6 replies to this topic

[alex_hawk]

Hi everyone! I'm courrently working on a treatment unit for some sour gas coming from an oil field. The first part of the plant, the compression unit, works at ambient T and low P, and at a certain point the material choosen for compressors and lines change from carbon steel to stainless steel, but I cannot understand why. Pressure changes from 3 to 11.5 barg, wile T changes from 44.7°C to 126.9°C. There is no condensation of water, even if there is a 0.56%, and there's an high concentration of sour gas (H2S=4.2%, CO2=7.05%). What's the effect of pressur in this kind of "hot" corrosion? Thanks! Alex

[ankur2061]

Alex,

You need to find out the partial pressure of H2S in the system where the material specification changes from CS to SS. Most likely it has been evaluated that the partial pressure of H2S in the system after the material specification break is equal to or greater than 0.3 kPa (0.05 psi), In such a case, according to NACE MR-0175 you need to provide Sulphide Stress Cracking (SSC) resistant steels. Some information can be found therough the web link related to NACE MR-0175 provided below:

^{http://pipelineandga...nment?page=show}

Check in your engineering documentation and most likely you will find the explanation provided above for switching from carbons steel to stainless steel at a certain point.

CO2 in your gas stream is also high and can form corrosive carbonic acid in presence of trace quantities of water.

Hope this helps.

Regards,
Ankur.

[alex_hawk]

Dear Ankur
Thank you for your reply.
According to NACE MR-0175 there is no reson for change CS to SS only for SCC.
In other words, once the selected material is CS NACE, there is no condition (high pressure or high temperature) for replace it with SS NACE.
Do you agree?

[ankur2061]

Alex,

If your Carbon Steel itself is NACE MR-0175 compliant for preventing SSC, then yes I agree that you don't need to change to stainless steel.

Regards,
Ankur.

[alex_hawk]

Dear Ankur,
thank you again for your kind reply.
I would ask you one more thing related to corrosion, which is: once stainless steel is chosen for high H2S concentrations in pipes, is there any necessity for a PWHT?
Thank you again!

[ankur2061]

Alex,

AS per NACE MR-0175, Austenitic Stainless Steels (SS316, SS304,SS309, SS310, SS209) do not require PWHT. Amongst Corrosion Resistant Alloys (CRAs) only martensitic stainless steels require PWHT (e.g. SS 410, SS 415, SS420, J91150, J91151, J91540, SS424).

Hope this helps.

Regards,
Ankur.

[alex_hawk]

_{Thank you very much Ankur!}