Archive for the ‘Extreme Fire Behavior’ Category

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.

My last two posts (Contra Costa County Line of Duty Deaths (LODD) Part 1 & Part 2) examined the conditions and circumstances involved in the incident that took the lives of Captain Matthew Burton and Engineer Scott Desmond while conducting primary search in a small residential structure in San Pablo, California early on the morning of July 21, 2007.

As identified in the Contra Costa County Investigation and NIOSH Death in the Line of Duty Report F2007-28, these line of duty deaths were the result of a complex web of events, circumstances, and actions.

These two reports identify the rapid fire progression that trapped Captain Burton and Engineer Desmond as a fire gas ignition (county and NIOSH reports) or ventilation induced flashover (NIOSH report). Both reports also point to ineffective or inappropriate use of positive pressure ventilation as a contributing factor in the occurrence of extreme fire behavior. However, neither report provides a substantive explanation of how and why this extreme fire behavior occurred.

Investigative Approach

Developing a reasonable explanation of the extreme fire behavior that occurred in this incident involved application of the scientific method as outlined in NFPA 921 Standard on Fire and Explosion Investigations (2008).

The following analysis is based on narrative data and photographic evidence provided in the Contra Costa County Fire Protection District Investigation Report: Michele Drive Line of Duty Deaths and the video taken by the Q76 Firefighter.

In that the district and NIOSH had already collected data, this effort focused on 1) analysis of the data contained in the incident reports, photographs, and video; 2) development of a hypothesis that provided an explanation for what occurred (deductive reasoning), 3) testing this hypothesis (inductive reasoning); 4) revising the hypothesis as necessary; and 5) selecting a final hypothesis.

Figure 1. Fire Development in Bedroom 2

Hypothesis

The fire originated in Bedroom 2, likely on or near the bed. In the growth stage, the fire extended through the hallway into the living room (see Figure 1). The fuel load in the living room and ventilation provided by the open front door permitted the fire to progress through flashover and become fully developed (see Figure 2).

Figure 2. Extension and Fire Development in the Living Room

The extent of fire in the living room consumed the oxygen supplied through the front door, resulting in an extremely ventilation controlled fire in the hallway and bedroom. Unburned flammable products of combustion and pyrolysis products from contents and structural materials accumulated in the upper layer in the bedrooms and hallway.

Figure 3. Fire Control and Development of a Gravity Current

Extinguishment of the fire in the living room allowed development of a gravity current and movement of oxygen through the living room to the hallway and bedrooms allowing flaming combustion in these areas to resume.

Figure 4. Positive Pressure Ventilation

Flaming combustion in the hallway or bedroom resulted in piloted ignition of a substantive accumulation of pyrolysis products and flammable products of incomplete combustion in the upper layer within the hallway and bedrooms. Application of positive pressure at the door on Side A influenced (or speeded up) this phenomena and may have increased the violence of this ignition (due to increased pressure and confinement) but likely aided in limiting the spread of flaming combustion from the hallway into the living room.

Figure 5. Fire Gas Ignition

Supporting Information

Information supporting the preceding hypothesis is divided into three categories: Known, suspected, and assumptions.

Known

The cause and origin  and line of duty death investigation conducted by the Contra Costa Fire Protection District and line of duty death investigation conducted by NIOSH identified and documented a range of data supporting this hypothesis. These data elements include physical evidence, and narrative data obtained from interviews with individuals involved in the incident.

• The fuel load in the bedroom included a bed, dresser, and other contents, exposed wood ceiling, carpet, and carpet pad.
• Fire originated in Bedroom 2 (on or near the bed)
• The female occupant exited the structure prior to making a 911 call to report the fire (via cell phone).
• The female occupant then reentered the building prior to the arrival of the first fire unit in an effort to rescue her husband. [Observations by bystanders included in the report]
• The fire in Bedroom 2 entered the growth stage and extended into the hallway and subsequently the living room. This fire spread was in part due to the combustible wood ceiling. [Information on the cause and origin investigation provided in the report]
• Windows other than the living room window on Side A were substantively intact until the occurrence of the extreme fire behavior event. [Observation by firefighters included in the report]
• E70 knocked down the fire in the living room prior to initiating primary search (without a hoseline). E70 used a left hand search pattern in which they would have moved into the hallway and bedrooms located on Side B of the residence.
• A blower was placed at the front door while E70 and E73 were conducting primary search. Due to the placement of the blower close to the door, it is possible that the air cone did not fully cover the door opening. There is no mention in the report regarding the air track at the door or living room window following placement of the blower. However, E73 reported increased visibility and temperature in the kitchen a short time after the blower was placed, and observed rollover from the hallway leading to the bedrooms.]
• The large window in the living room (if fully cleared of glass) would provide approximately equal area as the door on Side A used as an inlet. Given an equal sized inlet and outlet, efficiency of PPV is likely to be approximately 70%. However, given the location of the exhaust opening next to the inlet, the effectiveness of this ventilation at clearing smoke from compartments beyond the living room and kitchen would have been limited.
• Vertical ventilation was not completed until after the occurrence of the extreme fire behavior phenomena that trapped and killed Captain Burton and Engineer Desmond. The exhaust opening created in the roof had limited impact on interior conditions when it was completed due to the presence of the original roof.
• Fuel load in this compartment was more than sufficient to provide the heat release rate necessary to allow fire development to flashover. [This assessment is based on post-fire photos, room dimensions, and ventilation openings at the time of the ignition].
• Other bedrooms contained a similar fuel load.

Deductions

Several factors supporting the stated hypothesis are not directly supported by physical evidence or narrative data. These elements are deduced based on the design, construction, and configuration of the building and principles of fire dynamics in conjunction with known information.

• The front door remained open after the female occupant reentered. [E70 reported fire and smoke showing from the door and living room window on arrival, but no information provided in the report regarding the position of the door or extent to which the window had failed (fully or partially)]
• Use of the blower is likely to have increased mixing of air and hot, fuel rich fire gases in the hallway, particularly near the opening between the hallway and the living room. Ventilation of smoke from the living room and kitchen through the window on Side A, likely reduced the potential for flaming combustion to have extended from the hallway into the living room.
• Heat conducted through the tongue and groove wood roof/ceiling may have resulted in melting and gasification of asphalt roofing which may have been forced through gaps between the planks to add to the gas phase fuel resulting from pyrolysis and incomplete combustion of contents and structural surfaces within the involved compartments.
• The primary source of air for the fire was through the front door and the living room window. The bottom of the doorway was the lowest opening in the building, likely resulting in a bi-directional air track with smoke exiting out the top of the door and air entering at the bottom. While the sill of the living room window was higher than the door, a bi-directional air track likely developed at this opening as well, with the extreme lower portion of the window opening serving as an inlet while the top of the window functioned as an outlet for flames and smoke [No information about air track at the front door was provided in the report.]
• The fire in the living room reached the fully developed stage after the civilian occupant reentered and prior to the arrival of E70 [This deduction is based on the ability of the female occupant to enter and make her way to the kitchen and the presence of flames exiting the door and living room window on Side A when E70 arrived]

Assumptions

In addition to known and deduced information, the hypothesis is based on the following assumptions.

• The fully developed, ventilation controlled fire in the living room substantively utilized the atmospheric oxygen provided by the air entering through the front door, causing the fire in Bedroom 2 and the hallway to enter ventilation controlled decay. The decay stage fire and heat from the hot gas layer present in the hallway and adjacent rooms continued pyrolysis of fuel packages in this area, resulting in accumulation of a substantial concentration of gas phase fuel in the smoke.
• Control of the fully developed fire in the living room reduced oxygen demand from the fire. The bi-directional air track would have continued and gravity current would have increased air supply to the ventilation controlled decay stage fire in the hallway and bedroom(s).
• Establishment of positive pressure ventilation with the door on Side A serving as the inlet (or inlet and outlet) and the living room window serving as an outlet would have cleared smoke from the living room, but would not have influenced smoke movement from the hallway and bedrooms (as quickly).

Validation

Special thanks to Dr. Stefan Svensson of the Swedish Civil Contingencies Agency and Assistant Professor Greg Gorbett of Eastern Kentucky University for serving as critical friends and providing useful feedback in development of this analysis.

This hypothesis is supported by a range of evidence, deductions and assumptions. However, further validation would require use of other methods such as development of a computational fluid dynamics model and small or full scale fire tests.

More to Follow

My next post will examine the potential influence of positive pressure ventilation (PPV) in this incident as well as a broader look at potential hazards when PPV is used incorrectly or under inappropriate circumstances.

Ed Hartin, MS, EFO, MIFireE, CFO

References

Contra Costa County Fire Protection District.  (2008). Investigation report: Michele drive line of duty deaths. Retrieved February 13, 2009 from http://www.cccfpd.org/press/documents/MICHELE%20LODD%20REPORT%207.17.08.pdf

National Institute for Occupational Safety and Health (2009).  Death in the line of duty report 2007-28. Retrieved May 5, 2009 from http://www.cdc.gov/niosh/fire/pdfs/face200728.pdf.

National Fire Protection Association (NFPA) (2008) NFPA 821 Standard on fire and Explosion Investigations. Quincy, MA: Author.

This post continues examination of the incident that took the lives of Captain Matthew Burton and Engineer Scott Desmond early on the morning of July 21, 2007. Captain Burton and Engineer Desmond died while conducting primary search in a small, one-story, wood frame dwelling with an attached garage at 149 Michele Drive in San Pablo (Contra Costa County), California.

This post focuses on firefighting operations, key fire behavior indicators, and firefighter rescue operations implemented after Captain Burton and Engineer Desmond were discovered after rapid fire progression in the area in which they were searching.

Firefighting Operations

Based on the report of trapped occupants, E70 immediately placed a 150′ preconnected 1-3/4″ (45 m 45 mm) line into service using apparatus tank water. The officer of E70, seeing what he believed to be E74 arriving he passed command to the E74 officer. Unfortunately, the second arriving engine was E73 (using apparatus normally assigned to Station 74 and marked E74).

Note: This incomplete passing of command resulted in loss of command, control, and coordination of tactical operations until the arrival of BC7 at 0202 and formally assumed command at 0205. All tactical operations prior to 0205 were the result of independent action by first alarm companies.

The crew of E70 (officer and firefighter) initiated fire attack through the door on Side A and advanced 3′-5′ (0.9-1.5 m) through the door and quickly knocked down flaming combustion in the living room and through dispatch, requested the first arriving truck to establish vertical ventilation. Retrieving a thermal imaging camera (TIC) from the apparatus, the crew of E70 began a left hand search (towards the bedrooms), but left the hoseline just inside the door on Side A (see Figure 1)

Figure 1. Floor Plan-149 Michelle Drive

E73 hand stretched 200′ of 5″ (127 mm) supply line to a nearby hydrant. As he returned from the hydrant the firefighter from E73 observed a large volume of smoke from Side B. E73 officer tasked E70 engineer with placing a blower at the door on Side A. E73 (officer and firefighter) entered through the door on Side A and began a right hand search (taking the opposite direction from E70). E73 encountered poor visibility, but moderate temperature. While E73 conducted the search, E73 engineer shut off the natural gas service to the house.

E69 arrived at 0157 and prepared to perform vertical ventilation. The officer performed a size-up while the engineer obtained a chain saw and the firefighter placed a 14 ladder to provide access to the roof at the A/D corner. E70 engineer, asked the E69 officer about placing a blower to the front door (as previously ordered by the officer of E73) and he answered in the affirmative. The engineers from E70 and E73 placed a blower into operation 3′ (0.9 m) from the front door due to a half wall that partially enclosed the porch.

Note: No information is provided in the report regarding air track prior to or following pressurization of the building. The only substantive exhaust opening at the time the blower was placed into operation was the window in the living room immediately adjacent to the door on Side A.

E73 located the first civilian casualty, a female occupant in the kitchen (see Figures 2 and 5). As they removed the victim, both visibility and temperature increased dramatically. As they move the victim through the living room, they observed rollover coming from the hallway leading to the bedrooms (see Figures 2 and 5). The E73 officer briefly operated the hoseline left in the living room by E70 to control flaming combustion in the upper layer. The blower was turned 90^o to permit removal of the victim, but was then returned to its original operating position. E69 officer assigned the E69 firefighter to assist E73 with patient care on Side A.

The E69 officer and engineer proceeded to the roof and began making a vertical ventilation opening on Side A roof, over the hallway. At 0159 Q76 arrived and while the officer was donning his breathing apparatus (BA), the window in Bedroom 1 failed suddenly followed by a significant increase in flaming combustion from the windows in Bedroom 1 and 2 on Sides A and B.

The firefighter from E73 who was providing emergency medical care to the civilian fire victim observed that the window in Bedroom 1 which had been cracked with some discharge of smoke, failed violently with glass blowing out onto the lawn and a large volume of flames venting from the window for a period of 10 to 15 seconds (see Figure 2).

Figure 2. Extreme Fire Behavior

Note: Adapted from eight seconds of video was shot by Q76 firefighter from in front of Exposure D, looking towards the A/D corner of the fire building.

Figure 3. Post Fire Photo from in Front of Exposure D

Note: This screenshot from Google Maps Street View is from a similar angle as the video taken by Q76 firefighter and is provided to provide a point of reference and perspective for the video.

The E73 officer reentered the building and initiated fire attack using the hoseline left in the living room. E70 engineer stretched a second 150′ 1-3/4″ (45 m 45 mm) line to the front door. The second line was stretched into the building by Q76. Immediately after entering through the door on Side A, the Q76 met E73 officer who was exiting with low air alarm activation. Q76 took over the initial hoseline and worked their way down the hallway leading to the bedrooms, leaving the second line in the living room (see Figure 2) Q76 encountered poor visibility and high temperature with flames extending out of Bedrooms 1 and 2 and rollover in the hallway.

Shortly after exiting the building E73 officer advised E73 engineer that he was “out of air” [he was likely in a low air condition with low air alarm sounding rather than completely out of air] and expressed concern regarding E70’s air status.

Battalion 7 (BC7) arrived at 0202 and attempted to make face-to-face contact with Command (E70) as he had not heard E70 attempt to pass command to E74. At 0203, BC7 confirmed that a medic unit was responding and requested that the medic upgrade from Code 2 to Code 3. (Code 2 is a non-life threatening medical emergency requiring immediate response without the use of red lights or siren. Code 3 is a a medical emergency requiring immediate response with red lights and siren.) BC7 then attempted to contact E70 on the tactical channel and asked other crews operating at the incident about the status of E70. At 0205, BC7 ordered a second alarm and attempted to contact E70 on non-assigned tactical channels (in the event that their radios were inadvertently on the wrong channel). The second alarm added three engines (E74, E75, and E73) and a battalion chief (BC71) to the incident.

While BC7 was attempting to locate E70, Q76 was operating in the hallway and bedrooms in an effort to control the fire. They knocked the fire down in Bedroom 2 and controlled the rollover extending from Bedroom 1 down the hall. Q76 officer scanned Bedroom 2 with a TIC, but did not observe any victims. Q76 then advanced to Bedroom 1.

E69 completed a 6′ x 6′ (1.8 m x 1.8 m) ventilation opening in the roof on Side A, two thirds of the way from their access point at the A/D corner to Side B. Immediately after making the opening, they observed minimal smoke discharge (and were able to see items stored in the attic and the attic floor (original roof). They attempted to breach the attic floor, but were unable to do so (as it was constructed of 2″ x 6″ (51 mm x 152 mm) tongue and groove planks).

At 0206, after repeated unsuccessful attempts to contact E70, BC7 transmitted a report of a missing firefighter and assumed Command. Command requested an additional engine (E68) be added to the second alarm assignment. Battalion 64 (BC64) added himself to the incident and advised dispatch.

As E69 exited the roof they heard a loud pop and observed flames exiting the roof ventilation opening a distance of 8′-10′ (2.4-3.0 m). After knocking down the fire in Bedroom 1 Q76 moved back to Bedroom 2. Failure of the gypsum board on the wall between Bedrooms 1 and 2 allowed operation of the stream from their hoseline into both bedrooms.

While at the doorway of Bedroom 2, Q76 observed a substantial volume of fire in the attic through a small hole in the hallway ceiling (see Figure 4) and attempted to apply water into the attic. However, their stream was ineffective.

Figure 4. Hallway Ceiling.

Note: Adapted from Contra Costa Fire Protection District Photos, Investigation Report: Michele Drive Line of Duty Deaths. Brightness and contrast adjusted to increase clarity.

After exiting the roof, E69 proceeded counter clockwise around the building to Side C where they removed window screens and broke out several panes of glass, but did not observe an appreciable discharge of smoke. Continuing around the B/C corner, E69 observed flames from the window of Bedroom 2 and the attic.

At 0208 Command (BC7) repeatedly attempted to contact E70 by radio on the tactical channel. Unsuccessful, he requested an additional Code 3 ambulance and advised that the status of the missing firefighters was unknown.

E69 met with Command (BC7) and was assigned to continue primary search for the second reported occupant. E69 firefighter and engineer began the search while the officer replaced his SCBA cylinder. As they entered, they picked up a hoseline (second 1-3/4″ (45 mm) hoseline) and used it to extinguish small areas of fire as they moved towards the kitchen. Q76 handed off their TIC to E69 as they exited the building with low air alarms sounding.

Q76 replaced SCBA cylinders and was tasked with search for E70 on the exterior. While conducting this search, they observed flames 10′-15′ (3.0-4.6 m) in length issuing from the gable vent on Side B.

After E69 officer rejoined his crew in the kitchen, they located the second civilian casualty who was determined to be diseased (see Figure 2). Command (BC7) ordered E69 to defer removing the victim and continue searching for E70.

Firefighter Rescue Operations

E69 walked through the interior of the dwelling looking for E70 and used a hoseline to knock down fire still burning in the closet of Bedroom 2. E69 advised command that E70 was not inside, but was instructed to conduct a second search of the interior.

At 0127, Command (BC7) asked dispatch to conduct a “head count” [personnel accountability report (PAR)]. Second alarm resources arrived between 0218 and 0221.

E69 reentered the building and conducted a thorough search for E70. At 0221, Command (BC7) ordered companies to “evacuate” [withdraw from] the building. Based on the urgency of his assignment to locate E70, E69 officer decided to continue the search into Bedroom 2. At approximately 0222, E69 located Captain Burton (fire service casualty 1) under debris on the right side of the bed (see Figure 2). His facepiece was still in place and his low air alarm was ringing slowly. E69 attempted to remove the Captain, but were only able to move him to the doorway to Bedroom 2 before smoke conditions worsened and visibility decreased. Near exhaustion, one member of the crew experience low air alarm activation and became disoriented requiring assistance to exit to the door on Side A.

Command (BC7) assigned Q76 to assist with the search. As E69 exited, they advised Q76 that they had located one member of E70 in the bedroom. After exiting, E69 advised Command (BC7) that they had located one member of E70 and that he appeared to be diseased and that they were having difficulty in removing him. Q76 quickly located Captain Burton inside the doorway of Bedroom 2 and removed him to Side A at 0228. E73 attempted resuscitation, but quickly determined that the Captain’s injuries were fatal.

BC64 and E76 officer continued the search in Bedroom 2 and located Engineer Desmond (fire service casualty 2) on the left side of the bed (see Figure 2). E72 assisted in controlling the fire in Bedroom 2 and the removal of the second member of E70 on a backboard. Engineer Desmond was removed from the building at approximately 0224. After both members of E70 were removed, crews removed the deceased civilian occupant.

Timeline

Review the Michelle Drive Timeline (PDF format) to gain perspective of sequence and the relationship between tactical operations and fire behavior.

Questions

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

1. The E73 officer tasked E70 engineer with placement of a blower at the door on Side A (use of this tactic was reaffirmed by the E69 officer). What air track did this use of positive pressure create and what effect did this have on 1) conditions in the living room and kitchen and 2) in the hallway and bedrooms? Why do you think that this was the case?
2. What type of extreme fire behavior phenomena occurred in this incident? Do you agree with the Contra Costa County Fire Protection District report conclusion that this was a fire gas ignition or do you suspect that some other phenomenon was involved?
3. How did the conditions necessary for this extreme fire behavior event develop (address both the fuel and ventilation sides of the equation)?
4. What was the initiating event(s) that lead to the occurrence of the extreme fire behavior that trapped Captain Burton and Engineer Desmond? How did the use of positive pressure ventilation influence the occurrence of the extreme fire behavior (if in fact it did)?
5. What action could have been taken to reduce the potential for extreme fire behavior and maintain tenable conditions during primary search operations?
6. How did building design and construction impact on fire behavior and tactical operations during this incident?

Ed Hartin, MS, EFO, MIFireE, CFO

References

Contra Costa County Fire Protection District.  (2008). Investigation Report: Michele Drive Line of Duty Deaths. Retrieved February 13, 2009 from http://www.cccfpd.org/press/documents/MICHELE%20LODD%20REPORT%207.17.08.pdf

National Institute for Occupational Safety and Health (2009).  Death in the Line of Duty Report 2007-28. Retrieved May 5, 2009 from http://www.cdc.gov/niosh/fire/pdfs/face200728.pdf.

As discussed in previous posts, developing mastery of the craft of firefighting requires experience. However, it is unlikely that we will develop the base of knowledge required simply by responding to incidents. Case studies provide an effective means to build our knowledge base using incidents experienced by others.

Introduction

The deaths of Captain Matthew Burton and Engineer Scott Desmond in a residential fire were the result of a complex web of circumstances, actions, and events. This case study was developed using the Contra Costa County Fire Protection District Investigative Report and NIOSH Death in the Line of Duty Report 2007-28 and video taken by a Firefighter assigned to Quint 76 (Q76), the first alarm truck company. This case study focuses on the fire behavior and related tactical operations involved in this incident. However, there are a number of other lessons that may be learned from this incident and readers are encouraged to review both the fire district’s investigation and NIOSH report for additional information.

The Case

Early on the morning of July 21, 2007, Captain Matthew Burton and Engineer Scott Desmond were performing primary search of a single family dwelling in San Pablo, California. During their search, they were trapped by rapidly deteriorating conditions and died as a result of thermal injuries and smoke inhalation. Two civilian occupants also perished in the fire.

Figure 1. 149 Michele Drive-Alpha/Delta Corner

Note: Contra Costa Fire Protection District (Firefighter Q76) Photo, Investigation Report: Michele Drive Line of Duty Deaths. This photo illustrates conditions shortly after 0159 (Q76 time of arrival).

Building Information

The fire occurred in a 1,224 ft² (113.7 M2), one-story, wood frame dwelling with an attached garage at 149 Michele Drive in San Pablo (Contra Costa County), California. The house was originally built in 1953 and remodeled in 1991 with the addition of a pitched rain roof over the original (flat) roof.

This single story structure was of Type V, platform frame construction. The building was originally constructed with 4″ x 8″ (102 mm x 203 mm) beams supporting a flat roof with 2″ x 6″ (51 mm x 152 mm) tongue and groove planking with a built-up overlay consisting of several layers of tar and gravel. The pitched roof was constructed of 2″ x 8″ (51 mm x 203 mm) rafters covered with plywood and asphalt composite shingles. The ridge of the pitched roof was parallel to Side A. The gable ends on Sides B and D were constructed of plywood and fitted with a small gable vent.

Figure 2. Floor Plan-149 Michelle Drive

Note: This floor plan is based on data provided in the Contra Costa Fire Protection District Investigation Report and is not drawn to scale. The position of exterior doors and condition of windows as illustrated is based on the narrative or photographic evidence. Interior doors are shown as open as illustrated in the report. Fire service casualties are designated as follows: 1) Captain Burton, 2) Engineer Desmond.

All windows with the exception of the Living Room and Bedroom 1 (see Figure 2) were fitted with security bars (see Figure 3). The front door was the primary exit. In addition, an additional exit was provided from the kitchen through the garage to the exterior on Side D. The exterior door on Side D was fitted with a security grate.

Figure 3. View of Side C from the B/C Corner

Figure 4. Hallway and Bedroom 2

Note: Figures 3 & 4 adapted from Contra Costa Fire Protection District Photos (brightness and contrast adjusted to provide increased clarity).

Interior walls were gypsum board with wood veneer paneling on some of the walls (e.g., living room). All ceilings with the exception of the kitchen were exposed 2″ x 6″ (51 mm x 152 mm) tongue and groove planking (see Figure 4). The kitchen ceiling was covered with gypsum board. Ceiling height was 8′ (2.4 M).

Figure 5. Living Room

Note: Adapted from Contra Costa Fire Protection District Photos, Investigation Report: Michele Drive Line of Duty Deaths.

The Fire

Investigators determined that the fire likely originated on or near the east end of the bed in Bedroom 2 (see Figures 2 & 3). The likely source of ignition was improper discard of smoking materials. Developing into growth stage, the fire progressed from Bedroom 2 into the hallway (see Figures 2 & 4) leading to the living room, dining area, and kitchen (see Figures 2 & 5). It is likely that the door on Side A was closed at the time of ignition, but was opened by an occupant exiting some time after discovery of the fire.

Dispatch Information

Occupants discovered the fire and notified a private alarm company via two-way intercom at 0134. The alarm company notified the Contra Costa Regional Fire Communications Center of receipt of a fire alarm from 149 Michelle Drive at 0136 using the non-emergency telephone number. The alarm company did not indicate that they had talked to the resident who had reported a fire, but simply that they had received a fire alarm. The caller was placed on hold due to a higher priority 911 call. The dispatcher returned to the call from the alarm company at 0142 to obtain the address and callback information. Two attempts were made to call the incident location prior to dispatch of Engine 70 at 0144 to investigate the alarm. Contra Costa County Fire Protection District (CCCFPD) Engine 70 responded at 0145.

Shortly after Engine 70 responded, the communications center received a cell phone call from the female occupant at 149 Michelle Drive. This call was originally received by the California Highway Patrol and transferred to Contra Costa County Regional Fire Communications Center. The caller reported a residential fire and indicated that she had not been able to get her husband out of the building. Between the time that she spoke to the dispatcher and arrival of Engine 70, the female occupant reentered the building to attempt to rescue her husband (leaving the door on Side A open).

At 0146, the dispatcher upgraded the response to a residential fire and added two additional engines, a quint (as the truck company), and a battalion chief. Subsequent to the upgrade to a residential fire, additional 911 calls were received reporting a residential fire at 149 Michelle Drive.

Resources dispatched on the first alarm were as follows: Engine 70 (already responding on the initial dispatch for a residential alarm), Engine 69 (CCCFPD) as well as Rodeo-Hercules Fire Protection District Quint 76, and Battalion 7.  Richmond Fire Department Engine 68 was requested for automatic aid response through the Richmond Communications Center to fill out the first alarm assignment. Pinole Fire Department Engine 73 cleared a medical call a short distance away from the incident location and added themselves to the first alarm assignment. With the addition of Engine 73, the dispatcher canceled response of Engine 68 through Richmond Dispatch.

Note: Engine 73 was using an apparatus normally assigned at Station 74 which was marked with the designation Engine 74. This created some confusion during initial incident operations.

Weather Conditions

Conditions were clear, temperature was approximately 61^o F (16^o C), with a south to southeast (Side D to Side B) wind at between 2 and 6 mph (3.2 and 9.7 kph).

Conditions on Arrival

Shortly prior to arrival, Engine 70 reported “smoke showing a block out” and was advised by the dispatcher that the female occupant had been trying to get her husband out of the house and that it was uncertain if she had been successful. Engine 70 arrived at 0150, reported heavy smoke and fire from a single-story residential structure (flames and smoke were exiting from the open front door and large living room window on Side A), and established Command. Due to delays in the dispatch process, the time from the initial auomatic alarm until the arrival of E70 was approximately 16 minutes.(Refer to Contra Costa Fire Protection District, Investigation Report: Michele Drive Line of Duty Deaths for additional information regarding factors influencing the dispatch delay.

Questions

The following questions provide a basis for examining the first segment of this case study. You have an advantage that Captain Burton did not in that you are provided with a floor plan, photographs of Side C and the interior, and have knowledge of the eventual outcome. However, it is important that you place yourself in the situation encountered on arrival.

1. What stage(s) of fire and burning regime(s) were present in the building when E70 arrived? Consider potential differences in conditions in the living room, hallway, and bedrooms?
2. If you suspect that fire conditions in the living room were different than the hallway and bedrooms, why might this be the case? What evidence supports your position? What are your assumptions?
3. While limited information is available about the fire behavior indicators present during this incident, what Building, Smoke, Air Track, Heat, and Flame (B-SAHF) indictors did E70 observe when they arrived?
4. What B-SAHF indicators would you anticipate could have been observed on Sides B and C had this reconnaissance been conducted prior to making entry?
5. If you were faced with this situation, fire showing from the front door and window of a single family dwelling with persons reported, what actions would you take?
6. How do you think your selection of tactics would have influenced fire behavior and interior conditions?

Tactical Operations & Fire Behavior

My next post will examine tactical operations conducted by the first arriving companies and fire behavior encountered inside the building.

Ed Hartin, MS, EFO, MIFireE, CFO

References

Contra Costa County Fire Protection District.  (2008). Investigation Report: Michele Drive Line of Duty Deaths. Retrieved February 13, 2009 from http://www.cccfpd.org/press/documents/MICHELE%20LODD%20REPORT%207.17.08.pdf

National Institute for Occupational Safety and Health (2009).  Death in the Line of Duty Report 2007-28. Retrieved May 5, 2009 from http://www.cdc.gov/niosh/fire/pdfs/face200728.pdf.

The deaths of Captain Matthew Burton and Engineer Scott Desmond in a residential fire on July 27, 2001 were the result of a complex web of circumstances, actions, and events. The Contra Costa County Fire Protection District and National Institute for Occupational Safety and Health (NIOSH) both investigated this incident and have published reports that outline the sequence of events, contributing factors, lessons learned, and recommendations. Readers are encouraged to read the Contra Costa County Fire Protection District Report and National Institute for Occupational Safety and Health (NIOSH) Death in the Line of Duty Report F2007-28. Also have a look at Tim Sendelbach’s post In Their Honor at Firefighter Nation.

Incident Overview

Early on the morning of July 21, 2007, Captain Matthew Burton and Engineer Scott Desmond were performing primary search of a small, one-story, single family dwelling in San Pablo, California. During their search, they were trapped by rapidly deteriorating conditions and died as a result of thermal injuries and smoke inhalation. Two civilian occupants also perished in the fire.

The crews of the first arriving companies (two engines arrived almost simultaneously) faced significant challenges with a report of civilian occupants trapped in the building, flames from the door and a large window on Side Alpha and smoke throughout the structure. The two engines rapidly initiated fire attack, primary search, and placed a blower for positive pressure ventilation. During interior firefighting operations, Captain Burton and Engineer Desmond were trapped extremely rapid fire development in the hallway and bedrooms while conducting search without a hoseline.

Contributing Factors

NIOSH Report F2007-28 identifies eight factors that contributed to the tragic outcome of this incident.

• Failure by the alarm company to report a confirmed fire
• Inadequate staffing to effectively and safely respond to a structure fire
• The failure to conduct a size-up and transfer incident command
• Conducting a search without protection from a hoseline
• Failure to deploy a back-up hoseline
• Improper/inadequate ventilation
• Lack of comprehensive training on fire behavior
• Failure to initiate/deploy a Rapid Intervention Crew

NIOSH identified these factors as contributing, not causal factors. This reflects the complex and interrelated relationship between the factors that resulted in the occurrence of extreme fire behavior during primary search operations and inability of the search crew to escape from the building.

As you read the reports on this incident consider the contributing factors identified by NIOSH. Do you agree that these factors were contributory; if so, in what way; if not, why not?

NIOSH Recommendations

Based on analysis of this incident and the contributing factors, NIOSH made nine recommendations [emphasis added]:

• Ensure that fire and emergency alarm notification is enhanced to prevent delays in the alarm and response of emergency units
• Ensure that adequate numbers of staff are available to immediately respond to emergency incidents
• Ensure that interior search crews are protected by a staffed hose line
• Ensure that firefighters understand the influence of positive pressure ventilation on fire behavior and can effectively apply ventilation tactics
• Develop and implement standard operating procedures (S.O.P.’s) regarding the use of backup hose lines to protect the primary attack crew from the hazards of deteriorating fire conditions
• Develop and implement (S.O.P.’s) to ensure that incident command is properly established, transferred and maintained
• Ensure that a Rapid Intervention Crew is established to respond to fire fighters in emergency situations
• Implement joint training on response protocols with mutual aid departments

Additionally standard setting agencies, states, municipalities, and authorities having jurisdiction should:

• Consider developing more comprehensive training requirements for fire behavior to be required in NFPA 1001 Standard for Fire Fighter Professional Qualifications and NFPA102 1 Standard for Fire Officer Professional Qualifications and states, municipalities, and authorities having jurisdiction should ensure that fire fighters within their district are trained to these requirements

This final recommendation is extremely significant in that this is the first time that NIOSH has indicated that lack of effective fire behavior training in the US fire service is a systems problem. Fire training is often driven by the need to meet (rather than exceed) minimum standards. This is understandable, given the wide range of competencies required of today’s firefighters and fire officers. However, the need to develop a sound understanding of fire behavior and practical fire dynamics is critical. While this issue needs to be addressed in the professional qualification standards, we should not wait until this is accomplished. Firefighters and fire officers must become (or continue to be) students of fire behavior and develop proficiency in reading the fire and mitigation of the hazards presented by extreme fire behavior phenomena such as flashover, backdraft, smoke explosion, and flash fire.

Ed Hartin, MS, EFO, MIFireE, CFO

Application of the B-SAHF (Building, Smoke, Air Track, Heat, & Flame) organizing scheme for critical fire behavior indicators to photographs or video of structure fires provides an excellent opportunity to develop your knowledge of fire behavior and skill in reading the fire.

This video clip was recommended by Captain Virgil Hall, Tualatin Valley Fire & Rescue. Virgil is stationed at Station 64 and is one of TVF&R’s CFBT Instructors.

Residential Fire

Download and print the B-SAHF Worksheet. Consider the information provided in the short video clip. First, describe what you observe in terms of the Building, Smoke, Air Track, Heat, and Flame Indicators and then answer the following five standard questions?

1. What additional information would you like to have? How could you obtain it?
2. What stage(s) of development is the fire likely to be in (incipient, growth, fully developed, or decay)?
3. What burning regime is the fire in (fuel controlled or ventilation controlled)?
4. What conditions would you expect to find inside this building?
5. How would you expect the fire to develop over the next two to three minutes?

Review the video again, watch the indicators on Side A closely, and give some thought to the following questions posed by Captain Hall:

1. How did the smoke and flame indicators change?
2. What did this indicate?
3. Why did these changes occur (what were the influencing factors)?

Special thanks to Captain Hall for recommending this video clip. Please feel free to contribute to this process and share or recommend video clips or photographs that will help us develop our skill in reading the fire.

Remember the Past

While some firefighters have heard about the incidents involving multiple fatalities, others have not and most do not know the stories of firefighters who died alone. In an effort to encourage us to remember the lessons of the past and continue our study of fire behavior, I will occasionally be including brief narratives and links to NIOSH Death in the Line of Duty reports and other documentation in my posts. The first narrative in this post is incomplete as this incident, resulting in the death of two members of the Houston Fire Department occured last Sunday. It is important for us to continue our efforts to understand and mitigate the complex and interrelated factors that result in firefighter fatalities occuring during structural firefighting operations.

April 12, 2009
Captain James Harlow
Firefighter Damion Hobbs
Houston Fire Department, Texas

Captain James Harlow and Firefighter Damion Hobbs of the Houston, Texas Fire Department lost their lives in the line of duty while conducting primary search in a single family dwelling on the morning of April 12, 2009. Preliminary information indicates that Captain Harlow and Firefighter Hobbs were trapped by rapid fire progress, possibly influenced by wind. The Houston Fire Department, Texas State Fire Marshal, and National Institute for Occupational Safety and Health (NIOSH) are all investigating this incident. More information will be posted as it becomes available.

April 11, 1994
Lieutenant Michael Mathis
Private William Bridges
Memphis Fire Department, Tennessee

On April 11, Lt. Michael Mathis and Private William Bridges of the Memphis (TN) Fire Department were killed when they became trapped and overcome by smoke during a fire on the ninth floor of a high rise building. Two civilians also died in the arson fire. Lt. Mathis became disoriented when he was caught in rapidly spreading fire conditions on the fire floor, burning him and causing his SCBA to malfunction. He found his way into a room on the ninth floor were he was later discovered by other fire crews with his SCBA air depleted. Private Bridges, aware that Lt. Mathis was unaccounted for after several unsuccessful attempts to contact him by radio, left a safe stairwell where he had been attempting to fix a problem with his own SCBA. Investigators believe Bridges was trying to locate Lt. Mathis. Bridges became entangled in fallen cable TV wiring within a few feet of the stairwell, and died of smoke inhalation after depleting his SCBA supply. A Memphis Fire Department investigation found many violations of standard operating procedures by companies on the scene, including crews taking the elevator to the fire floor, problems with the incident command system and coordination of companies, operating a ladder pipe with crews still on the fire floor, and a failure of personnel, including Lt. Mathis and Private Bridges, to activate their PASS devices.

April 16, 2007
Firefighter-Technician I Kyle Robert Wilson
Prince William County Department of Fire and Rescue, Virginia

Technician Wilson was assigned to Tower 512, a ladder company. Tower 512 was dispatched to a reported house fire at 0603 hours. The Prince William County area was under a high wind advisory as a nor’easter moved through the area. Sustained winds of 25 miles per hour with gusts up to 48 miles per hour were prevalent in the area at the time of the fire dispatch.

Initial arriving units reported heavy fire on the exterior of two sides of the single-family house, and crews suspected that the occupants were still inside the house sleeping because of the early morning hour. A search of the upstairs bedroom was conducted by Technician Wilson and his officer. A rapid and catastrophic change of fire and smoke conditions occurred in the interior of the house within minutes of Tower 512’s crew entering the structure. Technician Wilson became trapped and was unable to locate an immediate exit. “Mayday” radio transmissions of the life-threatening situation were made by crews and by Technician Wilson. Valiant and repeated rescue attempts to locate and remove Technician Wilson were made by the firefighting crews during extreme fire, heat, and smoke conditions. Firefighters were forced from the structure as the house began to collapse on them and fire conditions worsened. Technician Wilson succumbed to the fire and the cause of death was reported by the medical examiner to be thermal and inhalation injuries.

An extensive report on this incident is available from the Prince William Department of Fire and Rescue: Technician Kyle Wilson LODD Report.

For additional information regarding this incident, please refer to NIOSH Fire Fighter Fatality Investigation and Prevention Program Report F2007-12.

Ed Hartin, MS, EFO, MIFireE, CFO

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