Diffusion process simulation and ventilation strategy for small-hole natural gas leakage in utility tunnels

The utility tunnel provides enough spaces, dry environment, better monitoring and maintenance for municipal pipelines. When gas pipelines are incorporated into the utility tunnel, the flammability and explosion property of the natural gas would threaten its safe operation. At present, the safety prevention technology and the safety design specifications for gas pipeline in utility tunnels are not mature. To investigate the gas diffusion characteristics in the utility tunnel, and to propose an emergency accident ventilation strategy, in this paper, the mechanism of small-hole gas leakage diffusion under natural and mechanical ventilation is studied, and the influential factors are simulated and analyzed in detail using the Fluent software. The results indicate that under the natural ventilation, the gas spreads to the whole compartment at the leakage time of 300 s, while under the normal ventilation scheme, the leaked natural gas fills the downstream half of the compartment at the time of 120 s. When the leak size and pressure increase, the alarm response time shortens, the concentration of test point increases, and the danger zone expands. To prevent alarm failure and gas gathering for the near-outlet leakage, it is recommended that the last detector and the air outlet should be placed as close as possible to the fireproof wall. In addition, when a leak occurs, increasing the air change rate can greatly reduce the overall gas concentration. When the leak size is no more than 5 mm, and the pipeline pressure is lower than 0.4 MPa, the minimum accident ventilation scheme could guarantee the safe operation of the utility tunnel. When the pipeline pressure grows to 0.8 MPa, the effective air change rates of the emergency accident ventilation are 15 times/h, 18 times/h, and 24 times/h, respectively for the leak sizes of 4 mm, 5 mm, and 6 mm. Moreover, for the pipeline pressure of 1.6 MPa, the effective air change rates are 21 times/h, 30 times/h, and 36 times/h, respectively for the leak sizes of 4 mm, 5 mm, and 6 mm.