Study On Inert Gas Replacement Method Of Natural Gas Spherical Tank

At present,the study of substitution of natural gas spherical tank is mainly focused on the substitution methods,the limit oxygen content of natural gas mixed gases,the substitution standards,and the precautions of the substitution process.It does not thoroughly investigate the distribution and variation rule of oxygen(or methane)in the spherical tank and the amount of nitrogen used.The limit oxygen content of methane explosion is 12%(volume fraction),but the substitution standard fails to reach consensus.The maximum oxygen content in the spherical tank must be less than 12%,it can ensure no explosion in the spherical tank.And the value of the average oxygen volume fraction at this time can be used as substitution criteria.In order to determine the state of the maximum oxygen content in the spherical tank is less than 12%,to establish a reasonable substitution standard,to understand the distribution of gas components in the spherical tank,so this article based on nitrogen as an intermediate medium to study natural gas spherical tank substitution.In order to achieve the research objectives,this project uses theoretical calculations and CFD simulations,and then compares the data with the substitution project of natural gas spherical tanks.Through the analysis of natural gas spherical tank substitution examples,a two-dimensional symmetrical model was established for the substitution of natural gas spherical tanks to simulate the process of replacing nitrogen with air when the natural gas spherical tank was put into operation.The inlet and outlet diameters of the spherical tanks were DN50,and the injected nitrogen flow was 1000Nm~3/h,that is,the mass flow rate was 0.3472 kg/s,which was simulated by one-time boost pressure substitution,graded boost pressure-substitution,and isobaric substitution.Under the condition of an example,isobaric substitution method is used to simulate A?B?C?D schemes of nitrogen or air substitution of methane during overhaul.The injected air flow is 1000Nm~3/h,which means the mass flow is 0.3583 kg/s.The simulation result of the example is the same as the theoretical calculation result.The error is within the allowable range,which proves that the example simulation is correct and feasible.Through the analysis of the simulation results,the following conclusions are drawn:(1)In the substitution of production,in the boost pressure-substitution process,the pressure increases linearly with time.The oxygen volume fraction and the inlet velocity decrease inversely with time,and the temperature tends to the ambient temperature.Graded boost pressure-substitution save nitrogen and time compared to one-time boost pressure substitution,and the amount of nitrogen injected at one time is preferably the capacity of the spherical tank(in the standard state).(2)During the boost pressure-substitution process of a 1000m~3 natural gas spherical tank,when the initial inlet velocity is higher than 60 m/s,all gases in the spherical tank participate in eddy current flow,the mixing of the gas components will be more uniform,and the difference between the maximum and the average value of the oxygen volume fraction is small(the isobaric substitution also has the same conclusion),so the boost pressure-substitution process can be directly dissipated without the regulated voltage mixing process.(3)Under the working conditions of this case in the article,when the initial inlet velocity is greater than 60 m/s whether it is boost pressure-substitution or isobaric substitution,it is necessary to consider a certain safety factor,and it is reasonable and safe to use the oxygen volume fraction of 10%as the substitution standard for natural gas spherical tank.(4)When using the inert gas substitution method,isobaric substitution is the best substitution method for substitution of natural gas spherical tanks.The D program for isobaric substitution is the best substitution process for the natural gas spherical tank maintenance substitution.