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Pressure values in pressure dependent demand analysis

Hi,

When I am performing pressure driven analysis (PDA), using piecewise linear function, negative pressure values are not obtained at any node in the system even when the water level at the reservoir is set to be lower than the elevation of most of the nodes. It seems that (at least in the case that I had run) when the actual demand in a node become zero the pressure head is forced to be equal to zero and can not become negative. 

To better clarify my problem, I run another scenario: I assign zero demand to all the nodes in the network. I run the case with both demand driven analysis (DDA) and PDA, with same the hydraulic and physical conditions. The pressure values in some nodes become negative using demand driven analysis while, the pressure values calculated for these nodes are zero and no negative values were observed using PDA. I was thinking maybe the way that the pressure demand function is defined is causing this limitation to PDA. I would like to know your idea about this issue and if there is any way to fix this issue?   

Thanks

  • Fatemeh,

    It might help us to see the model where you have your two scenarios compared PDD vs. DDA. There are two options for sharing your model files on Communities. If you would like the files to be visible to other members, compress the files into a zip file and upload them as an attachment using the ‘Advanced Reply editor’ before posting. If your data is confidential, you can follow the instructions in the link below to send it to us via Bentley Sharefile. Files uploaded to Sharefile can only be viewed by Bentley.

    communities.bentley.com/.../7079.be-communities-secure-file-upload

    Regards,
    Mark

    Mark

  • Dear Mark,

    Thank you for your fast reply. The files are uploaded. Please let me know if there is any unclear point.

    Best Regards,
    Fatemeh
  • Fatemeh,

    I'm seeing the same thing as you when running the pressure dependent demands (PDD) scenario and I believe that you are correct in your analysis that the PDD linear function table is what is limiting the demand from becoming negative. I'm going to discuss this issue with a few people tomorrow though to confirm this.

    Regards,
    Mark

    Mark

  • Fatemeh,

    After discussing this with a few people today it looks like what you have identified is probably due to the lack of having an unrealistic model setup. Let me explain, if you have a reservoir at an elevation of 50 ft. and at least one node at a higher elevation, 55 ft. in this case, you would need a pump to add enough head to meet any demands you might have there. A more realistic setup would be to have a pump right after your reservoir and at the time when there is no demand maybe you have a control to shut the pump off.

    Does the system that you are working with have a demand at a higher elevation than the water source? What would you expect to happen in the situation using PDD?


    Regards,
    Mark

    Mark

  • Hi,

    Thank you very much for your reply. But we can not say that this is due to an realistic model set up. As you know the main reason to perform PDA is modeling and predicting the realistic demand as well as other hydraulic parameters (such as pressure and flow) under pressure deficient conditions. In this scenario to created low pressure in the system I myself have decreased the water level at the reservoir to 50 m to see (1) what is happening to nodal based demand, and (2) how low the nodal pressure can go during abnormal pressure condition in that WDS. Therefore, I think there is no need to add a pump in this case. If as you suggest we add a pump right after the reservoir to meet any demands so what is the benefit of performing a pressure driven analysis?
    My main question was why in PDA in WaterGems I can not achieve any negative pressure under pressure deficient conditions? The lowest pressure when performing PDA is zero, however it is possible to have negative pressure in a DS under abnormal conditions. I wanted to know if there is any setting that I have not considered during performing PDA that cause this issue?

    Regards,
    Fatemeh