Hydrant Rated Flow at 20 psi (NFPA 291)
Hydrant rated capacity at 20 psi residual: QR = QF (hr/hf)^0.54, hf = static - residual, hr = static - 20, with the AA/A/B/C color class. Static 70, residual 50, QF 1000 gpm -> 1640 gpm, Class AA; static 65, residual 45, QF 800 -> 1240 gpm, Class A. A field/planning estimate; the water authority's data govern.
Formula and source
hf = static - residual; hr = static - 20; QR = QF (hr/hf)^0.54; class AA/A/B/C by QR.
The NFPA 291 hydrant rated-flow-at-20-psi projection and the color-class marking, by name.
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