Research On The Effect Of Wax Deposition On Shutdown And Restart Characteristics Of Crude Oil Pipeline

A mathematical model of shutdown and restart process containing wax layer of buried hot oil pipeline was established in this paper.And finite difference method,finite element method and the double characteristic method were used to solve the mathematical model.The wax deposition and shutdown-restart processes were simulated by writing a program,and the accuracy of this program has been verified.Zhong-Luo crude oil pipeline was simulated with this program,and the effect of wax deposition on shutdown temperature dropping,restart pressure and safe shutdown time was analyzed in detail.When buried hot oil pipeline contains wax layer,oil temperature in the pipeline will be elevated under normal operating conditions,while the soil temperature field around the pipeline decreases.So shutdown temperature dropping rate increases,which is not conducive to restart pipeline successfully.Within a certain shutdown time range,virtual crossing point of oil temperature distribution existed by comparing the wax deposition pipeline and no wax deposition pipeline.When outbound temperature were same under normal operating conditions,oil temperature distribution along wax deposition pipeline was lower than no wax deposition pipeline from outbound position to virtual crossing point,while oil temperature distribution along wax deposition pipeline was higher than no wax deposition pipeline from virtual crossing point to inbound position.With shutdown time extending,virtual crossing point appeared in different locations along the pipeline conveying direction in order.When the shutdown time exceeded the moment of the emergence of virtual crossing point at pipeline inbound,oil temperature distribution along the entire wax deposition pipeline was lower than no wax deposition pipeline.With wax layer thickness increasing,the effect of wax layer on the shutdown temperature dropping characteristics was more and more obvious,and the virtual crossing point was close to the outbound gradually.The restart pressure was developed because of wax layer,and the safe shutdown time was shortened to some extent.The effect of four different treatment methods of using actual wax thickness,average wax thickness along pipeline,average wax thickness after wax precipitation point and no wax thickness on shutdown and restart characteristics was analyzed in this paper.The shear stress on the wall of waxy crude oil pipeline had shearing action to the wax deposition layer in the process of restarting.This process was abstracted into shedding of wax deposition layer in this paper,in order to characterize the effect of shearing action.Simulation results indicated that with the extension of the restart time,the shedding of wax deposition layer occurred at different locations along the pipeline in sequence;the longer shutdown time was,the more difficult for wax deposition layer to fall off from pipe wall it was;the smaller the flow was,the smaller the shearing action of restart on the wax deposition layer was;different seasons also affected the shearing action on the wax deposition layer.