Page 243 - Handbook of Materials Failure Analysis

6 Conclusion 239

Seasonal temperature changes mainly influenced the susceptibility to failure
of distribution conduits. A clear correlation was observed in the case of cast
iron conduits: 70% of failure events occurred in autumn and winter. In winter
months, pipelines were damaged primarily due to transversal and longitudinal
cracking and to forcing out of seals. In summer months, in turn, they were
affected by longitudinal cracking, mechanical damages and transversal cracking.
Groundwater was affected by every fifth failure in winter and every sixth
failure in the summer.

Irrespective of the season, when a failure of the water supply network occurred:

Over a half of failure events concerned conduits laid in boulder clays, while
every fourth failure of the water supply network affected pipelines laid in
fluvioglacial and glacial sands. Other soil types did not show substantial
correlation with the susceptibility of water supply conduits to failure.
Over 80% of failure events affected conduits laid above the groundwater level
and only 18% affected conduits operating below groundwater table. This
tendency is confirmed for cast iron conduits and for conduits made of PVC.
The above analysis used the criterion for assessing susceptibility to failure presented
in Table 9.5. On the basis of performed calculations, it was discovered that arterial
conduits are characterized by low susceptibility to failure, while distribution
conduits—by medium susceptibility to failure. 0.06 failures per kilometer per year
affect the former category of conduits, while the latter category is affected by five
times more failures, that is, 0.31 failures per kilometer per year.
In terms of the material used to manufacture conduits, cast iron conduits are char-
acterized by the highest susceptibility to failure amounting to 0.43 failures/km year.
Next in the ranking of susceptibility to failure correlated with the length of pipelines
are conduits of asbestos cement and steel conduits. Conduits made of plastics—
namely PVC, PE, and PP—are characterized by the lowest susceptibility to failure.
According to the adopted classification scheme, only PE conduits fall into the cat-
egory of conduits with low susceptibility to failure.
The distribution of failures of water supply network as a function of the month of
a failure event shows that seasonal variations (of temperature) have an impact on
network failure frequency. The highest failure rate was observed in winter months,
that is, January, February, March, and April. The lowest number of failures per kilo-
meter was recorded in June.

6 CONCLUSION
The foregoing analysis based on the results acquired during network operations
yielded a highly diversified outcome for similar or even identical objects under
investigation. Therefore, the most objective approach consists in presenting the
range of values describing susceptibility of these materials to failure. In the case