Backflow Prevention Assemblies and Devices
Assembly types and functions
Candidates must know the mechanical and non-mechanical options and their applications:
- Air gap: physical separation; highest protection; requires no testing but needs inspection.
- RPZ (reduced pressure principle assembly): two check valves plus a differential relief valve; high hazard; backpressure and backsiphonage; must discharge from relief valve under failure.
- Double check valve assembly: two check valves; low hazard; backpressure and backsiphonage.
- Pressure vacuum breaker (PVB): check valve plus air inlet; backsiphonage only; must be installed above highest downstream outlet.
- Atmospheric vacuum breaker (AVB): backsiphonage only; no continuous pressure; air inlet opens at zero flow.
- Spill-resistant vacuum breaker (SVB): for continuous pressure; air inlet stays closed until needed.
Installation and testing concepts
Know required orientation (RPZ horizontal unless approved), clearance for relief discharge, drainage, freeze protection, and access for testing. Understand that assemblies must be tested at installation, after repair, and at required intervals (often annually). Know the difference between field test (verifies operation) and inspection (visual condition).
Traps
- Using a double check for high hazard.
- Installing a PVB where continuous pressure exists.
- Forgetting that RPZ relief discharge must be visible and drained.
- Assuming an AVB can be under pressure for long periods.
Worked rule
If the device has an air inlet that opens to atmosphere, it protects against backsiphonage only unless it is a spill-resistant type designed for continuous pressure. If it has two checks and a relief valve, it is an RPZ for high hazard.
Study tip
Memorize a matrix: device × hazard × flow direction × pressure condition × testing requirement.