A check valve is a valve in which the opening and closing member is a circular valve flap and acts on its own weight and medium pressure to block the reverse flow of the medium. It is an automatic valve, also known as a check valve, check valve, return valve or isolation valve. The flap motion mode is divided into lift type and swing type. The lift check valve is similar in construction to the shut-off valve and lacks only the valve stem that drives the flap. 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 valve disc weight and its flow resistance, the valve is opened. Conversely, the valve closes when the media is reversed. The swing check valve has a valve flap that is inclined and rotatable about the shaft, and operates in a similar manner to the lift check valve. Check valves are often used as bottom valves for pumping devices to prevent backflow of water. The combination of the stop valve and the shut-off valve provides a safe isolation. The disadvantage is that the resistance is large and the sealing is poor when closed.