Gas Pipeline faces Higher velocity in pipeline network.

[MIANCH]

Hi Als,
See attached screen shot of gas pipeline network, total length of pipeline is 335 km, many users are there, case was run for gas flow 650 MMSCFD and Pressure 740 psig. Pipeline ID is 33.25 inches. I cannot understand why gas velocity has increased after 160 km. Can anyone please explain this phenomenon.
Thanks

 

[LittleInch]

Well from that data it looks like the pressure drops from about 550 psi to 350 psi at the two point SLR07 and the Almergh tie in.

You need more data points or an understanding of what is happening at SLR07. Change of diameter or pressure?

 

[MIANCH]

SLR07 is sphere launcher & receiver in same area and located 98 km from starting point for power plant gas supply tie-in point. the only thing which is pipe facing downhill 80 meter and then uphill 180 meter after 170 km from the starting point.

 

[pierreick]

Hi,
Where are the data coming from? Calibrated sensors?
Did you perform a mass balance to confirm/consolidate the values?
Can you sketch your system for us to visualize better?
Do you have operation issue or it's just for clarification?
Pierre

 

[MIANCH]

Pierre,Thanks for asking such type of questions and hereunder are my explanations,
There is no calibrated sensors, only pressure gauges only. Pipeline is 335 km long and consumers tie-in points are shown. Pipeline design pressure is 780 psig for 400 MMSCFD gas. Operators want to operate at 740 psig and flow rate at 650 MMSCFD. I have run simulation on hysys and pipesime. both give me higher velocity and higher pressure drop as stated in previous post.

 

[katmar]

I'm afraid that my comment below is so trivial and obvious that all it will prove is that I have not understood the question. Perhaps once you have seen where I have missed the point you can re-ask the question more clearly.

If you put the gas through a pipe line, then all else being equal you would expect the velocity to be proportional to the flow rate in standard terms. You have not indicated where the 160 km point is so I have just compared the values at Mellitah and Alkhoms. The quantity of gas drops from 650 mmscfd to 451 mmscfd so you would expect the velocity to decrease to (100 x 451) / 650 = 69.4 % of the original velocity.

As the pressure decreases along the line the gas density will decrease and again, all else being equal, you would expect the velocity to be inversely proportional to the density. If we assume the density is a function of the pressure and temperature then the density over the same distance would decrease to (100 x 336.4 / 619.2) x (84.53 + 459.67) / (72.45 + 459.67) = 55.56 % of the original density. The velocity will therefore increase to 100/55.56 = 180.0 % of the original.

Combining these two effects gives an overall increase in velocity to 100 x 0.694 x 1.800 = 124.9 %.

 

[MIANCH]

Dear Katmar,

Many thanks for your clear and thorough mathematical explanations.

Best regards

 

[LittleInch]

So what is the issue? Your trying to stock 55 % more gas through the line at slightly lower pressure. Frictionallosses will increase so to get more flow pressure will need to do, density reduces, so actual velocity increases. 12 m/ sec is not particularly fast.j

 

[MIANCH]

My concern was this particular gas pipeline segment has more velocity whilst other section velocity remain between 7-8.5 m/s.

 

[Snickster]

Further to what Katmar stated, you can just use the ideal gas equation to determine what the velocity should be at any point in the line as follows:

P(144)(V)(A) = m Z R T

Where P = Pressure psia
V = Velocity ft/sec
A = pipe inside area ft^2
m = mass flowrate lbs/sec (lbs weight not mass)
R = universal gas constant = 1545/MW - where MW is the molecular weight of gas
Z = compressibility factor
T = flowing temperature deg Rankine

 

[LittleInch]

My concern was this particular gas pipeline segment has more velocity whilst other section velocity remain between 7-8.5 m/s.

It's long section and velocity follows an inverse square profile versus length. Nothing strange here as far a I can see.

 

[georgeverghese]

Max permissible gas velocity in corrosion inhibitor injected CS gas - condensate pipelines is approx 25m/sec in the gas phase to maintain inhibitor stability/ avoid film erosion.
Running at 740psig inlet pressure in a 780psig MAWP CS pipeline may be too high for normal operating pressure range - I would trip out the pipeline at this pressure.

 

[pierreick]

Hi,
Consider this document to support your analysis.
About formula of velocity in SI units, consider this one:
Velocity(m/s) = QM (kg/s) *Z*R*T (K)/ [Mw (kg/mol) *P(Pa)*A (m2)]
with R=8.314 (SI)
Mw molecular weight.
Pierre

 

[MIANCH]

Max permissible gas velocity in corrosion inhibitor injected CS gas - condensate pipelines is approx 25m/sec in the gas phase to maintain inhibitor stability/ avoid film erosion.
Running at 740psig inlet pressure in a 780psig MAWP CS pipeline may be too high for normal operating pressure range - I would trip out the pipeline at this pressure.

Hi George
Risk of pipe damage is 5% if operated at 740 psig. can you explain more on risk as you have vast experience.

 

[LittleInch]

Basically by the look of it unless you're adding compressor stations, the only way you're getting more flow than design is by lowering the arrival pressure, which increases velocity.

I think what George means is that you only have a small margin (40 psi or 6%) below trip pressure setting of max 780.

That is often difficult to achieve in practice.

Not sure what you mean by by risk of 5%??

 

[georgeverghese]

Trip pressure setting is somewhat arbitrary, but typically it is say 95% of design pressure if there is no overpressure relief requirement. If there is a possibility that the feed source can deliver pressure at or more than 780psig, then you may wish to trip out the source on PAHH at 90% of DP. Then the PSV (say on an upstream feed compressor) may be set at 780psig.
If you go for a pilot operated PSV at the compressor discharge, then you may go up to 95% of 780psig for the PAHH ( and maybe 92-93% of 780psig for PAH) to trip out the upstream compressor. Note that even in this case, the discharge pressure controls on the compressor would have kicked in by now to reduce feed to the pipeline.
Talk to the plant operations team and see what they have at the feed Mellitah gas plant and adjust feed pressure to this pipeline as required, so there is enough room for all these instrumented pressure control and alarm and protection devices to operate.

 

[MIANCH]

Trip pressure setting is somewhat arbitrary, but typically it is say 95% of design pressure if there is no overpressure relief requirement. If there is a possibility that the feed source can deliver pressure at or more than 780psig, then you may wish to trip out the source on PAHH at 90% of DP. Then the PSV (say on an upstream feed compressor) may be set at 780psig.
If you go for a pilot operated PSV at the compressor discharge, then you may go up to 95% of 780psig for the PAHH ( and maybe 92-93% of 780psig for PAH) to trip out the upstream compressor. Note that even in this case, the discharge pressure controls on the compressor would have kicked in by now to reduce feed to the pipeline.
Talk to the plant operations team and see what they have at the feed Mellitah gas plant and adjust feed pressure to this pipeline as required, so there is enough room for all these instrumented pressure control and alarm and protection devices to operate.

Apart from the safety systems already in place for overpressure protection, I would like to better understand the pressure behavior along the 100 km stretch starting from Mellitah Complex. The first user is located at 61 km and the second at 98 km, both segments are experiencing very high pressure. The third user, located at 229 km, is where higher gas velocity is developing. My primary concern is with the segments experiencing high pressure and the segment with elevated velocity and effects on pipeline integrity.

 

[LittleInch]

This is all normal stuff though. The pressure at the start is high, but not above design so what's the problem?

If you previously operated at a lower pressure and the condition of the pipe is aspect then you have a risk.

You are trying to get more gas through. Raising pressure at the start is the only way you're going to do this.