Large Ranges in Power Piping Girth Weld Creep Rupture Lives

Abstract

Stress analysis evaluations of high energy piping systems operating in the creep range have revealed that each piping system has a large range of ASME B31.1 Code stresses. It is typical that there are only a few locations of high stresses and many more locations of much lower Code stresses. Over the past 20 years, the authors have evaluated several hundred piping systems operating in the creep range, including main steam, hot reheat, high pressure, and intermediate pressure systems constructed of Grade 11 (1¼-Cr-½Mo-Si), Grade 22 (2¼Cr-1Mo), and Grade 91 (9Cr-1Mo-V) materials. Stress contour plots illustrate the significant range of Code stresses (sometimes factors greater than 2) at various piping system locations. This study also considered the variation of high stress locations for the initial as-designed piping stress analysis versus the as-found stresses associated with field anomalies. The stress contour plots also illustrate that field anomalies in sister units can result in different high stress locations from one unit to another. In addition, significant unintended field anomalies may result in as-found analysis high stress locations at low stress as-designed (expected) analysis locations. Since there is a large range of stresses in these power piping systems, the girth welds have a significant range of creep rupture lives. In Grade 11 material operating at 1000°F (538°C), an 18% stress increase results in 50% decrease in creep rupture life. In Grade 22 material operating at 1000°F, a 12% stress increase results in 50% decrease in creep rupture life. In Grade 91 material operating at 1060°F (571°C), an 8% stress increase results in 50% decrease in creep rupture life. For Grades 11, 22, and 91, the creep rupture times are a function of stress to the powers of 4, 6, and 9, respectively. Consequently, the evaluation of the large range of stresses in these piping systems revealed that the piping system girth welds can have creep rupture lives varying by more than a factor of 10. The large range of piping stresses and associated large range in creep rupture lives within a piping system are illustrated as stress histograms for several example piping systems. Four case studies illustrate successful selection of girth weldments with the most in-service related creep damage.