Civil Engineering Reference
In-Depth Information
SYSTEM DESIGN
Nonhydraulic Design Considerations
A number of factors unrelated to system hydraulics must be considered in water dis-
tribution system design. These nonhydraulic design considerations are summarized in
the following paragraphs.
The ability to isolate parts of the system is important, especially during emergency
operation conditions (e.g., main breaks). All water distribution systems should be pro-
vided with sufficient isolation and drain valves to permit necessary repairs without
undue interruption of service over any appreciable area.
Valves should be placed in numbers and locations that give the best possible control
of the system consistent with cost limitations. Valves in smaller mains are typically
more numerous than valves in larger mains. Valves should be located where they will
be readily accessible in the event of a main failure. Branch mains, connecting to larger
primary or secondary feeders that cross under arterial highways or streets, are usually
valved close to the larger main before the crossing.
In transmission lines, valve spacing is usually determined by operating require-
ments, and thus is a matter of individual design. In feeder mains, valves are usually
spaced so that each feeder loop can be effectively isolated. Usually this spacing will
not exceed 3,000 feet (914 m) in 16-inch (0.41-m) feeders or 4,000 feet (1.2 km) in
20-inch (0.51-m) feeders.
Service main valves will be spaced so that adequate shutdown capability is provided
without putting large numbers of customers out of service. In residential areas or
districts, valves in 6- and 8-inch (0. 15- and 0.20-m) mains should be spaced no more
than 1,000 feet (305 m) apart, and in 10- and 12-inch (0.25- and 0.31-m) mains no
more than 2,000 feet (610 m) apart. Normally, each smaller main will be valved in
intersections involving 10-inch and 12-inch (0.25-m and 0.31-m) mains connecting to
smaller mains. Service mains crossing creeks, railroads, and expressways will be
valved on each side of the crossing. Each fire hydrant branch should be equipped with
a control valve.
Valve sizes are normally the same as the water main in which they are installed,
except that in mains 30 inches (0.76 m) or larger, line valves are often one size smaller
than the main size.
Hydraulic Design Considerations
The design of water distribution systems may rely entirely on detailed calculations of
system hydraulics, or may depend in part on minimum design standards defined by
the owner of the distribution facilities. For instance, some utilities size transmission
lines based on limiting the headloss to 1 foot per 1,000 feet of length. Others specify
using a minimum-size pipe every mile or half-mile in a grid system. Design based on
minimum standards typically will meet or exceed hydraulic requirements under all the
ordinary system needs. Minimum design standards generally define materials and con-
struction methods, minimum pipeline sizes, maximum pipeline headloss or velocities,
and various criteria for ensuring adequate system flows and pressures without detailed
hydraulic analysis of system extensions.
An example of one such set of design standards is the Recommended Standards
for Water Works published by Health Education Services in Albany, N.Y.
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