Backflow Prevention Assembly Types, Design and Applications
Assembly selection
Selection begins with hazard classification and the direction of possible backflow. An air gap provides the highest protection when properly maintained and unobstructed. A reduced-pressure zone backflow preventer (RPZ) protects against both backsiphonage and backpressure and is used for high-hazard applications. A double check valve assembly (DCVA) also protects against both, but is limited to low-hazard use because it has no relief valve and no visible discharge. A pressure vacuum breaker (PVB) is for backsiphonage only and must not be used where backpressure can occur. An atmospheric vacuum breaker (AVB) cannot be under continuous pressure and must not have a shutoff valve downstream. A spill-resistant vacuum breaker may be used under continuous pressure but still not for backpressure. Detector assemblies protect fire sprinkler systems while allowing metered flow for alarms and testing.
Traps: questions mix up “high hazard” with “backpressure,” or assume every vacuum breaker is interchangeable. RPZ relief discharge is normal during certain pressure conditions; a DCVA has no relief discharge. A PVB installed downstream of a pump or boiler is a classic misapplication.
Worked example
A chemical plant needs protection against a booster pump that can exceed supply pressure. Because backpressure and a health hazard are both present, select an RPZ, not a PVB or DCVA. Rule of thumb: air gap first for the highest hazard; RPZ for high hazard with both flow directions; DCVA for low hazard with both; vacuum breakers only where backpressure is impossible.