Load factors for ASME VIII Div.2 5.3.3 Elastic-Plastic Analysis 2

[Bill4037]

Hello Guys,

I am designing a replacement unit for a heat exchanger. The previous unit has failed at the tube to tubesheet joints due to low cycle fatigue. The new design reduced the tubesheet thickness to reduce the stress at the tube to tubesheet joint. The design of the components without failure remained the same. I performed an elastic analysis in accordance with ASME VIII Div.2 Part 5. The design passed the limits on primary stress, fatigue assessment in accordance with 5.5.3 and ratcheting assessment in accordance with 5.5.6. However, the channel barrel to tubesheet joint has failed against the design limits on the primary plus secondary stress and also triaxial stress limit in accordance with 5.3.2. Most of the stress at this location is due to the differential expansion between the channel barrel and tubesheet. No failure has been reported at this joint on the previous unit. The previous design has shown similar stress levels at this location. Therefore, I do not think the overstress is due to the changed tubesheet thickness. The previous unit was designed to a different code using the elastic plastic analysis. Therefore, I am going to perform an elastic plastic analysis to ASME VIII Div.2.
my questions are:
1. I think both primary plus secondary stress limit and Triaxial stress limits from 5.3.2 are for protection against local failure. Therefore, the elastic plastic analysis should be performed in accordance with ASME VIII Div.2 5.3.3 to demonstrate the protection against local failure. Is my understanding correct?
2. In the table 5.5, there is no load case combinations including the thermal loads for local failure assessment. What load factor should I use for this analysis?

Your assistance would be greatly appreciated.


Kind regards,

Bill

 

[TGS4]

Regarding the Triaxial stress limit, please refer to the commentary in ASME PTB-1 (2014):

Two issues that are apparent is the use of an elastic stress basis for a local criterion and the
stress category that is used with this criterion. It is not apparent how pseudo elastic stresses, i.e. elastically calculated stresses that exceed the yield strength can be used to evaluate a local fracture strain of a ductile material with strain hardening. In addition, the type of stress used in the criterion (i.e. linearized or average values verse stress at a point) and stress category (i.e. primary, secondary and peak) needs to be resolved. Since local failure is the failure mode being evaluated, the type of stress and stress category used in VIII-3 would appear to more correct. For ductile materials, a local criterion based on elastic analysis may not meaningful and the elastic-plastic method that follows is recommended for all applications.

The elastic primary-plus-secondary stress limit is applicable for Protection Against Failure From Cyclic Loading: Ratcheting.

Focus on each failure mode by themselves - each one has different design margins.

Regarding your second question about Local Failure and Table 5.5 load factors (design margins), and the lack of inclusion of thermal loads - all that I can help you with is that the Code Committee could not determine an appropriate design margin for that load combination. Therefore, it is the responsibility of the User and the Designer to determine appropriate load factors for these circumstances.

 

[Bill4037]

Hi TGS4,

Thanks very much for your comprehensive response.

Kind Regards,

Bill