Low Head (Pump)

Question

I am working on a simple model with just 3 pump stations and an outfall. One of the iterations I have to do states that all pumps be off except 1. The are no errors in the model and the flow (pump) is fine, but the head (pump) is less than 1 ft (0.78 ft.). Why is the head pump so low? Can someone please help me. I've attached the model as reference. 
 Canterbury Crossing-Stone Investors Iteration 4.stsw

Jesse Dringoli · Answer

Hi Marie, 
 I assume you are using SewerCAD or SewerGEMS. 
 I see you attached a .STSW file but all of the model data is stored inside the .STSW.SQLITE file so we'll need that file too, in order to open the model. Please go to File > Save to Package and attach the resulting ZIP file or use the ShareFile method described here to upload it (then reply here with the file name and steps to reproduce). 
 Generally speaking, the pump operating point (head and flow) are based on the intersection between the pump characteristic curve and the system head curve. If you are using SewerCAD (GVF-Convex solver), try generating a System Head Curve for insight into why the head is low. Also check the pump definition to make sure the pump curve is correct. 
 If you are pumping to a downstream gravity system, a low pump head generally indicates that the static and dynamic lift are low. For example the pipes could be oversized and/or the difference between the upstream wetwell water surface elevation and the downstream gravity main is very low. Viewing a profile of the system from the wetwell through the pump, to the outfall, may also be helpful. See: 
 
 How are pumps handled in each OpenFlows product? 
 Understanding System Head Curves in WaterGEMS, WaterCAD, and SewerCAD 
 Options for creating, viewing and exporting profiles

Jesse Dringoli · Answer

Thanks for providing the model. See my previous reply for the explanation - the static lift is actually negative in this case - you are pumping downhill, from the wetwell elevation of 47 m, to the outfall elevation of 43 m. So, the pump only needs to add a very small amount of head to overcome the dynamic lift (friction losses from the pipes). Here is the profile: 
 
 When the other pumps are on, the pump head is much higher, because the extra flow from the other pumps increases the friction losses and thus raises up the head that the pumps need to add to overcome those losses. (the pipe diameters are small, 2-4 inches.)