Question: What does water hammering mean, and what causes it?

Water hammering is a pressure surge caused by a rapid change in flow velocity in the pipeline. This phenomenon is referred to as water hammering  because the pressure surges are frequently accompanied by a noise that sounds as if the pipeline were being pounded with a hammer.

In well installations, water hammering generally occurs if

• the non-return valve in the downpipe extending down from the pump is installed more than 9 meters (30') above the water level, or 
•  the non-return valve installed in the downpipe extending down from the pump is leaky, while a non-return valve fitted above that level stays tight. In both cases, a partial vacuum is created in the downpipe.

The next time the pump is started up, water fills the vacuum at high flow velocity and presses against the closed non-return valve and the stationary water column in the pipeline, thereby causing a pressure surge or water hammering. This pressure surge may cause pipelines to burst and pipe joints to break, and may damage the pump and/or the motor. If pressure surges do occur, the system should be switched off, and the pump installer should be brought in to resolve the problem. 

Submersible well pumps water hammering

For submersible well pumps, water hammering occurs when the water flow velocity is suddenly stopped, causing a pressure surge to bounce back and forth; hitting the stops so hard it sounds like hitting the pipe with a hammer. 

Example: The submersible pump is pumping and shuts off. Water flows back down the pipe towards the submersible pump. The non-return valve (check valve) imideatly slams shut, stopping the backward flow of water. The backward flow hits the closed non-return valve, and the pressure surge bounces back up the pipe and hits the closed faucet (valve, system). Then bounces back to the closed non-return valve. Each back and forth is diminished, so only hammers once or a few times quickly.