Pressure drop in compressible flow 10

[TomaszKruk]

Hello,

I have a problem to solve that seemed trivial at first, but kinda got out of hand quickly. I want to properly calculate pressure drop on a gas pipe on a chemical plant. I thought I'd find "plug and play" equations with iterations maybe, but after 2 days of reading I'm horribly confused.

Can someone point me into a direction of a legit source for compressible flow pressure drop equation? There will be no outlet to atmosphere, no flare - I need to calculate exact value of pressure drop on the line so the equipment downstream from my pipe won't work above design conditions. I did not select the working pressure or design the equipment downstream - all I got is obvious conclusions that I need a certain loss value or things might look bad.

I tried an older version (that is the one available for me) of the 410TP. Simple Darcy equation is easy enough to solve but untrustworhy. Darcy formula including Y factor is infuriating - you need to know the pressure drop in order to calculate Y, so the equation is useless to me.

I did solve a verion of the Isothermal flow equation provided, all good and well, but my spreadsheet is acting funny (probably excel finds it hard to iterate solution since I work with SI units and my pressure is around 40 bar - the equation works for Pa only). I'd like to verify the results. Especially that I suspect it works best for mass flow calculations, if the pressure loss is given.

I found a webpage with free calc, but it gave me an error I'm too dumb to understand (supersonic outflow?). I even went thru the process of learning to use "." instead of "," and the Imperial units for it

http://www.freecalc.com/gasfric.htm

I read a lot of sources but most of them I suspect are viable only for natural gas even though they do not state that directly (like Panhandle equation - I always believed it works for natural gas only, but there it was in a paper about gas flow thru pipes).

Gonna go with this one next:

https://www.chemengonline.com/a-novel-equation-for-isothermal-pipe-flow/?printmode=1

Please help guys - this is the thing I've been hoping to get my teeth into, but need a good source or good book / paper on piping / chemical engineer level. So far I found a lot of things that just aren't right.

Thanks,
Thomas

 

[DarcyDude]

Hello all,

For what its worth this is a pretty common item in aerospace. In my experience the preferred procedure is to model as "Fanno Flow", i.e. the equations goutam_freelance provided. This needs to be solved iteratively, and if setting up code to perform this procedure it is ideal to incorporate with REFPROP so that your script can be easily used for any fluid. At each new company I have worked I set one of these scripts up (using matlab, excel, or python), then just used it repeatedly as needed.

Some initial conditions can be difficult to get it to converge (need pretty small step size). I've been meaning to beef up my python script with dynamically updating step size but have never gotten around to it.

If a conservative value for pipe roughness is chosen, this will produce accurate/conservative dP values when compared to test data. On the few instrumented setups I have been part of building anyway. I typically pull rougness values from the appendix in Crane No. 410.

Maybe thats helpful. Good luck!

 

[katmar]

Quote (DBradley)
Also, the friction factor for liquids is fm and for gases it is ff. ff = 1/4 fm.


Ummm, fDarcy = 4 x fFanning.

I'm not sure about this fliquid / fgas stuff.

Good Luck,
Latexman

My experience with the various friction factors in use is similar to Latexman's. The Darcy and Fanning forms are both used for gases and for liquids. The form used (in my experience) has more to do with the industry, the country, the engineering discipline and history. In older books from the UK (eg Coulson and Richardson) there is a third form attributed to Stanton which is 1/8 of the Darcy (or Moody) factor.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[1503-44]

We need an ISO for this.