When should back flow check valves close?

Backflow check valves are designed to prevent the reverse flow of liquids or gases in a piping system, ensuring that the flow only goes in one direction. These valves are critical in maintaining system integrity and preventing contamination or hazardous situations.

The correct response indicates that backflow check valves close specifically when the flow is either stopped or reversed. This mechanism is crucial because it protects the system from potential back pressure or back siphonage, which could lead to contamination of a clean water supply or other safety hazards. When flow is interrupted or reversed due to changes in pressure or downstream conditions, the valve must function properly to seal off the line and ensure that the contents do not travel back upstream.

Understanding the operation of check valves is essential for safe installation and maintenance practices in propane and other gas systems. In contrast, the other options do not accurately reflect the operational purpose of backflow check valves. For instance, increased pressure may not be related to flow direction, temperature decreases do not directly dictate valve operation, and emptying a container poses different operational concerns not linked to the functionality of check valves.