Backflow prevention, cross-connection control, and potable water protection
Cross-connection control
Potable water must be protected from contamination and pollution. Backflow occurs by backsiphonage when supply pressure drops, or by backpressure when downstream pressure exceeds supply pressure. Protection is hierarchical: an air gap gives the highest protection; a reduced-pressure-zone assembly (RPZ) protects against both backpressure and backsiphonage; a double-check valve assembly protects lower hazards; a pressure vacuum breaker protects against backsiphonage only; an atmospheric vacuum breaker is limited to noncontinuous use with no downstream shutoff. Testable assemblies need annual testing and records.
Applications
Irrigation with fertilizer injection, boiler feed, chemical dilution, laboratory equipment, and hose bibs are common cross-connections. Provide air gaps for sinks and receptors; use RPZs for high-hazard or backpressure connections. Install devices where accessible for testing and draining, with no bypass. Remember that a backflow preventer can create a closed system and require thermal expansion control.
Worked example
A commercial irrigation system injects fertilizer. The hazard is high and backpressure is possible from the irrigation pump, so an air gap or RPZ is required. A pressure vacuum breaker is not acceptable because it does not protect against backpressure and cannot have downstream shutoff valves that create continuous pressure. The RPZ is installed horizontally where accessible, with drain and test cocks, and tagged for annual testing.
Traps
Using an AVB on a hose bib with a shutoff downstream; choosing a DCVA for high hazard; burying a testable assembly; omitting annual testing; no thermal expansion tank after a PRV; bypassing a backflow device.