When testing a double check valve assembly, which shutoff valve should be closed first?

When testing a double check valve assembly, the procedure typically dictates that the second shutoff valve should be closed first. This is significant because closing the second valve allows for the isolation of the assembly in a way that preserves the integrity of the test being conducted. It ensures that any potential backflow condition can be effectively assessed without interference from upstream or downstream flow.

By closing the second valve first, the tester can also confirm the operational integrity of the check valves within the assembly, as well as ascertain whether there are any leaks or failing components. Proper testing requires that the assembly be isolated appropriately to provide accurate readings and reliable results.

Closing the first shutoff valve before the second could inadvertently allow for residual flow which may not reveal issues within the assembly effectively. Furthermore, closing the third shutoff valve may not serve the purpose of thoroughly testing the backflow prevention capability of the device housed in the assembly.

Hence, closing the second shutoff valve is the correct approach within the context of backflow testing procedures, facilitating an effective evaluation of the double check valve's performance.