Cv (Flow Factors) for Compressed Air
FLUID POWER - Design Data Sheet 22
Design Data Sheet No. 17 showed charts for
sizing hydraulic valves by using Cv (flow factor) ratings. The
chart below gives the same information for compressed air. Sheet 17
should be reviewed for an explanation of Cv factor and other
The chart below; although calculated for a valve having a Cv of
1.00, can be used equally well for any size valve on which the
manufacturer has a published Cv factor, and will work for any
application within the pressure range shown.
A 2-way valve has only one flow path, so there will be only one
pressure drop across the valve from inlet to outlet.
A 4-way valve has two flow paths. If the Cv factor is the same
for both paths, there will be about the same pressure loss through
each. If the Cv factors are different, there will be a higher loss
through the path with lower Cv factor.
The chart shows the relation between inlet pressure, outlet
pressure and flow capacity for any orifice or valve passage which
has a Cv factor of 1.00. On valves or orifices with other Cv
factors, the flow capacity will be directly proportional to the Cv
rating. For example, a valve with Cv of 4.00 will pass 4 times as
much flow at the same pressure difference as shown in the
SCFM AIR FLOW THROUGH A
VALVE ORIFICE HAVING A Cv FACTOR OF 1.00
Notice that flow is not proportional to delta P (pressure
difference between inlet and outlet pressures). Flow does increase
as delta P increases until the critical pressure ratio is reached.
For air and other gases this critical ratio is reached when outlet
pressure drops to less than 50% of inlet pressure. Flow at the
critical pressure ratio is the maximum that can be passed through
the orifice regardless of how much more the delta P may become.
This can be seen in each column of the chart. When outlet pressure
falls to less than 50% of inlet pressure, the flow levels off to a
maximum for that value of inlet pressure. Only by increasing the
inlet pressure can the flow capacity of the valve be increased.
Calculating Other Pressure Conditions
The chart was calculated from a formula published by the Fluid
Controls Institute, Inc., 12 Bank St., Summit, NJ 07901, which,
when simplified for a Cv of 1.00, compressed air (specific gravity
of 1.00), and an air temperature of 80 to 100°F, reads as
Q is the air flow
P1 is the inlet pressure, PSIG
P2 is the outlet pressure, PSIG
Cv is understood to be 1.00 when using this
Other pressure conditions, within the above limitations can be
calculated from the formula. Remember that for outlet pressures
less than 50% of the inlet (below the critical ratio), use the
critical pressure for P2 in the formula. For other gases and other
ambient temperatures, use the original formula as published by the
Fluid Controls Institute.
Examples of Use of the Chart
Example: Determine the valve Cv factor
necessary to pass 55 SCFM at only a 5 PSIG pressure loss when valve
is connected to an 80 PSIG air line.
Solution: Look down the 80 PSIG inlet pressure
column and opposite the 75 PSIG outlet pressure line (a 5 PSIG drop
through the valve). The chart, which is based on a Cv of 1.00,
states that 18.8 SCFM will flow. To find necessary Cv for a 55 SCFM
flow: 55 ÷ 18.8 = 2.93. Select a valve with at least a 2. 93 Cv and
it will meet the flow conditions.
Example: Suppose a certain valve has a
published Cv factor of 3.75. If this valve is connected to a 125
PSIG air line, find the pressure loss through it when it is passing
a flow of 243 SCFM.
Solution: Since the chart is for a Cv of 1.00,
first convert the 243 SCFM to the equivalent flow expected on a
valve having a Cv of 1.00: Equiv. flow = 243 ÷ 3.75 = 64.8 SCFM.
Look down the 125 PSIG column of the chart. The value of 64.9 SCFM
opposite the 80 PSIG line comes very close. Since the chart shows
an outlet pressure of 80 PSIG, the pressure loss through the valve
is: 125 - 80 = 45 PSIG.
EXPLOSION PROOF SOLENOID VALVES
Availability of suitable valves may be a problem on air or
hydraulic applications which operate in hazardous locations and
require explosion proof solenoid valves.
For a valve to carry the official underwriters label (UL), the
entire valve assembly, with solenoid operator in place, must have
been submitted to the UL Laboratories for testing and approval.
Each valve size, type, and model must be separately submitted. Some
manufacturers who offer a large variety of sizes and types do not
feel that the high cost of UL testing on each model is justified by
the relatively small demand for such valves.
Some manufacturers have offered solenoid valves which they claim
are built to UL specifications but which have not been submitted
for approval and do not carry the label. Such valves may or may not
be acceptable in a given situation.
Explosion Proof Operator.
Figure 1. Miniature 3-way solenoid valves of the
type pictured are available from several manufacturers with the
official UL label of approval for certain classes and groups of
hazardous service. Most of these valves are built for use on
compressed air. They can be used as pilot operators for standard
4-way air valves, and for 4-way hydraulic valves which are equipped
with air pilots. The outlet (cylinder) port on the 3-way valve can
be connected to the end cap of the 4-way valve with ordinary
piping. Wiring to the 3-way valve coil should follow all
recommendations of the National Electrical Code, or of local
ordinances regarding conduit connections and sealing.
Explosion Proof Enclosures.
Valves and other electrical components can be enclosed, when
necessary, in an explosion proof box which is approved for the
particular type of service. Electrical and fluid lines can be
brought in through threaded connections provided in the box, Wiring
and sealing should conform to the National Electrical Code.
Hazardous Locations and Atmospheres
First, locations are classified according to the general nature of
the hazard, as follows:
Class I - Highly inflammable gases or
Class II - Combustible dust.
Class III - Combustible fibers or flyings.
Then, various atmospheric mixtures have been assigned ratings,
Groups A, B, C, D, E, F, and G, according to specific
characteristics such as flash point, explosion pressure, ignition
temperature, etc. It is necessary that equipment be approved not
only for the class of location (I, II, III), but also for the
specific gas, vapor, or dust (group) that will be present. Example:
Class 1, Group D.
For further information refer to the National Electrical Code.
Copies may be purchased from:
National Fire Protection Association
1 Batterymarch Park
Quincy, MA 02169-7471
Download a PDF of Fluid Power
Design Data Sheet 22 - Cv (Flow Factors) for
© 1990 by Womack Machine Supply Co. This
company assumes no liability for errors in data nor in safe and/or
satisfactory operation of equipment designed from this