When I put tank at the end to fill it in 24 hours or 2 days, I always get the negative pressure at the Junction just before the tank, no matter how much head I put at the pump, the junction always shows the negative pressure.

When I put tank at the end to fill it in 24 hours or 2 days, I always get the negative pressure at the Junction just before the tank, no matter how much head i put at the pump, the junction always shows the negative pressure, please guide me how can I resolve this issue or what's the reason behind this.

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  • Hello Noman,

    Kindly help me understand the issue by replying to the following questions - 

    - Are there too high demands in the system?

    If the pressure is negative at high flows, then the problem is loss of head due to friction somewhere between the tank and the junctions. It might happen that the system is trying to satisfy demands that are too high for the size of the piping network. You can fix this by lowering the demands, or by increasing the size of the pipes, or making them smoother so that there is less energy lost to friction.

    - Have you checked the HGL of the tank and the elevation of the junction in that zone?

    The HGL of the tank should be high enough, and if it is not then you will never have positive pressure. You can test this by raising the HGL of the tank to something above the elevation of the junctions and seeing if that fixes the problem.

    Kindly check more troubleshooting steps in the following wiki article- 

    Troubleshooting negative pressures at pumps, junctions, & other node elements

    If the above wiki does not help then kindly share your hydraulic model with us so that we can review. Save it in File > Save as Package and then forward it following the steps mentioned in the article below:

    Sharing Hydraulic Model Files on the OpenFlows

  • Flow is not so high, and sizing of pipes is also ok to cater that flow with very less losses in pipes. Problem is with HGL, my HGL of TANK is a little lower than the JUNCTION. I have fixed levels, how can I overcome this issue? Please advise.

  • Tanks represent boundary conditions so the model will solve the hydraulic grade based on your assumed tank water surface elevation. As seen in the profile, the hydraulic grade is "connected" to the tank water surface elevation. If the junction next to it is at a higher elevation, it is physically impossible to have positive pressure.

    Can you tell us more about what these elevations represent? Is the water surface elevation in the tank 131.2 (meters?) and if so, what is the elevation of the pipe next to it? Does the pipe actually travel uphill as you move away from the tank, and rise up to 135.2 m? If so, then in the real system you would likely have part-full flow (not pressurized) in that pipe. WaterCAD/WaterGEMS assume pipes are pressurized, which is why you see the negative pressure. If the last pipe is flowing part-full, consider ending the system at the high point (location of the junction at 135.2 m) with a demand, reservoir, emitter coefficient, D2A element. You could also consider using the air valve element as described here.

    If this does not help, please tell us more about what actually exists (or will exist) in this area of the system so we can help you model it.


    Regards,

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

  • My tank Maximum water surface elevation is 135.25 and my source is reservoir and I have to check that how much the tank will be filled in 24 hours, I am using pump after reservoir with 30m head, but pump is operating only at 8m head. Reservoir elevation is 130m and the length of the transmission line is approximately 3km.

Reply Children
  • The pump will operate at the point where the pump curve intersects the system head curve per this article. So, if the pump is adding less head than you expect and it is pumping from a reservoir to a tank, this would suggest that the friction losses in the pipes are less than you intended, or the difference in static head is smaller than you intended (130 to 135.25).

    However, even if the pump were to add 30 m or more of head, you would still see negative pressure at the junction right next to the tank as explained earlier. The hydraulic gradient (HGL slope) would need to be steeper than the physical slope of the pipe next to the tank, in order to keep the pressure positive at that junction. Therefore I think the solution/answer to the negative pressure concern will be based on a better understanding of the piping next to that tank and whether you expect it to be operating part-full.


    Regards,

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