Creep Life Prediction For High Energy Piping Girth Welds Case History: Cholla, Unit 2

Abstract

Creep damage of high energy piping (HEP) systems in fossil fuel power plants results from operation at creep range temperatures and stresses over many years. Thermal expansion stresses are typically below the yield stress and gradually relax over time. Consequently, the operating stresses in a piping system are typically below the yield stress and become load controlled. Conventional designs of HEP systems use the American Society of Mechanical Engineers B31.1 Power Piping Code. The Code is a general guideline for piping system design. Utilities typically determine examination sites by performing Code piping stress analyses and selecting locations that include the highest sustained longitudinal stress, highest thermal expansion stress, and terminal points. However, the Code does not address weldment properties, redistribution of thermal stresses and time-dependent life consumption due to material creep degradation. As an alternative, a high energy piping life consumption (HEPLC) methodology was used to predict maximum material damage locations. The methodology was used to prioritize expected creep damage locations, considering applicable affects such as weldment properties, field piping displacements, time-dependent operating stresses, and multiaxial piping stresses. This approach was applied to the main steam piping system at Cholla Unit 2. The locations of highest expected creep damage would not have been selected by a conventional site selection approach. Significant creep damage was found at the locations of maximum expected creep damage using the HEPLC methodology.