Fire and Smoke Understanding the Hazards (1986) / Chapter Skim
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6 Guidelines for Hazard Assessment: Case Studies
6 Guidelines for Hazard Assessment: Case Studies
Pages 105-130
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From page 105... ...
4. Specification of the fire and smoke properties of the product that are needed to provide the minimal acceptable TAE or to increase TAE.
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From page 106... ...
CASE STUDY 1: BURNING OF AN UPHOLSTERED CHAIR STEP 1: DEFINING SCENARIOS Environment The chair and the people exposed to the fire are assu2ed to be in a compartment of about 650 ft (about 60 m )
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From page 107... ...
Upholstered furniture is often ignited, not by flaming ignition, but by a dropped cigarette. Such ignitions usually lead to smoldering, a qualitatively different kind of combustion.
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From page 108... ...
One approach to approximating incapacitating dose would be simply to use, say, 20% of the lethal dose of combustion products as the n incapacitating dose. n STEP 2: COMPUTING TAE AS A FUNCTION OF FIRE AND SMOKE PROPERTIES Fire Model l The model selected for this computation was the Fifth Harvard Computer Fire Code (5.2)
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From page 109... ...
Burning of Upholstered Furniture The objective of the study is to relate fire conditions in the room to the fire performance of the fuel, so it is necessary to direct attention to how the heat release of the upholstered furniture is related to its burning in the room. It has been demonstrated that the peak heat release rate of a piece of upholstered furniture is proportional to its total mass and to the heat release per unit area, as measured in the laboratory.
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From page 110... ...
doorway as the vent would slow the growth of the hot layer substantially and would not yield a good prediction of its growth in the worst case. Just how thick the hot layer is in the late stages of the fire depends on the exact height chosen for the vent.
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From page 111... ...
Results of Calculations lThe Harvard code was run with the three simulated chair fires as input. The thickness of the upper layer and its temperature are calculated at 2-s intervals throughout the burning of the chair, and results are reported at 20-s intervals.
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From page 112... ...
100.0 80.0 ._ ~ 60.0 . ~7 a, o ~ 40.0 a, I o In 20.0 0.0 0.0 100.0 200.0 1 300.0 400.0 500.0 600.0 Time, s FIGURE 6-4 Smoke dose in adjacent room vs.
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From page 113... ...
Fires like those discussed above provide ample smoke density for rapid detector actuation, if the detector is situated so that it is exposed to the developing smoke layer at the earliest time, e.g., mounted on the ceiling or the top of the wall, not far below the ceiling. At TABLE 6- 2 Fire Growth Timetable: 25-kg Chair Burning in 7.7 x 7.7 x 2.4-m Room with Flaming Ignition Dimension Peak heat release rate, MW Time, s, when hot layer reaches 1 m Chair 1 85 275 2 50 110 above floor Time, s, when hot layer reaches 180°C Smoke dose encountered before onset of lethal temperature, g~min/m3 30 22 35 60
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From page 114... ...
Each of the three chair fires can be divided into intervals. First is a period of relatively unhindered escape between the time when the detector alarms and the time when the hot smoke layer extends down to 1 m above the floor.
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From page 115... ...
so and rate of mass loss increase, k. STEP 3: DECIDING ON MINIMAL ACCEPTABLE TAE In these relatively simple scenarios, the time needed ~~~ A
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From page 116... ...
Flaming fire -- occupants on other side of fire wall; 10% of smoke leaks through 3. Smoldering fire -- no heat; automatic detection uncertain STEP 4: SPECIFYING FIRE AND SMOKE PROPERTIES 4 60 60 Let it be supposed that all three of the situations discussed above are deemed equally important and that fire and smoke properties that meet each one should be specified.
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From page 117... ...
3. Flaming fire -- occupants of fire on other side wall; 10% of smoke leaks through Smoldering fire -- no heat; automatic detection uncertain Required Performance Heat release rate no more than 100 kW/m2 and L(Ct)
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From page 118... ...
The exercise is intended to illuminate the method, not to suggest a regulation. CASE STUDY 2: CONCEALED COMBUSTIBLE MATERIAL STEP 1: DEFINING SCENARIOS Assessment of the fire hazard associated with combustible material concealed behind a wall or ceiling is more complicated than the assessment in the previous example, which focused on upholstered furniture.
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From page 119... ...
Because the focus is on the combustible material behind the wall, details of ignition and spread of fire in the room are relatively unimportant. STEP 2: COMPUTING TAE AS FUNCTION OF FIRE AND SMOKE PROPERTIES Four tasks are required: determining the thermal conditions that the room contents (exposed fuel)
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From page 120... ...
on its precise location in the cavity. In an effort to circumvent this kind of uncertainty, we calculated the temperature rise in the cavity by assuming that it was empty and that it lost heat only by conduction through the outside gypsum board.
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From page 121... ...
Material performance is gauged by exposing the material in the laboratory to a range of imposed radiant flux qua (in which q denotes heat, the dot denotes its derivative with respect to time, and the double prime denotes "per unit area") and determining the rate of mass loss (m")
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From page 122... ...
The contribution of the concealed combustible material, f2(x) , is zero until
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From page 123... ...
The curve can be approximated by series of relatively linear regions: m, g/s Interval s , o 48 32 o L At, q,, i 20 / ;"~\''~ 80 0-180 180-1,080 1,080-1,800 71,800 (fuel exhausted) Knowing the mass loss in the room permits calculation of fl(x)
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From page 124... ...
6t A = area of concealed combustible material receiving energy In cavity, . L = apparent heat of vaporization of concealed combustible material, q"(t)
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From page 125... ...
Obviously, the wall eventually will be physically breached. The combustible material behind the wall is then exposed to the same thermal conditions as the interior of the room.
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From page 126... ...
Until the concealed combustible material becomes involved, the fire analysis is similar to the one in Case Study 1: the buildup is controlled by the flame inability properties of the room fuels. Note that conditions in the neighborhood of the fire become untenable in just a few minutes.
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From page 127... ...
The contribution of the concealed combustible material that is "acceptable" is arbitrary. Let it be supposed
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From page 128... ...
2. Less than 10% of toxic volume from pipe before collapse Less than 10% of toxic volume from pipe after collapse Required Performance _ q0 no less than 18 kW/m2 qo no less than 10 kW/m2 and [L x L(Ct)
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From page 129... ...
Requirements will eventually be set for flammability characteristics, as well as for smoke toxicity, so it is crucial that the model be able to provide quantitative relationships between a given set of fire and smoke properties and TAE. Specific models are discussed in detail in Chapter 3.
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From page 130... ...
SPECIFYING PRODUCT PERFORMANCE In principle, the hazards associated with smoke are controlled only if both the smoke toxicity and the properties that determine smoke production are controlled. The modeling process and selection of a TAE should make it apparent that various combinations of toxicity and smoke production can provide acceptable overall performance.
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