I am trying to understand why the pressure fluctuations during a hydraulic hammer phenomenon would go below the line pressure (Pmin < Pi) I thought there will just be a rise in pressure and then the pressure would came back to the settling pressure but thats not the case as the pressure seems to go well below the settling pressure too (which could cause cavitation if the pressure drops below vapor pressure). I know there are multiple peaks because of the pressure wake being reflected back and fourth in the hydraulic line (and that the pressure at any point in the pipe at any given time is a superposition of all the wave, incident and reflected, at the point). I am not able to understand how the pressure can go below the line pressure as surge is suppose to be a compression wave. The Pmin suggest that a expansion wave is travelling behind the compression wave. Any material to understand this better would also be helpful.