Shell plate Thickness 3/16"

[Chap123]

I'm right now checking a design of 32 ft high and 15 ft wide tank with 1000 bbl capacity.

After running Ametank software , i found shell thickness is 3/16" which is non compliant to API650 standard annex J , for shop assembled tanks.

My client was insisting to approve this design as lower shell plate is 1/4" as he think we will be ok.

I need to know any provision in code that accept this tank with that she'll thickness.

Here are other desgin data.

Specific gravity : 0.61
External pressure Vacuum: 0.4 0z
Internal pressure: 16 oz
Mdmt: -40

I think annex E and F tell something about suck tanks.

Any ideas?

 

[IFRs]

If possible, take the time to read API 650 and then run through the relatively simple calculations yourself. That said, I ran the calcs using your data plus 1/16" corrosion allowance, A516-60 material, joint efficiency of 70% and found that 3/16 is suitable for all rings except for the bottom ring which must be 1/4". See note 4 in section 5.6.1.1. These thicknesses are all minimums and exceed the thicknesses needed to resist the calculated hoop stress. I did not check for wind or seismic stresses.

 

[MJCronin]

How will the tank be pressure tested ?

Will the tank be pressure tested with water ?

With an operating SG of only 0.61, what is the required wall thickness for pressure testing ?

MJCronin
Sr. Process Engineer

 

[JStephen]

If it's an Annex J tank, minimum shell thickness is 1/4".
This is presumably based on shipping/handling considerations, not on in-use load conditions. For a field-erected tank, the 0.1875" would be adequate.
I'm not aware of any provision that allows you to waive the thickness requirement if it is in fact a shop-built tank.

 

[IFRs]

JStephen is, as usual, 100% correct - if this is an Annex J tank as the OP noted and I missed, then 1/4" is the minimum thickness for the tank shell. This minimum thickness is not based on stress but instead on fabrication, transportation and erection issues. Your client wishes to use 3/16" for the upper rings which will certainly be strong enough to resist hydrostatic loads and will in all likelihood not represent a high risk for them. The tank won't be in compliance with Annex J but might be in compliance with the main body of the standard. The tank should not have an API name plate that lists Annex J.

 

[Chap123]

Yes according to basic standard of the code, we are allowed to use 3/16" plate thickness but not with Annex J.

I have also checked my design using One foot method using both design fluid thickness and hydrostatic test thickness and both are less than client shell plate thickness 3/16".

Please find attached screenshot and let me know if it makes sense

 

[IFRs]

Your statement that "according to basic standard of the code, we are allowed to use 3/16" plate thickness" is not correct. The basic standard requires you to use 1/4" for the lowest shell course. You are correct that Annex J requires 1/4" for all courses. Your calculations are also correct as far as they go but they do not include the 1/4" minimum thickness called for in section 5.6.1.1 Note 4.

 

[Chap123]

Thanks IFRs,

Our lower plate shall thickness is 1/4" but upper course plates are 3/16" and that complies with 5.6.1.1.


Do you think we need to go for any material testing rwqyirmemt to approve that design .?

I also calculated noozle loads and they are not creating stress on the plates as well.

 

[IFRs]

So, you are making a shop built tank but not in accordance with Annex J. Are you otherwise adhering to the main body of the standard or Annex J? Your material for the tank shell should come with material certifications, I don't see any other testing needed. Other calculations would include wind and seismic stability, if anchors or a thickened annular bottom are required for uplift from pressure, wind or seismic, frangibility, intermediate shell stiffeners, lifting lugs, stresses due to lifting / tilting and transportation, normal and emergency venting (I'm probably missing some). You may run into dimensional tolerance issues using the thinner plate on a shop built tank - I would check these also after the tank is installed in the field. It looks like you are using a 1/4" self-supported cone roof - do the calculations support this thickness?