Available Pressure at a Three-Story Office
A three-story, 48,000 ft² office building is proposed on a site served by an 8 in. ductile-iron city main. A flow test at the hydrant nearest the tap yields 70 psi static and 48 psi residual at 1,150 gpm. The sprinkler demand at the base of the riser is 780 gpm at 52 psi, including 250 gpm hose allowance. The connection from the hydrant to the building is 550 ft of 8 in. main with C=100. The building will have a 6 in. reduced-pressure backflow preventer with a calculated loss of 7 psi at 780 gpm and a 4 in. meter with a 3 psi loss. The contractor's preliminary calc uses the 70 psi static pressure and concludes no fire pump is needed. The owner wants to avoid a fire pump if possible.
1. Using the flow test, estimate the available pressure at the base of the riser at 780 gpm. Show your steps and state whether the demand is met.
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Static pressure is not available at flowing demand. Use the flow-test curve:
P = 70 − (70 − 48) × (780/1,150)^1.85 P = 70 − 22 × (0.678)^1.85 ≈ 70 − 10.7 = 59.3 psi
Then subtract the supply losses at 780 gpm:
- 550 ft of 8 in. main, C=100: Hazen-Williams gives about 0.0081 psi/ft, so ≈ 4.5 psi.
- Backflow preventer: 7 psi.
- Meter: 3 psi.
Available at the base of the riser ≈ 59.3 − 4.5 − 7 − 3 = 44.8 psi.
The demand is 52 psi. The system is short by about 7.2 psi, so the demand is not met. The 250 gpm hose allowance is already included in the 780 gpm demand; it is not an additional pressure loss.
2. What additional information and decisions are needed before selecting a fire pump?
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You need the pump curve and suction conditions, not just a single pressure deficit. Confirm the flow test is reliable and represents seasonal low pressure; verify the actual backflow and meter losses; check the AHJ's required safety margin; and evaluate the pump's churn pressure so it does not overpressure the system.
The demand is 780 gpm. A 750 gpm rated pump is below the demand, so the next standard NFPA 20 rating is 1,000 gpm. The pump must add at least the 7.2 psi deficit at 780 gpm plus margin. Also compare alternatives: upgrade the city main, reduce the demand, or add storage. The final selection must satisfy NFPA 20 and NFPA 13 and be accepted by the AHJ.
3. The contractor reruns the calculation with C=130 for the 8 in. ductile iron to avoid the pump. Is that appropriate?
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No. C=130 is not justified for unlined ductile iron and ignores aging and tuberculation. Even if C=130 were allowed, the main loss would drop only from about 4.5 psi to about 3.2 psi, saving about 1.3 psi - not the roughly 7.2 psi deficit. It cannot make the demand.
Use C=100 or the value supported by the AHJ and manufacturer. If the main is cement-lined, provide documentation. The correct fix is a fire pump, a larger supply main, or a reduced demand - not changing C to avoid a pump.