Understanding Obstacle Fire Hazards

Fires occurring within industrial and commercial applications are commonly controlled and extinguished with portable fire extinguishers, provided those fires are caught early enough and the proper extinguisher is used. That’s why it’s important to understand the various characteristics associated with specific fire extinguisher models placed within such settings, as well as the types and configurations of class “B” fires that can be presented.

A typical application example involves a commercial aircraft ramp fire, where a fuel leak has spread and pooled completely around the rear wheels of the aircraft and responders instinctively grab the first fire extinguisher available. When the first extinguisher observed is taken from the cab of a nearby support vehicle, this quick response effort is only able to temporarily reduce and control a portion of the flame—until the extinguisher empties and flames again engulf the full area. However, when another responder selects and utilizes the fire extinguisher specifically installed on the nearby airport fueling cart for properly addressing such fire situations, this operator is able to effectively extinguish the fire.

With this type of flammable liquid release, the fire around the aircrafts rear wheels represented an obstacle fire situation requiring the right piece of equipment for extinguishment. While both fire extinguishers may have had identical 40B:C numerical ratings, the first smaller 5 pound extinguisher model pulled from the vehicle cab only had an agent discharge flow rate of .41 pounds per second (PPS).

The second larger 20 pound fire extinguisher installed on the fuel cart per the NFPA-10 (Portable Fire Extinguishers) and NFPA-407 (Aircraft Fuel Servicing) standards had an agent flow rate of 2.18 PPS. The significant and key difference being their agent discharge application rates, which will clearly produce different results.

Understanding how to properly identify and address special class “B” flammable liquid fire situations within various commercial and industrial applications with portable extinguishers, is something that the fire protection industry is now attempting to better communicate. The current 2018 edition of the NFPA-10 standard covering “Portable Fire Extinguishers” specifically addresses such special hazards with minimum equipment recommendations in section 5.5.1.

Back in 2007, the NFPA 10 standard added various new minimum equipment requirements for properly addressing class “B” obstacle fire situations and hazards. The petrochemical industry and fire equipment manufacturers have understood those special equipment needs for many years, however it’s seldom a topic that many have ever received any formal instruction or training on.

Fire Extinguisher Hardware and Numerical Ratings

While the numerical class “B” portable fire extinguisher rating system is commonly referenced within NFPA 10 for the selection and placement of extinguishers within various types of occupancies, there are important reasons why these ratings are not specifically referenced when addressing liquid fire situations beyond simple spills.

The relationship between numerical class “B” fire extinguisher ratings and the extended equipment discharge durations required to achieve them, has a direct impact on the extinguishing agent application rates being discharged and applied onto fires. Because agent application rates are necessary to successfully accomplish the extinguishment of many other common class “B” fire situations, minimum extinguisher agent capacities and agent flow rates are now being specified in lieu of simple numerical class “B” fire ratings.

To better understand this rationale, some history and knowledge of the existing ANSI/UL-711 class “B” fire extinguisher rating system recognized and referenced within NFPA is necessary.

The numerical class “B” fire rating system is only based on open square steel test pan fires, having various established surface areas. These test pans are filled with a 2-inch depth of heptane fuel, leaving a pre-established 6-inch portion of the pans side board exposed from the top lip of the test pan to the actual fuel surface.

Among other requirements, there is a minimum 1-minute pre-burn requirement before test firefighters are allowed to approach and fight the fire from only one side of the pan. It is important to point out that the burning liquid fuel surface is perfectly flat, open, and still. While this pan fire test criteria established back in 1955 may not initially appear very realistic or real-world, it was primarily only intended to provide a controlled, consistent, and repeatable burning configuration for measuring and checking various extinguisher agent and hardware performance characteristics.

Every fire extinguisher is required to have a minimum effective agent discharge duration time of at least 8 seconds, with numerical fire ratings higher than 20B requiring extended discharge durations. The fire test criteria extended these minimum equipment discharge durations in 1965, when the numerical rating system was expanded to include much larger outdoor fires and numerical ratings of 30B to 640B. At that time, extending the minimum discharge durations for the larger fires was believed necessary, to help dictate the selection and utilization of larger extinguisher sizes.

The class “B” numerical rating system additionally recognizes key firefighting differences between experienced and novice operators with a built-in safety factor, that’s based on some actual live fire test comparisons. The various numerical fire surface area references are essentially reduced to reflect 40 percent of the area that experts are required to repeatedly extinguish. So the given numerical reference number basically represents the anticipated square foot surface area of spill fire situations, that novice operators should be capable of extinguishing.

Thus, if the expert could repeatedly demonstrate the extinguishment of a 100-square-foot test pan fire with an extinguisher model having a minimum 13 second discharge duration, it could earn and display the 40B numerical fire rating. That 40B rating reference, reflects a potential 40 square-foot surface area spill fire situation that novice operators should be able to effectively address.

The following diagram helps summarize the class “B” requirements contained within the ANSI/UL-711 standard, showing the relationship between actual fire test pan sizes and minimum effective hardware discharge time requirements necessary for obtaining each numerical fire rating.

To obtain higher numerical ratings and extend equipment discharge durations for any given extinguisher size, the equipment manufacturers began to focus on various converging/diverging nozzle designs which reduced agent flow rates, extended ranges and increased discharge durations.

The reduced rates of extinguishing agent being discharged from those higher rated extinguisher models however started to generate problems, when fighting many real-world types of class “B” fire situations like those involving obstacles.

During the 1970s, the petro-chemical industry began to discover that fire extinguisher performance was much more effective when the larger and older equipment models having lower numerical class “B” fire ratings were being used. This led to additional research which ultimately identified just how important increased agent application rates were for successfully accomplishing the extinguishment of specific fire burning situations.

The equipment manufacturers then began to develop special “high flow” or “fast flow” models of fire extinguishers to specifically address such hazards. However, because of the shorter discharge durations associated with high flow models, they often will carry much lower numerical class “B” fire ratings than similar sized units.

Obstacle Fire Hazards and Extinguisher Recommendations

To help identify and recognize various potential obstacle fire hazards, it is necessary to first understand which class “B” situations can present them. Obstacle fire situations are presented whenever sizeable objects become surrounded by or directly involved within burning liquid fuel surfaces.

When burning liquid fuels surround obstacles, they present a condition where the utilization of the proper extinguishing agent and hardware become critical. While obstacle fire situations can occur with other fire classifications, the quick flame reaction and spread times associated with class “B” flammable liquid and gas fire situations generally represent the greatest challenge.

Some common forms of obstacles present within various commercial and industrial fire situations include fuel containers, vehicle wheels, fuel handling piping, pumps, machinery, hardware fixtures and drain grates, as well as other structure-related materials like concrete curbs, posts and pillars.

Whenever obstacles within class “B” fire situations are of a sufficient size to potentially block or hinder the application of non-vapor securing types of extinguishing agents like dry chemicals and vaporizing agents, only temporary control is possible unless the proper extinguishing equipment is utilized. Site hazard surveys can help identify and determine in advance, the need for this specialized equipment.

Because hand portable fire extinguishers have limited agent capacities and discharge times, its important to ensure they are properly selected and distributed to obtain the best possible chance of extinguishment. When smaller or low-flow extinguisher models are typically utilized on real-world obstacle fire situations, operators may initially gain some temporary control, only to ultimately have any hidden portion of flame behind the obstacle relight the exposed fuel surfaces when the discharge is exhausted.

To successfully address the extinguishment of obstacle fire situations, NFPA 10 paragraph 5.5.3 establishes three specific equipment recommendations for properly covering them.

The first recommendation is the use and application of a vapor suppressing fire extinguishing agent like foam. From a hand portable fire extinguisher standpoint, this is limited to extinguisher models containing aqueous film forming foam (AFFF) or film forming fluro protein (FFFP) pre-mixed foam agent solutions.

Because properly applied foam solutions remain on the surface of exposed horizontal liquid fuels and effectively suppress the release of combustible vapors, they allow operators the ability to move around obstacles to positions where the application of agent onto all exposed fuel surface areas is possible for extinguishment. The surface tension properties of applied foam solutions additionally allow them to float and spread over horizontal liquid fuel surfaces. The cooling and vapor suppression characteristics associated with foams are especially desirable whenever liquid fuel properties present any potential re-flash or auto-ignition concerns with extended pre-burn times beyond 1 to 2 minutes.

Only AFFF and FFFP pre-mixed foam fire extinguisher models having 6 liter, 2.5 gallon, and 33 gallon agent capacities are currently listed and approved. It is important to understand that while some models may utilize an alcohol resistant “AR” type of foam designated to address both hydrocarbon and polar solvent types of class “B” liquids, the specific fire extinguisher model may have only been fire tested with a hydrocarbon fuel like heptane.

Because of the reduced application rates associated with foam solutions delivered from hand portables, some “AR” formulations may not be as effective on given polar solvent/ alcohol resistant types of fuels. With polar solvent types of class “B” fuels, the specific foam extinguisher manufacturer should be consulted for additional performance test information to ensure proper selection and use.

The second recommendation is the simultaneous application of non-vapor suppressing types of fire extinguishing agents from multiple sources. This is usually accomplished by having two or more extinguisher operators approach and fight obstacle fires together. The simultaneous application of extinguishing agents from different angles can effectively eliminate any potential blind spots presented by the obstacle and accomplish total extinguishment.

With training and practice, this teamwork approach for fighting obstacle fires can be very effective, however it’s important to remember that it requires multiple responders and equipment to be available at the time of a fire.

The third recommendation is the selection of larger capacity fire extinguishers having higher agent discharge rates of flow. The use of these models in obstacle fire situations will essentially overwhelm and wrap around obstacles to eliminate blind spots and fully extinguish such fires. Extinguishers having higher agent discharge rates are able to address most common workplace-sized obstacles, such as a standing 55-gallon barrel displacing an approximate surface area foot print of 3 square feet. 

There are many acceptable high-flow dry chemical hand portable and wheeled fire extinguisher models available containing agent capacities from 10 to 350 pounds. Most high-flow extinguishers will show and reflect agent flow rate characteristics on their nameplates, as well as within the product literature published by the manufacturers.

The old rule of thumb associated with portable fire extinguisher training and discharge rates is that when you double the rate of agent being applied onto fire situations, you effectively triple your firefighting capability. Various teamwork-related fire extinguisher training evolutions clearly demonstrate how increased agent application rates can improve extinguishment objectives. The excess application of extinguishing agent directly onto fire situations will additionally also help compensate for any minor operator technique errors, making them especially ideal for use by operators having limited or minimal firefighting experience.

The 2018 edition of NFPA 10 paragraph 5.5.3 (3) identifies large capacity models having a minimum agent capacity of 10 pounds and a minimum agent discharge rate-of-flow of 1 pound per second (0.45 kg/sec) as being necessary to address class “B” obstacle fire situations.

Obstacle Applications and Training

Live fire demonstrations comparing agent flow rate performance differences, clearly reflect just how effective high-flow extinguisher models can be on class “B” fire situations. Fire testing conducted by the committee task group utilizing 50-square-foot test pans containing a 2 inch depth of heptane fuel and a standing 55-gallon barrel centered to represent a typical industrial obstacle example — demonstrated how the discharge of dry chemical from extinguisher models delivering a minimum 1 pound per second flow rate were able to repeatedly achieve extinguishment.

By comparison, the application of dry chemical from models having twice the size and numerical class “B” fire rating, but significantly lower agent flow rate could not achieve extinguishment, regardless of how they applied the agent.

Because most emergency response fire situations involving the accidental release of liquid fuels will likely present potential obstacle situations, the hand portable extinguisher models professional firefighters, fire brigades, and emergency response team members prefer will have agent discharge flow rates of between 1 to 2 pounds per second.

Most wheeled fire extinguisher models typically deliver agent discharge rates well over 1 pound per second, with some exceeding 4 pounds per second. This is why wheeled models are commonly specified and placed around class “B” fuel storage and transfer areas such as fuel farms, process areas, warehouses, rail yards, and flight lines.

Since NFPA-10 began referencing minimum extinguisher agent capacities and flow rates, other application-specific standards have started establishing similar recommendations for properly addressing anticipated class “B” hazards. Some examples include NFPA 410, Aircraft Maintenance; NFPA 610, Guide for Emergency and Safety Operations at Motorsports Venues; NFPA 59, Production, Storage, and Handling of Liquefied Natural Gas; and NFPA 385, Tank Vehicles for Flammable and Combustible Liquids.

Advanced industrial flammable liquid firefighting schools such as the international “HellFighter-U” evolutions conducted at the Texas A&M fire field stress and demonstrate the importance of agent application rates. Such schools utilize leading industry experts as instructors to effectively train thousands of municipal and industrial firefighters from around the world on how to properly address various flammable liquid fire situations with extinguishers.

There are other recognized fire extinguisher training school evolutions where similar live flammable liquid obstacle fire training opportunities can also be obtained.

Whenever attempting to address various potential forms of special class “B” hazard situations, it’s essential to understand why fire extinguisher agent discharge characteristics often become much more important than numerical ratings.

While more NFPA standards are beginning to establish various minimum fire extinguisher sizes and agent flow rate recommendations, many still simply reference NFPA-10 and rely upon the reader’s ability to understand and properly apply the special class “B” equipment recommendations.                                                                     

J.R. Nerat is the technical manager at Badger/Kidde Fire Protection and the NFPA Industrial Section representative on the NFPA 10 Technical Committee.