The Principle Operation of a Hose Connection Dual Check Valve Vacuum Breaker – Backflow Prevention Device – Backflow Prevention Backpressure Condition
If a backpressure condition were to occur at 4.33 psi or less, the second valve would close tight preventing a cross-connection incident. The atmospheric vent is closed and the first check valve is open. The HCDCV is providing protection.
If a backpressure condition were to occur at 4.33 psi or less and the second check did not close tight, the first check valve would close, if operating properly, provide protection and the atmospheric vent would be open. Water may discharge from atmospheric vent. The HCDCV is providing protection.
If a backpressure condition were to occur at 4.33 psi or less and both check valve were fouled open or did not close, there would be no protection even though the atmospheric vent is open. Water may discharge from atmospheric vent and non-potable water may also be entering the potable water system since the downstream pressure is greater than the line pressure. The atmospheric vent will most likely not be able to discharge the volume of water contained in the downstream piping. The amount of water that may enter the potable water system or discharge from the atmospheric vent is equal to the volume of water contained in the downstream piping system. The HCDCV is not providing protection when both check valves are not drip tight during a backpressure condition even though the atmospheric vent is open.