Torsional Vibration of Turbomachinery
Including application case studies and in-depth interpretation of computer-generated results, this book offers practical answers to preventing excessive damage, failure and noise due to vibrations in machines.
TABLE OF CONTENTS Torsional Vibration of Turbomachinery
Chapter 11: Machine Torsional Vibration Design Rules
Most producers of rotating machinery develop design criteria often based on operational experience. Design practices and criteria also evolve as the analytical state-of-the-art advances. Knowledge and procedures in this area are generally highly proprietary because they differentiate competitor s designs and drive product operational performance and ability to succeed in the marketplace.
It should be noted that design practices that are unduly conservative will generally escalate product costs and may be counterproductive for meeting other machine design requirements.
This chapter therefore by intent discusses possible design strategies only in general terms for achieving torsionally rugged machine designs and uses the turbine-generator class of machinery as an example.
11.1 Tuning of Torsional Natural Frequencies
Recognizing that there are significant generator airgap torque frequency components at the first two harmonics of the transmission system frequency and low damping of torsional modes of vibration, an obvious design strategy is to avoid resonance with these harmonics of system electrical frequency.
Torsional vibration modes in the vicinity of the first harmonic of the electrical system frequency are generally predictable to better than 3 percent. Hence a reasonable design guideline would be to separate the torsional natural frequencies from the system forcing frequency (60 Hz) by approximately 10 percent.
For those torsional vibration modes in the vicinity of twice the system frequency, for which the turbine blades can be considered to act as rigid connections to the rotors that support them, the design guideline for frequency separation needs to be guided by the mode shape.
For mode...
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