| With the paved area of pipe network extending continuously,especially the increasing of pipes constructed in complex and large-drop terrain year by year,lots of new problems for clearing tube had been brought out.During the drainage process of the large-drop natural gas pipeline after pressure-testing,it was a very complicated transient and multi-factor coupling problem for the prediction of pressure change between the back-end gas section and the front liquid section of pipeline pig and its movement.The main research contents of this paper include the unsteady motion law of pipeline pig,transient change law of gas and liquid segments,the separation of liquid column and the water hammer phenomenon in the drainage process after pressure-testing.The detailed content of this paper are shown as follows.(1)Firstly,comprehensive theoretical study for the drainage process after pressure-testing of large-drop natural gas pipeline was included in this paper.Several transient processes and their causes as well as affecting factors that may exist in the liquid section of the pipeline during the drainage process were analyzed and clarified.The above study includes the cause of pressure drop in the liquid section during the drainage process after the pressure-testing and the influencing factors of the gas release,the vaporization theory of the liquid and the dissolution mechanism of the gas in the liquid,how to judge the liquid column separation in the pipeline,the cause of water hammer after the above separation and the liquid flow regime as well as their transformation in the tube.Secondly,the transient behavior of pipeline pig in the drainage process after pressure-testing was analyzed including the force analysis and movement law of the pig in the flat stage,the uphill stage,the downhill stage and the accident-shut-valve stage,respectively.Based on the above study,the transient behavior of clearing pipe in the drainage process had been mastered on the macro level.Further more,the transient behavior of divided region and period had been analyzed,which could provide theoretical basis for the establishment of hydraulic transient mathematical model during the drainage process.(2)The basic differential equation of water hammer numerical calculation,the characteristic line method and the basic principle of finite difference method are briefly explained.After that,the above differential equation of water hammer is transformed and solved with characteristic line method.In addition,the initial conditions and boundary conditions suitable for the studied working condition were established,which could provide a reliable theoretical basis for solving the model.(3)The unsteady motion model for the upstream gas segment and downstream liquid segment was established and solved with characteristic line method.The dynamic analysis of pipe pig in the drainage process was carried out.The above unsteady motion model for gas segment and liquid segment were coupled with the dynamic model of pipeline pig with appropriate coupling method,respectively.Then,complete unsteady motion model for clearing tube in the drainage process after pressure-testing was established.(4)Based on the transient hypothesis of liquid flow and the occurrence mechanism of transient in the drainage process,the discrete cavity model and homogeneous foam flow model were established,respectively.In addition,the range of application,the basic assumptions and the corresponding solving process for the above two models were analyzed.(5)Based on the unsteady motion model,combined with the discrete cavity model and homogeneous foam flow model,a procedure for transient process of clearing tube was developed with FORTRAN language.The numerical simulation of drainage process after pressure-testing was carried out aimed at a certain large-drop pipe section and the relationship between the inlet and outlet boundary conditions and the transient process in the tube was analyzed emphatically. |
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