Patent ID: 8037899

Claim:
A pressure-reducing backflow preventer comprising: a first check valve; a second check valve; an intermediate chamber provided between said first check valve and said second check valve; a relief valve provided in said intermediate chamber; said first check valve comprising: a first casing having a first flow path formed therein and a first valve seat formed at an intermediate portion of said first flow path; a first valving element disposed in said first flow path, said first valving element movably positioned, at least partially in said first flow path, to selectively seat on and unseat from said first valve seat at one end thereof; a first urging member that urges said first valving element toward said first valve seat; a back-pressure chamber that movably receives at one end thereof at least a portion of said first valving element; a narrow flow path portion downstream of a position at which said first valving element abuts against said first valve seat, an upstream end of said narrow flow path portion being brought into and out of communication with an upstream side of said first flow path in response to said first valving element unseating from and seating on said first valve seat, a downstream end of said narrow flow path portion being constantly in communication with a downstream side of said first flow path; and a communicating passage providing fluidic communication between said narrow flow path portion and said back-pressure chamber said relief valve comprising: a discharge port formed in a casing of said intermediate chamber, said discharge port having a relief valve seat; a relief valving element movable to selectively seat on and unseat from said relief valve seat; a relief valve urging member that urges said relief valving element away from said relief valve seat; and a differential pressure applying mechanism that applies a differential pressure, determined by a pressure differential between a fluid pressure at an upstream side of said first check valve and a fluid pressure in said narrow flow path portion, to said relief valving element in a direction against said relief valve urging member, wherein said differential pressure applying mechanism comprises: a pressure chamber comprising a high-pressure chamber and a low-pressure chamber separated by a pressure-receiving movable member; and a coupling member that connects said pressure-receiving movable member and said relief valving element which move in interlocking relation to each other, wherein said pressure chamber is part of said intermediate chamber, and; said pressure-reducing backflow preventer further comprising: a high-pressure side communicating conduit providing fluidic communication between said high-pressure chamber and the upstream side of said first check valve; and a low-pressure side communicating conduit providing fluidic communication between said low-pressure chamber and said narrow flow path portion.