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March 2014: Flue Spaces in Racks - NFPA 13 vs. FM Global Data Sheet 8-9

Flue spaces are necessary for rack storage arrangements to allow heat from a fire to quickly vent vertically. This is critical in order to help ensure the ceiling-level sprinklers will operate as quickly as possible. When flue spaces are blocked, this leads to greater horizontal fire spread within the rack, thus increasing the fire area. Flue spaces are also necessary to allow sprinkler water penetration down through the racks. This is extremely important when ceiling-only automatic sprinkler protection is provided. Without sufficient water penetration to burning commodities stored on the lower tiers of the racks, fire suppression and/or control may not be achieved and extensive fire damage could result.

Advances in sprinkler technology (i.e., suppression mode or ESFR sprinklers) in the last 25 years or so have been effective in reducing the need for in-rack automatic sprinklers (IRAS) in storage racks in buildings as high as 45 ft. When ceiling-only automatic sprinkler protection is provided in buildings this high, it becomes even more of a necessity to maintain adequate flue spaces in racks. If the center-core of the sprinkler discharge is blocked, then fire suppression may not be achieved. Obstructions and blocked flue spaces can slow down the velocity of sprinkler discharge and reduce water droplet size to a point that it cannot penetrate the fire plume, thus making the sprinkler ineffective.

Both NFPA and FM Global recognize the importance of flue spaces and have addressed this in their respective rack storage standards for many years. Until about 5 years ago, the requirements or recommendations were mostly the same. Below are some of the highlights in the current editions of the applicable standards.

NFPA 13, Standard for the Installation of Sprinkler Systems (2013 edition):

Flue Space Requirements for Storage Up to and Including 25 ft. (7.6 m)

Flue Space Requirements for Storage Over 25 ft. (7.6 m)

Where multiple-row racks of any height have no longitudinal flue or where double-row racks with storage up to 25 ft. (7.6 m) in height have no longitudinal flue, the situation shall not be considered solid shelves where transverse flues exist at maximum 5 ft. (1.5 m) intervals and additional in-rack sprinklers shall not be required.

Annex C: Explanation of Test Data and Rack Storage Procedures states in Section C.13 that test results indicate flue spaces of about 3 in. (76.2 mm) allow reasonable passage of sprinkler water down through racks.

FM Global Property Loss Prevention Data Sheet 8-9, Storage of Class 1, 2, 3, 4 and Plastic Commodities (January 2012 edition):

Flue Spaces for Rack Storage Protected by Ceiling-Level Sprinklers Only:

The Main Differences between the two Standards:

To be effective as outlined above, flue spaces must be a minimum net width of 3 in. and ideally about 6 in. wide. A flue space's net width is a measure of its gross width minus any horizontal obstructions, such as rack uprights, located within the flue space.

Please read our July 2010 web article “Flue Spaces" for more details and definitions regarding flue spaces.

Risk Logic can help you ensure that adequate flue spaces are provided for the rack storage at your facility. Please contact us for additional details.