Aerodynamic Damping of Last Stage Rotating Blades of Low Pressure Steam Turbine Determined From Vibration Test Data

Abstract

Abstract In the past, power generation steam turbines were mainly operated at design conditions and at base load. Due to the greater share of fluctuating renewable energy in electricity grids, conventional power plants are increasingly subject to load cycling, which presents higher demands with respect to operational flexibility of turbines. In addition to uncompromised requirements on the efficiency at varying operating conditions, extending the operating range to higher mass flow densities is becoming more important. Often, such developments go beyond existing experience, which necessitates the proper assessment of instability phenomena such as flutter. Safe operation requires the knowledge of the damping behavior in the entire operating range in order to conclude on the stability of the rotating blades. The aggregate damping consists of mechanical and aerodynamic damping, whereby it is the latter that may lead to an unstable situation. This paper includes a systematic study on the suitability of various methods for obtaining reliable damping values from experimental test data under realistic operating conditions of steam turbines. In a first step, the influence of different parameters is investigated on a generic ideal signal. Among others, the sensitivity of derived damping values from model parameters is quantified. In a second step, the presented evaluation methods are applied to vibration test data acquired on a free-standing last stage blade row of a scaled steam turbine. The turbine features controlled blade excitation by means of AC electromagnets. The transferability from generic to real signals is analyzed. A robust method is presented to obtain reliable aggregate damping values at various operating conditions from real test data. For this aggregate damping, the proportion of mechanical and aerodynamic damping is determined by the evaluation of the aggregate damping at several operating points at constant flow rate. This procedure allows to determine the aerodynamic damping of the excited nodal diameters for each operating point. Finally, the experimental aerodynamic damping is compared to data from numerical calculations.