Real-time torsional vibration measurement and analysis
Rotating and reciprocating machines are prone to some degrees of torsional vibration, but however its presence is generally unaware since there are no permanent torsional vibration monitoring system installed in most cases unless there are some failures occurred and/or gearbox is installed in the drive train. Then, torsional vibration can be seen in radial/lateral vibration measurement system as torsional-lateral coupled vibration. This is very common in power generating unit as well as air/gas compressor train.
Torsional natural frequency with associated mode shape was calculated and identified during design process for new machines. Frequency interference/Campbell diagram can then be generated with possible exciting sources to identify potential resonance problem. There are still some torsional vibration problems in the field/operating machine trains as actual operating condition or environment is different from the design and/or some exciting sources were not identified and studied properly, for example, oscillating motor air-gap torque due to distorted supply current, torsional-lateral coupled vibration due to electrical faults/synchronization issues, zero backlash of double helical gear due to poor axial alignment, unexpected oscillating torque induced from variable frequency drive (VFD), uprated plant operation resulting in over torque to the coupling, etc. For old machine, there is another story as torsional analysis report is not available. The issue is suspected when user encountered frequent shaft/coupling broken!
Field torsional vibration measurement, analysis, and troubleshooting is challenging! We have been involved and used many techniques to measure and analyze torsional vibration over the past few years. However, those are post processing techniques, for examples, high-sampling rate data measured from single or multiple events per shaft revolution using phase marker probe or laser/optical sensor then the data is post processed to obtain torsional vibration data for analysis in either time waveform or spectrum format. This is quick and easy approach to prove if torsional vibration exists in the drive train or not. But this is not applicable in many cases where we need to correlate torsional vibration data with relevant process variables such as motor current, load control operation, or even lateral vibration data, etc. to identify possible root cause and further propose solution to fix the problem. This calls for “real-time” torsional vibration measurement and analysis. Strain gage and its telemetry is used in conjunction with vibration analyzer as data acquisition unit and present measured data in various plots. Gapless (raw) vibration data can also be recorded for playing back and post processing if required and this is supplementary to real-time torsional vibration data. We found this approach is very effective as we can identify the root cause and help customer to solve their machine problems.
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