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Why do i get that inflow in tank change with pattern ?

hi,

i got a reservoir and tank and junctions that take water from the tank,

i don't have pump or anything but a pipe between my reservoir and tank,

but i keep getting that the inflow in the tank changes when using ESP calculation (pattern) when i change time in time browser i get that the inflow change in a way that the inflow is the same as the outflow from the tank.

but isn't supposed that the inflow be constant that is coming from the reservoir to the tank ?? 

what does the inflow mean in the tank isn't it the flow that is coming to the tank ?

if the outflow is opposite to the inflow than i do not need my tank from the first place why isn't the inflow constant and the outflow change with time ?? because the outflow represent the total demands in the junctions that change with the pattern but the inflow must be constant because the reservoir and tank are in a fixed elevation !

  • Joseph,

    Flow (In net) and Flow (Out net) are always going to be equal values and opposite signs because of how they are defined. Flow (In net) = Flow In - Flow Out, and Flow (Out net) = Flow Out - Flow In. Typically you don't graph both together, you would pick the one you need.

    In your case, it sounds like you would be more interested in the actual values for Flow In and Flow Out, rather than the difference between them. To get that, I would graph the flow in the pipes, rather than the tank itself.

    One thing to check is whether the tank is completely full for the entire simulation. Your reservoir is 12 meters higher than the top of the tank, so unless there is a lot of headloss in the pipe it will tend to keep the tank completely full. If that is happening, as soon as any flow leaves the tank towards the junction, it is immediately replaced with flow from the reservoir, so the inflow and outflow values will be nearly identical.

    To check the tank level, right click on the tank, select Graph, and check the 'Level (Calculated)' option. If it is steady at 6 meters, than the tank is locked out full, and the hydraulic grade at the tank is actually higher than the top of tank elevation.

    A couple of notes:

    1) In general, for a reservoir going directly into a tank, the flow rate is not constant. It will vary depending on the hydraulic grade at both ends of the pipe (and the length/diameter/material of the pipe). The hydraulic grade at the reservoir is constant, but the hydraulic grade at the tank changes with the tank level. If the tank is completely full, the hydraulic grade can continue to rise above the top of the tank, and the tank becomes pressurized.

    2) If you graph the hydraulic grade of the tank itself and the tank is full and under pressure, WaterGEMS will show the hydraulic grade as being equal to the maximum tank elevation even though the true value is higher. I typically place a junction in the pipe very close to the tank, and set the elevation at ground elevation, and use that junction to check the hydraulic grade at the tank.

    3) I built a quick test model to mimic your setup and got strange behavior from the reservoir to the tank. After the tank filled, the reservoir continued sending more flow than the junction was demanding, and this flow seemed to be disappearing. If you are having that problem, try adding a junction in between the reservoir and the tank. That fixed the problem for me.

    4) For the model you have, the behavior of the system is going to be very dependent on the length and diameter of the pipe between the reservoir and tank. If the distance is small and/or the diameter large, the tank may almost always be completely full and pressurized. If the distance is long and/or the diameter is small, the flow in to the tank may not be able to keep up with demand and the tank could empty completely. If the distance and diameter are moderate, the tank level will rise and fall opposite demand. You'll want to carefully check those values to make sure it is a reasonable representation of your system.

    5) If you know that for your particular system there will be constant flow going in to the tank, you can model this by adding a Flow Control Valve (FCV) between the reservoir and tank.

    -Stephen

    Answer Verified By: Joseph Daou 

  • Thank you that was veryyyy helpful !