Vibration Analysis of Low Pressure Stages of Powerful Steam Turbines with ANSYS

High cycle fatigue (HCF) plays a significant role in a bulk of turbine blade failures. During operation, periodic fluctuations in the steam force occur at frequencies corresponding to the operating speed and harmonics and cause vibration of the bladed disks. Digital prototyping and optimization become the most practical and economical means for turbine stages design and for identification and solving HCF-caused failures associated with resonant conditions. During fabrication, 2-3% variation of blades thickness is considered acceptable. Such fluctuations can lead to deviation of the blades vibration from expected values within the range of 5%. More drastic difference of blades’ dynamic properties during turbine operation could occur as a result of blades erosion. Significant amount of research efforts went into developing strategies of grouping blades such that minimize adversary effect of blade variations. These techniques are illustrated with analysis of gas turbine working wheel with 34 buckets. Mistuning as modeled by blade material density variation. It was presumed that a half of the blades ware 5% heavier than another half. This leads to 2.5% difference in natural frequencies. Two simplest blading methods were considered: blades of the same type were arranged in either 4 interlaced sectors, or in 8 sectors.

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