Hydraulic Squeeze on a Warehouse Rack System
You are designing a new 40,000 sq ft single-story warehouse with 32 ft ceilings and 24 ft high double-row rack storage of Class III commodity in cartons on shelves with flue spaces. The building has a 6-inch dead-end public main at 55 psi static and 1,400 gpm available at 20 psi residual at the tap. The owner wants a wet system with 12 heads per branch and a 1,500 gpm fire pump already ordered but not yet installed. Preliminary calculations show the rack demand alone is 1,150 gpm at 45 psi at the base of the riser, and adding the 250 gpm hose allowance puts you at 1,400 gpm. The general contractor wants to submit plans in two weeks and has asked whether the pump is even needed.
1. Evaluate whether the 1,500 gpm pump is justified, and identify the hydraulic variables you must verify before agreeing to submit.
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Start by separating supply from demand. The available supply at the tap is 1,400 gpm at 20 psi residual, but that is at the tap, not the riser base. You must subtract elevation, meter, and underground friction losses to get the true residual at the riser base, then compare it against the 1,150 gpm at 45 psi demand plus 250 gpm hose. If the adjusted supply curve falls below the demand point, the system is undersized and the pump is justified. Even if the raw numbers appear close, a 20 psi safety margin and the pump's rated curve (not just churn) govern. Confirm the rack demand is correct for the actual storage height, aisle width, and flue space, because those drive the design density and remote area. Verify the hose allowance is applied at the correct point, and confirm whether the pump is rated at 1,500 gpm at 10 psi or another point on its curve. Do not submit until the supply curve, demand point, and pump curve are plotted on the same graph.
2. The GC proposes reducing branch lines from 12 to 8 heads to cut cost. Explain the hydraulic and code consequences of that change.
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Reducing heads per branch reduces friction loss on the branch line but does not reduce the required remote area or density for the hazard; those are set by commodity and storage configuration. Fewer heads per branch may actually increase the number of branch lines and fittings, and it changes the balance point of the tree. More importantly, the design must still deliver the required density over the entire remote area, so you cannot simply shrink the flowing head count to lower demand. Check that the smaller branch still meets minimum pipe size and velocity limits, and that the remote area calculation reflects the actual number of operating heads. If the change reduces total demand below the supply curve, document it with a new hydraulic calculation and confirm the pump selection is still appropriate. Any change to head spacing or branch layout must also satisfy spacing and coverage rules, so a cost-driven layout change requires a full recalculation, not a shortcut.
3. What information should you request from the water purveyor before finalizing the design?
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Request a current flow test report with static pressure, residual pressure, and flow, including the date and location of the test and the hydrant used. Ask for the elevation of the test hydrant relative to the building, the size and material of the main, and whether the main is dead-end or looped. Confirm whether the 1,400 gpm at 20 psi figure is a single-hydrant result or a combined supply, and whether the purveyor anticipates any main flushing, valve closures, or future demand that could reduce it. Obtain the meter size and anticipated pressure loss through the meter and backflow assembly, since those are often the largest unmodeled losses. Finally, ask for written confirmation that the supply is available year-round and that no booster pump restrictions apply. Without this, any hydraulic calculation is built on assumptions that may not survive plan review.