SUPPRESSION AGENT STRATEGIES, METHODS AND TECHNIQUES OF FIRE EXTINGUISHMENT
Fire, a good servant and a bad servant, can be tricky to handle when it is the master. In this regard, it is desirable to put out fire using the most appropriate extinguisher depending on the cause of the fire. Failure to match the cause of fire (or the fuel in this case) can result to increasing the severity of the disaster which has untold consequences. This paper will look at different agents of putting out fires, their most suitable environments, how they work and their application.
Aqueous agents (water) is the most standard fire extinguishing agent used by man. As a matter of fact, it is the first solution to a razing fire when a layman gets involved. Aqueous agents are most beneficial in instances where fire is of exceptionally high temperatures. It is also desirable if the environment is at stake. To be effective, water works by applying these three principles. First, water reduces temperatures by absorbing heat as it evaporates. Second, water puts out fires by soaking the fuel and making it impossible to burn. On this, the fuel must be able to soak hence it must be a solid e.g. wood or clothing. Third, water puts out fire by cutting out the supply of oxygen.
Enhanced water is water with additives. The additives are of special characteristics and in most case are chemical in nature. The additives provide the water with superior (enhanced) fire suppression capabilities through emulsification or encapsulation. Emulsified water is used for class B fires. This types of enhanced water are able to trap flammable vapors hence cutting off fuel supply. To achieve emulsification, the enhanced water is applied using high pressure nozzles. It is worth to note that this kind of fire suppression strategy is not effective with large fires especially those with bulk liquid fuels (Friedman, 1998). Enhanced water can be delivered in any liquid ready firefighting equipment that has the ability to deliver high pressured water.
Aqueous foams are used for fire suppression as they are able to absorb heat and the same time prevent fire from coming into contact with the fuel by coating the fuel. It is of particular importance to note that aqueous foams need to be matched with the type of fire, for instance, class A fires are put out using class A foams. To properly dissect how foams work, this paper will look at the various types of foams.
Aqueous film forming foam (AFFF).
AFFFs are concentrates which are manufactured from materials such as hydrocarbon surfactants, solvents, foam stabilizers and fluoro-chemical surfactants. AFFF works by forming a film on the surface of the flammable liquid. Jet fuel fires are the best to put out using AFFF from high pressure nozzles. The film is able to flow and float on the surface of hydrocarbon fuels. According to Ranjbar & Shahraki (2013), AFFF puts out fire through the interruption of the fire tetrahedron by chemical means.
They contain natural proteins which act as the foaming agents. Protein foams are bio-degradable hence environmentally friendly, spread slowly and are capable of providing a blanket that is heat resistant and more durable. To deliver the foam, it is required that it be delivered using an air aspirating discharge device. The foam should be applied gently especially on flammable liquid surfaces. Protein foam is known to provide post fire security of exceptional level as it is capable of providing a uniform foam blanket that is stable and with superb drainage characteristics and heat resistance.
This is an improvement of protein foam through the addition of flurochemical surfactants. It is able to interrupt the fire chain reaction much quicker. In collaboration with dry-chemical extinguishing agents, fluroprotein foams are more compatible and offer high resistance to the pick-up of fuels. Fluoroprotein foams are appropriate for hydrocarbon fueled and some oxygenated fuel additives. It is however drastic to note that ethanol-gasoline fires with more than 10% ethanol are not suitable with fluoroprotein foams.
Medium expansion foam
Synthetic foam concentrates are used to produce medium expansion foams which expand up to 200 times. The foams are of high volume and low weight and are able to extinguish fires by separating, repression and cooling. To produce the foam, a water/foam mixture in a branch pipe is swirled with drawn-in air which is expanded even more by hitting it on a mesh placed inside the branch pipe. 50-100 expansion times foams are suitable for plastic, liquid and tire fires while 100-200 expansion times are suitable with flat surfaces that can be flooded.
High expansion foams.
These are able to achieve high expansion rates of more than 200 times. These types of foams are of exceptionally low weight and high volume. It is able to achieve fire suppression by separating fuel and oxygen as well as insulating the fuel. They are able to minimize water damage as high expansion foams have minimal content. High expansion foams are suitable with enclosed environment, for instance, in warehouses and in airplane hangars.
Non-aqueous fire extinguishing agents are said to be composed of a dry powder fire extinguishing agent released in an organic liquid. Improvements to the non-aqueous agents may be facilitated by adding bromodichlromethane and dichlorotrifluorethane. The no-aqueous agents used in fire extinguishing include:
Dry chemicals agents known to possess unique capabilities in putting off fire. On the class B fire, the dry chemicals have a superior flame knock-down above other agents. The chemicals in this category include: monoammonium phosphate which acts on class A, B, and C fires; potassium carbonate (Purple-K), potassium chloride (Super-K) (Voelkert, 2009). Carbon dioxide is a more efficient agent more than steam. It is commonly used fixed fire-fighting installations. Its use leads to minimal damage of property as well as less corrosion to the equipment (Zhang, 2000). Hot aerosols are more preferred as they require no pressurized gases and they capabilities to extinguish class A, B, C, and K fires. The hot aerosol technology applies alkaline or alkaline earth metal nitrates. (Zhang et al. 2015)
Dry chemicals accomplish the fire extinguishing for class B fires due to particle size and decomposition; smaller particle sizes works best, decomposition on the other hand leads to smothering effect where CO2 is produced and also cooling occurs which breaks the barriers between fuel surface and radiant heat on the liquid fuel (Voelkert, 2009). Carbon dioxide puts off fire by decreasing the supply of oxygen to a point where combustion can no longer occur. The hot aerosols act on fire both physically (where particles forms barriers) and chemically (where the oxidant combust to produce micro particles which recombine with fire supporting radical thereby reducing their concentration (Zhang et al. 2015).
Inert gases are naturally occurring gases and are known to be the most stable elements. They include neon, argon, xenon, krypton and radon. Most inert gases fire extinguishing agents are mixed with other gases to enhance buoyancy. Such gases to create this blend include nitrogen and carbon dioxide which are also fairly unreactive. Inert gases are environment friendly and are also not toxic to human beings. They are mainly stored in gaseous state in canisters. Since these elements are stable in their configuration they do not undergo thermal decomposition and thus no by-products are formed in the fire extinguish process. The inert gases agents are required in large quantities. They are stores as pressurized gases in cylinders leading to space and weight implications (Kim, 2011).
Inert gases suppress fire by suffocation. For fire to thrive, oxygen levels need to be above 15%. Using inert gases to suffocate fire by cutting oxygen concentration may also be toxic to live of human beings. The balance between fire extinguishing and sustaining life is attained at 12% oxygen concentration. At this level, combustion cannot occur while it is only toxic when oxygen falls below 10% (Snyder, 2008).
Active halogenated agents
The most common active halogenated agents applied in the process of extinguishing fire are bromotrrifluoromethane, bromochlorodifluormethane, bromochloromethane, carbon tetrachloride, dibromotetrafluoroethane, dibromodifluoroethane, and methyl bromide (Voelkert, 2009).the above named agents, possesses distinctive physical properties relating to vapor pressure, boiling point, and specific gravity. The halogenated agents are known to decompose in temperatures exceeding 9000 F (5100C). The use of halon agents is not recommended their negative effects to the environment especially the destruction of ozone layer.
The active halogenated agents put off fire by disorienting the chemical reaction in the combustion process. The interruption of the fire tetrahedron occurs chemically. When the agent decomposes in fire it releases bromine which merges with the free radicals responsible for supporting combustion. Halogenated agents are preferred due to their good extinguishing capabilities, require less space, and are efficient in cost. The use of halogenated products for fire extinguishing process requires not clean up. The agents do not cause secondary damage to property and thus preferred for environments related to electronics, data processing, jet engines and high tech appliances (Voelkert, 2009).
Mode of application
Carbon dioxide and halogenated agents fire extinguishers applies portable extinguishers and total flooding systems in the process of putting fire. The use of dry chemical requires use of portable extinguishers, fire trucks as well as fixed systems.
In conclusion, it can be said, without fear of contradiction, that we have different fires depending on the fuel and as such, different fires require different approaches, methods and agents to suppress.
Friedman, R. (1998). Principles of fire protection chemistry and physics. Jones & Bartlett Learning.
Kim, A. (2011). Advances in Fire Suppression Systems. Retrieved from http://www.nrc-cnrc.gc.ca/ctu-sc/ctu_sc_n75
Ranjbar, H., & Shahraki, B. H. (2013). Effect of Aqueous Film‐Forming Foams on the Evaporation Rate of Hydrocarbon Fuels. Chemical Engineering & Technology, 36(2), 295-299.
Snyder, L. (2008). Halon Replacements: Inert Gases, Clean Agents and Water Mist Sytems. Retrieved from http://www.facilitiesnet.com/firesafety/article/Halon-Replacements-Inert-Gas-Clean-Agents-And-Water-Mist-Systems-Facilities-Management-Fire-Safety-Feature–10303
Voelkert, C. (2009). Fire and Fire Extinguishment. Retrieved from http://www.amerex-fire.com/wp-content/uploads
Zhang, S. (2000, Digital Repository of the World Maritime University). Fire Protection onboard: Enhance fire safety by design.
Zhang, X., Ismail, M., Ahmud, F., Abdullah, N., & Hee, C. (2015). Hot Aerosol Fire Extinguishing Agents and The Associated Technologies: A review. Brazilian Journal of Chemical Engineering.
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