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System Head Curves

I would like to know how I can to show the equation of the system head curve?

Thanks!

 Erick

Parents
  • The system head curve we show is not an equation but a series of poitns conencted by a straight line. The only time it woudl fit a nice equation is the case where you have a single pipe between the pump and the discharge tank with no users, branches, loops, other pumps, etc. between the two ends. In that case it would fit a line given by

    H = ho + aQ^b

    Where ho is the static lift, b = 1.85 for Hazen Willians or 2 for Manning or Dacrcy Weisbach and a depends on pipes size, length roughenss, etc.

    You can take the points from our system head curve and try to run some kind of polynomial regression through them.

    Remember also that any system head curve is the curve for that set of tank levels and demands. There is not a single system head curve but a band of system head curves.

  • Thanks very much for your help!

    I have another doubt, I am trying to run my system with EPS and in the "User's Notifications" appears this message:

    41355 2009 (N/A) -1 (N/A) DISCONNECTED DEMAND NODES DETECTED (Hydraulic results are likely to be compromised). 0,000 Pressure Engine

    What is this? How I can to correct this fail?

    Thanks!

    Erick

Reply
  • Thanks very much for your help!

    I have another doubt, I am trying to run my system with EPS and in the "User's Notifications" appears this message:

    41355 2009 (N/A) -1 (N/A) DISCONNECTED DEMAND NODES DETECTED (Hydraulic results are likely to be compromised). 0,000 Pressure Engine

    What is this? How I can to correct this fail?

    Thanks!

    Erick

Children
  • Hello Erick,

    That message means that you have demands in your system that do not have an open hydraulic path to a water source. There should be other user notifications referring to specific node labels. Take a look at the path between those nodes and your tank/reservoir(s) and look for closed pipes, closed valves, or defects in the drawing (such as a gap between junctions that overlap) which could cause a 'break' in the pipeline, If your source is a tank, make sure it's not empty. WaterCAD and WaterGEMS are demand-based, so all entered demands must be satisfied. To model a demand that should become zero when disconnected, I believe you would need to use an emitter coefficient or pressure dependent demand (PDD).

    If you need further help, I would suggest submitting your model to technical support via the online ticket manager .

    I hope this helps.

    Regards,

    Jesse


    Regards,

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

  • Hello Jesse,

    you mention in your reply to Erick " believe you would need to use an emitter coefficient or pressure dependent demand (PDD)", Please may you explain what that means? and how to do it?

    Thanks,

    Mohamad Azzam

  • Mohamad,

    Emitter coefficients are seen in the properties of a junction and are used to represent outflow that varies significantly with pressure, such as a sprinkler system. You can read more about this, including some common coefficients, in the help documentation and the Advanced Water Distribution Modeling and Management book.

    Similarly, Pressure Dependent Demands (PDD) are demands that vary depending on the pressure. PDD gives you more flexibility in defining the pressure vs. demand relationship. You can read more about PDD in the help documentation and in this technote:

    communities.bentley.com/.../setting-up-pressure-dependent-demand-tn.aspx

    The reason why this might be used in the case discussed below is because if the pressure drops to zero, the demand/outflow drops to zero. Without using PDD, the program will always try to satisfy the user-entered demand, even if that node is disconnected from a water source and the pressure drops to zero. Of course, if you are using a scenario to model a case where something like a closed valve cuts off a demand, you could also simply use a new demand alternative for that scenario and make the demand for that node zero.

    I hope this helps.


    Regards,

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