The Influence of Differential Pressure on Stability of Vertical Rotor-Seal System

Abstract

Abstract The rotordynamic (RD) fluid force generated in fluid elements such as seals in turbomachinery affects the stability of turbomachinery and causes shaft vibrations. Various studies have been conducted to clarify the effects of seals on the stability of rotor systems. Many studies have been conducted on the rotor dynamics of horizontal shaft systems, considering the RD fluid force generated in the seals, and in these studies the stability of horizontal shaft systems has been assessed by eigenvalue analysis using RD coefficients. However, few studies have been conducted on vertical shaft systems. The dynamic behavior of vertical shafts differs significantly from that of horizontal shafts owing to their different structural arrangements. Vertical shaft systems are generally prone to instability because of the fluid film whirl, and the amplitude of shaft whirl tends to be large. When the amplitude is large, the RD fluid force cannot be linearized around the equilibrium point using RD coefficients. Therefore, destabilization and stabilization phenomena that appear in vertical shaft systems cannot be predicted by eigenvalue analysis. To predict such phenomena, Fluid-Structure Interaction (FSI) analysis is required, which considers the interaction between the shaft vibration and the RD fluid force generated in seals. This study used FSI analysis to investigate the effects of unbalance and differential pressure on the stability of a vertical shaft system subjected to RD fluid force generated in the seal.