5 replies to this topic

[emma14778]

I have been trying to figure out how to calculate compressible fluid flow. I have been doing some research and have worked through a derivation for isothermal compressible flow. I keep finding contradicting equations, ones which have gravity included and ones which do not.

 

I have attached two pictures. One is an isothermal equation from crane TP-10 which includes gravity and the other is from an article by Robert Kern which does not include gravity.

 

I can follow the derivation in the article up until the point where they simply change units, at this point the gravity term disappears with no mention of it. I have seen this in several different references so I doubt it is just a typo. Can anyone explain why we can neglect the gravity term?

 

Thanks in advance!

 

 

Attached Files

•  Robert Kern Chemical Engineering.png   107.98KB   4 downloads
•  Crane TP-410.png   58.28KB   0 downloads

[IGC]

Assumption number 7 in the crane paper - the pipe line is straight and horizontal.  

 

www.cchem.berkeley.edu/cbe150a/mom/CompressableFlow.ppt

 

That ppt has an example of isothermal flow + an energy balance to show why it is neglected.  

[breizh]

Hi ,

You may find pointers in this paper.

Breizh

[Bayo Alabi]

Can anyone explain why we can neglect the gravity term?

 

 

 

nmoverley,

 

For all practical purposes, gravity term or potential-energy term is not considered should the density of the gas due to high pressure or high molecular weight be greater than 5 lb/ft3 (80 k/m3) with a difference in elevation of about 50 ft (15 m)

 

Bayo

[breizh]

Hi ,

let you consider this resource too.

Breizh

[Bayo Alabi]

I have been trying to figure out how to calculate compressible fluid flow. I have been doing some research and have worked through a derivation for isothermal compressible flow. I keep finding contradicting equations, ones which have gravity included and ones which do not.

 

I have attached two pictures. One is an isothermal equation from crane TP-10 which includes gravity and the other is from an article by Robert Kern which does not include gravity.

 

I can follow the derivation in the article up until the point where they simply change units, at this point the gravity term disappears with no mention of it. I have seen this in several different references so I doubt it is just a typo. Can anyone explain why we can neglect the gravity term?

 

Thanks in advance!

 

nmoverley,

 

I made mistake in my last post. The densities of most common gases and vapours are sufficiently low that differences in potential energy for normal piping configurations and flow conditions are negligible with respect to kinetic-energy changes and friction losses. That's why potential energy changes usually need not be included in the evaluation. However, if your gas density is greater than 80 kg/m3 with a difference in elevation greater than about 15 m, then you should consider potential-energy term.

 

Bayo