H2Doh! When Plumbing Goes Wrong: Featuring Water Hammer

Wayne Manton of Hawkins’ Bristol Office recently gave a webinar for the Chartered Institute of Loss Adjusters (CILA) on escapes of water. His presentation included what the common causes of plumbing failures are and why water hammer, in particular, can be so destructive.

The Association of British Insurers calculated that in 2018, escapes of water made up 31% of property damage claims, so we at Hawkins already know that water damage can be pretty common. But what are some of the most common causes behind an escape of water?

  • Burst pipes
  • Building defects/waterproofing failures
  • Basement drainage issues
  • Weather-related flooding
  • Plumbing Issues

Burst Pipe

The following specific causes (not an exhaustive list) of escapes of water are associated with plumbing issues:

  • Poorly installed joints
  • Manufacturing defects
  • Incorrect system design
  • In-service degradation of components
  • Abnormally high pressure

The final cause (abnormally high pressure) can sometimes be caused by hydraulic transients, or more vividly: Water Hammer. Since water is incompressible, its volume does not change no matter how much pressure you apply. This means that Water Hammer is a short-term, high-pressure event that occurs when water moving through pipework is abruptly stopped. You may even hear the effect of water ‘hammering’ into a closed valve if you quickly turn your tap’s running water off. It is essentially a shockwave travelling through the plumbing system created by a sudden change in the water’s momentum. And because water is actually quite dense, with 1m3 = 1000 litre = 1 tonne, there is no way to soften the blow.

An example of water hammer in action, taken from Practical Engineering’s YouTube video: https://www.youtube.com/watch?v=xoLmVFAFjn4

A fairly average-sized pipeline might be 1 metre in diameter and run for 100 kilometres, which means the mass of the water inside could be as much as 80 million kilograms! That is equivalent to about 10 freight trains. If you close a valve quickly, all the water slams into it, just like those freight trains slamming into a brick wall. The pressure spike that results can rupture the pipe or cause serious damage to other parts of the system. 

Water Hammer

An illustration of the weight of water against a valve, taken from Practical Engineering’s YouTube video: https://www.youtube.com/watch?v=xoLmVFAFjn4

Reducing the speed at which fluid moves through the pipe is one of the simplest ways to reduce the effect of water hammer. The profile of a water hammer pulse can actually be calculated with the Joukowsky Equation. Without having to do any calculations, however, it is easy to consider that when designing a pipeline, the flow rate might be fixed, but you can increase the diameter of your pipe to reduce the water’s velocity. Next, you can also increase the time over which the change in momentum occurs; one way of doing this is by adding flywheels to pumps so that they run down slowly rather than stopping suddenly, or that you simply close the valve slowly. The final parameter we can adjust is the speed of sound through the fluid, also known as the wave celerity (how quickly a pressure wave can propagate through the pipe). The addition of a shock arrestor will cushion the blow, reducing the pressure spike.

While the concept of water hammer is nothing new, Wayne notes that it is important to discuss its effect on systems that may have been shut down during the COVID-19 lockdown. Transient spikes can often be caused when equipment is restarted after a period of rest. Boosted systems, particularly those in commercial settings, are having their power restored as businesses are allowed to reopen. Once power is restored, control systems detect low pressure and the pumps start to re-pressure the system; this can cause pumps to accelerate to maximum speed and water to flow rapidly up before reaching and striking the end of the riser at the top of a building.

Wayne says the most practical advice for anyone dealing with an escape of water claim would be to photograph the source of the leak before a plumber arrives, retain the fitting or area of the pipework that has failed, and keep fractured surfaces apart rather than trying to reassemble separated components to avoid your own H2Doh

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