Archive for March, 2013

This post continues the discussion with Captain Mike Sullivan with the Mississauga Ontario Fire Department regarding fire attack methods. Captain Sullivan refined his definitions and explanation of direct, indirect, and combination fire attack, stating:

Direct Attack: Water droplets put out the fire (droplets land directly on the burning fuel and cool this fuel to put out the fire).

This is essentially correct, water applied directly to the burning fuel absorb energy as the water is heated and considerably more when vaporized into steam, this reduces the temperature of the fuel and extinguishes the fire. In the end, this is generally necessary regardless of what method of fire attack or fire control you begin with.

Indirect Attack: Steam puts out the fire (water droplets turn to steam and this expanded steam eventually makes its way to the area where the burning fuel is and continues to absorb heat from this burning fuel until the fire goes out. All this steam also reduces oxygen concentration which results in a reduced heat release rate).

This is close, but the process of steam production absorbs a tremendous amount of energy. So it might be more accurate to state that production of steam and that heating of the steam as the hot gases and steam reach a thermal equilibrium cool the fire environment. In addition, steam production reduces oxygen concentration that reduces heat release rate. These processes in combination control and in fewer cases may achieve extinguishment. Indirect attack almost always must be followed up with aggressive overhaul and direct attack to achieve extinguishment. This does not diminish the utility of indirect attack for control of fully developed fires or decay stage fires resulting from limited ventilation (where high temperatures exist).

Combination Attack: Water droplets put out the fire (the droplets act the same way here as in the direct attack).

As with direct attack this is essentially correct. The application of water to burning fuel results in extinguishment through cooling. With the combination attack, some of the water is vaporized in the upper layer, assisting with control of the fire environment as well as the process of extinguishment. However, this is often at the expense of disrupting thermal layering (less of an issue when well-coordinated tactical ventilation is provided in front of fire attack.

This discussion gave rise to several other questions from Captain Sullivan:

In the combination attack, although the hose stream is directed at the ceiling (indirect part of the combination attack) and creates steam it is not as effective at cooling the fuel as the direct part of the combination attack is (wow is that wordy), Therefore,the main purpose of the indirect part of the combination attack is to cool the overhead gases so the entire environment is cooler when firefighters enter and really doesn’t have much to do with extinguishing the fire. So would you say in this case the “indirect” part of the combination attack really isn’t a key contributor to extinguishment?

This would depend (another way of saying “it depends”). The indirect component of the combination attack is important in controlling flaming combustion in the upper layer (such as rollover). While control of the burning gases overhead alone will not achieve extinguishment (same as with gas cooling), it is an important component of the extinguishment process as the heat flux from burning gases overhead is significant both as a threat to firefighters and also as a mechanism for heating unignited fuel and continuing the combustion process of fuel that is already burning. However, in the end, it is the direct element of the combination attack that achieves extinguishment. As with indirect attack, combination attack is followed up with direct attack to achieve complete extinguishment.

If what I have said above is true then, although both the indirect and combination attack produce large amounts of steam, is the purpose of the steam production actually different (put out the fire vs. cool the overhead gases)?

The purpose of steam production in both cases is to take advantage of the high latent heat of vaporization of water to achieve cooling. In addition, indirect attack reinforces the cooling effects of steam production by reducing oxygen concentration and thus reducing heat release rate from the fire.

In his comments Stefan Svensson stated that “in order to put out the fire we need to hit it with water”, but from what we have discussed here, with an indirect attack it is not water putting out the fire but steam.

In the indirect attack, production of steam and related effects on oxygen concentration result in fire control, but not necessarily extinguishment. Consider the potential outcome if you used an indirect attack on a fire in a building an did not follow up with direct attack and thorough overhaul. Likely a return visit to the same building some time later for a rekindle. In the end, when dealing with Class A fuels typically found in buildings, it is necessary to put water on fuel that is burning.

When Nelson and Royer were doing their research on the Iowa Rate of Flow, did they use a combination attack or did they use and indirect attack and then develop the combination attack after their experiments?

The combination attack was developed during their experiments. Initial application of water was done using an indirect attack (similar to that described by Lloyd Layman in Attacking an Extinguishing Interior Fires (1955). The Iowa State Story: The Iowa Rate of Flow Formula and Other Contributions of Floyd W. (Bill) Nelson and Keith Royer to the Fire Service – 1951 to 1988 (Wiesman, J., 1998) provides an excellent overview of Nelson’s and Royer’s work (but their discussion of fire behavior is inconsistent with current theory and terminology). While out of print it is available (used) through Amazon and a number of other used book outlets.

There are many departments that will flow a straight steam ahead of them across the ceiling to cool the room as they make their way to the fire. I have read that this would be considered surface cooling and not gas cooling because a straight stream will pass right through the gas layer without cooling it and only cool the ceiling, upper wall and floor surfaces as the stream bounces off the ceiling and land on the floor. I have a few questions about this.

When straight stream from a combination nozzle or a stream from a solid bore nozzle deflects off the ceiling, does the stream get broken up enough that the droplets become reduced in size enough that they will cool the hot gas layer on the way down to the floor or are they still too large and therefore pass right through the hot gas layer without cooling it?

There is some cooling, but it is less efficient than when a fog pattern is used as the large droplets will be more difficult to vaporize. If temperature is extremely high, some cooling will occur as even large droplets may be vaporized. If the stream can reach the seat of the fire, this inefficiency may be less significant as the fire will likely be controlled by the direct element of the combination attack. When faced with hot gases or flaming combustion overhead with a fire that is shielded from direct attack, cooling the gases with pulsed water fog will be considerably more effective and efficient than use of a solid or straight stream.

When the stream cools the ceiling and upper walls are the ceiling and walls now able to absorb more heat from the upper gas layer ( so this actually would be gas cooling) and if so how effective is it at cooling these gases (how much heat can these cooled surfaces now absorb from the hot gas layer)?

As discussed in my last post, gypsum board (a typical compartment lining material) which has a specific heat of 1.017 kJ/kg (Manzello, Park, Mizukami, & Bentz, 2008). This is one quarter the specific heat of water and half the specific heat of steam. So the indirect cooling effect of removing energy from compartment linings is quite inefficient at cooling the fire environment.

Once you have created steam from applying water to the ceiling and upper walls—-does that steam not now effectively cool the gases? I know you need smaller droplets suspended in the gases to absorb heat from the upper gas layer and steam would certainly meet that criteria.

The specific heat of steam is 2.0 kJ/kg  as compared to the combined theoretical cooling capacity of 2.6 MJ/kg when water is heated from 20^o C to 100^o C and vaporized into steam. While steam will cool the hot gases until they reach thermal equilibrium, but to a lesser extent than water fog applied into the hot upper layer.

Stefan Svensson mentioned in his comments that “sometimes fog nozzles are the best way to apply water to fire and sometimes it’s straight streams”. You often hear blanket statements being made about straight streams producing less steam and only fog streams can cool the gas layer. I was wondering if you could expand on the misconceptions and highlight some of the better practices we need to know about using the different streams from a more scientific point of view? When approaching a fire are we better to use both straight and fog patterns to cool the room as we make our way to find the base of the fire?

In a conversation with John Wiseman, Keith Royer stated that “there is not just one tool that will solve all fire problems”. The perspective that there is only one way to approach structural firefighting is dogmatic. Dogma is a point of view or tenet put forth as authoritative without adequate grounds. The simplest answer to your question is that when cooling gases, a fog pattern is more effective and when applying water to surfaces, the pattern selected may depend on the distance from the surface. When far away and the stream must be applied through a hot atmosphere, a solid or straight stream will likely be most effective, when close, a straight stream or fog pattern may be equally effective.

A question unrelated to this discussion but I am sure you can help me with. I am sure you are quite aware (or involved with) that excellent video called “kill the flashover” that shows the effects of closing a door. I know that the temperature stops increasing due to the lack of oxygen for heat release. But,  not only does the temperature not increase but it actually quickly decreases. This decrease is due to the fact that the walls and ceiling are absorbing the heat causing it to drastically reduce so my question is this; if that fire was allowed to burn for long enough that the walls could no longer hold any more heat, then the door was closed, would the temperature drop have been less and would it have lowered more slowly?

The reason the temperature dropped so quickly was not due to the absorption of energy by compartment linings, but by reduction of heat release rate (HRR) due to consumption of oxygen within the compartment. So, it would not make a significant difference if the fire had been burning longer. The higher the HRR, the more quickly you would see an impact. This also influences the visible fire behavior indicators (smoke and air track) on the exterior. As demonstrated in the UL ventilation experiments and previous work by NIST, visible smoke and air track indicators decrease dramatically as the fire becomes ventilation controlled due to a reduction in temperature (and resulting reduction in pressure inside the building).

Thanks for the great questions, let’s keep the discussion going! The next set of questions comes from Garland TX regarding “Flashover Training”. A bit of controversy here in a number of areas!

References

Layman, L. (1955) Attacking and extinguishing interior fires. Boston: National Fire Protection Association (NFPA).

Wiesman, J. (1998).The Iowa state story: The Iowa rate of flow formula and other contributions of Floyd W. (Bill) Nelson and Keith Royer to the fire Service – 1951 to 1988. Stillwater, OK: Fire Protection Publications.

As I was beginning work on a post focusing on fire attack methods and fire stream effectiveness, I received an e-mail from Captain Mike Sullivan with the Mississauga Ontario Fire Department asking for help in clarifying indirect and combination fire attack methods and their impact on the fire environment.

Mike is particularly interested in how to explain the method of extinguishment in the various methods of fire attack discussed in the International Fire Service Training Association (IFSTA) Essentials of Firefighting.

As Mike’s current perspectives and explanation of the methods of fire attack are quite good, they serve as a good starting point for our examination of this topic:

 Direct Attack: This is fairly straight forward; water is applied directly to the burning fuel to cool it to the point where there is no longer pyrolysis (below its ignition temperature).

 As Mike explains, the concept and mechanism of direct attack application of water to burning fuel to cool it. However, it is important to remember that combustion does not necessarily cease when flaming combustion is no longer visible, surface combustion can continue unless sufficient cooling is accomplished to not only extinguish flaming and surface combustion, but also to cool the fuel to the point where it is no longer pyrolizing.

 Indirect Attack: Here is how I would like to explain it. This is used when the seat of the fire cannot be readily accessed. Water is applied from the exterior of a very hot compartment (1000 degrees [F]+ at the ceiling) with limited ventilation. The goal is to create as much steam as possible. To do so you can begin with a fog stream since it is the most effective at cooling therefore creates more steam. The fog stream should be directed at the ceiling where it is hottest. Due to the fact that the stream has limited reach you will then want to narrow your stream eventually using straight stream. The idea is to reach as much of the room as possible. When a straight stream hits the superheated walls and ceilings it will also create a huge amount of steam as it cools the surfaces (most people don’t consider that a straight stream can create a lot of steam). The goal is to do this very quickly then close the door or window and let the steam do its work. There is one main question I was hoping you could help me with here since I have read different theories. What is the main mechanism of extinguishment here, does the steam continue to absorb heat to cool the room down and extinguish the fire or is there so much steam created that it excludes the oxygen therefore smothering and not cooling the fire (I realize both are actually happening), basically does this technique mainly cool or smother the fire.

 This is a complex question in need of a simple answer. The simplest answer is that the primary method of extinguishment is cooling. The complexity is in that the cooling is accomplished by several mechanisms. First, water heated from 20^o C to 100^o C and vaporized into steam absorbs a tremendous amount of energy based on its specific heat (energy required to raise the temperature of a specific mass of water by one degree) and latent heat of vaporization (energy required to change a substance from liquid to gas phase with no increase in temperature).

Water has a specific heat of 4.2 kJ/kg and a latent heat of vaporization of 2260 kJ/kg. Heating a single kilogram of water from 20^o C to 100^o C and vaporized it into steam, requires 2.6 MJ of energy. In addition (and contrary to common belief in the fire service) steam produced in an environment above 100^o C continues to absorb energy and increase in temperature until the temperature of the steam and the surrounding environment is equalized. Steam has a specific heat of 2.0 kJ/kg. This compares to the specific heat of smoke of approximately 1.0 kJ/kg (Särdqvist, 2002) and gypsum board (a typical compartment lining material) which has a specific heat of 1.017 kJ/kg (Manzello, Park, Mizukami, & Bentz, 2008). Water converted to steam in an indirect attack absorbs a tremendous amount of energy and the steam continues to absorb energy as the temperature in the compartment moves towards equilibrium. As with gas cooling or direct attack, some of the water is vaporized in the hot upper layer and some is vaporized in contact with hot surfaces (compartment linings, burning fuel, etc.). As the specific heat of smoke and compartment lining materials are lower than the specific heat of water (as a liquid or steam) and considerably lower than the latent heat of vaporization of water, the temperature of the smoke and compartment linings will drop to a greater extent than the temperature of the steam will increase (for a more detailed discussion of the cooling effects of water along with a bit of math, see Gas Cooling Parts 1-5).

Steam produced in and enclosed space also reduces oxygen concentration. As oxygen is required for release of energy from fuel, this can also be considered an extinguishing method. Reduction in oxygen concentration results in decreased heat release rate (HRR), which correspondingly results in a decrease in temperature. So in reality it is all about cooling (largely accomplished by vaporization of water into steam along with reduction of oxygen concentration).

 Combination Attack: We seem to have a real problem with this one. When I ask for an explanation of this technique I usually get “T”, “O”, and “Z” pattern as an answer. As a matter of fact a neighbouring fire department has these 3 letters painted on their walls to practice the pattern, again we are dealing more with technique instead of method of extinguishment. My explanation is that these patterns are merely a way of creating steam by cooling all surfaces in the room as well as allowing the water land on the burning fuel to cool it. What is the main mechanism of extinguishment here is it the creation of steam (and again what is the steam doing, cooling or smothering) or is it the water on the fuel cooling it. Also, would you recommend using a fog stream to create steam as it cools the gases and nearby surfaces then switch to a straight stream to create steam as it hits more distant surfaces (walls ceilings).

 The combination attack is intended to both cool the hot upper layer and apply water to burning fuel (less so to cool compartment linings, although this is accomplished as well). The term “combination” refers to the combination of direct and indirect attack. As indirect attack is not applied in an occupied compartment due to steam production (on contact with compartment linings), it is critical ventilation be provided in front of and closely coordinated with fire attack. As with the other methods of fire attack, the principle method is cooling.

As to your second question regarding use of a fog stream to create steam as it cools the gases and nearby surfaces and then switch to a straight stream to cool more distance surfaces. A combination attack may be done with a narrow fog pattern, straight stream, or solid stream. Reach in this case is a good thing. Cooling of hot gases overhead (with a little cooling of compartment linings) is the basic concept used in gas cooling. This technique is most commonly used to control the fire environment when the fire is shielded from direct attack and is not an extinguishing method. This approach does not result in an increased volume of steam and smoke and related lowering of the upper layer. In fact if approximately 35% or more of the water is vaporized in the upper layer, the total volume will be reduced (see Gas Cooling Parts 1-5 for a more detailed explanation of why). This technique can be effectively combined with direct attack on burning fuel and painting of compartment linings to lower their temperature. Painting is a gentle application of water to cool without excess steam production.

I believe that the Fire Streams and Fire Control Chapters in the 6^th Edition of the International Fire Service Training Association (IFSTA) Essentials of Firefighting provide a more clear discussion of fire attack methods inclusive of direct, indirect, combination, and the technique of gas cooling.

References

International Fire Service Training Association (IFSTA). (2013). Essentials of firefighting (6^th ed). Stillwater, OK: Fire Protection Publications.

Manzello, S., Park¸S, Mizukami, T., & Bentz, D. (2008) Measurement of thermal properties of gypsum board at elevated temperatures. Retrieved March 23, 2013 from http://fire.nist.gov/bfrlpubs/fire08/PDF/f08023.pdf

Särdqvist, S. (2002) Water and other extinguishing agents. Karlstad, Sweden: Räddnings Verket

Upcoming Events

April 19-20, 2013 – Seminar and Workshop on Practical Fire Dynamics & 3D Firefighting in Winkler, MB

April 23-27, 2013 – Wind Driven Fires in Private Dwellings at Fire Department Instructors Conference, Indianapolis, IN

May 25-26, 2013 – Compartment Fire Behavior Training Workshop at the British Columbia Training Officers Conference, Penticton, BC

Updated with Additional Video

On March 10, 2013 five Harrison, New Jersey firefighters were injured in an explosion while working at a commercial fire at 600-602 Frank E. Rodgers Boulevard. The fire originated in a two-story commercial building at the corner of Frank E. Rodgers Boulevard North and Davis Street and extended into Exposures Charlie and Delta, two-story residential buildings.

Figure 1. Alpha/Bravo Corner and Exposure Charlie

Image from Google Maps, click on the link to walk around using Street View.

Reading the Fire

Before watching the video (or watching it again if you have already seen it), download and print the B-SAHF Worksheet. Using the pre-fire photo (figure 1) and observations during the video, identify key B-SHAF indicators that may have pointed to potential for extreme fire behavior in this incident.

Important! Keep in mind that there is a significant difference between focusing on the B-SAHF indicators in this context and observing them on the fireground. Here you know that an explosion will occur, so we have primed the pump so you can focus (and are not distracted by other activity).

Backdraft or Smoke Explosion

While smoke explosion and backdraft are often confused, there are fairly straightforward differences between these two extreme fire behavior phenomena. A smoke explosion involves ignition of pre-mixed fuel (smoke) and air that is within its flammable range and does not require mixing with air (increased ventilation) for ignition and deflagration. A backdraft on the other hand, requires a higher concentration of fuel that requires mixing with air (increased ventilation) in order for it to ignite and deflagration to occur. While the explanation is simple, it may be considerably more difficult to differentiate these two phenomena on the fireground as both involve explosive combustion.

1. Did you observe any indicators of potential backdraft prior to the explosion?
2. Do you think that this was a backdraft?
3. What leads you to the conclusion that this was or was not a backdraft?
4. If you do not think this was a backdraft, what might have been the cause of the explosion?

For more information in Backdraft, Smoke Explosion, and other explosive phenomena on the fireground, see:

• Smoke Explosion and Backdraft
• Explosions During Structural Firefighting
• Lima, Peru: Backdraft
• Chicago Extreme Fire Behavior: Analysis of Fire Behavior Indicators
• Sudden Blast
• Gas Explosions
• Extreme Fire Behavior: An Organizing Scheme
• Backdraft at 62 Watts Street
• Fire Gas Ignitions

Back at it!

I would like to say thanks to all of you who have sent e-mail or contacted me on Facebook inquiring about the status of the CFBT-US blog. The last several years have been extremely busy at Central Whidbey Island Fire & Rescue and my focus has been almost exclusively on the fire district. However, I am renewing my commitment to developing knowledge of practical fire dynamics throughout the fire service and will endeavor to return to posting on a regular basis. In addition, I am working on a series of short (10-minute) drills on fire dynamics that will be cross posted on the CFBT Blog and the Fire Training Toolbox.

Ed Hartin, MS, EFO, MIFIreE, CFO