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System curve for Direct Pumping

Shirantha Wickramatunga
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Hi All

Could you any one please explain me how to plot the system curve using WaterGEMS. This is the direct VFD pumping system with variable demand nodes with constant pressure control node. The given curves in the software is abnormal by the command 'Alternative' + 'System head curve'. Please explain.

  • 0 Offline
    WaterGEMS can perform this calculation for you.

    If you want to see the theory behind it, send me (tom.walski@bentley.com) your email address and I'll send you a copy.
  • 0 Online in reply to Tom Walski
    In addition to the theory data Tom has access to, the following link might have useful information as well: communities.bentley.com/.../24627.system-head-curves-in-watergems-and-watercad

    Thanks,
    Scott
  • 0 Offline
    Good Morning Tom,

    Thanks for the given support. If you din't mind could you please advise on the bellow.
    Quarry - Plotting of system curve
     
    1.      Requirement
     
    The followings are the requirement of plotting system curve.
     
    a)      To illustrate whether pump curve intersect the system curve throughout the frequency range of the pump.
    b)      To decide the Number pumps required to ensure pumps are operated within acceptable efficiency range.
    c)      Do the VFD programming according to the system curve.
     
    2.      Developing system curve
     
    As Water Cad/Water Gem do not generate system curve for close systems automatically, the following steps are followed to plot the system curve in Excel worksheet.
     
    a)      Run the system with VFD pump.
    b)      Tabulate the pump flow and pump head in a excel sheet for different calculated relative speed factor values.
    c)       Plot pump head against pump flow.
     
    Similarly calculated two system curves are given in the Excel worksheet attached (Annex 1)
     
     
    3.       Question 1
     
    a)      Is this system curve correct?
     
    b)      In network with VFD pump, if the Watercad create additional heads or reduce heads at nodes with change of demand pattern, to match relevant demands, automatically system will be changed.
     
    Does Watercad create or reduce heads as described above?
     
     
    4.       Question 2
     
    According to the attached Annex 1, different pump heads are visible at same pattern multiplier and same or close relative speed factor (calculated), keeping flow without changed.
     
    Is this due to approximations in the calculations?


    Many thanks
    Kind Regards

    Grashan


  • 0 Offline in reply to Shirantha Wickramatunga
    Good comments.

    Requirement
    a. This is a feature I've been wanting and possibly with your encouragement, we may get it some day.
    b. The engineer/operator needs to make the decision. Usually you want to run near full speed. Look at the pump combination display in the pump station element for guidance.
    c. What do you mean by "doing programming". Usually, in water distribution pumping, you set the VFD control to maintain a fixed discharge HGL.

    Developing system curve
    a. We do this already.
    b. This result would vary as the demand varies and would have a virtually infinite number of points.
    c. You have the information to do this in the WaterGEMS output. The problem is that this will vary with time of day as customers will have fewer or more faucets/orifices open.

    Question 1
    1. It is correct if you accept the assumptions on which it is based.
    2. This is controlled by pressure dependent demand setup.

    Question 2
    Not sure what you mean by this. For a given demand, you should get the same pump head. You may need to submit this model.
  • 0 Offline
    Many thanks for quick reply

    I'll keep in touch with you.


    Kind regards


    Grashan

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