I have a strange problem which has started about a year or so ago. We have two fans on one of our units. Both driven by 150 hp 1800 rpm motors...The fan blades are carbon steel about 4 feet in diameter, and they are pushing hot air around 350-400 degrees...


I'ts gotten to the point when every time we shut the unit down cold, and then heat it back up for start-up I have to continually re-balance the fans...I've been tracking the balance weights, and none of them have been falling off.

Could this be due to a blade wear issue, where the integrity of the metal is weakening. We have three fans in the plant which are doing this....Sort of has us scratching our heads. Anyone else experiencing this problem. This is getting old...!

Thanks,

RB
Baton Rouge, LA

Rodney--

Could the fan hub be slipping on the shaft due to differing metallurgy / rates of thermal / growth?

Just a thought--

Tony

If you're operating ~400F you should slow-roll upon shut-down to prevent a bow from developing. If the bow develops you're in trouble with the overhung fan expecially. You can do run-out checks and slow-roll attempts to get back to zero upon or at start-up or straighten the shaft and implement a shut-down procedure in the future. Is this reasonable per your circumstances?

Usually when we bting these down we have to do it all the way, since these fans are pushing an ignitable gas which feeds our dryers...We also checked these wheels last year for looseness on the shaft. They both appeared to be ok...

These are carbon steel blades, and they are about 6-8 years old, so I was sort of leaning on the old age and wear senario.. We are slowly converting all our fans to Stainless steel...

I may go out and take some phase readingS with the strobe light later...

Thanks Guys,

RB

Hi Rodney
I am inclined to agree with Sam on this with the info given.Have you changed anything in the way you shut them down now versus a year ago. The strange part is you say you have more than one unit that does this.And that they both started doing this at about the same time.Are the amplitudes of the imbalance the same on both units.And is it the same each time you shut it down.I would wait unitl the unit was down and cooled and mark the shaft position to see if this is where the imbalnace is when you start back up.It does not take much sag or bow to cause you grief.Especially on an overhung unit.

Howdy,

I don't like suggesting the obvious, but this same thing happened to me. Similay application, hot exhausts. What I found was the fans were rebuilt using two fixed bearings, not with an expansion bearing on the fan end. I balanced those babies until the cows came home. We installed the correct expansion bearing on the fan end of the shafts and the problems went away.

Rodney,

Could your problem be a growth problem due to high temperature and the fan blades do NOT return to their original configuration when being stopped ?

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.

I believe that you are faced with fan blades deformation caused by high temperature and of course combined with the amount of thrust they are submitted to.

Perhaps the stainless steel ones is a better choice.

Marko Leo

Those are all good answers. We do have a floating bearing on the #4 or inbd position, next to the blade. #3 is the ob bearing next to the sheave...

We've discussed this problem quite a bit over the past year, and the only logical solution I can come up with, (because of not having this problem before) is heat distortion making the phase change, and/or old age on the fan blade itself, which is a carbon steel unit, and it's losing it's integrity, delamination, etc.

They have been bringing these fans down the same way for years, so I don't suspect that being the problem.

I just took some strobe readings before lunch and documented the phase and amplitude. #3 & #4 HVEL are 186 and 188 perspectively.

The VVEL is a bit different...#3=183 degrees & #4=250... Thats a 77 degree difference on the vertical measurement. Maybe some dyanamic imbalance going on here too....

We plan on redoing these fans soon, going to stainless steel. I guess in the meantime all I can do is monitor them...

Thanks for all the input!

RB

Correction...I meant 260 degrees on the #4 VVEL...

Very good Miquel! We took a passing look at this about 2 years ago. Now I'm thinking we should get more serious about it! I very much like the concept, since we wouldn't be losing production...

Do you have a prototype of this in your place of work?

Thanks,

RB

Rodney

Do these overhung fans have a hub with cylindrical fit on the shaft secured by setscrew or do they have a tapered, interference fit hub?

I had this problem with three fans on top of a very hot methanol reformer. The balance changed when we shut down and cooled down, but the unbalance was always in the same plane relative to the keyway in the shaft (note: my fans were between bearings). The hubs were reseating on the shaft was all we could conlude. 5 other fans on the same roof never needed balance correction. We were never allowed to pull one of these fans apart but I bet we'd have found a loose hub/shaft fit or hubs up on the keys.

One way to test this is to impact test the wheel to determine the 1st natural wheel/shaft frequency, then turn the rotor and retest. If you see a change in frequency, the wheel is likely either loose or riding on a high key.

Ted

Ted,

The wheels we use are all secured on the shaft with almost zero tolerance with a keyway, and two large set screws.

We have three fans with the same problem when they cool down. The odds of all three having wheel looseness would probably be slim to none.

One thing I was concerned about, and informed our supervisors was every time they increase the fan speed to gain production, the fans cannot handle it. The begin to vibrate. Trying to squeeze to much out of things... Different problem but still makes the imbalance magnified on these hot fans...

Rod

Rodney,

I would use caution when changing to stainless steel. If your problem is related to thermal growth, it could be made worse by changing to stainless as it has a coefficient of thermal expansion that is about 4 x greater than carbon.

"almost zero tolerance".... so how much is that, exactly? Zero? +0.001" ? -0.001" ? +0.025" ?

Until you measure, you won't know... and I'm sure "they" don't know either.

If you have a perfectly balanced wheel (done on a stand, for instance) and you put it on a shaft that is 0.005" undersized, then you have displaced the center of mass by 0.0025". Depending on the mass of the wheel, that can be a lot. And for a given amount of imbalance, if you double the speed, then vibration will increase 400%.

So a variable speed machine, with a wheel mounted with an unknown clearance, could be problematic.

I also don't see why the wheel being "old" is a problem. Unless you are seeing "chunks" of metal coming off the wheel, it is not getting out of balance. These wheels run "clean", right? So buildup should not be a problem.

But it doesn't take much hub/shaft clearance to make the machine go out of balance. I would monitor the phase on all machines affected by taking readings at the beginning and end of each workday, but this needs to be done at the same speed. The phase should not vary that much during the day, or from day to day.

Posted by Rodney

quote:

One thing I was concerned about, and informed our supervisors was every time they increase the fan speed to gain production, the fans cannot handle it. The begin to vibrate.

This is a sign wich has to be carefully observed. If you have phase change while increase fan speed then you are next to a critical speed.

If the last is your case, and in the other hand, natural frequency is proportional to elastic module of the material wich can be reduced when temperature is increased (so fan has no vibration when is cold). Then,if natural frequency decrease due to high temperature and shaft speed (wich carry residual unbalance) increase , it will yield phase change wich mean the fan is next to critical speed.
If this problem is new, maybe can be related with high bearing clearance wich is an adition element that contribute to decrease natural frequency and you have the power to get control easily.

I am not sure what the material has to do with this issue. Rotor bow may be the problem as already suggested by Sam.

If you do go with stainless it would be advisable to review the material properties. I would be surprized to see aa large change in the coefficient of thermal expansion as a 4X difference.

Bill is correct. 1.5 x is more accurate.

Carbon Steel 6.33e-6 average, 304 stainless 9.9e-6 32-600 deg f.

I didn't know that the coefficient of expansion increased slightly with temperature either, but I came across that too.

Well, to be honest I am not a mechanical eng. The guy we had that started the program here in 2000 has left, so we picked up the baton...The standars for our plant at this point in time is too discard all our older carbon steel blades and go to a good grade of stainless. I think the idea behind this is for the durability of it, over the carbon stell, and also longer life. Maybe the thinking is wrong here, but all I know is the older these machines get the worse they become. Can never keep them balanced now, and it's just a matter of time before they crash and burn...!

7-8 years old, and running 24/7 can take it's toll on rotating equipment. Both of these fans already have about 55-60 balance weights on them, just to give you an idea on how bad the problem is...We even knocked all the weights off one of these fan a few years ago and started over...

Rod

quote:

55-60 balance weights

Doesn't make them go out of balance, but can make them very difficult to balance. In theory, it shouldn't matter, but it does.