High Speed Balance vs. Low Speed 1

[TangoCleveland]

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

 

[CESSNA1]

TANGOCLEVELAND:

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

 

[GregLocock]

Roughly 0.01 mm/sec, but I would not be in the least surprised to see any number, just about.

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.

 

[electricpete]

I think the question needs a lot of clarification.

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.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[TangoCleveland]

Our high speed residual unbalance, with stiffness considered, ended up at 6.4 and 15.3 gram-inches (there are two bearings). A shaft sleeve was replaced on the blind end of the rotor, and I'd expect to see similar residual unbalance at low speed. This rotor was completely assembled into the compressor, and the shaft sleeve failed on the first attempt to run the machine. Manufacturer replaced the sleeve; seems that the interference fit wasn't tight enough...

Larry

 

[alexit]

If it was balanced at 12,000 rpm before, and you've touched anything connected to the shaft or that applied a load to the bearings (locknuts, shaft collars etc.) you could have disturbed this result. If the machinery operates at 12,000 rpm, you better not low speed balance at 1,000 rpm, the life you save could be your own...

 

[GregLocock]

15 gram inches is 30 g cm, that's way outside our spec for a driveshaft at only 6000 rpm. Is this some huge great machine?

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.

 

[TangoCleveland]

We're visiting the low speed balance shop on Monday. The repeat high speed balance would cost us $14,000. This is a new unit that was tested successfully at the manufacturer's shop after balancing. The compressor is designed for air, 40 psig suction at 100F, 150 psig discharge, 38 lb/second flowrate. Rotor is 560 lb, 50.6" bearing span, and 19.1 inch diameter for the largest of 3 impellers.

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

 

[GregLocock]

30 g cm for a 200 kg machine is pretty good. I can't tell from those peaks whether you have a flexible rotor, I think not, in which case a low speed balance may be OK.

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.

 

[TPL]

If I have done the sums correctly, then your combined unbalance of 6.4 and 15.3 gram-inches corresponds to 550 gram-mm which, for a rotor of mass 254kg at 12000rpm looks like a balance grade of G2.5 as defined by ISO1940. Now G2.5 is alleged to be good for "Gas and Steam Turbines, Turbo-compressors" but experience has shown that aiming for better than G1.0 (a total of less than 220 gram-mm) is more realistic for high spped machines (and easily achievable - try 4W/N for a better tolerance).

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.

 

[TangoCleveland]

TLP,
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

 

[TPL]

Well its done now - its still not clear where the velocity readings that you quote came from - were these from the machine bearing housings after it had been rebuilt?

I may have missed it, but I don't recall API 617 specifying vibration limits in terms if velocity, only displacment mils or microns

 

[TangoCleveland]

TLP,

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

 

[TangoCleveland]

We ran our reassembled air compressor today, with 40 psig inlet and 135 psig discharge. Compressor vibrations were fine, until the thrust bearing end shaft sleeve crept down the shaft and contacted the bearing housing, resulting in a large shower of sparks. Same problem as original incident. Back to the shop.......

Larry