Returning to the foundations: fire behavior


CarolinaFireJournal - Mike Coffey
Mike Coffey
04/21/2013 -

By now, we have figured out to what ends our means is accomplished as we take the task of fire fighting to heart. Understanding what fire is and how to interpret its life signs and shadows are essential to all firefighters’ safety and survival. The ability to use the information assists in our gaining the advantage on fire itself within structures, vehicles and wildland operations. With civilian injuries and fatalities noted daily, including our own injuries and deaths — this creates the needed mindset change to continue to learn and understand the demon we dedicate to tame.

image

Good, old-fashioned physics and chemistry define substances into physical states of solids, liquids and gases. Solid fuels are defined as materials with definitive size and shape, with the substances reacting very differently to the heat energy applied to them. Some melt — as in aluminum — or change into other forms — as in a vapor or gas state. Liquid fuels have no definitive shape, but do have definitive mass. They conform to the container they are confined. Once released, they will flow downhill and pool in the lowest place available. Several terms are used in defining if the liquid fuels will burn or need to be heated for burning to happen. Gaseous fuels and vapors are the most dangerous. They are in a state that mixes readily with air and are defined with having mass, but no definitive shape or volume. These can rise in the atmosphere or seek low spots, depending on some conditions. They all oxidize in some form or fashion.

Some forms of oxidation — elements or compounds combining with oxygen — are slow processes. Rust is oxidation of iron oxide in the atmosphere with the open water vapor and oxygen. Fire is known as a rapid version of oxidation that evolves heat and light. The extreme side of the oxidation spectrum is an explosion. There are two terms associated with this phenomenon — deflagration and detonation. Deflagration is fire that happens below the speed of sound or subsonic in nature. Detonation is fire that happens faster than the speed of sound or supersonic in nature. Either way, both are quick to show themselves when they arrive.

The science behind fire — or the technical term of combustion —traditionally depicted with the long-held ideal of the fire triangle, dictates that only three things are needed for combustion to happen. A fuel — some form of solid matter, liquid material or gas/vapor — mixed with oxygen from the atmosphere (21 percent of the makeup of air) and heated in some way (with chemical, mechanical, electrical or in rarest form — nuclear fission or fusion) creates combustion. Upon further scientific study, the fire tetrahedron — depicted as a pyramid-type illustration — developed another way to interpret the chemical reaction of combustion. Still needed are fuel (now known as the reducing agent), oxygen (known as oxidizing agent) and heat (temperature). Now, the mixing of ingredients — called the chemical chain reaction — begins the process of burning. Once the chain is sustained, combustion continues until the fuel or oxygen is consumed, temperature is cooled below certain levels or the chemical chain reaction is broken in some manner.

Now that the chemistry lessons are explained and you are out there yawning, let us break down and define some terms that help define why fire behavior is so important. The first term to define is endothermic meaning a substance or process that absorbs heat. Exothermic materials or processes are those that produce heat. Combustion is an exothermic reaction, whereas our personal protective equipment is endothermic, for the most part, in absorbing heat. The term pyrolysis is a term used to describe decomposition or transformation of a usually solid compound caused by heat. Wood pyrolyzes into a vapor for combustion when heated. Vaporization is a process where a liquid releases vapors or gases that can be burned.

Solid materials have properties that either slow the combustion process or speed it up based on the size of the surface exposed — also known as surface-to-mass ratio. Smaller particles show more surfaces to the heat and that allows easier ignition and faster fire movement. Solids in fine particulate states have extraordinarily high surface-to-mass ratios with little capacity to absorb heat, which burns quickly up to the point of detonation.

Liquids must be atomized before they can be ignited; their vapor pressure pushes vapors off the liquid. The higher the vapor pressure means a greater amount of vapor is produced into the air. When burnable liquids have high vapor pressures, the more intense burning will be. Heating any liquid increases vapor pressure; this increases vapor production. Also with liquids, the boiling point or a temperature at which liquids will convert to a gas/vapor at a vapor pressure equal to or greater than atmospheric pressure applies. Water has a boiling point of 212°F, whereas LP gas has a boiling point of – 44°F and gasoline is ranged between 100° to 400°F. Flash point is defined as the lowest temperature that liquids off-gas an ignitable mixture that will simply flash and not sustain combustion with an outside spark/flame. Fire point would be defined as the lowest temperature that liquids off-gas an ignitable mixture that ignite and continue to burn at which sufficient vapors are given off to sustain combustion process. Liquid fuels that vaporize at temperatures under 100°F to burn present a significant hazard to us in the streets.

With the fuels ready to burn, adding the heat (temperature) leg of the fire tetrahedron is two-fold — piloted ignition is when a mixture of fuel and oxygen encounter external heat with enough energy to start combustion or autoignition that occurs without any external flame/spark to ignite fuel gases/vapors. The extra term of autoignition temperature is defined as a temperature at which the surface of a substance must be heated for ignition and self-sustained combustion to occur. This always will be higher than the piloted ignition temperature. Speaking of ignition temperature, this term is defined as the lowest temperature that any fuel will off-gas an ignitable mixture that can self-ignite and sustain burning. Who said chemistry wasn’t fun?

Since gases or vapors are already volatile, just looking for an ignition source and liquids mimic the same patterns; two definitions of specific gravity and vapor density mean the same thing as specific gravity applies to liquids and vapor density applies to gases or vapors. Vapor density means the weight of a gas compared to air where the weight of air is one at 70°F at one atmosphere or 14.7 psi atmospheric pressure at sea level. Gases with more than one in vapor density are greater than the value of air or sink in air; gases less than one are lesser than the value of air or rises up in the atmosphere. In a fire environment — heat present that can cause heavy gases to rise until heat is lost and falls back to earth. Specific gravity means a ratio of mass of a given volume of liquid compared with the same mass of an equal volume of water. Water is valued similar to air as being one; less than one means the liquid is lighter than water so it floats on the surface of water, more than one means the liquid is heavier than water so it sinks below the surface and settles. These values also have hazardous materials implications as well.

Once we ignite the vapors of the fuels involved and combustion begins, our fire now grows and spreads from the incipient stage or beginning fire into the free-burning stage. Fires move throughout the area using conduction, convection and radiation. Conduction uses a medium to transfer heat; convection is movement of air currents as hot air rises and cooler air sinks; radiation is energy waves moving through the atmosphere in all directions. During this time, the terms of rollover, flashover and backdraft apply in this stage of fire development. Rollover is a term that describes conditions where the unburned gases in the upper layers accumulate and ignite with flames moving through that layer and across ceiling. The main difference from flashover is rollover being the involvement of fire gases only and not the other fuel packages. These flames do add to temperature generated in the compartment but rollover is NOT flashover. The event of flashover is known to be a rapid transition between growth and fully developed stages of fire. There is no way to determine exactly when this phenomenon happens. A second definition of flashover states this is the stage of fire when all surfaces and objects within a space have been heated to their ignition temperature with flame breaking out almost at once over all surfaces of objects in that area. Since this can occur instantaneously, flashover moves faster than firefighters can attempt to escape and radiant heat generated is not survivable for more than a few seconds even with PPE and SCBA. Warning signs for flashover can be smoke leaving a room turbulently (highly agitated) and rollover are significant signs that flashover is imminent. Flashover does not occur in every compartment fire and must have two determining factors:

  • Fuels have sufficient heat energy to develop flashover conditions
  • Ventilation — the compartment must have sufficient oxygen to reach flashover; sealed rooms may not provide enough oxygen

Backdraft has been defined as a sudden and explosive ignition of pressurized, superheated and oxygen-deprived gases within a closed space caused by reintroduction of oxygen. With today’s construction being so airtight (energy-efficient) and the fire load within the structure that has doubled or tripled in BTU output, there is even more reason to vent early.

Now these events have happened, fire evolves into the fully developed stage of burning. This is maximum heat output and maximum burning. All available fuel has ignited with heat being produced at maximum rate. Thermal radiation extends in all directions around fire. It can move downward due to radiation and burning materials falling on unburned fuel, fire spreads upward due to thermal column of hot gases and can burn above fire. Any exterior exposures are in trouble at this point, meaning more protection lines are needed and the fire flow is added.

The decay stage is the fire’s dying stage. Fuel has been used up, thermal radiation decreases, hot gas production decreases because of less heat being available because of lesser volumes of fire; eventually flames become embers because fire goes out because of lack of fuel. It’s “marshmallow time” at this point.

Now we have progressed through all the definitions, stages of fire development and some serious events that happen to us on the fire ground. Staying vigilant to fire behavior and building construction allow us to become the “lion tamer” using attack lines as our whips to contain fire’s ravages. Being aware of the surroundings using our senses keeps us from becoming another number in a study. Be safe and be careful. Enjoy the vacation season!

Captain Coffey currently serves as one of the training officers of the High Point Fire Department. He has been an instructor with the NC Community College system and the Office of the State Fire Marshal since 1990. He can be reached at [email protected].
Comments & Ratings
rating
  Comments




Daily Fire / EMS News

A collection of Fire / EMS -related news from around the web!

Get Aggregated RSS


View the full Fire - Rescue - EMS News section
for more articles

About the Carolina Fire Rescue EMS Journal

Welcome to the Carolina Fire Rescue EMS Journal! We want to provide you with timely online information and breaking news that best equips you to meet today’s emergency challenges. Among our firefighting articles, you will find the latest in firefighter technology, firefighter training, leadership development and the newest products and services presented in an “Act Now” user friendly format.  We want to be your best online source for the fire and rescue information, resources and reviews you need.
 
Regional Impact, National in Scope
  • Delivered free of charge to ALL fire departments, ambulance bays, rescue squads and hazmat teams in North and South Carolina
  • Quarterly circulation includes: fire academies, industry related technical schools and colleges and all major apparatus manufacturers
  • Regional & National trade show distribution
  • Largest circulated regional industry trade publication subscription base