A question for my diagnostic friends:

Was called to investigate operator concerns of higher vibration on a river water pump used for cooling.
Operator noted increase in vibration following an event where ice buildup around reventment screens resulted in operation for number of hours with water level very low - just above the intake bowl. Following clearing of the ice buildup, the operator stated he notices no degradation of flow capacity.

The pump is 3-stage vertical, close-coupled (Floway J-line, 4000 GPM) pump driven by 250 HP, 4 pole motor.
The pump shaft is supported by bronze bearings at impeller/bowl assys and rubber bearings along line shaft.
The bowl assy is probably 30' long.

Vib data was taken at several locations on motor and the only two significant peaks were at running speed (~ 0.2 ips) and a larger sub-synchronous peak (~ 0.3 ips) approx. 57% of running speed.
I compared data against a baseline taken some years ago which showed no evidence of the sub-synch peak.

Question to me is what is causing the high vibration and what course of action to recommend.
I "suspect" the source of the sub-sync peak is some type of excessive recirculation in the line shaft, possibly the result of low flow condition (lack of fluid support - resulting brg wear ?) noted several months ago.
The assy is not well suited for prox probes so am trying to give best answer on data I have.
I plan to have data taken at several different flow conditions to see how sensitive sub-synch is to flow.
Other than the higher vibration noted, the pump seems to be doing its job.

A bit long winded but I appreciate any comments or suggestions from my experienced colleagues. Has anyone seen similar peaks above 0.5X ??

THANKS Jim P

Can rubbing and/or to much clearance give excitation of some kind that excite full shaft length resonance? Or a part that no longer is damped by rubber bearing support excited by flow? Do a waterfall just to check, have no better idea. Olov