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Flow Capacity, Cv

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What is the CV of a Valve?

The flow coefficient or Cv of a coupling is defined as the volume of water in US gallons per minute that will flow through the coupling with a pressure drop of 1 psi. In other words, flow capacity is a measure of efficiency at allowing liquid to flow. The use of the flow coefficient offers a method of of comparing valve capacities and sizes for a number of different industries. The flow coefficient can measure beyond water. It can measure liquids such as acetone, ethanol and gasses such as helium and hydrogen. More information on these gravitates are located near the bottom of the page.

Flow Capacity Cv values are often listed in manufacturer's catalogs because it is a convenient way of characterizing the flow capacity of the part. For convenience, flow capacities for the most commonly used couplings are presented on graphs. Just follow the link on the table below. Pressure drops for all couplings can be quickly calculated using the formulas under the table. Calculated pressure drops are estimates only.

Common Couplings
LP Series KL Series UF Series MD Series HP Series
06-003 KL-006 UF-006 SG-006 02-003
LP-002 KL-012 UF-007 MD-006 HP-004
LP-003 KL-030 UF-012 MD-007 HP-006
07-003   UF-019 MD-012 HP-010
LP-004   UF-032 MD-019 HP-016
LV-004     MD-025 HP-020
LP-006     MD-032 HP-032
SP-006     MD-050 HP-050
LP-007        
SP-009        
LP-012        
LP-019        
LP-032        
LP-050        

Estimating Pressure Drop Using Cv Values

Liquids

  • Q = flow rate in gpm (US gallons)
  • G = specific gravity of liquid
  • Dp = pressure drop = (inlet pressure – outlet pressure)
    Cv =       Q    
              Ö(Dp/G)
    or...
    Dp = G(Q/Cv)2

Approximate specific gravities of some common liquids are:

Acetone = 0.78 Benzene = 0.88
Ethanol = 0.79 Kerosene = 0.8
Water (fresh) = 1.00 Water (salt) = 1.03
Example:

An HP-010 coupling has a Cv of 1.85 flowing 8 gpm of benzine, the pressure drop will be approximately: 0.88 x (8/1.85)2 = 0.88 x 4.322 = 16.5 psi 

Note: this relationship is widely published and commonly used in industry. It provides reasonable estimates liquids with viscosities similar to water. Please contact us for estimates for other liquids. Also check data on specific couplings for maximum flow rate restrictions

Calculating Pressure Drop for Gases

Calculating pressure drops for gases is much more complicated because of the compressibility of gases.

  • Q = flow rate in SCFH (standard cubic feet per hour)
  • SG = specific gravity of gas (air = 1)
  • p1 = absolute inlet pressure (gauge pressure in psi + 14.7)
  • p2 = absolute outlet pressure (gauge pressure in psi + 14.7)
  • T = absolute temperature in Rankin (°F + 460)

When the outlet pressure is greater than 0.5 x inlet pressure:

Q = 963 x Cv x ((p1 - p2) x (p1 + p2))½
                          (SG x T)½

When the outlet pressure is less than 0.5 x inlet pressure:

Q = 963 x Cv x 0.87 x p1
                  (SG x T)½

Approximate specific gravities of some common gases are:

Air = 1.0 Ammonia = 0.6 Argon = 1.38 Butane = 2.1 CO2 = 1.53
Chlorine = 2.49 Ethylene = 0.97 Helium = 0.14 Hydrogen = 0.07 H2S = 1/19
Methane = 0.55 Nitrogen = 0.97 Oxygen = 1.15 Propane = 1.56 SO2 = 2.21
Notes:

Gas formulae are suitable for gas temperatures between 30 and 150°F. All pressure drop calculations of this type are approximations. Fluid properties can be very significantly affected by pressure and temperature.

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