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 a) the non-return valve in the downpipe extending down from the pump is installed more than 9 m above the water level, or b) 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.