Booster Fan Vibration Problems 1

[mtigos]

Can anybody help.? I am experiencing a repetitive imbalance problem on a two stage Booster Fan. We call experts to come in and they perform balancing (on the Impeller) and the following week the problem comes back again. I have been suspecting the Flue-gas. Currently the Bag-Filters are worn out and when you look into the Booster Fan Housing you can see the blades are wearing off. They are very sharp and I was suspecting that the Flue-gas has too much abrasive material from the Boiler, hence the wearing off of the impeller and probably resulting in increased vibration. BUT my superiors are suspecting the Support structure (Concrete base) and I JUST dont agree with them. The support structure was built in 1991, can the support structure have such a high level input into the Booster Fan getting out of balance?

 

[MotorVib]

I would look at the boiler, if it is some kind of abrasive material you should be able to see wear it is coming from or see more damage after the fan.

Chris

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

 

[rmw]

You haven't said much about what the process is or what the bag filters are letting get by but it sounds like in addition to abrasive wear (which usually occurs relatively symetrically-all blades suffer alike) you may be getting material build up which is throwing you off balance. With material build up, it can build up symetrically, but then some of it sloughs off and that WON'T happen symetrically and that can make imbalance appear suddenly. Sometimes the imbalance and resultant shaking will then dislodge more of the build up and somewhat alleviate the problem so the imbalance can come and go.

Let us know more about what it is that you are having to deal with with this fan.

rmw

 

[mtigos]

Thanks to everyone's responce, especially "rmw", the boiler FLUE-GASES go through the bag filters before being extracted through the stack. Some of the flue-gases are sucked by two Booster fans and channeled to a kind of scrubber where soft water and a bit of soda-ash are added to let CO2 escape alone while trapping the SO2. The CO2 is critical as it is needed by the MEA Plant before going on to the Carbonation Plant. I am working for a Soda Ash manufacturing plant. You right when you say the wear appears symmetrical, that I have noticed. The problem is the vibration levels just keep peaking on a weekly basis. The guy comes in a does a trial-and-error job of welding weights on the fan blade tips and it gets better, then the next week its out. What is going on here???? Please help.

 

[TPL]

We call experts to come in and they perform balancing (on the Impeller) and the following week the problem comes back again

The guy comes in a does a trial-and-error job of welding weights on the fan blade tips and it gets better, then the next week its out

They are not really experts are they?

As a start, I would suggest that you number the blades and take digital photos of every one each time they are balanced to 'prove' the wear theory.

I would suggest that you get your operations people to trend the fan vibration to see if it gradually increases over the week or goes up in a sequence of step changes?

Did the appearance of this problem coincide with the bag filters being worn out? What is your definition of worn out - are they physically damaged with holes in them or are they blocked? Why don't you just replace the filters with new?

This might not be a balance issue.

You really need to keep a log of how much weight is added and where - a proper balancing routine, would be useful, so you can track the amount of weight and the angle it was added at. This would allow you to calculate the influence coefficient (crudely how much weight corresponds to how much vibration, but this is a vector quantity and the angle is needed)- if this was truly a balance problem, then the influence coefficients would be relatively constant - if there is wide variation, then maybe unbalance is not the issue.

You need some data to figure this out, otherwise you will stay in the 'suck it and see' mode until the fan can no longer be rebalanced.

Here are some things you might want to consider that can produce these symptoms:

a) Cracked shaft
b) Cracked foundation
c) Impeller loose on shaft - not so loose that you can rattle it about, but loose enough to slowly slip around over a period of a week or so

 

[mtigos]

Thanks guys. You guys are the real experts. Working in Africa is a nightmare.!!!
I will take every point suggested and look deeper.
The vibration problem seems to have come about at the same time the bag filters failed, at least thats what the Technicians are telling me. I am new in the Reliability section but not the plant. I have never heard of this vibes problem until about towards the end of last year, hence my total disapproval of what some of my colleagues are suggesting, "that the support structure isnt stiff enough"....this is because there are no visible cracks on the concrete foundation and other steel structure supporting the Assembly.

TPL, you asked about the worn bag-filters, they failed, as in holes to the point where you can see dust coming out through the stack and tracks of boiler sand in the fan casing.
The bags have been ordered and they are simply not arriving, AFRICA.!!! I love this place!!!!!!!!!

 

[rmw]

Are your fan blades flat, curved, or do they have an airfoil profile?

rmw

 

[Strong]

I balance an induced draft fan (1500 hp) at a small power plant many times. I make sure the fan is thoroughly cleaned before balancing. This may take 4 to 8 hours. Balance can change quickly due to temperature change, if the rotor bowed after cold shutdown. Ocasionally the fan would get "foam" when the upstream scrubber level overflowed. Erosion is usually a long term effect on balance, but hot gas and wet upstream/downstream dust fouling can be short-term problems. There may be other issues affecting balance such as bearings, shaft alignment, loose wheel on shaft, cracked blades or shroud, water/dust inside hollow blades, balancing method, residual unbalance after balancing, and operating speed close to a resonant frequency.

Walt

 

[MAPower]

Do you have any of the vibration data available? Are the vibration levels the same each time the fan becomes imbalanced? Some data would help diagnose your problem.

It is possible you have a process problem rather than a mechanical problem with the fan. Such as rmw said about the material buildup on the wheel, this can appear as if you have an imbalance on the wheel itself, but is and imbalance being produced by the material buildup. Clean your wheel via soft sandblasting if you can. If this is your problem and it is related to the filter, then it will keep coming back until you get that new filter.

Do you have vibration spectrums and waveforms to look at? This would also be helpful.

 

[Tmoose]

I'd be interested in the approximate weight and correction radius that brings the 1X vibration from "problem" to OK.
(Assuming single plane balance correction)

If it is much less than 3 or 4 times what is recommended for ISO G6.3 I'd say the structure is weak, or worse.

If it is equal or greater than 3 or 4 times G6.3 I'd say the fan is losing material at a too rapid rate.

ISO G6.3 based on rotor weight and rpm can be determined on page 3 (inch) or 4 (metric) here -

http://www.irdbalancing.com/downloads/TechPaper1BalQualityReqmts.pdf.