API 650 appendix F & S 2

[kywlium]

Since addenda 2007 the minimum thickness for a stainless steel storage tank (A240 316L) the lower shell coarse must be 6 mm because appendix S refers to par. 5.6.1.1 note 4 (I believe). Previous addenda did not show this note so minimum thickness acc. table of this paragraph was 5 mm.
I can imagine this should be applicable for CS storage tanks (due to external corrosion) but not for SS storage tanks.
Acc. Appendix S the minimum thickness for a bottom plate is also 1 mm less (5 mm) compared to the min.thickness required for CS storage tanks.
In that case we can have SS tank of 6 m dia. with a bottom plate of 5 mm and a lower shell coarse of 6 mm.
This looks strange to me.

Even the European Code for storage tanks (EN 14015) shows a similar table with min. thicknesses for both carbon- and stainless steel tanks.
Acc. this table 3 mm is sufficient for a SS tank with dia. of 6 m.

Does anyone know what the reason is for this increase of thickness since SS is generally not sensitive for corrosion?

 

[Bonswa]

I dont think it has anything to do with corrosion. Mainly because that note only applies to a specified diameter range. If the increase was due to corrosion, it would apply to all diameters. This is only my educated opinion, but the fact that it only applies to a particular diameter range tells me that it has something to do with the joint efficiency of that shell-to-bottom weld. There could have been problems with that corner weld on tanks in that range with 5mm plate. Apparently, by raising the thickness 1mm, they've improved on that design.

Just a guess. You could write to API, if you dont mind waiting for an answer. They'll be able to tell you the reason for the increase in minimum thickness for that range.

 

[JoeTank]

The intent of the change was to address only carbon steel tanks. I think the API tank committee made a simple mistake by not revising Appendix S so 5mm was adequate.

Joe Tank

 

[Bonswa]

To follow up on what JoeTank said...

I dont think that the "NOTES" apply to the appendix. I've found many times that when I use the appendix, it's canceled out if you try to use the notes too.

 

[kywlium]

Thanks JoeTank and Bonswa for your quick answers.
Acc. Appendix S3.2 the thickness should be acc. formula "but not less than the nominal plate thickness LISTED in 5.6.1.1".
When I see the word "listed" does this mean all values given in the table?. When I check the dictionary (grouped items) you could say that note 4 is NOT part of that list.
Since I'm not that familiar with API-650 (In Europe more EN 14015)I want to know if there are indeed more examples
like Bonswa suggested.
At least the software does still give 5 mm shell thkn. as an answer, so I will officially ask the supplier why it is not 6 mm. When I know something more I 'll let you know.