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Pressure Vessels2026-07-059 min read

Pressure Relief Valve Sizing for Pressure Vessels and Reactors

Every ASME pressure vessel requires overpressure protection. Understanding the relief scenarios — fire, blocked outlet, runaway reaction, thermal expansion — helps you verify that the fabricator's PRV sizing is correct before installation.

pressure relief valvePRV sizingAPI 520fire caserelief valveoverpressure protection

ASME Section VIII requires every pressure vessel to be protected against overpressure by a pressure relief device (PRD) — typically a spring-loaded pressure relief valve (PRV) or rupture disc. The relieving capacity must handle the worst credible scenario. Undersized relief valves are a common finding in HAZOP and pre-startup safety reviews.

Common Relief Scenarios

ScenarioTypical ApplicationSizing Basis
Fire caseVessels containing flammable liquid in fire zoneAPI 521 — wetted surface area heat input
Blocked outletVessel with liquid or vapour inlet, outlet valve closedMaximum inlet flow at relieving conditions
Thermal expansionLiquid-filled vessel with blocked outlet, heatedLiquid expansion coefficient × volume × ΔT
Runaway reactionExothermic reactorReaction kinetics — requires process engineering input
External heat exchanger failureVessel heated by external sourceMaximum heat input from failed exchanger
Utility failureCooling water loss to condenserVapour generation rate at relieving pressure

Set Pressure and Accumulation

PRV set pressure is typically set at or below the vessel MAWP. ASME allows limited overpressure above MAWP during relief:

- Single PRV (unfired vessel): 10% accumulation above MAWP (i.e., relieving pressure = 1.1 × MAWP for set pressure = MAWP). - Multiple PRVs or fire case: up to 21% accumulation in some cases per ASME UG-155.

The vessel must be designed for the relieving pressure, not just MAWP — check that shell thickness calculations use the correct design pressure including accumulation allowance.

What to Request from the Fabricator

  • Relief valve sizing calculation per API 520 Part I (sizing) and API 521 (guidance on scenarios)
  • Relieving rate (kg/hr or m³/hr) and relieving pressure for each scenario
  • PRV tag number, set pressure, and orifice size (e.g., 3K4, 4M6)
  • Confirmation that PRV discharge piping back-pressure is within valve manufacturer's limits (built-up back pressure < 10% of set pressure for conventional valves)

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