Relief system — the fire case, on the flowsheet

Two vessels on one fire zone, each protected by a relief valve, both discharging into a shared flare header. The point is that nothing here is retyped: the relieving temperature, pressure, composition and molecular weight come from the solved streams, and the flare's load is the sum of what the valves actually send it.

The relief study, block by block. PSV_SEP is on the separator overhead: its scenario is the API 521 external fire, Q = C·F·A^0.82 over 45 m² of wetted surface, and the vapour rate is Q/λ with the latent heat taken from the property package at the relieving pressure rather than assumed. PSV_SURGE protects the surge drum on the classic blocked-outlet case — it relieves the whole inlet flow, which is a number the canvas already knows. Each reports its required orifice area with the API 526 letter above it.

Why they are drawn as lifted. Both carry relieving: true, which models the relief case rather than normal operation. Turn it off and each valve is shut — the process passes through untouched and only the sizing is reported — but then the flare has no load to size against, and says so instead of inventing one. A design load only becomes a flow when you say the valve is open.

What the flare adds that valve-by-valve sizing misses. A fire zone lifts every valve in it at once, so the header carries both loads together: that is what sets the built-up back pressure, and it is checked against the lowest-set valve on the header, because a back pressure a 20-barg PSV shrugs off will chatter a 6-barg one beside it. The tip is checked against the API 521 ~0.5 Mach blow-off limit, and the radiant flux at a receiver 60 m away against the 4.73 kW/m² personnel level, with the heat of combustion computed from the relief gas itself.

Bounds, stated. A single header (the multi-branch network with per-source back pressures is the Flare network tool); a point-source radiation model with no wind tilt; no knock-out or seal drum; and the fire case is API 520's peak instantaneous rate, not a depressuring transient — that is the Depressuring / blowdown tool.