Fire demand is the amount and rate of supply of water that are required to extinguish the largest probable fire that could occur in a community. Usually, a fire occurs in factories and stores. The quantity of water required for firefighting purposes should be easily available and always kept stored in the storage reservoir. When population increases number of building also increases and the risk also increases. Fire demand is considered in designing pumps, reservoirs and distribution system.
Following requirement must be met for fire demand.
- Three jet streams are simultaneously thrown from each hydrant; one on the burning property and one each on adjacent property on either sides of the burning property. The discharge of each stream should be about 1100 liters/minute.
- The minimum water pressure available at fire hydrants should be of the order of 100 – 150 kN/m2 and should be maintained even after 4 to 5 hours of constant use of fire hydrant.
In case of public water supply, fire demand is treated as a function of population and some of the empirical formulae, commonly used for calculating the fire demand are as follows.
1) Kuichling’s Formula
Q = 3182 √P
Where, Q = Quantity of water required in liters per minute
P = Population in thousands
2) Freeman Formula
Where, ‘P’ and ‘Q’ have the same meaning as above.
3) Buston’s Formula
Q = 5663 √P
Where, ‘P’ and ‘Q’ have the same meaning as above.
4) National Board of Fire under Writers Formula
A) For a central congested high valued city
(i) When population ≤ 200000
Q = 4637√P [1−0.01√P ]
Where, ‘P’ and ‘Q’ have the same meaning as above.
(ii) When population > 200000
A provision for 54600 liters per minute may be made with an additional provision of 9100 to 36400 liters/minute for a second fire.
(B) For a residential city
(i) For small or low buildings: Q = 2200 liters/minute.
(ii) For large or higher buildings: Q = 4500 liters/minute.
Example Question
Compute the ‘fire demand’ for a city of 200000 populations by all the above formulae.
Solution
1) By using Kuichling’s Formula
Q = 3182 √P
= 3182 √200
= 45000.275 l/min
2) By using Freeman Formula
3) By using Buston’s Formula
Q = 5663 √P
Q = 5663 √200
= 80086.914 l/min
4) By using National Board of Fire under Writers Formula
Q = 4637√P [1−0.01√P ]
= 4637√200 [1−0.01√200 ]
= 56303.080 l/min
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