Answer / muhammad ismael

When the downstream flow requirement of a compressor will increase, the pressure will decrease here and pressure on suction will increase. This may effect back flow in compressor or a physical damage also. On that time flow control system try to change the speed of Drive (Motor & Turbine) to match the new requirement of flow but speed of a large mass (Driver + Compressor) is not possible to change with in a short period of time. so, to avoid from surge a control valve used from suction to discharge or a valve used to blow the high pressure.

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