Page 226 - Water Engineering Hydraulics, Distribution and Treatment

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Chapter 6
Water Distribution Systems: Components, Design, and Operation
requirements, if the following corrective methods of instal-
5 ft (1.52 m) shall be provided over the pipe unless
lation may be used:
otherwise approved by the reviewing authority. When
1. Such deviation may allow installation of the water
crossing water courses, which are greater than 15 ft
main closer to a sewer, provided that the water main
(4.57 m) in width, the following shall be provided:
is laid in a separate trench or on an undisturbed earth
(a) the pipe shall be of special construction, having
shelf located on one side of the sewer at such an
flexible, restrained, or welded watertight joints; (b)
elevation that the bottom of the water main is at least
valves shall be provided at both ends of water cross-
18 in. (45.72 cm) above the top of the sewer.
ings so that the section can be isolated for testing or
2. The sewer materials shall be water works grade
repair, and the valves shall be easily accessible, and
150 psi (1,035 kPa) pressure rated pipe meeting
not subject to flooding; (c) permanent taps or other
AWWA standards or pipe approved by the review- 2. For the underwater crossings, a minimum cover of
provisions shall be provided to allow insertion of a
ing authority and shall be pressure tested to ensure small meter to determine leakage and obtain water
water tightness. samples on each side of the valve closest to the sup-
ply source.
Surface water crossings, whether over or under water, present
special problems. The following are the recommended Ten- 6.9.10 Head Loss of Water System Fittings
State Standards:
Appendix 25 shows the equivalent pipe length to the head
1. For the above-water crossings, the pipe shall be ade- loss in fittings. A design engineer may calculate the equiv-
quately supported and anchored, protected from van- alent pipe lengths of all major fittings for a detailed water
dalism, damage, and freezing, and accessible for distribution system analysis. Contact the manufacturers for
repair or replacement. the missing fittings not listed in Appendix 25.
EXAMPLE 6.7 DETERMINATION OF EQUIVALENT PIPE LENGTH FOR A STANDARD ELBOW
Determine the equivalent pipe length of a 6 in. (152.4 mm) standard elbow.
Solution:
The dotted line in Appendix 25 shows that the resistance (or head loss) of a 6 in. (152.4 mm) standard elbow is equivalent to
approximately 16 ft (4.88 m) of 6 in. (152.4 mm) standard pipe.
EXAMPLE 6.8 DETERMINATION OF EQUIVALENT PIPE LENGTH FOR A GATE VALVE
Determine the equivalent pipe lengths of a 6 in. (152.4 mm) gate valve when it is (a) fully open and (b) ∕ 4 closed.
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Solution:
a. From Appendix 25, the resistance of a 6 in. (152.4 mm) gate valve is equivalent to approximately 3.5 ft (1.07 m) of 6 in.
(152.4 mm) standard pipe if the gate valve is fully open.
b. The resistance of a 6 in. (152.4 mm) gate valve is equivalent to approximately 19 ft (5.79 m) of 6 in. (152.4 mm) standard
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pipe if the gate valve is ∕ 4 closed.
EXAMPLE 6.9 DETERMINATION OF EQUIVALENT PIPE LENGTH FOR A SUDDEN ENLARGEMENT
Determine the equivalent pipe length of a sudden enlargement from 6 in. (152.4 mm) to 12 in. (304.8 mm) in diameter.
Solution:
Use Appendix 25 to find the solution.
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d∕D = 6in.∕(12 in.) = 152.4mm∕(304.8mm) = ∕ 2.
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The resistance of the sudden enlargement (d∕D = ∕ 2) is equivalent to approximately 9 ft (2.74 m) of a 6 in. (152.4 mm) standard
pipe.