Reduced pressure zone device

A reduced pressure zone device (RPZD, RPZ, or RPZ valve) is a type of backflow prevention device used to protect water supplies from contamination. RPZDs may also be known as reduced pressure principle (RP), reduced pressure principle backflow prevention devices,reduced pressure zone assemblies (RPZA), or reduced pressure principle assembly (RPPA).

Backflow preventers are categorized into three groupings, an Assembly, a Device or a Method. With the exception of elimination, these are the only ways one can control backflow from taking place.

The eight (8) named backflow assemblies ALL have two resilient seated isolation/shut off valves with properly located test ports. These assemblies have the distinct advantage of being in-line serviceable and can be tested & repaired without having to remove an installed assembly. Special accredited courses are given to test & repair backflow assemblies and only certified testers may test backflow assemblies.

There are approximately fifteen (15) devices related to backflow which cannot be tested as they do NOT have isolation valves or test ports and there are no standards (Test Procedures) set in place to test for any device. This is what sets apart an assembly from a device, an assembly is testable, a device is not.

Lastly, an air gap or barometric loop are methods to prevent backflow from taking place. These are also non-testable.

A RPZD is considered suitable for significant hazard applications, that is, where the consequence of backflow into the water supply would cause significant harm, although not for the highest risks, such as human waste. They are considered suitable because they prevent both back pressure and back-siphonage, because of a redundant design (even with two check valves broken the device still provides protection), and because they are testable to verify correct operation.

RPZDs are often chosen instead of an air gap since the head loss across the RPZD is much smaller.

The device consists of two independent check valves, plumbed in series, with a pressure monitored chamber between. The chamber is maintained at a pressure that is lower than the water supply pressure, but high enough to be useful downstream. The reduced pressure is guaranteed by a differential pressure relief valve, which automatically relieves excess pressure in the chamber by discharging to a drain. Four test cocks are installed to allow an inspector to verify proper operation of the valve with a differential pressure meter. Two gate valves (one before and one after the RPZD) are provided to allow for testing and repair.

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