One of the most important factors when considering two-shifting operation is to keep the turbine temperatures as high as possible during shutdown. This means that the machine needs to be deloaded as rapidly as possible.
Figure 2.90 shows typical shutdown curves for a 660 MW turbine-generator, in which the machine is shut down within 20 minutes, with the last 50% of load being shed in approximately 2 minutes. Maintaining the turbine temperatures in such a way ensures a faster run-up and reduces thermal cycling of the turbine components. When starting, it is important to ensure that the steam inlet temperature is matched to the metal temperatures to avoid cooling the turbine unnecessarily. Steam conditions at the stop valves should allow for the temperature drop due to throttling at steam admission to the turbine (see Section 1.3 of this chapter). It is particularly important when carrying out hot starts, that the rate of run-up to speed and initial block loading is sufficient to prevent a reduction in turbine temperature. The range of temperature cycling should be controlled as far as possible to minimise the risk of thermal fatigue.
Figures 2.91 and 2.92 show typical run-up curves for a warm start (after a weekend shutdown) and a hot start (after an overnight shutdown) respectively. The rapid loading shown during the hot start indicates the importance of maintaining a steady rate of rise of temperature. Run-up procedures must be adhered to in order to minimise cyclic thermal stresses in the turbine and thus prevent thermal fatigue problems during the life of the turbine.