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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Temperature gradients can become important as well during shutdown of the unit. A forced cooling down can for instance be implemented prior to a maintenance stop to expedite the cooling down process. In some cases however,

Figure 18: Reduction of live and reheat steam temperatures to reduce the thermal stress levels during start-up

the normal shutdown procedure itself causes an important cooling down of the rotor. This is exemplified in Figure 19 where it can be seen that the control valves are still open while the steam temperature reduces. This causes important tensile stresses which add up to the compressive stress during the start, thereby increasing the overall fatigue damage. Tensile stress also cause a crack to propagate further. Adapting the shutdown procedure can therefore be used to mitigate rotor cracking.

Figure 19: Live steam temperature reduces while control valves are still open causing rotor to cool down

5.3. Machining of rotor surface Borescope inspection at the critical rotor locations is not always possible without dismantling the turbine which means that a crack can be left unnoticed for a long time. Consequently, cracks are typically only detected when it is too late. In most cases, this means that the crack has propagated to a large size forcing the unit to trip due to vibration issues (Figure 6). Eventual repair, if deemed possible, causes important unavailability of the unit which can easily take several months up to a full year. In the worst case, a new rotor is needed with an even larger delay.