For non-compressible fluids (liquids), fluid head resistance can be calculated by using the Darcy-Weisbach formula.


h fluid = f (L/D) x (v ²/2g)


f = friction factor

L = length of pipe work

D = inner diameter of pipe work

v = velocity of fluid

g = acceleration due to gravity


Fluid head loss calculated by the Pipe Flow Wizard software when using the non-compressible (liquid) calculation engine is based on the Darcy-Weisbach formula. 


This formula can be used for compressible (gas) calculations provided that the gas density at the start of the pipe is used to determine the fluid velocity, and provided that the pressure drop in the pipe is less than ten percent of the absolute pressure at the start condition. 


For compressible (gas) calculations where the pressure drop is more than ten percent, but less than forty percent of the absolute pressure at the start of the pipe, the equation gives reasonable results provided that the gas density at the average pressure condition in the pipe is used to calculate the average velocity of the gas.


The Pipe Flow Wizard software however, does not allow use of the Darcy-Weisbach equation for compressible gas calculations since it uses the more accurate specialist compressible gas flow equations, which provide more accurate results, and which are not limited to working with pressure drops of up to a certain percentage of the initial start pressure.