pressure drop for compressible fluids 1

[Ulther]

How to calculate pressure loss in a pipe for compressible fluid (gas)(without using dedicated engineering software). In case of incompressible fluids we can use Darcy-Weisbach equation, unfortunately it cannot be applied to gases.

Can anybody help?

 

[BigInch]

Break up the pipe into segments that yield pressure drops in each segment less than or equal to 10% of the inlet pressure and use DW, then chain the pressure drops together.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[zdas04]

If you "can't" use DW for gas, then look at the AGA equation and a copy of MathCad. If you "can't" get MathCad then Panhandle A or Weymouth are the gas equivalents of D'Arcy Weisbach.

The interesting thing about flow of compressible fluids is that below some very high velocity (typical values are quoted as between 0.3 and 0.6 Mach) gases act incompressible. The general rule of thumb alluded to above is that as long as downstream pressure is more than 90% of upstream pressure then the density can be assumed to be "constant" and incompressible arithmetic can yield acceptable results.

David

 

[BigInch]

Let's add density is assumed constant "along each segment".

Then you can use an average density for each segment, which is figured at a pressure of around Pin - 1/3 * the dP of the segment.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Ulther]

Thanks for yor reply. I used method with dividing pipe into segments of constant density. Simple and practical - that's exactly what I've been looking for.

 

[sheiko]

zdas04,

I do not endorse the use of ANY range of Mach Numbers as a determinant for this decision.

Read this quote from Green D.W., Perry R.H., "Perry's Chemical Engineers´Handbook", 8th Ed., 2008:
"Compressibility effects are important when the Mach number exceeds 0.1 to 0.2. A common error is to assume that compressibility effects are always negligible when the Mach number is small. The proper assessment of whether compressibility is important should be based on relative density changes, not on Mach number."


"We don't believe things because they are true, things are true because we believe them."

 

[zdas04]

Sheiko,
I've read it. I've got to conclude the D.W. Green has not been divinely inspired and is just expressing his interpretation of the data he's analyzed. It doesn't jibe with my interpretation of the data I've analyzed and I know what my uncertainty and processes were, so I think I'll ignore Mr/Ms/Dr Green.

The problem I have with the pedantic statement you've quoted (again) is that if you have a high dP at low flow then there are significant mechanical processes at work (lots of friction, a line partially blocked, etc.). I know that at high velocity, flow parameters are dominated by acoustic effects instead of mechanical flow resistance. At lower velocity the acoustic effects can be safely ignored.

On the planet where I live, if velocity is much above 0.6 M, then I get a significant deviation between values predicted by the AGA Gas Flow equation and observed values. Below that value the comparison is quite usable as long as the line is short enough (or has been broken into segments that are short enough) to keep the change in density from end to end below about 10% of the upstream value. Above 0.6M, nothing I do makes the turbulent-flow equations match observed conditions.

David