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Smooth Pipe & Rough Pipe Difference

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#1 kasri


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Posted 10 March 2009 - 11:10 PM

Dear All,
Will you please any one explain what is smooth pipe & rough pipe.In calculating friction factor how do determine this.Whether it indicates the inner surface smoothness, how do we measure.Please explain.

#2 katmar


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Posted 11 March 2009 - 02:09 AM

We should not talk of "smooth pipe" and "rough pipe" but rather of "smooth pipe flow" and "rough pipe flow" because the distinction between smooth and rough does not depend on only the pipe. I will get back to this distinction.

The "roughness" is a characteristic purely of the pipe. There are ways of measuring it directly, but for flow calculation purposes the roughness is the equivalent sand particle size that would have to be glued onto the inner surface of the pipe to give the same flow characteristics. I think the classic experiments in this regard were done by Nikuradse - your fluids book will give you more details.

The "relative roughness" is the absolute roughness (as measured by the sand particles) divided by the pipe inside diameter.

When a fluid flows through a pipe the layer closest to the pipe wall is stationary. This is known as the "boundary layer". Again, refer to your fluids text for details. If the boundary layer is thicker than the roughness then the moving fluid does not "see" the roughness because it is enclosed in the stationary layer. This is termed "smooth pipe flow" because the roughness has no influence on the flowing fluid.

As the flow rate increases (or more accurately, as the Reynolds number increases) the boundary layer gets thinner. This means that at higher flow rates (higher Reynolds number) the roughness is more likely to project through the boundary layer, thus influencing the flowing fluid and causing "rough pipe flow".

If you look at a Moody (or Fanning) Friction Factor Chart you will see that in the turbulent flow regime there is usually a thicker line drawn below all the other curves. This is the "smooth pipe flow" friction factor curve. You will see that at lower Reynolds numbers the curves for low relative roughnesses (usually written as e/d) join into the smooth flow curve because the boundary layer is relatively thick. To the right hand side of the Moody Chart where the Reynolds numbers are high there are more curves for low values of e/d because the boundary layer is thinner and the pipe roughness has more influence.

In order to determine whether you have smooth or rough pipe flow you need to calculate the Reynolds number and the relative roughness (e/d) and see where you fall on the Moody Chart. If you are calculating your friction factor by a formula rather than by looking on the Friction Factor Chart then the expressions for smooth pipe flow are simpler than for rough pipe flow because it is not necessary to include (e/d) in the expression.

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