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What Is the Valve Flow Coefficient (Cv)? | Piping Engineering Glossary
What Is the Valve Flow Coefficient (Cv)?
In piping engineering and process engineering, the valve flow coefficient, written Cv, describes how much flow a valve can pass for a given pressure drop. In US units, Cv is the number of US gallons per minute of water at about 60°F that flow through the valve with a pressure drop of 1 psi. The metric equivalent, Kv, is the flow in cubic meters per hour of water with a 1 bar pressure drop, and Cv is approximately 1.16 times Kv.
Using Cv to Size a Valve
For liquids, the basic sizing relationship is Q = Cv × √(ΔP / SG), where Q is the flow in gallons per minute, ΔP is the pressure drop across the valve in psi, and SG is the specific gravity. A valve with a Cv of 100 passing water at a 4 psi drop flows about 200 gpm. Gas and steam sizing use modified equations that account for compressibility.
Manufacturers publish Cv values at different openings, so engineers can see how capacity changes as a control valve travels from closed to fully open. Sizing standards such as ISA-75.01 and IEC 60534 refine the basic equation with factors for piping geometry and flow conditions.
Applications
Control Valve Sizing
Engineers calculate the required Cv at minimum, normal, and maximum flow, then select a valve that operates within a controllable part of its travel, often roughly 20 to 80 percent open.
Comparing Valve Types
Cv lets designers compare a globe valve, butterfly valve, or ball valve of the same size, since their capacities differ significantly.
System Pressure Drop
Plant engineers use Cv to estimate the pressure drop across manual and on/off valves in hydraulic calculations.
Benefits
Standard Comparison
Cv provides a common, published basis for comparing valves from different manufacturers.
Supports Good Control
Correct Cv selection avoids oversized valves that operate nearly closed and control poorly.
Simple Estimates
The basic equation allows quick hand checks before detailed sizing software is used. These checks help catch sizing errors early in a project.
Limitations
Ideal Conditions Only
The simple liquid equation does not account for cavitation, flashing, or choked flow, which can limit capacity.
Installed Performance Differs
Reducers, nearby fittings, and changing system pressure drop alter how a valve performs in place compared with its rated Cv.
Viscous Fluids
Highly viscous liquids require correction factors, because Cv values are based on water. Sizing standards include factors for laminar and transitional flow.
FAQ
What is the difference between Cv and Kv?
Cv uses US gallons per minute and psi, while Kv uses cubic meters per hour and bar. Cv is roughly 1.16 times Kv for the same valve.
Should a control valve be sized for full flow at full open?
No. Valves are usually selected so the maximum required flow occurs below full travel, leaving margin for control and future changes.
How does Cv relate to valve authority?
Control valve authority compares the valve’s pressure drop with the total system drop. A valve with too little pressure drop has low authority and controls poorly, regardless of its Cv.
Does a larger Cv always mean a larger valve?
Not necessarily. Valve type and trim design affect Cv, so a full-bore ball valve can have a much higher Cv than a globe valve of the same size.
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