US Patent 6851255 Normally open reverse flow flapper valve

A fluid flow actuated valve for controlling the fluid flow in a conduit comprises a support structure adapted to be mounted within the conduit and at least one flap member pivotally mounted to the support structure. The at least one flap member is pivotable between an first position for a minimum fluid flow and a second position for a maximum fluid flow. A torsion spring is attached to the at least one flap member to urge the same to pivot against a fluid pressure differential, from the first position to the second position when the fluid pressure differential is smaller than a predetermined level.