Designing fire systems for flammable, combustible liquids

02/21/2014


Common situations

Figure 2: A palletized storage arrangement of combustible and flammable liquids in cartoned and plastic containers is shown. Courtesy: Starr Tech Although this article is not intended as a design manual, here are some routine concepts and how to handle them.

Floor storage versus rack storage: The required ceiling densities are fairly similar for floor and rack storage, but rack storage increases the quantities per unit area. With floor storage, the ceiling sprinklers must absorb convective heat and also control the fire, whereas with rack storage, the in-rack sprinklers do most of the fire control. In most instances, the storage of liquids in racks will require the provision of in-rack sprinklers per NFPA 30, IFC, and FM. Some arrangements may only have a proven protection scheme for either floor or rack storage.

Foam-water sprinklers versus water-only sprinklers: Foam is generally preferred for the protection of flammable/combustible liquids over water-only. Foam discharged through sprinklers blankets the flammable liquid surface. This results in several advantages over water-only sprinklers. Foam typically results in fire extinguishment versus fire control, seals the liquid against re-ignition, and requires lower sprinkler design densities. The latter results in a lower total water demand, which can translate into cost savings. Generally, the expense of installing a foam-water system over water-only is not significantly higher and existing sprinkler systems can be retrofitted for foam-water relatively easily. Unfortunately, there are fewer situations for which proven protection schemes with foam-water have been tested. This does not mean that the application of foam protection is narrower than water-only. It simply means that there has been more testing to date with water-only than with foam. Chapter 16 of NFPA 30, Table 3404.3.6.6 of the IFC, and FM Data Sheet 7-29 contain the required densities for many storage arrangements.

Spill containment: This is a key component in limiting the spread of pool fires. NFPA 30 takes the least restrictive approach. There are several situations for which spill containment is not required, the main ones being containers under 10 gal in volume and Class IIIB liquids (refer to Figure 16.8.1). The IFC takes a more restrictive stance but also exempts Class IIIB liquids from containment. This is addressed in sections 2704.2 and 3403.4.

The FM Global containment requirements are a bit different. Liquids with flash points of 500 F or greater and in containers 40 gal or greater do not require containment under most conditions. There is a separate data sheet for containment, 7-83. Just because containment is not required does not mean that it should not be provided. The majority of flammable liquid fires require substantial volumes of water to control and extinguish. This fire protection water, mixed with products of combustion and possibly the stored chemicals, will flow to the lowest point—typically the storm sewer. Such discharge can result in unwanted environmental impacts. Some individuals use the latter as a case against sprinklers. This is a misguided approach as water quantities discharged as the result of manual fire extinguishment significantly exceed the quantities discharged by sprinklers. The best approach is to provide containment whether or not it is required. In many cases, innovative means of containment, such as using truck loading wells, offer low-cost solutions. One aspect that is not typically addressed by the codes is the design of the containment. Containment areas should be arranged to provide localized containment. In other words, the goal should be to keep a liquid pool as small as possible and prevent it from traveling to other areas. Drainage of the liquid to a safe area is generally preferred over impoundment because the fuel is removed from the fire area rather than being merely held.

Limited quantities: NFPA 30 and the IFC both provide for an exemption of flammable/combustible liquid protection requirements up to a volume threshold. This is referred to as maximum allowable quantities (MAQ). These quantities vary from 30 to 13,200 gal, depending on the liquid class (refer to NFPA 30 Table 9.6 and IFC Table 2703.1.1(1)). These quantities can generally be doubled if kept in a building protected with automatic sprinklers. They can also be doubled if the liquids are kept in flammable liquid storage cabinets, although the practicality of such becomes questionable on a larger scale.

The big advantage with the MAQ method is that as long as the quantities are not exceeded, the stricter protection requirements do not apply. The use of this exemption should be made judiciously, as a fire involving hundreds of gallons of liquids could still result in a total loss of the building, even if allowed by code. Again, complying with minimum code requirements is no guarantee against a major loss. With the exception of mercantile, industrial, storage, and other special occupancies, neither IFC nor NFPA 30 limits the quantities of Class IIIB liquids in a sprinkler-protected building, providing that the sprinkler protection is adequate for such storage. This means that Class IIIB liquid storage does not need to meet the other requirements for a hazardous occupancy under these specific conditions.

Relieving-style containers: NFPA 30 and FM Global give protection credit for relieving-style containers (refer to NFPA 30, Table A.16.2.3 or FM DS 7-29, Appendix A), whereas non-relieving-style containers get no such credit. A relieving-style container is a metal container arranged to relieve the expanding internal pressure when exposed to fire. Typically, the difference is that relieving-style containers are provided with plastic plugs/caps that melt when the container is exposed to heat. This may appear to be counter-intuitive, but a 2-in. opening is better than a burst drum that turns into a fireball. The IFC does not differentiate between these types of containers.

Plastic containers: The low cost and chemical resistance of plastic containers versus that of steel ones make them attractive to product manufacturers. The IFC generally defers to NFPA 30 concerning allowable container types and volume limits. Both NFPA 30 and the IFC allow flammable and combustible liquids to be packaged in plastic containers with size restrictions; however, the indoor protection of liquids in plastic containers is one of the most challenging situations in the industry. There are few proven sprinkler protection schemes for this situation.

The IFC is silent on the container type (i.e., plastic or metal) in its indoor protection criteria. In the author’s opinion, this is a significant oversight that should not be ignored. NFPA 30 does address the protection of liquids in plastic containers. Presently, the only acceptable protection method per NFPA 30 is to store plastic containers in racks while providing the racks with face and flue sprinklers and horizontal fire barriers. Other methods that meet NFPA 30 include the storage of plastic containers in one or more inside cutoff rooms, each not exceeding a floor area of 500 sq ft. Each room can hold a maximum of 5,000 gal of flammable liquids, regardless of container type. The ceiling sprinkler system should be suitable for a 0.60 gpm/sq ft density over the area of the cutoff room. FM Global’s general position is that liquids in plastic containers cannot be protected; only that the provision of the recommended protection will prevent the building from burning down. The protection requirements are similar to but more specific than those in NFPA 30 (refer to FM DS 7-29, Tables 12 through 18 for details). If none of the above solutions is feasible, plastic containers should be stored outdoors, subject to the appropriate pile and height limitations and spatial separations to buildings, property lines, and public ways.

Outdoor storage: Although some products are sensitive to extreme temperatures, most flammable/combustible liquids are not subject to freezing. Thus, storing containers outdoors in a yard offers an attractive and low-cost alternative to a warehouse. In general, the protection requirements for outdoor storage are lax. The goal is that a fire should not expose buildings, public streets, or neighbors and can be reasonably controlled and extinguished by the fire department; however, the fire protection practitioner should understand that all storage within a single fire area (i.e., not separated by a large open space) will likely be subject to a total loss. Noncombustible shelter structures without sprinklers with open sides are acceptable to shield the containers from the weather. Minimum 20-ft-wide access ways are required for the fire department. NFPA 30, Table 15.3 also contains limitations on pile size and height, as well as on separation distances between piles, buildings, property lines, and streets. This table assumes that “protection for exposures” is provided. Such protection typically consists of manual or automatic monitor nozzles that cover the storage area. Spill containment is required for the outdoor storage area. Although not recommended, a limited amount of liquids in containers is allowed to be stored adjacent to buildings.

Detached low-value buildings: When no proven protection schemes exist and the products must be stored indoors, an option is to store the containers in a detached unprotected building. The idea is that in case of fire, the building and contents are lost, but the loss is limited to a “sacrificial” building rather than more important areas. The NFPA 30 requirements are that the building must be separated by at least 200 ft and the storage pile quantities and heights are limited; otherwise, there is no limitation on the overall quantity of liquids.

Unfortunately, the NFPA 30 allowances for omitting sprinklers conflict with the IBC, which essentially does not allow nonsprinklered indoor liquid storage areas. If such a situation arises, it should be clearly explained to the fire prevention authority that no proven protection schemes exist. The authority may still insist that sprinkler protection be provided under the code requirements, but in that case, the professionals charged with the risk evaluation need to clearly understand that despite sprinklers, the entire building and contents will likely be a loss.

Three things to remember

  1. The protection requirements should be determined before attempting to lay out storage arrangements. It is better to base the storage layout on protection restrictions beforehand than to find out that an entire storage plan must be changed because the original arrangement cannot be protected.
  2. A position of “meeting code” does not necessarily mean that the building won’t burn down or that stakeholders (i.e., insurance companies) will have a warm and fuzzy feeling that the building is adequately protected. A major loss as the result of poor engineering decisions will not be soon forgotten.
  3. Additional protection options will likely become available in the future as more full-scale tests continue to be conducted; however, this will take time. In the meantime, decision-makers, designers, and approving agencies should use caution in merely “meeting the code.” There is no partial credit with flammable liquid fires.

Dominique Dieken is a senior fire protection engineer with Starr Technical Risks Agency Inc. (Starr Tech), a member of the C. V. Starr & Co. Inc. group of companies. Starr Tech is an insurance agency serving the power generation, petrochemical, chemical, energy, and oil and gas industries and other complex occupancies with property insurance coverages and technical loss control support.

The author would like to acknowledge Timothy Burchett for his diligent input into this writing. Burchett, also at Starr Tech, has nearly 20 years of experience with FM Global in evaluating and improving the protection of flammable and combustible liquid storage arrangements.


Additional References

Nugent, D. P., Directory of Fire Tests Involving Storage of Flammable and Combustible Liquids in Containers, Schirmer Engineering Corp., September 2004 (available from the Society of Fire Protection Engineers, Bethesda, Md.).

Nugent, D. P., Developing Sprinkler System Design Criteria for Flammable and Combustible Liquid Storage, AON Risk Consultants, April 2006.



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