Damping Identification and its Comparison for Various Types of Blade Couplings

Regarding steam turbine blade vibrations, damping of blade as well as bladed disc mode shapes is one of the most important parameters in terms of steam turbine operation. A value of the parameter depends on properties of material used for manufacturing and construction elements of the blades and the discs such as blade roots, shrouds, tiebosses (snubbers) and dampers.This article deals with a comparison of damping of mode shapes for particular blade couplings and shows which methods are suitable for determination of the damping in individual cases. The whole identification procedure of the damping together with its specifics is also presented. At first, an identification technique of material damping ratio is introduced and its results are given for different materials. The material damping ratio is assessed as material strain dependent. Subsequently, damping ratio of bladed disc mode shapes under bladed disc rotation is identified taking into account two alternatives. The alternatives differ in such a way that blades have been free for the first time and then coupled with friction dampers.Outcomes presented in the article illustrate good agreement between damping ratio of bladed disc mode shapes with free blades and material used for manufacturing of the blades. On the other hand, damping ratio of bladed disc mode shapes with friction dampers is significantly different and strongly dependent on blade vibration amplitudes as well as nodal diameters of bladed disc mode shapes. Finally, nonlinear behavior of the bladed disc has been revealed along large blade vibration amplitudes and higher nodal diameters of the disc. The non-linear behavior manifests itself in such a way that values of natural frequencies of the disc have become dependent on blade vibration amplitudes.Copyright © 2013 by ASME