Selecting A Backflow Preventer

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What Type of Backflow Preventer Should Be Used?

Backflow prevention devices and assemblies include Atmospheric Vacuum Breakers, Dual Check Valves, Pressure Vacuum Breakers, Double Check Valve Assemblies, and Reduced Pressure Principle Backflow Assemblies. The proper selection of backflow preventer is crucial to insure that the devices works properly and is providing adequate protection for the specific application. It must be stressed that these devices are not all equally acceptable as protection against all types of hazards and other factors must be taken into account with each application. In general, there are four elements to consider when designing your backflow prevention program: System Characteristics, Degree of Hazard, Type of Application, and Type of Installation.

System Characteristics

Each backflow device is designed to handle a specific group of system characteristics. System characteristics include the hydraulic requirements of the specific backflow prevention design. Factors such as the need for continuous pressure versus non-continuous pressure, backpressure versus back-siphonage, and flow-requirements versus velocity, must be specified to determine the type of device to be used.

Existing conditions of pressure loss must be taken into account prior to the installation of a backflow device. All plumbing systems experience pressure loss. Pressure loss begins with the friction of the water against the walls of the pipe and increases with each additional fitting added to the system. Many of the fittings and components in a plumbing system can be identified as fixed orifice items such as pipes. fittings, gate valves and ball valves. The pressure loss for fixed orifice items can be identified as a constant value and is characteristic of each type and size of valve. Once this value is established for a particular valve, the pressure loss through that valve or fitting can be calculated by simply knowing the specific gravity of the fluid running through the pipe and the flow rate (in gallons per minute).

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Elevation in the plumbing system is also a very large factor. Approximately 1 psi for every 28 inches of elevation can amount to almost 7 psi loss in just a 2-story building. A highrise building with an elaborate fire sprinkler system cannot tolerate losses in supply pressure and is therefore highly concerned with the effects of elevation in the supply system.

Degree of Hazard

The ultimate purpose of control and prevention of backflow is to protect the public health. Should a backflow condition occur, the "Degree of Hazard" must be understood in order to provide the proper Backflow Preventor to stop the hazard on hand. The degree of protection is determined based on the degree of hazard.A Low Hazard application is when potential backflow can pollute the drinking water. Pollution is defined as materials that can cause undesirable effects to the water, such as discoloration, smell or taste, but will not cause sickness or death. A High Hazard application is one where the potential backflow can contaminate the water supply. Contamination is defined as any impairment to the water quality such that contact with this water can result in illness or death.

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Text courtesy of Wilkins a Zurn company.