Development of a generalized mixed integer nonlinear programming model for assessing and improving the operational flexibility of water network designs

In designing and operating any water network, a typical issue that must be addressed is concerned with the uncertain process conditions. A systematic procedure is proposed in this paper to enhance the operational flexibility of a given structure. Specifically, two design options have been investigated: (1) relaxation of the allowed maximum freshwater consumption rate in the nominal design and (2) installation of auxiliary pipelines and/or elimination of existing ones. The flexibility index model proposed by Swaney and Grossmann (1985) has been modified and formulated in a generalized format to evaluate the impact of introducing various modifications into an existing network. Since this model is a mixed integer nonlinear program (MINLP), it is in general very difficult to locate the global optima in the iterative solution processes. A simple and effective initialization procedure has also been devised in this work to facilitate successful convergence. The effectiveness of the proposed approach is demonstrated with two examples in this paper.