General Creep Question

[jgibbs22]

Hey Everyone:

Had a general question regarding Creep concerns. If I have a SA-516 Gr. 70 vessel with a allowable temperature of 650 deg. F that runs for 45 hours at 800 deg. F:

1) IF the vessel will no longer see temperatures above 650 deg F, is there any concern for creep failure?
2) Does API 579 discuss any analysis methods to determine if running that hot for that time will be detrimental to the long term life of the vessel?

Thanks!

 

[boofi]

As far as question 2 goes, API 579 part 10 is for assessment of components operating in the creep range, including remaining life assessments.

 

[metengr]

You will have time dependent (creep deformation) exposure based on your stated conditions. The question is how much and is it significant? What was the actual membrane stress on the vessel during this time period? Based on an assume creep deformation rate for time dependent behavior, you can determine both creep deformation and using the LMP method a reduced creep life for comparison.

 

[jgibbs22]

I would have to calculate the membrane values, but I believe they were around 17ksi. So if we operated int he creep range for 44 hours and had a certain amount of creep deformation, then no longer operated in the creep range, is there still a concern for long term creep damage? Or would the effect be that for those 44 hours of operation, we in essence lowered the allowable stress the shell can see per the Code?

I know these are broad strokes, just trying to understand some of the general concerns with creep range operation. Thanks!

 

[metengr]

Most likely you did not sustain enough creep damage to effect remaining life based on the temperature you stated and short exposure time because you are not operating continously in the time dependent (creep) regime.

If anything, I would perform an inspection during the next outage and look for any local deformation and obtain wall thickness readings and hardness. This would be more valuable to me than any calculation.

 

[EdStainless]

If the short HT operation either changed the material properties or caused distortion (which could change stresses) then you have an issue.
Otherwise you should be OK. But an assessment is needed.

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P.E. Metallurgy, Plymouth Tube