With a pump, it will shift on its curve based on the system conditions. When using a negative demand on a junction, it will always inject that flow rate regardless of the system conditions. A negative is like a known in the equation, similar to fixed positive demands and fixed boundary conditions - you tell the program the flow you want to leave (positive demands) the flow you want to inject (negative demands) and the hydraulic grade at reservoirs and tanks. The program then solves the rest of the results around those "knowns" in the equation, physical properties and controls. Let's say you have a model like this: Negative demand pipe reservoir at 100 m. If the negative demand that you entered results in a headloss in that given pipe of 2.0 meters, then the model will solve the hydraulic grade at the negative demand location as 102 m. This is the HGL is needs to be in order to satisfy those "knowns" (reservoir at 100 m, the negative demand you entered, pipe size/roughness).
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