Estimating main steam piping circumferential weldment creep life consumption

Abstract

A high energy piping (HEP) program is important for the safety of plant personnel and reliability of the piping systems. These piping systems have been designed considering allowable stresses with some safety margin at 100,000 hours. Unfortunately, many of the main steam (MS) piping systems evaluated by the author have operated beyond 350,000 hours, for which the ASME B31.1 safety margin may no longer exist. Since creep and fatigue is a typical failure mechanism, the probability of HEP failures increases with unit age.The MS piping system is one of the most critical HEP systems. Weldment failures are typically due to a combination of high temperature creep and fatigue. The circumferential weld stresses for MS piping systems operating in the creep regime should include a weldment performance factor that is both multiaxial and time dependent. Application of the actual field stresses, considering malfunctioning supports, to weldment life consumption has resulted in more accurate weldment creep – fatigue rankings and remaining life estimates. This paper demonstrates that the evaluation of actual piping stresses and variations in the actual pipe wall thickness can have a significant effect on the estimates of remaining lives of the girth weldments.