How to use this calculator
- Enter required flow. Enter the free-air flow rate through the valve.
- Enter pressures. Enter upstream and downstream gauge pressures at the valve.
- Set gas properties. Use temperature and gas specific gravity; air is SG = 1.
- Read Cv and pressure ratio. Check whether the pressure ratio is still in the subcritical range.
How it works
For dry air in the subcritical range, a common Cv shortcut is: Q = 22.67 · Cv · √((P₁² − P₂²) / (SG · T)) where Q is SCFM, P₁ and P₂ are absolute pressures in psia, SG is gas specific gravity relative to air and T is absolute temperature in degrees Rankine. The calculator solves this for Cv.
The pressure ratio matters. When P₂/P₁ falls near about 0.53 for air, flow is in or near choked-flow territory, so a simple subcritical Cv estimate becomes a rough screen rather than a final selection.
Worked example
Verified against the live calculator
For 1400 Nl/min of air (about 49 SCFM), 6.2 bar upstream, 5.5 bar downstream, 20 °C and SG = 1, the calculator converts pressures to absolute psi and returns a required Cv of about 1.1. The pressure ratio is above the choked range, so the subcritical shortcut is reasonable for screening.
Frequently asked questions
How do I estimate valve Cv for air?
For a subcritical dry-air estimate, use Q = 22.67·Cv·sqrt((P1²−P2²)/(SG·T)), with Q in SCFM, pressures in psia and temperature in degrees Rankine. The calculator rearranges this for Cv.
What is choked pneumatic flow?
When the downstream/upstream absolute pressure ratio is low enough, the flow reaches sonic velocity at the restriction. Past that point, more pressure drop does not increase flow according to the subcritical equation.
Is this enough to select a production valve?
Use it for screening. Final selection should use the valve maker flow curves or ISO 6358 / ISA sizing data, especially near choked flow or for non-air gases.
Why do I enter gauge pressure?
Shop air gauges read gauge pressure. The calculator converts to absolute pressure by adding atmospheric pressure before using the gas equation.
Method & assumptions
- Dry gas, ideal-gas behaviour, subcritical flow shortcut.
- Uses gauge pressure inputs but absolute pressure in the equation.
- Does not model valve-specific recovery, sonic conductance, fittings, silencers or actuator response time.
- Near choked flow, use manufacturer data or ISO/ISA sizing equations.