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Fire Flow Calibration - Question

I'm completing a fire flow calibration, and I've noticed some differences depending on how its completed. For our calibration, I've been applying demand to the hydrants. According to the literature, when completing a fire flow the hydrant setting should be changed for "open," however when this is done in addition to applying a demand the flow significantly exceeds the supplied demand. If demand is being applied to the hydrant in this way, is the hydrant supposed to be closed? From what I understand, the hydrant could also just be set to open without a demand and the flows should (hypothetically) match what was seen in the field, but its been a challenge to get this to match with so many additional factors throughout the model.

 

Additionally, when the demand is applied to the nearest node instead of the hydrant itself the pressure drop is fairly different. Is this difference only due to headloss in the hydrant?

Thanks!

  • When you make a hydrant "open", it calculates the fire flow based on the emitter coefficient and the local pressure. If you are also applying a known demand to the hydrant, then you are double counting the hydrant flow.

    If you measured the flow during the hydrant flow test, it is best to use that value rather than trying to dial in the emitter coefficient.

  • To add to Tom's reply, the following link has information for setting up and viewing results from an automated fire flow calculation: Understanding Automated Fire Flow Results.

    For the LoadBuilder question, if you are including lateral losses, there will be a difference in the results for the pressure. Another possible reason for the difference is a change in elevation. 

    Regards,

    Scott

  • Madison - note that the "hydrant status" field only applies to the Emitter Coefficient field. If you have also added demands to the hydrant via the "demand collection" or "unit demand collection" field, those demands will always be satisfied regardless of the hydrant status. See more here:

    How to use the "Hydrant Status" property in WaterGEMS and WaterCAD


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.

  • Hello,

    This question is a bit off-topic from the original, but suppose I have data from several fire hydrant flow tests, including measured flow at the 'open' hydrant and measured residual pressures at another hydrant downstream. I'm not sure what model results I should be comparing my field data to. For example, if I open a fire hydrant in the field and measure 1000-gpm, this isn't necessarily the 'Available Fire Flow' that the model calculates during an automated Fire Flow Analysis, correct? Available Fire Flow at a node depends on the pressure constraints in the calculation options; but in the field, the hydrant will flow what is available at that time based on tank levels, pump status, etc. Therefore, I don't know what parameter I should be trying to calibrate my model to.

  • Here's the approach I take.

    1. Compare the static HGL (Pressure) between the model and the hydrant flow test.

    2. It should be close. If not, find the problem and correct it. Check things like pressure zone boundaries, tank water level, gage elevation, PRV settings. Don't try to adjust roughness when the velocities are low.

    3. Once you've eliminated those issues and are confident in the model for normal demands, place a demand corresponding to the flow test on the nearest junction, and compare the HGL (pressure) at the residual hydrant wit the model results.

    4. If there are discrepancies, determine the cause and correct the model. It may be closed valves, network connectivity or roughness. If most of the model has fairly consistent pipe roughness but one area requires a lower value, there is usually a closed valve. Darwin Calibrator can help find the closed valve.