Original post found at: http://firefighterworld.org/fire-behavior-forgotten-teachings/
I live in a state that is literally surrounded by water. As a child. I had often times sat next to a river and watched objects slowly float past, often times wondering why they don’t float back the other direction. I was curious and began to get deeper and deeper into the science of why this phenomenon occurred. Once I realized that gravity played a huge role in the direction of flow, I moved on to my next extemporaneously unanswered question.
Why do I relate this story? In the fire service we learn a vast amount about fire and its behavior during our first couple of year and especially during our pursuit for minimum standards. But have you ever realized that we never go back and review our teachings? I have heard many people speak of this “going back to basics” mentality but do they really go all the way back? Why would you go back to something unless you have forgotten it or are not using it? The basics are not something to go back to but to build on. We should always practice the basics, therefore never having to go back to them. But I digress. Let’s go back to the subject at hand: Fire Behavior.
During our infancy in the fire service, we learn about the science behind fire. We start with identifying the components of the fire triangle and graduating to the fire tetrahedron. We learn about the different sources of heat energy. We also learn about the phases of fire and how fire progresses from stage to stage. We learn about the condition each of these stages produce in the environment surrounding the fire. We also learn about thermal layering and how it occurs in a fire. We learn that disturbing this layering of heat is bad for us and should be avoided. We learn how we need to put “the wet stuff on the red stuff”. We learn that proper ventilation can help the firefighters inside to find the seat of the fire and help with visibility within the structure and even help with the survivability of those trapped victims. We learn about the advantages and effects of vertical ventilation and signs, causes and effects of backdrafts and flashovers. So you’re sitting there now probably thinking “yeah, yeah, I know all that”. But do you know it has all changed before your eyes, especially if you’ve been in the fire service for five or more years?
Why do I say that? Let’s look at the traditional teaching versus some of the new technology and studies that may affect how you view fire behavior. Traditionally we were taught that once a fire started and begins to grow it will reach the fully developed stage. The speed of growth is dependent on how quickly the consumables within a certain compartment reach the ignition temperatures and “light” off. Once the fire has reached the fully developed stage, it should progress to the decay phase and die out. The fire can go back to the growth phase if one of the contributing factors are added, such as more fuel or air (oxygenation). So far everything seems exactly like what we learned from our essentials book on fire behavior. But the change is the fuels and the compartmentalization of the new styles in building construction. My mom’s wooden furniture is not what I find in the big box stores today. Her stuff would burn clean with little smoke and would burn slowly. Not true for today’s furniture and room contents. Today’s furniture is almost 100% synthetics and burns completely differently. The amount of smoke and the heat generated by it is way more than that of before. So what does this mean for those fighting fires today?
The recent article. “Interrupting The Flow Path”, published by UL and Newscience, gives us some alarming facts. “In modern structure fires, the time between tactical ventilation and flashover istwo minutes, compared to eight minutes in traditional fire, giving firefighters 25% of the time they previously had to recover from poorly timed ventilation.”(UL, 2014) Wow! Think about that last statement. We now have 75% less time to recover from a dummy venting all the windows because he likes to break things. That can be the difference between life and death for some of us. Therefore, having a thorough understanding about fire behavior is paramount in today’s fire service.
The UL article discusses the positives about knowing the flow path and performing effective ventilation practices. But I’m more of a “show me” person. Below you will find a video attached. I will map out a timeline and things to look for during this timeline. The fire was lit and only straw and pallets were used in a semi-sealed compartment. No openings were created until later in the burn, which you will see in the video. This is a great example of a vent-limited fire and the effects of poor ventilation practices.
- Notice the temperature is about 180 to 200 degrees at the ceiling and the door temperature is only 75 degrees.
- At the 0:36 second mark, you can see a good picture of the thermal layering. This shows us that those fire gases are slowly banking down but they are not turbulent which means the fire is oxygen starved and is looking to breathe.
- At the 0:40 second mark they open the bedroom door. Notice the turbulence created and how it sucks (draws in) the air into the bedroom. The door behind them is quickly closed before it has an effect on the fire.
- At the 0:49 second mark you can see the temperature at the seat of the fire is a little more than 350 degrees and the fire doesn’t seem to be growing, but instead dying down. A second later the exterior bedroom window id ventilated
- At the 0:55 second mark you can see a better view of the fire.
- Between the 1:04 and the 1:22 mark you see significant change in the fire behavior. The fire has re-entered the growth phase and the temperature has doubled.
- At the 1:30 mark you can see how the fire has now created a flowpath by sucking in air from the window while breathing out hot fire gasses. The flames can be seen trying to escape from the window, while at the bottom of the same window, air can be seen drawing in and feeding the fire.
The video is just a sample depiction of how fire behavior is not just a simple PowerPoint lesson that we can watch once and store away in the back of our mind. The study of fire behavior is not a onetime thing; it has to be a continual learning process that is reviewed and studied regularly. Technology has become a huge help for us in the fire service. But technological advances in polymers and synthetics have also made our jobs a little more complicated. If we continually strive to stay on top of these changes in modern construction, we can keep from making fire behavior a forgotten teaching. In my next article I will cover a little more about flow path as part two of the fire behavior series.