Chapter 24

APPLICATIONS
There are many potential applications for real-time analysis, ranging from advanced vibration analysis to structural and process analyses. However, the major applications for this diagnostic technique are transient analysis, complex-machine analysis using synchronous time averaging and narrowband zoom, multichannel analysis, and torsional analysis.

TRANSIENT ANALYSIS
Many of the machines and process systems found in manufacturing and process plants are periodically subjected to events that directly affect their operating dynamics, reliability, and useful life. These events, called transients, may be part of the normal operating mode or they may be an external incident or variable not uniquely associated with the machine or system. Specific applications that require transient analysis include variable-speed machines, process or product variations, and random or periodic impacts.
A transient is a short-duration event that periodically occurs in a machine or process system. As an example, when a variable-speed machines running speed coincides with its rotors critical speed, that event is considered to be a transient. In normal practice, the duration of this type of event is--or should be-relatively short. Because they generally occur so quickly, conventional vibration-monitoringinstruments do not have the processing speed required to capture the impact of the event and, therefore, a real-time analyzer is required to perform the proper diagnostics.

Figure 24.l(a) illustrates a transient captured in a time waveform and Figure 24.l(b) illustrates the same transient in a waterfall or cascade frequency-domain plot. These capture-and-displaycapabilities account for much of the diagnostic power of RTA.

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Figure 24.1 (a) Time waveform of transient event. (b) Same transient in waterfall format.

Variable-Speed Machines

Paper machines and steam turbines are examples of variable-speed machines that operate over a wide range of speeds and are excellent applications for RTA. Because the dynamics of the machine and its components change during the transitional period associated with any substantial speed change, the ability to measure and evaluate these changes is critical to any reliability program.

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RTA permits accurate capture of real-time, multichannel vibration and process parameter data as the machine accelerates to, or decelerates from, full speed. Because these variations are rapid, the increased data-processing speed of a real-time analyzer is essential.
Process or Product Variations

Machines or process systems that are subject to a range of product- or process-related variables are also ideal applications for RTA. Proper use of data-acquisition and analysis techniques permits the analyst to fully define and quantify the cause-and-effect of the variables on the operating dynamics, reliability, and useful life of the machine or system component.
Random or Periodic lmpacfs

Many of the machine-trains and process systems that make up a plant are subjected to apparently random or periodic impacts, or transients, that may affect their reliability or useful life. Conventional vibration monitoring will not detect these events except by accident. The combination of digital tape recorders and RTA can be used to document and evaluate these occurrences. Using a digital tape recorder to acquire vibration and process data over a long period of time is a cost-effective way to capture those random or long-interval periodic events that may affect a machines reliability. When combined with the tremendous data-processing speed of a real-time analyzer, this provides the analyst with the means to evaluate these events quickly to isolate their root cause.

COMPLEX-MACHINE ANALYSIS
Not all machines or process systems within a plant are simple. Many are comprised of complex components that all contribute to the vibration waveform or signature. As a result, analysis of the vibration profile is often very difficult. For example, a triplereduction gear box, coupled to a variable-speed steam turbine and variable-pitch fan, generates a very complex vibration profile. Each of the components in the driver, gearbox, and fan contributes to the time waveform or frequency-domain signature. Conventional vibration meters cannot separate the individual contributions of these components from the composite profile. Real-time analyzers can be used to break up the individual frequencies generated by various machine or system components because of its data-processing speed and signal-conditioning logic. Two RTA techniques are especially useful for complex machines: synchronous time averaging and narrowband zoom.

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Synchronous Time Averaging

When synchronous time averaging is.coupled with the real-time analyzers ability to provide parallel, multiple data channels and real-time processing, RTA can be used to evaluate extremely complex vibration profiles. This technique removes unwanted frequencies that may be generated by other components either within the same machine, from other machines, or from unknown outside sources. Simply stated, this technique nullifies or removes any vibration frequencies that are not absolutely synchronous to the shaft or turning speed being investigated.
Narrowband Zoom

One of the limitations of conventional, general-purpose vibration instruments is their inability to provide high-resolution signatures of specific frequency components (e.g., gear-mesh, vane-pass, or bearing modulation). Because these systems have a fixed lower data acquisition frequency limit (i.e., zero), they cannot provide resolution on any set of frequencies that has a lower limit greater than zero. As a result, the analyst may not be able to visually fully evaluate the profile generated by a specific machine component. Most real-time analyzers can provide a true high-resolution signature for any set of frequency components within the machine vibration profile. This ability, called narrowband zoom, is a powerful diagnostic tool. It is especially useful for complex machines where multiple components may require close evaluation. As an example, narrowband zoom can be used to radically increase the resolution of the gear-mesh frequency of a gear set. The analyst can establish a narrowband-zoom window by selecting the gear-mesh as the center frequency and defining a bandwidth that includes the gears modulation frequencies. The resultant display provides a high-resolution signature that can be used to fully analyze the gears condition.

MULTICHANNEL ANALYSIS
Multichannel analysis is not unique to RTA. It can be performed with conventional, general-purpose vibration instruments using a digital tape recorder and good discipline. However, this approach is more difficult and does not provide the diagnostic power of RTA. Conventional vibration-monitoring systems are limited by their processing speed and functional capabilities. Therefore, they cannot provide all of the tools needed for full multichannel analysis. In addition, using them requires the raw data to be acquired using multichannel tape recorders and then played back through the general-purpose meter. As a result, playback timing becomes a real problem. There is no guarantee that all data are time synchronized or that the displayed data are meaningful.

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Use of a real-time analyzer, with or without a digital tape recorder, eliminates these problems. The multichannel,parallel-processingcapabilities of the analyzer provide a quick, positive means of retrieving and displaying data that are absolutely time synchronized.

TORSIONAL ANALYSIS
Torsional vibration of rotating elements is the rapid fluctuation of angular shaft velocity, and its basic units are either radians or degrees. A machine will often increase or decrease speed over some period of weeks, days, or seconds. As a machine changes speed, torque is applied-tothe shaft in one direction or the other. Torsional vibration is not a simple parameter to analyze. Transducer requirements are stringent and shaft access may be limited. Above all, however, there is a peculiar mystique engulfing torsional vibration. Therefore, this module attempts to dispel its mystique by providing a basic understanding of torsional motion, what it means, and how it can be interpreted.