Posts Tagged ‘backdraft’

Fire Behavior Indicators – A Quick Review

The B-SAHF (Building, Smoke, Air Track, Heat, & Flame) organizing scheme for fire behavior indicators provides a sound method for assessment of current and potential fire behavior in compartment fires. The following provides a quick review of each of these indicator types.

Figure 1. B-SAHF

Building: Many aspects of the building (and its contents) are of interest to firefighters. Building construction influences both fire development and potential for collapse. The occupancy and related contents are likely to have a major impact on fire dynamics as well.

Smoke: What does the smoke look like and where is it coming from? This indicator can be extremely useful in determining the location and extent of the fire. Smoke indicators may be visible on the exterior as well as inside the building. Don’t forget that size-up and dynamic risk assessment must continue after you have made entry!

Air Track: Related to smoke, air track is the movement of both smoke (generally out from the fire area) and air (generally in towards the fire area). Observation of air track starts from the exterior but becomes more critical when making entry. What does the air track look like at the door? Air track continues to be significant when you are working on the interior.

Heat: This includes a number of indirect indicators. Heat cannot be observed directly, but you can feel changes in temperature and may observe the effects of heat on the building and its contents. Remember that you are insulated from the fire environment, pay attention to temperature changes, but recognize the time lag between increased temperature and when you notice the difference. Visual clues such as crazing of glass and visible pyrolysis from fuel that has not yet ignited are also useful heat related indicators.

Flame: While one of the most obvious indicators, flame is listed last to reinforce that the other fire behavior indicators can often tell you more about conditions than being drawn to the flames like a moth. However, that said, location and appearance of visible flames can provide useful information which needs to be integrated with the other fire behavior indicators to get a good picture of conditions.

It is important not to focus in on a single indicator, but to look at all of the indicators together. Some will be more important than others under given circumstances.

Getting Started

Considering the wide range of different building types and occupancies, developing a concept map of the factors and interrelationships that influence fire behavior is no simple task. As you begin this process, keep in mind that it is important to move from general concepts to more specific details. For example, you might select construction type, contents, size, ventilation profile, and fire protection systems as the fundamental factors as illustrated in Figure 2. (However, you also might choose to approach this differently!).

Figure 2. Basic Building Factors

Remember that this is simply a draft (as will each successive version of your map)! Don’t get hung up on getting it “right”. The key is to get started and give some thought to what might be important. After adding some detail, you may come back and reorganize the map, identifying another basic element. For example, early versions of this map listed Fire Suppression Systems (e.g., automatic sprinklers) as one of the core concepts. However, after adding some detail, this concept was broadened to Fire Protection Systems (e.g., automatic sprinklers, fire detection, and other types of inbuilt fire protection).

Developing the Detail

Expanding the map requires identification of additional detail for each of the fundamental concepts. If an idea appears to be obviously related to one of the concepts already on the map, go ahead and add it. If you are unsure of where it might go, but it seems important, list it off to the side in a staging area for possible additions. For example, area and height are important concepts related to size. However, compartmentation may be related to size or it may be a construction factor. If you are unsure of where this should appear on the map, place it in the Staging Area for now.

Figure 3. Expanding the Map

Next Steps

Remember that the process of contracting your own map is likely as important as the (never quite) finished product. The following steps may help you expand and refine the building factors segment of the map:

• Look at each of the subcategories individually and brainstorm additional detail. This works best if you collaborate with others.
• Take your partially completed map and notes and visit several different types of buildings. Visualize how a fire might develop and what building features would influence this process.
• Examine the incident profiled in the Remember the Past segment of this post and give some thought to how building factors may have influenced fire behavior and the outcome of this incident.

In addition, I am still posing questions related to B-SAHF using Twitter. Have a look [http://twitter.com/edhartin] and join in by responding to the questions. While this is not a familiar tool to most firefighters, I think that it has great potential.

Thanks

I would also like to thank Senior Instructor Jason Collits of the New South Wales (Australia) Fire Brigades and Lieutenant Matt Leech of Tualatin Valley Fire and Rescue (also an Instructor Trainer with CFBT-US, LLC) for their collaborative efforts on extending and refining our collective understanding of the B-SAHF indicators. Jason and Matt have been using Bubbl.us to develop and share their respective maps and I will be integrating their work into future posts on Fire Behavior Indicators.

Figure 4 Jason Collits and Matt Leech

Remember the Past

Yesterday was the eighth anniversary of a tragic fire in New York City that claimed the lives of three members of FDNY as a result of a backdraft in the basement of a hardware store.

June 17, 2001
Firefighter First Grade John J. Downing, Ladder 163
Firefighter First Grade Brian D. Fahey, Rescue 4
Firefighter First Grade Harry S. Ford, Rescue 3
Fire Department City of New York

Fire companies were dispatched to a report of a fire in a hardware store. The first- arriving engine company, which had been flagged down by civilians in the area prior to the dispatch, reported a working fire with smoke venting from a second-story window.

A bystander brought the company officer from the first-arriving engine company to the rear of the building where smoke was observed venting from around a steel basement door. The first-arriving command officer was also shown the door and ordered an engine company to stretch a line to the rear of the building. A ladder company was ordered to the rear to assist in opening the door; Firefighter Downing was a member of this company. The first-due rescue company, including Firefighters Fahey and Ford, searched the first floor of the hardware store and assisted with forcible entry on the exterior.

The incident commander directed firefighters at the rear of the building to open the rear door and attack the basement fire. Firefighters on the first floor were directed to keep the interior basement stairwell door closed and prevent the fire from extending. The rear basement door was reinforced, and a hydraulic rescue tool was employed to open it. Once the first door was opened, a steel gate was found inside, further delaying fire attack.

Firefighters Downing and Ford were attempting to open basement windows on the side of the building, and Firefighter Fahey was inside of the structure on the first floor.

An explosion occurred and caused major structural damage to the hardware store. Three fire-fighters were trapped under debris from a wall that collapsed on the side of the hardware store; several firefighters were trapped on the second floor; firefighters who were on the roof prior to the explosion were blown upwards with several firefighters riding debris to the street below; and fire-fighters on the street were knocked over by the force of the explosion.

The explosion trapped and killed Firefighters Downing and Ford under the collapsed wall; their deaths were immediate. Firefighter Fahey was blown into the basement of the structure. He called for help on his radio, but firefighters were unable to reach him in time.

The cause of death for Firefighters Downing and Ford was internal trauma, and the cause of death for Firefighter Fahey was listed as asphyxiation. Firefighter Fahey’s carboxyhemoglobin level was found to be 63%.

In addition to the three fatalities, 99 firefighters were injured at this incident. The fire was caused when children – two boys, ages 13 and 15 – knocked over a gasoline can at the rear of the hard-ware store. The gasoline flowed under the rear doorway and was eventually ignited by the pilot flame on a hot water heater.

For additional information on this incident, see the following:

NIOSH Death in the Line of Duty Report F2001-23,

Simulation of the Dynamics of a Fire in the Basement of a Hardware Store

Incident Photos by Steve Spak

Ed Hartin, MS, EFO, MIFireE, CFO

References

Grimwood, P., Hartin, E., McDonough, J., & Raffel, S. (2005). 3D firefighting: Training, techniques, & tactics. Stillwater, OK: Fire Protection Publications.

Hartin, E. (2007) Fire behavior indicators: Building expertise. Retrieved June 17, 2009 from www.firehouse.com.

Hartin, E. (2007) Reading the fire: Building factors. Retrieved June 17, 2009 from www.firehouse.com.

National Institute for Occupational Safety and Health (NIOSH). (2003) Death in the line of duty report F2001-23. Retrieved June 18, 2009 from http://www.cdc.gov/niosh/fire/pdfs/face200123.pdf

Bryner, N. & Kerber, S (2004) Simulation of the dynamics of a fire in the basement of a hardware store – New York, June 17, 2001 NISTR 7137. Retrieved June 18, 2009 from http://www.fire.nist.gov/bfrlpubs/fire06/PDF/f06006.pdf

United States Fire Administration (USFA) Firefighter fatalities in 2001. Retrieved June 18, 2009 from http://www.usfa.dhs.gov/downloads/pdf/publications/fa-237.pdf

As discussed in my last post, lack of an adequate exhaust opening is a common factor when use of positive pressure ventilation causes or increases the severity of extreme fire behavior. Unfortunately there has not been a great deal of research examining why this is the case. Part of the challenge in conducting a scientific investigation of this issue is the tremendous variability in building configuration and fire conditions. Control of these variables becomes more difficult as building configuration becomes more complex and multiple fire scenarios are considered. However, this does not preclude improvement of our understanding of this important issue.

Burning Regime

How an increase in ventilation influences fire behavior is largely (but not entirely) dependent on burning regime. If the fire is fuel controlled, fire development is dependent on the characteristics, configuration and amount of fuel. When a compartment fire becomes ventilation controlled, fire development is limited by the available oxygen. In the ventilation controlled burning regime, increased ventilation results in increased heat release rate. See my earlier post Fuel and Ventilation for additional information on burning regime.

In most ventilation controlled fires, the concentration of gas phase fuel (i.e., unburned pyrolyzate and flammable products of incomplete combustion) is not sufficient to present threat of backdraft. In these cases, increased ventilation will generally result in one of the following outcomes:

• Increase in heat release rate that is not sufficient to result in a rapid transition to a fully developed fire (flashover)
• Rapid increase in heat release rate that results in flashover and a fully developed fire.
• Intervention by firefighters to control the fire before ventilation induced flashover can occur.

If the concentration of gas phase fuel is sufficient to present threat of backdraft, increased ventilation may result in a backdraft…or not (depending on the extent of mixing of air and smoke, presence of an adequate ignition source, etc.).

The greater the extent to which the fire is ventilation controlled and the higher the concentration of gas phase fuel, the greater the potential for extreme fire behavior following increases in ventilation. Positive pressure ventilation influences this process in several ways, if effective, gas phase fuel is removed from the structure (often burning outside the exhaust opening). If PPV is not effective, increased air flow is accompanied with turbulence and resultant mixing of fuel an air which increases the probability of ignition and rapid fire progression. In addition, pressure applied at the outlet increases confinement which may increase the violence of extreme fire behavior phenomena such as backdraft.

Fluid Dynamics

Movement of fluids (liquids and gases) should be of significant interest to firefighters. Both fireground hydraulics and tactical ventilation require an understanding of fluid dynamics. In examining the influence of inadequate exhaust opening size on the effectiveness of PPV and potential for extreme fire behavior, I found some parallels with fireground hydraulics.

Laminar Flow: Smooth movement of a fluid in parallel layers with little disruption between the layers. The following video clip illustrates laminar flow in a pipe.

Turbulent Flow: Fluid flow characterized by eddies and vortexes disrupting smooth movement. The following video clip illustrates turbulent flow in a pipe.

A number of characteristics influence flow characteristics when a fluid moves through a conduit such as a pipe, hoseline, or even a building. These include fluid characteristics such as viscosity and density, the roughness of the conduit, restrictions to flow, and velocity of the fluid.

For example, friction loss in 1-1/2″ (38 mm) hose is higher than that in 1-3/4″ (45 mm) hose at the same flow rate. Why? Velocity must be higher to move the same flow rate through the smaller hose. This results in increased turbulence and resulting loss in pressure. If a discharge gate is partially closed, this obstructs the waterway, creating turbulence and increasing friction loss. As illustrated in this example, increased velocity and the presence of obstructions both increase turbulence. How does this apply to PPV?

The extent of turbulence as air and fire effluent (smoke and fire gases) move through a building is influenced by the configuration of the building (e.g., walls, doorways), obstructions (e.g., furniture), and velocity. Turbulence increases mixing of fire effluent and air. If the concentration of unburned pyrolizate and flammable products of incomplete combustion is high, turbulence increases the potential of a flammable mixture. In addition, increased oxygen concentration and air movement across surfaces can result in transition from surface to flaming combustion, providing a source of ignition for the flammable mixture of fire effluent and air.

Outlet/Inlet Ratio

When using natural ventilation, the size of the inlet opening(s) should be larger than the exhaust opening(s). However, with positive pressure ventilation this is reversed. When using PPV. exhaust opening(s) should be at least as large and preferably two to three times as large as the inlet opening as illustrated in Figure 1.

Figure 1. PPV Efficiency Curve

Note: Adapted from Fire Ventilation (Svensson, 2000, p. 71)

For a detailed examination of the physics and mathematical explanation of how the positive pressure ventilation efficiency curve is derived, see Stefan Svensson’s excellent text Fire Ventilation.

If the outlet size is adequate, a unidirectional ventilation flow from inlet to outlet is created. If opening size is inadequate, turbulence is increased as fire effluent and air seeks an exit path. If no opening is made or if the opening is extremely small, fire effluent may push back out the inlet opening.

Watch the following video clip and focus your attention on the exhaust opening on Side B (at approximately 0:19) and fire behavior indicators immediately after the blower is placed at the door on Side A and started (at approximately 3:00)

Find more videos like this on firevideo.net

Even though there was an exhaust opening, it was of inadequate size. While this fire was likely progressing towards a ventilation induced flashover due to the effects of natural horizontal ventilation, increased airflow and turbulence caused by ineffective PPV  likely was a contributing factor in the way that this extreme fire behavior phenomena occurred.

Important: Implementation of PPV after entry and before the fire has been located and controlled presents a significant risk to firefighters. Risk can be minimized by either using positive pressure attack (implementing PPV prior to entry) or locating and controlling the fire before implementing PPV.

Next Steps

In the Education vs. Training in Fire Space Control, Kris Garcia (2008) wrote that we need to increase our focus on ventilation education, rather than simply training on ventilation skills. Effective use of PPV to support fire attack or following fire control requires an understanding of fire and fluid dynamics as well as skill in creating openings and the placement and operation of blowers.

My next post will examine review Positive Thinking, an article by Watch Manager Gary West of the Lancashire Fire Rescue Service (UK) published in the August 2008 issue of Fire Risk Management. In this article, Gary provides an excellent overview of the approach to PPV training and implementation taken by the UK fire service.

References

Svensson, S. (2005). Fire ventilation. Karlstad, Sweden: Swedish Rescue Services Agency.

Garcia, K. (2008, September). Education vs. training in fire space sontrol. Fire Engineering. Retrieved May 21, 2009 from http://positivepressureattack.com/images/pdfs/EdVsTng-GarciaFESept08.pdf

West, G. (2008, August). Positive thinking. Fire Risk Management, 46-49.

Effective use of positive pressure ventilation aids in fire control and provides increased tenability throughout the fire building. However, inappropriate or ineffective use of this tactic has resulted in numerous near misses, injuries, and more than a few line of duty deaths. In many of these cases, positive pressure was applied with an inadequate exhaust opening.

Find more videos like this on firevideo.net

Did you ever wonder why the size and location of the exhaust opening is critical to safe and effective use of positive pressure ventilation? If not, maybe you should!

A Quick Review

As discussed in an earlier post (see Language and Understanding: Extreme Fire Behavior), common language and definitions are critical to developing a shared understanding. To that end, I want to start this examination of positive pressure ventilation (PPV) with a brief review of terminology used in this post.

Ventilation: The exchange of the atmosphere inside a compartment with the atmosphere outside the compartment. Ventilation is ongoing in all habitable spaces. Under fire conditions, this involves exit of smoke and intake of fresh air (if smoke is visible, ventilation is occurring).

Tactical Ventilation: Planned, systematic, and coordinated removal of heat, smoke, and fire gases (fire effluent) and their replacement with fresh air. There are three important parts of this definition, 1) tactical ventilation is part of the overall tactical plan and is coordinated with other fireground operations (particularly fire control), 2) hot fire effluent is removed, and 3) fresh (cooler) air is introduced into the compartment.

Note: I gave a bit of thought to use of the terms smoke and fire effluent in this discussion of ventilation. The International Standards Organization (ISO) definition of smoke focuses on the visible products of combustion while fire effluent includes all gaseous, aerosol, and particulates generated by combustion. The National Fire Protection Association (NFPA) definition of smoke is comparable to the ISO definition of fire effluent. Given that the traditional definition of (tactical) ventilation refers to “heat, smoke, and fire gases” (IFSTA, 2008, p. 541), I will use the term fire effluent as the broader, more encompassing term (inclusive of smoke and fire gases).

Natural Ventilation: Use of pressure and density differences generated by the higher temperature of gases inside the compartment than outside and ambient wind conditions to accomplish the exchange of hot fire effluent and air.

Assisted Ventilation: These tactics use mechanical or hydraulically generated pressure to influence and increase the exchange of fire effluent and air. Assisted ventilation includes the use of fog streams and fans to reduce pressure at the exhaust opening (negative pressure ventilation) and use of fans or blowers to increase pressure at the inlet opening (positive pressure ventilation).

Positive Pressure Ventilation (PPV): Use of a blower at the inlet opening to increase the pressure differential between the inlet and exhaust opening to control and increase the exchange of fire effluent and air.

Positive Pressure Attack (PPA): This term was coined by Garcia, Kauffmann, & Schelble (2006) to differentiate positive pressure ventilation initiated prior to fire attack from use of this tactic following fire control operations. From a physics perspective, PPV and PPA are the same, the term PPA simply designates the sequence in which the tactic is performed.

Exhaust Opening: The opening(s) used for removal of fire effluent. Note that this opening may be created by unplanned ventilation due to fire effects, civilians, or freelancing responders or it may be created as the result of tactical action. Remember that any location where flames and/or smoke is visible is an exhaust opening.

Inlet Opening: The opening(s) used to introduce fresh air into the compartment. As with exhaust openings, inlet openings may be unplanned or planned. Openings may serve simply as an inlet or may serve as both an inlet and outlet with fire effluent exiting at the top and air entering at the bottom (bi-directional air track).

Smoke Movement in Buildings

Fluids (like fire effluent) flow from areas of higher pressure to areas of lower pressure. In a compartment fire, energy released by combustion raises the temperature of the fire effluent and entrained air. As temperature increases, gases expand and become less dense (more buoyant). However, when gases are confined, increased temperature results in increased pressure. These differences in density and pressure result in movement of smoke out of the compartment and inward movement of air from outside the compartment. This exchange may be through normal building leakage, unplanned ventilation, or tactical ventilation.

The pressure generated by a fire inside a compartment is dependent on the heat release rate, ventilation (openings), and resulting temperature inside the compartment. However, NFPA 92A Standard for Smoke-Control Systems Utilizing Barriers and Pressure Differences (NFPA, 2006) specifies pressure differences in non-sprinklered buildings of between 12.5 Pascal (Pa) and 44.8 Pa to overcome the pressure resulting from hot gases at a temperature of 927^o C (1700^o F) next to the smoke barrier (these pressures include a 7.4 Pa safety factor). If the safety factor is removed, the pressure generated by a fire in a non-sprinklered occupancy would likely be between 5 Pa and 37.3 Pa. All very interesting, but what is a Pascal?

While firefighters in the United States are generally familiar with pounds per square inch (psi) as a unit of measure for pressure, the standard international unit for pressure is the Pascal (P). A Pascal is an extremely small unit (1 psi = 6895 Pa) roughly equivalent to the pressure exerted by a sheet of writing paper laying on a flat surface. As you can see, the pressure generated by the fire is quite small, but more than adequate to result in significant movement of fire effluent!

Two key points that influence movement of fire effluent and ventilation under fire conditions:

• If the temperature of fire effluent is higher than that of the ambient air it will tend to rise.
• Fire effluent flows from areas of higher pressure to areas of lower pressure.

PPV Basic Concepts

Many firefighters think that they understand positive pressure ventilation and how it should (and should not) be used on the fireground. Some do. However, there are a number of common misconceptions and a great deal of misunderstanding when it comes to effective application of this tactic.

A good starting point is to examine the fundamental purpose of the use of positive pressure in tactical ventilation and anti-ventilation. “The purpose of the positive pressure ventilation fan is to create pressures higher than that of the fire to manage where the smoke and hot gases flow” (Kerber & Madrzykowski, 2008). When used in tactical ventilation, positive pressure can be used to control air track and speed the removal of fire effluent from the compartment. In anti-ventilation (e.g., pressurization of a stairwell or attached exposure), positive pressure is used to confine the fire effluent.

The basic sequence of positive pressure tactical ventilation is as follows

1. Size-up and dynamic risk assessment (ongoing)
2. Determination that positive pressure is indicated (and not contraindicated)
3. Identification of appropriate and adequate exhaust openings
4. If necessary creating or enlarging exhaust openings
5. Application of positive pressure at the inlet
6. Verification that positive pressure ventilation is working

Positive pressure ventilation is an extremely powerful tool that can rapidly clear smoke logged areas of the building. However, if used without thinking and understanding the influence of ventilation on fire behavior, it can cause extreme fire behavior even more quickly. The following criteria should be met for safe and effective use of positive pressure ventilation:

• Firefighters understand the use of PPV and are skilled in its use
• The required tools are available
• Location and extent of the fire is known Svensson, 2000). This is not an absolute requirement, but influences the most appropriate location for the exhaust opening)
• A charged hoseline is in place for fire control (Svensson, 2000)
• Backdraft conditions are not present (Svensson, 2000; Garcia, Kauffmann, & Schelble, 2006).
• Victims or firefighters are not between the fire and the exhaust opening (Svensson, 2000)
• Victims or firefighters are not in the exhaust opening (Garcia, Kauffmann, & Schelble, 2006)
• Ventilation openings can be controlled and an adequate exhaust (preferably 2 to 3 times the size of the inlet) opening is provided (Svensson, 2000).
• Positive control of the blower (the ability to start and stop positive pressure immediately)
• Ventilation is coordinated with fire attack (Svensson, 2000; Garcia, Kauffmann, & Schelble, 2006). This requires communication with personnel at the outlet, inlet, interior working positions, and Command.

Common Problems

Kriss Garcia, co-author of Positive Pressure attack for ventilation & firefighting indicates that most situations where use of positive pressure ventilation resulted in occurrence of extreme fire behavior or some other adverse outcome generally involve one or more of the following (personal communication, May 2006):

• Lack of an exhaust opening
• Inadequate exhaust opening size
• Lack of command, control, & coordination

More to Follow

My next post will get to into the nuts and bolts of exhaust opening size and why use of positive pressure with an inadequate exhaust opening can result in extreme fire behavior.

References

Garcia, K., Kauffmann, R. & Schelble, R. (2006). Positive pressure attack for ventilation & firefighting. Tulsa, OK: Penwell.

International Fire Service Training Association (IFSTA). (2008). Essentials of firefighting (5^th ed.). Stillwater, OK: Fire Protection Publications.

Kerber, S. & Madrzykowski, D. (2008).Evaluating positive pressure ventilation In large structures: school pressure and fire experiments. Retrieved May 17, 2009 from http://www.fire.nist.gov/bfrlpubs/fire08/PDF/f08016.pdf.

National Fire Protection Association (NFPA). (2006). NFPA 92A. Standard for smoke-control systems utilizing barriers and pressure differences. Quincy, MA: Author.

My last post (Gas Explosions) examined flammability and ignition of fuel/air mixtures as related to gas explosions. Deflagration of a fuel/air mixture can result in a significant energy release, when confined, this results in a significant pressure increase.

Pressure

If a confined gas is heated, pressure will increase as indicated in Gay-Lussac’s Law.

Gay-Lussac’s Law: When the volume of a gas remains the same and temperature is increases, pressure increases in proportion to the absolute temperature of the gas.

Pressure generated in a gas explosion is dependent on the speed with which flames move through the fuel and the degree to which expanding hot gases are confined.

The speed with which flames propagate through unburned pyrolysis and flammable combustion products is subsonic (slower than the speed of sound), making this a deflagration. Flame propagation in backdraft may be several meters per second (Guigay, G., Eliasson, J., Gojkovic, D., Bengtsson,L., & Karlsson, B., 2008). The pressure generated by this type of explosion inside a compartment or building can easily break windows (changing the ventilation profile) and in many cases can be sufficient to result in structural damage.

When pre-mixed fuel and air is ignited, it pushes unburned bas ahead of the flame, producing turbulence. Flame propagation into this turbulent, pre-mixed fuel will result in an increased rate of combustion, increasing velocity and turbulence even further. This feedback loop results in acceleration of flaming combustion and high pressure from expansion of hot gases. When this reaction is confined (e.g., ventilation is limited to a single opening such as a door or window), pressure can increase to an even greater extent.

In an explosion of unburned pyrolysis and combustion products and air, the severity of the reaction will depend on the total mass and concentration of fuel, location of the ignition point, strength of the ignition source, and extent of confinement. While it is not possible to evaluate these factors under fire conditions, understanding the variables aids in understanding the processes involved. For example, as illustrated in Figures 1 and 2, ignition that occurs inside a compartment can expel a mass of unburned fuel which may subsequently ignite (note that the opening may not be to the exterior, but may simply be to another interior compartment, stairwell, etc.).

Figure 1. Influence of Ignition Location

Expulsion of unburned gas phase fuel from a compartment in a backdraft results in a characteristic spherical mass of fuel (as illustrated in Figure 2) which subsequently ignites, resulting in a fireball.

Figure 2. Expulsion of Gas Phase Fuel in a Backdraft

How might ignition location have influenced the nature and duration of flaming combustion in the stairwell in the Watts Street incident discussed in 15 Years Ago: Backdraft at 62 Watts Street and 62 Watts Street: Modeling the Backdraft?

Thermal and Structural Effects of Explosions

Explosions and the resulting pressure increase occur extremely rapidly, this makes the force that is applied to structures a dynamic load. How a structure responds to this type of dynamic load depends on the magnitude of the load, design, and condition of the structure before the load was applied. In addition to the pressure generated by an explosion, movement of gas at high velocity also adds to the dynamic load imposed on the structure. Figure 4 illustrates the rapid changes in pressure resulting from an explosion in a compartment.

Figure 4. Explosion Time-Pressure Curve

Note: Adapted from the Gas Explosion Handbook (GexCon, 2006).

Backdraft and smoke explosion can generate considerably more pressure and flow than is necessary to cause structural damage. Even if the pressure from an explosion is limited, it will generally be sufficient to cause failure of window glazing or damage to other building openings, resulting in a significant change in ventilation profile. When the fire is ventilation controlled, this will lead to increased heat release rate and potential for rapid transition to a fully developed fire.

Ed Hartin, MS, EFO, MIFireE, CFO

References

GexCon. (2006) Gas explosion handbook. Retreived March 20, 2009 from http://www.gexcon.com/index.php?src=handbook/GEXHBchap4.htm

Guigay, G., Eliasson, J., Gojkovic, D., Bengtsson,L., & Karlsson, B. (2008) The Use of CFD Calculations to Evaluate Fire-Fighting Tactics in a Possible Backdraft Situation. Fire Technology

Extreme fire behavior can be categorized as a step event which results in a sustained increase in heat release rate or a transient event that results in a brief increase in heat release rate. Transient events involve combustion of unburned combustion and pyrolysis products. The speed of this combustion process can vary widely depending on the concentration of fuel and oxygen, extent of mixing, confinement and a number of other factors. Transient events may simply involve rapid combustion (e.g., flash fire) or they may be explosive (e.g., backdraft, smoke explosion), resulting in a significant pressure increase within the compartment or building.

My next few posts will provide brief overview of gas explosions in general to provide a foundation for understanding explosive extreme fire behavior phenomena such as backdraft and smoke explosion.

Introduction

The Gas Explosion Handbook (GexCon, 2006) defines a gas explosion as a process where combustion of premixed gas phase fuel and an oxidizer (e.g., fuel and air) causes a rapid increase in pressure. The fuel in a gas explosion may result from release of a flammable gas normally used as a fuel or in industrial processes (e.g., methane, cyclohexane) or from accumulation of unburned pyrolysis and combustion products in a compartment fire.

An explosion involving unburned pyrolysis and combustion products in a compartment fire may occur in one of two ways: 1) air is mixed with a rich fuel/air mixture and subsequently undergoes auto or piloted ignition (backdraft), or 2) a pre-mixed, flammable, fuel/air mixture undergoes piloted ignition (smoke explosion). Exploring the basic processes involved in a gas explosion will lay a foundation for understanding these two important extreme fire behavior phenomena.

Flammable Fuel/Air Mixtures in Compartment Fires

Compartment fires generally involve combustion of natural and synthetic organic (carbon containing) materials such as wood, paper, and plastics. In order for flaming combustion to occur, fuel must be transformed into the gas phase through vaporization or pyrolysis. Incomplete combustion of organic fuels results in production of carbon monoxide, soot, and a wide range of other products of combustion (many of which are flammable). Smoke is comprised of not only the products of incomplete combustion, but also unburned pyrolysis products. As illustrated in Figure 1, Smoke is Fuel!

Figure 1. Smoke is Fuel

Gas phase fuel in smoke may ignite and burn in the plume or ceiling jet or it may burn as it exits through a ventilation opening. However, unburned gas phase fuel may also accumulate inside the compartment or building, mixing with air to form a potentially flammable mixture. In ventilation controlled fires, concentration of gas phase fuel increases and may become too rich to burn without introduction of additional air. In addition, flammable products of combustion and pyrolysis products may infiltrate into uninvolved compartments or attached exposures and mix with air to form a flammable atmosphere.

Review of Flammability

Combustion requires fuel and oxygen in the proper concentration. Under normal conditions, air contains approximately 21% oxygen and 79% nitrogen and trace amounts of other gases. The nitrogen, other gases, and water vapor are passive agents as they are not chemically part of the combustion reaction (but as energy is required to raise the temperature of passive agents, they do influence combustion).

Figure 1. Methane Flammability Diagram

At first glance, the flammability diagram in Figure 2 appears to be extremely complex. However, it simply represents the relationship between fuel, oxygen, and passive agents. In this triangular diagram, the total of the concentration of fuel, oxygen, and passive agents equals 100%. In the case of Figure 2, the triangular diagram shows all possible mixtures of methane, oxygen, and nitrogen passive agents (predominantly nitrogen in the air). The blue (air) line indicates oxygen concentration from normal 21% (by volume) to 0%. The red (stoichiometric) line indicates the ideal mixture of oxygen and fuel for complete combustion. The gray shaded area indicates the mixtures of methane, oxygen, and passive agents that will be flammable.

The area of this diagram that is of greatest interest in most compartment fires is the region to the right of the Air Line (flammable limits under normal conditions and the minimum oxygen concentration that will allow combustion). This is because most compartment fires are dependent on ambient air as a source of oxygen. If the concentration of fuel increases, it must be offset by a corresponding reduction in oxygen and passive agents and may make the mixture too rich to burn. However, if fuel escapes and is replaced with air, the concentration of the mixing gases may reenter the flammable range.

The flammability diagram applies to pre-mixed fuel and air (oxygen and passive agents). In a compartment fire, the concentration and mixing of fuel and air varies considerably due to differences in temperature and resulting density of smoke and air. Consequently, there may be pockets of fuel and air that are within the flammable envelope, while other areas may be too rich or lean to burn.

Ignition of Fuel/Air Mixtures

If smoke is flammable, why doesn’t it always ignite and burn? Ignition is dependent on having sufficient fuel and oxygen as well as an adequate ignition source. Ignition of a mixture of pre-mixed air and fuel requires an ignition source with sufficient strength. The minimum amount of energy required to initiate combustion is the minimum ignition energy. Factors that affect the minimum ignition energy include:

• Type of fuel
• Mixture of fuel and air
• Temperature
• Total energy supplied
• Rate at which energy is supplied (energy per unit time)
• Area over which energy is delivered

The minimum ignition energy for a given fuel generally corresponds to the stoichiometric (ideal) mixture of fuel and air. As concentration increases or decreases within the flammable range, ignition energy increases (i.e., ignition energy at the Lower and Upper Flammable Limits will be higher than for the stoichiometric concentration).

The concentration and specific gas species of flammable combustion and pyrolysis products is complex and will influence the energy required for ignition. The concept of ignition energy and the influence of concentration of fuel in air is important in understanding why a flammable mixture of combustion and pyrolysis products may not be ignited by surface combustion, but may be ignited by the higher energy provided by flames.

More to Follow

My next post will continue with a look at other factors that influence explosive ombustion in compartment fires.

Ed Hartin, MS, EFO, MIFireE, CFO

References

GexCon. (2006) Gas explosion handbook. Retreived March 20, 2009 from http://www.gexcon.com/index.php?src=handbook/GEXHBchap4.htm

Two incidents recently point to the hazards presented by explosions which may occur during firefighting operations.

Pittsburgh, PA

On March 25, 2009, firefighters in Pittsburgh, Pennsylvania were operating at a fire in a three-story apartment building of ordinary construction when an explosion occurred on Floor 2 while WPXI was videotaping fireground operations. Watch the video and see what you think?

• Did you observe any indicators of potential backdraft prior to the explosion?
• Do you think that this was a backdraft?
• What leads you to the conclusion that this was or was not a backdraft?
• If you do not think this was a backdraft, what might have been the cause of the explosion?

A news reporter quotes a chief officer, providing the following explanation: [Backdrafts] occur when a fire causes a buildup of pressure inside a building. When a firefighter enters a pressurized area, an influx of oxygen can cause the fire to explode. Note: comments reported in the press are not always an accurate representation of what was said.

While the comments reported are not completely inaccurate, they do not accurately describe the mechanism by which a backdraft occurs.

Cleveland, OH

On April 2, 2009, in Cleveland, Ohio an explosion occurred while firefighters were operating at a fire in a 2-1/2 story, wood frame dwelling. The fire, which had originated on the exterior of the structure, extended into the building and to the upper floors through void spaces in the balloon frame walls. According to news reports, the explosion occurred shortly after firefighters conducting primary search opened an attic door. The force of the explosion blew the two firefighters down the stairs to the second floor. Both firefighters received burns to the neck and face. News reports represented the phenomena involved in this event as a smoke explosion or backdraft.

• Based on the limited information provided in the news reports, which of these phenomena (backdraft or smoke explosion) do you think was most likely?
• What leads you to the conclusion as to which of these phenomena was most likely to have occurred?

A WKYC news report quoted a chief officer as stating “When they opened up the door to the attic that flow of oxygen allowed that fire to ignite, and it actually explodes.” Watch the video of this interview. This is a simple, but incomplete explanation of how a backdraft occurs. However, it does not explain the smoke explosion phenomena.

While smoke explosion and backdraft are often confused, there are fairly straightforward differences between these two extreme fire behavior phenomena. A smoke explosion involves ignition of pre-mixed fuel (smoke) and air that is within its flammable range and does not require mixing with air (increased ventilation) for ignition and deflagration. A backdraft on the other hand, requires a higher concentration of fuel that requires mixing with air (increased ventilation) in order for it to ignite and deflagration to occur. While the explanation is simple, it may be considerably more difficult to differentiate these two phenomena on the fireground as both involve explosive combustion.

While definitions are often ambiguous and the lines between various extreme fire behavior phenomena are a bit fuzzy, it is useful to examine even the limited information provided in news reports and give some thought to what might have happened. Are reported conditions consistent with the reported phenomena and what alternative theories might explain what happened?

Ed Hartin, MS, EFO, MIFIreE, CFO

In Fire Gas Ignitions and Language & Understanding: Extreme Fire Behavior, I pointed out the ambiguity in definition of terms related to extreme fire behavior. In the structural firefighting context, the term extreme fire behavior is used to identify phenomena that result in rapid fire progression and present a significant threat to firefighters. Rapid fire progression may involve transition to a fully developed fire (e.g., flashover) or it may involve a brief, but significant increase in energy release (e.g., backdraft, flash fire, smoke explosion).

One way to begin the process of reducing the ambiguity surrounding extreme fire behavior phenomena is to establish a framework for organizing and classifying extreme fire behavior phenomena.

Organizing Concepts

The organization and classification framework presented in this post is based on the following general concepts:

• Extreme fire behavior involves a rapid increase in heat release rate (HRR).
• The increase in HRR can be sustained or it may be relatively brief.
• Brief increases in HRR may or may not result in overpressure inside a compartment or building.
• Extreme fire behavior may occur in a fuel or ventilation controlled burning regime
• Concentration (mass fraction) of fuel in the gas phase influences the nature of extreme fire behavior.
• Depending on existing or developing conditions, extreme fire behavior may be initiated by reaching critical HRR, an increase in ventilation, or a source of ignition.

It is likely that there are additional concepts or criteria that may prove useful in the process of organizing and classifying extreme fire behavior. However, these concepts provide a starting point for this process and discussion.

Classification by Outcome

At the highest level, extreme fire behavior phenomena are classified on the basis of the duration of increased HRR. If increased HRR is sustained and the fire enters a (relatively) steady state of combustion, the phenomena would be classified as a Step Event. However, if the increase in HRR is brief and not sustained, the phenomena would be classified as a Transient Event.

A rapid increase in HRR results in increased temperature of the atmosphere inside the compartment. As temperature increases, the gas (i.e., air and smoke) volume within the compartment will expand. If the gas volume inside the compartment is confined and cannot expand, pressure will increase, in some cases significantly! Transient events are classified as Explosive (resulting in a significant overpressure) or Non-Explosive (not resulting in a significant overpressure). Explosiveness is in part a result of the mixture of gas phase fuel and air present in the compartment and the extent to which combustion is confined.

Classification of extreme fire behavior phenomena on the basis of outcome are illustrated graphically in Figure 1.

Figure 1. Outcome Classification

Classification by Conditions

Additional clarity can be obtained by examining extreme fire behavior phenomena on the basis of requisite conditions for occurrence. However, it is important to keep in mind that conditions are rarely uniform in structure fires. Different compartments (e.g., habitable spaces, voids) can have dramatically different conditions in burning regime, fuel concentration, oxygen concentration, and temperature.

In a compartment with sufficient openings, flashover can occur prior to fire growth becoming significantly limited by available ventilation. However, a majority of extreme fire behavior phenomena occur when the fire is in a ventilation controlled burning regime. As compartment fire development becomes limited by ventilation, not all of the gas phase fuel resulting from pyrolysis is burned. This excess pyrolizate increases both the mass and concentration of fuel within the compartment (and other compartments as smoke spreads through the building). Concurrently, with increased fuel concentration, oxygen concentration decreases.

Provided a source of ignition with sufficient energy, gas phase fuel/air mixtures within the flammable range can be ignited. However, if the fuel/air mixture is too rich, additional air must be introduced and mixed with the fuel in order for combustion to occur.

For extreme fire behavior phenomena occurring within a ventilation controlled burning regime, the following factors can be used to further define the nature of the phenomena:

• Fuel Concentration
• Oxygen Concentration
• Extent of Confinement

The combination of fuel/air mixture and extent of confinement define what type of initiating event (contact with source of ignition, increase in ventilation, or both) will be necessary for the extreme fire behavior to occur.

Graphical Representation

It is often easier to see how things are organized using a visual model or diagram. However, it is not so simple to capture a high level of complexity in a simple drawing. Figure 2 illustrates the concepts presented in this post regarding classification of extreme fire behavior phenomena.

This is a work in progress and feedback is greatly appreciated!

Ed Hartin, MS, EFO, MIFireE, CFO

On March 24, 1994 Captain Drennan and Firefighters Young and Seidenburg of the FDNY were trapped in the stairwell of a three-story apartment building  by rapid fire progression that occurred as other companies forced entry into the fire apartment on the floor below. The FDNY requested assistance from National Institute for Standards and Technology (NIST) in modeling this incident to develop an understanding of the extreme fire behavior phenomena that occurred in this incident.

Brief Review

A short case study of the 62 Watts Street incident was presented in my last post. As a brief review, FDNY companies responded to 62 Watts Street for a report of smoke and sparks coming from the chimney (see Figure 1). On arrival, there was no indication of a serious fire in the building. Companies opened the scuttle over the stairwell and stretched a line to the first floor apartment while Captain Drennan and the other members of the Ladder 5’s inside team proceeded to the second floor to search for occupants. When the door to the first floor apartment was opened, air rushed in and then warm smoke pushed out. This pulsation in the air track at the door was followed by a flaming combustion filling the upper portion of the door and almost immediately filling the stairwell. Firefighters on the first floor were able to escape, while Captain Drennan and Firefighters Young and Seidenburg were trapped on floor 2.

Figure 1. 3D Cutaway View of 62 Watts Street

Analysis and Computer Modeling

FDNY asked NIST to assist in developing a computerized model to aid developing an understanding of the fire behavior phenomena that occurred during this incident.

Hypothesis: The fire burned for over an hour under severely ventilation controlled conditions resulting in production of a large quantity of unburned pyrolyzate and products of incomplete combustion. Opening the apartment door allowed exhaust of warm fire gases and inflow of cooler ambient air, resulting in a combustible fuel/air mixture. Bukowski (1995) does not identify a source of ignition. However, it is likely that the combustible fuel/air mixture underwent piloted ignition as flaming combustion resumed in the apartment. Once the gas phase fuel was ignited, flaming combustion extended from the door through the stairwell to the ventilation opening at the roof.

Richard Bukowski of the NIST Building and Fire Research Laboratory modeled the fire using CFAST to determine if a sufficient mass of gas phase fuel could have accumulated in the apartment to account for the severity and duration of flaming combustion that occurred. CFAST is a two-zone fire model used to predict the distribution of smoke and fire gases and temperature over time in a multi-compartment structure subjected to a fire. A two-zone model is based on calculations that describe conditions in the upper and lower layers (see Figure 2). While there are obvious differences in conditions within each of these zones, these differences are relatively small in comparison to the differences between the two zones (Jones, Peacock, Forney, & Reneke, 2005).

Figure 2. Upper and Lower Layers in Two Zone Models

Bukowski’s (1995) model of the Watts Street fire divided the involved area of the structure into three compartments. The apartment was defined as a single 6.1 m (20′) x 14 m (46′) x 2.5 m (8’3″) compartment. The stairwell was defined as a second 1.2 m (4′) x 3 m (10′) x 9.1 m (30′) compartment connected to the apartment by a closed door and having a roof vent with a cross sectional area of 0.84 m² (9 ft²). The fireplace flue was defined as a vertical duct with a cross section of 0.14 m (1.5 ft2) x 10 m (33′).

The heat release rate in the initial growth phase of a compartment fire is nearly always accelerating with energy release as the square of time (t²). Multiplying t² by a factor ?, various growth rates (e.g., ultra-fast, fast, medium, slow) can be simulated (Karlsson & Quintiere, 2000).

Based on experimental data from burning trash bags, Bukowski (1995) estimated the initial heat release rate at 25 kW with the fire transitioning to a medium t² fire (typical of residential structure contents) which would have had a peak HRR of 1 MW, but did not reach this HRR due to limited ventilation.

Figure 3. Heat Release Rate of Growth Phase t² Fires.

Note: Adapted from CFAST – Consolidated model of fire growth and smoke transport (Version 6).

Results of the computer model indicated that the HRR of the fire in the apartment grew to a heat release rate of 0.5 MW (see Figure 4) and then HRR decreased rapidly as oxygen concentration dropped below 10% (see Figure 5).

As the fire continued to burn under extremely ventilation controlled conditions, the concentration of unburned pyrolizate and flammable products of incomplete combustion in the apartment continued to increase.

Figure 4. Heat Release Rate

Note: Adapted from Modeling a Backdraft: The 62 Watts Street Incident.

Research indicates that the concentration of gas phase fuel (e.g., total hydrocarbons, carbon monoxide) is a critical determinant in the likelihood of backdraft occurrence. In small scale, methane fueled compartment fire experiments, Fleischmann, Pagni, & Williamson (1994) found that a total hydrocarbon concentration >10% was necessary for occurrence of a backdraft.  At lower concentrations, flame travel is slow and compartment overpressure is lower. As total hydrocarbon concentration increased, the overpressure resulting from backdraft increased. Similarly, Weng & Fan (2003) found mass fraction (concentration by mass) of unburned fuel to be the critical determinant in the occurrence and severity of backdraft. In their small scale, methane fueled experiments, increases in mass fraction of unburned fuel resulted in increased overpressure and more severe backdraft explosions.

Both of these research projects involved use of a methane burner in a compartment and the researchers identified the need for ongoing research using realistic, full scale compartment configurations and fuel loads.

Figure 5. Oxygen Concentration

Note: Adapted from Modeling a Backdraft: The 62 Watts Street Incident.

Figure 6. Temperature

Note: Adapted from Modeling a Backdraft: The 62 Watts Street Incident.

Estimating the time that fire companies forced the door to the apartment, the front door in the simulation was opened at 2250 seconds. As in the actual incident, there was an outflow of warm air from the upper part of the doorway, followed by inward movement of ambient air in the lower part of the doorway. Almost immediately after this air track pulsation, the heat release rate in the stairwell increased to nearly 5.0 MW (see Figure 5), and raising temperature in the stairwell to in excess of 1200^o C (2200^o F).

Theory and Practice

Output from the CFAST model was consistent with the observation and conditions encountered by the companies operating at 62 Watts Street on March 28, 1994.  The model showed that sufficient fuel could have accumulated under the ventilation controlled conditions that existed in the tightly sealed apartment to result in the extended duration and severity of flaming combustion that occurred in the stairwell.

Following this investigation, FDNY identified a number of similar incidents that had occurred previously, but which had gone unreported because no one had been injured. Remember that it is important to examine near miss incidents as well as those which result in injuries and fatalities.

Questions

The following questions focus on fire behavior, influence of tactical operations, and related factors involved in this incident.

1. Examine the oxygen concentration and temperature curves (Figures 5 & 6) up to the time that the door of the apartment was opened (2250 seconds). How does this data fit with the observations of the company making entry into the first floor apartment and your conception of conditions required for a backdraft?
2. How might the temperature in the apartment have influence B-SAHF indicators visible from the exterior an when performing door entry during this incident?
3. In Modeling a Backdraft Incident: The 62 Watts St (NY) Fire, Bukowski (1995) states “as buildings become better insulated and sealed for energy efficiency such hazards [e.g., ventilation controlled fires, increased concentration of gas phase fuel, backdraft] may become increasingly common. Thus, new operational procedures need to be developed to reduce the likelihood of exposure to flames of this duration” (p. 5) What operational procedures and practices would be effective in reducing risk and mitigating the hazards presented by ventilation controlled fires in energy efficient buildings? Consider size-up and dynamic risk assessment as well as strategies and tactics.
4. The often oversimplified tactical approach to dealing with potential backdraft conditions is to ventilate vertically. In this case, existing roof openings were used to ventilate the stairwell, but this had no impact on conditions in the apartment. How can tactical ventilation be used effectively (or can it) when faced with potential backdraft conditions on a lower floor or in a basement?
5. Another, less common approach to dealing with potential backdraft conditions is to cool the atmosphere and  inert the space with steam to reduce the potential for ignition. Examine the temperature curve prior to opening of the door (2250 seconds) and determine if this was a viable option?
6. Bukowski’s (1995) paper did not speak to the door entry procedures used by the companies at the apartment door. How might good door entry procedures have reduced risk in this incident?

Ed Hartin, MS, EFO, MIFIreE, CFO

References

Bukowski, R. (1996). Modeling a backdraft: The 62 Watts Street incident. Retrieved March 14, 2009 from http://fire.nist.gov/bfrlpubs/fire96/PDF/f96024.pdf

Fleischmann, C., Pagni, P., & Williamson, R. (1994) Quantitative backdraft experiments. Retrieved March 15, 2009 from http://www.fire.nist.gov/bfrlpubs/fire94/art135.html

Jones, W., Peacock, R., Forney, G., & Reneke, P. (2005). CFAST – Consolidated model of fire growth and smoke transport (Version 6) Retrieved March 15, 2009 from http://cfast.nist.gov/Documents/SP1026.pdf.

Karlsson, B. & Quintiere, J. (2000). Enclosure fire dynamics. New York: CRC Press.

Weng, W. & Fan, W. (2003). Critical condition of backdraft in compartment fires: A reduced scale experimental study. Journal of Loss Prevention in the Process Industries, 16, 19-26.

Fifteen years ago tomorrow, three members of the Fire Department of the City of New York (FDNY) lost their lives while conducting search in a three story apartment building located at 62 Watts Street in Manhattan. Captain Drennan and Firefighters Young and Seidenburg were trapped in a stairwell by rapid fire progression that occurred as other companies forced entry into the fire apartment on the floor below.

The Case

This case study was developed using a paper written by Richard Bukowski (1996) of the National Institute for Standards and Technology (NIST) Building and Fire Research Laboratory (BFRL). The FDNY requested the NIST assistance in modeling this incident to develop an understanding of the extreme fire behavior phenomena that took the lives of Captain Drennan and Firefighters Young and Seidenburg.

At 1936 hours on March 28, 1994, FDNY responded to a report of heavy smoke and sparks from a chimney of a three-story apartment building at 62 Watts Street (see Figure 1) in Manhattan. On arrival companies observesd smoke from the chimney, but no other evidence of fire. The first due engine and truck companies stretched a hoseline to the first floor unit and vertically ventilated over the stairwell.

Figure 1. 62 Watts Street-Side A

Working as the inside team of the second due truck company, Captain John Drennan (Ladder 5), Firefighter James Young, and Firefighter Christopher Seidenburg (both detailed from Engine 24 to Ladder 5) went to the second floor to begin primary search of the upper floors. At the doorway to the second floor apartment unit they were trapped by an explosion and rapid fire progression from the first floor apartment up the common stairwell. Both firefighters died within 24 hours as a result of thermal injuries. Captain Drennan survived for 40 days in the burn unit before succumbing to his injuries.

Building Information

The fire occurred in a 6.1 m (20′) x 14 m (46′), 3 ½ story apartment building of ordinary (Type III) construction, containing four dwelling units (the basement apartment was half below grade). Each unit had a floor area of slightly less than  81.7 m² (880 ft²). The basement unit had its own entrance and the units on Floors 1-3 were served by a common stairwell on Side D of the building (see Figure 1). Exposure B was an attached building identical to the fire structure. Exposure D was a similar structure. Neither exposure was involved.

Figure 2. Floor Plan-First Floor Apartment

Note: Adapted from Modeling a Backdraft Incident: The 62 Watts St. (NY) Fire.

The building was originally built in the late 1800s and had undergone numerous renovations. Recent renovations involved replacement of plaster and lath compartment linings with drywall over wood studs and lowering of the ceiling height from 2.8 m (9’3″) to 2.5 m (8’4″). All apartments had heavy wood plank flooring. During the latest renovation, windows and doors were replaced and extensive thermal insulation added to increase energy efficiency. The building was originally heated with the use of multiple fireplaces in each apartment. However, most of these had been sealed shut. However, the fireplace in the living room of the first floor apartment (unit of origin) was operable and had a 0.209 m² (2.25 ft²) flue.

All apartments had similar floor plans (differences resulting from location of the stairwell). The floor plan of the first floor apartment (unit of origin) is illustrated in Figure 2. Each apartment consisted of a living room, kitchen, bathroom, and bedroom. The first floor unit had an office constructed within the bedroom.

The structure had a flat roof with a scuttle and skylight over the stairwell.

The Fire

The occupant left the first floor apartment at 1825 hours, leaving a plastic trash bag on top of the gas fired kitchen range (see Figure 2). Investigators deduced that the bag was ignited by heat from the pilot light. Fire extended from the bag of trash to several bottles of high alcohol content liquor located on the counter adjacent to the stove. The fire progressed into the growth stage, involving other fuel packages within the apartment. The apartment was tightly sealed with the only sources of ventilation being the open fireplace flue and minimal normal building ventilation.

Weather Conditions

The weather was 10^o C (50 ^o F) with no appreciable wind.

Conditions on Arrival

On arrival companies observed smoke from the chimney of the apartment building, but no other signs of fire from the exterior.

Firefighting Operations

The outside team from the first due truck went to the roof and opened the scuttle over the stairwell while the first arriving engine company stretched a hoseline to the interior and prepared to make entry into the first floor apartment along with the inside team from the ladder company. Ladder 5 was the second due truck. The inside team from Ladder 5, Captain Drennan, Firefighter Young, and Firefighter Seidenburg, went to the second floor to begin primary search.

When the first due engine and truck forced the door to the first floor apartment they observed a pulsing air track consisting of an inward rush of air followed by an outward flow of warm (not hot) smoke. This single pulsation was followed by a large volume of flame from the upper part of the door and extending up the stairwell.

Figure 3. 3D Cutaway View of 62 Watts Street

Note: Adapted from Modeling a Backdraft: The 62 Watts Street Incident.

The crews working on Floor 1 were able to escape the rapid fire progression, but Ladder 5’s inside team was engulfed in flames which filled the stairwell. Flames extended from the doorway of the first floor apartment through the stairwell and vented out the scuttle opening and skylight. This flaming combustion continued in excess of 6 minutes 30 seconds. The intense fire in the stairwell severely damaged the stairs and melted the wired glass in the skylight.

Questions

The following questions focus on fire behavior, influence of tactical operations, and related factors involved in this incident.

1. Other than smoke and sparks from the chimney, what B-SAHF indicators might have been present and visible from the exterior or at the doorway that may have provided an indication of conditions inside the apartment?
2. What do you make of the observations of the company making entry to the first floor apartment for fire attack? Is this consistent with your understanding of backdraft indicators? Why or why not?
3. What steps can you take when making entry if you suspect that the fire is ventilation controlled? How would this change if you suspected or saw indicators of potential backdraft conditions?
4. Firefighters often identify vertical ventilation when given a scenario where backdraft indicators are present. If there is value (savable people or property) and the fire is on a lower floor (as it was in the Watts Street incident), what tactical options are available to mitigate the hazards of potential backdraft conditions?

Analysis and Computer Modeling

My next post will examine the results of this investigation and how the computer modeling performed by NIST contributes to our understanding of the events that took the lives of Captain Drennan and Firefighters Young and Seidenburg.

Ed Hartin, MS, EFO, MIFIreE, CFO

Language is Important

Language has a substantial influence on what and how we think. “What a man cannot state he does not perfectly know, and conversely the inability to put his thoughts into words sets a boundary to his thought” (Newbolt, Bailey, Baines, Boas, Davies, Enright, et al., 1921, p. 20).

While the authors of this statement were focused on English language education in English schools in the 1920’s, the underlying concept applies equally well today. Language is the foundation of understanding. While this is true in day-to-day life, it is equally (or even more) important when dealing with scientific concepts and phenomena related to firefighting.

While construction and fuel loading vary to some extent, fire services around the world are challenged by similar fire problems in the built environment. Each of us faces the same processes of compartment fire development and extreme fire behavior phenomena such as flashover, backdraft, and smoke explosion. However, our understanding and communication about these important processes and phenomena are limited by lack of a common language. In many cases terms have more than one definition. In addition, definitions are often unclear and imprecise.

Shared Concepts

In philosophy, ontology is the study of the nature of reality, categories of being, and their relations; what entities can exist and how they can be grouped, related within a hierarchy, and divided based on their similarities and differences. Ontology is a system of concepts that provides a shared vocabulary that can be used to describe and think about a particular domain.

We do not really have an ontology that encompasses fire behavior phenomena such as flashover, backdraft, smoke explosion, and the like. As Dr. Stefan Svennson so astutely observes, it is complicated and there may not always be a clearly defined differences between phenomena. However, going back to the opening paragraph of this post, I contend that a shared language is necessary for us to understand and mitigate the hazards we face as a result of rapid fire progress. Hopefully this post will engage you in this ongoing effort.

Extreme Fire Behavior

Terms such as flashover, backdraft, and smoke explosion are often used to describe phenomena involving rapid fire progression in compartment fires. Currently accepted definitions provide a starting point for developing improved clarity. As a starting point, I have examined definitions of extreme fire behavior phenomena from the following sources:

1. International Standards Organization (ISO)
2. National consensus standards organizations (e.g., National Fire Protection Association, Fire Protection Association)
3. International or national professional associations (e.g., Institution of Fire Engineers, Society of Fire Protection Engineers)
4. Recognized texts

Consider the similarities and differences in the following definitions and give some thought to the questions that follow.

Flashover: 1) Stage of fire transition to a state of total surface involvement in a fire of combustible materials within an enclosure’ (ISO 13943, 2008, 4.156). 2) A transitional phase in the development of a compartment fire in which surfaces exposed to thermal radiation reach ignition temperature more or less simultaneously and fire spreads rapidly throughout the space resulting in full room involvement or total involvement of the compartment or enclosed area (NFPA 921-2007).

Discussion: This transition is often assumed to take place between the growth and fully developed stages. However, neither the ISO nor NFPA definition specifies this. In addition, while the NFPA definition indicates that this transition is extremely rapid (i.e., more or less simultaneously), the ISO definition does not describe the speed with which the transition to total surface involvement occurs.

• Is the occurrence of flashover limited to the transition between growth and fully developed stages of fire development?
• Can flashover result from increasing ventilation to a ventilation controlled fire (vent induced flashover)? If yes, how does this differ from backdraft?
• Can a fire reach the fully developed stage without transitioning through flashover?

Backdraft: 1) Rapid flaming combustion caused by the sudden introduction of air into a confined oxygen-deficient space that contains hot products of incomplete combustion. In some cases, these conditions can result in an explosion (ISO 13943, 2008, 4.21). 2) A deflagration resulting from the sudden introduction of air into a confined space containing oxygen-deficient products of incomplete combustion (NFPA 921, 2008, 3.3.14).  3) A phenomenon that occurs when a fire takes place in a confined area such as a sealed aircraft fuselage and burns undetected until most of the oxygen within is consumed. The heat continues to produce flammable gases, mostly in the form of carbon monoxide. These gases are heated above their ignition temperature and when a supply of oxygen is introduced, as when normal entry points are opened, the gases could ignite with explosive force (NFPA 402, 2008).

Discussion: The ISO definition is considerably more broad than that specified in NFPA 921 and as such would be inclusive of phenomena such as ventilation induced flashover as well deflagration resulting from introduction of air to an extremely ventilation controlled fire. The definition of backdraft in NFPA 402, Guide for Aircraft Rescue and Firefighting Operations illustrates the common misconception that carbon monoxide is the primary gas phase fuel in a backdraft. There is no scientific evidence that this is the case. Both NFPA definitions indicate that backdraft is explosive in nature (e.g., deflagration) while the ISO definition indicates that this is a possibility, but not a requisite outcome.

• How does backdraft differ from a vent induced flashover? This is essentially the same question as before, but this time, think about it from the backdraft perspective.
• If there is a difference between vent induced flashover and backdraft, what is different (about the nature of the phenomena, requisite conditions, and initiating event(s))?
• Many firefighters believe that backdraft requires high temperature (resulting in auto-ignition following an increase in ventilation), yet this is not mentioned in any of the definitions. Is this the case?
• Is a backdraft always an explosive event?

Fire Gas Ignition: Ignition of accumulated unburned pyrolysis products and flammable products of incomplete combustion existing in or transported into a flammable state (Grimwood, Hartin, McDonough, & Raffel, 2005)

Discussion: In 3D Firefighting, Grimwood uses the term Fire Gas Ignition as a broad category of phenomena including smoke (fire gas) explosion, flash fire, and a number of other fire behavior phenomena.

• What differentiates phenomena classified as fire gas ignitions from backdraft, or for that matter flashover?
• If there is a common theme, is it useful to have an overarching category such as fire gas ignition?

Smoke Explosion: 1) See Backdraft (NFPA 921, 2008). 2) When unburnt gases from an under-ventilated fire flow through leakages into a closed space connected to the fire room, the gases there can mix very well with air to form a combustible gas mixture. A small spark is then enough to cause a smoke gas explosion (Karlsson & Quintiere, 2000). 3) A smoke gas explosion results from ignition of a confined mass of smoke gases and air that fall within the flammable range. This may result in a significant increase in pressure within the compartment (paraphrased from Bengtsson, 2001).

Discussion: In the past, the terms smoke explosion and backdraft were frequently used synonymously (and still used this way within NFPA 921). However, smoke explosion is a substantively different phenomenon as evidenced by the definitions provided by Karlsson & Quintiere (2000) and Bengtsson (2001). Drysdale (1998) also discusses this phenomenon, and while not providing a definition per say, delineates the difference between smoke explosion and backdraft as different phenomena.

• How are smoke explosion and backdraft different?
• What differentiates smoke explosion from flash fire?
• The phenomenon of smoke explosion as defined in various texts requires a mixture of fuel and air within the flammable range. If this flammable mixture is achieved by an increase in ventilation (adding air to a rich mixture of air and fuel), would piloted ignition result in a smoke explosion or backdraft?

Flash Fire: A fire that spreads rapidly through a diffuse fuel, such as dust, gas, or the vapors of an ignitable liquid, without the production of damaging pressure (NFPA 921, 2008, 3.3.72)

Discussion: While this definition appears reasonably clear when taken by itself, how does this differ from rollover, or for that matter flashover?

• What differentiates flash fire from other phenomena such as rollover (flameover) where fire spread rapidly through gas phase fuel in the upper layer?
• While the term “flash” infers a brief occurrence, the definition does not clearly define the duration of this phenomenon. Is this different from the rapid transition to a fully developed fire that results from flashover?
• What differentiates flash fire from a smoke explosion (the NFPA definition of flash fire provides a fuzzy hint, but is this clear enough)?

For a longer and more detailed examination of the definitions of flashover and backdraft, see The Current Knowledge and Training Regarding Flashover, Backdraft, and Other Rapid Fire Progression Phenomenon (Gorbett & Hopkins, 2007).

What Next?

Over the next couple of months, I will be working to develop a discussion (in a variety of formats) to develop a common framework and working definitions that will aid us in talking about fire behavior phenomena that present a significant threat to firefighters (i.e., extreme fire behavior). I invite you to be part of this process! More information will be provided in subsequent posts.

Ed Hartin, MS, EFO, MIFireE, CFO

References

Bengtsson, L. (2001). Enclosure Fires. Karlstad, Sweden: Räddnings Verket.

Drysdale, D. (2000). An introduction to fire dynamics. Chichester, England: John Wiley & Sons.

Gorbett, G. & Hopkins, R. (2007). The Current Knowledge and Training Regarding Flashover, Backdraft, and Other Rapid Fire Progression Phenomenon. Retrieved March 19, 2009 from http://www.kennedy-fire.com/backdraft%20paper.pdf.

Grimwood, P., Hartin, E., McDonough, J., & Raffel, S. (2005). 3D firefighting: Training , techniques, and tactics. Stillwater, OK: Fire Protection Publications.

Karlsson, B. & Quintiere, J.G. (2000). Enclosure fire dynamics. Boca Raton, FL: CRC Press.

National Fire Protection Association. (2008) NFPA 402 Guide for aircraft rescue and fire-fighting operations. Quincy, MA: Author.

National Fire Protection Association. (2008) NFPA 921 Guide for fire and explosion investigations. Quincy, MA: Author.

Newbolt, H., Bailey, J., Baines, K., Boas, F., Davies, H., Enright, D., et al. (1921). Teaching of English in England.  Retrieved March 17, 2009 from http://ia340921.us.archive.org/2/items/teachingofenglis00greaiala/teachingofenglis00greaiala.pdf