Leak Testing When Revising Operating, Upset, and Design Pressures in Pressure Piping

Abstract

A pressure equipment operator initiated a program to achieve compliance with a jurisdiction’s requirements for Overpressure Risk Assessment updates. The program was also initiated to provide clarifications and improvements in the pressure piping documentation where overpressure allowances were inherent in the design of heritage piping and equipment. Most of these heritage piping systems were designed between 1975 to 1995. During that period of time, it was a common industry practice to take advantage of the provision for variations per ASME B31.3 ¶302.2.4. As per this provision, it is acceptable for occasional, infrequent and short-in-duration upset events to exceed the design condition provided that all the requirements in ¶302.2.4 are met. The Overpressure Risk Assessment review of a large number of existing piping OPPSD systems recognized higher operating cases and higher overpressure upset cases than those in the original documentation. In most cases, the main reason for this inconsistency between the original and the recently calculated values is due to changes in API 520 / API 521 upset cases. Additionally, operating history, since facility start-up, provides data that demonstrates that upset events have occasionally exceeded the original values which are currently presented in the LDT. Updates to the original LDT are necessary to properly capture the experienced upset events, operating cases and, in some cases, design conditions. For heritage (pre-2013) pressure piping (PP) systems that require updates of the design pressure, the traditional margin provided between the new design pressure and the original leak test pressure as required by ASME B31.3 ¶345.4.2 will not be fully available to support the required rerate. The objective of this paper is to discuss whether another leak test, at a margin of 1.5 times new design pressure, would provide any additional value in terms of incremental safety. This is discussed in the context of pressure piping systems that have been in continuous successful service for between 25 and 43 years. The mechanical integrity of these systems is being ensured by monitoring and assessment activities that have been carried out within a comprehensive Pressure Equipment Integrity Program. The paper evaluates four different cases of the pressure piping systems that are in the scope of the program, discusses the purpose of leak testing in both construction and post-construction and lists potential risks associated with re-performing leak tests. The paper also provides recommendations for when a prior leak test is sufficient to demonstrate that a rerated piping system with a successful service history is suitable for the new service conditions.