The dangers of water hammer

By "Apollo" Valves
March 26, 2018

The first sign that you've got a water hammer problem is usually a sudden banging in the pipes. Then you might notice a pulsating flow and see the pipes shaking. It only lasts a few seconds, but it can do a lot of damage, especially if allowed to happen over and over. Here we'll explain what causes water hammer – which can occur anywhere from your washing machine to your kitchen sink – and what you can do about it.

“Solid” fluids

Strike a metal bar with a hammer and it rings as the shock wave passes through it. Something very similar happens with a liquid. There's no room between the molecules for it to compress, so if water in a pipe is pushed at one end, that force is transmitted through to the other end. If it didn't behave in this incompressible manner, hydraulic power wouldn't exist.

Shock waves and their causes

Water hammer isn't caused by something striking the water, but by the water striking something. Like the ringing of that metal bar, water hammer is a shock wave passing through the fluid.

These shock waves are caused by changes in flow velocity. The simplest example is a check valve suddenly swinging shut. Water that was moving through at several inches a second is stopped almost instantly. The result is a pressure wave that travels back up the pipe in the opposite direction to the flow. Bends in the piping bear the brunt of this shock wave, changing the wave into vibration.

Any type of fast-acting valve can cause this effect, but non-return valves are an especially common cause because, by design, they operate quickly. Pressure valves and any quarter-turn valves also can be responsible.

Sudden pump starts or stops also can create water hammer when the fluid velocity changes rapidly. A particular problem is when a pipe runs vertically straight from a pump. And don't forget that any liquid, not just water, can create water hammer.


The banging noise is inconvenient, but really it's just a symptom of a bigger problem: a sudden change in pressure. That makes pipes move, which can damage supports and cause joints to leak. The shock wave itself can damage pumps and in extreme cases may even burst pipes.


Water hammer is a complex phenomenon that’s hard to predict, even with sophisticated computer programs. The key to preventing it, or at least minimizing its impact, is to slow the rate at which flow velocity changes. That means eliminating non-return valves as far as possible and installing multi-turn rather than quarter-turn valves.

Larger diameter pipe helps, as this reduces velocity while maintaining the same flow rate. Another approach is to install surge tanks, air chambers, or water arrestors. These act as dampers by transferring some of the energy of the shock wave to air. If high pressure is at the root, a water pressure regulator may help.

Act quickly

Water hammer is more than just irritating. It damages plumbing systems and can lead to leaks and floods. If observed, take action promptly to minimize negative consequences.

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