ESFR Warehouse Design and a Water Supply That Will Not Support It
A fire sprinkler contractor licensed in Illinois is preparing plans for a new 180,000 ft² single-story distribution warehouse in a Chicago suburb. The building has a 32 ft ceiling deck. The owner wants five-high rack storage of cartoned, unexpanded Group A plastic housewares to 24 ft, plus a 12 ft solid-piled floor area of the same commodity. The contractor's criteria sheet proposes ESFR pendent sprinklers, K = 25.2, 12 sprinklers operating at 35 psi minimum, with a 250 gpm hose allowance included. The public water supply flow test at the tap shows 68 psi static and 44 psi residual at 1,180 gpm through a 6 in. ductile service. The existing 4 in. backflow preventer has a published friction loss of 9 psi at 1,500 gpm. The project superintendent has asked whether the fire pump can be avoided by "running the numbers at 20 psi and 9 heads," and whether in-rack sprinklers are truly unnecessary. The AHJ has adopted NFPA 13 (2016 edition) without local amendments.
1. Is the proposed ESFR concept defensible for this commodity and storage height? What must be verified before it is committed to the plans?
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Concept. ESFR is a legitimate control strategy for cartoned, unexpanded Group A plastics stored to about 25 ft under a ceiling within the listing limits, and it is precisely the strategy that avoids in-rack sprinklers. So the idea is defensible only if the listing supports it.
Verify, in this order:
- Manufacturer's listing - maximum storage height, maximum ceiling height, minimum operating pressure, and number of operating sprinklers for that K-factor and deflector type. "12 heads at 35 psi" is a listing value, not a code choice; if the listing for this arrangement requires 40 psi or a different head count, the criteria sheet is wrong.
- Commodity and arrangement - cartoned unexpanded plastics are commonly listed; exposed expanded plastics (foam cups, loose foam) generally are not, and would force a different design. Confirm the actual packaging, rack depth, flue spaces, and aisle widths, and that the solid-pile area is within the listed height.
- Building geometry - ceiling slope, obstructions, and any mezzanine or duct within the ESFR obstruction rules.
- Hose allowance - 250 gpm must remain in the calculation; it is not optional and cannot be deleted to make the supply work.
2. Compute the required flow and evaluate the adequacy of the public supply. Show your arithmetic and state the conclusion.
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Required flow at the base of the riser
- Per head: Q = K√P = 25.2 × √35 = 25.2 × 5.916 ≈ 149 gpm
- Sprinkler demand: 12 × 149 ≈ 1,789 gpm
- Plus hose allowance: 1,789 + 250 = 2,039 gpm
Pressure available at 2,039 gpm (using the 1.85 exponent on the flow test):
- Ratio = 2,039 ÷ 1,180 = 1.728
- 1.728^1.85 ≈ 2.75
- ΔP = (68 − 44) × 2.75 = 24 × 2.75 ≈ 66 psi
- Available ≈ 68 − 66 = about 2 psi
Subtract the backflow preventer (roughly 12–15 psi at this flow) and the supply is not merely short - it is effectively zero. The 1,200 gpm-class service cannot feed an ESFR demand of this size at any usable pressure.
Conclusion. A fire pump (or a substantially upgraded supply, such as a new main plus a storage tank) is mandatory. A pump in the neighborhood of 2,000–2,250 gpm adding roughly 65–75 psi of net boost is the ballpark, but the final selection must come from a complete hydraulic calculation using the actual piping, the backflow curve, elevation to the most remote head, and the pump curve with its churn point checked against component pressure ratings. Never select a pump from a hand estimate.
3. The owner refuses a fire pump because of cost and space. What design alternatives would you present, and how would you check that one of them actually works against this supply?
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Present options that reduce demand rather than pretending the supply is adequate.
Option A - reduce storage height and use a control-mode design. If the arrangement is brought down to the height where NFPA 13's miscellaneous-storage provisions apply (verify the commodity/height limits allowed in the adopted edition's applicable chapter - do not assume it is 12 ft for Group A plastics), a density/area design may be used. Example check: 0.20 gpm/ft² over 1,500 ft² = 300 gpm, plus 250 gpm hose = 550 gpm. Supply at 550 gpm: ratio 550/1,180 = 0.466; 0.466^1.85 ≈ 0.244; ΔP = 24 × 0.244 ≈ 5.9 psi; available ≈ 62 psi. That demand is comfortably supportable. The trade-off is real: less cube, more floor area.
Option B - in-rack sprinklers with a control-mode ceiling design. Sometimes lower total demand than ESFR, but far more heads, more piping, and much higher maintenance burden.
Option C - upgrade the supply. New service, dedicated fire main, or a storage tank and pump.
Test every option the same way: total demand (sprinklers + hose) against the supply curve adjusted for backflow and elevation, then confirm every component's pressure rating. Reject any proposal that reduces the number of operating heads below the listing value without a listed basis - that is not value engineering, it is an unprotected building.