Hotel Combined System: Supply-vs-Demand at the Riser Base
A 5-story, 120,000 sq ft hotel (light hazard throughout, residential-style guest floors) is being laid out with a combined wet sprinkler and Class I standpipe system fed from a 6 in. city main. The only water supply data is a hydrant flow test at the tap: static 68 psi, residual 40 psi while flowing 1,150 gpm. The site is flat, the pump room is at grade, and the topmost 2½ in. hose valve is 58 ft above grade. The layout technician's worksheet applies 0.10 gpm/sq ft over 1,500 sq ft, adds 100 gpm of hose allowance, adds 500 gpm of standpipe flow, and concludes that a 500 gpm, 40 psi booster pump is "more than enough" because the test still showed 40 psi at 1,150 gpm. The owner has already released that pump for purchase. The AHJ has flagged the hydraulic calculation and is asking for a supply-versus-demand analysis at the base of the riser before the submittal can be approved.
1. Rebuild the supply-versus-demand comparison at the base of the riser and state whether the specified 500 gpm, 40 psi booster will work.
Read the worked answer
Supply side. Convert the one-point test into a curve. Flow at 20 psi: 1,150 x sqrt((68-20)/(68-40)) = 1,150 x 1.309 = about 1,505 gpm. Available pressure at any flow: P = 68 - 28 x (Q/1,150)^2. At the combined demand of 650 gpm (150 gpm sprinkler + 500 gpm standpipe): P = 68 - 28 x (0.565)^2 = 68 - 8.9 = 59 psi.
Demand side. NFPA 14 requires 100 psi residual at the topmost 2½ in. outlet while 500 gpm flows. Add elevation (58 ft x 0.433 = 25 psi) plus riser, cross-main and hose-valve friction at 500 gpm (about 20 psi), giving a base-of-riser requirement near 145 psi.
Do not double count the hose stream. The 500 gpm standpipe flow is the hose stream; adding the 100 gpm light-hazard hose allowance on top of it inflates the demand without basis.
Gap. 145 - 59 = 86 psi at 650 gpm. A 40 psi boost leaves the system roughly 45 psi short, and the pump cannot even pass 650 gpm at its rated point. The correct order of magnitude is roughly 750 gpm at 90 to 95 psi, with final selection plotted above the demand point on the pump curve, not merely touching it at rated flow.
2. The mechanical contractor responds with a 1,000 gpm pump rated at 120 psi and says the extra capacity covers everything. What must you check before that pump is accepted?
Read the worked answer
A larger pump solves the top-of-building deficit but creates a lower-floor problem, so check four things.
Churn pressure. At 120 psi rated, churn is about 110 percent, near 132 psi, plus 68 psi city static = about 200 psi at the pump discharge with no flow. Check it against the 175 psi rating of valves, fittings, and especially hose valves on the lower floors. A hose valve at the first floor would see roughly 195 psi, which is not acceptable; pressure-reducing valves or a pressure-limiting pump arrangement are required, and the PRV locations must be computed floor by floor, not assumed.
Suction condition. At 1,000 gpm the city curve gives only about 47 psi. Verify the pump's NPSH requirement against that, and confirm the 6 in. main and the tap can actually pass 1,000 gpm without excessive velocity and friction that would flatten the curve you used.
The 150 percent point. Confirm the pump still delivers at least 65 percent of rated pressure at 1,500 gpm; this is where undersized pumps usually fail and where the city supply may be exhausted.
Test validity. Confirm the flow test was taken at the tap elevation, with no large users on, and with the same main configuration that will serve the building.
Oversizing the pump without PRVs simply moves the failure from the top floor to the bottom floor.
3. The architect asks whether the building can be built with no fire pump at all. Which options are legitimate and which are shortcuts you should refuse?
Read the worked answer
Legitimate options:
- Ask the water purveyor to upsize the tap or main, or to confirm a higher-pressure main nearby; rerun the flow test at the new tap and redo the analysis.
- Provide a suction tank and pump, or a ground-level tank with a booster, if the purveyor cannot deliver.
- Re-examine the standpipe configuration only within code limits (for example, whether the building genuinely requires Class I outlets everywhere, or whether a Class II/III arrangement with 1½ in. stations is permitted by the adopted code and the AHJ) - this reduces demand only if the code truly allows it.
Shortcuts to refuse:
- Deleting the 500 gpm standpipe flow or cutting the residual below the 100 psi minimum because "the building is fully sprinkled."
- Counting the standpipe flow as the hose allowance in one place and adding the 100 gpm sprinkler hose allowance in another, or dropping hose allowance to make the numbers close.
- Lowering the design density below 0.10 gpm/sq ft or shrinking the remote area without a code basis.
- Using the 40 psi residual at 1,150 gpm as proof of adequacy; pressure at a much lower demand is a different point on the curve.
If none of the legitimate paths close the 86 psi gap, the pump is not optional - it is the design.