Reduced pressure zone device connected to a fire hydrant
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,[1][2]reduced pressure zone assemblies (RPZA),[3] or reduced pressure principle assembly (RPPA).[4]
ASSE Standard 1013 - Reduced Pressure Backflow Assembly
ASSE Standard 1015 - Double Check Valve Assembly
ASSE Standard 1020 - Pressure Vacuum Breaker
ASSE Standard 1047 - Reduced Pressure Detector Assembly
ASSE Standard 1047 - Reduced Pressure Detector Assembly Type II
ASSE Standard 1048 - Double Check Detector Assembly
ASSE Standard 1048 - Double Check Detector Assembly Type II
ASSE Standard 1056 - Spill Resistant Vacuum Breaker
Backflow preventers are categorized into three groupings: Assembly, Device or Method. With the exception of elimination, these are the only ways one can control backflow from taking place.
The eight 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 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 an assembly apart from a device. An assembly is testable, but a device is not.
Lastly, an air gap or barometric loop are methods to prevent backflow from taking place. These are also non-testable.
Description
A RPZD is considered suitable for significant hazard applications,[note 1] 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.[5] 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 (also known as the zone). 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 valves (one before and one after the RPZD) are provided to allow for testing and repair.
Facilities that require constant water pressure must install two RPZDs 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 (this is why check one has a minimum passing value of relief +3. It is designed to handle the pressure fluctuations).
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 RPZD 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 is a variety of manufacturers of these gauges, but most utilize either a three-valve or a five-valve design to test the RPZD in different ways.
A valve is a device or natural object that regulates, directs or controls the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically fittings, but are usually discussed as a separate category. In an open valve, fluid flows in a direction from higher pressure to lower pressure. The word is derived from the Latin valva, the moving part of a door, in turn from volvere, to turn, roll.
A fire sprinkler system is an active fire protection method, consisting of a water supply system providing adequate pressure and flowrate to a water distribution piping system, to which fire sprinklers are connected. Although initially used only in factories and large commercial buildings, systems for homes and small buildings are now available at a cost-effective price.
A check valve, non-return valve, reflux valve, retention valve, foot valve, or one-way valve is a valve that normally allows fluid to flow through it in only one direction.
A siphon is any of a wide variety of devices that involve the flow of liquids through tubes. In a narrower sense, the word refers particularly to a tube in an inverted "U" shape, which causes a liquid to flow upward, above the surface of a reservoir, with no pump, but powered by the fall of the liquid as it flows down the tube under the pull of gravity, then discharging at a level lower than the surface of the reservoir from which it came.
A tap is a valve controlling the release of a fluid.
A backflow prevention device is used to protect potable water supplies from contamination or pollution due to backflow.
A double check valve or double check assembly (DCA) is a backflow prevention device designed to protect water supplies from contamination. It is different from the two-way check valves used in air brake systems on heavy trucks which select from the highest pressure source.
Sometimes called a check valve or a backflow preventer, a chemigation valve is an apparatus designed to protect water supplies from agricultural chemicals used during chemigation, the application of chemicals such as fertilizers and pesticides through irrigation water. Most chemigation valves consist of a spring-loaded check valve, a low pressure drain, an air and vacuum relief valve, and an injection port for introducing the chemicals downstream of the check valve. Many chemigation valves also have a 4-inch inspection port so that a person can reach inside and feel if the check valve is still functional. Some governments require the use of two chemigation valves installed in series if hazardous chemicals are to be injected.
A ballcock is a mechanism or machine for filling water tanks, such as those found in flush toilets, while avoiding overflow and backflow. The modern ballcock was invented by José Antonio de Alzate y Ramírez, a Mexican priest and scientist, who described the device in 1790 in the Gaceta de Literatura Méxicana. The ballcock device was patented in 1797 for use in steam engines by Edmund Cartwright.
Hydronics is the use of liquid water or gaseous water (steam) or a water solution as heat-transfer medium in heating and cooling systems. The name differentiates such systems from oil and refrigerant systems.
An air gap, as related to the plumbing trade, is the unobstructed vertical space between the water outlet and the flood level of a fixture. Air gaps of appropriate design are legally required by water health and safety regulations in many countries. An air gap is the simplest form of a backflow prevention device.
A hydrostatic loop, though not often used in plumbing practice, is an arrangement of pipes formed into a vertical loop to prevent backflow of water within the plumbing potable water system. Since a siphon has a maximum height that it can work, a hydrostatic loop is built higher than 33 feet. There are several ways to prevent siphonage and an undesirable backflow of the water in a plumbing system.
An Atmospheric Vacuum Breaker (AVB) is a backflow prevention device used in plumbing to prevent backflow of non-potable liquids into the drinking water system.
A vacuum breaker is an attachment commonly placed on a bibcock valve or toilet or urinal flush valve, that prevents water from being siphoned backward into the public drinking water system. This prevents contamination should the public drinking water system's pressure drop. A vacuum breaker is also used in steam distribution systems to prevent collapse of steam coils and pipes by letting in air when the pipe pressure becomes sub-atmospheric.
A pressure vacuum breaker (PVB) is a type of backflow prevention device, used to keep non-potable water from entering the water supply. A PVB is similar to an atmospheric vacuum breaker (AVB), except that the PVB contains a spring-loaded poppet. This makes it acceptable for applications that are high hazard or where valves are downstream. Pressure vacuum breakers must be protected from freezing when installed outdoors. PVBs usually have test cocks, to which specially-calibrated gauges are attached, in order to ensure that they are functioning properly.
A metering pump moves a precise volume of liquid in a specified time period providing an accurate volumetric flow rate. Delivery of fluids in precise adjustable flow rates is sometimes called metering. The term "metering pump" is based on the application or use rather than the exact kind of pump used, although a couple types of pumps are far more suitable than most other types of pumps.
A backwater valve is a backflow prevention device used to prevent outbound water through a dwelling's drain pipes from re-entering -- "back flowing"—into a home. The valve contains a flap that allows water to exit the home, but closes to prevent the back flow into the home.
Backflow is a term in plumbing for an unwanted flow of water in the reverse direction. It can be a serious health risk for the contamination of potable water supplies with foul water. In the most obvious case, a toilet flush cistern and its water supply must be isolated from the toilet bowl. For this reason, building codes mandate a series of measures and backflow prevention devices to prevent backflow.
The mechanism of diving regulators is the arrangement of components and function of gas pressure regulators used in the systems which supply breathing gases for underwater diving. Both free-flow and demand regulators use mechanical feedback of the downstream pressure to control the opening of a valve which controls gas flow from the upstream, high-pressure side, to the downstream, low-pressure side of each stage. Flow capacity must be sufficient to allow the downstream pressure to be maintained at maximum demand, and sensitivity must be appropriate to deliver maximum required flow rate with a small variation in downstream pressure, and for a large variation in supply pressure, without instability of flow. Open circuit scuba regulators must also deliver against a variable ambient pressure. They must be robust and reliable, as they are life-support equipment which must function in the relatively hostile seawater environment, and the human interface must be comfortable over periods of several hours.
References
↑ In the UK they are considered suitable for Category 4 'Significant hazard' e.g. antifreeze, but not Category 5 'Serious health risk', e.g. human waste
University of Southern California, Foundation of Cross-Connection Control and Hydraulic Research (USC-FCCCHR) Manual of Cross-Connection Control - 10th Edition.
International Association of Plumbing and Mechanical Officials (IAPMO) Backflow Prevention Reference Manual - 3rd Edition
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