Brennan Q&A October 2004

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TOM BRENNAN has more than 35 years of fire service experience having responded to 33,000 fire alarms. His career spans more than 20 years with the Fire Department of New York as well as four years as chief of the Waterbury (Conn.) Fire Department. He has a bachelor of science degree, summa cum laude, John Jay College; Alumnus of the Year Award, John Jay College; chairman of the Connecticut Fire Chiefs Association and a charter member of the National Fire Protection Association, Fire Service Section. He has delivered courses and seminars throughout the United States and has instructed at the National Fire Academy. He was the editor of Fire Engineering Magazine for eight years, is currently a technical editor, and his column “Random Thoughts,” is a regular monthly feature. He is co-editor of The Fire Chief’s Handbook, Fifth Edition. He is the recipient of the 1998 Fire Engineering Lifetime Achievement Award.

OCTOBER 2004

Question: While compiling training video clips, I stumbled across a Chicago clip from the early '90's showing a fire in a night club. The fire looks well vented, but you can see smoke pushing under pressure, then stopping, and pushing again. This repeats several times. A short time later a fireball erupts out the door in an explosive manner. I want to use this clip to teach, but I don't want to teach the wrong thing. I was wondering about your thoughts on the subject. If you could reply with any and all conclusions that you can come up with, it will be very helpful.

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Answer: Interesting question. In my day (I was also in FDNY in the '60’s, '70’s, and '80’s), I began to question a couple of pieces of information that were in the study texts as “gospel,” mostly because there were so many fires in which certain phenomena that could have been no more than annual events actually occurred more often.

One was that a brick wall should fall no more than one-third of their height. We lost pieces of apparatus and close calls to members when walls fell all of their height!

The other was the explosive extension of flame within structure fires that were “already vented.”

The only explanation was that SMOKE EXPLOSION and BACKDRAFT were not the same thing, at least not as “backdraft” (smoke explosion) was defined in all texts that were available at that time.

I had never seen a true backdraft in the true definition of the term, with all the colors and inward rushes and true explosions that are witnessed more often today (though much rarer than the witnesses believe).

Backdraft needs only one thing to “go.” It has all the heat necessary! It has all the fuel necessary! It needs only air, and it gets it so fast upon the first opening that it explodes with speed and force sufficient to cause structural integrity damage and to “blow torch” out of the openings that were made by fire or firefighter.

Once a building is opened for entry, or ventilation or interior attempt at firefighting, it cannot be a true backdraft; nor is it ever violent (time that energy is released). The phenomenon caught in the Chicago video is much slower than backdraft and there is much more building-and-contents fire spread than is the case with rollover (products of combustion ignition).

What a smoke explosion needs is two things, in my estimation:

A little more air.
Mostly a higher temperature ignition source (a flame tip).

This occurs as:

Unseen flame is allowed to grow as interior advance pushes air and water towards the trapped explosive cache.
Improper ventilation allows flame from below to extend into enclosed areas both horizontal in construct as well as vertical.

a. We pushed fire during extinguishment through a commercial occupancy in Brownsville that we did not know was connected by a small door to a side 2 (B) exposure forming an “L” shaped structure. As the flame entered, the force of ignition blew down the roll-down doors that the second truck was trying to open. Backdraft? No! Forceful? Yes!

b. Another example unfortunately cost the lives of firefighters in my day and recently also. Top-floor fires should be vented to the outer air earlier. The firefighters down below may believe that it can be held up, but the building needs it. Trapped and explosive products that have been forced into the cockloft are presented with an ignition source of flame as the ceilings are pulled and fire extends up and into the trapped area (cold smoke explosion) and blows down the entire cockloft membrane with the resultant fireball that resembles flash over.

I have looked at the clip a number of times. The front and roof are vented by firefighters and flame. The fire is in the rear as seen from early glow. It is burning so fiercely that it does not get enough air and pulses, as is seen in the front door.

Eventually the right combination (of air and flame) is attained, and the fire erupts explosively into a violent rollover effect. Note that the flame does not stay, once it becomes too rich to burn in the rear again.

So my answer is that you have a smoke explosion that reacts to the camera site like rollover (or flame-over, as Dunn likes to say).

At least that is my explanation of what I saw here and in many other experiences.

Only my opinion though.

Video courtesy of Chicagoland Video.

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Write if you agree or don’t agree or on anything. Tbrennan@firenuggets.com

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