Flow Test vs. Demand: Is the City Supply Enough?
Your crew is installing a wet-pipe system in a four-story office building (light hazard, 0.10 gpm/ft² over 1,500 ft²). The hydraulic calculation shows a system demand of 1,150 gpm at 58 psi at the base of the riser. The water department's flow test was run at a hydrant on the same main, at the same elevation as the riser flange, 350 ft away, and gave: static 68 psi; residual 52 psi while flowing 950 gpm. There is no fire pump in the contract, and the private main from the meter to the riser is short, with negligible friction loss. The project manager wants to submit the calculation as-is: "Static is 68 and we only need 58, so we have 10 psi to spare."
1. Using the flow test results, determine the pressure available at 1,150 gpm. Is the supply adequate? Show your work.
Read the worked answer
Static pressure is irrelevant to adequacy; only the pressure at the demand flow matters.
- Drop at the test point: 68 − 52 = 16 psi at 950 gpm.
- Friction loss in the main scales as Q^1.85: (1,150 / 950)^1.85 = (1.2105)^1.85 ≈ 1.42.
- Drop at demand flow: 16 × 1.42 ≈ 22.8 psi.
- Available at 1,150 gpm: 68 − 22.8 ≈ 45 psi.
Demand requires 58 psi, so the supply is short by roughly 13 psi (about 22%). The system as calculated cannot be supported by the city main. Note also that a normal design cushion (on the order of 10% in flow plus 10 psi) would make the shortfall worse, not better.
2. The PM suggests "tightening up the pipe layout and re-running the calc." What are the legitimate ways to close a 13 psi gap, and what is not legitimate?
Read the worked answer
Legitimate:
- Reduce demand within the code - confirm the calculation already uses the smallest allowed design area (1,500 ft² is the light-hazard floor; quick-response area reductions cannot take it below that), verify the density and hose allowance are correct and not padded, and check that fittings and C-factors are realistic rather than conservative guesses.
- Reduce losses - upsize the underground/service main, relocate or upsize the tap, or replace a high-loss backflow assembly with a lower-loss listed device.
- Increase supply - add a listed fire pump (a contract change requiring owner approval, electrical provisions, and AHJ acceptance), or obtain a larger/looped connection from the water department.
Not legitimate: submitting the calculation based on static pressure, deleting the hose allowance or the safety cushion to make the numbers work, or "field adjusting" the density. Each of those produces a system that will not deliver its design density under fire conditions.
3. Before recommending a fire pump to the owner, what would you verify about the flow test itself?
Read the worked answer
- Elevation: gauge elevation versus the riser base; correct with 0.433 psi/ft if they differ.
- Location relative to losses: the test hydrant must be downstream of the same meter, backflow preventer, and main the building will use - a test run upstream of a high-loss device overstates what the system sees.
- Conditions at the time: date, season, and time of day; a test run at 3 a.m. in winter can look far better than a summer afternoon peak.
- Nearby large users: irrigation, industrial draws, or a flushing program within the test window can depress residual pressure.
- Test quality: static held before the flow, pitot/gauge calibration, and whether the flowing hydrant and the residual hydrant are on the same main.
- What the demand includes: hose allowance, sprinkler-to-riser friction, and the riser elevation gain already accounted for.
Only after those checks would I conclude the deficit is real and a pump is the answer.