Relief Valve Sizing

API 520 Part I sizing (vapour/gas · liquid · steam) + API 526 orifice-letter selection · validated <0.5% (methodology · relief-valve guide).

Relieving scenario (accumulation)

Required flow, W

Set pressure (gauge)

Temperature, T

Molar mass, M

Specific-heat ratio, k

Compressibility, Z

Total backpressure P₂ (abs)

Discharge coeff, Kd

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%)

PASS

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)