Pressure Relief Devices (PRDs)

API RBI for PRDs

Considered to be the critical 'Layer of Protection' for pressurized systems, properly functioning Pressure Relief Devices (PRDs) are essential to avoid unexpected overpressure events that can cause equipment damage, loss of containment and, ultimately, plant shutdowns.

The regular inspection, testing and overhaul of PRDs is needed to ensure proper function upon demand. But when and how often do they need to be inspected? Most inspection programs do an adequate job of managing the unreliability of PRDs, without giving much consideration to the potential consequences of PRD failure to open or leakage. Many plants inspect PRDs on preset intervals as recommended by inspection Codes, such and NB-23 and API 510. These Codes allow increases in inspection intervals based on the historical performance of the PRD, but significant increases in intervals over the preset intervals can take many years.

The use of API RBI allows optimum intervals to be set immediately based on the probability and consequences associated with PRD failure.

To improve prediction accuracy and gain industry acceptance, an API RBI technical working group, chaired by Equity Engineering’s Phil Henry, has developed ground-breaking technology that includes a highly quantitative approach for determining individual PRD inspection intervals, allowing owner-users to plan maintenance programs based on measured risk. This approach systematically prioritizes maintenance and inspection resources by:

• Targeting the most critical PRDs
• Allowing deferral of lower-risk PRDs
• Linking PRDs to the damage state of protected equipment
• Considering other 'Layers of Protection'
• Optimizing intervals based on plant risk targets

This API PRD RBI methodology has been incorporated into API RBI. Typically, the PRD module is linked directly to the protected equipment, but, PRD RBI can be performed without fixed equipment RBI.

The API PRD Difference

API PRD RBI methodology evaluates the criticality of pressure relief devices, and sets inspection and testing frequencies accordingly. This quantitative approach, is consistent with API 510 Pressure Vessel Inspection Code, and employs extensive computational analysis by considering the following:

• PRD Reliability/Failure Rate Database – Considers the fluid service, duration in service, type of device, and provides default PRD failure rates backed by an extensive database of bench tests.
• Criticality – Considers the overpressure scenarios (loss of cooling, fire, blocked discharge, etc.) applicable to each PRD.
• Integrated Consequence Analysis Package – Considers the fluid severity and hazard potential (flammable and toxic consequences) when evaluating the loss of containment as a result of PRD failure to open, or the consequences of PRD leakage.
• Integrated System – Ties directly to the damage state of the equipment protected by the PRD. The criticality of the PRD increases as the condition of the protected equipment deteriorates over time.
• Documented Inspection History – Updates the default Probability of Failure using the historical PRD inspection data for each PRD.

This is a more logical approach to PRD RBI than the traditional condition-based PRD inspection program. The calculations are not "black box," as the technology has been validated and approved by the industry committee for API RBI. API PRD RBI has the potential to significantly reduce maintenance costs without jeopardizing the safety of the plant.

PRD Expertise

Equity is uniquely qualified to implement the new API PRD RBI technology because we are recognized industry leaders in the four areas that were merged to create it: Risk-Based Inspection; Pressure Relieving Systems; damaged equipment Fitness-For-Service; and software engineering. Philip Henry, P.E., an expert in pressure relief systems, led the development of the API PRD RBI methodology as Chairman of the API RBI Technical Working Group. Phil is also the API PRS Subcommittee chairman of TF520, responsible for standards associated with the sizing, selection, and installation of pressure relief devices.