Page 279 - Intro Predictive Maintenance

270 An Introduction to Predictive Maintenance

affected by system variations, no matter how simple or complex. For example, a com-
parison of vibration profiles acquired from a centrifugal compressor operating at 100
percent load and at 50 percent load will clearly be different. The amplitude of all rota-
tional frequency components will increase by as much as four times at 50 percent
load. Why? Simply because more freedom of movement occurs at the lower load.
As part of the compressor design, load was used to stabilize the rotor. The designer
balanced the centrifugal and centripetal forces within the compressor based on the
design load (100 percent). When the compressor is operated at reduced or excessive
loads, the rotor becomes unbalanced because the internal forces are no longer equal.
In addition, the spring constant of the rotor-bearing support structure also changes
with load: It becomes weaker as load is reduced and stronger as it is increased.

In more complex systems, such as paper mills other continuous process lines, the
impact of the production process is much more severe. The variation in incoming
product, line speeds, tensions, and a variety of other variables directly impacts the
operating dynamics of the system and all of its components. The vibration profiles
generated by these system components also vary with the change in the production
variables. The vibration analyst must adjust for these changes before the technology
can be truly beneficial as either a maintenance scheduling or plant improvement tool.

Because most predictive maintenance programs are established as maintenance tools,
they ignore the impact of operating procedures and practices on the dynamics of
system components. Variables such as ramp rate, startup and shutdown practices, and
an infinite variety of other operator-controlled variables have a direct impact on both
reliability and the vibration profiles generated by system components. It is difficult,
if not impossible, to accurately detect, isolate, and identify incipient problems without
clearly understanding these influences. The predictive maintenance program should
evaluate existing operating practices; quantify their impact on equipment reliability,
effectiveness, and costs; and provide recommended modifications to these practices
that will improve overall performance of the production system.

13.1.4 Training Limitations
In general, predictive maintenance analysts receive between 5 and 25 days of train-
ing as part of the initial startup cost. This training is limited to three to five days of
predictive system training by the system vendor and about five days of vibration or
infrared technology training. In too many cases, little additional training is provided.
Analysts are expected to teach themselves or network with other analysts to master
their trade. This level of training is not enough to gain even minimal benefits from
predictive maintenance.

Vendor training is usually limited to use of the system and provides little, if any, prac-
tical technology training. The technology courses that are currently available are of
limited value. Most are limited to common failure modes and do not include any train-
ing in machine design or machine dynamics. Instead, analysts are taught to identify
simple failure modes of generic machine-trains.