Which type of valve allows water to flow freely in one direction while preventing backflow?

A clapper valve is specifically designed to allow fluid to flow in one direction while preventing any backflow. It contains a hinged disc or flap that opens when water flows in the intended direction. When the flow reverses, the flap closes against the seat, effectively blocking the reverse flow. This mechanism is critical in applications like fire protection systems, where preventing backflow is essential for maintaining pressure and ensuring water availability.

The other valve types serve different functions: a gate valve is primarily used to start or stop the flow of water but does not prevent backflow; a butterfly valve facilitates flow control but isn't specifically designed for backflow prevention; and a pressure relief valve is used to automatically release pressure from a system to ensure safety, rather than controlling the direction of flow. Thus, the clapper valve stands out as the appropriate choice for preventing backflow while allowing forward flow.