Stability of Allowable Flaw Angles for High Toughness Ductile Pipes Subjected to Bending Stress in the ASME Code Section XI

Abstract

Allowable stresses for pipes with circumferential flaws are provided by the ASME Code Section XI. The stresses for the high toughness pipes are determined by fully plastic collapse stresses and safety factors. The plastic collapse stresses are estimated by limit load criteria, which are also provided by the ASME Code Section XI. When applied stresses of the pipes at the flaw locations are less than the allowable stresses, the flaws are acceptable for the end-of-evaluation period. The allowable stresses are categorized for various service-level conditions of the plant operation. When pipe walls are thin, part-through flaws can easily develop into through-wall flaws, and the likelihood of coolant leakage is high. The ASME Code Section XI provides final allowable flaw angles of through-wall flaws that are a limit on the length of part-through wall flaws for thin-walled pipes. The final allowable angles are currently applied to pipes in order to maintain structural tolerance if the part-through flaws become through-wall flaws. To ensure that this stability is not compromised, plastic collapse stresses for through-wall flaws are combined with the allowable stresses. However, the final allowable angles of through-wall flaws are not identified for thin-walled pipes. This paper compares plastic collapse stresses of through-wall flaws and allowable stresses of part-through flaws for pipes. The comparison of these stresses is used to derive the final allowable angles of through-wall flaws. The angles can be expressed either in the form of exact solutions or as conventional options that are appropriate for various service-level conditions.