• Please to View Article

    Flammable Liquid Fire Hazards: Are You Prepared?

    By Mark Conroy

    When faced with a fire, having a fire extinguisher in close proximity is critical to limiting property damage and injuries. But more important, is having the right fire extinguisher available.

    Those in the fire protection industry know that the selection of a fire extinguisher is based on several factors. Generally the main concerns are the type of hazard in the area and the size of fire likely to occur. Other factors include physical conditions, health and safety concerns, agent properties and their effects and specific hazards. Situations can be complicated and present unique hazards, so a thorough understanding of all of the conditions and how they interrelate is essential.

    Considered to be the most challenging fires to extinguish, Class B fires – pressure, three-dimensional and obstacle fires – pose great challenges to those that have to plan for as well as extinguish them.

    Pressurized flammable liquid fires, pressurized flammable gas fires and three-dimensional flammable liquid fires have been identified as unique hazards since the 1984 edition of NFPA 10, standard for Portable Fire extinguishers. In 2007, obstacle fires were added to this category. Up until the 2007 edition of NFPA 10, the standard only acknowledged that the system used to rate fire extinguishers on Class B fires (flammable liquids in depth) was not directly applicable to these types of hazards.

    Taking the lead on this issue, the NFPA Technical Committee on Portable Fire extinguishers proposed the first minimum criteria for type extinguishers. The standard now requires large capacity dry chemical extinguishers of 10 lbs (4.54 kg) or greater, with a discharge rate of 1 lb/sec (0.45 kg/sec) or more, for the protection of hazards involving pressurized flammable liquids, pressurized flammable gases, and three-dimensional flammable liquids. For obstacle fire hazards, dry chemical extinguishers (10 lbs or greater) are now considered equivalent to foam extinguishers. (Note though, that this size extinguisher is a minimum standard and more severe hazards will necessitate larger extinguishers with even higher flow rates.)

    With the establishment of these specific requirements, extinguisher manufacturers had a directive for what to produce. On April 22, 2010, Badger Fire Protection announced that they were launching a “new fire extinguisher line ideally suited for commercial and industrial applications that handle, transport, process or store flammable liquids and gases.” They also said that their extinguishers “exceed the NFPA-10 recommendations (requirements) for Class B obstacle, pressure, gravity and three dimensional fires” because their fire extinguishers release more than 1 lb of extinguishing agent per second.

    The introduction of new products provides the marketplace the opportunity to evaluate their merit, while learning more about these specific hazards.

    Here’s Why the rating System Can’t be Used

    Any extinguisher that is rated for Class B flammable liquid fires must pass a laboratory fire test. The Underwriters Laboratory (UL) system for rating extinguishers utilizes pan fires of flammable liquids. A small extinguisher with a 1-B rating is based on an expert extinguishing a 2½ ft² pan fire. Larger extinguisher ratings are multiples of the 1-B test, so a 10-B extinguisher will extinguish a 25 ft² pan fire. Since an expert can typically extinguish 2½ times as much fire as an average person, the 1-B and 10-B ratings mean that an average person can extinguish 1 ft² and 10 ft² fires, respectively, using those extinguishers. (The system includes requirements for discharge times, which means low flow rates.) The results are the basis for Table in NFPA 10, Standard for Portable Fire Extinguishers, which establishes the minimum size and placement of extinguishers for light, ordinary and extra Class B hazards. The extinguisher requirements of Table also only apply to fires similar to the pan fires. When different fire hazard are encountered, special considerations are needed that are beyond the scope of Table

    Here’s Why Foam Extinguishers Won’t Work

    Often considered the best extinguishers for flammable liquid fires, foam extinguishers are only good for blanketing the horizontal surface of liquids. While excellent for handling pan-type and spill fires involving obstacles, foam extinguishers are not appropriate for pressurized fires where the flammable liquid is spewing out of a pressurized pipe. They are also inappropriate for three-dimensional Class B fires involving flammable liquids that are flowing, pouring or running.

    Foam extinguishers are a perfect match for the UL rating system and NFPA 10, Table, requirements for flammable liquid fires, but additional considerations are needed for pressurized or three-dimensional fires. (Based on comparative evidence, dry chemical extinguishers have proven to be more effective on these specific hazards.)

    Here’s Why Large, High flow-rate Dry Chemical Extinguishers Work

    Unique hazards that may result in pressurized or three-dimensional fires require special extinguishers as well as basic training on their proper use. This is critical for a successful outcome during a pressurized or three-dimensional fire.

    Dry chemical has been proven to be extremely effective for quickly extinguishing flammable liquid fires (if applied at an adequate rate and with the proper technique). That is because they penetrate the flames to the liquid surface, chemically interrupting the flaming process with a flooding atmosphere. Large-capacity dry chemical extinguishers with specially designed nozzles and higher rates of agent application have been specifically designed for these applications. Large-capacity dry chemical extinguishers (10 lbs or more, with a discharge rate of at least 1 lb/sec) are also required by NFPA 10 for these types of hazards.

    Learning and practicing proper flame extinguishment techniques are necessary for effective control of pressurized or three-dimensional fires. For a pressurized fire, where a flammable liquid is being released under pressure from a hose, pipe, flange, fitting, pump, cylinder or tank, the extinguishing agent is released at the flammable liquid source and discharged in the direction of flow. For a three-dimensional fire, the agent is applied at the bottom (lowest elevation) and worked upward. These methods are the only way to knock down the flames and keep them from re-flashing on flowing liquids.

    Although dry chemical extinguishers are effective on both moving liquid and pool fires, it’s always best to followup with a foam extinguisher for the resulting pool fire in the event an ignition source is in the area. Aqueous Film Forming Foam (AFFF) or Film Forming Fluoroprotein Foam (FFFP) that are used in foam extinguishers are compatible with dry chemicals, so foam and dry chemical can be applied simultaneously. (Technically, the dry chemical knocks out the flames on the moving flammable liquid while the foam blankets the surface of the resulting pool fire, securing the vapors and preventing re-ignition.)

    Obstacle fires are Unique hazards now Identified in NFPA 10

    Dry chemical extinguishers are also more appropriate where a large quantity of flammable liquid could spill on a flat, horizontal surface containing combustible or noncombustible materials, and where spilled containers can create obstacles to the fire fighting activity, too.

    NFPA 10 requires a foam extinguisher, multiple non-vapor-suppressing agent extinguishers, or a dry chemical extinguisher of at least 10 lbs (4.54 kg) with a discharge rate of at least 1 lb/sec (0.45 kg/sec) for hazards that could become obstacle fires. The use of foam extinguishers works because the foam can float around objects, sealing the flammable liquid surface area.

    Another approach for flame extinguishment is simultaneously discharging dry chemical or halogenated agent from several angles. This rapidly knocks down the flames surface wide on flammable liquids. (The concern is that obstacles work to block the extinguishing agent from reaching flaming fuel. The objective would be to anticipate this and to cover the possibility with an application from another direction.)

    A third option that is just as effective but more economical is to employ a large-capacity/high-flow-rate dry chemical extinguisher. Where a dry chemical extinguisher is provided, NFPA 10 requires that it be at least 10lbs (4.54 kg) with a discharge rate of at least 1 lb/sec (0.45 kg/sec) for this type of fire. These extinguishers are effective with objects as large as 55-gallon drums. They work because the dry chemical creates a cloud on the surface of the flaming liquid (even hidden from view). In essence, a “total flooding” atmosphere is created that disrupts the flaming process.Training with specially designed dry chemical extinguishers, and practice attacking and gaining control of these types of fires, is highly recommended.

    Is the hazard real?

    It’s easy to say, “ideally suited for commercial and industrial applications that handle, transport, process or store flammable liquids and gases”, but what are the real life examples of hazards that need these extinguishers? Flammable liquid storage tanks are most often gravity fed to a piping network. At the tank, a flange connects the tank to the piping. If the piping is compromised, there could be a leak resulting in a pressurized fire. The same applies to pressurized pipe containing a flammable liquid.

    On November 25, 1990, a fire erupted at the tank farm at Stapleton International Airport, Denver, Colorado. From the time firefighting efforts were initiated, until the fire was extinguished, a total of 634 firefighters, 47 fire units and 4 contract personnel expended 56 million gallons of water and 28,000 gallons of foam concentrate. The fire burned for about 48 hours and about 3 million gallons of fuel stored in the tanks were either consumed by the fire or lost as a result of leakage from the tanks. Total damage was estimated by United Airlines to have been between $15 and $20 million. No injuries or fatalities occurred as a result of the fire, but United Airlines' flight operations were disrupted because of the lack of fuel to prepare aircraft for flight.¹

    To extinguish the tank farm fire, dry chemical was used at much larger quantities and flow rates than are outlined in NFPA 10, but the same concepts applied. Dry chemical was the only effective agent for the pressurized fires at the tank flanges.Another ignition source, where dry chemical extinguishers would be essential, is pallets of flammable liquid containers that are moved by forklifts. Also possible are three-dimensional fires in rack storage areas where forklifts can puncture a drum or container of flammable liquid.

    Summary and Conclusion

    Foam extinguishers are the best for blanketing horizontal surfaces and preventing re-flash of flammable liquids. Dry chemical is the most effective extinguishing agent for pressurized liquid and three-dimensional flowing liquid fires. They are also preferred for many large spill fires where obstacles are within the spill area. For pressurized liquid and three-dimensional flowing liquid fires, large capacity dry chemical extinguishers with specially designed nozzles and high flow rates must be employed. These extinguishers are also necessary where a dry chemical extinguisher is preferred for spill fires with obstacles. With some basic training on proper techniques and planning, a successful outcome can be expected on unique hazards using large capacity, high-flow dry chemical extinguishers.

    The NFPA has taken the lead in establishing minimum requirements for flow rates and manufacturers are producing equipment that meets the criteria. As the public becomes more familiar with this type of hazard and has the right equipment to respond to a flammable liquid pressurized fire, more successful outcomes can be expected.

    ¹ National Transportation safety Board, Aircraft Accident report, PB91-910408, NTSB/AAr-91/07, Aircraft Accident Fuel Farm Fire at Stapleton international Airport, Denver, Co, november 25, 1990.

    The above article is the opinion of the author and does not necessarily represent the position of an nFPA technical committee or the NFPA, and may not be considered to be or relied upon as such. Mark Conroy is an engineer with Brooks equipment Company and a principal member of the NFPA Technical Committee on Portable Fire extinguishers. Copyright 2010, all rights reserved.

    Click Here to view a PDF version