Introduction Applications
6
The term ‘head’ is usually expressed in feet whereas pressure is usually expressed in pounds-per-square-inch. The
formula for converting these factors follows:
(A rough equation is 3 feet of gasoline head equals 1 psi).
The history of industry practice concerning location of the submersible pump in relation to the dispenser indicates
that static head is not normally a problem. The general operating parameters for the leak detector dictate that it will
trip or reset at approximately 3.5 to 4 psi or below, and the approximate 11 feet of liquid head or excess of 3.5 to 4
psi is not usually experienced by the leak detector. Typically, FX model leak detectors will overcome static heads of
3.5 to 4 psi.
However, if the static head and subsequent pressure experienced by the leak detector is in excess of the
threshold, the leak detector may not operate. (see the three-step test).
Important Points to Remember:
1. Leaks occurring at a height above the leak detector that exert more fluid pressure than the threshold value
will keep the leak detector from entering the leak sensing position. Such leaks will not be detected.
2. The threshold value for the reset pressure is a result of varying mechanical characteristics in the leak
detector and may vary. Our experience shows that it is possible to encounter leak detectors that will work
in a given situation when others won’t because they have a slightly higher threshold.
The effect of excessive static pressure can be observed when a simulated leak is placed into the line above the
threshold height and then closed before the pump is turned on. Under normal operations, the leak detector will
hesitate at the metering pressure for several seconds before opening up to full pump pressure. With excessive
static head present, the pressure will increase immediately to full pump pressure. If under these conditions a leak
is present, the pump is turned on and full pump pressure is realized immediately, the leak detector is not operating
properly.
The amount of pressure experienced by the leak detector can be determined by installing a pressure gauge on the
line test port of the pump. The gauge must remain at the level of the line test port and should have a low pressure
range (i.e., 0 to 30 psi) to get accurate readings.
After the pump is turned off, use the valve on the test apparatus to bleed the pressure from the highest vertical
point in the system, typically located at the dispenser shear valve. The gauge on the line test port will read the
static head present on the leak detector.
Possible solutions to the problem of excessive static head are:
1. Test the operation of leak detectors. Replace the leak detector with a low threshold attempting to find one with
a higher threshold by observing test results.
2. Modify the system by raising the leak detector to decrease its static head. This can be done by installing a
longer riser pipe between the pump and the tank. Note that this will raise the pump inlet further from the
bottom of the tank by the distance added to the riser.
In all cases, operation of the mechanical leak detector must be verified by inducing a leak and confirming correct
operation of the unit after it is installed at the site.
Pressure
(Lbs./Sq. In.)
Head (feet) x Specific Gravity
2.31
Pressure (psi) x 2.31
Specific Gravity
Head (feet)
=
=
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