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Gas pipeline capacity

Gas pipeline capacity

(OP)
Hello gents;
Does Anyone knows a methods for the calculation of a pipeline Maximum Flow capacity that it could transport, Data available: e.g 24 in, inlet P=40 barg, out P=20 Barg, lenght 100 Km?

Thanks

RE: Gas pipeline capacity

I'm going to assume that we are talking about gas flow. For liquid flow, you have a concern about erosional velocity that is described in one of the API standards. I have seen people set the maximum velocity for a liquid line at 1/2 the errosional velocity, but that is pretty arbitrary.

For the conditions you describe, you are allowing 20 kPa/km. With that dP, in new pipe, with methane using the Isothermal gas flow equation you would see 118 MMSCF/day [3350 kSCm/day], but that does not necessarily define any sort of maximum flow.

Maximum flow is an economic consideration, not a technical one. Basically you are defining how much compression hp you are willing to waste in friction. It is common to specify a maximum pressure drop per unit length (e.g., for nominal 1000 psig [6.8 MPa] lines people frequently use 5 psi/mile [20 kPa/km] as a reasonable maximum). For gathering and multiphase lines it is common to use 15 psi/mile [64 kPa/km], but by no means universal.

Other people use a maximum velocity, but that is very arbitrary. I've seen numbers ranging from 50 ft/sec to 120 ft/s [15-40 m/s], but none of the justifications for the number ever make much sense when evaluated on purely technical merit.

In your example, it happens that you have maximum flow if your economic limit is 20kPa/km. If you allow 40 kPa/km you could move 5640 kSCm/day

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Gas pipeline capacity

The simplest compressible flow expression to use for this would be eqn. 6-114 in Perry Chem Engg Handbook 7th edn. This is adequate for isothermal gas transport for cases where total dp > 10% of inlet pressure.

RE: Gas pipeline capacity

For those conditions, I end up at a capacity of 231 MMSCFD using several of the correlations available in Crane TP-410, assuming gas with MW=18.85 or thereabouts, flowing at 20-25 C.

RE: Gas pipeline capacity

That's quite a big pressure drop in percent terms (50%). Hence at the end compared to the start point density will be about half, actual velocity double, friction drop per unit length 4 times.

Hence I think you need to look carefully at the limitations of any single graph or equation. Many don't include the effect of temperature or temperature drop which can be significant and impact the results substantially. Especially start temperature.

If you chop this down into at least 10 segments and then make a spread sheet so that the end results of one section feed into the next one, then as the mass flow stays the same (your only constant), then you can play around with inlet flow until your overall pressure drop equals your start and end condition.

Without using a proper pipeline flow analysis program with gas composition (most people here are assuming you mean methane) temperature data, soil temp etc, I think your number will have an accuracy of 20% or more. You already have two answers above which vary from 118 mmscfd to 231 mmscfd.

Steady state gas flow is quite simple to plug into an analysis program.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Gas pipeline capacity

(OP)
Dear All;
I did exactly what I am looking for, the say that the actual flow is 1 MSCM/d, what would be the maximum flow that could be transported via the pipeline,the DP is 90 Barg.

RE: Gas pipeline capacity

1 MSCF/d is a flow unit?
But if you are looking for free engineering thats another story.

I think the message is that for gas pipelines the flow rate is a economical optimization between cost of pipeline (mainly CAPEX) and cost of re-compression (mainly CAPEX+OPEN) and that some companies have internal guidelines/rules of thumb that will simplify this optimization. You have to remember that for gas the dP is proportinal with the flow rate squared - so that doubling you flow rate will quadruple your dP - and thus your need for compression.

RE: Gas pipeline capacity

One caution to using LittleInch's method of breaking the line up into smaller segments is that that technique is only more accurate if you use an equation that recalculates friction factor for each segment. If you are using one of the special case equations (e.g, AGA Fully Turbulent, Panhandle A, Weymouth, etc.) then you will get the same answer doing the calculation in one step as doing it in 100 steps. This is true because all of these closed form equations make some major simplifying assumptions about friction factors and then replace the "transmission factor" (i.e., 1/ff^0.5) with a constant function of pipe diameter (for example).

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Gas pipeline capacity

(OP)
Dear MortenA
1 million STD cubic meter is a flow unit of course, I did not understand your answer!

RE: Gas pipeline capacity

I'm confused...

The inlet pressure was 40 barg, the outlet pressure was 20 barg and the DP is 90 barg?

RE: Gas pipeline capacity

SNORGY,
Good catch, that 90 bar has to be a typo.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Gas pipeline capacity

(OP)
Operating P=40 barg
Design Pressure 90 barg???

What is wrong here?!

RE: Gas pipeline capacity

Oh, sorry petroabbes1980...I read DP (Design Pressure) as dP (differential pressure) and referred back to your original post.

RE: Gas pipeline capacity

petroabbes1990,
What is wrong is text-message speak does not adequately communicate technical subjects. Both SNORGY and I took the "DP" to mean "differential pressure" without ever considering an alternate meaning. Differential pressure is an important concept in flow calculations. Design Pressure is generally not an important concept in flow calculations (by the time you are talking about flow, the design pressure is set in stone).

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Gas pipeline capacity

(OP)
Sorry for the confusion,

Generally process Engineer use DP as Design Pressure and dP as differential pressure or pressure drop.


RE: Gas pipeline capacity

To get back to the key question, there's something not right here.

You're saying the actual flow with a 20 bzr pressure drop is 1 million scm pdr day??? = 35.3mmscfd??

Seems far too low.

So what exactly is this new data. What is it based on?

What are the maximum inlet and outlet pressures??

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Gas pipeline capacity

The compressible flow equation accounts for varying density along the transport duct in a single equation. The OP hasnt mentioned gas mol wt here or confirmed SNORGY's assumption of 18.9 or given an exact value for the pipe ID. Also is there a risk of condensate dropout here in this gas ? What is the gas hydrocarbon dewpoint at 40barg or at pipeline max packing pressure (at startup)?

RE: Gas pipeline capacity

I wrote a MathCad program to bust the problem into segments.

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 150E-06 ft)
  • No water standing in line
  • Temp constant at 520R
Steps:
  1. Guess segment downstream pressure
  2. Calculate average pressure with the guess (using front-end loaded average)
  3. Calculate compressibility at average and standard pressure
  4. Calculate density at average pressure and standard pressure
  5. Calculate viscosity at average pressure
  6. Calculate Reynolds Number
  7. Calculate Fanning Friction Factor
  8. Using Isothermal Gas Flow Equation to calculate downstream pressure
  9. If calculated downstream pressure more than 100 Pa from guess, iterate 1-9.
  10. Move segment downstream pressure to upstream pressure and repeat 1-10
I ran the program for a number of different segments:

Seg Length......Max Flow rate for 20 Bar dP and 100 km length
100 km.................243.725 MMSCF/day
50 km..................243.756 MMSCF/day
25 km..................243.760 MMSCF/day
10 km..................243.765 MMSCF/day
2 km...................243.765 MMSCF/day
1 km...................243.765 MMSCF/day
500 m..................243.765 MMSCF/day
250 m..................243.765 MMSCF/day
100 m..................243.765 MMSCF/day
10 m...................243.765 MMSCF/day


I found it interesting that between 100 km and 10 km (i.e. one segment and 10 segments) the flow rate increased slightly and then held constant. I played around with longer and shorter segments and I'm not sure what conclusions I can draw from it.

What I get from this is using rigorous calculations, breaking a line up into segments does little improve the results for relatively low flow scenarios. I'll probably re-run it for double the dP (80 bar upstream and 40 bar downstream) just out of curiosity.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Gas pipeline capacity

David, Assuming z=1.0, I get flow = 252mmscfd or 202e3 kg/hr with the isothermal compressible flow expression in Perry with your input info. 3% deviation in results - okay.

RE: Gas pipeline capacity

For a front-end weighted average pressure on the one-segment case using the Hall Yarborough correlation I get Z=0.881.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Gas pipeline capacity

It's still a lot more than the ops 35mmscfd......

How was the"front end loaded average" worked out?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Gas pipeline capacity

I suspect Equation #1-39 in Crane TP-410M (2011):

P'(avg) = 2/3*{[P'1^3-P'2^3]/[P'1^2-P'2^2]}

RE: Gas pipeline capacity

No, the one I use (from GPSA) is:

Pavg=(2/3)(P1+P2-(P1*P2)/(P1+P2)

With just a bit of algebra (it is easy to find the steps if you know the end result) you can factor Crane into exactly the GPSA equation, but I find the GPSA equation to be easier to explain in class.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Gas pipeline capacity

Using the GPSA expression for average P, I get P average at 32bar abs, and at 16degC, z = 0.95 at this press from NBS tabulations for pure methane in Perry.
Including this z value in the isothermal expression, mass rate is 201 e3 kg/hr or 251.1mmscfd.

RE: Gas pipeline capacity

zdas04,

Interesting. The GPSA equation that you state is the one I ultimately used when I developed my spreadsheet tool years ago, and the one it still uses. Then I recently got a copy of the 2011 Crane TP410M and decided I'd look at it and - to be honest - came across #1-39 for the first time. It made me wonder if I had been doing it wrong all these years. As you state, a bit of algebra probably would have gotten me there. But in any event, as always, I learned something new in this thread, namely, the Hall-Yarborough correlation. I had never heard of it until today. When I built my spreadsheet tool (and several others) years ago, I had used four of the cubic equations of state in the back of GPSA to estimate Z (R-K, S-R-K, P-R and VDW) and I could toggle between whichever one I wanted. I usually use VDW to avoid having to worry about boiling point properties and acentric factors that would force me to dive into a lot more detail to accurately characterize the gas which, for what I need on a routine basis, isn't necessary for me. (I'm mechanical, not process, by background). It also aligns with my approach towards predicting J-T, which is premised on VDW. In any case, when I adjusted the input in my tool to more closely match the characterization you appear to have in your calculation, my compressibility factor (via Peng-Robinson) was Z = 0.879 and my flow rate ended up at about 247 MMSCFD; my assumed gas viscosity was 0.011 cP. So, at the end of the day, there does not appear to be much disagreement between you and georgeverghese and me.

Thanks for pointing me towards Hall-Yarborough. Never heard of it until today. Because of that, I'm just a little bit better today than I was yesterday.

RE: Gas pipeline capacity

Snorgy, If z=0.88, flow remains at 251mmscfd using the isothermal compressible flow routine on my mathcad calc.

RE: Gas pipeline capacity

Should we be debating small changes in the compressibility factor (Z) when we don't know the composition of the gas and we are throwing a 95% efficiency at the problem? Nor have we defined the standard conditions.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: Gas pipeline capacity

ZDAS04, it is very interesting that the length of the segments affects the result. The Reynolds Number and friction factor should remain constant for the whole length of the pipe (assuming constant viscosity). If those are also varying with the segment size it may give you a pointer to the cause of the flow result variations.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: Gas pipeline capacity

Katmar,
The conditions I assumed are normal for commodity pipeline gas in the U.S. (e.g., what is traded on the commodities market under the label "Henry Hub" or NYMEX Natural Gas). The pressure of this line is kind of intermediate between normal raw-gas pressures and some short-haul commodity pipeline gas so I guessed commodity. Assuming raw-gas just gives you a more complicated multi-phase flow environment that was way outside the OP's question.

For liquids, the Reynolds Number and friction factor remain constant, but for gas both density and viscosity are a function of pressure (I recalculated both for each segment) and the Reynolds Number changes considerably as you go down the pipe.

I've found that with new steel pipe the design calculations I've done over the years using a 95% efficiency factor have tended to match start-up conditions within about 2% of dP. Over time, the accumulation of liquids in the line required me to lower that number to match field conditions (which is the indicator I used to assess pigging schedules quarterly, a line that was getting worse needed to be pigged more often, a line that was above 90% could probably be pigged less often).

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Gas pipeline capacity

georgeverghese,

I think that's simply because your model is probably better and more accurate than mine, or I might have entered something wrong.

katmar also raises a good point regarding standard conditions. I used the international standard of 288.15 K and 101.325 kPa.

Carefully re-checking my input and using better data for critical pressure and temperature for methane, my calculated Z (P-R) is now 0.92 and flow rate is 252.9 MMSCFD. Not sure what other input data was wrong on my end because I didn't save the spreadsheet with the last data I used, but now the result is getting closer to what georgeverghese is getting. I am not carrying anything in the way of an efficiency factor. My calculated friction factor is 0.0116, and I suspect that is where the slight variance comes from.

That said, I think zdas04's rationalization for using an efficiency factor is well-founded:

http://www.ewp.rpi.edu/hartford/~ernesto/S2009/EP/...

RE: Gas pipeline capacity

The density of the gas will decrease along the length of the pipe, but the velocity increases in proportion to the reciprocal of the density and the product of density and velocity (basically the mass flowrate) remains constant. The decrease in density therefore has no impact on the Reynolds Number.

The data I found for the viscosity (Younglove and Ely) indicates a decrease in viscosity of about 4% in going from 40 bar to 20 bar. This would translate directly into a 4% change in Reynolds Number, but a less than 0.1% change in friction factor. This is a similar order of magnitude to the change in flowrate you calculated from the largest to smallest section length and I would suspect that the reason for the variation in your results is in the way you have averaged the viscosity for each section.

I have no quibble with applying a 95% efficiency. My point was that if we apply what is effectively a 5% safety factor we shouldn't fuss over 2% variations between different ways of calculating the flowrate. However the variation resulting from the change in section length is interesting because the same calculation method is being applied in all cases.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: Gas pipeline capacity

Katmar,
I modified my program to output Reynolds Number and Fanning friction factor for the 10,000 step case. Reynolds Number changed by 4% (change is dominated by the change in compressibility since as you say density and velocity largely offset each other), resulting in a change in friction factor of 0.1%. In the 2 step case, Reynolds Number changed almost 10% and Friction Factor changed by nearly 1%.

My point is that the fewer factors that you assume to be constant, the smaller your total uncertainty is. In this example using the friction factor from step one would have resulted in no change to the outcome, that conclusion does not say that in the next example it won't lead you to selecting a different pipe size by including a redetermination of every parameter at every step. When you accept 4% uncertainty on viscosity, 2% uncertainty on friction factor, 10% uncertainty on efficiency, 20% uncertainty by using nominal instead of actual ID, etc., sooner rather than later you reach a point where you would be better off using a pipe-capacity vs. pressure chart generated at HVAC.com (I don't know if that is a real web site, but the HVAC guys seem to be the ones most content with ignoring compressibility and treating viscosity as a function of gas composition rather than composition, pressure, and temperature, mostly because their dP is consistently very low).

When I was learning this stuff we used slide rules (yep, I'm THAT old). Sensible engineers back then would pull together all of the terms that are acceptably constant for that problem (and the threshold for "constant" was pretty low) into one number and only change the few parameters that they believed actually made a difference for that step. With MathCad (or to a lesser extent Excel and calculators) redoing friction factor for every segment is no more difficult than adding a line and a function call to the program. I feel that adding that line (which admittedly rarely makes a meaningful difference to the outcome) increases my confidence in that the program will lead me to a decision that will result in a higher chance of a project performing like I predicted, which after all is the reason for the exercise.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Gas pipeline capacity

If the Reynolds number is changing by more than the change in the viscosity then there is a mistake in your program. In a pipe of constant diameter the density and velocity don't "largely" offset each other. They perfectly offset each other.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

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