Modeling of Pressure-Deficient WDNs in the Presence of Air Intrusion Based on Fictitious Source Nodes and Pipe Roughness Corrections

This paper presents a novel methodology for the simulation of water distribution networks (WDNs) in the presence of pipes operating in free surface flow in the case of low service pressure. Under the realistic assumption of pipe permeability to air, it overcomes the problems encountered by most hydraulic solvers, which can yield unrealistically negative pressure heads at demand nodes in scenarios causing low service pressure, such as segment isolations and flow control valve activations. Compared to state of the art, it presents the advantage of simplicity and adaptability to all hydraulic solvers, as it does not require changes in the basic equations. In fact, it is based on the conversion of pressure-deficient demand nodes into fictitious source nodes and on the conductance/roughness correction on pipes fed by these nodes, which actually operate under free surface flow conditions, for an accurate evaluation of flow and head drop on these pipes. Applications to three case studies of growing complexity prove the numerical effectiveness and efficiency of the novel methodology, which is implemented in the EPANET 2.2 environment. Some remarks about the nonuniqueness of the solution to pressure-deficient WDN modeling, depending on the chosen criterion for placement of fictitious source nodes, are also made.