RELIABILITY IMPROVEMENTS IN DESIGN OF WATER DISTRIBUTION NETWORKS RECOGNIZING VALVE LOCATION

Water distribution networks can fail either by the actual demand at one or more nodes exceeding the design demands, or by a pipe between two nodes failing. The implications of each type of failure can be assessed by the shortfall in supply caused by a failure event together with the probability of occurrence of the event, and can be represented by the expected volume of deficit. Converting the implications of the two failure types into these commensurate units permits them to be added directly to give a single consistent measure of reliability. The assessment of shortfall for the pipe failure mode is derived from the observation that when a pipe breaks, a section of pipe must be isolated by valves to permit the repair to be made. Isolating the pipe also isolates the customers who withdraw water from that section of pipe. Thus the shortfall in supply in this case is based on the amount of supply (number of customers) cut off by isolating the pipe for repair. The measure extends previous reliability parameters by recognizing that in reality demand occurs along links rather than being concentrated solely at nodes at the ends of the links which is the normal assumption for both simulation and optimization models. If reliability of the network is found to be unsatisfactory, it can be improved in two ways. One is to increase the design demand at nodes so that the probability of actual demand's exceeding the design value is reduced. The other is to add more valves so that the length of pipe which has to be isolated in order to repair a pipe is reduced, thereby, reducing the number of customers who must have their supply cut off during a repair. Use of the two methods to determine and, if necessary, to improve reliability is demonstrated by their application to an example network.