What is water hammer?

Water hammer is a sudden pressure surge in a pipe that occurs when a moving column of water is abruptly stopped or set in motion. You hear it as a loud bang or knock in the pipework, for example when a tap shuts quickly or the washing machine stops filling. Water is almost incompressible, so its kinetic energy is converted within a fraction of a second into a pressure wave that travels back and forth along the pipe.

How big is the surge?

For near-instant closure the Joukowsky equation applies:

Δp = ρ × c × Δv

• ρ = density of water (about 1,000 kg/m³)
• c = speed of the pressure wave in the pipe
• Δv = change in water velocity in m/s

In metal pipes c is in the order of 1,000 m/s or more. If water flowing at 1 m/s is stopped almost instantly, the surge is about 1,000 × 1,000 × 1 = 1,000,000 Pa, roughly 10 bar, on top of the normal mains pressure. In plastic pipes c is lower and the surge smaller, but not negligible.

Causes

• Fast-closing taps such as single-lever mixers and push taps.
• Solenoid valves on washing machines, dishwashers and coffee machines.
• High supply pressure, which means faster flow.
• Pipes that are loose in their clips and slam against the structure with every surge.

Risks

Repeated water hammer stresses joints, taps, flexible hoses and appliances, and over time can cause leaks at compression fittings and unions.

Fixes

1. Fit a water hammer arrestor as close as possible to the appliance or tap causing the bang.
2. Clip pipes securely with enough brackets so they cannot move.
3. Have the pressure measured. If it is high, a pressure reducing valve after the meter can help.
4. Close taps more gently where they are operated by hand.

Note: gentle ticking from hot water pipes is usually thermal expansion, not water hammer.

Related terms

• Water hammer arrestor
• Compression fitting
• Union nut
• Pressure pipe
• Expansion vessel

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