Investigation On Natural Gas Pipeline Drying Technology

Before the natural gas pipeline is put into production,it is necessary to carry out hydraulic pressure test,and water is often used as a test medium.After this process,most of the water is often removed by pig,but there is still a small amount of free water.Free water will lead to corrosion of the inner wall of the pipeline,the formation of hydrate clogging pipeline and reduce the transmission efficiency of pipeline,so it is necessary to dry the natural gas pipeline before putting into production.At present,the commonly used pipe drying methods include dry air drying,vacuum drying and desiccant drying three method,when selecting drying method,inaccurate drying process parameter prediction will lead to the selection of drying methods and process parameters unreasonable.Therefore,according to the process parameter determination and drying method selection of natural gas pipeline,the following research is carried out:(1)Mass transfer in the drying of natural gas pipelineMass transfer law of single-phase inbound and gas-liquid in dry air drying,vacuum drying and desiccant drying are analyzed.Aiming at the interphase mass transfer of dry air drying method,the free water evaporation rate model in tube based on Chilton-Colbum analogy method is modified on the basis of considering the mass transfer resistance of gas-liquid interface,in which the interface resistance is described by Schrage model.The single mass(diffusion of water vapor in dry air)in dry air drying method uses GRI model to calculate its mass transfer coefficient.According to the vacuum drying method,the contact surface of free water boiling in liquid film and tube airflow is analyzed,and the free water boiling rate model is established by combining Schrage model.According to the drying method of desiccant,because the diffusion and mass transfer process of desiccant is mainly affected by eddy current diffusion and convection diffusion,the mass transfer coefficient is calculated by Sittel correlation formula in this paper.The establishment of mass transfer rate model lays a foundation for the research of drying process simulation.(2)Simulation of dry air drying processDry air drying process into three stages,namely,initial purge,water absorption and drying.Firstly,based on the principle of mass,momentum and component conservation,the mathematical model of the initial purge stage of dry air drying is established,and the discrete model of the total implicit finite volume method is adopted,in which the mass and momentum equations adopt the first-order windward format,the component equation adopts the second-order QUICK format,and the solution strategy of the discrete equation based on the Secondly,according to the principle of component conservation,the mathematical model of water absorption stage is established,and the finite volume method is also used to solve the model.Finally,the validity of the model is verified by two cases.For the initial purge stage,the calculation results of the change of dry air concentration and the average deviation of the experimental results are 8.4%and 8.2%respectively,and the dynamic change process of the parameters such as water film thickness,pressure,mass flow rate and steam concentration during dry air drying is analyzed,as well as the flow rate,ambient temperature,The effect of water film thickness and tube length on the initial sweep drying process.In addition,the variation law of water vapor concentration in the suction stage and the re-drying stage is also analyzed.(3)Simulation of vacuum drying processFirstly,based on the law of mass,momentum and energy conservation,a mathematical model of vacuum drying of natural gas pipeline is established.Secondly,the model is discretized by the all-implicit finite volume method,in which the mass and momentum equations adopt the first-order windward format,the energy equation adopts the second-order windward format,and the Newton-Lafferson method is used to solve the discrete equation.Finally,the correctness of the model was verified by three cases,in three which the calculated drying time was 17.7 days,17.2 days and 14 days,respectively,and the relative deviations were 1.7%,1.2%and 5.7%,respectively.In addition,the dynamic process of parameters such as water film thickness,pressure,mass flow rate and temperature is analyzed.The effects of pumping rate,ambient temperature,water film thickness and tube length on the drying process during vacuum drying were also analyzed.(4)Simulation of desiccant drying processWhen establishing desiccant drying model,pipeline is divided into 3 segments:The upstream section,the train section and the downstream section.Based on the principles of mass conservation,momentum conservation and component conservation,respectively,the drying model of these three segments is established.Then the coupling of the model is realized by the moving boundary of the train.The upstream fluid flow model of the train,the downstream fluid flow model of the cleaning train and the plug component model of the Desiccant section are discretized by the finite volume method,and the train motion model of the pipe cleaning is discretized by the finite difference method,and the discrete model is solved by Newton method.Secondly,through the drying example of the sea pipe of Liwan 3-1 gas field,the correctness of the model is verified,and the drying process of the desiccant is analyzed from two aspects of the operation Law of the train and the evaluation of the drying effect.Finally,the influence of the number of desiccant plugs and the amount of desiccant on the drying effect is discussed.(5)Selection of natural gas pipeline drying methodThrough the simulation method of dry air drying,vacuum drying and desiccant drying process,the quantitative prediction of investment cost,drying efficiency and drying effect is realized,and the drying scheme is evaluated according to the factors of safety risk and environment,which solves the problem of drying method selection and proeess parameter determination of natural gas pipeline.