Backflow Prevention Assembly Types and Operation
The assemblies
Know each device by its mechanical operation and its approved use. An air gap is the only non-mechanical protection and provides the highest degree of protection when properly sized and maintained. A barometric loop relies on a 35-foot rise and protects only against backsiphonage. A PVB (pressure vacuum breaker) has a loaded check and an air inlet that opens on loss of supply pressure; it must be installed above the highest downstream outlet and cannot have continuous pressure on the downstream side. An AVB (atmospheric vacuum breaker) has a float check and air inlet, must be installed above the highest outlet, and cannot be under continuous pressure. A double check valve assembly (DCVA) has two independently acting check valves and protects against non-health hazards only. A reduced pressure zone assembly (RPZ) has two checks plus a relief valve that discharges when the differential across the first check falls, providing health-hazard protection. A spill-resistant vacuum breaker (SVB) uses a check and air inlet with a relief valve and can be under continuous pressure.
Traps
- DCVA is not acceptable for health hazards; RPZ is.
- PVB and AVB must be installed above the highest downstream outlet; a shutoff downstream of an AVB is prohibited.
- RPZ relief discharge is normal during testing and during certain flow conditions; it is not automatically a failure.
- Air gap sizing depends on the effective opening of the receiving vessel, not the pipe size alone.
Rule of thumb
Match device to hazard: health hazard, use RPZ or air gap; non-health hazard, DCVA is acceptable; backsiphonage only, consider PVB/SVB.