Theoretical and Conceptual Framework - Pattern Recognition for Reliability Assessment of Water Distribution Networks - page 9

Table 2.1 Definitions of water distribution system reliability

Source Definitions

Cullinane et al. (1992) 'Reliability is the ability of the system to provide service with an acceptable level

of interruption in spite of abnormal conditions of water distribution system to meet
the demand that are placed on it.'

Gouter (1995) 'Reliability is the ability of a water distribution system to meet the demands that are

placed on it where such demands are specified in terms of the flows to be supplied
(total volume and flow rate) and the range of pressures at which the flows must be
provided.'

Xu & Goulter (1999) 'Reliability is the ability of the network to provide an adequate supply to the

consumers, under both regular and irregular operating conditions.'

Tanyimboh et al. (2001) 'Reliability is the time-averaged value of the flow supplied to the flow required.'

Lansey (2002) 'Reliability is the probability that a system performs its mission under a specified

set of constraints for a given period of time in a specified environment.'

Low pressure is unanimously considered as the primary indicator of poor hydraulic
performance/level of service, leading to almost certain demand reduction. This can be a result
of inadequate operation, for instance an insufficient pumping caused by electricity failure.
Furthermore, in many distribution systems, low pressures occur as a result of scarce water
source. Common denominator for both of these cases is that the consumer demand has
exceeded the supply. On a longer term, this can also happen from ageing of the system and/or
the growth of population. If, on the other hand, a certain level of service is not satisfied due to
poor condition of the network resulting in frequent failures of its components, such situation
will be described by so-called mechanical reliability . Component failures in distribution
systems involve pipe bursts, blockage of valves, failure of pumping stations, etc., which all
reduce the delivery capacity of the network such that it is no longer able to meet the required
service level. Pipes as major network components are subject to structural deterioration
because of physical, environmental and operational stresses leading to a failure, as shown in
Table 2.2 (Source: NGSMI, 2002).

Mathematically, the mechanical reliability R(t) of a component is defined as the probability
that the component experiences no failures during an interval from time 0 to time t . In other
words, the reliability is the probability that the time to the failure T exceeds t . The formula for
R(t) is:

∞

R ( t ) =

∫

f ( t ) dt

2.1

t

where R(t) is the reliability factor, having the values between 0 and 1, and f(t) is the
probability density function of the time to the failure, which can be developed from the
failure records.

This concept of reliability is meant for so called non-repairable components, in which the
component has to be replaced after it fails. Nevertheless, most of the components in water
distribution systems are generally repairable and can be put back into operation. It seems
therefore more appropriate to use the concept of component availability as a surrogate.