Water Hammer and Arrestors
A plain-English explanation of water hammer — the pressure shock created when fast-moving water is stopped suddenly — and the arrestors and design measures that absorb it before it damages pipes and fittings.
Water hammer (more correctly, hydraulic surge) is the loud banging or knocking you sometimes hear in pipes when a tap or appliance valve shuts quickly. It happens because water has mass and momentum: when a fast-flowing column of water is stopped almost instantly, its energy of motion cannot simply vanish and is converted into a high-pressure shock wave that travels back along the pipe.
The surge is brief but violent. It can spike the pressure in a pipe to several times the normal working value, loosen joints, crack fittings, damage valve seats and water heaters, and shorten the life of the whole installation. In the UAE, where buildings rely on pressurised booster systems and many quick-closing solenoid valves in washing machines, dishwashers and flush valves, controlling water hammer is a routine part of good plumbing design.
How it works
The cause — momentum stopped suddenly. Water flowing in a pipe carries kinetic energy proportional to its mass and the square of its velocity. When a valve closes faster than the water can decelerate, the moving column slams into the closed valve. Because water is almost incompressible, the energy is released as a pressure wave rather than absorbed by compression.
The pressure wave — back and forth. The shock wave races back up the pipe at the speed of sound in water (roughly 1,200–1,400 m/s in steel pipe), reflects off a tank or a change in section, and travels back again. The pipe walls flex with each pass, producing the characteristic hammering noise and repeated stress until friction finally damps the oscillation out.
Why velocity matters most. The size of the surge depends mainly on the flow velocity and how quickly the valve closes. Doubling the velocity roughly doubles the surge, which is why keeping pipe design velocities sensible (commonly around 1.5–2.4 m/s for water) is the first line of defence. Long straight runs and rigid materials make the effect worse because there is more momentum to stop and less give in the pipe.
The cure — give the energy somewhere to go. The classic solution is to provide a cushion of air or gas that can compress when the surge arrives, absorbing the energy harmlessly. A purpose-made water hammer arrestor contains a sealed gas charge behind a piston or bellows; an older air chamber is simply a capped vertical pipe stub holding trapped air. Slow-closing valves, correct pipe sizing and securely clipped pipework complete the strategy.
Placement and maintenance. Arrestors work best installed close to the quick-closing valve causing the surge — at washing-machine and dishwasher connections, solenoid-valve groups and the ends of long branches. Modern sealed arrestors are maintenance-free, whereas plain air chambers eventually waterlog as the air dissolves into the water and must be drained to recharge them.
Main types
In the UAE
- UAE buildings commonly use booster pump sets and pressurised distribution, so high velocities and quick-closing solenoid valves make water hammer a frequent concern that design must address.
- Plumbing works follow the local authority and water-utility design rules (for example ADDC/Al Ain and Abu Dhabi requirements), with sound engineering practice on velocity limits and surge control expected for approval.
- High ambient temperatures and the use of thermoplastic pipes (PPR, PE) change how surge behaves versus metal pipe, so velocity control and correctly rated arrestors are specified to protect joints over the long term.
How GPR applies this
GPR designs plumbing systems with sensible pipe velocities and installs sealed water hammer arrestors at washing machines, dishwashers, flush-valve groups and the ends of long branches across Abu Dhabi projects. We coordinate arrestor selection with the booster set and any pressure-reducing valves, restrain pipework correctly, and test the installation so occupants get quiet, durable plumbing free of banging and premature fitting failure.
Frequently asked questions
What actually causes water hammer?
A fast-moving column of water is stopped almost instantly by a quick-closing valve; its momentum converts into a high-pressure shock wave that travels back along the pipe.
Is water hammer just annoying, or does it cause damage?
Both. Besides the banging noise, the repeated pressure spikes can crack fittings, loosen joints, damage valve seats and water heaters, and shorten the life of the system.
What is a water hammer arrestor?
A small sealed device with a gas or air cushion behind a piston or diaphragm; it compresses when the surge arrives and absorbs the energy harmlessly.
Where should arrestors be installed?
As close as possible to the quick-closing valve causing the surge — typically at washing-machine and dishwasher connections, solenoid valves and the ends of long branch pipes.
Why do old air chambers stop working?
A plain capped air chamber slowly fills with water as the trapped air dissolves into it; once waterlogged it gives no cushion and must be drained to recharge, which is why sealed arrestors are preferred.