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Thread: Low Design Flow During Fire Flow

  1. #11

    Join Date
    Nov 2016
    Posts
    23
    I re-drew the network removing the short pipes and extra nodes and setting one node to supply to services. I increased the pipes from 8" to 10" to 12" and still did not achieve design flows over 1500 gpm. I'm attaching the results from the latest fire flow analysis.
    Attachment 388Attachment 389Attachment 390
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  2. #12

    Join Date
    Nov 2016
    Posts
    23
    please find the screen capture of the analysis
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  3. #13
    Forum Moderator

    Innovyze Employee



    Innovyze Employee



    Join Date
    May 2015
    Posts
    417
    David,

    Would you be able to contact us at support@innovyze.com so we can set up a WebEx and look at your model together?

    If you can't contact support try running a manual fireflow by placing a demand on the fireflow node in question and look to see where the system headloss is occurring.

    While increasing the pipe size is resulting in slight changes in the available fireflow, there appears to be something else that is your limitation that is difficult to tell with the information provided. A 10 or 12 inch line should eliminate the majority of the pipeline headloss so it should no longer be that. Another possibility is that your pump curve is the issue in that at higher flows the head drops or that the elevation of junction J10 may be severely limiting what head you have to "burn" in the fireflow. But it is definitely something besides the pipe size that appears to be limiting. Run a manual FF and look at what the pumps are doing and where the headloss is occurring. That is the only way to really identify what needs to be adjusted.


    If you do email support please refer to this forum and direct the email to me and I can get with you.

    Patrick Moore

  4. #14
    Forum Moderator

    Innovyze Employee



    Innovyze Employee



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    May 2015
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    417
    David,

    Glad we could meet and discuss this further via WebEx. For the sake of the forum I will post a few notes for others

    1) Change in water supply. Since you had a flow test from the system and there are no existing pumps we set the supply reservoir at the HGL of the static pressure in the flow test. We then created a general purpose valve that had Headloss vs. flow based on the flow test hydrant curve by calculating the total headloss at several flows as the Static Pressure minus the residual pressure at that flow for several points to get a relatively smooth curve.
    2) We then ran a manual FF to see what the pressure downstream of the valve would be at peak day (i.e. max day) demands plus your required FF with the manual FF of 1500 gpm at the farthest junction. The pressure downstream of the valve indicated an expected pressure of 34.5 psi. This mean we could only have 14.5 total psi headloss (~33 ft) in your system during a fire and still remain above 20 psi.
    3) To get a guess at the likely pipe size needed we used the hydraulic calculator in the InfoWater-> Tools-> Hydraulic Calculator and knowing the max main length was around 600 ft with C= 120, Q = 1900 gpm (1500 ff + 400 gpm Peak day demand) we calculated the expected headloss in 8, 10, and 12 inch lines and found 8 would likely be too much (> 33 ft HL), while a 10 should be fine(around 14-15 ft hl) and 12 seemed a bit oversized at around 4 ft HL.
    4) You were then going to change the pipe diameters and to verify using a FF run that your desired criteria would be met with these changes. This would also take into account the elevation change between the valve and the farthest node as this would also limit the maximum headloss available.

    One thing we should have also checked if we had time was to verify what the static pressure at the farthest node would be when the discharge valve was at 34.5 psi because any positive elevation difference between the valve and the farthest node you will supply fireflow will reduce the head available for fireflow. If we could have 33 ft of headloss but had 30 feet of elevation rise at the farthest node for fireflow you would then only have a maximum of 3 ft of headloss that could occur and to still remain at 20 psi. The one good news is that you may not need to have a hydrant at the end of the 600 ft capable of doing that as local fire codes (you should verify) may only require you to provide a fire hydrant within 500 ft of each home capable of meeting the required FF. This could reduce you pipe length from 600 ft down to maybe 100-200 ft which could make a huge difference.

    If the elevation change ends up being the constraint, then the pipe improvements may need to occur upstream of your new development such as upsizing an 8 inch main to a 10 in order to increase the available head at the Max day plus FF demand in your development. What you are trying to find is where the pipes are constraining you (i.e. as we saw the 8 inch supply lines really seem to be too small) and this may simply be upstream of your new development.

    Patrick

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