Accurate Demand Modeling for Surge Analysis

Surge modeling is important to safeguard against breaches in water distribution systems integrity. Its primary use is to predict and eliminate potential weak spots. To be effective, application of surge modeling in decision making requires accurate formulation of nodal demands, which directly affect both flow and pressure conditions in the distribution system. In this paper, a comparative study of two distinct demand formulations for surge analysis is presented. The first formulation assumes that the demands are independent of local pressures and can be met under all operating conditions. This approach is normally referred to as demand-driven (or pressure insensitive demand) analysis. In contrast, the second formulation expresses nodal demands as a function of nodal pressures and is termed head-driven (or pressure- sensitive) analysis. Each method is encoded into an existing distribution system transient model and applied to an example network to effectively compare transient analysis results. Results show that under transient conditions both positive and negative pressure surges can drastically alter the local pressures and affect the demand magnitude that can be extracted at the network nodes. The demand-driven formulation proved to be intrinsically inaccurate and may lead to overdesign of surge protection devices resulting in unnecessary additional costs. It is concluded that a pressure-sensitive demand formulation should be utilized for surge analysis to properly assess the impact of pressure changes and produce more accurate and useful transient results.