Air Gap Drain
An air gap drain prevents backflow by separating the supply line from the receiving line; in this system, the two pipes don't meet at all. The air gap between the pipes breaks pressure between the two. The gap between the two lines needs to be wide enough to prevent possible contamination. Industry standards state that the gap should never be less than an inch and should measure at least twice the diameter of the supply line.
The double-check valve device comprises two separate valves attached to pipes via hinges held open by water pressure. During normal flow, the valves remain open. When pressure drops or ceases, the hinges close to prevent backflow. Small particles of dirt or sand can become lodged in the hinges, causing them to stay open, and may allow a small amount of backflow to enter the system.
Reduced Pressure Principle Assembly
The purpose of a reduced pressure principle assembly is to keep a pressure gradient between the supply and receiving ends of a line, preventing backflow. A pressure relief valve placed between two check valves creates an area between the two check valves that always has a lower pressure. The two check valves are open during normal operation, but when pressure changes, the pressure relief valve introduces air into the system, breaking the vacuum.
Pressure Vacuum Breaker
During a backflow event, the pressure vacuum breaker engages to break the vacuum. A spring-loaded valve closes when water pressure drops or ceases, and an air relief valve opens to introduce air and break the vacuum to prevent backflow. Pressure vacuum breakers prevent backflow from entering the main line, but under constant pressure, the air inlet valve may become stuck closed, posing a potential risk.
Atmospheric Vacuum Breaker
The atmospheric vacuum breaker device has a moving element inside that seals the device and prevents water from spilling during normal pressure conditions. When water pressure drops or ceases, the device drops, providing an opening for spillage of contaminated backflow. The main line remains protected because contaminants spill out rather than entering the system.