extra hazard occupancy classifications) and will
spread until it is confined by either the physical
construction of the structure or operation of the
fire sprinkler system. The area of fire spread
corresponds to the empirically derived area where
fire sprinklers are expected to activate and subse-
quently contain the fire, referred to as the "design
area" or "most remote area". The simultaneous
flow of sprinklers within the design area produces
a hydraulic demand of both the flow of water
(gallons per minute � gpm) and the system
pressure (pounds per square inch � psi) needed to
produce the proper sprinkler discharge density and
droplet pattern over the design area.
Much of a sprinkler system's overall perfor-
mance relative to controlling the fire is dictated by
the available water supply, assuming that the
proper design was applied to the hazard being
protected. The basic principle of a hydraulically
designed water-based fire sprinkler system is that
the peak flow and pressure demand is no greater
than the available supply. If an inadequate flow
and/or system pressure is delivered, the sprinkler
system is likely to operate improperly, possibly
allowing the fire to spread out of control.
The hydraulic demand at a system reference
point, such as the base of riser (BOR), can be
graphically compared to the available supply to
determine the adequacy of the water supply. A
hypothetical system demand is shown in Figure 1
where the system demand point is less than the
supply curve (plotted as a line on semi-log graph).
The design margin or "buffer" is the pressure
difference in the available supply curve and the
system hydraulic demand point, in psi. The model
codes governing the design of automatic sprinkler
systems are silent regarding the minimum design
buffer, leaving the size of the margin to the
discretion of the designer. The buffer should
account for foreseeable variations in water system
strength such as seasonal effects and peak water
consumption hours.
Water supply evaluation
Accurately characterizing the flow
and pressure of the available water
supply is paramount not only
during the initial design and instal-
lation of a fire sprinkler system, but
also for the continued protection
of the building. The following
actions are provided for considera-
tion for the ITM of water-based
wet pipe fire sprinkler systems:
1 Perform periodic testing of
the fire sprinkler system
NFPA 25, Standard for the Inspec-
tion, Testing and Maintenance
of Water-Based Fire Protection
Systems, is a reference commonly
adopted for enforcement. NFPA 25
offers a schedule of preventive ITM
activities. In particular, with respect
to water supply, Section 13.2.5 of
NFPA 25 states that a main drain
test shall be conducted annually at
each water-based fire protection
system riser to determine whether
there has been a change in the
condition of the water supply,
supply piping, and control valves.
Although it is customary to consider the main
drain test as an "annual" event, more frequent
tests are required depending on the water supply
configuration. The frequency of the main drain
test is increased to a quarterly basis where the sole
source of the water supply is through a backflow
preventer and/or pressure reducing valves.
Individuals responsible for the maintenance of
sprinkler systems should verify these tests are
being conducted on a timely basis in accordance
with NFPA 25 or other applicable requirements.
During the main drain test, the static system
pressure is recorded from the system riser gauge
as shown in Figure 2. The main drain valve is
opened fully and the residual (i.e. flowing)
pressure is recorded when the flow stabilizes. After
the residual pressure is recorded, the main drain
valve is closed and the system pressure is recorded
70 INTERNATIONAL FIRE PROTECTION
Figure 1: Water supply
curve showing the
available water supply
and sprinkler demand
MANAGING CHANGE OF FIRE SPRINKLER SYSTEM WATER SUPPLIES
SPRINKLER TECHNOLOGY
Figure 2: Diagram of typical main drain
arrangement on a sprinkler riser
Water Supply Flow Test
120
110
100
90
80
70
60
50
40
30
20
10
0
100 200 300 400 500 600 700 800 900 1000
Pressure(PSI)
Sprinkler Demand 600 GPM at 40 PSI
Available Water Supply 800 GPM at 40 PSI
Q1.85 Flow (GPM)
To System
Sprinkler
Riser
Pressure
Gauge
Angle
Valve
Drain
Pipe
From Supply
Inspector's
1/4 Test Plug

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