What is Water Hammer?

Water hammer is a pressure surge or wave caused when a fluid in motion, usually a liquid but sometimes also a gas, is forced to stop or change direction suddenly; a momentum change. This phenomenon commonly occurs when a valve closes suddenly at an end of a pipeline system, and a pressure wave propagates in the pipe.

What is Water Hammer?

Cause and effect

Water flowing through a pipe has momentum. If the moving water is suddenly stopped – such as by closing a valve downstream of the flowing water, the pressure can rise suddenly with a resulting shock wave. This shock wave is experienced in domestic plumbing as a loud banging resembling a hammering noise. Water hammer can cause pipelines to break if the pressure is sufficiently high. Air traps or stand pipes (open at the top) are sometimes added as dampers to water systems to absorb the potentially damaging forces caused by the moving water.


There are a number of ways to prevent water hammer, including:

  • Using a water hammer arrestor, which is a device that absorbs the shock waves that can be created when a valve is suddenly closed.
  • Installing a surge tank, which is a large tank of water that helps to absorb the shock waves.
  • Designing the plumbing system to minimize the effects of water hammer.
  • Avoiding sudden changes in water flow.

The Physics…

The physics of water hammer can be explained by the following principles:

  • Momentum: Momentum is the product of mass and velocity. When water is flowing through a pipe, it has momentum.
  • Inertia: Inertia is the tendency of an object to resist a change in its motion. When a valve is suddenly closed, the water in the pipe has inertia and continues to move forward.
  • Elasticity: The walls of a pipe are elastic and can stretch slightly. When the water in the pipe hits the closed valve, it causes the pipe walls to stretch.
  • Reflection: When a wave hits a barrier, it is reflected back in the opposite direction. When the water in the pipe hits the closed valve, it is reflected back towards the source of the water.

The combination of these principles leads to the formation of a pressure wave in the pipe. The pressure wave travels back and forth between the closed valve and the source of the water. The higher the velocity of the water, the higher the pressure wave will be.


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