Figure 1 at right 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 at right 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.
