The New Natural Gas Pipeline Replacement Process Dynamic Simulation Analysis

With the. peak period of construction of gas pipeline coming, how to use and perfect the long natural gas pipeline replacement and production technology and how to ensure the smooth operation of gas pipeline has been a growing concerned question.Make sure to replace the air in the pipeline before natural gas coming into the pipeline and ensure the safe operation. There are four methods of gas pipeline replacement:the method without pig or N2(natural gas-air), the method of pig without N2(natural gas-pig-air), the method of N2 without pig (natural gas-N2-air, also means "gas pushing gas"), the method of pig and N2(natural gas-pig-N2-pig-air). Analyzing of the process, features and security of these four methods, we can get that it is easy to control the speed and the replacement resistance is small when use the method of N2 without pig. What is important that the impact of external conditions on the replacement effects are small and manageable. Additionally, the replacement cost is low, and operation is simple. So the best choice of the natural gas pipeline replacement is the method of N2 without pig.Based on the theory of mixed gas molecular diffusion and convective mass transfer, construct a mathematical model which is describing the mixed gas law of natural gas pipeline replacement. For the laminar flow, construct a mathematical model of replacement by the composition continuity equation and the Fick diffusion law, and use the two-step method which means characteristic method and the difference method to solve the model. For the turbulent flow, we can make it into three layers:the bottom layer, buffer layer and turbulent core area, and construct the mathematical models respectively. Use the f implicit difference method to solve the model.To the PingChang-PuGuang gas pipeline case study, use the mathematical model and Visual Basic to make the software of natural gas pipeline replacement. Obtain the results as follows:(1) use the model to calculated the mixed gas length and aeration of the natural gas pipeline replacement (2) Analyze and get the nitrogen molar concentration distribution along the axial and radial and the changes of gas flow rate in the pipeline. Use Fluent software to simulate the nitrogen replacement process of natural gas pipeline and verify the correctness of the model. (3) Analyze five factors of the mixed gas length:back pressure, flow pattern, velocity, diameter and pipeline length. The mixed gas length increases with the increasing of gas flow rate, diameter, back pressure and pipeline length; try to avoid the presence of back pressure and vent to reduce the back pressure; make sure that the replacement is in a turbulent state and control the flow rate to ensure safety of the process.(4) Determine the calculation method of the parameters of natural gas pipeline nitrogen replacement and the safety requirements.