Product(s):	SewerCAD, SewerGEMS
	Version(s):	08.11.XX.XX and higher
	Area:	Modeling

 

Problem

In SewerCAD or when using the GVF-Convex solver in SewerGEMS, the system head curve for a pump appears to be different from expected, with very large numbers seen for head and generally not following a typical parabolic shape.

Or, the System head curve jumps up and down unexpectedly.

Solution

This can occur if there is a closed pipe or air valve between the wet well and the node that the flow is draining to (such as a manhole or outfall). Please check each element between to make sure that the element is connected, active and open.

Also check the status of the wet well upstream of the pump. If the wet well is empty during the simulation, this will essentially close the pipe between the wet well and the pump and throw off the results of the system head curve. If the wet well is closed, make sure to make adjustments to the wet well properties and/or the pump controls.

Lastly, this issue has been known to occur in a model with downstream air valves where the adjacent pipe orientation is backwards. Check for such a condition, try reversing the pipe orientation to fix it, and if that does not help, contact Technical Support. Related reference # 477468, related forum discussion here.

Note: in SewerGEMS, System Head Curves can only be generated when using the GVF-Convex (SewerCAD) solver. See: Generating a System Head Curve in SewerGEMS

What if the system head curve jumps around, rising and falling unexpectedly?

This can happen if the status of elements are changing at specific flows. For example as the flow increases, another pump station discharging into the same pressure zone may suddenly be unable to supply flow or head, and the sudden decrease in flow can cause an up-and-down shape for the pump curves. 

To investigate, make the pump inactive or delete it and the adjacent pipes, then run a series of steady state simulations, varying an inflow into the upstream pressure junction and observing the change in HGL at that pressure junction. This also enables you to see what else is going on in the system at those specific flows.