Water hammer is a pressure surge or wave resulting when water in motion is forced to stop or change direction suddenly. Water hammer commonly occurs when pipe is suddenly closed at the outlet, the mass of water before the closure is still moving forward with some velocity, building up a high pressure and a pressure wave propagates in the pipe. This pressure wave can cause major problems, from noise and vibration to pipe collapse.

In the home, water hammer often occurs when a dishwasher, washing machine or toilet shuts off water flow, resulting in a loud bang or continuous banging sound. Air traps or stand pipes (open at the top) are sometimes added as dampers to water systems to provide a cushion to absorb the force of moving water in order to prevent damage to the system.

WIKIPEDIA ENTRY FOR WATER HAMMER.