There are significant economic benefits in continuing the operation of aging pressure vessels, piping, and tankage that contain flaws or damage. API 579-1/ASME FFS-1 was developed to provide standardized Fitness-For-Service Assessment procedures that can be used to evaluate these flaws (i.e., thinning, pitting, and crack-like flaws) or damaged conditions (i.e., creep and fire damage) that are found in equipment after commissioning. In this context, Fitness-For-Service is defined as the ability to demonstrate the structural integrity of an in-service component containing a flaw or damage for continued safe operation. The Fitness-For-Service Assessment procedures in API 579-1/ASME FFS-1 are intended to supplement and augment the requirements in the current in-service inspection codes NBIC, API 510, API 570 and API 653: (i) to ensure safety of plant personnel and the public while older equipment continues to operate; (ii) to provide technically sound Fitness-For-Service Assessment procedures to ensure that different service providers furnish consistent life predictions; and (iii) to help optimize maintenance and operation of existing facilities, maintain availability of older plants, and enhance long-term economic viability.
A comparison of methods for evaluating Local Thin Areas (LTA) in cylinders subject to pressure and net-section forces and moments is described in this publication. This is a particularly important subject matter for refinery and chemical plant facilities due to the complexity in layout and operation of on-site piping systems where significant forces and moments result from weight, thermal expansion, and occasional loads such as wind and earthquake ground motion. It is shown that the current method in API 579-1/ASME FFS-1, Part 5 is sufficiently conservative for the evaluation of LTAs in complex piping systems subject to pressure and net-section forces and moments. In addition, recommendations are made to remove some conservatism in the current assessment method based on a comparison of results to both experimental data and numerical simulations.
This publication is part of a series of WRC Bulletins that contain the technical background to the assessment procedures utilized in API 579-1/ASME FFS-1. Note that these documents are updated as required and the latest edition should be used.