mothballing a generator 1

[Yuma]

Dear all,

We are thinking of mothballing a generator and as far as I know, you can use dry air, nitrogen or carbon dioxide for that. Can anybody comment on the pros and cons of the three methods? Is there any difference if the generator is water cooled or hydrogen cooled?

 

[electricpete]

I have never been involved in either of these approaches for long-term storage. Some thoughts fwiw:

nitrogen or carbon dioxide - Con - potentially deadly atmosphere for humans. Either inside the enclosure when it's opened and people poke their heads in (long after you're retired). Also for CO2 if escapes for some reason can collect in lower point to form suffocating environment There are longshot scenario's, but the stakes are high. Pro's... in theory better at corrosion protection since oxygen is excluded, but shouldn't be an issue as long as the environment is dry.

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[electricpete]

If the setup is prone to small leaks, I imagine air is better (dry air bottle with regulator to keep continuous slight positive pressure). If you are going to be able to get a hermetic seal and lock in the gas, than the CO2 or N2 might be more attractive.

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[electricpete]

By the way, how big is the machine? Will the rotor be in the machine? For large high speed machines, there is a tendency for manufacturers to recommend periodic rotation during storage. Need to give some thought to bearings.

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[edison123]

Nothing special. Protect it against elements, power-up space heaters inside the machine and have a mechanism to turn the rotor periodically with bearings properly lubicated.

Muthu

http://www.edison.co.in

 

[mns15451]

The generator space heaters should be ON all the times. Ideally speaking,if it is a bigger generator I mean 5 Mva or more, then we should consider giving a quarter turn to the shaft after 2 - 3 days. For that we must have a prper lubrication system.
However, we had a break-down/fire in the transmission sub-station that forced all the 18 Gas-turbine generators (ranging from 20 to 60) Mw out for over a year. All we did was to that we kept the space heaters of all the machine in order AND THATS IT. After normalization, the machine started without any trouble.
Larger Generator rotors (over 10 MVA} are kept in stores in SEALED CAPSULE presurized with nitrogen.
Hope this give u an idea to decide.

 

[Yuma]

Thanks to all for your input. We are talking about big (>300 MW) turbogenerators and yes, the rotor would stay in the machine. We have turbogenerators of different manufacturers and cooling type, and we have of course asked the manufacturers, and still waiting for an answer. But in the meantime, I appreciate to have some unbiased information from you guys

As for the periodic rotation: yes, I have heard of giving a complete turn from time to time, and leaving the rotor 90 degrees apart from last position. I have even found - Googling - one opinion that suggests leaving it on turning gear, which I think would be terrible for the rotor for more than a couple of days or so.

But going back to the gas of choice, from a chemical point of vue, do you think one of the gases is better than the others with regard to the insulation, wedges, etc. I've heard that CO2 increases conductivity and lowers pH, but probably this effects are negligible.

 

[ScottyUK]

>300MW? Is this a hydrogen cooled machine?

As it is being mothballed, I assume it is going to remain in-situ so you will have lube oil available and, if it is a hydrogen machine, you'll also have seal oil. If you have seal oil then you'll have no problem containing the dry gas.

I'm not sure why leaving it on barring gear would be bad for the rotor in any way provided you maintain jacking oil to the bearings.

Nitrogen would, in my non-chemist opinion, be a better choice than CO2 because it does not have any tendency to form an acidic solution if it encounters any water.


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[electricpete]

I am in agreement 24-7 jacking gear for years at a time cannot be good for bearings.

http://www.me.utexas.edu/~bryant/courses/me383s/DownloadFiles/LectureNotes/BoundaryLubrication.pdf

Stribeck curve with very low speed will but you on the left of the graph (boundary lubrication) where wear rate is high.

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[electricpete]

Typo Correction in bold:
Stribeck curve with very low speed will Put you on the left of the graph (boundary lubrication) where wear rate is high.

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[electricpete]

The curve I linked is hydrodynamic, not consider effect of jacking oil, so not the full story. Still I think jacking may not establish perfect lubrication.

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[Yuma]

Actually, when I said that I don't think that being permanently on turning gear is good for the rotor, I was worried about the insulation of the field winding, particularly in the zones under the retaining rings. Also I remember a paper by GE reporting a rotor that had been on turning gear for thousands of hours, and eventually developed cracks in the rotor teeth.

And being on turning gear doesn't seem to have any advantage over periodic rotation from time to time. Or maybe I'm missing something...

 

[edison123]

At that size, I would store the rotor separately in an enclosure with space heaters and support it liberally on its core portion to avoid permanent set.

Muthu

http://www.edison.co.in

 

[ejrampe]

Yuma

I work for a generation utility and we recently mothballed 4 x 40 yr old Parsons 60 MW, hydrogen cooled only turbogenerators. The seal and lube oil systems were de-commissioned while the underbelly heaters remained in-service. We left the rotors in situ and did not seal the generators in an enclosure or anything like that, we only used a (semi - because of air leaks) closed air loop with a de-humidifier to continuously pump dry air through the generators. We did not rotate the rotors at any stage.
After a period of 18 months, two of the units were re-commissioned without any problems to the generator.

Why not nitrogen? Big $$ to maintain the nitrogen volume because of all the leaks.
Why not CO2? As ScottyUK mentioned, CO2 will begin to form an acid with any moisture present which is not good for your winding insulation and especially your retaining rings. CO2 should really only be used as a purging gas and should be left in the generator for as little time as possible.

There are likely many other reasons that I have not included above.

One important point. You are correct regarding the problem of continuously operating the rotor on turning gear. It is not a good idea at all. The problem you have described, we commonly referred to as "copper dusting".
At the low speed of turning gear, the rotor windings are continuously pounded due to their own weight and this may lead to the creation of copper dust. The copper dust may result in a short circuit between turns of the winding or to ground. As you mentioned, this problem is most likely in the end winding regions of the field winding and is made even worse if the clearances between the winding and the slot insulation are large.

While we have not experienced a failure of our machines as a result of this, I have read many papers describing the problem of others.
A good source of information on this subject is "Inspection of Large Synchronous Machines - Checklists, Failure Identification and Troubleshooting" by Isidor Kerszenbaum in the IEEE Power Engineering Series.

Hope this helps

 

[ejrampe]

I should have also mentioned that the problem of "copper dusting" as described above is not a problem at rated speed because the copper windings are held in position by the large centrifugal forces pushing them onto the wedges and retaining rings.

 

[edison123]

I have seen and rectified copper dusting problem in 150 MW machines which were running on full speed. In my case, it was due to two conductors in a turn rubbing against each other due to differential expansion.

Muthu

http://www.edison.co.in

 

[Yuma]

Many thanks to all for sharing your experience. This thread is getting really interesting!

ejrampe:
I agree with you that CO2+H2O = H2CO3, but is it not also possible that N2 + H2O = HNO3? Or maybe that doesn't happen at ambient temperature. My Chemistry knowledge is a bit rusted, you know...

The thing is, there are some people out there that are using CO2 for mothballing generators, and while it was a surprise for me - I only knew of dry air or nitrogen for that - there has to be some advantage in using CO2 if people uses it.

By the way, I'm still waiting for an answer from the manufacturers...

 

[ScottyUK]

Nitrogen molecules don't readily dissociate to form HNO3, at least not a room temperature, so the simple equation doesn't give the whole picture.

Interesting comments on copper dusting, I hadn't considered that when I made my previous comment. I'll have a look at the book referenced above - thanks.


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[mns15451]

I think we are going in too much chemistry.
If the generator is in-situ and I have the facility to make barring or put on turning gear I would alter the turning gear controls to make the turning of shaft for about 5-10 minutes after every 48 hours and keep the space heaters ON. Thats it.
Please note that the turning or barring the shaft is to avoide rotor bowing.
I have spend life in the power plants and have done this several times.
Good luck

 

[Yuma]

By the way, going back to the copper dusting issue that ejrampe explained so well, another source on copper dusting is GE's TIL 965-3.
(Obviously copper dusting can happen also in generators made by others, not only GE's designs)