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Issue Date: November 22, 1995
Revision Date: September 22, 2009
Tanknology
VacuTect
NON-VOLUMETRIC TANK TIGHTNESS
TEST METHOD (VACUUM)
| Certification |
Leak rate of 0.1
gph with PD = 100% and PFA = 0%. |
| Leak
Threshold |
A tank system
should not be declared tight when:
sonic emission of air ingress is detected in ullage area and/or; sonic
emission of bubbles formed by air ingress is detected in product-filled
portion of the tank and/or; water ingress is detected by the water sensor. |
| Applicability |
Gasoline, diesel,
aviation fuel, fuel oil #4, waste oil.
Water miscible products limit the effectiveness of water ingress
detection. |
| Tank
Capacity |
Maximum of 75,000
gallons.
Test may be performed at any product level, including empty, if total ullage volume
does not exceed 20,000 gallons.
Maximum of 30,000 gallons per tank and 60,000 gallons cumulative capacity
for manifolded tank systems with microphone (hydrophone) and water sensor
in each tank. |
| Waiting
Time |
None between
delivery and testing. |
| Test
Period |
Minimum not
specified to declare a tank "non-tight".
There must be no dispensing or delivery during test.
When water level in tank excavation backfill is below bottom of tank:
Minimum of 1 hour to declare tank tight.
When water level in tank excavation backfill is above bottom of tank:
Minimum to declare a tank tight may need to be extended because a water
sensor must be used.
When water level in tank excavation backfill is not determined:
Minimum to declare a tank tight may need to be extended because a water
sensor must be used.
To determine test period, water level in tank excavation backfill must be
assumed to be just above bottom of tank.
When using a water sensor, the test
period is determined based on tank size, water level inside tank prior to
test, tank tilt, type of water sensor, water sensor location in the tank,
and water level in tank excavation backfill relative to bottom of tank.
Tank tilt is determined by an inclinometer, or by measuring tank bottom
elevations at two points within tank. Water sensor is placed in low end of
tank or in pre-existing cross section of water inside tank.
For example:
For a printed circuit board water sensor, the test period to detect a 0.1
gph leak rate in a 10,000 gallon (96"dia x 324"lg) level tank
without a striker plate and without water in the tank is 51 minutes for
water ingress to contact the water sensor plus 38 minutes to allow the
sensor to detect the "minimum detectable change in water level"
(see "Water Sensor" section below).
For example:
For a magnetostrictive water sensor, the test period to detect a 0.1 gph
leak rate in a 10,000 gallon (96"dia x 324"lg) level tank
without a striker plate and without water in the tank is 37 minutes for
water ingress to contact the water sensor plus 2 minutes to allow the
sensor to detect the "minimum detectable change in water level"
(see "Water Sensor" section below). |
| Test
Pressure |
Required test
pressure is a function of tank construction, burial depth, product level
in the tank, and water level in tank excavation backfill.
Pressure differential across tank wall at the bottom of the tank must be
at least 0.5 psi.
Pressure differential across the tank wall is equal to the absolute value
of vacuum applied to tank, plus the pressure of the tank excavation
backfill on tank, plus groundwater pressure on tank, minus pressure of
liquid in tank.
If water level in tank excavation backfill is not determined by wells,
probes or pump boxes in the tank excavation backfill, test pressure
calculation must account for both:
1) water level just above bottom of tank to achieve minimum 0.5 psi at
worst case condition, and
2) tank completely submerged to assure tank is not damaged from over
pressurization. |
| Temperature |
Sonic emission
and water ingress are independent of product temperature. |
| Water
Sensor |
Must be used if
water level in tank excavation backfill is above tank bottom or if water
level in tank excavation backfill was not determined.
Printed circuit board sensor minimum detectable water level is 0.022 inch,
and minimum detectable change in water level is 0.016 inch.
Magnetostrictive sensor minimum detectable water level is 0.017 inch, and
minimum detectable change in water level 0.001 inch. |
| Groundwater |
If groundwater
level in tank excavation backfill is above bottom of tank, or if
groundwater level in tank excavation backfill is not determined, test time
must be sufficient to detect water ingress using one of the above water
sensors. |
| Comments |
Evaluated using
gasoline, diesel, Jet-A, and JP-4.
Microphone (hydrophone) should be located within 60 feet of any possible
leak source.
Vacuum test method may not be effective in some tank excavation backfill
(such as clay) because it may plug holes in tank.
If free product is present in tank excavation backfill, a leak in the free
product zone may not be detected by a vacuum test method.
A well point or observation well in the tank excavation backfill can help
identify the presence of free product, tank excavation backfill material,
and water elevation in the tank excavation backfill.
More that 4 psi pressure differential across the tank wall at any location
in the tank could damage tank. |
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Tanknology
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Evaluator:
Ken Wilcox Associates
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11000 North MoPac Expressway, Suite 500
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Tel:
(816) 443-2494
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Austin, TX 78759
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Dates of Evaluations:
10/28/91,
09/18/92, 02/20/92
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Tel: (800) 800-4633
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01/18/94, 02/23/96
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