Development and Preliminary Validation of a New Strategy to Model the Interaction Between Rotating Machines and Elastic Supporting Structure

Abstract

The accurate modelling of the complicated dynamic phenomena characterizing rotors and support structures represents a critical issue in rotor dynamics field. A correct prediction of the whole system behavior is fundamental to identify safe operating conditions and to avoid instabilities that may lead to erroneous project solutions or possible unwanted consequences for the plant. Although a generic rotating machine is mainly composed by four components (rotors, bearings, stators and supporting structures), many research activities are often more focused on single components rather than on the whole system. The importance of a combined analysis of rotors and elastic supporting structures arises with the continuous development of turbo machinery applications, in particular in the Oil and Gas field where a wide variety of structurally optimized solutions with reduced weight on off-shore installations or modularized turbo-compression and turbo-generator trains, requires a more complete analysis not only limited to the rotor-bearing system. Complex elastic systems, in some cases, strongly affect the entire shaft line rotor dynamic response such as mode shapes, resonance frequencies, unbalance response and critical speeds. The aim of the study is a development of a new efficient methodology based on FEM approach to model the complete rotating machinery systems (rotors, bearings, stators and supporting structures), by means of appropriate transfer functions matrix. Taking advantage from the matrix of transfer functions H(ω) obtained through PSD analysis, the baseplate dynamic behavior can be timely and CPU efficiently computed, avoiding computationally expensive harmonic sweeps. The appropriate usage of undocumented ANSYS command ‘TFUN’ has been pursued in order to extract the required components of the transfer functions matrix at the bearing location. With such a solution the full dynamic interaction between the system components was accurately accounted. The outcome of the new methodology was successfully tested in a real field issue where evidences of structure to rotor interaction emerged at the proximity probe measurement during machine start-up.