Click here for a printable version of this article.

July 2010: Flue Spaces

You might have heard someone mention flue spaces for rack storage, and wondered what flue spaces are and why they are needed.

Flue spaces are clear lines of sight from the floor up to the ceiling within rack storage areas. There are two types of flue spaces - transverse flues and longitudinal flues. Transverse flue spaces are spaces between rows of storage parallel to the direction of loading. Longitudinal flue spaces are spaces between rows of storage perpendicular to the direction of loading.

Flue spaces are necessary for rack storage arrangements to allow heat from a fire to vent vertically. This allows the ceiling-level sprinklers (and in-rack sprinklers if provided) to operate as quickly as possible, and slows down the horizontal fire spread within the rack. Flue spaces are also necessary to allow sprinkler water to reach the base of the fire down through the racks. Without enough water reaching the base of a fire on the lower tiers of the racks, the fire may not be controlled.

Unless in-rack sprinklers are provided at every storage tier, transverse flue spaces are always needed in rack storage arrangements regardless of building and storage height.

Longitudinal flue spaces are optional for double-row rack storage up to and including 25 ft. (7.6m) high per NFPA 13, Standard for the Installation of Sprinkler Systems. However, these flue spaces are needed for storage exceeding 25 ft (7.6 m) in height. FM Global 8-9, Storage of Class 1, 2, 3, 4, and Plastic Commodities, does not have this height requirement and makes these flue spaces optional for any storage height. However, longitudinal flue spaces are needed at least every 16 ft. in multiple-row racks. If longitudinal flue spaces are present, provide a minimum 3 in. (75 mm) net clear width (6 in. [150 mm] for multiple-row racks) vertically throughout the height of the rack.

Ensure flue spaces are a minimum net width of from 3 in. (75 mm) to no more than 24 in. (600 mm) wide (6 in. [150 mm] is preferred). 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. In other words, a rack upright (typically 3 in. [75 mm] wide) is not considered a flue space, due to the cross bracing used. If the space between two rack structures is more than 24 in. (600 mm), treat the space as an aisle for in-rack sprinkler design and installation purposes.

Maintain vertical alignment of flue spaces as closely as possible.

Ensure rolled commodities stored in racks are arranged so they cannot bulge into or encroach over flue spaces. If flue spaces are blocked during a fire, sprinkler water will not be able to penetrate through the racks and fire may spread horizontally.

NFPA 13 indicates transverse flue spaces should be provided between loads and at the rack uprights. However, a specific width is not listed. FM Global Data Sheet 8-9 indicates that 3 in. net transverse flue spaces should be provided at least every 4.5 ft. and 6 in. net transverse flue spaces should be provided at least every 9 ft. to qualify as open-framed. If these flue spaces are not present, then this storage should be protected as solid-shelf storage. Solid-shelf storage protection would require a more robust sprinkler system design.

Flue spaces are typically fairly easy to maintain with a little bit of operator training when rack storage is on pallets. However, when items are hand-stacked in racks, these flue spaces become more difficult to maintain; visual cues and even physical stops are often needed to ensure proper flue spaces are provided.

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