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When designing a closed system with only demands downstream (no storage), how can the program help me select a pump? The system head curve doesn't look right in this case.
There are a few options to consider:
1) Insert a pump node element with an estimated pump curve (it doesn't matter since the system head curve is independent), then run the system head curve tool (right click the pump > System head curve) with downstream demands set as Pressure Dependent (PDD). See below articles for more information on PDD and why it is needed in this case, if wanting to see a system head curve:
Using Pressure Dependent Demands
System head curves with no downstream storage
2) Look at the sum of the downstream demands for typical conditions (you can see this as "flow demanded" in the Calculation Summary), then select a pump curve whose best efficiency point is near that flow, with a corresponding head that would provide adequate system pressures. Try the pump curve in the model and check the results.
3) Consider a variable speed pump.
There is some additional advice on this on page 312-313 of the book Advanced Water Distribution Modeling and Management.
General Pump Selection Process
System head curves with no downstream storage (Solution 500000056093)