Application of Turbine Generator Train Rotordynamic Analysis for Setting Residual Modal Unbalance Requirements for High Speed Balancing of a Single Rotor

Abstract

Assessment of high speed balance quality of a rotor with residual modal unbalance (RMU) has many advantages over traditional residual vibration method. In addition to be practically independent from support stiffness variations, it allows utilization of full train rotordynamic analysis for setting balance limits for a single rotor. Rotordynamic calculations of a turbine-generator train identify eigenmodes that can roughly be separated into two categories: single rotor and system modes. Large units with many rotors may have over 40 modes. The challenge is to select eigenmodes relevant for each rotor in the train and generate appropriate RMU requirements for high speed balancing. The other challenge is correlating eigenmodes of a single uncoupled rotor in a balancing facility to eigenmodes of a turbine-generator train and vice versa. The paper demonstrates that unbalance response calculations for turbine-generator train using appropriate modal weight sets exciting particular modes can allow for identification of modes relevant for high speed balancing facility conditions [1]. These modes can then be targeted for setting appropriate RMU limits. The emphasis is given to operating speed range and modes above 2nd critical speed. The paper offers an example of setting RMU tolerance for high speed balance for a large generator rotor based on rotordynamic analysis of turbine-generator train. Field vibration data is presented to demonstrate the applicability of the approach.