Archive for the ‘Reviews’ Category

Fire Ventilation

Monday, November 10th, 2008

Fire Ventilation by Stefan Svensson was originally written to support ventilation training delivered by the Swedish Rescue Services Agency (Rddnings Verket). However, the English translation of this text is an excellent resource for any firefighter or fire officer.

Stefan does an excellent job of integrating practical fireground experience and the underlying science of thermodynamics and fluid dynamics that are essential to really understanding ventilation. Many of the concepts presented in this text will be familiar to firefighters anywhere in the world. Topics include:

  1. Fire Ventilation
  2. Fire Behavior
  3. Fire Gases
  4. The Spread of Fire Gases
  5. Working with Fire Ventilation
  6. Creating Openings
  7. Safety When Working at high Altitudes
  8. Openings in Different Roof Structures
  9. Tactics
  10. Examples of Firefighting Situations

The chapters on fire ventilation, fire behavior, and fire gases, were a necessary introduction to the topic, but other texts provide a more comprehensive examination of these important subjects. The chapters that I found most useful were The Spread of Fire Gases and Working with Fire Ventilation. Stefan’s explanation of influences on smoke movement, influence of inlet and outlet opening size, and other factors that impact the effectiveness and efficiency of ventilation operations is excellent. The narrative is simple and straightforward and shaded boxes highlight mathematical formula and calculations necessary for those who want to engage with this topic at a deeper level.
For individuals without an engineering background, the mathematical explanations of the underlying principles and engineering applications may seem a bit daunting. More detailed explanation and worked examples would provide better support of this content. However, this is a minor issue which does not significantly compromise the utility of this text to a wide range of audiences.
Fire Ventilation is available for on-line purchase from the Swedish Rescue Services Agency for 120 SEK (around $16.00) plus shipping. The agency will invoice for payment Swedish Kroner after your purchase (which necessitates using a bank that can produce a check in foreign currency).

NIOSH Public Meeting

On November 19, 2008, the National Institute for Occupational Safety and Health will be conducting a public stakeholder meeting regarding the Firefighter Fatality Investigation and Prevention Program. This meeting will be held at 1000 hours at the Crown Plaza Hotel at Chicago’s O’Hare Airport. My next post will provide a preview of my presentation at this meeting and written testimony submitted for inclusion in the public docket. Take a minute to review NIOSH Report F2007-29 before Thursday’s post.

Ed Hartin, MS, EFO, MIFireE, CFO

It’s the GPM!

Thursday, November 6th, 2008

I recently read an article in the October issue ofFire Engineering magazine titled Improving Preconnect Function and Operation. The author, LT Bob Shovald, described how his department approached the process of improving operations with small, preconnected handlines and focused on three critical factors in effective engine company operations: 1) Hose diameter and flow rate, 2) nozzle selection, and 3) hoseloads. LT Shovald made a number of good points, but misconnected on the basic science behind effective and efficient use of water for fire control.

Flow Rate

LT Shovald makes a case for high flow handlines based on changes in the built environment that influence potential fire behavior.

Primarily it comes down to one important factor, gallons per minute (gpm). Using 95- and 125-gpm attack lines is outdated and dangerous.

  • Because of the huge increase of synthetic materials in modern homes and businesses, including foams, plastics, vinyl, and volatile coatings, we are now experiencing fires with higher rates of release than ever before.
  • Because of the high cost of energy, more homes and businesses have improved insulation. In a fire, this seals that increased heat inside the structure.
  • As a result of more effective fire prevention programs, we arrive on-scene much sooner than in years past, in large part thanks to inexpensive smoke detectors.

What this adds up to is that we are getting on-scene sooner to hotter, more aggressive fires, often just before flashover conditions or self-ventilation. To fight the beast, today we need a bigger gun with bigger bullets (i.e., proving the greater gpm and thus more water faster at the start of our interior attacks). The gpm not the pressure and not the steam kill the beast.

LT Shovald’s argument for high flow handlines sounds reasonable. However, there are a few problems once you look past the surface.

Fire Power vs. Firefighting Power

LT Shovald correctly makes the connection between heat release rate and flow rate necessary for fire control. All too often, firefighters think that it takes “gpm to overcome Btu”. However, British thermal units (Btu) like Joules (J), are a measure of energy, not its release rate. Heat release rate is expressed in units of energy per unit of time, such as Btu/minute or watts (J/s).

Heat release rate is the most critical factor compartment fire development. If heat release rate is insufficient (e.g., a small fire in a metal trash can) the fire will not flashover or reach the fully developed stage. On the other hand, if the fire involves a recliner or couch, heat release rate is likely to be sufficient for the fire to grow and rapidly transition through flashover to the fully developed stage.

However, there is another critical factor in this scenario. Oxygen is required for the fire to release the chemical potential energy in the fuel. If doors are closed and windows are intact, the fire may quickly consume much of the available oxygen. If this occurs, heat release rate is limited by ventilation and fire growth slows.

LT Shovald states that “it’s the gpm, not the pressure, and not the steam” that extinguishes the fire. Flow rate is critical, but this is not entirely correct. Water is an excellent extinguishing agent because it has a high specific heat (energy required to raise its temperature) and high latent heat of vaporization (energy required to change it from water to steam). Of these two factors, conversion of water to steam is most significant as it absorbs 7.5 times more energy than heating water from 20o C ( 68o F) to its boiling point. The firefighters power is not simply related to flow rate, but flow rate effectively applied to transfer heat from hot gases and surfaces by changing its phase from liquid (water) to gas (steam). Extinguishing a compartment fire generally involves converting a sufficient flow (gpm or lpm) of water to steam. So while the “steam” itself does not generally extinguish the fire, the energy absorbed in turning the water to steam has the greatest impact on fire extinguishment.

Changes in the Built Environment

LT Shovald is correct that many of the synthetic fuels used in today’s buildings have a higher heat of combustion (potential chemical energy) and given sufficient ventilation have a higher heat release rate when compared to materials such as wood and paper. True to their design, modern, energy efficient buildings retain energy during fire development, speeding the process. However, this type of building also controls normal ventilation (the building is not as “leaky” as older structures) and energy efficient windows are far less likely to fail and change the ventilation profile. As a consequence, the fire department is likely to encounter ventilation controlled fires where heat release rate is limited by the available oxygen. Early detection may also influence fire conditions as firefighters may arrive to find a pre-flashover growth stage fire when heat release has not yet peaked.

The key here is that flow rate must be sufficient to meet or exceed the fires heat release rate. Arriving earlier in the fires growth and building characteristics leading to a ventilation controlled fire, do not necessarily lead to the need for a higher flow rate, on the contrary, the required flow rate during the growth stage is actually lower than that for a fully developed fire (when heat release rate is at its maximum). However, firefighters must also consider potential increase in heat release rate that result from tactical ventilation or unplanned changes in the ventilation profile (e.g., failure of a window).

One excellent point in supporting the argument for high flow handlines that LT Shovald did not raise is the large volume (floor area and ceiling height) and limited compartmentation encountered in many contemporary homes. Older homes generally had smaller rooms and were more highly compartmented. Many new homes have spacious and open floor plans, in some cases with multi-level atriums and high ceilings. In addition to frequently having open floor plans, many of these buildings are also have an extremely large floor area. This type of structure presents a significantly different fire problem and often requires a much higher flow rate than a more traditional, highly compartmented residence.

Tactical Flow Rate

While I agree with LT Shovald regarding the value of high flow handlines, his statement that 95 and 125 gpm are “outdated and dangerous” is unsupported. Safe, effective and efficient fire control requires:

  • Water application to control the fire environment as well as direct attack on the fire
  • Appropriate flow rate for the tactical application (cooling hot, but unignited gases may be accomplished at a lower flow rate than direct attack on the fire)
  • Direct attack to exceed the critical flow rate based on the fires heat release rate
  • Sufficient reserve (flow rate) be available to control potential increases in heat release rate that may result from changes in ventilation
  • Water application in a form appropriate to cool its intended target (i.e., small droplets to cool hot gases or to cover hot surfaces with a thin film of water).
  • Water to reach its intended target (fog stream to place water into the hot gas layer and a straight or solid stream to pass through hot gases and flames and reach hot surfaces)
  • Control of the fire without excessive use of water

A flow rate of 95 or 125 gpm is only dangerous if firefighters attempt to use it to control a fire which requires (or has the potential to require) a higher flow rate. While a high flow rate will quickly extinguish a small fire, this generally results in use of considerably more water as illustrated below.

Critical and Optimal Flow Rate

Effective and efficient fire control requires that we match the flow rate to the task at hand. At the simplest level this means using 1 ” (38 mm) or 1 ” (45 mm) handlines for smaller fires and 2″ (50 mm) or 2 ” (64 mm) handlines for larger fires. It may also mean placing control of flow rate in the nozzle operators hands by using a variable flow or automatic nozzle and letting the firefighter select the flow rate based on the tactical situation.

Ed Hartin, MS, EFO, MIFireE, CFO

Ventilation Tactics: Understanding and Application

Thursday, October 30th, 2008

Second only to the great solid stream versus fog debate, ventilation strategies seem to create the most discussion and disagreement among fire service practitioners. Vertical or horizontal; natural, negative, or positive pressure; vent before, during or after fire control? These are all good questions (many of which have more than one answer).

The Importance of Why

BC Kriss Garcia recently published an interesting article titled Education vs. Training in Fire Space Control (Fire Engineering, September 2008) examining the difference between training and education, in particular as it relates to ventilation strategies. Kriss emphasized that we train to improve performance and efficiency, but use education to develop our ability to think and understand not only how, but when, why, and why not. Both are critical to today’s firefighters and fire officers.

Space Control

Firefighters sometimes perform ventilation operations by routine, executing tactics simply based on common practice without thought to the influence of these actions on the fire environment and fire behavior. Kriss emphasizes the importance of understanding the effect of changing the ventilation profile and its relationship to fire control, stating:

Absolute control of the space you are opening is necessary for a safe and effective fire attack. If firefighters cannot control the space with enough direct application of [British thermal unit] Btu -quenching water, they should not be opening the space, encouraging additional free burning.

The concepts included in this brief statement are critical, but could be expanded and clarified a bit.

  • Developing and maintaining control of the space is critical to offensive firefighting operations and the survival of civilian fire victims who may be trapped in the building.
  • Increasing the air supply to a ventilation controlled fire will increase the heat release rate. Heat release rate is measured in kilowatts (kW) or British thermal units (Btu)/m.
  • Water application in liters or gallons per minute (lpm or gpm) must exceed the critical rate of flow based on the heat release rate (kW or Btu/min) developed by the fire.

There are two key differences in this expanded outline of the importance of understanding the influence of changing the ventilation profile: 1) Recognizing and understanding the dominant influence on current fire development, fuel or ventilation. Heat release rate from a fire burning in the ventilation controlled burning regime will increase if the fire receives additional air. 2) Water application (lpm or gpm) must be sufficient to overcome the heat release rate from the fire. While it is common to hear firefighters say that gpm must overcome Btu, this is not completely correct. Btu is a measure of energy much the same as liters or gallons is a measure of the volume of water. Kilowatts (1000 joules/second) or Btu/minute are a measure of heat release rate as lpm or gpm are a measure of water application rate.

Tactical ventilation is the other element of space control. Smoke contains unburned pyrolizate and flammable products of incomplete combustion, and as such is fuel. Hot fuel gases overhead can be cooled, providing a buffer zone around the nozzle team, but only when smoke is removed through tactical ventilation is this hazard fully mitigated.

Understanding is Critical

The difficulty that some firefighters have in accepting positive pressure ventilation or positive pressure attack is frequently rooted in a lack of understanding. In some cases, this based on dogmatic attachment to other tactical approaches. In other cases, it is a result of too much training (how to do it) and not enough education (why, why not, and when). Kriss emphasizes the value of positive pressure ventilation and the need to balance training and education to develop both skills proficiency and understanding.

Friendly Criticism

The concluding paragraph of Kriss’s article Education vs. Training in Fire Space Control (Fire Engineering, September 2008) makes two strong statements.

Regardless of the approach we use to safely control fires, we must maintain as the basis of all discussions our ability to control the fire space prior to opening it. The most dramatic means of accomplishing this is through control of the interior environment with [positive pressure attack] PPA and direct water application.

I am fully in agreement with the first sentence. Maintaining control of the fire space is absolutely critical to safe and effective offensive operations. However, the second sentence, which so emphatically supports PPA integrated with direct attack without qualifying the conditions under which this tactic should or should not be used, could be a bit misleading. Under many conditions, PPA and direct attack will be extremely effective. In other circumstances, these tactics are not appropriate. For example, Positive Pressure Attack for Ventilation and Firefighting by Garcia, Kauffmann, and Schelble, identifies several contraindications to use of PPA, inclusive of victims in the exhaust opening or other area which may be threatened and extremely ventilation controlled fire conditions which may present risk of backdraft.

The metaphor of the silver bullet applies to any straightforward solution perceived to have extreme effectiveness. The phrase typically appears with an expectation that some new technology or practice will easily cure a major prevailing problem. (Wikipedia)

In firefighting there are no silver bullets. Increased understanding of the theoretical foundations of fire behavior and the influence of ventilation and applied research such as that done by the National Institute for Standards and Technology are the key to effective use of ventilation strategies and improving the safety and effectiveness of fireground operations.

Firefighters should not uncritically accept current practice. Neither should firefighters accept new or different approaches without the same thoughtful and critical examination. Not just what and how, but why! Kriss’s Positive Pressure Attack website has a wide range of resources related to positive pressure ventilation and positive pressure attack. As Kris advises, both education and training are critical to safe and effective firefighting. Positive pressure attack is an extremely powerful tool when used correctly, be a student of your craft and learn not just what and how, but why!

Kris also published an article titled The Power of Negative Thinking in October issue of FireRescue magazine. This article takes a look at how pressure differences inside and outside the fire building influence ventilation. This interesting article will be the focus of a future post.

Ed Hartin, MS, EFO, MIFireE, CFO

Hazard of Ventilation Controlled Fires

Thursday, October 9th, 2008

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

Positive Pressure Ventilation: Theory and Practice

Sunday, October 5th, 2008

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.

NIST School PPV Test

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

On-Line Ventilation and Fire Behavior Course

Sunday, August 31st, 2008

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