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Dear Everybody,

I have a case of abnormal vibration at both side of steam turbine which I hope to receive your comment/ideal of the root cause on this issue (pls see in file attachment for your information).
Some information as below:
The pump is boiler feed water pump which is driven by a small steam turbine (Dresser Drand as OEM, turbine is impulse turbine) through a reducer gearbox.
On 14/11/2014 the pump was taken into operation after conducting overhaul for all pump trains. The vibration at pump and gearbox is normal but in both side of steam turbine, it is abnormal with vibration fluctuating unstably (as the vibration trend below).
As operation confirmation that the operation parameter like flow, pressure of steam is normal.
All clearance is in acceptance as OEM specification.

Thank you in advance.

N.V.Đ

Attachments

Original Post
quote:
Originally posted by herovn:
At steady status there is no problem, at abnormal status orbit and spectra indicate unbalance problem. You should check any abnormal operating conditions


I believe the individual looking for the steady state polar would like to see of there is perhaps a spiral pattern while the machine acts up. It is in your interest to provide the plots we may request.
In my view, I think that the rubbing could be occurred but unbalance is not a case. The thermal growth could be a case.
The information as you required please see attached file.
Hope to receive more comment on the issues.

Attachments

Shaft end carbone seals can demonstrate similar symptoms specially during the first startups after an overhaul.

Tight clearances and case distortion due to thermal expansion or alignment (external and internal) can lead to rubbing.
Hi Shurafa,
thank for your comment.
You are right that the steam turbine is sealing by cacbon seal. But why the cacbon seal will act like the symptom in this case? do you know that?

For the alignment parameter, i will get and let you know then.
Hi Bill,

I have met a case with the other steam turbine which have a fluctuating vibration caused oil coking and i see something similar to this case. But in this case, i misunderstand about the coking. can you explain more about this, Bill?

Thanks,
I believe I have seen an article from the Texas A&M Turbo-Symposium on this. Look for that.

The worse I saw (and have pictures) was a large turbine shaft that must have rubbed over a decade or two. The shaft broke apart when doing maintenance.

It takes oil, probably some dirt, and heat. Moisture may help.
quote:
Originally posted by Ngo Dinh:
Hi Bill,

I have met a case with the other steam turbine which have a fluctuating vibration caused oil coking and i see something similar to this case. But in this case, i misunderstand about the coking. can you explain more about this, Bill?

Thanks,


"Coking" as we are referring to it here is the same as that discussed at
http://maintenanceforums.com/e...0533373#56620533373Q. It is generally the breakdown of a lubricant to its basic component carbon when exposed to high temperatures.

As someone mentioned new carbon seals can also present similar symptoms due to minimal clearances. I don't see excessively high clearances so I would be inclined to continue running and re-evaluate in a few weeks.
Coke builds up, reduces the clearance, and causes a rub. Because coke is very hard, it can wear away the shaft. One may see a grove in the shaft.

In the case that I referenced, the rub would cut shaft, clear the rub, grow again, and cut more shaft. This went on for years according to the vibration data - cyclic vibration amplitudes. Plus, the pictures showed cutting in the general same area, but not the exact same place; this would have taken many years to advance to the point that the shaft broke during maintenance.
With coke buildup, the constant rubbing on the shaft can generate enough temperature in the shaft to anneal and soften the material over time also, to the point where the shaft can be condemned due to being too soft. I've seen this on a gas turbine rotor.
Thanks John, Bill and Rechard Heiden,

I agree with John that you could mention to the tight clearance between Carbon seal with shaft which make a abnormality of vibration about 2 days after starting up. And then, when clearance become bigger the shaft can rotate smoothly, the vibration is absent.
The coking as Bill and Rechard Heiden mentioned I have met in an other steam turbine which caused by leaking oil with high temperature condition and I solved by injecting a buffer air to prevent the oil leaking and cool down the temperature and it is good up to now.

Thanks for your comments.
quote:
Originally posted by Ngo Dinh:
Hi Shurafa,
thank for your comment.
You are right that the steam turbine is sealing by cacbon seal. But why the cacbon seal will act like the symptom in this case? do you know that?

For the alignment parameter, i will get and let you know then.



Carbone rings, in my limited experience, may be a bit tighter on the ID for variety of reasons and hence result in contact between the stationary and rotating parts. In such cases, we allow the turbine to develop high vibration and even to trip a few times to “rub and shape” the ring profiles so the rotor moves in a more stable way.

Saying that, it is not necessary your specific case but it could be. Repair and installation reports need to be carefully reviewed as a first and fundamental step.


Regards- Ali M. Al-Shurafa
When starting up a steamer after an overhaul we have had to run the unit first at 500 rpm and then 1000 rpm for many hours to seat in new packing and seals before bringing it up to speed. This obvoiusly avoids high vibrations and trips during the start up, but also help seat in the seals and packing properly. I don't have much experience on smaller units in this kind of service, but is that not normal practice for these units too?
We have got many steam turbines provided by different OEM's and I haven't seen any such procedure provided by any of them .

However 02 years ago we have created one such procedure with the name of "Carbon seals Break-in procedure".

What I understand is that carbon rings do comes in slight contact with shaft even during normal turbine operation as carbon rings are radially free to move inside its housing along with the shaft and carbon ring to shaft clearance is usually less as compared to the radial clearance of journal bearings. Only difference is during start up (after installation of new carbon rings) shaft has not made it's enough path/ impact on carbon rings due to which we might face vibrations issue however with he passage of time ID of carbon rings become smooth/ grind. Probably that is why shaft of Steam Turbine is hardened and surface finished at carbon seal area....

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