For industrial gas turbine plant, Reliability, Availability & Maintainability (RAM) are the critical parameters that yield maximum production. As gas turbines work in high temperature environment, periodic maintenance of the machine is important for life evaluation & to detect any early failures. During major inspection, all critical components need to be disassembled, which reduces the availability of gas turbine & impacts the plant production. Minimizing the duration of the scheduled outage helps to increase the availability of a gas turbine unit. During every major inspection, it’s necessary to ensure that the turbine is cooled below the allowable temperature. Gas turbine maintenance phases are often delayed because of slow natural convection cooling during normal shutdown. To reduce the cool-down time, it’s necessary to accelerate the cooling & one of industry practices is to spin the unit at a certain speed for an extended period of time which enables low temperature ambient air circulation. However, this forced cooling of the machine imposes additional thermal stresses on the components that may result in overall life reduction.

This paper compares the thermal behaviour of normal cool down and forced cool down phenomena by simulating the critical gas turbine components. A 2D axi-symmetric flange to flange thermal modelling approach is used to predict the temperature distribution of the gas turbine components. Conduction, convection and radiation effects are captured using GE in-house methodology. Compressor and turbine casings as well as rotors are studied in detail to understand the transient thermal gradients. The results show that a high rim to bore gradient exists during forced cool down, which results in high stresses in the compressor and turbine wheel regions.

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