Pressure relief valve (PRV) — Failure Modes & Failure Rate

Spring-loaded valve that opens at a set pressure to vent overpressure to a safe location. Mandated by ASME BPVC Section VIII for any pressure vessel, and a foundational independent protection layer (IPL) in LOPA. Failure rate dominated by the stuck-closed (fail-dangerous) mode — the PRV is the last line against vessel rupture.

λ typical

5.0×10-6 / h

Range

1.0×10-6 – 2.0×10-5

Source

OREDA / Exida

Failure modes

Stuck closed (fail-dangerous)

Root causes: Corrosion bonding the disk to the seat; polymer / asphaltene build-up gluing the nozzle shut; spring corrosion / set-pressure drift upward; mechanical jam from debris.
Detection: Pop test during scheduled maintenance; periodic on-line testing with hydraulic pop-test rig (TREVITEST or similar); difficult to detect during operation without a dedicated test.
Mitigation: Periodic pop-testing per API 510 / ASME PCC-3 schedule; clean / refurbish at every test interval; redundant PRVs (50%+50% or 100%+100%) to mitigate single-PRV dangerous failure. See <a href="/templates/process_pressure_vessel_rupture">vessel-rupture template</a>.

Leak past seat

Root causes: Seat erosion from chatter (PRV cycling against itself); soft-seat compression set; thermal expansion mismatching disk and seat metals.
Detection: Process-fluid loss to the relief header; flare-load above expected; downstream-temperature rise on a leaking PRV.
Mitigation: Set-pressure margin ≥10% above operating pressure to avoid chatter; soft-seat for tight shutoff in clean service; metal-seat for hot or abrasive service.

Set-pressure drift (high)

Root causes: Spring relaxation over multi-year service; corrosion-induced loss of cross-section in the spring; calibration error after refurbishment.
Detection: Pop-test result above the original set point; impossible to detect during normal operation.
Mitigation: Refurbishment and re-set per ASME PCC-3 at the inspection interval; tamper-proof seal on the bonnet to detect post-refurbishment adjustment; PRV vendor's certified test report retained.

Premature opening (low set)

Root causes: Spring fatigue from repeated chatter; wrong spring fitted at refurbishment; back-pressure variations affecting balanced-bellows designs.
Detection: PRV opens at known pressure below the set point during operation; flare-load alarm; product loss to relief.
Mitigation: Balanced-bellows or pilot-operated PRV for variable back-pressure service; chatter analysis at design stage; bonnet vent inspected during proof-test.

Typical applications

Pressure-vessel overpressure protection (universal in process plant); compressor / pump discharge relief; thermal-relief on isolated liquid-filled segments; fired-equipment relief; pipeline pressure protection.

How to model in a fault tree

In LOPA, the PRV is typically credited as an IPL with a Probability of Failure on Demand (PFD) of ~10⁻² for a single PRV with normal inspection — the IEC 61511 RRF target then drives the SIL of any electronic SIS layer above it. For FTA work, model the PRV stuck-closed mode as a basic event under the vessel-rupture top OR, with λ_DU calibrated to the actual proof-test interval. Redundant PRVs (50/50 or 100/100) are an AND structure under the PRV branch — but apply Beta-factor CCF for shared-fluid plugging modes that affect both simultaneously.