ASME VIII DIV 2 - PART 5 DBA 1

[td_eng]

Hi guys,

I have a question regarding ASME VIII Div 2 (2010):

1) Are the load case combinations in Table 5.5 applicable for the elastic-plastic fatigue assessment (section 5.5.4), as it doesn't actually reference the load case combinations in the section. In comparison, elastic-plastic protection against local failure (section 5.3.3) references Table 5.5 in the first step. - Although the loading conditions overview (section 5.1.3) does specify 'load case combinations shall be considered for elastic, limit load, and elastic-plastic analysis'.

This question relates to a pressure vessel I'm designing. During the hydrostatic pressure test analysis, stress and strain values are high in a sharp corner (radius or no radius), which leads to fatigue damage failure

Thanks in advance.

 

[cbPVme]

Why are you designing a new vessel to the 2010 edition of Div 2? This is several editions out of date. 2019 is the current edition. There are new load case combinations in the latest code that should clarify your questions.

 

[TGS4]

Hold on - let me get this straight - you are performing a fatigue analysis using the design load combinations (which are factored), even though the rules tell you to perform the fatigue analysis using the actual operating conditions?

 

[td_eng]

Thank you both for your replies.

I've acquired a copy of the 2019 edition.

TGS4, yes that's right. My question would be, is that correct? .. As the general requirements state to use design load combinations (factored) for any elastic-plastic analysis; however, the fatigue analysis section doesn't actually specify design load combinations. I take it that design load combinations shouldn't be used for fatigue analysis?

 

[TGS4]

Absolutely not. A fatigue analysis is intended to evaluate the fatigue performance of the equipment under ACTUAL operating conditions. The design margin is still built in you the fatigue curved. The rules after very clear on this point.

 

[IdanPV]

@TGS4,
Example E5.5.4 of PTB-3 stated that the load to be considered is a full internal pressure cycle from 0 psig to 1000 psig although the internal pressure of the vessel (Figure E5.2.1-7) is only 420 psig.
Isn't it means that the loads for the fatigue assessment need to by multiplied by some kind of factor? or, am I missing something?

 

[TGS4]

@IdanPV
That seems to me to be a really bad example. I will make a note that when PTB-3 is re-written that a better example needs to be provided. It is indeed misleading.

Take a look at the Assessment procedure in 5.5.3.2, Step 1. It references the operating loads.

Also, take a look at API 579-1, 14.4.3.3. In 14.4.3.3.1 a) STEP 1, again it references operating loads, and there is no mention of an factors to be applied.

When the 2023 Edition of VIII-2 is issued, the fatigue rules are being rewritten to align more closely with API 579-1/ASME FFS-1. I will make sure that this point is more clearly laid out. In the meantime, if it would be helpful, an Interpretation request could be submitted, making specific reference to PTB-3 Example 5.5.4. If anyone makes such a request, please let this forum know the ASME tracking number for that Interpretation Request, and I will make sure to shepherd it through the system.

 

[BJI]

The PTB-3 example states the following under step 1:

Per the User’s Design Specification (see Example E5.5.2), a full internal pressure cycle is the only applicable event to be considered. To better illustrate the method and generate plastic strain in the model, the operating pressure will be taken as 1000 psig for this example.