Hazen-Williams NFPA

Some in industry use an alternate form of the Hazen-Williams formula which has this form (NFPA 13, 1999NFPA 13, Installation of Sprinkler Systems, 1999 Edition, page 13-139.):

(1)

where:

p = Pressure drop gradient (psi per foot)
C = friction loss coefficient
Q = Volumetric flow rate (gpm)
d = Internal diameter (inches)

This differs slightly from the Hazen Willams version used in AFT Fathom.

While both Equation 6 (and its variant Equation 7) in topic Pressure Drop in Pipes and Equation 1 (in this topic) are used heavily in industry, surprisingly they are not equivalent. The reason relates to Equation 1 being in terms of pressure drop but not having a fluid density in the equation. Equation 1 is intended for water, but it says that the pressure drop is independent of fluid density. This is contrary to Equation 2 and almost all other references.

The definition of pressure loss as it relates to head loss is

If this equation is used to equate Equations 1 and 2, the density must be 62.0933 lbm/ft3 to achieve equality. Hence any other density used in AFT Fathom will result in a pressure drop which does not equal to that of the Equation 1 NFPA formulation. Using the ASME Steam Tables, a density of 62.0933 lbm/ft3 at 1 standard atmosphere occurs at a temperature of 91.78 F.

Matching AFT Fathom Hazen Williams to NFPA Hazen-Willams

Users who want their results to match the NFPA version in Equation 1 can do so by one of the following two options:

Use a density of 62.0933 lbm/feet3 in all calculations, or
Use the desired density and then apply a design factor on each pipe to correct the pressure drop for density. The design factor should be equal to 62.0933 lbm/feet3 divided by the density on the Fluid panel in Analysis Setup. If variable properties are used, the design factor must be varied on a pipe-by-pipe basis.