Enclosure fires

More documents

Recommendations

Info

Limited ventilation can lead to a fi re producing smoke gases containing signifi cant proportions of unburnt gases. If these accumulate then the admission of air when an opening is made to the compartment can lead to a sudden defl agration, moving through the apartment and out of the opening. This defl agration is known as a “backdraught”. 20 The National Fire Protection Association (NFPA) has the following defi nition: Backdraft is the burning of heated gaseous products of combustion when oxygen is introduced into an environment that has a depleted supply of oxygen due to fi re. This burning often occurs with explosive force. 20 The Institute of Fire Engineers (IFE) proposes the following: An explosion of greater or lesser degree, caused by the inrush of fresh air from any source or cause, into a burning building, in which combustion has been taking place in a shortage of air. 20 The defi nitions are similar and it is not really very important which one is used. In some defi nitions the word “explosive” is used, which indicates an explosion could take place. But this does not necessarily need to be the case; a backdraught often occurs in a fairly quiet way. We use our own defi nition in Sweden, based on the others. This states: A backdraught is the combustion of unburnt smoke gases, which can occur when air is introduced into a room where the oxygen content is signifi cantly reduced due to the fi re. Combustion can then occur more or less rapidly. A backdraught presupposes that a certain part of the content is premixed, otherwise the incident is not classifi ed as a backdraught. 31 A backdraught includes the following features: • Unburnt gases accumulate • An air-rich current • There is a well-mixed area of unburnt gases and air • An ignition source ignites the gases in the well-mixed area • A turbulent defl agration occurs in the room • A fi re ball is ejected out of the room. 125
Figure 90. The picture shows a ventilationcontrolled fi re, which has returned to being a smouldering fi re. Unburnt smoke gases accumulate. 126 A backdraught can occur when the fi re is ventilation controlled at a very early stage, for instance, if the room is closed from the outset or if there are only limited openings (see Figure 89). When the oxygen level drops the temperature in the room falls too. If the door to the room is then opened the smoke gases can ignite and cause a backdraught (see Figure 89). When a backdraught has occurred the whole room can be involved in fl ames, thereby resulting in a fully developed compartment fi re. A backdraught can cause signifi cant damage in the area outside, as there can be a very high pressure build-up there. 6.5.2 A typical backdraught scenario A fi re breaks out in a room. Combustion takes place either as fl aming combustion or a smouldering fi re. The room has a limited vent opening, e.g. normal leakage paths. The fi re grows until the level of oxygen in the room becomes restricted. During the same period, a hot smoke gas layer will have formed beneath the ceiling and it sinks downwards. As the smoke gas layer grows, the air, which is being drawn into the top part of the fi re’s plume, will contain ever-increasing quantities of unburnt gases and an ever-decreasing proportion of oxygen. This process results in incomplete combustion. As the heat release rate falls due to oxygen defi ciency, the unburnt gases accumulate in the hot smoke gas layer. Then, the fi re will either continue to smoulder or spontaneously go out, depending on the amount of fuel available and the size of the ventilation opening. As time goes on, the concentration of unburnt gases will increase. This produces a fuel-rich atmosphere in the room. The type of fuel plays a crucial role in determining the amount of gases which will have accumulated. Some materials emit pyrolysis products more easily than others. A door or window is then opened in the room. This can happen, for instance, when fi refi ghters enter an apartment or a window cracks. The hot smoke gases escape through the opening and air will be sucked in through the bottom part of the opening (see Figure 91). This infl ow of cold air is known as a gravity current. 32 When
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 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: Figure 89. A backdraught entails a
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?