Posts Tagged ‘flow path’

After reading National Institute for Occupational Safety and Health (NIOSH) Death in the line of duty…2012-28, I was left scratching my head. For many years I have been a supporter of the Firefighter Fatality Investigation and Prevention Program and have served as an expert reviewer for several reports involving fatalities resulting from extreme fire behavior. As a friendly critic I have encouraged the NIOSH staff to improve their investigation and analysis of fire behavior related fatalities. Over the last several years there has been considerable improvement However, this latest report leaves a great deal to be desired. That said, there are a number of important lessons that can be drawn from this incident.

Discussion of Fire Behavior

The Fire Behavior section of the report identified the attic as the origin of the fire and that the fire burning in the attic was ventilation limited. The report also identified that the enclosed rear porch was substantially involved. However, the report failed to discuss how the fire may have extended from the attic to the lower area of the porch (other than a statement that the BC notices “fire raining down in the enclosed porch area”.

The report correctly described the influence of the addition of air to a ventilation limited fire; increased heat release rate and potential to transition through flashover to a fully developed stage. However, the report failed to clearly articulate that there are two sides to the ventilation equation, air in and hot smoke and fire gases out. Flow path is critical to fire development and extension, and in this incident was likely one of the most significant factors in creating untenable conditions in the 2^nd floor hallway.

It would have been useful to examine how the changes in ventilation resulting from opening of doors at the first floor level, existing openings in the attic (windows at the front and rear), opening of the door at the 2^nd floor to extend the hoseline, and failure of the rear door may have influenced the flow path. While, the National Institute of Standards and Technology (NIST) modeling of this incident will shed considerable light on this subject, the physical evidence present at the fire scene could have informed discussion of flow path in the report.

Recommendation #1 states “Fire departments should ensure that fireground operations are coordinated with consideration given to the effects of horizontal ventilation on ventilation-limited fires”. This is a reasonable recommendation, but fails to speak to the importance of understanding flow path and the thermal effects of operating in the flow path downstream from the fire. In addition, while speaking to the importance of coordination, the report neglects to define exactly what that means; water on the fire concurrent with or prior to performing tactical ventilation.

Failure of the rear door established a flow path through the narrow, question mark shaped hallway and kitchen to the front stairway. Given the narrow width of this hall and its complex configuration, it is likely that there would be considerable mixing of hot smoke (fuel) and air providing conditions necessary for combustion. The dimensions of the space may also have influenced the velocity of the hot gases, increasing convective heat transfer.

The report did not speak to conditions initially observed in the kitchen and hallway or observed changes in conditions by members of other companies or the Engine 123 firefighter, prior to Captain Johnson’s collapse.

Things to Think About: Conditions on floor 2 were quite tenable prior to failure of the 2^nd floor rear door, but changed extremely quickly in the hallway when the door failed. It is important to consider potential changes in flow path resulting from tactical operations and fire effects. It is unclear if the crews working on the 2^nd floor were aware of the extent or level of the fire in the rear porches (having observed conditions indicating an attic fire on approach). The BC addressed the fire in the rear, but the it is uncertain if the line stretched to the back of the building was in operation before door failed or if application through the attic window would have significantly impacted the fire in the lower areas of the porch.

Structure

The section of the report addressing the Structure provided a reasonably good overview of the construction of this building and identified that the 2^nd floor ceiling had multiple layers. However, there was no discussion of what influence these multiple layers may have had (e.g., reducing the thermal signature of the fire burning above). One significant element missing from discussion of the structure was the open access between the rear porch and the attic that allowed ready extension of fire to the rear porches.

The report also failed to discuss the type of door between the 2^nd floor living area and the rear porch, other than to mention in passing that it was metal. Closed doors frequently provide a reasonable barrier to fire spread, but in this case, the door failed following an undetermined period of fire exposure. This was likely a significant factor in changing the flow path and creation of untenable conditions on the 2^nd floor.

Things to Think About: Closed doors can provide a significant fire barrier in the short term. However, it would be useful to examine door performance in greater depth to understand what happened in this incident.

Training and Experience

The section of the report addressing training and experience is exhaustive, providing an overview of state training requirements implemented in 2010 (well after the Captain would have attended recruit training). It was unclear if these requirements were implemented on a retroactive basis. The number of hours of training for various personnel involved in the incident were provided, but with little specificity as to content of that training.

These observations are not intended to infer that the training of the members involved was or may have been inadequate, but simply that if NIOSH is investigating a fire behavior related incident, it would be useful to speak to training focused on fire behavior, rather than a generic discussion of training.

It was also interesting to note that while the report spoke well of the Chicago Fire Department training program, it failed to mention that the CFD has been heavily involved in fire dynamics research with both NIST and Underwriters Laboratories (UL) for many years.

Things to Think About: If you are reading this, you likely are plugged into current research in fire dynamics and tactical operations. Share the knowledge and build a strong connection between theory and practical application on the fireground.

Other Observations

While the floor plan of the 2^nd floor is useful in understanding the layout of that space, it does not provide a good basis to visualize the flow paths and changes in flow paths that influenced the tragic outcome of this incident. Providing a simple three dimensional drawing with ventilation openings would have significantly increased the clarity of the information provided.

Things to Think About: Don’t be a passive user of NIOSH reports. For a host of reasons, NIOSH does not include the names of Firefighters who have died in the line of duty, the agency they worked for, or the location of the incident (other than the state). However, this information is readily available and can provide additional information to help you understand the incident. In this case accessing the address of this incident (2315 W 50th Place, Chicago) allows the use of Google Maps satellite photos and street view to gain a better perspective of the exterior layout of the building and configuration of openings.

Final Thoughts

The NIOSH Firefighter Fatality Investigation and Prevention Program is an important and valuable resource to the fire service. Developing an understanding of causal factors related to firefighter fatalities is an important element in extending our knowledge and reducing the potential for future line of duty deaths. Firefighters often observe that NIOSH reports simply say the same thing over and over again. To some extent this is true, likely because Firefighters continue to die from the same things over and over again.

The fire service across the United States is making progress towards developing improved understanding of fire dynamics and the influence of tactical operations on fire behavior. This is in no small part due to the efforts of the UL Firefighter Safety Research Institute, NIST, and agencies such as the Chicago Fire Department and Fire Department of the City of New York (FDNY). However, we need to look closely at near miss incidents, those involving injury, and fatalities resulting from rapid fire progression and seek to develop a deeper understanding of the contributing and causal factors. The NIOSH Firefighter Fatality Investigation and Prevention Program can be a tremendous asset in this process, but more work needs to be done.

What’s Next

I just spent the last two days at UL’s Large Fire Lab for the latest round of Attic Fire Tests and will be headed to Lima, Peru the first week of December. While on the road I will be working on my thoughts and observations related to attic fire tactics. The simple answer is that there is no single answer, but these recent tests presented a few surprises and have given me a great deal to think about.

Ed Hartin, MS, EFO, MIFireE, CFO

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

Photo courtesy of Curt Isakson, County Fire Tactics

UL Focus on Flow Path

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.

• Fire Behavior Case Study Townhouse Fire: Washington, DC
• Townhouse Fire: Washington DC-What Happened
• Townhouse Fire: Washington DC-Extreme Fire Behavior
• Townhouse Fire: Washington DC-Computer Modeling
• Townhouse Fire: Washington DC-Computer Modeling Part 2

Wind Driven Fires & Flow Path

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:

• Wind Driven Fires
• NIST Wind Driven Fire Experiments: Establishing a Baseline
• NIST Wind Driven Fire Experiments: Anti-Ventilation and Wind Control Devices
• NIST Wind Driven Fire Experiments: Anti-Ventilation and Wind Control Devices Part 2

Discussion

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.

Kerber, S. (2011). Impact of ventilation on fire behavior in legacy and contemporary residential construction. Retrieved July 16, 2011 from http://www.ul.com/global/documents/offerings/industries/buildingmaterials/fireservice/ventilation/DHS%202008%20Grant%20Report%20Final.pdf

Madrzykowski, D. & Kerber, S. (2009). Fire Fighting Tactics Under Wind Driven Conditions. Retrieved (in four parts) February 28, 2009 from http://www.nfpa.org/assets/files//PDF/Research/Wind_Driven_Report_Part1.pdf; http://www.nfpa.org/assets/files//PDF/Research/Wind_Driven_Report_Part2.pdf;http://www.nfpa.org/assets/files//PDF/Research/Wind_Driven_Report_Part3.pdf;http://www.nfpa.org/assets/files//PDF/Research/Wind_Driven_Report_Part4.pdf.

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

National Institute for Occupational Safety and Health (NIOSH). (1999). Death in the line of duty, Report 99-21. Retrieved August 31, 2009 from http://www.cdc.gov/niosh/fire/reports/face9921.html