Pressure‐dependent demand under pressure‐deficient conditions

This article shows that for pressure‐dependent demand (PDD) modeling, as pressures become very low, not only does the decrease in pressure affect outflow from orifices (faucets, showers, leaks), but it can leave some orifices above the hydraulic grade. The orifices above the hydraulic grade line would not have flowed at all. This has implications for the PDD function used in some hydraulic models in that a simple orifice equation cannot accurately describe such nodal demands. This article presents the results of lab experiments that measure the effect of very low pressures on demands. An equation that accounts for the influence of very low pressure and orifice elevation is proposed and verified. The impact of this equation on modeling low‐pressure situations is presented.