Since it was identified as a killer of high power compressors in the 1960’s, “half speed whirl” and “whip” have been understood as rotor vibration that is unstable. It is common knowledge this scary situation can only be dealt with safely by rapidly reducing speed, dramatically reducing load, or outright tripping of the machine in question. But even if this saves the machine, it can be very costly to plant production. Furthermore, a permanent fix typically requires a significant reduction of “cross-coupled stiffness” of the rotor system, involving difficult and expensive design modifications to bearings, labyrinth seals, or impeller side-passages.
Therefore, if the so-called “subsynchronous” (in other words, at a frequency below running speed) vibration turns out in fact to not be a rotordynamic instability, this is very important to know. Otherwise, after a good deal of time and expense, the wrong fix has been implemented, and the original problem still persists!
So, is all subsynchronous vibration an instability? Also, is it always at or close to half running speed? In any event, what physical phenomena would drive such below-running-speed vibration?
The short answer is that, no, not all subsynchronous vibration is a rotordynamic instability.
Why do I say this? Stay tuned for the next installment!
About the Author:
Mr. Marscher has spent his career of nearly 40 years involved in the design, development, and troubleshooting of mechanical components and systems, including pumps, fans, compressors, and turbines. His capabilities and experience include finite element analysis, rotordynamic analysis, experimental modal analysis, vibration testing, predictive maintenance, the mechanical design of turbomachinery, and the development and application of advanced (including magnetic) bearings and seals. In the experimental area, Mr. Marscher is known for his approach of combining advanced test techniques with computer analysis to solve mechanical problems in electrical components and rotating machinery. His innovative vibration test procedures won the $5,000 Dresser Creativity Award in 1986. His rotating machinery rub analysis method won the ASLE Hodson Award in 1983. Mr. Marscher also has pioneered the use of finite element analysis in the prediction of casing stress and split flange leakage, and the effects of piping nozzle loads. He has developed computer programs which automatically set up finite element models for complicated centrifugal machinery components, such as double suction casings, impellers, and entire machinery assemblies. Mr. Marscher has been active for over thirty years in the field of vibration analysis and testing, and he is the originator of the “time averaged pulse” (TAP™) vibration test procedure, which has led to the solution of pump, compressor, turbine, and motor problems in over 300 power plant, water, wastewater, chemical, and API installations.
