Effects of Secondary Creep Formulation on API 579-1 Residual Life Evaluation

Although several ad hoc procedures are codified into main international standards, the creep life prediction remains a critical phase of each Fitness-For-Service assessment. Commonly, either a time-fraction or a ductility exhaustion approach can be used. In both cases, conservative predictions within a factor of 2 or 3 are expected [1]. However, since the procedures to determine the creep damage are based upon the results of a stress analysis, the residual life evaluation can be affected by the adopted creep formulation. The choice to use a simple modeling, only accounting for the dislocational creep range, could lead to overestimate the component creep life at low stresses, and this is also subtly true even at concentration points if triaxiality or deformation-controlled loading lead to marked stress relaxation over time. In this paper, the tube to header and the header to hemispherical end joints of a HRSG assembly were assessed by the API 579-1/ASME FFS-1 [2] Level 3 procedure, via inelastic FEA, changing the creep formulation to compare the results. The classical Norton's law was replaced by more sophisticated secondary creep models to account for the complex time-dependent stress-field. In particular, the primary and secondary stress re-distribution/relaxation in the creep range were investigated in order to evaluate the impact of the steady-state creep constitutive equation on the residual life prediction.