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Pump Motor Vibration

08/14/2009 5:03 PM

Hello,

We have a Worthington horizontal centrifugal pump with a 2000 hp Siemens electric driver used to pump raw municipal water. Rated speed is 720 RPM, but we operate it with an ASD and limit it to a maximum of 675 RPM. I recently performed vibration assessments at each pump and motor bearing housing in the horizontal, vertical and axial directions with the pump operating at N=672 RPM. The vibration spectra showed normal levels for the pump, with the predominant vibration at the blade pass frequency (7N) with magnitude up to 0.04 in/s RMS. Some measurements had small peaks at various harmonics (2N, 3N) but nothing measureable at 1N.

On the other hand, the motor measurements shown an unusual change from a previous assessment performed just over a month ago. Most of the peaks for the motor were normal, but one large peak was evident in the horizontal plane for both motor bearings at 2.2N (1485 CPM) with a magnitude of 0.18 in/s RMS. Axial and vertical measurements did show the same frequency but at a much lower magnitude (0.02-0.05 in/s RMS). I do have some 2N peaks so I know the instrument was operating properly, so I don't understand what would cause a 2.2N vibration. Any ideas what might cause this?

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#1

Re: Pump Motor Vibration

08/14/2009 7:33 PM

Was it a clean 2.2 spike, or was there a haystack of frequencies around 2N?

What was your Fmax set to and how many lines of resolution?

This may be a figment of the FFT process. If you change Fmax and or increase the number of lines of resolution (say from 400 to 800) it may go away.

This also may be a side band assosiated with the motor speed control, or it may be a torsional being excited.

Finally it is also possible that this is a natural frequency of the motor in the horizontal direction. Try running the motor slower or faster to see if it goes away. Also look for look hold down or bearing bracket bolts, cracked supports, or broken foundation indications.

Hope that helps, let me know what you find.

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#6
In reply to #1

Re: Pump Motor Vibration

08/17/2009 11:14 AM

The horizontal measurements had a haystack around the 2.2N peak (~1450-1530 CPS), but it did not tail out across the 2N frequency (1342 CPS). The vertical and axial measurements also had a smaller haystack at 2.2N peaks, but there was a clean 2N peak with distinct separation between the two.

Fmax is 12,000 CPS with 1600 lines.

I thought about it being a natural frequency, but since it wasn't present in previous measurements and there is no excitation harmonic, the only thing I could think of was some sort of internal looseness. But I would have expected (perhaps erroneously) that looseness would show up on a harmonic frequency. I will inspect the motor closely and try running it at a slower speed to see what happens.

Thanks.

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#2

Re: Pump Motor Vibration

08/14/2009 9:05 PM

What length of the line from the last bend/elbow or valve to the inlet to the pump.

(pump manufacturer would have the minimum requirement)

If length is not sufficient, may have had slight cavitation that was at first unnoticeable, until at a point that enough of the impeller was eroded away, causing the vibration.

phoenix911

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#7
In reply to #2

Re: Pump Motor Vibration

08/17/2009 11:29 AM

We have about 30 ft of straight 34" pipe on the pump inlet, which is borderline but unfortunately that's the way the plant was built. We do get some minor cavitation under certain flow conditions, possibly due to low NPSHA, but we have never had any issues with impeller erosion on other pumps.

If the vibration originated at the pump due to impeller erosion, wouldn't I see stronger vibration at the pump bearings instead of the motor bearings? I saw much lower vibration at the 2.2N peak on the pump, which I expect was transmitted through the rigid gear coupling.

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#9
In reply to #7

Re: Pump Motor Vibration

08/17/2009 11:49 AM

Like your company, I have design using minimum requirements for inlets before, because of no other way.

That may be an issue, that should be looked at. Remember, erosion due to cavitation can happen immediate, be results or effects can show up later as its eroded away.

phoenix911

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#3

Re: Pump Motor Vibration

08/16/2009 2:23 AM

Hi !!! I am not an expert on Vibration Anaylsis on Machines , but would like to suggest to change the material of Base Frame on which Pump & motor are mounted.

Perhaps you may be aware that Polymer / Epoxy Mineral Castings have around 6 to10 times better vibration damping property compared to Cast Iron and about 40 to 45 times better than steel

Ploymer / Epoxy Mineral Quartz grout can also be filled in Mild Steel fabricated Bases to enhance vibration damping properties .

Perhaps this might help you reducing Vibration . you can know more about this material on website www.rampf-group.com or on www.cptllc.com or www.granitek.co.uk

Udayan Patel

INDIA

udayan121049@yahoo.co.in

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#4

Re: Pump Motor Vibration

08/16/2009 8:54 PM

Well... I know you are worried because it changed but 0.12 ips is very OK. I like to think in terms of severity, not % of change. Ex: If a 0.005" changed to a 0.01" it is double ... "Vibes went up 100% ... !!!! " , but severity is still low.

Make a rough sketch of the horizontal profile of the unit from the foundation up, like looking at the unit, foundation, grout and machine from the end bell of the motor. Probe it horizontally each 1 inch or at each structure transition (foundation to grout ... grout to skid ... skid to motor mount) and graph the vibe levels at each point. You might just find the trouble area where the vibes jump. The graph helps to visualise 'looseness' or deformation. I drew cute little ppt and made a pdf of it but can't seem to embed it here. I can send it by email.

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#5
In reply to #4

Re: Pump Motor Vibration

08/16/2009 10:35 PM

Instead of saving the ppt as a pdf, try saving it as a jpeg and posting it here. I have saved individual slides as jpegs from powerpoint presentations and posted in comments and it has worked without a problem.

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#8
In reply to #4

Re: Pump Motor Vibration

08/17/2009 11:49 AM

I agree that the vibration severity is still OK, but I am mostly concerned with the appearance of this particular frequency, which wasn't evident about a month ago. Also, since it jumped up to about 2/3 of the ISO limit of 0.28 in/s I suspect that the condition could rapidly deteriorate and become a problem. I will try taking additional measurements as you suggested and see if I can identify where the problem is located.

Thanks.

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Guru

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#10
In reply to #8

Re: Pump Motor Vibration

08/18/2009 10:41 AM

Assuming that you have a "flexible" gear coupling, not a "rigid gear" coupling and assuming that you're pumping "cool" raw municipal water that doesn't vary much in temperature or corrosiveness, I don't see erosion at either inlet vane( loss of NPSH ) or discharge tips (creating an imbalance) as being as much an issue. The coupling should have been selected so as to prevent any such transmission of vibration issues to the motor. I would suggest that when the opportunity arises to remove the pump from service, disconnect the pump from the motor and run the motor without any load through your spectrum of desired speeds and see if the vibraton changes are still there. If you have ever thrown a wheel balance weight on your car and noticed that there are certain limited speed ranges at which you have a shimmy then you'll know where this is going.

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Users who posted comments:

Anonymous Poster (1); dac1267 (3); Jaxy (1); PetroPower (1); phoenix911 (2); Spinco (1); Steve S. (1)

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