Creep-fatigue interaction of P91 steam piping subjected to typical start-up and shutdown cycles

Abstract

The possibility of creep–fatigue interaction occurring in steam pipes of power generation plants during operation has been on the front burner for years. Often, failure of the pipes during operation is attributed to either creep alone or fatigue alone. Of recent, some failures are speculated to be due to simultaneously coupled interaction between creep and fatigue, especially when the failure occurs earlier than anticipated. The literature shows that such studies are very limited and indeed under-researched. Thus, there is a dire need to systematically investigate this coupled creep–fatigue phenomenon and provide clarity as the failure of high-pressure steam piping has consequential and very significant effects on the suppliers and end users. In this work, a special Fortran user subroutine script of a phenomenological modified hyperbolic sine creep model was developed and implemented in Abaqus CAE/2019 finite element code to initially determine the creep behavior of a P91 steam piping network subjected to a typical daily start-up and shutdown cycle. Subsequently, fe-safe/TURBOlife software was employed to investigate whether the failure induced by the start-up and shutdown cycles was due to fatigue alone, creep alone or due to creep–fatigue interaction. Interestingly, the study showed that the failure of the piping network under the specified operating conditions is specifically due to creep alone. Furthermore, the intrados of the elbow in the piping network was identified as the region most prone to failure, and the piping network will only survive a total of 7.1 and 7.7 years under these operating conditions for both machined and fine-machined surfaces, respectively. These results were thereafter analytically validated, and it showed a strong correlation with the numerically determined creep rate.