API 650 Annex F - Error in formula for Maximum Design Pressure 2

[sajk14]

Hello All,

I hope everyone is well and staying safe during these strange and difficult times.

Just wanted some opinions regarding some terminology used in ANNEX F of API 650 13th Edition...

The formula given to determine the Maximum Design Pressure (clause F.4.1) states that the value of 'A' for the participating area of the roof-to-shell joint is to be based on the corroded thickness.

However the term 'DLR' is then stated as the nominal weight of the roof plates i.e. the ordered/uncorroded thickness?

My understanding is that if we are using the corroded area in the formula then we should also use the corroded weight for the roof plates and not the nominal weight? Is this an error in the code?

The same query would also apply to clause F.5.1 when calculating the Required Compression Area.

Your thoughts/comments on this would be appreciated.

Thanks & Best Regards

 

[JStephen]

You could logically view it either way, and I assume it is stated as it is intended to be.
If the corrosion considered in CA is uniform over the entire plate surface, and if there is some mechanism to remove the corrosion products, then it makes sense to use the corroded weight.
If the corrosion is isolated areas, pits, etc., or if the corrosion products remain in place, then the weight doesn't change appreciably when the corrosion takes place.
Either assumption could be reasonably used, and which is actually considered in the wording is one of those items that gets voted on by a committee at some point.
Note that while vessel work and tank work have a lot of similarities, there are points like this where the thinking is somewhat different, and in vessel work, you'd generally treat it like the CA was an entire layer of metal that uniformly disappeared.

 

[IFRs]

I think the thought was that the failure mechanism is probably going to be local and could easily be affected by corrosion, while the DLR is unlikely to be uniformly corroded. Therefore it is reasonable to use the corroded section for strength and the uncorroded weight for the load.

 

[HTURKAK]

sajk14 (Mechanical);

Both IFR (Petroleum) and JStephen (Mechanical) have explained the probable reasons for the assumptions of API CA , and relevant corrosion based calculations..

I agree with them and pink stars for these posts ..I just want to add,

- The corrosion could be local ,if local corrosion ( pits etc) still the strength calculations will be adversely affected .So the calculations shall be based on the corroded thk. You can see the same approach for shell thk. calculation , rafters for roof etc..

- If the corroded portion is not removed or cleaned etc. the corroded steel weight will be more than the ordered / uncorroded wt.

 

[r6155]

See 5.10.2.2 .....For frangible roof tanks, where a corrosion allowance is specified, the design must have frangible characteristics in the nominal (uncorroded) condition.

REgards

 

[sajk14]

Thank you all for your comments.

Best Regards