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CHAPTER 3. Assembling the model
Modeling a pumping station Don’t add pumps at this stage even if they are included in the network. This is because the pipes are dimensioned first and the network stabilized. Once this is done the suitable pumps are added so they work for that specific network. Wherever the pump goes put instead an imaginary reservoir at a certain height and proceed as if it was a gravity system. Work with the pipes until you can lower the tank as far down the hill as possible. The height of the hill will be the water pumping head (the height the pump should be able to lift), at it will be more economical to pump to lower altitudes. Look at the image of the section Skeletonization further ahead, and this is exactly what has been done, and why there are so many reservoirs. Here is an example highlighting some points: •
The type of pump (centrifuge, horizontal, submersible) does not affect the simulation if the intake of the pump has enough water pressure so that the pump does not suck in air or produce it (google NPSH for more info).
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If there are big demand variations, several pumps of different sizes are used instead of a bigger one so that they can be turned on and off depending on the required discharge of the moment.
Modeling a submersible pump for a borehole Let’s design a very simple network in a flat area, let’s say in Eritrea. The water is taken from a borehole and is directly distributed to four points. 1. Start drawing this network without connecting the borehole (it will be modeled as a reservoir). 2. Enter the following data: 0m for elevations, 100m for pipe lengths and, to start, 200 mm for pipe diameters. 3. Assign the maximum pressure you want for the system to the reservoir, above the height of the lowest point. Let’s say that I want 3 bars, equivalent to 30m of head. Because the lowest height is 0, the reservoir elevation is 0 m + 30 m = 30 m.
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