Creep design above 100,000 hours?

[MrPDes]

For time dependent properties, the stress values in ASME II Part D are for up to 100,000 hours only. I need to calculate an allowable stress for 150,0000 hours using the rules in ASME II Part D Mandatory Appendix 1. I have figured out how to derive Minimum and Average Rupture Strength from API 530, however I am having trouble deriving the "Average Stress to produce a Creep Rate of 0.01% per 1000 hours".

API 579-1/ASME FFS-1 Fitness for service looks like the place to derive this stress. However in this spec when I look up a creep damage rate of 10^-7 (equal to 0.01%/1000hours) I get a corresponding stress that is very close to 0.666 times the stress in ASME II Tables.

Also the allowable Creep Damage/Strain at the end of a components life in ASME FFS-1 shall be no greater than 0.25 = 25%.
For ASME II it is 0.01 = 1%.

Why is there a difference? How do you convert information in ASME FFS-1 to an allowable stress in ASME II. Is ASME FFS-1 the correct code to use?

(I need this allowable stress for a ASME VIII Div 1 vessel.)

 

[metengr]

The difference is the stress values published in ASME Section II, Part D are minimum values to ensure meeting either a minimum of 100,000 operating hours without creep rupture or achieving 1% creep strain in time dependent service, whichever is lowest. This does not mean the creep life of the component is 100,000 hours or when reaching 1% creep strain the component has reached end of creep life. In design, you need to have some basis to ensure minimum requirements have been met. In most cases, safe operation can/will extend well above 100,000 hours in creep service because of design margins and good engineering practices.

 

[MrPDes]

Thanks, I agree with you, however this will involve additional inspections to extend the equiments life beyond 100,000 hours.

I don't wish to introduce these additional inspections. I wish for a vessel designed from the start with the same design margins for 150,000 hours. I am having trouble re-deriving the "Average Stress to produce a Creep Rate of 0.01% per 1000 hours" from its original source.

 

[metengr]

Your question can be answered by is this for an in-service vessel or for design for construction?

 

[MrPDes]

I'm designing for construction.

"API 579-1/ASME FFS-1 Fitness for service" is for in-service inspection so probably is not the correct standard to use to calculate the ASME II average creep rate allowable stress but it is all I can find at the moment.

Do you know if this source standard for ASME II Part D is different from ASME FFS-1?

 

[metengr]

You can use the 100,000 hr or creep rate allowable stress from Section II, Part D and using the Larson Miller parametric equation to approximate a reduced allowable stress for 150,000 hours or x% creep rate.

 

[MrPDes]

You're suggesting I use existing stress values from Section II, Part D and reduce it by the percentage change of the Larson Miller parametric equation when going from 100000hrs to 150000hrs?

I am sort of already doing this type of approximation with "API 579-1/ASME FFS-1 Fitness for service" graphs.

 

[metengr]

Yes, I am. Design by analysis.