Page 291 - Corrosion Engineering Principles and Practice

264 C h a p t e r 8 C o r r o s i o n b y W a t e r 265

A pipeline must first be selected for analysis before any additional
work can be done on it.
A second important point is that the maintenance and inspection
strategy should differ between distribution and transmission systems
to reflect the much higher failure consequences for transmission
pipelines compared to those of a typical distribution line. Approaches
that may not be economically viable for a distribution system could
be employed advantageously in a transmission system to prevent
failures. Essentially, the low consequences associated with a single
distribution failure also mean that the emphasis in distribution
systems should be on failure management in order to minimize
life-cycle costs while a more proactive failure prevention approach
would be better suited for transmission systems due to the high
consequences of associated failures [6].

8.2.3 Condition Assessment Techniques
The pipes in water distribution and transmission systems are difficult
to inspect for damage due to their location below the surface of the
ground. This difficulty has led water utilities to rely on techniques
such as breakage records, leak detection, and water audits to
determine the health of their systems. While these techniques have
been shown to be very useful in prioritizing repairs and replacements,
they have the disadvantage of being reactive in nature. In each case,
problems with the water system only become apparent after the pipes
have failed in some manner [7].
Table 8.3 provides a comparative listing of the different techniques
for inspecting metallic water pipes. Each technique is named and the
relative advantages and disadvantages of the technique are given.

Technique Advantages Disadvantages
Zone water Cheap Does not give the precise
audits Covers large areas of a location of leaks
city quickly Requires isolation of zones
Allows for a comparison Work must be performed at
of water losses between night
individual districts Only gives an overview of
Useful as a screening current problems
process for other
techniques
Can be used to evaluate
the effectiveness of
repair programs

TABLE 8.3 Comparison of Diagnostic Techniques for Metallic Water Mains [7]