Backsiphonage Backflow – Relief Valve Opened – First Check Valves Fouled Open and Second Check Valve Closed
During a backsiphonage situation with the upstream and downstream valves open, the relief valve will open since there is no supply or inlet pressure keeping it closed. The second check valve if not fouled will close tight. The pressure downstream of the second check valve is positive. The first check valve during a backsiphonage situation will close tight if it is not fouled open. If the first check valve is fouled open during a backsiphonage situation atmospheric pressure will be pulled into the RPZ through the relief valve port of the opened relief valve and back through the fouled first check valve creating an air gap separation in the piping system. The zone pressure is sub-atmospheric as is the pressure upstream of the first check valve since it is fouled open. The RPZ is providing protection by preventing non-potable water from enter the zone and the potable water system, since the second check valve is closed tight and the relief valve is open. Since the second check valve is closed tight, only a small amount of water (the water in the zone) will discharge out of the relief valve.
