Leakage detection of urban water supply pipe network is an important topic in the water industry, and it is also a worldwide problem. Therefore, it is necessary to comprehensively use multidisciplinary methods, combine advanced and cutting-edge technical concepts, and carry out high-quality and efficient pipeline network leakage detection work in an all-round way according to the characteristics of abnormal points of water supply network leakage. This paper will focus on introducing new methods for pipeline network leakage detection based on other principles, including:
(1) Flow monitoring method based on the principle of water balance;
(2) Pressure monitoring method based on the principle of pressure change;
(3) Gas tracer method based on the principle of gas concentration change;
(4) pipeline endoscopic method based on closed-circuit television imaging principle;
(5) Ground-penetrating radar method based on the principle of electromagnetic wave detection;
(6) Surface temperature measurement method based on the principle of infrared thermal imaging;
(7) Radar satellite imaging method based on the principle of electromagnetic wave detection;
(8) Pipeline under-pressure detection technology based on the principles of acoustics, closed-circuit television imaging, optical cable communication, and temperature sensing;
It is hoped that through the series of elaborations in this article, readers will be helped to establish an understanding of the international advanced and cutting-edge pipeline network leakage detection methods and science and technology, and provide diversified solutions for pipeline network leakage detection.
1.1 Gas tracer method
1.1.1 Basic mechanism
The gas tracer method is to make the tracer gas fill the water supply pipeline to be tested, and the leak detector uses the gas sensor to detect the concentration of the tracer gas along the upper part of the pipeline, and then detects the location of the water leakage point. Because the relative molecular mass of the tracer gas is small, after a period of time, the tracer gas will escape from the leak of the water supply pipeline and spread to the ground. Usually, the point of the leak is below the position where the tracer gas concentration is the highest Location.
1.1.2 Scope of application
Because the operation process of the gas tracer method is relatively cumbersome, the gas tracer method is not suitable for the general inspection of water supply pipe network leakage, but is suitable for the precise location of abnormal points of water leakage and the leak detection of newly installed water supply pipelines. In addition, the gas tracer method, with its unique mechanism of action, can be used as a powerful supplementary technology for water leakage detection, such as small water leakage abnormalities and plastic pipe leakage abnormalities, which are difficult to solve by sound detection methods.
1.1.3 Technical points
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The selection of tracer gas should meet the following requirements:
First, in order to prevent the tracer gas from affecting the water quality and the service life of the water supply pipeline, the tracer gas should be non-toxic, colorless, odorless, non-corrosive, non-flammable and explosive;
The second is to ensure that the tracer gas can be detected and detected, the tracer gas should be stable and not easily absorbed by the medium around the pipeline such as soil;
Third, in order to improve the detection efficiency of the gas tracer method, the tracer gas should have the characteristics of low relative density, free upward, strong penetrability, and easy detection;
Fourth, the density difference between the tracer gas density and the air should not be too large to ensure that there will be no obvious stratification of the vertical distribution of the tracer gas concentration;
Fifth, in order to avoid the interference of atmospheric components, the background concentration of tracer gases in the atmosphere should be kept at a low level;
Sixth, the difficulty of obtaining tracer gas should not be too high, and the cost of obtaining it should not be too high. Usually, a mixture of 5% hydrogen and 95% nitrogen can be used as the tracer gas.
2. Before using the gas tracer method for detection, the volume of the water supply pipeline to be tested should be calculated according to formula (11-2), and sufficient tracer gas should be prepared.
P cylinder ▪ V cylinder = P pipe ▪ V pipe
3. Before injecting the tracer gas into the water supply pipeline to be tested, the corresponding valve should be closed, and the valve body, valve screw and related interfaces should be sealed without leakage.
4. In order to improve the application effect of the gas tracer method, when using the gas tracer method, the properties of the road surface can be observed first, mainly the ground material and the degree of dryness and humidity of the ground. Because the ground material, degree of dryness and humidity and other factors will affect the air permeability of the ground, which in turn will affect the escape time of the tracer gas and the concentration of the detected gas. If there are various ground materials above the water supply pipeline to be tested, or if the water supply pipeline to be tested is buried deep, it is advisable to drill holes along the direction of the pipeline to detect the concentration of the tracer medium when conditions permit, and the water supply pipeline to be tested must not be damaged when drilling.
5. The optimal detection period of the gas tracer method should be determined according to factors such as the buried depth of the pipeline, the type of medium around the pipeline, the nature of the road surface, and the time for the tracer medium to escape from the leak point to the surface.
6. In order to avoid the adverse effects of the harsh environment on the gas tracer method, it is not suitable to use the gas tracer method under wind and rainy weather conditions.
7. When using the gas tracer method, the sensitivity of the instrument sensor used should be better than 1mg/L.
11.4 Application process and precautions
1. Application process
Introduce the tracer gas into the pipeline to be tested through openings, exhaust valves, water meters or fire hydrants and other interface points. The tracer gas should be injected along the direction of water flow. It is recommended to use the introduction rod to inject the tracer gas into the pipe at the middle or bottom of the pipe.
2. Precautions
(1) When a mixture of 5% hydrogen and 90∼2. nitrogen is used as the tracer gas, a gas tank with a volume of 40L and an output pressure of 10MPa can be selected, and a pressure reducing valve of 0~2.5MPa can be installed externally.
(2) In order to ensure that the tracer gas can normally spread to the remote end in the pipeline to be tested, the tracer gas concentration at the end of the pipeline should be detected in real time.
(3) When there are multiple ground materials above the water supply pipeline to be tested, attention should be paid to the change of the ground material. Due to the difference in air permeability of different ground materials, the tracer gas escapes from the soft road first. Where the gas concentration is higher, larger deviations may occur.
(4) When the flow rate of the fluid in the water supply pipeline to be tested is high, and the leakage of the abnormal water leakage point is known to be large through other detection methods, it can be considered to add tracer gas bubbles to save a large amount of tracer gas.
(5) Before using the gas tracer method to detect, the airtightness of the water supply pipeline should be tested. Before measuring the gas concentration, check whether the status of the instrument sensor is normal.