Control valve calculator

Introduction to Control Valve Sizing and Flow Coefficients

Selecting the correct control valve size for a given application requires an understanding of the actual process conditions the valve will encounter. In the control valve industry, flow coefficient (Cv or Kv) and flow characteristics are standard parameters used to determine valve performance.

Flow Coefficient (Cv and Kv)

In the UK and USA, the Cv coefficient is commonly used. It represents the flow rate of water (in gallons per minute, gpm) at 60°F that results in a 1 psi pressure drop across the valve. The basic formula for liquid flow through a valve is:

$\mathrm{Cv}=\frac{q}{\sqrt{\frac{\mathrm{\Delta P}}{S}}}$

Where:
• Cv = Flow coefficient [gpm]
• q = Flow rate [gpm]
• ΔP = Pressure drop [psi]
• S = Specific gravity (relative density)

In Europe, the Kv coefficient is widely used. It represents the flow rate of water (in cubic meters per hour, m³/h) that results in a 1 kg/cm² pressure drop across the valve (1 kg/cm² ≈ 0.980665 bar). The equation for Kv is:

$\mathrm{Kv}=\frac{q}{\sqrt{\frac{\mathrm{\Delta P}}{S}}}$

Where:
• Kv = Flow coefficient [m³/h]
• q = Flow rate [gpm]
• ΔP = Pressure drop [psi]
• S = Specific gravity (relative density)

Valve Sizing and Considerations

Manufacturers use standardized test facilities, such as those established by the Fluid Control Institute (FCI), to ensure uniform measurement of liquid flow coefficients (Cv). When selecting a valve, it is crucial to account for the viscosity of the fluid—as higher viscosity reduces valve capacity. Additionally, Cv and Kv can be converted using the following formula:

$\mathrm{Cv}=\mathrm{Kv}\times 1.156$

$\mathrm{Kv}=\mathrm{Cv}\times 0.864$

Proper control valve sizing relies on calculating the flow coefficient for a given pressure drop and flow rate, ensuring optimal valve performance under actual operating conditions.