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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 ?

Determine the Flow Rate:
Determine the required flow rate for the application in units of gallons per minute (GPM).
Measure the Differential Pressure:
Measure the pressure drop across the valve, also known as the differential pressure, in units of pounds per square inch (PSl).
Determine the Specific Gravity:
Determine the specific gravity (SG) of the fluid being controlled

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

union valve butterfly valve

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.

union valve butterfly valve

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.

Size Cv

DN/mm

10°

20°

30°

40°

50°

60°

70°

80°

90°

40

0

1

4

7

13

21

35

62

69

50

0

2

6

14

25

41

69

123

137

65

0

5

13

29

54

89

147

264

294

80

1

8

25

54

96

160

267

480

532

100

1

13

36

82

145

242

404

727

807

125

2

27

80

178

319

532

888

1598

1776

150

4

41

126

283

509

848

1415

2547

2829

200

6

85

257

686

1049

1730

2884

5178

5767

250

11

159

480

1078

1960

3267

5444

9799

10888

300

14

209

532

1197

2177

3629

6049

10888

12098

350

14

209

635

1425

2594

4369

7281

13105

14561

400

16

239

516

1622

2950

4969

8281

14906

16561

450

21

323

978

2198

3999

6736

11226

20207

22452

500

27

421

1279

2872

5226

8803

14826

26688

29652

600

36

545

1652

3710

6751

11371

19151

34471

38301

700

43

660

2001

4494

8176

13772

23194

41749

46388

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.

About the Author
Profile Picture
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.
    Posted in Cv

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