12.4 Hydrant Classification, Color Coding & Available Flow Extrapolation at 20 psi Residual

Measured test flow and predicted available flow

The flow actually discharged during a hydrant test is not automatically the flow available at the chosen residual pressure. NFPA 291 (2022) provides a common method for projecting available flow from measured static pressure, residual pressure, and total test flow. Twenty psi is the usual classification datum, subject to the applicable requirements and local water authority. The result describes the tested network conditions; it is not a guarantee of supply under every future demand or source configuration.

Use simultaneous residual and flow observations, a suitable test layout, and enough pressure reduction for a meaningful estimate. The 2022 recommendation is at least a ten-percent reduction from static pressure. A tiny pressure difference makes the calculation sensitive to gauge error. Do not promise precise available flow from a 1 psi drop on coarse instruments, or extrapolate through an unrealistic pressure range without qualification.

The projection formula

For projection to 20 psi residual:

$Q_{20}=Q_F\left(\frac{P_S-20}{P_S-P_R}\right)^{0.54}$

Here, $Q_F$ is total measured test flow in gpm, $P_S$ is static pressure in psi at the residual point, and $P_R$ is simultaneous residual pressure there. The exponent comes from the empirical flow/head-loss relationship. It is not a linear ratio, and it differs from the square-root relation used to calculate a single outlet's flow from pitot pressure. Keep the two formulas and their pressure inputs separate.

The denominator must represent a real positive test pressure drop. If static equals residual, the expression does not provide a valid finite estimate. If the test data show impossible relationships, check timing, instruments, units, and source operation instead of forcing the calculator to produce a positive result. Also confirm that static pressure is above the desired projection datum and that the method is applicable to the actual supply.

Work an example without premature rounding

Assume static pressure is 75 psi, residual pressure is 55 psi, and measured flow is 950 gpm. The available drop to the datum is 75 − 20 = 55 psi. The test drop is 75 − 55 = 20 psi. Their ratio is 2.75. Raising 2.75 to the 0.54 power gives approximately 1.7268, and multiplying by 950 gives approximately 1,640 gpm. Retain calculator precision until the final reported value.

A second example has 60 psi static, 42 psi residual, and 720 gpm test flow. The ratio is 40/18, and the projected flow is about 1,108 gpm. A third example with 50 psi static, 30 psi residual, and 550 gpm measured flow gives 550 × (30/20)^0.54, or about 685 gpm. A reported estimate should match the accuracy of the original measurements; many decimal places do not make field data more accurate.

Classification and recommended marking

NFPA 291 is a recommended practice. Its familiar capacity classification uses light blue for Class AA at 1,500 gpm or more, green for Class A from 1,000 to less than 1,500 gpm, orange for Class B from 500 to less than 1,000 gpm, and red for Class C below 500 gpm. The capacity colors apply to bonnets and caps under the recommended scheme. Chrome is not an alternative capacity color for Class AA.

Thus, the first example is Class AA, the second Class A, and the third Class B. A 1,250 gpm available-flow result is Class A even if the actual test discharged only 800 gpm. Classification is based on the capacity at the prescribed datum, not simply the number printed on a nozzle chart during a smaller test.

Body colors identify other aspects of the local hydrant scheme. Confirm the water authority/AHJ's adopted marking rules, including private, nonpotable, and out-of-service identification. Do not invent one universal bag color or nonpotable body color, and do not paint over operating directions or required identification. Marking should reflect a defensible current record and authorized maintenance.

Understand the datum's limits

Maintaining adequate distribution pressure helps protect water-system reliability and reduces risks associated with severe depressurization. Pressure below 20 psig does not instantly cause every pump to cavitate, every pipe to collapse, or every backflow path to contaminate the main. Those effects depend on actual conditions. Follow the water authority's test limits and stop criteria; do not deliberately pull a main toward vacuum to improve the calculated flow.

A color does not prescribe fireground tactics or guarantee a connected sprinkler demand. Document raw readings, projected value, classification, date, layout, and qualifications. Compare future tests under equivalent conditions and investigate deterioration. Restore the hydrants safely and keep the capacity record available to the responsible owner and authority.

Source checked October 19, 2026: NFPA 291 (2022), flow projection and hydrant marking provisions.