Annular bottom plate thickness per API 650

[alienitmeca]

hello,

while calculating the thickness of annular bottom plates, i needed to determine the stress in first shell course which can be calculated using the formula in table 5.1a (5.1b).



the formula in table 5.1a (5.1b) states two thicknesses in both product and hydrostatic test stress

Question:
i can't figure out the which thickness to use, apparently there are both the same, which does not make sense

thank you in advance for your help

 

[fegenbush]

The stresses/thicknesses should not be the same, as the allowable stresses for product and hydrostatic test are different.

 

[IFRs]

Sort out the thickness question but I think that is a separate issue. Your next question was which stress to use when determining the annular plate thickness. Note B in table 5.1 should clear that up.

 

[alienitmeca]

IFRs,

i did read the note B in table 5.1, it didn't clear anything up:

for example:

product stress= (td - CA/corroded t)*(Sd)

i can't determine the difference between : (td-CA) and (corroded t), aren't they the same thing ? if not could you please explain!!

thank you

 

[IFRs]

I think the order of operations wold be ( td - (CA/corroded t) ) * Sd

 

[alienitmeca]

IFRs,

whatever was my question, you are always there !! i can't thank you enough,

i really apreciate your help !!

thank you

 

[mariog123]

In my understanding, td is the required minimum thickness of shell plates, calculated according to the paragraph 5.6.3.2; let's say you calculate it based on the formula 5.6.3.2 and the result is 19.13 mm, for CA=2 mm.
We can say that td-CA corresponds mathematically to Sd stress.


t_ordered is the thickness ordered for plate; in my example it could be 20 mm.
t_ordered-CA= t_corroded corresponds to the "Product Stress" in your tank, in the end of the tank life.


By mathematical structure of the hoop stress ("Stress" is proportional with inverse of "t") it follows that
Product Stress= [(td-CA)/(t_ordered-CA)]*Sd= [(td-CA)/t_corroded]*Sd

 

[sajk14]

My understanding of this is the same as per mariog123's explanation.

i.e. product stress = [(calculated design thickness 'td' - C.A) / (ordered plate thickness - C.A)] x Sd

Also take note with regards to the placement of the brackets in the formula between the 11th Edition and the latest API 650 12th edition...

11th Edition shows...............Product Stress = (td-CA/corroded t)(Sd)

12th Edition has corrected this to.........Product Stress = ((td-CA)/corroded t)(Sd)