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What are the reasons for high vibrations in turbines?

Until now I've only seen two causes for high vibrations: bad shaft alignment and bad flame dynamics (if it is a gas turbine) or gone wrong warm-up sequence (if it is a steam turbine).

You want also to check your vibration sensors and the settings in the vibration control panel.

If you lose a blade or 2 you will also get vibrations

Vibrations reasons (w.r.t gas turbines) -

1. Lube oil film failure
2. Low oil header temperature
3. Shaft misalignment
4. Water in oil
5. Startup without sufficient ratcheting
6. Sudden change in combustion dynamics
7. Abnormal closure of bleed air valves
8. IGV Blade or compressor blade shear and carryover in gas stream
9. rotor unbalacnce
10. Wear of load gear teeth
11. Faulty measuring device(pickup or cable problem)

Usually GT bearings are equipped with radial, axial and seismic vibration probes. Standard GE HD GT's are equipped with seismic probes as prime vibration protection. For condition monitoring purposes, these additional probes might be required by thr end-user. Sometimes the protection philosophy is based on fast response vibration detection. The radial vibration probes are excellent example of this kind of application: The radial probes will detect the vibration faster as the seismic. Why? Because the oil film and the bearing casing is a kind of barrier which damp the vibration before reaching the seismic. In general, the vibration detection by seismic should be proportional to the radial vibrations.
The following scenarios might cause high vibration to a rotating equipment:

1) Bearing stiffness
2) Misalignment
3) Resonance
4) Aerodynamic forces
5) Tooth wear, couplings
6) Disturbed bearing lubrication

Unbalance is the usual cause of high vibration. This can be caused due to rotor bow due to unequal cool down, and/or damaged blades and buckets, and/or deposits embedded on the axial compressor of the GT. One of the best methods for analyzing of these vibrations is using the so called FFT analyzers. With these analyzers, various trends can be plotted for further analyses. Sometimes even spending millions for high tech conditioning monitoring systems, you still need to open the machine to see where the problems are. I would suggest, spend more money for good bearings and parts then for conditioning monitoring.

Docendo Discumus

Thrust balance issues. Caused from tbv or lack of cdp/hpc bleed.

Hi,

We have experienced high vibration in the Turbines during the start up followed by a forced shutdown to replace IGV servo valve. During this time the cooling was not enough. During the start up vibration went upto 20 mm/sec and it was shutdown immediately. After putting it on barring gear for some more time,then vibration was normal in the start up.

We had another experience.We have 2 similar 7FA turbines. But during the summer time, it is observed that one of the GT's seismic vibration increases with the ambient but still within alarm limits. It happens only to one turbine only even though the LO temp. are same.

Thanks,
MKVISCE

Mr.MKVISCE,

What comes to my mind is the excessive vibration may be due to improper allignment of turbine to gear box and if the coupling is toothed coupling the excessive backlash will cause severe vibrations.

DHAYANANDHAN.S,
INDIA,
sd610 [at] rediffmail.com

98% of the time on GE Industrial Gas Turbines the cause of higher than normal vibration levels is imbalance or mis-alignment. Imbalance can be caused by FOD, IOD, errosion, corrossion, etc. Mis-alignmnet is self explanatory

Mr.Dhayanandhan,

But how the vibrations are going up when the ambient is going up and that too on only one machine. If it is hardware problem, it should be continuous. It reaches 7 mm/sec in the day time and 5 in night time.

Agreed that the case as you say is true (fluctuation in vibrations during day and night), but have you noticed any other changes viz. change in frequency, bearing metal temperatures, return oil temperatures, loading pattern, electrical parameters like exciter voltage etc.

A lot of parameters need to be considered rather than just the timing.

Next, is it sequential (on a daily basis)or is an intermittent happening

Do not over look the importance of checking that all bleed valves are operating properly. Assure they close and open "when" they are suppose to.

I have exactly the same problem here with a Frame 6 that has higher than normal vibration levels which go higher daytime than night time. I don't know you location, but here we get > 40 Deg. C daytime and 20-25 at night. Two problems, your L.O. header will probably be above spec daytime and, if your units are outside, you tend to get a slight alignment change daytime with heat on one side of the machine. The alignment can still be within spec. but changes. Both of these can affect vibration levels.

> What are the reasons for high vibrations in turbines? <

Does anyone have experience with AXIAL pedestal vibrations?
These are vibrations picked up on the bearing pedestal looking parallel to the shaft.

After a major steam turbine and Generator Outage we had big (16mm/sec) AXIAL vibrations on the last Bearing, Generator NDE. One Vibration expert stated that the Rotor may not be in its electrical field (axially)

Hasadam...

1) Is vibration period sympathetic with rpm.

2) Does vibration amplitude change with Gen Load?

3) Is vibration present when TG on turning gear, or when TG-set is slowing down?

Regards, Phil Corso (cepsicon [at] aol.com)