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Thread: Design Fire Flow, MDD vs. ADD

  1. #1

    Join Date
    Nov 2018
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    1

    Design Fire Flow, MDD vs. ADD

    This may be more of a hydraulics question than an InfoWater software question but here it goes. When I run a fire flow simulation with Max day demand, the available (or design) flow is greater than if I run a fire flow simulation with average day demand. Intuitively I would think the available fire flow would be less during max day demand since the demand on the system is greater but this is not what the results are suggesting. Any thoughts?

    Thanks!

  2. #2
    Forum Moderator

    Innovyze Employee



    Innovyze Employee



    Join Date
    May 2015
    Posts
    417
    Katie,

    Demands are just one factor in determining what is available. Tank levels and if boosters are running will also play a very large role in determining the available and or design Fireflow values. If your system boundary conditions are not exactly identical for both runs it is very possible that you could see results just as you did, although I would still check my system setup to make sure there was not any errors in the scenario setup as well that were contributing to the differences as one should always check that.

    One easy way to check is to compare the system Head (HGL) before the fire at the same location in both scenarios. If the starting HGL during Max day was higher than the starting head during the ADD FF run it is very likely the Max day run may give a higher fireflow. The more head there is the more flow that can be provided.

    This is one of the reasons many engineers prefer to not run fireflow on an EPS run where you choose a specific hour as you have very little control over tank levels, system demands, and pump operation. All of these factors will definitely influence the fireflow results as they will impact the hydraulic grade line available to "push" the additional fireflow supply into the system to satisfy the fireflow demand. Many engineers get around this by setting up specific steady state scenarios to run fireflow where you can directly control all tank levels, what pumps are allowed to operate and also get "exact" maximum day demands at every point in the system. When you set up fireflow scenarios this way it can be much easier to compare the impact of just changing demands has on the available fireflow. If the system conditions are not identical (besides the demand) it is harder to truly see the impact the demand has on the results.

    I would still check your model setup to make sure the differences are not due to a simple mistake somewhere as this can easily occur with all of the data in any given model, but it certainly is not out of the realm of possibility to get the results you are seeing if the conditions are different for each fireflow run besides just the demands.

    I was able to confirm that Utilities Kingston has a current InfoCare with us, so if you would like to have a call or WebEx with one of our team if you are unable to verify the differences, please email us at support@innovyze.com and please refer to this thread on the user forum in your inquiry for background.

    Patrick Moore


    Quote Originally Posted by KatieM View Post
    This may be more of a hydraulics question than an InfoWater software question but here it goes. When I run a fire flow simulation with Max day demand, the available (or design) flow is greater than if I run a fire flow simulation with average day demand. Intuitively I would think the available fire flow would be less during max day demand since the demand on the system is greater but this is not what the results are suggesting. Any thoughts?

    Thanks!

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