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These are similar to wall-mounted gates, except that the gate is closed in its "up" position, and as it is lowered, the opening is uncovered, to allow the flow over the top to be controlled.
A different seal configuration is used, with either "J" or "Triangular" type Neoprene seals being fixed to the frame instead of the gate.
Downward-opening weir gates can be either four-sided sealing, if the head is greater than the height of the opening, and it is necessary to shut off the flow completely, or three-sided sealing, when no top seal is required.
Occasionally a weir is required to have a width much greater than its height. As this may cause the gate to jam in the guides, the effective height can be increased by means of side extensions to the gate. If this is not practical, twin lifting spindles with two inter-connected gearboxes are used, but depending on gate size and the travel required, a tilting weir may be preferable to a downward-opening type.
Tilting weir gates are used for wide openings where the vertical travel required is not more than about 500 mm. The gate is hinged at the bottom, and is raised and lowered by means of a centrally mounted spindle which can be operated either manually, or by an actuator. As friction is very low, operating effort for tilting weirs is minimal.
Sealing plates can be fitted to the end walls if required, whilst a flat Neoprene seal across the width of the weir prevents leakage along the hinged section.
Scum baffles, located upstream of the weir, are optional.