A flap valve is simple, one-way valve that allows the flow of gases or fluids in one direction while preventing flow in the opposite direction. The valve accomplishes this objective with a spring loaded or weighted flap placed across the opening of a pipe, tube, or duct. The valve flap releases in the direction of the desired flow and is either forced open by the flow or by remote means. When the flow ceases or the remote actuation releases, the spring or weight of the flap closes it against a seal, effectively preventing fluid or gas from flowing back into the pipe. The flap valve also prevents undesirable foreign objects from entering the pipe or channel to which it is attached.
Flap valves are cost effective and efficient one-way seals and have many different applications ranging from large water treatment plant valves to small flush valves used in household toilets. Due to their inherent simplicity and lack of complex actuation systems, they are also among the simplest and cheapest valves to maintain. This makes the flap valve a particularly attractive option for systems that work hard and are seldom subjected to very high pressures. This simplicity also makes flap valves very effective for applications in remote areas where regular maintenance is impractical.
One of the most common domestic applications of the flap valve is the household toilet. This type of flap valve is remotely actuated by the flush push button or lever and lifts to allow water in the cistern to flow down into the bowl to flush the toilet. Once the cistern empties, the valve is released to seal the outlet pipe and to allow the cistern to fill once again.
Flap valves come in a wide range of sizes and designs which range from small plastic valves one or two inches (about 3 to 5 cm) in diameter to huge stainless steel versions several yards across. The basic principle of operation remains the same; as such, large flap valves remain as simple and easy to maintain as their smaller counterparts. Large valves are, however, more likely to be opened by pumped fluid flows at higher pressure than the natural gravity drainage frequently encountered with small domestic valves.
The actual flap component of a flap valve may also be made of a flexible material such as neoprene or silicone. The only real requirement for the effective operation of this type of valve is that the flap opens when subjected to the normal flow through the duct and forms a good seal when closed. The valves in the human heart are classic examples of a flap valve, amply demonstrating the great simplicity and reliability of the design.