ASME Pressure Vessels Without PRDs – Is That Allowed?

Introduction

Overpressure protection by system design in lieu of pressure relief devices (PRDs) as a method of pressure vessel overpressure protection has been used in industry for a long time, and only recently has it been added to Section VIII, Division 1, of the ASME Boiler and Pressure Vessel Code (ASME Code) as paragraph UG-140 [1]. As part of the recent restructuring of the ASME Code, most of the requirements associated with UG-140 are now located in ASME Section XIII, Part 13 [2].

Refiners and chemical companies have used instrumentation as part of their overpressure protection systems in situations where a PRD is unreliable due to the fouling or plugging nature of the relieving fluid, where the required capacity (i.e., number of relief devices) is unreasonable, or where PRDs introduced other dangers due to increased emissions.

Instrumentation can be incorporated into the system design to eliminate certain overpressure scenarios and reduce the size and number of PRDs required. However, many operators are still not applying the system design philosophy as intended by the ASME Code. Part of the reason for this could be due to significant confusion about how it is to be applied.

Background on Overpressure Protection by System Design

Industry leaders from the American Society of Mechanical Engineers (ASME) and American Petroleum Institute (API) recognized that the industry had been applying a system design philosophy inconsistently, so they worked together to establish minimum requirements for applying system design in lieu of PRDs. These rules were initially published in 1996 as a Code Case to the ASME Code: Code Case 2211, “Pressure Vessels with Overpressure Protection by System Design.” The Code Case was reaffirmed in 1999 as CC2211-1.

In the 12 years between the initial publication of CC2211 and the incorporation of UG-140 into the ASME Code, a Welding Research Bulletin, WRC 498, was published in 2005 with the goal of providing the industry with practical guidance on applying the Code Case and ensuring consistency in the way it was being applied [3].

In addition to providing guidance to owner/operators on how to apply the Code Case, WRC 498 also recommended several improvements for the ASME Code to consider when eventual incorporation occurred. Firstly, WRC 498 recommended that the ASME Code allow similar allowable overpressures (accumulations) when traditional PRD overpressure protection is supplied (i.e., 10% above the maximum allowable pressure MAWP for normal process overpressure scenarios). CC2211 was originally written to ensure that overpressure by system design applications limit the pressure rise under all circumstances to the vessel’s MAWP. When system design was eventually incorporated into the ASME Code, UG-140 allowed accumulation of a pressure of 116% of the MAWP when instrumentation is used in the design of the overpressure protection system.

Secondly, WRC 498 recommended that the Code consider both the probability and the consequences (i.e., risk) when assessing the credibility of potential overpressure scenarios. This is where some of the confusion occurs. WRC 498 interpreted CC2211 to require that all potential overpressure scenarios, regardless of the overpressure potential (probability/likelihood), be accompanied by a credibility analysis that defines credibility as any event that occurs more frequently than 1 x 10^-4 events per year (or 1 event per 10,000 years). Some users will reduce this to 1 x 10^-5 (1/100,000 years) for catastrophic events. As written, then, even if the overpressure were to exceed the MAWP by only 20% (below hydro), for example, high-integrity instrumentation (quite expensive) might be required to reduce the event frequency to the tolerable 1 x 10^-4 events per year. Vessels are typically hydrotested at 1.3 times the MAWP, so an overpressure of 20% clearly should not be considered high risk. A risk-based approach that evaluates both likelihood and consequences using a company’s risk matrix might not require high-integrity instrumentation for low-consequence overpressure scenarios.