Relief valve sizing calculator (API 520 / 526)
Size pressure-relief valves for vapour/gas, liquid and steam service straight in the browser. The calculator builds the relieving pressure from your set pressure and accumulation scenario, applies the API 520 Part I method — critical vs sub-critical vapour flow, the liquid Eq. 27 with viscosity correction, and the steam Napier equation with superheat correction — then selects the API 526 orifice letter (D–T) and shows the next size up. Every result is clause-cited and validated to <0.5% against the API method.
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Relieving pressure P₁ = Pset·(1 + accumulation) + atmospheric, built internally (API 520 Pt I §3.6).
Selected API 526 orifice
M+21.72%
Required A 1908.06 mm² · next N (+46.75%)
Critical (choked) flow: P2/P1=0.084 ≤ 0.55 — sized on Eq. 3 (area independent of backpressure).Selected orifice M (2322.58 mm²) — 21.7% above required 1908.1 mm².Next size N (2799.99 mm²) — 46.7% above required (turndown/future duty).
P1_kPa1201.325
P2_kPa101.325
pressureRatio0.0843
accumulation0.1
C0.025053
F20
W_kgh18000
Kd0.975
Kb1
Kc1
relief-vapour · api-520-1:2020 — area→API 520 Part I §5.6.3 Eq. 3 (critical) · critical→API 520 Part I §3.6 (conventional-valve critical-flow rule, P2/P1 ≤ 0.55)
Keep going
- Relief valve sizing guide — the API 520/521/526 method, critical-flow rule and orifice selection explained.
- Methodology & V&V — the equations, editions and validation cases behind every number.
- Control valve sizing · DP flow element sizing — the rest of the instrumentation suite.