Shell Thickness against External Pressure - Duplex vs Carbon Steel

[Matrix_93]

Hello,
I was checking a shell of pressure vessel under external pressure using two different materials at 150 °C and Full Vacuum (103,4 kPa).
I don't understand why, using ASME VIII div.1, with duplex material I have a thickness higher than Carbon Steel material due to external pressure only.
If I check Compressive allowable Stress, duplex has higher allowable than carbon steel and for me this is correct; but If I check MAEP (maximum allowable external pressure) Duplex has a lower value than Carbon Steel due to B factor, in order to have 103,4 kPa.
Why? I don't understand; usually if a material has higher compressive allowable it would have lower thickness and not the reverse!

Thank you very much

 

[SnTMan]

Matrix_93, comparing Charts CS-2 and HA-5 it appears that for a given A value at a given temperature, HA-5 yields a larger B value. This should give a higher allowable external pressure for the duplex material.

From what you've given it is hard to say more than that, but it seems something is out of order. More detail might allow a better understanding...

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[r6155]

Maybe you used corrosion in CS. Check it.

Regards

 

[JStephen]

The stainless steels have differences in strength and in modulus of elasticity. You could perhaps be in a region where one is governing one loading and the other governs the other loading.
Also, some of the stainless steels have lower yield point but higher ultimate compared to carbon.
I would more nearly expect either an error like the CA, or just some sort of variation due to various steels being lumped together.

 

[csk62]

If you read code case 2286, now part of div 2 I believe, in external pressure scenarios there is an elastic buckling failure mode for certain material+geom combinations while in others, plastic failure (yield strength dependent) occurs first. Not sure if this info helps for this particular scenario.