Reduced pressure principle assembly (RP)

A reduced pressure principle assembly, commonly called an RP or RPZ, is a mechanical backflow preventer built around two independently acting, spring-loaded check valves with a hydraulically operated relief valve located between them. The assembly also includes shutoff valves on each end and test cocks for field testing. In normal operation the zone between the checks is kept at a lower pressure than the supply, which is the principle that gives the assembly its name.

The RP is regarded as the highest level of mechanical protection because of how it fails safe. If either check valve fouls or leaks, or if pressure conditions reverse, the relief valve opens and discharges water from the zone to the atmosphere rather than allowing it to pass backward. That visible discharge is both a safeguard and a signal that something needs attention. Because it protects against both backpressure and backsiphonage, the RP is the standard choice for health hazard connections.

Testers and property owners should plan for the RP's practical needs. It must be installed where the relief port can discharge freely and drain safely, which usually means adequate drainage and protection from flooding and freezing. Occasional relief discharge can be normal during pressure fluctuations, but continuous discharge indicates a fouled check or relief problem that warrants service. Like other testable assemblies, it requires periodic testing, typically annual where required, set by the water authority.

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