Relief valve assembly for use with backflow preventers

Claim 1

A backflow preventer apparatus having an inlet adapted to be connected to a supply pipe and an outlet adapted to be connected to a service pipe, said backflow preventer apparatus comprising: a body member having a flow passage therethrough from the inlet to the outlet; a first upstream check valve and a second downstream check valve in series in said passage and defining a zone therebetween, said first check valve and said second check valve being operable to open position to permit flow of liquid from said supply pipe to said service pipe upon demand for liquid in said service pipe and closed to oppose reverse flow of liquid from said service pipe to said supply pipe; and a relief valve assembly operable in response to changes in differential pressure between line pressure of said supply pipe and pressure in said zone, said relief valve assembly being mounted beneath said body member and including a housing having a passage therein with an opening in communication with said zone, said opening in the passage of the housing being totally below both said first check valve and said second check valve, said opening of the passage of the housing defining an inlet for discharge of liquid from said zone to the passage and an outlet for discharge of air from said passage to said zone, said passage having an outlet for liquid opening downwardly to atmosphere, valve means positioned in said passage above said liquid outlet, said valve means including a valve seat lying in a substantially vertical plane and a valve element cooperating with said seat, said valve element operating on a substantially horizontal axis, said passage downstream of said valve seat having an enlarged chamber surrounding said valve element and communicating with said liquid outlet, at least one air port in said housing above said liquid outlet and communicating with said chamber above a lower portion of said valve means, said at least one air port opening downwardly to atmosphere and permitting air to be drawn into an upper portion of said chamber and discharged through said passage and the opening of the same into said zone when said valve element is open and while liquid is flowing from the zone through said opening of said passage of the housing but not filling said passage and being discharged through said liquid outlet, rolling diaphragm means in said housing, means providing communication between the passage in said body member upstream of said first check valve and one side of said diaphragm means, means providing communication between the zone and the other side of said diaphragm means, means connecting said valve element to said diaphragm means for movement thereby, spring means normally urging said valve element toward open postion, said diaphragm means normally urging said valve element towards closed position when there is a predetermined pressure across said first check valve and toward open position in conjunction with said spring means when there is a pressure drop below said predetermined closing differential pressure across said first check valve.