Page 115 - An Introduction To Predictive Maintenance

Predictive Maintenance Techniques 105

Vibration monitoring must be focused on the critical production systems. Each of these
systems must be evaluated as a single machine and not as individual components. For
example, a paper machine, annealing line, or any other production system must be
analyzed as a complete machine—not as individual gearboxes, rolls, or other compo-
nents. This methodology permits the analyst to detect abnormal operation within the
complex system. Problems such as tracking, tension, and product-quality deviations
can be easily detected and corrected using this method.

When properly used, vibration monitoring and analysis is the most powerful predic-
tive maintenance tool available. It must be focused on critical production systems, not
simple rotating machinery. Diagnostic logic must be driven by the operating dynam-
ics of machinery—not simplified vibration failure modes.

The proof is in the results. The survey conducted by Plant Services in July 1999 indi-
cated that less than 50 percent of the vibration-monitoring programs generated enough
quantifiable benefits to offset the recurring cost of the program. Only 3 percent gen-
erated a return on investment of 5 percent. When properly used, vibration-based pre-
dictive maintenance can generate return on investment of 100:1 or better.

6.2 THERMOGRAPHY
Thermography is a predictive maintenance technique that can be used to monitor the
condition of plant machinery, structures, and systems, not just electrical equipment.
It uses instrumentation designed to monitor the emission of infrared energy (i.e.,
surface temperature) to determine operating condition. By detecting thermal anom-
alies (i.e., areas that are hotter or colder than they should be), an experienced techni-
cian can locate and define a multitude of incipient problems within the plant.

Infrared technology is predicated on the fact that all objects having 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,
temperature 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.
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.

Variations in surface condition, paint or other protective coatings, and many other vari-
ables can affect the actual emissivity factor for plant equipment. In addition to
reflected and transmitted energy, the user of thermographic techniques must also con-
sider the atmosphere between the object and the measurement instrument. Water vapor