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Pipelines, Piping and Fluid Mechanics engineering FAQ
Flow in Pipe
When should I segment my pipeline flow calculations? by zdas04
Posted: 28 May 16

It has always been an article of faith that if the density of the downstream piping were within 10% of the density of the upstream piping then the incompressible flow equations would provide adequate results in an inherently compressible fluid. When the downstream density is less than 90% of the upstream density then common practice was to segment the line until that 90% was realized. To get a feel for how valid this assumption is I wrote a MathCad program to bust up a flow problem with downstream density around 50% of upstream density for a single segment and then matched the (pipeline inlet  pipeline outlet) dP while segmenting the line finer and finer.
Assumptions:
 100% CH4 (MW 16.043, SG 0.5539)
 Std weight 24 inch new steel pipe (ID 23.25 in, efficiency 0.95, absolute roughness 150E06 ft)
 No water standing in line
 Temp constant at 520R
 Upstream pressure 580 psia [40 bara]
 Downstream pressure 290 psia [20 bara]
 Line length 62 mi [100 km]
Steps:
 Guess segment downstream pressure
 Calculate average pressure with the guess (using frontend loaded average)
 Calculate compressibility at average and standard pressure
 Calculate density at average pressure and standard pressure
 Calculate viscosity at average pressure
 Calculate Reynolds Number
 Calculate Fanning Friction Factor
 Using Isothermal Gas Flow Equation to calculate downstream pressure
 If calculated downstream pressure more than 100 Pa from guess, iterate 19.
 Move segment downstream pressure to upstream pressure and repeat 110
I ran the program for a number of different segments:
Seg Length...........Max Flow rate..........Density Ratio (worst segment)
100 km.................243.725 MMSCF/day...............48.1%
50 km..................243.759 MMSCF/day...............61.5%
25 km..................243.759 MMSCF/day...............74.3%
10 km..................243.759 MMSCF/day...............87.0%
2 km...................243.765 MMSCF/day...............96.9%
1 km...................243.765 MMSCF/day...............98.4%
500 m..................243.765 MMSCF/day...............99.2%
250 m..................243.765 MMSCF/day...............99.6%
100 m..................243.765 MMSCF/day...............99.8%
10 m...................243.765 MMSCF/day...............99.9+%
I found it interesting that above 90% the flow rate was unchanging. I reran this several times for flow rates increasing and found that below 90%, the flow rate changed with increasing segmentation, and above 90% a given dP gave the same flow rate. Changes above 30% were pretty small (more than the 0.016% above, but much smaller than the +/10% accuracy we expect to get from this equation). My conclusion is that I will continue to use the 90% check, but certainly understand why someone would ignore it for pipeline calculations. 
Back to Pipelines, Piping and Fluid Mechanics engineering FAQ Index
Back to Pipelines, Piping and Fluid Mechanics engineering Forum 

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