How does the Automated Fire Flow analysis work?

Applies To 
Product(s): WaterGEMS, WaterCAD
Version(s): CONNECT Edition, V8i
Area:  Modeling
Original Author: Jesse Dringoli & Mark Pachlhofer, Bentley Technical Support Group


How does the Automated Fire Flow analysis feature work? 


Note: For a demonstration, see this Webinar recording

Fire flows are computed at each node by iteratively assigning demands and computing system pressures. In short, it looks at each node one at a time, and determines the amount of flow available without violating the desired constraints. When you compute a fire flow analysis, the program will:

  1. Calculate a steady-state simulation for all nodes designated as fire-flow nodes. At each node, it begins by running a Steady-State simulation using only non-fire demands, to ensure that the fire flow constraints (e.g., minimum residual pressure, minimum zone pressure) that have been set can be met without withdrawing any Fire Flow from any of the nodes.
  2. Evaluate the Fire Flow Upper Limit and Available Fire Flow at each of the fire-flow nodes. Assuming the fire flow constraints were met in the initial run, the program performs a series of steady-state runs in which flow is applied to each specified fire-flow node and results are evaluated against fire-flow constraints. Note that the fire flow for each individual node is evaluated using a separate analysis (i.e., needed fire flow is not applied simultaneously to all fire-flow nodes).
    1. The program performs a series of steady-state analyses in which the Fire Flow Upper Limit discharge is applied to each node in turn. If the fire flow constraints are met for the Fire Flow Upper Limit discharge, the node satisfies the fire flow constraints and no further analysis is required for that node.
    2. The program then performs a series of steady-state analyses in which it iteratively assigns lesser demands to nodes that do not meet Fire Flow Upper Limit constraint to determine the Avalable Fire Flow. The Available Fire Flow is the maximum fire flow that each node can supply without violating fire flow constraints.
    3. If the Available Fire Flow is greater than or equal to Needed Fire Flow, the node satisfies the fire flow contraints. If Available Fire Flow is less than Needed, it does not.
  3. Run a final Steady-State calculation that does not apply Fire Flow demands to any of the junctions. This provides a baseline of calculated results that can then be compared to the Fire Flow conditions, which can be determined by viewing the results presented on the Fire Flow tab of the individual junction editors, or in the Fire Flow Tabular Report. Do not confuse the baseline results such as "Pressure" (where no fire flow is applied) with the fire flow results such as "Pressure (Calculated Zone Lower Limit)" (where the fire flow is applied).


  • For assessing tank capacity for fire fighting, see Running a fire flow analysis in EPS (Extended Period Simulation). However, there is some uncertainty here, as seen in this forum discussion.
  • Pressure at customer meters is not currently part of the zone or system pressure constraint check, nor is it included in the auxiliary results (for use with the fireflow results browser). In a typical model with a large number of customer meters, this would cause too much data to be stored and increase the complexity of the fireflow calculations. Instead, you would need to rely on the pressure at nearby junctions which would typically be at a similar elevation.

Videos / Training

See Also

Understanding Automated Fire Flow Results

WaterGEMS and WaterCAD Automated Fire Flow FAQ  

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