Fan Vibration Changing by Inlet Gas Temperature.

Dear Experts

One of our Exhaust Fan has an interesting vibration behavior. The Vibration of this fan changes with inlet gas temperature change! (It change between 1 to 7 in horizontal vibration at point 4) But this changing is not in same direction, sometimes the vibration increases with temperature increase and some times it is decreases. The Velocity spectrum shows 1X is dominant. Another interesting thing is the magnitude of horizontal and vertical vibration: for point 4 the horizontal vibration is 7 but the vertical vibration is 0.2 mm/s RMS!

Please download the attachment file, it contains some vibration graph and technical data about fan.

I will appreciate you, if you can help me to find the problem.

Attachments

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Original Post

Wow! What a presentation. It should be published on the top of the site as a good example.  Going back to the theme. I think you partly answered to the original question in the ˝brief history˝. I hope you have already tried to balance the impeller after that thermal shock happened. IMO the increased temperature caused some structural changes, most possible on the weakest part - the impeller. It is also very possible that the shaft got some eccentricity while being stopped for 8 h at 250°C. In those conditions the rotor should took some rotation after some short interval!

The vibration difference between H and V direction could be due to some loseness (have you checked the bolts?) or due to difference in structural stiffness between those two directions gained by the increased unbalance/eccentricity.

The vibration difference between different temperature conditions are not so unusual specially in your case of possible previous structural changes. The operating temperatures are quite high.

Anyway I have some thoughts on your vibration alarm limits. 4,5 mm/s for such a brutal machine? I would definitely go to 11,2. So from this point of view I would ask myself if your condition is really so critical at all and would make some additional measurements and monitor the trend.

I would definitely:

- check all the main bolts

- try to balance

- measure shaft and impeller eccentricity

Becar posted:

Wow! What a presentation. It should be published on the top of the site as a good example.  Going back to the theme. I think you partly answered to the original question in the ˝brief history˝. I hope you have already tried to balance the impeller after that thermal shock happened. IMO the increased temperature caused some structural changes, most possible on the weakest part - the impeller. It is also very possible that the shaft got some eccentricity while being stopped for 8 h at 250°C. In those conditions the rotor should took some rotation after some short interval!

The vibration difference between H and V direction could be due to some loseness (have you checked the bolts?) or due to difference in structural stiffness between those two directions gained by the increased unbalance/eccentricity.

The vibration difference between different temperature conditions are not so unusual specially in your case of possible previous structural changes. The operating temperatures are quite high.

Anyway I have some thoughts on your vibration alarm limits. 4,5 mm/s for such a brutal machine? I would definitely go to 11,2. So from this point of view I would ask myself if your condition is really so critical at all and would make some additional measurements and monitor the trend.

I would definitely:

- check all the main bolts

- try to balance

- measure shaft and impeller eccentricity

Dear Mr. Becar

Thanks for your respond. I checked  main bolts and found another interesting thing. Please download the attachment file, it is a fig. Refer to the point 4 fig. the vibration of "A" is 0.2 mm/s but the vibration of "C" is 1.87 mm/s! and the phase between A - B is 120* and C-D is 4*!

how it can be happened?

Attachments

Photos (1)

Ptayebi, have you checked bolt tightness or bolt vibration only? Have you found any bolt looseness or soft foot? Any construction cracks around bearings? If not, I would say shaft eccentricity could be the source of that vertical angle difference. 

Rotating Guy posted:

Hi,

Refer to below picture, can you give me the phase reading? Put the probe at bolt when you’re getting phase for B & D

 

083FDAC3-089F-4547-AA70-13DF033D2991

Dear rotating guy

Thanks for your respond. 

A-B: was not stable,first time:-160* and second time:104*

B-C: was not stable,first time :-110* and second time :170*

A-D: was stable, 3*

D-E: was stable,4*

Becar posted:

Sorry, previously I didn't see everything correctly... now I am quite sure you are dealing with bolt looseness or ˝soft foot˝ on that particular place. Be sure to check bolts correctly because you have safety nut involved!

Dear Becar

If we have any soft foot,  how it can change with inlet gas temperature? The outside of fan and pillowblock temperature is not going so high! Some times the vibration reaches to 1 mm/s!

I didn't mean the connection between soft foot and the temperature. I am talking about the difference between H - V vibration level in connection with bolt looseness or soft foot. Your measured phase angles are just one more indicator for this.

The temperature shock could influence overall vibration levels but is not connected with the soft foot. These are two different issues.

Although it is possible the increased levels (higher unbalance/eccentricity) made bolts to loose.

Ptayebi posted:
Rotating Guy posted:

Hi,

Refer to below picture, can you give me the phase reading? Put the probe at bolt when you’re getting phase for B & D

 

083FDAC3-089F-4547-AA70-13DF033D2991

Dear rotating guy

Thanks for your respond. 

A-B: was not stable,first time:-160* and second time:104*

B-C: was not stable,first time :-110* and second time :170*

A-D: was stable, 3*

D-E: was stable,4*

What is your reference point, you do xphase but i forgot you to get the A-A, that will be your reference, A-A should be zero or +-20 degree tolerance, 340 - 360(0), or 360(0) - 20, then that will be the result of A-B or within that range, if it is something loose, if there is, try to tigthen the bolt and see the changes of phase, same with BD subsequently in A-D and D-E, remember that these forces will serve to amply nor to excite whatever forces in your machine. In my perception, due to these forces your rotor is rotating eccentric that is why it gives you a time signal of sinusoidal at horizontal, why horizontal? It is due to stiffness, as i remember in rotor dynamics. Anyhow, if you do impact test to this it will give you near at resonant range which will assump you on structural issue but in reality you have a softfoot/looseness. 

I agree on what Mr. Becar perception.

Iregards, 

RG

simple dont worry

you have good equipment vibration analyser

   Fan DE

Vertical            -  Velocity (reading mm/s)

Horizontal     - Velocity(reading mm/s)  ,Acceleration(reading m/s2)

Acceleration - Displacement.(microns)

 Fan NDE

Vertical            -  Velocity (reading mm/s)

Horizontal     - Velocity(reading mm/s)  ,Acceleration(reading m/s2)

Acceleration - Displacement.(microns)

 Motor  DE

Vertical            -  Velocity (reading mm/s)

Horizontal     - Velocity(reading mm/s)  ,Acceleration(reading m/s2)

Acceleration - Displacement.(microns)

Motor NDE

Vertical            -  Velocity (reading mm/s)

Horizontal     - Velocity(reading mm/s)  ,Acceleration(reading m/s2)

Acceleration - Displacement.(microns)

fill and send back as per my requirement i will give feedback 

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