Hello members

Question arise about the horizantal vibration in the foundation of the FDF (force draft fan) .the boiler have two FDF one left and the other right .There is acable chanal between them in that chanal we have upnormal horizantal vibration .the fans was balance as per specifcation .
The question is that what the case induce vibration in the chanal side (horizantal vibration).

some indication may be useful
1-The aerodynamic flow make that vibration.
2-The profile of the fans blade make that deffect.
3-The clearance between the blades tips and the fan case have effect on vibration .
4-The eccenteric between the fan and the case have effect .
5-The concret foundation is rigid and have no crack and we have no vibration before that time .
I need some comment about this case and how to improve it .
thank you very much and god bless.

jamil a alshahed nama
hartha power station

it is necessary to know The frequency of the vibration(s) of interest.

If #3 is a statement that you have determined that the fan blade tip clearance, I'd expect the predominant vibration to be at "blade pass" frequency.

If the vibration on the fan bearings is acceptable, and the high vibration is on the cable "channel" (cable tray?) then the mounting of the cable tray is suspect, regardless of the frequency.

.the fans was balance as per specifcation

Jamil -I would strongly suggest that you revisit this statement and let us know what the specification was, how the balancing was done (workshop or in-situ) and at what speed.

ISO 1940 recommends that a balance grade of G6.3 is OK for fans - I would suggest that G1.0 is much more appropriate and just as easily achieved (Gx refers to the acceptable residual unbalance and is based on machine operating speed and rotor mass) - and that is assuming that the people doing the balancing actually did the job properly. I have lost count of the number of times I have been told that
"after maintenance including balancing, vibration was much higher than before" only to find that the balance was badly carried out.

Other than balance, what about alignment - how was that carried out? Poor alignment can result in higher vibration in one direction?

Are thre any dampers or other flow control mechanisms in the air path? Are these set up correctly?

As TMoose said, yu need to know the frequency of the problem vibration.



 

First, allow me to summarize my understanding of the question.  There is a high vibration in some steel structure connected to two fans. This vibration did not exist in the past, but does now.  You believe that the fans have been properly balanced.  

Both of the previous replies are absolutely correct and appropriate.  But, based on my interpretation of the problem, I feel I can make a few comments.  

If a vibration has increased, then something changed.  The change could be an increase in the excitation or a change in the response of the steel structure.  The excitation could come from imbalance, misalignment, surging, speed change, mechanical damage or a number of other causes.  If there are known changes in any of these, then that should suggest that they may be the cause.  If the blower was recently rebalanced, then TPL is absolutely correct. The balance may not have been performed correctly or the balance specification may not have been adequate.  The same point could be made regarding alignment.  If the operation of the system has changed (flow rate, louver operation or speed), then the new operating conditions could be causing a new excitation.  We have some fans that experience very high vibration at low flow.  For one particular fan, we were able to establish that when the louvers closed below 20%, the fan would experience surge condition.

If there was no change in the excitation, then there could be a change in the response of the steel structure.  Any change that could have altered the mass or stiffness could have changed the natural frequency of the structure so that it is now resonant at the same frequency as some vibration produced by the fan (fan running speed, two times fan running speed, vane pass frequency, gear mesh frequency, driver running speed, etc.)  This could be checked by changing the mass or stiffness of the structure.  Depending on how it's made, that can be very easy.  If you place a bag of sand on top of the structure and the vibration stops, then it would suggest it was resonating and the extra mass de-tuned it so it does not resonate at that frequency any more.  

But, as noted in the first reply by Tmoose, you need to know that the frequency content of the vibration is.  That would eliminate at lot of the guess work.  
 

Johnny Pellin

Can you isolate one fan at a time and check for vibration?
Check which fan is responsible for the vibration.
As mentioned you can tell from the frequency if you have a clearance issue, bearings issue, etc.

Your description of this boiler system is not so clear. What's is the temperature these fans "see" from the boiler's side?  

 

Hello
My great thanks to all member reply to this case .All replys are useful and have good recomendation .It gives us good information to anlyies the case of vibration.

I attachthe last reading of vibration and some illustration drawing of fans and the location of the chanal my be useful in anlyises of the case .
Thank you very much and god bless .

jamil a alshahed nama
hartha power station

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I looked at your numbers first what are your units and take a look are you coulping the Axial reading looks way to high on FDF-6B on 19/01/2009.  As a generial rule Axial should be far less then all the other reading unless there is something that you know is causing an Axial vibration.  In an ideal world I like Axial to see something about 10% to 20% of max reading.

Chris

"In this house, we obey the laws of thermodynamics." Homer Simpson

Check bearing caps torquing values (or is it the bases only? anyway check them too).

What is the motor speed at each test?
Number of balls in each bearing?

The impeler is the fan, correct?

What are the units of vibration? I would guess microns.

Horizontal vibration on the 19th is much lower than on the 19th - - any idea why this might be? Was the speed different? Was any maintenance work carried out between these dates? Were the fan impellers washed/cleaned? Does the vibration change of its own accord or is it load/temperature related?

Bearing number 4, nearest the fan impeller shows highest vibration, suggesting that unbalance is a likely cause of the problem.

You cannot go any further with this issue unless you have some solid data to allow a diagnosis to take place.

If these were my fans I would be loking to do the following:

Trend overall amplitudes and 1X data (amplitudes and phase) over a couple of days to see what happened.

Monitor a shutdown to identify resonances/critical speeds

Horizontal vibration on the 19th is much lower than on the 19th - that doesn't make much sense does it? I meant to type lower on the 19th than 18th

Dear Jamil,

I'll recomend you first to balance your fans in place (try to reach fan casing and try to weld some mass, not much, just to see influence on bearing casings and in cable channel). If you see some influence on cable channel horizontal vibrations, then balance your fan as low as possible. If this fan has got vibration dampers or isolators, you shoul'd check them.

Hope this helps,

Neven