Resonant Frequencies in Pump Operating Range - How Far to Stay Away 1

[cdavidso]

Hello All,

I am interested in hearing resoning for the minimum range to stay away from resonant frequencies in a pump's operating range.

For some context, the pumps I am interested in are vertically mounted centrifugal pumps on the order of 900 HP, used to pump water (in a wastewater application). The pumps are having vibration issues on the motor non-drive end. I believe this is the "reed" frequency.

I have read some literature that says to stay at least 10% of full operating range away from known resonants. I have read some other lit that says 25%.

What is the industry standard?

Can anybody quote any standards (ISO, etc.)?

Any anecdotal evidence.

Thank you,
Chris

 

[1gibson]

Industry standard depends on the industry

ISO13709:2009 (aka API 610-11th)

"Generally, a 20 % margin of separation should be maintained between
the natural frequency of the motor support structure and the operating speed."


You said operating RANGE, is it on a VFD? Can you avoid the vibration with a change in speed?

 

[cdavidso]

Thanks for the info, that is one of the standards I am currently reviewing!

Yes, it is on a VFD, and we can adjust speed. Right now, the speed varies according to inflow, and can sometimes trip out the pump on vibration.

The operators have a few work arounds, but as you surmised, we are trying to develop operational protocals to keep the pump away from its resonant frequencies (i.e. do not operate pump from 45-85%).

I forgot to mention, this is actually a group of 5 pumps that all work together.

Thanks again,
Chris

 

[1gibson]

Here's a good read:

http://www.pump-zone.com/pumps/vert...ral-resonance-problems-on-vertical-pumps.html

 

[cdavidso]

Wow, that is a good read, thank you! I was not aware of spring plates.

 

[electricpete]

Another possible strategy for addressing running close to resonance (besides adding stiffness or math) is to add a tuned dynamic absorber:

thread384-264106


=====================================
(2B)+(2B)' ?

 

[1gibson]

Pumps in barrels, or a sump? If a sump, you could have some interaction at the suction of the pumps based on the inlet flowrate, and sump geometry. The pump speed when that happens could be secondary.

Did you confirm the resonant frequency (hit pumps with a mallet and record vibration spectrum) or do you just know that it sometimes trips at a given pump speed? Which could also mean it trips at a given inlet flowrate. Or it could mean that it trips on Sunday morning, the point being: keep a broad perspective while gathering information.

 

[Artisi]

I agree with 1gibson, it could well be inlet / flow / sump / water level configuration and should be investigated.

Does it happpen on individual pumps or is it on multi-pump operation, does sump level change during operation etc etc.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[cdavidso]

Sorry for the slow response...

The pumps are all in a sump.

This problem happens when all the pumps are running.

I will keep an open mind, the research continues...

Thank you for all the help!

 

[Artisi]

If all pumps are in a common sump and it only happens when multiple pumps are operating then it would appear to be inlet conditions, not a pump frequency problem.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[SNORGY]

That kind of screams "vortex" to me...

Regards,

SNORGY.

 

[Artisi]

Vortex is one likely problem, however it could be uneven flow or disturbed flow to the pump inlets etc etc -

Once again it's was the pump/s at fault. Why is the first call always " there is something wrong with the pump/s" when a problem comes up - doesn't anyone ever consider that a lot of these pumps, designs and hydraulics have been round since Adam was a boy and run perfectly well under the conditions they were designed for and quiet often run well away from the design conditions.

I would wager a bet that if all the "something wrong with my pump/s" posted here were analysed, the majority would be other than pump problems.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[electricpete]

Yes, it is on a VFD, and we can adjust speed

This problem happens when all the pumps are running.

So, you're saying the vib occurs when all pumps are running, but not in other conditions when pumps are not running. Is the speed the same in the conditions you're comparing ? Or closer to the suspect resonant frequency when all pumps running?

=====================================
(2B)+(2B)' ?

 

[electricpete]

should've been "...but not in other conditions when pumps are not all running..."

=====================================
(2B)+(2B)' ?

 

[cdavidso]

Artisi/Snorgy, sounds like I may have a combination of resonant frequencies, and sump flow dynamics.

Electricpete, Yes the problem vibration occurs when all pumps are running together. Yes, the problem vibration occurs under the same loading condition (i.e. at a certain total flow over all the pumps).

The work around has been to shut down one pump and run only 4 at a higher rate to avoid the "problem-zone" of operation.

I am trying to organize some run-up/run-up down tests (at varying flows), and will hopefully have some more info.

Thank you all for the comments, you have given me alot to consider.

Thanks,
Chris

 

[Artisi]

I would put money on the sump.

Is it a sump or a forebay configuration, how are the pumps arranged - a sketch or some photo's might help throw a bit of light on the problem.


It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)