What is Cv? A Guide to Valve Flow Coefficient Explained

What Is Cv?

Cv is the flow coefficient of a valve, which measures the flow capacity of a control valve at full opening relative to the pressure differential across the valve.

Specifically, it is defined as the flow rate of water (measured in gallons per minute, GPM) that passes through a fully opened valve with a pressure differential of 1 psi at 60°F.

For example, a valve with a Cv value of 1.0 indicates that under a pressure differential of 1 psi, the flow rate through the valve would be 1 gallon per minute (with water having a specific gravity of 1).

Why Is Valve Cv Important?

It is an indicator of the valve's ability to control fluid flow and an important parameter in control valve sizing design.

The Cv value of a control valve is typically determined by the manufacturer according to standardized testing procedures and is provided in the product documentation. Valve manufacturers usually base the Cv value on water as the medium, with a specific gravity (G) of 1.0. Other liquid media have different specific gravities, which affect both the flow resistance and the Cv value.

If the Cv value of a control valve is too low or too high, it can lead to a series of problems, such as inefficient system operation, excessive energy consumption, or even damage to the valve and system components.

1.Valve Sizing: Cv is one of the key factors in selecting the correct valve size. For a given fluid system, engineers choose a valve with an appropriate flow coefficient based on the system's flow and pressure drop requirements to ensure the required flow rate is achieved.

2.Flow Control: The Cv value of a valve determines the flow rate of fluid that can pass through the valve under a specific pressure differential. By selecting the right flow coefficient, precise control of fluid flow can be achieved.

3.System Stability: If the Cv value is incorrectly selected, it may cause instability in the fluid system, such as oscillations or pressure fluctuations.

4.Energy Efficiency: Choosing a valve with an appropriate Cv value helps reduce energy loss in the fluid system, leading to energy savings and a reduction in emissions.

To properly select the control valve size for a specific application, the required Cv value must be determined based on the system's flow and pressure drop requirements.

The following is the formula for calculating Cv:

Cv = flow coefficient

Q = rate of flow

SG = specific gravity of the fluid

△P = pressure drop across the valve

How To Select Valve Size Based On The Desired Cv Value ?

Calculate the Cv:

Using the formula Cv = Q /√(△P/SG], substitute the values of Q, △P and SG to calculate the Cv value.

Here is an example calculation using the following values:

Flow rate (Q)= 50 GPM

Differential pressure (△P)= 15 PSI

Specific gravity (SG〕= 0.9

Cv=Q/√(△P/SG)

Cv=50/√(15/0.9)Cv = 12.24

This value can be used to select a control valve that is appropriately sized

Factors Affecting The Cv Value Of A Control Valve

1.Valve Size and Shape

The size and shape of the valve orifice can affect the Cv value as it determines the maximum flow rate that can pass through the valve.

A larger orifice size generally results in a higher Cv valve,allowing for a higher flow rate, while a smaller orifice size results in a lower Cv valve and lower fow rate.

2.Valve Design

The valve's design can affect the Cv value, particularly the valve's flow characteristic and valve trim.

The flow characteristic refers to how the valve's flow rate changes in relation to the valve's opening position.

3.Fluid Properties

The properties of the fluid being controlled, such as viscosity, density, and specific gravity, can impact the Cv value.

Fluids with higher viscosity and density typically result in lower Cv values and flow rates due to increased resistance to flow

4.Differential Pressure

A higher differential pressure results in a higher Cv value and flow rate, while a lower differential pressure results in a lower Cv valve and flow rate

5.Process Temperature

As the temperature increases, the viscosity of the fluid generally decreases, which increases the Cv value, as the fluid can flow more easily through the valve.

However, if the temperature becomes too high, it may cause the fluid to vaporize or undergo phase changes, leading to issues such as flashing or cavitation. 

6.Piping Conditions Upstream and Downstream of the Valve

The Cv value can be influenced by the conditions of the piping around the valve. Factors such as obstructions inside the valve or the presence of reducers, expanders, or other irregularities in the upstream or downstream piping can alter the flow dynamics.

These conditions can create turbulence or pressure losses, which may reduce the effective Cv value of the valve.

7.Valve Material

The material of the valve also plays a crucial role in determining its flow coefficient (Cv), as different materials can affect the friction between the fluid and the valve's internal surfaces.

Practical Application of Control Valve Cv

Here are some common examples of Cv values for various types of valves:

Gate Valve: Gate valves typically have a lower Cv due to their more complex internal structure, which creates more resistance to flow.

Ball Valve: Ball valves have a higher Cv, especially full-port ball valves, because their flow path is unobstructed, resulting in less resistance to fluid flow.

Butterfly Valve: The Cv of a butterfly valve depends on the angle of the valve disc; it is highest when fully open.

Globe Valve: The Cv of control valves can be optimized through design to meet different flow control requirements.

Please note, the specific Cv values should be obtained from the manufacturer’s data, as they vary based on the valve’s design and size.

Below is the Cv chart for Union Valve's butterfly valves.

These data may contain inaccuracies or discrepancies. For more detailed information, it is recommended to directly contact the Union Valve supplier. The supplier can provide comprehensive information and data to help you select the best valve according to your specific needs.

Overall,selecting a control valve with the appropriate Cv is critical to ensuring optimal system performance and minimizing the risk of cavitation,excessive noise, and other problems associated with poor valve selection.

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• Recommend the Top ten Butterfly Valve Manufacturers With the Best Quality in China

About the Author

Tina Wei

Tianjin Union Valve Co., Ltd Sales Managers

More than 8 years of experience in the valve sales industry. Have extensive qualifications and experience.