I am working on a complex dynamic wastewater model, and there is one area where there are two separate lift stations that are connected by a gravity line. When one lift station on the downstream end of the gravity line gets backed up due to high flows, flows should move through that gravity line and enter the other lift station, which still has capacity.
When modelling this scenario however, this doesn't occur. At some point in this gravity line, the hydraulic grade line will drop significantly at one of the manholes. It will go from the high (overflowing) HGL of the backed up lift station, to the lower HGL of pipes upstream of the lift station with capacity.
This is using the implicit solver. I understand this is a very complex interaction, and the software likely has trouble coming to a solution. I was wondering if there was a potential workaround, or way to represent this interaction in a form that the software can more easily solve.
Interconnection:
Profile between the two lift stations:
Currently I have the conduits set for the low end to be the "start" and the high end to be the "stop". If I have them reversed, the last conduit before the lift station with capacity would suddenly peak to a flow of 3000 L/s after the other lift station backed up, with no other conduit seeing anywhere close to those flows.I will upload the model using the secure upload. Zip file will be called "Wastewater Lift Station Interconnection". Scenario being ran is Future System - Pre Annexation Wet Weather
Hello Jamie,
I see something similar. Is there a scenario where this issue doesn't occur? What is supposed to happen in this part of the system?
Regards,
Scott
Hi Scott,I don't have a similar scenario, as the other scenarios either don't have high enough loads for the east lift station backup, or have upgrades that prevent the backups.What should happen, in my estimation, is once the HGL gets above the highest invert on the west side, I should start seeing flows from east to west. Somewhere in the realm of the difference between the peak flow coming into the east lift station, and its pumping capacity, so roughly 100-150 L/s?. The peak flows going to those two lift stations are higher than their summed pumping capacity, so I expect surcharge upstream of both lift stations. What I want to see is the extent of surcharge during the peak flow that would occur with this interconnection. Right now the surcharge is only occurring upstream of the east lift station, up to where that sudden drop in HGL occurs.
Good Afternoon Scott,I just wanted to follow up with this issue. Is there any more information I can provide in order to help find the root cause here? This is a bit of a time sensitive project, so I would like to be able to get some feedback or a resolution this week, if at all possible.
Thanks,
Jamie
I'm still doing some testing with the Implicit solver, but I did notice that the Explicit solver with a time step of 1 second (and after globally editing the conduit fill depth to zero) appears to have a profile that looks more inline with what you are expecting.
I will keep testing but please see if the Explicit solver will work.
Thanks Scott,
I did do a test run with the explicit solver and I agree that the results in that section are much more in line with what I was expecting. This did bring up other issues though, such as an extremely high non-convergence amount (>90%). I'm not very familiar with the SWMM engine so I'm not sure what the implications of that are, and how switching solvers would impact my model moving forward. I spent a lot of time ensuring the calculation settings in the implicit solver gave good results across all of my scenarios, and would like to retain that if at all possible.Thanks,