Surge prevention control system for dynamic compressors

A control system for avoiding surge in a dynamic compressor, the compressor having a variable operating point definable on a compressor map which includes a stable region, a surge region, a surge line separating said regions, and a surge control line in the stable region near but displaced from the surge line, the control system comprising, in combination:

• an anti-surge valve coupled to the compressor and responsive to an electrical control signal for controllably increasing compressor flow;
• a multiple module controller responsive to a control variable indicative of operating point of the compressor for producing said control signal, the multiple module controller including:
• a closed loop PID control module responsive to the control variable for exerting control action on the operating point of the compressor when said operating point is between the surge control line and the surge line;
• a closed loop rate control module responsive to a signal indicative of the rate of approach of the operating point to the surge control line for exerting control action on the operating point of the compressor anywhere in the compressor map when the rate of approach exceeds a given level, and
• selector means for passing control of the anti-surge valve between the PID control module and the rate control module.

A method for preventing surge in a dynamic compressor, the dynamic compressor having a variable operating point definable on a compressor map which includes a surge region and a stable region separated by a surge line, and a surge control line in the stable region displaced from the surge line, the method comprising the steps of:

• providing an anti-surge valve having an adjustable opening for increasing flow through the dynamic compressor;
• sensing process conditions in the dynamic compressor to produce a control variable indicative of the operating point of the dynamic compressor;
• operating a first closed loop PID to generate a first output signal for controlling the anti-surge valve to control the operating point about the surge control line;
• monitoring a rate of approach of the operating point to the surge control line;
• operating a second closed loop PID to generate a second output signal for controlling the anti-surge valve to control the rate of approach;
• selecting among the first and second output signals as a control signal;
• and
• adjusting the opening of the anti-surge valve according to the control signal.

A method for preventing surge in a dynamic compressor, the dynamic compressor having a variable operating point definable on a compressor map which includes a surge region and a stable operation region separated by a surge line, and a surge control line in the stable region displaced from the surge line, the method comprising the steps of:

• providing an anti-surge valve having an adjustable opening for increasing flow through the dynamic compressor;
• sensing process conditions of the dynamic compressor to produce a control variable indicative of the operating point of the dynamic compressor;
• performing a closed loop PID control on the anti-surge valve using the control variable as a process variable and the surge control line as a setpoint;
• monitoring a rate of approach of the operating point toward the surge control line;
• and
• in the event of high rate of approach, taking control of the anti-surge valve by performing a closed loop PID control using the rate of approach as a process variable.