A check valve refers to a valve whose opening and closing parts are circular discs and rely on their own weight and medium pressure to act to block the reverse flow of the medium. It belongs to the automatic valve category, also known as check valve, one-way valve, return valve or isolation valve. The movement mode of the valve disc is divided into lifting type and swing type. The structure of the lift check valve is similar to that of the globe valve, except that it lacks the valve stem that drives the valve disc. The medium flows in from the inlet end (lower side) and flows out from the outlet end (upper side). When the inlet pressure is greater than the sum of the weight of the valve disc and its flow resistance, the valve is opened. On the contrary, the valve is closed when the medium flows back. The swing check valve has an inclined disc that can rotate around its axis, and its working principle is similar to that of the lift check valve. Check valves are often used as foot valves in water pumping devices to prevent the backflow of water. Check valves and stop valves are used in combination to provide safe isolation. The disadvantage is that the resistance is large and the sealing is poor when closing.