Material selection for sour/acidic service

Nauman Khursh

I am facing some problems regarding material selection for sour/acidic service of
raw natural gas due to the presence of H2S & CO2 gas. According to the common
practice if the raw gas contains <4 ppmv H2S & <6 mol% CO2 then we consider this
as a sweet service and we can use carbon steel (SA-516 Gr. 70) for both piping and
vessel materials. However if the recommended limits exceed then we have some
concerns regarding material selection.
1. In the presence of free water and CO2 (above 6 mol%) in the raw gas please tell
me the material selection criteria. Either we can use normalized carbon steel or we
have to go for SS material due to the acidic attack of CO2 to the material . Please
also tell me is there any option to go for simple carbon steel upto specific temp and
pressure conditions in this acidic conditions.
2. If both H2S & CO2 present in the raw gas then please tell me the material
selection criteria for this sour service nature. Please recommend the material in that
condition where H2S lie in SSC (Sulphide stress cracking) region according to
NACE MR0175.
For these two conditions please tell me what material should be used in
regeneration section of Amine Unit (Where highly corrosive envoirment exists due to
CO2 or H2S in presence of free water).
Please note that I am asking the material selection for gas sweetening and gas
dehydration plants.

Your cooperation will be highly anticipated.

Regards,

Nauman khurshid