API 650 12th - F.5 - Required Compression Area at the Roof-to-Shell Junction

[Jacobass80]

Hi everyone,
regarding Appendix F, F.5.1 "Required Compression Area at the Roof-to-Shell Junction", I'd like to clarify what DLR does it mean.
As per code description, it should be the "nominal weight of roof plate plus any attached structural", please find below my doubts:
- corroded weight?
- in case of supported roof, the structure weight (usually not welded to the roof plates) should be considered or not?

Thank you.

 

[JStephen]

My interpretation:
"nominal" means as-built, not corroded.
"attached structural" means rafters that are under the roof plate and welded to it, or rafters that are on top of the roof plates. Standard structural that is not attached to the roof plate would not be considered.

 

[Jacobass80]

I agree with your literal interpretation of "nominal weight", but I'm afraid that it is not a conservative approach.

 

[JStephen]

You asked what it means, not what it ought to say. Different standards approach this in different ways. There are other ways in which API is unconservative as well, but certainly nothing to prevent you from designing more conservatively if you so elect.

I think API has flipflopped on some of this. If the corrosion in your case is normally uniform corrosion over the entire surface, then you would more appropriately consider the corroded weight. If corrosion is somewhat localized, then it would make sense to neglect the loss of weight in the structure. And in some cases, the corroded weight could actually increase- the iron oxide is heavier than the iron, so until they get separated, the tank is getting heavier.

 

[gr2vessels]

IMO, the intent of the standard was to use the weight of the uncorroded roof, as opposed to the corroded roof with less weight, to determine the minimum, safe required area of compression. I don't believe the code was interested in evaluating the specific gravity of the rust and speculate on the amount of rust on the roof.
I recommend sticking to JStephens interpretation as being the conservative approach.
Cheers,
gr2vessels

 

[Jacobass80]

I got your point, thank you for your support!