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An atmospheric vacuum breaker (AVB), also called an atmospheric-type vacuum breaker, prevents contaminated water from flowing back through a water system and contaminating clean water. Valves in the atmospheric vacuum breaker open and close in response to pressure in the water line, to ensure that water flows only in the correct direction. Some applications of AVBs include sprinkler and irrigation systems, commercial dishwashers, and water systems with hose outlets.
Atmospheric vacuum breakers include an inlet, an outlet, and a vent with a valve seal. The inlet and outlet are perpendicular to one another, while the vent and valve seal are located about the inlet, so that it can be sealed off when necessary. AVBs are installed in line with the water supply line, near the inlet control for the system. If the system has multiple inlet controls, an AVB must be installed at each one.
Under normal operating conditions, water flows into the inlet and then out the outlet of the atmospheric vacuum breaker. The valve seal is open to allow flow through the system. This is because the system has positive pressure, that is the pressure is forcing the water in the normal flow direction for the system.
When negative pressure — pressure that forces the water in the wrong direction — starts to build up in the system, a condition called backsiphonage occurs. This causes the flow in the water system to be reversed. If there is no valve to prevent the backward flow, used, contaminated water flows back through the system and pollutes the clean water supply. When an atmospheric vacuum breaker is installed, however, the negative pressure in the system comes through the AVB vent and forces the valve seal closed, obstructing the flow of the contaminated water. The vent also allows air to come into the system and restore the correct pressure.
AVBs are simple, inexpensive types of backflow-preventing devices, but they also have some limitations. An atmospheric vacuum breaker must be installed vertically, and at least six inches higher than the outlet of the water system in order to function properly. It also cannot be subjected to more than 12 hours of continual pressure or the valve seal may become stuck. Not only does this mean that the system should not run for more than 12 hours at a time, it also means that no valves can be installed downstream of an AVB, as they put pressure on it, even when the system is not running.