Existing Pressure Vessels--Allowable Stress Range for Primary + Secondary Stresses 1

[pvrerate]

Is it appropriate to apply the criterion of ASME Section VIII, Division 2 paragraph 5.5.6.1(d)(2) to vessels built prior to the 2001 Code Edition? (If I am correct this additional requirement was added in 2001.)

For rerating existing equipment we follow API-510. For analyses of loadings on nozzles and attachments we utilize FEPipe/NozzlePro, which follows the Rules of Division 2, to analyze local stresses. The conservative approach for heritage equipment is not to take credit for the additional criterion as mentioned in paragraph 5.5.6.1(d)(2). However, such an approach may show that a component is failing perhaps unnecessarily. In order to decide whether or not this requirement can be applied to heritage vessels it would be most beneficial if someone could answer the following:

What brought about this change in the Code that allows designs to be less conservative? Was it that our understanding of material properties and behavior have gotten better? Was it because material properties have improved through the years or that the predictability of material properties has improved or the quality of manufacturing and testing has improved or was it something else? In essence, what was the justification for this change in the Code or said another way, what was the driving force behind approving this change?

Thanks.

 

[TGS4]

In my opinion, yes it would be acceptable. For those reading this, the OP is referring to the "new" option for the limit on primary+secondary membrane+bending of 2Sy (actually the limit, Sps, is defined as the greater of 3S or 2Sy).

If you look back to the Criteria Document written in 1972, you will find that the definition of a ratcheting limit is actually 2Sy. However, when Division 2 was originally written, they only wanted rules that were multiples of S. So, this was really implementing a rule that should have been written in the first place.

HOWEVER, I will caution you that this limit is placed on the stress range over the worst case load ranges. You are NOT to use the Design Load Combinations as described in Table 5.3. (This is a common mistake that many engineers make...).

 

[pvrerate]

Thanks TGS4 for your reply. You provided the clarification that we were seeking.