ASTM SA312 TP321 maximum allowable stress

[nihili]

Hi everyone,
I am doing some calculations with piping ASTM SA312 TP321. I am having some troubles deciding which is the maximum allowable stress since ASME 31.1 allows 2 different values for 750 ºK: 12.7 and 17.2 ksi. The diffrence between them is that for the value 17.2 the following note applies:

"Due to relatively low yield strength of these materials, these higher allowable stress values were established at temperatures where the short time tensile properties govern to permit the use of these alloys where slightly greater deformation is acceptable. These stress values exceed 67% but do not exceed 90% of the yield strength at temperature. Use of these stress values may result in dimensional changes due to permanent strain. These values should not be used for the flanges of gasketed joints or other applications where slight amounts of distortion can cause leakage or malfunction".

Can someone explain what this note implies/means? When the note says that some dimensional change can occur, are we talking about big ones or infinitesimal ones?

Thanks a lot

 

[metengr]

What the note simply means is that if your design permits or allows slight permanent deformation under service stresses, you can take advantage of higher allowable stresses for design. For flanges, which rely on having no local deformation, lower allowable stresses are used to ensure no leakage.

 

[nihili]

Thanks metengr. One more doubt regarding this issue. This higher allowable stress, can be used under permanent conditions? What I mean is that in my design I am going to work permanently at 750 ºK, can I used 17.2 ksi if I allow some permanent deformation?

 

[metengr]

Yes, if your design permits it.