Recent advances in the understanding and numerical modelling of column separation in pipelines

Abstract

The possibility of the coliapse of a vapour cavity generated during column separation in a pipeline during a water hammer event has always been of major concern to designers. A commonly used numerical model for describing water column separation is the discrete vapour cavity model. Studies have shown that the results from this numerical model exhibit numerical spikes of unrealistic pressure rises following the collapse of vapour cavities in pipelines. The cause of these numerical spikes is attributed to the intermittent formation and collapse of vapour cavities at computational sections. The discrete vapour cavity model does not specifically differentiate between localized vapour cavities and distributed vaporous cavitation. This paper considers the importance of the formation of distributed vaporous cavitation zones on the pressure generated by the collapse of a vapour cavity at a valve. An alternative numerical model has been developed that describes water hammer regions, localized vapour cavities and distributed vaporous cavitation regions separately. The formation of intermediate cavities is also accounted for in the model. The implementation of this numerical model is described in this paper. Examples of results from this alternative numerical model and results from the discrete vapour cavity model are compared algainst experimental data collected at The University of Michigan for a 36 meter long pipeline connecting two reservoirs.