Project & Hydrant Locations
Static Pressure & Residual Reading Both measured at the residual hydrant (R).
Flow Hydrant Readings One row per flowing outlet. Total flow QF is the sum across all rows, observed at the residual reading above.
| # | # Outlets | Orifice Size (in) | Outlet Type / Coefficient | Pitot (psi) | Flow per row (gpm) | Comments |
|---|
Results
Projection per Hazen-Williams: Qtarget = QF · ((PS − Ptarget) / (PS − PR))0.54.
Water Supply Curve N1.85 paper, per § 4.13.4
Static point at Q=0 (square), flow test point at (QF, PR) (circle), and projected point at (Qtarget, Ptarget) (triangle). The supply line is straight on N1.85 paper because P = PS − k·Q1.85.
Hydrant Test Layouts Figure 4.4.4 — R = residual hydrant, F = flow hydrant. Arrows show flow direction.
Method
Discharge from each outlet (§ 4.9.3)
For each flowing hydrant butt, discharge is computed from the orifice diameter, the outlet coefficient, and the pitot (velocity head) reading:
Q (gpm) = 29.84 · c · d² · √p [d in inches, p in psi]Outlet coefficient c (Figure 4.9.1)
- 0.90 — outlet smooth and rounded
- 0.80 — outlet square and sharp
- 0.70 — outlet square and projecting into the barrel
- 0.95 — flow tubes / stream straighteners (§ 4.9.2)
Pumper outlets (§ 4.10)
When a pumper outlet is used without flow tubes, the result of Eq. 4.9.3 is multiplied by an additional coefficient from Table 4.10.2, selected by pitot pressure:
- 2 psi → 0.97
- 3 psi → 0.92
- 4 psi → 0.89
- 5 psi → 0.86
- 6 psi → 0.84
- ≥ 7 psi → 0.83
Best results are obtained with pumper outlets when pitot pressure is between 5 and 10 psi (§ 4.10.1).
10% pressure-drop check (§ 4.4.6)
The pressure drop at the residual hydrant should be at least 10% of the static pressure to give a reliable basis for projection. When the system is reinforced by automatic pumps that resist showing a real drop, an artificial 10% drop in static pressure may be assumed to construct a theoretical supply curve.
Projection to target residual (Hazen-Williams)
Available flow at a target residual pressure Ptarget (typically 20 psi for fire-protection design) is computed from:
Qtarget = QF · ( (PS − Ptarget) / (PS − PR) )0.54The exponent 0.54 = 1/1.85 corresponds to the Hazen-Williams friction-loss relationship hf ∝ Q1.85, which is also the basis for the N1.85 plotting paper used in § 4.13.4. The supply curve plots as a straight line on this paper.
Engineering aid only. Orifice coefficient, pitot accuracy, hydrant-outlet geometry, and elevation differences between hydrants must all be assessed by the engineer of record. Real water supplies vary with the day, season, and operating state of municipal pumps and storage. This tool implements the calculation and plotting procedures of NFPA 291 but does not substitute for proper field practice or independent judgement.