Reliability/availability analysis of municipal water distribution networks: Case studies

Reliability/availability analyses are performed on two large-scale municipal water distribution networks in the Tucson Metropolitan Area in Arizona. Reliability is defined as the probability of satisfying nodal demands and pressure heads for various possible pipe failures (breaks) in the water distribution system at any given time. An existing reliability model based on a minimum cut-set method is linked to a steady-state simulation model that implicitly solves the continuity and energy equations. The results from the simulation model are used in the reliability model to define minimum cut sets and determine the values of system and nodal reliability. Availability is the proportion of time the system adequately satisfies the demands upon it. This more practical measure combines hydraulic and mechanical availability in a form that defines the proportion of the time that the system will satisfactorily fulfill its function. In addition to a simple discrete failure relationship with absolute failure if pressure heads fall below a prescribed minimum, hydraulic availability is modelled with a continuous increasing acceptability with higher pressures. Comparisons of results illustrate the similarities and differences in the design of each system under varying operating conditions. Design alernatives to improve reliability are discussed.