Water Supply Margin at a Memory-Care Conversion
An existing 4-story, 96,000 sq ft hotel is being converted to a memory-care facility. The new wet-pipe system demand at the base of the riser is 850 gpm at 65 psi, including elevation and sprinkler piping losses. A flow test on the 8 in city main produced a static pressure of 78 psi and a residual pressure of 55 psi at 1,000 gpm. The connection from the main to the riser is 120 ft of 6 in ductile iron; the meter and reduced-pressure backflow preventer are estimated to lose 14 psi at 850 gpm, and the underground supply line loses 2 psi at that flow. The contractor's hydraulic calculation shows a demand of 850 gpm at 65 psi and a supply of 61 psi at 850 gpm, and concludes the system is adequate because 61 psi is close to 65 psi. The AHJ requires a fire flow test for hydrants, but the owner does not want a fire pump.
1. Reconstruct the available pressure at the riser and determine whether the system demand is met. Show the calculation logic and state the conclusion.
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Start with the flow test, not the static pressure. The test drop is 78 - 55 = 23 psi at 1,000 gpm. At 850 gpm, the drop scales approximately as (850/1,000)^1.85, or about 0.74, so the drop is about 17 psi. Available pressure at the main at 850 gpm is therefore about 78 - 17 = 61 psi.
That 61 psi is not available at the riser. The meter and reduced-pressure backflow preventer lose about 14 psi at 850 gpm, and the 120 ft underground line loses about 2 psi. Available pressure at the base of the riser is approximately 61 - 14 - 2 = 45 psi.
The demand at the base of the riser is 65 psi. The supply is short by about 20 psi, so the system is not adequate. The contractor's calculation appears to have compared the main pressure directly with the riser demand and omitted the meter, backflow preventer, and underground losses. The fact that 61 psi is 'close' to 65 psi is not a code basis for acceptance; the available pressure must meet or exceed the demand at the calculated flow. A fire pump, a larger service, or a redesigned system is required.
2. The owner asks whether a small booster pump can solve the problem. What factors must be evaluated before selecting a fire pump, and what code and plan-review issues arise?
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A booster pump is possible, but it must be sized as a fire pump under NFPA 20, not selected as a plumbing booster. The pump must provide the required pressure at the system demand of 850 gpm, not just add a nominal pressure boost. The designer must evaluate the suction pressure at the pump, the city main's ability to supply the pump plus fire flow, the backflow preventer location and pressure loss, and the pump's rated flow and churn pressure.
Other issues include the pump controller and power source, a test header and drain, fire pump room enclosure and access, and the AHJ's acceptance requirements. The water purveyor must confirm that the pump will not create an unacceptable suction condition or reduce hydrant fire flow. The owner should also understand that a fire pump increases maintenance and testing obligations.
The plan reviewer will expect a hydraulic calculation showing the pump curve, suction pressure, and system demand, plus a fire pump submittal. If the pump takes suction from a main that is also needed for fire flow, the design must demonstrate that both demands can be met. Do not let a small 'booster' label hide the fact that this is a fire pump installation.