ETL 1110-3-481
31 Mar 97
Closed Fire Extinguishing Systems
4.2
Closed fire protection systems are basically overhead wet pipe or
preaction sprinkler systems and have closed-head sprinklers.
This is in contrast with open-head or deluge sprinkler systems in
which all sprinkler heads are open and discharge upon system
activation. An inherent feature of the closed fire extinguishing
systems is that each individual sprinkler head must be actuated
by the heat of a fire before it will begin discharging. Closed-
head sprinkler systems, whether wet-pipe or preaction, are not
susceptible to false discharges due to system malfunction or
inadvertent actuation. Their inherent safeguard against "false
dumps" makes closed head systems the preferred system for most
DoD hangar facilities.
DESIGN GUIDANCE
5.
Most Probable Worst Case
5.1
a. Containment systems will be designed to contain the most
probable worst case AFFF discharge. The most probable worse case
AFFF discharge is the maximum discharge likely to occur in a non-
catastrophic event. Most probable worst cases are different for
closed fire extinguishing systems and for open extinguishing
systems.
b. Foam discharges associated with major fires are not
considered the most probable worst case event for two reasons.
First, a major fire in a hangar would be considered a
catastrophic event. It is impractical to design a containment
system for a catastrophic event due to the infinite number of
variables associated with such an event. Secondly, an occurrence
of a major fire in a well protected hangar is not considered a
probable event. In an event of a fire, an installed AFFF fire
suppression system would control the fire and would not produce
significant amounts of AFFF.
c. It should be also noted that significantly less AFFF
discharge would be produced in a protected hangar, than would be
produced if a fire occurred in an unprotected hangar. To fight a
fire in a unprotected hangar, a much larger amount of AFFF would
be applied by the fire department hose streams. The fire in an
unprotected hangar would pose a significant environmental impact.
d. Open Fire Extinguishing Systems. Open systems are
oscillating and fixed nozzle systems, as well as deluge sprinkler
systems which discharge foam by activation of detectors or manual
A-7
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