Numerical investigation of rapid filling in bypass pipelines

The rapid filling behaviour of a bypass pipeline is analysed numerically using an elastic model combined with the polytropic law for the air phase. These explorations show that the system can be readily filled when the entrapped air in the bypass line could be released into the atmospheric air, but that large pressure surges are predicted due to the collision of the filling fronts coming from the opposite ends of the bypass line. As expected, both the distance to the upstream reservoir and the size of the bypass line influence the magnitude of resulting overpressures. The magnitude of these predicted surges is naturally reduced by modelling the presence of the compressible air, the presence of which tends to cushion the impact of the filling fronts. A decrease in the volume of initially entrapped air increases the magnitude of the pressure surge and decreases the period of oscillation. In general, entrapped air is beneficial during rapid filling; however, such air is certainly problematic for conditions of normal flow and thus should be routinely removed using air valves.