Section 6 .
EVALUATION OF LOSSES IN COMPRESSED AIR SYSTEMS
1 COMPRESSED AIR SYSTEM LEAKS. Leakage of compressed air is a problem at
industrial installations and if uncorrected, will result in significant
monetary losses. Leakage can result from corrosion in underground piping,
damaged joints, a n d defective fittings and valves. A relatively simple test
has been devised which will rapidly and economically determine whether a
d i s t r i b u t i o n line is leaking and if so, the magnitude of the losses.
2 TEST METHOD. This test requires that a segment of a compressed air
d i s t r i b u t i o n line be pressurized, sealed, and checked by use of a pressure
g a u g e to determine if the line is leaking. If there is no pressure decrease,
there is no leakage. If the pressure does decrease, a leak is indicated. The
amount of leakage can be determined and a graph prepared that will determine
the loss at operating pressure.
The equipment is very simple and relatively inexpensive:
3 TEST EQUIPMENT.
4 TEST PRECAUTIONS. T h i s test will produce valid results as long as the
r e l a t i o n s h i p between pressure and flow rate is linear. Assuming a sonic exit
v e l o c i t y , the relationship will be linear as long as the ratio of the
a t m o s p h e r i c pressure to the line pressure exceeds the critical pressure ratio
of 0.53 for gases. Thus, to ensure maximum accuracy, test data should not be
used if the pressure gauge registers less than 20 psig.
5 TEST PROCEDURES. The following steps must be performed to complete a
pressure decay test.
( a ) Obtain scale drawings of the section to be tested. Verify drawings
in the field and calculate the volume of the section to be tested.
( b ) Install a pressure gauge at a convenient location.
( c ) Secure all loads the line supplies.
(d) Isolate the line from the compressed air system.
(e) Immediately begin taking readings at the pressure gauge but do not
try to start the moment the valve is closed. Observe the pressure gauge and
begin timing when the pointer passes a convenient mark. Example: On a
100-psig system, wait for the pressure gauge to reach 95 psig before starting
( f ) Note the time at convenient pressure intervals (5 or 10 psi
increments). Continue data recording until 20 psig is reached.
( g ) Calculate Q from equation 5.1(a).
(h) On graph paper, plot Q on the Y axis and P on the X axis.