Industrial Gas Turbines

Behavior and lifetime of the gas turbine components are especially influenced by the instationary operation. Cyclic loads due to time dependent, rotational speed and temperature distribution influence the gas turbine components, as described below:

In the hot parts of the turbine, but also in the rear of the compressor, (air temperatures of more than 600°C), high temperature gradients are formed between the rim and the bore (2.2-Fig.5 and "Ill. 3.3-7") at transient condition, during start.

The thermal stresses thus induced, superimpose the disc loads by the centrifugal force ( "Ill. 2.2-4") and contribute decidedly to the low cyclical load (low cycle fatigue = LCF, "Ill. 3.1.2.1-0") of the rotors. Modern gas turbines often have rotors where at least discs of individual stages have a limited LCF life. The more time the disc has for warming at a relatively low, circumferential temperature, the less the life reduction. If LCF is the life depending factor, the start cycle of the rotor can be far more important than the resulting long, running load in steady state operation, by which a comparatively lower temperature gradient between the disc rim and the hub dominates. Instantaneous starts are especially long life consuming. The faster the rim heats up during start, all the more higher are the tension stresses in the hub. Such a sequence can correspond a multiple for normal life time cycles and can be comparable to a long stationary operation (see Chapter 2.2.4).