Fact or Fiction? "Half-Running-Speed Vibration Is Always Rotordynamic Instability" Part 1

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!