Step 1 — Scenario. Before sizing anything, ask whether a fire is even credible for this vessel. API five-twenty-one defines a fire envelope of about seven-point-six metres above grade. If the vessel sits entirely above that height, or there's no realistic pool-fire source nearby, the fire case may not apply at all — and another scenario, like a blocked outlet or control-valve failure, might govern instead. Don't reach for "fire case" just because it sounds like the worst one; confirm it's physically credible first.

Step 2 — Wetted area. Only the part of the shell touching liquid, and within the fire envelope, counts as wetted area. For a horizontal drum, that's the surface below the liquid level over the wetted length. A good habit: sketch the cross-section and shade the wetted region by hand before you compute anything — it heads off the most common geometry mistakes.

Step 3 — Heat input. Now estimate the heat absorbed from the fire using the API five-twenty-one equation: Q equals C, times F, times area to the power zero-point-eight-two. C depends on drainage, F is the environment factor. For bare, unprotected steel, F equals one. Good drainage and fire-resistant insulation can drop it to zero-point-three or lower — always state the assumption you've made.

Step 4 — Relief load. The absorbed heat boils off liquid, so divide Q by the latent heat of vaporisation to get the vapour mass flow the valve must pass. Use the latent heat at relieving conditions — not storage conditions — because for light hydrocarbons the difference is significant.

Step 5 — Orifice area. With the relief rate known, apply the API five-twenty vapour sizing equation to get the minimum required effective discharge area. For the fire case, the relieving pressure is one-point-two-one times the set pressure — that's the standard twenty-one per cent overpressure allowance — expressed in absolute units.

Step 6 — Select and check. Round up to the next standard API five-twenty-six orifice letter. Then verify the valve's rated capacity exceeds your relief rate, confirm the inlet and outlet lines won't choke the flow, and check that backpressure stays within the limit for your valve type. Remember: a bigger orifice isn't automatically safer — oversizing causes chatter. Pick the smallest standard orifice that meets the requirement.

Step 7 — Exemptions and mitigations. It's worth asking whether the fire case can be reduced or excluded. Opinion is split — some engineers argue it can be excluded for low-design-temperature equipment, since gaskets would melt and relieve pressure before the vessel fails. But standard practice is clear: if the fire is credible, size for it. Any exclusion needs documented, peer-reviewed justification. Genuine mitigations include fireproof insulation to lower F, elevating equipment above the fire envelope, and adequate drainage and spacing.

Step 8 — Layered protection. A relief valve is only one layer. Robust protection combines active and passive fire protection — fireproofing on supports, thermal insulation, and water deluge at around nine-point-eight litres per minute per square metre — with emergency depressurisation that brings the vessel to roughly half its design pressure within fifteen minutes. Add vacuum protection to prevent implosion during rapid cooling, and containment such as bunding and firewalls to limit escalation. The objective throughout: prevent rupture, collapse, or structural failure by controlling heat, limiting pressure, guarding against vacuum, and reducing fire spread.