What is rotor dynamics in vibration analysis?
The characteristics of the high speed rotor system are very important to define in evaluating the performance of the gas turbine. The vendor should provide a damped unbalanced response analysis for the prototype of each gas turbine model.
The damped unbalance response analysis should be based on but not limited to the following considerations of the turbine characteristics:
Support (base, flame, and beating housing) stiffness, mass, and damping characteristics, including effects of rotational speed variation. The vendor should state the support system values and the basis of these values.
2. Bearing stiffness and damping values used in the analysis. The basis of these values and the assumptions made in calculating these values.
3. Rotational speeds, including various starting speeds, operating speeds, critical speeds, and the trip speed. Start-up and coast down conditions indicating bleed valve closures and openings, respectively, must be fully documented.
4. Rotor masses including the mass moment of coupling halves, stiffness, and damping effects (such as accumulated fit tolerances, damping, frame effects).
5. Rotor system response to trim balancing in the field.
The analysis should consist of the following charts and tables:
1. A Nyquist and Bode chart showing the frequency phase and amplitude through the entire range of operation.
2. Identification of each critical speed from zero to trip.
3. Identification of mode shapes at each critical speed from zero to trip.
4. Tables showing the acceptable vibration level at various frequencies.
5. A detailed description of the rotor system including the number of stages, number of vanes and blades at each stage, number of gear teeth, and other geometric components that would affect the rotor characteristics of the turbine.
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