Archive for the ‘Tactical Ventilation’ Category

In the Grading the Fireground on a Curve, published in the September issue of Firehouse magazine, Battalion Chief Mark Emery warned of the hazards of assuming that limited volume and velocity of visible smoke indicates a growth stage fire. He correctly identified that compartment fires may enter the decay phase as fuel is consumed or due to a lack of oxygen.

Emery cites National Institute for Occupational Safety and Health (NIOSH) Death in the Line of Duty reports 98-F07 and F2004-14, in which firefighters initiated offensive fire attack in commercial buildings and encountered rapidly deteriorating fire conditions due to changes in the ventilation profile. Concluding the introduction to his article, Emery observes “Unless you know which side of the fire growth curve you are entering, advancing into zero-visibility conditions is really a bad idea”.

I agree with BC Emery’s basic premise that appearances can be deceiving. However, this article points to two interrelated issues. The hazards presented by ventilation controlled fires and the dangerous conditions presented by enclosed buildings. In Smoke Burns,originally published on Firehouse.com I discussed the hazards of ventilation controlled fire and the relationship of burning regime to extreme fire behavior phenomena such as flashover and backdraft. The hazards presented by ventilation controlled fires are compounded when the fire occurs in an enclosed structure (a building with limited means of access and egress). Captain Willie Mora has written extensively on Enclosed Structure Disorientation on Firehouse.com.

BC Emery illustrated how appearances can be deceiving using data and still images from a full scale fire test in a warehouse in Phoenix, Arizona conducted by the National Institute for Standards and Technology (NIST). NIST conducted these tests as part of a research project on structural collapse. However, the video footage and temperature data from this test is extremely useful in studying the influence of ventilation on fire behavior and fire behavior indicators (Building, Smoke, Air Track, Heat, and Flame (B-SAHF)). The full report and video from this test is available on-line from the NIST Building Fire Research Laboratory (BFRL).

As an oxidation reaction, combustion requires oxygen to transform the chemical potential energy in fuel to thermal energy. If a developing compartment fire becomes ventilation controlled, with heat release rate limited by the oxygen available in the compartment, pyrolysis will continue as long as temperature in the compartment is above several hundred degrees Celsius. Pyrolysis products in smoke are gas phase fuel ready to burn. Increased ventilation at this point, may cause the fire to quickly transition to the fully developed stage (ventilation induced flashover). However, if the fire continues to burn in a ventilation controlled state and the concentration of gas phase fuel (pyrolysis products and flammable products of incomplete combustion) increases sufficiently, increased ventilation may result in a backdraft.

I take issue with BC Emery’s illustration of the growth side of the fire development curve as the value side of the cure and the decay side of the curve as the no value side of the curve. Depending on resources, a fire on the growth side of the curve may exceed the offensive fire control capability of the fire department. Conversely, a fire on the decay side of the curve which is limited to a single compartment or series of compartments may be effectively controlled using an appropriate tactics in an offensive strategic mode. However, Emery’s discussion of the more subtle indicators of burning regime that may warn firefighters of a ventilation controlled fire is right on track. For more information on fire behavior indicators and fire development, see Fire Behavior Indicators and Fire Development Parts I and II on Firehouse.com.

Ed Hartin, MS, EFO, MIFireE, CFO

Many firefighters consider Positive Pressure Ventilation (PPV) to be a new tactical approach, despite practical application in the United States since the 1980s. Since its inception, PPV has strong advocates and equally strong opponents. In many cases these opinions sprang from observation of inappropriate application of PPV without a sound understanding of how this tactic actually works. Early on there was little scientific research integrated with practical application of PPV tactics.

However, over the last six years the National Institute of Standards and Technology (NIST) has been conducting an ongoing program of research to identify how PPC works, factors influencing effectiveness in varied applications, and best practices in the application of this tactic. Steve Kerber served as principal investigator on this project. Steve is a fire protection engineer (who also serves as a volunteer Chief Officer in Prince Georges County, Maryland). Steve authored an excellent article titled NIST Goes Back to School published in the September/October issue of NFPA Journal.

This article provides a brief overview of the NIST research on PPV to date and outlines a series of tests conducted in a two-story, 300,000 ft²(27,871 m²) retired high school in Toledo, Ohio, to examine the ability of PPV fans to limit smoke spread or to remove smoke from desired areas in a large low-rise structure.
Steve pointed out the effectiveness of appropriate use of PPV as demonstrated in this series of tests, observing:

In this limited series of experiments the pressure was increased sufficiently to: reduce temperatures, giving potential occupants a more survivable environment and increase fire fighter safety, limit smoke spread, keeping additional parts of the structure safe for occupants and undamaged and reducing the scale of the emergency for the fire fighters, and increase visibility, allowing occupants a better chance to self evacuate and providing fire fighters with an easier atmosphere to operate in. Positive pressure ventilation is a tool the fire service can utilize to make their job safer and more efficient.

However, Steve also provided the following cautionary advice:

Ventilation of oxygen limited or fuel rich fires, either naturally or mechanically, can cause rapid fire growth. Ventilation is not synonymous with cooling. Venting was most effective when coordinated with other operations on the fire ground.

Strong advocates of PPV and positive pressure attack (PPA) such as Battalion Chiefs Kris Garcia and Reinhard Kauffmann, authors of Positive Pressure Attack for Ventilation and Firefighting also caution against use of positive pressure ventilation under extremely ventilation controlled/fuel rich conditions due to backdraft potential.

However, there is no clear line defining when fire conditions are sufficiently ventilation controlled to preclude safe and effective use of positive pressure as a ventilation tactic. Safe and effective use of this tactic requires a sound understanding of practical fire dynamics and the potential influence of tactical operations. This reinforces the ongoing need for scientific research and integration of theory and practical fireground experience in defining best practices in tactical ventilation.

NIST Technical Note 1498, Evaluating Positive Pressure Ventilation in Large Structures: School Pressure and Fire Experiments as well as reports related to NIST’s prior PPV research are available at the Fire.Gov web site. Downloadable video footage is also available for each of these NIST PPV tests.

Ed Hartin, MS, EFO, MIFireE, CFO

While fire investigators are the target audience for this course, it provides a good overall look at the influence of ventilation on fire behavior regardless of your interest in compartment fire behavior. The instructional presentation is particularly strong in its examination of building and environmental factors (e.g., wind and temperature differential effects), drawing heavily on Dr. Stefan Svensson’s text Fire Ventilation.

While solid in its examination of influences on ventilation, this course fails to adequately address the influence of unplanned and tactical ventilation on fire behavior. The course outlines potential positive effects of tactical ventilation, but discussion of potential for ventilation induced extreme fire behavior is limited to a brief mention of potential for backdraft in ventilation controlled conditions. In addition, there was no discussion of the potential impact of incorrect tactical ventilation such as establishment of positive pressure with no outlet or inadequate outlet area or failure to coordinate tactical ventilation with fire control. These issues are of more immediate concern to firefighters than investigators, the potential influence on fire behavior (and subsequent investigation) may be significant. A more detailed discussion of fuel and ventilation controlled burning regime and the potential influence of ventilation under each of these conditions would be a useful addition.

The use of multiple choice questions in the mid course and final assessment was generally effective in checking learner comprehension of the concepts presented. However, there were a few problems with the two assessment instruments. The mid-course assessment included one question addressing a topic covered in the second segment of the course. In the final assessment there were two true-false questions in which both answers are arguably correct (although it was fairly easy to discern which answer was “correct” based on course content. In addition, there were a number of questions in the final assessment that would accurately assess learner understanding if worded differently.

Overall, this is a worthwhile training program for compartment fire behavior instructors and others interested in compartment fire behavior. However, as always you should maintain a critical perspective. This training program is offered (free) at http://www.cfitrainer.net

Ed Hartin, MS, EFO, MIFireE, CFO