looking for a reference for relative roughness

[SandCounter]

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.

 

[JohnBreen]

SandCounter,

Are you looking for the flow coefficient (Cv)?

Maybe page thre of this document:

http://www.parker.com/stratoflex/products/catalog/sso.pdf

 

[SandCounter]

Thank you for your time Mr. Breen. I was looking for something more like this:

http://www.fluidedesign.com/download-free/pipe_rough_values.pdf

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.

 

[stanier]

Losses through hoses are usually higher than adjacent pipe as the inside diameter is reduced compared to the pipe material. The surface roughness is low as the manufacturing process produces a smooth bore akin to PVC.

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.

 

[katmar]

This data is indeed difficult to find. The program SF Pressure Drop gives an absolute roughness of 0.0016 mm. The program Pipe Flow Wizard gives a value for neoprene as 0.0818 mm. This sounds high because it is about double the value for commercial steel pipe. In my own hand-written notes I found the value 0.005 mm, but without a reference. My bad. The Fluide Design reference you gave had a value of 0.025 mm.

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

 

[SandCounter]

Thank you Katmar and Stanier, that gives me a good starting point.

 

[JLSeagull]

ASME 846.1-2002, SURFACE TEXTURE (SURFACE 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

 

[rconner]

Not an expert in these matters, but suspect the pressure drop through some elastomeric or reinforced elastomeric type hoses might vary some depending on specific design and among manufacturers (along with the surface and whatever other variables are out there, I wouldn't be surprised to see a hose/design that swells a little more/larger than anotherresult in at least a little less lesser pressure drop). There appears to be a lot of information out there on the web for amount of pressure drop for various at leat smaller sizes of various "rubber hoses" at various flow/pressure rates etc. (understandable as so many of such hoses are used in critical fire protection etc.); thus, perhaps you could back-calculate the apparent "roughness" from this data? Once you do this, however, I wouldn't be surprised to see some scatter in this data from many sites!

 

[proinwv2]

A caution here. If this is a critical situation, then I suggest that you run lab tests to confirm any calculations.

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

http://www.ostand.com

 

[Mech85]

The book "Internal Flow Systems" by D.S.Miller which is part of the BHRA fluid engineering series quotes the following for rubber hose:

'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.