Surge is a situation that can destroy a compressor. It is a critical factor in design of the compressor and its control system. It is also a critical operating limit.
Surge is a condition of unstable flow within the compressor, resulting in flow reversal and pressure fluctuations in the system. This occurs when the head (pressure) developed by the compressor is less than that required to overcome downstream system pressure. At surge, continuous “forward” flow is interrupted.
While surge is caused by aerodynamic instability in the compressor, interaction with the system sometimes produces violent swings in flow, accompanied by pressure fluctuations and relatively rapid temperature increase at the compressor inlet. Surge affects the overall system and is not confined to only the compressor. Therefore, an understanding of both the external causes and the machine design is necessary to apply an adequate anti-surge system.
The compressor surge region was previously identified in Figure 200-13. In Figure 200-17 lines depicting three typical system operating curves have been added. The shapes of these curves are governed by the system friction, and pressure control in the particular system external to the compressor.
A compressor will operate at the intersection of its curve and the system curve.
To change the point at which the compressor operates:
1. Change the speed or variable geometry of the compressor, thus relocating the compressor curve; or
2. Change the system curve by repositioning a control valve or otherwise altering the external system curve.