Gas pipeline capacity 2

[petroabbes1980]

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

 

[zdas04]

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

 

[georgeverghese]

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.

 

[SNORGY]

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.

 

[LittleInch]

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.

 

[petroabbes1980]

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.

 

[MortenA]

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.

 

[zdas04]

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

 

[petroabbes1980]

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

 

[SNORGY]

I'm confused...

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

 

[zdas04]

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

 

[petroabbes1980]

Operating P=40 barg
Design Pressure 90 barg???

What is wrong here?!

 

[SNORGY]

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

 

[zdas04]

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

 

[petroabbes1980]

Sorry for the confusion,

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

 

[LittleInch]

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.

 

[georgeverghese]

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)?

 

[zdas04]

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

Assumptions:
[ul]
[li]100% CH4 (MW 16.043, SG 0.5539)[/li]
[li]Std weight 24 inch new steel pipe (ID 23.25 in, efficiency 0.95, absolute roughness 150E-06 ft)[/li]
[li]No water standing in line[/li]
[li]Temp constant at 520R[/li]
[/ul]

Steps:
[ol 1]
[li]Guess segment downstream pressure[/li]
[li]Calculate average pressure with the guess (using front-end loaded average)[/li]
[li]Calculate compressibility at average and standard pressure[/li]
[li]Calculate density at average pressure and standard pressure[/li]
[li]Calculate viscosity at average pressure[/li]
[li]Calculate Reynolds Number[/li]
[li]Calculate Fanning Friction Factor[/li]
[li]Using Isothermal Gas Flow Equation to calculate downstream pressure[/li]
[li]If calculated downstream pressure more than 100 Pa from guess, iterate 1-9.[/li]
[li]Move segment downstream pressure to upstream pressure and repeat 1-10[/li]
[/ol]

I ran the program for a number of different segments:

[tt]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[/tt]

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

 

[georgeverghese]

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.

 

[zdas04]

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