Turbine Compressor Surge

A gas turbine compressor surge takes place when the main flow through the compressor reverses its direction, flowing from the exit to the inlet for short time intervals. It is the lower limit of a steady and normal operation as it involves the reversal of the flow. If nothing is done to fix it, this abnormal process could cause serious damage to the machine and it might not be repaired. Although compressor surge is a very interesting phenomenon, it cannot fully be understood.

The turbine compressor surge occurs when there is a kind of aerodynamic instability within the system, with a part of the compressor being usually the cause of this aerodynamic instability. A surge can be detected by noticing excessive vibration and a perceivable noise. It is evident that the underlying cause of this reversal of flow is aerodynamic stall, which may occur in either the impeller or the diffuser.

When the impeller seems to be the cause of surge, the inducer section is where the flow separation begins. Either a decrease in the mass flow rate, or an increase in the rotational speed of the impeller, or both, can cause the compressor to surge, which can be started in the diffuser by flow separation that takes place at the diffuser’s entrance. A diffuser usually consists of a vaneless space, with the pre-diffuser section before the throat which contains the initial portion of the vanes in a vaned diffuser.

The vaneless space accepts the velocity generated by the centrifugal impeller and diffuses the flow so that it enters the vaned diffusser passage at a lower velocity as it avoid any shock losses and the separation of the flow. When the vaneless diffuser stalls, the flow will not get into the throat. Then a separation takes place, causing the flow to finally reverse, which surge the compressor.