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603.3.7 Reduced-Pressure Principle Backflow Prevention Assembly (RP). A reduced-pressure principle backflow prevention assembly consists of two independently acting internally loaded check valves, a differential pressure-relief valve, four properly located test cocks, and two isolation valves.

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A field test of an RP is required on installation, at least annually thereafter, after any repair or when relocated. The assembly must be installed a minimum of 12 inches above the surrounding ground or floor.

As water flows through the first check valve, a pressure drop is created. The assembly's relief valve determines if the supply pressure is greater and will remain closed; if the difference in pressure between the supply and zone is reduced, the relief valve will open. With a loss of supply pressure and both checks closing, the relief valve will open to maintain a minimum pressure of two psi less than the supply pressure of the fluid in the zone and downstream line. If there is a complete loss of supply pressure, the pressure between the two check valves will become atmospheric because the spring has opened the relief valve.

An RP's relief valve has the potential to discharge a significant amount of water. Care should be taken as to its location and provision for indirect drainage. The drainage piping should be sized adequately to take the full discharge of the system. Always refer to the manufacturer's installation requirements.

The RP may be used at actual or potential high- or lowhazard applications. As previously mentioned, the assembly provides backpressure and backsiphonage protection.

The name of this assembly is derived from a zone of reduced pressure needed for the operation of the relief or vent valve. This device is an assembly of two internally loaded, specially designed and independently operating check valves that have a relief valve installed between the primary valves, specifically designed to maintain a zone of relative differential pressure between the two check valves unless one or both of the check valves are fouled. This assembly has a tightly closing upstream and a tightly closing downstream shutoff valve (see Figure 603.3.7).

The reduced pressure principle backflow prevention assembly is referred to as the RP, RPPZ, RPZ as well as other acronyms. The RP is the best mechanical backflow preventer available today. It is considered as proper protection for high- or low-hazard applications, backpressure or backsiphonage backflow and containment or isolation protection.