Fluid Mechanics

Valve Cv / Kv Sizing Calculator with Cavitation and Choked Flow Check

Calculate Cv, Kv and valve opening using flow rate, pressure drop, fluid density and valve type.

Unit-aware inputs Deterministic calculation Engineering interpretation
Calculation workspace

Enter the known values and review the calculated result

Deterministic calculation
01
Parameters

Input parameters

Use consistent values and select the intended engineering units.

Fluid properties

Flow conditions

Valve configuration

02
Output

Results

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Engineering reference

Method, application and limitations

Review the calculation method, intended application and engineering assumptions before using the result in a design decision.

01
Method

Formula and calculation method

Cv, Kv and opening formula:

ΔP = P1 − P2

ρrel = ρ / 1000

For liquid:

Kv = Q / √(ΔP / ρrel)

For gas:

x = ΔP / P1

Y = max(0.67, 1 − 0.41 · x)

Kv = Q / (Y · √((P1 · x) / T))

Flow coefficient conversion:

Cv = Kv · 1.156

Valve opening calculation:

Kvmax = Kv / 0.7

Opening = ln(Kv / Kvmax) / ln(1 / 50) · 100

Openingfinal = max(0, min(100, Opening))

where:

  • Cv — flow coefficient, US scale
  • Kv — flow coefficient, metric scale
  • Opening — valve opening (%)
  • Q — volumetric flow rate
  • P1 — upstream pressure
  • P2 — downstream pressure
  • ΔP — pressure drop
  • ρ — density
  • ρrel — relative density based on 1000 kg/m³
  • T — temperature
  • x — pressure drop ratio
  • Y — gas expansion factor
  • Kvmax — estimated maximum valve flow coefficient
02
Application

When to use this calculator

When to use this calculator:

  • Size a valve flow coefficient from required volumetric flow rate and pressure drop.
  • Convert the calculated metric valve coefficient to the US valve coefficient.
  • Estimate valve opening from the calculated valve flow coefficient using an equal-percentage characteristic.
  • Check whether a liquid case reaches cavitation or flashing risk based on vapor pressure and valve type.
  • Check whether a gas case reaches choked flow based on upstream-to-downstream pressure ratio.
03
Decision support

How to interpret the result

Cv is defined as the US valve flow coefficient calculated from the metric valve flow coefficient. Kv is defined as the metric valve flow coefficient required to pass the entered volumetric flow rate at the entered pressure conditions.

Valve opening is defined as the estimated opening percentage from an equal-percentage valve characteristic with rangeability equal to 50 and sizing at approximately 70% of the calculated maximum valve coefficient.

  • Safe — the flow is not choked and the calculated valve opening is not greater than 80%.
  • Limit — the flow is not choked, but the calculated valve opening is greater than 80%.
  • Warning — liquid cavitation or flashing risk exists, or gas choked flow exists, while valve opening is not greater than 85%.
  • Unsafe — liquid cavitation or flashing risk exists, or gas choked flow exists, and valve opening is greater than 85%.
  • Invalid — required input conditions are not satisfied, including positive flow rate, positive pressures, upstream pressure greater than downstream pressure, positive density, valid fluid type and valid valve type.

For liquid flow, cavitation or flashing risk exists when the cavitation index is lower than the critical cavitation index. For gas flow, choked flow exists when the upstream-to-downstream pressure ratio is greater than the calculated critical pressure ratio.

04
Worked case

Calculation example

Example:

A user wants to size a globe valve for liquid flow and check whether the selected pressure drop creates cavitation risk.

  • Fluid type: Liquid
  • Q — Volumetric flow rate: 0.02
  • P1 — Upstream pressure: 500000 Pa
  • P2 — Downstream pressure: 300000 Pa
  • ρ — Density: 1000 kg/m³
  • Pv — Vapor pressure: 2300 Pa
  • Valve type: Globe valve

ΔP = 200000 Pa, Kv = 0.0000447, Cv = 0.0000517, Opening = 9.12%.

05
Model boundaries

Assumptions and limitations

  • The fluid type is limited to liquid or gas.
  • The valve type is limited to globe, ball or butterfly.
  • The liquid calculation uses density relative to 1000 kg/m³.
  • The gas calculation uses a simplified expansion factor equal to max(0.67, 1 − 0.41 · pressure drop ratio).
  • The gas choked-flow check uses γ = 1.4.
  • The valve opening model uses an equal-percentage characteristic with rangeability equal to 50.
  • The maximum valve coefficient is estimated from the required valve coefficient divided by 0.7.
06
Questions

Frequently asked questions

How to calculate Cv, Kv and valve opening?
Kv is calculated from volumetric flow rate and pressure drop. For liquid flow, it also depends on density relative to 1000 kg/m³. For gas flow, it depends on upstream pressure, pressure drop ratio, temperature and gas expansion factor. Cv is calculated by multiplying Kv by 1.156. Valve opening is calculated from the ratio between the required valve coefficient and the estimated maximum valve coefficient.
What affects Cv, Kv and valve opening the most?
The flow coefficient increases when required volumetric flow rate increases. For liquid flow, a larger pressure drop decreases the required flow coefficient. For gas flow, the result changes with upstream pressure, downstream pressure and temperature because these values define the pressure drop ratio and expansion factor.
When is the Cv, Kv and opening formula not valid?
The calculation is invalid when fluid type is not liquid or gas, flow rate is not greater than zero, upstream pressure is not greater than downstream pressure, density is not greater than zero, vapor pressure is missing for liquid flow, temperature is missing for gas flow, or valve type is not globe, ball or butterfly.
Can this calculator be used for liquid and gas valve sizing?
It can be used for liquid cases when density, vapor pressure, flow rate, upstream pressure, downstream pressure and valve type are provided. It can be used for gas cases when density, temperature, flow rate, upstream pressure, downstream pressure and valve type are provided.
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Engineering notice. Verify input data, assumptions, applicable standards and required safety factors before using results for design release or safety-critical decisions.