High Speed Balance vs. Low Speed
High Speed Balance vs. Low Speed
(OP)
We had a new compressor rotor high speed balanced at 12,000 rpm, with a final vibration reading of 0.14 mm/sec velocity. Some work was done on a shaft seal, and we are doing a low speed balance at about 1,000 rpm to save money. If balance is identical before and after the repair, what kind of velocity would we expect compared to the 0.14 mm/sec at 12,000 rpm?
Larry





RE: High Speed Balance vs. Low Speed
It depends on the balancing machine and the residual unbalance that is specifed. I have seen people try to balance to .001 in-oz in a 5 pound rotor on the same machine that they used to balance several hundred pound generator rotors. NO WAY - you can not get that kind of residual unbalance accuracy on a large balancing machine.
From my handy-dandy vibration computer I find that at 12000 rpm a 14 mm/sec velocity equates to about 1.8 G and is in the "rough" part of the scale. The "OK" part of the scale is less than 4 mm/sec
You should find out the residual unbalance specification from the manufacturer and then find a balancing machine that can meet that requirement.
Regards
Dave
RE: High Speed Balance vs. Low Speed
Dave you misread the first note, it is 0.14 at 12000
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: High Speed Balance vs. Low Speed
The machine was balanced at 12,000 rpm and "final" vibration was 0.14 mm/sec. Is that still on the balance machine? Why do you express it as a velocity instead of a calculated residual unbalance using stiffness coefficient calculated during the balance run?
You can calculate that stiffness coefficient also during the 1,000rpm, which would in a simple way allow comparing the unbalance condition... except for the fact that a low-speed balance may not be satisfactory for a flexible rotor which operates above 75% of it's first critical. In that case, the flexing of the rotor near operating speed isn't recreated at low speed.
Forgetting all that and focusing on a question: "what happens if a machine has unbalance causing 0.14 mm/sec at 12,000rpm and I slow it down to 1,000rpm". The answer to that in general we can't tell since we don't know where the resonance or critical speed is. If we assumed both speeds far below resonance, we could assume velocity of vibration from unbalance proportional to speed^3. If we assumed both speeds far above resonance, we could assume velocity of vibration from unbalance proportional to speed^1. Many other possibilities when one speed near resonance or both speeds on opposite sides of resonance.
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RE: High Speed Balance vs. Low Speed
Larry
RE: High Speed Balance vs. Low Speed
RE: High Speed Balance vs. Low Speed
Have you plotted first order vibration vs speed?
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: High Speed Balance vs. Low Speed
Vibration vs. speed at final balance with stiffening showed a peak of about 0.2 mm/sec at 6,000 and 9,000 rpm, about 0.1 mm/sec at 12,000 rpm.
I'm grateful for the details of balancing that you've all provided. You've given me lots of discussion items for the manufacturer and the balance shop. By the way, the compressor manufacturer is a very old and respected company, but I don't want to mention names in public.
I'll let you know how things look at the shop. Thanks again!
Larry
RE: High Speed Balance vs. Low Speed
I guess the cheap solution is to low speed balance it and then try it out. If it vibrates too much then tear it apart again and do a high speed balance.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: High Speed Balance vs. Low Speed
What do you mean when you say “with stiffness considered”? – I have not heard this terminology before. Unbalance is unbalance in terms of oz-inches, gram-inches or gram-mm – the amount of vibration velocity, as measured on the casing that you see when the rotor is built into its casing will be determined by the casing stiffness, in general, the higher the stiffness the lower the vibration (for a given unbalance).
In this case, its not a question of whether or not the rotor is flexible but a question of 'how flexible' in as much as which mode it adopts when at operating speed. If in its 1st mode (banana shape)you have a fighting chance, but in its 2nd mode I would not rate your chances very highly, hence the need for a high speed balance. With a relatively light and long rotor with impellers at 19inch diameter, I wouldn't like to guess which mode you are in at operating speed, but you should ask the manufacturer.
What might help is if any balance adjustments were made only on the sleeve that was replaced and nothing was done to any other part of the rotor.
What is not clear is why you are liable for the cost of a high speed balance when the manufacturer said that the interference fit of the sleeve was incorrect: shouldn’t they be responsible for putting it right?
BTW: stating that the compressor company is old and respected doesn’t mean a thing in this day and age.
RE: High Speed Balance vs. Low Speed
We're a support contractor for the Government. The Government is working the issue of liability, since they bought the equipment. Here's the trip report I sent out, edited for anonymity:
Low speed balancing was performed at 700 rpm on a calibrated Hoffman balancing machine. Residual unbalance limits were calculated as required in API Std. 617, "Axial and Centrifugal Compressors and Expander-compressors for Petroleum, Chemical, and Gas Industry". The results were acceptable, and residual unbalance was comparable to the results of high speed balancing done in November, 2004 by the manufacturer. Government will be provided with a formal balance report by the manufacturer.
The original high speed balance resulted in vibration of 0.21 and 0.14 mm/sec at each bearing at 12,000 rpm (API617 requires less than 1 mm/sec). Residual unbalances were 6.4 and 15.3 gram-inches at each bearing at 1,000 rpm. API617 requires low speed residual unbalance readings during high speed balancing for future comparison in a low speed balancing machine. That is exactly the situation we have here. During this low speed balance, the target residual unbalance was 5.5 gram-inches at each bearing, which is less than the high speed test numbers. Balancing with residual unbalance is essentially independent of rotational speed, so the low speed balance on 10/24/05 will be acceptable.
Larry
RE: High Speed Balance vs. Low Speed
I may have missed it, but I don't recall API 617 specifying vibration limits in terms if velocity, only displacment mils or microns
RE: High Speed Balance vs. Low Speed
The original high speed balance resulted in vibrations of 0.21 and 0.14 mm/sec at each bearing. Residuals were 6.4 and 15.3 gram-inches. Our low speed balance was on residuals, targeting 5.5 gram-inches or less.
API 617, paragraph 2.6.8.5, gives vibration acceptance criteria for opersting-speed balancing.
We're reassembling now, and will probably run on Tuesday afternoon. We have Bently-Nevada displacement probes on the shaft, and we'll also use our IRD 885 for local readings on casing vibrations. I'll continue to report progress.
Thanks all for your valuable information.
Larry
RE: High Speed Balance vs. Low Speed
Larry