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ivibr8

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Reposted into this Topic area.....thanks John

Thought I would share a recent field balancing job and ask if any of our members have seen something similar.

Two days ago I was contracted to field balance a baghouse fan at a stone plant. This fan is used to remove lime dust from stone quarry mix. It is an overhung fan assy belt-driven by a 250HP motor. It had just failed its second set of bearings within a month. The bearings themselves did not fail but the interface between the shaft and inner race became loose (setscrew design). New heavy duty dual-row angular roller bearings installed with locking taper collar.

In short, I was not successful in balancing the fan. There was significant amount of axial vibration (27 mils) while the radial vibration was varying between 1.5 mils to 3.5 mils and phase very erratic.
The entire fan assy base plate was loose and causing the entire structure to be rocking: I measured at various locations along the vertical length of the structure (getting linearly higher as I measured up and all in phase).

 I really didn't expect to be successful but I tried to use the "average" values from the "O" and "O+T" data, but the two shots were not very effective. I essentially had a non-linear system.

What was confusing to me was that the the operators indicated the base has been loose for years and earlier in the year, the client's vibration guy was able to get levels down to less than a mil and according to him, it responded well to shots.  I struggled with trying to explain how the vibration increased when the balance condition had not changed. The speed is always the same. The damper position is always the same.  There were a number of things we looked at but no smoking gun.

Today, I went back to the Plant to retrieve some gear I left behind (yes, age has something to do with it). But I was glad I did. The fan (now in operation) was running very smooth!!!???  The operator thought it was because it was in normal operation. I didn't think this was likely as we changed damper settings from full open to full closed and I only measured minor effect. I could not think of any other force in play.

What I did notice was the the foundation plate had significantly lower movement. Why?
The only thing I could think of was last night's rain had soaked into the ground and around the foundation.  Could the rain actually act as a "coupling" agent between the interfaces of the known looseness?  Could this have changed the system response that much?
I asked the operator to notice whether the vibration comes back at the end of the day when it becomes "unloaded" or whether the vibration remains low.

So members.....has anyone seen or experienced something similar? Does it seem plausible to you that rain water can act as a good coupling agent, even if its temporary as drier conditions return?

Sorry for the long post.....but thought it be a nice topic for discussion and learning on my part

Regards
Jim Powers
dnk

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Never saw that. Was there a large difference in the air temp going through the fan?
ivibr8

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No.  According to the operators the air temp is about 100 to 120 deg F during the summer.
The other thing I should mention is the soil around the fan assy is heavy clay-like.  There is a fair amount of lime
deposit in the entire area. I would think much of it gets absorbed into the ground over years.
I think time will tell on this case history. As I learn more, I will follow up.

JP   
Edwin

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First: I have enough experience to realise that "anything" is possible in our business.
Also I have equipment that responds to rain. We are on soft grounds here with clay soil below sea level. And ground water level effects some equipment's response. So could be in your case also.
 

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Curran919

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On compressor skid piles, whether the ground was frozen or not is the largest effect of vibration. On our vertical pumps, minor changes in water level in the sump can have a large affect. We had one pump that reacted bad in direct sunlight because of the resulting thermal gradient in the pipework. Putting a canopy over the pump fixed the misalignment induced vibe. These aren't directly relevant, but it just goes to show that intermittent weather can effect your machinery in funny ways. We have half a dozen weather stations (wind temp and precipitation) that we can hook up to our long term analysis DAQs just for this reason.

Bob Eisenmann has a case study in his book about a fan with intermittent vibration due to ground water. It involves freezing, so it may not be applicable, but I've scanned and included it anyway since you'd probably find it interesting. Because the graphs got cut off, the first x-axis is rpm (100-800) and second is horizontal 1x casing vibration amplitude in mils p-p (0-7).

 
Attached Files
pdf Eisenmann Case Study 27.pdf (1.59 MB, 18 views)

JuddJones

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This would be a great application for a motion amplification camera if you know anyone in your area who owns one.
ivibr8

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The Eisenmann article was interesting. It goes to show that my assumption has more than a hint of plausibility and that even subtle changes in the boundary conditions on bases below the surface can have quite an effect.
I am still not 100% sure yet....am waiting for feedback from the plant operators.

Interesting thought about the motor amplification.
At the end of the day with all of us scratching our heads trying to decide what to do next...their management were discussing plans to change out the shaft and impeller assy.  Based on what I saw, I was not convinced that would solve their problems and I told them not to be disappointed if the high vibration continues (remember at this time, I didn't have knowledge of the low vibration levels that I saw two days later).  

I had suggested and wrote up in my report that either an ODS or visual amplification study would help identify the true cause. I could do the ODS but I know Tony DeMatteo fairly well and his company (4XDiagnostics) are fully qualified on the  IRIS-M motion amplification process.

Thanks again for the replies

Best regards
Jim Powers
RustyCas

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Reply with quote  #8 
Were you able to record coastdown amplitude and phase plots? I do that on my reference run in most every balance job. Easiest way to know where the machine is on the resonance curve. It's possible the fan is near a resonance, likely because of the loose base. Water under the base, or even the clay may act as a dampener. Wet clay can create quite a "suction"?effect.

As an aside, where I grew up some of the farmland had a high clay content. They called this type soil "gumbo" (no idea why). My dad was a linetruck driver for the utility company. He said once he "stuck" it in gumbo which gripped the tires so tight that he twisted the axle into when he went to low range (lots of torque).


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Curran919

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Quote:
Originally Posted by RustyCas
Wet clay can create quite a "suction"?effect. As an aside, where I grew up some of the farmland had a high clay content. They called this type soil "gumbo" (no idea why). My dad was a linetruck driver for the utility company. He said once he "stuck" it in gumbo which gripped the tires so tight that he twisted the axle into when he went to low range (lots of torque).


Many (not all) water-saturated clays behave as non-Newtonian fluids and will change in stiffness depending on the frequency and strength of the vibration (think cornstarch in water). They can be very difficult to model/predict.
ivibr8

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Quote: Were you able to record coastdown amplitude and phase plots? I do that on my reference run in most every balance job. Easiest way to know where the machine is on the resonance curve. It's possible the fan is near a resonance, likely because of the loose base. Water under the base, or even the clay may act as a dampener. Wet clay can create quite a "suction"?effect. 

Rusty....Sadly, I didn't think to do a coastdown plot until I got back home to write up my report; which I included as part of my go-forward actions. I did take data at various speeds (motor is VFD controlled) and yes, I did notice some changes as we got up to full speed. A big problem I had was that both the amplitude and the phase were wildly erratic. However, now that the fan assy is much smoother (at least as of a few days ago), the whole idea of performing a coastdown seems even more useful.  Hell, this thing is so interesting to me I am willing to go there gratis just to see for myself.

Another frustrating point: I have contacted the contractor who hired me (twice) and I have called both the operator and the engineer at the Plant and asked them to get back to me about what they found when normal operations are secured (but the fan is still running). They still have not contacted me.......very frustrating as I think its important clue in understanding the root-cause.    NO FEEDBACK.      Come on guys....help me help you.
Does this lack of feedback occur to you also??  

JP
 
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