During the growth stage of a fire (fig. 7-2), the
As the available fuels and combustibles in the
average space temperature is low and the fire is
space are consumed the fire begins to decay. In the
localized near the area where it started. It is hot in the
decay stage, combustion slows down (decays) and
immediate vicinity of the fire, and rising heat and
finally the fire goes out.
smoke create a hot upper level in the compartment.
There are significant exposure thresholds for
human tolerance to heat as shown in table 7-2, along
with other temperature characteristics that may help
DOOR
HOT SMOKE LAYER
you put them in perspective.
If a fire goes out quickly due to a lack of oxygen,
such as in a tightly sealed compartment, fuel vapors
may still be formed from any flammable liquid that is
heated above its flashpoint. If fresh air is allowed into
the space before this fuel vapor cools below its
flashpoint, this mixture can ignite explosively. This is
COOL AIR
FIRE
known as backdraft, and fortunately, is an unusual
occurrence.
Figure 7-2. Growth stage of a compartment fire.
Fire Spread
In what is called the "rollover" of a compartment
I f s p a c e p e r s o n n e l a t t a c k a fi r e e a r l y a n d
fire, a flame front of burning gases is formed across the
efficiently, it can be confined to the area in which it
overhead of the space. Rollover takes place in the
started. If the fire continues to burn unchecked, it can
growth stage when unburned combustible gases from
generate great amounts of heat that will travel away
the fire mix with fresh air in the overhead and begin
from the fire area, starting more fires wherever fuel and
burning at some distance from the fire. Rollover differs
oxygen are available. Steel bulkheads and decks and
from flashover in that only gases are burning in the
other fire barriers can delay but not prevent heat
space.
transfer.
The flashover stage is the period of transition from
W h e n a f u l l y - d eve l o p e d fi r e ex i s t s i n a
the growth stage to the fully-developed fire stage. It
compartment, the fire is most quickly spread to other
occurs in a short period of time and may be considered
compartments through openings such as doorways,
an event, much as ignition is an event in a fire. It
vent ducts, and unsealed cableways. It will also spread
normally occurs at the time the temperature of the
upper smoke layer reaches 1100F (600C). The most
to adjacent compartments by heat conduction through
obvious characteristic of flashover is the sudden
the bulkheads. Fires normally spread faster upward to
spreading of flame to all remaining combustibles in the
the space above than to adjacent horizontal spaces
space. Personnel still in the compartment when
simply because heat rises.
flashover occurs are not likely to survive.
Tests have been developed to provide typical
In the fully-developed fire stage all flammable
temperatures, radiant heat flux, and length of time for
materials in the compartment have reached their
material ignition by conduction through steel
ignition temperature and are burning. The rate of
bu l k h e a d s f r o m a f u l l y - d eve l o p e d fi r e . T h e
combustion will normally be limited by the amount of
compartments tested were 8-foot x 8-foot x 8-foot steel
oxygen available in the air to provide combustion.
cubes with bare metal surfaces. These typical values
Flames may emerge from any opening; hatches, open
shown will differ based on the space that the fire is in,
ventilation ducting, etc. Unburned fuel vapor in the
d u e t o fa c t o r s s u c h a s bu l k h e a d i n s u l a t i o n ,
smoke may flash when it meets fresh air in adjacent
c o m p a r t m e n t d i m e n s i o n s , ve n t i l a t i o n , s p e c i fi c
compartments. There may be structural damage to
material characteristics, and water application and
bulkheads or decks when exposed to these extreme
cooling. These figures, as shown in table 7-2, are
temperatures. A compartment may reach the
p r ov i d e d t o s h ow y o u t h e c h a r a c t e r i s t i c s o f
fully-developed fire stage very quickly during
conduction, and should be of particular interest to your
machinery space flammable-liquid fires or during
boundarymen.
enemy weapon-induced fires.
7-5
