Mitigating the risk for creep-fatigue cracking in steam turbine rotors: an end user's perspective

Abstract

Over the last few years, cracks appeared at an early stage of operation on several steam turbine rotors due to low cycle fatigue damage induced by transient thermal events. These cracks led to expensive repair and associated unavailability of the units. A proper risk assessment is therefore key, especially given the expected increase in transient operation required from the conventional power plants in the future.To manage the risk for the ENGIE fleet, with a wide variety of different steam turbine rotors, ENGIE Laborelec has developed a fleet risk management program. At the heart of this are numerical and finite element (FE) models, along with damage assessment methodologies that can offer estimates on the past and future lifetime consumption. Experience with cracked rotors provided the unique opportunity to calibrate the approach, enabling a more accurate prediction for the rest of the ENGIE fleet.Based on the outcome of the lifetime study, different mitigation actions are proposed. Besides the reduction of operational gradients, a very interesting approach is to pro-actively machine the rotor surface before the onset of cracking.