Rodney, if you have not already, make a bump test to find the critical stand still cold. Bump with a soft hand at 6 o´clock at the rim and measure just aside, also axially. First major peak? Then repeat at 3 or 9 o´clock. More damped, shifted to what frequency? Actual operation speed - nominally and today´s demands?

One more effect of the heat can be that the hub main disk stretches and opens the shaft hole to a variable play situation. Do not trust the solidity of the shaft fit from the cold situation.

If you find the crit too near, I have tuning suggestions. The last thing I would do is to change material. A plain waste of time and dollars, - been there, seen that mistake already.

Markoleo wrote :

quote:

In the Aircraft industry, impellers, turbines and other rotating pieces of equipment are running in extreme temperature, yet that phenomena does not happen.
However, they are using very sophisticated and controlled material.

Rodney :
There are people specilized about.
Contact :
.William Research Corporation and
.Pratt and whitney

Rodney, do you have any amplitude and phase info for the bearings in the axial direction? If so, could you post this?

Does the vertical and horizontal phase difference seem to be the same (more or less) for these fans?

Could you possibly perform a bump test on the bearing pedastals in the vertical direction?

Is is possible that that any of the mounting bolts for the pedastals could be loose?

Yes, Matter of fact I took our phase strobe light out and took some readings Thursday on the fan.

I didn't get the axial, readings but did take the horizonal and vertical on both bearings...The amplitudes are higher on the horizonal, than the vertical, which is typical for unbalance, but the difference is that the phase differential at the vertical positions are excactly 77 degrees..They are the same at the horizonal position...

Looking like I may have a dyanamic balance problem. Imbalance on both planes of the wheel. No surprise because I can actually see some wobble on one of these troublesome fans. Which would most likely give me high axial readings...

I'm at home now, but perhaps Monday I can send some data over the forum...

Thanks for your interest!

Rodney

Miquel,

I spent 7 years in the Air Force, and 8 years in the general aviation industry. I was an aircraft mechanic and avionics tech. Spent alot of time inspecting compressor blades on multi-stage axial-flow jet engines. Titanium was and probably still is the metal of choice due to it's light weight and strength. Definately has to be well balanced, and if one of those rows of blades becomes damaged, you'll know it immediately. God help anyone standing around or flying on an airplane when one of those disentigrates!! Oh, BTW I can't hear anymore, hehe...

R. Bell
USAF 1971-77
Mech. T-39...T-38...F-4D,E's
Vietnam Vet

For material suggest you can ask :
[/URL]link

This message has been edited. Last edited by: Miguel Kovac,

Rodney, you certainly have a lot to go on now with all the input from the other guys. Here's my two cents, have you taken a dial indicator runout on the shafts. Near the wheel, is it possible that the shaft may have sagged and is now whipping?

Sure did Yvan...I changed the bearings on one of those problem fans. Put a dial indicator on it on both ends. Looked okay. Less than .001

Rod

Rodney,

I find myself in the same situation you were/are in with some hot tail gas fans. We balanced them cold, and when put back into production, they have gone back up. One of them worse than before, the other two a little better than before, but worse than when we finished balancing them.

Did you ever arrive at any conclusions, or find a cause/cure?

Dave

Hello Dave,

Matter of fact, on Monday we are having a little two day shutdown, we will replace that wheel and shaft with a new one, then install some Dodge Imperial bearings, instead of the traditional SKF bearings.

Also I recommended slowing down the fan like it was before we started having these problems. Hopefully that will solve the problem. These overhung fans are just too large to run up to speeds of 26 to 2800 RPM...And with that heat you know these fans as old as they are probably are developing a wobble, the way they hang off at the back.

Here lately with these fans you don't know what to do about the balance, and when to do it. Seems nothing helps.These fans ran good for years, now all of a sudden these problems develope. Thats why I'm thinking the integrity of the metal has sowly been delaminating, and has lost it's structural soundness...

I'm anxious to get my hands on it...I'll let you know how it turns out...

See Ya,

Rod

Rodney,

I just jumping in at this point. If fan speed is higher than in the past, then the current speed may be closer to a natural frequency associated with fan shaft, fan wheel, or support/foundation. Structural vibration tests could easily identify natural frequencies. I recommend measuring 1xSS amplitude and phase during coastdown and also at startup going from cold to hot. If the unbalance vector change is consistant from cold to hot, then it may be possible to balance cold; such that when hot it runs smooth (like hot alignment offset). I do believe there is one or more natural frequencies, and your idea of reducing fan speed is a good one.

Walt

Thanks Rodney,

The speed of these buggers worries me. They are running in the 2900-3000 CPM range, and I suspect near critical, from the shakes. They are sitting on isolators, of which some are collapsed.
We plan on doing some bumps on them first opportunity, and I will have my dial indicators with me the next time (phase didn't suggest it around the bearing, but ....), since I now know what I'm dealing with. I do not believe they run them until the dryers cool off, just shut them off. Hard to tell what we'll find.
Keep posting.

Dave

Rodney,

I have been out of the field for a while so I hope this response makes sense and is of some help. I have balanced a few overhung fans in my day...

As the fans once ran OK I would focus upon what has changed:

1) The addition of 25-35 lbs. on each rotor due to the addition of balance weights.
2) The increased operational speed due to production demands.

I suspect both changes are interrelated. The rotor weight addition has changed the system resonance situation as speed increases to meet production demands.

Two things are clear: if the fans are balanced before shutting them down (under .5 mils), they should be balanced upon restarting the system. I don't know what increased vibration levels we are talking about (observed increase at start-up), but if possible, allow the system to come back up to operating temperature where the system vibration should decrease to levels seen before shutdown. As Sam has suggested previously, a ˜bow' in the shaft may develop as the system cools down. It also should go away at system operating temps.

Removing all rotor weights from previous balancing iterations may or may not help. You need, as others have suggested, to define system resonate frequencies and go from there. Your management recommendation may be, based upon your findings, to stay away from increased speeds but...

As clear to me is the reality that if production demands warrant it, production considerations will rule at the end of the day regardless of the impact on these poor fans! When the fans need to run at higher speeds, chose to run your vibration routes in another area of the plant! As I was once told: ˜if these fans can run another hour without self destructing, the money they will make us in production will pay for 10 new fan units!'

For what it is worth.

Gary

Gary has given my advice already. Remove the
weights that have been added during the trim
balancing. Do more bump test. One on the keyway
and one 90 deg. either way. If you don't have
a problem with it could you mention the
manufacture and model number.

All good advice! The company has decided to pull the shaft tomorrow on that shutdown we're having. The blades have been getting some wear, and are banged up a bit so they want to start fresh.

I still have another hot tailgas fan right next to the one we're pulling. I may do some bump testing on that one, but my feeling on that one is the same. It's just about worn out, and the housings are corroded as well. Even have duct tape on one of those flanges on the outlet. These are 24 inch pipes...That is one of the problems with these fans. They push a highly corrosive flammable gas which we used in our dryers..Sulfur is also emitted and eats on them as well.

I do like the suggestions, some of which I have already tried before posting this problem.

Thanks guys for all your help...Good to have alot of years, and experience from this group. It is highly valued by me...

Ya'll enjoy your Sunday!

Rod

we work on a lot of hot fans, i.e. oxidizers, printing press dryers, etc. We have are contually balancing them except those that have modified their controls to run the fans until the temperature drops below 150 degrees F. If you shut them down hot, they will sag.

Rick,

What size weight, are these fans your talking about? The ones we're having trouble with run about 450-500 pounds. 6-blade fans...Temps at running temps are 375-400 degrees.

Rod

Rodney,
Those are the same fans (weight wise and blade count) that we have been having trouble with also. I did find what I think is piping strain on one of them. I have to wait until they have a shutdown and measure the clearance between a flange on the inlet pipe, which when up and running is hard over against a wall. The wall is shaking, and I am almost positive it is in a bind. The pipe makes a 90 deg. bend right at the wall, and the expansion joint is above this bend. There is no expansion joint next to the fan. The flanges at the joint are offset 2" or so. In 7', I calculate about .200" of thermal growth. This fan is not on isolators either.
The others I will wait for your solution, unless I spy something on their piping too.

Dave

Know what you mean about pipe strain. Some of our fans also have resonance problems. I can always tell by the cracks in the welds, and they just keep cracking. These need to be replaced, as they are getting very old now, and corroded. Gets to a point where so much of the steel flakes off over the years, and the integrity of the structure weakens...

Rod

Rodney,
It does seem that the ones with the worst problems are 6 and 8 vane, straight radial wheels and about the same weights (400 to 600) lbs. Several of our bad-actors have been behaving themselves now that our customers are not shutting them down hot. If you think about it, this design is very flexy compared to backward inclined ones that have short blades sandwiched between backplate and shroud. Usually the straight radial wheels are quite wide as well,welded to the backplate and only short welds to the hub. Of all the wheels, straight radial moves the most air.