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Problem with my demand?

Hello there,

I am an engineering tech student and am working on a project which involves creating a water distribution model for a small town in WaterCAD V8i and doing a basic pressure/flow analysis on it. I've created the model with the appropriate data however when I run the analysis I am getting some strange results. The first thing I notice is that when I run the analysis and check the graph of my demands it showing that there is a flow demanded which follows my demand pattern however its saying that there is no flow supplied. I have a feeling that maybe I am inputting incorrect demands. I am taking the average daily demand of 340 L/Day per person and then muliplying that by a population of 2278 people and then dividing it by the 34 evenly spaced nodes to get a total of 22,780 Litres a day as my base demand that showing for each node. Does my base demand sound like its somewhere in the ballpark of where it should be? Im running an EPS and pairing this base demand with a common hydraulic pattern in the demand control center. The other problem I'm noticing is that when I run the analysis is that my flows are almost non existent however my nodes are showing that I have pressure at the ranges they should be at. Could the problem be with my pump curves perhaps? Any ideas what I might be doing wrong here? Thanks for any help!

  • Hi Styxic,

    It would help if you are able to attach the model to this forum post - then someone might be able to look at it for you.

    Here are a couple of comments though:

    1. In the calculation summary 'flow supplied' means flow that comes from a Reservoir element. If you only have tanks in your model then this will always be zero. Most models have at least one Reservoir element.

    2. Your demands seem in the right ballpark. There is a Unit Demand library under Engineering Libraries with some example values that you can look at for guidance (though they are suited more for North America. In places like Australia the typical demands per resident tend to be much lower)

    3. If you're demands are entered correctly (check your units!) then the system will  supply those demands regardless of what the pump curve or pipe capacity is (provided you aren't using the advanced Pressure Dependent Demand option). So when you say you're flows are almost non-existent what do you mean? Again be careful of units because the flow could be in L/s while the demand is L/day.

    I would also recommend that you open up and study one of the example files that ships with the software. For example the file called Example5.wtg sounds similar to what you are trying to produce. You can find it here:

    32 bit : C:\Program Files\Bentley\WaterGEMS\Samples

    64 bit: C:\Program Files (x86)\Bentley\WaterGEMS\Samples

    Regards,

    Mal

          

    Answer Verified By: Craig Knudsen 

  • Hi Mal,

    Thanks for your quick reply. You are right about my model not having a Reservoir element, I have a tank supplying the network with an exaggerated diameter so that it doesn't undergo draw down during the EPS so that would explain why there is no flow supplied indicated. There is a well source obviously supplying the main storage after undergoing treatment but I have no information regarding the rate that it is stored so I left it out for the sake of simplifying the model.

    After I read your post I realized that when I was looking at my flow through the pipes under the flex table I had my units set to L/S which were reading as 0 - after changing it to L/day I had better results.

    One last problem I am noticing is that my velocities are quite low - I think the highest was .08 m/s when they should be within a range of .6 - 1.5 m/s. I'm not sure why that may be.

    I tried using the advanced reply editor to attach my model to this reply but it wouldn't allow me to upload the file for some reason because it had a .wtg and .sqlite extension. Would I be able to email you a copy of it instead?

    Thanks again for your help,

    Craig

  • Ive attached the model to this post along with files that accompany it. Thanks!

    MC Model.zip
  • I took a quick look at the model. The low velocities don't surprise me because the pipes are probably sized to supply fire flows on a peak day (and those demands would be much higher than the demands have in your model).

    You might consider adding some controls to the pumps though, or setting them automatically vary their speed. Currently they both run full speed all of the time so they are mostly running a long way from their best efficiency point.  To see this you can right click on a pump and look at the 'Pump Curve'.

    Regards,

    Mal

          

    Answer Verified By: Craig Knudsen 

  • Thanks Mal, this answers alot of questions.... your help has appreciated!