The diameter of an
orifice equivalent to a Cv of 1 is .1833 or the diameter
squared is .034 for a Cv of 1.
1.
Multiply the Cv of the valve you have chosen by
.034.
2.
Take the square root of that number and label it “Inside
Diameter Minimum”
3.
Make double dawg sure that the inside diameter of the pipe,
tubing, fittings etc. meets or exceeds this diameter.
For illustration: We
have selected a valve with a Cv or 1.3 because it
was the first valve slightly larger than the Cv number
that we calculated. Then Cv x .034 is equivalent to 1.3
x .034 = .044. The square root of .044 = .210. Do not use any item
with an inside diameter less than .210” ID Min. in the flow
path of this application.
“I intended to use
¼” Nylon tube with .170 inside diameter. You can’t tell me it makes
that much difference.”
Don’t be stuck on
silly. Go to 3/8” nylon tubing with .275 inside diameter and live
happily ever after. If you use the tubing with an ID of .170 (.170^2
= .029, .029/.034 = .85 Cv) your system will not
exceed a Cv value of .85. Your carefully
selected valve with a Cv of 1.33 is limited to a Cv
of .85 by the ¼” tubing.
For flow control
valves, shuttle valves, quick dump, check valves etc. do not assume
that they meet or exceed the flow or Cv required to match
the valves you are using. Drop a dime and call the distributor or
manufacturer or surf the web.
It is high time that
the manufacturers of these devices publish better flow data.
One parting shot. Some
catalog flow data for valves and similar devices is given as flow to
atmosphere instead of Cv.
Should you find a ¼”
ported valve that lists 68 SCFM at 100 psig as the flow capability
think it through. A ¼” ported valve should have about a Cv
of 1(.5 to 1.5). If we use the rule of thumb 90psig
at10psid = 30scfm,
at 90 psig with a 10 psi delta P a device with a Cv or 1
will flow 30 SCFM, it appears that the valve in question is rated
abnormally high or with flow to atmosphere. .
Write this in your
crib notes. For a Cv of 1 flow to atmosphere, Qscfm
= .4874 x P1psia
Q = .4874 x 114.7 =
55.9scfm Then 68 / 55.9 =
1.22 Cv for the mystery valve if it’s flow
is rated to atmosphere.
I suggest the all
pneumatic valves be rated at the flow to atmosphere with the inlet
pressure. P1psia specified.
All the valves would
be rated in the same, simple way. They could be checked in 5 minutes
with a flowmeter.
Remember that with
pneumatics bigger is almost always better. Use generously sized
plumbing and accessories.
