Backflow Prevention and Cross-Connection Control
Definitions drive device selection
A cross-connection is any actual or potential link between potable and nonpotable water. Backsiphonage is caused by negative pressure in the supply; backpressure is caused by downstream pressure exceeding supply pressure. The hazard is classed as a health hazard (contamination) or a non-health hazard (pollution) - that classification, plus whether the condition is continuous or non-continuous, picks the device.
The device ladder
- Air gap: the highest protection, a physical separation (commonly twice the effective pipe diameter, minimum 1 in).
- RPZ (reduced pressure zone): protects against both backsiphonage and backpressure; health hazard; requires testing and has relief discharge.
- Double check valve assembly (DCVA): backpressure and backsiphonage for non-health hazards; testable.
- Pressure vacuum breaker (PVB): backsiphonage only, non-continuous pressure; must be above the highest downstream outlet.
- Atmospheric vacuum breaker (AVB): backsiphonage only, no continuous pressure, no downstream shutoff.
- Hose-bibb vacuum breaker: individual outlet protection.
Program concepts
Containment protects the public main at the service; isolation protects internal fixtures. Testable assemblies require annual testing by a certified tester, and records kept. Devices must be accessible, not buried, and protected from freezing.
Worked example
A boiler make-up line is connected to potable water with a DCVA and the boiler chemicals are toxic. Because the hazard is a health hazard and backpressure is possible, an RPZ (or air gap) is required - not a DCVA.
Traps
Using an AVB or PVB where backpressure or continuous pressure exists; burying test cocks; and confusing containment with isolation.