Water hammer is what happens when moving liquid is stopped abruptly. The whole column's momentum piles up behind the closed valve, firing a pressure wave back along the pipe that can rattle and even burst the walls.
It is the bang and shock you get when a flowing liquid is stopped suddenly. The moving column's momentum becomes a pressure surge, and the pipe rings like a struck bell.
From outside you only hear the bang and see a gauge jump. A cutaway shows the moving column, the abrupt stop, and the pressure wave travelling back through the liquid, which is what actually hammers the walls.
With the valve open, the whole column of liquid moves along the pipe at speed, carrying momentum.
A fast-closing valve stops the flow at the downstream end almost instantly. There is no time for the liquid behind to slow down gradually.
The liquid still moving suddenly runs into the wall of the shut valve. Its momentum squeezes it, so the pressure right behind the valve spikes.
The high pressure does not stay put: it travels back up the pipe as a wave, the way a compression travels along a spring. This wave is what makes the bang.
At the reservoir the wave reflects and races back, so the pressure at the valve oscillates high and low until it damps out. That repeated hammering can crack joints and burst the pipe.
Suddenly stopping a moving liquid. When a valve closes quickly, the momentum of the whole column has nowhere to go, and the pressure behind the valve jumps, sending a pressure wave along the pipe.
Because the pressure wave strikes the pipe wall like a hammer blow, producing a loud bang and a physical jolt that can be felt through the pipework.
By closing valves slowly, fitting a surge chamber or accumulator to absorb the energy, using a slower-closing valve, and keeping flow speeds moderate. An air chamber gives the surge somewhere to go.
Yes. The pressure spike can be many times the working pressure, enough to crack joints, split pipes and damage fittings, especially in long lines with fast-closing valves.
The pressure wave travels up the pipe, reflects at the reservoir and returns, so it bounces back and forth. Each pass fades a little until friction damps it out.