Isothermal Tail Gas Calc - Darcy Friction Factor? 1

[Bill3752]

I am using Jan 07 API 521 isothermal equation (25) pg 105 for tail gas calcs. The std state friction factor = Moody, but doesn't state whether Darcy or Fanning. The std states another method uses Fanning.

I am assuming Darcy for isothermal, but am not 100% sure this is correct. Does anyone know which to use?

 

[MortenA]

Darcy and Fanning are "all the same". For some reason Fanning included a factor 4 in his equation whereas Moody and Darcy dosnt.

So Fanning:

dP=4*f*L/D*den*(V^2)/2

Darcy

dP=f*L/D*den*(V^2)/2

Of the two, the Darcy factor is more commonly used by civil and mechanical engineers, and the Fanning factor by chemical engineers, but care should be taken to identify the correct factor regardless of the source of the chart or formula.

Best regards

Morten

 

[insult2injury]

I believe that the factor of 4 is a result of how the characteristic hydraulic diameter is defined.

I2I

 

[MortenA]

Re-read the original post: Just use the one that you are most coftable with - and use the corrct chart/formula

The method presented is a "suggestion" it was not haded down to Moses with burning bushes and everything

Best regards Morten

 

[Bill3752]

This is not a situation of which I feel more comfortable with. The API isothermal vent calcs basically compute an upstream pressure which is a function of fL/D, where f is the friction factor - but is not defined as Fanning or Darcy. Since there is a factor of 4 difference between the two, use of the incorrect friction factor will lead to an incorrect dP. So does anyone know which friction factor is used in the API 521 isothermal equations?

 

[lizking]

API521 uses the Moody friction factor as used with the Darcy pressure drop formula (see Crane p. 1-6). See note after equation 33 in API521 for an example of Fanning friction factor usage.

 

[Latexman]

While it would be great if API would document the basis of f in plain sight, my instincts are telling me they want K for the segment of the system you are working on. So:

K = f_DarcyL/D = 4f_FanningL/D

Sound reasonable?

Good luck,
Latexman

 

[Bill3752]

thanks Latexman. But I don't know if their equations use Fd or Ff. Do you know?

 

[MortenA]

Read section 7.3.1.3.3: where its says that you can use a number of methodes - and that one folling that sentense is suggested if you dont have anything else! Its a little unclear if ANY method can be used or if the choise is restricted to the one listed in the bibligraphy - I read it as the first however as long as you can argue for your choise.

So you can use a formula for Darcy if you got a Darcy and a fanning if you got a Fanning map. if you got a Fanning map and no other fomula - then i go for Latexman: Its Darcy

Best regards

Morten

 

[lizking]

There's no such thing as a "Darcy friction factor." Equation 25's variables are clearly explained below the equation: f= Moody friction factor. See Crane TP-410 for graphs of Moody friction factors.

 

[Latexman]

http://en.wikipedia.org/wiki/Darcy_friction_factor

suggests there is a "Darcy friction factor."

Bill - why don't you test it out based on an example out of Crane (Chapter 4) or another source or create a ficticious, easy problem, like 100 feet of 2", Sch. 40 pipe with enough flow of 60 F and 100 psig air to have about a 10% pressure drop (from Table B-15 in Crane that's 166.6 cfm at 60 F and 100 psig for a 10.1 psi drop). Which friction factor gives you about the right answer? That's the one!

Good luck,
Latexman

 

[skearse]

I took the API University course last year & the course notes refer to Darcy/Crane. For all of the course examples they simply gave the K values from Crane to use.

 

[Bill3752]

lizking, after my last post I actually did what you suggested (different pipe configuration, but same idea)and using the compressible flow equations in Crane showed the dP to be comparable to the API 521 isothermal dP using Darcy, not Fanning friction factor.

skearse, I appreciate your confirmation on this.

API is confusing, however, because the alternate tail gas methodology (the shortcut Lapple calc) calls for Fanning. That fact is what led me to be cautious about this.

Thanks to all for your input!!!

Bill

 

[MortenA]

I would still like to say to others that read this tread:

You are not limited to what is printed in API521 - as long as you can argue that its just as good or better! And its even printed in the API. A standard is not an excuse for not thinking imo! I have had clients that though the other way around ;-) and all respect to the large amount of work and experience that goes into creating these standards - but sometimes they also have to make allowance for the fact that some project call for "big guns" whereas others can reach the target using simpler and more conservative approaches.

Best regards

Morten

 

[Bill3752]

MortenA, good point. In this case the isothermal equations are generally conservative, and provide a simple approach for computing back pressure, at least for the vent headers I am analyzing. The aforementioed Lapple approach is pretty good also, but once I set up my "isothermal excel model" that method seemed to be best for me. Thanks.

 

[MortenA]

Another point is that if you are in a desgin phase of a new facility then using a conservative approach may benefit you enormesly later on. Quite often systems gets more complex over time and more sources are added. Then a conservative approach with margins are good later where you can use more sophisticated tools e.g. transiet analysis to find margins for more sources.

Best regards

Morten