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Tanks next to each other

I have to tanks thta are right next to each other in the system that i am modeling. It is a steady state model so I modeled the 2 tanks as reservoirs. But as you can see by the flow direction one tank is being filled by the other which does not seem logical to me both reservoirs should be sending flow out. 

  • Hello Christen,

    This may be a flow balance issue. If the elevation of one reservoir is lower than the other, you may see flow into the reservoir.

    If the elevation is the same, it may be related the issue in this wiki: Rapid flow oscillation between hydraulically close tanks. This link includes some possible solutions, including simply combining the sources. Since these are reservoirs and not tanks, having only one reservoir may be a suitable solution.

    Regards,

    Scott

    Answer Verified By: Christen Crique 

  • In a given steady state, energy is balanced based on boundary conditions (known hydraulic grade at reservoirs and tanks) and other constraints like demands and physical parameters. For a steady state this concept is the same between tanks and reservoirs (both establish a boundary condition based on the reservoir elevation or the tank initial elevation).

    In your example case the flow direction of each reservoir is based on what the hydraulic grade is at the junction between them. Consider the two below illustrations for example. In the first example on the left side, the HGL from the outflow of the left tank/reservoir is still higher than the right side tank elevation at the junction location, so the right side tank fills. In the second example on the right side the headloss in the left side pipe is higher so by the time flow reaches the junction, the HGL is lower than the elevation of the right tank, so both tanks experience outflow.

    Check the elevations of your reservoirs, the headloss in the connecting pipes and look at the profile view. If they represent hydraulically close tanks, combine them into a single tank (or place a reservoir at the junction point with the elevation set to the assumed HGL that the tanks would supply). This general idea is documented in the following article: Rapid flow oscillation between hydraulically close tanks


    Regards,

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

    Answer Verified By: Christen Crique