UGB vibration high

[zkz]

Sir,
We have a 130MW generator (375RPM vertical hydro generator) in a powerhouse. When I took charge of Electrical Maintenance Engineer in that power house, I noted that the UGB vibration reading channel of Vibration monitor of that unit was found blocked. I also found that the LGB vibration channel and TGB vibration channel are in service . I enabled UGB vibration channel and found that its value is higher than other two channels. At rated RPM in unexcited mode, the unit showed the following values:
TGB: 2.2 mils; LGB: 1.6mils; UGB: 6.0 mils.
When the unit was excited, the UGB vibration increased to 7.0mils.
Some engineers in that powerhouse said me that UGB vibration alone can not increase without increase in vibrations of LGB and TGB. They explained that UGB channel was showing 'abnormal' values only because of vibration monitor fault and not actual vibration. That is why they blocked that channel in order to avoid unnecessary tripping. I checked UGB vibration with another portable vibration measuring equipment and confirmed that UGB vibration is high.
We are planning to carry out rotor balancing of the unit. We have already checked the tightness of rotor pole keys and wedges. Checked 'centering' (eccentricity) of sliprings with the help of dial gauges. Checked maximum travel of shaft (run out)at UGB end. The shaft diameter is 30 inches. Shaft clearance we got both in X and Y directions were 15mils. Eccentricity of sliprings were 20 mils.
I am kindly inviting your attention for valuable comments about the causes of high vibration and remedies.
Thanks in advance.
Regards,
appunni

 

[wolf39]

zkz:

This is an interesting thread but it should have been better placed in the "Mechanical Accoustics/Vibration Engineering" forum.

Nevertheless: My question is about the location of the thrust bearing. Is it placed below or above the generator rotor?

It looks to me as if you don't give us vibration figures but shaft movement data. Shaft movement figures are not very representative for judging the balance status of your generator rotor.

To give you an example: Your generator is running at rated speed and unexcited. If the rotor is perfectly balanced, the shaft is wobbling around inside the guide bearing in an unpredictable manner, consuming all the bearing clearances, except for the dynamic oil film thickness (this oil film thickness is not constant but changes with temperature). If a certain unbalance is present, this wobble turns into a shaft displacement of about the same magnitude as before but the shaft displacement now is rotating with rated speed. For balancing rotors it is common to use vibration pick-ups placed at the outside of generator guide bearings. The vibration usually is measured as a vibration displacement in single peak micrometers or peak-to-peak micrometers or as vibration speed in mm/s, depending on the type of equipment used.

So, before the forum can give you hints on your vibration issue, let us have more information on the type of vibration equipment installed in the generator and the portable equipment you've used for checking.

Wolf

http://www.hydropower-consult.com

 

[wolf39]

zkz:

The proper term is "vibration velocity", expressed in mm/s, instead of vibration speed.

Regards,

Wolf

http://www.hydropower-consult.com

 

[zkz]

dear sir,

Ours is overhung rotor ie the thrust bearng is above the rotor. The portable instrument we use is IRD Mechanalysis model 350 vibration analyser and Sendig 903 FFT analyzer.
We measure the casing vibration ( support vibration )using the latter one. The body vibration was less than 50 microns at UGB.
regards
appunni

 

[wolf39]

appunni:

With your IRD and Sendig equipment you should use the VEL (velocity) option for balancing. The vibration velocity mm/s figures are telling you how much vibration energy is present. This is the basis for vibration judgement in ISO standards. For hydraulic machinery you have to refer to standard ISO 10816 - 5, where you can find permissible vibration figures.

For literature I can recommend you to go to

http://www.irdbalancing.com.

Then select the technical papers link. Especially the two technical papers at the bottom of the list are very useful.

One more thing: The location where the vibration sensors have to be installed must be selected very carefully. Usually it is the outer oil pot housing wall. However, make sure that the spot you select is exactly behind one of the radial beams (stiffeners) which physically is in line with one of the guide bearing pads. Otherwise you may measure not only the vibration originating from the shaft but the vibration of the oil pot wall as well. In order not to have vibration interference from outside, make sure that no other hydro generator is in operation when balancing.

Hope this helps.

Wolf

http://www.hydropower-consult.com

 

[edison123]

zkz

What sort of vibration monitoring system doe the machine have ? Is it a proximity probe ? At 375 RPM, a casing vibration using a velocity probe may not give right vibrations since their sensitivity falls below 600 RPM.

Also, checking the concentricity of the shaft area is more indicative than that of slipring, which could have worn out unevenly.

 

[wolf39]

zkz:

Before starting with the unit re-balancing you have to make sure that the shaft-plumb, shaft straightness and shaft runout are still within standard tolerances. The question arises when the generator was commissioned. There are quite a few cases where the power house and dam structures changed over the years, resulting in deterioration of turbine and generator vibration levels. So, how old is the unit?

Slipring concentricity is also present when the slipring-to-shaft insulation thickness is unsymmetrical over the shaft circumference. Better measure the shaft concentricity in the vicinity of the slipring arrangement.

Regards,

Wolf

http://www.hydropower-consult.com

 

[zkz]

Sir,

We use proximity sensor for the IRD 5806 online monitor and Direct Prod Velocity Pickup (Moving Coil Type) for IRD 350 analyser.

The manufacturer says the direct prod pickup can be used for speed as low as 50cpm

Now we take vibration displacement. There is option for measuring vibration velocity also.

Regards
appunni

 

[wolf39]

zkz:

Are we talking about one of the Idikki generators?

Regards

Wolf

http://www.hydropower-consult.com

 

[zkz]

Sir,
You are right. We are talking about one of Idukki generators which is approximately 30 years old.
regards,
appunni

 

[marks1080]

I think a harmonic analysis could be warrented here. Do you know if there shaft has been heating up as well?

If purely vibrations, I would keep an eye out for negative sequence harmonics. If you are getting increased thermal activity you will likely see zero sequence harmonics as well.

marks

 

[zkz]

dear sir,
Sorry for belated reply. There was no noticeable heat in the shaft.
Recently we balanced the rotor. Now UGB vibraion shows 1.6 mils (originally it was 6.2mils) , LGB vibration is 3.0 mils (originally it was 1.8mils) and TGB vibration is 2.6mils (originally it was 2.6 mils). We will excite the generating unit after some days as other maintenace works are going on.
Your comments are welcomed..
regards,
appunni