Preventing contamination of the water supply is essential to safeguarding public health. One possible situation that can lead to contamination is backflow, where a dramatic drop in pressure draws dirty water back into the system. A pressure vacuum breaker is a device designed to prevent backflow. Another similar device that performs basically the same function is called an atmospheric vacuum breaker.
In a pressure vacuum breaker, the water travels through a check valve and causes a poppet to raise up a guide into an air inlet. The air inlet also has a valve which ordinarily remains closed. Both air inlet valve and check valve are spring-loaded; when water is flowing, the spring on the check valve is compressed. The water pressure basically holds the check valve open in spite of the pressure exerted against it by the spring.
Various accidents like a break in a pipe can cause the water pressure to drop suddenly. If it does, water could easily be drawn backward through the line, creating suction that could potentially draw contaminated water back into the line somewhere further downstream. With a pressure vacuum breaker, however, a drop in the pressure of the water enables the spring to snap the check valve shut. As soon as the water pressure drops, the check valve closes and backflow does not occur.
In the meantime, the drop in pressure causes the air inlet valve to open, allowing air to enter the system. This air also helps to offset the drop in pressure and prevent backflow. As soon as normal water pressure is restored, the flow of water will force the check valve back open, while the air inlet valve will close again. The basic principles of operation are similar to atmospheric vacuum breakers, except atmospheric breakers rely on atmospheric pressure rather than a spring.
In order to function properly, the pressure vacuum breaker needs to be mounted at least 12 inches above the highest point of use. You may have seen pressure vacuum breakers without realizing it; the pipe leading into and out of a pressure vacuum breaker forms three sides of a square with a hood or cap on one corner. Breakers are not foolproof and can fail, especially if dirt and other gunk accumulates in the valves. Unlike atmospheric vacuum breakers, however, they can be tested to ensure they are operating properly.