Enter the relief load and conditions — get the required orifice area and the API 526 orifice letter.
Open the calculator →A pressure safety valve (PSV) must pass the governing relief load without over-pressuring the equipment. EnggiBasis computes the required effective orifice area for your phase and conditions per API 520, then picks the standard API 526 orifice letter — the number and the hardware, together.
A = W / (C · Kd · P₁ · Kb · Kc) · √(T · Z / M)
where W is the relief mass flow, C a coefficient from the ratio of specific heats, Kd the discharge coefficient, P₁ the relieving pressure (set + overpressure + atmospheric), Kb and Kc correction factors, and T, Z, M the relieving temperature, compressibility and molecular weight.
Compare the required area against the standard API 526 orifice areas (D → T) and pick the first that exceeds it. EnggiBasis returns the letter directly.
Getting the load right matters most. API 521 governs the contingency — fire case, blocked outlet, control-valve failure, thermal expansion. EnggiBasis includes a contingency helper so you size for the governing scenario, not just the obvious one.
For a vapour relief of a given W, P₁, T, Z and M, the API 520 equation returns a required area — say 1.9 cm². That falls between API 526 orifices "F" (1.98 cm²) and the next down, so you select a "F" orifice valve. Run it with your real relief case →
Find the governing relief load, compute the required orifice area from the API 520 equation for the phase, then pick the next-larger API 526 orifice.
A = W/(C·Kd·P₁·Kb·Kc) · √(T·Z/M) for critical flow.
Relief and flare sizing is part of EnggiBasis Pro (₹99/month). Open the app free to explore.
API 520 area + API 526 orifice + contingency helper · Pro (₹99/month)
Open the calculator →Related: Control valve sizing · Line sizing · Guides