What is the function of an air/vacuum relief valve?

• A. Purges gas from the system
• B. Permits entrance of air to prevent a vacuum and allows air to escape when filling
• C. Acts as a safety relief valve for overpressure
• D. Diverts flow to relief line

Answer: (B)

The main idea is that this valve manages air in the pipeline to prevent problems caused by air pockets and vacuum. As the system fills, the valve opens to vent trapped air so water can push through smoothly. If pressure in the line drops or a vacuum could form, the valve allows air to enter, preventing collapse or slow filling. Once water reaches it, the valve closes to stop continuous air leakage, keeping the system efficient.

That makes it the best choice because it describes both venting air during filling and admitting air to prevent a vacuum, which is exactly what an air/vacuum relief valve is designed to do. Other options describe functions like protecting against overpressure or flushing gas, which are roles of different valve types and don’t capture the specific air-management purpose of this valve.