FIGHTING FLAMMABLE LIQUID FIRES: A PRIMER, PART 2
Photo by Josh Weggeland.
This is the second article in a series on Class B, flammable liquid firefighting foams. The first article (Fire Engineering, January 1993) intro- duced the “family of firefighting foams”—the different types of foams, their characteristics, and their recommended uses. This article focuses on foams used to control flammable and combustible liquid fires.the Coast Guard Reserve and a firefighting instructor at the U. S. Navy Firefighting School, Treasure island. California He coauthored the “Aircraft Fire Protection. Rescue, and Incident Management Training Program” for the state of California and helped rewrite IFSTA 206,Aircraft Fire Protection and Rescue, and NFPA 1405, Recommended Procedures for Land Based Firefighters that Respond to Vessel Fires.
Before you can begin to understand and safely, effectively, and efficiently use Class B foams, you must have a good working knowledge of the enemy-flammable and combustible liquids. Most of the liquids capable of burning, and the ones that firefighters usually will encounter, are refined from crude oil. Many of the major fires requiring the use of large quantities of foam have involved crude oil itself.
Crude oil is the raw petroleum material formed underground from decomposition, compression, and other geological actions of organic materials over a period of millions of years. Crude oil is pumped from oil wells and transported by tankship or pipeline to oil refineries, where it is “cracked” or broken down into various flammable materials or refined petroleum products. From this “crude” is derived liquid petroleum gases (LPGs) such as propane and butane; low-test fuels such as gasoline and naphtha; high-test fuels such as diesel, kerosene, and jet fuels; heating oils; lubricating oils; and tars and road oils such as asphalt.
They are called “hydrocarbons” because their molecules are predominantly composed of hydrogen atoms bonded to chains or branches of carbon atoms. When five or more carbon atoms are connected together, the resulting material takes on a liquid form. The more carbon atoms in the hydrocarbon molecule, the thicker or more viscous as well as the higher the flashpoint or less volatile (less flammable) the material will be.
NONPOLAR VS. POLAR MATERIALS
These common hydrocarbons usually are easily recognizable and exhibit similar characteristics when on fire. They burn vigorously with a bright orange flame. They also burn incompletely, giving off large quantities of thick, dirty, black smoke. These basic hydrocarbons usually are colored, either naturally or with dye. Their vapor densities are greater than one, meaning that they are heavier than air. Gasoline vapors, for instance, are more than three times heavier than air. Vapors generated from hydrocarbons naturally will move along the ground and collect in low areas. Always try to approach and operate from upwind and uphill of a flammable or combustible liquid spill.
These basic hydrocarbons have specific gravities of less than one, meaning that they are lighter than water. Furthermore, since water is a polar material and basic hydrocarbons are nonpolar, hydrocarbons are nonmiscible (cannot be mixed) with and nonsoluble in water Because of these qualities, nonpolar hydrocarbons will float on a water surface, with a definite demarcation between the two materials.
(This and following photos by author.)
The main objective of Class B foam is to create a situation where the water can do just the opposite: float on the fuel surface. This is accomplished by mixing a foam concentrate with water and blowing bubbles. The bubbles hold the water in suspension and float on top of the hydrocarbon surface. The bubble is the vehicle that allows the water to control the fuel fire. Unfortunately, it does not last forever. The bubbles will pop, releasing the solution, and the water drains back through and to the bottom of the fuel. This is a very simplified description of how Class B foams work. As we addressed in the first article (Fire Engineering, January 1993), some Class B foams have other qualities, such as aqueous films and polymeric layers, that also are involved in and affect the extinguishment process.
Any Class B foam will extinguish a fire involving nonpolar hydrocarbons, such as gasoline, diesel, or kerosene. Not all Class B foams will control a fire involving polar hydrocarbons or polar solvents, such as alcohol, acetone, or methyl ethyl ketone. Multipurpose foam (also known as alcohol-resistant foam) is needed in this case. A polar solvent is usually a hydrocarbon (bonded hydrogen and carbon) molecule with a functional group containing an added oxygen atom or atoms. Oxygen is a very electrovalent and reactive element. The addition of oxygen can make the basic hydrocarbon polar instead of nonpolar, giving it the ability to mix with water. Some common polar functional groups, which are bonded to hydrocarbon backbones and their resultant polar solvent materials, are Oil (alcohols), CO (ketones), CO2 (esters), COH (aldehydes), and O (ethers). Certain polar solvents also can be distilled by bacterial decomposition of plant material. An example of this would be creating ethanol (an alcohol) from corn.
What makes a liquid polar or nonpolar is a difficult concept to understand. The atoms of hydrogen and carbon in the nonpolar hydrocarbon are covalently bonded, or share electrons. This gives the nonpolar hydrocarbon molecule a neutral charge. 1’he opposite is true with polar hydrocarbons, where the entire molecule has an electrical charge or polarity. With alcohol, this addition of the negative ion (OH) to one side of the molecule gives it a negative side or pole and a positive side or pole. A similar situation occurs in the water molecule, making it a polar material also. Like substances will dissolve in like substances, so alcohol will mix with water. The wide variety of flammable polar materials also will exhibit from weak to strong characteristics.
The key point to remember is that polar hydrocarbons or solvents love to mix with water. A foam blanket is almost entirely water, with a small amount of foam concentrate added. A polar solvent immediately will mix with and pull the water out of an AFFF or fluoroprotein foam blanket. Once the water is gone, there is nothing left of the foam blanket; the foam blanket will be dissolved and destroyed as fast as firefighters can apply it. A multipurpose foam concentrate contains a polymer that immediately reacts with the polar solvent to form a polymeric layer or barrier between the spill and the foam blanket, ‘litis tough layer protects the foam blanket and prevents the polar solvent front mixing with its water contents.
Other indications will warn firefighters that they are dealing with polar solvents at a spill or tire: Polar solvents often burn much cleaner and give off less smoke than nonpolar hydrocarbons. Some of the alcohols used in race cars are almost invisible when they burn Others will burn with a blue, green, lighter orange, or obviously chemical-colored flame. They are often a clear liquid that looks much like water. Because they mix with water, they can be flooded and diluted until they will not burn. This can take a considerable amount of water to accomplish and usually will not be a logical tactic to use.
FLAMMABLE VS. COMBUSTIBLE LIQUIDS
The biggest hazard presented by any liquid capable of burning is its vapors. The primary objective of Class B foams is to control vapor emission. Liquids that burn often are categorized or classified by their flash points. Flash point is the lowest temperature at which a liquid will give off sufficient vapors to flash momentarily (when an ignition source is introduced) but will not continue to burn.
Flammable liquids have flash points below 100°F. Most of the common flammable liquids will have very lowtemperature flash points. The flash point of gasoline is — 45°F, w hile that of acetone is — 4°F. Flammable liquids have a strong tendency to vaporize. ‘ITiey are alw-ays ready to burn, no matter what the weather conditions. Thev are equivalent to dynamite and made to explode.
There is a constant threat of ignition, reignition, and flashback with flammable liquids. Every incident scene can present numerous sources of ignition, including—but not limited to—friction, electrical arcs, apparatus electrical systems and hot engine surfaces, electrical and motorized tools, static electricity, lighting equipment, cameras, video equipment, a dropped flashlight or tool, flares, smoking materials, as well as most radios and telephones, which are not intrinsically safe. Once ignited, they burn fast with a flame spread of 12 feet per second. This is faster than you can run. A good rule with flammable liquids is to “shoot foam first and ask questions later.” Again, approach from upwind and uphill, and maintain a thick blanket of foam over the entire spill surface. Constantly check for vapor concentrations with monitoring equipment. The exception to this is in a closed container where there almost always will be too much vapor (too rich) and not enough oxygen to support combustion.
Combustible liquids have flash points higher than 100°F. Kerosene and diesel, the two most common combustible liquids, have flash points ofbetween 95°F and 16()°F. They have a low tendency to vaporize and usually will not be ready to burn. Under normal temperatures or atmospheric conditions, combustible liquids usually will not be warm enough to give off sufficient vapors to flash. They often are difficult to ignite even with a match or flare. When kerosene jet fuels are used for training burns, several gallons of gasoline commonly are used to get the pit burning. Their flame spread is approximately one foot per second—still nothing a firefighter should consider having a race with.
Under certain situations a combustible liquid can be just as dangerous and hazardous its a flammable liquid. The warmer it is, the more vapor will be produced. As a combustible liquid is heated, its vapor pressure increases. During the summer months, it is not uncommon for the ambient temperature to go above 100°F. Under these conditions, black asphalt pavement could be hot enough to cause a combustible liquid to give off dangerous amounts of explosive vapors. Another potentially dangerous scenario is that in which a combustible liquid fire has just been extinguished by the fire department. The ground, container, and fuel still are very hot—perfect for a reflash.
“Wicking” is a situation in which a combustible liquid can act similar to a flammable liquid. Similar in concept to a burning candle, “wicking” occurs when a small, localized hot spot in a combustible liquid spill heats enough fuel to ignite and eventually spread fire across the entire surface. An example of this would be hot aircraft engine parts in an unignited fuel spill after a plane crash.
Finally, all combustible liquids are explosively flammable—at any temperature—under mist conditions. This is common during aircraft crash impact conditions, when wing tanks break apart, throwing up a mist of fuel. This also can occur with ruptured high-pressure fuel, lubrication, or hydraulic lines. Do not get casual with combustible liquids. Plan for the worst-case scenario, and treat all significant spill situations as if they were flammable liquids.
OTHER IMPORTANT CHARACTERISTICS
The tendency for flammable liquids to form vapor clouds cannot be overemphasized. If conditions are right, they will move along the ground for hundreds of feet and collect in lowlying areas or depressions. They will actively seek an ignition source. These vapors usually arc invisible. In some rare situations, in super-rich vapor formations, a shimmering or a shadow can be seen on the ground. Almost all petroleum vapors are narcotics or anesthetics and will numb the sense of smell after several minutes of exposure. A firefighter’s normal senses may not provide adequate warning of danger in flammable vapor situations.
They usually are toxic to some degree. Many fuels contain tetraethyl lead, benzene, and other toxic inhibitors, lubricants, dyes, and additives. Much of the lead in motor fuels has been replaced with some very toxic oxygenates and polar solvents. The TI.V-TWA for gasoline is 300 ppm. Inhalation of vapors can cause chemical pneumonia and other respiratory problems. Do not get it on your skin. Always wear full protective turnouts or bunkers, including rubber boots, flash hood, gloves, fully secured coat, pants, helmet, and SCBA. Proximity gear is nice but not a necessity. Stay upwind and uphill, maintain a proper foam blanket, enter a hot zone only if properly protected, and have a good reason for being there. Do not forget to perform decontamination.
Although combustible liquids are more difficult to ignite under normal conditions, they burn just as hot as flammable liquids. Gasoline, a flammable liquid, burns at 19,000 Btus (British thermal units) per pound, and kerosene, a combustible liquid, burns at 18,500 Btus. Containerized flammable liquids, because of their higher vapor pressure, will burn off at a rate of approximately 12 inches per hour, compared with eight inches for combustible liquids. Firefighters may encounter quantities of fuel in measurements other than gallons. The petroleum industry often uses the measurement of a barrel, which represents 42 gallons of liquid. In aircraft situations, fuel quantities may be listed in pounds. A gallon of aviation gasoline weighs 5.7 pounds, compared with 6.7 pounds for kerosene grade jet fuels. A good rule of thumb when dealing with jet fuels is 1,000 pounds will represent approximately 150 gallons of fuel.
Class B firefighting foams are effective on low-vapor-pressure liquids such as gasolines and alcohols. Highvapor-pressure liquids or compressed gases, such as LFG and liquid natural gas (LNG), have tremendous vapor pressures and desire to go from a liquid to a gas. They readily will escape through any type of foam blanket. With respect to these materials, if the gas release cannot be stopped or the container cannot be kept sufficiently cool, evacuate the surrounding area immediately per recommended guidelines.
HOW CLASS B FOAMS WORK
Fire occurs when each side of the fire triangle—fuel, oxygen (or oxidizer), and heat—is established and maintained. Foam eliminates or controls all three sides of the fire triangle. The foam blanket smothers by excluding air from mixing with the flammable vapors, separating the fuel from the oxidizing source; it eliminates the fuel side of the triangle by reducing and suppressing vapor release from the flammable liquid surface; and it eliminates the heat side of the fire triangle by acting as a flame barrier and separating the heat and flames from the flammable liquid, cooling and absorbing heat from the flammable liquid and adjacent surfaces because it consists mostly of water.
However, foam does not inhibit the combustion chain reaction (fire tetrahedron). When hydrocarbon flammable liquids burn, the carbon, hydrogen, and other atoms in the fuel molecule begin to react with the oxygen atoms in the air. The hotter and bigger the fire or flame area, the faster this chemical reaction occurs. Only halon or dry chemical extinguishing agents will stop this combustion chain reaction of the chemical free radicals in the fire. Therefore, foam usually does not work on spraying, moving, flowing, pressure-fed flammable liquid fires. These threedimensional fire situations need an agent that inhibits the combustion chain reaction. Foam works primarily on flat, two-dimensional (length times width) flammable liquid fires.
QUALITIES OF A GOOD FIREFIGHTING FOAM
What qualities do you want in a Class It firefighting foam? What criteria are used to measure and evaluate foams? There is no perfect foam; each type has strengths and weaknesses, positive and negative features.
A foam should flow freely around objects and be able to gain access to hard-to-reach areas in the fire. It should be capable of resealing or rehealing itself if disturbed. Modern synthetic foams are very mobile and can easily flow considerable distances across a fuel surface. Older organic or protein foams are stiffer and do not flow as well. You want a foam light enough to float on the surface of a flammable liquid, yet dense enough to resist disruption by wind. heat, flames, and thermal updrafts. It should form a tough, cohesive blanket and resist fuel pickup or fuel attack. It also should hold water in bubble suspension for as long as possible. These latter characteristics are more a function of the expansion or aeration of the foam solution than of the type of Class B foam, although much information on Class B foams written over the years states that a protein-based foam makes a stiffer, longer lasting, more flameresistant blanket.
“Expansion ratio”—the ratio of the volume of air to foam solution in the finished foam —is one way to measure foam effectiveness. The thicker or more aerated and expanded the foam, the longer the blanket lasts. Expansion or aeration of Class B foams is determined primarily by the nozzle or discharge device. Fire knockdown or extinguishing ability is another way to evaluate foam. Because of the presence of an aqueous film, synthetic foams usually are more effective than traditional protein-based foams in this area. Another quality is burnback resistance—the amount of time the foam will resist heat and flames before it breaks down. Again, this is primarily a function of the expansion, aeration, and type of discharge device used.
Shelf life is the length of time over which a foam concentrate remains stable, without significant changes in performance characteristics. This basically asks the question, “How long will the stuff last in storage?” Foam is a big investment for a fire department. It costs from $10 to $30 a gallon, depending on the type of foam and the quantity purchased. Synthetic foam concentrates last much longer than organic protein-based concentrates. AFFF will last 20 to 30 years and even longer under certain conditions. Concentrates will last the longest when left in their original sealed containers and stored indoors. As soon as the cap is taken off the foam container, the foam starts to degrade. It gets worse when the foam is put in a concentrate tank on a foam apparatus, in which case it constantly is exposed to the drying effects of air and can become contaminated. Shelf life is further shortened by heat, sunlight, cold, freezing, and agitation. Freezeprotected foam concentrates, in which a glycol or antifreeze additive is mixed in with the concentrate, are available.
The last criterion on which to evaluate foam is its “23 percent drain time,” also called its “foam quarter life”—the time it takes for 23 percent of the water in the foam solution to drain out, or the time it takes to lose 23 percent of the foam’s effectiveness. The foam bubble will hold water in a bubbly suspension for a limited time. Bubbles will pop and release their water, which then sinks through the flammable liquid. The better the foam is aerated or expanded, the longer it will take to drain out. The more foam applied to the fuel surface, the longer the blanket will last.
Under laboratory conditions, the 23 percent drain time of AFFF has been observed to be as low as three minutes. At an incident scene with wind, heat, and foot traffic factors with which to contend, the drain time may be as short as one minute. At an incident, consider replenishing the foam blanket every five minutes. Have in the incident command system a foam officer whose sole job is to manage the foam operation. Continually adding fresh foam while fire personnel are working in the spill area is a good procedure. Firefighters must always see a thick, unbroken blanket of white stuff over the entire flammable liquid surface. No open spaces or gaps should be visible in the blanket I like a foam blanket that covers the toes of my boots when I step into the spill area. More next month