POWER November 2011 - 23

prioritize repairs prior to outages, to evaluate completed repair
work, and to check new installations prior to startup.
Spot checking is used primarily when maintenance or operations
staff suspect that a problem exists, or prior to planned
maintenance to identify developing problems and prioritize the
work. One advantage of thermographic analysis of electrical
equipment is that the scan provides an immediate and quantifiable
indication of the temperature difference between a properly
functioning circuit or connection, and one that is overheating.
This means that the technician can often determine the seriousness
of the electrical problem without having to review historical
patterns or past analyses. This quantitative analysis can also
be helpful in determining how long the equipment can continue
to operate before there is a catastrophic failure.
Routine PdM Requirement
Additional information can be obtained by performing thermographic
analysis on a periodic basis, for example once a year, or after each
major overhaul or plant outage. Periodic thermographic analysis can
provide a more subtle indication of electrical deterioration, bearing
or coupling wear, roof deterioration or leakage, and refractory cracking.
This can allow plant staff to project acceptable performance for
the coming six months or one year. Advance notice of developing
problems means that they can be resolved or repaired during normal
shutdowns, rather than allowing a catastrophic failure to occur. Because
problems are detected when they are relatively minor, they are
usually much less expensive to repair.
One difficulty with IR surveys is documenting the results.
Data typically are stored today in digital form or in photographs,
neither of which clearly and conveniently depicts the overall
situation to work planners and upper management. To remedy
this, some specialists generate thermal maps, which depict temperatures
throughout the unit and color-code problem areas. The
thermal map shows elevation levels and the location of recognizable
components such as buckstays and manways.
IR scanners can only measure the temperature of visible, radiating
surfaces, which means they cannot take readings through
glass or metal housings or covers, unless the metal covering is
thin enough for secondary heat patterns to appear. Readings
are also affected by the reflective nature of the surface being
measured, and this reflectivity value must be taken into account
when scanning different objects or surface materials. The equipment
cost is moderate to expensive, particularly when many of
the more sophisticated options are included.
IR scanners vary in their accuracy, sensitivity, temperature
range, resolution, and portability. Options include a variety of
graphic displays, video/audio capabilities, zoom, interchangeable
lenses, battery operation, data logger/printer, image storage,
and digital outputs.
Scanner operation is relatively easy; however, interpretation
of the scanned images requires a moderate amount of training
and experience. Training and certification can be obtained from
either the scanner manufacturer or one of several independent
certification companies. For companies with limited budgets,
there are service companies that will perform thermographic
analysis on a contract basis.
More Coming
In the next segment of " Predictive Maintenance That Works, "
we'll continue our discussion of specific NDT-related conditionmonitoring
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.
November 2011 | POWER
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