Page 224 - Water Engineering Hydraulics, Distribution and Treatment

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Chapter 6
multidirectional flow design, adequate valving for flow and
and saw him fall to the ground. The police and emergency
pressure controls, and provisions for air release and ade-
medical services personnel were called and arrived within a
quate flushing. Water systems should be designed according
few minutes of the fall and began treating the victim, who
to the federal, state, and/or county standards to maximize
was still breathing. During this time the shaken coworker
water turnover and to minimize residence times while deliv-
was lowered to the ground on the material hoist line. The
ering acceptable water flows, and pressures. In the absence
victim was transported to the local medical center where he
of any government standards, design criteria and materi-
was pronounced dead.
Investigators concluded that these guidelines should be
als meeting applicable water professional standards (such as
followed in order to prevent similar incidents in the future:
American Water Works Association standards and National
Sanitation Foundation standards) can be accepted by the
1. Employers must thoroughly plan all work and per-
reviewing authority for adoption.
form a job hazard analysis of the site prior to starting
work. Water Distribution Systems: Components, Design, and Operation
2. When practical, employers should provide and 6.9.1 Minimum Design Period Requirements
require the use of a stable work platform for working Water mains should have a minimum size based on a
at elevated worksites. hydraulic analysis utilizing 20-year design for a specified
3. Employers should provide a system of fall protection water demand. The projected land uses and water demand
that protects employees at all times when working at based on full service area development should be considered.
elevations. The specified water demand selected for design depends on
4. Employers should ensure that fall protection equip- the area to be serviced and the type of water main (feeder,
ment is appropriate and maintained in good condi- arterial, distribution).
tion. Employees should inspect fall protection equip-
ment before each use to ensure that all components 6.9.2 Water Pressure Requirements
are in operational order.
The water system shall be designed to maintain a minimum
5. Employers should ensure that material hoists are not
of 20 psi (140 kPa) at ground level at all points in the water
used for raising or lowering employees to or from the
distribution system under all conditions of flow (such as fire
worksite.
flow conditions), at any hydrant or any point in the water sys-
6. Employers should ensure that electrical safety prac- tem. The normal working pressure in the water distribution
tices are followed when welding. system should be approximately 60–80 psi (410–500 kPa),
7. Owners of water towers and similar structures should and no less than 35 psi (240 kPa). The State of Florida Hills-
design and install a permanent static safety line sys- borough County requires a minimum water pressure of 35 psi
tem on the tower to facilitate the use of fall protection (240 kPa) in a water transmission line, under any flow con-
devices. ditions. The State of Iowa recommends that individual or
system pressure reducing devices be used when operating
pressure exceeds 100 psi (625 kPa).
6.9 PRACTICAL DESIGN AND ANALYSIS OF
WATER DISTRIBUTION SYSTEMS
6.9.3 Minimum Size Requirements
The technical information presented in this section is impor-
tant to practical design of a new water distribution system All water mains should be sized large enough to provide
from scratch, or analysis of an existing (or a newly designed) existing and future residential, commercial, and industrial
system. A design engineer or a government reviewer must water demands and fire protection flows to the area to be
design/review a water distribution in accordance with appro- served. The Ten-State Standards state that (a) the minimum
priate government requirements and/or professional stan- size of a water main which provides for fire protection and
dards. The authors selected the Recommended Standards serving fire hydrants shall be 6 in (152.4 mm) diameter; and
for Water Works (i.e., Ten-State Standards; 2007), the State (b) the minimum size of water main in the water distribution
of Iowa’s Statewide Urban Design and Specifications – system where fire protection is not to be provided should be
Design Manual (2012), and the State of Florida Hillsborough a minimum of 3 in. (26.2 mm) in diameter. The minimum
County’s subdivision review guidelines, Drinking Water Dis- pipe sizes for distribution mains shall be 4 in. (101.6 mm) in
tribution Systems (2011) as the typical examples of regional, diameter in the State of Florida Hillsborough County, but 8 in.
statewide, and countywide standards. A professional engi- (203.2 mm) in the State of Iowa, unless otherwise approved
neer must carefully select appropriate standards for design by the jurisdictional authority.
and/or review of a water distribution system. Arterial or feeder mains, typically 12 in. (304.8 mm) and
Water distribution systems shall be designed to main- larger, should conform to an existing grid pattern. Readers
tain treated water quantity and quality. Special consideration are referred to Section 6.2 for more information on other
should be given to sizing of distribution main, providing for common standards for sizing pipes.