Effects of geometry of a local thin area defect on remaining strength and fitness of pressure equipment

Abstract

As an important equipment for production, pressure vessels are important equipment in the factory and can be dangerous when there is structural damage. In recent years, the development of applicability assessment technology has enabled various defects to be properly evaluated. The local thinning defect (LTA) basically uses the RSF as an indicator to measure the eligibility of the device. In the evaluation process using API 579 Level 1 and Level 2, different LTAs have the same residual strength results, so the availability evaluation results may have doubts. However, when using API 579 Level 1 and Level 2 for defect evaluation, it was found that two defects of the same length but different widths had the same residual strength. Therefore, this study explores its causes in depth.In addition, the practice of treating the defect thickness profile in CTP mode in API 579 Level 2 is considered to be relatively conservative and hence be safer. However, reordering the defect profile in fact smooth out the stress concentration effect and offers an overly optimistic estimate on the fitness of the device.First of all, the experiment started off with a set of real defect data, which was taken from measuring a localized thin area on a cylindrical pressure vessel. By changing the width of the LTA, a list of defects were generated and finite element analysis was performed. What follows is to compare results of finite element models with defects modeled with true thickness profile, critical thickness profile(CTP), and parabolic profile.For a square LTA, results from API 579 Level 2 approach is quite close to those from finite element analysis. However, the variation increases as the aspect ration of width to length of LTA becomes large. Contrary to the API 579 Level 2, the width dimension of LTA does have influence RSF, and should be taken into account. Also, in API 579, a rearranged CTP was used instead of the true profile of LTA on a belief that results from using CTP is more conservative, which later was found to be incorrect. Hence, the true thickness profile is preferred in most cases.