Carbon Deposits on Turbine Vibration Probe

I don't think this has anything to do with static.

Just to confirm this probe came out of the generator, not the turbine?

Is the bearing oil isolator seal a carbon ring? Is the air box shaft seal a carbon ring? Wear from either seal might explain it, if they are carbon rings.

The oil near this probe did indeed get hot, as can be seen by the varnish on the shank of the probe holder. What kind of oil are you using? Any possibility that it came from the oil? Any sign of oil filter clogging or gunk building up in the reservoir?

Yes, probe came out of the generator.

No carbon isolators, labyrinth type bearing isolator. Air box seal is separated from the bearing by about 10" and not suspected.

Agree with your comment about the oil getting hot. We are utilizing Shell VSI 32 lubricating oil and last 4 quarterly lubrication analysis came back pristine (new oil following overhaul in November 2008).

We had a major electrical fault event in our refinery in February where we saw a bearing metal temperature rise of 30F over 45 minutes, then subside when we took the machine offline for inspection. Things are now pointing to a potential bearing fire that extinguished itself at that time, but was unnoticed at the time. This is grasping at straws, but as you mention and others have observed, the oil got hot in this area.

Oil fire would do it. I think that is your best explaination.

Oh and BTW thankyou for using Shell products. Much appriciated.

It is nice topic, the problem mentioned is not very common. Most probale cause for the in-bearing fire can be vapors, which accumulated there.

Its not uncommon to find deposits on the bearing pads(if a thrust bearing, which I think it is in your case). You need to check the lube oil being used. Normally the anti-wear additive (of the type of Succinic acid anhydride) has a tendency to react with elements around it and form deposits. Just a thought.......................

Any chance that probe may have come loose and contacted the shaft?

No chance it contacted the shaft as the end of the probe is similarly coated with black carbon-type deposits. If it had contacted, it would have shown some wear. When we cleaned the end of the probe off, it looked pristine. Thanx.

are you sure it's carbon? is it possible that it's a powder coating applied to the device at the factory to prevent corrosion during warehousing? Are you sure it wasn't on the probe when it was installed, possibly a form of lubrication to prevent wear (could be carbon or molybdenum disulfide) during use? Do you have the original materials that came with the item when you bought it? It may be the wearing away of the graphite or Molybdenum disulfide that resulted in the increase in space that made the probe improperly detect vibration. Second issue: stray voltages in the generator. If the probe were inserted into a metallic socket and even a small amount of current was passing through, you'd have localized arcs and sparks, localized smoke, localized carbonization of greases or oil vapors floating by. Third, is the socket into which the thing is inserted made of graphite or lined with graphite?

Not poweder coated as this powder residue is unique to the two probes in this bearing. All 6 others and the stores spares are bare stainless steel.

We are considering the stray voltages as a possibility, although, this machine is equipped with shaft grounding brushes that provide a 5 ohm impedence path to ground. Shaft grounding current monitor does not show significant grounding current, but there is some. We do have issues with condesation at the back end of this backpressure turbine that may cause some static charge to build on the turbine rotor.

No graphite located in the socket, bare threads into the bearing cap. there is a small amount of graphite based anti-sieze used to keep the threads lubricated, but nowhere near the amounts we are seeing on the probe. Thanx.

I am surprised that your 200 MW machine is a back pressure machine. My guess would be it tops down to some fairly large condensing unit or you have a very large process demand.

If the problem were stray currents we normally look for pitting not deposits. That probe very simply looks like it was overheated for whatever reason. Did you figure out what tripped the machine?

It is actually a 20MW machine. We have 2 of these and yes, process demands are rather large (1.2 MMlb/hr). Appreciate the tip to look for pitting. There is none evident on the probe and we will inspect the bearing at next opportunity unless CBM tools warrant sooner. The trip was due to the probe failing. Y-axis (failed) showed very erratic for 6 hours prior to trip, while X-axis was rock solid at 0.7 mils. Considering changing trip logic to 2 our of 2 voting with probe test quality logic to eliminate false trips (2 in the last 2 years).