Study on vibration characteristics of steam turbine vanes with large capacity (Vibration response characteristics of half ring structure)

Abstract

A vane used in a low pressure end of steam turbine is usually fixed to an inner shroud and an outer ring by welding both ends. In such a vane structure, the damping in loading operation is comprised of the material damping and the aerodynamic damping, because the structural damping is very small. In addition, steam turbine vanes are sometimes manufactured in a half-ring structure, where all vanes in a upper half and a lower half are connected by the shroud. In such a half-ring structure, the vane exhibits closely spaced natural frequencies of Toop (Tangential out-of-phase) modes, in which each vane in a half-ring vibrates like a fixed-fixed beam in a different phase. In such a steam turbine vane, the forced vibration and the self-excited vibration of Toop modes may become a serious issue. Therefore, in the design of the steam turbine vane, it becomes indispensable to evaluate the resonant response and the stability of Toop modes under the high loading condition. In this study, first, the vibration characteristics of steam turbine vane with a half-ring structure are studied by use of the results of FE analysis. Second, the reduced order model (the equivalent spring-mass model) of a half-ring structure is assembled, based on the results of FE analysis. Finally, the forced response analysis of the mistuned system of a half-ring structure is carried out, using the reduced order model and the Monte Carlo simulation. From these results, the vibration response characteristics of steam turbine vane with a half-ring structure are clarified.