Compressor and pump controls are normally used to manage the injection of a fluid (compressible and incompressible) into a reservoir or pipeline. Some of the fluids that are injected include water, methanol, carbon dioxide, natural gas and other gases.
When injecting compressible fl uids, large centrifugal single and multi-stage compressors are used. However, correct control of these large units is essential if the operation is to be fault-free.
A concern with these units can be controlling the rotational speed, to prevent the compressor turbine from reaching a stall condition. If the gas supply to the compressor inlet is suddenly cut, the compressor’s RPM increases rapidly until it goes into “surge” or until it reaches a potentially destructive RPM and shuts down. The surge control’s set-point is approximately 25% below the design operating flow rate for the compressor. When a surge situation occurs, the signal from the compressor flowmeter is registered in the control system, which sends a signal to the anti-surge valve to open. The anti-surge valve, mounted in a recycle line around the compressor, opens in less than a second. The valve remains open until the gas supply to the compressor is re-established. Then the anti-surge valve will close slowly.