Backflow Prevention Assembly Types, Design, and Operation
The testable assemblies
- Air gap: physical separation, at least twice the effective pipe diameter and never less than 1 in. Absolute protection; the reference standard for health hazards.
- RPZ (reduced pressure principle): two independently acting check valves plus a differential relief valve between them. Protects against both backsiphonage and backpressure; for health hazards. Must be installed horizontally, with the relief port terminating at least 12 in above grade and draining to a safe discharge; never in a pit subject to flooding.
- DCVA (double check valve): two check valves, no relief. Non-health hazard only; tolerates mild backpressure.
- PVB (pressure vacuum breaker): one check plus an air inlet, for backsiphonage only. Must be 12 in above the highest downstream piping or flood level rim, with no downstream shutoff and no continuous pressure over 12 hours.
- SVB (spill-resistant vacuum breaker): permits downstream shutoff valves and continuous pressure, but still requires elevation above flood level.
- AVB (atmospheric): no continuous pressure, no downstream shutoff, mounted at least 6 in above the highest downstream outlet.
Traps to expect
- PVB versus SVB limitations (shutoffs, continuous pressure) are the highest-frequency confusion.
- RPZ orientation and pit restrictions.
- Air gap sizing: 2× diameter, minimum 1 in - items swap in "same as diameter."
- DCVA is never for a health hazard.
Rule of thumb
Match the device to the hazard first, then to the flow condition: health hazard plus backpressure demands an RPZ or air gap. Worked example: a 3 in line feeding a chemical mixing tank needs an air gap of 6 in (2 × 3 in).