Page 28 - Buried Pipe Design

6 Chapter One

small concentration of solids found in sanitary or storm sewage is
not sufficient to make it behave significantly different from water
hydraulically. Thus, sewage is accepted to have the same hydraulic
flow characteristics as water. Of course, the design engineer must
be aware of the possibility of the deposition of solids and hydrogen
sulfide gas generation in sanitary sewers. These considerations are
not within the scope of this text. In either case, pressure flow or open-
channel flow, the fluid encounters frictional resistance. This resistance
produces head loss, which is a function of the inside surface finish or
pipe roughness. The smoother the inside surface, the better the flow.
Many theories and empirical equations have been developed to
describe flow in pipes. The solution of most flow problems requires
experimentally derived coefficients which are used in conjunction with
empirical equations. For pressure flow, the Hazen-Williams equation
is widely accepted. Another equation that has a more theoretical basis
is attributed to Darcy and Weisback. For open-channel flow, the
Manning equation is normally used. These equations, or others, are
used to calculate head loss as a function of flow or vice versa.

Water Systems
Water systems are lifelines of communities. They consist of such items
as valves, fittings, thrust restraints, pumps, reservoirs, and, of course,
pipes and other miscellaneous appurtenances. The water system is
sometimes divided into two parts: the transmission lines and the dis-
tribution system. The transmission system is that part of the system
which brings water from the source to the distribution system.
Transmission lines have few, if any, interconnections. Because of this,
flow in such a line is usually considered to be quasi-steady with only
relatively small transients. Such lines are normally placed in fairly
shallow soil cover. The prime design consideration is internal pressure.
Other design considerations include longitudinal stresses, ring deflec-
tion, buckling, and thrust restraints.
The distribution piping system distributes water to the various
users. It includes many connections, loops, and so forth. The design is
somewhat similar to that of transmission lines except that a substan-
tial surge allowance for possible water hammer is included in the pres-
sure design. Also, greater care is usually taken in designing the
backfill for around the pipe, fittings, and connections. This is done to
prevent longitudinal bending and differential settlement. Distribution
systems are made up of an interconnected pipe network. The hydraulic
analysis of such a system is almost impossible by “hand” methods, but
is readily accomplished using programming methods via digital
computers.