This is from one of the local municipalities, describing their requirement for double check valves.
http://www.cityofkeller.com/CD/PDF/S...ionHandout.pdf
This is a description from Watts
http://www.watts.com/pdf/1915345.pdf
This is from the Texas State administrative code
See method number 3
SUBCHAPTER DSTANDARDS FOR LANDSCAPE IRRIGATIONRULE §344.73Backflow Prevention Methods
All irrigation systems connected to a public or private potable water supply must be properly connected through one of the following backflow prevention methods:
(1) Atmospheric vacuum breakers. Atmospheric vacuum breakers are designed to prevent only back siphonage. Therefore, atmospheric vacuum breakers must not be used in any irrigation systems where back-pressure may occur. There cannot be any shutoff valves downstream from an atmospheric vacuum breaker. Where atmospheric vacuum breakers may be used, they must be installed at least six inches above any downstream piping and the highest downstream opening. Where local topography effectively prohibits such installation, the executive director shall be consulted for alternative acceptable installation criteria. Such alternative criteria must provide equivalent protection to the potable water supply. In addition, continuous pressure on the supply side of an atmospheric vacuum breaker is prohibited. Where atmospheric vacuum breakers are used in an irrigation system, a separate atmospheric vacuum breaker must be installed on the discharge side of each water control valve, between the valve and all of the sprinkler heads which the valve controls.
(2) Pressure-type vacuum breakers. Pressure-type vacuum breakers are designed to prevent back siphonage and can operate under continuous pressure. Pressure vacuum breakers must be installed at least 12 inches above any downstream piping and the highest downstream opening. Where local topography effectively prohibits such installation, the executive director shall be consulted for alternative acceptable installation criteria. Such alternative criteria must provide equivalent protection to the potable water supply.
(3) Double check valve assembly backflow preventors. Double check valve assembly backflow preventors are designed to prevent back pressure and back siphonage of water not containing any toxic substance. They may be used where water supply pressure and back pressure on the backflow prevention device may continuously exist. If a double check valve assembly is installed below grade, there must remain adequate space for testing and repair of the device. Test cock plugs must be of non-ferrous material. Test cocks shall not be used as supply connections and must be plugged except when being tested.
(4) Reduced pressure principle backflow prevention assemblies. Reduced pressure principle assemblies are designed for water containing toxic or non-toxic substances and for back pressure and back siphonage. They must be installed 12 inches above grade in a location so as to insure that the device will not be submerged. In addition, adequate provisions must be made for any water which may be discharged through the assembly relief valve.
(5) Air Gap. An air gap, when used must be installed and maintained in accordance with the standards established in the American Waterworks Association M14 Manual on Cross Connection Control.
