Article Archive
Risk Assessment
A Must Read On 3-D Fire Fighting
Vol 21 No 4

If you have been around for a while, most of the new books on the market probably give you a few new ideas or make you think about something you already know in a different way. A book that has completely new ideas is uncommon but one was recently published entitled 3D Training, Techniques, and Tactics by Grimwood, Hartin, McDonough, and Raffel and published by Fire Protection Publications of Oklahoma State University. The book can be found at www.ifsta.org. I started reading it one evening and stayed up almost all night before I could put it down.

This book is directed at municipal firefighters and not specifically at the industrial audience. There are; however, several applications for the industrial firefighter. The most obvious is structural firefighting. Most industrial firefighters will have some kind of structural responsibility and many serve as volunteers in their home community.

Besides presenting a method of attack called a 3D (three dimensional) attack, it provides an excellent discussion of whichever side of the fog versus straight-stream debate you find yourself. The 3D attack is based on offensive cooling the hot fire gases to reduce the risk of flashover while extinguishing the fire. The concept has been is use in Europe for some time now. The idea is just starting to catch on in the United States. Details will be left to the book. There is not space in this column to do the concept justice so no more will be said. To learn more, visit www.firetactics.com

The text takes a scientific view of the amount of water needed to extinguish a fire. It is the first major publication geared toward the fire service that expresses heat release rates in the engineering term "megawatt." Most firefighters in the United States are taught about fire loads in terms of British Thermal Units (BTUs.) They will say something like "A facility has a high BTU load" or "The fire was giving off more BTUs than the fire department could absorb."

In fire protection engineering applications, the energy content of a fuel is based on joules per unit mass of fuel. What is more important than the heat energy potential of a fuel is the rate at which that heat energy is released. This would be expressed in BTU/seconds or joule/seconds. One joule/second is equal to one watt. Since a watt as a measure of heat energy release is relatively small, most fire heat release rates are expressed in terms of megawatts (MW) or one million watts. To put things in perspective, the peak heat release rate of a burning couch is about 3 MW. Per NFPA 92B, Standard for Smoke Management Systems, a furnished living room (heat at open door) has a heat release rate of 4 - 8 MW.

Once the heat release rate of a potential fire is known, estimates can be made of the amount of water needed. The reason these are estimates is that a lot depends on the efficiency at which the water can be applied. Another reason is that theoretically, it is not necessary to absorb all of the heat being released, just enough to stop the fire from being self-sustaining.

The heat release rates for many industrial scenarios can be found in the following NFPA standards:
? 72, National Fire Alarm Code
? 92B, Standard for Smoke Management Systems
? 204, Standard for Smoke and Heat Venting
* These lists are mostly duplicates so if you have any of these three standards you can get started

Some representative heat release rates are given in Tables 1 and 2.

For high fire loads in buildings, it quickly becomes apparent why a properly designed sprinkler system is needed. It is simply not practical to develop the large hose stream flows needed before the building is destroyed. For example, the 13 foot (4 meter) stack of polystyrene jars shown above has a heat release rate of 144 MW. Using the values on page 89 of the text, approximately 2760 GPM would be needed to absorb the heat from this pile of commodity.

Think about how difficult that flow rate would be to deliver by hose streams in the center of a large warehouse. While the portable master streams are being set up, the fire will grow even larger. A steel bar joist roof will probably collapse. A properly designed sprinkler system is the proper way to handle the fire.

Another important subject the book addresses is Thornton's Rule. This rule states that the amount of heat released during the consumption of a given quantity of oxygen is relatively constant for most combustibles. This means that the heat released per unit of oxygen consumed is about the same for wood or plastic. This is a scientific fact but it could lead to some misunderstanding.

In a ventilation-controlled fire, where the amount of air entering through openings in a room governs the fire, the heat release rate in the room cannot exceed what the available air supply will support. The text (p 109) points out that the air supply may limit the heat release rate in the compartment but that unburned gases (those that could not burn in the room) can burn outside of the compartment.

Of concern to the industrial firefighter are large open buildings. Here, a fire is fuel controlled for a longer period of time meaning that there is sufficient air to support the combustion of fuel packages that have high heat release rates1.

Thornton's rule still applies. The reason fire protection engineers know that Group A plastic commodities,2 for example, burn at higher heat release rates than paper commodities is by measuring the amount of oxygen consumed in a device called an oxygen consumption calorimeter. The heat release rate is then deduced from the rate of oxygen consumption. In NFPA 13, Installation of Sprinkler Systems, a greater sprinkler discharge density (more water flow per square foot or square meter) is needed as the plastic content of a commodity and its packaging increases3.

In closing, the in depth analysis of water application to structure fires is why the text 3D Training, Techniques, and Tactics is one that belongs on every industrial fire department's bookshelf.

Endnotes:
1. A fire in a large open building such as a warehouse can eventually become ventilation controlled. This is of limited interest to an industrial firefighter because by the time this happens, the sprinklers have probably failed and the fire will be well beyond manual control. Usually the only thing left to do is to try to make a stand at the firewalls if it can be done safely.
2. Group A plastics are defined in NFPA 13. Examples include polyethylene, polypropylene, and polystyrene.
3. There are cases where wood and paper require greater sprinklers densities than Group A plastics because of the physical arrangement of the fuel. Rolled tissue paper and idle wood pallets are among the most challenging cellulose based products. But for standard commodities in cardboard boxes, Group A plastic commodities require greater sprinkler densities.
Follow-up to the last article: Better the Enemy You Know, Dr. Vytenis Babrauskas has generously offered to e-mail copies of research articles he has prepared on metal deck roof fires. To obtain a copy, contact John_Frank@swissre.com

Swiss Re is the world's leading and most diversified global reinsurer. The company operates through offices in over 30 countries. Founded in Zurich in 1863, Swiss Re offers financial services products that enable risk-taking essential to enterprise and progress. Swiss Re's traditional reinsurance products and related services for property and casualty, as well as the life and health business are complemented by insurance-based corporate financial solutions and supplementary services for comprehensive risk management.

 
 

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