Enclosure Fires - Chapter 1
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The aim of this book is to provide a deeper understanding of how fire behaves during enclosure fires. This book has been written primarily with firefighters in mind.
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Every year around a hundred people die throughout Sweden as a result of fire, with most dying in house fires. Fire also causes extensive damage to property, with the insurance sec- tor putting an estimated figure of 3.4 billion Swedish kronor (approx. £260 million) on the value of the property destroyed every year.
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This is why it is important to prevent fire. If considerable resources are channelled into fire prevention this can reduce both the number of deaths and the cost. When a fire breaks out it is vital that it can be tackled at an early stage, taking the appropriate action.
The aim of this book is to help provide a deeper under- standing of how fire behaves during enclosure fires. It focuses on understanding the processes involved in an enclosure fire. The main purpose, however, is not to look at how to actually fight this type of fire, by using smoke venting or applying a particular extinguishing medium, for instance, even though appropriate actions like these will be discussed in some sections. When discussing firefighting measures, reference will be made instead to relevant manuals dealing with smoke venting and extinguishing media.
Nor does this book claim to provide an exhaustive insight into this area. For instance, a fire’s behaviour in industrial premises is different, compared to in house fires. This book mainly describes fires in smaller areas such as flats or houses. Firefighters need to have basic knowledge about the factors controlling the behaviour of these types of fire. In this situation, as with other operations, there is a great deal of pressure in terms of taking the right course of action. It is also obviously very important to be familiar with and look out for the warning signs which can be picked up during an operation.
During the last 10 years great strides have been made in the research into these areas. This has led to a change in the approach to compartment fires in certain respects. Some of this research provides the basis for this book, along with the practical experience of the local fire service.
The book is intended to provide the basic material for teaching about fire development in Sweden’s Rescue Services Agency colleges. This is the group which the book is mainly targeted at. It may also be of interest, however, to people in other professions who encounter problems relating to enclosure fires. I hope that the book will manage to serve this purpose, both in terms of being used as a teacher’s guide and as a basic handbook helping to increase the knowledge about fire development.
I am sincerely grateful for all the assistance I received at the various stages in writing this book. There have been contributions from many people, in fact too many for me to name them individually. But I would like to express my heartfelt thanks to them, one and all.
A fire can break out and develop in many different ways. It is impossible to describe and predict every specific type of fire development, but we can provide a general understanding of how a compartment fire develops. A fire’s development is mainly affected by the quantity of combustible material and its arrangement in the fire room. The oxygen supply is another crucial factor.
If the compartment where the fire starts is closed its intensity will gradually decrease, which means that the temperature of the smoke gases in the compartment will drop. In some cases, a window may crack, for instance, and the oxygen supply provided as a result will give the fire new impetus. Concepts such as mass loss rate of fuel and heat release rate are important.
We usually use a fire growth curve to describe a fire’s development, as shown in Figure 1. This figure is vitally important and will be used in a number of places in the book.
The horizontal axis specifies time and the vertical axis the temperature of the smoke gases accumulated under the ceiling – this is assumed to be an average temperature. The figure shows possible paths for the fire’s development.
The period from ignition to flashover is referred to as the early stage of fire develop- ment. We will start by looking at this development.
During the early fire development stage (see Figure 1), the temperature will gradually increase if there is an opening, such as a window or door, in the compartment where the fire starts. This compartment can be a normally furnished apartment. The fire can progress to flashover, which means that any combustible surfaces in the area will emit pyrolysis products. The flames produced will completely fill the entire room space, which will generate very high levels of radiation. A per- son cannot survive a flashover. Wearing protective clothing allows you to withstand this only for a few seconds. This means that from the point of view of saving lives, one essential task is to prevent the fire from reaching flashover.
In the event of a flashover, the heat released by the fire in- creases dramatically and the fire can then be very difficult to put out. This makes the damage much worse. This is another reason why it is so important to fight the fire so that a flashover does not occur. It is certainly not the case that all fires progress to flashover.
In fact, according to statistics from the Swedish Rescue Services Agency, this happens in only a few percent of cases.
After a flashover occurs, it is mainly the access to oxygen which controls the heat release rate. This stage is known as a fully developed compartment fire. This stage in the fire’s behaviour is important in terms of the calculations for the building components’ bearing and partitioning capabilities.
When all the material in the compartment has been burning for quite a long time, the mass loss rate of fuel and consequently, the heat release rate decreases. This stage is known as the decay period.
The mechanisms which control flashovers, such as flame spread and re-radiation from the smoke gas layer, are discussed in detail in chapters 2 and 3. It is very important to learn to recognise the signs of an imminent flashover in order to min- imise the risks involved in the fire service’s operation. Recognising these signs can be completely crucial to the outcome of the rescue operation.
If there are only leakage paths in a compartment, which is otherwise closed, the fire does not progress to a flashover due to the lack of oxygen. The fire’s intensity diminishes before a flashover can occur. This can happen in many different ways, as shown in Figure 2 and is discussed in detail below.
Ventilation control means that the extent of the fire is determined by limiting the amount of oxygen. In many cases the fire will be ventilation controlled when the fire service arrives on the scene. The time it takes for a fire to reach ventilation control varies. Let us take the example of a TV set catching fire.
A burning TV set can generate between around 200 and 500 kW. This means that the oxygen in a normal-size room will already be used up 3–6 minutes after the fire has started. After that, the intensity of the fire will diminish and it then usually just smoulders or spontaneously goes out.
A large number of the house fires which occur in Sweden are ventilation controlled when the fire service arrives on the scene. But the problems remain as the firefighters open the door to the premises. Air will then rush into the room where the fire is, which may result in the smoke gases in the room igniting. However, this does not happen very often, only in a few per- cent of all fires (represented by line 3 in Figure 2).
If firefighters wearing breathing apparatus take the appropriate action and immediately cool down the smoke gases, the risk of the smoke gases igniting is kept to a minimum.
Figure 3. Percentage distribution indicating the extent of a building fire by the time the fire service arrives at the scene. Note that the fire has only spread in a few cases to other rooms when the fire service arrives.
In a few cases, the smoke gases may ignite very quickly and the flames shoot out of the room at a speed of a couple of metres per second. This phenomenon is known as a backdraught and is dealt with at great length in Chapter 6. Back- draughts entail major risks, which can even, in some instances, result in firefighters getting killed.
This is why it is very important to learn to recognise the signals warning of an imminent backdraught. A backdraught is illustrated by line 1 in Figure 2. In many apartment fires the impact of the fire is limited to a few objects and there is only slight smoke damage.
The fire is often still burning the first object when the fire service arrives.
There are mainly two common scenarios in this case. In the first scenario, the fire spontaneously goes out due to a lack of oxygen. It has hardly spread from the initial object. This is indicated by line 2 in Figure 2. The temperature in this case is fairly low, but there can be a lot of smoke in the apartment.
In the other common scenario, the fire is still fuel controlled when the fire service arrives. In this case, air is freely accessible and the fire is controlled by the amount of fuel. Fuel control may be due to the total fire load being small, which means that the fire does not release sufficient heat to cause a flashover. It may also be due to the fact that the fuel arrangement in the compartment, i.e. the combustible objects’ locations in relation to each other, means that the fire cannot spread from the initial object.
A fire’s behaviour is controlled mainly by the room’s geometry, the presence of any openings and their size, the type of fuel and the fuel’s arrangement in the room. Other contributory factors include enclosure surfaces’ thermal properties, such as density and heating capacity.
The apartment may be fairly full of smoke, but the temperature is often fairly low. This scenario is shown by line 4 in Figure 2. In order to be able to determine how far a fire will spread, it is important to have a good knowledge about flame spread and ignition.
During 1999 the fire service in Sweden was called out to around 11,000 fires in buildings. Some 6,000 of them were house fires. Statistics show that most fires can be tackled with- out any major problems. But there are a few fires which involve major risks. It is precisely for these fires that it is important to be properly prepared.
You should also remember that situations which are easy for the fire service to handle may be lethal for people who are in the fire room. In many fires people die as a result of smoke gas inhalation.
We have now described a variety of scenarios which could arise in the fire room. This should obviously not be interpret- ed as meaning that these are the only scenarios which can occur. The reality is much more complicated and the way in which a specific type of fire behaviour develops is controlled by a load of different factors.
We have already mentioned a few of them, such as the amount of fuel, the fuel’s arrangement and access to air. Other significant factors include the proper- ties of enclosure materials (e.g. walls, glass), especially heat conductivity.
To be able to understand a compartment fire, you need to have a good knowledge of the physical and chemical processes which control a fire’s development. This book contains descriptions of these elements, thereby providing a good basis for understanding the different types of fire development and their warning signs.