RPZ Backflow Prevention

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Backflow preventers work, simply, by allowing water to flow in one direction while preventing water flow backwards in the reverse, unintended direction. Thus, the water supply is protected. RPZ (Reduced Pressure Zone) devices are implemented where backflow of water into the supply would cause significant harm. You can imagine that you would not want the waste from your house or business re-entering the water supply. This is especially important for industrial facilities that work with harmful chemicals. 


An RPZ device consists of two checked valves plumbed in series, with a pressure monitored chamber between (aka “the zone”). The chamber’s pressure is maintained to be lower than the water supply pressure, but high enough to be useful in the water supply. The differential pressure relief valve maintains the reduced pressure, automatically relieving excess pressure via discharge to a drain. This relief valve actually dumps the backflowing water out, protecting the water supply. These systems have four test cocks installed, which allow plumbers or inspectors to verify appropriate operation of the valve using a differential pressure meter. Two valves on either side of the RPZD allow for testing and repair of the device. 


Cities all over the USA are beginning to require RPZ to prevent backflow. Due to their high level of protection, many domestic and fire service lines are implementing the devices as a requirement. The RPZD is suitable for hazard applications. These devices suitably prevent back pressure and back-siphonage, preventing harmful substances from entering the usable water supply. They incorporate a redundant design, meaning even when the check valves are broken, the device still protects the water supply. Having the redundant design of the RPZ also means they are testable, verifying correct operation. The RPZD is often the prime choice rather than an air gap device due to RPZD’s head loss being much less. Read about the difference of backflow prevention options here


If a facility requires constant water pressure, two RPZDs must be installed, in parallel. This allows one device to be available to provide the facility during testing or repair of the other. The zone of pressure indicates whether or not the aforementioned relief valve is working properly. If there is no water dumping out, the backflow preventer device is functioning correctly. If the relief valve does emit water, then something in the system is not quite right, which requires maintenance. This makes it easy for municipalities and engineers to check the water supply lines. 


If both valves are faulty simultaneously, it is possible for backflow to occur, if and only if the reverse flow rate exceeds the capacity of the relief valve. Due to this theoretical possibility, it is vital for the RPZ backflow device to be mounted where the drain will not become flooded.


So where could backflow be hazardous? Well, inside underground sewerage pumping stations where well washing devices are located. Untreated sewerage can contain gases that can damage concrete walls, eventually breaking them down. Well washers are utilized to spray down the contaminated concrete walls with water, effectively washing them. These well washers must be installed with RPZ devices. If a pumping station breaks, allowing sewerage levels to rise above the well washer, backflow could result within the water supply line. 


The differential gauge used to test reduced pressure zone devices attaches to the test cocks of the RPZD. The gauge then checks the check valve and relief valve springs, verifying correct operation. The typical design of these differential gauges are three-valves or five-valves, allowing RPZDs to be tested in various ways. In several locations, the RPZD valve must be tested on an annual basis, by an accredited tester. 


If you are interested in installing or maintaining your RPZ devices or backflow systems, contact us here.