Backflow preventer

Abstract

A reduced pressure principle backflow preventer includes downstream and upstream check valves separating an inlet and an outlet from an intermediate zone. A differential pressure relief valve maintains the intermediate zone at a pressure lower than the inlet pressure during potential backflow conditions and vents the zone during potential back siphon conditions. A unitary body member supports a pair of resilient annular valve members and is inserted into a vented housing bore and retained by friction to form the complete backflow preventer. Both valve members are retained on shoulders on the body member and are held between the shoulders and the bore wall. One valve member includes an axially movable portion cooperating with a downstream valve seat on the body to form the downstream check valve. The second valve member is cylindrical in form and is radially movable inward against an upstream valve seat on the body to form the upstream check valve. Radially outward movement of the second valve member closes the vent ports so that this valve member performs the relief valve function as well. An annular spring surrounding the second valve member loads the member in opposition to the force applied by inlet pressure to establish the pressure differential operation of the relief valve, and to provide a pressure drop across the upstream check valve during forward flow. The unitary body includes a group of elongated slots in a cylindrical wall defining a strainer at the inlet of the backflow preventer.