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Thread: Fire Hydrant Headloss

  1. #1

    Join Date
    Nov 2014

    Fire Hydrant Headloss

    Hi all

    Our current residential fire flow standard specifies a minimum residual pressure of 200kPa at the outlet of the hydrant for a fire flow of 10L/s. We use DN100 mushroom sealing and spring force closer hydrants.

    To ensure this standard is achieved we wish to incorporate the headloss across the hydrant into our design modelling standards.

    Has anyone ever undertaken any field tests to determine the actual headloss across this type of hydrant for a fire flow of 10 L/s? What hydrant headloss figures do other modellers use?


  2. #2
    Forum Moderator

    Innovyze Employee

    Innovyze Employee

    Join Date
    May 2015

    It looked like no one had replied to your thread, so I thought I would chime in as I recently shared something similar in a different thread.

    In the US,most hydrants (if not all) in the US are governed by the AWWA C502-94 standard. This standard states the following:

    I would suspect there are similar standards used in the UK which specify a maximum headloss for a hydrant at specified flowrates for a DN100 hydrant. US manufacturers will also test their specific hydrant products to show they meet the applicable standards and will usually include these materials in any hydrnats included in a construction project submittal packet.

    One user, had wanted to add a small pipe at the end of a hydrant node to account for the hydrant loss. I noted that if this type of loss was included the user should keep in mind the following:

    1) The headloss should not exceed the maximums noted in the applicable standards

    2) Except for instances where you are checking just a few hydrants in the model, Adding a short element to each hydrant would be time consuming considering the high number of hydrants in a typical system model). So just be aware if incorporating system wide, adding a short pipe could be time intensive.

    3) Using a minor loss coefficient on the hydrant lead can be used to replicate the expected headloss at a specific flow, but will only be accurate at the flow used to develop it since the headloss characteristics for hydrants usually varies with flow and will depend on the nozzle used to develop it. To accurately model a specific headloss, one could add a valve with a specific headloss vs. flow curve and this would most accurately replicate the varied loss, but would be time intensive to incorporate.

    4) The last challenge is that most utilities use more than one hydrant manufacturer and more than one Hydrant model type and to be most accurate, and headloss characteristics for each type would need to be used, unless the utility developed a generic type for design purposes.

    I think for many modelers (at least in the US), they are comfortable enough with the standard results that they do not account for these additional losses especially given the challenges associated with adding short pipes or minor loss coefficients to each hydrant lateral. Certainly most models I am personally familiar with and industry standard practice does not typically account for any specific loss at the hydrants. In fact many models don't even include the hydrant laterals which can often have more headloss than the outlets would. But if modeling the losses is important, there are ways it can be done. The key is understandiThe suggestions above can provide a few ways it could be done.

    Example calculations can be found in this thread if interested:

    Unfortunately there are not really any modeling standards that have been developed to provide guidance on what to include or leave out in a fireflow analysis, so the modeler must use a lot of engineering judgment to decide what is critical and what is not. A lot of times, I found you focus mainly on the mainline pipe capacity and ignore the losses at the hydrant, as right now it is not too easy to incorporate specific nozzle losses given the variability in hydrants and the difficulties in knowing what nozzles are used alone or in combination which would also impact the potential headloss expected. I suspect, due to the high number of variables, no one has been able to add this type of feature to a model automatically. But a specific user can always add a specific headloss vs flow when needed and so it has been left for the Modeler to decide how best to deal with it.


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    Patrick Moore

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