Plan discharge area for emergency pressure events. Choose gas or liquid mode, add factors easily. See orifice and diameter, then export files instantly here.
This calculator estimates the required orifice area using simplified discharge relations with correction factors. Use absolute upstream pressure for gas/vapor and pressure drop for liquid.
m = (Kd·Kb·Kc) · A · P₀ · √(k/(Z·R·T)) · (2/(k+1))^((k+1)/(2(k−1)))
Where m is mass flow rate, A is orifice area, P₀ is absolute relieving pressure, T is absolute temperature, R is specific gas constant, k is heat capacity ratio, and Z is compressibility.
Q = (Kd·Kb·Kc) · A · √(2·ΔP/ρ)
Where Q is volumetric flow, ΔP is upstream minus downstream absolute pressure, and ρ is liquid density.
Example values are illustrative. Your results depend on exact standards and factors.
| Case | Set (bar g) | Over (%) | Back (bar g) | T (°C) | Flow | MW | k | Z | Area (mm²) |
|---|---|---|---|---|---|---|---|---|---|
| Gas/Vapor | 10 | 10 | 0 | 50 | 1000 kg/h | 28.97 g/mol | 1.40 | 1.00 | ≈ 540 |
| Liquid | 8 | 10 | 1 | 25 | 10 m³/h | — | — | — | ≈ 295 |
Relief devices protect vessels, piping, and rotating equipment from overpressure events such as blocked outlets, fire exposure, thermal expansion, runaway reactions, and utility failures. An undersized orifice can allow pressure to rise above design limits, while an oversized valve may chatter, increase emissions, and raise cost. Many facilities size valves using defined scenarios and allowable overpressure (often 10% for common cases, with different limits for fire). This calculator estimates the minimum orifice area so you can shortlist candidate standard orifices and record assumptions clearly.
The tool treats entered set pressure and backpressure as gauge values. It then converts to absolute pressure by adding atmospheric pressure. The relieving pressure is computed as set pressure multiplied by the selected overpressure allowance, then converted to absolute. Using absolute pressure is essential because the gas/vapor equation scales directly with upstream absolute pressure, while liquid sizing depends on the pressure drop between upstream and downstream conditions.
For gas/vapor, the calculator applies a choked-flow form that assumes sonic conditions at the throat. Required area increases with mass flow rate and temperature, and decreases with higher relieving pressure. The heat capacity ratio (k) shapes the choked-flow factor. Compressibility (Z) adjusts for real-gas effects; values near 1.0 often apply at moderate pressures, while higher-pressure systems may require a more accurate Z value from an equation of state.
For liquids, the calculator uses an incompressible orifice equation: flow is proportional to area and to the square root of pressure drop divided by density. If the calculated pressure drop is small, the required area can become large. If the liquid may flash or cavitate, real two-phase behavior can dominate and a dedicated flashing method should be used instead of the simple incompressible relation.
The effective factor is the product of Kd, Kb, and Kc. Kd represents discharge characteristics of the valve or nozzle. Kb accounts for backpressure effects, especially for balanced designs or installations with significant discharge header pressure. Kc is used for combination installations such as rupture disk upstream of a relief valve. Enter values that match your governing specification or vendor documentation.
This calculator provides unit conversions for common pressure, flow, density, and temperature choices. Use a relieving temperature that reflects the worst credible scenario, not ambient. If results look unrealistic, confirm that mass flow is not entered as volumetric flow, that the correct unit is selected, and that atmospheric pressure has not been double-counted. The output includes area in m², mm², and in², plus an equivalent diameter for quick comparison.
The computed area is the minimum theoretical orifice area needed to pass the specified load under the selected assumptions and factors. In practice, select the next larger standardized orifice size offered by the manufacturer and verify capacity at rated conditions. If backpressure is high, confirm whether flow remains choked and whether additional correction is required. This tool does not model two-phase flashing, detailed piping losses, or lift behavior, so finalize design with certified standards and vendor data.
Set pressure is the nominal opening point. Relieving pressure is the expected operating pressure during discharge, usually set pressure plus allowed overpressure or accumulation. The calculator uses relieving pressure to estimate required flow area.
Gas mass flow depends on the absolute upstream pressure available to drive choked discharge. Gauge pressure alone ignores atmospheric pressure, which would understate the true pressure and distort the calculated orifice area.
If downstream pressure is high relative to upstream, the flow may become subsonic. Significant backpressure, long discharge piping, or high header pressure can reduce the pressure ratio enough that choked assumptions need verification.
Use values from your governing specification, applicable code guidance, or manufacturer documentation for your valve type and installation. If uncertain, use conservative factors and confirm with vendor-rated capacity data.
For preliminary work, Z≈1 may be acceptable at modest pressures. For higher pressures or non-ideal gases, estimate Z from an equation of state or property package at relieving conditions to avoid sizing errors.
Not reliably. Flashing and two-phase discharge require specialized correlations and property calculations. Use a method designed for two-phase relief and validate results against recognized standards and vendor guidance.
For incompressible flow, discharge velocity scales with √(ΔP/ρ). Lower density or larger pressure drop increases velocity, reducing required area for the same volumetric flow. Accurate ΔP and density improve the estimate.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.