PSI - Issue 57

Malik Spahic et al. / Procedia Structural Integrity 57 (2024) 833–847 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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5.2. Reduction of operational gradients The lifetime of a steam turbine rotor is mainly driven by the thermal stress during start and stop. Reducing the operational gradients and hence the thermal stress levels is the most direct way to reduce lifetime consumption and consequently the risk for cracking. This can be achieved by lowering down the load gradient setpoint. The setpoint typically increases with increasing initial temperature of the rotor as shown in Figure 16 for a single shaft CCGT.

Figure 16: Load gradient setpoint as a function of the turbine temperature

Further lowering down the setpoint reduces the heat input and hence the temperature gradient with which the rotor is heated. This is similar to the main functioning of the RSE which automatically reduces the load gradient when the thermal stress becomes too high. If an RSE is out of operation, reduced load gradient setpoints are typically imposed by the OEM as shown on Figure 16 to avoid excessive lifetime consumption of the rotor. Besides reducing the load gradient setpoint, thermal stress can also be reduced by reducing the steam temperature gradients. To what extent this is possible depends on the possibilities of the power plant to control the steam temperature entering the steam turbine during the start-up. In some cases, a final desuperheater is present enabling a direct control of the steam temperature as shown on Figure 17. In other cases, the gas turbine load needs to be controlled to enable a lower steam temperature gradient. For very cold starts, a lower steam temperature setpoint can

Figure 17: A final desuperheater enables a direct control of the steam temperature gradient and hence on the resulting thermal stress levels

reduce the large difference between steam and rotor temperature when steam starts entering the turbine. An example is shown in Figure 18 where both live and reheat steam temperature reductions are implemented. The reductions are limited as the steam needs to be sufficiently superheated.