NonSynchronous Vibration at Vertical Cantilever Pump

[jsasada]

Hi all,

We got high vibration on 1.66X shaft speed of a vertical cantilever pump (at 2250 cpm). The RMS of Motor NDE bearing is 13 mm/s (being the highest). The motor is variable speed with rated of 1475 rpm. During the measurement the speed was 1349 rpm. Any thoughts on why this is happening? This has been a steady problem since a long time. But it is not detected with online vibration of DCS because they only measure the RMS on pump DE bearing horizontal. For more detail, I attach below the spectra.

The time waveform:

The picture of the pump itself:

Looking forward for any thoughts and advices.
Best regards,

Yosep

 

[Tmoose]

Does the vibration remain high if the speed is increased 9% or decreased 10-15% ?

The position labels on the spectra are too small to read.
I //think// the vibration is not just a simple reed motion of the pump system above the floor

Regardless, I believe it is self excited ( by flow ) vibration at one of the natural frequencies of the LONG thin poorly supported pump assembly.
Are there other identical pumps on site that do not have that vibration?

I'd investigate several aspects of the installation.
1 - The design of the sump vs ANSI / Hydraulic Institute Standards, and observe the sump while the pump is online looking for vortices etc.
2 - There are several mechanical interfaces that can seriously muck with the system natural frequencies. Mostly lower them numerically compared to any of the manufacturer's simplified modeling and calculations.
Motor to pump, pump to motor support, motor support to pump head, pump head to sole plate, sole plate (grouted and bolted) to floor.
2A - I'd make a vibration map of the sole plate and floor and add liquid to sole plate to floor joints to test for motion/pumping there. Using nuts on anchor bolts on the underside of the sole plate to level the pump before grouting is pretty much guaranteed to end up with a weak, flexible joint no matter how hard the anchor bolt nuts are tightened.

h

https://www.gouldspumps.com/getattachment/Products/VIT/vit_ff_pl_cutaway.jpg.aspx