Definition - What does Water Hammer mean?
A water hammer is a surge in pressure that occurs in a pipeline or drainage system when the valve controlling it is shut off. A water hammer is not limited to water, and can be used to describe the surge in pressure of any liquid contained in a pipeline.
The surge in pressure caused by water hammer can have several negative effects. If the pressure caused by the water hammer is greater than the yield limit of the pipeline material then a blow out or pipeline rupture can occur. Another consequence of water hammer is the sound of the pressure increase, which can be particularly annoying in a home or office environment.
Corrosionpedia explains Water Hammer
Water hammer can be controlled using several methods. One way to prevent water hammer is to decrease the liquid's velocity. If the same volume of liquid needs to be transported per unit time then a larger diameter pipe can be used to allow for a slower velocity. The rate at which the valve is shut off also plays a role in inducing water hammer. Slowly closing a valve will help eliminate the negative effects of water hammer.