Which device prevents backward flow in a system?

• A. Check valve
• B. Block valve
• C. Throttle valve
• D. Relief valve

Answer: (A)

Preventing backward flow in a system is achieved with a device that allows flow in only one direction. A check valve uses a movable element, such as a disc or ball, that is pushed open by forward flow and automatically seats to block the passage when flow tries to reverse. This passive, one-way design protects pumps and downstream equipment, prevents contamination, and helps maintain system pressure without needing an operator or actuator.

A block valve is a manual shutoff that can stop flow when closed, but it doesn’t automatically prevent backflow if conditions change or if it’s left open. A throttle valve restricts flow to control rate, not to stop reverse movement. A relief valve releases pressure to the environment when it’s too high, not to stop backward flow.