Cv Calculator — Valve Flow Coefficient

Use our C_v calculator to calculate the flow coefficient of a valve and determine its appropriate size based on the specific volume of fluid flowing through it.

A valve's flow coefficient (C_v) is a convenient way to represent its flow capacity for various fluids and process parameters. The C_v calculator calculates either the C_v or the flow rate using the additional parameters provided regarding the fluid, inlet and outlet pressure, and temperature (for gases).

Keep reading and discover:

• What is the flow coefficient (C_v);
• How to calculate the flow coefficient;
• How to use the valve C_v calculator; and
• Some quick examples of C_v calculations.

Try our C_v calculator now to select a valve with an appropriate flow coefficient, ensure proper flow control, prevent leaks, and maintain desired process conditions!

The flow coefficient of a valve (C_v) measures the speed at which a fluid (liquid or gas) can pass through a valve. The C_v value is the flow coefficient in the imperial system. It is a valuable tool for selecting the appropriate valve size, ensuring all fluids flow at the desired pressure. The C_v value is indicated in the product description or valve specification label.

For example, if a given valve has a flow coefficient of 2, it will allow 2 GPM of water to pass through with a pressure drop of 1 psia. To allow 3 GPM of water to pass through, choose another valve with a flow coefficient of 3. The higher the C_v value of the valve, the greater the valve's flow capacity.

When calculating the valve flow coefficient, it is essential to consider various factors such as fluid properties, valve characteristics, and desired flow rate.

• Specific gravity: A fluid's specific gravity (SG) affects its density and, therefore, its flow rate. Specific gravity is the ratio of a fluid's density to the density of water at a given temperature. It's a dimensionless unit that compares the relative densities of different fluids. Water has a density of 1.00 at 60 °F. To know more about SG, check out our specific gravity calculator.

• Flow rate: Flow rate (Q) measures the amount of fluid passing through the valve at a given moment. It is usually expressed in gallons per minute (GPM). To determine the flow rate, divide the volume of fluid passing through the valve by the time it takes for that volume to pass through. For example, if it takes 7 minutes for 50 gallons of water to pass through the valve, the flow rate is 7.1 GPM.

• Pressure drop: The best way to calculate pressure drop is to use standard charts. The user must know the type of pipe, the inside diameter, and the flow rate. Using these values, locate the pressure drop (usually represented in PSIA — absolute pounds per square inch) corresponding to these values on the chart. You can also calculate pressure drop using the difference between the upstream and downstream fluid pressure.

The coefficient of flow (C_v) is a formula used to determine a valve’s flow under various conditions and select the correct valve for a flow application. The C_v was designed for use with liquid flows. However, this same C_v value can be used to determine gas flows through a valve. The formula becomes more intricate for gases, as gases are compressible fluids and are thus affected by temperature.

C_v formula for liquids

To find the C_v value for liquids, use the following formula:

where:

• $C_v$ — Valve flow coefficient;
• $Q$ — Flow rate in US gallons per minute (GPM) at 60 °F;
• $SG$ — Specific gravity of the liquid (dimensionless); and
• $\Delta P = P_1 - P_2$ — Pressure drop across the valve in pounds per square inch (psi), with:
  • $P_1$ — Inlet pressure in pounds per square inch absolute (psia); and
  • $P_2$ — Outlet pressure in psia.

The C_v value is the flow rate of water in US gallons per minute (GPM) at a temperature of 60 °F with a pressure drop of 1 psi across the valve. For example, a valve with a C_v of 14 will allow 14 GPM of liquid to pass through the valve with a pressure drop of 1 psi at 60 °F.

C_v formulae for gases

Gas flow through a valve can change from subcritical to critical flow. Here are the corresponding formulae:

Subcritical C_v formula

The downstream pressure (P2) is greater than half the upstream pressure (P1), i.e. $\frac{P_1}{2} \lt P_2$. Therefore, changes in downstream pressure can affect the flow rate.

Critical C_v formula

The downstream pressure is less than half the upstream pressure, i.e. $\frac{P_1}{2} \gt P_2$. Variations in downstream pressure do not affect the flow rate. This type of flow is also known as choked flow.

where:

• $C_v$ — Valve flow coefficient;
• $Q$ — Volumetric flow rate in standard cubic feet per hour (SCFH, also written as ft³/hr);
• $SG$ — Specific gravity of the gas (dimensionless);
• $P_1$ — Inlet pressure in pounds per square inch absolute (psia);
• $P_2$ — Outlet pressure in psia; and
• $T$ — Temperature in Rankine (remember: °R = °F + 459.67 or you can use our temperature converter).

The C_v calculator helps you size a valve or determine flow characteristics for liquid and gas applications. It can either calculate the valve flow coefficient (C_v) from known conditions or the expected flow rate from a known C_v. It also supports reverse calculations for certain variables (e.g., finding inlet pressure from C_v and flow rate).

Here's a quick step-by-step explanation on how to use our valve C_v calculator:

1. Select the Calculation type:

   • Cᵥ: Choose this option if you want to determine the valve's flow coefficient; or
   • Flow: Choose this option if you know the C_v and want to find the flow rate.

2. Select the Medium type:

   • Liquid; or
   • Gas.

3. Depending on the mode and medium types:

   • Enter the Inlet pressure (P₁) and Outlet pressure (P₂) in your preferred pressure units (psi, bar, kPa, etc.);
   • Choose the liquid or gas of your choice from the System medium dropdown menu. You can also select Other and enter a custom Specific gravity;
   • For gases, input the Temperature, in °F or °C (the flow coefficient C_v calculator converts it internally to Rankine); and finally...
   • Enter the Flow Rate (Q), in your preferred units (GPM, SCFH, L/min, m³/h, etc.); or
   • The Cᵥ (Valve flow coefficient).

4. Finally, the result appears instantly!

Interested in other flow-related calculations? Check out our pipe flow calculator.

Let's take a look at two easy examples for C_v calculations!

Example of C_v calculation for liquids

Let's calculate the C_v value of a valve for a water flow rate of 18 GPM (US gallons per minute) with an upstream (inlet) pressure of 12 psia and a downstream pressure (outlet) of 3 psia. Water has a specific gravity of 1.

The required $C_v$ value is $6$. Use this value to select an appropriate valve size.

Example of C_v calculation for gases

Let's calculate the C_v value of acetylene with a specific gravity of 0.907, an inlet pressure of 100 psig, an outlet pressure of 95 psig, and a flow rate equal to 15,000 SCFH at a temperature of 70 °F.

First, convert $P_1$ and $P_2$ expressed in psig to psia.

If you don't want to do it by hand, use our psig to pisia converter.

Then, convert the temperature from Fahrenheit to Rankine.

Half of the inlet pressure is less than the outlet pressure ($\frac{114.7}{2} \lt109.7$), so we use the subcritical formula:

We get a $C_v$ of $10.203$. You can verify this result using our flow coefficient C_v calculator!