Surge Protection for Axial Compressor

wikipedia.org has an excellent description of axial compressors; the 'Design' section's final paragraph talks about how bleed valves protect axial compressor during off-speed operation. Accelerating from zero speed to rated speed is considered off-speed operation.

Compressor bleed valves, such as those used on GE-design heavy duty gas turbines, really serve to extract air from middle stages and direct the flow to the gas turbine exhaust area. This helps to reduce the mass flow during acceleration (below rated speed) to prevent damage to the axial compressor.

The compressor bleed valves of GE-design heavy duty gas turbines are also open during deceleration from rated speed--for the same protection. The valves are closed during normal operation at rated speed which increases the efficiency and compression ratio of the axial compressor to the design capability.

Surge and stall are also well-described in the wikipedia axial compressor article.
 
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