Page 182 - An Introduction To Predictive Maintenance

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THERMOGRAPHY

Thermography is a predictive maintenance technique that can be used to monitor the
condition of plant machinery, structures, and systems. It uses instrumentation designed
to monitor the emission of infrared energy (i.e., temperature) to determine operating
condition. By detecting thermal anomalies (i.e., areas that are hotter or colder than
they should be), an experienced surveyor can locate and define incipient problems
within the plant.

8.1 INFRARED BASICS
Infrared technology is predicated on the fact that all objects with a temperature above
absolute zero emit energy or radiation. Infrared radiation is one form of this emitted
energy. Infrared emissions, or below red, are the shortest wavelengths of all radiated
energy and are invisible without special instrumentation. The intensity of infrared
radiation from an object is a function of its surface temperature; however, tempera-
ture measurement using infrared methods is complicated because three sources of
thermal energy can be detected from any object: energy emitted from the object itself,
energy reflected from the object, and energy transmitted by the object (Figure 8–1).
Only the emitted energy is important in a predictive maintenance program. Reflected
and transmitted energies will distort raw infrared data. Therefore, the reflected and
transmitted energies must be filtered out of acquired data before a meaningful analy-
sis can be completed.

The surface of an object influences the amount of emitted or reflected energy. A perfect
emitting surface, Figure 8–2, is called a “blackbody” and has an emissivity equal to
1.0. These surfaces do not reflect. Instead, they absorb all external energy and re-emit
it as infrared energy.

Surfaces that reflect infrared energy are called “graybodies” and have an emissivity
less than 1.0 (Figure 8–3). Most plant equipment falls into this classification. Careful

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