Research On Heat Transfer Characteristics Of Buried Heat Tracing System During Shutdown

China produces three high crude oil and widely adopts temperature raising method to improve the fluidity of crude oil.With the development of old oilfields entering the middle and late period,the heat tracing system often occurs such phenomena as pipeline leakage and corrosion damage,which leads to the shutdown of the heat tracing system.Heat tracing system shutdown involves heating pipe,heating medium heat transfer,phase change of crude oil,filling medium,the main line and thermal insulation layer,soil field coupling physical process and the flow of crude oil changes,cross function of many factors result in heat transfer is extremely complex,therefore research on heat transfer heat tracing system shutdown process,accurate grasp different shutdown condition,the director of the oil temperature change rule,It is of great significance to reasonably control the shutdown maintenance time and formulate the restart plan.When the temperature of crude oil is lower than the temperature of wax precipitation point,the precipitated paraffin crystals deposit on the inner wall of the pipeline,resulting in restart failure.Therefore,the key to restart safely and stably is to accurately grasp the thermal change of the shutdown heat tracing system,reasonably control the medium temperature of the heat tracing pipeline and formulate the safe shutdown time.In this paper,based on the waxy crude oil three-phase zoning method,a three-dimensional heat transfer model of buried shutdown pipeline tracing system is established,and the finite volume method is used to discretize and solve the equation.The phase change heat transfer characteristics of waxy crude oil were analyzed under two working conditions of simultaneous shutdown of heat tracing pipeline and crude oil pipeline,and the heat tracing pipeline was not stopped and crude oil pipeline was stopped.The temperature field,flow field and growth law of condensate layer of main crude oil under no thermal working condition were clarified.Through the comparison of the influence of thermal factors,it is clear that the heat tracing tube is placed directly below the main pipe,which can significantly improve the heat tracing effect;Heat tracing effect can be greatly improved by filling thermal conductive cement between the main pipe and the heat tracing pipe due to its increased heat storage and thermal conductivity.In addition,the influence of pipeline specification,medium flow rate and medium temperature on the heat tracing effect is quantitatively compared and analyzed.According to the variation characteristics of average crude oil temperature curve,this paper proposed that the cut-off time corresponding to average crude oil temperature dropping to 5℃ above freezing point was taken as the limit to evaluate the heat tracing effect and give the optimal cut-off time range.The effects of double tube heat tracing and triple tube heat tracing were compared,and the influence factors of flow rate and temperature of heat tracing medium,diameter of heat tracing pipe and laying position were analyzed.