Got another senario here on another blower...

This blower repaired back in Jan 2005, been running remarkebly well, then for no apparent reason last data, the thing went up into the red and yellow alert, on the #3 & #4 VVEL...Bearings are okay. This is all 1X vibration in the vertical direction. Checked all feet, they are good, haven't looked at the rubber pads under the frame, but I don't think thats it. Doesn't look like alignment.

Thing went from .125ips to .470ips from one month to the next. Caught this on my monthly route last week. Looks like something came loose inside, or this blower swallowed something. Don't know for sure, and probably won't at this point.

Anyone ever experienced this before? And if so, what did you find? Coing up on only 3 years since the last repair. Not a long enough life span...I'm really not expecting to solve this one in house, but I'm curious as to what may have casued this. I've attached one of the trends...

Thanks,

I would not discount the coupling. A large number of Hoffman blowers use Thomas couplings. They are great if the machine runs almost all the time. Cycling through the thermal changes and you get fatigue.

Do you not have spectral data? And/or more data and info?

Has HORIZINTAL reading at NDE also increaded in the same proportion? What about other points? How about checking for resonance? Possibly something invisible has changed bringing it to resonance.

I have seen this happen when a crack forms in the wheel or loose/broken rivet/rivets on one of the wheels. I know that the aluminum wheels are riveted. Can you take phase readings to see if the rotor is bowed or unbalance is the problem?

I'm not sure of the size but smaller Hoffman blowers have aluminum wheels and I have seen a number of cracks or damage due to moisture (not sure of this specific application).

Has anything changed in the process it supplies air too? The Hoffman Blowers I am familiar with have a vent system so if the process does not need the air the blower can produce it allows air to pass and not dead head the blower. Process change can cause a change in the 1XRPM.

Check the key in the keyway. I have seen keys migrate beyond the end of the shaft and make contact with the other side.

Good luck

We repair these on a regular basis for one customer. There filtration is not at the level it needs to be and material builds up on the wheels. Usually fairly evenly. One day some will come off and its out of balance. That's my bet.

We have already had an issue with one of the RTD holders which was too tight against a floating outer bearing race, and the resulting load was inducing the vibration. We searched for a long time before finding the cause...

Alec,

Do you mean that the floating bearing did not float? If not, could you please elaborate little more?

Dave

That's it, the floating bearing did not float. After backing off the outlet blower bearing RTD on this Hoffman blower, the vibration level around 180hz had fallen dramatically and we could almost hear the flies fly; as simple as that!

Alec,
What harmonic number was 180 HZ?

In fact, there was the presence of the third harmonic (178hz) of the rotation speed (3558 rpm) indicating a misalignment and there was also a frequency at 184hz which was predominant in the spectrums. An outsourcing firm who had studied this case on the site were told that this frequency was related to a bending mode of the rotor assembly.

Several years ago we had a similar 1X issue with a Hoffman Blower (5-Stage / 3600RPM). The shop found 2 impellers with significant damage and a pipe nipple laying inside the blower on the inlet side. The pipe nipple had come loose somewhere upstream and made it's way through getting into the impellers.

Hoffman_Exhauster_with_Damaged_Impeller.pdf (43 Kb, 27 downloads)

Have you checked the expansion joint on the discharge piping? I have seen them hang, causing a large increase in horizontal vibration.

Good morning to all,

I was out friday for a while so let me respond to these comments.

All good suggestions. I can rule out coupling problems, checked that out last week. I did indeed consider something going through the blower (ingested) and causing damage to the impellers. This would make sense, because everything was good, then this sudden increase...It's like something just snapped or let go.

I think I will take some phase readings this morning soon as I warm up a bit. Maybe I can see something going on there across the coupling or in the blower.

All indications appearto be imbalance looking at the basically flat spectrum with the huge 1X peak. See attached file. This is the worse point on the machine. Horizonal positions aren't so bad, it's all vertical...

Upon studying the internal blower design it appears that there is what they call a balance piston on the ODE which keeps the axial movement in check. Not sure how it does that. The axial readings were a bit high as well but still in the green.

Really don't know what else to add other than that. I'm still monitoring daily...

Thanks to all of your for your input on this...

Good day!