Process Equipment and Plant Design
A cylindrical storage tank can have a self supported conical roof,

If its diameter is less than 15-20metres
If the thickness of the roof is more than that of the cylindrical shell
If its diameter is more than 50 meters
Irrespective of its diameter

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Process Equipment and Plant Design
Seamless pipe is made

Both B and C
From rolled strips formed into cylinders and seam-welded
By forging a solid round, piercing it by simultaneously rotating & forcing it over a piercer point and further reducing it by rolling gas drawing
By extrusion & casting into static and centrifugal molds

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