6 replies to this topic

[Bill B]

Do the isothermal equations for tail gas dP calcs in API 521 for fl/D use the Fanning or Darcy friction factor? I have been assuming Darcy, but I am not sure this is correct.

[Art Montemayor]

Bill:

The issue to always keep in mind when doing fluid flow resistance calculations is that the Darcy equation is the principal and favorite Version that is employed in industry. Why Chemical Engineering instructors continue to expound the Fanning factor is beyond me. In my 48 years as a practicing professional, I have found it extremly difficult to find Fanning Factor curves for the Friction Factor.

The so-called "Moody" charts are really Darcy Friction Factor-based. My advice is to ALWAYS MAKE SURE THAT YOUR REFERENCE OR BACK-UP INFORMATION IS FULLY IDENTIFIED AS TO WHETHER THE FRICTION FACTOR USED IS THE DARCY OR THE FANNING. The Crane Technical Paper #410 is based - like most other published data - on the Darcy Factor. In it you will note on Page 1-1: "the Darcy formula is also known as the Weisbach faormula or the Darcy-Weisbach formula; also, as the Fanning formula, sometimes modified so that the friction factor is one-fourth the Darch friction factor."

[Bill B]

Thanks Art, I have been using the Darcy friction factor. But my problem is that API 521 doesn't specifically cite Darcy under the section for isothermal calcs (Section 7.3.1.3, Jan 07 Ed). On page 105, the std state f = Moody friction factor. I am using equation 26.

Later, in Sections 7.3.1.3.5 for 2 phase flow, the standard more specifically says to use the fanning friction factor.

So my gut feel says Moody on page 105 must be Darcy, but I can't say that with 100% confidence. So yes I agree that one must define which is being used and that is what I am trying to accomplish. So ... is it your opinion that the isothermal equations use Darcy or Fanning??

[Art Montemayor]

Bill:

You are wise to be wary of sloppy engineering on the part of others. It is sad that some of our colleagues - especially the so-called "educated" elite - are so sloppy and inept in their writings and texts, but that is a reality of life today.

The way that I have consistently verified exactly WHAT friction factor is being given is by referring to the plot (chart) of the f factor versus the Reynolds Number. If the curve of the LAMINAR REGION (a straight, sloped line) has a slope value of "64", then the f factor is the DARCY FACTOR. This check method has never failed me.

The equation for the Darcy laminar curve is: f = 64/Reynolds Number;

The equation for the Fanning laminar curve is: f = 16/Reynolds Number.

[JoeWong]

Darcy.

More in http://en.wikipedia....factor_formulae

The equation for two phase flow in Sections 7.3.1.3.5 is based on the works done by Dr. Joseph Leung, the OMEGA method. I have quick checked on the original paper, the fanning friction factor is used.

[Bill B]

Art, yes I learned that trick about figuring out whether the chart was fanning or friction a while back - this is good for everyone to know!

My problem however is that the API-521 equations call for estimation of upstream pressure based on discharge pressure, as well as other variables including friction factor. However, I can't find in the standard whether it is using the fanning or darcy f. Since one is 4 x greater than the other, it affects the dP calculations.

So I am trying to figure out which to use. Once I know that, I can choose the correct number from the Moody chart.

[sheiko]

Hello,

For info. http://webwormcpt.bl...w-chart-in.html