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Industrial gas turbines are designed to operate safely during startup, shutdown, and over a wide range of normal load and speed conditions. Within the gas turbine, the axial compressor does have aerodynamic surge and choke stability limits and, clearly, operation beyond these limits must be avoided.
The choke limit is unlikely to be a problem in a two-shaft gas turbine because its fixed first-stage turbine nozzle will assure a positive pressure ratio across the compressor for all running cases while single shaft engines at part load may operate in choke. However, the speed lines of an axial compressor map are relatively steep and, although the compressor is typically designed with sufficient surge margin at full load, any significant impedance to the flow through the gas turbine can move the compressor’s operating point into surge.
Thus, there are scenarios when the axial compressor can potentially surge and, although this is not a common event, when it happens it results in violent low-frequency axial vibrations that are often catastrophic to the gas turbine. Specifically, during axial compressor surge, significant damage potential exists for the seals, thrust bearings and even the blades.
In a surge event, hot combustion gases may flow backwards through the compressor, which is not designed for the resulting temperatures. These surge events can usually be avoided through proper maintenance practices and diagnostic monitoring of the gas turbine performance.
Thus, the most prominent causes for gas turbine axial compressor surge are discussed below:
The above are the primary reasons that cause gas turbine surges. By properly maintaining the gas turbine, using appropriate inlet filtration, and monitoring the degradation of compressor, turbine, and overall performance of the gas turbine, these dangerous and damaging surge events can be almost completely eliminated.
As a minimum the overall gas turbine power loss due to performance degradation should be monitored. If the output power is reduced by more than 7% to 8% when compared to its original performance, the gas turbine is not just operating inefficiently, but there is a good chance that the axial compressor’s surge margin is severely reduced.