If the valve is suddenly closed (closing time tcl = 0) with stationary outflow at velocity w1 and if the pipe run is thus completely shut off, the fluid column is abruptly decelerated from w1 to w = 0. This produces a pressure wave which passes through the medium against the flow direction at the speed of sound "a". The pressure wave is reflected at the tank outlet (see Figure), moves back to the valve inlet and is reflected there again, etc. Superimposition of the pressure waves against the flow direction and the reflected pressure waves results in pressure piling, the maximum value of which can be calculated in accordance with the following equation:
This pressure piling in accordance with equation 10 is referred to as pressure surge or water hammer. It can be many times the operating pressure and can thus lead to bothersome noises and, above all, increased pressure stressing of the system.