Page 120 - Water Engineering Hydraulics, Distribution and Treatment

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Quantities of Water Demand
Chapter 4
Variations in per capita domestic water consumption
Table 4.12
in Riyadh, Saudi Arabia
on exposed buildings, add 500 gpm (1,893 Lpm) to
the above flows.
Ratio to
3. High-risk areas. Multifamily, commercial, and indus-
(Lpcd)
Description
trial areas are considered high-risk areas. The fire
Overall population
flows available for these areas require special con-
310
1.00
Average day
sideration. The distribution and arterial mains in the
Maximum day
1,323
4.27
high-risk areas are to accommodate the required fire
0.13
41
Minimum day
flows in those areas.
Income level
1.09
Low income (average day)
339
4. Typically the water main system must be able to meet
219
Medium income (average day)
0.71
the flow requirements of (a) peak day demands plus
1.39
432
High income (average day) Consumption average day 2. For wood-shingle roof coverings on the building or
fire flow demands as a minimum for any water main
House occupants (average day) 343 1.11 design and (b) instantaneous peak demands for spe-
Building occupants (average day) 221 0.71
cial water mains from water source, water treatment
Conversion factor: 1 gpcd = 3.785 Lpcd. plant, and/or water storage facilities.
5. Water storage facilities should have sufficient capac-
ity, as required by the Recommended Standards for
Water Works (2007 edition), to meet domestic aver-
4.4.2 Fire Demands
age daily water demands and, where fire protection
Height, bulk, area, congestion, fire resistance, type of con- is provided, fire flow demands.
struction, and building occupancy determine the rate at which
water should be made available at neighboring hydrants, Fire demand can also be estimated according to the com-
either as hydrant or engine streams, to extinguish localized munity size and realty subdivision development by using the
fires and prevent their spread into areal or citywide con- following empirical equation from the National Board of Fire
flagrations. The needed fire flow (NFF) is defined as the Underwriters:
water flow rate, measured at a residual pressure of 20 psi 0.5 0.5
Q = 1,020(P ) [1 − 0.01(P ) ] (US customary units)
(138 kPa) and for a given duration, which is required for fire k k
fighting a fire in a specific building. Analysis of water (4.22a)
demands actually experienced during fires in communities 0.5 0.5
Q fire = 3,860.7(P ) [1 − 0.01(P ) ] (SI units) (4.22b)
k
k
of different sizes underlies the formulation of the general
standards. The American Water Works Association (1998) where Q fire is the fire demand, in gpm for US customary units
in its M31 Manual describes three methods for calculat- and L/min for SI units, and P is the population, in thousands.
k
ing the fire flow requirements that were developed by these The National Board of Fire Underwriters requires provi-
organizations: sion for a 5-hour fire flow in places with populations of less
1. Insurance Services Office Inc. (ISO) than 2,500 and provision for a 10-hour flow in larger places.
The ISU method is relatively simple and quick to use
2. Illinois Institute of Technology Research Institute
but yields low fire flow requirements, whereas the IITRI
(IITRI)
method produces excessively high rates. The ISO method-
3. Iowa State University (ISU)
ology calculates values in between the other two. The most
The following general information is taken from the recent International Fire Code (International Code Coun-
2006 edition of the ISO’s Guide for Determination of Needed cil [ICC], 2006) recommends a minimum fire flow of 1,000
Fire Flow: gpm (3,785 L/min) for one- and two-family dwellings hav-
2
2
ing an area that does not exceed 3,600 ft (344 m ). Fire
1. For one- and two-family dwellings not exceeding flow and flow durations for larger buildings having areas in
two stories in height, the following needed fire flows excess of 3,600 ft (344 m ) are not to be less than what
2
2
should be used: is shown in Table 4.13. Type I-A structures are typically
concrete-frame buildings made of noncombustible materi-
als. All of the building elements—structural frame, bearing
Distance between buildings, ft (m) Fire flow, gpm (L/min) walls, floors, and roofs—are fire resistance rated. Type V-B
construction is typically wood-frame construction, which is
Over 100 (over 30) 500 (1,893)
very common because it does not require a fire rating.
31–100 (9.1–30) 750 (2,839)
The benefits of early fire suppression are acknowledged
11–30 (3.1–9) 1,000 (3,785)
by the acceptance that the firefighting water requirement can
10 or less (3 or less) 1,500 (5,678)
be reduced by 50% for one- and two-family dwellings and