Hello,
I am modelling a collection system flowing to a screw pump (Archimede's screw) which lifts sewage to a treatment plant. I am not modelling anything downstream of the screw pump, and have the screw pump defined as an Outfall with an elevation-flow curve boundary condition. The maximum flow rate of the screw pump is 59.5-CFS at elevation 918.52. During high flow events, when inflow into the screw pump exceeds pump capacity, the screw pump should continue to discharge at a constant 59.5-CFS above elevation 918.52 as sewage begins to back-up into the collection system. Instead, SewerGEMS, is allowing the Outfall node to pass flow in excess of 59.5-CFS while artificially lowering the HGL at the outfall.
See the attached image for flow and HGL at the Outfall node along with the screw pump rating curve. Any idea what is going on here? Is the Outfall attempting to lower the HGL to the normal or critical depth elevation in the outfall pipe? My goal is have the screw pump discharge at a constant 59.5-CFS whenever the outfall elevation is above 918.52 so I can study the back-up in the collection system under various storm events.
Thank you.
Hello Zak,
It may be better to model the screw pump as a pump instead of an outfall. The following link has information on modeling these in SewerGEMS: Modeling a screw pump in SewerGEMS and SewerCAD. The workflow will be a little different depending on what solver you are using.
Regards,
Scott
Thanks for the reply, Scott.
I referred to your link earlier in the project and attempted to model the screw pump with the Depth-Flow pump definition. This resulted in an unstable pump discharge hydrograph (jagged) which I attribute to the very flat screw pump performance curve. Artificially increasing the volume of the upstream wet well helped buffer this out and provide a more realistic pump discharge, however, the artificial storage volume will impact my model results.
I had a similar issue attempting to model the screw pump rating curve as a pond with outlet structure. The outlet structure had difficulty accurately representing the elevation-flow curve without a large upstream storage volume.
I'm open to any suggestions.
This can be considered as two separate systems, flow till upstream can be modeled as an outfall and for the downstream condition you can simply add the inflow at the manhole. There will be no physical connection between the systems. This approach can help you appropriately model the positive displacement pump. This option is also provided in the article Scott shared before. Did you try this as well?
Yashodhan Joshi
One other idea is to use the depth-flow pump definition but try a smaller timestep, which may help with the instability you saw from the fast changes. This and other suggestions related to stability are found in these articles (assuming you're using the Implicit or Explicit solver):
Troubleshooting unstable SewerGEMS and CivilStorm results using the implicit solver
Troubleshooting unstable SewerGEMS and CivilStorm model results using the Explicit SWMM Solver
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
Yashodhan,
Essentially, this is what I'm doing. I'm not interested in anything downstream of the screw pump, so my model terminates at the Outfall. I just don't know why the Outfall does not follow the prescribed Elevation-Flow curve as inflow approaches the pump's maximum discharge rate.