NFPA 291 Fire-Flow — Method
A hydrant flow test measures three things: the static pressure (no flow), the residual pressure (while a measured flow is discharged), and that test flow Q_F (often from a pitot reading on the flowing hydrant). The available flow at any target residual — usually 20 psi — is projected with QR = QF × (hr^0.54 / hf^0.54), where hr is the pressure drop to the target (static − target) and hf is the measured drop (static − residual). The 0.54 exponent comes from the relationship between flow and friction/velocity head in the supply.
Frequently Asked Questions
Use NFPA 291: QR = QF × (hr^0.54 / hf^0.54), where QF is the measured test flow, hr = static − 20, and hf = static − residual. It projects the tested flow to a 20 psi residual.
Open one hydrant to flow water while reading the residual pressure at a nearby hydrant and the flow (via a pitot gauge on the flowing outlet). Static is read with no flow. These three values feed the NFPA 291 formula.
It models how flow relates to the available pressure (friction/velocity head ∝ flow^1.85), so the inverse projection of flow versus pressure drop uses the 0.54 power.
NFPA 291 colour-codes hydrant caps by available flow at 20 psi: blue ≥1500 gpm, green 1000–1499, orange 500–999, red below 500 gpm.
Yes. The residual must fall measurably below static. If the drop is too small, open more flow (more hydrants/outlets) so the projection is accurate.
Hydrant Fire-Flow Testing (NFPA 291)
Before designing a sprinkler or hydrant system you must know what the water supply can actually deliver. NFPA 291 standardises the flow test and the maths to project the result to the 20 psi residual that most designs assume.
The three readings
Static pressure (no flow), residual pressure (while flowing a known quantity), and the test flow itself. From these the available flow at any residual is projected with the 0.54-power formula.
Using the result
The available flow at 20 psi is compared with the sprinkler or hydrant demand (from the sprinkler hydraulic calculator). If supply is short, a tank and fire pump make up the difference.
Related: Sprinkler Hydraulic, Fire Pump, Pipe Sizing.