Add a secondary pressure constraint to identify the junction with the lowest pressure within a search range
A Design Fireflow simulation allows you to add a secondary pressure constraint to identify the junction with the lowest pressure within a search range. The hydrant available flow calculated in the standard fireflow simulation is recalculated based on this critical node, so its pressure does not drop below the minimum pressure constraint. This new value is called the hydrant design flow.
To restrict the analysis to the selected domain and configure the Design Fireflow simulation:
While simulating a hydrant flowing, the software will check the pressures at all junctions within the search range. It will also recalculate the available flow at the hydrant until none of the pressures drop below the minimum pressure constraint (20 psi in this example).
The Fireflow Design report opens automatically in the Report Manager.
IMPORTANT: Any junctions within your search range with a standard simulation pressure less than the minimum pressure constraint will not be in the results. This is because pressure is reduced as the flow is increased. A standard simulation would be lower flows than fireflow, where demands are increased by 1000+ gpm. Therefore, if a junction's pressure is less than the constraint before large demands are applied, adding that increased flow is only going to further reduce pressure. So essentially, if a junction cannot maintain the minimum pressure constraint before fireflows are applied, then there is no way it could handle those intended fireflows.
The column headers show the resulting hydrant and critical node data. The Critical Node Pressure at Fire Demand field is identifying the lowest pressure in the search range when the fireflow demand is applied, which is the most constraining pressure in the search range. If this is greater than or equal to your pressure constraint—in this case, 20 psi for these units—then the entire system can support what is required.
