High Pressure H2S/ CO2 Pipeline Material 8

[seniorprojectmanager]

We are designing a buried 3" gas line some 200 m long to carry 70% H2S, 29% CO2, 0.0059% H2O, and the balance of C1 and C2, all in mol fractions. Design pressure is 1800 psig,(ANSI 900# RTJ flanges chosen) and design temp is 54C. Looking at NACE requirements and the fact that corrosion inhibition injection is uneconomical, we are chosing 316 SS pipe and valves.Is this the best solution or are there better alternatives?

 

[Unotec]

I think yes. Expensive, but should withstand it no problem.

 

[strider6]

If you are referring to NACE / ISO 15156 you can't use SS 316. The partial pressure limit for 316 in sour service is 1 bar (15 psig). In your case you've 1260 psig. IS better if you make some calculation of the corrosion due to CO2 on carbon steel and then evaluate other materials.

S:

http://www.corrosionist.com

 

[seniorprojectmanager]

Helo Strider 6,

Thanks for your input. But please refer to Table A2 of NACE MR0175/ISO 15156-3:2003 Technical Corrigendum 2, dated 2005-09-01. This corrigendum has removed the pp H2S restiction on the use of Austenitic SS for sour service.
Thanks

 

[zdas04]

The thing you didn't mention is if there is a reasonable expectation of moisture being present. If this vile gas stream is dehydrated then your corrosion risks go way down. If not then a careful understanding of the materials issues become much more inportant.

David

 

[strider6]

seniorprojectmanager

NACE didn't remove the pH2S limits on SS 316. It has introduced the concept that if the Chloride conc is below 50 mg/l then there are no limits on pH2S , temperature and in situ pH, no limits are set on individual parameters but some combination of these parameters might not be acceptable.
These means that you've to assess if your combinations of parameters is acceptable making test or based on previous experience or you accept the possibility of a failure.this is up to you......
And regarding the moisture, if a gas is dry didn't means that the requiremnts on Sour Service go away because you've to consider also the upset and shutdown conditions when water can condense.

S.

http://www.corrosionist.com

 

[dcasto]

looks like acid gas injection line to me. I use 304 SS for my line to the welhead, then plastic coated in the tubing.

In theory it could all be carbon steel because there will be no freewater to corrode the line at the temperature and pressures you indicated. In practcal terms, during upsets you can get freewater.

 

[rustbuster]

I would not bury 300 series stainless unless with an excellent external coating. As decasto stated, CS should be fine in those conditions (externally coated/insulated). You may want to design the line for pigging and or drying at depressured conditions where water could condense.

 

[strider6]

are you sure no condensation will occour in the line?
304/316 SS in this sour service conditions??

S.

http://www.corrosionist.com

 

[seniorprojectmanager]

The gas analysis shows a small percentage of water. 316 has been chosen over 304, because of the susceptibility of 304 to stress corrosion cracking in wet hydrogen sulfide service. Please refer to Page 36 of the book Materials Engineering 1, by McGraw Hill, edited by Kenneth McNaughton and the staff of Chemical Engineering in 1980. The particular paper was presented by Robert E. Moore of United Engineers & Constructors.

 

[strider6]

I don't want to insist but:
in terms of resistance to SSC, sulphide stress cracking, there is no difference between 304SS and 316 SS for NACE / ISO.
In NACE / ISO 15156 table A.2 of part 3 you can see that the limits are valid for a class of Autenitic Stainless Steel (304 and 316).

S.

http://www.corrosionist.com

 

[dcasto]

I'd question the water. As you compress H2S and CO2, they become hydroscopic and suck up more water into the harmless vapour phase.

 

[maddocks]

In designing our AGI schemes, here's the general criteria:

1.) If you have liquid water present, the use of stainless is recommended. The only caution on this is using an external coating to prevent chloride attack from soils and ground water. This is usually a field applied mastic with careful backfilling of the ditch. You need to consider the fact that at some point, the line will cool down to ambient conditions so it's not enough to assume that the line will run warm all the time. We've used 304, 304L, and 316L depending on the service and client.

2.) If no liquid water, we look at using carbon steel. Typically a select chemistry A333 Gr6 seamless steel line with MTR's, component control, hardness controlled welding, and external yellow jacket coating.

Hope this helps.....

 

[seniorprojectmanager]

This is particularly a useful answer as it is comprehensive and is based on actual experience. Reading all the opinions on this topic,I can summarise that for acid gas injection lines, if there is no water present or likelihood of condensation, A 333 Gr 6 pipe can be used. When there is moisture,Stainless Steel 304, 304L or 316L should be used. For this project, our client had for a previous project used 304 stainless steel, and wanted to stay with stainless steel. Since we could not find the chemistry of the gas for the previous pipeline, we looked at the current gas analysis, and the current NACE requirements, and in light of what Materials Engineering Book 1 (Referred to earlier) said about stress corrosion cracking for 304 in wet service, 316 L was selected.
I think we have had enough discussion on this matter, and I thank all participants for their kind advice in this matter; I would like to close this topic for now. Thanks.

 

[PAN]

Sorry for my late reply "after close".

Should we also consider external corrosion of buried stainless steel pipe? If yes, this link may be useful for you.

http://www.euro-inox.org/pdf/map/CorrResist_SoilsConcrete_EN.pdf

Note :
-No. 1.4301 is equivalent to Type 304.
-No. 1.4401 is equivalent to Type 316.