Civil Engineering Reference
In-Depth Information
lication is commonly referred to as the Ten States Standards. The Ten States Standards,
or similar standards, can be used if detailed hydraulic engineering studies are not a
viable option for the utility.
System Pressures
It is important to establish criteria for minimum and maximum system pressures oc-
curring during the peak hour demand. It is also desirable to establish maximum pres-
sure fluctuations within the water distribution system. Typically, minimum acceptable
water system pressures are 35 to 40 psi (240 to 275 kPa), and maximum pressures are
100 to 120 psi (690 to 825 kPa). A minimum system pressure of 35 to 40 psi (240 to
275 kPa) ensures adequate flows to the individual consumers and allows for reasonable
operation of home-type irrigation / sprinkler systems. A more desirable low system
pressure may be in the range of 50 psi (345 kPa). This level of low system pressure
provides adequate flows and working pressures for most typical residential and com-
mercial uses. Maximum pressure limitations are desirable to minimize the additional
cost of providing piping materials with adequate strength to cope with the high pres-
sure. In addition, high pressures can injure existing residential and commercial plumb-
ing systems. Where main pressures exceed 100 to 110 psi (690 to 760 kPa), individual
pressure-reducing valves should be installed on each service.
Minimizing the pressure fluctuations in the distribution system provides reliability
for the consumer. Irrigation or in-house water-based appliances operate more consis-
tently when pressure fluctuations do not exceed certain limits. Wide system pressure
variations make proper design and selection of these appliances difficult and can also
create operational difficulties. Typical goals for pressure variation limitation at any
single point in a distribution system is 10 to 15 psi (69 to 103 kPa).
Water system design may be based entirely on a hydraulic analysis that selects pipe
sizes to ensure adequate pressures within all areas of the water distribution system
under a set of system demand or loading conditions. For some water systems, a detailed
hydraulic analysis may be done for every development area or main extension. In other
systems, design may be based on a combination of minimum design standards and
hydraulic analysis. Hydraulic analyses may be as complex as extensive modeling of
the water distribution system, or as simple as a hydraulic evaluation of a single pipe
utilizing standard hydraulic tables.
WATER DISTRIBUTION SYSTEM MODELING AND HYDRAULIC ANALYSIS
Modeling Overview
Water distribution system modeling and hydraulic analysis are based on fluid mechan-
ics. The analysis and the simulations rely on basic equations used to determine friction
headloss. Equations that have been applied in hydraulic analysis and simulation include
the Darcy-Weisbach, Hazen-Williams, Manning, and Darcy-Weisbach / Colebrook-
White formula pipe flow equations. The fundamentals of fluid mechanics are described
in several basic texts.
3-5
The design of a single pipe or main may be accomplished using any of the above
equations or by the use of standard hydraulic tables or nomographs based on the above
equations.
6
A multiple-pipe network branching to form complex, looped systems re-
