Heat exchanger tube vibration

[C4Reactor]

What is the best way to determine the vibration of tubes in a shell and tube exchanger. Is just monitoring vibration on the shell external wall gives a valid data? What is the normal vibration figure and the max allowable. If you guys have any experience in this, appreciate any sharing of ideas.

Thanks

 

[vanstoja]

I don't have any direct experience in shell in tube heat exchanger vibration measurements but my general impression is that external shell measurements would not provide reliable data on internal tube vibrations because of the many non-tube component responses likely to be present in accelerometers mounted on the shell. I think you would have to get acceleration measurements directly on the tubes or at least the tubesheets supporting the tube ends. A recent text that treats heat exchanger vibration analysis extensively,is "Au-Yang,M.K. (2001), "Flow Induced Vibration of Power and Process Plant Components - A Practical Workbook", ASME Press, ISBN: 0-7918-1066-7. It cites experimental work but doesn't address test instrumentation. In Sect. 12.5 on "Heat Exchanger Acoustics", Au-Yang shows and describes resonance maps developed by Zaida etal (1989)in the Journal of Fluids and Structures Vol. 3 pp. 293-314 and 315-324. The resonance parameter G uses fluid and acoustic Reynolds numbers with staggered and in-line tube array dimensional parameters to define resonant and non-resonant conditions. Au Yang notes that two-phase flow and inhomogeneous single phase flow tend to disorganize and randomize tube vortex shedding and structural lock-in effects. Another source addressing the same issues of heat exchanger tube vibrations is Lucas,M.J. etal(1997), "Handbook of Acoustic Characteristics of Turbomachinery Cavities", ASME Press, ISBN 0-7918-0054-7 where Chapter 9 is on "Vortex Shedding from Bluff Bodies"

 

[VibEngr]

vanstoja - Those are good references. Blevins also has a good book on flow induced vibration and vortex shedding.

hfzmm - We have performed several field investigations of large heat exchangers. Tests we have used include impact tests of the tubes while the unit is down, strain gages inside the heat exchanger while the unit is running, and pressure measurements.

Impact tests will tell you if there are potential modes of concern. The main reason you care about tube vibration is if it causes high stress, cracks, and failures. The strain gages will help identify these problems.

We have observed that at various flow conditions, acoustic modes will be excited. Vortex shedding is usually the cause, but an acoustic mode can significantly amplify the pressure pulsation, which can be measured with pressure transducers.

The big problems occur when mechanical and acoustic natural frequencies are coincident. For more information on high frequency acoustic modes you may download the following paper at

http://www.engdyn.com/download/84.pdf

 

[hacksaw]

Agree with other posts, you are not going to get meaniful data from external measurements, however, if you have external noise and/or vibration then you may have a problem in the tube-bundle.

Blevins and varous heat exchanger authorities such as the ASME and the Heat Exchanger Inst. have design guidelines for avoiding tube bundle resonances.

It is not a simple or straight forward process. Your supplier is a good starting point.

Good luck,