looking for a reference for relative roughness
looking for a reference for relative roughness
(OP)
I am looking for a reference for relative roughness of different materials. I ultimately want to fine an equivalent line length for a 14" rubber hose. I have found some resources, but most have numbers for only steel, iron, PVC, and even wood, however I can find nothing on rubber. I have browsed some manufacturer's websites, but have found nothing to help. Does anyone have any suggestions or links that I might find this sort of information?
Thank you for your help.
Thank you for your help.





RE: looking for a reference for relative roughness
Are you looking for the flow coefficient (Cv)?
Maybe page thre of this document:
http://
RE: looking for a reference for relative roughness
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I found it after a few hours of searching, there is not much out there for rubber.
I would still love a cross reference, the roughness value for rubber seems a bit low. Maybe it is a fortunate suprise.
RE: looking for a reference for relative roughness
There is a fallacy that materials such as PVC have a much lower pressure drop because of the smooth bore. When you do the numbers you will find that the PVC has athicker wall and the smaller diameter impacts more than the surface roughness.
RE: looking for a reference for relative roughness
From this data I would guess that a realistic value is somewhere between 0.0016 mm and 0.025 mm. Perhaps that is how I came to a value of 0.005 for my own use.
This seems like a very wide range and it may appear that a sound design is not possible. But in fact its not bad at all. It is one of those cases where going back to an old fashioned hand calculation would give a lot of comfort. Luckily for you, because you have a fairly large pipe when you convert the absolute roughness to a relative roughness by dividing by the pipe diameter you get very small numbers. If you plot these on the Moody chart you will see that the lines are all bunched up at the low e/D values, and there is little effect on the friction factor.
As an example, taking a flowrate of 1,000 m3/h would give a Reynolds number close to 1,000,000 and a velocity of almost 3 m/s. This is probably fairly typical for an actual use. In this case the friction factor for e/D values of 0.0016 and 0.025 mm varies by only 13%. For a fixed flowrate your pressure drop estimate will therefore vary by this 13%, and for a fixed pressure drop your flowrate will vary by the square root, i.e. about 7%. This is probably less uncertainty than some of your other variables.
If the flowrate was less than 1,000 m3/h the variation would be even less.
With a rubber pipe I would include a fairly conservative safety factor to take into account kinking and wrinkling, couplings and so on. So although it may look as though there is a lot of uncertainty in the data, I believe you can still generate a confident design.
Good luck
Harvey
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: looking for a reference for relative roughness
RE: looking for a reference for relative roughness
WAVINESS, AND LAY)
1.1 General
1.1.1 Scope. This Standard is concerned with the geometric
irregularities of surfaces. It defines surface texture
and its constituents: roughness, waviness, and lay. It also
defines parameters for specifying surface texture.
The terms and ratings in this Standard relate to surfaces
produced by such means as abrading, casting, coating,
cutting, etching, plastic deformation, sintering,
wear, erosion, etc.
...
This Standard is to be used in conjunction
with ASME Y14.36M-1996, Surface Texture
Symbols
RE: looking for a reference for relative roughness
RE: looking for a reference for relative roughness
I have never seen a tube or pipe manufactured to a roughness spec., so realize that any number you have is relativem, but likely not universal.
Put adequate FS into your design.
Best of luck.
Paul Ostand
www.ostand.com
RE: looking for a reference for relative roughness
'Flexible straight rubber pipe with a smooth bore' roughness value = 0.025mm. This correlates with the value in the Fluide Design page. Hope this helps.