Venturi valves for steam turbines and improved design considerations

As a turbine governing valve, the venturi valve has been widely used in large turbines to regulate inlet flow for about 40 years. It is favoured in terms of low total pressure loss because of the converging-diverging configurations of the valve passage. However, as turbines become larger and larger, a number of valve failure incidents have been reported, and there is a great demand for improved designs. Yet, because of the complicated nature of the fluid-structure interaction mechanisms, the basic mechanism causing valve vibration and failure is still far from being fully understood. Most of the available literature relies heavily on experiments before the 1980s. There are several improved designs by the trial and error method, but governing rules, or even a clear direction for improvement, are almost non-existent. There has still seen no published investigation using computational fluid dynamics (CFD) tools. As CFD is increasingly recognized as a powerful tool for understanding complicated fluid phenomena, a two-dimensional numerical investigation was performed in the present work. The study revealed that valve plug vibration is due to hydraulic forces acting on the plug at its balanced position and fluid-induced excitation as the plug vibrates in the lateral and vertical directions. All this relates to unexpected asymmetric flow patterns. By changing the plug shape, the flow patterns can be made much more symmetric, which reduces the intensity of steady forces and fluid plug interaction.