Pressure Relief Area Calculator

Design vent openings for safer equipment and piping. Compare liquid and gas relief sizing. Generate exportable results for audits and documentation.

Calculator
Choose a model, enter inputs, then compute required area.
CSV Export
Use choked gas when downstream pressure is sufficiently low.
Common range: 0.60–0.80 for sharp-edged orifices.
Applied to calculated area. Typical: 1.00–1.50.
Converted internally to m³/s.
Water near room temperature is about 998–1000.
Use available differential pressure across the opening.
Total required relieving mass flow.
Absolute pressure at the inlet to the opening.
Converted internally to Kelvin.
Air is roughly 1.40; many hydrocarbons are lower.
Air ≈ 28.97; CO₂ ≈ 44.01.
Exports include both metric and chosen display.

Formula used

Liquid (incompressible)
A = Q / (Cd · √(2ΔP/ρ))
Q in m³/s, ΔP in Pa, ρ in kg/m³. A is multiplied by the safety factor.
Gas (ideal, choked)
ṁ = Cd · A · P₀ · √(γ/(RspecT₀)) · (2/(γ+1))((γ+1)/(2(γ−1)))
Rspec = Ru/M. Solved for A, then multiplied by the safety factor.
Equivalent diameter
D = √(4A/π)

How to use this calculator

  1. Select the relief model that matches your scenario.
  2. Enter flow, pressure, and fluid properties with stated units.
  3. Set Cd and a safety factor based on your standards.
  4. Press Calculate to show results above the form.
  5. Use CSV or PDF to attach results to your checklist.
Engineering note
This tool is for preliminary sizing. Confirm with applicable codes, vendor data, and full relieving conditions before final design.

Example data table

Case Mode Key inputs Computed area (mm²) Equiv. diameter (mm)
A Liquid Q=12 m³/h, ρ=1000, ΔP=50 kPa, Cd=0.62, SF=1.10 ≈ 2,785 ≈ 59.56
B Liquid Q=4 m³/h, ρ=870, ΔP=30 kPa, Cd=0.70, SF=1.20 ≈ 1,510 ≈ 43.82
C Gas (choked) ṁ=0.8 kg/s, P₀=900 kPa abs, T₀=35°C, γ=1.33, M=44.01, Cd=0.75, SF=1.15 ≈ 1,886 ≈ 49.00

Relief area sizing for liquid discharge

For incompressible flow, the calculator applies an orifice relationship linking volumetric flow, density, and available differential pressure. Flow is entered in m³/h and converted to m³/s. Pressure drop is entered in kPa and converted to Pa. With density near 1000 kg/m³, a 50 kPa drop can produce high velocities, so required area often lands in a few thousand mm² for moderate duties.

Choked gas relief assumptions and limits

For gases, the tool uses an ideal, choked-flow form where mass flow depends on upstream absolute pressure, temperature, heat-capacity ratio, and molecular weight. Choking occurs when downstream pressure is low enough for the throat Mach number to reach one. Use upstream pressure in kPa absolute, not gauge. Temperature is converted from °C to K, and the specific gas constant is derived from the universal constant divided by molecular weight.

Discharge coefficient and safety factor selection

The discharge coefficient (Cd) accounts for contraction and losses through the opening. Sharp-edged orifices commonly fall around 0.60–0.80, while well-rounded nozzles may be higher. The safety factor multiplies computed area to add margin for uncertainty in properties, fouling, and installation effects. Many teams start with 1.10–1.25 for screening, then refine Cd and margin using vendor data and internal standards.

Interpreting results and equivalent diameter

Results are shown as area in m² and mm² plus an equivalent circular diameter. The diameter is derived from D = √(4A/π), which helps when comparing to standard drilling sizes or vent fittings. Treat the diameter as equivalent; multiple holes can provide the same total area but may change flow behavior. When selecting hardware, match the net flow area after accounting for screens and mounting constraints.

Documentation and review workflow

After calculation, export options support design reviews and traceability. The CSV export captures the selected mode, inputs, and outputs for quick comparison across cases. The PDF export creates a concise report for attaching to a relief sizing note. For consistency, record the relieving scenario, the basis for Cd and safety factor, and the pressure reference (absolute versus gauge) alongside the exported results. Include units, revision date, and reviewer initials to streamline future revalidation and change management efforts.

FAQs

1) When should I use the liquid model?

Use it for incompressible liquids where density changes are negligible during discharge, such as water, many oils, and process liquids under moderate pressure drops.

2) What does “choked gas” mean here?

Choked flow means the gas reaches sonic velocity at the opening, so mass flow depends mainly on upstream conditions and throat area, not downstream pressure.

3) Why is upstream pressure required as absolute?

The choked-flow equation uses thermodynamic absolute pressure. If you enter gauge pressure, the calculated area may be underestimated, especially at lower operating pressures.

4) How do I choose Cd?

Start with published ranges for your geometry, then refine using manufacturer data or test results. Screens, sharp edges, and short inlets generally reduce effective Cd.

5) What does the safety factor change?

It multiplies the computed area. Use it to add margin for uncertainty, installation losses, and variability in relieving conditions without changing the physics model.

6) Can I size multiple openings?

Yes. Calculate the total required area, then distribute it across openings. Ensure each opening’s net area and layout do not introduce extra losses beyond your chosen Cd.

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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.