Seventh-Floor Office Fire: Dry Riser Pump Pressure and a Smoke-Logged Stair
Fire is reported on the seventh floor of a nine-storey office block with 3.5 m floor-to-floor height. On arrival, smoke is issuing from one seventh-floor window and occupants are walking down the single protected stair. The building has a dry riser with two 65 mm ground inlets and 65 mm landing valves; the booster pump and fire pumps are out of service, so the riser must be fed from the appliance. The seventh-floor landing valve is 24.5 m above the pump. The plan is one branch from that valve, two 30 m lengths of 45 mm hose to the fire room, delivering 400 L/min at 4 bar at the branch. Friction loss is 0.25 bar per 30 m length of 45 mm hose; riser, appliance and fitting losses are estimated at 0.6 bar. A cleaner has wedged open the seventh-floor stair door and smoke is now entering the stair. The nearest hydrant will give 1,200 L/min at 3 bar residual.
1. Calculate the pump pressure needed for the first branch, then decide whether a second branch of 400 L/min can be added from the same landing valve with the available water supply.
Read the worked answer
Pump pressure = branch + elevation + hose loss + riser/appliance loss
- Branch pressure required: 4.00 bar
- Static head: 24.5 m ÷ 10 = 2.45 bar
- Hose friction: 2 lengths × 0.25 = 0.50 bar
- Riser, appliance and fittings: 0.60 bar
Total = 4.00 + 2.45 + 0.50 + 0.60 = 7.55 bar, round to about 7.6 bar at 400 L/min.
Adding a second branch: the second branch has the same elevation, its own hose and its own 4 bar requirement, so the pump pressure stays near 7.6 bar; riser friction rises with the doubled flow, so allow perhaps 0.2–0.4 bar more - set about 7.8–8.0 bar and confirm at the branch gauge.
Water supply: combined demand is 800 L/min. The hydrant delivers 1,200 L/min at 3 bar residual, so 800 L/min leaves useful margin. Verify by watching the compound gauge: if residual pressure falls sharply (in the order of 1 bar) when the second branch opens, the supply is being overtaxed and the second branch should be fed from a second appliance or a second hydrant.
Also check the riser inlet plate for its rated working pressure before pushing the inlet above the calculated figure, and confirm the landing valve and non-return are functioning by charging with the branch open.
2. Smoke is entering the stair through a wedged-open seventh-floor door while occupants from floors eight and nine still have to descend past that level. Set out your priorities.
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This single stair is the only escape route for the upper floors, so the wedged door has converted a fire problem into a life-safety problem for two floors above the fire.
Priority 1 - door control. A crew in BA closes the seventh-floor stair door immediately and keeps it closed; the door is the smoke barrier and it works only when shut. This is faster and more reliable than any fan.
Priority 2 - protect the descending occupants. The smoke layer in the stair is being fed from the seventh floor; with the door closed, natural leakage and any residual flow can still leave the stair contaminated. Consider a fan at the ground floor entrance to pressurise the stair, keeping the seventh-floor door shut so the pressure holds - this protects the escape route rather than ventilating the fire.
Priority 3 - manage floors eight and nine. If the stair above the seventh floor is already smoke-logged, do not push occupants into it. Tell them to stay in their compartments behind closed fire doors (defend in place), give a clear instruction by floor warden or PA, and clear the stair before moving them. Lifts are not used.
Priority 4 - attack. The seventh-floor lobby is the bridgehead; the branch goes in from there with the stair door closed behind the crew. Firefighting from the stair itself will destroy the only escape route.
3. Compare feeding the branch from the dry riser with laying 45 mm hose up the stair, and state which you would use with reasons.
Read the worked answer
Riser route: two 30 m lengths from the landing valve to the fire room. Pump at the inlet at ground level, 7.6 bar, branch running within a minute or two of the inlet being charged. The stair stays clear for evacuation and for the second crew, and the hose is not lying in the path of descending occupants.
Stair lay: with 3.5 m floors and a 9-storey building, reaching the seventh floor by stair needs roughly two lengths per floor, about 12–14 lengths (over 350 m of 45 mm hose). The friction loss alone would be 14 × 0.25 ≈ 3.5 bar on top of the 2.45 bar head and 4 bar at the branch - about 10.5 bar at the pump before appliance losses - and the lay has to be made up the stair by crews in BA, taking many minutes while the fire grows. It also fills the only stair with hose, obstructing evacuation and the second crew's advance.
Decision: use the dry riser. It is precisely what the installation is for. Conditions: feed the inlet at ground level and keep an operator on it, confirm the landing valve and non-return work, keep the inlet below the riser's rated working pressure, and have a backup in case the riser is damaged or blocked - the stair lay remains the fallback, started early if there is any doubt.