Fire and Smoke Understanding the Hazards (1986) / Chapter Skim
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3 Status of Fire Hazard Models and Test Methods
3 Status of Fire Hazard Models and Test Methods
Pages 45-61
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From page 45... ...
A major class of fire scenarios consists of fires that continue to burn, with or without growth, beyond the point where lethal concentrations of fire products (the products of both combustion and pyrolysis) reach occupant locations and escape paths.
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From page 46... ...
For each specific case, a mathematical model must be run to determine the degree of benefit. Of course, some fire scenarios include an extremely rapidly growing fire, perhaps involving flammable liquids, in which TAE is much less than TNE.
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From page 47... ...
. If it is assumed that the fire grows at the original rate, but that the combustible material produces a less easily detectable type of smoke and that the detector is designed with substantial resistance to smoke entry, then the detector might not respond until ~ he F ; r" ; c ==v ~ times as larae (500 kW)
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From page 48... ...
is used. It should calculate the accumulation of hot fire products under the ceiling, the loss of heat from these products to the ceiling, the deepening of the hot layer to below the top of the doorway that leads to the adjacent compartment, the mixing and dilution of the hot gases as they flow through the doorway, and the eventual filling of the adjacent compartment.
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From page 49... ...
In summary, the fluid-mechanical aspects of the fire are approximated rather roughly. The outputs of the model consist of a number of timedependent quantities, including the rate of deepening of the hot layer in the compartment, the ignition time of objects after the first object, the rate of gas flow out of the compartment, and the concentration of species in the outflowing gas.
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From page 50... ...
It allows for energy loss from the hot layer to the compartment in an ad hoc manner. With these restrictions, it computes the time available before the smoke layer deepens to reach the occupant, if burning rate is known.
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From page 51... ...
It did not do well at predicting areas of fire spread. It predicted toxic gas concentrations in the hot layer (CO, hydrogen cyanide, hydrogen chloride, and hydrogen fluoride)
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From page 52... ...
0 s Fire actuates smoke detector in hall +73 s Upper temperature in living room is untenable (183°C) +91 s Visibility is lost in hall (smoke 1 m -- 3.3 ft-above floor)
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From page 53... ...
However, if the postulated rescuer arrived sooner or later than that narrow interval, the results would be essentially unaffected by the reduction in LC50. If the LC50 were reduced by a factor of 10, instead of by a factor of 2, lethal conditions would be predicted to occur in the hallway before loss of visibility, and TAE would be reduced from 91 s (based on visibility)
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From page 54... ...
MODELS FOR TIME NEEDED FOR ESCAPE Once a fire is detected, the time needed for escape is controlled by a combination of psychologic, physiologic, and physical factors. Some models, such as EVACNET+,1 21 deal with physical factors -- specifically, the escape paths available, the time needed to traverse each path, the flow capacity of each path, and the initial locations of the occupants.
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From page 55... ...
However, no standard small-scale tests or any known combination of them is adequate for predicting the full-scale burning rate of an item made from the tested material. One exception to this generalization would be a noncharring combustible material uniformly ignited over a single horizontal surface, for example, a dish of heptane 1 m across or a horizontal slab of polymethyl methacrylate 1.5 m square.
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From page 56... ...
or the fullsize burning rate.2~9 However, by applying an empirically arrived at radiant heat flux of around 50 kW/m2 to the small-scale sample or by burning the small-scale sample in an atmosphere artificially enriched in oxygen, one can make the small-scale sample burn at about the same rate as the full-scale sample. Then, by using the same small-scale test conditions for other noncharring horizontal materials, one can predict fullscale burning rates.
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From page 57... ...
However, the fire models can predict local concentrations of any species of interest, such as CO or hydrogen chloride (HC1) , if test methods can provide the needed inputs, specifically grams of the species of interest yielded per gram of burned material.
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From page 58... ...
no standard test method is available for obtaining such data on the fire products of a given composite substance. IGNITABILITY In many fire scenarios, the original ignition is a "given," and the task of a model is to describe the history of the fire after ignition.
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From page 59... ...
The small-scale rate-of-heat-release methods previously mentioned25 2~8 have also been used to measure ignitability in cases of radiative exposure. Ignitability of flammable fabric is measured by a standard test involving a 3-s exposure to a small flame.9~ The Setchkin furnace98 is standardized as ASTM Test D-1929, which measures the furnace temperature at which a small sample will just ignite in an airstream.
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From page 60... ...
Theoretically, sublethal effects of toxic fire products can affect TNE (e.g., the possible deleterious effect of CO on judgment or ambulation in hindering escape)
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From page 61... ...
However, comparisons with realistic fire tests have shown order-of-magnitude agreement with model predictions in a number of cases. Furthermore, the relative influence of the various parameters should be generally correct.
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