Design against corrosion 1

[Corgas]

Can anybody tell me where in ASME Section VIII, Div I, are guidelines or recommendations given to prevent crevise corrosion (especially inside vessels)?

Thanks,

 

[metengr]

Corgas;
There are no such guidelines or recommendations for preventation of crevice corrosion in ASME Section VIII, Div 1. This is a design detail that must be considered by the Design Engineer (or firm) working with a Materials Engineer.

 

[psafety]

Why not eliminate the crevice.

 

[vesselfab]

Some seal weld, some full penetration weld.

NACE should give you help

ASME is mechanical code, not corrosion

 

[Corgas]

psafety ;

Vessels are already built!!!

 

[gr2vessels]

Corgas, no need to shout;- is not the ASME code regulating the fine detail design of the pressure containing equipment.
If you are looking for help, please come back with specific questions, particularly if the equipment has already been built. As kindly mentioned above, other standards and codes are applicable to your problem, usually listed in the design specification provided by the Client to the fabricator.
Cheers,
gr2vessels

 

[chicopee]

What mechanism is creating crevice corrosion?

 

[Corgas]

Chicopee:

In this particular case (service), during upset conditions the normal iron oxide protective layer (tower is SA-516 Gr 70 PWHT) is destroyed and gets into the crevices, I think right term is under-deposit corrosion.Before you ask, I'll tell you that we do have oxygen in the system everytime we add "make up solvent".

Going bact to what metengr wrote, ASME Section 8 Div 1 Appex E gives some guidelines but this appex is NON-MANDATORY.

 

[doct9960]

Corgas,

Hire a professional who does Fitness-For-Service assessments.

http://www.fitness4service.com/publications/pdf_downloads/ReliabilityConfJaskePaper.pdf

 

[SJones]

Review the degradation mechanisms detailed in API RP 571 to see which ones apply to the equipment then try to work out whether the duration of the conditions giving rise to the degradation will lead to unacceptable metal loss over the intended service life. API RP 581 can help you with this. If this will be the case you have the three standard corrosion control options:
1. Change the environment (prevent oxygen ingress if it is detrimental; inject chemicals)
2. Change the material
3. Isolate the material from the environment by coatings, claddings etc.

Steve Jones
Materials & Corrosion Engineer

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

 

[chicopee]

What I wanted to know, how is the crevice created? Flexing of metal? accidental gouge in metal? weld undercutting?

 

[Corgas]

Sjones:

you're always on the money!!!. I got process engineering already working on effects of oxygen scavengers in the process.

Can't change material. Tower is already built.

Our "in-house" spec calls for seal weld on all internal attachments that's the reason coating was not considered at first.

 

[Corgas]

chicopee (Mechanical):

Lack of seal welding on internal attachments...leaving crevices along the way!!

 

[chicopee]

I dont get it. Seal welding is a non strength weld for the purpose of preventing leakage around let's say a fire or water tube expanded into a tube sheet. Internals within let's say a steam drum are fillet welded and if improperly done could result in undercutting at the toes of the weld but this is not refered as crevices. To me crevice is a gap between parts where deposits of chemicals promote corrosion suggesting poor design or lousy workmanship.
There is in the codes provisions for corrosion allowance but does not address your problem.