Station Smoke Screening User Guide

1What the tool does

The tool answers one question for a group of underground stations: which stations have the least favourable smoke reservoir, and should therefore be studied in detail with CFD?

For each station it runs a simple transient smoke-filling calculation. A design fire grows, its plume feeds a smoke layer under the ceiling, and the layer deepens until it spills under the downstands and descends towards the platform. The tool records how long that takes and compares it with the two NFPA 130 evacuation times: 4 minutes to evacuate the platform and 6 minutes to reach a point of safety.

Preliminary analysis only. The tool is a screening aid for ranking stations against each other. It does not replace CFD analysis, an egress calculation or a tenability assessment, and its results must not be used to demonstrate code compliance or for design.

2Screen layout

The page reads from top to bottom in three numbered steps, followed by Help and the method notes.

The tool as it opens, with one station — **The tool as it opens.** One example station is loaded so every part of the page shows a result straight away.

Area	What it is for
Header and intro	The fixed header with the tool name, then a short introduction with the two NFPA 130 time limits the results are checked against.
1 Stations	The station list on the left, and the selected station's inputs, results and animated section on the right.
2 Scenario	Assumptions that apply to every station: fire, time limits, extract, downstand, services blockage and selection rule.
3 Compare and select	Summary numbers, the vulnerability ranking and the layer-height chart for all stations.
Help	A five-line quick start and the link to this guide.
Method	The equations, the NFPA 130 limits and what is not modelled. Click the line to open it.

Every result updates as soon as you change a value. There is no Calculate button.

3Entering stations

Edit the first station

The tool opens with Station 1 filled with example values. Click in a field and type the station's own data. The results and the section redraw while you type.

**Step 1 with two stations.** The list on the left shows each station as a stop on the line. The highlighted stop is the one being edited on the right.

Field	What to enter
Station name	Any name. It appears in the list, the ranking and the chart key.
Reservoir plan area (m²)	Plan area of the smoke zone bounded by the downstands or walls, usually the platform public area within one smoke zone.
Floor to ceiling (m)	Clear height from platform floor to the underside of the slab or ceiling that holds the smoke.
Downstand depth (m)	Depth of the smoke downstand below the ceiling. Greyed out when one depth is set for all stations in step 2.
Services blockage (%)	Share of the reservoir volume taken by ducts, cable trays and other services. Higher blockage means less storage and faster filling.
Extract rate (m³/s)	Smoke extract rate serving this zone. Only used when smoke extract is switched on in step 2, otherwise greyed out.

Add, select and remove stations

+ Add station creates the next station with typical values and puts the cursor in its name field.
Click a stop in the list to select that station. Its section plays from ignition.
Remove this station deletes the selected station. The last remaining station cannot be removed.
Start over with one station clears the list and resets every scenario value to its default.

Paste a whole list

To load many stations at once, open Paste a station list and paste one station per line in this order:

name, plan area, floor to ceiling, downstand depth, blockage %, extract rate

Values can be separated by commas or tabs, so six columns copied straight from Excel work. Press Replace list. Lines that do not have a name and five numbers are skipped and counted in the message under the button.

The paste box with six stations — **Pasting six stations.** Replace list swaps the whole list for the pasted one.

What the stops show

Filled orange: the layer reaches the critical height within 4 minutes.
Orange ring: within 6 minutes.
Black ring: holds beyond 6 minutes.
CFD The station is selected for CFD under the rule in step 2.
Under each name: the time to critical height and, with more than one station, its rank.

4Reading the station section

The right-hand panel in step 1 shows everything about the selected station.

**The station panel during filling.** Inputs at the top, the four results, the time controls, the animated section and the live readouts.

The four results

Result	Meaning
Reservoir full, spill under downstand	Time at which the layer reaches the bottom of the downstand. After this, smoke can pass to the next zone or floor.
Layer reaches critical height	Time at which the layer base descends to the critical clear height set in step 2. This is the value used for ranking.
Margin on 4 min platform limit	Time to critical height minus 240 s. Negative means the layer gets there before the platform evacuation time.
Margin on 6 min point of safety limit	Time to critical height minus 360 s.

A result box turns orange when it is unfavourable. The pill beside the station name gives the overall check: Below 4 min Below 6 min Holds 6 min

Play and the time slider

Play button and time slider — **Time controls.** Play runs from the current time to the end of the simulated duration. Drag the slider to stop on any moment.

Play runs the animation. The button becomes Pause while it runs and Replay at the end.
Drag the slider to move to a time. The number beside it is seconds from ignition.
When you select a station it plays from zero. When you edit a value, the section jumps to the moment the layer reaches the critical height.

What is drawn

The black bar at the top is the ceiling, with a downstand at each end of the reservoir. Heights are to scale; widths are not.
The hatched block under the ceiling is the share of the reservoir taken by services.
The grey moving cloud is the smoke layer. Its wavy lower edge is the layer base, labelled with its height on the right.
The flames grow with the heat release rate, and puffs rise in the plume to the layer.
Orange arrows and drifting puffs at the downstands appear when the reservoir is full and smoke is spilling.
The dashed line is the critical clear height. It turns orange and thicker once the layer crosses it.
Top right, the section says whether the current time is within the 4 minute or 6 minute limit, or past both.

The figures

Figures orange — **Orange figures.** The layer base is at or below the critical height.

Figures red — **Red figures.** The layer base has reached head height.

Black: the layer is above the critical height.
Orange: the layer base is at or below the critical height.
Red: the layer base is at or below head height, taken as 1.75 m.

The colours are indicators from the layer height only. The tool does not calculate visibility distance, temperature exposure or toxicity.

Live readouts

The six boxes under the section give the values at the time shown: time, heat release, plume mass flow, layer temperature, layer height and state. The state reads Fire growing, Reservoir filling, Spilling under downstand, Below critical height, or Smoke at head height.

5Setting the scenario

Step 2 holds the assumptions shared by all stations. Changing any of them recalculates every station.

**The scenario band.** Five groups, from the fire on the left to the selection rule on the right.

Setting	Default	What it does
Peak heat release rate	2000 kW	Size of the design fire once fully grown.
Fire growth	Fast	t² growth rate up to the peak: slow, medium, fast or ultra-fast.
Convective fraction	0.7	Share of the heat release carried up in the plume.
Layer heat loss	0.3	Share of the convective heat lost from the layer to the structure. More loss gives a cooler, denser layer that fills more slowly.
Ambient temperature	30 °C	Air temperature before the fire.
Critical clear height	2.5 m	Height above the floor the layer must stay above. Drives the ranking.
NFPA 130 limits	240 s, 360 s	Fixed. Platform evacuation and evacuation to a point of safety.
Simulated duration	1800 s	How long each station is run.
Include smoke extract	Off	Off gives the pure filling time. On removes smoke at each station's extract rate.
Extract start delay	180 s	Time from ignition until extract starts, covering detection and fan run-up.
Extraction efficiency	100%	Share of the extracted flow that is smoke from the layer. Lower it to allow for clear air drawn up from below.
Downstand depth for all stations	Blank	Blank uses each station's own depth. Enter a value to test one depth across the whole line.
Blockage min and max	20%, 50%	Range for assigning services blockage when it has not been surveyed.
Assign blockage to all stations		Overwrites every station's blockage with a value from a normal distribution centred on the middle of the range. The same list always gets the same values.
Selection threshold	540 s	A station is selected for CFD when its time to critical height is at or below this.
Maximum stations to select	10	Upper limit on the number selected, taken from the top of the ranking.

6Comparing and selecting

**Step 3 with six stations.** Summary tiles, the ranking on the left and the layer-height chart on the right.

Summary tiles

The four tiles give the number of stations screened and selected, how many reach the critical height within 4 minutes and within 6 minutes, and the shortest time with the station it belongs to.

Vulnerability ranking

Rank 1 is the station whose layer reaches the critical height soonest.
Stations where the layer never reaches the critical height are placed last, ordered by the lowest layer height they reach.
Shaded rows with an orange edge are selected for CFD.
Click a row to open that station in step 1 and play its section.
Copy as CSV copies the table to the clipboard for pasting into Excel or an email.

Layer-height chart

Each station has its own line style and carries its rank number in a circle on the curve.
Orange lines are selected for CFD; black lines are not. The thick line with the filled marker is the station open in step 1.
The horizontal dashed line is the critical height. The dotted vertical lines are the 4 and 6 minute limits, and the shaded band covers the first 6 minutes. The orange dashed line is the selection threshold.
Point at a station in the key under the chart, or at a ranking row, to fade the others. Click to open that station.

7A typical session

Collect, for every station, the reservoir plan area, floor to ceiling height, downstand depth, services blockage and extract rate.
Paste the list in step 1, or add stations one at a time.
In step 2, set the design fire and critical height from the project basis of design. Leave extract off for a first pass.
If blockage has not been surveyed, set the range and press Assign blockage to all stations.
Read the ranking in step 3. Click the top stations and play their sections to see why they rank high: low ceiling, shallow downstand, small area or heavy blockage.
Adjust the selection threshold or maximum number until the shaded set matches the number of CFD cases the project can run.
Switch extract on and repeat, to see which stations still fall short with the system running.
Copy the ranking as CSV and record the scenario values used alongside it.

8Saving and sharing

The station list and scenario are kept in the browser on this computer and reappear when the tool is opened again in the same browser. They are not stored inside the HTML file.
Sending the HTML file to someone gives them the tool with one example station, not your data. Send the copied CSV, or the pasted station list, alongside it.
The tool is two files: index.html and this guide, help.html, plus the assets folder holding the logo and icon. Keep them together in the same folder so the links between them work.
Both files run offline. Nothing is sent over the internet.
To keep a record of a section or chart, use the browser's print to PDF, or a screen capture.

9Common questions

Why does a result say "Not reached"?

The layer did not get that low within the simulated duration. This usually happens with extract on, where the layer settles at a steady height. The station is then ranked by its lowest layer height.

Why is the extract rate field greyed out?

Smoke extract is switched off in step 2. Tick Include smoke extract to use it.

Why is the downstand depth field greyed out?

A single downstand depth is set for all stations in step 2. Clear that field to return to each station's own depth.

Why did my blockage values change?

Assign blockage to all stations overwrites them. Type the surveyed values back in, or paste the list again.

Are the 4 and 6 minutes the station's actual evacuation times?

No. They are the NFPA 130 limits used as fixed yardsticks. The tool does not calculate an egress time for each station.

Why does nothing move?

If the computer is set to reduce motion, the smoke is drawn still and Play jumps straight to the moment the layer reaches the critical height. The slider still works.

How accurate are the times?

They are comparative. The calculation treats each station as a single well-mixed smoke zone with a plume from NFPA 92, and ignores spill to other zones, tunnel air movement, inlet air and ceiling jet travel time. Use the times to rank stations, then confirm the selected ones with CFD.