Elevation Difference for Compressible Fluid

[RJB32482]

Hello,
I am using Equation 3-20 in the Crane Technical Paper 410 to calculate flow rate of nitrogen out of a pipe which is about 30 ft long with 90 and 180 degree bends. P1= 3 psig (after a pressure reducer) and P2=0 psig. The line size is 4" Schedule 40 S.S. How using equation 3-20, it does not take in account elevation differences (which in this case is about 10 feet). I want to find flow rate of nitrogen. Can I ignore the elevation difference or do I need another calculation method? If another method is needed, is it in Crane Manual or can someone send me a link or explain it in here?

Thank You.

 

[Zapster]

3 psig nitrogen at room temp has a density of 0.096 lbm/ft^3. I haven’t looked at the equation in Crane; however, I don’t see how an elevation consideration for something with this type of density would be of concern to any typical flow problem.

 

[Zapster]

Change the 0.096 to 0.090 as stated above. I am getting old and need glasses for fine print.

 

[StoneCold]

Ignore the elevation difference. Elevation differences only make a difference in liquids and I suppose super critical fluids.

Regards
StoneCold

 

[TBP]

I believe it's actually in the Crane publication somewhere that you can ignore elevation changes with compressible fluids. I have worked in district heating operations where the steam pressure on the 10th floor of a commercial building was the same as what was showing on the gauge in the underground parking garage. You can see the same thing with gas lines in multi-story buildings. Only the equivalent length of pipe matters - whether it's going up, down or sideways. Liquids are another matter.

 

[Montemayor]

Using Equation 3-20 in Crane's Tech Paper #410 to find the flow rate of N2 with pressure drop input gained from pressure gages or transmitters is going to give you an answer that has an accuracy value that is such that a 10-ft hydrostatic head of N2 gas won't make a differance that you can measure in the field. In other words, with the accuracy of the flow prediction being at least within 10-15% (at best), you shouldn't be concerned with the trivial elevation.

 

[MortenA]

You can igonore elevation changes for most applications with gasses.

I have done one job involving a 100 km pipeline operating at 220 barg->80 barg with a elevation difference from 0 to 2200 m below the sea surface and back to 0. Here it DID matter but that was a first.

Best regards

Morten

 

[katmar]

The static head due to an elevation difference of 10 ft with a density of 0.09 lb/ft³ is less than 0.006 PSI, or 0.2% of the 3 PSI available - so I agree with the earlier posters that it can be safely neglected.

However, there is another factor which I would like to raise for discussion here. Crane Eq 3-20 includes the factor "K" which represents the total resistance of the line. This would normally include the entrance and exit effects, which are especially important in a short line like this.

As you have an inline pressure reducer you can neglect the entrance effect. The K value for your 30 ft of pipe plus bends will be about 2.6 (assuming 1 each of 90 and 180^o bends), and the K value for a free exit is 1.0, giving a total K of 3.6.

If you neglect the exit effect you will overstate the flow by a factor of (3.6/2.6)^0.5, or roughly 18%, which is a lot more significant than the static head.

But here comes the bit on which I would like to invite some comment. In this particular case I believe that we could make an argument for neglecting the exit effect as well. The inline pressure reducer will control the static pressure, and will not see the velocity head. As the reason for including an exit effect is to compensate for the velocity head not being recovered, and in this case we did not include the velocity head anyway, it would be wrong to subtract something that was not included to start with. Comments?

regards
Harvey