A check valve including a valve housing defining a pair of valve openings; a pair of flappers pivotably mounted for rotation relative to the housing between an open position in which they permit fluid flow through the respective valve openings and a closed position in which they prevent fluid flow through the valve openings; and a stop element arranged such that the flappers will contact the stop element in their open positions. Each flapper includes a primary flapper element pivotally mounted to a hinge pin that extends across the valve, and a secondary flapper element. The primary flapper element further includes at least one flapper opening formed therethrough. The secondary flapper element is pivotally mounted such that it may rotate relative to the primary flapper element for opening and closing the at least one flapper opening in the primary flapper element.

A passive backflow prevention apparatus includes a tubular base that has a non-planar end defining an opening. The passive backflow prevention apparatus also includes a flap assembly that includes a non-planar flap. The passive backflow prevention apparatus further includes a hinge that pivotably couples together the tubular base and the flap assembly such that the flap assembly is pivotable from an open position, in which the non-planar flap of the flap assembly is disengaged with the non-planar end to enable a fluid, flowing in a first direction, through the opening, to a closed position, in which the non-planar flap of the flap assembly is engaged with the non-planar end to prevent a fluid, flowing in a second direction, through the opening.

A flow-controlling unit provided in a flow channel is disclosed. The flow-controlling unit provided in a flow channel, according to one embodiment of the present invention, comprises: a plate which is disposed perpendicular to the moving direction of a fluid in a flow channel, and on which are formed multiple fluid through-holes through, which the fluid passes; and one or more flow-control ling pieces which are provided in a position corresponding to the fluid through-holes, form an inclination with respect to the plate in the direction of the inflow of the fluid, are capable of tilting on the plate, and control the degree of opening of the fluid through-holes by being tilted due to pressure applied from hydraulic pressure of the fluid.