Periodic refurbishment of critical motors 3

[electricpete]

I am trying to evaluate/justify the need for periodic refurbishment of motors.

Our population of critical motors includes some large (8000hp), some small (1.5hp), some sleeve bearing, some ball bearing, some indoor, some outdoor. We have 15 years of operating history. Motors come in groups of 6 or 8 identical machins. Mostly class F VPI insulation systems at class B temperature rise.

We have a pretty good predictive/monitoring program, but we recognize it is not perfect and it may be unreasonable to attempt to run critical motors 40 years without refurbishment, regardless of predictive maintenance information.

I am working on tabulating failure history, running load, running winding temperatures, starting patterns, environmental/enclosure factors, past cleanliness of refurbished motors to prioritize which groups of motors most deserve refurbishment.

Are there any other suggestions on means to justify proactive refurbishment and classify the necessity by motor design and operating characteristics?

* Are motor refurbishments identified in any standard industry documents?
* Typically I have seen that people will refurbish large motors but not refurbish or even replace small motors. It makes sense if the cost of motor failure is limited to rewind, but not if the failure is disruptive to the plant. Does anyone have program to refurbish or replace small motors?
* Bearing vendors suggest that even if a bearing is periodically regreased, it must eventually repacked by hand. We have a program for this (I suspect this is same as others). Is the limited life associated with lubricant and/or balls/races in rolling element bearings a good justification for motor replacement/refurbishment at perhaps 20 year interval?

 

[DougMSOE]

The most effective way to get through what you are facing is use statistical models.
I have recently used this method for a large mining concern.
First, Using Doble Power Factor testing and the failures that had occurred, a Weibull Distribution was constructed. From this information, reasonable accurate (2 months-) were made.
They then knew,
1)When to test
AND
2)What to test

 

[electricpete]

Thanks Doug. I can see where you are coming from and I believe our approach is similar.

At this point I am working on developing a methodical means to prioritize refurbishment. I have identified some of the factors we plan to use in prioritization process above. We will also use the motor critical-ness, which is determined by a probabilistic model of our plant. I am looking for other criteria.

An example of a useful criteria:
Vane axial fans located within duct-work have very limited ability for monitoring vibration to detect bearing and other problems. That will push these machines toward the top of our list.

 

[GusD]

Hello Electricpete

About your axial fans.Could the installation of permanent tranducers help in this case?
We have been installing a lot of online vibration on most of our motors,gearboxes,pumps,blowers and anything that we have deemed critical enough.
DougMsoe suggestion of using statistical determination by the use of Weibull Distribuition analyses is probably a step saver with good results.They have a great site with some new Reliability software approaches that were not available a few years back.

On Standards for refurbishement of electric motors.
There are papers on the subject,I don't think they would
classify as Standards.Each plant environment dictates a different set of rules for its particular equipment.

Repairing Large motors and forget about small ones.
My feeling is, if a "small" motor causes the shutdown of a large Utility Generator,or causes our Oil Distillation Cocker shutdown,it is a pretty big motor.I have to monitor this drive like I would any critical piece of equipment.

Your "proactive refurbishement".This is a tough one.
It is a big dilema for our P.Groups ,that justified Millions of dollars on the ability to monitor equipment till it can no longer run safely or efficiently and remove it just before breakdown.How do I deal with it?

Let assume that I have a 1500 hp motor,it operated 10 to 15 years without problems and now the Pump,Blower,Gearbox or other,has to be removed for extensive work.
This opportune time,gives me a chance to evaluate my motor.
Will it last another 10 years ,without a bearing change,a
Winding Dip or some other problem.If I answer yes to my questions than I leave it in.On the contrary,if I don't believe it can last another10 years;than,I have a good reason and the opportune time to do a refurbishing.

EPete,in the old days,when motors were designed with ample HP room to drive the load,we could have 30/40 years runs.
The reality of today's industry demands,coupled with motor design to a "T" specs,VFDs and God Knows,makes a 10 to 20 year run a better reality.
It may still be possible on the very large utility equip.

We don't have a program for small motors.We try to treat motors by "how critical"they are.So having said that,it makes sense that I always have to have the most reliable small motor possible in place.A new one may not necessarily be the best motor.Statistics says that "Infante
moratlity " could do it in.

Caution(Weibull Dist Analyses) says that"Infant mortality"
will cause 30% of the failures, of new and repaired motors in the first 3 years (a broad generalization).If they survive the 3 yers, they have a good chance of running for 10 years.A few Factors to be weighed in determining,should I repair,replace or let it run.
Your own plant environment will dictate the course.

Epete.In general Bearings account for 40% of motor failures.It may not be the bearings that fail,housing fits and journals get sloppy,depending on the application.
The motor may have to be removed just because of that.

Bearing Vendors suggestion that even if bearing is periodically regreased,it "must eventually be repacked by hand".
I have a problem with Vendors above suggestion.
If they sell me a bearing that has an L10 life of 50,000 hrs for Example,they don't tell me that I have to repack it by hand at 25,000 or so.Every bearing has a regreasing capability that when done properly ,should take 90% of the bearings to their L10 lifecycle design.
Pete ,I am not saying that repacking by hand would be a bad idea,unfortunately ;it is not a very practical one to do.I personally know or heard of of any plant, that uses that in their bering program.

Your 20 year interval may not be able to be applied acoss the board.Your plant criteria (Weibull Dist)should help you in determining that.

Try the Weibull site,they seem to have a lot of good ideas
Epete Hope I helped the cause



GusD

 

[electricpete]

Good comments, thanks. Regarding manual repacking, SKF provides the following recommendation:

"One of the two procedures described below should be used, depending on the relubrication interval tf obtained:

1 – if the relubrication interval is shorter than 6 months, it is recommended that the grease fill in the bearing arrangement be replenished (topped up) at intervals corresponding to 0,5 tf; the complete grease fill should be replaced after three replenishments, at the latest;
2 – when relubrication intervals are longer than 6 months it is recommended that all used grease be removed from the bearing arrangement and replaced by fresh grease.

The six-month limit represents a very rough guideline recommendation and may be adapted to fall in line with lubrication and maintenance routines for the particular machine or plant."

I agree this recommendation does not reflect common practice. I suspect that if you asked SKF what is required to assure L10 life they would advice you to follow their lub procedures including this one.

What is the real impact on reliability of not following this particular recommendation for a long period of time? I don't know but I'm interested to find out. I can imagine that if the machine design and/or regrease procedure do not lead to effective ejection of used grease, it will not have much effect initially, but will eventually catch up. Initially pack cavity 1/2 full... add an ounce per year (6313 bearing)... I'm guessing 10-20 years until the cavity gets full (?). I have seen one of our machines found after many years of operation with cavities packed full, in spite of our attempts to use good greasing practices.

Why am I concerned about this failure mode? I think it can result in rapid failure from bearing overheating with little vib warning as with many lubrication-related failures. We don't routinely monitor our bearing housing temperatures on small motors. I'm not sure how much warning time would be expected if housing temperatures were monitored.

 

[jbartos]

Suggestion: List of items pertaining to the motor refurbishing
1. Predictive approach to motor wear
2. Refurbishing cost estimations
3. Refurbishing firms pool including their tooling and expertise
4. Categorization of refurbishing by motor type, size, voltage, cost, downtime length, manufacturer, etc.
5. Speed or time of refurbishing
6. Content or type of refurbishing
7. Etc.

 

[GusD]

Hello electricpete

I will take my chances with the SKF people.I don't think they would press too hard on this issue.Some things would be nice to do,but for the most part, they are impractical in the real world.If you had to follow this practice,in most cases,you would have to remove the motors,disassemble the and remove NonDr End shroud,remove cooling fan and open bearing caps.On some DEnds caps you don't even have
enough room to remove the cap because of the coupling.If you don't remove the motor from its location,you could be doing a great deal of harm to the bearings,by exposing'em to what in some plants is very dirty environments.
Bearings greatest enemy is dirt.
EPete,I am sure that most of us have heard about motors in storage, that they should have their shafts rotated every so often so as to not damage the bearings.No one ever said,that it is not a good idea.I personally doubt, that anyone with 50 motors or more in storage,ever bother to do that.It is still a good practice.
As for replenishing the grease on most bearings,if your motors have drain plugs,and plugs can be removed during regreasing,than use the recommended amount of grease(the right amount) until new grease shows in the drain plug.The motor should continue to run with the plug removed until the new grease can expand and exit housing.Bearing should not be overgreased if the procedure is followed.
We have large 1250 hp drives that come without drain plugs.
The worldly reknown manufacturer ,believes that bearing housings are large enough to accomodate the grease from repeated regreasings until the next overhaul.It must work,I don't think we ever lost this type of motor due to overgrease.We did lose a few for lack of grease,but that's a different story.
Epete, I don't know your particular plant or its environment.Relubricating bearings is hardly something that we do to motors every so often.It is a little more complicated than that.
If you have 2 similar motors in a particular area and one motor is on Standby most of the time,it is easy to understand that you shouldn't regrease the 2 motors using the same frequency and amounts.
I do believe,if one has done some basic work in finding out what your motors do, and what their duties are,a simple criteria can be establish to provide reliable relubricating programmes.
Adequate lubrication is one of the most important tools for motor long life.

You mention Richard L Nailen "Managing Motors" in a previous post.He has some basic information on relubricating motor bearings.
Epete.
I am curious as to which motors in your plant seem to give you the most problems(greasing).

So far, you have not mention anything about "shock pulse" or Spike Energy Vibration analyses and how they relate to bearing lubrication.(some people think so)

GusD

 

[electricpete]

I think there is a misunderstanding of my purpose in bringing up the SKF recommendation for grease replacement. The only reason I bring it up is to get to the question of the possible need for refurbishement.

The SKF recommendation leads us to consider that a greased rolling bearing may requires some form of attention beyond regreasing during it's lifetime. Like you I don't consider grease replacement practical but would consider refurbishment including bearing replacement. You have mentioned a motor with no drain but enough housing capacity to get between overhauls. If it is agreed that overhaul is to be performed, than the question of grease purging is somewhat irrelevant in my mind. I realize that SKF's time interval is tremendously shorter than the overhaul intervals, but as mentioned above I believe 10-20 years is the time interval during which imperfect greasing (failure to purge) can have their effect.

To stir the pot a little more, how does a bearing eject grease from the drain? My answer is that it very rarely happens unless the bearing cavity is completely full. (Most motors don't eject grease during run after re-lub). When full, the grease is expelled by hydraulic pressure of thermal expansion. But really, when the grease cavity is completely full, the bearing has already been overlubricated by most standards, hasn't it? This is just my way of looking at it. I have heard people suggest that maybe the grease melts and flows out like a liquid during running after grease, but I don't buy it.

 

[jbartos]

Suggestion: All lubricants experience some breakdown after some time. The lubrication is probably most perfected in the automobile industry. The motor high rpm needs special oils.

 

[Laplacian]

We are currently assessing the health of all our large critical motors (> 1MW) with motors up to 15MW. For 13.2kV motors, we use either a company proprietary partial discharge monitor or the IRIS bus coupler partial discharge monitoring system to predict insulation degradation.

For MV motors in outdoor service that have WP enclosure, we have upgraded the filters to high efficiency gas turbine designs. These filters give us 15 years of continuous runtime with a forced lube oil system and regular filter changes.

We use the WinSmith Visual (weibull) software to predict motor failures given at least 4 data points in a population.


The greatest reliability of motor class are the TEWAC enclosed motors. Bearings are usually the first failure.


To truly gauge you refurbishment priorities, I would evaluate the following for each motor you are considering:
Trend vibration data
Trend winding temperature
Trend bearing temperatures
Evaluate enclosure vs. environment
Look at start frequency
Trend stator current
Evaluate motor's PM or maintenance history
Pedestal vs. internal mounted bearings (oil ingress potential)
Prioritize based on highest cost of outage; maintenance
plus lost production

 

[electricpete]

Thx Laplacian, that makes sense.

I have thought a little about which motors have potential for oil ingress. I'm not sure what you mean by pedestal vs internal mounted bearings.

We have Iris pd as well. You mention company proprietary partial discharge, must be a large company. Ontario Hydro? AEP?

 

[jbartos]

Suggestion: Visit

http://www.oilanalysis.com/learning...p?articleid=469&relatedbookgroup=Lubrication2

for "Shock Pulse Meter"

 

[jbartos]

Suggestion: Visit

http://www.oilanalysis.com/learning...sp?articleid=209&relatedbookgroup=Lubrication

for: Hybrid Bearing Grease and the Continuing Challenge