API RBI for PRDs
Considered to be the ultimate 'Layer of Protection' for pressurized systems, properly functioning Pressure Relief Devices (PRDs) are essential to avoid unexpected overpressure events that can cause equipment damage and loss of containment – and result in costly plant shutdowns.
The regular inspection, testing and overhaul of pressure relief devices is needed to ensure proper PRD function upon demand. But when and how often do you need to inspect? 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 leak. 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 interval based on the historical performance of the PRD, but significant increases in intervals over the preset values 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 new technology that includes a highly quantitative approach for determining individual PRD inspection intervals, allowing owner-users to design maintenance programs based on measured risk. This approach systematically prioritizes maintenance and inspection dollars by:
- Targeting the most critical PRD's
- Allowing deferral of lower-risk PRD's
- Linking PRDs to the damage state of protected equipment
- Considering other 'Layers of Protection'
- Optimizing intervals based on plant risk tolerance
This API PRD RBI methodology has been incorporated into API RBI software as well as our VCESage™ software. Typically, the PRD module is linked directly to the protected equipment. However, 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 approach, which is highly quantitative is consistent with API 510 Pressure Vessel Inspection Code, employs extensive analysis in its calculations.
- Extensive PRD Reliability/Failure Rate Database – Considers the fluid service, duration in service, and 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 individual PRD.
- Integrated Consequence Analysis Package – Considers the fluid severity and hazard potential (flammable and toxic consequences) when evaluating loss of containment as a result of PRD failure to open. Also evaluates the consequence 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 accurate approach to PRD RBI that provides in-depth documentation for your inspection plan. The calculations are not "black box" as the technology has been validated and approved by the industry committees. API RBD RBI has the potential to significantly reduce maintenance costs without jeopardizing safety of the plant.
PRD Expertise
E2G is uniquely qualified to implement the new API PRD RBI technology because we're 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.
