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