Damper seals (such as honeycomb and hole pattern seals) have been widely used in the turbomachinery industry to counterbalance destabilizing aerodynamic forces acting on the rotor system and to provide the necessary amount of damping for stable operation of the machine. Recent experience and research developments have focused the interest of the turbomachinery community on the dynamic characteristics of the divergent-taper damper seal. In addition to offering a review of the current literature on the subject of interest (in order to provide a comprehensive vision of the overall phenomenon), this paper extends the discussion for a better understanding of the mechanism and impact of damper seal clearance divergence on the rotordynamics of a rotor-bearing-seal system (as applied to centrifugal compressors). The analysis of a damper seal alone is insufficient to assess the influence of this component on the rotordynamic stability of a turbocompressor. This paper will show the variation of a complete rotor-bearing-seal system’s logarithmic decrement as a function of the hole pattern seal clearance divergence for a sample centrifugal compressor application, as analyzed with the proprietary, state-of-the-art rotordynamic software suite from the author’s company. This divergence-based transition of system logarithmic decrement from positive to zero (thereby implying the onset of instability) leads to the definition of a “damper seal divergence stability threshold.” Divergence in a damper seal can originate from numerous sources, including taper produced during manufacture, and pressure or thermal-driven distortion of the seal under operation. While small damper seal divergence may first produce an increase of the system logarithmic decrement, additional seal divergence has a dramatic effect on the first forward whirling mode natural frequency, as well as on the logarithmic decrement of that mode. This paper describes the analytical methods used to derive the stability of the rotor-bearing-seal system. It also presents a practical experience that stresses the necessity for a sound approach to properly evaluate the impact of a divergent damper seal on the stability of centrifugal compressors.

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