Impact of air valves on air–water dynamics during pump startup and shutdown

This study aims to evaluate the effectiveness of air valves in mitigating pressure surges induced by trapped air pockets during rapid filling operations. A locally transparent pipeline with a high point is constructed to directly observe air–water interactions under varying air volumes and three inlet velocities. Experiments reveal that entrapped air can trigger the most severe pressure surges with volumes below 8 l (1.1 % of pipeline volume) in the absence of an air valve. With air valve installed, the pressure response can be distinctly divided into two characteristic processes: the expulsion process and the compression-leading process, which transition point is objectively identified by a sudden change in the pressure derivative. Key parameters such as water front velocity and air exhaust rate are quantified, and the influence of valve installation position on the pressure history and peak values is systematically analyzed. These findings provide experimental evidence and theoretical guidance for the optimal design and safe operation of air valves in water conveyance systems.