Study On Pigging Law Of The Discharge Pig

Wax plug often occurs in front of the conventional pig during the pigging process,resulting in blockage of the pipeline.The jet produced by the discharge pig can blow the downstream wax crumbs away,thus preventing the formation of the wax plug.However,due to lack of theoretical support and other reasons,wax clogs the discharge hole and immerses the pig frequently;and with the continuous blending of the wax particles and the crude oil in front of the pigs,the physical properties of the oil in front of the pig will change,which may also affect the safety of pigging.Therefore,it is of great significance to study the pigging law of discharge pigs and the changing rules of the physical properties of wax slurry in front of the pig.In this paper,firstly,the pigging mechanism and the forces of the pig are analyzed theoretically.The conditions for the effective operation of the discharge pig are given.The quantitative relationship between the pig speed,the pressure difference at both ends and the discharge area is deduced.Newton’s second law is applied to establish the force model of the pig.The quantitative relationship between the friction resistance and the interference of the pig is obtained,and it is proved that the shear resistance of the wax layer on the wall has little effect on the operation of the pig.Secondly,a motion model of the discharge pig is developed.This model applies overlapping grid technology and integrates the dynamics control equations into Fluent to control the operation of pigs by means of UDF interface.The model can be used to obtain parameters such as running speed,force and flow field of pigs.The relative error of the model’s prediction of the pig’s speed and the measured value on site is within 10.18%.The motion of the pig in the simulated section is divided into the initial stage,the transitional stage,and the stable operation phase.In the stable operation stage,the pig’s running speed,the driving force,and the flow field change little.Thirdly,the influences of bypass rate and interference on operating speed,turbulence intensity,and discharge flow of the pig are systematically studied,and the selection strategy of the bypass ratio is given.The research shows that the operating speed of the pig decreases with the increase of the bypass ratio and the amount of interference;the turbulence intensity is greatly affected by pigs,and it increases first and then decreases with the increase of the bypass rate;the discharge flow of the pig presents an increasing trend with the increase of the bypass ratio;the selection of bypass rate should be considered from the aspects of pig running speed,turbulence intensity and discharge flow.Finally,the indoor experiment is done to simulate the dissolution behavior of the oil in front of the pig on wax deposits.By controlling the wax blending ratio,melting wax temperature,stirring time and other conditions,the dissolution effect of crude oil on wax and the rheological properties of the resulting slurry are studied,and the prediction method of the physical properties of the oil in front of the pig is given.