Some Interesting Flow Characteristics of a Heavy Crude Pipeline

In this paper the hydraulic characteristics of a heavy crude pipeline were discussed in detail. The crude oil to be transported is very heavy and highly viscous, with the specific density as high as 0.965 (API degree is about 14.7) at 20°C and the kinematic viscosity as high as 920mm²/s at 50°C, and its viscosity increases sharply with decreasing temperature. Due to its poor flow-ability the heavy crude is to be heated in the heating-pumping stations of the heat-insulated pipeline during its pipelining process. It is interesting that the pipeline does not behave like a usual hot oil pipeline in two ways. First, given a heating temperature and a pressure drop for a section of the pipeline, the trial-and-error process to solve for the flow-rate of the section is likely to fall into the trap of an unstable flow-rate interval, in which the friction loss of a pipe section decreases with increasing flow-rate, just contrary to usual oil pipelines. Secondly, the flow of heavy crude through the pipeline is probably in the critical regime between laminar flow and turbulent flow, so it is possible that a proper flow-rate can not be found corresponding to some heating temperature and pressure drop of the pipeline. For a hot oil pipeline, the necessary condition for occurrence of an unstable flow-rate interval is to fix the discharge temperature of a heating station, additionally, a steep curve of viscosity versus temperature and good heat transfer condition between the pipeline and its surroundings will be activating factors. Based on the hydraulic calculations of Xinmei heavy crude pipeline at different flow-rates, the curves of friction loss versus flow-rate were plotted, from which the unstable flow-rate intervals may be determined. From the viewpoint of safe operation, the unstable flow-rate interval should be avoided. For the hydraulic calculations of an oil pipeline, a critical Reynolds number of 2000 is usually used to divide flow regimes into laminar and turbulent flow. Because of different friction factors determined respectively from laminar flow and turbulent flow for the Reynolds number 2000, for some values of friction loss of a pipe section, the corresponding flow-rates do not exist, in this case, the problem for finding out the flow-rate of a pipe section is not solvable. On the other hand, the critical Reynolds number is sensitive to many factors related to pipe flow, so for the stable operation of a pipe section, its Reynolds number should be far from the Reynolds number 2000.