Allowable from 1A for Stress Analysis ? 3

[BPVFEA]

We are frequently facing this situation. The Vessel as per Div.1. Since some part of it was not possible to Design By Rules we carry out stress analysis by Div-2. We linearise Von Misses stress and compare it with allowable from table 5A (Von Misses allowable).
However customer/third party insists that allowable should be taken from Table 1A since it is division-1 vessel.
As per my opinion, we should take allowable from 5A since it is EQUIVALENT VON MISSES stress. Similarly for old code, I would take allowable from 2A since it was
Linearised STRESS INTENSITY.
I find it inappropriate to Linearise Von Misses Stress and compare it with Allowable DIRECT Stress.
Had it been linearised DIRECT stress (Radial/Tangential), it would have been appropriate to take from Table 1A.
When there is enough safety margin, it may be conservative to take from 1A. But what to do when stresses are more than 1A allowables?
Please tell me what is technically correct to use?
Thanks in advance

 

[prex]

You should use Div.1 allowables.
First thing to be said is that, as a concept, Div.1 and Div.2 allowables are equivalent, they are used to limit the stresses based on similar principles, what you say about direct stresses on one side and equivalent stresses on the other one makes no sense to me.
So they are equivalent in principle, but of course the values are different. In general Div.2 allowables are higher that for Div.1, and this is due to the more detailed analysis required (on the whole vessel, not only on those parts that you cannot justify by Div.1) and on generally more stringent requirements concerning materials, quality and tests.
So, as long as you stay within Div.1 with your vessel, you need to use the corresponding allowables, just as you use Div.1 criteria for examinations, permitted materials, and all that.
Seen from another perspective: the equivalent VonMises stress is just a method for determining a uniaxial stress for 2D and 3D distributions, that is then compared with values obtained from uniaxial material tests. Nothing more, nothing less.

prex

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[TGS4]

To add to what prex said, you need to pay special attention to the Code paragraphs that allow you to "go to Div. 2". That would be U-2(g), which states

This Division of Section VIII does not contain rules to cover all details of design and construction. Where complete details are not given, it is intended that the Manufacturer, subject to acceptance of the Inspector, shall provide details of design and construction which will be as safe as those provided by the rules of this Division.

In order to prove to the Inspector that your design is as safe as it would otherwise be in Div. 1, the generally-accepted engineering principle says use the Div. 1 allowable stresses.

As a side note about your post - please note that you are NOT linearizing the equivalent stresses. You are calculating an equivalent stress based on the linearized component stresses. The two are NOT the same, and you should be very very skeptical of any FEA software that claims to linearize stresses - my experience with several of the commercial software is that they are not in compliance with Annex 5.A.

 

[BPVFEA]

Thank you.I got it now. The strigent testing requirements in Div-2 justfy for higher allowable while less strigent testing requirement in Div-1 require high allowable stress.