API 571 Creep Threshold

[A_Alrumaidh]

Dear,

I was studying the API 571 about damage mechanisms and specifically, Creep & Stress Rupture. One thing that caught my attention was Table 4-3 "Threshold Temperatures for Creep". In this table, different materials are listed with corresponding minimum temperatures at which creep becomes a "concern". This table gives reference to API 579 about FFS.

I went to API 579 and found the screening and damage curves. What I'm having doubt in is what was the criterion at which API 571 exported a set of curves into a single value. Creep is greatly impacted by stress. So, what was the stress considered in this extraction?

Thanks,

 

[TGS4]

You will note that there is small discrepancy between the values in API 571 Table 4-3 (identical to API 579-1/ASME FFS-1 Table 4.1), and the values at which the design allowable stress is governed by time-dependent properties (typically ASME Section II, Part D, shown in italics in the allowable stress tables).

If you look at the MPC Project Omega curves, below the temperatures in these two API documents, the stress levels need to be well above twice yield to have reduced life. Based on my experience, I would treat these API tables as thresholds - at or above, creep needs to be considered (although whether it governs indeed depends on stress).

 

[A_Alrumaidh]

TGS4,

I agree with you in that the temperature limits between ASME and API 579 are somewhat similar.
In ASME, it clearly states the criteria on which these limits were specified (0.01%/1,000hr or 100,000 minimum rupture life).
API 579, assessment level 1, specifies a maximum damage of 25%. Assuming a design life of 100,000 hr, the creep rate would be 0.25%/1,000hr, which is 25 times the criterion specified in ASME II-D!!

That's why I'm having a doubt. How can the temperature limits in API and ASME be similar if the difference in criteria is high?

 

[TGS4]

Don't conflate the temperature threshold at which creep becomes relevant with a damage limit. The two are very different measures, and not at all related.

 

[A_Alrumaidh]

In that case, on what basis did API 579-1 specify the temperature thresholds in Table 4.1? Is there a criterion similar to the one written in ASME 2D?