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 5.1 Test Formula.
In determining the air losses, use the following mass loss
formula.
Q = 35.852 V
(a)
(PI-PF)
(T+460)(t F-tI)
Where: Q
= volumetric airflow (scfm)
= volume of tank, ft3
V
T
= temperature, F
P
= pressure, psig
t
= time, minutes
I
= initial
F
= final
(b) Although the regression equation can be calculated by hand, the
calculations are quite laborious. It is strongly recommended that an
inexpensive hand-held calculator with statistics capability or an in-house
computer program be used as the information can then be rapidly and accurately
calculated.
5.2 Example.
The following example illustrates the pressure decay test
procedure.
(a) A section of l0-inch compressed air line is suspected of leakage.
The line is located on drawings and verified by a field inspection. Using an
engineering scale and the drawing, the length of line is found to be 1,000
feet. Calculating the volume of the line:
(1102.7in/ft ) x 1,000 ft
π d2 1
5 in
1π x
V=
=
4
4
V = 630.3 ft3
(b) A pressure gauge is installed on the line at an outlet valve, and
all loads on the line are secured. With a person watching the gauge, the line
is isolated from the central air distribution system.
(c) The pressure gauge, which had indicated 96 psig, begins to fall
immediately. When the gauge reaches 90 psig, the stopwatch is started. Time
is recorded at l0-psi intervals as shown in table B-2. (A stopwatch with a
lap counter makes this easier.)
(d) Assuming an ambient temperature of 68F, the losses can be
calculated for each pressure interval, using equation 5.1(a). Results are
shown in table B-3.
(e) The data can be plotted as shown in figure B-l to determine how well
the test data fits a straight line. Although test data from an actual test
will normally be offset from a straight line to some degree, severe deviations
will require that the test be repeated.
B-5
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