No abstract

In modern gas turbines, SC (Single Crystal) and DS (Directionally Solidified) nickel alloys are applied which, compared to CC (Conventionally Casting) alloys, hold a higher cyclic life and a significantly improved creep rupture strength. Because SC and DS alloys feature a significant directionally dependence of material properties, the vibration analysis of the SC and DS blade has to be carried out, taking account of the anisotropy of material properties. In the vibration analysis by FEA (Finite Element Analysis), the DS blade has to be modeled approximately as transverse orthotropic material, while the SC blade can be modeled exactly as orthotropic material in lattice directions. In order to design the SC and DS blade with high reliability, it is necessary to establish the analysis model and to clarify the influence of the anisotropy of the material properties on the vibration characteristics of the blade. In this paper, first, the effect of the anisotropy of elastic constants on the vibration characteristics of the SC and DS blade is investigated. Second, the validity of the assumption of the transverse isotropy for the DS blade, which is applied in the current blade design, is examined by Monte Carlo simulation. Finally, the frequency deviation of the SC and DS blade is analyzed by the sensitivity analysis method, and is compared to that of the CC blade.

The 3 000 rpm 48 inch blade for steam turbine was developed as one of the new standard series of LP end blades. The new LP end blades are characterized by the ISB (Integral Shroud Blade) structure. In the ISB structure, blades are continuously coupled by blade untwist due to centrifugal force when the blades rotate at high speed. Therefore, the number of the resonant vibration modes can be reduced by virtue of the vibration characteristics of the circumferentially continuous blades, and the resonant stress can be decreased due to the additional friction damping generated at shrouds and stubs. In order to develop the 3 000 rpm 48 inch blade, the latest analysis methods to predict the vibration characteristics of the ISB structure were applied, after confirming their validity to the blade design. Moreover, the verification tests such as rotational vibration tests and model turbine tests were carried out in the shop to confirm the reliability of the developed blade. As the final verification test, the field test of the actual steam turbine was carried out in the site during the trial operation, and the vibration stress of the 3 000 rpm 48 inch blade was measured by use of telemetry system. In the field test, the vibratory stress of the blade was measured under various operating conditions for more than one month. This paper first presents the up-to-date design technology applied to the design of the 3 000 rpm 48 inch blade. In the second place, the results of the various verification tests carried out in the shop are presented as well as their procedure. Lastly, the results of the final verification tests of 3 000 rpm 48 inch blade carried out in the site are presented.