Which device is used to prevent back siphonage only?

• A. Gate valves
• B. Drain valves
• C. Angle valves
• D. Vacuum Breakers

Answer: (D)

Back siphonage happens when system pressure drops and water is siphoned back into the potable supply. An atmospheric vacuum breaker does exactly what’s needed: it vents to the atmosphere whenever pressure falls, admitting air and breaking the siphon so contaminated water cannot be drawn back into the supply. Because it relies on air entry rather than blocking backflow with a valve, it cannot resist backpressure, so it protects against siphonage only, not against backflow caused by elevated pressure in the irrigation system. The other options are simply shut‑off or drain valves and do not provide siphonage protection.