Reduced Pressure Principle Backflow Prevention Device Installed on Service Line Backflow Condition – Fouled Second Check Valve

Reduced pressure principle backflow prevention device assemblies (RPZ) installed on service lines may allow non-potable water to enter the domestic potable water system from unprotected indirect cross-connections. If a negative pressure conditions occurs in the service line upstream of the RPZ, the first and second check valves in the RPZ will close and the relief valve will open. Any water in the zone between the check valves will discharge out of the relief valve. The water pressure in the domestic water system in the building downstream of the RPZ will be atmospheric. If a tap on a lower level of the building upstream from the unprotected indirect cross-connection and downstream of the RPZ is opened, a siphon condition would be created and non-potable water would be siphoned into the potable domestic water system within the building. A cross-connection incident was created within the building. A similar situation would occur if the second check valve in the RPZ were fouled open. If the second check valve were fouled open, water would discharge out of the relief valve causing a siphon condition in the domestic water system within the building. Just like opening a tap upstream of the cross-connection, non-potable water is now entering the domestic potable water system within the building and draining out of the relief valve. A cross-connection incident was created in the domestic potable water system by the fouled second check valve and the opening of the relief valve.