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Water Hammer Wave Reflection and Valve Closure Time

http://www.fluidmechanics.co.uk/hydra... When the flow rate in a pipeline system is rapidly changed or stopped it will induce water hammer or a pressure surge wave into the pipework. This is the second video in the series on water hammer. In the first video the mechanism by which water hammer occurs and its magnitude was explained. In this video the propagation of the pressure wave and fluid movement in the pipeline is explained. When the pressure and fluid velocity waves reach the ends of the pipe they are reflected. These reflections can keep on bouncing off either end of the pipe for a number of cycles. Over time the magnitude of these pressure and velocity waves die down due to friction between the pipe and fluid and because the elastic deflections in the pipe wall are not fully reversible. If at one end of the pipe there is a flow source held at a constant pressure (for example by a header tank) and the other end is open to atmosphere. This will provide the initial flow conditions in the pipe. If the open end of the pipe is then suddenly closed this will induce a water hammer wave front which travels up the pipe. On one side of the front the pressure and flow are the same as the initial conditions, on the other side the flow will stop and surge or water hammer pressures are induced. The magnitude of this pressure rise is defined by the Joukowsky equation. When the wave reaches the other end of the pipe the flow will have stopped all the way along the pipe and the fluid will be at the elevated pressure determined by the Joukowsky equation. This situation is not stable and induces the first of 4 wave reflections before the process is repeated. The time it takes to repeat these 4 reflections determines the forcing frequency of the water hammer pressure waves. In the video the maximum closure time required to induce water hammer is discussed. It is shown that as long as the closure time is less than it takes for the pressure wave to travel two lengths of the pipe then the pipe closure can be classified as rapid and water hammer will occur.