DESCRIPTION:  The reduced-pressure principle backflow-prevention assembly consists of two independently acting, approved check valves together with a hydraulically operating, mechanically independent pressure differential relief valve located between the check valves and below the first check valve.  These units are located between two tightly closing resilient-sealed shutoff valves.


     FUNCTION:  During normal operation, the first internally loaded check valve creates a reduced-pressure zone between the two check valves and under flow conditions both check valves open, allowing water to flow to the downstream piping.  The relief valve is held closed by the supply pressure acting on a diaphragm within the relief valve. In a no-flow or static-pressure condition both check valves will close and the supply pressure will hold the relief valve shut. If the supply pressure drops, the relief valve will maintain a minimum pressure in the zone between check valve 1 and check valve 2 of 2 psi, lower than the supply pressure, by releasing sufficient water to maintain the required difference in pressure.  If the supply pressure becomes less than 2 psi, the relief valve opens, discharging the material in the reduced-pressure zone to the atmosphere. In the event that pressure increases downstream of the assembly, tending to reverse the direction of flow, both check valves in the assembly should close tightly and prevent backflow.  However, if the second check valve does not close tightly, leakage into the reduced-pressure zone will increase the pressure, which will cause the relief valve to open. If the supply pressure drops to atmospheric pressure or within 2 psi of the reduced-pressure zone, the relief valve will open creating an internal air gap.  Any leakage past the second check valve would then be discharged through the open relief valve.


     APPLICATION:  The RPZ is effective against backflow caused by backpressure and backsiphonage. The RPZ is used to isolate health hazards.  The main advantage is that there is visible flow if failure of the assembly or system occurs. The RPZ is normally used in locations where an approved air gap is impractical.  It is important to remember that RPZs are mechanical devices and must be tested and serviced regularly to maintain positive protection.