Cold Service--Allowable Primary and Secondary Stresses 1

[spayette]

An existing carbon steel vessel, which was not impact tested, is being evaluated for suitability. Desired MDMT is -150 F. When operation will occur at this temperature general membrane stresses are such that the ratio as shown in Figure UCS-66.1 is less than 0.35.

Local stresses at nozzles and saddles will be evaluated using FEA and following Division 2 rules, but using the Sm value from Division 1.

Question: For Primary and Primary plus Secondary Stresses is it appropriate in this case to use 1.5 times Sm and 3 times the average of Sm (cold) and Sm (hot) as prescribed by the Code for the allowable local stresses?

 

[MJCronin]

Ummmmmm,,, Carbon steel at -150F ..???

-MJC

 

[gr2vessels]

1.5 and 3 times are in accordance with the code and normal practices, I would however consider them too optimistic and use some conservative 1 and 2-2.5 times Sm.
Is your carbon steel vessels actual thickness less than 0.3"? If not, refer to MJC reply...
cheers,
gr2vessels

 

[spayette]

gr2vessels,

Thanks for your response. It seems reasonable, however, is there anything in technical literature to support your recommendation?

To answer your guestion, the vessel was originally impact tested at -55F. Only during infrequent upset conditions would it be possible for the vessel to be exposed to the minus 150F temperature. When this occurs the internal pressure is very low and the thickness ratio per UCS 66.1 is much less than .35. The nominal thickness of the equipment is 0.5".

 

[TGS4]

spayette,

That is actually a very astute question that you have raised. It raises issues of a failure mode (brittle fracture) that the 1.5Sm and 3Sm limits do not address. I would be very hesitant to use any limit in excess of that given by UCS-66.

Given the severity of the failure mode (brittle/rapid fracture), I would actually suggest that you address this question directly to the ASME Section VIII Subgroup Design Code Committee. The contact information is here -

http://cstools.asme.org/csconnect/CommitteePages.cfm?Committee=N20110300.

The requirements for asking the question are located in Appendix 16.

 

[TGS4]

After thinking about this some more, I realized that what you are doing is more of a re-rate - therefore, for your particular case, I would recommend that you refer to API-579/ASME FFS-1, Part 3 "Assessment of Existing Equipment for Brittle Fracture".

 

[MJCronin]

Sprayette...

Unless I am mistaken, all simple carbon steels will experience a transition to brittle behavior (and failure) at some nill-ductility transition temperature.

http://steel.keytometals.com/Articles/Art98.htm

And this is why some choose to pay a premium and use austenitic materials at low temperatures.

Are we only simply arguing about which set of rules that will apply to your case..??

-MJC

 

[spayette]

TGS4,
Thanks for your replies. I appreciate your suggestion about forwarding this issue to ASME.

The owner has performed an API-579 Level 2 assessment and has concluded that the vessel is suitable for the -150F design temperature. I spent some time looking through API-579 but I could not find any guidance on allowable stress for local primary and secondary loadings other than what ASME mentions. I may follow your suggestion and go to the design code committee. Thanks again.