As stated before I am new to this so here I go again. I have a boiler I.D. fan that we have had problems with goimg out of balance with due to ash build up on the fan blades. This fan is a 66" new york Bi-Forty fan with a 5-7/16 shaft 71" long running at 1170 r.p.m. The last four days we have had to stop and clean this fan once a day were as previously it would go almost a week. All the opperators are crying bearing and kind of have me second guessing myself. We just got done cleaning the fan and my readings were as follows befor cleaning I was at 1.2 ip/21mils-pk to pk in twf was 1.81 all one times and 100% synch energy. After cleaning the fan I am now at .2 ips/3.5 mils-pk tp pk twf is .763 95 sync. energy. I believe that I am corrcet in saying that there is no problem with the bearing but I am loosing my confidence real FAST!!!!

wpjlg1,
Hang in there bud, believe in your instruments. You won't get good readings immediately after a cleaning if a bearing fault is causing 1.2 ips. What I want to know is why you have to clean it so often. And to think I was concerned because we can't make 3 months between cleaning. Has anyone looked into why you get buildup so fast?

David Eason

Good Evening
A couple of questions.....

Have you been able to observe the fan before cleaning to compare it with how dirty it used to be when cleaning was required? In other words is it actually getting as dirty now in less time? If it is have ya'll changed anything like fuel type, firebox temps, changed air flow distribution in the combustor, Nox suppression, anything that could explain why it is getting dirty faster.

If not is it possible that you have a resonance condition that is causing the fan to be more sensitive to imbalance. Taking less weight to effect the vibration as resonance acts as an amplifier for any forcing function. In this case a small amount of imbalance that would previously not have made a dramatic effect in the vibration now does. Is the ration between your vertical to horizontal vibration greater than 3:1? Have you checked the phase vert-horz-axial? Is this an over hung or center hung fan?
Check the hold down bolts on the housings and the foundation for cracks or other signs of weakness. This kind of stuff can cause the natural frequency of your fan to change causing a resonance as it may be near the running speed.

Hope this helped

If it has not developed a resonance at running speed, I think you have a flyash problem. Something is leaking through, like maybe your airlock rotors are not catching the flyash as they should(if your boiler is equipped with them). Maybe you have a tube leak that is allowing water to be sucked in with the fly ash and sticking to the blades.

Is this a constant speed or variable speed fan?

wpjlg1,
Take the advise written in the replies.List them down.Check them off one by one.They are spot on. As David mentioned believe in your instruments and have confidence in your diagnostics.Just remember to look at what's causing consistant problems. Think positive and think of this as a good opportunity to prove cynics wrong.After all it is all black magic to alot of people.And I know starting out can be very hard to gain confidance in people.I'm sure we've all been there.

Thanks for the advise guys. Yes there is a problem with our pre-heater tubes and we are hoping to get a package boiler in here so we can do the maintenance that is desperatly needed on our 20+ year old boiler. I was in here at 5:00 am this morning and went around and tightened all the mounting bolts on the base and bearing housings. This didn't come to me until after a couple of beers last night thats how frustrated they had me yesterday. I believe that was my problem as I found the OB bearing mount bolts were a little loose and it calmed the fan down after I tightened them. By the way what is meant by the ratio between hor and vert being greater than 3:1 and if it is what does this mean? Thanks again for the advise guys I just wish that I would have found this site 1 year and 2 1/2 months ago!!!

wpjlg1,
If you keep coming in at 5:00, somebody's going to put you on steady.

Suppose you collected your data and you got a peak at running speed in all your readings. The peak in the horizontal point is .6 ips while the peak in the vertical point is at .2 ips. That would be a ratio of 3:1. If you have an unbalanced fan, the peaks should be roughly the same, but since one is so much higher than the other, this SUGGESTS that the energy is mainly in one direction. Resonance is highly directional and MAY produce the high ratio that everybody is asking about.

David Eason

Well I just took some more readings and here's what I've got, IBH .578 ips and IBV .246 ips and OBH .621 ips and OBV .222 ips not exactly 3:1 but real close the difference being .06 ips IB and .015 ips OB to be exactly 3:1 ratio. With this said I am running at or near resonance correct and I believe that this is being caused from base fractures that were found this afternoon and possible bearing bolts being (slightly)loose the bolts need to be replaced on our next down day and also a fair amount of welding and stiffening done to the outboard end of the fan base itself where a crack was found burried in dust. Am I on the right path or not??

WPJLG1,
Sounds like you are on the right track. One thing you can do to determine for sure if you have resonance condition at or near your run speed is to do a bump test. Basically you attach your accelerometer to the bearing housing and use an object to strike the bearing in the direction of the accel. I like to use a piece of 4x4 and strike with the but end. You should not have to hit it hard to get a measurable effect. The peaks that are then in your spectrum are the natural frequencies of that bearing housing. If these are near or at your run speed then this is a resonance condition. These frequencies can change or be changed based upon the stiffness, dampening, and/or mass of the bearing housing/foundation. This test should be done with the machine off.

With CSI analyzers I can do this with the machine on using a negative averaging function. I have never laid eyes on an Empac so maybe someone out there can tell you if this can be done on line using Empac.

You said that you have a problem with preheater tubes could this be causing an increase in the build up on the ID Fan?

Finding the hold down bolts loose was a very good thing and to have this positively effect your amplitude is definitely a step in the right direction. You see with these bolts loose it changed the stiffness of the machine thus changing the natural frequencies. Fixing the other problems should also have a positive effect on the overall vibration.

Our ID fans are very large two speed units that are center hung, we have very little trouble with them. Most run below .05 Ips. We have more trouble with our overhung PA fans.

Keep your head up and know that this has verified your feelings that this was not a bearing issue as operations were so sure about.

When I was working at the Paper Mill there was a supervisor there with 35 Plus years at the mill. A very knowledgeable fellow about all of the equipment and how it worked. One day the fan pump began making noise. Spectrum showed a mound of energy indicating a cavatation condition. It sounded like cavitaion with a strange bumping. Peakvue showed no bearing faults. This supervisor laid his hand on the outboard bearing and declared that bearing was bad. Over my recommendations they took a forced outage of the mill and spent $25000.00 to replace this pump. I asked for the bearings when the pump was tore down. Well the bearings looked like brand new. What we found was that a nozzle in the couch pit had broke off and was lodged in the suction of the fan pump causing the cavitation and the bumping.

Believe your analyzer, know your equipment and believe in yourself. It is always better than a calibrated hand no matter how much experience it has believe me

I have a problem with an I.D. fan building up
with saltcake that has to be cleaned every three
weeks to a month but I have never seen a build up on our ash boiler I.D. fan.Right now the recovery boiler fan is running at 12 mils because the coupling was not lined up properly.
Nobody wants to put it down to do it right.When it reached 3mils before it was slated to be cleaned but now after cleaning if it gets down
to 3 mils it's a miracle.The coupling is a fluid drive scoop control type SCR.14.
It seems to me even before I got into vibration
monitoring that as soon as there is a problem
bearing are the first culprit as far as operations are concerned.

H:V=3:1 or even higher ratio is far from being sufficient to declare a resonance. In combination with high unbalance sensitivity it just warrants a resonance test either by performing a bump test with phase or a coastdown test with phase. Erratic phase reading at operating speed will also be an indication of resonance proximity.

Excessive clearance can be a problem, but I don't see how it causes imbalance. If this were so, then a 20 mil clearance in a fluid film bearing with a 100,000 lb rotor would create incredible forces, which doesn't happen.

I would expect excessive clearance to show as looseness (multiples of 1x)

If you have enough clearance to shift the center of mass it will be an imbalance force. I tried to balance a fan once that had excessive bearing clearance and every time I would stop and start the fan after I got it close to being balanced the mass center would shift to a different location and the phase would be completely different.

Please post your spectra in acceleration @2kHz.

New York blowers are typically good economical blower IMHO. They are extremely sensitive to setup, imbalance and resonance. Have you calculated the weight of the build-up? Also measure the distortation of the housing and case around the bearings. I think you will find distoration which will in turn cuase a high 1X and appear as imbalance when it's binding at the bearings. Have you monitored temperature of the bearings? I think you will find an increase in temp.

Consider stiffners!

Some years back, we cleaned our big ID fans on line by making up a hopper connected to the fan inlet box. River sand was shovelled into the hopper, drawn into the fan, and must have had some effect in cleaning ash off the blades as the vibration went down.

I used to joke that one day the hopper shape at the bottom of the stack would collapse with the weight of sand collected there over the years - but it never has! We must have blessed the surrounding area with finely spread out sand - but I know of no complaints.

Worth a try? Ray Beebe

I have often wondered why you couldn't just use a permanent manifold to direct high pressure air (or sand) onto the wheel. Heavy wall pipe with nozzles welded in would work -- you'd want to extend it through the housing on either side and weld circumferentially inside, and outside.

Rusty,
That's how the steam engines used to do it. The fireman would shovel a couple of scoops of sand into the boiler. They were generally careful to do this away from laundry hanging on the lines because of the soot that would be expelled.

Sorry, I meant firebox not boiler.

quote:

Originally posted by Rusty:
Machines with fluid film bearings are heavy enough that clearance does not create imbalance. But I've seen smaller 3600 rpm motors with sleeve bearings that showed imbalance symptoms when there was excessive clearance (wear) in the bearings.

Physics is physics. Clearance affects the stiffness and damping (i.e. restraints) and not the imbalance, which is rotor related. One of the first symptoms expected from a bearing with too much clearance is an increased 1x vibration. This in no way means that the imbalacne changed. In these cases without extenuating circumstances, correct the clearance and geometry of the bearing and the issue is corrected, no need to balance.