API 650 Annular Plate Function 2

[99491]

Will someone please tell me what is the exact function the annular ring under the tank shell. According to API 650 sect 5.5 an annular ring is soley dependant on the stress in the bottom course. According to BS 2654 a tank over 12.5m dia must have an annular ring. I would like to know why the annular is necessary? Because butt welded the completely flat and even surface makes the shell to bottom weld much stronger but there must be more. Please assist.

 

[JoeTank]

In API-650 the annular rings are required for tanks designed with higher strength material. These materails are normally used only for larger diameter tanks. The higher stresses and larger diameters lead to significantly greater rotations of the shell-to-bottom joint. That leads to higher bending stresses in the annular plate... hence the requirements for special design, constructiona nd NDE provisions. There are also cyclic considerations built into the API annular plate tables.

Joe Tank

 

[JStephen]

The bottom plate acts as a radial restraint on the shell, and is subject to some radial and/or hoop stress as the tank size and design stress increases.

 

[99491]

Thanks for the reply.

Why would you than ever have annulars the same thickness as the bottom should the stress not exceed the parameters set out in 5.5.3. For example if you have a bottom course of , 19mm and a stress < 220 the annular plates are 6mm. How is this annular ring assisting in any way if the bottom plate is also 6mm.

 

[JoeTank]

The shell plate carries the tensile HOOP stress of the product. The annular plate is subjected to a very low radial load but a very high radial bending load. The API procedures look at secondary bending in the annular plate, considers cyclic conditions and specifies an appropriate thickness, which is sometimes only 6mm. Selection is a function how much shell rotation occurs, which is dependent upon hoop stress level in the shell and tank diameter.

Joe Tank