4 replies to this topic

[Alia1969]

Hello everybody!

 

I'm the newest member and I'm a learner of these engineering forums.

 

Anyway, I'm interested to find a roughness database from main piping materials. More specificaly, I'd be grateful to know the value from Ductile Iron pipe roughness in case of a new pipe and, let's say, 10 years old.

 

Thanks in advance,

 

Alia1969 

[Art Montemayor]

Alia1969:

 

What is your purpose in finding information on piping roughness?  You should already know that the existing, real-time roughness of all piping depends on such unknown, varying, and uncontrolled factors such as:

• The actual fluid(s) used in the piping (in this case, over a time of 10 years!);
• The actual velocities imposed on the fluids during the time of service;
• The actual composition (including impurities, such as solids) of the fluids;
• The potential (and actual) chemical reactions occurring between the fluids and the piping Material of Construction;
• Method of piping fabrication;
• The actual material of construction - and its method of fabrication (welded or seamless);
• The amount of dissolved gases (especially oxygen) in the fluids;
• The temperature(s) of the fluids being transported;
• Piping roughness is not a fixed, stable value; it will vary with time and use - sometimes differentially, sometimes spontaneously.  There is a roughness growth rate - as reported by Colebrook and White in 1937;  seawater is notorious for forming incrustations and other growth;
• Not all sections of a piping run will exhibit the same, identical roughness; fluid turbulence and velocities can cause roughness differences in the same piping run.

And even if you were able to identify and control all these factors with accuracy, you still will be unable to measure the actual “absolute” roughness of the piping with any degree of credible accuracy while the piping is installed.  This all leads to a solution based on accepting what engineers have been doing - and continue to do - in predicting the acceptable piping diameter for a system or the expected pressure drop.  The friction factor is currently determined by resorting to accepted “roughness” values developed many years ago - most of which fails to be identified in much of the literature.

 

If you are doing Fluid Mechanics studies and research, that’s fine.  You can dwell in theory and research in much of your work.  However, if you are applying Fluid Mechanics to practical, industrial applications then my years of experience begs to inform you that you should dwell on practicality rather than theory when it comes to predicting the RMS (Root Mean Square).  All professional engineers I’ve worked with have relied more on their experience and learnings for selecting the proper RMS value to employ in their fluid flow process calculations.  Until a radical and improved method to the Darcy-Weisbach relationship is found, I can’t see how any better and more practical procedure can be used.  In my opinion, an engineer has to use experience and practical sense in applying any equation - especially the Darcy-Weisbach.  For example, refer to the attached listing of commonly found and employed pipe roughness values.  I consider it totally incomprehensible to have an engineer employ the following roughness values:

• Riveted steel
• Wood stave
• Galvanized iron
• Cast iron - asphalt dipped

My father was a riveter and I personally worked in a shipyard where riveting was last employed in 1957 - and that was only for repairing a very old ship’s deck.  I know of no one person today that could pick up a riveting tool and apply it to piping.  In fact, how (and why?) could anyone justify the cost, labor, and deficiencies of such a pipe when seamless and welded piping is readily available all over the world?

 

What chemical engineer would undertake to employ a wood stave pipe?  This is another leftover from the 19^th century.  Even the Chinese would no longer employ wood or bamboo piping in an engineering project today.

 

I have never seen or heard of any pipe that is galvanized for fluid flow.  Note that the roughness of a pipe only applies to its INTERIOR WALL - not its exterior.  Therefore, how would one go about (and again, why?) coating the total, 100% interior wall of a pipe with galvanization?  What is even a worse question is how could you ever expect the galvanization to remain intact 100% and all of the time?  Even a minute pore in the coating would defeat it.  The answer is you can’t; and that is why this fictional application is ridiculous and unbelievable.

 

The same comments apply to the asphalt dipped interior wall of the cast iron pipe.  Cast Iron inherently is selected because of it relative inertness.  Any asphalt sprayed or coated to its interior wall is going to result in defective coatings and/or release of coating pieces while in use.  This is an application waiting to fail - and without any process advantage to speak of.  What all this means is that the published values for these applications are, at best, very suspicious.

 

My whole intent is writing this lengthy post is to advise all young engineers that the best effort that they can make in selecting a practical and workable roughness value to find the friction factor in a pipe is to always consult with an experienced process design engineer and thoroughly discuss the actual application and its scope of work.  The result of such a consultation should be a reliable and dependable value on which you can hang your hat (and reputation) on.  For a practical and interesting discussion on predicting the roughness values, refer to the attached 1988 paper on the subject.

 

 

I hope this experience helps you direct your interest and work effort in a useful direction.

 Pipe Absolute Roughness Values.docx   20KB   58 downloads

 Predicting Internal Piping Roughness in Water Mains.pdf   1.05MB   53 downloads

[Alia1969]

Dear Art,

 

Thanks a lot for your so detailled reply! As a Pump Product Manager, sometimes I must calculate TDH's due to customer's requests and thereby I'm interested to find a reliable roughness database. Anyway, let me contact you in the future for further information.

 

Happy New Year 2015!

 

Alia1969

[Art Montemayor]

Alia1969:

 

Thank you for the New Year greetings - and I pass on my sincere wishes to all our members towards a Happy and Prosperous New Year in 2015.

 

Your application is not only interesting but also a practical one.  I presume that what you supply your clients is a “best professional effort calculation”, based on sound and conservative values - and NOT a guaranteed prediction of how the Total Developed Head in a piping system is going to result.

 

This topic is one of profound interest and importance in the Chemical Engineering field (as well as in Mechanical and Civil Engineering).  One individual, Professor Glenn Brown of Oklahoma State University, is a civil engineer who has written a lot of papers on the Friction Factor and the related Darcy-Weisbach Equation.  You have probably already had prior knowledge of this, but I am enclosing two papers on the topic as well as a PowerPoint presentation by professor Brown for you to refer to and also for the benefit of our members who take personal interest in this topic.  In order to explain my comments on the identification of credible and accurate rugosity and friction factor values to be used in process piping, please look at Henry Darcy’s quaint and candid remarks on the values of these coefficients found on page 25 of the PowerPoint Presentation.  My experience concurs with his opinion when it comes to selecting these values.  It is a challenging endeavor to identify an accurate friction factor in any piping system with experience being our best weapon of employment.  My best advice on this subject is to know your strengths - but also be aware of your weakness on the subject.

 

I hope this helps you out.

 History of Darcy-Weisbach Equation.docx   28.26KB   36 downloads

 History of the Darcy-Weisbach Eq.pdf   284.21KB   39 downloads

 The History of the Darcy-Weisbach Equation.pptx   2.32MB   32 downloads

 

[erj]

Dear

 

Mr. Art Montemayor,

 

Thank you very much for sharing this information about Professor Glenn Brown of Oklahoma State University and his technical papers with all of us, Pump and Pipe sizing is one of my kind interest, my work is also dependent on to these calculations.

 

Regards,

 

Rachit