11 replies to this topic

[Process Engineer 90]

Dear all

How can I choose the gas plant pressure if i am the designer .

what are the factors.

 

thank you 

Edited by Osama Umaier, 15 January 2017 - 03:11 PM.

[rdcrags]

I am assuming this gas plant is in a refinery. Vapor and liquid feed streams may be coming from various units and have varying flow rates and compositions. There will be sufficient hydrogen in the feed to require an absorber as the first column , with the overhead from it feeding a sponge absorber for adequate recovery of C3+.  The pressure of that column combination is selected such that the off gas can go to the fuel system without a compressor. For the other columns, which have overhead condensers, the pressures are set to be slightly above the dew points of the overhead lproduct compositions, based on air and water temperatures. That provides minimum utilities consumption.

[Process Engineer 90]

The first column is an absorber with TEG to my products to the treatment plant which is 70 KM away so what is the best pressure to set it out for my station .

[rdcrags]

Is the pipe line existing? Design pressure?

[Bobby Strain]

Gas plants come in many varieties. So, what is the process you are using for the gas plant? And what is the pressure, flow, and composition from the TEG absorber?

 

Bobby

[Process Engineer 90]

At first production separator then a TEG contactor to remove the water content and lower the dew point to send the products (gas+condensate) 70 Km to make further operations like Sale gas + LPG + Condensate .

FTHP= 108 bar

Flow= 88 MMSCF/D

Edited by Osama Umaier, 16 January 2017 - 03:10 PM.

[Bobby Strain]

It's going to take a long time for us to give you any help when you supply information only bit by bit when requested. Give us a complete picture in the form of a PFD at least. And you didn't answer whether the pipeline is existing. A gas composition would be most helpful. Along with information about the condensate. Your function for the gas plant is also necessary so we can understand what LPG is. So, put together all the information and post it. And, I don't know what FTHP is, although I can understand 108 bar, and at this pressure it doesn't matter whether you meant gauge or absolute. But you should always be specific. Bad habits are hard to break. You should refrain from using such acronyms and abbreviations.

 

Bobby

[Process Engineer 90]

Mr. Bobby 

The files you requested .

Attached Files

•  There are 5 wells each well initial flow 27 MMSCF.pdf   495.21KB   51 downloads

[Fletch]

I can appreciate the question, it can seem like any design pressure could be picked.

Considering nominal pipe class pressure ratings can help you eliminate a 1000 possibilities and focus towards 2-3 nominal design pressures. Each with pros and cons which you consider along with the other constraints e.g. boundary conditions.

 

Like the other guys have mentioned, your boundary conditions can help determine your design pressure.

What is the design pressure of the pipeline or the downstream treatment plant, this can help govern your design.

You may maintain a high design pressure for an inherently safe design. But you will likely run into a pressure break at a boundary somewhere that will likely need protecting.

 

Another factor that can factor in is economics.

For instance, your conditions might allow you to use 900lb class piping and maintain a high operating pressure through out the plant, but this comes at a cost of materials, heavy walled pipe and vessels etc.

Dropping the design pressure down to a 600lb rating would reduce piping costs and a thinner walled vessel (your contactor might be a little larger in diameter).

And for 27 MMSCFD economics might be a factor

 

Another factor could be technology limits, a certain gas treatment might not operate well at such high pressures you would likely drop the design pressure to suit. For your case TEG is OK for the likely pressure range.

 

Less design pressure related and more plant design question.

Do you need compression to transfer the gas to the treatment plant and meet the arrival conditions?

This might mean the required export pressure pushes you to a higher design pressure for the back end of your plant.

 

Your pressure decline might also factor in.

Are you always going to operate at a high pressure and then step down to a LP operation? or boost the inlet pressure later on with compression?

Or drop the operating pressure down from the very start of production and recompress your gas back up to pressure you need for export. If you aren't operating up at high pressures you can have a lower design pressure through out... (but it does seem like a waste to just blow all that pressure and you will have to protect it from the high pressure of the wells).

Edited by Fletch, 19 January 2017 - 07:56 AM.

[Process Engineer 90]

Fletch

My man thank you for your detailed answer . But I'm not asking about the design pressure of the mechanical equipment , What I'm asking about is the operating pressure of the gathering station how to choose it so if you can help me I appreciate it so much.

[Fletch]

True, looking back it did sound like you were after operating pressure, however most of those considerations still hold.

• Boundary conditions
• Process design configuration
• Gas pressure profile

What is your boundary/battery limit pressure need to be?

Working back from the treatment plants operating pressure and the line losses (for your max flow case) in the pipeline, this will give you your boundary pressure you need to achieve.

 

If this is lower than your late field life (declined pressure) e.g. 30barg. Then you could set this as your operating pressure and always control down to this pressure for your contactor and free flow to the treatment plant.

(its unlikely this will be the case and also your pipeline would be larger due to the higher volumetric rate of operating at a lower pressure, but it just illustrates the lower limit operating pressure).

 

If your required boundary pressure falls somewhere in between your early and late life pressures, then you should consider an intermediate pressure to operate at e.g. 80 barg

This will allow you to control the pressure down initially and boost the pressure up to it as the well pressure falls. This gives you distinct bands/stages to operate in.

This will allow you to have a stable operating pressure for your contactor for many years and allow you to plan for compression in the future.

Your boundary pressure could be this intermediate pressure.

 

If your required boundary pressure is above your early field life pressure e.g. >130 barg then you need compression of some form from the very beginning.

Again, this boundary pressure could still be your operating pressure, you could provide inlet compression to have your contactor operate at this higher pressure and flow directly into the pipeline.

 

As long as you don't run into any technical constraints for any technologies at these higher pressures. If you do then you will need to operate your contactor at a intermediate pressure and boost your export gas to the boundary pressure.

 

If you are providing inlet compression then you are likely to be operating your manifold at a lower pressure to provide a constant suction pressure (like in the early scenario when you have distinct pressure stages). When you get to the lower pressure/late field life (lower flowrate and more compression required) then you can reconfigure your compressor (e.g. re-wheel a centrifugal compressor) to have a higher head with the lower flowrate.

 

In summary, you should look for an intermediate pressure to provide distinct stages of operation, one that you will be able to control down to in early field life and then in later field life as well pressure declines be able to boost up to this pressure. Providing stable conditions and time to plan for provision of future requirements.

All of which hangs around knowing your boundary conditions.

 

Additionally, always keep consideration to what your effective design pressure will be (Max operating + margins) and therefore what piping class you will have, too not unnecessarily have a higher rated pipe that will likely hurt your economics.

 

I hope this helps to get closer to answering your question.

Edited by Fletch, 21 January 2017 - 07:46 AM.

[Process Engineer 90]

Thank you dear Fletch