Before you get into anything as sophisticated as Darwin Calibrator, you need to understand how the model uses pressure data. Pressure is not an input to WaterCAD. WaterCAD calculates pressure as an output based on the system description. If you measure 25 psi, the model result for that point should be close to 25 psi. If the model gives you a much different value, you need to determine why. It could be an inaccurate elevation, pump curve, or pipe roughness for example. You need to use your knowledge of hydraulics to determine why the values differ and make the appropriate correction. In your case, it sounds as if you are designing a pump station to tap into an existing force main. You really should model the entire force main system. If you do, you will find that the pressure at the tapping point varies all over depending on which other pumps are running in that force main. It will not be a constant value. You really should consider the full range of condition that your new pump station will see. For example, at night on a dry day, you may be pumping against 2 psi while at peak hour on a wet day, all the other pumps on the force main may be running and you may be pumping against 40 psi. On the other hand the variation in pressure may be 22 psi to 27 psi. Unless you understand the dynamics of the force main, you may end up with a poor pump selection. A lot depends on the scale of the problem. If you are talking about a 10 gpm pump, just make sure the pump is big enough. If you are dealing with 1000 gpm pump, then energy costs and efficiency become very important. You may need to use a variable speed pump if the range of discharge pressures is large. The way I would analyze this is to generate a full range of system head curves for the new station and select pumps that can run efficiently over that range.
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