High-Piled Storage Demand Versus City Main Supply
A 180,000 sq ft single-story warehouse with a 32 ft ceiling is being converted from Class III palletized storage to 24 ft high rack storage of Group A plastics. The existing wet-pipe system has 2½ in risers. A hydraulic analysis for the new storage shows a demand of 1,900 gpm at 70 psi at the base of the riser, excluding the underground piping and backflow preventer. The city main at the street provides 1,200 gpm at 55 psi residual and 70 psi static. The underground piping and backflow preventer add 25 psi at 1,900 gpm. The building has no fire pump or water tank, and the water authority will not allow a connection larger than 6 in. The owner wants to avoid a new tank and asks whether the city main can be used with a booster pump only.
1. Determine whether the city main can satisfy the new demand with a booster pump only. Compare pressures at the same reference point.
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Convert the sprinkler demand to the street reference point: 70 psi at the base of the riser plus 25 psi for underground piping and backflow prevention equals 95 psi required at 1,900 gpm at the street. The city main can supply only 1,200 gpm at 55 psi residual, and no 1,900 gpm point is available. A booster pump increases pressure, but it cannot create additional flow. If a booster pump took suction from the city main at 1,900 gpm, the suction pressure would fall below the water authority’s minimum and could become negative. NFPA 20 requires a reliable suction supply; a city main that cannot deliver the required flow is not acceptable as the sole source. Therefore a booster pump alone is not adequate. The owner must provide a storage tank or another reliable source sized for the demand and duration, plus a fire pump. The city main can then serve as a fill line, not as the pump suction source, unless a hydraulic analysis demonstrates 1,900 gpm at an acceptable residual pressure.
2. If a suction tank and fire pump are added, what rated flow and pressure should the pump have, and what suction-side checks are required?
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Select a pump rated at least 1,900 gpm; standard ratings would make this a 2,000 gpm pump. The rated pressure must satisfy the system demand at 1,900 gpm, including the 70 psi at the base of the riser plus any discharge-side backflow preventer and fittings, so a rated pressure near 80 psi may be needed rather than 70 psi. Verify the pump curve: at 150% of rated flow, pressure must be at least 65% of rated pressure, and at churn it must not exceed 140% of rated pressure. On the suction side, confirm the tank can deliver 2,000 gpm without vortexing, that the suction pipe is sized for the flow, and that net positive suction head is adequate. Check the required duration; for a 2-hour demand, 1,900 gpm requires 228,000 gallons of usable storage. Also verify the tank refill rate and low-water protection so the pump cannot draw air.