What is Water Hammer?

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What is Water Hammer?

Water Hammer or Hydraulic shock is a pressure surge or wave caused when a liquid or gas in motion is forced to stop or change direction suddenly. This commonly occurs when a valve suddenly closes at the end of a pipeline system, this then causes a pressure wave to grow in the pipe.

This pressure wave can cause major problems, from noise and vibration to pipe collapse.

Damaged pipe

Water hammer in steam systems have resulted in serious and, in many cases, fatal injuries in steam plants and is most frequently caused when steam is introduced into cold pipework that has not been sufficiently drained.

The result of this can cause one of the following to occur;

1. Condensate driven by steam

When steam is admitted via an isolating valve into cold pipework containing water, the steam - which is travelling faster than the water - causes the water to form a plug which is accelerated along the pipework until it meets the next downstream closed valve or obstruction. The water hits the closed valve or obstruction like a hammer and rebounds back within the pipework into the vacuum created by the condensing steam.

2. Condensate moving into a vacuum

When steam is admitted into a cool space, or if it is in contact with water, it may condense rapidly and create a vacuum. If the steam has been trapped - e.g. against an isolating valve - condensate may be drawn into the vacuum at a speed high enough to deliver a hammer blow to the valve.

Controlling the speed in which an isolation valve is opened to introduce steam into a cold pipeline is important to help stop water hammer from occurring. If an isolating valve is opened too quickly, without allowing the cold pipework to warm through gradually, you have a higher risk of water hammer.

It is important that Isolation valves made from brittle materials, such as cast iron, be changed for a more ductile material. Failure from water hammer is highly likely to cause these valves to shatter becoming extremely dangerous, even fatal to anyone in the vicinity.

How to minimise the hazards of water hammer

Full training to boiler operators to avoid the risk of thermal shock and recognise the significance of loud banging noises from the systems.
Operators should be trained to operate manual by-pass drains correctly.
Ensure pipework has a suitable fall in the direction of the steam flow and that drainage points are situated at appropriate positions; checking that the system permits complete drainage when cold.
Reduce or eliminate points where condensate could collect - eg sagging lengths of pipework, vertical legs, changes of slope, dead ends, fittings in pipes etc. Where such features are unavoidable, fit suitable drainage.
Maintain steam traps in accordance with the manufacturer's instructions and test them regularly to ensure they are operating correctly.