System Head Curves in SewerCAD

Question

I am modeling a duplex configuration in a wet well, with a force main that discharges to a creek. I am confused why the system head curve for one pump is different than the system head curve calculated for the pump station. 
 The results provided in the Flex tables align with my own manual calculations, as well as the system head curve for one pump. I am unsure why the system curve changes when selecting one pump vs the pump station. 
 Any insight would be appreciated!

Tom Walski · Accepted Answer

I assume you are using combination pump curves. 
 When you run one pump, the velocity is lower than when you run two. When you run two, the head loss increases as the velocity squared. 
 Is that what you are observing?

Jesse Dringoli · Answer

Hi Charlie, 
 Without seeing the model plus steps to reproduce I cannot say for sure, but most likely you are encountering the situation that is the topic of the following article: 
 System head curve changing depending on status of other pumps 
 When you have multiple pumps in parallel and are looking at the system head curve for one, it is routing flow through that particular pump but is still respecting the status of everything else in the modeling including any other pumps in parallel. If one of those other pumps is set to be initially on, the head provided by that pump will impact the system head curve of the other pump. It is best to turn off all other pumps in parallel. 
 If you are referring to the Combination Pump curve tool where you select a "representative pump", see the section near the bottom of this article .

System Head Curves in SewerCAD

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