Modeling Local Water Storages Delivering Customer-Demands in WDN Models

Water distribution network (WDN) models account for customer-demands as water withdrawals concentrated in nodes. Customer-demands can be assumed constant or varying with nodal head/pressure entailing demand-driven or pressure-driven simulation, respectively. In both cases, the direct connection of customer properties to the hydraulic system is implicitly assumed. Nonetheless, in many technical situations, the service pipe fills a local private storage (e.g. a roof tank or a basement tank) from which the water is actually delivered to customers by gravity or pumping systems. In such contexts, the service pipe fills the local tank by means of a top orifice. Consequently, what is really connected to the hydraulic system is a tank, which is subject to a filling/emptying process while supplying water to customers. Therefore, since modeling this technical situation in WDN analyses is necessary, the paper develops a formulation for nodal water withdrawals in WDN models accounting for the filling/emptying process of inline tanks between the hydraulic network and customers. The formulation is also introduced in a widely used method for steady-state WDN modeling, the Global Gradient Algorithm, and its effectiveness to increase the hydraulic accuracy of results is discussed using a simple case study and a small network.