Which device prevents backflow in a piping system by allowing flow in one direction only?

• A. Check valve
• B. X-Valve
• C. Gate valve
• D. Ball valve

Answer: (A)

A device that prevents backflow by allowing flow in one direction only is a check valve. It stays open when fluid moves forward, but when pressure tries to push the fluid backward, the valve seat closes (often aided by gravity or a spring), stopping reverse flow. This automatic action protects the system from contamination and cross-connection.

The other options aren’t built primarily for automatic one-way flow protection. A gate valve and a ball valve are manual shut-off devices used to start or stop flow; they don’t inherently block reverse flow unless they’re closed. The X-Valve isn’t a standard plumbing component and would be considered a distractor.