4-Run Method of Balancing 1

[mtigos]

Can anybody help me with the procedure of carrying out a 4-Run Balancing Method using Mils/sec as the vibration parameter. I am only familiar with using Mils instead. Also, please give me the equations for calculating the correction mass

 

[GregLocock]

It is a bit of a slippery fish. As it happens the B&K blue book on vibrations does include an explanation and a diagram.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[electricpete]

I have provided something of a graphical proof of the method attached to my message 19 Feb 09 21:27

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[MachEng]

mtigos,

I have read all of the discussions and I have not seen mentioned a very important step. This is a flu gas fan, a very large one. It was mentioned that it is always going out of balance.

If you have already thought of this please forgive me, but it strikes me that the first thing before balancing would be to clean the fan of all accummulated dirt on the impellar blades. After cleaning run the fan again to check the balance if it is still unbalanced excessively remove all weights and start from scratch. Usually the fan is very close to being balancd from the factory (it may have come with a trim wieght which in that case you will probably be putting a balance wieght back in one of the spots). Balance your clean fan and then if it becomes unbalanced again clean it first and you should solve your problem without balancing. Balancing a dirty will always end up out of balance again soon.

Also, it was never mentioned whether the fan is an overhung impellar. If this is the case then you may be dealing with a two plane balance problem almost in all cases. There is a special procedure on the Emmerson/CSI web site under Dr. Know.

I trust this is helpful for you.


DANIEL C. SMAISTRLA
judasmai@optusnet.com.au

Equipment: Centrifugal Pumps and compressors; Steam and Gas Turbines; Reciprocation Engines and
Compressors

Specialties: Machinery diagnostics, design application of compressors, FEED,Detail Design,construction,commissioning and maintenance.

 

[fredt]

Having balanced many large fans it always surprises me that many engineers consider that it was all black magic when it really was just following the theory. However, there are many pitfalls. You do need an analyser but it is vital that you understand the vector method or things can go awry if the response is not exactly what the analyser expects. You can rent analysers. Although many out of balance problems can be corrected in the single plane the more difficult ones usually turn out to be two plane. I recommend correcting the static or single plane first hence the need to understand what is happening - the analyser computer will try to do both in one step which can lead to big problems in tricky cases. On large fans removing a couple (two plane) may require very large balance weights which must be exactly equal and diametrically opposite - several kg on each plane are not unusual. Be careful -especially with weight attachment - make sure that you know what material the impeller is made from and use appropriate welding. In your case the my guess would be that you do have an issue with the support structure stiffness and are running close to resonance (which can make balancing very difficult with conventional methods). Fixing the foundation could remove the problem long term. There is another problem that occurs on a few large hot fans - expansion may not be symmetrical and the fan may have to be balanced hot for best results. Check for other sources of vibration - impeller contact with the inlet cone or shaft seals when the fan is running at temperature. Hope this is helpful. In many cases you will need more than the theoretical minimum four runs.

 

[fredt]

As a PS to MachEng comments. Be careful about removing all balance weight as this could result in dangerous out-of-balance when you try to run the fan. Large balance weights are often applied by the manufacturer to correct fabrication asymmetry which is not unusual on big fans. Therefore don't assume that the impeller with all weights removed will be close to being in balance.