The Hydrant That Isn't There
You are planning water supply for a large warehouse fire on the edge of town. Your pump carries 1,800 litres. The nearest hydrant is marked on the map 400 metres away, but when you send a firefighter to check, the hydrant is capped and the cover is seized. A second hydrant is 900 metres away and is reported working. Your main jet uses 450 litres per minute at the branch. A second jet, if you run one, uses another 350 litres per minute. The water tender can shuttle 2,000 litres per trip and takes 6 minutes per round trip including filling and dumping. You need to sustain at least one jet for 20 minutes before the second appliance arrives.
1. Can you sustain one jet for 20 minutes on your own water alone? Show the arithmetic.
Read the worked answer
No.
- One jet at 450 L/min for 20 minutes = 450 × 20 = 9,000 litres.
- Your pump carries 1,800 litres.
- 1,800 ÷ 450 = 4 minutes of one jet on pump water alone.
So you need the tender shuttle running from the start, not after the pump runs dry. Plan the shuttle before you commit the jet.
2. With the tender shuttling from the working hydrant 900 m away, can you run one jet continuously? Show the arithmetic.
Read the worked answer
Yes, but only just, and you must not add a second jet.
- Tender delivers 2,000 L every 6 minutes = 2,000 ÷ 6 ≈ 333 L/min.
- One jet needs 450 L/min.
- Deficit = 450 − 333 = 117 L/min, drawn from your 1,800 L pump buffer.
- Buffer lasts 1,800 ÷ 117 ≈ 15.4 minutes.
So one jet is sustainable for about 15 minutes on the buffer plus the shuttle, and the second appliance must arrive before then or you must reduce the jet. A second jet at 350 L/min would need 800 L/min total - the shuttle cannot come close, and the buffer would last about 2 minutes. Do not run two jets.
Conclusion: one jet, shuttle running immediately, second appliance requested now, and tell the incident commander the water plan in those numbers.