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Calcimator

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

%

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

How the result changes with Mass flow rate W (lb/hr)

Mass flow rate W (lb/hr)Required orifice area (in²)Selected API 526 orifice area (in²)
10,000,000820.8626
35,000,0002,873.0326
65,000,0005,335.6226
90,000,0007,387.7826

What each input means

Mass flow rate W (lb/hr)
Required relieving mass flow rate.
Set pressure (psig)
Valve set pressure (gauge).
Overpressure allowance (%)
Typically 10% for single valve, 16% for fire case.
Back pressure (psia)
Downstream pressure at valve outlet.
Relieving temperature (°F)
Temperature of the fluid at relieving conditions.
Molecular weight (g/mol)
Molecular weight of the gas (air = 28.97, methane = 16.04).
Specific heat ratio k (Cp/Cv)
Ratio of specific heats (air = 1.4, steam ≈ 1.33).
Compressibility factor Z
Gas compressibility (1.0 for ideal gas).

What each result means

Required orifice area (in²)
Minimum effective orifice area per API 520.
Required orifice area (mm²)
Same area in metric units.
Selected API 526 orifice area (in²)
Next standard orifice size that meets the requirement.
Relieving pressure P₁ (psia)
Set pressure plus overpressure plus atmospheric.
Coefficient C
Flow coefficient derived from specific heat ratio k.
Critical pressure ratio
Pressure ratio below which flow is critical (choked).
Critical flow? (1=yes, 0=no)
Whether flow through the valve is choked at these conditions.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Mass flow rate W (lb/hr) = 10000, Set pressure (psig) = 150, Overpressure allowance (%) = 10, Back pressure (psia) = 14.7 = 8 input(s) provided
  2. Calculate Required orifice area
    Required orifice area = (massFlow * sqrt(tempR * compressibility)) /
    0.8209 = 0.8209
  3. Calculate Selected API 526 orifice area
    1.287 = 1.287
  4. Calculate Required orifice area
    Required orifice area = areaIn2 * 645.16
    529.59 = 529.59
  5. Calculate Relieving pressure P₁
    Relieving pressure P₁ = setPressure * (1 + overpressurePct / 100) + 14.696
    179.7 = 179.7

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