Enclosure fires

More documents

Recommendations

Info

6.1 Pulsations In a ventilation-controlled fi re the heat release rate is limited by the amount of oxygen fl owing into the compartment. This can sometimes cause the fi re to “draw breath”. Pulsations start as a result of the heat release rate falling due to the limited amount of oxygen. When the temperature drops the volume of gas in the fi re room will also drop, thereby creating a certain amount of negative pressure. Air can be drawn into the fi re room again and when the oxygen reacts with the combustible gases, combustion occurs. When the smoke gases ignite the volume increases, which generates positive pressure. Smoke gases are then forced out through the available openings. This causes a lack of oxygen again, which limits the release rate, thereby triggering the next pulsation. Fire pulsations are shown in Figure 79. It is diffi cult to determine in which situations a fi re may start to pulsate. One of the factors causing pulsations is the size of the openings in relation to the heat released. 6.2 Fire has spontaneously gone out If the fi re continues burning in an oxygen-defi cient environment it is very likely that gradually, it will spontaneously go out or start smouldering. This probably happens before the temperature in the fi re room has risen suffi ciently to trigger signifi cant pyrolysis of other fuel surfaces in the room. If the fuel surface is also limited the chances of the gaseous mass being able to ignite will be low. When the temperature drops the pressure will also fall in the fi re room, which means that so many smoke gases will not be forced out. As a result, it can take a long time for the fi re to be detected. This situation is very common in apartment fi res. Often the apartments are affected by quite extensive smoke damage, but there are very often no problems at all in extinguishing the fi re. In this situation too, the conditions in the fi re room will be very dangerous for people. The smoke gases contain a number of hazardous substances, such as carbon monoxide, which can mean that someone who is asleep will not wake up when the Temp Figure 80. The fi re has spontaneously gone out due to oxygen defi ciency. 117 Oxygen deficiency
Figure 81 (above). The fi re is about to spontaneously go out in a closed room. The smoke gases sink down above the seat of the fi re. The only clear visibility is at fl oor level. Figure 82 (right). The smoke gases contain a number of toxic products. 118 CO2 HCN CO2 CO C HCN NO2 CO H2 fi re is burning. In this situation fi re detectors are vital. They can really save lives. Figure 83 shows the result of a compartment fi re. The fi re starts in a closed room. As you can see, the fi re becomes ventilation controlled after about 300 seconds, when the temperature and heat release rate start to drop. The temperature falls quickly and pyrolysis of the material stops fairly rapidly. In this instance, there are not enough combustible gases available for the smoke gases to be able to ignite. But people without any breathing apparatus can still die in the room due to the toxic smoke gases. When the door C HCl CO
Page 1 and 2:
Lars-Göran Bengtsson Enclosure fi
Page 3 and 4:
The content of this book may not be
Page 5 and 6:
4.4 Risk assessment 99 Smoke gases
Page 8 and 9:
Foreword The aim of this book is to
Page 10:
Overview This book assumes that its
Page 13 and 14:
Figure 1. Fire growth curve featuri
Page 15 and 16:
Fire extinguished/burnt out Just sm
Page 18 and 19:
chapter 2 How a fi re starts People
Page 20 and 21:
smoke gases in the compartment, but
Page 22 and 23:
Pyrolysis gases material is subject
Page 24 and 25:
Ignition time Ignition time can als
Page 26 and 27:
Smouldering fi res can therefore re
Page 28 and 29:
2.4.1 Thermal inertia kρc The fl a
Page 30 and 31:
grow to 50 cm, then a 1 m high fl a
Page 32 and 33:
Flammability in solid materials is
Page 34:
6. The fl ame spread rate varies wi
Page 37 and 38:
Figure 22. Smoke gases start to acc
Page 39 and 40:
Figure 25. The different sections i
Page 41:
The production of unburnt gases is
Page 44 and 45:
Heat release rate from a fuel surfa
Page 46 and 47:
Carbon monoxide (CO) is the next mo
Page 48 and 49:
ment to accelerate, but this is obv
Page 50 and 51:
is far too high for a premixed fl a
Page 52 and 53:
is fi lled with fuel molecules, con
Page 54 and 55:
Figure 33. Flames on the under side
Page 56 and 57:
Flammability limits Vent For a prem
Page 58 and 59:
CH4 + 2 + 2 + + 2O2 = CO2 + 2H2O +
Page 60 and 61:
Temperature impact on fl ammability
Page 62 and 63:
energy is required, which can be ex
Page 64 and 65:
ustion takes place in under-ventila
Page 66 and 67:
If turbulence affects the system th
Page 68 and 69: Neutral plane The differences in pr
Page 70 and 71: Where the differences in temperatur
Page 72 and 73: Positive pressure Negative pressure
Page 74 and 75: Dp = 353(1/Ta - 1/Tg)gh Let us take
Page 76 and 77: Figure 51 (top). The pressure condi
Page 78 and 79: Vent just during the early stage of
Page 80 and 81: In Figure 57 calculations have not
Page 82 and 83: 3.4 Summary In many situations the
Page 84: oom and in a fairly closed room (on
Page 87 and 88: Figure 59. The fi re has spread to
Page 89 and 90: Figure 60. Flashover occurrence ove
Page 91 and 92: 90 precisely the exact moment when
Page 93 and 94: The amount of energy released when
Page 95 and 96: Figure 63. Thermal balance on a fue
Page 97 and 98: Figure 64. Flame spread under the c
Page 99 and 100: Access to fuel Figure 67. Fire’s
Page 101 and 102: 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
Page 111 and 112: 110
Page 113 and 114: 112
Page 115 and 116: 114
Page 117: Figure 78. Fire pulsating. Figure 7
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 154 and 155: ignition source required to ignite
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
show all

Enclosure fires

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?