Frequent Replacement of Babbitt Bearing on Hydro Turbine 1

[engrdeepee]

Our company is currently maintaning and operating this 60 year old mini 1.80 MW hydroelectric power plant. For the past few years, it has underwent changes and improvements. However, we haven't rectified this problem of frequent bearing replacement. Realignment has been done which costs $10,000. Lubrication systems are being modified to fight off contamination on the lube oil (Regal 68) . However, I have come to consider a change of Babbitt grade from a ASTM B23 Grade 2 Babbitt to a ASTM B23 Grade Babbitt would suffice. I have checked and compared the mechanical properties for both. If I were to consider Grade 3, it has 5% lesser tin, 5% more antimony and 6% copper. Any similar instances from this post? Would appreciate any useful comments. Thanks.

 

[gruntguru]

A change of bearing material can only help reduce wear under under conditions of boundary lubrication or worse. The real goal should be to eliminate or at least minimise the occurence of these conditions ie why is viscous lubrication breaking down? Is the shaft frequently stopped/started?

Your post suggests lube contamination is the likely cause and I assume you mean water contamination. If that is the case you need to eliminate the contamination. If that is not possible you can dramatically improve the situation with the right oil. There may be others but I can personally vouch for Royal Purple Synfilm 68. For water tolerance and boundary lubrication the comparison to R&O mineral oil is chalk and cheese.

je suis charlie

 

[Tmoose]

Is it possible to perform oil analysis with the unit on line?
Are the bearings fitted with pairs of "prox probes" like large turbines frequently are

At what point did bearing wear become a problem, relative to the "changes and improvements" ?
What are the changes made?

Is it this style turbine, or something different ?

http://www.esru.strath.ac.uk/EandE/Web_sites/01-02/RE_info/casehydro1.jpg

Do the failed bearings resemble any of these?

https://www.johncrane.com/~/media/J...ument/Babbitted_Bearing_Health_Assessment.pdf

http://www.bearingsplus.com/uploads/literature/turbomachinery/tms_vol2016_1991.pdf

Shaft geometry, surface finish, and even the direction of final polishing can be a major issue causing bearing wear in heavily loaded (overloaded) automotive engine journal bearings. I'm guessing the turbine designers did not venture anywhere near automotive operating loads, but I'd want to quantify the condition of the bearing journals.

Example of particle contamination recommendations for Journal Bearings here -

http://www.eaton.com/ecm/groups/public/@pub/@eaton/@hyd/documents/content/ct_233707.pdf

(high speed) 17/15/13 >400 RPM
(low speed) 18/16/14 <400 RPM

 

[romke]

what were the exact causes for replacing the bearings? the way the bearings look when worn may give a clue to what caused it. whatever the cause, if it is wear due to contact of the shaft with the bearing and thus a situation where no hydrodynamic lubrication was achieved. that may be due to several causes: to low viscosity, oil contaminated with water, speed to low to create a suitable filmthickness.

changing the bearing material may change the loadbearing capacity somewhat, but given the fact that the machine has functioned correctly over years it is unlikely that the load would be too high.

a journal bearing in a turbine that is sufficiently lubricated with clean oil of a suitable viscosity should last forever, since there will be no contact between shaft and bearing once the normal operating speed has been reachd.

 

[ronfrend]

Well said Romke.
If the bearing has failed due to fatigue then you will see cracks (or bruising). Changing to a higher strength babbitt will help in that case but the rule of thumb is to make sure the hardness of the babbitt must be no more than 1/3 the hardness of the journal. If you do have a lube problem then the harder babbitt will end up kissing the journal and (if it's too hard) will damage the journal. EXPENSIVE.
As Romke said, if the problem is lubricant related, have a good look at the oil.
If the problem is bearing fatigue you need to find out why the shaft loading has increased so much.

Ron Frend

http://www.predicon.net

 

[engrdeepee]

Attached is a photo of a newly removed bearing. I believe that the thrust shoe which is attached to the shafting might contribute to bearing damage due to metal to metal to contact. I am just curious that is it either strong axial force might contribute into this or because oil starvation has took place causing absence of oil film lubrication to take place.

 

[romke]

A few remarks. In the image it looks that quite some scoring has taken place, which looks that some abrasive particles have been "grinding" in the bearing surface. Whether external particles that entered between the bearing and its opposing surface caused this or that internal wear particles were generated that subsequently have been spinning around is not clear.

The crack is remarkable. Given the fact that the bearing was relatively short lived, I wonder how well the bearing pad was supported by the underlying hardware. It more or less looks the bearing was exposed to a form of bending stress that caused it to crack after a relative small number of revolutions.

Have cracks been seen in previous damages and if so, on the same spot?

Could you give some more information how the shaft is lubricated and especially the thrust bearing? I assume that when not in use the full weight of the impeller is on the thrust bearing. Another question is whether water could find it's way into the thrust bearing. That might cause the lubricant film to collapse locally and thus induce local stress, heat and wear.

Do you have more images? I have been tinkering with the image in Photoshop to either lighten or darken it to see more details, but I would like to see more.

 

[JJPellin]

I am not familiar with this machine type. This bearing design is unusual. The fact that the thrust bearing is a fixed pad design that is built as part of the radial bearing seems strange to me. We would normally only use a bearing like this if there was very little predicted thrust. We would call it more of a thrust bumper that you might see on the ends of the radial bearings in a double-helical gearbox with no net thrust.

I see a couple of things in the photo that draw my attention. The pressure dam in the radial bearing seems to be packed with something dark. This suggests contamination in the lube oil. The thrust face damage could be suggestive of overload, inadequate lubrication, contaminated lubrication, poor oil selection. I would check for any sources of unexpected or excessive thrust loads. Without any knowledge about this machine, I would consider coupling spacing, differential thermal expansion, balance piston/disk design or balance line routing.

I can’t see how oil gets into this bearing. I see a pressure dam in the radial bearing, but no oil distribution grooves or a port for introduction of oil into the bearing. I would expect to see some ports or passages to introduce oil between the pads on the thrust face.

I would check the oil system for filter design and rating. I would check for piping that would allow oil to bypass the filter. I would verify that the oil used is the correct viscosity for this machine design and speed.

I would not expect to solve this problem with a change to babbitt grade.

If you could provide details about the machine configuration and the oil system, we would have a better chance to help you understand this.


Johnny Pellin

 

[electricpete]

I agree with everyone that the lubrication of this thrust face is problematic.

How are we supposed to get hydrodynamic lubrication? Small amount of oil may be swept from the radial channel to the pad but there is no chance for wedge to form if the surfaces are parallel. There is not even a radius/bevel on the "leading edge" of the pad (which still wouldn't create a wedge but at least would help feed oil in).

So as mentioned it may be suitable for brief temporary load (easier for oil to find it's way in while unloaded… rely on that residual oil for brief periods of load), but I don't think it can carry load for long.

What is the actual loading? Intermittent or continual.

Putting aside the above, there are some aspects that do not look like lubrication problem. If cause was insufficient lub of continuously loaded face, then I'd expect to see signs of overheating (brown staining/varnishing)… don't see any. Was it cleaned with solvent prior to photograph?

The divets and cracks on the working surface resemble "fatigue" damage from heavy loading. It there an axial play where the machine can slam onto this face?

Also seems like worse damage around 9:00 / 10:00 / 11:00 then around the rest. Brings to mind misalignment although I'd think that would be more distinctive clues.

The pattern shows a difference along the id of the pads (shinier, more uniform)... what is it telling us? Beats me.


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(2B)+(2B)' ?

 

[engrdeepee]

As part of Preventive Maintenance Procedure, we are considering to perform shaft alignment along with improvement in lubrication outlet system. We have minimized the leaks which come out from the stuffing box and therefore contamination is not the case anymore. Another thing is that I am considering technical personnel from our lubrication supplier to further evaluate if the existing lubricant is appropriate our should we consider to upgrade. I will be posting some pictures and hopefully hear from you guys again.