| 摘要 |
PROBLEM TO BE SOLVED: To provide a flap valve capable of opening a valve element slightly to discharge water securely even if an amount of flowing water is small at normal time and being speedily closed when a water level on a water discharge tip side rises. SOLUTION: This flap valve has a valve casing 16 connected with a gutter pipe 11, a flap valve element 20 opening and closing a water drain port 19 of the valve casing 16, a support member supporting the flap valve element 20 so as to oscillate freely around a valve bar 21, and a counterweight 24 energizing the flap valve element 20 in the direction of opening. Moment M2 in the direction of opening acts on the flap valve element 20 due to self-weight of the counterweight 24, and moment M1 in the direction of closing acts on the flap valve element 20 due to self-weight of the flap valve element 20. When a water level A of a river 12 rises, the moment M2 in the direction of opening is reduced due to buoyancy acting on the counterweight 24 to close the flap valve element 20.
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