MIL-HDBK-1164
10.3.2
Class II--Moderate Degree of Hazard. If a backflow were to occur, the
esthetic qualities of the water supply would change significantly. The foreign
substance must be nontoxic to humans.
10.3.3
Class III--High Degree of Hazard. If a backflow were to occur, the effect
on the water supply could cause illness or death if the water were consumed by
humans. The foreign substance may be toxic to humans either from a chemical,
bacteriological, or radiological standpoint. Effects of these contaminants may result
from short- or long-term exposure.
10.4
Approved Backflow Prevention Devices. Devices that protect the potable
water supply from these backflow hazards are listed in Table 32 and discussed below.
Additional descriptions of approved backflow prevention devices can be found in the
references listed in par. 11.2.
Table 32
Approved Backflow Devices
Degree of Hazard
Allowed Approved Devices
Class I
Air gap
Atmospheric type vacuum breaker
Pressure type vacuum breaker
Double check valve assembly
Reduced pressure principle device
Class II
Air gap
Double check valve assembly
Reduced pressure principle device
Class III
Air gap
Reduced pressure principle device
10.4.1
Air Gap. An approved air gap may be used under any and all conditions
of hazard and pressure conditions.
10.4.2
Vacuum Breakers. Pressure and atmospheric vacuum breakers are
primarily in-plant or end-of-service line solutions to cross-connection. They are not
used in water service connections. They are placed at the end of a line, and at fixtures
or equipment that discharge to atmospheric pressure. These do not protect against
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