Water hammer is a pressure surge or wave caused when a fluid in motion is forced to stop or change direction suddenly.  Water hammer commonly occurs when a valve closes suddenly at the end of a pipeline system, and a pressure wave propagates in the pipe. It’s also called hydraulic shock.  Water hammer can cause piping to shake, bang, make noise and in extreme cases damage valves within the piping system.  A water hammer arrestor helps to stop water hammer by helping absorb the wave of water or pressure surge in the piping system.