29.83 (0.0667766) is a constant derived from the physical laws relating water velocity, pressure, and conversion factors that conveniently leave the answer in gallons per minute (liters per minute). The metric formula is not a direct conversion of the U.S. formula.
This formula was derived by assigning a coefficient of 1.0 for an ideal frictionless discharge orifice. An actual hydrant orifice or nozzle will have a lower coefficient of discharge, reflecting friction factors that will slow the velocity of flow. The coefficient will vary with the type of hydrant outlet or nozzle used. When using a hydrant orifice, the operator will have to feel the inside contour of the hydrant orifice to determine which one of the three types is being used (slide #7). When a nozzle is used, the coefficient of discharge will depend on the type. Some common nozzle types and their respective coefficients of friction are given on (Slide #4).
FPT 102 - Fire Prevention and Inspection // Instructor: Kevin M. Kolb Slide #2