Life Assessment of an HP-IP Rotor Under Creep Service Conditions

Abstract

A CrMoV high and intermediate-pressure (HP-IP) rotor exhibited in-service cracking in the blade groove walls. Cracks initiated from a notch and propagated under creep conditions to a depth of 0.255 in. (6.5 mm). Since the unit was base loaded, the contribution of cyclic loading (startups and shutdowns) was not significant. The total life of the groove walls is predicted by applying a life prediction approach that accounts for both crack initiation and propagation. Crack initiation life is based on short-term creep tests extrapolated to long times using a time-temperature parameter. Crack propagation time is estimated using a newly developed time-dependent fracture mechanics concept. A parameter Ct is used to express the crack growth rate under creep loading conditions. Pertinent material properties such as creep deformation and creep crack growth rate were obtained by testing the material from the subject rotor. The results obtained by this analysis show that about 50 percent of the estimated life was spent in crack propagation. Predicted service life time compares very favorably with the actual operating life of the rotor.