How this calculator works
Liquid Cv follows the US gallon-per-minute and psi definition. Gas flow uses absolute inlet and outlet pressures, upstream absolute temperature and gas specific gravity relative to air. The simplified gas equation caps its pressure-drop fraction at one half, representing choking; real valve choking factors can differ. Gas volume is a standard-reference volume on the coefficient convention, rather than actual line volume. Use manufacturer data for the reference conditions, viscous liquids, flashing, steam or gases near condensation.
Formula
Liquid: Q_gpm = Cv sqrt(delta_p_psi/SG). Gas: Q_scfm = 22.67 Cv p1_psia (1-2u/3) sqrt(u/(SG T_R)), where u = min(delta_p/p1,0.5), T_R = 1.8 T_K.
Worked example
For water with Cv = 10 and a one psi pressure drop, flow is 10 US gal/min or 0.000630901964 m3/s. A four psi drop gives 20 US gal/min.
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Frequently asked questions
How is this different from Cv flow?
This page includes compressible gas flow and its simplified choking limit. Cv flow focuses on inverse liquid pressure and density calculations.
Can I enter gas gauge pressures?
Convert them to absolute pressure first by adding the local atmospheric pressure.
Why does lowering gas outlet pressure stop increasing flow?
The simplified model reaches its choked-flow cap at an outlet-to-inlet pressure ratio of one half.