Page 203 - Water Engineering Hydraulics, Distribution and Treatment

Chapter6
Water Distribution Systems: Components,
Design, and Operation
6.1 DISTRIBUTION SYSTEMS that supply water to the congested,or high-value, district
from at least two directions. This more or less doubles the
Apart from a few scattered taps and takeoffs along their feeder
delivery of the grid (Fig. 6.2c). In large systems, feeders are
conduits, distribution systems for public water supplies are
constructed as pressure tunnels, pressure aqueducts, steel
networks of pipes within networks of streets. Street plan,
pipes, or reinforced-concrete pipes. In smaller communities
topography, and location of supply works, together with ser-
the entire distribution system may consist of ductile-iron
vice storage, determine the type of distribution system and
pipes. Ductile iron is, indeed, the most common material
the type of flow through it. Although service reservoirs are
for water mains, but plastics, in general, in the case of small
often placed along lines of supply, where they may usefully
supplies, are also important.
reduce conduit pressures, their principal purpose is to satisfy
network requirements. Accordingly they are, in fact, com-
ponents of the distribution system, not of the transmission 6.1.3 Pipe Grids
system.
The gridiron system of pipes stretching over all but the outly-
ing sections of a community (Fig. 6.2) may consist of single
6.1.1 One- and Two-Directional Flow or dual mains. In the Northern Hemisphere, single mains are
The type of flow creates four systems, as sketched in Fig. 6.1. customarily laid on the north and east sides of streets for
Hydraulic grade lines and residual pressures within the areas protection against freezing. In the Southern Hemisphere, the
served, together with the volume of distribution storage, gov- south and east sides are used. Valves are generally installed
ern the pipe sizes within the network. It is plain that flows as follows: three at crosses, two at tees, and one on single-
from opposite directions increase system capacity. With two- hydrant branches. In dual-main systems, service headers are
added on the south (north in Southern Hemisphere) and
directional flow in the main arteries, a pumped or gravity
west sides of streets, and piping is generally placed beneath
supply,ora service reservoir, feeds into opposite ends of the
distribution system or through the system to elevated storage the sidewalks. Hydraulically, the advantages of dual-main
in a reservoir, tank, or standpipe situated at the far end of systems over single-main systems are that they permit the
the area of greatest water demand. Volume and location of arrangement of valves and hydrants in such ways that breaks
service storage depend on topography and water needs. in mains do not impair the usefulness of hydrants and do not
dead-end mains.
Dual-main systems must not be confused with dual-
6.1.2 Distribution Patterns water supplies: a high-grade supply for some purposes and a
low-grade supply for others.
Two distribution patterns emerge from the street plan: (a)
a branching pattern on the outskirts of the community,
in which ribbon development follows the primary arteries
6.1.4 High and Low Services
of roads (Fig. 6.2a), and (b) a gridiron pattern within the
built-up portions of the community where streets crisscross Sections of the community too high to be supplied directly
and water mains are interconnected (Figs. 6.2b and 6.2c). from the principal, or low-service, works are generally incor-
Hydraulically, the gridiron system has the advantage of porated into separate distribution systems with independent
delivering water to any spot from more than one direction piping and service storage. The resulting high services are
and of avoiding dead-ends. The system is strengthened by normally fed by pumps that take water from the main sup-
substituting for a central feeder a loop or belt of feeders ply and boost its pressure as required. Areas varying widely

Water Engineering: Hydraulics, Distribution and Treatment, First Edition. Nazih K. Shammas and Lawrence K. Wang.
© 2016 John Wiley & Sons, Inc. Published 2016 by John Wiley & Sons, Inc.

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