Load factors of local criteria in Elastic-Plastic analysis

[Mm.Kaiser]

Hi,
According to table 5.5 of ASME VIII-2 we can see two different load factor for Local Criteria (in brief):
1.7*P
2.3*P (in Hydrostatic test)

I want to know since we have Hydrostatic test with 2.3*P and calculating "εL:limiting triaxial strain" in that load case, does it have a meaning to rerun the analysis with 1.7*P ?!

Is it true that 1.7*P will be the basis of design whenever we don't have a Hydrostatic test?

The other question is why we shouldn't calculate εL in the load case of 2.4*P (Global Criteria) although we have a lots of discontinuity and concentrations such as fillets in our equipment.

Best Regards

 

[TGS4]

The only Design Load Case Combination required for demonstrating Protection Against Local Failure is shown in Table 5.5 as 1.7*(P+Ps+D). That's it. That's all.

The writers of the Code determined:
1) That a design margin of 1.7 was appropriate for this failure mode. This design margin is different from the design margin for Protection Against Plastic Collapse. That's fine - different design margins are used for different failure modes throughout the Code.
2) That this was the sole Design Load Case Combination necessity to be evaluated for this failure mode.

The other Design Load Case Combination that you mention regarding the hydrostatic test condition is a further extension of the Design Load Case Combinations that are necessary to be checked in order to demonstrate Protection Against Plastic Collapse. You will note that this Design Load Case Combination does not actually referenced the chosen/selected/actual hydrostatic test pressure in the current rules. Although this will remain so when the 2015 Edition is released, that will change in the 2017 Edition (hopefully).

 

[Mm.Kaiser]

That's right,
But if in Local criteria just there is one design load case 1.7*(P+Ps+D), why in table 5.5 under Hydrostatic test conditions, it wrote "Global and Local criteria" ? It could be only "Global criteria"! I've searched in ASME-VIII-2 and couldn't find any word like "Local criteria" except in 5.3.
Thanks

 

[TGS4]

That was not the intent. I will include fixing that error when we fix the hydrostatic test condition load combinations that will reference the actual test pressure.

 

[Mm.Kaiser]

Could you please explain more?

 

[TGS4]

Your interpretation of the word in Table 5.5 (and similar words in Table 5.4) and not entirely unreasonable. However, test interpretation is incorrect and not (in my opinion) consistent with the intent of the Code.

I am member of WG-DBA (VIII) and the project manager for a code item to fix the description of the hydrostatic test condition load cases in Tables 5.3, 5.4, and 5.5. Since you have pointed out a potential interpretation issue with the words in the tables, I will include in my proposal to fix that.

Clear?

 

[Mm.Kaiser]

Right. I got it.
According to your Statement, the procedure of E-P analysis will be so (in brief):

1-Global Criteria; Perform E-P analysis with 2.4*(P+Ps+D) to achieve convergence
2-Hydrostatic test; Perform E-P analysis with max[2.3,2(St/S)]*(P+Ps+D)+Wpt to achieve convergence (so if we don't have Wpt and St=S, this calculation will be omitted)
3-Local Criteria; Perform E-P analysis with 1.7*(P+Ps+D) and calculate εL

If this procedure is correct, so i am satisfied.

Kind Regards

 

[TGS4]

You got it. Of course for step 1, you should be doing ALL of the Design Load Case Combinations listed in Table 5.5.

 

[Mm.Kaiser]

Roger. Thanks.

As you mentioned, We have to consider all of the design load case combinations, but my project is a Kind of Blind Flange (which attached to the vessel) so i think i shouldn't consider the effect of Wind load, Earthquake load and Snow load to that Flange although those Forces were considered in design of Main part(Vessel) in PVElite according to VIII-1.

Am i Right? If not, does it have a meaning and how i can consider for example Earthquake load for a Flange? Instead of those load and for more safety, i have increased my thicknesses so they can tolerate up to 2.8*(P+Ps+D)!

Regards.

 

[TGS4]

My preference is to leave the engineering judgement of what loads are appropriate to the design engineer (and the jurisdiction, as applicable).

 

[Mm.Kaiser]

I respect your preference.
But i want you as a consultant to help me as a design engineer: how should i consider Earthquake load for a Flange in FEA? I decide to consider all load combinations but if applicable. what is your opinion about my last post?

 

[TGS4]

Unfortunately, in this type of forum, this is about all of the assistance that I can provide. Perhaps off-line more assistance could be rendered.

 

[beastgod]

One more question please.

Is the pressure in global criteria of design condition of table 5.5 equal the pressure of hydrostatic conditions???
I always think that hydrostatic pressure greater.

so
"1-Global Criteria; Perform E-P analysis with 2.4*(P+Ps+D) to achieve convergence
2-Hydrostatic test; Perform E-P analysis with max[2.3,2(St/S)]*(P+Ps+D)+Wpt to achieve convergence (so if we don't have Wpt and St=S, this calculation will be omitted)"

if Wpt=0 and we got factor 2.3 then we should check this "max[2.3,2(St/S)]*(P+Ps+D)+Wpt" anyway.

Am I right?

Thanks!

 

[TGS4]

The hydrostatic test condition is, as noted above, poorly written. It should be fixed for 2017 Edition...

Your interpretation of the rules, as written, is, in my opinion, not unreasonable.