natural gas compressibility factor question 4

[mod231]

Hello all, I am sizing a small natural gas line with a starting pressure of 40psi for an industrial application. It will need to flow 8300cfh, the pipe run is 3500'. I was going to use the Renouard equation, then I read in a piping design handbook that the compressibility factor "Z" doesn't really need to be accounted for on this short of a run, and with pressures this low. Is this correct? Is there a more appropriate formula for this situation?

Best regards.

 

[katmar]

ione, well spotted - I should use my own software more often! I was using a chart from a GPSA book that I got from a page I downloaded from another forum and although it is clearly labelled as kPa, because of the US origin of the document I read it as psi. Mea culpa. If I read the GPSA document properly for your example then I get close to Z=0.95 and in many cases this will not be negligible.

This GPSA NG chart was for a MW of 18.85, so I think the value for pure methane should be very close. If anyone wants access to the original GPSA book the single page I have is labelled FIG 23-6 and page 23-14.

Thanks for pointing this out - it certainly underlines the need to double check anything you read on the internet! My apologies to anyone else who was confused by my mistake.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[zdas04]

I developed the attached chart for a class I'm putting together. The data is from NIST REFPROP.exe.

The methane line is self-explanatory, CBM is 8% CO2, 92% methane. "Sweet Gas" is a 1173 BTU/SCF mix of hydrocarbons. Sour Gas has 3% H3S and 7% CO2 (963 BTU/SCF).

I'm not sure what EOS REFPROP uses but it has consistently given me good results. UCONEER uses Peng-Robinson which is also very good For the conditions the Ione entered, REFPROP yields 0.96752 instead of UCONEER's 0.9598 so the two methods are within 0.7% of each other. I find the attached graph useful for getting orders of magnitude quickly, but an EOS or something like Hall-Yarborough are a lot better for the heavy lifting.

David

 

[ione]

For methane and conditions listed in my previous post, the Soave-Redlich-Kwong EOS gives the compressibility factor posted by zds04 (anyway I don’t know whether REFPROP works on that EOS)