What is the Cv of a control valve?

The flow coefficient (Cv) is a relative measure of a valve's efficiency at allowing fluid flow. It is defined as the number of US gallons per minute (GPM) of 60°F water that will flow through a valve with a pressure drop of 1 psi.

How do you calculate Cv for gas flow?

For gas flow, Cv is calculated using the ISA-75.01 formula, which considers inlet pressure, pressure drop, gas gravity, and temperature. It also accounts for choked (critical) flow when the pressure drop exceeds a certain threshold.

What is choked flow in a control valve?

Choked flow occurs when the fluid velocity at the narrowest point in the valve (vena contracta) reaches the speed of sound. At this point, further decreasing the outlet pressure will not increase the flow rate.

Control Valve Sizing Calculator | Cv & Flow Coefficient

Service Type

Flow Rate

GPM

Flow Rate

MMSCFD

Specific Gravity

Viscosity (optional)

For heavy oils, glycol. Leave blank for water-like fluids.

Vapor Pressure (optional)

psia

For cavitation/flashing check. Leave blank to skip.

Gas Gravity

Temperature

°F

Pressure

Inlet Pressure (P1)

psig

Outlet Pressure (P2)

psig

Valve Type

Trim Characteristic

Pipe Size

Cv Oversizing Factor

Calculated Sizing Parameters

Valve Coefficients:
Required Cv (Flow Coefficient), Target Cv (Oversized), Selected Valve Size (NPS), Valve Opening %.

Flow Analysis:
Choked Flow Check, Piping Geometry Factor (Fp), Reynolds Number Factor (FR), Valve/Pipe Velocity.

Industry Standards:
ISA-75.01 (IEC 60534) equations, ANSI/FCI 70-2 leakage classes, Cavitation/Flashing risk analysis.

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Valve Sizing Results

Required Cv

Target Cv (with oversizing) -

Selected Valve Size -

Valve Cv -

Valve Opening -

Pipe Velocity -

Valve Velocity -

Noise Level -

Flow Status -

Piping Factor (Fp) -

Viscosity Factor (FR) -

Sizing Equations

Liquid: Cv = Q × √(G / ΔP)

Gas: Cv = Q × √(G × T) / (963 × P₁ × √ΔP)

Q = Flow rate (GPM or SCFH)

G = Specific gravity

ΔP = Pressure drop (psi)

Standards & References

ISA-75.01
Flow Equations for Sizing Control Valves
IEC 60534
Industrial-process Control Valves
ANSI/FCI 70-2
Control Valve Seat Leakage

Important Notes

Cv = flow rate (GPM) at 1 psi drop for water at 60°F
Select valve with Cv 20-30% higher than calculated
Check for choked flow conditions in gas service
Cavitation risk when ΔP > 0.5 × P1 for liquids
Kv (metric) = Cv × 0.865

Control Valve Sizing Calculator

Service Type

Pressure

Valve Type

Calculated Sizing Parameters

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Valve Performance Curve

⚠️Sizing Warnings

Valve Sizing Results

Sizing Equations

Standards & References

Important Notes

Control Valve Sizing Calculator

Service Type

Pressure

Valve Type

Calculated Sizing Parameters

📚 Learn the Theory

Valve Performance Curve

⚠️Sizing Warnings

Valve Sizing Results

Sizing Equations

Standards & References

Important Notes

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