Page 302 - Water Engineering Hydraulics, Distribution and Treatment

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Cross-Connection Control
Chapter 9
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 aux-
iliary piping. A submerged inlet may be created in numerous
ways, and its detection in some of these subtle forms may be
Nonpotable
difficult.
(a) Potable is used as an aid in diffusion, dispersion, and agitation of the
9.3.3 Backflow
City supply Condenser
Backflow refers to reversed flow due to backpressure other
than siphonic action. Any interconnected 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 pres-
sure differential. The flow will occur from the zone of higher
pressure to the zone of lower pressure. This type of back-
flow 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. Occasion-
Sanitary sewer ally, a booster pump is used. The auxiliary system is often
(b) pressurized by a centrifugal pump, although backpressure
may be caused by gas or steam pressure from a boiler. A
Figure 9.17 Valved connections between potable water and (a)
reversal in differential pressure may occur when pressure
nonpotable fluid; (b) sanitary sewer.
in the potable system drops, for some reason, to a pressure
lower than that in the system to which the potable water is
supply and the source of pollution. Two basic types of con- connected.
nections are used in piping systems: The most positive method of avoiding this type of back-
flow is the total or complete separation of the two systems.
1. The solid pipe with a valved connection
Other methods used involve the installation of mechan-
2. The submerged inlet ical devices. All methods require routine inspection and
maintenance.
Figures 9.17a and 9.17b illustrate solid connections. This
Dual piping systems are often installed for extra pro-
type of connection is often installed where it is necessary
tection in the event of an emergency or possible mechanical
to1ay an auxiliary piping system from the potable source. It
failure of one of the systems. Fire protection systems are an
is a direct connection of one pipe to another pipe or recepta-
example. Another example is the use of dual water connec-
cle. Solid pipe connections are often made to continuous or
tions to boilers. These installations are sometimes intercon-
intermittent waste lines where it is assumed that the flow will
nected, thus creating a health hazard.
be in one direction only. An example of this would be used
cooling water from a water jacket or condenser as shown
in Fig. 9.17b. This type of connection is usually detectable,
9.4 METHODS AND DEVICES FOR THE
but creating a concern on the part of the installer about the
PREVENTION OF BACKFLOW AND
possibility of reversed flow is often more difficult.
BACKSIPHONAGE
Submerged inlets are found on many common plumbing
fixtures and are sometimes necessary features of the fix- A wide variety of devices are available that can be used
tures if they are to function properly. Examples of this type to prevent backsiphonage and backpressure from allowing
of design are siphon-jet urinals or water closets, flushing contaminated fluids or gases into a potable water supply
rim slop sinks, and dental cuspidors. Old-style bathtubs and system. Generally, the selection of the proper device to use is
lavatories had supply inlets below the flood-level rims, but based on the degree of hazard posed by the cross-connection.
modern sanitary design has minimized or eliminated this Additional considerations are based on piping size, location,
hazard in new fixtures. Chemical and industrial process vats and the potential need to periodically test the devices to insure
sometimes have submerged inlets where the water pressure proper operation.