Penstock is a pressure conduit that runs between the turbine scroll case and the open water source. The open water can be a canal, river, free flow tunnel, surge tank or a reservoir. They are hydraulically efficient, as to conserve available head and structurally safe to prevent failure that would result in property and loss of life. Penstocks can be manufactured from a variety of materials however the flexibility and strength of steel makes it best suited material for the range of pressure fluctuations that arises in the turbine operation.
Designing and construction of penstocks are majorly governed by appropriate codes which specify certain practices and safety rules that have to be followed while designing the penstocks. Until a special penstock code is formulated, steel penstocks are fabricated in accordance with the American Society of Mechanical Engineers (ASME) Codes. These codes are subject to periodic revision to keep it abreast of new developments in the materials, construction, design and inspection of pressure vessels. Current design standards and construction practices are developed gradually following the emergence of welded construction, and are the results of the enhancements in the production of welding-quality steels especially in welding processes and procedures, and in the testing and inspection of welds.
The position and arrangement of penstocks are determined through the type of dam, location of the intake and outlet, relative location of the dam and the process of river diversion carried out during dam construction. Dams that require tunnel for diversion of the river flow during construction, the penstocks are positioned in the tunnels only when diversion has been discontinued and the intake of the tunnel has been plugged. Such type of arrangement was carried out for the first time in 30-foot lower Arizona and Nevada penstock on Hoover Dam. In low head dams, penstocks are manufactured in the centre of the dam. Large concrete dams have both longitudinal and transverse contraction joints such as Glen Canyon Dam. Steel type penstocks are required to provide the water tightness in the concrete and at the contraction joints.
Penstocks that are fixed in concrete dams or are installed in tunnels are filled with concrete to transmit some of the radial thrust arising from internal water pressure. Typically, such penstocks are fabricated to resist the full internal pressure. In all cases, the shell poses sufficient amount of thickness to provide the sufficient rigidity required during handling and fabrication. Buried or embedded penstock shells are also provided with adequate stiffness to withstand any type of external hydrostatic pressure. In Shasta dam, the upstream portions of the long penstocks are mounted in the dam whereas the downstream portions are exposed above the ground.
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