Stages Of Fire Development in building- You should know
Fires
evolve through several stages as the fuel and oxygen available
are consumed. Each stage has its own characteristics and hazards that
should be understood by safety managers and
fire fighting personnel.
Incipient Stage:
The incipient stage
is the first or
beginning stage of a fire. In this stage, combustion has
begun. This stage is identified by an ample supply of fuel and oxygen. The
products of combustion that are released during this stage normally include water vapour, carbon dioxide,
and carbon
monoxide. Temperatures at the seat of the fire may have reached
1000°F, but room
temperatures are still close to normal.
Free Burning Stage:
The free-burning
stage follows the incipient stage. At this point, the self-sustained chemical reaction is
intensifying. Greater amounts of heat are emitted and
the fuel and oxygen supply is rapidly consumed. Room temperatures
can rise to over 1300°F. In an enclosed compartment, the free-burning stage can
become dangerous.
Because of the heat intensity, the contents within a compartment are heated. At
some point, if the compartment is not well ventilated, compartment contents
will reach their ignition temperature. A flash over occurs when the contents
within a compartment simultaneously reach their ignition temperature and
become involved in flames. It is not uncommon for room temperatures to exceed
2000°F following a flash over. Human survival, even for properly protected
fire fighters, is difficult if not impossible for a few seconds within a
compartment following a flash over.
The smouldering stage
follows the free-burning stage. As a free-burning fire continues to burn, the
chemical reaction will eventually consume the available oxygen within the
compartment and ultimately convert it into carbon monoxide and carbon dioxide.
This causes the
oxygen concentration within the compartment to decrease. When the
oxygen concentration decreases to 15% by volume, the chemical reaction will not have sufficient oxygen to
support free-burning combustion. Visibly, the flames subsist and the
fuel begins to glow. A smoldering fire is identified by a sufficient amount
of fuels and lower oxygen concentrations. Smoldering fires, especially when
insulated within a compartment, can continue the combustion process for hours.
Room temperatures can range from 1000–1500°F. The bi products of combustion
also fill the compartment and human survival is impossible. During the smoldering
stage, an extreme
hazard, called a back draft, can develop. A back draft
occurs when oxygen is introduced into a smoldering compartment fire. The
immediate availability of sufficient oxygen in the presence of sufficient fuel,
heat, and chemical chain reactions causes flaming combustion again. In some
cases, the back draft is so violent that an explosion will
occur. Human survival, even of properly protected fire fighters, is usually not
possible.
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