Page 239 - Handbook of Materials Failure Analysis

5 Examples of Serious Failures of Water Supply and Wastewater 235

According to the adopted classification of researched objects, arterial conduits (A)
and distribution conduits (D) were distinguished within the water supply network.
Within the network, 1921 failures were recorded during the entire observation
period. The majority of failures (96%) occurred on distribution conduits, while only
4% affected arterial conduits. The number of failures alone is insufficient to perform
a comprehensive assessment; although it is an important piece of general information
about susceptibility of a water supply network to failure. The susceptibility of a net-
work is also a function of its length. Therefore, the average unit number of failures
per kilometer per year is decidedly a better index for the purposes of assessment. The
failure rate amounted to 0.06 for 244 km of arterial conduits and to 0.31 for 1184 km
of distribution conduits.
An analysis was performed of types and effects of damages to conduits related the
material implemented to manufacture them. A preliminary data analysis showed that
damages to conduits made of thermoplastics. (PVC, PE, and polypropylene; PP) and
of asbestos cement required mainly the shutting-off of a damaged section, which
resulted in discontinuation of water supply to consumers. Due to limited data avail-
ability, the relations of types and effects of damages are presented for the conduits
that are most susceptible to failure, that is, for cast iron and steel conduits (Tables 9.9
and 9.10).
It follows from the analysis of types and effects of damages to cast iron and steel
conduits that:
Conduits made of steel were primarily affected by corrosion (74% of failures),
and only every fourth failure affecting these conduits did not result in a
discontinuation of water supply to consumers.
The highest number of failures was recorded for cast iron conduits. The failures
were caused by a wide variety of damages, primarily longitudinal and transversal
cracking responsible for the highest number of failures. Every fifth reported
failure was caused by forced-out seals, while every seventh failure resulted from
a mechanical damage. In 88% of cases, the damages resulted in excluding a
conduit from operation.

The analysis of types of failures of other conduits allows one to formulate the fol-
lowing conclusions:

PVC conduits were affected by mechanical damages (over 3/4 of all failures).
To a much lower extent were they affected by transversal and longitudinal
cracking and by forcing the sealing material out of pipe sockets. In 99% of
cases, the failures affecting PVC conduits resulted in excluding a conduit
section from operation.
PE pipelines were affected by mechanical damages as well as longitudinal and
transversal cracking.
Pipelines made of asbestos cement were affected primarily by damages in the
form of transversal and longitudinal cracks (51% and 31%, respectively). Apart
from that, cases of forced-out seals and mechanical damages were recorded.