Fire Physics: The New Offensive Fire Attack?


CarolinaFireJournal - Trey Smith
Trey Smith
07/21/2017 -

By now, you may have heard, there is a paradigm shift underway within the U.S. fire service — an old fire tactic from yesteryear that was resurrected in 2011 and is still being argued today. I am speaking, of course, about the “Transitional Fire Attack.” This technique was used for many years until the mid-1980s when several publications and textbooks citied exterior fire stream applications resulted in “pushing fire” throughout a structure.

image

Inerting a heavy fire in the attic space from an exterior stream in the garage door windows. Notice the steam issuing from outlets.

The transitional fire attack has undergone tremendous scrutiny and chastising between “seasoned traditionalists” and “fire nerds” across the nation. What is all of the mud slinging about anyway? Why has this exterior hit on the fire caused such a controversy among the practitioners in the fire service? Why have we found ourselves pitting brother against brother over a topic intended to provide a safer fire environment? The answer — it has the appearance of threatening the very foundation of the fire service — the interior fire attack.

The Transition Attack

Historically, tactics manuals have referred to the transitional attack when the declaration of changing — or preparing to change — from one operational mode to another. (i.e. offensive to defensive mode or vice versa). The term “transitional” implies when a particular tactic has commenced and changing towards another for a favorable outcome with the potential for the incident quickly deteriorating if not immediately implemented. The “stream delivery location and type” discussion has generated heated discussions among suburban and urban departments citing available manpower as a justification for this tactic. The transitional attack was introduced to the U.S, Fire Service over the past few years by many creditable international instructors to prove there is an alternative fire attack method in hopes of reducing firefighter injuries or deaths from flashovers. Our mission is simple — introduce fire dynamics education and why flashovers occur, explain how air flows contribute to fire growth and implement tactics towards mitigating the potential for radical fire events.

The philosophy of the transitional attack is to deliver water remotely from an exterior position — front door, window, yard, etc. — of a structure to “reset” the fire back in time while an interior attack line is also deployed to initiate the interior assault — or use the same line. International research has validated fires successfully hitting the source of the fire via the exterior “temporarily” interrupts the combustion process by the introduction of water that converts to steam — which is an inert or non-flammable molecule — into the fire environment. This results in the displacement of flammable gases such as smoke with non-flammable gases such as steam. By cooling and creating an inert environment, the fire decays and slows — not stops — the combustion process which results in an immediate reduction in heat energy by several hundred degrees Fahrenheit.

“Inerting” is commonly utilized in hazardous materials or confined space rescue events when an ignitable flammable mixture is intentionally displaced with non-flammable gases in an effort to prevent or remove an ignitable mixture. The concept of temporarily transitioning the fire from a flammable to non-flammable environment from the exterior is a similar tactic. It significantly reduces the potential for fuel sources over our heads and pyrolyzing around us from igniting reducing the risk of a flashover. With today’s fuels comprised primarily of hydrocarbons, the likelihood of seeing a flashover develop — as we observe in training fires in such a fuel-rich (smoke) or vent-limited environment — is highly unlikely, so every attempt should be made to cool the fuel and replace it with inert molecules.

Many other fire departments have instituted a transitional attack method so they can get water onto the fire source quickly then move to an interior fire attack with good results and increased survivability of their members and potential victims. Despite its success for many years, unfortunately, many departments prohibit it because it is believed this method of attack results “pushing fire” into tenable areas of a structure and onto victims.

As several thousand international experiments, directing toward the ceiling with a straight or solid stream to cool resulted in no fire extending beyond the fire room. It was only when the nozzle pattern was opened to a wider fog pattern that an interruption in the flow path occurred. Research burns shown indicate no temperature spikes in remote rooms, especially the rooms adjacent to the fire room. Fire growth is actually reduced by water application and that external water application had no negative impact on occupant survivability. The moral of the story: “Things get better when a solid or narrow straight stream was applied from the outside.” The problem of appearing to push fire occurs when a wider fog pattern is directed into a flow path opening and that is serving as an outlet.

Understanding the Flow Path:

The transitional fire attack is not overly complicated, but it does require a full understanding of flow paths in order to know when and why the transitional attack is executed.

Flow paths are created when there are changes in the internal air pressures within a compartment. As combustion occurs, there are multi-dimensional changes in air pressures that occur immediately surrounding the fire origin and remotely — horizontally and vertically — from the source when openings are created due to window failures or door openings. Let us accept that air and fire gases are considered mass, so when mass leaves a compartment it is replaced by another mass as the compartment attempts to seek a neutral balance of air pressures.

As hot buoyant gases rise, they are seeking the path of least resistance. Like a hot air balloon, these hot gases attempt to go upward and their velocity slows as they cool and transform into what we recognize as smoke. This smoke is unburned fuel — still heated but not superheated enough to burn and not in an ideal mixture with air to support combustion. As this heat energy and gases travel along the ceiling they will begin banking toward the floor as the fire compartment fills with gases (smoke). As these positively pressurized gases (overpressure) flow along the ceiling a demand for air at the base of the fire exists resulting in a drop the air pressure (under pressure).

The two competing air pressures reveal themselves in the neutral zone where the two pressures meet (See photo #2).

Photo #2

This is distinguishable by the layer of smoke or fire issuing from the upper portion — outlet — and air entering in the lower portion — inlet — of an opening such as a window or doorway. This type of flow path is called a Bi-directional Flow Path (See photo #3). This flow path is less efficient compared to other flow paths due to the restricted inlet and competing flow paths.

Photo #3 Bio-Directional Flow Path

The Uni-directional flow path is designated as a single or dedicated flow path for both the inlet and outlet. It is readily distinguishable by the entire opening serving as either an inlet or outlet.

When a bi-directional flow path is observed and a wider fog pattern is directed into the upper portion of the ceiling (overpressure) the gases traveling along the outlet suddenly find their exit blocked due to a higher air pressure being drawn behind the fog nozzle into the opening. Essentially this is the equivalent of shoving a cork into the outlet and the exiting gases have no outlet to escape from and begin backing up into the structure seeking an alternate flow path. This is what gives the appearance of pushing fire because the gases bank down to the ground since they are unable to overcome the higher air pressure being brought from the outside. The smaller nozzle pattern enables for the outlet to remain but becomes less efficient.

Please do not misunderstand me. I am not indicating interior attack is not an appropriate method of attack nor is an exterior attack used in every situation. After all, we can hit it hard from the front door too! The fact remains, to fully extinguish an interior fire you must go interior. But shouldn’t we make every attempt to ensure the fire corridor along the flow path we are operating does not flash? The benefits of improving conditions inside the structure for both occupants and firefighters should be our sole mission as nozzle crews.

It is essential upon us as fire service leaders to strive to learn more about current trends in the fire service. For those salty traditional folks who have been around for a while, this is a “not so new change” from the way we have done business in the past. It’s just incorporating an extra step to reduce the potential for flashover or a radical fire event. The transition fire attack debate — although not perfectly understood of its intentions — provides some perspective into the future of a “temporary offensive fire attack” combination of exterior and interior fire attack in the aspirations of making the interior safer. NIST/UL, and many other international fire research organizations are very credible organizations and this method of attack is backed up by science and physics but we owe it to ourselves to gain an understanding about when and how the transition attack should be commenced.

Before you put this technique into practice with your organization, I highly recommend that you follow the process as you would with any new technique or tool to get it approved. There is a significant amount of research literature and training opportunities available to help in your efforts towards making your interior fire attack safer. The transitional fire attack was introduced to address the threat of flashovers on America’s Bravest. The misconception that exterior fire streams are a replacement for an internal fire attack is grossly inaccurate. Fire is a very complex thing and there is no one fire attack method for every fire.

Every firefighter and officer must understand that until the first attack line is deployed and operating onto the origin of the fire, conditions will continue to deteriorate exponentially. Rapid-fire growth and heat release rates equates to earlier flashover potentials, compromising structural integrity found in today’s construction.

As fire research continues, the presence of modern fuels and larger open-spaced homes with lighter weight construction will be a game changer towards our fire attack strategies. The flow path — or lack of air — influencing the fire growth proves controlling the air inlet, as well as, applying “fast water” water can make a significant impact on reducing heat energy production and reducing the threat from extreme fire events.

I observed several tests conducted by both U.L. and NIST in Spartanburg, South Carolina in 2014 that showed significant problems in attacking a fire with a wider fog pattern directed towards the top of a window. These findings resulted in major changes to the transitional fire model. More emphasis on using straight stream applications directed towards the top of the window has become the law of the land. Another technique is to direct a fog pattern into the under pressure or lower portion of a window with the intention of allowing the droplets of water to follow the flow path and keep the outlet intact.

The facts concerning the transitional attack are:

  1. It will NOT fully extinguish a fire. Unless the fire stream reaches the source of the heat energy, significant cooling or reduction of heat energy may not take place as intended.
  2. The resetting of the fire is temporary. As inert molecules are introduced and allowed to ventilate from openings in the structure the influx of air will permit fire re-growth.
  3. It is NOT used in every fire situation nor solely from the front yard, window or doorway.
  4. Is it a great tactic when staffing issues do not permit for safe entry into the fire corridor? Or when structural or fire conditions do not permit for the safe entry of interior crews.
  5. Directing a fog pattern into the outlet will result in fire gases being blocked from venting and will “back up” and seek an alternate lower pressure route from the structure giving the appearance of pushing fire.

Structural fires are multi-dimensional, directing the straight stream solely into the ceiling may not always be effective. As my good friend Lars Axelsson “The Swedish Fire Nerd” says, “A Moving Nozzle is a Happy Nozzle.” The nozzleman must understand to reset the fire effectively, will require the manipulation of the fire stream from a variety of positions onto the source of the fire to achieve surface cooling and inerting to take place. Otherwise, the intent of reducing heat energy may not be achieved. Ask yourself if you see the reduction in fire and steam conversion? If not, consider moving to a different position. But keep the overpressure intact! Using a fog nozzle for the transition attack has the potential for a fog pattern to be incorrectly applied. My advice is to PRACTICE, PRACTICE, PRACTICE under live fire training conditions. See what works and doesn’t work. Don’t be so quick to scrutinize unless you have trained on it and have a working knowledge. Simply watching a video doesn’t make you an expert!

Sources:

Smoke Burns; Author John Taylor

International Association of Fire Service Instructors; Principles of Modern Fire Attack

Kill the Flashover Project; Joe Starnes

Underwriters Laboratory; Fire Research

National Institute of Standards & Technology

 

 

 

Trey Smith is the Founder of Street Smart Tactics and Flow Path Management Training Experience. He is a 35 year veteran of the fire service. Captain Smith also serves as an active member of the Kill the Flashover Research Project and International Society of Fire Service Instructors assisting in the delivery of the Principles of Modern Fire Attack and several other custom live fire training courses. He is also the owner of Rescue North Carolina LLC. (http://rescuenc.com) where he specializes in delivering swiftwater and other technical rescue programs through the N.C. Office of the State Fire Marshall, Rescue 3 International, as well as his customized industrial safety and rescue courses. He can be reached at [email protected] or 704-507-6152.

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Issue 32.2 | Fall 2017

Keeping First Responders Safe
Ideas to improve safety on the job, leadership, serving our community and keeping the desire to serve others...