One of the key problems in the discussion of surge control is the definition of what is being controlled. This is due to two reasons: First, surge is a system phenomenon, that only exists due to the interaction of a system of pipes, valves and volumes (in short “the piping system”), and a compressor.
The second reason is that the concepts of stall, rotating stall, mild surge, and violent surge are often used inaccurately, or interchangeably. So, let us start with a few definitions: For the purpose of this discussion, surge is considered a situation where the flow through the entire compressor is reversed intermittently, in other words gas is flowing from the discharge nozzle to the suction nozzle.
This is what is often referred to as violent surge, and the intermittent nature of the flow reversal creates large forces capable of damaging compressor bearings, seals, and other rotating elements. There is also something called mild surge. This is also an instability, but without the full reversal of flow. The essential fact is that the same compressor can develop mild surge or violent surge, depending on the geometry (especially the volume) of the piping system upstream and downstream of the compressor. Similarly, the surge frequency depends on the piping system.
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