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PROCEEDINGS OF THE EIGHTH INTERNATIONAL CONFERENCE ON CIVIL AND STRUCTURAL ENGINEERING COMPUTING
Edited by: B.H.V. Topping
Dynamic Analysis of a Steam Turbine Support Structure
V. Karthigeyan+*, G.K.V. Prakhya$ and K. Vekaria*
+Offshore Safety Division, Health and Safety Executive, London, United Kingdom
V. Karthigeyan, G.K.V. Prakhya, K. Vekaria, "Dynamic Analysis of a Steam Turbine Support Structure", in B.H.V. Topping, (Editor), "Proceedings of the Eighth International Conference on Civil and Structural Engineering Computing", Civil-Comp Press, Stirlingshire, UK, Paper 124, 2001. doi:10.4203/ccp.73.124
Keywords: dynamic analysis, finite element, foundation, harmonic analysis, piles, soil-structure interaction.
Steam turbines and their alternators are normally supported on concrete tables in order that the steam condenser can be located immediately below. Conventionally dynamic analysis of this arrangement was carried out using lumped mass models assuming the foundation of the table as supported on rigid strata and the table top as rigid in its plane. This simplifies the number of dynamic degrees of freedom (DOF), and enables dynamic analysis to be carried out using lumped 2 or 3 DOF models to capture predominant modes. The conventional models do not take into account the complex nature of the machine assembly, load paths and flexural modes of vibrations.
The availability of finite element (FE) programs with dynamic analysis capability allows realistic modes to be evaluated and amplitudes of vibrations to be calculated accurately. Modelling using plate, shell and some solid elements are considered necessary for the irregular shaped machinery components and concrete sections and also to faithfully produce all the relevant modes and amplitudes due to loads such as out of balance, turbine blade-off, and generator short-circuit.
The authors have presented a finite element model for a steam turbine table top using a combination of beam and plate elements. More importantly they have illustrated how connectivity conditions of the shafts to casing and casing to concrete table and complex configurations can be modeled in order that the loads are applied correctly with due regard for load paths to the table. The modeling also allowed for accurate location of COGs and realistic mass moment of inertia of the machine components such as turbine casing, rotor, alternator stator, rotor, and condenser.
The reinforced concrete table top considered in the paper was supported on piles. The stiffness and damping matrices required in the dynamic analysis to cater for soil-structure interaction effects were computed using DYNA software and were input as support conditions for the FE model. A selective use of stiffness and damping matrices was necessary in order to account for variations arising due to shear wave velocity of soil, and mode shape of the pile cap.
It was observed that the frequencies for corresponding modes in conventional models and FE model are different. The analysis which included sensitivity studies with respect to soil stiffness revealed predominant frequencies in flexure modes close to operational speeds that could never be detected using lumped mass models. Harmonic analysis was also carried out to compute the amplitudes of vibration for the out-of-balance machine loads and to limit them to a suitable acceptance criterion. The study highlighted that the amplitudes computed from the FE model are higher than that computed from lumped mass models due to the participation of flexural modes.
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