Today’s power generation market requests high efficiency for steam turbines. One key factor in fulfilling these demands is the increase of exhaust area: efficiency is increased by reducing exhaust losses and lower costs are reached by a lower number of flows. The 54″ last stage blade for a steam turbine has been developed with the application of modern design features. It was designed with integral cover, mid-span tie-boss connection, and straight fir-tree dovetail. Blades are continuously coupled by the blade untwist due to the centrifugal force, so vibration control and increased structural damping are provided.
This paper describes in detail the design characteristics used in the development of the blade including non-linear structural analysis methods. The blade airfoil was optimized from point of view of minimization of its centrifugal force. The blade was well tuned in order to have natural frequencies safely away from possible excitation. Because of connection members, the number of the resonant vibration modes can be reduced by virtue of the vibration characteristics of the circumferentially continuous blades. Coupled rotor-blade frequencies were also analysed.
Extensive material tests were performed to check titanium properties, which showed lower erosion and low cycle fatigue properties. New procedures were developed to improve titanium characteristics, so the material behaviour is acceptable for last stage blade construction.
To validate the results, full scale rotational vibration test in vacuum chamber was carried out and suitable dynamic properties as well as mechanical integrity of the blade were confirmed.