Bearings are a key factor in achieving a good rotor dynamics performance for turbo machinery. Large compressors, steam and gas turbines for industrial applications are generally equipped with journal bearings either as tilting pad or multi-lobe bearing type. Here bearing parameters such as bearing geometry, bearing load or oil viscosity significantly alter bearing behavior and influence the rotor dynamics of the entire rotor-bearing system. In order to find an optimal set of bearing parameters for a given rotor-bearing system a nonlinear parameter optimization approach is employed. The rotor-bearing system is parameterized using bearing width, clearance and preload as design variables, since they represent design parameters that can be modified without significantly influencing the rotor design as a whole. The set of design variables is further constraint to stay within feasible limits of bearing design. The objective function is defined as a quantitative measure of rotor dynamic performance evaluating the distance from required separation margins with respect to rotor critical speeds based on API 617 7th Ed. In order to compute the objective function based on the design variables the bearing code ALP3T, solving Reynolds equations for the bearing fluid film, is used to compute the required stiffness and damping coefficients as input to the rotor dynamics program. The rotor dynamics performance is then evaluated using the rotor dynamics code SR3 based on the transfer matrix method. Both programs have been developed by the University of Braunschweig and are defacto industry standard within the German turbo machinery industry. The two programs are coupled and the nonlinear constraint optimization problem is solved using MATLAB’s optimization toolbox. The feasibility of this method is discussed based on an example of an axial flow compressor using two-lobe bearings. It is shown that a significant improvement in rotor dynamic performance can be achieved when compared to previous bearing selections for similar compressor designs and that the approach is suitable for a real-life engineering environment.

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