Comparison of API 510 pressure vessels inspection planning with API 581 risk-based inspection planning approaches

Highlights

• RBI calculated inspection intervals are as long as twice of API 510 inspection code.

• Two case studies verified the advantage of RBI in inspection planning.

• RBI is a more reliable methodology when evaluating multiple damage mechanisms.

• Damage factor calculations can be used for determining RSFa value in FFS assessments.

Abstract

To ensure mechanical integrity, all pressure vessels shall be inspected at the intervals provided in inspection codes or based on a risk-based inspection (RBI) assessment. The RBI assessment may allow previously established inspection intervals to be extended.

This paper describes the methodology, analysis and results of two RBI studies conducted on 293 pressure vessel components in two crude oil distillation units. Based on API RBI methodology in API 581 (2008), risk target concept was used for determining inspection dates. It was shown that when thinning is the major active damage, the RBI recommended intervals are as long as twice the API 510 intervals. This paper summarizes that, as a fundamental step in the risk calculation, RBI has a more defined methodology for evaluating equipment for multiple damage mechanisms and a more defined approach to specify the use of other inspection technologies beyond the traditional visual, ultrasonic, and radiography tests.

Introduction

Based on jurisdictional requirements all pressure containing equipment must be inspected according to appropriate inspection code to ensure its mechanical integrity for continuing service [1], [2]. The main reason of inspection is the activity of damage mechanisms which leads to loss of mechanical integrity of equipment over the time. One of the most important issues in pressure vessel inspection is “when to inspect pressure vessels”.

Setting the intervals/due dates between inspections has evolved over time. With the need to periodically verify equipment integrity, organizations initially resorted to time-based or “calendar-based” intervals/due dates. With advances in inspection approaches, and better understanding of the type and rate of deterioration, inspection intervals/due dates became more dependent on the equipment condition (i.e. condition-based inspection), rather than what might have been an arbitrary calendar date. Codes and standards such as API 510 (Pressure Vessel Inspection Code), API 570 (Inspection, Repair, Alteration, and Rerating of In-service Piping Systems), and API 653 (Tank Inspection, Repair, Alteration, and Reconstruction) evolved to an inspection philosophy with elements such as inspection intervals/due dates based on some percentage of equipment life (such as half-life), on-stream inspection in lieu of internal inspection based on low deterioration rates, internal inspection requirements for damage mechanisms related to process environment induced cracking and consequence based inspection intervals/due dates.

RBI represents the next generation of inspection approaches and interval/due date setting, recognizing that the ultimate goal of inspection is the safety and reliability of operating facilities. RBI, as a risk-based approach, focuses attention specifically on the equipment and associated damage mechanisms representing the most risk to the facility. In focusing on risks and their mitigation, RBI provides a better linkage between the mechanisms that lead to equipment failure (loss of containment) and the inspection approaches that will effectively reduce the associated risks [3].

In this study, the results of implementation of RBI technology in two distillation units are presented. In addition, the inspection intervals are calculated based on API 510 inspection code. Then the results are compared to each other. Both distillation units convert crude oil to lighter products such as kerosene, naphtha, diesel, etc. in atmospheric and vacuum distillation towers with a capacity of about 150,000 BPD. The general information about the distillation units under study is shown in Table 1.