Cv and Flow Rate Calculator

Cv & Flow Rate Calculator (Liquids)

Understanding Valve Flow Coefficient (Cv)

In fluid dynamics and valve sizing, Cv (Valve Flow Coefficient) is a critical metric used to determine the capacity of a valve to allow fluid to pass through it. It is defined as the number of US gallons of water per minute (GPM) that will flow through a valve with a pressure drop of 1 PSI at 60°F.

Why is Calculating Cv Important?

Properly sizing a valve ensures system efficiency and safety. If a valve's Cv is too small for the required flow rate, it will cause a significant pressure drop, leading to reduced flow downstream (starvation) or cavitation. If the Cv is too large, the valve may be oversized, leading to poor control resolution and unnecessary costs.

The Formulas

For non-compressible fluids (liquids like water), the relationships between Flow Rate ($Q$), Valve Coefficient ($C_v$), Pressure Drop ($\Delta P$), and Specific Gravity ($SG$) are governed by the following equations:

• To Calculate Cv: $$ C_v = Q \sqrt{\frac{SG}{\Delta P}} $$
• To Calculate Flow Rate (Q): $$ Q = C_v \sqrt{\frac{\Delta P}{SG}} $$
• To Calculate Pressure Drop ($\Delta P$): $$ \Delta P = SG \left(\frac{Q}{C_v}\right)^2 $$

Key Definitions

• Q (Flow Rate): Measured in US Gallons Per Minute (GPM).
• $\Delta P$ (Pressure Drop): The difference between the inlet pressure and the outlet pressure across the valve, measured in PSI.
• SG (Specific Gravity): The ratio of the density of the fluid to the density of water. Water has an SG of 1.0. Oils are typically lighter ( 1.0.

Example Calculation

Imagine you need to select a control valve for a cooling system. You require a flow rate of 100 GPM of water ($SG = 1.0$), and the allowable pressure drop across the valve is 5 PSI.

Using the formula for Cv:

$$ C_v = 100 \times \sqrt{\frac{1.0}{5}} = 100 \times 0.447 = 44.7 $$

You would need to select a valve with a Cv rating of at least 44.7 to maintain the desired flow at the specified pressure drop.