Maintworld 3/2017

CONDITION MONITORING
How Do I Get Started?
There are two common questions that
many first-time users of ultrasound
have. The first is, “How do I set baselines?”
The second is, “How do I know if
what I’m listening to is good or bad?”
The Comparison Method
One way to get a quick idea as to what is
good and what is bad is by using the comparison
approach. With this method, the
inspector simply compares the decibel
level readings at identical points on
identical machines. Using this method,
the inspector also begins to “train”
their ear as to what rotating equipment
sounds like, and it will become obvious
that a bearing with a particular fault
such as an inner race, or outer race defect,
will sound much different than a
bearing that is in a “good” condition.
The baseline can then be set based on
an average of decibel levels at the compared
points. The software may even
default to the first reading taken and
downloaded. The baseline can then be
changed as more readings are collected.
The Historical Method
The historical method is the preferred
method for establishing baselines and
alarm levels in bearing condition monitoring
routes. Using this method, the inspector
first establishes a route or database
in the ultrasound software. The database
is then loaded into the ultrasonic
instrument. Data is then collected at the
various points along the route. When the
initial round of data has been collected,
it may be necessary to collect data more
frequently than needed in order to build
Figure 4. Pump 4 MTR OB from the ultrasound instrument.
Notice the distinct 175.8Hz harmonics detected.
the history, and get an idea if the decibel
readings are remaining similar in the
historical readings.
For example, when collecting the
initial data for setting the baseline, the
readings may need to be taken once per
week for 4-5 weeks. Once the baseline is
set, the readings need only be taken only
once per month, or every other month
depending on asset criticality and equipment
runtime.
Ultrasound Imaging
Through advancements in ultrasound
instruments and software, the user can
obtain an “image” of the sound that is
being heard to analyse, diagnose, and
confirm mechanical fault conditions in
rotating equipment.
Examples of
Ultrasound Imaging
Let’s take a two motor and pump combination
as an example: 60hp motors
powering water pumps.
While collecting data, both decibel
readings and sound files were recorded.
In Figures 1 and 2 screen shots from the
spectral analysis software show a comparison
between the points “PUMP 3
MTROB 007” and the “PUMP 4 MTROB
010.”
Notice the difference between the
two points. Both motors are operating
under the same conditions, but the
Pump 4 MTR OB point has a much different
spectrum. If you were listening
through the headset of the ultrasound
instrument, it would also have a much
different sound.
Another image of the Pump 4
MTROB point, captured from on board
the ultrasound instrument, can be seen
in Figure 3.
The spectrum analysis software used
has a built-in bearing fault frequency
calculator. By entering in the speed
(rpm) and the number of balls (bearings),
an outer race, inner race, ball pass,
and cage frequency are calculated. For
this particular motor, the speed was
1750rpm and the type and number of
WITH A SHORT LEARNING
CURVE, EASE OF COLLECT-
ING DATA, AND REMOTE
MONITORING SOLUTIONS,
ULTRASOUND CAN BECOME
ANOTHER VALUABLE TOOL TO
USE FOR YOUR CONDITION
MONITORING EFFORTS.
bearings was confirmed and the number
of bearings was 10. The fault frequency
calculated by the spectrum analysis software
that was of interest was an inner
race fault at 175Hz. This is the same fault
harmonic detected on the ultrasound
instrument. Another interesting point
was the fact that the vibration analysis
data was collected two days later, and did
confirm an inner race fault on the Pump
4 motor outboard point.
Conclusion
Implementing ultrasound for condition
monitoring applications is easier
than you think. With a short learning
curve, ease of collecting data, and remote
monitoring solutions, ultrasound can
become another valuable tool to use for
your condition monitoring efforts.
Lubrication PM’s can also become
more effective because ultrasound
trends will show which bearings need
to be lubricated. Therefore, instead of
greasing everything on a time-based lube
route, only the points that are currently
in the lubrication alarm from ultrasound
trends are greased until the decibel level
drops back down to the baseline dB.
If you are only using ultrasound as a
leak detector, I would encourage you to
take a more in depth look into condition
monitoring with ultrasound.
46 maintworld 3/2017