Size of the Fire
Before we discuss fire growth, it is important to understand how we define the size of a fire. Combustion generates energy, referred to as a ‘heat release rate’ (HRR) and it is measured in Watts (W), or more typically for building fires; Kilowatts (kW = 1000 Watts) or Megawatts (MW = 1 million Watts).
To understand what a 'Watt' or 'megawatt' of fire energy looks like, here are some examples of typical fire sizes:
Candle flame - 80W
Small rubbish bin (wastepaper basket) - 100 kW
Single upholstered chair - 1 MW
Timber pallets stacked 3m high - 7 MW
The heat release rate determines the size of the fire. Therefore, the fire is measured not by its temperature, but by the amount of energy it releases.
Fire Growth
The development of fire is different when it happens in a compartment or an open environment. Most of our studies are focused on fires in a compartment for building safety.
The development of fire in a compartment has the following 6 stages:
1 - Incipient:
This is the stage before the ignition. The ignition is always preceded by heating of potential fuel. During this stage, it often produces smoke and other (toxic) products of combustion. This stage can be detected before ignition by gas or smoke alarms, and even by occupants smelling smoke. There is not enough heat to activate a sprinkler system or heat detector.
The incipient stage can have a duration of a few milliseconds or several days depending on different factors, such as the fuel involved, ambient conditions and heat source.
2 - Ignition:
The process of fire initiation involves ignition and the development of a chemical chain reaction (self-sustained combustion reaction). The ignition can be caused by direct flame contact or heat transfer via conduction, convection and/or radiation. Its source often has low energy and is very small, but its effect is usually sufficient to start a fire. Its origin might be deliberate or accidental and usually takes place in one of three ways:
· Piloted Ignition: Flame is initiated by a ‘pilot’, such as an electrical spark, ember or flame from a match.
· Spontaneous Ignition is where the flame develops spontaneously in the absence of a pilot flame or spark.
· Spontaneous Combustion in Bulk Fuels: Less common and is caused by self-heating of solids resulted from a chemical reaction, biological process or heating through oxidation of drying oils.
3 - Combustion (Flame spread):
After ignition, smouldering combustion can occur, which results in slow fire development and can continue for hours or days. It may die out or develop into flaming combustion. Flaming combustion may also begin immediately after ignition.
It is essential to understand that combustion always occurs where the fuel is in a gas state. Then the question is, what about wood, which is solid and gasoline which is liquid, aren’t they fuel?
Yes, they are! What happens is that liquid fuel evaporates converting liquid into gases. For solid fuel, the principle is called pyrolysis, which is the conversion of the substance to generate gaseous fuels. In both cases, the fuel is broken down into a combustible vapour. This vapour, when mixed with air and at the presence of heat, causes combustion.
However, each fuel has different miscibility, which is the capacity of mixing with other elements, and gaseous fuels will only ignite within a range of concentration limits. See the graph below.
Figure 1 - Combustion Mixture (Source: Principles of Fire Behaviour, J.G. Quintiere, 2017)
· Lower Flammability Limit (LFL) - the concentration of gaseous fuel and air that allows flame propagation with a small sufficient heat source (pilot)
o Below the lower flammability limit, there is too much air and not enough fuel (fuel-lean) to burn.
· Upper Flammability Limit (UFL) - the highest concentration of gaseous fuel and air capable of producing a flash of fire in the presence of an ignition source
o Above the upper flammability limit, there is too much fuel (fuel-rich) and not enough air to burn.
· Flashpoint (TFP) - Fuel surface temperature of the lower flammability limit (LFL), at which piloted ignition can occur
· Autoignition temperature (AIT) - Temperature at which gas can spontaneously ignite
· Fire point - is the minimum liquid surface temperature needed to sustain a diffusion flame
4 - Compartment Fire Development (Fire Growth Stage):
This can be split into three parts:
· Smoke filling: Fire growth to flashover (pre-flashover phase);
· Flashover; and
· Fire Development post flashover.
The understanding of the growth stage pre and post flashover is fundamental for fire engineering and fire safety. Due to smoke and heat produced in the pre-flashover, it is the initial risk for life safety. The post-flashover is most important when considering property protection, structural stability and fire spreading.
BElow we discuss these parts with more detail..
5 - Building Fire Spread
The building fire spread may be within the building or to an adjacent property. This stage usually happens on a fully developed fire (burning stage).
A fully developed fire is a fire that has reached its maximum extent of growth, usually extending throughout the fire compartment. The fire is in the fully-developed stage once flashover occurred.
6 - Decay:
As the fuel is consumed and is unable to sustain the maximum burning rate, the fire intensity decreases. It is called the decay stage and usually is defined when 80% of the fuel has been consumed. It is the longest stage of a fire, and two common dangers during this stage are the existence of non-flaming combustibles (which can start another fire) and the danger of a backdraft.
The image below provides a typical fire development.
Figure 2 Typical fire development (Source: Fire Engineering Design Guide - Michael Spearpoint, 2008)
Note: If sufficient cooling power is available, intervention with firefighting resources can control or extinguish the fire at any stage. The stages of fire development described above are not considering any firefighting intervention by automatic or manual systems.
As explained above, for a Fire Engineering Design, understanding the growth stage pre- and post-flashover is fundamental as this will provide information to determine the required fire safety systems in a building.
Fire Growth to Flashover
The fire growth to flashover is the stage where the ignition has already happened, and the size of the fire increases as it burns the fuel in the room. It can last for a few minutes or several hours. This pre-flashover stage is where automatic sprinkler systems are designed to operate, while the fire is small enough to be controlled or extinguished with a moderate amount of water.
Figure 2 - Growth Stage (Source: Fire Engineering Design Guide - Michael Spearpoint, 2008)
The growth stage is where the fire spread is governed by geometry, combustibility, and arrangement of the fuel, increasing the burning area and heat release rate until it is limited by either available fuel or air(oxygen) supply.
Figure 3 - Compartment room growth Stage (Source: Fire Engineering Design Guide - Michael Spearpoint, 2008)
During an early stage of fire growth, it is assumed that there is enough air at ambient temperature, and the fire's development is governed by fuel availability and geometry. It is termed the ‘fuel-controlled’ stage.
Smouldering combustion may happen during this early growth stage for an extended period, and the fire may grow in a short period when fresh air is introduced. Sometimes this growth occurs instantly, which is called a backdraft.
Flashover
Some building fires can progress to an event called flashover.
Figure 4- Fire Growth HRR x time (source: SFPE Handbook 5th edition, chapter 37, M.J. Hurley and E.R. Rosenbaum)
A flashover is the near-simultaneous ignition of most of the directly exposed combustible material in an enclosed area. The flames are no longer confined to the initially burning items, but it becomes a general burning of items everywhere in the compartment.
For a better understanding of this event, imagine a chair burning in a room. A few things are happening at the same time:
As explained previously, in a real fire scenario, there is incomplete combustion. Among other combustion products, the smoke generated by the fire contains unburned gases.
During the growth stage, this fire will grow as the fuel is consumed. This fire will heat all the surfaces in the room (i.e. furniture, walls).
When all the fuels in the room reach ignition temperature, all the combustible items (i.e. furniture, unburned gases in the smoke) will ignite at the same time. This phase is called flashover.
At the flashover:
There is a rapid increase in the heat release rate
The fire is then considered to be in a fully developed stage.
Because the fire is now too big, there is not enough oxygen in the room, and it limits the size of the fire. This new stage of fire growth is termed ventilation controlled fire.
The flame propagates through the smoke (unburned gases) under the ceiling
Some conditions which can lead to flashover:
The ventilation through doors and windows allows fresh air to enter the room, feeding the fire with oxygen and forcing the smoke towards the ceiling. As oxygen decreases, the smoke descends, and with increased heat, all combustible materials can ignite.
The fire loading keeps generating heat, the temperature rises, and all the other surfaces in the room are heated from radiation from the flames and hot smoke layer to reach ignition temperature.
The room temperature is the driving factor for the flashover as it usually occurs at 500-600°C when all surfaces of the room ignite at the same time.
Occupants in the compartment cannot survive after a flashover due to the lack of oxygen, high temperatures, and carbon monoxide concentration. The signs that a flashover is about to happen are:
A dramatic and rapid rise in compartment temperature and heat from gases at ceiling level
Fuel surfaces giving off fumes (pyrolysis)
Smoke rapidly descending to the floor
Tongues of flame visible in the smoke layer
Ignition of fuel vapours within the over-pressure zone
Other surfaces giving off fumes
Fire development post-flashover
The post-flashover stage is where there are relatively uniform conditions throughout the compartment space. The fire is in the fully-developed stage (high HRR and high temperatures), and the burning rate inside the compartment is controlled by airflow into space. Flames outside a window is an easy way to identify a post-flashover stage.
Figure 2 – Post-Flashover Stage (Source: Fire Engineering Design Guide - Michael Spearpoint, 2008)
At Nelligan, we have the expertise to help with the specification and detailing of your Fire Engineer Design, considering the fire growth and its stages and how to best protect against them.