Both rigid column theory and elastic or water hammer analysis will be applied in this
chapter to the solution of unsteady flows in pipe networks. Simplified solutions that
ignore both inertial and elastic effects in pipe networks were covered in Chapter 6 under the
name "extended time simulations."
What conditions require the full consideration of inertial effects, and what situations
will make the elastic properties of the liquid and pipe so important that a full water
hammer network analysis is necessary? When is an extended time simulation sufficient?
There are no precise answers for these questions. The next few paragraphs mention some
relevant factors in making such a decision, but in the end professional judgment and
personal experience are also factors.
An elastic analysis is required whenever the changes in velocity are sufficiently rapid to
cause substantial changes in the flow variables over time intervals that are less than several
times the value of L/a for the pipe(s) under investigation. Examples are the rapid closure
of a valve, the filling of a pipeline with liquid that moves at high velocity and forces air
from the lines, an abrupt change in the operation of pumps, and in general any event that
is sufficiently rapid to prevent the fluid throughout the network from gradually
accommodating the change. However, the occurrence of a rapid change in a single pipe
does not necessarily mean that an elastic analysis of the entire network is warranted. When
demands are changing throughout a large distribution system, large pressure changes will
alter the system demands so the pressure wave is rapidly absorbed. In this case the need for
an elastic analysis may be restricted to that pipe, or possibly to it and a few nearby pipes.
Rigid column theory can be applied to situations in which the demands on an elastic
pipe network change rather rapidly but not instantaneously, causi