Throttle valves in steam turbines often operate at very small lift positions during turbine startup. The large pressure differentials across these valves, combined with the very small openings at the valve seat, result in large pressure drops across these valves and high local steam Mach numbers. A steam turbine throttle valve operated under these conditions was found to be undergoing self-excited vibration. Stress and structural dynamic finite element analyses (FEA) were performed to identify the structural mode for the valve oscillations. A three-dimensional transient computational fluid dynamics (CFD) analysis of the valve revealed an unsteady fluid dynamics phenomenon in the pressure balancing arrangement that served as a forcing function for this vibration. Valve modifications were implemented as a result of these analyses. The improved valve has performed successfully, and the design modifications have been incorporated in other production valves.

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