Pressure characteristics and identification method of water hammer in single phase and two phases with entrapped air during pump startup

Since the entrapped air in pipes conveying fluid has an impact on water hammer characteristics, it is of great significance to study the similarities and differences of water hammer characteristics in single phase and two phases with entrapped air for presence identification and risk assessment of the entrapped air. In this research, an experimental system is set up for water hammer investigation during pump startup in pump-pipe system, and the corresponding numerical model is established based on the improved elastic water model and computational fluid dynamics method. The water hammer under different startup conditions is experimentally studied and numerically simulated to get the pressure oscillation characteristics in single phase and two phases with entrapped air. It is found that the peak pressure and main frequency of single-phase water hammer are significantly higher than that in two phases. Based on the negative correlation between air volume and main frequency, the prediction equation for air volume is proposed with a considerable accuracy verified by experiments. The identification method is presented to distinguish between single-phase and two-phase water hammer during pump startup. It can serve as the theoretical basis for the safety evaluation and operation reliability of pump-pipe system.