Application of an Advanced Creep–Fatigue Procedure for Flexible Design of Steam Turbine Rotors Based on Fracture Mechanics Methods

Abstract

The demand for steam turbine components is driven not only by high efficiency but also by high plant operational flexibility. Steam turbine rotors are therefore exposed to increased temperatures and increased number of stress cycles. These aspects should be considered for lifetime prediction. Fracture mechanics methods are usually applied when crack like defects are detected not only for new rotors but also for rotor components in service. Based on the findings, a decision has to be made with respect to acceptability considering high temperature effects as well as the expected future operating regime. For defect analysis in the high temperature range, crack initiation and crack propagation under combined creep and fatigue loading need to be taken into account. Based on fracture mechanics methods and longterm testing data, an advanced creep–fatigue procedure for the evaluation of crack initiation and crack growth has been developed within the German Creep Group W14 for creep crack growth (CCG) behavior. Furthermore, recent studies show that the crack size for creep crack initiation (CCI) depends on material ductility and creep strain in the ligament. This paper demonstrates the industrial application of the abovementioned method for steam turbine rotor assessment, which has a focus on crack initiation and crack growth under creep–fatigue conditions. For crack initiation, a simplified approach based on defect size and material ductility is compared to a standard approach—twocriteriadiagram (2CD). For the advanced evaluation concept, the CCI criterion is combined for analysis with a creep–fatigue crack growth (CFCG) procedure. The benefit of the method especially for ductile material will be highlighted.