Sulfidic corrosion on washoil drum - extremely high rate

[Onurb85]

Hi,
On the past weeks a failure happened on a washoil drum (Vaccum unit) due to sulfidic corrosion (bottom area of the drum). This drum was never replaced (original from 1993 - P5 metallurgy) and until 2014 had very low corrosion rates even with high sulfur crudes (2.5-3.5% sulfur on washoil). Only after 2014, the corrosion rate increased up to 0.6mm/year. However, in the last 5 months, the corrosion rate increased to 6-7mm/year. Does anyone have any similar experiences on other refineries? Which kind of crudes were processed on those refineries? Any similar experiences on the HVGO section?

Based on the real process values (operating temperature, sulfur and acidity), the McConomy curves show a corrosion rate on that area of 0.5-0.6mm/year. What could increase the corrosion to 6-7mm/year?
Thanks in advance.

 

[NBrink]

I'm not familiar with the terminology "washoil drum", but that sort of jump in a vacuum gasoline system screams napthenic acid corrosion to me.

Doesn't even necessarily need to be tied to a change in crude slate (although the napthenic acid in different crudes can vary wildly in corrosivity, even at the same acid number). Have heard of a case where tower internal damage changed where the acids separated out from a 317 line to a 9 cr line that had been fine for decades causing a massive spike in corrosion rates.

Would start with looking at TAN and reactive sulfur levels for the gasoil.

Nathan Brink

 

[Onurb85]

The washoil is a product that is heavier than HVGO and is used to "wash" the bottom section of the vaccum column. So,on the vaccum column you produce destillates, LVGO, HVGO and washoil (can be called also "slopwax"). Each of these streams has a drum (except the distillate).

The crude slates are all analysed in the lab and all presented TAN < 0.2 since October 2016. So I am sure that was not corrosion by naphtenic acids. Im thinking more about the sulfur and how it decomposes in the drum. Do you know any article where mentions the sulfur speciation and how affects corrosion? Any experience where sulfidic corrosion happened at this rate?
Thanks

 

[NBrink]

Would agree at that low of a TAN, NAC is unlikely. There are a number of NACE papers regarding sulfur speciation, and API 939C should have a few mentions. Generally H2S and Mercaptans are considered to be the most aggressive common species.

Nathan Brink

 

[Chumpes]

Yo,
McConomy and Couper&Gorman can estimate the order of magnitude of corrosion rates but these models does not provide precise value of corrosion rate with reference to API 939C. What corrosion rate is calculated througth Couper&Gorman in your system?

also, recent development in the API shows that 5%Cr corrodes as fast as carbon steel material : there is no significative difference between 5%Cr and carbon steel with regard to high temperature sulfidation

I am not surprised that the corrosion rate calculated can be multiplied by a 10 factor.
Have a look at API 939C that demonstrates the difficulty in predicting corrosion rates in sulfur containing environments.

0.6 or 6mm/y are both unacceptable corrosion rates because >> 0.1-0.25mm/y