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This post was last edited by yinkuilin6868 on 2015-12-14 at 10:13. Classification of detection technologies: Research on leak detection in gas pipelines is currently very advanced around the world, with a variety of detection methods available. These methods can generally be divided into two categories: direct leak detection and indirect leak detection. Direct leak detection in this context refers to responding to pipeline leaks through manual inspection or the use of portable instruments; it is primarily used to detect minor leaks, and can be carried out both when the pipeline is in operation and when it is not. Indirect leak detection refers to the identification of leaks in pipelines by utilizing physical parameters such as sound waves, pressure waves, eddies, and flow rates, as well as material balance relationships. Conventional leak detection techniques include: schlieren imaging detection method, intelligent crawling detection method, odor sensor technology, gas detection method, ground-penetrating radar, stress wave detection technology, flow or mass balance method, statistical leak detection method, and dynamic model analysis method. Most of these methods for detecting leaks in gas pipelines suffer from issues such as insufficient detection accuracy and weak capability to detect minor leaks. 2 New Leak Detection Technologies 2.1 Infrared Imaging Leak Detection Technology When a gas pipeline leaks, it causes changes in the temperature of the surrounding soil. Infrared imaging technology makes use of this phenomenon to detect leaks by comparing the temperature with that of normal soil. This technology was invented by the American company OILTON. It involves using a helicopter equipped with precise infrared cameras to fly along the gas pipelines, thereby recording the irregular geothermal radiation patterns around them. Spectral analysis is then used to determine whether there is a pipeline leak and to identify the exact location of the leak. The former Soviet Union once used long-range laser analyzers developed by the United States, mounted on helicopters, to fly along gas pipelines. A gas cloud several meters in size with an ethane volume fraction of just one percent was successfully detected. This technology offers advantages such as fast detection speed, wide detection range, and high accuracy, but it still faces certain difficulties when detecting pipes that are buried deep. According to relevant records, when the aircraft is flying at an altitude of 300 m, the burial depth of the pipeline should be within 6 m, and this method does not allow for continuous monitoring of the pipeline. 2.2 Negative pressure wave leak detection technology: In recent years, the negative pressure wave detection method has attracted considerable attention internationally as a technique for detecting pipeline leaks. Its principle is that when a gas leak occurs, the gas density around the leak site in the pipeline decreases, resulting in a sudden drop in pressure; since gas is continuous, this creates a pressure difference around the leak site. This pressure difference drives the high-pressure gas upstream and downstream to flow toward the low-pressure gas at the leakage point, creating a pressure wave centered on the leakage point, which is mechanically referred to as a negative pressure wave. Due to the waveguide effect of the tube walls, negative pressure waves experience little attenuation as they propagate, allowing them to travel over long distances. By using pressure sensors installed at both ends of the pipeline, it is possible to detect signals of pressure fluctuations, and the exact location of the pipeline leak can be determined based on the time difference between the negative pressure waves detected at the two ends. The advantage of using negative pressure waves to detect pipeline leaks is their high positioning accuracy and fast response speed. Its drawback is that it is difficult to detect small leaks; it can only be detected when there is a significant leak in the pipeline. Due to the complex operating conditions of gas transmission pipelines, pressure sensors are often subjected to various interference signals, resulting in a decrease in detection accuracy. 2.3 Magnetic Flux Leakage Detection Technology Magnetic flux leakage occurs when a pipeline is magnetized by an external magnetic field; if the pipeline is intact without any defects, the magnetic field lines are distributed evenly within it. However, when defects appear on the inner or outer wall of the pipeline, the magnetic field lines become distorted, and some of them penetrate through the pipe wall, resulting in magnetic flux leakage. Flux leakage detection technology involves using probes placed in close contact with the pipe wall to detect the magnetic flux leakage. The detected signals are then processed through filtering, amplification, and analog-to-digital conversion before being stored in the memory of the detector. Finally, software is used to analyze these signals in order to determine whether any defects are present. At present, magnetic flux leakage testing is recognized both domestically and internationally as one of the most effective methods for inspecting pipelines. It can not only detect leaks in pipelines but also assess the degree of corrosion, providing a basis for pipeline maintenance. This approach **reduces the randomness in pipeline repairs and saves costs. 2.4 Distributed fiber optic leak detection technology Fiber optic sensing technology came to attract attention in the 1970s, with the rapid development of fiber optic communications. Fiber-optic leak detection technology relies primarily on changes in the distribution of backscattered light from the fiber to analyze and detect variations in measured quantities such as strain and temperature around the fiber. Backscattered light is further divided into elastic scattering light and inelastic scattering light, among which elastic scattering light includes Rayleigh scattering light, while inelastic scattering light includes Raman scattering light and Brillouin scattering light. Various sensors can be developed by utilizing the different properties of different scattered lights. For example: OTDR sensors developed using backscattering Rayleigh scattering, B-OTDR and B-OTDA sensors developed using Brillouin scattering, R-OTDR sensors developed using Raman scattering, and so on. The FBG sensor is a new type of all-fiber passive device that was developed in the 1990s. It enables online monitoring of gas pipelines, and this sensing technology primarily employs long-wavelength modulation to avoid issues such as the influence of light intensity and the ambiguity in phase measurement in interferometric fiber sensors. Taking advantage of the ability of quasi-distributed fiber Bragg gratings for wide-area detection, an optical cable can be laid along the pipeline, with FBGs used as sensors to acquire strain signals along the natural gas pipeline. By analyzing and processing these signals, it is possible to determine the location of leaks in the gas transmission pipeline. The advantage of distributed fiber leak detection is that any point on the sensing fiber has the ability to sense, allowing for continuous information acquisition. It is able to locate the source of the leak with high accuracy. It has a long detection range and high sensitivity. Although distributed fiber leak detection has many advantages, there are still some pressing issues that need to be addressed. (1) The effect of the acoustic signal pressure at the leakage point on light; (2) The sensitivity of optical fibers to acoustic signals; (3) The aging issue of optical fiber coating materials; (4) Problems related to the processing algorithms for detection signals; (5) Light intensity loss in the optical path. At present, research on fiber-optic leak detection technology in our country is still in its initial stages. Moreover, most leaks in gas transmission pipelines are minor leaks, for which OTDR technology is essentially ineffective. In addition, with such extensive fiber-optic coverage, factors such as material aging, service life, and cost all need to be taken into consideration. Therefore, distributed fiber sensing technology still has great potential for development. 2.5 Odor sensor leak detection technology: This technology makes use of sensors that respond to the chemical components in natural gas to generate signals; by processing these signals, it is possible to determine whether there is a leak in the pipeline. By placing odor sensors at regular intervals along the pipeline to form a sensor network, real-time monitoring of the pipeline can be achieved; when there is a gas leak, the sensors will respond. However, research on this technology is still in its initial stages; it is an immature technology, so it has not yet been applied in production. 2.6 Intelligent anti-corrosion layer leak detection technology: The intelligent anti-corrosion layer method involves adding an anti-theft detection circuit to the anti-corrosion layer of gas transmission pipelines; a leak will inevitably damage the anti-corrosion layer, and whether the layer remains intact can be determined by the resistance value of the circuit. The advantages of this detection method are: (1) high sensitivity, (2) high positioning accuracy, (3) low transmission and sensing costs, and (4) easy system recovery. The disadvantages are: (1) This detection method is highly sensitive to environmental factors; changes in temperature and humidity can affect the resistivity, leading to false alarms. (2) Installing the system requires high costs. 2.7 Leak detection technology based on acoustic waves in pipes In recent years, acoustic waves have become a focus of research in the field of pipeline leak detection due to their unique signal characteristics. Research has shown that the low-frequency characteristic signals generated by pipeline leaks can travel over long distances within the pipeline, and their signal characteristics differ from those of signals produced by operational disturbances. As early as the 1990s, Jolly and others used sound waves to detect pipeline leaks. In 2009, Hua Ke and others developed a sound-wave-based system for detecting and locating leaks in gas pipelines, achieving excellent detection results. The principle of leak detection based on acoustic waves in pipes is as follows: acoustic wave signals collected by sensors installed upstream and downstream of the pipe are utilized, and specific analysis of these signal characteristics is carried out to determine whether a pipe leak has occurred. Experiments have shown that this method offers good positioning accuracy, is simple to implement, and can detect minor leaks. However, in real-world working environments, the high level of noise can affect the detection accuracy, and its performance is not optimal for long pipelines; as a result, it has not yet been widely used in practical applications. 2.8 Gas imaging leak detection technology: The principle of gas imaging relies on the difficulty of imaging gases using optical techniques to determine whether there are leaks in the pipes. This technology can detect leaks in gas pipelines on the surface, as well as those underground. 2.9 Field Image Leak Detection Technology, originally named field-signature method, abbreviated as FSM]. This technology can not only detect pipeline leaks, but also determine the degree of corrosion in the pipelines before leaks occur, as well as identify any minor damages such as cracks. The principle is that when current flows into a pipe, an electric field is generated; this electric field has a unique \"electrical fingerprint\", which is what is known as the field pattern. Damage and leaks in the pipeline are determined based on changes in the detected electric field fingerprint. This technology can detect the degree of pipeline corrosion; it features high sensitivity and a wide detection range, making it suitable for inspecting long-distance gas pipelines. 3 Conclusion As global oil reserves gradually decline and energy demand continues to rise, natural gas will inevitably become the main energy source in the future. Leak detection for natural gas transmission pipelines is particularly important. With the progress and development of technology, leak detection methods in the future will become more reliant on artificial intelligence, and there will be an increasing demand for higher sensitivity in equipment. The combined use of various detection methods to compensate for the shortcomings of individual methods makes pipeline leak detection technology more sophisticated. The detection and localization of small leaks in long-distance natural gas pipelines remains a key focus for future research. With the development of science and technology, a large number of new technologies are being applied to pipeline leak detection, which will surely bring new momentum to the research on gas pipeline leaks.