Thickness of ASME Section II - Part D Materials

[torsani]

Hi

Why does some materials in ASME sec. II - Part D has the Allowable Stress varying with its thickness?
Do allowable stress is different in the new vessel and in the corroded vessel?

Thanks in advance.

Torsani

 

[Shmulik]

Hi,
To my opinion it doesn’t have anything to do with corroded or uncorroded material.

Stress in any component of a vessel is one of three categories: primary secondary and peak stress. Primary stress, such as hoop stress in a cylinder due to internal pressure. (Secondary stress is developed when the deformation of a component due to applied loads is restrained by other component).
This primary stress is divided into two subcategories: primary membrane stress and primary bending stress. Longitudinal and circumferential stresses in a cylinder due to internal pressure are classified as primary membrane stress.
Combination of these membrane plus bending stresses becomes more significant when thickness of material increasing. That what I see the reason to a lower allowable stress as the thickness is greater.

 

[twjag]

In general, the variation of allowable stress versus thickness in Section IID matches variaton in specified minimum tensile strength versus thickness in the material specification.

The answer to your second question is no, there is no difference in allowable stresses in new and corroded vessels.