Dual Check Valve Backflow Prevention Device – Backflow Backpressure Condition – First or Second Check Valve Not Drip Tight

When the DuCV is in a normal flow condition and there is a demand downstream, the checks valves are open. If a backpressure condition were to occur due to the activation of a pump for example, that created a pressure downstream greater than the supply or inlet pressure; the second check valve would close tight if it were not fouled and operating properly. If the downstream shut-off valve was open and there was no demand or no-flow downstream, and backpressure condition was created by thermal expansion of the water in the downstream piping system for example, the second check valve would close tight if it were not fouled or operating properly. When backpressure occurs, the second check valve will close before the first check valve due to the increase in pressure downstream of the second check valve. The first check valve will close after the second check valve, since the second check valve closing due to the backpressure condition creates a no-flow or static condition. If the first check valve is fouled and does not close, the DuCV is still providing protection.If the second check valve was fouled with debris and could not close, the first check would hold tight and the device would still provide protection.

If both check valves were fouled as well under a backpressure condition, the device would not offer protection. The status or condition of the check valves cannot be determined since the DuCV is not inline testable. Since the condition of the DuCV cannot be evaluated inline, it should not be used to control hazardous cross-connections but should only be used for hydraulic purposes such as service line protection. Service line protection prevents the flow of a potable water system into another potable water system.