ABSTRACT
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:
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.
