Sizing Butterfly Valves

Overview

Engineering reference data for Sizing Butterfly Valves in fluid mechanics.

Key Formulas

Reynolds Number

$Re = \frac{\rho v D}{\mu}$

Ratio of inertial to viscous forces — determines flow regime.

Bernoulli's Equation

$P + \frac{1}{2}\rho v^2 + \rho g h = \text{const}$

Conservation of energy for steady, inviscid, incompressible flow.

Continuity Equation

$A_1 v_1 = A_2 v_2$

Conservation of mass for incompressible flow.

Darcy-Weisbach

$\Delta P = f \frac{L}{D} \frac{\rho v^2}{2}$

Pressure drop due to friction in a pipe.

Variables

Symbol	Description	Unit
$Re$	Reynolds number	—
$\rho$	Fluid density	kg/m³
$v$	Flow velocity	m/s
$D$	Characteristic dimension	m
$\mu$	Dynamic viscosity	Pa·s
$P$	Pressure	Pa
$f$	Darcy friction factor	—

CV Calculation Principles

The valve flow coefficient ($C_v$) quantifies a valve's flow capacity. For liquid service, it is defined as the volume flow of water at 60°F (in US gallons per minute) that creates a 1 psi pressure drop across the valve.

The general sizing formula for liquids is: $C_v = Q \sqrt{\frac{G}{\Delta P}}$

Where:

• $C_v$ = Valve flow coefficient
• $Q$ = Volumetric flow rate (GPM)
• $G$ = Specific gravity of fluid (relative to water)
• $\Delta P$ = Pressure drop across the valve (psi)

Proper selection requires comparing the calculated $C_v$ against the valve manufacturer's published $C_v$ ratings for the specific valve size and opening angle.

Additional Resources

For a detailed tutorial on sizing butterfly valves and calculating $C_v$, refer to the following technical document: Sizing Butterfly Valves Tutorial (PDF) (Johnson Controls)

References

• Original Source