Environmental Engineering Reference
In-Depth Information
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figure 2.17 Pipes in series
Unfortunately, this ideal single-pipe scenario is not practical for real-world
applications. Instead of a single piping system, a network of pipes is laid
under the streets. Each of these piping networks is composed of different
materials that vary (sometimes considerably) in diameter, length, and age.
These networks range in complexity to varying degrees, and each of these
joined-together pipes contributes energy losses to the system.
2.11.1.1 Energy Losses in Pipe Networks
Wastewater flow networks may consist of pipes arranged in series,
parallel, or some complicated combination. In any case, an evaluation of
friction losses for the flows is based on energy conservation principles
applied to the flow junction points. Methods of computation depend on
the particular piping configuration. In general, however, they involve
establishing a sufficient number of simultaneous equations or employ-
ing a friction loss formula where the friction coefficient depends only on
the roughness of the pipe (e.g., see Equation 2.22, the Hazen-Williams
equation).
Note: Demonstrating the procedure for making these complex computa-
tions is beyond the scope of this text. We only present the operator “need
to know” aspects of complex or compound piping systems in this text.
2.11.1.2 Pipes in Series
When two pipes of different sizes or roughnesses are connected in
series (see Figure 2.17), head loss for a given discharge, or discharge for
a given head loss, may be calculated by applying the appropriate equa-
tion between the bonding points, taking into account all losses in the
interval. Thus, head losses are cumulative. Series pipes may be treated
as a single pipe of constant diameter to simplify the calculation of fric-
tion losses. The approach involves determining an “equivalent length” of
a constant-diameter pipe that has the same friction loss and discharge
characteristics as the actual series pipe system. In addition, application
of the continuity equation to the solution allows the head loss to be
expressed in terms of only one pipe size.
Note: In addition to the head loss caused by friction between the water
and the pipe wall, losses are also caused by obstructions in the line,
changes in directions, and changes in flow area. In practice, the method
of equivalent length is often used to determine these losses. The method
of equivalent length uses a table to convert each valve or fitting into an
equivalent length of straight pipe.
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