Valve CV Flow Coefficient Calculator
ISA-75.01 compliant sizing for liquids, gases & steam. Calculate required CV, check for choked flow & cavitation, and download a professional PDF report.
Enter your process conditions and click Calculate CV to see results
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What is Valve CV (Flow Coefficient)?
The flow coefficient CV is the most important parameter for sizing a control valve. Defined by ISA-75.01, CV represents the flow rate in US gallons per minute (GPM) of 60°F water that passes through a fully open valve with a 1 psi pressure drop across it.
In metric systems the equivalent is Kv (m³/h per bar½). The relationship is: CV = 1.156 × Kv.
Liquid CV Formula (ISA-75.01)
Q = GPM, ΔP = pressure drop (psi), SG = specific gravity
Choked ΔP = Fl² × (P1 − Ff × Pv)
Fl = pressure recovery factor, Ff = liquid critical pressure ratio factor
Gas & Vapor CV Formula (ISA-75.01)
Q = SCFH, P1 = psia, T1 = °R, Gg = sp. gravity vs air
Y = 1 − x/(3 × Fk × xT) [choked when x ≥ Fk × xT]
Steam CV Formula
W = lb/hr, ρ1 = inlet density (lb/ft³), P1 = psia
Low Flow, Standard & High Flow Applications
For low flow (small CV, high viscosity), laminar conditions may exist (Re < 10,000) requiring a viscosity correction factor FR — this calculator flags when that applies. High flow / choked flow is automatically detected using the valve's Fl factor. Rotary valves (ball, butterfly) have lower Fl (0.55–0.70) vs. globe valves (0.85–0.95).
Frequently Asked Questions
What safety factor should I apply to CV?
Size the valve to operate 20–80% open at design flow — typically select a valve with a rated CV of 125–150% of the calculated required CV.
What does "choked flow" mean?
Choked flow occurs when ΔP exceeds a maximum limit (set by Fl). Further pressure drop increases do not increase flow. For liquids, this also risks cavitation damage.
What is cavitation?
Local pressure drops below vapor pressure, forming vapor bubbles that collapse violently — causing noise, vibration, and rapid valve damage. Use anti-cavitation trim or reduce ΔP.
Linear vs. equal percentage trim?
Linear: flow proportional to travel — ideal for constant ΔP. Equal %: equal % flow change per % travel — best for varying ΔP, better controllability across wide range.
What is the Fl pressure recovery factor?
Fl represents how much pressure recovers downstream of the vena contracta. Globe valves: high Fl ≈ 0.90. Ball/butterfly: lower Fl ≈ 0.55–0.70.