In gas turbine operation we often hear the terms surge and stall.
What exactly is the difference between the two terms or are they the same meaning? Are both reversal of air flow and what are the reasons.

 

Stall: When a compressor is operated away from its design point, the gas flow into the aerodynamic components (impellers, blades, diffusers, etc.) deviates from its design direction. If the angle of deviation (or incidence angle) is large, flow separation occurs. At higher incidence angles, the flow fully separates at the impeller leading edge or diffuser inlet, and the flow is said to be stalled. The rotating stall is a special form of stall, where one or multiple flow regions in the diffuser (or impeller) are stalled but where other regions of the same impeller or diffuser are not stalled yet. The stall regions usually travel in the direction of the rotation at a speed that is fractionally lower than the rotating speed of the compressor. Stall and flow separation may be precursors to surge, but not necessarily so. Many compressors operate with local flow separations over parts of their operating range and function perfectly fine.

Surge: compressors will surge when forward flow through the compressor can no longer be maintained due to an increase in pressure across the compressor, and a momentary flow reversal occurs. Once a surge occurs, the reversal of flow reduces the discharge pressure or increases the suction pressure, thus allowing forward flow to resume again until the pressure rise again reaches the surge point. This surge cycle continues at a low frequency until some change is made in the process or compressor conditions.