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Old 11-21-2012, 08:37 PM
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cjohn2000 cjohn2000 is offline
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Join Date: Jun 2010
Location: Tacoma, WA
Posts: 492
I don't know what area your from, but over here the most common bp is the doublecheck, 2 spring loaded check valves, one after the other, usually mounted in a box below grade.

In areas where it doesn't freeze they use PVBs, pressure vacuum breakers.
PVB has single check valve protecting the incoming water. Any water trying to "Flow back" goes out the top of the unit, since the top is vented. Another form is the little screw on deals that can be added to outdoor faucets.

Atmospheric vacuum breaker is like the PVB except the check isn't spring loaded and the unit is not testable. Usually they are molded with the solenoid control valve. In our area ( as far as I know) Atmospheric vacuum breakers can be used but one for every solenoid control valve and they need to be mounted 6-12" above the highest head.

Last is the Reduced pressure backflow preventer like a double check except their is a vent that opens between the two checks should the second check fail.

All my own understanding of the different devices.
 
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