Page 24 - Intro Predictive Maintenance

Impact of Maintenance 15

part of generate the vibration profile that is acquired and stored for analysis, both the
data acquisition and analysis processes must always include all of the process vari-
ables, such as incoming materials, pressures, speeds, temperatures, and so on, that
define the operating envelope of the system being evaluated.

Generally, the first five to ten measurement points defined for a machine-train
should be process variables. Most of the microprocessor instruments that are used
for vibration analysis are actually data loggers. They are capable of either directly
acquiring a variety of process inputs, such as pressure, temperature, flow, and so
on, or permitting manual input by the technician. These data are essential for
accurate analysis of the resultant vibration signature. Unless analysts recognize the
process variations, they cannot accurately evaluate the vibration profile. A simple
example of this approach is a centrifugal compressor. If the load changes from 100
percent to 50 percent between data sets, the resultant vibration is increased by a
factor of four. This is caused by a change in the spring constant of the rotor system.
By design, the load on the compressor acts as a stabilizing force on the rotat-
ing element. At 100 percent load, the rotor is forced to turn at or near its true
centerline. When the load is reduced to 50 percent, the stabilizing force is reduced by
one-half; however, spring constant is a quadratic function, so a 50 percent reduction
of the spring constant or stiffness results in an increase of vibration amplitude of 400
percent.

Infrared Technologies. Heat and/or heat distribution is also an essential tool
that should be used for all electromechanical systems. In simple machine-trains, it
may be limited to infrared thermometers that are used to acquire the temperature-
related process variables needed to determine the machine or system’s operating enve-
lope. In more complex systems, full infrared scanning techniques may be needed
to quantify the heat distribution of the production system. In the former technique,
noncontact, infrared thermometers are used in conjunction with the vibration
meter or data logger to acquire needed temperatures, such as bearings, liquids
being transferred, and so on. In the latter method, fully functional infrared cameras
may be needed to scan boilers, furnaces, electric motors, and a variety of other
process systems where surface heat distribution indicates the system’s operating
condition.

The Total Package. The combination of these three technologies or methods is the
minimum needed for an effective predictive maintenance program. In some instances,
other techniques, such as ultrasonics, lubricating oil analysis, Meggering, and so on,
may be needed to help analysts fully understand the operating dynamics of critical
machines or systems within the plant. None of these technologies can provide all of
the data needed for accurate evaluation of machine or system condition; however,
when used in combination and further augmented with a practical knowledge of
machine and system dynamics, these techniques can and will provide a predictive
maintenance program that will virtually eliminate catastrophic failures and the need
for corrective maintenance. These methods will also extend the useful life and mini-
mize the life cycle cost of critical production systems.