Restriction Orifice Sizing Calculator
Size a restriction orifice (RO) to limit flow or drop pressure: find the bore diameter for a target flow, for liquids or for gases with the critical (choked) flow check.
- Q = Cd ÷ √(1 − β⁴) × π/4 × d² × √(2ΔP ÷ ρ), solved for d by iteration
- d = 8.047 mm
Restriction Orifice Equations
Gas, choked (P₂/P₁ ≤ (2/(k+1))k/(k−1)): ṁ = Cd × A × P₁ × √[kM ÷ (ZRT) × (2/(k+1))(k+1)/(k−1)]
Gas, subcritical: ṁ = Cd × A × √[2ρ₁P₁ × k/(k−1) × (r2/k − r(k+1)/k)]
Cd is the discharge coefficient (about 0.6 for a thin sharp edged plate; higher for thick plates and rounded entries), A the bore area, β = d/D, ρ the liquid density, P absolute pressures, r = P₂/P₁, k the specific heat ratio, M the molecular weight and R = 8314 J/kmol·K. These are standard orifice equations; vendors use their own coefficients for final sizing.
Worked example
To pass 5 m³/h of water with 10 bar drop in a 2" line, the bore is about 8.0 mm. 500 kg/h of air from 10 bar a to 2 bar a is choked and needs about 11.2 mm.
Frequently Asked Questions
What is a restriction orifice?
A plate with a calibrated bore used to limit flow or drop pressure, not to measure flow.
What is choked flow?
When the downstream to upstream pressure ratio falls below the critical ratio, the gas reaches sonic velocity in the bore and flow stops increasing as downstream pressure falls.
When is a multi stage orifice needed?
For large liquid pressure drops (cavitation risk) or large gas drops where noise and vibration matter.
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