API 650 AP S x EN 14015 STAINLESS STEEL ATMOSPHERIC TANKS - SHELL MINIMAL THICKNESS FOR CONSTRUCTION

[mancini]

Hi everybody,

In our projects of small diameter atmospheric tanks (Diameter < 4000mm x Height < 4000 mm) supplied in one piece ("complete shop assembly and delivery to the installation site in one piece"), stainless steel A240 TP 304L, we have used the API 650 code, from the code in the Annex S, item S.3.2.1.1 it is informed to be considered (after calculations,...) the minimal nominal thickness to be used on shell courses as 5 mm (3/16") based on the item 5.6.1.1. , but in this specific item it is informed in the note 2 that "The thicknesses specified are based on erection requirements" and besides our construction experience show that the smaller thickness like 4 mm do not have any impact on quality of our construction,... for many years we have used the minimal shell nominal thickness as defined in the API 650.

Otherwise in the Code EN 14.015 it´s informed in the item 9.1.5 at Table 16 that for stainless steel tanks diameter < 4000 mm the "Minimum specified nominal shell thickness" is 2 mm and in the Note 1 is informed that "These specified thickness requirements are needed for construction purposes and ..."

So, as there are differences from these Codes in the minimal nominal thickness for shell, based on construction, the doubts are:

1) Do you know the specific reason for this definition of 5 mm (3/16") from API 650 which is also applied to the small dimension SS atmospheric tanks?
2) Do you know if there is any alternative from the API 650 in order to use on the small atmospheric tanks the smaller nominal shell thickness (like the use of tck 4 mm)

Thanks in advance

 

[JStephen]

Requirements like that are usually judgment issues, that may also be influenced by material availability, welding procedures, or the practices of some of the larger fabricators.
There isn't any provision in API-650 for reduced thicknesses.
I believe UL-142 allows some reduced thicknesses below what API-650 allows.