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

The check valve is specifically designed to allow fluid to flow in only one direction. This unique functionality is crucial in various plumbing and fluid systems as it prevents backflow, which can lead to contamination or system failure. Check valves operate automatically, relying on the pressure of the fluid to open and close, ensuring that when the fluid attempts to flow backward, the valve will close and block that reverse flow.

In contrast, the other types of valves mentioned serve different purposes. A ball valve can regulate flow by rotating a ball inside the valve body, allowing for both on and off control but does not inherently prevent backflow without specific design features. A gate valve operates by lifting a gate out of the flow path and is typically used for fully open or closed applications rather than flow direction control. The butterfly valve uses a rotating disc to manage flow and can allow for bidirectional flow depending on its design. Thus, the check valve is uniquely suited for applications where the prevention of backflow is essential.