Abstract:
A method and an apparatus for finding the location of one or more holes in a fluid flow system such as pipe, duct, or conduit using one or more tracers that interact with or can be chemically or physically differentiated from those of the liquid or gas contents located outside the system and pulled back into the system when the system is placed under a vacuum. The preferred embodiment of this detection and location method uses one or more interactive tracers that are injected into a pipe or duct that may be leaking at one location and then monitor the concentration of these tracers at the same or another location in the pipe or duct. Detection and location are accomplished by analysis of the characteristic features of measured curves of tracer concentration. Various types of interactive tracers may be used, including partitioning and reactive tracer gaseous tracers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/509,316 filed Oct. 6, 2003, and U.S. Provisional Application No. 60/509,312 filed Oct. 6, 2003, the disclosures of which are incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   A method and an apparatus for finding the location of one or more holes in a fluid flow system such as a pipe, duct, or conduit, using one or more tracers that interact with or can be chemically or physically differentiated from the liquid or gas contents located outside the system and pulled back into the system when the system is placed under a vacuum. The invention is particularly applicable to underground pipes. This invention is related to the invention disclosed in U.S. patent application Ser. No. 10/960,653, filed Oct. 6, 2004, titled Method and Apparatus for Detecting and Locating Explosives, Biological, and Chemical Substances in Ducts and Structures Using Tracers. 
   2. Brief Description of the Prior Art 
   Locating holes in piping, especially underground or buried piping is a challenging and important problem. It has both environmental and economical implications. A method for accurately locating a leak in underground pressurized piping is of immediate benefit to the petroleum, chemical, and nuclear industries. 
   This specification describes a method and an apparatus for finding the location of one or more holes in a fluid flow system such as a pipe, duct, or conduit, using one or tracers that are injected into the system and that interact with or can be differentiated chemically or physically from the liquid or gas contents outside the system when pulled back into the system under a vacuum. PCUT is an acronym for Pipeline Characterization Using Tracers, and characterization includes detection, location, and quantification of a contaminant in the pipe. The PCUT technology was demonstrated in a pipe using a thin layer of diesel fuel trapped at the bottom of the low point of the pipe as the contaminant. The method of the present invention differs from the method for detecting and locating contamination in a pipe, because the contamination was already present in the pipe. In the present invention, a vacuum is generated in the pipe to pull in the released liquid (or gas) contents of the pipe that was released or leaked through a hole in the pipe wall. Once the released contents are pulled back into the pipe, the PCUT technology applies, using the released contents that were pulled back into the pipe as a marker of the location of the leak hole. 
   There are a number of methods used to locate a hole in a pipe. These methods have been used to find holes in pipelines containing petroleum fuel, natural gas, dangerous chemicals, and radioactive liquids. The most conventional approach is to insert a tracer into the pipe and search for it outside the pipe. If the tracer is detected, and if it is unique (i.e., not part of the background environment), the presence of the tracer outside the pipe is an indication that the pipe has a hole. The location of the tracer is an indication of the location of the hole. For example, helium is often used as a tracer. 
   When the pipe is buried underground, which is the most common need for the method of the present invention, a carefully designed sampling procedure is required to detect the presence of the tracer. While liquid tracers can be used, gaseous tracers or liquid tracers that volatize when released into the ground are the most often used, because gaseous tracers rise to the ground surface where their presence can be detected. Closely spaced vapor samples along the length of the pipe with a hole are required. Liquid tracers are not as common or cost effective for this application, because once the tracer is released into the soil, it is not easily sampled. The use of gaseous tracers is expensive to implement and has many technical pitfalls. It is expensive to implement, because samples must be collected at 5- to 20-ft intervals to avoid missing the presence of the tracer, and the tracer material is generally expensive. Technically, the location of the underground pipe must be well known for the method to work. If the samples are too far off the centerline of the pipe, the presence of any tracer can be missed. A larger problem is that the subsurface soil is not homogenous and the tracer will tend to follow underground pathways and rise to the surface off the centerline of the pipe. Finally, the performance of the method depends on the nature of the tracer. For best performance, the tracer must be unique and not be present in the environment. In some cases, constituents in the pipe fluid are used. These have not been very effective because previous releases and spills of the pipe contents contaminate the soil. Adding a tracer to the pipe that is unique and is not present in the pipe fluid or the surrounding soil is the most effective. However, both types of tracers have resulted in false alarms because of their presence in the surrounding soil. While there are technical pitfalls, this technique has been widely applied. 
   Another approach is to use passive acoustics. Two acoustic sensors are attached to the pipe in such a way that the sensors bracket the hole. The pipe is then pressured so that an acoustic signal is generated at the hole. This technique will work with both liquid- and gas-filled pipes. However, the acoustic leak signal generated in a gas-filled pipe is much weaker than the acoustic signal generated in a liquid-filled pipe. Advanced signal processing using correlation and coherence analysis algorithms are required for accurate location estimates. The distance between the sensors on the pipe, and a good estimate of the acoustic propagation velocity are required to make accurate location estimates. In general, for small holes, sensor separations must not be too much greater than 100 ft for gas-filled pipe and 500 ft for liquid-filled pipe. Also, access to the pipe is required to mount the sensors on the pipe. This can be expensive if the pipeline to be interrogated is long and the pipe has to be excavated at many locations to obtain the required spacing to implement the method. This technique is also widely used, but is best used in pipe applications where the pipe is on the order of 500 to 1,000 ft or where access to the pipe exists (e.g., at valve pits). 
   There are other approaches to locating a hole in a buried pipe. There are infrared sensing methods that detect the presence of the released pipe contents from a change in temperature between the contaminated and uncontaminated regions. This approach has not been very effective. 
   There are other approaches to locating a hole in a buried pipe. There are infrared sensing methods that detect the present of the release pipe contents from a change in temperature between the contaminated and uncontaminated regions. This approach has not been very effective. 
   There is a need for a method that does not require sampling at closely spaced intervals at the surface or excavation to the top of the pipe for implementation of the method. There is a need for a method with higher performance than the tracer, acoustic and infrared systems now in use. Stated differently, there is a need for a method that can be used on long sections of pipe that is both more reliable and more accurate than these three methods. Finally, there is a need for a method that is less expensive than these current methods. 
   The method of the present invention uses a gaseous tracer method developed by the inventors to locate a hole in a pipe. While the invention can also detect the presence of a hole, there are more reliable, accurate, less expensive methods, and commercially available methods, such as volumetric and pressure-based methods, that are used that can perform this task. These detection methods outperform and are less expensive than the tracer, acoustic, and infrared methods mention above. The method is called PCUT (Pipeline Characterization Using Tracers). A gaseous tracer that interacts with the contaminant of interest is used. Both partitioning and reactive tracers can be used for this application. The method described here is very similar to the one described in two patent applications submitted by the inventors for characterizing (detection, location, and quantification) contamination in pipes, ducts, and other fluid flow systems, and for find dangerous and hazardous substances like explosive devices in ducts, buildings and the like. However, the application objective is very different, but the technology for location, which is required for this application, is very similar. 
     FIG. 1  is a simplified illustration of the use of PCUT for detecting and locating the presence of a liquid contaminant, such as diesel fuel, in a pipe, using a conservative tracer and two partitioning tracers. As illustrated in  FIG. 1 , the method was implemented by injecting a “slug” of two tracers  120  and  130  into a pipe  10  or other fluid flow system, with different partitioning coefficients (K i ). One of the tracers is a conservative tracer, i.e., it will not dissolve, adhere, or interact with the hazardous substance of interest. The other tracers are selected so they will dissolve, adhere or interact with the hazardous substance of interest. The tracers are transported or advected from the injection point  150  (at one location in the pipe) to one or more extraction points  250  (at other locations in the pipe) by a gas flow field established in the pipe prior to the injection of the tracers  120  and  130 . The gas flow field used to transport the tracers is typically nitrogen or air, because they do not generally interact with the tracers or the hazardous substances in the fluid flow system. The velocity of the advection flow field is selected so that the tracers have enough time to fully dissolve, adhere or interact with the hazardous substance before the leading edge of the tracer reaches the extraction point and is measured with a gas chomatograph (GC) or some other sensing means that can measure the magnitude of the tracers reaching the measurement point  250 . At that point, no more tracer is introduced into the line. By measuring the time history of the concentration  70  of the partitioning  72 ,  74  and the conservative  76  tracers at the extraction point in the pipe, the presence and amount of the contaminant within the pipe or duct can be determined. 
   Detection and quantification can be accomplished using the difference in the mean arrival time of the partitioning and conservative tracers, or the difference in the levels of concentration between the conservative and partitioning tracers. The location of the contaminant can be determined by introducing a perturbation to the advection flow field. This can be accomplished by flushing (i.e., removing) the conservative and partitioning tracers in the line, and then measuring the mean time of arrival of the partitioning tracers that are still being eluted from the contamination in the system. Alternatively, this can be accomplished by introducing enough partitioning tracer at the beginning of the duct test to cover the entire duct, then stop the flow, and allow the tracer to interact with dangerous or hazardous substance. After a period of time, an advection flow field is established, and GC samples are collected and analyzed. 
   A series of over 25 laboratory experiments were conducted to demonstrate the capability of PCUT for detecting, locating, and quantifying a contaminant using one or more interactive tracers, and for some measurements, one or more conservative tracers. The location capability of the PCUT technology was demonstrated in the laboratory using the 116-ft, long-pipe illustrated in  FIG. 12 . The long pipeline  900  (116 ft) is comprised of 66-ft of 2-in.-diameter pipe  910  between the inlet  902  and the first 4-ft 3-in.-diameter section of PVC pipe  912  and 45-ft of 2-in. PVC pipe  914 ,  916 ,  918  between the first 3-in.-diameter PVC pipe section  912  and the outlet  904 . The second 3-in.-diameter PVC pipe section, which could be used as a contamination point, was not used in the location tests presented herein. 
   A 3-ft by 1.625-in. rectangular tray was inserted into the 4-ft section of 3-in. diameter PVC pipe  912  whose center position was located 47.5 ft from the outlet end of the pipe  904  where the GC measurements were made. The shallow tray was used to hold 300 ml of aged diesel fuel. Two partitioning tracers (C 7 F 14  and C 8 F 16 ) were used for these tests. The advection gas was nitrogen. 
     FIG. 13   a  shows the time history of the concentration curves of the two partitioning tracers  392 ,  394  and the conservative tracer  390  from one of the location tests (Pipe Test # 8 ). Superimposed on these curves is the flow rate of the advection gas  385 ,  387 ,  389 . The location measurement is made after the detection measurement. The data required for detection is the same as for previous tests except only enough data needs to be collected to define the peak of the conservative tracer. This allows a comparison between the partitioning tracers and the conservative tracer for detection and also allows sufficient time for the tracers to partition into the contaminant. The next step is to rapidly flush  387  the conservative and partitioning tracers through the pipe and then to re-establish the advection flow stream  388  at a known velocity  389 . As shown in  FIG. 13   a , the line was flushed at 350 ml/min  387 , which is over 10 times the initial flow rate of the measurements  385 . Once the flow rate is re-established  388  at a flow rate of approximately 20 ml.min  389 , the partitioning tracers  393 ,  395  in the diesel fuel re-enter the flow stream and are advected to the end of the line at a known flow rate  389 . The location of the contamination was then determined from the advection velocity  389  and the arrival time of the tracers. The arrival time is the time between the end of the flush  388  at the high flow rate  387  and the arrival of the partitioning tracers  393 ,  396  being emitted from the contaminant.  FIG. 13   b  shows the two partitioning tracers  398 ,  399  arriving at about 63 hours; the advection flow field  397  was re-established at 44 hours  396 . 
   Table 1 summarizes the location results for the tests conducted in the long pipe  900  illustrated in  FIG. 12 . Two methods were used to locate the contamination. Both methods used the time of arrival of the leading edge of the tracer concentration curves. (The average time of arrival can also be used.) 
   
     
       
             
           
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               PCUT estimation of the location of the 300 ml of diesel fuel 
             
             
               contamination. 
             
           
        
         
             
                 
               Method 1 
               Method 2 
             
           
        
         
             
               Test 
               Location (ft) 
               Error (%) 
               Location (ft) 
               Error (%) 
             
             
                 
             
             
               Pipe Test #8  
               51.3 
               3.3% 
               53.2 
               4.9 
             
             
               Pipe Test #10 
               38.1 
               6.4% 
             
             
               Pipe Test #12 
               45.4 
               0.1% 
             
             
               Pipe Test #23 
               44.7 
               0.6% 
             
             
               Average 
                 
               2.6% 
             
             
                 
             
             
               * The actual location of the contamination is centered 47.5 ft from the outlet end of the 116-ft pipe. 
             
           
        
       
     
   
   The first method, which requires a priori information about the diameter or geometry of the pipe, uses the maximum velocity of the advection fluid within the pipe and the time of arrival of the tracer(s) after flushing. The average velocity is computed by dividing the measured volumetric flow rate by the diameter of the pipe; the maximum velocity (for laminar flow in a pipe) is twice this value. The second method, which does not require a priori information about the diameter or geometry of the pipe, only the total length of the pipe, utilizes the ratio of the time of arrival of the leading edge of the first tracer pulse  392  or  394 , which traveled over the full length of the pipe (i.e., 116 ft)  900  and the time of arrival of the second tracer pulse  393  or  395 , which traveled only the distance from the contamination  912  to the outlet end  904  of the pipe  900 . After weighting the arrival times by the mean of the measured flow rates, the distance from the outlet end  904  of the pipe to the contamination  912  can be determined. Both methods were applied to Pipe Test #8 and only the first method was applied to Pipe Tests # 10, #12, and #23. 
   The location test was also repeated (Pipe Test #12) using a dried glue sample of approximately 20 grams (10 ml). For this test the pipeline was flooded with tracer overnight and then flushed with 350 ml/min of the advection gas. After the flush, an advective flow stream was established and used to determine the location of the dried glue specimen. The distance from the end of the pipe to the glue sample was calculated to be 49.2 ft which is less than 10% error on the actual value of 47.5 ft. 
   SUMMARY OF THE INVENTION 
   It is the object of this invention to provide a method and an apparatus for locating holes in pipes, ducts, conduit, or other type of fluid flow systems without having to physically, robotically, or mechanically enter the system. The method and apparatus can be used for both underground and aboveground fluid flow systems. 
   Another object of this invention is to provide a method and an apparatus for locating holes in underground pipes, ducts, conduit, or other type of fluid flow systems. 
   It is also an object of this invention to provide a method and an apparatus for locating holes in long pipes, ducts, conduit, or other type of fluid flow systems. 
   It is another object of this invention to provide a method and an apparatus for locating holes in pipes, ducts, conduit, or other type of fluid flow systems without having to collect samples on the ground above the pipe. 
   Still another object of this invention is to provide a method and an apparatus for locating holes in pipes, ducts, conduit, or other type of fluid flow systems without having to excavate pits along the pipe to attach sensors to the pipe. 
   It is a further object of this invention to provide a method and an apparatus for locating holes in pipes, ducts, conduit, or other type of fluid flow systems with access to the pipe at only two locations. 
   Another object of this invention is to provide a method and an apparatus for detecting holes in pipes, ducts, conduit, or other type of fluid flow systems without having to physically, robotically, or mechanically enter the system. 
   For the purposes of this specification, the method and its alternative embodiments will be described first in terms of locating a hole in an underground pipe containing petroleum fuels (e.g., diesel fuel), a very real problem. This application will be particularly illustrative, because the PCUT technology for detecting, locating, and quantifying contamination in a pipe was demonstrated using diesel fuel as the contaminant. However, it must be understood that the method and apparatus are not limited to this application and will work equally well for other liquids or gases and other pipe configurations. For example, the method can also be used for locating holes in aboveground pipes and pipes in buildings and other structures. In addition, the method can be used for pipes containing both liquids and gases provided that all liquids are removed from the pipe before the method is applied. Also, while the method is described first in terms of pulling back into the pipe the contents that were released through the hole in the pipe due to the leak (e.g., diesel fuel), gases and liquids in the ambient environment or gases in the ambient environment surrounding the pipe, either resident or placed there for the leak-location measurement, can also be used. The method will be described in terms of locating only a single hole in the pipe, but the method can be used to find multiple holes. Finally, the method will be described in terms of one tracer, but multiple tracers can also be used. Applications for ducts and conduits follow directly for someone skilled in the art. 
     FIG. 2  shows a simplified illustration of the present invention  300  for location of a hole  30  in an underground pipe  10  that has released the liquid contents of the pipe  10 , such as diesel fuel, into the ground  15  around the pipe  10 . The plume  20  of the released contents in the vicinity of the hole  30  in the pipe  10  is also shown in  FIG. 2 . The tracer injection system  100  with a vacuum pump  110  is shown on one end of the pipe, and the tracer measurement system  200  is shown at another location of the pipe downstream of the leak hole  30 . The tracer measurement system  300  is identical to the one used for the application of PCUT for finding contamination in a pipe except for the vacuum pump  110 . The tracer injection system  100 , which attaches to the pipe  10  at  150 , is comprised of a tracer injection container  130  and a valve  132 , a pressured bottle of inert gas  120  and a valve  122 , a vacuum pump  110  and a valve  112 , and a valve  142  on the communication conduit  140  to isolate the pipe  10  from the tracer injection system  100 . The tracer measurement system  200 , which attaches to the pipe  10  at  250 , is comprised of a gas chromatograph (GC)  210 , or some other analytical measurement system, and a valve  212 , a valve  222  on the communication conduit  220  to isolate the pipe  10  from the tracer injection system  200 . 
   The preferred method of hole-location using the present invention is applied to an empty pipe or section of pipe that can be isolated with valves or blinds. If the pipe contains a liquid, then it must be emptied and cleaned before the method can be applied. The overall approach is to draw a vacuum on the pipe so that any of the released fluid from the pipe will be brought back into the pipe at the location of the hole so that it can interact with the tracer or tracers used for detection and location. The first step is to flood the pipe with a gaseous partitioning tracer selected to interact with the released pipe contents (i.e., the contaminant). The second step is to draw a vacuum over an established time period to pull back into the pipe any of the liquid or gas released from the pipe, which may reside in the surrounding soil in the immediate vicinity of the hole. Once the contaminant is back in the pipe, the third step is to allow sufficient time is allowed for the tracer to interact with the contaminant. The fourth step is to establish a known advection flow in the pipe by introducing a gas that does not interact with the contaminant or tracer at the inlet side of the pipe. The advection flow is slow enough to transport the fluid in the pipe without significant mixing. The fifth step is to extract the gas in the pipe at the outlet side of the pipe and to measure the concentration of the tracer as a function of time with a gas chromatograph or another analytical instrument. The location of the hole is determined from the time of arrival of the lower concentration tracer gas that interacted with the contaminant and the velocity of the advection flow field. 
   This method can be more robust if a partitioning tracer is used. If a partitioning tracer is used, all of the gas in the pipe can be flushed from the line after allowing the tracer to partition into the contaminant and before establishing an advection field. In this case, only the presence and time of arrival of the partitioning tracer that has been emitted from the hole-contaminant need be detected. This is more robust because edge mixing with the tracer in the first implementation is avoided. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified illustration of the method for characterizing contamination in pipes and ducts. The time history of the elution curves of tracer concentration for both the conservative and the partitioning tracers are shown. 
       FIG. 2  is a simplified illustration of the method of the present invention for locating a hole in an underground pipe after a release or leak has occurred, the liquid in the pipe has been removed, and the inside of the pipe has been cleaned. 
       FIG. 3  is a simplified illustration of the method of the present invention for locating a hole in an underground pipe that has leaked and after the pipe has been inundated with one or more partitioning tracers. 
       FIG. 4  is a simplified illustration of the method of the present invention for locating a hole in an underground pipe that has leaked and after the pipe has been inundated with one or more partitioning tracers, and a vacuum has been pulled on the pipe. The released material pulled back into the pipe and the interaction of the tracer with this material are shown. 
       FIG. 5  is a simplified illustration of the method of the present invention illustrated in  FIG. 4  after the tracers in the pipe are being advected with an inert gas toward the tracer measurement point. 
       FIG. 6  is a simplified illustration of the method of the present invention for locating a hole in an underground pipe shown in  FIG. 4  after the tracer in the pipe and the slug of tracer that has interacted with the released material pulled back into the pipe have been removed from the pipe by rapidly flushing these tracers gases through the pipe with an inert gas. 
       FIG. 7  is a simplified illustration of the method of the present invention for locating a hole in an underground pipe after the tracer material has been flushed from the pipe, as shown in  FIG. 6 , and the partitioned tracer in the released material pulled back the pipe has begun eluting tracer back into an advection flow stream whose velocity is known. 
       FIG. 8  is a simplified illustration of the method of the present invention for locating a hole in an underground pipe that has leaked and after a vacuum has been pulled on the pipe. The released material pulled back into the pipe is shown. 
       FIG. 9  is a simplified illustration of the method of the present invention for locating a hole in an underground pipe that has leaked and after a vacuum has been pulled on the pipe, and a “slug” of one or more partitioning tracers have been introduced into the pipe. 
       FIG. 10  is a simplified illustration of the method of the present invention for locating a hole in an underground pipe illustrated in  FIG. 9  as the “slug” of tracer is being advected along the pipe towards the measurement location. 
       FIG. 11  is a simplified illustration of the method of the present invention for locating a hole in an underground pipe illustrated in  FIG. 10  as the tracers have reached and interacted with the released material pulled back into the hole in the pipe. For determining the location of the hole, the pipe is rapidly flushed with an inert gas, as illustrated in  FIG. 6 , and then, as illustrated in  FIG. 7 , after the advection flow stream of the inert gas is reduced to a known velocity, the tracer being eluted back into the flow stream from the released material pulled back into the pipe, is transported to the measurement point at a known velocity. 
       FIG. 12  illustrates a schematic of a pipeline used in laboratory testing. 
       FIG. 13   a  illustrates test results from a test to determine the location of contamination within the long pipe using 300 ml of diesel as the contaminant located at two thirds of the pipe length. 
       FIG. 13   b  illustrates a tracer concentration curve that re-emerges from the contaminant after the line has been flooded and flushed. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The preferred embodiment of the method and apparatus shown in  FIG. 2  requires the interaction of one or more gaseous tracers  130  injected into a pipe  10  (see  FIG. 3 ) and the liquid or gas contents pulled back into pipe from the plume  20  at the location of a hole  30  when the pipe is placed under a vacuum (see  FIG. 4 ). The location of the hole  30  will be marked by the material pulled back into the pipe from the contamination plume  20  created by the leak. The preferred embodiment of the present invention illustrated in  FIG. 2  further requires that the line be empty  40  before the start of the location measurement. This may require pumping out all of the liquid or gas from the pipe. In the case of a diesel fuel pipeline, the diesel fuel can be pumped back into the underground or aboveground storage tank used to supply fuel to the pipe. If the tracer used is designed to interact with the pipe contents and the slug of material pulled back into the pipe is the same as the contents that were released, it might be necessary to flush the pipe with an inert gas, water, or some other cleaning solvent before injecting the tracer into the pipe. The pipe should be cleaned sufficiently so that the original contents of the pipe will not interact with the tracer. This may not be true if water or air is pulled back into the pipe and this is the quantity to detect. 
   As illustrated in  FIG. 3 , a tracer  42 , which will interact with the liquid or gas contents located outside  20  and in the immediate vicinity of the hole  30  in the pipe  10 , is injected into the pipe  10  being tested in such a way that the entire line is filled with the tracer  42 . Then, the pipe is placed under a vacuum, as illustrated in  FIG. 4 , so that a slug of liquid (or gas) material  20  located in the plume outside the hole  30  in the pipe is pulled back into the pipe through the hole  30  in the pipe  10 . This slug of liquid material  22  pulled back into the pipe interacts with the tracer in the pipe in such a way that it changes the physical or chemical properties of the tracer  50  in the immediate vicinity of the hole in the pipe. In general, this interaction results in a lower concentration of the interactive tracers. As illustrated in  FIG. 5 , the entire contents of the pipe are then advected to a monitoring point  250  located downstream of the hole in the pipe using an inert gas, such as nitrogen or air, that does not interact with the material  22  pulled back into the pipe  10 . Both reactive and partitioning tracers can be used. The advection velocity  60  is selected so that the tracer in the immediate vicinity of the hole will be mix back into the surrounding tracer gases such that it is undetectable at the downstream measurement point. If a location estimate is desired, then the advection velocity should be at a known flow velocity. Careful control of the flow velocity is not required if only detection is desired. 
   Alternatively, if partitioning tracers are used, then the tracer gas or gases in the pipe shown in  FIG. 4  can be quickly flushed at a high advection velocity  62 , as illustrated in  FIG. 6 , using the advection gas (e.g., nitrogen). Once the pipe is free of tracers, then the only tracer that will be in the pipe is the tracer that partitions from the material  22  that was brought back into the pipe from the external plume  20  of the released pipe contents. As illustrated in  FIG. 7 , this tracer eluting  50  from the material  22  is then advected down the pipe at a controlled flow velocity. 
   The location of the hole in the pipe is determined from the time of arrival of the tracer or tracers that interacted with the slug of material  22  pulled back into the pipe  10  through the hole  30  in the pipe  10 . Using the known geometry of the pipe and the known advection velocity, the location in terms of distance can be determined. 
   The tracer measurement can also be accomplished at the same location that the tracers were introduced when an inert advection gas is not used to advect the tracers down the line to the measurement point; this would require, however, that a vacuum be pulled on the line to transport the tracers back to the injection point. 
   The selection of the tracer and the duration of the test depends on whether or not (1) a liquid or gaseous contaminant is pulled back into the pipe that interacts chemically with the tracer and changes the concentration of the tracer in the immediate vicinity of the hole, or (2) air, water, or the released pipe contents that do not interact with the tracer is pulled back into the pipe and displaces a volume of the gaseous tracer with a non-interactive substance or liquid contents of the pipe with an immiscible liquid (e.g., fuel and water). In both cases, the concentration of the tracer in the immediate vicinity of the hole is lowered. The advection velocity must be selected so that this slug of lower concentration tracer moves along the pipe to the measurement point downstream of the hole without getting mixed back into the tracer located on either side of the slug. A gas chromatograph  210 , located at the measurement point  250 , is used to monitor the concentration of the transported tracer. As stated above, the location of the hole can be determined from the time of arrival of the tracer with lower concentration, and the distance from the inlet or outlet can be determined from the time of arrival and the transport velocity. 
   As illustrated in  FIGS. 1 and 2 , the apparatus needed to apply the leak-location method is the same one used for finding contamination in a pipe. A GC is required if the concentration of the tracer needs to be measured at the downstream monitoring point. If the material pulled back into the pipe is inert such as air or water, which is distinguished from the tracer placed into the pipe, because it displaces a large enough volume of tracer to be detected, then a simpler detection measurement system than a GC that samples for water or air can be used. A simpler device can also be used if it has the capability of sensing the presence of the tracer or tracers used in the test. 
   DESCRIPTION OF ALTERNATIVE EMBODIMENTS 
   First Alternative Embodiment 
   The above method, in which the entire line is initially flooded with a tracer, can be used to regardless of what type of material, liquid or gas, is ingested back into the pipe at the location of the hole. Alternatively, if the liquid contents released from the line are pulled back into the line when the vacuum is placed on the line, then only a slug of tracer gases are required for detection and location. Once the released liquid is pulled back into the line, the PCUT method and apparatus is then used to locate the leak hole using the contamination as an indicator of the hole location. 
   In this alternative embodiment, the pipe shown in  FIG. 2  is first cleaned by flushing with an inert gas or water or some other cleaning substance. The pipe should be cleaned sufficiently so that the original contents of the pipe will not interact with the tracer. Then, as illustrated in  FIG. 8 , a vacuum is pulled on the pipe and any liquid in the plume  20  in the vicinity of the hole  30  is pulled back into the pipe  10 . A tracer is then selected that will partition into the released contaminant when such contamination is pulled back into the pipe at the location of the hole. In contrast to the preferred embodiment, the vacuum is pulled on the pipe before the tracer is injected into the line and only a slug of tracer is used. The entire fluid volume of the pipe does not need to be inundated with tracer. 
   For detection and location, the method requires the injection of a slug of partitioning tracer  44  upstream of the hole, as illustrated in  FIG. 9 . Then, as illustrated in  FIG. 10 , tracer slug  46  is transported at a known velocity  60  with an inert gas that does not interact with either the tracers or the contaminant to be located to a monitoring location  250  downstream of the hole  30 . The partitioning tracer is selected so that when it passes by the liquid contaminant on the bottom of the pipe in the region of the holes it partitions into the contaminant as it passes by the hole. If detection is required, then the test continues until the lower magnitude or the longer time of arrival of the partitioning tracer compared to the conservative tracer is determined at the measurement point. For location, however, as illustrated in  FIG. 6 , the pipe must be flushed with an inert gas (i.e., the advection gas) of all the tracer in the pipe once the slug of partitioning tracers have passed this hole region. Then, as illustrated in  FIG. 11 , the velocity of the inert gas is adjusted to a known rate  60  to transport the tracer  50  that partitioned into the contaminant during the first pass and is now partitioning out of the contaminant and re-entering the advection flow stream. The location of the hole  30  is then determined from the time of arrival of the tracer coming out of the contamination and reaching the monitoring point  250 . 
   Second Alternative Embodiment 
   The same approach will work if the pipe contains a liquid and the liquid is not removed from the pipe before the location measurement is made provided that the pipe is underground, the liquid contents are immiscible with water, and the hole is located beneath the groundwater table. In this case, groundwater is pulled back into the pipe when the vacuum is drawn on the pipe. The advection velocity is selected so that the fuel or water will be transported to a measurement point. The location of the hole is determined from the time of arrival of the water at the measurement point. 
   Third Alternative Embodiment 
   The water-tracer approach, described as an alternative embodiment of the second alternative embodiment, will also work if the pipe contains a gas that is not emptied before the location test is initiated provided that the chemical properties of this gas can be distinguished from the air pulled back into the line. In this case, the tracer is air, and the pipe can be located underground or aboveground. 
   This approach can also be used if the pipe is in a location in which another tracer gas besides air, such as helium, can be supplied to the environment surrounding the pipe. This is accomplished for an underground pipe by filling the pipe with helium under pressure so that the helium will be released into the soil and backfill surrounding the pipe in the immediate vicinity of the hole. Then the line is flushed to remove the helium gas. Once the helium gas has been removed from the line, the line is placed under a vacuum using the vacuum pump and any helium gas in the soil or backfill in the vicinity of the hole is pulled back into the line and then is transported at a known velocity using another inert gas (such as nitrogen) that can be distinguished from helium. For a pipe in which access to the outside surroundings of the pipe is possible, the helium gas can be placed in the environment directly. In this instance, helium does not need to be placed in the pipe. This application of the method works for pipe that may be located in a room or building, or the pipe in the inner pipe of a double-walled pipe.