Effects of free outflow in rising mains with air chamber

Air chamber design is usually accomplished by the use of charts or numerical models based on the classic boundary condition of a constant-level downstream reservoir. However, a constant-level downstream reservoir typically does not exist in currently operating facilities. The downstream end of the rising main is usually designed with the outlet at a level above the maximums allowed in the receiving reservoir. Such a scheme is usually chosen in order to avoid the loss of stored water in case of leakage or breakdown of the pipeline. For this reason, the final section of the discharge pipeline is often fitted with a vertical pipe with free outflow. This particular downstream condition influences the transient state that occurs following a power failure in the pump. In this paper, we focus on the variations of maximum and minimum pressures and water-level oscillations that arise in the terminal section of the rising main, by means of complete transient analysis. The rigid water column model is used to provide design charts for preliminary design. A numerical investigation using dimensionless parameters is performed to investigate the system behavior within the significant ranges of system-characteristic quantities. Field experiments were performed on a pump-pipeline discharge system located in a water supply system in southern Italy. The recorded pressure traces are compared. with results obtained from a model that accounts for this particular downstream boundary condition.