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Photo of a fire test taken shortly after sprinkler operation.

Protection of exposed, expanded Group A plastics stored in racks

The 2013 and prior editions of NFPA 13, Standard for the Installation of Sprinkler Systems, did not include sprinkler system design criteria for exposed (uncartoned), expanded Group A plastics stored in racks, since this type of protection was referenced as being outside the scope of Chapter 17 (the chapter in NFPA 13 that addresses the rack storage of plastics). Recognizing this glaring gap in sprinkler system design requirements, NFPA’s Fire Protection Research Foundation (FPRF) coordinated an effort to undertake research that would develop technical substantiation for designing a sprinkler system to protect this challenging fire risk using only ceiling sprinklers.

A few examples of commodities that may be characterized as exposed expanded Group A plastics include cushioned furniture, mattresses, foam plastic containers and trays used for packaging raw meat in retail facilities. Some of the characteristics of this type of fire that make it very challenging to protect with fire sprinklers include the following:

  • Fast development with high heat release rate (HRR) generated within a short period of time
  • Rapid fire travel (both vertical and horizontal) within the storage array
  • Effectiveness of pre-wetting from sprinkler discharge is reduced due to limited absorption of water by the exposed expanded plastic commodity

To provide technical oversight for this research initiative, the FPRF formed a Technical Panel which was comprised of several experts within the fire sprinkler community. The Technical Panel pursued a very effective strategy to address the challenging characteristics of this fire.

During this test series, the vertical fire growth was so rapid that the fire typically impinged on the ceiling within 45 seconds after the fire was initiated at the base of the storage array. To address the high HRR generated within a short period of time, the Technical Panel chose to use large K-factor ESFR technology. A K363 (K 25.2) pendent ESFR sprinkler discharging 4.1 bar (60 psig) which correlates to a discharge density of approximately 80 mm/min (1.95 gpm/ft2) discharge density was used for the majority of this testing.

To address the rapid horizontal fire travel within the storage array, the Technical Panel incorporated the use of vertical barriers within the array that extend from the bottom to top of the storage. These barriers are to be spaced a maximum distance of 5 m (16.5 ft.) apart and assisted in containing the fire to an area that could be effectively attacked by the sprinkler system.

Finally, to address the reduced effectiveness of sprinkler pre-wetting, the Technical Panel conducted the testing using 2.4 m (8 ft.) aisles to reduce the potential for the fire to jump from the storage bay where the fire was originated to an adjacent storage bay.

Based upon the data generated as a part of this extensive research effort, the 2016 Edition of NFPA 13 includes new design criteria for this challenging commodity stored in racks under ceilings up to 12.2 m (40 ft.) high. A total of 10 large scale fire tests were conducted at UL’s facilities in Northbrook, Illinois to validate the new sprinkler system design criteria. The first series of fire tests was initiated in July 2012, and the last series of tests was completed in October 2014.

An overview of the new sprinkler design criteria, which utilizes a combination of vertical barriers to mitigate horizontal fire travel and nominal K=363 lpm/(bar)1/2 (25.2 gpm/(psi)1/2 ) Early Suppression Fast Response (ESFR) sprinkler technology, is described in the following table. The reports prepared as a part of this research effort can be downloaded free of charge from NFPA’s FPRF website at nfpa.org/research/fire-protection-research-foundation.

For more information, go to www.ul.com

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Kerry M. Bell is Principal Engineer, Fire Sprinkler and Pump Equipment for UL LLC.

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