Purge time calculation on gas turbine?

Purge times are calculated from many factors, including the air flow through the gas turbine's axial compressor during purging, air flow from zero speed to purge speed, and the volume of the exhaust--including heat recovery steam generator (HRSG; boiler); exhaust stack volume, any exhaust duct work between the turbine and HRSG and stack, etc. Some installations have a "bypass stack" that can be used when the gas turbine exhaust is not to be used to make steam. In this case the purge time is usually shorter. The accepted industry standard is to have three to five "exchanges" of air through the entire unit (axial compressor, turbine, exhaust/HRSG/stack) to ensure any combustible gases are purged before fuel/spark are admitted to the gas turbine. Some manufacturers have done some very creative things with "purging" recently because owners/operators have demanded shorter and shorter purge times. If you are having difficulty or more specific questions, we can try to help. Remember: Not every Frame 7F/FA is exactly like every other Frame 7F/FA. Yes they all suck and squeeze and burn and blow; but they can have very different axial compressors and exhaust configurations and operating characteristics.