Page 23 - An Introduction To Predictive Maintenance

Impact of Maintenance 13

On systems that use computer-based or processor logic control (PLC), the parameters
or variables that define their operating envelopes are automatically acquired and then
used by the control logic to operate the system. The type and number of variables vary
from system to system but are based on the actual design and mode of operation for
that specific type of production system. It is a relatively simple matter to acquire these
data from the Level I control system and use it as part of the predictive diagnostic
logic. In most cases, these data combined with traditional predictive technologies
provide all of the data an analyst needs to fully understand the system’s performance.

Manually operated systems should not be ignored. Although the process data is more
difficult to obtain, the reliability or predictive analyst can usually acquire enough data
to permit full diagnostics of the system’s performance or operating condition. Analog
gauges, thermocouples, strip chart recorders, and other traditional plant instrumenta-
tion can be used. If plant instrumentation includes an analog or digital output, most
microprocessor-based vibration meters can be used for direct data acquisition. These
instruments can directly acquire most proportional signal outputs and automate the
data acquisition and management that is required for this expanded scope of predic-
tive technology.

Because most equipment used in domestic manufacturing, production, and process
plants consists of electromechanical systems, our discussion begins with the best
methods for this classification of equipment. Depending on the plant, these systems
may range from simple machine-trains, such as drive couple pumps and electric
motors, to complex continuous process lines. Regardless of the complexity, the
methods that should be used are similar.

In all programs, the primary focus of the predictive maintenance program must be on
the critical process systems or machine-trains that constitute the primary production
activities of the plant. Although auxiliary equipment is important, the program must
first address those systems on which the plant relies to produce revenue. In many
cases, this approach is a radical departure from the currently used methods in tradi-
tional applications of predictive maintenance. In these programs, the focus is on simple
rotating machinery and excludes the primary production processes.

Electromechanical Systems
Predictive maintenance for all electromechanical systems, regardless of their com-
plexity, should use a combination of vibration monitoring, operating dynamics analy-
sis, and infrared technologies. This combination is needed to ensure the ability to
accurately determine the operating condition, to identify any deviation from accept-
able operations, and to isolate the root-cause of these deviations.

Vibration Analysis. Single-channel vibration analysis, using microprocessor-based,
portable instruments, is acceptable for routine monitoring of these critical production
systems; however, the methods used must provide an accurate representation of the
operating condition of the machine or system. The biggest change that must be made
is in the parameters that are used to acquire vibration data.