Last week’s blog focused on Double Check assemblies. Today, my series will address how vacuum breaker backflow preventers work-what they do, and where they’re likely to be installed. I’ll be looking at both Pressure Vacuum Breakers (PVB) and Atmospheric Vacuum Breakers (AVB). These two types of cross-connection devices are only used in irrigation, but never in hazardous places, such as chemigation.
Atmospheric devices rely on -you guessed it- the difference of atmospheric pressure. When water backsiphons, from the water pressure going out being higher than the water flow, vacuum breakers have a small vent that is pushed out to allow air to essentially divide the two water souces, stopping any siphonage. AVB’s are vented, but have no check valve so can’t actually be tested, and are only useful for siphon situations. Unlike a Double-Check backflow preventer, with it’s tell-tale name, the PVB has a single check valve. Rather than the check valve responding to backflow of water though, a PVB’s vent opens for air whenever the pressure in the device approaches the atmospheric pressure, breaking any siphoning–and is only useful in back-siphon situations.
Both vacuum breakers–pressure and atmospheric–are useful only where back-siphonage could occur : not backflow. Because of this, AVB’s and PVB’s are installed a minimum of six inches above the highest outlet of water. Your area may vary. Sometimes regulations require them to be placed 12 inches or higher above ground. In sprinkler systems you often see them because they are above ground–the difference of where they are installed is that AVB’s are directly before the valve to turn on the water. Because of this, if your irrigation line have several valves, you would also need several Atmospheric Vacuum Breakers, which can be less economical the more valves you have. Vacuum breakers are intrinsically useless if they are installed underground, or any place where they could be submerged under water, simply because they rely on atmospheric pressure and contact with air to function correctly.
Why would pressure breakers be used, when a double check valve could be used underground, and can protect against both backsiphon and backflow of water?
Good question. For one reason, it’s incredibly easy to access if the cross-connection device is above ground. Another reason is they are simple, which means if something goes awry, it’s easier to fix or replace. In the end, aside from a natural air-gap, they are the cheapest option for back-siphon.
How do an atmospheric vacuum breaker and a pressure vacuum breaker differ from one another?
Basically, a PVB has an overflow valve–which means it can be under constant pressure, unlike an AVB. It may spit out water if back-siphon begins to occur as it opens it’s vent. Although handy and cheap, Atmospheric Vacuum assemblies don’t have any way to be tested, which is one of the reason the PVB was developed. PVB’s, because of their check valve and ability to be under constant pressure, are allowed to be downstream of a valve (although never below it) where as an AVB must always be upstream of it since it has one single vent.