Surge has been defined as peak head. However, actual "hard surge" is a condition where the discharge pressure of the compressor overcomes the dynamics of the compressor and flow is reversed from the discharge to the inlet of the compressor. Unless something is done to correct the situation, the system will continue to cycle until something breaks. Surge can be a mild stall or an intense heavy hard reversing flow. For centrifugal compressors, if the impellers are not damaged from the reverse pressure gradients, the diaphragms may crack. Surge is best avoided by a good anti surge system that accurately monitors compressor inlet and discharge conditions and flow rate. The surge point should be verified by actually surging the compressor in the field at several speeds (for variable speed machine) as the surge points supplied by the OEM is approximate and very hard to predict. In fact, the surge point is somewhat affected by the system in which the compressor is installed and location of the anti-surge line and check valve can be crucial. Even very mild surging or stall can cause serious damage to a compressor, resulting in high rotor vibration or internal damage. Methods of detecting surge and incipient surge will be presented.
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