Page 232 - Handbook of Materials Failure Analysis

228 CHAPTER 9 Reliability and failure analysis of wastewater systems

Theoretically, as a result of material wear and constant impact of physical and chem-
ical factors as well as internal and external loads, mechanical properties of a conduit
can be expected to undergo natural deterioration in time, thereby increasing conduit
susceptibility to failure. This rule, however, is not confirmed by observations. An
increase of the surface roughness of internal walls is also an important factor related
to the actual service life, although this correlation is not unequivocal. Much research
attempted to demonstrate a correlation between susceptibility of conduits to failure
and their actual age, yet the results were not apparent.
It is worth pointing out that it is difficult to unequivocally determine the impact of
all the factors on the susceptibility of conduits to failure, which is due to the fact that
these factors most often operate jointly and generate interdependent chain of events.
It is helpful to use GIS databases that allow one to create datasets that describe the
status of damages, their causes and effects, as well as to collect indispensable doc-
umentation that allows one to characterize failures in a more detailed manner.
The analysis of susceptibility to failure attempts to consider the most important
factors that influence the susceptibility of the water supply and wastewater disposal
networks to failure. The results of research acquired up to now provide an overview
of the susceptibility of conduits to failure, correlated with the type of implemented
materials, as the acquired failure rates reflect a cumulative impact of various factors
on the susceptibility of a single element (e.g., a conduit) to failure.

4 RELIABILITY OF WATER SUPPLY AND WASTEWATER
DISPOSAL INFRASTRUCTURE
Conduits of both water supply and wastewater disposal networks operate in highly
variable hydraulic conditions. This fact contributes to an increase in their suscepti-
bility to failure, as well as to the diversity of conduit damages to be faced, compared
to transit (transmission) conduits.
In the case of steel pipes, the following defects were often identified: cracks, per-
forations, and connection leakages. Similar defects were identified in conduits made
of gray cast iron: connection leakages were identified more often in higher diameter
conduits, while more cracks and perforations were identified on smaller diameter
conduits. PVC conduits—both pipes and fittings—suffered most often from longi-
tudinal and transversal cracking. Failures of hub and spigot joints were found to suf-
fer damages at equal frequency (due to defective seals). In the case of PE conduits
assembled at the beginning of the 1990s, the research identified weld cracks and spo-
radic fitting cracks. Table 9.3 presents the most frequent failures occurring in a water
supply network, as correlated with implemented materials.
Susceptibility of conduits to failure is clearly related to the implemented struc-
tural and material solution that is defined by the used material and the implemented
conduit connection method.
In Table 9.4, one can also notice a widespread of the failure rates of water supply
conduits made of the same material and installed in different towns. This is due to