Page 363 - Intro Predictive Maintenance
P. 363
354 An Introduction to Predictive Maintenance
vations can automatically be recorded concurrent with data acquisition of vibration
data.
Process Dynamics
A true understanding of plant condition cannot be accomplished without knowing the
operating efficiency of every machine or system in the plant. For example, how do
you know the operating condition of a shell-and-tube heat exchanger without knowing
the efficiency and fouling factor? The calculations required to determine these
two critical factors is extremely simple, but you must first know the actual process
parameters (i.e., flow, pressure and temperature) on both the primary and secondary
side of the heat exchanger. Six simple measurements will provide the data required to
periodically calculate both the efficiency and fouling factor.
Monitoring process parameters usually require the addition of some plant instru-
mentation. Few plants have working instruments that monitor all of the variables
required to determine the operating condition of critical systems; however, advance-
ments in instrumentation technology have developed nonintrusive methods of acquir-
ing most of the required process variables without the expense of installing permanent
instrumentation. Several techniques have been developed to monitor process flow—
the most difficult process parameter to measure—without installing a pitot or vortex-
shedding flowmeter. These new instruments are commercially available and can
often be read by the microprocessor-based, vibration-based predictive maintenance
systems.
A few of the microprocessor-based, vibration-monitoring systems provide the ability
to directly acquire process data from permanently installed instruments and allow for
manual entry of analog gauges. This capability provides the means to automatically
acquire process parameters in conjunction with routine acquisition of vibration data.
In addition, some of these systems can automatically calculate unknown process para-
meters (e.g., efficiency, fouling factors). These systems record the process parameters
that can be directly measured and then automatically calculate, store, and trend the
unknown in the same manner as parameters that are acquired directly. This ability
greatly enhances the predictive maintenance system’s benefits and eliminates both
the manual effort required to calculate unknowns and the potential errors that manual
calculation may create.
Thermography
Implementing a full thermographic program is usually not cost effective; however,
many of the vibration-based systems will permit direct acquisition of infrared data
through a point-of-use scanner. This feature should be incorporated into every pre-
dictive maintenance program. The scanner can be used to acquire several process para-
meters that will augment the program. Typical applications for this technique include
bearing cap temperatures, spot checks of process temperatures, motor winding
temperatures, spot checks of electrical equipment, and many more.