Abstract
Steam control valves in power plants are operated under wide range of valve openings and pressure ratios during startup and shutdown transients of the plants. It has been clarified by several researchers that transonic flows and flow fluctuations occur on these valves at a certain range of the operation condition. The transonic flow fluctuation can cause intense pressure fluctuation in the valve and in associating piping. The pressure fluctuation on the valve head yields unsteady fluid forces which can cause forced or self-excited vibrations. In the present study, measurements of the fluid forces are carried out. Using a fixed and misaligned valve, the correlation between the fluid forces and the pressure fluctuations on the valve head surface is confirmed. The time mean fluid force acts as restoring force against the valve head misalignment. Using a valve head exciter, unsteady fluid forces associated with the valve head vibration are measured. The result shows that the unsteady fluid force can provide negative damping force which causes the self-excited vibration of the valve head.
