TIME DEPENDENT PROPERTIES -EXAMPLE SA 240 316

[brkmech1234]

The note no T8 of Table 1A of ASME II Part D mentions "Allowable stresses for temperatures of 750F and above are values obtained from time dependent properties". How this will affect the vessel design?

 

[muld0020]

In time dependant material properties Creep failure is an additional concern. When you are in time dependent properties a detailed fatigue analysis is required. That anaylsis is time consuming and can be expensive. Depending on design temperature like say 800F, it is generally easier to just change the material from carbon steel (SA516 Gr70) to a CrMoly with a higher limit for time-dependant properties.

 

[TGS4]

When you are in time dependent properties a detailed fatigue analysis is required.

That's an interesting claim. Care to provide a reference to that?

brkmech1234 - It means that that allowable based on the lower of:

(1) 100% of the average stress to produce a creep rate of 0.01%/1,000 hr
(2) 100*Favg% of the average stress to cause rupture at the end of 100,000 hr
(3) 80% of the minimum stress to cause rupture at the end of 100,000 hr
(Where Favg is defined as: At 1500°F and below, Favg=0.67. Above 1500°F, it is determined from the slope of the log time-to-rupture vs log stress plot at 100,000 hr such that Favg=1/n, but it may not exceed 0.67.

What this means from a design perspective is, in my opinion, the following:
a) Not much - vessels are designed in the creep regime all the time without much difficulty. However, the life will be limited, depending on the OPERATING stress levels and OPERATING temperature.
b) A fatigue analysis to ASME Section VIII, Division 2, Part 5 is not permitted, as the current rules are explicitly in the non-creep regime. If you will be cycling pressure or temperature or both, then I would recommend that you contact someone with creep-fatigue evaluation experience.
c) Evaluation of nozzle loads may or may not be more complicated. If you stick with WRC 537 (previously WRC 107), and the allowable stresses then, based on my experience, you'll be fine.

 

[brkmech1234]

Thank you TGS4.