Rack Storage of Group A Plastics in a Class III Warehouse
A 20,000 sq ft single-story warehouse has a 30-ft ceiling and an existing wet-pipe system designed for Class III commodity stored 12 ft high. The design is 0.25 gpm/sq ft over 2,000 sq ft with K=8.0 pendent sprinklers on 12-ft by 12-ft spacing, and the hydraulic calculation shows 20 psi at the base of the riser. A hydrant flow test gives 70 psi static and 50 psi residual at 1,500 gpm. A new tenant wants to install double-row rack storage of unexpanded Group A plastics to a height of 20 ft, with 8-ft aisles and 6-in transverse flue spaces. The existing system has no in-rack sprinklers and 18-in clearance between the top of storage and the ceiling. The contractor proposes to keep the same layout but increase the design density to 0.30 gpm/sq ft over 2,000 sq ft. The AHJ has not yet approved the change and asks for your evaluation of the existing system and the contractor's proposal.
1. Can the existing sprinkler layout and K=8.0 heads be retained for the new storage? Explain using the commodity classification and storage height.
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No. The existing system is designed for Class III commodity at 12 ft. Unexpanded Group A plastics are a high-challenge commodity, and rack storage at 20 ft is a different design basis. The existing layout has no in-rack sprinklers, uses K=8.0 heads at 12-ft by 12-ft spacing, and provides only 18-in clearance to the ceiling. Those features may not be listed or approved for the new commodity and height.
Increasing the density alone does not change the K-factor, spacing, storage arrangement, or the need for in-rack sprinklers. The design must be redone using the applicable NFPA 13 rack-storage provisions, which may require ESFR sprinklers or a control-mode density/area design with in-rack sprinklers. The 18-in clearance also needs to be checked against the listing of the selected sprinkler; ESFR and some storage sprinklers require greater clearance. The existing layout cannot be approved as-is.
2. The contractor says 0.30 gpm/sq ft over 2,000 sq ft is more than the old 0.25, so it is conservative. Evaluate that reasoning and identify what else controls the design.
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The reasoning is incomplete. Density alone does not make a storage design conservative. The design is controlled by the commodity classification, storage height and arrangement, aisle width, flue spaces, ceiling height, sprinkler type and K-factor, maximum coverage area, clearance to the ceiling, and whether in-rack sprinklers are required. A higher density applied to an arrangement that is not listed for the hazard may still fail to control a fire.
A 0.30 gpm/sq ft density over 2,000 sq ft gives 600 gpm of sprinkler demand before hose allowance. With K=8.0 heads at 12-ft by 12-ft spacing, each head would flow about 43.2 gpm at about 29 psi, but that does not establish code compliance for Group A plastics at 20 ft. The contractor must submit a complete storage design with the correct design basis, not just a density increase.
3. If the owner selects ESFR, what major layout and supply checks must be completed?
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First, verify that the selected ESFR sprinkler is listed for unexpanded Group A plastics at the proposed storage height and arrangement. Confirm the maximum coverage area, minimum pressure and number of design sprinklers, and the required clearance below the deflector. ESFR typically requires about 36 in of clearance below the deflector, so the existing 18-in clearance is likely a problem and the storage height or ceiling may need to change.
Check the ceiling construction, slope, obstructions, aisle widths, and flue spaces against the listing. Then perform a hydraulic calculation using the ESFR demand plus hose allowance and compare it with the supply curve. For example, if the listing requires 50 psi at the base and the flow test gives 70 psi static and 50 psi residual at 1,500 gpm, the available pressure at the ESFR flow must be calculated, not assumed. Also verify that the fire pump, if any, and the underground main can deliver the required flow. Submit the listing, calculations, and plans to the AHJ before installation.