| 摘要 |
PROBLEM TO BE SOLVED: To fully open a flap valve element normally and to quickly close it when the water level of a drainage destination side is increased by providing a pair of guide means guiding the flap valve element to the fully closing position and fully opening position of the drain port of a valve casing. SOLUTION: A flap valve element 17 floated on the external water surface is arranged below a valve casing 14. A pair of guide bars 18 guiding the flap valve element 17 to a fully closing position and a fully opening position are provided on the valve casing 14. When the water level of a river 12 is increased by rainwater at an abnormality, for example, the flap valve element 17 is raised to approach a drain port 16 as the external water surface rises, and the drain port 16 is fully closed quickly at the upper limit water level UL to prevent the backflow of river water when the external water surface reaches the drain port 16. When the water level of the river 12 is lowered and the external water surface becomes lower than the upper limit water level UL, the flap valve element 17 is opened to resume drainage as the external water surface is lowered.
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