From Ken Hawkins 46

“Since 1950, my vocation has involved rotating equipment - pumps, blowers, compressors plus their drivers and systems.

Beginning in 1970, the focus became vibration problems involving balance, alignment and resonant amplification of normal exciting forces. It has taken me to every continent but Antarctica.

In July or 2006, I was sent to a Georgia water treatment plant to check vibration of a 1250HP, vertical turbine, variable speed, can pump that vibrated unacceptably at ‘2100’ cycles per minute while rotating at ‘1350’ revolutions per minute.

The 1st cantilever resonance critical frequency was 744CPM and the second was 2325CPM—from floor to the motor cap is approximately 16 feet. Note that there was normal vibration of the variable speed pump below 1200 revolutions per minute and above 1450 revolutions per minute.

It was apparent that the second critical was being activated by an outside source as even the motor turns at only 1750RPM.

The exciting noise, originating in a tee entering a large manifold—but very close to a blind flange backed by a concrete thrust block, was the culprit. With a vibration analyzer I found that, while an adjoining pump tee produced low value vibration, the offending tee produced a single spike at the correct frequency at 100 times more power.

Correcting the tee would be quite expensive (plant shutdown) so I made two tries to increase the cantilevered rigidity (and critical frequencies) but failed for reasons I’ve not yet determined. Lowering the rigidity would require lifting a 13,000 pound motor plus the variable speed drive but no crane is available. However using this method, there is a good possibility that the problem might be displaced but not corrected.

The tentative plan is tee correction sometime this winter.”