Vibrational Stresses in the Last-Stage Blades of a Powerful Steam Turbine Under Kinematic Excitation of Oscillations. Part 2. Investigation of System with Cyclic Symmetry Violations

Abstract

The influence of kinematic excitation of vibrations on vibration stress in a disk–blade system with a violation of cyclic symmetry when one blade is damaged is evaluated. To assess the trouble-free operation, it is relevant to determine their stress state when the blade shape changes due to erosion damage. The results of calculations of the maximum stresses in the blades under power and kinematic excitation of oscillations are presented. The three-dimensional finite element models of the disk–blade system and the corresponding mathematical software are used to determine the parameters of stationary vibrations and blade stresses. The force effect of a steam flow with a frequency of 2100 Hz (the number of guide blades is 42) and kinematic excitation when the center of the disk moves along an elliptical trajectory in its plane with a frequency of 50 Hz, which is caused by rotor vibration in sliding bearings in stationary operating conditions, is taken into account. The load from the steam flow on each blade was set to be linearly variable from zero at the root of the blades to 1 and 5 kPa at the periphery and for a uniformly distributed 2.5 kPa along their length, acting normally at the points of the blade working surfaces. It is assumed that the physical and mechanical properties of the damaged blade material are preserved after repair and surface treatment. The change in the maximum equivalent stresses in the impeller blades for different loading conditions is determined. The amplitude-frequency characteristics for the maximum stresses in the region of rotational speeds and the action of the load on the blades are given. The results are compared for the system with and without kinematic excitation of oscillations. The studies confirmed the practicality of considering the influence of kinematic excitation when assessing the stress state of the last stage blades of a powerful steam turbine.