Station Smoke Reservoir Screening
Underground stations, fire and life safety
Underground stations, fire and life safety
See how fast the smoke layer descends at each station, check it against the NFPA 130 evacuation times, and pick the stations that go forward to CFD.
A simplified zone-model screen for comparing stations and prioritising them for study. It does not replace CFD analysis, an egress calculation or a tenability assessment, and must not be used to demonstrate compliance or for design.
Enter a station and watch its reservoir fill. Add the next station when you are ready.
Heights are to scale, with the figures 1.75 m tall. Widths are schematic. The hatched block is the share of the reservoir taken by services.
Assumptions applied to every station. Change one and all results update.
Off by default, which gives the pure filling time.
| Rank | Station | Effective reservoir (m³) | Spill under downstand (s) | Critical height (s) | Margin on 4 min (s) | Margin on 6 min (s) | Lowest layer (m) | NFPA 130 time check |
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Shaded rows are selected for CFD. Click a row to open that station in step 1. Stations where the layer never reaches the critical height are ranked last, by lowest layer height.
Each station has its own line style and carries its rank number. Orange lines are selected for CFD. Point at a station in the key or the ranking to isolate its curve.
How to operate the tool. The full user guide opens as a separate page.
Keep help.html in the same folder as this file.
Time limits. NFPA 130 (stations, means of egress, Section 5.3.3) requires enough egress capacity to evacuate the platform occupant load from the platform in 4 minutes or less, and evacuation from the most remote point on the platform to a point of safety in 6 minutes or less. This tool uses those two times, 240 s and 360 s, as the limits the smoke layer is checked against. Confirm the clause numbers and values against the edition adopted on the project.
Calculation. Each station is one smoke zone bounded by its downstands. The fire grows as Q = α t² up to the peak. The plume entrains air up to the layer base using the axisymmetric plume relation in NFPA 92, with z the clear height from floor to layer base and Qc the convective heat release in kW:
z > zl: m = 0.071 Qc^(1/3) z^(5/3) + 0.0018 Qc kg/sz ≤ zl: m = 0.032 Qc^(3/5) z kg/s, with zl = 0.166 Qc^(2/5)Layer temperature is Ts = T0 + (1 − loss) Qc / (m cp), and the volume added each second is m Ts / 353. Extract, when included, removes its volume rate multiplied by the extraction efficiency after the start delay. The efficiency is the share of the extracted flow that is smoke from the layer, the rest being clear air drawn from below. Inside the downstand depth the plan area is reduced by the services blockage. Where the blockage is not known, it can be assigned from a normal distribution truncated to the chosen range (20 to 50% by default), centred on the midpoint with the range taken as six standard deviations. Below the downstand the full plan area applies, which assumes the smoke stays within the same footprint.
Not modelled. Smoke spill to adjacent zones and floors, ceiling jet transport time, tunnel and piston air movement, inlet air effects, non-uniform layers, visibility and toxicity. The figure colours are indicators only: orange when the layer base is at or below the critical height, red when it is at or below a 1.75 m head height. The evacuation times are the NFPA 130 limits, not a calculated egress time for each station.