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Negative demand node with a fixed pattern is fluctuating its demand

Hi,

I am modeling a negative demand node upstream of a storage tank to represent the water treatment plant's capacity. i.e it is assumed that the water treatment plant continually produces a constant flow of treated water to their storage tanks. From thee water is pumped into the distribution system. I have this set up for different scenarios in the model which correspond to different phases of the project and therefore different water treatment plant capacities. For every other phase/scenario this node (J-600) produces a constant negative flow to fill the tank, except for model scenario "P1.4 Max Da EPS 24 hr" which has a zero demand at certain hours of the day, like it s following a demand pattern except the pattern for that node/demand is the same in every scenario. 

It looks a little different when I view the data tab. Side note: can someone explain to me what it means to be (N/A)?

I am not sure what is causing this , but it seems to be affecting the pumped flow:

Here are these two graphs together: 

Can someone please help me understand what is happening? I want to see a constant negative demand coming from J-600.

I have gone ahead and uploaded the model file.

Thank you, 

Nastassja

Parents
  • Hello Nastassja,

    From my preliminary understanding, it seems your tank is quickly filling up causing the upstream pipe P-600 to close. Thus your negative demand from J-600 drops to zero during some times. See this graph below which compares tank's percent full vs the flow in P-600;

    When the tank gets full the built in altitude valve automatically closes the upstream pipe since the tank cannot store any more water thus stopping the inflow. See this article which explains more about this: What happens when a tank becomes empty or full? (altitude valve)

    I also checked the parent scenario of this scenario where we see such results but the inflow (negative demand) at J-600 in that case is significantly smaller and thus the tank is never getting completely full (percent full = 100%) and you see a constant inflow towards the tank since both the upstream and downstream pipes are never closed.

    Also since your downstream pump is VSP type with target pressure of 80 psi, the flow out through the tank (the flow the pump would pull) is controlled and the tank is not emptying fast enough thereby remaining full for some time before the levels are dropping. Try using "Target Head" instead of pressure and specify the target hydraulic grade and see if that helps. I used the corresponding hydraulic grade to 80 psi and got some better results (i.e., flow out of J-600 was constant except for a couple hours).


    Regards,

    Yashodhan Joshi

Reply
  • Hello Nastassja,

    From my preliminary understanding, it seems your tank is quickly filling up causing the upstream pipe P-600 to close. Thus your negative demand from J-600 drops to zero during some times. See this graph below which compares tank's percent full vs the flow in P-600;

    When the tank gets full the built in altitude valve automatically closes the upstream pipe since the tank cannot store any more water thus stopping the inflow. See this article which explains more about this: What happens when a tank becomes empty or full? (altitude valve)

    I also checked the parent scenario of this scenario where we see such results but the inflow (negative demand) at J-600 in that case is significantly smaller and thus the tank is never getting completely full (percent full = 100%) and you see a constant inflow towards the tank since both the upstream and downstream pipes are never closed.

    Also since your downstream pump is VSP type with target pressure of 80 psi, the flow out through the tank (the flow the pump would pull) is controlled and the tank is not emptying fast enough thereby remaining full for some time before the levels are dropping. Try using "Target Head" instead of pressure and specify the target hydraulic grade and see if that helps. I used the corresponding hydraulic grade to 80 psi and got some better results (i.e., flow out of J-600 was constant except for a couple hours).


    Regards,

    Yashodhan Joshi

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