fire sprinkler main and backflow devise.

I have a customer that needs a 3" water service for a fire sprinkler system with a backflow devise. not sure what backflow devise or type of pipe to use.

They usually use a double check with rising stem gate valves. IT must be approved for that application by the proper testing bureau. The double check is installed ABOVE the ground at the required elevation, with the proper supports and bracing. Your fire department should have descriptions of the REQUIRED installation standards. BUT, why are YOU installing it? The fire sprinkler companies usually REQUIRE that they install it themselves, if for no other reasons than liability and performance.