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Air Gap (AG)
An approved air gap is the unobstructed vertical distance through air between the lowest point of a water supply outlet and the flood level rim of the fixture or vessel into which the fluid discharges. This vertical, physical separation must be at least twice (2x) the diameter of the water supply outlet, never less than 25 mm (1 in).
Note: In theory, a well-designed and properly maintained air gap is the best means available for protection against backflow. However, an air gap is not always practical and is vulnerable to bypass arrangements that can nullify its effectiveness. In extremely hazardous installations, an air gap separation is recommended. Extending the outlet pipe to avoid splashing nullifies the effectiveness of the air gap.
Reduced Pressure Principle Assembly (RP)
The RP Backflow Prevention Device consists of two independently acting, internally loaded, approved check valves separated by a reduced pressure zone having an approved pressure differential relief valve. This device is installed as a unit between two tightly closing shut off valves and each device is fitted with test cocks for period testing.
Typical RP Backflow-Prevention Device
Double Check Valve Assembly (DCVA)
The DCVA Backflow Prevention Device consists of two internally loaded check valves, either force-loaded or internally weighted, as shown in Figure 4. With the two check valves in series, the device prevents backflow even if one check valve fails to close tight, but no visible warning is given of this failure. Test cocks are provided for periodic testing.
Typical DCVA Backflow-Prevention Device
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