For years, we’ve all heard the mantra of gpm (gallons per minute) vs. Btus (British thermal units). We’ve heard and seen this so many times that it must be true, right? Although this axiom has served us well, and I couldn’t agree more with the tenet that sufficient water puts out fire, the truth is that it’s not entirely accurate.
Comparing Apples to Oranges
So why am I throwing stones at this long-held adage? Because it’s inaccurate and the math is wrong. When we compare gpm to Btus, we’re comparing apples to oranges. A simple change in our terminology can both enhance our understanding of fire behavior and reinforce the importance of speed on the fireground.
To clarify, Btu measures the energy produced by the fire, whereas gpm refers to our fire flow, or the power absorbed by the water. Power (P) equals the amount of heat energy (E) per unit time (t) that the water can absorb, or P = E/t. As we can see from the equation above, power does not equal energy-they’re not the same thing. Although they’re related, they’re still two completely different physical quantities. Energy measures the total quantity of work done-not how fast it gets done. Consider the difference between running a mile and walking a mile. It takes about the same amount of energy to move your body the 5,280 feet, but it requires greater power when you run. This is the reason you’re more tired and sweat more when you run.
If we’re only concerned with comparing energy to energy, the correct units would be gallons vs. joules (J), since in the fire service and throughout the entire scientific community, the joule has largely replaced the Btu (1 Btu = 1,055 J). Although energy is important to understand on the fireground, it doesn’t have a place when we’re talking about fire flows. As an example, it’s pretty obvious to anyone reading this that 150 gallons (10 gpm for 15 minutes) is much different than 150 gpm. The undeniable fireground truth: Gallons do not equal gpm. If we’re comparing gpm to Btus, we’re comparing apples to oranges or rather power to energy, which doesn’t make any sense.
Comparing Apples to Apples
To remedy this, we must compare apples to apples and replace Btus with a measure of power, such as watts or joules/second (J/s or Jps). Whether you realize it or not, we’re all familiar with power on the fireground, since we all now know the importance of heat release rate (HRR). HRR is a measure of power. HRR is not just another buzzword; understanding HRR is necessary to fully grasp the complexities of the fireground. Because of the constantly changing construction materials and fuels found on the fireground, our environment is always evolving. This is the reason for another fireground axiom: You’re not fighting your father’s fires anymore.
Seconds Matter
Although we usually see HRR measured in watts (W, kW, or MW), you can also measure it in Jps (1 W = 1 Jps). I prefer Jps to W because I like the consistency it brings since we already speak of Joules when referring to the energy that fire creates. I also like it because it expresses the importance of time on the fireground. Time has never been more important than it is today-lightweight construction materials/methods and synthetic fuels are robbing us of time and increasing the HRR. Appreciating that the modern fireground is much more powerful than in generations past means that it’s inherently less forgiving and, therefore, we have a smaller margin of error. This speaks to the fact that we need to be faster, more proficient, and more efficient than ever before.
Thanks to the work of Escambia County (FL) Fire Rescue Battalion Chief Curt Isakson, I actually prefer gps (gallons per second) to gpm. Speaking in terms of gps will signify the importance of fast water with your crew. The fireground operational clock has been sped up, and fast water is vital for us and for the civilians we protect. Fast fires need fast water. In our profession, seconds matter, and these seconds can ultimately mean the difference between a rescue and a recovery. I also prefer the continuity gps brings to the equation. So when we compare gps vs. Jps, we’re comparing our fire flow to the fire’s HRR, power to power, or apples to apples. This isn’t a paradigm shift; it’s just semantics, but words are important.
Fast Water Wins
Although we can’t directly control the HRR a fire will produce prior to our arrival, arguably the best way to limit the HRR after we’re on scene is to flow water; we could also limit the air entrained into the building (or remove the fuel or stop the chemical chain reaction), but that’s for another article. To succeed on the fireground, we need to make sure that our gps literally overpowers the Jps. To do this, we need to be proficient at the basics, and it doesn’t get any more basic than flowing water-and doing it quickly. It’s important to note that the water also needs to definitively impact the power source-the seat of the fire. The importance of quickly getting the correct line to the correct location, using the correct technique, and having the correct flow (gps) cannot be overstated.
Fires are getting faster-are we? This question, like most regarding the fireground, is answered by sweat on the training ground. Hopefully, we all see the value in constantly pulling lines and flowing water on the training ground, but are we emphasizing the necessity of steadily increasing our tempo and speed on the fireground? Training can help us steal back some of the time the fire has stolen from us and the civilians. The goal of every training session should be for members to leave a little smarter, a little quicker, a little more efficient, and a little more confident than before.
We all know that proper training and a copious amount of sweat will assist us in quickly getting the correct line to the correct location using the correct technique; but flowing the correct gps requires a slightly different approach. Although many of us assume that we know how much water we’re flowing on the fireground, the truth is that too many of us are doing just that-assuming. There are so many variables between the pump and the nozzle that it makes it nearly impossible to accurately calculate your flows using formulas alone. There’s only one way to definitively know your flows, and that’s to flow test your equipment (apparatus, hose, and pipe). Some incredibly smart firefighters have written on this topic, so if you need more information on flow testing, just do some research.
Changing the terminology and, consequently, our thinking from gpm vs. Btus to gps vs. Jps emphasizes the importance of understanding both power and speed on the fireground. By recognizing that our fire flow must exceed the firepower to successfully extinguish the fire, we highlight the fact that just as big fires need big water, so, too, do fast fires need fast water. To translate this conceptual understanding to the fireground requires us to stress the importance of quickly getting the correct line to the correct location with the correct technique and flowing the correct gps. Although this is as basic as it gets, that’s okay-the basics still work.
NICK LEDIN is a firefighter paramedic with the Eau Claire (WI) Fire Department, assigned to Engine 5/Medic 5. He’s a member of the Fire Engineering Blog Network, co-host of The International Perspective on Fire Engineering Blog Talk Radio, and a technical panel member for the UL Fire Safety Research Institute PPV Study. Ledin has been working in emergency services for 12 years, is a fire service instructor with Chippewa Valley Technical College, and is the president of the Northland F.O.O.L.S.
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