GB2331152A - Leakage testing - Google Patents

Abstract

A method of detecting leakage passing along a leakage path 18 extending from a known external leakage point 14 through an internal lining 6 of a fluid storage tank 4, said method comprising: applying a pressure differential between an inner surface of said lining and an outer surface of said storage tank; moving acoustic transducing means (Fig.4 or Fig.5) over said external surface; and detecting, with said acoustic transducing means, acoustic signals characteristic of said leakage path during said movement to identify the course of said leakage path. In one embodiment (Fig.4), the transducing means comprises a pair of microphones (72, 74) spaced apart on a hand-held support (76). In another embodiment (Fig.5), the transducing means comprises a rigid rod (86) with a cavity (94) for conducting sound form the storage tank to a microphone (92).

Description

LEAKAGE TESTING This invention relates to leakage testing, including a method of identifying the course of a leakage path extending from a known external leakage point through an internal lining of a fluid storage tank, and apparatus for locating fluid leakage. The invention is particularly, but not exclusively, applicable in relation to the leakage testing of fliel tanks in aircraft wings.

Aircraft fuel tanks, particularly wing tanks, are designed and built as an integral part of the air frame structure. On completion of the fabrication of the wing section, the inside of the wing is sealed with resins formulated to withstand rapid temperature changes of +/- SOC, whilst remaining sufficiently flexible to withstand the flexing of the wing, during flight.

After a period of service small cracks begin to appear in some of the wing tank linings. Aviation fuel then begins to seep between the liner and the metallic wing skin until it eventually finds an egress point. Such egress points are typically a loose rivet or an overlap joint. The aviation fuel spreads over the surface of the wing creating what is termed in the industry as "wingwet".

As the problem becomes more severe, a drip or trickle of aviation fuel emerges from the wing tank when frill.

Once the leak has been identified externally, a problem is trying to locate the breach within the resin liner, which is usually remote from the visible leakage point external to the wing structure.

Two methods are currently known for identifying the location of the internal breach in the resin liner. According to one method, a vacuum is induced in the empty wing tank and a penetrating dye is applied to the known external leakage point. With a limited possible vacuum allowed within the wing tank, the process of drawing the dye along the leak path and to the inner breach can last a number of days. Once the dye has reached the breach, the dye is visibly identified through a series of inspection covers located on the underside of the wing tank.

A second and less preferred method is to apply a vacuum to the fuel tank and to introduce helium gas to the outer leak site. With access through the inspection covers, a helium detector is used to attempt to identify first of all the wing tank compartment and then the actual site of the breach in the resin lining. Again, this process has a duration of a number of days and is not always successful.

US patent 5,341,670 describes a method for locating leakage from an above ground tank. The method proposed includes receiving acoustic signals at a first set of points, determining a phase delay between the acoustic signals to define a first arc of possible leak locations, receiving acoustic signals at a second set of points, determining the phase delay between the acoustic signals received at the second set of points to define a second arc of possible leak locations and locating the actual position of the leak by interception of the first and second arcs. This proposed method, in common with other proposed methods for determining the existence of a leak within a storage tank by acoustic sensing, is unsuitable for identifying the course of a leakage path extending from a known external leakage point through an internal lining or a fluid storage tank.

In accordance with an aspect of the present invention there is provided a method of detecting leakage passing along a leakage path extending from a known external leakage point through an internal lining of a fluid storage tank, said method comprising: applying a pressure differential between an inner surface of said lining and an outer surface of said storage tank; moving acoustic transducing means over said external surface; and detecting, with said acoustic transducing means, acoustic signals characteristic of said leakage path during said movement to identify the course of said leakage path.

By means of this aspect of the invention, the course of a leakage path even in a storage tank having a complex internal construction and having limited internal access can be accurately identified. By identifjring the course of the leakage path, a repair along the entire course of the leakage path from the external leakage point to one or more internal breach points can be readily and reliably effected.

In accordance with a fUrther aspect of the invention there is provided apparatus for locating fluid leakage, said apparatus comprising a device to be held by a user, said device comprising two acoustic transducers arranged to separately pick up acoustic signals emanating from mutually spaced points on a test surface when the device is held against said test surface.

This aspect of the invention provides apparatus suitable for following the course of a leakage path being identified below the test surface. By balancing the signals emanating from the points at which the acoustic transducers pick up signals as the device is moved over the test surface, the course of the leakage path can be identified.

In accordance with a further aspect of the invention there is provided apparatus for locating fluid leakage, said apparatus comprising a rigid rod member to be held by a user, an acoustic transducer attached thereto and monitoring means responsive to said acoustic transducer, said acoustic transducer being arranged to pick up vibrations transmitted via said rod member in preference to atmospheric vibrations.

This aspect of the invention provides a highly sensitive means for identifying a point or a path of leakage by contacting the device with a test surface.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is a cross-section of an aircraft wing which leaks fuel; Figure 2 is a schematic illustration of components of a test kit used in this embodiment; Figure 3 is a schematic illustration of further components of the test kit being used in this embodiment; Figure 4 is a cross-section of an embodiment of acoustic sensor which may be used to detect the course of a leak path in accordance with this embodiment; and Figure S is a cross-section of a frirther embodiment of acoustic sensor which may be used to detect the course of the leak path.

Referring to Figure 1, a common structure of aircraft wing 2 includes an outer metal skin 4, which is internally lined with a resin composition layer 6.

The wing is divided into several fuel tank compartments 8, by a series of baffles 10, which include connecting passages 12 to allow the transfer of friel between the compartments at a slow rate. The purpose of the baffles is to prevent a rush of fuel and thereby an imbalance or large transfer of weight from one side of the aircraft to the other during banked flying operations. The compartments when full normally contain a majority of liquid friel 8, and a small gas-filled headspace.

Figure 1 shows an example of a typical leakage path which has developed in the wing structure alter a period of service. An external leakage point 14, for example a loose rivet on the underside of the wing allows fuel to escape from the tank via an internal breach 16 in the resin lining 6 and a leakage path 18. The leakage path 18 lies between the resin lining 6 and the outer skin 4 of the wing.

Figure 2 shows components of a test kit according to this embodiment of the invention. A control case 70 houses a vacuum pump 24, connected to an exhaust outlet 26, a manually-operable vacuum regulator 28, a manuallyoperable pressure regulator 30, a mechanical positive/negative pressure gauge 32, and two electronic pressure transducers 34, 36. A canister 38 of oxygenfree nitrogen, or other suitable inert gas, is connected to an inlet port 40 of the control case. The inlet port 40 is connected to a manually-operable three position selector valve 42. In a first position, the selector valve 42 connects the inlet port 40, via the vacuum regulator 28, to the vacuum pump 24. In a second poSition, the selector blanks off the inlet port 40. In a third position, the selector valve 42 connects the inlet port 40 to the pressure regulator 40. A manually-operable two position selector valve 44 connects an outlet port 46 either to the vacuum pump 24 or the pressure regulator 30, depending on its position. The pressure gauge 32 is connected to the outlet port 46.

One of the pressure transducers 34 is connected to a first pressure return port 48 via a manually-operable open/close selector valve 52, depending on its pOsition. The other pressure transducer 36 is connected to a second pressure return port 54 via a further manually-operable open/close selector valve 56, depending on its position.

The pressure transducers 34, 36 are electronic pressure transducers having a displayed read out accurate to 0. l Mb, such as a Druck (trade mark) pressure transducer, Model No DPI 700 IS. A coil of neoprene tubing 58, 60, connects each pressure transducer 34, 36 to its respective selector valve 52, 56. The purpose of this tubing coil is to provide a damping action in relation to the incoming pressure data, such that the coils absorb instantaneous transient fluctuations in pressure which would destabilize the reading on the pressure transducers 34, 36. The coils move, i.e. expand and contract, to provide the damping action. A similar effect could be achieved by using coils of other flexible materials, such as copper, or by arranging for an amount of liquid to be present in the pressure return path close to the pressure transducers 34, 36, for example liquid mercury, which would damp such incoming pressure fluctuations.

In the first stage of the testing procedure the outlet port 46 and the first pressure return port 48 are connected, via a T-junction and flexible pipelining, to the fuel tank vent outlet in the underside of the wing. This vent outlet is connected to a vent box, located centrally in the fuselage, via a vent line, which in turn vents the gas pressure in each of the compartments of the fusel storage tank in each wing. The vent port thereby serves two purposes. First, in normal use, the vent serves to equalise the gas pressure within the fuel storage tanks with atmospheric pressure. Secondly, during testing, the vent port is used to alter the pressure within the fuel storage tanks with respect to atmospheric pressure.

During testing, the second pressure return port 54 of the control case 20 is connected, by means of a specially-adapted connector to the refUelling port, thereby to sense the liquid pressure within the fuel storage tanks, at the base of the liquid. The specially-adapted connector is similar to a standard fuelling nozzle, but connects a pressure return line to the fuelling port rather than a fuelling hose.

Once connected as described, the control case 20 is set, using the selector switch 42, the pressure regulator 30 and two-way selector switch 44, to generate a positive pressure at the outlet port 46. In this manner, the gas pressure within the fuel storage tank is increased to a predetermined positive pressure, in this embodiment 200 Mb. As stated above, the fuel tank is full to its maximum capacity with their normal contents, in this case aviation fuel.

The fuel storage tank is sealed for a monitoring period, to determine a rate of leakage. The gas pressure within the storage tank is monitored on the first pressure transducer 34, whilst the liquid pressure at the base of the tank may be monitored with the second pressure transducer 36. The rate at which the excess pressure within the fuel storage tank is lost is indicative of the size of the leak. A calibration with respect to the known gas volume within the fuel storage tank at maximum capacity provides a calibrated leak rate.

Furthermore, by comparing the rate of pressure loss within the gasfilled head space with the pressure change observed in the liquid-filled portion of the tank (the respective readouts of the first pressure transducer 34 and the second pressure transducer 36), it can be identified whether the internal breach 16 is within the gas-filled headspace in the storage tank or within the liquid filled space. If the internal breach is within the gas-filled headspace, the decrease in pressure monitored on both of the pressure transducers 34, 36 is equal. On the other hand, if the internal breach is in the liquid-filled space, the pressure drop monitored on the second pressure transducer 36 (that indicating the pressure change in the liquid-filled space) is considerably greater than the pressure drop within the gas-filled headspace. This is because the pressure monitored on the second pressure transducer 36 is the sum of the pressure within the gas-filled space and the weight of the liquid above the point of monitoring. As liquid escapes, both the pressure within the gas-filled headspace and the weight of liquid above the monitoring point decrease.

In order to identify the actual height of the inner breach within the tank, the pressure within the gas-filled headspace of the tank is reduced, by appropriate setting of the switches 42, 44 and the vacuum regulator 28 in the control case 20, to a negative pressure of-200 Mb. The tank is then sealed, and the gradual reduction in gas pressure is monitored on the first pressure transducer 34. In the case of a single internal breach 16, the pressure will gradually decrease and tend towards a given negative pressure, which is lower than atmospheric pressure. At this pressure, the height of the fuel above the internal breach 16 counter-balances the negative pressure within the gas-filled headspace, and no further ingress of air occurs. The end vacuum pressure is proportionate to the depth of the breach below the liquid surface, which may thus be readily calculated.

It is possible that more than one breach 16 exists within the lining of the tank. In the case of a number of internal breaches (the leak path may be branched) the initial leakage rate under vacuum consists of a combination of the leakages into each of the internal breaches 16. As the vacuum gradually decays, the lowest breach will at one point stop admitting air, due to the liquid height/gas pressure balancing effect described above. Meanwhile, the higher internal breach will continue admitting air. Thus, by monitoring the rate of leakage over a period of time it is possible to distinguish internal breaches at a plurality of different heights within the fuel storage tank. Around each of the above-described balance-point vacuums, a charge in the gradient of the rate of leakage will indicate the presence of an additional internal breach 16. Thus, the approximate depth of each breach can readily be calculated.

Once these internal procedures have been carried out, the following information will have been identified: (a) a total leakage rate, indicating the seriousness of the leakage from the tank; (b) whether the internal breach is in a part of the tank normally containing liquid or normally containing air; (c) in the case of a liquid leak, the depth(s) of one or more internal breaches within the liquid.

In the case of a combination of breaches spaced throughout the inside of the storage tank, the various rates of decay at both positive and negative test pressures and above the liquid surface and below the liquid surface may be used by a test operator, using the indications referred to above, to identify the various breaches 16 and their heights within the tank.

The next stage of the testing procedure concerns identifying the course of the leakage path 18 within the resin lining 6. In this stage of the testing procedure, the control case 20 is set to produce a constant negative pressure, for example -200Mb, within the gas-filled headspace of the fliel storage tank.

Thus, the fusel storage tank is not sealed during the second stage of the testing procedure, but rather is subject to constant vacuuming by the vacuum pump 24, at a pressure set by manipulation of the vacuum regulator 28.

The further components of the test kit used in this stage of the procedure, referring to Figure 3, are a dual-microphone acoustic sensor 62 and/or a sensing rod 84, a two channel signal receiver and processor 64, a pair of stereo headphones 66, a signal recorder 68 and a two channel oscilloscope 70.

As shown in Figure 4, the dual-microphone acoustic sensor 62 includes two microphones 72, 74 picking up signals from a test surface, in this case the wing skin 4. The microphones are spaced apart on two sides of a hand grip 76. A circular rubber flange 78 surrounds the space in front of each microphone 72, 74, in order to block atmospheric sound from the listening space defined between the microphone 72 and the test surface 4. In addition, the space adjacent to and behind each microphone 72, 74 is filled with a silicone gel 80, which also tends to block atmospheric sounds reaching each microphone 72, 74. Both microphones 72, 74 are connected to a two channel radio transmitter 82.

The receiver 64 contains a two channel amplifier and a variable frequency selector. By varying the frequency range selected by the receiver 64, specific frequencies characteristic of the leakage path can be selected for monitoring on the headphone 66, the signal recorder 68 and the oscilloscope 70.

In particular, when the fuel tank, having a breach as illustrated in Figure 1, is depressurised, the sound of incoming air flowing along the leakage path 18 connecting the external leakage point 14 with the internal breach 16 has a characteristic frequency which is relatively high (between 10-20 kH depending on the materials fonning the leakage path 18).

In order to identify this characteristic frequency, the acoustic sensor 62 is first placed adjacent to the known external leakage point 14, and the frequency selector is tuned until the characteristic frequency of the incoming air flow is detected. Once the receiver 64 is tuned to a frequency range including this characteristic frequency, the acoustic sensor 62 is moved to track the course of the internal leakage path 18. The course of the path is tracked by balancing the signals picked up by each respective microphone 72, 74 whilst the test operator holds the acoustic sensor 62 against the wing skin 4 as illustrated in Figure 4. The balancing of the signals picked up by the two microphones 72, 74 whilst the acoustic sensor 62 is held orthogonal to the presumed course of the leak path 18 acts to maintain the leak path 18 centrally between the two microphones 72, 74.

If, as the operator moves the acoustic signal 62 along the leakage path 18 being charted, the volume of sound heard in one side of the headphones 66 exceeds the volume of sound heard in the other, and/or correspondingly one of the signals shown on the oscilloscope display has a greater amplitude than the other, this indicates that the leak path is no longer central between the two microphones 72 and 74. The operator correspondingly adjusts the position of the acoustic sensor 62 to maintain balance between the two signals. During identification of the course of the leakage path 18, the operator can mark the course, for example with soft chalk, on the outside of the wing 2. The course is then charted until the internal breach 16 is reached, at which point the magnitude of the signals decreases. By, for example, rotating the acoustic sensor 62 about an axis normal to the wing surface, the test operator can readily identify that the end of the leakage path has been found. Alternatively, the receiver 64 can be tuned to a different frequency range, somewhat lower than the previous frequency range, to listen to a signal characteristic of bubbles of air detaching from the inner surface of the tank within the liquid content, also indicating the end of the leakage path.

Optimally, the microphones 72, 74 are spaced by approximately 20 cm, although other spacings are also possible.

A further embodiment of acoustic sensor which may be used for confirming the course of the leakage path 18 is illustrated in Figure 5. This is a sensing rod 84 having a microphone which is also be connected to the arrangement of the radio transmitter 82, the receiver 64, headphones 66, recorder 68 and oscilloscope 70 as described in relation to the dualmicrophone acoustic sensor 62. In this case, only one channel is required as only a single signal is to be sensed.

This sensing rod 84 includes a cylindrical body 86 having a solid end portion 88 with a rounded base 90. The rounded base 90 is designed to pick up acoustic vibrations by direct contact with a test surface. The opposite end of the body 86 receives a microphone 92, connected as described above to a radio transmitter 82. Between the microphone 92 and the solid end 88, a resonance cavity 94 extends along the length of the body 86. The microphone is solidly and rigidly threaded into the body 86, such that vibrations transmitted along the body 86 are transmitted directly to and picked up by the microphone.

Furthermore, the microphone 92 is directed towards the cavity 94 within the body 86 such that sounds transmitted along the resonance cavity 94 are picked up with great sensitivity. On the other hand, atmospheric sounds emanating from outside the body 86 are not readily picked up by the microphone 92.

The sensing rod 84 provides a means of increased sensitivity for detecting vibrations within the test surface 4 of small amplitude. The sensing rod 84 is highly sensitive to the characteristic sound of air flowing along the leak path 18. With the receiver 64 tuned to select a frequency range spanning the characteristic frequency of the air flow along the leakage path 18, a test operator can readily detect proximity of the leakage path by contacting the rounded base with the wing surface and identify the course of the leakage path 18 by moving the sensing rod 84 over the outer surface of the wing skin 4, preferably starting from the known external leakage point 14 and ending at the internal breach 16.

It is preferred that both the dual-microphone acoustic sensor 62 and the sensing rod 84 are used in combination in order to positively identify the course of the leakage path 18. First, the dual-microphone sensor 62 is used to coarsely plot the leakage path 18. Once so plotted, the sensing rod 84, with its higher sensitivity, is used to diagonally criss-cross in a zig-zag fashion across the coarsely charted leakage path, to finely locate the path of leakage.

Once the leakage path has been identified along the external surface of the tank, a corresponding part of the inner lining can be cut out and repaired with great confidence that the leakage path 18 will have been repaired along its whole length, and that then no further leakage will occur from the external leakage point 14.

In the above-described procedure, the characteristic signal monitored on the acoustic sensors is that of air travelling along the leakage path 18. It is possible to increase the volume of the characteristic signals by applying liquid (preferably aviation fiel) to the external leakage point 14 during acoustic Sensing, to ensure that the fluid passing along the leakage path consists of a mixture of air and liquid. This may be achieved, for example, by spraying the liquid onto the wing surface at the external leakage point during acoustic monitoring.

Other modifications and variations are possible within the scope of the invention Furthermore, arrangements similar to the embodiments of acoustic sensor described may be used in other types of leakage sensing operations, for example underground pipeline leakage detection and location.

Claims (25)

1. A method of detecting leakage passing along a leakage path extending from a known extemal leakage point through an internal lining of a fluid storage tank, said method comprising: applying a pressure differential between an inner surface of said lining and an outer surface of said storage tank; moving acoustic transducing means over said external surface; and detecting, with said acoustic transducing means, acoustic signals characteristic of said leakage path during said movement to identify the course of said leakage path.

2. A method according to claim 1, comprising moving said acoustic transducing means from said external leakage point to the location of a breach in said lining by monitoring said characteristic signals.

3. A method according to claim 1 or 2, comprising following said leakage path during said movement.

4. A method according to any preceding claim, wherein said transducing means comprises two spaced microphones, said method comprising comparing the acoustic signals picked up by said microphones.

S. A method according to any preceding claim, comprising repeatedly moving said acoustic transducing means across said leakage path during said movement.

6. A method according to any preceding claim, comprising applying a vacuum to said storage tank during said movement.

7. A method according to claim 6, comprising drawing gas into said storage tank during said movement.

8. A method according to claim 6 or 7, comprising applying liquid to said external leakage point during said movement.

9. A method according to claim 8, wherein said liquid is a normal content of said storage tank.

10. A method according to any preceding claim, wherein said storage tank comprises an aircraft wing.

11. Apparatus for locating fluid leakage, said apparatus comprising a device to be held by a user, said device comprising two acoustic transducers arranged to separately pick up two acoustic signals respectively emanating from mutually spaced points on a test surface when the device is held against said test surface.

12. Apparatus according to claim 11, comprising surrounding means for blocking sounds from the surrounding atmosphere when so held.

13. Apparatus according to claim 12, wherein said surrounding means comprises a flexible flange.

14. Apparatus according to claim 11, 12 or 13, comprising monitoring means for separately monitoring said acoustic signals emanating from said points.

15. Apparatus according to claim 14 wherein said monitoring means comprise a loudspeaker and/or a signal display means.

16. Apparatus according to claim 14 or 15, wherein said monitoring means comprises tuning means for altering the frequency response of said monitoring means.

17. Apparatus for locating fluid leakage, said apparatus comprising a rigid rod member to be held by a user, an acoustic transducer attached thereto and monitoring means responsive to said acoustic transducer, said acoustic transducer being arranged to pick up acoustic vibrations transmitted via said rod member in preference to atmospheric vibrations.

18. Apparatus according to claim 17, said acoustic transducer being rigidly attached within a cavity in said rod member.

19. Apparatus according to claim 17 or 18, said rod member comprising a solid portion forming a point of contact for picking up acoustic vibrations at one end of said rod member.

20. Apparatus according to claim 19, wherein said acoustic transducer is located at the other end of said rod member.

21. Apparatus according to any of claims 17 to 20, said monitoring means comprising tuning means for altering the frequency response of said monitoring means.

22. A method of locating fluid leakage using the apparatus of any of claims 11 to 20, said method comprising holding said apparatus against a test surface, and moving said apparatus to detect selected acoustic signals indicative of said fluid leakage.

23. A method of detecting the depth of a leakage point within a liquid-filled storage tank, said method comprising creating a vacuum within said storage tank, changing the amount of said vacuum and monitoring a characteristic varying with differing rates of gas ingress due to said change to determine a vacuum pressure at which said leakage point no longer admits gas, thereby to determine the depth of said leakage points below the liquid surface.

24. A method according to claim 23, comprising detecting the depths of a plurality of different leakage points within said tank using said method.

25. Apparatus, or a method, substantially as hereinbefore described, in particular with reference to the accompanying drawings.