Combined Riser: The Standpipe That Never Made It Into the Calculation
A five-story office building has 60 ft of elevation from the base of the riser to the top floor landing. The system is light hazard, wet, with quick-response sprinklers, and the 4 in. riser is combined: it feeds the sprinkler system and a Class I standpipe with 2½ in. outlets at every landing. The submitted calculation shows 0.10 gpm/ft² over 1,500 ft² plus a 100 gpm hose allowance - 250 gpm at 71 psi at the base of the riser - and states 'supply is adequate.' The standpipe does not appear in the calculation. The city flow test at the riser reads 70 psi static and 60 psi residual at 900 gpm. The contractor wants the calculation accepted as submitted to avoid buying a pump.
1. What is missing from the calculation, and what does the standpipe actually require?
Read the worked answer
The calculation covers half the system. Because the riser is combined, the supply must satisfy the sprinkler demand and the standpipe demand together.
The standpipe requirement is 250 gpm at 100 psi residual at the hydraulically most remote 2½ in. outlet - at the outlet, not at the base of the riser. Expressed at the base:
- Elevation: 60 ft × 0.434 psi/ft ≈ 26 psi
- Friction in the riser and the outlet at 250 gpm: on the order of 10–20 psi
- Base-of-riser requirement for the standpipe alone: 100 + 26 + friction ≈ 135–145 psi
In addition, the required number of simultaneously flowing standpipe outlets per the NFPA 14 table depends on the number of standpipes and on whether the building is fully sprinklered. That is a documented design decision, not an assumption that one outlet flows.
What the 71 psi at 250 gpm proves: that the sprinkler system alone works. It says nothing about the standpipe.
Consequence: the calculation must be redone with the sprinkler remote area and the standpipe demand supplied simultaneously - on the order of 250 gpm sprinkler plus 250 gpm standpipe at the base, with the pressure fixed by the 100 psi at the top outlet.
2. Using the flow test, determine whether the supply can meet the requirement.
Read the worked answer
Supply curve. From 70 psi static and 60 psi residual at 900 gpm, the available pressure at 500 gpm is approximately:
70 − (70 − 60)(500/900)^1.85 = 70 − 10(0.556)^1.85 ≈ 70 − 3.4 ≈ 67 psi
Demand. The base of the riser needs the top outlet's 100 psi plus 26 psi of elevation plus the friction of the combined flow - roughly 140 psi at 500 gpm.
Conclusion: the supply is short by a factor of about two - 67 psi available versus roughly 140 psi required. No amount of pipe resizing fixes a 70 psi static supply being asked to deliver 100 psi at 60 ft of height; the elevation alone consumes 26 psi of it.
Options, in order:
- Fire pump sized for the combined sprinkler + standpipe demand at the required pressure. This is almost certainly the answer.
- Storage plus a pump if the city main cannot support the pump suction - check the suction margin and the net positive suction requirements.
- Demand reduction only if NFPA 14 permits fewer or lesser flowing outlets for a fully sprinklered building - verify before claiming the credit. Do not assume it.
Message to the owner: this is a code minimum at the outlet, not a calculation error to be argued away.
3. If a fire pump is added, what new problem appears and what must the submittal show?
Read the worked answer
Too much pressure at the bottom.
- A fire pump rated at, say, 150 psi at 500 gpm churns at up to 140% of rated head = 210 psi at no flow.
- Add the suction pressure (~70 psi) and the lower-floor hose valves can see over 250 psi at churn.
- NFPA 14 requires a pressure-reducing device where the outlet pressure exceeds 175 psi, so the first - and probably the second - floor hose valves now need PRVs set to deliver the required residual.
Knock-on items the submittal must show:
- NFPA 13 requires a pressure relief valve where system pressure can exceed 175 psi; lower-floor sprinkler heads and gauges see the same churn pressure.
- Pump test header, relief valve, and sensing line locations, coordinated with the mechanical room.
- The FDC arrangement - a pump downstream of the FDC connection changes what the fire department sees, and the layout must be shown to the AHJ.
- The pump sized on the combined sprinkler + standpipe curve, not the 250 gpm sprinkler number, with the driver sized for the 150% flow point.
Judgement: the pump is the right answer, but the submittal that gets approved is the one that also carries the PRVs, the relief valve, and the revised lower-floor pressure check. Approving the pump without them just moves the failure two floors down.