Which device is used to start or stop flow in a pipeline?

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

Answer: (A)

To start or stop flow in a pipeline, you need an isolation valve that can fully close off the line. That’s why the block valve is used: it provides a complete shutoff, allowing the segment of pipe to be isolated for maintenance or safety. A throttle valve, by contrast, is meant to regulate flow and is typically left partially open to control rate rather than to stop flow entirely. A check valve prevents reverse flow but doesn’t offer intentional on/off control for starting or stopping flow. A relief valve is a safety device that releases pressure when it gets too high, not a tool for stopping or starting the flow. So, the device best suited to starting or stopping flow is the block valve.