All,

It appears like there are 1/3 orders ( see attached). They are present in other directions as well. They reside in sub-synchronous range only and seems to be there forever.

It is a direct driven fan with a grid type coupling. There is known offset and angular misalignment 2 times exceeding norm. Spherical roller bearings mounted on a tapered sleeve support the shaft. The fan is located on a metal frame mezzanine.

What do you make of it?

Thanks,
David

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

0.33_harmonics_sys_fan3.doc (88 Kb, 82 downloads)

I have heard that 1/3 and 2/3 fan speed vibration often seem to be linked to suction conditions like blockage in the suction. I'm not sure why that is.

Hi

1/3X Or 1/2X harmonics are indication for sever rotating looseness

Dave,

"always been there" there is only 10-months of data with this characteristic. I agree with the loose bearing comment. Vibration levels are low and unchaged, so I would expect that no action is necessary.

Have you:
Felt any "thumping" on bearing cap?
Listened to vibration for impacting sound?
Measured demodulation or PeakVue for detecting impacting?
Measured ultrasound for dB-level and sound characteristic?

Walt

I can't rule out that 1/3x and its harmonics represents possible looseness.

But please don't rule out flow and suction conditions as a possible cause. More info here:

http://maintenanceforums.com/eve/forums/a/tpc/f/3751089...621013751#8621013751

There is some info about this at the link that I posted in that thread on 07 August 2005 09:28 PM. Also some later comments from Tony on similar case. And finally the resolution of the problem as reported in the last post on that thread came through opening dampers (although in this case, discharge dampers, not suction dampers... perhaps change in discharge damper changes flow which again changes the suction conditions). It was easy for him to check when the dampers were repositioned further open.

It is a little mysterious to me, but apparently not too uncommon. Would be interested to hear any explanations why the particular pattern of harmonics of 1/3x seem to be tied to flow conditions (rotating stall).

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

Another link providing some anecdotal discussion of rotating stall in fans:
http://www.achrnews.com/Articles/Feature_Article/ac51bc...CM100000f932a8c0____

quote:

Rotating Stall

This is a special case of stall that normally only occurs in high-efficiency, backwardly inclined centrifugal fans. Most observers also report that inlet vanes are involved.
These fans are encased in a scroll-type housing that helps generate the fan’s pressure. The pressure around the periphery of the fan wheel varies relative to how near it is to the fan outlet (where it is highest). These fans have several blades, typically nine to 12.

We will call the passageway between each blade a cell. The flow through each cell can vary since the pressure around the periphery varies. Near the stall point it becomes possible for most of the cells to have the normal forward flow, while one or two cells have reverse flow.

The air that “squirts” backward through these cells has nowhere to go so it moves to an adjacent cell, deflecting the air which was already traveling through it. This change of attack angle now forces this cell to stall. It then also has reverse flow, passes on its bubble of air, and on and on around the fan wheel.

Most researchers have reported that the frequency of travel of this rotating stall occurs at about two-thirds of the fan rotational rpm. Some have observed two traveling cells at once generating a four-thirds-rpm frequency.

There are other reports of rotating stall ranging from two-thirds to more than 90% of the operating frequency. This frequency will show up in both sound and vibration measurements, but it is normally found by complaints of noise.

Note that rotating stall is also discussed often in connection wtih centrifugal compressors (vs fans). There the vibration is usually described as subsyncronous (for example 0.62) , rather than subharmonic (for example 0.66667). I have no idea what drives the differences.

Also I have read that if the machine has a hump in the head vs dp curve, rotating stall can occur when you operate to the left of the peak of that hump.

Here is an update. No new readings, just more thorough analysis of old data.

Firstly, I ruled looseness out at the very beginning since other typical vibration symptoms when looseness is present, such as multiple 1x harmonics did not show up.

Secondly, what was perceived as 2/3x turned out to be background vibration (see attached )

Thirdly, a higher res. spectrum, which facilitates noise reduction, has eliminated those sub synch vibration in question.

In other words, that was a false alarm IMO!

Pete,
Thanks for your insight of stall condition.

Walt,
Why do you suggest listening ( which was impossible in this noisy environment) to the audible "thumping" noise, if this thump, if present, probably should manifest itself in normal vibration data ( taken in this case with Fmax=2 kHZ) or in the PeakVue data sampled at 100 KHZ? Nothing showed up in neither of these cases.

David

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

0.33_harmonics_sys_fan3_updated.doc (207 Kb, 14 downloads)

I see many many harmonics (14+) of the 1200 frequency (2/3X) on the spectrum in your figure 4. Does your nearby machine running at 1200 also have lots of 1200 harmonics on its vib spectrum? And how close is it? Typically those higher frequencies are attenuated more than lower frequencies and don't travel as well.

I don't doubt your conclusions at all. Especiallyi if you have enough resolution to clearly seaparte the frequencies of interest. And I don't particularly expect a lot of harmonics to be caused by this rotating whirl thing. But it would be interesting if you get a chance to monitor vib when one or the other machine is shut down (to confirm/disprove your theory) or to open the dampers further (to confirm/disprove my theory).

Dave,

I simply listed several methods for detecting if there was looseness in bearings, since some form of impacting would have to be present. Your data in your first posting did not include any PeakVue data, so that is why I suggested that along with audio monioring.

I did not think that the symptoms (1/3xSS) indicated a dynamic pressure problem such as rotating stall. I have measured fans with dymanic pressure faults and found that fan case vibrations and radiated sound provided a better indication than bearing housing vibrations.

Walt

Walt - Just to clarIfy,

You didn't think it could be stall in this particular case based on absence of reported fan case vibration and radiated sound? (and other info David posted about nearby machine). I can buy that.

Or

Are you disagreeing with my comments that in general harmonics of 1/3x in bearing vib (and particular 2/3x) on bearing housing vibration can be caused by rotating stall due to suction conditions and operating at too low flow?

At least four different sources that have suggested this:
1 - Post "Fan vibrations - side bands on run speed harmonics". The author showed us his bearing housign spectrum with 2/3x and 4/3x and then later experimented by opening dampers - pattern went away.
http://maintenanceforums.com/eve/forums/a/tpc/f/3751089...621013751#8621013751
2 - Comments by Tony near the end of the thread linked above.
3 - Engineering dynamics article at http://www.engdyn.com/papers/abstracts/ab9.htm
(Look for the pdf icon near upper right-hand corner and right-click and save-as to save to your hard drive... 3.1MB file).
Case #3 is the one I was talking about (pages 17 thru of the pdf or pages 2-12 thru 2-14 according to number at bottom of the page). Here is an excerpt: "The predominant pulsation and strain frequencies were 20hz. Rotating stall generally occurs at multiples of 1/3 shaft speed. This fan ran at 15hz and the pulsations were at 20hz which was 4/3 shaft speed. The stall was due to marginal flow or improper preswirl conditions at the fan inlet which caused the air to impinge on the fan blade at a poor angle of attack."
4 - http://www.achrnews.com/Articles/Feature_Article/ac51bc...CM100000f932a8c0____
"Most researchers have reported that the frequency of travel of this rotating stall occurs at about two-thirds of the fan rotational rpm. Some have observed two traveling cells at once generating a four-thirds-rpm frequency."

El'Pete,

Vibrations and sound at 2/3xSS and 4/3xSS may be from rotating stall that is typically caused by choked inlet, but not at 1/3xSS. If bearing vibrations 1/3xSS was present, then I would expect mechanical looseness or rubbing.

Walt

Thanks Walt.

As far as "listening" as Walt suggested, I ALWAYS listen to the bearings when I suspect a problem, first with a 'yellow-handled analyzer' (mine is a file with a plastic handle), and then with headphones (CSI with 629 adpater). My reasoning, is not "Why listen?" but rather "Why not?" You lay your hand on it, don't you?

You simply can't 'trust' vibration data.... I do everything that is reasonably possible to 'verify' the data when I suspect a problem. Listening is one of the easiest things you can do to verify what the data seems to be saying. The spectrum and waveform are just digital "interpretations" of what is physically taking place, and as such, are subject to error.

quote:

You simply can't 'trust' vibration data.

I now have a new question to ask of potential vibration consultants. Can you trust your vibration data?

Rusty, I see it but can't believe it. My understand is that you're getting paid to function as a PdM/vibration consultant but don't trust your data or system or method of data acquisition?

What song does the bird sing? Your hearing must be excellent! A 6312 brg may be housed in a massive housing w/machine mount on sole plate or skid or???? Produt and common piping may present a new set of 'feel' problems in an enviroment of 100dB ambient noise.

If you can't trust your data; you have a problem! Either you don't believe in the science or discipline or your system is inadequate.

I would suggest evaluation of your system. Transducers: I use a variety of casing transducers from Kistler, PCB to CTC at various resolutions and sensitivities. There is no magic. There's a voltage signal that represnets machine components - 1X RPM must be valid and a signal above the transducers noise floor. Integrating that voltage per given frequency to velocity doesn't magically make it better. I would argue the converse; there's a loss. Is a volt a volt, is a g actually one g? Gotta go; sermon is over.

Good thing Rusty isn't running for Political Office, because people like Bill & Sam would take quotes out of context and get all upset! I read Rusty's whole posting, and I have no problem with it. My take is that vibration data may be only one of several available pieces of information (useful or not) to make a fault diagnosis and recommendations for corrective actions. I don't use a screw driver for listening to vibrations, since a carry a sound level meter and ultrastound meter as part of my "audio" tools. I know how to use a screw driver for that purpose (learned from an old gear man), and I would do so if necessary.

This whole Posting Thread is getting way off the original question.

Walt

Great Walt; and, good point but I can't see it is taken out of context except not to include all context. If all verbage were included it would take into account more than this particular thread.

I've had so many machines where the analyzer says it's bad, but all senses say no problem here - I'll trust the analyzer over ears and feel sensitivity everytime.

I have tech's gather data and re-gather data monthly and I write reports and call shots for a planned/scheduled maintenance program. I have a plant where not all is paid attention to; however, the next survey or the time frame given for repair ---------- not adhered to - find a new motor or pump sitting there. The guys will come in and say, "what did you say about that pump last time". I'll look - and they'll say, "well, new pump there. They didn't listen". I've had three plants listen and do exactly per schedule given and sadly can say, not all listen but when they don't you can see results of your report. I usually say, "if you don't believe the report just sit on it and see what happens".

But, I'll have to add - this posting is not about Rusty! It is about the discipline and my livelihood. "Are those 1/3 orders real?" The question is based on data. If I see a 1/2 or sub-harmonic or sub-frequency - why is it there? Can I trust my data or must I use osmosis? The 120 Hz component; is it real or imagined or does it really have anything to do with the machine?

We all make typo's and have many short answers and reply off the cuff on a question that really isn't understood at the time. Having said that, we should all have some understanding. It is those that think they're perfect that upset those of us who really are Can you poke your finger in a bucket of water and leave a hole? The one who walked on the water will usher in perfection - until then, I'll wait.

This message has been edited. Last edited by: Sam Pickens,

Like Walt, I read Rusty's comments to mean that he doesn't rely solely on his data collector to make calls. It is another tool and we all know about the bucket that Rusty totes. If he needs it and it's not in the bucket, it is probably in his head.

I went back and looked again at the original data. The cursor was at 1/3xSS with harmonic markers on. There was no significant peak at 1/3xSS. There were peaks at 2/3xSS and 3/3xSS=1xSS, so I would not call that a group of harmonics of 1/3xSS. The only frequency out of the ordinary is 2/3xSS. Even this frequency can have more that one cause (fault) for a motor-fan. One would have to add more information to the single spectrum provide to make a definitive diagnosis. I can poke a hole in a bucket of water with my finger, if the bucket is made of styrofoam! If water skiiing counts as walking on water, then I have done that. Sam, you are correct, "this posting is not about Rusty".

Walt

I re-read the posts, but I don't see anything gout of context that I said. Perhaps, Rusty meant to write a different context - I don't know.

I have seen machines die a hard death, because people didn't believe the instrumentation. Operations can be reluctant to believe a measurement if it gets in the way of production. Not only could this cause damage or increased damage, but it could present hazards to people or the envioronment.