Double Check Valves consists of two check valves assembled in series. This uses two operating principles: first, one check valve will still be able to act, even if the other is jammed wide open. Secondly the closure or failure of one valve reduces the pressure differential across the other, allowing for a more reliable seal, and avoiding major leakage.
Smaller valves can be as small as 1/4″ in diameter and may be so compact that they are barely noticeable particularly when they are integrated into the bodies of existing taps (faucets). Larger check valves can commonly be as big as 12″ diameter and may be installed with ball valves at the ends, for isolation and testing. Often, test cocks, smaller ball valves, are in place for attaching gauges and other test equipment for evaluating the pressure of each chamber to see whether the double check assembly is still functional.
Double check valve assemblies are suitable for prevention of back pressure and back siphonage. It is commonly used on lawn irrigation, fire sprinkler and combi-boiler systems. If the hazard is higher, even a relatively low hazard such as using antifreeze in the fire sprinkler system or the usage of chemicals and cleaning of tanks, then a reduced pressure zone device which is a more reliable check valve may be needed.