Which type of valve allows fluid to flow in only one direction?

Check valves are specifically designed to allow fluid to flow in only one direction, preventing backflow in a system. They operate automatically with the changing pressure within the pipeline. The internal mechanism typically consists of a disc or flap that opens when the fluid flows in the intended direction, and closes when there is reverse flow. This functionality is crucial in many applications to maintain system pressure and prevent potential damage caused by backflow.

In contrast, plug valves, globe valves, and butterfly valves do not inherently function to prevent backflow. Plug valves primarily control flow and can allow fluid to flow in both directions depending on their orientation. Globe valves are mainly used for regulating flow rather than ensuring unidirectional flow, and butterfly valves can also permit flow in either direction as they are used for isolation as well as throttling purposes. Thus, it is the check valve that fundamentally serves the primary purpose of unidirectional flow control.