Valve Cv Calculator

Calculate the valve flow coefficient (Cv) for sizing control valves, determine flow rate from a known Cv value, or find the pressure drop across a valve. Supports both liquid and gas/steam applications.

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How It Works

The Valve Cv Calculator determines the flow coefficient (Cv) of a valve, which represents the volume of water in US gallons per minute (GPM) that will flow through a valve with a pressure drop of 1 psi at 60°F. Cv is the primary sizing parameter used by engineers to select the correct valve for a given application.

Liquid Cv Formula

For incompressible liquids, the valve Cv is calculated using:

Cv = Q × √(SG / ΔP)

Where:

  • • Cv = Valve flow coefficient
  • • Q = Volumetric flow rate (GPM)
  • • SG = Specific gravity relative to water at 60°F
  • • ΔP = Pressure drop across the valve (P1 - P2) in psi

Gas Cv Formula

For compressible gases and steam, the valve Cv is calculated using:

Cv = Q / (963 × √(ΔP × (P1 + P2) / (SG × T)))

Where:

  • • Q = Gas flow rate (SCFH - Standard Cubic Feet per Hour)
  • • ΔP = Pressure drop across the valve (psi)
  • • P1 = Inlet pressure (psia)
  • • P2 = Outlet pressure (psia)
  • • SG = Specific gravity relative to air
  • • T = Absolute temperature (Rankine = °F + 460)

Cv to Kv Conversion

Kv is the metric flow coefficient: the flow of water in m³/h at a 1 bar pressure drop. The calculator reports both.

Kv = 0.865 × Cv    Cv = 1.156 × Kv

A valve with Cv = 22.36 has Kv = 0.865 × 22.36 = 19.34. European catalogues list Kv (sometimes Kvs for the fully open value); North American catalogues list Cv.

Worked Example: Liquid

Water at 100 GPM through a valve with 100 psi at the inlet and 80 psi at the outlet (the calculator's defaults):

  • • ΔP = 100 − 80 = 20 psi, SG = 1.0
  • • Cv = 100 × √(1.0 / 20) = 100 × 0.2236 = 22.36
  • • Kv = 0.865 × 22.36 = 19.34

Select a valve rated 20–30% above this, so a Cv of roughly 27–29: a 1" ball valve (Cv 30–40) at partial opening or a 1" butterfly valve (Cv 35–45).

Worked Example: Gas

Air at 10,000 SCFH, 100 psia in, 80 psia out, 60°F:

  • • T = 60 + 460 = 520 °R, SG = 1.0 (air)
  • • √(ΔP × (P1 + P2) / (SG × T)) = √(20 × 180 / 520) = √6.923 = 2.631
  • • Cv = 10,000 / (963 × 2.631) = 3.95

Enter gas pressures as absolute (psia = psig + 14.7): the P1 + P2 term is an absolute-pressure term, and the calculator uses the values as typed. The equation is the subcritical form; it applies while P2 is above about half of P1.

Typical Cv Values by Valve Size

Fully open values for common valve sizes. These are the same ranges the calculator uses for its sizing recommendation; above Cv 125 it points you to a 3" or larger valve and the manufacturer's catalogue.

Valve size Type Cv (fully open) Kv
1/2" Globe 4 - 6 3.5 - 5.2
1" Globe 10 - 15 8.7 - 13
1" Ball 30 - 40 26 - 35
1" Butterfly 35 - 45 30 - 39
1" Gate 20 - 30 17 - 26
2" Globe 40 - 50 35 - 43
2" Ball 100 - 125 86 - 108
2" Butterfly 100 - 120 86 - 104

FAQ

Here you will find the answers to the frequently asked questions about valve Cv and valve sizing.

Frequently Asked Questions

What is valve Cv and why is it important?

Valve Cv (flow coefficient) is a dimensionless number that represents the flow capacity of a valve. It is defined as the number of US gallons per minute of water at 60°F that will flow through a fully open valve with a pressure drop of 1 psi. Cv is the primary parameter used to size valves for a specific application, ensuring the valve can pass the required flow at the available pressure drop.

How do I size a control valve using Cv?

To size a control valve, first calculate the required Cv using the flow rate and available pressure drop. Then select a valve with a rated Cv that is 20-30% larger than the calculated Cv to allow for normal operating variations. The valve should operate between 10% and 80% of its travel range for optimal controllability. Always verify that the selected valve can handle the system's minimum and maximum flow requirements.

What ISA standards apply to valve sizing?

The primary standards for control valve sizing are ISA-75.01 (IEC 60534-2-1) for incompressible fluids and ISA-75.01 (IEC 60534-2-3) for compressible fluids. These standards define standardized methods for calculating Cv, accounting for factors such as critical pressure ratio, liquid vapor pressure, pipe geometry, and choked flow conditions. Following these standards ensures consistent and accurate valve selection across the industry.

What is critical flow and how does it affect valve sizing?

Critical flow (also called choked flow) occurs when the flow rate through a valve reaches a maximum and cannot increase further regardless of increasing pressure drop. For liquids, this happens when the pressure at the vena contracta drops below the fluid's vapor pressure, causing flashing or cavitation. For gases, critical flow occurs when the velocity reaches the speed of sound. In critical flow conditions, the standard Cv equations must be modified to account for the flow limitation.

What is the difference between Cv and Kv?

Cv and Kv are both valve flow coefficients but use different unit systems. Cv uses US gallons per minute and psi (imperial), while Kv uses cubic meters per hour and bar (metric). The conversion between them is Kv = 0.865 × Cv, or equivalently Cv = 1.156 × Kv. North American manufacturers typically use Cv, while European manufacturers often use Kv. Both values represent the same physical property of the valve's flow capacity.

How do I calculate Cv with GPM?

Cv = GPM × √(SG / ΔP). Take the flow in US gallons per minute, the specific gravity (1.0 for water) and the pressure drop across the valve in psi. For 100 GPM of water with a 20 psi drop: 100 × √(1 / 20) = 22.36. If your flow is in L/min or m³/h, convert to GPM first (multiply L/min by 0.264172, m³/h by 4.40287); the calculator does this when you pick the unit.

How do I calculate Cv for gas?

For gas and steam, Cv = Q / (963 × √(ΔP × (P1 + P2) / (SG × T))), with Q in SCFH, P1 and P2 in psia, SG relative to air and T in degrees Rankine (°F + 460). Set the fluid type to Gas/Steam and the calculator switches to this form and adds the temperature input. For 10,000 SCFH of air from 100 to 80 psia at 60°F the result is Cv = 3.95. Use absolute pressures, and note the equation is for subcritical flow (P2 above roughly half of P1).

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