SOUR CRUDE OIL

[GabrieleB]

I'm working on a 16" carbon steel pipeline that transfer crud oil at 90°C.
This Crude Oil has the following characteristics:

Specific gravity at 35°C = 0.95
Sulphur content = 3,5%
Viscosity = 220 Cst
Water sediment = 1%

I read a lot of articles that indicate 1% as a limit for sulphur on crude oil and that above this value means the necessity of NACE requirements.
The NACE ISO 15156-2 doesn't mention any limit of sulphur. How can I decide if the Oil is sour or not?
In the above mentioned case should I apply NACE materials?

Thanks in advance

 

[Chumpes]

Hello

Sour service (wet H2S) requires free water and H2S (extended to wet HF and wet HCN that also produce 'wet H2S' type cracking).
The presence of free water is confirmed (1%) in your case.
If your crude has high sulfur content, can these sulfur compounds transform to H2S ? if yes I would specify NACE XXX requirements (which NACE document do you refer to?)
I ve never seen this sulfur limit 1% that you mentionned... what is the source for this 1% figure ?

Some users refer to 50ppm(w) H2S into the free water phase to start application of NACE XXX requirements, some use ppH2S > 0.003bar, some others consider NACE XXX requirements as soon as H2S is present together with a free water phase, whatever the concentration is.
You can refer to NACE MR0175 and NACE Pub 8X194 for more information on sour service environments.

Regards

 

[GabrieleB]

I know that the NACE ISO 15156-2 gives the limit of 0.3 KPa of partial pressure of H2S in the gas phase and that NACE 0103 mention 50ppm in the water phase.
But in my case having crude oil and no other indications, I have no idea about how can I compare my analysis with the above.
On the other hands I have the concern that the sulfur of 3% can generate H2S. In fact I have find some articles that considering the sulfur a potential risk, and mention 0.5% or 1% as a limit for sour crude oil. In any case I have no reference in the NACE ISO 15156-2 and NACE MR 0103 so I cannot verify this limitation as true.

 

[Chumpes]

if your analyse of the environmental conditions is that wet conditions occur together with H2S (low but unknown concentration) then you can take the decision to apply NACE MR0103 to your carbon steel system for controlling the risks of wet H2S cracking (PWHT).
if you can demonstrate that the H2S level dissolved in water remains well below 50ppm and that the partial pressure H2S remain well below 0.3kPa, also because 1% water is dispersed in HC and induces low corrosion rates and lower SSC/HIC susceptibility, then you have arguments for allowing simple carbon steel (no PWHT).

 

[cavbilly]

Whatever the limits are, with free water and that much sulfur, you can make a safe bet this is sour, and needs treatment to prevent cracking corrosion of the steel.

 

[LittleInch]

GabrieleB

The issue here is that the H/C industry has a different meaning for the same word.

Upstream you tend to look at "Sour" as meaning H2S limits which impact on corrosion, issues with steel cracking and at high levels of H2S a toxic gas.

Downstream producers tend to look at elemental sulphur and somewhere around 0.5 to 1% is the difference between low sulphur and high sulphur crude oils which then gets translated as "sour".

The two things are completely different. To determine if the fluid is "sour" to NACE you need to know the level of H2S in your Crude Oil and water, not Sulphur.

I have to say that is some pretty heavy gloopy Crude Oil you have there.

At the end of the day deciding something is sour to NACE isn't that big a deal unless this translates into having to heat treat every weld.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[brimmer]

Gabriele - as suggested by LittleInch you need to try and get the H2S content in your oil, the amount of sulfur does not matter (as far as classification of sour or not). Typically with crude or LVP lines, there is no gas phase, so this gets a little more complicated, your oil probably limited to 0.5% BS&W as well. Where I am we define sour liquid lines (gas free, <0.5% BS&W) as: the H2S content partial pressure exceeds 0.3kPa at the bubble point absolute pressure. We use NACE MR0175 Annex C.