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LOCAL AND WEATHER AFFECT
In a
Cleveland laboratory or other location, test results
may be corrected mathematically to the accepted sea
level standard, for documentation, specifications,
catalog information, warranty data etc.
The
manufacturing engineer in Detroit, salesman in
Seattle, buyer in Burbank may calculate their
required flow and select a valve or widget based
upon similar formulas that are approximately correct
for standard air on an ideal day at sea level.
However even “Standard” may only be an approximation
for the “Standard” might have been related to ASME,
ISO, Fluid Power, National Aeronautics and others.
The average barometric pressure
and weather in Detroit is 29.26 In Hg, 58 degrees F
and 62% relative humidity. Seattle is 29.50 in Hg,
59 Deg. F, and 70% RH. Chicago is 29.27 In Hg, 59
Deg. F and 61% RH.
(From Compressed air and gas data by
Ingersoll Rand) The prevalent weather conditions in
most US cities do not match so called Standard
conditions
A formula for SCFM will produce a
close approximation in most locations.
The approximate results from SCFM
formulas and calculations are normally close enough
to serve our need.
Full instrumentation with empirical
tests are required for precise results at any time
and place.
FORMULA FALLIBILITY
Professor Phil O. Sophize suggested
common formulas for flow and
Cv
are most accurate in the top 10% of the Delta P
range. Many morphed the suggested “10% most accurate
range” into a Delta P of 10% in all cases.
Others took that one step further and
use 10 PSID for the Delta P for every calculation
which below 100 psig is greater than 10%. 10% or
more is too wasteful for most applications. In the
interest of better and more efficient pneumatic
applications the flow formulas provided below will
save you time and reduce mistakes. Additionally the
first are based upon a very conservative Delta P of
2 psi. The second formula is based on a more liberal
pressure drop of 5 psi. For a greater pressure drop
a 10 psi Delta P formula is included.
LIGHTEN YOUR LOAD
-
Orifice, Qscfm = 14.5 x Dia.2
x (PSIG + 14.7) To atmosphere, over 15 psig.
-
Cv
= Qscfm / √ [1.907 x (P2 + 14.7)] When Delta
P equals 2
2a. Cv
= Qscfm / √ [4.768 x (P2 + 14.7)] When Delta
P equals 5
2b.
Cv
= Qscfm / √ [9.537 x (P2 + 14.7)] When
Delta P equals 10
-
Qscfm (Max)
= .4874 x Cv x (P1+14.7) SONIC FLOW
See table at right |
