COMPREHENSIVE BACKFLOW PROGRAM COMPONENTS

Types of Backflow Preventers

There are 6 distinct types of mechanical assemblies considered to be backflow prevention assemblies.  They are not all equally acceptable as protection against all types of hazards.  To determine the type of backflow prevention assembly most applicable to the situation, the degree of hazard must be assessed along with the type of cross- connection present.  An overview of the five types of backflow prevention assemblies as described by the Foundation for Cross-Connection Control, and Hydraulic Research, University of Southern California, Article “An Introduction to Cross-Connection Control is provided as follows:

 1. Air Gap

“An Air Gap is a physical separation of the supply pipe by at least two pipe diameters (never less than one inch) vertically above the overflow rim of the receiving vessel.  In this case line pressure is lost resulting in the need for a booster pump downstream, unless the flow of the water by gravity is sufficient for the after use.  With an air gap there is no direct connections between the supply main and the equipment.  An air gap may be used to protect against a contaminant or a pollutant and will protect against both back-siphonage and backpressure.  An air gap is the only acceptable means of protecting against lethal hazards.”

 2. Atmospheric (Non-Pressure) Type of Vacuum Breaker (AVB)

“The AVB is always placed downstream from all shutoff valves.  Its air inlet valve closes when the water flows in the normal direction.  But, as water ceases to flow the air inlet valve opens, thus interrupting the possible back-siphonage effect.  If piping or a hose is attached to this assembly and run to a point of higher elevation, the backpressure will keep the air inlet valve closed because of the pressure created by the elevation of water.  Hence, it would not provide the intended protection.  Therefore, this type of assemble must always be installed at least six inches above all downstream piping and outlets.

 Additionally, this assembly may not have shut-off valves or obstructions downstream.  A shut-of valve would keep the assembly under pressure and allow the air inlet valve (or float check) to seal against the air inlet port, thus causing the assembly to act as an elbow, not a backflow preventer.   The AVB may not b under continuous pressure for this same reason.  An AVB must not be used for more than twelve out of any twenty-four hour period.  It may be used to protect against either a pollutant or a contaminant, but may be only be used to protect against a back-siphonage condition.”

 3. Pressure Vacuum Breaker (PVB)

“The PVB includes a check valve which is designed to close with the aid of a spring when flow stops.  It also has an air inlet valve, which is designed to open when the internal pressure is one psi above atmospheric pressure so that no non-potable liquid may be siphoned back into the potable water system.  Being spring-loaded it does not rely upon gravity, as does the atmospheric vacuum breaker.  This assembly includes resilient seated shut=off valves and test-cocks.  The PVB must be installed twelve inches above all downstream piping and outlets.  The PVB may be used to protect against a pollutant or contaminant; however, it may only be used to protect against back-ssiphonage.  It is not acceptable protection against back-pressure.”

4. Double check Valve Assembly (DCVA)

“The Double Check Valve Assembly consists of (2) internally loaded, independently operating check valves together with tightly closing resilient seated shut-off valves upstream of the check valves.  Additionally, there are resilient seated test-cocks for testing of the assembly.  The DC may be used to protect against a pollutant only.  However, this assembly is suitable for protection against wither back-siphonage or back-pressure.”

5. Reduced Pressure Zone Assembly (RPZA)

“This assembly consist of (2) internally loaded independently operating check valves and a mechanically independent, hydraulically dependent relief valve located between the check valves.  This relief valve is designed to maintain a zone of reduced pressure between the two check valves at all times.  The RP also contains tightly closing, resilient seated shut-off valves upstream and downstream of the check valves along with resilient seated test-cocks.  This assembly is used for the protection of the potable water supply from either pollutants or contaminants and may be used to protect against either back-siphonage or back-pressure.”

 6. Double Check Detector Assembly (DCDA)

“The DCDA is composed of a line-sized double check valve assembly with a specific bypass meter and meter-sized double check valve assembly.  The meter registers accurately for very low flow rates to detect any unauthorized use of water.  This assembly is used when the protection of a double check valve assembly is required, yet where the added requirement of detecting any leakage or unauthorized use of water exists.  Normally their assemblies are reserved for use on fire sprinkler lines.”

 Backflow Testing Assembly Information Table.

Equipment Used For Conducting Assembly Testing

The two types of equipment used for conducting assembly testing are described as follows:

Manual- is attached to the assembly by a technician and the testing series is conducted.  The accuracy rating of the units are within a 2/100ths rating (well within acceptable standards).  A manual unit is preferred by testers due to the cost, and to avoid the possibility of the unit getting wet and effecting its accuracy.  Manual models are all equal in testing accuracy.

 Electronic - is attached manually by a technician and the testing series and results are preformed and calculated by a computer.  These units are within a 1/100^th to 2/100ths accuracy rating (well within acceptable standards).

Both the manual and electronic equipment requires calibration.  The equipment must be repaired, re-certified, and re-calibrated annually by a private company.  Then, the certifications are re-submitted to the appropriate city and/or county Backflow Prevention Department.