Why Creep Life analysis?

Turbine life usage manifests itself by the creep damage. When turbine materials are subjected to stress and operating above a certain temperature, they will suffer from plastic deformation known as Creep. When a 0.2% plastic deformation has resulted then the material is assumed to have failed in creep.

The time taken for failure in creep depends on the operating stress and temperature. Engine manufacturers normally quote engine life when the engine is operating at 100% power at ISO conditions. Typically, this time corresponds to 20,000 hours for an aero derivative and longer for an industrial gas turbine. However, gas turbines rarely operate at ISO ratings. If an engine operates at 90% of this power at ISO conditions then the creep life of the turbine will be 5 times that of 100% power (See figure below), increasing the creep life to 100,000 fired hours. However a 2% loss in compressor efficiency will only achieve 3 times the life at ISO rating, giving only 60,000 hours of creep life at 90% ISO power.

Performance deterioration results in an increase in operating temperature and certain types of performance deterioration also results in an increase in operating speed (e.g. Compressor Fouling). Performance deterioration always results in accelerated creep life usage.

Effect of power output and performance deterioration on Turbine Creep Life XCREEP PROGRAM

The creep life used is dependent on many factors (e.g. power output, ambient conditions and performance deterioration). Without proper monitoring it is difficult to access the creep life used. XCREEP monitors key parameters and calculates the creep life used. It also displays the life used based on fired hours.

Display from XCREEP showing the actual life used when performance deterioration is present; the life used based on fired hours and the life used if no performance deterioration is also shown