Study On Working Characteristics Of Wax - Free Hot Oil Pipeline System

Most of China’s production of crude oil is waxy crude oil, and the process of pipelining waxy crude oil often is heating. In the process of heating transportation for waxy crude oil, wax deposition can bring a great security risk to pipeline, also can cause great loss of energy (heat and pressure). Therefore, researching the law of wax deposition along pipeline between the heating stations, figuring out what wax deposition cause impact on the characteristics of pipe and pump station system, building a model to predict the distribution of wax deposition in the pipeline, these has important practical significance to improve the transporting efficiency of pipeline system, to ensure the safety of transporting crude oil and to formulate reasonable pigging scheme. To this end, in this paper, the waxy hot oil pipeline system as the research object, carried out the following main research contents.In the actual operational process of waxy hot oil pipeline between the heating stations, crude oil properties and frictional characteristics will vary along the oil temperature reducing and the effective flow area and the heat transfer coefficient of the pipeline will decrease continuously with increase of thickness of wax layer. Taking all these factors above, reckoning in the influence of the frictional heat at the same time, the hydraulics average wax layer thickness algorithm which is calculated inversely by means of pressure drop is improved, and the corresponding calculation programs are compiled by using MATLAB7.0.A model of long-distance pipeline system with the same pipe distance between stations in flat area is set up by OLGA, and then under the situation of the same external conditions and of transporting the same product for each pipeline between stations, the wax deposition process is simulated in turn by different ways of pressure rise at pump station which uses fixed speed single pump, fixed speed two pumps series and variable speed single pump. Based on the simulation results, this paper analyses the influence derived from wax deposition on the operating characteristics of different types of centrifugal pumps, and studies the law of the distribution of wax deposition in the pipeline and the influence from pump type selection on the distribution of wax deposition.On the basis of the data that the physical properties of Daqing crude oil vary with temperature, constant terms in the formulas are obtained in regression by using the least square method. According to running parameters of the crude pipeline between Tieling station and Anshan station in the four seasons, using the model presented before in this paper to calculate the average thickness of wax layer in wax section of the pipeline under different seasons, different operating periods, meanwhile combined with OLGA simulation results and the model of predicting wax deposition rate of Daqing crude oil, the law of wax deposition along pipeline between Tieling station and Anshan station and impact caused by wax deposition on the operating characteristics of the piping system were analyzed.Through the above analysis and researches, getting the following conclusions:firstly wall shear stress, oil viscosity and the radial temperature gradient after wax precipitation peak point is the main factors influencing the wax deposition rate, the radial temperature gradient of which is the primary influence factors, and when ground temperature rise, the effect of the radial temperature gradient on wax deposition rate reduce, and ground temperature rise the more, the greater the effect amplitude decrease. Secondly the distribution of wax deposition along pipeline between stations is directly related to the pump type selected by pump station, and for waxy hot oil long-distance pipeline, when pump station choose a pump, should more choose a pump whose characteristic curve about head and flow is relatively steep. Thirdly temperature range about40℃, where wax precipitation peak zone of Daqing crude oil between stations is in, is part of interval II(between wax precipitation point and abnormal point), and in all the year round, if the longer the length of the interval II is, the more serious the situation about wax deposition is. In addition, wax deposition rates in the wax precipitation peak zone were similar, therefore, the longer the peak zone, the closer the distribution of wax deposition as a shape like ladder.