A fire water system has three numbers: how much it holds, how fast it delivers and how far it reaches. A bucket line delivers 50-80 L/min from a reserve of at least 200 L per cluster, while a tank 10 m above the nozzle could deliver up to 412 L/min through a 25 mm hose, 5 to 8 times as fast, but would empty 5,000 L in about 12 minutes, so pressure buys rate and not volume.
Part of Fire Prevention in Settlements
A fire water system is the combination of stored water, the means of moving it to a fire and the means of applying it, planned together so that rate, volume and reach match the job.
The Reserve, The Route And The Nozzle Each Set A Limit
A system is judged by three numbers: volume, rate and reach.
The fire guide sets the reserve (at least 200 L per cluster of 4-8 houses, ideally 1,000 L or more, at least 5,000 L for 150 people, within 30 m of every building) and the way it is moved: a bucket brigade of two lines, passing 50-80 L a minute. The water-point article turns those into minutes of brigade time (Water Points for Fire Fighting); the layout guide adds a pressurised source in each zone. Where water already flows by gravity (Gravity-Fed Plumbing, about 0.1 bar of pressure for each metre of height), it can serve a fire hose or a fill tap as well as a household tap.
Worked Example: What A 10 m Head Can Do
Inputs: tank surface 10 m above the nozzle; water density 1,000 kg/m³; g = 9.81 m/s²; a nozzle 25 mm across; reserve 5,000 L. The result is the ideal case, with no hose friction or losses; real flow is less, and it should be measured by timing the filling of a barrel.
- Pressure: 1,000 × 9.81 × 10 = 98,100 Pa, about 0.98 bar.
- Jet speed: v = √(2 × 9.81 × 10) = 14.0 m/s.
- Nozzle area: π × 0.0125² = 4.91 × 10⁻⁴ m².
- Flow: 14.0 × 4.91 × 10⁻⁴ = 6.87 × 10⁻³ m³/s = 6.87 L/s = 412 L/min.
- Compared with a bucket line: 412 ÷ 80 = 5.2 times to 412 ÷ 50 = 8.2 times faster.
- How long 5,000 L lasts: 5,000 ÷ 412 = 12 minutes, against 5,000 ÷ 80 = 62.5 to 5,000 ÷ 50 = 100 minutes for a brigade.
- Reach, ignoring air drag: a jet at 45 degrees travels v² ÷ g = 14.0² ÷ 9.81 = 20 m.
- Reaction on the person holding the nozzle: ρ × Q × v = 1,000 × 6.87 × 10⁻³ × 14.0 = 96 N, a push equal to the weight of about 10 kg.
The same head fills a 12 L bucket in 12 ÷ 6.87 = 1.7 s from a tap at the ideal flow, and in about 17 s at a tenth of it. The fire guide says filling is the hardest work of a bucket line; a pressurised tap removes it. Pressure does not add water, so the volume still has to be stored.
What You Need
- Stored water in covered tanks or cisterns, kept separate from drinking supplies (Cisterns and Rainwater Storage); metal roofs refill it better than thatch (a runoff coefficient of 0.9 against 0.5)
- A height difference between the tank and the buildings, if a hose is to be used
- A valve at each fill tap and a tee on the main, following Water Distribution Routes
- Hose or flexible pipe and a nozzle, if made
- Buckets at every water point, for the cases where nothing else works
Method: Match The System To The Job
Step 1: Size the volume first, from the largest wall or roof you may need to wet (Water Reserve Sizing).
Step 2: Place the water so every building is within 30 m of a point, and the points are on different sides.
Step 3: Add a head where terrain allows: a tank on a rise, with a pipe to taps at the settlement.
Step 4: Test the flow by timing the filling of a 200 L barrel from the tap and by trying the nozzle at the real distance.
Step 5: Keep the system fed and frost-free: a drain valve at each low point and a plan for freezing weather (Drain Points).
Common Mistakes
- Counting only volume. A big tank with a trickle outlet is a bucket line.
- A hose from a tank with the pipe too narrow. Friction eats the head.
- Connecting fire water to the drinking supply. It can contaminate treated water.
- No refill rule. The reserve was used last month and nobody topped it up.
Safety
Drowning, whiplash and wet floors
Open tanks and barrels drown small children, so cover or fence them. A hose under pressure whips if it is dropped, and the nozzle pushes back with about 96 N at the flow worked out above; hold it with two people and a clear footing. Do not point a jet at a person, and do not stand under a wall or roof you are wetting while it burns.
What’s Next
- Learn next: Water Reserve Sizing — how many litres the job needs
- Combine with: Distribution & Access Points — where the taps and barrels go