Studies on the determination of natural frequencies of industrial turbine blades

Abstract

A steam turbine blade is a very complex structure. It has geometric complexities such as variation of twist, taper, width and thickness along its length. For most of the time, these variations are not uniform. Apart from these geometric complexities, the blades are coupled by means of lacing wire, lacing rod or a shroud. Blades are attached to a flexible disc which contributes to the dynamic behavior of the blade. Root fixity also plays an important role in this behavior. There is considerable variation in the frequencies of blades of newly assembled turbines and in frequencies after some hours of running. Again because of manufacturing tolerances, there can be some variation in the blade to blade frequencies. The determination of natural frequencies of the blade is therefore a very critical job. Problems associated with typical industrial turbine bladed discs of a 235 MW steam turbine are highlighted in this paper.