“False” Rotordynamic Instability Measurements

Rotordynamic instability is not common, but can be disastrous for the operation of high speed turbomachinery. Most ‘instabilities” are due to de-stabilizing cross coupled forces from variable fluid dynamic pressure around a rotor component, acting in the direction of the forward whirl to produce sub-synchronous orbiting of the rotor. However, not all sub-synchronous vibration measurements indicate instability. Methods to diagnose the potentially unstable signatures from the benign ones are critical to the verifying the health and safety of rotor-bearing systems.

Methods to demarcate between the two were developed in studies conducted by Dr. Vance and Rahul Kar at Texas A&M University. It was shown that orbit shape, frequency tracking, and agreement of sub-synchronous frequencies with known eigenvalues can be used as diagnostic tools. It was also shown that a change in synchronous phase angle produced by de-stabilizing cross coupled forces can be used as a definitive indicator of incipient instability. Typical signatures of sub-synchronous benign vibrations induced from non-linear stiffness of the rotor- bearing system were examined analytically and through experiments.

In one of the tests, a rotor rig was modified to have nonlinear bearing support stiffness as shown by the measurements in Figure 1. The measured frequency spectrum of rotor vibration at twice the first critical speed is shown in Figure 2. The 0.5X vibration is large, and does not track, but it is stable and does not grow larger. There is no cross-coupled stiffness in this test. A waterfall plot obtained from run-up and coast-down of the rotor illustrated that the sub-synchronous vibration disappears when the speed is increased. The experiments were repeated for a linear support system after removing the nonlinear stiffener and no sub-synchronous response was noted.

The component at exactly 0.5X in a spectrum should always be suspected to be possibly benign. It is certainly benign if it tracks the running speed. If it does not track, it may just be due to nonlinear support stiffness.

More details and more methods can be found in the reference below the figures.

Figure 1: Stiffness measurement at the bearing support

Figure 2: Order spectrum when running at twice the first natural frequency

Reference

Vance, J.M. and Kar, R., “Subsynchronous Vibrations In Rotating Machinery -

Methodologies To Identify Potential Instability”, Paper # GT2007-27048, Proceedings of GT2007 - ASME Turbo Expo 2007: Power for Land, Sea and Air, May 14-17, 2007, Montreal, Canada.

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