off or recycled. The operating control would have to be shifted from adjustable inlet guide vane control to blow-off control. Suction Throttling Constant Pressure Control Suction throttling control, which is also known as intake throttling or capacity modulation control, is usually used in situations in which the compressor is not equipped with inlet guide vanes and is driven by a constant-speed drive. Suction throttling is more efficient than discharge throttling by approximately 3 to 5%. This control is also applied in plant and instrument air compressor systems when the demand for air is relatively constant. The system usually includes a large air receiver, which allows large volume draws to affect major pressure changes in the receiver pressure so that the air compressor can modulate the flow with relatively small pressure variations. Compressors with this type of control system have a single pressure-volume characteristic curve. Figure 3 shows a typical suction throttling constant pressure control system, and it also shows the associated characteristic curves for a constantspeed compressor.

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Figure 3. Suction Throttling Constant-Pressure Control System and Characteristic Curves The suction throttle valve (STV) is normally included as part of the compressor package. Starting the system, especially with a motor driver, with the suction throttle closed and the discharge anti-surge vent valve open, will develop a vacuum on the inlet to the impellers. Although this type of startup reduces the motor starting torque and the horsepower requirements, it must be avoided. SAES-K-402 states that suction throttling must not result in subatmospheric pressure and risk of air ingestion into the process streams. Starting torque is not critical with a steam turbine driver, where the compressor Saudi Aramco DeskTop Standards

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package will be brought up to speed much slower. The discharge anti-surge vent valve will still be open, however, for turbine startup. Suction throttling should be used when compressor flow and/or discharge pressure may vary as required by the process. The suction throttling valve may receive actuation signals from the flow sensing device, from discharge pressure, or from suction pressure 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 the suction knockout drum and the anti-surge recycle return line. The preferred location of the recycle return line is upstream of the knockout drum. Such a location ensures good mixing of the recycle stream with the main suction stream prior to reaching the compressor. The preferred location of the suction throttle valve is close to the compressor suction. When electric motors are used as constant speed drivers, the centrifugal compressor is normally controlled through use of a suction throttling valve. Butterfly valves are typically used as suction throttling valves because they minimize flow disturbance. Throttling the suction results in a slightly lower suction pressure than the pressure for which the machine is designed and, therefore, a higher total head is required if the discharge pressure must remain constant. The increase in total head can be matched to the compressor head-capacity curve, i.e., higher head at reduced flow. In throttling the inlet, the density of the gas is reduced, which results in a matching of the required weight flow to the compressor inlet-volume capabilities at other points on the head/capacity curve. In the control system that is shown in Figure 3, the value of pressure is sensed by the pressure transmitter (PT). The pressure transmitter converts this signal to a signal that is proportional to the process pressure, and it sends a signal to the pressure controller (PC). The pressure controller amplifies the transmitter signal and sends a modified signal to the control element. Depending on system requirements, the controller may require additional correction factors, which are called reset and 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 process flow requirements, the pressure would increase to Y1 on the unthrottled characteristic curve. The increase in pressure would be sensed by the pressure transmitter, and a control signal would be sent by the pressure controller to the STV to modulate the valve. By throttling across the STV, the inlet pressure can be reduced, and, although the compressor is operating at the pressure ratio and volume of point Y1, the discharge pressure and volume flow to the process will be equivalent to point Y. To further explain the operation, the following example, which assumes that the STV is fully open, should be considered: Qw = 100%,

P3 = 2.0 P 1w

Qy1 = 80%,

P3 1 Y = 2.1 P1

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)Y1, the pressure ratio is 2.10. To maintain the discharge pressure of 29.4 psia, the inlet pressure (P2) must be reduced to 14.0 psia (29.4/2.10). The volume to the compressor Y1 (at pressure P2) is 80%, but the equivalent volume Y (at pressure P1) is less than the ratio of 80% x (14.0/14.7), or 76.3%, which is the actual volume at pressure P1 that is delivered to the process. Anti-surge controls, which are discussed later in this module, will prevent the operating point from moving to the left past point X. Like the variable-speed constant-pressure control, if the process required a flow rate of point Z, the excess flow would need to be blown off or recycled. Discharge Throttling Constant Pressure Control Discharge pressure throttling for constant pressure is less efficient than suction throttling; however, it may be more economical from the standpoint of requiring a smaller throttling valve and flanges. The discharge throttling valve is located

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downstream of the anti-surge recycle supply line. If a main discharge aftercooler is used to cool the recycle gas, then the throttle valve is located downstream of this cooler, which may be a considerable distance from the compressor. If the recycle gas has a dedicated cooler, then the throttle valve can be located upstream of the aftercooler. As with suction throttling, only one pressure-volume characteristic curve is associated with discharge throttling for a constant-speed compressor. Figure 4 shows a typical discharge throttling constant pressure control system and the associated characteristic curves for a constant-speed compressor. Pressure control is maintained by throttling the actual compressor discharge pressure to the desired setpoint along the characteristic curve. Discharge throttling requires more power than suction throttling for the same flow. For example, if the process requires 80% flow with discharge throttling, the compressor must operate at Y1, and the gas must be throttled to the desired pressure. A comparison of this scenario with suction throttling shows that the compressor would operate at W1 with a lower pressure ratio. The actual inlet volume to the compressor would be higher with suction throttling, but the weight flow to the process is the same. Because the pressure ratio is lower with suction throttling than with the same conditions with discharge throttling, the horsepower that is required for suction throttling would be lower. The example shows that the advantage of suction throttling depends on the shape of the dynamic compressor curve. The steeper the curve, the greater the advantage. If the characteristic curve is a flat, horizontal line, there is no advantage to suction throttling.

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

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Blow-Off (Recycle) Constant Pressure Control Blow-off constant-pressure control is the least efficient method of control, and it is used to extend control range with only the more efficient control methods. As previously shown in Figure 4, if only blow-off control is used, the compressor would always operate at point W, regardless of the process requirements. The difference in flow between the process requirements and QW would have to be blown off, and all of the work expended on the extra flow would be wasted. For flows that are less than the surge limit, blow-off (recycle) control must be used. This type 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-capacity control methods as pressure control. In a flow control system, a flow transmitter (FT) senses the process flow, converts the signal to a signal proportional to the process flow, and sends the signal to the flow controller (FC). The flow controller amplifies the transmitter signal and sends a modified signal to the control element. Variable-Speed Constant Flow Control Figure 5 shows a typical variable-speed constant-flow control system, and it also shows the associated characteristic curve. The characteristic curve is highlighted with the constant flow requirements. If the compressor is operating at point Y and the head required increases, the operating point will move up and left along the specific speed characteristic curve as the flow decreases. The flow transmitter will sense the decrease in flow, and the flow controller will send a proportional signal to the turbine governor. The governor will increase the speed of the compressor (through an increase in turbine speed), and it will increase the system flow back to the flow setpoint at the higher resistance. The new operating point, Y1, would be located on the desired flow line but at a higher pressure. The opposite reaction will occur if process resistance decreases Saudi Aramco DeskTop Standards

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with subsequent flow increase: the compressor speed will be reduced. Any desired flow may be chosen and controlled within the shaded area of the curve. If the compressor has a flow-oriented anti-surge control system, the flow transmitter and controller that are used for system control can be the same as what is used in the anti-surge system. Once the system operating requirements fall within or to the left of the surge line, the anti-surge protection system takes over and flow control of the process is lost. If flow control were required in the area that is located to the left of the surge line, separate flow transmitters and controllers would be required: one flow transmitter and controller to serve the process control and the other flow transmitter and control to serve the anti-surge system.

Figure 5. Variable-Speed Constant-Flow Control System

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and Characteristic Curve An alternate variable-speed constant-flow control system configuration is shown in Figure 6. In this arrangement, the flow element (FE) is located at the compressor suction. The operation of this system is identical to the operation on the control system that was previously shown in Figure 5.

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

Adjustable Inlet Guide Vane Constant Flow Control Figure 7 shows two, typical, adjustable, inlet guide vane, constant-flow control systems and it also shows the associated characteristic curves for a constant-speed compressor. One control system measures flow on the discharge of the compressor, and the other control system measures flow on the compressor inlet. The control element is the compressor guide vane mechanism. The guide vanes are adjusted through the use of a positioning cylinder. 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 process resistance decreases, the flow will begin to increase, and the operating point moves to the right and downward along the characteristic curve for the given inlet guide vane position. The flow transmitter will sense the increase in flow, and it will send a 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. The SRV will reposition the inlet guide vanes to a greater prerotation vane angle, which decreases gas flow through the compressor back to the desired flow setpoint. The new operating point, Y1, would be located on the desired flow line but at a lower pressure. The desired flow setpoint can be anywhere to the right and below the surge line. Like the other control systems that are discussed in this module, operation in the surge region is controlled through the use of the anti-surge control system.

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

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Suction Throttling Constant Flow Control Suction throttling constant flow control operates very similarly to the suction throttling constant pressure control. Figure 8 shows 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 unthrottled characteristic curve and if there is a reduction in the head required, the flow would increase to Y1 on the unthrottled characteristic curve. The increase in flow would be sensed by the flow transmitter, which would send a corresponding signal to the flow controller, which would then send the required control signal to the STV to modulate the valve. The STV will modulate until the desired flow, Y, is reached. The pressure ratio at the compressor flanges for points W and Y is equal because the compressor suction pressure (after the throttle valve) is reduced to satisfy the flow setpoint.

Figure 8. Suction Throttling Constant Flow Control System and Characteristic Curve

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Discharge Throttling Constant Flow Control Constant flow control can be accomplished with discharge throttling; however, as with the discharge throttling constant pressure control system, it is less efficient and it requires more power for the same flow than suction throttling. In Figure 8, if the compressor is operating at point W and if a reduction in process resistance occurs, the flow will increase toward Y1 until the process resistance is matched. The control system senses the increase in flow, and it modulates the discharge valve to reduce flow and force the compressor’s operating point back up along the characteristic curve to point W. With discharge throttling, the compressor will operate at a maximum power level, regardless of the process resistance. Blow-Off Constant Flow Control As with blow-off constant pressure control, blow-off constant flow control is only used to extend the operating range and as anti-surge protection for the more efficient control methods. In Figure 8, the compressor will always operate at point W with blow-off control. If the operating point for the required flow is point Z, the flow QW - QZ will 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 this module, is to prevent equipment from operating under damaging conditions. When a monitored process variable approaches a value that could cause damage to the equipment, the protection system takes action either to restore the variable to an acceptable value or to shut down the affected equipment. One of the main functions of a dynamic compressor protection system is to provide protection against operation in the surge area of the performance curve. Compressor surge is a large pressure and flow fluctuation that occurs when the compressor is operated at a higher pressure ratio than the design maximum. Surge typically occurs below 50% to 70% of the rated flow through the compressor, however, the surge limit can be reached from a stable operating point through a reduction in flow, a reduction in gas density, a decrease in suction pressure, or an increase in discharge pressure. An anti-surge system senses conditions approaching surge, and it maintains the compressor pressure ratio below the surge limit by recycling some of the discharge flow to the compressor suction. Because of the heat that is generated by compression, a method of cooling the recycled gas flow must be used to prevent overheating of the compressor. In addition to preventing compressor surge, dynamic compressor protection systems may include controls to prevent stonewall. For a constant speed compressor with fixed suction conditions, a decrease in process resistance or an increase in gas density will cause the operating point to move along the performance curve to the right, eventually reaching a point of maximum flow and minimum head. Beyond this point, a further reduction in the process resistance or an increase in gas density will not increase the flow rate. This point is referred to as the choke point or stonewall. Stonewall is not particularly damaging to single-stage centrifugal compressors, but it can affect the rotors and blades of multi-stage centrifugal and axial compressors. To maintain a suitable process resistance and to prevent compressor stonewall, an anti-choke controller may be used to operate an anti-choke control valve. An anti-choke controller is not usually required because most process systems provide sufficient resistance to prevent choke.

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

Surge Protection Every centrifugal or axial compressor has (at a given rotational speed and at given inlet conditions) a characteristic combination of maximum head and minimum flow beyond which it will surge. Prevention of this damaging phenomenon is one of the most important tasks of a dynamic compressor control and protection system. The purpose of the surge system is to prevent the low velocity gas (low flow) from entering the compressor. Surging is an operating condition that is caused by stall in the compressor’s impeller, stator, or diffusers. Stall is described as flow separation that results from low gas velocities. When a compressor experiences stall, the energy that is produced by the compressor (head) decreases. The result is backflow through the compressor from the process, which is known as surge. The following are some of the many harmful effects of surge 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 can even cause catastrophic failure. Protection systems are installed that will trip the compressor and cause an emergency shutdown if any of these effects are detected. The function of the surge system is to continuously monitor the compressor operating point and to open the surge control valve before the compressor surges. Surge control is effected through use of the 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 down and/or to the right of the operating curve, away from the surge control line. Because surge conditions can be defined by inlet pressure, discharge pressure, inlet temperature, speed, compressibility, and molecular weights, surge control systems can monitor a variety of variables to determine whether a compressor surge condition is imminent. Typical surge control systems use flow, pressure, differential pressure, density, differential temperature, and motor power, or combinations of these parameters. The most dependable and widely used method of surge control is an increase in the throughput of the compressor by opening the surge control valve. The surge control valve is essentially a bypass valve that either recycles gas around the compressor or blows the excess gas off to the atmosphere. Opening the surge control valve will reduce the process system resistance and allow the compressor to operate at a flow rate high enough to that will prevent surge; however, because bypassing or venting of the gas wastes power, surge flow should be determined as accurately as possible to avoid unnecessary bypassing or venting while maintaining safe compressor operation. The surge control setpoint is usually 5 to 10% from the actual surge line. Flow Systems A basic, volume-controlled, anti-surge system for compressors with constant speed drivers and constant inlet conditions is shown in Figure 9. The flow transmitter (FT) senses the process flow through use of an orifice or venturi that serves as the primary flow element (FE). The FT produces a signal that is proportional to the process flow, and it sends the signal to the surge controller (SC). The surge controller compares the transmitted signal to its setpoint signal. If the setpoint signal is exceeded, the surge controller sends a signal to the surge control valve (SCV). The SCV releases the pressure buildup at the discharge of the compressor in response to the demands of the surge controller. The discharge of the SCV is directed to a flare on an open suction compressor, and back to the compressor suction through a cooler, or, for air compressors, to the atmosphere through a silencer.

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As flow decreases to less than the minimum volume setpoint, a signal from the surge controller will cause the surge control valve to modulate to keep a minimum volume flowing through the compressor.

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

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Surge Control on a Constant-Speed Compressor with Suction Throttling A typical compressor installation is a plant and instrument air compressor that is driven by a constant-speed electric motor. The compressor is required to maintain a constant pressure in the discharge piping header. Figure 10 shows a typical capacity and surge control system, and it also shows the associated performance curve for a constant speed compressor with suction-throttling. In this scenario, the pressure transmitter (PT) and the pressure controller (PC) maintain a constant discharge pressure by throttling the suction valve. A flow transmitter (FT) and a surge controller (SC) are used to measure the gas flow through the compressor. The operation of the system is identical to the operation of the suction-throttling, constant-pressure control system that was previously discussed. As system demand decreases, the suction throttle valve will throttle close, which decreases flow through the compressor. The decrease in flow through the compressor is sensed by means of the inlet flow transmitter. As the compressor gas flow approaches the surge control point, the surge controller will modulate the surge control valve and vent the excess gas flow. As a result of the venting (or recycle), the discharge pressure will decrease due to the throttling of the suction throttle valve. The PC will signal the suction throttle valve to open slightly in order to maintain the system pressure. The pressure and the surge controller react independently from each other, but they will seek a balance of maintaining system pressure while maintaining minimum compressor gas flow.

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

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Constant speed dictates that the compressor will always follow the performance curve. A point can be selected on the performance curve at a predetermined distance from the surge curve at which the surge control valve will modulate and prevent compressor flow from decreasing past that point. The surge control point will be the setpoint for the surge controller. As discussed in previous modules, the performance and surge curves are not single lines, but they will move with inlet (ambient) pressure, temperature, and molecular weight, as shown in Figure 11. The air compressor for the compressor map that is shown in Figure 11 must provide 220 psig to the discharge header with the mass air flow delivered at the controlled pressure varying with the suction pressure and temperature (assuming constant molecular weight). The suction pressure will vary with the throttling of the inlet throttle valve. Temperature will vary with the change from summer to winter or with changes in the installation facility ambient temperature. The operation of the compressor will change from performance curve to performance curve with the pressure and temperature changes. Each performance curve will have its own surge curve or surge point for a single-speed machine. The point of convergence of the surge curves is shown 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 compressor discharge pressure to compensate for the changes in the surge point. 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 the surge control system that was shown previously in Figure 10 with the exception of the pressure compensation. The surge control system summer (Σ) performs calculations through use of the inlet flow (which will vary with changes in inlet conditions) and the discharge pressure (which is a constant for a pressure control system). A ratio relay (R) is used to set the pressure signal gain and bias. The surge control system summer provides a compensated measured variable (inlet flow) to the surge controller to compensate for the different inlet conditions. Another option to compensate for the changes in the surge point is through use of suction flow rate, temperature, and pressure sensors to provide the necessary values to calculate the actual cubic feet per minute flow rate that enters the compressor. Calculation of the actual cubic feet per minute is typically performed when the compressed gas molecular weight is fairly constant. If the compressed gas molecular weight Saudi Aramco DeskTop Standards

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varies significantly, a density transmitter, which is located on the compressor inlet, is used to compensate the surge control system for changes in inlet conditions. A density transmitter is a pressure transmitter configured to translate a pressure signal for density. Because of the following reasons, flow measurement in the discharge of the compressor is often preferable to flow measurement at the compressor suction:

• The pressure gradient in the suction is too small to achieve a reliable flow signal. • Flowmeter permanent pressure loss is not as objectionable in the discharge header. • The inlet pipe diameter is so large that the required straight run of piping that is needed for accurate flow measurement would not be practicable. • A discharge flow measurement is already required for process reasons. Compressor discharge flow rate must be corrected for inlet conditions for use in the surge control system, which is shown in Figure 13. The configuration in Figure 13 is based on the measurement of mass flow in the discharge and the fact that mass flow into the compressor equals mass flow out of the compressor. The discharge flow is compensated to mass flow, and the mass flow is compensated to inlet volumetric conditions.

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Figure 13. Discharge Mass Flow Rate Measurement Compensated to Inlet Volumetric Flow Rate Variable-Speed Compressor Based on Delta Pressure and Flow Figure 14 shows a typical configuration for a surge control system on a variable-speed compressor that is based on the pressure difference across the compressor and the gas flow rate. This system can be designed for constant flow or constant pressure control for normal operation. The speed transmitter (XT) and the flow transmitter (FT) or the high pressure side (discharge) signal of the differential pressure transmitter (DPT) can be used as setpoint to the variable-speed controller (XIC). For the surge system, the FT provides the measured variable. The compensated compressor pressure signal is provided by the DPT. Together, FT and DPT define the operating point or the input to the surge controller. As the operating point approaches the surge control point, the surge controller will open the surge control valve to maintain the required protection flow through the compressor.

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Figure 14. Surge Control System for a Variable-Speed Compressor Based on Differential Pressure and Gas Flow Rate Variable-Speed Multisection Compressors Centrifugal compressors that use multiple sections that are equipped with an interstage cooler capable of accommodating gas removal or addition between the sections can be described as two separate compressors that perform different duties but that are driven by a single shaft. Each section of the compressor has its own set of performance curves and its own surge line. Figure 15 shows a basic, multisection, variablespeed compressor with an anti-surge valve that protects the

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entire compressor. The system that is shown in Figure 15 is only suitable if the process gas does not contain any constituents that will condense at compressor section 1 discharge conditions. If the process gas contains constituents that will condense at compressor section 1 discharge conditions, condensate will be drained from the intercooler, and the molecular weight to the second section will change. In this case, a separate surge protection system must be used for each compressor section.

Figure 15. Multisection, Variable-Speed Compressor with Surge Control Valve that Protects the Entire Compressor Figure 16 shows the performance curve and surge line for each section of the compressor. The operation of this surge control system is identical to the surge control for a variable-speed Saudi Aramco DeskTop Standards

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

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

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Multisection compressor anti-surge control systems are typically 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 the other section and possibly place that section in a surge condition. Surge control on multisection compressors must be more stringent than surge control of a single section compressor. Frequently, a separate surge control system is required on each section to protect that section from surge, as shown in Figure 17. The surge control system for each section of the compressor operates like the surge control for a variable-speed compressor based on delta pressure and flow, but each section’s surge control system operates independently to prevent a surge condition in each section.

Figure 17. Multisection, Variable-Speed Compressor with a Surge Control Valve for Each Section Saudi Aramco DeskTop Standards

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The mismatch in pumping capacities among the compressor sections at lower speeds will occasionally result in a surge condition in the first compressor section during compressor startup. During startup, the first section will not have reached the design pressure ratio, and the density of the gas that enters the second section will be less than the density for which the section is designed. Although the second stage is pumping the expected volumetric flow, it is not pumping away the expected mass flow rate. For this reason, volumetric flow through the first section is less than expected, which results in a surge condition. If the surge in the first section is severe enough to cause damage, a remotely operated valve (H) can be installed in a recycle around the first section, as shown in Figure 18. During a startup, the remotely operated control valve is opened to recycle the first section gas back to the suction. A recycle cooler cools the recycled gas to prevent overheating in the first section. When the compressor is up to speed, the remotely operated control valve is gradually closed, and it is then left closed during normal operation. In multistage compressors that have a surge control valve for each section, the surge controller for the first compressor section may have a manual function mode, which would eliminate the need for a separate remotely operated control valve.

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Figure 18. Multisection, Variable-Speed Compressor with Remotely Operated Control on the First Section It is good engineering practice to require that all automatic antisurge control systems be equipped with a manual override.

System Arrangements Multiple compressor systems are assembled with either seriesor parallel-connected dynamic compressors. Compressors that are connected in series provide the higher pressures that are required in some petrochemical applications. Compressors that are connected in parallel provide higher flow rates at the same pressure, and the configuration provides greater rangeability and reliability.

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The number of variables to be considered increases dramatically with the number of compressors in the configuration. As previously mentioned in this module, compressor control and protection systems for single compressors have two objectives: to provide stable control of the compressor at all required operating conditions and to provide protection against operation in the surge area of the performance curve. Multiple compressor systems have two more objectives: to balance the load among dissimilar compressors and to safely start up and shut down the compressors. Series Several surge control system designs are available for compressors that are installed in series. One design calls for a complete surge control system for each compressor or compressor section, as shown in Figure 19. This system uses a constant pressure controller that senses the final discharge header pressure and that controls the suction throttle valve to maintain capacity control. The surge control system on the first compressor uses the discharge flow rate, the pressure, and the temperature for a mass flow and discharge pressure system. The surge controller measurement is a compensated flow signal, and the setpoint is a biased discharge pressure signal. The first surge control valve releases to flare or recycles through a cooler (not shown) back to the compressor suction. The second compressor surge control system uses an inlet flow and the differential pressure across the compressor as variables to the surge controller. When the second compressor surge control system actuates, the discharge from the second compressor is recycled back to the compressor suction. Typical installations have the recycle line installed after the gas cooler (not shown) and the return line connected upstream of the suction knockout drum (not shown). This arrangement allows the gas and compressor temperature to be maintained by means of a single heat exchanger. In some cases, a separate recycle heat exchanger (not shown) is used. Each surge controller operates independently of the other; however, the action of each surge controller will directly affect the operation of the other surge control system because the parameters that are measured on the compressor systems are affected by each compressor’s 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 integrated surge control system for two compressors in series. Like the control system that was shown in Figure 19, both compressors have an independent surge control system. Both compressors that are shown in Figure 20 use compressor differential pressure and inlet flow as process variables for the surge control system. Conflicting interaction between the two surge control systems is minimized through transmission of the

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changes in output of any one surge controller to the other surge controller. Each surge controller uses this information to protect 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 of controlling each compressor independently must be provided. Parallel compressors should have identical characteristics, but the compressors that are purchased with the same specification data usually will not have identical characteristics. Individual compressor operating characteristics vary due to manufacturing and assembly tolerances, which would have some effect on their individual performance; therefore, parallel operation should have a single discharge pressure sensor in the common header and a flow sensing device for each compressor. A single controller would receive the common pressure signal and the individual compressor flow signals, and it would provide an output signal that would actuate the specific control element for each compressor. Figure 21 shows a basic pressure control system for constant-speed compressors that are arranged in parallel.

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Figure 21. Discharge Pressure Control of Constant-Speed Parallel Compressors with Dissimilar Operating Characteristics The most effective parallel compressor operating strategy is to simultaneously load and unload the compressors equally as required. Simultaneous, equal loading and unloading will improve the efficiency and rangeability of the parallel compressor configuration. In some installations, the control system should be set up so that the compressors in the parallel system will sequentially load and unload. When the control system is set up for sequential loading/unloading, the least efficient compressor should be loaded last and unloaded first. The control system for compressors in a parallel configuration should unload the compressors so that all compressors in the parallel system will reach their control lines simultaneously.

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The surge control system for compressors that operate in parallel is more complex than the system for compressors that operate in series. When two or more centrifugal compressors operate in parallel, the greatest surge protection and efficiency results from all compressors operating equidistant from their respective surge control lines. The compressor and instrumentation industries have adopted a criterion to measure the angular distance between the operating point and the surge control line. The criterion is known as the “S-Criterion.” The S value is a dimensionless number that is relative. The absolute value of the number has, therefore, no meaning; however, compressors that have the same S number will be operating equidistantly from their respective surge lines. A surge control system 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 following equation: S=

∆p c + b ∆p o

Where:

∆pc = The differential pressure across the compressor b

= The surge margin

∆po = The differential pressure across the flow element The S-Criterion will be less than 1 when the operating point is safely away from surge, and it will be equal to 1 when the operating point is on the surge line control line. Figure 22 shows a parallel compressor configuration with suction pressure control, load sharing control based on the deviation of the compressor operating point from the surge control line (S 1), and anti-surge control. The anti-surge controllers calculate the S value for the compressors, and they monitor the compressor parameters to detect the approach to surge condition. The two load-sharing controllers, one for each compressor, perform a calculation to ensure that all compressors will simultaneously reach their surge lines.

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

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POSITIVE-DISPLACEMENT COMPRESSOR CONTROL SYSTEMS Unlike centrifugal compressors, positive-displacement compressors cannot self-regulate capacity against a given discharge pressure. A positive-displacement compressor will simply keep displacing gas until indication is received to control it otherwise. Capacity control for positive-displacement compressors is usually accomplished in steps, either automatically or manually, through the use of suction valve unloaders, clearance pockets or slider valves, or a bypass valve. These basic control system options for positivedisplacement compressors are used to maintain constant suction pressure, constant discharge pressure, or a desired flow rate through the compressor. The control systems that are selected for use are dependent upon the operating requirements of the compressor. Suction valve unloading, which is the most commonly used, loads or unloads cylinders. Clearance pockets are commonly used for small capacity adjustment of cylinders with no power change; however, when clearance pockets are used, they reduce cylinder efficiency. Bypass valves are useful in placing a compressor under load during a process system startup or shutdown; however, because the energy of compression is wasted, bypass valves do not provide an efficient method for loading or unloading a compressor. Bypass valves may be used in conjunction with unloaders or clearance pockets to exactly obtain the desired capacity values. Variable-speed control is not a preferred method for positive-displacement process compressors because the use of variable-speed control may result in problems with valve design and rod reversal. Variable-speed control for positive-displacement compressors will not be discussed in this module.

Valve Unloading Suction valve unloaders, as shown in Figure 23, are the most commonly used capacity control device. An unloader holds the cylinder suction valve open during the suction and compression piston strokes; so, suction gas is only pushed back and forth in the cylinder. The cylinder continues to take in gas normally; however, instead of completing the normal cycle of compression and discharge, the cylinder will simply pump the gas, 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 for simple one- or two- cylinder services in which the process does not require automatic control and in which sufficient time for operation is available. When automatic control is required, the unloader is fitted with a piston or diaphragm. A signal from a control device (either the air or the gas being processed) depresses the diaphragm. The diaphragm is connected to fingers that open the suction valve.

Figure 23. Suction Valve Unloader

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All valve unloader types can be manually operated, or they can be actuated through use of a pneumatic cylinder. When pneumatically actuated, these devices can be designed to load or unload upon either application on removal of air pressure. The advantages of pneumatic operation are the ability to remotely control the capacity of the compressor or even to automate the control. In applications that use cylinder lubrication, the unloaders are usually timed to prevent excessive accumulation of lube oil in the cylinder. On double-acting cylinders, for example, after approximately 30 minutes of operation, the head-end unloader will briefly close, and the crank-end unloader will briefly open to drain excessive oil. Finger-type unloaders are shown in Figure 24. There are three types of pneumatically operated finger-type unloaders: (a) direct-acting (air-to-unload); (b) reverse-acting or fail-safe (airto-load), which automatically unloads the compressor in the event of control air failure; and (c) manual operation. The finger-type unloaders consist of a series of small fingers that are housed in the valve crab assembly and that are actuated through use of a push rod from an outside actuator. To unload the valve, the fingers are lowered so that they depress the valve-sealing components and hold the valve in the open position. The pathway between the cylinder bore and the gas passage is through these open suction valves. Fingertype unloaders will typically be mounted on each suction valve so that the flow area of the unloaded pathway is maximized. Also, because the fingers simply hold open the existing suction valves, no special valve design is required. Actuation of fingertype unloaders can be manual (through the use of a handwheel and screw or lever arrangement to lower the fingers) or automatic (through the use of a small air cylinder on the top of the 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-type unloaders is the potential for damaging the valve-sealing elements with the fingers. The force that is generated by pneumatically actuated fingers, as they are driven down against the valve-sealing components, can contribute to premature valve failure. In accordance with SAES-K-403 and in order to minimize maintenance and to increase valve life, reduced unloader pressure settings will be used whenever possible. Pressure settings must be compatible with the ESD system set pressure, which is determined by the minimum acceptable system pressure that is required for safe plant operation. Pressure settings must also provide sufficient receiver storage capacity to allow startup of the standby compressor. Unloader controls must be set to maintain a 100 kPa (ga; 15 psig) pressure differential from loading to unloading. If air is used to operate the unloader, external operators with a vent chamber between the diaphragm and the vent packing are mandatory for flammable gas service. Manufacturers’ standard automatic control may be either on/off or step unloading. On/off control is acceptable for small process air or gas compressors in intermittent service, but the driver must be sized for frequent on-load starting. Automatic or manual step unloading may be accomplished through the use of either suction valve unloaders, clearance pockets, or a combination of both. Five-step unloading must provide capacities of 100 percent, 75 percent, 50 percent, 25 percent, and 0 percent; three-step unloading must provide capacities of 100 percent, 50 percent, and 0 percent; and two-step unloading must provide capacities of 100 percent and 0 percent. If a cylinder is unloaded to 0 percent, special precautions must be taken to prevent overheating in the cylinder. In general, suction valve unloading is an excellent method to control capacity. The devices are simple and easy to maintain and operate. Suction valve unloaders are efficient, and they are very good for startup unloading so that starting torque requirements are extremely low.

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Clearance Pockets Clearance pockets, which are shown in Figure 25, are pockets or reservoirs that are attached to the cylinders. For reduced capacity operation, the clearance pocket valve is opened, and the cylinder capacity is reduced by the effect of this added clearance on the volumetric efficiency. The gas is compressed into the pockets on the compression stroke, and the gas expands into the cylinder on the suction stroke to reduce the intake of additional gas. Clearance pockets provide an additional volume to the fixed clearance volume of a cylinder. This additional volume reduces the amount of gas that is introduced during the suction stroke of the piston. The reduction of the amount of gas that is introduced results in a reduced capacity of the compressor.

Figure 25. Clearance Pockets Saudi Aramco DeskTop Standards

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The gas in the volume at the end of the compression stroke is at the elevated discharge pressure. During the suction stroke of the piston, the volume expands in the cylinder to a pressure that is lower than the suction pressure to the particular cylinder. The expansion volume is constant for any cylinder with a constant stroke length; therefore, if additional volume of highpressure clearance gas is available due to the clearance pocket, the pressure level of the expanded gas at the end of the suction stroke will be higher. Higher pressure results in less suction pressure gas being introduced into the cylinder and in a reduction in the compressor capacity. The cylinder volume increased by the clearance pockets does not have an effect on the power that is required for the compression stroke. The cylinder pressure at the beginning and end of the compression stroke is the same, and the stroke volume remains unchanged. Clearance pockets are usually of a fixed volume, and they are sized to reduce flow precisely to a predetermined level. Typically, the use of multiple fixed-volume clearance pockets that allow for numerous reduced-capacity steps of control are used. In accordance with SAES-K-403, fixed-volume clearance pockets that allow the capacity to be reduced through an increase of the clearance volume of the cylinders may be manually or automatically operated. Variable volume pockets must not be used.

Bypass Operation A bypass valve system places a bypass valve in a line from the compressor discharge back to the compressor suction to route some or all of the compressor discharge to the suction. A bypass valve may be used as the sole means of control, but it is usually employed in combination with other control methods. The bypass valve controls capacity by directing the compressed gas back to the compressor’s suction. Directing the compressed gas back to the compressor suction is accomplished by piping from the compressor’s discharge line, through a control valve, back to the compressor’s suction line. To reduce the flow to process, the bypass valve is opened, and the excess flow is diverted back to the compressor’s suction. In addition to being simple, this system also has the advantage of being infinitely controllable (within the limitation of the size of the bypass line). Saudi Aramco DeskTop Standards

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Use of the bypass valve for continuous capacity control requires that the bypass gas stream be provided with a cooler to remove the heat of compression prior to returning to the suction. 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 small degrees of fine capacity control or for limited duration start-up unloading, where a simple loop around the compressor can be opened for a short period of time to relieve the initial compression load.

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POSITIVE-DISPLACEMENT COMPRESSOR PROTECTION SYSTEMS Typical positive-displacement compressor protection systems consist of relief valves, startup bypass valves, and high process temperature indication and control. Individually or combined, these protection devices help to ensure safe and reliable operation of the compressor system.

Relief Valve (Stage) A relief valve (PZV) is an automatic pressure-relieving device that is actuated by the static pressure upstream of the valve. When the static pressure upstream of the PZV exceeds its allowable value by a specified amount, the PZV actuates to relieve the pressure. Conventional PZVs are the most common, and they open fully when actuated. Pilot-operated PZVs are less common, and they modulate when actuated. In accordance with SAES-J-600, PZV(s) must be provided for positive-displacement compressors where the pressure at a closed discharge can exceed safe limits. For positivedisplacement compressors, interstage PZVs, as well as discharge PZVs, must be provided. The pressure setpoint must exceed the rated discharge pressure by 10 percent or 175 kPa (ga), whichever is greater. For reciprocating compressors, a greater differential than 10 percent may be required due to pressure surges. Interstage PZVs must be set at or above the compressor’s settling-out pressure to avoid lifting at shutdown. In addition, the PZV capacity must equal compressor’s capacity, and it must discharge to a safe area or flare and not to the compressor suction. The relative setting of the relief valves in each stage of a typical three-stage reciprocating compressor is basically the same. The stage discharge piping and components are protected from overpressurization by the PZV, which is set at approximately 10% above the stage discharge pressure.

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

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driver torque that is available during acceleration is not exceeded. The need for an unloaded startup is required because starting a positive-displacement compressor fully loaded can have a 3:1 peak-to-mean torque ratio. This peak torque requirement, coupled with breakaway friction, means that the driver now must have as much as a 350 percent starting torque capability. Typical motors are designed to have only 40 to 60 percent starting torque capability. Both manual and automatic compressor startup unloading is used. Common methods of unloading during startup include discharge venting, discharge to suction bypass, and cylinder unloading.

High Process Temperature In accordance with 31-SAMSS-002, the high temperature shutdown device is required to safely shut down the compressor. At a minimum, a high temperature shutdown device must be installed in the final stage discharge gas stream, and a high temperature shutdown device must be installed downstream of the aftercooler. Additional high temperature shutdown devices may be installed for high lube oil temperature. On some compressors, a temperature switch may be unsuitable due to high vibration levels at or near the cylinder head. Thermocouples or RTDs should be used as a means of temperature measurement. API-618 specifies that the maximum discharge temperature of 300°F can be exceeded for compressors with non-lubricated cylinders. Temperature control for non-lubricated compressors must comply with the requirements of 31-SAMSS-002. Note that SAES-K-403 requires that compressors used in hydrogen service limit discharge temperature to 275ºF (135°C).

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GLOSSARY aftercooler

Heat exchanger for cooling air or gas discharged from compressors. Aftercoolers also provide a means of removing moisture from compressed air and gases.

actual capacity

Quantity of gas actually compressed and under actual pressure and temperature conditions.

clearance pocket

An auxiliary volume that may be opened to the clearance space to increase the clearance, usually temporarily, to reduce the volumetric efficiency and, therefore, actual capacity 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) in order to control another variable (the control variable).

performance curve

A plot of expected operating characteristics, such as head or discharge pressure versus inlet capacity.

stonewall

A point of maximum flow and minimum head or discharge pressure on a dynamic compressor operating curve and beyond which a reduction in process resistance will not increase gas flow rate.

surge limit

The volume flow below which dynamic compressor operation becomes unstable.

throttling

Manipulating a variable to a higher or lower value.

aftercooler

Heat exchanger for cooling air or gas discharged from compressors. Aftercoolers also provide a means of removing moisture from compressed air and gases.

actual capacity

Quantity of gas actually compressed and under actual pressure and temperature conditions.

clearance pocket

An auxiliary volume that may be opened to the clearance space to increase the clearance, usually temporarily, to reduce the volumetric efficiency and, therefore, actual capacity of the compressor.

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