Hydroelectric power plants are used worldwide to cover varying electricity demands. The Salime hydroelectric power plant, which is located at Asturias, Spain, has four hydro turbines with a total design capacity of 128 MW. Each turbine has a ball valve with two maintenance seals to ensure a closure as complete as possible when the group ceases to operate. Unfortunately, at some occasions the seals did not really perform their sealing function properly, but started to develop periodic vibrations of indefinite duration. This phenomenon generates periodic leakage flow as well as high amplitude pressure fluctuations in the penstocks, which are not acceptable. This phenomenon corresponds to the field of flow-induced vibrations, in particular to the type of self-excited vibrations. The purpose of the research now reported was to develop a simplified theoretical model that can explain the excitation mechanism for the seal vibrations and that can estimate the behavior of the hydro-mechanical system depending on the relevant geometrical and physical parameters. In order to calculate the pressure and flow rate fluctuations, the energy equation for unsteady, unidirectional, incompressible and viscous flow has been applied along each pipe of the hydraulic system, together with continuity considerations at each pipe junction and the seal equation of motion. The perturbation technique has been used to solve the system variables. The mathematical model was solved by means of a specially designed MATLAB code, which allows simulating the time evolution of the annular seal vibration as well as the unsteady flow and pressure induced throughout the system for different system configurations. The results show that the system stability depends on the behavior of the hydraulic pressure force acting on the seal and the gap flow rate after system disturbance. Besides, the results obtained support that, at standing group situation, seal vibrations are less prone to occur when operating at either low reservoir energy level or very large reservoir energy level.
In practice, vibrations in the turbine inlet valves can develop flow rate and pressure fluctuations in the penstocks of the hydropower plants that can risk the safe operation of the power plant. Accordingly, this study has two aims: the first aim is to explain and simulate the valve’s periodic vibrations while considering the power plant’s relevant physical and geometrical parameters. The second aim is to provide recommendations to improve the dynamic stability of the valve’s vibrations. The theoretical model developed to explain the phenomenon comprises the nonlinear unsteady energy equations representing the fluid flow at the pipelines, the continuity equation modelling fluid flow through different junctions, and the valve’s seal equation of motion to compute the valve’s vibrations. The system governing equations are solved nonlinearly using the MATLAB toolbox SIMULINK. The study demonstrated that the origin of the valve’s vibrations is the leakage of the valve’s service seal. The presented model results exhibited that the unstable valve vibrations (of increasing amplitudes) are more prone to occur at higher input reservoir energy levels. Also, installing a valve that can be closed to a certain degree at the pilot pipeline can enhance the dynamic stability of the valve’s vibrations.
The main function of the turbine inlet valve (TIV) in a hydroelectric power plant is to prevent flow of water to the turbine whenever the turbine is not operating. Usually, the valve responsible for this operation is spherical with annular seals to perform the sealing function. Occasionally, when the valve is set into a closed orientation, the annular seal may not execute its sealing function properly though develop periodic oscillations accompanied by periodic leakage flows. These seal vibrations cause pressure fluctuations in the penstock pipeline, which risks the plant’s reliable and safe operation. So, the primary goal of this research is to present a simplified theoretical model, able to clarify the excitation mechanism of the periodic seal vibration and simulate the plant's transient behavior. Afterwards, develop some recommendations to enhance the stable operation of the (TIV). The system governing equations comprises the water hammer equations to model the water flow through the various pipelines, the vibrating seal equation of motion, and the system boundary conditions. Results revealed that the dynamic instability of the (TIV) vibrations is more likely to arise at higher input reservoir energy levels and at the 1st harmonic of the seal oscillation. Also, modifying the (TIV) by increasing pilot pipeline head losses and reducing its diameter can eliminate the (TIV) vibrations and warrant the plant’s safe operation.
Purpose The main function of the turbine inlet valve (TIV) in a hydroelectric power plant is to prevent flow of water to the turbine whenever the turbine is not operating. Usually, the valve responsible for this operation is spherical with annular seals to perform the sealing function. Occasionally, when the valve is set into a closed orientation, the annular seal may not execute its sealing function properly though develop periodic oscillations accompanied by periodic leakage flows. These seal vibrations cause pressure fluctuations in the penstock pipeline, which risks the plant’s reliable and safe operation. Therefore, the primary goal of this research is to present a simplified theoretical model, able to clarify the excitation mechanism of the periodic seal vibration and simulate the plant’s transient behavior. Afterwards, develop some recommendations to enhance the stable operation of the (TIV). Methods The system governing equations comprises the water hammer equations to model the water flow through the various pipelines, the vibrating seal equation of motion, and the system boundary conditions. Results The dynamic instability of the (TIV) vibrations is more likely to arise at higher input reservoir energy levels and at the first harmonic of the seal oscillation. In addition, modifying the (TIV) by increasing pilot pipeline head losses and reducing its diameter can eliminate the (TIV) vibrations and warrant the plant’s safe operation. Conclusion Results revealed that fluid compressibility and acoustic transmission have a decisive effect on the fluid-dynamic forces acting on the seal and the (TIV) stability. In addition, the origin of the (TIV) vibrations is the valve leakage flow through the service seal.
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