The seventh tactical implication identified in the Underwriters Laboratories study of the Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction (Kerber, 2011) is the influence of changes in ventilation on flow path.
“Every new ventilation opening provides a new flow path to the fire and vice versa. This could create very dangerous conditions when there is a ventilation limited fire” (Kerber, 2011).
Air Track and Flow Path
Air track and flow path are closely related and provide an excellent framework for understanding the influence of changes in ventilation on fire development and flow path.
Air Track: Closely related to flow path, air track is the movement of air and smoke as observed from the exterior and inside the structure. Air track is used to describe a group of fire behavior indicators that includes direction of smoke movement at openings (e.g., outward, inward, pulsing), velocity and turbulence, and movement of the lower boundary of the upper layer (e.g., up, down, pulsing).
Observation of air track indicators may provide clues as to the potential flow path of air and hot gases inside the fire building. As discussed in previous posts in this series (Part 1, Part 2, Part 3, Part 4, Part 5, Part 6), movement of air to the fire has a major impact on fire development. Movement of hot gases away from the fire is equally important!
Flow Path: In a compartment fire, flow path is the course of movement hot gases between the fire and exhaust openings and the movement of air towards the fire.
Both of these components of flow path are important! Movement of hot gases between the fire an exhaust openings is a major factor in heat transfer outside the compartment of origin and presents a significant thermal threat to occupants and firefighters. When the fire is in a ventilation controlled burning regime, movement of air from to the fire provides the oxygen necessary for fire growth and increased heat release rate (impacting on conditions in the flow path downstream from the fire.
Flow path can significantly influence fire spread and the hazard presented to occupants and firefighters.
Reading the Fire
Before engaging in the meat of this UL Tactical Implication, quickly review essential air track indicators used in the Building, Smoke, Air Track, Heat, and Flame (B-SAHF) fire behavior indicators organizing scheme.
Figure 1. Air Track Indicators
As illustrated in Figure 1, key indicators include wind direction and velocity (consider this before you even arrive on-scene), directions in which the air and smoke are moving, and the velocity and flow of smoke and air movement.
Take a look at Figure 2. Consider all of the B-SAHF indicators, but pay particular attention to Air Track. What is the current flow path? How might the flow path change if one or more windows on Floor 2 Side A are opened prior to establishing fire control?
Figure 2. Residential Fire in a 1 ½ Story Wood Frame Dwelling
Tactical implications related to flow path identified in Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction (Kerber, 2011) focus on creation of additional openings and changes in flow path as a result of “crews venting as the go” (p. 296). This is only one issue related to flow path!
The UL experiments showed that increasing the number of flow paths resulted in higher peak temperatures, a faster transition from decay to growth stage and more rapid transition to flashover. However, this is not the only hazard!
As previously discussed in the series of posts examining the fire in a Washington DC townhouse that took the lives of Firefighters Anthony Phillips and Louis Matthews, operating in the flow path presents potential for significant thermal hazard.
In this incident, the initial attack crew was operating on the first floor of a two-story townhouse with a daylight basement. When crews opened the sliding glass doors in the basement (on Side C), a flow path was created between the opening at the basement level on Side C, up an open interior stairway to the first floor, and out the first floor doorway (on Side A). Firefighters working in this flow path were subjected to extreme thermal stress, resulting in burns that took the lives of Firefighters Phillips and Mathews and serious injuries to another firefighter.
Figure 1. Perspective View of 3146 Cherry Road and Location of Slices
Note: From Simulation of the Dynamics of the Fire at 3146 Cherry Road NE Washington D.C., May 30, 1999, NISTR 6510 (p. 15) by Dan Madrzykowski and Robert Vettori, 2000, Gaithersburg, MD: National Institute for Standards and Technology.
Figure XX illustrates thermal conditions, velocity and oxygen concentration at various locations within the flow path.
Figure 10. Perspective Cutaway, Flow/Temperature, Velocity, and O2 Concentration
The temperature of the atmosphere (i.e., smoke and air) is a significant concern in the fire environment, and firefighters often wonder or speculate about how hot it was in a particular fire situation. However, gas temperature in the fire environment is a bit more complex than it might appear on the surface and is only part of the thermal hazard presented by compartment fire.
Convective heat transfer is influenced by gas temperature and velocity. When hot gases are not moving or the flow of gases across a surface (such as your body or personal protective equipment) is slow, energy is transferred from the gases to the surface (lowering the temperature of the gases, while raising surface temperature). These lower temperature gases act as an insulating layer, slowing heat transfer from higher temperature gases further away from the surface. When velocity increases, cooler gases (which have already transferred energy to the surface) move away and are replaced by higher temperature gases. When velocity increases sufficiently to result in turbulent flow, hot gases remain in contact with the surface on a relatively constant basis, increasing convective heat flux.
For a more detailed discussion of this incident and the influence of radiative and convective heat transfer in the flow path, see the prior posts on the Washington DC Townhouse Fire Case Study.
While operating in the flow path presents serious risk, when fire behavior is influenced by wind, conditions in the flow path can be even more severe. In experiments conducted by the National Institute of Standards and Technology (NIST) demonstrated that under wind driven conditions, both temperature and heat flux, which were twice as high in the “flow” portion of the corridor as opposed to the “static” portion of the corridor (where there was no flow path). See the previous posts on Wind Driven Fires for more information on flow path hazards under wind driven conditions:
The sixth and seventh tactical implications identified in the UL Horizontal Ventilation Study are interrelated and can be expanded to include the following key points:
Heat transfer (convective and radiative) is greatest along the flow path between the fire and exhaust opening.
Exhaust openings located higher than the fire will increase the velocity of gases along the flow path (further increasing convective heat transfer).
Flow of hot gases from the fire to an exhaust opening is significantly influenced by air flow from inlet openings to the fire (the greater the inflow of air, the higher the heat release rate and flow of hot gases to the exhaust opening).
Flow path can be created by a single opening that serves as both inlet and exhaust (such as an open door or window).
Thermal conditions in the flow path can quickly become untenable for both civilian occupants and firefighters. As noted in an earlier NIST Study examining wind driven fires, under wind driven conditions this change can be extremely rapid.
Closing an inlet, exhaust opening, or introducing a barrier (such as a closed door) in the flow path slows gas flow and reduces the hazard downstream from the barrier.
When the fire is ventilation controlled, limiting inflow of air (e.g., door control) can slow the increase in heat release rate and progression to a growth stage fire.
Multiple openings results in multiple flow paths and increased air flow to the fire, resulting in more rapid fire development and increased heat release rate.
What’s Next?
The next tactical implication identified in the UL Horizontal Ventilation study examines an interesting question: Can you vent enough (to return the fire to a fuel controlled burning regime)? This question may also be restated as can you perform sufficient natural horizontal ventilation to improve internal conditions. The answer to this question will likely be extended through the Vertical Ventilation Study that will be conducted by UL in early 2012!
References
District of Columbia (DC) Fire & EMS. (2000). Report from the reconstruction committee: Fire at 3146 Cherry Road NE, Washington DC, May 30, 1999. Washington, DC: Author.
Madrzykowski, D. & Vettori, R. (2000). Simulation of the Dynamics of the Fire at 3146 Cherry Road NE Washington D.C., May 30, 1999, NISTR 6510. August 31, 2009 from http://fire.nist.gov/CDPUBS/NISTIR_6510/6510c.pdf
The sixth tactical implication identified in the Underwriters Laboratories study of the Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction (Kerber, 2011) identifies potential hazards and risks related to the tactic of Vent Enter Search (VES).
Kerber (2011) provides a straightforward explanation of this tactic.
It can be described as ventilating a window, entering through the window, searching that room and exiting out the same window entered…A primary of objective of VES is to close the door of the room ventilated to isolate the flow path…
Origins and Context
While it is difficult to identify or isolate the origins of many fireground tactics, VES has been practiced by FDNY for many years and is described in detail in the Firefighting Procedures Volume 3, Book 4: Ladder Company Operations at Private Dwellings manual (FDNY, 1997). As described in Ladder Company Operations at Private Dwellings, FDNY truck companies are staffed with an officer, apparatus operator, and four firefighters and are divided into two teams; inside team and outside team. While tactics are dependent on the type of structure and fire conditions; VES is performed by the outside team while the inside team works in conjunction with an engine company, supporting fire attack and searching from the interior. In this context, VES is part of a coordinated tactical operation.
It is also important to recognize the impact of changes in the fire environment since the development of this tactic (likely in the 1960s). Changes in the speed of fire development are graphically illustrated in the Underwriters Laboratories (UL) test of fire development with modern and legacy furnishings.
The tremendous fuel load, development of ventilation limited conditions, and rapid transition from tenable to untenable conditions for firefighters following increased ventilation (without initiation of fire control), reduce the time for Firefighters to make entry and control the door when performing VES in the modern fire environment.
Influence on Fire Behavior
The following incident illustrates the rapid changes in conditions that can result during VES operations. This information was originally presented in the post titled Criticism Versus Critical Thinking. This incident involved VES at a residential structure where rapid fire progress required the Captain conducting the search to perform emergency window egress from a second floor window onto a ladder.
Companies were dispatched to a residential fire at 0400 hours with persons reported. On arrival, cars were observed in the driveway and neighbors reported the likely location of a trapped occupant on the second floor.
Given fire conditions on Floor 1, the Captain of the first in truck, a 23 year veteran, determined that Vent, Enter, and Search (VES) was the best option to quickly search and effect a rescue.
The following video clip illustrates conditions encountered at this residential fire:
In his, vententersearch.com post Captain Van Sant provided the following information about his observations and actions:
When we vent[ed] the window with the ladder, it looks like the room is burning, but the flames you see are coming from the hallway, and entering through the top of the bedroom doorway. Watch it again and you’ll see the fire keeps rolling in and across the ceiling.
When I get to the window sill, the queen-sized bed is directly against the window wall, so there is no way to “check the floor” … Notice that you continue to see my feet going in, because I’m on the bed.
Believe me, in the beginning, this was a tenable room both for me and for any victim that would have been in there…
My goal was to get to the door and close it, just like VES is supposed to be done. We do it successfully all the time.
When I reached the other side of the bed, I dropped to the floor and began trying to close the door. Unfortunately, due to debris on the floor, the door would not close [emphasis added].
Conditions were still quite tenable at this point, but I knew with the amount of fire entering at the upper level, and smoke conditions changing, things were going to go south fast…
I kept my eyes on my exit point, and finished my search, including the closet, which had no doors on it. Just as I was a few feet from the window, the room lit off…
Tactical Considerations
Is VES an appropriate tactic for primary search in private dwellings? This question must be placed into operational context bounded by fire dynamics, resources and staffing, experience level of the firefighters involved, potential for survivable occupants, and the fireground risk management philosophy of the department. Consider the following:
There have been instances where VES has resulted in saving of civilian life.
There have been instances where VES has resulted in significant thermal injury to firefighters.
The UL ventilation tests (Kerber, 2011) demonstrate that conditions rapidly become untenable for civilian occupants in rooms with open doors. Rooms with closed doors remain tenable for civilian occupants for a considerable time.
VES may result in rapid search of specific threatened areas.
VES is a high risk tactic that involves working alone (but if a second firefighter remains at the entry point this is similar to oriented search).
VES (as normally practiced) involves working without a hoseline.
VES changes the ventilation profile and places firefighters in the flow path between the fire and an exhaust opening (unless or until the door to the compartment is closed)
As demonstrated in the UL ventilation tests (Kerber, 2011), thermal conditions change from a tenable operating environment for firefighters to untenable and life threatening in a matter of seconds.
Based on these factors, you may determine that VES is not an appropriate tactic for primary search under any circumstances, or you may determine that it might be appropriate under specific circumstances. The following tactical scenarios may provide a framework for discussion of these issues.
Scenario 1:You have responded to a fire in a medium sized, two-story, wood frame, single-family dwelling at 02:13 hours. You observe a smoke issuing at moderate velocity from the eaves and condensed pyrolizate on the inside of window glazing. A dull reddish glow can be observed through several adjacent windows on the Charlie Side (back of the house), Floor 1.
Given your normal first alarm assignment and staffing Is VES an appropriate option for primary search given the conditions described and potential for possible occupants? Why or why not?
Scenario 2: You have the same building, smoke, air track, heat, and flame indicators as in Scenario 1, but a female occupant meets you on arrival and reports that her husband is trying to rescue their daughter who was sleeping in a bedroom on Floor 2 at the Alpha/Bravo corner of the house.
Given your normal first alarm assignment and staffing Is VES an appropriate option for primary search given the conditions described and reported occupants? Why or why not?
Scenario 3: You have the same building, smoke, air track, heat, and flame indicators as in Scenario 1, and observe two occupants, an adult male and a female child in a window on Floor 2 at the Alpha/Bravo corner of the house. Smoke at low velocity is issuing from the open window above the occupants. However, before you can raise a ladder to rescue the occupants in the window, they disappear from view and the volume and velocity of smoke discharge from the window increases.
Given your normal first alarm assignment and staffing Is VES an appropriate option for primary search given the conditions described and initial observation of occupants? Why or why not?
Vertical Ventilation Study
UL has commenced a study on the Effectiveness of Vertical Ventilation and Fire Suppression Tactics using the same legacy and contemporary residential structures used in their study of horizontal ventilation. This research project will examine a range of vertical ventilation variables including the size, location, and timing of openings. In addition, further research will be conducted on the effectiveness of exterior streams and their impact on interior conditions.
Preliminary design parameters for the study were developed in conjunction with a technical panel representing a wide range of jurisdictions and types of fire service agencies, including:
Atlanta Fire Department (GA)
Central Whidbey Island Fire & Rescue (WA)
Chicago Fire Department (IL)
Cleveland Fire Department (OH)
Coronado Fire Department (CA)
Fire Department of the City of New York (FDNY) (NY)
Loveland-Symmes Fire Department (OH)
National Institute of Standards and Technology (NIST) (MD)
Northbrook Fire Department (IL)
Milwaukee Fire Department (WI)
Lake Forest Fire Department (IL)
Phoenix Fire Department (AZ)
Full scale tests are anticipated to begin in January 2012. I will provide updates as this research project progresses.
References
Fire Department of the City of New York (FDNY). (1997) Firefighting prodedures volume 3 book 4: Ladder company operations at private dwellings. New York: Author.
In the experiments conducted by UL, both the single and two story dwellings filled rapidly with smoke with the smoke layer reaching the floor prior to ventilation. This resulted in zero visibility throughout the interior (with the exception of the one bedroom with a closed door). After ventilation, the smoke layer did not lift (as many firefighters might anticipate) as the rapid inward movement of air simply produced a tunnel of clear space just inside the doorway.
Put in the context of the Building, Smoke, Air Track, Heat, and Flame (B-SAHF) fire behavior indicators, these phenomena fit in the categories of smoke and air track. Why did these phenomena occur and what can firefighters infer based on observation of these fire behavior indicators?
Smoke Versus Air Track
There are a number of interrelationships between Smoke and Air Track. However, in the B-SAHF organizing scheme they are considered separately. As we begin to develop or refine the map of Smoke Indicators it is useful to revisit the difference between these two categories in the B-SAHF scheme.
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.
Smoke Indicators
There are a number of smoke characteristics and observations that provide important indications of current and potential fire behavior. These include:
Location: Where can you see smoke (exterior and interior)?
Optical Density (Thickness): How dense is the smoke? Can you see through it? Does it appear to have texture like velvet (indicating high particulate content)?
Color: What color is the smoke? Don’t read too much into this, but consider color in context with the other indicators.
Physical Density (Buoyancy): Is the smoke rising, sinking, or staying at the same level?
Thickness of the Upper Layer: How thick is the upper layer (distance from the ceiling to the bottom of the hot gas layer)?
As discussed in Reading the Fire: Smoke Indicators Part 2, these indicators can be displayed in a concept map to show greater detail and their interrelationships (Figure 1).
Figure 1. Smoke Indicators Concept Map
Air Track
Air track includes factors related to the movement of smoke out of the compartment or building and the movement of air into the fire. Air track is caused by pressure differentials inside and outside the compartment and by gravity current (differences in density between the hot smoke and cooler air). Air track indicators include velocity, turbulence, direction, and movement of the hot gas layer.
Direction: What direction is the smoke and air moving at specific openings? Is it moving in, out, both directions (bi-directional), or is it pulsing in and out?
Wind: What is the wind direction and velocity? Wind is a critical indicator as it can mask other smoke and air track indicators as well as serving as a potentially hazardous influence on fire behavior (particularly when the fire is in a ventilation controlled burning regime).
Velocity & Flow: High velocity, turbulent smoke discharge is indicative of high temperature. However, it is essential to consider the size of the opening as velocity is determined by the area of the discharge opening and the pressure. Velocity of air is also an important indicator. Under ventilation controlled conditions, rapid intake of air will be followed by a significant increase in heat release rate.
Discharge of smoke at openings and potential openings (Building Factors) is likely the most obvious indicator of air track while lack of smoke discharge may be a less obvious, but equally important sign of inward movement of air. Observation and interpretation of smoke and air movement at openings is an essential part of air track assessment, but it must not stop there. Movement of smoke and air on the interior can also provide important information regarding fire behavior.
An Ongoing Process
Reading the fire is an ongoing process, beginning with reading the buildings in your response area prior to the incident and continuing throughout firefighting operations. It is essential to not only recognize key indicators, but to also note changing conditions. This can be difficult when firefighters and officer are focused on the task at hand.
UL Experiment 13
This experiment examined the impact of horizontal ventilation through the door on Side A and one window as high as possible on Side C near the seat of the fire. The family room was the fire compartment. This room had a high (two-story) ceiling with windows at ground level and the second floor level (see Figure 3).
Figure 3. Two-Story Dwelling
In this experiment, the fire was allowed to progress for 10:00 after ignition, at which point the front door (see Figure 3) was opened to simulate firefighters making entry. Fifteen seconds after the front door was opened (10:15), an upper window in the family room (see Figure 3) was opened. No suppression action was taken until 12:28, at which point a 10 second application of water was made through the window on Side C using a straight stream from a combination nozzle.
As with all the other experiments in this series fire development followed a consistent path. The fire quickly consumed much of the available oxygen inside the building and became ventilation controlled. At oxygen concentration was reduced, heat release rate and temperature within the building also dropped. Concurrently, smoke and air track indicators visible from the exterior were diminished. Just prior to opening the door on Side A, there was little visible smoke from the structure (see Figure 4).
Figure 4. Experiment 13 at 00:09:56 (Prior to Ventilation)
As illustrated in Figure 5, a bi-directional air track was created when the front door was opened. Hot smoke flowed out the upper area of the doorway while air pushed in the bottom creating a tunnel of clear space inside the doorway (but no generalized lifting of the upper layer.
Figure 5. Experiment 13 at 00:10:14 (Door Open)
As illustrated in Figure 6, opening the upper level window in the family room resulted in a unidirectional air track flowing from the front door to the upper level window in the family room. No significant exhaust of smoke can be seen at the front door, while a large volume of smoke is exiting the window. However, while the tunneling effect at floor level was more pronounced (visibility extended from the front door to the family room), there was no generalized lifting of the upper layer throughout the remainder of the building.
Figure 6. Experiment 13 at 00:10:21 (Door and Window Open)
With the increased air flow provided by ventilation through the door on Side A and Window at the upper level on Side C, the fire quickly transitioned to a fully developed stage in the family room. The heat release rate (HRR) and smoke production quickly exceeded the limited ventilation provided by these two openings and the air track at the front door returned to bi-directional (smoke out at the upper level and air in at the lower level) as shown in Figure 7.
Figure 7. Experiment 13 at 00:11:22 (Door and Window Open)
What is the significance of this observation? Movement of smoke out the door (likely the entry point for firefighters entering for fire attack, search, and other interior operations) points to significant potential for flame spread through the upper layer towards this opening. The temperature of the upper layer is hot, but flame temperature is even higher, increasing the radiant heat flux (transfer) to crews working below. Flame spread towards the entry point also has the potential to trap, and injure firefighters working inside.
Gas Velocity and Air Track
A great deal can be learned by examining both the visual indicators illustrated in Figures 4-7 and measurements taken of gas velocity at the front door. During the ventilation experiments conducted by UL, gas velocities were measured at the front door and at the window used for ventilation (see Figure 3). Five bidirectional probes were placed in the doorway at 0.33 m (1’) intervals. Positive values show gas movement out of the building while negative values show inward gas movement. In order to provide a simplified view of gas movement at the doorway, Figure 8 illustrates gas velocity 0.33 m (1’) below the top of the door, 0.33 m (1’) from the bottom of the door, and 0.66 m (2’) above the bottom of the door.
A bidirectional (out at the top and in at the bottom) air track developed at the doorway before the door was opened (see Figure 8) as a result of leakage at this opening. It is interesting to note variations in the velocity of inward movement of air from the exterior of the building, likely a result of changes in combustion as the fire became ventilation controlled. The outward flow at the upper level resulted in visible smoke on the exterior of the building. While not visible, inward movement of air was also occurring (as shown by measurement of gas velocity at lower levels in the doorway.
Creation of the initial ventilation opening by opening the front door created a strong bidirectional air track with smoke pushing out the top of the door while air rapidly moved in the bottom. Had the door remained the only ventilation opening, this bidirectional flow would have been sustained (as it was in all experiments where the door was the only ventilation opening).
Opening the upper window in the family room resulted in a unidirectional flow inward through the doorway. However, this phenomenon was short lived, with the bidirectional flow reoccurring in less than 60 seconds. This change in air track resulted from increased heat release rate as additional air supply was provided to the fire in the family room.
While not the central focus of the UL research, these experiments also examined the effects of exterior fire stream application on fire conditions and tenability. Each experiment included a 10 second application with a straight stream and a 10 second application of a 30o fog pattern. Between these two applications, fire growth was allowed to resume for approximately 60 seconds.
The straight stream application resulted in a reduction of temperature in the fire compartment and adjacent compartments (where there was an opening to the family room or hallway) as water applied through the upper window on Side C (ventilation opening) cooled the compartment linings (ceiling and opposite wall) and water deflected off the ceiling dropped onto the burning fuel. As the stream was applied, air track at the door on Side A changed from bidirectional to unidirectional (inward). This is likely due to the reduction of heat release rate achieved by application of water onto the burning fuel with limited steam production.
When the fog pattern was applied, there was also a reduction of temperature in the fire compartment and adjacent compartments (where there was an opening in the family room or hallway) as water was applied through the upper window on Side C (ventilation opening) cooled the upper layer, compartment linings, and water deflected off the ceiling dropped onto the burning fuel. The only interconnected area that showed a brief increase in temperature was the ceiling level in the dining room. However, lower levels in this room showed an appreciable drop in temperature. Air track at the door on Side A changed from bidirectional to unidirectional (outward) when the fog stream was applied. This effect is likely due to air movement inward at the window on Side C and the larger volume of steam produced on contact with compartment linings as a result of the larger surface area of the fog stream.
The effect of exterior streams will be examined in more detail in a subsequent post.
Important Lessons
The fifth tactical implication identified in the Underwriters Laboratories study of the Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction (Kerber, 2011) is described as failure of the smoke layer to lift following horizontal natural ventilation and smoke tunneling and rapid air movement in through the front door.
Additional lessons that can be learned from this experiment include:
Ventilating horizontally at a high point results in higher flow of both air and smoke.
Increased inward air flow results in a rapid increase in heat release rate.
The rate of fire growth quickly outpaced the capability of the desired exhaust opening, returning the intended inlet to a bi-directional air track (potentially placing firefighters entering for fire attack or search at risk due to rapid fire spread towards their entry point).
Tactical applications of this information include:
Ensure that the attack team is in place with a charged line and ready to (or has already) attack the fire (not simply ready to enter the building) before initiating horizontal ventilation.
Cool the upper layer any time that it is above 100o C (212o F) to reduce radiant and convective heat flux and to limit potential for ignition and flaming combustion in the upper layer.
Note that this research project did not examine the impact of gas cooling, but examination of the temperatures at the upper levels in this experiment (and others in this series) point to the need to cool hot gases overhead.
What’s Next?
I am on the hunt for videos that will allow readers to apply the tactical implications of the UL study that have been examined to this point in conjunction with the B-SAHF fire behavior indicators. My next post will likely provide an expanded series of exercises in Reading the Fire.
The next tactical implication identified in the UL study (Kerber, 2011) examines the hazards encountered during Vent Enter Search (VES) tactical operations. A subsequent post will examine this tactic in some detail and explore this tactical implication in greater depth.
The fourth tactical implication identified in the Underwriters Laboratories study of the Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction (Kerber, 2011) is that fire attack and (tactical) ventilation must be coordinated. This recommendation has been repeated in National Institute for Occupational Safety and Health (NIOSH) Death in the Line of Duty Reports for many years. In fact, most reports on firefighter fatalities related to rapid fire progression contain this recommendation.
Importance of Coordination
Coordination of (tactical) ventilation and fire attack as a tactical implication is closely related to the first two tactical implications identified in the UL study; potential changes in fire behavior based on stages of fire development, burning regime, and changes in ventilation profile that increase oxygen supplied to the fire.
If air is added to the fire and water is not applied in the appropriate time frame the fire gets larger and the hazards to firefighters increase. Examining the times to untenability provides the best case scenario of how coordinated the attack needs to be. Taking the average time for every experiment from the time of ventilation to the time of the onset of firefighter untenability conditions yields 100 seconds for the one-story house and 200 seconds for the two-story house. In many of the experiments from the onset of firefighter untenability until flashover was less than 10 seconds. These times should be treated as very conservative. If a vent location already exists because the homeowner left a window or door open then the fire is going to respond faster to additional ventilation openings because the temperatures in the house are going to be higher at the time of the additional openings (Kerber, 2011, p. 289-290)
The Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction Underwriters Laboratories (UL) on-line course and report provide an example of firefighters are at risk when ventilation is performed prior to entry, fire attack is delayed, and other tactical operations such as primary search are initiated.
In UL’s hypothetical example, the firefighters make entry into the one-story house, search the living room (fire compartment), the kitchen, and dining room shortly after forcing the door and ventilating a large window in the fire compartment. Consider a somewhat different scenario, with the same fire conditions.
Companies respond to a residential fire with persons reported during the early morning hours. A truck and engine arrive almost simultaneously and while the engine lays a supply line from a nearby hydrant, the truck company forces entry, ventilates a window on Side A, and begins primary search (anticipating that the engine crew will be right behind them to attack the fire). The engine completes a forward lay and begins to stretch an attack line after the search team has made entry.
Figure 1. Timeline and Progression of Primary Search
Figure 2. View of the Living Room (Fire Compartment) from the Door on Side A
As illustrated in Figure 3, visible flaming combustion when the door is opened at 08:00 is limited to a small flame from the top of the couch just inside the door on Side A. However, in the 30 seconds that it takes for the search team to make entry, flaming combustion has resumed and flames are near or at the ceiling above the couch. The search team may estimate that they have time to complete a quick search of the bedrooms (likely location of the reported persons). However, fire development progresses to untenable conditions within a minute, trapping the crew on Side D of the house.
Figure 3. Fire Progression in the Living Room 00:08:00 to 00:10:00
As the search team completes primary search of Bedroom 2 and moves towards Bedroom 3 in the hallway, conditions have deteriorated to an untenable level. Figure 4 illustrates the change in temperature at the 3’ level in the Living Room (fire compartment). Shortly before the search team reached Bedroom 2, fire conditions in the living room began to change dramatically, with temperature at the 3’ level transitioning from ordinary to extreme, quickly becoming untenable in the living room, hallway and adjacent compartments. In addition to this significant change in temperature, flames (with temperatures higher than the gas temperature at the 3’ level) significantly increase radiant heat transfer (flux) to the surface of both fuel packages and firefighters protective equipment.
Figure 4. Temperature at the 3’ Level
Note: Figure 4 illustrates temperature conditions starting eight minutes after ignition. The fire previously progressed through incipient and growth stages before beginning to decay due to lack of ventilation.
Why the Dramatic Change in Conditions?
As discussed in UL Tactical Implications Part 1, Fires in the contemporary environment progress from ignition and incipient stage to growth, but often become ventilation controlled and begin to decay, rather than continuing to grow into a fully developed fire. This ventilation induced decay continues until the ventilation profile changes (e.g., window failure due to fire effects, opening a door for entry or egress, or intentional creation of ventilation openings by firefighters. When ventilation is increased, heat release rate again rises and temperature climbs with the fire potentially transitioning through flashover to the fully developed stage (see Figure 4 and 5).
Figure 5. Fire Development in a Compartment
Captain James Mendoza of the San Jose (CA) Fire Department and CFBT-US Lead Instructor demonstrates the influence of ventilation on fire development using a small scale prop developed by Dr. Stefan Svensson of the Swedish Civil Contingencies Agency.
The prop used in this demonstration is a small, single compartment with a limited ventilation opening on the right side (which in a full size building could be represented by normal building leakage or a compartment opening that is restricted such as a partially open door or window). The front wall of the prop is ceramic glass to permit direct observation of fire conditions within the compartment.
As you watch this demonstration, pay particular attention to how conditions change as the fire develops and then enters the decay stage. In addition, observe how quickly the fire returns to the growth stage and develops conditions that would be untenable after the window is opened at 12:17.
Fire development and changes in conditions following ventilation in this demonstration mirror those seen in the full scale experiments conducted by UL. Increasing ventilation to a ventilation controlled fire, results in increased heat release rate and transition from decay to the growth stage of fire development.
The same phenomena can be observed under fireground conditions in the following video clip of a residential fire in Dolton, Illinois (this is a long video, watch the first several minutes to observe the changes in fire behavior).
It appears that the front door was open at the start of the video clip and the large picture window on Side A was ventilated at approximately 00:47. Fire conditions quickly transition to the growth stage with flames exiting the window and door, causing firefighters on an uncharged hoseline that had been advanced into Floor 1, to quickly withdraw.
Fires that have progressed beyond the incipient stage are likely to be ventilation controlled when the fire department arrives.
Ventilation controlled fires may be in the growth, decay, or fully developed stage.
Regardless of the stage of fire development, when a fire is ventilation controlled, increased ventilation will always result in increased HRR.
Firefighters and fire officers must recognize that the ventilation profile can change (e.g., increasing ventilation) as a result of tactical action or fire effects on the building (e.g., window failure).
Firefighters and fire officers must anticipate potential changes in fire behavior related to changes in the ventilation profile and ensure that fire attack and ventilation are closely coordinated.
Coordinated Tactical Operations
Understanding how fire behavior can be influenced by changes in ventilation is essential. But how can firefighters put this knowledge to use on the fireground and what exactly does coordination of tactical ventilation and fire attack really mean?
Tactical ventilation can be defined as the planned, systematic, and coordinatedremoval of hot smoke and fire gases and their replacement with fresh air. Each of the elements of this definition is important to safe and effective tactical operations.
Ventilation (both tactical and unplanned) not only removes hot smoke, but it also introduces fresh air which can have a significant effect on fire behavior.
Tactical ventilation must be planned; these two elements speak to the intentional nature of tactical ventilation. Tactics to change the ventilation profile must be intended to influence the fire environment or fire behavior in some way (e.g., raise the level of the upper layer to increase visibility and tenability). The ventilation plan must also consider the flow path (e.g., vent ahead of, not behind, the attack team; vent in the immediate area of the fire, not at a remote location).
Tactical ventilation must be systematic, exhaust openings should generally be made before inlet openings (particularly when working with positive pressure ventilation or when taking advantage of wind effects).
And as pointed out in the UL Study (Kerber, 2011), tactical ventilation must be coordinated. Coordination of ventilation and other tactical operations requires consideration of sequence and timing:
Sequence: Ventilation may be completed before, during, or after fire attack has been initiated. Sequence will likely depend on the stage of fire development, burning regime, time required to reach the fire.
If the fire is small and staffing is limited, it may be appropriate to control the fire and then effect ventilation (e.g., hydraulic ventilation performed by the attack team). This approach minimizes potential fire growth,
In general, when the fire is ventilation controlled (as those beyond the incipient stage are likely to be), ventilation should not be completed unless the attack line(s) can quickly apply water to the seat of the fire. In a small, single family dwelling this may mean that the attack team is on-air, the line is charged, and the entry door is unlocked or has been forced and is being controlled (held closed). In a larger building, this may mean that the attack line has entered the structure and is in position to move onto the fire floor or into the fire area.
The key questions that must be answered prior to implementing tactical ventilation are:
What influence will these ventilation tactics have on fire behavior?
Are charged and staffed attack line(s) in place?
Will the attack team(s) be able to quickly reach the fire?
How will this impact crews operating on the interior of the building?
Coordination requires clear, direct communication between companies or crews assigned to ventilation, fire attack, and other tactical functions that are or will be taking place inside the building.
Important: While not a tactical implication directly raised by the UL study, another important consideration is the hazard of working without or ahead of the hoseline. While a controversial topic in the US fire service (where truck company personnel generally work on the interior without a hoseline), searching with a hoseline provides a means of protection and a defined exit path. Staffing is another key element of the operational context. If you do not have enough personnel to control the fire and search; in most cases it is likely the best course of action to control the fire and ensure a safer operating environment for search operations.
What’s Next?
The next tactical implication identified in the UL study (Kerber, 2011) examines information that may be obtained by reading the air track at the entry point opening. This implication will be expanded with a broader discussion of air track indicators and how related hazards can be mitigated to improve firefighter safety.
Note: Figure 4 illustrates temperature conditions starting eight minutes after ignition. The fire previously progressed through incipient and growth stages before beginning to decay due to lack of ventilation.
Why the Dramatic Change in Conditions?
As discussed in UL Tactical Implications Part 1 [LINK], Fires in the contemporary environment progress from ignition and incipient stage to growth, but often become ventilation controlled and begin to decay, rather than continuing to grow into a fully developed fire. This ventilation induced decay continues until the ventilation profile changes (e.g., window failure due to fire effects, opening a door for entry or egress, or intentional creation of ventilation openings by firefighters. When ventilation is increased, heat release rate again rises and temperature climbs with the fire potentially transitioning through flashover to the fully developed stage (see Figure 4 and 5).
Developing and maintaining proficiency in reading the Fire using the B-SAHF (Building, Smoke, Air Track, Heat, and Flame) organizing scheme for fire behavior indicators, requires practice. This post provides an opportunity to exercise your skills using a video segment shot during a commercial fire.
Residential Fire
This post examines fire development during a residential fire in New Chicago, Indiana.
Watch the first 30 seconds (0:30) of the video. First, describe what you observe in terms of the Building, Smoke, Air Track, Heat, and Flame Indicators; then answer the following five standard questions?
What additional information would you like to have? How could you obtain it?
What stage(s) of development is the fire likely to be in (incipient, growth, fully developed, or decay)?
What burning regime is the fire in (fuel controlled or ventilation controlled)?
What conditions would you expect to find inside this building?
How would you expect the fire to develop over the next two to three minutes
In addition, consider how the answers to these questions impact your assessment of the potential for survival of possible occupants.
Now watch the video clip from 0:30 until firefighters make entry at 3:05. Now answer the following questions:
Did fire conditions progress as you anticipated?
What changes in the B-SAHF indicators did you observe?
How do you think that the stage(s) of fire development and burning regime will change over the next few minutes?
What conditions would you expect to find inside this building now?
How would you expect the fire to develop over the next two to three minutes
The crews working in this video appeared to achieve fire control fairly quickly and without incident. However, consider the following tactical and task related questions:
It did not appear that any member of the first arriving companies performed a 360o recon and size-up (they may have, but this was not visible in the video). Why might this be a critical step in size-up at a residential fire?
It appeared that two lines were run simultaneously (the first line to the door ended up as the back-up line, possibly due to a slight delay in charging the line). How should fire attack and backup roles be coordinated?
Fire attack was initiated from the interior (unburned side). What would have been the impact of the first line darkening the fire from the exterior (prior to entry)?
Were there any indicators of potential collapse (partial) of the roof? How would you manage this risk when working in a lightweight wood frame residence with observed extension into the trussloft? What factors would influence your decision-making and actions?
Reading the Fire
See the following posts for more information on reading the fire:
UL’s third tactical implication is that there may be little smoke showing when a fire initially enters the decay stage as a result of limited ventilation. These fire conditions may present similar indicators to an incipient fire. However, fire conditions and the hazards presented to firefighters are considerably different.
Visible Indications of Fire Development
In Reading the Fire: B-SAHF, I introduced Building, Smoke, Air Track, Heat, and Flame (B-SAHF) as an organizing scheme for fire behavior indicators. Use of a standardized and organized approach to reading the fire can improve our ability to assess current fire conditions and predict likely fire development and changes that may occur.
Station Officer Shan Raffel of Queensland (Australia) Fire Rescue recently published an excellent article titled The Art of Reading Fire on the FirefighterNation website that provides another view of the B-SAHF indicators and Reading the Fire.
Building factors (particularly the normal ventilation profile, size and compartmentation, and thermal characteristics) can have a significant impact on fire development and how fire conditions present from the exterior of the building. However, this UL tactical implication relates most closely to Smoke and Air Track as well as somewhat indirectly to Heat (but this is the key to understanding what is happening). First a quick review of these key indicators
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.
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).
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. Visual clues such as crazing of glass and visible pyrolysis from fuel that has not yet ignited are also useful heat related indicators. Important: Temperature influences smoke and air track indicators such as volume and velocity of smoke discharge.
For a more detailed look at B-SAHF and reading the fire, see the following posts:
Stages of Fire Development, Burning Regime, Smoke, Air Track & Heat
While the “stages of fire” have been described differently in fire service textbooks the phenomenon of fire development is the same. For our purposes, the stages of fire development in a compartment will be described as incipient, growth, fully developed and decay (see Figure 1). Despite dividing fire development into four “stages” the actual process is continuous with “stages” flowing from one to the next. While it may be possible to clearly define these transitions in the laboratory, in the field it is often difficult to tell when one ends and the next begins.
Understanding the stages of fire development is important, but this only provides a limited picture of fire development in a compartment. Conversion of chemical potential energy from fuel depends on availability of adequate oxygen for the combustion reaction to occur. As the ambient air in the compartment provides adequate oxygen, in incipient stage and early growth stage, heat release rate is limited by the chemical and physical characteristics of the fuel. This condition is known as a fuel controlled burning regime. In a compartment fire, combustion occurs in an enclosure where the air available for combustion is limited by 1) the volume of the compartment and 2) ventilation. Ventilation in a compartment fire is limited (particularly if doors and windows are closed and intact), as the fire grows and heat release rate increases, so too does demand for oxygen. When fire growth is limited by the available oxygen, heat release rate is slowed and then diminishes. This condition is known as a ventilation controlled burning regime.
Many if not most fires that have progressed beyond the incipient stage when the fire department arrives are ventilation controlled. This means that the heat release rate (the fire’s power) is limited by the existing ventilation. If ventilation is increased, either through tactical action or unplanned ventilation resulting from effects of the fire (e.g., failure of a window) or human action (e.g., exiting civilians leaving a door open), heat release rate will increase (see Figure 1)
Figure 1. Fire Development Curve (Fuel and Ventilation Controlled Regimes)
Several things happen as a compartment fire develops: Heat release rate increases, smoke production increases, and pressure within the compartment increases proportionally to the absolute temperature. These conditions result in a number of fire behavior indicators that may be visible from the exterior of the building. As a fire moves from the Incipient to the Growth Stage, an increasing volume of smoke may be visible from the exterior (Smoke Indicator) and the velocity of smoke discharge will likely increase (Air Track Indicator and indirect Heat Indicator).
It is a reasonably logical conclusion that a smaller volume of smoke and lower velocity of smoke discharge will be observed in incipient and early growth stage fires and the volume and velocity of smoke discharge will increase as the fire develops. However, what happens when the fire becomes ventilation controlled?
Influence of Ventilation on Residential Fire Behavior
Earlier this year, Underwriters Laboratories (UL) conducted a series of full-scale experiments to determine the influence of ventilation on fire behavior in legacy and contemporary residential construction (see Did You Ever Wonder? and UL Ventilation Course).
These tests were conducted in full-scale one and two-story, wood-frame structures constructed inside the UL laboratory in Northbrook, IL. Fires in the one-story structure were all started in the living room (see Figure 2) and involved typical contents found in a single-family home.
Figure 2: One-Story Structure and Floor Plan
As discussed in UL Tactical Implications Part 1and Part 2, each of the fires during these tests quickly became ventilation controlled due to the fuel load within the buildings and limited ventilation provided by closed and intact doors and windows.
As each fire developed, the volume of smoke visible from the exterior and velocity of smoke discharge increased. This is consistent with fire development within the structure and increasing heat release rate, temperature, and volume of smoke production from the developing fire. Figure 3 illustrates exterior conditions at 05:05 during Test 5 conducted in the one-story residence.
Figure 3: Conditions at 05:05 (UL Test 5)
Interestingly, as the fire became ventilation controlled (as determined by both measurement of oxygen concentration and the heat release rate in the building), the volume and velocity of smoke discharge decreased to a negligible level as illustrated in Figure 4.
Figure 4: Conditions at 05:34 (UL Test 5)
This change occurred within a matter of 30 seconds! How might this influence firefighters’ perception of fire conditions inside the building if they arrived at 05:34 rather than 05:04? While presenting much the same as an incipient or early growth stage fire, conditions within the building at 05:34 are significantly ventilation controlled and increased ventilation resulted in rapid fire development and transition through flashover to a fully developed fire.
NIST Phoenix Warehouse Tests
In the Hazard of Ventilation Controlled Fires, I discussed a series of tests conducted by the National Institute for Standards and Technology (NIST) at an ordinary constructed warehouse in Phoenix, AZ. These tests were intended to develop information about performance of ordinary constructed buildings related to structural collapse. However, they also provided some interesting information regarding fire behavior.
One of the tests involved a fire in the front section of the warehouse that measured 50’ x 90’ (15.2 m x 27.4 m) with a height of 15’ (4.6 m) to the top of the pitched truss roof. The fuel load for this test included four stacks of 10 wood pallets and the interior finish and combustible structural elements of the building. All doors and windows were closed at the start of the test.
Figure 5 illustrates a large volume of dark gray to black smoke discharging from the roof of the structure and flames visible from roof ventilators at 03:57. The B-SAHF indicators visible n this photo indicate a significant growth stage fire within this building.
Figure 5. Conditions at 03:57 (NIST Warehouse Test)
However, at 05:31 conditions visible on the exterior are quite different. The color and volume of smoke discharge (Smoke Indicators) as well as the velocity of discharge (Air Track Indicator) may lead firefighters to believe that this is an incipient or early growth stage fire. Nothing could be further from the truth. This fire is in the decay stage as a result of limited ventilation and any increase in ventilation will result in a rapid and significant increase in heat release rate!
Figure 6. Conditions at 05:31 (NIST Warehouse Test)
Heat release rate and temperature drop as the fire becomes ventilation controlled. Volume and velocity of smoke discharge are a function of pressure (given a constant opening size). Reduction in temperature and corresponding reduction in pressure will result in a smaller volume and lower velocity of smoke discharge.
When the temperature is the same, the velocity of discharge will likely be similar. Figure 1 shows that the temperature inside a compartment may be the same during the growth and decay stages of the fire. If the ventilation profile (number, size, and location of openings) remains the same, similar Smoke and Air Track indicators can be present.
Decay stage incidators may be subtle. Consider the full range of B-SAHF inciators that may be observed under ventilation controlled, decay stage conditions (see Figure 7.
Figure 7. B-SAHF Decay Stage Indicators.
Durango, Colorado Commercial Fire
CFBT-US developed a case study examining an extreme fire behavior event that occurred during a commercial fire in Durango, CO in 2008 injuring nine firefighters and fire officers. The reporting party indicated that there was a large amount of dark smoke coming from the roof of the building. However, when firefighters arrived, they found nothing showing but a small amount of light colored smoke. Why might this have been the case?
Download a copy of the Fire Behavior Case Study: Durango CO Commercial Fireand see if this may have been a result of similar fire development and presentation of fire behavior indicators as seen in the UL and NIST tests!
Stroup, D., Madrzykowski, D., Walton, W., & Twilley, W. (2003). Structural collapse fire tests: Single story, ordinary construction warehouse, NISTIR 6959. Retrieved July 16, 2011 from http://www.nist.gov/customcf/get_pdf.cfm?pub_id=861215
A recent video posted on the firevideo.net [http://firecamera.net/] web brought to mind a number of painful lessons learned regarding live fire training in acquired structures. When watching video of fire training or emergency incidents, it is essential to remember that video provides only one view of the events. This video, titled Probationary Live House Burn shows a live fire evolution from ignition through fire attack with the comment “Burnin up the probies… LOL”.
This video shows multiple fire locations and an extremely substantial fire load (well in excess of what is necessary to bring typical residential compartments to flashover). I am uncertain if the comment posted with the video “burnin up the probies…LOL [laughing out loud]” was posted by an instructor or learner. Likely this is considered as just a joke, but comments like this point to our collective cultural challenges in providing safe and effective live fire training.
Fuel Load & Ventilation in Live Fire Training
NFPA 1403 Standard on Live Fire Training is reasonably explicit regarding the nature of acceptable fuel, extent of fuel load, as well as number and location of fires used for live fire training in acquired structures.
4.3.1 The fuels that are utilized in live fire training evolutions shall have known burning characteristics that are as controllable as possible.
4.2.17 Combustible materials, other than those intended for the live fire training evolution, shall be removed or stored in a protected area to preclude accidental ignition.
4.3.3* Pressure-treated wood, rubber, and plastic, and straw or hay treated with pesticides or harmful chemicals shall not be used.
A.4.3.3 Acceptable Class A materials include pine excelsior, wooden pallets, straw, hay, and other ordinary combustibles.
Fuel materials shall be used only in the amounts necessary to create the desired fire size.
A.4.3.4 An excessive fuel load can contribute to conditions that create unusually dangerous fire behavior. This can jeopardize structural stability, egress, and the safety of participants.
4.3.5 The fuel load shall be limited to avoid conditions that could cause an uncontrolled flashover or backdraft.
4.4.15 Only one fire at a time shall be permitted within an acquired structure.
4.4.16 Fires shall not be located in any designated exit paths.
While quite explicit regarding fuel requirements and limitations, NFPA 1403 (2007) has little to say about the ventilation with the exception of a brief mention that roof ventilation openings that are normally closed but may be opened in an emergency are permitted (not required as many believe). However, the Appendix has a much more important statement regarding the importance of ventilation to fire development:
A.4.3.7 The instructor-in-charge is concerned with the safety of participants and the assessment of conditions that can lead to rapid, uncontrolled burning, commonly referred to as flashover. Flashover can trap, injure, and kill fire fighters. Conditions known to be variables affecting the attainment of flashover are as follows:
(1) The heat release characteristics of materials used as primary fuels
(2) The preheating of combustibles
(3) The combustibility of wall and ceiling materials
(4) The room geometry (e.g., ceiling height, openings to rooms [emphasis added])
In addition, the arrangement of the initial materials to be ignited, particularly the proximity to walls and ceilings, and the ventilation openings [emphasis added] are important factors to be considered when assessing the potential fire growth.
The building in this video appeared to have been used for multiple evolutions prior to the one depicted in the video. A number of the windows appeared to be damaged, providing increased ventilation to support combustion. The fuel load of multiple pallets and excelsior or straw (acceptable types of fuel) provided an excess of fuel required to reach flashover in typical residential rooms (which may have been an intended outcome and level of involvement given the transitional attack (defense to offense)). If in fact the sets were in multiple rooms, this would be inconsistent with the provisions of NFPA 1403 limiting acquired structure evolutions to a single fire.
It is essential for those of us who conduct live fire training to remember that most of the provisions of NFPA 1403 (2007) are based on line-of-duty deaths of our brothers and sisters. Safe and effective live fire training requires that instructors be technically competent, well versed in the requirements or relevant regulations and standards, and that individually and organizationally we have an appropriate attitude towards safe and effective learning and the process of passing on the craft of firefighting.
One useful case to focus discussion of these issues is the death of Firefighter/Paramedic Apprentice Rachael Wilson of the Baltimore City Fire Department:
At 4:56 in the video, accumulation of a layer of smoke is clearly visible under the porch roof. No comment is made about this by the instructors and no action is taken to mitigate the hazard. At 5:55, flames exiting a broken window to the left of the door ignite the smoke layer just prior to when the attack team opens the door.
Figure 1. Fire Gas Ignition Sequence
It is essential to recognize that smoke is fuel and that ignition of this gas phase fuel overhead results in a rapid and signfiicant increase in radiant heat flux (which is dependent largely on temperature and proximity). Cooling the gases overhead and use of good door entry technique can minimize risk of this thermal insult to firefighters and potential for transition to other types of extreme fire behavior such as flashover.
Fire Streams
This video also shows some interesting aspects of fire stream application. A solid (or straight) stream can be quite effective in making a direct attack on the fire. However, when the fire is shielded, the effectiveness of this type of stream is limited. While limited steam production is often cited as an advantage of solid (and straight) streams, initial application of water through the doorway in this video results in significant steam production and limited effect on the fire. This is likely due to shielding of the burning fuel by interior configuration and compartmentation. Remember than no single type of fire stream is effective for all applications.
Perspective
Consider the question posed in the title of this post: Was this a safe and effective live fire training session or a near miss? I suspect that the learners in the video enjoyed this live fire training session and that the instructors desired to provide a quality learning experience. It is even likely that this evolution was conducted substantively (but likely not completely) in compliance with the provisions of NFPA 1403. Like most training exercises and emergency incidents, it is easy to watch a video and criticize the actions of those involved. I do not question the intent of those involved in this training exercise, but point to some issues that we (all of us) need to consider and reflect on as we go about our work and pass on the craft to subsequent generations of firefighters.
What’s Next?
I am working hard at getting back into a regular rhythm of posting and hope to have a post looking at another of the Tactical Considerations from the UL ventilation study up within the next week.
Ed Hartin, MS, EFO, MIFireE, CFO
References
National Fire Protection Association. (2007). NFPA 1403 Standard on live fire training. Quincy, MA: Author.
Is making entry with a hoseline for fire attack, ventilation? Is entering through a doorway when conducting search, ventilation? While many firefighters do not think about ventilation when performing these basic fireground tasks, the answer is a resounding yes!
Making Entry is Ventilation
While the Essentials of Firefighting (IFSTA, 2008) defines ventilation as “the systematic removal of heated air, smoke, and fire gases from a burning building and replacing them with cooler air” [emphasis added] (p. 541), the main focus of most ventilation training is on the exhaust opening. In discussing compartment fire development, the 6th Edition of Essentials (IFSTA, 2008) includes a discussion of the concept of fuel and ventilation controlled burning regimes. In addition, the section of the text addressing the positive effects of ventilation such as reducing potential for flashover and backdraft, Essentials (IFSTA, 2008) cautions that increasing ventilation to ventilation limited fires may result in rapid fire progression. However, these concepts were not included in earlier additions and the connection between openings made for the purpose of ventilation and openings made for other reasons is often overlooked.
Ventilation versus Tactical Ventilation
Despite the definitions given in fire service text that describe ventilation in terms of actions taken by firefighters, ventilation is simply the exchange of the atmosphere inside a building with that which is outside. Normal air exchange between the interior and exterior of a building is expressed as the number of complete air exchanges (by volume) per hour and varies depending on the purpose and function of the space. In residential structures, the air in the building is completely exchanged approximately four times per hour. In commercial and industrial buildings this rate may be significantly higher, depending on use. When firefighters arrive to find smoke issuing from a building, ventilation is occurring and when firefighters open a door to make entry, the ventilation profile changes as ventilation has been increased. Remember:
If smoke exits the opening (air is entering somewhere else) ventilation is occurring.
If air enters the opening (smoke is exiting somewhere else), ventilation is occurring.
If smoke exits and air enters the opening ventilation is occurring.
The entry point is a ventilation opening and if the fire is ventilation controlled, any ventilation opening will increase heat release rate (HRR)!
Ventilation Controlled Fires
As discussed in Influence of Ventilation in Residential Structures: Tactical Implications Part 1 [LINK], compartment fires that have progressed beyond the incipient stage are likely to be ventilation controlled when the fire department arrives. Firefighters and fire officers must recognize the potential for a rapid increase in HRR when additional atmospheric oxygen is provided to ventilation controlled fires. This is particularly important when considering door entry and door control during fire attack, search, and other interior operations. The Underwriters Laboratories (UL) research project Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction (Kerber, 2011) examined fire behavior in a small, single-story, wood frame house and a larger, two-story, wood frame house (see Figures 1 & 2) Figure 1. Single-Story Legacy Dwelling
Figure 2. Two-Story Contemporary Dwelling
Each of the fires in these experiments occurred in the living room (one-story house) or family room (two-story house). While the fuel load was essentially the same, the family room had a much greater volume as it had a common cathedral ceiling with an atrium just inside the front door. Experiments one and two examined fire behavior in each of these structures with the front door being opened at the simulated time of arrival of the fire department. Figure 3 illustrates the changes in temperature during UL Experiment 1 (Single Story-Door as Vent Opening) and Experiment 2 (Two-Story-Door as Vent Opening). Figure 3. Living/Family Room Temperature Curves-Door as Ventilation Opening
As illustrated in Figure 3, temperature conditions changed dramatically and became untenable shortly after the front door was opened. In the one-story experiment (Experiment 1) temperature:
Was 180 °C (360 °F) at ventilation (480 s),
Exceeded the firefighter tenability threshold of 260 °C (500 °F) at 550 s
Reached 600 °C (1110 °F) at 650 s and transitioned through flashover to a fully developed fire in the living room
In the two-story experiment (Experiment 2) temperature:
Was 220 °C (430 °F) at ventilation (600 s)
Exceeded the firefighter tenability threshold of 260 °C (500 °F) at 680 s
Reached 600 °C (1110 °F) at 780 s and transitioned through flashover to a fully developed fire in the family room
When the door is opened the clock is ticking! HRR will increase and the tenability within the fire compartment and adjacent spaces will quickly deteriorate unless water can be applied to control the fire. Keeping the door closed until ready to make entry delays starting the clock. Closing the door after entry (leaving room for passage of your hoseline) slows fire development and buys valuable time to control the fire environment, locate the fire, and achieve fire control.
Just as in the UL experiments on the influence of ventilation in residential structures, heat release rate will increase and fire conditions can change dramatically when a door is opened for access and entry.
In 2008, two firefighters from the Riverdale Volunteer Fire Department in Prince Georges County Maryland recently were surprised by a flashover in a small, single family dwelling. In the first photo, firefighters from Engine 813 and Truck 807 prepare to make entry. Note that the front door is closed, the glass of the slider and windows are darkened, and smoke can be observed in the lower area of the front porch. Six seconds later it appears that the front door has been opened, flames are visible through the sliding glass door, and the volume of smoke in the area of the porch has increased. However, the smoke is not thick (optically dense). Forty eight seconds later, as the crew from Truck 807 makes entry to perform horizontal ventilation the volume of smoke from the front door increases and thickens (becomes more optically dense).
Figure 4. Ventilation Induced Flashover-Door as a Ventilation Opening
Note: Photos by Probationary Firefighter Tony George, PGFD The crew from Engine 813 experienced a burst hoseline, delaying fire attack. Two minutes after the first photo, and shortly after the crew from Truck 807 made entry, flashover occurred. For additional information on this incident, see Situational Awareness is Critical.
Door Control
The issue of door control presents a similar (and related) paradox as ventilation. Ventilation is performed to improve interior tenability and to support fire control, but when presented with a ventilation controlled fire, increased air supply increases HRR and can result in worsening tenability and potential for extreme fire behavior. Firefighters often chock doors open to provide ease of hoseline deployment and an open egress path, but when the fire is ventilation controlled, this (ventilation) opening starts the clock on increased HRR and rapid fire development. It is useful to consider door control in two phases, door entry procedures and control of the door after entry.
Size-Up: Door entry begins with a focused size-up as you approach the building. Assessment of conditions is not only the incident commander or officer’s job. Each member entering the building should perform a personal size-up and predict likely conditions. When making entry, size-up becomes more closely focused on conditions observed at or near the door and includes an assessment of potential forcible entry requirements as well as B-SAHF (Building-Smoke, Air Track, Heat, and Flame) indicators. If available, a thermal imaging camera (TIC) can be useful, but remember that temperature conditions may be masked by the thermal characteristics of the building. If a thermal imaging camera is not available, application of a small amount of water to the door may indicate temperature and the level of the hot gas layer (water will vaporize on contact with a hot door).
Size-up begins as you exit the apparatus and approach the building, but continues at the door and after you make entry!
At the door, pay close attention to air track and heat (door temperature) indicators as these can provide important clues to conditions immediately inside the building!
Prior to Entry: If the door is open, close it. If it is closed, don’t open it until you are ready. If the door is unlocked, control is generally a simple process (see Nozzle Techniques and Hose Handling: Part 3 for detailed discussion of door entry when the door is unlocked).
If the door is locked and must be forced, this adds an element of complexity to the door entry process. In selecting a forcible entry method, consider that the door must remain intact and on its hinges if you are going to maintain control of the air track at the opening.
This is fairly easy with outward opening doors. Inward opening doors present a greater challenge. A section of webbing or rope can be used to control an inward door by placing a cinch hitch around the door knob (see Figure 5). As the the door is forced, it can be pulled closed. However, if the door was not locked with a deadbolt, it may re-latch when pulled closed. Figure 5. Door Control with Web or Utility Rope
Figure 5. Door Control with Webbing or Utility Rope
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Alternately, a Halligan or hook may be used to capture the door and pull it substantially closed after it is forced (see Figure 6).
Figure 6. Door Control with a Tool
If B-SAHF (Building, Smoke, Air Track, Heat, & Flame) indicators point to hazardous conditions on the other side of the door, forcible entry must be integrated with good door entry procedure to control potential hazards. After Entry: The most effective way to control the door after entry and provide ease of egress is to have a firefighter remain at the opening to control the door and feed hose to the hose team working inside (see Figure 7).
Figure 7. Door Control After Entry
Note: The Firefighter maintaining door control would be wearing complete structural firefighting clothing and breathing apparatus (this is simply an illustration of door control with a hoseline in place)!
Unfortunately, many companies do not have sufficient staff to maintain a nozzle team of two and leave another firefighter at the door. In these cases, it may be possible for the standby firefighters (two-out) to control the door for the crew working inside until additional resources are available.
Nozzle Technique & Hose Handling
Prior posts on nozzle technique and hose handling included a series of drills to develop proficiency in critical skills.
Review these nine drills and then extend your proficiency by integrating forcible entry with good door entry procedure by maintaining control of the door.
Drill 10-Door Entry-Forcing Inward Opening Doors: Many doors (particularly interior and exterior residential) open inward. In this situation forcible entry, door control, and nozzle operation must bee closely coordinated. Practicing these techniques under a variety of conditions (e.g., wall locations, compartment sizes) is critical to developing proficiency.
Drill 11-Door Entry-Forcing Outward Opening Doors: Commercial (and some interior residential doors) open outward. While less complex, Firefighters must develop skill in integration of forcible entry, door control, and nozzle technique in this situation as well.
In both of these drills, focus on maintaining control of the door during forcible entry and limit air intake by keeping the door as closed as possible while passing the hoseline as it is advanced.
UL Research on the Impact of Ventilation on Fire Behavior
Earlier this year, Underwriters Laboratories (UL) conducted a series of full-scale experiments to determine the influence of ventilation on fire behavior in legacy and contemporary residential construction (see Did You Ever Wonder?).
This series of posts will examine twelve tactical implications identified by UL in the research report and on-line course.
Burning Regime and Stages of Fire Development
As illustrated in Figure 1, the traditional fire development curve used to illustrate changes in heat release rate and temperature over time as a compartment fire develops shows ignition and the incipient stage of fire development followed by growth, a fully developed fire, and decay. While this is reasonably representative of combustion of a single fuel package under unconfined and well ventilated conditions, it often does not reflect the reality of today’s fire environment as illustrated by a near miss in Springfield, VA on January 22, 2011. Three firefighters from Fairfax County Fire Rescue were caught in flashover which occurred after initial firefighting operations were commenced in a two-story townhouse.
On February 4, 2011, five firefighters in New Hyde Park, NY were caught by rapid fire progression while advancing a hoseline into a basement fire in a single family dwelling (more information here).
These incidents point to the importance of a sound understanding of practical fire dynamics and the influence of ventilation on fire behavior.
In the contemporary fire environment, fires progress from ignition and incipient stage into growth, but here is where things change. Instead of progressing through growth to the fully developed stage, fires often become ventilation controlled and begin to decay. Decay continues until the ventilation profile changes (e.g., window failure due to fire effects, opening a door for entry or egress, or intentional creation of ventilation openings by firefighters. When ventilation is increased, heat release rate again rises and temperature climbs with the fire potentially transitioning through flashover to a the fully developed stage.
Figure 1. Fire Development in a Compartment
Oxygen Consumption and Energy Release
That the energy released by combustion is related to the oxygen consumed in the reaction is not a new idea: “The door should be kept shut while the water is being brought, and the air excluded as much as possible, as the fire burns exactly in proportion to the quantity of air which it receives” (Braidwood, 1866, p. 64). For the time, James Braidwood, first Chief of the City of London Fire Brigade had a remarkable understanding of combustion. Despite this practical understanding of oxygen and release of energy through combustion, it wasn’t until 50 years later that this relationship was quantified. In 1917, British scientist W.M. Thornton discovered that while the heat of combustion of various types of organic (carbon based) fuel varies widely, the amount of oxygen required for release of a given amount of energy remains remarkably consistent (Thornton, 1917).
While the heat release of 13.1 MJ/kg (13.1 kJ/g) of oxygen consumed during combustion is often referred to as Thornton’s Rule, discovery of this concept and quantification of this value under a variety of conditions was the work of a number of individuals. For example, in the 1970’s, researchers at the National Bureau of Standards (now the National Institute of Standards and Technology, NIST) independently discovered the same thing and extended this work to include many other types of organic materials and examined both complete and incomplete combustion (Parker, 1977; Huggett, 1980).
Heat release during combustion is dependent on oxygen. However, the atmosphere is comprised of only 21% oxygen. Examining the relationship between consumption of atmospheric oxygen and energy release requires adaptation of Thornton’s Rule based on oxygen concentration. Multiplying 13.1 MJ/kg of oxygen by 21% gives a value of 2.751 MJ/kg of air. The Society of Fire Protection Engineering (SFPE) Handbook of Fire Protection Engineering (SFPE, 2002) rounds this value to 3.0 MJ/kg of air. While it is easy to understand that air has mass, it may be a bit more difficult to visualize a kilo of air! The density of dry air at sea level and at a temperature of 20o C is 1.2 kg/m3 (0.075 lbs./ft3). Air density decreases as temperature or moisture content of the air increases, but this provides a starting point for visualizing the relationship between volume and mass at normal temperature and pressure.
As illustrated in Figure 1, multiplying the mass of a cubic meter of air (1.2 kg) by the energy released per unit mass of air (3.0 KJ/kg) provides an approximation of the energy released when the oxygen in one cubic meter of air is consumed in a combustion reaction.
Figure 2. Energy Release per Cubic Meter of Dry Air
Oxygen to support energy release resulting from combustion occurring within a closed compartment is substantially (but not entirely) limited to the mass of air in the compartment. Normal air exchange between the interior and exterior of a building is expressed as the number of complete air exchanges (by volume) per hour and varies depending on the purpose and function of the space. In residential structures, the air in the building is completely exchanged approximately four times per hour. In commercial and industrial buildings this rate may be significantly higher, depending on use.
Designed air exchange and leakage provide additional oxygen that can support ongoing combustion, but this is generally not a major factor in buildings where the windows and doors are closed and intact.
Oxygen Concentration and Ventilation Controlled Fires
Energy release as a result of combustion is directly proportional to the oxygen consumed in the reaction. However, when a fire is burning in an oxygen limited environment such as an enclosed space, not all of the oxygen can be used to support flaming combustion. As observed by Mowrer, “A diffusion flame immersed in a vitiated [oxygen limited] atmosphere will extinguish before consuming all the available oxygen from the atmosphere” (McGrattan, Hostikka, Floyd, Baum, & Rehm, 2008, p. 85).
As oxygen within a compartment is consumed, fire growth becomes limited by ventilation (inclusive of the air within the compartment at ignition and the ongoing air exchange). Ventilation becomes the dominant factor in fire development when the oxygen concentration is between 14 and 16 %.
Oxygen Concentration and Flaming Combustion
As temperature increases, the oxygen concentration required to support flaming combustion decreases. Figure 3 illustrates the relationship between gas temperature and the concentration of oxygen required to support flaming combustion. Keeping in mind that temperature within involved and adjacent compartments can vary considerably, flaming combustion may be possible in some areas and not in others.
Figure 3. Oxygen Concentration Required for Flaming Combustion
Note: Adapted from Fire Dynamics Simulator (Version 5) Technical Reference Guide (p. 25), by K. McGrattan, S. Hostikka, J. Floyd, H Baum, & R. Rehm, 2008, National Institute of Standards and Technology.
Oxygen concentration required to support flaming combustion varies over a wide range based on temperature. However, in examining fire development in a single compartment or a residential structure, it is reasonable to use the value of 10.5% as the concentration required to support flaming combustion based on the fairly consistent temperatures of between 500o C and 600o C developed prior to ventilation (window failure, opening a door for access, or tactical ventilation operations). This assumption is based on analysis of the data from full scale residential fire tests conducted by Underwriters Laboratories in representative legacy and contemporary structures (Kerber, 2011).
Limitations on Fire Development in a Single Compartment
A fire in a single, small compartment with no openings (window and door closed) provides a simple example of the impact of limited oxygen on fire development. To estimate the potential for development of a ventilation limited conditions, the available oxygen must be compared to the oxygen that would likely be consumed during combustion of typical contents.
Figure 4. Single Compartment
As illustrated in Figure 4, a 3 m x 4 m x 2.44 m (9’ 10” x 13’ 1” x 8’) compartment contains 30 m3 (1059.44 ft3) of air. Given the oxygen concentration required for flaming combustion under conditions typically encountered in a developing fire (10.5 % O2 at a temperature of 600o C), half of this volume, 15 m3 (529.72 ft3) would be available to support flaming combustion in a rapidly developing fire. Multiplying 15 m3 x 3.6 MJ/m3 of dry air indicates that the total energy release before flaming combustion is substantially reduced or ceases is approximately 46.8 MJ.
A single wood and polyurethane upholstered chair (such as a recliner) is likely to have a heat of combustion of approximately 18 MJ/kg and have a mass of 28 kg (Bukowski, 1985). Multiplying the heat of combustion by the mass identifies the total potential energy of this single fuel package as being approximately 504 MJ, well in excess of the potential energy release provided by the oxygen available in the 3 m x 4 m x 2.44 m (9’ 10” x 13’ 1” x 8’) compartment.
While a single fuel package such as an upholstered chair provides sufficient fuel to develop a ventilation limited fire, the fuel load in a normal residential compartment is likely to be substantially greater. International Fire Engineering Guidelines indicate the 95% fractile fuel load for dwellings may be estimated as 970 MJ/m2 (95% of dwellings will have a value at or below this level) (Bukowski, 2006). Multiplying the compartment floor area 12 m2 by 970 MJ/m2 provides an estimated total fuel load of 11,640 MJ. This indicates that there is far less atmospheric oxygen in the compartment than required to fully oxidize the likely fuel load.
Joules are a measure of energy, in this case a measure of total energy released by the combustion reaction. This is important, but even more important is the energy released per unit of time or heat release rate (HRR).
Some fires develop slowly, consuming oxygen at a correspondingly slow rate. However, as illustrated in Figure 3 fires involving many contemporary furnishings develop quite quickly.
Figure 5. T2 Fire Development Curves
At a HRR of 0.5 MW (500 kW), a fire in the 3 m x 4 m x 2.44 m (9’ 10” x 13’ 1” x 8’) compartment illustrated in Figure 4, would become ventilation limited to the point where flaming combustion would be significantly diminished or cease in less than two minutes. As the fire develops and HRR increases, the time for the fire to become ventilation controlled and enter the decay stage becomes considerably less.
Calculation of the energy release or time to become ventilation controlled and enter the decay stage based on the volume of the room does not account for normal building ventilation (exchange of the atmosphere inside the building with that on the outside). With an exchange of four times per hour, the 3 m x 4 m x 2.44 m (9’ 10” x 13’ 1” x 8’) compartment containing 30 m3 (1059.44 ft3) of air would have an air exchange rate of 0.033 m3/s (1.65 ft3/s). Multiplying the 0.033 m3 of air exchanged per second by 3.6 MJ/m3 (of dry air) indicates that the normal air exchange would support an ongoing HRR of 118.8 kW (roughly the same peak heat release rate as combustion of a small trash can).
Important: Rapid transition from growth to decay stage is dependent on ventilation being limited to normal building air exchange with the door and windows in the compartment remaining closed and intact. Should the door be open, windows fail, or a combination of both, the fire may become ventilation limited, but continue in the growth stage and potentially transition through flashover to a fully developed fire.
Limitations on Fire Development in Multiple Compartment
Examination of fire development in a single, closed compartment provides a simple illustration of how compartment fire development is influenced by compartment volume and normal building ventilation. However, firefighters most commonly encounter fires in buildings comprised of multiple, interconnected compartments. Interior doors (particularly in residential occupancies) are frequently open, providing additional atmospheric oxygen for fire development and a pathway for smoke and fire travel from the compartment of origin into adjacent compartments.
In order to assess the potential for development of a ventilation limited, decay stage fire in the residential structures used in the UL experiments on the Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction, researchers conducted a series of heat release rate experiment using a 5.49 m x 3.96 m x 2.44 m (18’ x 13’ x 8’) compartment, similar to the living rooms in the experimental houses. The opening in the front of the room was 3.66 m x 2.13 m (12’ x 7’) simulating the interconnection between the living room and other areas of the experimental houses.
During the HRR experiment, 3647.01 MJ of thermal energy was released over the 19 minute experiment with 2268.44 MJ released in the first 10 minutes after ignition. The total atmospheric oxygen in the single-story legacy dwelling was sufficient to support release of 846.2 MJ of thermal energy if it was completely consumed. If, as in our single compartment example, the fire becomes ventilation limited at an oxygen concentration of 10.5%, a fire in this dwelling would become ventilation limited after a release of just over 423 MJ of thermal energy. To put this in perspective in relation to HRR, in the UL living room fuel package tests, HRR reached 1 MW within 4 minutes 30 seconds and 10 MW in 5 minutes 30 seconds (Kerber, 2011).
The full-scale burns in the single-story, legacy dwelling in Experiments 1 and 3 in UL’s research provide a graphic example of the influence of ventilation on fire behavior fire development and burning regime. Experiments 1 and 3 were conducted in the one-story 111.48 m2 (1200 ft2) ranch house (see Figure 6). While the floor plan was of legacy design, the dwelling was furnished with contemporary contents including furnishings, carpet, and carpet pad.
Figure 6. Configuration of the Single Story Legacy Dwelling
In each of the experiments conducted in this structure the fire was located in the Living Room. The only major variable was the location, sequence, and total area of horizontal ventilation openings provided. There was also variation in water application method (straight versus fog stream), but this was not the primary research focus.
The living room was furnished with two sofas, armoire, television, end table, coffee table, two pictures, lamp with shade, and two curtains. The floor was covered with polyurethane foam padding and polyester carpet. Additional detail on the fuel load used during these experiments (e.g., bedrooms, kitchen, and dining room) is provided in UL’ research report Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction(Kerber, 2011).
All of the experiments began with the windows and exterior door closed and all of the interior doors in the same position (open with the exception of the door to Bedroom 3 which was closed during all experiments). This ventilation profile remained constant until eight minutes (8:00) into the experiment at which point tactical ventilation was initiated. This timeframe was based on three factors: time to achieve ventilation limited conditions, potential fire service response and intervention time, and potential window failure time. After ventilation, fire development was allowed to progress to flashover or perceived maximum burning rate (based on temperature readings and observation of exterior and interior conditions).
At the conclusion of each experiment, a hand held hose stream was applied through a ventilation opening for 10 seconds. Two patterns were used, a straight stream and a 30o fog pattern positioned upward 30o to 40o from horizontal (e.g., directed towards the ceiling). The purpose of this manual water application was not to extinguish the fire completely, but to control flaming combustion in the upper layer and determine the impact of this type of water application on conditions in adjacent rooms. After manual water application with the hoseline, sprinklers were manually activated to complete the extinguishment process.
Experiment 1: This experiment was designed to simulate firefighters making entry through the front door by opening the front door eight minutes after ignition. No other ventilation openings were made in the time between opening the door and manual fire suppression with a hoseline. After manual fire suppression, the right half of the window in the living room (fire compartment) was opened.
Figure 7 illustrates the temperature and oxygen concentration at 1.24 (5’) meters above the floor throughout Experiment 1. What can be learned by examining the relationship between the temperature and oxygen concentration?
Figure 7. Experiment 1: Oxygen Concentration and Temperature at 1.24 m (5’)
As illustrated in Figure 7, it is apparent that the fire became ventilation controlled at approximately 325 seconds as oxygen concentration decreased to below 15%. However, understanding what happened next requires some thought. As previously illustrated in Figure 3, the concentration of oxygen required to support flaming combustion is dependent on temperature. The higher the temperature, the less oxygen that is required for flaming combustion. Limited ventilation can result in the fire progression into the decay stage, as it did in Experiment 1 prior to opening of the door on Side A (see Figure 6). However, increased ventilation and transition through flashover to a fully developed fire, results in burning that is still ventilation limited, as evidenced by oxygen concentrations that do not exceed 13% throughout the remainder of the experiment. The implications of a ventilation limited, but fully developed fire are a rapid, further increase in HRR (and temperature) if additional ventilation is provided. This is illustrated by the rapid temperature rise and significantly higher temperature following opening of the left half of the living room window (see Figure 7)
Experiment 3: This experiment was designed to simulate firefighters making entry through the front door and having a ventilation opening made shortly after near the seat of the fire. In this experiment the front door was opened eight minutes after ignition and the living room window was opened 15 seconds later. At the conclusion of the experiment, water from a hoseline was applied through the living room window for 10 seconds prior to manual activation of the sprinkler system.
Figure 8 illustrates the temperature and oxygen concentration at 1.24 (5’) meters above the floor throughout Experiment 3. In what ways is this similar to the relationship of oxygen concentration and temperature in Experiment 1? In what ways is it different? What can be learned by comparing the results of these two experiments?
Figure 8. Experiment 3: Oxygen Concentration and Temperature at 1.24 m (5’)
Fires that have progressed beyond the incipient stage are likely to be ventilation controlled when the fire department arrives.
Ventilation controlled fires may be in the growth, decay, or fully developed stage.
Regardless of the stage of fire development, when a fire is ventilation controlled, increased ventilation will always result in increased HRR.
Firefighters and fire officers must recognize that the ventilation profile can change (e.g., increasing ventilation) as a result of tactical action or fire effects on the building (e.g., window failure).
Firefighters and fire officers must anticipate potential changes in fire behavior related to changes in the ventilation profile and ensure that fire attack and ventilation are closely coordinated.
These key points do not mean that (planned, systematic, and coordinated) tactical ventilation is inappropriate. Simply that firefighters and fire officers must recognize the potential for a rapid increase in HRR when additional atmospheric oxygen is provided to ventilation controlled fires. This necessitates anticipating unplanned ventilation as a result of fire effects and close coordination of fire attack and tactical ventilation operations.
Any opening that introduces air or allows smoke to escape is a ventilation opening! Often, the most significant ventilation opening during initial operations is the door through which firefighters make entry. The next post in this series will examine tactical consideration of the entry point as a ventilation opening.
Society of Fire Protection Engineers (SFPE). (2002). The SFPE handbook of fire protection engineering. Quincy, MA: National Fire Protection Association.
There were multiple near miss incidents and injuries involving flashover during the month of December. These incidents point to the importance of understanding fire dynamics and reading the fire as part of initial size-up and ongoing dynamic risk assessment. Each member operating on the fireground must maintain a high level of situational awareness and communicate key fire behavior indicators and potential for extreme fire behavior phenomena.
Flashover Disrupts Firefighters’ Rescue Effort
Firefighters attempting to rescue a victim from a burning Portsmouth (VA) house on Thursday were forced to abandon the rescue attempt and exit a window when a flashover occurred.
Firefighters first entered the home through the front door, but were repelled by flames. They then made entrance through the front bedroom windows when the flashover occurred. After escaping, firefighters tried to reenter through the back of the house, but they could not.
“An Ottawa firefighter had to be rescued from a burning basement after he was caught in a possible flashover yesterday afternoon. We don’t know what happened, and we haven’t had a chance yet to look into exactly what the details were, but we have a feeling that it might have been a flashover,” department spokesman Marc Messier said.
“Flames were coming up from the basement and out of the windows when crews arrived at the Dana Avenue house fire. There was a flashover, and fire crews quickly evacuated the duplex. Two firefighters were injured in the flashover, Battalion Chief David Whiting” said.
When they arrived, flames were coming from the first and second story of the house, firefighters said.
Kansas City, Mo., Fire Chief Smokey Dyer tells KMBC 9’s Justin Robinson what happened in a fire early Saturday that left three firefighters injured
Kansas City Fire Chief Smokey Dyer said crews went inside and started to go up the stairs, when conditions inside the house suddenly changed. He said it burned the fire hose and left the firefighters completely surrounded by flames. The firefighters sent out a mayday call for help.
“In the past 10 years, every significant firefighter injury that we have sustained in fire combat has been a result of a rapid change of conditions,” [emphasis added] Dyer said.
Incidents such as these point to the need for continued emphasis on developing firefighters’ understanding of practical fire dynamics and effective strategies and tactics to control the fire environment and prevent, rather than react to occurrence of fire phenomena such as flashover.
Flashover is Just Flashover
In a recent discussion with a number of international colleagues, we were challenged to think about language, terminology, and precision when describing fire phenomena. While this is a more obvious challenge when working with firefighters, researchers, and scientists who have different first languages, it is also a day to day problem for firefighters with a common native language (e.g., English).
Consider two recognized definitions for flashover:
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).
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)
This transition is often assumed (and in many cases explicitly stated) 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.
In some respects, flashover is always a transition between the growth and fully developed stage (as increasing heat release rate is necessary). However, this may be a bit misleading. In the modern fire environment a compartment fire may follow an alternate path, often transitioning from growth to decay prior to flashover due to limited ventilation as illustrated in Figure 1.
Figure 1. Fire Development in a Compartment
As illustrated in Figure 1, the traditional fire development curve shows fire progressing neatly through incipient and growth stages, with occurrence of flashover resulting in transition to the fully developed stage and then decay as fuel is consumed.
The path of fire development is often quite different in the modern fire environment. The nature of common building contents provides a rapid increase in heat release rate (HRR) and corresponding oxygen consumption, resulting in the fire becoming ventilation controlled. With heat release limited by ventilation, the fire begins to decay (HRR and temperature are reduced). Uninterrupted this may cause the fire to self-extinguish. However, should an opening be created (as a result of window failure due to fire effects or opening of a door), the fire re-enters the growth stage and transitions through flashover to the fully developed stage. This is sometimes described as ventilation induced flashover (but in some respects, flashover is simply flashover).
In a spirited debate, some of my international colleagues have stated that “all flashover is ventilation induced” as ventilation is necessary to develop sufficient HRR for flashover to occur. Others have said that “flashover is temperature driven” as sufficient upper layer temperature is required. None have specifically said that flashover is a fuel dependent phenomenon, but this is true as well (given that the fuel that is burning must have sufficient energy and heat release rate for flashover to occur). In addition, flashover is dependent on compartment size and configuration, as a given fire will reach flashover in one compartment (generally a smaller one) and not in another). So, what’s the answer? It Depends!
This really boils down to being able to recognize what is important for firefighters to understand about fire development and flashover (as well as other extreme (i.e., extremely rapid changes in) fire behavior.
What We Know and Why It Matters
There are a number of things that we know about compartment fire behavior that are significant when considering how and why flashover occurs:
Fire behavior is completely predictable if you have the necessary information and the time to analyze it (but on the fireground you seldom do). Predicting fire behavior is really saying: This is what I think is likely to happen.
Changes in the built environment have influenced fire development (but there are a number of variables that may vary from nation to nation). In the US, modern building contents have increased heat of combustion and heat release rate, resulting in more rapid fire development than in the past.
If ventilation is adequate, the typical room (e.g., bedroom, living room, family room) has well in excess of the amount of fuel (both in heat of combustion and peak heat release rate) to allow a fire to progress to flashover.
Smoke is fuel. This is not dependent on the size or occupancy of the building. Smoke always presents a potential flammability hazard and as the concentration of fuel and energy in the smoke increases (think temperature, even though this is not the same as energy), the hazard increases.
When a compartment fire becomes ventilation controlled, pyrolysis continues, adding additional gas phase fuel to the smoke in the upper layer.
Building configuration and ventilation profile has a significant impact on fire development. However, despite increased compartment size and open floor plans, fires in modern single family dwellings are likely to be ventilation controlled when the fire department arrives.
Increasing the air supplied to a ventilation controlled fire will result in an increased heat release rate (unless you immediately put the fire out) and this can occur quickly. Where you ventilate in relation to the fire, the existing heat release rate, and energy in the upper layer will all influence how quickly these changes occur.
Creating an opening for entry is ventilation! This change in the ventilation profile often influences development of ventilation controlled fires by increasing air supply and providing a flow path for fire travel from the current area of involvement to the entry point (watch for a bi-directional air track with air in at the bottom and smoke out at the top of the opening).
Adding additional openings will further increase the HRR and speed fire growth (unless you put the fire out). This is true even if the openings are near the seat of the fire.
It is unlikely that you can tactically create sufficient ventilation to return a ventilation controlled fire to a fuel controlled burning regime (meaning that as you continue to increase ventilation, HRR will continue to rise). This does not mean that ventilation is bad as you may influence fire spread and the level of the upper layer, but recognize that the fire will get larger (increased HRR).
Wind can have a significant influence on fire behavior. Consider wind direction, velocity, and how fire behavior (e.g., HRR, flow path) may change if the ventilation profile changes.
Given what we know, how should this inform our choice of strategies and tactics? Remember that strategies and tactics are context dependent. If you arrive with a single resource and two firefighters, your capabilities are different than if you arrive with six resources and 24 firefighters. Resources change some of your tactical options and the potential for concurrent operations. However, resources and their capability do not change the chemistry and physics of fire dynamics. It is important to recognize potential fire behavior, the scope and magnitude of the problems presented by the incident and the capabilities of the resources at hand.
Recognize that there are no simple answers to the questions of how much risk is too much and what actions are appropriate in a given circumstances. That said the following are steps you can take to reduce the potential of being caught or trapped by rapid fire progress:
Recognize the indicators of flashover potential and communicate these observations to the members of your crew. Company officers (crew/team leaders) should communicate observation of flashover indicators to their immediate supervisor (e.g., Command, Division or Group Supervisor).
Ensure that fire attack (or any other operation that involves working inside a burning building) and tactical ventilation is coordinated. In more explicit terms this means that ventilation occurs when companies or crews assigned to fire attack can quickly put water on the fire (not when they are ready to call for water or are simply ready to enter the building).
Ensure that you are working on a hoseline (or are protected by one) if you are working in a smoke filled environment. Without a charged hoseline you have no defense (you cannot outrun flashover or other rapid fire development phenomena).
Take positive actions to reduce the threat. If there are hot gases overhead, cool them. If you can put water directly on the fire, do it. If you put the fire out, things will generally improve! When you can control the fire ventilate to remove the smoke and remove the hazard.
Consider the effects of wind on potential fire behavior. Consider exterior attack and avoid advancing lines in the potential flow path when the potential for wind driven fire conditions exits. Use caution when entering from the windward side and control inlet openings (or provide adequate exhaust).
Clearly understand when you are taking a reasonable and calculated risk and when you are gambling. Think about this before you are engaged in a firefight. Make it a conscious decision and not simply a default choice. Field Marshal Erwin Rommel made this distinction between taking risks and gambling: “With a risk, if it doesn’t work, you have the means to recover from it. With a gamble, if it doesn’t work you do not. Normally, to succeed you must take risks. On occasion you have to make a gamble” (Clancy, 1997, p. 152).
This training program is of critical importance to anyone fighting fires in today’s buildings. All firefighters and fire officers should complete this training program before the end of January 2011! Take the time and get your head around the implications of this research on what we do on the fireground. This takes a bit of effort as we need to question our assumptions and standard practices, but the outcome is worth the work.
Be a student of our craft, be safe and look out for the firefighters and fire officer that work with you. Have a great New Year!
References
Clancy, T. Into the storm: A study in command. New York: G. F. Putnam & Sons
Note: Adapted from 
