19 October 2011

Flashover in a Bottle

One of the most feared words in the vocabulary of modern firefighters is "backdraft." We fear the very thought of one, and with good reason. Despite what it is called in the fire service, the scientists that study fire and it's development simply see this as another form of a flashover, which by it's definition is a rapid change in a developing room fire to a full room fire. So, a backdraft would fit the classification of a flashover.

The backdraft phenomena is often classified into a group of dramatic fire behavior situations called smoke explosions. I would propose to you that a smoke explosion is simply a "bottled flashover."

Explanation

During a house fire, generally, the percentage of available oxygen to support combustion will drop until the fire self-ventilates. What you get as the level of oxygen drops the self sustaining chemical chain reaction will become less efficient and less complete, however the fire will continue to burn openly. When the fire self ventilates, either through burning through the area above the fire or by a window failure it will become less and less ventilation limited and more fuel limited.

If the fire is completely contained (such as in concrete buildings, building with energy efficient windows, or without windows at all) the heat will remain but will not trigger a reaction with the fuel in the air simply because the oxygen concentration is too low. The fire has become ventilation limited, and the fire enters a "hot smoldering" phase.

At this point, the heat remains very high at the ceiling level but may drop off as it gets closer to the floor. The smoke layer will be very low to the floor but may be cooler than expected. The rapid progression of the fire to flashover has been slowed down by the exclusion of oxygen by combustion products.This where the flashover becomes bottled, unable to find another oxygen source, it sits and it waits like a shaken soda bottle until someone comes along to open it.

With the opening of a secondary oxygen source (say during a poorly coordinated horizontal ventilation effort, or plain ignorance through improper size up) and the percentage of oxygen suddenly rising, the fire now has free reign to continue open burning as it is now no longer ventilation limited. The result as the fuel-oxygen ratio drops rapidly back into the explosive range is an equally rapid oxidation reaction.

Mitigation

The best way to handle the danger is to vertically ventilate as high as possible. It makes sense to ventilate superheated smoke and gas up and out of a structure. This removes the threat of explosion as you make entry into the structure by not only removing heat but also allowing the gases to exit. Horizontal ventilation is not to be used where you can help it.

Sometimes, however, you may be forced to use horizontal techniques to access a structure where these events are seeming to be occurring. John Norman (FDNY, ret) in his book Fire Officer’s Handbook of Tactics (2005) had an excellent example, and suggests creating a small penetration of the space and large amounts of water immediately directed in, utilizing an indirect attack method FROM THE EXTERIOR. The superheated gas will vaporize the water into steam, creating a barrier and absorbing the heat necessary to trigger an explosion as well as cool the seat of the original fire if it can be reached.

Since we are discussing the use of an indirect attack, it should be mentioned that at this point you are no longer concerned about rescuing occupants as the heat and fire gases will probably have already killed them. An indirect attack should only be used in situations where the rescue of occupants cannot be undertaken because of fire progression or structural instability.

Sources

Norman, J. (2005). Fire Officer's Handbook of Tactics (3rd ed). Tulsa, OK: Penwell Publishing.

Quintiere, J. (1998). Principles of Fire Behavior. Albany, NY: Delmar Publishers.

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15 October 2011

Starting From The Beginning

The candle post was a bit cryptic, but it is probably the simplest form of a flame I can muster and easily explain afterwards.

It is a great way to start to explain concepts that affect fire spread in a confined space, and why fire behaves the way it does. The best way to start is from the beginning on what combustion is, what it does, and later on I will launch into an explanation of tactics and strategies with these things in mind.

Simply, combustion is a sequence of chemical reactions that create heat, light, and the conversion of chemicals. The candle does a very good job of explaining this process because you can observe it safety.

The candle flame is created by the heating of the wick by an outside source. The heat begins the process of pyrolysis, which is the conversion of a solid into a gas by heating.

What happens as the heat rises into a plume, is that it creates air entrainment. Fresh air comes in at the bottom of the plume, enters the intermix zone where heated fire gases are chemically combining with oxygen to oxidize and burn, and the hot gases rises away from the plume. Generally, a single flame separate from other fuel sources isn't dangerous. It is when the heat plume is allowed to contact other fuels that it becomes dangerous.

See, when a fuel source is ignited and gives off heat, it will mushroom at the level where the gases are too cool to continue to rise. Put that in a box, and now you have a problem. The heat plume will mushroom way too early, causing the heat to bank back down to floor level. Through radiation alone the heat flux to surrounding fuels will not be enough to heat them to their ignition temperatures, but with the plume interrupted the heat will bank back down and multiply the heat flux, and things in the room will heat through convention.

As the inside temperature approaches 1000 degrees F, auto-ignition occurs and results in a flashover. The fire transition from ventilation limited (limited only by air flow) to fuel limited (limited only to available fuel sources).

10 September 2011

Swedish Expert Slams American Fire Service

Now, if you're following me over from hybridmedic.com, you know that at times I can be abrasive. So when I read a summary article on Dr. Stefan Svensson's speech at FRI in Atlanta, it got an equally abrasive response.

Not saying the guy is ALL wrong, but some of it stands to reason EXACTLY when he had the run in with Captain Obvious.
 "I can see it's a big problem but I find it hard to understand, as from my point of view it's obvious that firefighters need to be in good shape, but it does not seem to be like that over here," he said.
To quote a fellow firefighter: "well no screamin' eagle shit."

All one needed do is look at the USFA, NIST, IAFF, NVFC, and NFPA data on firefighter deaths and see that American firefighters are notoriously out of shape, leading to high incidence of cardiovascular disease. I mean seriously, all you need do is look at American waist lines at large to see that as a society we are getting fatter, and have developed a culture based around high fat and cholesterol diets and low activity.

The fire service feeds into this perfectly. Come in, eat a greasy breakfast, do some light housework and some paper training, then sleep all day occasionally making a call here and there. So what is Dr. Svensson's solution? Medical testing. You mean the NFPA physical we got when we were hired, and the annual we get at our family physician? Truthfully, not many will go to their family doctor for those vital check ups and top down physicals for many reasons: time, insurance, or a combination of the the two.

The next point he hits on is knowledge. Now he mainly talks about fire behavior, since that is his level of expertise, but really how much different is fighting fire in Sweden than it is in the United States? Well, considering things like building types, construction, regulatory differences, weather patterns, and the fact that the United States is much MUCH larger than Sweden, he have a diverse land area and a different construction type and material for each one.

What I am getting at is that we can't possibly design a standard teaching curriculum that reasonably covers ALL bases and levels of secondary education expected to properly understand the material, and the prospective firefighter have a chance of learning. We have tried in EMS education as well, but always run into the problem that the training time is too long or that the material is too difficult.

His final point comes with culture, but he gives no specific examples to what he thinks should be changed, just that we "do some things well." Do we reject that which does not work for us? Absolutely. Do we embrace something wholeheartedly without skepticism or reservation? Absolutely not. Is he suggesting that because we do not run to the next greatest trend that we have something wrong with us?

The injuries and deaths come as a breakdown in a system caused by many different factors. The key is strengthening that system to that failure becomes less likely. The best way is with prevention, both traditional fire prevention, but also prevention of poor health and poor decisions through teaching about nutrition and exercise, as well as a more stringent standard of education.

Read The Fire Critic's opinion here

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10 August 2011

Ventilation

It is the job of the truck at a fire to perform ventilation tasks during an active fire fight, either inside or outside the fire building. It is common for a company officer to split his crew into multiple parts with a different task, a couple for roof ventilation, others for interior ventilation and salvage, and some others for forcible entry. Some places don't have a true truck, or understaff a truck, so they have engine company personnel performing truck tasks. This is one of the most common truck jobs at a fire, so we'll just cover the ventilation team.

The ventilation team is what most people associate truck work with, it is the removal of the smoke and heat from the fire building using heavy equipment. Those are the people you see cutting holes in roofs and setting up positive pressure fans. Ventilation is accomplished either vertically or horizontally, with the easiest being horizontal, but no less complex. When handling vertical ventilation, the crew must "go to the roof" and it is primarily natural ventilation. Horizontal ventilation can be either mechanical (or forced), natural, or hydraulic.
Successful vertical ventilation starts before you even get to the fire. You must ensure that your power saws are fueled and will start up when you need them to and your tools are in their correct places. On the scene is not the time to be running around the truck looking for tools or fueling the saw. Remember the 5 P's: Prior Preparation Prevents Poor Performance

In order to perform this task, you need ladders. Of course, this is a ladder truck we are talking about, so there should be no shortage. I learned a tip from Tom Brennan many years ago that you should pick the length of ladder that starts with the number of the floor you want to go to, starting with the 2nd floor as 1. Most single ladders of 14' will reach the roof with plenty of length to spare. From there, you should sound the roof listening and looking for instability. It is recommended that you use a roof ladder to ensure stable footing in case of collapse. If you have sure footing, you may proceed without one, but you should consult your local SOP's before attempting.

Select a site to cut that is as directly over the fire as you can safely get, and as close. In order to gain footing on the roof without a ladder, you may use a hand tool like a axe or a halligan that has been driven into the roof deck. You now have a stable platform to use a power saw or an axe to open the roof for ventilation. Text books teach you to cut a 4' by 4' hole (and many SOP's as well), but you should use judgment and make the hole large enough to effect the proper ventilation. Too small and you won't have effective smoke and gas flow upwards, so usually bigger is better. I prefer a louvered cut, where the sections of roof are cut allowing them to rotate 90 degrees. This is because of overhaul and salvage, you can close the louvers when you are finished.

Once you cut the hole to proper dimension, you should open the louvers using a pike pole. You don't want to be standing anywhere near the hole when those are opened, just due to the release of heat and gas. Fire Engineering's Training Minutes has excellent videos here on vertical ventilation under Truck Company Operations.

Sometimes you will not be able to use louvered cuts to accomplish vertical ventilation. This could be due to roof construction, fire conditions, etc. You will still need to accomplish the mission, so you will need to adapt to those circumstances.

Horizontal ventilation doesn't require as many tools, but it requires more skill. You will need to coordinate the horizontal ventilation with the interior attack and manage which openings to open and when, because if you pressurize the structure with the fan or open the wrong window at the wrong time you can push fire and smoke back towards the attack team or on to victims who may still be viable saves.

Forced ventilation is just that, forced. You will be using a mechanical means to pressurize the structure to remove smoke and heat from the structure. Positive pressure ventilation (PPV) fans are the most common sight when you are performing forced ventilation. It is typically placed at the front door or a common entry point and creates a draft through the structure that removes the smoke. The cone of air created by the fan should cover most of the opening and you should open a window at the opposite end away from the fan. The wondow should be cleaned out completely, no screen, no frame, nothing. These things can reduce the amount of air flow at the point of discharge up to 60%.

Natural ventilation is using wind and natural airflow to ventilate a structure. You should open windows on the both the leeward and windward sides of the structure and use the cross ventilation to move smoke out. This is a very slow and inefficient method because the wind speed is not constant, so you will not get a very dramatic or fast change of conditions.

Hydraulic ventilation is where you would use water to create airflow. This is only really effective when you are using a fog nozzle. This works when you are directing the stream out of an opening, forcing the air following the water to entrain smoke and heat out of the window. You can use this method when you do not have personnel to perform ventilation right away, and it is typically the initial attack line that performs it in the room of origin, to clear the air for the next arriving crews to perform salvage and overhaul.

That is a quick overview of ventilation operations and by no means inclusive, and there are definitely more than one way to accomplish this task. You should always refer to your local SOP's and perform ventilation as instructed. However, you should keep these principles in mind while working, as a technique or procedure you have practiced will save you time when it comes times to use it and give much needed relief to the companies operating inside.

04 August 2011

Did We Create Our Own Problem?

I recently stumbled across a forum post on the Fire Engineering Community entitled "Safe and Aggressive?" After reading and giving some thought, I'll reply.

Bobby Halton is right, many teachers of firefighter safety have either intentionally or unintentionally made aggressive synonymous with reckless and careless. I think the best way to examine the vilification of aggressive operations is to look at what causes those teachers of overly cautious approaches to think that way.

The decline in fires over the past 20 years has led to a drying up of base knowledge of the "why" component of a lot of what we do, so we get concepts taught but not understood or fully explained, and hard lessons learned. I think this is partially to blame for the "Risk Nothing, Ever" type of safety models we see emerge. Knee jerk reactions to hard lessons learned and people are scared to make the same decision that had a negative result, but like the military we occasionally have to send people to and be sent into harms way. This lacks in fire officer training and firefighter training, that there is always a risk but through proper training we can reduce the risk by producing a standard response. The military teaches soldiers to kill by recreating the battlefield in a controlled environment, yet we teach people how to be firefighters in situations that are nothing like the real thing.

So as a result of training that is not so realistic, we get firefighters hurt by sending them into environments that they are neither ready for nor are they experienced enough to encounter. We get imprinting, they begin following the lead of the older generation with little explanation as to why they do what they do. Young firefighters assume that they must act this way every time, and so they do. What was aggressive operations is still aggressive, but is not done with skill, finesse, or professionalism of the previous generation. Recklessness ensures, more injuries occur. The chiefs above, seeing more and more injuries, become more cautious and what they see as an increase in danger potential is actually a generational gap forming.

I think aggressive should be augmented with other words. Fire operations should be aggressive, as well as coordinated, intelligent, and well practiced. The only way to get to those other words is through proper education and training.

01 August 2011

Abandoned Homes

A hazard for all firefighters are those neglected rentals and abandoned houses in our territories. They may be empty but that doesn't make then not hazardous. I have made many fires in abandoned houses and gotten a surprise each time, let me share one with you.

The Situation

You are dispatched to building fire in your first due territory. It's one of those pesky "next door to" calls, which usually means someone saw some steam from a drier outlet, thought it was smoke and did the neighborly thing to call for their neighbor. These usually end up in confused and panicked homeowner because now three fire engines and a truck are arriving in front of their house with all the red lights sirens. That's how it goes, usually.



Today doesn't seem to be much different. When your engine pulls up you don't see any smoke in the air, or any flames sillouetting a building against the night sky. However, there are several people jumping and pointing to what seems to be a clearing as the pop open the door and exit with full gear, and begin a trot up a long lane with the officer to where the people are pointing. You then see an abandoned house, boarded up,  dark gray smoke billowing from a gable end above another lower roof line, embers floating harmlessly into the sky.

Turning, the officer rushes past you to begin a 360 degree walk-around the building, while you relay back to the driver and hook up man to back the engine down the lane. This takes a few minutes as you assist the driver down the lane. "This is good," you think, avoiding a long lay of 2-1/2 inch wyed lines.

The main body of the house is average for your area, about 800 square feet, ordinary construction with signs of decent upkeep. There is a smaller add on structure on the BRAVO division. The boards make it difficult to determine how much smoke is coming out, but it becomes apparent what is going down when your lieutenant kicks in the front door and thick black smoke rolls out. The hook up man backs the engine into position and hands off a long line, a preconnected 200' lay of 1-3/4 inch with a 200 GPM nozzle. Advancing the flat loaded hose towards the door is easier than usual.


A step is missing to the front porch, so you pull slack on the line, step up, and pull on your mask and hood down then drop your helmet on top. Holding your breath, you turn the knob on your bottle and get airflow through the regulator with your first breath. You click your chest flashlight on. You can feel the heat as you walk forward. Heat kisses at the space where the mask and hood are supposed to cover.

"Charge my line"

You find a doorway and kneel in it as the water comes blasting from the fully open nozzle. The straight stream plays out as your whip the nozzle in a circle. You advance into the doorway with the nozzle still open, chasing the orange glow back towards another room. You feel the 2nd in engine company guys come in behind you, and the truck guys start on the ceiling and directing you towards the back room. You knock out the last of the main fire in that room and begin wetting down hot spots, including a burning door.

As the ceiling in the smaller part of the room is exposed, more fire appears in the attic. It is quickly knocked out. The foam that is mixed into the line at the pump makes quick work of the deep seated embers in the insulation. As the PPV fan roars to life and the smoke clears, you notice several key building components that made this fire and structure potentially dangerous...

Hidden Dangers

Several key findings were made, before, during, and after firefighting operations.

Construction types - the main building used a kind of stucco construction for interior walls, making it very sturdy. Lucky not much of the original structure was involved and extension into the attic was cut off by the fire attack, and just in time. The occupants that built onto the house cut a hole in the original exterior gable end that was covered by the roof over the addition for heating and air ducts.

Boarded windows and doors - made early fire detection by the people that lived on this small lane impossible, and the fire was allowed to grow unchecked and undetected for some time and created flashover conditions in the addition. Had there been a greater fire load (if the building was occupied) it would definitely have destroyed the addition completely and taken most of the original structure with it, but fire detection would have been much sooner.

Entanglement Hazards - the addition used lightweight ductwork for heat and air, and when exposed to fire, the wires that hold that ductwork open become exposed. Thankfully there were only two stretches of this ductwork in the involved area, so the amount of wires were kept at a minimum.

Other construction shortcuts - The roof over the addition was very flimsy, and this is because of the roof construction. The truss lacked the cross members you would normally see in a premanufactured roof truss. Also, the ceiling fans and other appliances were poorly attached to the ceiling, and they fell when exposed to fire. The original back door in the kitchen had been covered completely and made into a window. You could not tell from the exterior but it was fairly obvious outside. All in all, the add-on appeared to be do-it-yourself project with very little in the way of safety margins.

Lessons Learned

Be aware of the abandoned buildings in your area. Be observant to what kind of activity goes on around them, often times the homeless will use them for shelter or drug addicts as a place to get high out of public view. They are often the cause of fires in these structures.

Be prepared to preserve evidence in these structures, as they could be a crime scene or part of an arson investigation later on. An arrest may hinder on you doing your part to preserve evidence that is not in the way of accomplishing your task.

Fundamental firefighting techniques are crucial to efficient completion of the task. Nozzle control and practice deploying hose lines goes a long way in ensuring that there is no delay in getting water on the fire and then once water is on the fire, making it go out quickly.

Be mindful of the possibility of construction hazards but keep up an aggressive attack, and always have an escape plan.

Remember these things, and you will keep your culture of extinguishment intact while maintaining safe fireground operations.