Brittle Fracture of Layered Vessels

[PVDean]

I have a question regarding the thickness used in MDMT calculations for layered vessels. The minimum design metal temperature is related to wall thickness in the ASME temperature exemption curves. For layered vessels, do you sum the thicknesses or use the thickest layer (usually inside shell) for calculating your MDMT?

TIA.

 

[desertfox]

Hi PVDean

I would use the stress and temperature of the weaker material assuming your using two different materials.

desertfox

 

[gr2vessels]

I believe you have to use the inner shell thickness, not the total thickness (refer to ASME VIII Div 3, clause KD-810).
Cheers,
gr2vessels

 

[PVDean]

Thanks for the responses.

I still don't understand how I am going to lower my MDMT to our site CET by taking credit for the stress ratio at a lower pressure. It almost seems like I'm conservative for using effective thickness, ULW-16, and just assuming the layered vessel acts as monobloc. Ugh.

This analysis is simply a tool for my boss to judge if he wants to acquire a set of layered firebox vessels from the early 60's for use as ambient temperature accumulators.

 

[desertfox]

Hi PVDean

Why are you worried about temperature if there at ambient?
Can you not just calculate the stresses at ambient for a compound cylinder and be done with it?
Sadly I have no ref to ASME so I can't see what your refering to.

regards

desertfox

 

[PVDean]

Because for firebox material, which has been normalized, at a thickness over 2 inches the MDMT is somewhere above 60 degrees F. We have to be sure the metal temperature does not drop below that (for brittle fracture analysis since the vessels will see full pressure at ambient), but the site will see temps as low as the high 30s. So we take these old vessels and lower the pressure (stress) and reduce the MDMT by a stress ratio approach. This sort of analysis has been done on monobloc vessels, and is being asked for layered ones now.

So I have a vessel with a 3.75 inch inner layer surrounded by 12 0.25 inch layers, of higher toughness. I can find the MDMT at 3.75, but I have difficulty in the stress ratio approach since the MAWP requires those other layers. If I use the equivalent thickness, based on toughness of the two materials, the vessel comes out to be 5.22 inch thick. I can work with that if I assume it's monobloc. It just doesn't seem physically correct, unless those layers somehow contribute to brittle fracture.

 

[unclesyd]

You might want to get a copy of this book as it appears that there is some information on layer and wrapped vessels.

http://books.google.com/books?id=SZ...v=onepage&q=concrete pressure vessels&f=false