Theoretical and Conceptual Framework - Pattern Recognition for Reliability Assessment of Water Distribution Networks - page 11

of the system components. Evaluating the pipe itself, it could be considered as a non-
repairable component if it has to be replaced by a new one, while in the evaluation of the pipe
as a component of the water distribution system, it is repairable i.e. available component.

The availability of a component can also be expressed as a percentage of the time during
which the component is in operational state. The remaining is the time when the component
is not functional, most frequently due to a failure or some maintenance. There are two basic
categories of maintenance events:
-

Corrective maintenance , which includes repair after a breakdown or unscheduled
maintenance resulting from the equipment failure. This additional information should be
available from the maintenance records, depending on the pipe material and its condition;
the records can be summarised in the form of a diagram as shown in Figure 2.1 (source:
Vreeburg and van den Hoven, 1994).

- Preventive maintenance , which includes regular activities to prevent breakdowns before

they occur.

Figure 2.1 Pipe bursts frequency; example from The Netherlands (Vreeburg and van den Hoven, 1994)

On annual basis, the component availability can be calculated as:

8760 − CMT − PMT

A =

2.2

8760

where A stands for component availability, CMT is annual corrective maintenance time in
hours, and PMT is annual preventive maintenance time in hours.

1

0.9

0.8

0.7

Availability - A(t)

0.6

0.5

0.4

0.3

0.2

Reliability - R(t)

0.1

0

0 10 20 30 40 50 60

Time - t (years)

Figure 2.2 Reliability and availability trends (adapted from Tung, 1996)