Field of the invention

The present invention relates to a method for precise pin-pointing of leaks in underground gas pipes in accordance with the preamble of claim 1.

Background of the invention

Regulations throughout the world require regular survey of gas networks to check for unknown leakage. The reason for this is primarily to protect the public from fires and explosions resulting from gas collecting in buildings and other confined spaces. The leak survey is typically carried out by moving a detector, sensitive to one of the main components of the gas, over the surface of the ground above the pipe carrying the gas. If leakage is detected in an underground gas line, the location of the leak must be pin-pointed to reduce the excavation work to a minimum. This has long posed difficult problems for repair crews. Although the gas line itself is often located, the soil is often saturated with gas, which makes it difficult to pin-point the actual location of the leak. As a result, it is not uncommon for large stretches of a surface, including paved roadways, to require excavation to precisely locate the leak - a situation that can be costly, time consuming, and potentially hazardous.

The most common pin-pointing procedure on hard coated surfaces is roughly the following :

Probe holes are drilled through the hard cover at 1 to 2 m distance in the area where the gas leak was detected. The holes are drilled right above and along the route of the presumably leaking pipe.

The holes are left to ventilate for a while. A detector "sniffer" is placed in the holes and measures the gas concentration pumped out of the hole by the detector.

Further and more closely spaced holes are drilled around the hole or holes showing the highest concentration until the crew feel that accuracy is satisfactory or that better accuracy cannot be achieved.

There are two reasons for making the holes:

Reducing the influence of ground and surface conditions on the position of the highest concentration making it appear somewhere else than right above the actual leak, and to allow venting out the gas from the soil so that the gas concentration profile is in a steady state and thereby improving precision of pin-pointing. To achieve a steady state situation it is vital that the holes are going most or all of the way down to the leaking pipe. It is also important that the holes are big enough to allow proper ventilation of the accumulated gas.

The process of drilling holes requires special equipment such as a drilling machine and a power source for the drill. The power source is typically either a compressed air unit or an electric power generator. In either case it adds to the complexity of the whole operation.

In addition to the time and effort spent there are also two serious risks involved :

When drilling a whole there is a risk that a large amount of gas is released that could be ignited by sparks formed when drilling or by some other ignition source, and the pipe may be damaged from the drilling which can cause substantial leakage with associated fire and explosion risks.

The current method is time consuming and not as accurate as desired. The main reason for the limited accuracy is that the leak is typically rather old meaning the gas has had time to spread quite a bit sideways. Typically gas is detected in an area stretching 5 to 20 meters along the route of the pipe. Considerable larger areas are often encountered and it is also common that gas is detected not directly above the pipe but instead beside the pipe at the edge of the road or sidewalk or along the curb. This also means that it is quite common to find more than one gas area from one leak.

Typical accuracy using the probe hole procedure is +/- 2 m but it is common that two or more holes show peak concentrations forcing the crew to dig in the wrong place or to dig more than would be necessary if more accurate pin-pointing was possible. Summary of the invention

It is an object of the present invention to provide an improved method for pinpointing leaks in buried gas pipes by injecting a tracer gas in the pipe shortly before performing the pin-pointing operation.

This is achieved in accordance with the characterizing portion of claim 1.

By injecting a lighter than air tracer gas, for which there is a selective and sensitive detector, into the leaking pipe pin-pointing the leak will become much more accurate and considerably easier to perform as the gas will not have time to spread widely in the ground before the pin-pointing work begins.

This is achieved by following phenomena :

The spread of the added gas is minimal as it started leaking just recently (hours before pin-pointing) . Hydrogen readily rises to the surface and penetrates most surface layers without the need for drilling probe holes.

The two most advantageous tracer gases to use are hydrogen and helium. There are selective detectors for both these gases. The addition of helium would reduce the energy content of the gas and hence affect the efficiency of the appliances using the gas.

By choosing hydrogen the influence on the gas will be minimal and there are suitable selective detectors on the market.

The method according to the present invention requires the use of a highly selective detector that will react much stronger to the added tracer gas than to the gas originally in the pipe. The selectivity factor should preferably be several orders of magnitude for optimal employment of this method. Detectors with this quality are manufactured and sold by INFICON AB in Sweden. The selectivity of this detector for hydrogen as compared to methane is more than 5 orders of magnitude.

Furthermore, the need for drilling probe holes will be greatly reduced when employing the present invention.

The method further requires access to suitable injection points which are possible points for connecting and injecting the tracer gas into the pipe.

An embodiment of the invention is described with reference to the drawings. shows the arrangement for performing the method of the invention under use of mere tracer

Fig. 2 shows the tracer gas concentration at the surface,

Fig. 3 shows the arrangement of Fig. 1 by use of mere utility gas without tracer gas,

Fig. 4 shows the utility gas concentration profile at the surface,

Fig. 5 shows the arrangement according to Fig. 1 by use of a utility gas and the tracer gas of the invention and

Fig. 6 shows the gas concentrations at the surface.

Fig. 1 shows a gas pipe 12 below the surface 14 of the ground 16. The pipe 12 is thus entirely surrounded by ground material 16. The pipe 12 is positioned below the ground surface 14. The pipe 12 carries the tracer gas 18 in the direction of arrow 20 in Fig. 1. The pipe 12 contains a tracer gas 18 only and no utility gas 26. The tracer gas 18 enters the pipe 12 upstream of the leak 22 and exits the pipe 12.

Fig. 1 shows how the tracer gas 18 spreads out of the leak 22 and within the ground 16. A fairly narrow plume 24 of tracer gas 18 forms within the soil. The plume 24 is very narrow at the leak position 22 and expands until it reaches a surface 14.

Fig. 2 shows the tracer gas concentration at the surface 14. It can be realized that the diameter of the tracer gas concentration at the surface 14 is about one meter. This is typical in the case of hydrogen and helium. The diameter of the tracer gas plume 24 at the surface 14 will increase over time. Therefore, it is desirable to perform the leak detection or pin-pointing shortly after introduction of the tracer gas 18 to the pipe 12.

Fig. 3 shows the case where the pipe 12 only carries a utility gas 26 but no tracer gas 18. The gas is carried in the direction of arrow 20 in Fig. 3. This is not an embodiment of the invention, but rather shown for illustration purposes. The utility gas 26 will exit the pipe 12 at the position of the leak 22 and it will expand throughout the surrounding ground material 16 in an area which is much wider than the area of the tracer gas plume 24 in Fig. 1.

Fig. 4 shows an example of the concentration profile of the utility gas 26 at the surface 14 which will form according to Fig. 3. The leaking utility gas 26 will form a cloud within the ground 16 and reach an outer diameter typically in the range between 5 and 25 meters at the surface 14. It can be immediately realized that it will be very hard and imprecise to detect the position of the leak 22 only from measuring the gas 18 at the surface 14. Fig. 5 now shows the embodiment of the invention where the pipe 12 carries a utility gas 26, such as methane and a tracer gas 18 such as hydrogen or helium. Fig. 5 shows that both the utility gas 26 and the tracer gas 18 escape from the leak 22 into the ground 16. The ground 16 is saturated with a large volume of utility gas 26 which expands within a very large area of the ground surface 14, as explained above with regard to Figs. 3 and 4.

The hydrogen or helium tracer gas 18, however, is much lighter than air and therefore ascends within the ground 16 from the leak 22 up to the surface 14 without expanding as much as the utility gas 26. The tracer gas 18 is detected with a sniffing probe 28 above the surface 14. The sniffing probe 28 is at least ten times more sensitive to the tracer gas 28 than to the utility gas 26.

Fig. 6 shows the concentration profile on the surface 14 of the utility gas 26 and of the tracer gas 18.

When using the gas detector 28 according to the method of the invention which is much more sensitive to the tracer gas 18 than to the utility gas 26, the gas detector 28 will selectively detect the tracer gas 18 within its much more narrow expansion area 30 at the surface 14 compared to the expansion area 29 of the utility gas 26. Consequently, the possible area of the leak 22 of the gas pipe 12 is considerably smaller when using the method of the invention compared to leak detection methods which require detecting the utility gas 26 at the ground surface 14.