Development Of Gas Optical Fiber Sensing System For Subway Tunnel Application

Gas is one of the sources which are the major disaster such as coal mine and underground transport hub. If stratum have gas that would be a serious threat to the subway tunnel construction and its following safe. At present,international and domestic gas detection and monitoring of targets are mainly for coal,and rarely for the subway system. However, the gas detection between subway hub and mine are very different.Therefore,it is very important to measure the distribution of methane concentration during construction and operation of subway tunnel in real-time.Based on the principle of gas near-infrared spectral absorption, this article develops the first optical fiber gas sensor detection system for domestic subway tunnel, demonstrate that the system can be applied in the subway tunnel field. In this article we focus on the research of the detection of CH4, and develop the four-channel fiber-optic gas sensors and optical detector; analysis and calculation Hangzhou Metro tunnel gas distribution and concentration; design the engineering drawings such as gas detector layout and installation.It can be measured methane concentration during the period of subway tunnel construction and follow-up operation. Besides,it can be alarmed at the threshold of limitation.The main work in this article include the aspect as follows:1) Some research were done on the working principle of the fiber optic sensor which based on tunable diode laser absorption spectroscopy.Besides,use the wavelength modulation and second harmonic detection technology to improve detection sensitivity.2) According to the theoretical analysis, the light path and circuit were designed. In the light path design, we designed the system sensor and detector to separated. It means one sensor instrument connected four intrinsic safety detector by optical fiber. Therefore, it reduces the cost,and is suitable for subway gas detection system which has long distance, multi-point testing requirements. In circuit design, we designed a Phase-locked demodulator circuit which based on analog multiplier MLT04, and the phase locked amplify circuit eliminate the effects of the phase difference between the test signal and the reference signal which causes by the delay of the optical fiber transmission and the circuit itself.3) In view of working environment for the subway tunnel, the structure of the sensor's optical detector was designed specially, and adopted dust-proof and waterproof measures.The optical detector basically do not need to adjust,so that it satisfies for the special requirements during the metro operation.4) Some experiments were done, such as structural reliability, temperature stability and detection performance and so on. So it can be better used in gas detection during the period of subway tunnel construction and operation.5) According to the theoretical analysis, we had calculated gas distribution and concentration during the operation period of Hangzhou No.1 Metro.6) According to the two-lane tunnel for Hangzhou No.1 Metro,we had investigated the distance of detector distribution, and designed the detaile engineering drawings such as gas detector layout and installation and so on.