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Backflow Prevention Devices

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1    An air gap is a vertical, physical separation between the end of a water supply outlet and the flood-level rim of a receiving vessel. This separation must be at least twice the diameter of the water supply outlet and never less than 1 inch. 

An air gap is effective against back-pressure  backflow and back-siphonage and may be used to isolate health or non-health hazards.
 2   Reduced-pressure Principle Backflow Assembly (RPBA) includes two independently acting, spring-loaded check valves with a hydraulically operating, mechanically independent, spring-loaded pressure differential relief valve in between. Testing is required for this type of BPA. An RPBA is effective against back-pressure  backflow and back-siphonage and may be used to isolate health or non-health hazards.
 3   Pressure Vacuum Breaker (PVB) consists of an independently acting, spring-loaded check valve and an independently acting, spring-loaded air inlet valve. This assembly requires testing. A PVB may be used to isolate health or non-health hazards but is effective against back-siphonage only.
 4   Double Check Valve Assembly (DCVA) consists of two independently acting, spring-loaded check valves. This type of assembly requires testing. A DCVA is effective against back-pressure  backflow and back-siphonage but should be used only to isolate non-health hazards.