Cv Flow Factor - Flow Coefficient for Valves

Flow Factor sells Solenoid valves, Jet Pumps and Control Valves.

The Flow Coefficient or Flow Factor is used to size and compare the flow rate through valves and other devices used for moving liquids and gases.

The flow coefficient - C_v - let us compare flow capacities of valves at different sizes, types and manufacturers. The flow coefficient is in general determined experimentally and express the flow capacity in imperial units - GPM (US gallons per minute) of water that a valve will pass for a pressure drop of 1 lb/in² (psi).

The flow factor - Kv - is also in common use, but express the capacity in SI-units.

The flow coefficient - C_v - required for a specific application can be estimated by using specific formulas for the different fluids or gases. With the estimated C_v value - the correct valve can be selected from the manufacturers catalogues.

In more practical terms, the flow coefficient C_v is the volume (in US gallons) of water at 60°F that will flow per minute through a valve with a pressure drop of 1 psi across the valve.

The use of the flow coefficient offers a standard method of comparing valve capacities and sizing valves for specific applications that is widely accepted by industry.

Flow Coefficient - C_v - for Liquids

For liquids the flow coefficient - C_v - expresses the flow capacity in gallons per minute (GPM) of 60^oF water with a pressure drop of 1 psi (lb/in²).

Flow expressed by volume

C_v = q (SG / dp)^1/2         (1)

where

q = water flow (US gallons per minute)

SG = specific gravity (1 for water)

dp = pressure drop (psi)

Flow Coefficient - C_v - for Air and other Gases

Note! - there is a difference between critical and non critical pressure drops. 

For critical pressure drop, where the outlet pressure - p_o - from the control valve is less than 53% of the inlet pressure - p_i, the flow coefficient can be expressed as:

C_v = q [SG (T + 460)]^1/2/ 660 p_i         (5)

where

q = free gas per hour, standard cubic feet per hour (Cu.ft/h)

SG = specific gravity of flowing gas gas relative to air at 14.7 psia and 60^oF

T = flowing air or gas temperature (^oF)

p_i = inlet gas absolute pressure (psia)

 For non critical pressure drop, where the outlet pressure - p_o - from the control valve is greater than 53% of the inlet pressure - p_i, the flow coefficient can be expressed as:

C_v = q [SG (T + 460)]^1/2/ [1360 (dp p_o)^1/2]         (5b)

where

dp = (p_i - p_o)

p_o = outlet gas absolute pressure (psia)

Specific Gravities of common materials and mediums

Alcohol	0.82
Mercury	13.95
Paraffin	0.80
Petrol	0.72
Water (4oC)	1.00
Sea water	1.02
Aluminum	2.72
Brass	8.48
Cadmium	8.57
Chromium	7.03
Copper	8.79
Cast iron	7.20
Lead	11.35
Nickel	8.73
Nylon	1.12
PVC	1.36
Rubber	0.96
Steel	7.82
Tin	7.28
Zinc	7.12
Acetylene	0.0017
Dry air	0.0013
Carbon dioxide	0.00198
Carbon monoxide	0.00126
Hydrogen	0.00009
Nitrogen	0.00125
Oxygen	0.00143

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