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Have a question I need some help with...

When doing a modal analysis on a vertical pump, there is no water in the columns or the bowl assembly when the pump is not running. Just to clarify, we will be instrumenting this pump both above and below grade. The water will be drained from the sump the pump takes suction from during the Modal analysis portion. An ODS will be performed following the modal.

This lack of water will effect the stiffness and the damping of the components, not to mention it may shift the frequencies of the resonance(s) identified by an unknown amount.

With the ODS being done while the pump is running and filled with water, you should be able to see the mode shape of the components (ie; column) if they are at resonance which will be beneficial and will help narrow down how much of a change there may have been. I am thinking that we would also be able to look at the spectrums collected during the ODS (and the frequency range of the resonance identified) in log scale to then help determine the amount they (NF's) may have shifted with the water now in the pump and the associated changes in frequency/damping.

How do you account for these unknown values of change when you are entering the modal in ME' Scope and with the conclusion? How do you know how much of a shift you may see in the resonant frequencies given this situation? I am curious how people have handled this in the past.

Please let me know.... I would appreciate any help I can get on this topic as I have an upcoming project that this will be an issue with.

Thanks so much,

KB

Tags: Vertical, Analysis, NF, Modal, Pump, ODS

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If I'm not mistaken, Vibrant Technologies has a version of MeScope that allows the user to input variables (e.g. mass of water) and make assessments of what you are looking for. You might want to contact them directly.

For the modal test, are you impacting above ground and obtaining response data from the support column below ground? I've not done that but would be very curious how well the data comes out. 

So when the ODS is performed (now there is the added mass of the water in the wetted piping) you are hoping you might see a change in Fn. You're right in that you may or may not even excite or see the mode(s) depending on location of Fn to running speed. You could try to "jog" the pump to provide an impulse to the whole structure in the hopes it might provide enough energy to see the modes.

If you do find solution to this testing, please share

Best regards

Jim Powers

RM

Hi Jim,

Thanks for looking at this and replying.

I will most certainly look into the mass of water/ME' Scope issue. I am curious about this as well. The guy we've hired to do this testing is one of the best at Modal/ODS that I know of and has years and years of experience so I am the results we will get are as accurate as possible.

Like you said, we will be using a combination of coast down testing, ODS, and Modal data to try to narrow down where those frequencies are and how much they shift with and without water.

I will look into the ME'Scope possibility and get back to you. Will also share the amount we find the NF's shift with and with out water.

Thanks again for your help.

 

KB

RM

I have done this type of testing on large vertical pumps, but not with ME-Scope. Water inside and outside the pump column adds mass. A rule-of-thumb is to use twice the volume of water inside with same sump level to account for the mass of water outside the column.  There can also be a difference in natural frequencies with the impeller support conditions between wet and dry. Pray that the stop logs hold back the water when you are standing knee deep in water, muscles, and gunk while impacting the pump inlet bell!

Walt

RM

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