U.S. patent number RE31,884 [Application Number 06/405,492] was granted by the patent office on 1985-05-14 for method for leakage measurement.
This patent grant is currently assigned to Hunter Environmental Services, Inc.. Invention is credited to William B. Hansel, Earl W. Smith.
United States Patent |
RE31,884 |
Hansel , et al. |
May 14, 1985 |
Method for leakage measurement
Abstract
A method of high sensitivity for measuring leakage of liquid
from a storage tank which comprises introducing a sensor into the
liquid in the tank, the sensor being coupled to means for sensing
displacement of mass, measuring the displacement over a known
period of time and calculating the leakage rate. The sensor is
designed to compensate for evaporative losses and temperature
changes.
Inventors: |
Hansel; William B. (Media,
PA), Smith; Earl W. (Wilmington, DE) |
Assignee: |
Hunter Environmental Services,
Inc. (Malvern, PA)
|
Family
ID: |
26770744 |
Appl.
No.: |
06/405,492 |
Filed: |
August 5, 1982 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
084241 |
Oct 12, 1979 |
04300388 |
Nov 17, 1981 |
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Current U.S.
Class: |
73/49.2;
73/309 |
Current CPC
Class: |
G01F
23/0023 (20130101); G01M 3/3245 (20130101); G01F
25/0046 (20130101); G01F 23/20 (20130101) |
Current International
Class: |
G01F
25/00 (20060101); G01M 3/32 (20060101); G01F
23/20 (20060101); G01F 23/00 (20060101); G01M
003/32 (); G01F 023/20 () |
Field of
Search: |
;73/49.2,309 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1371910 |
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Apr 1964 |
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FR |
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127330 |
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Jan 1963 |
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NZ |
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276474 |
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Jul 1970 |
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SU |
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Primary Examiner: Goldberg; Gerald
Assistant Examiner: Roskos; Joseph W.
Attorney, Agent or Firm: Parkhurst & Oliff
Claims
The invention claimed is:
1. A method of high sensitivity for measuring leakage of liquid
into or out of a storage tank .Iadd.normally at atmospheric
pressure .Iaddend.which comprises introducing a freely suspended
sensor into the liquid in the tank .Iadd.through the fill-pipe or
other access hole of said tank.Iaddend., said sensor being coupled
to means for sensing mass displacement, .[.and.]. observing the
change of displacement of said sensor over a period of time to
determine a change of liquid mass in said tank.Iadd., and
subsequently removing said sensor from said tank.Iaddend..
2. A method of high sensitivity for measuring leakage of liquid
into or out of a storage tank, which comprises introducing a freely
suspended sensor into the liquid in the tank, said sensor being
coupled to means for sensing mass displacement, calibrating said
mass sensing displacement means to determine the amount of liquid
per unit change, measuring the units of change of displacement of
said sensor over a period of time and calculating the rate of
change of liquid in said tank.
3. A method of high sensitivity for measuring leakage of liquid
into or out of a storage tank whereby effects of evaporation of
said liquid in said tank are compensated, which comprises
introducing a sensor into the liquid in the tank, said sensor
having liquid from said storage tank in liquid holding means at its
top .[.and extending above the level of liquid in said
tank.]..Iadd., said sensor being disposed in the tank liquid such
that said liquid holding means extends above the tank liquid level,
.Iaddend.the cross-sectional area of said liquid surface in said
liquid holding means being essentially equal to the cross-sectional
area of the .[.outside dimension of.]. said sensor .Iadd.in contact
with the tank liquid.Iaddend., said sensor being coupled to means
for sensing mass displacement, calibrating said mass sensing
displacement means to determine the amount of liquid per unit
change, measuring the units of change of displacement of said
sensor over a period of time and calculating the rate of change of
liquid in said tank.
4. The method of claim 3 where the storage tank is underground.
5. The method of claim 3 wherein the tank is filled with liquid and
the sensor extends to near the bottom of said tank whereby the
effects of temperature are minimized.
6. A method of high sensitivity for measuring leakage of liquid
into or out of a storage tank whereby effects of evaporation of
said liquid in said tank are compensated, which comprises
introducing a sensor into the liquid in the tank, said sensor
having liquid from said storage tank in liquid holding means at its
top .[.and extending above the level of liquid in said
tank,.]..Iadd., said sensor being disposed in the tank liquids such
that said liquid holding means extends above the tank liquid level,
.Iaddend.the cross-sectional area of said liquid surface in said
liquid holding means being essentially equal to the cross-sectional
area of .[.the outside dimension of.]. said sensor .Iadd.in contact
with the tank liquid.Iaddend., said sensor being coupled to the arm
of a balance for sensing mass displacement and movement of said
balance arm being detected by a differential transformer,
calibrating the output of said differential transformer to
determine the amount of liquid per unit change of displacement,
measuring the units of change of displacement of said sensor over a
period of time and calculating the rate of change of liquid in said
tank.
7. The method of claim 6 where the storage tank is underground.
8. The method of claim 6 wherein the output of said differential
transformer is coupled to a recording device. .Iadd.
9. A method of improved sensitivity for measuring leakage of liquid
into or out of a storage tank having a tank atmosphere whereby
effects of evaporation of the tank liquid are compensated, said
method comprising the steps of:
introducing into the tank liquid a sensor containing liquid
segregated from the tank liquid and exposed to the tank
atmosphere,
sensing at least one buoyancy-dependent sensor condition which is
responsive to the level of the tank liquid in the tank;
observing the change in the at least one sensor condition to
determine a change of liquid mass in the tank. .Iaddend. .Iadd.10.
The method of claim 9 wherein said at least one sensor condition
comprises displacement of
said sensor from a predetermined initial position. .Iaddend.
.Iadd.11. The method of claim 9 wherein the cross-sectional area of
the surface of said sensor liquid is substantially equal to the
cross-sectional displacement area of said sensor. .Iaddend.
.Iadd.12. The method of claim 11 wherein said sensor liquid is
liquid taken from said tank liquid. .Iaddend. .Iadd.13. The method
of claim 12 wherein said sensor has liquid holding means at its top
for holding at least some of said sensor liquid. .Iaddend.
.Iadd.14. The method of claim 13 wherein said sensor is introduced
into the tank such that said liquid holding means is disposed above
the level of the tank liquid. .Iaddend.
Description
This invention relates to a highly sensitive method to determine
the rate of leakage of liquid from a storage tank and is
particularly directed to determining the leakage and rate of
leakage of fuel hydrocarbons in installed underground storage
tanks. Leakage into and out of the tank can be determined by the
method of the invention.
There are thousands of underground storage tanks at the numerous
fuel service stations throughout the world and over a period of
time leaks inevitably occur causing fuel hydrocarbons such as
gasoline, diesel oil and the like to flow into the ground. Such
leaks are, of course, environmental hazards since they could lead
to ground water contamination. Also, the leaking fluid can gather
in buildings and result in hazardous and toxic hydrocarbon
vaporization in the confined air space. Leaks into storage tanks
also occur, most often water, which contaminates the tank contents.
Thus, it is essential that such leaks be determined as soon as
possible so that corrective measures can be taken. In order that
early detection can be made it is essential that the detection
method be extremely sensitive (in the range of 0.05 gal. per hour)
so as to determine very slight leakage in a relatively short period
of time. This is necessary for several reasons including the time
value of the test itself, the need to remove the storage tank from
dispensing operations for a minimum amount of time, and the like.
Heretofore no such suitable method has been available.
It is known in the art to measure the mass of liquid in a storage
tank by use of Archimedes Buoyancy Principle using a float in the
liquid. However, none of the devices or methods known are adaptable
to a highly sensitive, rapid method for leak determination in
difficulty accessable storage tanks (e.g. already installed
underground tanks). For example, U.S. Pat. No. 967,378 discloses a
storage tank fitted with a hydrometer-type weighing device to
estimate the weight of liquids stored in the tank. U.S. Pat. No.
988,342 shows a similar device fitted to the storage tank, but
where the scale is on the ground for easy access.
In the method of the present invention a sensor of appropriate
design is inserted into the storage tank through the fillpipe or
other access hole and is coupled to a sensing device to measure
mass displacement over a period of time. As will be further
explained, the preferred sensor design is such as to compensate for
changes due to vaporization of the liquid and changes occuring in
density and the level of the liquid due to temperature variations.
The sensing device is calibrated to determine the amount of liquid
per unit of change and after a short period of time, usually only
several minutes, the change in mass displacement due to the leak is
noted. In this way the rate of liquid loss can be determined by
simple calculation.
Reference is now made to the drawings:
FIG. 1 is a broad overall view of an underground hydrocarbon
storage tank with the sensor and detector device in position.
FIG. 2 is a detailed view of the detector system.
FIGS. 3 and 4 are detailed views of preferred sensors.
FIG. 4a shows the quick release mechanism of FIG. 4 in the open
position.
FIGS. 5 and 6 show the insertion of a preferred sensor into a
tank.
FIGS. 7 and 8 show the removal of a preferred sensor from the
tank.
FIG. 1 shows an underground filled hydrocarbon storage tank 11 with
the sensor 12 in position in the liquid. The top of the sensor is
above the liquid level and the bottom of the sensor is within about
1 to 2 inches from the bottom of the tank. The sensor has a cap 13
and also a cup portion 14 (detailed in FIG. 3) to which supports of
wire or string 15 are attached which, in turn, are connected to
line 16 which supports the sensor from the weighing arm 17 of a
balance shown generally as 18. A coupler 22 may be used for
convenience in connecting line 16 to the balance arms. Any change
in mass in the tank will result in a liquid level change, which in
turn, will result in a change of weight of the sensor. This change
of mass detected by the sensor is also sensed by a differential
transformer 24 which surrounds the support line 16. The balance 18
may be supported on a box 19 which in turn is supported by a base
20 having leveling screws 21. The box 19 may also contain the
desired instrumentation and/or be provided with input and output
jacks for connection to appropriate devices which are discussed
later. The balance may be shielded from wind by means of a cover 23
as shown.
Referring now to FIG. 2, line 16 is attached to the balance beam 17
by a detachable coupler 22 and passes through a differential
transformer (i.e. a displacement transducer) 24. Attached to line
16 and between the coils of the transducer is a ferromagnetic
material 25 whose movement changes the electromagnetic flux of the
transducer which is detected by the electronic circuitry shown at
26 powered by a power supply 27. It will be understood that the
transducer need not necessarily be around the line 16, but may be
placed in any convenient position, the only requirement being that
it is responsive to movement of the balance beam. A suitable
transducer is described in U.S. Pat. No. 3,179,193 where it is used
in conjunction with a cantilever type support for a fuel tank to
weigh fuel removed from the tank and fed to an internal combustion
engine. The electronic circuitry 26 will also contain an amplifier
to send an amplified signal to a digital readout device 28 which,
if desired, may have a strip chart recorder or other printer 29
attached thereto for obtaining a printed record of the
measurements.
In FIG. 3 which is a section taken on line 3--3 of FIG. 1, the
preferred sensor having cap 13 and cup portion 14 is shown in
detail. The sensor is filled with liquid from the tank and extends
from above the top of the liquid 30 in fill-pipe 31 to just above
the bottom of the storage tank 11. As is evident from the drawing
the inside diameter of the cup section 14 is essentially equal to
the outside diameter of the sensor in order to obtain compensation
for evaporation of the hydrocarbon in the tank. The entire sensor
system is readily supported by line 16 by appropriate wires or
strings 15 through coupling means 22.
Compensation for evaporation results from the fact that when the
hydrocarbon is in the cup portion, any evaporation that occurs will
occur from both the surface of the liquid in the tank and from the
surface of liquid in the cup. The buoyancy of the hydrocarbon
liquid on the sensor is a function of the sensor diameter, but
since the inside diameter of the cup is essentially the same as the
outside diameter of the sensor, the evaporative loss of liquid from
the cup will compensate for the buoyancy change due to evaporation
of the liquid in the tank. Thus, when the liquid hydrocarbon in the
tank evaporates, its buoyant force is reduced and the sensor
becomes heavier. But the evaporation of the liquid in the cup
reduces the weight of the sensor by exactly the same amount of the
buoyant force lost and no change in weight of the sensor occurs. It
will be understood of course, that instead of the sensor being
tubular (i.e. circular in cross-section) it may be of another
configuration and the cup-like section then will be made to have
the same .[.cross-section configuration..]. .Iadd.interior
cross-sectional area as the cross-sectional area of the sensor in
contact with the tank liquid. .Iaddend.
By positioning the sensor near the bottom of the storage tank
completely filled with liquid and having it extend through
essentially the entire height of the liquid in the tank and riser
31 as shown in FIG. 1 the effects of any change in temperature are
minimized. This is because the sensor fluid temperature is
essentially the same as the tank liquid temperature with the same
gradient, if any. As indicated, it is desired that the system
measure a change of about 0.05 gallons per hour which is equivalent
to about 200 cc per hour. Since the measurements made in accord
with the invention are made within a few minutes it is unlikely
that there will be a temperature change of more than a fraction of
one degree. Tests show that with a tank filled with gasoline using
a sensor with a five-eighth inch diameter in a two-inch pipe riser,
a one degree (.degree.F.) change effects a change in mass
equivalent to about 0.3 cc, which 0.3 cc change represents an
insignificant error of 0.15%. Thus, temperature changes are of no
significance when the measurements are performed on an essentially
full tank and using a sensor extending essentially to the bottom.
It will be understood, of course, that where no temperature
problems are anticipated, the sensor need not extend to near the
bottom of the tank and a shorter sensor may be used. Also, if
temperature compensation is not desired, it is not necessary to
make the measurement with a full tank of liquid.
In carrying out the procedures of the invention with temperature
compensation it is desirable to plug off any conduits in the tank
system such as risers and the like to reduce the area of such
risers in order to maintain sensitivity during leak measurement.
After the necessary plugging is done, the balance is erected over
the fillpipe hole, gauge hole or other access hole to the tank, the
sensor filled with liquid from the tank and inserted through the
hole. The sensor suspension wire is then attached to the beam of
the balance. Care must be taken to see that the sensor and its
suspension wire do not touch the pipe or tank. The balance is then
leveled, and appropriate electrical corrections made to the power
supply, transducer, and recorder. It is also often desirable to
cover the balance to protect it against gusts of wind or other
interferences. Calibration is then carried out by adding a known
amount of liquid to the tank and observing the change in the
recording instrument. Observation is then made to the recorder
device to note changes. With a strip chart recorder, which is
preferred, the pen will trace a straight line if there are no
leaks. If a leak is present, the pen will deflect and the angle of
deflection is indicative of the leak rate. From the number of
divisions on the chart paper that the pen has deflected, the time
of the trace, and the deflection per unit of liquid obtained from
the calibration step, the leak rate is readily calculated. Also, by
starting the pen at the center of the paper and standardizing the
direction of deflection, the leak can be determined to be into or
out of the tank.
As indicated, it is possible to carry out the method with partially
filled tanks when temperature compensation is not considered to be
critical. In such measurements it is not necessary to plug off the
risers and other conduits from the tank and it is preferred to use
a different shaped sensor. It will be understood that the
sensitivity of the sensor is dependent upon the liquid level in the
tank and the shape of the tank. The smaller the area of liquid, the
greater the response to change of the sensor. Thus, when the tank
is filled with liquid in a riser section, the sensor is most
sensitive. At mid-point of a cylinderical tank where the area of
liquid level is greater, sensitivity is lowest, and sensitivity
will be between these extremes at other positions. Thus, when a
partially filled tank is measured, a sensor with a relatively large
displacement is desired to increase sensitivity. Also, a more
sensitive balance may be used for increased sensitivity.
The preferred sensor for less than a full tank system is shown in
FIG. 4 and may be in the shape of a flat board 32 with a dished out
upper surface 33 and having supporting wires 34 and 35. The board
32 will be sufficiently narrow so as to pass through a pipe into
the liquid in the tank. The sensor is suspended on the liquid
surface in a horizonal attitude with some liquid in the dished out
portion 33 and the cross-sectional area of the liquid in the dished
out surface is essentially the same as the cross-sectional area in
contact with the liquid in the tank. In this way the compensation
for evaporation is accomplished in the same manner as described
above. The means by which the narrow sensor is introduced into and
removed from the tank is illustrated by FIGS. 4 through 8. The
sensor is provided with means to enter the tank and enable it to
function in a horizontal position on the surface of the liquid.
These means must also enable the sensor to be removed from the tank
through the same opening through which it was introduced. Such
means shown generally as 48 in FIGS. 4 and 4a embodies a quick
release mechanism to enable the sensor to be easily removed. As
seen in FIG. 4 the release means comprises a solid cylindrical core
member 36 surrounded by an outer cover 37 made of sheet metal,
plastic or other suitable material. An angular bore hole in the
core member is fitted with a pushing spring 38 and push rod 39. The
support line 16 passes through the center top of the outer cover 37
and is fixed to a small cylindrical fitting 40 with an annular
groove 41 which is held in a fixed position within core 36 by a
restraining set screw 42. The core also is fitted with a screw eye
43 or other device to which line 34 is attached. A conical spring
42 is positioned between the top of core 36 and outer cover 37. The
outer cover is provided with a port 44 in alignment with push rod
39. The edge of the outer cover 37a formed by the port 44 acts as a
catch for a pin 45 with an annular groove 46 which is attached to
line 35. The port 44 is about twice as long as the diameter of pin
45. Outer cover 37 also may be provided with limiting stubs 47 to
hold the assembly together. Before introduction of the sensor into
the tank, the pin 45 is inserted through port 44 against push rod
39 and the groove 46 in the pin is positioned at the catch edge 37a
of the outer cover. The effect of spring 42 is to hold the outer
cover in a fixed position and because of this and spring 38, the
pin 45 remains firmly fixed in the catch 37a. The sensor is then
placed in the tank through a fill pipe or other access hole as
shown in FIG. 5. When it is completely through the pipe the sensor
opens as shown in FIG. 6 and after dipping it in the liquid to
place liquid in its tray portion it is allowed to rest on the
surface of the liquid as shown in FIG. 7. After the determination
for leakage is made and it is desired to retrieve the sensor, a
weight 50 surrounding line 16 is dropped down the fillpipe 31 onto
the quick release system 48. As shown in FIG. 4a, the weight
impacts upon the outer cover 37 pushing it downward, which in turn
removes the catch 37a from groove 46 of pin 45. This causes the
cooperating push spring 38 and push rod 39 to push pin 45 outwardly
and causes the sensor to take the position shown in FIG. 8. Removal
of the sensor is then simply made by pulling it upwardly through
the fill-pipe riser 31.
It will be understood that the above described leak measuring
system may be used in numerous applications for both above and
below ground installation. Also numerous variations may be made to
the system. For example, it is possible to replace the displacement
transducer with other means for determining displacement; e.g.
capacitance devices and the like. In another alternative embodiment
the displacement of the sensor may be replaced by the technique of
measuring the change in mass necessary to keep the balance at its
null point. Since such a technique will require servo-mechanisms it
will be more expensive to build the equipment and therefore it is
not a preferred method. Another option is to use telemetering means
so as to remotely measure any changes in mass. Other variations and
embodiments will be obvious to the skilled art worker.
* * * * *