Which valve type is used to prevent backflow by closing when flow reverses?

• A. Gate valve
• B. Globe valve
• C. Stop check valve
• D. Relief valve

Answer: (C)

Backflow prevention relies on a valve that automatically seals when flow tries to move in the opposite direction. A check valve does this: it opens with forward flow, then a reverse force pushes the disc (or ball) onto a seat, stopping any backflow. A stop check valve is a type of check valve that also offers a manual shutoff, but its core function—closing when flow reverses to prevent backflow—remains the same. The other valves serve different roles: a gate valve is for full isolation and isn’t relied on to prevent backflow automatically; a globe valve is optimized for throttling and flow control; a relief valve opens to vent excess pressure rather than block reverse flow.