Study On Process Problems In Commissioning Of Large-slope Pipelines

Air pockets are easy to entrap in commissioning of large-slope oil pipelines,which seriously threatens the safe operation of the pipelines.Multiple exhaust methods have been used to solve the problem of air pockets,including sending pigs,installing air exhaust valves near high points,and "hydraulic air exhaustion".Some scholars have studied various exhaust methods,so far no related research has performed CFD software simulations on the gas-liquid flow law in commissioning of large-slope oil pipelines,and no scholar has modeled and analyzed the high-point exhaust process.Therefore,depth researches have been conducted on the air exhaustion in commissioning of large-slope oil pipelines.Firstly,the CFD software Fluent is used to simulate the gas-liquid distribution of a single U-pipe and a continuous U-pipe during the liquid-gas replacement process,and the flow patterns in this process are mainly gas-liquid stratified flow,bubble flow,and slug.The transient simulation results provide technical support for the prediction of gas-liquid flow patterns during the commissioning of large-slope pipelines.Secondly,based on the flow in the open channel,a gas accumulation model for large-slope pipelines is established,which can be used to calculate the initial liquid holdup,initial air void and other parameters.Meanwhile,based on the gas-liquid two-phase stratified flow and two-fluid model,a model of accumulated gas compression was established,which can be used to calculate changes in parameters such as the pressure and length of the gas accumulation section during commissioning.The gas accumulation model can make a preliminary judgment of the gas pockets at different locations,which is beneficial to the realization of pipeline exhaust.In addition,the post-processing software CFD-Post was used to compare and analyze the initial gas void and the pressure calculated by Fluent.Then,a high-point exhaust model was established based on the isentropic flow principle of the shrinking nozzle.According to whether the high point exhaust position is in the U-shaped pipe section where the current head is located,two exhaust models are established respectively.Based on the principle of energy conservation,the pressure of each gas pocket during continuous water filling can be calculated.The establishment of the high-point exhaust model has laid a theoretical foundation for the realization of high-point exhaust.Finally,the established model was used to calculate the Mangshi-Baoshan section of the first phase of the China-Myanmar crude oil pipeline project(domestic section),and the parameters such as exhaust pressure,exhaust flow,and residual gas quality under three exhaust conditions are analyzed.In addition,the effects of the flow rate,pipe diameter and back pressure on the compression process of the gas pockets are analyzed,and the effects of the exhaust valve diameter and the initial exhaust pressure on the exhaust process are analyzed.This paper is of reference significance for operation safety in commissioning of large-slope oil pipelines.