The design of blades in the last stage of a steam turbine is one of the most demanding engineering tasks in the turbomachinery field. Increasing turbine efficiency has led to the designing of higher tip-to-hub ratios. Slender blading conforms to reliability requirements, such as high blade stiffness and a high first mode natural frequency.
Several high vibration amplitude problems were reported regarding a slender last stage blading of a commercial low-pressure steam turbine. During maintenance it was decided that the blades would be geometrically mistuned to prevent self-excitation.
This paper presents a numerical study of LP steam turbine last stage bladed disk mistuning. Two different approaches to mistuning were applied and numerically compared: geometrical and material. The mode shapes and natural frequencies of the steam turbine bladed disk were calculated on the basis of an FEM model. The smallest range of mistuning (0,5Hz) in a bladed disk contaminates nodal diameters up to the fourth series. This should be taken into account when tip-timing method is adapted for steam turbine operation monitoring.