Which Surface Roughness value to use for pipe flow friction factor

In summary, the conversation discusses using a surface profilometer to measure surface roughness of different materials and the values for various roughness variables. The speaker is using calculations based on "Absolute Roughness" and pipe diameter to determine the Moody Friction Factor and head loss, but is unsure which roughness value corresponds to the "Absolute Roughness." The suggested solution is to look up the Colebrook equation and use Rq as the closest descriptor. Moody's data was based on Nikuradse's work, which used laquered sand-grains and Rz seems to be the most accurate descriptor.
  • #1
jamespb
2
0
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
 
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  • #2
Look up the Colebrook equation, which will relate friction factor to roughness. I would suggest using Rq, which is the RMS roughness.
 
  • #3
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).
 

1. What is surface roughness and how does it affect pipe flow friction factor?

Surface roughness refers to the irregularities or roughness of the inner surface of a pipe. It can affect the friction factor of pipe flow by increasing the resistance to flow, which in turn affects the pressure drop and flow rate.

2. How is surface roughness measured and what units are used?

Surface roughness is measured using a device called a profilometer, which measures the height and depth of surface irregularities. The units used to measure surface roughness are typically micrometers (μm) or microinches (μin).

3. What is the recommended surface roughness value for pipe flow friction factor calculations?

The recommended surface roughness value varies depending on the type of pipe material and the fluid being transported. Generally, a value between 0.001-0.01 mm is used for most common pipe materials.

4. How does the surface roughness value affect the accuracy of pipe flow friction factor calculations?

The surface roughness value plays a significant role in determining the accuracy of pipe flow friction factor calculations. A higher roughness value will result in a higher friction factor, leading to a higher pressure drop and lower flow rate. Therefore, it is important to use an appropriate roughness value for accurate calculations.

5. Can the surface roughness value be determined experimentally?

Yes, the surface roughness value can be determined experimentally by measuring the roughness of a sample of the pipe material using a profilometer. However, it is more common to use a standard roughness value based on the type of pipe material and the fluid being transported.

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