Wet H2S Corrosion

I'm not familiar with NACE standards or the standard you are citing but I do recognize your numbers. 33<55 This is true. Therefore unless you expect the H2S deviations to exceed 50% from this waste water concentration reading or you see a trend raising the H2S level this already meets the criteria.

It does make sense that the waste products of any refining process will vary as the content of the raw material changes. I do not recommend you ignore this concentration. I recommend that you monitor this concentration and respond accordingly if it gets close to this threshold.

There's got to be a piping standard at this refinery for you to follow. What worries me is that either you haven't found it, or you haven't looked. Which is it, please?

You need to consult an expert!!

The release water may have 33ppm, but if there is turbulence and gas traps, there may be areas where that gas accumulates with significantly higher concentrations.

Sewer mains are a classic example for this.

There are many other relevant questions that will all contribute to the correct answer.

Is the line constant flow or intermittent? (Allows stagnation)

Is there air in the line? (Allows other chemical interaction and also allows presence of H2S digesting bugs that release sulphuric acid as a byproduct.)

Is there any turbulence? (enhances release of the gas from solution.)

What else is in the line? (Other nutrient sources for digestions.)

What if the flow velocity? (possible scouring of surfaces if too fast, possible slime build if too slow.)

ETC!!!!

Find an expert!!

I think it is elemental sulphur content. Am I right?

I hope neither NACE specifies about the limitation of elemental sulphur in percentage/ppm nor API specifies. It only states there will be increased corrosion if sulphur like oxidants are present. But I must say if there is any sulfur content specified in metal chemistry, then it will aggravate the sensitivity towards cracking. Since you need to follow the NACE material standard, I think no need to consider the elemental sulfur content as you have already interpreted the data regarding H2S concentration or partial pressure which is in the acceptable region. If requires, you may consider and correlate the in situ pH of the waste water as well with the H2S partial pressure, for acceptability.

http://www.documentation.emersonprocess.com/groups/public/documents/articles_articlesreprints/nace_standard_mr103_bush.pdf