Fire Relief Load Calculator

Formula overview

SI: Aws in m², Q in W, λ in J/kg, W in kg/s. The exponent 0.82 and the C constants are dimensional — they belong to this unit set.

API 521 para 4.4.13.2.4 — wetted-area fire case:

  Q = C · F · Aws^0.82        heat absorbed, W
  W = Q / λ                   vapour generated, kg/s

  C = 43 200  with adequate drainage and firefighting
  C = 70 900  without

  F = 1.0 bare;  below 1.0 for insulation, credited by its
      conductance and its ability to survive the fire

Worked example

Vessel with 50 m² of wetted area, bare (F = 1.0), adequate drainage
and firefighting available, λ = 2 000 kJ/kg at relieving pressure

Aws^0.82 = 50^0.82 = 24.7
Q = 43 200 × 1.0 × 24.7 = 1 068 000 W = 1 068 kW
W = 1 068 000 / 2 000 000 = 0.534 kg/s = 1 923 kg/h

Without drainage credit, C rises to 70 900 and the load becomes
3 156 kg/h — 64% more, which can be a whole orifice letter.

Common mistakes

• Taking the total vessel surface as the wetted area. Only the surface in contact with liquid, and below the fire elevation — usually 7.6 m above grade — absorbs heat into the liquid. Dry wall above the level heats the vapour instead and is treated separately.
• Claiming an insulation credit that will not survive. The F factor for insulation requires it to stay in place during the fire and remain effective when hosed with water — jacketing, banding, and support have to be specified accordingly, or F reverts to 1.0.
• Using latent heat at normal operating pressure. λ falls as pressure rises and goes to zero at the critical point, so the value must be taken at relieving conditions; near-critical fluids need a different treatment entirely.

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