Said invention concerns a device to achieve maximal flow and minimal fall in pressure in check valves, which sees that liquids and gases do not flow in an unwanted direction. Said invention can be used for air, combustion gases, against bad odours, flooding via drains, rat invasion via drainpipes, etc. Check valves according to said invention are self-sealing and cheap to manufacture and can be used for many different applications. The principle allows straight laminar flow area without impedimentary parts. Said invention comprises inside a pipe (1) axially skewed spinnaker-like membrane (6) of reinforced foil. Two of the arch-shaped membranes (6) sides (7) are attached to opposite pipe walls with starting position from the pipes (1) periphery to be level with the centre of the pipe. Said membranes (6) protruding short side (8) at the middle of the pipe (1) is entirely free and can under pressure from fluids protrude and touch opposite pipe wall and there form a C-like shape for round pipes. In this way the flow is blocked totally in one direction, but can flow freely in the opposite direction when said membrane (6) can bend away entirely in the other direction on reverse pressure of fluid and thus liberate a fully open flow area without disturbing edges and impedimentary details.Linvention concerne un dispositif qui permet dobtenir un écoulement maximal et une chute de pression minimale dans des clapets de non-retour, ledit dispositif empêchant que des liquides et des gaz ne sécoulent dans la direction non voulue. Ladite invention peut être utilisée pour de lair et des gaz de combustion, contre les mauvaises odeurs, contre une inondation par des tuyaux dévacuation, contre une invasion de rats par des tuyaux dévacuation, etc. Des clapets de non-retour selon ladite invention sont à auto-étanchéité et peu coûteux à fabriquer, et peuvent être utilisés pour de nombreuses applications différentes. Le principe permet davoir une zone découlement laminaire droite sans parti
