By Jason Sowders
The United States military is our nation’s most elite group of individuals; they have led our nation through many battles and won many victories, but none of these victories have come without a precise and well-thought-out method of attack.
In war, many hours are spent in reconnaissance training to figure out the enemy’s strengths and weaknesses before engaging them. A ground war is fought and won not just by using a minimum amount of artillery but with overwhelming force designed to obliterate the enemy.
Firefighters are now engaged in a war, like it or not, that has already invaded our homes and is ready to show itself in a very hostile manner: petrochemicals. Your favorite chair, sofa, bed, etc. may seem quite harmless, but these pieces of furniture are helping to kill firefighters in a subtle fashion.
HEAT RELEASE RATES
The heat release rate (HRR) of petrochemicals is without a doubt catching today’s firefighters off guard. They need to be treated as highly flammable sources of fuel.
The HRR is a measurement of heat energy released over a specific unit of time. A fuel has a specific heat of combustion or a total amount of energy released when burned. Different materials possess a given amount of potential energy–a bottle rocket has a low potential energy compared to a stick of dynamite. This is of concern to firefighters when this energy, measured in kilojoules per gram, is released as kinetic energy by the combustion process. How fast this energy can be released is the HRR, which is measured in kilojoules per second. The potential energy of a synthetic material such as polystyrene yields energy of 39.58 kJ/g; this is almost identical to propane, which yields 46.45 kJ/g. The HRR is an exponential phenomenon, meaning that heat makes more heat. A high HRR directly indicates a high threat to life, rapid temperature changes, faster flashover times, and an increase in the products of combustion. A single overstuffed chair made out of polystyrene has enough energy to drive a 10x 10 room to flashover, and the HRR can be viewed as the engine driving the fire.
We should not depend on flow rates any less than 150 gpm for interior structural fires. Flow rates below 150 gpm are inadequate to overwhelm compartments that are filled with synthetic materials. The HRR given off by these synthetics must be controlled with the correct stream and volume of flow to overwhelm such rapid fire events.
You enter a structure with a 200-foot preconnect handline combination nozzle set on straight stream flowing 150 gpm with 170 psi at your pump panel. Before making entry, your captain wants the engineer to just back the pressure down to 130 psi so they can move it around easier (sound familiar?) Are you really receiving adequate flow? You have just dropped your psi by 25 percent, so you now have 115 gpm, which is woefully inadequate and not a safe practice. In such a case, you may be merely containing the fire, not extinguishing it.
Let’s take a look at streams produced from the combination nozzle in comparison to the solid streams produced by the smooth bore. The combination offers you two stream choices; let’s start with the fog stream.
The 1950s brought about the use of the fog stream application by Chief Lloyd Layman, who, at the time, conducted studies based on Coast Guard and U.S. Navy experiments. In these studies, the fog pattern had great heat absorption abilities. Two requirements for success with this stream were 1) ceiling temperatures of at least 1,000º, and 2) the fire being attacked had to be located in a sealed compartment. He also stated that the stream should be remotely injected to ensure no injuries to firefighters from super-heated smoke and steam, a method called indirect attack.
In none of Layman’s findings did he mention using this technique in an area where firefighters or victims would be. The fog stream has a much larger surface ratio and little if any of the broken stream makes contact with solid surfaces or fuel source. Remember, our goal is to apply water to the fuel source, not to just cool off the thermal layer. Many advocates of the combination nozzle state that they prefer this type of nozzle for the versatility to adjust the pattern, narrow or wide. Others also state that they would never actually open up to a fog pattern when they are operating inside because of steam conversion. Then why take the fog nozzle into the structure to start with?
The answers to this usually include the desire to hydraulically ventilate, the need for the extra protection, or just in case. Fog streams flowing 150-180 gpm produce more than 6,000-10,000 cubic feet per minute (cfm) compared to a solid tip 15/16, which produces 510 cfm, but our main focus here is fire suppression. If our main focus is stretching the initial attack line for ventilation purposes, we need to reevaluate our thinking. Let’s leave the ventilation to the truck companies. Our main focus for the initial stretch should be extinguishment.
We have been fooled for many years believing that a curtain of water between you and the fire is protection. What is occurring is that you are pushing heat, fire, smoke and other products of combustion out in front of you–not a bad thing if you are fighting a vehicle fire outside. The pushing effect gives you a false sense of security because what you have pushed away is likely to return to you. Fire, heat, and smoke will seek the path of least resistance and potentially extend your fire vertically and laterally.
I have witnessed in several training evolutions combination nozzles advanced into structures and rotated accidentally by whatever they come in contact with. One would not like to be on the receiving end of the fog stream that has been injected into the thermal layer. While I personally advocate the use of solid streams, if a combination nozzle is the only option, it should be used in a straight stream position with an adequate rate of 150 gpm. A solid or straight stream will provide a rapid knockdown with less violent disruption of the thermal layer and is more likely to reach the seat of the fire because of less premature conversion to steam or the stream getting carried away by convection currents.
Although their use in direct attack is similar, straight and solid streams have distinct differences. A straight stream, in essence, is a very narrow fog stream. It is produced by a combination nozzle and is made up of millions of tiny water droplets divided by air entrained within the stream. Narrow streams produced by combination nozzles are not “solid streams,” which are produced by a smooth bore orifice and entail a compact, solid cylinder of water as it leaves the nozzle. Proper tip pressure will allow this stream to remain compact for a considerable distance before friction with the air; gravity, and other factors destroy the quality of the stream. One important reason solid streams are more effective than straight streams in interior fire attack concerns the water droplets.
When a solid stream is deflected off the walls and ceiling, it produces droplets of sufficient mass and size to reach the burning fuel without being carried away by the thermal currents or prematurely vaporized by the heat. Straight streams created by fog nozzles are the result of changing the direction of water travel inside the nozzle by striking the stream against the deflector. This consists of countless droplets that are now even smaller when colliding with the ceiling and upper walls. These small droplets are drawn into and propelled out of the thermal column of the fire, never to reach the burning fuel.
Most fire departments throughout the country are aware of the harmful effects of fog application and are teaching their recruits to use straight stream water application for interior structural firefighting. It is unfortunate that we still see a combination fog nozzle at the end of hoselines. Some will say “we only use a straight stream pattern.” But then what is your reason for having a fog nozzle on the end of your attack line for interior structural firefighting? How about using the smooth bore? This lowers the nozzle pressure, which in turn reduces nozzle reaction, provides greater gpm, reach, and penetration. It is less work on your firefighters and your water is delivered in the most proper form to achieve extinguishment of the fire.
We must be ready for battle with effective hoseline selection, nozzle selection, and flow rates to overwhelm the driving force of fuel. It is our duty to be proactive when it comes to the constant changes our profession brings. Firefighters owe this to ourselves and to the citizens that we serve.
These recommendations are related to interior structural firefighting and are techniques that are proven to be effective and provide the greatest chance for survival and extinguishment.
Dr. Vytenis Babrgoskas. “Heat Release Rate: A Brief Primer,” http://www.doctorfire.com/hrr_prmr.html
Andrew A. Fredericks. “Return of the Solid Stream,” http://www.fireengineering.com/articles/print/volume-148/issue-9/features/return-of-the-solid-stream.html
“Methods of Structure Fire Attack,” http://www.fireengineering.com/articles/print/volume-148/issue-9/features/methods-of-fire-attack.html
“Handline Selection and Stretching the Initial Attack Line”
“Why Fires Are More Dangerous Today”
David Fornell. Firestream Management Handbook
Jason Sowders is a 13-year veteran and a company commander/EMT with the Bowling Green (KY) Fire Department. He also serves as the training officer and has served as Firefighter I and II, a fire service instructor, state fire instructor, incident safety officer, and hazmat technician.