I’m doing a series for the curious customer–breaking down how each of the various backflow preventer devices When I think of backflow, I think of double check assemblies. Part of this is because my first understanding of backflow devices comes from when I did work for an irrigation company–it seemed every backflow test report I saw was double-check (aka DC or DCVA–double check valve assembly). To me, when I think backflow, I automatically think of “double check” more often than not.
How does a DC work?
Double-checks are suitable for basic backflow and back-siphonage. The backflow preventer is pretty basic–not as basic as an airgap, but not far from it. It operates by utilizing two independent check valves. As with a RPZ the first section of the DC will fill with water before the next check valve will push forward–letting the water flow. As with any backflow preventer, if water flows back or in the case of back-siphonage, the pressure of this water closes the check valve. In case there is something lodged in either of the check valves, the other one will still close. (For instance a tiny rock in the case of irrigation.)
They lack the relief valve of a RPZ so they aren’t suitable for toxic water flow.
Where are Double Check assemblies typically used?
These backflow preventers are, as I mentioned earlier, mostly used on lawn irrigation. However, if there is chem-irrigation being used, they aren’t installed–this is where an RPZ takes the cake. This is one reason that they are used in lawn irrigation but not typically larger agricultural areas. DC’s are also installed on fire sprinklers (a slightly varieted version known as a DCDA) and combi-boiler systems.
Next week, let’s take a look at PVB’s–Pressure Vacuum Breakers.