CA1185693A - Hydrocarbon leakage detection system and apparatus - Google Patents
Hydrocarbon leakage detection system and apparatusInfo
- Publication number
- CA1185693A CA1185693A CA000372936A CA372936A CA1185693A CA 1185693 A CA1185693 A CA 1185693A CA 000372936 A CA000372936 A CA 000372936A CA 372936 A CA372936 A CA 372936A CA 1185693 A CA1185693 A CA 1185693A
- Authority
- CA
- Canada
- Prior art keywords
- tank
- sensing
- pressure
- perforation
- bubbles
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B67—OPENING, CLOSING OR CLEANING BOTTLES, JARS OR SIMILAR CONTAINERS; LIQUID HANDLING
- B67D—DISPENSING, DELIVERING OR TRANSFERRING LIQUIDS, NOT OTHERWISE PROVIDED FOR
- B67D7/00—Apparatus or devices for transferring liquids from bulk storage containers or reservoirs into vehicles or into portable containers, e.g. for retail sale purposes
- B67D7/06—Details or accessories
- B67D7/32—Arrangements of safety or warning devices; Means for preventing unauthorised delivery of liquid
- B67D7/3209—Arrangements of safety or warning devices; Means for preventing unauthorised delivery of liquid relating to spillage or leakage, e.g. spill containments, leak detection
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B67—OPENING, CLOSING OR CLEANING BOTTLES, JARS OR SIMILAR CONTAINERS; LIQUID HANDLING
- B67D—DISPENSING, DELIVERING OR TRANSFERRING LIQUIDS, NOT OTHERWISE PROVIDED FOR
- B67D7/00—Apparatus or devices for transferring liquids from bulk storage containers or reservoirs into vehicles or into portable containers, e.g. for retail sale purposes
- B67D7/06—Details or accessories
- B67D7/78—Arrangements of storage tanks, reservoirs or pipe-lines
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M3/00—Investigating fluid-tightness of structures
- G01M3/02—Investigating fluid-tightness of structures by using fluid or vacuum
- G01M3/04—Investigating fluid-tightness of structures by using fluid or vacuum by detecting the presence of fluid at the leakage point
- G01M3/24—Investigating fluid-tightness of structures by using fluid or vacuum by detecting the presence of fluid at the leakage point using infrasonic, sonic, or ultrasonic vibrations
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M3/00—Investigating fluid-tightness of structures
- G01M3/02—Investigating fluid-tightness of structures by using fluid or vacuum
- G01M3/26—Investigating fluid-tightness of structures by using fluid or vacuum by measuring rate of loss or gain of fluid, e.g. by pressure-responsive devices, by flow detectors
- G01M3/32—Investigating fluid-tightness of structures by using fluid or vacuum by measuring rate of loss or gain of fluid, e.g. by pressure-responsive devices, by flow detectors for containers, e.g. radiators
- G01M3/3236—Investigating fluid-tightness of structures by using fluid or vacuum by measuring rate of loss or gain of fluid, e.g. by pressure-responsive devices, by flow detectors for containers, e.g. radiators by monitoring the interior space of the containers
- G01M3/3245—Investigating fluid-tightness of structures by using fluid or vacuum by measuring rate of loss or gain of fluid, e.g. by pressure-responsive devices, by flow detectors for containers, e.g. radiators by monitoring the interior space of the containers using a level monitoring device
Landscapes
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Engineering & Computer Science (AREA)
- Mechanical Engineering (AREA)
- Examining Or Testing Airtightness (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A method and apparatus for detecting perforations in an underground fuel storage tank, employs the creation of a partial vacuum in the tank and a pressure difference across the tank wall, to cause air to pass through the perforation, and a hydrophone to sense the acoustic waves produced by the formation of air bubbles in the fuel. The acoustic waves of the bubbles caused by the air passage are sensed by a hydrophone and processed to provide an electrical signal indicative of the presence of the perforation. A water level sensor is also used to detect the level of water beneath hydrocarbon fuel in the tank. The method and apparatus can also be used with above ground fuel storage tanks and other types of above and below ground storage tanks.
A method and apparatus for detecting perforations in an underground fuel storage tank, employs the creation of a partial vacuum in the tank and a pressure difference across the tank wall, to cause air to pass through the perforation, and a hydrophone to sense the acoustic waves produced by the formation of air bubbles in the fuel. The acoustic waves of the bubbles caused by the air passage are sensed by a hydrophone and processed to provide an electrical signal indicative of the presence of the perforation. A water level sensor is also used to detect the level of water beneath hydrocarbon fuel in the tank. The method and apparatus can also be used with above ground fuel storage tanks and other types of above and below ground storage tanks.
Description
FIELD OF THE lNVENTION
The present invention relates to a method of ana apparatus for detectiny a perforation in a tank containing a liquid.
~ACKGR~UND O~ T~ INVENTION
Gasoline, diesel fuel and heating oil are the most widely used hydrocarbons and are commonly stored undergrouna at retail service stations, bulk service stations, municipal garages, etc. ~uch liquid hydrocarbons are normally storea in underground tanks and there is a risk that the hydrocarbons may escape into the ground from perforations which develop in these underground storage tanks.
Tank leakage problems are recogniæed by the petroleum marketing industry and by governmental and environmental agencies.
Furthermore, a leaking tank is generally undiscovered until a flagrant appearance of hydrocarbons is traced to it.
The tracing procedure can be long and costly because of factors such as unusual soil strata, a network of backfilled trenches, large and frequent variations in the height o~ the water table or a dense concentration of underground tanks.
Perforations in tanks usually develop from corrosion;
internal corrosion is less prevalent than external corrosion, but does occur, especially in a narrow band along tne tank bottom~ 'L'his is because water, whlch separates from the hydrocarbons is more dense than the hydrocarbon and sinks to form a layer at the bottom cf the tank. Internal corrosion is .,, ~
~ ., aggravated un~er the fill pipe because protective rust is removed by the flow of liquid against the tank bottom during filling or by the impact and movement of a gauge stick DE~SCRIPTION OF T~IE PRIOR_ARI' It has previously been proposed to detect perforations in a tank by detecting the sound generated by the egress of liquid through the perforation and by separating this sound from background noise, which is frequently stronger than the sound being sought.
A method utiiizing acoustic detection has been proposed, requiring the use of two or more hydrophones on a rotatable arm. Irhis method was aesiyneo primarily for large above-ground tanks and intended to detect, by triangulation ano phase correlation~ the source of the sound generated by fluid leaking tnrough the perforation. One of the disadvantages of this prior proposal is that, while the passase of fluid through a leak creates random noise, the intensity of which increases with pressure, this noise may be lost in background noise when the pressure is only that of the fluid head within the tank.
Most underyround storage tanks are unable to withstand the high internal pressure of the magnitu~e required to ~evelop a sound level discernible from backgrouna noise by the passage of fluid through a leakage nole.
OBJECT Oh' Tf:IE INVENTI()N
It is accordingly an oDject of tne present invention to obviate or mltigate the disadvantages associated with the aforesaid method ana apparatus.
~, ~p BRIEF SU~ARY OF THE INVENTION
According to the present invention, there is proviaed a method of detecting a perforation in a tank containing a liqui~, the method eomprising reducing the pressure in the interior of the tank to cause the passage of a gas into the tank through the perforation due to a static pressure differential between the interior anci the exterior of the tank, acoustically sensing the formation cf the bubbles of the air in the liquid in the tank eonverting the acoustic information into an electrical signal and processing the eleetrieal signal to provide an indication of the existerlce of the perforation.
The formation of a bubble emits a pattern of acoustic energy until it breaks at the liquid surface, and this acoustic energy is sensed by a hydrophone. Tne pattern of acoustic energy is known as an acoustic signatureO
It has been found tnat, as the bubbles form they become detached from the edge of the perforation, ancl as they rise towards the surface of the liquid they change shape due to a combination of forces acting on tne ~ubble, an~ these cnanges of shape emlt acoustic waves or volume pulsation of substantially constant frequency. This volume pulsation results in a characteristic souna signature of a particular trequency and duration, ancl these soun~ signatures can be sensed by a hydrophone immersed within tne liquid, identifled and distinguished from background nolse.
Tne invention also provides apparatus for detecting a perforation in a tank containing a liqui~ comprising, means for sealing the tank where the sensor apparatus is inserted, means for reducing the pressure within the sealed tank to cause the passage of a gas into the tank, a perforation in the tank due to a pressure differential between the interior and the exterior of the tank, means for acoustically detecting the formation of bubbles of the gas in the liquid in tne tank, ana signal processing means for converting the acoustic information to produce an electrical signal, and means fvr processing the electrical signal to provide an indication of the existence of the nole.
~RI~F D~SCkIPTIO~ OF TH~ D~AWINGS
_ _ _ _ The invention will be more readily understood from the following aescription of a preferred embodiment thereof given, by way of example, with reference to the accompanying drawings in which:
~ igure 1 shows a diagrammatic cross-section tnrough an unaerground storage tank;
Fiyure 2 shows a diagrammatic view, in perspective and partly broken away, of the tank of Figure 1 provide~ with leakage detecting apparatus embodyiny the present invention;
~ igure 3 SllOWS a dlagralnmatic view, in greater detail, of some parts o~ tne apparatus of ~igure 2;
Figures 4 and ~ show upper and lower portions respectively, of a proDe shown in Figure 2;
Figure 6 SIlOWS an end view of parts of a water level sensor taKen along tne line VI-VI of Figure 5;
3~
Figures 7 and 8 show opposite sides of a printed circuit board forming part of the probe of E`igure 6;
E'igures 91 10 an~ 11 show alternative electrode shapes for tAe side of the printed circuit boara shown in E~igure 7;
E'igure 12 shows a block circuit diagram of the water level sensor;
Flgure 13 shows a diagrammatic broken-away cross-section tnrough the part of a tank wall and illustrates the formation of a bubble at a perforation in the tank wall;
Figures 14 and 15 show oscilloscope traces of the acoustic signatures of the formation of bubbles from two different perforations; ana Figure 16 snows a pair of tanks connected together to a single leakage detecting apparatus.
DESCRIPTION OF T}lE PR~FEKR~ EM~ODIivlENT
As snown in Figure 1, an unaergroun~ fuel storage tank 10 is buried in ground 11 and surrounàea by backfill 1~.
The tank 10 contains a body 14 of liquid hydrocarbon fuel, for example gasoline, having a liquid surface 15. A water table 16 is present in the grouna 11 an~ the back fill 12.
hubbles 17 are shown rising frolll d perforation 18 in the tank 10 above the water table 16 ana a body o~ water 19 is shown beLow the fue:L 14, the water 19 having entere~ through a perforation ~0 in tne tank 1~ located oelow the water tahle 16.
It wlll be appreciatea that the fuel 14 will ten~ to leak outwardly of the tank througn tne noles 1~ an~ 20, since pressure in the tank 10 is greater than the outside, so that the bubbles 17 will not normally be present, although the condensea water 19 may collect at the bottom of the tank as a result of condensation at the top of the tank, the denser water sinKing and collecting at the bottom rather than pass through the hole 20. As aescribea in greater detail hereinafter, the present method and apparatus are intended tO reduce the pressure within the tank 20 below pressure outsiae tne tank in order to deliberately induce the inf~ow of air in tne form of bubbles 17 in the liquid thereon.
Referring now to E'igure 2 a probe 22 is immersed in the tank 10 ana located near the bottom lOa.
The probe 22 is suspenaea within the tank 10 by a cable 24 passing tnrough a fill pipe 25 for~ing part of tne tank 10.
The cable 14 is suspended from a closure 26 whicn hermetically seals the fill pipe 25.
From the closure 26, the cable is connected to a control ana ~isplay unit 27, which is connected in turn by a cable ~8 to an oscilloscope 29 ana a cable 30 to a pair of headphones 31.
The tank 10 also irlcludes a vent pipe 33, to which a flexible hose 34 is connected by a couple 32.
The flexible hose 34 is connected to an evacuation control unlt 35, whlch lS in turn connectea by a flexible ~ose 36 to a vacuum pump 37, which is driven by an electric motor 38. The evacuator control unit 35 is also connectea by a flexible hose 40 to a gas cylinder 41 contalning nitrogen.
As aescribed hereina~ter in greater aetail, the probe 22 includes a hydrophone, for detecting the acoustic signatures of bubbles, as well as other sensors, each of the sensors being connected through the cable 24 to the control display unit 27.
The various components connected to the vent pipe 33 are shown diagrammatically in greater detail in Figure 30 The coupling 32 serves to prevent the entry of atmospheric air into the tank 10 and the hose 34 connects the vent pipe 33 via the evacuation control unit 35 to the pump 37 and to the gas cylinder 41.
The evacuation control unit 35 includes a wire mesh flame arrestor 45, an electrically operated vaive 46 and a check valve 47 connected between the hose 34 and the pump 37 and an electrically operated valve 49 which connects the unit 35 to the gas cylinder 41. A pressure regulator 50 is connected between the gas cylinder 41 and the electrically operated valve 49.
The electrlcally controlled valve 46 is a normally closed valve so that, in case of power failure or disconnection, the connection between the hose 34 and the vacuum pump 37 is automatically interrupted. Closure of the valve 46 enables loss of vacuum to be prevented when the operation of the vacuum pump 37 is interrupted to avoid ground noise interference during operation of the hydrophone as described below.
The check valve 47 serves to prevent flashback to the tank 10, and the electric motor 38 is brushless and controlled by a solid state, arcless switch (not shown) to reduce hazard in the environment of the hydrocarbon storage tank lOo . ~ ., Pressure sensitive transducers 51 and 52 are provi~ed before and after, respectively, the vacuum pump 37 for sensing the pump inlet and outlet pressures and are connected to the control and display unit 27 and, likewise, temperature sensitive transducers 53 and 54 for sensing the air/fuel vapour mixture at the inlet and outlet of the pump 37 are connected to the control display unit 277 Also9 semi-conductor gas sensor 56 is provided before the inlet to the pump 37 an~ connected to t~le control and display unit 27 for providiny an indication of the air/fuel vapour ratio of the mixture being pumped by the pump 37, this information together with the temperature measurement of the mixture being used to indicate risk of explosion.
A mecAanical pressure gauge 58 is provided immediately before the pump inlet for providing an indication of the pressure of the mixture.
'~'he outlet of the pump 37 is connected through a flow sensor 60 to a portable exhaust stack 61 proviaed with a flame arrestor 62.
~ eferring now to Figures 4 an~ 5, whicn show the probe 22 in greater aetaiL and, more particularly, an upper probe portion in~icatea yenerally by reference numeral 66 ana a lower probe portion in~lcated generally by reference numeral 67. The upper probe portion 66 comprises a cylinarical housing 68 having an ena portion 69 of reducea cross section ~or interengagement with a correspondingly widened end portion 70 of a cylindrical housing 71 of the lower housing portion 67.
The cylindrical housing 68 is closed at one end by a closure pluy 73t secured by screws 74 and providea with O-ring seals 75 for sealing the closure plug 73 to the internal surace of the cylindrical housing 68.
The opposite end of the cylindrical housing 68 is closed by a diaphragm body member 76, which is sealed to the internal surface of tne cylindrical housing 68 by O-rlng seals 77.
TAe outermost end of the body member 76 is formed witn an isolation chamber in the form of a cylindrical recess 79, ana a diaphragm 80 extends across the recess 79 ana is clamped at its periphery by an annular oiaphragm cap 81.
A pressure sensitive transaucer 82 is exposea to the pressure prevailing in the cylindrical space 79 through a pipe fitting 83, the diaphragm 80 serving to isolate the pressure sensitive transducer 82 from the liquid in the tank 10.
The end closure plug 73 and tne diaphragm boay member 76 support between them a generally cylindrical inner barrel 85 which, in turn, throuyh radial supports 86 supports a pair of printed circuit boards 87 on whlcn are provide~ the circuits of the pressure sensitive transducer 82 ana tne other sensors described hereina~ter.
The lower probe housing 67 contains the hydrophone, which is inalcate~ by reference numeral ~0 an~ a water level sensor mounting 91.
The hydrophone 90, wnich is responsive to soun~ over an angle of 360 transversely of the ~ongitu~inal axis of the ~35~
probe, is fixedly supporteci in the lower cylindrical housing b7 by a pair of annular supports ~3 an~ 94.
l'he water level sensor mounting 91, however, is mounted for limited sliding movement relative to ana longitudinally of the lower probe housing 67 and is urged outwardly of the righthand end of the lower prcbe housing 67, as viewed in ~igure 5, by ~elleville springs 96 accomnlodated between a shoulder 97 on the water level sensor mounting 91 and an annular support member 98, relative to which tne water level sensor mounting Yl is slidable.
l'he ~elleville springs 96 serve to absorb sAock when the probe 22 is lowered onto the bottom of the tank 10 ~uring the beginning of a test procedure.
A temperature sensor ~ is mounted on an annular support ~8 for providing intormation with respect to the temperature in the tank 10, this information being useful to the operator of the apparatus but not being directly related to the detection of the bubbles, the acoustic signatures of which are not temperature depencient.
The lower probe portion nousing 67 is provided with slots extending tnrough the wall thereof, only one of these slots being snown in the drawings and ~eing indicated by reference numeral 100. These slots lUO allow the fuel within the tank 10 to enter the lower probe portion housing 67 anc~ thus allow the acoustic signatures of the ~ub~les to be ~etectec~ by the hydrophone.
Eigure 6 shows an end vlew o~ tne water level sensor mounting, which has a cylindrical portion 102 defining a nollow interior space 103, the wall 102 being provided witn opposeo slots 10~i througn which tAe fuel in the interior of the tank 10, and any water such as the body of water 19 of ~igure 1, can enter tne hollow interior 103.
A printed circuit board 106 extends transversely across the hollow interior 103 and has opposite longitudinal edges thereof recei~ed in recesses 107 in the cylindrical wall 102.
Opposite sides of the printed circuit ~oard 106 are shown in Fiyures 7 ana 8.
Referring firstly to Fiyure 7, one side or the board 106 has printed thereon three electrodes 108, 109 ana 110, tne latter being spaced from the electrodes i08 and 109 by gaps 111. Conouctors 112 an~ 113 are provided for connecting tne electrodes 108 and 110, respectively, to the circuitry to be describe~ hereinafter.
The opposite side of the board 106 is formed witn electrodes 115, 116 ana 117 whicn are interconnected by conductor 118. Electrodes 115 and 116 are a~so connected through the board to the electrodes 109 and 108 respectively.
Electrodes 119 are connected to a conductor 120 for connection, in turn, to the circuitcy described hereinafter.
The electcodes 108 an~ 109, being connected through electro~es llS and 116 and conductor 118, act as a single eLecto~e space~ by a gap from the electrode 110, which exten~s parallel to the axis oE the probe. Upon immersion vf the lower end of the proDe 22 in tne water 19, the electrical resistance across the gap will vary in accordance with the depth of the water 19. This electrical resistance is calibrated to represent the depth of the water 19. Of course, it is evident that as the water level 'minimises', the electrical resistance measured between the electrodes changes, until the electrodes are completely immersed. Thereafter the reading remains substantially constant~
In order to linearize the variation in this resistance the nature of the inter-electrode spacing is variable.
The electrodes 115, 116 and 117 on the back of the board 10~ act as a common first reference electrode, and the electrodes 119 act as a common second reerence electrode separated by a reference gap from the first reference electrode. The first and second reference electrodes, and their gap, are located at the lowermost end of the printed circuit board 106 so ~hat, when the water level sensor is in use, these reference electrodes and their gap are disposed below a minimum water level of the body of water 19, the depth of which is to be measured.
Figure 12 shows a block circuit diagram of the water level sensor, in which the varying resistance of the effective electrode gap at the side of the printed circuit board 106 shown in Figure 7 is represented by Rv and the resistance of the reference gap at the opposite side of the printed circuit board is represented by Rr.
As shown in Figure 12, the water level sensor circuit includes an oscillator 130 the output of which is connected to a 5,' ~,~, ,~.'.,, ~'i ?3 controlled variable gain amplifier 131, which supplies an a.c.
voltage to the primary winding o a coupling transformer 132 The transformer 132 is connected, together with two further coupling transformers 133 and 134 and the resistances Rv and Rr, in a bridge circuit indicated generalLy by reference numeral 135.
The secondary winding of the coupling transfer 133 is connected to an absolute value circuit 134, which converts the sinusoidal AC output signal from the couplinq transformer 133 into a full wave rectified signal, which is passed through a low pass filter 135 to an error amplifier 136, the output of which is connected to the variable gain amplifier 1310 The voltage fed from the amplifier 131 to the coupling transformer 132 is adjusted to compensate for the variation in the voltage drop across the coupling transformer 132 resulting from changes in the impedance level of the bridge 135.
The secondary winding of the coupling transformer 134 is connected through an absolute value circuit 137 and a low pass filter 138 to a converter 139, which effects output voltage current conversion.
The output of the converter 139 is connected to the control and display unit 27 through the cable 24.
The coupling transformers 132, 133 and 134 isolate the printed circuit board 106 from the remainder of the circuitry of the water level sensor in order to eliminate effects of a current leakage from the printed circuit board 106 to ground.
l'he printed circuit board 106 has its surfaces gold-plated to inhibit corrosion and the buildup of non-conductive films.
The operation of the above-described apparatus is as follows:
The tank 10 is firstly almost filled with the fuel 14, which is then alLowed a settling time of 3 or 4 hours to permit any possible emulsification of water and fuel within the tank 10 to dissipate. Emulsification can be produced when the tank is filled due to mixing of the fuel with any water at the bottom of the tank as the fuel is fed into the tank.
The probe 22 is then inserted into the tank 10 through the fill pipe 25 and positioned as shown in Figure 2, i.e. near the bottom of the tank 10, and the fill pipe 25 is hermetically sealed by the closure 26.
~ he flexible hose 34 and attached apparatus are connected to the vent pipe 33 by the coupling 32, and the water level sensor in the probe 22 is operated to provide an initial reading indicating the surface leveL of the body of water 19, if any water is present in the tank 10 at this time.
The pump 37 is then driven in stages by the motor 38 in order to decrease the air/vapour pressure in the ullage in the tank 10 above the fuel surface 15 by discrete amounts.
At each stage, the hydrophone is used to effect an acoustical scan of the interior of the tank 10, with the water level sensor output being continuously monitored and recorded.
The pressure in the tank ullage is reduced incrementally in this way until the pressure difference at the ~?
tank bottom, between the interior and the exterior of the tank~
is such that the minimum permissible perforation size can be detected.
When the external pressure exceeds the combined pressures of the ullage gases and the static head of the fuel 14 above the leakage hole 18, air passes through the perforation, creating bubbles 17.
As the bubbles 17 break away from the internal wall surface of the tank 10 they emit the characteristic sounds or acoustic signatures which are detected by the hydrophone, which produces repetitive signals. These acoustic signatures are produced as the formed bubbles change shape as they rise towards the surface. The pressure forces, buoyancy forces and the surface tension of the fuel caused the bubble shape to deform as it rises, and these changes in shape or "volume pulsationsl' emit acoustic waves which are detected by the hydrophone. These acoustic signatures tend to be constant for a particular perforation, and to differ from perforation to perforation as illustrated in Fig. 14 and Fig. 15. These signals are applied by the control and display unit 27 to the oscilloscope 29 and/or to the headphones 31, where they can be recogniæed by the operator of the apparatus.
When the tank is at least partly below the level of the water table 16, the difference between interior and exterior pressure may also cause water to pass into the tank through the hole 20, and this is detected as a variation in the level of the body of water 19 by the water level sensor in the probe 22.
When the above-described testing of the tank 10 has been completed, the control unit 35 is operated to discontinue the pumping of air and vapour from the tank ullage and, instead, is used to supply nitrogen gas from the cylinder 41 into the ullage.
The present method of leakage detection has a number of significant advantages. It is relatively fast, and can be completed within about one hour, which is an economically acceptable time periodO Pin hole perforations can be detected as easily as larger perforations and fuel is not expelled from the tank through any perforation during the testing procedure.
The relative size of perforations can be deduced from the frequency of the acoustic signatures. The size of a perforation is not increased by the present method, which is also unaffected by mechanical or dimensional instability, it does not require thermal stabilization, fluid agitation or temperature compensation and it is not rendered inoperable by water table fluctuations.
Reference is now made to Figure 13 of the accompanying drawings, which illustrates the formation of a bubble 17A at an ideal, i.e. circular orifice 18A in a tank wall lOA as air flows into the tank through the orifice 18A.
The minimum pressure difference (aP) to form the bubble 17A at the orifice may be expressed as aPmin=(Pb Pi) where S6~3 = surface tension of the liquid r = radius of the orifice 18A
Pb = pressuee in the bubble 17A
Pi = static pressure in the li~uia at the orifice When the minimum pressure difference required to form the bu~ble 17A i9 exceeded, the bubble 17~ grows as excess pressure energy is converted into surface energy at the air/liquid interface. Eventually, the buoyancy force acting on the bubble 17A exceeds the surface tension force hoiding the bubble 17A to the orifice 18A. The equilibrium condition is expressed ~s Ro = ~ - p~ - where = bu~ble radius at break away from the perforation p = density of liquid It will be seen from tnese equations that the surface tension of the liquid, ~, has a marked effect on the differential pressure re-luired to detach tne ~ubble from the orifice. For a given orifice size, a smaller bubble will be formed in a hydrocaxbon than will be formed in water because the ratio of surface tension of water to the surface tension of, say octane is 3.5. Also, the minimum pressure differential required to form a bubble for a given perforation is much higher for water than for a hydrocarbon.
~ u~ble noise is generated by the volume pulsation of the bubble which occurs lmmediately after the bubble detacnes from the orifice until the ~ubbles break at tne surface of the liquid. ~ubbles formed un~er iaeal condltions will exhi~it souna due to volume pulsations at frequencies (f) given by tne equation ~ R
where ~ = ratio of specific heat capacity of air at constant pressure, cp, to specific heat capacity at constant volume, Cv (about 1.4).
Ro is raàius of bubble. Consequently f is directly proportional to tne bubble radius for a given liquid. That is, the bubble repetition rate is a function of tne pressure dit`ference across the orifice.
Figures 14 and 15, by way of example, the acoustic signatures of two tank leakage bubbles in the form oE scans on the oscilloscope 29. Such signatures are of overall short duration (approximately 15ms) ana substantially constant volume pulsation frequency and can therefore be distinguishea from background noise.
Various modif`ications may be made to tne preferrea emboaiment without departing from the scope of the invention.
For example, altnough the gas cylinder perferably contains nitrogen it can also be any other inert gas such as krypton, argon or car~on ~ioxide.
The electrode gap in the pre~errea emboaiment is substantially constant although in fact this nee~ not oe the case; Figs. 9, 10 ana 11 illustrate three di~erent tapering inter-electroae gaps whlcn coul~ also be usea.
It snoula also ~e understood tnat the present inventlon is not restricted to the detection of leakage ln undergrouna tanks ana is equally applica~le for use With above ground tan~s.
A pair of tanks containing the same liqul~ are ,~ ~.
" ~i, sometimes connected together as shown in Figure 16. l'wo tanks lOA and lOB are connected by a siphon line 140, which is also connected through a check valve 141 to a suction line 142.
Whell two or more tanks are tested by the above-described method and apparatus it is important to ensure that the same partial vacuum i5 simultaneously applied to the vent pipe of each tank.
The present invention relates to a method of ana apparatus for detectiny a perforation in a tank containing a liquid.
~ACKGR~UND O~ T~ INVENTION
Gasoline, diesel fuel and heating oil are the most widely used hydrocarbons and are commonly stored undergrouna at retail service stations, bulk service stations, municipal garages, etc. ~uch liquid hydrocarbons are normally storea in underground tanks and there is a risk that the hydrocarbons may escape into the ground from perforations which develop in these underground storage tanks.
Tank leakage problems are recogniæed by the petroleum marketing industry and by governmental and environmental agencies.
Furthermore, a leaking tank is generally undiscovered until a flagrant appearance of hydrocarbons is traced to it.
The tracing procedure can be long and costly because of factors such as unusual soil strata, a network of backfilled trenches, large and frequent variations in the height o~ the water table or a dense concentration of underground tanks.
Perforations in tanks usually develop from corrosion;
internal corrosion is less prevalent than external corrosion, but does occur, especially in a narrow band along tne tank bottom~ 'L'his is because water, whlch separates from the hydrocarbons is more dense than the hydrocarbon and sinks to form a layer at the bottom cf the tank. Internal corrosion is .,, ~
~ ., aggravated un~er the fill pipe because protective rust is removed by the flow of liquid against the tank bottom during filling or by the impact and movement of a gauge stick DE~SCRIPTION OF T~IE PRIOR_ARI' It has previously been proposed to detect perforations in a tank by detecting the sound generated by the egress of liquid through the perforation and by separating this sound from background noise, which is frequently stronger than the sound being sought.
A method utiiizing acoustic detection has been proposed, requiring the use of two or more hydrophones on a rotatable arm. Irhis method was aesiyneo primarily for large above-ground tanks and intended to detect, by triangulation ano phase correlation~ the source of the sound generated by fluid leaking tnrough the perforation. One of the disadvantages of this prior proposal is that, while the passase of fluid through a leak creates random noise, the intensity of which increases with pressure, this noise may be lost in background noise when the pressure is only that of the fluid head within the tank.
Most underyround storage tanks are unable to withstand the high internal pressure of the magnitu~e required to ~evelop a sound level discernible from backgrouna noise by the passage of fluid through a leakage nole.
OBJECT Oh' Tf:IE INVENTI()N
It is accordingly an oDject of tne present invention to obviate or mltigate the disadvantages associated with the aforesaid method ana apparatus.
~, ~p BRIEF SU~ARY OF THE INVENTION
According to the present invention, there is proviaed a method of detecting a perforation in a tank containing a liqui~, the method eomprising reducing the pressure in the interior of the tank to cause the passage of a gas into the tank through the perforation due to a static pressure differential between the interior anci the exterior of the tank, acoustically sensing the formation cf the bubbles of the air in the liquid in the tank eonverting the acoustic information into an electrical signal and processing the eleetrieal signal to provide an indication of the existerlce of the perforation.
The formation of a bubble emits a pattern of acoustic energy until it breaks at the liquid surface, and this acoustic energy is sensed by a hydrophone. Tne pattern of acoustic energy is known as an acoustic signatureO
It has been found tnat, as the bubbles form they become detached from the edge of the perforation, ancl as they rise towards the surface of the liquid they change shape due to a combination of forces acting on tne ~ubble, an~ these cnanges of shape emlt acoustic waves or volume pulsation of substantially constant frequency. This volume pulsation results in a characteristic souna signature of a particular trequency and duration, ancl these soun~ signatures can be sensed by a hydrophone immersed within tne liquid, identifled and distinguished from background nolse.
Tne invention also provides apparatus for detecting a perforation in a tank containing a liqui~ comprising, means for sealing the tank where the sensor apparatus is inserted, means for reducing the pressure within the sealed tank to cause the passage of a gas into the tank, a perforation in the tank due to a pressure differential between the interior and the exterior of the tank, means for acoustically detecting the formation of bubbles of the gas in the liquid in tne tank, ana signal processing means for converting the acoustic information to produce an electrical signal, and means fvr processing the electrical signal to provide an indication of the existence of the nole.
~RI~F D~SCkIPTIO~ OF TH~ D~AWINGS
_ _ _ _ The invention will be more readily understood from the following aescription of a preferred embodiment thereof given, by way of example, with reference to the accompanying drawings in which:
~ igure 1 shows a diagrammatic cross-section tnrough an unaerground storage tank;
Fiyure 2 shows a diagrammatic view, in perspective and partly broken away, of the tank of Figure 1 provide~ with leakage detecting apparatus embodyiny the present invention;
~ igure 3 SllOWS a dlagralnmatic view, in greater detail, of some parts o~ tne apparatus of ~igure 2;
Figures 4 and ~ show upper and lower portions respectively, of a proDe shown in Figure 2;
Figure 6 SIlOWS an end view of parts of a water level sensor taKen along tne line VI-VI of Figure 5;
3~
Figures 7 and 8 show opposite sides of a printed circuit board forming part of the probe of E`igure 6;
E'igures 91 10 an~ 11 show alternative electrode shapes for tAe side of the printed circuit boara shown in E~igure 7;
E'igure 12 shows a block circuit diagram of the water level sensor;
Flgure 13 shows a diagrammatic broken-away cross-section tnrough the part of a tank wall and illustrates the formation of a bubble at a perforation in the tank wall;
Figures 14 and 15 show oscilloscope traces of the acoustic signatures of the formation of bubbles from two different perforations; ana Figure 16 snows a pair of tanks connected together to a single leakage detecting apparatus.
DESCRIPTION OF T}lE PR~FEKR~ EM~ODIivlENT
As snown in Figure 1, an unaergroun~ fuel storage tank 10 is buried in ground 11 and surrounàea by backfill 1~.
The tank 10 contains a body 14 of liquid hydrocarbon fuel, for example gasoline, having a liquid surface 15. A water table 16 is present in the grouna 11 an~ the back fill 12.
hubbles 17 are shown rising frolll d perforation 18 in the tank 10 above the water table 16 ana a body o~ water 19 is shown beLow the fue:L 14, the water 19 having entere~ through a perforation ~0 in tne tank 1~ located oelow the water tahle 16.
It wlll be appreciatea that the fuel 14 will ten~ to leak outwardly of the tank througn tne noles 1~ an~ 20, since pressure in the tank 10 is greater than the outside, so that the bubbles 17 will not normally be present, although the condensea water 19 may collect at the bottom of the tank as a result of condensation at the top of the tank, the denser water sinKing and collecting at the bottom rather than pass through the hole 20. As aescribea in greater detail hereinafter, the present method and apparatus are intended tO reduce the pressure within the tank 20 below pressure outsiae tne tank in order to deliberately induce the inf~ow of air in tne form of bubbles 17 in the liquid thereon.
Referring now to E'igure 2 a probe 22 is immersed in the tank 10 ana located near the bottom lOa.
The probe 22 is suspenaea within the tank 10 by a cable 24 passing tnrough a fill pipe 25 for~ing part of tne tank 10.
The cable 14 is suspended from a closure 26 whicn hermetically seals the fill pipe 25.
From the closure 26, the cable is connected to a control ana ~isplay unit 27, which is connected in turn by a cable ~8 to an oscilloscope 29 ana a cable 30 to a pair of headphones 31.
The tank 10 also irlcludes a vent pipe 33, to which a flexible hose 34 is connected by a couple 32.
The flexible hose 34 is connected to an evacuation control unlt 35, whlch lS in turn connectea by a flexible ~ose 36 to a vacuum pump 37, which is driven by an electric motor 38. The evacuator control unit 35 is also connectea by a flexible hose 40 to a gas cylinder 41 contalning nitrogen.
As aescribed hereina~ter in greater aetail, the probe 22 includes a hydrophone, for detecting the acoustic signatures of bubbles, as well as other sensors, each of the sensors being connected through the cable 24 to the control display unit 27.
The various components connected to the vent pipe 33 are shown diagrammatically in greater detail in Figure 30 The coupling 32 serves to prevent the entry of atmospheric air into the tank 10 and the hose 34 connects the vent pipe 33 via the evacuation control unit 35 to the pump 37 and to the gas cylinder 41.
The evacuation control unit 35 includes a wire mesh flame arrestor 45, an electrically operated vaive 46 and a check valve 47 connected between the hose 34 and the pump 37 and an electrically operated valve 49 which connects the unit 35 to the gas cylinder 41. A pressure regulator 50 is connected between the gas cylinder 41 and the electrically operated valve 49.
The electrlcally controlled valve 46 is a normally closed valve so that, in case of power failure or disconnection, the connection between the hose 34 and the vacuum pump 37 is automatically interrupted. Closure of the valve 46 enables loss of vacuum to be prevented when the operation of the vacuum pump 37 is interrupted to avoid ground noise interference during operation of the hydrophone as described below.
The check valve 47 serves to prevent flashback to the tank 10, and the electric motor 38 is brushless and controlled by a solid state, arcless switch (not shown) to reduce hazard in the environment of the hydrocarbon storage tank lOo . ~ ., Pressure sensitive transducers 51 and 52 are provi~ed before and after, respectively, the vacuum pump 37 for sensing the pump inlet and outlet pressures and are connected to the control and display unit 27 and, likewise, temperature sensitive transducers 53 and 54 for sensing the air/fuel vapour mixture at the inlet and outlet of the pump 37 are connected to the control display unit 277 Also9 semi-conductor gas sensor 56 is provided before the inlet to the pump 37 an~ connected to t~le control and display unit 27 for providiny an indication of the air/fuel vapour ratio of the mixture being pumped by the pump 37, this information together with the temperature measurement of the mixture being used to indicate risk of explosion.
A mecAanical pressure gauge 58 is provided immediately before the pump inlet for providing an indication of the pressure of the mixture.
'~'he outlet of the pump 37 is connected through a flow sensor 60 to a portable exhaust stack 61 proviaed with a flame arrestor 62.
~ eferring now to Figures 4 an~ 5, whicn show the probe 22 in greater aetaiL and, more particularly, an upper probe portion in~icatea yenerally by reference numeral 66 ana a lower probe portion in~lcated generally by reference numeral 67. The upper probe portion 66 comprises a cylinarical housing 68 having an ena portion 69 of reducea cross section ~or interengagement with a correspondingly widened end portion 70 of a cylindrical housing 71 of the lower housing portion 67.
The cylindrical housing 68 is closed at one end by a closure pluy 73t secured by screws 74 and providea with O-ring seals 75 for sealing the closure plug 73 to the internal surace of the cylindrical housing 68.
The opposite end of the cylindrical housing 68 is closed by a diaphragm body member 76, which is sealed to the internal surface of tne cylindrical housing 68 by O-rlng seals 77.
TAe outermost end of the body member 76 is formed witn an isolation chamber in the form of a cylindrical recess 79, ana a diaphragm 80 extends across the recess 79 ana is clamped at its periphery by an annular oiaphragm cap 81.
A pressure sensitive transaucer 82 is exposea to the pressure prevailing in the cylindrical space 79 through a pipe fitting 83, the diaphragm 80 serving to isolate the pressure sensitive transducer 82 from the liquid in the tank 10.
The end closure plug 73 and tne diaphragm boay member 76 support between them a generally cylindrical inner barrel 85 which, in turn, throuyh radial supports 86 supports a pair of printed circuit boards 87 on whlcn are provide~ the circuits of the pressure sensitive transducer 82 ana tne other sensors described hereina~ter.
The lower probe housing 67 contains the hydrophone, which is inalcate~ by reference numeral ~0 an~ a water level sensor mounting 91.
The hydrophone 90, wnich is responsive to soun~ over an angle of 360 transversely of the ~ongitu~inal axis of the ~35~
probe, is fixedly supporteci in the lower cylindrical housing b7 by a pair of annular supports ~3 an~ 94.
l'he water level sensor mounting 91, however, is mounted for limited sliding movement relative to ana longitudinally of the lower probe housing 67 and is urged outwardly of the righthand end of the lower prcbe housing 67, as viewed in ~igure 5, by ~elleville springs 96 accomnlodated between a shoulder 97 on the water level sensor mounting 91 and an annular support member 98, relative to which tne water level sensor mounting Yl is slidable.
l'he ~elleville springs 96 serve to absorb sAock when the probe 22 is lowered onto the bottom of the tank 10 ~uring the beginning of a test procedure.
A temperature sensor ~ is mounted on an annular support ~8 for providing intormation with respect to the temperature in the tank 10, this information being useful to the operator of the apparatus but not being directly related to the detection of the bubbles, the acoustic signatures of which are not temperature depencient.
The lower probe portion nousing 67 is provided with slots extending tnrough the wall thereof, only one of these slots being snown in the drawings and ~eing indicated by reference numeral 100. These slots lUO allow the fuel within the tank 10 to enter the lower probe portion housing 67 anc~ thus allow the acoustic signatures of the ~ub~les to be ~etectec~ by the hydrophone.
Eigure 6 shows an end vlew o~ tne water level sensor mounting, which has a cylindrical portion 102 defining a nollow interior space 103, the wall 102 being provided witn opposeo slots 10~i througn which tAe fuel in the interior of the tank 10, and any water such as the body of water 19 of ~igure 1, can enter tne hollow interior 103.
A printed circuit board 106 extends transversely across the hollow interior 103 and has opposite longitudinal edges thereof recei~ed in recesses 107 in the cylindrical wall 102.
Opposite sides of the printed circuit ~oard 106 are shown in Fiyures 7 ana 8.
Referring firstly to Fiyure 7, one side or the board 106 has printed thereon three electrodes 108, 109 ana 110, tne latter being spaced from the electrodes i08 and 109 by gaps 111. Conouctors 112 an~ 113 are provided for connecting tne electrodes 108 and 110, respectively, to the circuitry to be describe~ hereinafter.
The opposite side of the board 106 is formed witn electrodes 115, 116 ana 117 whicn are interconnected by conductor 118. Electrodes 115 and 116 are a~so connected through the board to the electrodes 109 and 108 respectively.
Electrodes 119 are connected to a conductor 120 for connection, in turn, to the circuitcy described hereinafter.
The electcodes 108 an~ 109, being connected through electro~es llS and 116 and conductor 118, act as a single eLecto~e space~ by a gap from the electrode 110, which exten~s parallel to the axis oE the probe. Upon immersion vf the lower end of the proDe 22 in tne water 19, the electrical resistance across the gap will vary in accordance with the depth of the water 19. This electrical resistance is calibrated to represent the depth of the water 19. Of course, it is evident that as the water level 'minimises', the electrical resistance measured between the electrodes changes, until the electrodes are completely immersed. Thereafter the reading remains substantially constant~
In order to linearize the variation in this resistance the nature of the inter-electrode spacing is variable.
The electrodes 115, 116 and 117 on the back of the board 10~ act as a common first reference electrode, and the electrodes 119 act as a common second reerence electrode separated by a reference gap from the first reference electrode. The first and second reference electrodes, and their gap, are located at the lowermost end of the printed circuit board 106 so ~hat, when the water level sensor is in use, these reference electrodes and their gap are disposed below a minimum water level of the body of water 19, the depth of which is to be measured.
Figure 12 shows a block circuit diagram of the water level sensor, in which the varying resistance of the effective electrode gap at the side of the printed circuit board 106 shown in Figure 7 is represented by Rv and the resistance of the reference gap at the opposite side of the printed circuit board is represented by Rr.
As shown in Figure 12, the water level sensor circuit includes an oscillator 130 the output of which is connected to a 5,' ~,~, ,~.'.,, ~'i ?3 controlled variable gain amplifier 131, which supplies an a.c.
voltage to the primary winding o a coupling transformer 132 The transformer 132 is connected, together with two further coupling transformers 133 and 134 and the resistances Rv and Rr, in a bridge circuit indicated generalLy by reference numeral 135.
The secondary winding of the coupling transfer 133 is connected to an absolute value circuit 134, which converts the sinusoidal AC output signal from the couplinq transformer 133 into a full wave rectified signal, which is passed through a low pass filter 135 to an error amplifier 136, the output of which is connected to the variable gain amplifier 1310 The voltage fed from the amplifier 131 to the coupling transformer 132 is adjusted to compensate for the variation in the voltage drop across the coupling transformer 132 resulting from changes in the impedance level of the bridge 135.
The secondary winding of the coupling transformer 134 is connected through an absolute value circuit 137 and a low pass filter 138 to a converter 139, which effects output voltage current conversion.
The output of the converter 139 is connected to the control and display unit 27 through the cable 24.
The coupling transformers 132, 133 and 134 isolate the printed circuit board 106 from the remainder of the circuitry of the water level sensor in order to eliminate effects of a current leakage from the printed circuit board 106 to ground.
l'he printed circuit board 106 has its surfaces gold-plated to inhibit corrosion and the buildup of non-conductive films.
The operation of the above-described apparatus is as follows:
The tank 10 is firstly almost filled with the fuel 14, which is then alLowed a settling time of 3 or 4 hours to permit any possible emulsification of water and fuel within the tank 10 to dissipate. Emulsification can be produced when the tank is filled due to mixing of the fuel with any water at the bottom of the tank as the fuel is fed into the tank.
The probe 22 is then inserted into the tank 10 through the fill pipe 25 and positioned as shown in Figure 2, i.e. near the bottom of the tank 10, and the fill pipe 25 is hermetically sealed by the closure 26.
~ he flexible hose 34 and attached apparatus are connected to the vent pipe 33 by the coupling 32, and the water level sensor in the probe 22 is operated to provide an initial reading indicating the surface leveL of the body of water 19, if any water is present in the tank 10 at this time.
The pump 37 is then driven in stages by the motor 38 in order to decrease the air/vapour pressure in the ullage in the tank 10 above the fuel surface 15 by discrete amounts.
At each stage, the hydrophone is used to effect an acoustical scan of the interior of the tank 10, with the water level sensor output being continuously monitored and recorded.
The pressure in the tank ullage is reduced incrementally in this way until the pressure difference at the ~?
tank bottom, between the interior and the exterior of the tank~
is such that the minimum permissible perforation size can be detected.
When the external pressure exceeds the combined pressures of the ullage gases and the static head of the fuel 14 above the leakage hole 18, air passes through the perforation, creating bubbles 17.
As the bubbles 17 break away from the internal wall surface of the tank 10 they emit the characteristic sounds or acoustic signatures which are detected by the hydrophone, which produces repetitive signals. These acoustic signatures are produced as the formed bubbles change shape as they rise towards the surface. The pressure forces, buoyancy forces and the surface tension of the fuel caused the bubble shape to deform as it rises, and these changes in shape or "volume pulsationsl' emit acoustic waves which are detected by the hydrophone. These acoustic signatures tend to be constant for a particular perforation, and to differ from perforation to perforation as illustrated in Fig. 14 and Fig. 15. These signals are applied by the control and display unit 27 to the oscilloscope 29 and/or to the headphones 31, where they can be recogniæed by the operator of the apparatus.
When the tank is at least partly below the level of the water table 16, the difference between interior and exterior pressure may also cause water to pass into the tank through the hole 20, and this is detected as a variation in the level of the body of water 19 by the water level sensor in the probe 22.
When the above-described testing of the tank 10 has been completed, the control unit 35 is operated to discontinue the pumping of air and vapour from the tank ullage and, instead, is used to supply nitrogen gas from the cylinder 41 into the ullage.
The present method of leakage detection has a number of significant advantages. It is relatively fast, and can be completed within about one hour, which is an economically acceptable time periodO Pin hole perforations can be detected as easily as larger perforations and fuel is not expelled from the tank through any perforation during the testing procedure.
The relative size of perforations can be deduced from the frequency of the acoustic signatures. The size of a perforation is not increased by the present method, which is also unaffected by mechanical or dimensional instability, it does not require thermal stabilization, fluid agitation or temperature compensation and it is not rendered inoperable by water table fluctuations.
Reference is now made to Figure 13 of the accompanying drawings, which illustrates the formation of a bubble 17A at an ideal, i.e. circular orifice 18A in a tank wall lOA as air flows into the tank through the orifice 18A.
The minimum pressure difference (aP) to form the bubble 17A at the orifice may be expressed as aPmin=(Pb Pi) where S6~3 = surface tension of the liquid r = radius of the orifice 18A
Pb = pressuee in the bubble 17A
Pi = static pressure in the li~uia at the orifice When the minimum pressure difference required to form the bu~ble 17A i9 exceeded, the bubble 17~ grows as excess pressure energy is converted into surface energy at the air/liquid interface. Eventually, the buoyancy force acting on the bubble 17A exceeds the surface tension force hoiding the bubble 17A to the orifice 18A. The equilibrium condition is expressed ~s Ro = ~ - p~ - where = bu~ble radius at break away from the perforation p = density of liquid It will be seen from tnese equations that the surface tension of the liquid, ~, has a marked effect on the differential pressure re-luired to detach tne ~ubble from the orifice. For a given orifice size, a smaller bubble will be formed in a hydrocaxbon than will be formed in water because the ratio of surface tension of water to the surface tension of, say octane is 3.5. Also, the minimum pressure differential required to form a bubble for a given perforation is much higher for water than for a hydrocarbon.
~ u~ble noise is generated by the volume pulsation of the bubble which occurs lmmediately after the bubble detacnes from the orifice until the ~ubbles break at tne surface of the liquid. ~ubbles formed un~er iaeal condltions will exhi~it souna due to volume pulsations at frequencies (f) given by tne equation ~ R
where ~ = ratio of specific heat capacity of air at constant pressure, cp, to specific heat capacity at constant volume, Cv (about 1.4).
Ro is raàius of bubble. Consequently f is directly proportional to tne bubble radius for a given liquid. That is, the bubble repetition rate is a function of tne pressure dit`ference across the orifice.
Figures 14 and 15, by way of example, the acoustic signatures of two tank leakage bubbles in the form oE scans on the oscilloscope 29. Such signatures are of overall short duration (approximately 15ms) ana substantially constant volume pulsation frequency and can therefore be distinguishea from background noise.
Various modif`ications may be made to tne preferrea emboaiment without departing from the scope of the invention.
For example, altnough the gas cylinder perferably contains nitrogen it can also be any other inert gas such as krypton, argon or car~on ~ioxide.
The electrode gap in the pre~errea emboaiment is substantially constant although in fact this nee~ not oe the case; Figs. 9, 10 ana 11 illustrate three di~erent tapering inter-electroae gaps whlcn coul~ also be usea.
It snoula also ~e understood tnat the present inventlon is not restricted to the detection of leakage ln undergrouna tanks ana is equally applica~le for use With above ground tan~s.
A pair of tanks containing the same liqul~ are ,~ ~.
" ~i, sometimes connected together as shown in Figure 16. l'wo tanks lOA and lOB are connected by a siphon line 140, which is also connected through a check valve 141 to a suction line 142.
Whell two or more tanks are tested by the above-described method and apparatus it is important to ensure that the same partial vacuum i5 simultaneously applied to the vent pipe of each tank.
Claims (23)
1. A method of detecting a perforation in the wall of a tank containing a stored liquid comprising:
inserting acoustic sensing means into the tank at an insertion point;
hermetically sealing the insertion point to be substantially gas-tight;
reducing gas pressure in the tank to a level sufficient to produce a pressure difference across the wall of the tank causing fluid to enter the tank through the perforation;
detecting the acoustic signal produced by bubbles of said f1uid entering said tank from said perforation, separating from said signal a component attributable to the volume pulsation of said bubble and using said separated component to provide a signal indicative of the existence of the perforation.
inserting acoustic sensing means into the tank at an insertion point;
hermetically sealing the insertion point to be substantially gas-tight;
reducing gas pressure in the tank to a level sufficient to produce a pressure difference across the wall of the tank causing fluid to enter the tank through the perforation;
detecting the acoustic signal produced by bubbles of said f1uid entering said tank from said perforation, separating from said signal a component attributable to the volume pulsation of said bubble and using said separated component to provide a signal indicative of the existence of the perforation.
2. A method as claimed in claim 1 including reducing the pressure in the interior of said tank in successive steps, sensing the formation of said bubbles at each of said stages in the stored liquid to produce a plurality of discrete acoustic signals, connecting each acoustic signal to an electrical signal and detecting said component each of said electrical signals.
3. A method as claimed in claims 1 or claim 2, including sensing the entry of liquid through a perforation by monitoring the level of a body of water beneath the stored liquid in the tank.
,
,
4. A method as claimed in claim 1 or claim 2 including restoring the pressure within the tank to atmospheric pressure subsequent to sensing the volume pulsation during the formation of bubbles by disposing an inert gas into the tank.
5. A method of detecting a perforation in a tank containing a stored liquid comprising:
inserting acoustic sensing means into said tank;
sealing the insertion location to be substantially gas-tight in said tank;
reducing the gas pressure in said tank to a level sufficient to cause a pressure difference between the interior and the exterior of the tank, the pressure difference forcing gas to enter the tank through the perforation, the gas forming bubbles in the stored liquid, the volume pulsation of said bubble formation producing acoustic wave energy as the bubbles rise from the perforation to the liquid surface;
using said acoustic sensing means to acoustically sense the volume pulsation during the formation of said bubbles, converting the acoustic wave energy sensed to produce an electrical signal representative of the volume pulsation of said bubble; and processing said detected electrical signal to provide a signal indicative of the existence of the perforation.
inserting acoustic sensing means into said tank;
sealing the insertion location to be substantially gas-tight in said tank;
reducing the gas pressure in said tank to a level sufficient to cause a pressure difference between the interior and the exterior of the tank, the pressure difference forcing gas to enter the tank through the perforation, the gas forming bubbles in the stored liquid, the volume pulsation of said bubble formation producing acoustic wave energy as the bubbles rise from the perforation to the liquid surface;
using said acoustic sensing means to acoustically sense the volume pulsation during the formation of said bubbles, converting the acoustic wave energy sensed to produce an electrical signal representative of the volume pulsation of said bubble; and processing said detected electrical signal to provide a signal indicative of the existence of the perforation.
6. A method as claimed in claim 5, including:
filtering the electrical signal to counteract background noise.
filtering the electrical signal to counteract background noise.
7. A method as claimed in claim 5 or claim 6, wherein the insertion of said hydrophone into said tank comprises lowering a probe through a fill pipe on said tank, said probe including said acoustic sensing means, said method further comprising the step of using said probe to detect the level of a body of water at the bottom of said tank.
8. A method as claimed in claim 5, which includes detecting the level of water within said tank by immersing a pair of spaced electrodes in the stored liquid and thereafter measuring the electrical resistance between said spaced electrodes and comparing the measured value with a reference resistance valve, using the difference between the measured and reference values to compute the level of water.
9. A method as claimed in claim 5 or claim 6 including restoring the pressure within the tank to atmospheric pressure subsequent to sensing the volume pulsation during formation of the bubbles by releasing an inert gas into the tank.
10. Apparatus for detecting a perforation in the wall of a tank containing a stored liquid, the apparatus comprising:
means for substantially providing a gas-tight seal at an inlet point of said tank;
means for reducing the gas pressure within said sealed tank sufficient to cause a pressure difference across the wall of the tank, the pressure difference being sufficient to force a a fluid to pass through the perforation in the tank wall;
acoustic sensing means for sensing the formation of bubbles of said gas entering the stored liquid in the tank through the perforation, said acoustic sensing means sensing acoustic wave energy produced by the volume pulsation during the formation of bubbles, and said acoustic sensing means also converting the acoustic wave energy into an electrical signal;
and means for separating the component of the signal attributable to the volume pulsation of said bubble; and means for detecting and processing said component of the electrical signal to provide an indication of the existence of said perforation.
means for substantially providing a gas-tight seal at an inlet point of said tank;
means for reducing the gas pressure within said sealed tank sufficient to cause a pressure difference across the wall of the tank, the pressure difference being sufficient to force a a fluid to pass through the perforation in the tank wall;
acoustic sensing means for sensing the formation of bubbles of said gas entering the stored liquid in the tank through the perforation, said acoustic sensing means sensing acoustic wave energy produced by the volume pulsation during the formation of bubbles, and said acoustic sensing means also converting the acoustic wave energy into an electrical signal;
and means for separating the component of the signal attributable to the volume pulsation of said bubble; and means for detecting and processing said component of the electrical signal to provide an indication of the existence of said perforation.
11. Apparatus as claimed in claim 10, wherein said acoustic sensing means includes a probe and means for suspending said probe from the inlet point in the interior of the tank, said probe having a hydrophone responsive to the acoustic wave energy produced by the volume pulsation during formation of the bubbles.
12. Apparatus as claimed in claim 11 including a water level sensor for sensing the level of a body of water within said tank, the body of water being disposed below the stored liquid and means responsive to said water level sensor for providing an indication of the level of the body of water.
13. Apparatus as claimed in claim 11 or claim 12, including pressure sensitive means for sensing pressure at the bottom of the interior of said tank and means responsive to said pressure sensitive means for providing an indication of said pressure.
14. Apparatus as claimed in claim 10, wherein said pressure reducing means comprises means for pumping gas from the ullage of the tank, duct means for connecting said pumping means to said tank; check valve means for preventing return flow of said gas along said duct means to said tank and spark arrestor means for preventing combustion of said gas through said duct means.
15. Apparatus as claimed in claim 14, further including vent means for venting to the atmosphere said gas pumped from said ullage, the spark arrestor means being associated with said vent means for preventing explosion therein.
16. Apparatus as claimed in claim 10, further including means for supplying an inert gas into said tank after the sensing of the formation of said bubbles.
17. Apparatus as claimed in claim 12, wherein said water level sensor comprises:
first and second elongate sensing electrodes adapted to be immersed in a general vertical direction in the stored liquid, with ends of the first and second sensing electrodes being disposed lowermost;
said first and second sensing electrodes defining an elongate gap therebetween;
first and second reference electrodes spaced apart to define a reference gap therebetween;
said first and second reference electrodes being located in the vicinity of said lowermost ends of the first and second sensing electrodes; and means for measuring the difference between electrical resistance across said sensing gap and across said reference gap, the difference being a function of the length of said sensing gap which is immersed in the water.
first and second elongate sensing electrodes adapted to be immersed in a general vertical direction in the stored liquid, with ends of the first and second sensing electrodes being disposed lowermost;
said first and second sensing electrodes defining an elongate gap therebetween;
first and second reference electrodes spaced apart to define a reference gap therebetween;
said first and second reference electrodes being located in the vicinity of said lowermost ends of the first and second sensing electrodes; and means for measuring the difference between electrical resistance across said sensing gap and across said reference gap, the difference being a function of the length of said sensing gap which is immersed in the water.
18. Apparatus as claimed in claim 17, wherein said measuring means includes means for applying an a.c. voltage to said first sensing electrode to counteract polarization of said electrodes.
19. Apparatus as claimed in claim 12, further including means for resiliently supporting said acoustic sensing means on the bottom of said tank.
20. Apparatus as claimed in claim 11, wherein said probe includes means for sensing the pressure in the environment of said probe.
21. Apparatus as claimed in claim 20, wherein said probe has a liquid-tight isolation chamber, a flexible diaphragm forming one wall of said isolation chamber, a second watertight chamber and a passageway connecting said second chamber to said isolation chamber and said pressure sensing means comprise pressure responsive transducer means in said second chamber.
22. Apparatus as claimed in claim 11, wherein said probe includes means sensing the temperature in the environment of said probe.
23. A method as claimed in claim 5 including reducing the level of the pressure in discrete steps, the component of said signal produced by the volume pulsation during the formation of bubbles being acoustically sensed at each step, and processing the detected electrical signals to determine the sensitivity of the perforation to different pressure differences between the exterior and the interior of the tank.
Priority Applications (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000372936A CA1185693A (en) | 1981-03-13 | 1981-03-13 | Hydrocarbon leakage detection system and apparatus |
| US06/252,212 US4462249A (en) | 1981-03-13 | 1981-04-08 | Tank leakage detection method |
| EP84305140A EP0169283B1 (en) | 1981-03-13 | 1984-07-27 | Method and apparatus for detecting leakage in tanks |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000372936A CA1185693A (en) | 1981-03-13 | 1981-03-13 | Hydrocarbon leakage detection system and apparatus |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1185693A true CA1185693A (en) | 1985-04-16 |
Family
ID=4119442
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000372936A Expired CA1185693A (en) | 1981-03-13 | 1981-03-13 | Hydrocarbon leakage detection system and apparatus |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1185693A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| USRE48204E1 (en) | 2012-08-22 | 2020-09-15 | Franklin Fueling Systems, Inc. | Method and apparatus for limiting acidic corrosion in fuel delivery systems |
-
1981
- 1981-03-13 CA CA000372936A patent/CA1185693A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| USRE48204E1 (en) | 2012-08-22 | 2020-09-15 | Franklin Fueling Systems, Inc. | Method and apparatus for limiting acidic corrosion in fuel delivery systems |
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