NG pipeline packing calculation improve 1

[Julio8287]

Hi Everybody:

I would like to know about Linepack calculation, In 408 km natural gas pipeline 34 inches in diameter, there are stations every 30 Km with PT's and TT's, I did the calculation in segments, I would like to improve this standar volume calculation adding gas composition.

Thanks in advance

 

[zdas04]

I don't know what you mean by "standard" calculation, but I don't know of any way to determine line pack without density. And I don't know how to calculate density without either specific gravity or gas composition. And I don't know how to get specific gravity without gas composition. In short, if you are doing a "line pack calculation" without including gas compostion you might as well be playing Sudoku with those numbers since they don't represent anything physical.

When I've done line pack calcs I:
1. Determined the total volume of the pipe
2. Assumed that my lowest allowable compressor suction pressure was the "line pack pressure"
3. Determined the gas density at the line pack pressure
4. Multiplied the gas density times the volume determined in #1 to get mass.
5. Divided the mass by the density of the gas at the contract STP to get volume at standard conditions. The answer is line pack.

Breaking the line into segments is a waste of time since that is only useful in dynamic calcs like compressor performance or fluid friction. Line Pack is a static calculation.

David

 

[BigInch]

If the pipeline is long, it can be very helpful to divide the line up in segments. All computer analysis programs capable of transient gas analysis do it. Whether you need to break your line up into segments, depends on the conditions you have in the pipeline at the time. With 400 km, I think its a good idea to use segments. Its easy in Excel to do it. Over that length you probably have gas with greatly different pressures and temperatures, which will have different compressibility factors and the line pack will vary in each segment. If you had a diameter change, then you would have to break into segments.

I'd use the average gas pressure in each segment, if each were "short", and note that average pressure is not (P1+P2)/2, but more closely follows,

Pavg = 2/3 * (P1 + P2 - (P1 * P2)/(P1 + P2))
Then calculate the compressibility using that pressure and the average temperature (T1+T2)/2
Pressures and temperatures must be absolute.

Just to see if I've been wasting my time talking about this, take only 1 segment of 408 km and using the max and min pressures, calculate the line pack. Then calculate it again using each of your segments and tell us what the difference is. I think you'll easily have a difference of 10% or more.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

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

 

[zdas04]

BigInch,
I'm not sure we have the same definition of line pack. The definition that I've always used is the amount of gas that must be present in the line to begin operations. It sounds like you are talking about the amount of gas that is in the line for steady state operations.

With my definition, if every compressor station has the same minimum suction pressure then that pressure represents the minimum amount of gas that must be present to begin operations. If the various compressors have different minimum suction pressures then you have to treat each segment upstream of a compressor as its own line pack.

With the other definition you must use segments and average pressures. Typically you would start with compressor discharge and run the segment to compressor suction. If outlet is less than 90% of inlet, then the segment is too big and should be further reduced so you get a meaningful compressibility.

David

 

[vzeos]

BigInch's definition is the correct one.

 

[BigInch]

Line pack for me is just the amount of gas in the pipeline at any given time. Its important to gas transmission pipelines, as line pack can be an easy source of gas used to supply short term high load flows at various terminals during a cold snap, to provide flow during compressor station maintenance, or when a line segment upstream must be taken out of service for repairs, etc., then the line pack is built up again over the next several days, or weeks. Using excess line pack can be easier than activating a storage cavern or bringing on additional wells just for a short term load. Its also used as a baseline for leak detection algorithms.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

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

 

[Julio8287]

Hi Folks:

David, I assumed 100% of Methane density, I have the compossition now and I've found this for Z-Factor Calculation:

http://www.mechengcalculations.com/jmm/pipe015_process.jsp

How is your Density Calculation?

Biginch, the diference is 2.5% without segments, I would like to know about your compressibility calculation.

Thanks!

 

[BigInch]

Do you want to know how I calculate compressibility to find the densities?

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

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

 

[MortenA]

I would use transient computer simulation e.g. Pipeline Studio from energy solutions.

Best regards

Morten

 

[Julio8287]

Biginch, yes, I want to know your density calculation, I hope that you can help me to solve this issue.

Thanks in advance.

 

[BigInch]

What issues?
------------------------------
For determining density see,

http://en.wikipedia.org/wiki/Equation_of_state

Any one of these will do for starters 4.1, 4.2, 4.3, 4.4


**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

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

 

[dcasto]

density is PV = znRT or n/V = P/zRT

Tc =169.01+314.001*Spgr
Pc = 708.75-57.5*Spgr
Pr = PSIA / Pc
Tr = T rankin / Tc
z = 1 + (0.0703 *Pr / Tr) * (1 - 6 / Tr ^ 2)