API 521 / API 520 / API 526 · Engineering tool
PSV fire-case sizing (API 521)
Q = 34500 · F · A^0.82
Wetted-vessel pool-fire relief rate and PSV orifice per API 521 / API 520 — heat input, vapor relief rate, and the API 526 letter.
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How it works
A pressure vessel wetted by liquid and exposed to an open pool fire absorbs heat through its wetted surface. API 521 gives the heat absorbed as Q = 34,500·F·A^0.82 (Q in BTU/hr, A the wetted surface in ft², F an environmental factor) — the consolidated form used for screening.
The relief rate is the heat input divided by the liquid latent heat of vaporisation at the relieving pressure, W = Q/λ. The vapor then sets the PSV orifice through the API 520 vapor/gas equation, using the fluid molecular weight, specific-heat ratio, and relieving temperature.
This tool reports the heat input, the vapor relief rate, and the next standard API 526 orifice. Fire-case relief must be checked against the current API 521 edition — including adequate-drainage credit, the actual wetted surface (liquid inventory), and proper fluid properties.
Assumptions & limitations
- Wetted-vessel pool-fire case (not unwetted, jet fire, or cold-side). Environmental factor F accounts for drainage/insulation credit (1.0 bare steel).
- Heat input Q = 34,500·F·A^0.82 BTU/hr (A in ft²) — the API 521 consolidated fire-equation form; verify against the current edition.
- Relief rate W = Q/λ uses the liquid latent heat at relieving pressure; vapor is treated as ideal gas in the API 520 orifice equation with Kd = 0.975.
- Latent heat, k, MW, and relieving temperature are user inputs from a fluid property source; condensable/heavy components need a rigorous evaluation.
Related reading
Companion articles
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Emergency depressurisation and blowdown — the API 521 15-minute rule
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Flare network sizing and API 521 blowdown
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Jose CampinsNeed a fire-case PSV sizing, a relief and flare system study, or an independent review?
A principal will respond within one business day with an initial fit assessment and a practical next step.