A check valve is a device that permits the flow in only one direction. Check valves for fire protection prevent this unwanted reverse flow of water, known as backflow, from occurring.
Check valves are equipped with a disc that swings on a hinge or shaft. The flap swings away from the seat to allow forward flow of the media, and when the flow stops, the flap swings back to the seat to stop reverse flow. This type of valve should generally be installed horizontally in the pipeline.