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Water hammer

Water hammer is a phenomenon that can occur in any piping system where valves are used to control the flow of liquids or steam. Water hammer is the result of a pressure surge, or high-pressure shock wave that propagates through a piping system when a fluid in motion is forced to change direction or stop abruptly. This shockwave is also commonly referred to as a hydraulic shock or hydraulic surge, and may be characterized by a marked banging or knocking sound on the pipes immediately after shutoff.


Water hammer can occur when an open valve suddenly closes, causing the water to slam into it, or when a pump suddenly shuts down and the flow reverses direction back to the pump. Since water is incompressible, the impact of the water results in a shock wave that propagates at the speed of sound between the valve and the next elbow in the piping system or within the column of water after the pump.

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