Science for Dummies Like Me, Part 4

The Shot Heard ‘Round the World

It was a few years back when fire researchers Dan Madrzykowski from the National Institute of Standards and Technology (NIST) and Stephen Kerber from Underwriters Laboratories took the stage at the Fire Department Instructors Conference and presented the slide below.

There is no doubt about what the slide says: Water in a Window Does Not Push Fire. With our modern-day hyper-speed communications, this statement was spread throughout our industry and in some cases condensed to, “You can’t push fire.” Some immediately dismissed this as crazy based on their prior experiences and, of course, the way we were all taught to fight fire from the unburned side. Others jumped on it immediately as scientific fact and repeated emphatically, “You can’t push fire with water!” This was discussed and debated at dinner tables, on conference calls, on golf courses, at other fire conferences, in tailboard training sessions, and of course on the Internet.

The slide was used to explain the effects of the exterior straight stream through a window on a fire started in the middle room of the structure (Governors Island experiments) with no openings other than the window in the kitchen where the water was being introduced. Because the only air intake (vent) was the kitchen window, the fire spread to the kitchen, and then the flames extended out of the window. Because of a lack of oxygen supply, flaming combustion ceased in the middle room. In that experiment, the only exhaust vent was the kitchen window, so with 14 seconds of water the fire did not move or “get pushed” out of the kitchen room. All exhaust expelled back through the kitchen window as the water turned to steam and expanded. In retrospect, the slide and message may have been better served being titled, “Water in a Window Did Not Push Fire.” The slide was later updated with the following, “Fire Cannot Be Pushed - No Flow Path, No Oxygen for Flaming Combustion Except for Kitchen.”

To break that down in terms I could understand, I had to ask for a better explanation. The explanation for me (the dummy) was, if the pressure is equalized or there is no opening, there is no flow and it can’t move. I relate this back to the use of the positive-pressure fan for smoke removal. If you set up a fan at the front door and start it up (preferably after you have extinguished the fire), there is a quick inward flow for just a few seconds while the structure becomes pressurized. Once it’s pressurized, you cannot feel any air movement in the structure except directly in front of the fan. It is only when you create an opening that the pressure changes - thus, the creation of air flow.

Under Pressure

Pressure is the cause of the movement of the products of combustion. Smoke moves from an area of higher pressure to an area of lower pressure. Most of us understand that a container filled with water creates pressure. Pressure would force water out of an opening in that container. If the opening was large enough to let all the pressure out, the level of water would not rise higher than the opening. If the opening was small and the amount of water created more pressure than could be relieved out of the small opening, the pressure would build and the level water in the container would rise past the opening. The water exiting the hole would gain pressure and velocity as the water rises higher and higher in the container. Same thing with pressure caused by combustion in fire. In the experiment on the slide, the fire did not move or was not pushed into the rear room. Now, had a door or window in the rear room been open, creating a vent to the lower pressure outside environment, the fire would have started to spread in that direction even before any water was flowed.

FDIC presentation slide. (Image by Dan Madrzykowski and Stephen Kerber.)

You CAN Push Fire with a Nozzle

All researchers I have spoken with agree that you “can push fire with a nozzle.” I think instead of focusing on whether you can push fire, we should look at how we can push fire.

Any flowing nozzle in a fog pattern increases the amount of air introduced with the water. We already know that many of the fires we are responding to are starved for oxygen. Fog streams are like fans; they move air, which increases pressure and provides oxygen for the fires to grow bigger, which creates more pressure - a scientific double whammy! A solid or straight stream that is whipped around and around frantically in effect moves air just like the fog stream.

This also introduces more air and thus more pressure. Just like with a positive-pressure fan, things will move around until the pressure equalizes. If the pressure is great enough and an opening is made, then we all know it’s all coming to that opening. So, to prevent your nozzle from having the opportunity to inject air into the situation, use a solid or straight stream with limited movement. This applies to whatever attack method you choose to use and whether you are inside or outside. The important point is to understand this movement and control and use it to your advantage. If you need those heated gases out of the way, then control where they go by coordinating ventilation with attack.

NIST Test

NIST tested many of these nozzle flows in a test at Spartanburg, South Carolina, with the International Society of Fire Service Instructors. Some refer to this video as the “the video that they don’t want you to see,” but the fact is it was posted on the NIST YouTube site back in July 2014 and is still there for your viewing pleasure (www.youtube.com/watch?v=P1kjCf_8vFo). The video clearly shows that in certain applications the nozzle flow did not push fire but did in others, especially streams using fog patterns, which most certainly pushes fire and pushes it most efficiently. Each line flowed 150 gpm, one on straight stream and the next on fog.

Old School Pseudo Truths

As I have stated before, I was always taught, and taught others, that the nozzle would push fire and that you always tried to attack from the unburned side and push the fire back into the burned part of the structure. This is neither 100 percent right nor 100 percent wrong because it depends on the size of the fire (amount of pressure created), the opening(s) on the flow path, the amount of water you have to deliver, and how you deliver it (amount of pressure created).

We established that the combustion process creates pressure. If you can deliver enough water to overwhelm the combustion and stop the building of pressure then you don’t push fire because you have eliminated the pressure. In this case, it does not matter if the water is applied from top, bottom, side, inside, or outside. This explains why all of us have been accidentally successful when we have been forced, out of necessity, to hit the fire from areas that went against our training (ooops, the fire went out).

So once again we find that the use of “always” is rarely accurate for anything. My goal in writing this article is to let you know that the issue is not as easy as “Does it or doesn’t it push fire?” The answer is yes and no. It can, but it is not always; there are many variables that prevent pushing fire and many that cause it. It’s never the water’s fault, though it is always the introduction of air and combustion’s fault that cause the pressure. You are much more likely to cause the movement of fire by creating openings in the structure than you are with a properly used solid or straight stream. Both openings in the structure and hose stream are required, so coordination of both is essential!

I will close with a quote from an old friend: “They say you can’t push fire now. Maybe they are right, but you can sure scare the hell out of it and make it run down the stairs!” - Bob Pressler

That’s all I have to say about that.

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