Civil Engineering Reference
In-Depth Information
180
160
140
120
Average Daily Flow
100
80
60
40
20
0
0
2 4 6 8 0 2 4 6 8 0 2 4
Midnight
Noon
Midnight
Time of Day
Fig. 7-5.
Sample diurnal flow variation
zone within the water service area. Usually the local fire marshal establishes required
fire flows based upon building and fire code standards and insurance agency rating
requirements.
Fire flow requirements are typically defined in terms of a rate of flow at a minimum
pressure for a specified duration. For example, in a residential area the minimum fire
flow may be defined as 1,000 gpm (63 L / s) for 2 hr; a commercial area or school
may require 4,000 gpm (250 L / s) for 4 hr. During a fire demand, a water system
should be capable of maintaining a minimum pressure of 20 psi (140 kPa) at the point
of fire flow delivery as well as throughout the rest of the system. In some systems, a
high fire demand at one location can cause very low or even negative pressures in
other parts of the system. These low or negative pressures can cause problems with
other services and future water quality. Water system planners and designers should
use a network model to simulate design fires at key locations to determine impacts on
the distribution system. Alternative improvements to fix low-pressure problems can
then be evaluated.
A practical limit on fire flow for typical midsized municipal water systems is be-
tween a minimum of 1,000 gpm (63 L / s) and a maximum of 5,000 gpm (320 L / s).
Larger systems may need to provide for much higher fire flows. For typical systems,
fire flows may be critical in determining minimum water main sizes. In addition,
depending upon the criteria a utility uses, emergency fire flow volumes can account
for a significant percentage of volume required in water storage reservoirs.
The American Water Works Association (AWWA) Manual M31,
Distribution Sys-
tem Requirements for Fire Protection
,
2
provides an excellent source of information
regarding planning for fire protection. This manual references other materials from the
Insurance Services Office and the National Fire Protection Association.
