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316 Chapter 9 Cross-Connection Control
industrial process vats sometimes have submerged inlets where the water pressure is used
as an aid in diffusion, dispersion, and agitation of the vat contents. Even though the supply
pipe may come from the floor above the vat, backsiphonage can occur because the siphon
action can raise a liquid such as water almost 34 ft (10.4 m). Some submerged inlets that
are difficult to control are those that are not apparent until a significant change in water
level occurs or where a supply may be conveniently extended below the liquid surface by
means of a hose or auxiliary piping. A submerged inlet may be created in numerous ways,
and its detection in some of these subtle forms may be difficult.

9.3.3 Backflow
Backflow refers to reversed flow due to backpressure other than siphonic action. Any in-
terconnected fluid systems in which the pressure of one exceeds the pressure of the other
may have flow from one to the other as a result of the pressure differential. The flow will
occur from the zone of higher pressure to the zone of lower pressure. This type of backflow
is of concern in buildings where two or more piping systems are maintained. The potable
water supply is usually under pressure directly from the city water main. Occasionally, a
booster pump is used. The auxiliary system is often pressurized by a centrifugal pump, al-
though backpressure may be caused by gas or steam pressure from a boiler. A reversal in
differential pressure may occur when pressure in the potable system drops, for some rea-
son, to a pressure lower than that in the system to which the potable water is connected.
The most positive method of avoiding this type of backflow is the total or complete
separation of the two systems. Other methods used involve the installation of mechanical
devices. All methods require routine inspection and maintenance.
Dual piping systems are often installed for extra protection in the event of an emer-
gency or possible mechanical failure of one of the systems. Fire protection systems are an
example. Another example is the use of dual water connections to boilers. These installa-
tions are sometimes interconnected, thus creating a health hazard.

9.4 METHODS AND DEVICES FOR THE PREVENTION OF BACKFLOW
AND BACKSIPHONAGE

A wide variety of devices are available that can be used to prevent backsiphonage and
backpressure from allowing contaminated fluids or gases into a potable water supply sys-
tem. Generally, the selection of the proper device to use is based on the degree of hazard
posed by the cross-connection. Additional considerations are based on piping size, loca-
tion, and the potential need to periodically test the devices to insure proper operation.
The six basic types of devices that can be used to correct cross-connections are as follows:
1. Air gaps
2. Barometric loops
3. Vacuum breakers—both atmospheric and pressure type
4. Double check valves with an intermediate atmospheric vent
5. Double check valve assemblies
6. Reduced pressure principle devices.
In general, all manufacturers of these devices, with the exception of the barometric
loop, produce them to one or more of three basic standards, thus ensuring that dependable
devices are being utilized and marketed. The major standards in the industry are devised by