Figure 1 below depicts a double check backflow preventer. This device is commonly used to protect against low to medium hazard installations such as food processing steam kettles and apartment projects. It may be used under continuous pressure and protect against both backsiphonage and backpressure conditions.
A double check valve is essentially two single check valves coupled within one body and furnished with test cocks and two tightly closing gate valves.
The test capability feature gives this device a big advantage over the use of two independent check valves in that it can be readily tested to determine if either or both check valves are inoperative or fouled by debris.
Figure 1 - Double Check Backflow Preventer
Each check is spring loaded closed and requires approximately a pound of pressure to open. This spring loading provides the ability to “bite” through small debris and still seal—a protection feature not prevalent in unloaded swing check valves.
Figure 2 depicts a reduced pressure zone backflow preventer (RPZ). These devices are utilized when the risks from contamination are high. Maximum protection is achieved against backsiphonage and backpressure conditions utilizing reduced pressure principle backflow preventers.
These devices are essentially modified double check valves with an atmospheric vent capability placed between the two checks and designed such that this “zone” between the two checks is always kept at least two pounds less than the supply pressure.
Figure 2 - Reduced Pressure Zone Backflow Preventer
With this design criteria, the reduced pressure principle backflow preventer can provide protection against backsiphonage and backpressure when both the first and second checks become fouled. They can be used under constant pressure and at high hazard installations. They are furnished with test cocks and gate valves to enable testing and are available in sizes ¾-inch through 10 inch.