Study On Environmental Impact Of Leakage And Diffusion Of Urban Directly Buried Gas Pipeline

Due to people’s attention to the environment,natural gas as a clean energy has been vigorously developed,urban gas utilization rate is increasing,urban underground gas pipe network is densely distributed,many urban gas pipeline has been in the stage of high incidence of accidents.Through the statistical analysis of gas leakage accidents,it is found that the accidents are mainly divided into two categories: the first type is the gas pipeline leakage in the soil holes,accumulation in the confined space,fire,explosion and other accidents when encountered fire source;The second type is due to third-party damage caused by pipeline leakage,gas directly leaked into the ground space,in the densely populated residential areas and streets and other places such as explosions and other accidents.This paper mainly studies these two typical gas leakage accidents and analyzes the law of gas leakage and diffusion in soil and environment through experiments and simulations.The research results can provide a theoretical basis for ensuring the safe operation of urban gas pipe network and emergency treatment of accidents.First of all,according to the characteristics of the first type of gas pipeline leakage accident,an experimental study was carried out on the small hole leakage of buried pipeline in the soil,and the gas diffusion distribution law when the gas pipeline leakage occurs under different pipeline pressures was studied.The results show that at the same measuring point position,the higher the pipeline pressure,the greater the concentration of methane entering the measuring point,and the greater The Times of gas displacement.Secondly,according to the size of the experimental platform is established gas diffusion through a hole leakage occurred in the soil of the three dimensional numerical model,in the gas pipe in the soil hole leakage diffusion process is simulated,the contrast analysis of the spread in the different time,different pipe pressure,the changing rule of the gas diffusion distribution under different soil types.The results show that the high concentration zone,medium concentration zone and low concentration zone of gas at both ends of the leakage hole are roughly symmetrical.The gas concentration is the largest near the leak hole,and gradually decreases from the leak hole to the surrounding gas concentration.The diffusion rate of gas is obviously accelerated after it enters the atmosphere through the soil,and there is a big difference between the gas mass fraction at both ends of the boundary between the soil and the atmosphere.With the increase of the pipeline pressure,the range and risk of gas leakage also increase when the gas pipeline is slightly leaking.Soil types have an important effect on the natural gas leakage and diffusion process.When the soil type is clay,due to the small porosity of clay,the resistance of gas leakage diffusion is great.The gas concentration area is mainly concentrated in a small area near the leak hole,and the diffusion scope is the least at the same time,followed by sandy loam and silty sand.Then,according to the characteristics of the second type of accident,the physical and mathematical models of gas pipeline leakage and diffusion in different street building layouts are established and solved from the starting point of no street building layout in the city.In the establishment of the physical model,the calculation domain of700 m×650m×300 m and three layouts of determinant,oblique column and enclosed type were selected.The mathematical model includes three equations of conservation of mass,conservation of momentum and conservation of energy as well as gas state equation.The reliability of the model is verified by comparative experiments,and the correctness of the model is proved.Finally,the gas leakage diffusion model is used to simulate and analyze the influence of gas pipeline leakage diffusion in different street layouts.The results show that with the increase of the pressure of the leakage pipeline,the area of the gas reaching the explosion warning becomes larger in the same time of the leakage diffusion.Through the analysis of the simulation results of gas leakage diffusion of three different street layouts,it can be concluded that the enclosed layout has the worst gas circulation,the gas is the least easy to dilute and diffuse,and the highest risk,followed by the diagonal type,and the determinant type gas circulation has the best effect and the lowest risk.Gas will diffuse together with the airflow.The larger the ambient wind speed,the larger the diffusion distribution range: the possibility of gas aggregation is the least when the ambient wind speed is 3m/s,and the diffusion range is the largest and the risk is the highest when the ambient wind speed is 8m/s.The danger range mainly concentrates on the upper part of the building in the downwind direction,the building wall near the leakage port and the street cavity in the airflow roundabout area.In these areas,people should be evacuated in time to eliminate the hidden danger after the occurrence of gas leakage accident.By comparing and analyzing the simulation results of gas leakage diffusion of three different street layouts under the same leakage pressure and the same background wind speed,it can be seen that the enclosed layout has the worst gas circulation,the gas is the least easy to dilute and diffuse,and the risk is the highest,followed by the diagonal layout,and the determinant gas circulation has the best effect and the risk is the lowest.