Modeling Of Rotor Bow During Hot Restart In Centrifugal Compressors

Abstract

Rotor bow during hot restart is a common cause of high vibrations in turbomachinery. It may occur after the shutdown of a machine operating under load, if the rotor is left cooling in standstill condition (no turning gear) with a subsequent restart of the machine performed before the end of the thermal transient. A complete model of the phenomenon was developed, in order to predict the amplitude of rotor vibrations during hot restart. The thermal distribution across the rotor is calculated basing on fluid-dynamic theory, and is used as boundary condition for a finite element analysis (FEA) that returns the deformed shape of the rotor. Vibration amplitude during compressor restart is then calculated with the traditional tools used for rotordynamic analysis, by unbalancing the rotor according to FEA results. The developed model has been used to simulate a real case for which experimental data were available. Calculation results showed very good agreement with experimental measures, confirming the expected features of the phenomenon, its trend with time and the effectiveness of slow roll in straightening the rotor and reducing vibration amplitude.