Dear all,

I’m monitoring a 40kW (2975rpm 2 pole) vertical lube oil pump. Please refer to the attached vibration data for the pump. On the 26th Feb the coupling (Jaw coupling) was visually inspected and found the spider completely damaged and the coupling was replaced.  Based on the spectra and time waveform  is there any case for concern? What could be causing the modulation at around 3.5Hz? The pump bearing was replaced on 22/11/18 and the coupling was replaced at the time as well. Is there anything in the data that could explain the coupling failure in 3 months? What would be causing the sub synchronous vibration still present after the coupling replacement on 26th Feb??

All help is much appreciated 

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Original Post
Kishore kumar posted:

Dear Thimba ,

Primary suspects are Vertical shaft alignment and Spider coupling clearances .

Hi Kishore Kumar,

Thankx heaps for your input. I found the below paper that seems to suggest the low frequency modulation could be related to severe misalignment and coupling.

http://files.engineering.com/d...of_Misalignments.pdf

I am however still  puzzled about the vibration around the   1/2 x. My understanding is that these sub synch vibrations may be caused by rotating looseness. Any thoughts on this? 

John from PA posted:

Do you have sufficient information to rule out a possible torsional issue?

Thankx John for your comment. I'm not sure if I do. Do you mean there's some resonance due to extrication at the torsional natural frequency. How would I go about detecting/ ruling out a torsional issue. I am relatively new to vibration analysis and haven't come across this issue.  

 

Dear Thimba ,

I suspects  coupling excessive clearances, there are peaks to 1x, 2x, 3x, 4x rpm.

this kind of behavior in vertical pump near to coupling (Motor DE) is associated to coupling looseness.

the sub harmonic peak is not cause of any concern.

Best Regards.

RRR

Yousuf Masood posted:

Sub synchronous peak of 3.5 Hz is not a cause of concern but there is room for improvement in shaft alignment. 

Thankx heaps for the reply.. It was the 1/2 peak+ harmonics and modulation and sidebands I was worried about at around 3-4Hz. And as you suggested it turned out the alignment was actually off.

Thank you all for your inputs on my conundrum. We decided to stop the pump on the 5th March and check alignment and coupling and found there were some misalignment and some wear in the jaw coupling spider. Pump was aligned and the coupling was changed to a grid coupling as per the original design. I noticed the 1/2x and the harmonics have somewhat disappeared but the amplitude modulation and the sidebands remained @ around 3-4Hz .

Since the coupling change I have been monitoring this daily and it looks like the 1/2x + harmonics are back and the 1/2x seems higher than the running speed peak.

I have also noticed some high frequency peaks and harmonics with 2xLF sidebands in the acceleration spectrum but I do not know the RBPF or winding slot frequency.

Does anyone have any thoughts in this?  What could be causing the 1/2x and harmonic? And what might be causing the modulation?

On a much older thread here there was suggestion from @Rusty Cas that this is most likely rotating looseness due to excessive bearing clearance.

https://www.maintenance.org/to...ed-harmonics-but-why  

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1/2x vibration is because the coupling/shaft tolerances are not precise.  1) Check the OEM recommendations 2) Measure shaft  OD and coupling ID and verify the limits are within the OEM specs 3) Monitor coupling mounting method and the use of correct coupling grease ( Ideally NLGI-3) , also grease should be well packed in the coupling.

ramonruiz posted:

Dear Thimba,

can you share picture about the pump and motor (pump model and brand or sectional drawing).

also, peaks 1/2x are related with looseness..

and now we need to know where they coming.

RRR

Dear Ramonruiz,

The pump we got is a buffalopumps 3308 type. I have attached a sectional drawing here as well plus a photo of the motor name tag and a picture of the vertical pump. Hope this helps. 

The measured vibration levels are higher at the motor NDE but now sure if this is the case as there's poor access for measurement at the motor DE and pump DE as marked on the picture.mceclip0

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OLI posted:

Don't measure on the thin metal shroud on NDE measure on fins or solid stator.

Thankx Oli, 

My preference was to measure on the solid stator but its curved both horizontally and vertically. My measurement point is actually on a solid lifting lug connected to the motor housing near the NDE bearing.  I have also measured on the fins and the spectra are similar with slight changes in amplitude (dominant 1/2x peak with some modulation on the TWF) 

Ok, that is also ok. I just saw the picture in principle pointing to the shroud and that is not so good. In worst case I even use the grease zerk if there are such if all the rest is plastic and thin aluminium. Just checking. By the way, the 0.5X and harmonics to me indicate severe looseness. So are the coupling hubs worn so they eat up the new inserts and you get looseness again PDQ? Or anything else including hubs mount on the shafts?

Dear Thimba,

Still you can follow the equipment vibration trend .
Next opportunity check as follow:
1.- Axial displacement of pump shaft trying to push up the shaft ( make reading using dial indicator).
2.- check tighten all bolts around base plate, also bolts of pump DE area.
3.- Radial clearance in DE side of motor (pushing in radial direction and taking reading using dial indicador)
4.- Run-out of hubs at motor and pump.


Rotor Bar Frequency (RBF – 38x RPS= 1886 Hz) and 2XLF sidebands, could be due
to the fact that each rotor bar passes slight disruptions in the magnetic field due to the current's path through the rotor and stator, but in this case still no represent problem because the amplitude is still acceptable.


But 2x RBF ( 3772 Hz) and 2xLF side bands, could be related to 1/2x? 

Internal looseness in motor? 

I Wait also for comments.

RRR

Attachments

This looks like the standard lube oil pump from an old Westinghouse 501 gas turbine. I had issues with these pumps (and couplings) on an earlier assignment. I will look at some of the old data to see if there is any similarity. I recorded data at the same locations also and included a baseplate axial reading to see if the support was solid. 

OLI posted:

Ok, that is also ok. I just saw the picture in principle pointing to the shroud and that is not so good. In worst case I even use the grease zerk if there are such if all the rest is plastic and thin aluminium. Just checking. By the way, the 0.5X and harmonics to me indicate severe looseness. So are the coupling hubs worn so they eat up the new inserts and you get looseness again PDQ? Or anything else including hubs mount on the shafts?

The jaw coupling hubs were slightly worn at the time of failure on 26/02/19. after replacing the inserts and the hubs, the vibration improved for a couple of days and then went back to something similar to previous spectrum (1/2 x harmonics, ~3-4Hz side-bands, RBPF peak, 2xRBPF with 2xLF sidebands on acceleration and up to 4x peaks on demodulation spectrum). So we realigned the shafts which were off specs and replaced the coupling with a grid type  coupling with exact clearances. but the 1/2x still remains  

rgf12 posted:

This looks like the standard lube oil pump from an old Westinghouse 501 gas turbine. I had issues with these pumps (and couplings) on an earlier assignment. I will look at some of the old data to see if there is any similarity. I recorded data at the same locations also and included a baseplate axial reading to see if the support was solid. 

Thankx heaps RGF12. This is a GE SC5 steam turbine. We do have  2x AC lube oil pumps + a DC pump as backup. But any issues with either of the AC pumps, we go for a safe shutdown. This one's already tripped on 26/2/19 due to coupling insert damage and high vibration which cost a good 6 hours in downtime!! The coupling was only replaced on 22/11/18 when pump was taken out of service due to Pump DE bearing damage. So any help to prevent this from happening again would be much appreciated!!   

ramonruiz posted:

Dear Thimba,

Still you can follow the equipment vibration trend .
Next opportunity check as follow:
1.- Axial displacement of pump shaft trying to push up the shaft ( make reading using dial indicator).
2.- check tighten all bolts around base plate, also bolts of pump DE area.
3.- Radial clearance in DE side of motor (pushing in radial direction and taking reading using dial indicador)
4.- Run-out of hubs at motor and pump.

But 2x RBF ( 3772 Hz) and 2xLF side bands, could be related to 1/2x? 

Internal looseness in motor? 

I Wait also for comments.

RRR

Thankx again Ramonruiz, I will try your remedies/ checks and if nothing found there and the trend continues, I will try swapping the motor to newly refurbished one in the workshop. 

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