Remaining Life Assessment of an In-service Exposed High Pressure Valve Casing

Most critical steam turbine components in power plants operate under high pressure and at high temperature. As a result a material degradation.. crack initiation and growth under steady state and transient operational condition over the long operational time occur. The crack initiation and/or crack growth time define the inspection frequency as a constrictive criterion concerning the component availability. The paper shows an industrial application case of the remaining life assessment of a high pressure valve casing with cracks found during a non-destructive inspection. The damage qualification and quantification (creep and fatigue) is done in the frame of the remaining life analysis combining the non-destructive inspection, material modelling and thermo-mechanical computational analysis: Metallographic replica testing is a common method for creep damage monitoring which classifies the material degradation; additionally the crack. depth and wall thickness in the crack area was assessed using ultrasonic inspection. The thermo-mechanical analysis is based on real operational conditions and process parameter courses and takes into account the creep behaviour of the component material, creep damage evolution and influence of the porosity on the crack growth rate in the ligament in front of the crack tip. To judge if the crack is acceptable with reference to the further safe operations, a synthetic time dependent limit curve diagram accounting for the material in-service exhaustion was developed. Based on the results of the analysis, the valve was released for the further operation.