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Flow total in wet well

Bentley Community,

Using SewerGEMS, in a wet well, the total flow (results flow) is different from the total flow entering from the pipes. In my case two pipes are entering in the wet well, one is carrying 5,683.82 m3/d and the second 1,674.98 m3/d so the total flow in the wet well should be 7,358.8 m3/d. In reality, the wet well showed 7,216.51 m3/d.

Parents
  • Hello Muhammad, 

    Do you mean there are two different branch pipes coming into a wet well with combined flow of 7,358.8 m3/d, however flow (total in) at well is 7,216.51 m3/d and not 7,358.8 m3/d ? Or do you mean flow going out of wet well is not equal to flow coming into wet well, it appears to be case 1 though. 

    Could you please attach a screenshot of your setup? Is it steady state analysis or EPS? What type of loads have you entered, sanitary loads or inflows? 

    I tested in a sample models with two incoming pipes in a wet well one with sanitary load and one with inflow hydrograph, the Flow ( Total In ) for a wet well = flow combined of two pipes at a given time step. 

    What is the depth of flow at wet well? Do you observe any overflow at wet well after computing the model. Is pump working as expected? 

    You may upload model files for our review if required. Sharing model files.  

    Regards,

    Sushma Choure

    Bentley Technical Suppport

  • Yes, Its first scenario. Kindly check the shared model, W-4 wet well in peak scenario 2050.

Reply Children
  • Muhammad,

    The reason is because a higher population results in a smaller extreme flow factor. See: Troubleshooting Extreme Flow Results with Count based Unit loads

    This is a steady state simulation with the GVF-Convex (SewerCAD) solver in which you are using unit loads and the extreme flow method to peak your flows. When the flow from the two upstream conduits combines at the wetwell, the inflow is recalculated based on the extreme flow method you have configured to use. In your case the "Peak_2050" scenario is using an extreme flow setup in which all unit loads are using the Babbitt equation.

    If you look at the "Population (system sanitary)" result field in the properties of the two upstream conduits, you will see that they have a population of 8114 capita and 285 capita, for a total of 8399 capita reported at the wetwell. If you use 8.399 in the Babbitt equation (the population needs to be divided by 1000 since the Babbitt equation uses a population unit of capita x 10^3), you will get an extreme flow factor of 3.2668. If you follow this same procedure for the population in the upstream two pipes, you will get a higher extreme flow factor since the population in each of those contributing pipes is lower than the combined total population.

    Since all loads are using the same unit loads, and that unit load has a base value of 0.21 m^3/day, multiply 0.21 times the total population of 8399, times the extreme flow factor of 3.2668 for a total sanitary flow of 5762 m^3/day. This is the "Flow (System Sanitary)" that you see in the wetwell properties. Since you have wet weather loads and pumped loads upstream, those are all added together for the total wetwell inflow: 78.8 + 2.53 + 1,373.20 + 5762 = 7216.5 m^3/day

    In short, as the population increases, the extreme flow factor decreases and therefore the flow downstream of two combined pipes will be less than the sum of each pipe flow. For more background information on the theory behind extreme flow methods, see the Help topic, chapter 6.3 of our wastewater book and this article: Using Extreme Flow Factors (Peaking Factors) in SewerCAD and SewerGEMS


    Regards,

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

    Answer Verified By: Sushma Choure