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Fire flow analysis and EPS Snapshot question.

I'm trying to reconcile some fire flow results I'm seeing vs. the EPS results.

Namely, I have multiple junctions that reach low pressures (below 20 psi) during certain time steps of an EPS run.

For instance, at 1:24 am, one particular junction exhibits a pressure of roughly 17 psi as shown in the pic below. Considering how low this pressure is during regular, non-fire flow demands, I would expect the fire flow results for this node to fail during this time step, since I have a min. residual pressure of 20 psi set as the lowest limit in the fire flow alternative.

When I run the fire flow scenario at 1:24 am (EPS Snapshot), this junction passes as shown below. 

Maybe I'm not understanding the mechanics behind the fire flow scenario and EPS Snapshot, but if this node is struggling to provide 16 gpm @ 20 psi in the EPS scenario, how can it provide 1,000 gpm in the fire flow scenario at the same time step?

      

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  • Hello David,

    It would be useful to see a copy of the model files to look into this.

    Regards,

    Scott

  • It's uploaded Scott. The EPS and fire flow scenarios in question are 2019 PDD and 2019 PDD+FF, respectively. 

  • Hello David,

    Thanks for providing the model. To test this, I put a demand of 1,085.37 gpm on TRIL_J-41 and then ran the steady state run using EPS snapshot. The pressure results at TRIL_J-41 were reasonably close to 20 psi, and the pressure at TRIL_J-40 was just a little above 20 psi, as seen when running a fire flow analysis. 

    I did need to remove TRIL_J-41 from the selection set that it is in. This is because there is a demand adjustment on the selection set that multiplies the demand results by 1.8. This would have made a much larger demand compared to the fire flow analysis. If you did not do this in your test, please give that a try and see if that helps.

    Regards,

    Scott

  • Scott - thank you for your reply. While your test lends validity to the fire flow results, there's still the question of how pressure at a node could be lower during the EPS run than the FF run.

    Discussing this with someone else pointed toward the tank levels. Since the FF scenario doesn't fluctuate tank levels like the EPS scenario, even when using EPS Snapshot, it's possible to have higher tank levels in the FF scenario and vice versa, depending upon which time step is chosen. That appears to be what is happening in this case.

    During the EPS run at 1:24 am, the tank closest to this node is almost empty, but its initial level is 1/3 full, which is what the FF scenario is using.

    Does that sound feasible?

  • Hello David,

    The the conditions are not the same as the EPS Snaphot, like the tank level or a pump status, the results will be different. With the tank at a lower elevation, it would be unable to produce as much head as during the fire flow run. So that could have an impact on the results. 

    Regards,

    Scott

  • To close the loop on this, I have added a section to the following article called "Comparing Automated Fireflow Results to EPS"

    Running a fire flow analysis in EPS (Extended Period Simulation)


    Regards,

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

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