Cavitation is one of the recurring issues with Dredging. It occurs both in CSD’s and TSHD’s and can cause quite some damage. In this week’s Knowledge Update we will discuss what it is, why it is an issue, how you can identify it and, most importantly, how to prevent it.
So what is cavitation?
Simply put cavitation is boiling water. When the water pressure reduces, water will boil at a lower and lower temperature. Maybe you heard of the fact that water will boil at a much lower temperature when you boil it on a high mountain. This is due to the lower pressure.
With cavitation in dredging the same thing is happening. Due to the vacuum created in front of a pump the boiling point of water is lowered, until at a certain vacuum the water will spontaneously start boiling. And boiling means the forming of bubbles of water vapour.
This also means it is not a gradual progress. The shift from no cavitation at all and a lot of it is very sudden. We can use that to our advantage.
Why is cavitation an issue?
There are two issues with cavitation. The first is a lower production and the second is damage to the pump.
A centrifugal pump only works when there is fluid inside of it. This is also the reason why a centrifugal pump needs to be filled with water before you can start it up. With cavitation there are water vapour bubbles, a gas, in the pump and because of that the pump doesn’t function as it should. You will see a big drop in pump pressure and mixture velocity. That means a lower production.
The damage of the pump is the result of what happens after the water vapour bubbles are formed. These bubbles are a result of very low pressures. When the pressure in the pump increases these bubbles will turn back to water. This goes very quickly, so these bubbles implode. These implosions cause very high stresses in the immediate surroundings. These forces can chip away pieces of the impeller, so one of the effects of cavitation is a very high wear rate of the impeller. The localized high forces also cause vibrations on the impeller and subsequently in the axis, bearings and gearbox. In the long run it can lead to metal fatigue, which in turn can lead to broken shafts and damaged bearings.
How to identify cavitation?
Luckily cavitation is easy to spot. A few indicators are a sudden drop in discharge pressure and a decreasing mixture velocity. This in combination with a high vacuum is an almost sure indicator of cavitation.
Cavitation is usually not a constant factor either, so you will see the discharge pressure fluctuate a lot.
When the pump is above water you can usually hear and feel it as well. When cavitating a pump produces a rustling noise. When you touch it you can feel the vibrations.
How to prevent or remedy cavitation?
Cavitation is easily remedied, which is why it’s such a shame it is such a recurring issue. The major cause for cavitation is a high vacuum. When you lower the vacuum, the cavitation will automatically disappear. The best way to reduce the vacuum is lowering the mixture velocity. That can be achieved by lowering the pump revolutions or, if that is not possibly, by increasing the discharge resistance by putting a restrictor at the end of the discharge pipeline. Also make sure the suction line has no unnecessary restrictions in it.
A slight decrease in vacuum with some simple measures will already remove the cavitation issue, prevent the damage and increase the production.
If you have any cavitation issues I would love to hear about them. Please leave a comment in below comment box.
To Long Term Great Results,