Two Check Valvesin series with a zone between them are mounted between the supply pipe and discharge pipe, and a relief valve vents the zone to atmosphere under predetermined pressure conditions in the pipes. A valve poppet in the upstream Check Valvescooperates with its seat and an enclosing stationary barrel to form a first chamber. An ejector nozzle receives liquid from this first chamber when the valve poppet is open, and discharges it into the zone. The action of the ejector nozzle aspirates a second chamber in the upstream check valve to reduce the pressure drop when the valve is open. The velocity head of the discharge from the ejector nozzle is directed against a valve poppet in the downstream check valve, which also includes a pressure affected piston area. In each check valve the flow restriction across the valve seat decreases more rapidly than the downstream flow restriction. Also, the outer surface of the valve poppet in the upstream check valve defines, with its stationary barrel enclosure, a series of ribs and cavities which direct flow of fluid from the CHECK VALVESthrough the ejector nozzle while minimizing flow into the aspirated chamber. A relief valve device connected to vent the zone to atmosphere has a valve stem subjected to a pressure differential between the inlet pressure to the upstream check valve and the reduced pressure in its aspirated chamber.

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