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Why is it not possible to use a "constant power" pump definition type with a variable speed pump? Why doesn't the program allow the model to be computed in this case?
As the name implies, a constant power pump curve needs to work with a constant speed pump.
Constant power pump curves are less accurate and should be avoided whenever possible, as mentioned in the Help. These types of pump curves are a crude approximation that don't work very well with the Affinity laws, so it is not supported with variable speed pumps. It has a concave shape, where it goes off to infinity if flow or head approach zero.
This type of pump curve was available in EPANET and KYPIPE, so it has been available in WaterCAD and WaterGEMS to support users who may need to use it for a quick check. If you need to model a variable speed pump, you will need to enter the real, full pump curve using one of the other methods such as 3-point or multi-point.