API 579: A Comprehensive Fitness-For-Service Guide

by David A. Osage, P.E., ASME Fellow / December 2000

 

This article presents an overview of the recently published American Petroleum Institute (API) Recommended Practice 579, which covers fitness-for-service assessment of pressure equipment in petrochemical and other industries. Although API 579 covers a wide range of flaws and damage mechanisms, including local metal loss, pitting corrosion, blisters, weld misalignment, and fire damage, the emphasis of the present article is on the assessment of crack-like flaws. The API 579 procedure for evaluating cracks incorporates a failure assessment diagram (FAD) methodology very similar to that in other documents, such as the British Energy R6 approach and the BS 7910 method. The API document contains an extensive compendium of K solutions, including a number of new cases generated specifically for API 579. In the initial release of the document, API has adopted existing reference stress solutions for the calculation of Lr in the FAD procedure. In a future release, however, API plans to replace these solutions with values based on a more rational definition of reference stress. These revised reference stress solutions will incorporate the effect of weld mismatch. In addition to the Appendices of K and reference stress solutions, API 579 includes appendices that provide guidance on estimating fracture toughness and weld residual stress distributions. Over the next few years these appendices will be enhanced with advances in technology. Recently, API has entered into discussions with the American Society of Mechanical Engineers (ASME) to convert API 579 into a joint API/ASME fitness-for-service guide.

 

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