For installation with all 4A, 4An and RPS 40 Series reduced pressure principle backflow preventers. The Apollo® air gap drain (AGD) is designed to funnel minor relief valve discharges, due to line pressure fluctuations and/or minor check valve fouling, into the drainage system. Drain piping is easily attached to the drain's threaded bottom. Note: The AGD is designed to collect expected minor discharges due to fouled checks or pressure fluctuations but not the full discharge capacity of the relief valve.