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I am trying to represent a pump that I do not have a pump curve for. I have one person advocating for simply using a negative demand but I am afraid this won't capture the actual pump performance as determined by the system conditions that will vary. Rather, I am thinking it would be more appropriate to use a one point design curve that has been somewhat "calibrated" to match hydrant tests/system pressures. Which would be the correct or more accurate way to do this in WaterCAD?
There is nothing better than real data. Doesn't anybody in your organization have a pressure gage? You can estimate the flow if you can do a draw down from a tank.
If you can't do that, you should at least know the nominal flow and head and you can use the nominal rated capacity when the pump was new and use a one point pump curve. Yo can adjust that during calibration.
The negative demand approach may be acceptable to the extent that conditions in the system remain constant over time.