Ask Greg McMillan - What role do you see dynamic simulation playing in the future of best understanding of compressor surge oscillations?

Greg's Response:

Equations have been developed to show the compressor map to the left of the surge point not commonly known that is the key to understanding the oscillations. In figure above the operating point starts at point A and moves to the left along the com­pressor characteristic curve as the throttle valve closes. When the operating point reaches point B, which is where the compressor characteristic curve slope is zero, the operating point jumps to point C. This jump corresponds to the pre­cipitous drop in flow that signals the start of the surge cycle.

The operating point cannot follow the positive sloped portion of the compressor curve because the reduced flow out of the plenum, caused by the closing throttle valve, requires that the plenum pressure increase instead of decrease per the curve. (The plenum pres­sure must increase if the flow into the plenum exceeds the flow out of the plenum.)

The flow pattern around the impeller breaks up, and the flow reverses direction to backward flow from the discharge volume to the compressor suction. After this jump to point C, the operating point follows the compressor curve from point C to point D as the plenum volume is emptied due to reverse flow. When the oper­ating point reaches point D, which is where the compressor characteristic slope is zero again, the operating point jumps to point A.

The operating point cannot follow the positive sloped portion of the compressor curve because the reduced forward flow into the plenum volume requires that the plenum pressure decrease instead of increase per the curve. (The plenum pressure must decrease if the flow out of the plenum exceeds the flow into the plenum.)

The flow pattern around the impeller is established and the flow forward rapidly increases. After the jump to point A, the operating point follows the compressor curve to point B as the ple­num volume is filled. The surge cycle repeats itself unless the throttle valve or a surge control valve is opened.

The operating point follows the compressor curve only during the peaks and valleys of the surge flow oscillations. The jumps on the compressor map correspond to the rapid transition between the peaks and valleys.

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