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Discussion in 'Irrigation' started by zliminator, Jun 19, 2006.
can anyone tell me why anti-siphon valves have to be mounted above ground?
Because any device with an opening to the atmosphere is supposed to be mounted with a foot or more of clearance between it and the ground. Besides, any device with a vacuum breaker is working on gravity, and must be located at an elevation higher than any pipe or sprinkler head it feeds.
Also, it prevents the siphonage of water from the irrigation system back into the potable water supply. By being above the highest outlet on the line, when the vacuum is broken water cannot travel past a point above itself by gravity.
Think of a soda straw. Works good for moving liquids from one place to another. If you were to cut holes in the sides of the straw equal to the area of the straw it would not work. Same with the AVB. It allows air in and breaks the vacuum.
And if you punch a small hole in a soda can near the opening the suction is lost as the user dribbles all over himself.
From what I've read, one of the reasons Anti-siphon valves, PVB, and RPZs have to be installed above ground is to insure they don't become flooded. Should they ever become submerged, they lose thier ability to prevent backflow.
An RPZ is sometimes found in an underground vault, and it will function even if submerged. Still, one would worry whether the combination of a totally-fouled RPZ and a flooded valve pit, and loss of supply pressure could lead to contamination. These uncommon exceptions to the accepted practice of above-grade RPZs would never be located in a flood plain.