MIL-HDBK-1164
back pressure, only against back siphonage. Valves should not be located
downstream from an atmospheric type vacuum breaker.
Note: Vacuum breakers are permitted on irrigation systems
regardless of hazard class for protection from back siphonage only.
10.4.3
Reduced Pressure Principle Device. The reduced pressure (RP) principle
device protects against both back pressure and back siphonage and can be used for
any degree of hazard.
10.4.4
Double Check Valve Assembly. This device works in a back pressure or
back siphonage mode. This device neither discharges water nor provides a visual sign
of backflow or unit malfunction. Therefore, it does not offer the degree of protection
provided by the reduced pressure principle device.
10.5
Selection and Installation of Backflow Preventers. Selecting proper
devices is very important. However, having the proper device on the connection is not
sufficient; the device also needs to be installed correctly. Guidance on selecting and
installing backflow prevention devices is provided in par. 2.2.10. Critical potable water
supplies should have parallel installation of the proper approved backflow prevention
device. This avoids interruption to water service when maintenance or testing is
required. This type of installation also provides higher flow capacity than is provided
by one backflow preventer. Methods and devices occasionally promoted for backflow
prevention include the single check valve, the swivel connection, the removable section
or spool, and the barometric loop. None of these methods is approved for use in
military water systems. Reasons for their unacceptability are discussed below:
a)
A single check valve offers no visual or mechanical means of
determining malfunctioning. Since all such mechanical devices are subject to wear and
interference resulting from deposits and other factors, the single check valve is not
considered an adequate backflow preventer.
b)
The swivel connection and removal section or spool are too easily
allowed to remain in place to be considered an acceptable means of backflow
prevention.
c)
A barometric loop consists of a vertical section of pipe, extending
at least 35 feet (11 m) above the highest fixture. The principle is that a complete
vacuum cannot raise water to an elevation greater than 33.9 feet (10.3 m). The device
does not protect against backflow because of back pressure, and installing a pipe loop
of this height is usually costly and impractical.
10.6
Inspection and Testing Schedule. Each installation should create a
schedule for inspecting and testing all backflow protection devices, including air gaps.
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