Gas pipeline venting flow calculation

[nesamani1983]

We have a 20 inch, 41 km of natural gas pipeline, which needs to be pigged by ILI tool.

The pipeline has no gas flow. Hence pigging is planned by venting the gas at receiver end.
The ILI vendor informed that the pipeline shall be filled to 15 bar, before launching the PIG. venting is planned by 8'' vent pipe.
Tool velocity has to be 1 to 3 m/s. Is there any calculation to position the 8'' valve, to simulate the flow required in the pipeline for the required PIG speed.
Differential pressure required across the PIG is 3 bar.

Thanks in advance.

 

[1503-44]

A velocity of 3m/s in a 20" equals a Displacement volumetric Flow Rate of 1.52 m3/s.
That volume displacement rate must be held at any pipe internal pressure.

Standard Flow Rate at the 8"valve needed to maintain 3m/s gas velocity in the pressurized pipeline.

At 15 barg, the std flow of gas will be 1.52 m3/s /1 bara x (15+1) bara = 24.4 m3/s.
At 12.5 barg, 1.52 x (17.5+1) = 20.5 m3/s standard flow rate
At 10 barg, 1.52 x (10+1) = 16.75 m3/s
At 5 barg, 1.52 x 6 = 9.1 m3/s
At 1 barg, 1.52 m3/s x 2 = 3 m3/s

Volume of 41km 20", V1 = 20,500 m3 P1 = 1 B
P2 = 15 B Driving Pressure Upstream of pig and initial entire pipeline pressure.
Std M3 at 15 barg = V1 x (15 +1) = 328,000 m3

Roughly, the procedure is
As you vent gas volume Vv , your pipe internal pressure drops.
pipeline pressure downstream of pig drops to
P2 = P1 V1 /V2, where V2 = V1 -Vv

Pipeline pressure upstream of pig depends on how far your pig has moved.
It should move according to its velocity as we calculated above from flow rate, so the upstream and downstream pressures should decrease equally, although the upstream pressure could be up to 3 bars higher.

At each of those flow rates there will be a pressure drop inside the pipeline from 8" vent to pig from the gas moving at that glow rate. Calculate that pressure drop, Pd
Pressure at the vent + Pd + 3 bar must always be less than P2, your driving pressure at any given moment.
If it is not, then the initial pressure must be raised to at least equal that amount.

When you understand that, we can get into some refinements.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[pmover]

"pipeline has no gas flow. Hence pigging is planned by venting the gas at receiver end. . ."

not feeding gas into pipeline at location where pig is inserted, your method will not work as there is likely insufficient gas to expand to push pig along the pipe.

 

[1503-44]

No flow in? I forgot that bit. You're right.

Why do an ILI, if you have NO gas? You must have a source that can maintain the pressure upstream.

Find some gas or buy some N2?


--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[1503-44]

You may only need 82,000 Std m3 of additional gas or N2, if the pipeline now is full of gas at 15 berg.

At 15 barg, 82,000 Std m3 will occupy 5125 m3 of pipeline. 1/4th of the pipeline's volume.
Reducing its pressure to 7.5 barg, it will expand to 10,250 m3 = 1/2 PL vol.
Reducing it again to 3.25 barg it will expand to 20,500 m3, = Total PL vol.

You would inject the N2 at the beginning of the pipeline at about the same rate it was being vented until the pig reached 10.25 km. Then stop the N2 injection. That should be enough N2 to expand behind the pig until itreaches the end of the pipeline at 3.5 barg of pressure.

Buy some extra N2 in case the pig needs some extra push here and there.



--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

This sounds like someone hasn't put brain into gear.

Agree with all the comments above, but who came up with 3 bar across the pig??

No way. Work out the force that is on a 20" pipe (It's about 6 tonnes of force). Your cups flip over / pass at about 0.5 to 1 bar differential. You just can't seal it and the acceleration would be huge. There is more resistance for a ILI pig, but 3 bar?

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

 

[1503-44]

Vertical pipe?

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[georgeverghese]

Info I see on the net indicates pig speed of 2.5-3.5m/sec, while flowing dp seems to vary between the 2 studies - one indicates a pig flowing diff press of 2-3psi, while another about 7psi. One study says a dp of 1bar is required initially to kick the pig into position from pig launcher into pipeline.
May be better to get a a few runs done on a dynamic simulation program for pipelines for this.

 

[nesamani1983]

Thank you all for your replies. The pipeline is kept idle. But integrity assessment is need to be done. So we are going to do the ILI with gas venting.
can anyone share me the excel file with formula & data input.

 

[georgeverghese]

This is not something that you can estimate with a set of formulae. Dynamic simulation programs use complex time dependent algorithms that run several hundreds of iterations in the blink of an eye. You've got to show the right color of money to a specialist dynamic simulation consultant Co if you want a decent job of this done - speak to your Engg Manager to hire some one who can prepare a scope of work and supervise this exercise with the selected external consultant.

 

[LittleInch]

nesamani,

I not sure you actually read the notes above?

Just because if you blow this line down you get a gas velocity doesn't mean you can use it to drive a pig when the pig is at one end.

You need to introduce a large quantity of gas, be it natural gas or nitrogen - probably about 50% of its length, then you can de-pressurise in front of the pig which will then drive it to the far end.

All the simulations or calculations in the world won't get you pig to move until you obtain approx 50% of the line volume at 15 bar to push the pig along.

Until you get your head around this we won't make any progress here.

I am quite sure the ILI vendor doesn't realise that you're not talking about flowing gas into the pipeline.

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

 

[1503-44]

Actually we are not sure if you are still talking about moving the pig only by venting gas in front of it. That will result only in depressuring the pipeline. That little piggy never gets to market. He's happy at home, sitting in the launching pen. I thought that was clear a long time ago.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[pierreick]

Hi,
Consider this link and video.

https://www.dexon-technology.com/pipeline-services/intelligent-pigging/in-line-inspection/

You may want to contact them
Pierre