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Valve CV Calculation

Cv is the volume of water in gallons per minute at 60°F that passes through a valve with a pressure drop of 1 psi, calculated using the formula Cv = Q * √(SG / ΔP). Key factors affecting Cv include fluid properties, pressure drop, temperature, valve design, and piping conditions. The document also explains the relationship between Cv and Kv, the metric flow factor, and emphasizes the importance of accurate Cv calculations for optimal valve selection.

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71 views2 pages

Valve CV Calculation

Cv is the volume of water in gallons per minute at 60°F that passes through a valve with a pressure drop of 1 psi, calculated using the formula Cv = Q * √(SG / ΔP). Key factors affecting Cv include fluid properties, pressure drop, temperature, valve design, and piping conditions. The document also explains the relationship between Cv and Kv, the metric flow factor, and emphasizes the importance of accurate Cv calculations for optimal valve selection.

Uploaded by

sudeer
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as DOCX, PDF, TXT or read online on Scribd
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Defining Cv

Cv is defined as the volume of water in US gallons per minute (gpm) at 60°F that will
pass through a valve with a pressure drop of 1 pound-force per square inch (psi). While
water is the base for this calculation, Cv can be used for gases and other liquids by
accounting for their properties.

The Cv calculation formula


The basic formula for calculating Cv for liquids is:

Cv = Q * √(SG / ΔP)

Where:

 Q: Flow rate in gallons per minute (gpm).

 SG: Specific gravity of the fluid (1.0 for water).

 ΔP: Pressure drop across the valve in psi.

Step-by-step example
Let's say you have a liquid flowing at 10 gpm with a specific gravity of 1 (water), and you
want to ensure a pressure drop of 0.11 psi across the valve.

1. Determine the variables:

1. Q = 10 gpm

2. SG = 1.00 (water)

3. ΔP = 0.11 psi

2. Plug values into the formula:

1. Cv = 10 * √(1 / 0.11)

3. Solve the equation:

1. Cv = 10 * √(9.0909)
2. Cv = 10 * 3.0151134457776

3. Cv ≈ 30.15

This indicates that a valve with a Cv of approximately 30.15 is needed for this
application.

Key considerations in Cv calculation


 Fluid Properties: Viscosity, density, specific gravity, and compressibility can affect flow
and Cv values. For gases, different formulas are used based on critical vs. subcritical
flow conditions.

 Pressure Drop: This is often the most complex variable to determine, and using charts,
software, or direct measurement is recommended.

 Temperature: Fluid temperature affects viscosity and density, impacting Cv calculations.

 Valve Design: The physical dimensions, internal geometry (e.g., port design), and even
valve type (ball, gate, globe, etc.) influence the Cv.

 Piping Conditions: Obstructions, reducers, or expanders near the valve can alter flow
and affect the effective Cv.

Cv and Kv
The flow coefficient also exists in the metric system as the flow factor (Kv). Kv
measures the flow rate of water in cubic meters per hour at a pressure drop of 1 bar.
The conversion between Cv and Kv is:

 Cv = 1.156 * Kv

 Kv = 0.865 * Cv

By understanding and accurately calculating the valve Cv, engineers and technicians
can select the correct valve size for optimal system performance, avoiding potential
issues like cavitation, inadequate flow capacity, and energy inefficiencies.

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