Pressure Relief Valve Sizing

API 520 Part I orifice area calculation for gas/vapour, steam, and liquid service. Suggests the next standard API orifice designation.

Service

Pressures

Relieving pressure P₁ (set + accumulation, absolute)

Typically set pressure × 1.10 for process vessels

Backpressure Pb (absolute)

Gas / vapour properties

Relief flow (kg/h)

Molar mass M (g/mol)

Inlet temperature (°C)

Ratio Cp/Cv γ

Compressibility Z

Discharge coeff. Kd

Back-pressure factor Kb

Common gases: Air M=28.96 γ=1.40 · N₂ M=28 γ=1.40 · CO₂ M=44 γ=1.30 · CH₄ M=16 γ=1.31 · Steam M=18 (use Steam tab)

This calculator implements the SI form of the API 520 sizing equations. It does not replace a full relief-load study or code-compliance review.

For balanced-bellows or pilot-operated valves, Kb may differ from 1.0 — consult manufacturer data or API 520 Appendix C.

Relieving pressure P₁ = set pressure × 1.10 for fire/emergency cases (10 % accumulation on ASME vessels).