How to design fire sprinkler systems for high-piled storage

Fire sprinkler insights

• There are several types of fire suppression systems available and fire sprinklers are widely used.
• Warehouse and storage buildings have unique requirements for fire suppression, specifically when it comes to high-piled storage.

The use of fire sprinkler systems for protection of storage materials has been around for more than 100 years. The first recognized patent for a sprinkler system was issued in early 1700s and the National Fire Protection Association (NFPA) was established in New York City in 1896. A few years later, NFPA standards were endorsed and employed in the United States and Canada. NFPA 13: Standard for the Installation of Sprinkler Systems  continued its evolution and during the 1950s, the “Red Book” (NFPA 13) was the to-go reference for installation and inspection of fire sprinkler systems.

Fire sprinkler systems are one of the most effective and economic options for protection of property and people against fire hazards. They are also the first choice for protection of warehouses due to their simple mechanism, fast responsiveness, efficiency, high success rate and vast availability for alternatives and equipment selection.

No other type of fire protection system can perform comparable when considering overall benefits and effectiveness of fire sprinkler systems for warehouse and storage protection applications.

Fire sprinkler reference tools

There are several applicable codes, standards and references as it relates to fire sprinklers and suppression in general.

• Computational Modeling of Fire Sprinkler Spray Characteristics Using Fire Dynamics Simulator by Matthew J. Bourque, Thomas A. Svirsky.

• International Building Code (IBC).

• International Fire Code (IFC).

• NFPA 13: Standard for the Installation of Sprinkler Systems.

Significant progress has been made in warehouse and storage industry with the ongoing evolution of advanced structures and modern construction materials. New generations of warehouse buildings have morphed into much taller and larger areas for racking up increasing amounts of goods in more easily accessible arrangements.

As a result, fire protection schemes for distribution and warehousing industry sometimes equates to an extremely challenging task due to the complexity of assemblies and frameworks.

What fire sprinklers are needed in storage areas?

Standard spray sprinklers perform an outstanding job for protection of most of the commercial light and ordinary occupancies, including business and residential buildings and have served greatly to cover this necessity for many decades.

However, for challenging warehouse fires they fall short due to physical characteristics of water spray droplets, the size and consequently insufficient velocity that give rise to momentum for penetration into fire plumes. Droplet velocity is an important spray characteristic when determining if the sprinkler will be able to extinguish a fire. Fire plumes have an upward velocity and if the water droplet does not have enough downward vertical velocity and momentum, it will not be able to penetrate the plume and reach the base of the fire. Moreover, the longer it takes the droplet to penetrate the fire plume, the higher the chance the droplet will evaporate.

Because rooms with higher ceiling will have higher fire plumes, the velocity will need to be larger than in rooms with lower ceilings. Contrary to standard sprinklers, storage application sprinklers with larger orifices and resulting larger K-factors and advanced deflector designs, have been developed and tested to provide enhanced water distribution, increased drop sizes and amplified velocities for producing higher densities of water at the same or very close pressures as standard sprinklers perform.

Pick modules and conveyors warehousing systems and ceiling fire sprinkler protection. Courtesy: MAG Material Handling

Based on the performed tests on spray drops velocities and water spray pattern development, it has been determined that droplet diameter has a direct correlation with terminal velocity and Reynolds number: the larger the droplet diameter, the higher the terminal velocity and the higher the Reynolds number can be obtained. As a reminder, the Reynolds number represents the effect of air (friction) on the droplet and the terminal velocity is an important characteristic to determine if the droplet will end up with enough velocity to penetrate the upward plume velocity.

These fire sprinklers with larger K-factors of 11.0 through 34.0 are designed and listed for storage applications. Considering their advanced design and significantly larger orifices, storage sprinklers can reduce the call for higher water pressures generally required for the high-density systems demand.

The sprinkler types used in high-piled storage applications include the following:

• Early suppression fast response (ESFR).

• Control mode specific application (CMSA).

• Control mode density area (CMDA).

Early suppression fast response

ESFR sprinklers requirements for storage applications can be tracked down in Chapter 23 of NFPA 13. In addition to specific guidelines found in Chapter 23, general criteria for ESFR sprinklers must be practiced in accordance with Chapter 20 of NFPA 13 standard.

Purpose: In the 1980s, ESFR sprinklers were developed and designed to extinguish the fire and eliminate the need for extinguishment and final mop-up by firefighters.

Design considerations: ESFR sprinklers are listed for use in wet systems only. They are also recognized to protect light and ordinary hazard occupancies, as well as any storage setup designed per ordinary hazard I and II and extra hazard I and II design criteria. In addition, ESFR sprinklers are the favored choice for protection of high-piled storage of class I-IV commodities, including solid piled, palletized, rack storage; and group A plastics, rubber tires, rolled paper, plastic motor vehicle components and high-bay record storage applications.

The ceiling-only ESFR sprinkler are not allowed to protect storage on solid shelf racks in ceiling-only applications and storage with open-top containers. In case of installation of ESFR sprinklers adjacent to systems with standard-response sprinklers, a draft curtain of noncombustible construction and at least 2 feet in depth must separate two sprinkler systems and a clear aisle of 4 feet, centered below the draft curtain, required for systems separation. Lastly, the clearance between the ESFR sprinkler deflector and the top of storage shall be 36 inches.

Design area and hydraulic calculations: According to Section 14.2.8 of NFPA 13, the maximum area of coverage of any ESFR sprinkler cannot exceed 100 square feet, unless existing obstructions created by structural members result in moving a sprinkler along the branch line not more than 1 foot from its allowable spacing. The consequent area of coverage cannot exceed 110 square feet per sprinkler and average actual floor area protected by the moved sprinkler and adjacent sprinkler head should not exceed 100 square feet.

Additionally, adjacent branch lines will maintain the same pattern of spacing and spacing between two sprinklers never exceed 12 feet. By the same token, the minimum permitted protection area of coverage for ESFR sprinklers cannot be less than 64 square feet.

For the purpose of establishing design area, Section 27.2.4.4 of NFPA 13 standard is the proper reference; design area for ESFR sprinklers shall consist of the most hydraulically demanding area of 12 sprinklers, consisting of four sprinklers on each of the three branch lines, unless other specific numbers of design sprinklers are indicated in other sections of the standard (e.g., rubber tire protection with 20 sprinklers in four adjacent branch lines).

For design densities, sprinkler minimum pressure and design area requirements, Chapter 20 and 23 must be consulted.

The design area will be comprised of the hydraulically most demanding in accordance with Chapters 19 and 20 in most cases, but for special design approaches, the outlined directions per Chapter 26 shall be sought. For ESFR gridded systems, and all other gridded fire sprinkler systems, a minimum of two additional sets of hydraulic calculations at immediately adjacent areas on each side along the same branch lines will be produced to demonstrate peaking of most demanding area friction loss referencing the main design area.

In case of using computerized hydraulic calculations programs, rather than providing two sets of hydraulic calculations, a single set of calculations is acceptable. Notes that include peaking information for calculations performed by computer program, must be among the hydraulic calculations “Summary Sheet” information.

Fire sprinkler protection under storage pick module system. Courtesy: MAG Material Handling

Control mode specific application

CMSA sprinklers specific requirements for storage applications can be found in Chapter 22 of NFPA 13. Additionally, general design criteria for CMSA sprinklers is scattered through the different sections of Chapter 20.

Purpose: Contrary to ESFR sprinklers, CMSA sprinklers have been designed and developed to control the fire; they do not necessarily extinguish the fire. In essence, they provide greater flows with lower pressures with larger K-factors and as a result, the minimum allowable K-factor for CMSA sprinklers is K-11.2 to achieve better plume penetration.

Design considerations: CMSA sprinklers can be used in wet, dry or preaction systems and must be installed in accordance with their listings. Quick-response CMSA sprinklers can be used to protect light and ordinary hazard occupancies, whereas standard-response CMSA sprinklers are permitted for use in ordinary hazards in addition to storage occupancies. Both quick- and standard-response sprinklers are allowed to be used to meet criteria in Chapter 20 through Chapter 25, including general storage (Chapter 20), high-piled storage (Chapter 21), CMSA-specific requirements (Chapter 22), alternative systems (Chapter 24) and rack storage systems (Chapter 25).

Storage conditions that can influence greatest water demand in a building include pile height, clearance to ceiling, pile stability and array of storage and shall be considered for the design of CSMA sprinkler systems. To a great extent, CMSA sprinklers rely on direct attack to gain rapid control of both the burning fuel and ceiling temperatures. Therefore, interference with the discharge pattern and obstructions to the distribution should be avoided.

Design area and hydraulic calculations: The design area for CMSA sprinklers must comply with the specific requirements of Section 27.2.4.3.1. Per this section, the design area will consist of a rectangular area having a dimension parallel to the branch lines at least 1.2 times square root of the area protected by the number of sprinklers to be included in the design area. The design area protected by the number of sprinklers to be used by the 1.2 rule shall be based on the maximum allowable area per sprinkler and any fractional sprinkler shall be carried to the next higher whole sprinkler.

In systems having branch lines with an insufficient number of sprinklers to comply with 1.2 requirement, the design area will be extended to include sprinklers in adjacent branch lines supplied by the same cross main. Moreover, the minimum area of coverage of any CMSA sprinkler may not be less than 80 square feet and the maximum area may not be more than 130 square feet. Design must account for minimum operating pressure and number of sprinkler heads in the design area obtained from Chapter 22.

Control mode density area

Specific requirements for the storage applications of CMDA sprinklers can be found in Chapter 21 of NFPA 13. Additionally, general design criteria for CMDA sprinklers can be found in assorted sections of Chapter 20.

Purpose: Like CMSA sprinklers, CMDA sprinklers have been designed and developed to control the fire; they do not extinguish the fire like ESFR sprinklers.

Fire and building code requirements

Chapter 32 of International Fire Code (IFC) presents conditions for recognizing various classes of commodities and other features of fire and life safety systems regarding to high-piled storage occupancies as defined in Chapter 2 of the IFC. It also sets down requirements for protection of high piled storage on pallets, in racks or on shelves where the top of storage is greater than 12 feet in height or 6 feet for high-hazard commodities.

High-piled rack storage system. Courtesy: MAG Material Handling

It should be noted that in addition to the requirements of Chapter 32, the material-specific conditions should comply with the following chapters (or with NFPA 13 for paper records):

• Aerosols: Chapter 51.

• Flammable and combustible liquids: Chapter 57.

• General storage of combustible material: Chapter 3.

• Hazardous materials: Chapter 50.

• Storage of combustible fibers: Chapter 37.

• Storage of combustible paper records: NFPA 13.

Commodity classification in Chapter 32 of IFC lines up with NFPA 13 standard’s high-piled class I, II, III, IV, storage classifications and plastics in A, B and C grouping. In addition, the amount of group A plastics in class I, II, III and IV commodities without resulting in a Group A plastic classification is limited to the quantities specified in Section 3203.9.

However, to regulate the main commodity classification of the products and materials, IFC Table 3203.8 must be referenced. Table 3203.8 considers the product and the packaging if listed with the item. Products with additional packaging consisting of Group A plastics shall be classified in accordance with Section 3203.9. The system designer or consulting engineer should keep in mind that in case of conflicts occurring between requirements of fire code and NFPA 13 standard, requirements of the IFC applies.

High-piled storage areas, considered for storage of a different commodity class than adjacent areas, must be designed and correctly contain Class I, II, III, IV or high-hazard commodities. The selection of a high-piled storage area must also be according to the highest hazard commodity class stored in the area with exception to engineering analysis referenced in Section 3204.2.

Fire protection and life safety features for high-piled storage areas are referenced in Section 3206. Table 3206.2 specifies type(s) of protection required based on the commodity classification, size of storage area, whether they are open to public (e.g., department stores), storage type, pile dimension, storage height and pile volume (application to solid-piled, shelf and palletized storage).

How big can a high-piled storage area be?

The size of each high-piled storage area includes the footprint of the actual high-piled storage racks, shelves or piles. It also includes the interior aisles within the footprint of the storage area, and an aisle around the perimeter of the footprint with a minimum width as indicated in Section 3206.10.1 or the dimension to a wall or full height wall, whichever is less.

If a building contains multiple high-piled storage areas, the aggregate of all high-piled storage areas may be used for the application of Table 3206.2 unless the high-piled storage areas are separated by fire barriers with a minimum fire-resistance-rating of one hour. In the case of sprinklered buildings, high-piled storage areas must be separated by 100 feet or more of free of high-piled combustible storage area. In addition to general requirements of Section 3206, for specific requirements of high-piled storage areas, the following sections of IFC shall be consulted:

• Solid-piled and shelf storage: Section 3207.

• Rack storage: Section 3208.

• Automated storage: Section 3209.

• Specialty storage: Section 3210.

Fire sprinkler protection overhead and below obstructions wider than 4 feet. Courtesy: MAG Material Handling

Managing change and code advantages

Unexpected changes and scheduled modifications are anticipated in storage occupancies. It is a common practice to frequently alter the storage arrangement within storage warehouses. The storage layout and heights are always changing and, as a result, a more adaptable sprinkler protection that affords flexibility is highly desired.

Storage application sprinklers with notably large K-factors permit ceiling-only sprinkler system installations (e.g., ESFR sprinklers), which provide a greater water density without a significant increase in minimum required listed discharge pressure. This is a great advantage over design and installation of complex in-rack sprinkler systems and puts the facility owners in more workable position for their new storage setup and providing maximum operational flexibility while minimizing risk of accidental discharge.

In addition to change in storage layout, the ceiling sprinkler system densities can be regulated and changed through implementation of sprinklers with alternative larger K-factors closely designed for same application with an increased density requirement.

As an example, replacing of ESFR K-14.2 sprinkler heads with ESFR K-16.8 sprinklers after the removal of permission of K-14 sprinklers use in buildings greater than 35 feet in NFPA 13-2016 and later editions, allows the implementation of K-16.8 through direct replacement for smaller K factors (K-14.2) for least financial impact and same hydraulic requirements.

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