So, How do Fire Starts & Spreads? A question we need to understand and appreciate the dynamics of fire life cycle.

So, How do Fire Starts & Spreads? A question we need to understand and appreciate the dynamics of fire life cycle.

The figure on the right depicts a typical life cycle of a fire: its 4 stages of fire induction, growth, fully developed, and finally its decay stage. At the induction stage (also known as the initiation stage), the essential components of the fire triangle interact. The fuel in the correct proportion and physically mixed in the presence of oxygen and heat initiated the combustion chemical reaction: The fire is started The fire combustion reaction then begins to produce combustion products such as heat and smoke. It is uncertain at this point to know if the fire or combustion reaction can be sustained; this is very much dependent if the fuel is continuously feed into the chemical reaction.

At the growth stage (also known as the propagation stage), the chemical combustion process is sustained or self-sustained. Fuel can be feeding into the combustion process in 2 ways: (1) We call a Sustained Combustion, where we purposely feed the combustion process with a continuous supply of fuel, such as the provision of LP-Gas at the cooking stove, use of Bunsen burner at the laboratory workbench. Such Sustained-Fires are usually by design and well controlled, especially in the rate of reaction. We are quite familiar with all these sustained fires in our daily life, don t we? (2) A Self-sustaining Combustion, where the heat produced by the initial ignition process is sufficiently high enough to cause the surrounding fuels to also start to burn; thereby creating a chain reaction, which begins to grow in a selfsustaining way. Such Self-sustained-Fires are usually not by design; hence we are not in control of therate of reaction. In both cases, the fire is growing

Note that in the growth stage, a designed sustained combustion is usually well controlled in its rate of reaction and they proceed quickly into a fully developed fire bythe design fuel supply. However, due to the uncertain rate of reaction in the Self-sustaining Combustion process, Zone (a) is what we called a Flashover Zone in the growth stage. As the self-sustaining fire grows, more heat, smokes, and combustible fire gases are produced and trapped within the burning compartment. As thetemperature continue to rise and combustible fire gases continue to fill the compartment, there will come a point when the temperature is hot enough to cause an instantaneous ignition of all items within thecompartment A Flashover has occurred. A flashover is therefore a temperature-dependent fire phenomenon that will accelerate the rate of reaction rapidly and elevate the temperature of the fire within a short span of time, involving the burning of all items within the compartment at the same time; moving the combustion process into the next stage.

When the self-sustaining fire reaction reaches its near-steady state, we say that the fire has reached its Fully Developed Stage. At this stage, the fire continues to burn violently, consuming the available fuels and is characterized by the massive flames and high temperature in excess of 500 o C: The fire is spreading

At the decay stage, the combustion reaction begins to lose its steam. As the amount of fuel decreases, the rate of reaction also slowed down and eventually extinguished itself when thefuels are depleted. The fire is dying

Let s review the fire life cycle with the visual of a compartment fire and appreciate how fire starts and spread. From the visual, you would have noticed the interaction of the fire triangle at the seat of the sofa. As the fire reaction picks up momentum, air is draw in to support the combustion process, producing heat and smokes as products of combustion A fire plume is developing. As the heat and smoke layer rises to the ceiling, they are diverted horizontally left and right towards the edge of the compartment.

As the heat accumulates at the ceiling, it began to radiate downward towards the other contents of the room. As a result, gaseous pyrolysis products such as phosgene, hydrogen chloride, carbon monoxide, chlorine, etc. are emitted by the furniture and other contents within the compartment forming the deckhead combustible gas layer near the ceiling of the compartment.

As the combustible deckhead gases become superheated, they get ignited with the increasing ceiling temperature, a fire phenomenon called rollover will be observed to be burning across the deckhead within the smoke layer across the ceiling. Rollover is an indicator of eminent flashover occurring. When the compartment temperature reaches about 500 o C, all combustible gases will ignite and all combustible materials within the compartment will flash into flame A flashover has occurred, consuming all materials within the compartment instantaneously

After flashover, the compartment fire will developed into a fully developed fire involving the whole room or compartment; and eventually decay and extinguished itself when thecontents of thecompartment are depleted, as shown. So, you can see that the life cycle of a compartment fire can be described in the similar 4 stages of fire induction, growth, fully developed, and decay.

But how do heat moves from one place to another? We now look at the principles behind fire spread As in water that flow from high to low level; heat moves from high to low intensity and is transferred from a hot to cool surface. This is done by 3 key methods called conduction, convection, and radiation. Conduction is a method of heat transferred that involved direct contact of the surface material. In fire within buildings, fire spread by this method mostly involved solid building materials and elements of structures that are in contact with one another. It is not the most prevalent method of fire spread because most building materials are limited by their thermal conductivity before they are approved for use.

Convection is a method of heat transferred that involved indirect contact of the surface material. In fire within buildings, fire spread by this method mostly involved movement of gas-layer or liquid-layer from the hot area to a cooler area, thereby transferring the heat to the exposed cooler surface that came in contact with the convection medium such as the hot smoke-layer in a burning building. It is the most common method of fire spread within building and structure, limited by the cooling effect of as thesmoke-layer moves and theexposed combustible surfaces.

Radiation is a method of heat transferred that involved no contact of material at all. In fire within buildings, fire spread by this method mostly involved solid or liquid materials being exposed to the thermal Infrared (IR) electromagnetic radiation. Absorbed infrared radiation raises the temperature of the solid and liquid exposed, thereby transferring the heat from the distant source to the exposed materials. Radiation is the most common and prevalent method of fire spread within the building and even across structure at distant away; a very good example is the daily sunshine and warmth that we get from the sun. As long there is a line of sight and the electromagnetic wave is strong enough to reach, thermal radiation you will absorb.

Alright, what we had just discussed are the 3 basic scientific methods of heat transfer. But in the event the exposed material is directly below or near the fire, it may be subjected to combination of the 3 methods, we called Direct Burning (or also known asflame Impingement). We also need to appreciate that how fire spreads within a building and across structure is also influenced by other factors such as the initial design and intended use of the premise. These factors contribute to the eventual fuel load and occupant load of the building; and they make a big different if the intended use of the building is to be a warehouse or commercial building. The required protection for the elements of structures and ventilation needs can also vary vastly. All these factors do influence the resultant fire spread

Let's stop again for a quick look at the big picture. Insofar, we had begun to understand how fires start, spread, and how heat transfer methods played a part in thewhole fire life cycle. We had discussed the compartment fire dynamics, know the associated fire phenomena such as the rollover indicator, flashover point that led to a full-blown developed fire but there is one more that we did not discuss - a backdraft, or sometime called a smoke explosion that can occur at the decay stage. We also need to understand that in the industry, due to the nature and quantity of fuel and the cocktails of flammable materials that refinery, chemical plants, warehouses and factories are handling, the fire phenomena you may face may include pressurised jet fire, spill pool fire, a boiling liquid expanding vapour explosion, and a possible fireball. Again, it is not the intent of this topic to go into details and in-depth knowledge. But what is important for you as prospective fire safety manager to appreciate the various

fire phenomena and associated hazards when you operate in different workplace setting. With this understanding, you would see the important to provide effective training to your workplace Evacuation Warden Team, and the Company Emergency Response Team called CERT in short.