Study On Multi-stage Coupling Leakage And Diffusion Hazard Law Of Buried Gas Pipeline

With the rapid development of gas industry and the acceleration of pipe network construction,the gas supply capacity is continuously enhanced,and urban gas pipeline has become the energy main artery of national economic development.The gas pipeline must be paid great attention because it carries flammable and explosive gas.The major gas explosion accidents in recent years have attracted the attention of the national government and all sectors of society.Due to the bad and concealment of the buried environment,the leakage accidents of buried pipelines are more frequent and difficult to detect than those of aboveground pipelines.Currently,the research on leakage and diffusion of buried gas pipeline considers less about the pipeline buried environment,and focuses on the simulation analysis of single gas flow stage with simple boundary conditions.In essence,gas leakage and diffusion is a multi-stage coupled complex gas flow process constrained by the boundary of real buried environment.In addition,the pipeline buried environment is diverse and closely related to regional attributes,which has a serious impact on gas leakage and diffusion.Based on the above discussion,the multi-stage coupling leakage and diffusion hazard law of gas considering the real buried environment of pipeline was studied.The experimental platform of buried pipeline leakage gas diffusion was built,and the effects of various parameters on the diffusion process of methane gas in soil were studied.The soil characteristic parameters of sandy soil used in the buried pipeline leakage gas diffusion experiment were measured and calculated by using gas seepage experiment and ring knife soil cutting experiment.The experimental results provided data support for the accuracy verification of subsequent numerical simulation methods and the setting of soil porous media parameters.Based on the basic equations of fluid mechanics and the flow process of buried pipeline leaked gas,the multi-stage control equations of gas flow in pipeline,soil and atmosphere were given,which provided a theoretical basis for numerical simulation research.Aiming at the large-scale model of long-distance high-pressure gas pipeline leakage which can not be studied by experimental and numerical simulation methods,the one-dimensional simplified calculation small-hole model,large-hole model and pipeline model of pipeline leakage rate were deduced and established by using the theoretical analysis method,and the calculation models selection and definition were realized by using the aperture ratio(d/D).In view of the gas flow characteristics of the buried pipeline leakage and the potential safety hazards caused by entering the underground confined space,the flow field distribution of the gas leakage and diffusion in the soil and the diffusion characteristics of the gas entering the confined space through the soil were studied by using the transient numerical simulation method,and the analysis of gas-soil-gas coexistence process of was realized.The concentration distribution of gas leakage and diffusion in the confined space was studied by using the steady-state numerical simulation method,the confined space harmful boundary calculation model of buried gas pipeline leakage was established,and the harmful boundary plate was drawn,which provided the basis for safe distance design of the buried gas pipeline construction.The buried gas pipeline leakage was affected by the soil characteristics of buried environment,and there was no critical flow distribution at the leakage hole.At the initial stage of leakage,the internal and external pressure and velocity of the pipeline at the leakage hole were unstable,and reach the equilibrium state after a period of time,and then the pipeline downstream of the leakage hole produced reversal flow phenomenon.In view of the influence of ground conditions and soil characteristics in the pipeline buried environment on the two processes of gas leakage and diffusion,the ground was divided into hardened surface ground(HSG),unhardened surface ground(UHSG)and semi-hardened surface ground(SHSG),and the effective characterization of different soil characteristics was realized by using soil resistance coefficient,saturation and porosity.The gas leakage rate was used to analyze the leakage process,the gas invision distance(GID)and early warning boundary(EWB)were defined to study the gas diffusion process.Based on the least square method and multiple regression theory,the leakage rate direct calculation model of buried gas pipeline and the calculation models of GID and EWB under HSG and UHSG conditions considering the real buried environment were established,and the systematic calculation method of gas leakage and diffusion was formed,which provided a universal calculation method for the buried gas pipeline leakage hazards calculation under different ground condition and soil characteristics.Aiming at the gas diffusion soil-atmosphere coupling process of buried pipelines leakage under UHSG,considering the adsorption of gas by soil porous media,the calculation model of buried pipeline leakage gas emission in the atmosphere was established.The user defined function(UDF)was used to realize the definition of the exponential relationship between the wind velocity at the side of the atmospheric wind inlet and the height.The distribution characteristics of the first danger zone(FDZ)and the second danger zone(SDZ)between atmosphere buildings cavity after gas leakage through the soil were studied.The downstream of the FDZ was the high-risk side of the gas distribution,and the bottom end was the high-risk point.The upstream of the SDZ was the high-risk side of the gas distribution,and the top end was the high-risk point.According to the influence of underground and aboveground factors on the gas concentration distribution in the dangerous zone,the gas concentration prediction model on high-risk side of the FDZ was established,the gas concentration distribution plate was drawn,and the quantitative evaluation of gas concentration distribution in the atmosphere was realized.The research results provide a theoretical basis and reference for leakage hazards quantitative analysis of urban medium pressure buried gas pipelines and reduction of secondary explosion accidents.Meanwhile,it has important value and engineering practical significance for the flow guarantee of buried gas pipeline,the emergency decision-making of leakage accident,and the improvement of gas pipeline design and maintenance specifications.