WOULD YOU LIKE TO UNDERSTAND HOW CAVITATION WORKS AND HOW IT CAN AFFECT THE OPERATION OF A WATER PUMP?

¿Have you ever wondered how a water pump works? Believe it or not, this handy device  keeps inside some interesting stuff that might surprise you and that I bet you’ll find  worth knowing. Now, don’t you shrink off from reading if I use some technical wording: I promise I’ll keep it to the very minimum. Courage and go on reading.

To make it very simple, a water pump works transferring liquid from one place to another and, most important of all, in a very fast, time-saving way.

The process starts when water is suctioned through a tube into the main area of the pump. Here, a motor keeps the fluid turning around, pushed by blades. As the rotation is very fast, it creates pressure over the water which is expelled with force, whereas new fluid enters the pump, so that it is never empty.

Quite a few times, cavitation comes up… to the dismay of those operating the pump. But let’s start from the beginning.

Cavitation ¿What’s that?  

To put it simply, this occurs when the water moving inside the pump starts making little vapour bubbles as if it were boiling –I’ll explain why in a moment-, only that in this case we are talking about room temperature. Hang on a moment, didn’t we learn at school that water must reach 212 ºF (100ºC) before turning into vapour, at sea-level pressure?

Precisely. Pressure plays an important role in the whole process. If water pressure drops below a certain level –for instance because something obstructs the suction mechanism, making the water intake low and creating vacuum areas – the temperature needed to transform water into vapour will drop too. Even to the point of room temperature. And this is more frequent than you might expect.

OK; <I know it sounds kind of weird all that stuff about water turning into vapour at room temperature; but why should it be so terrible?

Well, the problem is what happens next. Those little vapour bubbles are short-lived as such. They collapse as fast as they are created, going back to their former liquid state, only that the energy released generates a tiny explosion that works very much like a hammer chipping tiny bits of metal off.  Actually, a hammering-like sound can be clearly heard coming from inside the pump.

¿How does cavitation make trouble?

It is obvious from all what we said above, that if the inner parts of a pump undergo a continuous hammering, they will lose their functionality, water will not move as intended and eventually the pump will be so damaged that it will need replacing.

Now, there is a difference in the amount of damage, depending on how cavitation takes place.  If it is due to an insufficient suction head, vapour locks will appear inside the pump but if, on the other hand, water evacuation is not fast enough, the excess of water will move in the wrong places.

Devastation is deeper in the first case but both can be clearly visible when the pump is taken to pieces. It goes without saying: when this happens, the pump is beyond repair.

Well, then ¿can cavitation be prevented?

Whatever the cause of cavitation, engineers can use some specific calculations to work out the exact amount of liquid and its corresponding pressure the pump should be dealing with at each moment. They are used both during the design of the device and to monitor its operation so as to prevent the risk of falling into cavitation, by introducing the corrections needed.

Now, I know you are eager to know what formulae to use, but I’m just going to mention them, this time.

NPSH (Net Positive Suction Head) is the difference in pressure between the entry point and that with the lowest pressure level inside the pump. If this value falls below the evaporation pressure level, the pump will go into cavitation.

NPSHR (Net Positive Suction Head Required) is the lowest pressure level a pump should have at the liquid-entry point to avoid cavitation.

NPSHR (Net Positive Suction Head Available) shows how close the fluid inside the pump is to cavitation.

Centrifugal pump manufacturers must provide their customers with the recommended data information for each case.

If you made it up to here, congratulations! Now you know something that surely will be useful to you some time in the future.

Now, on a more serious note, once more, DIKOIN INGENIERÍA has developed its own equipment to empower students who can observe and learn by doing; so much so, that principles become alive through unforgetable  experiments.

 

MH 05.1 Eng VISUALIZATION NPSH EQUIPMENT to download DataSheet,

 

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