We have a 3-stage centrifugal plant air Elliott Compressor that was overhauled in the month of July, 2006. 1st stage shaft turning speed is 37,333 RPM and 2nd & 3rd stage shaft turning speeds are 51,940 RPM. After overhauling, Compressor Casing axial vibrations were below 3.0 mm/sec RMS and radial vibrations were below 2.0 mm/sec RMS with Portable Vibration Analyser (CSI 2120).

On 28 Oct, 2006, 2nd stage axial vibrations (with Portable Analyser) increased to 7.2 mm/sec. However, there was no significant increase in the vibrations of other bearing locations of the Compressor. Also, there were no change in the Online Vibration values i.e. High speed- 15 Um & Low Speed- 7.6 Um of the Compressor. In view of axial vibrations we suspected suction filters plugging, Suction filters were replaced with new filters. Differential Pressure across suction is 3.5 inches of water. But there was improvement in the vibrations.

We corresponded this problem to M/S CICB (representative of Elliott Compressor in India). As per their recommendations, we have checked piping strains. Small piping strain was found and it was corrected. After correction of piping strains, there was no decrease in the Vibrations.

Compressor 2nd stage axial vibrations increased to 12.5 mm/sec on Dec 9, 2006. But, there is no change in the Online Vibration values i.e. High speed- 15 Um & Low Speed- 7.6 Um of the Compressor. Dominating vibration’s peak is observed at 6xRPM (of 2nd Stage speed). Compressor is having radial-cum-thrust bearings with Radial Pads – 5nos. and thrust pads-8 nos.
Any suggestion on this on this problem.
Thanks.

Munesh,

Could you post vibration spectra of the compressor and the driver? What is the driver and some info on the base/skid would help us approach the problem better informed?
I take it your online vibration sensor is proximity probe, where are these located?

Regards....Rajan

Really need to know at what frequency the vibration amplitude increased. Also what was done on the overhaul? were gear elements replaced? I have seen in this kind of machine high casing vibration after installing a new pinion but leaving the original bull gear. This kind of machine likes the gears to be matched. That would come up as a gear mesh frequency, so again, need to know the frequencies....

Driver is Induction Motor, RPM of motor: 2975

Motor is directly coupled to Bull Gear i.e. Bull Gear RPM is 2975.

1st Stage Compressor RPM: 37330
2nd & 3rd Stage Compressor RPM: 51940

Online Proximity Probes Location:
1st stage: On thrust collar
2nd Stage: Between Pinion & thrust collar



Vibrations of 311640 CPM (5194 Hz) are increased which is equal to 6x RPM of 2nd Stage. The vibrations are increased in axial direction at all compressor points (mainly 2nd stage).


Exactly gear meshing frequency is not known. Vibration peak is at 6xRPM of pinion; pinion will have more than 6 no. of teeth so gear meshing frequency will be more that 311640 CPM. Vibrations of frequency higher than 311640 CPM are in low magnitude.

During Overhaul of the Compressor, compressor casing, pinions bearings, seals and other parts were removed for inspection.

In overhaul following parts were replaced:

1. Pinion bearing 4 nos.
2. Carbon seal repair kit 2 nos.
3. Shim pack pinion bearing 2 set
4. Vitriolic gasket 5” 3 no
5. Vitriolic gasket 6” 1 nos.
6. Rear & front gasket 2 set

Remaining old parts were reused.
Spectrums of Motor and Compressor are attached.

Regards...Munesh

Spectrums.rtf (691 Kb, 14 downloads) Vibration Spectrums

Could you post pictures of the Impeller and the thrust bearing?

Looks to me like your transducer is in resonance at that freq. What type transducer are you using, and what mounting. If it is magnet mount, it very possibly is resonating at that speed.
I monitor some Clark Isopac and Centacs. I had the same type freq. response when using a high freq. trans. on a small pencil erasor size magnet. I permanent mounted the transducer and the peaks, although still there, dropped in amplitude by a factor of 10.
Just a thought.

Dave