Practical Uses of Advanced Rotor Dynamics Tools to Ensure Trouble Free Operation of an Overhung Turbo-Expander Rotor

Abstract

The intent of this paper is to illustrate how rotor dynamics analysis can be an effective tool to improve rotor dynamic characteristics. Analytical methods, such as undamped critical speeds, unbalance response and stability (damped eigenvalues) analysis are used to evaluate the vibration characteristics of rotating machinery. Advanced rotor dynamics software is capable of generating and analyzing rotor models. By removing undesirable sources of instability and ensuring reliable separation margin, rotor-stability improvement is validated. This study presents an overhung rotor with a proven stable operating history; however, higher power requirements and changes in components resulted in a potential resonant condition of concern. Based on the rotor dynamics analysis outputs, design changes were implemented to ensure the rotor will meet API’s vibration acceptance limits. After evaluating different design alternatives using computer modeling, a change in the inboard bearing diameter proved to be the best solution for attaining smooth operation. Although this paper focuses mainly on the analysis performed on an overhung rotor, the methodology used can be applied to any rotating equipment. It is clear that rotor dynamics analysis is a powerful instrument for predicting operating behavior, thus allowing for necessary changes in analytical modeling and ensuring more stable machines.