Volume 5, Issue 9 (October 2008)
Carrying Capacity Prediction of Steam Turbine Rotors with Operation Damage
This study is concerned with assessing the integrity of cracked steam turbine rotor which operates under creep conditions. Damage accumulation and growth for turbine rotor took place on the inner surface of hole in a shaft. In this case the crack front was nearly half-elliptical shape. The model based on the critical distance concept is presented for expressing crack growth rate in terms of creep damage accumulation in a process zone ahead of the crack tip. An engineering approach to the prediction of residual lifetime of a turbine rotor which is sensitive to the loading history at maintenance is proposed. Full-size stress-strain state analysis of turbine rotor is represented for different stages of lifetime under considering loading conditions. As a result accumulated creep strain in critical zones of turbine rotor depending on time of loading is defined. The creep fracture mechanics parameters that are found numerically are used to characterize the local strain rate and stress fields at any instant around the crack tip in a turbine rotor subjected extensive creep conditions. The meaning of the critical creep ductility appropriate to the stress state at the crack tip is defined as the difference between permissible and accumulated creep strains at different times of loading. Approximate estimations of carrying capacity are presented for the different stress-strain state of a steam turbine rotor at the operation. It is found that the individual mean lifetime of a turbine rotor can be predicted with reasonable accuracy from knowledge of the stress-strain distributives and the material uniaxial creep ductility and creep rupture strength.