Flow Test Says 52 psi at 900 gpm: Pump Sizing and the 150% Suction Rule
A new 90,000 sq ft ordinary hazard Group 2 manufacturing building has a calculated sprinkler demand of 1,050 gpm at 72 psi at the base of the riser, including the hose stream allowance. The only supply is a 6 in dead-end city main. The flow test used two 2 1/2 in hydrant nozzles with a pitot reading and produced 76 psi static and 54 psi residual flowing 940 gpm, corrected for hydrant elevation and the test header. The mechanical contractor has already ordered a horizontal split-case fire pump rated 1,250 gpm at 50 psi, and the owner likes the extra capacity 'for future expansion.' A 4 in double-check backflow preventer will be installed on the domestic/fire service, and the manufacturer's submittal shows 8 psi friction loss at 1,000 gpm. The AHJ requires a 10 psi safety margin in the hydraulic calculation. The project is in a county where the water purveyor will not guarantee pressures above the tested values.
1. Using the flow test, determine the pressure available at the system demand point and state whether the pump as ordered will work.
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Use the 1.85-power relationship between flow and pressure drop, since the test establishes the drop at one flow. Here the drop is 76 - 54 = 22 psi at 940 gpm. To find available pressure at 1,050 gpm: (1,050/940)^1.85 = (1.117)^1.85, which is about 1.23, so the drop grows to about 27 psi and the available pressure is 76 - 27 = 49 psi at 1,050 gpm from the city, before adding the 8 psi backflow loss at the service. Subtract that loss and roughly 41 psi is available at the riser. The demand is 72 psi plus the AHJ's 10 psi margin, or 82 psi. The pump must therefore add about 41 psi at 1,050 gpm, and any pump selected must be checked at or above the demand flow, not merely at its rating. A 1,250 gpm pump at 50 psi rated would appear to close the gap at the duty point, but rated-point arithmetic is not the test.
2. Explain why the 150% of rated flow check may disqualify this pump and what alternatives exist.
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A fire pump is selected against its suction supply at 150% of rated capacity, because the pump will run far out on its curve and the suction side must still deliver positive pressure. For a 1,250 gpm pump, 150% is 1,875 gpm. Back to the city curve: (1,875/940)^1.85 is about 3.6, so the drop from static would be roughly 79 psi against a 76 psi static, driving the suction pressure slightly negative. That is not an acceptable installation; the pump would cavitate and the supply would be unreliable at the worst possible moment. Options: reduce the pump rating so that 150% of rating falls inside the supply's capability - but a pump rated below the 1,050 gpm demand is poor practice and pushes the operating point past its rating - or reduce the system demand through larger mains, a relocated riser, or a lower remote area, or accept a suction supply that is not the city main. The defensible answer here is a fire-resistive suction tank sized for the duration and a pump drawing from it, or an upgrade of the city main confirmed in writing by the purveyor. Do not let 'future expansion' drive a pump the water supply cannot feed.
3. What effect does the backflow preventer have on the final selection, and how should it be represented in the calculation?
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The double-check assembly is a point loss that sits between the supply and everything downstream, so it must be deducted from available pressure at the flow being evaluated, not from the static pressure alone. Its loss rises with flow: the submittal's 8 psi at 1,000 gpm becomes roughly 8 x (1,050/1,000)^1.85, about 8.8 psi at the demand flow, and grows further at 150% of pump rating - approximately 8 x (1,875/1,000)^1.85, on the order of 26 psi. That last number matters because at 150% flow the pump is already starved for suction pressure. Treat the device as a fixed element in the hydraulic model with a manufacturer's curve, confirm the listing for fire service use and the approving authority's acceptance, and include the friction in both the demand-side and suction-side calculations. Where a bypass or a detector check with different friction is proposed, the lower-loss device does not remove the obligation - the supply-side check at 150% still has to close, and the AHJ will want the submittal sheet attached to the calculation.