by Philip A. Henry, P.E. / Valerie Magyari / Jan 01, 2008

Properly functioning Pressure Relief Devices (PRDs) are essential to protect plant personnel and equipment, as unexpected overpressure events can cause equipment damage and loss of containment, and result in costly plant shutdowns. PRDs are considered to be the ultimate layer of protection of pressurized systems. The inspection and testing of relief devices is needed to ensure PRD functionality on demand. But when and how often should PRDs be inspected? Many plants apply inspection intervals to PRDs in general agreement with API 510 and NBIC, without considering historical performance or the potential hazards associated with the relief device application. Previously, the primary focus of Risk-Based Inspection (RBI) programs has been to create inspection plans that reduce risk and optimize total maintenance and inspection expenses for pressurized static equipment. This approach has been expanded to include a quantitative methodology for PRDs. The PRD RBI technology provides a system approach (includes the damage state of the protected equipment) and evaluates the probability (reliability) and the consequence (criticality) of pressure relief valve failure and sets the appropriate inspection frequency based on risk. Results of recent studies have demonstrated the benefits of the new approach. Compared to a typical API 510 approach, the risk-based plan can significantly reduce the overall risk in the plant while at the same time a significant reduction in inspection costs can be realized. A case study of implementation in a refinery process unit illustrates the methodology has been successfully used to establish risk-based inspection intervals for PRDs.

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