CA1310719C - Leak detection system for storage tanks - Google Patents

Abstract

LEAK DETECTION SYSTEM FOR STORAGE TANKS
ABSTRACT OF THE INVENTION
A leak detection system for detecting the leakage of fluids from storage tanks uses a reference container, such as a standpipe, having its bottom adjacent the level of the bottom of the tank, which is initialized to the same hydrostatic head pres-sure as the tank, and then closed off. The differential pressure between the hydrostatic head pressures of the standpipe and the tank is monitored over time in an externally mounted differential pressure gauge in order to detect the leakage of fluid from the storage tank. Shut off of valves are provided in a tank pressure pipe and a reference pressure pipe to isolate both sides of the differential pressure gauge. The very high sensitivity of avail-able differential pressure gauges allows the detection of very small leaks and very low leakage rates. A leak detection system of unitary construction may be provided in which the standpipe is provided with an equalizing valve so that the system may be in-stalled through a single hole in the top of a liquid storage tank. In alternate embodiments the standpipe is mounted inter-nally and external to the tank and in an external extension of the tank. In another embodiment a vacuum system is used to draw fluid equally through the differential pressure gauge from both the tank and the standpipe to develop equal negative pressures on each side of the gauge.

Description

L~I~LD OF THE INVENTION
This invention relates generally to the field of fluid measurement systems, and pertains more particularly to systems for measuring the rate of leakage of fl~lid from storage tanks.

B KC.ROUND OF THE INV~NTION
Detecting leakage from fluid storage tanks is a very important problem. For example, leaks from large liquid fuel storage tanks can be very damaging, and detecting such leaks is important in order to allow repair or preventive measures.
Detecting leaks when the leak itself is very small or the leakage rate is slow is very difficult using presently available techniques.
The use of pressure transducers to measure the level of a liquid in a tank by measuring the hydrostatic head pressure of the tank is known. However, detecting a small leak by observing changes in the indicated tank level is very difficult using such pressure measurement techniques. This difficulty is caused by the fact that only very small changes in tank level will be observed, and thus the change in the measured hydrostatic head pressure will be very small due to leakage. Also, temperature changes and level changes due to evaporation may be falsely interpreted as leakage indications. The leakage of fluid from tan~s may be very undesirable for a number of reasons, including the wastage of costly tank contents and pollution produced by leakage that creates environmental contamination.
U.S. Patent No. 3,538,746 of Jacobs shows a s~stem with a submerged dif~erential pressure gauge and electrical conductor 36 subject to hazard if used with combustible fluid. Equal head pressure is not maintained. Also, the bottom of pressure chamber 32 is substantially above the bottom of tank 12 creating temperature errors.
U.S. Patent No. 4,646,560 of Maresca, Jr., et al, maintains equal head pressures but again the DPU is submerged.
U.S. Patent No. 3~460,386 of Guignard, U.S. Patent No.
4,627,281 of Tavis, U.S. Patent No. 4,389,888 of Morooka, U.S.

~3tQ7lq 72~32-36 Patent No. 3,537,298 of Knap~f, U.S. Patent No. 3,921,436 of Plegat, U.S. Patent No. 3,939,383 of Alm and U.S. Patent No.
3,527,909 of Torre are considered less pertinent.

OBJE~TS OF T~IE INVENTION
Therefore, it is an object of this invention to provide a lealc detection system which may be used with fluid storage tanks for detecting leakage.
Another object of the invention is to provide a leak detection system for fluid storaye tanks in which very small leaks may be detected, and which is relatively insensitive to temperature changes or evaporation of the fluid.
Further objects of this invention are to provide a leak detection system for fluid tanks with a very high sensitivity, great repeatability, low cost, low level of co~plexity, and being easy to maintain and install.

SUMMARY 0~ THE INVENTION
According to a prin~ipal aspect of the present invention, there is provided a leak detection system to measure the rate of leakage from a tank by detecting the differential fluid pressure between the hydrostatic head pressure of the tank and a reference standpipe pressure. The standpipe is connected to the tank through an equalizing valve so that fluid may be allowed to flow from the tank into the standpipe through the equalizing valve in order to establish a reference hydrostatic head pressure inside the standpipe, which therefore makes the reference pressure equal to the hydrostatic head of the tank at that particular point in time. Next, the equalizing valve is closed in order to seal off the standpipe, and then the differential pressure between the standpipe and the tank is measured for a period of time in order to detect the rate of ,~, .. ...

1 3 1 07 1 q 7~43~-~6 leakage of fluid from the tank. The standpipe may be positloned inside the tank ltself or may be mounted externally. The standpipe and equalizlng valve may be constructed as an integral unit for mountin~ through a single hole in the top of a liquid storage ~ank. Standpipe and tank shutoff valves ma~ be connected either to the input sides of a differential pressure gauge or to opposite sides to create an equal negative pxessure thereon.
In accordance with the present invention there is provided a leak detection system for detecting the leakage o~ a fluid from a storage tank having a bo~tom comprising in combination: a differential pressure gauye mounted external to said tank having a reference pressure port and a tank pressure port; a standpipe being mounted in conjunction with said tank, said standpipe having a bottom at a level adjacent that of the bottom of said tank and having an internal reference volume of said fluid; a standpipe shut-off valve being mounted in a reference pressure pipe coupled between said standpipe and said reference pressure port; a tank shut-off valve being mounted in a tank pressure pipe coupled between said tank and said tank pressure port; an equalizing valve coupled between said standpipe valve and said tank valve, said equalizing valve being open for a period of time to allow the fluid volume in said standpipe to raach the same level as the fluid level inside said tank, and said e~ualizing valve belng closed after equilibrium is reached; means responsive to indicate the dif~erence detected by said gauge in the fluid pressure between said standpipe and tank with said standpipe shut-o~f valve and tank shut-off valve being in the open ~`~ 4 131071~

conditionr whereby the indic~tion ls a measure of the leakage of fluid from said tank; and one side of said s~andpipe shut~off valve and one side of said tank pressure valve are mounted in said reference pressure pipe and said tank pressure pipe respectively on a side of differential pressure transducer away from said reference pressure plpe and said tank respectively and the opposite side of said standpipe and tank shut-off valves are each connec~ed through one side of a bleeder valve to a liquid trap, sald liquid trap having a vacuum pump connected thereto in order to draw a vacuum which will draw liquid from both standpipe and tank into said trap to create an equal negative pressure on both sides of said differential pressure gauge.

BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a side cut-away view of a liquid storage tank with an internally mounted standpipe and the external leak detection system components shown in symbolic form;
Fig. 2 is a side cut-away view of a liquid storage tank and externally mounted standplpe assembly having both tank fluid and reference fluid levels with leak detection system components shown symbolically;
Fig. 3 ls a cut-away side view of a storage tank and externally mounted standpipe assembly having only a reference fluid level with leak detection components shown symbolically; and Fig. 4 is a side cut-away view of a standpipe assembly and tank e~ualizing valve in accordance with an alternate embodiment.

4a 1 3 1 07 1 q DESCRIPTION OF THE PREFERRED EM~ _IMENTS
Referrlng now to the drawingæ in detail, and first to Fig. 1, a leak detection system 10 is shown which is designed for use with a fuel storage tank 12. The system 10 is constructed to detect the leakage of a liquid fuel 14 ~rom inside the -tank 12.
The construction of the leak detection system 10 is such thak leaks will be detected irrespective of the level of the liquid fuel 14 inside the tank 12, and irrespective of ambient temperature changes or evaporation of the liquid fuel 14 inside the tank 12.
The leak detection system 10 uses a differential pressure gauge 16 for detec~ing small differences in hydrostatic : ; 4b 131071q 7243~-3 pressure. The gau~e 16 is pre~erably an ITT Barton model 6001 di~ferentlal pressure transml~er, a commerclally a~ailable pro-duct. The gauge 16 produces an output electrlcal signal on a sig-nal wire 18 lndicating the measured dlf~erential pressure. For application with most llquid ~uels, it is preferable that the gau~e 1~ have an lnput hydrostatlc ~lead pressure dlf~erential range o~ from -0.2 to +0.2 lnches o~ water column (W.C.) For this lnput pressure range, it ls pre~erable that the gauge 16 produce an electrical slgnal output on the signal wire 18 ln the forrn of l~ electrlcal current spannlng a range of 4 to 20 mllllamperes ~mA).
The preferred dlfferentlal pressure gauge 16 has a sensitivity to changes at least as small as 0.001 inch W.C. for the small hydro-static head pressure changes to be measured and it may be prefer-able for some applicatlons to have a sensltlvlty of 0.0001 lnch W.C .
The slgnal wire 18 is connected to the measurement instrument 20 ln order to allow electrlcal measurements to be made to detect the presence of leaks ln the tank lZ and the rate of leakage of the llquld 14 from the tank 12. Varlous designs may be used for the constructlon of the measurement lnstrument 20, de-pending on the partlcular way in whlch the leak detection system 10 ls to be used. The measurement lnstrument 20 is located out-side the tank 12 in order to avold problems assoclated wlth elec-trlclty withln tank 12 particularly if liquid 14 is combustlble.
The measurement instrument 20 may consist oE a conven-tional electrical indicator, such as a moving coll meter driven by the electrical signal on the wire 18, so that the full scale range 7243~-36 o~ the meter pointer deflectlon represents a span of -0.2 to +0.2 inches of W.C. detected by the gauge 16. Uslng such an electrical lndlcator for the instrument 20 wlll involve visual observatlon by the operator, who will manual:Ly note the before and after meter indicator readings, and the time elapsed between readlngs, from which the leakage rate from the tank 12 may be manua:lly cal-culated. In place of or ln addltlon to an electrlcal lndlcator for the measurement lnstrument 20, a recorder, such as a conven-tional moving pen strip chart recorder, may be used to provide a graphlc record of measured differential pressure changes by the gauge 16 agalnst a tlme base provlded by the recorder.
In p.lace of or ln combinatlon with an electrical indica-tor and recorder for the measurement instrument 20, a mlcro-processor hased recordlng/computing devlce programmed for varlous tlme perlods and engineerlng unlts may be used to perform the re-cordlng and leak rate calculatlon functlons. Commercially avall-able recording/computing devlces such as the ITT Barton MC 3000 or MC 1000 products may be used for this purpose, and may be inter-faced with commerclally avallable prlnters. Qne recordlng\-computlng devlce may be used as the measurement instrument 20 toprocess electronlc slgnal data recelved from several dlfferent leak detectlng pressure gauges, such as gauge 16, ln a large system ln whlch leaks are to be detected from several tanks.
The plplng connectlon arrangement ls a very lmportant part of the leak detectlon system 10. In Flg. 1, a l" dlameter standpipe 22 ls mounted to e~tend vertlcally upwards lnside the fuel storage tank 12 substantially from the bottom thereof. The 13tU119 standplpe 22 ls connecte~ by a reference pressure plpe 24 e~tend-ing through the sidewall o~ the tank 12, and through a standpipe shutoff valve 26 to the low pressure slde port 28 of the gauge 16.
A tank pressure plpe 30 extends through the sidewall of the tank 12 and through the tank shuto~f valve 32 to the high pressure side port 34 of the gauge 1~. The pipe 24 hydraullcally couples the lnslde of the standplpe 22 to the port 28, and the plpe 30 hydraulically couples the lnside of the tank 12 to the port 34 both at polnts substantlally on the bottom of tank 12 and at the same fluld level. The gauge 16 measures the differences ln pressure ~etween the ports 34 and 28. An equallzlng plpe 36 extends through an equallzing valve 38 in order to connect the reference pressure plpe 24 and the tank pressure pipe 30. The equalizing valve 38 may be a manually operated type or lt may be an electrlcal solenoid valve which may be remotely operated by a manually operated swltch (not shown) or b~ clrcultry ln a recorder/computer devlce used for the measurement lnstrument 20.
Referrlng next to Flg. 2, a leak detectlon system 40 is shown whlch is simllar in purpose to the system 10 of Flg. 1 and in~ludes analogous parts including a tank 42, liquld fuel 44, differentlal pressure gauge 46, slgnal wlre 48, measurement ln-strument 50, standpipe 52, reference pressure pipe 54, standpipe shutoff valve 56, low pressure side port 58, tank pressure pipe 60, tank shutoff valve 62, a hlgh pressure slde port 64, an equal-izlng plpe 66, and an equallzlng valve 68.
The leak detectlon system 40 o~ Fig. 2 also includes a measurement tank 70 whlch ls mounted externally to the tank 42 and 72~3~ 36 which contains the standplpe 52. A tank transfer plpe 72 and a trans~er shutoff valve 74 are provided between the tanks 42 and 70 ~n or~er to allow fuel 44 to flow from the tank ~2 into the tank ~0. An upper vent p:lpe 76 sloped slightly downward toward tank 42 is provided to allow the ~ree flow of vapors between the upper head spaces of tanks 42 and 70. In the event tank 42 has a floatlng head on the top of fuel ~4, both tanks 42 and 70 are vented to atmosphere. The liquid fuel 78 in the tank 70 is allow-ed to rlse to the same level as the ~uel 44 in tank 42 when the transfer shutoff valve 74 is open. The gauge 46 and measurement lnstrument 50 operate ln the same way as gauge 16 and measurement lnstrument 20, respectively.
The system 40 of Fig. 2 may be advantageous in situa-tions where the system 10 of Fig. 1 may be unsuitable. For example, the system 40 would eliminate some of the costs assocla-ted with the lnstallation of the system 10. That is, the system 40 may be installed on an existing tank 42, lncludlng connectlon of the pipes 72 and 76 without requirlng the complete drainlng of the tank 42 of its fuel 44 contents, wlthout re~uirlng the removal ~0 of residual fuel traces from lnside the tank 42, and without having to send workmen inside the tank 42 for lnstallation of the leak detection system.
Referring next to Fig. 3, a leak detectlon system 84 is shown whlch is similar in function to the systems 10 and 40 shown in Figs. 1 and 2 respectlvely. The system includes a standpipe 86 connected to tank 88 at the bottom by equalizer pipe 90 having an equalizln~ valve 92. Again an upper vent pipe 94 sloped slightly downward toward tank 8~ assures the same vapor pressure in the heads of standplpe 86 and tank 88. A DPU 96 is connected to the bottoms of standplpe 86 and tank 88 through con~ui~s g8 and 100 respectlvely which have shutoff valves 102 and 104 respectlvely.
Referring ne~t to Fig. 4, there ls illustrated a flange 106 used to mount standpipe 108 through the top of a tank contain-ing liquid (not ~llustrated). A vent 110 connects the top space of standpipe 108 to the top space of the tank to equalize vapor pressures therein. An equalizing valve 112 may be pneumatically actuated through conduit 114 to open and equallze the fluid levels in standpipe 108 and the tank, as e~plained previously, through port 116. Fluid conduits 118 an~ 120 are connecte~ from the bottoms of standpipe 108 and the tank throuqh port 122 through the two sides 124 and 126 of DPU 128 respectively and thence through shutoff valves 130 and 132 respectively and are ~oined at 134 to one side of shutoff valve 136. The other side of valve 136 is connected by conduit 138 to liquid trap 140 which is connected in turn through conduit 142 to a vacuum pump 144.
OPERATION OF THE INVENTION
Using the leak detection system 10 of Fig. 1, with the level of the liquid fuel 14 at some point above the pipes 24 and 30; the equalizer valve 38, and shutoff valves 26 and 32 are open-ed. The shutoff valves 26 and 32 are kept open during measure-ments, but the equalizer valve 38 is only kept open for a suffi-cient perlod of tlme to allow the llquid inslde the standpipe 22 to reach the same level as the llquld fuel 14 lnslde the tank 12.
When this equillbrlum is reached, the output current electrlcal 7~3~--36 slgnal on the slynal wire 18 ~rom yauge 16 will be 12 mA, repre-sentln~ zero differential pressure between the ports 28 and 34 of the gauge 16.
After the appropriate tlme has passed to reach the equi-librium condltlon descrlbed above, the equalizer valve 38 i5 clos-ed, locking or trapping the liquid in the standpipe 22 so that no liquid is allowed to flow into or out o~ the standplpe 22 throuyh the reference pressure plpe 24. At the instant that the equalizer valve 38 ls closed, the hydrostatlc head pressure ls the same on both the high and low pressure sl~e (34 and 2~ respectlvely) of the gauge 16; i.e. zero dlfferential pressure exlsts across the gauge 16, and the signal output on the wlre 18 is 12 mA. When the system lO ls placed ln thls leak detectlon mode wlth the standplpe 22 filledl valves 2Ç and 32 open, and the equalizlng valve 38 closedl the system lO will detect any small changes produclng dlfferences between the llquid level ln the standpipe 22 and the llquld level in the tank 12 by measuring the dlfferentlal pressure across the gauge 16. Small changes ln the llquid level of liquid 14 ln the tank 1~l when not matched by slmilar chanyes ln the liquld level in the standplpe 22l will produce a pressure dlffer-ence detected by gauge 16 lndicatlng leakage from the tank 12.
The gauge 16 acts to measure the dlfference between: the llquld column hydrostatlc head pressure of the tank 12 taken at the ele-vatlon of the gau~e 16; and the llquld column hydrostatlc head pressure of the standpipe 22 taken at the elevation of the gauge 16. Because of the high sensitivity of the gauge 16l very small leaks and low leakage rates from the tank 12 may be detected.

1 3 1 ()~ 1 ~
7~32-36 One dynamic condltlon tha~ should be considerecl b~ the user of the ~ystern lo ls that a~dlng to or drawln~-off the ll~uid fuel 14 ~rom the tank 12 ~ill produce a change ln the level of fuel 14 lnside the tank 12, an~ wll:L produce a ~alse indicatlon of leakage by creatiny a differential pressure across the gauge 16.
Temperature changes of the fuel 14 lnslde the tank 12 will effect the speclfic gravity of the fuel 14, resultlng ln level changes in both the standplpe 22 and the tank 14. However, both columns of llquid are affected eyually with the two liquld levels rising by the same amounts since they are of substantially the same helght, with llquid inside the standpipe 22 experiencing the same speclfic gravlty changes as the liquld 14 inside the tank 12, therefore, no erroneous indication of leakàge by the syskem 10 ls expected due to any temperature changes of the ll~uld 14.
If the temperatures and pressures are the same on the upper surface of the llquid (ln the head space) lnslde the stand-plpe 22 and on the upper surface of the llquld fuel 14 (ln the head space) lnslde the tank 12, evaporatlon effects inslde the standplpe 22 and the tank 12 will be equal. The standplpe 22 has lts upper end 23 vented lnto the head space 15 of tank 12 ln order to make sure that the amblent pressures and temperatures applled to the llqulds ln the standpipe 22 and tank 12 are equal. There-fore, no erroneous lndlcatlon of leakage by the system 10 due to evaporatlon of the fuel 14 ls expected.
As ls true wlth most electronlc lnstrumentatlon, amblent temperature changes may produce changes ln the performance of the lfferential pressure gauge 16. There are ways of reducing such ~ , 131071q 72~2-36 temperature effects, such as the use of an amblent temperature sensor (not shown), whose output signal would be used as an lnput to the measurement system 20 ln order to allo~ for c~mpensatlon of errors resultlng from amblent temperature changes.
The shutoff valves 26 and 32 are usually kept open con-tinuously during the use of the system 10, ~ut may be ~urned off for malntenance of the gauge 16. The equallzlng valve 38 ls kept closed durlng the measurement of leakage ~rom tank 12, but is opened when the tank 12 ls belng fllled or emptled of llquld.
The leak detectlon system 40 of Flg.2 is operated in much the same way as the system 10, proceedlng wlth the use of the shutoff valves 56 and 62, and the equalizing valve 68. The transfer shutoff valve 74 ls kept open during operatlon of the leak detectlon system 40.
Fluid flow through the pipe 72 makes the liquid level inslde the tanks 70 and 42 the same. Of course, lf the tempera-ture and other conditlons ln the tanks 70 and 42 are or become dlfferent, then there may be extraneous effects on the measurement of the level of tank 42 lndlcated by the instrument 50. The shut-off valve 74 ls ordinarlly kept open, but ls closed for malnten-ance of the tank 70. The upper vent plpe 76 connects the head spaces of tank 42, tank 70 and standplpe 52 so that llqulds lnside each will be exposed to the same amblent temperature and pressure or ln the case of a floatlng top on tank 42 to atmosphere.
The leak detectlon system 80 of Flg. 3 operates ln a manner very slmllar to the systems 10 and 40 of Figs. 1 and 2.
The equallzer valve 92 ls closed unless actuated, and ls opened by 1 3 1 07 1 q an alr slgnal for example allowing the fluld in the tank ~8 to move ln or out of the standpipe ~6 through the condult 9o, until the fluld level lnside the standplpe 86 ls equal to the fluid level ln the tank 88~ When the ~wo ~luld levels are e~ual, the equallzlng valve 92 ls closed by removlng the air pressure. If the fluld level in the tank subsequently drops due to a leak, the hydrostatic pressure change ~ill be sensed on the pressure gauge 96.
After the installation of standplpe 108 of ~lg. 4 ln the tank e~uallzlng valve 112 ls opened by a remote alr signal through valve 146 to allow the fluid ln the standplpe 108 to equallze wlth the level ln the tank, then valves 130, 132 and 136 are opened.
Vacuum pump 144 ls then operated until enough fluld appears in trap 140 indlcatlng both senslng lines 118 and 120 are full. Valve 136 is then closed to allow the system to stablllze. E~uallzer valve 112 ls then closed lsolatlng the fluld ln standplpe 108 from the tank.
Then valves 130 and 132 are closed creatlng an equal negatlve pressure on both sldes of DPU 128. Any change ln the level of the tank wlll then create a change in the pressure on the tank side 126 of the DPU 128. The output changes of the DPU 128 can then be monltored for the requlred time period indlcatlng a leak1 lf any.
The baslc operatlng theory for the leak detectlon systems of thls lnventlon ls that measurlng the leakage of a fluid from a tank may be accomplished by trapplng a reference volume of the fluid at the same pressure as the initial tank pre~sure, and 12a 1 ~ 1 (3~
7243~-36 subsequently measurln~ the dif~erence between the reference volume pressure and the kank pressure as the tank leaks. The avail-ablllty of highly sensitive dl~ferentlal pressure gauges makes possible the detectlon of very small leaks and very small leakage rates. In the case of liquids, leakage is detected by measuring the level of the llquld ln a tank, wlth a falling liqul~ level lndlcatlve of leakage from the tank. The llquld le~el measurement ls not measured directly, but is measured wlth reference to an lnltlal level, thls belng accompllshed by measuring the hydro-static head pressure dlfference between the tank and a referencestandplpe. Hlghly sensltlve dlfferentlal pressure gauges allow such measurements of small pressure changes due to leaks even in the presence of hlgh common-mode hydrostatlc pressures.
The leak detection systems of thls lnventlon may be used to meet a need only recently recognlzed, that ls, the need to periodlcally check the performance of storage tanks. FGr example, in order to prevent dangerous leakage of toxlc materlals, some governmental agencles requlre scheduled measurements of leakage rates from tanks used for toxic llquld storage. Speclfically, the annual measurement of leakage from underground gasollne storage tanks at servlce statlons may be requlred. The temporary lnstal-latlon of a leak detectlon system as shown ln Flg. 4 ls partlcu-larly valuable for thls purpose. Such a system may be slmply low-ered from the top of the tank into the tank, and left in place only long enough to make the deslred measurement. Such a system, as in Flg. 4, ls both convenient, and portable. Electrlcal func-tions are also kept out~ide the tank to avoid problems associated therewlth lf the fluid is combustlble.

12b ~ .

Claims (2)

1. A leak detection system for detecting the leakage of a fluid from a storage tank having a bottom comprising in combination:
a differential pressure gauge mounted external to said tank having a reference pressure port and a tank pressure port;
a standpipe being mounted in conjunction with said tank, said standpipe having a bottom at a level adjacent that of the bottom of said tank and having an internal reference volume of said fluid;
a standpipe shut-off valve being mounted in a reference pressure pipe coupled between said standpipe and said reference pressure port;
a tank shut-off valve being mounted in a tank pressure pipe coupled between said tank and said tank pressure port;
an equalizing valve coupled between said standpipe valve and said tank valve, said equalizing valve being open for a period of time to allow the fluid volume in said standpipe to reach the same level as the fluid level inside said tank, and said equalizing valve being closed after equilibrium is reached;
means responsive to indicate the difference detected by said gauge in the fluid pressure between said standpipe and tank with said standpipe shut-off valve and tank shut-off valve being in the open condition, whereby the indication is a measure of the leakage of fluid from said tank; and one side of said standpipe shut-off valve and one side of said tank pressure valve are mounted in said reference pressure pipe and said tank pressure pipe respectively on a side of differential pressure transducer away from said reference pressure pipe and said tank respectively and the opposite side of said standpipe and tank shut-off valves are each connected through one side of a bleeder valve to a liquid trap, said liquid trap having a vacuum pump connected thereto in order to draw a vacuum which will draw liquid from both standpipe and tank into said trap to create an equal negative pressure on both sides of said differential pressure gauge.

2. The combination of claim 1 in which said standpipe is mounted internally in said tank.