FFS Forum: Updates to the New & Improved API 579 Standard

Introduction

The current edition of API 579/FFS-1, Fitness-for-Service, was released in 2016. Five years later, the newest edition is expected in December 2021, so it should have been released by publication of this article.

There are many excellent changes and updates in this new edition, including significant technical changes and improvements, and hundreds of smaller editorial changes and corrections. In this FFS Forum article I’ll go through many of what I believe to be the most important changes. The document itself includes a more detailed list of important changes, which is worth a read.

I’ve organized this article by Parts, so you can focus on your particular areas of interest. I’ve skipped sections where I feel there are no major changes.

Part 2, Fitness-for-Service Engineering Assessment Procedure

MAWP by Analysis

This part includes what I perceive to be a fairly significant change to FFS procedures. All FFS assessments are cast in terms of evaluating the deficiency at the Maximum Allowable Working Pressure (MAWP), and the MAWP has always been calculated by the Code formula (where one exists, e.g., shells and heads). The new edition of API 579 includes a provision to allow the MAWP to be calculated by the stress analysis procedure of Annex 2D, which lists procedures for elastic, limit load, and plastic collapse analysis. This means that in an FFS context, the MAWP can be calculated by elastic-plastic stress analysis methods. This could increase the rated MAWP of components and have a corresponding effect on the allowable pressure or thickness from an FFS assessment.

Division 2 Allowables for Division 1 Equipment

In the 2016 Edition, Annex C, paragraph 2C.2.4(c) stated:

“If a pressure vessel was constructed to VIII-1 and the flaw is located in the base material of a cylindrical shell, conical, or spherical shell outside the weld band, the allowable stress may be established in accordance with VIII-2.”

This paragraph has been deleted in the latest revision. No more using VIII-2 allowables! This was an artifact of API 510 before the ASME Div 2 safety factor changed to 2.4 (from 3.0).

Updated Rambert-Osgood Model

For those of you who use elastic-plastic analysis to obtain FFS assessments, you’ll find the Ramberg-Osgood model has been revised. The Ramberg-Osgood model is used to approximate the shape of the full stress-strain curve based on basic material properties, such as yield and ultimate stress.