It is best to have an ______ to prevent the collapse of a pipe when it is being emptied.

• A. Air and vaccum relief valve
• B. Vent stack
• C. Pressure reducing valve
• D. Check valve

Answer: (A)

When a pipe is emptied, water leaving the line can create a vacuum inside. If air can’t enter to fill the void, the internal pressure drops and the pipe can be damaged or collapse. An air and vacuum relief valve is designed to admit air into the line whenever negative pressure begins to develop, keeping the pressure from dropping too far and preventing collapse. It stays closed when the line is full and opens as water drains, which is exactly what’s needed during emptying. A vent stack isn’t a dedicated vacuum-relief device for a distribution main, a pressure reducing valve lowers downstream pressure but doesn’t prevent vacuum, and a check valve prevents backflow rather than venting air.