Page 230 - Handbook of Materials Failure Analysis

226 CHAPTER 9 Reliability and failure analysis of wastewater systems

higher the reliability. This index is credible, because it can be assessed on the basis of
data collected during system service.
The data required to carry out the analysis can be broken down into two groups:
data concerning damages and data identifying damaged objects. The first group
covers the date and time of a failure event, duration of the failure, type of failure,
and its result. The second group covers data concerning the damaged element: the
conduit type, its location, the material, or diameter of a damaged conduit.
The type, effect, and cause of the susceptibility of a conduit to failure result not
only from the method of its utilization, but they often result from errors in workman-
ship as well as in design. Apart from that, damages may be caused by internal causes,
that is, originating inside a conduit, or external ones that operate from outside of the
network. However, the effects of all such causes that take the final form of conduit
damages generally become manifest at the final stage, that is, during the process of
conduit operation. Assessment of susceptibility of conduits to failure is based on
operational tests of their reliability that take the form of a natural experiment, carried
out in real conditions of networks functioning and operational. However, analyses of
data concerning susceptibility to failure often face the problem of absence or incom-
pleteness of relevant data. This is related to various deficiencies of datasets, such as
impossibility of distinguishing between conduits made of similar materials, missing
updated data on failure duration, uncertain identification of a failure event yielding
an attempted interpretation. Different situations are described utilizing the same
code, which makes it impossible for a failure event to be properly classified. More-
over, it is impossible to determine the age of damaged conduits, etc. [14,18,19].
As the data concerning damages is incomplete, there arises a need to create sep-
arate subsets of data required for particular analyses. Therefore, particular analyses
are based on datasets that consist of different numbers of elements and do not have to
balance the datasets of higher order.
Generally, it is proposed to classify the factors that influence susceptibility of
conduits to failure into the following categories according to [14]:

A: conduit-related factors:
• Assembly quality (especially the quality of connections between conduit sections)
• Material and method of anticorrosion protection
• Function and diameter
• Number and type of connections and fittings
• Age
• Pressure and pressure fluctuations (water supply system); fill-level (wastewater
system)
• Water quality (corrosively); corrosiveness, aggressiveness (wastewater)
B: factors related to conduit environment:
• Soil type, aggressiveness, and humidity
• External loads (dynamic and static)
• Ground instability (seismic or mining damage)