Today, i took the vibration of a pump whose dominating frequncy was 0.61 X. The pump is a vertical type variable speed which is having two angular contact ball bearings at the top and a hydroststic beaaing at the bottom lubricated by pumping fluid itself.The pump is side inlet and bottom dicharge type.The vibration in the bearing area was 1.6 mm/sec and the dominating vibration was 0.7 mm at 0.61 RPM. As it is not possible to measure bottom bearing vibration, the discharge line vibration was measured and found 6.1 mm/sec.The dominating vibration was 4.8 mm/sec at 0.61X .I changed the speed( to eliminate the confirmance of external excitation), flow (to eliminate flow induced vibration)and fluid temperature ( To eliminate oil whirl and whip )but the dominant frequency was remained between 0.61 - 0.64 X only.What could be the other reasons for 0.61- 0.64 X vibration.

In the side and out the bottom? Correct rotation? Lubricant is product?

Collect additional data on the pump and coast down data as well; report findings if possible. Also ring test (bump test). Operational parameters are:____________________________? Stable and normal?

I've seen this subsychronous vibe on several vertical pumps over the years, and each time it's meant we lost the top pump bearing (down in the hole). I'm not enough of an engineer to explain it mathematically, but it's held true.

I believe you are having a wiped out bottom bearing. Its reduced stiffness caused resonance condition. Is there a lot of random impacting at the location of the bottom bearing?

Please post the spectrum, TWF, and your findings.

We have had long-standing vibration on some of our vertical condensate pump motors in the neighborhood 0.35x. Upon decreasing flow the magnitude at 0.35x gets higher. It has been suggested to us it may be a combination of worn seal rings, opening radial bearings, and resonance of the pump column. This frequency stays right where it is when we vary flow but the magnitude increases as low flow. These motors have Kingsbury bearing on top, water-lubd pump sleeve radial bearings. No partner sideband above 1x on these.

On another group of vertical pumps/motors (our LPHD pumps), I posted a lot of info in a thread entitled something like "0.57x sidebands around 1x". One similarity to your situation is we have on top of the motor a standard angle contact bearing and a 4-point contact (bidirectional thrust) angle contact bearing. What we see is these subsync vibration frequencies have high magnitude high during either of the following conditions:
1 - the motor is uncoupled.
OR
2 - the pump is at low flow about 50% B.E.P.
The non-sync vib goes away at normal BEP flow. Discussion with our motor repair shop indicates it is likely some kind of whirl due to the fact that upper bearing is much less stiff radially when you take away the axial downthrust. If you have a partner sideband somewhere around 1.4x maybe you have a similar phenomenon.