Water Hammer Created by Joshua Toebbe NOHS 2015. Water hammer What is water hammer? .

Fluid Mechanics Applied

Water Hammer

Created by Joshua Toebbe NOHS 2015

Water hammer

What is water hammer?

http://processengineering.theengineer.co.uk/home/pumps-and-valves/hammer-action/1018283.article

Not exactly… Created by Joshua Toebbe NOHS 2015

Water hammerWhen water is flowing it has kinetic energy. We know that energy has the ability to do work, and

work is the application of a force over a distance, so energy can be considered as a measure of force applying potential (per unit distance)

Take a look at the units below

The only difference between energy and force is the extra distance over witch the force is applied.


Water hammerSo as water is flowing it is carrying with it a

tremendous amount of energy, and thus the ability to apply a force over a distance.

When a valve or gate is closed, the water must suddenly stop moving.

According to Newton’s 1st Law, and object in motion will want to remain in motion. So the kinetic energy of the water must be dissipated some how.

This dissipation of energy is known as water hammer, from the characteristic hammer like noise it makes. (also known as fluid shock)Created by Joshua Toebbe NOHS 2015

Water hammer

Basically the flowing water pushes against the pipe or vessel and transfers its kinetic energy to the walls of the pipe as a force.

Since pipes are generally inflexible, the distance over which the force is applied is relatively small, so the force must be great to dissipate all the kinetic energy.


Water hammer demo

http://www.sciencefirst.com/Water-Hammer.html


Water hammerSo what about large dams?Lets consider a dam with a 1 square meter penstock

that is 15 meters long, containing water moving at an average of 15 m/s.

Using the equation for kinetic energy

We get a kinetic energy of KE=1688000 joules.If we have a larger dam with a larger penstock the

water can potentially carry a 100,000,000 joules of energy or more. Created by Joshua Toebbe NOHS 2015

Water hammer solutions

That much energy can easily break a valve, which means the dam has to be shut down for maintenance and the energy production goes off-line.

So how can this be prevented?(take a second to think about it, and write it down)


Water hammer solutions

Water hammer is the result of rapid dissipation of kinetic energy.

So this energy must be dissipated gradually, and not be applied to the valve itself.

But how can we dissipate that much kinetic energy?(take a second to think about it, and write it down)


Water hammer solutionsWell where is that energy coming from? In a hydraulic dam, the energy is coming

from the height of the falling water.So we can send it back were it came from.Surge drums


Water hammer solutionsWhen the valve closes the excess energy

pushes the water up a surge drum.As the water rises it converts the KE back

into PEThis prevent the force from being applied to

the valve closure.


Water hammer solutionsSo what about all the other cases of water

hammer, like the loud clunk that happens when the toilet or dishwasher shuts off?

How could water hammer be averted on a smaller scale?(take a second to think about it, and write it down)


Water hammer solutionsIts called a water hammer arrestor, and its

basically like a tiny surge drum.

When the valve shuts the energy carries water into the arrestor or “tiny surge drum”.

Why do you think that piston has to be there?Created by Joshua Toebbe NOHS 2015

Water hammer solutionsOkay so the water doesn’t push against the

valve.What happens to the water on the other side

that keeps flowing?

Cavitation!But we will look at that

tomorrow…


The EndDon’t forget to go with the flow… (I know, I

know…)

(and turn in your practice problems please)


Water Hammer Created by Joshua Toebbe NOHS 2015. Water hammer What is water hammer? .

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