Fire-Case Relief Load

Wetted-area fire heat input and vapour relief rate per API 521 — feeds PRV orifice sizing

Ready to calculate

Enter values and press Calculate to see results.

Engineering Tips

Estimates the relief load from an external pool fire enveloping a wetted vessel or line — the load an emergency relief device must pass. Feed the result into the PRV orifice-sizing calculator.

Fire heat input (SI):

• Q = C·F·Aws^0.82 (W, Aws in m²)

• C = 43,200 drainage + firefighting

• C = 70,900 otherwise

• W = Q/λ — vapour relief rate

Recommended values

Wetted area: Count wetted surface only up to the credited height (≈7.6 m / 25 ft above grade or fire source).
Drainage credit: Default to “No” (C = 70,900) unless drainage and prompt firefighting are demonstrably adequate.
Latent heat: Use λ at relieving pressure. Light HC ≈ 200–400 kJ/kg; water ≈ 2257 kJ/kg (atm).

Limitations

Fire case only: Evaluate non-fire contingencies (blocked outlet, thermal, tube rupture) separately — the governing case sizes the device.
Latent-heat method: For near-critical / supercritical relief where λ → 0, use the API 521 heat-balance method instead.

About the standards

API 521 (7th ed., 2020): Pressure-relieving and Depressuring Systems — para 4.4.13.2.4 wetted-area fire.
API 520 Part I: orifice sizing that consumes this relief load.

Fire-Case Relief Load

Input Parameters

Fire Exposure

Relieving Fluid