Relief Valve Sizing Calculator

Size pressure relief valves per API 520/526 for gas/vapor service. Calculates required orifice area and selects the standard API 526 orifice designation.

Inputs

Mass flow rate W (lb/hr)

Set pressure (psig)

Overpressure allowance (%)

Back pressure (psia)

Relieving temperature (°F)

Molecular weight (g/mol)

Specific heat ratio k (Cp/Cv)

Compressibility factor Z

Tab to move between fields

Results

Required orifice area (in²)

0.82

Selected API 526 orifice area (in²)

1.29

Required orifice area (mm²)529.59

Relieving pressure P₁ (psia)179.7

Coefficient C356.06

Critical pressure ratio0.53

Critical flow? (1=yes, 0=no)1

Area Cm25.3

Selected Letter Code74

How to Use This Calculator

Enter Mass flow rate W (lb/hr), Set pressure (psig), and Overpressure allowance (%).
Set Back pressure (psia), Relieving temperature (°F), and Molecular weight (g/mol).
Adjust Specific heat ratio k (Cp/Cv), Compressibility factor Z as needed.
Review Required orifice area (in²) and Selected API 526 orifice area (in²).
Use Required orifice area (mm²) and Relieving pressure P₁ (psia) to inform your decision.

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