Multidisciplinary methodology for turbine overspeed analysis

Abstract

In this paper, an integrated approach to turbine overspeed analysis is presented, taking into account the secondary air system dynamics and mechanical friction in a turbine assembly following an unlocated high-pressure shaft failure. The axial load acting on the rotating turbine assembly is a governing parameter in terms of overspeed protection since it governs the level of mechanical friction which acts against the turbine acceleration due to gas torque. The axial load is dependent on both the force coming from secondary air system cavities surrounding the disc and the force on the rotor blades. It is highly affected by secondary air system dynamics because rotor movement modifies the geometry of seals and flow paths within the network. As a result, the primary parameters of interest in this study are the axial load on the turbine rotor, the friction torque between rotating and static structures and the axial position of the rotor. Following an initial review of potential damage scenarios, several cases are run to establish the effect of each damage scenario and variable parameter within the model, with comparisons being made to a baseline case in which no interactions are modelled. This allows important aspects of the secondary air system to be identified in terms of overspeed prevention, as well as guidelines on design changes in current and future networks that will be beneficial for overspeed prevention.