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Flow Diversion at Treatment Plant

I'm trying to model a flow diversion system at a wastewater treatment plant (see attachment for schematic).  Wet Well A is a flat channel that, for all intents and purposes, serves as a storage basin between two treatment processes at the plant.  Wet Well B is an actual wet well at a pump station that's used for flow diversion when the plant flow gets too high.  This wet well is at a lower elevation than Wet Well A, and therefore the flow from Wet Well A to Wet Well B is by gravity.  Wet Well C is a holding tank that's at a higher elevation than the rest of the plant.  The green line in the attachment represents the flow path of water when it's being diverted from the treatment process to the holding tank.  The orange path represents the flow path of water when it's being conveyed, by gravity, from the holding tank back to the treatment process.  Just to be clear, the green and orange lines for Segments 1 and 3 represent the same pipe, only with the flow being in opposite directions.

Is it possible to model this system in SewerCAD?  If so, do I need to have both a pressure pipe and a conduit in the model for certain pipe segments?  Would it be easier to model this diversion system in WaterCAD?

Flow Diversion Diagram.pdf

  • Hello Robert,

    Modeling multiple wet-wells is not possible in SewerCAD. The solver associated with SewerCAD; GVF-Convex can have only one outfall or wet-well per gravity network. So when multiple wet-wells are detected during validation, an error is generated. You can learn more about his in the following article;

    SewerCAD Error

    However, the Implicit Solver within SewerGEMS can have multiple wet-wells as well as outfalls. Hence, it is possible to create the model and analyze within SewerGEMS using the Implicit Solver.

    If you don't have access to SewerGEMS, you can create the model in WaterCAD. However your entire model would be consisting of pressure flow as gravity flow cannot be simulated in WaterCAD.

    A few observations on your model with the different solvers involved;

    1. Although the model can be created and run in SewerGEMS (Implicit Solver), the logical controls associated with the pump in SewerCAD (GVF-Convex Solver) are not available. You can define the "On" & "Off" elevations as per upstream wet-well elevations in Implicit Solver. As per your schematic and description, you want the pump to stop when the Wet-well C is completely filled and the wastewater flows to Wet-well A.

    2. It is not possible to use the same pipe for "Segment 3" for both inflow & outflow, as the outflow pipe from pump is a pressure pipe connected to the discharge end of the pump. A better option would be to provide a separate pipe from Wet-well C to intersection of Segment 1. In this case you can have the pump filling the holding tank (Wet-well C) from top, and the discharge from holding tank can be carried by bottom opening.

    3. A better model can be prepared in WaterCAD where you can control the pump on / off conditions with the governing HGL of Wet-well C. So, when the holding tank is completely filled, pump can be shut off and flow from holding tank to Wet-well A will occur. Only drawback is, you won't be able to simulate gravity flow. Also a model in WaterCAD will require number of valves, FCV, TCV etc. to control the filling and emptying of tanks as per your requirement.

    I am sharing a sample I created in SewerGEMS based on your schematic which might be useful.

    Sample 1.stswpkg.zip

    Hope this helps,


    Regards,

    Yashodhan Joshi

  • Just to add to my colleague Yashodhan's reply -

    1) There is a distinction between open channel flow by gravity and pressure flow by gravity. WaterCAD (and WaterGEMS) can model gravity flow if the pipes are under pressure, based on the difference in boundary condition HGL (HGL in two tanks for example). So, if all of these pipes are flowing full, then I agree that WaterCAD may be the best solution to model this.

    2) When the pump is on, what prevents water from flowing to both the holding tank and wetwell A? If there is a valve closing, then that can be modeled as a TCV or a control on a pipe.

    3) What prevents wetwell A from becoming empty as it drains to wetwell B? Similar to #2 above, controls may be needed to close an open the pipe for segment 1

    4) Are you modeling the flow to/from Wetwell A, per the arrows you drew to the left and right of it in the figure? What controls this? A specific inflow can be modeled in WaterCAD with a negative demand.

    Here is a diagram of how this might be modeled in WaterCAD:


    Regards,

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

  • It appears to me that there two distinct hydraulic systems at work here.

    1. The system that pumps water to the holding tank (A to C)
    2. The system that drains water from the holding tank back to the plant (C to A).

    I would set up two different models or at least two different topological alternatives in different scenarios.

    1. Case 1 (Green system) would cover the mode when the pump is running. Don’t include segment 2 in the model and make segment 3 a pressure pipe. Make tank A a know inflow.
    2. Case 2 (Orange system) would cover the mode where holding tank C is draining to wet well A. Don’t include the pump or tank B in the model but include Segment 2 and Segment 3 as gravity pipes.

    Is this a design study or an existing system?

  • All of the gravity flow in this system is pressurized flow and not open channel flow.  It sounds like it would be better for me to model this system in WaterCAD instead of SewerCAD, so I might do that.  However, a few follow-up questions about SewerCAD have come to mind:

    a) If I were to have a conduit going from Wet Well C to the intersection of Segment 1 (which presumably would also be a conduit), would I need to insert a manhole there to allow flow from Wet Well C to Wet Well A and have the rim elevation at this manhole be higher than the maximum water surface elevation in Wet Well A?

    b) Is there a way to open and close a conduit in SewerCAD (or add a valve to that line) like there is with pressure pipes?

    c) If I were to convey water from Wet Well C to Wet Well A via conduits (as described in Item (a)), is there any way to set up the controls such that water will only start to flow through these conduits when the inflow/loading into Wet Well A drops below a certain value?

    Thanks,

    Rob

  • In answer to your specific questions

    1. You would need a manhole (bolted cover) there or you could just eliminate the pipe from that point to tank B.
    2. You can apply control to pressure pipes in SewerCAD.
    3. You can open the pipe from A to B based on the level in A or the flow in the pipe coming in to A. (I thought you would open it if the flow was above a certain value?)