Page 244 - Handbook of Materials Failure Analysis

240 CHAPTER 9 Reliability and failure analysis of wastewater systems

of water supply networks, the lowest degree of susceptibility to failure (irrespective
of the number of factors affecting this value) is assigned to transit conduits and some
arterial conduits. The corresponding range of values of λ is 0.04-0.1 failure/km year.
Distribution conduits, in turn, are characterized by high susceptibility to failure that
oscillates around 1 failure/km year. When the conduit type is correlated with its func-
tion, one gets a correlation between failure rate and conduit diameter. This correla-
tion tends to assume decreasing values as the diameter increases. Irrespective of the
actual age and diameter of conduits, an increase of susceptibility to failure is also
correlated with changes of the temperature of the ground and water (or sewage, in
case of wastewater disposal systems) and to changes of air temperature. On the other
hand, the failure rate drops, when the pressure in a water supply network is reduced.
An example based on the data from Wroclaw: pressure reduction by 40% resulted in
the reduction of failure rate by about 41%, while pressure reduction by 20% in
Oles ´nica resulted in the reduction of the failure rates of conduits made of gray cast
iron by 53% and by 63% in the case of PVC conduits.
In terms of reliability, conduits made of thermoplastics (PE and PVC) have the
best results. They are characterized by the lowest failure rate, compared to conduits
made of steel and grey cast iron. It is worth underscoring that the investigated failure
rates of PE conduits in Poland—falling within the range between 0.02 and 0.69
failure/km year—describes the first generation of these pipelines, installed in the
period 1981-90. In other countries, the failure rate for pipelines made of the same
material falls within the range of 0.02-0.18 failure/km year, with very similar values
for PVC conduits.
The assessment the susceptibility of water supply and wastewater disposal
systems to failure was based on data relating to the vulnerability of particular
elements operating in real systems. The level of the system susceptibility to failure
is acceptable.

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