The Principle Operation of a Dual Check Valve with Intermediate Atmospheric Vent – Backflow Prevention Device – Check Valve and Atmospheric Vent

The DCVwIV has two independently acting spring loaded check valves force loaded to a normally closed position in the direction of flow. The intermediate atmospheric vent is kept closed by inlet pressure and opens when the inlet pressure is atmospheric or sub-atmospheric. Each check valve spring is loaded to a minimum of 1 psi. The check valves in the DCVwIV provide primary backflow protection. The intermediate atmospheric vent provides additional protection during a backflow condition when the inlet pressure is atmospheric or sub-atmospheric. ASSE Standard #1012-2009 states, “Under a backpressure condition, failure of the atmospheric vent to open when the supply pressure is not less than 20% of the downstream pressure shall result in a rejection of the device.” If the second check valve does not close tight and there is a backpressure condition, the intermediate atmospheric vent will open when the supply pressure is 20% less than the backpressure. For example, if the backpressure is 100 psi, the intermediate atmospheric vent will open when the supply pressure is 80 psi or less (20% of the downstream pressure). If the backpressure is 100 psi, the intermediate atmospheric vent will not open if the supply pressure is 81 psi or greater. Backflow protection is not provided if the differential pressure between the supply pressure and backpressure is not within the specified percentage. The intermediate atmospheric vent opening requirement makes the DCVwIV only suitable for use on low-hazard cross-connections.