OK, here is one for you that I cant quite figure out. We have a 500 MW generator, steam turbine driven with an exciter at the end. A nine bearing set-up. The exciter is enclosed in its own housing. The vibration on the exciter bearing is 7 mills( 1x-1800cpm fundamental). Here is my question; when we keep the temperature in the exciter hut around 110 degrees or warmer the vibration will reduce to 5 mills. If we allow the temp to drop to 100 we are at 7 mills. The oil temperature stays constant.
Why would changing the air temperature this small amount have this effect on 1x vibration??

I suspect that the actual change in the temperature distribution of the rotor is causing a variation in the thermal bow of the rotor. You could try plotting the inlet and exit air temps vs. the 1X amp and phase. Take maybe 2 deg steps and allow the vibes to stabilize. Plot it in increasing and decreasing steps to determine if the vibes are repeatable. Evaluate how the vectors are changing. If they are repeatable, balancing the exciter might allow the unit to run smoother with varying temperature.

Michael Titone

What does the phase relation between the bearings do when the temp drops to 100? What happens if the temp drops to say 80 or 90? If it is increased to 120?
Why does it have hut over it, because it will not run smooth without it? Which bearing increases with temp change or based on the original post, you referred to the "exciter's BEARING", does it only have one bearing? Is the coupling end held up by the coupling?

Jason,

I have seen this quite a few times, while Mike might be right, changes in vibration are either influenced by changes in the excitation provided by the rotor or changes in the support structure.

You might want to consider that the exciter is under the influence of its first balance resonance at running speed The posted critical speed for these exciter rotors is 1750-1950 rpm. The rotor is overhung from the generator supported only by the #9 bearing. The critical speed can be influenced significantly by the vertical elevation or loading of the #9 bearing. The critical is not well damped, meaning that the critical is very sharp. Any any change in the bearing loading can result in a larger than would be expected change in the response. If you have bearing metal temp there you might see some interesting correlation. This can be mitigated with a balance shot and likely corrected with an alignment change.

We share your curiosity at the causes of vibration changes at our exciter bearing. It seems very sensitive to changes in seal oil temperature, reactive generator load, real generator load, and occasionally hotwell temperature. Vibration commonly cycles daily, as some of the parameters that seem to cause it to change go through daily cycles as well (eg reactive gen load, or ambient temperature effects on seal oil or condenser cooling water temperature).

So...here's my two guesses:
1. If the exciter cubicle temperature cycles at the same period as any of the other parameters just mentioned, maybe the effect is not directly from the cubicle temperature, but from another parameter. It the exciter cubicle temperature does not swing with other parameters, then forget what I said here...and let me try another guess.
2. Increases in cubicle temperature could cause some structural thermal growth that either loads or unloads the exciter bearing. An unloaded bearing may allow the shaft in that location to swing around a little more. We experienced some degradation at the grouted support of our exciter pedestal. It took several buckets of epoxy grout to fill the voids. Potential thermal changes, along with exciter pedestal degradation, could (by my guess) be a possible cause.

Let me guess, this is a Westinghouse Turbine exciter? It sounds like you are experiencing is extremely common problem. Mark Diamond is right on the money with his comments about the 1st critical being excited. The higher temperature in the exciter dog house, causes the #9 bearing pedistal to "grow" increasing load on the bearing, increasing shaft stiffness, and raising the natural frequency. You likely have this problem because either the bearing was not loaded enough during the last alignment (bottom coupling gap too narrow near bearing 8) or the exciter foundation has shifted and gotten soft. If you experiment, you will likely also find that increasing seal oil temperatures will also reduce the #9 bearing vibration. Some plants have been known to run air and hydrogen side seal oil temperatures at 125 to 130 deg. F. to keep vibration low at the #9 bearing. (if you do this, increased hydrogen leaking can result due to increased absorbtion of hydrogen by the oil) Increasing these seal oil temperatures changes stiffness of the shaft at the seals and affects the bending moment which shifts the natural frequency.
So, mitigating actions are to keep exciter air temperature above 110 deg. F. (we run ours between 115 and 120), keep seal oil temperatures above 115 deg and below 125, keep lube oil temperature between 110 and 115 deg.
Final fixes for the problem will involve properly loading the #9 bearing by increasing the bottom coupling gap and/or shimming of the #9 support. (Note that the gap recommended at the coupling by the OEM is rarely sufficient.) Experience has shown that if the bearing is loaded enough to produce bearing metal temperatures of ~150 deg. or higher, then the sensitivity to exciter air and seal oil temperatures significantly decreases.

quote:

Let me guess, this is a Westinghouse Turbine exciter?

Ding, we have a winner. That is exactly what it is. It is reassuring that there are similiar problems out there. I know we are going to work on it in the next year so we can take a look at it then. Thanks for all the input. After reviewing the post and discussing this issue with our turbine engineer it seems like some of our oil temperatures are increased for this issue as well.

Jason,
I have attached an ODMI we used to control our vibration until the problem was corrected. It might be helpful.

ANO-1_Main_Turbine_Brg_9_vibration_ODMI_rev6a.doc (87 Kb, 39 downloads) 9bearingODMI

Ed,
Thanks for the ODMI. That should help our our system engineer get this taken care of until we can make changes. Were you able to correct the vibration by raising the bearing pedestal putting more load on the bearing?

Yes, loading the bearing, thus stiffening the shaft raised the natural frequency enough above running speed that it is not a problem. It is still pretty close to the 1st critical right now and a little sensitive to temperature changes but not enough to concern anyone. Currently running around 3 mils max vibes with seal oil at 115 deg. and exciter air ~110. These temps can vary by as much as 10 deg and vibes will only move around a mil or so up and down. We had to shim the bearing pedistal and put in a 9 mil gap on the coupling. Looking at doing even more next outage, because our bearing temps are still only 142 deg... and our calculated load on the bearing is only 12,500 lbs. The rotor is ~20,000 lbs, and it is necessary to get at least 12,000 lbs of load on the bearing for the 1st critical to be above running speed. I would like to have close to 15000 lbs load on the bearing and temperature up above 150, which would completely eliminate sensitivity to temperatures and provide the lowest vibration. We have run in that range before, but then screwed up the loading after a generator rotor out inspection which is what got us into the condition where we needed the Operating Instructions.
Please note that our unit is 900MW, thus rotor weights will likely be different than yours. I've had a lot of questions about these westinghouse exciters and have found that both the 9 bearing and 11 bearing machines are affected by the problem and everything from 500 to 1200 MW. The overhung exciter is apparently a problem in all sizes if not loaded enough.

This message has been edited. Last edited by: Ed Hudson,