Analysis of natural gas leakage diffusion characteristics and prediction of invasion distance in utility tunnels

With the continuous development of urbanization, the number of underground supplies and demanded pipelines is increasing. In order to reasonably plan the construction of urban underground space, underground utility tunnels emerge, and show potential to reduce the difficulty of pipeline maintenance and repair. For natural gas pipelines laying in utility tunnels, however leakage accident can result in serious consequences. In this study, the numerical simulation method was used to analyze the leakage and diffusion characteristics of natural gas in utility tunnels under various working conditions, and the single-factor change and multi-factor change of different pressures, leakage diameters, ventilation conditions and leakage locations were investigated. The re-sults showed that: the increase of pipeline operating pressure and leakage diameter could accelerate the diffusion velocity of methane in the utility tunnel. According to the prediction equation of methane invasion distance (MID) under natural ventilation condition, the average error was 8.29%. The mechanical ventilation condition accelerated the methane discharge and greatly reduced the distribution range and concentration of methane in the utility tunnel. In the single-factor change analysis, when the mechanical ventilation frequency was above 15times/h, the methane concentration at each monitoring point was below the lower explosion limit. No matter where the leakage occurs, the mechanical ventilation will prevent methane from diffusing upstream direction, and the downstream was at higher risk for methane distribution. The larger the coordinate value x of leakage location was, the smaller the distribution area of methane in utility tunnel was. When x = 90 m, the methane concentration at each monitoring point was 0. The prediction equation of MID under natural ventilation con-dition can provide reference for the installation distance of methane leakage alarm device. The research on methane diffusion characteristics under mechanical ventilation conditions provides suggestions for daily main-tenance and accident prevention of gas pipelines in the utility tunnel.