2-1/2" Backflow Preventer, APOLLO 4ALF20902 Reduced Pressure Principal, Lead-Free with Flanged NRS Gate Valves2 1 2" Backflow Preventer, Reduced Pressure Principal, Lead Free , Stainless Steel Body with Flanged Non Rising Stem (NRS) Gate Valves APOLLO 4ALF20902 DESCRIPTION The Apollo Model RPLF 4A Reduced Pressure Principle Backflow Preventers consist of two independently acting, TriForce center stem guided check valves with a differential pressure relief valve located between the check valves. The unit is designed to give maximum protection against backflow