Stainless Steel minimum bottom course thickness

[pbc825]

I'm working on an API 650 tank being fabricated from A240-316L plate with 18' diameter. Design thickness of the bottom course is 0.0835" and API clause 5.6.1.1 recommends a minimum shell thickness of 1/4" for the first course and 3/16" for the upper courses. Appendix S does not note minimum design thickness. Vendor is proposing to make all courses (bottom included) from 3/16" plate. I'm in favor, but I want to ensure I'm not breaking any API 650 rules by doing so. I'm also interested in any commentary (philosophical or otherwise) regarding minimum API 650 thicknesses and why in some instances the bottom course is recommended to be thicker that the top courses. Does it have anything to do with nozzle loads from tank deflections and rigid piping on the lower course?

Thank you in advance for any replies.

 

[JStephen]

See S.3.2.1.1, last clause "Note 4 does not apply", and it is that Note 4 that specified the 0.25" first ring thickness. So the 0.1875" is in compliance with API-650.

I don't know the exact reasoning for the thickness requirements. It is a more critical area due to bending at the restrained joint, due to nozzles, due to anchor chair attachments, due to water corrosion, etc., so the extra thickness isn't a bad idea there.

 

[pbc825]

Very good JStephan. Thank you for your help. It is much appreciated.

PC