Large Steam Turbine Response Due to Sudden Coupling Failure

While turbine coupling failures are rare events, the consequences of a coupling failure are potentially catastrophic for both plant equipment and personnel near the machine.  Critical to assessing turbine reliability and designing an effective safety control system for tripping the turbine is the ability to accurately predict the response of the turbine due to a sudden loss of load and driven inertia.  This paper analyzes the peak overspeed of a large, mechanical-drive steam turbine using a numerical technique for dynamic simulation and compares this to the API 612 energy balance method (PTC 20.2-1965). For the energy balance method to accurately predict peak overspeed, the change in efficiency of the energy conversion must be considered during the trapped steam expansion time period.  Using both solution techniques, the influence of turbine rotor inertia, trip delay, trapped steam volume, and trip point setting are investigated.  Most of these parameters are beyond the control of the systems engineer, but modern digital governors and overspeed trip systems provide the precise speed control and quick detection and actuation capabilities necessary to reduce the magnitude of an overspeed.  By reducing the trip point setting and using an acceleration limit, the peak overspeed of the turbine can be significantly reduced.