Hydraulic Supply Shortfall on an Office-to-Storage Conversion
A 24,000 sq ft single-story office building is being converted to Class III storage with a 12 ft ceiling. The design is a wet-pipe system at 0.20 gpm/sq ft over 1,500 sq ft, plus a 250 gpm hose allowance. The contractor's calculation shows 850 gpm at 55 psi at the base of the riser. A flow test at a nearby hydrant produced a static pressure of 55 psi and a residual of 20 psi at 1,200 gpm. The test hydrant is at the same elevation as the riser, but the service main is 180 ft of 6-inch ductile iron with a meter and a 4-inch reduced-pressure backflow preventer. The contractor says the static pressure alone proves the supply is adequate and proposes to skip a fire pump. The building official is asking for a defensible supply analysis before plan approval.
1. Using the hydrant test, estimate the available pressure at 850 gpm and determine whether the proposed system can be supplied without a fire pump. State your assumptions.
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Use the square-root supply relationship, Q = K times the square root of delta P, where delta P is the drop from static to residual pressure. At the test point, delta P = 55 minus 20 = 35 psi at 1,200 gpm, so K = 1,200 divided by the square root of 35 = 202.8. At 850 gpm, delta P = (850 divided by 202.8) squared = 17.6 psi. Available residual at 850 gpm = 55 minus 17.6 = 37.4 psi. The demand at the riser is 850 gpm at 55 psi, so the supply is short by about 17.6 psi before adding the service-main and backflow losses. The static pressure alone is not a supply test. The test hydrant must also be adjusted for elevation and for friction loss between the hydrant and the riser; if the test was at a hydrant on the same main, those adjustments may be small, but they cannot be ignored. Conclusion: without a pump or a larger supply, the design does not close. The contractor must either provide a pump, upsize the service, or obtain a new flow test at the actual tap.
2. The contractor proposes to add the 4-inch RPZ with an 8 psi loss at 850 gpm and to install a 500 gpm fire pump rated at 20 psi. Evaluate that proposal.
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An RPZ adding 8 psi at 850 gpm raises the demand to 63 psi. The deficit becomes 63 minus 37.4 = 25.6 psi. A 500 gpm pump rated at 20 psi is not a 20 psi boost at 850 gpm; a centrifugal pump curve falls as flow increases. At 850 gpm the boost may be only 10 to 15 psi, so the pump may not close the gap. Also, a 500 gpm rated pump operating at 850 gpm is at 170 percent of rated flow, beyond the normal allowable operating range and possibly outside its listed curve. The correct approach is to select a pump whose curve, not just its rated point, provides at least the required pressure at the actual demand flow, and then check the pump at 150 percent of rated flow. Add a safety margin for gauge error and daily supply variation, and confirm the pump shutoff pressure does not exceed component ratings.
3. The building official asks whether the 250 gpm hose allowance can be deleted because the building has no standpipe. How should the designer respond?
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No. The hose allowance is a water-supply component of the sprinkler demand, not a standpipe requirement. For this storage occupancy the allowance is typically 250 gpm; it represents fire-department hose use through a fire-department connection or outside hydrants. Deleting it would understate the demand and produce a nonconservative calculation. The allowance could be reduced only if the applicable standard and the AHJ specifically permit a lower allowance for the occupancy and the building has no fire-department connection, which is unusual for a 24,000 sq ft storage building. Even then, the calculation must show the basis. The designer should keep the allowance and size the supply or pump accordingly.