There are five main types of backflow prevention devices, depending on the degree of hazard in the specific locations.
1) Air gaps are the physical separation of the potable water and non-portable system by an air space.
2) Atmospheric type vacuum breakers are used on connections to non-potable systems where there is no back pressure, no continuos pressure and is on the discharge side of the last control valve.
3) Pressure type vacuum breakers are used on connections to non-potable systems under continuous pressure that are not subject to backpressure. They must be installed a minimum of 12” above the point of use.
4) Double check valve assemblies are used on direct connections to systems where high concentrations of foreign materials could enter the potable water system, such as lawn sprinkler systems and swimming pools.
5) Reduced pressure zone valves are used in situations where the connection of the potable water system is connected to a source of material that can cause a potential health hazard. Some examples are in chemical factories or doctors offices where x-ray development is being performed.
An example for why backflow preventers are needed:
May 1973 Insecticide Chlordane
This one happened in New Jersey and was reported in the New Jersey AWWA publication, PIPELINE.
A breakdown in a 24” water distribution main on May 16, 1973, triggered an event which endangered residents of the Morganville and Wickatunk section of Marlboro Township Municipal Utilities Authority.
An exterminating contractor created an illegal cross connection by diluting a quantity of the highly toxic insecticide, CHLORDANE, by means of a submerged garden house into a drum of this material. The water main break, which occurred during this operation, caused a negative pressure in the distribution system resulting in the siphonage of the entire contents of the drum, through the house service connection and into the distribution system.
Although extensive flushing of the main was affected on the orders of the Bureau of Potable Water, the velocity provided by the standard hydrant ports was insufficient to properly flush out the chlordane from the 20” main. The required velocity was achieved by attaching a fire pumper to a 4” port on the hydrant. Subsequently, additional samples taken by Mr. Wiseman of the Bureau of Potable Water demonstrated the success of the procedure.
This incident exemplifies the danger of illegal cross connections, the need for public awareness of their dangers, and indicates the need for the provision of check valves or vacuum breakers on all outside hose bibs.
Some Common Questions:
1) What is back-siphonage?
Back-siphonage is the reversal of the normal flow in a system caused by a negative pressure in supply piping. Water tends to flow in the direction of least resistance or towards lower pressure.
2) What factors can cause back-siphonage? Back-siphonage can be created when there is a stoppage of the water supply due to nearby fire-fighting, repairs or breaks in the city main.
3) What is a cross connection?
Across connection is a direct arrangement of a piping line, which allows the potable water supply to be connected to a line that contains a contaminant. The most common cross connection is a garden hose in a container of hazardous materials, such as a cesspool or bucket of house cleaners.
4) Are my hose connections safe?
Caution should be taken that hose ends are left in swimming pools or with chemical feeders on them. A good safety feature is hose bib vacuum breakers installed on the hose bib connection.
5) Can backflow preventers produce any effects on the rest of my plumbing system?
Yes. Thermal expansion caused by water heating in a potable hot water system may cause a relief valve to release water. A potable water expansion tank will need to be added to the system to stop the release.
For more info and examples see www.wattsreg.com
