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196 Chapter 6 Water Distribution Systems: Components, Design, and Operation
mains are customarily laid on the north and east sides of streets for protection against
freezing. In the Southern Hemisphere, the south and east sides are used. Valves are
generally installed as follows: three at crosses, two at tees, and one on single-hydrant
branches. In dual-main systems, service headers are added on the south (north in
Southern Hemisphere) and west sides of streets, and piping is generally placed
beneath the sidewalks. Hydraulically, the advantages of dual-main systems over
single-main systems are that they permit the arrangement of valves and hydrants in
such ways that breaks in mains do not impair the usefulness of hydrants and do not
dead-end mains.
Dual-main systems must not be confused with dual-water supplies: a high-grade
supply for some purposes and a low-grade supply for others.

6.1.4 High and Low Services
Sections of the community too high to be supplied directly from the principal, or low-
service, works are generally incorporated into separate distribution systems with
independent piping and service storage. The resulting high services are normally fed
by pumps that take water from the main supply and boost its pressure as required.
Areas varying widely in elevation may be formed into intermediate districts or zones.
Gated connections between the different systems are opened by hand during emergen-
cies or go into operation automatically through pressure-regulating valves. Because
high-service areas are commonly small and low-service areas are commonly large,
support from high-service storage during breakdowns of the main supply is generally
disappointing.
Before the days of high-capacity, high-pressure, motorized fire engines, confla-
grations in the congested central, or high-value, district of some large cities were
fought with water drawn from independent high-pressure systems of pipes and hy-
drants (Boston, Massachusetts, still maintains a separate fire supply). Large industrial
establishments, with heavy investments in plant, equipment, raw materials, and fin-
ished products, that are concentrated in a small area are generally equipped with high-
pressure fire supplies and distribution networks of their own. When such supplies are
drawn from sources of questionable quality, some regulatory agencies enforce rigid
separation of private fire supplies and public systems. Others prescribe protected
cross-connections incorporating backflow preventers that are regularly inspected for
tightness.

6.1.5 Service to Premises
Water reaches individual premises from the street main through one or more service
pipes that tap into the distribution system. The building supply between the public main
and the takeoffs to the various plumbing fixtures or other points of water use is illus-
trated in Fig. 6.3. Small services are made of cement-lined iron or steel, brass of vary-
ing copper content, copper, and plastics such as polyethylene (PE) or polyvinyl chlo-
ride (PVC). Because lead and lead-lined pipes may corrode and release lead to the
water, they are no longer installed afresh. For large services, coated or lined ductile-
iron pipe is often employed. For dwellings and similar buildings, the minimum desir-
able size of service is 3/4 in. (19 mm). Pipe-tapping machines connect services to the
main without shutting off the water. They also make large connections within water dis-
tribution systems.