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Impact of CO2 % increase in natural gas recip compressors 2

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RMNKBR

Mechanical
Oct 26, 2004
7
I work in a gas compression field that and mainly use 2 stages or 3 stages reciprocating compressors with capacities range from 4 to 25 MMSCFD (Suction pressures from vacuum to 50 psig @ 110°F and Discharge is 450 psig).

Normal composition of the natural gas in our gas gathering system is 75% C1, 10% C2, 5% C3, 0.1% N2 and 3% CO2 (rest is IC4, C4, IC5, C5 and C6).

It is possible that CO2 injection happens in the field and CO2% will certainly increase in the gas gathering system. I'm evaluating the imapact of CO2% increase on the recip compressor performance and mechanical condition of components.

Assuming 20% CO2 in the performance runs the 1st stage and 2nd stage discharge temperature are around 250°F (increase of 5°F from 3% CO2). Assuming extreme case of 92% CO2 result is 260°F (Total rod load increased only 6%).

I found acceptable conditions in terms of temperatures and rod loads but the other concern is about corrosion and therefore need of replacement of internal components due to CO2% increase.

I'd appreciate comments or links to technical papers about the same subject. At this time our units have 98% availability and we need to identify all possible problems thta CO2 will bring.

Thanks very much,



 
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CO2 can create a corrosion hazard. Sometimes. The reaction requires that the CO2 be disolved in water and that the pH be in a pretty narrow range. If you have enough water coming into your compressors to pose a continious-phase liquid with acidic pH then corrosion is the least of your problems (liquid in the cylinder of a recip has a tendency to redecorate the landscape with pieces of metal).

The difference in "k" between your original analysis and the increased CO2 scenario is about right for the temp increase you are expecting--it shouldn't be any big deal.

You didn't say if your compressors were engine driven or motor driven. If they're engine driven, you'll reach a point at about 450 BTU/SCF where the engines won't run on the gas. Down to that point you will have to change the jets in the carb at around 800 BTU/SCF and at 600 BTU/SCF you'll need to change the carb to a digester carb.



David Simpson, PE
MuleShoe Engineering
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Thanks for the prompt reply. We have both types (gas engine and electric motor driven) and fuel gas is supply from a residue gas header from the plant with no CO2.

I'll check then with each vendor about components that may be upgraded considering corrosion.
 
RMNKBR:

I’ve done what you essentially describe: converting an oil patch reciprocating compressor from natural (or associated) gas service to pure CO2 service. I’ve done it with IRs, Worthingtons, Clarks, Chicago-Pneumatics, and others – all of them American-built. I’ve never done it with a non-American compressor. I’ve also done it with water-saturated CO2 and pure, dry CO2. I’ve done it with machines up to 3-stages, in tandem, and horizontal.

You state “It is possible that CO2 injection happens in the field” and I’ll assume this to mean that you intend to inject pure CO2 although you don’t mention whether your CO2 is saturated with water (like that recovered from an Amine unit) or if it is 100% pure and absolutely dry.

The main items that merited attention as far as material of construction were: the cylinder valves, the piston rings, the intercooler tubes, and the rod packing. The compressor proper was essentially cast iron construction. I found that all I basically had to change was the piston rings: from bronze to cast iron. I had no problem with bronze intercooler tubes.

Even with totally water-saturated CO2, I never experienced any corrosion or detrimental effects. I simply drained all condensed water in between stages. It is only Carbonic Acid (water-saturated CO2) that is to be feared for corrosion; pure CO2 is not corrosive at all. I’ve found evidence of Carbonic Acid corrosion in carbon steel – but only at temperatures above ambient, and not in cast iron.

You state you’ll be compressing from 15 psia to 465 psia as a maximum – and in a 2-stage machine. This is, in my opinion, pushing the compression ratio (5.5) if you intend to operate the machines 24 hr/day, 7 days/wk. At 110 oF suction, your discharge temperature will be running around 365 oF and that’s a little too hot for me to run continuously. I prefer to keep my compression ratio below 4.0. I’ve found that the “k” for methane is practically the same as that for CO2, so for the same compression conditions the two gases exhibit the same approx. discharge temperature.

I consider the 3-stage machine much better suited for the maximum, over-all compression ratio of 31. With balanced stages you might get individual compression ratios of around 3.2 -3.3. This would yield much better, balanced results. The 2- stage machine(s) are better suited for the 65 psia to 465 psia compression. Depending on your piston speeds and valve velocities, you might have to change out your valves for a different type or design. Your compressor fabricator should be able to help out on recommending the right valves. You don’t have to resort to Stainless Steel construction for this type of conversion.

Hope this experience helps.
 
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