Corrosion 2

[ejrg]

Hi,

We have found high concentrations of H2S (800 - 1400ppm) in our system were the pH is 7. We are injecting 100% of sea water on the whellhead which could be causing this high values of pH. I have calculate the partial pressure of H2S and it is greater than 0.05psi (maximun pH2S before SSC could happen according to Nace standars)

I would be interested in any advice you could provide on the suitability of sour corrosion for vessels and piping upstream.

Thanks for any input.

 

[gr2vessels]

ejrg,
I'm not sure where did you get the generic 0.05 psi limit from, but for this one you should refer to NACE MR0175, which lists various limits up to 100 psi partial pressure for H2S in upstream production (high alloys;- eg. Table A8). Regarding the pH, the lower limit is usually specified for various material selection (not the upper), hence your 7 looks pretty good. However, the note 1 in Table 1 recommends minimum pH of 10, check if that applies to you). For materials suitability advice, note first the Clause 1 (Scope) of the NACE standard, particularly the Caution note. The materials suggested as suitable for sour service are not imune, but are resisting / minimizing the damage inflicted upon the material by the adverse environment. Also, the recommendations provided by the standard are by all means indicative only and not prescriptions for imunity to the environment. Complete understanding of the corrosive environment, no shortcuts in the design stage, proper engineering judgement for material selection, proper fabrication control are the secrets of limiting the damage to the equipment. Also, this forum is a wealth of information regarding your topic, use it in full, search for specific questions and important, ask specific questions, because vague generic questions like you started with, will only get you vague answers.
cheers,
gr2vessels

 

[strider6]

I think that the 0.05 psi limit refer to the minimum value of pH2S above which NACE MR 0175 classify a service as sour.
Now, since 2003, the standard is a NACE / ISO 15156 and some changes has been introduced.
Is not possible to say something about the suitability of your vessel and piping materials 'cause as gr2 said the assessment of the suitability of a materials is a complex task that must take into account many factors of which the pH2S is only one. Other factors are water composition, presence of sand and solids, presence of elemental sulphur, chloride content, CO2 content and so on..
My suggestion is that you start looking at NACE MR 0175 / ISO 15156 and have an idea if your materials are within the suggested limits.

S.

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