Hello All,

Been a few months since posting. Things have been doing pretty well.

My question to you is...

I suspect I'm having a problem with critical running speed on one of our hot fans which was just recently speeded up from 1920 rpm to 2261 rpm...The fan is shaking itself literally to pieces.

It's an overhung fan belt driven, 8 blade about 45 in in diameter, weighs in about 680 pounds. Temps running in the 350-400 degree range...150 hp motor 1785rpm

I've recommended to supervision that the fan should be slowed back down, which they say they will do this week. Reluctantly I might add..I know there is modal testing which you can do to determine where that critical speed is, and that the fan should not be running when doing this.

Can you give me a brief rundown how to do this if possible. I'm only a level 1 vib tech striving to get to the next level, but I haven't had this problem much, so am not sure how to do this...We use the SKF Microlog CMVA65

I'm typing this from home so you can reply, and I'll get back to you here, and at the plant tomorrow.

Thanks guys n gals,

How about a coastdown test (recording magnitude and phase)? It can help give you a pretty good idea on whether you are running near a resonance/critical.

Oh BTW,I forgot to mention that this fan has a real bad 3X dominant peak. Boss says, "try to balance it" I told him it was not a balance problem. But we tried anyway, and while it was coming up to speed I noticed it was smooth just before it came up to running speed. So I know it will probably run good 2 or maybe 3 hundred rpm slower...Wish I had checked that now.

If you recorded amplitude and phase data from your balance attempt, we can look at that and tell you how close to resonance you are running. Would need data from the 'reference' run + at least the first trial run, but the more data (runs) the better. Do you have that data available?

Yes, the SKF saves that in the database, but we restarted a few times. I'll get that out in the morning when I get to work, but I do remember the original reference run. It was .29 amplitude at 259 phase angle...Problem is since it is not a balance problem the phase didn't change that much if I remember right.

Overall was .97 The 3X dominates the spectrum as I mentioned earlier.

I'll get the accurate data in the morning.

Also, it seemed that all the trial weights were added at the 45 and 90 degree positions

Here is the run data on that balance job:

REFERENCE RUN

RPM 2177
Amplitude 1x .293 Overall Amplitude .985
Phase 181

TRIAL RUN Added 3.0 oz to blade 0

RPM 2177
AMPLITUDE 1X .3047 Overall .852
PHASE 338

ADD WEIGHT 1.49 @ 12 DEGREES PHASE ANGLE

Trim Run

RPM 2178

Amplitude 1X .235 Still 1.1 overall
Phase 46 degrees

Not a balance problem

quote:

Originally posted by rustythevibeguy:
If you recorded amplitude and phase data from your balance attempt, we can look at that and tell you how close to resonance you are running. Would need data from the 'reference' run + at least the first trial run, but the more data (runs) the better. Do you have that data available?

How'd you speed up the fan? Is it on a VFD and you could speed up the motor or did you have to change sheaves?

I'm guessing it's worse in the horizontal. Have you roved around with the accel to see where the highest/lowest amplitudes are? Is it sitting on a steel a-frame or concrete base? Anything loose/cracked/missing?

Have you got a Coastdown Plot of the Amplitude vs RPM.

Is it correct to assume (which one should never do) that it was running ok prior to the speed up and there was no 3x present?

Sheave was changed to speed it up. Larger drive sheave. Fan is running 2261...Used to run 1920 or so. It's just a standard dog house and pedestal. Nothing special. Readings are worse in the horizonal position...

I think we are going to try a run up and or coast down on it to see where this needs to run. This will be new ground for me.

quote:

Originally posted by lawrencep:
How'd you speed up the fan? Is it on a VFD and you could speed up the motor or did you have to change sheaves?

I'm guessing it's worse in the horizontal. Have you roved around with the accel to see where the highest/lowest amplitudes are? Is it sitting on a steel a-frame or concrete base? Anything loose/cracked/missing?

Correct Ralph. It was running okay before the speed up, so we know what it is. Just don't know where to run it. I wish this did have a VFD on it. Sure would make it alot easier.

quote:

Originally posted by Ralph Stewart:
Have you got a Coastdown Plot of the Amplitude vs RPM.

Is it correct to assume (which one should never do) that it was running ok prior to the speed up and there was no 3x present?

Can you post a horizontal spectrum for us to look at?

Does not look like you are operating at a critical speed to me. The balance attempt was not overly sensitive and your weight was close to 180 out from the initial vibration. This suggests that you are below the 1st critical. These are just observations from the data available and I could be wrong.

However you may have a structural resonance at the 3X. Would like to see the spectrum. Lots of others things could be going on. The speed change was significant so you are pushing a lot more air. Do you have dampers that control the air flow? Maybe you have three of them?

Ralph asked if the 3X was present at the lower speed. I would also like to know that answer.

Sorry I wasn't able to post earlier. Our compuiter was having some problems. I'm home now so am unable to send a spectrum.

I can say that there is definately a resonance problem, because the fan structure is starting to get some small cracks, but we think this is because of the fan natural frequency being excited by the critical speed.

Haven't performed the run-up test yet because the fan is still in production mode. Soon as we get the fan we'll do that. I was reading up on that this morning on how to set up the microlog, and set the low speed and high speed.

quote:

Originally posted by Ralph Stewart:
Does your Coastdown Plot resemble any of these?

quote:

Haven't performed the run-up test yet because the fan is still in production mode.

Hmmm! I thought sure that would have been done when the fan was stopped to add the trial weight since everything was setup for balancing. IMO, the coastdown plot capture is first during the "next" shutdown after adding the tape. But that's just my thinking, which sometimes seems to be way out in left field when it should be somewhere else.

Why all the discussion about critical speed? The vibration level at 3xSS is 3x higher than at 1xSS! The vibration level at 1xSS is 0.3 in/sec, and that is rough for a fan. A coastdown that typically captures 1xSS amplitude and phase would be of marginal value given that most of the vibration is at 3xSS. A cascade or waterfall spectra plot during coastdown would be more usefull.

I have seen similar symptoms on a single inlet fan with overhung rotor and 350-F air temperature. I would consider that vibration is from a combination of rotor unbalance and looseness with possible resonant structure adding to the problem. Looseness could be in the bearings or the wheel is loose on the shaft. I would also inspect closely for wheel rub on casing at the inlet. Make sure wheel is centered at inlet cone. Do a better balance job.

Walt

quote:

quote from Walt.
Why all the discussion about critical speed? The vibration level at 3xSS is 3x higher than at 1xSS!

quote:

Quote from Rocking Rod.
It was running okay before the speed up, so we know what it is. Just don't know where to run it.

May be from what Rod said.

OK, I am not used to looking at velocity data when balancing, but I'll take a shot at this (you guys correct me if I flub this up).

Velocity leads displacement by 90 degs, so displacement lags velocity by 90. So if you were reading displacement, your initial phase angle would be 181 + 90 = 271 degs. The only other thing you need to know is the calculated position of the final balance weight, which I calculate to be 352 degs. This puts your original heavy spot at 172 degs, so the actual lag is 99 degs. (271 - 172). If you were running exactly at resonance, your lag would be 90 degs. So you are running just above resonance. Typically, when balancing near resonance, the 'required' weight calculated by the balance program will be too high. And this is just what we see from your data.

Your balance data actually looks fairly reasonable to me. Your trial weight was obviously too heavy by a factor of 2 since you had essentially the same reading as your reference run, but with a 157 deg. phase shift. Nearly the right spot, but just too much. And the solution of 1.5 oz @ 12 deg. supports that. You removed the 3 oz. trial weight and put 1.5 oz. at 12 degs, right? You did get a reduction in amplitude, though not as much as expected. The solution would be 0.9 oz at 352 degs, which seems pretty reasonable. Did you try that?

Sometimes balance 'solutions' will give you perfect results, but often they don't. As long as the amplitude is decreasing, I keep giving the machine what it wants. Obviously with the high 3x you have something else going on with the machine which is also likely creating some 1x as well.

But as long as the response is fairly linear, you can often balance a machine just fine in the presence of other problems. Once had a hammer mill that would give you very different balance data when you just started it up and shut it down repeatedly. But 5 runs would yield 3 sets of similar data and 2 sets that were all over the map. So I'd toss the bad data and average the good. 5 runs per trial took quite a while, but it balanced OK. (They didn't have the time or the parts to fix the looseness problem).

I think you are running near a resonance. You need to either stiffen the fan, or reduce the speed. Balancing will help quite a bit as well.

By far the easiest and fastest test for resonance is a simple bump test with the machine off.

This message has been edited. Last edited by: rustythevibeguy,

Wow, lots of information here! Let me see if I can shed more light on this. BTW, I am off today, so I won't be able to get any data for you.

I just learned yesterday that these Dodge Imperial bearings which ARE on this fan are rated for a speed of 2200cpm...Fan is running right at that...2177 (bearing stiffness)

There is also (blade pass frequency) going on in there. Housing is very tight for the blade, and not much wiggle room axially. I noticed some blade wobble on coast down the other day while balancing. Could be causing some of that 3X vibration??? Not sure...

Getting back to the balance job, with a 1X of only .25 which is well within the acceptable limit, we felt we were wasting time balancing with that 3X going on. Maybe that 3X is coming from the blade wobble, just speculating.

I do want to reiterate that when the fan was coming up to speed it smoothed out just before it reached it's running speed. This is what leads me to believe we are running in the critical speed curve. I looked at the history of this fan yesterday as far as the running speeds.

From 2001 to 2004 it ran 1900cpm, then engineering wanted to speed it up to 2561. The vibrating started then but not as bad as now. 2005 we backed down to 2180cpm where we are now.

Getting back to the resonance issue and cracks, this fan housing is very old and I feel is losing it's structural integrity with all the rust and corrosion, and metal flaking away on the foundation.

I was thinking a combination of this, the speed, and perhaps that wobble, but mainly the fan running in critical speed is exciting this whole machine, and causing it to resonate like a tuning fork...

We're supposed to shut this fan down next week if it doesn't crash first. We'll do a run up and coast down on it. Hopefully that will tell us what we need to do.

Thanks Guys,

Are you able to due a coast down while monitoring the phase of the 3rd order of running speed. If I am reading this right this is the frequency that is exciting the resonance in the system.