Lessons learned from a crack assessment of a 36-inch steam header operating at high pressure and temperature

Abstract

A leaking crack in a high-pressure steam header, manufactured from carbon steel, was assessed for acceptability according to the precepts of the API 579-1/ASME FFS-1 2021 Fitness-For-Service standard. A finite element model, with shell elements in the line section, combined with solid elements in the region of interest, was constructed to gain insight into the through-thickness stress distribution. A basic linear elastic methodology, utilizing a lower-bound estimate of fracture toughness, indicated that the crack was acceptable for continued operation, albeit with a small margin of safety. Five other evaluations, four of which were based on ductile tearing criteria, were performed and it was found that the limiting crack size was in fact larger than the assessed crack with a substantial margin. The most accurate assessment performed used ductile tearing instability as the limiting criterion, with the tearing analysis supported by test results obtained from service-exposed samples. A J-integral tearing resistance (J-R) curve was experimentally established at the operational temperature, using a section cut from the header. The outcomes of the six assessments were critically compared with regard to damage tolerance and conclusions were drawn from these comparisons.