Water Behavior: Firefighting Operations

Water Behavior

Over the past few years, the fire service has undergone a renaissance in our understanding of our environment. This is mainly due to an increase in fire dynamics research from multiple agencies, including Underwriters Laboratories (UL), the National Institute of Standards and Technology (NIST), the International Society of Fire Service Instructors (ISFSI), the South Carolina Fire Academy (SCFA), and Kill the Flashover (KTF). Many firefighters now have a much better conceptual understanding of flow paths, door control, coordinated ventilation, vent-enter-isolate-search (VEIS), heat release rate, etc. With the increase in information and understanding of our workplace, new questions have arisen, and some old questions have taken on a new importance. As is often the case, it seems that the more you know, the more you realize you don't know.

UL and NIST have done a lot of burns, and they have millions of data points and thousands of pages of technical reports to prove it. In their research, they have measured temperature, heat flux, pressure, gas concentration (CO, O2, and CO2), and gas velocity at numerous locations throughout the fire buildings. They also have hours and hours of thermal imaging and video recording so we can visualize nearly every angle possible in our studies. This data has been dissected by some of the brightest minds in the fire service, but we seem to be becoming more polarized in our views on what's best for the unprotected. The truth is we're still not 100% sure how the environment will affect survivability. When the topics of softening the target, pushing products of combustion/extinguishment, and attacking from the burned vs. unburned side are brought up, we seem to be operating with some missing information. Complicating things even further is the fact that subject matter experts in our field have opposing views of many of our tactics: While some claim a given tactic to be blasphemy, others see it as the grand unified theory of firefighting.


There seems to be one glaring measurement that's missing from all of these studies: moisture content-and for good reason. The reason that moisture content hasn't been measured is because there isn't a monitor that can get an accurate measurement inside a fire building, especially after water has been applied. UL has long recognized this missing link and, along with the University of Illinois, recently received funding to quantify the moisture content (along with the previously mentioned measurements) on the fireground in the Interior/Exterior Streams Study.

THE IMPACT OF STEAM

This brings me to one of the old questions on everyone's mind: How bad is steam for civilians? There is an obvious lack of quantitative data regarding the effects of steam on the human respiratory tract. Understandably, not too many people want to volunteer to take a couple of deep breaths of the air (wet or dry) found inside a fire building. The Moritz Study is often quoted about the effects of steam on the human respiratory tract, although the methodology leaves plenty of room for interpretation. The Moritz Study was a set of 18 experiments on dogs (and two on pigs) examining "The Effects of Inhaled Heat on the Air Passages and Lungs." There were three groups of experiments performed on dogs under anesthetic: one group (n = 7) breathing (both passive and active respiration) ordinary hot air at different temperatures (350°C or 500°C); another group of five where flame and products of combustion from a blast burner were inhaled; and a third group of six where a live steam and air mixture were breathed. In all of the experiments, the hot air was conveyed directly to the trachea by means of an insulated trans-oral cannula, placed inferior to the vocal cords.

So with this lack of quantitative data, all we have to go off of is anecdotal "evidence." Blurring the lines and confusing the conversation even more is that there is no shortage of subjective evidence that firefighters have to offer. If you've been on the job for any length of time, you have undoubtedly felt the effects of a steam burn. Some assume that because we can feel the steam through our modern PPE then it has to be lethal for the unprotected. Others will disagree, stating that we go into steam rooms all time at the gym for their health benefits, so steam is obviously harmless and we're overthinking things.

We deserve a more intelligent dialogue. So how can we start speaking the same language and make sure that we have the best information available? The first step is to review some facts and dispel some common myths about steam and its precursor, water. If we truly want to be masters of our domain, we must first understand our tools and our environment. As firefighters, the most fundamental tool to our success is our water, and we owe it to our community to learn as much as we can about water behavior. Seriously, we spend countless hours learning about fire behavior but barely even mention water behavior. As students of the job, it's mandatory that we learn as much as we can about how water functions in our environment. The way water behaves can and should dictate where and how you will attack a fire.

HIGH-QUALITY H2O

Water equals life. We all carry water on our engines, quints, tenders, and brush trucks. We use water to extinguish fires for a few key reasons: It's relatively cheap, it's ubiquitous, and it's great at absorbing heat. I'm not going to focus on the cost or ease of use of water for fire department operations, but I am going to focus on what makes water so great at absorbing heat energy. According to William Clark, "Water has the ability to absorb heat more than any element except mercury."

A molecule of water is made up of two hydrogen atoms and an oxygen atom held together by covalent bonds. Any molecule of H₂O can also "interact" with up to four other H₂O molecules via hydrogen bonding. The term hydrogen bond is actually a misnomer, as it is not a true bond but a particularly strong dipole-dipole attraction. Hydrogen bonds are an electrostatic attractive interaction between polar molecules. I know, I know, just bear with me for a minute. A single hydrogen bond isn't all that strong compared to covalent or ionic bonds, but there is strength in numbers. Because every water molecule can form up to four hydrogen bonds, an elaborate network of molecules is formed. That means in one gallon of water (at 39.2°F to make the math simple) there are 1.266 x 10ˆ26 water molecules (that's over 126 septillion … I had to look that one up), which means that there are even more hydrogen bonds. When water is converted to steam, the hydrogen bonds are broken, freeing the steam to go where it wants (flow path). The sheer number of "bonds" alone means that it takes a lot of energy to change the temperature and, most importantly (for our purposes), the phase of water.

Now that we have an understanding of why water is so efficient at absorbing heat, let's begin to examine how much heat energy water can absorb. We know that energy is needed to heat any given quantity of water. To quantify this, a couple of figures need to be found: the specific heat of water, the latent heat of vaporization of water, and the specific heat of steam.

SPECIFIC HEAT OF WATER

To measure the amount of heat that's needed to raise the water temperature, we're going to need a little math. Stay with me; it'll be worth it. Let's assume that we are trying to raise the temperature of one gallon of water from a comfortable 70°F (294°K) to its boiling point (212°F or 373°K). We're going to need the following formula:

• Q = mcT

–Q is the amount of heat energy needed [joule (J) or megajoule (MJ)].
–M is the mass of the water [kilogram (kg)].
–c is the specific heat of water (a constant; 4,186 J/kg K for water).
–T is the change in temperature of the water (°K).

That means that the amount of energy needed to raise the temperature of a gallon of water from 70°F to 212°F is ≈1,250 kJ.

LATENT HEAT OF VAPORIZATION OF WATER

Next we need to figure out the latent heat of vaporization of water, which will answer how much energy is needed to change a gallon of water (at 212°F) to steam (still at 212°F). So we're going to need the following formula:

• Q = mL

–Q and m are the same as in the above equation.
–L is the latent heat of vaporization of water (a constant; 2,260 kJ/kg for water).

That means that the amount of energy needed to change the phase of a gallon of water from liquid to gas (steam) is ≈8,550 kJ. Notice that the amount of energy needed to change the same volume of water to steam is ≈6.8 times higher than the amount needed to change it from room temperature (70°F) to boiling. Remember earlier when I was putting you to sleep with the hydrogen bond talk? That's precisely why it takes so much longer for a boiling pot of water to completely evaporate than it does to get the water to boil.

If steam is absorbing all that energy, it's a good thing, right? Well, it depends. The energy that was absorbed is still present; it's just changed its clothes. To clarify, even if we reduce the temperature in a fire building by applying water, the energy is still there (now in the form of steam) until it travels along the flow path and exits the fire building. This too can be easily proven by the law of conservation of energy.

Another important point to remember is that if water absorbs heat energy when it converts to steam, it must then release its energy when it condenses back to water. If there is no one inside the fire building and our only goal is to extinguish the fire, then it's most definitely a good thing. But if there is a known, or suspected, person (or even a fully protected firefighter) inside the fire building, then it gets a little more complicated.

SPECIFIC HEAT OF STEAM

One of the biggest myths in the fire service is that steam is 212°F [assuming pressure is 1 atm (atmospheric pressure)]; this is not always the case. Just like water is liquid from 32°F to 212°F and ice is solid <32°F, steam is a gas >212°F. So we need to figure out the specific heat of steam, which will answer how much energy is needed to change a "gallon" of steam from 212°F to 500°F and also to 1,000°F.

• Q = mcT

–Q is the amount of heat energy needed (J or MJ).
–m is the mass of the steam (kg).
–c is the specific heat of steam (a constant; 2,080 J/kg °K for steam).
–T is the change in temperature of the steam (°K).

Doing some quick math will reveal that the amount of energy needed to raise the temperature of a "gallon" of steam from 212°F to 500°F is ≈2,775 kJ and from 500°F to 1,000°F is ≈4,805 kJ.

VOLUME

When water converts to steam, it expands. We're all familiar with the fact that water expands ≈1,700 times its volume when it converts to steam at 1 atm [≈101.3 KPa (kilopascals pressure)]. What happens to the volume of steam at higher temperatures? Is the relationship linear, exponential, quadratic, etc.? Using the Universal/Ideal Gas Law (and making a couple of assumptions), we can break it down.

• pV=nRT or V=nRT/p

–p is the pressure (Pa).
–V is the volume of steam (m³).
–n is the amount of a steam (a constant; 210 moles in a gallon of water/steam).
–R is the Universal Gas Constant (8.314 J/kg °K).
–T is the temperature of the steam (°K).

I mentioned a couple of assumptions were needed to understand the relationship between the temperature and the volume of steam. The first assumption is that steam is an ideal gas, when in actuality no gas is "ideal" (an ideal gas is strictly theoretical and does not exist). The second assumption is that the pressure inside the fire building will remain constant and uniform-another impossibility. If we didn't use these assumptions, we could not attempt to quantify or qualify how our actions will affect the fireground.

This means that volume and temperature are directly proportional (linear) to each other, meaning that as the temperature of the steam doubles (in °K), so too will the volume of the steam produced. We are concerned with the relationship between temperature and pressure (p ∝ T), not the actual numbers. The numbers are inaccurate and irrelevant, but just to satisfy my OCD, the volume of steam produced at 500°F is ≈2,430 m³ and at 1,000°F is ≈3,696 m³.

It must also be noted that as water converts to steam, the temperature of the room will likely decrease, causing the gases found in the smoke to contract and decrease in volume, which is precisely why gas cooling works. To attempt to accurately quantify this decrease in volume is nearly impossible because of the hundreds of chemicals present in the smoke (not to mention the aerosols and particulates); but we can again make the same assumptions as above and find that the relationship will again be approximately directly proportional. So as the temperature is cut in half, so too will the volume of gas in the smoke.

ADDITIONAL THREAT

Wake up! We're done with the math. So what did we learn here?

  •  Water is great at absorbing heat.
  •  Steam has a lot of energy in it.
  •  Steam releases its energy when it condenses back to water.
  •  Steam can be > 212°F.

As the temperature of the steam increases, so will the volume of the steam, and at a directly proportional rate. Conversely, as the temperature of the fire building decreases, so too will the volume of the gas in the smoke.

Couple this "new" information with our understanding of flow paths, and now the fireground isn't so simple. While we still don't fully understand how dangerous steam is to the unprotected, we now know that it's not benign and should be considered along with all of our other concerns on the fireground. Knowing that steam is the primary product of extinguishment, we must be concerned with where that steam is going. This means that we need to be concerned with where our hoselines are operating from, how we're operating them, and when and where we're ventilating in relation to the survivable space.

Our primary concern isn't the fire; it's the life inside the building. So, how bad is steam for civilians? We don't have the evidence to definitively answer this question, but hopefully we're a little smarter now, which will undoubtedly lead to better decisions on our next job. Now take this information to the training ground and the fireground; your crews and your community will thank you for it.

Resources

Clark, William E, Firefighting Principles & Practices, PennWell, 1974, 15.

Hartin, Ed. Gas Cooling: Parts 1 – 5, Blog, http://cfbt-us.com.

Moritz Alan, Henriques, Frederick, McLean Regina, "The Effects of Inhaled Heat on the Air Passages and Lungs: An Experimental Investigation," American Journal of Pathology, PMC, 1945, 311–331.



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