Importance

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Abstract

A liquid supply system in which liquid is only supposed to flow from a high-pressure upstream portion to a low-pressure downstream portion is provided with a backflow preventer and upstream thereof with an isolating valve assembly. The backflow preventer opens completely whenever the pressure upstream of itself drops below a predetermined minimum, thereby draining the conduit downstream of itself to prevent reverse flow in the line. The isolating valve assembly continuously monitors the pressure in the upstream and downstream conduit portions and shuts a cutoff valve between these portions whenever the pressure differential between the two portions falls below a predetermined minimum, while simultaneously depressurizing or draining the downstream portion. The operation of the isolating valve assembly therefore ensures that in case of a sudden pressure surge downstream of the backflow preventer the cutoff valve will close until the downstream portion has been depressurized sufficiently to actuate the backflow preventer. The isloating valve assembly has a pair of pistons that operate the cutoff valve either directly or through a solenoid, and which are isolated from each other in separate cylinders to eliminate mixing of the liquids in the upstream and downstream conduit portions. A pilot valve may be connected to the cylinder of one of these pistons for applying to this one cylinder the relatively high pressure in the upstream conduit portion when the valve is supposed to operate, and the other cylinder is normally continuously pressurized with this upstream pressure.

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First Claim