What is Pressure Relief Device Risk-Based Inspection (PRD RBI)?
Just as Risk-Based Inspection (RBI) is used on fixed equipment in industrial facilities, it can also be applied to some of the components that help protect them. Properly functioning Pressure Relief Devices (PRDs) are critical for protection of plant equipment and personnel. PRD RBI does not differ in the fundamental concepts of fixed equipment RBI in that you are still assigning a risk based on Probability of Failure (POF) and Consequence of Failure (COF) calculations. Ultimately you are aiming to achieve the same goals of increased safety, improved compliance, and reduction in maintenance and operations costs.
How Is PRD RBI Different Than Fixed Equipment RBI?
For PRDs the main input that we have for probability is inspection results. Have you seen galling, plugging, or failures to lift or seat properly? The environment is also evaluated for PRD RBI. Is there excessive vibration or is it properly sized for the scenario it has been placed? In terms of consequence, you still cover environmental, health and safety, and economic impact.
PRD RBI has not been widely adopted the way RBI on pressure containing vessels has been. Part of the reason being the increased complexity of PRD RBI. With RBI on a pressure vessel, we determine the risk of failure of that vessel separately. PRD RBI adds another level of detail because if the PRD does not open at the set pressure, the surrounding equipment is also in danger. As a result, before PRD RBI can be performed, RBI must be performed on the equipment protected by the PRD and the inputs and outputs of said protected equipment are required for the PRD RBI analysis. In addition to input data related to protected equipment, data related to the PRD, including inspection data, service data and sizing all come into play.
PRD failures are evaluated for two different modes during normal operation. The first being a leak and the second a failure to open (FTO). This “leak” is not a pinhole leak as defined for fixed assets but rather a leak past the seat of the valve where it is wedged open or no longer able to create an efficient seal. In the instance of a leak a few questions have to be asked. How much and what kind of product is being leaked and what are the consequences of the leak? Can it continue to operate or does it need to be taken out of service for repair?
FTO refers to a scenario when the valve fails to open when called upon. This could be caused by the valve being galled shut, the inlet or outlet piping being plugged, or another scenario that prevents the valve from relieving pressure when it needs to. In addition to the risks associated with leaks, the risk of the valve not opening when needed is known as Probability of Failure on Demand (POFOD). This risk calculation takes into account performance of pop tests, service, valve size, and the equipment being protected. Typically, the consequence of an FTO occurrence would be significantly higher than a leak because it would likely lead to an overpressure event on the equipment that it is protecting.
The risk associated with Leak and FTO scenarios are evaluated separately and then added together at the end of the calculation to determine risk and inspection interval. Different methodologies have been seen across the industries when it comes to determining acceptable/unacceptable risk and applying it to risk matrices. Some facilities will use the stair step method while others a linear iso-risk line.
What is the Value of PRD RBI?
PRDs serve as the last line of defense for pressure vessels, boilers, and other pressurized containment in order to prevent loss of containment, equipment damage, and costly unplanned downtime. It is vital to have a reliable inspection program in place at your facility to ensure the availability of PRDs to mitigate risk in case of overpressure events. It’s also important that inspection dollars are not spent inspecting low-risk PRDs as better focus of resources means better safety and reliability.
With PRD RBI models it allows a facility to increase some inspection intervals based on evaluation of low-risk areas or reduce allocations to those areas. It also helps to identify areas of high-risk where you may need to increase inspection intervals or add additional support to increase facility safety.
What Makes Collaborating with Pinnacle Different?
PRD RBI is not a widely adopted methodology in the industry so when a facility is ready to adopt a new program, they are looking to utilize an industry accepted approach and in this case that means an API RP 581 methodology.
Watch Lynne Kaley, Vice President of Research and Development at Pinnacle, talk about how Pinnacle Subject Matter Experts (SME) took the time to break down the calculations to create the only tool that uses the most up to date API 581calculations. In creating the tool, inconsistencies were found in the old methodology and the changes were balloted and accepted as the new official API RP 581 methodology and will be released as part of the 4th edition.