## 1 Introduction  
Mine air compressors are important equipment for guaranteeing safety. Compressed air is transported underground through seamless steel pipes. On one hand, compressed air provides power for wind hammers, air pumps and other mine pneumatic equipment [1]. On the other hand, it can ensure that there is enough oxygen underground to protect the lives of coal miners [2]. Due to the high-pressure characteristics of compressed air, compressed air often leaks from the gas delivery pipeline during operation. According to statistics, there is a serious loss of compressed air during production and utilization, which accounts for about 25% to 30% of the total demand for compressed air [3]. In some specific systems, the loss rate even reaches 30% to 60% [4]. Leakage is the most obvious and important reason for the loss of compressed air, which can reach 20% to 40% of the total gas usage [5]. If the compressed air leaks in a very short time, it will not only cause huge economic losses, but also cause equipment damage and endanger personal safety. At present, the detection of such faults mainly depends on worker's hearing and observation to determine whether the pipeline is leaking. Obviously, the detection efficiency is very low and the leak fault cannot be found in time. In addition, the air temperature and noise in the compressor room is very hostile. It is extremely critical to discover and eliminate the compressed air leak faults early when the faults occur. Therefore,the research of compressed air leak fault diagnosis technology for mine pipes is of great significance and practical value. In recent decades, many experts and researchers have proposed two general leak fault monitoring methods. The first is to detect the substance that escapes from the pipe. A sensor is used to detect the leaking substance, which should be contacted with this substance and be close to the leak. This requires a dense concentration of sensors. Thus, for a long pipeline, the method will yield a large number of remote units and high costs. The second way is to detect certain leak-related properties such as pressure drops, acoustic emissions, volume balance changes and temperature changes. Temperature monitoring and volume balance comparison (comparing the amount of material injected into the pipe to the amount received from the pipe) techniques are useful for the detection of large or catastrophic leaks, and have been widely used in many fields. However, these systems are not sensitive to small leaks. At present, the detection methods for pipeline gas leakage are mainly based on acoustic, ultrasonic and infrared technologies. [PERSON] et al. [6] used an infrared camera to detect the leakage of SF6 gas, improving the detection efficiency. [PERSON] et al. [7] used ultrasound and infrared thermography technology to detect the leakage of compressed air, and found that thermography can offer good results for the leakage quantification from orifices greater than 1.0 mm and ultrasound should be used for leakage detection for all dimensions of orifices. [PERSON] et al. [8] conducted an experimental study on the detection and location of natural gas pipeline leaks based on acoustic methods. The wavelet transform was used to reduce the noise of the acoustic signal collected by the sensor, which improved the recognition sensitivity of the system. [PERSON] [9] used ultrasound to study the pipeline leak detection technology. However, this method cannot perform real-time inspection on the pipeline at work, and is only suitable for inspection before the pipeline is used. [PERSON] [10] used the basic principle of ultrasonic detection of pressure vessel leakage, wavelet analysis and BP neural network to identify the gas leakage signal. [PERSON] et al. [11] proposed a method for detecting and locating natural gas pipeline leaks based on the sonic method. It can be seen that the traditional gas leak detection methods cannot effectively detect the gas leak on time and cannot meet the accuracy demand of fault diagnosis.