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6.4 System Pressure 201
The general firefighting requirements according to the IFC are summarized in
Chapter 4, Section 4.4.2 and Table 4.13. To these requirements for firefighting must be
added a coincident demand of 40 to 50 gpcd in excess of the average consumption rate for
the area under consideration. In small communities or limited parts of large-distribution
systems, pipe sizes are controlled by fire demand plus coincident draft. In the case of
main feeder lines and other central works in large communities or large sections of met-
ropolitan systems, peak hourly demands may determine the design.

6.4 SYSTEM PRESSURE

For normal drafts, water pressure at the street line must be at least 20 psig (140 kPa) to
let water rise three stories and overcome the frictional resistance of the house-distribu-
tion system, but 40 psig (280 kPa) is more desirable. Business blocks are supplied
more satisfactorily at pressures of 60 to 75 psig (420 to 520 kPa). To supply their upper
stories, tall buildings must boost water to tanks on their roofs or in their towers and,
often, also to intermediate floors.
Fire demand is commonly gauged by the standard fire stream: 250 gpm (946 L/min)
issuing from a 1 ⁄8-in. (28.6 mm) nozzle at a pressure of 45 psig (312.3 kPa) at the base of
1
1
the tip. When this amount of water flows through 2 ⁄2-in. (63.5 mm) rubber-lined hose, the
frictional resistance is about 15 psi per 100 ft of hose (3.42 kPa/m). Adding the hydrant re-
sistance and required nozzle pressure of 45 psig (312.3 kPa) then gives the pressure needs
at the hydrant, as shown in Table 6.2. A standard fire stream is effective to a height of 70 ft
(21.34 m) and has a horizontal carry of 63 ft (19.20 m).
Because hydrants are normally planned to control areas within a radius of 200 ft (61 m),
Table 6.2 shows that direct attachment of fire hose to hydrants (hydrant streams) calls for a
residual pressure at the hydrant of about 75 psig (520.5 kPa).
To maintain this pressure during times of fire, system pressures must approach 100 psig
(694 kPa). This has its disadvantages, among them danger of breaks and leakage or waste of
water approximately in proportion to the square root of the pressure. Minimum hydrant
pressures of 50 psig (347 kPa) cannot maintain standard fire streams after passing through
as little as 50 ft (15 m) of hose.
Motor pumpers commonly deliver up to 1,500 gpm (5,677 L/min) at adequate pres-
sures. Capacities of 20,000 gpm (75,700 L/min) are in sight, with single streams discharg-
ing as much as 1,000 gpm (3,785 L/min) from 2-in.(50-mm) nozzles. To furnish domestic
and industrial draft and keep pollution from entering water mains by seepage or failure
under a vacuum, fire engines should not lower pressures in the mains to less than 20 psig
(140 kPa). For large hydrant outlets, the safe limit is sometimes set at 10 psig (70 kPa). In
a real way, modern firefighting equipment has eliminated the necessity for pressures much
in excess of 60 psig (420 kPa), except in small towns that cannot afford a full-time, well-
equipped fire department.

Table 6.2 Hydrant Pressures for Different Lengths of Fire Hose
Length of Hose (ft) Required Pressure (psig)
100 63
200 77
300 92
400 106
500 121
600 135

Conversion factors: 1 ft 0.3048 m; 1 psig gage pressure 6.94 kPa