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Water Reserve Sizing

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Water Systems for Fire Fighting
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Water Reserve Sizing

Fire water has a hard physical floor: a litre that is heated from 20 C to 100 C and boiled absorbs at most 2.59 MJ, so a brigade line of 50-80 L/min can absorb at most 2.2-3.5 MW of fire, and wetting a wall that receives 12.5 kW/m² needs at least 0.29 L/min per m². Real water absorbs only a fraction of that, so at one-third efficiency the brigade holds about 0.7-1.2 MW, and a 5,000 L reserve wets 100 m² for anything from 10 to 172 minutes depending on how efficiently it is applied.

Part of Fire Prevention in Settlements

Water reserve sizing is the calculation of how many litres, and at what rate, a settlement must hold to fight a small fire and to wet the neighbours of a large one, using the heat that water can absorb.

Water Fights Fire By Absorbing Heat, So The Limit Is Physical

The fire guide asks for at least 200 L per cluster, ideally 1,000 L or more, and at least 5,000 L for a settlement of 150 people. It says 200 L cannot extinguish a fully involved structure but can contain a small fire and wet adjacent buildings. This article tests those numbers against the one thing that is certain: water works by taking up heat, and a litre can only take up so much. The standard values used (specific heat of water 4.18 kJ per kg per K, heat of vaporisation 2,257 kJ/kg) are recalled from physical tables and not read from this corpus.

Worked Example: Rate, Wall And Reserve

Heat absorbed per kilogram of water heated from 20 C to 100 C (80 K) and boiled: 80 × 4.18 + 2,257 = 2,591 kJ/kg = 2.59 MJ/kg. One litre is one kilogram. This is the ideal case in which every drop reaches the fire and evaporates; real efficiency η is below 1 and is the unknown.

What a brigade can absorb.

  1. 50 L/min = 0.833 kg/s; × 2.59 MJ/kg = 2.16 MW. 80 L/min = 1.333 kg/s; × 2.59 = 3.45 MW.
  2. At η = 1/3, one-third (an assumed round figure): 0.72 to 1.15 MW, about 0.7 to 1.2 MW.

So the brigade can at best hold a fire of 2.2 to 3.5 MW and realistically about one. The building-spacing article’s burning wall is a larger source than that, which is why the fire guide says to wet the neighbour and not fight the burning building. The charcoal forge’s own fire, about 42 kW at 5 kg an hour, is far inside the brigade’s reach (Fire Safety & Suppression).

Wetting a wall. Flux q on the wall in kW/m²; ideal water flow per m² = q ÷ 2,591 kJ/kg.

  1. At 12.5 kW/m² (the wood ignition level used in the spacing article, Fire Breaks & Building Spacing): 12.5 ÷ 2,591 = 0.00482 kg/s = 0.29 L/min per m². At 23 kW/m²: 0.53 L/min per m².
  2. A wall of 15 m² (5 m by 3 m) at 12.5 kW/m²: 15 × 0.29 = 4.3 L/min ideal, so 200 L lasts 46 minutes; at η = 1/3, 13 L/min and 15 minutes.

The efficiency problem. As I recall, exposure-protection sprinkler design uses on the order of 5 L/min per m² (about 0.1-0.15 US gallons a minute per square foot); I have not checked this against a standard. Against the ideal 0.29 at 12.5 kW/m² it implies η = 0.29 ÷ 5 = 6%, because most water runs off or falls where it absorbs nothing. Using it:

  1. The same 15 m² wall: 15 × 5 = 75 L/min, and 200 L lasts 2.7 minutes.
  2. 100 m² of wall and roof (an assumed exposed area) for 30 minutes: ideal 0.29 × 100 × 30 = 870 L; at η = 1/3, 2,600 L; at 5 L/min per m², 15,000 L.
  3. How long 5,000 L wets 100 m²: ideal 5,000 ÷ 29 = 172 minutes; at η = 1/3, 58 minutes; at 5 L/min per m², 5,000 ÷ 500 = 10 minutes.

The 5,000 L reserve is therefore adequate if water is placed well and marginal if it is not. That is a result about technique, which no formula can supply; it is measured in drills.

What You Need

  • The exposed area of each building a fire could threaten, in m², from the site plan
  • A bucket or hose and a timer for drills
  • A volume gauge: a marked barrel
  • The flux from the spacing article for the distance in question

Method: Size And Then Test

Step 1: Take the floor: exposed area × ideal L/min per m² × minutes you must protect. This is the least you could ever need.

Step 2: Divide by a pessimistic η (start at 1/3, and test lower).

Step 3: Drill. Wet a wall in the sun with a real bucket line for 10 minutes. Note how much water was used and whether the wall stayed wet.

Step 4: Set the reserve from the drill result, never below the fire guide’s 200 L per cluster and 5,000 L for 150 people, and set the refill rule.

Step 5: Place the water within 30 m of every building (Water Systems for Fire Fighting).

Common Mistakes

  • Taking the heat-absorption figure as the working rate. It is the lower bound.
  • Counting the reserve as usable in full. Some is dead volume, and a frozen barrel is nothing.
  • Sizing for one fire. Several roofs may need wetting at once in an ember storm.
  • Skipping the drill. Without it the efficiency is a guess.

Safety

A reserve is not a reason to stay

Having water does not make a well-alight building safe to fight. The numbers say a brigade holds a small fire and not a large one. If the fire is past the first minutes, get everyone out, count them and wet the neighbours from a distance (Response Protocols & Training).

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