Theoretical and Conceptual Framework - Pattern Recognition for Reliability Assessment of Water Distribution Networks - page 10

Table 2.2 Factors that contribute to water system deterioration (Source: NGSMI, 2002)

Physical factors Impact /Cause

Pipe material Pipes made from different materials fail in different ways.*

Pipe wall thickness Corrosion will penetrate thinner walled pipe more quickly.

Pipe age Effects of pipe degradation become more apparent over time.

Pipe vintage Pipes made at a particular time and place may be more vulnerable to failure.

Pipe diameter Small diameter pipes are more susceptible to beam failure.

Type of joints Some types of joints have experienced premature failure.

Thrust restraint Inadequate restraint can increase longitudinal stresses.

Pipe lining and coating Lined and coated pipes are less susceptible to corrosion.

Dissimilar metals Dissimilar metals are susceptible to galvanic corrosion.

Pipe installation Poor installation practices can damage pipes, making them vulnerable to failure.

Pipe manufacture Defects in pipe walls produced by manufacturing errors can make pipes

vulnerable to failure.

Environmental factors Impact /Cause

Pipe bedding Improper bedding may result in premature pipe failure.

Trench backfill Some backfill materials are corrosive or frost susceptible.

Soil type Some soils are corrosive; some soils experience significant volume changes in

response to moisture changes, resulting in changes to pipe loading. Presence of
hydrocarbons and solvents in soil may result in some pipe deterioration.

Groundwater Some groundwater is aggressive toward certain pipe materials.

Climate Climate influences frost penetration and soil moisture.

Pipe location Migration of road salt into soil can increase the rate of corrosion.

Disturbances Underground disturbances in the immediate vicinity of an existing pipe can lead

to actual damage or changes in the support and loading structure on the pipe.

Stray electrical currents Stray currents cause electrolytic corrosion.

Seismic activity Seismic activity Seismic activity can increase stresses on pipe and cause pressure

surges.

Operational factors Impact /Cause

Internal water pressure,
transient pressure

Changes to internal water pressure will change stresses acting on the pipe.

Leakage Leakage erodes pipe bedding and increases soil moisture in the pipe zone.

Water quality Some water is aggressive, promoting corrosion.

Flow velocity Rate of internal corrosion is greater in unlined dead-ended mains.
Backflow potential Cross connections with systems that do not contain potable water can
contaminate water distribution system.
O & M practices Poor practices can compromise structural integrity and water quality.
* More details can be found in Brandon (1984)

Whereas the reliability is the probability that the component experiences no failures during
the time interval 0 - t , the availability of a component is the probability that the component is
functional at time t , assuming that the component is as good as when it was new at time 0.
For example, after a segment of broken pipe is replaced, this pipe will function again as one