ABSTRACT
The high-head gates and valves differ from surface (crest) gates in that they are always under water, and they close the opening from all sides and do not allow overflowing of water at the upper side. In the case of dams, they are employed with outlet works, supply structures, intake structures, and also with deep or high-head spillway structures. The submersion of the gate or valve may be from several metres up to several dozens of metres. The characteristic service features of high-head gates and valves are such that they are subjected to a high water-pressure, while they are less accessible for control in comparison to surface gates, and handling them is less frequent. Velocities of flow in the water-conveyance structures or conduits, which are closed by these highhead gates and valves, are considerable and they can reach 30 m/s. The above-cited features impose the need for the construction of these gates and valves to be firm and stable, while the operation safe even in the hardest conditions of service. In order to be protected from coarse objects and solids, rough trash racks are usually placed in front of them. And since they close important structures, the high-head gates and valves are manufactured as either double type, one after another, or an overhaul-emergency gate or valve is anticipated, in addition to the service gate or valve.
