9 replies to this topic

[James_Rotating]

Dear All,

Currently, the design for a Flash Gas Compressor is a 3-stage reciprocating compressor as per the following data:

1st Stage:
Compressor Inlet Gas Flow : 3.5 MMSCFD
Compressor Inlet Pressure : 5 bara
Compressor Discharge Pressure : 11.4 bara
Compressor Inlet Temperature : 48 C
Compressor Inlet Gas Mol. Wt. : 55
Compressor Outlet Temperature : 90 C
Max. Gas Temperature d/s of Cooler : 54 C

2nd Stage
Compressor Inlet Gas Flow : 5.5 MMSCFD
Compressor Inlet Pressure : 10.7 bara
Compressor Discharge Pressure : 32 bara
Compressor Inlet Temperature : 30 C
Compressor Inlet Gas Mol. Wt. : 99
Compressor Outlet Temperature : 30 C
Max. Gas Temperature d/s of Cooler : 54 C

3rd Stage
Compressor Inlet Gas Flow : 5.5 MMSCFD
Compressor Inlet Pressure : 31.4 bara
Compressor Discharge Pressure : 71 bara
Compressor Inlet Temperature : 55 C
Compressor Inlet Gas Mol. Wt. : 29
Compressor Outlet Temperature : 132 C
Max. Gas Temperature d/s of Cooler : 54 C

If you notice, the discharge temperature from the 3rd stage is already 132 C, meaning that, if I were to design it to 2-stage recip compressor, the compression ratio will be higher which is approximately 3.8 (5 bara to 71 bara for 2-stage). When we have higher compression ratio, definately we'll have higher discharge temperature.

My questions are:
1) Is there a chance that the discharge temperature at final stage will not exceed 150 C for a 2-stage (as per API 618)

2) If the compresion ratio is 3.8, is too high for a compression ratio?

Thanks.

Edited by James_Rotating, 13 February 2011 - 06:18 AM.

[ankur2061]

James,

The term too high is a very relative term. What is too high compression ratio for a compressor?

Compression stages are determined on the following overriding concerns including the compression ratio:

1. Limit the amount of mechanical work for a stage to avoid excessive wear and tear per stage

2. Arrive at a stage configuration in such a manner that an optimization is available between the first cost CAPEX) (too huge a machine) and the operating cost (OPEX) (power consumption and maintenance) for the selected multi-stage compressor. Remember that more the number stages for a given compression ratio lower is the power consumption but the cost of the machine increases.

3. As already provided in your post the temperature rise due to the heat of compression for a given compression ratio.

The link I am providing you is an excellent discussion on this very forum where Art Montemayor has discussed in length about the logic and reasoning for the limitations of compression ratio and the number of stages.

http://www.cheresour...atio-per-stage/

Hope this helps.

Regards,
Ankur.

[Art Montemayor]

James:

My response to your two basic questions is:

• I can’t answer your specific question without having correct and checked basic data. Your submitted, listed data does not make any sense. Please refer to my attached, detailed PFD that should clearly define what your data problem is. You basically have not given us all the specific details – and those that you report don’t make much sense.
• A compression ratio can’t be treated as cast in solid steel. It all depends on what you are compressing, your scope of work, and the process conditions you have to adapt to. You haven’t given us all of these items and those that you have volunteered are clearly wrong.

Please review my workbook and feel free to make your comments on it, submitting it as Rev1.

I continue to repeat what I have expounded in the past: failure to resort to clear, detailed, graphical engineering descriptions often leads to mistakes, misinterpretations, misunderstandings, and a waste of time laboring with the written word.

Await your reply.

Attached Files

•  Flash Gas Compression in a Recip.xls   982KB   94 downloads

[paulhorth]

Art,

Just a guess... but if the second stage discharge temp of 30 C and the mol weight of 99 are interchanged, to give a discharge temp of 99 C and a mol wt of 30, the data makes much more sense.

Just a slip of the fingers, I suspect.

Paul

[Art Montemayor]

Paul:

Yes, that would be my guess too. However, I don't believe we should be guessing when all the OP has to do is furnish the correct information and basic data - and this is always much better expressed and understood with a sketch whenever possible. I have done that part (which isn't my job) in order to make my point. Let's see if the point gets across.

[James_Rotating]

Dear All,

I apologize for the late reply due to limited internet access at construction site. I also apologize for giving the wrong information about the Compressor Inlet Gas MW and Outlet Temperature at 2nd stage.

Attached is the revised excel from Art.

Basically, the maximum compression ratio of 3 has been considered in this design to keep the stage discharge temperature limit of 140 C. So I guess it is quite impossible to reduce to 2-stage and maintain temperature below 140 C.

Please advise.

Thanks

James.

Attached Files

•  Flash Gas Compression in a Recip rev01.xlsx   59.23KB   35 downloads

[Art Montemayor]

James:

Judging from the little basic data you have given us, I can only surmise that you can’t reduce the number of compression stages from 3 to 2 due to the higher expected discharge temperatures out of the 1^st and 2^nd stage. However, discharge temperatures are usually not the only criteria used to decide on the number of stages. There are other factors that enter into that decision, such as side streams (as in your application), the use of polymers or plastics in the compressor (such as Teflon, Peek, etc.), and the desire to economize on the power consumed for compression (more stages means less total compression power required).

Additionally, you may not want the higher cylinder temperatures because of tar or polymer formation – especially if you have reactive or double- and trip-bonded hydrocarbons in the vapor being compressed. The higher, reactive hydrocarbons will form tars and/or polymers inside the cylinder and practically ruin the compressor. That is why I don’t like operating the 3^rd stage suction with 55 ^oC. I would cool down the suction gas to each stage as much as possible because I believe the higher molecular weight gas contains a lot of unsaturated hydrocarbons and these will form a lot of tars and solids if compressed too hot. But since we haven’t been give the gas compositions, I can’t comment.

I would specify the compressor configuration exactly as it appears on your Rev 01 – a 3-stage reciprocating compressor with a side stream entering at the 2^nd state suction. Depending on your application and needs, you may opt for one low pressure compressor on the first stage and handle the rest of the compression in a 2-stage machine. It depends on your process and what the selected manufacturer has as available cylinders and frames to maximize the effiiciency and minimize the capital cost.

[James_Rotating]

Dear Art,

Attached is a rev02 which I have included the gas composition in each stage.

1) What is the recommended temperature for gas prior entering the compressor suction? is 55C too high?

James

Attached Files

•  Flash Gas Compression in a Recip rev02.xlsx   61.83KB   16 downloads

[Art Montemayor]

James:

Thank you for submitting the additional composition information. You fail to identify if the identified streams are the proposed INLETS TO THE RELATED COMPRESSION STAGES of if they are inlets to the interstage separators (which I hope you are planning to incorporate) and so I have to assume that the compositions are for the gases entering each of the compression stages.

That being the case, I comment and recommend as follows:

• I consider the suction gases to the 1st stage as relatively hot at 48 ^oC. I would cool all the suction gas streams as cool as possible (as stated before). I would try to obtain 30 ^oC as a maximum on the suction gas to each stage. I understand perfectly that this means cooling water (and it may not be available) and capital costs in bigger coolers. But the compressor valves and cylinders are being subjected to unsaturated hydrocarbons (your analysis doesn’t specify it, but I suspect there is ethylene and propylene in the C2 & C3 noted) such as benzene and others. These gases will have a tendency to breakdown or polymerize under compression and heat-up. Cooling not only reduces that tendency, it also condenses some of these and allows for removal as valuable liquids in the interstage separators and also allows for these same liquids to scrub and absorb some of the H₂S and CO₂. I believe all these features are favorable to your operation and are more than worth the extra investment. The better and kinder you treat a reciprocating compressor, the better, trouble-free service it will yield.
• As I stated previously, I consider the suction temperature to the 3rd stage as too high at 55 ^oC. I would stress what I have stated above: higher temperatures mean more risk in unsaturated compounds reacting and fails to condense the heavier compounds to allow easier and convenient handling.
• Don’t fail to take into account the fact that you will require some degree of capacity control. I hope it isn’t valve unloaders. If you contemplate applying a recycle stream as control, then you are aggravating the temperature situation in each cylinder because you are recycling hot discharge gas back to suction.
• I don’t know how you obtained the discharge temperatures out of each stage. I presume the simulation program spit them out. You should check out the adiabatic discharge temperatures out of each stage using the Excel spreadsheet to make the calculations and we can check them.
• If you are applying a non-lube design, then your suction temperatures (which affect your discharge temperatures) should be as cool as you can obtain. We don’t know anything about this feature or the type, model, make, piston speed, valve type, etc., etc. so we can’t comment on further details.

[James_Rotating]

Dear Art,

Thanks for the recommendations.

I will stick to my initial design, Gas Engine Driven 3-Stage Reciprocating Compressor

• "I consider the suction gases to the 1st stage as relatively hot at 48 ^oC. I would cool all the suction gas streams as cool as possible (as stated before). I would try to obtain 30 ^oC as a maximum on the suction gas to each stage. I understand perfectly that this means cooling water (and it may not be available) and capital costs in bigger coolers. But the compressor valves and cylinders are being subjected to unsaturated hydrocarbons (your analysis doesn't specify it, but I suspect there is ethylene and propylene in the C2 & C3 noted) such as benzene and others. These gases will have a tendency to breakdown or polymerize under compression and heat-up. Cooling not only reduces that tendency, it also condenses some of these and allows for removal as valuable liquids in the interstage separators and also allows for these same liquids to scrub and absorb some of the H₂S and CO₂. I believe all these features are favorable to your operation and are more than worth the extra investment. The better and kinder you treat a reciprocating compressor, the better, trouble-free service it will yield". Art, I have attached another list of gases (from simulation) at each stage. Regarding the cooling water, unfortunately, I won't be using that as a cooler.

• "Don't fail to take into account the fact that you will require some degree of capacity control. I hope it isn't valve unloaders. If you contemplate applying a recycle stream as control, then you are aggravating the temperature situation in each cylinder because you are recycling hot discharge gas back to suction".
  Art, based on your previous post, you have mentioned a few types capacity control:
• Recycling discharge gas back to suction;
  
• Suction Valve Finger Unloaders;
  
• Clearance Pockets and Plug Valves;
  
• Varying the compressor rpms.

I'm thinking to install 1 capacity control for stage 1 and the other capacity control for stage 2 & 3. I haven't decided on which type yet.

• "I don't know how you obtained the discharge temperatures out of each stage. I presume the simulation program spit them out. You should check out the adiabatic discharge temperatures out of each stage using the Excel spreadsheet to make the calculations and we can check them."

The discharge temperature is based on simulation.

For adiabatic discharge temperature:
1st Stage : 85.3C
2nd Stage: 82.5C
3rd Stage: 123.7C

• If you are applying a non-lube design, then your suction temperatures (which affect your discharge temperatures) should be as cool as you can obtain. We don't know anything about this feature or the type, model, make, piston speed, valve type, etc., etc. so we can't comment on further details.

Art, kindly see the attached file for more detail on the simulation

Thanks.

James

Attached Files

•  Flash Gas Compression in a Recip rev03.xlsx   1.29MB   40 downloads