Hydrodynamic self-excited vibrations in leaking spherical valves with annular seal

Abstract

Occasionally, annular seals of ball valves in hydro power plants may not perform their sealing function adequately but develop periodic vibration with periodic leakage rates causing high amplitude pressure fluctuations in the penstocks. The aim of the present study is to develop a simplified model that can explain the excitation mechanism and estimate the behavior of the hydro-mechanical system depending on the geometrical and physical parameters. The stability of the system has been studied in order to yield recommendations for best operating conditions. The governing equations are nonlinear and the perturbation technique has been adopted to solve the system variables. Following, a dimensionless analysis was developed to gain more knowledge on the parameters affecting this phenomenon. A MATLAB code was designed and implemented to study the system stability for different input reservoir energy levels and different system configurations. The results show that the system stability depends on the relation between the main line inertia and the leakage kinetic energy, the leakage reduced velocity, the seal surface area ratio, the pilot line geometry and the hydraulic losses. Also, solving the hydro-mechanical model nonlinearly brings about oscillation amplitudes that exceed the gap clearance as it is very small and the seal will hit the ball surface going backward and forward.

Keywords

• Flow induced vibration
• Self-excited vibrations
• Salime hydroelectric power plant
• Periodic leakage flow
• Pressure fluctuation generation
• Perturbation technique