Pressure Dependent Demand Accounting for Customer Properties

When using a hydraulic model with pressure dependent demands (PDD), the PDD function relating demand vs. pressure must represent a collection of orifices (e.g. sinks, toilets, showers, etc.) with a wide variety of orifice coefficients located at different elevations operating at different times. Modeling every orifice for every user is impractical, but some type of effective PDD function relating pressure and water use must be provided. This paper presents several approaches for creating functions that represent water use at the model node and reasonably approximate actual water consumption. When the elevation of the water using orifices differs from the elevation of the model node, the function that represents water use tends to vary from the expected exponent of 0.5, especially at low pressures. This paper explains those findings using both manual calculations and a hydraulic model.