Reduced pressure zone device
Encyclopedia
A Reduced Pressure Zone Device (RPZD, RPZ or RPZ valve) is a type of backflow prevention device
Backflow prevention device
A backflow prevention assembly is used to protect water supplies from contamination or pollution.In water supply systems, water is normally maintained at a significant pressure to enable water to flow from the tap, shower etc...

 used to protect water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 supplies from contamination. RPZDs may also be known as reduced pressure principle backflow prevention devices, or reduced pressure zone assemblies.

Description

A RPZD is considered suitable for significant hazard applications,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 that is, where the consequence of backflow into the water supply
Water supply
Water supply is the provision of water by public utilities, commercial organisations, community endeavours or by individuals, usually via a system of pumps and pipes...

 would cause significant harm, although not for the highest risks, such as human waste. 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 valve
Check valve
A check valve, clack valve, non-return valve or one-way valve is a mechanical device, a valve, which normally allows fluid to flow through it in only one direction....

s, 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
Relief valve
The relief valve is a type of valve used to control or limit the pressure in a system or vessel which can build up by a process upset, instrument or equipment failure, or fire....

 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.

In the UK
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

, New York
New York
New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...

, Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...

, as well as St. Louis city and county, the RPZ valve must be tested at least every 12 months by an accredited tester.

In the UK all RPZ Valve Testers have to be WRAS Accredited

See also

  • backflow prevention device
    Backflow prevention device
    A backflow prevention assembly is used to protect water supplies from contamination or pollution.In water supply systems, water is normally maintained at a significant pressure to enable water to flow from the tap, shower etc...

  • Air gap
  • Double check valve
    Double check valve
    A double check valve or double check assembly is a backflow prevention device designed to protect water supplies from contamination. It is also a valve used in air brake systems on heavy trucks.Usage in Water Supply:...

  • Atmospheric vacuum breaker
    Atmospheric vacuum breaker
    An Atmospheric Vacuum Breaker is a backflow prevention device used in plumbing to prevent backflow of non-potable liquids into the drinking water system. It is usually constructed of brass and resembles a 90-degree elbow with a hood on its top to allow air to enter the water system if a siphon...


External links

  • http://www.watts.com/pdf/ES-009.pdf
  • http://www.zurn.com/operations/wilkins/pdfs/specsheets/BF-375A.pdf
  • http://www.iapmo.org/iapmo/news/IAPMO_pubs_available.pdf
  • http://www.wrasaccredited.co.uk/WRAS%201.pdf - UK Guidance on RPZ Valve Installation
  • http://www.honeywellukwater.com/Products/Backflow-Preventers/BA298F/
  • http://www.honeywellukwater.com/Products/Backflow-Preventers/BA295/
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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