API 650 is for tanks with internal pressure approximating atmospheric pressure (not exceeding 2.5 psig), whose entire bottom is uniformly supported and whose operating temperature is lower than 200DegF [90DegC}.  API 620 is for tanks designed for higher internal pressures as well as other conditions outside of 650.  If your tank is low pressure, use 650.

IRF,

This is an atmospheric, API code, 14 ft dia tank and I'm concerned about fitting it within a specific code so that it can be stamped. I question the use of API-650 only because it is not a flat-bottom tank. I can not find a clear definition for API-650 tank bottoms. In Appendix J (J.3.2) of the API-650 code it refers to flat bottoms and in 3.5 it refers to annular bottoms (assuming flat). What you are saying is that since it is low pressure and ground supported it should be 650 even though the bottom is cone-shaped, annular bottom (1:12 pitch)?

This is a grey area in my mind - can anyone clarify this code?

clp

clp,
If the tank bottom is supported by a foundation or grillage it fits within the basic construction scope of API-650.  Tank bottoms almost always have a slope for drainage or process reasons.  We still call them flat bottom tanks eventhough bottom is only nominally flat.  I see no problem with a 1:12 slope if it is uniformly supported by the foundation.

Steve Braune
Tank Industry Consultants
www.tankindustry.com