What is the difference between a check and a relief valve?

• A. A check valve prevents backflow; a relief valve regulates pressure and temperature.
• B. A check valve allows flow in one direction; a relief valve prevents excessive pressure.
• C. A check valve is used only on sludge lines; a relief valve is never used.
• D. A check valve is automatic; a relief valve is manual.

Answer: (B)

The essential idea is how each device protects the system: when flow should only go one way, a check valve acts as a one-way gate that automatically closes if flow tries to reverse, preventing backflow. A relief valve, on the other hand, automatically opens when system pressure gets too high and vents fluid to reduce pressure, protecting equipment and piping from overpressure. In wastewater applications, a check valve might stop sewage from flowing backward toward a pump or upstream pipes, while a relief valve guards against pressure buildup in tanks or lines. Temperature is not the primary control for a relief valve; its purpose is to regulate pressure.