Low pressure gas transfer 1

[Rauf30]

Hi all,
I'll really appreciate some help regarding equipment selection.
We are about to design an Associated Petroleum Gas Treatment plant. The plant will be located approximately 1.5 km from Gathering Station, and it is planned to lay two pipelines (high pressure 7 barg and low pressure) from GS to APGT.
Gas pressure at tie-in point to (inside of GS) low pressure pipeline is almost atmospheric (200 - 600 Pa), with max flow 4000 sm3/h. We are now considering the following options to transfer AP gas to the APGT plant.
1. Liquid ring compressor (preferably installed at APGT)
2. Blower or fan installed at GS and screw compressor or sliding vane at APGT
3. Ejector at GS (probably multiple ejectors) and centrifugal at APGT
4. Your option
Any experience sharing in transferring low pressure gas is highly welcomed.

 

[1503-44]

What is the flow rate of the hi pressure pipeline?

Lo P gas pipelines are not efficient and will be subject to liquids collecting inside. I hope there is a good reason that the two gas streams cannot be combined by compressing the lo P stress at the GS and then building only one pipeline.

 

[Rauf30]

Hi 1503-44,

Flow rate of the high pressure pipeline is around 9000 scm3/h.

[highlight #FCE94F]I hope there is a good reason that the two gas streams cannot be combined by compressing the lo P stress at the GS and then building only one pipeline.[/highlight]

Pipelines can be combined, I am proposing option 3 above (ejectors). Is there any other way to combine two different pressure pipelines?

 

[1503-44]

Not really. Obviously they must operate at the same pressure.

Do you know what pipeline diameters you are planning to use?
Is it natural gas?
Are there liquids, water, or condensates in the gas stream coming from the GS? How much?
Usually you have liquids in pipes before arriving at the treating facility.

Due to the very poor efficiency of atmospheric pressure gas pipelines, I suggest using the higher pressure 7 barg range, if possible. I have not worked out the gas velocity, as you did not mention pipe diameters, however it is usually very important to have a velocity that will push any liquids along the pipeline, especially in gathering systems where you normally get water and condensates from the wells.

I'm not sure about using ejectors. My impression is that they like higher pressures, but I do not know anything about them.

 

[Rauf30]

Diameter of LP pipeline 20", HP pipeline 10". Actually we are in the design stage and diameters can be changed.

Yes, there is liquids in pipes. During treatment about 250kg/h water will be returned to GS for reinjection.

Probably you right, ejectors will require higher pressure, but we can add something like recycle compressor to increase motive gas pressure.

What about LRV compressor? Any experience using LRV compressors in low pressure pipeline?

 

[1503-44]

OK, so you can move your 4000 SCM/H through a 250mm pipeline for 1.5 km with only a 0.1 bar pressure drop at 12m/s.

But I am confused because I thought the 200-600 pressure was kPa, but now I see that you wrote Pa. Can you confirm the pressure unit.

 

[Rauf30]

Yes, that is the problem. There is only just 1.002 bara at the tie-in point.

 

[1503-44]

Well let's just call that 0 kPag.
Yeah. Thats a problem.
So you need like a 15 kW compressor.
I dont know about any equipment with that small power.
Maybe someone else can help.

So, why 2 pipelines?

 

[LittleInch]

A fart has more pressure than that...

Seriously, just compress the LP gas into the high pressure supply at the GS

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

 

[georgeverghese]

How about a multistage centrifugal compressor (second preference recip compressor) to inject LP gas into 700kapg HP line at GS - much more efficient than an LR compressor or ejectors, and less troublesome too - see operating map in the GPSA. You can install 2x50% machines ( ie 2x2000sm3/hr machines) running on electric motors with VFD (or gas engines, if you must).
Besides, a dedicated 1.5km LP line at these pressures will be next to impossible to pig.

 

[Rauf30]

1503-44,

There is no pipelines yet, just two tie-ins. We need to increase LP gas pressure after tie-in, and yes you right, inject gas to HP pipeline.

 

[Rauf30]

LittleInch,

[highlight #FCE94F]just compress the LP gas into the high pressure supply at the GS[/highlight]

That is the area I need you guys expert opinion. I need to know which type compressor will work at 1.002 bara suction pressure and flow of 4000 scm/h.

 

[1503-44]

George is saying
1) centrifugal
2) reciprocating compressor

Gas flow is always better at higher pressure so install the compressor at the gathering station if possible.


LI, its great you found your sense of humor, but now I have an image of a really unusual PI that I can't seem to get rid of.

 

[Rauf30]

Georgeverghese,

As per operating map of the GSPA rotary-screw seems the best option for flow of 4000scm/h and discharge pressure 7 barg.

I am not sure about centrifugal compressors, normally they require higher suction pressure. Plus there will be some pressure loses till compressor.

 

[Rauf30]

1503-44,

From control point of view it is preferred to install compressor at APGT, but if there no other option compressor will be installed at GS.

 

[georgeverghese]

Rotary screws may be troublesome to operate in fouling service. Cleaning up the gas upstream would present other disposal and operating issues. Centrifugals would be best from a reliability point of view, but will most likely be higher CAPEX. Recip machines are also troublesome but are lower CAPEX like rotary screws. Project managers may prefer the cheapest option, especially when Big Brother JV partner is watching how you spend their money, but if you can convince them of lowest life cycle cost with centrifugals, that would be better.

Operations staff often have the short end of the stick when it comes to machine selection, and end up having to nurse a troublesome child for 20years, while the design phase project manager gets a feather in his cap for low cost project completed on time and on schedule, and then he runs off in a hurry to mess up the next project.

 

[1503-44]

If located at the treater, the gas will expand greatly, especially at near vacuum levels, as pressure reduces along the pipe. At 1Barg, gas volume doubles with only a 1 bar pressure drop, velocity doubles and pressure drop along the pipe is squared, gas density is less so you pay a lot to transport very little gas. It will work for now, but increasing flow rate in the future could become very difficult. Increasing flow often happens with gathering systems. If that is not expected, locating the compressor at the treater station could be OK. You will have to catch any liquids in a slug catcher before they can hit the compressor.

BTW Is the high pressure pipeline the 500mm, or the 250?

 

[Rauf30]

1503-44,

Diameter of LP pipeline is 500mm, HP 250.

4000 scm/h is the maximum expected flow. You right, we need to catch liquids before the compressor.

Separation of liquids from gas is another challenge.

Gents, any idea except slug catcher?

 

[1503-44]

Which one depends on what comes out of the well. Water, gas liquids, condensates, sand?
I think you will need a slug catcher for gas with 250kg/h of liquids. Thats like 5-8 m3/day.
Whats the elevation profile of the 1.5 kn run.
Up hill, down hill, both or flat? How many meter difference in elevation?


Some seperator options here,

https://www.youtube.com/watch?v=-xWQMWJD8hg

 

[1503-44]

As I understand your project,

You only need 1 pipeline.

You can run 13,000 m3/h through the 500mm line at 0.15 Barg at a velocity of 18m/s and 16kW compression.

Or

You can run 13,000 m3/h through the 250mm line at 3.7 Barg at a velocity of 23m/s and 187kW compression. Any lower pressure than that will increase velocity too high.