The purpose of the backflow preventer is to keep the water in your irrigation system isolated from your potable house water. There are a number of valves designed to keep water from moving backward toward the source, single check valves, double check valves, atmospheric antisyphon valves etc. However, with regards to an irrigation system, where pesticides, fertilizers, and other chemicals may be used, it is essential that a proper vacuum breaker is used. Consult with your supplier to ascertain which is appropriate for your situation. In most cases, a ¾” pressure vacuum breaker (PVB) is sufficient.
This is installed above ground using copper and/or brass pipe and fittings. If attached to galvanized pipe (not recommended) install a dielectric union at the connection. Galvanized pipe will, over time, corrode inside, the corrosion can flake off, and stick in the valve not allowing it to close properly. This does not occur with either copper or brass, thus keeping your water bill lower and giving you significantly fewer maintenance calls to clean, flush, and replace valve diaphragms.