Protection against water hammer

What is water hammer?

Water hammer is a rapid rise in pressure that occurs when there is a sudden change in fluid velocity after the sudden opening/closing of or a sudden pump startup/stoppage. This can cause major problems, like backflows, an increase in flow rate and banging water pipes.

This pressure surge can be significant. It often translates into a characteristic noise and can lead to burst pipes in large facilities because of the amount of fluid in motion. This problem can be resolved by installing a water hammer arrestor or a shock absorber. The water hammer arrestor is used to avoid a burst tank and pipes, vibrations and noise.

The force of the water hammer can be compared to a true explosion. The shock wave falls into the pipeline until it hits a curve (a pipe elbow or a larger pipe, for example) and flows back instantly to the initial point of impact.

Average shock-wave speed of water hammer: 1,300 metres/second (4,265 ft/s)

It is possible to create a device to pump fluid to a certain height without any other energy than the speed of the fluid itself by using the water hammer phenomenon. This is called hydraulic energy.

When a pipe is suddenly closed, the mass of liquid before the closure is still moving at a certain speed, generating a pressure surge, as well as a shock wave. In common plumbing, this is experienced by a loud noise, resembling a hammering sound. Water hammer can cause pipes to burst if the pressure reached becomes too high. Air chambers can be added to the piping system to absorb the shock wave and to protect the system.

At home, water hammer may occur when a washing machine or a dishwasher shuts off the water flow. This usually results in a loud “bang”.

Other causes of water hammer may be pump failure or a check valve slamming shut.

Finally, it is necessary to have several water hammer arrestors to prevent all sudden backflow problems. Every arrestor is designed to absorb shock waves that may be generated by a certain number of appliances and, by extension, of the entire piping system.


Preventive measures:

Water hammer can cause accidents, but usually the damage is limited to ruptured pipes or equipment connected to the piping system. The pipelines carrying hazardous fluids require careful design, construction and operation.

The following measures may reduce or eliminate water hammer:

  • Reduce the pressure of the water supply by fitting a pressure regulator;
  • Reduce fluid velocity in the pipes. To significantly reduce the shockwave, some sizing guides recommend a flow velocity equivalent or inferior to 1.5 m/second;
  • Install slow-closure faucets;
  • Use start-up and shut-down procedures on an existing installation
  • Install a mechanical device to prevent water shock, also called a shock-absorber or a water hammer arrestor;
  • Install an air chamber;
  • Reduce the length of straight pipes by adding elbows or expansion loops. Elbows reduce the influence of pressure waves;
  • Use plumbing elements designed to reduce pressure (expensive solution);
  • Install a flywheel on the pump;
  • Install a pumping station bypass;
  • Use an automatic pump valve.


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