Exceptional demands are placed on control systems applied to centrifugal compressors. Not only must these systems regulate the delivery of process gas or air at specified pressures or flow rates, but they must also effectively prevent surge and its attendant problems. The surge phenomenon adversely affects the quality of control, machine life, and plant operating costs.
Surge is a phenomenon associated with axial and centrifugal compressors. It occurs when, at any given speed, guide vane angle, or inlet valve position, flow in the system decreases sufficiently to cause momentary flow reversal in the compressor. Flow reversal occurs at an instant when the pressure developed by the compressor no longer exceeds the pressure in the downstream system. This is an unstable condition, which triggers self-oscillation between flow and pressure, resulting in erratic compressor capacity.
Surge appears as rapid pulsations in the flow and discharge pressure, which invariably causes damage to the compressor and associated piping, and upsets to the process. Broadly stated, an improperly protected compressor plant can incur increased running costs, expensive equipment repairs, more frequent compressor overhauls, and expensive plant downtimes, as well as representing a danger to plant personnel.
Anti-surge and capacity controls are the main elements of compressor control. Anti-surge prevents surge by maintaining a safe minimum flow through the compressor. This is accomplished by manipulating a blow-off or recycle valve. The capacity control is generally based on a pressure or flow by manipulating a suction or discharge valve, guide vanes, or rotational speed.
During steady-state operation, the capacity control of the compressor can conflict with the anti-surge control, since each attempts to vary the flow through the compressor in opposite directions. Therefore, the control system must also decouple the capacity control with the anti-surge control to avoid possible instability.
A sound anti-surge system will prevent surge with a surge control line. To maximize compressor efficiency, the control margin between the surge and the surge control lines must be minimized. To accommodate the minimized control margin, the surge and the surge control lines must be calculated dynamically from the operating point of the compressor. The safety line and adaptive gain further enhance this control algorithm. An optional surge detector serves as a back-up for the anti-surge control system.
To meet process requirements, the capacity of the compressor must be controlled. This is accomplished by manipulating a discharge valve, suction valve, inlet guide vanes, or rotational speed. The choice of which process variable will be controlled and the manipulated variable are often dictated by the process dynamics.
DECOUPLING OF ANTI-SURGE AND CAPACITY CONTROL
Both of the above systems control the mass balance around the compressor. Therefore strong interaction between these two functions can be expected. Of the two control systems, antisurge must take precedence over capacity control, because of the possible damages caused by surge. Repairs on possible damages and downtimes caused by surge can be expensive.
To minimize the effect of the interaction between the two systems, they must be effectively decoupled. Decoupling will reduce the response of one system with respect to the other system, which will minimize unwanted side effects caused by the interaction.
AUTOMATIC START-UP AND SHUTDOWN
Experience had shown that most surges happen during start-up or shut down. One of the most common factors is inconsistency of the operator.
The possibility of surges during an automated start-up or shutdown is dramatically reduced, because the compressor is controlled exactly the same way during every start-up or shut down.
Compressors will interact when they are operated in parallel to a common discharge header. This interaction must be optimized to minimize the interaction, particularly when the flow is reduced towards the surge line.
Compressors are often operated in series because of the required pressure ratio. The compressors can be driven by the same shaft or separately.
Each one of the compressors should be seen as separate and be controlled accordingly. The compressors and the anti-surge control will interact, which could lead to instability. This interaction must also be optimized to minimize the interaction, particularly when the flow is reduced towards the surge line.
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