A check valve is a type of valve that uses a circular valve disc and its own weight, along with the pressure of the medium, to prevent the reverse flow of the medium. It belongs to the category of automatic valves and is also known as a non-return valve, one-way valve, backflow preventer, or shutoff valve. The movement of the valve disc is divided into lifting and tilting types. The lifting type check valve is similar in structure to a stop valve, but lacks the valve stem that drives the valve disc. The medium flows into the inlet (lower side) and out of the outlet (upper side). When the inlet pressure exceeds the sum of the valve disc's weight and its flow resistance, the valve opens. Conversely, when the medium reverses, the valve closes. The tilting type check valve has a slanted valve disc that can rotate around an axis, with a working principle similar to that of the lifting type. Check valves are commonly used as the bottom valve in pumping units to prevent water backflow. When used in combination with a stop valve, they can act as a separator. The downside is that they have high resistance and poor sealing when closed.