Why do I get a negative pressure at a high point in my system? Shouldn the pump add enough head to push the water over the hill?
By default, pumps only consider the boundary conditions (reservoirs and tank elevations) in your system. So, the pump will add enough head to lift the water to the downstream known hydraulic grade. It does not consider junction elevations inbetween. If you are using WaterCAD or WaterGEMS V8i, you should add an Air Valve element at the high point to properly model this situation. By placing an air valve at the high point, the pump sees the air valve elevation as its downstream boundary condition for instances in which pressure would have otherwise been negative at the high point. For any air valve that is expected to be open in this way, ensure that you select “false” for the “Treat air valve as junction?” attribute.
