Investigation On Nitrogen Gas Replacement In Natural Gas Pipeline

With the coming of peak period of our country's gas pipeline construction, how to use and perfect the Natural Gas Long Transportation Replacement Commissioning Technique to guarantee successful replacement of natural gas transmission line has increasingly become a focused problem. On the basis of theoretical derivation and mathematical calculation, in this paper, investigation on the replacement method of using nitrogen gas to push the air in natural gas pipeline is carried out.Firstly, the replacement methods used in process of nitrogen gas replacement in the field are introduced. Using the basic diffusion theory, the development of gas-mixing section in process of nitrogen gas replacement is described, and the integrated relational expressions of gas-mixing concentration, gas-mixing capacity and gas-mixing length are derived from the convection and diffusion equation.Secondly, the mathematical model for the replacement in natural gas pipeline is presented, and three-dimension steady as well as unsteady numerical calculation are carried out, and the variation regularity of the mole concentration of nitrogen gas, velocity and pressure are analyzed. In the steady numerical calculation, the relationships between the pipeline diameter, velocity, the pipeline length and gas-mixing length are discussed, and the relational expressions of gas-mixing length with the pipeline diameter, velocity and the pipeline length are derived from non-linear curvefitting. In the unsteady numerical calculation, the relationships between length-diameter ratio, velocity and the gas-mixing length are discussed, and the results are consistent with the former steady numerical calculation.Finally, the results of theoretical derivation and mathematical calculation are compared and verified with the site data. The comparison and verification shows that the research can optimize the nitrogen gas input in the process of nitrogen gas replacement, and can decrease the unnecessary nitrogen gas loss and reduce operation cost and invest. This research can also provide a theoretical reference in the project establishment of field production run.