The Principle Operation of a Laboratory Faucet Dual Check Vacuum Breaker – Backflow Prevention Device – Continuous Pressure and Backpressure
The LFDC is not suitable for continuous pressure applications. Since the LFDC is equipped with an atmospheric vent mechanism similar to that of a hose bibb vacuum breaker, the LFDC should not be used for more than twelve hours of continuous use in a twenty-four-hour period. Continuous use for more than twelve hours may cause the atmospheric vent diaphragm to stick to the vent openings, making the potable water system vulnerable to backflow hazards. The LFDC shall not be used where head pressure could be created in the hose or where continues use would exceed twelve hours. If downstream shut-off valve are to be used on the end of a hose connection, such as spray nozzle, the water supply shall be closed at the sill cock upstream of the LFDC and the pressure in the hose released by opening the spray nozzle. Pressurized hoses may create a constant pressure for more than twelve hours which may cause the atmospheric diaphragm to seal to the atmospheric vent opening causing the LFDC to malfunction. If the atmospheric vent is stuck closed or malfunctions, the LFDC is inoperative and not providing protection.