POWER October 2012 - 28

Wear-particle analysis can be used to determine the number, size,
composition, and shape of ferrous and some nonferrous wear particles.
Some techniques also permit visual examination of the collected
particles, which can include additional nonferrous materials
such as seal or gasket material. The presence of large ferrous wear
particles can indicate severe wear; and the presence of large quantities
of small wear particles may be important if there are closetolerance
valves whose surfaces can become eroded. One analysis
technique calculates the ratio of small to large particles, noting that
when this ratio changes significantly, a severe wear process has begun.
Another calculates the total weight of wear particles present
in the sample and trends that value as an indicator of the severity
of the wear process. A third technique measures the concentration
of ferrous wear particles, noting that an increase in concentration is
often indicative of the start of a severe wear process.
Spot-checking is used primarily when someone suspects that
the equipment has a severe wear problem. The resultant data can
be compared with data from similar machines that have previously
been identified as having a wear problem and those that
are operating normally. If the suspected machine has a sufficient
number of large or severe wear particles, further examination and
corrective action should be taken immediately. Continuing to operate
equipment with high levels of wear particles can substantially
increase component wear and contribute to a severe failure.
Periodic analysis can provide a more subtle indication of gradual
component wear and the transition from normal wear to a
critical or severe wear process. This advance notice of potentially
serious wear problems means that repairs or replacements can be
planned in advance and be made at a convenient time.
Monitor Machine Wear Trends
Monitoring oil condition over six or eight sample periods can
indicate improper maintenance or repair practices. These can
include: improper installation or removal of bearings, inadequate
lubrication, the failure to properly flush out a system
after repair, improper installation or repair of seals and
packings, improper fluid- or lubricant-handling procedures
that introduce water or dirt contamination, or improper filterhandling
or replacement techniques. Consistently excessive
wear in the same machine or machine type can also indicate
a poor design. This can include bearings or gears that are not
adequately sized for the speed or load, inadequate lubrication,
or inadequate filtration.
Wear-particle analysis is limited by the consistency and timeliness
of the sample. The value of trended information diminishes
quickly if samples are not taken from the same place on the machine
each time, and the longer a sample sits before it is shipped
and analyzed, the less significant the data will be. These concerns
can be diminished by using an onsite lab or a diagnostic
system with a permanently mounted sample port or sensor. For
critical or expensive equipment, the added hardware and training
cost can be a very cost-effective investment.
More Coming
In the next segment of " Predictive Maintenance That Works, "
we'll continue our discussion of specific nondestructive testing
condition-monitoring techniques used at power plants
and why each should be a part of your PdM program. ■
-Dr. Robert Peltier, PE is POWER's editor-in-chief.
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POWER October 2012

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