[en] A non-return flap valve is patented for the pipe systems of nuclear power plants or other installations with forced multiple circulation where it is required that at zero flow of the medium at least half of the valve area should be open and that complete closure should take place at the preset pressure gradient in reverse direction on the valve disc. The valve disc is attached to a pin slidably mounted in the bearing bush on the arm which revolves around the axis placed next to the valve seat. During the flow of the medium the arm with the disc are elevated. The interruption of the flow will cause the arm to drop into vertical position and the free end of the arm will rest on a stop. Between the disc and the saddle there will be a gap securing the required size of the valve area. In case of back flow the force acting on the reverse side of the disc will overcome the resistance of the spring inserted between the arm and the pin retainer. The valve disc will rest on the seat and will stop back flow. (Ha)