Numerical and experimental investigation of air hammer and associated hydraulic transients in sewer pipes

Municipal sewerage systems are normally designed to operate without surcharging. However, under some conditions, such a heavy storm or pump failure, the flow in sewers may experience a transient from free surface flow to full pipe flow or vice versa. The fluid transients induced pressure rise could cause damage to the system. The trapped air pressure was also found to be sufficient to pop off manhole covers. In this study, a mathematical model that solves the continuity, momentum and energy equations is developed to simulate the hydraulic transients in sewers. The method of characteristics is used for numerical computation. The determination of surge velocity and pressure change under the conditions of pressurization and depressurization is carried out to examine the phenomena of hydraulic transients. A 1:6 scaled physical model consisting of a mainline, a drop structure, and a manhole structure was constructed to quantify the pressure surge and to identify the occurrence of the air-hammer induced manhole cover pop-off. Pressure transducers were placed along the pipeline and on the manhole cover for pressure head measurements. The LabView data acquisition system was set up to record the time series data. The numerically predicted pressure rises or pressure drops under various surge conditions are found to agree reasonably well with those recorded from physical model tests.;To explore ways to mitigate the air hammer phenomenon, model tests by considering the effects of valve closure time and air releasing from vent holes on the manhole cover is constructed. The study indicates that an acceptable solution for preventing the manhole cover from popping off is to place a ventilation system in a manhole near the downstream end to effectively reduce the air pressure for the whole pipeline, even though other manholes remain completed closed.