7 replies to this topic

[uetian98]

hi,

I am confused between using a correct correlation for determining pressure drop for Compressible Fluids.

1. I have seen people using Darcy Weisbach for compressible too
this is only good if Dp is less than 10% (GPSA_Section 10).
2. Some recommend Panhandle & Weymouth 's correlation which is easy but inaccurate too.
3. Some use Fline Software method based on Adiabatic rather than Isothermal Pressure drop.
4. ProII gives you Beggs-Brill, Dukler Eaton, Oilmens, Mukerjee-Brill, Gray or Hagedorn Brown Correlations.

Now which one is correct (or more realistic). I have a huge network for which I am planning to incorporate piping details in an Excel Spreadsheet to have it done in 1 go (I know it would be numerous iteration across each pipe segment before it reaches the final solution).
little help would be appreciable.

Thanks.

[pleckner]

There are only two condions where one can treat a gas/vapor as an incompressible fluid.

(1) The total pressure drop in the system is less than 10% of the upsteam pressure. In this case, the density will be fairly constant and this is why you can treat the gas/vapor as an imcompressible fluid.

(2) The total pressure drop in the system is less than 40% of the upstream pressure. In this case you must use the average density between the upstream and downstream conditions.

My advise is to use the Isothermal gas flow equations whenever dealing with a gas/vapor. These equations are straight forward, as easy to use as incompressible fluid equations (e.g. DARCY) and available in every book on fluid mechanics and thermodynamics.

[pleckner]

I want to add a reference article that all may find interesting:

F. C. Yu, "Compressible Fluid Pressure Drop Calculations - Isothermal Versus Adiabatic", Hydrocarbon Processing, May 1999.

For the most part, stick with the isothermal equations, they are easier to use unless of course you are using a computer program, then it really doesn't matter. Read the article!

[M S Miglani]

Another way to tackle the problem is to divide the network in smaller lengths so that the pressure drop does not exceed the 10% of the starting pressure. By this way you can use the darcy equation.

[Radionise]

Awais,

Pandhandle and Weymouth's are, normally, used for dP calculations of gas transmission pipelines. If you are calculating dP for piping, i.e. in process plants, dividing your piping into segments with dP of less than 10% as suggested by Miglani is a good way.

[pleckner]

I'm going to repeat myself because I think too many people are afraid to use the gas flow equations. Using the Isothermal gas equation is as easy as using the Darcy equation and you don't have to worry about breaking up your line into segments. How many segments? It is a trial-and-error procedure whereas the isothermal gas equation is a one-shot deal in most cases.

[ardni]

Question: If we were to use the isothermal gas equation, which pressure and temperature should we use to get the fluid density?

[pleckner]

Start at the reservoir (source or upstream) conditions.