Date of Award:
Master of Science (MS)
Civil and Environmental Engineering
Michael C. Johnson
In many hydroelectric projects there is a need to safely dissipate the energy associated with the elevation of the water surface. When the flow is not passing through the turbines, bypass valves are often used. A valve that is commonly used is the fixed-cone valve. Fixed-cone valves, also known as Howell-Bunger valves, are devices often used to safely reduce flow energy at dams with medium to high heads. The valve directs the outflow into a conical hollow jet, which requires a large area for energy dissipation. The flow is controlled by an adjustable sleeve, also known as the gate which surrounds the valve and requires minimal power for operation even for large valves. Depending on the installation, the conical jet may need to be controlled by installing a fixed stationary hood or other structure to contain and direct the conical jet. While the hood reduces the spray, the use of the hood causes the formation of a concentrated hollow jet having a high velocity. To eliminate the hollow jet and dissipate much of the associated energy, the hood can have interior baffles. If the hood is not precisely placed relative to the valve, a phenomenon, known as backsplash, will occur. Backsplash is when a significant amount of water exits the upstream end of the hood. Backsplash is a concern for operators because it can prevent access to the valve during operation and can flood valve vaults. Because the use of fixed-cone valves and baffled-hoods are becoming more popular, the need for guidelines to correctly position the hood relative to the valve will benefit both engineers and contractors. In some hydroelectric sites, submerging the fixed-cone valve is used to control the spray and dissipate energy. Submerging the valve can have can produce violent flow conditions which can cause damage to a structure or heavy erosion. The use of a submerged fixed-cone valve is rarely used, and a submerged valve used with a baffled-hood has never been constructed. The study performed shows that the use of a baffled hood with a fixed-cone valve in submerged conditions performs well. The results may lead the way for more submerged fixed-cone valves in the future.
Prettyman, Barry Jacob, "Considerations for Hood Placement and Design Downstream from a Fixed-Cone Valve" (2014). All Graduate Theses and Dissertations. Paper 2119.
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