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Control valve sizing: Cv and Kv

Enter fluid, flows, pressures and valve type to calculate Cv and Kv per IEC 60534-2-1 / ISA-75.01.01, check choked flow and see the selected valve’s travel at minimum, normal and maximum flow.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

Pre-sizing
Fluid

Default: water at 77 °F (vapor pressure 0.46 psia, critical 3,200 psia).

Pressures

The drop is treated as constant at the three flows; in a real installation it changes with the system curve.

Valve
—Required Cv at maximum flow
—

Valve travel at the three flows

Characteristic curve of the selected valve with minimum, normal and maximum flow points. The recommended range is 20% to 80% travel.

Tap the chart to see the values.

Results by flow

The report gathers this page's inputs, results, equations, assumptions and references, with fields for project, engineer and revision. In the print dialog, choose Save as PDF.

Method, assumptions and references

Equations used

Liquid: Kv = Q · √((ρ ÷ ρ0) ÷ Δp) [Q in m³/h, Δp in bar]; Cv = 1.156 · Kv
Choked drop: Δpmax = FL² · (p1 − FF · pv), with FF = 0.96 − 0.28 · √(pv ÷ pc)
Gas and steam: Kv = W ÷ (31.6 · Y · √(x · p1 · ρ1)) [W in kg/h, p in bar(a), ρ in kg/m³]
x = Δp ÷ p1; Fγ = γ ÷ 1.40; critical x = Fγ · xT; Y = 1 − x ÷ (3 · Fγ · xT), with x capped at critical
Ideal gas: ρ1 = p1 · M ÷ (Z · R · T1); saturated steam: ρ1 from IAPWS-IF97, γ = 1.33
Travel: equal percentage Cv ÷ Cvrated = R^(x − 1), with R = 50; linear Cv ÷ Cvrated = x

Assumptions and limits

  • No reducers or expanders at the valve (Fp = 1) and turbulent flow; for a valve smaller than the line, apply Fp per IEC 60534-2-1.
  • Typical FL and xT by valve type; use the manufacturer’s values for final design.
  • Idealized inherent characteristic; the installed characteristic depends on how pressure drop is distributed in the system.
  • Validation: results match the IEC 60534 implementation in the Python fluids library (Caleb Bell) for the same inputs.

Standards and technical documents

  • IEC 60534-2-1:2011: Industrial-process control valves — Flow capacity — Sizing equations for fluid flow under installed conditions
  • ANSI/ISA-75.01.01-2012 (60534-2-1 Mod): Industrial-Process Control Valves — Flow Capacity — Sizing Equations
  • IEC 60534-2-4: Inherent flow characteristics and rangeability; ISA-RP75.23: Considerations for evaluating control valve cavitation
  • IEC 60534-8-3 / 8-4: Noise considerations for aerodynamic and hydrodynamic flow

U.S. regulations

  • OSHA 29 CFR 1910.119 (PSM): process safety information must document design codes for valves in covered processes
  • ASME B16.34: valve pressure-temperature ratings; NFPA 70 (NEC) Article 500 for electric actuators in classified areas

Frequently asked questions

What is the difference between Cv and Kv?

Cv is water flow in US gpm at 1 psi drop; Kv is water flow in m³/h at 1 bar. Cv = 1.156 × Kv.

When does cavitation occur?

In liquids, when pressure at the vena contracta falls below vapor pressure. Per IEC 60534, flow chokes when the drop exceeds FL² · (p1 − FF · pv). Incipient cavitation can start before that.

What travel is ideal?

Size the valve to run between 20% and 80% travel from minimum to maximum flow. Too open loses control authority; too closed is unstable and erodes the trim.