Which Surface Roughness value to use for pipe flow friction factor

[jamespb]

I am trying to predict pressure drop in a pipeline with a range of different materials.

I have used a surface profilometer to measure the surface roughness of the different materials and I have values for various surface roughness variables - Ra Rz Rmax Rq Rm S Sm R3z Wt and a range of other variables.

The calculations I am using are based on using an "Absolute Roughness" (units mm) and the pipe diameter (units mm) to calculate a "Relative Roughness" and then from this calculate the Moody Friction Factor and head loss.

My issue is I have no idea which surface roughness value corresponds to the "Absolute Roughness" as used by the Moody Chart.

Thanks in advance for any assistance

 

[boneh3ad]

Look up the Colebrook equation, which will relate friction factor to roughness. I would suggest using Rq, which is the RMS roughness.

 

[SmwhrInSpaceX]

I believe Rz is the closest modern descriptor. There's some interesting discussion in this recent paper: http://www.rit.edu/~w-taleme/Papers/Conference%20Papers/C084.pdf

Moody's data was based on work by Nikuradse, who laquered sand-grains to the inside of pipes. Rz is the average peak-to-peak roughness and seems to be the closest descriptor. The mathematical average roughness, Ra, will usually be way too low (close to a factor of 4).