Application Discussion
AD128
June 15, 2003

Compressor Anti-Surge protection is required on all continuous-flow (centrifugal and axial) compressors. These units are used in natural gas pipeline, LNG refrigerant, and chemical / petrochemical processes. Sizes range from small electric motor driven units to those driven by large gas turbines.

Anti-Surge protection is provided by a control valve that opens allowing gas from the compressor outlet to re-circulate to the inlet. Compressor surge happens when the downstream flow demand changes and doesn’t match the current compressor operating point. The flow control system will change the driver (turbine or motor) speed if possible to adjust to the new flow rate, but the speed of the large rotating mass (driver + compressor) often can't be changed quickly enough to match the change in operating point. When the compressor speed and corresponding design flow rate don't match the actual system demand, flow reversal can occur in the compressor. Without outside intervention the flow reversal cycle will continue placing seals and other internal parts under great stress. Complete compressor failure is possible.

So this is a critical, control valve application. There are two typical requirements for this valve. The first is a fast stroking speed. The valve must provide recycle flow in the same time frame as the flow transient. In this compressible-flow system pressure pulses can travel at the speed of sound, so the valve reaction must be very quick. This requires special actuator sizing and instrumentation, typical required stroking speed is full open in 750 msec to 2 seconds. The second typical requirement is low valve noise, usually 85 dBA. This usually requires Whisper Trim.