Hydraulic Calculation Dispute on a 13R System
A four-story, 60-unit apartment building is being sprinklered under NFPA 13R. The contractor's calculations use a design density of 0.05 gpm/sq ft over 1,000 sq ft, a 100 gpm hose allowance, and a 30-minute duration. The water supply flow test shows 70 psi static, 55 psi residual at 1,000 gpm. The building has a 6-inch service and a booster pump rated at 500 gpm at 40 psi. The plan reviewer notes that the building has combustible concealed spaces and a standpipe system. The contractor argues that 13R allows the lower density and that the pump is adequate because 'the numbers work out.'
1. Are the design density, area, and duration appropriate for a 13R system in this building?
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13R permits reduced densities and areas compared with 13, but the values must match the standard's tables for the occupancy and construction. A 0.05 density over 1,000 sq ft may be within 13R for certain light-hazard residential occupancies, but the contractor must confirm the specific table entry for the construction type and the presence of combustible concealed spaces. Duration for 13R is typically 30 minutes, which matches. The reviewer should require the contractor to cite the exact table and show that the building's construction and occupancy fit it; if combustible concealed spaces require protection, the design basis may change.
2. Is the booster pump adequate? Walk through the reasoning.
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Not necessarily. The pump is rated 500 gpm at 40 psi, but the system demand includes the sprinkler demand plus the 100 gpm hose allowance plus the standpipe demand if the standpipe is part of the same supply. The flow test shows 55 psi residual at 1,000 gpm, which is a relatively flat curve, but the pump's 500 gpm rating may be below the combined demand. The contractor must plot the pump curve against the system demand curve, including the hose and standpipe allowances, and verify the pump operates near its rated point without exceeding its maximum. If the combined demand exceeds 500 gpm, the pump is undersized or the demand must be reduced by design changes.
3. What standpipe considerations affect the hydraulic calculations?
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If the standpipe is supplied by the same system, the calculations must include the standpipe demand (flow and pressure at the topmost outlet) in addition to the sprinkler demand, and the pump and supply must handle the greater of the two or the combined demand as required. The reviewer should require the contractor to show the standpipe demand separately and demonstrate that the pump and service can meet it. The 6-inch service and the pump rating must be checked against the combined demand, not just the sprinkler demand.