Design of Branched Pipe Networks Using Reliability and Total Annual Cost

The branched and looped type networks are the two basic configurations of water distribution. The branched pipe networks are generally designed by traditional and optimization methods. In the traditional method, a pipeline is designed by balancing the available energy against the loss of energy due to friction, whereas in the optimization methods, the cost is minimized. For ensuring regular water supply, it is essential to consider the reliability of the water distribution network, which depends on the failure rate of pipes, quality and quantity of water available at the source, and pressure requirements at demand nodes. Therefore, the reliability of the system needs to be incorporated while minimizing the total cost of the distribution system. In this paper, an enumerative method using both the reliability and total annual cost is developed for designing the branched pipe networks. The method satisfies pressure and discharges requirements along with the recommended values of the minimum and maximum velocities in the pipes. In this method, different sets of pipe sizes are taken to calculate the reliability and cost of the network, and that set of the pipe network is selected, which has the highest ratio of reliability to cost. To calculate the ratio of reliability to cost, the capital cost or the total annual cost can be used. The total annual cost is the sum of the annual fixed and annual energy costs. The applicability of the developed method is illustrated with the help of a design example. It was found that the reliability of the branched network varied with the total annual cost, as well as the capital cost, of the network for different sets of pipe diameters in the network. Further, it was found that the reliability for the designed network using total annual cost was higher than using capital cost. Therefore, the total annual cost should be preferred over the capital cost in the design to establish the balance between reliability and economy. The presented method provides a straightforward approach for water supply engineers to determine economical as well as reliable branched pipe networks.