Determination of Safe Pumping Temperature for Waxy Crude Pipelines

Flow risk of a hot waxy crude pipeline mainly comes from restart failure, i.e. oil gelation resulted from prolonged pipeline shutdown, and unstable operation at low flow rate. Once the unstable operation happens, the friction loss of the pipeline increases with decreasing flow rate and finally flow may cease if treated improperly. To avoid these flow risks, the pumping temperature of the crude is generally required to be kept above a minimum allowable temperature, and conventionally the pour point temperature is taken. This practice is effective but quite rough. Obviously, to control the inlet temperature of a heating station at the pour point temperature implies different safety margin for winter and summer operation. For large throughput hot oil pipelines, reduction of the heating temperature even by a little bit may save a great amount of fuel. Therefore, how to save fuel while ensuring safe operation has been a valuable topic for long time. On the other hand, many factors impacting the flow safety are stochastic and with uncertainty, so analysis without considering this feature can hardly yield convincible results, though this has been the common case for many years. In this paper, by taking the stochastic feature into account, a Stable Operation Index (SOI) and a Pipeline Restartability Index (PRI) were proposed to assess the flow safety of a pipeline concerning the low-flowrate stable operation and restartability after shutdown. Combining these two indexes, a Pipeline Flow Safety Index (PFSI) was adopted to assess the flow risks of hot waxy crude pipelines. On this basis a new approach to quantitatively determining the safe pumping temperature was developed and illustrated by a case study. Encouraging results show that this new approach has the potential to replace the simple rule of pour point as a guide to determining the safe pumping temperature of waxy crude pipelines.