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The drainage process of a large drop natural gas pipeline after a pressure test is a complicated transient flow process. This process is a gas-phase and liquid-phase flow process connected by a pig, in which the pressure inside the tube, and the flow velocity of gas and liquid, and the movement state of the pig are constantly changing. At the same time, there are also a variety of complicated working conditions such as liquid evaporation and water hammer. Based on the pig transient model, a transient simulation model is established for the upstream gas section and downstream liquid sections of the pipeline in this paper, and the discrete vapor cavity model is used to simulate the drainage process. Taking a large drop pipe section of natural gas pipeline as an example, the paper studied the influences of key parameters such as flow velocity and pressure on the inlet and outlet of the pipeline on the drainage process and analyzed the complicated conditions such as discrete cavity and water hammer in the pipeline during the drainage process. In this paper, the law of pressure variation in the pipe is obtained, and the method of adjusting the inlet and outlet pressure and flow velocity of the pipe section by segmentation is proposed to avoid the occurrence of complex working conditions such as discrete cavity and water hammer.

Simulation Model and Case Study of Drainage Process After Pressure Test of Large Drop Natural Gas Pipeline