Corrosion Allowance for CS in H2S containing environment 1

[newerbee]

Hello everyone,

I am new in this forum and hoping to get some help for the following.
I am checking a pipeline corrosion assessment study as part of my training. the pipeline is for a multiphase oil production line including 120ppm H2S and 3% CO2. I understand that the primary corrosion is due to CO2 and presence of H2S complicates things as it reduces the pH, introduces cracking and leads to pitting.

I understand that pitting corrosion rates are high. so my questions are
[ul]how do we deal with pitting risk during Carbon Steel pipeline design? is the only option a CRA?[/ul]
[ul]do corrosion inhibitors stop pitting once it starts?[/ul]
[ul]if can be calculated, do we add pitting corrosion rates to corrosion allowance? as I understand only general corrosion rates after inhibition are added to CA calculations[/ul]

thanks in advance
Mati.

 

[weldstan]

Sometimes it (H2S) increases the corrosion rate in CO2 sytems, sometimes it deceases the corrosion rate. Minimum corrosion allowance that I have seen is 1/8". Without inhibitor additions, I recorded pitting rates at 3/32" per year in a similar system.

 

[brimmer]

Corrosion pitting rates will be difficult (if not impossible) to calculate, will only be able to get a general rate. Even if you could calcualte pitting rate, you would find the rate high enough that adding a corrosion allowance based on this rate would be impossbile.

Corrosion inhibiotrs can stop (or limit) corrosion once it starts, however the difficult part is cleaning out the internal corrosion pits (by pigging) once they start (as they will have corrosion product inside). If they can't be compeltely cleaned, and you can't get inhibitor to the pipe wall (rather than sitting on top of the corrosion product), the inhibitor won't work.

CRA may be an option for you and your company, however for what you are describing typically carbon steel is used with a maintenance strategy. So operations will conduct maintenance pigging at a specified frequency, you would put corrosion inhibition on the line, you would monitor/inspect the line for corrosion and make repairs before a leak occurred. If you have H2S and sour conditions, you will need steel that meets sour service requirement (hardness levels <22HRC).

 

[newerbee]

thank you for the Answers.

So we don't try to calculate the pitting rates in design stage to increase the calculated Corrosion allowance but we rely on maintenance strategy. Inspection results will give the actual risk of pitting and the required strategy.

 

[SJones]

http://www.eng-tips.com/search.cfm?...2&searchtype=1&searcharea=1&d=1&action=search

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.