Page 115 - Intro Predictive Maintenance

106 An Introduction to Predictive Maintenance

and other gases absorb infrared radiation. Airborne dust, some lighting, and other vari-
ables in the surrounding atmosphere can distort measured infrared radiation. Because
the atmospheric environment is constantly changing, using thermographic techniques
requires extreme care each time infrared data are acquired.

Most infrared-monitoring systems or instruments provide filters that can be used to
avoid the negative effects of atmospheric attenuation of infrared data; however, the
plant user must recognize the specific factors that affect the accuracy of the infrared
data and apply the correct filters or other signal conditioning required to negate that
specific attenuating factor or factors.

Collecting optics, radiation detectors, and some form of indicator are the basic ele-
ments of an industrial infrared instrument. The optical system collects radiant energy
and focuses it on a detector, which converts it into an electrical signal. The instru-
ment’s electronics amplifies the output signal and processes it into a form that can be
displayed.

6.2.1 Types of Thermographic Systems
Three types of instruments are generally used as part of an effective predictive main-
tenance program: infrared thermometers, line scanners, and infrared imaging systems.

Infrared Thermometers
Infrared thermometers or spot radiometers are designed to provide the actual surface
temperature at a single, relatively small point on a machine or surface. Within a pre-
dictive maintenance program, the point-of-use infrared thermometer can be used in
conjunction with many of the microprocessor-based vibration instruments to monitor
the temperature at critical points on plant machinery or equipment. This technique is
typically used to monitor bearing cap temperatures, motor winding temperatures, spot
checks of process piping temperatures, and similar applications. It is limited in
that the temperature represents a single point on the machine or structure; however,
when used in conjunction with vibration data, point-of-use infrared data can be a
valuable tool.

Line Scanners
This type of infrared instrument provides a one-dimensional scan or line of com-
parative radiation. Although this type of instrument provides a somewhat larger
field of view (i.e., area of machine surface), it is limited in predictive maintenance
applications.

Infrared Imaging
Unlike other infrared techniques, thermal or infrared imaging provides the means to
scan the infrared emissions of complete machines, process, or equipment in a very