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gazine · 02/08
Bearingproblemsoninstrumentair
compressoratLNGPlant
The instrument air compressor
serves a critical role in an LNG
plant as well as in other indus-
tries. It provides clean, dry air
that is used in a range of con-
trol and shutdown valves.
In this case story a bearing
fault was detected and
diagnosed while
the compressor
was still under
warranty.
T
here are three instrument air
compressors per train at the
LNG plant.
The 3-stage oil-free centrifugal com-
pressor is driven by a 90 kW mo-
tor though a gearbox and uses tilting
pad journal bearings.
The units were relatively new and
still under warranty at the time of this
case story.
Monitoring Strategy
All the instrument air compressors at
the LNG plant are instrumented both
for safety and condition monitoring.
The following measurements are
monitored for safety monitoring:
n
Axial thrust DC displacement
n
First and third stage impeller bear-
ings - Overall bandpass measure-
ments from the radial displace-
ment sensor (10-5k Hz, RMS P-P,
mm)
The safety monitoring system has du-
plicate sensors and monitoring mod-
ules for redundancy.
There are a number of condition
monitoring measurements that are
monitored for the entire compressor
train. Those that are monitored from
the displacement sensors of the first
and third stage impeller bearings in-
clude the following:
n
DC measurement for shaft posi-
tion
n
1/2 magnitude for bearing stability
monitoring (RMS P-P, mm)
n
First order vector (magnitude and
phase, RMS P-P, mm)
n
2nd, 3rd and 4th order magnitude
(RMS P-P, mm)
n
6% constant percentage band-
width spectrum, (CPB, 16.3-
1.03kHz, RMS, mm)
n
FFT spectrum (500Hz span, P-P,
mm)
n
Dual time function (orbit)
Because of limited access there is no
displacement sensor on the second
stage impeller bearing. Instead, an
accelerometer is used on the casing
of the impeller. The measurements
from this sensor are similar to those
monitored by displacement sensors
on the first and third stage impeller
bearings.
Observations
The bandpass vibration level on the
inboard bearing of the first stage im-
peller had exceed the alert alarm lim-
its, so special attention was given to
monitor its progression. As seen in
Fig. 2 the vibration trend had been in-
creasing steadily for about one year.
After a few months the vibration level
subsided again but after two
t
Instrument Air Compressor
Date put in service
April 2005
Power
90 kW
Flow capacity
200 Tonnes/day
Outlet pressure
10 bar
Speed
50 Hz