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 backflow prevention devices, or reduced pressure zone assemblies.
An RPZD is considered suitable for high hazard applications, that is, where the consequence of backflow into the water supply would cause significant harm. They are considered suitable because they prevent both backflow 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.
RPZD are often chosen instead of an air gap since the head loss across the RPZ 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 RPZ) are provided to allow for testing and repair.
Facilities which require constant water pressure must install two RPZs in parallel so that one device will be available to supply the facility while the other is tested or repaired.
Discharge from the relief device is an indication that either of the two check valves is 'passing' (leaking past their shutoff seats) or the relief valve itself is faulty. Some normal water use conditions may cause short episodes of discharge.
In the case of the upstream check valve passing, the differential pressure (higher supply pressure compared with the lower chamber pressure) causes any flow to occur only in one direction. A buildup of pressure in the chamber in such a case would be relieved to the drain.
In the case of the downstream check valve passing, the differential pressure relief valve prevents the possibility of the chamber pressure from exceeding the supply pressure.
It is theoretically possible that, when both valves are faulty, backflow could occur, should the reverse flow rate exceed the capacity of the relief valve.
Because certain combinations of check valve failure and/or system backpressure cause the relief valve to discharge, the device must be mounted in a location where the drain will not become flooded.
An example of where backflow would harm the water supply is the use of well washing devices inside underground sewerage pumping stations. At times untreated sewerage may contain a variety of harmful gases that will effectively break down and deteriorate concrete wells, hence well washers are utilised to spray water and wash down contaminated concrete walls of a well. All well washers are installed with RPZ Devices in case a pumping station breaks down, and the sewerage level rises above the well washer causing backflow down the water supply line.
A reduced pressure zone device is tested with a device called a differential gauge. This gauge uses hoses attached to the various test cocks of the RPZ and checks to be certain that the check valve springs as well as the relief valve spring are not fouled and are working properly. There are a variety of manufacturers of these gauges, but most utilize either a three valve or a five valve design to test the RPZ in different ways.