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.
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