by chemebabak

Many folks calculate 4 times greater head loss (or 4 times less) than the actual friction loss. This comes from confusion between Moody and Fanning Friction factors. Some friction factor graphs are for Moody Friction factor, which is 4 times Fanning friction factor. That is, f = 64/Re is Moody and f = 16/Re is Fanning.

Be careful with your hydraulic calcs. It is easy to mix the two and calculate 400% greater (or 25% less) head loss. The calculation for head loss in feet is:

using Moody Friction factor -

h(friction) = f(M) * (L/D) * v^2 / (2 * g)

using Fanning Friction factor -

h(friction) = 4*f(F) * (L/D) * v^2 / (2 * g)

where,

h(friction) = head loss by friction in feet

f(M) = Moody Friction factor

f(F) = Fanning Friction factor

L = length in feet

D = pipe inside diameter in feet

v = velocity in ft/s

g = 32.174 ft/s^2, acceleration due to gravity

The Colebrook-White equation is an iterative method that calculates Fanning friction factor.

f(F)^2 = 1 / ( -4 * Log(eps / (3.7 * D) + 1.256 / (Re * √f(F) )

where,

eps = pipe roughness in feet

Re = Reynold’s number