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Pressure pipe with two outfalls

 Hello,

Background: I am modeling a sanitary sewer system that consists of 20 lift stations using SewerCAD V8i (SELECTseries3).  All of the lift stations pump into a primary force main.  This force main has two discharges (one at each end of the FM).  Typically there is an isolation valve that splits the primary FM into two distinct systems.  Let's call them the north and south systems.  Occasionally there is a large demand in the north system which in the past has caused a traffic jam for other lift stations pumping into this system.  To combat this issue, the maintenance team opens the isolation valve and lets flow go to either the north or south system.  This has be done for many years now and has prevented long pump run times and potential overflows. 

My question -

When analyzing this scenario in the model, I get a large negative flow rate at the north system discharge pressure pipe (-200 gpm) as a base flow rate (at Time = 0 or when no stations are operating). This pressure pipe discharges into a gravity manhole and pipe to the outfall.  The gravity pipe flow starts at 0 gpm as expected, there is no flow assigned to the manhole. 

Furthermore, on the south system discharge pressure pipe the base flow is +200 gpm when there is nothing in operation.  This pressure pipe discharges directly to an outfall and the outfall flow matches the pipe flow. 

It is almost like there is a ghost flow of 200 gpm going from the north outfall to the south outfall. 

When stations are operating, the each discharge location shows positive flow, so I know flow is travelling to each discharge location.  how it is splitting between the two discharge locations, I don't know. 

As for model set up, I am using the GVF-Convex solver and an running a EPS for 24 hours. The model is able to achieve convergence.  I have input multiple design points for each pump and have created controls to operate the pumps.  Also, when the north and south yard systems are isolated I am getting accurate model results compared to observed data.  so I am confident that the data I have enter is an accurate representation of the actual system. 

Any suggestions or feedback is very much appreciated. 

Thanks,

Scott

  • Along with the explanation, uploading the model also would help.


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    Regards,

    Sushma Choure

    Bentley Technical Suppport

  • Thanks for the feedback. I am seeking permissions to upload the model from my client. Hopefully they will let me do so.
  • Hi Scott,

    In the meantime, perhaps some screen captures of the relevant input and output would help show the problem.


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.

  • I have uploaded the model to the Bentley secure file site.

    Additional information that might be helpful.

    - lift station known flows are based on actual data. Due to demographics of area, there are stations that have very little or no flow right now.

    - I have altered the initial way the south outfall is represented since I initially put out this help request. In going through the model I noticed that pressure pipe connection to the south outfall was set to match the outfall invert, whereas I thought I had this value set to false (and the actual value was input) as the discharge occurs well above the invert. In trying to update this, the pressure pipe would not let me change the "set invert to stop?" from True to False (still don't know why). Therefore I added a transition node and a short section of gravity pipe so prior to discharging to the MH. This has help reduce this mystery flow in half. however when running the model with zero flow there is still a -100 gpm flowing from the northern most pressure pipe to the southern most pressure pipe.

    -the model is built using as-builts and other available drawings, so elevations, pump curves, minor losses*, diameters, etc. are as accurate as can be. *in a few locations I have added 'dummy' minor losses to adjust model results to observed results.

    Please feel free to contact me privately if you need additional information/ details as I have shared as much as I can here.

    Thanks in advance for reviewing this model. I am anxious to see what you all are able to determine.

    Thanks,
    Scott
  • Hello Scott,

    You are using an older version, so it's possible upgrading may help. I did a quick review of the model and was having trouble finding areas with negative flow. Could you give an example of where this is happening?

    Regards,
    Scott