The happy zone

Now wc can see the importance of the concentric ellipses of efficiency on the pump family curve. As much as possible we should find a pump whose primary efficiency arc covers the needs of the system. Certainly the needs of the system should fall within the second or third efficiency arcs around the pump's BEP. If the system's needs require the pump to consistently run too far to the left or right extremes on its curve, it may be best to consider pumps in parallel, or series, or a combination of the two, or some other arrangement, possibly a PD pump. We'll see this later.

As elevations change in the process of draining one tank and filling another, the pump moves on its curve from one elevation extreme to the other. If we've selected the right pump for the system, it will move from one extreme of its happy zone, through the BEP to the other extreme.


This is the beginning of many problems with pumps. A pump is specified with the BEP at one set of system coordinates. Then the system (the TDH) goes dynamic, changing, and the pump moves on its curve away from its BEP out to one or the other extreme. It is necessary to determine the maximum and minimum elevations in the system and design the pump within these elevations. If the system continues to change on the pump, you'll either have to modify the system or modify or change the pump, unless you really like to change bearings and seals.

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