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RustyCas

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Reply with quote  #1 
I received the "questions" below from a customer, sent as a questionnaire to all their balancing guys across multiple sites.  I have a feeling it may be about "does Rusty know what he's doing" since it originated at the site I cover, and we lost a large fan there about a year ago (supposedly due to a shroud failure, however I think the failure originated with a 'pitched' blade liner).

I know my answers to all the questions (some of which are obviously coming from a perspective of knowing nothing about balancing or vibration), but I was curious if any of you have strong feelings about any of them.

Feel free to address (by number) any or all questions that interest you.  You, of course, will not be quoted.

1. Can you determine if there is a defect in the rotating unit that has made it unsafe to operate from a balancing standpoint?

2. When should you take existing weights off and start over?

3. Are there field conditions you avoid for quality reasons (temperature, wind, etc.)?

4. What is the maximum amount of weight you should apply?

5. What are your rules of thumb for locations of attaching weights?

6. What records do you keep for yourself, what do you leave with the customer?

7. How do you calculate the weight size and locations. Do you depend on equipment for that, if so, then how do you know the equipment is calibrated correctly?

8. How do you decide when the balance is good enough?

9. What standards do you use?

10. What damper position do you use when balancing?

11. If it’s a VFD, what speed (RPM) do you balance at?

12. When would you use vibration info for balancing purposes?

13. What thickness, shape, type of steel should you use for weights?

14. Are there any indicators in the harmonics of a fan that would indicate end of life for the fan wheel?





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Danny Harvey

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Reply with quote  #2 
I'm certainly not an expert on balancing but I'll take a quick run at it because, like you say, some of these questions need some work.

1. No.
2. Only as a last resort after at least two other people have been stumped.
3. Most of the field conditions that I would avoid for quality conditions would have driven me away for safety concerns before they would affect quality, I think.
4. It depends on the speed, radius or rotation and rotor mass.  60 lbs. is the biggest I've ever seen.
5. As far away from the center of rotation as is secure makes for smaller weights. Welded weights should be low profile and be welded to the back side of the blades. Remove trial weights to minimize the number of weights in the fan.
6. All of them
7. With math or a program. You don't unless you check the sensors at the time of use. But then you don't know for sure that your NIST traceable shaker is still accurate.  All you know is that it's certificate hasn't expired.  So I guess you can't know for sure.  Who knew that the world wasn't perfect.
8. By standards.
9. Those specified by the client or those specified by accepted standards such as ISO or the nomograph developed by IRD years ago.
10. I don't know but if it is a concern, you can test at several settings and see if there is significant change in vibration. Then balance in the worst case.  Adding more variables such as speed, temperature, relative humidity, etc would make it much more difficult to determine the proper conditions.
11. You can do a run up-coast down to determine system natural frequencies but generally speaking, maximum speed because that's where the force due to unbalance will be at the maximum.
12.  Prior to the balance job to identify and correct other issues. If it includes phase data that is in accordance with the balance setup, then it can serve as your reference run to avoid a start cycle.
13.  Already answered except that like metals should be used, I guess.
14. Saved the best for last didn't they? There are plenty that in retrospect might have shown that but they almost all are more likely something else. "End of life for fan wheel" are words I have never typed in a report. It seems like there are many other, better ways to find that out.



Ron Stiemsma

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Reply with quote  #3 
  1. Only by a good visual inspection and maybe NDE testing.

  2. If the machine is not responding to trail weight as it should.  This usually means there is another problem other than balance that needs to be corrected first.  There have been cases where a single plane solution was not adequate so I had to regroup and do a dual plane balance.  This requires going back and taking some other initial data.

  3. Not for quality but for safety.

  4. Depends on the machine.  I have used a couple of grams to over 3000 lbs.

  5. I like to go on the end planes of fans if possible or on heavy blades.  Inside lip if it is available so the rotating force helps hold it on.  Not always possible.  Mainly depend on the equipment.  I like to look and try to see where it was balance before and how the weights were installed.

  6. I keep the speed direction of rotation radius of weights and all the moves and responds for myself.  Makes it easier to balance the equipment next time.  Customer gets before and after and how it meets any spec they requested.

  7. First trial weight I use the formula for 10% the force to the fan.  Rest of the moves are based on calculated formulas.  I draw everything out so I know if it is responding as it should.  Vibration amplitude is the only calibrated equipment I need but if you use the same meter that is not critical.

  8. When it meets spec.

  9. Several based on what the customer wants.

  10. I like about 20% air flow on fans.

  11. As fast as possible but not in a resonance area.

  12. Almost all the time.  I do have one set of machines where I use motor amps instead of vibration.

  13. Most are carbon steel.  It is “C” channel with a set screw.  I have also used bolts, washers and nuts. Again, depends on the machine.

  14. No not to my knowledge.

Alex

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Reply with quote  #4 
I could answer to all questions, but Rusty, I rather advise you to run away from those people (if not some very serious money is in the game). [smile] My experience is, when the customer complicates this way even before the job done, they will always find something wrong even when there isn't anything.
Vibe-Rater

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Reply with quote  #5 
Quote:
Originally Posted by RustyCas
I received the "questions" below from a customer, sent as a questionnaire to all their balancing guys across multiple sites.  I have a feeling it may be about "does Rusty know what he's doing" since it originated at the site I cover, and we lost a large fan there about a year ago (supposedly due to a shroud failure, however I think the failure originated with a 'pitched' blade liner).

I know my answers to all the questions (some of which are obviously coming from a perspective of knowing nothing about balancing or vibration), but I was curious if any of you have strong feelings about any of them.

Feel free to address (by number) any or all questions that interest you.  You, of course, will not be quoted.

1. Can you determine if there is a defect in the rotating unit that has made it unsafe to operate from a balancing standpoint?
Only history will tell. If rotational stuff has not changed for years then unlikely to do so now despite amplitudes maybe being on the high side. So sort of no but maybe.

2. When should you take existing weights off and start over?
In my case I normally balance paper machine rolls which have discrete balance weight locations so it is best to start clean slate. So I always say, remove old balance weights first because we can't be welding everywhere.

3. Are there field conditions you avoid for quality reasons (temperature, wind, etc.)?
Only if I had too many beans the night before.
4. What is the maximum amount of weight you should apply?
From memory there is a rule of thumb??...?? 10%... ?

5. What are your rules of thumb for locations of attaching weights?
Normally to reduce residual vibration. Not to increase it.

6. What records do you keep for yourself, what do you leave with the customer?
MS reports forward to customer - in my case Excel. And all vibration data is kept - because customer doe not have vibe software.

7. How do you calculate the weight size and locations. Do you depend on equipment for that, if so, then how do you know the equipment is calibrated correctly? portable equipment is calibrated annually.

8. How do you decide when the balance is good enough? G1.0 is fantastic anything close to that is normally good too. Especially for low speed.

9. What standards do you use? ISO

10. What damper position do you use when balancing? N/A in my case.

11. If it’s a VFD, what speed (RPM) do you balance at? I would do so at normal running speed but I don't get involved with fans. 

12. When would you use vibration info for balancing purposes? Stupid question.

13. What thickness, shape, type of steel should you use for weights? Whatever fits.

14. Are there any indicators in the harmonics of a fan that would indicate end of life for the fan wheel?  Harmonics would indicate clearance / looseness so a potential maintenance situation. Next question.




OLi

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Reply with quote  #6 

I do second the opinion that it is maybe a trouble customer indication and it may be fun to prove them wrong but it should be taken in to account.
1. Only by general vibration analysis and visual inspection if all mountings are loose and it vibrate 200mm/s or other similar problems that dominate should be fixed first.
2. When the balancing results misbehave or there are no room for more weights.
3. Some sites have restrictions on working above some height above some wind speed.
4. Some OEM have limitations as 1Kg on a 25T rotor in field but otherwise as much as required and possible to fit like 35-40Kg on a 15T worn crusher rotor.
5. Where possible and safe.
6. I write down the data and produce a report depending on what the customer like to have, some 14 pages, mostly 1 page.
7. Mostly software on occasion supported by vectorplot. Actually the precision is based on a good scale all the other is just required to be repeatable and reproducible including the object. I use a stainless kitchen scale from a famous Swedish 4 letter outlet.
8. When customer is happy, when vibration is within standard, when balancing quality is within ISO 1940-1 definition for the unit or it is as good as it gets governed by the bearing clearance.
9. ISO, API whatever customer throws at you.
10. Normal operation if possible r whatever makes it possible to operate.
11. Off resonance as near normal operation speed as possible.
12. Always
13. Any suitable for the purpose.
14. High vibration, looseness multiples, alignment indication, cracked blades, welds, foundation. I refused to balance a fan once, a stainless hi temp with 2" crack propagation where the blade had started to bend over....
All indicating things to be fixed before balancing not end of life.


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VibGuy~5

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Reply with quote  #7 
This sounds like a future court case.
Are you sure their lawyers weren't involved in framing the questions?
(First rule of law-only sue people that have money[biggrin])
RustyCas

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Reply with quote  #8 
If they only sue people with money, then I'm in the clear. Turns out this is a "task force" apparently formed with no vibration expert onboard. They literally have no idea what they are talking about. I suspect some of the questions were fed by a well-meaning supervisor who has little understanding of vibration or balancing. There are many common misconceptions, even within our profession.

I had a knowledgeable fan repair guy (large US shop) say they never "strip" an excess accumulation of balance weights in the field because it would then be "way out of balance" (I just map the weights, calculate, and then resolve all the weights to one (or two if 2 balance planes). Also he doesn't understand why I say when using large weights on large fans, you always split the weights between the 2 planes (shrouds) instead of just putting the weight on the side with the highest vibration. Seems obvious to me.

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Shoveldr

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Reply with quote  #9 
Quote:
Originally Posted by RustyCas
I had a knowledgeable fan repair guy (large US shop) say they never "strip" an excess accumulation of balance weights in the field because it would then be "way out of balance"


I've done way to many balance jobs where the issue was uneven product build up or, more likely, uneven product slough off.  The rare instances where they have actually cleaned the fan my first move is to do a gentle roll to confirm the field weights are on the bottom and then remove them.
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