Fitness for Service of Aboveground Storage Tanks – Applicability of API 579

Introduction

The accumulation of damage is an inevitable part of the life cycle of industry fixed equipment. Aboveground storage tanks (ASTs) take no exception to this rule as they can accumulate a wide range of damage—damage that is both intended for in the initial design as well as unanticipated due to service, operational upsets, environment, etc. If unanticipated damage is discovered, repair or replacement of the affected components may be the initial reaction; however, a more favorable alternative could be a fitness-for-service (FFS) assessment to qualify continued service in the damaged condition. FFS is a recognized industry practice for assessing equipment mechanical integrity and can often be used to extend service life while minimizing downtime and avoiding costly repairs.

API 579-1/ASME FFS-1, Fitness-For-Service (API 579) is the go-to standard for assessment of FFS of fixed equipment in the petrochemical industry. ASTs designed to API 650, Welded Tanks for Oil Storage and API 620, Design and Construction of Large, Welded, Low-Pressure Storage Tanks are specifically included in the scope of API 579. The assessment procedures in API 579 may also be applied to pressure-containing equipment constructed to other recognized codes and standards, including international and internal corporate standards. In this article, we will examine the application of these API 579 FFS procedures to ASTs.

API 579 Applicability to ASTs

API 653, Tank Inspection, Repair, Alteration, and Reconstruction is the main in-service standard for ASTs. There are a few, limited FFS methodologies provided in API 653, primarily for shell thinning and pitting, and shell and bottom settlement. In situations where API 653 has no FFS rules or when API 579 is specifically referenced, API 653 provides for the use of API 579.

API 579 is organized around damage mechanisms and the corresponding FFS methodology to evaluate each of the covered damage mechanisms. Parts 3 through 14 of API 579 provide FFS procedures for a dozen different damage mechanisms. Of these damage mechanisms, all but a few apply to ASTs. In particular, brittle fracture and crack-like flaw assessments (Parts 3 and 9), metal loss assessments (Parts 4, 5, and 6) and distortion assessments (Part 8) are the most applicable. Other damage mechanisms covered in API 579 like hydrogen damage – blisters, HIC and SOHIC (Part 7), creep (Part 10), and dents & gouges in pipelines (Part 12) generally do not apply to ASTs. A few damage mechanisms covered in API 579 such as fire damage (Part 11), laminations (Part 13), and fatigue (Part 14) have limited applicability to ASTs. In the following section, we will examine the half dozen sections of API 579 most applicable to ASTs.

AST Thinning Assessments (Parts 4, 5, and 6 of API 579)