"Pressure pipe stop invert is higher than the connected wet-well base elevation..."

  Applies To 
  Product(s): SewerCAD, SewerGEMS
  Version(s): 08.11.04.54
  Area:  Layout and Data Input
  Original Author: Jesse Dringoli, Bentley Technical Support Group

Problem Description

What does the following User Notification mean and what can I do to work around it?

"Pressure pipe stop invert is higher than the connected wet-well base elevation so a pipe stop invert set equal to wet-well base elevation"

Steps to Resolve

This notification is there to warn you of a limitation in this situation. During calculations, the program will override your selection of the stop invert of the pressure pipe and set it equal to the bottom of the wetwell. It doesn't mean that there's a problem, but rather an assumption to be aware of so that you can decide how to proceed. The model will still calculate, and the results will be reliable, assuming that you're OK with the assumption of the invert being set to the bottom.

The implications of this are the same as if you manually connected the pressure pipe to the bottom of the wetwell. This means that the upstream pump will need to add less head than it would have. Depending on the pump curve and system head curve, this may not make a significant difference. This is likely why this assumption is in place, as most systems like this would see a negligible difference.

If you feel that the operating point of the upstream pump and the information you ultimately need might be effected by this assumption, a manhole workaround can be considered - insert a "fake" manhole just upstream of the wetwell at the desired elevation, then connect it to the wetwell with the short, smooth, large diameter conduit. Although the manhole isn't there in the real system, the manhole suggestion is a workaround in order to allow the hydraulics to behave as they would with the invert connected at a higher elevation. If your goal is to make sure that the existing system is working OK, then this should not be a concern - if the pipe between the "fake" manhole and the wetwell is hydraulically insignificant (large and smooth), it will not effect the results, and the model should act just as it would in the field where the upstream pressure pipe is connected directly to the wetwell at an elevation higher than the bottom. From doing this, you could also compare the results to that with the pipe connected directly to the bottom of the wetwell, as a sensitivity analysis. If the end results you need are not noticeably different, then you might conclude that the manhole workaround is not necessary.

We will look into improving SewerCAD in the future to allow a pressure pipe to be connected directly to the upstream side of a wetwell at an elevation higher than the bottom (reference # 136510).

 

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