Fire case relief load (API 521)

How this calculator works

For a vessel holding liquid, the fire case is set by the heat a pool fire puts into the wetted wall. API Standard 521 gives Q = C·F·A^0.82 W (A in m²), with C = 43,200 where there is adequate drainage and prompt fire-fighting and 70,900 where there is not, and an environment factor F for insulation or burial. The wetted area counts only the wall below the liquid level and no higher than the fire height — 7.6 m (25 ft) above grade or any surface that can hold a pool fire. The calculator integrates the exact wetted area of the shell and of 2:1 ellipsoidal, hemispherical, torispherical or flat heads, for horizontal and vertical vessels. The relief load is Q divided by the latent heat at relieving conditions.

Example: a horizontal drum 2.5 m in diameter and 8 m between tangents, 2:1 heads, half full, its bottom 1 m above grade, bare, with drainage and fire-fighting: wetted area 38.2 m² (31.4 m² shell, 6.8 m² heads), heat input 856 kW, and with a latent heat of 300 kJ/kg a relief load of 10,280 kg/h. With 50 mm of fireproof insulation (k = 0.05 W/m·K, contents at 150 °C) F falls to 0.011 and the load to 116 kg/h. The wetted areas were checked against the exact partial-surface functions of the open-source fluids library for 24 vessels and levels: within 0.0005 %.

Limits: the wetted-wall case only. Gas-filled vessels (unwetted walls), wall overheating and depressuring, spheres, columns with tray holdup, and the latent heat of mixtures near their critical point need the full API 521 treatment. Insulation earns credit only if it stays in place in a fire and under hose streams. Size the valve with the PSV sizing calculator at 21 % overpressure. References: API Standard 521, Pressure-relieving and Depressuring Systems, 6th ed. (2014), 4.4.13 and Table 5; API Standard 520 Part I.