Experimental study of air-pocket-induced transient pressures in pipelines

Entrapped air pockets can induce severe transient pressures in pressurized pipelines, yet most previous studies have focused on atmospheric or weakly pressurized air in relatively simple pipe configurations. As a result, the transient motion of air pockets and the associated pressure-oscillation mechanisms in more complex systems remain insufficiently understood. In this study, the dynamic behaviour of pressurized air pockets in a gravity-driven DN100 pipeline system composed of galvanized steel pipes and Perspex observation sections was experimentally investigated. Under normal operating conditions, corresponding to a fixed terminal-valve opening, a control-variable approach was used to analyse the effects of flow rate (25–40 m3 h−1) and air-pocket volume (0.055–0.219 m3, corresponding to 5–20 s of aeration) on pressure peaks. The results showed that flow rate and air-pocket volume were the key controlling factors affecting the pressure response, and that both exhibited an upward-convex parabolic relationship with the maximum pressure, with a peak value of 18.52 mH2O at a flow rate of 35 m3 h−1 and an aeration time of 10 s. Under terminal valve-closure conditions, corresponding to a valve-induced transient, the air-entrapped condition produced a markedly higher transient pressure peak than the full-water condition. The present results suggest that, under both normal operating conditions and terminal valve-closure conditions, the presence of entrapped air pockets may have adverse implications for the safe and stable operation of the pipeline. These findings provide an important reference for risk assessment in pressurized pipelines.