LPG storage — fire case, PSV sizing and flare radiation

An LPG storage and truck-loading facility, built so the relief study that follows it is the real one: rundown from the fractionator is cooled, let down to storage pressure, held in a sphere, and pumped to a loading line. The process itself is deliberately simple — the demonstration is what you do with it next, using the Analysis panel's API 520/521 tools, which is the workflow HYSYS sells its Depressuring and Flare utilities on.

Fire case (Analysis > Fire-case relief). The governing scenario for a pressurised LPG vessel. A 160 m2 wetted sphere in a pool fire with adequate drainage takes 2.77 MW of absorbed heat (API 521 environment factor 1.0); at a relieving pressure of 18 bar abs the stored 70/30 propane-butane boils at 338 K with a latent heat of 270 kJ/kg, so the PRV must pass 10.3 kg/s — an API 526 M orifice.

Reaction force (Analysis > Relief reaction force). That same 10.3 kg/s leaving a 150 mm tailpipe chokes at 9.5 bar and 260 m/s, putting 17.7 kN on the pipe — the load the tailpipe supports have to carry, and the number that decides whether the discharge piping needs bracing.

Flare radiation (Analysis > Flare radiation). Burning that relief load (46 MJ/kg, 30% radiated) releases 473 MW. From a 40 m radiant centre the API 521 exclusion zones come out at 64 m horizontal for continuous exposure, 18 m for emergency personnel access, and zero for equipment. The actionable result: a receiver 60 m away sees 2.5 kW/m2 — comfortably under the 4.73 kW/m2 personnel limit, but above the 1.58 kW/m2 continuous limit, so a permanently manned building there needs shielding or relocation.

Thermal relief (Analysis > Thermal relief). A second, entirely different scenario on the same facility: the loading line blocked in full of liquid and warmed by the sun. 12 kW into trapped LPG (cubic expansion coefficient 3.9e-3 /K at 526 kg/m3) needs only 0.017 kg/s — a D orifice, the smallest API 526 size. Sizing this line for the fire case instead would oversize the valve by three orders of magnitude in area, which is exactly the mistake the separate tool exists to prevent.

Honesty notes. These are screening calculations to the API 520/521 equations, not a stamped relief study: the wetted area, drainage credit, heat of combustion and radiant-centre height are engineering inputs you supply, and the vessel inventory is not modelled dynamically (a depressuring transient is a different calculation from the steady relief rate sized here). The flowsheet's own numbers — 250 mol/s of liquid at 310 K, the 0.15 bar loading-line drop, an erosional-velocity ratio of 0.2 against the API 14E limit — come from the solve, not the prose.