On gas transport in downward slopes of sewerage mains

pressure mains are subject to gas pockets in declining sections. These gas pockets cause an additional head loss and an associated capacity reduction, which cannot be predicted with sufficient accuracy. The number of parameters, affecting downward gas transport in sewerage mains, is overwhelming. Therefore, the development of a numerical model is inevitable, starting from the knowledge base in the available literature. This paper includes a critical review of the literature on gas transport by flowing water in declining pipes. This review will show that the wide spread in the correlations for the clearing velocity, as reported by various investigators, is mainly caused by a subtle misinterpretation of the original data. Furthermore, new information from a few old references will be presented and integrated into a synthesis of the available literature. Finally, a new dimensionless velocity parameter is introduced, which is shown to be a more appropriate scaling parameter than the existing velocity scaling.