In all the years I've been working with pumps, I've never seen cavitation cause problems with bearings, unless the damage done to the wet end of the pump created other problem ie balance issue. My question is can cavitation ALONE damage bearings. Thanks for the input

I've never really thought about cavitation having a direct affect on the pump bearings. I've always been concerned about its destructive effect on the impeller. I don't see how it could directly affect the bearings except as you said by creating an imbalance from impeller damage. Interesting question though.

Let's say we measure 20 G's acceleration on the bearing housing in the range of cavitation. The question(s) would be, how much of that is being transmitted thru the impeller/shaft/bearing and how much is just the pump casing reacting to the implosions?

Now imagine a small diameter hammer head of the roller contact surface area hitting the bearing races in various locations many times at that G level. I dont know if this can be calculated into the L10 rating of a bearing, but I'm pretty sure it doesn't prolong bearing life . Not sure how much is shortens it the life tho.

Heinz Bloch says cavitation can damage bearings.

Process Plant Machinery (Second Edition)
by Heinz P. Bloch, Claire Soares 1998 ISBN: 0750670819

Appendix 7A under heating "Net Positive Suction Head (NPSH) and Cavitation" states:

quote:

The accompanying noise is the easiest way to recognize cavitation. Besides impeller damage, cavitation normally results in reduced capacity due to the vapor present in the pump. Also, the head may be reduced and unstable and the power consumption may be erratic. Vibration and mechanical damage such as bearing failure can also occur as a result of operating in cavitation.

Sam Pickens also says cavitation can and will damage bearings under given circumstances; re: BS123 first edition

I have come upon one and monitor monthly that had a filter installed a few feet in front on the pump and reduction to the pump was via a conical reducer. The noise is very broad and spreads all the way down to 1X at times. So when you can get into the shaft's fundamental and its harmonics or shake the shaft you're in trouble - page 2. it's Friday!

I have got to believe that cavitation imparts an impulsive thrust load on the rotor. Kinda like hitting the impeller face with a hammer. Impulsive loading can't be good for the bearings....

I have setup pumps and monitored shaft deflection via eddy current probes mounted back near the bowl and record readings with known bearing clearances.

If you have 2 mils brg clearance at each end in brg C/L and 6 mils at the probes or 6 at one and 8 at the other or??? you can graph this out on paper if you want and enhance with phase info. Sometimes you'll see deflection will remove clearance in angular contact bearings. And after the break, the rest of the story and deranged? It's Friday!!! You wouldn't know I have to work all weekend would you.

I would think that in most cases if you have cavitation severe enough to damage the bearings, something else is going to fail before they do.

From my point of view , cavitation alone can damage bearings. In case of severe cavitation in a 3 stage centrifugal pump, I have measured vibration change from 2.5 mm/sec to nearly 8 mm/sec on bearing housing. Obviously, this is not going to do any good to the bearings.

Very interesting and thought provoking comments. This problem is occuring on one of our Unit 3 circ water booster pumps. The pump is definiely cavitating. They just tore it down, but unfortunately with me being assigned to a different unit, I did not get to see the impeller or the bearings. One of the mechanics from U3 did tell me there was some considerable "frosting" on the ob bearing outer race. He said the bearing clearances were set a little tight. I'll have to see if the majority of noise went away or is it still there. Thanks for the comments,

For me I can imagine it makes a big difference what type of pump your talking about. Massive rotor and long shaft between pump and bearings -> cavitation seems not likely to damage the bearings. Small rotor and short pump shaft supported by bearings very close -> easier to imagine.

For an example of the first type of pump, I am thinking of our vertical CW pump. 3,500 hp, 324 rpm. It is a massive axial flow impeller, very long shaft (30'), large motor and thrust bearings at the top of the motor. That amounts to a whole lot of mass that has to move to respond to a cavitation bubble implosion impact in order to load the bearings. I tend to think cavitation is much less likely to affect the bearings on this type of machine (massive rotor, long shaft between pump and thrust bearing) unless the cavitation were extremely violent.

For an example of the second type of pump, I am imagning a small overhung pump 100 hp, 3600 rpm single radial flow pump supported by back-to-back angle contact bearings adjacent to the pump and deep groove just outboard of that. Then there is not a lot of mass of rotor to move, the location of the impact is closer to the bearings, and it's easier to imagine the cavitation damaging the bearing.