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)
- In double-row and multiple-row racks without solid shelves, a longitudinal (back-to-back clearance between loads) flue space shall not be required.
- Nominal 6 in. (152.4 mm) transverse flue spaces between loads and at rack uprights shall be maintained in single-row, double-row, and multiple-row racks.
- Random variations in the width of flue spaces or in their vertical alignment shall be permitted.
Flue Space Requirements for Storage Over 25 ft. (7.6 m)
- Nominal 6 in. (152.4 mm) transverse flue spaces between loads and at rack uprights shall be maintained in single-row, double-row, and multiple-row racks.
- Nominal 6 in. (152.4 mm) longitudinal flue spaces shall be provided in double-row racks.
- Random variations in the width of the flue spaces or in their vertical alignment shall be permitted.
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:
- Maintain all transverse flue spaces at a minimum net width of 3 in. (75 mm) throughout the vertical height of the rack. Provide transverse flue spaces a maximum of every 4 1/2 ft. (1.4 m) horizontally when their net width is less than 6 in. (150 mm) or when their vertical alignment cannot be maintained. The maximum horizontal distance between well-maintained, vertically aligned transverse flue spaces can be increased to 9 ft. (2.7 m) when their net width is 6 in. (150 mm) or greater.
- Longitudinal flue spaces are not required in double-row racks; however, if they are provided, maintain a minimum net width of 3 in. (75 mm) throughout the vertical height of the rack. Provide net 6 in. (150 mm) wide longitudinal flue spaces within multiple-row racks a maximum of every 16 ft. (4.8 m) horizontally.
- Treat storage racks not having flue spaces as described above as racks with solid shelves. Base the size of the shelf area on the presence of well-maintained, vertically aligned flue spaces having a minimum net width of 3 in. (75 mm). If well-maintained, vertically aligned flue spaces having a minimum net width of 3 in. (75 mm) cannot be provided, protect the racks based on the presence of solid shelves greater than 64 sq. ft. (6.0 sq. m) in area.
The Main Differences between the two Standards:
- NFPA 13 allows random variations in the width of the flue spaces or in their vertical alignment and FM Global Data Sheet 8-9 does not. It should be noted that previous versions of FM Global Data Sheet 8-9 (as recent as May 2006) allowed random variations in the width of the flue spaces or in their vertical alignment. The January 2008 edition of the Data Sheet changed the recommendation to “Maintain vertical alignment of flue spaces as closely as possible.”
- FM Global Data Sheet 8-9 does not require longitudinal flue spaces in double-row racks to any height, while NFPA 13 requires nominal 6 in. longitudinal flue spaces in double-row racks over 25 ft.
- FM Global is more prescriptive with regards to transverse flue widths and horizontal spacing.
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.