APPARATUS AND METHOD FOR DETERMINING RATE OF LEAKAGE OF LIQUID FROM AND INTO TANKS

DISPOSITIF ET METHODE POUR DETERMINER LE RYTHME DES FUITES DE LIQUIDE D'UNE CITERNE, OU DE L'INFILTRATION DE LIQUIDE DANS UNE CITERNE

English Abstract

ABSTRACT
Apparatus and method for measuring leakage
of liquid into or from a storage tank such as a buried
gasoline tank. The storage tank should have rigid
walls. A reference tank is immersed into liquid
stored in the storage tank and is filled to the same
height and with the same liquid as contained in the
storage tank, and having sealed and rigid thermal
energy conducting walls at least where it is in
contact with the liquid, but having vapor space above
the liquid in the reference tank vented to the vapor
space in the storage tank. The cross-sectional area
of the reference tank is desirably (but not
necessarily) proportional to the cross-sectional area
of the storage tank at least wherever it is in contact
with liquid. The material of the reference and
storage tanks should have similar temperature
coefficients of linear expansion. A sensing structure
is located at approximately the same levels within the
storage tank and within the reference tank for sensing
differential hydrostatic pressure between liquids in
the two tanks. The volume of any liquid lost from or
gained by the storage tank over a period of time is
determined from an algorithm.

Claims

The embodiments of the invention in which
an exclusive property or privilege is claimed are
defined as follows:
1. Apparatus for measuring leakage of
liquid from a liquid storage tank having rigid liquid
containing walls comprising:
(a) a reference tank for immersion into
the storage tank, the reference tank being rigid
walled wherever it is in contact with liquid, and
having thermal energy conducting walls, sealed within
the liquid of the storage tank, and being fabricated
of material whereby the reference and storage tank
have similar temperature coefficients of linear
expansion,
(b) means for filling the reference tank
with the same liquid and to the same height as the
liquid in the storage tank,
(c) means for sensing the change in
differential hydrostatic pressure of the liquid at
about the same level within the storage and reference
tanks, whereby the volume of any liquid lost from or
gained by the storage tank (.DELTA.V) during a time interval
.DELTA.T may be determined by operating the algorithm
.DELTA.V=<IMG>
where .DELTA.P is a signal representing the sensed change
in differential hydrostatic pressure
between the storage and reference tanks
during the time interval .DELTA.T,
A is a signal representing the cross-sectional
area at the level of the liquid surface of
the storage tank,
? is a signal representing the density of the
liquid in the tank, and
g is a signal representing the gravitational
constant.
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2. Apparatus as defined in claim 1 in
which the space above the reference tank is vented to
the space above the storage tank.
3. Apparatus as defined in claim 2
including computing apparatus for receiving signals
from said sensing means corresponding to said
pressures and providing an output display and/or
signal representative of the volume of said liquid
lost or gained by storing digital data signals
representative of said cross-sectional area, said
density of the liquid, and said gravitational
constant, and operating a computer program
implementing said algorithm.
4. Apparatus as defined in claim 3 in
which said computing apparatus includes means for
controlling said sensing means and thereby receiving
signals representative of said pressure at
predetermined intervals over time and providing said
output display and/or signals at predetermined
intervals.
5. Apparatus as defined in claim 4 in
which said computing apparatus includes means for
providing an output display and/or signals
representative of the liquid leak rate by operating a
computer program operating an algorithm
<IMG>
where .DELTA.t is a signal representing the
elapsed time during which the change in differential
hydrostatic pressure .DELTA.P was sensed.
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6. Apparatus as defined in claim 2 in
which said same level at which the pressures are
sensed is adjacent the bottom of both tanks.
7. Apparatus for measuring leakage of
liquid into or from a storage tank comprising:
(a) a liquid containing storage tank
having rigid walls,
(b) a reference tank having similar
coefficient of thermal expansion as the storage tank
immersed into liquid stored in the storage tank,
filled with the same liquid as contained in the
storage tank, and having sealed and rigid heat
conducting walls at least where it is in contact with
the liquid, the cross-sectional area of the reference
tank being proportional to the cross-sectional area of
the storage tank at least wherever it is in contact
with liquid, the vapor space above the liquid in the
reference tank being vented to the vapor space above
the liquid in the storage tank,
(c) means at the same level within the
liquid in the storage tank and within the liquid in
the reference tank for sensing change in differential
hydrostatic pressure of the liquid over a time
interval .DELTA.T,
whereby the volume of any liquid lost from
or gained by the storage tank (.DELTA.V) during the time
interval .DELTA.T may be determined from the algorithm
.DELTA.V=<IMG>
where .DELTA.P is the sensed change in differential
hydrostatic pressure between the storage
and reference tanks during the time
interval .DELTA.T,
A is the cross-sectional area at the level of
the liquid surface of the storage tank,
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? is the density of the liquid in the tank,
and
g is the gravitational constant.
8. Apparatus as in claim 7 including a
valve in a wall of the reference tank located at a
level for controllably introducing liquid from the
storage tank into the reference tank prior to sensing
said pressures.
9. Apparatus as defined in claim 7 in
which the storage tank is a cylinder having a vertical
axis and the reference tank is a cylindrical tube
having a sealed bottom extending from the top to the
bottom of the storage tank.
10. Apparatus as defined in claim 9 in
which the storage tank contains a filler hole at a top
surface and in which the reference tube extends
downwardly from the filler hole.
11. Apparatus as defined in claim 9
including computing apparatus for receiving signals
from said sensing means corresponding to said pressure
and providing an output display and/or signals
representative of the volume of said liquid lost or
gained, by storing data signals representative of said
cross-sectional area, said density of the liquid, and
said gravitational constant, and operating a computer
program processing said algorithm.
12. A method for determining leakage of
liquid from or into a rigid walled storage tank
comprising detecting hydrostatic pressure at a level
within the liquid within the storage tank, detecting
hydrostatic pressure ? at about the same level, of
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liquid contained within a rigid walled reference tank,
which reference tank has thermal energy conducting
sealed walls, and which is immersed within the liquid
of the storage tank and is filled to the same level as
the storage tank with the same liquid, and has the
vapor space above its liquid vented to the vapor space
above the liquid in the storage tank, and providing a
liquid volume leakage signal .DELTA.V representing leakage
over a time interval .DELTA.T by operating a computer
program to process the algorithm
.DELTA.V=<IMG>
where .DELTA.P is the change in detected differential
hydrostatic pressure between the storage
tank and the reference tank during a time
interval .DELTA.T,
A is the cross-sectional area of the storage
tank at the level of the liquid surface,
? is the density of the liquid in the tank,
and
g is the gravitational constant.
13. A method as defined in claim 12
including the step of providing a leakage rate signal
by operating said computer program to process the
algorithm
<IMG>
where .DELTA.t is the time interval over which the pressure
change .DELTA.P is determined.
14. Apparatus for measuring leakage of
liquid from a liquid storage tank having rigid liquid
containing walls comprising:
(a) a reference tank for immersion into
the storage tank, the reference tank being rigid
-14-

walled wherever it is in contact with liquid, having
cross-sectional area proportional at every height in
contact with the liquid to that of the storage tank,
and thermal energy conducting walls, sealed within the
liquid of the storage tank, and being fabricated of
material whereby the reference and storage tank have
similar temperature coefficients of linear expansion,
(b) means for filling the reference tank
with the same liquid and to the same height as the
liquid in the storage tank,
(c) means for venting the vapor space
above the liquid of the reference tank,
(d) means for sensing the change in
differential hydrostatic pressure of the liquid at
about the same level within the storage and reference
tanks, whereby the volume of any liquid lost from or
gained by the storage tank (.DELTA.V) during a time interval
.DELTA.T may be determined by operating the algorithm
.DELTA.V=<IMG>
where .DELTA.P is a signal representing the sensed change
in differential hydrostatic pressure
between the storage and reference tanks
during the time interval.DELTA.T,
A is a signal representing the cross-sectional
area at the level of the liquid surface of
the storage tank,
? is a signal representing the density of the
liquid in the tank, and
g is a signal representing the gravitational
constant.
15. Apparatus as defined in claim 1, 2, 7
or 14 in which the storage tank and reference tank are
open to atmospheric pressure.
16. Apparatus as defined in claim 1, 2 or
3 in which the storage tank is a cylinder having a
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horizontal axis and the reference tank is a
cylindrical tube having a sealed bottom extending from
the top to the bottom of the storage tank.
17. Apparatus as defined in claim 1, 2 or
3 in which the storage tank is a cylinder having a
horizontal axis and the reference tank is a
cylindrical tube having a sealed bottom extending from
the top to the bottom of the storage tank, and in
which the storage tank contains a filler hole at a top
surface and in which the reference tube extends
downwardly from the filler hole.
18. Apparatus as defined in claim 1, 2 or
3 in which the storage tank is a cylinder having a
horizontal axis and the reference tank is a
cylindrical tube having a sealed bottom extending from
the top to the bottom of the storage tank, and further
including computing apparatus for receiving signals
from said sensing means corresponding to said pressure
and providing an output display and/or signals
representative of the volume of said liquid lost or
gained, by storing data signals representative of said
cross-sectional area, said density of the liquid, and
said gravitational constant, and operating a computer
program processing said algorithm.
19. Apparatus as defined in claim 1, 2, 7
or 14 in which the storage tank and reference tank are
vented to the atmosphere via a pressure relief valve.
20. A method for determining leakage of
liquid from or into a rigid walled storage tank
comprising detecting hydrostatic pressure at a level
within the liquid within the storage tank, detecting
hydrostatic pressure, at about the same level, of
-16-

liquid contained within a rigid walled reference tank,
which reference tank has thermal energy conducting
sealed walls and which is immersed within the liquid
of the storage tank and is filled to the same level as
the storage tank with the same liquid, and has the
vapor space above its liquid vented to the vapor space
above the liquid in the storage tank, and has
cross-sectional area proportional to the
cross-sectional area of the storage tank at every
level wherever it is in contact with liquid, and
providing a liquid volume leakage signal .DELTA.V
representing leakage over a time interval .DELTA.T by
operating a computer program to process the algorithm
.DELTA.V=<IMG>
where .DELTA.P is the change in detected differential
hydrostatic pressure between the storage
tank and the reference tank during a time
interval .DELTA.T,
A is the cross-sectional area of the storage
tank at the level of the liquid surface,
? is the density of the liquid in the tank,
and
g is the gravitational constant.
21. A method for determining leakage of
liquid from or into a rigid walled storage tank which
is open to the atmosphere comprising detecting
hydrostatic pressure at a level within the liquid
within the storage tank, detecting hydrostatic
pressure, at about the same level, of liquid contained
within a rigid walled reference tank, which reference
tank has thermal energy conducting sealed walls, which
is open to the atmosphere and which is immersed within
the liquid of the storage tank and is filled to the
same level as the storage tank with the same liquid,
-17-

and providing a liquid volume leakage signal .DELTA.v
representing leakage over a time interval .DELTA.T by
operating a computer program to process the algorithm
.DELTA.V = <IMG>
where .DELTA.P is the change in detected differential
hydrostatic pressure between the storage
tank and the reference tank during a time
interval .DELTA.T,
A is the cross-sectional area of the storage
tank at the level of the liquid surface,
? is the density of the liquid in the tank,
and
g is the gravitational constant.
22. A method for determining leakage of
liquid from or into a rigid walled storage tank which
is open to the atmosphere comprising detecting
hydrostatic pressure at a level within liquid within
the storage tank, detecting hydrostatic pressure, at
about the same level, of liquid contained within a
rigid walled reference tank, which reference tank has
thermal energy conducting sealed walls, which is open
to the atmosphere. and which is immersed within the
liquid of the storage tank and is filled to the same
level as the storage tank with the same liquid, and
has cross-sectional area proportional to the
cross-sectional area of the storage tank at every
level wherever it is in contact with liquid, and
providing a liquid volume leakage signal .DELTA.V
representing leakage over a time interval .DELTA.T by
operating a computer program to process the algorithm
.DELTA.V=<IMG>
where .DELTA.P is the change in detected differential
hydrostatic pressure between the storage
-18-

tank and the reference tank during a time
interval .DELTA.T,
A is the cross-sectional area of the storage
tank at the level of the liquid surface,
? is the density of the liquid in the tank,
and
g is the gravitational constant.
23. A method as defined in claim 20, 21
or 22 including the step of providing a leakage rate
signal by operating said computer program to process
the algorithm
<IMG>
where .DELTA.t is the time interval over which the pressure
change .DELTA.P is determined.
24. Apparatus for measuring leakage of
liquid from a liquid storage tank having rigid liquid
containing walls comprising:
(a) a reference tank for immersion into
the storage tank, the reference tank being rigid
walled wherever it is in contact with liquid, and
having thermal energy conducting walls, sealed within
the liquid of the storage tank, and being fabricated
of material whereby the reference and storage tank
have similar temperature coefficients of linear
expansion,
(b) means for filling the reference tank
with the same liquid and to the same height as the
liquid in the storage tank,
(c) means for sensing the change in
differential hydrostatic pressure of the liquid at
about the same level within the storage and reference
tanks, and
(d) means for determing the volume of
liquid lost from the storage tank from any sensed
change in differential hydrostatic pressure.
-19-

25. A method for determining leakage of
liquid from or into a rigid walled storage tank
comprising detecting hydrostatic pressure at a level
within the liquid within the storage tank, detecting
hydrostatic pressure at about the same level, of
liquid contained within a rigid walled reference tank,
which reference tank has thermal energy conducting
sealed walls, and which is immersed within the liquid
of the storage tank and is filled to the same level as
the storage tank with the same liquid, and has the
vapor space above its liquid vented to the vapor space
above the liquid in the storage tank, and determining
the volume of liquid leakage from the change in
detected differential hydrostatic pressure between the
storage tank and the reference tank over a time
interval.
-20-

Description

;5~
01 This invention relates to apparatus and
02 to a method for determining the rate of leakage of
03 liquid out of or into a storage tank, and i5
04 particularly useful with petroleum storage tanks.
05 For economic as well as environmental
06 hazard reasons it has become increasingly important to
07 determine whether liquid is leaking from storage
08 tanks. ~or exa~ple, the accurate measurement of the
09 amount of gasoline stored in gasoline storage tanks at
automobile service stations can help determine the
ll value of any lost gasoline and also can determine
12 whether gasoline is leaking and thus possibly
13 polluting ground water. It is also important to
14 determine whether surEace or ground water is leaking
into such storage tanks, in order to ensure that the
16 stored liquid su~h as gasoline is not becoming
17 polluted.
18 It is common to measure the amount of
19 gasoline stored in a tank by measuring the volume of
liquid in the tank at two times separated typically by
21 sev~ral hours. The amount of leakage is determined by
22 subtracting one measurement value from the other.
23 However inaccuracies in the result are
24 introduced by changes in temperature which change the
volume of stored liquid and also change the volume of
26 the tank during the interval between the two
27 measurements, and also by imperfections in the
28 measuring instrumentation. Further the leakage
29 determination is subject to the inaccuracies in the
dlfference between two large, nearly equal quantities,
31 each beinc~ determined under the inaccurate conditions
32 noted above.
33 The present invention is a structure and
34 method for avoiding the aforenoted problems and has
been shown to be capable of accurately detec-ting very
36 small amounts of leakage in relatively large tanks.
37 Instead of attempting to measure the total
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01 volume oE liquid at two different times within the
02 tank, it has been found that the measurement problem
03 is greatly simpliEied if the level of liquid in ~he
04 tank suspected of leaking is compared to the level of
05 liquid in a reference tank having a particular
06 structure. In general the reference tank should be
07 made of the same material as the main tank suspected
08 of leakage and should be filled to the same level as
09 the main tank and with an identical liquid. rrhe
reference tank is subjected to the identical
11 temperature influences as the main tank under study.
12 The liquid level in the reference tank will as a
13 result always be at the level that the main tank would
14 be if there is no leakage. The re~erence tank need
not be identical in size to the tank under study, but
16 can instead be a scale model. The desired conditions
17 regarding identical fluid and identical temperature
18 can be satisfied by inserting the reference tank
1~ inside the tank under study. A measurement o~ the
difference in liquid levels between the two tanks
21 after some time has elapsed will be indicative of the
22 rate of leakage.
23 More particularly a preferred embodiment
24 of the invention for measuring leakage of liquid into
or from a storage tank is comprised of a liquid
26 containing storage tank having rigid walls, and a
27 reference tank immersed into liquid stored in the
28 storage tank and filled with the same liquid as
29 contained in the storage tank, and having sealed and
rigid thermal energy conducting walls at least whe~e
31 it is in contact with the liquid, but having vapor
32 space above the liquid in the reEerence tank vented to
33 the vapor space in the storage tank. rrhe
34 cross-sectional area of the reference tank is
desirably (but not necessarily) proportional to the
36 cross~sectional area of the storage tank at least
37 wherever it is in contact with liquid. A sensing
38 - 2 -
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., : .: , : : .
.: ... . .. . . .. .
1 . ' .. . , : ~
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I structure is located at approximately the same levels
2 within the storage tank and within the reference tank
3 for sensing differential hydrostatic pressure between
4 liquids in the two tanks. The material of the
S reference and storage tanks should have similar
6 temperature coef~icients of linear expansion. The
7 reference tank should be filled to the same height as
8 the liquid in the ~torage stank. The volume of any
9 liquid lost from or gained by the storage tank ( V)
over a period of time (~T) is determined from the
11 expression
12
13 ~V = -
pxg
14
where ~P is the ssnsed change in differential in
16 hydrostatic pressure between the storage
17 and refer~nce tanks over the time period
18 ~T,
19 A is the cross-sectional area at the level
of the liquid surface of the storiny
21 tank,
22 p is the density of the li~uid in the tank,
23 and
24 g is the gravitational constant.
2S Another embodiment of the invention is a
26 method for determining leakage of liquid from or into
27 a rigid walled storage tank comprising detecting
2~ hydrostatic pressure at a level within the liquid
29 within the storage tank, detecting hydrostatic
pressure, at about the same level, of liquid contained
31 within a rigid walled reference tank, which reference
32 tank has thermal energy conducting ~ealed walls, and
33 which is immersed within the liquid of the storage
34 tank and is filled to the same level as the storage
tank with the same liquid, and has the vapor space
36 above its liquid vented to the vapor space above the
37 liquid in the storage tank, and determining the volume
38 of liquid leakage from the change in detected
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:. ~ . . - ,. ., :- . , . - :
- ~ , ., .:
,
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2 diPferential hydrostatic pressure between the storage
3 tank and the reference tank over a time interval.
4 Another embodiment of the invention is
S apparatus for measuring leakage of liquid from a
6 liquid storage tank having rigid liquid containing
7 walls which comprise a reference tank for immersion
8 into the storage tank r the reference tank being rigid
9 walled wherever it is in contact with liquid, and
having thermal energy conducting walls, sealed within
11 the liquid of the storage tank, and being fabricated
12 of material whereby the reference and storage tank
13 have similar temperature coefficients of linear
14 expansion, apparatus for Eilling the reference tank
lS with the same liquid and to the same height as the
16 liquid in the storage tank, apparatus for sensing the
17 change in differential hydrostatic pressure of the
18 liquid at about the same level within the storage and
19 re~erence tanks, and apparatus for determining the
volume of liquid lost from the storage tank from any
21 sensed change in differential hydrostatic pressure.
22 A better understanding of the invention will
23 be obtained by reference to the detailed description
24 below, in conjunction with the accompanying drawings,
in which:
26 Figure 1 i5 a cross-sectional schematic in
27 the vertical plane illustrating the invention, and
28 Figure 2 is a detail illustrating the
29 preferred embodiment of a pressure gauge.
Turning to the figures, a liquid storage tank
31 1 is shown which contains a liquid 2. A reference
32 tank 3 i5 inserted into the storage tank. The
33 reference tank 3 is filled to the same level w.ith
34 liquid 2 as the liquid storage tank 1. This may be
conveniently done by fitting a valve 4 into a wall of
36 the reference tank 3, preferably near the bottom of
37 the reference tank. The valve 4 may be operated by
38 remote control in a well known manner, to fill the
39 - 3a -
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s~
01 reference tank 3 from the storage tank 1, then close.
02 Pressure sensors 5 are located at about
03 the same level, one near the botto~ of the storage
04 tank and one near the bottom of the reference tank,
05 whereby the liquid pressure at the level of the
06 pressure sensor within the reference tank may be
07 determined and the liquid pressure about the same
08 depth within the storage tank may be separatel~
09 determined (or their differential). In the preferred
embodiment only one pressure sensor 5' is used,
11 connected between the two tanks, as shown in Figure 2.
12 The reference tank should be made of
13 material which conducts thermal energy easily, in
14 order that -the temp0rature of the liquid in the
storage tank should be conducted to the liquid
16 contained in the reference tank, and their
17 temperatures should remain the same. Furthermore, the
18 material of the reference tank should be such that its
19 temperature coefficient of linear expansion is similar
to that of the storage tank. The cross-sectional area
21 of the reference tank is preferably, but optionally
22 proportional at every height which is in contact with
23 the liquid to that of the storage tank.
24 The reference tank should be sealed
25 wherever it is in contact with the liquid. However
26 the vapor spaced above the reference tank should be
27 ve~ted to vapor space above the liquid in the storage
28 tank. The vapor spaced above both can be open to the
29 atmosphere, and may be vented to the atmosphere via a
30 pre~tsure relief valve.
31 It is preferred, but it is not necessary,
32 that should khe outer tank have straight vertical
33 walls, so should the inner tank. If the outer tank
3~ has walls which are bowed outwardly, 60 preferabl~
should the inner tank. For measurement of leakage
36 from petroleum storage tanks having a cylindrical.
37 cross-section and a vertical axis, the above may be
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01 accomplished by fabricating a tube having a sealed
02 bottom (with the ancillary filler valve if desired as
03 described above), the tube being made out of the same
04 material (e.g. steel) as the storage tank. The
05 reference tank can then be inserted into the liquid of
06 the storage tank through its open filler hole at the
07 top of the storage tank. The filler valve may then be
0~ closed by mechanical or solenoid means in a manner
09 known in the art. However the storage tank often will
be cylindrical with a horizontal axis, and in this
11 case the reference tank can be a cylindrical tube
12 having a vertical axis.
13 A frame carrying pressure sensors may be
14 used to lower pressure sensors one on each side of -the
outer wall of the reference tank into the liquid to
1~ the bottom of the tanks. One pressure sensor thus
17 determines the liquid pressure within the reference
18 tank and the other pressure sensor determines the
19 liquid pressure at about the same height within the
storage tank. However it is preferred that a single
21 pressure sensor 5' should be connected through the
22 wall of the reference tank in order to sense the
23 differential pressure between the liquid in the
24 reference tank and in the storage tank.
Because the reference tank is in very
26 intimate thermal contact with ~he liquid in the
27 storage ~ank, the temperature of the liquid in the two
28 tanks will be very nearly identical. Small changes in
29 the temperature of the liquid in the outer tank will
be transmitted to the reference tank and the liquid
31 therein. Similarly changes in temperature of the
32 storage tank will be transmitted by the liquid to the
33 reEerence tank and the liquid contained therein.
34 Temperature changes causing thermal expansion or
contraction of the liquid and/or the tanks has been
36 found to cause the liquid level in both tanks to rise
37 and fall in almost exactly the same manner. A
38
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01 differential measurement of the hydrostatic pressures
02 at a convenient level below the liquid sur~ace of each
03 tank would show no changes over the time period T
04 unless liquid was leaking into or out of the storage
05 tank.
06 The volume of any liquid lost from or
07 gained ~y the storage tank (aV) over a period of -time
08 aT is determined by apparatus processing the algorithm
a V = ~PxA
~xg
11 where aP is a signal representing the sensed change
12 in differential hydrosta-tic pressure between
13 the storage and reference tanks during the
14 lapsed time ~T,
A is a signal representing the cross-sectional
16 area at the level of the liquid surface of
17 the storage tank,
18 ~is a signal representing the density of the
19 liquid in the tank, and
g is a signal representing the gravitational
21 constant.
22 The density o~ the liquid in the tanks may
23 be estimated to sufficient accuracy for most
24 applications, or may be determined by other means
which is not part of the present invention.
26 The algorithm noted above may be processed
27 by applying signals representing the sensed pressure
28 differential generated by the pressure sensors 5 or
29 pressure sensor 5' to the input of a microcomputer
or other processor, which preferably has a display 7
31 associated with it. The display may be located at a
32 remote operator position, and may be Eor example, a
33 cathode ray or other instantaneous display, or may be
34 a printer, and may include signal storage mean~ Eor
storing output signals from the microcomputer for
36 later absolute or comparative display. The pressure
37 signals could be transmitted to the microcomputer or
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01 other signal processing apparatus located at a remote
02 location.
03 The pressure sensors 5 or 5' can be
04 sensors which translate liquid pressure to electrical
05 signals, which can be transmitted directly to an
06 interface (not shown) connected to microcomputer 6.
07 Preferably however the pressure readings may
08 accurately be obtained by use of bubble tubes as
09 described in Canadian patent 1,201,532 which issued
March 4th, 1986, in conjunction with bubble plates
11 described in Canadian patent application 500,905,
12 Eiled January 31st, 1986.
13 The chanye in volume ~V can be converted
14 to a leakage rate over time ~at) by processing the
algorithm dV/~t. This will provide an indication of
16 the severity of the leak. This algorithm may also be
17 processed by microcomputer 6 or by other means
18 provided to operate it.
19 It should be noted that if the leak rate
need not be known to a high degree of accuracy, it is
21 not necessary to know ~P, A,f, g or at -to a very high
22 degree of accuracy; yet the existence o~ a leak
23 becomes evident very clearly using the present
24 invention. This distinguishes from systems which
involve measuring the total volume in the storage tank
26 over a period of time in which the exact volume must
27 be known to a high degree of accuracy.
28 It is important in this invention that the
29 pressure sensors should be sensitive enough to
indicate a small change in pressure. ~Iowever they
31 need not be very linear, nor very accurate.
32 Since typical tank materials, e.~. steel,
33 do not change dimensions very much ~ith temperat~re,
3~ some difference in rela~ive temperature of the two
tanks can be tolerated, with benefits of the present
36 invention still being obtained. For cylindrical steel
37 tanks having a vertical axis as shown in the figure,
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01 it has been found that differences in relative
02 temperature of up to + 0.4C. can be tolerated and the
03 apparatus still resolve leakage to within one part in
04 lOo,ooo.
05 Because the volume of most liquids has
06 been found to vary rather strongly with temperature,
07 it is important that the liquid within the two tanks
08 should be subjected to the same temperature changes.
09 The intimate contact between the liquids in the
reference tank and storage tank ensures that this will
11 be the case.
12 The two pressure sensing points, one in
13 each tank, do not have to be exactly at the same
14 level. A difference would result in an initial value
of di~ferential pressure which is not ~ero. The
16 pressure change a P to be used in the fundamental
17 algorithm would be the amount by which the
18 differential pressure changes over the duration o~ the
19 test.
Difference in height of the two pressure
21 sensing points will make the operation somewhat ~ore
22 sensitive to temperature a-ffects, but this has been
23 determined as not being critically so. For example in
2~ a typical underground storage tank a height
di~ferential of 1 millimeter would cause the observed
26 leak rate to be in error by about 1% for every degree
27 Celcius that the temperature of the liquid changed
28 during the course o~ ~he test. This has been ~ound to
29 ~e a neyligible error for most purposes.
The present invention has been found to be
31 highly useul to service station operators and in
32 other petroleum storage facilities. It is of course
33 not limited thereto, and can be used for such
34 applications as water storage tanks, milk storage
tanks, heavy oil storage tanks, liquid chemical
36 storage tanks, etc.
37 The entire structure is also highly
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01 adaptible to remote control sensing. Signals from the
02 pressure sensors can be transmitted via a network to a
03 cen-tral computer which receives similar signals from a
04 large number of facilities, providing constant
05 monitoring of leakage by a central control computer,
06 for display to management personnel.
07 The reference tank with differential
08 pressure gauge can be made portable and used as needed
09 in connection with different storage tanks.
A person skilled in the ar-t understanding
11 the present invention may now conceive of variations
12 or other embodiments using the concepts descr;.bed
13 herein. All are considered to be within the sphere
14 and scope of the invention as defined in the claims
appended hereto.
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Representative Drawing