Page 258 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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218 Chapter 6 Water Distribution Systems: Components, Design, and Operation
Figure 6.17 Cross-
Connection between Gate M Gage B Gage C Gate N
Municipal Water Supply and
Private (Industrial) Water
Supply Protected by Double From public 3 To private
Check-Valve Installation. water main Gage A 4 ˝ test system
To test installation: (1) close drain D 3 4 ˝ test drain E
gates M and N; (2) open test Check valve G
drain B, and observe gages A Check valve F
and B; (3) open test drain E,
and observe gage C. If check
valves F and G are tight, gage Brick or concrete pit Steel foothold inserts
A will drop to zero; gages B Plan
and C will drop slightly Indicator post
owing to compression of
optional
rubber gaskets on check
valves F and G. Manhole
Conversion factors: 1
1 in. 25.4 mm; 1 ft
0.3048 m
Wood cover
Steel foothold
inserts
Gage A
Gage B Gage C
Blowoff
Sleeve Spacer 3 to 5 ft long
F G
3
Cast-iron M D 3 4 ˝ test drain 4 ˝ test E N Cast-iron
pipe drain pipe

Siphon ejector or drain
Floor pitched to drain
Concrete support
Section

current: 100 to 200 amperes at 3 to 10 volts for small pipes and up to several thousand
amperes at 55 or 110 volts for large mains. The current applied is varied with the electri-
cal resistance and the melting point of the pipe metals. Nonmetallic jointing and caulking
compounds and or plastic pipes obstruct current flow. Electric grounds on interior water
piping, or the piping itself, must be disconnected during thawing operations. Grounds are
needed but are an annoyance when they carry high voltages into the pipes and shock
workmen. Pipes and hydrants can also be thawed with steam generated in portable boilers
and introduced through flexible block-tin tubing.
Loss of water by leakage from distribution systems and connected consumer premises
should be kept under control by leakage surveys.
Remember that the best way to prevent problems and accidents is to minimize hazards
and “design them out” early in the design process (see Chapter 20). Three illustrative ex-
amples from the National Institute for Occupational Safety and Health (NIOSH) are sum-
marized in the following subsections.