Fitness-For-Service (FFS) assessment is a crucial technology to manage asset integrity of pressure vessels, tanks and piping. The present paper describes the role of the FFS assessment technology in guidelines for asset integrity management published by American Institute of Chemical Engineers. Also, the article summarizes significant changes in the latest API 579-1⁄ASME FFS-1 published in 2016.
API 579⁄ASME FFS-1 is the major post construction code widely recognized all over the world. As the code covers variety of damage mode that requires appropriate expertise in accordance with the assessment level, it is important to understand the basic pattern of the assessment procedure, its required organizational elements including its roll and responsibilities premised in the code. In the following, the outline of API 579⁄ASME FFS-1 Part 1 and Part 2 will be discussed. In addition, the summary of Part 3 will be reviewed and explained to illustrate the overall picture of the FFS assessment.
Fitness-for-service assessment procedures for various types of flaws are presented in API 579-1⁄ASME FFS-1. This report addresses the assessment of metal loss resulting from corrosion⁄erosion and⁄or mechanical damage. There are two different approaches to evaluate the integrity of pressure equipment subjected to metal loss in this code. Part 4 provides the procedure of general metal loss assessment. This evaluation is carried out based on a thickness averaging approach. On the other hand, Part 5 provides the procedure to evaluate local metal loss. In this part, the concept of a remaining strength factor is utilized to define the acceptability of pressure equipment for continuous operation.
A guideline of fitness-for-service assessment to pitting damage is introduced to API 579-1⁄ASME FFS-1 Code. Pitting is defined as localized regions of metal loss that can be characterized by a pit diameter on the order of the plate thickness or less, and a pit depth. Assessment procedures are provided to evaluate both widespread and localized pitting in a component with or without a region of local metal loss. The Level 1Assessment technique utilizes standard pit charts and the maximum pit depth in the area being evaluated to estimate a Remaining Strength factor, RSF. Level 2 Assessment accounts for the orientation of the pit-couple with respect to the maximum stress direction.
A guideline of fitness-for-service assessment to hydrogen damage is introduced to API 579-1⁄ASME FFS-1 Code. Fitness-For-Service (FFS) assessment procedures are provided in this Part for low strength ferritic steel pressurized components with hydrogen induced cracking (HIC) and blisters, stress oriented HIC (SOHIC) damage, and components with laminations that Part 13 directs that Part 7 shall be used (e. g. the component operates in a hydrogen charging environment or closely spaced laminations at different depths in through-thickness direction) . These forms of damage are further described below and in NACE Standard SP0296-10 Detection, Repair, and Mitigation of Cracking in Refinery Equipment in Wet H2S Environments. This Part excludes sulfide stress cracking (SSC) and hydrogen embrittlement of high strength steels which generally occur in steels with a hardness above Rockwell C 22 (Brinell237) or with tensile strengths above 793 MPa (115 ksi) . The technical basis for Part7 is summarized in Annex 7A FFS
Weld misalignment and shell distortions might decrease in strength and lead accident in plant. Fitness-For-Service (FFS) assessment procedures for pressurized components with weld misalignment and shell distortions, including out-of-roundness and bulges, are provided in API 579-1 Part 8. This paper provides a general instruction of the assessment procedure with an example covered both Level 1 and Level 2 assessment.
A guideline of fitness-for-service assessment of crack-like flaws is introduced to API 579-1⁄ASME FFS-1 Code. Since the assessment of crack-like flaws is based on the failure assessment diagram with the modeling of flaw to be detected and the calculations of stress intensity factor and reference stress, the predictions of material fracture toughness and yield strength for the equipment to be evaluated will be essential. Based on the case of shutdown inspection and replacement of the hydroprocessing reactor, the assessment procedure where the material properties obtained from the retired hydroprocessing reactor and the material fracture toughness reproducing the embrittlement phenomenon due to the service environment are applied is described in this paper.