Relay pumping to a village factory fire
It is 14:20 on a hot, dry afternoon. You are the station officer commanding a pump ladder from a rural station. A fire is reported in a single-storey garment factory in a village 600 m from the nearest hydrant. The hydrant is on a 100 mm dead-end main with poor pressure. A static concrete tank beside the village hall holds an estimated 80,000 L and has a hard-suction point. Your hose lay to the factory gate is 20 lengths of 70 mm hose, and the nozzle is 20 m above the tank outlet. Your friction-loss card gives 0.2 bar per 30 m length at 1,000 L/min. You plan a nozzle pressure of 5 bar and allow 1 bar for appliance and nozzle losses. Two 10,000 L tankers are available from neighbouring stations. Each needs 8 min to fill at the village tank, 20 min round trip, and 4 min to dump. The fire needs at least 1,000 L/min to avoid extension.
1. Calculate the required pump discharge pressure at the static tank and decide whether a single pump can supply the nozzle. State any assumptions.
Read the worked answer
Pressure build-up at 1,000 L/min
- Hose friction: 20 lengths x 0.2 bar = 4.0 bar.
- Static elevation: 20 m ÷ 10 m per bar = 2.0 bar (taking 1 bar ≈ 10 m of water).
- Nozzle pressure required: 5.0 bar.
- Appliance and nozzle allowance: 1.0 bar.
Total pump outlet pressure = 4.0 + 2.0 + 5.0 + 1.0 = 12.0 bar.
A single pump can meet the demand only if its performance curve gives at least 12 bar at 1,000 L/min after allowing for suction lift, hose kinks and any relay losses. The calculation proves the demand, not the pump's capability. The hard-suction point must be checked for adequate water depth and to avoid drawing air; if the pump is more than about 3 m above the water, suction performance falls. The answer should therefore be: the lay requires 12 bar at the pump outlet, and a single pump is adequate only if its rated performance at that flow is above 12 bar with margin. If not, place a relay pump or reduce the hose lay.
2. How long will the 80,000 L tank sustain 1,000 L/min without tankers? With the two tankers, what is the net drawdown rate, and how many more tankers are needed?
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Tank duration without tankers
80,000 L ÷ 1,000 L/min = 80 min.
Tanker delivery rate
Each tanker cycle = 8 min fill + 20 min travel + 4 min dump = 32 min.
Delivery per tanker = 10,000 L ÷ 32 min = 312.5 L/min.
Two tankers deliver 2 x 312.5 = 625 L/min.
Net drawdown from the static tank = 1,000 - 625 = 375 L/min.
With two tankers, the tank lasts 80,000 ÷ 375 ≈ 213 min, but this assumes perfect cycling and no fireground reserve. To replace the full 1,000 L/min, the number of tankers required is 1,000 ÷ 312.5 = 3.2, so 4 tankers are needed. Two are available, so 2 more are required to stop the tank falling. The practical message is that two tankers only slow the decline; they do not sustain the fire flow.
3. At minute 50 the tank is down to 30,000 L and the fire has extended. Set out a water-supply plan and a trigger for withdrawing crews.
Read the worked answer
At minute 50, 50,000 L has been used at 1,000 L/min. With only two tankers, the net drawdown is 375 L/min, so 30,000 L gives about 30,000 ÷ 375 = 80 min before the tank is empty. That is a planning figure, not a安全 margin: suction may fail before the tank is empty, and fireground flow often exceeds the plan.
Water-supply plan
- Appoint a water-supply officer and record tank level every 5 min.
- Request the two additional tankers immediately, plus a second alarm if the fire has extended.
- Identify a second water source, such as a distant hydrant relay or a larger static source, rather than relying on the village tank.
- Reduce branch flow where it does not compromise firefighting; a 1,000 L/min fire flow is a minimum, not a target to exceed unnecessarily.
- Keep a reserve for BA emergency and withdrawal.
Withdrawal trigger
Set a trigger at 15,000 L remaining, or 15 min of reserve at the planned flow, whichever comes first. At that point the incident commander should move to defensive operations unless a confirmed alternative supply is flowing. No crew should be withdrawn on a single sector officer's judgement alone; the trigger must be communicated to all sectors and to Control.