Principles of fi re and explosion
7.4 Principles of fi re spread
Once a fi re has started and there is suffi cient fuel and oxygen to sustain it, there are three recognised ways in which it can spread within the building: convection, con-duction and radiation. There are also the effects of direct burning or heat transfer to take into account.
7.4.1 Convection
Due to its properties hot air rises; this can be seen graph-ically when smoke from a bonfi re rises and disperses within the atmosphere or a fi re is started within a grate and rises up through the chimney.
The convection process begins when combust ible materials are subject to excessive levels of heat and they give off a vapour which in turn ignites. When these vapours are heated they expand and become less dense than air. As they rise they leave an area of low pressure which is replaced instantly by cooler unheated air. This fresh air is then mixed with the vapour and heated, assisting in the development of greater temperatures. The process is cyclic, continuing to support the fi re process.
Convection is the most common cause of fi re spread within buildings and structures. During a fi re hot gases and vapours (smoke and heated air) will rise vertically through stairwells, lift shafts and service risers to the highest level available. They then form a layer at that height, from which they spread out horizontally until checked.
As the temperature of the smoke and now the toxic gas/vapour layer increases, heat is radiated back downwards and may ignite other combustible mater ials in the vicinity. Modern research also shows that the Figure 7.13 Class ‘C’ fi res
Class D – fi res that involve metals such as sodium, lithium, manganese and aluminium when in the form of swarf or powder.
Class F – fi res that involve cooking mediums such as vegetable or animal oil and fats in cooking appliances.
Such fi res are particularly diffi cult to extinguish as they retain considerable heat allowing the chemical reaction to restart.
Figure 7.15 Class ‘F’ fi res
It is worthy of note that there is no classifi cation for electrical fi res; this is due to the fact that electricity does not actually involve any fuels which can be extin-guished. Fires that involve electrical circuits and appli-ances, cables, etc. can either be extinguished with a non-conductive extinguishing medium, or the supply can be isolated and the actual material (given its own ‘class’) can be extinguished.
Figure 7.16 Boxes containing matches being stored adjacent to a light fi tting
Figure 7.14 Class ‘D’ fi res
Principles of fi re and explosion
unburnt fuel (unburnt products of pyrolysis) in the smoke can reignite on reaching its spontaneous combustion point causing a substantial rise in temperature. This rise in temperature, if provided with a fresh source of oxygen (window glazing fails), has the ability to cause a fl ashover or explosion.
Clearly there is a substantial difference in a fi re starting in a confi ned area such as a building, in compari-son to one that starts in the open air and it is this issue that will need to be addressed when designing buildings that minimise smoke and fi re spread enabling people to escape safely.
Convection and the effects of smoke
Although it is important to restrict the spread of fi re within a building, it is equally important to consider the speed of spread and effect of the smoke created by the fi re as it burns.
As a fi re develops it will create large quantities of smoke which will, usually, spread ahead of the fi re quickly fi lling a building. The effect of this is to present a toxic and/or asphyxiant hazard to people within the building.
Smoke also reduces visibility and obscures escape routes; this linked to people’s natural reluctance to walk into or through smoke can lead to, or increase, panic, which in turn leads to disorientation reducing the chances of safe escape.
considerably according to the type of material, e.g. metal is a much better conductor than brick. It should be noted that conduction may occur in solids, liquids or gases; however, in relation to fi res within buildings it is most prevalent in solids.
The thermal conductivity (the ability to conduct heat) varies between materials and is a key element in building design and construction, which will be con sidered in Chapter 9.
A fi re in one room can spread to adjacent rooms by heat being conducted through the fabric of the building (walls/ceilings, etc.), especially via metal pipes or frames used in building construction. The heat can then ignite materials in direct contact with the surface, or radiate out from the surface. This can raise the temperature of materials in the adjacent room to their spontaneous combustion temperature, thus spreading the fi re.
The relative conductivity of building materials is therefore an important factor in the fi re resisting ability of a structure or building. This issue is considered within the Building Regulations Approved Document B.
Figure 7.17 Smoke spread – convection 7.4.2 Conduction
Conduction is the movement of heat through a mater-ial. The ability of conductors to transfer heat varies
Figure 7.18 Fire spread through a fi re resisting wall by conduction along a steel pipe
7.4.3 Radiation
Radiation is the transfer of heat energy as electro-magnetic waves, which heat solids and liquids (but not gases) encountered in its path.
Fire radiation paths do not require any contact between bodies and move independently of any material in the intervening space. If not absorbed by fi re resist-ant material the electromagnetic radiation can radiate through glazing causing fi res to spread and involve a number of compartments/rooms. As with the example of a heater or open fi re, fi res can spread to combust ible
items left to dry or from building to building when heat from a fi re may be radiated to an adjacent building by passing through windows, and igniting combustible contents in the second building.
heated to an ignition temperature by coming into con-tact with a burning material causes fi re to spread.
7.4.5 Fire growth
The rate at which a fi re grows will depend upon numer-ous factors and it should be noted that a single factor on its own may not promote fi re growth but interreacting factors may develop a fi re more rapidly. A rapid growth rate will have an effect upon areas such as the stability of the building and the effectiveness of the emergency plan to ensure that people can leave the building safely.
The fi re growth rate is generally recognised as the rate at which it is estimated that a fi re will grow; this includes spread of fl ame over surfaces and behind linings, and within any part of the contents. Fire growth rates may be categorised in accordance with Table 7.2.
Factors that may affect the growth rate include the:
➤ Construction and layout of the building
➤ Ventilation into, throughout and out of the building
➤ Use of the building (including the types of activity being undertaken)
➤ Fire loading within the building.
Construction and layout of the building
How the building or structure is constructed in terms of its materials and the quality in which the materials have been used within the building has an effect upon any potential fi re growth rate. Clearly, buildings constructed of wood have the potential to speed the fi re growth rate;
however, in itself, due to the nature of wood (strengthens when it is burnt) the fi re growth rate would be affected more by voids between fl oors, ceilings and roofs in wooden buildings than the use of wood itself.
The size and layout of a building also has the potential to affect the growth rate. When a building has high ceilings, such as in the case of atria (found in shopping malls), fi re growth is likely to be much slower than those buildings with low ceilings, or those that
Category Fire growth rate Examples
1 Slow Open plan offi ce – with limited combustible materials, stored or used 2 Medium Warehouse – which is likely to have stacked cardboard boxes, wooden pallets 3 Fast Production unit/warehouse – baled thermoplastic chips for packaging, stacked plastic
products, baled clothing awaiting delivery
4 Ultra-fast Production unit/warehouse – fl ammable liquids, expanded cellular plastics and foam Manufacturing, processing, repairing, cleaning or otherwise treating any hazardous goods or materials
Table 7.2 Fire growth rates
Heater Towel drying
Figure 7.19 Fire spread by radiation from heater to com-bustible material
Figure 7.20 Fire spread by radiation from one building to another
7.4.4 Direct burning
When combustible materials come in direct contact there is a physical transfer of heat from the ignition source to the material which in turn releases vapours which ignite and propagate the fi re. It is true to say that direct burning makes use of one or more of the previ-ously discussed methods of heat transfer; however, it is appropriate to mention this method as a reasonable proportion of fi res are started in this way and when
Principles of fi re and explosion
include basements. On these occasions, refl ected radiated heat, due to the lack of height available, will develop the fi re more rapidly and raise the growth rate.
The use of sandwich panel walling, containing polystyrene (used as thermal insulation for areas such as cold storage facilities), has historically resulted in rapid fi re growth, due to the substantial heat release of the building material, i.e. the polystyrene. The failure of component parts, due to the rapid fi re growth rate of the materials, also has the ability to destabilise the panels and cause premature collapse.
Whether included as part of the building construc-tion or as part of the contents of a building, wall and surface lining materials also have a direct effect upon fi re growth rates.
Approved Document B of the Building Regulations classifi es performance of internal linings. These will be discussed in future chapters; however, the principles involve ensuring that the internal linings should adequately resist the spread of fi re over their surfaces and, if ignited, a rate of heat release which should be reasonable in the circumstances.
In this paragraph ‘internal linings’ means the mater ials lining any partition, wall, ceiling, or other internal lining, such as plasterboard, plaster wall coverings including paper. Clearly hessian and materials such as polystyrene tiles have the ability to release large volumes of heat and thus would greatly affect fi re growth, whereas plaster, plasterboard, etc. release heat very slowly and would not adversely affect the fi re growth rate.
Ventilation
As previously discussed the supply of oxygen is critical to the development and spread of a fi re. Fire growth rate
is inextricably linked to the supply of oxygen and there-fore affected by the ventilation of a building. Air condi-tioning and air circulation systems provide a fi re with a ready source of oxygen via forced ventilation, which will aid fi re growth. The presence of dust, vapours and fumes within the atmosphere also have the ability to affect the fi re growth rate in so far as they provide a rich source of fuel. It should also be noted that appropriate levels of dust in the atmosphere may cause an explo-sion; this subject is discussed later in this chapter.
When mechanical smoke extraction systems are utilised within a building the effect of fi re growth is notably less. The smoke laden air containing particles of fuel and substantial quantities of heat is removed from the atmosphere by the extraction system reducing the speed at which a fi re grows, while assisting smoke ventilation and allowing persons clear, smoke-free, escape routes.
Use
The use of a building is directly related to the type of occupancy. Building use can be categorised in the following groups:
➤ Offi ces and retail premises
➤ Factories and warehouse storage premises
➤ Sleeping accommodation such as hotels, boarding houses, etc.
➤ Residential and nursing homes
➤ Teaching establishments
➤ Small and medium places of assembly – public houses, clubs, restaurants, etc.
➤ Large places of assembly (more than 200 persons) – shopping centres, conference centres, etc.
➤ Theatres and cinemas
Figure 7.21 Cross-section of sandwich panel
Fire Fire stop
Floor or Wall
Possible route of fire spread if fire stop absent
Sandwich wall panel with combustible core
External facing or structural support member Combustible core capable of giving off toxic combustion products, e.g. foamed plastic
Fire-protecting inner lining, e.g. plasterboard
Fire
(a) (b)
Junctions between sandwich panels and fire separating walls or floors should
always be fire-stopped
Use wall panels with an effectively fixed fire-protecting inner facing such as plasterboard
which does not decompose, disintegrate or shatter in fire
➤ Healthcare premises
➤ Transport networks.
Clearly, the use to which a building is put will greatly infl uence the contents within. In addition the quantity of combustible and fl ammable materials within a build-ing will also refl ect its use. The nature of the contents of a building is a key factor in determining the rate of fi re growth and spread in the event of fi re.
Fire loading within the building
The contents of the building and therefore the growth rate of a fi re will vary according to the materials and activities being used or undertaken, for example where a signifi cant amount of dangerous substances or prepar-ations, e.g. substances or preparations that have a fast or ultra-fast fi re growth rate or are classifi ed as explo-sive, oxidising, extremely fl ammable, highly fl ammable under the Chemicals (Hazard Information and Packaging for Supply) Regulations 2002, are stored and/or used, the area is considered to be of high fi re risk.
When large quantities of readily combustible products are stored or displayed under a large open plan mezzanine or gallery with a solid fl oor (as in some DIY outlets) there is always a risk of rapid fi re growth resulting in fl ames spreading beyond the edge of the mezzanine or gallery fl oor, hence posing a threat to life safety, particu-larly when the occupants of the building are members of the public and are likely to be unfamiliar with the emergency plan (builders’ merchants, DIY stores, etc).
Figure 7.22 Example of fi re load from storage containers in a warehouse
Not only should the contents and activities be considered in relation to fi re growth, but also the arrangements for the storage, of materials with potential for high heat release.
Heat release from materials is measured in megawatts per square metre (MW/m2). Some mater ials generate much greater MW/m2 than others. For example, wooden pallets that are stacked 1.5 metres high are likely to release 5.2 MW/m2 whereas polystyrene jars packed in cartons stacked at the same height have a heat release of 14 MW/m2.
Table 7.3 shows some examples of common commodities together with their known fi re loading in terms of megawatts per square metre. When consid-ering the fi re risk associated with stored materials it is important to think about the heat that may be generated when materials are involved in a fi re.
It is also widely recognised that the containers within which some materials are stored add signifi cantly to the fi re loading within buildings, most notably bulk warehouses.
The Hazardous Installations Directorate report on chemical warehouse hazards states that fl ammable liquids in plastic, intermediate bulk containers (IBCs) present a very high risk because they inevitably fail in the case of a fi re releasing their contents, adding, as a rule of thumb, around 3 MW/
m2 to the total rate of heat release each.
As can be seen from the examples above and the issues discussed in both construction and ventila-tion, fi re growth is not based on any one element but a combination of all. This will need to be taken into account when undertaking fi re risk assessments and any subsequent action plans.
Table 7.3 Common commodities together with their known fi re loading
Commodity Heat release
MW/m2
Wood pallets, stack 0.46 m high 1.4 (5–12% moisture)
Wood pallets, stack 1.5 m 5.2
high (5–12% moisture)
Wood pallets, stack 3.1 m high 10.6 (5–12% moisture)
Polyethylene rubbish litter bins in 2 cartons, stacked 4.6 m high
Polystyrene jars packed in cartons, 14 compartmented, stacked 1.5 m high
Polystyrene tubs nested in cartons, 5.4 stacked 4.3 m high
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