Answer / sandip goswami

'Surging' is 'A momentary back-flow' through the compressor from the discharge to the suction. This can occur when the mass flow of gas to the compressor falls below a critical level with a high pressure difference across the machine. When there is not enough gas to replace that being pushed forward, discharge gas will flow backwards through the compressor towards the suction side. This back-flow will tend to decrease the speed of rotation while the speed controller will try to maintain the correct speed. The gas flowing backward provides more volume to the suction side and the compressor picks up and begins to push gas forward again. The machine speed will tend to increase and the governor will again try to maintain the correct, set speed. All of this takes place very quickly; the backward and forward gas flow together with the speed control action causes rapid fluctuations in the flow and pressure of the system. This 'Surging' in the machine can be very damaging to the compressor and associated piping and equipment due to heavy vibrations set up in the system. Generally, if the suction flow drops too low, a 'Low-flow Trip' will shut down the machine. Also, surging can cause the machine to 'Overspeed' before the control system can react.
These problems of Surging are usually prevented by an 'Anti-surge' system and control valve which, externally, recycles discharge gas back to the suction side in order to maintain a 'Minimum Flow Rate' to the machine.

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