Pick the service, enter the governing relief flow and relieving conditions, then press Calculate. The required area is matched to the smallest API 526 orifice.
Gas: A = W/(C·Kd·P₁·Kb·Kc)·√(TZ/M)
Steam: A = W/(51.5·P₁·Kd·Kb·Kc·KN·KSH)
Liquid: A = Q/(38·Kd·Kw·Kc·Kv)·√(G/ΔP)
Engineering tips
- API 526 Orifices (in²):
D:0.110
E:0.196
F:0.307
G:0.503
H:0.785
J:1.287
K:1.838
L:2.853
M:3.600
N:4.340
P:6.380
Q:11.05
R:16.00
T:26.00
- Critical vs subcritical: gas is critical when P₂/P₁ ≤ (2/(k+1))^(k/(k−1)); above that, back-pressure cuts capacity.
- Kd basis: 0.975 vapor/steam, 0.65 liquid for certified valves — leave Kd blank to use these defaults.
- Back-pressure: above ~10% built-up back-pressure a conventional valve needs Kb or a balanced-bellows design.
Recommended values
- Overpressure: 10% for a single relief (non-fire), 21% for the fire case, 16% for multiple valves.
- Back-pressure: Conventional valve only up to ~10% built-up; above that use balanced-bellows or pilot-operated.
- Discharge coeff.: Kd ≈ 0.975 gas/steam, ≈ 0.65 liquid — always use the valve-certified value.
- Orifice selection: Round the required area up to the next API 526 letter (D…T).
Limitations
- Steady-state: Single-phase, single-valve sizing only — no two-phase / flashing (use API 520 / DIERS).
- Not included: Inlet 3% pressure-loss rule, outlet built-up back-pressure, reaction force and piping are checked separately.
About the standards
- API 520 Part I: Sizing and Selection of Pressure-Relieving Devices.
- API 526: Flanged Steel Pressure-Relief Valves (standard orifice designations D–T).