Office Building Water Supply Shortfall
A two-story, 20,000 ft² office building is proposed with an NFPA 13 wet-pipe system. The sprinkler contractor classifies the occupancy as Ordinary Hazard Group 2 and calculates a 1,500 ft² remote area at 0.20 gpm/ft². A 250 gpm hose allowance is added at the riser. The calculated system demand at the base of the riser is 550 gpm at 65 psi. A hydrant flow test near the service shows a static pressure of 62 psi and a residual pressure of 48 psi flowing 1,200 gpm. The contractor proposes to connect directly to the 8-inch city main without a fire pump, arguing that the static pressure is close to the demand and that the building is only two stories. The fire official asks for a water supply analysis before plan approval.
1. Calculate the available flow at 20 psi and the available pressure at the 550 gpm system demand.
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Use the flow-test relationship Q = Qr × sqrt((Ps − P)/(Ps − Pr)). At 20 psi, Q20 = 1,200 × sqrt((62 − 20)/(62 − 48)) = 1,200 × sqrt(3.0) = 2,078 gpm. At the demand flow of 550 gpm, P = Ps − (Ps − Pr)(Q/Qr)² = 62 − 14(550/1,200)² = 62 − 2.94 = 59.1 psi. So the supply can deliver 550 gpm at only about 59 psi at the test hydrant elevation, before any elevation or fitting losses to the riser. The demand at the riser is 65 psi, so the direct connection is short by about 6 psi. Many authorities also require a safety margin, which makes the deficit larger.
2. Should the fire official approve the direct connection? What alternatives or documentation are needed?
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No. The available pressure at 550 gpm is about 59 psi, below the calculated demand of 65 psi. The contractor's argument that static pressure is close to demand is wrong because pressure falls as flow increases along the system curve. Options include a fire pump sized for the demand plus margin, an upgraded service main, or a suction tank. Reducing demand is acceptable only if the occupancy and design allow a lower density, remote area, or hose allowance. The reviewer should require a hydraulic calculation that includes elevation, backflow preventer, fittings, and the hose allowance, and should not approve until the supply curve is at least equal to the demand at the riser.