(41y) Evaluating Overpressure Scenario Credibility

When considering overpressure protection, it is appropriate to typically consider any overpressure scenario that can occur, regardless of likelihood or credibility. This is an intentionally conservative approach, to try to ensure that no overpressure scenarios are overlooked.

Over the years however, the use of likelihood or credibility has increased, as it relates to overpressure scenarios. For example, API Standard 521 7th edition, addresses:

• Use of Administrative Controls if Corrected Hydrotest Pressure Not Exceeded
• Double Jeopardy
• Latent Failures
• Operator Error/Effect of Operator Response
• Role of Instrumentation in Overpressure Protection

All of these factors can have an impact on the likelihood of an overpressure scenario occurring.

When it comes to determining overpressure scenario credibility, a very useful reference document is Bulletin 498, published in 2005 by the Welding Research Council. The title of this bulletin is "Guidance on the application of Code Case 2211 - Overpressure Protection by System Design".

Despite Code Case 2211 having been superseded, first by UG-140 in ASME Section VIII, and then by Part 13 in ASME BPVC Section XIII; Bulletin 498 is still an important and relevant reference when evaluating overpressure scenario credibility and applying the concepts of Overpressure Protection by System Design. In fact, it is directly referenced in ASME BPVC XIII for this specific reason.

This paper considers the importance of evaluating overpressure scenario likelihood. It reviews the guidance provided in WRC Bulletin 498 and demonstrates how the credibility of an overpressure scenario can be determined and applied.