Transient conditions in the transition from gravity to surcharged sewer flow

Abstract

One of the least understood aspects of flow in sewers is the nature of the transition from gravity to pressure or surcharged flow. A complete design of a storm sewer should consider both gravity and surcharged conditions. The available design and/or simulation models can handle gravity (open-channel) flow with various degrees of sophistication, whereas some consider surcharged flow. None of the available stormwater computer models include an adequate treatment of the transient pressures associated with surges that can occur at the transition from gravity to pressure flow. During the transition period there is a further complication because there is a mixture of air and water in the pipe.This paper deals with transients that occur when gravity flow is suddenly changed to pressure flow by the occurrence of a surge in the line. The pressure head fluctuations associated with this transient have been studied. Some of the factors affecting the pressure transients are: pipe size, pipe shape, flow velocity, Froude number, relative depth of flow, alignment of the pipe, pipe material, venting arrangements, and boundary conditions such as pumps, interceptors, and drop pipes. The paper also suggests a theory to predict the excess pressure rise due to these transients. Keywords: Fluid transients, gravity flow, instability, pipe flow, sewers, surcharged flow, surges, two-phase flow.