The 4.4 psi Gap: Reading a Flow Test Correctly

No - the supply is short by roughly 4 psi, and the static pressure is irrelevant to the comparison.

1. Build the supply curve. A flow test follows Q = K x sqrt(Ps - Pr). Here Ps = 64 psi and Pr = 46 psi at Q = 920 gpm, so delta P = 18 psi and K = 920 / sqrt(18) = 920 / 4.243 = 216.8.
2. Solve for the available pressure at the actual demand flow (550 gpm), not at 920 gpm.
   • Q / K = 550 / 216.8 = 2.536
   • delta P = 2.536^2 = 6.4 psi
   • Pavail = 64 - 6.4 = 57.6 psi
3. Compare apples to apples. Demand = 62 psi at 550 gpm; available = 57.6 psi at 550 gpm. Deficiency = 4.4 psi. Comparing the static pressure (64 psi) to the demand pressure (62 psi) is a common and fatal error: static pressure exists only at zero flow.
4. Confirm the reference point. Both numbers must be at the point of connection, at the same elevation. If the test hydrant is downstream of the tap, the pressure at the tap is higher by the friction loss in that 250 ft segment - about 1.7 psi at 920 gpm and less than 1 psi at 550 gpm on 8-inch cement-lined ductile iron (C = 130). That does not close a 4.4 psi gap, but it must be documented so the reviewer can follow the arithmetic.
5. Verify nothing is missing from the calc. The run from the tap to the riser must include the meter assembly, the RPZ backflow preventer, the private main, and any elevation change between the tap and the base of the riser. A missing RPZ loss (often 5-10 psi) is the usual reason a calc 'almost' passes.

It is not permitted, even though the arithmetic would appear to bail the project out.

• NFPA 13 requires a hose stream allowance based on the occupancy hazard classification, added to the sprinkler demand at the point of connection to the water supply. For Ordinary Hazard the allowance is typically 250 gpm. The allowance exists because the responding fire department will flow water from the FDC and nearby hydrants simultaneously with the sprinkler system, and the supply must be able to serve both. It is not contingent on the building having a standpipe.
• Why it 'works' numeriCALLY: dropping the hose allowance drops the demand from 550 gpm to 300 gpm. On the same supply curve, the available pressure at 300 gpm is 64 - (300/216.8)^2 = 64 - 1.9 = 62.1 psi, while the calculated required pressure at 300 gpm is far lower than the 62 psi required at 550 gpm. The calculation would show compliance - but it describes a system that cannot be supported under real fire conditions.
• What a reviewer should do: reject the substitution and require either a revised, compliant design or a documented variance from the AHJ. A plan that quietly deletes a required allowance is a design deficiency, not a value-engineering choice, and it transfers an unachievable performance expectation to the fire department.
• Note the symmetry: the hose allowance should also not be added twice if a standpipe is supplied by the same system - the combined demand rules are applied at the point of connection, once, based on the hazard.

The 6 a.m. retest is not legitimate as a design basis. A water supply must be evaluated at its worst-case (peak-demand) condition, not at the hour when the system looks best. A favorable-hour test would understate the friction loss and understate future demand as the main is built out; the AHJ is entitled to reject a test that does not represent minimum supply.

Legitimate options, roughly in order of cost and disruption:

1. Audit the calculation first. Errors in equivalent length, C-factors, elevation, or omitted devices (RPZ, meter, detector check) frequently account for more than 4 psi. Confirm the sprinkler K-factors and the actual coverage per head match the drawing.
2. Check the point of connection. Tapping the 12-inch main rather than the 8-inch, or relocating the tap closer to the source, can recover several psi at little cost if the site geometry allows it.
3. Re-examine the design, if the edition in force allows it. Quick-response sprinklers in a wet system may permit a reduced remote area under limited conditions (ceiling height and concealed space criteria must be met). Confirm against the adopted edition and the AHJ - this is an edition-specific allowance, not a universal 30% discount.
4. Reduce losses in the private main. Upsizing a short, undersized segment between the tap and the riser is often cheaper than a pump.
5. Add a fire pump only if 1-4 cannot close the gap; then size it to the actual demand and confirm the suction conditions and the churn pressure against the system component ratings.

A retest is useful only to confirm a genuinely changed condition (main valve opened, new main in service) or to verify gauge calibration - not to shop for a better number.