Pump killers: How to fight the 13 most common centrifugal pump failures? Number 4.

In the series “Pump killers” we investigate the 13 most common causes of centrifugal-pump failure and take a look at the measures that can be taken to fight these ‘killers’. Every week we will share a new, even bigger, killer. Our goal: to reduce the number of victims by sharing our knowledge.

This week's pump killer really needs no introduction. As "the mother of all pump failures", at least: people in the field sometimes want you to believe, she is blamed for everything. This is because often only "something" needs to be done with a pump or this "buzzword" is thrown. There will be people who might have expected this pump killer to be number one, but this would be wrong. This week, we're going to try to put an end to all the mystery surrounding this pump killer for good. We are of course talking about this: cavitation, or the lack of an NPSH calculation.

Photo: "Capacity versus NPSHr relationship graph" - Credit Oveducon.

For our story about cavitation we have to start with the chart books. We have already added a number of articles to this and this 'killer' also finds its roots here. Many pump users will be aware that there are chart books available that can be used, among other things, to select a pump. Depending on the brand, up to four graphs per pump may be available on one page. These graphs indicate the following relationships:

- Capacity versus head;

- Capacity versus efficiency;

- Capacity versus absorbed power;

- Capacity versus NPSHr.

In practice it appears that the latter in particular is an unknown graph for many people. A pity, because this graph is necessary to calculate the system and determine whether a pump is going to cavitate or not. But what about this then?

Photo: "Cavitation damage to impeller" - Credit Oveducon.

How does this phenomenon arise? A centrifugal pump can only function properly if the pump is able to draw the medium in easily. Do the test yourself with a glass and a straw:

Fill a glass full of water and suck up this water with the straw in your mouth and nose closed. Now repeat the same experiment again and then, instead of water, put pouring syrup in the glass and try to suck it up in the same way. Which option required more effort?

I often use this example to illustrate it, as it makes it very clear why the suction conditions of a pump are so important. A pump can be so good, but give this pump an incorrectly dimensioned suction pipe, so that the pump has to pull hard on its medium, and the pump will not meet the expected capacity and head.

Photo: "Crater profile caused by cavitation" - Credit Oveducon.

What are the consequences? If the pump has to pull too hard on the medium, bubbles form in the liquid (underpressure), which implode again in the pump housing (overpressure). We call this phenomenon cavitation. Cavitation is accompanied by very large shock waves capable of removing a piece of material from the fan, provided the bubbles implode against the surface of the fan.

Photo: "Cast iron impeller with cavitation damage (preserved with clear coat)" - Credit Oveducon.

Especially with cast iron this is clearly visible from the crater profile which is caused by this. It must be said here: the smoother the surface of the impeller, the less quickly a cavitation bubble will be able to get a grip on the surface. All in all, the damage that cavitation causes is great and so are the costs for the repairs:

- Damage to Impellers;

- Impellers that fall apart;

- Impeller blades that break out of the impeller;

Photo: "Pump shaft broken by cavitation" - Credit Oveducon.

- Pump shafts that break;

- Bearings that fail quickly.

In short, cavitation damage shows up in the weakest part of the pump at that time. Because, here too the saying applies: a chain is only as strong as its weakest link!

Photo: "Fallen stainless steel impeller" - Credit Oveducon.

So if you think you are smart and you choose a stainless steel impeller, which has a higher material resistance to cavitation, then the damage will still occur elsewhere in your pump. That way you do not solve the problem.

How can this be controlled? This pump killer can also be avoided when calculating the system and selecting the pump. So remember the NPSH calculation and don't forget to give the suction pipe the correct dimensions, because this can prevent cavitation.

Photo: "Manometer" - Credit Oveducon.

In addition, it is possible to place a vacuum pressure gauge in the suction line, so that any change in the pre-pressure of the pump is immediately visible. There are also "special" vacuum gauges where the system can even be alarmed or switched off if the pump is at risk due to low NPSH.

Cavitation: a "magic word". In this article, we've tried to unravel a little bit of the mystery surrounding this concept, but let's be honest, this doesn't mean we've said all about it. There are several forms of cavitation, but we will come back to this in a later article. We are now again curious about everyone's experiences with this 'killer'!

Next week we will be back in the top 3.

Until then!