GB2282892A - Method of detecting a leak in a barrier between two regions - Google Patents

Abstract

A method of detecting the presence of a leak in a barrier, e.g. a floor membrane 16 of a building or the wall of an underground a pipeline 61, comprises creating a pressure differential across the barrier, e.g. by introducing superambient air at a plurality of spaced points into the region 14 beneath the building or by creating a vacuum within a sealed section 63 of the pipeline, and detecting the presence of toxic or noxious soil gases G on the lower pressure side of the barrier (i.e. in region 15 or in the sealed section 63) within a limited period of time from the creation of the pressure differential. <IMAGE>

Description

METHOD OF DETERMINING THE INTEGRITY OF A BARRIER BETWEEN ONE REGION AND ANOTHER The present invention relates to a method of determining the integrity of a barrier between one region and another, and to apparatus for use in the method.

The invention also extends to and includes the use of a tracer gas to detect breaches in the barrier through which migration of gases may occur from a region to one side of the barrier into that on the other, thus establishing the integrity of the said barrier.

As used in this specification, the term gas or "gases" will be understood to include a vapour or vapours.

As used in this specification, the term "region" will be understood to refer to a volume bounded along one face by the barrier, extending to the side of the said barrier, and otherwise bounded or unbounded.

A current problem experienced in, for example, buildings close to or on sites used for disposal of urban refuse, is the presence of high levels of unwanted gases, especially toxic or noxious gas, such as methane gas arising from the breakdown of the refuse: in other cases the problem may arise from gases emanating from subterranean sources, in either case where gases are able to enter regions at least partly defined by a barrier, they may become concentrated thereby posing a potential health risk.

The presence of unwanted hazardous gases in such regions often results from breaches in the integrity of the barrier. The need to establish and maintain a gas impermeable barrier between a region to be protected and the source of the gases is important where there is a risk of contamination with hazardous gases.

Various methods are known for testing the integrity of a barrier. Known methods of testing a barrier for integrity are based on testing for the presence of certain identified gases by sensors located on the side of the barrier expected to be protected from the ingress of said gases. These known methods are useful but have several limitations. Probably the most significant of these results from the very low concentration levels of many of the unwanted gases and consequently the extended sensing times which are required for the attainment of a viable result. There are various environmental factors distorting the data obtained by such methods, such as changes in temperature, wind and water tables, as well as human induced factors such as dumping of toxic waste into rivers or oceans.

There has thus been a need for an economic, accurate and effective means of testing the integrity of such barriers.

The present invention seeks to provide a new technique for testing barriers for breaches, which will avoid many of the problems and inaccuracies encountered by prior art techniques.

Embodiments of the invention may enable breaches in a barrier to be ascertained with greater certainty. Without prejudice to the generality of the invention, non limitative examples of barriers to which the present invention may be applied include the floor of a building, the wall of a tunnel, or an industrial pit lining. From accurate data it should then be possible to make assessments and decisions which are important for pollution control, environmental and economic reasons.

According to the present invention there is provided a method of determining the integrity of a barrier between one region and another, comprising the steps of creating a pressure differential across the said barrier and determining the presence of a selected gas in one region defined by the barrier within a limited period of time from the creation of the said pressure differential.

In performing the method of the invention the pressure differential may be created by supplying a gas into the said other region. The subsequent increase in pressure within the said other region forces movement of any gas present through any breaches in the barrier, into the said one region. A variation in concentration of a selected gas in the said one region can then be detected more readily than would be the case in the absence of the pressure differential.

The method of the invention is based on a realisation that the variation in concentration of the selected gas must be established within a time limit as there is a limited volume of that gas available for detection. This is due to the fact that the pressure field created by the increase in pressure inhibits the migration of gas into the region thus pressurised so that only that volume of gas present at the commencement of pressurisation can traverse the barrier.

Delay in detection of the selected gas may therefore produce inaccurate results. In another aspect the present invention therefore seeks to provide an improved technique for determining the movement of gases through breaches in a barrier, which avoids many of the problems and inaccuracies encountered when there is delay in detection of the selected gas.

The invention envisages the introduction of at least one detectable tracer gas as the said selected gas into the other region on the side of the barrier opposite the said one region prior to the said detection step.

The concentration of the selected gas at least at one point within the said one region is sensed, whereby a change in the said concentration is detected, and an indication of the occurrence of such a change is provided.

The step of sensing a variation in the concentration of the said selected gas may be performed with respect to either time or distance. This step may be performed with a sensor stationary within the said one region over a period of time such that a temporal variation in concentration is detected. However, the said step may also be performed by displacing a sensor within the said one region, such that a spatial variation in concentration is detected.

Having established a method whereby a selected gas is detected, points of maximum concentration of the said selected gas may be sought within the said one region, thereby locating points of likely breach of the barrier.

The invention extends to a method in which a plurality of distinguishable tracer gases are introduced to the said other region and the presence of a tracer gas in the said one region is detected. The tracer gases may also be introduced at a plurality of entry points into the said other region. This may allow the movement of gases of different characteristics to be assessed under the same pressure conditions, thus enabling analogies to be drawn with natural or industrial gases in the environment.

Alternatively the present invention may include a method in which tracer gases are sequentially introduced at a single entry point into the said other region.

In another aspect of the invention, the movement of tracer gases may be tested under a plurality of pressures. Thus embodiments of the present invention may include a step of varying the pressure differential across the said barrier. This enables comparisons to be made with different gases under varying pressure conditions, induced, say, by temperature, wind, moisture level or other factors.

An advantage provided by the ability to vary the pressure input is that experiments to detect gas ingress through breaches in a barrier may be conducted at a variety of pressures with a variety of different time limits thereby compensating for environmental factors.

A further advantage of the method of the invention is that it allows a consistent pressure field of desired pressure to be sustained if the results are likely to be influenced by environmental factors (eg. temperature).

In another aspect, the present invention provides a method in which the said pressure differential is maintained substantially constant over a selected period of time and the said detection step is repeated whereby to establish the effect of any variation in environmental conditions during the said selected period of time. This is especially important if prolonged or continual monitoring of integrity is required, for example a method of protecting buildings by detecting breaches in a barrier, such as a floor structure or sub-floor membrane.

One example of the present invention is a method in which the said barrier separates the ground structure of a building from the superstructure thereof. A second such example is a method in which the said barrier defines a pit such as a refuse infill site. A third such example is a method in which the said barrier defines a tunnel or a pipe or a tube.

The above examples of application of the method of the invention are equally applicable in circumstances where it may be desired to monitor the integrity upon creation of a barrier and to monitor the integrity of the barrier on a continual basis.

Another aspect of the present invention comprises a method including the steps of determining a pressure field plan of the pressure field on the said other side of the barrier resulting from the creation of the pressure differential across the barrier. This enables assessment and monitoring of the pressure differential to be undertaken.

In another aspect, the invention provides apparatus for use in determining the integrity of a barrier, comprising means for creating a pressure differential across the said barrier, gas sensor means for detecting the presence of a selected gas in a region defined at least in part by the said barrier, and means for determining a time period within which the gas sensor is operative.

The apparatus of the invention may provide means for delivering gas at superambient pressure into the said other region. Alternatively, the apparatus of the invention may provide means for reducing the pressure of gas in the said one region to a subambient pressure, this may be achieved by extracting gas from the said one region.

A preferable embodiment of the present invention includes apparatus in which the means for introducing the said gas at superambient pressure comprise pump means for elevating the pressure of the said gas, and a distribution unit in communication with the said pump means, the said distribution means delivering the said gas to a plurality of locations in the said other region.

Advantageously the distribution unit may be controlled by a pressure sensor arranged to regulate the operation of the pump. The apparatus of the present invention therefore preferably includes a further pressure sensor on the said other side of the barrier, and means sensitive to an output signal from the said further sensor, arranged to regulate the pressure of the gas introduced to the said- other region in response to variation in the said output signal. The invention therefore provides means for maintaining an economic pressure for continual monitoring or monitoring in response to changes of pressure over time.

The apparatus as hereinabove described may include a plurality of means for introducing the said gas.

Further, the apparatus as hereinabove described may include apparatus in which the sensitivity of the sensor means is adjustable.

The present invention may also include apparatus in which the said means for determining the said time period is adjustable.

According to another aspect of the present invention, there is provided apparatus as hereinabove described including means for determining a pressure field plan of the pressure field in the said other region which is generated upon the creation of the pressure differential across the barrier. The pressure field plan may be obtained by introducing tracer gas and detecting its arrival time at the said means for determining the pressure field plan. Once a known quantity of tracer gas has been introduced, any further addition of the tracer gas is stopped. The pressure differential is maintained at a substantially constant value and the time taken for the complete dispersal of the tracer gas is used to calculate the potential rate of migration at normal pressure.

Further, the embodiment of the invention incorporating means for determining the pressure field plan may include means for detecting local pressure in a plurality of different locations within the said other region without breaching the said barrier. Alternatively, one specific embodiment may have one or more sampling means which may pass through the barrier. These, of course, should preferably be capable of being sealed after use. However, one or more permanent sampling ducts may exist as prelaid sampler tubes located below the barrier. It is preferred that these tubes be fitted with hydrophobic membranes to protect against obstruction by water.

Embodiments of the invention will now be described, by way of non-limitative example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a cross section of a building to which a first embodiment of the invention has been applied; Figure 2 is a cross section, on an enlarged scale through the floor membrane of the building shown in Figure 1; Figure 3 is a plan view from above of the pressure field created underneath the floor membrane of the building shown in Figure 1; Figure 4 is a schematic perspective view showing a distribution system for creating a superambient pressure field below the membrane of a building in which a second embodiment of the invention is incorporated; Figure 5 is a schematic cross sectional view of a second embodiment of the invention in which a low pressure field is created within a closed region such as a tunnel; ; Figure 6 is a diagram illustrating the variation in concentration of a gas over time during the performance of the method of the invention; Figure 7 is a diagram illustrating an exemplary spatial variation in concentration of a gas detected using the method of the invention; Figure 8 is a flow chart showing the major steps in conducting an assessment of the integrity of a barrier by the method of the invention; and Figure 9 is a flow chart showing an alternative assessment technique.

With reference to the drawings, Figure 1 is a cross section through a building, with footings 11 carrying walls 12 supporting a roof 13.

The cross section also shows a porous subfloor region 14 which is separated from the dry enclosures of the dwelling by an impermeable floor membrane 16.

A problem experienced in a building such as the one described above, is that unwanted toxic and noxious gases, emanating from the soil beneath the dwelling, may concentrate at the subfloor region 14 and penetrate through any breaches in the floor membrane 16 into the enclosed region 15 of the dwelling, thereby posing a health risk.

In Figure 1 the means by which the present invention is employed to determine the integrity of the floor membrane 16 are shown.

Although the movement of soil gases G is multidirectional, greater concentrations of soil gases G will be found in the subfloor region 14 due to entrapment.

As can be seen from Figures 1 and 3 pumps 18 and 33 are located close to the building at diagonally opposite corners. Since they are substantially the same, only the pump 18 and its associated pipework will be described in detail. The delivery outlet of the pump 18 is connected to a pipe 19 leading to three branch delivery pipes 20, 21, 22 which lie on the surface of the ground. The pipes 20, 21 and 22 lead to vertical delivery sections 23, 24, 27, 28 which line vertical bore holes sunk in the ground to a depth slightly greater than the maximum depth of the footings 11. The vertical delivery sections 23, 24, 27, 28 terminate in respective delivery outlets from which air (or other gas) pumped from pump 19 can be delivered into the porous subfloor region as indicated by the arrows A. AS can be seen in Figure 1, the vertical delivery section 24 terminates in a delivery outlet 26.

In use, a pressure differential is created across the floor membrane 16 by the introduction of air at superambient pressure into the subfloor region through the pipes 20, 21, 22 and operation of the pump 18 to create a pressure field 17, the general form of which is illustrated in Figure 3. Soil gases G which have migrated into the subfloor region 14 are urged to move across the pressure differential, into the lower pressured enclosed region 15, through any breaches which may exist in the floor membrane 16. The presence of soil gases G within the enclosed region 15 is detected by a sensor 27 having associated detector and display apparatus 28 thereby establishing a lack of integrity of the floor membrane.The superambient pressure field 17 created in the subfloor region 14 and the effect on soil gases G of the consequent pressure differential across the floor membrane 16 is illustrated in more detail in Figure 2.

Figure 2 illustrates the superambient pressure field 17 created in the subfloor region 14 of the dwelling as described in Figure 1, and further illustrates the effect on soil gases G within the subfloor region 14, of the pressure differential created across the floor membrane 16.

The superambient pressure field 17 is created by the introduction of air, at superambient pressure, into the partly defined subfloor region 14. The subfloor region 14 is therefore at greater pressure than the enclosed region 15 of the building, which remains at atmospheric pressure. The effect of the pressure differential across the membrane is that soil gases G within the pressure field 17 of the subfloor region 14 are urged to move towards the lower pressure enclosed region 15 of the building through any breaches in the floor membrane 16.

Figure 3 is a detailed illustration of the pressure field created in the subfloor region 14. Air, at superambient pressure, is pumped by pump 18 and delivered into the subfloor region 14 and disperses from each delivery pipe outlet 25 to 28 and is pumped by pump 33 to delivery outlets 29 to 32 at sufficient superambient pressure to create a high pressure region over the whole of the area defined by the membrane 16. One such outlet is shown in Figure 1, from which air, pumped at superambient pressure, disperses in all directions creating a high pressure region centred on the outlet and decreasing in pressure with distance from the outlet. As can be seen in Figure 3 individual regions surrounding the outlets 25 to 32 merge to form the overall pressure held in the subfloor region 14.

As can be seen in Figure 3, the two pumps 18, 33 are positioned at diagonally opposed corners of the dwelling in order to provide sufficient capacity for air to be pumped at superambient pressure into the entire subfloor region 14. Delivery pipes 19, 34 are connected to each pump 18, 33 respectively. The pumps 18, 33 and their delivery pipes 19, 34 are positioned closely around the dwelling such that the floor membrane and the subfloor region 14 are surrounded.

The delivery pipes 19, 34 extend from the pumps 18, 36 to strategic points 25-32, such that the dwelling is evenly surrounded, and further extend them into the ground to a level where the subfloor region 14 can be adequately pressurised, without impairing the integrity of the floor membrane.

Figure 4 on the other hand shows a system which can be installed at the time the dwelling is being built. The advantage of this lies in the fact that pipes can be laid under the floor and an economical arrangement of pipes and delivery points devised allowing air to be delivered at an optimum superambient pressure, via an economic delivery pipe arrangement while maintaining a substantially homogenous pressure field.

Figure 4 illustrates an assembly for the distribution of air, at superambient pressure, into the subfloor region 40, and the consequent creation of a pressure field 41 of sufficient pressure to create a pressure differential across the floor membrane such that the air, carrying any soil gases present within the pressure field, will move through any breaches in the floor membrane, into the enclosed region of the dwelling.

The assembly comprises a pump 42 having two delivery pipes 43, 44 leading from the pump to two pairs of branch pipes 45, 46 and 47, 48 having outlets 49, 50 and 51, 52 respectively. The outlets 49, 50 and 51, 52 are located within the subfloor region 40 at points such that the pressure field 41, which is created when the pump 42 is operated to deliver air at superambient pressure to the outlets 49, 50 and 51, 52 extends entirely over the whole area under the floor at a sufficiently high minimum value for the pressure differential to be considered substantially uniform over the whole of the membrane.

Figure 5 illustrates a second embodiment of the invention showing the application of the invention to a subterranean pipe structure. In this embodiment the pipe structure is a water pipe 61 the impermeable wall 60 of which acts both to carry water without leakage and to protect the water from contamination by the ingress of toxic or noxious soil gases G.

Because the region around the pipe is unbounded it is not practical to create a superambient pressure around the pipe, and if such superambient pressure is created within it, detection would be difficult because of the surrounding soil. Therefore in this embodiment of the invention a subambient pressure is created within the pipe 61. Figure 5 illustrates the means by which such a subambient pressure field 7 is created in the inner region 63 of the pipe so as to create a pressure differential across the wall of the pipe 61. A portion of the pipe to be tested is sealed at both ends 64, 65 to create a fixed volume within the pipe structure, the integrity of which is to be determined, such that either gas ingress from the soil into the pipe or gas egress from the pipe into the soil, (if, for example, the pipe is carrying a gas supply), can be identified.

A pump 66 is connected by an associated suction pipe 67 to the inner region 63 of the pipe, at a junction point 68. Air is extracted from the inner region 63 of the pipe by the pump 66, thereby creating a region of low or subambient pressure within. If there are any breaches in the wall of the pipe 61 gases G migrate from the outer region 62 across the pressure differential into the inner region 63 of the pipe, that is to the subambient pressure region. The presence of the gases within the pipe is detected by a sensor 69 having associated detector and display apparatus 70, located in the inner region 63 of the pipe, thereby establishing a lack of integrity of the pipe wall 60.

Time is not such an important consideration in this situation as it is in the embodiment illustrated in Figure 1, nevertheless, time is still limited by the fact that Gases G will migrate into the subambient pressure region 63 more quickly than they will be replaced in the outer region 62 so a natural depletion over time would be expected.

Figure 6 is a diagram illustrating the detection of concentrations of a selected gas over time. The concentrations C of a selected gas which are detected are plotted along the ordinate, ranging from 0% (no selected gas detected) to 100% (maximum selected gas detected).

These concentrations are plotted against time T, measured in seconds along the abscissa. The commencement of detection is taken to be at 0 seconds and the detection will continue for a time such that sufficient concentrations of the selected gas are detected for reliable and consistent results. The diagram shows a base level B of normal concentration volumes of the selected gas present in the atmosphere from escape into the surrounding surface. Any detected concentration C1 over and above the base concentration B is indicative of a breach in the integrity of a barrier. By plotting a diagram of this kind, the rate of migration of the selected gas can be determined. The rate of migration of the selected gas can be calculated from the starting time of detection at 0 seconds to the time tl, of maximum concentration detected at a constant distance d.

Figure 7 is a diagram illustrating the detection of concentration of a selected gas over a given distance.

The concentrations of a selected gas which are detected are plotted on the ordinate ranging from 0% (no selected gas detected) to 100% (maximum selected gas detected).

These concentrations are plotted against distance, measured in metres along the abscissa. The commencement of detection is taken to be at 0 metres at a point A and the detection will continue for a given distance to a point B. The diagram shows as before a base level B of normal concentration volumes of a selected gas. Any detection in concentration Cl over and above the normal base concentration B is indicative of a breach in the integrity of a barrier, and is further indicative of where the breach is situated. The point dl shows the most likely position of a breach.

Figure 8 is a flow chart showing the principal steps in conducting an assessment of the integrity of a barrier (as illustrated in Figure 1). Steps 1 and 2 relate to the creation of a greater pressure region relative to the said one region on the opposite side of the barrier. The pressure differential may alternatively be formed by creation of a lower pressure region relative to the said other region.

Monitoring of the pressure field created is covered by step 3; this is an optional step and may be achieved by assessing the pressure at selected points by means of ducts which traverse a barrier, or by means of a pressure sensor situated within the pressure field area itself and connected electrically to the display apparatus.

Step 4 relates to the sensing of the variation in concentrations of a selected gas by sensor means, which variation may be temporal, spatial or both.

Step 5 illustrates the need for the process of step 4 to be performed within a limited period of time. If the process of creating a greater pressure field is used the time required for sensing the selected gas is limited due to the inability of the selected gas to enter the positive pressure field area, once it is created, such that there is only a limited amount of the selected gas, already within the greater pressure field upon its creation, which will be available to move into the lower pressured region.

Step 6 shows that by using both temporal and spatial sensing methods, points of increased concentration of the selected gas may be established, thereby indicating a particular breach in the integrity of the barrier, and further the rate at which the movement of the selected gases occurs across the pressure differential may be determined thereby establishing the extent or emergency of the breach.

Figure 9 illustrates the steps in an alternative method from that shown in Figure 8, whereby the selected gas is a tracer gas which is introduced into the system at a known volume or pressure and so has the advantage of being easily detectable at very low percentage volumes and furthermore, can be easily introduced into the greater pressure field region, thus enabling repeated assessment of the integrity of the barrier.

Claims (27)

CLAINS

1. A method of determining the integrity of a barrier between one region and another, comprising the steps of creating a pressure differential across the barrier and detecting the presence of a selected gas in one region defined by the barrier within a limited period of time from the creation of the said pressure differential.

2. A method as claimed in claim 1 including the steps of introducing at least one detectable tracer gas as the said selected gas into the other region on the side of the barrier opposite to the said one region prior to the said detection step.

3. A method as claimed in claim 1 or 2, in which the concentration of the selected gas at least at one point within the said one region, is sensed whereby a change in the said concentration is detected, and providing an indication of the occurrence of such a change is provided.

4. A method as claimed in claim 3, in which the step of sensing the concentration of the said selected gas is performed with a sensor stationary with respect to the said one region over a period of time such that a temporal variation in concentration is detected.

5. A method as claimed in claim 3 in which the step of sensing the concentration of the said selected gas is performed by displacing a sensor within the said one region, such that a spacial variation in concentration is detected.

6. A method as claimed in claim 3 whereby points of maximum concentration of the said sensed gas are located within the said one region, thereby locating points of likely breach of the barrier.

7. A method as claimed in claim 2, in which a plurality of distinguishable tracer gases are introduced to the said other region and the presence of the tracer in thee said one region is detected.

8. A method as claimed in claim 2 or claim 7, in which tracer gases are introduced at a plurality of entry points into the said other region.

9. A method according to claim 2 or claim 7, in which tracer gases are sequentially introduced at a single entry point into the said other region.

10. A method as claimed in any preceding claim further including the step of varying the pressure differential across the said barrier.

11. A method as claimed in any preceding claim, in which the said pressure differential is substantially constant over a selected period of time and the said detection step is repeated whereby to establish the effect of any variation in environmental conditions during the said selected period of time.

12. A method as claimed in any preceding claim, in which the said barrier defines a pit such as a refuse infill site.

13. A method as claimed in any preceding claim, in which the said barrier defines a tunnel or a pipe or tube.

14. A method as claimed in any preceding claim, in which the said barrier separates the ground structure of a building from the superstructure thereof.

15. A method as claimed in claim 1 including the steps of determining a pressure field plan of the pressure field on the other side of the barrier from the said one side, resulting from the creation of the pressure differential across the barrier.

16. Apparatus for use in determining the integrity of a barrier, comprising means for creating a pressure differential across the said barrier, gas sensor means for detecting the presence of a selected gas in a region defined at least in part by the said barrier, and means for determining a time period within which the gas sensor is operative.

17. Apparatus as claimed in claim 16 in which there are provided means for introducing gas at superambient pressure into the said other region.

18. Apparatus as claimed in claim 16 or claim 17, in which there are provided means for reducing the pressure of gas in the other region to a subambient value.

19. Apparatus as claimed in claim 17, in which the means for introducing the said gas at superambient pressure comprise pump means for elevating the pressure of the said gas and a distribution unit in communication with the said pump means, the said distribution means delivering the said gas to a plurality of locations in the said other region.

20. Apparatus as claimed in claim 19, including a further pressure sensor on the said other side of the barrier, and means sensitive to an output sensor from the said further sensor, arranged to vary the pressure of the gas introduced to the said other region in response to variation in the said output signals.

21. Apparatus as claimed in claims 16 to 20, including a plurality of means for introducing the said gas.

22. Apparatus as claimed in claims 16 to 21, in which the sensitivity of the sensor means is adjustable.

23. Apparatus as claimed in claims 16 to 22, in which the said means for determining the said time period is adjustable.

24. Apparatus as claimed in claims 16 to 23, including means for determining a pressure field plan of the pressure field in the other region on the side of the barrier opposite to the said one region which occurs upon the creation of the pressure differential across the barrier.

25. Apparatus as claimed in claim 25, in which the said means for determining the pressure field plan include one means for detecting local pressure in a plurality of different locations within the said other region without breaching the said barrier.

26. Apparatus for use in determining the integrity of a barrier, substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

27. A method for use in determining the integrity of a barrier, substantially as hereinbefore described with reference to the accompanying drawings.