Odd (Electrolosis) Marks on Bearing and Shaft

I will have to add pictures from home, won't work from my phone...

Drew K

Any large voltage sources nearby?

Are you running a VFD on this pump?

I have no personal experience with it but have seen reports in the MRO magazine before relating to bearing etching with them.

No VFD, or observed electric potential on the motor when it isn't running (and none when running).

This pump sits directly under the condensor which is attached to the low pressure turbine of a 110MW generator.

Drew K

Not knowing much about 110MW generators, I'm just going to throw ideas/thoughts your way; which may be complete and utter nonsense; just a bit of brainstorming. So please don't take the P.

What sort of lubricant is being used on the bearings? Is there any possibility there's the smallest amount of contamination?

How long has this equipment been running and, has this problem just appeared? Have you changed any procedure, consumables/supplier of these consumables (i.e. lubricant) in your maintenance procedure?

What sort of magnetic field does the 110MW generator give out?

Could it be inducing some sort of current (very small) to flow from shaft to bearing to ground, just as if the pump was being driven by VFD?

Could the pump be sited in a different position away from the generator?

It would be nice to know what was causing this problem, if you could let us know any findings/causes/solutions.

Hope you do get to the bottom of this one.

The generator is 2 stories up with the entire mass of the steel condensor between the motor and generator.

There are oil slingers that show in one of the pictures that distribute oil to the bearing. Oil has been sampled and found clean, this is a well maintained pump / motor in a plant that is held to high standards.

This pump has been running since the 50's with a few rebuilds and repairs through the years. From what the mechanic told me we rebabbited the bearing (because of disbonding) about 6 months ago. It was run for the summer and fall then the unit was shut down for overhaul. The marks were observed during this overhaul. The only thing changed was the babbit in the bearing, no service or lubricant modifications.

Not practical to attempt to move the pump and it is doubtful that location has much of a factor (unless it is stray current from nearby equipment in which case the stray current would be fixed instead of moving the pump :) .

Thanks for the input...no piss taking here!

Drew

Do these marks cover the complete circumference of the inner/outer faces of the bearing/shaft? It looks as if they don't in the pictures and, it's as if they're concentrated in one area.

Marks are only on 2 locations on the shaft and appear to mirror the mark on the bearing surface and only occur on the bottom of the bearing half. It appears that the marking happened then the shaft was rotated andht occured again.

Drew K

A reading of below 2 gauss magnetism is acceptable . The shaft and the bearing lack typical washboard pattern and hence may not be a case of fluting.

Can the possibility of brinelling be discounted?

Brinelling is very uncommon on sleeve bearings. The vibration sensors did not record any undue stresses and the pump is not overloaded.

The babbit was replaced 6 months ago because of high temp alarms.

Drew K

It doesn't appear to be electrical fluting, which generally occurs to a VFD motor when running and would be completely encircling the inner or outer race..

It may be due to a stray current when the motor is not running, since it appears localized on the bearing surface.

I suspect it might be stray current but I am having trouble backing up that theory.

The pump and motor are both connected to the massive grounding grid which should protect from most stray ground currents. The piping and framework surrounding the pump and motor are all grounded to the same grid. All the interconnecting metal and grounding grid should provide a lower resistant path for any current.

Perhaps this isn't electrolosis but I don't know what else it could be.

Drew K

Since the pump is situated below the condenser, are you positive there is no condition in the area surrounding the pump and condenser that could produce condensation from the atmosphere? For instance, a steam leak nearby, the condenser out of service (cold), and the pump stationary. If liquid water forms containing carbon dioxide, I suspect a localized galvanic corrosion between babbit bearing and pump/motor shaft.

I have a hard time seeing how this really should occur if the bearing sleeve is a close to tight fit with the shaft, and is supposedly operating under oil film conditions, right?

Are you sure the oil is not being over cooled and taking on moisture, as this could also be the source of a conductive corrosion reaction.

I just spoke with my mechanic, there was no evidence of water in the oil or rusting on the housing anywhere.

He spoke with the bearing supplier that had rebabbited it and they said it looks like eddy current damage but none of us can fathom where the current could have come from.

We will continue to watch the potential between the pump and motor to see if we can track it down.

Drew K

Could any welding nearby caused earthling through this bearing?

We considered the welder option because welding was done nearby but were able to discount it after talking to the welders.

The pump is either on or off, it never turns slowly enough to not oil the bearing.

Drew K

I've observed condensate pumps which were on stand-by (not the running pumps) which were on slow rolling do have digging in of the bearing bottom halves. This is explained due to the unsuccessful establishment of oil-wedge due to low speed. Similar digging happens in the bottom halves, but the marks on the shaft would be circular/continuous, unlike the patches in your picture.

I myself have wondered if there is some magnetic coupling of the armature magnetic fields and the motor shaft in such instances that could produce an "unreadable" eddy current. I do not expect a problem with the build up of static charges on such equipment that is well grounded. But just to be sure, perhaps you should test your earth grounds in the plant to make sure they are up to speed, and that you are not carrying current in some odd way back through the bearing/shaft. I do not think this has anything to do with loss of oil film.

"and only occur on the bottom of the bearing half."

Where moisture/electrolytes would accumulate during down time.

True, but the mechanic foreman in charge of this piece of equipment is the sort who has been working for the plant for nearly 30 years but is smart enough to know how much is left out there to learn. He is the type of mechanic who perfectly couples decades of experience with the common sense to know just because it has always been that way doesn't mean it is right.

I completely trust his statement that there was no evidence of moisture / electrolytes within the bearing housing anywhere.

I did look for signs of white oil or corrosion droplets indicative of condensation in and around the housing before I asked the foreman.

Galvanic corrosion was our first suspect for causing this damage and after thoroughly investigating it we ruled it out.

The determination made during the staff meeting was that it was caused by eddy current and we will monitor the bearing for future events in an effort to track down how it happened.

Drew K

Being from a non mechanical background, please forgive me if my next observations are incorrect.

Am I correct in thinking that this babbit bearing effectively floats on a very thin layer of oil/lubricant? How is this gap maintained? You said that this is a bearing between motor and pump and, the babbit was renewed 6 months ago. Could there be the possibility of miss alignment?

In the first picture you posted there's the babbit with the marks and what I can see as 3 holes. I'm assuming that these are the lubrication feed (2off) and return (1off) holes. So does this mean the lubricant enters at the 2 holes and while rotating is distributed around the circumference of the shaft/babbit and, flows out of the one hole? Because these marks look as if they've built up between these holes, while I'd say in a non rotational state. Does the bearing lubricant still flow when in the non rotational state? If so, would it be possible for this lubricant to be lifting the shaft slightly under pressure, then dropping it back onto the babbit once the lubricant finds a realise and, the pressure drops off, on a continuous cycling basis?

As I've said I'm just trying to throw ideas your way, in a hope that they may give you some sort of inspiration, for you to solve this problem. Please feel free to disregard if complete nonsense.

Oiling is accomplished by slinger rings, you can see the edge of one of the brass rings in one of the pictures. The holes are to allow oil to flow and keep the bearing cool. This is a central bearing on a pump motor unit where there is a flexible shaft union to allow for slight imperfections in alignment.

I am not 100% on how the shaft begins to float on the bearing at start-up. This is not a heavily loaded pump, it pushes water through the feedwater heaters going toward the boiler feed pumps.

Tolerance gap is checked on installation to ensure the bearing is fitted to the shaft properly. There is a condition where a loose bearing with a shaft that shifts around, but it is usually due to poor alignment (but blamed on balance). There are other causes for that sort of shaft movement but all would result in a different shaped marking on the shaft and bearing than what we have.

The conclusion we have arrived at is that it is eddy current damage caused by an unknown source.

Drew K

I visited with one of the maintenance staff at my plant, and he suggests water intrusion, similar to that I alluded to earlier. It simply might not take much water to cause these marks.