Enclosure fires

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ignition source required to ignite a premixed gaseous mass can be very small (see Chapter 3). There is an alternative scenario for a smoke gas explosion, which can possibly occur in the fi re room. If there is only a small opening in the fi re room, i.e. if the opening is not big enough to cause a fl ashover in the fi re room, the fi re will continue to burn with a depleted supply of oxygen. This will cause a large quantity of unburnt smoke gases to accumulate in the fi re room. The fi re in the room may even seem to spontaneously go out. If the temperature remains high in the room a lot of unburnt gases can accumulate in the smoke gas layer, especially if there is a lot of combustible material at the top of the room. If the room is well insulated the temperature can remain high for a long time. 35 This may seem harmless, but after a long time the room starts to cool down and air is sucked in. The fuel-rich atmosphere combines with the air. The oxygen content rises to around 10% of the volume, which is roughly the amount needed for the gases to be able to combust. The gases in the room have now mixed very well, but they do not ignite as there is no ignition source present. The ignition source, e.g. embers that fl y up from the initial fi re, can appear later and ignite a more or less premixed gaseous mass. 36 This will cause a powerful smoke gas explosion. The chance of the ignition source appearing when the gaseous mass is premixed must be considered, however, as being small. It is diffi cult to predict smoke gas explosions, which means it is also diffi cult to guard against them. They occur only a few times a year in Sweden. Unfortunately, they are so dangerous that they can have fatal consequences. 40 7.2.1 Factors affecting the power of a smoke gas explosion Size of ventilation opening(s) The larger the opening, the easier it is for the pressure to drop in the room. If the room is almost completely closed the pressure will be much higher if the gaseous mass ignites. Theoretically, the pressure could reach as much as eight bar in a totally closed room. 153
Table 8. Pressure resistance for various structures. 12 154 Premixed proportion in room volume The larger the premixed proportion in the room volume, the greater the increase in pressure. It only requires a small volume percentage of the smoke gases to be premixed for the pressure to become very high, see Chapter 3 earlier on. Building components’ pressure resistance The weakest building component collapses fi rst, after which the pressure will decrease. If the building was entirely closed and the building components could cope with the increases in pressure, the pressure could reach as much as 8 bar. 12 In most buildings, however, there is, for example, a window, which means that the increase in pressure is very often moderate. The table below indicates the approximate values for the amount of pressure various building components can withstand: Typical pressure at which various building structures will collapse Structure Pressure (mbar) Pressure (Pa) Glass windows 20–70 2000–7000 Room doors Lightweight walls 20–30 2000–3000 (wooden framework and wooden boards) 20–50 2000–5000 Double plasterboard 30–50 3000–5000 10 cm brick wall 200–350 20000–35000 Burning velocity (Su) The higher the burning velocity, the faster the expansion. The rate depends on the substance involved in the combustion and also varies according to where the gaseous mass ignites in relation to the stoichiometric point. The burning velocity is highest when the stoichiometric point is reached. There is a common misconception that the mixture must be very close to the stoichiometric point to achieve large increases in pressure. But the pressure will be very high, even when the gas mixture is close to the fl ammability limits. The burning veloc-
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Lars-Göran Bengtsson Enclosure fi
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The content of this book may not be
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4.4 Risk assessment 99 Smoke gases
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Foreword The aim of this book is to
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Overview This book assumes that its
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Figure 1. Fire growth curve featuri
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Fire extinguished/burnt out Just sm
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chapter 2 How a fi re starts People
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smoke gases in the compartment, but
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Pyrolysis gases material is subject
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Ignition time Ignition time can als
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Smouldering fi res can therefore re
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2.4.1 Thermal inertia kρc The fl a
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grow to 50 cm, then a 1 m high fl a
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Flammability in solid materials is
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6. The fl ame spread rate varies wi
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Figure 22. Smoke gases start to acc
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Figure 25. The different sections i
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The production of unburnt gases is
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Heat release rate from a fuel surfa
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Carbon monoxide (CO) is the next mo
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ment to accelerate, but this is obv
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is far too high for a premixed fl a
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is fi lled with fuel molecules, con
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Figure 33. Flames on the under side
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Flammability limits Vent For a prem
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CH4 + 2 + 2 + + 2O2 = CO2 + 2H2O +
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Temperature impact on fl ammability
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energy is required, which can be ex
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ustion takes place in under-ventila
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If turbulence affects the system th
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Neutral plane The differences in pr
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Where the differences in temperatur
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Positive pressure Negative pressure
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Dp = 353(1/Ta - 1/Tg)gh Let us take
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Figure 51 (top). The pressure condi
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Vent just during the early stage of
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In Figure 57 calculations have not
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3.4 Summary In many situations the
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oom and in a fairly closed room (on
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Figure 59. The fi re has spread to
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Figure 60. Flashover occurrence ove
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90 precisely the exact moment when
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The amount of energy released when
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Figure 63. Thermal balance on a fue
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Figure 64. Flame spread under the c
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Access to fuel Figure 67. Fire’s
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The signs indicating that a fl asho
Page 103 and 104: Figure 69. Different signs indicati
Page 105 and 106: Smoke gas temperature 700 600 500 4
Page 107 and 108: 106 clear that we need to reach the
Page 109 and 110: This example illustrates how crucia
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Page 117 and 118: Figure 78. Fire pulsating. Figure 7
Page 119 and 120: Figure 81 (above). The fi re is abo
Page 121 and 122: Figure 85. The fi re resumes its de
Page 123 and 124: 122 is suffi ciently big. A fi re
Page 125 and 126: Figure 89. A backdraught entails a
Page 127 and 128: Figure 90. The picture shows a vent
Page 129 and 130: 128 The current which results in sm
Page 131 and 132: 130 fans should not be used. You sh
Page 133 and 134: Figure 97. A larger part is premixe
Page 135 and 136: Figure 101. There are still combust
Page 137 and 138: Figure 103. Sauna. A common scenari
Page 139 and 140: Fires in enclosed spaced with minim
Page 141 and 142: Figure 107. The smoke gases are str
Page 143 and 144: Figure 110. The fi xed lance versio
Page 145 and 146: Four fi re scenarios: 1. The fi re
Page 147 and 148: 146 to occur. If a backdraught occu
Page 149 and 150: 148 Fire at 62 Watts Street The New
Page 152 and 153: chapter 7 Smoke gas explosion Throu
Page 156 and 157: ity is also affected by turbulence
Page 158 and 159: 7.3.2 Course of action If smoke gas
Page 160 and 161: Test your knowledge! 1. Why do smok
Page 162: Corridor Space in between Sauna app
Page 165 and 166: 164 gases the fl ames would come ou
Page 167 and 168: Figure 119. Plan drawing of the bas
Page 169 and 170: 168 Glossary Adiabatic fl ame If al
Page 171 and 172: Heat of combustion, This measures t
Page 173 and 174: 172 Suggested solutions to test que
Page 175 and 176: 174 13. In a closed room there will
Page 177 and 178: 176 6. By cooling down the smoke ga
Page 179 and 180: Sample calculations Flammability li
Page 181 and 182: You should bear in mind that this c
Page 183 and 184: Backdraught Sample calculation: The
Page 185 and 186: E = (T f/T i)( N b/N u) Tf Ti Nb Nu
Page 187 and 188: v* = v/(g × h × b) 0.5 , which gi
Page 189 and 190: 188 gram), Institutionen för brand
Page 191 and 192: Index 62 Watts street 148 Adiabatic
Page 193 and 194: 192 List of fi gures Drawn illustra
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Enclosure fires

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