Understanding friction loss and cavitation

Whenever you pump water through pipes at a high rate, there is a risk of friction loss and cavitation.

In this module, we’re going to introduce you to these phenomena. We’ll explain how friction loss can cause a decrease in the efficiency, how cavitation can cause damage to the pipework and pumps and not least how you can reduce the risk of both.

Let’s start with friction loss. In simple terms, friction loss refers to the pressure lost by liquid as a result of contact between the moving liquid and the enclosure in which it is moving.

When you transport water in a water pumping system, it creates friction between the surfaces that the water touches. Ultimately, this leads to a loss of energy and pressure, decreasing the efficiency throughout your system.

No part of your system is exempted from friction loss – it occurs in the pipes, the elbows and the valves.

But how do you know the level of friction loss in your system? Well, that depends on a variety of parameters including:

1. The flow in the system
2. The viscosity of the fluid

The pipe’s diameter and length – the smaller and shorter the pipe, the greater the viscosity and thus the greater the friction loss.

And finally, the surface of the pipes. If, for instance the walls of the pipes are smooth, it is easier to pump water through them, reducing the level of friction loss.

So, the level of friction loss can be determined by the speed at which the water is pumped, and the pipe size.

In order to reduce friction losses, you can do two things:

1. reduce the flow and…
2. increase the size of the pipe.

While increasing the pipe size also increases the initial system costs, it will, on the other hand, reduce your total life cycle costs, making it a sensible long-term solution.

Typically, you can obtain friction loss data from the manufacturers of pipes, elbows, t-joints and any other components.

Now, let’s turn our attention to cavitation.

Cavitation is defined as the rapid formation and collapse of air bubbles in the water, caused by boiling water.

Normally, water boils at 100 degrees. At this temperature, water turns into vapour.

But if the system pressure drops, so too does the boiling point.

For instance, if the pressure in an area drops to 0.1 bar, the water in this area will start to boil at 45 degrees instead of 100 degrees.

And as soon as the pressure in these areas rises beyond the local boiling points again, the vapourised molecules implode and return to their liquid forms. This is called cavitation.

You’ll typically be able to hear it as the implosions create a loud noise within the pipes.

Cavitation is one of the most common issues within any pipework in which water is being pumped, and it is one of the main sinners when it comes to pump and pipework damage.

Fortunately, there are a few ways to reduce the risk of cavitation. Just make sure to do the following:

1. Lower the pump inlet and increase the inlet pressure
2. Reduce the friction loss in the suction pipe
3. Reduce the pump’s flow
4. Increase the elevation of the suction water level

That covers our training module on friction loss and cavitation.

Thanks for watching.