Page 58 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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1.9 Distribution Works 21
Hydraulically, the gridiron system has the advantage of delivering water to any spot
from more than one direction and of avoiding dead-ends. Gridiron systems are strength-
ened by substituting for a central feeder a loop or belt of feeders that supplies water to the
congested, or high-value, district from at least two directions. This more or less doubles
the delivery of the grid. In large systems feeders are constructed as pressure tunnels, pres-
sure aqueducts, steel pipes, or reinforced concrete pipes. In smaller communities the entire
distribution system may consist of cast-iron pipes. Cast iron is, indeed, the most common
material for water mains, but asbestos-cement, in general, and plastics, in the case of small
supplies, are also important.

1.9.1 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 emergencies or go into operation automat-
ically by means of pressure-regulating valves.

1.9.2 Fire Supplies

Before the days of high-capacity, high-pressure, motorized fire engines, conflagra-
tions in the congested central, or high-value, district of some large cities were fought
through independent high-pressure systems of pipes and hydrants. Taking water from
the public supply and boosting its pressure by pumps in power stations whenever an
alarm was rung in, these systems performed well. For extreme emergencies, rigorously
protected connections usually led to independent sources of water: rivers, lakes, or
tidal estuaries. Large industrial establishments, with heavy investments in plant,
equipment, raw materials, and finished products, concentrated in a small area, are gen-
erally equipped with high-pressure fire supplies and distribution networks of their
own. Because such supplies may be drawn from sources of questionable quality, some
regulatory agencies enforce rigid separation of private fire supplies and public sys-
tems. Others prescribe protected cross-connections that are regularly inspected for
tightness. Ground-level storage and pumping are less advantageous.

1.9.3 Pressures
In normal municipal practice, pressures of 60 to 75 psig (416 to 520 kPa) are main-
tained in business blocks and 40 psig (278 kPa) in residential areas. Higher pressures,
such as 100 psig (694 kPa) or more, delivering adequate amounts of water for fire-
fighting through hoses attached directly to fire hydrants are no longer important.
Instead, modern motor pumpers can discharge thousands of gallons per minute at even
greater pressures. Moreover, low operating pressures make for low leakage from
mains and reduce the amount of water that is unaccounted for. To supply their upper
stories, tall buildings boost water to tanks at various elevations and on their roofs or in
towers. In individual industrial complexes, the water pressure may be raised during
fires by fixed installations of fire pumps.