Hello SoltanHamid
The manufacturing process for plate and pipe are different. Plate is produced by passing the hot billet through rollers, and pipe is oftem made by an extrusion process with a floating mandrel. The thickness of the plate can be fairly closely controlled. However, in the case of pipe, the floating mandrel can slip to one side, and the pipe as a consequence can be thinner on one side than the other. The standard tolerance for pipe is 12.5%, which means that the pipe wall can be only about 0.875 times the nominal thickness. Therefore, when pipe is selected for the main shell of a pressure vessel, the nominal thickness must be reduced - So, if say you specify a 12" Schedule Standard pipe, the nominal wall thickness is 0.375", but the code requires that the actual finished thickness be adjusted to 0.375 x 0.875 = 0.328". In the case of PVElite and CodeCalc, when you select a pipe (eg SA-106 Grade B) for the shell, the programme automatically deducts the 12.5% in the calculation.
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Sincerely,
Ray Delaforce
CADWorx & Analysis Solutions
Hexagon PPM