The headloss in a pipe downstream of a parallel VSP is higher than expected

Product(s): WaterGEMS, WaterCAD, SewerGEMS, SewerCAD
Version(s): 10.XX.XX.XX, 08.11.XX.XX
Area: Output and Reporting


The headloss in a pipe downstream of a parallel VSP is higher than expected.


This is related to the way parallel variable speed pumps are modeled in the program. The algorithm for parallel VSPs routes the flow through a single pipe when calculating the headloss. For typical pipes in a pump station, the pipes are large enough that this workflow results in a minimal impact. If relatively small pipes are used, this impact will be somewhat larger.

There are a couple of things you can try in the model to achieve more reasonable results. First, you could try using a variable speed pump battery, which basically is a single element that models parallel VSPs. Since there is a single element representing the parallel VSPs, there will be a single link from the element. This downstream pipe should be the equivalent size representing all pipes leaving the individual pumps. In other words, for a case with three VSPs in parallel each with 6 inch pipes directly downstream of the pumps, you would have a single pipe representing these three pipes.

The other option would be to resize the downstream pipes connected to the VSP so they are a little larger. If the problem is caused by excessive head loss in the pump station piping, making the pipes to the lead pump larger using equivalent pipes typically works well. Usually, most of the head loss is outside the pumping station so this should not be significant.

Note that the results downstream of the pump station will have accurate head losses based on the flows.

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