During the start-up phase of a 1 750 MW large scale turbine unit

several fatigue fracture cases occurred in by-pass valve components due to valve vibrations

which affected the safety operation of the units. The goal of this paper is to reveal the vibration mechanism and law of by-pass valve

and provide references for its structure and operation mode optimization. Simulation is performed to investigate the fluid dynamics inside the valve. The results show that

there are obvious vortex shedding and boundary layer separation phenomena in the valve fluid medium

and the flow characteristics change significantly with the changing valve opening degree. Based on the simulation results of fluid structural dynamics and vibration test results

the fluid structure coupling comprehensive analysis is carried out. The analysis results indicate that

the vibration induced by vortex shedding in the valve is the main cause of the fracture of the valve stem and the coupling. The valve structure optimization decreases the turbulence and vortex shedding degree of the fluid medium. However

when the flowrate is regulated frequently at large valve opening degrees

the higher vibration level of valve stem and coupling will still be excited. The vortex induced vibration of the valve with large opening degrees is the main reason for the damage of turbulence hood.