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We have a Frame 5 Gas Turbine driving a Centrifugal Compressor thorugh a gearbox. Since approx 2 months, Gas Turbine exhaust end bearing (Bearing# 2) has been experiencing intermittent vibration peaks as high as 2.0 to 2.7 mils. Normal running vibration is 0.8 to 1 mil. The vibration peaks appear after 18 to 24 hours gap. The vibration increase and decreases over a period of 10 to 20 minutes. When these vibration peaks appear, the Gas Turbine inlet end beraing( Bearing# 1) also shows vibration peaks but with a lot lesser amplitutde. This means that the problem is at the Bearing # 2 area. Machine is fitted with proximity probes and data has been collected using ADRE. Unfortunately, one out of the 2 probes installed at Bearing # 2 is not functional, so the vibration data collected from Bearing # 2 is limited. However from the one working probe, the spectrum shows that it is primarily the 1X component that increases and results in an overall amplitude increase. During the vibration increase period, the phase angle changes by about 30 degrees or so. The vibration peak events are not linked with any RPM or load change. Also these are correlation with ambient temperature change in day or night. The lube oil pressure or temperature is constant.
When the vibration is running normal, the spectrum shows following peaks:
Overall amplitude = 0.98 mils
1X = 0.37 mils
2X = 0.23 mils
3X = 0.2 mils
4X = 0.11 mils
5X = 0.15 mils and so on.

During the high vibration event, only the 1X increases, while the other times speed peaks have negliginle rise.
Original Post
Coupling lock could be a probability, however then it should also show as considerable vibration peaks on the gearbox, which in this case is not. Therefore I would keep this cause as a very low porbability

Light rub is definetly in consideration, however we cannot confirm it till we open the machine for inspection.

Morton Effect does not seem a likely reason, as the idications or conditions for a Morton effect are not present.

Some additional information is given below:
1- The Gas Turbine both bearings are eliptical bore (lemon bore)
2- Gas Turbine is driving a centrifugal compressor through a speed increasing gearbox. Gas Turbine is coupled with the Gearbox through a gear coupling.
Thanks Valve. Oil temperature was amongst the first few items that we played with and found that it has no impact.

Till now I am more and more convined on the partial rubbing phenomenon. The vibration behaviour is more or less similar to that of figure 3 in the attachment with exception of time gap in vibration peaks which could be a function of rate of coke buildup in oil deflector (Labyrinth seals). If I look at the vibration trend, I see a small increasing trend each time the vibration peak is faced. I mean from 0.8, the vibration increase to 1.1 or so mils and then the peak comes (3.0 mils or so)and after the peak the vibration again goes down to 0.8 mils. The phenomenon repeats each time with a differenmt time gap of 18 to 24 hrs.

The question we are being faced with is how severe the problem is and do we need to shutdown machine immediately or can we wait for a month or two.

I will post trends and vibration plots later today, however sinve the 2nd probe is faulty, therefore the vibration plots give limited data, thereby impairing our ability to perform a complete analysis.

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I think this symptom related to oil temperature or pressure if the vibration is not constant. From vibration trend it seem your vibration amplitude happened once at 12 o’clock you have to provide a trend for one week or one month with oil temperature , oil pressure , ambient temperature and inlet gas flow. Also you have to take by ADRE vibration to get orbit, shaft center line and others plots.
quote:
Originally posted by monitor:
Seeing the trend plot it suggest to be a rub, but(!) Direct Orbit plot will give us more info. Can you send us the direct orbits ont its hight vib amplitude and before and after the highest amplitude?


I believe that what you are asking for is not possible since the other proximity probe is malfunctioning.

Ali, I wish you the best to troubleshoot such a problem with the experts here, I am new in the field of turbo-machinery analysis, and I am getting a lot out of what you are talking and discussing.

Regards..

Mohammad
quote:
When these vibration peaks appear, the Gas Turbine inlet end beraing( Bearing# 1) also shows vibration peaks but with a lot lesser amplitutde. This means that the problem is at the Bearing # 2 area.


In general, this is bad logic, not consistent with the physics.

Decide if your informaton suffices. If furhter information is needed, design the test.

Have you looked for correlations with other data and operating conditions (other than ambient temperature)? Does the ambient temperature affect the turbine controls?

You mention only two bearings. So, I assume this is a single shaft frame 5. Correct?

Have you looked at seismic vibration? Temperature can affect the thermal growth of the machine.

I would not rule out rubs (as suggested above), yet. I would be surprised if this relates to the Morton Effect on a single shaft frame 5. The Morton Effect does have similarities to the type of rub investigated by Newkirk (not the Kirk in the references to your article).

I haven't heard of this on a two shaft fram 5, but the situration may be more favorable - wouldn't be my first thougt. The intermittent nature is more consistent with a rub.
Thankyou ALI, Monitor, Mohammad and Bill for practical questions and comments. I will answer/comment against your posts one by one as given below:

Ali, as I mentioned that the 2nd proximity probe is bad (out of service), therefore we are unable to collect orbit or shaft center line data. However I am attaching a 10 days trend from PI on 1st page of the attachment. Regrading your question of vibration plot vs oil temperature, oil pressure, ambient temperature and inlet gas flow, I will answer one by one on each parameter. Oil pressure is constant and trend of oil pressure with vibration shows no co-relation. Ambient temperature has also no effect. The 2nd page of the attachment shows a plot of time at which a maximum vibration peak was recorded. You will notice that the peak vibration time is spread out in 24 hrs of the day. Inlet gas (Gas Turbine fuel) has also been checked and plotted against vibration and no co-relation was observed. Infact, since the problem was noticed, the machine speed and load has been made constant with negligible variation. Regarding oil temperature, we increased oil temperature in early January of this year from 120 to 125 Defgree F and found to effect. However on 21st Feb, the oil temperature was increased to 140 F. After this the vibration peaks didn't go away, however their magnitude has decreased by 1 mils, i.e. earlier the vibration going as high as 6.3 mils goes now as high as 5.3 mils or so.

Monitor, as mentioned above, the 2nd proximity probe is bad and therefore we cannot get orbit.

Mohammad, thankyou for your wishes. Hope all of us will learn form the analysis of this problem.

Bill, you are right this is a single shaft Gas Turbine. Co-relation with all possible process and machine parameters have been checked and found nothing, other then the 20F oil temperature rise decreasing the peak amplitude from 6.3 mils to 5.3 mils as described above.
For the seismic probe data, we don't have siesmic probes installed and niether is it possible to collect data from a portable data collector as the bearing# 2 of the machine(problemetic bearing) is installed in a high temperature tunnel like area with no possible physical access.

One point to note is that the bearings of this turbine are elliptical bearings (lemon bore)

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Hello Ali,

1- The first suspect should be the vibration probes; proximity probes are highly sensitive to electrical interference and can give false high 1X vibration without showing a classic interefernce spectrum, seen this myself on several occasions on Bently equipment. This electrical interference can affect the probes at wiring, at the probe itself [due to high magnetization of shaft], or at the proximeters. Source can be poor grounding attachement at Junction box, poor electrical ground in earth, or poor cable insulation, not grounding the outer signal wire proeprly.

Has someone done welding on the skid before these high vibration events?

In addition, to confirm if its a real vibration or not, you will have to install seismic pickups to mesure vibration on the other cooler bearing #1 , and note does this pickup notice a real change in seismic vibration when the Exhaust bearing #2 x-y probes register high readings?

2- Second possibility is that the Exhaust bearing is loose in its bearing housing, and some intermittant force, maybe a torsional vibration or gear coupling lock-up, is acting to move the shaft which then moves the loose bearing [lack of crush], leading to unstable shaft vibration.

3- If the bearing is relatively old, the vibration could be an unusual vibration we saw on very old and heavy Sulzer gas turbines: Fatigue of the Babbit metal under the shaft causes cracking of babbit and small bits break off, causing a sudden high vibration event, and then the shaft friction melts them back into place. Strange phenomena but we saw it with our own eyes on two occasions...... Eeker

4- A heavy buildup of loose sand or dirt particles has caused similar unusual vibration in GE-Frame 5 turbines, this buildup can be in the Turbine wheel or other rotor locations. On one turbine, this was only discovered after repetaed balancing on a shop balncing machine failed to balance the rotor; when disassembled, two kilograms of sand were found inside the rotor trapped between the Disks.


Thats it, Check for all these possibilities.

Good Luck,

Abdul
Umar,

I had a similar situation several years ago. Frame 5 coupling to helper turbine. In my case, I had two probes where I could get an orbit. This appeared on the outboard of the steam turbine. The vibration was about .5 mils. The vibration would increase to about 2 mils for a few minutes then go back to a .5 mil round orbit. The orbit would go from round orbit to a flat, straight up and down line. It did this several times over several days. The probes were mounted per API (45 degrees of TDC). I thought that I was having a hardware issue with either the electronics or the probe stinger hardware (resonance). I was on the deck investigating the electronics and the hardware when the (gear) steam turbine side coupling hub let loose, went over my head into the cooling tower behind me. (It was over before I knew it happened. The root cause for the hub to come loose was due to a pit that was started in the coupling hub due to lube oil contamination. It turned into a crack that worked its way around the bolt holes on the hub until it couldn't take the torque. To say the least, I will never forget it and have since then shut down machines and inspected couplings (and have found cracks) when I see a intermittent phase shift and amplitude increase. It's probably a torsional response. Hope this helps.
Thnakyou "Rotate" and "Abdul20" for valuable comments. I will respond one by one against each point.

Abdul20, regarding your 1st point of electrical interference, the vibration amplitude and intervals get effected by the oil temperature as mentioned in the discussion above. Therefore I would tend to rule out electrical interference.

On your 2nd point of loose bearing being excited intermittently, yes it could be a possibility, although a loose bearing (inadequate crush) would result in an increase in normal vibration at this bearing which is not the case on this machine. So the probability of this cause would be low.

Your 3rd point of old bearing facing fatigue failure of babbit material can be taken as moderate probability cause as the bearing is infact a couple of years old. We will find if this is the cause or not by end of this month when we are scheduling this machine to be shutdown for inspection.

Your 4th point of sand or dirt getting accumulated in the machine has low probaibility as we have 4 frame 5 gas turbines with us and have never faced this issue ever. In other words, our inlet filtartion is working properly and not allowing any dirt or sand to go inside the machine.

Rotate, your point will make me affraid to go near the machine in this condition Smiler However you have raised a valid point, however in my opinion a cracked coupling hub related vibration will not react to oil temperature. What is your opinion on this.
The coupling we have is also gear type and is connecting the Gas Turbine to a Gearbox.

I have also attached a trend from PI of vibration at 120 F lube oil supply temp and at 140~145 F lube oil supply temp. Notice that after the increase in oil temp, the interval has increased by twice. Also initially the amplitude also decreased.

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Umar,

Have you seen any response from the gearbox?

Increasing the lube oil temp increases (or lowering the viscosity of the oil) the frequency of the occurance increases. Umm. From my experience, it points to the coupling teeth hanging up. The Frame 5 gear type coupling has a tendancy to sludge up. The oil sludges due to the high radiant temperature in the load tunnel. The sludge blocks the oil from getting through the little holes in the back of the hub that supply oil to the gear teeth.

That is why many FR 5 users have switched over to dry type couplings. That creates it's own set of problems with the axial thermal growth, residual oil etc.

Let us know what you find during the inspection.

Rotate
Yes Rotate, coupling lock was one of the very first things that we suspected, however coudn't get any indications pointing towards it. Vibrations on the load gearbox bearing show a slight variation (0.1~0.2 mils only) when the high vibration peak comes on bearing # 2 of the of the turbine.

For lubrication of the coupling gear teeth, there are nozzles installed at each end of the coupling guard that direct oil spray onto the rotating coupling's gear teeth.

For conversion of the gear coupling to dry type, you are right that the dry coupling brings its own problems. In past on 2 of our steam turbine driven compressor trains, we have replaced 6 gear coupling with dry couplings. While 5 never gave us a problem, however the 6th became one big issue for us. This is why we haven't replaced the gear couplings on this gas turbine driven compressor train and some other machines, although we have thought about it a couple of times.

Yup, I will share the findings once the machine gets inspected.

Dear Ex_Mach,

Have you had any change related to process, cooling air system or any action near the vibration has just fluctuated as you said it could be 2 months ago. Have you check the cooling air from the 4th and 10th stage (I suspected as our gas turbine)of compressor? is it normal? Is it any oil leaking from bearing #2?

I agreed that the light rub could happen and I suspected more on Coke deposit  on oil seal. If able, you could change the oil pressure a little bit and see what change, inspect the cooling air flow for any stuck, orifice of cooling air. I think you should collect all data based on the same time scale to see what wrong and what need to focus more.

Regards,

 

 

 

Dear Ex-Mach,

What was concluded back then? What did you find during maintenance / inspection activities after shutdown of machine?

I am facing the same problem in our gas turbine. There are events of high vibration during steady state operation in which vibration increases at inboard bearing whereas 90 deg phase change is observed at  outboard bearing. The event lasts for about 30 minutes.

 

Regards,

 

Certainly sounds like a rub. We had a similar thing on our 6FA where the 1x vibration was intermittent at base load, and with 90 deg phase change at bearing #1. The rub was due to an oil leak at Bearing #2 due to tunnel insulation particles blocking the ventilation holes at the seal. This disrupted the circulation flow and allowed oil to leak out the seal and into the stub shaft wheel space where it cocked up and caused the rubs. Over time the friction of rubbing cause annealing of the stub shaft wheel and put the unit at risk.

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Dear Richard!

Thank you for your response,

We collected steady state and shutdown data with ADRE 208. No evidence of rub found in orbit plot.Only 02 such events were captured where vibration became high at inboard bearing and 90 deg phase change occurred at outboard bearing. Both events lasted for about 30 minutes. I have also attached waterfall and trend plots

 

We are also facing high exhaust spread temperature issues. Could this be a problem?

 

Regards,

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Images (8)
  • Trend 1X
  • Trend 1Y
  • Trend 2X (Generator Side)
  • Trend 2Y (Generator side)
  • 1X waterfall
  • 1Y Waterfall
  • 2X Waterfall (Generator Side)
  • 2Y Waterfall (Generator Side)

Dear JR RE,

If convenient, please share the details of the gas turbine model, make and construction.

What is the frequency of occurrence of this vibration change and any correlation with the specific time duration during a day?

There are few interesting things in your provided data like,

Without the vibration peak, both bearings 1x & 2x phase angles are ~same & similarly 1y & 2y phase angles are ~same which means that the machine rotor is operating in a well colinerar position; probably a rigid rotor.

Well at the time of peak, bearing# 1 phase angle changes by 90degress in the decreasing direction along with the decrease in vibration. However, at the bearing# 2 vibration increases along with the increase in phase angle.

Normally, such symptoms and provided the time span of 30minutes which is considerable duration can be due to oil charring in oil seals. In its aggravated forms sharp corners can be seen in the orbit. I would appreciate if you would share the orbits both during the normal vibration and during vibration excursion. Please check the history of the machine if there were any findings of oil charring on oil seals.

Furthermore, these symptoms can be due to torsional vibration which might be due to some coupling problem. Well you can check the vibration across coupling also to rule out the same.

Last but not the least, what and when is the last maintenance performed on the machine? This might also give some clues.

Regards.