Transient Analysis: A Method to the Madness

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WHITE PAPER A Learning Publication from Full Spectrum Diagnostics

TRANSIENT ANALYSIS: A METHOD to the MADNESS

TRANSIENT ANALYSIS: A METHOD to the MADNESS Dan Ambre, Full Spectrum Diagnostics, PLLC, Plymouth, Minnesota

Vibration analysis with Transient Techniques are a form of “digging a little deeper” when simple spectral analysis and monthly trending don’t resolve your rotating machinery problem(s). The term “transient” in itself alone means something is changing, such as speed, load, pressure, or mass flow. By measuring the vibration signatures (time and frequency) during these transient periods, the analyst can isolate problems that are intermittent, unexpected, or only occur during process parameter changes. Thus, the techniques are suitable for many industrial applications and manufacturing processes.

Some popular transient methods include:

Figure 1.0 Motor Startup Waterfall Map

Long Time (Time Waveform) Capture or Recording Transient Capture Based on Speed Changes (∆RPM) Transient Capture Based on Time Intervals (∆t) Transient Phase Response (bode) Transient Orbit Analysis

The analysis method and transient technique chosen by the analyst is typically dictated by constraints inherent in the problem machine itself. Some of these include: • • • • • •

The Analysts’ ability to Change and/or Control Speed (RPM) Triggering Capability The Timing of the Event (day / night) Duration of the Transient Event Repeatability of the Event Capability of the Analysts’ Equipment

If the machine requires a scheduled shutdown, the analyst may only have a single shot at data capture. If the speed varies as a function of its operation, i.e., winding operations, the setup and capture of data can be attached from a more of a trial and error perspective.

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Just as there are numerous methods for performing transient data capture, as one would expect, there are also numerous plotted formats to make the data more understandable. Analyzer and Data Collector options that create and display Bode, Polar Plots, Spectral Map, Waterfall Map, or Cascade Diagrams are designed for short or long term transient events. The analyst can experiment with frequency ranges, resolution, and signal processing techniques to capture the hidden signals. The “standard” FFT spectrum is acquired not just once, but in rapid succession to create a format that documents the transition of vibration sources as they change with time and speed, or are amplified by resonant operation.

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Figure 2.0 Generator Oil Whip Event Contour Map Typical AC Induction Motors will start-up very quickly and even though the coast-down may take several minutes to come to a complete stop, the really interesting stuff usually happens in the first 5-10 seconds after you hit the stop button.

Some events are notorious for occurring in the middle of the night or on the weekend. These transient s may require remote and/or long time capture and amplitude triggering schemes to wrangle. Speaking of triggering, your analysis will likely rely on you knowing the speed, shaft position, or phase angle to define the problem. For this you will need some type of speed reference or data collection trigger.

Laser Tachometers and optical Tachometers are part of a good analysis plan. Finally, your ability to take the data and solve the problem will rely on your equipment capabilities. Real Time Analyzers are

designed to perform transient data capture. Portable data collectors have come a long way in the last 10 years in transient capability as well, but it might require purchasing an add-on option. Some Transient Analysis techniques are outlined in the following paragraphs with some brief descriptions.

Long Time Capture Methods are suited for uncontrolled, rapid, short-term transients. Especially for events at very low operating speeds where the analyst battles with the need for high resolution which entails long sampling

times. The FFT requires enough resolution to define the individual peaks, but also must sample quickly to detect changes. Sometimes the event is over before you know it! On the other hand, in the case of beat phenomena, these transients require long time waveform capture to accommodate the very long sampling periods to see the event unfold or to determine if it is repeatable. When the transient event is recorded via long time capture methods, it can be re-processed later on at the analyst’s leisure where multiple signal processing techniques can be explored.

Figure 3.0 Long Time Capture of a Cement Plant Kiln Gear Transient (3 revolutions shown)

Figure 4.0 Variations in Gear Mesh Frequency Amplitude with Stepped Speed Change RPM or Time-Based Capture Methods are applicable for controlled or uncontrolled smooth transients. This method is common in an uncontrolled format for machinery start-up or shut-down events occur in medium to higher speed applications. When some form of speed control is present the presentation takes on a “stair-step�

profile of a series of steady-state events overlaid on one another (Figure 4.0).

Peak-Hold Capture Techniques are not really a special transient capability or analyzer option, rather a signal processing (averaging) trick used when a more formal

method is not available. It’s the standby method when need some quick and dirty data right now (see Figure 5.0).

The drawback of this technique is that the measurement is “live” during the entire collection period.

Each average stores the highest amplitude in each frequency bin. Unfortunately if the transducer cable is bumped or if someone drops a wrench, the data may be ruined. This is quite frustrating, especially if you are reaching the end of a 20-minute coast down.

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change in these frequencies can indicate changes bearing stiffness that might be a result of shifts in load (alignment problems) or a loss of oil film properties.

Another presentation similar to the Bode format is the Polar Analysis, or Polar Plot format (Figure 7.0). Polar plots show radial amplitude and phase during transient events but have limited speed reference information.

Where the Bode plot shows changes in phase shift with frequency, the Polar plot displays the phase vector directly on the specially marked paper

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Figure 6.0 Bode Coast-down Diagram 130

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Figure 5.0 Peak Hold Averaging 140

On-line (Overall) Capture is a time-based sampling method that typically records an overall vibration level on critical machinery. The data is sampled and stored as a long time trend of a vibration parameter that can be compared to other plant process variables (pressure, temperature, flow, etc.). Some alarm capability typically exists, but since only a single value is stored (no spectrum) the method has its limits.

A Bode Analysis is a transient method whereby the parameter’s change in amplitude is compared to its change in phase (with respect to a fixed reference). This method is usually limited to start-up and shutdown applications where the residual rotating unbalance traverses a natural frequency as the speed is changed (see Figure 6.0). Most turbine-generator monitoring or protection systems create bode plots on start and shutdown to verify the locations of critical speeds. A

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Figure 7.0 Polar Plot format during a Machine Start-up

Transient analysis can also be combined with other analysis methods to provide unique characterizations of machinery.

Combining with Orbit Analysis via the measurement of proximity or eddy current probe outputs during changes in rotating speed can indicate transitions through natural frequencies, rubs, or other non-linear events.

As with other advanced analysis techniques, the setup and approach to any given problem is not a one size fits all prospect. Training and practical experience in the field are required to be effective. The training gives you the background on methods and techniques.

Field experience hones your trouble-shooting skills. Vibration problems rarely provide us with quick easy solutions. Nor do they present themselves in ways that allow the analyst to define the entire dynamic system with a single measurement.

Thus, we need specialized techniques to “squeeze” information from these problem machines, digging down deep to get to that elusive root cause. ______________________________________________ Dan Ambre is the founder of Full Spectrum Diagnostics, a full service Predictive Maintenance company specializing in advanced vibration diagnostics and vibration analysis training.

Full Spectrum Diagnostics has recently published the “Vibration Analysis Techniques Guide”. This 108-page paperback pocket reference introduces the analyst to numerous analysis techniques and methods to make the most of your diagnostic capabilities. It also makes an effective reference resource for virtually any vibration analysis training course. Visit us at www.fullspec.net, drop us a note at modalguy@aol.com, or give us a call at (763) 577-9959 to get more information, or to order your guide!