Pressure relief devices (PRDs) play a crucial role in maintaining process safety by preventing overpressure conditions that could lead to catastrophic equipment failures. PRDs include pressure relief valves (PRVs), vacuum breakers, rupture disks, and relief hatches. However, their reliability as an independent protection layer (IPL) depends on several key factors.
Conditions for PRDs to Be Considered Dependable
For PRDs to function effectively as an IPL, all the following conditions must be met:
- Proper Sizing: PRDs must be adequately sized for the specific case under consideration, assuming that all other layers of protection (LOPs) fail, especially check valves.
- Adequate Downstream Handling: Equipment downstream of the PRD, such as headers, dump tanks, and flare systems, must be capable of safely handling the relieved material at the expected temperature and maximum specified flow rate under prevailing conditions. This includes considering:
- Two-phase flow dynamics
- Relief from other PRDs that may activate simultaneously (e.g., during a sitewide power failure event)
- Sufficient Storage and Pumping Capacity: If liquid relief is possible, the storage and pumping capacity of downstream equipment, such as flare knockout drums, must be sufficient to accommodate the relieved liquid.
- Material Compatibility: The materials of construction for downstream equipment must be compatible with the expected relief fluid composition and temperature. Special consideration should be given to potential cooling effects due to gas expansion through the relief device, which may lead to low-temperature exposure.
- Safe Discharge Location: PRDs must relieve to a location that can either safely contain or destroy the hazardous release or release it in a way that does not pose a significant risk (e.g., venting to a safe location).
- Prevention of Fouling and Isolation: PRDs must not be prone to fouling or inadvertent isolation. Any block valves on the inlet and outlet must be locked open to fensure reliability.
Probability of Failure on Demand (PFD) Considerations
A typical Probability of Failure on Demand (PFD) for a single relief valve in a clean service (non-fouling process) is often taken as 0.01. However, for multiple PRDs, the PFD value should be adjusted based on redundancy and required functionality:
- Parallel Relief Valves: If multiple PRDs are required to function together for full relief capacity (e.g., two 50% capacity valves), a higher PFD, such as 0.03, should be considered.
- Redundant PRDs: If PRDs are installed redundantly (e.g., two 100% capacity valves always online), the PFD can be reduced to 0.005. However, additional redundancy may not significantly reduce risk further due to common-cause failures.
Potential Secondary Risks of PRD Operation
While PRDs protect against overpressure, their activation can introduce secondary risks. For example, the release of flammable materials into the atmosphere could lead to fire or explosion hazards. If such secondary risks are significant, it may not be reasonable to consider the PRD as an effective IPL, as the overall risk reduction may not be sufficient.
In complex scenarios, a more detailed risk analysis may be required, utilizing Fault Tree Analysis and Event Tree Analysis to evaluate the true effectiveness of PRDs as an IPL.
References:
Functional Safety from Scratch by Peter Clarke
