High Axial Vibration on Alternator

Hi Team,

Recently I have attended an offshore oil rig site.

There was a main diesel engine EMD with an alternator in operation. 

Engine make : EMD, 900 RPM

Alternator:  Synchronous type- single bearing alternator, 2525Amp, 60 HZ, 600V, , 900 RPM, 


Observations:

1. Engine DE & NDE side vibrations were in range of 9 -10 mm/s rms

2. High axial vibrations on Alternator NDE side observed

                            H           V        A

DE                       9.3        5.3       16.7

NDE                   7.8       16.0      21.8

Spectrum: High peaks at 1X & 4X along with tiny 2X & 3X

(Attached for reference)

Phase relations : erratic. (Not constant at H,V,A)

We have ruled out possibility of misalignment as we have done laser shaft alignment, but still the vibrations are on higher side with same above. 

Please suggest.

Regards,

Chetan. 

Attachments

Photos (6)
Original Post

As I see you are using EI software, so you are also capable of providing gE data information (envelope). Be sure to insert the correct rpm value, otherwise the envelope will be pointless. From the data sent so far I see the dominant amplitude is at 1x in axial direction. There are some other amplitudes at 2x, 3x, 4x. I would say the most possible issue is rotor eccentricity combined with some unbalance at the alternator side. I would appreciate to get the envelope data. 

First check your measurement system for correct and stable phase measurement capability. If using an optical tachometer, then make sure it has a stable low-vibration mount and reliable detection of reflective marker.

I have seen phase changes from rotor-stator rubbing, loose rotor mass, severely worn shaft coupling, and engine torsional damper. 

I agree with Becar about resonance. This is somewhat supported by the high NDE vibration levels in both vertical and axial directions ( possibly from rocking/pitching motion).

Loose base/foundation bolts, exhaust pipe, and cooling water pipes should be inspected for looseness and excessive vibration. 4xSS can be an indication of looseness, but it may be engine firing (combustion) frequency depending on #cycles and #cylinders.

Do you go into a dark room without a flashlight?

Walt

Thank you so much for your efforts in helping me out with this issue.

I'm quite confirmed with the looseness of the bolts as I had thoroughly checked the bolts and tightened them. Considering the case of instability of the optical sensor, I'm not sure as their might be a chance of fluctuation in the movement of optical sensor. 

The room where the measurements were taken didn't have ample of light source. Does the amount of surrounding light affect the performance of optical sensor?

"The room where the measurements were taken didn't have ample of light source. Does the amount of surrounding light affect the performance of optical sensor?"

The optical sensor has its own light source. It can work well in a dark room. Bright sunlight or interior light can cause problems when there is not enough reflected light contrast between shaft surface and reflective tape. Change the sensor distance, change light angle to the reflective tape, add contrast color on shaft (black marker or paint), or block offending background light. 

Walt

Thanks Walt. 

Dear Team, 

This is for my general understanding. I have some doubts and want clarifications on those.

With similar kind of machine train (EMD diesel engine with an alternator) if an alternator is having High vertical as well as high axial vibrations with 1X dominant frequency then what are the possible causes for such high vibrations?

 

"With similar kind of machine train (EMD diesel engine with an alternator) if an alternator is having High vertical as well as high axial vibrations with 1X dominant frequency then what are the possible causes for such high vibrations?"

Structural Resonance of alternator and base frame with natural frequency close to 1xSS or other frequency with high vibrations is indicated. Typical confirming tests include: variable speed test (run engine to overspeed limit and below), operating deflection shape (ODS) test with 2-accelerometers and dual channel analyzer with machine running, impulse-response (impact) test with machine running of off (preferred).

If any or all of the tests are negative for indicating resonant structure, then consider field balancing alternator rotor or send to shop for balancing. Do the easy things/tests first and preferably while machine is in service or at least operational!

I had a project to evaluate and correct extreme vibrations of diesel generators on fishing vessel. The resonant structure was corrected by stiffening the support structure and replacing the vibration isolator mounts. It went from extreme to smooth!

Walt

Add Reply

×
×
×
×