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gazine · 02/08
Diagnostic Analysis
One of the first observations in the
spectrum plot shown in Fig. 3 was
that the third order harmonic of the
running speed from the inboard bear-
ing of the first stage impeller start-
ed to increase more than two years
before the shutdown. The band-
pass vibration level increased for a
while, decreased, and then increased
again. This could be the result of a
bearing fault that initally smoothed it-
self out for a while and then the fault
re-developed again. The DC vibration
displacement shown in Fig. 2 shows
a relatively linear change in shaft po-
sition over the course of three years.
This indicates gradual wear of the
bearing.
Because the compressor was still un-
der warranty, the manufacturer was
contacted to investigate the bearing
deterioration.
Root Cause Analysis
The manufacturer has determined
that the fretting on the first stage
bearing (Fig. 4) was due to poor
bonding of the bearing surface as a
result of a manufacturing defect. This
evaluation is based on the fact that
the high vibration levels had already
started an upward trend not long af-
ter commissioning.
The scratched bearing surface for the
second stage impeller (Fig. 5) was
most likey due to to foreign object
that entered the bearing during com-
missioning. The burnt oil on the bear-
ing surface from the 3rd stage impel-
ler (Fig. 6) was most likey due to car-
ry-over water ingress.
t
Figure 3. The 3rd order harmonic
dominates the vibration response in
this spectrum plot (FFT). The 460 Hz
signal component from a gearbox
pinion remains unchanged, thus pre-
cluding the possibility of a fault with
this portion of the gearbox (higher
frequencies not shown in this plot).
Figure 4. Fretting on the first stage
impeller bearing, presumedly caused
by a manufacturing defect.