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What Is the Valve Flow Coefficient?
The valve flow coefficient (Cv) is a dimensionless number that quantifies the flow capacity of a control valve. It is defined as the number of US gallons per minute of water at 60 degrees F that will flow through a fully open valve with a pressure drop of 1 psi across it. A larger Cv indicates a valve that can pass more flow with less pressure drop.
Cv is the primary parameter used to size control valves in process industries. Engineers select valves whose Cv rating matches the required flow capacity, ensuring the valve can handle the system flow while maintaining adequate control range. Typically, a valve is sized so that it operates between 20% and 80% open at normal conditions.
Cv Formula
Typical Valve Cv Values
| Valve Size | Globe Valve | Ball Valve | Butterfly |
|---|---|---|---|
| 1" | 10-15 | 30-50 | 35-60 |
| 2" | 40-60 | 100-200 | 150-250 |
| 4" | 150-250 | 500-900 | 700-1200 |
| 6" | 350-500 | 1200-2000 | 1500-3000 |
Frequently Asked Questions
What is the difference between Cv and Kv?
Cv is the US standard using gallons per minute and psi. Kv is the metric equivalent using cubic meters per hour and bar. The conversion is Kv = 0.865 x Cv.
How do I select the right valve size?
Calculate the required Cv for your maximum flow condition with the available pressure drop. Select a valve whose full-open Cv is 1.25 to 1.5 times your required Cv to ensure the valve operates at 60-80% open at design conditions.
Does fluid viscosity affect Cv?
Yes. The standard Cv formula assumes turbulent flow with water. For viscous fluids above 100 centistokes, a correction factor must be applied as provided by ISA/IEC standards.